SIPPING Working Group                                       G. Camarillo
Internet-Draft                                                  Ericsson
Expires: May 18, July 13, 2004                                  January 13, 2004                                  November 18, 2003

     The Early Session Disposition Type for the Session Initiation
                             Protocol (SIP)

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

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Copyright Notice

   Copyright (C) The Internet Society (2003). (2004). All Rights Reserved.


   This document defines a new disposition type (early-session) for the
   Content-Disposition header field in SIP. The treatment of
   "early-session" bodies is similar to the treatment of "session"
   bodies. That is, they follow the offer/answer model. Their only
   difference is that session descriptions whose disposition type is
   "early-session" are used to establish early media sessions within
   early dialogs, as opposed to regular sessions within regular dialogs.

Table of Contents

   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2. Terminology  . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3. Issues Related to Early Media Session Establishment  . . . . . . 3
   4. The Early Session Disposition Type . . . . . . . . . . . . . . . 4
   5. Preconditions  . . . . . . . . . . . . . . . . . . . . . . . . . 5
   6. Option tag . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   7. Example  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   8. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . . 7
   9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 7 8
      Normative References . . . . . . . . . . . . . . . . . . . . . . 8
      Informational References . . . . . . . . . . . . . . . . . . . . 8
      Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
      Intellectual Property and Copyright Statements . . . . . . . . . 9

1. Introduction

   A SIP [2] user agent establishing an INVITE dialog may need to
   exchange media with the destination user agent (or user agents if the
   INVITE forks) or with application servers in the path before the
   dialog is established. Media exchanged this way is referred to as
   early media.

   Section 3 describes the current approach to establish early media
   sessions in SIP and discusses its problems. Section 4 defines the
   "early-session" disposition type to resolve those problems.

2. Terminology

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

3. Issues Related to Early Media Session Establishment

   Traditionally, early media sessions have been established in the same
   way as regular sessions. That is, using an offer/answer exchange
   where the disposition type of the session descriptions is "session".
   Application servers perform an offer/answer exchange with the UAC to
   exchange early media exclusively, while UASs use the same offer/
   answer exchange, first to exchange early media, and once the regular
   dialog is established, to exchange regular media. There are issues
   related to both, application servers and UASs using this mechanism.

   Application servers may not be able to generate an answer for an
   offer received in the INVITE. The UAC created the offer for the UAS,
   and so, it may have applied end-to-end encryption or have included
   information (e.g., related to key management) that the application
   server is not supposed to use. Therefore, application servers need a
   means to perform an offer/answer exchange with the UAC which is
   independent from the offer/answer exchange between both UAs.

   UASs using the offer/answer exchange that will carry regular media to
   send and receive early media can cause media clipping, as described
   in Section 2.1.1 of [6]. Some UACs cannot receive early media from
   different UASs at the same time. So, when an INVITE forks and several
   UASs start sending early media, the UAC mutes all the UASs but one
   (which is usually randomly chosen). If the UAS that accepts the
   INVITE (i.e., sends a 200 OK) was muted, a new offer/answer exchange
   is needed to unmute it. This usually causes media clipping.
   Therefore, UASs need a means to perform an offer/answer exchange with
   the UAC to exchange early media which is independent from the offer/
   answer exchanged used to exchange regular media.

   A potential solution to this need would be to establish a different
   dialog using a globally routable URI to perform an independent offer/
   answer exchange. This dialog would be labelled as a dialog for early
   media and would be related to the original dialog somehow at the UAC.
   However, performing all the offer/answer exchanges within the
   original dialog has many advantages:

      It is simpler.

      It does not have synchronization problems, because all the early
      dialogs are terminated when the session is accepted.

      It does not require globally routable URIs.

      It does not introduce service interaction issues related to
      services that may be wrongly applied to the new dialog.

      It makes firewall management easier.

4. The Early Session Disposition Type

   We define a new disposition type for the Content-Disposition header
   field: early-session. User agents MUST use early-session bodies to
   establish early media sessions in the same way as they use session
   bodies to establish regular sessions, as described in RFC 3261 [2]
   and in RFC 3264 [3]. Particularly, early-session bodies MUST follow
   the offer/answer model and MAY appear in the same messages as session
   bodies do with the exceptions of 2xx responses for an INVITE and
   ACKs. Nevertheless, it is NOT RECOMMENDED to include early offers in
   INVITEs because they can fork, and the UAC could receive multiple
   early answers establishing early media streams at roughly the same
   time. It is also NOT RECOMMENDED to use the same transport address
   (IP address plus port) in a session body and in an early-session
   body. Using different transport addresses (e.g., different ports) to
   receive early and regular media makes it easy to detect the start of
   the regular media.

   If a UA needs to refuse an early-session offer, it MUST to so by
   refusing all the media streams in it. When SDP [5] is used, this is
   done by setting the port number of all the media streams to zero.

      This is the same mechanism that UACs use to refuse regular offers
      that arrive in a response to an empty INVITE.

   An early media session established using early-session bodies MUST be
   terminated when its corresponding early dialog is terminated or it
   transitions to a regular dialog.

   It is RECOMMENDED that UAs generating regular and early session
   descriptions use, as long as it is possible, the same codecs in both.
   This way, the remote UA does not need to change codecs when the early
   session transitions to a regular session.

5. Preconditions

   RFC 3312 [4] defines a framework for preconditions for SDP.
   Early-sessions MAY contain preconditions, which are treated in the
   same way as preconditions in regular sessions. That is, the UAs do
   not exchange media and the called user is not alerted until the
   preconditions are met.

6. Option tag

   We define an option tag to be used in Require and Supported header
   fields. Its name is early-session. A UA adding the early-session
   option tag to a message indicates that it understands the
   early-session disposition type.

7. Example

   Figure 1 shows the message flow between two UAs. INVITE (1) has an
   early-session option tag in its Supported header field and the body
   shown in Figure 2. The UAS sends back a response with two body parts
   ,as shown in Figure 3; one of disposition type session and the other
   early-session. The session body part is the answer to the offer in
   the INVITE. The early-session body part is an offer to establish an
   early media session. When the UAC receives the 183 (Session Progress)
   response, it sends the answer to the early-session offer in a PRACK,
   as shown in Figure 4. This early media session is terminated when the
   early dialog transitions to a regular dialog. That is, when the UAS
   sends the (5) 200 (OK) response for the INVITE.

        A                           B
        |                           |
        |--------(1) INVITE-------->|
        |            offer          |
        |                           |
        |<--(2) Session Progress----|
        |       early-offer         |
        |       answer              |
        |                           |
        |---------(3) PRACK-------->|
        |             early-answer  |
        |                           |
        |<--------(4) 200 OK--------|
        |                           |
        |  *                     *  |
        | ************************* |
        |*       Early Media       *|
        | ************************* |
        |  *                     *  |
        |                           |
        |<--------(5) 200 OK--------|
        |                           |
        |----------(6) ACK--------->|
        |                           |

                         Figure 1: Message flow

   Content-Type: application/sdp
   Content-Disposition: session

   o=alice 2890844730 2890844731 IN IP4
   c=IN IP4
   t=0 0
   m=audio 20000 RTP/AVP 0

                            Figure 2: Offer

   Content-Type: multipart/mixed; boundary="boundary1"
   Content-Length: 401

   Content-Type: application/sdp
   Content-Disposition: session

   o=Bob 2890844725 2890844725 IN IP4
   c=IN IP4
   t=0 0
   m=audio 30000 RTP/AVP 0

   Content-Type: application/sdp
   Content-Disposition: early-session

   o=Bob 2890844714 2890844714 IN IP4
   c=IN IP4
   t=0 0
   m=audio 30002 RTP/AVP 0


                    Figure 3: Early offer and answer

   Content-Type: application/sdp
   Content-Disposition: early-session

   o=alice 2890844717 2890844717 IN IP4
   c=IN IP4
   t=0 0
   m=audio 20002 RTP/AVP 0

                            Figure 4: Answer

8. IANA Considerations

   This document defines a new Content-Disposition header field
   disposition type (early-session) in Section 4. This value should be
   registered in the IANA registry for Content-Dispositions with the
   following description:

         early-session   the body describes an early communications
                         session, for example, an RFC 2327 SDP body

   This document defines a SIP option tag (early-session) in Section 6.
   It should be registered in the SIP parameters registry (http:// under "Option Tags", with
   the following description.

      A UA adding the early-session option tag to a message indicates
      that it understands the early-session content disposition.

9. Acknowledgements

   Francois Audet and Christer Holmberg provided useful comments on this

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

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

Informational References

   [5]  Handley, M. and V. Jacobson, "SDP: Session Description
        Protocol", RFC 2327, April 1998.

   [6]  Camarillo, G. and H. Schulzrinne, "Early Media and Ringback Tone
        Generation in the Session Initiation  Protocol",
        draft-camarillo-sipping-early-media-02 (work in progress), July

Author's Address

   Gonzalo Camarillo
   Hirsalantie 11
   Jorvas  02420


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