2004/id/draft-ietf-urn-syntax-03.txt



Internet-Draft                                                Ryan Moats
draft-ietf-urn-syntax-03.txt                                        AT&T
Expires in six months                                         March 1997


                               URN Syntax
                 Filename: draft-ietf-urn-syntax-03.txt


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

   Uniform Resource Names (URNs) are intended to serve as persistent,
   location-independent, resource identifiers. This document sets
   forward the canonical syntax for URNs.  A discussion of both existing
   legacy and new namespaces and requirements for URN presentation and
   transmission are presented.  Finally, there is a discussion of URN
   equivalence and how to determine it.

1. Introduction

   Uniform Resource Names (URNs) are intended to serve as persistent,
   location-independent, resource identifiers and are designed to make
   it easy to map other namespaces (which share the properties of URNs)
   into URN-space. Therefore, the URN syntax provides a means to encode
   character data in a form that can be sent in existing protocols,
   transcribed on most keyboards, etc.


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2. Syntax

   All URNs have the following syntax (phrases enclosed in quotes are
   REQUIRED):

                     <URN> ::= "urn:" <NID> ":" <NSS>

   where <NID> is the Namespace Identifier, and <NSS> is the Namespace
   Specific String.  The leading "urn:" sequence is case-insensitive.
   The Namespace ID determines the _syntactic_ interpretation of the
   Namespace Specific String (as discussed in [1]).

   RFC 1630 [2] and RFC 1737 [3] each presents additional considerations
   for URN encoding, which have implications as far as limiting syntax.
   On the other hand, the requirement to support existing legacy naming
   systems has the effect of broadening syntax.  Thus, we discuss the
   acceptable syntax for both the Namespace Identifier and the Namespace
   Specific String separately.

2.1 Namespace Identifier Syntax

   The following is the syntax for the Namespace Identifier. To (a) be
   consistent with all potential resolution schemes and (b) not put any
   undue constraints on any potential resolution scheme, the syntax for
   the Namespace Identifier is:

   <NID>         ::= <let-num> [ 1,31<let-num-hyp> ]

   <let-num-hyp> ::= <upper> | <lower> | <number> | "-"

   <let-num>     ::= <upper> | <lower> | <number>

   <upper>       ::= "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"

   <lower>       ::= "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"

   <number>      ::= "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |
                     "8" | "9"


   This is slightly more restrictive that what is stated in [4] (which


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   allows the characters "." and "+").  Further, the Namespace
   Identifier is case insensitive, so that "ISBN" and "isbn" refer to
   the same namespace.

   To avoid confusion with the "urn:" identifier, the NID "urn" is
   reserved and MUST NOT be used.

2.2 Namespace Specific String Syntax

   As required by RFC 1737, there is a single canonical representation
   of the NSS portion of an URN.   The format of this single canonical
   form follows:

   <NSS>         ::= 1*<URN chars>

   <URN chars>   ::= <trans> | "%" <hex> <hex>

   <trans>       ::= <upper> | <lower> | <number> | <other> | <reserved>

   <hex>         ::= <number> | "A" | "B" | "C" | "D" | "E" | "F" |
                     "a" | "b" | "c" | "d" | "e" | "f"

   <other>       ::= "(" | ")" | "+" | "," | "-" | "." |
                     ":" | "=" | "@" | ";" | "$" |
                     "_" | "!" | "*" | "'"

   Depending on the rules governing a namespace, valid identifiers in a
   namespace might contain characters that are not members of the URN
   character set above (<URN chars>).  Such strings MUST be translated
   into canonical NSS format before using them as protocol elements or
   otherwise passing them on to other applications. Translation is done
   by encoding each character outside the URN character set as a
   sequence of one to six octets using UTF-8 encoding, and the encoding
   of each of those octets as "%" followed by two characters from the
   <hex> character set above. The two characters give the hexadecimal
   representation of that octet.

2.3 Reserved characters

   The remaining character set left to be discussed above is the
   reserved character set, which contains various characters reserved
   from normal use.  The reserved character set follows, with a
   discussion on the specifics of why each character is reserved.

   The reserved character set is:

   <reserved>    ::= '%" | "/" | "?" | "#"


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2.3.1 The "%" character

   The "%" character is reserved in the URN syntax for introducing the
   escape sequence for an octet.  Literal use of the "%" character in a
   namespace must be encoded using "%25" in URNs for that namespace.
   The presence of an "%" character in an URN MUST be followed by two
   characters from the <hex> character set.

   Namespaces MAY designate one or more characters from the URN
   character set as having special meaning for that namespace.  If the
   namespace also uses that character in a literal sense as well, the
   character used in a literal sense MUST be encoded with "%" followed
   by the hexadecimal representation of that octet.  Further, a
   character MUST NOT be "%"-encoded if the character is not a reserved
   character.  Therefore, the process of registering a namespace
   identifier shall include publication of a definition of which
   characters have a special meaning to that namespace.

2.3.2 The other reserved characters

   RFC 1630 [2] reserves the characters "/", "?", and "#" for particular
   purposes. The URN-WG has not yet debated the applicability and
   precise semantics of those purposes as applied to URNs. Therefore,
   these characters are RESERVED for future developments.  Namespace
   developers SHOULD NOT use these characters in unencoded form, but
   rather use the appropriate %-encoding for each character.

2.4 Excluded characters

   The following list is included only for the sake of completeness.
   Any octets/characters on this list are explicitly NOT part of the URN
   character set, and if used in an URN, MUST be %encoded:

   <excluded> ::= octets 1-32 (1-20 hex) | "\" | """ | "&" | "<"
                  | ">" | "[" | "]" | "^" | "`" | "{" | "|" | "}" | "~"
                  | octets 127-255 (7F-FF hex)

   In addition, octet 0 (0 hex) should NEVER be used, in either
   unencoded or %-encoded form.

   An URN ends when an octet/character from the excluded character set
   (<excluded>) is encountered.  The character from the excluded
   character set is NOT part of the URN.

3. Support of existing legacy naming systems and new naming systems

   Any namespace (existing or newly-devised) that is proposed as an
   URN-namespace and fulfills the criteria of URN-namespaces MUST be


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   expressed in this syntax.  If names in these namespaces contain
   characters other than those defined for the URN character set, they
   MUST be translated into canonical form as discussed in section 2.2.

4. URN presentation and transport

   The URN syntax defines the canonical format for URNs and all URN
   transport and interchanges MUST take place in this format. Further,
   all URN-aware applications MUST offer the option of displaying URNs
   in this canonical form to allow for direct transcription (for example
   by cut and paste techniques).  Such applications MAY support display
   of URNs in a more human-friendly form and may use a character set
   that includes characters that aren't permitted in URN syntax as
   defined in this RFC (that is, they may replace %-notation by
   characters in some extended character set in display to humans).

5. Lexical Equivalence in URNs

   For various purposes such as caching, it's often desirable to
   determine if two URNs are the same without resolving them. The
   general purpose means of doing so is by testing for "lexical
   equivalence" as defined below.

   Two URNs are lexically equivalent if they are octet-by-octet equal
   after the following preprocessing:

           1. normalize the case of the leading "urn:" token
           2. normalize the case of the NID
           3. normalizing the case of any %-escaping

   Note that %-escaping MUST NOT be removed.

   Some namespaces may define additional lexical equivalences, such as
   case-insensitivity of the NSS (or parts thereof).  Additional lexical
   equivalences MUST be documented as part of namespace registration,
   MUST always have the effect of eliminating some of the false
   negatives obtained by the procedure above, and MUST NEVER say that
   two URNs are not equivalent if the procedure above says they are
   equivalent.

6. Examples of lexical equivalence

   The following URN comparisons highlight the lexical equivalence
   definitions:

           1- URN:foo:a123,456
           2- urn:foo:a123,456
           3- urn:FOO:a123,456


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           4- urn:foo:A123,456
           5- urn:foo:a123%2C456
           6- URN:FOO:a123%2c456
   URNs 1, 2, and 3 are all lexically equivalent.  URN 4 is not
   lexically equivalent any of the other URNs of the above set.  URNs 5
   and 6 are only lexically equivalent to each other.

7. Functional Equivalence in URNs

   Functional equivalence is determined by practice within a given
   namespace and managed by resolvers for that namespeace. Thus, it is
   beyond the scope of this document.  Namespace registration must
   include guidance on how to determine functional equivalence for that
   namespace, i.e. when two URNs are the identical within a namespace.

8. Security considerations

   This document specifies the syntax for URNs.  While some namespaces
   resolvers may assign special meaning to certain of the characters of
   the Namespace Specific String, any security consideration resulting
   from such assignment are outside the scope of this document.  It is
   strongly recommended that the process of registering a namespace
   identifier include any such considerations.

9. Acknowledgments

   Thanks to various members of the URN working group and <<your name
   here!!>> for comments on earlier drafts of this document.  This
   document is partially supported by the National Science Foundation,
   Cooperative Agreement NCR-9218179.

10. References

   Request For Comments (RFC) and Internet Draft documents are available
   from <URL:ftp://ftp.internic.net> and numerous mirror sites.

         [1]         K. R. Sollins, "Requirements and a Framework for
                     URN Resolution Systems," Internet Draft (work in
                     progress),  November 1996.


         [2]
          T. Berners-Lee, "Universal Resource Identifiers in WWW," RFC
          1630, June 1994.


         [3]         K. Sollins and L. Masinter,  "Functional Require-
                     ments for Uniform Resource Names," RFC 1737.


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                     December 1994.


         [4]         T. Berners-Lee, R. Fielding, L. Masinter, "Uniform
                     Resource Locators (URL)," Internet Draft (work in
                     progress),  December 1996.

11. Editor's address

   Ryan Moats
   AT&T
   15621 Drexel Circle
   Omaha, NE 68135-2358
   USA

   Phone:  +1 402 894-9456
   EMail:  jayhawk@ds.internic.net


Appendix A. Handling of URNs by URL resolvers/browsers.

   The URN syntax has been defined so that URNs can be used in places
   where URLs are expected.  A resolver that conforms to the current URL
   syntax specification [3] will extract a scheme value of "urn:"
   rather than a scheme value of "urn:<nid>".

   An URN MUST be considered an opaque URL by URL resolvers and passed
   (with the "urn:" tag) to an URN resolver for resolution.  The URN
   resolver can either be an external resolver that the URL resolver
   knows of, or it can be functionality built-in to the URL resolver.

   To avoid confusion of users, an URL browser SHOULD display the com-
   plete URN (including the "urn:" tag) to ensure that there is no con-
   fusion between URN namespace identifiers and URL scheme identifiers.


              This Internet Draft expires September 30, 1997.


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1
2
3
4
5
6	Internet-Draft Ryan Moats
7	draft-ietf-urn-syntax-03.txt AT&T
8	Expires in six months March 1997
9
10
11	URN Syntax
12	Filename: draft-ietf-urn-syntax-03.txt
13
14
15	Status of This Memo
16
17	This document is an Internet-Draft. Internet-Drafts are working
18	documents of the Internet Engineering Task Force (IETF), its
19	areas, and its working groups. Note that other groups may also
20	distribute working documents as Internet-Drafts.
21
22	Internet-Drafts are draft documents valid for a maximum of six
23	months and may be updated, replaced, or obsoleted by other
24	documents at any time. It is inappropriate to use Internet-
25	Drafts as reference material or to cite them other than as ``work
26	in progress.''
27
28	To learn the current status of any Internet-Draft, please check
29	the ``1id-abstracts.txt'' listing contained in the Internet-
30	Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net
31	(Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East
32	Coast), or ftp.isi.edu (US West Coast).
33
34
35	Abstract
36
37	Uniform Resource Names (URNs) are intended to serve as persistent,
38	location-independent, resource identifiers. This document sets
39	forward the canonical syntax for URNs. A discussion of both existing
40	legacy and new namespaces and requirements for URN presentation and
41	transmission are presented. Finally, there is a discussion of URN
42	equivalence and how to determine it.
43
44	1. Introduction
45
46	Uniform Resource Names (URNs) are intended to serve as persistent,
47	location-independent, resource identifiers and are designed to make
48	it easy to map other namespaces (which share the properties of URNs)
49	into URN-space. Therefore, the URN syntax provides a means to encode
50	character data in a form that can be sent in existing protocols,
51	transcribed on most keyboards, etc.
52
53
54
55
56
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58
59
60
61
62
63	INTERNET DRAFT URN Syntax March 1997
64
65
66	2. Syntax
67
68	All URNs have the following syntax (phrases enclosed in quotes are
69	REQUIRED):
70
71	<URN> ::= "urn:" <NID> ":" <NSS>
72
73	where <NID> is the Namespace Identifier, and <NSS> is the Namespace
74	Specific String. The leading "urn:" sequence is case-insensitive.
75	The Namespace ID determines the _syntactic_ interpretation of the
76	Namespace Specific String (as discussed in [1]).
77
78	RFC 1630 [2] and RFC 1737 [3] each presents additional considerations
79	for URN encoding, which have implications as far as limiting syntax.
80	On the other hand, the requirement to support existing legacy naming
81	systems has the effect of broadening syntax. Thus, we discuss the
82	acceptable syntax for both the Namespace Identifier and the Namespace
83	Specific String separately.
84
85	2.1 Namespace Identifier Syntax
86
87	The following is the syntax for the Namespace Identifier. To (a) be
88	consistent with all potential resolution schemes and (b) not put any
89	undue constraints on any potential resolution scheme, the syntax for
90	the Namespace Identifier is:
91
92	<NID> ::= <let-num> [ 1,31<let-num-hyp> ]
93
94	<let-num-hyp> ::= <upper> \| <lower> \| <number> \| "-"
95
96	<let-num> ::= <upper> \| <lower> \| <number>
97
98	<upper> ::= "A" \| "B" \| "C" \| "D" \| "E" \| "F" \| "G" \| "H" \|
99	"I" \| "J" \| "K" \| "L" \| "M" \| "N" \| "O" \| "P" \|
100	"Q" \| "R" \| "S" \| "T" \| "U" \| "V" \| "W" \| "X" \|
101	"Y" \| "Z"
102
103	<lower> ::= "a" \| "b" \| "c" \| "d" \| "e" \| "f" \| "g" \| "h" \|
104	"i" \| "j" \| "k" \| "l" \| "m" \| "n" \| "o" \| "p" \|
105	"q" \| "r" \| "s" \| "t" \| "u" \| "v" \| "w" \| "x" \|
106	"y" \| "z"
107
108	<number> ::= "0" \| "1" \| "2" \| "3" \| "4" \| "5" \| "6" \| "7" \|
109	"8" \| "9"
110
111
112
113	This is slightly more restrictive that what is stated in [4] (which
114
115
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119
120
121
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123	INTERNET DRAFT URN Syntax March 1997
124
125
126	allows the characters "." and "+"). Further, the Namespace
127	Identifier is case insensitive, so that "ISBN" and "isbn" refer to
128	the same namespace.
129
130	To avoid confusion with the "urn:" identifier, the NID "urn" is
131	reserved and MUST NOT be used.
132
133	2.2 Namespace Specific String Syntax
134
135	As required by RFC 1737, there is a single canonical representation
136	of the NSS portion of an URN. The format of this single canonical
137	form follows:
138
139	<NSS> ::= 1*<URN chars>
140
141	<URN chars> ::= <trans> \| "%" <hex> <hex>
142
143	<trans> ::= <upper> \| <lower> \| <number> \| <other> \| <reserved>
144
145	<hex> ::= <number> \| "A" \| "B" \| "C" \| "D" \| "E" \| "F" \|
146	"a" \| "b" \| "c" \| "d" \| "e" \| "f"
147
148	<other> ::= "(" \| ")" \| "+" \| "," \| "-" \| "." \|
149	":" \| "=" \| "@" \| ";" \| "$" \|
150	"_" \| "!" \| "*" \| "'"
151
152	Depending on the rules governing a namespace, valid identifiers in a
153	namespace might contain characters that are not members of the URN
154	character set above (<URN chars>). Such strings MUST be translated
155	into canonical NSS format before using them as protocol elements or
156	otherwise passing them on to other applications. Translation is done
157	by encoding each character outside the URN character set as a
158	sequence of one to six octets using UTF-8 encoding, and the encoding
159	of each of those octets as "%" followed by two characters from the
160	<hex> character set above. The two characters give the hexadecimal
161	representation of that octet.
162
163	2.3 Reserved characters
164
165	The remaining character set left to be discussed above is the
166	reserved character set, which contains various characters reserved
167	from normal use. The reserved character set follows, with a
168	discussion on the specifics of why each character is reserved.
169
170	The reserved character set is:
171
172	<reserved> ::= '%" \| "/" \| "?" \| "#"
173
174
175
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179
180
181
182
183	INTERNET DRAFT URN Syntax March 1997
184
185
186	2.3.1 The "%" character
187
188	The "%" character is reserved in the URN syntax for introducing the
189	escape sequence for an octet. Literal use of the "%" character in a
190	namespace must be encoded using "%25" in URNs for that namespace.
191	The presence of an "%" character in an URN MUST be followed by two
192	characters from the <hex> character set.
193
194	Namespaces MAY designate one or more characters from the URN
195	character set as having special meaning for that namespace. If the
196	namespace also uses that character in a literal sense as well, the
197	character used in a literal sense MUST be encoded with "%" followed
198	by the hexadecimal representation of that octet. Further, a
199	character MUST NOT be "%"-encoded if the character is not a reserved
200	character. Therefore, the process of registering a namespace
201	identifier shall include publication of a definition of which
202	characters have a special meaning to that namespace.
203
204	2.3.2 The other reserved characters
205
206	RFC 1630 [2] reserves the characters "/", "?", and "#" for particular
207	purposes. The URN-WG has not yet debated the applicability and
208	precise semantics of those purposes as applied to URNs. Therefore,
209	these characters are RESERVED for future developments. Namespace
210	developers SHOULD NOT use these characters in unencoded form, but
211	rather use the appropriate %-encoding for each character.
212
213	2.4 Excluded characters
214
215	The following list is included only for the sake of completeness.
216	Any octets/characters on this list are explicitly NOT part of the URN
217	character set, and if used in an URN, MUST be %encoded:
218
219	<excluded> ::= octets 1-32 (1-20 hex) \| "\" \| """ \| "&" \| "<"
220	\| ">" \| "[" \| "]" \| "^" \| "`" \| "{" \| "\|" \| "}" \| "~"
221	\| octets 127-255 (7F-FF hex)
222
223	In addition, octet 0 (0 hex) should NEVER be used, in either
224	unencoded or %-encoded form.
225
226	An URN ends when an octet/character from the excluded character set
227	(<excluded>) is encountered. The character from the excluded
228	character set is NOT part of the URN.
229
230	3. Support of existing legacy naming systems and new naming systems
231
232	Any namespace (existing or newly-devised) that is proposed as an
233	URN-namespace and fulfills the criteria of URN-namespaces MUST be
234
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238
239
240
241
242
243	INTERNET DRAFT URN Syntax March 1997
244
245
246	expressed in this syntax. If names in these namespaces contain
247	characters other than those defined for the URN character set, they
248	MUST be translated into canonical form as discussed in section 2.2.
249
250	4. URN presentation and transport
251
252	The URN syntax defines the canonical format for URNs and all URN
253	transport and interchanges MUST take place in this format. Further,
254	all URN-aware applications MUST offer the option of displaying URNs
255	in this canonical form to allow for direct transcription (for example
256	by cut and paste techniques). Such applications MAY support display
257	of URNs in a more human-friendly form and may use a character set
258	that includes characters that aren't permitted in URN syntax as
259	defined in this RFC (that is, they may replace %-notation by
260	characters in some extended character set in display to humans).
261
262	5. Lexical Equivalence in URNs
263
264	For various purposes such as caching, it's often desirable to
265	determine if two URNs are the same without resolving them. The
266	general purpose means of doing so is by testing for "lexical
267	equivalence" as defined below.
268
269	Two URNs are lexically equivalent if they are octet-by-octet equal
270	after the following preprocessing:
271
272	1. normalize the case of the leading "urn:" token
273	2. normalize the case of the NID
274	3. normalizing the case of any %-escaping
275
276	Note that %-escaping MUST NOT be removed.
277
278	Some namespaces may define additional lexical equivalences, such as
279	case-insensitivity of the NSS (or parts thereof). Additional lexical
280	equivalences MUST be documented as part of namespace registration,
281	MUST always have the effect of eliminating some of the false
282	negatives obtained by the procedure above, and MUST NEVER say that
283	two URNs are not equivalent if the procedure above says they are
284	equivalent.
285
286	6. Examples of lexical equivalence
287
288	The following URN comparisons highlight the lexical equivalence
289	definitions:
290
291	1- URN:foo:a123,456
292	2- urn:foo:a123,456
293	3- urn:FOO:a123,456
294
295
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303	INTERNET DRAFT URN Syntax March 1997
304
305
306	4- urn:foo:A123,456
307	5- urn:foo:a123%2C456
308	6- URN:FOO:a123%2c456
309	URNs 1, 2, and 3 are all lexically equivalent. URN 4 is not
310	lexically equivalent any of the other URNs of the above set. URNs 5
311	and 6 are only lexically equivalent to each other.
312
313	7. Functional Equivalence in URNs
314
315	Functional equivalence is determined by practice within a given
316	namespace and managed by resolvers for that namespeace. Thus, it is
317	beyond the scope of this document. Namespace registration must
318	include guidance on how to determine functional equivalence for that
319	namespace, i.e. when two URNs are the identical within a namespace.
320
321	8. Security considerations
322
323	This document specifies the syntax for URNs. While some namespaces
324	resolvers may assign special meaning to certain of the characters of
325	the Namespace Specific String, any security consideration resulting
326	from such assignment are outside the scope of this document. It is
327	strongly recommended that the process of registering a namespace
328	identifier include any such considerations.
329
330	9. Acknowledgments
331
332	Thanks to various members of the URN working group and <<your name
333	here!!>> for comments on earlier drafts of this document. This
334	document is partially supported by the National Science Foundation,
335	Cooperative Agreement NCR-9218179.
336
337	10. References
338
339	Request For Comments (RFC) and Internet Draft documents are available
340	from <URL:ftp://ftp.internic.net> and numerous mirror sites.
341
342	[1] K. R. Sollins, "Requirements and a Framework for
343	URN Resolution Systems," Internet Draft (work in
344	progress), November 1996.
345
346
347	[2]
348	T. Berners-Lee, "Universal Resource Identifiers in WWW," RFC
349	1630, June 1994.
350
351
352	[3] K. Sollins and L. Masinter, "Functional Require-
353	ments for Uniform Resource Names," RFC 1737.
354
355
356
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358
359
360
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362
363	INTERNET DRAFT URN Syntax March 1997
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365
366	December 1994.
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369	[4] T. Berners-Lee, R. Fielding, L. Masinter, "Uniform
370	Resource Locators (URL)," Internet Draft (work in
371	progress), December 1996.
372
373	11. Editor's address
374
375	Ryan Moats
376	AT&T
377	15621 Drexel Circle
378	Omaha, NE 68135-2358
379	USA
380
381	Phone: +1 402 894-9456
382	EMail: jayhawk@ds.internic.net
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387
388	Appendix A. Handling of URNs by URL resolvers/browsers.
389
390	The URN syntax has been defined so that URNs can be used in places
391	where URLs are expected. A resolver that conforms to the current URL
392	syntax specification [3] will extract a scheme value of "urn:"
393	rather than a scheme value of "urn:<nid>".
394
395	An URN MUST be considered an opaque URL by URL resolvers and passed
396	(with the "urn:" tag) to an URN resolver for resolution. The URN
397	resolver can either be an external resolver that the URL resolver
398	knows of, or it can be functionality built-in to the URL resolver.
399
400	To avoid confusion of users, an URL browser SHOULD display the com-
401	plete URN (including the "urn:" tag) to ensure that there is no con-
402	fusion between URN namespace identifiers and URL scheme identifiers.
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404
405	This Internet Draft expires September 30, 1997.
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417	Expires 9/30/97 [Page 7]
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