2004/id/draft-ietf-urlreg-guide-01.txt


INTERNET-DRAFT                                          Larry Masinter
<draft-ietf-urlreg-guide-01.txt>                  Harald T. Alvestrand
December 9, 1997                                           Dan Zigmond

                  Guidelines for new URL Schemes

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 obsoleted by other
   documents at any time.  It is inappropriate to use Internet-Drafts
   as reference material or to cite them other than as ``work in
   progress.''

   To learn the current status of any Internet-Draft, please check the
   ``1id-abstracts.txt'' listing contained in the Internet-Drafts
   Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net
   (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East
   Coast), or ftp.isi.edu (US West Coast).

Abstract

   A Uniform Resource Locator (URL) is a compact string representation
   of the location for a resource that is available via the Internet.
   This document provides guidelines for the definition of new URL
   schemes.

1. Introduction

   A Uniform Resource Locator (URL) is a compact string representation
   of the location for a resource that is available via the Internet.
   [URI-SYNTAX] defines the general syntax and semantics of URIs, and,
   by inclusion, URLs. URLs are designated by including a "scheme"
   and then a "scheme-specific part". Many URL schemes are already
   defined.

   This document provides guidelines for the definition of new URL
   schemes, for consideration by those who are defining and
   registering or evaluating those definitions.

   The process by which new URL schemes are registered or defined
   is not defined here.

2. Guildelines for new URL schemes

   Because new URL schemes potentially complicate client software, new
   schemes must have demonstrable utility and operability, as well as
   compatibility with existing URL schemes. This section elaborates
   these criteria.

2.1 Syntactic compatibility

   New URL schemes should follow the same syntactic conventions of
   existing schemes when appropriate. 

2.1.1 Use of initial "//" for top level

   Many proposed new URL schemes seem to use "://" as a kind of
   indicator that what follows is a URL. However, the use of "//"
   indicates a "top level" for schemes that support relative
   URLs, and is not necessary (and just confusing) for schemes
   that have no relative forms. URL schemes without relative
   forms (such as mailto, cid, mid) do not use an initial "//".

2.1.2 Compatibility with relative URLs

   URL schemes should use the generic-URL syntax if they are intended
   to be used with relative URLs.  A description of the allowed
   relative forms should be included in the scheme's definition.
   Many applications use relative URLs extensively.

   o Can it be parsed according to [URI-SYNTAX] - that is, if the
     tokens "//", "/", ";", "?" and "#" are used, do they have the
     meaning given in RFC URI-SYNTAX?

   o Does it make sense to use it in relative URLs like those
     [URI-SYNTAX] specifies?

   o If something is designed to be broken into pieces, does it
     document what those pieces are, why it should be broken in this
     way, and why the breaks aren't where URI-SYNTAX says that they
     usually should be?
     
   o If it has a hierarchy, does it go left-to-right and with slash
     separators like [URN-SYNTAX]? If not, why not?
   
2.2 Is the scheme well defined?

   It is important that the semantics of the "resource" that a URL
   "locates" be well defined. This might mean different things
   depending on the nature of the URL scheme.

2.2.1 Clear mapping from other name spaces

   In many cases, new URL schemes are defined as ways to translate
   other protocols and name spaces into the general framework of
   URLs. The "ftp" URL scheme translates from the FTP protocol, while
   the "mid" URL scheme translates from the Message-ID field of
   messages.

   In either case, the description of the mapping must be complete,
   must describe how character sets get encoded or not in URLs, must
   describe exactly how all legal values of the base standard can be
   represented using the URL scheme, and exactly which modifiers,
   alternate forms and other artifacts from the base standards are
   included or not included. These requirements are elaborated
   below.

2.2.2 URL schemes associated with network protocols

   Most new URL schemes are associated with network resources that
   have one or several network protocols that can access them. The
   'ftp', 'news', and 'http' schemes are of this nature.  For such
   schemes, the specification should completely describe how URLs are
   translated into protocol actions in sufficient detail to make the
   access of the network resource unambiguous.  If an implementation
   of of the URL scheme requires some configuration, the configuration
   elements must be clearly identified. (For example, the 'news'
   scheme, if implemented using NTTP, requires configuration of the
   NTTP server.)

2.2.3 Character encoding

   When describing URL schemes in which (some of) the elements of
   the URL are actually representations of sequences of characters,
   care should be taken not to introduce unnecessary variety in the
   ways in which characters are encoded into octets and then into
   URL characters. Unless there is some compelling reason for a
   particular scheme to do otherwise, translating character sequences
   into UTF-8 [RFC2044] and then subsequently using the %HH encoding
   for unsafe octets is recommended.
   
2.2.4 Definition of non-protocol URL schemes
   
   In some cases, URL schemes do not have particular network protocols
   associated with them, because their use is limited to contexts
   where the access method is understood. This is the case, for
   example, with the "cid" and "mid" URL schemes. For these URL
   schemes, the specification should describe the notation of the
   scheme and a complete mapping of the locator from its source.
   
2.2.5 Definition of URL schemes not associated with data resources

   Most URL schemes locate Internet resources that correspond
   to data objects that can be retrieved or modified. This is the
   case with "ftp" and "http", for example. However, some URL schemes
   do not; for example, the "mailto" URL scheme corresponds to an
   Internet mail address.
   
   If a new URL scheme does not locate resources that are data
   objects, the properties of names in the new space must be clearly
   defined.

2.2.6 Definition of operations

   In some contexts (for example, HTML forms) it is possible to
   specify any one of a list of operations to be performed on a
   specifc URL. (Outside forms, it is generally assumed to be
   something you GET.)

   The URL scheme definition should describe all well-defined
   operations on the URL identifier, and what they are supposed to
   do.
        
   Some URL schemes (for example, "telnet") provide location
   information for hooking onto bidirectional data streams, and don't
   fit the "infoaccess" paradigm of most URLs very well; this should
   be documented.

   NOTE: It is perfectly valid to say that "no operation apart from
   GET is defined for this URL". It is also valid to say that "there's
   only one operation defined for this URL, and it's not very
   GET-like". The important point is that what is defined on this type
   is described.

2.3 Demonstrated utility

   URL schemes should have demonstrated utility.  New URL schemes are
   expensive things to support. Often they require special code in
   browsers, proxies, and/or servers.  Having a lot of ways to say the
   same thing needless complicates these programs without adding value
   to the Internet.

   The kinds of things that are useful include:

      o Things that cannot be referred to in any other way.

      o Things where it is much easier to get at them using this
        scheme than (for instance) a proxy gateway.


2.3.1 Proxy into HTTP/HTML

   One way to provide a demonstration of utility is via a gateway
   which provides objects in the new scheme for clients using an
   existing protocol. It is much easier to deploy gateways to a new
   service than it is to deploy browsers that understand the new URL
   object.

   Things to look for when thinking about a proxy are:

   o Is there a single global resolution mechanism whereby any proxy can
     find the referenced object?
   o If not, is there a way in which the user can find any object of
     this type, and "run his own proxy"?
   o Are the operations mappable one-to-one (or possibly using
     modifiers) to HTTP operations?
   o Is the type of returned objects well defined?
      * as MIME content-types?
      * as something that can be translated to HTML?
   o Is there running code for a proxy?

2.4 Are there security considerations?

   Above and beyond the security considerations of the base mechanism
   a scheme builds upon, one must think of things that can happen in
   the normal course of URL usage.

   In particular:

   o Does the user need to be warned that such a thing is happening
     without an explicit request (GET for the source of an IMG tag,
     for instance)?  This has implications for the design of a proxy
     gateway, of course.

   o Is it possible to fake URLs of this type that point to different
     things in a dangerous way?

   o Are there mechanisms for identifying the requester that can be
     used or need to be used with this mechanism (the From: field in a
     mailto: URL, or the Kerberos login required for AFS access in the
     AFS: url, for instance)?

   o Does the mechanism contain passwords or other security
     information that are passed inside the referring document in the
     clear (as in the "ftp" URL, for instance)?

2.5 Does it start with UR?

   Any scheme starting with the letters "U" and "R", in particular if
   it attaches any of the meanings "uniform", "universal" or
   "unifying" to the first letter, is going to cause intense debate,
   and generate much heat (but maybe little light).

   Any such proposal should either make sure that there is a large
   consensus behind it that it will be the only scheme of its type, or
   pick another name.

2.6 Non-considerations

   Some issues that are often raised but are not relevent to new URL
   schemes include the following.

2.6.1 Are all objects acessible?

   Can all objects in the world that are validly identified by a
   scheme be accessed by any UA implementing it?

   Sometimes the answer will be yes and sometimes no; often it will
   depend on factors (like firewalls or client configuration) not
   directly related to the scheme itself.

3. Security considerations

   New URL schemes are required to address all security considerations
   in their definitions.

4. IANA considerations

   The process by which URL schemes are defined and registered
   is not defined in this document.

5. References

 [RFC2044] F. Yergeau, "UTF-8, A Transformation Format of Unicode
    and ISO 10646", Alis Technologies, October 1996.

 [URI-SYNTAX] T. Berners-Lee, R. Fielding, L. Masinter, "Uniform
    Resource Identifiers (URI): Generic Syntax and Semantics",
    <draft-fielding-uri-syntax-*>.

6. Author's Addresses

   Larry Masinter
   Xerox Corporation
   Palo Alto Research Center
   3333 Coyote Hill Road
   Palo Alto, CA 94304
   Fax: +1-415-812-4333
   EMail: masinter@parc.xerox.com

   Harald T. Alvestrand
   UNINETT A/S
   Postboks 6683 Elgeseter 7002
   Trondheim, Norway
   Tel: +47 73 59 70 94
   EMail: Harald.T.Alvestrand@uninett.no

   Dan Zigmond
   Wink Communications
   1001 Marina Village Parkway
   Alameda CA 94610
   Fax: +1-510-337-2960
   Phone: +1-510-337-6359
   Email: dan.zigmond@wink.com

1	wakaba	1.1
2			INTERNET-DRAFT Larry Masinter
3			<draft-ietf-urlreg-guide-01.txt> Harald T. Alvestrand
4			December 9, 1997 Dan Zigmond
5
6			Guidelines for new URL Schemes
7
8			Status of this Memo
9
10			This document is an Internet-Draft. Internet-Drafts are working
11			documents of the Internet Engineering Task Force (IETF), its areas,
12			and its working groups. Note that other groups may also distribute
13			working documents as Internet-Drafts.
14
15			Internet-Drafts are draft documents valid for a maximum of six
16			months and may be updated, replaced, or obsoleted by other
17			documents at any time. It is inappropriate to use Internet-Drafts
18			as reference material or to cite them other than as ``work in
19			progress.''
20
21			To learn the current status of any Internet-Draft, please check the
22			``1id-abstracts.txt'' listing contained in the Internet-Drafts
23			Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net
24			(Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East
25			Coast), or ftp.isi.edu (US West Coast).
26
27			Abstract
28
29			A Uniform Resource Locator (URL) is a compact string representation
30			of the location for a resource that is available via the Internet.
31			This document provides guidelines for the definition of new URL
32			schemes.
33
34			1. Introduction
35
36			A Uniform Resource Locator (URL) is a compact string representation
37			of the location for a resource that is available via the Internet.
38			[URI-SYNTAX] defines the general syntax and semantics of URIs, and,
39			by inclusion, URLs. URLs are designated by including a "scheme"
40			and then a "scheme-specific part". Many URL schemes are already
41			defined.
42
43			This document provides guidelines for the definition of new URL
44			schemes, for consideration by those who are defining and
45			registering or evaluating those definitions.
46
47			The process by which new URL schemes are registered or defined
48			is not defined here.
49
50			2. Guildelines for new URL schemes
51
52			Because new URL schemes potentially complicate client software, new
53			schemes must have demonstrable utility and operability, as well as
54			compatibility with existing URL schemes. This section elaborates
55			these criteria.
56
57			2.1 Syntactic compatibility
58
59			New URL schemes should follow the same syntactic conventions of
60			existing schemes when appropriate.
61
62			2.1.1 Use of initial "//" for top level
63
64			Many proposed new URL schemes seem to use "://" as a kind of
65			indicator that what follows is a URL. However, the use of "//"
66			indicates a "top level" for schemes that support relative
67			URLs, and is not necessary (and just confusing) for schemes
68			that have no relative forms. URL schemes without relative
69			forms (such as mailto, cid, mid) do not use an initial "//".
70
71			2.1.2 Compatibility with relative URLs
72
73			URL schemes should use the generic-URL syntax if they are intended
74			to be used with relative URLs. A description of the allowed
75			relative forms should be included in the scheme's definition.
76			Many applications use relative URLs extensively.
77
78			o Can it be parsed according to [URI-SYNTAX] - that is, if the
79			tokens "//", "/", ";", "?" and "#" are used, do they have the
80			meaning given in RFC URI-SYNTAX?
81
82			o Does it make sense to use it in relative URLs like those
83			[URI-SYNTAX] specifies?
84
85			o If something is designed to be broken into pieces, does it
86			document what those pieces are, why it should be broken in this
87			way, and why the breaks aren't where URI-SYNTAX says that they
88			usually should be?
89
90			o If it has a hierarchy, does it go left-to-right and with slash
91			separators like [URN-SYNTAX]? If not, why not?
92
93			2.2 Is the scheme well defined?
94
95			It is important that the semantics of the "resource" that a URL
96			"locates" be well defined. This might mean different things
97			depending on the nature of the URL scheme.
98
99			2.2.1 Clear mapping from other name spaces
100
101			In many cases, new URL schemes are defined as ways to translate
102			other protocols and name spaces into the general framework of
103			URLs. The "ftp" URL scheme translates from the FTP protocol, while
104			the "mid" URL scheme translates from the Message-ID field of
105			messages.
106
107			In either case, the description of the mapping must be complete,
108			must describe how character sets get encoded or not in URLs, must
109			describe exactly how all legal values of the base standard can be
110			represented using the URL scheme, and exactly which modifiers,
111			alternate forms and other artifacts from the base standards are
112			included or not included. These requirements are elaborated
113			below.
114
115			2.2.2 URL schemes associated with network protocols
116
117			Most new URL schemes are associated with network resources that
118			have one or several network protocols that can access them. The
119			'ftp', 'news', and 'http' schemes are of this nature. For such
120			schemes, the specification should completely describe how URLs are
121			translated into protocol actions in sufficient detail to make the
122			access of the network resource unambiguous. If an implementation
123			of of the URL scheme requires some configuration, the configuration
124			elements must be clearly identified. (For example, the 'news'
125			scheme, if implemented using NTTP, requires configuration of the
126			NTTP server.)
127
128			2.2.3 Character encoding
129
130			When describing URL schemes in which (some of) the elements of
131			the URL are actually representations of sequences of characters,
132			care should be taken not to introduce unnecessary variety in the
133			ways in which characters are encoded into octets and then into
134			URL characters. Unless there is some compelling reason for a
135			particular scheme to do otherwise, translating character sequences
136			into UTF-8 [RFC2044] and then subsequently using the %HH encoding
137			for unsafe octets is recommended.
138
139			2.2.4 Definition of non-protocol URL schemes
140
141			In some cases, URL schemes do not have particular network protocols
142			associated with them, because their use is limited to contexts
143			where the access method is understood. This is the case, for
144			example, with the "cid" and "mid" URL schemes. For these URL
145			schemes, the specification should describe the notation of the
146			scheme and a complete mapping of the locator from its source.
147
148			2.2.5 Definition of URL schemes not associated with data resources
149
150			Most URL schemes locate Internet resources that correspond
151			to data objects that can be retrieved or modified. This is the
152			case with "ftp" and "http", for example. However, some URL schemes
153			do not; for example, the "mailto" URL scheme corresponds to an
154			Internet mail address.
155
156			If a new URL scheme does not locate resources that are data
157			objects, the properties of names in the new space must be clearly
158			defined.
159
160			2.2.6 Definition of operations
161
162			In some contexts (for example, HTML forms) it is possible to
163			specify any one of a list of operations to be performed on a
164			specifc URL. (Outside forms, it is generally assumed to be
165			something you GET.)
166
167			The URL scheme definition should describe all well-defined
168			operations on the URL identifier, and what they are supposed to
169			do.
170
171			Some URL schemes (for example, "telnet") provide location
172			information for hooking onto bidirectional data streams, and don't
173			fit the "infoaccess" paradigm of most URLs very well; this should
174			be documented.
175
176			NOTE: It is perfectly valid to say that "no operation apart from
177			GET is defined for this URL". It is also valid to say that "there's
178			only one operation defined for this URL, and it's not very
179			GET-like". The important point is that what is defined on this type
180			is described.
181
182			2.3 Demonstrated utility
183
184			URL schemes should have demonstrated utility. New URL schemes are
185			expensive things to support. Often they require special code in
186			browsers, proxies, and/or servers. Having a lot of ways to say the
187			same thing needless complicates these programs without adding value
188			to the Internet.
189
190			The kinds of things that are useful include:
191
192			o Things that cannot be referred to in any other way.
193
194			o Things where it is much easier to get at them using this
195			scheme than (for instance) a proxy gateway.
196
197
198			2.3.1 Proxy into HTTP/HTML
199
200			One way to provide a demonstration of utility is via a gateway
201			which provides objects in the new scheme for clients using an
202			existing protocol. It is much easier to deploy gateways to a new
203			service than it is to deploy browsers that understand the new URL
204			object.
205
206			Things to look for when thinking about a proxy are:
207
208			o Is there a single global resolution mechanism whereby any proxy can
209			find the referenced object?
210			o If not, is there a way in which the user can find any object of
211			this type, and "run his own proxy"?
212			o Are the operations mappable one-to-one (or possibly using
213			modifiers) to HTTP operations?
214			o Is the type of returned objects well defined?
215			* as MIME content-types?
216			* as something that can be translated to HTML?
217			o Is there running code for a proxy?
218
219			2.4 Are there security considerations?
220
221			Above and beyond the security considerations of the base mechanism
222			a scheme builds upon, one must think of things that can happen in
223			the normal course of URL usage.
224
225			In particular:
226
227			o Does the user need to be warned that such a thing is happening
228			without an explicit request (GET for the source of an IMG tag,
229			for instance)? This has implications for the design of a proxy
230			gateway, of course.
231
232			o Is it possible to fake URLs of this type that point to different
233			things in a dangerous way?
234
235			o Are there mechanisms for identifying the requester that can be
236			used or need to be used with this mechanism (the From: field in a
237			mailto: URL, or the Kerberos login required for AFS access in the
238			AFS: url, for instance)?
239
240			o Does the mechanism contain passwords or other security
241			information that are passed inside the referring document in the
242			clear (as in the "ftp" URL, for instance)?
243
244			2.5 Does it start with UR?
245
246			Any scheme starting with the letters "U" and "R", in particular if
247			it attaches any of the meanings "uniform", "universal" or
248			"unifying" to the first letter, is going to cause intense debate,
249			and generate much heat (but maybe little light).
250
251			Any such proposal should either make sure that there is a large
252			consensus behind it that it will be the only scheme of its type, or
253			pick another name.
254
255			2.6 Non-considerations
256
257			Some issues that are often raised but are not relevent to new URL
258			schemes include the following.
259
260			2.6.1 Are all objects acessible?
261
262			Can all objects in the world that are validly identified by a
263			scheme be accessed by any UA implementing it?
264
265			Sometimes the answer will be yes and sometimes no; often it will
266			depend on factors (like firewalls or client configuration) not
267			directly related to the scheme itself.
268
269			3. Security considerations
270
271			New URL schemes are required to address all security considerations
272			in their definitions.
273
274			4. IANA considerations
275
276			The process by which URL schemes are defined and registered
277			is not defined in this document.
278
279			5. References
280
281			[RFC2044] F. Yergeau, "UTF-8, A Transformation Format of Unicode
282			and ISO 10646", Alis Technologies, October 1996.
283
284			[URI-SYNTAX] T. Berners-Lee, R. Fielding, L. Masinter, "Uniform
285			Resource Identifiers (URI): Generic Syntax and Semantics",
286			<draft-fielding-uri-syntax-*>.
287
288			6. Author's Addresses
289
290			Larry Masinter
291			Xerox Corporation
292			Palo Alto Research Center
293			3333 Coyote Hill Road
294			Palo Alto, CA 94304
295			Fax: +1-415-812-4333
296			EMail: masinter@parc.xerox.com
297
298			Harald T. Alvestrand
299			UNINETT A/S
300			Postboks 6683 Elgeseter 7002
301			Trondheim, Norway
302			Tel: +47 73 59 70 94
303			EMail: Harald.T.Alvestrand@uninett.no
304
305			Dan Zigmond
306			Wink Communications
307			1001 Marina Village Parkway
308			Alameda CA 94610
309			Fax: +1-510-337-2960
310			Phone: +1-510-337-6359
311			Email: dan.zigmond@wink.com
312