2004/id/draft-ietf-uri-relative-url-01.txt

Uniform Resource Identifiers Working Group                R.T. Fielding
INTERNET-DRAFT                                                UC Irvine
Expires April 28, 1995                                 October 28, 1994


                  Relative Uniform Resource Locators
                 <draft-ietf-uri-relative-url-01.txt>


Status of this Memo

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Abstract

   Uniform Resource Locators (URLs) are a compact representation of the
   location and access method for a resource available via the Internet.
   When embedded within a base document, a URL in its absolute form may
   contain a great deal of information which is already known from the
   context of that base document's retrieval, including the access
   scheme, network location, and parts of the url-path.  In situations
   where the base URL is well-defined and known to the parser (human or
   machine), it is useful to be able to embed URL references which
   inherit that context rather than re-specifying it in every instance.
   This document defines the syntax and semantics for such Relative
   Uniform Resource Locators.


1.  Introduction

   This work is derived from concepts introduced by the World-Wide Web
   global information initiative, whose use of such objects dates from
   1990 and is described in "Universal Resource Identifiers in WWW",
   RFC 1630 [1].  This document is a companion to the Internet-Draft
   "Uniform Resource Locators (URL)" [2], which specifies the
   syntax and semantics of absolute URLs.  A URL is "absolute" if it
   can be interpreted consistently and unambiguously, with global scope,
   independent of any other URL.

   This document describes the syntax and semantics for "relative"
   Uniform Resource Locators (relative URLs): a compact representation
   of the location and access method for a resource available via the
   Internet relative to an absolute base URL.  The name space of
   relative URLs is a superset of that defined in [2] for Uniform
   Resource Locators, in that all absolute URLs can be interpreted
   consistently relative to any Internet-accessible resource.  For the
   sake of clarity, however, this document will only term "relative"
   those URLs which obtain global scope only when interpreted relative
   to a separate base URL.

   A primary use for Uniform Resource Locators is to embed them within
   a document (referred to as the "base" document) for the purpose of
   identifying other Internet-accessible resources.  This is
   particularly true of hypertext documents, where URLs can serve as
   the identifiers for hypertext link destinations. 

   It is often the case that, where a group or "tree" of documents
   serves a common purpose, the vast majority of URLs within those
   documents point to locations within that tree rather than outside
   of it.  Similarly, documents located at a particular Internet site
   are much more likely to refer to other resources at that site than
   to resources at remote sites.

   Absolute URLs contain a great deal of information which may already
   be known from the context of the base document's retrieval, 
   including the access scheme, network location, and parts of the
   URL path.  In situations where the base URL is well-defined and
   known to the parser (human or machine), it is useful to be able to
   embed a URL reference which inherits that context rather than
   re-specifying it within each instance.

   In addition to the space saved, relative addressing of URLs allows
   document trees to be partially independent of their location and/or
   access scheme.  For instance, if they refer to each other using
   relative URLs, it is possible for a single set of documents to be
   simultaneously accessible and, if hypertext, traversable via each
   of the "file", "http", and "ftp" access schemes. Furthermore,
   document trees can be moved, as a whole, without changing any of
   the embedded URLs.  Experience within the World-Wide Web has
   demonstrated that the ability to perform relative referencing is
   necessary for the long-term usability of embedded URLs.

2.  Relative URL Syntax

   The syntax for relative URLs is the same as that for absolute URLs
   [2], with the exception that portions of the URL may be missing, and
   certain path components ("." and "..") have a special meaning when
   interpreting a relative URL path.  Although this document does not
   seek to define the overall URL syntax, some discussion of it is
   necessary in order to describe the parsing of relative URLs.

2.1.  URL Syntactic Components

   The relative form relies on a property of the URL syntax that
   certain characters ("/") and certain path segments ("..", ".") have
   a significance reserved for representing a hierarchical space.
   Additional reserved characters are sometimes used to separate the
   URL path from other components, including object parameters (";"),
   query information ("?"), and fragment identifiers ("#").

   Like absolute URLs, relative URL syntax is dependent upon the access
   scheme.  Some schemes use "?" and ";" to indicate special reserved
   components, while others just consider them to be part of the path.
   However, there is enough uniformity in the syntax to allow a parser
   to resolve relative URLs based upon a few syntactic categories.
   These categories are described in Section 2.3.

   In general, the relative URL syntax consists of six components:

      <scheme>://<net_loc>/<path>;<params>?<query>#<fragment>

   each of which may be absent or may be disallowed by a particular
   scheme.  They are defined as follows (a complete BNF is provided in
   Section 2.2):

      scheme ":"   ::= access scheme name, as per Section 2.1 of [2].

      "//" net_loc ::= network location and login information, as per
                       Section 3.1 of [2].

      "/" path     ::= URL path, as per Section 3.1 of [2].

      ";" params   ::= object parameters (e.g. ";type=a" as in 
                       Section 3.2.2 of [2]).

      "?" query    ::= query information, as per Section 3.3 of [2].

      "#" fragment ::= fragment identifier (currently only used within
                       the World-Wide Web initiative).

   The order of the components is important.  If both <params> and
   <query> are present, the <query> information must occur after the
   <params>.  Relative components are resolved from left-to-right,
   according to the rules given in Section 4.

2.2.  BNF for Relative URLs

   This is a BNF-like description of the Relative Uniform Resource
   Locator syntax, using the conventions of RFC 822 [7], except that
   "|" is used to designate alternatives, and brackets "[]" are used
   around optional or repeated elements. Briefly, literals are quoted
   with "", optional elements are enclosed in [brackets], and elements
   may be preceded with <n>* to designate n or more repetitions of the
   following element; n defaults to 0.

   Because relative URLs are parsed within the context of the base URL,
   this BNF is not sufficient to completely specify the allowed syntax
   within any given context.  Section 2.4 describes a context-sensitive
   parsing algorithm which disambiguates the grammar.


   relativeURL = [ absoluteURL | location | abs_path | rel_path ]
                 [ "#" fragment ]

   absoluteURL = scheme ":" [ location | abs_path | rel_path ]
   location    = "//" net_loc [ "/" rel_path ]
   abs_path    = "/"  rel_path
   rel_path    = [ path ] [ ";" params ] [ "?" query ]

   path        = segment *[ "/" segment ]
   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
   hex         = digit | "A" | "B" | "C" | "D" | "E" | "F" |
                         "a" | "b" | "c" | "d" | "e" | "f"

   alpha       = lowalpha | hialpha
   lowalpha    = "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" | "i" |
                 "j" | "k" | "l" | "m" | "n" | "o" | "p" | "q" | "r" |
                 "s" | "t" | "u" | "v" | "w" | "x" | "y" | "z"
   hialpha     = "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" | "I" | 
                 "J" | "K" | "L" | "M" | "N" | "O" | "P" | "Q" | "R" | 
                 "S" | "T" | "U" | "V" | "W" | "X" | "Y" | "Z"

   digit       = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |
                 "8" | "9"

   safe        = "$" | "-" | "_" | "." | "+"
   extra       = "!" | "*" | "'" | "(" | ")" | ","
   national    = "{" | "}" | "|" | "\" | "^" | "~" | "[" | "]" | "`"
   reserved    = ";" | "/" | "?" | ":" | "@" | "&" | "="
   punctuation = "<" | ">" | "#" | "%" | <">


2.3.  Specific Schemes and their Syntactic Categories

   Each URL access scheme has its own rules regarding the presence or
   absence of the syntactic components described in Section 2.1 and 2.2.
   However, there is enough commonality among the schemes to be able
   to group them into just a few categories.  These categories are
   sufficiently general to allow new schemes to be added without 
   substantial changes to the algorithm for resolving relative URLs.

   Within this section, we include as examples only those schemes 
   which have a defined URL syntax in [2].  This includes:

      ftp        File Transfer Protocol [3]
      http       Hypertext Transfer Protocol [4]
      gopher     Gopher and Gopher+ Protocols [5, 6]
      mailto     Electronic Mail [7]
      news       USENET news [8]
      nntp       USENET news using NNTP access [9]
      telnet     TELNET Protocol for Interactive Sessions [10]
      wais       Wide Area Information Servers Protocol [11,12]
      file       Host-specific Files
      prospero   Prospero Directory Service [13]

   It is recommended that new schemes include a description of their
   membership in the following categories when they are registered,
   as per Section 4 of [2].  Membership in the five categories is 
   described in terms of named sets: Uses-Netloc, Non-Hierarchical,
   Uses-Params, Uses-Query, and Uses-Fragment.

2.3.1  The Uses-Netloc Set

   The Uses-Netloc set includes those access schemes which use the
   Common Internet Scheme Syntax described in Section 3.1 of [2], where
   the network location and/or login information starts with a
   double-slash "//" to indicate its presence, and continues until the
   following slash "/", if any.

      Uses-Netloc = {ftp, http, gopher, nntp, telnet, wais,
                     file, prospero}

2.3.2  The Non-Hierarchical Set

   The Non-Hierarchical set includes those access schemes which do not
   use "/" to indicate hierarchical segments in the URL path.

      Non-Hierarchical = {gopher, wais, mailto, news, telnet, prospero}

   When resolving relative paths for schemes not in the Non-Hierarchical
   set, the complete path segments ".." and "." have a significance
   reserved for representing the path hierarchy, indicating up-one-level
   and current-level, respectively.

2.3.3  The Uses-Params Set

   The Uses-Params set includes those access schemes which allow the
   semicolon ";" character to separate object parameters from the
   URL path.  There may be more than one parameter, each being
   separated by a semicolon ";".

      Uses-Params = {ftp, http, prospero}

2.3.4  The Uses-Query Set

   The Uses-Query set includes those access schemes which allow the
   question mark "?" character to separate query information from the
   URL path.

      Uses-Query = {http, wais}

2.3.5  The Uses-Fragment Set

   The Uses-Fragment set includes those access schemes which allow the
   crosshatch "#" character to separate a fragment identifier from
   the rest of the URL.  Within systems that use fragment identifiers,
  
      Uses-Fragment = {ftp, http, gopher, news, nntp, wais,
                       file, prospero}

   Unlike the other sets, however, the fragment identifier is only
   reserved within systems which use it.  Outside of those systems,
   Uses-Fragment is equal to the empty set.

2.3.6.  Summary of Categories by Scheme

                 Uses-   Non-Hier  Uses-    Uses-    Uses-
                 Netloc  archical  Params   Query   Fragment
               .--------------------------------------------.
      ftp      |  XXXX  |        |  XXXX  |        |  XXXX  |
      http     |  XXXX  |        |  XXXX  |  XXXX  |  XXXX  |
      gopher   |  XXXX  |  XXXX  |        |        |  XXXX  |
      mailto   |        |  XXXX  |        |        |        |
      news     |        |  XXXX  |        |        |  XXXX  |
      nntp     |  XXXX  |        |        |        |  XXXX  |
      telnet   |  XXXX  |  XXXX  |        |        |        |
      wais     |  XXXX  |  XXXX  |        |  XXXX  |  XXXX  |
      file     |  XXXX  |        |        |        |  XXXX  |
      prospero |  XXXX  |  XXXX  |  XXXX  |        |  XXXX  |
               `--------------------------------------------'

2.4.  Parsing a URL

   An accepted method for parsing URLs is necessary to disambiguate the
   relative URL syntax of Section 2.2 and to describe the algorithm for
   resolving relative URLs presented in Section 4.  This section
   describes the parsing rules for breaking down a URL (relative or
   absolute) into the component parts described in Section 2.1.  The
   rules assume that the URL has already been separated from any
   surrounding text and copied to a "parse string".  The rules are
   listed in the order in which they must be applied by the parser.

2.4.1.  Parsing the Scheme

   If the parse string contains a colon ":" after the first character
   and before any characters not allowed as part of a scheme name
   (i.e. any not an alphanumeric, plus "+", period ".", or hyphen "-"),
   the scheme of the URL is the substring of characters up to but not
   including the first colon.  These characters and the colon are then
   removed from the parse string before continuing. 
 
2.4.2.  Parsing the Fragment Identifier

   If the scheme is not a member of the Uses-Fragment set, this section
   is skipped.

   If the parse string contains a crosshatch "#" character, then the
   substring after the last (right-most) crosshatch "#" and up to the
   end of the parse string is the fragment identifier.  If the
   crosshatch is the last character, or no crosshatch is present, then
   the fragment identifier is empty.  The matched substring, including
   the crosshatch character, is removed from the parse string before
   continuing.

   Note that the fragment identifier is not considered part of the URL.
   However, since it is often attached to the URL, parsers must be able
   to recognize and set aside fragment identifiers as part of the
   process.

2.4.3.  Parsing the Network Location/Login

   If the scheme is not a member of the Uses-Netloc set, this section
   is skipped.

   If the parse string begins with a double-slash "//", then the
   substring of characters after the double-slash and up to, but not
   including, the next slash "/" character is the network location/login
   (<net_loc>) of the URL.  If no trailing slash "/" is present, the
   entire remaining parse string is assigned to <net_loc>.  The 
   double-slash and <net_loc> are removed from the parse string before
   continuing. 
 
2.4.4.  Parsing the Query Information

   If the scheme is not a member of the Uses-Query set, this section
   is skipped.

   If the parse string contains a question mark "?" character, then the
   substring after the first (left-most) question mark "?" and up to the
   end of the parse string is the query information.  If the question
   mark is the last character, or no question mark is present, then the
   query information is empty.  The matched substring, including the
   question mark character, is removed from the parse string before
   continuing.

2.4.5.  Parsing the Parameters

   If the scheme is not a member of the Uses-Params set, this section
   is skipped.

   If the parse string contains a semicolon ";" character, then the
   substring after the first (left-most) semicolon ";" and up to the
   end of the parse string is the parameters (<params>).  If the
   semicolon is the last character, or no semicolon is present, then
   <params> is empty.  The matched substring, including the semicolon
   character, is removed from the parse string before continuing.

2.4.6.  Parsing the Path

   After the above steps, all that is left of the parse string is
   the URL path and the slash "/" that may precede it.  Even though
   the initial slash is not part of the URL path, the parser must
   remember whether or not it was present so that later processes
   can differentiate between relative and absolute paths.  Often this
   is done by simply storing the preceding slash along with the path.

3.  Establishing a Base URL

   In order for relative URLs to be usable within a base document,
   the absolute "base URL" of that document must be known to the
   parser.  There are three methods for obtaining the base URL of
   a document, listed here in order of precedence.

3.1.  Base URL within Document Content

   Within certain document content-types, the base URL of the document
   can be embedded within the content itself such that it can be
   readily obtained by a parser.  This can be useful for descriptive
   documents, such as tables of content, which may be transmitted to
   others through schemes which do not support relative addressing
   (e.g. E-Mail or USENET news).

   It is beyond the scope of this document to specify how, for each
   content-type, the base URL can be embedded.  However, an example of
   how this is done for the Hypertext Markup Language (HTML) [14] is
   provided in an Appendix (Section 10).

3.2.  Base URL within Message Headers

   For access schemes which make use of message headers like those
   described in RFC 822 [7], a second method for identifying the base
   URL of a document is to include that URL in the message headers.
   It is recommended that the format of this header be:

      Base-URL: absoluteURL

   where "Base-URL" is case-insensitive.  For example,

      Base-URL: http://www.ics.uci.edu/Test/a/b/c

   would indicate that any relative URLs found within the document
   should be parsed relative to <URL:http://www.ics.uci.edu/Test/a/b/c>.
   In situations where both an embedded base URL (as described in
   Section 3.1) and a "Base-URL" message header are present, the
   embedded base URL takes precedence.

3.3.  Base URL from the Retrieval Context

   If neither an embedded base URL nor a "Base-URL" message header
   is present, then, if a URL was used to retrieve the base document,
   that URL shall be considered the base URL.  Note that if the
   retrieval was the result of a redirected request, the last URL used
   (i.e., that which resulted in the actual retrieval of the document)
   is the base URL.

3.4.  Default Base URL

   If none of the conditions described in Sections 3.1 -- 3.3 apply,
   then the base URL is considered to be the empty string and all
   embedded URLs within that document shall be interpreted as absolute.
   It is the responsibility of the distributor(s) of a document
   containing relative URLs to ensure that the base URL for that
   document can be established.  It must be emphasized that relative
   URLs cannot be used reliably in situations where the object's base
   URL is not well-defined.

4.  Resolving Relative URLs

   This section describes an example algorithm for resolving URLs
   within a context in which the URLs may be relative, such that the
   result is always a URL in absolute form.  Although this algorithm
   cannot guarantee that the resulting URL will equal that intended
   by the original author, it does guarantee that any valid URL
   (relative or absolute) can be consistently transformed to an
   absolute form given a valid base URL.

   The following steps are performed in order:

   Step 1: The base URL is established according to the rules of
           Section 3.

   Step 2: If the base URL is the empty string (unknown), the embedded
           URL is interpreted as an absolute URL and we are done.

   Step 3: Both the base and embedded URLs are parsed into their
           component parts as described in Section 2.4.

           a) If the embedded URL starts with a scheme name, it is
              interpreted as an absolute URL and we are done.

           b) Otherwise, the embedded URL inherits the scheme of
              the base URL.

   Step 4: If the scheme is a member of the Uses-Netloc set
           (Section 2.3.1), then

           a) If the embedded URL's <net_loc> is non-empty, we skip to
              Step 8.

           b) Otherwise, the embedded URL inherits the <net_loc> of the
              base URL.

   Step 5: If the embedded URL path is preceded by a slash "/", the
           path is not relative and we skip to Step 8.

   Step 6: If the embedded URL path is empty (and not preceded by a
           slash), then

           a) The embedded URL inherits the base URL path; and,

           b) If the embedded URL's <params> is empty, it
              inherits the <params> of the base URL (if any); and,

           c) If the embedded URL's <query> is empty, it inherits
              the <query> of the base URL (if any); and,

           d) We skip to Step 8.

   Step 7: The last path segment of the base URL's path (anything
           following the rightmost slash "/", or the entire path if no
           slash is present) is removed and the embedded URL's path is
           appended in its place.  The following operations are
           then applied, in order, to the new URL path:
 
           a) All occurrences of "./", where "." is a complete path
              segment, are removed.

           b) If the URL path ends with "." as a complete path segment,
              that "." is removed.

           c) All occurrences of "<segment>/../", where <segment> and
              ".." are complete path segments, are removed.  Removal of
              these path segments is performed iteratively, removing the
              leftmost matching pattern on each iteration, until no
              matching pattern remains.

           d) If the URL path ends with "<segment>/..", that
              "<segment>/.." is removed.

   Step 8: The resulting URL components, including any inherited from
           the base URL, are recombined to give the absolute form of
           the embedded URL.

   Parameters, regardless of their purpose, do not form a part of the
   URL path and thus have no effect on the resolving of relative paths.
   In particular, the presence or absence of the ";type=d" parameter
   on an ftp URL has no effect on the interpretation of paths relative
   to that URL.  Fragment identifiers are never inherited from the
   base URL.

5.  Examples and Recommended Practice

   Within an object with a well-defined base URL of

      <URL:http://a/b/c/d>

   the relative URLs would be resolved as follows: 

5.1.  Normal Examples

      g:h        = <URL:g:h>
      g          = <URL:http://a/b/c/g>
      ./g        = <URL:http://a/b/c/g>
      g/         = <URL:http://a/b/c/g/>
      /g         = <URL:http://a/g>
      //g        = <URL:http://g>
      ?y         = <URL:http://a/b/c/d?y>
      g?y        = <URL:http://a/b/c/g?y>
      g?y/./x    = <URL:http://a/b/c/g?y/./x>
      .          = <URL:http://a/b/c/>
      ./         = <URL:http://a/b/c/>
      ..         = <URL:http://a/b/>
      ../        = <URL:http://a/b/>
      ../g       = <URL:http://a/b/g>
      ../..      = <URL:http://a/>
      ../../g    = <URL:http://a/g>

5.2.  Abnormal Examples

      ../../../g = <URL:http://a/../g>
      ./../g     = <URL:http://a/b/g>
      ./g/.      = <URL:http://a/b/c/g/>
      /./g       = <URL:http://a/./g>
      g/./h      = <URL:http://a/b/c/g/h>
      g/../h     = <URL:http://a/b/c/h>
      http:g     = <URL:http:g>
      http:      = <URL:http:>

   Note that, although the abnormal examples are not likely to occur
   for a normal relative URL, all URL parsers should be capable of
   resolving them consistently.

5.3.  Recommended Practice

   Authors should be aware that path names which contain a colon
   ":" character cannot be used as the first component of a relative
   URL path (e.g. "this:that") because they will likely be mistaken for
   a scheme name.  It is therefore necessary to precede such cases with
   other components (e.g., "./this:that"), or to escape the colon
   character (e.g., "this%3Athat"), in order for them to be correctly
   parsed.  The former solution is preferred because it has no effect
   on the absolute form of the URL.

6.  Security Considerations

   None.

7.  Acknowledgements

   This work is derived from concepts introduced by Tim Berners-Lee and
   the World-Wide Web global information initiative.  Relative URLs are
   described as "Partial URLs" in RFC 1630 [1].  That description was
   expanded for inclusion as an appendix for the Internet-Draft
   "Uniform Resource Locators (URL)" [2].  However, after further
   discussion, the URI-WG decided to specify Relative URLs separately
   from the primary URL draft.

   This document is intended to fulfill the requirements for Internet
   Resource Locators as stated in [15].  It has benefited greatly from
   the comments of all those participating in the URI-WG.  Particular
   thanks go to Larry Masinter, Michael A. Dolan, Guido van Rossum, and
   Dave Kristol for identifying problems/deficiencies in earlier drafts.

8.  References

   [1] Berners-Lee, T., "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,
       <URL:ftp://ds.internic.net/rfc/rfc1630.txt>, June 1994.

   [2] Berners-Lee, T., Masinter, L., and McCahill, M., Editors,
       "Uniform Resource Locators (URL)", Internet-Draft (work in
       progress), <URL:ftp://ds.internic.net/internet-drafts/
       draft-ietf-uri-url-08.txt>, October 1994.

   [3] Postel, J. and Reynolds, J.K., "File Transfer Protocol (FTP)",
       RFC 959, <URL:ftp://ds.internic.net/rfc/rfc959.txt>, October 1985.

   [4] Berners-Lee, T ., "Hypertext Transfer Protocol (HTTP)" ,
       CERN, <URL:ftp://info.cern.ch/pub/www/doc/http-spec.txt.Z>,
       November 1993.

   [5] Anklesaria, F., McCahill, M., Lindner, P., Johnson, D.,
       Torrey, D., and Alberti, B., "The Internet Gopher Protocol:
       A distributed document search and retrieval protocol",
       RFC 1436, <URL:ftp://ds.internic.net/rfc/rfc1436.txt>,
       March 1993.

   [6] Anklesaria, F., Lindner, P., McCahill, M., Torrey, D.,
       Johnson, D., and Alberti, B., "Gopher+: Upward compatible
       enhancements to the Internet Gopher protocol",
       University of Minnesota, <URL:ftp://boombox.micro.umn.edu
       /pub/gopher/gopher_protocol/Gopher+/Gopher+.txt>, July 1993.

   [7] Crocker, D. H., "Standard for the Format of ARPA Internet Text
       Messages", RFC 822, <URL:ftp://ds.internic.net/rfc/rfc822.txt>,
       April 1982.

   [8] Horton, M. and Adams, R., "Standard For Interchange of USENET
       messages", RFC 1036, <URL:ftp://ds.internic.net/rfc/rfc1036.txt>,
       December 1987.

   [9] Kantor, B. and Lapsley, P., "Network News Transfer Protocol:
       A Proposed Standard for the Stream-Based Transmission of News",
       RFC977, <URL:ftp://ds.internic.net/rfc/rfc977.txt>,
       February 1986.

  [10] Postel, J. and Reynolds, J., "TELNET Protocol Specification",
       RFC 854, <URL:ftp://ds.internic.net/rfc/rfc854.txt>, May 1983.

  [11] Davis, F., Kahle, B., Morris, H., Salem, J., Shen, T., Wang, R.,
       Sui, J., and Grinbaum, M., "WAIS Interface Protocol Prototype
       Functional Specification", (v1.5), Thinking Machines Corporation, 
       <URL:ftp://quake.think.com/pub/wais/doc/protspec.txt>,
       April 1990.

  [12] St. Pierre, M, Fullton, J., Gamiel, K., Goldman, J., Kahle, B.,
       Kunze, J., Morris, H., and Schiettecatte, F.,
       "WAIS over Z39.50-1988", RFC 1625,
       <URL:ftp://ds.internic.net/rfc/rfc1625.txt>, June 1994.

  [13] Neuman, B.C., and Augart, S. "The Prospero Protocol",
       USC Information Sciences Institute, <URL:
       ftp://prospero.isi.edu/pub/prospero/doc/prospero-protocol.PS.Z>,
       June 1993.

  [14] Berners-Lee, T., Connolly, D., et al. "HyperText Markup Language
       Specification -- 2.0", HTML-WG draft (work in progress),
       <URL:ftp://www.ics.uci.edu/pub/ietf/html/>, October 1994.

  [15] Kunze, J., "Functional Requirements for Internet Resource
       Locators", Internet-Draft (work in progress),
       <URL:ftp://ds.internic.net/internet-drafts/
       draft-ietf-uri-irl-fun-req-01.txt>, July 1994.

9.  Author's Address

   Roy T. Fielding
   Department of Information and Computer Science
   University of California
   Irvine, CA  92717-3425
   U.S.A.

   Tel: +1 (714) 824-4049
   Fax: +1 (714) 824-4056
   Email: fielding@ics.uci.edu

   This Internet-Draft expires April 28, 1995.


10.  Appendix - Embedding the Base URL in HTML documents.

   It is useful to consider an example of how the base URL of a
   document can be embedded within the document's content.  In this
   appendix, we describe how documents written in the Hypertext Markup
   Language (HTML) [14] can include an embedded base URL.  This appendix
   does not form a part of the relative URL specification and should not
   be considered as anything more than a descriptive example.

   HTML defines a special element "BASE" which, when present in the
   "HEAD" portion of a document, signals that the parser should use
   the BASE element's "HREF" attribute as the base URL for resolving
   any relative URLs.  The "HREF" attribute must be an absolute URL.
   Note that, in HTML, element and attribute names are case-insensitive.
   For example:

      <!doctype html public "-//IETF//DTD HTML//EN">
      <HTML><HEAD>
      <TITLE>An example HTML document</TITLE>
      <BASE href="http://www.ics.uci.edu/Test/a/b/c">
      </HEAD><BODY>
      ... <A href="../x">a hypertext anchor</A> ...
      </BODY></HTML>

   A parser reading the example document should interpret the given
   relative URL "../x" as representing the absolute URL

      <URL:http://www.ics.uci.edu/Test/a/x>

   regardless of the context in which the example document was obtained.
