MMUSIC R. Gilman
Internet-Draft Independent
Intended status: Standards Track R. Even
Expires: September 1, 2011 Gesher Erove Ltd
F. Andreasen
Cisco Systems
February 28, 2011
SDP Media Mapabilities Negotiation
draft-ietf-mmusic-sdp-media-capabilities-11
Abstract
Session Description Protocol (SDP) capability negotiation provides a
general framework for indicating and negotiating capabilities in SDP.
The base framework defines only capabilities for negotiating
transport protocols and attributes. In this document, we extend the
framework by defining media capabilities that can be used to
negotiate media types and their associated parameters.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 1, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
Gilman, et al. Expires September 1, 2011 [Page 1]
Internet-Draft SDP Media Capabilities Negotiation February 2011
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Gilman, et al. Expires September 1, 2011 [Page 2]
Internet-Draft SDP Media Capabilities Negotiation February 2011
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. SDP Media Capabilities . . . . . . . . . . . . . . . . . . . . 6
3.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Solution Overview . . . . . . . . . . . . . . . . . . . . 7
3.3. New Capability Attributes . . . . . . . . . . . . . . . . 13
3.3.1. The Media Format Capability Attribute . . . . . . . . 13
3.3.2. The Media Format Parameter Capability Attribute . . . 15
3.3.3. The Media-Specific Capability Attribute . . . . . . . 17
3.3.4. New Configuration Parameters . . . . . . . . . . . . . 19
3.3.5. The Latent Configuration Attribute . . . . . . . . . . 20
3.3.6. Enhanced Potential Configuration Attribute . . . . . . 23
3.3.7. Substitution of Media Payload Type Numbers in
Capability Attribute Parameters . . . . . . . . . . . 26
3.3.8. The Session Capability Attribute . . . . . . . . . . . 27
3.4. Offer/Answer Model Extensions . . . . . . . . . . . . . . 31
3.4.1. Generating the Initial Offer . . . . . . . . . . . . . 31
3.4.2. Generating the Answer . . . . . . . . . . . . . . . . 32
3.4.3. Offerer Processing of the Answer . . . . . . . . . . . 32
3.4.4. Modifying the Session . . . . . . . . . . . . . . . . 32
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.1. Alternative Codecs . . . . . . . . . . . . . . . . . . . . 33
4.2. Alternative Combinations of Codecs (Session
Configurations) . . . . . . . . . . . . . . . . . . . . . 36
4.3. Latent Media Streams . . . . . . . . . . . . . . . . . . . 36
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 39
5.1. New SDP Attributes . . . . . . . . . . . . . . . . . . . . 39
5.2. New SDP Option Tag . . . . . . . . . . . . . . . . . . . . 40
5.3. New SDP Capability Negotiation Parameters . . . . . . . . 40
6. Security Considerations . . . . . . . . . . . . . . . . . . . 41
7. Changes from previous versions . . . . . . . . . . . . . . . . 42
7.1. Changes from version 10 . . . . . . . . . . . . . . . . . 42
7.2. Changes from version 09 . . . . . . . . . . . . . . . . . 42
7.3. Changes from version 08 . . . . . . . . . . . . . . . . . 42
7.4. Changes from version 04 . . . . . . . . . . . . . . . . . 43
7.5. Changes from version 03 . . . . . . . . . . . . . . . . . 43
7.6. Changes from version 02 . . . . . . . . . . . . . . . . . 44
7.7. Changes from version 01 . . . . . . . . . . . . . . . . . 44
7.8. Changes from version 00 . . . . . . . . . . . . . . . . . 44
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 45
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 46
9.1. Normative References . . . . . . . . . . . . . . . . . . . 46
9.2. Informative References . . . . . . . . . . . . . . . . . . 46
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 47
Gilman, et al. Expires September 1, 2011 [Page 3]
Internet-Draft SDP Media Capabilities Negotiation February 2011
1. Introduction
Session Description Protocol (SDP) capability negotiation [RFC5939]
provides a general framework for indicating and negotiating
capabilities in SDP[RFC4566]. The base framework defines only
capabilities for negotiating transport protocols and attributes.
The [RFC5939] document lists some of the issues with the current SDP
capability negotiation process. An additional real life case is to
be able to offer one media stream (e.g. audio) but list the
capability to support another media stream (e.g. video) without
actually offering it concurrently.
In this document, we extend the framework by defining media
capabilities that can be used to indicate and negotiate media types
and their associated format parameters. This document also adds the
ability to declare support for media streams, the use of which can be
offered and negotiated later, and the ability to specify session
configurations as combinations of media stream configurations. The
definitions of new attributes for media capability negotiation are
chosen to make the translation from these attributes to
"conventional" SDP [RFC4566] media attributes as straightforward as
possible in order to simplify implementation. This goal is intended
to reduce processing in two ways: each proposed configuration in an
offer may be easily translated into a conventional SDP media stream
record for processing by the receiver; and the construction of an
answer based on a selected proposed configuration is straightforward.
Gilman, et al. Expires September 1, 2011 [Page 4]
Internet-Draft SDP Media Capabilities Negotiation February 2011
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [RFC2119] and
indicate requirement levels for compliant RTP implementations.
"Base Attributes": Conventional SDP attributes appearing in the base
configuration of a media block.
"Base Configuration": The media configuration represented by a media
block exclusive of all the capability negotiation attributes defined
in this document, the base capability document[RFC5939], or any
future capability negotiation document. In an offer SDP, the base
configuration corresponds to the actual configuration as defined in
[RFC5939].
"Conventional Attribute": Any SDP attribute other than those defined
by the series of capability negotiation specifications.
"Conventional SDP": An SDP record devoid of capability negotiation
attributes.
"Media Capability": A media encoding, typically a media subtype such
as PCMU, H263-1998, or T38.
Gilman, et al. Expires September 1, 2011 [Page 5]
Internet-Draft SDP Media Capabilities Negotiation February 2011
3. SDP Media Capabilities
The SDP capability negotiation [RFC5939] discusses the use of any SDP
[RFC4566] attribute (a=) under the attribute capability "acap". The
limitations of using acap for fmtp and rtpmap in a potential
configuration are described in [RFC5939]; for example they can be
used only at the media level since they are media level attributes.
The [RFC5939] does not provide a way to exchange media-level
capabilities prior to the actual offer of one or more media streams.
This section provides an overview of extensions providing an SDP
Media Capability negotiation solution offering more robust
capabilities negotiation. This is followed by definitions of new SDP
attributes for the solution and its associated updated offer/answer
procedures [RFC3264]
3.1. Requirements
The capability negotiation extensions described herein are described
as follows.
REQ-01: Support the specification of alternative (combinations of)
media formats (codecs) in a single media block.
REQ-02: Support the specification of alternative media format
parameters for each media format.
REQ-03: Retain backward compatibility with conventional SDP.
Ensure that each and every offered configuration can be easily
translated into a corresponding SDP media block expressed with
conventional SDP lines.
REQ-04: Ensure the scheme operates within the offer/answer model in
such a way that media formats and parameters can be agreed upon
with a single exchange.
REQ-05: Provide the ability to express offers in such a way that
the offerer can receive media as soon as the offer is sent. (Note
that the offerer may not be able to render received media prior to
exchange of keying material.)
REQ-06: Provide the ability to offer latent media configurations
for future negotiation.
REQ-07: Provide reasonable efficiency in the expression of
alternative media formats and/or format parameters, especially in
those cases in which many combinations of options are offered.
Gilman, et al. Expires September 1, 2011 [Page 6]
Internet-Draft SDP Media Capabilities Negotiation February 2011
REQ-08: Retain the extensibility of the base capability negotiation
mechanism.
REQ-09: Provide the ability to specify acceptable combinations of
media streams and media formats. For example, offer a PCMU audio
stream with an H264 video stream, or a G729 audio stream with an
H263 video stream. This ability would give the offerer a means to
limit processing requirements for simultaneous streams. This
would also permit an offer to include the choice of an audio/T38
stream or an image/T38 stream, but not both.
Other possible extensions have been discussed, but have not been
treated in this document. They may be considered in the future.
Three such extensions are:
FUT-01: Provide the ability to mix, or change, media types within a
single media block. Conventional SDP does not support this
capability explicitly; the usual technique is to define a media
subtype that represents the actual format within the nominal media
type. For example, T.38 FAX as an alternative to audio/PCMU
within an audio stream is identified as audio/T38; a separate FAX
stream would use image/T38.
FUT-02: Provide the ability to support multiple transport protocols
within an active media stream without reconfiguration. This is
not explicitly supported by conventional SDP.
FUT-03: Provide capability negotiation attributes for all media-
level SDP line types in the same manner as already done for the
attribute type, with the exception of the media line type itself.
The media line type is handled in a special way to permit compact
expression of media coding/format options. The lines types are
bandwidth ("b="), information ("i="), connection data ("c="), and,
possibly, the deprecated encryption key ("k=").
3.2. Solution Overview
The solution consists of new capability attributes corresponding to
conventional SDP line types, new parameters for the pcfg, acfg, and
the new lcfg attributes extending the base attributes from [RFC5939],
and a use of the pcfg attribute to return capability information in
the SDP answer.
Three new attributes are defined in a manner that can be related to
the capabilities specified in a media line, and its corresponding
rtpmap and fmtp attributes.
Gilman, et al. Expires September 1, 2011 [Page 7]
Internet-Draft SDP Media Capabilities Negotiation February 2011
o A new media attribute ("a=mcap") defines media capabilities in the
form of a media subtype (e.g. "PCMU"), and its encoding
parameters (e.g. "/8000/2"). Each resulting media format type/
subtype capability has an associated handle called a media
capability number. The encoding parameters are as specified for
the rtpmap attribute defined in [RFC4566], without the payload
type number part.
o A new attribute ("a=mfcap") specifies media format parameters
associated with one or more media capabilities. The mfcap
attribute is used primarily to associate the formatting
capabilities normally carried in the fmtp attribute.
o A new attribute ("a=mscap") that specifies media parameters
associated with one or more media capabilities. The mscap
attribute is used to associate capabilities with attributes other
than fmtp or rtpmap, for example, the rtcp-fb attribute defined in
[RFC4585].
o A new attribute ("a=lcfg") specifies latent media stream
configurations when no corresponding media line ("m=") is offered.
An example is the offer of latent configurations for video even
though no video is currently offered. If the peer indicates
support for one or more of offered latent configurations, the
corresponding media stream(s) may be added via a new offer/answer
exchange.
o A new attribute ("a=sescap") is used to specify an acceptable
combination of simultaneous media streams and their configurations
as a list of potential and/or latent configurations.
New parameters are defined for the potential configuration (pcfg),
latent configuration (lcfg), and accepted configuration (acfg)
attributes to associate the new attributes with particular
configurations.
o A new parameter type ("m=") is added to the potential
configuration ("a=pcfg:") attribute and the actual configuration
("a=acfg:") attribute defined in [RFC5939], and to the new latent
configuration ("a=lcfg:") attribute. This permits specification
of media capabilities (including their associated parameters) and
combinations thereof for the configuration. For example, the
"a=pcfg:" line might specify PCMU and telephone events [RFC4733]
or G.729B and telephone events as acceptable configurations. The
"a=acfg:" line in the answer would specify the configuration
chosen.
Gilman, et al. Expires September 1, 2011 [Page 8]
Internet-Draft SDP Media Capabilities Negotiation February 2011
o A new parameter type ("pt=") is added to the potential
configuration, actual configuration, and latent configuration
attributes. This parameter associates RTP payload type numbers
with the referenced media capabilities, and is appropriate only
when the transport protocol uses RTP.
o A new parameter type ("mt=") is used to specify the media type for
latent configurations.
Special processing rules are defined for capability attribute
arguments in order to reduce the need to replicate essentially-
identical attribute lines for the base configuration and potential
configurations.
o A substitution rule is defined for any capability attribute to
permit the replacement of the (escaped) media capability number
with the media format identifier (e.g., the payload type number in
audio/video profiles).
o Replacement rules are defined for the conventional SDP equivalents
of the mfcap and mscap capability attributes. This reduces the
necessity to use the deletion qualifier in the pcfg a= parameter
in order to ignore rtpmap, fmtp, and certain other attributes in
the base configuration.
o An argument concatenation rule is defined for mfcap attributes
which refer to the same media capability number. This makes it
convenient to combine format options concisely by associating
multiple mfcap lines with multiple media capabilities.
This document extends the base protocol extensions to the offer/
answer model that allow for capabilities and potential configurations
to be included in an offer. Media capabilities constitute
capabilities that can be used in potential and latent configurations.
Whereas potential configurations constitute alternative offers that
may be accepted by the answerer instead of the actual
configuration(s) included in the "m=" line(s) and associated
parameters, latent configurations merely inform the other side of
possible configurations supported by the entity. Those latent
configurations may be used to guide subsequent offer/answer
exchanges, but they are not part of the current offer/answer
exchange.
The mechanism is illustrated by the offer/answer exchange below,
where Alice sends an offer to Bob:
Gilman, et al. Expires September 1, 2011 [Page 9]
Internet-Draft SDP Media Capabilities Negotiation February 2011
Alice Bob
| (1) Offer (SRTP and RTP) |
|--------------------------------->|
| |
| (2) Answer (RTP) |
|<---------------------------------|
| |
Alice's offer includes RTP and SRTP as alternatives. RTP is the
default, but SRTP is the preferred one (long lines are folded to fit
the margins):
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
a=creq:med-v0
m=audio 3456 RTP/AVP 0 18
a=tcap:1 RTP/SAVP RTP/AVP
a=rtpmap:0 PCMU/8000/1
a=rtpmap:18 G729/8000/1
a=fmtp:18 annexb=yes
a=mcap:1,4 g729/8000/1
a=mcap:2 PCMU/8000/1
a=mcap:5 telephone-event/8000
a=mfcap:1 annexb=no
a=mfcap:4 annexb=yes
a=mfcap:5 0-11
a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32 \
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=pcfg:1 m=4,5|1,5 t=1 a=1 pt=1:100,4:101,5:102
a=pcfg:2 m=2 t=1 a=1 pt=2:103
a=pcfg:3 m=4 t=2 pt=4:18
The required base and extensions are provided by the "a=creq"
attribute defined in [RFC5939], with the option tag "med-v0", which
indicates that the extension framework defined here, must be
supported. The Base level support is implied since it is required
for the extensions.
The "m=" line indicates that Alice is offering to use plain RTP with
PCMU or G.729B. The media line implicitly defines the default
transport protocol (RTP/AVP in this case) and the default actual
configuration.
The "a=tcap:1" line, specified in the base protocol, defines
transport protocol capabilities, in this case Secure RTP (SAVP
Gilman, et al. Expires September 1, 2011 [Page 10]
Internet-Draft SDP Media Capabilities Negotiation February 2011
profile) as the first option and RTP (AVP profile) as the second
option.
The "a=mcap:1,4" line defines two G.729 media format capabilities,
numbered 1 and 4, and their encoding rate. The capabilities are of
media type "audio" and subtype G729. Note that the media subtype is
explicitly specified here, rather than RTP payload type numbers.
This permits the assignment of payload type numbers in the media
stream configuration specification. In this example, two G.729
subtype capabilities are defined. This permits the declaration of
two sets of formatting parameters for G.729.
The "a=mcap:2" line defines a G.711 mu-law capability, numbered 2.
The "a=mcap:5" line defines an audio telephone-event capability,
numbered 5.
The "a=mfcap:1" line specifies the fmtp formatting parameters for
capability 1 (offerer will not accept G.729 Annex B packets).
The "a=mfcap:4" line specifies the fmtp formatting parameters for
capability 4 (offerer will accept G.729 Annex B packets).
The "a=mfcap:5" line specifies the fmtp formatting parameters for
capability 5 (the DTMF touchtones 0-9,*,#).
The "a=acap:1" line specified in the base protocol provides the
"crypto" attribute which provides the keying material for SRTP using
SDP security descriptions.
The "a=pcfg:" attributes provide the potential configurations
included in the offer by reference to the media capabilities,
transport capabilities, attribute capabilities and specified payload
type number mappings. Three explicit alternatives are provided; the
lowest-numbered one is the preferred one. The "a=pcfg:1 ..." line
specifies media capabilities 4 and 5, i.e., G.729B and DTMF, or media
capability 1 and 5, i.e., G.729 and DTMF. Furthermore, it specifies
transport protocol capability 1 (i.e. the RTP/SAVP profile - secure
RTP), and the attribute capability 1, i.e. the crypto attribute
provided. Lastly, it specifies a payload type number mapping for
media capabilities 1, 4, and 5, thereby permitting the offerer to
distinguish between encrypted media and unencrypted media received
prior to receipt of the answer.
Use of unique payload type numbers is not required; codecs such as
AMR-WB [RFC4867] have the potential for so many combinations of
options that it may be impractical to define unique payload type
numbers for all supported combinations. If unique payload type
Gilman, et al. Expires September 1, 2011 [Page 11]
Internet-Draft SDP Media Capabilities Negotiation February 2011
numbers cannot be specified, then the offerer will be obliged to wait
for the SDP answer before rendering received media. For SRTP using
SDES inline keying [RFC4568], the offerer will still need to receive
the answer before being able to decrypt the stream.
The second alternative ("a=pcfg:2 ...") specifies media capability 2,
i.e. PCMU, under the RTP/SAVP profile, with the same SRTP key
material.
The third alternative ("a=pcfg:3 ...") offers G.729B unsecured; it's
only purpose in this example is to show a preference for G.729B over
PCMU.
The media line, with any qualifying attributes such as fmtp or
rtpmap, is itself considered a valid configuration; it is assumed to
be the lowest preference.
Bob receives the SDP offer from Alice. Bob supports G.729B, PCMU,
and telephone events over RTP, but not SRTP, hence he accepts the
potential configuration 3 for RTP provided by Alice. Bob generates
the following answer:
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
t=0 0
a=csup:med-v0
m=audio 4567 RTP/AVP 18
a=rtpmap:18 G729/8000
a=fmtp:18 annexb=yes
a=acfg:3 m=4 t=2 pt=4:18
Bob includes the "a=csup" and "a=acfg" attributes in the answer to
inform Alice that he can support the med-v0 level of capability
negotiations. Note that in this particular example, the answerer
supported the capability extensions defined here, however had he not,
he would simply have processed the offer based on the offered PCMU
and G.729 codecs under the RTP/AVP profile only. Consequently, the
answer would have omitted the "a=csup" attribute line and chosen one
or both of the PCMU and G.729 codecs instead. The answer carries the
accepted configuration in the m line along with corresponding rtpmap
and/or fmtp parameters, as appropriate.
Note that per the base protocol, after the above, Alice MAY generate
a new offer with an actual configuration ("m=" line, etc.)
corresponding to the actual configuration referenced in Bob's answer
(not shown here).
Gilman, et al. Expires September 1, 2011 [Page 12]
Internet-Draft SDP Media Capabilities Negotiation February 2011
3.3. New Capability Attributes
In this section, we present the new attributes associated with
indicating the media capabilities for use by the SDP Capability
negotiation. The approach taken is to keep things similar to the
existing media capabilities defined by the existing media
descriptions ("m=" lines) and the associated "rtpmap" and "fmtp"
attributes. We use media subtypes and "media capability numbers"
instead of payload type numbers to link the relevant media capability
parameters. This permits the capabilities to be defined at the
session level and be used for multiple streams, if desired. Payload
types are then specified at the media level (see Section 3.3.4.2).
A media capability merely indicates possible support for the media
type and media format(s) in question. In order to actually use a
media capability in an offer/answer exchange, it must be referenced
in a potential configuration.
Media capabilities can be provided at the session-level and/or the
media-level. Media capabilities provided at the session level may be
referenced in any pcfg or lcfg attribute at the media level
(consistent with the media type), whereas media capabilities provided
at the media level may be referenced only by the pcfg or lcfg
attribute within that media stream only. In either case, the scope
of the <med-cap-num> is the entire session description. This enables
each media capability to be uniquely referenced across the entire
session description (e.g. in a potential configuration).
3.3.1. The Media Format Capability Attribute
Media subtypes can be expressed as media format capabilities by use
of the "a=mcap" attribute, which is formatted as follows:
a=mcap:<media-cap-num-list> <encoding-name>/<clock-rate>
[/<encoding-parms>]
where <media-cap-num-list> is a (list of) media capability number(s)
used to number a media format capability, the <encoding name> is the
media subtype e.g. H263-1998 or PCMU, <clock rate> is the encoding
rate, and <encoding parms> are the media encoding parameters for the
media subtype;. All media format capabilities in the list are
assigned to the same media type/subtype. Each occurrence of the mcap
attribute MUST use unique values in its <media-cap-num-list>; the
media capability numbers must be unique across the entire SDP
session. In short, the mcap attribute defines media capabilities and
associates them with a media capability number in the same manner as
the rtpmap attribute defines them and associates them with a payload
type number. Additionally, the mcap attribute allows multiple
Gilman, et al. Expires September 1, 2011 [Page 13]
Internet-Draft SDP Media Capabilities Negotiation February 2011
capability numbers to be defined for the media format in question.
This permits the media format to be associated with different media
parameters in different configurations.
In ABNF, we have:
media-capability-line = "a=mcap:" media-cap-num-list
1*WSP encoding-name "/" clock-rate
["/" encoding-parms]
media-cap-num-list = media-cap-num-element
*["," media-cap-num-element]
media-cap-num-element = media-cap-num
/ media-cap-num-range
media-cap-num-range = media-cap-num "-" media-cap-num
media-cap-num = 1*10(DIGIT)
encoding-name = token ; defined in RFC4566
clock-rate = 1*10(DIGIT)
encoding-parms = token
The encoding-name, clock-rate and encoding-params are as defined to
appear in an rtpmap attribute for each media type/subtype. Thus, it
is easy to convert an mcap attribute line into one or more rtpmap
attribute lines, once a payload type number is assigned to a media-
cap-num (see Section 3.3.5).
The "mcap" attribute can be provided at the session-level and/or the
media-level. There can be more than one mcap attribute at the
session or media level. Each media-cap-num must be unique within the
entire SDP record; it is used to identify that media capability in
potential, latent and actual configurations, and in other attribute
lines as explained below. When used in a potential, latent or actual
configuration it is, in effect, a media level attribute regardless if
it is specified at the session or media level. In other words, the
media capability applies to the specific media description associated
with the configuration which invokes it.
For example:
v=0
a=mcap:1 L16/8000/1
a=mcap:2 L16/16000/2
a=mcap:3,4 H263-1998/90000
m=audio 54320 RTP/AVP 0
a=pcfg:1 m=1|2, pt=1:99,2:98
m=video 66544 RTP/AVP 100
a=rtpmap:100 H264/90000
a=pcfg:10 m=3 pt=3:101
Gilman, et al. Expires September 1, 2011 [Page 14]
Internet-Draft SDP Media Capabilities Negotiation February 2011
3.3.2. The Media Format Parameter Capability Attribute
This attribute is used to associate media format specific format
parameters with one or more media capabilities. The form of the
attribute is:
a=mfcap:<media-caps> <list of parameters>
where <media-caps> permits the list of parameters to be associated
with one or more media capabilities and the format parameters are
specific to the type of media format. The mfcap lines map to a
single traditional SDP fmtp attribute line (one for each entry in
<media-caps>) of the form
a=fmtp:<fmt> <list of parameters>
where <fmt> is the media format description defined in RFC 4566
[RFC4566], as appropriate for the particular media stream. The mfcap
attribute MUST be used to encode attributes for media capabilities,
which would conventionally appear in an fmtp attribute. The existing
acap attribute MUST NOT be used to encode fmtp attributes.
The mfcap attribute adheres to [RFC4566] attribute production rules
with
media-format-capability = "a=mfcap:" media-cap-num-list 1*WSP
fmt-specific-param-list
fmt-specific-param-list = text ; defined in RFC4566
3.3.2.1. Media Format Parameter Concatenation Rule
The appearance of media subtypes with a large number of formatting
options (e.g., AMR-WB [RFC4867]) coupled with the restriction that
only a single fmtp attribute can appear per media format, suggests
that it is useful to create a combining rule for mfcap parameters
which are associated with the same media capability number.
Therefore, different mfcap lines MAY include the same media-cap-num
in their media-cap-num-list. When a particular media capability is
selected for processing, the parameters from each mfcap line which
references the particular capability number in its media-cap-num-list
are concatenated together via ";", in the order the mfcap attributes
appear in the SDP record, to form the equivalent of a single fmtp
attribute line. This permits one to define a separate mfcap line for
a single parameter and value that is to be applied to each media
capability designated in the media-cap-num-list. This provides a
compact method to specify multiple combinations of format parameters
when using codecs with multiple format options. Note that order-
dependent parameters MAY be placed in a single mfcap line to avoid
Gilman, et al. Expires September 1, 2011 [Page 15]
Internet-Draft SDP Media Capabilities Negotiation February 2011
possible problems with line rearrangement by a middlebox.
Format parameters are not parsed by SDP; their content is specific to
the media type/subtype. When format parameters for a specific media
capability are combined from multiple a=mfcap lines which reference
that media capability, the format-specific parameters are
concatenated together and separated by "; " for construction of the
corresponding format attribute (a=fmtp). The resulting format
attribute will look something like the following:
a=fmtp:<fmt> <fmt-specific-param-list1>;
<fmt-specific-param-list2>;
...
where <fmt> depends on the transport protocol in the manner defined
in RFC4566. SDP cannot assess the legality of the resulting
parameter list in the "a=fmtp" line; the user must take care to
ensure that legal parameter lists are generated.
The "mfcap" attribute can be provided at the session-level and the
media-level. There can be more than one mfcap attribute at the
session or media level. The unique media-cap-num is used to
associate the parameters with a media capability.
As a simple example, a G.729 capability is, by default, considered to
support comfort noise as defined by Annex B. Capabilities for G.729
with and without comfort noise support may thus be defined by:
a=mcap:1,2 audio G729/8000
a=mfcap:2 annexb:no
Media format capability 1 supports G.729 with Annex B, whereas media
format capability 2 supports G.729 without Annex B.
Example for H.263 video:
a=mcap:1 video H263-1998/90000
a=mcap:2 video H263-2000/90000
a=mfcap:1 CIF=4;QCIF=2;F=1;K=1
a=mfcap:2 profile=2;level=2.2
Finally, for six format combinations of the Adaptive MultiRate codec:
a=mcap:1-3 AMR/8000/1
a=mcap:4-6 AMR-WB/16000/1
a=mfcap:1,2,3,4 mode-change-capability=1
a=mfcap:5,6 mode-change-capability=2
Gilman, et al. Expires September 1, 2011 [Page 16]
Internet-Draft SDP Media Capabilities Negotiation February 2011
a=mfcap:1,2,3,5 max-red=220
a=mfcap:3,4,5,6 octet-align=1
a=mfcap:1,3,5 mode-set=0,2,4,7
a=mfcap:2,4,6 mode-set=0,3,5,6
So that AMR codec #1, when specified in a pcfg attribute within an
audio stream block (and assigned payload type number 98) as in
a=pcfg:1 m=1 pt=1:98
is essentially equivalent to the following
m=audio 49170 RTP/AVP 98
a=rtpmap:98 AMR/8000/1
a=fmtp:98 mode-change-capability=1; \
max-red=220; mode-set=0,2,4,7
and AMR codec #4 with payload type number 99,depicted by the
potential configuration:
a=pcfg:4 m=4, pt=4:99
is equivalent to the following:
m=audio 49170 RTP/AVP 99
a=rtpmap:99 AMR-WB/16000/1
a=fmtp:99 mode-change-capability=1; octet-align=1; \
mode-set=0,3,5,6
and so on for the other four combinations. SDP could thus convert
the media capabilities specifications into one or more alternative
media stream specifications, one of which can be chosen for the
answer.
3.3.3. The Media-Specific Capability Attribute
Media-specific attributes, beyond the rtpmap and fmtp attributes, may
be associated with media capability numbers via a new media-specific
attribute, mscap, of the following form:
a=mscap:<media caps> <att field> <att value>
Where <media caps> is a (list of) media capability number(s), <att
field> is the attribute name, and <att value> is the value field for
the named attribute. In ABNF, we have:
Gilman, et al. Expires September 1, 2011 [Page 17]
Internet-Draft SDP Media Capabilities Negotiation February 2011
media-specific-capability = "a=mscap:"
media-caps ; defined in 3.3.2
1*WSP att-field ; from RFC4566
1*WSP att-value ; from RFC4566
Given an association between a media capability and a payload type
number as specified by the pt= parameters in an lcfg or pcfg
attribute line, a mscap line may be translated easily into a
conventional SDP attribute line of the form
a=<att field>":"<fmt> <att value> ; <fmt> defined in [RFC4566]
A resulting attribute that is not a legal SDP attribute as specified
by RFC4566 MUST be ignored by the receiver.
A single mscap line may refer to multiple media capabilities; this is
equivalent to multiple mscap lines, each with the same attribute
values, one line per media capability.
Multiple mscap lines may refer to the same media capability, but,
unlike the mfcap attribute, no concatenation operation is defined.
Hence, multiple mscap lines applied to the same media capability is
equivalent to multiple lines of the specified attribute in a
conventional media record.
Here is an example with the rtcp-fb attribute, modified from an
example in [RFC5104] (with the session-level and audio media
omitted). If the offer contains a media block like the following,
m=video 51372 RTP/AVP 98
a=rtpmap:98 H263-1998/90000
a=tcap:1 RTP/AVPF
a=mcap:1 H263-1998/90000
a=mscap:1 rtcp-fb ccm tstr
a=mscap:1 rtcp-fb ccm fir
a=mscap:* rtcp-fb ccm tmmbr smaxpr=120
a=pcfg:1 t=1 m=1 pt=1:98
and if the proposed configuration is chosen, then the equivalent
media block would look like
m=video 51372 RTP/AVPF 98
a=rtpmap:98 H263-1998/90000
a=rtcp-fb:98 ccm tstr
a=rtcp-fb:98 ccm fir
a=rtcp-fb:* ccm tmmbr smaxpr=120
Gilman, et al. Expires September 1, 2011 [Page 18]
Internet-Draft SDP Media Capabilities Negotiation February 2011
3.3.4. New Configuration Parameters
Along with the new attributes for media capabilities, new extension
parameters are defined for use in the potential configuration, the
actual configuration, and/or the new latent configuration defined in
Section 3.3.5.
3.3.4.1. The Media Configuration Parameter (m=)
The media configuration parameter is used to specify the media
encoding(s) and related parameters for a potential, actual, or latent
configuration. Adhering to the ABNF for extension-config-list in
[RFC5939] with
ext-cap-name = "m"
ext-cap-list = media-cap-num-list
[*(BAR media-cap-num-list)]
we have
media-config-list = ["+"]"m=" media-cap-num-list
[*(BAR media-cap-num-list)]
; BAR is defined in RFC5939
; media-cap-num-list is defined above
Alternative media configurations are separated by a vertical bar
("|"). The alternatives are ordered by preference, most-preferred
first. When media capabilities are not included in a potential
configuration at the media level, the media type and media format
from the associated "m=" line will be used. The use of the plus sign
("+") is described in RFC5939.
3.3.4.2. The Payload Type Number Mapping Parameter (pt=)
The payload type number mapping parameter is used to specify the
payload type number to be associated with each media type in a
potential, actual, or latent configuration. We define the payload
type number mapping parameter, payload-number-config-list, in
accordance with the extension-config-list format defined in
[RFC5939]. In ABNF:
payload-number-config-list = ["+"]"pt=" media-map-list
media-map-list = media-map *["," media-map]
media-map = media-cap-num ":" payload-type-number
; media-cap-num is defined in 3.3.1
payload-type-number = 1*3(DIGIT) ; RTP payload type number
/ "*" ; dummy
Gilman, et al. Expires September 1, 2011 [Page 19]
Internet-Draft SDP Media Capabilities Negotiation February 2011
The example in Section 3.3.7 shows how the parameters from the mcap
line are mapped to payload type numbers from the pcfg "pt" parameter.
The use of the plus sign ("+") is desribed in RFC5939.
The "*" value for payload-type-number is used in cases such as BFCP
[RFC4583] in which the fmt list in the m-line is ignored.
A latent configuration represents a future capability, hence the pt=
parameter is not directly meaningful in the lcfg attribute because no
actual media session is being offered or accepted; it is permitted in
order to tie any payload type number parameters within attributes to
the proper media format. A primary example is the case of format
parameters for the RED payload, which are payload type numbers.
Specific payload type numbers used in a latent configuration may be
interpreted as suggestions to be used in any future offer based on
the latent configuration, but they are not binding; the offerer
and/or answerer may use any payload type numbers each deems
appropriate. The use of explicit payload type numbers for latent
configurations can be avoided by use of the parameter substitution
rule of Section 3.3.7 . Future extensions are also permitted.
3.3.4.3. The Media Type Parameter
When a latent configuration is specified (always at the session
level), indicating the ability to support an additional media stream,
it is necessary to specify the media type (audio, video, etc.) as
well as the format and transport type. The media type parameter is
defined in ABNF as
media-type = ["+"] "mt=" media; media defined in RFC4566
At present, the media-type parameter is used only in the latent
configuration attribute, and the use of the "+" prefix to specify
that the entire attribute line is to be ignored if the mt= parameter
is not understood, is unnecessary. However, if the media-type
parameter is later added to an existing capability attribute such as
pcfg, then the "+" would be useful. The media format(s) and
transport type(s) are specified using the media configuration
parameter ("+m=") defined above, and the transport parameter ("t=")
defined in [RFC5939], respectively.
3.3.5. The Latent Configuration Attribute
One of the goals of this work is to permit the exchange of
supportable media configurations in addition to those offered or
accepted for immediate use. Such configurations are referred to as
"latent configurations". For example, a party may offer to establish
a session with an audio stream, and, at the same time, announce its
Gilman, et al. Expires September 1, 2011 [Page 20]
Internet-Draft SDP Media Capabilities Negotiation February 2011
ability to support a video stream as part of the same session. The
offerer can supply its video capabilities by offering one or more
latent video configurations along with the media stream for audio;
the responding party may indicate its ability and willingness to
support such a video session by returning a corresponding latent
configuration.
Latent configurations returned in SDP answers must match offered
latent configurations (or parameter subsets thereof). Therefore, it
is appropriate for the offering party to announce most, if not all,
of its capabilities in the initial offer. This choice has been made
in order to keep the size of the answer more compact by not requiring
acap, mcap, tcap, etc. lines in the answer.
Latent configurations may be announced by use of the latent
configuration attribute, which is defined in a manner very similar to
the potential configuration attribute. The latent configuration
attribute combines the properties of a media line and a potential
configuration. The media type (mt=) and the transport protocol(s)
(t=) MUST be specified since the latent configuration is independent
of any media line present. In most cases, the media configuration
(m=) parameter MUST be present as well (see Section 4 for examples).
The lcfg attribute is a media level attribute and, like a media line,
it ends the session level of the session description if it appears
before any media line.
Each media line in an SDP description represents an offered
simultaneous media stream, whereas each latent configuration
represents an additional stream which may be negotiated in a future
offer/answer exchange. Session capability attributes may be used to
determine whether a latent configuration may be used to form an offer
for an additional simultaneous stream or to reconfigure an existing
stream in a subsequent offer/answer exchange.
The latent configuration attribute is of the form:
a=lcfg:<config-number> <latent-cfg-list>
which adheres to the [RFC4566] "attribute" production with att-field
and att-value defined as:
att-field = "lcfg"
att-value = config-number 1*WSP lcfg-cfg-list
config-number = 1*10(DIGIT) ; defined in RFC5234
lcfg-cfg-list = media-type 1*WSP pot-cfg-list
; as defined in RFC5939
; and extended herein
Gilman, et al. Expires September 1, 2011 [Page 21]
Internet-Draft SDP Media Capabilities Negotiation February 2011
The media-type (mt=) parameter identifies the media type (audio,
video, etc.) to be associated with the latent media stream, and MUST
be present. The pot-cfg-list MUST contain a transport-protocol-
config-list (t=) parameter and a media-config-list (m=) parameter.
The pot-cfg-list MUST NOT contain more than one instance of each type
of parameter list. As specified in [RFC5939], the use of the "+"
prefix with a parameter indicates that the entire configuration MUST
be ignored if the parameter is not understood; otherwise, the
parameter itself may be ignored.
Media stream payload numbers are not assigned by a latent
configuration. Assignment will take place if and when the
corresponding stream is actually offered via an m-line in a later
exchange. The payload-number-config-list is included as a parameter
to the lcfg attribute in case it is necessary to tie payload numbers
in attribute capabilities to specific media capabilities.
If an lcfg attribute invokes an acap attribute that appears at the
session level, then that attribute will be expected to appear at the
session level of a subsequent offer when and if a corresponding media
stream is offered. Otherwise, acap attributes which appear at the
media level represent media-level attributes. Note, however, that
mcap, mfcap, mscap, tcap attributes may appear at the session level
because they always result in media-level attributes or m-line
parameters.
The configuration numbers for latent configurations do not imply a
preference; the offerer will imply a preference when actually
offering potential configurations derived from latent configurations
negotiated earlier. Note however that the offerer of latent
configurations MAY specify preferences for combinations of potential
and latent configurations by use of the sescap attribute defined in
Section 3.3.8. For example, if an SDP offer contains, say, an audio
stream with pcfg:1, and two latent video configurations, lcfg:2, and
lcfg:3, then a session with one audio stream and one video stream
could be specified by including "a=sescap:1 1,2|3". One audio stream
and two video streams could be specified by including "a=sescap:2
1,2,3" in the offer. In order to permit combinations of latent and
potential configurations in session capabilities, latent
configuration numbers MUST be different from those used for potential
configurations. This restriction is especially important if the
offerer does not require cmed-v0 capability and the recipient of the
offer doesn't aupport it. If the lcfg attribute is not recognized,
the capability attributes intended to be associated with it may be
confused with those associated with a potential configuration of some
other media stream.
If a cryptographic attribute, such as the SDES "a=crypto:" attribute
Gilman, et al. Expires September 1, 2011 [Page 22]
Internet-Draft SDP Media Capabilities Negotiation February 2011
[RFC4568], is referenced by a latent configuration through an acap
attribute, any key material REQUIRED in the conventional attribute,
such as the SDES key/salt string, MUST be included in order to
satisfy formatting rules for the attribute. The actual value(s) of
the key material SHOULD be meaningless, and the receiver of the lcfg
attribute MUST ignore the values.
3.3.6. Enhanced Potential Configuration Attribute
The present work requires new extensions (parameters) for the pcfg:
attribute defined in the base protocol [RFC5939] The parameters and
their definitions are "borrowed" from the definitions provided for
the latent configuration attribute in Section 3.3.5. The expanded
ABNF definition of the pcfg attribute is
a=pcfg: <config-number> [<pot-cfg-list>]
where
config-number = 1*DIGIT ;defined in [RFC5234]
pot-cfg-list = pot-config *(1*WSP pot-config)
pot-config = / attribute-config-list / def in [RFC5939]
transport-protocol-config-list / ;defined in [RFC5939]
extension-config-list / ;[RFC5939]
media-config-list / ; Section 3.3.4.1
payload-number-config-list ; Section 3.3.4.2
Except for the extension-config-list, the pot-cfg-list MUST NOT
contain more than one instance of each parameter list.
3.3.6.1. Returning Capabilities in the Answer
Potential and/or latent configuration attributes may be returned
within an answer SDP to indicate the ability of the answerer to to
support alternative configurations of the corresponding stream(s).
For example, an offer may include multiple potential configurations
for a media stream and/or latent configurations for additional
streams; the corresponding answer will indicate (via an acfg
attribute) the configuration accepted and used to construct the base
configuration for each active media stream in the reply, but the
reply MAY also contain potential and/or latent configuration
attributes, with parameters, to indicate which other offered
configurations would be acceptable. This information is useful if it
becomes desirable to reconfigure a media stream, e.g., to reduce
resource consumption.
When potential and/or latent configurations are returned in an
answer, all numbering MUST refer to the configuration and capability
Gilman, et al. Expires September 1, 2011 [Page 23]
Internet-Draft SDP Media Capabilities Negotiation February 2011
attribute numbering of the offer. The offered capability attributes
need not be returned in the answer. The answer MAY include
additional capability attributes and/or configuratons (with distinct
numbering). The parameter values of any returned pcfg or lcfg
attributes MUST be a subset of those included in the offered
configurations or those added by the answerer; values may be omitted
only if they were indicated as alternative sets, or optional, in the
original offer. The parameter set indicated in the returned acfg
attribute need not be repeated in a returned pcfg attribute. The
answerer may return more than one pcfg attribute with the same
configuration number if it is necessary to describe selected
combinations of optional or alternative parameters.
Similarly, one or more session capability attributes (a=sescap) may
be returned to indicate which of the offered session capabilities is/
are supportable by the answerer (see Section 3.3.8.)
Note that, although the answerer MAY return capabilities beyond those
included by the offerer, these capabilities MUST NOT be used to form
any base level media description in the answer. For this reason, it
seems advisable for the offerer to include most, if not all,
potential and latent configurations it can support in the initial
offer. Either party MAY later announce additional capabilities by
renegotiating the session in a second offer/answer exchange.
3.3.6.2. Payload Type Number Mapping
When media capabilities defined in mcap attributes are used in
potential configuration lines, and the transport protocol uses RTP,
it is necessary to assign payload type numbers to them. In some
cases, it is desirable to assign different payload type numbers to
the same media capability when used in different potential
configurations. One example is when configurations for AVP and SAVP
are offered: the offerer would like the answerer to use different
payload type numbers for encrypted and unencrypted media so that it
(the offerer) can decide whether or not to render early media which
arrives before the answer is received. This association of distinct
payload type number(s) with different transport protocols requires a
separate pcfg line for each protocol. Clearly, this technique cannot
be used if the number of potential configurations exceeds the number
of possible payload type numbers.
3.3.6.3. Processing of Media-Format-Related Conventional Attributes for
Potential Configurations
In cases in which media capabilities negotiation is employed, SDP
records are likely to contain conventional attributes such as rtpmap,
fmtp, and other media-format-related lines, as well as capability
Gilman, et al. Expires September 1, 2011 [Page 24]
Internet-Draft SDP Media Capabilities Negotiation February 2011
attributes such as mcap, mfcap, and mscap which map into those
conventional attributes when invoked by a potential configuration.
In such cases, it MAY be appropriate to employ the a delete-
attributes option in the attribute configuration list parameter in
order to avoid the generation of conflicting fmtp attributes for a
particular configuration. Any media-specific attributes in the media
block which refer to payload type numbers not used by the potential
configuration MUST be ignored.
For example:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
a=creq:med-v0
m=audio 3456 RTP/AVP 0 18 100
a=rtpmap:100 telephone-events
a=fmtp:100 0-11
a=mcap:1 PCMU/8000
a=mcap:2 g729/8000
a=mcap:3 telephone-events/8000
a=mfcap:3 0-15
a=pcfg:1 m=2,3|1,3 a=-m pt=1:0,2:18,3:100
a=pcfg:2
In this example, PCMU is media capability 1, G729 is media capability
2, and telephone-event is media capability 3. The a=pcfg:1 line
specifies that the preferred configuration is G.729 with extended
dtmf events, second is G.711 mu-law with extended dtmf events, and
the base media-level attributes are to be deleted. Intermixing of
G.729, G.711, and "commercial" dtmf events is least preferred (the
base configuration provided by the "m=" line, which is, by default,
the least preferred configuration). The rtpmap and fmtp attributes
of the base configuration are replaced by the mcap and mfcap
attributes when invoked by the proposed configuration.
If the preferred configuration is selected, the SDP answer will look
like
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
a=csup:med-v0
m=audio 6543 RTP/AVP 18 100
Gilman, et al. Expires September 1, 2011 [Page 25]
Internet-Draft SDP Media Capabilities Negotiation February 2011
a=rtpmap:100 telephone-events/8000
a=fmtp:100 0-15
a=pcfg:1 m=2,3|1,3 a=-m pt=1:0,2:18,3:100
a=pcfg:2
a=acfg:1 m=2,3 pt=1:0,2:18,3:100
3.3.7. Substitution of Media Payload Type Numbers in Capability
Attribute Parameters
In some cases, for example, when an RFC 2198 redundancy audio subtype
(RED) capability is defined in an mfcap attribute, the parameters to
an attribute may contain payload type numbers. Two options are
available for specifying such payload type numbers. They may be
expressed explicitly, in which case they are bound to actual payload
types by means of the payload type number parameter (pt=) in the
appropriate potential or latent configuration. For example, the
following SDP fragment defines a potential configuration with
redundant G.711 mu-law:
m=audio 45678 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=mcap:1 PCMU/8000
a=mcap:2 RED/8000
a=mfcap:2 0/0
a=pcfg:1 m=2,1 pt=2:98,1:0
The potential configuration is then equivalent to
m=audio 45678 RTP/AVP 98 0
a=rtpmap:0 PCMU/8000
a=rtpmap:98 RED/8000
a=fmtp:98 0/0
A more general mechanism is provided via the parameter substitution
rule. When an mfcap, mscap, or acap attribute is processed, its
arguments will be scanned for a payload type number escape sequences
of the following form (in ABNF):
ptn-esc = "%m=" media-cap-num "%" ; defined in 3.3.1
If the sequence is found, the sequence is replaced by the payload
type number assigned to the media capability number, as specified by
the pt= parameter in the selected potential configuration. The
sequence "%%" (null digit string) is replaced by a single percent
sign and processing continues with the next character, if any.
For example, the above offer sequence could have been written as
Gilman, et al. Expires September 1, 2011 [Page 26]
Internet-Draft SDP Media Capabilities Negotiation February 2011
m=audio 45678 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=mcap:1 PCMU/8000
a=mcap:2 RED/8000
a=mfcap:2 %m=1%/%m=1%
a=pcfg:1 m=2,1 pt=2:98,1:0
and the equivalent SDP is the same as above.
3.3.8. The Session Capability Attribute
The session capability attribute provides a means for the offerer
and/or the answerer to specify combinations of specific media stream
configurations which it is willing and able to support. Each session
capability in an offer or answer MAY be expressed as a list of
required potential configurations, and MAY include a list of optional
potential and/or latent configurations.
The choices of session capabilities may be based on processing load,
total bandwidth, or any other criteria of importance to the
communicating parties. If the answerer supports media capabilities
negotiation, and session configurations are offered, it MUST accept
one of the offered configurations, or it MUST refuse the session.
Therefore, if the offer includes any session capabilities, it SHOULD
include all the session capabilities the offerer is willing to
support.
The session capability attribute is described by:
"a=sescap:" <session num> <list of configs>
which corresponds to the standard attribute definition with
att-field = "sescap"
att-value = session-num 1*WSP list-of-configs
[1*WSP optional-configs]
session-num = 1*DIGIT ; defined in RFC5234
list-of-configs = alt-config *["," alt-config]
optional-configs = "[" list-of-configs "]"
alt-config = config-number *["|" config-number]
; config-number defined in RFC5939
The session-num identifies the session; a lower-number session is
preferred over a higher-numbered session. Each alt-config list
specifies alternative media configurations within the session;
preference is based on config-num as specified in [RFC5939]. Note
that the session preference order, when present, takes precedence
over the individual media stream configuration preference order.
Gilman, et al. Expires September 1, 2011 [Page 27]
Internet-Draft SDP Media Capabilities Negotiation February 2011
Use of session capability attributes requires that configuration
numbers assigned to potential and latent configurations MUST be
unique across the entire session; [RFC5939] requires only that pcfg
configuration numbers be unique within a media description.
As an example, consider an endpoint that is capable of supporting an
audio stream with either one H.264 video stream or two H.263 video
streams with a floor control stream. The SDP offer might look like
the following:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
a=creq:med-v0
a=sescap:2 1,2,3,5
a=sescap:1 1,4
m=audio 54322 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=pcfg:1
m=video 22344 RTP/AVP 102
a=rtpmap:102 H263-1998/90000
a=fmtp:102 CIF=4;QCIF=2;F=1;K=1
i= main video stream
a=label:11
a=pcfg:2
a=mcap:1 H264/90000
a=mfcap:1 profile-level-id=42A01E; packetization-mode=2
a=acap:1 label:13
a=pcfg:4 m=1 a=1 pt=1:104
m=video 33444 RTP/AVP 103
a=rtpmap:103 H263-1998/90000
a=fmtp:103 CIF=4;QCIF=2;F=1;K=1
i= secondary video (slides)
a=label:12
a=pcfg:3
m=application 33002 TCP/BFCP *
a=setup:passive
a=connection:new
a=floorid:1 m-stream:11 12
a=floor-control:s-only
a=confid:4321
a=userid:1234
a=pcfg:5
If the answerer understands MediaCapNeg, but cannot support the
Binary Floor Control Protocol, then it would respond with:
Gilman, et al. Expires September 1, 2011 [Page 28]
Internet-Draft SDP Media Capabilities Negotiation February 2011
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.22
t=0 0
a=cusp:med-v0
a=sescap:1 1,4
m=audio 23456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=acfg:1
m=video 41234 RTP/AVP 104
a=rtpmap:100 H264/90000
a=fmtp:104 profile-level-id=42A01E; packetization-mode=2
a=acfg:4 m=1 a=1 pt=1:104
a=pcfg:2
m=video 0 RTP/AVP 103
a=acfg:3
m=application 0 TCP/BFCP *
a=acfg:5
An endpoint that doesn't support Media capabilities negotiation, but
does support H.263 video, would respond with one or two H.263 video
streams. In the latter case, the answerer may issue a second offer
to reconfigure the session to one audio and one video channel using
H.264 or H.263.
Session capabilities can include latent capabilities as well. Here's
a similar example in which the offerer wishes to initially establish
an audio stream, and prefers to later establish two video streams
with chair control. If the answerer doesn't understand Media CapNeg,
or cannot support the dual video streams or flow control, then it may
support a single H.264 video stream. Note that establishment of the
most favored configuration will require two offer/answer exchanges.
h
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
a=creq:med-v0
a=sescap:1 1,3,4,5
a=sescap:2 1,2
a=sescap:3 1
a=mcap:1 H263-1998/90000
a=mfcap:1 CIF=4;QCIF=2;F=1;K=1
a=tcap:1 RTP/AVP TCP/BFCP
Gilman, et al. Expires September 1, 2011 [Page 29]
Internet-Draft SDP Media Capabilities Negotiation February 2011
m=audio 54322 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=label:11
a=pcfg:1
m=video 22344 RTP/AVP 102
a=rtpmap:102 H264/90000
a=fmtp:102 profile-level-id=42A01E; packetization-mode=2
a=label:11
a=content:main
a=pcfg:2
a=lcfg:3 mt=video t=1 m=1 a=31,32 i=3
a=acap:31 label:12
a=acap:32 content:main
a=lcfg:4 mt=video t=1 m=1 a=41,42 i=4
a=acap:41 label:13
a=acap:42 content:slides
a=lcfg:5 mt=application m=51 t=51
a=tcap:51 TCP/BFCP
a=mcap:51 *
a=acap:51 setup:passive
a=acap:52 connection:new
a=acap:53 floorid:1 m-stream:12 13
a=acap:54 floor-control:s-only
a=acap:55 confid:4321
a=acap:56 userid:1234
In this example, the default offer, as seen by endpoints which do not
understand capabilities negotiation, proposes a PCMU audio stream and
an H.264 video stream. Note that the offered lcfg lines for the
video streams don't carry pt= parameters because they're not needed
(payload type numbers will be assigned in the offer/answer exchange
that establishes the streams). Note also that the three mcap, mfcap,
and tcap attributes used by lcfg:3 and lcfg:4 are included at the
session level so they may be referenced by both latent
configurations. As per Section 3.3, the media attributes generated
from the mcap, mfcap, and tcap attributes are always media-level
attributes. If the answerer supports Media CapNeg, and supports the
most desired configuration, it would return the following SDP:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.22
t=0 0
a=csup:med-v0
a=sescap:1 1,3,4,5
a=sescap:2 1,2
a=sescap:3 1
Gilman, et al. Expires September 1, 2011 [Page 30]
Internet-Draft SDP Media Capabilities Negotiation February 2011
m=audio 23456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=acfg:1
m=video 0 RTP/AVP 102
a=pcfg:2
a=lcfg:3 mt=video t=1 m=1 a=31,32
a=lcfg:4 mt=video t=1 m=1 a=41,42
a=lcfg:5 mt=application t=2
This exchange supports immediate establishment of an audio stream for
preliminary conversation. This exchange would presumably be followed
at the appropriate time with a "reconfiguration" offer/answer
exchange to add the video and chair control streams.
3.4. Offer/Answer Model Extensions
In this section, we define extensions to the offer/answer model
defined in RFC3264 [RFC3264] and [RFC5939] to allow for media
capabilities, bandwidth capabilities, and latent configurations to be
used with the SDP Capability Negotiation framework.
The [RFC5939] provides a relatively compact means to offer the
equivalent of an ordered list of alternative media stream
configurations (as would be described by separate m= lines and
associated attributes). The attributes acap, mscap, mfcap and mcap
are designed to map somewhat straightforwardly into equivalent m=
lines and conventional attributes when invoked by a pcfg, lcfg, or
acfg attribute with appropriate parameters. The a=pcfg: lines, along
with the m= line itself, represent offered media configurations. The
a=lcfg: lines represent alternative capabilities for future use.
3.4.1. Generating the Initial Offer
When an endpoint generates an initial offer and wants to use the
functionality described in the current document, it should identify
and define the codecs it can support via mcap, mfcap and mscap
attributes. The SDP media line(s) should be made up with the
configuration to be used if the other party does not understand
capability negotiations (by default, this is the least preferred
configuration). Typically, the media line configuration will contain
the minimum acceptable capabilities. The offer MUST include the
level of capability negotiation extensions needed to support this
functionality in a "creq" attribute.
Preferred configurations for each media stream are identified
following the media line. The present offer may also include latent
configuration (lcfg) attributes, at the session level, describing
media streams and/or configurations the offerer is not now offering,
Gilman, et al. Expires September 1, 2011 [Page 31]
Internet-Draft SDP Media Capabilities Negotiation February 2011
but which it is willing to support in a future offer/answer exchange.
A simple example might be the inclusion of a latent video
configuration in an offer for an audio stream.
3.4.2. Generating the Answer
When the answering party receives the offer and if it supports the
required capability negotiation extensions, it should select the
most-preferred configuration it can support for each media stream,
and build its answer accordingly. The configuration selected for
each accepted media stream is placed into the answer as a media line
with associated parameters and attributes. If a proposed
configuration is chosen, the answer must include the supported
extension attribute and each media stream for which a proposed
configuration was chosen must contain an actual configuration (acfg)
attribute to indicate just which pcfg attribute was used to build the
answer. The answer should also include any potential or latent
configurations the answerer can support, especially any
configurations compatible with other potential or latent
configurations received in the offer. The answerer should make note
of those configurations it might wish to offer in the future.
3.4.3. Offerer Processing of the Answer
When the offerer receives the answer, it should make note of any
capabilities and/or latent configurations for future use. The media
line(s) must be processed in the normal way to identify the media
stream(s) accepted by the answer, if any. The acfg attribute, if
present, may be used to verify the proposed configuration used to
form the answer, and to infer the lack of acceptability of higher-
preference configurations that were not chosen. Note that the base
specification [RFC5939] requires the answerer to choose the highest
preference configuration it can support, subject to local policies.
3.4.4. Modifying the Session
If, at a later time, one of the parties wishes to modify the
operating parameters of a session, e.g., by adding a new media
stream, or by changing the properties used on an existing stream, it
may do so via the mechanisms defined for offer/answer [RFC3264]. If
the initiating party has remembered the codecs, potential
configurations, and latent configurations announced by the other
party in the earlier negotiation, it may use this knowledge to
maximize the likelihood of a successful modification of the session.
Alternatively, the initiator may perform a new capabilities exchange
as part of the reconfiguration. In such a case, the new capabilities
will replace the previously-negotiated capabilities. This may be
useful if conditions change on the endpoint.
Gilman, et al. Expires September 1, 2011 [Page 32]
Internet-Draft SDP Media Capabilities Negotiation February 2011
4. Examples
In this section, we provide examples showing how to use the Media
Capabilities with the SDP Capability Negotiation.
4.1. Alternative Codecs
This example provide a choice of one of six variations of the
adaptive multirate codec. In this example, the default configuration
as specified by the media line is the same as the most preferred
configuration. Each configuration uses a different payload type
number so the offerer can interpret early media.
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
a=creq:med-v0
m=audio 54322 RTP/AVP 96
rtpmap:96 AMR-WB/16000/1
a=fmtp:96 mode-change-capability=1; max-red=220; \
mode-set=0,2,4,7
a=mcap:1,3,5 audio AMR-WB/16000/1
a=mcap:2,4,6 audio AMR/8000/1
a=mfcap:1,2,3,4 mode-change-capability=1
a=mfcap:5,6 mode-change-capability=2
a=mfcap:1,2,3,5 max-red=220
a=mfcap:3,4,5,6 octet-align=1
a=mfcap:1,3,5 mode-set=0,2,4,7
a=mfcap:2,4,6 mode-set=0,3,5,6
a=pcfg:1 m=1 pt=1:96
a=pcfg:2 m=2 pt=2:97
a=pcfg:3 m=3 pt=3:98
a=pcfg:4 m=4 pt=4:99
a=pcfg:5 m=5 pt=5:100
a=pcfg:6 m=6 pt=6:101
In the above example, media capability 1 could have been excluded
from the first mcap declaration and from the corresponding mfcap
attributes, and the pcfg:1 attribute line could have been simply
"pcfg:1".
The next example offers a video stream with three options of H.264
and 4 transports. It also includes an audio stream with different
audio qualities: four variations of AMR, or AC3. The offer looks
something like:
Gilman, et al. Expires September 1, 2011 [Page 33]
Internet-Draft SDP Media Capabilities Negotiation February 2011
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=An SDP Media NEG example
c=IN IP4 192.0.2.1
t=0 0
a=creq:med-v0
a=ice-pwd:speEc3QGZiNWpVLFJhQX
m=video 49170 RTP/AVP 100
c=IN IP4 192.0.2.56
a=maxprate:1000
a=rtcp:51540
a=sendonly
a=candidate 12345 1 UDP 9 192.0.2.56 49170 host
a=candidate 23456 2 UDP 9 192.0.2.56 51540 host
a=candidate 34567 1 UDP 7 10.0.0.1 41345 srflx raddr \
192.0.2.56 rport 49170
a=candidate 45678 2 UDP 7 10.0.0.1 52567 srflx raddr \
192.0.2.56 rport 51540
a=candidate 56789 1 UDP 3 192.0.2.100 49000 relay raddr \
192.0.2.56 rport 49170
a=candidate 67890 2 UDP 3 192.0.2.100 49001 relay raddr \
192.0.2.56 rport 51540
b=AS:10000
b=TIAS:10000000
b=RR:4000
b=RS:3000
a=rtpmap:100 H264/90000
a=fmtp:100 profile-level-id=42A01E; packetization-mode=2; \
sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==; \
sprop-interleaving-depth=45; sprop-deint-buf-req=64000; \
sprop-init-buf-time=102478; deint-buf-cap=128000
a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF
a=mcap:1-3,7-9 H264/90000
a=mcap:4-6 rtx/90000
a=mfcap:1-9 profile-level-id=42A01E
a=mfcap:1-9 aMljiA==
a=mfcap:1,4,7 packetization-mode=0
a=mfcap:2,5,8 packetization-mode=1
a=mfcap:3,6,9 packetization-mode=2
a=mfcap:1-9 sprop-parameter-sets=Z0IACpZTBYmI
a=mfcap:1,7 sprop-interleaving-depth=45; \
sprop-deint-buf-req=64000; sprop-init-buf-time=102478; \
deint-buf-cap=128000
a=mfcap:4 apt=100
a=mfcap:5 apt=99
a=mfcap:6 apt=98
a=mfcap:4-6 rtx-time=3000
a=mscap:1-6 rtcp-fb nack
Gilman, et al. Expires September 1, 2011 [Page 34]
Internet-Draft SDP Media Capabilities Negotiation February 2011
a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_80 \
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|220|1:32
a=pcfg:1 t=1 m=1,4 a=1 pt=1:100,4:97
a=pcfg:2 t=1 m=2,5 a=1 pt=2:99,4:96
a=pcfg:3 t=1 m=3,6 a=1 pt=3:98,6:95
a=pcfg:4 t=2 m=7 a=1 pt=7:100
a=pcfg:5 t=2 m=8 a=1 pt=8:99
a=pcfg:6 t=2 m=9 a=1 pt=9:98
a=pcfg:7 t=3 m=1,3 pt=1:100,4:97
a=pcfg:8 t=3 m=2,4 pt=2:99,4:96
a=pcfg:9 t=3 m=3,6 pt=3:98,6:95
m=audio 49176 RTP/AVP 101 100 99 98
c=IN IP4 192.0.2.56
a=ptime:60
a=maxptime:200
a=rtcp:51534
a=sendonly
a=candidate 12345 1 UDP 9 192.0.2.56 49176 host
a=candidate 23456 2 UDP 9 192.0.2.56 51534 host
a=candidate 34567 1 UDP 7 10.0.0.1 41348 srflx \
raddr 192.0.2.56 rport 49176
a=candidate 45678 2 UDP 7 10.0.0.1 52569 srflx \
raddr 192.0.2.56 rport 51534
a=candidate 56789 1 UDP 3 192.0.2.100 49002 relay \
raddr 192.0.2.56 rport 49176
a=candidate 67890 2 UDP 3 192.0.2.100 49003 relay \
raddr 192.0.2.56 rport 51534
b=AS:512
b=TIAS:512000
b=RR:4000
b=RS:3000
a=maxprate:120
a=rtpmap:98 AMR-WB/16000
a=fmtp:98 octet-align=1; mode-change-capability=2
a=rtpmap:99 AMR-WB/16000
a=fmtp:99 octet-align=1; crc=1; mode-change-capability=2
a=rtpmap:100 AMR-WB/16000/2
a=fmtp:100 octet-align=1; interleaving=30
a=rtpmap:101 AMR-WB+/72000/2
a=fmtp:101 interleaving=50; int-delay=160000;
a=mcap:14 ac3/48000/6
a=acap:23 crypto:1 AES_CM_128_HMAC_SHA1_80 \
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|220|1:32
a=tcap:4 RTP/SAVP
a=pcfg:10 t=4 a=23
a=pcfg:11 t=4 m=14 a=23 pt=14:102
This offer illustrates the advantage in compactness that arises if
Gilman, et al. Expires September 1, 2011 [Page 35]
Internet-Draft SDP Media Capabilities Negotiation February 2011
one can avoid deleting the base configuration attributes and
recreating them in acap attributes for the potential configurations.
4.2. Alternative Combinations of Codecs (Session Configurations)
If an endpoint has limited signal processing capacity, it might be
capable of supporting, say, a G.711 mu-law audio stream in
combination with an H.264 video stream, or a G.729B audio stream in
combination with an H.263-1998 video stream. It might then issue an
offer like the following:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
a=creq:med-v0
a=sescap:1 2,4
a=sescap:2 1,3
m=audio 54322 RTP/AVP 18
a=rtpmap:18 G729/8000
a=fmtp:18 annexb=yes
a=mcap:1 PCMU/8000
a=pcfg:1 m=1 pt=1:0
a=pcfg:2
m=video 54344 RTP/AVP 100
a=rtpmap:100 H263-1998/90000
a=mcap:2 H264/90000
a=mfcap:2 profile-level-id=42A01E; packetization-mode=2
a=pcfg:3 m=2 pt=2:101
a=pcfg:4
Note that the preferred session configuration (and the default as
well) is G.729B with H.263. This overrides the individual media
stream preferences which are PCMU and H.264 by the potential
configuration numbering rule.
4.3. Latent Media Streams
Consider a case in which the offerer can support either G.711 mu-law,
or G.729B, along with DTMF telephony events for the 12 common
touchtone signals, but is willing to support simple G.711 mu-law
audio as a last resort. In addition, the offerer wishes to announce
its ability to support video in the future, but does not wish to
offer a video stream at present. The offer might look like the
following:
Gilman, et al. Expires September 1, 2011 [Page 36]
Internet-Draft SDP Media Capabilities Negotiation February 2011
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
a=creq:med-v0
m=audio 23456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=mcap:1 PCMU/8000
a=mcap:2 g729/8000
a=mcap:3 telephone-event/8000
a=mfcap:3 0-11
a=pcfg:1 m=1,3|2,3 pt=1:0,2:18,3:100 a=lcfg:10 mt=video t=1
m=10|11
a=mcap:10 H263-1998/90000
a=mcap:11 H264/90000
a=tcap:1 RTP/AVP
The lcfg attribute line announces support for H.263 and H.264 video
(H.263 preferred) for future reference. The m-line and the rtpmap
attribute offer an audio stream and provide the lowest precedence
configuration (PCMU without any DTMF encoding). The mcap lines
define the media capabilities (PCMU, G729, and telephone-event) to be
offered in potential configurations. The mfcap attribute provides
the format parameters for telephone-events, specifying the 12
commercial DTMF 'digits'. The pcfg attribute line defines the most-
preferred media configuration as PCMU plus DTMF events and the next-
most-preferred configuration as G.729B plus DTMF events.
If the answerer is able to support all the potential configurations,
and also support H.263 video (but not H.264), it would reply with an
answer like:
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
t=0 0
a=csup:med-v0
m=audio 54322 RTP/AVP 0 100
a=rtpmap:0 PCMU/8000
a=rtpmap:100 telephone-event/8000
a=fmtp:100 0-11
a=acfg:1 m=1,3 pt=1:0,3:100
a=pcfg:1 m=2,3 pt=2:18,3:100 a=lcfg:1 mt=video t=1 m=10
The lcfg attribute line announces the capability to support H.263
video at a later time. The media line and subsequent rtpmap and fmtp
Gilman, et al. Expires September 1, 2011 [Page 37]
Internet-Draft SDP Media Capabilities Negotiation February 2011
attribute lines present the selected configuration for the media
stream. The acfg attribute line identifies the potential
configuration from which it was taken, and the pcfg attribute line
announces the potential capability to support G.729 with DTMF events
as well. If, at some later time, congestion becomes a problem in the
network, either party may, with expectation of success, offer a
reconfiguration of the media stream to use G.729 in order to reduce
packet sizes.
Gilman, et al. Expires September 1, 2011 [Page 38]
Internet-Draft SDP Media Capabilities Negotiation February 2011
5. IANA Considerations
5.1. New SDP Attributes
The IANA is hereby requested to register the following new SDP
attributes:
Attribute name: mcap
Long form name: media capability
Type of attribute: session-level and media-level
Subject to charset: no
Purpose: associate media capability number(s) with
media subtype and encoding parameters
Appropriate Values: see Section 3.3.1
Attribute name: mfcap
Long form name: media format capability
Type of attribute: session-level and media-level
Subject to charset: no
Purpose: associate media format attributes and
parameters with media format capabilities
Appropriate Values: see Section 3.3.2
Attribute name: mscap
Long form name: media-specific capability
Type of attribute: session-level and media-level
Subject to charset: no
Purpose: associate media-specific attributes and
parameters with media capabilities
Appropriate Values: see Section 3.3.3
Attribute name: lcfg
Long form name: latent configuration
Type of attribute: media-level
Subject to charset: no
Purpose: to announce supportable media streams
without offering them for immediate use.
Appropriate Values: see Section 3.3.5
Attribute name: sescap
Long form name: session capability
Type of attribute: session-level
Subject to charset: no
Purpose: to specify and prioritize acceptable
combinations of media stream configurations.
Appropriate Values: see Section 3.3.8
Gilman, et al. Expires September 1, 2011 [Page 39]
Internet-Draft SDP Media Capabilities Negotiation February 2011
5.2. New SDP Option Tag
The IANA is hereby requested to add the new option tag "med-v0",
defined in this document, to the SDP Capability Option Negotiation
Capability registry created for [RFC5939].
5.3. New SDP Capability Negotiation Parameters
The IANA is hereby requested to expand the SDP Capability Negotiation
Potential Configuration Parameter Registry established by [RFC5939]
to become the SDP Capability Negotiation Configuration Parameter
Registry and to include parameters for the potential, actual and
latent configuration attributes. The new parameters to be registered
are the "m" for "media", "pt" for "payload type number", and "mt" for
"media type" parameters. Note that the "mt" parameter is defined for
use only in the latent configuration attribute.
Gilman, et al. Expires September 1, 2011 [Page 40]
Internet-Draft SDP Media Capabilities Negotiation February 2011
6. Security Considerations
The security considerations of [RFC5939] apply for this document.
The addition of negotiable media encoding, bandwidth attributes, and
connection data in this specification can cause problems for
middleboxes which attempt to control bandwidth utilization, media
flows, and/or processing resource consumption as part of network
policy, but which do not understand the media capability negotiation
feature. As for the initial CapNeg work, the SDP answer is
formulated in such a way that it always carries the selected media
encoding and bandwidth parameters for every media stream selected.
Pending an understanding of capabilities negotiation, the middlebox
should examine the answer SDP to obtain the best picture of the media
streams being established.
As always, middleboxes can best do their job if they fully understand
media capabilities negotiation.
Gilman, et al. Expires September 1, 2011 [Page 41]
Internet-Draft SDP Media Capabilities Negotiation February 2011
7. Changes from previous versions
7.1. Changes from version 10
o Defined the latent configuration attribute as a media-level
attribute because it specifies a possible future media stream.
Added text to clarify how to specify alternative configurations of
a single latent stream and/or multiple streams.
o Improved the definition of the session capability attribute to
permit both required configurations and optional configurations -
latent configurations cannot be required because they have not yet
been offered.
o Removed the special-case treatment of conflicts between base-level
fmtp attributes and fmtp attributes generated for a configuration
via invoked mcap and mfcap attributes.
o Removed reference to bandwidth capability (bcap) attribute.
o Changed various "must", etc., terms to normative terms ("MUST",
etc.) as appropriate, in Section 3.3.5Section 3.3.6.1
Section 3.3.6.3 and Section 3.3.8
o Attempted to clarify the substitution mechanism in Section 3.3.7
and improve its uniqueness.
o Made various editorial changes, including changing the title in
the header, and removing numbering from some SDP examples.
7.2. Changes from version 09
o Additional corrections to latent media stream example in
Section 4.3
o Fixed up attribute formatting examples and corresponding ABNF.
o Removed preference rule for latent configurations.
o Various spelling and other editorial changes were made.
o updated crossreferences.
7.3. Changes from version 08
The major change is in Section 4.3, Latent Media Streams, fixing the
syntax of the answer. All the other changes are editorial.
Gilman, et al. Expires September 1, 2011 [Page 42]
Internet-Draft SDP Media Capabilities Negotiation February 2011
7.4. Changes from version 04
o The definitions for bcap, ccap, icap, and kcap attributes have
been removed, and are to be defined in another document.
o Corrected formatting of m= and p= configuration parameters to
conform to extension-config-list form defined in [RFC5939]
o Reorganized definitions of new parameters to make them easier to
find in document.
o Added ability to renegotiate capabilities when modifying the
session (Section 3.4.4).
o Made various editorial changes, clarifications, and typo
corrections.
7.5. Changes from version 03
o A new session capability attribute (sescap) has been added to
permit specification of acceptable media stream combinations.
o Capability attribute definitions corresponding to the i, c, b, and
k SDP line types have been added for completeness.
o Use of the pcfg: attribute in SDP answers has been included in
order to conveniently return information in the answer about
acceptable configurations in the media stream offer.
o The use of the lcfg: attribute(s) in SDP answers has been
restricted to indicate just which latent configuration offers
would be acceptable to the answerer.
o A suggestion for "naive" middleboxes has been added to the
Security Considerations.
o Various editorial changes have been made.
o Several errors/omissions have been corrected.
o The description of the mscap attribute has been modified to make
it clear that it should not be used to generate undefined SDP
attributes, or to "extend" existing attributes.
o <ms-parameters> are made optional in the mscap attribute
definition.
Gilman, et al. Expires September 1, 2011 [Page 43]
Internet-Draft SDP Media Capabilities Negotiation February 2011
o "AMR" changed to "AMR-WB" in cases in which the sample rate is
16000.
7.6. Changes from version 02
This version contains several detail changes intended to simplify
capability processing and mapping into conventional SDP media blocks.
o The "mcap" attribute is enhanced to include the role of the "ecap"
attribute; the latter is eliminated.
o The "fcap" attribute has been renamed "mfcap". New replacement
rules vis-a-vis fmtp attributes in the base media specification
have been added.
o A new "mscap" attribute is defined to handle the problem of
attributes (other than rtpmap and fmtp) that are specific to a
particular payload type.
o New rules for processing the mcap, mfcap, and mscap attributes,
and overriding standard rtpmap, fmtp, or other media-specific
attributes, are put forward to reduce the need to use the deletion
option in the a= parameter of the potential configuration (pcfg)
attribute.
o A new parameter, "mt=" is added to the latent configuration
attribute (lcfg) to specify the media stream type (audio, video,
etc.) when the lcfg is declared at the session level.
o The examples are expanded.
o Numerous typos and misspellings have been corrected.
7.7. Changes from version 01
The documents adds a new attribute for specifying bandwidth
capability and a parametr to list in the potential configuration.
Other changes are to align the document with the terminolgy and
attribute names from draft-ietf-mmusic-sdp-capability-negotiation-07.
The document also clarifies some previous open issues.
7.8. Changes from version 00
The major changes include taking out the "mcap" and "cptmap"
parameter. The mapping of payload type is now in the "pt" parameter
of "pcfg". Media subtype need to explictly definesd in the "cmed"
attribute if referenced in the "pcfg"
Gilman, et al. Expires September 1, 2011 [Page 44]
Internet-Draft SDP Media Capabilities Negotiation February 2011
8. Acknowledgements
This document is heavily influenced by the discussions and work done
by the SDP Capability Negotiation Design team. The following people
in particular provided useful comments and suggestions to either the
document itself or the overall direction of the solution defined
herein: Cullen Jennings, Matt Lepinski, Joerg Ott, Colin Perkins, and
Thomas Stach.
We thank Ingemar Johansson and Magnus Westerlund for examples that
stimulated this work, and for critical reading of the document.
Gilman, et al. Expires September 1, 2011 [Page 45]
Internet-Draft SDP Media Capabilities Negotiation February 2011
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5939] Andreasen, F., "SDP Capability Negotiation", RFC 5939,
September 2010.
9.2. Informative References
[RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol (SDP) Security Descriptions for Media
Streams", RFC 4568, July 2006.
[RFC4583] Camarillo, G., "Session Description Protocol (SDP) Format
for Binary Floor Control Protocol (BFCP) Streams",
RFC 4583, November 2006.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
July 2006.
[RFC4733] Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF
Digits, Telephony Tones, and Telephony Signals", RFC 4733,
December 2006.
[RFC4867] Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie,
"RTP Payload Format and File Storage Format for the
Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband
(AMR-WB) Audio Codecs", RFC 4867, April 2007.
[RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
"Codec Control Messages in the RTP Audio-Visual Profile
with Feedback (AVPF)", RFC 5104, February 2008.
Gilman, et al. Expires September 1, 2011 [Page 46]
Internet-Draft SDP Media Capabilities Negotiation February 2011
Authors' Addresses
Robert R Gilman
Independent
3243 W. 11th Ave. Dr.
Broomfield, CO 80020
USA
Email: bob_gilman@comcast.net
Roni Even
Gesher Erove Ltd
14 David Hamelech
Tel Aviv 64953
Israel
Email: ron.even.tlv@gmail.com
Flemming Andreasen
Cisco Systems
Edison, NJ
USA
Email: fandreas@cisco.com
Gilman, et al. Expires September 1, 2011 [Page 47]