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Network Working Group                                           Kutscher
Internet-Draft                                                       Ott
Expires: August 30, 2002                                         Bormann
                                                TZI, Universitaet Bremen
                                                          March 01, 2002


             Session Description and Capability Negotiation
                     draft-ietf-mmusic-sdpng-04.txt

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
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   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
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   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 August 30, 2002.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This document defines a language for describing multimedia sessions
   with respect to configuration parameters and capabilities of end-
   systems.

   This document is a product of 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 mmusic@ietf.org and/or the authors.

Document Revision



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   $Revision: 4.23 $

Table of Contents

   1.      Introduction . . . . . . . . . . . . . . . . . . . . . . .  4
   2.      Terminology and System Model . . . . . . . . . . . . . . .  6
   3.      SDPng  . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   3.1     Conceptual Outline . . . . . . . . . . . . . . . . . . . .  9
   3.1.1   Definitions  . . . . . . . . . . . . . . . . . . . . . . .  9
   3.1.2   Components & Configurations  . . . . . . . . . . . . . . . 11
   3.1.3   Constraints  . . . . . . . . . . . . . . . . . . . . . . . 13
   3.1.4   Session Attributes . . . . . . . . . . . . . . . . . . . . 14
   3.1.4.1 Owner  . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   3.1.4.2 Session Identification . . . . . . . . . . . . . . . . . . 15
   3.1.4.3 Time Specification (SDP 't=', 'r=', and 'z=' lines)  . . . 16
   3.1.4.4 Component Semantic Specification . . . . . . . . . . . . . 17
   3.2     Syntax Definition Mechanisms . . . . . . . . . . . . . . . 18
   3.3     Referencing Definitions  . . . . . . . . . . . . . . . . . 20
   3.3.1   The sdpng:use Element Type . . . . . . . . . . . . . . . . 21
   3.3.2   Properties . . . . . . . . . . . . . . . . . . . . . . . . 22
   3.3.3   Definition Groups  . . . . . . . . . . . . . . . . . . . . 23
   3.3.4   Usage of Child Elements and Attributes of sdpng:use
           Elements . . . . . . . . . . . . . . . . . . . . . . . . . 26
   3.4     External Definition Packages . . . . . . . . . . . . . . . 28
   3.4.1   Profile Definitions  . . . . . . . . . . . . . . . . . . . 28
   3.4.2   Library Definitions  . . . . . . . . . . . . . . . . . . . 29
   3.5     Mappings . . . . . . . . . . . . . . . . . . . . . . . . . 30
   4.      Capability Negotiation . . . . . . . . . . . . . . . . . . 32
   4.1     Outline of the Negotiation Process . . . . . . . . . . . . 32
   4.2     The Collapsing Algorithm . . . . . . . . . . . . . . . . . 34
   4.2.1   Collapsing Two Configurations  . . . . . . . . . . . . . . 35
   4.2.1.1 Collapsing of Attributes . . . . . . . . . . . . . . . . . 35
   4.2.1.2 Collapsing two Elements  . . . . . . . . . . . . . . . . . 38
   4.2.1.3 Collapsing nested Elements . . . . . . . . . . . . . . . . 39
   4.2.2   Deriving an actual Configuration . . . . . . . . . . . . . 41
   5.      Formal Specification . . . . . . . . . . . . . . . . . . . 42
   5.1     XML Schema as a Definition Mechanism . . . . . . . . . . . 42
   5.2     SDPng Schema . . . . . . . . . . . . . . . . . . . . . . . 43
   5.3     Profiles . . . . . . . . . . . . . . . . . . . . . . . . . 44
   5.4     SDPng Documents  . . . . . . . . . . . . . . . . . . . . . 45
   5.5     Libraries  . . . . . . . . . . . . . . . . . . . . . . . . 46
   5.6     Details on the use of specific XML Mechanisms  . . . . . . 47
   5.6.1   Default Namespace  . . . . . . . . . . . . . . . . . . . . 47
   5.6.2   Qualified Locals . . . . . . . . . . . . . . . . . . . . . 47
   5.6.3   Fixed Namespace Prefixes . . . . . . . . . . . . . . . . . 48
   5.7     SDPng Schema Definitions . . . . . . . . . . . . . . . . . 48
   5.7.1   SDPng Base Definition  . . . . . . . . . . . . . . . . . . 48
   5.7.2   Audio Codec Profile  . . . . . . . . . . . . . . . . . . . 55



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   5.7.3   RTP profile  . . . . . . . . . . . . . . . . . . . . . . . 56
   5.8     Issues . . . . . . . . . . . . . . . . . . . . . . . . . . 59
   6.      Use of SDPng in conjunction with other IETF Signaling
           Protocols  . . . . . . . . . . . . . . . . . . . . . . . . 60
   6.1     The Session Announcement Protocol (SAP)  . . . . . . . . . 60
   6.2     Session Initiation Protocol (SIP)  . . . . . . . . . . . . 61
   6.3     Real-Time Streaming Protocol (RTSP)  . . . . . . . . . . . 67
   6.4     Media Gateway Control Protocol (MEGACOP) . . . . . . . . . 68
   7.      Open Issues  . . . . . . . . . . . . . . . . . . . . . . . 69
           References . . . . . . . . . . . . . . . . . . . . . . . . 70
           Authors' Addresses . . . . . . . . . . . . . . . . . . . . 71
   A.      Base SDPng Specifications for Audio Codec Descriptions . . 72
   A.1     DVI4 . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
   A.2     G.722  . . . . . . . . . . . . . . . . . . . . . . . . . . 73
   A.3     G.726  . . . . . . . . . . . . . . . . . . . . . . . . . . 73
   A.4     G.728  . . . . . . . . . . . . . . . . . . . . . . . . . . 73
   A.5     G.729  . . . . . . . . . . . . . . . . . . . . . . . . . . 73
   A.6     G.729 Annex D and E  . . . . . . . . . . . . . . . . . . . 74
   A.7     GSM  . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
   A.7.1   GSM Full Rate  . . . . . . . . . . . . . . . . . . . . . . 74
   A.7.2   GSM Half Rate  . . . . . . . . . . . . . . . . . . . . . . 74
   A.7.3   GSM Enhanced Full Rate . . . . . . . . . . . . . . . . . . 74
   A.8     L8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
   A.9     L16  . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
   A.10    LPC  . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
   A.11    MPA  . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
   A.12    PCMA and PCMU  . . . . . . . . . . . . . . . . . . . . . . 75
   A.13    QCELP  . . . . . . . . . . . . . . . . . . . . . . . . . . 75
   A.14    VDVI . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
   B.      SDPng Library for Audio Codec Definitions  . . . . . . . . 76
   C.      SDPng Library for RTP Payload Format Definitions . . . . . 77
   D.      Change History . . . . . . . . . . . . . . . . . . . . . . 78
           Full Copyright Statement . . . . . . . . . . . . . . . . . 79


















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1. Introduction

   Multiparty multimedia conferencing is one of the applications that
   require 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 or for broadcast scenarios, 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 such as session directories can configure media tools upon
   startup.  This procedure however fails to work for conferences
   initiated spontaneously including Internet phone calls or ad-hoc
   multiparty conferences.  Fixed settings for parameters such as 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-
   system.

   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 [2] 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:



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

   o  to describe the capabilities of a system and possibly provide a
      choice between a number of alternatives (which SDP was not
      designed for).

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

   This document is based upon a set of requirements specified in a
   companion document [1].  In the following, we first introduce a model
   for session description and capability negotiation as well as the
   basic terms used throughout this specification (section 2).  Next, we
   outline the concept for the concepts underlying SDPng and introduce
   the syntactical components step by step in section 3.  In section 4,
   we provide a formal definition of the SDPng session description
   language.  Finally, we overview aspects of using SDPng with various
   IETF signaling protocols in section 5.  In Appendix A, we provide
   basic audio codec and payload type definitions that are subsumed in
   SDPng libraries in Appendix B and Appendix C.

   The next version of this draft will only contain the formal
   specification of the language itself.  Requirements and the
   description of the system model will be moved to a separate document.


























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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 use of these components in a conference.

   Each component can be characterized at least by (a) its intended use
   (i.e. the function it shall provide) and (b) 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
      white-board.  For each of these cases, additional parameters may
      exist, variations of which lead to additional configurations (see
      below).

   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



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      could H.261 encoded QCIF images.  All three cases constitute
      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 modelling multimedia sessions, 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 protocols).

   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.




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

      *  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-based [9] 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 to determine the subset
   of allowable potential configurations is deterministic to reduce the
   number of required round trips before a session can be established.
   For instance, in order to be used with SIP, the capability
   negotiation is required to work with the offer/answer model that is
   for session initiation with SIP -- limiting the negotiation to
   exactly one round trip.

   The requirements for the SDPng specification, subdivided into general
   requirements and requirements for session descriptions, potential and
   actual configurations as well as negotiation rules, are captured in a
   companion document [1].




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3. SDPng

   This section introduces the underlying concepts of the Session
   Description Protocol - next generation (SDPng).  The focus of this
   section is on the concepts of the capability description and
   negotiation language with a stepwise introduction of the various
   syntactical elements.  Note that this section does only examples
   accompanied by explanations -- a full formal specification is
   provided in section 4.

3.1 Conceptual Outline

   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.

   The 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); see Section 3.1.1.

      Potential or Actual Configurations; see Section 3.1.2.

      Constraints; see Section 3.1.3.

      Session attributes; see Section 3.1.4.


3.1.1 Definitions

   The "Definitions" section specifies a number of basic abstractions
   that are later referenced to avoid repetitions in more complex
   specifications and allow for a concise representation.  Definition
   elements are labelled with an identifier by which they may be
   referenced.  They may be elementary or compound (i.e.  combinations
   of elementary entities).  Examples of definitions that could occur in
   "Definitions" sections include (but are not limited to) codec
   definitions, redundancy schemes, transport mechanisms and payload
   formats.

   Elementary definition elements do not reference other elements.  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)
   configurations.  A mechanisms for overriding definitions is specified
   below.



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   For the moment, elementary abstractions are defined for media types
   (i.e. codecs) and for media transports mechanisms.  For each
   transport and for each codec to be used, the respective attributes
   need to be defined.  This definition may either be provided within
   the "Definitions" section itself or in an external document (similar
   to the audio-video profile or an IANA registry that defines payload
   types and media stream identifiers).

   It is not required to define all codecs and transport mechanisms in a
   "Definitions" sections and reference them when specifying potential
   and actual configurations.  Instead, a syntactic mechanism is defined
   that allows to give some definitions directly in a configurations
   section.

   Examples for elementary definitions:

   <audio:codec name="audio-basic" encoding="PCMU"
                sampling="8000" channels="1"/>


   <audio:codec name="audio-L16-mono" encoding="L16"
                sampling="44100" channels="1"/>

   The element type "audio:codec" is used in these examples to define
   audio codec configurations.  The configuration parameters are given
   as attribute values.

   Definitions may have default values specified along with them for
   each attribute (as well as for their contents).  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.  In the following example the definition of audio-
   L16-mono is re-used for the defintion of the corresponding stereo
   codec.  Appendix A provides a complete set of corresponding
   audio:codec definitions of the codecs used in RFC 1890 [4].

   <audio:codec name="audio-L16-stereo" ref="audio-L16-mono"
                channels="2"/>

   The example shows how existing definitions can be referenced in new
   definitions.  This approach allows to create simple as well as more
   complex definitions in an extensible set of reference documents.
   Section 3.4 specifies the mechanisms for external references.

   Besides definitions of audio codecs other definitions such as RTP
   payload formats and specific transport mechanisms are suitable to be
   defined in a definition section for later referencing.  The following



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   example shows how RTP payload types are defined using a pre-defined
   codec.

   <rtp:pt name="rtp-avp-0" pt="0" format="audio-basic"/>
   <rtp:pt name="rtp-avp-11" pt="11" format="audio-L16-mono"/>

   In this example, the payload type "rtp-avp-11" is defined with
   payload type number 11, referencing the codec "audio-L16-mono".
   Instead of referencing an existing definition it is also possible to
   define the format "inline":

   <rtp:pt name="rtp-avp-10" pt="10">
    <audio:codec encoding="L16" sampling="44100" channels="2"/>
   </rtp:pt>

   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.
   Again, see Section 3.4 for complete discussion of external
   definitions.

   The "Definitions" section may be empty if all transport, codecs, and
   other pieces needed to the specify Potential and Actual
   Configurations (as detailed below) are either included by referencing
   external definitions or are explicitly described within the
   Configurations themselves.

3.1.2 Components & Configurations

   The "Configurations" section contains all the components that
   constitute the multimedia application (IP telephone call, real-time
   streaming application, multi-player gaming session 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
   configure media streams after negotiation (e.g. with RTSP) or in
   session announcements (e.g. via SAP).  A potential and the actual
   configuration of a component may be identical.

   Each component is labelled with an identifier so that it can be
   referenced, 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 alternative way to
   realize the functionality of the respective component.

   Each configuration (potential as well as actual) is labelled with an



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   identifier.  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.  In addition, it is also possible to provide definition
   elements inline, inside the definition of a configuration.

   Note: Not all protocol environments and their respective operation
   allow to explicitly distinguish between Potential and Actual
   Configurations.  Therefore, SDPng so far does not provide for
   syntactical identification of a Configurations as being a Potential
   or an Actual one.  The semantics of configurations are to be
   determined from the requirements of the specific protocol that uses
   SDPng to express capabilities and configurations.

   The following example shows how RTP sessions can be described by
   referencing payload definitions.

   <cfg>
     <component name="interactive-audio" media="audio">
       <alt name="AVP-audio-0">
         <rtp:session format="rtp-avp-0">
          <rtp:udp addr="224.2.0.53" rtp-port="7800" rtcp-port="7801"/>
         </rtp:session>
       </alt>

       <alt name= AVP-audio-11">
         <rtp:session format="rtp-avp-11">
          <rtp:udp addr="224.2.0.53" rtp-port="7800" rtcp-port="7801"/>
         </rtp:session>
       </alt>
      </component>
   </cfg>

   For example, an IP telephone call may require just a single component
   "name=interactive-audio" with two possible ways of implementing it.
   The two corresponding configurations are "AVP-audio-0" without
   modification, the other ("AVP-audio-11") uses linear 16-bit encoding.
   Typically, transport address parameters such as the port number would
   also be provided.  In this example, this information is given by the
   "rtp:udp" element.  Of course, it must be possible to specify other
   transport mechanisms as well.  See Section 3.2 for a discussion of
   extension mechanisms that allow applications to use non-standard
   transport (or other) specifications.

   During/after the negotiation phase, an actual configuration is chosen
   out of a number of alternative potential configurations, the actual
   configuration may refer to the potential configuration just by its



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   "id", possibly allowing for some parameter modifications.
   Alternatively, the full actual configuration may be given.

   Instead of referencing existing payload type definitions it is also
   possible to provide the required information "inline".  The following
   example illustrates this:

   <cfg>
     <component name="audio1" media="audio">
       <alt name="AVP-audio-0">
         <rtp:session>
          <rtp:pt pt="0">
           <audio:codec name="audio-basic" encoding="PCMU"
                        sampling="8000" channels="1"/>
          </rtp:pt>
          <rtp:udp addr="224.2.0.53" rtp-port="7800" rtcp-port="7801"/>
         </rtp:session>
       </alt>
      </component>
   </cfg>

   The UDP/IPv4 multicast transport that is used in the examples is a
   simple variant of a transport specification.  More complex ones are
   conceivable.  For example, it must also be possible to specify the
   usage of source filters (inclusion and exclusion), Source Specific
   Multicast, the usage of multi-unicast, or other parameters such as
   QoS parameters.  Therefore it is possible to extend the definition of
   transport mechanisms by providing the required information in the
   element content.  An example:

   <cfg>
     <component name="audio1" media="audio">
       <alt name="AVP-audio-0">
         <rtp:session format="rtp-avp-0">
          <rtp:udp addr="224.2.0.53" rtp-port="7800" rtcp-port="7801">
           <option name="ssm" sender="sender.example.com"/>
          </rtp:udp>
         </rtp:session>
       </alt>
      </component>
   </cfg>

   Additional transport mechanisms and options will be defined in future
   versions of this document.

3.1.3 Constraints

   Definitions specify media, transport, and other capabilities, whereas



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   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 as
   CPU cycles, DSP resources 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 may exist across application
   boundaries.  Also, in many cases, expressing such constraints is
   simply not necessary (as many uses of the current SDP show), so
   additional overhead 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.  The following
   example shows the definition of a constraints that restricts the
   maximum number of instantiation of two alternatives (that would have
   to be defined in the configuration section before) when they are used
   in parallel:

   <constraints>
     <par>
       <use-alt ref="AVP-audio-11" max="5">
       <use-alt ref="AVP-video-32" max="1">
     </par>
   </constraints>

   As the example shows, constraints are defined by defining limits on
   simultaneous instantiations of alternatives.  They are not defined by
   expressing abstract end-system resources, such as CPU speed or memory
   size.

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

3.1.4 Session Attributes

   The fourth and final section of the SDPng syntax addresses session
   layer attributes.  These attributes largely include those defined by
   SDP [RFC2327] (which are explicitly indicated in the following
   specification) to describe originator, purpose, and timing of a
   multimedia session among other characteristics.  Furthermore, SDPng
   includes attributes indicating the semantics of the various
   Components in a teleconference or other session.  This part of the
   specification is open ended with an IANA registry to be set up to



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   register further types of components; only a few of the examples are
   listed here.

   A session-level specification for connection information (SDP "c="
   line), bandwidth information (SDP "b=" line), and encryption keys
   (SDP "k=" lines) is deliberately not provided for in SDPng.  The
   relevant information can be specified directly in the Configuration
   section for individual alternatives.

   Session level attributes as defined by SDP still have to be examined
   and adopted for SDPng in a future revision of this specification.

3.1.4.1 Owner

   The owner refers to the creator of a session as defined in RFC2327
   ("o=" line).  The syntax is as follows:

   <owner user="username" session-id="session-id" version="version"
          nettype="IN" addrtype="IP4" addr="130.149.25.97"/>

   The owner element must be present if SDPng is used with SAP.  For all
   other protocols, the owner element is not necessarily required.  The
   attributes listed above match those from the SDP specification; all
   attributes must be present and they are used following the rules of
   RFC2327.

   The owner element is an empty element.

3.1.4.2 Session Identification

   The "session" element is used to identify the session and to provide
   a description and possible further references.  It provides the
   following attributes:

   name: The session name as it is to appear e.g. in a session
      directory.  This is equivalent to the SDP "s=" line.

   The session element can contain arbitrary text of any length (but
   authors are encouraged to keep the inline description brief and
   provide additional information via URLs using the info element
   described below.  This text is used to provide a description of the
   session; it is the equivalent of the SDP "i=" lines.

   Additionally, the session element can contain other elements of the
   following types to provide further information about the session and
   its creator:

   info: The info element is intended to provide a pointer to further



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      information on the session itself.  It is an empty element and
      provides the attribute xlink:href that is used to specify an URI.
      Info elements are optional, they may occur any number of times.

   contact: The contact element provides contact information on the
      creator of the session.  It is an empty element and provides an
      attribute xlink:href that is used to specify an URI.  Any URI
      scheme suitable to reach a person or a group of persons is
      acceptable (e.g. sip:, mailto:, tel:).  Contact elements are
      optional, they may occur any number of times.


   <session name="An SDPng seminar">
       And here comes a long description of the seminar indicating what
       this might be about and so forth. But we also include further
       information -- as additional elements:
       <info xlink:href="http://www.ietf.org/"/>
       <contact xlink:href="mailto:joe@example.com"/>
       <contact xlink:href="mailto:bob@example.com"/>
       <contact xlink:href="tel:+49421218"/>
       <contact xlink:href="sip:joe@example.com"/>
       <contact xlink:href="sip:bob@example.com"/>
   </session>


3.1.4.3 Time Specification (SDP 't=', 'r=', and 'z=' lines)

   The time specification for a session follows the same rules as in
   SDP.  Time specifications are usually only meaningful when used in
   conjunction with SAP and are optional.  SDPng uses the following
   elements and attributes to specify timing:

   The element "time" is used to indicate a schedule for the session;
   time has two optional attributes:

   start: The starting time of the first occurrence of the session as
      defined in RFC2327.

   end: The ending time of the last occurrence of the session as defined
      in RFC2327.

   The time element can contain the following elements:

   repeat: This element specifies the repetition pattern for the
      schedule.  There may be zero or more occurrences of this element
      within the time element.  "repeat" has two mandatory and one
      optional attribute and is an empty element; the attributes are as
      defined in SDP:



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      interval: The duration between two start times of the session.
         This attribute is always present.

      duration: The duration for which the session will be active
         starting at each repetition interval.  This attribute is always
         present.

      offset: The offset relative to "start" attribute at which this
         repetition of the session is start.  This attribute is
         optional; if it is absent, a default value of "0" is assumed.

      Formatting of the attribute values follows the rules defined in
      RFC2327.

   zone: The zone element specifies one or more time zone adjustments as
      defined in RFC2327.  This element has zero or more occurrences in
      the time element.  It is an empty element and has two attributes
      as defined in SDP:

      adjtime: The time at which the next adjustment will take place.

      delta: The adjustment offset (typically +/- 1 hours).


   The example from RFC2327, page 16, expressed in SDPng:

   <time start="3034423619" stop="3042462419">
     <repeat interval="7d" duration="1h"/>
     <repeat interval="7d" duration="1h" offset="25h"/>
   </time>

   The time element can occur multiple times.

3.1.4.4 Component Semantic Specification

   Another important session parameter is to specify - ideally in a
   machine-readable way but at least understandable for humans - the
   function of the various components in a session.  Typically, the
   semantics of the streams are implicitly assumed (e.g. a video stream
   goes together with the only audio stream in a session).  There are,
   however, scenarios in which such intuitive understanding is not
   sufficient and the semantics must be made explicit.

   <info name="audio-interactive" function="speaker">
       Audio stream for the different speakers
   </info>

   The above example shows a simple definition of the semantics for the



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   component "audio-interactive".  Further options may be added to
   provide additional information, e.g. language, and other functions
   may be specified (e.g. "panel", "audience", "chair", etc.).

3.2 Syntax Definition Mechanisms

   In order to allow for the possibility to validate session
   descriptions and in order to allow for structured extensibility,
   SDPng relies on a syntax framework that provides concepts as well as
   concrete procedures for document validation and extending the set of
   allowed syntax elements.

   SGML/XML technologies allow for the creation 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.  XML DTDs however, cannot easily be extended.  It is not
   possible to alter to content models of element types or to add new
   element types after the DTD has been specified.

   For SDPng, a mechanism is needed that allows the specification of a
   base syntax -- for example basic elements for the high level
   structure of description documents -- while allowing extensions, for
   example elements and attributes for new transport mechanisms, new
   media types etc. to be added on demand.  Still, it has to be ensured
   that extensions do not result in name collisions.  Furthermore, it
   must be possible for applications that process descriptions documents
   to distinguish extensions from base definitions.

   For XML, mechanisms have been defined that allow for structured
   extensibility of a model of allowed syntax: XML Namespace and XML
   Schema.

   XML Schema mechanisms allows to constrain the allowed document
   content, e.g. for documents that contain structured data and also
   provide the possibility that document instances can conform 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



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   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, endpoints 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.

   Section 3.4 describes profile definitions and library definition.  A
   detailed definition of how the formal SDPng syntax and the
   corresponding extension mechanisms is provided in Section 5.

   The example below shows how the definition of codecs, transport-
   variants and configuration of components as well as a conference
   description are realized in SDPng.

   <def>
    <audio:codec name="audio-basic" encoding="PCMU"
                 sampling="8000" channels="1"/>

    <audio:codec name="audio-L16-mono" encoding="L16"
                 sampling="44100" channels="1"/>

    <rtp:pt name="rtp-avp-0" pt="0" format="audio-basic"/>
    <rtp:pt name="rtp-avp-11" pt="11" format="audio-L16-mono"/>

   </def>

   <cfg>
     <component name="interactive-audio" media="audio">
       <alt name="AVP-audio-0">
         <rtp:session format="rtp-avp-0">
          <rtp:udp addr="224.2.0.53" rtp-port="7800" rtcp-port="7801"/>
         </rtp:session>
       </alt>

       <alt name="AVP-audio-11">
         <rtp:session format="rtp-avp-11">



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          <rtp:udp addr="224.2.0.53" rtp-port="7800" rtcp-port="7801"/>
         </rtp:session>
       </alt>
      </component>
   </cfg>

   <constraints>
     <par>
       <use-alt ref="AVP-audio-11" max="1">
     </par>
   </constraints>

   <conf>
    <owner user="joe@example.com" id="foobar" version="1" nettype="IN"
                         addrtype="IP4" addr="130.149.25.97"/>
    <session name="An SDPng seminar">
     This seminar is about SDPng...
     <info xlink:href="http://www.ietf.org/"/>
     <contact xlink:href="mailto:joe@example.com"/>
     <contact xlink:href="sip:joe@example.com"/>
    </session>

    <time start="3034423619" stop="3042462419">
     <repeat interval="7d" duration="1h"/>
     <repeat interval="7d" duration="1h" offset="25h"/>
    </time>

    <info name="interactive-audio" function="speaker">
       Audio stream for the different speakers
    <info>

   </conf>


   Section 5 specifies the formal Schema definition that this example is
   conforming to.

   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.

3.3 Referencing Definitions

   This section specifies some generic mechanisms for referencing
   existing definitions.  Referencing existing definition allows to
   contruct definitions without having to include all parameters inline.
   By using these mechanisms, complex definitions can be derived by



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   combining multiple basic mechanisms.  Common parameters that occur in
   different configurations do not have to be repeated but can be
   defined once and then be referenced as often as they are needed.


3.3.1 The sdpng:use Element Type

   The element type "sdpng:use" is a generic reference mechanisms that
   allows to refer to arbitrary definition within another definition or
   configuration element.  "sdpng:use" is an element type with one
   mandatory attribute called "href".  The value of that attribute is
   the name of the definition to be referenced.  An example:

   <def>
      <rtp:udp name="endpoint-addr-1" rtp-port="7800" addr="224.2.0.53"/>
   </def>
   <cfg>
          <c name="interactive-audio" media="audio">
        <alt name="alt-avp-audio-10">
          <rtp:session format="rtp-avp-10">
               <use href="endpoint-addr-1"/>
          </rtp:session>
        </alt>
         </c>
   <cfg>


   In this example, an element "rtp:udp" is used in the definitions
   section to define some transport parameters that should later be re-
   used by referencing this definition using the specified name
   "endpoint-addr-1".  Within the element "rtp:session" in the
   configurations section the definition is referenced using the "use"
   element.

   An implementation that processes this SDPng document and wants to
   evaluate the configuration for the alternative "rtp-avp-10" MUST
   replace the "use" element by the referenced element.  If the
   referenced element contains "use" elements itself, those MUST also be
   dereferenced.

   When applying this algorithm to the sample SDPng document, the
   following result SDPng document is generated:









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   <def>
      <rtp:udp name="endpoint-addr-1" rtp-port="7800" addr="224.2.0.53"/>
   </def>
   <cfg>
          <c name="interactive-audio" media="audio">
           <alt name="alt-avp-audio-10">
             <rtp:session format="rtp-avp-10">
                <rtp:udp name="endpoint-addr-1" rtp-port="7800" addr="224.2.0.53"/>
             </rtp:session>
           </alt>
         </c>
   <cfg>


   For the purpose of comparing configurations, both SDPng documents are
   equal.

3.3.2 Properties

   The element type "sdpng:prop" can be used to add properties to
   definitions.  "sdpng:prop" has two attributes:

   name: the name of the property

   value: the value for the named property

   For example:

   <def>
      <audio:codec name="g722" encoding="G722" channels="1" sampling="16000"/>
      <rtp:pt name="rtp-avp-9" pt="9" format="g722"/>
      <rtp:udp name="endpoint-addr-1" rtp-port="7800" addr="224.2.0.53"/>
   </def>
   <cfg>
          <c name="interactive-audio" media="audio">
           <alt name="alt-avp-audio-9-4">
             <rtp:session format="rtp-avp-9">
              <use href="endpoint-addr-1"/>
             </rtp:session>
             <prop name="foo" value="bar"/>
           </alt>
         </c>
   <cfg>


   For comparing and collapsing elements, all sdpng:prop element that
   are contained in a parent element (like alt in the example above)
   MUST be transformed to attributes of the containing element.  If the



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   parent element already provides a corresponding attribute its value
   MUST be overwritten.

   The example above would thus be transformed to:

   <def>
      <audio:codec name="g722" encoding="G722" channels="1" sampling="16000"/>
      <rtp:pt name="rtp-avp-9" pt="9" format="g722"/>
      <rtp:udp name="endpoint-addr-1" rtp-port="7800" addr="224.2.0.53"/>
   </def>
   <cfg>
          <c name="interactive-audio" media="audio">
           <alt name="alt-avp-audio-9-4" foo="bar">
             <rtp:session format="rtp-avp-9">
              <use href="endpoint-addr-1"/>
             </rtp:session>
           </alt>
         </c>
   <cfg>


   The main purpose of the sdpng:prop element type is to provide a
   mechanism by which attributes of referenced elements can be modified
   by the referring element.  An application for this is described in
   Section 3.3.4.

3.3.3 Definition Groups

   Using the sdpng:group element arbitrary definition can be combined
   and defined as a group with a specific name.  Using this name, the
   definitions contained in the group can be referenced with the
   sdpng:use element and embedded into other elements.

   An example for the use of the sdpng:group element:

















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   <def>
      <audio:codec name="g722" encoding="G722" channels="1" sampling="16000"/>
      <rtp:pt name="rtp-avp-9" pt="9" format="g722"/>
      <rtp:udp name="endpoint-addr-1" rtp-port="7800" addr="224.2.0.53"/>

      <group name="g1">
       <prop name="foo" value="bar"/>
       <prop name="xyz" value="uvw"/>
      </group>

   </def>
   <cfg>
          <c name="interactive-audio" media="audio">

           <alt name="alt-avp-audio-9-4">
             <rtp:session format="rtp-avp-9">
              <use href="endpoint-addr-1"/>
             </rtp:session>
             <use href="g1"/>
           </alt>
         </c>
   <cfg>


   This example shows how a group that has been defined in the
   definitions section is referenced using the sdpng:use element.  The
   group element contains two sdpng:prop elements.

   For comparing and collapsing elements, all references to sdpng:group
   element MUST be replaced by the content of the corresponding
   sdpng:group element.  The example above would thus be transformed to:




















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   <def>
      <audio:codec name="g722" encoding="G722" channels="1" sampling="16000"/>
      <rtp:pt name="rtp-avp-9" pt="9" format="g722"/>
      <rtp:udp name="endpoint-addr-1" rtp-port="7800" addr="224.2.0.53"/>

      <group name="g1">
       <prop name="foo" value="bar"/>
       <prop name="xyz" value="uvw"/>
      </group>

   </def>
   <cfg>
          <c name="interactive-audio" media="audio">

           <alt name="alt-avp-audio-9-4">
             <rtp:session format="rtp-avp-9">
              <use href="endpoint-addr-1"/>
             </rtp:session>
             <prop name="foo" value="bar"/>
             <prop name="xyz" value="uvw"/>
           </alt>
         </c>
   <cfg>


   In this example the content of the sdpng:group element named g1 has
   been embedded into the alt element that contained the sdpng:use
   element referencing the group element.

   According to the rules in Section 3.3.2 the sdpng:prop elements are
   transformed in a second step to yield the following final decription:




















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   <def>
      <audio:codec name="g722" encoding="G722" channels="1" sampling="16000"/>
      <rtp:pt name="rtp-avp-9" pt="9" format="g722"/>
      <rtp:udp name="endpoint-addr-1" rtp-port="7800" addr="224.2.0.53"/>

      <group name="g1">
       <prop name="foo" value="bar"/>
       <prop name="xyz" value="uvw"/>
      </group>

   </def>
   <cfg>
          <c name="interactive-audio" media="audio">

           <alt name="alt-avp-audio-9-4" foo="bar" xyz="uvw">
             <rtp:session format="rtp-avp-9">
              <use href="endpoint-addr-1"/>
             </rtp:session>
           </alt>
         </c>
   <cfg>


   As a general rule, all references MUST be resolved before sdpng:prop
   elements are processed and transformed into attribute values.

3.3.4 Usage of Child Elements and Attributes of sdpng:use Elements

   It is also possible to provide arbitrary other elements within a
   sdpng:use element (depending on the specific application).  All
   elements that occur in a sdpng:use element MUST be transfomed to
   child elements of the referenced element when resolving a sdpng:use
   reference.  If the reference already provides child elements, the
   child elements of the sdpng:use element are added to the list of
   child elements of the referenced element.

   Any existing elements of a referenced element with the same GI as an
   element in the corresponding sdpng:use element MUST be replaced by
   the element of the sdpng:use element.  This mechanism allows to
   extend and to change referenced elements in a simple way.

   In the following we give an example of using an sdpng:prop element
   within a sdpng:use element which has the semantics of adding
   properties to the referenced element.  The semantics and processing
   requirements for the sdpng:prop element are specified in Section
   3.3.2.

   Example for the usage of an sdpng:use element containing an



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   sdpng:prop element:

   <def>
      <rtp:udp name="endpoint-addr-1" rtp-port="7800" addr="224.2.0.53"/>
   </def>
   <cfg>
          <c name="interactive-audio" media="audio">
           <alt name="alt-avp-audio-10">
             <rtp:session format="rtp-avp-10">
               <use href="endpoint-addr-1">
                <prop name="foo" value="bar"/>
               </use>
             </rtp:session>
           </alt>
         </c>
   <cfg>


   This will be transformed to:

   <def>
      <rtp:udp name="endpoint-addr-1" rtp-port="7800" addr="224.2.0.53"/>
   </def>
   <cfg>
          <c name="interactive-audio" media="audio">
           <alt name="alt-avp-audio-10">
             <rtp:session format="rtp-avp-10">
                <rtp:udp name="endpoint-addr-1" rtp-port="7800" addr="224.2.0.53">
                  <prop name="foo" value="bar"/>
                </rtp:udp>
             </rtp:session>
           </alt>
         </c>
   <cfg>


   In a second step, the sdpng:prop element would be transformed to an
   attribute of its parent element (rtp:udp in this case) according to
   the rules specified in Section 3.3.2.

   As an abbreviation, the properties for the referenced element do not
   have to be specified using sdpng:prop elements within the sdpng:use
   element but can also specified directly as attributes of the
   sdpng:use element, as shown in the following example:







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   <def>
      <rtp:udp name="endpoint-addr-1" rtp-port="7800" addr="224.2.0.53"/>
   </def>
   <cfg>
          <c name="interactive-audio" media="audio">
           <alt name="alt-avp-audio-10">
             <rtp:session format="rtp-avp-10">
               <use href="endpoint-addr-1" name="foo" value="bar"/>
             </rtp:session>
           </alt>
         </c>
   <cfg>


   In this example, the sdp:use element has no child element sdpng:prop
   but provides the property "foo" directly as an attribute.  All
   attributes of a sdpng:use element other than href MUST be transformed
   to attributes of the referenced elements.

   If the referenced element is a definition group (see Section 3.3.3),
   any child elements of an sdpng:use element MUST be transformed to
   child elements of the parent element of the sdpng:use element.  Any
   properties (either explicit sdpng:prop elements or attributes of the
   sdpng:use element) MUST be transformed to properties of the parent
   element of the sdpng:use element.

3.4 External Definition Packages

   There are two types of external definitions:

   Profile Definitions (Section 3.4.1) define rules for specifying
      parameters that are not covered by the base SDPng specification.

   Library Definitions (Section 3.4.2) contain definitions that can be
      referenced in SDPng documents.


3.4.1 Profile Definitions

   In order to allow for extensibility it must be possible to define
   extensions to the basic SDPng configuration options.

   For example, if some application requires the use of a new transport
   protocol, endpoints must be able to describe their configuration with
   respect to the parameters of that transport protocol.  The mandatory
   and optional parameters that can be configured and negotiated when
   using the transport protocol will be specified in a definition
   document.  Such a definition document is called a "profile".



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   A profile contains rules that specify how SDPng is used to describe
   conferences or end-system capabilities with respect to the parameters
   of the profile.  The concrete properties of the profile definitions
   mechanism are still to be defined.

   An example of such a profile would be the RTP profile that defines
   how to specify RTP parameters.  Another example would be the audio
   codec profiles that defines how specify audio codec parameters.

   SDPng documents can reference profiles and provide concrete
   definitions, for example the definition for the GSM audio codec.
   (This would be done in the "Definitions" section of an SDPng
   document.) An SDPng document that references a profile and provides
   concrete definitions of configurations can be validated against the
   profile definition.

3.4.2 Library Definitions

   While profile definitions specify the allowed parameters for a given
   profile, SDPng "Definitions" sections refer to profile definitions
   and define concrete configurations based on a specific profile.

   In order for such definitions to be imported into SDPng documents,
   "SDPng libraries" may be defined and referenced in SDPng documents.
   A library is a set of definitions that is conforming to one or more
   profile definitions.

   The purpose of the library concept is to allow certain common
   definitions to be factored-out so that not every SDPng document has
   to include the basic definitions, for example the PCMU codec
   definition.  SDP [2] uses a similar concept by relying on the well
   known static payload types (defined in RFC1890 [4]) that are also
   just referenced but never defined in SDP documents.

   An SPDng document that references definitions from an external
   library has to declare the use of the external library.  The external
   library, being a set of configuration definitions for a given
   profile, again needs to declare the use of the profile that it is
   conforming to.  A library itself can make reference to other external
   libraries.

   There are different possibilities of how profiles definitions and
   libraries can be used in SDPng documents:

   o  In an SPDng document, a profile definition can be referenced and
      all the configuration definitions are provided within the document
      itself.  The SDPng document is self-contained with respect to the
      definitions it uses.



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   o  In an SPDng document, the use of an external library can be
      declared.  The library references a profile definition and the
      SDPng document references the library.  There are two alternatives
      how external libraries can be referenced:

      by name: Referencing libraries by names implies the use of a
         registration authority where definitions and reference names
         can be registered with.  It is conceivable that the most common
         SDPng definitions be registered that way and that there will be
         a baseline set of definitions that minimal implementations must
         understand.  Secondly, a registration procedure will be
         defined, that allows vendors to register frequently used
         definitions with a registration authority (e.g., IANA) and to
         declare the use of registered definition packages in conforming
         SDPng documents.  Of course, care should be taken not to make
         the external references too complex and thus require too much a
         priori knowledge in a protocol engine implementing SDPng.
         Relying on this mechanism in general is also problematic
         because it impedes the extensibility, as it requires
         implementors to provide support for new extensions in their
         products before they can inter-operate.  Registration is not
         useful for spontaneous or experimental extensions that are
         defined in an SDPng library.

      by address: An alternative to referencing libraries by name is to
         declare the use of an external library by providing an address,
         i.e., an URL, that specifies where the library can be obtained.
         While this allows the use of arbitrary third-party libraries
         that can extend the basic SDPng set of configuration options in
         many ways, in introduces additional complexity that could
         result in in higher latency for the processing of a description
         document with references to external libraries.  In addition,
         there are problems if the referenced libraries cannot be
         accessed by all communication partners.

   o  Because of these problematic properties of external libraries, the
      final SDPng specification will have to provide a set of
      recommendations under which circumstances the different mechanisms
      of referring to external definitions should be used.


3.5 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 for the basic definitions.




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   In addition, mappings to H.245 will be defined in order to support
   applications like SIP-H.323 gateways.

















































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4. Capability Negotiation

   SDPng is a description language for both potential configurations
   (i.e. capabilities) of participants in multimedia conferencers and
   for actual configurations (i.e. final specifications of parameters).
   Capability negotiation is the process of generating a usable set of
   potential configurations and finally an actual configuration from a
   set of potential configurations provided by each potential
   participant in a multimedia conference.

   SDPng supports the specification of endpoint capabilities and defines
   a negotiation process: In a negotiation process, capability
   descriptions are exchanged between participants.  These descriptions
   are processed in a "collapsing" step which results in a set of
   commonly supported potential configurations.  In a second step, the
   final actual configuration is determined that is used for a
   conference.  This section specifies the usage of SDPng for capability
   negotiation.  It defines the collapsing algorithm and the procedures
   for exchanging SDPng documents in a negotiation phase.

   The description language and the rules for the negotiation phase that
   are defined here are (in general) independent of the means by which
   descriptions are conveyed during a negotiation phase (a reliable
   transport service with causal ordering is assumed).  There are
   however properties and requirements of call signalling protocols that
   have been considered to allow for a seamless integration of the
   negotiation into the call setup process.  For example, in order to be
   usable with SIP, it must be possible to negotiate the conference
   configuration within the three-way-handshake of the call setup phase.
   In order to use SDPng instead of SDP according to the offer/answer
   model defined in [15] it must be able to determine an actual
   configuration in a single request/response cycle.

4.1 Outline of the Negotiation Process

   Conceptually, the negotiation process comprises the following
   individual steps (considering two parties, A and B, where A tries to
   invite B to a conference).  Please note that is describes the steps
   of the negotiation process conceptually -- it does not specify
   requirements for implementations.  Specific procedures that MUST be
   followed by implementations are given below.

   1.  A determines its potential configurations for the components that
       should be used in the conference (e.g. "interactive audio" and
       "shared whiteboard") and sends a corresponding SDPng instance to
       B.  This SDPng instances is denoted "CAP(A)".

   2.  B receives A's SDPng instance and analyzes the set of components



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       (sdpng:c elements) in the description.  For each component that B
       wishes to support it generates a list of potential configurations
       corresponding to B's capabilities, denoted "CAP(B)".

   3.  B applies the collapsing function and obtains a list of potential
       configurations that both A and B can support, denoted
       "CAP(A)xCAP(B) = CAP(AB)".

   4.  B sends CAP(B) to A.

   5.  A also applies the collapsing function and obtains "CAP(AB)".  At
       this step, both A and B know each other capabilities and the
       potential configurations that both can support.

   6.  In order to obtain an actual configuration from the potential
       configuration that have been obtained, both particpants have to
       pick a subset of the potential configurations should actually be
       used in the conference and generate the actual configuration.  It
       should be noted that it depends on the specific application
       whether each component must be assigned exactly one actual
       configuration (one sdpng:alt element) or whether it is allowed to
       list multiple actual configurations.  In this model we assume
       that A selects the actual configuration, denoted CFG(AB).

   7.  A augments CFG(AB) with the transport parameters it intends to
       use, e.g., on which endpoint addresses A wishes to receive data,
       obtaining CFG_T(A).  A sends CFG_T(A) to A.

   8.  B receives CFG_T(A) and adds its own transport parameters,
       resulting in CFG_T(AB).  CFG_T(AB) contains the selected actual
       configurations and the transport parameters of both A and B (plus
       any other SDPng data, e.g., meta-information on the conference).
       CFG_T(AB) is the complete conference description.  Both A and B
       now have the following information:

       CAP(A) A's supported potential configurations

       CAP(B) B's supported potential configurations

       CAP(AB) The set of potential configurations supported by both A
          and B.

       CFG(AB) The set of actual configurations to be used.

       CFG_T(AB) The set of actual configurations to be used augmented
          with all required parameters.

   In this model, the capability negotiation and configuration exchange



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   process leads to a description that represents a global view of the
   configuration that should be used.  This means, it contains the
   complete configuration for all participants including per-participant
   information like transport parameters.

   Note that the model presented here results in four SDPng exchanges.
   As an optimization, this procedure can be abbreviated to two
   exchanges by including the transport (and other) parameters into the
   potential configurations.  A embeds its desired transport parameters
   into the list of potential configurations and B also sends all
   required parameters in the response together with B's potential
   configurations.  Both A and B can then derive CFG_T(AB).  Transport
   parameters are usually not negotiable, therefor they have to be
   distingiushed them from other configuration information.

   Specific procedures for re-negotiation and multi-party negotiation
   will be defined in a future version of this document.

4.2 The Collapsing Algorithm

   The following procedure MUST be used for the collapsing of two SDPng
   document instances into one:

   CAP(A) and CAP(B) are the two SDPng description document instances.
   For each component (sdpng:c element) in CAP(A) there is a
   corresponding component in CAP(B).  Components MAY be empty
   (containing no sdpng:alt elements) which means that there is no
   potential configuration and the component should not be used in the
   conference.

   Let cfg_AB be the result configuration element, initialized to an
   empty sdpng:cfg element.

   1.  For each component (sdpng:c element) in CAP(A) named c_A

       *  Let c_AB be the current result component, initialized to an
          empty sdpng:c element.

       *  For each alternative (sdpng:alt element) in c_A named a_A

          +  For each session element (name depends on the profile being
             used) in a_A named s_A

             -  Resolve any reference to definition elements recursively
                and obtain s1_A, the standalone media session
                description.  (Refer to Section 4.2.1 for a description
                of how to resolve references.)




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             -  Locate the component element that matches c_A in CAP(B)
                named (c_B).

             -  Let a_AB be the current result alternative, initialized
                to an empty sdpng:alt element.

             -  For each alternative (sdpng:alt element) in c_B named
                a_B

                o  For each session element (name depends on the profile
                   being used) in a_B named s_B

                   *  Let s1_AB be the computed result media session
                      configuration.

                   *  Resolve any reference to definition elements
                      recursively and obtain s1_B, the standalone media
                      session description.

                   *  Apply collapse(s1_A,s2_B) to compute s1_AB, the
                      collapsed media session configuration.

                   *  If s1_AB is not empty, add s1_AB to a_AB, the set
                      of sessions for the current result alternative.

             -  If a_AB is not empty, add a_AB to c_AB.

       *  If c_AB is not empty, add c_AB to cfg_AB.

   The collapsing function for collapsing two elements is specified in
   Section 4.2.1.

4.2.1 Collapsing Two Configurations

   Before two media session configuration element can be collapsed as
   described in Section 4.2 all references to definitions MUST be
   resolved.  This MUST be performed recursively, i.e.  references in
   definitions MUST also be resolved.  For resolving references, the
   algorithm specified in Section 3.3 MUST be used.

   By resolving all references two intermediate session configuration
   elements are obtained that can then be collapsed according to the
   algorithm specified in the following sections.

4.2.1.1 Collapsing of Attributes

   In SDPng, capabilities are specified in attributs of XML elements.
   Three different types of capabilities with different collapsing rules



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   are defined.  The type of a capability is encoded in the attribute
   value.

   Set of symbols:
      An attribute can specify a set of symbols.  When two attributes
      are collapsed the result is the intersection of the two sets.

      The following examples shows how two elements (with one attribute
      representing a set of symbols) originated from two capability
      descriptions from participants A and B are collapsed:

                      Element x in A's capability description:
                      <x a="[FOO, BAR, 3, 5, 8]"/>

                      Element x in B's capability description:
                      <x a="[3, 6, 8]"/>

                      Result:
                      <x a="[3, 8]"/>


      If the intersection result in an empty set the collapsing process
      has failed and there is no common set of values.  If the
      collapsing of one of an element's attributes with the type "set of
      symbols" has failed, the collapsing process of the element itself
      MUST be considered to have failed as well.

   Numerical ranges:
      An attribute can also specify a numercial range.  When two
      attributes are collapsed the result is the range of values that
      represents the intersection of the set of values that is included
      in both ranges.

      The following examples shows how two elements (with one attribute
      representing a numerical range) originated from two capability
      descriptions from participants A and B are collapsed:

                      Element x in A's capability description:
                      <x a="(2,8)"/>

                      Element x in B's capability description:
                      <x a="(5,10)"/>

                      Result:
                      <x a="(5,8)"/>


      A numerical range is represented by a tuple of comma-separated



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      numbers in brackets.  The first number represents the lower bound
      of the range and the second number represents the upper bound.
      Let MIN(a,b) be a function that returns the minimum of a and b and
      MAX(a,b) be a function that returns the maximum of a and b.  Given
      two ranges (minA, maxA) and (minB, maxB), the collapsed new range
      MUST be calculated using this algorithm:

                      (MAX(minA, minB), MIN(maxA, maxB))

       If this process results in a range with a smaller first value,
      the range is invalid and the collapsing has failed since there is
      no common range.  If the collapsing of one of an element's
      attributes with the type "numerical range" has failed, the
      collapsing process of the element itself MUST be considered to
      have failed as well.

   Optional parameters:
      A failure of collapsing attributes of the types "set of symbols"
      and "numerical range" results in a failure of collapsing the
      corresponding element.  There is a third type named "optional
      parameter" defined, that provides different collapsing rules.  An
      optional parameter is an attribute with an arbitrary value.  When
      collapsing two attributes of this type, their values MUST be
      tested for equality.  If they are equal, the collapsing has been
      successful and the attribute MUST appear as is in the result
      description.  If the attributes' values are different, the
      collapsing is considered to have failed and the attribute MUST not
      appear in the result description.  However, a failure in
      collapsing an attribute of type "optional parameter" does not
      affect the collapsing of the containing element.

      An example for a successful collapsing:

                      Element x in A's capability description:
                      <x a="foo"/>

                      Element x in B's capability description:
                      <x a="foo"/>

                      Result:
                      <x a="foo"/>

       An example for an unsuccessful collapsing:








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                      Element x in A's capability description:
                      <x a="foo"/>

                      Element x in B's capability description:
                      <x a="bar"/>

                      Result:
                      <x/>


4.2.1.2 Collapsing two Elements

   In order to collapse two elements with multiple attributes, the
   following algorithm specified below MUST be applied.  In general, the
   collapsing of two elements (if successful) yields a result element
   that contains the collapsed attributes.  If the collapsing of two
   elements has failed, no result element is generated.

   1.  For each attribute, determine the type and collapse the attribute
       by applying the algorithm for the corresponding attribute type.

   2.  If an attribute with a different type than "optional parameter"
       does not occur in both elements, the collapsing for this element
       MUST be considered to have failed.

   3.  If the collapsing of any attribute with a different type than
       "optional parameter" has failed, the collapsing of the element
       itself MUST be considered to have failed.

   4.  If the collapsing has been successful, obtain the result element
       by using the same element name (GI) and the attributes with their
       collapsed values.  Exclude any attribute of type "optional
       parameter" that has failed to collapse.

   An example:

                  Element x in A's capability description:
                  <x a="[FOO, BAR, 3, 5, 8]" b="(2,8)" c="foo"/>

                  Element x in B's capability description:
                  <x a="[3, 6, 8]" b="(5,10)" c="bar"/>

                  Result:
                  <x a="[3, 8]" b="(5,8)"/>







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4.2.1.3 Collapsing nested Elements

   In order to collapse nested elements the following algorithm MUST be
   applied:

   In analogy to attributes representing optional parameters there is
   also the possibility to mark elements as optional for the negotiation
   process.  Elements MAY provide an attribute names "status" that
   contains a symbol or a comma-separated list of symbols as its value.
   If the value "opt" occurs in the list of a "status" attribute of both
   elements to be collapsed, the elements to be collapsed are treated as
   optional.  This means, if the collapsing of the attributes has failed
   (according to the rules specified in Section 4.2.1.2), the collapsing
   process does not yield a result element but is still treated as
   "successful", i.e., further collapsing operation on other elements
   can continue.  The semantics of optional elements are that they
   describe optional features that may be supported and selected during
   a negotiation phase but do not neccessarily have to be supported by
   all participants in order to achieve interoperability.  The example
   below shows how to generate a result element in the presence of
   optional child elements that have failed to collapse.

   The collapsing algorithm for nested elements:

   1.  Let x be an element that occurs in the capability description of
       two participants A and B and that should be collapsed.

   2.  Collapse the attributes of the element x using the algorithm
       specified in Section 4.2.1.2.  If the collapsing has failed
       according to the rules of Section 4.2.1.2 and if the elements to
       be collapsed are not marked as optional, the collapsing of the
       element and all of its children MUST be considered to have
       failed.  The collapsing MUST be stopped.  If the collapsing has
       failed and both elements have been marked as optional, the child
       elements MUST NOT be processed.  In this case, the collapsing
       process does not yield a result element but the collapsing of
       other elements (sibling or parent elements) MUST be continued.

   3.  If the collapsing has been successful according to the rules of
       Section 4.2.1.2, the child elements of A's and B's x element MUST
       be processed.  If there are no child elements in both A's and B's
       content the collapsing has been successful and can be terminated.
       If either A's or B's x element provides child elements, apply the
       following algorithm to each child element named c of participant
       A's element x:

       1.  Find a corresponding element (same GI) in the set of
           participant B's child elements.  If no matching element has



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           been found, the collapsing of element x MUST be considered to
           have failed.

       2.  If a matching element has been found, apply the collapsing
           algorithm recursively.  As long as the collapsing is
           successful, the result of collapsing each element is
           transferred to the result element, such that the resulting
           element tree is isomorphic to both A's and B's element tree.

       If there are elements in B's x element that have not been
       processed (because there is no corresponding element in A's x
       element), the collapsing MUST be considered to have failed and
       MUST be stopped.

   An example:

                  Element x in A's capability description:
                  <x a="[FOO, BAR, 3, 5, 8]" b="(2,8)" c="foo">
                    <y b="[UVW, XYZ]"/>
                  </a>

                  Element x in B's capability description:
                  <x a="[3, 6, 8]" b="(5,10)" c="bar">
                    <y b="[RST, XYZ]"/>
                  </a>

                  Result:
                  <x a="[3, 8]" b="(5,8)">
                    <y b="[XYZ]"/>
                  </a>

   An example for collapsing optional elements:

                  Element x in A's capability description:
                  <x a="[FOO, BAR, 3, 5, 8]" b="(2,8)" c="foo">
                    <y status="opt" b="[UVW, XYZ]"/>
                  </a>

                  Element x in B's capability description:
                  <x a="[3, 6, 8]" b="(5,10)" c="bar">
                    <y status="opt" b="[RST]"/>
                  </a>

                  Result:
                  <x a="[3, 8]" b="(5,8)"/>






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4.2.2 Deriving an actual Configuration

   The result of a capability negotiation process is a potential
   configuration, i.e., a description potentially containing multiple
   alternatives per component.  The alternative themselves may provide
   elements that represent collapsed capabilities.  In order to derive
   an actual configuration, the following problems must be addressed:

   1.  For each component (sdpng:c element) an appropriate alternative
       (sdpng:alt element) has to be selected.  It is conceivable that
       the order of the alternatives in the description is used as a
       preference indicator.  More details have to be specified in a
       future version of this document.

   2.  If the description of the selected alternatives contains
       attributes with numerical ranges or sets of symbols with more
       than one entry, those attributes either have to be transformed
       that they represent a single value or participants have to agree
       that an actual configuration may contain ranges and sets of
       symbols.  The semantics of these variable actual configurations
       will have to specified in later versions of this document.  For
       example, for certain applications it may be desireable to agree
       on ranges of values for certain attributes during a capability
       negotiating meaning that any of the values of the range are
       supported (and have to be supported).

   The specific procedures to determine an actual configuration have to
   be defined in a later version on this document.























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5. Formal Specification

   This section defines the SDPng syntax and the use of XML mechanisms,
   such as XML Namespace and XML Schema.  Section 5.1 defines the
   relation between SDPng and XML Schema, Section 5.2 specifies general
   requirements for documents and profile definitions that are
   conforming to the SDPng schema, Section 5.3 list requirements for
   profile definitions, Section 5.4 specifies specific requirements for
   conforming documents and Section 5.5 lists requirements for the
   definition of SDPng libraries.

   Section 5.7 defines the SDPng base schema, Section 5.7.2 defines the
   profile for audio codec definitions and Section 5.7.3 defines the
   profile for RTP payload type definitions.

5.1 XML Schema as a Definition Mechanism

   SDPng documents reference profile schema definitions and libraries.
   Profile schema definitions contain schema definitions of SDPng
   document elements.  For example, the general structure is specified
   by a schema definition and extensions to SDPng for specific
   applications are specified as schema definitions as well.

   The baseline SDPng specification consists of a profile (a schema
   definition) and a library of commonly used definitions.

   SDPng uses XML-Schema [13][14] for defining the possible logical
   structures of SDPng documents for the following reasons:

   Extensibility: XML-Schema provides mechanisms that allow to extend
      existing definitions allowing to uniquely identify element types
      (by relying on XML namespaces [11]).

   Modularity: XML-Schema provide mechanisms that allow to organize
      schema definitions in multiple components.

   Expressiveness: XML-Schema provides many data types, that can be
      refined by user-supplied definitions.

   SDPng documents MUST be schema instances of the SDPng schema as
   defined in Section 5.7.  The following example shows how a Schema
   definition can be referenced in a document instance.

   Beginning of an SDPng-document:







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   <?xml version="1.0" ?>
   <sdpng:desc xmlns:sdpng="http://www.iana.org/sdpng"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-Instance"
            xsi:schemaLocation="http://www.iana.org/sdpng sdpng.xsd">

   XML-Schema specifies that documents can assign a namespace when
   referencing a schema definition.  A SDPng namespace is defined for
   this purpose.  The name of this namespace is
   "http://www.iana.org/sdpng".  A well-known namespace prefix is used
   for the SDPng schema definition, in order to allow for very simple
   implementations.  The well-known SDPng namespace prefix is "sdpng".
   Conforming Documents, profile definition and libraries MUST use this
   namespace name and this namespace prefix.

   For SDPng documents, this initial declaration can be added implicitly
   by applications, so that declarations like the one above do not have
   to be included in every description document.  Details are to be
   defined in a later version of this document.

5.2 SDPng Schema

   The basic SDPng schema definitions specifies the general document
   structures, e.g., one "Definitions" section followed by one
   "Configurations" sections, followed by one "Constraints" sections
   followed by a "Conference" section (for meta-information).  Each
   document MUST provide the elements for definitions, configurations,
   constraints and conference information in exactly this order, whereby
   only the configurations section is MANDATORY.  Refer to Section 5.7
   for a formal definition of the SDPng base schema and the specific
   element types for definitions, configurations, constraints and
   conference information.

   The SDPng base schema also specifies "abstract" base data types (by
   means of XML-Schema type definitions) for elements that MUST be used
   by documents in the corresponding sections.  The base data types
   provide common required attributes, e.g. a "name" attribute for
   naming definition elements.

   Example:
   The following example shows the definition of the base type for
   definition elements:

   <xsd:complexType name="Definition" abstract="true" mixed="false">
   <xsd:attribute name="name" type="xsd:string"/>
   </xsd:complexType>

   Profiles can then define specific types that augment the base type
   definitions.  Common attributes or content models, that have been



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   defined by this base definition, do not have to be provided by those
   concrete type definitions.  The type definitions can be identified as
   allowed element types for the content models that are specified in
   the base SDPng schema definition.  This allows for automatic
   validation of profile definitions and facilitates the extension of
   SDPng.

5.3 Profiles

   The baseline SDPng specification consists of a profile (a schema
   definition) and a library of commonly used definitions.

   The library of commonly used definitions provides data types for IP
   (and other) addresses.

   A profile definition MUST import (using the XML-Schema import
   mechanism) the base SDPng schema definition and MUST provide an
   extension definition, e.g., specializations of base element types.  A
   profile definition MUST also provide a target namespace name for its
   definitions.  For well-known (registered) profiles, the namespace
   name will be registered by IANA.  Proprietary profiles will use other
   namespace names, for example, based on domain names, that are
   registered by vendors providing a profile.

   Example:
   The following example shows such a declaration at the beginning of a
   profile definition:

   <xsd:schema targetNamespace="http://www.iana.org/sdpng/audio"
            xmlns:xsd="http://www.w3.org/2001/XMLSchema"
            xmlns:sdpng="http://www.iana.org/sdpng"
            xmlns:audio="http://www.iana.org/sdpng/audio">

     <xsd:import namespace="http://www.iana.org/sdpng"
                 schemaLocation="sdpng.xsd"/>

   In this example, the namespace prefix "audio" is defined and later
   used in schema definitions.  (The example profile provides definition
   mechanisms for audio codecs.)

   The following example shows, how a derived type for "definition"
   elements can be specified with XML-Schema mechanisms.  In this case,
   the abstract type "Definition" (fully qualified as
   "sdpng:Definition") is augmented by three attributes that are useful
   for defining audio codecs.

   Example:




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   <xsd:complexType name="AudioCodec" mixed="false">
     <xsd:complexContent>
       <xsd:extension base="sdpng:Definition">
        <xsd:attribute name="encoding" type="xsd:string"/>
        <xsd:attribute name="sampling" type="xsd:positiveInteger"/>
        <xsd:attribute name="channels" type="xsd:positiveInteger"/>
       </xsd:extension>
      </xsd:complexContent>
   </xsd:complexType>

   This type definition is then used to define an XML element type
   called "codec".

   Example:

            <xsd:element name="codec" type="AudioCodec"/>

   When used by SDPng documents, the general identifier is qualified
   with a namespace prefix, for example as in: "audio:codec".

5.4 SDPng Documents

   SDPng documents MUST reference the employed profiles and provide
   namespace prefixes for the namespace names of the profiles as shown
   in the following example.

   Example:

   <sdpng:desc xmlns:sdpng="http://www.iana.org/sdpng"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-Instance"
            xsi:schemaLocation="http://www.iana.org/sdpng sdpng.xsd"
            xmlns:audio="http://www.iana.org/sdpng/audio"
            xmlns:rtp="http://www.iana.org/sdpng/rtp">

   For well-known registered profiles, the namespace name AND the used
   namespace prefix SHOULD be registered to allow for simple basic
   implementations that can match identifiers by using fixed fully
   qualified names without having to interpret namespace declarations
   (see Section 5.6.3).  There is one issue with declaring used XML-
   Schema definitions in documents (see Section 7 below).

   The general structure of an SDPng documents MUST conform to the basic
   SDPng schema definition and MAY provide a "def" element for
   definitions; it MUST provide a "cfg" element for the configuration
   section; it MAY provide a "constraints" and a "conf" element.

   Example:
   The following example shows a sample definition section where the



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   element "codec" of the "audio codec profile" is used (plus the
   element type "pt" of an "RTP profile"):

       <def>
         <audio:codec name="dvi4" encoding="DVI4" channels="1"
                      sampling="8000"/>
         <audio:codec name="g722" encoding="G722" channels="1"
                      sampling="16000"/>
         <audio:codec name="g729" encoding="G729" channels="1"
                      sampling="8000"/>

         <rtp:pt name="rtp-avp-18" pt="18" format="g729"/>
       </def>

   It can be seen how the attribute name (provided by the base type for
   definition elements) and the profile specific attributes "encoding",
   "channels" and "sampling" are used together.

   The element "rtp:pt" is used to defined a payload type.  "rtp:pt"
   would have been defined in another profile, again using a type
   derived from the base definition type.  "rtp:pt" provides attribute
   for referencing other definitions, e.g., the definition of audio-
   codes as seen before.

5.5 Libraries

   SDPng libraries are collections of definitions that are referenced by
   documents.  Libraries are thus independent, valid SDPng documents.

   For example, the definition of the different audio codecs as shown in
   the previous example could be provided by a library that can be
   referenced by documents without having to define such common codecs
   in every document.

   The XML mechanism XInclude [12] is used for referencing libraries in
   SDPng documents.  XInlcude works at the XML Information Set
   ("infoset") level, i.e.  the mechanisms allows to have an integrating
   document reference fragment documents, while these fragments are
   well-formed (and, if applicable, valid) documents themselves.  By
   resolving XInclude directives in integrating documents the documents'
   infosets are "merged" together, enabling applications to operate on
   the resulting infosets as if it had been generated by parsing a
   single, monolithic document.

   Inclusion at the XML infoset level has the advantage that documents
   are standalone -- they can be validated independently.  Another
   advantage is that is relatively easy to generate a "merged" infoset
   for applications that are not able to resolve references to libraries



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   themselves.

   An alternative for XInclude would be to use references that are
   resolved by applications.  For XML, this would probably mean to use
   an XLink-based approach.  This solution would require the definition
   of an SDPng link element type and require applications to support
   XLink (or at least the SDPng-relevant subset thereof).  The inclusion
   at the application level is however problematic, because it does not
   result in a common integrated XML document infoset but would require
   applications to handle multiple infosets, i.e. multiple documents.

5.6 Details on the use of specific XML Mechanisms

   This section specifies the use of specific XML mechanisms for SDPng.
   In order to allow for efficient parsing and processing, not all
   features of XML Schema are allowed.  Some variable information is set
   to fixed values to allow the development of simplistic servers.

5.6.1 Default Namespace

   SDPng document instances MUST use the SDPng namespace
   "http://www.iana.org/sdpng".  That means, the general SDPng
   identifiers can be used without namespace prefixes.

5.6.2 Qualified Locals

   XML Schema allows to specify qualification of elements and
   attributes.  It is possible to use non-qualified element and
   attribute names in Schema definitions and document instances for so-
   called "local definitions" (this is the default setting).  "Local
   Definitions" are contained within "global definitions" in an XML
   schema definition.  In order to simplify parsing and processing of
   SDPng document instances, all elements MUST be fully qualified.
   Attribute names MUST NOT be fully qualified, they are considered to
   have the same namespace as their corresponding elements.

   This means, the SDPng Schema definition contains the following
   attributes for the "schema" element, that MUST also be used by SDPng
   profiles:

   o  elementFormDefault="qualified"
      This means that "locally defined" elements that are used within
      the scope of fully-qualified elements MUST always be fully
      qualified as well.

   o  attributeFormDefault="unqualified"
      This means that attribute names do not have to be fully qualified.
      Implementations MUST infer the namespace for attributes from the



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      namespace of the element that the attribute belongs to.  Note that
      the specification of XML Namespaces [11] defines that default
      namespaces do not apply to attributes.  In profile definitions,
      all attributes MUST be defined locally.  The same holds for the
      base SDPng schema.

   These rules make SDPng document instances process-able by non-Schema-
   aware XML parsers by requiring all element names to be fully
   qualified (no "local elements").

5.6.3 Fixed Namespace Prefixes

   In order to facilitate the development of basic implementations, a
   few commonly used namespaces names are associated with fixed
   prefixes, i.e. document instances and libraries MUST always use these
   prefixes.  These prefixes MUST NOT be used for namespaces names than
   the ones that are assigned to them.  In order to ensure the
   uniqueness of namespace prefixes, namespace prefixes will be have to
   registered together with the corresponding namespace names.

   The namespace prefix for the SDPng namespace is "sdpng".

5.7 SDPng Schema Definitions

   This section provides the definition of the base SDPng XML Schema.

   1.  Section 5.7.1 contains the base definition that provides the
       general element types for SDPng.

   2.  Section 5.7.2 contains a profile for audio codecs.

   3.  Section 5.7.3 contains a profile for RTP payload type
       definitions.


5.7.1 SDPng Base Definition

   This schema definition defines the general structure of SDPng
   document instances.  It defines the top-level element type "desc"
   that can have a sequence of "def", "cfg", "constraints" and "conf"
   elements as element content.

   In addition, "extensions hooks" are provided that can be used by
   extension profiles providing definitions for specific applications.
   In general, these extension are implemented by deriving profile
   definitions from SDPng base definitions.  The deployed XML Schema
   mechanisms are "deriving by extension" and "substitution groups".
   The SDPng base definition provides different base types (as



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   complexType definitions) for elements that are to be used in "def",
   "cfg" and "conf" sections.  In addition, it also defines specific
   element types as "head elements" with assigned types that are used
   for defining the content model of, e.g., the "def" element type.

   Profiles that provide new element types for specific applications
   will define types that are derived from the base types and provide
   the additional attributes and element content definitions that are
   required for the application.  The XML element types that are defined
   in a profile are declared as valid substitutes for the base elements
   ("head elements") by setting the "substitutionGroup" attribute to the
   name of the "head element" type.

   For an extension-profile that provides new definition element types,
   e.g. for codec definitions, a new complexType would be defined that
   extends sdpng:Definition (see below).  An element type definition
   that assigns that new type must then be declared to be in the
   substitutionGroup "sdpng:d".

   This mechanism allows common rules for attributes and content models
   to be defined in base element definition and re-used by extension
   profiles and it also allows validating parsers to identify the
   correct type of elements that have been defined by profile
   definitions.

   The SDPng Base Definition:

   <xsd:schema targetNamespace="http://www.iana.org/sdpng"
            xmlns:sdpng="http://www.iana.org/sdpng"
            xmlns:xsd="http://www.w3.org/2001/XMLSchema"
            elementFormDefault="qualified"
            attributeFormDefault="unqualified">

     <xsd:annotation>
       <xsd:documentation>
         This schema definition defines the general structure of SDPng
         document instances. It provides base type and base element
         definition for elements to occur in the different sections (def,
         cfg, constraints, conf) to be derived from in extension-profile
         definitions.

         For an extension-profile that provides new definition element
         types, e.g. for codec definitions, a new complexType would be
         defined that extends sdpng:Definition (see below). An element
         type definition that assigns that new type must then be declared
         to be in the substitutionGroup "sdpng:d".
       </xsd:documentation>
     </xsd:annotation>



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     <xsd:element name="desc">
       <xsd:annotation>
         <xsd:documentation>
        The top-level element of an SDPng document. It defines the
        overall structure of an SPDng document.
         </xsd:documentation>
       </xsd:annotation>
       <xsd:complexType mixed="false">
         <xsd:sequence>
        <xsd:element ref="sdpng:def" minOccurs="0"/>
        <xsd:element ref="sdpng:cfg"/>
        <xsd:element ref="sdpng:constraints" minOccurs="0"/>
        <xsd:element ref="sdpng:conf" minOccurs="0"/>
         </xsd:sequence>
       </xsd:complexType>
     </xsd:element>

   <!-- +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->

     <xsd:element name="def">
       <xsd:annotation>
         <xsd:documentation>The definitions section</xsd:documentation>
       </xsd:annotation>
       <xsd:complexType mixed="false">
         <xsd:sequence>
        <xsd:element ref="sdpng:d" maxOccurs="unbounded"/>
         </xsd:sequence>
       </xsd:complexType>
     </xsd:element>

   <!-- +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->

     <xsd:element name="cfg">
       <xsd:annotation>
         <xsd:documentation>The configurations section</xsd:documentation>
       </xsd:annotation>
       <xsd:complexType mixed="false">
         <xsd:sequence>
        <xsd:element ref="sdpng:c" maxOccurs="unbounded"/>
         </xsd:sequence>
       </xsd:complexType>
     </xsd:element>

   <!-- +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->

     <xsd:element name="constraints">
       <xsd:annotation>
         <xsd:documentation>The constraints section</xsd:documentation>



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       </xsd:annotation>
       <xsd:complexType mixed="false">
         <xsd:sequence>
        <xsd:element ref="sdpng:cn" maxOccurs="unbounded"/>
         </xsd:sequence>
       </xsd:complexType>
     </xsd:element>

   <!-- +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->

     <xsd:element name="conf" type="sdpng:Conference">
       <xsd:annotation>
         <xsd:documentation>The conference section</xsd:documentation>
       </xsd:annotation>
     </xsd:element>

   <!-- +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->

     <xsd:element name="d" type="sdpng:Definition">
       <xsd:annotation>
         <xsd:documentation>
        Placeholder base element for a definition element in the
        definitions section. To be derived from by specific definition
        element type definitions.
         </xsd:documentation>
       </xsd:annotation>
     </xsd:element>

   <!-- +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->

     <xsd:element name="c" type="sdpng:Component">
       <xsd:annotation>
         <xsd:documentation>
        Placeholder base element for a configuration element in the
        configurations section. To be derived from by specific
        configuration element type definitions.
         </xsd:documentation>
       </xsd:annotation>
     </xsd:element>

   <!-- +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->

     <xsd:element name="cn" type="sdpng:Constraint">
       <xsd:annotation>
         <xsd:documentation>
        Placeholder base element for a contraint element in the
        contraints section. To be derived from by specific constraint
        element type definitions.



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         </xsd:documentation>
       </xsd:annotation>
     </xsd:element>

   <!-- +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->

     <xsd:complexType name="Definition" abstract="true" mixed="false">
       <xsd:annotation>
         <xsd:documentation>
        The base type for definition. Defines a attribute "name" for
        naming definitions.
         </xsd:documentation>
       </xsd:annotation>
       <xsd:attribute name="name" type="xsd:string"/>
     </xsd:complexType>

   <!-- +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->

     <xsd:complexType name="Component" mixed="false">
       <xsd:annotation>
         <xsd:documentation>
        The specification of a component consists of a sequence of
        alternatives.
         </xsd:documentation>
       </xsd:annotation>
       <xsd:sequence>
         <xsd:element ref="sdpng:alt" minOccurs="1" maxOccurs="unbounded"/>
       </xsd:sequence>
       <xsd:attribute name="name" type="xsd:string"/>
       <xsd:attribute name="media" type="xsd:string"/>
     </xsd:complexType>

     <xsd:element name="alt">
       <xsd:annotation>
         <xsd:documentation>
        Each alternative consists of a "sc" (session configuration)
        element. The "sc" element is a base element of base type
        "sdpng:Session" that is used to derive specific session types
        in extension profiles.
         </xsd:documentation>
       </xsd:annotation>
       <xsd:complexType mixed="false">
         <xsd:sequence>
        <xsd:element ref="sdpng:sc" minOccurs="1" maxOccurs="1"/>
         </xsd:sequence>
         <xsd:attribute name="name" type="xsd:string"/>
       </xsd:complexType>
     </xsd:element>



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     <xsd:element name="sc" type="sdpng:SessionConfig"/>


     <xsd:complexType name="SessionConfig" abstract="true" mixed="false">
       <xsd:annotation>
         <xsd:documentation>
        The (abstract) base type for session elements. To be derived
        from in extension profiles.
         </xsd:documentation>
       </xsd:annotation>
     </xsd:complexType>

   <!-- +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->

     <xsd:complexType name="Constraint" mixed="false">
       <xsd:annotation>
         <xsd:documentation>
        The current content model for constraints is a sequence of
        "sdpng:par" elements. In each "par" element a sequence of
        "use-alt" elements may be used to specific the definitions
        that may used in parallel. Each "use-alt" element can define
        the number of minimum and maximum instantiations.
         </xsd:documentation>
       </xsd:annotation>
       <xsd:sequence>
         <xsd:element ref="sdpng:par"/>
       </xsd:sequence>
     </xsd:complexType>

     <xsd:element name="par">
       <xsd:complexType mixed="false">
         <xsd:sequence>
        <xsd:element ref="sdpng:use-alt">
        </xsd:element>
         </xsd:sequence>
       </xsd:complexType>
     </xsd:element>


     <xsd:element name="use-alt">
       <xsd:complexType mixed="false">
         <xsd:attribute name="ref" type="xsd:string"/>
         <xsd:attribute name="min" type="xsd:positiveInteger"
                     use="optional"/>
         <xsd:attribute name="max" type="xsd:positiveInteger"
                     use="optional"/>
       </xsd:complexType>
     </xsd:element>



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   <!-- +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->

     <xsd:complexType name="Conference" mixed="false">
       <xsd:sequence>
         <xsd:element name="meta" type="sdpng:ConfItem"/>
       </xsd:sequence>
       <!-- TBD -->
     </xsd:complexType>

     <xsd:complexType name="ConfItem" abstract="true" mixed="false">
       <xsd:annotation>
         <xsd:documentation>
        The base type for conference meta inforformation
        element. Currently, there is no common content model defined.
         </xsd:documentation>
       </xsd:annotation>
     </xsd:complexType>

     <!-- +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->

     <xsd:element name="owner">
       <xsd:complexType mixed="false">
         <xsd:complexContent mixed="false">
        <xsd:extension base="sdpng:ConfItem">
          <xsd:attribute name="user" type="xsd:string"/>
          <xsd:attribute name="session-id" type="xsd:string"/>
          <xsd:attribute name="version" type="xsd:string"/>
          <xsd:attribute name="nettype" type="xsd:string"/>
          <xsd:attribute name="addrtype" type="xsd:string"/>
          <xsd:attribute name="addr" type="xsd:string">
            <!-- FIXME: re-use common address type! -->
          </xsd:attribute>
        </xsd:extension>
         </xsd:complexContent>
       </xsd:complexType>
     </xsd:element>

   <!-- +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->

     <xsd:complexType name="SimpleLink" mixed="false">
       <xsd:attribute name="xlink:type" type="xsd:string" fixed="simple"/>
       <xsd:attribute name="xlink:role" type="xsd:string"/>
       <xsd:attribute name="xlink:arcrole" type="xsd:string"/>
       <xsd:attribute name="xlink:title" type="xsd:string"/>
       <xsd:attribute name="xlink:show" type="xsd:string" fixed="none"/>
       <xsd:attribute name="xlink:actuate" type="xsd:string" fixed="none"/>
       <xsd:attribute name="xlink:href" type="xsd:string"/>
     </xsd:complexType>



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     <xsd:element name="session">
       <xsd:complexType mixed="false">
         <xsd:complexContent mixed="false">
        <xsd:extension base="sdpng:ConfItem">
          <xsd:sequence>
            <xsd:element name="description" type="xsd:string"/>
            <xsd:element name="info" type="sdpng:SimpleLink"/>
            <xsd:sequence minOccurs="0">
              <xsd:element name="contact" type="sdpng:SimpleLink"/>
            </xsd:sequence>
          </xsd:sequence>
        </xsd:extension>
         </xsd:complexContent>
       </xsd:complexType>
     </xsd:element>
   </xsd:schema>



5.7.2 Audio Codec Profile

   The following profile defines an element type that can be used for
   specifying audio codec characteristics.  The element "audio:codec" is
   of type "audio:AudioCodec" which is derived from the SDPng base type
   "sdpng:Definition".  The element "audio:codec" is declared to have
   the substitution group "sdpng:d" (the "head element" of the SDPng
   base definition).

   This means, "audio:codec" element can be used as child elements in
   "sdpng:def" elements.  In addition to the attributes specified here
   "audio:codec" elements will also have to provide a "name" attribute
   as defined by "sdpng:Definition".



















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   <xsd:schema targetNamespace="http://www.iana.org/sdpng/audio"
            xmlns:audio="http://www.iana.org/sdpng/audio"
            xmlns:sdpng="http://www.iana.org/sdpng"
            xmlns:xsd="http://www.w3.org/2001/XMLSchema"
            elementFormDefault="qualified"
            attributeFormDefault="unqualified">

     <xsd:import namespace="http://www.iana.org/sdpng"
              schemaLocation="sdpng.xsd"/>

     <!-- AudioCodecs extends the abstract type "Definition" -->
     <!-- The data types for the attributes could be more restrictive... -->
     <xsd:complexType name="AudioCodec" mixed="false">
       <xsd:complexContent mixed="false">
         <xsd:extension base="sdpng:Definition">
        <xsd:attribute name="encoding" type="xsd:string"/>
        <xsd:attribute name="sampling" type="xsd:positiveInteger"/>
        <xsd:attribute name="channels" type="xsd:positiveInteger"/>
         </xsd:extension>
       </xsd:complexContent>
     </xsd:complexType>


   <xsd:element name="codec" substitutionGroup="sdpng:d">
       <xsd:complexType>
         <xsd:complexContent>
        <xsd:extension base="audio:AudioCodec"/>
         </xsd:complexContent>
       </xsd:complexType>
     </xsd:element>
   </xsd:schema>



5.7.3 RTP profile

   The following profile defines element types that can be used for
   specifying RTP payload types and RTP session configurations.  The
   element "rtp:pt" is of type "rtp:PayloadType" which is derived from
   the SDPng base type "sdpng:Definition".  The element "rtp:pt" is
   declared to have the substitution group "sdpng:d" (the "head element"
   of the SDPng base definition).

   The element "rtp:session" is of type "rtp:Session" which is derived
   from the SDPng base type "sdpng:SessionConfig".  The element
   "rtp:session" is declared to have the substitution group "sdpng:sc"
   (the "head element" of the SDPng base definition).




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   The RTP profile in turn defines base types for the specification of
   transport parameters that are to be derived from by profiles that
   define rules for elements that can be used to specify parameters for
   specific transport mechanisms.

   <xsd:schema targetNamespace="http://www.iana.org/sdpng/rtp"
            xmlns:rtp="http://www.iana.org/sdpng/rtp"
            xmlns:sdpng="http://www.iana.org/sdpng"
            xmlns:xsd="http://www.w3.org/2001/XMLSchema"
            elementFormDefault="qualified"
            attributeFormDefault="unqualified">

     <xsd:import namespace="http://www.iana.org/sdpng"
              schemaLocation="sdpng.xsd"/>

     <xsd:complexType name="PayloadType" mixed="false">
       <xsd:annotation>
         <xsd:documentation>
        PayloadType, the element for payload type definitions is
        derived from "sdpng:Definition". Inside an element of this
        type, more definitions may be given (derived from
        sdpng:Definition themselves). If no definition is given in the
        content, a definition may be referenced using the "format
        attribute".
         </xsd:documentation>
       </xsd:annotation>
       <xsd:complexContent mixed="false">
         <xsd:extension base="sdpng:Definition">
        <xsd:sequence>
          <xsd:element ref="sdpng:d" minOccurs="0" maxOccurs="unbounded"/>
        </xsd:sequence>
        <xsd:attribute name="pt" type="xsd:unsignedByte"/>
        <xsd:attribute name="format" type="xsd:string">
          <!-- IDREF? Issue: unique names for definitions!-->
        </xsd:attribute>
         </xsd:extension>
       </xsd:complexContent>
     </xsd:complexType>

     <xsd:element name="pt" type="rtp:PayloadType" substitutionGroup="sdpng:d"/>

   <!-- ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->

     <xsd:element name="session" type="rtp:Session" substitutionGroup="sdpng:sc"/>

     <xsd:complexType name="Session" mixed="false">
       <xsd:complexContent mixed="false">
         <xsd:extension base="sdpng:SessionConfig">



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        <xsd:sequence>
          <xsd:element name="transport" type="rtp:Transport"/>
        </xsd:sequence>
        <xsd:attribute name="format" type="xsd:string"/>
         </xsd:extension>
       </xsd:complexContent>
     </xsd:complexType>

     <xsd:complexType name="Transport" abstract="true" mixed="false">
       <xsd:complexContent>
         <xsd:extension base="sdpng:Definition">
        <xsd:attribute name="role" type="xsd:string"/>
        <xsd:attribute name="endpoint" type="xsd:string"/>
        <xsd:attribute name="rtp-port" type="xsd:unsignedShort" use="optional"/>
        <xsd:attribute name="rtcp-port" type="xsd:unsignedShort" use="optional"/>
         </xsd:extension>
       </xsd:complexContent>
     </xsd:complexType>

     <xsd:simpleType name="IPAddr">
       <xsd:restriction base="xsd:string"/>
     </xsd:simpleType>

     <xsd:simpleType name="IP4Addr">
       <xsd:restriction base="rtp:IPAddr"/>
     </xsd:simpleType>

     <xsd:simpleType name="IP6Addr">
       <xsd:restriction base="rtp:IPAddr"/>
     </xsd:simpleType>

     <xsd:complexType name="UDP" mixed="false">
       <xsd:complexContent mixed="false">
         <xsd:extension base="rtp:Transport">
        <xsd:choice>
          <xsd:element name="option">
            <!-- define options -->
          </xsd:element>
        </xsd:choice>
        <xsd:attribute name="addr" type="rtp:IP4Addr"/>
         </xsd:extension>
       </xsd:complexContent>
     </xsd:complexType>

     <xsd:element name="udp" type="rtp:UDP"/>

   </xsd:schema>




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5.8 Issues

   o  Libraries provide partially specified definitions, i.e. without
      transport parameters.  How can SDPng documents reference the
      definitions and augment them with specific transport parameters?

   o  Referencing extension profiles: XML-Schema does not support the
      declaration of multiple schemas via the schemaLocation attribute.
      Conceivable solution: When extension profiles are used, the SDPng
      description is a "multi-part" object, that consists of an
      integrating schema definition (that references all necessary
      profiles and the base definition) and the actual description
      document that is a schema instance of the integrating schema.

   o  Uniqueness of attribute values: When libraries are used they will
      contain definition elements with "name" attributes for later
      referencing.  How to avoid name clashes for those identifiers?
      When an SDPng document uses libraries from different sources they
      could be incompatible because of name collisions.  Possible
      solution: Prefix such IDs with a namespace name (either explicitly
      or implicitly by interpreting applications).  The explicit
      prefixes have the advantage that no special knowledge would be
      required to resolve links at the cost of very long ID values.




























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6. Use of SDPng in conjunction with other IETF Signaling Protocols

   The SDPng model provides the notion of Components to indicate the
   intended types of collaboration between the users in e.g. a
   teleconferencing scenario.

   Three different abstractions are defined that are used for describing
   the properties of a specific Component:

   o  a Capability refers to the fact that one of the involved parties
      supports one particular way of exchanging media -- defined in
      terms of transport, codec, and other parameters -- as part of the
      media session.

   o  a Potential Configuration denotes a set of matching Capabilities
      from all those involved parties required to successfully realize
      one particular Component.

   o  an Actual Configuration indicates the Potential Configuration
      which was chosen by the involved parties to realize a certain
      Component at one particular point in time.

   As mentioned before, this abstract notion of the interactions between
   a number of communicating systems needs to be mapped to the
   application scenarios of SDPng in conjunction with the various IETF
   signaling protocols: SAP, SIP, RTSP, and MEGACO.

   In general, this section provides recommendations and possible
   scenarios for the use of SDPng within specific protocols and
   applications.  Is does not specify normative requirements.

6.1 The Session Announcement Protocol (SAP)

   SAP is used to disseminate a previously created (and typically fixed)
   session description to a potentially large audience.  An interested
   member of the audience will use the SDPng description contained in
   SAP to join the announced media sessions.

   This means that a SAP announcement contains the Actual Configurations
   of all Components that are part of the overall teleconference or
   broadcast.

   A SAP announcement may contain multiple Actual Configurations for the
   same Component.  In this case, the "same" (i.e.  semantically
   equivalent) media data from one configuration must be available from
   each of the Actual Configurations.  In practice, this limits the use
   of multiple Actual Configurations to single-source multicast or
   broadcast scenarios.



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   Each receiver of a SAP announcement with SDPng compares its locally
   stored Capabilities to realize a certain Component against the Actual
   Configurations contained in the announcement.  If the intersection
   yields one or more Potential Configurations for the receiver, it
   chooses the one it sees fit best.  If the intersection is empty, the
   receiver cannot participate in the announced session.

   SAP may be substituted by HTTP (in the general case, at least), SMTP,
   NNTP, or other IETF protocols suitable for conveying a media
   description from one entity to one or more other without the intend
   for further negotiation of the session parameters.

   Example from the SAP spec.  to be provided.

6.2 Session Initiation Protocol (SIP)

   SIP is used to establish and modify multimedia sessions, and SDPng
   may be carried at least in SIP INVITE and ACK messages as well as in
   a number of responses.  From dealing with legacy SDP (and its
   essential non-suitability for capability negotiation), a particular
   use and interpretation of SDP has been defined for SIP.

   One of the important flexibilities introduced by SIP's usage of SDP
   is that a sender can change dynamically between all codecs that a
   receiver has indicated support (and has provided an address) for.
   Codec changes are not signaled out-of-band but only indicated by the
   payload type within the media stream.  From this arises one important
   consequence to the conceptual view of a Component within SDPng.

   There is no clear distinction between Potential and Actual
   Configurations.  There need not be a single Actual Configuration be
   chosen at setup time within the SIP signaling.  Instead, a number of
   Potential Configurations is signaled in SIP (with all transport
   parameters required for carrying media streams) and the Actual
   Configuration is only identified by the payload type which is
   actually being transmitted at any point in time.

   Note that since SDPng does not explicitly distinguish between
   Potential and Actual Configurations, this has no implications on the
   SDPng signaling itself.

   SIP relies on an "offer/answer" model for the exchange of capability
   and configuration information.  Either the caller or the callee sends
   an initial session description that is processed by the other side
   and returned.  For capability negotiation, this means that the
   negotiation follows a two-stage-process: The "offerer" sends its
   capability description to the receiver.  The receiver processes the
   offerers capabilities and his own capabilities and generates a result



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   capability description that is sent back to the offerer.  Both sides
   now know the commonly supported configurations and can initiate the
   media sessions.

   Because of this strict "offer/answer" model, the offerer must already
   send complete configurations (i.e. include transport addresses) along
   with the capability descriptions.  The answer must also contain
   complete configuration parameters.  The following figure shows, how
   SDPng content can be used in an INVITE request with a correspong 200
   OK message.

   Simple description document with only one alternative:

      F1 INVITE A -> B

      INVITE sip:B@example.com SIP/2.0
      Via: SIP/2.0/UDP hostA.example.com:5060
      From: A <sip:A@example.com>
      To: B <sip:B@example.com>
      Call-ID: 1234@hostA.example.com
      CSeq: 1 INVITE
      Contact: <sip:UserA@192.168.1.1>
      Content-Type: application/sdpng
      Content-Length: 685

   <def>
    <audio:codec name="audio-basic" encoding="PCMU"
                 sampling="8000" channels="1"/>

    <rtp:pt name="rtp-avp-0" pt="0" format="audio-basic"/>
   </def>

   <cfg>
     <component name="interactive-audio" media="audio">
       <alt name="AVP-audio-0">
         <rtp:session format="rtp-avp-0">
          <rtp:udp role="receive" endpoint="A" addr="192.168.1.1"
                rtp-port="7800"/>
         </rtp:session>
       </alt>
      </component>
   </cfg>

   <conf>
    <owner user="A@example.com" id="98765432" version="1" nettype="IN"
                         addrtype="IP4" addr="192.168.1.1"/>
    <session name="SDPng questions">
    </session>



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    <info name="interactive-audio" function="voice">
     Telephony media stream
    <info>
   </conf>

   ==================================================

      F2 (100 Trying) B -> A

      SIP/2.0 100 Trying
      Via: SIP/2.0/UDP hostA.example.com:5060
      From: A <sip:A@example.com>
      To: B <sip:B@example.com>
      Call-ID: 1234@hostA.example.com
      CSeq: 1 INVITE
      Content-Length: 0

   ==================================================

      F3 180 Ringing B -> A

      SIP/2.0 180 Ringing
      Via: SIP/2.0/UDP hostA.example.com:5060
      From: A <sip:A@example.com>
      To: B <sip:B@example.com>;tag=987654
      Call-ID: 1234@hostA.example.com
      CSeq: 1 INVITE
      Content-Length: 0

   ==================================================

      F4 200 OK B -> A

      SIP/2.0 200 OK
      Via: SIP/2.0/UDP hostA.example.com:5060
      From: A <sip:A@example.com>
      To: B <sip:B@example.com>;tag=987654
      Call-ID: 1234@hostA.example.com
      CSeq: 1 INVITE
      Contact: <sip:B@192.168.1.2>
      Content-Type: application/sdpng
      Content-Length: 479

   <def>
    <audio:codec name="audio-basic" encoding="PCMU"
                 sampling="8000" channels="1"/>

    <rtp:pt name="rtp-avp-0" pt="0" format="audio-basic"/>



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   </def>

   <cfg>
     <component name="interactive-audio" media="audio">
       <alt name="AVP-audio-0">
         <rtp:session format="rtp-avp-0">
          <rtp:udp role="receive" endpoint="A" addr="192.168.1.1"
                rtp-port="7800"/>
          <rtp:udp role="receive" endpoint="B" addr="192.168.1.2"
                rtp-port="9410"/>
         </rtp:session>
       </alt>
      </component>
   </cfg>
   ==================================================

   ACK from A to B omitted







   In the INVITE message, A sends B a description document, that
   specifies exactly one component with one alternative (the PCMU audio
   stream).  All required transport parameters all already contained in
   the description.  The rtp:udp element provides an attribute "role"
   with a value of "receive", indicating that the specified endpoint
   address is used by the endpoint to receive media data.  The element
   also provides the attribute "endpoint" with a value of "A",
   denominating the endpoint that can receive data on the specified
   address.  This means, the semantics of specified transport addresses
   in configuration descriptions are the same as for SDP (when used with
   SIP): An endpoint specifies where it wants to receive data.

   In the 200 OK message, B sends an updated description document to A.
   For the sake of conciseness, the conf element (containing meta
   information about the conference) has been omitted.  B supports the
   payload format that A has offered and adds his own transport
   parameters to the configuration information, specifying the endpoint
   address where B wants to receive media data.  In order to
   disambiguate its transport configurations from A's, B sets the
   attribute "endpoint" to the value "B".  The specific value of the
   "endpoint" attribute is not important, the only requirements are that
   a party that contributes to the session description, must use a
   unique name for the endpoint attribute and that a contributing party
   must use the same value for the endpoint attributes of all elements



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   it adds to the session description.

   The following example shows a capability description that provides
   two alternatives for the audio component.

   Description document with two alternatives:

      F1 INVITE A -> B

      INVITE sip:B@example.com SIP/2.0
      Via: SIP/2.0/UDP hostA.example.com:5060
      From: A <sip:A@example.com>
      To: B <sip:B@example.com>
      Call-ID: 1234@hostA.example.com
      CSeq: 1 INVITE
      Contact: <sip:UserA@192.168.1.1>
      Content-Type: application/sdpng
      Content-Length: 935

   <def>
    <audio:codec name="audio-basic" encoding="PCMU"
                 sampling="8000" channels="1"/>

    <audio:codec name="g729" encoding="G729" channels="1" sampling="8000"/>

    <rtp:pt name="rtp-avp-0" pt="0" format="audio-basic"/>
    <rtp:pt name="rtp-avp-18" pt="18" format="g729"/>

    <rtp:udp name="A-rcv" role="receive" endpoint="A" addr="192.168.1.1"
          rtp-port="7800"/>
   </def>

   <cfg>
     <component name="interactive-audio" media="audio">
       <alt name="AVP-audio-0">
         <rtp:session format="rtp-avp-0" transport="A-rcv""/>
       </alt>
       <alt name="AVP-audio-18">
         <rtp:session format="rtp-avp-18" transport="A-rcv"/>
       </alt>
      </component>
   </cfg>

   <conf>
    <owner user="A@example.com" id="98765432" version="1" nettype="IN"
                         addrtype="IP4" addr="192.168.1.1"/>
    <session name="SDPng questions">
    </session>



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    <info name="interactive-audio" function="voice">
     Telephony media stream
    <info>
   </conf>

   ==================================================

      F2 (100 Trying) B -> A

      SIP/2.0 100 Trying
      Via: SIP/2.0/UDP hostA.example.com:5060
      From: A <sip:A@example.com>
      To: B <sip:B@example.com>
      Call-ID: 1234@hostA.example.com
      CSeq: 1 INVITE
      Content-Length: 0

   ==================================================

      F3 180 Ringing B -> A

      SIP/2.0 180 Ringing
      Via: SIP/2.0/UDP hostA.example.com:5060
      From: A <sip:A@example.com>
      To: B <sip:B@example.com>;tag=987654
      Call-ID: 1234@hostA.example.com
      CSeq: 1 INVITE
      Content-Length: 0

   ==================================================

      F4 200 OK B -> A

      SIP/2.0 200 OK
      Via: SIP/2.0/UDP hostA.example.com:5060
      From: A <sip:A@example.com>
      To: B <sip:B@example.com>;tag=987654
      Call-ID: 1234@hostA.example.com
      CSeq: 1 INVITE
      Contact: <sip:B@192.168.1.2>
      Content-Type: application/sdpng
      Content-Length: 479

   <def>
    <audio:codec name="audio-basic" encoding="PCMU"
                 sampling="8000" channels="1"/>

    <audio:codec name="g729" encoding="G729" channels="1" sampling="8000"/>



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    <rtp:pt name="rtp-avp-0" pt="0" format="audio-basic"/>
    <rtp:pt name="rtp-avp-18" pt="18" format="g729"/>

    <rtp:udp name="A-rcv" role="receive" endpoint="A" addr="192.168.1.1"
          rtp-port="7800"/>

    <rtp:udp name="B-rcv" role="receive" endpoint="B" addr="192.168.1.2"
          rtp-port="9410"/>
   </def>

   <cfg>
     <component name="interactive-audio" media="audio">
       <alt name="AVP-audio-0">
         <rtp:session format="rtp-avp-0" transport="A-rcv B-rcv"/>
       </alt>
      </component>
   </cfg>
   ==================================================

   ACK from A to B omitted







   In the INVITE message, A sends B a description document, that
   specifies one component with two alternatives for the audio stream
   (PCMU and G.729).  Since A wants to use the same transport address
   for receiving media data regardless of the payload format, A provides
   the transport specification in the def element and references this
   definition in the rtp:session elements for both alternatives by using
   the attribute "transport".

   In the 200 OK message, B sends an updated description document to A.
   B does only support PCMU, so it removes the alternative for G.729
   from the description.  B also defines its transport address in the
   def element and references this definition by adding "B-rcv" to the
   attribute "transport" of the rtp:session element.  (B could also have
   used the rtp:udp element inside the rtp:session element, but this
   example intends to demonstrate how to reference multiple transport
   definitions by using the attribute "transport").

6.3 Real-Time Streaming Protocol (RTSP)

   In contrast to SIP, RTSP has, from its intended usage, a clear
   distinction between offering Potential Configurations (typically by



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   the server) and choosing one out of these (by the client), and, in
   some cases; some parameters (such as multicast addresses) may be
   dictated by the server.  Hence with RTSP, there is a clear
   distinguish between Potential Configurations during the negotiation
   phase and a finally chosen Actual Configuration according to which
   streaming will take place.

   Example from the RTSP spec to be provided.

6.4 Media Gateway Control Protocol (MEGACOP)

   The MEGACO architecture also follows the SDPng model of a clear
   separation between Potential and Actual Configurations.  Upon
   startup, a Media Gateway (MG) will "register" with its Media Gateway
   Controller (MGC) and the latter will audit the MG for its
   Capabilities.  Those will be provided as Potential Configurations,
   possibly with extensive Constraints specifications.  Whenever a media
   path needs to be set up by the MGC between two MGs or an MG needs to
   be reconfigured internally, the MGC will use (updated) Actual
   Configurations.

   Details and examples to be defined.





























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7. Open Issues

      Definition of baseline libraries

      A registry (reuse of SDP mechanisms and names etc.) needs to be
      set up.

      Negotiation mechanisms for multiparty conferencing need to be
      formalized.

      Mapping to other media description formats (SDP, H.245, ...)
      should be provided.  For H.245, this is probably a different
      document (belonging to the SIP-H.323 inter-working group).

      Implicit declaration of SDPng schema and default profiles




































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References

   [1]   Kutscher, D., Ott, J., Bormann, C. and I. Curcio, "Requirements
         for Session Description and Capability Negotiation", Internet
         Draft draft-ietf-mmusic-sdpng-req-01.txt, April 2001.

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

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

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

   [5]   Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
         Conferences with Minimal Control", draft-ietf-avt-profile-new-
         10.txt  (work in progress), March 2001.

   [6]   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.

   [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]   Handley, M., Perkins, C. and E. Whelan, "Session Announcement
         Protocol", RFC 2974, October 2000.

   [10]  World Wide Web Consortium (W3C), "Extensible Markup Language
         (XML) 1.0 (Second Edition)", Status W3C Recommendation, Version
         http://www.w3.org/TR/2000/REC-xml-20001006, October 2000.

   [11]  World Wide Web Consortium (W3C), "Namespaces in XML", Status
         W3C Recommendation, Version http://www.w3.org/TR/1999/REC-xml-
         names-19990114, January 1999.

   [12]  World Wide Web Consortium (W3C), "XML Inclusions (XInclude)
         Version 1.0", Status W3C Working Draft, Version
         http://www.w3.org/TR/2001/WD-xinclude-20010516, May 2001.

   [13]  World Wide Web Consortium (W3C), "XML Schema Part 1:
         Structures", Version http://www.w3.org/TR/2001/REC-xmlschema-1-
         20010502/, Status W3C Recommendation, May 2001.



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   [14]  World Wide Web Consortium (W3C), "XML Schema Part 2:
         Datatypes", Version http://www.w3.org/TR/2001/REC-xmlschema-2-
         20010502/, Status W3C Recommendation, May 2001.

   [15]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
         SDP", draft-ietf-mmusic-sdp-offer-answer-01.txt (work in
         progress), February 2002.


Authors' Addresses

   Dirk Kutscher
   TZI, Universitaet Bremen
   Bibliothekstr. 1
   Bremen  28359
   Germany

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


   Joerg Ott
   TZI, Universitaet Bremen
   Bibliothekstr. 1
   Bremen  28359
   Germany

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


   Carsten Bormann
   TZI, Universitaet Bremen
   Bibliothekstr. 1
   Bremen  28359
   Germany

   Phone: +49.421.218-7024, sip:cabo@tzi.org
   Fax:   +49.421.218-7000
   EMail: cabo@tzi.org









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Appendix A. Base SDPng Specifications for Audio Codec Descriptions

   [5] specifies a number of audio codecs including short name to be
   used as reference by session description protocols such as SDP and
   SDPng.  Those codec names, as listed in the first column of the above
   table, are used to identify codecs in SDPng.

   The following sections indicate the default values that are assumed
   if nothing else than the codec reference is specified.

   The following audio:codec attributes are defined for audio codecs:

   name: the identifier to be later used for referencing the codec spec

   encoding: the RTP/AVP profile identifier as registered with IANA

   mime: the MIME type; may alternatively be specified instead of
      "encoding"

   channels: the number of independent media channels

   pattern: the media channel pattern for mapping channels to payload

   sampling: the sample rate for the codec (which in most cases equals
      the RTP clock)

   Furthermore, options may be defined of the following format:

   <option id="name">value</option>

   if a value is associated with the option (note that arbitrary complex
   values are allowed), or alternatively:

   <option id="name"/>

   if the option is just a boolean indicator.

   Attributes for the "option" tag are the following:

   id: the identifier for the option (variable name)

   collaps: the collapsing rules for this optional element, defined as
      follows:

      min: for numeric values only

      max: for numeric values only




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      x: intersection of enumerated values, value lists


A.1 DVI4

   <audio:codec name="dvi4" encoding="DVI4" channels="1" sampling="8000">

   <rtp:pt name="rtp-avp-5" pt="5" format="dvi4"/>
   <rtp:pt name="rtp-avp-6" pt="6">
       <audio:codec encoding="DVI4" channels="1" sampling="16000">
   </rtp:pt>

   Note that there is no default sampling rate specified for DVI4 and
   hence a sampling rate MUST be specified.

A.2 G.722

   <audio:codec name="g722" encoding="G722" channels="1" sampling="16000"/>
   <rtp:pt name="rtp-avp-9" pt="9" format="g722"/>

   Note as per [5] that the RTP clock rate is 8000Hz rather than 16000
   Hz.

A.3 G.726

   <audio:codec name="g726-40" encoding="G726-40" channels="1" sampling="8000"/>
   <audio:codec name="g726-32" encoding="G726-32" channels="1" sampling="8000"/>
   <audio:codec name="g726-24" encoding="G726-24" channels="1" sampling="8000"/>
   <audio:codec name="g726-16" encoding="G726-16" channels="1" sampling="8000"/>

   <rtp:pt name="rtp-avp-5" pt="5" format="g726-32"/>


A.4 G.728

   <audio:codec name="g728" encoding="G728" channels="1" sampling="8000"/>
   <rtp:pt name="rtp-avp-15" pt="15" format="g728"/>


A.5 G.729

   G.729 Annex A: reduced complexity of G.729
   G.729 Annex B: comfort noise

   <audio:codec name="g729" encoding="G729" channels="1" sampling="8000"/>
   <rtp:pt name="rtp-avp-18" pt="18" format="g729"/>

   For further codec description, the following options (which carry no



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   values associated with them) MAY be included:

   <option id="annexA"/>
   <!-- to indicate the use of Annex A reduced complexity -->

   <option id="annexB"/>
   <!-- to indicate the use of Annex B comfort noise -->

   As stated in [5], the use of these options can be detected within the
   media stream.

A.6 G.729 Annex D and E

   <audio:codec name="g729d" encoding="G729D" channels="1" sampling="8000"/>
   <audio:codec name="g729e" encoding="G729E" channels="1" sampling="8000"/>

   The following option MAY be used with both Annexes D and E:

   <option id="annexB"/>
   <!-- to indicate the use of Annex B comfort noise -->


A.7 GSM

A.7.1 GSM Full Rate

   The GSM Full Rate codec is indicated as follows:

   <audio:codec name="gsm" encoding="GSM" channels="1" sampling="8000"/>
   <rtp:pt name="rtp-avp-3" pt="3" format="gsm"/>


A.7.2 GSM Half Rate

   The GSM Half Rate codec is indicated as follows:

   <audio:codec name="gsm-hr" encoding="GSM-HR" channels="1" sampling="8000"/>


A.7.3 GSM Enhanced Full Rate

   The GSM Enhanced Full Rate codec is indicated as follows:

   <audio:codec name="gsm-efr" encoding="GSM-EFR" channels="1" sampling="8000"/>







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A.8 L8

   <audio:codec name="l8" encoding="L8" channels="1" sampling="8000"/>


A.9 L16

   <audio:codec name="l16" encoding="L16" channels="1" sampling="8000"/>

   <rtp:pt name="rtp-avp-11" pt="11" format="gsm"/>
   <rtp:pt name="rtp-avp-10" pt="11" format="gsm">
     <audio:codec encoding="L16" channels="2" sampling="8000"/>
   </rtp:pt>


A.10 LPC

   <audio:codec name="lpc" encoding="LPC" channels="1" sampling="8000"/>


A.11 MPA

   <audio:codec name="mpa" encoding="MPA" channels="1" sampling="8000"/>
   <rtp:pt name="rtp-avp-14" pt="14" format="mpa"/>


A.12 PCMA and PCMU

   <audio:codec name="pcmu" encoding="PCMU" channels="1" sampling="8000"/>
   <audio:codec name="pcma" encoding="PCMA" channels="1" sampling="8000"/>

   <rtp:pt name="rtp-avp-0" pt="0" format="pcmu"/>
   <rtp:pt name="rtp-avp-8" pt="8" format="pcma"/>


A.13 QCELP

   <audio:codec name="qcelp" encoding="QCELP" channels="1" sampling="8000"/>
   <rtp:pt name="rtp-avp-12" pt="12" format="qcelp"/>


A.14 VDVI

   <audio:codec name="vdvi" encoding="VDVI" channels="1" sampling="8000"/>







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Appendix B. SDPng Library for Audio Codec Definitions

   This section contains an SDPng library with the audio codec
   definitions from Appendix A.

   <def xmlns="http://www.iana.org/sdpng"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://www.iana.org/sdpng/int integrated.xsd"
         xmlns:audio="http://www.iana.org/sdpng/audio"
         xmlns:rtp="http://www.iana.org/sdpng/rtp">

       <audio:codec name="dvi4" encoding="DVI4" channels="1" sampling="8000"/>
       <audio:codec name="g722" encoding="G722" channels="1" sampling="16000"/>
       <audio:codec name="g726-40" encoding="G726-40" channels="1" sampling="8000"/>
       <audio:codec name="g726-32" encoding="G726-32" channels="1" sampling="8000"/>
       <audio:codec name="g726-24" encoding="G726-24" channels="1" sampling="8000"/>
       <audio:codec name="g726-16" encoding="G726-16" channels="1" sampling="8000"/>
       <audio:codec name="g728" encoding="G728" channels="1" sampling="8000"/>
       <audio:codec name="g729" encoding="G729" channels="1" sampling="8000"/>
       <audio:codec name="g729d" encoding="G729D" channels="1" sampling="8000"/>
       <audio:codec name="g729e" encoding="G729E" channels="1" sampling="8000"/>
       <audio:codec name="gsm" encoding="GSM" channels="1" sampling="8000"/>
       <audio:codec name="gsm-hr" encoding="GSM-HR" channels="1" sampling="8000"/>
       <audio:codec name="gsm-efr" encoding="GSM-EFR" channels="1" sampling="8000"/>
       <audio:codec name="l8" encoding="L8" channels="1" sampling="8000"/>
       <audio:codec name="l16" encoding="L16" channels="1" sampling="8000"/>
       <audio:codec name="lpc" encoding="LPC" channels="1" sampling="8000"/>
       <audio:codec name="mpa" encoding="MPA" channels="1" sampling="8000"/>
       <audio:codec name="pcmu" encoding="PCMU" channels="1" sampling="8000"/>
       <audio:codec name="pcma" encoding="PCMA" channels="1" sampling="8000"/>
       <audio:codec name="qcelp" encoding="QCELP" channels="1" sampling="8000"/>
       <audio:codec name="vdvi" encoding="VDVI" channels="1" sampling="8000"/>

   </def>

















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Appendix C. SDPng Library for RTP Payload Format Definitions

   This section contains an SDPng library with the RTP payload format
   definitions from Appendix A.

   <def xmlns="http://www.iana.org/sdpng"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://www.iana.org/sdpng/int integrated.xsd"
         xmlns:audio="http://www.iana.org/sdpng/audio"
         xmlns:rtp="http://www.iana.org/sdpng/rtp"
         xmlns:xi="http://www.w3.org/2001/XInclude">

   <!-- import audio codec definitions here: -->

       <xi:xinclude href="http://www.iana.org/sdpng/audio/audio.sdpng" parse="xml"/>

       <rtp:pt name="rtp-avp-5" pt="5" format="dvi4"/>

       <rtp:pt name="rtp-avp-6" pt="6">
         <audio:codec encoding="DVI4" channels="1" sampling="16000">
       </rtp:pt>

       <rtp:pt name="rtp-avp-9" pt="9" format="g722"/>

       <rtp:pt name="rtp-avp-5" pt="5" format="g726-32"/>

       <rtp:pt name="rtp-avp-15" pt="15" format="g728"/>

       <rtp:pt name="rtp-avp-18" pt="18" format="g729"/>

       <rtp:pt name="rtp-avp-3" pt="3" format="gsm"/>

       <rtp:pt name="rtp-avp-11" pt="11" format="gsm"/>

       <rtp:pt name="rtp-avp-10" pt="11" format="gsm">
         <audio:codec encoding="L16" channels="2" sampling="8000"/>
       </rtp:pt>

       <rtp:pt name="rtp-avp-14" pt="14" format="mpa"/>

       <rtp:pt name="rtp-avp-0" pt="0" format="pcmu"/>

       <rtp:pt name="rtp-avp-8" pt="8" format="pcma"/>

       <rtp:pt name="rtp-avp-12" pt="12" format="qcelp"/>

   </def>




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Appendix D. Change History

   draft-ietf-mmusic-sdpng-04.txt

      *  New section on capability negotiation (Section 4).

      *  New section on referencing definitions (Section 3.3).

      *  New section on properties (Section 3.3.2).

      *  New section on definition groups (Section 3.3.3).

   draft-ietf-mmusic-sdpng-03.txt

      *  Extension of the SDPng schema (use of Xlinks etc.)

      *  Clarification in the text

      *  Fixed examples

      *  Added example libraries as appendices

      *  More details on usage with SIP, including examples.

   draft-ietf-mmusic-sdpng-02.txt

      *  Added a section on formal specification mechanisms (Section 5).

   draft-ietf-mmusic-sdpng-01.txt

      *  renamed section "Syntax Proposal" to "Syntax Definition
         Mechanisms".  More text on DTD vs.  schema.  Edited the example
         description.

      *  updated example definitions in section "Definitions" and
         "Components & Configurations"

      *  section "Session Attributes" replaces section "Session"

      *  new appendix on audio codec definitions











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Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
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   followed, or as required to translate it into languages other than
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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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