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Versions: (draft-camarillo-sip-rfc3312-update) 00 01 02 03 RFC 4032

Internet Engineering Task Force                                   SIP WG
Internet Draft                                              G. Camarillo
                                                                Ericsson
                                                              P. Kyzivat
                                                                   Cisco
draft-ietf-sip-rfc3312-update-00.txt
November 19, 2003
Expires: May, 2004


            Interactions of Preconditions with Session
         Mobility in the Session Initiation Protocol (SIP)

STATUS OF THIS MEMO

   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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Abstract

   This document describes how to use SIP preconditions in situations
   that involve session mobility. This document updates RFC3312, which
   defines the framework for SIP preconditions.












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                           Table of Contents



   1          Introduction ........................................    3
   2          Terminology .........................................    3
   3          Issues Related to Session Mobility ..................    3
   4          Update to RFC 3312 ..................................    4
   5          Desired Status ......................................    6
   6          Security Considerations .............................    6
   7          Authors' Addresses ..................................    6
   8          Normative References ................................    7
   9          Informative References ..............................    7



































G. Camarillo et. al.                                          [Page 2]


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1 Introduction

   RFC 3312 [1] defines the framework for SIP [2] preconditions and
   focuses on media sessions that do not move around. That is, media is
   sent between the same end-points throughout the duration of the
   session.

   However, media sessions established by SIP are not always static. SIP
   offers mechanisms to provide session mobility, namely re-INVITEs and
   UPDATEs  [5]. While existing implementations of RFC 3312 [1] can
   probably handle session mobility, there is a need to explicitly point
   out the issues involved and make a slight update to some of the
   procedures defined there. With the updated procedures defined in this
   document, messages carrying precondition information become more
   explicit about the current status of the preconditions.

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 RFC 2119 [3].

3 Issues Related to Session Mobility

   Section 5 of RFC 3312 [1] describes how to use SIP [2] preconditions
   with the offer/answer model [4]. RFC 3312 gives a set of rules that
   allow a user agent to communicate changes in the current status of
   the preconditions to the remote user agent.

   The idea is that a given user agent knows about the current status of
   some part of the preconditions (e.g., send direction of the QoS
   precondition) through local information (e.g., an RSVP RESV is
   received indicating that resource reservation was successful). The
   UAC informs the UAS about changes in the current status by sending an
   offer to the UAS. The UAS, in turn, could (if needed) send an offer
   to the UAC informing it about the status of the part of the
   preconditions the UAS has local information about.

        Note, however, that UASs do not usually send updates about
        the current status to the UAC because UASs are the ones
        resuming session establishment when all the preconditions
        are met. Therefore, rather than performing an offer/answer
        exchange to inform the UAC that all the preconditions are
        met, they simply send a 180 (Ringing) response indicating
        that session establishment has been resumed.

   While RFC 3312 [1] allows to update current status information using
   offers as described above, it does not allow to downgrade current



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   status values in answers, as shown in the third row of Table 3 of RFC
   3312. However, such downgrades are sometimes needed. Figure 1 shows
   an example where performing such a downgrade in an answer would be
   needed.



                    3pcc
         A       controller        B        C

         |            |            |        |
         |<-dialog 1->|<-dialog 2->|        |
         |            |            |        |
         | *********************** |        |
         |*         MEDIA         *|        |
         | *********************** |        |
         |            |            |        |
         |            |            |        |
         |<-dialog 1->|<------dialog 3----->|
         |            |            |        |
         | ******************************** |
         |*             MEDIA              *|
         | ******************************** |
         |            |            |        |
         |            |            |        |



   Figure 1: Session Mobility using 3pcc



   The 3pcc  [6] controller in Figure 1 has established a session
   between A and B using dialog 1 towards A and dialog 2 towards B. At
   that point, the controller wants A to have a session with C instead
   of B. To transfer A to C (configuration shown at the bottom of Figure
   1), the controller sends an empty (no offer) re-INVITE to A. Since A
   does not know that the session will be moved, its offer in the 200 OK
   states that the current status of the media stream in the send
   direction is "Yes". The controller, after contacting C establishing
   dialog 3, sends back an answer to A. This answer contains a new
   destination for the media (C) and should have downgraded the current
   status of the media stream to "No", since there is no reservation of
   resources between A and C.

4 Update to RFC 3312

   Below there are a set of new rules that update RFC 3312 [1] to
   address the issues above.



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   The rule below applies to offerers that are moving a media stream to
   a new address:

   When a stream is being moved to a new transport address, the offerer
   MUST set all the current status values it does not have local
   information about to "No".

   Note that for streams using segmented status (as opposed to end-to-
   end status), the fact that the address for the media stream at the
   local segment changes may or may not affect the status of the
   preconditions at the remote segment. However, moving an existing
   stream to a new location, from the preconditions point of view, is
   like establishing a new stream. Therefore, it is appropriate to set
   all the current status values to "No" and start a new precondition
   negotiation from scratch.

   The updated table and the rules below applies to an answerer that is
   moving a media stream. That is, the offerer was not aware of the move
   when it generated the offer.

   Table 3 of RFC 3312 [1] needs to be updated to allow answers to
   downgrade current status values. Table 1 below shows the result.


   Transac. status table  Local status table  New values transac./local
   ____________________________________________________________________
            no                    no                    no/no
            yes                  yes                   yes/yes
            yes                   no            depends on local info
            no                   yes            depends on local info


   Table 1: Possible values for the "Current" fields


   An answerer MUST downgrade the current status values that received in
   the offer if it has local information about them or if the media
   stream is being moved to a new transport address.

   Note that for streams using segmented status the address change at
   the answerer may or may not affect the status of the preconditions at
   the offerer's segment. However, as stated above, moving an existing
   stream to a new location, from the preconditions point of view, is
   like establishing a new stream. Therefore, it is appropriate to set
   all the current status values to "No" and start a new precondition
   negotiation from scratch.

   The new table below applies to an offerer that receives an answer
   that updates or downgrades its local status tables.


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   Offerers should update their local status tables when they receive an
   answer as shown in Table 2.


     Transac. status table  Local status table  New value Local Status
     _________________________________________________________________
              no                    no                    no
              yes                  yes                   yes
              yes                   no                   yes
              no                   yes                    no


   Table 2: Possible values for the "Current" fields after an answer


5 Desired Status

   The desired status that a UA wants for a media stream after the
   stream is moved to a new transport address may be different than the
   desired status negotiated for the stream originally. A UA, for
   instance, may require mandatory QoS over a low-bandwidth link but be
   satisfied with optional QoS when the stream is moved to a high-
   bandwidth link.

   If the new desired status is higher than the previous one (e.g.,
   optional to mandatory), the UA, following RFC 3312 procedures, may
   upgrade its desired status in an offer or in an answer. If the new
   desired status is lower that the previous one (e.g., mandatory to
   optional), the UA, following RFC 3312 procedures as well, may
   downgrade its desired status only in an offer (i.e., not in an
   answer.)

6 Security Considerations

   An attacker adding preconditions to a session description or
   modifying existing preconditions could keep sessions from being
   established. An attacker removing preconditions from a session
   description could force sessions to be established without meeting
   mandatory preconditions.

   It is thus STRONGLY RECOMMENDED that integrity protection be applied
   to the SDP session descriptions. S/MIME is the natural choice to
   provide such end-to-end integrity protection, as described in RFC
   3261 [2].

7 Authors' Addresses

   Gonzalo Camarillo
   Ericsson


G. Camarillo et. al.                                          [Page 6]


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

   Paul Kyzivat
   Cisco Systems
   1414 Massachusetts Avenue, BXB500 C2-2
   Boxborough, MA 01719
   USA
   electronic mail:  pkyzivat@cisco.com

8 Normative References

   [1] "Integration of resource management and session initiation
   protocol (SIP)," RFC 3312, Internet Engineering Task Force, Oct.
   2002.

   [2] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. R. Johnston, J.
   Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: session
   initiation protocol," RFC 3261, Internet Engineering Task Force, June
   2002.

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

   [4] J. Rosenberg and H. Schulzrinne, "An offer/answer model with
   session description protocol (SDP)," RFC 3264, Internet Engineering
   Task Force, June 2002.

9 Informative References

   [5] J. Rosenberg, "The session initiation protocol (SIP) UPDATE
   method," RFC 3311, Internet Engineering Task Force, Oct. 2002.

   [6] J. Rosenberg, J. L. Peterson, H. Schulzrinne, and G. Camarillo,
   "Best current practices for third party call control in the session
   initiation protocol," Internet Draft draft-ietf-sipping-3pcc-05,
   Internet Engineering Task Force, Oct. 2003.  Work in progress.



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