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rfc:rfc3326

Network Working Group H. Schulzrinne Request for Comments: 3326 Columbia University Category: Standards Track D. Oran

                                                                 Cisco
                                                          G. Camarillo
                                                              Ericsson
                                                         December 2002
 The 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 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 [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 in BCP 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 the

Schulzrinne, 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 Control

3.4 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 Interworking

4. IANA Considerations

 This document defines a new SIP header field, "Reason", according to
 Section 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
 under http://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 code               RFC 3261
 Q.850            Cause value in decimal    ITU-T Q.850
                  representation

5. 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.com

Schulzrinne, et. al. Standards Track [Page 7] RFC 3326 The Reason Header Field for SIP December 2002

9. 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]

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