RFC 3326 (rfc3326) - Page 2 of 8
The Reason Header Field for the Session Initiation Protocol (SIP)
Alternative Format: Original Text Document
RFC 3326 The Reason Header Field for SIP December 2002 Table of Contents 1. Introduction ............................................... 2 1.1. Terminology ................................................ 3 2. The Reason Request Header Field ............................ 3 3. Examples ................................................... 4 3.1. Call Completed Elsewhere ................................... 4 3.2. Refusing an Offer that Comes in a Response ................. 4 3.3. Third Party Call Control ................................... 5 3.4. ISUP interworking .......................................... 5 4. IANA Considerations ........................................ 6 5. Security Considerations .................................... 6 6. Acknowledgments ............................................ 7 7. Authors' Addresses ......................................... 7 8. Normative References ....................................... 7 9. Full Copyright Statement ................................... 8 1. Introduction The same SIP [1] request can be issued for a variety of reasons. For example, a SIP CANCEL request can be issued if the call has completed on another branch or was abandoned before answer. While the protocol and system behavior is the same in both cases, namely, alerting will cease, the user interface may well differ. In the second case, the call may be logged as a missed call, while this would not be appropriate if the call was picked up elsewhere. Third party call controllers sometimes generate a SIP request upon reception of a SIP response from another dialog. Gateways generate SIP requests after receiving messages from a different protocol than SIP. In both cases the client may be interested in knowing what triggered the SIP request. SIP responses already offer a means of informing the user of why a request failed. The simple mechanism in this document accomplishes something roughly similar for requests. An INVITE can sometimes be rejected not because the session initiation was declined, but because some aspect of the request was not acceptable. If the INVITE forked and resulted in a rejection, the error response may never be forwarded to the client unless all the other branches also reject the request. This problem is known as the "Heterogeneous Error Response Forking Problem", or HERFP. It is foreseen that a solution to this problem may involve encapsulating the final error response in a provisional response. The Reason header field is a candidate to be used for such encapsulation. Schulzrinne, et. al. Standards Track
