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Versions: (draft-johnston-mmusic-offer-answer-examples) 00 01 02 03 04 05 06 RFC 4317

MMUSIC Working Group                                         A. Johnston
Internet-Draft                                                       MCI
Expires: December 27, 2003                                     R. Sparks
                                                             dynamicsoft
                                                           June 28, 2003


           Session Description Protocol Offer Answer Examples
               draft-ietf-mmusic-offer-answer-examples-01

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.

   This Internet-Draft will expire on December 27, 2003.

Copyright Notice

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

Abstract

   This document gives examples of Session Description Protocol (SDP)
   offer answer exchanges.  Examples include codec negotiation and
   selection, hold and resume, and addition and deletion of media
   streams.  The examples show multiple media types, bidirectional,
   unidirectional, inactive streams and dynamic payload types. Common
   Third Party Call Control (3pcc) examples are also given.








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Table of Contents

   1.   Overview . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   Codec Negotiation and Selection  . . . . . . . . . . . . . .   3
   2.1  Audio and Video 1  . . . . . . . . . . . . . . . . . . . . .   3
   2.2  Audio and Video 2  . . . . . . . . . . . . . . . . . . . . .   4
   2.3  Audio and Video 3  . . . . . . . . . . . . . . . . . . . . .   5
   2.4  Two Audio Steams . . . . . . . . . . . . . . . . . . . . . .   6
   2.5  Audio and Video 4  . . . . . . . . . . . . . . . . . . . . .   7
   2.6  Audio Only 1 . . . . . . . . . . . . . . . . . . . . . . . .   8
   2.7  Audio and Video 5  . . . . . . . . . . . . . . . . . . . . .   9
   2.8  Audio and Video 6  . . . . . . . . . . . . . . . . . . . . .  10
   2.9  Asymmetric Codec Types . . . . . . . . . . . . . . . . . . .  11
   2.10 Asymmetric Attribute Values  . . . . . . . . . . . . . . . .  12
   3.   Hold and Resume Scenarios  . . . . . . . . . . . . . . . . .  13
   3.1  Hold and Unhold 1  . . . . . . . . . . . . . . . . . . . . .  13
   3.2  Hold with Two Streams  . . . . . . . . . . . . . . . . . . .  14
   4.   Addition and Deletion of Media Streams . . . . . . . . . . .  16
   4.1  Second Audio Stream Added  . . . . . . . . . . . . . . . . .  16
   4.2  Audio then Video Added . . . . . . . . . . . . . . . . . . .  17
   4.3  Audio and Video, then Video Deleted  . . . . . . . . . . . .  18
   5.   Third Party Call Control (3pcc)  . . . . . . . . . . . . . .  19
   5.1  No Media, then Audio Added . . . . . . . . . . . . . . . . .  20
   5.2  Hold and Unhold 2  . . . . . . . . . . . . . . . . . . . . .  20
   5.3  Hold and Unhold 3  . . . . . . . . . . . . . . . . . . . . .  22
   6.   Security Considerations  . . . . . . . . . . . . . . . . . .  23
   7.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  23
   8.   Changes since -00  . . . . . . . . . . . . . . . . . . . . .  23
        Informative References . . . . . . . . . . . . . . . . . . .  23
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  24
        Intellectual Property and Copyright Statements . . . . . . .  25




















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1. Overview

   This document describes offer answer examples of Session Description
   Protocol (SDP) based on RFC 3264 [1].  The SDP in these examples are
   defined by  RFC 2327 [2].  The offers and answers are assumed to be
   transported using a protocol such as Session Initiation Protocol
   (SIP) [3].

   Examples include codec negotiation and selection, hold and resume,
   and addition and deletion of media streams.  The examples show
   multiple media types, bidirectional, unidirectional, inactive streams
   and dynamic payload types.   Common Third Party Call Control (3pcc)
   [5] examples are also given.

   The following sections contain examples in which two parties, Alice
   and Bob, exchange SDP offers, answers, and, in some cases, additional
   offers and answers.

2. Codec Negotiation and Selection

2.1 Audio and Video 1

   This common scenario shows a video and audio session in which
   multiple codecs are offered but only one is accepted.  As a result of
   the exchange shown below, Alice and Bob may send only PCMU audio and
   MPV video.  Note: Dynamic payload type 97 is used for iLBC codec [6].



    [Offer]

      v=0
      o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 0 8 97
      a=rtpmap:0 PCMU/8000
      a=rtpmap:8 PCMA/8000
      a=rtpmap:97 iLBC
      m=video 51372 RTP/AVP 31 32
      a=rtpmap:31 H261/90000
      a=rtpmap:32 MPV/90000

   [Answer]

      v=0
      o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com



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      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49174 RTP/AVP 0
      a=rtpmap:0 PCMU/8000
      m=video 49170 RTP/AVP 32
      a=rtpmap:32 MPV/90000



2.2 Audio and Video 2

   Alice can support PCMU, PCMA, and iLBC codecs, but not more than one
   at the same time.  Alice offers all three to maximize chances of a
   successful exchange and Bob accepts two of them.  Audio only session
   is established in initial exchange between Alice and Bob using either
   PCMU or PCMA codecs (payload type in RTP packet tells which is being
   used).  Since Alice only supports one audio codec at a time, a second
   offer is made with just that one codec to limit the codec choice to
   just one.

   Note: the version number is incremented in both SDP messages in the
   second exchange.  Now only the PCMU codec may be used for media
   session between Alice and Bob.

   Note: The declined video stream still present in the second exchange
   of SDP with ports set to zero.



   [Offer]

      v=0
      o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 0 8 97
      a=rtpmap:0 PCMU/8000
      a=rtpmap:8 PCMA/8000
      a=rtpmap:97 iLBC
      m=video 51372 RTP/AVP 31 32
      a=rtpmap:31 H261/90000
      a=rtpmap:32 MPV/90000

   [Answer]

      v=0



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      o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49172 RTP/AVP 0 8
      a=rtpmap:0 PCMU/8000
      a=rtpmap:8 PCMA/8000
      m=video 0 RTP/AVP 31
      a=rtpmap:31 H261/90000

    [Second-Offer]

      v=0
      o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 51372 RTP/AVP 0
      a=rtpmap:0 PCMU/8000
      m=video 0 RTP/AVP 31
      a=rtpmap:31 H261/90000

   [Second-Answer]

      v=0
      o=bob 2808844564 2808844565 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49172 RTP/AVP 0
      a=rtpmap:0 PCMU/8000
      m=video 0 RTP/AVP 31
      a=rtpmap:31 H261/90000



2.3 Audio and Video 3

   Alice offers three audio and two video codecs, while Bob accepts with
   a single audio and video codec.  As a result of this exchange, Bob
   and Alice use iLBC for audio and H261 for video.

   Note: change of dynamic payload type from 97 to 99 between the offer
   and the answer is OK since it references same codec.


    [Offer]




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      v=0
      o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 0 8 97
      a=rtpmap:0 PCMU/8000
      a=rtpmap:8 PCMA/8000
      a=rtpmap:97 iLBC
      m=video 51372 RTP/AVP 31 32
      a=rtpmap:31 H261/90000
      a=rtpmap:32 MPV/90000

   [Answer]

      v=0
      o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49172 RTP/AVP 99
      a=rtpmap:99 iLBC
      m=video 51374 RTP/AVP 31
      a=rtpmap:31 H261/90000



2.4 Two Audio Steams

   Alice offers two separate streams, one audio with two codecs and the
   other with RFC 2833 [4] tones (for DTMF).  Bob accepts both audio
   streams choosing the iLBC codec and telephone-events.


    [Offer]

      v=0
      o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 0 97
      a=rtpmap:0 PCMU/8000
      a=rtpmap:97 iLBC
      m=audio 49172 RTP/AVP 98
      a=rtpmap:98 telephone-event
      a=sendonly




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   [Answer]

      v=0
      o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49172 RTP/AVP 97
      a=rtpmap:97 iLBC
      m=audio 49174 RTP/AVP 98
      a=rtpmap:98 telephone-event
      a=recvonly



2.5 Audio and Video 4

   Alice and Bob establish an audio and video session with a single
   audio and video codec.  In a second exchange, Bob changes his address
   for media and Alice accepts with the same SDP as the initial exchange
   (and as a result does not increment the version number).

    [Offer]

      v=0
      o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 97
      a=rtpmap:97 iLBC
      m=video 51372 RTP/AVP 31
      a=rtpmap:31 H261/90000

   [Answer]

      v=0
      o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49174 RTP/AVP 97
      a=rtpmap:97 iLBC
      m=video 49170 RTP/AVP 31
      a=rtpmap:31 H261/90000

   [Second-Offer]




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      v=0
      o=bob 2808844564 2808844565 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 newhost.biloxi.example.com
      t=0 0
      m=audio 49178 RTP/AVP 97
      a=rtpmap:97 iLBC
      m=video 49188 RTP/AVP 31
      a=rtpmap:31 H261/90000

   [Second-Answer]

      v=0
      o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 97
      a=rtpmap:97 iLBC
      m=video 51372 RTP/AVP 31
      a=rtpmap:31 H261/90000



2.6 Audio Only 1

   Alice wishes to establish an audio session with Bob using either PCMU
   codec or iLBC codec with RFC2833 tones, but not both at the same
   time.  The offer contains these two media streams.  Bob declines the
   first one and accepts the second one.  If both media streams had been
   accepted, Alice would have sent a second declining one of the
   streams, as shown in Section 4.3.



   [Offer]

      v=0
      o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 0
      a=rtpmap:0 PCMU/8000
      m=audio 51372 RTP/AVP 97 101
      a=rtpmap:97 iLBC
      a=rtpmap:101 telephone-events




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   [Answer]

      v=0
      o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 0 RTP/AVP 0
      a=rtpmap:0 PCMU/8000
      m=audio 49170 RTP/AVP 97 101
      a=rtpmap:97 iLBC
      a=rtpmap:101 telephone-events




2.7 Audio and Video 5

   Alice and Bob establish an audio and video session in the first
   exchange with a single audio and video codec.  In the second
   exchange, Alice adds a second video codec which Bob accepts which
   allows Alice and Bob to switch between the two video codecs without
   another offer/answer exchange.


    [Offer]

      v=0
      o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 99
      a=rtpmap:99 iLBC
      m=video 51372 RTP/AVP 31
      a=rtpmap:31 H261/90000

   [Answer]

      v=0
      o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49172 RTP/AVP 99
      a=rtpmap:99 iLBC
      m=video 51374 RTP/AVP 31
      a=rtpmap:31 H261/90000



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   [Second-Offer]

      v=0
      o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 99
      a=rtpmap:99 iLBC
      m=video 51372 RTP/AVP 31 32
      a=rtpmap:31 H261/90000
      a=rtpmap:32 MPV/90000

   [Second-Answer]

      v=0
      o=bob 2808844564 2808844565 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49172 RTP/AVP 99
      a=rtpmap:99 iLBC
      m=video 51374 RTP/AVP 31 32
      a=rtpmap:31 H261/90000
      a=rtpmap:32 MPV/90000



2.8 Audio and Video 6

   This scenario shows an audio and video offer that is accepted, but
   the answerer wants the video sent to a different address than the
   audio.  This is a common scenario in conferencing where the video and
   audio mixing utilizes different servers.  In this example, Alice
   offers audio and video and Bob accepts.



    [Offer]

      v=0
      o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 0 8 97
      a=rtpmap:0 PCMU/8000
      a=rtpmap:8 PCMA/8000



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      a=rtpmap:97 iLBC
      m=video 51372 RTP/AVP 31 32
      a=rtpmap:31 H261/90000
      a=rtpmap:32 MPV/90000

   [Answer]

      v=0
      o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49174 RTP/AVP 0
      a=rtpmap:0 PCMU/8000
      m=video 49172 RTP/AVP 32
      c=IN IP4 otherhost.biloxi.example.com
      a=rtpmap:32 MPV/90000



2.9 Asymmetric Codec Types

   This scenario shows a video and audio session where it is desired to
   agree on different audio and video codecs in each direction. An
   example for the need to establish an asymmetric session is when
   running over asymmetric bandwidth links such as ADSL.

   In this simplest case the logical association between the send-only
   and the receive-only streams is expressed by specifying the same port
   number in both m-lines. Because no explicit RTCP attribute [7] is
   included, usage of the port number in sendonly direction, which
   equals to the port number in recvonly direction, unambiguously hints
   that the two m-lines share the same RTCP bidirectional stream and
   belong to the same RTP session.

   Note: When the RTCP attribute needs to be used, alternative means for
   association between the m-lines will be used [8].

       [Offer]

         v=0
         o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
         s=
         c=IN IP4 host.atlanta.example.com
         t=0 0
         m=audio 49170 RTP/AVP 95
         a=rtpmap:95 iLBC
         a=sendonly



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         m=audio 49170 RTP/AVP 0 8
         a=rtpmap:0 PCMU/8000
         a=rtpmap:8 PCMA/8000
         a=recvonly
         m=video 51372 RTP/AVP 31 32
         a=rtpmap:31 H261/90000
         a=rtpmap:32 MPV/90000
         a=sendonly
         m=video 51372 RTP/AVP 34
         a=rtpmap:34 H263/90000
         a=recvonly

      [Answer]

         v=0
         o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
         s=
         c=IN IP4 host.biloxi.example.com
         t=0 0
         m=audio 49178 RTP/AVP 95
         a=rtpmap:iLBC
         a=recvonly
         m=audio 49178 RTP/AVP 0
         a=rtpmap:0 PCMU/8000
         a=sendonly
         m=video 51374 RTP/AVP 31
         a=rtpmap:31 H261/90000
         a=recvonly
         m=video 51374 RTP/AVP 34
         a=rtpmap:34 H263/90000
         a=sendonly


2.10 Asymmetric Attribute Values

   This scenario shows an audio session where it is desired to agree on
   audio codecs with different parameters (attributes) in each
   direction. An example for the need to establish an asymmetric session
   is when running over asymmetric bandwidth links such as ADSL.

   In this simplest case the logical association between the send-only
   and the receive-only streams is expressed by specifying the same port
   number in both m-lines. Because no explicit RTCP attribute [7] is
   included, usage of the port number in sendonly direction, which
   equals to the port number in recvonly direction, unambiguously hints
   that the two m-lines share the same RTCP bidirectional stream and
   belong to the same RTP session.




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   Note: When the RTCP attribute needs to be used, alternative means for
   association between the m-lines will be used [8].

       [Offer]

         v=0
         o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
         s=
         c=IN IP4 host.atlanta.example.com
         t=0 0
         m=audio 49170 RTP/AVP 97
         a=rtpmap:97 MPA
         a=fmtp: 97 layer=3 bitrate=10
         a=sendonly
         m=audio 49170 RTP/AVP 97
         a=rtpmap:97 MPA
         a=fmtp:97 layer=3 bitrate=60
         a=recvonly


      [Answer]

         v=0
         o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
         s=
         c=IN IP4 host.biloxi.example.com
         t=0 0
         m=audio 49178 RTP/AVP 97
         a=rtpmap:MPA
         a=fmtp:97 layer=3 bitrate=10
         a=recvonly
         m=audio 49178 RTP/AVP 97
         a=rtpmap:MPA
         a=fmtp:97 layer=3 bitrate=60
         a=sendonly


3. Hold and Resume Scenarios

3.1 Hold and Unhold 1

   Alice calls Bob, but Bob answers placing Alice on hold.  Bob then
   takes Alice off hold in the second offer.  Alice changes port number
   in the second exchange.  The media session between Alice and Bob is
   now active after Alice's second answer.  Note that a=sendrecv could
   be present in both second offer and answer exchange.  This is a
   common flow in 3pcc [5] scenarios.




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   [Offer]

      v=0
      o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 0 97
      a=rtpmap:0 PCMU/8000
      a=rtpmap:97 iLBC

   [Answer]

      v=0
      o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 placeholder.biloxi.example.com
      t=0 0
      m=audio 49172 RTP/AVP 97
      a=rtpmap:97 iLBC
      a=sendonly

   [Second-Offer]

      v=0
      o=bob 2808844564 2808844565 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49170 RTP/AVP 97
      a=rtpmap:97 iLBC

   [Second-Answer]

      v=0
      o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49178 RTP/AVP 97
      a=rtpmap:97 iLBC



3.2 Hold with Two Streams

   Alice sends but can not receive RFC2833 tones in a separate audio
   stream.  Bob accepts both audio streams.  Bob then puts Alice's audio



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   stream on hold but not the tone stream.  Alice responds with
   identical SDP to the initial offer.


    [Offer]

      v=0
      o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 0 97
      a=rtpmap:0 PCMU/8000
      a=rtpmap:97 iLBC
      m=audio 49172 RTP/AVP 98
      a=rtpmap:98 telephone-event
      a=sendonly

   [Answer]

      v=0
      o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49172 RTP/AVP 97
      a=rtpmap:97 iLBC
      m=audio 49174 RTP/AVP 98
      a=rtpmap:98 telephone-event
      a=recvonly

   [Second-Offer]

      v=0
      o=bob 2808844564 2808844565 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49172 RTP/AVP 97
      a=rtpmap:97 iLBC
      a=sendonly

      m=audio 49174 RTP/AVP 98
      a=rtpmap:98 telephone-event
      a=recvonly

   [Second-Answer]




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      v=0
      o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 0 97
      a=rtpmap:0 PCMU/8000
      a=rtpmap:97 iLBC
      m=audio 49172 RTP/AVP 98
      a=rtpmap:98 telephone-event
      a=sendonly



4. Addition and Deletion of Media Streams

   This section shows addition and deletion of media streams.

4.1 Second Audio Stream Added

   The second stream is added by Bob's media server (different
   connection address) to receive RFC 2833 telephone-events (DTMF
   digits, typically) from Alice.  Alice accepts.  Even though the 2nd
   stream is unidirectional, Alice receives RTCP packets on port 49173
   from the media server.


   [Offer]

      v=0
      o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 0 97
      a=rtpmap:0 PCMU/8000
      a=rtpmap:97 iLBC

   [Answer]

      v=0
      o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49170 RTP/AVP 97
      a=rtpmap:97 iLBC




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   [Second-Offer]

      v=0
      o=bob 2808844564 2808844565 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49170 RTP/AVP 97
      a=rtpmap:97 iLBC
      m=audio 48282 RTP/AVP 98
      c=IN IP4 mediaserver.biloxi.example.com
      a=rtpmap:98 telephone-events
      a=recvonly

   [Second-Answer]

      v=0
      o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 97
      a=rtpmap:97 iLBC
      c=IN IP4 host.atlanta.example.com
      m=audio 49172 RTP/AVP 98
      a=rtpmap:98 telephone-events
      a=sendonly



4.2 Audio then Video Added

   Audio only session is established in initial exchange between Alice
   and Bob using PCMU codec.  Alice adds a video stream which is
   accepted by Bob.



   [Offer]

      v=0
      o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 0
      a=rtpmap:0 PCMU/800




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   [Answer]

      v=0
      o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49172 RTP/AVP 0
      a=rtpmap:0 PCMU/8000

    [Second-Offer]

      v=0
      o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 0
      a=rtpmap:0 PCMU/8000
      m=video 49172 RTP/AVP 31
      a=rtpmap:31 H261/90000

   [Second-Answer]

      v=0
      o=bob 2808844564 2808844565 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49172 RTP/AVP 0
      a=rtpmap:0 PCMU/8000
      m=video 49168 RTP/AVP 31
      a=rtpmap:31 H261/90000



4.3 Audio and Video, then Video Deleted

   Alice and Bob establish an audio and video session.  In a second
   exchange, Bob deletes the video session resulting in an audio only
   session.

    [Offer]

      v=0
      o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com



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      t=0 0
      m=audio 49170 RTP/AVP 97
      a=rtpmap:97 iLBC
      m=video 51372 RTP/AVP 31
      a=rtpmap:31 H261/90000

   [Answer]

      v=0
      o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49174 RTP/AVP 97
      a=rtpmap:0 PCMU/8000
      m=video 49170 RTP/AVP 31
      a=rtpmap:31 H261/90000

   [Second-Offer]

      v=0
      o=bob 2808844564 2808844565 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49174 RTP/AVP 97
      a=rtpmap:97 iLBC
      m=video 0 RTP/AVP 31
      a=rtpmap:31 H261/90000

   [Second-Answer]

      v=0
      o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 97
      a=rtpmap:97 iLBC
      m=video 0 RTP/AVP 31
      a=rtpmap:31 H261/90000



5. Third Party Call Control (3pcc)

   This section shows examples common in Third Party Call Control (3pcc)
   flows [5].  Call hold and resume flows are also common in 3pcc.



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5.1 No Media, then Audio Added

   The first offer from Alice contains no media lines, so Bob accepts
   with no media lines.  In the second exchange, Alice adds an audio
   stream which Bob accepts.


    [Offer]

      v=0
      o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0

   [Answer]

      v=0
      o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0

   [Second-Offer]

      v=0
      o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 97
      a=rtpmap:97 iLBC

   [Second-Answer]

      v=0
      o=bob 2808844564 2808844565 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49172 RTP/AVP 97
      a=rtpmap:97 iLBC



5.2 Hold and Unhold 2

   The first offer from Alice contains the connection address 0.0.0.0



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   and a random port number, which means that Bob can not send media to
   Alice (the media stream is "black holed" or "bh"). Bob accepts with
   normal SDP.  In the second exchange, Alice changes the connection
   address, Bob accepts, and a media session is established.


    [Offer]

      v=0
      o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 0.0.0.0
      t=0 0
      m=audio 23442 RTP/AVP 97
      a=rtpmap:97 iLBC

   [Answer]

      v=0
      o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49170 RTP/AVP 97
      a=rtpmap:97 iLBC

   [Second-Offer]

      v=0
      o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 97
      a=rtpmap:97 iLBC

   [Second-Answer]

      v=0
      o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49170 RTP/AVP 97
      a=rtpmap:97 iLBC






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5.3 Hold and Unhold 3

   The first offer from Alice contains an audio stream, but the answer
   from Bob contains the connection address 0.0.0.0 and a random port
   number, which means that Alice can not send media to Bob (the media
   stream is "black holed" or "bh"). In the second exchange, Bob changes
   the connection address, Alice accepts, and a media session is
   established.


    [Offer]

      v=0
      o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 97
      a=rtpmap:97 iLBC

   [Answer]

      v=0
      o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 0.0.0.0
      t=0 0
      m=audio 9322 RTP/AVP 97
      a=rtpmap:97 iLBC

   [Second-Offer]

      v=0
      o=bob 2808844564 2808844565 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49172 RTP/AVP 97
      a=rtpmap:97 iLBC


   [Second-Answer]

      v=0
      o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0



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      m=audio 49170 RTP/AVP 97
      a=rtpmap:97 iLBC



6. Security Considerations

   SDP offer and answer messages can contain private information about
   addresses and sessions to be established between parties.  If this
   information needs to be kept private, some security mechanism in the
   protocol used to carry the offers and answers must be used.  For SIP,
   this means using TLS transport and/or S/MIME encryption of the SDP
   message body.

7. Acknowledgements

   Thanks to Orit Levin for the asymmetrical codec and attribute
   examples.

8. Changes since -00

   - Added asymmetrical codec and attribute examples

Informative References

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

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

   [3]  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.

   [4]  Schulzrinne, H. and S. Petrack, "RTP Payload for DTMF Digits,
        Telephony Tones and Telephony Signals", RFC 2833, May 2000.

   [5]  Rosenberg, J., Peterson, J., Schulzrinne, H. and G. Camarillo,
        "Best Current Practices for Third Party Call Control in the
        Session  Initiation Protocol", draft-ietf-sipping-3pcc-03 (work
        in progress), March 2003.

   [6]  Duric, A. and S. Andersen, "RTP Payload Format for iLBC Speech",
        draft-ietf-avt-rtp-ilbc-01 (work in progress), March 2003.

   [7]  Huitema, C., "RTCP attribute in SDP",
        draft-ietf-mmusic-sdp4nat-05 (work in progress), June 2003.



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   [8]  Camarillo, G., Eriksson, G., Holler, J. and H. Schulzrinne,
        "Grouping of Media Lines in the Session Description Protocol
        (SDP)", RFC 3388, December 2002.


Authors' Addresses

   Alan Johnston
   MCI
   100 South 4th Street
   St. Louis, MO  63102

   EMail: alan.johnston@mci.com


   Robert J. Sparks
   dynamicsoft
   5100 Tennyson Parkway
   Suite 1200
   Plano, TX  75024

   EMail: rsparks@dynamicsoft.com





























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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgment

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