Network Working Group Kutscher
Internet-Draft Ott
Expires: October 18, 2001 Bormann
TZI, Universitaet Bremen
April 19, 2001
Session Description and Capability Negotiation
draft-ietf-mmusic-sdpng-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2001). 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 confctrl@isi.edu and/or the authors.
Document Revision
$Revision: 1.8 $
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology and System Model . . . . . . . . . . . . . . . . 5
3. SDPng . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1 Conceptual Outline . . . . . . . . . . . . . . . . . . . . . 8
3.1.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1.2 Components & Configurations . . . . . . . . . . . . . . . . 10
3.1.3 Constraints . . . . . . . . . . . . . . . . . . . . . . . . 11
3.1.4 Session . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.2 Syntax Proposal . . . . . . . . . . . . . . . . . . . . . . 12
3.3 External Definition Packages . . . . . . . . . . . . . . . . 14
3.3.1 Profile Definitions . . . . . . . . . . . . . . . . . . . . 15
3.3.2 Library Definitions . . . . . . . . . . . . . . . . . . . . 15
3.4 Mappings . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . 18
References . . . . . . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 19
Full Copyright Statement . . . . . . . . . . . . . . . . . . 21
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1. Introduction
Multiparty multimedia conferencing is one application that requires
the dynamic interchange of end system capabilities and the
negotiation of a parameter set that is appropriate for all sending
and receiving end systems in a conference. For some applications,
e.g. for loosely coupled conferences, it may be sufficient to simply
have session parameters be fixed by the initiator of a conference.
In such a scenario no negotiation is required because only those
participants with media tools that support the predefined settings
can join a media session and/or a conference.
This approach is applicable for conferences that are announced some
time ahead of the actual start date of the conference. Potential
participants can check the availability of media tools in advance
and tools like session directories can configure media tools on
startup. This procedure however fails to work for conferences
initiated spontaneously like Internet phone calls or ad-hoc
multiparty conferences. Fixed settings for parameters like media
types, their encoding etc. can easily inhibit the initiation of
conferences, for example in situations where a caller insists on a
fixed audio encoding that is not available at the callee's end
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 [1] allows to specify multimedia sessions (i.e. conferences,
"session" as used here is not to be confused with "RTP session"!)
by providing general information about the session as a whole and
specifications for all the media streams (RTP sessions and others)
to be used to exchange information within the multimedia session.
Currently, media descriptions in SDP are used for two purposes:
o to describe session parameters for announcements and invitations
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(the original purpose of SDP)
o to describe the capabilities of a system (and possibly provide a
choice between a number of alternatives). Note that SDP was not
designed to facilitate this.
A distinction between these two "sets of semantics" is only made
implicitly.
In the following we first introduce a model for session description
and capability negotiation and define some terms that are later used
to express some requirements. Note that this list of requirements is
possibly incomplete. The purpose of this document is to initiate the
development of a session description and capability negotiation
framework.
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2. Terminology and System Model
Any (computer) system has, at a time, a number of rather fixed
hardware as well as software resources. These resources ultimately
define the limitations on what can be captured, displayed, rendered,
replayed, etc. with this particular device. We term features enabled
and restricted by these resources "system capabilities".
Example: System capabilities may include: a limitation of the
screen resolution for true color by the graphics board; available
audio hardware or software may offer only certain media encodings
(e.g. G.711 and G.723.1 but not GSM); and CPU processing power
and quality of implementation may constrain the possible video
encoding algorithms.
In multiparty multimedia conferences, participants employ different
"components" in conducting the conference.
Example: In lecture multicast conferences one component might be
the voice transmission for the lecturer, another the transmission
of video pictures showing the lecturer and the third the
transmission of presentation material.
Depending on system capabilities, user preferences and other
technical and political constraints, different configurations can be
chosen to accomplish the "deployment" of these components.
Each component can be characterized at least by (a) its intended use
(i.e. the function it shall provide) and (b) a one or more possible
ways to realize this function. Each way of realizing a particular
function is referred to as a "configuration".
Example: A conference component's intended use may be to make
transparencies of a presentation visible to the audience on the
Mbone. This can be achieved either by a video camera capturing
the image and transmitting a video stream via some video tool or
by loading a copy of the slides into a distributed electronic
whiteboard. For each of these cases, additional parameters may
exist, variations of which lead to additional configurations (see
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
could H.261 encoded QCIF images. All three cases constitute
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different configurations. Of course there are many more detailed
protocol parameters.
Each component's configurations are limited by the participating
system's capabilities. In addition, the intended use of a component
may constrain the possible configurations further to a subset
suitable for the particular component's purpose.
Example: In a system for highly interactive audio communication
the component responsible for audio may decide not to use the
available G.723.1 audio codec to avoid the additional latency but
only use G.711. This would be reflected in this component only
showing configurations based upon G.711. Still, multiple
configurations are possible, e.g. depending on the use of A-law
or u-Law, packetization and redundancy parameters, etc.
In this system model, we distinguish two types of configurations:
o potential configurations
(a set of any number of configurations per component) indicating
a system's functional capabilities as constrained by the intended
use of the various components;
o actual configurations
(exactly one per instance of a component) reflecting the mode of
operation of this component's particular instantiation.
Example: The potential configuration of the aforementioned video
component may indicate support for JPEG, H.261/CIF, and
H.261/QCIF. A particular instantiation for a video conference may
use the actual configuration of H.261/CIF for exchanging video
streams.
In summary, the key terms of this model are:
o A multimedia session (streaming or conference) consists of one or
more conference components for multimedia "interaction".
o A component describes a particular type of interaction (e.g.
audio conversation, slide presentation) that can be realized by
means of different applications (possibly using different
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.
* Potential configurations describe possible configurations that
are supported by an end system.
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* An actual configuration is an "instantiation" of one of the
potential configurations, i.e. a decision how to realize a
certain component.
In less abstract words, potential configurations describe what a
system can do ("capabilities") and actual configurations describe
how a system is configured to operate at a certain point in time
(media stream spec).
To decide on a certain actual configuration, a negotiation process
needs to take place between the involved peers:
1. to determine which potential configuration(s) they have in
common, and
2. to select one of this shared set of common potential
configurations to be used for information exchange (e.g. based
upon preferences, external constraints, etc.).
In SAP [9] -based session announcements on the Mbone, for which SDP
was originally developed, the negotiation procedure is non-existent.
Instead, the announcement contains the media stream description sent
out (i.e. the actual configurations) which implicitly describe what
a receiver must understand to participate.
In point-to-point scenarios, the negotiation procedure is typically
carried out implicitly: each party informs the other about what it
can receive and the respective sender chooses from this set a
configuration that it can transmit.
Capability negotiation must not only work for 2-party conferences
but is also required for multi-party conferences. Especially for the
latter case it is required that the process of determining the
subset of allowable potential configurations is deterministic to
reduce the number of required round trips before a session can be
established.
In the following, we elaborate on requirements for an SDPng
specification, subdivided into general requirements and requirements
for session descriptions, potential and actual configurations as
well as negotiation rules.
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3. SDPng
This section outlines a proposed solution for describing
capabilities that meets most of the above requirements. Note that at
this early point in time not all of the details are completely
filled in; rather, the focus is on the concepts of such a capability
description and negotiation language.
3.1 Conceptual Outline
Our concept for the description language follows the system model
introduced in the beginning of this document. We use a rather
abstract language to avoid misinterpretations due to different
intuitive understanding of terms as far as possible.
Our concept of a capability description language addresses various
pieces of a full description of system and application capabilities
in four separate "sections":
Definitions (elementary and compound)
Potential or Actual Configurations
Constraints
Session attributes
3.1.1 Definitions
The definition section specifies a number of basic abstractions that
are later referenced to avoid repetitions in more complex
specifications and allow for a concise representation. Definition
elements are labelled with an identifier by which they may be
referenced. They may be elementary or compound (i.e. combinations of
elementary entities). Examples of definitions of that sections
include (but are not limited to) codec definitions, redundancy
schemes, transport mechanisms and payload formats.
Elementary definition elements do not reference other elements. Each
elementary entity only consists of one of more attributes and their
values. Default values specified in the definition section may be
overridden in descriptions for potential (and later actual)
configurations. The concrete mechanisms for overriding definitions
are still to be defined.
For the moment, elementary elements are defined for media types
(i.e. codecs) and for media transports. For each transport and for
each codec to be used, the respective attributes need to be defined.
This definition may either be provided within the "Definition"
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section itself or in an external document (similar to the
audio-video profile or an IANA registry that define payload types
and media stream identifiers.
Examples for elementary definitions:
<audio-codec name="audio-basic" encoding="PCMU sampling_rate="8000 channels="1"/>
<audio-codec name="audio-L16-mono" encoding="L16 sampling_rate="44100 channels="1"/>
The element type "audio-codec" is used in these examples to define
audio codec configurations. The configuration parameters are given
as attribute values.
Compound elements combine a number of elementary and/or other
compound elements for more complex descriptions. This mechanism can
be used for simple standard configurations such as G.711 over
RTP/AVP as well as to express more complex coding schemes including
e.g. FEC schemes, redundancy coding, and layered coding. Again, such
definitions may be standardized and externalized so that there is no
need to repeat them in every specification.
An example for the definition of a audio-redundancy format:
<audio-red name="red-pcm-gsm-fec">
<use ref="audio-basic"/> <use ref="audio-gsm"/> <use ref="parityfec"/>
</audio-red>
In this example, the element type "audio-red" is used to define a
redundant audio configuration that is labelled "red-pcm-gsm-fec" for
later referencing. In the definition itself, the element type "use"
is used to reference other definitions.
Definitions may have default values specified along with them for
each attribute. Some of these default values may be overridden so
that a codec definition can easily be re-used in a different context
(e.g. by specifying a different sampling rate) without the need for
a large number of base specifications.
This approach allows to have simple as well as more complex
definitions which are commonly used be available in an extensible
set of reference documents. Section 3.3 specifies the mechanisms for
external references.
Note: For negotiation between endpoints, it may be helpful to define
two modes of operation: explicit and implicit. Implicit
specifications may refer to externally defined entities to minimize
traffic volume, explicit specifications would list all external
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definitions used in a description in the "Definitions" section.
Again, please see Section 3.3 for complete discussion of external
definitions.
3.1.2 Components & Configurations
The "Configurations" section contains all the components that
constitute the multimedia conference (IP telephone call, multiplayer
gaming session etc.). For each of these components, the potential
and, later, the actual configurations are given. Potential
configurations are used during capability exchange and/or
negotiation, actual configurations to configure media streams after
negotiation or in session announcements (e.g. via SAP). A potential
and the actual configuration of a component may be identical.
Each component is labelled with an identifier so that it can be
referenced, e.g. to associate semantics with a particular media
stream. For such a component, any number of configurations may be
given with each configuration describing an alternate way to realize
the functionality of the respective component.
Each configuration (potential as well as actual) is labelled with an
identifier. A configuration combines one or more (elementary and/or
compound) entities from the "Definitions" section to describe a
potential or an actual configuration. Within the specification of
the configuration, default values from the referenced entities may
be overwritten.
<cfg>
<component name="audio1" media="audio">
<alt name= AVP-audio-0">
<rtp transport="udp-ip" format="audio-basic">
<addr type="mc">
<ipv4>239.239.239.239</ipv4> <port>30000</port>
</addr>
</rtp>
</alt>
<alt name="AVP-audio-11">
<rtp transport="udp-ip" format="audio-L16-mono">
<addr type="mc">
<ipv4>239.239.239.239</ipv4> <port>30000</port>
</addr>
</rtp>
</alt>
</component>
</cfg>
For example, an IP telephone call may require just a single
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component id=interactive-audio with two possible ways of
implementing it. The two corresponding configurations are
"AVP-audio-0" without modification, the other ("AVP-audio-11") uses
linear 16-bit encoding. Typically, transport address parameters such
as the port number would also be provided. In this example, this
information is given by the "addr" element.
During/after the negotiation phase, an actual configuration is
chosen out of a number of alternative potential configurations, the
actual configuration may refer to the potential configuration just
by its "id", possibly allowing for some parameter modifications.
Alternatively, the full actual configuration may be given.
3.1.3 Constraints
Definitions specify media, transport, and other capabilities,
whereas configurations indicate which combinations of these could be
used to provide the desired functionality in a certain setting.
There may, however, be further constraints within a system (such as
CPU cycles, DSP available, dedicated hardware, etc.) that limit
which of these configurations can be instantiated in parallel (and
how many instances of these may exist). We deliberately do not
couple this aspect of system resource limitations to the various
application semantics as the constraints exist across application
boundaries. Also, in many cases, expressing such constraints is
simply not necessary (as many uses of the current SDP show), so
additional overhead can be avoided where this is not needed.
Therefore, we introduce a "Constraints" section to contain these
additional limitations. Constraints refer to potential
configurations and to entity definitions and express and use simple
logic to express mutual exclusion, limit the number of
instantiations, and allow only certain combinations. The following
example shows the definition of a constraints that restricts the
maximum number of instantiation of two alternatives (that would have
to be defined in the configuration section before) when they are
used in parallel:
<constraints>
<par>
<use ref="AVP-audio-11" max="5"> <use ref="AVP-video-32" max="1">
</par>
</constraints>
As the example shows, contraints are defined by defining limits on
simultaneous instantiations of alternatives. They are not defined by
expressing abstract endsystem resources, such as CPU speed or memory
size.
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By default, the "Constraints" section is empty (or missing) which
means that no further restrictions apply.
3.1.4 Session
The "Session" section is used to describe general meta-information
parameters of the communication relationship to be invoked or
modified. It contains most (if not all) of the general parameters of
SDP (and thus will easily be usable with SAP for session
announcements).
In addition to the session description parameters, the "Session"
section also ties the various components to certain semantics. If,
in current SDP, two audio streams were specified (possibly even
using the same codecs), there was little way to differentiate
between their uses (e.g. live audio from an event broadcast vs. the
commentary from the TV studio).
This section also allows to tie together different media streams or
provide a more elaborate description of alternatives (e.g. subtitles
or not, which language, etc.).
<conf>
<subject>SDPng test</subject>
<originator>joe@example.com</originator>
<about>A test conference</about>
<info name="audio1" function="speaker">
Video stream for the different speakers
<info>
</conf>
Further uses are envisaged but need to be defined in future versions
of this document.
3.2 Syntax Proposal
In order to allow for the possibility to validate session
descriptions and in order to allow for structured extensibility it
is proposed to rely on a syntax framework that provides concepts as
well as concrete procedures for document validation and extending
the set of allows syntax elements.
SGML/XML technologies allow for the preparation of Document Type
Definitions (DTDs) that can define the allowed content models for
the elements of conforming documents. Documents can be formally
validated against a given DTD to check their conformance and
correctness. For XML, mechanisms have been defined that allow for
structured extensibility of a model of allowed syntax: XML Namespace
and XML Schema.
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XML Schema mechanisms allows to constrain the allowed document
content, e.g. for documents that contain structured data and also
provide the possibility that document instances can conform to
several XML Schema definitions at the same time, while allowing
Schema validators to check the conformance of these documents.
Extensions of the session description language, say for allowing to
express the parameters of a new media type, would require the
creation of a corresponding XML schema definition that contains the
specification of element types that can be used to describe
configurations of components for the new media type. Session
description documents have to reference the non-standard Schema
module, thus enabling parsers and validators to identify the
elements of the new extension module and to either ignore them (if
they are not supported) or to consider them for processing the
session/capability description.
It is important to note that the functionality of validating
capability and session description documents is not necessarily
required to generate or process them. For example, endpoints would
be configured to understand only those parts of description
documents that are conforming to the baseline specification and
simply ignore extensions they cannot support. The usage of XML and
XML Schema is thus rather motivated by the need to allow for
extensions being defined and added to the language in a structured
way that does not preclude the possibility to have applications to
identify and process the extensions elements they might support. The
baseline specification of XML Schema definitions and profiles must
be well-defined and targeted to the set of parameters that are
relevant for the protocols and algorithms of the Internet Multimedia
Conferencing Architecture, i.e. transport over RTP/UDP/IP, the audio
video profile of RFC1890 etc.
The example below shows how the definition of codecs,
transport-variants and configuration of components could be
realized. Please note that this is not a complete example and that
identifiers have been chosen arbitrarily.
<def>
<audio-codec name="audio-basic" encoding="PCMU sampling_rate="8000 channels="1"/>
<audio-codec name="audio-L16-mono" encoding="L16 sampling_rate="44100 channels="1"/>
<fec name="parityfec"/>
<audio-red name="red-pcm-gsm-fec">
<use ref="audio-basic"/> <use ref="audio-gsm"/> <use ref="parityfec"/>
</audio-red>
</def>
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<cfg>
<component name="audio1" media="audio">
<alt name= AVP-audio-0">
<rtp transport="udp-ip" format="audio-basic">
<addr type="mc">
<ipv4>239.239.239.239</ipv4> <port>30000</port>
</addr>
</rtp>
</alt>
<alt name="AVP-audio-11">
<rtp transport="udp-ip" format="audio-L16-mono">
<addr type="mc">
<ipv4>239.239.239.239</ipv4> <port>30000</port>
</addr>
</rtp>
</alt>
</component>
</cfg>
<constraints>
<par>
<use ref="AVP-audio-11" max="5"> <use ref="AVP-video-32" max="1">
</par>
</constraints>
<conf>
<subject>SDPng test</subject>
<originator>joe@example.com</originator>
<about>A test conference</about>
<info name="audio1" function="speaker">
Video stream for the different speakers
<info>
</conf>
The example does also not include specifications of XML Schema
definitions or references to such definitions. This will be provided
in a future version of this draft.
A real-world capability description would likely be shorter than the
presented example because the codec and transport definitions can be
factored-out to profile definition documents that would only be
referenced in capability description documents.
3.3 External Definition Packages
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3.3.1 Profile Definitions
In order to allow for extensibility it must be possible to define
extensions to the basic SDPng configuration options.
For example if some application requires the use of a new esoteric
transport protocol endpoints must be able describe their
configuration with respect to the parameters of that transport
protocol. The mandatory and optional parameters that can be
configured and negotiated when using the transport protocol will be
specified in a definition document. Such a definition document is
called a "profile".
A profile contains rules that specify how SDPng is used to describe
conferences or endsystem capabilities with respect to the parameters
of the profile. The concrete properties of the profile definitions
mechanism are still to be defined.
An example of such a profile would be the RTP profile that defines
how to specify RTP parameters. Another example would be the audio
codec profiles that defines how specify audio codec parameters.
SDPng document can reference profiles and provide concrete
definitions, for example the definition for the GSM audio codec.
(This would be done in the "Definitions" section of a SDPng
document.) A SDPng document that references a profile and provides
concrete defintions of configurations can be validated against the
profile definition.
3.3.2 Library Definitions
While profile definitions specify the allowed parameters for a given
profile SDPng definition sections refer to profile definitions and
define concrete configurations based on a specific profile.
In order to such definitions to be imported into SDPng documents,
there will be the notion of "SDPng libraries". A library is a set of
definitions that is conforming to a certain profile definition (or
to more than one profile definition -- this needs to be defined).
The purpose of the library concept is to allow certain common
definitions to be factored-out so that not every SDPng document has
to include the basic definitions, for example the PCMU codec
definition. SDP [1] uses a similar concept by relying on the well
known static payload types (defined in RFC1890 [3]) that are also
just referenced but never defined in SDP documents.
An SPDng document that references definitions from an external
library has to declare the use of the external library. The external
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library, being a set of configuration definitions for a given
profile, again needs to declare the use of the profile that it is
conformant to.
There are different possibilities of how profiles definitions and
libraries can be used in SDPng documents:
o In an SPDng document a profile definition can be referenced and
all the configuration definitions are provided within the
document itself. The SDPng document is self-contained with
respect to the definitions it uses.
o In an SPDng document the use of an external library can be
declared. The library references a profile definition and the
SDPng document references the library. There are two alternatives
how external libraries can be referenced:
by name: Referencing libraries by names implies the use of a
registration authority where definitions and reference names
can be registered with. It is conceivable that the most common
SDPng definitions be registered that way and that there will
be a baseline set of definitions that minimal implementations
must understand. Secondly, a registration procedure will be
defined, that allows vendors to register frequently used
definitions with a registration authority (e.g., IANA) and to
declare the use of registered definition packages in
conforming SDPng documents. Of course, care should be taken
though not to make the external references too complex and
thus require too much a priori knowledge in a protocol engine
implementing SDPng. Relying on this mechanism in general is
also problematic because it impedes the extensiblity, because
it requires implementors to provide support for new extensions
in their products before they can interoperate. Registration
is not useful for spontaneous or experimental extensions that
are defined in an SDPng library.
by address: An alternative to referencing libraries by name is to
declare the use of an external library by providing an
address, i.e., an URL, that specifies where the library can be
obtained. While is allows the use of arbitrary third-party
libraries that can extend the basic SDPng set of configuration
options in many ways there are problems if the referenced
libraries cannot be accessed by all communication partners.
o Because of these problematic properties of external libraries,
the final SDPng specification will have to provide a set of
recommendations under which circumstances the different
mechanisms of externalizing definitions should be used.
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3.4 Mappings
A mapping needs to be defined in particular to SDP that allows to
translate final session descriptions (i.e. the result of capability
negotiation processes) to SDP documents. In principle, this can be
done in a rather schematic fashion.
Furthermore, to accommodate SIP-H.323 gateways, a mapping from SDPng
to H.245 needs to be specified at some point.
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4. Open Issues
Overriding
Sytnax for referencing profiles and libraries
Registry (reuse of SDP mechanisms and names etc.)
Negotiation
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References
[1] Handley, M. and V. Jacobsen, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[2] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobsen,
"RTP: A Transport Protocol for Real-Time Applications", RFC
1889, January 1996.
[3] Schulzrinne, H., "RTP Profile for Audio and Video Conferences
with Minimal Control", RFC 1890, January 1996.
[4] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., Handley,
M., Bolot, J., Vega-Garcia, A. and S. Fosse-Parisis, "RTP
Payload for Redundant Audio Data", RFC 2198, September 1997.
[5] Klyne, G., "A Syntax for Describing Media Feature Sets", RFC
2533, March 1999.
[6] Klyne, G., "Protocol-independent Content Negotiation
Framework", RFC 2703, September 1999.
[7] Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format for
Generic Forward Error Correction", RFC 2733, December 1999.
[8] Perkins, C. and O. Hodson, "Options for Repair of Streaming
Media", RFC 2354, June 1998.
[9] Handley, M., Perkins, C. and E. Whelan, "Session Announcement
Protocol", RFC 2974, October 2000.
Authors' Addresses
Dirk Kutscher
TZI, Universitaet Bremen
Bibliothekstr. 1
Bremen 28359
Germany
Phone: +49.421.218-7595
Fax: +49.421.218-7000
EMail: dku@tzi.uni-bremen.de
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Joerg Ott
TZI, Universitaet Bremen
Bibliothekstr. 1
Bremen 28359
Germany
Phone: +49.421.201-7028
Fax: +49.421.218-7000
EMail: jo@tzi.uni-bremen.de
Carsten Bormann
TZI, Universitaet Bremen
Bibliothekstr. 1
Bremen 28359
Germany
Phone: +49.421.218-7024
Fax: +49.421.218-7000
EMail: cabo@tzi.org
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