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rfc:rfc3311

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.

Rosenberg Standards Track [Page 1] RFC 3311 SIP UPDATE Method September 2002

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.

Rosenberg Standards Track [Page 2] RFC 3311 SIP UPDATE Method September 2002

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.

Rosenberg Standards Track [Page 3] RFC 3311 SIP UPDATE Method September 2002

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.

Rosenberg Standards Track [Page 4] RFC 3311 SIP UPDATE Method September 2002

    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

Rosenberg Standards Track [Page 5] RFC 3311 SIP UPDATE Method September 2002

 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.

Rosenberg Standards Track [Page 6] RFC 3311 SIP UPDATE Method September 2002

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.

Rosenberg Standards Track [Page 7] RFC 3311 SIP UPDATE Method September 2002

             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].

Rosenberg Standards Track [Page 8] RFC 3311 SIP UPDATE Method September 2002

         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.

Rosenberg Standards Track [Page 9] RFC 3311 SIP UPDATE Method September 2002

              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

Rosenberg Standards Track [Page 10] RFC 3311 SIP UPDATE Method September 2002

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.

Rosenberg Standards Track [Page 11] RFC 3311 SIP UPDATE Method September 2002

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

Rosenberg Standards Track [Page 12] RFC 3311 SIP UPDATE Method September 2002

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.

Rosenberg Standards Track [Page 13]

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