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rjsparks@nostrum.com

ART SIPCORE Working Group Internet-Draft The SIP Reason Header Field as defined in RFC 3326 allows only one Reason value per protocol value. Practice shows it is useful to allow multiple values with the same protocol value. This update to RFC 3326 allows multiple values for an indicated registered protocol when that protocol defines what the presence of multiple values means. Discussion Venues Discussion of this document takes place on the Session Initiation Protocol Core Working Group mailing list (sipcore@ietf.org), which is archived at . Source for this draft and an issue tracker can be found at .

Introduction The SIP Reason Header Field as defined in RFC 3326 allows only one Reason value per protocol value. Practice shows it is useful to allow multiple values with the same protocol value. This update to RFC 3326 allows multiple values for an indicated registered protocol when that protocol defines what the presence of multiple values means.

Conventions and Definitions 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 when, and only when, they appear in all capitals, as shown here.

Update to RFC3326 The last paragraph of section 2 of is replaced as follows: OLD: A SIP message MAY contain more than one Reason value (i.e., multiple Reason lines), but all of them MUST have different protocol values (e.g., one SIP and another Q.850). An implementation is free to ignore Reason values that it does not understand. NEW: A SIP message MAY contain more than one Reason value (i.e., multiple Reason lines). If the registered protocol for the Reason value specifies what it means for multiple values to occur in one message, more than one value for that protocol MAY be present. Otherwise, there MUST be only one value per protocol provided (e.g., one SIP and another Q.850). An implementation is free to ignore Reason values that it does not understand.

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

IANA Considerations This document has no IANA actions.

Normative References The REGISTER function is used in a Session Initiation Protocol (SIP) system primarily to associate a temporary contact address with an address-of-record. This contact is generally in the form of a Uniform Resource Identifier (URI), such as Contact: <sip:alice@pc33.atlanta.com> and is generally dynamic and associated with the IP address or hostname of the SIP User Agent (UA). The problem is that network topology may have one or more SIP proxies between the UA and the registrar, such that any request traveling from the user's home network to the registered UA must traverse these proxies. The REGISTER method does not give us a mechanism to discover and record this sequence of proxies in the registrar for future use. This document defines an extension header field, "Path" which provides such a mechanism. [STANDARDS-TRACK] In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.

Acknowledgments 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.