2004/id/draft-ietf-http-v11-spec-05.txt

HTTP Working Group                              R. Fielding, UC Irvine
INTERNET-DRAFT                                          J. Gettys, DEC
<draft-ietf-http-v11-spec-05>                         J. C. Mogul, DEC
                                                   H. Frystyk, MIT/LCS
                                               T. Berners-Lee, MIT/LCS
Expires December 7, 1996                                  June 7, 1996

                Hypertext Transfer Protocol -- HTTP/1.1

Status of this Memo

This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, and
its working groups. Note that other groups may also distribute working
documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or made obsolete by other documents at any
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To learn the current status of any Internet-Draft, please check the
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Distribution of this document is unlimited. Please send comments to the
HTTP working group at <http-wg@cuckoo.hpl.hp.com>. Discussions of the
working group are archived at
<URL:http://www.ics.uci.edu/pub/ietf/http/>. General discussions about
HTTP and the applications which use HTTP should take place on the <www-
talk@w3.org> mailing list.

Abstract

The Hypertext Transfer Protocol (HTTP) is an application-level protocol
for distributed, collaborative, hypermedia information systems. It is a
generic, stateless, object-oriented protocol which can be used for many
tasks, such as name servers and distributed object management systems,
through extension of its request methods. A feature of HTTP is the
typing and negotiation of data representation, allowing systems to be
built independently of the data being transferred.

HTTP has been in use by the World-Wide Web global information initiative
since 1990. This specification defines the protocol referred to as
"HTTP/1.1".


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Table of Contents


HYPERTEXT TRANSFER PROTOCOL -- HTTP/1.1................................1

1 Introduction.........................................................7
 1.1 Purpose ..........................................................7
 1.2 Requirements .....................................................7
 1.3 Terminology ......................................................8
 1.4 Overall Operation ...............................................11

2 Notational Conventions and Generic Grammar..........................13
 2.1 Augmented BNF ...................................................13
 2.2 Basic Rules .....................................................15

3 Protocol Parameters.................................................16
 3.1 HTTP Version ....................................................16
 3.2 Uniform Resource Identifiers ....................................17
 3.3 Date/Time Formats ...............................................19
 3.4 Character Sets ..................................................21
 3.5 Content Codings .................................................22
 3.6 Transfer Codings ................................................23
 3.7 Media Types .....................................................24
 3.8 Product Tokens ..................................................26
 3.9 Quality Values ..................................................26
 3.10 Language Tags ..................................................27
 3.11 Entity Tags ....................................................27
 3.12 Range Units ....................................................28

4 HTTP Message........................................................28
 4.1 Message Types ...................................................28
 4.2 Message Headers .................................................29
 4.3 Message Body ....................................................30
 4.4 Message Length ..................................................30
 4.5 General Header Fields ...........................................31

5 Request.............................................................32
 5.1 Request-Line ....................................................32
 5.2 The Resource Identified by a Request ............................34
 5.3 Request Header Fields ...........................................35

6 Response............................................................35
 6.1 Status-Line .....................................................36
 6.2 Response Header Fields ..........................................38

7 Entity..............................................................38
 7.1 Entity Header Fields ............................................38
 7.2 Entity Body .....................................................39

8 Connections.........................................................40

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 8.1 Persistent Connections ..........................................40
 8.2 Message Transmission Requirements ...............................43

9 Method Definitions..................................................44
 9.1 Safe and Idempotent Methods .....................................45
 9.2 OPTIONS .........................................................45
 9.3 GET .............................................................46
 9.4 HEAD ............................................................46
 9.5 POST ............................................................47
 9.6 PUT .............................................................48
 9.7 DELETE ..........................................................49
 9.8 TRACE ...........................................................49

10 Status Code Definitions............................................49
 10.1 Informational 1xx ..............................................50
 10.2 Successful 2xx .................................................50
 10.3 Redirection 3xx ................................................52
 10.4 Client Error 4xx ...............................................55
 10.5 Server Error 5xx ...............................................59

11 Access Authentication..............................................60
 11.1 Basic Authentication Scheme ....................................62
 11.2 Digest Authentication Scheme ...................................62

12 Content Negotiation................................................63
 12.1 Server-driven Negotiation ......................................63
 12.2 Agent-driven Negotiation .......................................64
 12.3 Transparent Negotiation ........................................65

13 Caching in HTTP....................................................65
 13.2 Expiration Model ...............................................70
 13.3 Validation Model ...............................................75
 13.4 Response Cachability ...........................................80
 13.5 Constructing Responses From Caches .............................81
 13.6 Caching Negotiated Responses ...................................83
 13.7 Shared and Non-Shared Caches ...................................84
 13.8 Errors or Incomplete Response Cache Behavior ...................84
 13.9 Side Effects of GET and HEAD ...................................85
 13.10 Invalidation After Updates or Deletions .......................85
 13.11 Write-Through Mandatory .......................................86
 13.12 Cache Replacement .............................................86
 13.13 History Lists .................................................86

14 Header Field Definitions...........................................87
 14.1 Accept .........................................................87
 14.2 Accept-Charset .................................................89
 14.3 Accept-Encoding ................................................89
 14.4 Accept-Language ................................................90
 14.5 Accept-Ranges ..................................................91
 14.6 Age ............................................................91
 14.7 Allow ..........................................................92

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 14.8 Authorization ..................................................92
 14.9 Cache-Control ..................................................93
 14.10 Connection ...................................................100
 14.11 Content-Base .................................................101
 14.12 Content-Encoding .............................................101
 14.13 Content-Language .............................................102
 14.14 Content-Length ...............................................102
 14.15 Content-Location .............................................103
 14.16 Content-MD5 ..................................................104
 14.17 Content-Range ................................................105
 14.18 Content-Type .................................................107
 14.19 Date .........................................................107
 14.20 ETag .........................................................108
 14.21 Expires ......................................................108
 14.22 From .........................................................109
 14.23 Host .........................................................109
 14.24 If-Modified-Since ............................................110
 14.25 If-Match .....................................................111
 14.26 If-None-Match ................................................112
 14.27 If-Range .....................................................113
 14.28 If-Unmodified-Since ..........................................114
 14.29 Last-Modified ................................................114
 14.30 Location .....................................................115
 14.31 Max-Forwards .................................................115
 14.32 Pragma .......................................................116
 14.33 Proxy-Authenticate ...........................................117
 14.34 Proxy-Authorization ..........................................117
 14.35 Public .......................................................117
 14.36 Range ........................................................118
 14.37 Referer ......................................................120
 14.38 Retry-After ..................................................121
 14.39 Server .......................................................121
 14.40 Transfer-Encoding ............................................122
 14.41 Upgrade ......................................................122
 14.42 User-Agent ...................................................123
 14.43 Vary .........................................................123
 14.44 Via ..........................................................125
 14.45 Warning ......................................................126
 14.46 WWW-Authenticate .............................................128

15 Security Considerations...........................................128
 15.1 Authentication of Clients .....................................128
 15.2 Offering a Choice of Authentication Schemes ...................129
 15.3 Abuse of Server Log Information ...............................130
 15.4 Transfer of Sensitive Information .............................130
 15.5 Attacks Based On File and Path Names ..........................131
 15.6 Personal Information ..........................................131
 15.7 Privacy Issues Connected to Accept Headers ....................132
 15.8 DNS Spoofing ..................................................132
 15.9 Location Headers and Spoofing .................................133


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16 Acknowledgments...................................................133

17 References........................................................134

18 Authors' Addresses................................................138

19 Appendices........................................................139
 19.1 Internet Media Type message/http ..............................139
 19.2 Internet Media Type multipart/byteranges ......................140
 19.3 Tolerant Applications .........................................140
 19.4 Differences Between HTTP Entities and RFC 1521 Entities .......141
 19.5 Changes from HTTP/1.0 .........................................144
 19.6 Additional Features ...........................................145
 19.7 Compatibility with Previous Versions ..........................149
 19.8 Notes to the RFC Editor and IANA ..............................150


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1 Introduction

1.1 Purpose

The Hypertext Transfer Protocol (HTTP) is an application-level protocol
for distributed, collaborative, hypermedia information systems. HTTP has
been in use by the World-Wide Web global information initiative since
1990. The first version of HTTP, referred to as HTTP/0.9, was a simple
protocol for raw data transfer across the Internet. HTTP/1.0, as defined
by RFC 1945 [6], improved the protocol by allowing messages to be in the
format of MIME-like messages, containing metainformation about the data
transferred and modifiers on the request/response semantics. However,
HTTP/1.0 does not sufficiently take into consideration the effects of
hierarchical proxies, caching, the need for persistent connections, and
virtual hosts. In addition, the proliferation of incompletely-
implemented applications calling themselves "HTTP/1.0" has necessitated
a protocol version change in order for two communicating applications to
determine each other's true capabilities.

This specification defines the protocol referred to as "HTTP/1.1". This
protocol includes more stringent requirements than HTTP/1.0 in order to
ensure reliable implementation of its features.

Practical information systems require more functionality than simple
retrieval, including search, front-end update, and annotation. HTTP
allows an open-ended set of methods that indicate the purpose of a
request. It builds on the discipline of reference provided by the
Uniform Resource Identifier (URI) [3][20], as a location (URL) [4] or
name (URN) , for indicating the resource to which a method is to be
applied. Messages are passed in a format similar to that used by
Internet mail as defined by the Multipurpose Internet Mail Extensions
(MIME) .

HTTP is also used as a generic protocol for communication between user
agents and proxies/gateways to other Internet systems, including those
supported by the SMTP [16], NNTP [13], FTP [18], Gopher [2], and WAIS
[10] protocols. In this way, HTTP allows basic hypermedia access to
resources available from diverse applications.


1.2 Requirements

This specification uses the same words as RFC 1123 [8] for defining the
significance of each particular requirement. These words are:


MUST
     This word or the adjective "required" means that the item is an
     absolute requirement of the specification.


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SHOULD
     This word or the adjective "recommended" means that there may exist
     valid reasons in particular circumstances to ignore this item, but
     the full implications should be understood and the case carefully
     weighed before choosing a different course.


MAY
     This word or the adjective "optional" means that this item is truly
     optional. One vendor may choose to include the item because a
     particular marketplace requires it or because it enhances the
     product, for example; another vendor may omit the same item.

An implementation is not compliant if it fails to satisfy one or more of
the MUST requirements for the protocols it implements. An implementation
that satisfies all the MUST and all the SHOULD requirements for its
protocols is said to be "unconditionally compliant"; one that satisfies
all the MUST requirements but not all the SHOULD requirements for its
protocols is said to be "conditionally compliant."


1.3 Terminology

This specification uses a number of terms to refer to the roles played
by participants in, and objects of, the HTTP communication.

connection
  A transport layer virtual circuit established between two programs
  for the purpose of communication.

message
  The basic unit of HTTP communication, consisting of a structured
  sequence of octets matching the syntax defined in section 4 and
  transmitted via the connection.

request
  An HTTP request message, as defined in section 5.

response
  An HTTP response message, as defined in section 6.

resource
  A network data object or service that can be identified by a URI, as
  defined in section 3.2. Resources may be available in multiple
  representations (e.g. multiple languages, data formats, size,
  resolutions) or vary in other ways.

entity
  The information transferred as the payload of a request or response.
  An entity consists of metainformation in the form of entity-header


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  fields and content in the form of an entity-body, as described in
  section 7.

representation
  An entity included with a response that is subject to content
  negotiation, as described in section 12. There may exist multiple
  representations associated with a particular response status.

content negotiation
  The mechanism for selecting the appropriate representation when
  servicing a request, as described in section 12. The representation
  of entities in any response can be negotiated (including error
  responses).

variant
  A resource may have one, or more than one, representation(s)
  associated with it at any given instant. Each of these
  representations is termed a `variant.' Use of the term `variant' does
  not necessarily imply that the resource is subject to content
  negotiation.

client
  A program that establishes connections for the purpose of sending
  requests.

user agent
  The client which initiates a request. These are often browsers,
  editors, spiders (web-traversing robots), or other end user tools.

server
  An application program that accepts connections in order to service
  requests by sending back responses. Any given program may be capable
  of being both a client and a server; our use of these terms refers
  only to the role being performed by the program for a particular
  connection, rather than to the program's capabilities in general.
  Likewise, any server may act as an origin server, proxy, gateway, or
  tunnel, switching behavior based on the nature of each request.

origin server
  The server on which a given resource resides or is to be created.

proxy
  An intermediary program which acts as both a server and a client for
  the purpose of making requests on behalf of other clients. Requests
  are serviced internally or by passing them on, with possible
  translation, to other servers. A proxy must implement both the client
  and server requirements of this specification.

gateway
  A server which acts as an intermediary for some other server. Unlike
  a proxy, a gateway receives requests as if it were the origin server

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  for the requested resource; the requesting client may not be aware
  that it is communicating with a gateway.

tunnel
  An intermediary program which is acting as a blind relay between two
  connections. Once active, a tunnel is not considered a party to the
  HTTP communication, though the tunnel may have been initiated by an
  HTTP request. The tunnel ceases to exist when both ends of the
  relayed connections are closed.

cache
  A program's local store of response messages and the subsystem that
  controls its message storage, retrieval, and deletion. A cache stores
  cachable responses in order to reduce the response time and network
  bandwidth consumption on future, equivalent requests. Any client or
  server may include a cache, though a cache cannot be used by a server
  that is acting as a tunnel.

cachable
  A response is cachable if a cache is allowed to store a copy of the
  response message for use in answering subsequent requests. The rules
  for determining the cachability of HTTP responses are defined in
  section 13. Even if a resource is cachable, there may be additional
  constraints on whether a cache can use the cached copy for a
  particular request.

first-hand
  A response is first-hand if it comes directly and without unnecessary
  delay from the origin server, perhaps via one or more proxies. A
  response is also first-hand if its validity has just been checked
  directly with the origin server.

explicit expiration time
  The time at which the origin server intends that an entity should no
  longer be returned by a cache without further validation.

heuristic expiration time
  An expiration time assigned by a cache when no explicit expiration
  time is available.

age
  The age of a response is the time since it was sent by, or
  successfully validated with, the origin server.

freshness lifetime
  The length of time between the generation of a response and its
  expiration time.

fresh
  A response is fresh if its age has not yet exceeded its freshness
  lifetime.

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stale
  A response is stale if its age has passed its freshness lifetime.

semantically transparent
  A cache behaves in a "semantically transparent" manner, with respect
  to a particular response, when its use affects neither the requesting
  client nor the origin server, except to improve performance. When a
  cache is semantically transparent, the client receives exactly the
  same response (except for hop-by-hop headers) that it would have
  received had its request been handled directly by the origin server.

validator
  A protocol element (e.g., an entity tag or a Last-Modified time) that
  is used to find out whether a cache entry is an equivalent copy of an
  entity.


1.4 Overall Operation

The HTTP protocol is a request/response protocol. A client sends a
request to the server in the form of a request method, URI, and protocol
version, followed by a MIME-like message containing request modifiers,
client information, and possible body content over a connection with a
server. The server responds with a status line, including the message's
protocol version and a success or error code, followed by a MIME-like
message containing server information, entity metainformation, and
possible entity-body content. The relationship between HTTP and MIME is
described in appendix 19.4.

Most HTTP communication is initiated by a user agent and consists of a
request to be applied to a resource on some origin server. In the
simplest case, this may be accomplished via a single connection (v)
between the user agent (UA) and the origin server (O).

          request chain ------------------------>
       UA -------------------v------------------- O
          <----------------------- response chain

A more complicated situation occurs when one or more intermediaries are
present in the request/response chain. There are three common forms of
intermediary: proxy, gateway, and tunnel. A proxy is a forwarding agent,
receiving requests for a URI in its absolute form, rewriting all or part
of the message, and forwarding the reformatted request toward the server
identified by the URI. A gateway is a receiving agent, acting as a layer
above some other server(s) and, if necessary, translating the requests
to the underlying server's protocol. A tunnel acts as a relay point
between two connections without changing the messages; tunnels are used
when the communication needs to pass through an intermediary (such as a
firewall) even when the intermediary cannot understand the contents of
the messages.


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          request chain -------------------------------------->
       UA -----v----- A -----v----- B -----v----- C -----v----- O
          <------------------------------------- response chain

The figure above shows three intermediaries (A, B, and C) between the
user agent and origin server. A request or response message that travels
the whole chain will pass through four separate connections. This
distinction is important because some HTTP communication options may
apply only to the connection with the nearest, non-tunnel neighbor, only
to the end-points of the chain, or to all connections along the chain.
Although the diagram is linear, each participant may be engaged in
multiple, simultaneous communications. For example, B may be receiving
requests from many clients other than A, and/or forwarding requests to
servers other than C, at the same time that it is handling A's request.

Any party to the communication which is not acting as a tunnel may
employ an internal cache for handling requests. The effect of a cache is
that the request/response chain is shortened if one of the participants
along the chain has a cached response applicable to that request. The
following illustrates the resulting chain if B has a cached copy of an
earlier response from O (via C) for a request which has not been cached
by UA or A.

          request chain ---------->
       UA -----v----- A -----v----- B - - - - - - C - - - - - - O
          <--------- response chain

Not all responses are usefully cachable, and some requests may contain
modifiers which place special requirements on cache behavior. HTTP
requirements for cache behavior and cachable responses are defined in
section 13.

In fact, there are a wide variety of architectures and configurations of
caches and proxies currently being experimented with or deployed across
the World Wide Web; these systems include national hierarchies of proxy
caches to save transoceanic bandwidth, systems that broadcast or
multicast cache entries, organizations that distribute subsets of cached
data via CD-ROM, and so on. HTTP systems are used in corporate intranets
over high-bandwidth links, and for access via PDAs with low-power radio
links and intermittent connectivity. The goal of HTTP/1.1 is to support
the wide diversity of configurations already deployed while introducing
protocol constructs that meet the needs of those who build web
applications that require high reliability and, failing that, at least
reliable indications of failure.

HTTP communication usually takes place over TCP/IP connections. The
default port is TCP 80, but other ports can be used. This does not
preclude HTTP from being implemented on top of any other protocol on the
Internet, or on other networks. HTTP only presumes a reliable transport;
any protocol that provides such guarantees can be used; the mapping of
the HTTP/1.1 request and response structures onto the transport data

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units of the protocol in question is outside the scope of this
specification.

In HTTP/1.0, most implementations used a new connection for each
request/response exchange. In HTTP/1.1, a connection may be used for one
or more request/response exchanges, although connections may be closed
for a variety of reasons (see section 8.1).


2 Notational Conventions and Generic Grammar


2.1 Augmented BNF

All of the mechanisms specified in this document are described in both
prose and an augmented Backus-Naur Form (BNF) similar to that used by
RFC 822 [9]. Implementers will need to be familiar with the notation in
order to understand this specification. The augmented BNF includes the
following constructs:


name = definition
     The name of a rule is simply the name itself (without any enclosing
     "<" and ">") and is separated from its definition by the equal "="
     character. Whitespace is only significant in that indentation of
     continuation lines is used to indicate a rule definition that spans
     more than one line. Certain basic rules are in uppercase, such as
     SP, LWS, HT, CRLF, DIGIT, ALPHA, etc. Angle brackets are used
     within definitions whenever their presence will facilitate
     discerning the use of rule names.


"literal"
     Quotation marks surround literal text. Unless stated otherwise, the
     text is case-insensitive.


rule1 | rule2
     Elements separated by a bar ("|") are alternatives, e.g., "yes |
     no" will accept yes or no.


(rule1 rule2)
     Elements enclosed in parentheses are treated as a single element.
     Thus, "(elem (foo | bar) elem)" allows the token sequences
     "elem foo elem" and "elem bar elem".


*rule
     The character "*" preceding an element indicates repetition. The
     full form is "<n>*<m>element" indicating at least <n> and at most

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     <m> occurrences of element. Default values are 0 and infinity so
     that "*(element)" allows any number, including zero; "1*element"
     requires at least one; and "1*2element" allows one or two.


[rule]
     Square brackets enclose optional elements; "[foo bar]" is
     equivalent to "*1(foo bar)".


N rule
     Specific repetition: "<n>(element)" is equivalent to
     "<n>*<n>(element)"; that is, exactly <n> occurrences of (element).
     Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three
     alphabetic characters.


#rule
     A construct "#" is defined, similar to "*", for defining lists of
     elements. The full form is "<n>#<m>element " indicating at least
     <n> and at most <m> elements, each separated by one or more commas
     (",") and optional linear whitespace (LWS). This makes the usual
     form of lists very easy; a rule such as
     "( *LWS element *( *LWS "," *LWS element )) " can be shown as
     "1#element". Wherever this construct is used, null elements are
     allowed, but do not contribute to the count of elements present.
     That is, "(element), , (element) " is permitted, but counts as only
     two elements. Therefore, where at least one element is required, at
     least one non-null element must be present. Default values are 0
     and infinity so that "#element" allows any number, including zero;
     "1#element" requires at least one; and "1#2element" allows one or
     two.


; comment
     A semi-colon, set off some distance to the right of rule text,
     starts a comment that continues to the end of line. This is a
     simple way of including useful notes in parallel with the
     specifications.


implied *LWS
     The grammar described by this specification is word-based. Except
     where noted otherwise, linear whitespace (LWS) can be included
     between any two adjacent words (token or quoted-string), and
     between adjacent tokens and delimiters (tspecials), without
     changing the interpretation of a field. At least one delimiter
     (tspecials) must exist between any two tokens, since they would
     otherwise be interpreted as a single token.


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2.2 Basic Rules

The following rules are used throughout this specification to describe
basic parsing constructs. The US-ASCII coded character set is defined by
ANSI X3.4-1986 [21].

       OCTET          = <any 8-bit sequence of data>
       CHAR           = <any US-ASCII character (octets 0 - 127)>
       UPALPHA        = <any US-ASCII uppercase letter "A".."Z">
       LOALPHA        = <any US-ASCII lowercase letter "a".."z">
       ALPHA          = UPALPHA | LOALPHA
       DIGIT          = <any US-ASCII digit "0".."9">
       CTL            = <any US-ASCII control character
                        (octets 0 - 31) and DEL (127)>
       CR             = <US-ASCII CR, carriage return (13)>
       LF             = <US-ASCII LF, linefeed (10)>
       SP             = <US-ASCII SP, space (32)>
       HT             = <US-ASCII HT, horizontal-tab (9)>
       <">            = <US-ASCII double-quote mark (34)>

HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all
protocol elements except the entity-body (see appendix 19.3 for tolerant
applications). The end-of-line marker within an entity-body is defined
by its associated media type, as described in section 3.7.

       CRLF           = CR LF

HTTP/1.1 headers can be folded onto multiple lines if the continuation
line begins with a space or horizontal tab. All linear white space,
including folding, has the same semantics as SP.

       LWS            = [CRLF] 1*( SP | HT )

The TEXT rule is only used for descriptive field contents and values
that are not intended to be interpreted by the message parser. Words of
*TEXT may contain characters from character sets other than ISO 8859-1
[22] only when encoded according to the rules of RFC 1522 [14].

       TEXT           = <any OCTET except CTLs,
                        but including LWS>

Hexadecimal numeric characters are used in several protocol elements.

       HEX            = "A" | "B" | "C" | "D" | "E" | "F"
                      | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT

Many HTTP/1.1 header field values consist of words separated by LWS or
special characters. These special characters MUST be in a quoted string
to be used within a parameter value.

       token          = 1*<any CHAR except CTLs or tspecials>

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       tspecials      = "(" | ")" | "<" | ">" | "@"
                      | "," | ";" | ":" | "\" | <">
                      | "/" | "[" | "]" | "?" | "="
                      | "{" | "}" | SP | HT

Comments can be included in some HTTP header fields by surrounding the
comment text with parentheses. Comments are only allowed in fields
containing "comment" as part of their field value definition. In all
other fields, parentheses are considered part of the field value.

       comment        = "(" *( ctext | comment ) ")"
       ctext          = <any TEXT excluding "(" and ")">

A string of text is parsed as a single word if it is quoted using
double-quote marks.

       quoted-string  = ( <"> *(qdtext) <"> )

       qdtext         = <any TEXT except <">>

The backslash character ("\") may be used as a single-character quoting
mechanism only within quoted-string and comment constructs.

       quoted-pair    = "\" CHAR


3 Protocol Parameters


3.1 HTTP Version

HTTP uses a "<major>.<minor>" numbering scheme to indicate versions of
the protocol. The protocol versioning policy is intended to allow the
sender to indicate the format of a message and its capacity for
understanding further HTTP communication, rather than the features
obtained via that communication. No change is made to the version number
for the addition of message components which do not affect communication
behavior or which only add to extensible field values. The <minor>
number is incremented when the changes made to the protocol add features
which do not change the general message parsing algorithm, but which may
add to the message semantics and imply additional capabilities of the
sender. The <major> number is incremented when the format of a message
within the protocol is changed.

The version of an HTTP message is indicated by an HTTP-Version field in
the first line of the message.

       HTTP-Version   = "HTTP" "/" 1*DIGIT "." 1*DIGIT

Note that the major and minor numbers MUST be treated as separate
integers and that each may be incremented higher than a single digit.

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Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is lower
than HTTP/12.3. Leading zeros MUST be ignored by recipients and MUST NOT
be sent.

Applications sending Request or Response messages, as defined by this
specification, MUST include an HTTP-Version of "HTTP/1.1". Use of this
version number indicates that the sending application is at least
conditionally compliant with this specification.

The HTTP version of an application is the highest HTTP version for which
the application is at least conditionally compliant.

Proxy and gateway applications must be careful when forwarding messages
in protocol versions different from that of the application. Since the
protocol version indicates the protocol capability of the sender, a
proxy/gateway MUST never send a message with a version indicator which
is greater than its actual version; if a higher version request is
received, the proxy/gateway MUST either downgrade the request version,
respond with an error, or switch to tunnel behavior. Requests with a
version lower than that of the proxy/gateway's version MAY be upgraded
before being forwarded; the proxy/gateway's response to that request
MUST be in the same major version as the request.

  Note: Converting between versions of HTTP may involve modification
  of header fields required or forbidden by the versions involved.


3.2 Uniform Resource Identifiers

URIs have been known by many names: WWW addresses, Universal Document
Identifiers, Universal Resource Identifiers , and finally the
combination of Uniform Resource Locators (URL)  and Names (URN) . As far
as HTTP is concerned, Uniform Resource Identifiers are simply formatted
strings which identify--via name, location, or any other characteristic-
-a resource.


3.2.1 General Syntax

URIs in HTTP can be represented in absolute form or relative to some
known base URI, depending upon the context of their use. The two forms
are differentiated by the fact that absolute URIs always begin with a
scheme name followed by a colon.

       URI            = ( absoluteURI | relativeURI ) [ "#" fragment ]

       absoluteURI    = scheme ":" *( uchar | reserved )

       relativeURI    = net_path | abs_path | rel_path


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       net_path       = "//" net_loc [ abs_path ]
       abs_path       = "/" rel_path
       rel_path       = [ path ] [ ";" params ] [ "?" query ]

       path           = fsegment *( "/" segment )
       fsegment       = 1*pchar
       segment        = *pchar

       params         = param *( ";" param )
       param          = *( pchar | "/" )

       scheme         = 1*( ALPHA | DIGIT | "+" | "-" | "." )
       net_loc        = *( pchar | ";" | "?" )
       query          = *( uchar | reserved )
       fragment       = *( uchar | reserved )

       pchar          = uchar | ":" | "@" | "&" | "=" | "+"
       uchar          = unreserved | escape
       unreserved     = ALPHA | DIGIT | safe | extra | national

       escape         = "%" HEX HEX
       reserved       = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+"
       extra          = "!" | "*" | "'" | "(" | ")" | ","
       safe           = "$" | "-" | "_" | "."
       unsafe         = CTL | SP | <"> | "#" | "%" | "<" | ">"
       national       = <any OCTET excluding ALPHA, DIGIT,
                        reserved, extra, safe, and unsafe>

For definitive information on URL syntax and semantics, see RFC 1738 [4]
and RFC 1808 [11]. The BNF above includes national characters not
allowed in valid URLs as specified by RFC 1738, since HTTP servers are
not restricted in the set of unreserved characters allowed to represent
the rel_path part of addresses, and HTTP proxies may receive requests
for URIs not defined by RFC 1738.

The HTTP protocol does not place any a priori limit on the length of a
URI. Servers MUST be able to handle the URI of any resource they serve,
and SHOULD be able to handle URIs of unbounded length if they provide
GET-based forms that could generate such URIs. A server SHOULD return
414 (Request-URI Too Long) status if a URI is longer than the server can
handle (see section 10.4.15).

  Note: Servers should be cautious about depending on URI lengths
  above 255 bytes, because some older client or proxy implementations
  may not properly support these lengths.


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3.2.2 http URL

The "http" scheme is used to locate network resources via the HTTP
protocol. This section defines the scheme-specific syntax and semantics
for http URLs.

       http_URL       = "http:" "//" host [ ":" port ] [ abs_path ]

       host           = <A legal Internet host domain name
                         or IP address (in dotted-decimal form),
                         as defined by Section 2.1 of RFC 1123>

       port           = *DIGIT

If the port is empty or not given, port 80 is assumed. The semantics are
that the identified resource is located at the server listening for TCP
connections on that port of that host, and the Request-URI for the
resource is abs_path. The use of IP addresses in URL's SHOULD be avoided
whenever possible (see RFC 1900 [24]). If the abs_path is not present in
the URL, it MUST be given as "/" when used as a Request-URI for a
resource (section 5.1.2).


3.2.3 URI Comparison

When comparing two URIs to decide if they match or not, a client SHOULD
use a case-sensitive octet-by-octet comparison of the entire URIs, with
these exceptions:

  .  A port that is empty or not given is equivalent to the default port
     for that URI;
  .  Comparisons of host names MUST be case-insensitive;
  .  Comparisons of scheme names MUST be case-insensitive;
  .  An empty abs_path is equivalent to an abs_path of "/".
Characters other than those in the "reserved" and "unsafe" sets (see
section 3.2) are equivalent to their ""%" HEX HEX" encodings.

For example, the following three URIs are equivalent:

      http://abc.com:80/~smith/home.html
      http://ABC.com/%7Esmith/home.html
      http://ABC.com:/%7esmith/home.html


3.3 Date/Time Formats


3.3.1 Full Date

HTTP applications have historically allowed three different formats for
the representation of date/time stamps:

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       Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
       Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036
       Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format

The first format is preferred as an Internet standard and represents a
fixed-length subset of that defined by RFC 1123  (an update to RFC 822
). The second format is in common use, but is based on the obsolete RFC
850  date format and lacks a four-digit year. HTTP/1.1 clients and
servers that parse the date value MUST accept all three formats (for
compatibility with HTTP/1.0), though they MUST only generate the RFC
1123 format for representing HTTP-date values in header fields.

  Note: Recipients of date values are encouraged to be robust in
  accepting date values that may have been sent by non-HTTP
  applications, as is sometimes the case when retrieving or posting
  messages via proxies/gateways to SMTP or NNTP.

All HTTP date/time stamps MUST be represented in Greenwich Mean Time
(GMT), without exception. This is indicated in the first two formats by
the inclusion of "GMT" as the three-letter abbreviation for time zone,
and MUST be assumed when reading the asctime format.

       HTTP-date    = rfc1123-date | rfc850-date | asctime-date

       rfc1123-date = wkday "," SP date1 SP time SP "GMT"
       rfc850-date  = weekday "," SP date2 SP time SP "GMT"
       asctime-date = wkday SP date3 SP time SP 4DIGIT

       date1        = 2DIGIT SP month SP 4DIGIT
                      ; day month year (e.g., 02 Jun 1982)
       date2        = 2DIGIT "-" month "-" 2DIGIT
                      ; day-month-year (e.g., 02-Jun-82)
       date3        = month SP ( 2DIGIT | ( SP 1DIGIT ))
                      ; month day (e.g., Jun  2)

       time         = 2DIGIT ":" 2DIGIT ":" 2DIGIT
                      ; 00:00:00 - 23:59:59

       wkday        = "Mon" | "Tue" | "Wed"
                    | "Thu" | "Fri" | "Sat" | "Sun"

       weekday      = "Monday" | "Tuesday" | "Wednesday"
                    | "Thursday" | "Friday" | "Saturday" | "Sunday"

       month        = "Jan" | "Feb" | "Mar" | "Apr"
                    | "May" | "Jun" | "Jul" | "Aug"
                    | "Sep" | "Oct" | "Nov" | "Dec"

  Note: HTTP requirements for the date/time stamp format apply only
  to their usage within the protocol stream. Clients and servers are


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  not required to use these formats for user presentation, request
  logging, etc.


3.3.2 Delta Seconds

Some HTTP header fields allow a time value to be specified as an integer
number of seconds, represented in decimal, after the time that the
message was received.

       delta-seconds  = 1*DIGIT


3.4 Character Sets

HTTP uses the same definition of the term "character set" as that
described for MIME:

  The term "character set" is used in this document to refer to a
  method used with one or more tables to convert a sequence of octets
  into a sequence of characters. Note that unconditional conversion
  in the other direction is not required, in that not all characters
  may be available in a given character set and a character set may
  provide more than one sequence of octets to represent a particular
  character. This definition is intended to allow various kinds of
  character encodings, from simple single-table mappings such as US-
  ASCII to complex table switching methods such as those that use ISO
  2022's techniques. However, the definition associated with a MIME
  character set name MUST fully specify the mapping to be performed
  from octets to characters. In particular, use of external profiling
  information to determine the exact mapping is not permitted.

  Note: This use of the term "character set" is more commonly
  referred to as a "character encoding." However, since HTTP and MIME
  share the same registry, it is important that the terminology also
  be shared.

HTTP character sets are identified by case-insensitive tokens. The
complete set of tokens is defined by the IANA Character Set registry
[19].

       charset = token

Although HTTP allows an arbitrary token to be used as a charset value,
any token that has a predefined value within the IANA Character Set
registry MUST represent the character set defined by that registry.
Applications SHOULD limit their use of character sets to those defined
by the IANA registry.


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3.5 Content Codings

Content coding values indicate an encoding transformation that has been
or can be applied to an entity. Content codings are primarily used to
allow a document to be compressed or otherwise usefully transformed
without losing the identity of its underlying media type and without
loss of information. Frequently, the entity is stored in coded form,
transmitted directly, and only decoded by the recipient.

       content-coding   = token

All content-coding values are case-insensitive. HTTP/1.1 uses content-
coding values in the Accept-Encoding (section 14.3) and Content-Encoding
(section 14.12) header fields. Although the value describes the content-
coding, what is more important is that it indicates what decoding
mechanism will be required to remove the encoding.

The Internet Assigned Numbers Authority (IANA) acts as a registry for
content-coding value tokens. Initially, the registry contains the
following tokens:


gzip An encoding format produced by the file compression program "gzip"
     (GNU zip) as described in RFC 1952 [25]. This format is a Lempel-
     Ziv coding (LZ77) with a 32 bit CRC.


compress
     The encoding format produced by the common UNIX file compression
     program "compress". This format is an adaptive Lempel-Ziv-Welch
     coding (LZW).

  Note: Use of program names for the identification of encoding
  formats is not desirable and should be discouraged for future
  encodings. Their use here is representative of historical practice,
  not good design. For compatibility with previous implementations of
  HTTP, applications should consider "x-gzip" and "x-compress" to be
  equivalent to "gzip" and "compress" respectively.

deflate The "zlib" format defined in RFC 1950[31] in combination with
     the "deflate" compression mechanism described in RFC 1951[29].

New content-coding value tokens should be registered; to allow
interoperability between clients and servers, specifications of the
content coding algorithms needed to implement a new value should be
publicly available and adequate for independent implementation, and
conform to the purpose of content coding defined in this section.


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3.6 Transfer Codings

Transfer coding values are used to indicate an encoding transformation
that has been, can be, or may need to be applied to an entity-body in
order to ensure "safe transport" through the network. This differs from
a content coding in that the transfer coding is a property of the
message, not of the original entity.

       transfer-coding         = "chunked" | transfer-extension

       transfer-extension      = token

All transfer-coding values are case-insensitive. HTTP/1.1 uses transfer
coding values in the Transfer-Encoding header field (section 14.40).

Transfer codings are analogous to the Content-Transfer-Encoding values
of MIME , which were designed to enable safe transport of binary data
over a 7-bit transport service. However, safe transport has a different
focus for an 8bit-clean transfer protocol. In HTTP, the only unsafe
characteristic of message-bodies is the difficulty in determining the
exact body length (section 7.2.2), or the desire to encrypt data over a
shared transport.

The chunked encoding modifies the body of a message in order to transfer
it as a series of chunks, each with its own size indicator, followed by
an optional footer containing entity-header fields. This allows
dynamically-produced content to be transferred along with the
information necessary for the recipient to verify that it has received
the full message.

       Chunked-Body   = *chunk
                        "0" CRLF
                        footer
                        CRLF

       chunk          = chunk-size [ chunk-ext ] CRLF
                        chunk-data CRLF

       hex-no-zero    = <HEX excluding "0">

       chunk-size     = hex-no-zero *HEX
       chunk-ext      = *( ";" chunk-ext-name [ "=" chunk-ext-value ] )
       chunk-ext-name = token
       chunk-ext-val  = token | quoted-string
       chunk-data     = chunk-size(OCTET)

       footer         = *entity-header

The chunked encoding is ended by a zero-sized chunk followed by the
footer, which is terminated by an empty line. The purpose of the footer
is to provide an efficient way to supply information about an entity

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that is generated dynamically; applications MUST NOT send header fields
in the footer which are not explicitly defined as being appropriate for
the footer, such as Content-MD5 or future extensions to HTTP for digital
signatures or other facilities.

An example process for decoding a Chunked-Body is presented in appendix
19.4.6.

All HTTP/1.1 applications MUST be able to receive and decode the
"chunked" transfer coding, and MUST ignore transfer coding extensions
they do not understand. A server which receives an entity-body with a
transfer-coding it does not understand SHOULD return 501
(Unimplemented), and close the connection. A server MUST NOT send
transfer-codings to an HTTP/1.0 client.


3.7 Media Types

HTTP uses Internet Media Types  in the Content-Type (section 14.18) and
Accept (section 14.1) header fields in order to provide open and
extensible data typing and type negotiation.

       media-type     = type "/" subtype *( ";" parameter )
       type           = token
       subtype        = token

Parameters may follow the type/subtype in the form of attribute/value
pairs.

       parameter      = attribute "=" value
       attribute      = token
       value          = token | quoted-string

The type, subtype, and parameter attribute names are case-insensitive.
Parameter values may or may not be case-sensitive, depending on the
semantics of the parameter name. Linear white space (LWS) MUST NOT be
used between the type and subtype, nor between an attribute and its
value. User agents that recognize the media-type MUST process (or
arrange to be processed by any external applications used to process
that type/subtype by the user agent) the parameters for that MIME type
as described by that type/subtype definition to the and inform the user
of any problems discovered.

  Note: some older HTTP applications do not recognize media type
  parameters. When sending data to older HTTP applications,
  implementations should only use media type parameters when they are
  required by that type/subtype definition.

Media-type values are registered with the Internet Assigned Number
Authority (IANA). The media type registration process is outlined in RFC
1590 [17]. Use of non-registered media types is discouraged.

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3.7.1 Canonicalization and Text Defaults

Internet media types are registered with a canonical form. In general,
an entity-body transferred via HTTP messages MUST be represented in the
appropriate canonical form prior to its transmission; the exception is
"text" types, as defined in the next paragraph.

When in canonical form, media subtypes of the "text" type use CRLF as
the text line break. HTTP relaxes this requirement and allows the
transport of text media with plain CR or LF alone representing a line
break when it is done consistently for an entire entity-body. HTTP
applications MUST accept CRLF, bare CR, and bare LF as being
representative of a line break in text media received via HTTP. In
addition, if the text is represented in a character set that does not
use octets 13 and 10 for CR and LF respectively, as is the case for some
multi-byte character sets, HTTP allows the use of whatever octet
sequences are defined by that character set to represent the equivalent
of CR and LF for line breaks. This flexibility regarding line breaks
applies only to text media in the entity-body; a bare CR or LF MUST NOT
be substituted for CRLF within any of the HTTP control structures (such
as header fields and multipart boundaries).

If an entity-body is encoded with a Content-Encoding, the underlying
data MUST be in a form defined above prior to being encoded.

The "charset" parameter is used with some media types to define the
character set (section 3.4) of the data. When no explicit charset
parameter is provided by the sender, media subtypes of the "text" type
are defined to have a default charset value of "ISO-8859-1" when
received via HTTP. Data in character sets other than "ISO-8859-1" or its
subsets MUST be labeled with an appropriate charset value.


3.7.2 Multipart Types

MIME provides for a number of "multipart" types -- encapsulations of one
or more entities within a single message-body. All multipart types share
a common syntax, as defined in section 7.2.1 of RFC 1521 [7], and MUST
include a boundary parameter as part of the media type value. The
message body is itself a protocol element and MUST therefore use only
CRLF to represent line breaks between body-parts. Unlike in RFC 1521,
the epilogue of any multipart message MUST be empty; HTTP applications
MUST NOT transmit the epilogue (even if the original multipart contains
an epilogue).

In HTTP, multipart body-parts MAY contain header fields which are
significant to the meaning of that part. A Content-Location header field
(section 14.15) SHOULD be included in the body-part of each enclosed
entity that can be identified by a URL.


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In general, an HTTP user agent SHOULD follow the same or similar
behavior as a MIME user agent would upon receipt of a multipart type. If
an application receives an unrecognized multipart subtype, the
application MUST treat it as being equivalent to "multipart/mixed".

  Note: The "multipart/form-data" type has been specifically defined
  for carrying form data suitable for processing via the POST request
  method, as described in RFC 1867 [15].


3.8 Product Tokens

Product tokens are used to allow communicating applications to identify
themselves by software name and version. Most fields using product
tokens also allow sub-products which form a significant part of the
application to be listed, separated by whitespace. By convention, the
products are listed in order of their significance for identifying the
application.

       product         = token ["/" product-version]
       product-version = token

Examples:

       User-Agent: CERN-LineMode/2.15 libwww/2.17b3
       Server: Apache/0.8.4

Product tokens should be short and to the point -- use of them for
advertising or other non-essential information is explicitly forbidden.
Although any token character may appear in a product-version, this token
SHOULD only be used for a version identifier (i.e., successive versions
of the same product SHOULD only differ in the product-version portion of
the product value).


3.9 Quality Values

HTTP content negotiation (section 12) uses short "floating point"
numbers to indicate the relative importance ("weight") of various
negotiable parameters. A weight is normalized to a real number in the
range 0 through 1, where 0 is the minimum and 1 the maximum value.
HTTP/1.1 applications MUST NOT generate more than three digits after the
decimal point. User configuration of these values SHOULD also be limited
in this fashion.

       qvalue         = ( "0" [ "." 0*3DIGIT ] )
                      | ( "1" [ "." 0*3("0") ] )

"Quality values" is a misnomer, since these values merely represent
relative degradation in desired quality.


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3.10 Language Tags

A language tag identifies a natural language spoken, written, or
otherwise conveyed by human beings for communication of information to
other human beings. Computer languages are explicitly excluded. HTTP
uses language tags within the Accept-Language and Content-Language
fields.

The syntax and registry of HTTP language tags is the same as that
defined by RFC 1766 [1]. In summary, a language tag is composed of 1 or
more parts: A primary language tag and a possibly empty series of
subtags:

        language-tag  = primary-tag *( "-" subtag )

        primary-tag   = 1*8ALPHA
        subtag        = 1*8ALPHA

Whitespace is not allowed within the tag and all tags are case-
insensitive. The name space of language tags is administered by the
IANA. Example tags include:

       en, en-US, en-cockney, i-cherokee, x-pig-latin

where any two-letter primary-tag is an ISO 639 language abbreviation and
any two-letter initial subtag is an ISO 3166 country code. (The last
three tags above are not registered tags; all but the last are examples
of tags which could be registered in future.)


3.11 Entity Tags

Entity tags are used for comparing two or more entities from the same
requested resource. HTTP/1.1 uses entity tags in the ETag (section
14.20), If-Match (section 14.25), If-None-Match (section 14.26), and If-
Range (section 14.27) header fields. The definition of how they are used
and compared as cache validators is in section 13.3.3. An entity tag
consists of an opaque quoted string, possibly prefixed by a weakness
indicator.

      entity-tag = [ weak ] opaque-tag

      weak       = "W/"
      opaque-tag = quoted-string

A "strong entity tag" may be shared by two entities of a resource only
if they are equivalent by octet equality.

A "weak entity tag," indicated by the "W/" prefix, may be shared by two
entities of a resource only if the entities are equivalent and could be


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substituted for each other with no significant change in semantics. A
weak entity tag can only be used for weak comparison.

An entity tag MUST be unique across all versions of all entities
associated with a particular resource. A given entity tag value may be
used for entities obtained by requests on different URIs without
implying anything about the equivalence of those entities.


3.12 Range Units

HTTP/1.1 allows a client to request that only part (a range of) the
response entity be included within the response. HTTP/1.1 uses range
units in the Range (section 14.36), and Content-Range (section 14.17)
header fields. An entity may be broken down into subranges according to
various structural units.

      range-unit       = bytes-unit | other-range-unit

      bytes-unit       = "bytes"
      other-range-unit = token

The only range unit defined by HTTP/1.1 is "bytes". HTTP/1.1
implementations may ignore ranges specified using other units. HTTP/1.1
has been designed to allow implementations of applications that do not
depend on knowledge of ranges.


4 HTTP Message


4.1 Message Types

HTTP messages consist of requests from client to server and responses
from server to client.

       HTTP-message   = Request | Response     ; HTTP/1.1 messages

Request (section 5) and Response (section 6) messages use the generic
message format of RFC 822 [9] for transferring entities (the payload of
the message). Both types of message consist of a start-line, one or more
header fields (also known as "headers"), an empty line (i.e., a line
with nothing preceding the CRLF) indicating the end of the header
fields, and an optional message-body.

        generic-message = start-line
                          *message-header
                          CRLF
                          [ message-body ]

        start-line      = Request-Line | Status-Line

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In the interest of robustness, servers SHOULD ignore any empty line(s)
received where a Request-Line is expected. In other words, if the server
is reading the protocol stream at the beginning of a message and
receives a CRLF first, it should ignore the CRLF.

  Note: certain buggy HTTP/1.0 client implementations generate an
  extra CRLF's after a POST request. To restate what is explicitly
  forbidden by the BNF, an HTTP/1.1 client must not preface or follow
  a request with an extra CRLF.


4.2 Message Headers

HTTP header fields, which include general-header (section 4.5), request-
header (section 5.3), response-header (section 6.2), and entity-header
(section 7.1) fields, follow the same generic format as that given in
Section 3.1 of RFC 822 [9]. Each header field consists of a name
followed by a colon (":") and the field value. Field names are case-
insensitive. The field value may be preceded by any amount of LWS,
though a single SP is preferred. Header fields can be extended over
multiple lines by preceding each extra line with at least one SP or HT.
Applications SHOULD follow "common form" when generating HTTP
constructs, since there might exist some implementations that fail to
accept anything beyond the common forms.

       message-header = field-name ":" [ field-value ] CRLF

       field-name     = token
       field-value    = *( field-content | LWS )

       field-content  = <the OCTETs making up the field-value
                        and consisting of either *TEXT or combinations
                        of token, tspecials, and quoted-string>

The order in which header fields with differing field names are received
is not significant. However, it is "good practice" to send general-
header fields first, followed by request-header or response-header
fields, and ending with the entity-header fields.

Multiple message-header fields with the same field-name may be present
in a message if and only if the entire field-value for that header field
is defined as a comma-separated list [i.e., #(values)]. It MUST be
possible to combine the multiple header fields into one "field-name:
field-value" pair, without changing the semantics of the message, by
appending each subsequent field-value to the first, each separated by a
comma. The order in which header fields with the same field-name are
received is therefore significant to the interpretation of the combined
field value, and thus a proxy MUST NOT change the order of these field
values when a message is forwarded.


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4.3 Message Body

 The message-body (if any) of an HTTP message is used to carry the
entity-body associated with the request or response. The message-body
differs from the entity-body only when a transfer coding has been
applied, as indicated by the Transfer-Encoding header field (section
14.40).

       message-body = entity-body
                    | <entity-body encoded as per Transfer-Encoding>

Transfer-Encoding MUST be used to indicate any transfer codings applied
by an application to ensure safe and proper transfer of the message.
Transfer-Encoding is a property of the message, not of the entity, and
thus can be added or removed by any application along the
request/response chain.

The rules for when a message-body is allowed in a message differ for
requests and responses.

The presence of a message-body in a request is signaled by the inclusion
of a Content-Length or Transfer-Encoding header field in the request's
message-headers. A message-body MAY be included in a request only when
the request method (section 5.1.1) allows an entity-body.

For response messages, whether or not a message-body is included with a
message is dependent on both the request method and the response status
code (section 6.1.1). All responses to the HEAD request method MUST NOT
include a message-body, even though the presence of entity-header fields
might lead one to believe they do. All 1xx (informational), 204 (no
content), and 304 (not modified) responses MUST NOT include a message-
body. All other responses do include a message-body, although it may be
of zero length.


4.4 Message Length

 When a message-body is included with a message, the length of that body
is determined by one of the following (in order of precedence):

1.   Any response message which MUST NOT include a message-body (such as
  the 1xx, 204, and 304 responses and any response to a HEAD request)
  is always terminated by the first empty line after the header fields,
  regardless of the entity-header fields present in the message.

2.   If a Transfer-Encoding header field (section 14.40) is present and
  indicates that the "chunked" transfer coding has been applied, then
  the length is defined by the chunked encoding (section 3.6).

3.   If a Content-Length header field (section 14.14) is present, its
  value in bytes represents the length of the message-body.

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4.   If the message uses the media type "multipart/byteranges", which is
  self-delimiting, then that defines the length. This Content-Type MUST
  NOT be used unless the sender knows that the recipient can parse it;
  the presence in a request of a Range header with multiple byte-range
  specifiers implies that the client can parse multipart/byteranges
  responses.

5.   By the server closing the connection. (Closing the connection cannot
  be used to indicate the end of a request body, since that would leave
  no possibility for the server to send back a response.)

For compatibility with HTTP/1.0 applications, HTTP/1.1 requests
containing a message-body MUST include a valid Content-Length header
field unless the server is known to be HTTP/1.1 compliant. If a request
contains a message-body and a Content-Length is not given, the server
SHOULD respond with 400 (bad request) if it cannot determine the length
of the message, or with 411 (length required) if it wishes to insist on
receiving a valid Content-Length.

All HTTP/1.1 applications that receive entities MUST accept the
"chunked" transfer coding (section 3.6), thus allowing this mechanism to
be used for messages when the message length cannot be determined in
advance.

Messages MUST NOT include both a Content-Length header field and the
"chunked" transfer coding. If both are received, the Content-Length MUST
be ignored.

When a Content-Length is given in a message where a message-body is
allowed, its field value MUST exactly match the number of OCTETs in the
message-body. HTTP/1.1 user agents MUST notify the user when an invalid
length is received and detected.


4.5 General Header Fields

There are a few header fields which have general applicability for both
request and response messages, but which do not apply to the entity
being transferred. These header fields apply only to the message being
transmitted.

       general-header = Cache-Control            ; Section 14.9
                      | Connection               ; Section 14.10
                      | Date                     ; Section 14.19
                      | Pragma                   ; Section 14.32
                      | Transfer-Encoding        ; Section 14.40
                      | Upgrade                  ; Section 14.41
                      | Via                      ; Section 14.44

General-header field names can be extended reliably only in combination
with a change in the protocol version. However, new or experimental

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header fields may be given the semantics of general header fields if all
parties in the communication recognize them to be general-header fields.
Unrecognized header fields are treated as entity-header fields.


5 Request

A request message from a client to a server includes, within the first
line of that message, the method to be applied to the resource, the
identifier of the resource, and the protocol version in use.

        Request       = Request-Line              ; Section 5.1
                        *( general-header         ; Section 4.5
                         | request-header         ; Section 5.3
                         | entity-header )        ; Section 7.1
                        CRLF
                        [ message-body ]          ; Section 7.2


5.1 Request-Line

The Request-Line begins with a method token, followed by the Request-URI
and the protocol version, and ending with CRLF. The elements are
separated by SP characters. No CR or LF are allowed except in the final
CRLF sequence.

       Request-Line   = Method SP Request-URI SP HTTP-Version CRLF


5.1.1 Method

The Method token indicates the method to be performed on the resource
identified by the Request-URI. The method is case-sensitive.

       Method         = "OPTIONS"                ; Section 9.2
                      | "GET"                    ; Section 9.3
                      | "HEAD"                   ; Section 9.4
                      | "POST"                   ; Section 9.5
                      | "PUT"                    ; Section 9.6
                      | "DELETE"                 ; Section 9.7
                      | "TRACE"                  ; Section 9.8
                      | extension-method

       extension-method = token

The list of methods allowed by a resource can be specified in an Allow
header field (section 14.7). The return code of the response always
notifies the client whether a method is currently allowed on a resource,
since the set of allowed methods can change dynamically. Servers SHOULD
return the status code 405 (Method Not Allowed) if the method is known
by the server but not allowed for the requested resource, and 501 (Not

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Implemented) if the method is unrecognized or not implemented by the
server. The list of methods known by a server can be listed in a Public
response-header field (section 14.35).

The methods GET and HEAD MUST be supported by all general-purpose
servers. All other methods are optional; however, if the above methods
are implemented, they MUST be implemented with the same semantics as
those specified in section 9.


5.1.2 Request-URI

The Request-URI is a Uniform Resource Identifier (section 3.2) and
identifies the resource upon which to apply the request.

       Request-URI    = "*" | absoluteURI | abs_path

The three options for Request-URI are dependent on the nature of the
request. The asterisk "*" means that the request does not apply to a
particular resource, but to the server itself, and is only allowed when
the method used does not necessarily apply to a resource. One example
would be

       OPTIONS * HTTP/1.1

The absoluteURI form is required when the request is being made to a
proxy. The proxy is requested to forward the request or service it from
a valid cache, and return the response. Note that the proxy MAY forward
the request on to another proxy or directly to the server specified by
the absoluteURI. In order to avoid request loops, a proxy MUST be able
to recognize all of its server names, including any aliases, local
variations, and the numeric IP address. An example Request-Line would
be:

       GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.1

To allow for transition to absoluteURIs in all requests in future
versions of HTTP, all HTTP/1.1 servers MUST accept the absoluteURI form
in requests, even though HTTP/1.1 clients will only generate them in
requests to proxies.

The most common form of Request-URI is that used to identify a resource
on an origin server or gateway. In this case the absolute path of the
URI MUST be transmitted (see section 3.2.1, abs_path) as the Request-
URI, and the network location of the URI (net_loc) MUST be transmitted
in a Host header field. For example, a client wishing to retrieve the
resource above directly from the origin server would create a TCP
connection to port 80 of the host "www.w3.org" and send the lines:

       GET /pub/WWW/TheProject.html HTTP/1.1
       Host: www.w3.org

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followed by the remainder of the Request. Note that the absolute path
cannot be empty; if none is present in the original URI, it MUST be
given as "/" (the server root).

If a proxy receives a request without any path in the Request-URI and
the method specified is capable of supporting the asterisk form of
request, then the last proxy on the request chain MUST forward the
request with "*" as the final Request-URI. For example, the request

       OPTIONS http://www.ics.uci.edu:8001 HTTP/1.1

would be forwarded by the proxy as

       OPTIONS * HTTP/1.1
       Host: www.ics.uci.edu:8001

after connecting to port 8001 of host "www.ics.uci.edu".

The Request-URI is transmitted in the format specified in section 3.2.1.
The origin server MUST decode the Request-URI in order to properly
interpret the request. Servers SHOULD respond to invalid Request-URIs
with an appropriate status code.

In requests that they forward, proxies MUST NOT rewrite the "abs_path"
part of a Request-URI in any way except as noted above to replace a null
abs_path with "*", no matter what the proxy does in its internal
implementation.

  Note: The "no rewrite" rule prevents the proxy from changing the
  meaning of the request when the origin server is improperly using a
  non-reserved URL character for a reserved purpose, since it is not
  feasible to fix all CGI scripts (or script authors) use URI syntax
  correctly. Implementers should be aware that some pre-HTTP/1.1
  proxies have been known to rewrite the Request-URI.


5.2 The Resource Identified by a Request

HTTP/1.1 origin servers SHOULD be aware that the exact resource
identified by an Internet request is determined by examining both the
Request-URI and the Host header field.

An origin server that does not allow resources to differ by the
requested host MAY ignore the Host header field value. (But see section
19.5.1 for other requirements on Host support in HTTP/1.1.)

An origin server that does differentiate resources based on the host
requested (sometimes referred to as virtual hosts or vanity hostnames)
MUST use the following rules for determining the requested resource on
an HTTP/1.1 request:


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  1.      If Request-URI is an absoluteURI, the host is part of the Request-
     URI. Any Host header field value in the request MUST be ignored.
  2.      If the Request-URI is not an absoluteURI, and the request includes
     a Host header field, the host is determined by the Host header
     field value.

  3.      If the host as determined by rule 1 or 2 is not a valid host on the
     server, the response MUST be a 400 (Bad Request) error message.

Recipients of an HTTP/1.0 request that lacks a Host header field MAY
attempt to use heuristics (e.g., examination of the URI path for
something unique to a particular host) in order to determine what exact
resource is being requested.


5.3 Request Header Fields

The request-header fields allow the client to pass additional
information about the request, and about the client itself, to the
server. These fields act as request modifiers, with semantics equivalent
to the parameters on a programming language method invocation.

       request-header = Accept                   ; Section 14.1
                      | Accept-Charset           ; Section 14.2
                      | Accept-Encoding          ; Section 14.3
                      | Accept-Language          ; Section 14.4
                      | Authorization            ; Section 14.8
                      | From                     ; Section 14.22
                      | Host                     ; Section 14.23
                      | If-Modified-Since        ; Section 14.24
                      | If-Match                 ; Section 14.25
                      | If-None-Match            ; Section 14.26
                      | If-Range                 ; Section 14.27
                      | If-Unmodified-Since      ; Section 14.28
                      | Max-Forwards             ; Section 14.31
                      | Proxy-Authorization      ; Section 14.34
                      | Range                    ; Section 14.36
                      | Referer                  ; Section 14.37
                      | User-Agent               ; Section 14.42

Request-header field names can be extended reliably only in combination
with a change in the protocol version. However, new or experimental
header fields MAY be given the semantics of request-header fields if all
parties in the communication recognize them to be request-header fields.
Unrecognized header fields are treated as entity-header fields.


6 Response

After receiving and interpreting a request message, a server responds
with an HTTP response message.

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       Response      = Status-Line               ; Section 6.1
                       *( general-header         ; Section 4.5
                        | response-header        ; Section 6.2
                        | entity-header )        ; Section 7.1
                       CRLF
                       [ message-body ]          ; Section 7.2


6.1 Status-Line

The first line of a Response message is the Status-Line, consisting of
the protocol version followed by a numeric status code and its
associated textual phrase, with each element separated by SP characters.
No CR or LF is allowed except in the final CRLF sequence.

       Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF


6.1.1 Status Code and Reason Phrase

The Status-Code element is a 3-digit integer result code of the attempt
to understand and satisfy the request. These codes are fully defined in
section 10. The Reason-Phrase is intended to give a short textual
description of the Status-Code. The Status-Code is intended for use by
automata and the Reason-Phrase is intended for the human user. The
client is not required to examine or display the Reason-Phrase.

The first digit of the Status-Code defines the class of response. The
last two digits do not have any categorization role. There are 5 values
for the first digit:


  .  1xx: Informational - Request received, continuing process

  .  2xx: Success - The action was successfully received, understood,
     and accepted

  .  3xx: Redirection - Further action must be taken in order to
     complete the request

  .  4xx: Client Error - The request contains bad syntax or cannot be
     fulfilled

  .  5xx: Server Error - The server failed to fulfill an apparently
     valid request
The individual values of the numeric status codes defined for HTTP/1.1,
and an example set of corresponding Reason-Phrase's, are presented
below. The reason phrases listed here are only recommended -- they may
be replaced by local equivalents without affecting the protocol.


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       Status-Code    = "100"   ; Continue
                      | "101"   ; Switching Protocols
                      | "200"   ; OK
                      | "201"   ; Created
                      | "202"   ; Accepted
                      | "203"   ; Non-Authoritative Information
                      | "204"   ; No Content
                      | "205"   ; Reset Content
                      | "206"   ; Partial Content
                      | "300"   ; Multiple Choices
                      | "301"   ; Moved Permanently
                      | "302"   ; Moved Temporarily
                      | "303"   ; See Other
                      | "304"   ; Not Modified
                      | "305"   ; Use Proxy
                      | "400"   ; Bad Request
                      | "401"   ; Unauthorized
                      | "402"   ; Payment Required
                      | "403"   ; Forbidden
                      | "404"   ; Not Found
                      | "405"   ; Method Not Allowed
                      | "406"   ; Not Acceptable
                      | "407"   ; Proxy Authentication Required
                      | "408"   ; Request Time-out
                      | "409"   ; Conflict
                      | "410"   ; Gone
                      | "411"   ; Length Required
                      | "412"   ; Precondition Failed
                      | "413"   ; Request Entity Too Large
                      | "414"   ; Request-URI Too Large
                      | "415"   ; Unsupported Media Type
                      | "500"   ; Internal Server Error
                      | "501"   ; Not Implemented
                      | "502"   ; Bad Gateway
                      | "503"   ; Service Unavailable
                      | "504"   ; Gateway Time-out
                      | "505"   ; HTTP Version not supported
                      | extension-code

       extension-code = 3DIGIT

       Reason-Phrase  = *<TEXT, excluding CR, LF>

HTTP status codes are extensible. HTTP applications are not required to
understand the meaning of all registered status codes, though such
understanding is obviously desirable. However, applications MUST
understand the class of any status code, as indicated by the first
digit, and treat any unrecognized response as being equivalent to the
x00 status code of that class, with the exception that an unrecognized
response MUST NOT be cached. For example, if an unrecognized status code
of 431 is received by the client, it can safely assume that there was

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something wrong with its request and treat the response as if it had
received a 400 status code. In such cases, user agents SHOULD present to
the user the entity returned with the response, since that entity is
likely to include human-readable information which will explain the
unusual status.


6.2 Response Header Fields

The response-header fields allow the server to pass additional
information about the response which cannot be placed in the Status-
Line. These header fields give information about the server and about
further access to the resource identified by the Request-URI.

       response-header = Age                     ; Section 14.6
                       | Location                ; Section 14.30
                       | Proxy-Authenticate      ; Section 14.33
                       | Public                  ; Section 14.35
                       | Retry-After             ; Section 14.38
                       | Server                  ; Section 14.39
                       | Vary                    ; Section 14.43
                       | Warning                 ; Section 14.45
                       | WWW-Authenticate        ; Section 14.46

Response-header field names can be extended reliably only in combination
with a change in the protocol version. However, new or experimental
header fields MAY be given the semantics of response-header fields if
all parties in the communication recognize them to be response-header
fields. Unrecognized header fields are treated as entity-header fields.


7 Entity

Request and Response messages MAY transfer an entity if not otherwise
restricted by the request method or response status code. An entity
consists of entity-header fields and an entity-body, although some
responses will only include the entity-headers.

In this section, both sender and recipient refer to either the client or
the server, depending on who sends and who receives the entity.


7.1 Entity Header Fields

Entity-header fields define optional metainformation about the entity-
body or, if no body is present, about the resource identified by the
request.

       entity-header  = Allow                    ; Section 14.7
                      | Content-Base             ; Section 14.11
                      | Content-Encoding         ; Section 14.12

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                      | Content-Language         ; Section 14.13
                      | Content-Length           ; Section 14.14
                      | Content-Location         ; Section 14.15
                      | Content-MD5              ; Section 14.16
                      | Content-Range            ; Section 14.17
                      | Content-Type             ; Section 14.18
                      | ETag                     ; Section 14.20
                      | Expires                  ; Section 14.21
                      | Last-Modified            ; Section 14.29
                      | extension-header

       extension-header = message-header

The extension-header mechanism allows additional entity-header fields to
be defined without changing the protocol, but these fields cannot be
assumed to be recognizable by the recipient. Unrecognized header fields
SHOULD be ignored by the recipient and forwarded by proxies.


7.2 Entity Body

The entity-body (if any) sent with an HTTP request or response is in a
format and encoding defined by the entity-header fields.

       entity-body    = *OCTET

An entity-body is only present in a message when a message-body is
present, as described in section 4.3. The entity-body is obtained from
the message-body by decoding any Transfer-Encoding that may have been
applied to ensure safe and proper transfer of the message.


7.2.1 Type

When an entity-body is included with a message, the data type of that
body is determined via the header fields Content-Type and Content-
Encoding. These define a two-layer, ordered encoding model:

       entity-body := Content-Encoding( Content-Type( data ) )

Content-Type specifies the media type of the underlying data. Content-
Encoding may be used to indicate any additional content codings applied
to the data, usually for the purpose of data compression, that are a
property of the requested resource. There is no default encoding.

Any HTTP/1.1 message containing an entity-body SHOULD include a Content-
Type header field defining the media type of that body. If and only if
the media type is not given by a Content-Type field, the recipient MAY
attempt to guess the media type via inspection of its content and/or the
name extension(s) of the URL used to identify the resource. If the media


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type remains unknown, the recipient SHOULD treat it as type
"application/octet-stream".


7.2.2 Length

The length of an entity-body is the length of the message-body after any
transfer codings have been removed. Section 4.4 defines how the length
of a message-body is determined.


8 Connections


8.1 Persistent Connections


8.1.1 Purpose

Prior to persistent connections, a separate TCP connection was
established to fetch each URL, increasing the load on HTTP servers and
causing congestion on the Internet. The use of inline images and other
associated data often requires a client to make multiple requests of the
same server in a short amount of time. An excellent analysis of these
performance problems is available [30]; analysis and results from a
prototype implementation are in [26].

Persistent HTTP connections have a number of advantages:

  .  By opening and closing fewer TCP connections, CPU time is saved,
     and memory used for TCP protocol control blocks is also saved.
  .  HTTP requests and responses can be pipelined on a connection.
     Pipelining allows a client to make multiple requests without
     waiting for each response, allowing a single TCP connection to be
     used much more efficiently, with much lower elapsed time.
  .  Network congestion is reduced by reducing the number of packets
     caused by TCP opens, and by allowing TCP sufficient time to
     determine the congestion state of the network.
  .  HTTP can evolve more gracefully; since errors can be reported
     without the penalty of closing the TCP connection. Clients using
     future versions of HTTP might optimistically try a new feature, but
     if communicating with an older server, retry with old semantics
     after an error is reported.
HTTP implementations SHOULD implement persistent connections.


8.1.2 Overall Operation

A significant difference between HTTP/1.1 and earlier versions of HTTP
is that persistent connections are the default behavior of any HTTP


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connection. That is, unless otherwise indicated, the client may assume
that the server will maintain a persistent connection.

Persistent connections provide a mechanism by which a client and a
server can signal the close of a TCP connection. This signaling takes
place using the Connection header field. Once a close has been signaled,
the client MUST not send any more requests on that connection.


8.1.2.1 Negotiation

An HTTP/1.1 server MAY assume that a HTTP/1.1 client intends to maintain
a persistent connection unless a Connection header including the
connection-token "close" was sent in the request. If the server chooses
to close the connection immediately after sending the response, it
SHOULD send a Connection header including the connection-token close.

An HTTP/1.1 client MAY expect a connection to remain open, but would
decide to keep it open based on whether the response from a server
contains a Connection header with the connection-token close. In case
the client does not want to maintain a connection for more than that
request, it SHOULD send a Connection header including the connection-
token close.

If either the client or the server sends the close token in the
Connection header, that request becomes the last one for the connection.

Clients and servers SHOULD NOT assume that a persistent connection is
maintained for HTTP versions less than 1.1 unless it is explicitly
signaled. See section 19.7.1 for more information on backwards
compatibility with HTTP/1.0 clients.

In order to remain persistent, all messages on the connection must have
a self-defined message length (i.e., one not defined by closure of the
connection), as described in section 4.4.


8.1.2.2 Pipelining

A client that supports persistent connections MAY "pipeline" its
requests (i.e., send multiple requests without waiting for each
response). A server MUST send its responses to those requests in the
same order that the requests were received.

Clients which assume persistent connections and pipeline immediately
after connection establishment SHOULD be prepared to retry their
connection if the first pipelined attempt fails. If a client does such a
retry, it MUST NOT pipeline before it knows the connection is
persistent. Clients MUST also be prepared to resend their requests if
the server closes the connection before sending all of the corresponding
responses.

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8.1.3 Proxy Servers

It is especially important that proxies correctly implement the
properties of the Connection header field as specified in 14.2.1.

The proxy server MUST signal persistent connections separately with its
clients and the origin servers (or other proxy servers) that it connects
to. Each persistent connection applies to only one transport link.

A proxy server MUST NOT establish a persistent connection with an
HTTP/1.0 client.


8.1.4 Practical Considerations

Servers will usually have some time-out value beyond which they will no
longer maintain an inactive connection. Proxy servers might make this a
higher value since it is likely that the client will be making more
connections through the same server. The use of persistent connections
places no requirements on the length of this time-out for either the
client or the server.

When a client or server wishes to time-out it SHOULD issue a graceful
close on the transport connection. Clients and servers SHOULD both
constantly watch for the other side of the transport close, and respond
to it as appropriate. If a client or server does not detect the other
side's close promptly it could cause unnecessary resource drain on the
network.

A client, server, or proxy MAY close the transport connection at any
time. For example, a client MAY have started to send a new request at
the same time that the server has decided to close the "idle"
connection. From the server's point of view, the connection is being
closed while it was idle, but from the client's point of view, a request
is in progress.

This means that clients, servers, and proxies MUST be able to recover
from asynchronous close events. Client software SHOULD reopen the
transport connection and retransmit the aborted request without user
interaction so long as the request method is idempotent (see section
9.1.2); other methods MUST NOT be automatically retried, although user
agents MAY offer a human operator the choice of retrying the request.
However, this automatic retry SHOULD NOT be repeated if the second
request fails.

Servers SHOULD always respond to at least one request per connection, if
at all possible. Servers SHOULD NOT close a connection in the middle of
transmitting a response, unless a network or client failure is
suspected.


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Clients that use persistent connections SHOULD limit the number of
simultaneous connections that they maintain to a given server. A single-
user client SHOULD maintain AT MOST 2 connections with any server or
proxy. A proxy SHOULD use up to 2*N connections to another server or
proxy, where N is the number of simultaneously active users. These
guidelines are intended to improve HTTP response times and avoid
congestion of the Internet or other networks.


8.2 Message Transmission Requirements

General requirements:

  .  HTTP/1.1 servers SHOULD maintain persistent connections and use
     TCP's flow control mechanisms to resolve temporary overloads,
     rather than terminating connections with the expectation that
     clients will retry. The latter technique can exacerbate network
     congestion.
  .  An HTTP/1.1 (or later) client sending a message-body SHOULD monitor
     the network connection for an error status while it is transmitting
     the request. If the client sees an error status, it SHOULD
     immediately cease transmitting the body. If the body is being sent
     using a "chunked" encoding (section 3.6), a zero length chunk and
     empty footer MAY be used to prematurely mark the end of the
     message. If the body was preceded by a Content-Length header, the
     client MUST close the connection.
  .  An HTTP/1.1 (or later) client MUST be prepared to accept a 100
     (Continue) status followed by a regular response.
  .  An HTTP/1.1 (or later) server that receives a request from a
     HTTP/1.0 (or earlier) client MUST NOT transmit the 100 (continue)
     response; it SHOULD either wait for the request to be completed
     normally (thus avoiding an interrupted request) or close the
     connection prematurely.
Upon receiving a method subject to these requirements from an HTTP/1.1
(or later) client, an HTTP/1.1 (or later) server MUST either respond
with 100 (Continue) status and continue to read from the input stream,
or respond with an error status. If it responds with an error status, it
MAY close the transport (TCP) connection or it MAY continue to read and
discard the rest of the request. It MUST NOT perform the requested
method if it returns an error status.

Clients SHOULD remember the version number of at least the most recently
used server; if an HTTP/1.1 client has seen an HTTP/1.1 or later
response from the server, and it sees the connection close before
receiving any status from the server, the client SHOULD retry the
request without user interaction so long as the request method is
idempotent (see section 9.1.2); other methods MUST NOT be automatically
retried, although user agents MAY offer a human operator the choice of
retrying the request.. If the client does retry the request, the client

  .  MUST first send the request header fields, and then

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  .  MUST wait for the server to respond with either a 100 (Continue)
     response, in which case the client should continue, or with an
     error status.
If an HTTP/1.1 client has not seen an HTTP/1.1 or later response from
the server, it should assume that the server implements HTTP/1.0 or
older and will not use the 100 (Continue) response. If in this case the
client sees the connection close before receiving any status from the
server, the client SHOULD retry the request. If the client does retry
the request, it should use the following "binary exponential backoff"
algorithm to be assured of obtaining a reliable response:

  1.      Initiate a new connection to the server
  2.      Transmit the request-headers
  3.      Initialize a variable R to the estimated round-trip time to the
     server (e.g., based on the time it took to establish the
     connection), or to a constant value of 5 seconds if the round-trip
     time is not available.
  4.      Compute T = R * (2**N), where N is the number of previous retries
     of this request.
  5.      Wait either for an error response from the server, or for T seconds
     (whichever comes first)
  6.      If no error response is received, after T seconds transmit the body
     of the request.
  7.      If client sees that the connection is closed prematurely, repeat
     from step 1 until the request is accepted, an error response is
     received, or the user becomes impatient and terminates the retry
     process.
No matter what the server version, if an error status is received, the
client

  .  MUST NOT continue and
  .  MUST close the connection if it has not completed sending the
     message.
An HTTP/1.1 (or later) client that sees the connection close after
receiving a 100 (Continue) but before receiving any other status SHOULD
retry the request, and need not wait for 100 (Continue) response (but
MAY do so if this simplifies the implementation).


9 Method Definitions

The set of common methods for HTTP/1.1 is defined below. Although this
set can be expanded, additional methods cannot be assumed to share the
same semantics for separately extended clients and servers.

The Host request-header field (section 14.23) MUST accompany all
HTTP/1.1 requests.


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9.1 Safe and Idempotent Methods


9.1.1 Safe Methods

Implementers should be aware that the software represents the user in
their interactions over the Internet, and should be careful to allow the
user to be aware of any actions they may take which may have an
unexpected significance to themselves or others.

In particular, the convention has been established that the GET and HEAD
methods should never have the significance of taking an action other
than retrieval. These methods should be considered "safe." This allows
user agents to represent other methods, such as POST, PUT and DELETE, in
a special way, so that the user is made aware of the fact that a
possibly unsafe action is being requested.

Naturally, it is not possible to ensure that the server does not
generate side-effects as a result of performing a GET request; in fact,
some dynamic resources consider that a feature. The important
distinction here is that the user did not request the side-effects, so
therefore cannot be held accountable for them.


9.1.2 Idempotent Methods

Methods may also have the property of "idempotence" in that (aside from
error or expiration issues) the side-effects of  N > 0 identical
requests is the same as for a single request. The methods GET, HEAD, PUT
and DELETE share this property.


9.2 OPTIONS

The OPTIONS method represents a request for information about the
communication options available on the request/response chain identified
by the Request-URI. This method allows the client to determine the
options and/or requirements associated with a resource, or the
capabilities of a server, without implying a resource action or
initiating a resource retrieval.

Unless the server's response is an error, the response MUST NOT include
entity information other than what can be considered as communication
options (e.g., Allow is appropriate, but Content-Type is not). Responses
to this method are not cachable.

If the Request-URI is an asterisk ("*"), the OPTIONS request is intended
to apply to the server as a whole. A 200 response SHOULD include any
header fields which indicate optional features implemented by the server
(e.g., Public), including any extensions not defined by this
specification, in addition to any applicable general or response-header

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fields. As described in section 5.1.2, an "OPTIONS *" request can be
applied through a proxy by specifying the destination server in the
Request-URI without any path information.

If the Request-URI is not an asterisk, the OPTIONS request applies only
to the options that are available when communicating with that resource.
A 200 response SHOULD include any header fields which indicate optional
features implemented by the server and applicable to that resource
(e.g., Allow), including any extensions not defined by this
specification, in addition to any applicable general or response-header
fields. If the OPTIONS request passes through a proxy, the proxy MUST
edit the response to exclude those options which apply to a proxy's
capabilities and which are known to be unavailable through that proxy.


9.3 GET

The GET method means retrieve whatever information (in the form of an
entity) is identified by the Request-URI. If the Request-URI refers to a
data-producing process, it is the produced data which shall be returned
as the entity in the response and not the source text of the process,
unless that text happens to be the output of the process.

The semantics of the GET method change to a "conditional GET" if the
request message includes an If-Modified-Since, If-Unmodified-Since, If-
Match, If-None-Match, or If-Range header field. A conditional GET method
requests that the entity be transferred only under the circumstances
described by the conditional header field(s). The conditional GET method
is intended to reduce unnecessary network usage by allowing cached
entities to be refreshed without requiring multiple requests or
transferring data already held by the client.

The semantics of the GET method change to a "partial GET" if the request
message includes a Range header field. A partial GET requests that only
part of the entity be transferred, as described in section 14.36. The
partial GET method is intended to reduce unnecessary network usage by
allowing partially-retrieved entities to be completed without
transferring data already held by the client.

The response to a GET request is cachable if and only if it meets the
requirements for HTTP caching described in section 13.


9.4 HEAD

The HEAD method is identical to GET except that the server MUST NOT
return a message-body in the response. The metainformation contained in
the HTTP headers in response to a HEAD request SHOULD be identical to
the information sent in response to a GET request. This method can be
used for obtaining metainformation about the entity implied by the
request without transferring the entity-body itself. This method is

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often used for testing hypertext links for validity, accessibility, and
recent modification.

The response to a HEAD request may be cachable in the sense that the
information contained in the response may be used to update a previously
cached entity from that resource. If the new field values indicate that
the cached entity differs from the current entity (as would be indicated
by a change in Content-Length, Content-MD5, ETag or Last-Modified), then
the cache MUST treat the cache entry as stale.


9.5 POST

The POST method is used to request that the destination server accept
the entity enclosed in the request as a new subordinate of the resource
identified by the Request-URI in the Request-Line. POST is designed to
allow a uniform method to cover the following functions:


  .  Annotation of existing resources;

  .  Posting a message to a bulletin board, newsgroup, mailing list, or
     similar group of articles;

  .  Providing a block of data, such as the result of submitting a form,
     to a data-handling process;

  .  Extending a database through an append operation.
The actual function performed by the POST method is determined by the
server and is usually dependent on the Request-URI. The posted entity is
subordinate to that URI in the same way that a file is subordinate to a
directory containing it, a news article is subordinate to a newsgroup to
which it is posted, or a record is subordinate to a database.

The action performed by the POST method might not result in a resource
that can be identified by a URI. In this case, either 200 (OK) or 204
(No Content) is the appropriate response status, depending on whether or
not the response includes an entity that describes the result.

If a resource has been created on the origin server, the response SHOULD
be 201 (Created) and contain an entity which describes the status of the
request and refers to the new resource, and a Location header (see
section 14.30).

Responses to this method are not cachable, unless the response includes
appropriate Cache-Control or Expires header fields. However, the 303
(See Other) response can be used to direct the user agent to retrieve a
cachable resource.

POST requests must obey the message transmission requirements set out in
section 8.2.

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9.6 PUT

The PUT method requests that the enclosed entity be stored under the
supplied Request-URI. If the Request-URI refers to an already existing
resource, the enclosed entity SHOULD be considered as a modified version
of the one residing on the origin server. If the Request-URI does not
point to an existing resource, and that URI is capable of being defined
as a new resource by the requesting user agent, the origin server can
create the resource with that URI. If a new resource is created, the
origin server MUST inform the user agent via the 201 (Created) response.
If an existing resource is modified, either the 200 (OK) or 204 (No
Content) response codes SHOULD be sent to indicate successful completion
of the request. If the resource could not be created or modified with
the Request-URI, an appropriate error response SHOULD be given that
reflects the nature of the problem. The recipient of the entity MUST NOT
ignore any Content-* (e.g. Content-Range) headers that it does not
understand or implement and MUST return a 501 (Not Implemented) response
in such cases.

If the request passes through a cache and the Request-URI identifies one
or more currently cached entities, those entries should be treated as
stale. Responses to this method are not cachable.

The fundamental difference between the POST and PUT requests is
reflected in the different meaning of the Request-URI. The URI in a POST
request identifies the resource that will handle the enclosed entity.
That resource may be a data-accepting process, a gateway to some other
protocol, or a separate entity that accepts annotations. In contrast,
the URI in a PUT request identifies the entity enclosed with the request
-- the user agent knows what URI is intended and the server MUST NOT
attempt to apply the request to some other resource. If the server
desires that the request be applied to a different URI, it MUST send a
301 (Moved Permanently) response; the user agent MAY then make its own
decision regarding whether or not to redirect the request.

A single resource MAY be identified by many different URIs. For example,
an article may have a URI for identifying "the current version" which is
separate from the URI identifying each particular version. In this case,
a PUT request on a general URI may result in several other URIs being
defined by the origin server.

HTTP/1.1 does not define how a PUT method affects the state of an origin
server.

PUT requests must obey the message transmission requirements set out in
section 8.2.


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9.7 DELETE

The DELETE method requests that the origin server delete the resource
identified by the Request-URI. This method MAY be overridden by human
intervention (or other means) on the origin server. The client cannot be
guaranteed that the operation has been carried out, even if the status
code returned from the origin server indicates that the action has been
completed successfully. However, the server SHOULD not indicate success
unless, at the time the response is given, it intends to delete the
resource or move it to an inaccessible location.

A successful response SHOULD be 200 (OK) if the response includes an
entity describing the status, 202 (Accepted) if the action has not yet
been enacted, or 204 (No Content) if the response is OK but does not
include an entity.

If the request passes through a cache and the Request-URI identifies one
or more currently cached entities, those entries should be treated as
stale. Responses to this method are not cachable.


9.8 TRACE

The TRACE method is used to invoke a remote, application-layer loop-back
of the request message. The final recipient of the request SHOULD
reflect the message received back to the client as the entity-body of a
200 (OK) response. The final recipient is either the origin server or
the first proxy or gateway to receive a Max-Forwards value of zero (0)
in the request (see section 14.31). A TRACE request MUST NOT include an
entity.

TRACE allows the client to see what is being received at the other end
of the request chain and use that data for testing or diagnostic
information. The value of the Via header field (section 14.44) is of
particular interest, since it acts as a trace of the request chain. Use
of the Max-Forwards header field allows the client to limit the length
of the request chain, which is useful for testing a chain of proxies
forwarding messages in an infinite loop.

If successful, the response SHOULD contain the entire request message in
the entity-body, with a Content-Type of "message/http". Responses to
this method MUST NOT be cached.


10 Status Code Definitions

Each Status-Code is described below, including a description of which
method(s) it can follow and any metainformation required in the
response.


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10.1 Informational 1xx

This class of status code indicates a provisional response, consisting
only of the Status-Line and optional headers, and is terminated by an
empty line. Since HTTP/1.0 did not define any 1xx status codes, servers
MUST NOT send a 1xx response to an HTTP/1.0 client except under
experimental conditions.


10.1.1 100 Continue

The client may continue with its request. This interim response is used
to inform the client that the initial part of the request has been
received and has not yet been rejected by the server. The client SHOULD
continue by sending the remainder of the request or, if the request has
already been completed, ignore this response. The server MUST send a
final response after the request has been completed.


10.1.2 101 Switching Protocols

The server understands and is willing to comply with the client's
request, via the Upgrade message header field (section 14.41), for a
change in the application protocol being used on this connection. The
server will switch protocols to those defined by the response's Upgrade
header field immediately after the empty line which terminates the 101
response.

The protocol should only be switched when it is advantageous to do so.
For example, switching to a newer version of HTTP is advantageous over
older versions, and switching to a real-time, synchronous protocol may
be advantageous when delivering resources that use such features.


10.2 Successful 2xx

This class of status code indicates that the client's request was
successfully received, understood, and accepted.


10.2.1 200 OK

The request has succeeded. The information returned with the response is
dependent on the method used in the request, for example:

GET  an entity corresponding to the requested resource is sent in the
     response;

HEAD the entity-header fields corresponding to the requested resource
     are sent in the response without any message-body;


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POST an entity describing or containing the result of the action;

TRACE   an entity containing the request message as received by the end
     server.


10.2.2 201 Created

The request has been fulfilled and resulted in a new resource being
created. The newly created resource can be referenced by the URI(s)
returned in the entity of the response, with the most specific URL for
the resource given by a Location header field. The origin server MUST
create the resource before returning the 201 status code. If the action
cannot be carried out immediately, the server should respond with 202
(Accepted) response instead.


10.2.3 202 Accepted

The request has been accepted for processing, but the processing has not
been completed. The request MAY or MAY NOT eventually be acted upon, as
it MAY be disallowed when processing actually takes place. There is no
facility for re-sending a status code from an asynchronous operation
such as this.

The 202 response is intentionally non-committal. Its purpose is to allow
a server to accept a request for some other process (perhaps a batch-
oriented process that is only run once per day) without requiring that
the user agent's connection to the server persist until the process is
completed. The entity returned with this response SHOULD include an
indication of the request's current status and either a pointer to a
status monitor or some estimate of when the user can expect the request
to be fulfilled.


10.2.4 203 Non-Authoritative Information

The returned metainformation in the entity-header is not the definitive
set as available from the origin server, but is gathered from a local or
a third-party copy. The set presented MAY be a subset or superset of the
original version. For example, including local annotation information
about the resource MAY result in a superset of the metainformation known
by the origin server. Use of this response code is not required and is
only appropriate when the response would otherwise be 200 (OK).


10.2.5 204 No Content

The server has fulfilled the request but there is no new information to
send back. If the client is a user agent, it SHOULD NOT change its
document view from that which caused the request to be sent. This

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response is primarily intended to allow input for actions to take place
without causing a change to the user agent's active document view. The
response MAY include new metainformation in the form of entity-headers,
which SHOULD apply to the document currently in the user agent's active
view.

The 204 response MUST NOT include a message-body, and thus is always
terminated by the first empty line after the header fields.


10.2.6 205 Reset Content

The server has fulfilled the request and the user agent SHOULD reset the
document view which caused the request to be sent. This response is
primarily intended to allow input for actions to take place via user
input, followed by a clearing of the form in which the input is given so
that the user can easily initiate another input action. The response
MUST NOT include an entity.


10.2.7 206 Partial Content

The server has fulfilled the partial GET request for the resource. The
request must have included a Range header field (section 14.36)
indicating the desired range. The response MUST include either a
Content-Range header field (section 14.17) indicating the range included
with this response, or a multipart/byteranges Content-Type including
Content-Range fields for each part. If multipart/byteranges is not used,
the Content-Length header field in the response MUST match the actual
number of OCTETs transmitted in the message-body.

A cache that does not support the Range and Content-Range headers MUST
NOT cache 206 (Partial) responses.


10.3 Redirection 3xx

This class of status code indicates that further action needs to be
taken by the user agent in order to fulfill the request. The action
required MAY be carried out by the user agent without interaction with
the user if and only if the method used in the second request is GET or
HEAD. A user agent SHOULD NOT automatically redirect a request more than
5 times, since such redirections usually indicate an infinite loop.


10.3.1 300 Multiple Choices

The requested resource corresponds to any one of a set of
representations, each with its own specific location, and agent-driven
negotiation information (section 12) is being provided so that the user


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(or user agent) can select a preferred representation and redirect its
request to that location.

Unless it was a HEAD request, the response SHOULD include an entity
containing a list of resource characteristics and location(s) from which
the user or user agent can choose the one most appropriate. The entity
format is specified by the media type given in the Content-Type header
field. Depending upon the format and the capabilities of the user agent,
selection of the most appropriate choice may be performed automatically.
However, this specification does not define any standard for such
automatic selection.

If the server has a preferred choice of representation, it SHOULD
include the specific URL for that representation in the Location field;
user agents MAY use the Location field value for automatic redirection.
This response is cachable unless indicated otherwise.


10.3.2 301 Moved Permanently

The requested resource has been assigned a new permanent URI and any
future references to this resource SHOULD be done using one of the
returned URIs. Clients with link editing capabilities SHOULD
automatically re-link references to the Request-URI to one or more of
the new references returned by the server, where possible. This response
is cachable unless indicated otherwise.

If the new URI is a location, its URL SHOULD be given by the Location
field in the response. Unless the request method was HEAD, the entity of
the response SHOULD contain a short hypertext note with a hyperlink to
the new URI(s).

If the 301 status code is received in response to a request other than
GET or HEAD, the user agent MUST NOT automatically redirect the request
unless it can be confirmed by the user, since this might change the
conditions under which the request was issued.

  Note: When automatically redirecting a POST request after receiving
  a 301 status code, some existing HTTP/1.0 user agents will
  erroneously change it into a GET request.


10.3.3 302 Moved Temporarily

The requested resource resides temporarily under a different URI. Since
the redirection may be altered on occasion, the client SHOULD continue
to use the Request-URI for future requests. This response is only
cachable if indicated by a Cache-Control or Expires header field.

If the new URI is a location, its URL SHOULD be given by the Location
field in the response. Unless the request method was HEAD, the entity of

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the response SHOULD contain a short hypertext note with a hyperlink to
the new URI(s).

If the 302 status code is received in response to a request other than
GET or HEAD, the user agent MUST NOT automatically redirect the request
unless it can be confirmed by the user, since this might change the
conditions under which the request was issued.

  Note: When automatically redirecting a POST request after receiving
  a 302 status code, some existing HTTP/1.0 user agents will
  erroneously change it into a GET request.


10.3.4 303 See Other

The response to the request can be found under a different URI and
SHOULD be retrieved using a GET method on that resource. This method
exists primarily to allow the output of a POST-activated script to
redirect the user agent to a selected resource. The new URI is not a
substitute reference for the originally requested resource. The 303
response is not cachable, but the response to the second (redirected)
request MAY be cachable.

If the new URI is a location, its URL SHOULD be given by the Location
field in the response. Unless the request method was HEAD, the entity of
the response SHOULD contain a short hypertext note with a hyperlink to
the new URI(s).


10.3.5 304 Not Modified

If the client has performed a conditional GET request and access is
allowed, but the document has not been modified, the server SHOULD
respond with this status code. The response MUST NOT contain a message-
body.

The response MUST include the following header fields:

  .  Date
  .  ETag and/or Content-Location, if the header would have been sent in
     a 200 response to the same request
  .  Expires, Cache-Control, and/or Vary, if the field-value might
     differ from that sent in any previous response for the same variant
If the conditional GET used a strong cache validator (see section
13.3.3), the response SHOULD NOT include other entity-headers. Otherwise
(i.e., the conditional GET used a weak validator), the response MUST NOT
include other entity-headers; this prevents inconsistencies between
cached entity-bodies and updated headers.


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If a 304 response indicates an entity not currently cached, then the
cache MUST disregard the response and repeat the request without the
conditional.

If a cache uses a received 304 response to update a cache entry, the
cache MUST update the entry to reflect any new field values given in the
response.

The 304 response MUST NOT include a message-body, and thus is always
terminated by the first empty line after the header fields.


10.3.6 305 Use Proxy

The requested resource MUST be accessed through the proxy given by the
Location field. The Location field gives the URL of the proxy. The
recipient is expected to repeat the request via the proxy.


10.4 Client Error 4xx

The 4xx class of status code is intended for cases in which the client
seems to have erred. Except when responding to a HEAD request, the
server SHOULD include an entity containing an explanation of the error
situation, and whether it is a temporary or permanent condition. These
status codes are applicable to any request method. User agents SHOULD
display any included entity to the user.

  Note: If the client is sending data, a server implementation using
  TCP should be careful to ensure that the client acknowledges
  receipt of the packet(s) containing the response, before the server
  closes the input connection. If the client continues sending data
  to the server after the close, the server's TCP stack will send a
  reset packet to the client, which may erase the client's
  unacknowledged input buffers before they can be read and
  interpreted by the HTTP application.


10.4.1 400 Bad Request

The request could not be understood by the server due to malformed
syntax. The client SHOULD NOT repeat the request without modifications.


10.4.2 401 Unauthorized

The request requires user authentication. The response MUST include a
WWW-Authenticate header field (section 14.46) containing a challenge
applicable to the requested resource. The client MAY repeat the request
with a suitable Authorization header field (section 14.8). If the
request already included Authorization credentials, then the 401

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response indicates that authorization has been refused for those
credentials. If the 401 response contains the same challenge as the
prior response, and the user agent has already attempted authentication
at least once, then the user SHOULD be presented the entity that was
given in the response, since that entity MAY include relevant diagnostic
information. HTTP access authentication is explained in section 11.


10.4.3 402 Payment Required

This code is reserved for future use.


10.4.4 403 Forbidden

The server understood the request, but is refusing to fulfill it.
Authorization will not help and the request SHOULD NOT be repeated. If
the request method was not HEAD and the server wishes to make public why
the request has not been fulfilled, it SHOULD describe the reason for
the refusal in the entity. This status code is commonly used when the
server does not wish to reveal exactly why the request has been refused,
or when no other response is applicable.


10.4.5 404 Not Found

The server has not found anything matching the Request-URI. No
indication is given of whether the condition is temporary or permanent.
If the server does not wish to make this information available to the
client, the status code 403 (Forbidden) can be used instead. The 410
(Gone) status code SHOULD be used if the server knows, through some
internally configurable mechanism, that an old resource is permanently
unavailable and has no forwarding address.


10.4.6 405 Method Not Allowed

The method specified in the Request-Line is not allowed for the resource
identified by the Request-URI. The response MUST include an Allow header
containing a list of valid methods for the requested resource.


10.4.7 406 Not Acceptable

The resource identified by the request is only capable of generating
response entities which have content characteristics not acceptable
according to the accept headers sent in the request.

Unless it was a HEAD request, the response SHOULD include an entity
containing a list of available entity characteristics and location(s)
from which the user or user agent can choose the one most appropriate.

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The entity format is specified by the media type given in the Content-
Type header field. Depending upon the format and the capabilities of the
user agent, selection of the most appropriate choice may be performed
automatically. However, this specification does not define any standard
for such automatic selection.

  Note: HTTP/1.1 servers are allowed to return responses which are
  not acceptable according to the accept headers sent in the request.
  In some cases, this may even be preferable to sending a 406
  response. User agents are encouraged to inspect the headers of an
  incoming response to determine if it is acceptable. If the response
  could be unacceptable, a user agent SHOULD temporarily stop receipt
  of more data and query the user for a decision on further actions.


10.4.8 407 Proxy Authentication Required

This code is similar to 401 (Unauthorized), but indicates that the
client MUST first authenticate itself with the proxy. The proxy MUST
return a Proxy-Authenticate header field (section 14.33) containing a
challenge applicable to the proxy for the requested resource. The client
MAY repeat the request with a suitable Proxy-Authorization header field
(section 14.34). HTTP access authentication is explained in section 11.


10.4.9 408 Request Timeout

The client did not produce a request within the time that the server was
prepared to wait. The client MAY repeat the request without
modifications at any later time.


10.4.10 409 Conflict

The request could not be completed due to a conflict with the current
state of the resource. This code is only allowed in situations where it
is expected that the user might be able to resolve the conflict and
resubmit the request. The response body SHOULD include enough
information for the user to recognize the source of the conflict.
Ideally, the response entity would include enough information for the
user or user agent to fix the problem; however, that may not be possible
and is not required.

Conflicts are most likely to occur in response to a PUT request. If
versioning is being used and the entity being PUT includes changes to a
resource which conflict with those made by an earlier (third-party)
request, the server MAY use the 409 response to indicate that it can't
complete the request. In this case, the response entity SHOULD contain a
list of the differences between the two versions in a format defined by
the response Content-Type.


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10.4.11 410 Gone

The requested resource is no longer available at the server and no
forwarding address is known. This condition SHOULD be considered
permanent. Clients with link editing capabilities SHOULD delete
references to the Request-URI after user approval. If the server does
not know, or has no facility to determine, whether or not the condition
is permanent, the status code 404 (Not Found) SHOULD be used instead.
This response is cachable unless indicated otherwise.

The 410 response is primarily intended to assist the task of web
maintenance by notifying the recipient that the resource is
intentionally unavailable and that the server owners desire that remote
links to that resource be removed. Such an event is common for limited-
time, promotional services and for resources belonging to individuals no
longer working at the server's site. It is not necessary to mark all
permanently unavailable resources as "gone" or to keep the mark for any
length of time -- that is left to the discretion of the server owner.


10.4.12 411 Length Required

The server refuses to accept the request without a defined Content-
Length. The client MAY repeat the request if it adds a valid Content-
Length header field containing the length of the message-body in the
request message.


10.4.13 412 Precondition Failed

The precondition given in one or more of the request-header fields
evaluated to false when it was tested on the server. This response code
allows the client to place preconditions on the current resource
metainformation (header field data) and thus prevent the requested
method from being applied to a resource other than the one intended.


10.4.14 413 Request Entity Too Large

The server is refusing to process a request because the request entity
is larger than the server is willing or able to process. The server may
close the connection to prevent the client from continuing the request.

If the condition is temporary, the server SHOULD include a Retry-After
header field to indicate that it is temporary and after what time the
client may try again.


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10.4.15 414 Request-URI Too Long

The server is refusing to service the request because the Request-URI is
longer than the server is willing to interpret. This rare condition is
only likely to occur when a client has improperly converted a POST
request to a GET request with long query information, when the client
has descended into a URL "black hole" of redirection (e.g., a redirected
URL prefix that points to a suffix of itself), or when the server is
under attack by a client attempting to exploit security holes present in
some servers using fixed-length buffers for reading or manipulating the
Request-URI.


10.4.16 415 Unsupported Media Type

The server is refusing to service the request because the entity of the
request is in a format not supported by the requested resource for the
requested method.


10.5 Server Error 5xx

Response status codes beginning with the digit "5" indicate cases in
which the server is aware that it has erred or is incapable of
performing the request. Except when responding to a HEAD request, the
server SHOULD include an entity containing an explanation of the error
situation, and whether it is a temporary or permanent condition. User
agents SHOULD display any included entity to the user. These response
codes are applicable to any request method.


10.5.1 500 Internal Server Error

The server encountered an unexpected condition which prevented it from
fulfilling the request.


10.5.2 501 Not Implemented

The server does not support the functionality required to fulfill the
request. This is the appropriate response when the server does not
recognize the request method and is not capable of supporting it for any
resource.


10.5.3 502 Bad Gateway

The server, while acting as a gateway or proxy, received an invalid
response from the upstream server it accessed in attempting to fulfill
the request.


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10.5.4 503 Service Unavailable

The server is currently unable to handle the request due to a temporary
overloading or maintenance of the server. The implication is that this
is a temporary condition which will be alleviated after some delay. If
known, the length of the delay may be indicated in a Retry-After header.
If no Retry-After is given, the client SHOULD handle the response as it
would for a 500 response.

  Note: The existence of the 503 status code does not imply that a
  server must use it when becoming overloaded. Some servers may wish
  to simply refuse the connection.


10.5.5 504 Gateway Timeout

The server, while acting as a gateway or proxy, did not receive a timely
response from the upstream server it accessed in attempting to complete
the request.


10.5.6 505 HTTP Version Not Supported

The server does not support, or refuses to support, the HTTP protocol
version that was used in the request message. The server is indicating
that it is unable or unwilling to complete the request using the same
major version as the client, as described in section 3.1, other than
with this error message. The response SHOULD contain an entity
describing why that version is not supported and what other protocols
are supported by that server.


11 Access Authentication

HTTP provides a simple challenge-response authentication mechanism which
MAY be used by a server to challenge a client request and by a client to
provide authentication information. It uses an extensible, case-
insensitive token to identify the authentication scheme, followed by a
comma-separated list of attribute-value pairs which carry the parameters
necessary for achieving authentication via that scheme.

       auth-scheme    = token

       auth-param     = token "=" quoted-string

The 401 (Unauthorized) response message is used by an origin server to
challenge the authorization of a user agent. This response MUST include
a WWW-Authenticate header field containing at least one challenge
applicable to the requested resource.

       challenge      = auth-scheme 1*SP realm *( "," auth-param )

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       realm          = "realm" "=" realm-value
       realm-value    = quoted-string

The realm attribute (case-insensitive) is required for all
authentication schemes which issue a challenge. The realm value (case-
sensitive), in combination with the canonical root URL (see section
5.1.2) of the server being accessed, defines the protection space. These
realms allow the protected resources on a server to be partitioned into
a set of protection spaces, each with its own authentication scheme
and/or authorization database. The realm value is a string, generally
assigned by the origin server, which may have additional semantics
specific to the authentication scheme.

A user agent that wishes to authenticate itself with a server--usually,
but not necessarily, after receiving a 401 or 411 response--MAY do so by
including an Authorization header field with the request. The
Authorization field value consists of credentials containing the
authentication information of the user agent for the realm of the
resource being requested.

       credentials    = basic-credentials
                      | auth-scheme #auth-param

The domain over which credentials can be automatically applied by a user
agent is determined by the protection space. If a prior request has been
authorized, the same credentials MAY be reused for all other requests
within that protection space for a period of time determined by the
authentication scheme, parameters, and/or user preference. Unless
otherwise defined by the authentication scheme, a single protection
space cannot extend outside the scope of its server.

If the server does not wish to accept the credentials sent with a
request, it SHOULD return a 401 (Unauthorized) response. The response
MUST include a WWW-Authenticate header field containing the (possibly
new) challenge applicable to the requested resource and an entity
explaining the refusal.

The HTTP protocol does not restrict applications to this simple
challenge-response mechanism for access authentication. Additional
mechanisms MAY be used, such as encryption at the transport level or via
message encapsulation, and with additional header fields specifying
authentication information. However, these additional mechanisms are not
defined by this specification.

Proxies MUST be completely transparent regarding user agent
authentication. That is, they MUST forward the WWW-Authenticate and
Authorization headers untouched, and follow the rules found in section
14.8.

HTTP/1.1 allows a client to pass authentication information to and from
a proxy via the Proxy-Authenticate and Proxy-Authorization headers.

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11.1 Basic Authentication Scheme

The "basic" authentication scheme is based on the model that the user
agent must authenticate itself with a user-ID and a password for each
realm. The realm value should be considered an opaque string which can
only be compared for equality with other realms on that server. The
server will service the request only if it can validate the user-ID and
password for the protection space of the Request-URI. There are no
optional authentication parameters.

Upon receipt of an unauthorized request for a URI within the protection
space, the server MAY respond with a challenge like the following:

       WWW-Authenticate: Basic realm="WallyWorld"

where "WallyWorld" is the string assigned by the server to identify the
protection space of the Request-URI.

To receive authorization, the client sends the userid and password,
separated by a single colon (":") character, within a base64  encoded
string in the credentials.

       basic-credentials = "Basic" SP basic-cookie

       basic-cookie   = <base64 [7] encoding of user-pass,
                        except not limited to 76 char/line>

       user-pass   = userid ":" password

       userid      = *<TEXT excluding ":">

       password    = *TEXT

Userids might be case sensitive.

If the user agent wishes to send the userid "Aladdin" and password "open
sesame", it would use the following header field:

       Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==

 See section 15 for security considerations associated with Basic
authentication.


11.2 Digest Authentication Scheme

Note for the RFC editor: This section is reserved for including the
Digest Authentication specification, or if the RFC editor chooses to
issue a single RFC rather than two RFC's, this section should be
deleted.


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12 Content Negotiation

Most HTTP responses include an entity which contains information for
interpretation by a human user. Naturally, it is desirable to supply the
user with the "best available" entity corresponding to the request.
Unfortunately for servers and caches, not all users have the same
preferences for what is "best," and not all user agents are equally
capable of rendering all entity types. For that reason, HTTP has
provisions for several mechanisms for "content negotiation" -- the
process of selecting the best representation for a given response when
there are multiple representations available.

  Note: This is not called "format negotiation" because the alternate
  representations may be of the same media type, but use different
  capabilities of that type, be in different languages, etc.

Any response containing an entity-body MAY be subject to negotiation,
including error responses.

There are two kinds of content negotiation which are possible in HTTP:
server-driven and agent-driven negotiation. These two kinds of
negotiation are orthogonal and thus may be used separately or in
combination. One method of combination, referred to as transparent
negotiation, occurs when a cache uses the agent-driven negotiation
information provided by the origin server in order to provide server-
driven negotiation for subsequent requests.


12.1 Server-driven Negotiation

If the selection of the best representation for a response is made by an
algorithm located at the server, it is called server-driven negotiation.
Selection is based on the available representations of the response (the
dimensions over which it can vary; e.g. language, content-coding, etc.)
and the contents of particular header fields in the request message or
on other information pertaining to the request (such as the network
address of the client).

Server-driven negotiation is advantageous when the algorithm for
selecting from among the available representations is difficult to
describe to the user agent, or when the server desires to send its "best
guess" to the client along with the first response (hoping to avoid the
round-trip delay of a subsequent request if the "best guess" is good
enough for the user). In order to improve the server's guess, the user
agent MAY include request header fields (Accept, Accept-Language,
Accept-Encoding, etc.) which describe its preferences for such a
response.

Server-driven negotiation has disadvantages:


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1.   It is impossible for the server to accurately determine what might be
  "best" for any given user, since that would require complete
  knowledge of both the capabilities of the user agent and the intended
  use for the response (e.g., does the user want to view it on screen
  or print it on paper?).

2.   Having the user agent describe its capabilities in every request can
  be both very inefficient (given that only a small percentage of
  responses have multiple representations) and a potential violation of
  the user's privacy.

3.   It complicates the implementation of an origin server and the
  algorithms for generating responses to a request.

4.   It may limit a public cache's ability to use the same response for
  multiple user's requests.

HTTP/1.1 includes the following request-header fields for enabling
server-driven negotiation through description of user agent capabilities
and user preferences: Accept (section 14.1), Accept-Charset (section
14.2), Accept-Encoding (section 14.3), Accept-Language (section 14.4),
and User-Agent (section 14.42). However, an origin server is not limited
to these dimensions and MAY vary the response based on any aspect of the
request, including information outside the request-header fields or
within extension header fields not defined by this specification.

HTTP/1.1 origin servers MUST include an appropriate Vary header field
(section 14.43) in any cachable response based on server-driven
negotiation. The Vary header field describes the dimensions over which
the response might vary (i.e. the dimensions over which the origin
server picks its "best guess" response from multiple representations).

HTTP/1.1 public caches MUST recognize the Vary header field when it is
included in a response and obey the requirements described in section
13.6 that describes the interactions between caching and content
negotiation.


12.2 Agent-driven Negotiation

With agent-driven negotiation, selection of the best representation for
a response is performed by the user agent after receiving an initial
response from the origin server. Selection is based on a list of the
available representations of the response included within the header
fields (this specification reserves the keyword Alternates, as described
in appendix 19.6.2.1) or entity-body of the initial response, with each
representation identified by its own URI. Selection from among the
representations may be performed automatically (if the user agent is
capable of doing so) or manually by the user selecting from a generated
(possibly hypertext) menu.


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Agent-driven negotiation is advantageous when the response would vary
over commonly-used dimensions (such as type, language, or encoding),
when the origin server is unable to determine a user agent's
capabilities from examining the request, and generally when public
caches are used to distribute server load and reduce network usage.

Agent-driven negotiation suffers from the disadvantage of needing a
second request to obtain the best alternate representation. This second
request is only efficient when caching is used. In addition, this
specification does not define any mechanism for supporting automatic
selection, though it also does not prevent any such mechanism from being
developed as an extension and used within HTTP/1.1.

HTTP/1.1 defines the 300 (Multiple Choices) and 406 (Not Acceptable)
status codes for enabling agent-driven negotiation when the server is
unwilling or unable to provide a varying response using server-driven
negotiation.


12.3 Transparent Negotiation

Transparent negotiation is a combination of both server-driven and
agent-driven negotiation. When a cache is supplied with a form of the
list of available representations of the response (as in agent-driven
negotiation) and the dimensions of variance are completely understood by
the cache, then the cache becomes capable of performing server-driven
negotiation on behalf of the origin server for subsequent requests on
that resource.

Transparent negotiation has the advantage of distributing the
negotiation work that would otherwise be required of the origin server
and also removing the second request delay of agent-driven negotiation
when the cache is able to correctly guess the right response.

This specification does not define any mechanism for transparent
negotiation, though it also does not prevent any such mechanism from
being developed as an extension and used within HTTP/1.1. An HTTP/1.1
cache performing transparent negotiation MUST include a Vary header
field in the response (defining the dimensions of its variance) if it is
cachable to ensure correct interoperation with all HTTP/1.1 clients. The
agent-driven negotiation information supplied by the origin server
SHOULD be included with the transparently negotiated response.


13 Caching in HTTP

HTTP is typically used for distributed information systems, where
performance can be improved by the use of response caches. The HTTP/1.1
protocol includes a number of elements intended to make caching work as
well as possible. Because these elements are inextricable from other
aspects of the protocol, and because they interact with each other, it

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is useful to describe the basic caching design of HTTP separately from
the detailed descriptions of methods, headers, response codes, etc.

Caching would be useless if it did not significantly improve
performance. The goal of caching in HTTP/1.1 is to eliminate the need to
send requests in many cases, and to eliminate the need to send full
responses in many other cases. The former reduces the number of network
round-trips required for many operations; we use an "expiration"
mechanism for this purpose (see section 13.2). The latter reduces
network bandwidth requirements; we use a "validation" mechanism for this
purpose (see section 13.3).

Requirements for performance, availability, and disconnected operation
require us to be able to relax the goal of semantic transparency. The
HTTP/1.1 protocol allows origin servers, caches, and clients to
explicitly reduce transparency when necessary. However, because non-
transparent operation may confuse non-expert users, and may be
incompatible with certain server applications (such as those for
ordering merchandise), the protocol requires that transparency be
relaxed

  .  only by an explicit protocol-level request when relaxed by client
     or origin server
  .  only with an explicit warning to the end user when relaxed by cache
     or client
Therefore, the HTTP/1.1 protocol provides these important elements:

  1. Protocol features that provide full semantic transparency when this
     is required by all parties.
  2. Protocol features that allow an origin server or user agent to
     explicitly request and control non-transparent operation.
  3. Protocol features that allow a cache to attach warnings to
     responses that do not preserve the requested approximation of
     semantic transparency.
A basic principle is that it must be possible for the clients to detect
any potential relaxation of semantic transparency.

  Note: The server, cache, or client implementer may be faced with
  design decisions not explicitly discussed in this specification. If
  a decision may affect semantic transparency, the implementer ought
  to err on the side of maintaining transparency unless a careful and
  complete analysis shows significant benefits in breaking
  transparency.


13.1.1 Cache Correctness

A correct cache MUST respond to a request with the most up-to-date
response held by the cache that is appropriate to the request (see
sections 13.2.5, 13.2.6, and 13.12) which meets one of the following
conditions:

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  1. It has been checked for equivalence with what the origin server
     would have returned by revalidating the response with the origin
     server (section 13.3);
  2. It is "fresh enough" (see section 13.2). In the default case, this
     means it meets the least restrictive freshness requirement of the
     client, server, and cache (see section 14.9); if the origin server
     so specifies, it is the freshness requirement of the origin server
     alone.
  3.       It includes a warning if the freshness demand of the client or the
     origin server is violated (see section 13.1.5 and 14.45).
  4.       It is an appropriate 304 (Not Modified), 305 (Proxy Redirect), or
     error (4xx or 5xx) response message.
and it is the most up-to-date response appropriate to the request the
cache has (see section 13.2.5, 13.2.6, and 13.12).

If the cache can not communicate with the origin server, then a correct
cache SHOULD respond as above if the response can be correctly served
from the cache; if not it MUST return an error or warning indicating
that there was a communication failure.

If a cache receives a response (either an entire response, or a 304 (Not
Modified) response) that it would normally forward to the requesting
client, and the received response is no longer fresh, the cache SHOULD
forward it to the requesting client without adding a new Warning (but
without removing any existing Warning headers). A cache SHOULD NOT
attempt to revalidate a response simply because that response became
stale in transit; this might lead to an infinite loop. An user agent
that receives a stale response without a Warning MAY display a warning
indication to the user.


13.1.2 Warnings

 Whenever a cache returns a response that is neither first-hand nor
"fresh enough" (in the sense of condition 2 in section 13.1.1), it must
attach a warning to that effect, using a Warning response-header. This
warning allows clients to take appropriate action.

Warnings may be used for other purposes, both cache-related and
otherwise. The use of a warning, rather than an error status code,
distinguish these responses from true failures.

Warnings are always cachable, because they never weaken the transparency
of a response. This means that warnings can be passed to HTTP/1.0 caches
without danger; such caches will simply pass the warning along as an
entity-header in the response.

Warnings are assigned numbers between 0 and 99. This specification
defines the code numbers and meanings of each currently assigned
warnings, allowing a client or cache to take automated action in some
(but not all) cases.

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Warnings also carry a warning text. The text may be in any appropriate
natural language (perhaps based on the client's Accept headers), and
include an optional indication of what character set is used.

Multiple warnings may be attached to a response (either by the origin
server or by a cache), including multiple warnings with the same code
number. For example, a server may provide the same warning with texts in
both English and Basque.

When multiple warnings are attached to a response, it may not be
practical or reasonable to display all of them to the user. This version
of HTTP does not specify strict priority rules for deciding which
warnings to display and in what order, but does suggest some heuristics.

The Warning header and the currently defined warnings are described in
section 14.45.


13.1.3 Cache-control Mechanisms

The basic cache mechanisms in HTTP/1.1 (server-specified expiration
times and validators) are implicit directives to caches. In some cases,
a server or client may need to provide explicit directives to the HTTP
caches. We use the Cache-Control header for this purpose.

The Cache-Control header allows a client or server to transmit a variety
of directives in either requests or responses. These directives
typically override the default caching algorithms. As a general rule, if
there is any apparent conflict between header values, the most
restrictive interpretation should be applied (that is, the one that is
most likely to preserve semantic transparency). However, in some cases,
Cache-Control directives are explicitly specified as weakening the
approximation of semantic transparency (for example, "max-stale" or
"public").

The Cache-Control directives are described in detail in section 14.9.


13.1.4 Explicit User Agent Warnings

Many user agents make it possible for users to override the basic
caching mechanisms. For example, the user agent may allow the user to
specify that cached entities (even explicitly stale ones) are never
validated. Or the user agent might habitually add "Cache-Control: max-
stale=3600" or "Cache-Control: reload" to every request. The user should
have to explicitly request either non-transparent behavior, or behavior
that results in abnormally ineffective caching.

If the user has overridden the basic caching mechanisms, the user agent
should explicitly indicate to the user whenever this results in the
display of information that might not meet the server's transparency

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requirements (in particular, if the displayed entity is known to be
stale). Since the protocol normally allows the user agent to determine
if responses are stale or not, this indication need only be displayed
when this actually happens. The indication need not be a dialog box; it
could be an icon (for example, a picture of a rotting fish) or some
other visual indicator.

If the user has overridden the caching mechanisms in a way that would
abnormally reduce the effectiveness of caches, the user agent should
continually display an indication (for example, a picture of currency in
flames) so that the user does not inadvertently consume excess resources
or suffer from excessive latency.


13.1.5 Exceptions to the Rules and Warnings

In some cases, the operator of a cache may choose to configure it to
return stale responses even when not requested by clients. This decision
should not be made lightly, but may be necessary for reasons of
availability or performance, especially when the cache is poorly
connected to the origin server. Whenever a cache returns a stale
response, it MUST mark it as such (using a Warning header). This allows
the client software to alert the user that there may be a potential
problem.

It also allows the user agent to take steps to obtain a first-hand or
fresh response. For this reason, a cache SHOULD NOT return a stale
response if the client explicitly requests a first-hand or fresh one,
unless it is impossible to comply for technical or policy reasons.


13.1.6 Client-controlled Behavior

While the origin server (and to a lesser extent, intermediate caches, by
their contribution to the age of a response) are the primary source of
expiration information, in some cases the client may need to control a
cache's decision about whether to return a cached response without
validating it. Clients do this using several directives of the Cache-
Control header.

A client's request may specify the maximum age it is willing to accept
of an unvalidated response; specifying a value of zero forces the
cache(s) to revalidate all responses. A client may also specify the
minimum time remaining before a response expires. Both of these options
increase constraints on the behavior of caches, and so cannot further
relax the cache's approximation of semantic transparency.

A client may also specify that it will accept stale responses, up to
some maximum amount of staleness. This loosens the constraints on the
caches, and so may violate the origin server's specified constraints on


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semantic transparency, but may be necessary to support disconnected
operation, or high availability in the face of poor connectivity.


13.2 Expiration Model


13.2.1 Server-Specified Expiration

HTTP caching works best when caches can entirely avoid making requests
to the origin server. The primary mechanism for avoiding requests is for
an origin server to provide an explicit expiration time in the future,
indicating that a response may be used to satisfy subsequent requests.
In other words, a cache can return a fresh response without first
contacting the server.

Our expectation is that servers will assign future explicit expiration
times to responses in the belief that the entity is not likely to
change, in a semantically significant way, before the expiration time is
reached. This normally preserves semantic transparency, as long as the
server's expiration times are carefully chosen.

The expiration mechanism applies only to responses taken from a cache
and not to first-hand responses forwarded immediately to the requesting
client.

If an origin server wishes to force a semantically transparent cache to
validate every request, it may assign an explicit expiration time in the
past. This means that the response is always stale, and so the cache
SHOULD validate it before using it for subsequent requests. See section
14.9.4 for a more restrictive way to force revalidation.

If an origin server wishes to force any HTTP/1.1 cache, no matter how it
is configured, to validate every request, it should use the "must-
revalidate" Cache-Control directive (see section 14.9).

Servers specify explicit expiration times using either the Expires
header, or the max-age directive of the Cache-Control header.

An expiration time cannot be used to force a user agent to refresh its
display or reload a resource; its semantics apply only to caching
mechanisms, and such mechanisms need only check a resource's expiration
status when a new request for that resource is initiated. See section
13.13 for explanation of the difference between caches and history
mechanisms.


13.2.2 Heuristic Expiration

Since origin servers do not always provide explicit expiration times,
HTTP caches typically assign heuristic expiration times, employing

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algorithms that use other header values (such as the Last-Modified time)
to estimate a plausible expiration time. The HTTP/1.1 specification does
not provide specific algorithms, but does impose worst-case constraints
on their results. Since heuristic expiration times may compromise
semantic transparency, they should be used cautiously, and we encourage
origin servers to provide explicit expiration times as much as possible.


13.2.3 Age Calculations

In order to know if a cached entry is fresh, a cache needs to know if
its age exceeds its freshness lifetime. We discuss how to calculate the
latter in section 13.2.4; this section describes how to calculate the
age of a response or cache entry.

In this discussion, we use the term "now" to mean "the current value of
the clock at the host performing the calculation." All HTTP
implementations, but especially origin servers and caches, should use
NTP [28] or some similar protocol to synchronize their clocks to a
globally accurate time standard.

Also note that HTTP/1.1 requires origin servers to send a Date header
with every response, giving the time at which the response was
generated. We use the term "date_value" to denote the value of the Date
header, in a form appropriate for arithmetic operations.

HTTP/1.1 uses the Age response-header to help convey age information
between caches. The Age header value is the sender's estimate of the
amount of time since the response was generated at the origin server. In
the case of a cached response that has been revalidated with the origin
server, the Age value is based on the time of revalidation, not of the
original response.

In essence, the Age value is the sum of the time that the response has
been resident in each of the caches along the path from the origin
server, plus the amount of time it has been in transit along network
paths.

We use the term "age_value" to denote the value of the Age header, in a
form appropriate for arithmetic operations.

A response's age can be calculated in two entirely independent ways:

  1. now minus date_value, if the local clock is reasonably well
     synchronized to the origin server's clock. If the result is
     negative, the result is replaced by zero.
  2. age_value, if all of the caches along the response path implement
     HTTP/1.1.
Given that we have two independent ways to compute the age of a response
when it is received, we can combine these as


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       corrected_received_age = max(now - date_value, age_value)

and as long as we have either nearly synchronized clocks or all-HTTP/1.1
paths, one gets a reliable (conservative) result.

Note that this correction is applied at each HTTP/1.1 cache along the
path, so that if there is an HTTP/1.0 cache in the path, the correct
received age is computed as long as the receiving cache's clock is
nearly in sync. We don't need end-to-end clock synchronization (although
it is good to have), and there is no explicit clock synchronization
step.

Because of network-imposed delays, some significant interval may pass
from the time that a server generates a response and the time it is
received at the next outbound cache or client. If uncorrected, this
delay could result in improperly low ages.

Because the request that resulted in the returned Age value must have
been initiated prior to that Age value's generation, we can correct for
delays imposed by the network by recording the time at which the request
was initiated. Then, when an Age value is received, it MUST be
interpreted relative to the time the request was initiated, not the time
that the response was received. This algorithm results in conservative
behavior no matter how much delay is experienced. So, we compute:

      corrected_initial_age = corrected_received_age
                            + (now - request_time)

where "request_time" is the time (according to the local clock) when the
request that elicited this response was sent.

Summary of age calculation algorithm, when a cache receives a response:

      /*
       * age_value
       *      is the value of Age: header received by the cache with
       *              this response.
       * date_value
       *      is the value of the origin server's Date: header
       * request_time
       *      is the (local) time when the cache made the request
       *              that resulted in this cached response
       * response_time
       *      is the (local) time when the cache received the
       *              response
       * now
       *      is the current (local) time
       */
      apparent_age = max(0, response_time - date_value);
      corrected_received_age = max(apparent_age, age_value);
      response_delay = response_time - request_time;

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      corrected_initial_age = corrected_received_age + response_delay;
      resident_time = now - response_time;
      current_age   = corrected_initial_age + resident_time;

When a cache sends a response, it must add to the corrected_initial_age
the amount of time that the response was resident locally. It must then
transmit this total age, using the Age header, to the next recipient
cache.

  Note that a client cannot reliably tell that a response is first-
  hand, but the presence of an Age header indicates that a response
  is definitely not first-hand. Also, if the Date in a response is
  earlier than the client's local request time, the response is
  probably not first-hand (in the absence of serious clock skew).


13.2.4 Expiration Calculations

In order to decide whether a response is fresh or stale, we need to
compare its freshness lifetime to its age. The age is calculated as
described in section 13.2.3; this section describes how to calculate the
freshness lifetime, and to determine if a response has expired. In the
discussion below, the values can be represented in any form appropriate
for arithmetic operations.

We use the term "expires_value" to denote the value of the Expires
header. We use the term "max_age_value" to denote an appropriate value
of the number of seconds carried by the max-age directive of the Cache-
Control header in a response (see section 14.10.

The max-age directive takes priority over Expires, so if max-age is
present in a response, the calculation is simply:

      freshness_lifetime = max_age_value

Otherwise, if Expires is present in the response, the calculation is:

      freshness_lifetime = expires_value - date_value

Note that neither of these calculations is vulnerable to clock skew,
since all of the information comes from the origin server.

If neither Expires nor Cache-Control: max-age appears in the response,
and the response does not include other restrictions on caching, the
cache MAY compute a freshness lifetime using a heuristic. If the value
is greater than 24 hours, the cache must attach Warning 13 to any
response whose age is more than 24 hours if such warning has not already
been added.


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Also, if the response does have a Last-Modified time, the heuristic
expiration value SHOULD be no more than some fraction of the interval
since that time. A typical setting of this fraction might be 10%.

The calculation to determine if a response has expired is quite simple:

      response_is_fresh = (freshness_lifetime > current_age)


13.2.5 Disambiguating Expiration Values

Because expiration values are assigned optimistically, it is possible
for two caches to contain fresh values for the same resource that are
different.

If a client performing a retrieval receives a non-first-hand response
for a request that was already fresh in its own cache, and the Date
header in its existing cache entry is newer than the Date on the new
response, then the client MAY ignore the response. If so, it MAY retry
the request with a "Cache-Control: max-age=0" directive (see section
14.9), to force a check with the origin server.

If a cache has two fresh responses for the same representation with
different validators, it MUST use the one with the more recent Date
header. This situation may arise because the cache is pooling responses
from other caches, or because a client has asked for a reload or a
revalidation of an apparently fresh cache entry.


13.2.6 Disambiguating Multiple Responses

Because a client may be receiving responses via multiple paths, so that
some responses flow through one set of caches and other responses flow
through a different set of caches, a client may receive responses in an
order different from that in which the origin server sent them. We would
like the client to use the most recently generated response, even if
older responses are still apparently fresh.

Neither the entity tag nor the expiration value can impose an ordering
on responses, since it is possible that a later response intentionally
carries an earlier expiration time. However, the HTTP/1.1 specification
requires the transmission of Date headers on every response, and the
Date values are ordered to a granularity of one second.

When a client tries to revalidate a cache entry, and the response it
receives contains a Date header that appears to be older than the one
for the existing entry, then the client SHOULD repeat the request
unconditionally, and include

       Cache-Control: max-age=0


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to force any intermediate caches to validate their copies directly with
the origin server, or

       Cache-Control: no-cache

to force any intermediate caches to obtain a new copy from the origin
server.

If the Date values are equal, then the client may use either response
(or may, if it is being extremely prudent, request a new response).
Servers MUST NOT depend on clients being able to choose
deterministically between responses generated during the same second, if
their expiration times overlap.


13.3 Validation Model

When a cache has a stale entry that it would like to use as a response
to a client's request, it first has to check with the origin server (or
possibly an intermediate cache with a fresh response) to see if its
cached entry is still usable. We call this "validating" the cache entry.
Since we do not want to have to pay the overhead of retransmitting the
full response if the cached entry is good, and we do not want to pay the
overhead of an extra round trip if the cached entry is invalid, the
HTTP/1.1 protocol supports the use of conditional methods.

The key protocol features for supporting conditional methods are those
concerned with "cache validators." When an origin server generates a
full response, it attaches some sort of validator to it, which is kept
with the cache entry. When a client (user agent or proxy cache) makes a
conditional request for a resource for which it has a cache entry, it
includes the associated validator in the request.

The server then checks that validator against the current validator for
the entity, and, if they match, it responds with a special status code
(usually, 304 (Not Modified)) and no entity-body. Otherwise, it returns
a full response (including entity-body). Thus, we avoid transmitting the
full response if the validator matches, and we avoid an extra round trip
if it does not match.

  Note: the comparison functions used to decide if validators match
  are defined in section 13.3.3.

In HTTP/1.1, a conditional request looks exactly the same as a normal
request for the same resource, except that it carries a special header
(which includes the validator) that implicitly turns the method
(usually, GET) into a conditional.

The protocol includes both positive and negative senses of cache-
validating conditions. That is, it is possible to request either that a


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method be performed if and only if a validator matches or if and only if
no validators match.

  Note: a response that lacks a validator may still be cached, and
  served from cache until it expires, unless this is explicitly
  prohibited by a Cache-Control directive. However, a cache cannot do
  a conditional retrieval if it does not have a validator for the
  entity, which means it will not be refreshable after it expires.


13.3.1 Last-modified Dates

The Last-Modified entity-header field value is often used as a cache
validator. In simple terms, a cache entry is considered to be valid if
the entity has not been modified since the Last-Modified value.


13.3.2 Entity Tag Cache Validators

The ETag entity-header field value, an entity tag, provides for an
"opaque" cache validator. This may allow more reliable validation in
situations where it is inconvenient to store modification dates, where
the one-second resolution of HTTP date values is not sufficient, or
where the origin server wishes to avoid certain paradoxes that may arise
from the use of modification dates.

Entity Tags are described in section 3.11. The headers used with entity
tags are described in sections 14.20, 14.25, 14.26 and 14.43.


13.3.3 Weak and Strong Validators

Since both origin servers and caches will compare two validators to
decide if they represent the same or different entities, one normally
would expect that if the entity (the entity-body or any entity-headers)
changes in any way, then the associated validator would change as well.
If this is true, then we call this validator a "strong validator."

However, there may be cases when a server prefers to change the
validator only on semantically significant changes, and not when
insignificant aspects of the entity change. A validator that does not
always change when the resource changes is a "weak validator."

Entity tags are normally "strong validators," but the protocol provides
a mechanism to tag an entity tag as "weak." One can think of a strong
validator as one that changes whenever the bits of an entity changes,
while a weak value changes whenever the meaning of an entity changes.
Alternatively, one can think of a strong validator as part of an
identifier for a specific entity, while a weak validator is part of an
identifier for a set of semantically equivalent entities.


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  Note: One example of a strong validator is an integer that is
  incremented in stable storage every time an entity is changed.

  An entity's modification time, if represented with one-second
  resolution, could be a weak validator, since it is possible that
  the resource may be modified twice during a single second.

  Support for weak validators is optional; however, weak validators
  allow for more efficient caching of equivalent objects; for
  example, a hit counter on a site is probably good enough if it is
  updated every few days or weeks, and any value during that period
  is likely "good enough" to be equivalent.

A "use" of a validator is either when a client generates a request and
includes the validator in a validating header field, or when a server
compares two validators.

Strong validators are usable in any context. Weak validators are only
usable in contexts that do not depend on exact equality of an entity.
For example, either kind is usable for a conditional GET of a full
entity. However, only a strong validator is usable for a sub-range
retrieval, since otherwise the client may end up with an internally
inconsistent entity.

The only function that the HTTP/1.1 protocol defines on validators is
comparison. There are two validator comparison functions, depending on
whether the comparison context allows the use of weak validators or not:

  .  The strong comparison function: in order to be considered equal,
     both validators must be identical in every way, and neither may be
     weak.
  .  The weak comparison function: in order to be considered equal, both
     validators must be identical in every way, but either or both of
     them may be tagged as "weak" without affecting the result.
The weak comparison function MAY be used for simple (non-subrange) GET
requests. The strong comparison function MUST be used in all other
cases.

An entity tag is strong unless it is explicitly tagged as weak. Section
3.11 gives the syntax for entity tags.

A Last-Modified time, when used as a validator in a request, is
implicitly weak unless it is possible to deduce that it is strong, using
the following rules:

  .  The validator is being compared by an origin server to the actual
     current validator for the entity and,
  .  That origin server reliably knows that the associated entity did
     not change twice during the second covered by the presented
     validator.
or

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  .  The validator is about to be used by a client in an If-Modified-
     Since or If-Unmodified-Since header, because the client has a cache
     entry for the associated entity, and
  .  That cache entry includes a Date value, which gives the time when
     the origin server sent the original response, and
  .  The presented Last-Modified time is at least 60 seconds before the
     Date value.
or

  .  The validator is being compared by an intermediate cache to the
     validator stored in its cache entry for the entity, and
  .  That cache entry includes a Date value, which gives the time when
     the origin server sent the original response, and
  .  The presented Last-Modified time is at least 60 seconds before the
     Date value.
This method relies on the fact that if two different responses were sent
by the origin server during the same second, but both had the same Last-
Modified time, then at least one of those responses would have a Date
value equal to its Last-Modified time. The arbitrary 60-second limit
guards against the possibility that the Date and Last-Modified values
are generated from different clocks, or at somewhat different times
during the preparation of the response. An implementation may use a
value larger than 60 seconds, if it is believed that 60 seconds is too
short.

If a client wishes to perform a sub-range retrieval on a value for which
it has only a Last-Modified time and no opaque validator, it may do this
only if the Last-Modified time is strong in the sense described here.

A cache or origin server receiving a cache-conditional request, other
than a full-body GET request, MUST use the strong comparison function to
evaluate the condition.

These rules allow HTTP/1.1 caches and clients to safely perform sub-
range retrievals on values that have been obtained from HTTP/1.0
servers.


13.3.4 Rules for When to Use Entity Tags and Last-modified Dates

We adopt a set of rules and recommendations for origin servers, clients,
and caches regarding when various validator types should be used, and
for what purposes.

HTTP/1.1 origin servers:

  .  SHOULD send an entity tag validator unless it is not feasible to
     generate one.
  .  MAY send a weak entity tag instead of a strong entity tag, if
     performance considerations support the use of weak entity tags, or
     if it is unfeasible to send a strong entity tag.

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  .  SHOULD send a Last-Modified value if it is feasible to send one,
     unless the risk of a breakdown in semantic transparency that could
     result from using this date in an If-Modified-Since header would
     lead to serious problems.
In other words, the preferred behavior for an HTTP/1.1 origin server is
to send both a strong entity tag and a Last-Modified value.

In order to be legal, a strong entity tag MUST change whenever the
associated entity value changes in any way. A weak entity tag SHOULD
change whenever the associated entity changes in a semantically
significant way.

  Note: in order to provide semantically transparent caching, an
  origin server must avoid reusing a specific strong entity tag value
  for two different entities, or reusing a specific weak entity tag
  value for two semantically different entities. Cache entries may
  persist for arbitrarily long periods, regardless of expiration
  times, so it may be inappropriate to expect that a cache will never
  again attempt to validate an entry using a validator that it
  obtained at some point in the past.

HTTP/1.1 clients:

  .  If an entity tag has been provided by the origin server, MUST use
     that entity tag in any cache-conditional request (using If-Match or
     If-None-Match).
  .  If only a Last-Modified value has been provided by the origin
     server, SHOULD use that value in non-subrange cache-conditional
     requests (using If-Modified-Since).
  .  If only a Last-Modified value has been provided by an HTTP/1.0
     origin server, MAY use that value in subrange cache-conditional
     requests (using If-Unmodified-Since:). The user agent should
     provide a way to disable this, in case of difficulty.
  .  If both an entity tag and a Last-Modified value have been provided
     by the origin server, SHOULD use both validators in cache-
     conditional requests. This allows both HTTP/1.0 and HTTP/1.1 caches
     to respond appropriately.
An HTTP/1.1 cache, upon receiving a request, MUST use the most
restrictive validator when deciding whether the client's cache entry
matches the cache's own cache entry. This is only an issue when the
request contains both an entity tag and a last-modified-date validator
(If-Modified-Since or If-Unmodified-Since).

  A note on rationale: The general principle behind these rules is
  that HTTP/1.1 servers and clients should transmit as much non-
  redundant information as is available in their responses and
  requests. HTTP/1.1 systems receiving this information will make the
  most conservative assumptions about the validators they receive.

  HTTP/1.0 clients and caches will ignore entity tags. Generally,
  last-modified values received or used by these systems will support

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  transparent and efficient caching, and so HTTP/1.1 origin servers
  should provide Last-Modified values. In those rare cases where the
  use of a Last-Modified value as a validator by an HTTP/1.0 system
  could result in a serious problem, then HTTP/1.1 origin servers
  should not provide one.


13.3.5 Non-validating Conditionals

The principle behind entity tags is that only the service author knows
the semantics of a resource well enough to select an appropriate cache
validation mechanism, and the specification of any validator comparison
function more complex than byte-equality would open up a can of worms.
Thus, comparisons of any other headers (except Last-Modified, for
compatibility with HTTP/1.0) are never used for purposes of validating a
cache entry.


13.4 Response Cachability

Unless specifically constrained by a Cache-Control (section 14.9)
directive, a caching system may always store a successful response (see
section 13.8) as a cache entry, may return it without validation if it
is fresh, and may return it after successful validation. If there is
neither a cache validator nor an explicit expiration time associated
with a response, we do not expect it to be cached, but certain caches
may violate this expectation (for example, when little or no network
connectivity is available). A client can usually detect that such a
response was taken from a cache by comparing the Date header to the
current time.

  Note that some HTTP/1.0 caches are known to violate this
  expectation without providing any Warning.

However, in some cases it may be inappropriate for a cache to retain an
entity, or to return it in response to a subsequent request. This may be
because absolute semantic transparency is deemed necessary by the
service author, or because of security or privacy considerations.
Certain Cache-Control directives are therefore provided so that the
server can indicate that certain resource entities, or portions thereof,
may not be cached regardless of other considerations.

Note that section 14.8 normally prevents a shared cache from saving and
returning a response to a previous request if that request included an
Authorization header.

A response received with a status code of 200, 203, 206, 300, 301 or 410
may be stored by a cache and used in reply to a subsequent request,
subject to the expiration mechanism, unless a Cache-Control directive
prohibits caching. However, a cache that does not support the Range and
Content-Range headers MUST NOT cache 206 (Partial Content) responses.

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A response received with any other status code MUST NOT be returned in a
reply to a subsequent request unless there are Cache-Control directives
or another header(s) that explicitly allow it. For example, these
include the following: an Expires header (section 14.21); a "max-age",
"must-revalidate", "proxy-revalidate", "public" or "private" Cache-
Control directive (section 14.9).


13.5 Constructing Responses From Caches

The purpose of an HTTP cache is to store information received in
response to requests, for use in responding to future requests. In many
cases, a cache simply returns the appropriate parts of a response to the
requester. However, if the cache holds a cache entry based on a previous
response, it may have to combine parts of a new response with what is
held in the cache entry.


13.5.1 End-to-end and Hop-by-hop Headers

For the purpose of defining the behavior of caches and non-caching
proxies, we divide HTTP headers into two categories:

  .  End-to-end headers, which must be transmitted to the ultimate
     recipient of a request or response. End-to-end headers in responses
     must be stored as part of a cache entry and transmitted in any
     response formed from a cache entry.
  .  Hop-by-hop headers, which are meaningful only for a single
     transport-level connection, and are not stored by caches or
     forwarded by proxies.
The following HTTP/1.1 headers are hop-by-hop headers:

  .  Connection
  .  Keep-Alive
  .  Public
  .  Proxy-Authenticate
  .  Transfer-Encoding
  .  Upgrade
All other headers defined by HTTP/1.1 are end-to-end headers.

Hop-by-hop headers introduced in future versions of HTTP MUST be listed
in a Connection header, as described in section 14.10.


13.5.2 Non-modifiable Headers

Some features of the HTTP/1.1 protocol, such as Digest Authentication,
depend on the value of certain end-to-end headers. A cache or non-
caching proxy SHOULD NOT modify an end-to-end header unless the
definition of that header requires or specifically allows that.


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A cache or non-caching proxy MUST NOT modify any of the following fields
in a request or response, nor may it add any of these fields if not
already present:

  .  Content-Location
  .  ETag
  .  Expires
  .  Last-Modified
A cache or non-caching proxy MUST NOT modify or add any of the following
fields in a response that contains the no-transform Cache-Control
directive, or in any request:

  .  Content-Encoding
  .  Content-Length
  .  Content-Range
  .  Content-Type
A cache or non-caching proxy MAY modify or add these fields in a
response that does not include no-transform, but if it does so, it MUST
add a Warning 14 (Transformation applied) if one does not already appear
in the response.

  Warning: unnecessary modification of end-to-end headers may cause
  authentication failures if stronger authentication mechanisms are
  introduced in later versions of HTTP. Such authentication
  mechanisms may rely on the values of header fields not listed here.


13.5.3 Combining Headers

When a cache makes a validating request to a server, and the server
provides a 304 (Not Modified) response, the cache must construct a
response to send to the requesting client. The cache uses the entity-
body stored in the cache entry as the entity-body of this outgoing
response. The end-to-end headers stored in the cache entry are used for
the constructed response, except that any end-to-end headers provided in
the 304 response MUST replace the corresponding headers from the cache
entry. Unless the cache decides to remove the cache entry, it MUST also
replace the end-to-end headers stored with the cache entry with
corresponding headers received in the incoming response.

In other words, the set of end-to-end headers received in the incoming
response overrides all corresponding end-to-end headers stored with the
cache entry. The cache may add Warning headers (see section 14.45) to
this set.

If a header field-name in the incoming response matches more than one
header in the cache entry, all such old headers are replaced.

  Note: this rule allows an origin server to use a 304 (Not Modified)
  response to update any header associated with a previous response
  for the same entity, although it might not always be meaningful or

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  correct to do so. This rule does not allow an origin server to use
  a 304 (not Modified) response to entirely delete a header that it
  had provided with a previous response.


13.5.4 Combining Byte Ranges

A response may transfer only a subrange of the bytes of an entity-body,
either because the request included one or more Range specifications, or
because a connection was broken prematurely. After several such
transfers, a cache may have received several ranges of the same entity-
body.

If a cache has a stored non-empty set of subranges for an entity, and an
incoming response transfers another subrange, the cache MAY combine the
new subrange with the existing set if both the following conditions are
met:

  .  Both the incoming response and the cache entry must have a cache
     validator.
  .  The two cache validators must match using the strong comparison
     function (see section 13.3.3).
If either requirement is not meant, the cache must use only the most
recent partial response (based on the Date values transmitted with every
response, and using the incoming response if these values are equal or
missing), and must discard the other partial information.


13.6 Caching Negotiated Responses

Use of server-driven content negotiation (section 12), as indicated by
the presence of a Vary header field in a response, alters the conditions
and procedure by which a cache can use the response for subsequent
requests.

A server MUST use the Vary header field (section 14.43) to inform a
cache of what header field dimensions are used to select among multiple
representations of a cachable response. A cache may use the selected
representation (the entity included with that particular response) for
replying to subsequent requests on that resource only when the
subsequent requests have the same or equivalent values for all header
fields specified in the Vary response-header. Requests with a different
value for one or more of those header fields would be forwarded toward
the origin server.

If an entity tag was assigned to the representation, the forwarded
request SHOULD be conditional and include the entity tags in an If-None-
Match header field from all its cache entries for the Request-URI. This
conveys to the server the set of entities currently held by the cache,
so that if any one of these entities matches the requested entity, the
server can use the ETag header in its 304 (Not Modified) response to

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tell the cache which entry is appropriate. If the entity-tag of the new
response matches that of an existing entry, the new response SHOULD be
used to update the header fields of the existing entry, and the result
MUST be returned to the client.

The Vary header field may also inform the cache that the representation
was selected using criteria not limited to the request-headers; in this
case, a cache MUST NOT use the response in a reply to a subsequent
request unless the cache relays the new request to the origin server in
a conditional request and the server responds with 304 (Not Modified),
including an entity tag or Content-Location that indicates which entity
should be used.

If any of the existing cache entries contains only partial content for
the associated entity, its entity-tag SHOULD NOT be included in the If-
None-Match header unless the request is for a range that would be fully
satisfied by that entry.

If a cache receives a successful response whose Content-Location field
matches that of an existing cache entry for the same Request-URI, whose
entity-tag differs from that of the existing entry, and whose Date is
more recent than that of the existing entry, the existing entry SHOULD
NOT be returned in response to future requests, and should be deleted
from the cache.


13.7 Shared and Non-Shared Caches

For reasons of security and privacy, it is necessary to make a
distinction between "shared" and "non-shared" caches. A non-shared cache
is one that is accessible only to a single user. Accessibility in this
case SHOULD be enforced by appropriate security mechanisms. All other
caches are considered to be "shared." Other sections of this
specification place certain constraints on the operation of shared
caches in order to prevent loss of privacy or failure of access
controls.


13.8 Errors or Incomplete Response Cache Behavior

A cache that receives an incomplete response (for example, with fewer
bytes of data than specified in a Content-Length header) may store the
response. However, the cache MUST treat this as a partial response.
Partial responses may be combined as described in section 13.5.4; the
result might be a full response or might still be partial. A cache MUST
NOT return a partial response to a client without explicitly marking it
as such, using the 206 (Partial Content) status code. A cache MUST NOT
return a partial response using a status code of 200 (OK).

If a cache receives a 5xx response while attempting to revalidate an
entry, it may either forward this response to the requesting client, or

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act as if the server failed to respond. In the latter case, it MAY
return a previously received response unless the cached entry includes
the "must-revalidate" Cache-Control directive (see section 14.9).


13.9 Side Effects of GET and HEAD

Unless the origin server explicitly prohibits the caching of their
responses, the application of GET and HEAD methods to any resources
SHOULD NOT have side effects that would lead to erroneous behavior if
these responses are taken from a cache. They may still have side
effects, but a cache is not required to consider such side effects in
its caching decisions. Caches are always expected to observe an origin
server's explicit restrictions on caching.

We note one exception to this rule: since some applications have
traditionally used GETs and HEADs with query URLs (those containing a
"?" in the rel_path part) to perform operations with significant side
effects, caches MUST NOT treat responses to such URLs as fresh unless
the server provides an explicit expiration time. This specifically means
that responses from HTTP/1.0 servers for such URIs should not be taken
from a cache. See section 9.1.1 for related information.


13.10 Invalidation After Updates or Deletions

The effect of certain methods at the origin server may cause one or more
existing cache entries to become non-transparently invalid. That is,
although they may continue to be "fresh," they do not accurately reflect
what the origin server would return for a new request.

There is no way for the HTTP protocol to guarantee that all such cache
entries are marked invalid. For example, the request that caused the
change at the origin server may not have gone through the proxy where a
cache entry is stored. However, several rules help reduce the likelihood
of erroneous behavior.

In this section, the phrase "invalidate an entity" means that the cache
should either remove all instances of that entity from its storage, or
should mark these as "invalid" and in need of a mandatory revalidation
before they can be returned in response to a subsequent request.

Some HTTP methods may invalidate an entity. This is either the entity
referred to by the Request-URI, or by the Location or Content-Location
response-headers (if present). These methods are:

  .  PUT
  .  DELETE
  .  POST


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In order to prevent denial of service attacks, an invalidation based on
the URI in a Location or Content-Location header MUST only be performed
if the host part is the same as in the Request-URI.


13.11 Write-Through Mandatory

All methods that may be expected to cause modifications to the origin
server's resources MUST be written through to the origin server. This
currently includes all methods except for GET and HEAD. A cache MUST NOT
reply to such a request from a client before having transmitted the
request to the inbound server, and having received a corresponding
response from the inbound server. This does not prevent a cache from
sending a 100 (Continue) response before the inbound server has replied.

The alternative (known as "write-back" or "copy-back" caching) is not
allowed in HTTP/1.1, due to the difficulty of providing consistent
updates and the problems arising from server, cache, or network failure
prior to write-back.


13.12 Cache Replacement

If a new cachable (see sections 14.9.2, 13.2.5, 13.2.6 and 13.8)
response is received from a resource while any existing responses for
the same resource are cached, the cache SHOULD use the new response to
reply to the current request. It may insert it into cache storage and
may, if it meets all other requirements, use it to respond to any future
requests that would previously have caused the old response to be
returned. If it inserts the new response into cache storage it should
follow the rules in section 13.5.3.

  Note: a new response that has an older Date header value than
  existing cached responses is not cachable.


13.13 History Lists

User agents often have history mechanisms, such as "Back" buttons and
history lists, which can be used to redisplay an entity retrieved
earlier in a session.

History mechanisms and caches are different. In particular history
mechanisms SHOULD NOT try to show a semantically transparent view of the
current state of a resource. Rather, a history mechanism is meant to
show exactly what the user saw at the time when the resource was
retrieved.

By default, an expiration time does not apply to history mechanisms. If
the entity is still in storage, a history mechanism should display it


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even if the entity has expired, unless the user has specifically
configured the agent to refresh expired history documents.

This should not be construed to prohibit the history mechanism from
telling the user that a view may be stale.

  Note: if history list mechanisms unnecessarily prevent users from
  viewing stale resources, this will tend to force service authors to
  avoid using HTTP expiration controls and cache controls when they
  would otherwise like to. Service authors may consider it important
  that users not be presented with error messages or warning messages
  when they use navigation controls (such as BACK) to view previously
  fetched resources. Even though sometimes such resources ought not
  to cached, or ought to expire quickly, user interface
  considerations may force service authors to resort to other means
  of preventing caching (e.g. "once-only" URLs) in order not to
  suffer the effects of improperly functioning history mechanisms.


14 Header Field Definitions

This section defines the syntax and semantics of all standard HTTP/1.1
header fields. For entity-header fields, both sender and recipient refer
to either the client or the server, depending on who sends and who
receives the entity.


14.1 Accept

The Accept request-header field can be used to specify certain media
types which are acceptable for the response. Accept headers can be used
to indicate that the request is specifically limited to a small set of
desired types, as in the case of a request for an in-line image.

       Accept         = "Accept" ":" #(
                             media-range
                             [ ( ":" | ";" )
                               range-parameter
                               *( ";" range-parameter ) ]
                            | extension-token )

       media-range    = ( "*/*"
                        | ( type "/" "*" )
                        | ( type "/" subtype )
                        ) *( ";" parameter )

       range-parameter = ( "q" "=" qvalue )
                       | extension-range-parameter

       extension-range-parameter = ( token "=" token )


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       extension-token = token

The asterisk "*" character is used to group media types into ranges,
with "*/*" indicating all media types and "type/*" indicating all
subtypes of that type. The range-parameter q is used to indicate the
media type quality factor for the range, which represents the user's
preference for that range of media types. The default value is q=1. In
Accept headers sent by HTTP/1.1 clients, the character separating media-
ranges from range-parameters MUST be a ":". HTTP/1.1 servers SHOULD be
tolerant of use of the ";" separator by HTTP/1.0 clients.

The example

       Accept: audio/*: q=0.2, audio/basic

SHOULD be interpreted as "I prefer audio/basic, but send me any audio
type if it is the best available after an 80% mark-down in quality."

If no Accept header is present, then it is assumed that the client
accepts all media types. If Accept headers are present, and if the
server cannot send a response which is acceptable according to the
Accept headers, then the server SHOULD send an error response with the
406 (not acceptable) status code, though the sending of an unacceptable
response is also allowed.

A more elaborate example is

       Accept: text/plain: q=0.5, text/html,
               text/x-dvi: q=0.8, text/x-c

Verbally, this would be interpreted as "text/html and text/x-c are the
preferred media types, but if they do not exist, then send the text/x-
dvi entity, and if that does not exist, send the text/plain entity."

Media ranges can be overridden by more specific media ranges or specific
media types. If more than one media range applies to a given type, the
most specific reference has precedence. For example,

       Accept: text/*, text/html, text/html;level=1, */*

have the following precedence:

       1) text/html;level=1
       2) text/html
       3) text/*
       4) */*

The media type quality factor associated with a given type is determined
by finding the media range with the highest precedence which matches
that type. For example,


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       Accept: text/*:q=0.3, text/html:q=0.7, text/html;level=1,
               text/html;level=2:q=0.4, */*:q=0.5

would cause the following values to be associated:

       text/html;level=1         = 1
       text/html                 = 0.7
       text/plain                = 0.3
       image/jpeg                = 0.5
       text/html;level=2         = 0.4
       text/html;level=3         = 0.7

  Note: A user agent may be provided with a default set of quality
  values for certain media ranges. However, unless the user agent is
  a closed system which cannot interact with other rendering agents,
  this default set should be configurable by the user.


14.2 Accept-Charset

The Accept-Charset request-header field can be used to indicate what
character sets are acceptable for the response. This field allows
clients capable of understanding more comprehensive or special-purpose
character sets to signal that capability to a server which is capable of
representing documents in those character sets. The ISO-8859-1 character
set can be assumed to be acceptable to all user agents.

       Accept-Charset = "Accept-Charset" ":"
                 1#( charset [ ";" "q" "=" qvalue ] )

Character set values are described in section 3.4. Each charset may be
given an associated quality value which represents the user's preference
for that charset. The default value is q=1. An example is

       Accept-Charset: iso-8859-5, unicode-1-1;q=0.8

If no Accept-Charset header is present, the default is that any
character set is acceptable. If an Accept-Charset header is present, and
if the server cannot send a response which is acceptable according to
the Accept-Charset header, then the server SHOULD send an error response
with the 406 (not acceptable) status code, though the sending of an
unacceptable response is also allowed.


14.3 Accept-Encoding

The Accept-Encoding request-header field is similar to Accept, but
restricts the content-coding values (section 14.12) which are acceptable
in the response.


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       Accept-Encoding  = "Accept-Encoding" ":"
                                 #( content-coding )

An example of its use is

       Accept-Encoding: compress, gzip

If no Accept-Encoding header is present in a request, the server MAY
assume that the client will accept any content coding. If an Accept-
Encoding header is present, and if the server cannot send a response
which is acceptable according to the Accept-Encoding header, then the
server SHOULD send an error response with the 406 (Not Acceptable)
status code.

An empty Accept-Encoding value indicates none are acceptable.


14.4 Accept-Language

The Accept-Language request-header field is similar to Accept, but
restricts the set of natural languages that are preferred as a response
to the request.

       Accept-Language = "Accept-Language" ":"
                         1#( language-range [ ";" "q" "=" qvalue ] )

       language-range  = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" )

Each language-range MAY be given an associated quality value which
represents an estimate of the user's preference for the languages
specified by that range. The quality value defaults to "q=1". For
example,

       Accept-Language: da, en-gb;q=0.8, en;q=0.7

would mean: "I prefer Danish, but will accept British English and other
types of English." A language-range matches a language-tag if it exactly
equals the tag, or if it exactly equals a prefix of the tag such that
the first tag character following the prefix is "-". The special range
"*", if present in the Accept-Language field, matches every tag not
matched by any other range present in the Accept-Language field.

  Note: This use of a prefix matching rule does not imply that
  language tags are assigned to languages in such a way that it is
  always true that if a user understands a language with a certain
  tag, then this user will also understand all languages with tags
  for which this tag is a prefix. The prefix rule simply allows the
  use of prefix tags if this is the case.

The language quality factor assigned to a language-tag by the Accept-
Language field is the quality value of the longest language-range in the

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field that matches the language-tag. If no language-range in the field
matches the tag, the language quality factor assigned is 0. If no
Accept-Language header is present in the request, the server SHOULD
assume that all languages are equally acceptable. If an Accept-Language
header is present, then all languages which are assigned a quality
factor greater than 0 are acceptable.

It may be contrary to the privacy expectations of the user to send an
Accept-Language header with the complete linguistic preferences of the
user in every request. For a discussion of this issue, see section 15.7.

  Note: As intelligibility is highly dependent on the individual
  user, it is recommended that client applications make the choice of
  linguistic preference available to the user. If the choice is not
  made available, then the Accept-Language header field must not be
  given in the request.


14.5 Accept-Ranges

The Accept-Ranges response header allows the server to indicate its
acceptance of range requests for a resource:

       Accept-Ranges     = "Accept-Ranges" ":" acceptable-ranges

       acceptable-ranges = 1#range-unit | "none"

Origin servers that accept byte-range requests MAY send

       Accept-Ranges: bytes

but are not required to do so. Clients MAY generate byte-range requests
without having received this header for the resource involved.

Servers that do not accept any kind of range request for a  resource MAY
send

       Accept-Ranges: none

to advise the client not to attempt a range request.


14.6 Age

The Age response-header field conveys the sender's estimate of the
amount of time since the response (or its revalidation) was generated at
the origin server. A cached response is "fresh" if its age does not
exceed its freshness lifetime. Age values are calculated as specified in
section 13.2.3.

        Age = "Age" ":" age-value

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        age-value = delta-seconds

Age values are non-negative decimal integers, representing time in
seconds.

If a cache receives a value larger than the largest positive integer it
can represent, or if any of its age calculations overflows, it MUST
transmit an Age header with a value of 2147483648 (2^31). HTTP/1.1
caches MUST send an Age header in every response. Caches SHOULD use an
arithmetic type of at least 31 bits of range.


14.7 Allow

The Allow entity-header field lists the set of methods supported by the
resource identified by the Request-URI. The purpose of this field is
strictly to inform the recipient of valid methods associated with the
resource. An Allow header field MUST be present in a 405 (Method Not
Allowed) response.

       Allow          = "Allow" ":" 1#method

Example of use:

       Allow: GET, HEAD, PUT

This field cannot prevent a client from trying other methods. However,
the indications given by the Allow header field value SHOULD be
followed. The actual set of allowed methods is defined by the origin
server at the time of each request.

The Allow header field MAY be provided with a PUT request to recommend
the methods to be supported by the new or modified resource. The server
is not required to support these methods and SHOULD include an Allow
header in the response giving the actual supported methods.

A proxy MUST NOT modify the Allow header field even if it does not
understand all the methods specified, since the user agent MAY have
other means of communicating with the origin server.

The Allow header field does not indicate what methods are implemented at
the server level. Servers MAY use the Public response-header field
(section 14.35) to describe what methods are implemented on the server
as a whole.


14.8 Authorization

A user agent that wishes to authenticate itself with a server--usually,
but not necessarily, after receiving a 401 response--MAY do so by
including an Authorization request-header field with the request. The

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Authorization field value consists of credentials containing the
authentication information of the user agent for the realm of the
resource being requested.

       Authorization  = "Authorization" ":" credentials

HTTP access authentication is described in section 11. If a request is
authenticated and a realm specified, the same credentials SHOULD be
valid for all other requests within this realm.

When a shared cache (see section 13.7) receives a request containing an
Authorization field, it MUST NOT return the corresponding response as a
reply to any other request, unless one of the following specific
exceptions holds:

  1. If the response includes the "proxy-revalidate" Cache-Control
     directive, the cache MAY use that response in replying to a
     subsequent request, but a proxy cache MUST first revalidate it with
     the origin server, using the request-headers from the new request
     to allow the origin server to authenticate the new request.
  2. If the response includes the "must-revalidate" Cache-Control
     directive, the cache MAY use that response in replying to a
     subsequent request, but all caches MUST first revalidate it with
     the origin server, using the request-headers from the new request
     to allow the origin server to authenticate the new request.
  3. If the response includes the "public" Cache-Control directive, it
     may be returned in reply to any subsequent request.

14.9 Cache-Control

The Cache-Control general-header field is used to specify directives
that MUST be obeyed by all caching mechanisms along the request/response
chain. The directives specify behavior intended to prevent caches from
adversely interfering with the request or response. These directives
typically override the default caching algorithms. Cache directives are
unidirectional in that the presence of a directive in a request does not
imply that the same directive should be given in the response.

  Note that HTTP/1.0 caches may not implement Cache-Control and may
  only implement Pragma: no-cache (see section 14.32).

Cache directives must be passed through by a proxy or gateway
application, regardless of their significance to that application, since
the directives may be applicable to all recipients along the
request/response chain. It is not possible to specify a cache-directive
for a specific cache.

       Cache-Control   = "Cache-Control" ":" 1#cache-directive

       cache-directive = cache-request-directive
                       | cache-response-directive

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       cache-request-directive =
                       | "no-cache" [ "=" <"> 1#field-name <"> ]
                       | "no-store"
                       | "max-age" "=" delta-seconds
                       | "max-stale" "=" [ delta-seconds ]
                       | "min-fresh" "=" delta-seconds
                       | "only-if-cached"
                       | cache-extension

       cache-response-directive =
                         "public"
                       | "private" [ "=" <"> 1#field-name <"> ]
                       | "no-cache" [ "=" <"> 1#field-name <"> ]
                       | "no-store"
                       | "no-transform"
                       | "must-revalidate"
                       | "proxy-revalidate"
                       | "max-age" "=" delta-seconds
                       | cache-extension

       cache-extension = token [ "=" ( token | quoted-string ) ]

When a directive appears without any 1#field-name parameter, the
directive applies to the entire request or response. When such a
directive appears with a 1#field-name parameter, it applies only to the
named field or fields, and not to the rest of the request or response.
This mechanism supports extensibility; implementations of future
versions of the HTTP protocol may apply these directives to header
fields not defined in HTTP/1.1.

The cache-control directives can be broken down into these general
categories:

  .  Restrictions on what is cachable; these may only be imposed by the
     origin server.
  .  Restrictions on what may be stored by a cache; these may be imposed
     by either the origin server or the user agent.
  .  Modifications of the basic expiration mechanism; these may be
     imposed by either the origin server or the user agent.
  .  Controls over cache revalidation and reload; these may only be
     imposed by an user agent.
  .  Control over transformation of entities.
  .  Extensions to the caching system.

14.9.1 What is Cachable

By default, a response is cachable if the requirements of the request
method, request header fields, and the response status indicate that it
is cachable. Section 13.4 summarizes these defaults for cachability. The
following Cache-Control response directives allow an origin server to
override the default cachability of a response:

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public
  Indicates that the response is cachable by any cache, even if it
  would normally be non-cachable or cachable only within a non-shared
  cache. (See also Authorization, section 14.8, for additional
  details.)

private
  Indicates that all or part of the response message is intended for a
  single user and MUST NOT be cached by a shared cache. This allows an
  origin server to state that the specified parts of the response are
  intended for only one user and are not a valid response for requests
  by other users. A private (non-shared) cache may cache the response.

  Note: This usage of the word private only controls where the
  response may be cached, and cannot ensure the privacy of the
  message content.

no-cache
  Indicates that all or part of the response message MUST NOT be cached
  anywhere. This allows an origin server to prevent caching even by
  caches that have been configured to return stale responses to client
  requests.

  Note: HTTP/1.0 caches will not recognize or obey this directive.


14.9.2 What May be Stored by Caches

The purpose of the no-store directive is to prevent the inadvertent
release or retention of sensitive information (for example, on backup
tapes). The no-store directive applies to the entire message, and may be
sent either in a response or in a request. If sent in a request, a cache
MUST NOT store any part of either this request or any response to it. If
sent in a response, a cache MUST NOT store any part of either this
response or the request that elicited it. This directive applies to both
non-shared and shared caches. "MUST NOT store" in this context means
that the cache MUST NOT intentionally store the information in non-
volatile storage, and MUST make a best-effort attempt to remove the
information from volatile storage as promptly as possible after
forwarding it.

Even when this directive is associated with a response, users may
explicitly store such a response outside of the caching system (e.g.,
with a "Save As" dialog). History buffers may store such responses as
part of their normal operation.

The purpose of this directive is to meet the stated requirements of
certain users and service authors who are concerned about accidental
releases of information via unanticipated accesses to cache data
structures. While the use of this directive may improve privacy in some
cases, we caution that it is NOT in any way a reliable or sufficient

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mechanism for ensuring privacy. In particular, malicious or compromised
caches may not recognize or obey this directive; and communications
networks may be vulnerable to eavesdropping.


14.9.3 Modifications of the Basic Expiration Mechanism

The expiration time of an entity may be specified by the origin server
using the Expires header (see section 14.21). Alternatively, it may be
specified using the max-age directive in a response.

If a response includes both an Expires header and a max-age directive,
the max-age directive overrides the Expires header, even if the Expires
header is more restrictive. This rule allows an origin server to
provide, for a given response, a longer expiration time to an HTTP/1.1
(or later) cache than to an HTTP/1.0 cache. This may be useful if
certain HTTP/1.0 caches improperly calculate ages or expiration times,
perhaps due to synchronized clocks.

  Note: most older caches, not compliant with this specification, do
  not implement any Cache-Control directives.  An origin server
  wishing to use a Cache-Control directive that restricts, but does
  not prevent, caching by an HTTP/1.1-compliant cache may exploit the
  requirement that the max-age directive overrides the Expires
  header, and the fact that non-HTTP/1.1-compliant caches do not
  observe the max-age directive.

Other directives allow an user agent to modify the basic expiration
mechanism. These directives may be specified on a request:

max-age
  Indicates that the client is willing to accept a response whose age
  is no greater than the specified time in seconds. Unless max-stale
  directive is also included, the client is not willing to accept a
  stale response.

min-fresh
  Indicates that the client is willing to accept a response whose
  freshness lifetime is no less than its current age plus the specified
  time in seconds. That is, the client wants a response that will still
  be fresh for at least the specified number of seconds.

max-stale
  Indicates that the client is willing to accept a response that has
  exceeded its expiration time. If max-stale is assigned a value, then
  the client is willing to accept a response that has exceeded its
  expiration time by no more than the specified number of seconds. If
  no value is assigned to max-stale, then the client is willing to
  accept a stale response of any age.


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If a cache returns a stale response, either because of a max-stale
directive on a request, or because the cache is configured to override
the expiration time of a response, the cache MUST attach a Warning
header to the stale response, using Warning 10 (Response is stale).


14.9.4 Cache Revalidation and Reload Controls

Sometimes an user agent may want or need to insist that a cache
revalidate its cache entry with the origin server (and not just with the
next cache along the path to the origin server), or to reload its cache
entry from the origin server. End-to-end revalidation may be necessary
if either the cache or the origin server has overestimated the
expiration time of the cached response. End-to-end reload may be
necessary if the cache entry has become corrupted for some reason.

End-to-end revalidation may be requested either when the client does not
have its own local cached copy, in which case we call it "unspecified
end-to-end revalidation", or when the client does have a local cached
copy, in which case we call it "specific end-to-end revalidation."

The client can specify these three kinds of action using Cache-Control
request directives:

End-to-end reload
  The request includes a "no-cache" Cache-Control directive or, for
  compatibility with HTTP/1.0 clients, "Pragma: no-cache". No field
  names may be included with the no-cache directive in a request. The
  server MUST NOT use a cached copy when responding to such a request.

Specific end-to-end revalidation
  The request includes a "max-age=0" Cache-Control directive, which
  forces each cache along the path to the origin server to revalidate
  its own entry, if any, with the next cache or server. The initial
  request includes a cache-validating conditional with the client's
  current validator.

Unspecified end-to-end revalidation
  The request includes "max-age=0" Cache-Control directive, which
  forces each cache along the path to the origin server to revalidate
  its own entry, if any, with the next cache or server. The initial
  request does not include a cache-validating conditional; the first
  cache along the path (if any) that holds a cache entry for this
  resource includes a cache-validating conditional with its current
  validator.

When an intermediate cache is forced, by means of a max-age=0 directive,
to revalidate its own cache entry, and the client has supplied its own
validator in the request, the supplied validator may differ from the
validator currently stored with the cache entry. In this case, the cache


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may use either validator in making its own request without affecting
semantic transparency.

However, the choice of validator may affect performance. The best
approach is for the intermediate cache to use its own validator when
making its request. If the server replies with 304 (Not Modified), then
the cache should return its now validated copy to the client with a 200
(OK) response. If the server replies with a new entity and cache
validator, however, the intermediate cache should compare the returned
validator with the one provided in the client's request, using the
strong comparison function. If the client's validator is equal to the
origin server's, then the intermediate cache simply returns 304 (Not
Modified). Otherwise, it returns the new entity with a 200 (OK)
response.

If a request includes the no-cache directive, it should not include min-
fresh, max-stale, or max-age.

In some cases, such as times of extremely poor network connectivity, a
client may want a cache to return only those responses that it currently
has stored, and not to reload or revalidate with the origin server. To
do this, the client may include the only-if-cached directive in a
request. If it receives this directive, a cache SHOULD either respond
using a cached entry that is consistent with the other constraints of
the request, or respond with a 504 (Gateway Timeout) status. However, if
a group of caches is being operated as a unified system with good
internal connectivity, such a request MAY be forwarded within that group
of caches.

Because a cache may be configured to ignore a server's specified
expiration time, and because a client request may include a max-stale
directive (which has a similar effect), the protocol also includes a
mechanism for the origin server to require revalidation of a cache entry
on any subsequent use. When the must-revalidate directive is present in
a response received by a cache, that cache MUST NOT use the entry after
it becomes stale to respond to a subsequent request without first
revalidating it with the origin server. (I.e., the cache must do an end-
to-end revalidation every time, if, based solely on the origin server's
Expires or max-age value, the cached response is stale.)

The must-revalidate directive is necessary to support reliable operation
for certain protocol features. In all circumstances an HTTP/1.1 cache
MUST obey the must-revalidate directive; in particular, if the cache
cannot reach the origin server for any reason, it MUST generate a 504
(Gateway Timeout) response.

Servers should send the must-revalidate directive if and only if failure
to revalidate a request on the entity could result in incorrect
operation, such as a silently unexecuted financial transaction.
Recipients MUST NOT take any automated action that violates this


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directive, and MUST NOT automatically provide an unvalidated copy of the
entity if revalidation fails.

Although this is not recommended, user agents operating under severe
connectivity constraints may violate this directive but, if so, MUST
explicitly warn the user that an unvalidated response has been provided.
The warning MUST be provided on each unvalidated access, and SHOULD
require explicit user confirmation.

The proxy-revalidate directive has the same meaning as the must-
revalidate directive, except that it does not apply to non-shared user
agent caches. It can be used on a response to an authenticated request
to permit the user's cache to store and later return the response
without needing to revalidate it (since it has already been
authenticated once by that user), while still requiring proxies that
service many users to revalidate each time (in order to make sure that
each user has been authenticated). Note that such authenticated
responses also need the public cache control directive in order to allow
them to be cached at all.


14.9.5 No-Transform Directive

Implementers of intermediate caches (proxies) have found it useful to
convert the media type of certain entity bodies. A proxy might, for
example, convert between image formats in order to save cache space or
to reduce the amount of traffic on a slow link. HTTP has to date been
silent on these transformations.

Serious operational problems have already occurred, however, when these
transformations have been applied to entity bodies intended for certain
kinds of applications. For example, applications for medical imaging,
scientific data analysis and those using end-to-end authentication, all
depend on receiving an entity body that is bit for bit identical to the
original entity-body.

Therefore, if a response includes the no-transform directive, an
intermediate cache or proxy MUST NOT change those headers that are
listed in section 13.5.2as being subject to the no-transform directive.
This implies that the cache or proxy must not change any aspect of the
entity-body that is specified by these headers.


14.9.6 Cache Control Extensions

The Cache-Control header field can be extended through the use of one or
more cache-extension tokens, each with an optional assigned value.
Informational extensions (those which do not require a change in cache
behavior) may be added without changing the semantics of other
directives. Behavioral extensions are designed to work by acting as
modifiers to the existing base of cache directives. Both the new

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directive and the standard directive are supplied, such that
applications which do not understand the new directive will default to
the behavior specified by the standard directive, and those that
understand the new directive will recognize it as modifying the
requirements associated with the standard directive.  In this way,
extensions to the Cache-Control directives can be made without requiring
changes to the base protocol.

This extension mechanism depends on a HTTP cache obeying all of the
cache-control directives defined for its native HTTP-version, obeying
certain extensions, and ignoring all directives that it does not
understand.

For example, consider a hypothetical new response directive called
"community" which acts as a modifier to the "private" directive. We
define this new directive to mean that, in addition to any non-shared
cache, any cache which is shared only by members of the community named
within its value may cache the response. An origin server wishing to
allow the "UCI" community to use an otherwise private response in their
shared cache(s) may do so by including

       Cache-Control: private, community="UCI"

A cache seeing this header field will act correctly even if the cache
does not understand the "community" cache-extension, since it will also
see and understand the "private" directive and thus default to the safe
behavior.

Unrecognized cache-directives MUST be ignored; it is assumed that any
cache-directive likely to be unrecognized by an HTTP/1.1 cache will be
combined with standard directives (or the response's default
cachability) such that the cache behavior will remain minimally correct
even if the cache does not understand the extension(s).


14.10 Connection

The Connection general-header field allows the sender to specify options
that are desired for that particular connection and MUST NOT be
communicated by proxies over further connections.

The Connection header has the following grammar:

       Connection-header = "Connection" ":" 1#(connection-token)
       connection-token  = token

HTTP/1.1 proxies MUST parse the Connection header field before a message
is forwarded and, for each connection-token in this field, remove any
header field(s) from the message with the same name as the connection-
token. Connection options are signaled by the presence of a connection-
token in the Connection header field, not by any corresponding

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additional header field(s), since the additional header field may not be
sent if there are no parameters associated with that connection option.

HTTP/1.1 defines the "close" connection option for the sender to signal
that the connection will be closed after completion of the response. For
example,

       Connection: close

in either the request or the response header fields indicates that the
connection should not be considered `persistent' (section 8.1) after the
current request/response is complete.

HTTP/1.1 applications that do not support persistent connections MUST
include the "close" connection option in every message.


14.11 Content-Base

The Content-Base entity-header field may be used to specify the base URI
for resolving relative URLs within the entity. This header field is
described as Base in RFC 1808, which is expected to be revised soon.

       Content-Base      = "Content-Base" ":" absoluteURI

If no Content-Base field is present, the base URI of an entity is
defined either by its Content-Location (if that Content-Location URI is
an absolute URI) or the URI used to initiate the request, in that order
of precedence. Note, however, that the base URI of the contents within
the entity-body may be redefined within that entity-body.


14.12 Content-Encoding

The Content-Encoding entity-header field is used as a modifier to the
media-type. When present, its value indicates what additional content
codings have been applied to the entity-body, and thus what decoding
mechanisms MUST be applied in order to obtain the media-type referenced
by the Content-Type header field. Content-Encoding is primarily used to
allow a document to be compressed without losing the identity of its
underlying media type.

       Content-Encoding  = "Content-Encoding" ":" 1#content-coding

Content codings are defined in section 3.5. An example of its use is

       Content-Encoding: gzip

The Content-Encoding is a characteristic of the entity identified by the
Request-URI. Typically, the entity-body is stored with this encoding and
is only decoded before rendering or analogous usage.

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If multiple encodings have been applied to an entity, the content
codings MUST be listed in the order in which they were applied.
Additional information about the encoding parameters MAY be provided by
other entity-header fields not defined by this specification.


14.13 Content-Language

The Content-Language entity-header field describes the natural
language(s) of the intended audience for the enclosed entity. Note that
this may not be equivalent to all the languages used within the entity-
body.

       Content-Language  = "Content-Language" ":" 1#language-tag

Language tags are defined in section 3.10. The primary purpose of
Content-Language is to allow a user to identify and differentiate
entities according to the user's own preferred language. Thus, if the
body content is intended only for a Danish-literate audience, the
appropriate field is

       Content-Language: da

If no Content-Language is specified, the default is that the content is
intended for all language audiences. This may mean that the sender does
not consider it to be specific to any natural language, or that the
sender does not know for which language it is intended.

Multiple languages MAY be listed for content that is intended for
multiple audiences. For example, a rendition of the "Treaty of
Waitangi," presented simultaneously in the original Maori and English
versions, would call for

       Content-Language: mi, en

However, just because multiple languages are present within an entity
does not mean that it is intended for multiple linguistic audiences. An
example would be a beginner's language primer, such as "A First Lesson
in Latin," which is clearly intended to be used by an English-literate
audience. In this case, the Content-Language should only include "en".

Content-Language may be applied to any media type -- it is not limited
to textual documents.


14.14 Content-Length

The Content-Length entity-header field indicates the size of the
message-body, in decimal number of octets, sent to the recipient or, in
the case of the HEAD method, the size of the entity-body that would have
been sent had the request been a GET.

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       Content-Length    = "Content-Length" ":" 1*DIGIT

An example is

       Content-Length: 3495

Applications SHOULD use this field to indicate the size of the message-
body to be transferred, regardless of the media type of the entity. It
must be possible for the recipient to reliably determine the end of
HTTP/1.1 requests containing an entity-body, e.g., because the request
has a valid Content-Length field, uses Transfer-Encoding: chunked or a
multipart body.

Any Content-Length greater than or equal to zero is a valid value.
Section 4.4 describes how to determine the length of a message-body if a
Content-Length is not given.

  Note: The meaning of this field is significantly different from the
  corresponding definition in MIME, where it is an optional field
  used within the "message/external-body" content-type. In HTTP, it
  SHOULD be sent whenever the message's length can be determined
  prior to being transferred.


14.15 Content-Location

The Content-Location entity-header field may be used to supply the
resource location for the entity enclosed in the message. In the case
where a resource has multiple entities associated with it, and those
entities actually have separate locations by which they might be
individually accessed, the server should provide a Content-Location for
the particular variant which is returned. In addition, a server SHOULD
provide a Content-Location for the resource corresponding to the
response entity.

       Content-Location = "Content-Location" ":"
                         ( absoluteURI | relativeURI )

If no Content-Base header field is present, the value of Content-
Location also defines the base URL for the entity (see section 14.11).

The Content-Location value is not a replacement for the original
requested URI; it is only a statement. of the location of the resource
corresponding to this particular entity at the time of the request.
Future requests MAY use the Content-Location URI if the desire is to
identify the source of that particular entity.

A cache cannot assume that an entity with a Content-Location different
from the URI used to retrieve it can be used to respond to later
requests on that Content-Location URI. However, the Content-Location can


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be used to differentiate between multiple entities retrieved from a
single requested resource, as described in section 13.6.

If the Content-Location is a relative URI, the URI is interpreted
relative to any Content-Base URI provided in the response. If no
Content-Base is provided, the relative URI is interpreted relative to
the Request-URI.


14.16 Content-MD5

The Content-MD5 entity-header field, as defined in RFC 1864 [23] is a
MD5 digest of the entity-body, for the purpose of providing an end-to-
end message integrity check (MIC) of the entity-body. (Note: a MIC is
good for detecting accidental modification of the entity-body in
transit, but is not proof against malicious attacks.)

        ContentMD5   = "Content-MD5" ":" md5-digest

        md5-digest   = <base64 of 128 bit MD5 digest as per RFC 1864>

The Content-MD5 header field may be generated by an origin server to
function as an integrity check of the entity-body. Only origin servers
may generate the Content-MD5 header field; proxies and gateways MUST NOT
generate it, as this would defeat its value as an end-to-end integrity
check. Any recipient of the entity-body, including gateways and proxies,
MAY check that the digest value in this header field matches that of the
entity-body as received.

The MD5 digest is computed based on the content of the entity-body,
including any Content-Encoding that has been applied, but not including
any Transfer-Encoding that may have been applied to the message-body. If
the message is received with a Transfer-Encoding, that encoding must be
removed prior to checking the Content-MD5 value against the received
entity.

This has the result that the digest is computed on the octets of the
entity-body exactly as, and in the order that, they would be sent if no
Transfer-Encoding were being applied.

HTTP extends RFC 1864 to permit the digest to be computed for MIME
composite media-types (e.g., multipart/* and message/rfc822), but this
does not change how the digest is computed as defined in the preceding
paragraph.

  Note: There are several consequences of this. The entity-body for
  composite types may contain many body-parts, each with its own MIME
  and HTTP headers (including Content-MD5, Content-Transfer-Encoding,
  and Content-Encoding headers). If a body-part has a Content-
  Transfer-Encoding or Content-Encoding header, it is assumed that
  the content of the body-part has had the encoding applied, and the

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  body-part is included in the Content-MD5 digest as is -- i.e.,
  after the application. The Transfer-Encoding header field is not
  allowed within body-parts.

  Note: while the definition of Content-MD5 is exactly the same for
  HTTP as in RFC 1864 for MIME entity-bodies, there are several ways
  in which the application of Content-MD5 to HTTP entity-bodies
  differs from its application to MIME entity-bodies. One is that
  HTTP, unlike MIME, does not use Content-Transfer-Encoding, and does
  use Transfer-Encoding and Content-Encoding. Another is that HTTP
  more frequently uses binary content types than MIME, so it is worth
  noting that, in such cases, the byte order used to compute the
  digest is the transmission byte order defined for the type. Lastly,
  HTTP allows transmission of text types with any of several line
  break conventions and not just the canonical form using CRLF.
  Conversion of all line breaks to CRLF should not be done before
  computing or checking the digest: the line break convention used in
  the text actually transmitted should be left unaltered when
  computing the digest.


14.17 Content-Range

When a server returns a partial response to a client, it must describe
both the extent of the range covered by the response, and the length of
the entire entity-body.

The Content-Range header is sent with a partial entity-body to specify
where in the full entity-body the partial body should be inserted. It
also indicates the total size of the full entity-body.

       Content-Range = "Content-Range" ":" content-range-spec

       content-range-spec      = byte-content-range-spec

       byte-content-range-spec = bytes-unit SP first-byte-pos "-"
                                 last-byte-pos "/" entity-length

       entity-length           = 1*DIGIT

Unlike byte-ranges-specifier values, a byte-content-range-spec may only
specify one range, and must contain absolute byte positions for both the
first and last byte of the range.

A byte-content-range-spec whose last-byte-pos value is less than its
first-byte-pos value, or whose entity-length value is less than or equal
to its last-byte-pos value, is invalid. The recipient of an invalid
byte-content-range-spec MUST ignore it and any content transferred along
with it.


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Examples of byte-content-range-spec values, assuming that the entity
contains a total of 1234 bytes:

  .  The first 500 bytes:
       bytes 0-499/1234

  .  The second 500 bytes:
       bytes 500-999/1234

  .  All except for the first 500 bytes:
       bytes 500-1233/1234

  .  The last 500 bytes:
       bytes 734-1233/1234

When an HTTP message includes the content of a single range (for
example, a response to a request for a single range, or to a request for
a set of ranges that overlap without any holes), this content is
transmitted with a Content-Range header, and a Content-Length header
showing the number of bytes actually transferred. For example,

       HTTP/1.1 206 Partial content
       Date: Wed, 15 Nov 1995 06:25:24 GMT
       Last-modified: Wed, 15 Nov 1995 04:58:08 GMT
       Content-Range: 21010-47021/47022
       Content-Length: 26012
       Content-Type: image/gif

When an HTTP message includes the content of multiple ranges (for
example, a response to a request for multiple non-overlapping ranges),
these are transmitted as a multipart MIME message. The multipart MIME
content-type used for this purpose is defined in this specification to
be "multipart/byteranges". See appendix 19.2 for its definition.

A client that cannot decode a MIME multipart/byteranges message should
not ask for multiple byte-ranges in a single request.

When a client requests multiple byte-ranges in one request, the server
SHOULD return them in the order that they appeared in the request.

If the server ignores a byte-range-spec because it is invalid, the
server should treat the request as if the invalid Range header field did
not exist. (Normally, this means return a 200 response containing the
full entity). The reason is that the only time a client will make such
an invalid request is when the entity is smaller than the entity
retrieved by a prior request.


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14.18 Content-Type

The Content-Type entity-header field indicates the media type of the
entity-body sent to the recipient or, in the case of the HEAD method,
the media type that would have been sent had the request been a GET.

       Content-Type   = "Content-Type" ":" media-type

Media types are defined in section 3.7. An example of the field is

       Content-Type: text/html; charset=ISO-8859-4

Further discussion of methods for identifying the media type of an
entity is provided in section 7.2.1.


14.19 Date

The Date general-header field represents the date and time at which the
message was originated, having the same semantics as orig-date in RFC
822. The field value is an HTTP-date, as described in section 3.3.1.

       Date  = "Date" ":" HTTP-date

An example is

       Date: Tue, 15 Nov 1994 08:12:31 GMT

If a message is received via direct connection with the user agent (in
the case of requests) or the origin server (in the case of responses),
then the date can be assumed to be the current date at the receiving
end. However, since the date--as it is believed by the origin--is
important for evaluating cached responses, origin servers MUST include a
Date header field in all responses. Clients SHOULD only send a Date
header field in messages that include an entity-body, as in the case of
the PUT and POST requests, and even then it is optional. A received
message which does not have a Date header field SHOULD be assigned one
by the recipient if the message will be cached by that recipient or
gatewayed via a protocol which requires a Date.

In theory, the date SHOULD represent the moment just before the entity
is generated. In practice, the date can be generated at any time during
the message origination without affecting its semantic value.

The format of the Date is an absolute date and time as defined by HTTP-
date in section 3.3; it MUST be sent in RFC1123 [8]-date format.


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14.20 ETag

The ETag entity-header field defines the entity tag for the associated
entity. The headers used with entity tags are described in sections
14.20, 14.25, 14.26 and 14.43. The entity tag may be used for comparison
with other entities from the same resource (see section 13.3.2).

      ETag = "ETag" ":" entity-tag

Examples:

      ETag: "xyzzy"
      ETag: W/"xyzzy"
      ETag: ""


14.21 Expires

The Expires entity-header field gives the date/time after which the
response should be considered stale. A stale cache entry may not
normally be returned by a cache (either a proxy cache or an user agent
cache) unless it is first validated with the origin server (or with an
intermediate cache that has a fresh copy of the entity). See section
13.2 for further discussion of the expiration model.

The presence of an Expires field does not imply that the original
resource will change or cease to exist at, before, or after that time.

The format is an absolute date and time as defined by HTTP-date in
section 3.3; it MUST be in RFC1123-date format:

      Expires = "Expires" ":" HTTP-date

An example of its use is

      Expires: Thu, 01 Dec 1994 16:00:00 GMT

  Note: if a response includes a Cache-Control field with the max-age
  directive, that directive overrides the Expires field.

HTTP/1.1 clients and caches MUST treat other invalid date formats,
especially including the value "0", as in the past (i.e., "already
expired").

To mark a response as "already expired," an origin server should use an
Expires date that is equal to the Date header value. (See the rules for
expiration calculations in section 13.2.4.)

To mark a response as "never expires," an origin server should use an
Expires date approximately one year from the time the response is sent.


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HTTP/1.1 servers should not send Expires dates more than one year in the
future.

The presence of an Expires header field with a date value of some time
in the future on an response that otherwise would by default be non-
cacheable indicates that the response is cachable, unless indicated
otherwise by a Cache-Control header field (section 14.9).


14.22 From

The From request-header field, if given, SHOULD contain an Internet e-
mail address for the human user who controls the requesting user agent.
The address SHOULD be machine-usable, as defined by mailbox in RFC 822
(as updated by RFC 1123 ):

       From   = "From" ":" mailbox

An example is:

       From: webmaster@w3.org

This header field MAY be used for logging purposes and as a means for
identifying the source of invalid or unwanted requests. It SHOULD NOT be
used as an insecure form of access protection. The interpretation of
this field is that the request is being performed on behalf of the
person given, who accepts responsibility for the method performed. In
particular, robot agents SHOULD include this header so that the person
responsible for running the robot can be contacted if problems occur on
the receiving end.

The Internet e-mail address in this field MAY be separate from the
Internet host which issued the request. For example, when a request is
passed through a proxy the original issuer's address SHOULD be used.

  Note: The client SHOULD not send the From header field without the
  user's approval, as it may conflict with the user's privacy
  interests or their site's security policy. It is strongly
  recommended that the user be able to disable, enable, and modify
  the value of this field at any time prior to a request.


14.23 Host

The Host request-header field specifies the Internet host and port
number of the resource being requested, as obtained from the original
URL given by the user or referring resource (generally an HTTP URL, as
described in section 3.2.2). The Host field value MUST represent the
network location of the origin server or gateway given by the original
URL. This allows the origin server or gateway to differentiate between


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internally-ambiguous URLs, such as the root "/" URL of a server for
multiple host names on a single IP address.

       Host = "Host" ":" host [ ":" port ]    ; Section 3.2.2

A "host" without any trailing port information implies the default port
for the service requested (e.g., "80" for an HTTP URL). For example, a
request on the origin server for <http://www.w3.org/pub/WWW/> MUST
include:

       GET /pub/WWW/ HTTP/1.1
       Host: www.w3.org

A client MUST include a Host header field in all HTTP/1.1 request
messages on the Internet (i.e., on any message corresponding to a
request for a URL which includes an Internet host address for the
service being requested). If the Host field is not already present, an
HTTP/1.1 proxy MUST add a Host field to the request message prior to
forwarding it on the Internet. All Internet-based HTTP/1.1 servers MUST
respond with a 400 status code to any HTTP/1.1 request message which
lacks a Host header field.

See sections 5.2 and 19.5.1 for other requirements relating to Host.


14.24 If-Modified-Since

The If-Modified-Since request-header field is used with the GET method
to make it conditional: if the requested variant has not been modified
since the time specified in this field, an entity will not be returned
from the server; instead, a 304 (not modified) response will be returned
without any message-body.

       If-Modified-Since = "If-Modified-Since" ":" HTTP-date

An example of the field is:

       If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT

A GET method with an If-Modified-Since header and no Range header
requests that the identified entity be transferred only if it has been
modified since the date given by the If-Modified-Since header. The
algorithm for determining this includes the following cases:


a)If the request would normally result in anything other than a 200
  (OK) status, or if the passed If-Modified-Since date is invalid, the
  response is exactly the same as for a normal GET. A date which is
  later than the server's current time is invalid.


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b)If the variant has been modified since the If-Modified-Since date,
  the response is exactly the same as for a normal GET.


c)If the variant has not been modified since a valid If-Modified-Since
  date, the server MUST return a 304 (Not Modified) response.

The purpose of this feature is to allow efficient updates of cached
information with a minimum amount of transaction overhead.

  Note that the Range request-header field modifies the meaning of
  If-Modified-Since; see section 14.36 for full details.

  Note that If-Modified-Since times are interpreted by the server,
  whose clock may not be synchronized with the client.

Note that if a client uses an arbitrary date in the If-Modified-Since
header instead of a date taken from the Last-Modified header for the
same request, the client should be aware of the fact that this date is
interpreted in the server's understanding of time. The client should
consider unsynchronized clocks and rounding problems due to the
different encodings of time between the client and server. This includes
the possibility of race conditions if the document has changed between
the time it was first requested and the If-Modified-Since date of a
subsequent request, and the possibility of clock-skew-related problems
if the If-Modified-Since date is derived from the client's clock without
correction to the server's clock. Corrections for different time bases
between client and server are at best approximate due to network
latency.


14.25 If-Match

The If-Match request-header field is used with a method to make it
conditional. A client that has one or more entities previously obtained
from the resource can verify that one of those entities is current by
including a list of their associated entity tags in the If-Match header
field. The purpose of this feature is to allow efficient updates of
cached information with a minimum amount of transaction overhead. It is
also used, on updating requests, to prevent inadvertent modification of
the wrong version of a resource. As a special case, the value "*"
matches any current entity of the resource.

       If-Match = "If-Match" ":" ( "*" | 1#entity-tag )

If any of the entity tags match the entity tag of the entity that would
have been returned in the response to a similar GET request (without the
If-Match header) on that resource, or if "*" is given and any current
entity exists for that resource, then the server MAY perform the
requested method as if the If-Match header field did not exist.


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A server MUST use the strong comparison function (see section 3.11) to
compare the entity tags in If-Match.

If none of the entity tags match, or if "*" is given and no current
entity exists, the server MUST NOT perform the requested method, and
MUST return a 412 (Precondition Failed) response. This behavior is most
useful when the client wants to prevent an updating method, such as PUT,
from modifying a resource that has changed since the client last
retrieved it.

If the request would, without the If-Match header field, result in
anything other than a 2xx status, then the If-Match header MUST be
ignored.

The meaning of "If-Match: *" is that the method SHOULD be performed if
the representation selected by the origin server (or by a cache,
possibly using the Vary mechanism, see section 14.43) exists, and MUST
NOT be performed if the representation does not exist.

A request intended to update a resource (e.g., a PUT) MAY include an If-
Match header field to signal that the request method MUST NOT be applied
if the entity corresponding to the If-Match value (a single entity tag)
is no longer a representation of that resource.  This allows the user to
indicate that they do not wish the request to be successful if the
resource has been changed without their knowledge. Examples:

       If-Match: "xyzzy"
       If-Match: W/"xyzzy"
       If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
       If-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
       If-Match: *


14.26 If-None-Match

The If-None-Match request-header field is used with a method to make it
conditional. A client that has one or more entities previously obtained
from the resource can verify that none of those entities is current by
including a list of their associated entity tags in the If-None-Match
header field. The purpose of this feature is to allow efficient updates
of cached information with a minimum amount of transaction overhead. It
is also used, on updating requests, to prevent inadvertent modification
of a resource which was not known to exist.

 As a special case, the value "*" matches any current entity of the
resource.

       If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag )

If any of the entity tags match the entity tag of the entity that would
have been returned in the response to a similar GET request (without the

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If-None-Match header) on that resource, or if "*" is given and any
current entity exists for that resource, then the server MUST NOT
perform the requested method. Instead, if the request method was GET or
HEAD, the server SHOULD respond with a 304 (Not Modified) response,
including the cache-related entity-header fields (particularly ETag) of
one of the entities that matched. For all other request methods, the
server MUST respond with a status of 412 (Precondition Failed).

See section 13.3.3 for rules on how to determine if two entity tags
match. The weak comparison function can only be used with GET or HEAD
requests.

If none of the entity tags match, or if "*" is given and no current
entity exists, then the server MAY perform the requested method as if
the If-None-Match header field did not exist.

If the request would, without the If-None-Match header field, result in
anything other than a 2xx status, then the If-None-Match header MUST be
ignored.

The meaning of "If-None-Match: *" is that the method MUST NOT be
performed if the representation selected by the origin server (or by a
cache, possibly using the Vary mechanism, see section 14.43) exists, and
SHOULD be performed if the representation does not exist. This feature
may be useful in preventing races between PUT operations.

Examples:

       If-None-Match: "xyzzy"
       If-None-Match: W/"xyzzy"
       If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
       If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
       If-None-Match: *


14.27 If-Range

If a client has a partial copy of an entity in its cache, and wishes to
have an up-to-date copy of the entire entity in its cache, it could use
the Range request-header with a conditional GET (using either or both of
If-Unmodified-Since and If-Match.) However, if the condition fails
because the entity has been modified, the client would then have to make
a second request to obtain the entire current entity-body.

The If-Range header allows a client to "short-circuit" the second
request. Informally, its meaning is `if the entity is unchanged, send me
the part(s) that I am missing; otherwise, send me the entire new
entity.'

        If-Range = "If-Range" ":" ( entity-tag | HTTP-date )


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If the client has no entity tag for an entity, but does have a Last-
Modified date, it may use that date in a If-Range header. (The server
can distinguish between a valid HTTP-date and any form of entity-tag by
examining no more than two characters.) The If-Range header should only
be used together with a Range header, and must be ignored if the request
does not include a Range header, or if the server does not support the
sub-range operation.

If the entity tag given in the If-Range header matches the current
entity tag for the entity, then the server should provide the specified
sub-range of the entity using a 206 (Partial content) response. If the
entity tag does not match, then the server should return the entire
entity using a 200 (OK) response.


14.28 If-Unmodified-Since

The If-Unmodified-Since request-header field is used with a method to
make it conditional. If the requested resource has not been modified
since the time specified in this field, the server should perform the
requested operation as if the If-Unmodified-Since header were not
present.

If the requested variant has been modified since the specified time, the
server MUST NOT perform the requested operation, and MUST return a 412
(Precondition Failed).

      If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date

An example of the field is:

       If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT

If the request normally (i.e., without the If-Unmodified-Since header)
would result in anything other than a 2xx status, the If-Unmodified-
Since header should be ignored.

If the specified date is invalid, the header is ignored.


14.29 Last-Modified

The Last-Modified entity-header field indicates the date and time at
which the origin server believes the variant was last modified.

       Last-Modified  = "Last-Modified" ":" HTTP-date

An example of its use is

       Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT


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The exact meaning of this header field depends on the implementation of
the origin server and the nature of the original resource. For files, it
may be just the file system last-modified time. For entities with
dynamically included parts, it may be the most recent of the set of
last-modify times for its component parts. For database gateways, it may
be the last-update time stamp of the record. For virtual objects, it may
be the last time the internal state changed.

An origin server MUST NOT send a Last-Modified date which is later than
the server's time of message origination. In such cases, where the
resource's last modification would indicate some time in the future, the
server MUST replace that date with the message origination date.

An origin server should obtain the Last-Modified value of the entity as
close as possible to the time that it generates the Date value of its
response. This allows a recipient to make an accurate assessment of the
entity's modification time, especially if the entity changes near the
time that the response is generated.

HTTP/1.1 servers SHOULD send Last-Modified whenever feasible.


14.30 Location

The Location response-header field is used to redirect the recipient to
a location other than the Request-URI for completion of the request or
identification of a new resource. For 201 (Created) responses, the
Location is that of the new resource which was created by the request.
For 3xx responses, the location SHOULD indicate the server's preferred
URL for automatic redirection to the resource. The field value consists
of a single absolute URL.

       Location       = "Location" ":" absoluteURI

An example is

       Location: http://www.w3.org/pub/WWW/People.html

  Note: The Content-Location header field (section 14.15) differs
  from Location in that the Content-Location identifies the original
  location of the entity enclosed in the request. It is therefore
  possible for a response to contain header fields for both Location
  and Content-Location. Also see section 13.10 for cache requirements
  of some methods.


14.31 Max-Forwards

The Max-Forwards general-header field may be used with the TRACE method
(section 14.31) to limit the number of proxies or gateways that can
forward the request to the next inbound server. This can be useful when

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the client is attempting to trace a request chain which appears to be
failing or looping in mid-chain.

       Max-Forwards   = "Max-Forwards" ":" 1*DIGIT

The Max-Forwards value is a decimal integer indicating the remaining
number of times this request message may be forwarded.

Each proxy or gateway recipient of a TRACE request containing a Max-
Forwards header field SHOULD check and update its value prior to
forwarding the request. If the received value is zero (0), the recipient
SHOULD NOT forward the request; instead, it SHOULD respond as the final
recipient with a 200 (OK) response containing the received request
message as the response entity-body (as described in section 9.8). If
the received Max-Forwards value is greater than zero, then the forwarded
message SHOULD contain an updated Max-Forwards field with a value
decremented by one (1).

The Max-Forwards header field SHOULD be ignored for all other methods
defined by this specification and for any extension methods for which it
is not explicitly referred to as part of that method definition.


14.32 Pragma

The Pragma general-header field is used to include implementation-
specific directives that may apply to any recipient along the
request/response chain. All pragma directives specify optional behavior
from the viewpoint of the protocol; however, some systems MAY require
that behavior be consistent with the directives.

       Pragma            = "Pragma" ":" 1#pragma-directive

       pragma-directive  = "no-cache" | extension-pragma
       extension-pragma  = token [ "=" ( token | quoted-string ) ]

When the no-cache directive is present in a request message, an
application SHOULD forward the request toward the origin server even if
it has a cached copy of what is being requested. This pragma directive
has the same semantics as the no-cache cache-directive (see section
14.9) and is defined here for backwards compatibility with HTTP/1.0.
Clients SHOULD include both header fields when a no-cache request is
sent to a server not known to be HTTP/1.1 compliant.

Pragma directives MUST be passed through by a proxy or gateway
application, regardless of their significance to that application, since
the directives may be applicable to all recipients along the
request/response chain. It is not possible to specify a pragma for a
specific recipient; however, any pragma directive not relevant to a
recipient SHOULD be ignored by that recipient.


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HTTP/1.1 clients SHOULD NOT send the Pragma request-header. HTTP/1.1
caches SHOULD treat "Pragma: no-cache" as if the client had sent "Cache-
Control: no-cache". No new Pragma directives will be defined in HTTP.


14.33 Proxy-Authenticate

The Proxy-Authenticate response-header field MUST be included as part of
a 407 (Proxy Authentication Required) response. The field value consists
of a challenge that indicates the authentication scheme and parameters
applicable to the proxy for this Request-URI.

       Proxy-Authentication  = "Proxy-Authentication" ":" challenge

The HTTP access authentication process is described in section 11.
Unlike WWW-Authenticate, the Proxy-Authenticate header field applies
only to the current connection and SHOULD NOT be passed on to downstream
clients. However, an intermediate proxy may need to obtain its own
credentials by requesting them from the downstream client, which in some
circumstances will appear as if the proxy is forwarding the Proxy-
Authenticate header field.


14.34 Proxy-Authorization

The Proxy-Authorization request-header field allows the client to
identify itself (or its user) to a proxy which requires authentication.
The Proxy-Authorization field value consists of credentials containing
the authentication information of the user agent for the proxy and/or
realm of the resource being requested.

       Proxy-Authorization     = "Proxy-Authorization" ":" credentials

The HTTP access authentication process is described in section 11.
Unlike Authorization, the Proxy-Authorization header field applies only
to the next outbound proxy that demanded authentication using the Proxy-
Authenticate field. When multiple proxies are used in a chain, the
Proxy-Authorization header field is consumed by the first outbound proxy
that was expecting to receive credentials. A proxy MAY relay the
credentials from the client request to the next proxy if that is the
mechanism by which the proxies cooperatively authenticate a given
request.


14.35 Public

The Public response-header field lists the set of methods supported by
the server. The purpose of this field is strictly to inform the
recipient of the capabilities of the server regarding unusual methods.
The methods listed may or may not be applicable to the Request-URI; the


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Allow header field (section 14.7) MAY be used to indicate methods
allowed for a particular URI.

       Public         = "Public" ":" 1#method

Example of use:

       Public: OPTIONS, MGET, MHEAD, GET, HEAD

This header field applies only to the server directly connected to the
client (i.e., the nearest neighbor in a chain of connections). If the
response passes through a proxy, the proxy MUST either remove the Public
header field or replace it with one applicable to its own capabilities.


14.36 Range


14.36.1 Byte Ranges

Since all HTTP entities are represented in HTTP messages as sequences of
bytes, the concept of a byte range is meaningful for any HTTP entity.
(However, not all clients and servers need to support byte-range
operations.)

Byte range specifications in HTTP apply to the sequence of bytes in the
entity-body (not necessarily the same as the message-body).

A byte range operation may specify a single range of bytes, or a set of
ranges within a single entity.

       ranges-specifier = byte-ranges-specifier

       byte-ranges-specifier = bytes-unit "=" byte-range-set

       byte-range-set  = 1#( byte-range-spec | suffix-byte-range-spec )

       byte-range-spec = first-byte-pos "-" [last-byte-pos]

       first-byte-pos  = 1*DIGIT

       last-byte-pos   = 1*DIGIT

The first-byte-pos value in a byte-range-spec gives the byte-offset of
the first byte in a range. The last-byte-pos value gives the byte-offset
of the last byte in the range; that is, the byte positions specified are
inclusive. Byte offsets start at zero.

If the last-byte-pos value is present, it must be greater than or equal
to the first-byte-pos in that byte-range-spec, or the byte-range-spec is
invalid. The recipient of an invalid byte-range-spec must ignore it.

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If the last-byte-pos value is absent, or if the value is greater than or
equal to the current length of the entity-body, last-byte-pos is taken
to be equal to one less than the current length of the entity-body in
bytes.

By its choice of last-byte-pos, a client can limit the number of bytes
retrieved without knowing the size of the entity.

       suffix-byte-range-spec = "-" suffix-length

       suffix-length = 1*DIGIT

A suffix-byte-range-spec is used to specify the suffix of the entity-
body, of a length given by the suffix-length value. (That is, this form
specifies the last N bytes of an entity-body.) If the entity is shorter
than the specified suffix-length, the entire entity-body is used.

Examples of byte-ranges-specifier values (assuming an entity-body of
length 10000):

  .  The first 500 bytes (byte offsets 0-499, inclusive):
       bytes=0-499

  .  The second 500 bytes (byte offsets 500-999, inclusive):
       bytes=500-999

  .  The final 500 bytes (byte offsets 9500-9999, inclusive):
       bytes=-500

  .  Or
       bytes=9500-

  .  The first and last bytes only (bytes 0 and 9999):
       bytes=0-0,-1

  .  Several legal but not canonical specifications of the second 500
     bytes (byte offsets 500-999, inclusive):
       bytes=500-600,601-999

       bytes=500-700,601-999


14.36.2 Range Retrieval Requests

HTTP retrieval requests using conditional or unconditional GET methods
may request one or more sub-ranges of the entity, instead of the entire
entity, using the Range request header, which applies to the entity
returned as the result of the request:

      Range = "Range" ":" ranges-specifier


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A server MAY ignore the Range header. However, HTTP/1.1 origin servers
and intermediate caches SHOULD support byte ranges when possible, since
Range supports efficient recovery from partially failed transfers, and
supports efficient partial retrieval of large entities.

If the server supports the Range header and the specified range or
ranges are appropriate for the entity:

  .  The presence of a Range header in an unconditional GET modifies
     what is returned if the GET is otherwise successful. In other
     words, the response carries a status code of 206 (Partial Content)
     instead of 200 (OK).
  .  The presence of a Range header in a conditional GET (a request
     using one or both of If-Modified-Since and If-None-Match, or one or
     both of If-Unmodified-Since and If-Match) modifies what is returned
     if the GET is otherwise successful and the condition is true. It
     does not affect the 304 (Not Modified) response returned if the
     conditional is false.
In some cases, it may be more appropriate to use the If-Range header
(see section 14.27) in addition to the Range header.

If a proxy that supports byte ranges receives a Range request, forwards
the request to an inbound server, and receives an entire entity in
reply, it SHOULD only return the requested range to its client. It
SHOULD store the entire received response in its cache, if that is
consistent with its cache allocation policies.


14.37 Referer

The Referer[sic] request-header field allows the client to specify, for
the server's benefit, the address (URI) of the resource from which the
Request-URI was obtained (the "referrer", although the header field is
misspelled.) The Referer request-header allows a server to generate
lists of back-links to resources for interest, logging, optimized
caching, etc. It also allows obsolete or mistyped links to be traced for
maintenance. The Referer field MUST NOT be sent if the Request-URI was
obtained from a source that does not have its own URI, such as input
from the user keyboard.

       Referer        = "Referer" ":" ( absoluteURI | relativeURI )

Example:

       Referer: http://www.w3.org/hypertext/DataSources/Overview.html

If the field value is a partial URI, it SHOULD be interpreted relative
to the Request-URI. The URI MUST NOT include a fragment.

  Note: Because the source of a link may be private information or
  may reveal an otherwise private information source, it is strongly

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  recommended that the user be able to select whether or not the
  Referer field is sent. For example, a browser client could have a
  toggle switch for browsing openly/anonymously, which would
  respectively enable/disable the sending of Referer and From
  information.


14.38 Retry-After

The Retry-After response-header field can be used with a 503 (Service
Unavailable) response to indicate how long the service is expected to be
unavailable to the requesting client. The value of this field can be
either an HTTP-date or an integer number of seconds (in decimal) after
the time of the response.

       Retry-After  = "Retry-After" ":" ( HTTP-date | delta-seconds )

Two examples of its use are

       Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
       Retry-After: 120

In the latter example, the delay is 2 minutes.


14.39 Server

The Server response-header field contains information about the software
used by the origin server to handle the request. The field can contain
multiple product tokens (section 3.8) and comments identifying the
server and any significant subproducts. The product tokens are listed in
order of their significance for identifying the application.

       Server         = "Server" ":" 1*( product | comment )

Example:

       Server: CERN/3.0 libwww/2.17

If the response is being forwarded through a proxy, the proxy
application MUST NOT modify the Server response-header. Instead, it
SHOULD include a Via field (as described in section 14.44).

  Note: Revealing the specific software version of the server may
  allow the server machine to become more vulnerable to attacks
  against software that is known to contain security holes. Server
  implementers are encouraged to make this field a configurable
  option.


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14.40 Transfer-Encoding

The Transfer-Encoding general-header field indicates what (if any) type
of transformation has been applied to the message body in order to
safely transfer it between the sender and the recipient. This differs
from the Content-Encoding in that the transfer coding is a property of
the message, not of the entity.

       Transfer-Encoding       = "Transfer-Encoding" ":" 1#transfer-
coding

Transfer codings are defined in section 3.6. An example is:

       Transfer-Encoding: chunked

Many older HTTP/1.0 applications do not understand the Transfer-Encoding
header.


14.41 Upgrade

The Upgrade general-header allows the client to specify what additional
communication protocols it supports and would like to use if the server
finds it appropriate to switch protocols. The server MUST use the
Upgrade header field within a 101 (Switching Protocols) response to
indicate which protocol(s) are being switched.

       Upgrade        = "Upgrade" ":" 1#product

For example,

       Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11

The Upgrade header field is intended to provide a simple mechanism for
transition from HTTP/1.1 to some other, incompatible protocol. It does
so by allowing the client to advertise its desire to use another
protocol, such as a later version of HTTP with a higher major version
number, even though the current request has been made using HTTP/1.1.
This eases the difficult transition between incompatible protocols by
allowing the client to initiate a request in the more commonly supported
protocol while indicating to the server that it would like to use a
"better" protocol if available (where "better" is determined by the
server, possibly according to the nature of the method and/or resource
being requested).

The Upgrade header field only applies to switching application-layer
protocols upon the existing transport-layer connection. Upgrade cannot
be used to insist on a protocol change; its acceptance and use by the
server is optional. The capabilities and nature of the application-layer
communication after the protocol change is entirely dependent upon the
new protocol chosen, although the first action after changing the

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protocol MUST be a response to the initial HTTP request containing the
Upgrade header field.

The Upgrade header field only applies to the immediate connection.
Therefore, the upgrade keyword MUST be supplied within a Connection
header field (section 14.10) whenever Upgrade is present in an HTTP/1.1
message.

The Upgrade header field cannot be used to indicate a switch to a
protocol on a different connection. For that purpose, it is more
appropriate to use a 301, 302, 303, or 305 redirection response.

This specification only defines the protocol name "HTTP" for use by the
family of Hypertext Transfer Protocols, as defined by the HTTP version
rules of section 3.1 and future updates to this specification. Any token
can be used as a protocol name; however, it will only be useful if both
the client and server associate the name with the same protocol.


14.42 User-Agent

The User-Agent request-header field contains information about the user
agent originating the request. This is for statistical purposes, the
tracing of protocol violations, and automated recognition of user agents
for the sake of tailoring responses to avoid particular user agent
limitations. User agents SHOULD include this field with requests. The
field can contain multiple product tokens (section 3.8) and comments
identifying the agent and any subproducts which form a significant part
of the user agent. By convention, the product tokens are listed in order
of their significance for identifying the application.

       User-Agent     = "User-Agent" ":" 1*( product | comment )

Example:

       User-Agent: CERN-LineMode/2.15 libwww/2.17b3


14.43 Vary

The Vary response-header field is used by a server to signal that the
response entity was selected from the available representations of the
response using server-driven negotiation (section 12). The Vary field
value indicates either that the given set of header fields encompass the
dimensions over which the representation might vary, or that the
dimensions of variance are unspecified ("*") and thus may vary over any
aspect of future requests.

       Vary  = "Vary" ":" ( "*" | 1#field-name )


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An HTTP/1.1 server MUST include an appropriate Vary header field with
any cachable response that is subject to server-driven negotiation.
Doing so allows a cache to properly interpret future requests on that
resource and informs the user agent about the presence of negotiation on
that resource. A server SHOULD include an appropriate Vary header field
with a non-cachable response that is subject to server-driven
negotiation, since this might provide the user agent with useful
information about the dimensions over which the response might vary.

The set of header fields named by the Vary field value is known as the
"selecting" request-headers.

When the cache receives a subsequent request whose Request-URI specifies
one or more cache entries including a Vary header, the cache MUST NOT
use such a cache entry to construct a response to the new request unless
all of the headers named in the cached Vary header are present in the
new request, and all of the stored selecting request-headers from the
previous request match the corresponding headers in the new request.

The selecting request-headers from two requests are defined to match if
and only if the selecting request-headers in the first request can be
transformed to the selecting request-headers in the second request by
adding or removing linear whitespace (LWS) at places where this is
allowed by the corresponding BNF, and/or combining multiple message-
header fields with the same field name following the rules about message
headers in section 4.2.

A Vary field value of "*" signals that unspecified parameters, possibly
other than the contents of request-header fields (e.g., the network
address of the client), play a role in the selection of the response
representation. Subsequent requests on that resource can only be
properly interpreted by the origin server, and thus a cache MUST forward
a (possibly conditional) request even when it has a fresh response
cached for the resource. See section 13.6 for use of the Vary header by
caches.

A Vary field value consisting of a list of field-names signals that the
representation selected for the response is based on a selection
algorithm which considers ONLY the listed request-header field values in
selecting the most appropriate representation. A cache MAY assume that
the same selection will be made for future requests with the same values
for the listed field names, for the duration of time in which the
response is fresh.

The field-names given are not limited to the set of standard request-
header fields defined by this specification. Field names are case-
insensitive.


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14.44 Via

The Via general-header field MUST be used by gateways and proxies to
indicate the intermediate protocols and recipients between the user
agent and the server on requests, and between the origin server and the
client on responses. It is analogous to the "Received" field of RFC 822
and is intended to be used for tracking message forwards, avoiding
request loops, and identifying the protocol capabilities of all senders
along the request/response chain.

      Via =  "Via" ":" 1#( received-protocol received-by [ comment ] )

      received-protocol = [ protocol-name "/" ] protocol-version
      protocol-name     = token
      protocol-version  = token
      received-by       = ( host [ ":" port ] ) | pseudonym
      pseudonym         = token

The received-protocol indicates the protocol version of the message
received by the server or client along each segment of the
request/response chain. The received-protocol version is appended to the
Via field value when the message is forwarded so that information about
the protocol capabilities of upstream applications remains visible to
all recipients.

The protocol-name is optional if and only if it would be "HTTP". The
received-by field is normally the host and optional port number of a
recipient server or client that subsequently forwarded the message.
However, if the real host is considered to be sensitive information, it
MAY be replaced by a pseudonym. If the port is not given, it MAY be
assumed to be the default port of the received-protocol.

Multiple Via field values represent each proxy or gateway that has
forwarded the message. Each recipient MUST append its information such
that the end result is ordered according to the sequence of forwarding
applications.

Comments MAY be used in the Via header field to identify the software of
the recipient proxy or gateway, analogous to the User-Agent and Server
header fields. However, all comments in the Via field are optional and
MAY be removed by any recipient prior to forwarding the message.

For example, a request message could be sent from an HTTP/1.0 user agent
to an internal proxy code-named "fred", which uses HTTP/1.1 to forward
the request to a public proxy at nowhere.com, which completes the
request by forwarding it to the origin server at www.ics.uci.edu. The
request received by www.ics.uci.edu would then have the following Via
header field:

       Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1)


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Proxies and gateways used as a portal through a network firewall SHOULD
NOT, by default, forward the names and ports of hosts within the
firewall region. This information SHOULD only be propagated if
explicitly enabled. If not enabled, the received-by host of any host
behind the firewall SHOULD be replaced by an appropriate pseudonym for
that host.

For organizations that have strong privacy requirements for hiding
internal structures, a proxy MAY combine an ordered subsequence of Via
header field entries with identical received-protocol values into a
single such entry. For example,

       Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy

        could be collapsed to

       Via: 1.0 ricky, 1.1 mertz, 1.0 lucy

Applications SHOULD NOT combine multiple entries unless they are all
under the same organizational control and the hosts have already been
replaced by pseudonyms. Applications MUST NOT combine entries which have
different received-protocol values.


14.45 Warning

The Warning response-header field is used to carry additional
information about the status of a response which may not be reflected by
the response status code. This information is typically, though not
exclusively, used to warn about a possible lack of semantic transparency
from caching operations.

Warning headers are sent with responses using:

       Warning    = "Warning" ":" 1#warning-value

       warning-value = warn-code SP warn-agent SP warn-text
       warn-code  = 2DIGIT
       warn-agent = ( host [ ":" port ] ) | pseudonym
                       ; the name or pseudonym of the server adding
                       ; the Warning header, for use in debugging
       warn-text  = quoted-string

A response may carry more than one Warning header.

The warn-text should be in a natural language and character set that is
most likely to be intelligible to the human user receiving the response.
This decision may be based on any available knowledge, such as the
location of the cache or user, the Accept-Language field in a request,
the Content-Language field in a response, etc. The default language is
English and the default character set is ISO-8599-1.

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If a character set other than ISO-8599-1 is used, it MUST be encoded in
the warn-text using the method described in RFC 1522 [14].

Any server or cache may add Warning headers to a response. New Warning
headers should be added after any existing Warning headers. A cache MUST
NOT delete any Warning header that it received with a response. However,
if a cache successfully validates a cache entry, it SHOULD remove any
Warning headers previously attached to that entry except as specified
for specific Warning codes. It MUST then add any Warning headers
received in the validating response. In other words, Warning headers are
those that would be attached to the most recent relevant response.

When multiple Warning headers are attached to a response, the user agent
SHOULD display as many of them as possible, in the order that they
appear in the response. If it is not possible to display all of the
warnings, the user agent should follow these heuristics:

  .  Warnings that appear early in the response take priority over those
     appearing later in the response.
  .  Warnings in the user's preferred character set take priority over
     warnings in other character sets but with identical warn-codes and
     warn-agents.
Systems that generate multiple Warning headers should order them with
this user agent behavior in mind.

This is a list of the currently-defined warn-codes, each with a
recommended warn-text in English, and a description of its meaning.

10 Response is stale
  MUST be included whenever the returned response is stale. A cache may
  add this warning to any response, but may never remove it until the
  response is known to be fresh.

11 Revalidation failed
  MUST be included if a cache returns a stale response because an
  attempt to revalidate the response failed, due to an inability to
  reach the server. A cache may add this warning to any response, but
  may never remove it until the response is successfully revalidated.

12 Disconnected operation
   SHOULD be included if the cache is intentionally disconnected from
  the rest of the network for a period of time.

13 Heuristic expiration
  MUST be included if the cache heuristically chose a freshness
  lifetime greater than 24 hours and the response's age is greater than
  24 hours.

14 Transformation applied
  MUST be added by an intermediate cache or proxy if it applies any
  transformation changing the content-coding (as specified in the

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  Content-Encoding header) or media-type (as specified in the Content-
  Type header) of the response, unless this Warning code already
  appears in the response. MUST NOT be deleted from a response even
  after revalidation.

99 Miscellaneous warning
  The warning text may include arbitrary information to be presented to
  a human user, or logged. A system receiving this warning MUST NOT
  take any automated action.


14.46 WWW-Authenticate

The WWW-Authenticate response-header field MUST be included in 401
(Unauthorized) response messages. The field value consists of at least
one challenge that indicates the authentication scheme(s) and parameters
applicable to the Request-URI.

       WWW-Authenticate  = "WWW-Authenticate" ":" 1#challenge

The HTTP access authentication process is described in section 11. User
agents MUST take special care in parsing the WWW-Authenticate field
value if it contains more than one challenge, or if more than one WWW-
Authenticate header field is provided, since the contents of a challenge
may itself contain a comma-separated list of authentication parameters.


15 Security Considerations

This section is meant to inform application developers, information
providers, and users of the security limitations in HTTP/1.1 as
described by this document. The discussion does not include definitive
solutions to the problems revealed, though it does make some suggestions
for reducing security risks.


15.1 Authentication of Clients

The Basic authentication scheme is not a secure method of user
authentication, nor does it in any way protect the entity, which is
transmitted in clear text across the physical network used as the
carrier. HTTP does not prevent additional authentication schemes and
encryption mechanisms from being employed to increase security or the
addition of enhancements (such as schemes to use one-time passwords) to
Basic authentication.

The most serious flaw in Basic authentication is that it results in the
essentially clear text transmission of the user's password over the
physical network. It is this problem which Digest Authentication
attempts to address.


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Because Basic authentication involves the clear text transmission of
passwords it SHOULD never be used (without enhancements) to protect
sensitive or valuable information.

A common use of Basic authentication is for identification purposes --
requiring the user to provide a user name and password as a means of
identification, for example, for purposes of gathering accurate usage
statistics on a server. When used in this way it is tempting to think
that there is no danger in its use if illicit access to the protected
documents is not a major concern. This is only correct if the server
issues both user name and password to the users and in particular does
not allow the user to choose his or her own password. The danger arises
because naive users frequently reuse a single password to avoid the task
of maintaining multiple passwords.

If a server permits users to select their own passwords, then the threat
is not only illicit access to documents on the server but also illicit
access to the accounts of all users who have chosen to use their account
password. If users are allowed to choose their own password that also
means the server must maintain files containing the (presumably
encrypted) passwords. Many of these may be the account passwords of
users perhaps at distant sites. The owner or administrator of such a
system could conceivably incur liability if this information is not
maintained in a secure fashion.

Basic Authentication is also vulnerable to spoofing by counterfeit
servers. If a user can be led to believe that he is connecting to a host
containing information protected by basic authentication when in fact he
is connecting to a hostile server or gateway then the attacker can
request a password, store it for later use, and feign an error. This
type of attack is not possible with Digest Authentication [32]. Server
implementers SHOULD guard against the possibility of this sort of
counterfeiting by gateways or CGI scripts. In particular it is very
dangerous for a server to simply turn over a connection to a gateway
since that gateway can then use the persistent connection mechanism to
engage in multiple transactions with the client while impersonating the
original server in a way that is not detectable by the client.


15.2 Offering a Choice of Authentication Schemes

An HTTP/1.1 server may return multiple challenges with a 401
(Authenticate) response, and each challenge may use a different scheme.
The order of the challenges returned to the user agent is in the order
that the server would prefer they be chosen. The server should order its
challenges with the "most secure" authentication scheme first. A user
agent should choose as the challenge to be made to the user the first
one that the user agent understands.

When the server offers choices of authentication schemes using the WWW-
Authenticate header, the "security" of the authentication is only as

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good as the security of the weakest of the authentication schemes. A
malicious user could capture the set of challenges and try to
authenticate him/herself using the weakest of the authentication
schemes. Thus, the ordering serves more to protect the user's
credentials than the server's information.

A possible man-in-the-middle (MITM) attack would be to add a weak
authentication scheme to the set of choices, hoping that the client will
use one that exposes the user's credentials (e.g. password). For this
reason, the client should always use the strongest scheme that it
understands from the choices accepted.

An even better MITM attack would be to remove all offered choices, and
to insert a challenge that requests Basic authentication. For this
reason, user agents that are concerned about this kind of attack could
remember the strongest authentication scheme ever requested by a server
and produce a warning message that requires user confirmation before
using a weaker one. A particularly insidious way to mount such a MITM
attack would be to offer a "free" proxy caching service to gullible
users.


15.3 Abuse of Server Log Information

A server is in the position to save personal data about a user's
requests which may identify their reading patterns or subjects of
interest. This information is clearly confidential in nature and its
handling may be constrained by law in certain countries. People using
the HTTP protocol to provide data are responsible for ensuring that such
material is not distributed without the permission of any individuals
that are identifiable by the published results.


15.4 Transfer of Sensitive Information

Like any generic data transfer protocol, HTTP cannot regulate the
content of the data that is transferred, nor is there any a priori
method of determining the sensitivity of any particular piece of
information within the context of any given request. Therefore,
applications SHOULD supply as much control over this information as
possible to the provider of that information. Four header fields are
worth special mention in this context: Server, Via, Referer and From.

Revealing the specific software version of the server may allow the
server machine to become more vulnerable to attacks against software
that is known to contain security holes. Implementers SHOULD make the
Server header field a configurable option.

Proxies which serve as a portal through a network firewall SHOULD take
special precautions regarding the transfer of header information that
identifies the hosts behind the firewall. In particular, they SHOULD

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remove, or replace with sanitized versions, any Via fields generated
behind the firewall.

The Referer field allows reading patterns to be studied and reverse
links drawn. Although it can be very useful, its power can be abused if
user details are not separated from the information contained in the
Referer. Even when the personal information has been removed, the
Referer field may indicate a private document's URI whose publication
would be inappropriate.

The information sent in the From field might conflict with the user's
privacy interests or their site's security policy, and hence it SHOULD
NOT be transmitted without the user being able to disable, enable, and
modify the contents of the field. The user MUST be able to set the
contents of this field within a user preference or application defaults
configuration.

We suggest, though do not require, that a convenient toggle interface be
provided for the user to enable or disable the sending of From and
Referer information.


15.5 Attacks Based On File and Path Names

Implementations of HTTP origin servers SHOULD be careful to restrict the
documents returned by HTTP requests to be only those that were intended
by the server administrators. If an HTTP server translates HTTP URIs
directly into file system calls, the server MUST take special care not
to serve files that were not intended to be delivered to HTTP clients.
For example, UNIX, Microsoft Windows, and other operating systems use
".." as a path component to indicate a directory level above the current
one. On such a system, an HTTP server MUST disallow any such construct
in the Request-URI if it would otherwise allow access to a resource
outside those intended to be accessible via the HTTP server. Similarly,
files intended for reference only internally to the server (such as
access control files, configuration files, and script code) MUST be
protected from inappropriate retrieval, since they might contain
sensitive information. Experience has shown that minor bugs in such HTTP
server implementations have turned into security risks.


15.6 Personal Information

HTTP clients are often privy to large amounts of personal information
(e.g. the user's name, location, mail address, passwords, encryption
keys, etc.), and SHOULD be very careful to prevent unintentional leakage
of this information via the HTTP protocol to other sources. We very
strongly recommend that a convenient interface be provided for the user
to control dissemination of such information, and that designers and
implementers be particularly careful in this area. History shows that
errors in this area are often both serious security and/or privacy

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problems, and often generate highly adverse publicity for the
implementer's company.


15.7 Privacy Issues Connected to Accept Headers

Accept request-headers can reveal information about the user to all
servers which are accessed. The Accept-Language header in particular can
reveal information the user would consider to be of a private nature,
because the understanding of particular languages is often strongly
correlated to the membership of a particular ethnic group. User agents
which offer the option to configure the contents of an Accept-Language
header to be sent in every request are strongly encouraged to let the
configuration process include a message which makes the user aware of
the loss of privacy involved.

An approach that limits the loss of privacy would be for a user agent to
omit the sending of Accept-Language headers by default, and to ask the
user whether it should start sending Accept-Language headers to a server
if it detects, by looking for any Vary response-header fields generated
by the server, that such sending could improve the quality of service.

Elaborate user-customized accept header fields sent in every request, in
particular if these include quality values, can be used by servers as
relatively reliable and long-lived user identifiers. Such user
identifiers would allow content providers to do click-trail tracking,
and would allow collaborating content providers to match cross-server
click-trails or form submissions of individual users. Note that for many
users not behind a proxy, the network address of the host running the
user agent will also serve as a long-lived user identifier. In
environments where proxies are used to enhance privacy, user agents
should be conservative in offering accept header configuration options
to end users. As an extreme privacy measure, proxies could filter the
accept headers in relayed requests. General purpose user agents which
provide a high degree of header configurability should warn users about
the loss of privacy which can be involved.


15.8 DNS Spoofing

Clients using HTTP rely heavily on the Domain Name Service, and are thus
generally prone to security attacks based on the deliberate mis-
association of IP addresses and DNS names. The deployment of DNSSEC
should help this situation. In advance of this deployment, however,
clients need to be cautious in assuming the continuing validity of an IP
number/DNS name association.

In particular, HTTP clients SHOULD rely on their name resolver for
confirmation of an IP number/DNS name association, rather than caching
the result of previous host name lookups. Many platforms already can
cache host name lookups locally when appropriate, and they SHOULD be

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configured to do so. These lookups should be cached, however, only when
the TTL (Time To Live) information reported by the name server makes it
likely that the cached information will remain useful.

If HTTP clients cache the results of host name lookups in order to
achieve a performance improvement, they MUST observe the TTL information
reported by DNS.

If HTTP clients do not observe this rule, they could be spoofed when a
previously-accessed server's IP address changes. As network renumbering
is expected to become increasingly common, the possibility of this form
of attack will grow. Observing this requirement thus reduces this
potential security vulnerability.

This requirement also improves the load-balancing behavior of clients
for replicated servers using the same DNS name and reduces the
likelihood of a user's experiencing failure in accessing sites which use
that strategy.


15.9 Location Headers and Spoofing

If a single server supports multiple organizations that do not trust one
another, then it must check the values of Location and Content-Location
headers in responses that are generated under control of said
organizations to make sure that they do not attempt to invalidate
resources over which they have no authority.


16 Acknowledgments

This specification makes heavy use of the augmented BNF and generic
constructs defined by David H. Crocker for RFC 822 . Similarly, it
reuses many of the definitions provided by Nathaniel Borenstein and Ned
Freed for MIME . We hope that their inclusion in this specification will
help reduce past confusion over the relationship between HTTP and
Internet mail message formats.

The HTTP protocol has evolved considerably over the past four years. It
has benefited from a large and active developer community--the many
people who have participated on the www-talk mailing list--and it is
that community which has been most responsible for the success of HTTP
and of the World-Wide Web in general. Marc Andreessen, Robert Cailliau,
Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois Groff, Phillip
M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob McCool, Lou Montulli,
Dave Raggett, Tony Sanders, and Marc VanHeyningen deserve special
recognition for their efforts in defining early aspects of the protocol.

This document has benefited greatly from the comments of all those
participating in the HTTP-WG. In addition to those already mentioned,
the following individuals have contributed to this specification:

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       Gary Adams                  Albert Lunde
       Harald Tveit Alvestrand     John C. Mallery
       Keith Ball                  Jean-Philippe Martin-Flatin
       Brian Behlendorf            Larry Masinter
       Paul Burchard               Mitra
       Maurizio Codogno            David Morris
       Mike Cowlishaw              Gavin Nicol
       Roman Czyborra              Bill Perry
       Michael A. Dolan            Jeffrey Perry
       David J. Fiander            Scott Powers
       Alan Freier                 Owen Rees
       Marc Hedlund                Luigi Rizzo
       Greg Herlihy                David Robinson
       Koen Holtman                Marc Salomon
       Alex Hopmann                Rich Salz
       Bob Jernigan                Allan M. Schiffman
       Shel Kaphan                 Jim Seidman
       Rohit Khare                 Chuck Shotton
       John Klensin                Eric W. Sink
       Martijn Koster              Simon E. Spero
       Alexei Kosut                Richard N. Taylor
       David M. Kristol            Robert S. Thau
       Daniel LaLiberte            Bill (BearHeart) Weinman
       Ben Laurie                  Francois Yergeau
       Paul J. Leach               Mary Ellen Zurko

Much of the content and presentation of the caching design is due to
suggestions and comments from individuals including: Shel Kaphan, Paul
Leach, Koen Holtman, David Morris, and Larry Masinter.

Most of the specification of ranges is based on work originally done by
Ari Luotonen and John Franks, with additional input from Steve Zilles.

Thanks to the "cave men" of Palo Alto. You know who you are.

Jim Gettys (the current editor of this document) wishes particularly to
thank Roy Fielding, the previous editor of this document, along with
John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen Holtman, John
Franks, Alex Hopmann, and Larry Masinter for their help.


17 References


[1]     H. Alvestrand. "Tags for the identification of languages." RFC

  1766, UNINETT, March 1995.


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[2]     F. Anklesaria, M. McCahill, P. Lindner, D. Johnson, D. Torrey,
  B. Alberti. "The Internet Gopher Protocol: (a distributed document

  search and retrieval protocol)", RFC 1436, University of Minnesota,
  March 1993.


[3]     T. Berners-Lee. "Universal Resource Identifiers in WWW." A

  Unifying Syntax for the Expression of Names and Addresses of Objects
  on the Network as used in the World-Wide Web." RFC 1630, CERN, June
  1994.


[4]     T. Berners-Lee, L. Masinter, M. McCahill.
  "Uniform Resource Locators (URL)." RFC 1738, CERN, Xerox PARC,

  University of Minnesota, December 1994.


[5]     T. Berners-Lee, D. Connolly.
  "HyperText Markup Language Specification - 2.0." RFC 1866, MIT/LCS,

  November 1995.


[6]     T. Berners-Lee, R. Fielding, H. Frystyk.
  "Hypertext Transfer Protocol -- HTTP/1.0." RFC 1945." MIT/LCS, UC

  Irvine, May 1996.


[7]     N. Borenstein, N. Freed.
  "MIME (Multipurpose Internet Mail Extensions) Part One: Mechanisms

  for Specifying and Describing the Format of Internet Message Bodies."
  RFC 1521, Bellcore, Innosoft, September 1993.


[8]     R. Braden.
  "Requirements for Internet hosts - application and support." STD 3,

  RFC 1123, IETF, October 1989.


[9]     D. H. Crocker.
  "Standard for the Format of ARPA Internet Text Messages." STD 11, RFC

  822, UDEL, August 1982.


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[10]    F. Davis, B. Kahle, H. Morris, J. Salem, T. Shen, R. Wang, J.
  Sui, M. Grinbaum. "WAIS Interface Protocol Prototype Functional
  Specification." (v1.5), Thinking Machines Corporation, April 1990.


[11]    R. Fielding. "Relative Uniform Resource Locators." RFC 1808, UC

  Irvine, June 1995.


[12]    M. Horton, R. Adams.
  "Standard for interchange of USENET messages." RFC 1036 (Obsoletes

  RFC 850), AT&T Bell Laboratories, Center for Seismic Studies,
  December 1987.


[13]    B. Kantor, P. Lapsley. "Network News Transfer Protocol." A

  Proposed Standard for the Stream-Based Transmission of News." RFC
  977, UC San Diego, UC Berkeley, February 1986.


[14]    K. Moore. "MIME (Multipurpose Internet Mail Extensions) Part Two

  : Message Header Extensions for Non-ASCII Text." RFC 1522, University
  of Tennessee, September 1993.


[15]    E. Nebel, L. Masinter. "Form-based File Upload in HTML." RFC

  1867, Xerox Corporation, November 1995.


[16]    J. Postel. "Simple Mail Transfer Protocol." STD 10, RFC 821,

  USC/ISI, August 1982.


[17]    J. Postel. "Media Type Registration Procedure." RFC 1590,

  USC/ISI, March 1994.


[18]    J. Postel, J. K. Reynolds. "File Transfer Protocol (FTP)." STD

  9, RFC 959, USC/ISI, October 1985.


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[19]    J. Reynolds, J. Postel. "Assigned Numbers." STD 2, RFC 1700,

  USC/ISI, October 1994.


[20]    K. Sollins, L. Masinter.
  "Functional Requirements for Uniform Resource Names." RFC 1737,

  MIT/LCS, Xerox Corporation, December 1994.


[21]    US-ASCII. Coded Character Set - 7-Bit American Standard Code for
  Information Interchange. Standard ANSI X3.4-1986, ANSI, 1986.


[22]    ISO-8859. International Standard -- Information Processing --
  8-bit Single-Byte Coded Graphic Character Sets --
  Part 1: Latin alphabet No. 1, ISO 8859-1:1987.
  Part 2: Latin alphabet No. 2, ISO 8859-2, 1987.
  Part 3: Latin alphabet No. 3, ISO 8859-3, 1988.
  Part 4: Latin alphabet No. 4, ISO 8859-4, 1988.
  Part 5: Latin/Cyrillic alphabet, ISO 8859-5, 1988.
  Part 6: Latin/Arabic alphabet, ISO 8859-6, 1987.
  Part 7: Latin/Greek alphabet, ISO 8859-7, 1987.
  Part 8: Latin/Hebrew alphabet, ISO 8859-8, 1988.
  Part 9: Latin alphabet No. 5, ISO 8859-9, 1990.


[23]    Meyers, M. Rose "The Content-MD5 Header Field." RFC 1864,

  Carnegie Mellon, Dover Beach Consulting, October, 1995.


[24]    B. Carpenter, Y. Rekhter, "Renumbering Needs Work." RFC 1900,

  IAB, February 1996.


[25]    P. Deutsch, "GZIP file format specification version 4.3." RFC

  1952, Aladdin Enterprises, May, 1996.


[26]    Jeffrey C. Mogul. "The Case for Persistent-Connection HTTP". In
  Proc. SIGCOMM '95 Symposium on Communications Architectures and
  Protocols, pages 299-313. Cambridge, MA, August, 1995. A longer, more
  comprehensive version of this paper is available on line at
  <URL
  :http://www.research.digital.com/wrl/techreports/abstracts/95.4.html


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  >, Digital Equipment Corporation Western Research Laboratory Research
  Report 95/4, May, 1995.,


[27]    Work in progress on content negotiation of the HTTP working
  group.


[28]    Mills, D, "Network Time Protocol, Version 3.", Specification,

  Implementation and Analysis RFC 1305, University of Delaware, March,
  1992.


[29]    P. Deutsch,
  "DEFLATE Compressed Data Format Specification version 1.3." RFC 1951,

  Aladdin Enterprises, May 1996.


[30]    S. Spero. "Analysis of HTTP Performance Problems"
  <URL:http://sunsite.unc.edu/mdma-release/http-prob.html>


[31]    P. Deutsch, J-L. Gailly,
  "ZLIB Compressed Data Format Specification version 3.3." RFC 1950,

  Aladdin Enterprises, Info-ZIP, May 1996.


[32]    Work In Progress for Digest authentication of the IETF HTTP
  working group.


18 Authors' Addresses

Roy T. Fielding

Department of Information and Computer Science
University of California
Irvine, CA 92717-3425, USA
Fax: +1 (714) 824-4056
Email: fielding@ics.uci.edu

Jim Gettys

MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, USA
Fax: +1 (617) 258 8682
Email: jg@w3.org

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Jeffrey C. Mogul

Western Research Laboratory
Digital Equipment Corporation
250 University Avenue
Palo Alto, California, 94305, USA
Email: mogul@wrl.dec.com

Henrik Frystyk Nielsen

W3 Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, USA
Fax: +1 (617) 258 8682
Email: frystyk@w3.org

Tim Berners-Lee

Director, W3 Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, USA
Fax: +1 (617) 258 8682
Email: timbl@w3.org


19 Appendices


19.1 Internet Media Type message/http

In addition to defining the HTTP/1.1 protocol, this document serves as
the specification for the Internet media type "message/http". The
following is to be registered with IANA .

       Media Type name:         message
       Media subtype name:      http
       Required parameters:     none
       Optional parameters:     version, msgtype

        version: The HTTP-Version number of the enclosed message
                 (e.g., "1.1"). If not present, the version can be
                 determined from the first line of the body.

        msgtype: The message type -- "request" or "response". If not
                 present, the type can be determined from the first
                 line of the body.

       Encoding considerations: only "7bit", "8bit", or "binary" are
                                permitted

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       Security considerations: none


19.2 Internet Media Type multipart/byteranges

When an HTTP message includes the content of multiple ranges (for
example, a response to a request for multiple non-overlapping ranges),
these are transmitted as a multipart MIME message. The multipart media
type for this purpose is called "multipart/byteranges".

The multipart/byteranges media type includes two or more parts, each
with its own Content-Type and Content-Range fields. The parts are
separated using a MIME boundary parameter.

       Media Type name:         multipart
       Media subtype name:      byteranges
       Required parameters:     boundary
       Optional parameters:     none

       Encoding considerations: only "7bit", "8bit", or "binary" are
                                permitted

       Security considerations: none

For example:

   HTTP/1.1 206 Partial content
   Date: Wed, 15 Nov 1995 06:25:24 GMT
   Last-modified: Wed, 15 Nov 1995 04:58:08 GMT
   Content-type: multipart/byteranges; boundary=THIS_STRING_SEPARATES

   --THIS_STRING_SEPARATES
   Content-type: application/pdf
   Content-range: bytes 500-999/8000

   ...the first range...
   --THIS_STRING_SEPARATES
   Content-type: application/pdf
   Content-range: bytes 7000-7999/8000

   ...the second range
   --THIS_STRING_SEPARATES--


19.3 Tolerant Applications

Although this document specifies the requirements for the generation of
HTTP/1.1 messages, not all applications will be correct in their
implementation. We therefore recommend that operational applications be
tolerant of deviations whenever those deviations can be interpreted
unambiguously.

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Clients SHOULD be tolerant in parsing the Status-Line and servers
tolerant when parsing the Request-Line. In particular, they SHOULD
accept any amount of SP or HT characters between fields, even though
only a single SP is required.

The line terminator for message-header fields is the sequence CRLF.
However, we recommend that applications, when parsing such headers,
recognize a single LF as a line terminator and ignore the leading CR.

The character set of an entity-body should be labeled as the lowest
common denominator of the character codes used within that body, with
the exception that no label is preferred over the labels US-ASCII or
ISO-8859-1.

Additional rules for requirements on parsing and encoding of dates and
other potential problems with date encodings include:

  .  HTTP/1.1 clients and caches should assume that an RFC-850 date
     which appears to be more than 50 years in the future is in fact in
     the past (this helps solve the "year 2000" problem).
  .  An HTTP/1.1 implementation may internally represent a parsed
     Expires date as earlier than the proper value, but MUST NOT
     internally represent a parsed Expires date as later than the proper
     value.
  .  All expiration-related calculations must be done in GMT. The local
     time zone MUST NOT influence the calculation or comparison of an
     age or expiration time.
  .  If an HTTP header incorrectly carries a date value with a time zone
     other than GMT, it must be converted into GMT using the most
     conservative possible conversion.

19.4 Differences Between HTTP Entities and RFC 1521 Entities

HTTP/1.1 uses many of the constructs defined for Internet Mail (RFC 822
) and the Multipurpose Internet Mail Extensions (MIME ) to allow
entities to be transmitted in an open variety of representations and
with extensible mechanisms. However, RFC 1521 discusses mail, and HTTP
has a few features that are different from those described in RFC 1521.
These differences were carefully chosen to optimize performance over
binary connections, to allow greater freedom in the use of new media
types, to make date comparisons easier, and to acknowledge the practice
of some early HTTP servers and clients.

At the time of this writing, it is expected that RFC 1521 will be
revised. The revisions may include some of the practices found in
HTTP/1.1 but not in RFC 1521.

This appendix describes specific areas where HTTP differs from RFC 1521.
Proxies and gateways to strict MIME environments SHOULD be aware of
these differences and provide the appropriate conversions where


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necessary. Proxies and gateways from MIME environments to HTTP also need
to be aware of the differences because some conversions may be required.


19.4.1 Conversion to Canonical Form

RFC 1521 requires that an Internet mail entity be converted to canonical
form prior to being transferred, as described in Appendix G of RFC 1521
. Section 3.7.1 of this document describes the forms allowed for
subtypes of the "text" media type when transmitted over HTTP. RFC 1521
requires that content with a type of "text" represent line breaks as
CRLF and forbids the use of CR or LF outside of line break sequences.
HTTP allows CRLF, bare CR, and bare LF to indicate a line break within
text content when a message is transmitted over HTTP.

Where it is possible, a proxy or gateway from HTTP to a strict RFC 1521
environment SHOULD translate all line breaks within the text media types
described in section 3.7.1 of this document to the RFC 1521 canonical
form of CRLF. Note, however, that this may be complicated by the
presence of a Content-Encoding and by the fact that HTTP allows the use
of some character sets which do not use octets 13 and 10 to represent CR
and LF, as is the case for some multi-byte character sets.


19.4.2 Conversion of Date Formats

HTTP/1.1 uses a restricted set of date formats (section 3.3.1) to
simplify the process of date comparison. Proxies and gateways from other
protocols SHOULD ensure that any Date header field present in a message
conforms to one of the HTTP/1.1 formats and rewrite the date if
necessary.


19.4.3 Introduction of Content-Encoding

RFC 1521 does not include any concept equivalent to HTTP/1.1's Content-
Encoding header field. Since this acts as a modifier on the media type,
proxies and gateways from HTTP to MIME-compliant protocols MUST either
change the value of the Content-Type header field or decode the entity-
body before forwarding the message. (Some experimental applications of
Content-Type for Internet mail have used a media-type parameter of
";conversions=<content-coding>" to perform an equivalent function as
Content-Encoding. However, this parameter is not part of RFC 1521.)


19.4.4 No Content-Transfer-Encoding

HTTP does not use the Content-Transfer-Encoding (CTE) field of RFC 1521.
Proxies and gateways from MIME-compliant protocols to HTTP MUST remove
any non-identity CTE ("quoted-printable" or "base64") encoding prior to
delivering the response message to an HTTP client.

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Proxies and gateways from HTTP to MIME-compliant protocols are
responsible for ensuring that the message is in the correct format and
encoding for safe transport on that protocol, where "safe transport" is
defined by the limitations of the protocol being used. Such a proxy or
gateway SHOULD label the data with an appropriate Content-Transfer-
Encoding if doing so will improve the likelihood of safe transport over
the destination protocol.


19.4.5 HTTP Header Fields in Multipart Body-Parts

In RFC 1521, most header fields in multipart body-parts are generally
ignored unless the field name begins with "Content-". In HTTP/1.1,
multipart body-parts may contain any HTTP header fields which are
significant to the meaning of that part.


19.4.6 Introduction of Transfer-Encoding

HTTP/1.1 introduces the Transfer-Encoding header field (section 14.40).
Proxies/gateways MUST remove any transfer coding prior to forwarding a
message via a MIME-compliant protocol.

A process for decoding the "chunked" transfer coding (section 3.6) can
be represented in pseudo-code as:

       length := 0
       read chunk-size, chunk-ext (if any) and CRLF
       while (chunk-size > 0) {
          read chunk-data and CRLF
          append chunk-data to entity-body
          length := length + chunk-size
          read chunk-size and CRLF
       }
       read entity-header
       while (entity-header not empty) {
          append entity-header to existing header fields
          read entity-header
       }
       Content-Length := length
       Remove "chunked" from Transfer-Encoding


19.4.7 MIME-Version

HTTP is not a MIME-compliant protocol (see appendix 19.4). However,
HTTP/1.1 messages may include a single MIME-Version general-header field
to indicate what version of the MIME protocol was used to construct the
message. Use of the MIME-Version header field indicates that the message
is in full compliance with the MIME protocol (as defined in RFC 1521).


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Proxies/gateways are responsible for ensuring full compliance (where
possible) when exporting HTTP messages to strict MIME environments.

       MIME-Version   = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT

MIME version "1.0" is the default for use in HTTP/1.1. However, HTTP/1.1
message parsing and semantics are defined by this document and not the
MIME specification.


19.5 Changes from HTTP/1.0

This section summarizes major differences between versions HTTP/1.0 and
HTTP/1.1.


19.5.1 Changes to Simplify Multi-homed Web Servers and Conserve IP
Addresses

The requirements that clients and servers support the Host request-
header, report an error if the Host request-header (section 14.23) is
missing from an HTTP/1.1 request, and accept absolute URIs (section
5.1.2) are among the most important changes defined by this
specification.

Older HTTP/1.0 clients assumed a one-to-one relationship of IP addresses
and servers; there was no other established mechanism for distinguishing
the intended server of a request than the IP address to which that
request was directed. The changes outlined above will allow the
Internet, once older HTTP clients are no longer common, to support
multiple Web sites from a single IP address, greatly simplifying large
operational Web servers, where allocation of many IP addresses to a
single host has created serious problems. The Internet will also be able
to recover the IP addresses that have been allocated for the sole
purpose of allowing special-purpose domain names to be used in root-
level HTTP URLs. Given the rate of growth of the Web, and the number of
servers already deployed, it is extremely important that all
implementations of HTTP (including updates to existing HTTP/1.0
applications) correctly implement these requirements:


  .  Both clients and servers MUST support the Host request-header.

  .  Host request-headers are required in HTTP/1.1 requests.

  .  Servers MUST report a 400 (Bad Request) error if an HTTP/1.1
     request does not include a Host request-header.

  .  Servers MUST accept absolute URIs.


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19.6 Additional Features

This appendix documents protocol elements used by some existing HTTP
implementations, but not consistently and correctly across most HTTP/1.1
applications. Implementers should be aware of these features, but cannot
rely upon their presence in, or interoperability with, other HTTP/1.1
applications. Some of these describe proposed experimental features, and
some describe features that experimental deployment found lacking that
are now addressed in the base HTTP/1.1 specification.


19.6.1 Additional Request Methods


19.6.1.1 PATCH

The PATCH method is similar to PUT except that the entity contains a
list of differences between the original version of the resource
identified by the Request-URI and the desired content of the resource
after the PATCH action has been applied. The list of differences is in a
format defined by the media type of the entity (e.g.,
"application/diff") and MUST include sufficient information to allow the
server to recreate the changes necessary to convert the original version
of the resource to the desired version.

If the request passes through a cache and the Request-URI identifies a
currently cached entity, that entity MUST be removed from the cache.
Responses to this method are not cachable.

The actual method for determining how the patched resource is placed,
and what happens to its predecessor, is defined entirely by the origin
server. If the original version of the resource being patched included a
Content-Version header field, the request entity MUST include a Derived-
From header field corresponding to the value of the original Content-
Version header field. Applications are encouraged to use these fields
for constructing versioning relationships and resolving version
conflicts.

PATCH requests must obey the message transmission requirements set out
in section 8.2.

Caches that implement PATCH should invalidate cached responses as
defined in section 13.10 for PUT.


19.6.1.2 LINK

The LINK method establishes one or more Link relationships between the
existing resource identified by the Request-URI and other existing
resources. The difference between LINK and other methods allowing links
to be established between resources is that the LINK method does not

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allow any message-body to be sent in the request and does not directly
result in the creation of new resources.

If the request passes through a cache and the Request-URI identifies a
currently cached entity, that entity MUST be removed from the cache.
Responses to this method are not cachable.

Caches that implement LINK should invalidate cached responses as defined
in section 13.10 for PUT.


19.6.1.3 UNLINK

The UNLINK method removes one or more Link relationships from the
existing resource identified by the Request-URI. These relationships may
have been established using the LINK method or by any other method
supporting the Link header. The removal of a link to a resource does not
imply that the resource ceases to exist or becomes inaccessible for
future references.

If the request passes through a cache and the Request-URI identifies a
currently cached entity, that entity MUST be removed from the cache.
Responses to this method are not cachable.

Caches that implement UNLINK should invalidate cached responses as
defined in section 13.10 for PUT.


19.6.2 Additional Header Field Definitions


19.6.2.1 Alternates

The Alternates response-header field has been proposed as a means for
the origin server to inform the client about other available
representations of the requested resource, along with their
distinguishing attributes, and thus providing a more reliable means for
a user agent to perform subsequent selection of another representation
which better fits the desires of its user (described as agent-driven
negotiation in section 12).

The Alternates header field is orthogonal to the Vary header field in
that both may coexist in a message without affecting the interpretation
of the response or the available representations. It is expected that
Alternates will provide a significant improvement over the server-driven
negotiation provided by the Vary field for those resources that vary
over common dimensions like type and language.

The Alternates header field will be defined in a future specification.


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19.6.2.2 Content-Version

The Content-Version entity-header field defines the version tag
associated with a rendition of an evolving entity. Together with the
Derived-From field described in section 19.6.2.3, it allows a group of
people to work simultaneously on the creation of a work as an iterative
process. The field should be used to allow evolution of a particular
work along a single path rather than derived works or renditions in
different representations.

       Content-Version = "Content-Version" ":" quoted-string

Examples of the Content-Version field include:

       Content-Version: "2.1.2"
       Content-Version: "Fred 19950116-12:26:48"
       Content-Version: "2.5a4-omega7"


19.6.2.3 Derived-From

The Derived-From entity-header field can be used to indicate the version
tag of the resource from which the enclosed entity was derived before
modifications were made by the sender. This field is used to help manage
the process of merging successive changes to a resource, particularly
when such changes are being made in parallel and from multiple sources.

       Derived-From   = "Derived-From" ":" quoted-string

An example use of the field is:

       Derived-From: "2.1.1"

The Derived-From field is required for PUT and PATCH requests if the
entity being sent was previously retrieved from the same URI and a
Content-Version header was included with the entity when it was last
retrieved.


19.6.2.4 Link

The Link entity-header field provides a means for describing a
relationship between two resources, generally between the requested
resource and some other resource. An entity MAY include multiple Link
values. Links at the metainformation level typically indicate
relationships like hierarchical structure and navigation paths. The Link
field is semantically equivalent to the <LINK> element in HTML .

       Link           = "Link" ":" #("<" URI ">" *( ";" link-param )


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       link-param     = ( ( "rel" "=" relationship )
                          | ( "rev" "=" relationship )
                          | ( "title" "=" quoted-string )
                          | ( "anchor" "=" <"> URI <"> )
                          | ( link-extension ) )

       link-extension = token [ "=" ( token | quoted-string ) ]

       relationship   = sgml-name
                      | ( <"> sgml-name *( SP sgml-name) <"> )

       sgml-name      = ALPHA *( ALPHA | DIGIT | "." | "-" )

Relationship values are case-insensitive and MAY be extended within the
constraints of the sgml-name syntax. The title parameter MAY be used to
label the destination of a link such that it can be used as
identification within a human-readable menu. The anchor parameter MAY be
used to indicate a source anchor other than the entire current resource,
such as a fragment of this resource or a third resource.

Examples of usage include:

       Link: <http://www.cern.ch/TheBook/chapter2>; rel="Previous"

       Link: <mailto:timbl@w3.org>; rev="Made"; title="Tim Berners-Lee"

The first example indicates that chapter2 is previous to this resource
in a logical navigation path. The second indicates that the person
responsible for making the resource available is identified by the given
e-mail address.


19.6.2.5 URI

The URI header field has, in past versions of this specification, been
used as a combination of the existing Location, Content-Location, and
Vary header fields as well as the future Alternates field (above). Its
primary purpose has been to include a list of additional URIs for the
resource, including names and mirror locations. However, it has become
clear that the combination of many different functions within this
single field has been a barrier to consistently and correctly
implementing any of those functions. Furthermore, we believe that the
identification of names and mirror locations would be better performed
via the Link header field. The URI header field is therefore deprecated
in favor of those other fields.

       URI-header    = "URI" ":" 1#( "<" URI ">" )


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19.7 Compatibility with Previous Versions

It is beyond the scope of a protocol specification to mandate compliance
with previous versions. HTTP/1.1 was deliberately designed, however, to
make supporting previous versions easy. It is worth noting that at the
time of composing this specification, we would expect commercial
HTTP/1.1 servers to:


  .  recognize the format of the Request-Line for HTTP/0.9, 1.0, and 1.1
     requests;

  .  understand any valid request in the format of HTTP/0.9, 1.0, or
     1.1;

  .  respond appropriately with a message in the same major version used
     by the client.
And we would expect HTTP/1.1 clients to:


  .  recognize the format of the Status-Line for HTTP/1.0 and 1.1
     responses;

  .  understand any valid response in the format of HTTP/0.9, 1.0, or
     1.1.
For most implementations of HTTP/1.0, each connection is established by
the client prior to the request and closed by the server after sending
the response. A few implementations implement the Keep-Alive version of
persistent connections described in section 19.7.1.1.


19.7.1 Compatibility with HTTP/1.0 Persistent Connections

Some clients and servers may wish to be compatible with some previous
implementations of persistent connections in HTTP/1.0 clients and
servers. Persistent connections in HTTP/1.0 must be explicitly
negotiated as they are not the default behavior. HTTP/1.0 experimental

implementations of persistent connections are faulty, and the new
facilities in HTTP/1.1 are designed to rectify these problems. The
problem was that some existing 1.0 clients may be sending Keep-Alive to
a proxy server that doesn't understand Connection, which would then
erroneously forward it to the next inbound server, which would establish
the Keep-Alive connection and result in a hung HTTP/1.0 proxy waiting
for the close on the response. The result is that HTTP/1.0 clients must
be prevented from using Keep-Alive when talking to proxies.

However, talking to proxies is the most important use of persistent
connections, so that prohibition is clearly unacceptable. Therefore, we
need some other mechanism for indicating a persistent connection is
desired, which is safe to use even when talking to an old proxy that

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ignores Connection. Persistent connections are the default for HTTP/1.1
messages; we introduce a new keyword (Connection: close) for declaring
non-persistence.

The following describes the original HTTP/1.0 form of persistent
connections.

When it connects to an origin server, an HTTP client MAY send the Keep-
Alive connection-token in addition to the Persist connection-token:

       Connection: Keep-Alive

An HTTP/1.0 server would then respond with the Keep-Alive connection
token and the client may proceed with an HTTP/1.0 (or Keep-Alive)
persistent connection.

An HTTP/1.1 server may also establish persistent connections with
HTTP/1.0 clients upon receipt of a Keep-Alive connection token. However,
a persistent connection with an HTTP/1.0 client cannot make use of the
chunked transfer-coding, and therefore MUST use a Content-Length for
marking the ending boundary of each message.

A client MUST NOT send the Keep-Alive connection token to a proxy server
as HTTP/1.0 proxy servers do not obey the rules of HTTP/1.1 for parsing
the Connection header field.


19.7.1.1 The Keep-Alive Header

When the Keep-Alive connection-token has been transmitted with a request
or a response, a Keep-Alive header field MAY also be included. The Keep-
Alive header field takes the following form:

       Keep-Alive-header = "Keep-Alive" ":" 0# keepalive-param

       keepalive-param = param-name "=" value

The Keep-Alive header itself is optional, and is used only if a
parameter is being sent. HTTP/1.1 does not define any parameters.

If the Keep-Alive header is sent, the corresponding connection token
MUST be transmitted. The Keep-Alive header MUST be ignored if received
without the connection token.


19.8 Notes to the RFC Editor and IANA

This section of the document should be DELETED! It calls for the RFC
editor and IANA to take some actions before the draft becomes a Proposed
Standard. After those actions are taken, please delete this section of
the specification.

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19.8.1 Charset Registry

The following names should be added to the IANA character set registry
under the category "Preferred MIME name" and this section deleted.

       "US-ASCII"
       | "ISO-8859-1" | "ISO-8859-2" | "ISO-8859-3"
       | "ISO-8859-4" | "ISO-8859-5" | "ISO-8859-6"
       | "ISO-8859-7" | "ISO-8859-8" | "ISO-8859-9"
       | "ISO-2022-JP" | "ISO-2022-JP-2" | "ISO-2022-KR"
       | "SHIFT_JIS" | "EUC-KR" | "GB2312" | "BIG5" | "KOI8-R"

Please also add the following new alias as the "preferred MIME name":

       "EUC-JP" for "EXTENDED_UNIX_CODE_PACKED_FORMAT_FOR_JAPANESE"


19.8.2 Content-coding Values

HTTP also defines a new class of registry for its content-coding values.
The initial set of values defined in this document are deflate, gzip and
compress. IANA should create a registry with those entries. The registry
should note that "x-gzip" and "x-compress" are used as content-codings
in HTTP but that their use is deprecated. The registry should note that
"specifications of the content coding algorithms needed to implement a
new value should be publicly available and adequate for independent
implementation, and conform to the purpose of content coding defined RFC
XXX." where RFC XXX is the number assigned to this document.


19.8.3 New Media Types Registered

This document defines two new media types which should be registered.
Specifically appendix 19.1 defines the Internet media type message/http
and appendix 19.2 defines multipart/byteranges.


19.8.4 Possible Merge With Digest Authentication Draft

Note that the working group draft for Digest Authentication may be
processed by the IESG at the same time as this document; we leave it to
the RFC editor to decide whether to issue a single RFC containing both
drafts (see section 11.2 for where it would be put); in any case, the
reference in the reference list will need to be either deleted, or made
to the appropriate RFC (and section 11.2 deleted).


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