Network Working Group Kutscher
Internet-Draft Ott
Expires: September 1, 2003 Bormann
TZI, Universitaet Bremen
March 03, 2003
Session Description and Capability Negotiation
draft-ietf-mmusic-sdpng-06.txt
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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: 6.9 $
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology and System Model . . . . . . . . . . . . . . . . 5
3. Capability Negotiation: Overview and Requirements . . . . . 9
3.1 Outline of the Negotiation Process . . . . . . . . . . . . . 9
3.2 The Negotiation Process . . . . . . . . . . . . . . . . . . 11
4. SDPng . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.1 Conceptual Outline . . . . . . . . . . . . . . . . . . . . . 12
4.1.1 Potential Configurations . . . . . . . . . . . . . . . . . . 13
4.1.2 Actual Configurations . . . . . . . . . . . . . . . . . . . 15
4.1.3 Constraints . . . . . . . . . . . . . . . . . . . . . . . . 18
4.1.4 Meta Information . . . . . . . . . . . . . . . . . . . . . . 18
4.2 Syntax Definition Mechanisms . . . . . . . . . . . . . . . . 18
4.3 Referencing Definitions . . . . . . . . . . . . . . . . . . 20
4.4 External Definition Packages . . . . . . . . . . . . . . . . 20
4.4.1 Package Definitions . . . . . . . . . . . . . . . . . . . . 20
4.4.2 Library Definitions . . . . . . . . . . . . . . . . . . . . 21
4.5 Mappings . . . . . . . . . . . . . . . . . . . . . . . . . . 22
5. Syntax Definition . . . . . . . . . . . . . . . . . . . . . 23
5.1 Potential Configurations . . . . . . . . . . . . . . . . . . 23
6. Specification of the Capability Negotiation . . . . . . . . 26
6.1 Offer/Answer . . . . . . . . . . . . . . . . . . . . . . . . 26
6.2 RFC2533 Negotiation . . . . . . . . . . . . . . . . . . . . 27
6.2.1 Translating SDPng to RFC 2533 Expressions . . . . . . . . . 27
6.2.2 Applying RFC 2533 Canonicalization . . . . . . . . . . . . . 29
6.2.3 Integrating Feature Sets into SDPng . . . . . . . . . . . . 30
7. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . 31
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32
References . . . . . . . . . . . . . . . . . . . . . . . . . 33
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 34
A. Use of SDPng in conjunction with other IETF Signaling
Protocols . . . . . . . . . . . . . . . . . . . . . . . . . 36
A.1 The Session Announcement Protocol (SAP) . . . . . . . . . . 36
A.2 Session Initiation Protocol (SIP) . . . . . . . . . . . . . 37
A.3 Real-Time Streaming Protocol (RTSP) . . . . . . . . . . . . 44
A.4 Media Gateway Control Protocol (MEGACOP) . . . . . . . . . . 44
B. Change History . . . . . . . . . . . . . . . . . . . . . . . 45
Full Copyright Statement . . . . . . . . . . . . . . . . . . 47
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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 B 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 [9] session announcements on the Mbone, for which SDP
was originally developed, the negotiation procedure is non-existent.
Instead, the announcement contains the media stream description sent
out (i.e. the actual configurations) which implicitly describe what
a receiver must understand to participate.
In point-to-point scenarios, the negotiation procedure is typically
carried out implicitly: each party informs the other about what it
can receive and the respective sender chooses from this set a
configuration that it can transmit.
Capability negotiation must not only work for 2-party conferences but
is also required for multi-party conferences. Especially for the
latter case it is required that the process to determine the subset
of allowable potential configurations is deterministic to reduce the
number of required round trips before a session can be established.
For instance, in order to be used with SIP, the capability
negotiation is required to work with the offer/answer model that is
for session initiation with SIP -- limiting the negotiation to
exactly one round trip.
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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).
Library
A library is a application specific collection of potential
configuration definition. For example, the RTP-AVP library would
include definitions for the audio and video codecs of the RTP
audio/video profile (AVP).
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. Capability Negotiation: Overview and Requirements
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 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 [16] it must be possible to determine an actual
configuration in a single request/response cycle.
3.1 Outline of the Negotiation Process
Conceptually, the negotiation process comprises the following
individual steps (considering two parties, A and B, where A tries to
invite B to a conference). Please note that 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
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(sdpng:c elements) in the description. For each component that B
wishes to support it generates a list of potential configurations
corresponding to B's capabilities, denoted "CAP(B)".
3. B applies the collapsing function and obtains a list of potential
configurations that both A and B can support, denoted
"CAP(A)xCAP(B) = CAP(AB)".
4. B sends CAP(B) to A.
5. A also applies the collapsing function and obtains "CAP(AB)". At
this step, both A and B know 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 (one sdpng:alt element) or
whether it is allowed to list multiple actual configurations. In
this model we assume that A selects the actual configuration,
denoted CFG(AB).
7. A augments CFG(AB) with the transport parameters it intends to
use, e.g., on which endpoint addresses A wishes to receive data,
obtaining CFG_T(A). A sends CFG_T(A) to A.
8. B receives CFG_T(A) and adds its own transport parameters,
resulting in CFG_T(AB). CFG_T(AB) contains the selected actual
configurations and the transport parameters of both A and B (plus
any other SDPng data, e.g., meta-information on the conference).
CFG_T(AB) is the complete conference description. Both A and B
now have the following information:
CAP(A) A's supported potential configurations
CAP(B) B's supported potential configurations
CAP(AB) The set of potential configurations supported by both A
and B.
CFG(AB) The set of actual configurations to be used.
CFG_T(AB) The set of actual configurations to be used augmented
with all required parameters.
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In this model, the capability negotiation and configuration exchange
process leads to a description that represents a global view of the
configuration that should be used. This means, it contains the
complete configuration for all participants including per-participant
information like transport parameters.
Note that the model presented here results in four SDPng 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.
Specific procedures for re-negotiation and multi-party negotiation
will be defined in a future version of this document.
3.2 The Negotiation Process
The algorithm for comparing two potential configurations and for
obtaining a commonly supported subset is application specific. For
some limited application scenarios, a application specific
offer/answer process may be employed such as the SDP offer/answer
model [16].
More advanced implementations require a generic capability
negotiation mechanism that allows for application-independent
negotiation of potential configuration with parameters from different
application domains. Capability negotiation frameworks such as RFC
2533 [18] can be employed for this purpose. In a future version of
this document, we will discuss of employing a RFC 2533 based
negotiation process for comparing and matching capability
descriptions in SDPng documents.
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4. SDPng
This section introduces the underlying concepts of the Session
Description Protocol - next generation (SDPng). The focus of this
section is on the concepts of the capability description language
with a stepwise introduction of the various syntactical elements.
Note that this section only provides examples accompanied by
explanations. The description elements used in this section are not
normative.
4.1 Conceptual Outline
In Section 2 we have distinguished between potential configurations
("capabilities") and actual configurations ("session descriptions").
SDPng provides the possibility to express potential configurations
and actual configurations in one document. A potential configuration
list is used to declare capabilities and an actual configuration list
is used to declare concrete configurations.
Potential configurations are described independently of actual
configurations. In a "potential configurations" section, a user
agent lists its capabilities as a list of named definitions. For
negotiating capabilities from different user agents, the individual
definitions are matched in order to determine a commonly supported
subset of capabilities. The data schema for potential configurations
is defined in "package definitions". An example for an element of a
potential configuration would be the definition of a supported audio
codec.
Actual configurations can refer to capabilities and specify concrete
parameters for application protocol sessions, including transport
parameters. Actual configurations cannot be negotiated.
When defining potential configurations, capabilities are never
expressed with respect to other potential configuration elements,
e.g., the definition of an audio codec capability does not limit the
capability of using other audio codec. Constraints like the
simultaneous usage of capabilities can be expressed separately from
the capabilities themselves.
In addition, information about the communication session itself, such
as scheduling, information on the semantics of application protocol
sessions and information on the user who has initiated a conference.
This information is expressed separately from the definition of
potential and actual configurations.
These different elements of session description are discussed in
detail in the following sections. There are four different elements:
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Potential Configurations; see Section 4.1.1.
Actual Configurations; see Section 4.1.2.
Constraints; see Section 4.1.3.
Session meta information; see Section 4.1.4.
4.1.1 Potential Configurations
A "Potential Configurations" section in an SDPng document lists
individual capabilities, e.g., codec capabilities. In a capability
negotiation process these potential configurations may be compared to
the potential configurations that are defined in an SDPng document
from another participant. The outline of such a negotiation process
is presented in Section 3.
Please note, that in the following examples, we use a straw-man
syntax in order to discuss the concepts. A final syntax will be
formally defined in a future version of this document.
These are two examples of elements in a potential configurations
section:
audio:codec
name=audio-basic
encoding="PCMU"
sampling="8000"
channels="[1]"
audio:codec
name="audio-L16-mono"
encoding="L16"
sampling="44100"
channels="[1,2,4]"
The following requirements can be stated for expressing potential
configurations:
o It must be possible to name potential configuration elements. In
the example above, this is achieved by the property "name". Names
MUST NOT be considered in a capability negotiation process.
o The potential configuration elements are referred to by this name
for specifying actual configurations. It MUST be ensured that
names that originate from different description documents reside
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in separate namespaces in order to avoid collisions.
o The properties of a given potential configuration element MUST
have a well-defined type. For example, codec type names are
expressed as strings, and for capability negotiation, two codec
names can be processed by applying a string comparison. A maximum
frame-rate would be expressed as a number that represents an upper
limit, and for capability negotiation, the minimum of two numbers
would be used as a commonly supported value for the frame-rate.
o User agents MUST be able to infer the type of a given property
without referring to an external schema definition, i.e., the type
must be specified either implicitly or explicitly. Note, that in
the example above, the type is not specified.
o In addition to the data type and its value a property can provide
other characteristics: Some properties that a package definition
defines for a certain application are mandatory, i.e., they must
be specified in configuration descriptions. In the example above,
this would apply to the encoding, the sampling-rate and the number
of channels. For this single potential configuration elements
these properties serve as constraints for a negotiation: The
capability description matches only those description from other
participants that provide the same encoding, the same sampling-
rate and either 1,2 or 4 channels. If a description did not
provide one of these properties, the negotiation would fail.
There are however properties that can represent optional
parameters, such as a codec parameter that can optionally be used.
If one participant specified such a property and another
participant did not, we would expect the resulting configuration
to not include that property, however, the negotiation itself
should be successful.
o Some capabilities such as codec capabilities may be associated
with additional constraints, e.g., the directionality of media
streams ('send-only', 'receive-only'). It will be defined in a
future version of this document whether the directionality is
specified as a capability (in a potential configuration) or
whether it is rather specified as an attribute of an actual
configuration.
With these requirements in mind, we add additional characteristics to
the properties in potential configuration descriptions (and change
the encoding for the second potential configuration element):
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audio:codec
name=audio-basic;type=name
encoding="PCMU";type=string
sampling="8000";type=maximum-limit
channels="[1]";type=set
featureX="200";type=maximum-limit;optional
audio:codec
name="audio-PCMU-44khz";type=name
encoding="PCMU";type=string
sampling="44100";type=maximum-limit
channels="[1,2,4]";type=set
featureY="200";type=string;optional
Note again, that these descriptions merely present examples in order
to present the data model that we use for potential configurations --
this is not the SDPng syntax. In these examples, we have added the
optional features 'featureX' and 'featureY'.
If we assume, that the two potential configurations are contributions
from different participants for a capability negotiation, a resulting
potential configuration, after a negotiation process as outlined in
Section 3, could look like this:
audio:codec
encoding="PCMU";type=string
sampling="8000";type=maximum-limit
channels="[1]";type=set
The name cannot be considered for a capability negotiation, the
optional properties 'featureX' and 'featureY' have only been provided
by one participant each and the other properties have been processed
by the negotiation algorithms (that will be specified in a future
version of this document in Section 3).
So far, we have only considered codec capabilities. Other
capabilities would include transport mechanisms, e.g., RTP/UDP/IPv4:
rtp-udp:transport
name="rtp-udp-ipv4";type=name
network="IP4;type=string
4.1.2 Actual Configurations
The "Actual Configurations" section lists all the components that
constitute the multimedia application (IP telephone call, real-time
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streaming application, multi-player gaming session etc.). For each
of these components, the actual configurations are given. Potential
configurations are used during capability exchange and/or
negotiation, actual configurations to configure media streams after
negotiation (e.g. with RTSP) or in session announcements (e.g. via
SAP).
Each component is labeled 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 actual configurations
may be given with each configuration describing an alternative way to
realize the functionality of the respective component.
The semantics of this are application dependent. For example, for
SIP applications using the SDP offer/answer model, providing multiple
alternatives for a component (a media type in SDP) means that the
offerer is prepared to receive at any of the specified addresses any
of the specified payload formats (for RTP applications). In this
example, the order of alternatives is used to specify a preference,
i.e., the first alternative is the most preferred one.
The following example provides two alternative configurations for a
component named "interactive-audio". Each alternative refers to the
RTP-transport capability named "rtp-udp-ipv4" and to an audio-codec
capability.
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component
name="interactive-audio"
alt
name="AVP-audio-0"
rtp-udp:transport
ref="rtp-udp-ipv4"
pt="96"
direction="send-receive"
addr="224.2.0.53"
rtp-port="7800"
rtcp-port="7801"
audio-codec
ref="audio-basic"
alt
name="AVP-audio-11"
rtp-udp:transport
ref="rtp-udp-ipv4"
pt="97"
direction="send-receive"
addr="224.2.0.53"
rtp-port="7800"
rtcp-port="7801"
audio-codec
ref="audio-L16-stereo"
For the RTP transport configuration, additional required parameters
are provided, such as the payload type number to be used (pt="97"),
the IP address and UDP port numbers for RTP and RTCP and the
directionality.
Note that in the example above, the actual configuration of the RTP
transport is identical for both alternatives -- with the exception of
the payload type number. In a final solution, this duplication
should be avoided by another level of indirection, i.e., by defining
these parameters once and referencing this definition where needed.
In order to determine the usable actual configurations after a
capability negotiation, a user agent has to traverse the references
in actual configurations to potential configurations and check
whether each capability is still supported after a negotiation
process. Only those alternatives that reference supported
capabilities can be considered for implementing the given component.
The semantics of specifying multiple alternatives for a component are
application specific -- for RTP configurations in SDP it means that
the endpoint is willing to receive any of the specified formats
without further out-of-band signaling and that the first
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configuration is preferred.
4.1.3 Constraints
Potential configurations are 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 resources available, dedicated hardware, etc.) that
limit which of these configurations can be instantiated in parallel
(and how many instances of these may exist). We deliberately do not
couple this aspect of system resource limitations to the various
application semantics as the constraints may exist across application
boundaries. Also, in many cases, expressing such constraints is
simply not necessary (as many uses of the current SDP show), so
additional overhead can be avoided where this is not needed.
The usage of constraints will be specified in a future version of
this document.
4.1.4 Meta Information
The fourth and final section of an SDPng description is used to
specify meta information such as session layer attributes. These
attributes largely include those defined by SDP [RFC2327] (which are
explicitly indicated in the following specification) to describe
originator, purpose, and timing of a multimedia session among other
characteristics. Furthermore, SDPng includes attributes indicating
the semantics of the various Components in a teleconference or other
session.
A session-level specification for connection information (SDP "c="
line), bandwidth information (SDP "b=" line), and encryption keys
(SDP "k=" lines) is deliberately not provided for in SDPng. The
relevant information can be specified directly in the Configuration
section for individual alternatives.
The section for meta information will provide for integrating and re-
using existing meta-information frameworks such as MPEG-7. Details
will be specified in a future version of this document.
4.2 Syntax Definition Mechanisms
In this section, we specify the syntax definition mechanisms for
SDPng.
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In order to allow for the possibility to validate session
descriptions and in order to allow for structured extensibility,
SDPng relies on a syntax framework that provides concepts as well as
concrete procedures for document validation and extending the set of
allowed syntax elements.
SGML/XML technologies allow for the creation of Document Type
Definitions (DTDs) that can define the allowed content models for the
elements of conforming documents. Documents can be formally
validated against a given DTD to check their conformance and
correctness. XML DTDs however, cannot easily be extended. It is not
possible to alter to content models of element types or to add new
element types by third-party definition packages without creating the
possibility of name collisions.
For SDPng, a mechanism is needed that allows the specification of a
base syntax -- for example basic elements for the high level
structure of description documents -- while allowing extensions, for
example elements and attributes for new transport mechanisms, new
media types etc. to be added on demand. Still, it has to be ensured
that extensions do not result in name collisions. Furthermore, it
must be possible for applications that process descriptions documents
to distinguish extensions from base definitions.
For XML, mechanisms have been defined that allow for structured
extensibility of document schemata: XML Namespace and XML Schema.
XML Schema mechanisms allow 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 extension 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
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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 packages must be well-
defined and targeted to the set of parameters that are relevant for
the protocols and algorithms of the Internet Multimedia Conferencing
Architecture, i.e. transport over RTP/UDP/IP, the audio video
profile of RFC1890 etc.
Section 4.4 describes package definitions and library definition.
4.3 Referencing Definitions
SDPng provides a referencing concept for definitions. For example,
in the specification of an actual configuration, we reference the
capabilities of the potential configurations section.
The concrete reference mechanism depends on the syntax in use and
will be specified in a future version of this document.
4.4 External Definition Packages
There are two types of external definitions:
Package Definitions (Section 4.4.1) define rules, i.e., a data
schema, for specifying parameters that are not covered by the base
SDPng specification.
Library Definitions (Section 4.4.2) contain definitions that can be
referenced in SDPng documents.
4.4.1 Package Definitions
In order to allow for extensibility it must be possible to define
extensions to the basic SDPng configuration options.
For example, if some application requires the use of a new transport
protocol, endpoints must be able to describe their configuration with
respect to the parameters of that transport protocol. The mandatory
and optional parameters that can be configured and negotiated when
using the transport protocol will be specified in a definition
document. Such a definition document is called a "package".
A package contains rules that specify how SDPng is used to describe
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conferences or end-system capabilities with respect to the parameters
of the package. The specific properties of the package definitions
mechanism are still to be defined.
An example of such a package would be the RTP package that defines
how to specify RTP parameters. Another example would be the audio
codec package that defines how specify audio codec parameters.
4.4.2 Library Definitions
While package definitions specify the allowed parameters for a given
profile, SDPng "Definitions" sections refer to package definitions
and define concrete configurations based on a specific package.
In order for such definitions to be imported into SDPng documents,
"SDPng libraries" may be defined and referenced in SDPng documents.
A library is a set of definitions that is conforming to one or more
package definitions.
The purpose of the library concept is to allow certain common
definitions to be factored-out so that not every SDPng document has
to include the basic definitions, for example the PCMU codec
definition. SDP [2] uses a similar concept by relying on the well
known static payload types (defined in RFC1890 [4]) that are also
just referenced but never defined in SDP documents.
An SPDng document that references definitions from an external
library has to declare the use of the external library. The external
library, being a set of configuration definitions for a given
package, again needs to declare the use of the package that it is
conforming to. A library itself can make reference to other external
libraries.
There are different possibilities of how package definitions and
libraries can be used in SDPng documents:
o In an SPDng document, a package 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 package 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
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can be registered with. It is conceivable that the most common
SDPng definitions be registered that way and that there will be
a baseline set of definitions that minimal implementations must
understand. Secondly, a registration procedure will be
defined, that allows vendors to register frequently used
definitions with a registration authority (e.g., IANA) and to
declare the use of registered definition packages in conforming
SDPng documents. Of course, care should be taken not to make
the external references too complex and thus require too much a
priori knowledge in a protocol engine implementing SDPng.
Relying on this mechanism in general is also problematic
because it impedes the extensibility, as it requires
implementors to provide support for new extensions in their
products before they can inter-operate. Registration is not
useful for spontaneous or experimental extensions that are
defined in an SDPng library.
by address: An alternative to referencing libraries by name is to
declare the use of an external library by providing an address,
i.e., an URL, that specifies where the library can be obtained.
While this allows the use of arbitrary third-party libraries
that can extend the basic SDPng set of configuration options in
many ways, in introduces additional complexity that could
result in in higher latency for the processing of a description
document with references to external libraries. In addition,
there are problems if the referenced libraries cannot be
accessed by all communication partners.
o Because of these problematic properties of external libraries, the
final SDPng specification will have to provide a set of
recommendations under which circumstances the different mechanisms
of referring to external definitions should be used.
4.5 Mappings
A mapping needs to be defined in particular to SDP that allows to
translate final session descriptions (i.e. the result of capability
negotiation processes) to SDP documents. In principle, this can be
done in a rather schematic fashion for the base specification and a
set of basic packages.
In addition, mappings to H.245 will be defined in order to support
applications like SIP-H.323 gateways.
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5. Syntax Definition
An SDPng description is an XML document with different element types
for the different sections. The SDPng base syntax specification
defines this overall document structure.
5.1 Potential Configurations
A section for potential configurations is an XML element that can
provide a list of child elements. Each child element represents an
individual capability as described in Section 4.1.1. Each property
is represented by an XML attribute. The element types are defined in
package definitions. XML Namespaces are used to disambiguate element
types and to allow for extensibility.
Each 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. If a prefix is specified, it MUST be
separated by a colon (':') from the name.
Each element represents a "feature set" (using the terminology of
[18]). Therefore, each attribute can provide a "range" of values --
not only a single value. For example, an attribute 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": An attribute can specify a set of
tokens or a numerical range.
Each property that is represented by an XML attribute has a well-
defined type that is specified in the package definition. The type
is encoded implicitly in the attribute value (similar to the syntax
in RFC 2533 [18]). The following types are distinguished:
Text strings, tokens
An attribute may provide a token (a symbolic name), e.g., for a
codec name.
An example for a corresponding attribute:
encoding="PCMU"
Token MUST be directly embedded into the attribute content, i.e.,
the token is the attribute value.
The complete formal syntax definition of tokens will be provided
in a later version of this document.
Token sets
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An attribute can specify a set of tokens (representing a list of
alternative values for a certain property).
An example for a corresponding attribute:
sampling="[8000,16000,44100]"
A token set MUST be specified as a list of tokens that are
separated by commas (',') and are enclosed by square brackets
('[',']').
The complete formal syntax definition of token sets will be
provided in a later version of this document.
Numbers and Numerical ranges
An attribute can specify a number or a range (minimum and maximum)
for numbers.
An example for an attribute specifying a number:
bitrate="(64)"
A number MUST be specified as a literal value in brackets ('(',
')').
An example for an attribute specifying a numerical range:
bitrate="(64,128)"
A numerical range MUST be specified as a pair of values. The
first value is the minimum value (included in the range) and the
seconds value is the maximum value (included in the range). The
values are separated by a comma (',') and are enclosed in ('(',
')'). One of the range limits MAY be omitted, i.e., either the
minimum or the maximum value, e.g., if the range has no upper or
lower limit.
An example for an attribute specifying a numerical range without
an upper limit:
bitrate="(64,)"
The complete formal syntax definition of numbers and numerical
ranges (and a definition of the exact number type) will be
provided in a later version of this document.
An example for an XML element describing an individual capability:
<audio:codec name="avp:pcmu" encoding="PCMU" channels="[1,2]" sampling="[8000,16000]"/>
Capability elements MAY also provide attribute from different XML
namespaces. For example, a video-codec capability MAY be described
with attributes declaring general video capabilities, and this
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element MAY provide a list of additional codec specific attributes,
as depicted in the following example:
<video:codec name="h263+-enhanced" resolution="QCIF" frame-rate="(,24)"
h263plus:A="foo" h263plus:B="bar" />
The definition of "optional properties" (properties that to do not
constitute a constraint but not optional enhancement -- see Section
4.1.1) will be provided in a future version of this document.
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6. Specification of the Capability Negotiation
The SDPng specification defines the syntax and the semantics of
capability declarations (potential configurations). The algorithms
that are used for processing descriptions and for comparing
capability descriptions from different participants are application
specific.
In this section we discuss two alternative algorithms for
implementations: A model that is base on the SDP offer/answer scheme
(Section 6.1 and a model that is based on the feature matching
algorithm that is specified in RFC 2533 [18] (Section 6.2).
6.1 Offer/Answer
The offer/answer model allows to 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 conceivable to realize these
components. The answerer indicates which components it supports
(e.g. disallow a video session and go with a 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 ("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 following
considerations apply to the SDPng documents:
o For each component to be used, all necessary parameters for at
least one actual configuration MUST be given, 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
identifiers.
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o Components that shall not be instantiated (i.e. that are refused
by the answerer) shall either be present but have all parameters
of the actual configuration removed (i.e. no transport addresses,
etc.) if they may be (re-)instantiated at a later stage. Or they
shall be removed entirely from the answer if the respective
component is not supported at all. In the latter case, the
corresponding configurations MUST be removed from both the
configuration section and the session section of the SDPng
document.
o For each component, the alternative potential configurations MUST
be listed in the order of preference. A certain preference
indicator ("q=" value) may be included in a future revision of
this document. The considerations of RFC 3264 to simply arriving
at symmetric codec use apply.
The rules for matching properties and determining answers based upon
the offers are similar to those specified in RFC 3264.
A future revision of this document will define the details for all
the attributes discussed in RFC 3264 that require special
considerations (e.g. the directionality attribute for media
streams).
6.2 RFC2533 Negotiation
SDPng potential configurations can be processed using the RFC 2533
algorithm as defined in [18]. This involves the following steps:
Translating SDPng potential configurations 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 actual configurations.
6.2.1 Translating SDPng to RFC 2533 Expressions
SDPng potential configurations 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 in an SDPng section for potential configurations is
represented as an XML element with a set of attributes. We first
describe how to translate a single capability element into a RFC 2533
feature set, and then consider the combination of multiple capability
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elements.
Basically, all attributes of an SDPng capability element and its
child elements MUST be transformed to a RFC 2533 expression, whereas
each attribute MUST be translated to a feature predicate. The
resulting feature predicate are combined using the '&' (AND)
operator. The name attributes MUST NOT be considered.
Each predicate MUST be encapsulated by brackets ('(', ')'). Each XML
attribute value is taken as a feature predicate value, i.e., the
quote are not considered. Each attribute 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 a RFC 2533 token.
Token set
A token set MUST be directly adopted as a RFC 2533 set.
Number
A single number in round brackets MUST be adopted as a RFC 2533
number. The brackets MUST be removed.
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 attribute bitrate="(64,128)" 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 attribute bitrate="(,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:
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<video:codec name="h263+-enhanced" resolution="QCIF" frame-rate="(,24)"
h263plus:A="foo" h263plus:B="bar" />
Transforming attributes to feature predicates:
(& (resolution=QCIF) (frame-rate<=24) (h263plus:A=foo) (h263plus:B=bar))
Note that in example above, the namespace name is not used for
feature tags, instead we use the namespace prefix (for abbreviation).
RFC 2533 uses the syntax rules of RFC 2506 for feature tags. An
additional requirement for transforming fully-qualified attribute
names (including namespace names) to feature tags will be specified
in a future version of this document.
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" encoding="PCMU" channels="[1,2]" sampling="[8000,16000]"/>
<video:codec name="h263+-enhanced" resolution="QCIF" frame-rate="(,24)"
h263plus:A="foo" h263plus:B="bar" />
Transforming attributes to feature predicates:
(|
(& (encoding=PCMU) (channels=[1,2]) (sampling=[8000,16000]))
(& (resolution=QCIF) (frame-rate<=24) (h263plus:A=foo) (h263plus:B=bar))
)
Additional requirements for processing "optional" parameters (see
Section 5) will be specified in a future version of this document.
6.2.2 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 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 [18].
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6.2.3 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 the common
subset of capabilities. 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
specified in Section 6.2.2.
1. For each capability element, generate the equivalent RFC 2533
feature set by applying the steps specified in Section 6.2.1.
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 [18].
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.
A future version of this document will specify requirements for
exchanging calculated capabilities and for selecting appropriate
actual configurations.
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7. Open Issues
Definition of baseline libraries
Libraries provide partially specified definitions, i.e. without
transport parameters. How can SDPng documents reference the
definitions and augment them with specific transport parameters?
Referencing extension packages: XML-Schema does not support the
declaration of multiple schemas via the schemaLocation attribute.
Conceivable solution: When extension packages are used, the SDPng
description is a "multi-part" object, that consists of an
integrating schema definition (that references all necessary
packages and the base definition) and the actual description
document that is a schema instance of the integrating schema.
Uniqueness of attribute values: When libraries are used they will
contain definition elements with "name" attributes for later
referencing. How to avoid name clashes for those identifiers?
When an SDPng document uses libraries from different sources they
could be incompatible because of name collisions. Possible
solution: Prefix such IDs with a namespace name (either explicitly
or implicitly by interpreting applications). The explicit
prefixes have the advantage that no special knowledge would be
required to resolve links at the cost of very long ID values.
A registry (reuse of SDP mechanisms and names etc.) needs to be
set up.
Implicit declaration of SDPng schema and default package
Should overwriting of child elements be allowed for referencing
existing definitions with the "ref" attribute?
We need a package definition language. XML-DTDs or XML-Schema is
not sufficient as we need ways to specify the type (string/symbol,
set, numerical range) and additional attributes (optional).
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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. Jacobsen,
"RTP: A Transport Protocol for Real-Time Applications", RFC
1889, January 1996.
[4] Schulzrinne, H., "RTP Profile for Audio and Video Conferences
with Minimal Control", RFC 1890, January 1996.
[5] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
Conferences with Minimal Control", draft-ietf-avt-profile-new-
10.txt (work in progress), March 2001.
[6] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., Handley,
M., Bolot, J., Vega-Garcia, A. and S. Fosse-Parisis, "RTP
Payload for Redundant Audio Data", RFC 2198, September 1997.
[7] Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format for
Generic Forward Error Correction", RFC 2733, December 1999.
[8] Perkins, C. and O. Hodson, "Options for Repair of Streaming
Media", RFC 2354, June 1998.
[9] Handley, M., Perkins, C. and E. Whelan, "Session Announcement
Protocol", RFC 2974, October 2000.
[10] World Wide Web Consortium (W3C), "Extensible Markup Language
(XML) 1.0 (Second Edition)", Status W3C Recommendation, Version
http://www.w3.org/TR/2000/REC-xml-20001006, October 2000.
[11] World Wide Web Consortium (W3C), "Namespaces in XML", Status
W3C Recommendation, Version http://www.w3.org/TR/1999/REC-xml-
names-19990114, January 1999.
[12] World Wide Web Consortium (W3C), "XML Inclusions (XInclude)
Version 1.0", Status W3C Candidate Recommendation, Version
http://www.w3.org/TR/2002/CR-xinclude-20020221, February 2002.
[13] World Wide Web Consortium (W3C), "XML Schema Part 1:
Structures", Version http://www.w3.org/TR/2001/REC-xmlschema-1-
20010502/, Status W3C Recommendation, May 2001.
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[14] World Wide Web Consortium (W3C), "XML Schema Part 2:
Datatypes", Version http://www.w3.org/TR/2001/REC-xmlschema-2-
20010502/, Status W3C Recommendation, May 2001.
[15] World Wide Web Consortium (W3C), "XML Linking Language (XLink)
Version 1.0", Version http://www.w3.org/TR/2001/REC-xlink-
20010627/, Status W3C Recommendation, June 2001.
[16] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
SDP", RFC 3264, June 2002.
[17] Hollenbeck, S., Rose, M. and L. Masinter, "Guidelines for The
Use of XML within IETF Protocols", draft-hollenbeck-ietf-xml-
guidelines-05.txt (work in progress), June 2002.
[18] Klyne, G., "A Syntax for Describing Media Feature Sets", RFC
2533, 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
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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. Use of SDPng in conjunction with other IETF Signaling
Protocols
The SDPng model provides the notion of Components to indicate the
intended types of collaboration between the users in e.g. a
teleconferencing scenario.
Three different abstractions are defined that are used for describing
the properties of a specific Component:
o a Capability refers to the fact that one of the involved parties
supports one particular way of exchanging media -- defined in
terms of transport, codec, and other parameters -- as part of the
media session.
o a Potential Configuration denotes a set of matching Capabilities
from all those involved parties required to successfully realize
one particular Component.
o an Actual Configuration indicates the Potential Configuration
which was chosen by the involved parties to realize a certain
Component at one particular point in time.
As mentioned before, this abstract notion of the interactions between
a number of communicating systems needs to be mapped to the
application scenarios of SDPng in conjunction with the various IETF
signaling protocols: SAP, SIP, RTSP, and MEGACO.
In general, this section provides recommendations and possible
scenarios for the use of SDPng within specific protocols and
applications. Is does not specify normative requirements.
A.1 The Session Announcement Protocol (SAP)
SAP is used to disseminate a previously created (and typically fixed)
session description to a potentially large audience. An interested
member of the audience will use the SDPng description contained in
SAP to join the announced media sessions.
This means that a SAP announcement contains the Actual Configurations
of all Components that are part of the overall teleconference or
broadcast.
A SAP announcement may contain multiple Actual Configurations for the
same Component. In this case, the "same" (i.e. semantically
equivalent) media data from one configuration must be available from
each of the Actual Configurations. In practice, this limits the use
of multiple Actual Configurations to single-source multicast or
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broadcast scenarios.
Each receiver of a SAP announcement with SDPng compares its locally
stored Capabilities to realize a certain Component against the Actual
Configurations contained in the announcement. If the intersection
yields one or more Potential Configurations for the receiver, it
chooses the one it sees fit best. If the intersection is empty, the
receiver cannot participate in the announced session.
SAP may be substituted by HTTP (in the general case, at least), SMTP,
NNTP, or other IETF protocols suitable for conveying a media
description from one entity to one or more other without the intend
for further negotiation of the session parameters.
Example from the SAP spec. to be provided.
A.2 Session Initiation Protocol (SIP)
SIP is used to establish and modify multimedia sessions, and SDPng
may be carried at least in SIP INVITE and ACK messages as well as in
a number of responses. From dealing with legacy SDP (and its
essential non-suitability for capability negotiation), a particular
use and interpretation of SDP has been defined for SIP.
One of the important flexibilities introduced by SIP's usage of SDP
is that a sender can change dynamically between all codecs that a
receiver has indicated support (and has provided an address) for.
Codec changes are not signaled out-of-band but only indicated by the
payload type within the media stream. From this arises one important
consequence to the conceptual view of a Component within SDPng.
There is no clear distinction between Potential and Actual
Configurations. There need not be a single Actual Configuration be
chosen at setup time within the SIP signaling. Instead, a number of
Potential Configurations is signaled in SIP (with all transport
parameters required for carrying media streams) and the Actual
Configuration is only identified by the payload type which is
actually being transmitted at any point in time.
Note that since SDPng does not explicitly distinguish between
Potential and Actual Configurations, this has no implications on the
SDPng signaling itself.
SIP relies on an "offer/answer" model for the exchange of capability
and configuration information. Either the caller or the callee sends
an initial session description that is processed by the other side
and returned. For capability negotiation, this means that the
negotiation follows a two-stage-process: The "offerer" sends its
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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
<def>
<audio:codec name="audio-basic" encoding="PCMU"
sampling="8000" channels="1"/>
<rtp:pt name="rtp-avp-0" pt="0">
<audio:codec ref="audio-basic"/>
</rtp:pt>
</def>
<cfg>
<component name="interactive-audio" media="audio">
<alt name="AVP-audio-0">
<rtp:session>
<rtp:pt ref="rtp-avp-0"/>
<rtp:udp role="receive" endpoint="A" addr="192.168.1.1"
rtp-port="7800"/>
</rtp:session>
</alt>
</component>
</cfg>
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<conf>
<owner user="A@example.com" id="98765432" version="1" nettype="IN"
addrtype="IP4" addr="192.168.1.1"/>
<session name="SDPng questions">
</session>
<info name="interactive-audio" function="voice">
Telephony media stream
<info>
</conf>
==================================================
F2 (100 Trying) B -> A
SIP/2.0 100 Trying
Via: SIP/2.0/UDP hostA.example.com:5060
From: A <sip:A@example.com>
To: B <sip:B@example.com>
Call-ID: 1234@hostA.example.com
CSeq: 1 INVITE
Content-Length: 0
==================================================
F3 180 Ringing B -> A
SIP/2.0 180 Ringing
Via: SIP/2.0/UDP hostA.example.com:5060
From: A <sip:A@example.com>
To: B <sip:B@example.com>;tag=987654
Call-ID: 1234@hostA.example.com
CSeq: 1 INVITE
Content-Length: 0
==================================================
F4 200 OK B -> A
SIP/2.0 200 OK
Via: SIP/2.0/UDP hostA.example.com:5060
From: A <sip:A@example.com>
To: B <sip:B@example.com>;tag=987654
Call-ID: 1234@hostA.example.com
CSeq: 1 INVITE
Contact: <sip:B@192.168.1.2>
Content-Type: application/sdpng
Content-Length: 479
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<def>
<audio:codec name="audio-basic" encoding="PCMU"
sampling="8000" channels="1"/>
<rtp:pt name="rtp-avp-0" pt="0">
<audio:codec ref="audio-basic"/>
</rtp:pt>
</def>
<cfg>
<component name="interactive-audio" media="audio">
<alt name="AVP-audio-0">
<rtp:session>
<rtp:pt ref="rtp-avp-0"/>
<rtp:udp role="receive" endpoint="A" addr="192.168.1.1"
rtp-port="7800"/>
<rtp:udp role="receive" endpoint="B" addr="192.168.1.2"
rtp-port="9410"/>
</rtp:session>
</alt>
</component>
</cfg>
==================================================
ACK from A to B omitted
In the INVITE message, A sends B a description document, that
specifies exactly one component with one alternative (the PCMU audio
stream). All required transport parameters all already contained in
the description. The rtp:udp element provides an attribute "role"
with a value of "receive", indicating that the specified endpoint
address is used by the endpoint to receive media data. The element
also provides the attribute "endpoint" with a value of "A",
denominating the endpoint that can receive data on the specified
address. This means, the semantics of specified transport addresses
in configuration descriptions are the same as for SDP (when used with
SIP): An endpoint specifies where it wants to receive data.
In the 200 OK message, B sends an updated description document to A.
For the sake of conciseness, the conf element (containing meta
information about the conference) has been omitted. B supports the
payload format that A has offered and adds his own transport
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parameters to the configuration information, specifying the endpoint
address where B wants to receive media data. In order to
disambiguate its transport configurations from A's, B sets the
attribute "endpoint" to the value "B". The specific value of the
"endpoint" attribute is not important, the only requirements are that
a party that contributes to the session description, must use a
unique name for the endpoint attribute and that a contributing party
must use the same value for the endpoint attributes of all elements
it adds to the session description.
The following example shows a capability description that provides
two alternatives for the audio component.
Description document with two alternatives:
F1 INVITE A -> B
INVITE sip:B@example.com SIP/2.0
Via: SIP/2.0/UDP hostA.example.com:5060
From: A <sip:A@example.com>
To: B <sip:B@example.com>
Call-ID: 1234@hostA.example.com
CSeq: 1 INVITE
Contact: <sip:UserA@192.168.1.1>
Content-Type: application/sdpng
Content-Length: 935
<def>
<audio:codec name="audio-basic" encoding="PCMU"
sampling="8000" channels="1"/>
<audio:codec name="g729" encoding="G729" channels="1" sampling="8000"/>
<rtp:pt name="rtp-avp-0" pt="0">
<audio:codec ref="audio-basic"/>
</rtp:pt>
<rtp:pt name="rtp-avp-18" pt="18">
<audio:codec ref="g729"/>
</rtp:pt>
<rtp:udp name="A-rcv" role="receive" endpoint="A" addr="192.168.1.1"
rtp-port="7800"/>
</def>
<cfg>
<component name="interactive-audio" media="audio">
<alt name="AVP-audio-0">
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<rtp:session format="rtp-avp-0">
<rtp:udp ref="A-rcv"/>
</rtp:session>
</alt>
<alt name="AVP-audio-18">
<rtp:session format="rtp-avp-18"/>
<rtp:udp ref="A-rcv"/>
</rtp:session>
</alt>
</component>
</cfg>
<conf>
<owner user="A@example.com" id="98765432" version="1" nettype="IN"
addrtype="IP4" addr="192.168.1.1"/>
<session name="SDPng questions">
</session>
<info name="interactive-audio" function="voice">
Telephony media stream
<info>
</conf>
==================================================
F2 (100 Trying) B -> A
SIP/2.0 100 Trying
Via: SIP/2.0/UDP hostA.example.com:5060
From: A <sip:A@example.com>
To: B <sip:B@example.com>
Call-ID: 1234@hostA.example.com
CSeq: 1 INVITE
Content-Length: 0
==================================================
F3 180 Ringing B -> A
SIP/2.0 180 Ringing
Via: SIP/2.0/UDP hostA.example.com:5060
From: A <sip:A@example.com>
To: B <sip:B@example.com>;tag=987654
Call-ID: 1234@hostA.example.com
CSeq: 1 INVITE
Content-Length: 0
==================================================
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F4 200 OK B -> A
SIP/2.0 200 OK
Via: SIP/2.0/UDP hostA.example.com:5060
From: A <sip:A@example.com>
To: B <sip:B@example.com>;tag=987654
Call-ID: 1234@hostA.example.com
CSeq: 1 INVITE
Contact: <sip:B@192.168.1.2>
Content-Type: application/sdpng
Content-Length: 479
<def>
<audio:codec name="audio-basic" encoding="PCMU"
sampling="8000" channels="1"/>
<audio:codec name="g729" encoding="G729" channels="1" sampling="8000"/>
<rtp:pt name="rtp-avp-0" pt="0">
<audio:codec ref="audio-basic"/>
</rtp:pt>
<rtp:pt name="rtp-avp-18" pt="18">
<audio:codec ref="g729"/>
</rtp:pt>
<rtp:udp name="A-rcv" role="receive" endpoint="A" addr="192.168.1.1"
rtp-port="7800"/>
<rtp:udp name="B-rcv" role="receive" endpoint="B" addr="192.168.1.2"
rtp-port="9410"/>
</def>
<cfg>
<component name="interactive-audio" media="audio">
<alt name="AVP-audio-0">
<rtp:session format="rtp-avp-0">
<rtp:udp ref="A-rcv"/>
<rtp:udp ref="B-rcv"/>
</rtp:session>
</alt>
</component>
</cfg>
==================================================
ACK from A to B omitted
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In the INVITE message, A sends B a description document, that
specifies one component with two alternatives for the audio stream
(PCMU and G.729). Since A wants to use the same transport address
for receiving media data regardless of the payload format, A provides
the transport specification in the def element and references this
definition in the rtp:session elements for both alternatives by using
the attribute "transport".
In the 200 OK message, B sends an updated description document to A.
B does only support PCMU, so it removes the alternative for G.729
from the description. B also defines its transport address in the
def element and references this definition by referencing the rtp:udp
element named "B-rcv".
A.3 Real-Time Streaming Protocol (RTSP)
In contrast to SIP, RTSP has, from its intended usage, a clear
distinction between offering Potential Configurations (typically by
the server) and choosing one out of these (by the client), and, in
some cases; some parameters (such as multicast addresses) may be
dictated by the server. Hence with RTSP, there is a clear
distinguish between Potential Configurations during the negotiation
phase and a finally chosen Actual Configuration according to which
streaming will take place.
Example from the RTSP spec to be provided.
A.4 Media Gateway Control Protocol (MEGACOP)
The MEGACO architecture also follows the SDPng model of a clear
separation between Potential and Actual Configurations. Upon
startup, a Media Gateway (MG) will "register" with its Media Gateway
Controller (MGC) and the latter will audit the MG for its
Capabilities. Those will be provided as Potential Configurations,
possibly with extensive Constraints specifications. Whenever a media
path needs to be set up by the MGC between two MGs or an MG needs to
be reconfigured internally, the MGC will use (updated) Actual
Configurations.
Details and examples to be defined.
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Appendix B. Change History
draft-ietf-mmusic-sdpng-06.txt
* Removed section on capability negotiation algorithm and section
on formal specification. Added Section 3.
* Removed specification of concrete XML syntax from Section 4.
Added requirements and theoretic considerations.
* Added clarification of term "actual configuration" in Section
2.
* Changed "profile" to "package".
* Added introducing text to Section 4.1.
* Added a list of terms with explanation at the end of Section 2.
* Removed audio and RTP packages from appendix.
* Added Section 5 ("Syntax Definition").
* Added Section 6 ("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 (Section 4.3). 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 (Section 4.3).
* New section on properties.
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* New section on definition groups.
draft-ietf-mmusic-sdpng-03.txt
* Extension of the SDPng schema (use of Xlinks etc.)
* Clarification in the text
* Fixed examples
* Added example libraries as appendices
* More details on usage with SIP, including examples.
draft-ietf-mmusic-sdpng-02.txt
* Added a section on formal specification mechanisms.
draft-ietf-mmusic-sdpng-01.txt
* renamed section "Syntax Proposal" to "Syntax Definition
Mechanisms". More text on DTD vs. schema. Edited the example
description.
* updated example definitions in section "Definitions" and
"Components & Configurations"
* section "Session Attributes" replaces section "Session"
* new appendix on audio codec definitions
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