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Versions: 00 01                                                         
Network Working Group                                           Kutscher
Internet-Draft                                                       Ott
Expires: January 12, 2001                                        Bormann
                                                TZI, Universitaet Bremen
                                                           July 14, 2000

    Requirements for Session Description and Capability Negotiation

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 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

     The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on January 12, 2001.

Copyright Notice

   Copyright (C) The Internet Society (2000). All Rights Reserved.


   This document defines some terminology and lists a set of
   requirements that are relevant for a framework for session
   description and endpoint capability negotiation in multiparty
   multimedia conferencing scenarios.

   This document is intended for discussion in the Multiparty
   Multimedia Session Control (MMUSIC) working group of the Internet
   Engineering Task Force.  Comments are solicited and should be
   addressed to the working group's mailing list at confctrl@isi.edu
   and/or the authors.

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Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.    Terminology and System Model . . . . . . . . . . . . . . . .  5
   3.    General Requirements . . . . . . . . . . . . . . . . . . . .  8
   3.1   Simplicity . . . . . . . . . . . . . . . . . . . . . . . . .  8
   3.2   Extensibility  . . . . . . . . . . . . . . . . . . . . . . .  8
   3.3   Firewall Friendliness  . . . . . . . . . . . . . . . . . . .  8
   3.4   Security . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   3.5   Text encoding  . . . . . . . . . . . . . . . . . . . . . . .  8
   3.6   Mapping (of a Subset) to SDP . . . . . . . . . . . . . . . .  8
   4.    Session Description Requirements . . . . . . . . . . . . . .  9
   4.1   Media Description  . . . . . . . . . . . . . . . . . . . . .  9
   4.1.1 Medium Type  . . . . . . . . . . . . . . . . . . . . . . . .  9
   4.1.2 Media Stream Packetization . . . . . . . . . . . . . . . . .  9
   4.1.3 Transport  . . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.1.4 QoS  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.1.5 Other parameters (media-specific)  . . . . . . . . . . . . . 10
   4.1.6 Naming Hierarchy and/or Scoping  . . . . . . . . . . . . . . 10
   5.    Requirements for Capability Description and Negotiation  . . 11
   5.1   Capability Constraints . . . . . . . . . . . . . . . . . . . 11
   5.2   Processing Rules . . . . . . . . . . . . . . . . . . . . . . 11
   6.    Remarks  . . . . . . . . . . . . . . . . . . . . . . . . . . 13
         References . . . . . . . . . . . . . . . . . . . . . . . . . 14
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 15
         Full Copyright Statement . . . . . . . . . . . . . . . . . . 16

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1. Introduction

   Multiparty multimedia conferencing is one application that requires
   the dynamic interchange of end system capabilities and the
   negotiation of a parameter set that is appropriate for all sending
   and receiving end systems in a conference. For some applications,
   e.g. for loosely coupled conferences, it may be sufficient to simply
   have session parameters be fixed by the initiator of a conference.
   In such a scenario no negotiation is required because only those
   participants with media tools that support the predefined settings
   can join a media session and/or a conference.

   This approach is applicable for conferences that are announced some
   time ahead of the actual start date of the conference. Potential
   participants can check the availability of media tools in advance
   and tools like session directories can configure media tools on
   startup.  This procedure however fails to work for conferences
   initiated spontaneously like Internet phone calls or ad-hoc
   multiparty conferences. Fixed settings for parameters like media
   types, their encoding etc. can easily inhibit the initiation of
   conferences, for example in situations where a caller insists on a
   fixed audio encoding that is not available at the callee's end

   To allow for spontaneous conferences, the process of defining a
   conference's parameter set must therefore be performed either at
   conference start (for closed conferences) or maybe (potentially)
   even repeatedly every time a new participant joins an active
   conference.  The latter approach may not be appropriate for every
   type of conference without applying certain policies: For
   conferences with TV-broadcast or lecture characteristics (one main
   active source) it is usually not desired to re-negotiate parameters
   every time a new participant with an exotic configuration joins
   because it may exclude the main source from media sessions. But
   conferences with equal ``rights'' for participants that are open for
   new participants on the other hand would need a different model of
   dynamic capability negotiation, for example a telephone call that is
   extended to a 3-parties conference at some time during the session.

   SDP [1] allows to specify multimedia sessions (i.e. conferences,
   "session" as used here is not to be confused with "RTP session"!)
   by providing general information about the session as a whole and
   specifications for all the media streams (RTP sessions and others)
   to be used to exchange information within the multimedia session.

   Currently, media descriptions in SDP are used for two purposes:

   o  to describe session parameters for announcements and invitations
      (the original purpose of SDP)

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   o  to describe the capabilities of a system (and possibly provide a
      choice between a number of alternatives). Note that SDP was not
      designed to facilitate this.

   A distinction between these two "sets of semantics" is only made

   In the following we first introduce a model for session description
   and capability negotiation and define some terms that are later used
   to express some requirements. Note that this list of requirements is
   possibly incomplete. The purpose of this document is to initiate the
   development of a session description and capability negotiation

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2. Terminology and System Model

   Any (computer) system has, at a time, a number of rather fixed
   hardware as well as software resources.  These resources ultimately
   define the limitations on what can be captured, displayed, rendered,
   replayed, etc. with this particular device.  We term features
   enabled and restricted by these resources "system capabilities".

      Example: System capabilities may include: a limitation of the
      screen resolution for true color by the graphics board; available
      audio hardware or software may offer only certain media encodings
      (e.g. G.711 and G.723.1 but not GSM); and CPU processing power
      and quality of implementation may constrain the possible video
      encoding algorithms.

   In multiparty multimedia conferences, participants employ different
   ``components'' in conducting the conference.

      Example: In lecture multicast conferences one component might be
      the voice transmission for the lecturer, another the transmission
      of video pictures showing the lecturer and the third the
      transmission of presentation material.

   Depending on system capabilities, user preferences and other
   technical and political constraints, different configurations can be
   chosen to accomplish the ``deployment'' of these components.

   Each component can be characterized at least by (a) its intended use
   (i.e. the function it shall provide) and (b) a one or more possible
   ways to realize this function.  Each way of realizing a particular
   function is referred to as a "configuration".

      Example: A conference component's intended use may be to make
      transparencies of a presentation visible to the audience on the
      Mbone.  This can be achieved either by a video camera capturing
      the image and transmitting a video stream via some video tool or
      by loading a copy of the slides into a distributed electronic
      whiteboard.  For each of these cases, additional parameters may
      exist, variations of which lead to additional configurations (see

   Two configurations are considered different regardless whether they
   employ entirely different mechanisms and protocols (as in the
   previous example) or they choose the same and differ only in a
   single parameter.

      Example: In case of video transmission, a JPEG-based still image
      protocol may be used, H.261 encoded CIF images could be sent as
      could H.261 encoded QCIF images.  All three cases constitute

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      different configurations.  Of course there are many more detailed
      protocol parameters.

   Each component's configurations are limited by the system
   capabilities.  In addition, the intended use of a component may
   constrain the possible configurations further to a subset suitable
   for the particular component's purpose.

      Example: In a system for highly interactive audio communication
      the component responsible for audio may decide not to use the
      available G.723.1 audio codec to avoid the additional latency but
      only use G.711.  This would be reflected in this component only
      showing configurations based upon G.711.  Still, multiple
      configurations are possible, e.g. depending on the use of A-law
      or u-Law, packetization and redundancy parameters, etc.

   In this model, we distinguish two types of configurations:

   o  potential configurations
      (a set of any number of configurations per component) indicating
      a system's functional capabilities as constrained by the intended
      use of the various components;

   o  actual configurations
      (exactly one per instance of a component) reflecting the mode of
      operation of this component's particular instantiation.

      Example: The potential configuration of the aforementioned video
      component may indicate support for JPEG, H.261/CIF, and
      H.261/QCIF.  A particular instantiation for a video conference
      may use the actual configuration of H.261/CIF for exchanging
      video streams.

   The key terms of this models:

   o  A multimedia session (or conference) consists of one or more
      conference components for multi media interaction.

   o  A component describes a particular type of interaction (e.g.
      audio conversation, slide presentation) that can be realized by
      means of different applications (possibly using different

   o  A configuration is a set of parameters that are required to
      implement a certain variation (realization) of a certain
      component. There are actual and potential configurations.

      *  Potential configurations describe possible configurations that
         are supported by an end system.

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      *  An actual configuration is an "instantiation" of one of the
         potential configurations, i.e. a decision how to realize a
         certain component.

      In less abstract words, potential configurations describe what a
      system can do ("capabilities") and actual configurations describe
      how a system is configured to operate at a certain point in time
      (media stream spec).

   To decide on a certain actual configuration, a negotiation process
   needs to take place between the involved peers

   1.  to determine which potential configuration(s) they have in

   2.  Out of this shared set of common potential configurations the
       peers need to select one to be used for information exchange
       (e.g. based upon preferences, external constraints, etc.).

   In SAP [11] -based session announcements on the Mbone, for which SDP
   was originally developed, the negotiation procedure is non-existent.
   Instead, the announcement contains the media stream description sent
   out (i.e. the actual configurations) which implicitly describe what
   a receiver must understand to participate.

   In point-to-point scenarios, the negotiation procedure is typically
   carried out implicitly: each party informs the other about what it
   can receive and the respective sender chooses from this set a
   configuration that it can transmit.

   Capability negotiation must not only work for 2-party conferences
   but is also required for multi-party conferences. Especially for the
   latter case it is required that the process of determining the
   subset of allowable potential configurations is deterministic to
   reduce the number of required round trips before a session can be

   In the following, we elaborate on requirements for an SDPng
   specification, subdivided into general requirements and requirements
   for session descriptions, potential and actual configurations as
   well as negotiation rules.

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3. General Requirements

3.1 Simplicity

   The SDPng syntax shall be simple to parse and the protocol rules
   shall be easy to implement.

3.2 Extensibility

   SDPng shall be extensible in a backward compatible fashion.
   Extensions should be doable without modifying the SDPng
   specification itself.  The spec should preclude two independent
   extensions from clashing with each other (e.g. in the naming of

3.3 Firewall Friendliness

   It should be theoretically possible for firewalls (and other network
   infrastructure elements) to process announcements etc. that contain
   SDng content. The concrete procedures have to be defined but if
   possible the processing of the SDPng content should be doable
   without interpretation of the textual descriptions.

3.4 Security

   SDPng should allow independent security attributes for parts of a
   session description.  In particular, signing and/or encrypting parts
   of a session description should be supported.

3.5 Text encoding

   A concise text representation is desirable in order to enhance
   portability and allow for simple implementations.

3.6 Mapping (of a Subset) to SDP

   It shall be possible to translate a subset of SDPng into standard
   SDP session description to enable a certain minimal degree of
   interoperability between SDP-based and SDPng-based systems.
   However, as SDPng will provide enhanced functionality compared to
   SDP, a full mapping to SDP is not possible.

   It is unclear at the moment whether syntactic backward compatibility
   to SDP as per RFC 2327 is a must.  Clearly, this is desirable but
   may not need to be achieved at all cost.

   Since several flavors of SDP have been developed (e.g., the MEGACO
   WG uses certain non-SDP enhancements) it needs to be discussed which
   of these flavors need to be considered for some kind of mapping.

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4. Session Description Requirements

   For now, we only consider requirements for media (stream)

4.1 Media Description

   It must be possible to express the following information with SDPng:

4.1.1 Medium Type

   Payload types and format parameters for audio and video are
   well-defined and the basic semantics are clear (as defined in
   RFC1889 [2] and RFC2327 [1]).

   Format description for text and whiteboard are currently only
   defined in the context of specific applications, this is probably
   going to change in the future (not an SDPng work item).

   Non-standard (in terms of defined as a non-standard payload type)
   codecs and format parameters can be accomplished by using dynamic
   payload type mappings. This is a crucial feature of SDP that needs
   to be preserved for the sake of interoperability with

   Current SDP only provides a= (a=fmtp) as means to specify codec
   parameters but actually gives little support on how to do this.
   Schemes for expressing more sophisticated parameters (e.g.
   supporting nesting) may be necessary. Nevertheless, it is imperative
   to keep the overall structure of a codec description manageable.

   Note that there is a conflict between the desire to be able to use
   any old SDP and translate it in SDPng and the desire to have a
   useful structure in the SDPng data.

4.1.2 Media Stream Packetization

   SDPng need to be able to take care of more sophisticated payload
   descriptions than simple payload type assignment. Audio/video
   redundancy coding schemes need to be supported as need other
   mechanisms for FEC (RFC 2733 [7]) and media stream repair (RFC 2354
   [8]).  Also, layered coding schemes need to be supported.

   Finally, a separation of the media encoding scheme, the
   packetization format, and possible repair schemes (and their
   respective parameters) is required.

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4.1.3 Transport

   Since session descriptions are not only used to describe sessions
   that use IPv4/RTP for media transport it must be possible to specify
   different transport protocols (and their corresponding mandatory
   parameters).  This means SDPng must support different address
   formats (IPv4, IPv6, E.164, NSAP, ...)  and different transport
   protocol stacks (RTP/UDP/IP, RTP/AAL5/ATM, ...).

   Additionally the requirement for expressing multiple addresses per
   actual configuration (layered coding support) has emerged, as well
   as the requirement for expressing multiple addresses per potential
   configuration (one port per payload type to simplify processing at
   the receiver). (A motivation has been provided by
   draft-camarillo-sip-sdp-00.txt [9].)

   In multi-unicast-scenarios it must be possible to specify more than
   one transport address for a single media stream in an actual
   configuration, i.e. by specifying address lists.

   In "broadcast"- or "lecture"-like sessions source filters might be
   needed that allow receivers to verify the source and apply filters
   in multicast sessions.

4.1.4 QoS

   QoS-Parameters for different protocol domains (e.g. traffic
   specification and flow specification or TOS bits for IP QoS) need to
   be specified. draft-ietf-mmusic-sdp-qos-00.txt [10] has provided a
   proposal for a syntax that can be used with SDP to describe network
   and security preconditions that have to be met in order to establish
   a session.

4.1.5 Other parameters (media-specific)

   Extension mechanisms that allow to describe arbitrary other
   parameters of media codecs and formats are mandatory. It is possibly
   required to distinguish between mandatory and optional extension

   In particular, it must be possible to introduce new (optional)
   parameters for a payload format and have old implementations still
   parse the parameters correctly.

4.1.6 Naming Hierarchy and/or Scoping

   Parameter names should be constructed in a way to avoid clashes and
   thereby simplify independent development of e.g. codec parameter
   descriptions in different groups.

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5. Requirements for Capability Description and Negotiation

5.1 Capability Constraints

   Capability negotiation is used to gain a session description (a
   actual configuration) that is compatible with the different end
   system capabilities and user preferences of the potential
   participants of a conference.

   A media capability description is the same as a potential
   configuration, as it contains a set of allowable configurations for
   different components that could be used to implement the
   corresponding component. A capability description should allow
   specifying a number of interdependencies among capabilities.
   Traditional SDP only supports alternative capabilities and the
   specification implicitly assumed that all capabilities could be
   combined and basically used at the same time (looking at the pure
   session description, at least).

   Processing power, hardware, link, or other resources may preclude
   the simultaneous use of certain configurations and/or limit the
   number of simultaneous instantiations of one or more configurations.
   This has led to a need to express in more detail constraints on
   combinations of configurations including the following constraints:

   o  grouping capabilities (-> capability set);

   o  expressing simultaneous capability sets;

   o  expressing alternative capability sets; and

   o  constraining the number of uses of a certain capability (set).

   It needs to be carefully investigated how much more sophistication
   (if any) than simply listing alternatives needs to go into a base
   specification of SDPng (and which extension mechanisms for certain
   applications or for future revisions should be allowed).

   Examples are known where complex capability descriptions are
   available but are simply not used (at least not at the level of
   sophistication that would be possible).  This strongly calls for
   keeping requirements on capability constraints rather modest (KISS).

5.2 Processing Rules

   The processing (i.e. collapsing, forwarding etc.) of different
   potential configurations in order to find a compatible subset must
   work without having to know the semantics of the individual
   parameters. This is a key requirement for extensibility.

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   Additionally it must be possible to make use of different
   negotiation policies in order to reflect different conference types.
   For example in a lecture-style conference it must be ensured that a
   capability collapsing process does not yield an actual configuration
   that excludes the main source (i.e. the lecturer and her end system)
   from the conference.

   Of course, the negotiation of configurations must not only work in
   peer-to-peer-conference scenarios but also be usable in multi party

   In order to allow for concise capability specification it will
   probably be required to group descriptions of, say, codecs and to
   establish a kind of hierarchy that allows to attach a certain
   attribute or parameter to a whole group of codecs.

   It might then also be required to have a naming scheme that allows
   to name definitions in order to be able to later reference them in
   subsequent definitions. This is useful in situations where some
   definition extends a previous definition by just one parameter or in
   situations where codecs are combined, for example for expressing
   redundancy or layered codings. Different models of re-use are

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6. Remarks

   Explicitly addressing the issue of capability negotiation when
   drafting the new session description language generates new sets of
   requirements, some of which might conflict with other important
   goals, such as simplicity, conciseness and SDP-compatibility.

   However, we think that it's worthwhile to sketch a complete and
   powerful solution first and then later develop a migration path from
   today's technology instead of imposing limits at the outset to
   minimize the possibly necessary changes.

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

   [2]  Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobsen,
        "RTP: A Transport Protocol for Real-Time Applications", RFC
        1889, January 1996.

   [3]  Schulzrinne, H., "RTP Profile for Audio and Video Conferences
        with Minimal Control", RFC 1890, January 1996.

   [4]  Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., Handley,
        M., Bolot, J., Vega-Garcia, A. and S. Fosse-Parisis, "RTP
        Payload for Redundant Audio Data", RFC 2198, September 1997.

   [5]  Klyne, G., "A Syntax for Describing Media Feature Sets", RFC
        2533, March 1999.

   [6]  Klyne, G., "Protocol-independent Content Negotiation
        Framework", RFC 2703, September 1999.

   [7]  Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format for
        Generic Forward Error Correction", RFC 2733, December 1999.

   [8]  Perkins, C. and O. Hodson, "Options for Repair of Streaming
        Media", RFC 2354, June 1998.

   [9]  Camarillo, G., Holler, J. and G. AP Eriksson, "SDP media
        alignment in SIP", Internet Draft
        draft-camarillo-sip-sdp-00.txt, June 2000.

   [10]  Rosenberg, J., Schulzrinne, H. and S. Donovan, "Establishing
         QoS and Security Preconditions for SDP Sessions", Internet
         Draft draft-ietf-mmusic-sdp-qos-00.txt, June 1999.

   [11]  Handley, M., Perkins, C. and E. Whelan, "Session Announcement
         Protocol", Internet Draft draft-ietf-mmusic-sap-v2-06.txt,
         March 2000.

   [12]  Kumar, R. and M. Mostafa, "Conventions for the use of the
         Session Description Protocol (SDP) for ATM Bearer
         Connections", Internet Draft
         draft-rajeshkumar-mmusic-sdp-atm-02.txt, July 2000.

   [13]  Quinn, B., "SDP Source-Filters", Internet Draft
         draft-ietf-mmusic-sdp-srcfilter-00.txt, May 2000.

   [14]  Beser, B., "Codec Capabilities Attribute for SDP", Internet

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         Draft draft-beser-mmusic-capabilities-00.txt, March 2000.

   [15]  Casner, S., "SDP Bandwidth Modifiers for RTCP Bandwidth",
         Internet Draft draft-ietf-avt-rtcp-bw-01.txt, March 2000.

Authors' Addresses

   Dirk Kutscher
   TZI, Universitaet Bremen
   Bibliothekstr. 1
   Bremen  28359

   Phone: +49.421.218-7595
   Fax:   +49.421.218-7000
   EMail: dku@tzi.uni-bremen.de

   Joerg Ott
   TZI, Universitaet Bremen
   Bibliothekstr. 1
   Bremen  28359

   Phone: +49.421.201-7028
   Fax:   +49.421.218-7000
   EMail: jo@tzi.uni-bremen.de

   Carsten Bormann
   TZI, Universitaet Bremen
   Bibliothekstr. 1
   Bremen  28359

   Phone: +49.421.218-7024
   Fax:   +49.421.218-7000
   EMail: cabo@tzi.uni-bremen.de

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Full Copyright Statement

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   This document and the information contained herein is provided on an


   Funding for the RFC editor function is currently provided by the
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