mmusic Kutscher
Internet-Draft Ott
Expires: April 26, 2004 Bormann
TZI, Universitaet Bremen
October 27, 2003
Session Description and Capability Negotiation
draft-ietf-mmusic-sdpng-07.txt
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document defines a language for describing multimedia sessions
with respect to configuration parameters and capabilities of
end-systems.
This document is a product of the Multiparty Multimedia Session
Control (MMUSIC) working group of the Internet Engineering Task
Force. Comments are solicited and should be addressed to the working
group's mailing list at mmusic@ietf.org and/or the authors.
Document Revision
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$Revision: 6.18 $
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology and System Model . . . . . . . . . . . . . . . . 6
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1 Outline of the Negotiation Process . . . . . . . . . . . . . 10
3.2 Capability Types . . . . . . . . . . . . . . . . . . . . . . 12
3.3 Application-specific Vocabulary . . . . . . . . . . . . . . 14
4. SDPng Syntax . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1 SDPng Base Syntax . . . . . . . . . . . . . . . . . . . . . 15
4.2 Capabilities . . . . . . . . . . . . . . . . . . . . . . . . 17
4.2.1 Tokens . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.2.2 Token Sets . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.2.3 Numerical Values . . . . . . . . . . . . . . . . . . . . . . 18
4.2.4 Numerical Ranges . . . . . . . . . . . . . . . . . . . . . . 18
4.2.5 Sample SDPng cap Element . . . . . . . . . . . . . . . . . . 19
4.2.6 Referencing Capability Elements . . . . . . . . . . . . . . 20
4.3 Definitions . . . . . . . . . . . . . . . . . . . . . . . . 20
4.4 Configurations . . . . . . . . . . . . . . . . . . . . . . . 22
4.5 Constraints . . . . . . . . . . . . . . . . . . . . . . . . 24
4.6 Session Information . . . . . . . . . . . . . . . . . . . . 24
4.7 Summary of SDPng XML-Syntax . . . . . . . . . . . . . . . . 24
5. Specification of the Capability Negotiation . . . . . . . . 26
5.1 Offer/Answer . . . . . . . . . . . . . . . . . . . . . . . . 26
5.2 RFC2533 Negotiation . . . . . . . . . . . . . . . . . . . . 28
5.2.1 Translating SDPng to RFC 2533 Expressions . . . . . . . . . 28
5.2.2 Applying RFC 2533 Canonicalization . . . . . . . . . . . . . 31
5.2.3 Integrating Feature Sets into SDPng . . . . . . . . . . . . 31
5.2.4 Processing Negotiation Results . . . . . . . . . . . . . . . 32
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . 33
7. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . 34
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 35
References . . . . . . . . . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 37
A. Formal Syntax Specifications . . . . . . . . . . . . . . . . 38
A.1 SDPng Base DTD . . . . . . . . . . . . . . . . . . . . . . . 38
A.2 SDPng XML-Schema Specification . . . . . . . . . . . . . . . 38
B. Sample Package Definitions . . . . . . . . . . . . . . . . . 45
B.1 Sample RTP Package Definition . . . . . . . . . . . . . . . 45
B.2 Sample Audio Package Definition . . . . . . . . . . . . . . 46
B.3 Sample Video Package Definition . . . . . . . . . . . . . . 46
C. Sample SDPng Description . . . . . . . . . . . . . . . . . . 48
D. Use of SDPng in Conjunction with other IETF Signaling
Protocols . . . . . . . . . . . . . . . . . . . . . . . . . 52
D.1 The Session Announcement Protocol (SAP) . . . . . . . . . . 52
D.2 Session Initiation Protocol (SIP) . . . . . . . . . . . . . 53
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D.3 Real-Time Streaming Protocol (RTSP) . . . . . . . . . . . . 58
D.4 Media Gateway Control Protocol (MEGACOP) . . . . . . . . . . 59
E. Change History . . . . . . . . . . . . . . . . . . . . . . . 61
Intellectual Property and Copyright Statements . . . . . . . 64
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1. Introduction
Multiparty multimedia conferencing is one of the applications that
require dynamic interchange of end-system capabilities and the
negotiation of a parameter set that is appropriate for all sending
and receiving end-systems in a conference. For some applications,
e.g. for loosely coupled conferences or for broadcast scenarios, it
may be sufficient to simply have session parameters be fixed by the
initiator of a conference. In such a scenario no negotiation is
required because only those participants with media tools that
support the predefined settings can join a media session and/or a
conference.
This approach is applicable for conferences that are announced some
time ahead of the actual start date of the conference. Potential
participants can check the availability of media tools in advance and
tools such as session directories can configure media tools upon
startup. This procedure however fails to work for conferences
initiated spontaneously including Internet phone calls or ad-hoc
multiparty conferences. Fixed settings for parameters such as media
types, their encoding etc. can easily inhibit the initiation of
conferences, for example in situations where a caller insists on a
fixed audio encoding that is not available at the callee's
end-system.
To allow for spontaneous conferences, the process of defining a
conference's parameter set must therefore be performed either at
conference start (for closed conferences) or maybe (potentially) even
repeatedly every time a new participant joins an active conference.
The latter approach may not be appropriate for every type of
conference without applying certain policies: For conferences with
TV-broadcast or lecture characteristics (one main active source) it
is usually not desired to re-negotiate parameters every time a new
participant with an exotic configuration joins because it may
inconvenience existing participants or even exclude the main source
from media sessions. But conferences with equal "rights" for
participants that are open for new participants on the other hand
would need a different model of dynamic capability negotiation, for
example a telephone call that is extended to a 3-parties conference
at some time during the session.
SDP [2] allows to specify multimedia sessions (i.e. conferences,
"session" as used here is not to be confused with "RTP session"!) by
providing general information about the session as a whole and
specifications for all the media streams (RTP sessions and others) to
be used to exchange information within the multimedia session.
Currently, media descriptions in SDP are used for two purposes:
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o to describe session parameters for announcements and invitations
(the original purpose of SDP) and
o to describe the capabilities of a system and possibly provide a
choice between a number of alternatives (which SDP was not
designed for).
A distinction between these two "sets of semantics" is only made
implicitly.
This document is based upon a set of requirements specified in a
companion document [1]. In the following, we first introduce a model
for session description and capability negotiation as well as the
basic terms used throughout this specification (Section 2). 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.
Appendix A provide formal specifications of SDPng such as XML DTD and
Schema definitions, Appendix D describes the usage of SDPng in
conjunction with IETF control protocol for multimedia communication
and Appendix E 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 and
quality of implementation may constrain the possible video
encoding algorithms.
In multiparty multimedia conferences, participants employ different
"components" in conducting the conference.
Example: In lecture multicast conferences one component might be
the voice transmission for the lecturer, another the transmission
of video pictures showing the lecturer and the third the
transmission of presentation material.
Depending on system capabilities, user preferences and other
technical and political constraints, different configurations can be
chosen to accomplish the use of these components in a conference.
Each component can be characterized at least by (a) its intended use
(i.e. the function it shall provide) and (b) one or more possible
ways to realize this function. Each way of realizing a particular
function is referred to as a "configuration".
Example: A conference component's intended use may be to make
transparencies of a presentation visible to the audience on the
Mbone. This can be achieved either by a video camera capturing the
image and transmitting a video stream via some video tool or by
loading a copy of the slides into a distributed electronic
white-board. For each of these cases, additional parameters may
exist, variations of which lead to additional configurations (see
below).
Two configurations are considered different regardless of whether
they employ entirely different mechanisms and protocols (as in the
previous example) or they choose the same and differ only in a single
parameter.
Example: In case of video transmission, a JPEG-based still image
protocol may be used, H.261 encoded CIF images could be sent, as
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could H.261 encoded QCIF images. All three cases constitute
different configurations. Of course there are many more detailed
protocol parameters.
Each component's configurations are limited by the participating
system's capabilities. In addition, the intended use of a component
may constrain the possible configurations further to a subset
suitable for the particular component's purpose.
Example: In a system for highly interactive audio communication
the component responsible for audio may decide not to use the
available G.723.1 audio codec to avoid the additional latency but
only use G.711. This would be reflected in this component only
showing configurations based upon G.711. Still, multiple
configurations are possible, e.g. depending on the use of A-law or
u-Law, packetization and redundancy parameters, etc.
In 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 (streaming or conference) consists of one or
more conference components for multimedia "interaction".
o A component describes a particular type of interaction (e.g. audio
conversation, slide presentation) that can be realized by means of
different applications (possibly using different protocols).
o A configuration is a set of parameters that are required to
implement a certain variation (realization) of a certain
component. There are actual and potential configurations.
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* Potential configurations describe possible configurations that
are supported by an end-system.
* An actual configuration is an "instantiation" of one of the
potential configurations, i.e. a decision how to realize a
certain component.
In less abstract words, potential configurations describe what a
system can do ("capabilities") and actual configurations describe
how a system is configured to operate at a certain point in time
(media stream spec).
To decide on a certain actual configuration, a negotiation process
needs to take place between the involved peers:
1. to determine which potential configuration(s) they have in
common, and
2. to select one of this shared set of common potential
configurations to be used for information exchange (e.g. based
upon preferences, external constraints, etc.).
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
for session initiation with SIP -- limiting the negotiation to
exactly one round trip.
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The requirements for the SDPng specification, subdivided into general
requirements and requirements for session descriptions, potential and
actual configurations as well as negotiation rules, are captured in a
companion document [1].
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 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 conferences 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 and processing rules that are applied to the
capability negotiation process. The rules specify how to process two
or more capability description in general in order to obtain an
interworking configuration.
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
negotiation rules are defined in this memo.
In the following, we provide an overview of the negotiation process
in Section 3.1 and describe the different capability types and the
corresponding 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
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
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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
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
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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.
The SDPng capability negotiation process is specified in Section 5.
3.2 Capability Types
The capability negotiation process relies on a fixed set of
processing rules for different types of capabilities. The following
types are defined:
1. Tokens (text strings)
Example:
<audio:encoding>PCMU</audio:encoding>
Processing rule:
Ascertain identity
2. Token lists
Example:
<audio:sampling-rate>8000 16000</audio:sampling-rate>
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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
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.
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3.3 Application-specific Vocabulary
While the SDPng specification defines the fundamental definition
types, 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 that
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 MUST be present in any SDPng description.
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 "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 so that type
information is encoded implicitly in the description document. Each
feature element MAY provide an attribute "status". If this attribute
is present it MUST provide one of the following values:
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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</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.
Section 5.2.4 provides implementation requirements for dealing with
references to capability elements after a capability negotiation
process.
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.
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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
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
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definition element take precedence over the feature elements of
the capability element.
Please note the implementation requirements for dealing with
references to capability elements after a capability negotiation
process provided in Section 5.2.4.
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 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 definition 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 SHOULD
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:
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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/
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.
Please note the implementation requirements for dealing with
references to capability elements after a capability negotiation
process provided in Section 5.2.4.
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">
<alt name"alt1">
<audio:codec ref="avp:pcmu"/>
<rtp:udp ref="rtp-cfg1"/>
</alt>
</component>
</cfg>
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<constraints>
[...]
</constraints>
<info>
[...]
</info>
</sdpng>
4.5 Constraints
The Constraints section allows to express constraints on the
combination of configurations that apply across different components.
The "constraints" element of an SDPng description is OPTIONAL.
The usage of constraints will be specified in a separate document.
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.
The "info" element is OPTIONAL and, if it is present, it MAY provide
a list of information elements. The element types are specified in
package definitions.
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
o component
o alt
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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. Specification of the Capability Negotiation
The SDPng specification defines the syntax and the semantics of
capability descriptions. The algorithms that are used for processing
descriptions and for comparing capability descriptions from different
participants are application specific.
In this section, we specify two alternative algorithms for
implementations: A model that is based on the SDP offer/answer scheme
(Section 5.1 and a model that is based on the feature matching
algorithm that is specified in RFC 2533 [15] (Section 5.2).
5.1 Offer/Answer
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 session part of the SDPng document using predefined
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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
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
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(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
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.
5.2 RFC2533 Negotiation
SDPng potential configurations can be processed using the RFC 2533
algorithm as defined in [15]. This involves the following steps:
Translating SDPng capability descriptions to RFC 2533 feature set
expressions;
Applying the RFC 2533 feature match algorithm; and
Integrating the resulting feature set expressions into the SDPng
selection of conference configurations.
5.2.1 Translating SDPng to RFC 2533 Expressions
SDPng capability descriptions can be translated to RFC 2533 feature
sets in a straightforward way, because SDPng uses a subset of the
mechanisms provided by RFC 2533 with a different syntax.
Each capability is represented as an XML element with a set of child
elements. We first describe how to translate a single capability
element into a RFC 2533 feature set, and then consider the
combination of multiple capability elements.
Basically, all attributes of an SDPng capability element and its
child elements MUST be transformed to an RFC 2533 expression, whereas
each child element MUST be translated to a feature predicate. The
resulting feature predicates are combined using the '&' (AND)
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operator. The name attributes MUST NOT be considered.
Each predicate MUST be encapsulated by brackets ('(', ')'). The value
or value range of each feature element is taken as a feature
predicate value. Each feature element name is directly adopted as a
feature tag, including the namespace name.
The SDPng data types map to RFC 2533 feature types as follows:
Token
A token MUST be directly adopted as an RFC 2533 token.
Token set
A token set MUST be adopted as an RFC 2533 set (a comma-separated
token list inside square brackets, such as
"video:channels=[1,2]").
Number
A single number in a "val" attribute of a feature elements of type
number MUST be adopted as an RFC 2533 number.
Numerical Ranges
A numerical range MUST be transformed to a feature set expression
with two feature predicates that are combined using the "&" (AND)
operator. The first predicate specifies the lower limit and the
second predicate specified the upper limit.
For example, the element <bitrate min="64" max="128"/> would be
transformed to the following feature set:
(& (bitrate>=64) (bitrate<=128))
A numerical range without a lower limit MUST be transformed to a
corresponding predicate with a '<=' operator and a numerical range
without a upper limit MUST be transformed to a corresponding
predicate with a '>=' operator.
For example, the element <bitrate max="128"/> would be transformed
to the following feature set:
(bitrate<=128)
The following sample SDPng potential configuration would be
transformed as follows:
Original SDPng expression:
<video:codec name="h263+-enhanced">
<video:resolution>QCIF</video:resolution>
<video:frame-rate max="24"/>
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<h263plus:A>foo</h263plus:A>
<h263plus:B>bar</h263plus:B>
</video:codec>
Transforming feature elements to feature predicates:
(& (video:resolution=QCIF) (video:frame-rate<=24)
(h263plus:A=foo) (h263plus:B=bar))
RFC 2533 uses the syntax rules of RFC 2506 [17] for feature tags.
Note that in example above, the namespace name is not used for
feature tags, instead we use the namespace prefix (for abbreviation).
It should be noted, that implementations MUST replace the namespace
prefix of SDPng elements with the namespace name when performing the
translation to an RFC 2533 expression. The following figure depicts
an corresponding expression for the previous example:
(& (http://www.iana.org/sdpng/video:resolution=QCIF)
(http://www.iana.org/sdpng/video:frame-rate<=24)
(http://www.example.com/h263plus:A=foo)
(http://www.example.com/h263plus:B=bar))
For this example, we assume that the prefix "video" has been assigned
to the namespace name "http://www.iana.org/sdpng/video" and that the
prefix "h263plus" has been assigned to the namespace name "http://
www.example.com/h263plus". In the following examples, we will use the
abbreviated form (using the namespace prefix only).
Multiple independent capability elements MUST each be transformed
using the specification above and then combined into a single RFC
2533 feature set by connecting the individual feature sets using the
'|' (OR) operator. For example, the following sample SDPng potential
configuration would be transformed as follows:
<audio:codec name="avp:pcmu">
<audio:encoding>PCMU</audio:encoding>
<audio:channels>1 2</audio:channels>
<audio:sampling>8000 16000</audio:sampling>
</audio:codec>
<video:codec name="h263+-enhanced">
<video:resolution>QCIF</video:resolution>
<video:frame-rate max="24"/>
<h263plus:A>foo</h263plus:A>
<h263plus:B>bar</h263plus:B>
</video:codec>
Transforming feature elements to feature predicates:
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(|
(& (video:encoding=PCMU) (video:channels=[1,2])
(video:sampling=[8000,16000]))
(& (video:resolution=QCIF) (video:frame-rate<=24)
(h263plus:A=foo) (h263plus:B=bar))
)
5.2.2 Applying RFC 2533 Canonicalization
After transforming different SDPng capability descriptions from
different participants into their equivalent RFC 2533 form, the
following steps MUST be performed to calculate the common subset of
capabilities:
1. The individual feature sets MUST be combined into a single
expression by creating a conjunction of the feature sets, i.e.,
the feature sets MUST be connected by the '&' (AND) operator.
2. The resulting expression MUST be reduced to disjunctive normal
form, i.e., the canonical from as specified by RFC 2533 [15].
5.2.3 Integrating Feature Sets into SDPng
A feature set that has been created by combining multiple independent
feature sets and by reducing the result for canonical form does not
indicate directly which of the capability elements belong to the
common subset of capabilities. SDPng uses the following approach:
After a "collapsing process" that has determined the commonly
supported capabilities, the resulting RFC 2533 expression is compared
to the original SDPng capability description. For this purpose, each
SDPng capability element is transformed to an RFC 2533 expression and
matched against the negotiation result (by constructing a conjunction
of the two feature sets). If the resulting canonical disjunctive form
is non-empty, the respective capability element represents a commonly
supported capability and can be adopted for the conference
configuration.
A future version of this document will specify how to adopt
individual values from the negotiation result for the SDPng
capability element.
The following steps MUST be performed to determine whether an
individual capability element (e.g., from one of the contributing
SDPng capability descriptions) belongs to the result feature set.
Let R be the result feature set obtained from the canonicalization as
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specified in Section 5.2.2.
1. For each capability element, generate the equivalent RFC 2533
feature set by applying the steps specified in Section 5.2.1. Let
C be the resulting feature set.
2. Combine R and C into a single feature set by building a
conjunction of the two feature sets (& R C). Let the result be
the feature set T.
3. Reduce T to disjunctive normal form by applying the
canonicalization as defined in RFC 2533 [15].
4. If the remaining disjunction is non-empty, the constraints
specified by capability element (the origin of C) can be
satisfied by R, i.e., C represents a commonly supported
capability.
5.2.4 Processing Negotiation Results
The capability negotiation results in an updated list of capability
elements of the SDPng "cap" element. The capability elements describe
the commonly supported capabilities. Capabilities that are not
supported by all end-systems have been removed.
Definition elements (inside the SDPng "def" element) and
configuration descriptions (inside the SDPng "alt" element) that
reference capability elements that have been removed after the
negotiation process, MUST be removed as well. Configuration
description (inside the SDPng "alt" element) that reference
non-existing definition elements (inside the SDPng "def" element")
MUST also be removed.
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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
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
>
<!ELEMENT alt ANY>
<!ATTLIST alt
name CDATA #REQUIRED
>
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
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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>
<!--
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>
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<!--
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">
<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>
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<!--
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 -->
<!-- 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="name" type="xsd:string" use="optional"/>
</xsd:complexType>
<!-- The base element for constraint element -->
<!-- FIXME: substitutionGroup? -->
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<xsd:element name="information" type="sdpng:Information" abstract="true">
</xsd:element>
<!-- ++++++++++++++++++++++++++++++++++++++++++++++++++ -->
<!--
SDPng document structure
-->
<xsd:element name="sdpng">
<xsd:complexType>
<xsd:sequence>
<xsd:element ref="sdpng:cap"/>
<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 mandatory "cap" element -->
<xsd:element name="cap">
<xsd:complexType mixed="false">
<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>
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</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"/>
</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"/>
</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 -->
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<xsd:element name="info">
<xsd:complexType mixed="false">
<xsd:sequence>
<xsd:element ref="sdpng:information" 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. Use of SDPng in Conjunction with other IETF Signaling
Protocols
This appendix is included temporarily and for informational purposes
only. Ultimately, it is up to each existing and evolving application
protocol to specify its use of SDPng.
The SDPng model provides the notion of Components to indicate the
intended types of collaboration between the users in e.g. a
teleconferencing scenario.
Three different abstractions are defined that are used for describing
the properties of a specific Component:
o a Capability refers to the fact that one of the involved parties
supports one particular way of exchanging media -- defined in
terms of transport, codec, and other parameters -- as part of the
media session.
o a Potential Configuration denotes a set of matching Capabilities
from all those involved parties that may be used for one
particular Component.
o an Actual Configuration indicates the Potential Configuration(s)
and its associated media session parameters which was/were chosen
by the involved parties to instantiate a certain Component.
As mentioned before, this abstract notion of the interactions between
a number of communicating systems needs to be mapped to the
application scenarios of SDPng in conjunction with the various IETF
signaling protocols, including (but not limited to) SAP, SIP, RTSP,
and MEGACO.
In general, this section provides recommendations and possible
scenarios for the use of SDPng within specific protocols and
applications. Is does not specify normative requirements.
D.1 The Session Announcement Protocol (SAP)
SAP is used to disseminate a previously created (and typically fixed)
session description to a potentially large audience. An interested
member of the audience will use the SDPng description contained in
SAP to join the announced media sessions.
This means that a SAP announcement contains the Actual Configurations
of all Components that are part of the overall teleconference or
broadcast.
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A SAP announcement may contain multiple Actual Configurations for the
same Component. In this case, the "same" (i.e. semantically
equivalent) media data from one configuration must be available from
each of the Actual Configurations.
Each receiver of a SAP announcement with SDPng compares its locally
stored Capabilities to realize a certain Component against the Actual
Configurations contained in the announcement. If the intersection
yields one or more Potential Configurations for the receiver, it
chooses the one it sees fit best. If the intersection is empty, the
receiver cannot participate in the announced session.
SAP may be substituted by HTTP (in the general case, at least), SMTP,
NNTP, or other IETF protocols suitable for conveying a media
description from one entity to one or more other without the intend
for further negotiation of the session parameters.
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.
D.2 Session Initiation Protocol (SIP)
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
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Actual Configurations at the syntax, this has no implications on the
SDPng signaling itself.
SIP relies on an "offer/answer" model for the exchange of capability
and configuration information. Either the caller or the callee sends
an initial session description that is processed by the other side
and returned. For capability negotiation, this means that the
negotiation follows a two-stage-process: The "offerer" sends its
capability description to the receiver. The receiver processes the
offerers capabilities and his own capabilities and generates a result
capability description that is sent back to the offerer. Both sides
now know the commonly supported configurations and can initiate the
media sessions.
Because of this strict "offer/answer" model, the offerer must already
send complete configurations (i.e. include transport addresses) along
with the capability descriptions. The answer must also contain
complete configuration parameters. The following figure shows, how
SDPng content can be used in an INVITE request with a correspong 200
OK message.
Simple description document with only one alternative:
F1 INVITE A -> B
INVITE sip:B@example.com SIP/2.0
Via: SIP/2.0/UDP hostA.example.com:5060
From: A <sip:A@example.com>
To: B <sip:B@example.com>
Call-ID: 1234@hostA.example.com
CSeq: 1 INVITE
Contact: <sip:UserA@192.168.1.1>
Content-Type: application/sdpng
Content-Length: 685
<?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"
>
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<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>
</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>
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<audio:sampling>8000</audio:sampling>
</audio:codec>
</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 all already contained in the respective
descriptions. 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
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it adds to the session description.
D.3 Real-Time Streaming Protocol (RTSP)
In contrast to SIP, 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). However,
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.
C->M: DESCRIBE rtsp://foo/audio-play RTSP/1.0
CSeq: 1
M->C: RTSP/1.0 200 OK
CSeq: 1
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">
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<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>
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.
D.4 Media Gateway Control Protocol (MEGACOP)
The MEGACO architecture also follows the SDPng model of a clear
separation between Potential and Actual Configurations. Upon startup,
a Media Gateway (MG) will "register" with its Media Gateway
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Controller (MGC) and the latter will audit the MG for its
Capabilities. Those will be provided as Potential Configurations,
possibly with extensive Constraints specifications. Whenever a media
path needs to be set up by the MGC between two MGs or an MG needs to
be reconfigured internally, the MGC will use (updated) Actual
Configurations.
Details and examples to be defined in a separate document.
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Appendix E. Change History
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
* 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".
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* 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 5 ("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.
* 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.
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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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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgment
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Kutscher, et al. Expires April 26, 2004 [Page 65]
