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Versions: 00 01 rfc3326                                                 
Internet Engineering Task Force                                   SIP WG
Internet Draft                                            H. Schulzrinne
                                                     Columbia University
                                                                 D. Oran
                                                            G. Camarillo
April 23, 2002
Expires: August 2002

      The Reason Header Field for the Session Initiation Protocol


   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   and may be updated, replaced, or obsoleted by other documents at any
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   For creating services, it is often useful to know why a 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.

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                           Table of Contents

   1          Introduction ........................................    3
   1.1        Terminology .........................................    3
   2          The Reason Request Header Field .....................    3
   3          Examples ............................................    5
   3.1        Call Completed Elsewhere ............................    5
   3.2        Refusing an Offer that Comes in a Response ..........    5
   3.3        Third Party Call Control ............................    5
   3.4        ISUP interworking ...................................    6
   4          IANA Considerations .................................    6
   5          Security Considerations .............................    7
   6          Acknowledgments .....................................    7
   7          Authors' Addresses ..................................    7
   8          Bibliography ........................................    7

H. Schulzrinne et. al.                                        [Page 2]

Internet Draft                    SIP                     April 23, 2002

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 have a means of informing the user of why a
   request failed. The simple mechanism in this draft accomplishes
   something roughly similar for requests.

   When an INVITE is rejected, not because the call is declined, but
   because some aspect of the request was not acceptable, if the INVITE
   was forked, the error response is not forwarded towards the UAC by
   the forking proxy. This problem is known as the "Heterogeneous Error
   Response Forking Problem", or HERFP. The header field defined in this
   draft allows encapsulating the final error response in a 155 (Update
   Requested) provisional response [2].

   Initially, the Reason header field defined here appears to be most
   useful for BYE and CANCEL requests, but it can appear in any request
   and in 155 (Update Requested) responses.

   When used in requests, 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",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [3] and
   indicate requirement levels for compliant SIP implementations.

2 The Reason Request Header Field

   The Reason header field can appear in any request and in 155 (Update
   Requested) responses. The syntax of the header field follows the
   standard SIP parameter syntax.

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        Reason            =  "Reason" HCOLON 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/2.0: The cause parameter contains a SIP status code.

        Q.850: The cause parameter contains an ITU-T Q.850 cause value
             in decimal representation.

   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 codes 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 values (i.e., multiple
   Reason lines), but all of them MUST have different protocol values
   (e.g., one SIP and another Q.850). A implementation is free to ignore

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   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 the CANCEL 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
   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

     A                Controller            B
     |   INV  no SDP     |                  |
     |<------------------|                  |
     |                   |                  |
     |    200 SDP A1     |                  |
     |-----------------> |                  |
     |                   |                  |
     |   ACK  SDP held   |                  |
     |<------------------|                  |
     |                   |                  |
     |                   |   INV no SDP     |
     |                   |----------------->|
     |                   |                  |
     |                   |  486 Busy Here   |
     |                   |<-----------------|
     |                   |                  |
     |                   |       ACK        |
     |                   |----------------->|

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Internet Draft                    SIP                     April 23, 2002

     |     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.

     A                Gateway               B
     |       IAM         |                  |
     |-----------------> |                  |
     |                   |     INVITE       |
     |                   |----------------->|
     |                   |                  |
     |                   |   100 Trying     |
     |                   |<-----------------|
     |     REL (16)      |                  |
     |-----------------> |                  |
     |                   | CANCEL (Q.850 16)|
     |                   |----------------->|
     |                   |      200 OK      |
     |                   |<-----------------|

   Figure 2: ISUP Interworking

4 IANA Considerations

   IANA registers new protocol values for the Reason header field.

   IANA also registers new values for the triggered parameter. These
   values MUST refer to either an ITU-T Recommendation number, an IETF
   protocol name or the recognized protocol identifier from another

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   standardization organization.

5 Security Considerations

   While spoofing or removing the Reason header field from a request has
   no impact on protocol operation, the user interface may change and
   end systems may provide services based on this header field. Spoofing
   or removing the Reason header field from a 155 (Update Requested)
   response can make 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.

6 Acknowledgments

   Jonathan Rosenberg and Rohan May provided helpful comments and

7 Authors' Addresses

   Henning Schulzrinne
   Dept. of Computer Science
   Columbia University
   1214 Amsterdam Avenue
   New York, NY 10027
   electronic mail:  schulzrinne@cs.columbia.edu

   David R. Oran
   Cisco Systems, Inc.
   Acton, MA
   electronic mail:  oran@cisco.com

   Gonzalo Camarillo
   Advanced Signalling Research Lab.
   FIN-02420 Jorvas
   electronic mail:  Gonzalo.Camarillo@ericsson.com

8 Bibliography

   [1] J. Rosenberg, H. Schulzrinne,  et al.  , "SIP: Session initiation
   protocol," Internet Draft, Internet Engineering Task Force, Feb.
   2002.  Work in progress.

   [2] J. Rosenberg, "The SIP UPDATE method," Internet Draft, Internet

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   Engineering Task Force, Mar. 2002.  Work in progress.

   [3] S. Bradner, "Key words for use in RFCs to indicate requirement
   levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.

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