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Versions: 00

Internet Engineering Task Force                     Gonzalo Camarillo
Internet draft                                               Ericsson
                                                            July 2000
                                                 Expires January 2000
                                    <draft-camarillo-3pcc-qos-00.txt>


            Third party call control with SDP preconditions


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
   other groups may also distribute working documents as Internet-
   Drafts. Internet-Drafts are draft documents valid for a maximum of
   six months and may be updated, replaced, or obsoleted by other
   documents at any time. It is inappropriate to use Internet- Drafts
   as reference material or to cite them other than as "work in
   progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt
   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.


Abstract

   This document describes how to perform third party call control in
   SIP when SDP preconditions are used. There are no new SIP extensions
   needed to accomplish this. The mechanism outlined is illustrated
   with an example in order to help understand it.

1. Introduction

   In third party call control the entity creating, modifying and
   terminating the session might not be involved in the media exchange.
   The controller uses SIP [1] to create sessions between other
   participants.

   Third party call control in SIP is addressed by [2] already.
   However, sessions that make use of SDP preconditions [3] are not
   taken into account. The next section describes the issues found when
   these two mechanisms are used together.

2. Third party call control and SDP preconditions

   In unicast sessions there is a number of media streams flowing
   between two entities. In order to perform resource reservation it is
   necessary to know the session descriptions from both parties. When

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           Third party call control with SDP preconditions


   third party call control is performed the information needed to
   establish the QoS required is not available from the beginning. The
   following call flow shows how the exchange of SDPs between both
   parties can be performed.


          Controller              A                  B

              |    (1) INVITE     |                  |
              |------------------>|                  |
              |    (2) 183 SDP A  |                  |
              |<------------------|                  |
              | (3) INVITE SDP A  |                  |
              |------------------------------------->|
              |    (4) 183 SDP B  |                  |
              |<-------------------------------------|
              | (5) PRACK SDP B   |                  |
              |------------------>|                  |
              | (6) 200 OK (PRACK)|                  |
              |<------------------|                  |
              |    (7) PRACK      |                  |
              |------------------------------------->|
              | (8) 200 OK (PRACK)|                  |
              |<-------------------------------------|
              |    (9) COMET      |                  |
              |<-------------------------------------|
              |(10) 200 OK (COMET)|                  |
              |------------------------------------->|
              |    (11) COMET     |                  |
              |<------------------|                  |
              |(12) 200 OK (COMET)|                  |
              |------------------>|                  |
              |    (13) COMET     |                  |
              |------------------>|                  |
              |(14) 200 OK (COMET)|                  |
              |<------------------|                  |
              |(15) 200 OK (INVITE)                  |
              |<------------------|                  |
              |    (16) COMET     |                  |
              |------------------------------------->|
              |(17) 200 OK (COMET)|                  |
              |<-------------------------------------|
              |(18) 200 OK (INVITE)                  |
              |<-------------------------------------|
              |    (19) ACK       |                  |
              |------------------>|                  |
              |    (20) ACK       |                  |
              |------------------------------------->|
              |                   |                  |

          Controller              A                  B



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           Third party call control with SDP preconditions


   The controller INVITEs A in (1). At this point of time there is no
   information available about codecs to be used port numbers or IP
   addresses. The SDP of this INVITE just contains SDP preconditions
   and the media stream types (audio, video, etc...).

   INVITE (3) contains the SDP received from A. This INVITE is sent to
   B. When B responses with (4) 183 it is ready to perform resource
   reservation. However, B will not start resource reservation until
   the PRACK (7) is received. This allows BÆs SDP to be sent to A in
   (5). This way both parties have all the information needed to
   perform resource reservation.

   The PRACK matching (2) is sent at (5). This PRACK is not sent before
   because it is used to send BÆs SDP to A. The controller does not get
   this information until (4).

   When the preconditions from B to the controller and from A to the
   controller are met two COMETs are received (9) and (11). At this
   point of time is up to the controller to let the session
   establishment go on sending a COMET to A (13). When A accepts
   joining the session (15), a COMET (16) is sent to B so B is alerted.

3. Example

   This example shows how a transcoding device can be implemented using
   the mechanism described previously. Transcoding devices are
   sometimes needed between two clients that do not have any common
   codec. This is a common situation when clients using a low rate
   access are involved in the communication. A commonly used codec such
   as PCM is not supported by the client because there are not 64 kbps
   available, and the low rate codec used for this concrete access is
   too expensive (IPR issues) to be supported by a normal client.
   Hopefully, a low rate codec that could be implemented "for free"
   will be available for all the clients in a future, making
   transcoding devices unnecessary.

   The mechanism used in this example allows the addition of a
   transcoding device in the media path by the caller. This addition is
   transparent to the callee.

   The transcoding device is modelled as a dial-up bridge in [4]. A
   client INVITEs the transcoding device,
   newsession@transcoder.provider3.com, and the transcoding device
   responses with a URL (in a contact header) that identifies the
   transcoding session: session3371@transcoder.provider3.com

   In this example, there are two media streams: audio and video. The
   audio media stream goes through the transcoding device while the
   video stream goes directly between A and B. This video stream
   illustrates how some media streams can traverse a certain path and
   others can traverse a different one.



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           Third party call control with SDP preconditions


   A wants to establish a session containing audio and video with B. A
   and B do not have common audio codecs. This has been discovered
   before with, for instance, an unsuccessful INVITE. A invokes
   transcoding services from T. In the call flows SDP TA represent TÆs
   SDP between T and A. SDP TB represents TÆs SDP between T and B.

   The transcoder lists all the codecs that it supports in (10) because
   it responses to an INVITE without codec information. In (2) the
   transcoder just lists codec 0 because it is a subset of what A sent
   in the INVITE (1).

   After the establishment of a session between A and T (8), the call
   flow is the same as in the previous section. Therefore, INVITE (1)
   in the previous section corresponds to INVITE (9) in this section.

              A                   T                  B

              | (1) INVITE SDP A  |                  |
              |------------------>|                  |
              |   (2) 183 SDP TA  |                  |
              |<------------------|                  |
              |    (3) PRACK      |                  |
              |------------------>|                  |
              |(4) 200 OK (PRACK) |                  |
              |<------------------|                  |
              |    (5) COMET      |                  |
              |------------------>|                  |
              | (6) 200 OK (COMET)|                  |
              |<------------------|                  |
              |(7) 200 OK (INVITE)|                  |
              |<------------------|                  |
              |     (8) ACK       |                  |
              |<------------------|                  |
              |    (9) INVITE     |                  |
              |------------------>|                  |
              |   (10) 183 SDP TB |                  |
              |<------------------|                  |
              |(11) INVITE SDP TB |                  |
              |------------------------------------->|
              |   (12) 183 SDP B  |                  |
              |<-------------------------------------|
              |(13) PRACK SDP B   |                  |
              |------------------>|                  |
              |(14) 200 OK (PRACK)|                  |
              |<------------------|                  |
              |   (15) PRACK      |                  |
              |------------------------------------->|
              |(16) 200 OK (PRACK)|                  |
              |<-------------------------------------|
              |   (17) COMET      |                  |
              |<-------------------------------------|
              |(18) 200 OK (COMET)|                  |
              |------------------------------------->|

Camarillo                                                            4


           Third party call control with SDP preconditions


              |    (19) COMET     |                  |
              |<------------------|                  |
              |(20) 200 OK (COMET)|                  |
              |------------------>|                  |
              |    (21) COMET     |                  |
              |------------------>|                  |
              |(22) 200 OK (COMET)|                  |
              |<------------------|                  |
              |(23) 200 OK (INVITE)                  |
              |<------------------|                  |
              |    (24) COMET     |                  |
              |------------------------------------->|
              |(25) 200 OK (COMET)|                  |
              |<-------------------------------------|
              |(26) 200 OK (INVITE)                  |
              |<-------------------------------------|
              |    (27) ACK       |                  |
              |------------------>|                  |
              |    (28) ACK       |                  |
              |------------------------------------->|
              |                   |                  |

              A                   T                  B


   Message details

   (1)
      INVITE sip:newsession@transcoder.provider3.com SIP/2.0
      Via: SIP/2.0/UDP ws1234.provider1.com:5060
      Require: 100rel
      From: UserA <sip:UserA@provider1.com>
      To: Transcoder <sip:newsession@transcoder.provider3.com>
      Call-ID: 12345678@ws1234.provider1.com
      CSeq: 1 INVITE
      Contact: UserA <sip:UserA@ws1234.provider1.com>
      Content-Type: application/sdp
      Content-Length: 154

      v=0
      o=UserA 2891234526 2891234526 IN IP4 ws1234.provider1.com
      s=Session SDP
      c=IN IP4 10.0.0.1
      t=0 0
      m=audio 20002 RTP/AVP 0
      a=qos:mandatory sendrecv

   (2)
      SIP/2.0 183 Session Progress
      Via: SIP/2.0/UDP ws1234.provider1.com:5060
      Require: 100rel
      RSeq: 112233
      From: UserA <sip:UserA@provider1.com>

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           Third party call control with SDP preconditions


      To: Transcoder <sip:newsession@transcoder.provider3.com>
   ;tag=314159
      Call-ID: 12345678@ws1234.provider1.com
      CSeq: 1 INVITE
      Contact: Transcoder <sip:session3371@transcoder.provider3.com>

      Content-Type: application/sdp
      Content-Length: 167

      v=0
      o=Transc 2891234526 2891234526 IN IP4 transcoder.provider3.com
      s=Session SDP
      c=IN IP4 10.0.0.3
      t=0 0
      m=audio 30000 RTP/AVP 0
      a=qos:mandatory sendrecv confirm

   (3)
      PRACK sip:session3371@transcoder.provider3.com SIP/2.0
      RAck: 112233 1 INVITE
      Via: SIP/2.0/UDP ws1234.provider1.com:5060
      From: UserA <sip:UserA@provider1.com>
      To: Transcoder <sip:newsession@transcoder.provider3.com>
   ;tag=314159
      Call-ID: 12345678@ws1234.provider1.com
      CSeq: 2 PRACK

   (4)
      SIP/2.0 200 OK
      Via: SIP/2.0/UDP ws1234.provider1.com:5060
      From: UserA <sip:UserA@provider1.com>
      To: Transcoder <sip:newsession@transcoder.provider3.com>
   ;tag=314159
      Call-ID: 12345678@ws1234.provider1.com
      CSeq: 2 PRACK

   (5)
      COMET sip:session3371@transcoder.provider3.com SIP/2.0
      Via: SIP/2.0/UDP ws1234.provider1.com:5060
      From: UserA <sip:UserA@provider1.com>
      To: Transcoder <sip:newsession@transcoder.provider3.com>
   ;tag=314159
      Call-ID: 12345678@ws1234.provider1.com
      CSeq: 3 COMET
      Content-Type: application/sdp
      Content-Length: 152

      v=0
      o=UserA 2891234526 2891234526 IN IP4 ws1234.provider1.com
      s=Session SDP
      c=IN IP4 10.0.0.1
      t=0 0
      m=audio 20002 RTP/AVP 0

Camarillo                                                            6


           Third party call control with SDP preconditions


      a=qos:success sendrecv

   (6)
      SIP/2.0 200 OK
      Via: SIP/2.0/UDP ws1234.provider1.com:5060
      From: UserA <sip:UserA@provider1.com>
      To: Transcoder <sip:newsession@transcoder.provider3.com>
   ;tag=314159
      Call-ID: 12345678@ws1234.provider1.com
      CSeq: 3 COMET

   (7)
      SIP/2.0 200 OK
      Via: SIP/2.0/UDP ws1234.provider1.com:5060
      From: UserA <sip:UserA@provider1.com>
      To: Transcoder <sip:newsession@transcoder.provider3.com>
   ;tag=314159
      Call-ID: 12345678@ws1234.provider1.com
      CSeq: 1 INVITE
      Contact: Transcoder <sip:session3371@transcoder.provider3.com>
      Content-Type: application/sdp
      Content-Length: 133

      v=0
      o=Transc 2891234526 2891234526 IN IP4 transcoder.provider3.com
      s=Session SDP
      c=IN IP4 10.0.0.3
      t=0 0
      m=audio 30000 RTP/AVP 0

   (8)
      ACK sip:session3371@transcoder.provider3.com SIP/2.0
      Via: SIP/2.0/UDP ws1234.provider1.com:5060
      From: UserA <sip:UserA@provider1.com>
      To: Transcoder <sip:newsession@transcoder.provider3.com>
      Call-ID: 12345678@ws1234.provider1.com
      CSeq: 1 ACK
      Contact: UserA <sip:UserA@ws1234.provider1.com>

   (9)
      INVITE sip:session3371@transcoder.provider3.com SIP/2.0
      Via: SIP/2.0/UDP ws1234.provider1.com:5060
      Require: 100rel
      From: UserA <sip:UserA@provider1.com>
      To: Transcoder <sip:newsession@transcoder.provider3.com>
      Call-ID: 87654321@ws1234.provider1.com
      CSeq: 1 INVITE
      Contact: UserA <sip:UserA@ws1234.provider1.com>
      Content-Type: application/sdp
      Content-Length: 156

      v=0
      o=UserA 2891234526 2891234526 IN IP4 ws1234.provider1.com

Camarillo                                                            7


           Third party call control with SDP preconditions


      s=Session SDP
      c=IN IP4 0.0.0.0
      t=0 0
      m=audio 0 RTP/AVP
      a=qos:mandatory sendrecv confirm

   (10)
      SIP/2.0 183 Session Progress
      Via: SIP/2.0/UDP ws1234.provider1.com:5060
      Require: 100rel
      RSeq: 223344
      From: UserA <sip:UserA@provider1.com>
      To: Transcoder <sip:newsession@transcoder.provider3.com>
   ;tag=123456
      Call-ID: 87654321@ws1234.provider1.com
      CSeq: 1 INVITE
      Contact: Transcoder <sip:session3371@transcoder.provider3.com>
      Content-Type: application/sdp
      Content-Length: 169

      v=0
      o=Transc 2891234526 2891234526 IN IP4 transcoder.provider3.com
      s=Session SDP
      c=IN IP4 10.0.0.3
      t=0 0
      m=audio 30002 RTP/AVP 0 8
      a=qos:mandatory sendrecv confirm

   (11)
      INVITE sip:UserB@ws4321.provider2.com SIP/2.0
      Via: SIP/2.0/UDP ws1234.provider1.com:5060
      Require: 100rel
      From: UserA <sip:UserA@provider1.com>
      To: UserB <sip:UserB@provider2.com>
      Call-ID: 11223344@ws1234.provider1.com
      CSeq: 1 INVITE
      Contact: UserA <sip:UserA@ws1234.provider1.com>
      Content-Type: application/sdp
      Content-Length: 268

      v=0
      o=UserA 2891234526 2891234526 IN IP4 ws1234.provider1.com
      s=Session SDP
      t=0 0
      m=audio 30002 RTP/AVP 0 8
      c=IN IP4 10.0.0.3
      a=qos:mandatory sendrecv confirm
      m=video 20000 RTP/AVP 31
      c=IN IP4 10.0.0.1
      a=rtpmap:31 H261/90000
      a=qos:mandatory sendrecv confirm

   (12)

Camarillo                                                            8


           Third party call control with SDP preconditions


      SIP/2.0 183 Session Progress
      Via: SIP/2.0/UDP ws1234.provider1.com:5060
      Require: 100rel
      RSeq: 334455
      From: UserA <sip:UserA@provider1.com>
      To: UserB <sip:UserB@provider2.com> ;tag=112233
      Call-ID: 11223344@ws1234.provider1.com
      CSeq: 1 INVITE
      Contact: UserB <sip:UserB@ws4321.provider2.com>
      Content-Type: application/sdp
      Content-Length: 246

      v=0
      o=UserB 2891234526 2891234526 IN IP4 ws4321.provider2.com
      s=Session SDP
      c=IN IP4 10.0.0.2
      t=0 0
      m=audio 40002 RTP/AVP 8
      a=qos:mandatory sendrecv confirm
      m=video 40000 RTP/AVP 31
      a=rtpmap:31 H261/90000
      a=qos:mandatory sendrecv confirm

   (13)
      PRACK sip:session3371@transcoder.provider3.com SIP/2.0
      RAck: 223344 1 INVITE
      Via: SIP/2.0/UDP ws1234.provider1.com:5060
      From: UserA <sip:UserA@provider1.com>
      To: Transcoder <sip:newsession@transcoder.provider3.com>
   ;tag=123456
      Call-ID: 87654321@ws1234.provider1.com
      CSeq: 2 PRACK
      Content-Type: application/sdp
      Content-Length: 162

      v=0
      o=UserA 2891234526 2891234526 IN IP4 ws1234.provider2.com
      s=Session SDP
      c=IN IP4 10.0.0.2
      t=0 0
      m=audio 40002 RTP/AVP 8
      a=qos:mandatory sendrecv confirm

   (14)
      SIP/2.0 200 OK
      Via: SIP/2.0/UDP ws1234.provider1.com:5060
      From: UserA <sip:UserA@provider1.com>
      To: Transcoder <sip:newsession@transcoder.provider3.com>
   ;tag=123456
      Call-ID: 87654321@ws1234.provider1.com
      CSeq: 2 PRACK

   (15)

Camarillo                                                            9


           Third party call control with SDP preconditions


      PRACK sip:UserB@ws4321.provider2.com SIP/2.0
      RAck: 334455 1 INVITE
      Via: SIP/2.0/UDP ws1234.provider1.com:5060
      From: UserA <sip:UserA@provider1.com>
      To: UserB <sip:UserB@provider2.com> ;tag=112233
      Call-ID: 11223344@ws1234.provider1.com
      CSeq: 2 PRACK

   (16)
      SIP/2.0 200 OK
      Via: SIP/2.0/UDP ws1234.provider1.com:5060
      From: UserA <sip:UserA@provider1.com>
      To: UserB <sip:UserB@provider2.com> ;tag=112233
      Call-ID: 11223344@ws1234.provider1.com
      CSeq: 2 PRACK

   (17)
      COMET sip:UserA@ws1234.provider1.com SIP/2.0
      Via: SIP/2.0/UDP ws4321.provider2.com:5060
      To: UserA <sip:UserA@provider1.com>
      From: UserB <sip:UserB@provider2.com>;tag=112233
      Call-ID: 11223344@ws1234.provider1.com
      CSeq: 1 COMET
      Content-Type: application/sdp
      Content-Length: 226

      v=0
      o=UserB 2891234526 2891234526 IN IP4 ws4321.provider2.com
      s=Session SDP
      c=IN IP4 10.0.0.2
      t=0 0
      m=audio 40002 RTP/AVP 8
      a=qos:success sendrecv
      m=video 40000 RTP/AVP 31
      a=rtpmap:31 H261/90000
      a=qos:success sendrecv


   (18)
      SIP/2.0 200 OK
      Via: SIP/2.0/UDP ws4321.provider2.com:5060
      To: UserA <sip:UserA@provider1.com>
      From: UserB <sip:UserB@provider2.com>;tag=112233
      Call-ID: 11223344@ws1234.provider1.com
      CSeq: 1 COMET

   (19)
      COMET sip:UserA@ws1234.provider1.com SIP/2.0
      Via: SIP/2.0/UDP transcoder.provider3.com:5060
      To: UserA <sip:UserA@provider1.com>
      From: Transcoder <sip:newsession@transcoder.provider3.com>
   ;tag=123456
      Call-ID: 87654321@ws1234.provider1.com

Camarillo                                                           10


           Third party call control with SDP preconditions


      CSeq: 1 COMET
      Content-Type: application/sdp
      Content-Length: 159

      v=0
      o=Transc 2891234526 2891234526 IN IP4 transcoder.provider3.com
      s=Session SDP
      c=IN IP4 10.0.0.3
      t=0 0
      m=audio 30002 RTP/AVP 0 8
      a=qos:success sendrecv

   (20)
      SIP/2.0 200 OK
      Via: SIP/2.0/UDP transcoder.provider3.com:5060
      To: UserA <sip:UserA@provider1.com>
      From: Transcoder <sip:newsession@transcoder.provider3.com>
   ;tag=123456
      Call-ID: 87654321@ws1234.provider1.com
      CSeq: 1 COMET

   (21)
      COMET sip:session3371@transcoder.provider3.com SIP/2.0
      Via: SIP/2.0/UDP ws1234.provider1.com:5060
      From: UserA <sip:UserA@provider1.com>
      To: Transcoder <sip:newsession@transcoder.provider3.com>
   ;tag=123456
      Call-ID: 87654321@ws1234.provider1.com
      CSeq: 3 COMET
      Content-Type: application/sdp
      Content-Length: 152

      v=0
      o=UserA 2891234526 2891234526 IN IP4 ws1234.provider2.com
      s=Session SDP
      c=IN IP4 10.0.0.2
      t=0 0
      m=audio 40002 RTP/AVP 8
      a=qos:success sendrecv

   (22)
      SIP/2.0 200 OK
      Via: SIP/2.0/UDP ws1234.provider1.com:5060
      From: UserA <sip:UserA@provider1.com>
      To: Transcoder <sip:newsession@transcoder.provider3.com>
   ;tag=123456
      Call-ID: 87654321@ws1234.provider1.com
      CSeq: 3 COMET

   (23)
      SIP/2.0 200 OK
      Via: SIP/2.0/UDP ws1234.provider1.com:5060
      From: UserA <sip:UserA@provider1.com>

Camarillo                                                           11


           Third party call control with SDP preconditions


      To: Transcoder <sip:newsession@transcoder.provider3.com>
   ;tag=123456
      Call-ID: 87654321@ws1234.provider1.com
      CSeq: 1 INVITE
      Contact: Transcoder <sip:session3371@transcoder.provider3.com>
      Content-Type: application/sdp
      Content-Length: 135

      v=0
      o=Transc 2891234526 2891234526 IN IP4 transcoder.provider3.com
      s=Session SDP
      c=IN IP4 10.0.0.3
      t=0 0
      m=audio 30002 RTP/AVP 0 8

   (24)
      COMET sip:UserB@ws4321.provider2.com SIP/2.0
      Via: SIP/2.0/UDP ws1234.provider1.com:5060
      From: UserA <sip:UserA@provider1.com> ;tag=112233
      To: UserB <sip:UserB@provider2.com>
      Call-ID: 11223344@ws1234.provider1.com
      CSeq: 3 COMET
      Content-Type: application/sdp
      Content-Length: 248

      v=0
      o=UserA 2891234526 2891234526 IN IP4 ws1234.provider1.com
      s=Session SDP
      t=0 0
      m=audio 30002 RTP/AVP 0 8
      c=IN IP4 10.0.0.3
      a=qos:success sendrecv
      m=video 20000 RTP/AVP 31
      c=IN IP4 10.0.0.1
      a=rtpmap:31 H261/90000
      a=qos:success sendrecv

   (25)
      SIP/2.0 200 OK
      Via: SIP/2.0/UDP ws1234.provider1.com:5060
      From: UserA <sip:UserA@provider1.com> ;tag=112233
      To: UserB <sip:UserB@provider2.com>
      Call-ID: 11223344@ws1234.provider1.com
      CSeq: 3 COMET

   (26)
      SIP/2.0 200 OK
      Via: SIP/2.0/UDP ws1234.provider1.com:5060
      From: UserA <sip:UserA@provider1.com>
      To: UserB <sip:UserB@provider2.com> ;tag=112233
      Call-ID: 11223344@ws1234.provider1.com
      CSeq: 1 INVITE
      Contact: UserB <sip:UserB@ws4321.provider2.com>

Camarillo                                                           12


           Third party call control with SDP preconditions


      Content-Type: application/sdp
      Content-Length: 178

      v=0
      o=UserB 2891234526 2891234526 IN IP4 ws4321.provider2.com
      s=Session SDP
      c=IN IP4 10.0.0.2
      t=0 0
      m=audio 40002 RTP/AVP 8
      m=video 40000 RTP/AVP 31
      a=rtpmap:31 H261/90000

   (27)
      ACK sip:session3371@transcoder.provider3.com SIP/2.0
      Via: SIP/2.0/UDP ws1234.provider1.com:5060
      From: UserA <sip:UserA@provider1.com>
      To: Transcoder <sip:newsession@transcoder.provider3.com>
   ;tag=123456
      Call-ID: 87654321@ws1234.provider1.com
      CSeq: 1 ACK

   (28)
      ACK sip:UserB@ws4321.provider2.com SIP/2.0
      Via: SIP/2.0/UDP ws1234.provider1.com:5060
      From: UserA <sip:UserA@provider1.com>
      To: UserB <sip:UserB@provider2.com> ;tag=112233
      Call-ID: 11223344@ws1234.provider1.com
      CSeq: 1 ACK


4. Open issues

   This document uses the following SDP format in an INVITE to indicate
   that preconditions are needed without providing a session
   description:

      v=0
      o=UserA 2891234526 2891234526 IN IP4 ws1234.provider1.com
      s=Session SDP
      c=IN IP4 0.0.0.0
      t=0 0
      m=audio 0 RTP/AVP
      a=qos:mandatory sendrecv confirm

   IP address, port number and codec type are not provided.

   It is assumed that the if the caller includes SDP preconditions in
   the INVITE, the callee requests confirmation of preconditions met
   (COMET from caller to callee) in the 183 response. However, the
   callee might rely on other mechanisms such as RSVP ResvConf messages
   to confirm that there are available resources for the session. A
   means in the INVITE for forcing the callee to ask for COMET from the
   caller is needed.

Camarillo                                                           13


           Third party call control with SDP preconditions



   The usage of the direction-tag in COMET indicating success is not
   clear in [3]. This document uses always "a=qos:success sendrecv".

5. Acknowledgements

   The author would like to thank Jan Holler for his input on this
   document.

6. References

   [1] M. Handley/H. Schulzrinne/E. Schooler/J. Rosenberg, "SIP:
   Session Initiation Protocol", RFC 2543, IETF; Mach 1999.

   [2] J. Rosenberg/J. Peterson/H. Schulzrinne, "Third Party Call
   Control in SIP", draft-rosenberg-sip-3pcc-00.txt, IETF; March 2000.
   Work in progress.

   [3] W. Marshall et al, "Integration of Resource Management and SIP
   ", draft-manyfolks-sip-resource-01.txt, IETF; June 2000. Work in
   progress.

   [4] H. Schulzrinne/J. Rosenberg, "SIP Call Control Services", draft-
   ietf-mmusic-sip-cc-01.txt, IETF; June 1999. Expired draft.


7. Author's Address

   Gonzalo Camarillo
   Ericsson
   Advanced Signalling Research Lab.
   FIN-02420 Jorvas
   Finland
   Phone: +358 9 299 3371
   Fax: +358 9 299 3052
   Email: Gonzalo.Camarillo@ericsson.com


















Camarillo                                                           14


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