draft-ietf-mmusic-sdp-media-capabilities-02.txt   draft-ietf-mmusic-sdp-media-capabilities-03.txt 
MMUSIC R. Gilman MMUSIC R. Gilman
Internet-Draft Avaya, Inc. Internet-Draft NDCI
Intended status: Standards Track R. Even, Ed. Intended status: Standards Track R. Even, Ed.
Expires: May 18, 2008 Polycom Expires: August 28, 2008 Polycom
F. Andreasen F. Andreasen
Cisco Systems Cisco Systems
November 15, 2007 February 25, 2008
SDP media capabilities Negotiation SDP media capabilities Negotiation
draft-ietf-mmusic-sdp-media-capabilities-02.txt draft-ietf-mmusic-sdp-media-capabilities-03.txt
Status of this Memo Status of this Memo
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Copyright Notice Copyright Notice
Copyright (C) The IETF Trust (2007). Copyright (C) The IETF Trust (2008).
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 indicating and negotiating capabilities in SDP.
framework defines only capabilities for negotiating transport The base framework defines only capabilities for negotiating
protocols and attributes. In this document, we extend the framework transport protocols and attributes. In this document, we extend the
by defining media capabilities that can be used to negotiate media framework by defining media capabilities that can be used to
types and their associated parameters. negotiate media types and their associated parameters. This
extension is designed to map easily to existing and future SDP media
attributes.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. SDP Media capabilities . . . . . . . . . . . . . . . . . . . . 6 3. SDP Media Capabilities . . . . . . . . . . . . . . . . . . . . 6
3.1. Solution Overview . . . . . . . . . . . . . . . . . . . . 6 3.1. Solution Overview . . . . . . . . . . . . . . . . . . . . 6
3.2. Capability Attributes . . . . . . . . . . . . . . . . . . 9 3.2. Design Goals . . . . . . . . . . . . . . . . . . . . . . . 10
3.2.1. Media Type and Subtype Capability Attribute . . . . . 10 3.3. New Capability Attributes . . . . . . . . . . . . . . . . 11
3.2.2. The Media Encoding Parameter Capability Attribute . . 11 3.3.1. The Media Encoding Capability Attribute . . . . . . . 11
3.2.3. The Media Format Parameter Capability Attribute . . . 12 3.3.2. The Media Format Parameter Capability Attribute . . . 12
3.2.4. The Bandwidth Capability Attribute . . . . . . . . . . 13 3.3.3. The Media-Specific Capability Attribute . . . . . . . 15
3.3. Extensions to the Potential Configuration Attribute . . . 15 3.3.4. The Bandwidth Capability Attribute . . . . . . . . . . 16
3.3.1. The Media Capability Extension to the Potential 3.4. Extensions to the Potential Configuration Attribute . . . 17
Configuration Attribute . . . . . . . . . . . . . . . 15 3.4.1. The Media Capability Parameter . . . . . . . . . . . . 17
3.3.2. The Payload Type Mapping Extension to the 3.4.2. The Payload Type Mapping Parameter . . . . . . . . . . 19
Potential Configuration Attribute . . . . . . . . . . 16 3.4.3. The Bandwidth Parameter . . . . . . . . . . . . . . . 20
3.3.3. The Bandwidth Extension to the Potential 3.5. Extensions to the Actual Configuration Attribute . . . . . 20
Configuration Attribute . . . . . . . . . . . . . . . 17 3.6. The Latent Configuration Attribute . . . . . . . . . . . . 21
3.4. Extensions to the Actual Configuration Attribute . . . . . 17 3.6.1. Cryptographic Attributes in Latent Configurations . . 23
3.5. The Latent Configuration Attribute . . . . . . . . . . . . 18 3.7. Offer/Answer Model Extensions . . . . . . . . . . . . . . 23
3.5.1. The crypto: Attribute in Latent Configurations . . . . 18 3.7.1. Generating the Initial Offer . . . . . . . . . . . . . 23
3.6. Offer/Answer Model Extensions . . . . . . . . . . . . . . 19 3.7.2. Generating the Answer . . . . . . . . . . . . . . . . 24
3.6.1. Generating the Initial Offer . . . . . . . . . . . . . 19 3.7.3. Offerer Processing of the Answer . . . . . . . . . . . 24
3.6.2. Generating the Answer . . . . . . . . . . . . . . . . 19 3.7.4. Modifying the Session . . . . . . . . . . . . . . . . 24
3.6.3. Offerer Processing of the Answer . . . . . . . . . . . 19 4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.6.4. Modifying the Session . . . . . . . . . . . . . . . . 20 4.1. Alternative Codecs . . . . . . . . . . . . . . . . . . . . 26
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 4.2. Latent Media Streams . . . . . . . . . . . . . . . . . . . 29
4.1. Alternative Codecs . . . . . . . . . . . . . . . . . . . . 21 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
4.2. Latent Media Streams . . . . . . . . . . . . . . . . . . . 21 6. Security Considerations . . . . . . . . . . . . . . . . . . . 32
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22 7. Changes from previous versions . . . . . . . . . . . . . . . . 33
6. Security Considerations . . . . . . . . . . . . . . . . . . . 23 7.1. Changes from version 02 . . . . . . . . . . . . . . . . . 33
7. Changes from previous versions . . . . . . . . . . . . . . . . 24 7.2. Changes from version 01 . . . . . . . . . . . . . . . . . 33
7.1. Changes from version 01 . . . . . . . . . . . . . . . . . 24 7.3. Changes from version 00 . . . . . . . . . . . . . . . . . 33
7.2. Changes from version 00 . . . . . . . . . . . . . . . . . 24 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 34
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26 9.1. Normative References . . . . . . . . . . . . . . . . . . . 35
9.1. Normative References . . . . . . . . . . . . . . . . . . . 26 9.2. Informative References . . . . . . . . . . . . . . . . . . 35
9.2. Informative References . . . . . . . . . . . . . . . . . . 26 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27 Intellectual Property and Copyright Statements . . . . . . . . . . 37
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 indicating and negotiating
SDP[RFC4566]. The base framework defines only capabilities for capabilities in SDP[RFC4566]. The base framework defines only
negotiating transport protocols and attributes. capabilities for negotiating transport protocols and attributes.
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. The capabilities include a simple form of media
Actual negotiation procedures taking advantage of such capabilities capabilities. RFC 3407 defines capability declaration only. Actual
have not been defined. The SDP capability negotiation framework adds negotiation procedures taking advantage of such capabilities have not
this required functionality. This document updates RFC3407 and new been defined. The SDP capability negotiation framework defined in
implementation SHOULD use the functionality defined in the current [SDPCapNeg] adds this required functionality, but does not define
draft to negotiate media capabilities. media capabilities. This document updates RFC3407 and [SDPCapNeg]
and new implementations SHOULD use the functionality defined in the
current document to negotiate media capabilities.
The [SDPCapNeg] document lists some of the issues with the current The [SDPCapNeg] document lists some of the issues with the current
SDP capability negotiation process. An additional real life case is SDP capability negotiation process. An additional real life case is
to be able to offer one media stream (e.g. audio) but list the to be able to offer one media stream (e.g. audio) but list the
capability to support another media stream (e.g. video) without capability to support another media stream (e.g. video) without
actually offering it currently. actually offering it currently.
In this document, we extend the framework by defining media In this document, we extend the framework by defining media
capabilities that can be used to negotiate media types and their capabilities that can be used to indicate and negotiate media types
associated parameters like bandwidth. This document also adds the and their associated parameters like bandwidth. This document also
capability to declare support for media streams that can be offered adds the ability to declare support for media streams, the use of
later rather than as a potential configuration for current use. which can be offered and negotiated later. 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.
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 "Base Configuration": the media configuration represented by a media
block exclusive of all the capability negotiation attributes defined
in this document and [SDPCapNeg]
The SDP media capability negotiation [SDPCapNeg] discusses the use of "Conventional SDP": all SDP lines exclusive of the capability
any SDP [RFC4566] attribute (a=) under the acap attribute. The negotiation attributes
limitations of using acap for fmtp and rtpmap as potential
configuration are listed, for example they can be used only at the 3. SDP Media Capabilities
media level since they are media level attributes. The [SDPCapNeg]
addresses the issue of bandwidth usage but does not offer a way to The SDP capability negotiation [SDPCapNeg] discusses the use of any
negotiate different bandwidth for different codecs and profiles. SDP [RFC4566] attribute (a=) under the attribute capability "acap".
This section provides an overview of extensions providing SDP media The limitations of using acap for fmtp and rtpmap in a potential
Capability negotiation solution offering more robust capabilities configuration are described in [SDPCapNeg]; for example they can be
used only at the media level since they are media level attributes.
The [SDPCapNeg] partially addresses the issue of bandwidth usage for
different configurations 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 and
Bandwidth negotiation solution offering more robust capabilities
negotiation. This is followed by definitions of new SDP attributes negotiation. This is followed by definitions of new SDP attributes
for the solution and its associated updated offer/answer procedures for the solution and its associated updated offer/answer procedures
[RFC3264] [RFC3264]
3.1. Solution Overview 3.1. Solution Overview
The solution consist of the following new attributes extending the The solution consists of five new attributes and four new parameters
base attributes from [SDPCapNeg]. for the pcfg attribute extending the base attributes from
[SDPCapNeg].
Four attributes are used to make up media capabilities in a manner Four new attributes are used to make up media capabilities in a
that can be related to the capabilities specified in a media line, manner that can be related to the capabilities specified in a media
its attributes and the bandwidth parameter lines. line, its attributes, and the bandwidth parameter lines.
o A new media attribute ("a=mcap") 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 of a subtype (e.g. "PCMU"), and its
more subtypes (e.g. "PCMU"), and associates a handle with each encoding parameters (e.g. "/8000/2"). Each resulting media format
subtype type / subtype capability has an associated handle. The encoding
parameters are as specified for the rtpmap attribute defined in
[RFC4566]
o A new attribute ("a=ecap") that lists encoding parameter o A new attribute ("a=mfcap") that specifies media format parameters
capabilities associated with a one or more media format associated with one or more media capabilities. The mfcap
capabilities. attribute is used to associate the formatting capabilities
normally carried in the fmtp attribute.
o A new attribute ("a=fcap") that lists media format parameter o A new attribute ("a=mscap") that specifies media parameters
capabilities associated with a one or more media format associated with one or more media capabilities. The mscap
capabilities. attribute is used to associate capabilities other than fmtp or
rtpmap attributes, for example the rtcp-fb attribute.
o A new attribute ("a=bcap") that is used to specify the bandwidth o A new attribute ("a=bcap") that is used to specify the bandwidth
parameter(s) for media capabilities. parameter capability.
o A new attribute ("a=lcfg") that specifies latent configurations o A new attribute ("a=lcfg") that specifies latent media stream
when no corresponding media line is offered. An example is a configurations when no corresponding media stream is offered. An
latent configuration for video even though no video is currently example is a latent configuration for video even though no video
offered. is currently offered.
o A new parameter type ("m=") to the potential configuration 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:") ("a=pcfg:") attribute and the actual configuration ("a=acfg:")
attribute defined in [SDPCapNeg], which permits specification of attribute defined in [SDPCapNeg], and to the new latent
configuration ("a=lcfg:") attribute 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 PCMU and telephone events or G.729B "a=pcfg:" line might specify PCMU and telephone events or G.729B
and telephone events as acceptable configurations. The "a=acfg:" and telephone events as acceptable configurations. The "a=acfg:"
line in the answer would specify the accepted choice. line in the answer would specify the accepted choice.
o A new parameter type ("pt=") to the potential configuration o A new parameter type ("pt=") is added to the potential
("a=pcfg:") attribute which associates RTP payload types with the configuration, actual configuration, and latent configuration
referenced media capabilities. attributes. This parameter associates RTP payload types with the
referenced media capabilities, and is appropriate only when the
transport protocol uses RTP.
o A new parameter type ("b=") used to specify bandwidth parameters o A new parameter type ("b=") is used to specify bandwidth
in a potential configuration. parameters in a potential configuration.
o A new parameter type ("mt=") is used to specify the MIME type for
latent configurations that are declared at the session level.
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) and associated parameters, latent
other side of possible configurations supported by the entity. Those configurations merely inform the other side of possible
latent configurations may be used to guide subsequent offer/answer configurations supported by the entity. Those latent configurations
exchanges. 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, The mechanism is illustrated by the offer/answer exchange below,
where Alice sends an offer to Bob: where Alice sends an offer to Bob:
Alice Bob Alice Bob
| (1) Offer (SRTP and RTP) | | (1) Offer (SRTP and RTP) |
|--------------------------------->| |--------------------------------->|
| | | |
| (2) Answer (RTP) | | (2) Answer (RTP) |
|<---------------------------------| |<---------------------------------|
skipping to change at page 7, line 48 skipping to change at page 8, line 22
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:med-v0 a=creq:med-v0
a=mcap:1 audio g729 iLBC PCMU g729
a=ecap:2 8000
a=fcap:1 annexb:no
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=tcap:1 RTP/SAVP
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 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=4,5|1,5 t=1 a=1 pt=1:100,4:101,5:102
a=pcfg:2 m=2 pt=2:103 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" The required base and extensions are provided by the "a=creq"
attribute defined in [SDPCapNeg], with the option tag "med-v0", 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=mcap:1" line defines four audio media subtype capabilities , The "m=" line indicates that Alice is offering to use plain RTP with
to be numbered consecutively starting with 1. Note that the media PCMU or G.729B. The media line implicitly defines the default
subtypes specified in the m-line (PCMU and G729) are explicitly transport protocol (RTP/AVP in this case) and the default actual
specified here. configuration.
The "a=ecap:2" line specifies the clock rate and encoding parmeters The "a=tcap:1" line, specified in the base protocol, defines a
(see [RFC4566]) for capability 2, iLBC. transport protocol capabilities, in this case Secure RTP (SAVP
profile) as the first option and RTP (AVP profile) as the second
option.
The "a=fcap:1" line specifies media format parameter capabilities for The "a=mcap:1,4" line defines two G.729 media format capabilities,
codec 1 ( no Annex B for G.729). numbered 1 and 4, and their encoding rate. The capabilities are of
subtype G729. Note that the media subtype is explicitly specified
here, rather than RTP payload type number. 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=fcap:2" line specifies a media format parameter for codec 2 The "a=mcap:2" line defines a G.711 mu-law capability, numbered 2.
(20ms packets for iLBC. [RFC3952]
The "a=tcap:1" line, specified in the base protocol, defines a The "a=mcap:5" line defines an audio telephone-event capability,
transport protocol capability, in this case Secure RTP. numbered 5.
The "m=" line indicates that Alice is offering to use plain RTP with The "a=mfcap:1" line specifies the fmtp formatting parameters for
PCMU or G.729. The media line implicitly defines the default capability 1 (no comfort noise packets).
transport protocol (RTP/AVP in this case) and the default actual
configuration.
The "a=acap:1" line ,specified in the base protocol provides the The "a=mfcap:4" line specifies the fmtp formatting parameters for
capability 4 (G.729B).
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 "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. Three
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 4 and 5, i.e., G.729B
and PCMU, or media capability 4 and 3, i.e., G.729B and PCMU. and DTMF, or media capability 1 and 5, i.e., G.729 and DTMF.
Furthermore, it specifies transport protocol capability 1 (i.e. the Furthermore, it specifies transport protocol capability 1 (i.e. the
RTP/SAVP profile - secure RTP), and the attribute capability 1, i.e. RTP/SAVP profile - secure RTP), and the attribute capability 1, i.e.
the crypto attribute provided. Lastly, it specifies, a payload type the crypto attribute provided. Lastly, it specifies, a payload type
mapping for codecs 1, 3, and 4 thereby permitting the offerer to mapping for media capabilities 1, 4, and 5, thereby permitting the
distinguish between encrypted media and unencrypted media received offeror to distinguish between encrypted media and unencrypted media
prior to receipt of the answer. For SRTP the offerer will still need received prior to receipt of the answer. Use of unique payload types
to receive the answer before being able to decrypt the stream. 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 types for all supported combinations. For SRTP
using SDES inline keying [RFC4568], the offeror will still need to
receive the answer before being able to decrypt the stream.
The second alternative specifies media capability 2, i.e. iLBC, under The second alternative specifies media capability 2, i.e. PCMU,
the default RTP/AVP profile. The media line, with any qualifying under the RTP/SAVP profile, with the same SRTP key material.
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 RTP, but not The third alternative offers G.729B unsecured; it's only purpose in
SRTP, and hence he accepts the actual configuration for RTP provided this example is to show a preference for G.729B over PCMU.
by Alice. Furthermore, Bob wants to use the iLBC codec and hence
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, and hence he accepts the
potential configuration 3 for RTP provided by Alice. Hence, Bob
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:med-v0 a=csup:med-v0
m=audio 4567 RTP/AVP 103 m=audio 4567 RTP/AVP 18
a=rtpmap:103 iLBC/8000 a=rtpmap:18 G729/8000
a=fmtp:103 mode=20 a=fmtp:18 annexb=yes
a=acfg:2 a=acfg:3 m=4 pt=4:18
Bob includes the "a=csup" and "a=acfg" attribute in the answer to 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 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 MAY generate Note that per the base protocol, after the above, Alice MAY 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. Design Goals
The design of the new elements added to the capability negotiation
framework by this document has been driven by two basic goals:
o The resulting SDP should be as compact as possible while
preserving the flexibility necessary to declare multiple options
for complex media encodings.
o The capability attributes defined should be easily mapped to and
from "conventional" SDP elements, that is, to and from one or more
SDP records that do not contain any capability attributes.
It is hoped that satisfaction of these goals will facilitate simple
implementations of capability negotiation in SDP.
3.3. New 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. We use media subtypes and "media capability numbers" attributes. We use media subtypes and "media capability numbers"
instead of payload types to link the relevant media capability instead of payload types to link the relevant media capability
parameters. This permits the capabilities to be defined at the parameters. This permits the capabilities to be defined at the
session level and used for multiple streams, if desired. Payload session level and be used for multiple streams, if desired. Payload
types are then specified at the media level (see Section 3.2.2). types are then specified at the media level (see Section 3.4.2).
A media capability merely indicates possible support for the media A media capability merely indicates possible support for the media
type and media format(s) in question. In order to actually use a type and media format(s) in question. In order to actually use a
media capability in an offer/answer exchange, it must be referenced media capability in an offer/answer exchange, it must be referenced
in a potential configuration (see Section 2.3.1). in a potential configuration (see Section 3.4.1).
Media capabilities can be provided at the session-level and/or the Media capabilities can be provided at the session-level and/or the
media-level. Media capabilities provided at the session level may be media-level. Media capabilities provided at the session level may be
referenced in an lcfg attribute at the session level, or by any pcfg referenced in an lcfg attribute at the session level, or by any pcfg
attribute at the media level, whereas media capabilities provided at attribute at the media level (consistent with the MIME type), whereas
the media level may be referenced by a pcfg attribute within that media capabilities provided at the media level may be referenced by a
media stream only. In either case, the scope of the <med-cap-num> is pcfg attribute within that media stream only. In either case, the
the entire session description. This enables each media capability scope of the <med-cap-num> is the entire session description. This
to be uniquely referenced across the entire session description (e.g. enables each media capability to be uniquely referenced across the
in a potential configuration) entire session description (e.g. in a potential configuration).
3.2.1. Media Type and Subtype Capability Attribute 3.3.1. The Media Encoding Capability Attribute
Media types and subtypes can be expressed as media format Media subtypes can be expressed as media encoding capabilities by use
capabilities by use of the "a=mcap" attribute, which is defined as of the "a=mcap" attribute, which is defined as follows:
follows:
a=mcap:<med-cap-num> <subtype>*[ <subtype>] a=mcap:<med-cap-num-list> <subtype>[/<encoding-parms>]
<med-cap-num-list> = <med-cap-num> *[COMMA <med-cap-num>]
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 and the included) used to number a media format capability, or a range of
<subtype> is the media subtype e.g. H263-1998, PCMU. The <med-cap- such integers and the <subtype> is the media subtype e.g. H263-1998
num> is the media capability number associated with the first subtype or PCMU. and <encoding-parms> are the media encoding parameters for
in the list, the number associated with the second subtype is one the <subtype>. All media format capabilities in the list are
higher, etc. Each occurrence of the attribute MUST use a different assigned to the same media type/subtype. Each occurrence of the mcap
value of <med-cap-num>. Furthermore, when an "mcap" attribute attribute MUST use unique values in its <med-cap-num-list>; the media
indicates more than one media format, the capability numbers implied capability numbers must be unique across the entire session or media
MUST NOT be used by any other "mcap" attribute in the entire session description. In short, the mcap attribute defines media capabilities
or media description (explicitly or implicitly). 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.
In ABNF, we have: In ABNF, we have:
media-capability-line = "a=mcap:" media-cap-num WSP media-type media-capability-line = "a=mcap:" media-cap-num-list
1*WSP media-cap *(WSP media-cap) 1*WSP media-cap
media-cap-num = 1*DIGIT ["/" clock-rate ["/" encoding-parms]]
media-cap-num-list = media-cap-num *[COMMA media-cap-num]
media-cap-num = 1*DIGIT | media-cap-range
media-cap-range = 1*DIGIT "-" 1*DIGIT
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 clock-rate = 1*DIGIT
encoding-parms = token
The clock-rate and encoding-params are as defined to appear in an
rtpmap attribute for each MIME type/subtype. Thus, it is easy to
convert an mcap attribute line into one or more rtpmap attribute
lines, once a payload type is assigned to a media-cap-num (see
section 3.4.2).
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 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 session or media level. The unique media-cap-num is used to identify
it in potential configurations. When used in a potential each media capability in potential and actual configurations. When
configuration it is a media level attribute regardless if it is used in a potential configuration it is a media level attribute
specified at the session or media level. capability 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 potential configuration in
question.
For example: For example:
v=0 v=0
a=mcap:1 iLBC a=mcap:1 L16/8000/1
a= mcap:2 H263-1998, H264 a=mcap:2 L16/16000/2
a=mcap:3,4 H263-1998/90000
3.2.2. The Media Encoding Parameter Capability Attribute
Media format capabilities may require additional encoding parameters,
such as sample rate, to be precisely defined. The "a=ecap" encoding
attribute is defined as
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
"a=rtpmap:" attribute defined in RFC 4566 [RFC4566].
The "ecap" attribute can be provided at the session-level and the
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 3.3.2. The Media Format Parameter Capability Attribute
desirable to specify the same subtype with, e.g., two different
encoding rates, then the subtype should be listed twice, and each
should be modified appropriately. For example:
a=mcap:1 L16 L16 This attribute is used to associate media-specific parameters with
a=ecap:1 8000 one or more media capabilities. The form of the attribute is:
a=ecap:2 16000/2
defines two low-data-rate codecs, codec 1 uses 8000 samples per a=mfcap:<med-cap-num-list> <list of parameters>
second, and codec 2 uses 16000 samples per second and 2 channels.
3.2.3. The Media Format Parameter Capability Attribute where <med-cap-num-list> permits the parameter(s) to be associated
with one or more media capabilities and the format parameters are
specific to the type of codec. The mfcap lines map to a single
traditional SDP fmtp attribute line (one per <media-cap-num>) of the
form
This attribute is used to associate media format parameters with a a=fmtp:<fmt> <list of parameters>
media capability. The form of the attribute is:
a=fcap:<med-cap-num-list> <list of format 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.
where <med-cap-num-list> permits the format parameter(s) to be The appearance of media subtypes with a large number of formatting
associated with one or more media capabilities, and the format options (e.g., AMR-WB [RFC4867]) coupled with the restriction that
parameters are specific to the type of codec(s), as described for the only a single fmtp attribute can appear per media format, suggests
fmtp: attribute defined in RFC 4566[RFC4566]. Unlike the fmtp that it is useful to create a combining rule for mfcap parameters
attribute, multiple fcap attributes may be associated with a which are associated with the same media capability number.
specified med-cap-num. This provides the ability to mix and match Therefore, different mfcap lines MAY include the same <med-cap-num>
format parameters when using codecs with multiple format options. in their <med-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 med-cap-num-list
are concatenated together via ";" 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 med-cap-num-list. This provides a
compact method to specify multiple combinations of format parameters
when using codecs with multiple format options.
The fcap attribute adheres to RFC 4566 attribute production rules The mfcap attribute adheres to RFC 4566 attribute production rules
with with
media-format-capability-line = "a=fcap:" <att-value> media-format-capability = "a=mfcap:"<media-caps> WSP
<fmt-specific-param-list>
med-caps = "*" ; wildcard: all media caps
/ <med-cap-num-list> ; defined in Section 3.3.1
format-specific-parameter-list = <format-specfic-parameter>
*[";"<format-specfic-parameter>] ;
format-specific-parameter = text
att-value = <med-cap-num-list> WSP <format-specific-parameter- Format parameters are not parsed by SDP; their content is specific to
list> 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):
med-cap-num-list = <med-cap-num> *["," <med-cap-num>] ; med-cap- a= fmtp:<fmt> WSP <format-specfic-parameter-list>
num is defined in Section 3.2.1 [1*[";"<format-specfic-parameter-list>] ;
format-specific-parameter-list = <format-specfic-parameter> where <fmt> depends on the transport protocol in the manner defined
*[";"<format-specfic-parameter>] ; format-specific-parameter- in RFC4566. SDP cannot assess the legality of the resulting
list are defined per codec/capability. parameter list in the "a=fmtp" line; the user must take care to
insure that legal parameter lists are generated.
The "fcap" attribute can be provided at the session-level and the The "mfcap" attribute can be provided at the session-level and the
media-level. There can be more than one fcap attribute at 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 identify session or media level. The unique media-cap-num is used to
it in potential configurations. When used in a potential associate the parameters with a media capability.
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 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 support comfort noise as defined by Annex B. Capabilities for G.729
with and without comfort noise support may thus be identified by: with and without comfort noise support may thus be defined by:
a=mcap:1 audio G729 G729 a=mcap:1,2 audio G729/8000
a=fcap:2 annexb:no 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: Example for H.263 video:
a=mcap:1 video H263-1998 H263-2000 a=mcap:1 video H263-1998/90000
a=fcap:1 CIF=4;QCIF=2;F=1;K=1 a=mcap:2 video H263-2000/90000
a=fcap:2 profile=2;level=2.2 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: Finally, for six format combinations of the Adaptive MultiRate codec:
a=mcap:1 audio AMR AMR AMR AMR AMR AMR a=mcap:1-3 AMR/8000/1
a=ecap:1,3,5 16000/1 a=mcap:4-6 AMR/16000/1
a=ecap:2,4,6 8000/1 a=mfcap:1,2,3,4 mode-change-capability=1
a=fcap:1,2,3,4 mode-change-capability=1 a=mfcap:5,6 mode-change-capability=2
a=fcap:5,6 mode-change-capability=2 a=mfcap:1,2,3,5 max-red=220
a=fcap:1,2,3,5 max-red=220 a=mfcap:3,4,5,6 octet-align=1
a=fcap:3,4,5,6 octet-align=1 a=mfcap:1,3,5 mode-set=0,2,4,7
a=fcap:1,3,5 mode-set=0,2,4,7 a=mfcap:2,4,6 mode-set=0,3,5,6
a=fcap:2,4,6 mode-set=0,3,5,6
So that AMR codec #1, when specified in a pcfg attribute (and So that AMR codec #1, when specified in a pcfg attribute within an
assigned payload type 98) as in audio stream block (and assigned payload type 98) as in
a=pcfg:1 m=1 pt=1:98 a=pcfg:1 m=1 pt=1:98
is essentially equivalent to the following is essentially equivalent to the following
m=audio 49170 RTP/AVP 98 m=audio 49170 RTP/AVP 98
a=rtpmap:98 AMR/16000/1 a=rtpmap:98 AMR/8000/1
a=fmtp:98 mode-change-capability=1; max-red=220; mode- a=fmtp:98 mode-change-capability=1; max-red=220; mode-
set=0,2,4,7 set=0,2,4,7
and AMR codec #4 with payload type 99, is essentially equivalent to and AMR codec #4 with payload type 99, is essentially equivalent to
the following: the following:
m=audio 49170 RTP/AVP 99 m=audio 49170 RTP/AVP 99
a=rtpmap:99 AMR/8000/1 a=rtpmap:99 AMR/16000/1
a=fmtp:99 mode-change-capability=1; octet-align=1; mode- a=fmtp:99 mode-change-capability=1; octet-align=1; mode-
set=0,3,5,6 set=0,3,5,6
and so on for the other four combinations. 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.
3.2.4. The Bandwidth Capability Attribute In some cases, particularly when an RFC 2198 redundancy audio subtype
(RED) capability is defined, the parameters to the mscap attribute
may contain payload type numbers. These numbers are bound to actual
payload types by means of the payload type parameter (pt=) in a
potential, actual, or latent configuration. See sections 3.4.2, 3.5,
and 3.6.
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, as follows:
media-specific-capability = "a=mscap:"
<media-caps> ; defined in 3.3.2
WSP <att-field> ; from [RFC4566]
WSP <ms-parameters>
ms-parameters = byte-string ; as defined per attribute.
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 attribute line of the form
a=<att-field>":"<fmt> <ms-parameters> ; fmt defined in [RFC4566]
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 example with the rtcp-fb attribute, modified from an example
in[I-D.ietf-avt-avpf-ccm] (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/AVP 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
3.3.4. The Bandwidth Capability Attribute
In some cases it is desirable to specify different bandwidth limits In some cases it is desirable to specify different bandwidth limits
for different media configurations. This may be done by use of the for different media configurations. This may be done by use of the
"a=bcap" attribute, which is defined as follows: "a=bcap" attribute, which is defined as follows:
a=bcap:<bw-cap-num> <bwtype>:<bandwidth> a=bcap:<bw-cap-num> <bwtype>:<bandwidth>
where <bw-cap-num> is an integer between 1 and 2^31-1 (both included) 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 used to identify the bandwidth capability, <bwtype> is the bandwidth
type, and <bandwidth> is the bandwidth value, as defined for the b= type, and <bandwidth> is the bandwidth value, as defined for the b=
skipping to change at page 14, line 18 skipping to change at page 17, line 4
used to identify the bandwidth capability, <bwtype> is the bandwidth used to identify the bandwidth capability, <bwtype> is the bandwidth
type, and <bandwidth> is the bandwidth value, as defined for the b= type, and <bandwidth> is the bandwidth value, as defined for the b=
line in RFC4566[RFC4566] line in RFC4566[RFC4566]
In ABNF, we have: In ABNF, we have:
media-bandwidth-cap-line = "a=bcap:" bw-cap-num 1*WSP media-bandwidth-cap-line = "a=bcap:" bw-cap-num 1*WSP
bwtype ":" bandwidth bwtype ":" bandwidth
where where
bw-cap-num = 1*DIGIT; the bandwidth "handle" bw-cap-num = 1*DIGIT; the bandwidth "handle"
bwtype = token; as defined in RFC4566 bwtype = token; as defined in RFC4566
bandwidth = 1*DIGIT; as defined in RFC4566 bandwidth = 1*DIGIT; as defined in RFC4566
The "bcap" attribute can appear at the session level, where it can be 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 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 it can be referenced by pcfg attributes. When invoked by a pcfg or
lcfg attribute, the resulting bandwidth line (b=) is to be lcfg attribute, the resulting bandwidth line (b=) is to be
interpreted at the media-level. There can be more then one bcap interpreted at the media-level for that configuration. There can be
attribute. The unique bw-cap-num is used to identify it in potential more than one bcap attribute. The unique bw-cap-num is used to
configurations. identify it in potential configurations. No provision has been made
for negotiation of total session bandwidth capabilities.
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 Bandwidth capabilities may be included in a potential configuration
via the "b=" parameter (see below). Any bandwidth capability via the "b=" parameter (see below). Any bandwidth capability
included replaces any media-level bandwidth of the same type declared included replaces any media-level bandwidth of the same type declared
in a "b=" SDP line. in a "b=" SDP line.
The following example offers a preferred potential configuration for The following example offers a preferred potential configuration for
H.263 QCIF at 360 Kbit/sec and a second 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 H.263 CIF at the offered 500 Kbit/sec
m=video 49170 RTP/AVP 99 m=video 49170 RTP/AVP 99
b=TIAS:500000 b=TIAS:500000
a=rtpmap:99 H263-1998/90000 a=rtpmap:99 H263-1998/90000
a=fmtp:99 CIF=4; QCIF=2 a=fmtp:99 CIF=4; QCIF=2
a=mcap:1 video H263-1998 H263-1998 a=mcap:1 video H263-1998/90000
a=fcap:1 QCIF=2 a=mcap:2 video H263-1998/90000
a=fcap:2 CIF=4; QCIF=2;F=1;K=1 a=mfcap:1 QCIF=2
a=mfcap:2 CIF=4; QCIF=2;F=1;K=1
a=bcap:1 TIAS:360000 a=bcap:1 TIAS:360000
a=pcfg:1 m=1 b=1 pt:100 a=pcfg:1 m=1 b=1 pt=1:100
a=pcfg:2 m=2 pt:101 a=pcfg:2 m=2 pt=2:101
3.3. Extensions to the Potential Configuration Attribute 3.4. Extensions to the Potential Configuration Attribute
The extension protocol of capabilities negotiation requires three new The extension protocol of capabilities negotiation requires three new
extensions for the pcfg: attribute defined in the base protocol. The extensions for the pcfg: attribute defined in the base protocol. The
first extension permits the specification of media capabilities, or first extension permits the specification of media capabilities, or
combinations thereof; the second 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 the third permits the specification of bandwidth limits for a media
stream. stream.
3.3.1. The Media Capability Extension to the Potential Configuration 3.4.1. The Media Capability Parameter
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
document, we define an extension parameter for the specification of document, we define an extension parameter for the specification of
media configurations in addition to the one specified on the media media configurations in addition to the one 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=mcap: 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)
; BAR is defined in [SDPCapNeg]
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
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=mcap attributes and hence MUST be numbers defined explicitly by a=mcap attributes and hence MUST be
between 1 and 2^31-1 (both included). Alternative potential media between 1 and 2^31-1 (both included). Alternative potential media
configurations are separated by a vertical bar ("|"). The configurations are separated by a vertical bar ("|"). The
alternatives are ordered by preference. When media capabilities are alternatives are ordered by preference. When media capabilities are
not included in a potential configuration at the media level, the not included in a potential configuration at the media level, the
media type and media format from the associated "m=" line will be media type and media format from the associated "m=" line will be
used. used.
When one or more media capabilities (a=mcap) are invoked in a
potential configuration (via m=), and associated with a payload type
number by default or by a payload type number parameter (pt=),
special processing must be invoked on the attributes associated with
that payload type. If the media capability is associated with any
mfcap or mscap attributes, then all corresponding conventional
attributes (e.g., fmtp or rtcp-fb attribute lines) in the media block
are ignored for that configuration. If no mfcap parameters are
specified, then an fmtp attribute line within the media block with
the correct payload type number, if any, will apply. Similarly, any
other media-specific attributes (e.g., rtcp-fb) in the media block
with the correct payload type number will apply unless there is an
applicable mscap attribute for the same attribute type (e.g.,
rtcp-fb), in which case all media-block attributes of the same type
and payload type number will be ignored. Any media-specific
attributes in the media block which refer to payload type numbers not
used by the potential configuration are to be ignored. These rules
are intended to avoid the need to duplicate attributes and use the
a=-m: form of invoking attributes in a potential configuration just
to replace an rtpmap or fmtp attribute.
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:med-v0 a=creq:med-v0
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=mcap:1 PCMU/8000
a=ecap:1,2,3 8000 a=mcap:2 g729/8000
a=fcap:3 0-15 a=mcap:3 telephone-event/8000
a=mfcap: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, PCMU is media capability 1, G729 is media capability
1, and events is media capability 3. The a=pcfg: line specifies that 2, and telephone-event is media capability 3. The a=pcfg: line
the preferred configuration is G.729 with dtmf events, second is specifies that the preferred configuration is G.729 with extended
G.711 mu-law with dtmf events. Intermixing of G.729, G.711, and dtmf dtmf events, second is G.711 mu-law with extended dtmf events.
events is least preferred (the actual configuration provided by the Intermixing of G.729, G.711, and "commercial" dtmf events is least
"m=" line, which is always the least preferred configuration). preferred (the base configuration provided by the "m=" line, which is
always 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.
3.3.2. The Payload Type Mapping Extension to the Potential 3.4.2. The Payload Type Mapping Parameter
Configuration Attribute
When media capabilities defined in mcap: 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, and the transport protocol uses RTP,
types to them. In some cases, it is desirable to assign different it is necessary to assign payload types to them. In some cases, it
payload types to media capabilities defined in the media line. One is desirable to assign different payload types to the same media
example of the latter is when configurations for AVP and SAVP are capability when used in different potential configurations. One
offered: the offerer would like the answerer to use different payload example is when configurations for AVP and SAVP are offered: the
types for encrypted and unencrypted media so that it (the offerer) offerer would like the answerer to use different payload types for
can decide whether or not to render early media which arrives before encrypted and unencrypted media so that it (the offerer) can decide
the answer is received. whether or not to render early media which arrives before the answer
is received. This association of distinct payload type(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
types.
We define the media type mapping configuration parameter, pt-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:
pt-media-map = "pt=" med-map *("," med-map); pt-media-map = "pt=" med-map-list
med-map = <med-cap-num> ":" <'payload-type> ; med-map-list = med-map *["," med-map]
med-cap-num is defined above med-map = med-cap-num ":" 'payload-type;
med-cap-num is defined in section 3.4.1
payload-type = 1*DIGIT ; RTP payload type 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
mcap 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 As described in section 3.4.1, the choice of payload type numbers is
especially important since they are also used to refer to base media
-specific attributes (e.g., fmtp or rtcp-fb) if mfcap or mscap
attributes are not specified for a media subtype.
3.4.3. The Bandwidth Parameter
A bandwidth parameter is added to the pcfg attribute in order to A bandwidth parameter is added to the pcfg attribute in order to
provide the flexibility to specify different bandwidth limits for provide the flexibility to specify different bandwidth limits for
different configurations. different configurations.
The bandwidth parameter, bw-param, is defined in accordance with the The bandwidth parameter, bw-param, is defined in accordance with the
following format: following format:
bw-param = "b=" bw-cap-list; bw-param = "b=" bw-cap-list;
bw-cap-list = bw-cap-num *("," bw-cap-num); bw-cap-list = bw-cap-num *("," bw-cap-num);
bw-cap-num = 1*DIGIT; as defined above bw-cap-num = 1*DIGIT; as defined by the bcap attribute
Note that it is considered an error condition if the same bandwidth Multiple bandwidth capabilities are permitted in case it is desired
type is referenced more than once in the bw-cap-list. to specify multiple bandwidth types. 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 If a bandwidth capability is referenced in a potentical
configuration, and that configuration is selected, then the bandwidth
capability will replace any bandwidth attribute of the same <bwtype>
which appears in the base media configuration. Bandwidth
specifications in the base configuration will apply to any potential
configuration for which no correspoinding bandwidth capability is
specified.
3.5. Extensions to the Actual Configuration Attribute
The Actual configuration attribute is specified in [SDPCapNeg]. The The Actual configuration attribute is specified in [SDPCapNeg]. The
actual configuration MUST list the potential configuration selected actual configuration MUST list the potential configuration selected
by the answerer. This section adds extensions parameters enabling by the answerer. This section adds extensions parameters enabling
the answerer to specify the potential configuration attributes the answerer to specify the potential configuration attributes
defined in this document and used in forming the answer. defined in this document and used in forming the answer.
We define actual configuration extensions based on the sel-extension- We define actual configuration extensions based on the sel-extension-
config specified in [SDPCapNeg]. The new parameters are: parameter config specified in [SDPCapNeg]. The new parameters are: parameter
act-media-config, act-med-map and act-bw-param in accordance with the act-media-config, act-med-map and act-bw-param in accordance with the
following ABNF: following ABNF:
act-media-config = "m=" med-cap-list ; defined in section act-media-config = "m=" med-cap-list ; defined in section 3.4.1
3.3.1 act-med-map = "pt=" med-map *("," med-map) ; defined in 3.4.2
act-med-map = "pt=" med-map *("," med-map) ; defined in act-bw-param = "b=" bw-cap-list ; defined in section 3.4.3
section 3.3.2
act-bw-param = "b=" bw-cap-list ; defined in section 3.3.3
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:med-v0 a=csup:med-v0
skipping to change at page 18, line 4 skipping to change at page 21, line 29
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:med-v0 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 pt=2:18,3:100 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, if any. The acfg attribute identifies to the offrer which answer, if any. The acfg attribute identifies to the offerer which
potential configuration and media encoding combination was selected potential configuration (and parameter combination) was selected by
by the answerer. the answerer. Note that payload types, if specified, refer to the
payloads to be received by the offeror; if the answerer wishes to
receive different payload types, those values MUST be specified in
the m= line of the SDP answer.
3.5. The Latent Configuration Attribute 3.6. 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
configurations in addition to those offered for immediate use. Such supportable media configurations in addition to those offered or
configurations are referred to as "latent configurations". For accepted for immediate use. Such configurations are referred to as
example, a party may offer to establish an audio session, and, at the "latent configurations". For example, a party may offer to establish
same time, announce its ability to support a video session and supply an audio session, and, at the same time, announce its ability to
its video capabilities. support a video session and supply its video capabilities by offering
one or more latent configurations; the responding party may indicate
its ability and willingness to support such a video session by
returning one or more latent configurations.
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-list>] ["t="<transport>] a=lcfg:<preference> ["mt="<media>]
["a="<attributes>] ["b="<bw-cap-list>] ["m="<media-caps-list>]
["t="<transport>]
["a="<attributes>]
["b="<bw-cap-list>]
["pt="<med-map-list>]
[future-extensions]
<media> is defined in [RFC4566] and future-extensions = extension-
config-list as defined in [SDPCapNeg].
The m=, t=, b= and a= parameters are identical in format and meaning The m=, t=, b= and a= parameters are identical in format and meaning
to those defined for the pcfg: attribute. Note that the pt= to those defined for the pcfg: attribute. Note that the media type
parameter is not permitted in the lcfg: attribute because no actual (mt=) and media capabilities list MUST be present if the latent
media session is being offered or accepted. configuration is defined at the session level. The mt= parameter
provides the MIME type (audio, video, etc.) for the configuration; it
is OPTIONAL when the latent configuration is declared at the media
level, but if present, MUST match the type of the m= line. The pt=
parameter is not directly meaningful in the lcfg: attribute because
no actual media session is being offered or accepted, but it is
included in order to tie any payload type parameters within
attributes to the media. 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 offeror
and/or answerer may use any payload type numbers each deems
appropriate. Future extensions are also permitted.
Latent Configurations MUST be specified at the session level when Latent configurations MUST be specified at the session level when
they represent an additional media stream to those in the offer or they represent an additional media stream to those in the offer or
answer. If an acap: attribute is declared at the session level for answer. If an acap: attribute is declared at the session level for
use in a session-level lcfg line, it SHOULD NOT be used in a pcfg use in a session-level lcfg line, it SHOULD NOT be used in a pcfg
line at the media level unless it is to become a session-level line at the media level unless it is to become a session-level
attribute in the answer. attribute in the answer.
3.5.1. The crypto: Attribute in Latent Configurations Latent configurations placed at the media level represent
configurations that are supportable, but are not desired for
immediate use. They represent potential future replacements for the
stream description in which they appear. For example, one party
might make an offer with several potential configurations (different
codecs, say). The answer can be made with one particular
configuration specified in the m= and "a=acfg" lines, and a number of
"a=lcfg" lines, one for each "a=pcfg" line which could be supported
by the answerer, but which was not chosen for this media stream.
If the sdescriptions crypto: attribute is necessary as part of any This permits the answerer to tell the offeror that, although a
latent configuration which announces sdescriptions capabilities, it specific codec will be used in the current media session, it can also
presents a slight problem in that the rather long key/salt string is support one or more of the offered alternatives. This could be
useless and should be ignored. This problem is avoided if we exclude useful, for example, in the case that PCMU audio encoding is chosen,
the a= parameters from the lcfg: attribute. [Editor's note: Should but subsequent network congestion suggests that the media stream
we define a new crypto: key-method, e.g. "latent", in which the key- should be renegotiated to G729; the offeror will know in advance that
salt portion of key-info is empty? I think it may be sufficient to the other party supports G729.
just include the RTP/SAVP transport to indicate SRTP capability since
there will be a new offer when the latent configuration will be
offered.]
3.6. Offer/Answer Model Extensions The rules for replacement of base-level attributes by mcap and mscap
attributes invoked within a latent configuration are the same as
specified for invocation in a potential configuration (see section
3.4)
3.6.1. Cryptographic Attributes in Latent Configurations
If a cryptographic attribute, such as the SDES "a=crypto:" attribute
[RFC4568], is referenced by a latent configuration, any key material
REQUIRED in the attribute, such as the SDES key/salt string, MUST be
included. The receiver of the lcfg: attribute MUST ignore any key
material associated with the latent configuration.
3.7. 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 [RFC3264] to allow for media capabilities to be defined in RFC3264 [RFC3264] and [SDPCapNeg] to allow for media
capabilities, bandwidth capabilities, and latent configurations to be
used with the SDP Capability Negotiation framework. used with the SDP Capability Negotiation framework.
3.6.1. Generating the Initial Offer The [SDPCapNeg] 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, bcap, 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.
When an endpoint generates an initial offer and want to use the 3.7.1. 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 functionality described in the current document, it should identify
and define the codecs it can support via mcap , ecap , and fcap and define the codecs it can support via mcap, mfcap and mscap
attributes. The SDP media line(s) should be made up with the default attributes. The SDP media line(s) should be made up with the
(least preferred) configuration. Typically, the media line configuration to be used if the other party does not understand
configuration will contain the minimum acceptable capabilities. The capability negotiations (by default, this is the least preferred
offer MUST include the required extensions needed to support this configuration). Typically, the media line configuration will contain
functionality. 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 Preferred configurations for each media stream are identified
following the media line. The present offer may also include latent following the media line. The present offer may also include latent
configuration (lcfg) attributes, at the session level, describing configuration (lcfg) attributes, at the session level, describing
media streams and/or configurations the offeror is not now offering, media streams and/or configurations the offeror is not now offering,
but which it is willing to support in a future offer/answer exchange. but which it is willing to support in a future offer/answer exchange.
A simple example might be the inclusion of a latent video A simple example might be the inclusion of a latent video
configuration in an offer for an audio stream. configuration in an offer for an audio stream.
3.6.2. Generating the Answer 3.7.2. Generating the Answer
When the answering party receives the offer and if it supports the When the answering party receives the offer and if it supports the
required extensions, it should select the most-preferred required capability negotiation extensions, it should select the
configuration for each media stream, and build its answer most-preferred configuration it can support for each media stream,
accordingly. Each selected media configuration is placed into the and build its answer accordingly. The configuration selected for
answer as a media line with associated parameters, It must include each accepted media stream is placed into the answer as a media line
the supported extension attribute and an actual configuration (acfg) 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 attribute to indicate just which pcfg attribute was used to build the
answer. The answer should may also include any latent configurations answer. The answer should also include any latent configurations the
the answerer can support, especially any configurations compatible answerer can support, especially any configurations compatible with
with latent configurations received in the offer. The answerer other proposed or latent configurations received in the offer. The
should make note of those configurations it might wish to offer in answerer should make note of those configurations it might wish to
the future. offer in the future.
3.6.3. Offerer Processing of the Answer 3.7.3. Offerer Processing of the Answer
When the offerer receives the answer, it should make note of any When the offeror receives the answer, it should make note of any
capabilities and/or latent configurations for future use. The media capabilities and/or latent configurations for future use. The media
line(s) must be processed in the normal way to identify 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 stream(s) accepted by the answer, if any. The acfg attribute, if
present, may be used to verify the proposed configuration used to present, may be used to verify the proposed configuration used to
form the answer, and to infer the lack of acceptability of higher- form the answer, and to infer the lack of acceptability of higher-
preference configurations that were not chosen. Note that the base preference configurations that were not chosen. Note that the base
specification [SDPCapNeg] requires the answerer to choose the highest specification [SDPCapNeg] requires the answerer to choose the highest
preference configuration it can support. preference configuration it can support.
3.6.4. Modifying the Session 3.7.4. Modifying the Session
If, at a later time, one of the parties wishes to modify the 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 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 stream, or changing the properties used on an existing stream, it may
via the mechanisms defined for offer/answer[RFC3264]. If the do so via the mechanisms defined for offer/answer[RFC3264]. If the
initiating party has remembered the codecs, potential configurations, initiating party has remembered the codecs, potential configurations,
and latent configurations announced by the other party in the earlier and latent configurations announced by the other party in the earlier
negotiation, it may use this knowledge to maximize the likelihood of negotiation, it may use this knowledge to maximize the likelihood of
a successful modification of the session. Alternatively, it may a successful modification of the session. Alternatively, it may
perform a new capabilities exchange as part of the reconfiguration. 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
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 so
the offeror can interpret early media.
1. v=0
2. o=- 25678 753849 IN IP4 192.0.2.1
3. s=
4. c=IN IP4 192.0.2.1
5. t=0 0
6. a=creq:med-v0
7. m=audio 54321 RTP/AVP 96
8. rtpmap:96 AMR/16000/1
9. a=fmtp:96 mode-change-capability=1; max-red=220; mode-
set=0,2,4,7
10. a=cmed:1,3,5 audio AMR/16000/1
11. a=cmed:2,4,6 audio AMR/8000/1
12. a=mfcap:1,2,3,4 mode-change-capability=1
13. a=mfcap:5,6 mode-change-capability=2
14. a=mfcap:1,2,3,5 max-red=220
15. a=mfcap:3,4,5,6 octet-align=1
16. a=mfcap:1,3,5 mode-set=0,2,4,7
17. a=mfcap:2,4,6 mode-set=0,3,5,6
18. pcfg:1 m=1 pt=1:96
19. pcfg:2 m=2 pt=2:97
20. pcfg:3 m=3 pt=3:98
21. pcfg:4 m=4 pt=4:99
22. pcfg:5 m=5 pt=5:100
23. pcfg:6 m=6 pt=6:101
In the above example, media capability 1 could have been excluded
from the cmed declaration in line 10 and from the mfcap attributes in
lines 12, 14, and 16. The pcfg line 18 could then 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:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=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
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
4.2. Latent Media Streams 4.2. Latent Media Streams
Consider a case in which the offeror 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 offeror 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:
1. v=0
2. o=- 25678 753849 IN IP4 192.0.2.1
3. s=
4. c=IN IP4 192.0.2.1
5. t=0 0
6. a=creq:med-v0
7. a=mcap:10 H263-1998/90000
8. a=mcap:11 H264/90000
9. a=lcfg:10 mt=video m=10|11
10. m=audio 23456 RTP/AVP 0
11. a=rtpmap:0 PCMU/8000
12. a=mcap:1 PCMU/8000
13. a=mcap:2 g729/8000
14. a=mcap:3 telephone-event/8000
15. a=mfcap:3 0-11
16. a=pcfg:1 m=1,3|2,3 pt=1:0,2:18,3:100
Lines 7-9 announce support for H.263 and H.264 video (H.263
preferred) for future reference. Lines 10 and 11 offer an audio
stream and provide the lowest precedence configuration (PCMU without
any DTMF encoding). Lines 12-14 define the media capabilities to be
offered: PCMU, G729, and telephone-event. Line 15 provides the
format parameters for telephone-events, specifying the 12 commercial
DTMF 'digits'. Line 16 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:
1. v=0
2. o=- 24351 621814 IN IP4 192.0.2.2
3. s=
4. c=IN IP4 192.0.2.2
5. t=0 0
6. a=csup:med-v0
7. a=mcap:10 H263-1998/90000
8. a=lcfg:1 mt=video m=10
9. m=audio 54321 RTP/AVP 0 100
10. a=rtpmap:0 PCMU/8000
11. a=rtpmap:100 telephone-event/8000
12. a=fmtp:100 0-11
13. a=acfg:1 m=1,3 pt=1:0,3:100
14. a=mcap:1 G729/8000
15. a=mcap:2 telephone-event/8000
16. a=mfcap:2 0-11
17. a=lcfg:2 m=1,2 pt=1:18,2:100
Lines 7 and 8 announce the capability to support H.263 video at a
later time. Lines 9-12 of the answer present the selected
configuration for the media stream. Line 13 identifies the potential
configuration from which it was taken, and lines 14-17 announce the
latent capability to support G.711 mu-law with DTMF events as well.
If, at some later time, congestion becomes a problem in the network,
either party may offer a reconfiguration of the media stream to use
G.729 in order to reduce packet sizes. Note that line 13 uses media
capability numbering as provided in the original offer, whereas lines
14-17 must use their own numbering.
5. IANA Considerations 5. IANA Considerations
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: bcap
Long form name: Bandwidth capability attribute
Type of attribute: Session-level or media-level
Subject to charset: No
Purpose: associate bandwidth limitations with potential
potential or latent configurations.
Appropriate Values: See Section 3.3.4
Attribute name: lcfg
Long form name: Latent Configuration
Type of attribute: Session-level or media-level
Subject to charset: No
Purpose: To announce supportable media configurations
without offering them for immediate use.
Appropriate Values: See Section 3.6
6. Security Considerations 6. Security Considerations
The security considertions of [SDPCapNeg] apply for this document.
No additional security considerations are introduced here.
7. Changes from previous versions 7. Changes from previous versions
7.1. Changes from version 01 7.1. 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.
7.2. Changes from version 01
The documents adds a new attribute for specifying bandwidth The documents adds a new attribute for specifying bandwidth
capability and a parametr to list in the potential configuration. capability and a parametr to list in the potential configuration.
Other changes are to align the document with the terminolgy and Other changes are to align the document with the terminolgy and
attribute names from draft-ietf-mmusic-sdp-capability-negotiation-07. attribute names from draft-ietf-mmusic-sdp-capability-negotiation-07.
The document also clarifies some previous open issues. The document also clarifies some previous open issues.
7.2. Changes from version 00 7.3. 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
in particular provided useful comments and suggestions to either the in particular provided useful comments and suggestions to either the
document itself or the overall direction of the solution defined document itself or the overall direction of the solution defined
herein: Cullen Jennings, Matt Lepinski, Joerg Ott, Colin Perkins, and herein: Cullen Jennings, Matt Lepinski, Joerg Ott, Colin Perkins, and
Thomas Stach. Thomas Stach.
We thank Ingemar Johansson and Magnus Westerlund for examples that
stimulated this work.
9. References 9. References
9.1. Normative References 9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[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-07 (work in draft-ietf-mmusic-sdp-capability-negotiation-08 (work in
progress), October 2007. progress), December 2007.
9.2. Informative References 9.2. Informative References
[I-D.ietf-avt-avpf-ccm]
Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
"Codec Control Messages in the RTP Audio-Visual Profile
with Feedback (AVPF)", draft-ietf-avt-avpf-ccm-10 (work
in progress), October 2007.
[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 [RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session
(RTP) Payload Format for internet Low Bit Rate Codec Description Protocol (SDP) Security Descriptions for Media
(iLBC) Speech", RFC 3952, December 2004. Streams", RFC 4568, July 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.
Authors' Addresses Authors' Addresses
Robert R Gilman Robert R Gilman
Avaya, Inc. NDCI
1300 West 120th Avenue Broomfield, CO 80020
Westminster, CO 80234
USA USA
Email: rrg@avaya.com Email: bob_gilman@comcast.net
Roni Even (editor) Roni Even (editor)
Polycom Polycom
94 Derech Em Hamoshavot 94 Derech Em Hamoshavot
Petach Tikva 49130 Petach Tikva 49130
Israel Israel
Email: roni.even@polycom.co.il Email: roni.even@polycom.co.il
Flemming Andreasen Flemming Andreasen
Cisco Systems Cisco Systems
Edison, NJ Edison, NJ
USA USA
Email: fandreas@cisco.com Email: fandreas@cisco.com
Full Copyright Statement Full Copyright Statement
Copyright (C) The IETF Trust (2007). Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors contained in BCP 78, and except as set forth therein, the authors
retain all their rights. retain all their rights.
This document and the information contained herein are provided on an This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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