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mmusic                                                          Kutscher
Internet-Draft                                                       Ott
Expires: August 21, 2005                                         Bormann
                                                TZI, Universitaet Bremen
                                                       February 20, 2005


             Session Description and Capability Negotiation
                     draft-ietf-mmusic-sdpng-08.txt

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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Copyright Notice

   Copyright (C) The Internet Society (2005). All Rights Reserved.

Abstract

   This document defines a language for describing multimedia sessions
   with respect to configuration parameters and capabilities of
   end-systems. The description language is independent of specific
   application scenarios (session announcement, session setup for
   interactive communication etc.) and is not limited to specific media
   types, capabilities, or configuration parameters.

   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.


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Document Revision

   $Revision: 6.21 $

Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.    Terminology and System Model . . . . . . . . . . . . . . . .  7
   3.    Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   3.1   Outline of the Negotiation Process . . . . . . . . . . . . . 11
   3.2   SDPng Data Types . . . . . . . . . . . . . . . . . . . . . . 13
   3.3   Application-specific Vocabulary  . . . . . . . . . . . . . . 15
   4.    SDPng Syntax . . . . . . . . . . . . . . . . . . . . . . . . 16
   4.1   SDPng Base Syntax  . . . . . . . . . . . . . . . . . . . . . 16
   4.2   Capabilities . . . . . . . . . . . . . . . . . . . . . . . . 18
   4.2.1 Tokens . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   4.2.2 Token Sets . . . . . . . . . . . . . . . . . . . . . . . . . 19
   4.2.3 Numerical Values . . . . . . . . . . . . . . . . . . . . . . 19
   4.2.4 Numerical Ranges . . . . . . . . . . . . . . . . . . . . . . 19
   4.2.5 Sample SDPng cap Element . . . . . . . . . . . . . . . . . . 20
   4.2.6 Referencing Capability Elements  . . . . . . . . . . . . . . 21
   4.3   Definitions  . . . . . . . . . . . . . . . . . . . . . . . . 21
   4.4   Configurations . . . . . . . . . . . . . . . . . . . . . . . 23
   4.5   Constraints  . . . . . . . . . . . . . . . . . . . . . . . . 25
   4.6   Session Information  . . . . . . . . . . . . . . . . . . . . 25
   4.7   Summary of SDPng XML-Syntax  . . . . . . . . . . . . . . . . 26
   5.    Usage of SDPng in Different Application Scenarios  . . . . . 28
   5.1   Broadcast/Announcement Scenarios . . . . . . . . . . . . . . 28
   5.2   Real-Time-Streaming  . . . . . . . . . . . . . . . . . . . . 29
   5.3   Two-Way Session Setup  . . . . . . . . . . . . . . . . . . . 31
   5.4   Multi-Party-Conferencing with Negotiation  . . . . . . . . . 38
   6.    IANA Considerations  . . . . . . . . . . . . . . . . . . . . 39
   7.    Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . 40
   8.    Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 41
         References . . . . . . . . . . . . . . . . . . . . . . . . . 42
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 43
   A.    Formal Syntax Specifications . . . . . . . . . . . . . . . . 44
   A.1   SDPng Base DTD . . . . . . . . . . . . . . . . . . . . . . . 44
   A.2   SDPng XML-Schema Specification . . . . . . . . . . . . . . . 45
   B.    Sample Package Definitions . . . . . . . . . . . . . . . . . 51
   B.1   Sample RTP Package Definition  . . . . . . . . . . . . . . . 51
   B.2   Sample Audio Package Definition  . . . . . . . . . . . . . . 52
   B.3   Sample Video Package Definition  . . . . . . . . . . . . . . 52
   C.    Sample SDPng Description . . . . . . . . . . . . . . . . . . 54
   D.    Change History . . . . . . . . . . . . . . . . . . . . . . . 58
         Intellectual Property and Copyright Statements . . . . . . . 61





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1. Introduction

   Different applications in the multimedia realtime communication
   domain require a means for session description and capability
   negotiation. For example, for establishing an interactive audio
   communication session between two participants, end-systems must be
   dynamically configured with respect to codec types, codec parameters,
   transport protocol parameters and other configuration details. All
   these parameters can be viewed as the configuration for a session,
   and a session description language is used to describe this
   configuration unambiguously.

   While the fundamental requirement to describe session parameters
   applies to all application scenarios, there are differences in how
   configurations are obtained and distributed among participants. For
   broadcast applications, e.g., sessions that are announced using SAP
   or IMG protocols, the sender is typically distributing a session
   description to potential receivers, describing the technical
   parameters (e.g., multicast addresses, port numbers, codecs etc.) and
   providing additional information about the session such as
   meta-information (about the content) and scheduling information.
   While a sender might provide alternative variants of one specific
   broadcast event (different language versions, different media codec
   configurations for different quality levels etc.) there is typically
   no negotiation process between sender and receiver in order to obtain
   the optimal configuration for a specific receiver. Instead, the
   sender may provide different potential configurations and the
   receiver would select the most appropriate one.

   Real-time-streaming applications such as "video-on-demand" provide
   similar but slightly different requirements. Still the sender is
   typically providing a set of different potential configurations, out
   of which the receiver may select the most appropriate one, so there
   is not really a negotiation of content and its representation (on the
   session description level). But, differently to the aforementioned
   scenarios, the receiver must tell the sender the endpoint address,
   i.e., where media data should be sent to. Depending on the control
   protocol, this may be specified using the session description
   language or other means (as it is done with RTSP).

   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 spontaneously
   initiated conferences 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



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   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 conferences with a fixed set of participants)
   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:

   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" was initially
   only made implicitly.  While numerous extensions to SDP were
   developed to account for various aspects of interactive session
   establishment (the offer/answer model in RFC 3264, grouping of media
   sessions in RFC 3388, and simple capability declaration in RFC 3407,
   among others), their expressiveness is naturally constrained by the
   simple (and veru limited) SDP syntax.

   SDPng, the session description languages specified in this document,
   provide a common, XML-based framework for session description and
   capability description for the aforementioned application scenarios.
   It allows for distributing "fixed" configuration descriptions in
   broadcast application scenarios but also supports the dynamic
   negotiation of session parameters for interactive multimedia



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   conferences.

   In order to support a broad range of different applications, SDPng
   itself is completely application-agnostic. The base specification
   does not define any application-specific vocabulary, e.g., media
   types, codecs and their configuration parameters etc., but is
   intended as an extensible framework that provides the necessary
   extensibility mechanisms for supporting both future applications and
   future transport and encoding mechanisms.

   With respect to capability negotiation, SDPng is based on the
   following principles:

      Capability negotiation is an optional feature, which MAY be
      employed for specific applications, e.g., session setup for
      interactive communication.

      SDPng provides the framework for describing capabilities and
      linking them to configurations of application sessions, but the
      base specification does not prescribe negotiation algorithms such
      as feature matching.

      The SDPng base specification provides the necessary support for
      application-independent capability negotiation, i.e., an SDPng
      processor that receives capability descriptions from one or
      multiple participants does not need to understand the capability
      semantics in order to process the different capability
      descriptions. For this purpose, SDPng provides a well-defined set
      of data types and defines a value representation that allows SDPng
      processors to infer data types without requiring access to schema
      definitions and without requiring application-specific knowledge.

   For most application classes, especially for broadcast applications,
   the session description will have to provide information about the
   session that is not used to configure end-systems but rather to
   facilitate content navigation for human users. For example, This
   meta-information can include author/source details, additional
   information about the whole session or individual media sessions,
   scheduling information and other, arbitrary information that is
   related to the session but not directly required to achieve
   interoperability between end-systems. SDPng provides fundamental
   mechanisms that support the inclusion of meta-information:

      Descriptions of application components (media sessions) and
      alternative configurations can be labeled to enable later
      referencing, i.e., for associating meta-information. For example,
      in a multi-language TV broadcast session, the language information
      could be provided by a meta-information fragment that assigns



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      language tags to media session descriptions by referencing them.

      SDPng itself does not define vocabulary for specifying
      meta-information, but allows for including arbitrary
      meta-information fragments, e.g., MPEG-7 descriptions. These
      description may either be included or inline or be referenced by
      URIs.

   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). In Section 3, we provide an overview
   of options for capability negotiation. Next, we outline the concept
   for the concepts underlying SDPng and introduce the syntactical
   components step by step in Section 4.  Section 5 describes how SDPng
   can be used in different application scenarios.

   Appendix A provide formal specifications of SDPng such as XML DTD and
   Schema definitions, Appendix B provides some sample package
   definitions, Appendix C provides a sample SDPng description, and
   Appendix D lists the change history.































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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,
      available licenses, and quality of implementation may constrain
      the possible video encoding algorithms.

   In multiparty multimedia conferences, participants employ different
   "components" in conducting the conference; similarly, a multimedia
   (rather than plain TV) broadcast may comprise several components that
   make up the full presentation.

      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 policy 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.




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



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      component. There are actual and potential configurations.

      *  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.).

   Note that the meaning of the term "actual configuration" is highly
   application-specific. For example, for audio transport using RTP, an
   actual configuration is equivalent to a payload format (potentially
   plus format parameters), whereas for other applications it may be a
   MIME type.

   In SAP-based [8] 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



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   for session initiation with SIP -- limiting the negotiation to
   exactly one round trip.

   The following list explains some terms used in this document:

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

   Component
      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).

   Package
      A package is an application specific data schema for expressing
      potential and actual configurations. For example, an audio package
      specifies the data schema for audio codecs.

   Potential Configuration
      Potential configurations describe possible configurations that are
      supported by an end-system ("capabilities").




























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3. Overview

   SDPng is a description language for both potential configurations
   (i.e. capabilities) of participants in multimedia conference 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 itself is an application-independent framework that defines a
   description syntax that are enable an (optional) capability
   negotiation process. It should be noted, that SDPng can be used
   without capability negotiation and that the specific negotiation
   algorithm is not specified in this document.

   A capability description for an endpoint is a set of individual
   capabilities, each of which provides a fixed type, e.g., a numeric
   value or a list value. The set of types and the corresponding
   abstract negotiation rules are defined in this memo. The SDPng data
   types are relevant to all SDPng application domains, while the
   processing rules are only applicable to application domains that rely
   on capability negotiation.

   In the following, we provide a conceptual overview of the negotiation
   process in Section 3.1 and describe the different capability types
   and the corresponding abstract negotiation rules in Section 3.2.

3.1 Outline of the Negotiation Process

   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 signaling 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



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   configuration within the two-way-handshake of the call setup phase.
   In order to use SDPng instead of SDP according to the offer/answer
   model defined in [13] it must be possible to determine an actual
   configuration in a single request/response cycle.

   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 this 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
       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 the capabilities of each other and
       the potential configurations that both can support.

   6.  In order to obtain an actual configuration from the potential
       configuration that has been obtained, both participants have to
       pick a subset of the potential configurations that 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 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



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       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.

   Note that the model presented here results in four SDPng messages. 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
   distinguished from other configuration information.

   Note also that, in case of multicast/broadcast scenarios, the sender
   may simply provide the full description of the alternative
   configurations including (transport) parameters. The potential
   receivers will decide based upon this information whether or not they
   are capable of receiving the respective media sessions and pick the
   most suitable configuration.

3.2 SDPng Data Types

   The description of actual configurations and the capability
   negotiation process rely on a fixed set of data types with
   corresponding processing rules. The following types are defined:

   1.  Tokens (text strings)

          Example:

                      <audio:encoding>PCMU</audio:encoding>

          Processing rule:
          Ascertain identity (compare strings)



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   2.  Token lists

          Example:

                      <audio:sampling-rate>8000 16000</audio:sampling-rate>

          Processing rule:
          Determine common subset

   3.  Numbers

          Example:

                      <audio:bitrate val="64"/>

          Processing rule:
          Ascertain equality

   4.  Numerical ranges

          Example:

                      <audio:bitrate min="6" max="64"/>

          Processing rule:
          Determine common subrange

   SDPng distinguishes between optional and mandatory capability
   definitions, with different processing rules for the negotiation
   process. Optional definitions are used for capabilities that can be
   provided by an entity but do not have to be supported by all
   participants. For example, an audio codec could provide optional
   codec parameters. The use of these parameters needs to be declared by
   a session description, but if the parameter is not understood by all
   implementations, a session can be established nevertheless.  As a
   result, the failure of a single processing step for a definition that
   has been marked as "optional" does not lead to a failure of the
   capability negotiation as a whole.

   A mandatory capability on the other hand has to be supported by all
   participants. For example, the specification of an audio codec for an
   audio capability is mandatory, and for obtaining an interoperable
   configuration, all participants must support the same audio codec or
   set of audio codecs.

   In addition to capabilities, a SDPng description can also provide
   parameters that are not negotionable, e.g., transport parameters. In
   SDPng, there is a distinction between capability definitions (that



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   are subject to a negotiation process) and parameters that are
   specified by each participant. In a description of alternative
   configurations for a specific component, capabilities and parameters
   can be referred to and describe the configurations.

3.3 Application-specific Vocabulary

   While the SDPng specification defines the fundamental types, abstract
   processing rules and the syntax definition for SDPng descriptions, it
   does not define any application-specific vocabulary.
   Application-specific vocabulary is defined in SDPng packages. An
   SDPng package defines a schema for application specific capability
   and parameter descriptions. Based on the description types specified
   by the SDPng base specification, a package definition specifies the
   capability and parameter definitions allowed for a specific
   application, the types of definitions and additional attributes,
   e.g., whether a definition element is optional with respect to the
   capability negotiation or not.

   The SDPng base specification does define some fundamental
   requirements for definition elements that are specified in package
   definitions, for example XML attributes for elements. Appendix A.2
   provides an XML Schema definition that specifies some base types to
   be used for package definitions.

   In order to allow for an application independent processing of SDPng
   description documents, SDPng descriptions are standalone, i.e., the
   package definition is not required to process a corresponding SDPng
   document. All information, e.g., the type of definitions and
   additional attributes are contained in the SDPng document itself. An
   SDPng implementation can thus be processed without access to the
   package definition.



















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4. SDPng Syntax

   This section specifies the SDPng base syntax. An SDPng description is
   an XML document consisting of up to five parts:

      Capabilities

      Definitions

      Configurations

      Constraints

      Session Information

   The Capabilities section provides a list of individual capabilities.
   In a capability negotiation process, these capabilities are matched
   against corresponding definitions of other participants' capability
   descriptions. This section is OPTIONAL for a SDPng description
   document.

   The Definitions section provides definitions of commonly used
   parameters for later referencing. This section is OPTIONAL for SDPng
   descriptions.

   The Configurations section provides the description of the different
   conference components (applications in a conference). Each component
   description can provide a list of alternative configurations. This
   section MUST be present in any SDPng description.

   The Constraints section provides contraints on combinations of
   configurations. This section is OPTIONAL for SDPng descriptions.

   The Session Information section provides meta information on the
   conferences and on individual components. This section is OPTIONAL
   for SDPng documents.

4.1 SDPng Base Syntax

   An SDPng description is an XML document.  The document root element
   MUST be an element of type "sdpng". The XML vocabulary for the SDPng
   base specification resides in the XML namespace "http://www.iana.org/
   sdpng". The root element of an SDPng description MUST define an XML
   default namespace "http://www.iana.org/sdpng". In addition, the
   "sdpng" element MUST map the namespace prefix "sdpng" to the
   namespace name "http://www.iana.org/sdpng". The "sdpng" element type
   provides the child elements "cap", "def", "cfg", "constraints", and
   "info" for the different sections of the SDPng description. The



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   default namespace is also applied to these elements.

   The encoding of the XML document MUST be UTF-8 (RFC2279, [16]).

   The following figure depicts the overall SDPng document structure.

   <?xml version="1.0" encoding="UTF-8"?>
   <sdpng xmlns="http://www.iana.org/sdpng"
          xmlns:sdpng="http://www.iana.org/sdpng">
     <cap>
       [...]
     </cap>
     <def>
       [...]
     </def>
     <cfg>
       [...]
     </cfg>
     <constraints>
       [...]
     </constraints>
     <info>
       [...]
     </info>
   </sdpng>

   Appendix A.1 provides a XML DTD that defines a corresponding document
   type.

   Note that the elements for the optional sections "Capabilities",
   "Definitions", "Contraints", and "Session-Level Information" are
   OPTIONAL.

   Application-specific vocabulary resides in its own namespace. For
   each namespace name of an SDPng package, a namespace prefix MUST be
   declared in the start tag of the "sdpng" element. The following
   figure depicts the declaration of namespace prefixes for two package
   namespaces:

   <?xml version="1.0" encoding="UTF-8"?>
   <sdpng xmlns="http://www.iana.org/sdpng"
          xmlns:sdpng="http://www.iana.org/sdpng"
          xmlns:rtp="http://www.iana.org/sdpng/rtp"
          xmlns:audio="http://www.iana.org/sdpng/audio">
       [...]
   </sdpng>





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4.2 Capabilities

   A section for capability descriptions is an XML element that can
   provide a list of child elements. The element type is called "cap"(in
   the "sdpng" namespace). Each child element represents an individual
   capability.

   Each capability element MUST provide an attribute "name". The value
   of this attribute SHOULD be composed of a prefix (representing a
   namespace-name) and a unique name for the corresponding capability
   within that namespace. The namespace-name designates a namespace for
   the source of the capability definition, e.g., for the participant of
   a conference. If a prefix is specified, it MUST be separated by a
   colon (':') from the name. The namespace MUST be declared in the
   respective element or in ancestor elements, e.g., the root "sdpng"
   element. The following figure depicts a capability element inside a
   "cap" element. Note that the child elements of "audio:codec" and the
   other sections of the SDPng description are not shown.

   <?xml version="1.0"?>
   <sdpng xmlns="http://www.iana.org/sdpng"
          xmlns:sdpng="http://www.iana.org/sdpng">
     <cap>
       <audio:codec name="avp:pcmu">
         [One or more feature elements]
       </audio:codec>

       [...]
     </cap>
   </sdpng>

   Each capability element provides a set of features.  Each feature is
   represented by a child element.  The element types are defined in
   package definitions. XML Namespaces are used to disambiguate element
   types and to allow for extensibility.  Each feature element can
   provide a "range" of values -- not only a single value. For example,
   a feature element can specify a set of supported alternative values
   for a given property, e.g., for the sampling rate of an audio codec.
   SDPng provides two different ways for representing "value ranges": A
   feature element can specify a set of tokens or a numerical range.

   Each feature that is represented by an XML feature element has a
   well-defined type that is specified in the package definition. The
   type determines the representation of the element values in XML so
   that type information is encoded implicitly in the description
   document. For example, a token set can be identified by the presence
   of whitespace separated list of token as element content of the
   respective feature element. Each feature element MAY provide an



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   attribute "status". If this attribute is present it MUST provide one
   of the following values:

      opt:
      This element describes an optional feature (as described by
      Section 3.2).

   The three different features types (as described in Section 3.2) are
   represented as described in the following sections. Section 4.2.5
   provides a complete example.

4.2.1 Tokens

   Token elements provide a single token as element content. The token
   is of type Nmtoken (name token) as defined by [9]. The following
   example depicts a feature element of type token.

                <audio:encoding>PCMU</audio:encoding>

   Boolean values SHOULD be represented as token elements with a values
   of either "true" or "false".

4.2.2 Token Sets

   Token set elements provide a token list as element content. The token
   is of type Nmtokens (name tokens) as defined by [9]. The following
   example depicts a feature element of type token set.

                <audio:sampling>8000 16000 32000</audio:sampling>


4.2.3 Numerical Values

   Elements for numbers provide an attribute "val" with a numerical
   value. The following example depicts a feature element of type
   numerical value.

                <audio:bitrate val="64"/>


4.2.4 Numerical Ranges

   Elements for numerical ranges can provide an attribute "min" and an
   attribute "max". Both attributes provide a numerical value. At least
   one of these attributes MUST be present.  The following example
   depicts a feature element of type numerical range.

                <audio:bitrate min="6" max="64"/>



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4.2.5 Sample SDPng cap Element


   <?xml version="1.0"?>
   <sdpng xmlns="http://www.iana.org/sdpng"
          xmlns:sdpng="http://www.iana.org/sdpng">
     <cap>
       <audio:codec name="avp:pcmu">
         <audio:encoding>PCMU</audio:encoding>
         <audio:channels>1 2</audio:channels>
         <audio:sampling>8000 16000</audio:sampling>
         <audio:bitrate min="6" max="64"/>
         <audio:silence-suppression status="opt">
           true
         </audio:silence-suppression>
       </audio:codec>

       [...]
     </cap>
   </sdpng>


   Capability elements MAY also provide elements from different XML
   namespaces. For example, a video-codec capability MAY be described
   with elements declaring general video capabilities, and this element
   MAY provide a list of additional codec specific feature elements, as
   depicted in the following example:

   <?xml version="1.0"?>
   <sdpng xmlns="http://www.iana.org/sdpng"
          xmlns:sdpng="http://www.iana.org/sdpng">
     <cap>
       <video:codec name="h263+-enhanced">
         <video:encoding>H.263+</video:encoding>
         <video:resolution>QCIF</video:resolution>
         <video:framerate max="30"/>
         <h263plus:A>foo</h263plus:A>
         <h263plus:B>bar</h263plus:B>
       </video:codec>

       [...]
     </cap>
   </sdpng>








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4.2.6 Referencing Capability Elements

   The capablity elements of a "cap" element can be referenced in later
   sections of the SDPng document. The fundamental model is that
   capability elements specify individual capabilities (without
   transport and other non-negotionable parameters) and that these
   elements are later augmented in Definitions and Configurations
   sections.

   When referencing a capability element, e.g., the element video:codec,
   the same element name (general identifier) is used. The referencing
   element MUST provide an attribute "ref", and the value of this
   attribute SHOULD provide the value of the attribute "name" of the
   referenced element.

   The referencing element MAY also provide additional feature elements
   (that have not been provided by the referenced capability element).
   The referencing element MAY also provide feature elements that have
   already been provided by the referenced element.

   The referencing element MAY provide an attribute "name". The
   semantics of a reference are defined in the corresponding sections
   where references to definitions are used, i.e., in Section 4.3 and in
   Section 4.4.

4.3 Definitions

   The Definitions section is an optional section that can provide
   definitions of fixed parameters that are not negotionable such as
   transport parameters. An SDPng description document MAY provide a
   "def" element that can provide a set of definitions as child
   elements.

   Each child element of a "def" element provides an element type
   specified in a package definition. Such child elements are referred
   to as "definition elements". Definition elements can provide a set of
   child elements, each of which specifies a specific configuration
   value. Syntactically, these child elements MUST be "feature elements"
   as specified in Section 4.2. Child elements of a definition element
   MUST be of type Token or of type Numerical Value. A definition
   element MUST provide an attribute "name" that is used to specify a
   unique name in the scope of the current SDPng description. A
   definition element MAY provide an attribute "ref" that is used to
   reference a capability element as specified in Section 4.2.

   The following example depicts a def element with one definition
   element of type "rtp:udp". This element is used to specify fixed
   parameters of an RTP session -- the allowable parameters would have



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   been specified in a corresponding SDPng RTP package.

   <?xml version="1.0"?>
   <sdpng xmlns="http://www.iana.org/sdpng"
          xmlns:sdpng="http://www.iana.org/sdpng">

     <cap>
       <audio:codec name="avp:pcmu">[...]</audio:codec>
       <rtp:udp name="rtpudpip6">[...]</rtp:udp>
     </cap>


     <def>
      <rtp:udp name="rtp-cfg1" ref="rtp:rtpudpip6">
        <rtp:ip-addr>::1</rtp:ip-addr>
        <rtp:rtp-port>9456</rtp:rtp-port>
        <rtp:pt>1</rtp:pt>
      </rtp:udp>
     </def>

     <cfg>
       [...]
     </cfg>
     <constraints>
       [...]
     </constraints>
     <info>
       [...]
     </info>

   </sdpng>


   A definition element SHOULD reference a capability element provided
   in the "cap" element, as depicted in the example. In the example, the
   definition named "rtp-cfg1" provides RTP transport parameters and
   references the RTP capability named "rtp:rtpudpip6". The semantics of
   referencing the capability element are as follows:

   o  An implementation MUST process the newly defined element by
      adopting the individual feature elements of the referenced
      capability element.

   o  For feature elements that are present in both the capability
      element and the description element, the feature elements of the
      definition element take precedence over the feature elements of
      the capability element.




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4.4 Configurations

   The Configurations section lists 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 actual configurations are given.

   An SDPng document MUST provide a "cfg" element that represents the
   Configurations section. The "cfg" element provides one or more
   "component" element describing alternative configurations for the
   component. The "cfg" element SHOULD provide at least one "component"
   element. Each "component" element MUST provide an attribute "name"
   that identifies the component uniquely in the scope of the SDPng
   description. Each "component" element MAY provide an attribute status
   of type CDATA that MAY be used to specify application specific status
   information.

   Each "component" element MUST provide one or more "alt" element, each
   of which describes an alternative configuration for the component.
   Each "alt" element MUST provide an attribute "name" that provides a
   unique identification for the alternative in the scope of the SDPng
   description.  In addition, each "alt" element MUST also provide an
   attribute "media" for specifying the media type for this particular
   alternative. Currently defined values for this attribute are "audio",
   "video", "application", "data", and "control". The semantics of these
   values are described in [2].

   Each "alt" element MUST provide one or more XML elements that
   describe the configuration parameters for the particular alternative
   configuration. The elements are defined by SDPng package
   specification and definitions from different packages can be mixed.
   The type of the elements and their order is application dependent.

   Each definition element that is contained in an "alt" element MAY
   provide an attribute "ref". The "ref" attribute is used to specify a
   reference to a capability element (from a "cap" section) or to a
   definition element (from a "def" section). The value of an "ref"
   element MUST provide the value of a "name" attribute of an existing
   capability or definition element. A definition element MAY provide
   child elements (for the specification of additional feature and
   configuration parameters) but it MAY also be an empty element.  The
   semantics of referencing the capability element are as follows:

   o  An implementation MUST process the newly defined element by
      adopting the individual feature elements of the referenced
      capability or definition element.

   o  For feature elements that are present in both the capability/



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      definition element and the current definition element, the feature
      elements of the current definition element take precedence over
      the feature elements of the referenced element.

   It should be noted that the inclusion of definition by reference is
   NOT MANDATORY. For certain applications such as session announcement,
   it may be sufficient to provide the configuration data directly
   inside an "alt" element.

   The following example depicts the description of a single
   configuration for a component named "interactive-audio". The
   description of the configuration references the "avp:pcmu" audio
   codec definition from the "cap" element and the "rtp-cfg1" RTP
   session definition from the "def" element. In this example, both
   elements of the "alt" element are empty elements that adopt the
   specified values from the referenced elements.

   <?xml version="1.0"?>
   <sdpng xmlns="http://www.iana.org/sdpng"
          xmlns:sdpng="http://www.iana.org/sdpng">

     <cap>
       <audio:codec name="avp:pcmu">[...]</audio:codec>
       <rtp:udp name="rtpudpip6">[...]</rtp:udp>
     </cap>


     <def>
      <rtp:udp name="rtp-cfg1" ref="rtp:rtpudpip6">
        <rtp:ip-addr>::1</rtp:ip-addr>
        <rtp:rtp-port>9456</rtp:rtp-port>
        <rtp:pt>1</rtp:pt>
      </rtp:udp>
     </def>

     <cfg>
       <component name="interactive-audio" media="audio" status="active">
        <alt name"alt1">
          <audio:codec ref="avp:pcmu"/>
          <rtp:udp ref="rtp-cfg1"/>
        </alt>
      </component>

     </cfg>
     <constraints>
       [...]
     </constraints>
     <info>



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       [...]
     </info>

   </sdpng>



4.5 Constraints

   The Constraints section allows to express constraints on the
   combination of configurations that apply across different components.
   This feature is intended for specialized devices with strict
   limitations on, e.g., parallel codec instantiations due to limited
   DSP resources. The SDPng base specification does not define the use
   of constraints. Instead, the usage of constraints will be specified
   in a separate document.

   The "constraints" element of an SDPng description is OPTIONAL.

4.6 Session Information

   The Session Information section is represented by an "info" element
   and is intended for meta information on the conference itself and on
   the individual components. In an SDPng description document, the
   "info" element MAY provide one or multiple "part" element, each of
   which may contain arbitrary meta-description content.

   The "info" element is OPTIONAL in an SDPng description document.

   Each "part" element inside an "info" element MUST provide a "type"
   attribute that specifies the type of the meta-information content,
   e.g., "MPEG-7", "TV-Anytime", "MPEG-21", "SDP".

   Each "part" element inside an "info" element MAY provide a "ref"
   attribute that can be used to specify an URI for the meta-information
   content. If a "ref" attribute is provided the "part" element MUST be
   empty.

   <?xml version="1.0"?>
   <sdpng xmlns="http://www.iana.org/sdpng"
          xmlns:sdpng="http://www.iana.org/sdpng">

     <cap>
       <audio:codec name="avp:pcmu">[...]</audio:codec>
       <rtp:udp name="rtpudpip6">[...]</rtp:udp>
     </cap>





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     <def>
      <rtp:udp name="rtp-cfg1" ref="rtp:rtpudpip6">
        <rtp:ip-addr>::1</rtp:ip-addr>
        <rtp:rtp-port>9456</rtp:rtp-port>
        <rtp:pt>1</rtp:pt>
      </rtp:udp>
     </def>

     <cfg>
       <component name="interactive-audio" media="audio" status="active">
        <alt name"alt1">
          <audio:codec ref="avp:pcmu"/>
          <rtp:udp ref="rtp-cfg1"/>
        </alt>
      </component>

     </cfg>
     <constraints>
       [...]
     </constraints>
     <info>
       <part type="SDP">
         o=jdoe 2890844526 2890842807 IN IP4 10.47.16.5
         s=SDP Seminar
         i=A Seminar on the session description protocol
         u=http://www.example.com/seminars/sdp.pdf
         e=j.doe@example.com (Jane Doe)
         t=2873397496 2873404696
       </part>
     </info>

   </sdpng>


4.7 Summary of SDPng XML-Syntax

   The SDPng base specification defines the following XML element types
   that reside in the SDPng namespace designated by the namespace name
   "http://www.iana.org/sdpng":

   o  sdpng

   o  cap

   o  def

   o  cfg




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   o  component

   o  alt

   o  constraints

   o  info

   Appendix A.1 provides an XML DTD that specifies the content model of
   the SDPng base elements.









































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5. Usage of SDPng in Different Application Scenarios

   This informative section describes how SDPng can be used in different
   application scenarios, namely broadcast/announcement,
   real-time-streaming, two-way-session-setup, and multiparty
   conferencing with negotiation.

5.1 Broadcast/Announcement Scenarios

   Broadcast and multicast application scenarios rely on session
   announcements to communicate information about multimedia sessions
   and their associated parameters. Historically, in the Mbone, the
   Session Announcement Protocol (SAP) [RF2974] was used to convey
   static session descriptions via multicast but the same information
   could also be advertized by means of email, NetNews, or web pages.
   For some years, digital television used Electronic Program Guides
   (EPGs) to convey programming information (movie schedule, metadata,
   channel information) to large audiences. More recently, streaming
   services for (3G and beyond) mobile networks make use of similar
   concepts to announce streaming services.  As a response to these
   needs, the generalized framework of Internet Media Guides (IMGs) has
   been devised to address conveying scheduling and media information to
   potentially large receiver groups.  This subsection addresses the use
   of SDPng with SAP and IMG-based session announcements.

   SAP and IMGs are 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
   communicated via SAP or IMGs to join the announced media sessions.
   While this is supported by MIME types identifying SDPng contents with
   implied semantics, the IMG framework explicitly suggests interactive
   retrieval using HTTP.  Furthermore, IMG has the concept of
   asynchronous notifications/updates to existing SDPng descriptions: it
   makes use of a (SIP-based) notification mechanism that allows
   interested parties to monitor the state of session descriptions and
   receive asynchronous updates whenever the description changes.

   SAP makes extensive use of the SDP session level attributes to
   provide a (limited) set of descriptive metadata for the session,
   including scheduling and subject information. Quite a bit of this
   information is application-specific and is therefore not defined in
   the baseline SDPng spec.  In particular, IMGs are expected to be used
   with more encompassing metadata description formats (such as MPEG-7)
   which will carry the respective information so that these need not be
   replicated in SDPng itself.

   While SAP only supports full SDP descriptions (which are minimal in
   size), IMGs allows in addition to the (potentially large)



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   (collections of) SDPng descriptions and associated metadata to carry
   delta information or pointers to contents (URIs).  The structured
   format of the XML-based SDP goes well with both concepts and allows
   to identify subparts of SDPng messages where and perform operations
   on them via well-defined means.

5.2 Real-Time-Streaming

   Real-time streaming provides an interactive way to offer real-time
   media to users.  In contrast to the announcement/broadcast based
   described in the previous section, where the communication is mostly
   unidirectional and interested receivers just "tune" into the
   respective media session, with RTSP an interactive session setup
   exists.  This also informs the media server that there are parties
   interested in a particular media stream and provides the opportunity
   to the client to obtain the most appropriate variant of a media
   stream.

   Similar to SAP and IMGs, RTSP has, from its intended usage, a clear
   distinction between offering a set of Potential Configurations (by
   the server) and choosing one out of these (by the client).  There is
   no capability negotiation process involved: the server provides a
   complete SDPng document describing all Components making up a
   presentation and includes detailed codec and transport parameters for
   each of there.  The client may only pick one out of alternatives for
   each of the offered Components but has no further option to negotiate
   parameters in depth.  Where some additional exchange is necesary --
   e.g. for the client's transport addresses and security parameters --,
   the respective parameters are no encoded in SDPng; instead,
   additional RTSP header fields and parameters are field for this
   purpose.

   Hence, SDPng is only used to describe alternatives to gain access to
   streaming media out of which the client has to choose.  No
   interaction takes place at the SDPng level.

   It should be noted that SAP or IMG-based announcements may also be
   used to point users to RTSP servers.  In such a case, the
   "negotiation" is a two-stage process: the media discovery takes place
   using SAP or IMG and leads to the RTSP server.  The actual streaming
   invocation process then takes place interactively between the RTSP
   client and server as described in this section.

        C->M: DESCRIBE rtsp://foo/audio-play RTSP/1.0
              CSeq: 1

        M->C: RTSP/1.0 200 OK
              CSeq: 1



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              Content-Type: application/sdp
              Content-Length: ...

      <sdpng xmlns="http://www.iana.org/sdpng"
             xmlns:audio="http://www.iana.org/sdpng/audio"
             xmlns:rtp="http://www.iana.org/sdpng/rtp"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://www.iana.org/sdpng sdpng-base.xsd
             http://www.iana.org/sdpng/audio sdpng-audio-pkg.xsd
             http://www.iana.org/sdpng/rtp sdpng-rtp-pkg.xsd"

          owner="A@example.com" id="98765432" version="1"
      >

        <cap>
                <audio:codec name="avp:pcmu">
                        <audio:encoding>PCMU</audio:encoding>
                        <audio:channels>1</audio:channels>
                        <audio:sampling>8000</audio:sampling>
                </audio:codec>
                <audio:codec name="avp:gsm">
                        <audio:encoding>GSM</audio:encoding>
                        <audio:channels>1</audio:channels>
                        <audio:sampling>8000</audio:sampling>
                </audio:codec>
        </cap>
        <def>
                <rtp:udp name="rtp-cfg1" ref="rtpudpip4">
                  <rtp:ip-addr>192.168.47.11</rtp:ip-addr>
                  <rtp:rtp-port>51400</rtp:rtp-port>
                </rtp:udp>
        </def>
        <cfg>
                <component>
                        <alt>
                                <audio:codec ref="avp:pcmu"/>
                                <rtp:udp ref="rtp-cfg1">
                                        <rtp:pt>0</rtp:pt>
                                </rtp:udp>
                        </alt>
                        <alt>
                                <audio:codec ref="avp:gsm"/>
                                <rtp:udp ref="rtp-cfg1">
                                        <rtp:pt>3</rtp:pt>
                                </rtp:udp>
                        </alt>
                </component>
        </cfg>



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      </sdpng>

        C->M: SETUP rtsp://foo/audio-play RTSP/1.0
              CSeq: 2
              Transport: RTP/AVP;unicast;client_port=8000-8001

        M->C: RTSP/1.0 200 OK
              CSeq: 2
              Transport: RTP/AVP;unicast;client_port=8000-8001;
                         server_port=51400-51401
              Session: 12345678


   To be continued with PLAY and, after the audio track has completed,
   finished with TEARDOWN.

5.3 Two-Way Session Setup

   The major application of SDP today is its use with the Session
   Initiation Protocol (SIP) [RFC3261].  Session descriptions are used
   configure the media streams between two parties involved in a
   multimedia call.

   SIP is used to establish and modify multimedia sessions, and SDPng
   may be carried at least in SIP INVITE, ACK and UPDATE 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,
   generalized in the offer/answer model documented in RFC 3264.

   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
   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 distinguish between Potential and
   Actual Configurations at the syntax, this has no implications on the
   SDPng signaling itself but is merely up to interpretation.



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   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
   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 corresponding
   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


      <?xml version="1.0" encoding="UTF-8"?>

      <sdpng xmlns="http://www.iana.org/sdpng"
             xmlns:audio="http://www.iana.org/sdpng/audio"
             xmlns:rtp="http://www.iana.org/sdpng/rtp"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://www.iana.org/sdpng sdpng-base.xsd
             http://www.iana.org/sdpng/audio sdpng-audio-pkg.xsd
             http://www.iana.org/sdpng/rtp sdpng-rtp-pkg.xsd"

          owner="A@example.com" id="98765432" version="1"
      >

        <cap>
                <audio:codec name="avp:pcmu">



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                        <audio:encoding>PCMU</audio:encoding>
                        <audio:channels>1</audio:channels>
                        <audio:sampling>8000</audio:sampling>
                </audio:codec>
                <audio:codec name="avp:gsm">
                        <audio:encoding>GSM</audio:encoding>
                        <audio:channels>1</audio:channels>
                        <audio:sampling>8000</audio:sampling>
                </audio:codec>
        </cap>
        <def>
                <rtp:udp name="rtp-cfg1" ref="rtpudpip4">
                  <rtp:ip-addr>192.168.47.11</rtp:ip-addr>
                  <rtp:rtp-port>51400</rtp:rtp-port>
                </rtp:udp>
        </def>
        <cfg>
                <component>
                        <alt>
                                <audio:codec ref="avp:pcmu"/>
                                <rtp:udp ref="rtp-cfg1">
                                        <rtp:pt>0</rtp:pt>
                                </rtp:udp>
                        </alt>
                        <alt>
                                <audio:codec ref="avp:gsm"/>
                                <rtp:udp ref="rtp-cfg1">
                                        <rtp:pt>3</rtp:pt>
                                </rtp:udp>
                        </alt>
                </component>
        </cfg>
      </sdpng>

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

         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

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




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         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

      <?xml version="1.0" encoding="UTF-8"?>

      <sdpng xmlns="http://www.iana.org/sdpng"
             xmlns:audio="http://www.iana.org/sdpng/audio"
             xmlns:rtp="http://www.iana.org/sdpng/rtp"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://www.iana.org/sdpng sdpng-base.xsd
             http://www.iana.org/sdpng/audio sdpng-audio-pkg.xsd
             http://www.iana.org/sdpng/rtp sdpng-rtp-pkg.xsd"

          owner="B@example.com" id="4595647" version="1"
      >

        <cap>
                <audio:codec name="avp:pcmu">
                        <audio:encoding>PCMU</audio:encoding>
                        <audio:channels>1</audio:channels>
                        <audio:sampling>8000</audio:sampling>
                </audio:codec>
                <audio:codec name="avp:gsm">
                        <audio:encoding>GSM</audio:encoding>
                        <audio:channels>1</audio:channels>
                        <audio:sampling>8000</audio:sampling>
                </audio:codec>



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        </cap>
        <def>
                <rtp:udp name="rtp-cfg1" ref="rtpudpip4">
                  <rtp:ip-addr>192.168.47.12</rtp:ip-addr>
                  <rtp:rtp-port>60006</rtp:rtp-port>
                </rtp:udp>
        </def>
        <cfg>
                <component>
                        <alt>
                                <audio:codec ref="avp:gsm"/>
                                <rtp:udp ref="rtp-cfg1">
                                        <rtp:pt>3</rtp:pt>
                                </rtp:udp>
                        </alt>
                </component>
        </cfg>
      </sdpng>

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

      ACK from A to B omitted



   In the INVITE message, A sends B a description document that
   specifies exactly one component with two alternatives (the PCMU and
   GSM audio streams).  The alternatives make reference to the
   capability section where the two codec types are defined.  All
   required transport parameters are already contained in the respective
   descriptions but they are kept separate from the capability part.
   The <def> element contains a definition for the RTP media sessions so
   that this needs not be repeated in the configuration of the single
   component.  Note that the semantics of the component is not
   explicitly specified (in an <info> element) but rather implied.

   In the 200 OK message, B sends an updated description document to A.
   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
   it adds to the session description.




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   The offer/answer model allows communicating peers to determine a
   (common) mode of operation to exchange media streams in a single
   round-trip.  Basically, the offerer proposes a set of components,
   providing one or more alternatives ("potential configurations") for
   each of these. From this offer, the answerer learns which components
   may be used and which configurations are applicable to realize these
   components.  The answerer indicates which components it supports
   (e.g. receiving a offer including audio and video, it may disallow
   the video session and go with an audio-only conversation) and also
   provides possible configurations to implement those components.
   Along with the media types and codec parameters, offerer and answerer
   specify which transport addresses to use and, in case of RTP, which
   payload types they want to use for sending. Offerer and answerer
   agree on a common set of media streams ("components") and on a
   possible set of codecs for each of these ("configurations") as well
   as the transport addresses and other parameters to be used.  However,
   they do not fix a certain configuration (unless only a single one is
   exchanged in each direction).  Instead, for each selected media
   stream, either peer may choose and dynamically switch to any of the
   configurations indicated by the other side in the respective offer or
   answer.

   For using SDPng with the offer/answer model (RFC 3264), the basic
   defined in RFC 3264 for generating offers and answers apply.  The
   following considerations specifically apply when using offer/answer
   with SDPng (instead of SDP) documents:

   o  For each component to be used, all necessary parameters MUST be
      given for at least one configuration per component, i.e. transport
      addresses and payload formats MUST be specified along with the
      capabilities.

   o  Matching of components is done based upon their identification in
      the info part of the SDPng document using predefined identifiers
      for certain session types.
      For simple sessions, where applications can implicitly derive the
      semantics of the the offered components, no such explicit mapping
      is necessary.  In this case, i.e. if the entire "<info>" element
      or the respective elements in the "<info>" element are absent, the
      order of appearance in the SDPng document is relevant as it is
      with SDP.

   o  For each component, the answerer performs a capability matching
      process as per then application's requirements
      For all components that are acceptable, the answerer determines
      whether or not to accept the offer.
      If the answerer decides to accept the offer for a certain
      component, it MUST accept at least one of the potential



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      configurations for the respective component.  It SHOULD indicate
      this by setting the "status" attribute of the component and of the
      selected configuration(s) to "active" (but it MAY also omit the
      status attribute in both cases).
      It is RECOMMENDED that the answerer selects exactly one
      configuration for each component as "active".

   o  The answerer MAY refuse individual configurations for a component
      from the offer in two ways.
      If the configuration shall not be used at all during a session,
      e.g. because the answerer does not support it or because the
      answere does not want to use this configuration at all, the
      answerer MUST set the "status" attribute of the respective
      component to "unused".  In this case, the answerer MAY omit all
      the elements contained in the respective configuration's elements.
      This is equivalent to setting the port parameter to "0" in SDP.
      If a configuration shall be accepted (i.e. the respective
      capability shall be indicated) but no media session shall be
      instantiated (not even on hold!), the answerer MUST set the
      "status" attribute of the respective configuration to "available"
      and omit all media-session-specific parameters the configuration.

   o  The answerer MAY refuse entire components that the offerer has
      included in two ways.
      If a component shall not be used at all during a session -- e.g.
      because the answerer does not support any of the configurations
      listed or because the answere does not want to use this component
      at all -- the answerer MUST set the "status" attribute of the
      respective component's to "unused".  In this case, the answerer
      MAY omit all the elements contained in the respective component
      elements.  This is equivalent to setting the port parameter to "0"
      in SDP.
      If a component shall be accepted (i.e. the respective capability
      shall be indicated) but no media session shall be instantiated
      (not even on hold!), the answerer MUST set the "status" attribute
      of the respective component to "available", omit all
      media-session-specific parameters from all acceptable
      configurations for the respective component.

   o  For each component, the alternative potential configurations MUST
      be listed in the order of preference.
      Within a configuration, alternatives (e.g. different codecs) MUST
      also be listed in the order of preference.
      The considerations of RFC 3264 to simply arriving at symmetric
      codec use apply.

   If a component shall be put on hold, the status attribute of the
   component MUST be set to "sendonly", "recvonly", or "inactive", as



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   appropriate.  In this case, the status attributes of all the
   contained configurations that were previously active MUST be set to
   indicate "sendonly", "recvonly", or "inactive", as appropriate.  The
   rules from RFC 3264 for putting media streams on hold SHALL apply.

5.4 Multi-Party-Conferencing with Negotiation

   The indipendent capability collapsing properties of SDPng allow to
   calculate the "intersection" of any number of SDPng documents
   independently and easily derive a common subset. Details are TBD in a
   separate document.








































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6. IANA Considerations

   The IANA should set up a registry for XML namespaces for SDPng and
   SDPng package definitions.

   The SDP parameter registry (http://www.iana.org/assignments/
   sdp-parameters) should be converted to SDPng package definitions.












































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7. Open Issues

      Meta-Information for SDPng description (author, version etc.)?

      Revise usage of terminology (potential configuration, actual
      configuration)

      Do we need an explicit mechanism to declare the used packages?
      E.g., <pkg ref="http://www.iana.org/sdpng/audio"/>

      Data model for audio package: sampling-rate vs. RTP clock rate

      Bib. references: distinguish normative and informational

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



































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8. Acknowledgements

   The authors would like to thank Teodora Guenkova, Goran Petrovic and
   Markus Nosse for their feedback and detailed comments.















































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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. Jacobson,
         "RTP: A Transport Protocol for Real-Time Applications", RFC
         3550, July 2003.

   [4]   Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
         Conferences with Minimal Control", RFC 3551, July 2003.

   [5]   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.

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

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

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

   [9]   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.

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

   [11]  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.

   [12]  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.

   [13]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
         SDP", RFC 3264, June 2002.




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   [14]  Hollenbeck, S., Rose, M. and L. Masinter, "Guidelines for the
         Use of Extensible Markup Language (XML) within IETF Protocols",
         BCP 70, RFC 3470, January 2003.

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

   [16]  Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
         2279, January 1998.

   [17]  Holtman, K., Mutz, A. and T. Hardie, "Media Feature Tag
         Registration Procedure", BCP 31, RFC 2506, March 1999.


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. Formal Syntax Specifications

A.1 SDPng Base DTD

   The following DTD specifies the SDPng base syntax. DTDs are not
   XML-Namespace aware, therefore the following DTD is for informational
   purposes only. Moreover, the content models for the element types
   "cap" and "def" have to be empty in this DTD as the specific element
   types for the allowed child elements are not defined by the base
   specification but by independent package definitions. Common
   requirements for these element types such as the "name" attribute
   cannot be expressed with XML DTDs.

   <!ELEMENT sdpng (cap?, def?, cfg, constraints?, info?)>

   <!ELEMENT cap ANY>

   <!ELEMENT def ANY>

   <!ELEMENT cfg (component+)>

   <!ELEMENT component (alt+)>
   <!ATTLIST component
     name   CDATA #REQUIRED
     media  CDATA #IMPLIED
     status CDATA #IMPLIED
   >

   <!ELEMENT alt ANY>
   <!ATTLIST alt
     name CDATA   #REQUIRED
     status CDATA #IMPLIED
   >

   <!ELEMENT constraints ANY>

   <!ELEMENT info (part+)>

   <!ELEMENT part ANY>
   <!ATTLIST part
     type CDATA #REQUIRED
     ref  CDATA #IMPLIED
   >








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A.2 SDPng XML-Schema Specification

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


   <!--

   A data type for the "status" attribute

     status=mandatory: feature match MUST be successful
     status=opt:       optional feature, feature match MAY fail
   -->

     <xsd:simpleType name="status">
       <xsd:restriction base="xsd:string">
         <xsd:enumeration value="mandatory"/>
         <xsd:enumeration value="opt"/>
       </xsd:restriction>
     </xsd:simpleType>


   <!-- Base type for definition elements -->

     <xsd:complexType name="Definition" abstract="true">
       <xsd:attribute name="name" type="xsd:string" use="optional"/>
       <xsd:attribute name="ref" type="xsd:string" use="optional"/>
     </xsd:complexType>


   <!--
   Data type for the content model of mandatory feature elements of type
   token
    -->

     <xsd:complexType name="token">
       <xsd:simpleContent>
         <xsd:extension base="xsd:NMTOKEN">
        <xsd:attribute name="status" type="sdpng:status" fixed="mandatory"/>
         </xsd:extension>
       </xsd:simpleContent>
     </xsd:complexType>




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   <!--
   Data type for the content model of optional feature elements of
   type token
    -->

     <xsd:complexType name="opttoken">
       <xsd:simpleContent>
         <xsd:extension base="xsd:NMTOKEN">
        <xsd:attribute name="status" type="sdpng:status" fixed="opt"/>
         </xsd:extension>
       </xsd:simpleContent>
     </xsd:complexType>


   <!--
   Data type for the content model of mandatory feature elements of type
   token list
   -->

     <xsd:complexType name="tokenlist">
       <xsd:simpleContent>
         <xsd:extension base="xsd:NMTOKENS">
        <xsd:attribute name="status" type="sdpng:status" fixed="mandatory"/>
         </xsd:extension>
       </xsd:simpleContent>
     </xsd:complexType>


   <!--
   Data type for the content model of optional feature elements of type
   token list
   -->

     <xsd:complexType name="opttokenlist">
       <xsd:simpleContent>
         <xsd:extension base="xsd:NMTOKENS">
        <xsd:attribute name="status" type="sdpng:status" fixed="opt"/>
         </xsd:extension>
       </xsd:simpleContent>
     </xsd:complexType>


   <!--
   Data type for the content model of mandatory feature elements of type
   numerical value
   -->

     <xsd:complexType name="numval">



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       <xsd:attribute name="status" type="sdpng:status" fixed="mandatory"/>
       <xsd:attribute name="val" type="xsd:decimal"/>
     </xsd:complexType>


   <!--
   Data type for the content model of optional feature elements of
   type numerical value
   -->

     <xsd:complexType name="optnumval">
       <xsd:attribute name="status" type="sdpng:status" fixed="opt"/>
       <xsd:attribute name="val" type="xsd:decimal"/>
     </xsd:complexType>


   <!--
   Data type for the content model of mandatory feature elements of type
   numerical range
   -->

     <xsd:complexType name="numrange">
       <xsd:attribute name="status" type="sdpng:status" fixed="mandatory"/>
       <xsd:attribute name="min" type="xsd:decimal"/>
       <xsd:attribute name="max" type="xsd:decimal"/>
     </xsd:complexType>


   <!--
   Data type for the content model of optional feature elements of
   type numerical range
   -->

     <xsd:complexType name="optnumrange">
       <xsd:attribute name="status" type="sdpng:status" fixed="opt"/>
       <xsd:attribute name="min" type="xsd:decimal"/>
       <xsd:attribute name="max" type="xsd:decimal"/>
     </xsd:complexType>


   <!-- Base type for definition elements -->

     <xsd:complexType name="Constraint" abstract="true">
       <xsd:attribute name="name" type="xsd:string" use="optional"/>
     </xsd:complexType>


   <!-- The base element for constraint element -->



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   <!-- FIXME: substitutionGroup? -->

     <xsd:element name="constraint" type="sdpng:Constraint" abstract="true">
     </xsd:element>


   <!-- Base type for information elements -->

     <xsd:complexType name="Information" abstract="true">
       <xsd:attribute name="type" type="xsd:string"/>
       <xsd:attribute name="ref" type="xsd:string" use="optional"/>
     </xsd:complexType>


   <!-- The nformation part element -->

     <xsd:element name="part" type="sdpng:Information" abstract="true">
     </xsd:element>

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

   <!--
   SDPng document structure
   -->

     <xsd:element name="sdpng">
       <xsd:complexType>
         <xsd:sequence>
        <xsd:element ref="sdpng:cap" minOccurs="0"/>
        <xsd:element ref="sdpng:def" minOccurs="0"/>
        <xsd:element ref="sdpng:cfg"/>
        <xsd:element ref="sdpng:constraints" minOccurs="0"/>
        <xsd:element ref="sdpng:info" minOccurs="0"/>
         </xsd:sequence>
       </xsd:complexType>
     </xsd:element>


   <!-- The base element for capability and definition elements -->
   <!-- FIXME: substitutionGroup? -->

     <xsd:element name="definition" type="sdpng:Definition" abstract="true">
     </xsd:element>

   <!-- The optional "cap" element -->

     <xsd:element name="cap">
       <xsd:complexType mixed="false">



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         <xsd:sequence>
        <xsd:element ref="sdpng:definition" maxOccurs="unbounded"/>
         </xsd:sequence>
       </xsd:complexType>
     </xsd:element>


   <!-- The optional "def" element -->

     <xsd:element name="def">
       <xsd:complexType mixed="false">
         <xsd:sequence>
        <xsd:element ref="sdpng:definition" maxOccurs="unbounded"/>
         </xsd:sequence>
       </xsd:complexType>
     </xsd:element>


   <!-- The mandatory "cfg" element -->

     <xsd:element name="cfg">
       <xsd:complexType mixed="false">
         <xsd:sequence>
        <xsd:element ref="sdpng:component" maxOccurs="unbounded"/>
         </xsd:sequence>
       </xsd:complexType>
     </xsd:element>

   <!-- The "component" element -->

     <xsd:element name="component">
       <xsd:complexType>
         <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" use="optional"/>
         <xsd:attribute name="status" type="xsd:string" use="optional"/>
       </xsd:complexType>
     </xsd:element>


     <xsd:element name="alt">
       <xsd:complexType mixed="false">
         <xsd:sequence>
        <xsd:element ref="sdpng:definition" maxOccurs="unbounded"/>
         </xsd:sequence>
         <xsd:attribute name="name" type="xsd:string"/>



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         <xsd:attribute name="status" type="xsd:string" use="optional"/>
       </xsd:complexType>
     </xsd:element>


   <!-- The optional "constraints" element -->

     <xsd:element name="constraints">
       <xsd:complexType mixed="false">
         <xsd:sequence>
        <xsd:element ref="sdpng:constraint" maxOccurs="unbounded"/>
         </xsd:sequence>
       </xsd:complexType>
     </xsd:element>


   <!-- The optional "info" element -->

     <xsd:element name="info">
       <xsd:complexType mixed="false">
         <xsd:sequence>
        <xsd:element ref="sdpng:part" maxOccurs="unbounded"/>
         </xsd:sequence>
       </xsd:complexType>
     </xsd:element>



   </xsd:schema>






















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Appendix B. Sample Package Definitions

B.1 Sample RTP Package Definition

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

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


     <xsd:complexType name="IPVersion">
       <xsd:simpleContent>
         <xsd:restriction base="sdpng:tokenlist">
        <xsd:enumeration value="IP4"/>
        <xsd:enumeration value="IP6"/>
         </xsd:restriction>
       </xsd:simpleContent>
     </xsd:complexType>


     <xsd:complexType name="RTPUDP">
       <xsd:complexContent>
         <xsd:extension base="sdpng:Definition">
        <xsd:all>
          <xsd:element name="network" type="rtp:IPVersion" minOccurs="0"/>
          <xsd:element name="ip-addr" type="sdpng:opttoken" minOccurs="0"/>
          <xsd:element name="rtp-port" type="sdpng:opttoken" minOccurs="0"/>
          <xsd:element name="rtcp-port" type="sdpng:opttoken" minOccurs="0"/>
          <xsd:element name="pt" type="sdpng:opttoken" minOccurs="0"/>
        </xsd:all>
         </xsd:extension>
       </xsd:complexContent>
     </xsd:complexType>


     <xsd:element name="udp" type="rtp:RTPUDP" substitutionGroup="sdpng:definition"/>

   </xsd:schema>








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B.2 Sample Audio Package Definition

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

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

     <xsd:complexType name="Codec">
       <xsd:complexContent>
         <xsd:restriction base="sdpng:Definition">
        <xsd:all>
          <xsd:element name="encoding" type="sdpng:token" minOccurs="0"/>
          <xsd:element name="channels" type="sdpng:tokenlist" minOccurs="0"/>
          <xsd:element name="sampling" type="sdpng:tokenlist" minOccurs="0"/>
        </xsd:all>
         </xsd:restriction>
       </xsd:complexContent>
     </xsd:complexType>

     <xsd:element name="codec" type="audio:Codec" substitutionGroup="sdpng:definition"/>

   </xsd:schema>



B.3 Sample Video Package Definition

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

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

     <xsd:complexType name="Codec">
       <xsd:complexContent>
         <xsd:restriction base="sdpng:Definition">
        <xsd:all>
          <xsd:element name="encoding" type="sdpng:token" minOccurs="0"/>
          <xsd:element name="sampling" type="sdpng:tokenlist" minOccurs="0"/>



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          <xsd:element name="framerate" type="sdpng:opttokenlist" minOccurs="0"/>
          <xsd:element name="size" type="sdpng:opttokenlist" minOccurs="0"/>
          <xsd:element name="bitrate" type="sdpng:optnumrange" minOccurs="0"/>
          <xsd:element name="min-quant" type="sdpng:optnum" minOccurs="0"/>
          <xsd:element name="max-quant" type="sdpng:optnum" minOccurs="0"/>
          <xsd:element name="gop-size" type="sdpng:optnum" minOccurs="0"/>
        </xsd:all>
         </xsd:restriction>
       </xsd:complexContent>
     </xsd:complexType>

     <xsd:element name="codec" type="video:Codec" substitutionGroup="sdpng:definition"/>

   </xsd:schema>





































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Appendix C. Sample SDPng Description

   <?xml version="1.0" encoding="UTF-8"?>

   <sdpng xmlns="http://www.iana.org/sdpng"
          xmlns:audio="http://www.iana.org/sdpng/audio"
          xmlns:video="http://www.iana.org/sdpng/video"
          xmlns:rtp="http://www.iana.org/sdpng/rtp"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="http://www.iana.org/sdpng sdpng-base.xsd
          http://www.iana.org/sdpng/audio sdpng-audio-pkg.xsd
          http://www.iana.org/sdpng/video sdpng-video-pkg.xsd
          http://www.iana.org/sdpng/rtp sdpng-rtp-pkg.xsd"
   >

        <cap>
                <audio:codec name="avp:pcmu">
                        <audio:encoding>PCMU</audio:encoding>
                        <audio:channels>1</audio:channels>
                        <audio:sampling>8000</audio:sampling>
                </audio:codec>
                <audio:codec name="avp:gsm">
                        <audio:encoding>GSM</audio:encoding>
                        <audio:channels>1</audio:channels>
                        <audio:sampling>8000</audio:sampling>
                </audio:codec>
                <audio:codec name="avp:g723">
                        <audio:encoding>G723</audio:encoding>
                        <audio:channels>1</audio:channels>
                        <audio:sampling>8000</audio:sampling>
                </audio:codec>
                <audio:codec name="avp:dvi4">
                        <audio:encoding>DVI4</audio:encoding>
                        <audio:channels>1</audio:channels>
                        <audio:sampling>8000 11025 16000 22050</audio:sampling>
                </audio:codec>
                <audio:codec name="avp:lpc">
                        <audio:encoding>LPC</audio:encoding>
                        <audio:channels>1</audio:channels>
                        <audio:sampling>8000</audio:sampling>
                </audio:codec>
                <audio:codec name="avp:g722">
                        <audio:encoding>G722</audio:encoding>
                        <audio:channels>1</audio:channels>
                        <audio:sampling>8000</audio:sampling>
                </audio:codec>
                <audio:codec name="avp:l16">
                        <audio:encoding>L16</audio:encoding>



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                        <audio:channels>1 2</audio:channels>
                        <audio:sampling>44100</audio:sampling>
                </audio:codec>
                <audio:codec name="avp:qcelp">
                        <audio:encoding>QCELP</audio:encoding>
                        <audio:channels>1</audio:channels>
                        <audio:sampling>8000</audio:sampling>
                </audio:codec>
                <audio:codec name="avp:cn">
                        <audio:encoding>CN</audio:encoding>
                        <audio:channels>1</audio:channels>
                        <audio:sampling>8000</audio:sampling>
                </audio:codec>
                <audio:codec name="avp:mpa">
                        <audio:encoding>MPA</audio:encoding>
                        <audio:channels>1</audio:channels>
                        <audio:sampling>32000 44100 48000</audio:sampling>
                </audio:codec>
                <audio:codec name="avp:g728">
                        <audio:encoding>G728</audio:encoding>
                        <audio:channels>1</audio:channels>
                        <audio:sampling>8000</audio:sampling>
                </audio:codec>
                <audio:codec name="avp:g729">
                        <audio:encoding>G728</audio:encoding>
                        <audio:channels>1</audio:channels>
                        <audio:sampling>8000</audio:sampling>
                </audio:codec>
                <audio:codec name="avp:g726-40">
                        <audio:encoding>G726-40</audio:encoding>
                        <audio:channels>1</audio:channels>
                        <audio:sampling>8000</audio:sampling>
                </audio:codec>
                <audio:codec name="avp:g726-32">
                        <audio:encoding>G726-32</audio:encoding>
                        <audio:channels>1</audio:channels>
                        <audio:sampling>8000</audio:sampling>
                </audio:codec>
                <audio:codec name="avp:g726-24">
                        <audio:encoding>G726-24</audio:encoding>
                        <audio:channels>1</audio:channels>
                        <audio:sampling>8000</audio:sampling>
                </audio:codec>
                <audio:codec name="avp:g726-16">
                        <audio:encoding>G726-16</audio:encoding>
                        <audio:channels>1</audio:channels>
                        <audio:sampling>8000</audio:sampling>
                </audio:codec>



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                <audio:codec name="avp:g729d">
                        <audio:encoding>G729D</audio:encoding>
                        <audio:channels>1</audio:channels>
                        <audio:sampling>8000</audio:sampling>
                </audio:codec>
                <audio:codec name="avp:g729e">
                        <audio:encoding>G729E</audio:encoding>
                        <audio:channels>1</audio:channels>
                        <audio:sampling>8000</audio:sampling>
                </audio:codec>
                <audio:codec name="avp:gsm-efr">
                        <audio:encoding>GSM-EFR</audio:encoding>
                        <audio:channels>1</audio:channels>
                        <audio:sampling>8000</audio:sampling>
                </audio:codec>
                <audio:codec name="avp:l8">
                        <audio:encoding>L8</audio:encoding>
                        <audio:channels>1 2</audio:channels>
                        <audio:sampling>8000 16000</audio:sampling>
                </audio:codec>
                <audio:codec name="avp:red">
                        <audio:encoding>RED</audio:encoding>
                        <audio:channels>1 2</audio:channels>
                        <audio:sampling>8000 16000</audio:sampling>
                </audio:codec>
                <audio:codec name="avp:vdvi">
                        <audio:encoding>RED</audio:encoding>
                        <audio:channels>1</audio:channels>
                        <audio:sampling>var</audio:sampling>
                </audio:codec>

                <video:codec name="avp:celb">
                 <video:encoding>CelB</video:encoding>
                 <video:framerate>4 6 8 12 16 20 24 30</video:framerate>
                </video:codec>

                <rtp:udp name="rtpudpip6">
                        <rtp:network>IP6</rtp:network>
                </rtp:udp>

        </cap>
           <def>
                <rtp:udp name="rtp-cfg1" ref="rtpudpip6">
                  <rtp:ip-addr>::1</rtp:ip-addr>
                  <rtp:rtp-port>9546</rtp:rtp-port>
                </rtp:udp>
           </def>
        <cfg>



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                <component>
                        <alt>
                                <audio:codec ref="avp:pcmu"/>
                                <rtp:udp ref="rtp-cfg1">
                                        <rtp:pt>0</rtp:pt>
                                </rtp:udp>
                        </alt>
                </component>
        </cfg>
   </sdpng>









































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Appendix D. Change History

   draft-ietf-mmusic-sdpng-08.txt

      *  Changed introduction: emphasis on non-negotiation scenarios
         (broadcast etc.), describe main concepts

      *  element "cap" is now OPTIONAL.

      *  new "status" attribute for "component" element type

      *  new "part" element for "info" element (meta-information)

      *  Removed section "Specification of the Capability Negotiation"

      *  Removed appendix "Use of SDPng in Conjunction with other IETF
         Signaling Protocols"

      *  Added section "Usage of SDPng in Different Application
         Scenarios"

      *  Updated DTD and XML-Schema-Definition wrt to afore-mentioned
         changes

   draft-ietf-mmusic-sdpng-07.txt

      *  New document structure:

         1.  Intro

         2.  Terminology and System Model

         3.  Overview

         4.  SDPng Syntax Specification

         5.  Negotiation Process

      *  Changes to Section 3: Describe all concepts

      *  Section 4 provides complete specification

      *  Changed XML syntax: Represent tokens and token list as element
         content (not attributes)

      *  a new element "def" for reusable definitions

      *  Adapted secion 5 accordingly



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      *  Sample DTD, schema definition and same SDPng document in
         appendix

      *  Updated section 5.1 (Offer/Answer)

      *  Updated appendix D (Use of SDPng in conjunction with other IETF
         Signaling Protocols)

   draft-ietf-mmusic-sdpng-06.txt

      *  Removed section on capability negotiation algorithm and section
         on formal specification. Added Section 3.

      *  Removed specification of concrete XML syntax from Section 4.
         Added requirements and theoretic considerations.

      *  Added clarification of term "actual configuration" in Section
         2.

      *  Changed "profile" to "package".

      *  Added a list of terms with explanation at the end of Section 2.

      *  Removed audio and RTP packages from appendix.

      *  Added a section "Syntax Definition".

      *  Added section "Specification of the Capability Negotiation".

   draft-ietf-mmusic-sdpng-05.txt

      *  Moved audio and RTP packages to appendix.

      *  Moved section "Use of SDPng in conjunction with other IETF
         Signaling Protocols" to appendix.

      *  Changed mechanism for references to definitions: Definition
         elements provide an attribute "ref" that can be used to
         referenced existing definitions. Removed other mechanisms for
         referencing (attributes "format" and "transport", element type
         "use").

      *  Corrections to schema definitions and examples

   draft-ietf-mmusic-sdpng-04.txt

      *  New section on capability negotiation.




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      *  New section on referencing definitions.

      *  New section on properties.

      *  New section on definition groups.

   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.

   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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   IPR Notice

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
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   made any independent effort to identify any such rights. Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
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   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

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   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard. Please address the information to the IETF at ietf-
   ipr@ietf.org.


Full Copyright Statement

   Copyright (C) The Internet Society (2005). This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on
   an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
   REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES,
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Acknowledgment

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






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