





Network Working Group                                       J. Rosenberg
Request for Comments: 3311                                   dynamicsoft
Category: Standards Track                                 September 2002


          The Session Initiation Protocol (SIP) UPDATE Method

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

   This specification defines the new UPDATE method for the Session
   Initiation Protocol (SIP).  UPDATE allows a client to update
   parameters of a session (such as the set of media streams and their
   codecs) but has no impact on the state of a dialog.  In that sense,
   it is like a re-INVITE, but unlike re-INVITE, it can be sent before
   the initial INVITE has been completed.  This makes it very useful for
   updating session parameters within early dialogs.























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Table of Contents

   1    Introduction ..............................................    2
   2    Terminology ...............................................    3
   3    Overview of Operation .....................................    3
   4    Determining Support for this Extension ....................    3
   5    UPDATE Handling ...........................................    4
   5.1  Sending an UPDATE .........................................    4
   5.2  Receiving an UPDATE .......................................    5
   5.3  Processing the UPDATE Response ............................    6
   6    Proxy Behavior ............................................    7
   7    Definition of the UPDATE method ...........................    7
   8    Example Call Flow .........................................    7
   9    Security Considerations ...................................   11
   10   IANA Considerations .......................................   11
   11   Notice Regarding Intellectual Property Rights .............   11
   12   Normative References ......................................   11
   13   Acknowledgements ..........................................   12
   14   Author's Address ..........................................   12
   15   Full Copyright Statement ..................................   13

1 Introduction

   The Session Initiation Protocol (SIP) [1] defines the INVITE method
   for the initiation and modification of sessions.  However, this
   method actually affects two important pieces of state.  It impacts
   the session (the media streams SIP sets up) and also the dialog (the
   state that SIP itself defines).  While this is reasonable in many
   cases, there are important scenarios in which this coupling causes
   complications.

   The primary difficulty is when aspects of the session need to be
   modified before the initial INVITE has been answered.  An example of
   this situation is "early media", a condition where the session is
   established, for the purpose of conveying progress of the call, but
   before the INVITE itself is accepted.  It is important that either
   caller or callee be able to modify the characteristics of that
   session (putting the early media on hold, for example), before the
   call is answered.  However, a re-INVITE cannot be used for this
   purpose, because the re-INVITE has an impact on the state of the
   dialog, in addition to the session.

   As a result, a solution is needed that allows the caller or callee to
   provide updated session information before a final response to the
   initial INVITE request is generated.  The UPDATE method, defined
   here, fulfills that need.  It can be sent by a UA within a dialog
   (early or confirmed) to update session parameters without impacting
   the dialog state itself.



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2 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.

3 Overview of Operation

   Operation of this extension is straightforward.  The caller begins
   with an INVITE transaction, which proceeds normally.  Once a dialog
   is established, either early or confirmed, the caller can generate an
   UPDATE method that contains an SDP offer [3] for the purposes of
   updating the session.  The response to the UPDATE method contains the
   answer.  Similarly, once a dialog is established, the callee can send
   an UPDATE with an offer, and the caller places its answer in the 2xx
   to the UPDATE.  The Allow header field is used to indicate support
   for the UPDATE method.  There are additional constraints on when
   UPDATE can be used, based on the restrictions of the offer/answer
   model.

4 Determining Support for this Extension

   The initiation of a session operates as specified in RFC 3261 [1].
   However, a UAC compliant to this specification SHOULD also include an
   Allow header field in the INVITE request, listing the method UPDATE,
   to indicate its ability to receive an UPDATE request.

   When a UAS compliant to this specification receives an INVITE request
   for a new dialog, and generates a reliable provisional response
   containing SDP, that response SHOULD contain an Allow header field
   that lists the UPDATE method.  This informs the caller that the
   callee is capable of receiving an UPDATE request at any time.  An
   unreliable provisional response MAY contain an Allow header field
   listing the UPDATE method, and a 2xx response SHOULD contain an Allow
   header field listing the UPDATE method.

   Responses are processed normally as per RFC 3261 [1], and in the case
   of reliable provisional responses, according to [4].  It is important
   to note that a reliable provisional response will always create an
   early dialog at the UAC.  Creation of this dialog is necessary in
   order to receive UPDATE requests from the callee.

   If the response contains an Allow header field containing the value
   "UPDATE", the UAC knows that the callee supports UPDATE, and the UAC
   is allowed to follow the procedures of Section 5.1.




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5 UPDATE Handling

5.1 Sending an UPDATE

   The UPDATE request is constructed as would any other request within
   an existing dialog, as described in Section 12.2.1 of RFC 3261.  It
   MAY be sent for both early and confirmed dialogs, and MAY be sent by
   either caller or callee.  Although UPDATE can be used on confirmed
   dialogs, it is RECOMMENDED that a re-INVITE be used instead.  This is
   because an UPDATE needs to be answered immediately, ruling out the
   possibility of user approval.  Such approval will frequently be
   needed, and is possible with a re-INVITE.

   The UAC MAY add optional headers for the UPDATE request, as defined
   in Tables 1 and 2.

   UPDATE is a target refresh request. As specified in RFC 3261 [1],
   this means that it can update the remote target of a dialog. If a UA
   uses an UPDATE request or response to modify the remote target while
   an INVITE transaction is in progress, and it is a UAS for that INVITE
   transaction, it MUST place the same value into the Contact header
   field of the 2xx to the INVITE that it placed into the UPDATE request
   or response.

   The rules for inclusion of offers and answers in SIP messages as
   defined in Section 13.2.1 of RFC 3261 still apply.  These rules exist
   to guarantee a consistent view of the session state.  This means
   that, for the caller:

      o  If the UPDATE is being sent before completion of the initial
         INVITE transaction, and the initial INVITE contained an offer,
         the UPDATE can contain an offer if the callee generated an
         answer in a reliable provisional response, and the caller has
         received answers to any other offers it sent in either PRACK or
         UPDATE, and has generated answers for any offers it received in
         an UPDATE from the callee.

      o  If the UPDATE is being sent before completion of the initial
         INVITE transaction, and the initial INVITE did not contain an
         offer, the UPDATE can contain an offer if the callee generated
         an offer in a reliable provisional response, and the UAC
         generated an answer in the corresponding PRACK.  Of course, it
         can't send an UPDATE if it has not received answers to any
         other offers it sent in either PRACK or UPDATE, or has not
         generated answers for any other offers it received in an UPDATE
         from the callee.





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      o  If the UPDATE is being sent after the completion of the initial
         INVITE transaction, it cannot contain an offer if the caller
         has generated or received offers in a re-INVITE or UPDATE which
         have not been answered.

   and for the callee:

      o  If the UPDATE is being sent before the completion of the INVITE
         transaction, and the initial INVITE contained an offer, the
         UPDATE cannot be sent with an offer unless the callee has
         generated an answer in a reliable provisional response, has
         received a PRACK for that reliable provisional response, has
         not received any requests (PRACK or UPDATE) with offers that it
         has not answered, and has not sent any UPDATE requests
         containing offers that have not been answered.

      o  If the UPDATE is being sent before completion of the INVITE
         transaction, and the initial INVITE did not contain an offer,
         the UPDATE cannot be sent with an offer unless the callee has
         sent an offer in a reliable provisional response, received an
         answer in a PRACK, and has not received any UPDATE requests
         with offers that it has not answered, and has not sent any
         UPDATE requests containing offers that have not been answered.

      o  If the UPDATE is being sent after the completion of the initial
         INVITE transaction, it cannot be sent with an offer if the
         callee has generated or received offers in a re-INVITE or
         UPDATE which have not been answered.

5.2 Receiving an UPDATE

   The UPDATE is processed as any other mid-dialog target refresh
   request, as described in Section 12.2.2 of RFC 3261 [1].  If the
   request is generally acceptable, processing continues as described
   below.  This processing is nearly identical to that of Section 14.2
   of RFC 3261 [1], but generalized for the case of UPDATE.

   A UAS that receives an UPDATE before it has generated a final
   response to a previous UPDATE on the same dialog MUST return a 500
   response to the new UPDATE, and MUST include a Retry-After header
   field with a randomly chosen value between 0 and 10 seconds.

   If an UPDATE is received that contains an offer, and the UAS has
   generated an offer (in an UPDATE, PRACK or INVITE) to which it has
   not yet received an answer, the UAS MUST reject the UPDATE with a 491
   response.  Similarly, if an UPDATE is received that contains an
   offer, and the UAS has received an offer (in an UPDATE, PRACK, or
   INVITE) to which it has not yet generated an answer, the UAS MUST



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   reject the UPDATE with a 500 response, and MUST include a Retry-After
   header field with a randomly chosen value between 0 and 10 seconds.

   If a UA receives an UPDATE for an existing dialog, it MUST check any
   version identifiers in the session description or, if there are no
   version identifiers, the content of the session description to see if
   it has changed.  If the session description has changed, the UAS MUST
   adjust the session parameters accordingly and generate an answer in
   the 2xx response.  However, unlike a re-INVITE, the UPDATE MUST be
   responded to promptly, and therefore the user cannot generally be
   prompted to approve the session changes.  If the UAS cannot change
   the session parameters without prompting the user, it SHOULD reject
   the request with a 504 response.  If the new session description is
   not acceptable, the UAS can reject it by returning a 488 (Not
   Acceptable Here) response for the UPDATE.  This response SHOULD
   include a Warning header field.

5.3 Processing the UPDATE Response

   Processing of the UPDATE response at the UAC follows the rules in
   Section 12.2.1.2 of RFC 3261 [1] for a target refresh request.  Once
   that processing is complete, it continues as specified below.  This
   processing is nearly identical to the processing of Section 14.1 of
   RFC 3261 [1], but generalized for UPDATE.

   If a UA receives a non-2xx final response to a UPDATE, the session
   parameters MUST remain unchanged, as if no UPDATE had been issued.
   Note that, as stated in Section 12.2.1 of RFC 3261 [1], if the non-
   2xx final response is a 481 (Call/Transaction Does Not Exist), or a
   408 (Request Timeout), or no response at all is received for the
   UPDATE (that is, a timeout is returned by the UPDATE client
   transaction), the UAC will terminate the dialog.

   If a UAC receives a 491 response to a UPDATE, it SHOULD start a timer
   with a value T chosen as follows:

      1. If the UAC is the owner of the Call-ID of the dialog ID
         (meaning it generated the value), T has a randomly chosen value
         between 2.1 and 4 seconds in units of 10 ms.

      2. If the UAC is not the owner of the Call-ID of the dialog ID, T
         has a randomly chosen value between 0 and 2 seconds in units of
         10 ms.

   When the timer fires, the UAC SHOULD attempt the UPDATE once more, if
   it still desires for that session modification to take place.  For
   example, if the call was already hung up with a BYE, the UPDATE would
   not take place.



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6 Proxy Behavior

   Proxy processing of the UPDATE request is identical to any other
   non-INVITE request.

7 Definition of the UPDATE method

   The semantics of the UPDATE method are described in detail above.
   This extension adds another value to the Method BNF described in RFC
   3261:

         UPDATEm  =  %x55.50.44.41.54.45 ; UPDATE in caps
         Method   =  INVITEm / ACKm / OPTIONSm / BYEm
                     / CANCELm / REGISTERm / UPDATEm
                     / extension-method

   Table 1 extends Table 2 of RFC 3261 for the UPDATE method.

   Table 2 updates Table 3 of RFC 3261 for the UPDATE method.

8 Example Call Flow

   This section presents an example call flow using the UPDATE method.
   The flow is shown in Figure 1.  The caller sends an initial INVITE
   (1) which contains an offer.  The callee generates a 180 response (2)
   with an answer to that offer.  With the completion of an offer/answer
   exchange, the session is established, although the dialog is still in
   the early state.  The caller generates a PRACK (3) to acknowledge the
   180, and the PRACK is answered with a 200 OK (4).  The caller decides
   to update some aspect of the session - to put it on hold, for
   example.  So, they generate an UPDATE request (5) with a new offer.
   This offer is answered in the 200 response to the UPDATE (6).
   Shortly thereafter, the callee decides to update some aspect of the
   session, so it generates an UPDATE request (7) with an offer, and the
   answer is sent in the 200 response (8).  Finally, the callee answers
   the call, resulting in a 200 OK response to the INVITE (9), and then
   an ACK (10).  Neither the 200 OK to the INVITE, nor the ACK, will
   contain SDP.













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               Header field          where   proxy  UPDATE
               ____________________________________________
               Accept                  R              o
               Accept                 2xx             o
               Accept                 415             c
               Accept-Encoding         R              o
               Accept-Encoding        2xx             o
               Accept-Encoding        415             c
               Accept-Language         R              o
               Accept-Language        2xx             o
               Accept-Language        415             c
               Alert-Info                             -
               Allow                   R              o
               Allow                  2xx             o
               Allow                   r              o
               Allow                  405             m
               Allow-Events           (1)             -
               Authentication-Info    2xx             o
               Authorization           R              o
               Call-ID                 c       r      m
               Call-Info                      ar      o
               Contact                 R              m
               Contact                1xx             o
               Contact                2xx             m
               Contact                3xx      d      o
               Contact                485             o
               Content-Disposition                    o
               Content-Encoding                       o
               Content-Language                       o
               Content-Length                 ar      t
               Content-Type                           *
               CSeq                    c       r      m
               Date                            a      o
               Error-Info           300-699    a      o
               Event                  (1)             -
               Expires                                -
               From                    c       r      m
               In-Reply-To                            -
               Max-Forwards            R      amr     m
               Min-Expires                            -
               MIME-Version                           o
               Organization                   ar      o

   Table 1: Summary of header fields, A--O ; (1) defined in [5].







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           Header field              where       proxy  UPDATE
           ____________________________________________________
           Priority                                       -
           Proxy-Authenticate         407         ar      m
           Proxy-Authenticate         401         ar      o
           Proxy-Authorization         R          dr      o
           Proxy-Require               R          ar      o
           RAck                        R                  -
           Record-Route                R          ar      o
           Record-Route             2xx,18x       mr      o
           Reply-To                                       -
           Require                                ar      c
           Retry-After          404,413,480,486           o
                                    500,503               o
                                    600,603               o
           Route                       R          adr     c
           RSeq                        -                  -
           Server                      r                  o
           Subject                     -                  -
           Subscription-State         (1)                 -
           Supported                   R                  o
           Supported                  2xx                 o
           Timestamp                                      o
           To                          c           r      m
           Unsupported                420                 m
           User-Agent                                     o
           Via                         R          amr     m
           Via                        rc          dr      m
           Warning                     r                  o
           WWW-Authenticate           401         ar      m
           WWW-Authenticate           407         ar      o


   Table 2: Summary of header fields, P--Z.

















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                Caller                        Callee
                   |                             |
                   |                             |
                   |(1) INVITE with offer 1      |
                   |---------------------------->|
                   |                             |
                   |                             |
                   |(2) 180 with answer 1        |
                   |<----------------------------|
                   |                             |
                   |                             |
                   |(3) PRACK                    |
                   |---------------------------->|
                   |                             |
                   |                             |
                   |(4) 200 PRACK                |
                   |<----------------------------|
                   |                             |
                   |                             |
                   |(5) UPDATE with offer 2      |
                   |---------------------------->|
                   |                             |
                   |                             |
                   |(6) 200 UPDATE with answer 2 |
                   |<----------------------------|
                   |                             |
                   |                             |
                   |(7) UPDATE with offer 3      |
                   |<----------------------------|
                   |                             |
                   |                             |
                   |(8) 200 UPDATE with answer 3 |
                   |---------------------------->|
                   |                             |
                   |                             |
                   |(9) 200 INVITE               |
                   |<----------------------------|
                   |                             |
                   |                             |
                   |(10) ACK                     |
                   |---------------------------->|
                   |                             |
                   |                             |
                   |                             |
                   |                             |

                     Figure 1: UPDATE Call Flow




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9 Security Considerations

   The security considerations for UPDATE are identical to those for
   re-INVITE.  It is important that the UPDATE be integrity protected
   and authenticated as coming from the same source as the entity on the
   other end of the dialog.  RFC 3261 [1] discusses security mechanisms
   for achieving these functions.

10 IANA Considerations

   As per Section 27.4 of RFC 3261 [1], this specification serves as a
   registration for the SIP UPDATE request method.  The information to
   be added to the registry is:

      RFC 3311: This specification serves as the RFC for registering
                the UPDATE request method.

      Method Name: UPDATE

      Reason Phrase: Not applicable.

11 Notice Regarding Intellectual Property Rights

      The IETF has been notified of intellectual property rights claimed
      in regard to some or all of the specification contained in this
      document.  For more information consult the online list of claimed
      rights.

12 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.

   [3] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with the
       Session Description Protocol (SDP)", RFC 3264, June 2002.

   [4] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional
       Responses in the Session Initiation Protocol (SIP)", RFC 3262,
       June 2002.

   [5] Roach, A.B., "Session Initiation Protocol (SIP)-Specific Event
       Notification", RFC 3265, June 2002.





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13 Acknowledgements

   The author would like to thank Jo Hornsby, Markus Isomaki, Rohan
   Mahy, and Bob Penfield for their comments.

14 Author's Address

   Jonathan Rosenberg
   dynamicsoft
   72 Eagle Rock Avenue
   First Floor
   East Hanover, NJ 07936

   EMail: jdrosen@dynamicsoft.com





































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15 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.



















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