2004/id/draft-ietf-urlreg-guide-03.txt


INTERNET-DRAFT                                           Larry Masinter
<draft-ietf-urlreg-guide-03.txt>                   Harald T. Alvestrand
August 7, 1998                                              Dan Zigmond
                                                             Rich Petke


                  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 view the entire list of current Internet-Drafts, please check
   the "1id-abstracts.txt" listing contained in the Internet-Drafts
   Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net
   (Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au
   (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu
   (US West Coast).

   Distribution of this memo is unlimited.

   This Internet Draft expires February 7, 1999.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

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.
   RFC [URI-SYNTAX] [1] 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 is defined in
   RFC [URL-PROCESS] [2].


2. Guidelines 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 Improper use of "//" following "<scheme>:"

   Contrary to some examples set in past years, the use of double
   slashes as the first component of the <scheme-specific-part> of a
   URL is not simply an artistic indicator that what follows is a URL:
   Double slashes are used ONLY when the syntax of the URL's
   <scheme-specific-part> contains a hierarchical structure as
   described in RFC [URI-SYNTAX].  In URLs from such schemes, the use
   of double slashes indicates that what follows is the top
   hierarchical element for a naming authority.  (See section 3 of RFC
   [URI-SYNTAX] for more details.)  URL schemes which do not contain a
   conformant hierarchical structure in their <scheme-specific-part>
   should not use double slashes following the "<scheme>:" string.


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.  Specifically,

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

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

   o If the scheme syntax is designed to be broken into pieces, does
     the documentation for the scheme's syntax specify what those
     pieces are, why it should be broken in this way, and why the
     breaks aren't where RFC [URI-SYNTAX] says that they usually should
     be?
     
   o If the scheme has a hierarchy, does it go left-to-right and with
     slash separators like RFC [URI-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 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 (RFC 2044) [3] 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
   specific 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 bi-directional 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 relevant to new URL
   schemes include the following.


2.6.1 Are all objects accessible?

   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 names are registered is specified
   in RFC [URL-PROCESS].


5. References

   [1] Berners-Lee, T., Fielding, R., Masinter, L., "Uniform Resource
       Identifiers (URI): Generic Syntax", RFC [URI-SYNTAX], August 1998

   [2] Petke, R., "Registration Procedures for URL Scheme Names",
       RFC [URL-PROCESS], August 1998

   [3] Yergeau, F., "UTF-8, A Transformation Format of Unicode and ISO
       10646", RFC 2044, October 1996.


6. Authors' 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 Tveit Alvestrand
   Maxware, Pirsenteret
   N-7005 Trondheim
   NORWAY
   Voice: +47 73 54 57 00
   EMail: harald.alvestrand@maxware.no

   Dan Zigmond
   WebTV Networks, Inc.
   305 Lytton Avenue
   Palo Alto, CA 94301
   USA
   Voice: +1-650-614-6071
   EMail: djz@corp.webtv.net 

   Rich Petke
   WorldCom Advanced Networks
   5000 Britton Road
   P. O. Box 5000
   Hilliard, OH 43026-5000
   Voice: +1-614-723-4157
   Fax: +1-614-723-1333
   EMail: rpetke@compuserve.net

1
2	INTERNET-DRAFT Larry Masinter
3	<draft-ietf-urlreg-guide-03.txt> Harald T. Alvestrand
4	August 7, 1998 Dan Zigmond
5	Rich Petke
6
7
8	Guidelines for new URL Schemes
9
10
11	Status of this Memo
12
13	This document is an Internet-Draft. Internet-Drafts are working
14	documents of the Internet Engineering Task Force (IETF), its
15	areas, and its working groups. Note that other groups may also
16	distribute working documents as Internet-Drafts.
17
18	Internet-Drafts are draft documents valid for a maximum of six
19	months and may be updated, replaced, or obsoleted by other
20	documents at any time. It is inappropriate to use Internet-
21	Drafts as reference material or to cite them other than as
22	"work in progress."
23
24	To view the entire list of current Internet-Drafts, please check
25	the "1id-abstracts.txt" listing contained in the Internet-Drafts
26	Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net
27	(Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au
28	(Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu
29	(US West Coast).
30
31	Distribution of this memo is unlimited.
32
33	This Internet Draft expires February 7, 1999.
34
35	Copyright Notice
36
37	Copyright (C) The Internet Society (1998). All Rights Reserved.
38
39	Abstract
40
41	A Uniform Resource Locator (URL) is a compact string representation
42	of the location for a resource that is available via the Internet.
43	This document provides guidelines for the definition of new URL
44	schemes.
45
46
47	1. Introduction
48
49	A Uniform Resource Locator (URL) is a compact string representation
50	of the location for a resource that is available via the Internet.
51	RFC [URI-SYNTAX] [1] defines the general syntax and semantics of URIs,
52	and, by inclusion, URLs. URLs are designated by including a
53	"<scheme>:" and then a "<scheme-specific-part>". Many URL schemes
54	are already defined.
55
56	This document provides guidelines for the definition of new URL
57	schemes, for consideration by those who are defining and
58	registering or evaluating those definitions.
59
60	The process by which new URL schemes are registered is defined in
61	RFC [URL-PROCESS] [2].
62
63
64	2. Guidelines for new URL schemes
65
66	Because new URL schemes potentially complicate client software, new
67	schemes must have demonstrable utility and operability, as well as
68	compatibility with existing URL schemes. This section elaborates
69	these criteria.
70
71
72	2.1 Syntactic compatibility
73
74	New URL schemes should follow the same syntactic conventions of
75	existing schemes when appropriate.
76
77
78	2.1.1 Improper use of "//" following "<scheme>:"
79
80	Contrary to some examples set in past years, the use of double
81	slashes as the first component of the <scheme-specific-part> of a
82	URL is not simply an artistic indicator that what follows is a URL:
83	Double slashes are used ONLY when the syntax of the URL's
84	<scheme-specific-part> contains a hierarchical structure as
85	described in RFC [URI-SYNTAX]. In URLs from such schemes, the use
86	of double slashes indicates that what follows is the top
87	hierarchical element for a naming authority. (See section 3 of RFC
88	[URI-SYNTAX] for more details.) URL schemes which do not contain a
89	conformant hierarchical structure in their <scheme-specific-part>
90	should not use double slashes following the "<scheme>:" string.
91
92
93	2.1.2 Compatibility with relative URLs
94
95	URL schemes should use the generic URL syntax if they are intended
96	to be used with relative URLs. A description of the allowed
97	relative forms should be included in the scheme's definition.
98	Many applications use relative URLs extensively. Specifically,
99
100	o Can the scheme be parsed according to RFC [URI-SYNTAX] - that is,
101	if the tokens "//", "/", ";", "?" and "#" are used, do they have
102	the meaning given in RFC [URI-SYNTAX]?
103
104	o Does the scheme make sense to use it in relative URLs like those
105	RFC [URI-SYNTAX] specifies?
106
107	o If the scheme syntax is designed to be broken into pieces, does
108	the documentation for the scheme's syntax specify what those
109	pieces are, why it should be broken in this way, and why the
110	breaks aren't where RFC [URI-SYNTAX] says that they usually should
111	be?
112
113	o If the scheme has a hierarchy, does it go left-to-right and with
114	slash separators like RFC [URI-SYNTAX]? If not, why not?
115
116
117	2.2 Is the scheme well defined?
118
119	It is important that the semantics of the "resource" that a URL
120	"locates" be well defined. This might mean different things
121	depending on the nature of the URL scheme.
122
123
124	2.2.1 Clear mapping from other name spaces
125
126	In many cases, new URL schemes are defined as ways to translate
127	other protocols and name spaces into the general framework of
128	URLs. The "ftp" URL scheme translates from the FTP protocol, while
129	the "mid" URL scheme translates from the Message-ID field of
130	messages.
131
132	In either case, the description of the mapping must be complete,
133	must describe how character sets get encoded or not in URLs, must
134	describe exactly how all legal values of the base standard can be
135	represented using the URL scheme, and exactly which modifiers,
136	alternate forms and other artifacts from the base standards are
137	included or not included. These requirements are elaborated
138	below.
139
140
141	2.2.2 URL schemes associated with network protocols
142
143	Most new URL schemes are associated with network resources that
144	have one or several network protocols that can access them. The
145	'ftp', 'news', and 'http' schemes are of this nature. For such
146	schemes, the specification should completely describe how URLs are
147	translated into protocol actions in sufficient detail to make the
148	access of the network resource unambiguous. If an implementation
149	of the URL scheme requires some configuration, the configuration
150	elements must be clearly identified. (For example, the 'news'
151	scheme, if implemented using NTTP, requires configuration of the
152	NTTP server.)
153
154
155	2.2.3 Character encoding
156
157	When describing URL schemes in which (some of) the elements of
158	the URL are actually representations of sequences of characters,
159	care should be taken not to introduce unnecessary variety in the
160	ways in which characters are encoded into octets and then into
161	URL characters. Unless there is some compelling reason for a
162	particular scheme to do otherwise, translating character sequences
163	into UTF-8 (RFC 2044) [3] and then subsequently using the %HH
164	encoding for unsafe octets is recommended.
165
166
167	2.2.4 Definition of non-protocol URL schemes
168
169	In some cases, URL schemes do not have particular network protocols
170	associated with them, because their use is limited to contexts
171	where the access method is understood. This is the case, for
172	example, with the "cid" and "mid" URL schemes. For these URL
173	schemes, the specification should describe the notation of the
174	scheme and a complete mapping of the locator from its source.
175
176
177	2.2.5 Definition of URL schemes not associated with data resources
178
179	Most URL schemes locate Internet resources that correspond
180	to data objects that can be retrieved or modified. This is the
181	case with "ftp" and "http", for example. However, some URL schemes
182	do not; for example, the "mailto" URL scheme corresponds to an
183	Internet mail address.
184
185	If a new URL scheme does not locate resources that are data
186	objects, the properties of names in the new space must be clearly
187	defined.
188
189
190	2.2.6 Definition of operations
191
192	In some contexts (for example, HTML forms) it is possible to
193	specify any one of a list of operations to be performed on a
194	specific URL. (Outside forms, it is generally assumed to be
195	something you GET.)
196
197	The URL scheme definition should describe all well-defined
198	operations on the URL identifier, and what they are supposed to
199	do.
200
201	Some URL schemes (for example, "telnet") provide location
202	information for hooking onto bi-directional data streams, and don't
203	fit the "infoaccess" paradigm of most URLs very well; this should
204	be documented.
205
206	NOTE: It is perfectly valid to say that "no operation apart from
207	GET is defined for this URL". It is also valid to say that "there's
208	only one operation defined for this URL, and it's not very
209	GET-like". The important point is that what is defined on this type
210	is described.
211
212
213	2.3 Demonstrated utility
214
215	URL schemes should have demonstrated utility. New URL schemes are
216	expensive things to support. Often they require special code in
217	browsers, proxies, and/or servers. Having a lot of ways to say the
218	same thing needless complicates these programs without adding value
219	to the Internet.
220
221	The kinds of things that are useful include:
222
223	o Things that cannot be referred to in any other way.
224
225	o Things where it is much easier to get at them using this scheme
226	than (for instance) a proxy gateway.
227
228
229	2.3.1 Proxy into HTTP/HTML
230
231	One way to provide a demonstration of utility is via a gateway
232	which provides objects in the new scheme for clients using an
233	existing protocol. It is much easier to deploy gateways to a new
234	service than it is to deploy browsers that understand the new URL
235	object.
236
237	Things to look for when thinking about a proxy are:
238
239	o Is there a single global resolution mechanism whereby any proxy
240	can find the referenced object?
241	o If not, is there a way in which the user can find any object of
242	this type, and "run his own proxy"?
243	o Are the operations mappable one-to-one (or possibly using
244	modifiers) to HTTP operations?
245	o Is the type of returned objects well defined?
246	- as MIME content-types?
247	- as something that can be translated to HTML?
248	o Is there running code for a proxy?
249
250
251	2.4 Are there security considerations?
252
253	Above and beyond the security considerations of the base mechanism
254	a scheme builds upon, one must think of things that can happen in
255	the normal course of URL usage.
256
257	In particular:
258
259	o Does the user need to be warned that such a thing is happening
260	without an explicit request (GET for the source of an IMG tag,
261	for instance)? This has implications for the design of a proxy
262	gateway, of course.
263
264	o Is it possible to fake URLs of this type that point to different
265	things in a dangerous way?
266
267	o Are there mechanisms for identifying the requester that can be
268	used or need to be used with this mechanism (the From: field in a
269	mailto: URL, or the Kerberos login required for AFS access in the
270	AFS: URL, for instance)?
271
272	o Does the mechanism contain passwords or other security
273	information that are passed inside the referring document in the
274	clear (as in the "ftp" URL, for instance)?
275
276
277	2.5 Does it start with UR?
278
279	Any scheme starting with the letters "U" and "R", in particular if
280	it attaches any of the meanings "uniform", "universal" or
281	"unifying" to the first letter, is going to cause intense debate,
282	and generate much heat (but maybe little light).
283
284	Any such proposal should either make sure that there is a large
285	consensus behind it that it will be the only scheme of its type, or
286	pick another name.
287
288
289	2.6 Non-considerations
290
291	Some issues that are often raised but are not relevant to new URL
292	schemes include the following.
293
294
295	2.6.1 Are all objects accessible?
296
297	Can all objects in the world that are validly identified by a
298	scheme be accessed by any UA implementing it?
299
300	Sometimes the answer will be yes and sometimes no; often it will
301	depend on factors (like firewalls or client configuration) not
302	directly related to the scheme itself.
303
304
305	3. Security considerations
306
307	New URL schemes are required to address all security considerations
308	in their definitions.
309
310
311	4. IANA considerations
312
313	The process by which URL schemes names are registered is specified
314	in RFC [URL-PROCESS].
315
316
317	5. References
318
319	[1] Berners-Lee, T., Fielding, R., Masinter, L., "Uniform Resource
320	Identifiers (URI): Generic Syntax", RFC [URI-SYNTAX], August 1998
321
322	[2] Petke, R., "Registration Procedures for URL Scheme Names",
323	RFC [URL-PROCESS], August 1998
324
325	[3] Yergeau, F., "UTF-8, A Transformation Format of Unicode and ISO
326	10646", RFC 2044, October 1996.
327
328
329	6. Authors' Addresses
330
331	Larry Masinter
332	Xerox Corporation
333	Palo Alto Research Center
334	3333 Coyote Hill Road
335	Palo Alto, CA 94304
336	Fax: +1-415-812-4333
337	EMail: masinter@parc.xerox.com
338
339	Harald Tveit Alvestrand
340	Maxware, Pirsenteret
341	N-7005 Trondheim
342	NORWAY
343	Voice: +47 73 54 57 00
344	EMail: harald.alvestrand@maxware.no
345
346	Dan Zigmond
347	WebTV Networks, Inc.
348	305 Lytton Avenue
349	Palo Alto, CA 94301
350	USA
351	Voice: +1-650-614-6071
352	EMail: djz@corp.webtv.net
353
354	Rich Petke
355	WorldCom Advanced Networks
356	5000 Britton Road
357	P. O. Box 5000
358	Hilliard, OH 43026-5000
359	Voice: +1-614-723-4157
360	Fax: +1-614-723-1333
361	EMail: rpetke@compuserve.net
362