Internet Engineering Task Force (IETF)                   V. Gurbani, Ed.
Request for Comments: 5954             Bell Laboratories, Alcatel-Lucent
Updates: 3261                                          B. Carpenter, Ed.
Category: Standards Track                              Univ. of Auckland
ISSN: 2070-1721                                             B. Tate, Ed.
                                                               BroadSoft
                                                             August 2010

   Essential Correction for IPv6 ABNF and URI Comparison in RFC 3261

Abstract

   This document corrects the Augmented Backus-Naur Form (ABNF)
   production rule associated with generating IPv6 literals in RFC 3261.
   It also clarifies the rule for Uniform Resource Identifier (URI)
   comparison when the URIs contain textual representation of IP
   addresses.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5954.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 2
   3.  Problem Statement . . . . . . . . . . . . . . . . . . . . . . . 2
     3.1.  Extra Colon in IPv4-Mapped IPv6 Address . . . . . . . . . . 2
     3.2.  Comparing URIs with Textual Representation of IP
           Addresses . . . . . . . . . . . . . . . . . . . . . . . . . 3
   4.  Resolution  . . . . . . . . . . . . . . . . . . . . . . . . . . 4
     4.1.  Resolution for Extra Colon in IPv4-Mapped IPv6 Address  . . 4
     4.2.  Clarification for Comparison of URIs with Textual
           Representation of IP Addresses  . . . . . . . . . . . . . . 5
   5.  Generating a Canonical IPv6 Textual Representation  . . . . . . 5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 6

1.  Introduction

   This document corrects the Augmented Backus-Naur Form (ABNF)
   production rule associated with generating IPv6 literals in RFC 3261
   [1].  It also clarifies the rule for Uniform Resource Identifier
   (URI) comparison when the URIs contain textual representation of IP
   addresses.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [2].

3.  Problem Statement

3.1.  Extra Colon in IPv4-Mapped IPv6 Address

   The ABNF [4] for generating IPv6 literals in RFC 3261 [1] is
   incorrect.  When generating IPv4-mapped IPv6 addresses, the
   production rule may actually generate the following construct:

   [2001:db8:::192.0.2.1] - Note the extra colon before the IPv4
   address.

   The correct construct, of course, would only include two colons
   before the IPv4 address.

      Historically, the ABNF pertaining to IPv6 references in RFC 3261
      was derived from Appendix B of RFC 2373 [7], which was flawed to
      begin with (see errata for RFC 2373 [8]).  RFC 2373 has been
      subsequently obsoleted by RFC 4291 [6].

   The ABNF for IPv6reference is reproduced from RFC 3261 below:

     IPv6reference  =  "[" IPv6address "]"
     IPv6address    =  hexpart [ ":" IPv4address ]
     IPv4address    =  1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT
     hexpart        =  hexseq / hexseq "::" [ hexseq ] / "::" [ hexseq ]
     hexseq         =  hex4 *( ":" hex4)
     hex4           =  1*4HEXDIG

   Note that the ambiguity occurs in the <IPv6address> production rule
   where the <IPv4address> non-terminal is prefixed by the ":" token.
   Because the <hexpart> production rule is defined such that two of its
   alternatives already include the "::" token, this may yield to the
   faulty construction of an IPv6-mapped IPv4 address with an extra
   colon when expanding those alternatives.

3.2.  Comparing URIs with Textual Representation of IP Addresses

   In SIP, URIs are compared for a variety of reasons.  Registrars
   compare URIs when they receive a binding update request, for
   instance.  Section 19.1.4 of RFC 3261 [1] provides the rules for
   comparing URIs.  Among other rules, it states that:

      For two URIs to be equal, the user, password, host, and port
      components must match.

   Does the above rule then imply that the following URIs are equal:

      sip:bob@[::ffff:192.0.2.128] = sip:bob@[::ffff:c000:280]?

      sip:bob@[2001:db8::9:1] = sip:bob@[2001:db8::9:01]?

      sip:bob@[0:0:0:0:0:FFFF:129.144.52.38] = sip:bob@
      [::FFFF:129.144.52.38]?

   In all of the above examples, the textual representation of the IPv6
   address is different, but these addresses are binary equivalents
   (implementers are also urged to consult Section 5 of this document
   for recommendations on IPv6 address text representations).  Section
   19.1.4 of RFC 3261 does not provide any rule for URIs containing
   different textual representations of IPv6 addresses that all
   correspond to the same binary equivalent.

      Note that the same ambiguity occurs for IPv4 addresses, i.e., is
      192.0.2.128 = 192.00.02.128?  However, IPv6, with its compressed
      notation and the need to represent hybrid addresses (like IPv4-
      mapped IPv6 addresses) makes the representation issue more acute.
      The resolution discussed in Section 4.2 applies to textual
      representations of both IPv6 and IPv4 addresses.

4.  Resolution

4.1.  Resolution for Extra Colon in IPv4-Mapped IPv6 Address

   The resolution to this ambiguity is simply to use the correct ABNF
   for the <IPv6address> production rule from Appendix A of RFC 3986
   [3].  For the sake of completeness, it is reproduced below:

     IPv6address   =                             6( h16 ":" ) ls32
                    /                       "::" 5( h16 ":" ) ls32
                    / [               h16 ] "::" 4( h16 ":" ) ls32
                    / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
                    / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
                    / [ *3( h16 ":" ) h16 ] "::"    h16 ":"   ls32
                    / [ *4( h16 ":" ) h16 ] "::"              ls32
                    / [ *5( h16 ":" ) h16 ] "::"              h16
                    / [ *6( h16 ":" ) h16 ] "::"

     h16           = 1*4HEXDIG
     ls32          = ( h16 ":" h16 ) / IPv4address
     IPv4address   = dec-octet "." dec-octet "." dec-octet "." dec-octet
     dec-octet     = DIGIT                 ; 0-9
                    / %x31-39 DIGIT         ; 10-99
                    / "1" 2DIGIT            ; 100-199
                    / "2" %x30-34 DIGIT     ; 200-249
                    / "25" %x30-35          ; 250-255

   Accordingly, this document updates RFC 3261 as follows:  the
   <IPv6address> and <IPv4address> production rules from RFC 3261 MUST
   NOT be used and instead, the production rules of the same name in RFC
   3986 (and reproduced above) MUST be used.  This will render
   <hexpart>, <hexseq>, and <hex4> production rules in RFC 3261
   obsolete; as such, these three production rules -- namely, <hexpart>,
   <hexseq>, and <hex4> -- from RFC 3261 MUST NOT be used.

   The use of the <IPv4address> production rule from RFC 3986 no longer
   allows syntactically valid -- though semantically invalid -- SIP URIs
   of the form "sip:bob@444.555.666.777".

4.2.  Clarification for Comparison of URIs with Textual Representation
      of IP Addresses

   The resolution to this ambiguity is a simple clarification
   acknowledging that the textual representation of an IP address
   varies, but it is the binary equivalence of the IP address that must
   be taken into consideration when comparing two URIs that contain
   varying textual representations of an IP address.

   Accordingly, the existing rule from the bulleted list in Section
   19.1.4 of RFC 3261 MUST be modified as follows:

   OLD:

   o  For two URIs to be equal, the user, password, host, and port
      components must match.

   NEW:

   o  For two URIs to be equal, the user, password, host, and port
      components must match.  If the host component contains a textual
      representation of IP addresses, then the representation of those
      IP addresses may vary.  If so, the host components are considered
      to match if the different textual representations yield the same
      binary IP address.

   In addition, the text in the following paragraph MUST be added to the
   existing list of examples in Section 19.1.4 of RFC 3261 in order to
   demonstrate the intent of the modified rule:

   The following URIs are equivalent because the underlying binary
   representation of the IP addresses are the same although their
   textual representations vary:

      sip:bob@[::ffff:192.0.2.128]
      sip:bob@[::ffff:c000:280]

      sip:bob@[2001:db8::9:1]
      sip:bob@[2001:db8::9:01]

      sip:bob@[0:0:0:0:0:FFFF:129.144.52.38]
      sip:bob@[::FFFF:129.144.52.38]

5.  Generating a Canonical IPv6 Textual Representation

   Implementers SHOULD generate IPv6 text representation as defined in
   RFC 5952 [5].

6.  Security Considerations

   This document does not introduce any new security considerations
   beyond those described in RFC 3261 [1].

7.  Acknowledgments

   The ABNF for IPv6 was developed by Roy T. Fielding and Andrew Main
   and published in RFC 3986.

   Jeroen van Bemmel, Peter Blatherwick, Gonzalo Camarillo, Paul
   Kyzivat, Jonathan Rosenberg, Michael Thomas, and Dale Worley provided
   invaluable discussion points on the SIP WG mailing list on the URI
   equivalency problem.  Alfred Hoenes urged the use of angle brackets
   (as specified in Section 2.1 of RFC 5234 [4]) to denote productions.

8.  References

8.1.  Normative References

   [1]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
        Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
        Session Initiation Protocol", RFC 3261, June 2002.

   [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [3]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
        Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
        January 2005.

   [4]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [5]  Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
        Address Text Representation", RFC 5952, August 2010.

8.2.  Informative References

   [6]  Hinden, R. and S. Deering, "IP Version 6 Addressing
        Architecture", RFC 4291, February 2006.

   [7]  Hinden, R. and S. Deering, "IP Version 6 Addressing
        Architecture", RFC 2373, July 1998.

   [8]  "RFC Editor Errata", <http://www.rfc-editor.org/errata.php>.

Authors' Addresses

   Vijay K. Gurbani (editor)
   Bell Laboratories, Alcatel-Lucent
   1960 Lucent Lane
   Room 9C-533
   Naperville, IL  60563
   USA

   Phone:  +1 630 224-0216
   EMail:  vkg@bell-labs.com

   Brian E. Carpenter (editor)
   Department of Computer Science
   University of Auckland
   PB 92019
   Auckland,   1142
   New Zealand

   EMail:  brian.e.carpenter@gmail.com

   Brett Tate (editor)
   BroadSoft

   EMail:  brett@broadsoft.com