SIP Preconditions Update: Update to the Session Initiation Protocol (SIP) Preconditions Framework

Network Working Group                                       G. Camarillo
Request for Comments: 4032                                      Ericsson
Updates: 3312                                                 P. Kyzivat
Category: Standards Track                                  Cisco Systems
                                                              March 2005


            Update to the Session Initiation Protocol (SIP)
                        Preconditions Framework

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 (2005).

Abstract

   This document updates RFC 3312, which defines the framework for
   preconditions in SIP.  We provide guidelines for authors of new
   precondition types and describe how to use SIP preconditions in
   situations that involve session mobility.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  2
   3.  Defining New Precondition Types  . . . . . . . . . . . . . . .  3
       3.1.  Precondition Type Tag  . . . . . . . . . . . . . . . . .  3
       3.2.  Status Type  . . . . . . . . . . . . . . . . . . . . . .  3
       3.3.  Precondition Strength  . . . . . . . . . . . . . . . . .  3
       3.4.  Suspending and Resuming Session Establishment  . . . . .  3
   4.  Issues Related to Session Mobility . . . . . . . . . . . . . .  4
       4.1.  Update to RFC 3312 . . . . . . . . . . . . . . . . . . .  5
       4.2.  Desired Status . . . . . . . . . . . . . . . . . . . . .  7
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  8
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
       8.1.  Normative References . . . . . . . . . . . . . . . . . .  8
       8.2.  Informational References . . . . . . . . . . . . . . . .  8
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  9
   Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 10

1.  Introduction

   RFC 3312 [3] defines the framework for SIP [2] preconditions, which
   is a generic framework that allows SIP UAs (User Agents) to suspend
   the establishment of a session until a set of preconditions are met.
   Although only Quality of Service (QoS) preconditions have been
   defined so far, this framework supports different types of
   preconditions.  (QoS preconditions are defined by RFC 3312 as well).

   This document updates RFC 3312,  provides guidelines for authors of
   new precondition types and explains which topics they need to discuss
   when defining them.  In addition, it updates some of the procedures
   in RFC 3312 for using SIP preconditions in situations that involve
   session mobility as described below.

   RFC 3312 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.  Nevertheless, 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 can
   probably handle session mobility, there is a need to explicitly point
   out the issues involved and make a slight update on some of the
   procedures defined there in.  With the updated procedures defined in
   this document, messages carrying precondition information become more
   explicit about the current status of the preconditions.

   Specifically, we now allow answers to downgrade current status values
   (this was disallowed by RFC 3312).  We consider moving an existing
   stream to a new location as equivalent to establishing a new stream.
   Therefore, answers moving streams to new locations set all the
   current status values in their answers to "No" and start a new
   precondition negotiation from scratch.

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
   described in BCP 14, RFC 2119 [1] and indicate requirement levels for
   compliant implementations.

3.  Defining New Precondition Types

   Specifications defining new precondition types need to discuss the
   topics described in this section.  Having clear definitions of new
   precondition types is essential to ensure interoperability among
   different implementations.

3.1.  Precondition Type Tag

   New precondition types MUST have an associated precondition type tag
   (e.g., "qos" is the tag for QoS preconditions).  Authors of new
   preconditions MUST register new precondition types and their tags
   with the IANA by following the instructions in Section 15 of RFC
   3312.

3.2.  Status Type

   RFC 3312 defines two status types: end-to-end and segmented.
   Specifications defining new precondition types MUST indicate which
   status applies to the new precondition.  New preconditions can use
   only one status type or both.  For example, the QoS preconditions
   defined in RFC 3312 can use both.

3.3.  Precondition Strength

   RFC 3312 defines optional and mandatory preconditions.
   Specifications defining new precondition types MUST describe whether
   or not optional preconditions are applicable, and in case they are,
   what is the expected behavior of a UA on reception of optional
   preconditions.

3.4.  Suspending and Resuming Session Establishment

   Section 6 of RFC 3312 describes the behavior of UAs from the moment
   session establishment is suspended, due to a set of preconditions,
   until it is resumed when these preconditions are met.  In general,
   the called user is not alerted until the preconditions are met.

   In addition to not alerting the user, each precondition type MUST
   define any extra actions UAs should perform or refrain from
   performing when session establishment is suspended.  The behavior of
   media streams during session suspension is therefore part of the
   definition of a particular precondition type.  Some precondition
   types may allow media streams to send and receive packets during
   session suspension; others may not.  Consequently, the following
   paragraph from RFC 3312 only applies to QoS preconditions:

      While session establishment is suspended, user agents SHOULD not
      send any data over any media stream.  In the case of RTP, neither
      RTP nor RTCP packets are sent.

   To clarify the previous paragraph, the control messages used to
   establish connections in connection-oriented transport protocols
   (e.g., TCP SYNs) are not affected by the previous rule.  So, user
   agents follow standard rules (e.g., the SDP 'setup' attribute [7]) to
   decide when to establish the connection, regardless of QoS
   preconditions.

   New precondition types MUST also describe the behaviour of UAs on
   reception of a re-INVITE or an UPDATE with preconditions for an
   ongoing session.

4.  Issues Related to Session Mobility

   Section 5 of RFC 3312 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 (User Agent Client) informs the UAS (User Agent Server) 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 allows updating current status information using the
   methods described above, it does not allow downgrading current status
   values in answers, as shown in the third row of Table 3 of RFC 3312.
   Figure 1 shows how performing such a downgrade in an answer would
   sometimes 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 (Third Party Call Control) [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".  After contacting C
   establishing dialog 3, the controller 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.

4.1.  Update to RFC 3312

   Below is a set of new rules that update RFC 3312 to address the
   issues above.

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

   When a stream is being moved to a new transport address, the offerer
   MUST set all current status values about which 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
   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 rules below apply to an answerer that is moving
   a media stream.  The offerer was not aware of the move when it
   generated the offer.

   Table 3 of RFC 3312 needs to be updated to allow answerers to
   downgrade current status values.  The following table 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

   An answerer MUST downgrade the current status values 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.

   Offerers should update their local status tables when they receive an
   answer as shown in the following table.


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

4.2.  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 (i.e., 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.)

5.  Security Considerations

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

   Thus, it is 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].

6.  IANA Considerations

   The IANA registration requirements for the preconditions framework
   are defined in RFC 3312.  Any new preconditions are governed by the
   IANA Considerations there.

7.  Acknowledgement

   Dave Oran and Allison Mankin provided useful comments on this
   document.

8.  References

8.1.  Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [2]  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.

   [3]  Camarillo, G., Marshall, W., and J. Rosenberg, "Integration of
        Resource Management and Session Initiation Protocol (SIP)", RFC
        3312, October 2002.

8.2.  Informational References

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

   [5]  Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
        Method", RFC 3311, October 2002.

   [6]  Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo,
        "Best Current Practices for Third Party Call Control (3pcc) in
        the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April
        2004.

   [7]  Yon, D. and Camarillo, G., "TCP-Based Media Transport in the
        Session Description Protocol (SDP)", Work In Progress, November
        2004.

Authors' Addresses

   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   EMail: Gonzalo.Camarillo@ericsson.com


   Paul Kyzivat
   Cisco Systems
   1414 Massachusetts Avenue, BXB500 C2-2
   Boxborough, MA  01719
   USA

   EMail: pkyzivat@cisco.com

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