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
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.
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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.
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  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:
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.
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RFC 5954 SIP IPv6 ABNF August 2010
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.
Accordingly, this document updates RFC 3261 as follows: the <IPv6address> and <IPv4address> production rules from RFC 3261MUST 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 3261MUST 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:email@example.com".
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RFC 5954 SIP IPv6 ABNF August 2010
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.
o For two URIs to be equal, the user, password, host, and port components must match.
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:
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 ) to denote productions.