7
Network Working Group K. Zeilenga
8
Request for Comments: 4518 OpenLDAP Foundation
9
Category: Standards Track June 2006
12
Lightweight Directory Access Protocol (LDAP):
13
Internationalized String Preparation
17
This document specifies an Internet standards track protocol for the
18
Internet community, and requests discussion and suggestions for
19
improvements. Please refer to the current edition of the "Internet
20
Official Protocol Standards" (STD 1) for the standardization state
21
and status of this protocol. Distribution of this memo is unlimited.
25
Copyright (C) The Internet Society (2006).
29
The previous Lightweight Directory Access Protocol (LDAP) technical
30
specifications did not precisely define how character string matching
31
is to be performed. This led to a number of usability and
32
interoperability problems. This document defines string preparation
33
algorithms for character-based matching rules defined for use in
40
A Lightweight Directory Access Protocol (LDAP) [RFC4510] matching
41
rule [RFC4517] defines an algorithm for determining whether a
42
presented value matches an attribute value in accordance with the
43
criteria defined for the rule. The proposition may be evaluated to
44
True, False, or Undefined.
46
True - the attribute contains a matching value,
48
False - the attribute contains no matching value,
50
Undefined - it cannot be determined whether the attribute contains
58
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RFC 4518 LDAP: Internationalized String Preparation June 2006
63
For instance, the caseIgnoreMatch matching rule may be used to
64
compare whether the commonName attribute contains a particular value
65
without regard for case and insignificant spaces.
67
1.2. X.500 String Matching Rules
69
"X.520: Selected attribute types" [X.520] provides (among other
70
things) value syntaxes and matching rules for comparing values
71
commonly used in the directory [X.500]. These specifications are
72
inadequate for strings composed of Unicode [Unicode] characters.
74
The caseIgnoreMatch matching rule [X.520], for example, is simply
75
defined as being a case-insensitive comparison where insignificant
76
spaces are ignored. For printableString, there is only one space
77
character and case mapping is bijective, hence this definition is
78
sufficient. However, for Unicode string types such as
79
universalString, this is not sufficient. For example, a case-
80
insensitive matching implementation that folded lowercase characters
81
to uppercase would yield different results than an implementation
82
that used uppercase to lowercase folding. Or one implementation may
83
view space as referring to only SPACE (U+0020), a second
84
implementation may view any character with the space separator (Zs)
85
property as a space, and another implementation may view any
86
character with the whitespace (WS) category as a space.
88
The lack of precise specification for character string matching has
89
led to significant interoperability problems. When used in
90
certificate chain validation, security vulnerabilities can arise. To
91
address these problems, this document defines precise algorithms for
92
preparing character strings for matching.
94
1.3. Relationship to "stringprep"
96
The character string preparation algorithms described in this
97
document are based upon the "stringprep" approach [RFC3454]. In
98
"stringprep", presented and stored values are first prepared for
99
comparison so that a character-by-character comparison yields the
102
The approach used here is a refinement of the "stringprep" [RFC3454]
103
approach. Each algorithm involves two additional preparation steps.
105
a) Prior to applying the Unicode string preparation steps outlined in
106
"stringprep", the string is transcoded to Unicode.
108
b) After applying the Unicode string preparation steps outlined in
109
"stringprep", the string is modified to appropriately handle
110
characters insignificant to the matching rule.
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RFC 4518 LDAP: Internationalized String Preparation June 2006
119
Hence, preparation of character strings for X.500 [X.500] matching
120
[X.501] involves the following steps:
126
5) Check Bidi (Bidirectional)
127
6) Insignificant Character Handling
129
These steps are described in Section 2.
131
It is noted that while various tables of Unicode characters included
132
or referenced by this specification are derived from Unicode
133
[Unicode] data, these tables are to be considered definitive for the
134
purpose of implementing this specification.
136
1.4. Relationship to the LDAP Technical Specification
138
This document is an integral part of the LDAP technical specification
139
[RFC4510], which obsoletes the previously defined LDAP technical
140
specification [RFC3377] in its entirety.
142
This document details new LDAP internationalized character string
143
preparation algorithms used by [RFC4517] and possible other technical
144
specifications defining LDAP syntaxes and/or matching rules.
146
1.5. Relationship to X.500
148
LDAP is defined [RFC4510] in X.500 terms as an X.500 access
149
mechanism. As such, there is a strong desire for alignment between
150
LDAP and X.500 syntax and semantics. The character string
151
preparation algorithms described in this document are based upon
152
"Internationalized String Matching Rules for X.500" [XMATCH] proposal
153
to ITU/ISO Joint Study Group 2.
155
1.6. Conventions and Terms
157
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
158
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
159
document are to be interpreted as described in BCP 14 [RFC2119].
161
Character names in this document use the notation for code points and
162
names from the Unicode Standard [Unicode]. For example, the letter
163
"a" may be represented as either <U+0061> or <LATIN SMALL LETTER A>.
164
In the lists of mappings and the prohibited characters, the "U+" is
170
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RFC 4518 LDAP: Internationalized String Preparation June 2006
175
left off to make the lists easier to read. The comments for
176
character ranges are shown in square brackets (such as "[CONTROL
177
CHARACTERS]") and do not come from the standard.
179
Note: a glossary of terms used in Unicode can be found in [Glossary].
180
Information on the Unicode character encoding model can be found in
183
The term "combining mark", as used in this specification, refers to
184
any Unicode [Unicode] code point that has a mark property (Mn, Mc,
185
Me). Appendix A provides a definitive list of combining marks.
187
2. String Preparation
189
The following six-step process SHALL be applied to each presented and
190
attribute value in preparation for character string matching rule
198
6) Insignificant Character Handling
200
Failure in any step causes the assertion to evaluate to Undefined.
202
The character repertoire of this process is Unicode 3.2 [Unicode].
204
Note that this six-step process specification is intended to describe
205
expected matching behavior. Implementations are free to use
206
alternative processes so long as the matching rule evaluation
207
behavior provided is consistent with the behavior described by this
212
Each non-Unicode string value is transcoded to Unicode.
214
PrintableString [X.680] values are transcoded directly to Unicode.
216
UniversalString, UTF8String, and bmpString [X.680] values need not be
217
transcoded as they are Unicode-based strings (in the case of
218
bmpString, a subset of Unicode).
220
TeletexString [X.680] values are transcoded to Unicode. As there is
221
no standard for mapping TeletexString values to Unicode, the mapping
222
is left a local matter.
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RFC 4518 LDAP: Internationalized String Preparation June 2006
231
For these and other reasons, use of TeletexString is NOT RECOMMENDED.
233
The output is the transcoded string.
237
SOFT HYPHEN (U+00AD) and MONGOLIAN TODO SOFT HYPHEN (U+1806) code
238
points are mapped to nothing. COMBINING GRAPHEME JOINER (U+034F) and
239
VARIATION SELECTORs (U+180B-180D, FF00-FE0F) code points are also
240
mapped to nothing. The OBJECT REPLACEMENT CHARACTER (U+FFFC) is
243
CHARACTER TABULATION (U+0009), LINE FEED (LF) (U+000A), LINE
244
TABULATION (U+000B), FORM FEED (FF) (U+000C), CARRIAGE RETURN (CR)
245
(U+000D), and NEXT LINE (NEL) (U+0085) are mapped to SPACE (U+0020).
247
All other control code (e.g., Cc) points or code points with a
248
control function (e.g., Cf) are mapped to nothing. The following is
249
a complete list of these code points: U+0000-0008, 000E-001F, 007F-
250
0084, 0086-009F, 06DD, 070F, 180E, 200C-200F, 202A-202E, 2060-2063,
251
206A-206F, FEFF, FFF9-FFFB, 1D173-1D17A, E0001, E0020-E007F.
253
ZERO WIDTH SPACE (U+200B) is mapped to nothing. All other code
254
points with Separator (space, line, or paragraph) property (e.g., Zs,
255
Zl, or Zp) are mapped to SPACE (U+0020). The following is a complete
256
list of these code points: U+0020, 00A0, 1680, 2000-200A, 2028-2029,
259
For case ignore, numeric, and stored prefix string matching rules,
260
characters are case folded per B.2 of [RFC3454].
262
The output is the mapped string.
266
The input string is to be normalized to Unicode Form KC
267
(compatibility composed) as described in [UAX15]. The output is the
272
All Unassigned code points are prohibited. Unassigned code points
273
are listed in Table A.1 of [RFC3454].
275
Characters that, per Section 5.8 of [RFC3454], change display
276
properties or are deprecated are prohibited. These characters are
277
listed in Table C.8 of [RFC3454].
282
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RFC 4518 LDAP: Internationalized String Preparation June 2006
287
Private Use code points are prohibited. These characters are listed
288
in Table C.3 of [RFC3454].
290
All non-character code points are prohibited. These code points are
291
listed in Table C.4 of [RFC3454].
293
Surrogate codes are prohibited. These characters are listed in Table
296
The REPLACEMENT CHARACTER (U+FFFD) code point is prohibited.
298
The step fails if the input string contains any prohibited code
299
point. Otherwise, the output is the input string.
303
Bidirectional characters are ignored.
305
2.6. Insignificant Character Handling
307
In this step, the string is modified to ensure proper handling of
308
characters insignificant to the matching rule. This modification
309
differs from matching rule to matching rule.
311
Section 2.6.1 applies to case ignore and exact string matching.
312
Section 2.6.2 applies to numericString matching.
313
Section 2.6.3 applies to telephoneNumber matching.
315
2.6.1. Insignificant Space Handling
317
For the purposes of this section, a space is defined to be the SPACE
318
(U+0020) code point followed by no combining marks.
320
NOTE - The previous steps ensure that the string cannot contain
321
any code points in the separator class, other than SPACE
324
For input strings that are attribute values or non-substring
325
assertion values: If the input string contains no non-space
326
character, then the output is exactly two SPACEs. Otherwise (the
327
input string contains at least one non-space character), the string
328
is modified such that the string starts with exactly one space
329
character, ends with exactly one SPACE character, and any inner
330
(non-empty) sequence of space characters is replaced with exactly two
331
SPACE characters. For instance, the input strings
332
"foo<SPACE>bar<SPACE><SPACE>", result in the output
333
"<SPACE>foo<SPACE><SPACE>bar<SPACE>".
338
Zeilenga Standards Track [Page 6]
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RFC 4518 LDAP: Internationalized String Preparation June 2006
343
For input strings that are substring assertion values: If the string
344
being prepared contains no non-space characters, then the output
345
string is exactly one SPACE. Otherwise, the following steps are
348
- If the input string is an initial substring, it is modified to
349
start with exactly one SPACE character;
351
- If the input string is an initial or an any substring that ends in
352
one or more space characters, it is modified to end with exactly
355
- If the input string is an any or a final substring that starts in
356
one or more space characters, it is modified to start with exactly
357
one SPACE character; and
359
- If the input string is a final substring, it is modified to end
360
with exactly one SPACE character.
362
For instance, for the input string "foo<SPACE>bar<SPACE><SPACE>" as
363
an initial substring, the output would be
364
"<SPACE>foo<SPACE><SPACE>bar<SPACE>". As an any or final substring,
365
the same input would result in "foo<SPACE>bar<SPACE>".
367
Appendix B discusses the rationale for the behavior.
369
2.6.2. numericString Insignificant Character Handling
371
For the purposes of this section, a space is defined to be the SPACE
372
(U+0020) code point followed by no combining marks.
374
All spaces are regarded as insignificant and are to be removed.
376
For example, removal of spaces from the Form KC string:
377
"<SPACE><SPACE>123<SPACE><SPACE>456<SPACE><SPACE>"
378
would result in the output string:
380
and the Form KC string:
381
"<SPACE><SPACE><SPACE>"
382
would result in the output string:
383
"" (an empty string).
385
2.6.3. telephoneNumber Insignificant Character Handling
387
For the purposes of this section, a hyphen is defined to be a
388
HYPHEN-MINUS (U+002D), ARMENIAN HYPHEN (U+058A), HYPHEN (U+2010),
389
NON-BREAKING HYPHEN (U+2011), MINUS SIGN (U+2212), SMALL HYPHEN-MINUS
390
(U+FE63), or FULLWIDTH HYPHEN-MINUS (U+FF0D) code point followed by
394
Zeilenga Standards Track [Page 7]
396
RFC 4518 LDAP: Internationalized String Preparation June 2006
399
no combining marks and a space is defined to be the SPACE (U+0020)
400
code point followed by no combining marks.
402
All hyphens and spaces are considered insignificant and are to be
405
For example, removal of hyphens and spaces from the Form KC string:
406
"<SPACE><HYPHEN>123<SPACE><SPACE>456<SPACE><HYPHEN>"
407
would result in the output string:
409
and the Form KC string:
410
"<HYPHEN><HYPHEN><HYPHEN>"
411
would result in the (empty) output string:
414
3. Security Considerations
416
"Preparation of Internationalized Strings ("stringprep")" [RFC3454]
417
security considerations generally apply to the algorithms described
422
The approach used in this document is based upon design principles
423
and algorithms described in "Preparation of Internationalized Strings
424
('stringprep')" [RFC3454] by Paul Hoffman and Marc Blanchet. Some
425
additional guidance was drawn from Unicode Technical Standards,
426
Technical Reports, and Notes.
428
This document is a product of the IETF LDAP Revision (LDAPBIS)
433
5.1. Normative References
435
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
436
Requirement Levels", BCP 14, RFC 2119, March 1997.
438
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
439
Internationalized Strings ("stringprep")", RFC 3454,
442
[RFC4510] Zeilenga, K., "Lightweight Directory Access Protocol
443
(LDAP): Technical Specification Road Map", RFC 4510,
450
Zeilenga Standards Track [Page 8]
452
RFC 4518 LDAP: Internationalized String Preparation June 2006
455
[RFC4517] Legg, S., Ed., "Lightweight Directory Access Protocol
456
(LDAP): Syntaxes and Matching Rules", RFC 4517, June
459
[Unicode] The Unicode Consortium, "The Unicode Standard, Version
460
3.2.0" is defined by "The Unicode Standard, Version
461
3.0" (Reading, MA, Addison-Wesley, 2000. ISBN 0-201-
462
61633-5), as amended by the "Unicode Standard Annex
464
(http://www.unicode.org/reports/tr27/) and by the
465
"Unicode Standard Annex #28: Unicode 3.2"
466
(http://www.unicode.org/reports/tr28/).
468
[UAX15] Davis, M. and M. Duerst, "Unicode Standard Annex #15:
469
Unicode Normalization Forms, Version 3.2.0".
470
<http://www.unicode.org/unicode/reports/tr15/tr15-
471
22.html>, March 2002.
473
[X.680] International Telecommunication Union -
474
Telecommunication Standardization Sector, "Abstract
475
Syntax Notation One (ASN.1) - Specification of Basic
476
Notation", X.680(2002) (also ISO/IEC 8824-1:2002).
478
5.2. Informative References
480
[X.500] International Telecommunication Union -
481
Telecommunication Standardization Sector, "The
482
Directory -- Overview of concepts, models and
483
services," X.500(1993) (also ISO/IEC 9594-1:1994).
485
[X.501] International Telecommunication Union -
486
Telecommunication Standardization Sector, "The
487
Directory -- Models," X.501(1993) (also ISO/IEC 9594-
490
[X.520] International Telecommunication Union -
491
Telecommunication Standardization Sector, "The
492
Directory: Selected Attribute Types", X.520(1993) (also
493
ISO/IEC 9594-6:1994).
495
[Glossary] The Unicode Consortium, "Unicode Glossary",
496
<http://www.unicode.org/glossary/>.
498
[CharModel] Whistler, K. and M. Davis, "Unicode Technical Report
499
#17, Character Encoding Model", UTR17,
500
<http://www.unicode.org/unicode/reports/tr17/>, August
506
Zeilenga Standards Track [Page 9]
508
RFC 4518 LDAP: Internationalized String Preparation June 2006
511
[RFC3377] Hodges, J. and R. Morgan, "Lightweight Directory Access
512
Protocol (v3): Technical Specification", RFC 3377,
515
[RFC4515] Smith, M., Ed. and T. Howes, "Lightweight Directory
516
Access Protocol (LDAP): String Representation of Search
517
Filters", RFC 4515, June 2006.
519
[XMATCH] Zeilenga, K., "Internationalized String Matching Rules
520
for X.500", Work in Progress.
562
Zeilenga Standards Track [Page 10]
564
RFC 4518 LDAP: Internationalized String Preparation June 2006
567
Appendix A. Combining Marks
569
This appendix is normative.
571
This table was derived from Unicode [Unicode] data files; it lists
572
all code points with the Mn, Mc, or Me properties. This table is to
573
be considered definitive for the purposes of implementation of this
576
0300-034F 0360-036F 0483-0486 0488-0489 0591-05A1
577
05A3-05B9 05BB-05BC 05BF 05C1-05C2 05C4 064B-0655 0670
578
06D6-06DC 06DE-06E4 06E7-06E8 06EA-06ED 0711 0730-074A
579
07A6-07B0 0901-0903 093C 093E-094F 0951-0954 0962-0963
580
0981-0983 09BC 09BE-09C4 09C7-09C8 09CB-09CD 09D7
581
09E2-09E3 0A02 0A3C 0A3E-0A42 0A47-0A48 0A4B-0A4D
582
0A70-0A71 0A81-0A83 0ABC 0ABE-0AC5 0AC7-0AC9 0ACB-0ACD
583
0B01-0B03 0B3C 0B3E-0B43 0B47-0B48 0B4B-0B4D 0B56-0B57
584
0B82 0BBE-0BC2 0BC6-0BC8 0BCA-0BCD 0BD7 0C01-0C03
585
0C3E-0C44 0C46-0C48 0C4A-0C4D 0C55-0C56 0C82-0C83
586
0CBE-0CC4 0CC6-0CC8 0CCA-0CCD 0CD5-0CD6 0D02-0D03
587
0D3E-0D43 0D46-0D48 0D4A-0D4D 0D57 0D82-0D83 0DCA
588
0DCF-0DD4 0DD6 0DD8-0DDF 0DF2-0DF3 0E31 0E34-0E3A
589
0E47-0E4E 0EB1 0EB4-0EB9 0EBB-0EBC 0EC8-0ECD 0F18-0F19
590
0F35 0F37 0F39 0F3E-0F3F 0F71-0F84 0F86-0F87 0F90-0F97
591
0F99-0FBC 0FC6 102C-1032 1036-1039 1056-1059 1712-1714
592
1732-1734 1752-1753 1772-1773 17B4-17D3 180B-180D 18A9
593
20D0-20EA 302A-302F 3099-309A FB1E FE00-FE0F FE20-FE23
594
1D165-1D169 1D16D-1D172 1D17B-1D182 1D185-1D18B
597
Appendix B. Substrings Matching
599
This appendix is non-normative.
601
In the absence of substrings matching, the insignificant space
602
handling for case ignore/exact matching could be simplified.
603
Specifically, the handling could be to require that all sequences of
604
one or more spaces be replaced with one space and, if the string
605
contains non-space characters, removal of all leading spaces and
608
In the presence of substrings matching, this simplified space
609
handling would lead to unexpected and undesirable matching behavior.
612
1) (CN=foo\20*\20bar) would match the CN value "foobar";
618
Zeilenga Standards Track [Page 11]
620
RFC 4518 LDAP: Internationalized String Preparation June 2006
623
2) (CN=*\20foobar\20*) would match "foobar", but
624
(CN=*\20*foobar*\20*) would not.
626
Note to readers not familiar with LDAP substrings matching: the LDAP
627
filter [RFC4515] assertion (CN=A*B*C) says to "match any value (of
628
the attribute CN) that begins with A, contains B after A, ends with C
629
where C is also after B."
631
The first case illustrates that this simplified space handling would
632
cause leading and trailing spaces in substrings of the string to be
633
regarded as insignificant. However, only leading and trailing (as
634
well as multiple consecutive spaces) of the string (as a whole) are
637
The second case illustrates that this simplified space handling would
638
cause sub-partitioning failures. That is, if a prepared any
639
substring matches a partition of the attribute value, then an
640
assertion constructed by subdividing that substring into multiple
641
substrings should also match.
643
In designing an appropriate approach for space handling for
644
substrings matching, one must study key aspects of X.500 case
645
exact/ignore matching. X.520 [X.520] says:
647
The [substrings] rule returns TRUE if there is a partitioning of
648
the attribute value (into portions) such that:
650
- the specified substrings (initial, any, final) match
651
different portions of the value in the order of the strings
654
- initial, if present, matches the first portion of the value;
656
- final, if present, matches the last portion of the value;
658
- any, if present, matches some arbitrary portion of the
661
That is, the substrings assertion (CN=foo\20*\20bar) matches the
662
attribute value "foo<SPACE><SPACE>bar" as the value can be
663
partitioned into the portions "foo<SPACE>" and "<SPACE>bar" meeting
664
the above requirements.
674
Zeilenga Standards Track [Page 12]
676
RFC 4518 LDAP: Internationalized String Preparation June 2006
681
[T]he following spaces are regarded as not significant:
683
- leading spaces (i.e., those preceding the first character
684
that is not a space);
686
- trailing spaces (i.e., those following the last character
687
that is not a space);
689
- multiple consecutive spaces (these are taken as equivalent
690
to a single space character).
692
This statement applies to the assertion values and attribute values
693
as whole strings, and not individually to substrings of an assertion
694
value. In particular, the statements should be taken to mean that if
695
an assertion value and attribute value match without any
696
consideration to insignificant characters, then that assertion value
697
should also match any attribute value that differs only by inclusion
698
nor removal of insignificant characters.
700
Hence the assertion (CN=foo\20*\20bar) matches
701
"foo<SPACE><SPACE><SPACE>bar" and "foo<SPACE>bar" as these values
702
only differ from "foo<SPACE><SPACE>bar" by the inclusion or removal
703
of insignificant spaces.
705
Astute readers of this text will also note that there are special
706
cases where the specified space handling does not ignore spaces that
707
could be considered insignificant. For instance, the assertion
708
(CN=\20*\20*\20) does not match "<SPACE><SPACE><SPACE>"
709
(insignificant spaces present in value) or " " (insignificant spaces
710
not present in value). However, as these cases have no practical
711
application that cannot be met by simple assertions, e.g., (cn=\20),
712
and this minor anomaly can only be fully addressed by a preparation
713
algorithm to be used in conjunction with character-by-character
714
partitioning and matching, the anomaly is considered acceptable.
721
EMail: Kurt@OpenLDAP.org
730
Zeilenga Standards Track [Page 13]
732
RFC 4518 LDAP: Internationalized String Preparation June 2006
735
Full Copyright Statement
737
Copyright (C) The Internet Society (2006).
739
This document is subject to the rights, licenses and restrictions
740
contained in BCP 78, and except as set forth therein, the authors
741
retain all their rights.
743
This document and the information contained herein are provided on an
744
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
745
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
746
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
747
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
748
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
749
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
751
Intellectual Property
753
The IETF takes no position regarding the validity or scope of any
754
Intellectual Property Rights or other rights that might be claimed to
755
pertain to the implementation or use of the technology described in
756
this document or the extent to which any license under such rights
757
might or might not be available; nor does it represent that it has
758
made any independent effort to identify any such rights. Information
759
on the procedures with respect to rights in RFC documents can be
760
found in BCP 78 and BCP 79.
762
Copies of IPR disclosures made to the IETF Secretariat and any
763
assurances of licenses to be made available, or the result of an
764
attempt made to obtain a general license or permission for the use of
765
such proprietary rights by implementers or users of this
766
specification can be obtained from the IETF on-line IPR repository at
767
http://www.ietf.org/ipr.
769
The IETF invites any interested party to bring to its attention any
770
copyrights, patents or patent applications, or other proprietary
771
rights that may cover technology that may be required to implement
772
this standard. Please address the information to the IETF at
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Funding for the RFC Editor function is provided by the IETF
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Administrative Support Activity (IASA).
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