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
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