draft-ietf-mmusic-sdp-media-capabilities-03.txt   draft-ietf-mmusic-sdp-media-capabilities-04.txt 
MMUSIC R. Gilman MMUSIC R. Gilman
Internet-Draft NDCI Internet-Draft NDCI
Intended status: Standards Track R. Even, Ed. Intended status: Standards Track R. Even
Expires: August 28, 2008 Polycom Expires: January 13, 2009 Polycom
F. Andreasen F. Andreasen
Cisco Systems Cisco Systems
February 25, 2008 July 12, 2008
SDP media capabilities Negotiation SDP media capabilities Negotiation
draft-ietf-mmusic-sdp-media-capabilities-03.txt draft-ietf-mmusic-sdp-media-capabilities-04.txt
Status of this Memo Status of this Memo
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aware will be disclosed, in accordance with Section 6 of BCP 79. aware will be disclosed, in accordance with Section 6 of BCP 79.
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This Internet-Draft will expire on August 28, 2008. This Internet-Draft will expire on January 13, 2009.
Copyright Notice Copyright Notice
Copyright (C) The IETF Trust (2008). 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 indicating and negotiating capabilities in SDP. general framework for indicating and negotiating capabilities in SDP.
The base framework defines only capabilities for negotiating The base framework defines only capabilities for negotiating
skipping to change at page 3, line 10 skipping to change at page 3, line 10
framework by defining media capabilities that can be used to framework by defining media capabilities that can be used to
negotiate media types and their associated parameters. This negotiate media types and their associated parameters. This
extension is designed to map easily to existing and future SDP media extension is designed to map easily to existing and future SDP media
attributes. 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. Requirements . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Design Goals . . . . . . . . . . . . . . . . . . . . . . . 10 3.2. Solution Overview . . . . . . . . . . . . . . . . . . . . 7
3.3. New Capability Attributes . . . . . . . . . . . . . . . . 11 3.3. New Capability Attributes . . . . . . . . . . . . . . . . 12
3.3.1. The Media Encoding Capability Attribute . . . . . . . 11 3.3.1. The Media Encoding Capability Attribute . . . . . . . 13
3.3.2. The Media Format Parameter Capability Attribute . . . 12 3.3.2. The Media Format Parameter Capability Attribute . . . 14
3.3.3. The Media-Specific Capability Attribute . . . . . . . 15 3.3.3. The Media-Specific Capability Attribute . . . . . . . 17
3.3.4. The Bandwidth Capability Attribute . . . . . . . . . . 16 3.3.4. Additional Capability Attributes . . . . . . . . . . . 18
3.4. Extensions to the Potential Configuration Attribute . . . 17 3.3.5. The Latent Configuration Attribute . . . . . . . . . . 20
3.4.1. The Media Capability Parameter . . . . . . . . . . . . 17 3.3.6. Enhanced Potential Configuration Attribute . . . . . . 24
3.4.2. The Payload Type Mapping Parameter . . . . . . . . . . 19 3.3.7. Substitution of Media Payload Type Numbers in
3.4.3. The Bandwidth Parameter . . . . . . . . . . . . . . . 20 Capability Attribute Parameters . . . . . . . . . . . 27
3.5. Extensions to the Actual Configuration Attribute . . . . . 20 3.3.8. The Session Capability Attribute . . . . . . . . . . . 28
3.6. The Latent Configuration Attribute . . . . . . . . . . . . 21 3.4. Offer/Answer Model Extensions . . . . . . . . . . . . . . 32
3.6.1. Cryptographic Attributes in Latent Configurations . . 23 3.4.1. Generating the Initial Offer . . . . . . . . . . . . . 33
3.7. Offer/Answer Model Extensions . . . . . . . . . . . . . . 23 3.4.2. Generating the Answer . . . . . . . . . . . . . . . . 33
3.7.1. Generating the Initial Offer . . . . . . . . . . . . . 23 3.4.3. Offerer Processing of the Answer . . . . . . . . . . . 34
3.7.2. Generating the Answer . . . . . . . . . . . . . . . . 24 3.4.4. Modifying the Session . . . . . . . . . . . . . . . . 34
3.7.3. Offerer Processing of the Answer . . . . . . . . . . . 24 4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.7.4. Modifying the Session . . . . . . . . . . . . . . . . 24 4.1. Alternative Codecs . . . . . . . . . . . . . . . . . . . . 35
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 4.2. Alternative Combinations of Codecs (Session
4.1. Alternative Codecs . . . . . . . . . . . . . . . . . . . . 26 Configurations) . . . . . . . . . . . . . . . . . . . . . 38
4.2. Latent Media Streams . . . . . . . . . . . . . . . . . . . 29 4.3. Latent Media Streams . . . . . . . . . . . . . . . . . . . 38
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 41
6. Security Considerations . . . . . . . . . . . . . . . . . . . 32 6. Security Considerations . . . . . . . . . . . . . . . . . . . 43
7. Changes from previous versions . . . . . . . . . . . . . . . . 33 7. Changes from previous versions . . . . . . . . . . . . . . . . 44
7.1. Changes from version 02 . . . . . . . . . . . . . . . . . 33 7.1. Changes from version 03 . . . . . . . . . . . . . . . . . 44
7.2. Changes from version 01 . . . . . . . . . . . . . . . . . 33 7.2. Changes from version 02 . . . . . . . . . . . . . . . . . 44
7.3. Changes from version 00 . . . . . . . . . . . . . . . . . 33 7.3. Changes from version 01 . . . . . . . . . . . . . . . . . 45
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 34 7.4. Changes from version 00 . . . . . . . . . . . . . . . . . 45
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 46
9.1. Normative References . . . . . . . . . . . . . . . . . . . 35 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 47
9.2. Informative References . . . . . . . . . . . . . . . . . . 35 9.1. Normative References . . . . . . . . . . . . . . . . . . . 47
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 36 9.2. Informative References . . . . . . . . . . . . . . . . . . 47
Intellectual Property and Copyright Statements . . . . . . . . . . 37 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 49
Intellectual Property and Copyright Statements . . . . . . . . . . 50
1. Introduction 1. Introduction
Session Description Protocol (SDP) capability negotiation [SDPCapNeg] Session Description Protocol (SDP) capability negotiation [SDPCapNeg]
provides a general framework for indicating and negotiating provides a general framework for indicating and negotiating
capabilities in SDP[RFC4566]. The base framework defines only capabilities in SDP[RFC4566]. The base framework defines only
capabilities for 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
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current document to negotiate media capabilities. 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 indicate and negotiate media types capabilities that can be used to indicate and negotiate media types
and their associated parameters like bandwidth. This document also and their associated stream parameters such as bandwidth. This
adds the ability to declare support for media streams, the use of document also adds the ability to declare support for media streams,
which can be offered and negotiated later. The definitions of new the use of which can be offered and negotiated later. The
attributes for media capability negotiation are chosen to make the definitions of new attributes for media capability negotiation are
translation from these attributes to "conventional" SDP [RFC4566] chosen to make the translation from these attributes to
media attributes as straightforward as possible in order to simplify "conventional" SDP [RFC4566] media attributes as straightforward as
implementation. This goal is intended to reduce processing in two possible in order to simplify implementation. This goal is intended
ways: each proposed configuration in an offer may be easily to reduce processing in two ways: each proposed configuration in an
translated into a conventional SDP media stream record for processing offer may be easily translated into a conventional SDP media stream
by the receiver; and the construction of an answer based on a record for processing by the receiver; and the construction of an
selected proposed configuration is straightforward. 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.
"Base Configuration": the media configuration represented by a media "Base Attributes": Conventional SDP attributes appearing in the base
configuration of a media block.
"Base Configuration": The media configuration represented by a media
block exclusive of all the capability negotiation attributes defined block exclusive of all the capability negotiation attributes defined
in this document and [SDPCapNeg] in this document, the base capability document[SDPCapNeg], or any
future capability negotiation document.
"Conventional SDP": all SDP lines exclusive of the capability "Conventional Attribute": Any SDP attribute other than those defined
negotiation attributes by the series of capability negotiation specifications.
"Conventional SDP": An SDP record devoid of capability negotiation
attributes.
"Media Capability": A media encoding, typically a media subtype such
as PCMU, H263-1998, or T38.
3. SDP Media Capabilities 3. SDP Media Capabilities
The SDP capability negotiation [SDPCapNeg] discusses the use of any The SDP capability negotiation [SDPCapNeg] discusses the use of any
SDP [RFC4566] attribute (a=) under the attribute capability "acap". SDP [RFC4566] attribute (a=) under the attribute capability "acap".
The limitations of using acap for fmtp and rtpmap in a potential The limitations of using acap for fmtp and rtpmap in a potential
configuration are described in [SDPCapNeg]; for example they can be configuration are described in [SDPCapNeg]; for example they can be
used only at the media level since they are media level attributes. used only at the media level since they are media level attributes.
The [SDPCapNeg] partially addresses the issue of bandwidth usage for The [SDPCapNeg] partially addresses the issue of bandwidth usage for
different configurations but does not offer a way to negotiate different configurations but does not offer a way to negotiate
different bandwidth for different codecs and profiles. This section different bandwidth for different codecs and profiles. This section
provides an overview of extensions providing SDP Media Capability and provides an overview of extensions providing SDP Media Capability and
Bandwidth negotiation solution offering more robust capabilities 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. Requirements
The solution consists of five new attributes and four new parameters The capability negotiation extensions described herein are described
for the pcfg attribute extending the base attributes from as follows.
[SDPCapNeg].
Four new attributes are used to make up media capabilities in a REQ-01: Support the specification of alternative (combinations of)
manner that can be related to the capabilities specified in a media media formats (codecs) in a single media block.
line, its attributes, and the bandwidth parameter lines.
o A new media attribute ("a=mcap") that lists media formats as REQ-02: Support the specification of alternative media format
capabilities in the form of a subtype (e.g. "PCMU"), and its parameters for each media format.
encoding parameters (e.g. "/8000/2"). Each resulting media format
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=mfcap") that specifies media format parameters REQ-03: Retain backward compatibility with conventional SDP.
Insure that each and every offered configuration can be easily
translated into a corresponding SDP media block expressed with
conventional SDP lines.
REQ-04: Insure the scheme operates within the offer/answer model in
such a way that media formats and parameters can be agreed upon
with a single exchange.
REQ-05: Provide the ability to express offers in such a way that
the offerer can receive media as soon as the offer is sent. (Note
that the offerer may not be able to render received media prior to
exchange of keying material.)
REQ-06: Provide the ability to offer latent media configurations
for future negotiation or acceptance in the current offer/answer
exchange.
REQ-07: Provide reasonable efficiency in the expression of
alternative media formats and/or format parameters, especially in
those cases in which many combinations of options are offered.
REQ-08: Retain the extensibility of the base capability negotiation
mechanism.
REQ-09: Provide the ability to specify acceptable combinations of
media streams and encodings. For example, offer a PCMU audio
stream with an H264 video stream, or a G729 audio stream with an
H263 video stream. This ability would give the offerer a means to
limit processing requirements for simultaneous streams. This
would also permit an offer to include the choice of an audio/T38
stream or an image/T38 stream, but not both.
REQ-10: Provide capability negotiation attributes for all media-
level SDP line types in the same manner as already done for the
attribute type, with the exception of the media line type itself.
The media line type is handled in a special way to permit compact
expression of media coding/format options.
Other possible extensions have been discussed, but have not been
treated in this document. They may be considered in the future. Two
such extensions are:
FUT-01: Provide the ability to mix, or change, MIMEmedia types
within a single media block. Conventional SDP does not support
this capability explicitly; the usual technique is to define a
media subtype that represents the actual format within the nominal
media type. For example, T.38 FAX as an alternative to audio/PCMU
within an audio stream is identified as audio/T38; a separate FAX
stream would use image/T38.
FUT-02: Provide the ability to support multiple transport protocols
within an active media stream without reconfiguration. This is
not explicitly supported by conventional SDP.
3.2. Solution Overview
The solution consists of new capability attributes corresponding to
conventional SDP line types, new parameters for the pcfg attribute
extending the base attributes from [SDPCapNeg], and a use of the pcfg
attribute to return capability information in the SDP answer.
Three new attributes are defined in a manner that can be related to
the capabilities specified in a media line, and its corresponding
rtpmap and fmtp attributes.
o A new media attribute ("a=mcap") defines media capabilities in the
form of a subtype (e.g. "PCMU"), and its encoding parameters
(e.g. "/8000/2"). Each resulting media format type / subtype
capability has an associated handle called a media capability
number. The encoding parameters are as specified for the rtpmap
attribute defined in [RFC4566]
o A new attribute ("a=mfcap") specifies media format parameters
associated with one or more media capabilities. The mfcap associated with one or more media capabilities. The mfcap
attribute is used to associate the formatting capabilities attribute is used primarily to associate the formatting
normally carried in the fmtp attribute. capabilities normally carried in the fmtp attribute.
o A new attribute ("a=mscap") that specifies media parameters o A new attribute ("a=mscap") that specifies media parameters
associated with one or more media capabilities. The mscap associated with one or more media capabilities. The mscap
attribute is used to associate capabilities other than fmtp or attribute is used to associate capabilities with attributes other
rtpmap attributes, for example the rtcp-fb attribute. than fmtp or rtpmap, for example, the rtcp-fb attribute.
o A new attribute ("a=bcap") that is used to specify the bandwidth o Several new capability attributes, corresponding to several SDP
parameter capability. line types (e.g., the bandwidth type "b="}, are defined for use in
capability negotiations.
o A new attribute ("a=lcfg") that specifies latent media stream o A new attribute ("a=lcfg") specifies latent media stream
configurations when no corresponding media stream is offered. An configurations when no corresponding media line ("m=") is offered.
example is a latent configuration for video even though no video An example is the offer of a latent configuration for video even
is currently offered. though no video is currently offered. If both parties indicate
support for one or more latent configurations, the corresponding
media stream(s) may be added via a new offer/answer exchange.
o A new attribute ("a=sescap") is used to specify an acceptable
combination of simultaneous media streams as a list of potential
and/or latent configurations.
New parameters are defined for the potential configuration (pcfg), New parameters are defined for the potential configuration (pcfg),
latent configuration (lcfg), and accepted configuration (acfg) latent configuration (lcfg), and accepted configuration (acfg)
attributes to associate the new attributes with particular attributes to associate the new attributes with particular
configurations. configurations.
o A new parameter type ("m=")is added to the potential configuration o A new parameter type ("m=") is added to the potential
("a=pcfg:") attribute and the actual configuration ("a=acfg:") configuration ("a=pcfg:") attribute and the actual configuration
attribute defined in [SDPCapNeg], and to the new latent ("a=acfg:") attribute defined in [SDPCapNeg], and to the new
configuration ("a=lcfg:") attribute which permits specification of latent configuration ("a=lcfg:") attribute. This permits
media capabilities (including their associated parameters) and specification of media capabilities (including their associated
combinations thereof for the configuration. For example, the parameters) and combinations thereof for the configuration. For
"a=pcfg:" line might specify PCMU and telephone events or G.729B example, the "a=pcfg:" line might specify PCMU and telephone
and telephone events as acceptable configurations. The "a=acfg:" events or G.729B and telephone events as acceptable
line in the answer would specify the accepted choice. configurations. The "a=acfg:" line in the answer would specify
the accepted choice.
o A new parameter type ("pt=") is added to the potential o A new parameter type ("pt=") is added to the potential
configuration, actual configuration, and latent configuration configuration, actual configuration, and latent configuration
attributes. This parameter associates RTP payload types with the attributes. This parameter associates RTP payload types numbers
referenced media capabilities, and is appropriate only when the with the referenced media capabilities, and is appropriate only
transport protocol uses RTP. when the transport protocol uses RTP.
o A new parameter type ("b=") is used to specify bandwidth o A new parameter type ("mt=") is used to specify the media type for
parameters in a potential configuration. latent configurations.
o A new parameter type ("mt=") is used to specify the MIME type for o New parameter types (e.g., "b=") are used to specify conventional
latent configurations that are declared at the session level. SDP parameters in potential or latent configurations.
The document extends the base protocol extensions to the offer/answer Special processing rules are defined for capability attribute
model that allow for capabilities and potential configurations to be arguments in order to reduce the need to replicate essentially-
included in an offer. Media capabilities constitute capabilities identical attribute lines for the base configuration and potential
that can be used in potential and latent configurations. Whereas configurations.
potential configurations constitute alternative offers that may be
accepted by the answerer instead of the actual configuration(s) o A substitution rule is defined for any capability attribute to
included in the "m=" line(s) and associated parameters, latent permit the replacement of the (escaped) media capability number
configurations merely inform the other side of possible with the media format identifier (e.g., the payload type number in
configurations supported by the entity. Those latent configurations audio/video profiles).
may be used to guide subsequent offer/answer exchanges, but they are
not part of the current offer/answer exchange. o Replacement rules are defined for the conventional SDP equivalents
of the mfcap, mscap, and bcap capability attributes. This reduces
the necessity to use the deletion qualifier in the pcfg a=
parameter in order to ignore rtpmap, fmtp, and certain other
attributes in the base configuration.
o An argument concatenation rule is defined for mfcap attributes
which refer to the same media capability number. This makes it
convenient to combine format options concisely by associating
multiple mfcap lines with multiple media capabilities.
This document extends the base protocol extensions to the offer/
answer model that allow for capabilities and potential configurations
to be included in an offer. Media capabilities constitute
capabilities that can be used in potential and latent configurations.
Whereas potential configurations constitute alternative offers that
may be accepted by the answerer instead of the actual
configuration(s) included in the "m=" line(s) and associated
parameters, latent configurations merely inform the other side of
possible configurations supported by the entity. Those latent
configurations may be used to guide subsequent offer/answer
exchanges, but they are not part of the current offer/answer
exchange.
The mechanism is illustrated by the offer/answer exchange below, 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) |
|<---------------------------------| |<---------------------------------|
| | | |
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 (long lines are folded to fit
the margins):
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 m=audio 3456 RTP/AVP 0 18
a=tcap:1 RTP/SAVP a=tcap:1 RTP/SAVP RTP/AVP
a=rtpmap:0 PCMU/8000/1 a=rtpmap:0 PCMU/8000/1
a=rtpmap:18 G729/8000/1 a=rtpmap:18 G729/8000/1
a=fmtp:18 annexb=yes a=fmtp:18 annexb=yes
a=mcap:1,4 g729/8000/1 a=mcap:1,4 g729/8000/1
a=mcap:2 PCMU/8000/1 a=mcap:2 PCMU/8000/1
a=mcap:5 telephone-event/8000 a=mcap:5 telephone-event/8000
a=mfcap:1 annexb=no a=mfcap:1 annexb=no
a=mfcap:4 annexb=yes a=mfcap:4 annexb=yes
a=mfcap:5 0-11 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
skipping to change at page 9, line 8 skipping to change at page 11, line 9
transport protocol (RTP/AVP in this case) and the default actual transport protocol (RTP/AVP in this case) and the default actual
configuration. configuration.
The "a=tcap:1" line, specified in the base protocol, defines a The "a=tcap:1" line, specified in the base protocol, defines a
transport protocol capabilities, in this case Secure RTP (SAVP transport protocol capabilities, in this case Secure RTP (SAVP
profile) as the first option and RTP (AVP profile) as the second profile) as the first option and RTP (AVP profile) as the second
option. option.
The "a=mcap:1,4" line defines two G.729 media format capabilities, The "a=mcap:1,4" line defines two G.729 media format capabilities,
numbered 1 and 4, and their encoding rate. The capabilities are of numbered 1 and 4, and their encoding rate. The capabilities are of
subtype G729. Note that the media subtype is explicitly specified media type "audio" and subtype G729. Note that the media subtype is
here, rather than RTP payload type number. In this example, two explicitly specified here, rather than RTP payload type numbers. In
G.729 subtype capabilities are defined. This permits the declaration this example, two G.729 subtype capabilities are defined. This
of two sets of formatting parameters for G.729. permits the declaration of two sets of formatting parameters for
G.729.
The "a=mcap:2" line defines a G.711 mu-law capability, numbered 2. The "a=mcap:2" line defines a G.711 mu-law capability, numbered 2.
The "a=mcap:5" line defines an audio telephone-event capability, The "a=mcap:5" line defines an audio telephone-event capability,
numbered 5. numbered 5.
The "a=mfcap:1" line specifies the fmtp formatting parameters for The "a=mfcap:1" line specifies the fmtp formatting parameters for
capability 1 (no comfort noise packets). capability 1 (no comfort noise packets).
The "a=mfcap:4" line specifies the fmtp formatting parameters for The "a=mfcap:4" line specifies the fmtp formatting parameters for
skipping to change at page 9, line 33 skipping to change at page 11, line 35
The "a=mfcap:5" line specifies the fmtp formatting parameters for The "a=mfcap:5" line specifies the fmtp formatting parameters for
capability 5 (the DTMF touchtones 0-9,*,#). capability 5 (the DTMF touchtones 0-9,*,#).
The "a=acap:1" line specified in the base protocol provides the The "a=acap:1" line specified in the base protocol provides the
"crypto" attribute which provides the keying material for SRTP using "crypto" attribute which provides the keying material for SRTP using
SDP security descriptions. SDP security descriptions.
The "a=pcfg:" attributes provide the potential configurations The "a=pcfg:" attributes provide the potential configurations
included in the offer by reference to the media capabilities, included in the offer by reference to the media capabilities,
transport capabilities, and associated payload type mappings. Three transport capabilities, and associated payload type number mappings.
explicit alternatives are provided; the first one, numbered 1 is the Three explicit alternatives are provided; the first one, numbered 1
preferred one. It specifies media capabilities 4 and 5, i.e., G.729B is the preferred one. It specifies media capabilities 4 and 5, i.e.,
and DTMF, or media capability 1 and 5, i.e., G.729 and DTMF. G.729B and DTMF, or media capability 1 and 5, i.e., G.729 and DTMF.
Furthermore, it specifies transport protocol capability 1 (i.e. the 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 media capabilities 1, 4, and 5, thereby permitting the number mapping for media capabilities 1, 4, and 5, thereby permitting
offeror to distinguish between encrypted media and unencrypted media the offeror to distinguish between encrypted media and unencrypted
received prior to receipt of the answer. Use of unique payload types media received prior to receipt of the answer. Use of unique payload
is not required; codecs such as AMR-WB [RFC4867] have the potential type numbers is not required; codecs such as AMR-WB [RFC4867] have
for so many combinations of options that it may be impractical to the potential for so many combinations of options that it may be
define unique payload types for all supported combinations. For SRTP impractical to define unique payload type numbers for all supported
using SDES inline keying [RFC4568], the offeror will still need to combinations. If unique payload type numbers cannot be specified,
receive the answer before being able to decrypt the stream. then the offerer will be obliged to wait for the SDP answer before
rendering received media. 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. PCMU, The second alternative specifies media capability 2, i.e. PCMU,
under the RTP/SAVP profile, with the same SRTP key material. under the RTP/SAVP profile, with the same SRTP key material.
The third alternative offers G.729B unsecured; it's only purpose in The third alternative offers G.729B unsecured; it's only purpose in
this example is to show a preference for G.729B over PCMU. this example is to show a preference for G.729B over PCMU.
The media line, with any qualifying attributes such as fmtp or The media line, with any qualifying attributes such as fmtp or
rtpmap, is itself considered a valid configuration; it is assumed to rtpmap, is itself considered a valid configuration; it is assumed to
be the lowest preference. be the lowest preference.
Bob receives the SDP offer from Alice. Bob supports G.729B, PCMU, 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 and telephone events over RTP, but not SRTP, hence he accepts the
potential configuration 3 for RTP provided by Alice. Hence, Bob potential configuration 3 for RTP provided by Alice. Bob generates
generates the following answer: 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 18 m=audio 4567 RTP/AVP 18
a=rtpmap:18 G729/8000 a=rtpmap:18 G729/8000
a=fmtp:18 annexb=yes a=fmtp:18 annexb=yes
skipping to change at page 10, line 44 skipping to change at page 12, line 48
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. 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 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 type numbers 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 be 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 3.4.2). types are then specified at the media level (see Section 3.3.1).
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 3.4.1). in a potential configuration.
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 (consistent with the MIME type), whereas attribute at the media level (consistent with the media type),
media capabilities provided at the media level may be referenced by a whereas media capabilities provided at the media level may be
pcfg attribute within that media stream only. In either case, the referenced by a pcfg or lcfg attribute within that media stream only.
scope of the <med-cap-num> is the entire session description. This In either case, the scope of the <med-cap-num> is the entire session
enables each media capability to be uniquely referenced across the description. This enables each media capability to be uniquely
entire session description (e.g. in a potential configuration). referenced across the entire session description (e.g. in a potential
configuration).
3.3.1. The Media Encoding Capability Attribute 3.3.1. The Media Encoding Capability Attribute
Media subtypes can be expressed as media encoding capabilities by use Media subtypes can be expressed as media encoding capabilities by use
of the "a=mcap" attribute, which is defined as follows: of the "a=mcap" attribute, which is defined as follows:
a=mcap:<med-cap-num-list> <subtype>[/<encoding-parms>] a=mcap:<med-cap-num-list> <subtype>[/<encoding-parms>]
<med-cap-num-list> = <med-cap-num> *[COMMA <med-cap-num>] <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>, or media capability number, is an integer
included) used to number a media format capability, or a range of between 1 and 2^31-1 (both included) used to number a media format
such integers and the <subtype> is the media subtype e.g. H263-1998 capability, or a range of such integers and the <subtype> is the
or PCMU. and <encoding-parms> are the media encoding parameters for media subtype e.g. H263-1998 or PCMU. and <encoding-parms> are the
the <subtype>. All media format capabilities in the list are media encoding parameters for the <subtype>. All media format
assigned to the same media type/subtype. Each occurrence of the mcap capabilities in the list are assigned to the same media type/subtype.
attribute MUST use unique values in its <med-cap-num-list>; the media Each occurrence of the mcap attribute MUST use unique values in its
capability numbers must be unique across the entire session or media <med-cap-num-list>; the media capability numbers must be unique
description. In short, the mcap attribute defines media capabilities across the entire SDP session. In short, the mcap attribute defines
and associates them with a media capability number in the same manner media capabilities and associates them with a media capability number
as the rtpmap attribute defines them and associates them with a in the same manner as the rtpmap attribute defines them and
payload type. associates them with a payload type number.
In ABNF, we have: In ABNF, we have:
media-capability-line = "a=mcap:" media-cap-num-list media-capability-line = "a=mcap:" media-cap-num-list
1*WSP media-cap 1*WSP media-cap
["/" clock-rate ["/" encoding-parms]] ["/" clock-rate ["/" encoding-parms]]
media-cap-num-list = media-cap-num *[COMMA media-cap-num] media-cap-num-list = media-cap-num *[COMMA media-cap-num]
media-cap-num = 1*DIGIT | media-cap-range media-cap-num = 1*DIGIT | media-cap-range
media-cap-range = 1*DIGIT "-" 1*DIGIT media-cap-range = 1*DIGIT "-" 1*DIGIT
media-cap = token ; Subtype name(PCMU, G729, etc.) media-cap = token ; Subtype name(PCMU, G729, etc.)
clock-rate = 1*DIGIT clock-rate = 1*DIGIT
encoding-parms = token encoding-parms = token
The clock-rate and encoding-params are as defined to appear in an 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 rtpmap attribute for each media type/subtype. Thus, it is easy to
convert an mcap attribute line into one or more rtpmap attribute convert an mcap attribute line into one or more rtpmap attribute
lines, once a payload type is assigned to a media-cap-num (see lines, once a payload type number is assigned to a media-cap-num (see
section 3.4.2). section 3.3.5).
The "mcap" attribute can be provided at the session-level and/or the 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. Each media-cap-num must be unique within the
each media capability in potential and actual configurations. When entire SDP record; it is used to identify that media capability in
used in a potential configuration it is a media level attribute potential, latent and actual configurations, and in other attribute
capability regardless if it is specified at the session or media lines as explained below. When used in a potential, latent or actual
level. In other words, the media capability applies to the specific configuration it is, in effect, a media level attribute regardless if
media description associated with the potential configuration in it is specified at the session or media level. In other words, the
question. media capability applies to the specific media description associated
with the configuration which invoke it.
For example: For example:
v=0 v=0
a=mcap:1 L16/8000/1 a=mcap:1 L16/8000/1
a=mcap:2 L16/16000/2 a=mcap:2 L16/16000/2
a=mcap:3,4 H263-1998/90000 a=mcap:3,4 H263-1998/90000
m=audio 54320 RTP/AVP 0
pcfg:1 m=1|2, pt=1:99,2:98
m=video 66544 RTP/AVP 100
a=rtpmap:100 H264/90000
a=pcfg:10 m=3 pt=3:101
3.3.2. The Media Format Parameter Capability Attribute 3.3.2. The Media Format Parameter Capability Attribute
This attribute is used to associate media-specific parameters with This attribute is used to associate media-specific format parameters
one or more media capabilities. The form of the attribute is: with one or more media capabilities. The form of the attribute is:
a=mfcap:<med-cap-num-list> <list of parameters> a=mfcap:<media-caps> <list of parameters>
where <med-cap-num-list> permits the parameter(s) to be associated where <media-caps> permits the parameter(s) to be associated with one
with one or more media capabilities and the format parameters are or more media capabilities and the format parameters are specific to
specific to the type of codec. The mfcap lines map to a single the type of codec. The mfcap lines map to a single traditional SDP
traditional SDP fmtp attribute line (one per <media-cap-num>) of the fmtp attribute line (one per <media-caps>) of the form
form
a=fmtp:<fmt> <list of parameters> a=fmtp:<fmt> <list of parameters>
where <fmt> is the media format description defined in RFC 4566 where <fmt> is the media format description defined in RFC 4566
[RFC4566], as appropriate for the particular media stream. The mfcap [RFC4566], as appropriate for the particular media stream. The mfcap
attribute MUST be used to encode attributes for media capabilities, attribute MUST be used to encode attributes for media capabilities,
which would conventionally appear in an fmtp attribute. which would conventionally appear in an fmtp attribute.
The mfcap attribute adheres to RFC 4566[RFC4566] attribute production
rules with
media-format-capability = "a=mfcap:"<media-caps>
1*WSP <fmt-specific-param-list>
media-caps = "*" ; wildcard: all media caps
/ <media-cap-num-list> ; defined in 3.3.1
fmt-specific-param-list = text ; defined in RFC 4566
3.3.2.1. Media Format Parameter Concatenation Rule
The appearance of media subtypes with a large number of formatting The appearance of media subtypes with a large number of formatting
options (e.g., AMR-WB [RFC4867]) coupled with the restriction that options (e.g., AMR-WB [RFC4867]) coupled with the restriction that
only a single fmtp attribute can appear per media format, suggests only a single fmtp attribute can appear per media format, suggests
that it is useful to create a combining rule for mfcap parameters that it is useful to create a combining rule for mfcap parameters
which are associated with the same media capability number. which are associated with the same media capability number.
Therefore, different mfcap lines MAY include the same <med-cap-num> Therefore, different mfcap lines MAY include the same <med-cap-num>
in their <med-cap-num-list>. When a particular media capability is in their <med-cap-num-list>. When a particular media capability is
selected for processing, the parameters from each mfcap line which selected for processing, the parameters from each mfcap line which
references the particular capability number in its med-cap-num-list references the particular capability number in its med-cap-num-list
are concatenated together via ";" to form the equivalent of a single are concatenated together via ";", in the order the mfcap lines
fmtp attribute line. This permits one to define a separate mfcap appear in the SDP record, to form the equivalent of a single fmtp
line for a single parameter and value that is to be applied to each attribute line. This permits one to define a separate mfcap line for
media capability designated in the med-cap-num-list. This provides a 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 compact method to specify multiple combinations of format parameters
when using codecs with multiple format options. when using codecs with multiple format options. Note that order-
dependent parameters MAY be placed in a single mfcap line to avoid
The mfcap attribute adheres to RFC 4566 attribute production rules possible problems with line rearrangement by a middlebox.
with
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
Format parameters are not parsed by SDP; their content is specific to Format parameters are not parsed by SDP; their content is specific to
the media type/subtype. When format parameters for a specific media the media type/subtype. When format parameters for a specific media
capability are combined from multiple a=mfcap lines which reference capability are combined from multiple a=mfcap lines which reference
that media capability, the format-specific parameters are that media capability, the format-specific parameters are
concatenated together and separated by "; " for construction of the concatenated together and separated by "; " for construction of the
corresponding format attribute (a=fmtp): corresponding format attribute (a=fmtp):
a= fmtp:<fmt> WSP <format-specfic-parameter-list> a= fmtp:<fmt> 1*WSP <fmt-specific-param-list> [1*[";" 1*WSP
[1*[";"<format-specfic-parameter-list>] ;
<fmt-specfic-param-list>] ;
where <fmt> depends on the transport protocol in the manner defined where <fmt> depends on the transport protocol in the manner defined
in RFC4566. SDP cannot assess the legality of the resulting in RFC4566. SDP cannot assess the legality of the resulting
parameter list in the "a=fmtp" line; the user must take care to parameter list in the "a=fmtp" line; the user must take care to
insure that legal parameter lists are generated. insure that legal parameter lists are generated.
The "mfcap" 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 mfcap 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 session or media level. The unique media-cap-num is used to
associate the parameters with a media capability. associate the parameters with a media capability.
skipping to change at page 14, line 38 skipping to change at page 16, line 41
Example for H.263 video: Example for H.263 video:
a=mcap:1 video H263-1998/90000 a=mcap:1 video H263-1998/90000
a=mcap:2 video H263-2000/90000 a=mcap:2 video H263-2000/90000
a=mfcap:1 CIF=4;QCIF=2;F=1;K=1 a=mfcap:1 CIF=4;QCIF=2;F=1;K=1
a=mfcap:2 profile=2;level=2.2 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:1-3 AMR/8000/1 a=mcap:1-3 AMR/8000/1
a=mcap:4-6 AMR/16000/1 a=mcap:4-6 AMR-WB/16000/1
a=mfcap:1,2,3,4 mode-change-capability=1 a=mfcap:1,2,3,4 mode-change-capability=1
a=mfcap:5,6 mode-change-capability=2 a=mfcap:5,6 mode-change-capability=2
a=mfcap:1,2,3,5 max-red=220 a=mfcap:1,2,3,5 max-red=220
a=mfcap:3,4,5,6 octet-align=1 a=mfcap:3,4,5,6 octet-align=1
a=mfcap:1,3,5 mode-set=0,2,4,7 a=mfcap:1,3,5 mode-set=0,2,4,7
a=mfcap:2,4,6 mode-set=0,3,5,6 a=mfcap:2,4,6 mode-set=0,3,5,6
So that AMR codec #1, when specified in a pcfg attribute within an So that AMR codec #1, when specified in a pcfg attribute within an
audio stream block (and assigned payload type 98) as in audio stream block (and assigned payload type number 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/8000/1 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 number 99, is essentially
the following: equivalent to the following:
m=audio 49170 RTP/AVP 99 m=audio 49170 RTP/AVP 99
a=rtpmap:99 AMR/16000/1 a=rtpmap:99 AMR-WB/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. SDP could thus convert and so on for the other four combinations. SDP could thus convert
the media capabilities specifications into one or more alternative the media capabilities specifications into one or more alternative
media stream specifications, one of which can be chosen for the media stream specifications, one of which can be chosen for the
answer. answer.
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 3.3.3. The Media-Specific Capability Attribute
Media-specific attributes, beyond the rtpmap and fmtp attributes, may Media-specific attributes, beyond the rtpmap and fmtp attributes, may
be associated with media capability numbers via a new media-specific be associated with media capability numbers via a new media-specific
attribute, mscap, as follows: attribute, mscap, as follows:
media-specific-capability = "a=mscap:" media-specific-capability = "a=mscap:"
<media-caps> ; defined in 3.3.2 <media-caps> ; defined in 3.3.2
WSP <att-field> ; from [RFC4566] 1*WSP <att-field> ; from [RFC4566]
WSP <ms-parameters> 1*WSP <ms-parameters>
ms-parameters = byte-string ; as defined per attribute. ms-parameters = byte-string ; as defined per attribute.
Given an association between a media capability and a payload type Given an association between a media capability and a payload type
number as specified by the pt= parameters in an lcfg or pcfg number as specified by the pt= parameters in an lcfg or pcfg
attribute line, a mscap line may be translated easily into a attribute line, a mscap line may be translated easily into a
conventional attribute line of the form conventional SDP attribute line of the form
a=<att-field>":"<fmt> <ms-parameters> ; fmt defined in [RFC4566] a=<att-field>":"<fmt> <ms-parameters> ; fmt defined in [RFC4566]
The resulting attribute SHOULD be a legal SDP attribute, otherwise it
MUST be ignored by the receiver.
A single mscap line may refer to multiple media capabilities; this is A single mscap line may refer to multiple media capabilities; this is
equivalent to multiple mscap lines, each with the same attribute equivalent to multiple mscap lines, each with the same attribute
values, one line per media capability. Multiple mscap lines may values, one line per media capability.
refer to the same media capability, but, unlike the mfcap attribute,
no concatenation operation is defined. Hence, multiple mscap lines Multiple mscap lines may refer to the same media capability, but,
applied to the same media capability is equivalent to multiple lines unlike the mfcap attribute, no concatenation operation is defined.
of the specified attribute in a conventional media record. 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 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 in [RFC5104] (with the session-level and audio media omitted). If
omitted). If the offer contains a media block like the following, the offer contains a media block like the following,
m=video 51372 RTP/AVP 98 m=video 51372 RTP/AVP 98
a=rtpmap:98 H263-1998/90000 a=rtpmap:98 H263-1998/90000
a=tcap:1 RTP/AVPF a=tcap:1 RTP/AVPF
a=mcap:1 H263-1998/90000 a=mcap:1 H263-1998/90000
a=mscap:1 rtcp-fb ccm tstr a=mscap:1 rtcp-fb ccm tstr
a=mscap:1 rtcp-fb ccm fir a=mscap:1 rtcp-fb ccm fir
a=mscap:* rtcp-fb ccm tmmbr smaxpr=120 a=mscap:* rtcp-fb ccm tmmbr smaxpr=120
a=pcfg:1 t=1 m=1 pt=1:98 a=pcfg:1 t=1 m=1 pt=1:98
and if the proposed configuration is chosen, then the equivalent and if the proposed configuration is chosen, then the equivalent
media block would look like media block would look like
m=video 51372 RTP/AVP 98 m=video 51372 RTP/AVPF 98
a=rtpmap:98 H263-1998/90000 a=rtpmap:98 H263-1998/90000
a=rtcp-fb:98 ccm tstr a=rtcp-fb:98 ccm tstr
a=rtcp-fb:98 ccm fir a=rtcp-fb:98 ccm fir
a=rtcp-fb:* ccm tmmbr smaxpr=120 a=rtcp-fb:* ccm tmmbr smaxpr=120
3.3.4. The Bandwidth Capability Attribute 3.3.4. Additional Capability Attributes
In some cases it is desirable to specify different bandwidth limits SDP [RFC4566] provides a number of line types which may be useful in
for different media configurations. This may be done by use of the negotiation of media streams. In particular, those line types that
"a=bcap" attribute, which is defined as follows: may be used at the media level, specifically media (m=), session
information (i=), connection (c=), bandwidth (b=), encryption key
(k=), and attribute (a=) may require a corresponding capability
negotiation attribute. The a= attribute line has been treated in
[SDPCapNeg] to produce the acap attribute. Consideration of the m=
media line leads to the pcfg attribute defined in [SDPCapNeg] and the
lcfg, mcap, mfcap, and mscap attributes defined in this document. A
CapNeg attribute for the connection line (c=) has been described in
[I-D.garcia-mmusic-sdp-cs]. It seems better to define such
attributes here, rather than have the definitions appear across a
number of documents not directly related to capabilities negotiation.
Without spending much effort to assess future utility, this section
defines a number of capability attributes that may prove useful in
the future.
SDP types that can appear only at the session level, and only one
time, are not expected to be negotiable. This applies to the
protocol version ("v="), origin ("o="), session name ("s="), and URI
("u=") types. Email ("e="), phone ("p="), and time/repeat/timezone
fields ("t=", "r=", and "z=") may appear multiple times in a session
description, but it's not clear why one might negotiate different
email addresses or phone numbers for a particular session. It might
be useful to negotiate different times for a session (a broadcast of
the CEOs speech, for example) depending on the video encoding to be
used (e.g., H.263 or H.264), but this could just as well be
accomplished by announcing two sessions, one for each encoding; in
practice, a single session will likely offer multiple encodings at
the same time. For the present, we adopt the position that the
session-level-only types need not be negotiable via CapNeg. The
following capability attributes are always to be interpreted as
media-level fields when invoked by a potential (pcfg) or latent
(lcfg) configuration. Note that the SDP encryption key type (k=) is
NOT RECOMMENDED in RFC 4566; use of the kcap attribute should be
limited to cases in which no convenient alternative exists. The
encryption key field should be included only when/if the key is
transported over a secure, trusted channel.
The general form of the capability attributes is the same for the
above SDP types. Given the SDP line (from [RFC4566])
<type> = <value>
we define the corresponding capability attribute as
a=<type-cap-attr> ":" <type-attr-num> 1*WSP <value>
where
<type-cap-attr> = <type> "cap"
<type> = %x69 / ;"i"
%x63 / ; "c"
%x62 / ; "b"
%x6b / ; "k"
%x61 / ; "a"
<type-attr-num> = 1*DIGIT ; integer, 1 to 2^31, inclusive
where <value> is as defined specifically for each <type> in [RFC4566]
and, possibly, extended in other RFCs. The definitions above
correspond to the acap definition in [SDPCapNeg] and the ccap
definition proposed in [I-D.garcia-mmusic-sdp-cs].
As an example, consider the negotiation of bandwidth. In some cases
it is desirable to specify different bandwidth limits for different
media configurations. This may be done by use of the "a=bcap"
attribute, which, from the above and [RFC4566], 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=
line in RFC4566[RFC4566] line in RFC4566[RFC4566]
In ABNF, we have:
media-bandwidth-cap-line = "a=bcap:" bw-cap-num 1*WSP
bwtype ":" bandwidth
where
bw-cap-num = 1*DIGIT; the bandwidth "handle"
bwtype = token; as defined in RFC4566
bandwidth = 1*DIGIT; as defined in RFC4566
The "bcap" attribute can appear at the session level, where it can be 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 for that configuration. There can be interpreted at the media-level for that configuration. There can be
more than one bcap attribute. The unique bw-cap-num is used to more than one bcap attribute. The unique bw-cap-num is used to
identify it in potential configurations. No provision has been made identify it in potential configurations. No provision has been made
for negotiation of total session bandwidth capabilities. for negotiation of total session bandwidth capabilities.
Bandwidth capabilities may be included in a potential configuration Bandwidth capabilities may be included in a potential configuration
skipping to change at page 17, line 30 skipping to change at page 20, line 38
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 H263-1998/90000 a=mcap:1,2 video H263-1998/90000
a=mcap:2 video H263-1998/90000
a=mfcap:1 QCIF=2 a=mfcap:1 QCIF=2
a=mfcap:2 CIF=4; QCIF=2;F=1;K=1 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=1:100 a=pcfg:1 m=1 b=1 pt=1:100
a=pcfg:2 m=2 pt=2:101 a=pcfg:2 m=2 pt=2:101
3.4. Extensions to the Potential Configuration Attribute 3.3.5. The Latent Configuration Attribute
The extension protocol of capabilities negotiation requires three new One of the goals of this work is to permit the exchange of
extensions for the pcfg: attribute defined in the base protocol. The supportable media configurations in addition to those offered or
first extension permits the specification of media capabilities, or accepted for immediate use. Such configurations are referred to as
combinations thereof; the second permits the assignment of payload "latent configurations". For example, a party may offer to establish
types to those capabilities when used in the specified configuration; a session with an audio stream, and, at the same time, announce its
the third permits the specification of bandwidth limits for a media ability to support a video stream as part of the same session. The
stream. offerer can supply its video capabilities by offering a latent video
configuration along with the media stream for audio; the responding
party may indicate its ability and willingness to support such a
video session by returning a corresponding latent configuration.
3.4.1. The Media Capability Parameter Latent configurations returned in SDP answers must match latent
configurations (or parameter subsets thereof). Therefore, it is
appropriate for the offering party to announce most, if not all, of
its capabilities in the initial offer. [Editor's note: this choice
has been made in order to keep the size of the answer more compact by
not requiring acap, mcap, tcap, etc. lines in the answer. However,
this restriction can be changed if it seems desirable.]
The potential configuration attribute ("a=pcfg") as defined in SDP Latent configurations may be announced by use of the latent
capabilities negotiation, permits alternate attributes to be configuration attribute, which is defined in a manner very similar to
associated with the media types defined in a media line. In this the potential configuration attribute. The media type (mt=) and the
document, we define an extension parameter for the specification of transport protocol(s) (t=) MUST be specified since there is no
media configurations in addition to the one specified on the media corresponding m-line for defaults. In most cases, the media
line. configuration (m=) parameter must be present as well (see section
3.3.8 for examples). The lcfg attribute is a session level
attribute, and all capability attributes referenced by lcfg attribute
parameters must appear at the session level in the SDP record.
We define the media capability configuration parameter, pot-media- latent-media-config = "a=lcfg:" <config-number>
config, in accordance with the following format: 1*WSP "mt=" <media> ;[RFC4566]
1*WSP transport-protocol-config-list
; defined in [SDPCapNeg]
[1*WSP media-config-list]
[1*WSP attribute-config-list]
; defined in [SDPCapNeg]
[1*WSP payload-number-config-list]
[1*WSP info-cap]
[1*WSP connection-config-list]
[1*WSP bandwidth-config-list]
[1*WSP key-config-list]
[1*(1*WSP extension-config-list)]
; defined in [SDPCapNeg]
The mt= parameter identifies the media type (audio, video, etc.) to
be associated with the latent media stream; the transport (t=) and
attribute (a=) parameters are identical in format and meaning to
those defined for the pcfg: attribute in [SDPCapNeg]. The media
subtype(s) and format parameters to be associated with the stream are
specified by the m= parameter.
The transport parameter (t=) is identical in format and meaning as
defined for the pcfg attribute in [SDPCapNeg]
We define the media configuration parameter, media-config-list, 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 (section 3.3.1).
In ABNF form (adhering to the ABNF for extension-config-list 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) media-config-list = "m=" med-cap-list *(BAR med-cap-list)
; BAR is defined in [SDPCapNeg] ; 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
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.
The attribute parameter (a=) is identical in format and meaning as
defined for the pcfg attribute in [SDPCapNeg].
We define the payload type number mapping parameter, pt-media-map, in
accordance with the following format:
payload-number-config-list = "pt=" med-map-list
med-map-list = med-map *["," med-map]
med-map = med-cap-num ":" payload-type-number
; med-cap-num is defined in section 3.4.1
payload-type-number = 1*DIGIT | "*" ; RTP payload type number
The example in the section 3.3.7 shows how the parameters from the
mcap line are mapped to payload type numbers from the pcfg "pt"
parameter. The "*" value is used in cases such as BFCP [RFC4583] in
which the fmt list in the m-line is ignored.
The session information parameter, info-cap, provides the capability
number of a session information (icap) capability attribute. In
accordance with RFC 4566, which permits no more than one i-line per
media stream, only one icap attribute may be specified. The format
of the parameter is defined as
info-cap = "i=" [delete] icap-num
delete = "-:" ; delete media-level i= line, if present
icap-num = 1*DIGIT ; cap-num from icap attribute
The connection data parameter, connection-config-list, may be used to
provide stream-specific connection data by reference to one or more
ccap attributes in the SDP record.
connection-config-list = "c=" [delete] full-ccap-list
full-ccap-list = mandatory-ccap-list ["," optional-ccap-list]
mandatory-ccap-list = ccap-list
optional-ccap-list = "[" ccap-list "]"
ccap-list = ccap-num [1*("," ccap-num)]
ccap-num = 1*DIGIT ; cap-num from ccap attribute
The bandwidth configuration parameter, bandwidth-config-list, may be
used to provide bandwidth specifications by reference to bcap
attributes described in section 3.3.4.
bandwidth-config-list = "b=" [delete] full-bcap-list
full-bcap-list = mandatory-bcap-list ["," optional-bcap-list]
mandatory-bcap-list = bcap-list
optional-bcap-list = "[" bcap-list "]"
bcap-list = bcap-num *("," bcap-num)
bcap-num = 1*DIGIT ; cap-num from bcap attribute
Although use of the encryption key line is NOT RECOMMENDED by RFC
4566, kcap attributes, defined in section 3.3.4, may be referenced
through the k= parameter.
key-config-list = "k=" [delete] full-kcap-list
full-kcap-list = mandatory-kcap-list ["," optional-kcap-list]
mandatory-kcap-list = kcap-list
optional-kcap-list = "[" kcap-list "]"
kcap-list = kcap-num *("," kcap-num)
kcap-num = 1*DIGIT ; cap-num from kcap attribute
A latent configuration represents a future capability, hence the pt=
parameter is not directly meaningful in the lcfg attribute because no
actual media session is being offered or accepted; it is permitted in
order to tie any payload type number parameters within attributes to
the proper media format. A primary example is the case of format
parameters for the RED payload, which are payload type numbers.
Specific payload type numbers used in a latent configuration may be
interpreted as suggestions to be used in any future offer based on
the latent configuration, but they are not binding; the offeror
and/or answerer may use any payload type numbers each deems
appropriate. The use of explicit payload type numbers for latent
configurations can be avoided by use of the parameter substitution
rule of section 3.3.7. Future extensions are also permitted.
Each latent configuration MUST be specified at the session level; it
represents an additional media stream to those in the media block(s}
of the offer or answer. If an acap: attribute is declared at the
session level for use in an lcfg line, it SHOULD NOT be used in a
pcfg line at the media level unless it is to become a session-level
attribute in the answer.
The configuration numbers for latent configurations share the same
preference rule as potential configurations: a lower-numbered
configuration is preferred over a higher-numbered configuration. If
latent configurations are used in session capability (sescap=)
attributes (section 3.3.8), the configuration numbers MUST be
different from those used for pcfg attributes.
If a cryptographic attribute, such as the SDES "a=crypto:" attribute
[RFC4568], is referenced by a latent configuration through an acap
attribute, any key material REQUIRED in the conventional attribute,
such as the SDES key/salt string, MUST be included in order to
satisfy formatting rules for the attribute. The actual value(s) of
the key material SHOULD be meaningless, and the receiver of the lcfg:
attribute MUST ignore the values.
3.3.6. Enhanced Potential Configuration Attribute
The present work requires new extensions (parameters) for the pcfg:
attribute defined in the base protocol [SDPCapNeg] The parameters and
their definitions are "borrowed" from the definitions provided for
the latent configuration attribute in section 3.3.5. The expanded
definition of the pcfg attribute is
a=pcfg: <config-number> [<pot-cfg-list>]
where
config-number = 1*DIGIT ;defined in [RFC5234]
pot-cfg-list = pot-config *(1*WSP pot-config)
pot-config = attribute-config-list /
;defined in [SDPCapNeg]
transport-protocol-config-list / ;defined in [SDPCapNeg]
extension-config-list / ;[SDPCapNeg]
media-config-list / ;sect. 3.3.5
payload-number-config-list / ; 3.3.5
info-cap / ;defined in section 3.3.5
connection-config-list / ; 3.3.5
bandwidth-config-list / ; 3.3.5
key-config-list ;section 3.3.5
The pot-cfg-list MUST NOT contain more than one instance of each
parameter list other than the extension-config-list.
3.3.6.1. Returning Capabilities in the Answer
Potential configuration attributes may be returned within the media
block(s) of an answer SDP to indicate the ability of the answerer to
to support alternative configurations of the corresponding stream(s).
For example, an offer may include multiple potential configurations
for a media stream; the corresponding answer will indicate (via an
acfg attribute) which configuration is accepted, but it MAY also
contain potential configuration attributes, with parameters, to
indicate which other offered configurations would be acceptable.
This information is useful if it becomes desirable to reconfigure a
media stream, e.g., to reduce resource consumption.
When potential configurations are returned in an answer, all
numbering MUST refer to the configuration and capability attribute
numbering of the offer. The referenced capability attributes MUST
NOT be returned in the answer. The parameter values of any returned
pcfg attributes MUST be a subset of those included in the offered
configurations; values may be omitted only if they were indicated as
alternative sets, or optional, in the original offer. The parameter
set indicated in the returned acfg attribute need not be repeated in
a returned pcfg attribute. The answerer may return more than one
pcfg attribute with the same configuration number if it is necessary
to describe selected combinations of optional or alternative
parameters.
Note that the answerer is unable to return additional capabilities
within an offered and answered media stream. For this reason, it
seems advisable for the offerer to include most, if not all,
potential capabilities in the initial offer. Additional capabilities
MAY be announced via one or more latent configurations (representing
a new media stream), or when renegotiating the session in a second
offer/answer exchange.
3.3.6.2. Payload Type Number Mapping
When media capabilities defined in mcap attributes are used in
potential configuration lines, and the transport protocol uses RTP,
it is necessary to assign payload type numbers to them. In some
cases, it is desirable to assign different payload type numbers to
the same media capability when used in different potential
configurations. One example is when configurations for AVP and SAVP
are offered: the offerer would like the answerer to use different
payload type numbers for encrypted and unencrypted media so that it
(the offerer) can decide whether or not to render early media which
arrives before the answer is received. This association of distinct
payload type number(s) with different transport protocols requires a
separate pcfg line for each protocol. Clearly, this technique cannot
be used if the number of potential configurations exceeds the number
of possible payload type numbers.
3.3.6.3. Processing of Media-Format-Related Conventional Attributes for
Potential Configurations
In cases in which media capabilities negotiation is employed, SDP
records are likely to contain conventional attributes such as rtpmap,
fmtp, and other media-format-related lines, as well as capability
attributes such as mcap, mfcap, and mscap which map into those
conventional attributes.
When one or more media capabilities (a=mcap) are invoked in a When one or more media capabilities (a=mcap) are invoked in a
potential configuration (via m=), and associated with a payload type potential configuration via m= arguments, each capability is
number by default or by a payload type number parameter (pt=), associated with a payload type number by default or by a payload type
special processing must be invoked on the attributes associated with number argument (pt=). Special processing MUST be invoked on
that payload type. If the media capability is associated with any conventional attributes associated with that payload type number. If
mfcap or mscap attributes, then all corresponding conventional the media capability is associated with one or more mfcap attributes,
attributes (e.g., fmtp or rtcp-fb attribute lines) in the media block then any corresponding conventional fmtp attribute in the media block
are ignored for that configuration. If no mfcap parameters are MUST be ignored for that configuration. If no mfcap attributes are
specified, then an fmtp attribute line within the media block with specified, then the fmtp attribute line within the media block with
the correct payload type number, if any, will apply. Similarly, any the matching payload type number, if any, will apply. Conventional
other media-specific attributes (e.g., rtcp-fb) in the media block fmtp attributes with payload type numbers not referenced in the
with the correct payload type number will apply unless there is an configuration MUST also be ignored. Similarly, any other
applicable mscap attribute for the same attribute type (e.g., conventional media-specific attributes (e.g., rtcp-fb) in the media
rtcp-fb), in which case all media-block attributes of the same type block with payload type number matching a mscap attribute will apply
and payload type number will be ignored. Any media-specific unless there is an applicable mscap attribute for the same attribute
attributes in the media block which refer to payload type numbers not type (e.g., rtcp-fb), in which case all base level attributes of the
used by the potential configuration are to be ignored. These rules same type and payload type number will be ignored. Any media-
are intended to avoid the need to duplicate attributes and use the specific attributes in the media block which refer to payload type
a=-m: form of invoking attributes in a potential configuration just numbers not used by the potential configuration will be ignored.
to replace an rtpmap or fmtp attribute. 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 0-11
a=mcap:1 PCMU/8000 a=mcap:1 PCMU/8000
a=mcap:2 g729/8000 a=mcap:2 g729/8000
a=mcap:3 telephone-event/8000 a=mcap:3 telephone-event/8000
a=mfcap:3 0-15 a=mscap:3 fmtp 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, PCMU is media capability 1, G729 is media capability In this example, PCMU is media capability 1, G729 is media capability
2, and telephone-event is media capability 3. The a=pcfg: line 2, and telephone-event is media capability 3. The a=pcfg: line
specifies that the preferred configuration is G.729 with extended specifies that the preferred configuration is G.729 with extended
dtmf events, second is G.711 mu-law with extended dtmf events. dtmf events, second is G.711 mu-law with extended dtmf events.
Intermixing of G.729, G.711, and "commercial" dtmf events is least Intermixing of G.729, G.711, and "commercial" dtmf events is least
preferred (the base configuration provided by the "m=" line, which is preferred (the base configuration provided by the "m=" line, which is
always the least preferred configuration). The rtpmap and fmtp always the least preferred configuration). The rtpmap and fmtp
attributes of the base configuration are replaced by the mcap and attributes of the base configuration are replaced by the mcap and
mfcap attributes when invoked by the proposed configuration. mscap attributes when invoked by the proposed configuration.
3.4.2. The Payload Type Mapping Parameter
When media capabilities defined in mcap attributes are used in
potential configuration lines, and the transport protocol uses RTP,
it is necessary to assign payload types to them. In some cases, it
is desirable to assign different payload types to the same media
capability when used in different potential configurations. One
example is when configurations for AVP and SAVP are offered: the
offerer would like the answerer to use different payload types for
encrypted and unencrypted media so that it (the offerer) can decide
whether or not to render early media which arrives before the answer
is received. This association of distinct payload type(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- 3.3.7. Substitution of Media Payload Type Numbers in Capability
map, in accordance with the following format: Attribute Parameters
pt-media-map = "pt=" med-map-list In some cases, for example, when an RFC 2198 redundancy audio subtype
med-map-list = med-map *["," med-map] (RED) capability is defined in an mfcap attribute, the parameters to
med-map = med-cap-num ":" 'payload-type; an attribute may contain payload type numbers. Two options are
med-cap-num is defined in section 3.4.1 available for specifying such payload type numbers. They may be
payload-type = 1*DIGIT ; RTP payload type expressed explicitly, in which case they are bound to actual payload
types by means of the payload type number parameter (pt=) in the
appropriate potential or latent configuration. For example, the
following SDP fragment defines a potential configuration with
redundant G.711 mu-law:
The example in the previous section shows how the parameters from the m=audio 45678 RTP/AVP 0
mcap line are mapped to payload type in the pcfg "pt" parameter. a=rtpmap:0 PCMU/8000
a=mcap:1 PCMU/8000
a=mcap:2 RED/8000
a=mfcap:2 0/0
a=pcfg:1 m=2,1 pt=2:98,1:0
As described in section 3.4.1, the choice of payload type numbers is The potential configuration is then equivalent to
especially important since they are also used to refer to base media m=audio 45678 RTP/AVP 98 0
-specific attributes (e.g., fmtp or rtcp-fb) if mfcap or mscap a=rtpmap:0 PCMU/8000
attributes are not specified for a media subtype. a=rtpmap:98 RED/8000
a=fmtp:98 0/0
3.4.3. The Bandwidth Parameter A more general mechanism is provided via the parameter substitution
rule:
A bandwidth parameter is added to the pcfg attribute in order to When an mfcap, mscap, or acap attribute is processed, its arguments
provide the flexibility to specify different bandwidth limits for will be scanned for sequences of the following form: "%" *DIGIT "%"
different configurations. If found, the digit string is interpreted as a media capability
number and the sequence is replaced by the payload type number
assigned to the media capability as specified by the pt= parameter in
the selected potential configuration. The sequence "%%" (null digit
string) is replaced by a single percent sign and processing continues
with the next character, if any.
The bandwidth parameter, bw-param, is defined in accordance with the For example, the above sequence could have been written as
following format:
bw-param = "b=" bw-cap-list; m=audio 45678 RTP/AVP 0
bw-cap-list = bw-cap-num *("," bw-cap-num); a=rtp map:0 PCMU/8000
bw-cap-num = 1*DIGIT; as defined by the bcap attribute a=mcap:1 PCMU/8000
a=mcap:2 RED/8000
a=mfcap:2 %1%/%1%
a=pcfg:1 m=2,1 pt=2:98,1:0
Multiple bandwidth capabilities are permitted in case it is desired and the equivalent SDP is the same as above. This technique is
to specify multiple bandwidth types. Note that it is considered an especially useful for latent configurations, for which it may not
error condition if the same bandwidth type is referenced more than otherwise be particularly useful to specify payload type numbers.
once in the bw-cap-list.
If a bandwidth capability is referenced in a potentical 3.3.8. The Session Capability Attribute
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 session capability attribute provides a means for the offerer
and/or the answerer to specify combinations of specific media stream
configurations which it is willing and able to support. Each session
capability in an offer is expressed as a list of potential and/or
latent configurations; in an answer, the session capabilities refer
to actual and/or latent media configurations. The session capability
attribute is described by:
The Actual configuration attribute is specified in [SDPCapNeg]. The session-capability = "a=sescap:" <session-num>
actual configuration MUST list the potential configuration selected 1*WSP <list-of-configs>
by the answerer. This section adds extensions parameters enabling
the answerer to specify the potential configuration attributes
defined in this document and used in forming the answer.
We define actual configuration extensions based on the sel-extension- session-num = 1*DIGIT ; defined in [RFC5234]
config specified in [SDPCapNeg]. The new parameters are: parameter list-of-configs = <alt-config> *["," <alt-config>]
act-media-config, act-med-map and act-bw-param in accordance with the alt-config = config-number *["|" config-number]
following ABNF: ; config-number defined in [SDPCapNeg]
The session-num identifies the session; a lower-number session is
preferred over a higher-numbered session. Each alt-config list
specifies alternative media configurations within the session;
preference is based on config-num as specified in [SDPCapNeg]. Note
that the session preference order, when present, takes precedence
over the individual media stream configuration preference order.
act-media-config = "m=" med-cap-list ; defined in section 3.4.1 Use of session capability attributes requires that configuration
act-med-map = "pt=" med-map *("," med-map) ; defined in 3.4.2 numbers assigned to potential and latent configurations be unique
act-bw-param = "b=" bw-cap-list ; defined in section 3.4.3 across the entire session; [SDPCapNeg] requires only that pcfg
configuration numbers be unique within a media description.
A response to the previous offer example in the above section might As an example, consider an endpoint that is capable of supporting an
be: audio stream with either one H.264 video stream or two H.263 video
streams with a floor control stream. The SDP offer might look like
the following:
v=0 v=0
o=- 24351 621814 IN IP4 192.0.2.2 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.2 c=IN IP4 192.0.2.1
t=0 0 t=0 0
a=csup:med-v0 a=creq:med-v0
m=audio 5432 RTP/AVP 18 100 a=sescap:2 1,2,3,5
a=rtpmap:100 events a=sescap:1 1,4
a=fmtp:100 0-15 m=audio 54322 RTP/AVP 0
a=acfg:1 m=2,3 pt=2:18,3:100 a=rtpmap:0 PCMU/8000
a=pcfg:1
Note that the capability numbers expressed in the acfg: attribute are m=video 22344 RTP/AVP 102
based on the offered capability numbering, not on those listed in the a=rtpmap:102 H263-1998/90000
answer, if any. The acfg attribute identifies to the offerer which a=fmtp:102 CIF=4;QCIF=2;F=1;K=1
potential configuration (and parameter combination) was selected by i= main video stream
the answerer. Note that payload types, if specified, refer to the a=label:11
payloads to be received by the offeror; if the answerer wishes to a=pcfg:2
receive different payload types, those values MUST be specified in a=mcap:1 H264/90000
the m= line of the SDP answer. a=mfcap:1 profile-level-id=42A01E; packetization-mode=2
a=acap:1 label:13
3.6. The Latent Configuration Attribute a=pcfg:4 m=1 a=1 pt=1:104
m=video 33444 RTP/AVP 103
One of the goals of this work is to permit the exchange of a=rtpmap:103 H263-1998/90000
supportable media configurations in addition to those offered or a=fmtp:103 CIF=4;QCIF=2;F=1;K=1
accepted for immediate use. Such configurations are referred to as i= secondary video (slides)
"latent configurations". For example, a party may offer to establish a=label:12
an audio session, and, at the same time, announce its ability to a=pcfg:3
support a video session and supply its video capabilities by offering m=application 33002 TCP/BFCP *
one or more latent configurations; the responding party may indicate a=setup:passive
its ability and willingness to support such a video session by a=connection:new
returning one or more latent configurations. a=floorid:1 m-stream:11 12
a=floor-control:s-only
a=confid:4321
a=userid:1234
a=pcfg:5
Latent configurations may be announced by use of the latent If the answerer understands MediaCapNeg, but cannot support the
configuration attribute, which is defined in a manner very similar to Binary Floor Control Protocol, then it would respond with:
the potential configuration attribute:
a=lcfg:<preference> ["mt="<media>] v=0
["m="<media-caps-list>] o=- 25678 753849 IN IP4 192.0.2.1
["t="<transport>] s=
["a="<attributes>] c=IN IP4 192.0.2.22
["b="<bw-cap-list>] t=0 0
["pt="<med-map-list>] a=cusp:med-v0
[future-extensions] a=sescap:1 1,4
m=audio 23456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=acfg:1
m=video 41234 RTP/AVP 104
a=rtpmap:100 H264/90000
a=fmtp:104 profile-level-id=42A01E; packetization-mode=2
a=acfg:4
a=pcfg:3
m=video 0 RTP/AVP 103
a=acfg:3
a=pcfg:3
m=application 0 TCP/BFCP *
a=acfg:5
<media> is defined in [RFC4566] and future-extensions = extension- An endpoint that doesn't support Media capabilities negotiation, but
config-list as defined in [SDPCapNeg]. does support H.263 video, would respond with one or two H.263 video
streams. In the latter case, the answerer may issue a second offer
to reconfigure the session to one audio and one video channel using
H.264 or H.263.
The m=, t=, b= and a= parameters are identical in format and meaning Session capabilities MAY include latent capabilities as well. Here's
to those defined for the pcfg: attribute. Note that the media type a similar example in which the offerer wishes to initially establish
(mt=) and media capabilities list MUST be present if the latent an audio stream, and prefers to later establish two video streams
configuration is defined at the session level. The mt= parameter with chair control. If the answerer doesn't understand Media CapNeg,
provides the MIME type (audio, video, etc.) for the configuration; it or cannot support the dual video streams or flow control, then it may
is OPTIONAL when the latent configuration is declared at the media support a single H.264 video stream. Note that establishment of the
level, but if present, MUST match the type of the m= line. The pt= most favored configuration will require two offer/answer exchanges.
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 v=0
they represent an additional media stream to those in the offer or o=- 25678 753849 IN IP4 192.0.2.1
answer. If an acap: attribute is declared at the session level for s=
use in a session-level lcfg line, it SHOULD NOT be used in a pcfg c=IN IP4 192.0.2.1
line at the media level unless it is to become a session-level t=0 0
attribute in the answer. a=creq:med-v0
a=sescap:1 1,3,4,5
a=sescap:2 1,2
a=sescap:3 1
a=mcap:1 H263-1998/90000
a=mfcap:1 CIF=4;QCIF=2;F=1;K=1
a=tcap:1 RTP/AVP TCP/BFCP
a=acap:31 label:12
a=acap:32 content:main
a=icap:3 primary video channel
a=lcfg:3 mt=video t=1 m=1 a=31,32 i=3
a=acap:41 label:13
a=acap:42 content:slides
a=icap:4 secondary video channel (slides)
a=lcfg:4 mt=video t=1 m=1 a=41,42 i=4
a=tcap:5 TCP/BFCP
a=acap:51 setup:passive
a=acap:52 connection:new
a=acap:53 floorid:1 m-stream:12 13
a=acap:54 floor-control:s-only
a=acap:55 confid:4321
a=acap:56 userid:1234
a=lcfg:5 mt=application t=2
m=audio 54322 RTP/AVP 0
i= voice
a=rtpmap:0 PCMU/8000
a=label:11
a=pcfg:1
m=video 22344 RTP/AVP 102
i= default video
a=rtpmap:102 H264/90000
a=fmtp:102 profile-level-id=42A01E; packetization-mode=2
a=label:11
a=content:main
a=pcfg:2
Latent configurations placed at the media level represent In this example, the default offer, as seen by endpoints which do not
configurations that are supportable, but are not desired for understand capabilities negotiation, proposes a PCMU audio stream and
immediate use. They represent potential future replacements for the an H.264 video stream. Note that the offered lcfg lines for the
stream description in which they appear. For example, one party video streams don't carry pt= parameters because they're not needed
might make an offer with several potential configurations (different (payload type numbers will be assigned in the offer/answer exchange
codecs, say). The answer can be made with one particular that establishes the streams. The lcfg line for the BFCP connection
configuration specified in the m= and "a=acfg" lines, and a number of doesn't even use a media-configuration (m=) parameter because the
"a=lcfg" lines, one for each "a=pcfg" line which could be supported protocol is specified in the transport parameter. If the answerer
by the answerer, but which was not chosen for this media stream. supports Media CapNeg, and supports the most desired configuration,
it would return the following SDP:
This permits the answerer to tell the offeror that, although a v=0
specific codec will be used in the current media session, it can also o=- 25678 753849 IN IP4 192.0.2.1
support one or more of the offered alternatives. This could be s=
useful, for example, in the case that PCMU audio encoding is chosen, c=IN IP4 192.0.2.22
but subsequent network congestion suggests that the media stream t=0 0
should be renegotiated to G729; the offeror will know in advance that a=csup:med-v0
the other party supports G729. a=sescap:1 1,3,4,5
a=sescap:2 1,2
a=sescap:3 1
a=lcfg:3 mt=video t=1 m=1 a=31,32 i=3
a=lcfg:4 mt=video t=1 m=1 a=41,42 i=4
a=lcfg:5 mt=application t=2
m=audio 23456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=acfg:1
m=video 0 RTP/AVP 102
a=pcfg:2
The rules for replacement of base-level attributes by mcap and mscap This exchange supports immediate establishment of an audio stream for
attributes invoked within a latent configuration are the same as preliminary conversation. This exchange would presumably be followed
specified for invocation in a potential configuration (see section at the appropriate time with a "reconfiguration" offer/answer
3.4) exchange to add the video and chair control streams.
3.6.1. Cryptographic Attributes in Latent Configurations [Editors' note: We have considered a form of the lcfg: attribute that
would permit an offerer to indicate that the configuration is
available for immediate acceptance via an answer with a corresponding
(new) m-line in the answer. This would permit the establishment of
the media streams to take place in one SDP exchange; at the same
time, non-negotiating endpoints could be offered a simple
configuration as in the above example. Ultimately, when all
endpoints understand the current specification, this would permit an
offer with latent configurations only, and an answer with the m-lines
for the accepted media streams.
If a cryptographic attribute, such as the SDES "a=crypto:" attribute The choices of session capabilities may be based on processing load,
[RFC4568], is referenced by a latent configuration, any key material total bandwidth, or any other criteria of importance to the
REQUIRED in the attribute, such as the SDES key/salt string, MUST be communicating parties. If the answerer supports media capabilities
included. The receiver of the lcfg: attribute MUST ignore any key negotiation, and session configurations are offered, it must accept
material associated with the latent configuration. one of the offered configurations, or it must refuse the session.
Therefore, if the offer includes any session capabilities, it should
include all the session capabilities the offerer is willing to
support.
3.7. Offer/Answer Model Extensions 3.4. 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] and [SDPCapNeg] to allow for media defined in RFC3264 [RFC3264] and [SDPCapNeg] to allow for media
capabilities, bandwidth capabilities, and latent configurations to be capabilities, bandwidth capabilities, and latent configurations to be
used with the SDP Capability Negotiation framework. used with the SDP Capability Negotiation framework.
The [SDPCapNeg] provides a relatively compact means to offer the The [SDPCapNeg] provides a relatively compact means to offer the
equivalent of an ordered list of alternative media stream equivalent of an ordered list of alternative media stream
configurations (as would be described by separate m= lines and configurations (as would be described by separate m= lines and
associated attributes). The attributes acap, bcap, mscap, mfcap and associated attributes). The attributes acap, bcap, mscap, mfcap and
mcap are designed to map somewhat straightforwardly into equivalent mcap are designed to map somewhat straightforwardly into equivalent
m= lines and conventional attributes when invoked by a pcfg, lcfg, or m= lines and conventional attributes when invoked by a pcfg, lcfg, or
acfg attribute with appropriate parameters. The "a=pcfg:" lines, acfg attribute with appropriate parameters. The "a=pcfg:" lines,
along with the m= line itself, represent offered media along with the m= line itself, represent offered media
configurations. The "a=lcfg:" lines represent alternative configurations. The "a=lcfg:" lines represent alternative
capabilities for future use. capabilities for future use.
3.7.1. Generating the Initial Offer 3.4.1. Generating the Initial Offer
When an endpoint generates an initial offer and wants to use the 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, mfcap and mscap and define the codecs it can support via mcap, mfcap and mscap
attributes. The SDP media line(s) should be made up with the attributes. The SDP media line(s) should be made up with the
configuration to be used if the other party does not understand configuration to be used if the other party does not understand
capability negotiations (by default, this is the least preferred capability negotiations (by default, this is the least preferred
configuration). Typically, the media line configuration will contain configuration). Typically, the media line configuration will contain
the minimum acceptable capabilities. The offer MUST include the the minimum acceptable capabilities. The offer MUST include the
level of capability negotiation extensions needed to support this level of capability negotiation extensions needed to support this
functionality in a "creq" attribute. 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.
3.7.2. Generating the Answer 3.4.2. 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 capability negotiation extensions, it should select the required capability negotiation extensions, it should select the
most-preferred configuration it can support for each media stream, most-preferred configuration it can support for each media stream,
and build its answer accordingly. The configuration selected for and build its answer accordingly. The configuration selected for
each accepted media stream is placed into the answer as a media line each accepted media stream is placed into the answer as a media line
with associated parameters and attributes. If a proposed with associated parameters and attributes. If a proposed
configuration is chosen, the answer must include the supported configuration is chosen, the answer must include the supported
extension attribute and each media stream for which a proposed extension attribute and each media stream for which a proposed
configuration was chosen must contain an actual configuration (acfg) 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 also include any latent configurations the answer. The answer should also include any latent configurations the
answerer can support, especially any configurations compatible with answerer can support, especially any configurations compatible with
other proposed or latent configurations received in the offer. The other proposed or latent configurations received in the offer. The
answerer should make note of those configurations it might wish to answerer should make note of those configurations it might wish to
offer in the future. offer in the future.
3.7.3. Offerer Processing of the Answer 3.4.3. Offerer Processing of the Answer
When the offeror 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.
3.7.4. Modifying the Session 3.4.4. 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 the properties used on an existing stream, it may stream, or by changing the properties used on an existing stream, it
do so via the mechanisms defined for offer/answer[RFC3264]. If the may do so via the mechanisms defined for offer/answer[RFC3264]. If
initiating party has remembered the codecs, potential configurations, the initiating party has remembered the codecs, potential
and latent configurations announced by the other party in the earlier configurations, and latent configurations announced by the other
negotiation, it may use this knowledge to maximize the likelihood of party in the earlier negotiation, it may use this knowledge to
a successful modification of the session. Alternatively, it may maximize the likelihood of a successful modification of the session.
perform a new capabilities exchange as part of the reconfiguration. Alternatively, it may perform a new capabilities exchange as part of
the reconfiguration.
4. Examples 4. Examples
In this section, we provide examples showing how to use the Media In this section, we provide examples showing how to use the Media
Capabilities with the SDP Capability Negotiation. Capabilities with the SDP Capability Negotiation.
4.1. Alternative Codecs 4.1. Alternative Codecs
This example provide a choice of one of six variations of the This example provide a choice of one of six variations of the
adaptive multirate codec. In this example, the default configuration adaptive multirate codec. In this example, the default configuration
as specified by the media line is the same as the most preferred as specified by the media line is the same as the most preferred
configuration. Each configuration uses a different payload type so configuration. Each configuration uses a different payload type
the offeror can interpret early media. number so the offeror can interpret early media.
1. v=0 1. v=0
2. o=- 25678 753849 IN IP4 192.0.2.1 2. o=- 25678 753849 IN IP4 192.0.2.1
3. s= 3. s=
4. c=IN IP4 192.0.2.1 4. c=IN IP4 192.0.2.1
5. t=0 0 5. t=0 0
6. a=creq:med-v0 6. a=creq:med-v0
7. m=audio 54321 RTP/AVP 96 7. m=audio 54322 RTP/AVP 96
8. rtpmap:96 AMR/16000/1 8. rtpmap:96 AMR-WB/16000/1
9. a=fmtp:96 mode-change-capability=1; max-red=220; mode- 9. a=fmtp:96 mode-change-capability=1; max-red=220; mode-
set=0,2,4,7 set=0,2,4,7
10. a=cmed:1,3,5 audio AMR/16000/1 10. a=macp:1,3,5 audio AMR-WB/16000/1
11. a=cmed:2,4,6 audio AMR/8000/1 11. a=mcap:2,4,6 audio AMR/8000/1
12. a=mfcap:1,2,3,4 mode-change-capability=1 12. a=mfcap:1,2,3,4 mode-change-capability=1
13. a=mfcap:5,6 mode-change-capability=2 13. a=mfcap:5,6 mode-change-capability=2
14. a=mfcap:1,2,3,5 max-red=220 14. a=mfcap:1,2,3,5 max-red=220
15. a=mfcap:3,4,5,6 octet-align=1 15. a=mfcap:3,4,5,6 octet-align=1
16. a=mfcap:1,3,5 mode-set=0,2,4,7 16. a=mfcap:1,3,5 mode-set=0,2,4,7
17. a=mfcap:2,4,6 mode-set=0,3,5,6 17. a=mfcap:2,4,6 mode-set=0,3,5,6
18. pcfg:1 m=1 pt=1:96 18. pcfg:1 m=1 pt=1:96
19. pcfg:2 m=2 pt=2:97 19. pcfg:2 m=2 pt=2:97
20. pcfg:3 m=3 pt=3:98 20. pcfg:3 m=3 pt=3:98
21. pcfg:4 m=4 pt=4:99 21. pcfg:4 m=4 pt=4:99
22. pcfg:5 m=5 pt=5:100 22. pcfg:5 m=5 pt=5:100
23. pcfg:6 m=6 pt=6:101 23. pcfg:6 m=6 pt=6:101
In the above example, media capability 1 could have been excluded In the above example, media capability 1 could have been excluded
from the cmed declaration in line 10 and from the mfcap attributes in from the mcap declaration in line 10 and from the mfcap attributes in
lines 12, 14, and 16. The pcfg line 18 could then have been simply lines 12, 14, and 16. The pcfg line 18 could then have been simply
"pcfg:1". "pcfg:1".
The next example offers a video stream with three options of H.264 The next example offers a video stream with three options of H.264
and 4 transports. It also includes an audio stream with different and 4 transports. It also includes an audio stream with different
audio qualities: four variations of AMR, or AC3. The offer looks audio qualities: four variations of AMR, or AC3. The offer looks
something like: something like:
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s=SDP Media NEG example s=An SDP Media NEG example
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=ice-pwd:speEc3QGZiNWpVLFJhQX a=ice-pwd:speEc3QGZiNWpVLFJhQX
m=video 49170 RTP/AVP 100 m=video 49170 RTP/AVP 100
c=IN IP4 192.0.2.56 c=IN IP4 192.0.2.56
a=maxprate:1000 a=maxprate:1000
a=rtcp:51540 a=rtcp:51540
a=sendonly a=sendonly
a=candidate 12345 1 UDP 9 192.0.2.56 49170 host a=candidate 12345 1 UDP 9 192.0.2.56 49170 host
skipping to change at page 29, line 5 skipping to change at page 37, line 51
a=fmtp:100 octet-align=1; interleaving=30 a=fmtp:100 octet-align=1; interleaving=30
a=rtpmap:101 AMR-WB+/72000/2 a=rtpmap:101 AMR-WB+/72000/2
a=fmtp:101 interleaving=50; int-delay=160000; a=fmtp:101 interleaving=50; int-delay=160000;
a=mcap:14 ac3/48000/6 a=mcap:14 ac3/48000/6
a=acap:23 crypto:1 AES_CM_128_HMAC_SHA1_80 a=acap:23 crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|220|1:32 inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|220|1:32
a=tcap:4 RTP/SAVP a=tcap:4 RTP/SAVP
a=pcfg:10 t=4 a=23 a=pcfg:10 t=4 a=23
a=pcfg:11 t=4 m=14 a=23 pt=14:102 a=pcfg:11 t=4 m=14 a=23 pt=14:102
4.2. Latent Media Streams This offer illustrates the advantage in compactness that arises if
one can avoid deleting the base configuration attributes and
recreating them in acap attributes for the potential configurations.
4.2. Alternative Combinations of Codecs (Session Configurations)
If an endpoint has limited signal processing capacity, it might be
capable of supporting, say, a G.711 mu-law audio stream in
combination with an H.264 video stream, or a G.729B audio stream in
combination with an H.263-1998 video stream. It might then issue an
offer like the following:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
a=creq:med-v0
a=sescap:1 2,4
a=sescap:2 1,3
m=audio 54322 RTP/AVP 18
a=rtpmap:18 G729/8000
a=fmtp:18 annexb=yes
a=mcap:1 PCMU/8000
pcfg:1 m=1 pt=1:0
pcfg:2
m=video 54344 RTP/AVP 100
a=rtpmap:100 H263-1998/90000
a=mcap:2 H264/90000
a=mfcap:2 profile-level-id=42A01E; packetization-mode=2
a=pcfg:3 m=2 pt=2:101
a=pcfg:4
Note that the preferred session configuration (and the default as
well) is G.729B with H.263. This overrides the individual media
stream preferences which are PCMU and H.264 by the potential
configuration numbering rule.
4.3. Latent Media Streams
Consider a case in which the offeror can support either G.711 mu-law, 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 or G.729B, along with DTMF telephony events for the 12 common
touchtone signals, but is willing to support simple G.711 mu-law touchtone signals, but is willing to support simple G.711 mu-law
audio as a last resort. In addition, the offeror wishes to announce 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 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 offer a video stream at present. The offer might look like the
following: following:
1. v=0 1. v=0
skipping to change at page 30, line 5 skipping to change at page 39, line 44
answer like: answer like:
1. v=0 1. v=0
2. o=- 24351 621814 IN IP4 192.0.2.2 2. o=- 24351 621814 IN IP4 192.0.2.2
3. s= 3. s=
4. c=IN IP4 192.0.2.2 4. c=IN IP4 192.0.2.2
5. t=0 0 5. t=0 0
6. a=csup:med-v0 6. a=csup:med-v0
7. a=mcap:10 H263-1998/90000 7. a=mcap:10 H263-1998/90000
8. a=lcfg:1 mt=video m=10 8. a=lcfg:1 mt=video m=10
9. m=audio 54321 RTP/AVP 0 100 9. m=audio 54322 RTP/AVP 0 100
10. a=rtpmap:0 PCMU/8000 10. a=rtpmap:0 PCMU/8000
11. a=rtpmap:100 telephone-event/8000 11. a=rtpmap:100 telephone-event/8000
12. a=fmtp:100 0-11 12. a=fmtp:100 0-11
13. a=acfg:1 m=1,3 pt=1:0,3:100 13. a=acfg:1 m=1,3 pt=1:0,3:100
14. a=mcap:1 G729/8000 14. a=mcap:1 G729/8000
15. a=mcap:2 telephone-event/8000 15. a=mcap:2 telephone-event/8000
16. a=mfcap:2 0-11 16. a=mfcap:2 0-11
17. a=lcfg:2 m=1,2 pt=1:18,2:100 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 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 later time. Lines 9-12 of the answer present the selected
configuration for the media stream. Line 13 identifies the potential configuration for the media stream. Line 13 identifies the potential
configuration from which it was taken, and lines 14-17 announce the 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. 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, If, at some later time, congestion becomes a problem in the network,
either party may offer a reconfiguration of the media stream to use 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 G.729 in order to reduce packet sizes. Note that line 13 uses media
capability numbering as provided in the original offer, whereas lines capability numbering as provided in the original offer, whereas lines
14-17 must use their own numbering. 14-17 must use their own numbering.
skipping to change at page 31, line 12 skipping to change at page 41, line 12
capability numbering as provided in the original offer, whereas lines capability numbering as provided in the original offer, whereas lines
14-17 must use their own numbering. 14-17 must use their own numbering.
5. IANA Considerations 5. IANA Considerations
The IANA is hereby requested to register the following new SDP The IANA is hereby requested to register the following new SDP
attributes: attributes:
Attribute name: mcap Attribute name: mcap
Long form name: media capability Long form name: media capability
Type of attribute: Session-level and media-level Type of attribute: session-level and media-level
Subject to charset: No Subject to charset: no
Purpose: associate media capability number(s) with Purpose: associate media capability number(s) with
media subtype and encoding parameters media subtype and encoding parameters
Appropriate Values: See Section 3.3.1 Appropriate Values: see Section 3.3.1
Attribute name: mfcap Attribute name: mfcap
Long form name: media format capability Long form name: media format capability
Type of attribute: Session-level and media-level Type of attribute: session-level and media-level
Subject to charset: No Subject to charset: no
Purpose: associate media format attributes and Purpose: associate media format attributes and
parameters with media format capabilities parameters with media format capabilities
Appropriate Values: See Section 3.3.2 Appropriate Values: see Section 3.3.2
Attribute name: mscap Attribute name: mscap
Long form name: media-specific capability Long form name: media-specific capability
Type of attribute: Session-level and media-level Type of attribute: session-level and media-level
Subject to charset: No Subject to charset: no
Purpose: associate media-specific attributes and Purpose: associate media-specific attributes and
parameters with media capabilities parameters with media capabilities
Appropriate Values: See Section 3.3.3 Appropriate Values: see Section 3.3.3
Attribute name: bcap Attribute name: bcap
Long form name: Bandwidth capability attribute Long form name: bandwidth capability
Type of attribute: Session-level or media-level Type of attribute: session-level or media-level
Subject to charset: No Subject to charset: no
Purpose: associate bandwidth limitations with potential Purpose: associate bandwidth limitations with potential
potential or latent configurations. or latent configurations.
Appropriate Values: See Section 3.3.4 Appropriate Values: see Section 3.3.4
Attribute name: icap
Long form name: session information capability
Type of attribute: media-level
Subject to charset: no
Purpose: associate session information with potential
or latent configurations.
Appropriate Values: see Section 3.3.4
Attribute name: ccap
Long form name: connection information capability
Type of attribute: media-level
Subject to charset: no
Purpose: associate connection information with potential
or latent configurations.
Appropriate Values: see Section 3.3.4
Attribute name: kcap
Long form name: encryption key capability
Type of attribute: media-level
Subject to charset: no
Purpose: associate encryption key with potential
or latent configurations.
Appropriate Values: see Section 3.3.4
Attribute name: lcfg Attribute name: lcfg
Long form name: Latent Configuration Long form name: latent configuration
Type of attribute: Session-level or media-level Type of attribute: session-level or media-level
Subject to charset: No Subject to charset: no
Purpose: To announce supportable media configurations Purpose: to announce supportable media configurations
without offering them for immediate use. without offering them for immediate use.
Appropriate Values: See Section 3.6 Appropriate Values: see Section 3.3.5
Attribute name: sescap
Long form name: session capability
Type of attribute: session-level
Subject to charset: no
Purpose: to specify and prioritize acceptable
combinations of media stream configurations.
Appropriate Values: see Section 3.3.8
6. Security Considerations 6. Security Considerations
The security considertions of [SDPCapNeg] apply for this document. The security considertions of [SDPCapNeg] apply for this document.
No additional security considerations are introduced here.
The addition of negotiable media encoding, bandwidth attributes, and
connection data in this specification can cause problems for
middleboxes which attempt to control bandwidth utilization, media
flows, and/or processing resource consumption as part of network
policy, but which do not understand the media capability negotiation
feature. As for the initial CapNeg work, the SDP answer is
formulated in such a way that it always carries the selected media
encoding and bandwidth parameters for every media stream selected.
Pending an understanding of capabilities negotiation, the middlebox
should examine the answer SDP to obtain the best picture of the media
streams being established.
As always, middleboxes can best do their job if they fully understand
media capabilities negotiation.
7. Changes from previous versions 7. Changes from previous versions
7.1. Changes from version 02 7.1. Changes from version 03
o A new session capability attribute (sescap) has been added to
permit specification of acceptable media stream combinations.
o Capability attribute definitions corresponding to the i, c, b, and
k SDP line types have been added for completeness.
o Use of the pcfg: attribute in SDP answers has been included in
order to conveniently return information in the answer about
acceptable configurations in the media stream offer.
o The use of the lcfg: attribute(s) in SDP answers has been
restricted to indicate just which latent configuration offers
would be acceptable to the answerer.
o A suggestion for "naive" middleboxes has been added to the
Security Considerations.
o Various editorial changes have been made.
o Several errors/omissions have been corrected.
o The description of the mscap attribute has been modified to make
it clear that it should not be used to generate undefined SDP
attributes, or to "extend" existing attributes.
o <ms-parameters> are made optional in the mscap attribute
definition.
o "AMR" changed to "AMR-WB" in cases in which the sample rate is
16000.
7.2. Changes from version 02
This version contains several detail changes intended to simplify This version contains several detail changes intended to simplify
capability processing and mapping into conventional SDP media blocks. capability processing and mapping into conventional SDP media blocks.
o The "mcap" attribute is enhanced to include the role of the "ecap" o The "mcap" attribute is enhanced to include the role of the "ecap"
attribute; the latter is eliminated. attribute; the latter is eliminated.
o The "fcap" attribute has been renamed "mfcap". New replacement o The "fcap" attribute has been renamed "mfcap". New replacement
rules vis-a-vis fmtp attributes in the base media specification rules vis-a-vis fmtp attributes in the base media specification
have been added. have been added.
skipping to change at page 33, line 37 skipping to change at page 45, line 23
attribute. attribute.
o A new parameter, "mt=" is added to the latent configuration o A new parameter, "mt=" is added to the latent configuration
attribute (lcfg) to specify the media stream type (audio, video, attribute (lcfg) to specify the media stream type (audio, video,
etc.) when the lcfg is declared at the session level. etc.) when the lcfg is declared at the session level.
o The examples are expanded. o The examples are expanded.
o Numerous typos and misspellings have been corrected. o Numerous typos and misspellings have been corrected.
7.2. Changes from version 01 7.3. 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.
7.3. Changes from version 00 7.4. Changes from version 00
The major changes include taking out the "mcap" and "cptmap" The major changes include taking out the "mcap" and "cptmap"
parameter. The mapping of payload type is now in the "pt" parameter parameter. The mapping of payload type is now in the "pt" parameter
of "pcfg". Media subtype need to explictly definesd in the "cmed" of "pcfg". Media subtype need to explictly definesd in the "cmed"
attribute if referenced in the "pcfg" attribute if referenced in the "pcfg"
8. Acknowledgements 8. Acknowledgements
This document is heavily influenced by the discussions and work done This document is heavily influenced by the discussions and work done
by the SDP Capability Negotiation Design team. The following people by the SDP Capability Negotiation Design team. The following people
skipping to change at page 35, line 19 skipping to change at page 47, line 19
[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.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[SDPCapNeg] [SDPCapNeg]
Andreasen, F., "SDP Capability Negotiation", Andreasen, F., "SDP Capability Negotiation",
draft-ietf-mmusic-sdp-capability-negotiation-08 (work in draft-ietf-mmusic-sdp-capability-negotiation-08 (work in
progress), December 2007. progress), December 2007.
9.2. Informative References 9.2. Informative References
[I-D.ietf-avt-avpf-ccm] [I-D.garcia-mmusic-sdp-cs]
Wenger, S., Chandra, U., Westerlund, M., and B. Burman, Garcia-Martin, M. and S. Veikkolainen, "Conventions for
"Codec Control Messages in the RTP Audio-Visual Profile the Use of the Session Description Protocol (SDP) for
with Feedback (AVPF)", draft-ietf-avt-avpf-ccm-10 (work Circuit-Switched Bearer Connections in the Public Switched
in progress), October 2007. Telephone Network (PSTN)", draft-garcia-mmusic-sdp-cs-01
(work in progress), April 2008.
[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.
[RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session [RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol (SDP) Security Descriptions for Media Description Protocol (SDP) Security Descriptions for Media
Streams", RFC 4568, July 2006. Streams", RFC 4568, July 2006.
[RFC4583] Camarillo, G., "Session Description Protocol (SDP) Format
for Binary Floor Control Protocol (BFCP) Streams",
RFC 4583, November 2006.
[RFC4867] Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie, [RFC4867] Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie,
"RTP Payload Format and File Storage Format for the "RTP Payload Format and File Storage Format for the
Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband
(AMR-WB) Audio Codecs", RFC 4867, April 2007. (AMR-WB) Audio Codecs", RFC 4867, April 2007.
[RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
"Codec Control Messages in the RTP Audio-Visual Profile
with Feedback (AVPF)", RFC 5104, February 2008.
Authors' Addresses Authors' Addresses
Robert R Gilman Robert R Gilman
NDCI NDCI
Broomfield, CO 80020 Broomfield, CO 80020
USA USA
Email: bob_gilman@comcast.net Email: bob_gilman@comcast.net
Roni Even (editor) Roni Even
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
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