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INTERNET-DRAFT                                                    J. Ott
Expires: January 2005                                                TZI
                                                            12 July 2004

               SDP Attributes for T.120 Data Conferencing

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   This document is a contribution to the Multiparty Multimedia Session
   Control (MMUSIC) working group of the Internet Engineering Task
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   This memo specifies the use of the Session Description Protocol in
   combination with the SDP Offer/Answer exchange to setup and teardown
   data conferecing sessions based upon the ITU-T T.120 Series of
   Recommendations, thereby particularly enabling application sharing as
   defined in ITU-T T.128.

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1. Introduction

   SDP [1] is widely used in the Internet today to describe multimedia
   sessions between two or more endpoints.  The offer/answer exchange
   [2] allows two endpoints to negotiate the media streams to be
   established, e.g. in the context of a SIP dialog [4].  While fax and
   text conversation have been defined for use with SIP, the primary
   media types in use today are still audio and video.  What is
   currently lacking is the support for application-sharing, shared
   editing, whiteboard and similar teleconferencing tools.

   This memo defines a minimal set of SDP attributes to enable
   standardized setup of T.120 data conferencing sessions -- and
   particularly application sharing as defined in T.128 and implemented
   e.g. in NetMeeting.

   While SDP is defined for use with many signaling protocols, the use
   of T.120 is probably only meaningful for SIP calls and conferences
   and hence this memo focuses on its use with SIP.

2. Overview of T.120

   T.120 defines multipoint conferencing protocols and services for data
   applications including whiteboard (T.126), file transfer (T.127),
   application sharing (T.128), and text conversation (T.134, T.140),
   among others [5].

   In T.120, communication takes place in a hierarchical structure of
   T.120 entities interconnected by T.120 connections.  In the simplest
   case, this may be just two entities interconnected via a T.120
   connection, in a slightly more sophisticated one a single conference
   bridge (MCU) may be used to interconnect more than two entities.
   Each point-to-point T.120 connection uses TCP as underlying transport
   and TPKT (RFC1006) framing on top [6].  Up to four TCP connections
   may be between any two T.120 entities to provide independent flow
   control for different transmission priorities.

   The lowest layer above the point-to-point transport is the Multipoint
   Communication Service (MCS) [7][8].  The MCS defines a connection
   setup procedure that allows to associate different transport
   connections with the same MCS Domaina and to organize the MCS
   "providers" in a tree structure during connection setup.  In the
   resulting tree, the MCS provides a multiplex for application data
   (using up to 64K channels) and simple means for synchronizations

   On top of MCS, the Generic Conference Control (GCC) [9] is
   responsible for controlling the connection setup and their
   association with a conference.  Furthermore, GCC manages conference
   resources, provides capability exchange, allows for floor control,
   and provides some kind of a conference-wide registry.  In GCC,
   conferences are identified by means of an octet string that is mapped
   to the MCS Domain identifier.  GCC allows to create and destroy

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   conferences as well as to inquire for, join, and leave existing ones.

   T.120 data applications make use of the MCS communication platform to
   exchange information and of the GCC services to learn about each
   others existence and to find each other.  In particular, GCC's
   capability exchange mechanism is used to discover commonly available
   applications and their respective features (with many tiny details
   being negotiable).

   The purpose of this memo is to define a minimal meaningful set of SDP
   attributes that allows two nodes to learn about their respective
   T.120 capabilities and enable them to set up a single T.120
   connection.  Whether a single or more TCP connections are used and
   how those are associated if entirely left up to the respective T.120
   entities, as is most of the T.120-specific capability exchange.

3. T.120 Setup Procedure

3.1. Use of the m= line

   A T.120 session is based upon TCP as underlying transport.
   Therefore, the rules for connection-oriented media as defined in [3]
   MUST be followed.

   The media part on the m= line MUST indicate "data", the port number
   indicate the port number to connect to (for one to four transport
   connections, again following the rules of [3]), the proto part MUST
   indicate "TCP" and the format MUST indicate "t120".  The
   registered port number for T.120 is 1503.

3.2. T.120-specific Attributes

   T.120 connections established within the context of an SDP
   offer/answer exchange may need to be associated with a SIP dialog or
   a SIP conference.  Therefore, the media-level "confname" attribute
   is introduced.  The content of the confname attribute MUST be copied
   to the ConferenceName field used by the GCC entity.  For a simple SIP
   dialog, the confname attribute SHOULD contain the SIP From tag, To
   tag, and Call-ID, concatenated with the comma (",") as a separator.
   For a SIP dialog in a conference, the confname attribute SHOULD
   contain the focus URI.

   T.120 defines a number of applications.  To determine from the
   offer/answer exchange whether or not at least the target
   application(s) are available at a peer, the optional "t120apps"
   attribute is introduced.  If present, this attribute SHOULD contain a
   list of T.120 application protocols supported by the respective peer.
   The following application protocols are defined: "t126", "t127",
   "t128", "t134", "t136", and "t137".  The fine-grained

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   negotiation of capabilities within these application protocols is
   left up to the GCC operation after setup of a T.120 connection.

3.3. Offer/Answer Operation

   The offerer wishing to set up a T.120 session MUST include an m= line
   according to section 3.1 and include an appropriate c= line.  The
   offerer MUST choose a new "connid" and MAY choose to specify a
   "setup" attribute of "active", "passive", or "actpass".  The
   offerer MUST provide the "confname" attribute as specified in
   section 3.2 and SHOULD include the list of supported T.120
   application protocols.

   The answerer MUST examine that it supports T.120 protocol.  If it
   does not support T.120 or does not wish to use T.120 in the present
   communication relationship, it MUST refure the media section by
   setting the port number to "0" in the answer.  If the answerer
   supports T.120, it MUST validate the "confname" attribute.  If no
   matching SIP dialog or focus URI can be found, the media section
   SHOULD be refused by setting the port number to "0" in the answer.
   If a matching conference is found and the "t120apps" attribute is
   not present, the answerer MAY decide whether to accept the session
   and proceed with the transport connection setup as per [3] or whether
   to refuse the media section.  If the "t120apps" attribute is
   present, the answerer MUST examine its contents and match the
   applications listed by the offerer against its own capabilities.  If
   the intersection of capabilities is empty, the answerer SHOULD refuse
   the media section.  If the intersection is not empty, the answerer
   SHOULD return the intersecting capabilities (in the order provided by
   the offerer) and then proceed with the transport connection setup as
   per [3].

   After successful establishment of a TCP connection as per [3], the
   respective entities MUST proceed with the T.120 connection setup as
   defined in [6], [8], and [9].

4. Formal SDP Attribute Specification

   This section provides the formal SDP attribute specification:

           confname      = "a=confname:" token

           t120apps      = "a=t120apps:" t120appid *("," t120appid)
           t120appid     = "t126" / "t127" / "t128" / "t134" /
                           "t136" / "t137"

5. Example

   In the following example, an node A wants to set up a telephone call
   to a node B and use supportive data applications (whiteboard, file
   transfer, and application sharing).  Node A expects to initiate
   connection setup to node B and therefore uses port number 9 [3] and

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   sets the setup attribute to "active".

           c=IN IP4
           m=audio 52000 UDP/AVP 96 97
           a=rtpmap:96 PCMU/8000
           a=rtpmap:97 L16/16000
           m=data 9 TCP t120

   Node B accepts the T.120 media session in its answer.  However, as
   node B does not support whiteboard and file transfer, it only
   confirms application sharing (T.128) in its answer.  Node B offers
   the registered T.120 port number to connect to.

           c=IN IP4
           m=audio 52000 UDP/AVP 96
           a=rtpmap:96 PCMU/8000
           m=data 1503 TCP t120

6. IANA Considerations

   This document defines two media level SDP attributes: "confname"
   and "t120apps". Their format is defined in Section 4. These two
   attributes should be registered by the IANA on


      under "att-field (at the media level only)".

7. Security Considerations


8. Author's Addresses

   Joerg Ott <jo@tzi.org>
   Universitaet Bremen
   MZH 5180
   Bibliothekstr. 1
   D-28359 Bremen

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9. Normative References

   [1]  Mark Handley, Van Jacobson, Colin Perkins, "SDP: Session
        Description Protocol," Internet Draft draft-ietf-mmusic-sdp-
        new-18.txt, Work in Progress, June 2004.

   [2]  Jonathan Rosenberg and Henning Schulzrinne, "An Offer/Answer
        Model with SDP," RFC 3264, June 2002.

   [3]  David Yon and Gonzalo Camarillo, "Connection-Oriented Media
        Transport in SDP," Internet Draft draft-ietf-mmusic-sdp-
        comedia-07.txt, Work in Progress, June 2004.

   [4]  J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J.
        Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: Session
        Initiation Protocol," RFC 3261, June 2002.

   [5]  ITU-T Recommendation T.120, "Data Protocols for Multimedia
        Conferencing", 1996.

   [6]  ITU-T Recommendation T.123, "Network Specific Data Protocol
        Stacks for Multimedia Conferencing", 1996.

   [7]  ITU-T Recommendation T.122, "Multipoint Communication Service
        for Audiographics and Audiovisual Conferencing Service
        Definition," 1993.

   [8]  ITU-T Recommendation T.125,."Multipoint Communication Service
        Protocol Specification," 1994.

   [9]  ITU-T Recommendation T.124, "Generic Conference Control,",

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