draft-ietf-mmusic-sdp-media-capabilities-01.txt   draft-ietf-mmusic-sdp-media-capabilities-02.txt 
MMUSIC R. Gilman MMUSIC R. Gilman
Internet-Draft Avaya, Inc. Internet-Draft Avaya, Inc.
Intended status: Standards Track R. Even, Ed. Intended status: Standards Track R. Even, Ed.
Expires: August 24, 2007 Polycom Expires: May 18, 2008 Polycom
F. Andreasen F. Andreasen
Cisco Systems Cisco Systems
February 20, 2007 November 15, 2007
SDP media capabilities Negotiation SDP media capabilities Negotiation
draft-ietf-mmusic-sdp-media-capabilities-01.txt draft-ietf-mmusic-sdp-media-capabilities-02.txt
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Copyright Notice Copyright Notice
Copyright (C) The IETF Trust (2007). Copyright (C) The IETF Trust (2007).
Abstract Abstract
Session Description Protocol (SDP) capability negotiation provides a Session Description Protocol (SDP) capability negotiation provides a
general framework for negotiating capabilities in SDP. The base general framework for negotiating capabilities in SDP. The base
framework defines only capabilities for negotiating transport framework defines only capabilities for negotiating transport
protocols and attributes. In this document, we extend the framework protocols and attributes. In this document, we extend the framework
by defining media capabilities that can be used to negotiate media by defining media capabilities that can be used to negotiate media
types and their associated parameters. types and their associated parameters.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. SDP Media capabilities . . . . . . . . . . . . . . . . . . . . 5 3. SDP Media capabilities . . . . . . . . . . . . . . . . . . . . 6
3.1. Solution Overview . . . . . . . . . . . . . . . . . . . . 5 3.1. Solution Overview . . . . . . . . . . . . . . . . . . . . 6
3.2. Capability Attributes . . . . . . . . . . . . . . . . . . 8 3.2. Capability Attributes . . . . . . . . . . . . . . . . . . 9
3.2.1. Media Type and Subtype Capability Attribute . . . . . 8 3.2.1. Media Type and Subtype Capability Attribute . . . . . 10
3.2.2. The Capability Encoding Parameters Attribute . . . . . 9 3.2.2. The Media Encoding Parameter Capability Attribute . . 11
3.2.3. The Media Format Parameter Capability Attribute . . . 10 3.2.3. The Media Format Parameter Capability Attribute . . . 12
3.3. Extensions to the Potential Configuration Attribute . . . 11 3.2.4. The Bandwidth Capability Attribute . . . . . . . . . . 13
3.3. Extensions to the Potential Configuration Attribute . . . 15
3.3.1. The Media Capability Extension to the Potential 3.3.1. The Media Capability Extension to the Potential
Configuration Attribute . . . . . . . . . . . . . . . 11 Configuration Attribute . . . . . . . . . . . . . . . 15
3.3.2. The Payload Type Mapping Extension to the 3.3.2. The Payload Type Mapping Extension to the
Potential Configuration Attribute . . . . . . . . . . 12 Potential Configuration Attribute . . . . . . . . . . 16
3.4. Extensions to the Actual Configuration Attribute . . . . . 12 3.3.3. The Bandwidth Extension to the Potential
3.5. The Latent Configuration Attribute . . . . . . . . . . . . 13 Configuration Attribute . . . . . . . . . . . . . . . 17
3.5.1. The crypto: Attribute in Latent Configurations . . . . 14 3.4. Extensions to the Actual Configuration Attribute . . . . . 17
3.6. Offer/Answer Model Extensions . . . . . . . . . . . . . . 14 3.5. The Latent Configuration Attribute . . . . . . . . . . . . 18
3.6.1. Generating the Initial Offer . . . . . . . . . . . . . 14 3.5.1. The crypto: Attribute in Latent Configurations . . . . 18
3.6.2. Generating the Answer . . . . . . . . . . . . . . . . 14 3.6. Offer/Answer Model Extensions . . . . . . . . . . . . . . 19
3.6.3. Offerer Processing of the Answer . . . . . . . . . . . 14 3.6.1. Generating the Initial Offer . . . . . . . . . . . . . 19
3.6.4. Modifying the Session . . . . . . . . . . . . . . . . 14 3.6.2. Generating the Answer . . . . . . . . . . . . . . . . 19
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3.6.3. Offerer Processing of the Answer . . . . . . . . . . . 19
4.1. Alternative Codecs . . . . . . . . . . . . . . . . . . . . 15 3.6.4. Modifying the Session . . . . . . . . . . . . . . . . 20
4.2. Latent Media Streams . . . . . . . . . . . . . . . . . . . 15 4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 4.1. Alternative Codecs . . . . . . . . . . . . . . . . . . . . 21
6. Security Considerations . . . . . . . . . . . . . . . . . . . 17 4.2. Latent Media Streams . . . . . . . . . . . . . . . . . . . 21
7. Changes from version 00 . . . . . . . . . . . . . . . . . . . 18 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19 6. Security Considerations . . . . . . . . . . . . . . . . . . . 23
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20 7. Changes from previous versions . . . . . . . . . . . . . . . . 24
9.1. Normative References . . . . . . . . . . . . . . . . . . . 20 7.1. Changes from version 01 . . . . . . . . . . . . . . . . . 24
9.2. Informative References . . . . . . . . . . . . . . . . . . 20 7.2. Changes from version 00 . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25
Intellectual Property and Copyright Statements . . . . . . . . . . 22 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
9.1. Normative References . . . . . . . . . . . . . . . . . . . 26
9.2. Informative References . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27
Intellectual Property and Copyright Statements . . . . . . . . . . 28
1. Introduction 1. Introduction
Session Description Protocol (SDP) capability negotiation [SDPCapNeg] Session Description Protocol (SDP) capability negotiation [SDPCapNeg]
provides a general framework for negotiating capabilities in provides a general framework for negotiating capabilities in
SDP[RFC4566]. The base framework defines only capabilities for SDP[RFC4566]. The base framework defines only capabilities for
negotiating transport protocols and attributes. In this document, we negotiating transport protocols and attributes.
extend the framework by defining media capabilities that can be used
to negotiate media types and their associated parameters.
SDP Simple Capability Declaration (simcap) is defined in RFC 3407 SDP Simple Capability Declaration (simcap) is defined in RFC 3407
[RFC3407]. It defines a set of SDP attributes that enables a limited [RFC3407]. It defines a set of SDP attributes that enables a limited
set of capabilities to be described at a session level or on a per set of capabilities to be described at a session level or on a per
media stream basis. RFC 3407 defines capability declaration only - media stream basis. RFC 3407 defines capability declaration only.
actual negotiation procedures taking advantage of such capabilities Actual negotiation procedures taking advantage of such capabilities
have not been defined. The SDP capability negotiation framework adds have not been defined. The SDP capability negotiation framework adds
this required functionality. This document updates RFC3407 and new this required functionality. This document updates RFC3407 and new
implementation SHOULD use the functionality defined in the current implementation SHOULD use the functionality defined in the current
draft to negotiate media capabilities. draft to negotiate media capabilities.
The [SDPCapNeg] 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 currently.
In this document, we extend the framework by defining media
capabilities that can be used to negotiate media types and their
associated parameters like bandwidth. This document also adds the
capability to declare support for media streams that can be offered
later rather than as a potential configuration for current use.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [RFC2119] and document are to be interpreted as described in RFC2119 [RFC2119] and
indicate requirement levels for compliant RTP implementations. indicate requirement levels for compliant RTP implementations.
3. SDP Media capabilities 3. SDP Media capabilities
In this section we first provide an overview of the SDP media The SDP media capability negotiation [SDPCapNeg] discusses the use of
Capability negotiation solution. This is followed by definitions of any SDP [RFC4566] attribute (a=) under the acap attribute. The
new SDP attributes for the solution and its associated updated offer/ limitations of using acap for fmtp and rtpmap as potential
answer procedures [RFC3264] configuration are listed, for example they can be used only at the
media level since they are media level attributes. The [SDPCapNeg]
addresses the issue of bandwidth usage but does not offer a way to
negotiate different bandwidth for different codecs and profiles.
This section provides an overview of extensions providing 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. Solution Overview 3.1. Solution Overview
The solution consist of the following new attributes extending the The solution consist of the following new attributes extending the
base attributes from [SDPCapNeg]. base attributes from [SDPCapNeg].
Three attributes are used to make up media capabilities Four attributes are used to make up media capabilities in a manner
that can be related to the capabilities specified in a media line,
its attributes and the bandwidth parameter lines.
o A new media attribute ("a=cmed") that lists media formats as o A new media attribute ("a=mcap") that lists media formats as
capabilities in the form a media type (e.g. "audio") and one or capabilities in the form a media type (e.g. "audio") and one or
more subtypes (e.g. "PCMU"), and associates a handle with each more subtypes (e.g. "PCMU"), and associates a handle with each
subtype subtype
o A new attribute ("a=cenc") that lists encoding parameter o A new attribute ("a=ecap") that lists encoding parameter
capabilities associated with a particular media format capability. capabilities associated with a one or more media format
capabilities.
o A new attribute ("a=cfmt") that lists media format parameter o A new attribute ("a=fcap") that lists media format parameter
capabilities associated with a particular media format capability. capabilities associated with a one or more media format
capabilities.
A new attribute ("a=lcfg") that specifies latent configurations when o A new attribute ("a=bcap") that is used to specify the bandwidth
no corresponding media line is offered. An example is a latent parameter(s) for media capabilities.
configuration for video even though no video is currently offered.
A new parameter type ("m=") to the potential configuration o A new attribute ("a=lcfg") that specifies latent configurations
when no corresponding media line is offered. An example is a
latent configuration for video even though no video is currently
offered.
o A new parameter type ("m=") to the potential configuration
("a=pcfg:") attribute and the actual configuration ("a=acfg:") ("a=pcfg:") attribute and the actual configuration ("a=acfg:")
attribute defined in [SDPCapNeg], which permits specification of attribute defined in [SDPCapNeg], which permits specification of
media capabilities (including their associated parameters) and media capabilities (including their associated parameters) and
combinations thereof for the configuration. For example, the combinations thereof for the configuration. For example, the
"a=pcfg:" line might specify PCUM and telephone events or G.729B and "a=pcfg:" line might specify PCMU and telephone events or G.729B
telephone events as acceptable configurations. The "a=acfg:" line in and telephone events as acceptable configurations. The "a=acfg:"
the answer would specify the accepted choice. line in the answer would specify the accepted choice.
A new parameter type ("pt=") to the potential configuration o A new parameter type ("pt=") to the potential configuration
("a=pcfg:") attribute which associates RTP payload types with the ("a=pcfg:") attribute which associates RTP payload types with the
referenced media capabilities. referenced media capabilities.
o A new parameter type ("b=") used to specify bandwidth parameters
in a potential configuration.
The document extends the base protocol extensions to the offer/answer The document extends the base protocol extensions to the offer/answer
model that allow for capabilities and potential configurations to be model that allow for capabilities and potential configurations to be
included in an offer. Media capabilities constitute capabilities included in an offer. Media capabilities constitute capabilities
that can be used in potential and latent configurations. Whereas that can be used in potential and latent configurations. Whereas
potential configurations constitute alternative offers that may be potential configurations constitute alternative offers that may be
accepted by the answerer instead of the actual configuration(s) accepted by the answerer instead of the actual configuration(s)
included in the "m=" line(s), latent configurations merely inform the included in the "m=" line(s), latent configurations merely inform the
other side of possible configurations supported by the entity. Those other side of possible configurations supported by the entity. Those
latent configurations may be used to guide subsequent offer/answer latent configurations may be used to guide subsequent offer/answer
exchanges. exchanges.
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| | | |
Alice's offer includes RTP and SRTP as alternatives. RTP is the Alice's offer includes RTP and SRTP as alternatives. RTP is the
default, but SRTP is the preferred one: default, but SRTP is the preferred one:
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
a=creq:v1 a=creq:med-v0
a=cmed:1 audio g729 iLBC PCMU g729 a=mcap:1 audio g729 iLBC PCMU g729
a=cenc:2 8000 a=ecap:2 8000
a=cfmt:1 annexb:no a=fcap:1 annexb:no
a=ctrpr:1 RTP/SAVP a=fcap:2 mode=20
a=tcap:1 RTP/SAVP
m=audio 3456 RTP/AVP 0 18 m=audio 3456 RTP/AVP 0 18
a=capar:1 a=crypto:1 AES_CM_128_HMAC_SHA1_32 a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=pcfg:1 m=1,3|4,3 t=1 a=1 pt=1:100,4:101,3:102 a=pcfg:1 m=1,3|4,3 t=1 a=1 pt=1:100,4:101,3:102
a=pcfg:2 m=2 pt=2:103 a=pcfg:2 m=2 pt=2:103
The required base and extensions are provided by the "a=creq" The required base and extensions are provided by the "a=creq"
attribute defined in [SDPCapNeg], with the option tag "v1", which attribute defined in [SDPCapNeg], with the option tag "med-v0", which
indicates that the extension framework defined here, must be indicates that the extension framework defined here, must be
supported. The Base level support is implied since it is required supported. The Base level support is implied since it is required
for the extensions. for the extensions.
The "a=cmed:1" line defines four audio media subtype capabilities , The "a=mcap:1" line defines four audio media subtype capabilities ,
to be numbered consecutively starting with 1. Note that the media to be numbered consecutively starting with 1. Note that the media
subtypes specified in the m-line (PCMU and G729) are explicitly subtypes specified in the m-line (PCMU and G729) are explicitly
specified here. specified here.
The "a=cenc:2" line specifies the clock rate and encoding parmeters The "a=ecap:2" line specifies the clock rate and encoding parmeters
(see [RFC4566]) for capability 2, iLBC. (see [RFC4566]) for capability 2, iLBC.
The "a=cfmt:1" line specifies media format parameter capabilities for The "a=fcap:1" line specifies media format parameter capabilities for
codec 1 ( no Annex B for G.729). codec 1 ( no Annex B for G.729).
The "a=ctrpr:1" line, specified in the base protocol, defines a The "a=fcap:2" line specifies a media format parameter for codec 2
(20ms packets for iLBC. [RFC3952]
The "a=tcap:1" line, specified in the base protocol, defines a
transport protocol capability, in this case Secure RTP. transport protocol capability, in this case Secure RTP.
The "m=" line indicates that Alice is offering to use plain RTP with The "m=" line indicates that Alice is offering to use plain RTP with
PCMU or G.729. The media line implicitly defines the default PCMU or G.729. The media line implicitly defines the default
transport protocol (RTP/AVP in this case) and the default actual transport protocol (RTP/AVP in this case) and the default actual
configuration. configuration.
The "a=capar:1" line ,specified in the base protocol provides the The "a=acap:1" line ,specified in the base protocol provides the
"crypto" attribute which provides the keying material for SRTP using "crypto" attribute which provides the keying material for SRTP using
SDP security descriptions. SDP security descriptions.
The "a=pcfg:" attributes provide the potential configurations The "a=pcfg:" attributes provide the potential configurations
included in the offer by reference to the media capabilities, included in the offer by reference to the media capabilities,
transport capabilities, and associated payload type mappings. Two transport capabilities, and associated payload type mappings. Two
explicit alternatives are provided; the first one, numbered 1 is the explicit alternatives are provided; the first one, numbered 1 is the
preferred one. It specifies media capabilities 1 and 3, i.e. G.729 preferred one. It specifies media capabilities 1 and 3, i.e. G.729
and PCMU, or media capability 4 and 3, i.e., G.729B and PCMU. and PCMU, or media capability 4 and 3, i.e., G.729B and PCMU.
Furthermore, it specifies transport protocol capability 1 (i.e. the Furthermore, it specifies transport protocol capability 1 (i.e. the
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Bob receives the SDP offer from Alice. Bob supports RTP, but not Bob receives the SDP offer from Alice. Bob supports RTP, but not
SRTP, and hence he accepts the actual configuration for RTP provided SRTP, and hence he accepts the actual configuration for RTP provided
by Alice. Furthermore, Bob wants to use the iLBC codec and hence by Alice. Furthermore, Bob wants to use the iLBC codec and hence
generates the following answer: generates the following answer:
v=0 v=0
o=- 24351 621814 IN IP4 192.0.2.2 o=- 24351 621814 IN IP4 192.0.2.2
s= s=
c=IN IP4 192.0.2.2 c=IN IP4 192.0.2.2
t=0 0 t=0 0
a=csup:v1 a=csup:med-v0
m=audio 4567 RTP/AVP 103 m=audio 4567 RTP/AVP 103
a=rtpmap:103 8000 a=rtpmap:103 iLBC/8000
a=fmtp:103 mode=20
a=acfg:2 a=acfg:2
Bob includes the "a=csup" and "a=acfg" attribute in the answer to Bob includes the "a=csup" and "a=acfg" attribute in the answer to
inform Alice that he can support the v1 level of capability inform Alice that he can support the med-v0 level of capability
negotiations. Note that in this particular example, the answerer negotiations. Note that in this particular example, the answerer
supported the capability extensions defined here, however had he not, supported the capability extensions defined here, however had he not,
he would simply have processed the offer based on the offered PCMU he would simply have processed the offer based on the offered PCMU
and G.729 codecs under the RTP/AVP profile only. Consequently, the and G.729 codecs under the RTP/AVP profile only. Consequently, the
answer would have omitted the "a=csup" attribute line and chosen one 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 or both of the PCMU and G.729 codecs instead. The answer carries the
accepted configuration in the m line along with corresponding rtpmap: accepted configuration in the m line along with corresponding rtpmap:
and/or fmtp: parameters, as appropriate. and/or fmtp: parameters, as appropriate.
Note that per the base protocol, after the above, Alice should Note that per the base protocol, after the above, Alice MAY generate
generate a new offer with an actual configuration ("m=" line, etc.) a new offer with an actual configuration ("m=" line, etc.)
corresponding to the actual configuration referenced in Bob's answer corresponding to the actual configuration referenced in Bob's answer
(not shown here). (not shown here).
3.2. Capability Attributes 3.2. Capability Attributes
In this section, we present the new attributes associated with In this section, we present the new attributes associated with
indicating the media capabilities for use by the SDP Capability indicating the media capabilities for use by the SDP Capability
negotiation. The approach taken is to keep things similar to the negotiation. The approach taken is to keep things similar to the
existing media capabilities defined by the existing media existing media capabilities defined by the existing media
descriptions ("m=" lines) and the associated "rtpmap" and "fmtp" descriptions ("m=" lines) and the associated "rtpmap" and "fmtp"
attributes, but using "media capability numbers" instead of payload attributes. We use media subtypes and "media capability numbers"
types to link the relevant media capability parameters. instead of payload types to link the relevant media capability
parameters. This permits the capabilities to be defined at the
session level and used for multiple streams, if desired. Payload
types are then specified at the media level (see Section 3.2.2).
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 (see Section 2.3.1).
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 an lcfg attribute at the session level, or by any pcfg
attribute at the media level, whereas media capabilities provided at
the media level may be referenced by a pcfg 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)
3.2.1. Media Type and Subtype Capability Attribute 3.2.1. Media Type and Subtype Capability Attribute
Media types and subtypes can be expressed as media format Media types and subtypes can be expressed as media format
capabilities by use of the "a=cmed" attribute, which is defined as capabilities by use of the "a=mcap" attribute, which is defined as
follows: follows:
a=cmed:<med-cap-num> <type> <subtype>*[ <subtype>] a=mcap:<med-cap-num> <subtype>*[ <subtype>]
where <med-cap-num> is an integer between 1 and 2^31-1 (both where <med-cap-num> is an integer between 1 and 2^31-1 (both
included) used to number the media format capabilities, <type> is a included) used to number the media format capabilities and the
media type (e.g., audio or video), and the <subtype> is the media <subtype> is the media subtype e.g. H263-1998, PCMU. The <med-cap-
subtype e.g. H263-1998, PCMU (Editors' note: can specify in cmed num> is the media capability number associated with the first subtype
anything that can be specified in an m-line?). The <med-cap-num> is in the list, the number associated with the second subtype is one
the media capability number associated with the first subtype in the higher, etc. Each occurrence of the attribute MUST use a different
list, the number associated with the second subtype is one higher, value of <med-cap-num>. Furthermore, when an "mcap" attribute
etc. Each occurrence of the attribute MUST use a different value of indicates more than one media format, the capability numbers implied
<med-cap-num>. Furthermore, when a "cmed" attribute indicates more MUST NOT be used by any other "mcap" attribute in the entire session
than one media format, the capability numbers implied MUST NOT be or media description (explicitly or implicitly).
used by any other "cmed" attribute in the entire session or media
description (explicitly or implicitly). In ABNF, we have:
media-capability-line = "a=cmed:" media-cap-num WSP media-type In ABNF, we have:
WSP media-cap *(WSP media-cap)
media-capability-line = "a=mcap:" media-cap-num WSP media-type
1*WSP media-cap *(WSP media-cap)
media-cap-num = 1*DIGIT media-cap-num = 1*DIGIT
media-type = token ; Type name (audio, video, etc.)
media-cap = token ; Subtype name(PCMU, G729, etc.) media-cap = token ; Subtype name(PCMU, G729, etc.)
The "mcap" attribute can be provided at the session-level and the
media-level. There can be more than one mcap attribute at the
session or media level. The unique media-cap-num is used to identify
it in potential configurations. When used in a potential
configuration it is a media level attribute regardless if it is
specified at the session or media level.
Media subtypes identified in "a=cmed" lines may be qualified via the For example:
attributes, "a=cenc" and "a=cfmt", in much the same way as media
formats in "m=" lines payload types can have their clock rate and
encoding parameters qualified by an "a=rtpmap" line and media format
specific parameters can be provided by "a=fmtp" lines.
3.2.2. The Capability Encoding Parameters Attribute v=0
a=mcap:1 iLBC
a= mcap:2 H263-1998, H264
3.2.2. The Media Encoding Parameter Capability Attribute
Media format capabilities may require additional encoding parameters, Media format capabilities may require additional encoding parameters,
such as sample rate, to be precisely defined. The "a=cenc" encoding such as sample rate, to be precisely defined. The "a=ecap" encoding
attribute is defined as attribute is defined as
a=cenc:<med-cap-num> <clock rate>[/<encoding parameters>] a=ecap:<med-cap-num-list> <clock rate>[/<encoding parameters>]
where
med-cap-num-list = <med-cap-num> *[COMMA<med-cap-num>]
The clock rate and other encoding parameters are as defined for the The clock rate and other encoding parameters are as defined for the
"a=rtpmap:" attribute defined in RFC 4566 [RFC4566]. For example, a "a=rtpmap:" attribute defined in RFC 4566 [RFC4566].
capability for low-bit-rate encoding at 8000 samples per second could
be specified by
a=cmed:1 iLBC The "ecap" attribute can be provided at the session-level and the
a=cenc:1 8000 media-level. There can be more than one ecap attribute at the
session or media level. The unique media-cap-num is used to identify
it in potential configurations. When used in a potential
configuration it is a media level attribute regardless if it is
specified at the session or media level.
For example, a capability for low-bit-rate encoding at 8000 samples
per second could be specified by
a=mcap:1 iLBC
a=ecap:1 8000
The encoding becomes part of the media capability. Thus, if it is The encoding becomes part of the media capability. Thus, if it is
desirable to specify the same subtype with, e.g., two different desirable to specify the same subtype with, e.g., two different
encoding rates, then the subtype should be listed twice, and each encoding rates, then the subtype should be listed twice, and each
should be modified appropriately. For example: should be modified appropriately. For example:
a=cmed:1 L16 L16 a=mcap:1 L16 L16
a=cenc:1 8000 a=ecap:1 8000
a=cenc:2 16000/2 a=ecap:2 16000/2
defines two low-data-rate codecs, codec 1 uses 8000 samples per defines two low-data-rate codecs, codec 1 uses 8000 samples per
second, and codec 2 uses 16000 samples per second and 2 channels. second, and codec 2 uses 16000 samples per second and 2 channels.
[EDITOR'S NOTE: I'm thinking that it might be better to put the
encoding information in the cmed line pcfg "pt", along with the media
subtype, and eliminate the cenc attribute. This would make the above
example look like:
a=cmed:1 L16/8000 L16/16000/2 3.2.3. The Media Format Parameter Capability Attribute
or
a=pcfg:1 m=1,3|4,3 t=1 a=1 pt=1:100/8000, 4:101/16000/2, 3:102 This attribute is used to associate media format parameters with a
media capability. The form of the attribute is:
This can be done so long as the encoding attributes never contain a=fcap:<med-cap-num-list> <list of format parameters>
whitespace. Is this the case? or instead use semi colon as
separator]
A media capability merely indicates possible support for the media where <med-cap-num-list> permits the format parameter(s) to be
type and media format(s) in question. In order to actually use a associated with one or more media capabilities, and the format
media capability in an offer/answer exchange, it must be referenced parameters are specific to the type of codec(s), as described for the
in a potential configuration (see Section 2.3.1. ). fmtp: attribute defined in RFC 4566[RFC4566]. Unlike the fmtp
attribute, multiple fcap attributes may be associated with a
specified med-cap-num. This provides the ability to mix and match
format parameters when using codecs with multiple format options.
Media capabilities can be provided at the session-level and the The fcap attribute adheres to RFC 4566 attribute production rules
media-level. Media capabilities provided at the session level apply with
to the session description in general, whereas media capabilities
provided at the media level apply to 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 referenced
across the entire session description (e.g. in a potential
configuration.)
3.2.3. The Media Format Parameter Capability Attribute media-format-capability-line = "a=fcap:" <att-value>
This attribute is used to associate media format parameters with a att-value = <med-cap-num-list> WSP <format-specific-parameter-
media capability. The form of the attribute is: list>
a=cfmt:<med-cap-num> <list of format parameters> med-cap-num-list = <med-cap-num> *["," <med-cap-num>] ; med-cap-
num is defined in Section 3.2.1
where the format parameters are specific to the type of codec, as format-specific-parameter-list = <format-specfic-parameter>
described for the fmtp: attribute defined in RFC 4566. As an *[";"<format-specfic-parameter>] ; format-specific-parameter-
example, a G.729 capability is, by default, considered to support list are defined per codec/capability.
comfort noise as defined by Annex B. Capabilities for G.729 with and
without comfort noise support may thus be identified by:
a=cmed:1 audio G729 G729 The "fcap" attribute can be provided at the session-level and the
a=cfmt:2 annexb:no media-level. There can be more than one fcap attribute at the
session or media level. The unique media-cap-num is used to identify
it in potential configurations. When used in a potential
configuration it is a media level attribute regardless if it is
specified at the session or media level.
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 identified by:
a=mcap:1 audio G729 G729
a=fcap:2 annexb:no
Example for H.263 video: Example for H.263 video:
a=cmed:1 video H263-1998 H263-2000 a=mcap:1 video H263-1998 H263-2000
a=cfmt:1 CIF=4;QCIF=2;F=1;K=1 a=fcap:1 CIF=4;QCIF=2;F=1;K=1
a=cfmt:2 profile=2;level=2.2 a=fcap:2 profile=2;level=2.2
Finally, for six format combinations of the Adaptive MultiRate codec:
a=mcap:1 audio AMR AMR AMR AMR AMR AMR
a=ecap:1,3,5 16000/1
a=ecap:2,4,6 8000/1
a=fcap:1,2,3,4 mode-change-capability=1
a=fcap:5,6 mode-change-capability=2
a=fcap:1,2,3,5 max-red=220
a=fcap:3,4,5,6 octet-align=1
a=fcap:1,3,5 mode-set=0,2,4,7
a=fcap:2,4,6 mode-set=0,3,5,6
So that AMR codec #1, when specified in a pcfg attribute (and
assigned payload type 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/16000/1
a=fmtp:98 mode-change-capability=1; max-red=220; mode-
set=0,2,4,7
and AMR codec #4 with payload type 99, is essentially equivalent to
the following:
m=audio 49170 RTP/AVP 99
a=rtpmap:99 AMR/8000/1
a=fmtp:99 mode-change-capability=1; octet-align=1; mode-
set=0,3,5,6
and so on for the other four combinations.
3.2.4. The Bandwidth Capability Attribute
In some cases it is desirable to specify different bandwidth limits
for different media configurations. This may be done by use of the
"a=bcap" attribute, which is defined as follows:
a=bcap:<bw-cap-num> <bwtype>:<bandwidth>
where <bw-cap-num> is an integer between 1 and 2^31-1 (both included)
used to identify the bandwidth capability, <bwtype> is the bandwidth
type, and <bandwidth> is the bandwidth value, as defined for the b=
line in RFC4566[RFC4566]
In ABNF, we have:
media-bandwidth-cap-line = "a=bcap:" bw-cap-num 1*WSP
bwtype ":" bandwidth
where
bw-cap-num = 1*DIGIT; the bandwidth "handle"
bwtype = token; as defined in RFC4566
bandwidth = 1*DIGIT; as defined in RFC4566
The "bcap" attribute can appear at the session level, where it can be
referenced by lcfg or pcfg attributes, or at the media level, where
it can be referenced by pcfg attributes. When invoked by a pcfg or
lcfg attribute, the resulting bandwidth line (b=) is to be
interpreted at the media-level. There can be more then one bcap
attribute. The unique bw-cap-num is used to identify it in potential
configurations.
Note: The session level bandwidth gives a maximum total values for
all the media in the session. Currently the authors suggest that
enforcing a potential limit can be done at the media level and by
using re-INVITE for the session level bandwidth.
Bandwidth capabilities may be included in a potential configuration
via the "b=" parameter (see below). Any bandwidth capability
included replaces any media-level bandwidth of the same type declared
in a "b=" SDP line.
The following example offers a preferred potential configuration for
H.263 QCIF at 360 Kbit/sec and a second potential configuration for
H.263 CIF at the offered 500 Kbit/sec
m=video 49170 RTP/AVP 99
b=TIAS:500000
a=rtpmap:99 H263-1998/90000
a=fmtp:99 CIF=4; QCIF=2
a=mcap:1 video H263-1998 H263-1998
a=fcap:1 QCIF=2
a=fcap:2 CIF=4; QCIF=2;F=1;K=1
a=bcap:1 TIAS:360000
a=pcfg:1 m=1 b=1 pt:100
a=pcfg:2 m=2 pt:101
3.3. Extensions to the Potential Configuration Attribute 3.3. Extensions to the Potential Configuration Attribute
The extension protocol of capabilities negotiation requires two new The extension protocol of capabilities negotiation requires three new
extensions for the pcfg: attribute defined in the base protocol. One extensions for the pcfg: attribute defined in the base protocol. The
extension permits the specification of media capabilities, or first extension permits the specification of media capabilities, or
combinations thereof; the other permits the assignment of payload combinations thereof; the second permits the assignment of payload
types to those capabilities when used in the specified configuration. types to those capabilities when used in the specified configuration;
the third permits the specification of bandwidth limits for a media
stream.
3.3.1. The Media Capability Extension to the Potential Configuration 3.3.1. The Media Capability Extension to the Potential Configuration
Attribute Attribute
The potential configuration attribute ("a=pcfg") as defined in SDP The potential configuration attribute ("a=pcfg") as defined in SDP
capabilities negotiation, permits alternate attributes to be capabilities negotiation, permits alternate attributes to be
associated with the media types defined in a media line. In this associated with the media types defined in a media line. In this
extension (this document), we define an extension parameter for the document, we define an extension parameter for the specification of
specification of media configurations in addition to the one media configurations in addition to the one specified on the media
specified on the media line. line.
We define the media capability configuration parameter, pot-media- We define the media capability configuration parameter, pot-media-
config, in accordance with the following format: config, in accordance with the following format:
m=<med-cap-list> *["|"<med-cap-list>] m=<med-cap-list> *["|"<med-cap-list>]
where <med-cap-list> is a comma-separated list of media capability where <med-cap-list> is a comma-separated list of media capability
numbers (media-cap-num) as defined by a=cmed: lines and media lines. numbers (media-cap-num) as defined by a=mcap: lines and media lines.
In ABNF form (adhering to the ABNF for pot-extension-config in In ABNF form (adhering to the ABNF for pot-extension-config in
[SDPCapNeg]: [SDPCapNeg]:
pot-media-config = "m=" med-cap-list *(BAR med-cap-list) pot-media-config = "m=" med-cap-list *(BAR med-cap-list)
med-cap-list = med-cap-num *("," med-cap-num) med-cap-list = med-cap-num *("," med-cap-num)
med-cap-num = 1*DIGIT ; defined in SDP med-cap-num = 1*DIGIT ; defined in SDP
BAR = *WSP "|" *WSP BAR = *WSP "|" *WSP
Each potential media configuration is a comma-separated list of media Each potential media configuration is a comma-separated list of media
capability numbers where med-cap-num refers to media capability capability numbers where med-cap-num refers to media capability
numbers defined explicitly by a=cmed attributes or implicitly by the numbers defined explicitly by a=mcap attributes and hence MUST be
media line, and hence MUST be between 1 and 2^31-1 (both included). between 1 and 2^31-1 (both included). Alternative potential media
Alternative potential media configurations are separated by a configurations are separated by a vertical bar ("|"). The
vertical bar ("|"). The alternatives are ordered by preference. alternatives are ordered by preference. When media capabilities are
When media capabilities are not included in a potential configuration not included in a potential configuration at the media level, the
at the media level, the media type and media format from the media type and media format from the associated "m=" line will be
associated "m=" line will be used. used.
For example: For example:
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
a=creq:v1 a=creq:med-v0
a=cmed:1 audio PCMU g729 telephone-event
m=audio 3456 RTP/AVP 0 18 100 m=audio 3456 RTP/AVP 0 18 100
a=rtpmap:100 telephone-events a=rtpmap:100 telephone-events
a=fmtp:100 0-15 a=fmtp:100 0-15
a=mcap:1 audio PCMU g729 telephone-event
a=ecap:1,2,3 8000
a=fcap:3 0-15
a=pcfg:1 m=2,3|1,3 pt=1:0, 2:18, 3:100 a=pcfg:1 m=2,3|1,3 pt=1:0, 2:18, 3:100
In this example, G729 is media capability 2, PCMU is media capability In this example, G729 is media capability 2, PCMU is media capability
1, and events is media capability 3. The a=pcfg: line specifies that 1, and events is media capability 3. The a=pcfg: line specifies that
the preferred configuration is G.729 with dtmf events, second is the preferred configuration is G.729 with dtmf events, second is
G.711 mu-law with dtmf events. Intermixing of G.729, G.711, and dtmf G.711 mu-law with dtmf events. Intermixing of G.729, G.711, and dtmf
events is least preferred (the actual configuration provided by the events is least preferred (the actual configuration provided by the
"m=" line, which is always the least preferred configuration). "m=" line, which is always the least preferred configuration).
3.3.2. The Payload Type Mapping Extension to the Potential 3.3.2. The Payload Type Mapping Extension to the Potential
Configuration Attribute Configuration Attribute
When media capabilities defined in cmed: attributes are used in When media capabilities defined in mcap: attributes are used in
potential configuration lines, it is necessary to assign payload potential configuration lines, it is necessary to assign payload
types to them. In some cases, it is desirable to assign different types to them. In some cases, it is desirable to assign different
payload types to media capabilities defined in the media line. One payload types to media capabilities defined in the media line. One
example of the latter is when configurations for AVP and SAVP are example of the latter is when configurations for AVP and SAVP are
offered: the offerer would like the answerer to use different payload offered: the offerer would like the answerer to use different payload
types for encrypted and unencrypted media so that it (the offerer) types for encrypted and unencrypted media so that it (the offerer)
can decide whether or not to render early media which arrives before can decide whether or not to render early media which arrives before
the answer is received. the answer is received.
We define the media type mapping configuration parameter, pot-media- We define the media type mapping configuration parameter, pt-media-
map, in accordance with the following format: map, in accordance with the following format:
pot-media-map = "pt=" med-map-num *("," *WSP med-map-num) pt-media-map = "pt=" med-map *("," med-map);
med-map-num = 1*DIGIT ":" 1*DIGIT ; med-map = <med-cap-num> ":" <'payload-type> ;
med-cap-num is defined above
payload-type = 1*DIGIT ; RTP payload type
The example in the previous section shows how the parameters from the The example in the previous section shows how the parameters from the
cmed line are mapped to payload type in the pcfg "pt" parameter. mcap line are mapped to payload type in the pcfg "pt" parameter.
3.3.3. The Bandwidth Extension to the Potential Configuration Attribute
A bandwidth parameter is added to the pcfg attribute in order to
provide the flexibility to specify different bandwidth limits for
different configurations.
The bandwidth parameter, bw-param, is defined in accordance with the
following format:
bw-param = "b=" bw-cap-list;
bw-cap-list = bw-cap-num *("," bw-cap-num);
bw-cap-num = 1*DIGIT; as defined above
Note that it is considered an error condition if the same bandwidth
type is referenced more than once in the bw-cap-list.
3.4. Extensions to the Actual Configuration Attribute 3.4. Extensions to the Actual Configuration Attribute
We define an actual configuration extension parameter act-media- The Actual configuration attribute is specified in [SDPCapNeg]. The
config in accordance with the following ABNF: actual configuration MUST list the potential configuration selected
by the answerer. This section adds extensions parameters enabling
the answerer to specify the potential configuration attributes
defined in this document and used in forming the answer.
We define actual configuration extensions based on the sel-extension-
config specified in [SDPCapNeg]. The new parameters are: parameter
act-media-config, act-med-map and act-bw-param in accordance with the
following ABNF:
act-media-config = "m=" med-cap-list ; defined in section
3.3.1
act-med-map = "pt=" med-map *("," med-map) ; defined in
section 3.3.2
act-bw-param = "b=" bw-cap-list ; defined in section 3.3.3
a=acfg:<med-cap-num> *WSP "m=" <med-cap-list>
A response to the previous offer example in the above section might A response to the previous offer example in the above section might
be: be:
v=0 v=0
o=- 24351 621814 IN IP4 192.0.2.2 o=- 24351 621814 IN IP4 192.0.2.2
s= s=
c=IN IP4 192.0.2.2 c=IN IP4 192.0.2.2
t=0 0 t=0 0
a=csup:v1 a=csup:med-v0
m=audio 5432 RTP/AVP 18 100 m=audio 5432 RTP/AVP 18 100
a=rtpmap:100 events a=rtpmap:100 events
a=fmtp:100 0-15 a=fmtp:100 0-15
a=acfg:1 m=2,3 a=acfg:1 m=2,3 pt=2:18,3:100
Note that the capability numbers expressed in the acfg: attribute are Note that the capability numbers expressed in the acfg: attribute are
based on the offered capability numbering, not on those listed in the based on the offered capability numbering, not on those listed in the
answer. The acfg identify to the offrer which potetial configuration answer, if any. The acfg attribute identifies to the offrer which
was selected by the answerer. potential configuration and media encoding combination was selected
by the answerer.
3.5. The Latent Configuration Attribute 3.5. The Latent Configuration Attribute
One of the goals of this work is to permit the exchange of media One of the goals of this work is to permit the exchange of media
configurations in addition to those offered for immediate use. Such configurations in addition to those offered for immediate use. Such
configurations are referred to as "latent configurations". For configurations are referred to as "latent configurations". For
example, a party may offer to establish an audio session, and, at the example, a party may offer to establish an audio session, and, at the
same time, announce its ability to support a video session. same time, announce its ability to support a video session and supply
its video capabilities.
Latent configurations may be announced by use of the latent Latent configurations may be announced by use of the latent
configuration attribute, which is defined in a manner very similar to configuration attribute, which is defined in a manner very similar to
the potential configuration attribute: the potential configuration attribute:
a=lcfg:<preference> ["m="<media-caps>] ["t="<transport>] a=lcfg:<preference> ["m="<media-caps-list>] ["t="<transport>]
["a="<attributes>] ["a="<attributes>] ["b="<bw-cap-list>]
The m=, t= and a= parameters are identical in format and meaning to
those defined for the pcfg: attribute. Note that the pt= parameter
is not permitted in the lcfg: attribute because no actual media
session is being offered or accepted.
Latent Configurations may be specified at the session level in offers The m=, t=, b= and a= parameters are identical in format and meaning
and in answers. to those defined for the pcfg: attribute. Note that the pt=
parameter is not permitted in the lcfg: attribute because no actual
media session is being offered or accepted.
[Editor's note: Do you have a good example of an offer of an audio Latent Configurations MUST be specified at the session level when
stream with a latent video stream?][Editor's note (rrg): What if, as they represent an additional media stream to those in the offer or
Roni suggested, we exclude the a= parameters from the lcfg: lines? answer. If an acap: attribute is declared at the session level for
We can include the t= parameter as the 'cheapest' way to indicate use in a session-level lcfg line, it SHOULD NOT be used in a pcfg
SRTP. That conveys most of the necessary info; the details can be line at the media level unless it is to become a session-level
firmed up in the actual offer when/if it's actually made.] attribute in the answer.
3.5.1. The crypto: Attribute in Latent Configurations 3.5.1. The crypto: Attribute in Latent Configurations
If the sdescriptions crypto: attribute is necessary as part of any If the sdescriptions crypto: attribute is necessary as part of any
latent configuration which announces sdescriptions capabilities. then latent configuration which announces sdescriptions capabilities, it
it presents a slight problem in that the rather long key/salt string presents a slight problem in that the rather long key/salt string is
is useless and should be ignored. This problem is avoided if we useless and should be ignored. This problem is avoided if we exclude
exclude the a= parameters from the lcfg: attribute. [Editor's note: the a= parameters from the lcfg: attribute. [Editor's note: Should
should we define a new crypto: key-method, e.g. "latent", in which we define a new crypto: key-method, e.g. "latent", in which the key-
the key-salt portion of key-info is empty? I think it may be salt portion of key-info is empty? I think it may be sufficient to
sufficient to just include the RTP/SAVP transport to indicate SRTP just include the RTP/SAVP transport to indicate SRTP capability since
capability] there will be a new offer when the latent configuration will be
offered.]
3.6. Offer/Answer Model Extensions 3.6. Offer/Answer Model Extensions
In this section, we define extensions to the offer/answer model In this section, we define extensions to the offer/answer model
defined in RFC3264 to allow for media capabilities to be used with defined in RFC3264 [RFC3264] to allow for media capabilities to be
the SDP Capability Negotation framework. used with the SDP Capability Negotiation framework.
3.6.1. Generating the Initial Offer 3.6.1. Generating the Initial Offer
When an endpoint generates an initial offer and want to use the
functionality described in the current document, it should identify
and define the codecs it can support via mcap , ecap , and fcap
attributes. The SDP media line(s) should be made up with the default
(least preferred) configuration. Typically, the media line
configuration will contain the minimum acceptable capabilities. The
offer MUST include the required extensions needed to support this
functionality.
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 offeror is not now offering,
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.
3.6.2. Generating the Answer 3.6.2. Generating the Answer
When the answering party receives the offer and if it supports the
required extensions, it should select the most-preferred
configuration for each media stream, and build its answer
accordingly. Each selected media configuration is placed into the
answer as a media line with associated parameters, It must include
the supported extension attribute and an actual configuration (acfg)
attribute to indicate just which pcfg attribute was used to build the
answer. The answer should may also include any latent configurations
the answerer can support, especially any configurations compatible
with latent configurations received in the offer. The answerer
should make note of those configurations it might wish to offer in
the future.
3.6.3. Offerer Processing of the Answer 3.6.3. 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 [SDPCapNeg] requires the answerer to choose the highest
preference configuration it can support.
3.6.4. Modifying the Session 3.6.4. 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 changing a codec 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, it may
perform a new capabilities exchange as part of the reconfiguration.
4. Examples 4. Examples
In this section, we provide examples showing how to use the Media In this section, we provide examples showing how to use the Media
Capabilities with the SDP Capability Negotiation. Capabilities with the SDP Capability Negotiation.
4.1. Alternative Codecs 4.1. Alternative Codecs
4.2. Latent Media Streams 4.2. Latent Media Streams
5. IANA Considerations 5. IANA Considerations
6. Security Considerations 6. Security Considerations
7. Changes from version 00 7. Changes from previous versions
7.1. 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.
7.2. Changes from version 00
The major changes include taking out the "mcap" and "cptmap" The major changes include taking out the "mcap" and "cptmap"
parameter. The mapping of payload type is now in the "pt" parameter parameter. The mapping of payload type is now in the "pt" parameter
of "pcfg". Media subtype need to explictly definesd in the "cmed" of "pcfg". Media subtype need to explictly definesd in the "cmed"
attribute if referenced in the "pcfg" attribute if referenced in the "pcfg"
8. Acknowledgements 8. Acknowledgements
This document is heavily influenced by the discussions and work done This document is heavily influenced by the discussions and work done
by the SDP Capability Negotiation Design team. The following people by the SDP Capability Negotiation Design team. The following people
skipping to change at page 20, line 21 skipping to change at page 26, line 21
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, with Session Description Protocol (SDP)", RFC 3264,
June 2002. June 2002.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006. Description Protocol", RFC 4566, July 2006.
[SDPCapNeg] [SDPCapNeg]
Andreasen, F., "SDP Capability Negotiation", Andreasen, F., "SDP Capability Negotiation",
draft-ietf-mmusic-sdp-capability-negotiation-02 (work in draft-ietf-mmusic-sdp-capability-negotiation-07 (work in
progress), February 2007. progress), October 2007.
9.2. Informative References 9.2. Informative References
[RFC3407] Andreasen, F., "Session Description Protocol (SDP) Simple [RFC3407] Andreasen, F., "Session Description Protocol (SDP) Simple
Capability Declaration", RFC 3407, October 2002. Capability Declaration", RFC 3407, October 2002.
[RFC3952] Duric, A. and S. Andersen, "Real-time Transport Protocol
(RTP) Payload Format for internet Low Bit Rate Codec
(iLBC) Speech", RFC 3952, December 2004.
Authors' Addresses Authors' Addresses
Robert R Gilman Robert R Gilman
Avaya, Inc. Avaya, Inc.
1300 West 120th Avenue 1300 West 120th Avenue
Westminster, CO 80234 Westminster, CO 80234
USA USA
Email: rrg@avaya.com Email: rrg@avaya.com
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