The SIP Reason header field as defined in RFC 3326 allows only one Reason value per protocol value. Experience with more recently defined protocols shows it is useful to allow multiple values with the same protocol value. This document updates RFC 3326 to allow multiple values for an indicated registered protocol when that protocol defines what the presence of multiple values means.

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The SIP Reason header field as defined in RFC 3326 allows only one Reason value per protocol value. Experience with more recently defined protocols shows it is useful to allow multiple values with the same protocol value [STIRREASONS]. This document updates RFC 3326 to allow multiple values for an indicated registered protocol when that protocol defines what the presence of multiple values means. It does not change the requirement in RFC 3326 restricting the header field contents to one value per protocol for those protocols that do not define what multiple values mean.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC 2119] [RFC 8174] when, and only when, they appear in all capitals, as shown here.

The last paragraph of Section 2 of RFC 3326 is replaced as follows: OLD:

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NEW:

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This document adds no security considerations to the use of SIP. The security considerations in [RFC 3326] and those in any registered protocols used in Reason header field values should be considered.

This document has no IANA actions.

[RFC2119]

S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC3326]

H. Schulzrinne, D. Oran, and G. Camarillo, "The Reason Header Field for the Session Initiation Protocol (SIP)", RFC 3326, DOI 10.17487/RFC3326, December 2002,
<https://www.rfc-editor.org/info/rfc3326>.

[RFC8174]

B. Leiba, "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017,
<https://www.rfc-editor.org/info/rfc8174>.

[STIRREASONS]

Somos Inc., C. Wendt, "Identity Header Errors Handling for STIR", Internet-Draft draft-ietf-stir-identity-header-errors-handling-08, February 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-stir-identity-header-errors-handling-08>.

This text is based on discussions at a STIR Working Group interim meeting. Jean Mahoney and Russ Housley provided suggestions that vastly improved the first attempts at assembling these words. Christer Holmberg, Dale Worley, Brian Rosen, Chris Wendt, and Paul Kyzivat provided constructive discussion during SIPCORE Working Group adoption.

Robert Sparks