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Versions: 00 01                                                         
Network Working Group                                           Kutscher
Internet-Draft                                                       Ott
Expires: May 25, 2001                                            Bormann
                                                TZI, Universitaet Bremen
                                                       November 24, 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 May 25, 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   Session vs. Media Description  . . . . . . . . . . . . . . .  9
   3.7   Mapping (of a Subset) to SDP . . . . . . . . . . . . . . . .  9
   4.    Session Description Requirements . . . . . . . . . . . . . . 10
   4.1   Media Description  . . . . . . . . . . . . . . . . . . . . . 10
   4.1.1 Medium Type  . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.1.2 Media Stream Packetization . . . . . . . . . . . . . . . . . 10
   4.1.3 Transport  . . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.1.4 QoS  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   4.1.5 Resource Utilization . . . . . . . . . . . . . . . . . . . . 11
   4.1.6 Dependencies . . . . . . . . . . . . . . . . . . . . . . . . 11
   4.1.7 Other parameters (media-specific)  . . . . . . . . . . . . . 11
   4.1.8 Naming Hierarchy and/or Scoping  . . . . . . . . . . . . . . 12
   5.    Requirements for Capability Description and Negotiation  . . 13
   5.1   Capability Constraints . . . . . . . . . . . . . . . . . . . 13
   5.2   Processing Rules . . . . . . . . . . . . . . . . . . . . . . 13
   6.    Remarks  . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   7.    SDPng: A Strawman Proposal . . . . . . . . . . . . . . . . . 16
   7.1   Conceptual Outline . . . . . . . . . . . . . . . . . . . . . 16
   7.1.1 Definitions  . . . . . . . . . . . . . . . . . . . . . . . . 16
   7.1.2 Components & Configurations  . . . . . . . . . . . . . . . . 17
   7.1.3 Constraints  . . . . . . . . . . . . . . . . . . . . . . . . 18
   7.1.4 Session  . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   7.2   Syntax Proposal  . . . . . . . . . . . . . . . . . . . . . . 19
   7.3   Mappings . . . . . . . . . . . . . . . . . . . . . . . . . . 21
         References . . . . . . . . . . . . . . . . . . . . . . . . . 22
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 23
         Full Copyright Statement . . . . . . . . . . . . . . . . . . 24

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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 inconvenience existing participants or even 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

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      (the original purpose of SDP)

   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 of 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 participating
   system's 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 system 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.

   In summary, the key terms of this model are:

   o  A multimedia session (streaming or conference) consists of one or
      more conference components for multimedia "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
       common, and

   2.  to select one of this shared set of common potential
       configurations 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

   Note that the order in which these requirements are presented does
   not imply their relative importance.

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

   Along with extensibility comes the requirement to identify certain
   extensions as mandatory in a given context while others as optional.

3.3 Firewall Friendliness

   It should be theoretically possible for firewalls (and other network
   infrastructure elements) to process announcements etc. that contain
   SDPng 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. At run time, size
   of encoded packets should be minimized, processing as well.

   A language that allows specifications to be formally validated is

   A tendency to use XML as basis for the specification language has
   been expressed repeatedly.

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3.6 Session vs. Media Description

   In many application scenarios (particularly with SIP and
   MEGACO/H.248), only media descriptions are needed and there is no
   need for session description parameters.  SDPng should make
   parameter sets optional where it is conceivable that not all
   application will need them.

3.7 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.

   Note: Backwards compatibility to the SDP syntax has been discussed
   and it was found that this is not goal for SDPng, as it is felt that
   RFC 2327 is too limiting.

   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 descriptions 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 RTP applications.

   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 needs 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.

4.1.3 Transport

   Since session descriptions are not only used to describe sessions

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   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, ...), multiplexing schemes (e.g.
   to identify a channel on a TDM link), and different transport
   protocol stacks (RTP/UDP/IP, RTP/AAL5/ATM, ...).  Potential further
   parameters and interdependencies for multiplexed transports should
   be considered.

   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. Similarly, for SSM, the transport address
   includes an (Sender,Group) pair of IP addresses.

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 Resource Utilization

   A requirement debated (but not yet agreed upon) was whether abstract
   terms should be found to describe resource requirements (in terms of
   CPU cycles, DSPs, etc.)

4.1.6 Dependencies

   Certain codes may depend on other resources being available (e.g. a
   G.723.1 audio codec may need a DTMF codec as well while a G.711
   codec does not).  Such interdependencies need to be expressed.

4.1.7 Other parameters (media-specific)

   Extension mechanisms that allow to describe arbitrary other

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   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.

4.1.8 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 (an
   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 of potential configurations includes the process of
   "collapsing" sets of potential configurations offered by
   participants, i.e. the computation of the intersection of these
   potential configurations.

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   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.

   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 the policy might be to
   ensure 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.

   Preferences may also be considered in the negotiation process.  This
   may need to be considered at the SDPng level (e.g. to express
   preferences, priorities).

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

   Negotiation of capabilities should take no longer than two or three
   message exchanges.  The description format must enable such

   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 reasonably
   complete and powerful solution first and then later develop a
   migration path from today's technology instead of imposing
   limitations at the outset to minimize the possibly necessary

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7. SDPng: A Strawman Proposal

   This section outlines a proposed solution for describing
   capabilities that meets most of the above requirements.  Note that
   at this early point in time not all of the details are completely
   filled in; rather, the focus is on the concepts of such a capability
   description and negotiation language.

7.1 Conceptual Outline

   Our concept for the description language follows the system model
   introduced in the beginning of this document.  We use a rather
   abstract language to avoid misinterpretations due to different
   intuitive understanding of terms as far as possible.

   PLEASE NOTE that the examples in the following are given for
   illustrative purposes only; they are not meant to be syntactically
   complete or consistent.  For a more real example refer to the end of
   this section.

   Our concept of a capability description language addresses various
   pieces of a full description of system and application capabilities
   in four separate "sections":

      Definitions (elementary and compound)

      Potential or Actual Configurations


      Session attributes

7.1.1 Definitions

   The definition section specifies a number of "entities" that are
   later referenced to avoid repetitions in more complex specifications
   and allow for a concise representation. Entities are identified by
   an "id" by which they may be referenced.  Entities may be elementary
   or compound (i.e. combinations of elementary entities).

   Elementary entities do not reference other entities.  Each
   elementary entity only consists of one of more attributes and their
   values.  Default values specified in the definition section may be
   overridden in descriptions for potential (and later actual)

   For the moment, elementary entities are defined for media types
   (i.e. codecs) and for media transports.  For each transport and for
   each codec to be used, the respective attributes need to be defined.

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   This definition may be either within the "Definition" section itself
   or in an external document (similar to the audio-visual profile or
   an IANA registry that define payload types and media stream

   Examples for elementary entities include "{media=audio, coding=PCM,
   compression=ulaw, rate=8000}" to be identified by id="PCMU" and
   "{transport=UDP, framing=RTP, network=IPv4, ...}" to be identified
   by id="AVP".

   Compound entities combine a number of elementary and/or other
   compound entities for more complex descriptions. This mechanism can
   be used for simple standard configurations such as G.711 over
   RTP/AVP as well as to express more complex coding schemes including
   e.g. FEC schemes, redundancy coding, and layered coding.  Again,
   such definitions may be standardized and externalized so that there
   is no need to repeat them in every specification.

   An example for a redundant audio payload format (following RFC 2198)
   could be "{media=audio, coding=rfc2198, primary=ref:PCMU,
   secondary=ref:GSM, pattern=1:2, pt=97}" referred to by
   id="G711-Red".  Standard uncompressed IP telephony audio could be
   "{transport=ref:AVP, codec=ref:PCMU}" identified by id="IPTEL-UNC".

   Both types of entities may have default values specified along with
   them for each attribute.  Some of these default values may be
   overridden so that a codec definition can easily be re-used in a
   different context (e.g. by specifying a different sampling rate)
   without the need for a large number of base specifications.

   This approach taken here allows to have simple as well as more
   complex definitions which are commonly used be available in an
   extensible set of reference documents. Care should be taken though
   not to make the external references too complex and thus require too
   much a priori knowledge in a protocol engine implementing SDPng.

   Note: For negotiation between endpoints, it may be helpful to define
   two modes of operation: explicit and implicit. Implicit
   specifications may refer to externally defined entities to minimize
   traffic volume, explicit specifications would list all external
   definitions used in a description in the "Definitions" section.

7.1.2 Components & Configurations

   The "Configurations" section contains all the components that
   constitute the multimedia conference, IP telephone call, etc.  For
   each of these components, the potential and, later, the actual
   configurations are given. Potential configurations are used during
   capability exchange and/or negotiation, actual configurations to

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   configure media streams after negotiation or in session
   announcements (e.g. via SAP).  A potential and the actual
   configuration of a component may be identical.

   Each component has an identifier ("id") so that it can be referred
   to, e.g. to associate semantics with a particular media stream.  For
   such a component, any number of configurations may be given with
   each configuration describing an alternate way to realize the
   functionality of the respective component.

   Each configuration (potential as well as actual) is identified by an
   "id".  A configuration combines one or more (elementary and/or
   compound) entities from the "Definitions" section to describe a
   potential or an actual configuration.  Within the specification of
   the configuration, default values from the referenced entities may
   be overwritten.

   For example, an IP telephone call may require just a single
   component id=interactive-audio with two possible ways of
   implementing it.  The two corresponding configurations are id=1
   "{ref=IPTEL-UNC}" without modification, the other uses redundancy
   coding by PCMU as both primary and secondary encoding: id=2
   "{codec=ref:G711-Red;secondary=PCMU, transport=ref:AVP}". Typically,
   transport address parameters such as the port number would also be
   provided but are omitted here for brevity.

   During/after the negotiation phase, an actual configuration is
   chosen of out a number of alternative potential configurations, the
   actual configuration may refer to the potential configuration just
   by its "id", possibly allowing for some parameter modifications.
   Alternatively, the full actual configuration may be given.

   If, from the above example, potential configuration #1 is chosen,,
   this could be expressed either in short form as "config=ref:1" or
   fully specified as id=1 "{ref=IPTEL-UNC}".

7.1.3 Constraints

   Definitions specify media, transport, and other capabilities,
   configurations indicate which combinations of these could be used to
   provide the desired functionality in a certain setting.

   There may, however, be further constraints within a system (such a
   CPU cycles, DSP available, dedicated hardware, etc.) that limit
   which of these configurations can be instantiated in parallel (and
   how many instances of these may exist).  We deliberately do not
   couple this aspect of system resource limitations to the various
   application semantics as the constraints exist across application
   boundaries.  Also, in many cases, expressing such constraints is

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   simply not necessary (as many uses of the current SDP show), so
   additional baggage can be avoided where this is not needed.

   Therefore, we introduce a "Constraints" section to contain these
   additional limitations.  Constraints refer to potential
   configurations and to entity definitions and express and use simple
   logic to express mutual exclusion, limit the number of
   instantiations, and allow only certain combinations.

   By default, the "Constraints" section is empty (or missing) which
   means that no further restrictions apply.

7.1.4 Session

   The "Session" section is used to describe general parameters of the
   communication relationship to be invoked or modified.  It contains
   most (if not all) of the general parameters of SDP (and thus will
   easily be usable with SAP for session announcements).

   In addition to the session description parameters, the "Session"
   section also ties the various components to certain semantics.  If,
   in current SDP, two audio streams were specified (possibly even
   using the same codecs), there was little way to differentiate
   between their uses (e.g. live audio from an event broadcast vs. the
   commentary from the TV studio).

   This section also allows to tie together different media streams or
   provide a more elaborate description of alternatives (e.g. subtitles
   or not, which language, etc.).

   Further uses are envisaged but need to be defined.

7.2 Syntax Proposal

   In order to allow for the possibility to validate session
   descriptions and in order to allow for structured extensibility it
   is proposed to rely on a syntax framework that provides concepts as
   well as concrete procedures for document validation and extending
   the set of allows syntax elements.

   SGML/XML technologies allow for the preparation of Document Type
   Definitions (DTDs) that can define the allowed content models for
   the elements of conforming documents. Documents can be formally
   validated against a given DTD to check their conformance and
   correctness.  For XML, mechanisms have been defined that allow for
   structured extensibility of a model of allowed syntax: XML Namespace
   and XML Schema.

   XML Schema allows to constrain the allowed document content, e.g.

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   for documents that contain structured data and also provide the
   possibility that document instances can be conformant to several XML
   Schema definitions at the same time, while allowing Schema
   validators to check the conformance of these documents.

   Extensions of the session description language, say for allowing to
   express the parameters of a new media type, would require the
   creation of a corresponding XML schema definition that contains the
   specification of element types that can be used to describe
   configurations of components for the new media type. Session
   description documents have to reference the non-standard Schema
   module, thus enabling parsers and validators to identify the
   elements of the new extension module and to either ignore them (if
   they are not supported) or to consider them for processing the
   session/capability description.

   It is important to note that the functionality of validating
   capability and session description documents is not necessarily
   required to generate or process them. For example, end-points would
   be configured to understand only those parts of description
   documents that are conforming to the baseline specification and
   simply ignore extensions they cannot support. The usage of XML and
   XML Schema is thus rather motivated by the need to allow for
   extensions being defined and added to the language in a structured
   way that does not preclude the possibility to have applications to
   identify and process the extensions elements they might support. The
   baseline specification of XML Schema definitions and profiles must
   be well-defined and targeted to the set of parameters that are
   relevant for the protocols and algorithms of the Internet Multimedia
   Conferencing Architecture, i.e. transport over RTP/UDP/IP, the audio
   video profile of RFC1890 etc.

   The example below shows how the definition of codecs,
   transport-variants and configuration of components could be
   realized. Please note that this is not a complete example and that
   identifiers have been chosen arbitrarily.

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            <codec id="audio-basic"

            <codec id="audio-L16-stereo"

            <transport id="transport-RTP"

            <component id="c1">
              <config id="c1.1" ref="audio-basic transport-RTP">
                <override udp:port="6789"

   The example does also not include specifications of XML Schema
   definitions or references to such definitions. This will be provided
   in the next version of this draft.

   A real-world capability description would likely be shorter than the
   presented example because the codec and transport definitions can be
   factored-out to profile definition documents that would only be
   referenced in capability description documents.

7.3 Mappings

   A mapping needs to be defined in particular to SDP that allows to
   translate final session descriptions (i.e. the result of capability
   negotiation processes) to SDP documents. In principle, this can be
   done in a rather schematic fashion.

   Furthermore, to accommodate SIP-H.323 gateways, a mapping from SDPng
   to H.245 needs to be specified at some point.

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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.org

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Full Copyright Statement

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