Network Working Group Kutscher
Internet-Draft Ott
Expires: October 16, 2001 Bormann
TZI, Universitaet Bremen
Curcio
Nokia Mobile Phones
April 17, 2001
Requirements for Session Description and Capability Negotiation
draft-ietf-mmusic-sdpng-req-01.txt
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Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document defines some terminology and lists a set of
requirements that are relevant for a framework for session
description and endpoint capability negotiation in multiparty
multimedia conferencing scenarios.
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
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$Revision: 2.1 $
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . 3
2. Use Cases . . . . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . 6
4. General Requirements . . . . . . . . . . . . . . . . . . 9
4.1 Simplicity . . . . . . . . . . . . . . . . . . . . . . . 9
4.2 Extensibility . . . . . . . . . . . . . . . . . . . . . 9
4.3 Firewall Friendliness . . . . . . . . . . . . . . . . . 9
4.4 Authentication, Authorization, Media Keys . . . . . . . 9
4.5 Text encoding . . . . . . . . . . . . . . . . . . . . . 10
4.6 Session vs. Media Description . . . . . . . . . . . . . 10
4.7 Mapping (of a Subset) to SDP . . . . . . . . . . . . . . 10
5. Session Description Requirements . . . . . . . . . . . . 11
5.1 Media Description . . . . . . . . . . . . . . . . . . . 11
5.1.1 Medium Type . . . . . . . . . . . . . . . . . . . . . . 11
5.1.2 Media Stream Packetization . . . . . . . . . . . . . . . 11
5.1.3 Transport . . . . . . . . . . . . . . . . . . . . . . . 11
5.1.4 QoS . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1.5 Resource Utilization . . . . . . . . . . . . . . . . . . 12
5.1.6 Dependencies . . . . . . . . . . . . . . . . . . . . . . 13
5.1.7 Other parameters (media-specific) . . . . . . . . . . . 13
5.1.7.1 Media Codec Bit Rates . . . . . . . . . . . . . . . . . 13
5.1.7.2 Advanced codec modes . . . . . . . . . . . . . . . . . . 13
5.1.8 Naming Hierarchy and/or Scoping . . . . . . . . . . . . 13
5.1.9 Profiles . . . . . . . . . . . . . . . . . . . . . . . . 14
5.1.10 Registrations of Names . . . . . . . . . . . . . . . . . 14
5.1.11 Meta-Information . . . . . . . . . . . . . . . . . . . . 14
5.1.11.1 Scheduling . . . . . . . . . . . . . . . . . . . . . . . 14
5.1.11.2 Optional Meta-Information Packages . . . . . . . . . . . 15
6. Requirements for Capability Description and Negotiation 16
6.1 Capability Description . . . . . . . . . . . . . . . . . 16
6.2 Capability Constraints . . . . . . . . . . . . . . . . . 16
6.3 Processing Rules . . . . . . . . . . . . . . . . . . . . 17
7. Open Issues . . . . . . . . . . . . . . . . . . . . . . 19
8. Remarks . . . . . . . . . . . . . . . . . . . . . . . . 20
References . . . . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . 22
Full Copyright Statement . . . . . . . . . . . . . . . . 24
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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. Use Cases
This is an initial list of use cases:
And endpoint is a device attached to an IP network via a fixed or
wireless connection (LAN, WLAN, GPRS, IMT-2000, etc.).
Case 1: Point-to-point connection
Case 2: Point-to-point connection with use of proxy/CPS
Case 3: 1-to-n connection (multicast distribution such as a lecture
or video streaming or music)
Case 4: n-party connection (such as a speech and/or video and/or
data call with a variable number of participants over time)
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3. Terminology
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 [11] 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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4. General Requirements
Note that the order in which these requirements are presented does
not imply their relative importance.
4.1 Simplicity
The SDPng syntax shall be simple to parse and the protocol rules
shall be easy to implement.
4.2 Extensibility
SDPng shall be extensible in a backward compatible fashion.
Extensions should be doable without modifying the SDPng
specification itself. The spec should preclude two independent
extensions from clashing with each other (e.g. in the naming of
attributes).
Along with extensibility comes the requirement to identify certain
extensions as mandatory in a given context while others as optional.
It should be possible to define subsets or profiles of the SDPng so
that simple implementations understands only minimal parts of SDPng,
and are able to interwork with more refined and complex SDPng
implementations.
4.3 Firewall Friendliness
It should be theoretically possible for firewalls (and other network
infrastructure elements) to process announcements etc. that contain
SDPng content. The concrete procedures have to be defined but if
possible the processing of the SDPng content should be doable
without interpretation of the textual descriptions.
In general, there should not be any problem for a gateway or a proxy
server to execute the capability computation, and this operation has
not to be limited to the endpoints only.
4.4 Authentication, Authorization, Media Keys
SDPng should allow independent security attributes for parts of a
session description. In particular, signing and/or encrypting parts
of a session description should be supported.
The originator of the session may authenticate the session
description or parts of it, or encrypt it so that only authorized
users may access it.
In addition to the media encryption keys, the session description
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should include also authentication keys, or include ways for
authorized session participants to derive these keys.
In order to support rapid re-keying, the session description should
include way to specify multiple encryption contexts and indicate how
and when these encryption contexts will be used. For instance, the
session description would indicate the current key and destination
group, and then that the key will be changed at 20010401Z084000, and
the media encoded with the new key will be sent to group
243.243.12.8.
4.5 Text encoding
A concise text representation is desirable in order to enhance
portability and allow for simple implementations. At run time, size
of encoded packets should be minimized, processing as well.
A language that allows specifications to be formally validated is
desirable.
4.6 Session vs. Media Description
In many application scenarios (particularly with SIP and
MEGACO/H.248), only media descriptions are needed and there is no
need for session description parameters. SDPng should make parameter
sets optional where it is conceivable that not all application will
need them.
4.7 Mapping (of a Subset) to SDP
It shall be possible to translate a subset of SDPng into standard
SDP session description to enable a certain minimal degree of
interoperability between SDP-based and SDPng-based systems. However,
as SDPng will provide enhanced functionality compared to SDP, a full
mapping to SDP is not possible.
Note: Backwards compatibility to the SDP syntax has been discussed
and it was found that this is not goal for SDPng, as it is felt that
RFC 2327 is too limiting.
Since several flavors of SDP have been developed (e.g., the MEGACO
WG uses certain non-SDP enhancements) it needs to be discussed which
of these flavors need to be considered for some kind of mapping.
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5. Session Description Requirements
For now, we only consider requirements for media (stream)
descriptions.
5.1 Media Description
It must be possible to express the following information with SDPng:
5.1.1 Medium Type
Payload types and format parameters for audio and video are
well-defined and the basic semantics are clear (as defined in
RFC1889 [2] and RFC2327 [1]).
Format descriptions for text and whiteboard are currently only
defined in the context of specific applications, this is probably
going to change in the future (not an SDPng work item).
Non-standard (in terms of defined as a non-standard payload type)
codecs and format parameters can be accomplished by using dynamic
payload type mappings. This is a crucial feature of SDP that needs
to be preserved for RTP applications.
Current SDP only provides a= (a=fmtp) as means to specify codec
parameters but actually gives little support on how to do this.
Schemes for expressing more sophisticated parameters (e.g.
supporting nesting) may be necessary. Nevertheless, it is imperative
to keep the overall structure of a codec description manageable.
Note that there is a conflict between the desire to be able to use
any old SDP and translate it in SDPng and the desire to have a
useful structure in the SDPng data.
5.1.2 Media Stream Packetization
SDPng needs to be able to take care of more sophisticated payload
descriptions than simple payload type assignment. Audio/video
redundancy coding schemes need to be supported as need other
mechanisms for FEC (RFC 2733 [7]) and media stream repair (RFC 2354
[8]). Also, layered coding schemes need to be supported.
Finally, a separation of the media encoding scheme, the
packetization format, and possible repair schemes (and their
respective parameters) is required.
5.1.3 Transport
Since session descriptions are not only used to describe sessions
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that use IPv4/RTP for media transport it must be possible to specify
different transport protocols (and their corresponding mandatory
parameters). This means SDPng must support different address formats
(IPv4, IPv6, E.164, NSAP, ...), multiplexing schemes (e.g. to
identify a channel on a TDM link), and different transport protocol
stacks (RTP/UDP/IP, RTP/AAL5/ATM, ...). Potential further parameters
and interdependencies for multiplexed transports should be
considered.
Additionally the requirement for expressing multiple addresses per
actual configuration (layered coding support) has emerged, as well
as the requirement for expressing multiple addresses per potential
configuration (one port per payload type to simplify processing at
the receiver). See also Section 6.1 for a requirement to separate
alternative configurations and simultaneous media sessions.
In multi-unicast-scenarios it must be possible to specify more than
one transport address for a single media stream in an actual
configuration, i.e. by specifying address lists.
In "broadcast"- or "lecture"-like sessions source filters might be
needed that allow receivers to verify the source and apply filters
in multicast sessions. Similarly, for SSM, the transport address
includes an (Sender,Group) pair of IP addresses.
The definition of codecs and the definitions of transport parameters
should be strictly separated from each other. See also Section
5.1.9.
5.1.4 QoS
QoS-Parameters for different protocol domains (e.g. traffic
specification and flow specification or TOS bits for IP QoS) need to
be specified. draft-ietf-mmusic-sdp-qos-00.txt [10] has provided a
proposal for a syntax that can be used with SDP to describe network
and security preconditions that have to be met in order to establish
a session.
5.1.5 Resource Utilization
Capability descriptions should not be based on available resources
and resource requirements (in terms of CPU cycles, DSPs, etc.) for
the following reasons:
o Device manufacturers might not like to let hardware information
go out from their devices.
o The resource utilization function is not bijective since, for
example, to support a certain media, a device A could require a
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quantity X of resources, while another device B of a different
manufacturer could require a quantity Y of resource, where X <>
Y. This is an implementation dependent issue, and it is related
with how efficiently/inefficiently a manufacturer is able to
implement a feature in its device.
5.1.6 Dependencies
Certain codes may depend on other resources being available (e.g. a
G.723.1 audio codec may need a DTMF codec as well while a G.711
codec does not). Such interdependencies need to be expressed.
5.1.7 Other parameters (media-specific)
Extension mechanisms that allow to describe arbitrary other
parameters of media codecs and formats are mandatory. It is possibly
required to distinguish between mandatory and optional extension
parameters.
In particular, it must be possible to introduce new (optional)
parameters for a payload format and have old implementations still
parse the parameters correctly.
Some audio/video specific parameters can be defined as suggested in
[16].
5.1.7.1 Media Codec Bit Rates
There should be the possibility to configure ranges of bit rates
(for example 32-64 kbps) or a discrete set of rates (i.e. 24, 32,
48, 64, 128, ... kbps). This is to allow an efficient resource
allocation and allow as well interworking with systems where only a
number of discrete bit rates is available. Resource reservation and
QoS configuration mechanisms in general should available as optional
packages (see Section 5.1.4). It is conceivable that there be a
separation into generic and transport specific QoS packages.
5.1.7.2 Advanced codec modes
Special advanced codec modes may be announced depending, for
example, on the network conditions, to activate special settings in
order to preserve good quality of media and to reduce the
probability of call dropping.
5.1.8 Naming Hierarchy and/or Scoping
Parameter names should be constructed in a way to avoid clashes and
thereby simplify independent development of e.g. codec parameter
descriptions in different groups.
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5.1.9 Profiles
The configuration options for the different aspects of session
descriptions (codec definitions, transport parameters etc.) should
be defined in different orthogonal profiles ("packages"). Two
different types of profiles are required:
o A profile can define the data structures and collapsing rules for
a specific function, e.g., for transport parameters. Capability
and session descriptions can refer to such a profile and define
concrete values for the profile parameters.
o Instantiations of such profiles, that already contain concrete
parameters, say for specific codec definitions, can be referenced
by capability and session descriptions in order to allow for
combining different aspects to a final description that is
synthetic and of lower computational complexity.
An open issue is the question whether profiles should be referenced
by name, i.e., by creating a well-known registry, or whether
profiles should be referenced by address, i.e., by creating the
possibility to retrieve them on demand. It is conceivable that a
combination of both is useful: Some basic definitions are registered
and well known and some other, uncommon definitions can be
referenced by URIs.
5.1.10 Registrations of Names
SDP uses top-level MIME content types [15] for media names. These
convention should be adopted in order to avoid the unneccessary
creation of a new namespace.
SDP also defines a registration procedure that allows to register
new names for "media", "proto", "fmt", "bwtype", "nettype", or
"addrtype" field names (defined in [1]). If possible, the names that
are already defined should be re-used. The definition of SDPng
should contain a specification that states the registration
procedures for SDPng.
5.1.11 Meta-Information
5.1.11.1 Scheduling
In order to be usable for conference announcements, e.g. by means of
SAP [11] it also required to provide a means for expressing
scheduling information, i.e. to express the date (or the recurring
dates) when a conference is taking place.
Two alternatives for implementing scheduling functions are
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conceivable:
o SDP-style (using the SDP model that is implemented as t= and r=
lines); and
o Using ICalender [14].
5.1.11.2 Optional Meta-Information Packages
Location Meta-Information: In case of usage of SAP [11] as content
or channel directory, the session description should include the
location information, including physical location and the L1/L2
addressing information required to access the session. L1/L2
info may include things like transmission media used,
frequencies, L2 multiplexing information etc. This makes it
possible to broadcast session descriptions sent using broadband
radio on, e.g., narrowband radio network. The recipients can
derive their location from the location sent in the SAP session
description, and then decide if and how they can receive the
media sessions.
Pricing Information: The session description need to specify the
pricing information for the session, if participating in the
session requires payment.
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6. Requirements for Capability Description and Negotiation
6.1 Capability Description
When describing the capabilities of endsystem by providing a list of
different potential configurations, it must be possible to
distinguish alternatives (different potential configuration) from
different simlutaneous sessions of a conference. A clear separation
of these two concepts must be made that should also be realized in
the description language.
6.2 Capability Constraints
Capability negotiation is used to gain a session description (an
actual configuration) that is compatible with the different end
system capabilities and user preferences of the potential
participants of a conference.
A media capability description is the same as a potential
configuration, as it contains a set of allowable configurations for
different components that could be used to implement the
corresponding component. A capability description should allow
specifying a number of interdependencies among capabilities.
Traditional SDP only supports alternative capabilities and the
specification implicitly assumed that all capabilities could be
combined and basically used at the same time (looking at the pure
session description, at least).
Processing power, hardware, link, or other resources may preclude
the simultaneous use of certain configurations and/or limit the
number of simultaneous instantiations of one or more configurations.
This has led to a need to express in more detail constraints on
combinations of configurations including the following constraints:
o grouping capabilities (-> capability set);
o expressing simultaneous capability sets;
o expressing alternative capability sets; and
o constraining the number of uses of a certain capability (set).
It needs to be carefully investigated how much more sophistication
(if any) than simply listing alternatives needs to go into a base
specification of SDPng (and which extension mechanisms for certain
applications or for future revisions should be allowed).
Examples are known where complex capability descriptions are
available but are simply not used (at least not at the level of
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sophistication that would be possible). This strongly calls for
keeping requirements on capability constraints rather modest (KISS).
The description of capabilities and the specification of capacity
limits (maximum numbers of instantiations at a time) should be
separated. This allows for greater modularity, since the common
descriptions of capabilities can be referenced and imported, while
the constraints (that are usually unique for a specific endsystem)
can be provided inline and can be applied across singe capability
definitions. In order to allow for simple basic implementations,
this also allows to treat the constraints as optional sections that
do not have to be processed by an implementations.
The capabilities should be expressed as limitations on codec
support, transport capability restrictions but not implicitely as
limitations on machine resources, such as CPU type, clock rates,
memory etc., that describe internal limitations in order to infer
the supported capabilities.
6.3 Processing Rules
The processing of potential configurations includes the process of
"collapsing" sets of potential configurations offered by
participants, i.e. the computation of the intersection of these
potential configurations.
The processing (i.e. collapsing, forwarding etc.) of different
potential configurations in order to find a compatible subset must
work without having to know the semantics of the individual
parameters. This is a key requirement for extensibility. Endsystems,
conference bridges, proxies and gateways are thus only required to
understand the basic SDPng semantics of session and capability
description in order to compute the supported subset of capablities
for a conference.
Additionally it must be possible to make use of different
negotiation policies in order to reflect different conference types.
For example in a lecture-style conference the policy might be to
ensure that a capability collapsing process does not yield an actual
configuration that excludes the main source (i.e. the lecturer and
her end system) from the conference.
Preferences may also be considered in the negotiation process. This
may need to be considered at the SDPng level (e.g. to express
preferences, priorities).
Of course, the negotiation of configurations must not only work in
peer-to-peer-conference scenarios but also be usable in multi party
scenarios. The collapsing rules should work commutatively and
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associatively, that means if given 3 end systems A,B,C the result
for computing the supported configurations should be same when
computing (A*B)*C and (B*C)*A (let "*" be the collapsing function).
Negotiation of capabilities should take no longer than two or three
message exchanges. The description format must enable such
efficiency.
In order to allow for concise capability specification it will
probably be required to group descriptions of, say, codecs and to
establish a kind of hierarchy that allows to attach a certain
attribute or parameter to a whole group of codecs.
It might then also be required to have a naming scheme that allows
to name definitions in order to be able to later reference them in
subsequent definitions. This is useful in situations where some
definition extends a previous definition by just one parameter or in
situations where codecs are combined, for example for expressing
redundancy or layered codings. Different models of re-use are
conceivable.
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7. Open Issues
This section contains a list of items that need further work and/or
discussion:
It needs to distinguished more precisely between mandatory baseline
functionality and optional extensions.
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8. Remarks
Explicitly addressing the issue of capability negotiation when
drafting the new session description language generates new sets of
requirements, some of which might conflict with other important
goals, such as simplicity, conciseness and SDP-compatibility.
However, we think that it's worthwhile to sketch a reasonably
complete and powerful solution first and then later develop a
migration path from today's technology instead of imposing
limitations at the outset to minimize the possibly necessary
changes.
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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] Camarillo, G., Holler, J. and G. AP Eriksson, "SDP media
alignment in SIP", Internet Draft
draft-camarillo-sip-sdp-01.txt, November 2000.
[10] Rosenberg, J., Schulzrinne, H. and S. Donovan, "Establishing
QoS and Security Preconditions for SDP Sessions", Internet
Draft draft-ietf-mmusic-sdp-qos-00.txt, June 1999.
[11] Handley, M., Perkins, C. and E. Whelan, "Session Announcement
Protocol", RFC 2974, October 2000.
[12] Kumar, R. and M. Mostafa, "Conventions for the use of the
Session Description Protocol (SDP) for ATM Bearer
Connections", Internet Draft draft-ietf-mmusic-sdp-atm-05.txt,
February 2001.
[13] Casner, S., "SDP Bandwidth Modifiers for RTCP Bandwidth",
Internet Draft draft-ietf-avt-rtcp-bw-02.txt, November 2000.
[14] Dawson, F., Stenerson, D., MISSING ORGANIZATION and ,
"Internet Calendaring and Scheduling Core Object Specification
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(iCalendar)", RFC 2445, November 1998.
[15] Freed, N., Klensin, J. and J. Postel, "Multipurpose Internet
Mail Extensions (MIME) Part Four: Registration Procedures",
BCP 13, RFC 2048, November 1996.
[16] Levin, O., "SIP Requirements for support of Multimedia and
Video", Internet Draft draft-levin-sip-for-video-00.txt,
February 2001.
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.org
Joerg Ott
TZI, Universitaet Bremen
Bibliothekstr. 1
Bremen 28359
Germany
Phone: +49.421.201-7028
Fax: +49.421.218-7000
EMail: jo@tzi.org
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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Igor Curcio
Nokia Mobile Phones
P.O. Box 83 (Visiokatu 1)
33721 Tampere
Finland
Phone: +358.3.272.5372
Fax: +358.10.505.7662
EMail: igor.curcio@nokia.com
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