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Network Working Group                                     H. SchulzrinneRequest for Comments: 3326                           Columbia UniversityCategory: Standards Track                                        D. Oran                                                                   Cisco                                                            G. Camarillo                                                                Ericsson                                                           December 2002The Reason Header Field for the Session Initiation Protocol (SIP)Status of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2002).  All Rights Reserved.Abstract   For creating services, it is often useful to know why a Session   Initiation Protocol (SIP) request was issued.  This document defines   a header field, Reason, that provides this information.  The Reason   header field is also intended to be used to encapsulate a final   status code in a provisional response.  This functionality is needed   to resolve the "Heterogeneous Error Response Forking Problem", or   HERFP.Schulzrinne, et. al.        Standards Track                     [Page 1]

RFC 3326            The Reason Header Field for SIP        December 2002Table of Contents1.   Introduction ...............................................21.1. Terminology ................................................32.   The Reason Request Header Field ............................33.   Examples ...................................................43.1. Call Completed Elsewhere ...................................43.2. Refusing an Offer that Comes in a Response .................43.3. Third Party Call Control ...................................53.4. ISUP interworking ..........................................54.   IANA Considerations ........................................65.   Security Considerations ....................................66.   Acknowledgments ............................................77.   Authors' Addresses .........................................78.   Normative References .......................................79.   Full Copyright Statement ...................................81. 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                     [Page 2]

RFC 3326            The Reason Header Field for SIP        December 2002   Initially, the Reason header field defined here appears to be most   useful for BYE and CANCEL requests, but it can appear in any request   within a dialog, in any CANCEL request and in any response whose   status code explicitly allows the presence of this header field.   Note that the Reason header field is usually not needed in responses   because the status code and the reason phrase already provide   sufficient information.   Clients and servers are free to ignore this header field.  It has no   impact on protocol processing.1.1 Terminology   In this document, the key words "MUST", "MUST NOT", "REQUIRED",   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",   and "OPTIONAL" are to be interpreted as described inBCP 14,RFC 2119   [2] and indicate requirement levels for compliant SIP   implementations.2. The Reason Header Field   The Reason header field MAY appear in any request within a dialog, in   any CANCEL request and in any response whose status code explicitly   allows the presence of this header field.  The syntax of the header   field follows the standard SIP parameter syntax. Reason            =  "Reason" HCOLON reason-value *(COMMA reason-value) reason-value      =  protocol *(SEMI reason-params) protocol          =  "SIP" / "Q.850" / token reason-params     =  protocol-cause / reason-text                      / reason-extension protocol-cause    =  "cause" EQUAL cause cause             =  1*DIGIT reason-text       =  "text" EQUAL quoted-string reason-extension  =  generic-param   The following values for the protocol field have been defined:      SIP: The cause parameter contains a SIP status code.      Q.850: The cause parameter contains an ITU-T Q.850 cause value           in decimal representation.Schulzrinne, et. al.        Standards Track                     [Page 3]

RFC 3326            The Reason Header Field for SIP        December 2002   Examples are:      Reason: SIP ;cause=200 ;text="Call completed elsewhere"      Reason: Q.850 ;cause=16 ;text="Terminated"      Reason: SIP ;cause=600 ;text="Busy Everywhere"      Reason: SIP ;cause=580 ;text="Precondition Failure"   Proxies generating a CANCEL request upon reception of a CANCEL from   the previous hop that contains a Reason header field SHOULD copy it   into the new CANCEL request.   In normal SIP operation, a SIP status code in a response provides the   client with information about the request that triggered the   response, the session parameters, or the user.  For example, a 405   (Method not allowed) response indicates that the request contained an   unsupported method.  A 488 (Not Acceptable Here) indicates that the   session parameters are unacceptable and a 486 (Busy Here) provides   information about the status of the user.   Any SIP status code MAY appear in the Reason header field of a   request.  However, status codes that provide information about the   user and about session parameters are typically useful for   implementing services whereas status codes intended to report errors   about a request are typically useful for debugging purposes.   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.3. Examples   This section contains a number of examples that illustrate the use of   the Reason header field.3.1 Call Completed Elsewhere   A proxy forks an INVITE request and one of the branches returns a 200   (OK).  The forking proxy includes this status code in a Reason header   field in the CANCEL request that it sends to the rest of the   branches.3.2 Refusing an Offer that Comes in a Response   A client sends an empty INVITE and receives an unacceptable offer in   a 200 (OK) response.  The client sends an ACK with a correctly   formatted answer and immediately sends a BYE to terminate theSchulzrinne, et. al.        Standards Track                     [Page 4]

RFC 3326            The Reason Header Field for SIP        December 2002   session.  The client includes a 488 (Not Acceptable Here) status code   in a Reason header field.3.3 Third Party Call Control   The third party call controller of figure 1 tries to establish a   session between A and B.  However, user B is busy.  The controller   sends a BYE with the status code 486 (Busy Here) in a Reason header   field.      A                Controller            B      |   INV  no SDP     |                  |      |<------------------|                  |      |                   |                  |      |    200 SDP A1     |                  |      |-----------------> |                  |      |                   |                  |      |   ACK  SDP held   |                  |      |<------------------|                  |      |                   |                  |      |                   |   INV no SDP     |      |                   |----------------->|      |                   |                  |      |                   |  486 Busy Here   |      |                   |<-----------------|      |                   |                  |      |                   |       ACK        |      |                   |----------------->|      |     BYE (486)     |                  |      |<------------------|                  |      |                   |                  |      |     200 OK        |                  |      |-----------------> |                  |      |                   |                  |         Figure 1: Third Party Call Control3.4 ISUP interworking   The PSTN gateway of figure 2 generates an INVITE that has to be   CANCELed when a REL (release) message is received from the ISUP side.   The CANCEL request contains the Q.850 cause value (16 Normal Call   Clearing) of the REL message.Schulzrinne, et. al.        Standards Track                     [Page 5]

RFC 3326            The Reason Header Field for SIP        December 2002      A                Gateway               B      |       IAM         |                  |      |-----------------> |                  |      |                   |     INVITE       |      |                   |----------------->|      |                   |                  |      |                   |   100 Trying     |      |                   |<-----------------|      |     REL (16)      |                  |      |-----------------> |                  |      |                   | CANCEL (Q.850 16)|      |                   |----------------->|      |                   |      200 OK      |      |                   |<-----------------|             Figure 2: ISUP Interworking4. IANA Considerations   This document defines a new SIP header field, "Reason", according toSection 27 of RFC 3261.   Protocol values (and their associated protocol cause) to be used with   this header field are registered by the IANA into a new sub-registry   underhttp://www.iana.org/assignments/sip-parameters, labeled "Reason   Protocols".  Reason protocols MUST refer to either an ITU-T   Recommendation number, an IETF protocol name or the recognized   protocol identifier from another standardization organization.   Protocol cause describes the source of the 'cause' field in the   Reason header field.   The only entries in the registry for the time being are:   Protocol Value   Protocol Cause            Reference   --------------   ---------------           -----------   SIP              Status codeRFC 3261   Q.850            Cause value in decimal    ITU-T Q.850                    representation5. Security Considerations   Spoofing or removing the Reason header field from a response in a   HERFP scenario can make it impossible for a client to update properly   its previous request, making therefore session establishment   impossible. Thus, it is RECOMMENDED that this header field is   protected by a suitable integrity mechanism.Schulzrinne, et. al.        Standards Track                     [Page 6]

RFC 3326            The Reason Header Field for SIP        December 2002   properly its previous request, making therefore session establishment   impossible. Thus, it is RECOMMENDED that this header field is   protected by a suitable integrity mechanism.6. Acknowledgments   Jonathan Rosenberg, Rohan Mahy and Vijay K. Gurbani provided helpful   comments and suggestions.8. 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 1997.7. Authors' Addresses   Henning Schulzrinne   Dept. of Computer Science   Columbia University   1214 Amsterdam Avenue   New York, NY 10027   USA   EMail:  schulzrinne@cs.columbia.edu   David R. Oran   Cisco Systems, Inc.   Acton, MA   USA   EMail:  oran@cisco.com   Gonzalo Camarillo   Ericsson   Advanced Signalling Research Lab.   FIN-02420 Jorvas   Finland   EMail:  Gonzalo.Camarillo@ericsson.comSchulzrinne, et. al.        Standards Track                     [Page 7]

RFC 3326            The Reason Header Field for SIP        December 20029.  Full Copyright Statement   Copyright (C) The Internet Society (2002).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Schulzrinne, et. al.        Standards Track                     [Page 8]