draft-ietf-mmusic-sdp-media-capabilities-00.txt   draft-ietf-mmusic-sdp-media-capabilities-01.txt 
MMUSIC F. Andreasen MMUSIC R. Gilman
Internet-Draft Cisco Systems Internet-Draft Avaya, Inc.
Expires: August 16, 2007 R. Gilman Intended status: Standards Track R. Even, Ed.
Avaya, Inc. Expires: August 24, 2007 Polycom
R. Even, Ed. F. Andreasen
Polycom Cisco Systems
February 12, 2007 February 20, 2007
SDP media capabilities Negotiation SDP media capabilities Negotiation
draft-ietf-mmusic-sdp-media-capabilities-00.txt draft-ietf-mmusic-sdp-media-capabilities-01.txt
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Abstract Abstract
Session Description Protocol (SDP) capability negotiation provides a Session Description Protocol (SDP) capability negotiation provides a
general framework for negotiation capabilities in SDP. The base general framework for negotiating capabilities in SDP. The base
framework defines only capabilities for negotiation transport framework defines only capabilities for negotiating transport
protocols and attributes. In this document, we extend the framework protocols and attributes. In this document, we extend the framework
by defining media capabilities that can be used to negotiate media by defining media capabilities that can be used to negotiate media
types and their associated attributes. types and their associated parameters.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. SDP Media capabilities . . . . . . . . . . . . . . . . . . . . 5 3. SDP Media capabilities . . . . . . . . . . . . . . . . . . . . 5
3.1. Solution Overview . . . . . . . . . . . . . . . . . . . . 5 3.1. Solution Overview . . . . . . . . . . . . . . . . . . . . 5
3.2. Capability Attributes . . . . . . . . . . . . . . . . . . 8 3.2. Capability Attributes . . . . . . . . . . . . . . . . . . 8
3.2.1. Media Type and Subtype Capability Attribute . . . . . 8 3.2.1. Media Type and Subtype Capability Attribute . . . . . 8
3.2.2. The Capability Encoding Parameters Attribute . . . . . 9 3.2.2. The Capability Encoding Parameters Attribute . . . . . 9
3.2.3. The Capability Format Parameters Attribute . . . . . . 10 3.2.3. The Media Format Parameter Capability Attribute . . . 10
3.2.4. The Media Line Capabilities Attribute . . . . . . . . 11
3.3. Extensions to the Potential Configuration Attribute . . . 11 3.3. Extensions to the Potential Configuration Attribute . . . 11
3.3.1. The Media Capability Extension to the Potential 3.3.1. The Media Capability Extension to the Potential
Configuration Attribute . . . . . . . . . . . . . . . 11 Configuration Attribute . . . . . . . . . . . . . . . 11
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.3.2. The Payload Type Mapping Extension to the
4.1. Alternative Codecs . . . . . . . . . . . . . . . . . . . . 16 Potential Configuration Attribute . . . . . . . . . . 12
4.2. Latent Media Streams . . . . . . . . . . . . . . . . . . . 16 3.4. Extensions to the Actual Configuration Attribute . . . . . 12
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 3.5. The Latent Configuration Attribute . . . . . . . . . . . . 13
6. Security Considerations . . . . . . . . . . . . . . . . . . . 18 3.5.1. The crypto: Attribute in Latent Configurations . . . . 14
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19 3.6. Offer/Answer Model Extensions . . . . . . . . . . . . . . 14
8. Normative References . . . . . . . . . . . . . . . . . . . . . 20 3.6.1. Generating the Initial Offer . . . . . . . . . . . . . 14
3.6.2. Generating the Answer . . . . . . . . . . . . . . . . 14
3.6.3. Offerer Processing of the Answer . . . . . . . . . . . 14
3.6.4. Modifying the Session . . . . . . . . . . . . . . . . 14
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1. Alternative Codecs . . . . . . . . . . . . . . . . . . . . 15
4.2. Latent Media Streams . . . . . . . . . . . . . . . . . . . 15
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
6. Security Considerations . . . . . . . . . . . . . . . . . . . 17
7. Changes from version 00 . . . . . . . . . . . . . . . . . . . 18
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
9.1. Normative References . . . . . . . . . . . . . . . . . . . 20
9.2. Informative References . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
Intellectual Property and Copyright Statements . . . . . . . . . . 22 Intellectual Property and Copyright Statements . . . . . . . . . . 22
1. Introduction 1. Introduction
Session Description Protocol (SDP) capability negotiation provides a Session Description Protocol (SDP) capability negotiation [SDPCapNeg]
general framework for negotiation capabilities in SDP[RFC4566]. The provides a general framework for negotiating capabilities in
base framework defines only capabilities for negotiation transport SDP[RFC4566]. The base framework defines only capabilities for
protocols and attributes. In this document, we extend the framework negotiating transport protocols and attributes. In this document, we
by defining media capabilities that can be used to negotiate media extend the framework by defining media capabilities that can be used
types and their associated attributes. to negotiate media types and their associated parameters.
SDP Simple Capability Declaration (simcap) is defined in RFC 3407. SDP Simple Capability Declaration (simcap) is defined in RFC 3407
It defines a set of SDP attributes that enables capabilities to be [RFC3407]. It defines a set of SDP attributes that enables a limited
described at a session level or on a per media stream basis. RFC set of capabilities to be described at a session level or on a per
3407 defines capability declaration only - actual negotiation media stream basis. RFC 3407 defines capability declaration only -
procedures taking advantage of such capabilities have not been actual negotiation procedures taking advantage of such capabilities
defined. The SDP capability negotiation framework adds this required have not been defined. The SDP capability negotiation framework adds
functionality. This document updates RFC3407 and new implementation this required functionality. This document updates RFC3407 and new
SHOULD use the functionality defined in the current draft to implementation SHOULD use the functionality defined in the current
negotiate media capabilities. draft to negotiate media capabilities.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [RFC2119] and document are to be interpreted as described in RFC2119 [RFC2119] and
indicate requirement levels for compliant RTP implementations. indicate requirement levels for compliant RTP implementations.
3. SDP Media capabilities 3. SDP Media capabilities
In this section we first provide an overview of the SDP media In this section we first provide an overview of the SDP media
Capability negotiation solution. This is followed by definitions of Capability negotiation solution. This is followed by definitions of
new SDP attributes for the solution and its associated updated offer/ new SDP attributes for the solution and its associated updated offer/
answer procedures [RFC3264] answer procedures [RFC3264]
3.1. Solution Overview 3.1. Solution Overview
The solution consist of the following new attributes extending the The solution consist of the following new attributes extending the
base attributes from (reference) base attributes from [SDPCapNeg].
A new media attribute ("a=cmed") that lists potential media types and Three attributes are used to make up media capabilities
subtypes
A new attribute ("a=cenc") that lists potential encoding parameters o A new media attribute ("a=cmed") that lists media formats as
capabilities in the form a media type (e.g. "audio") and one or
more subtypes (e.g. "PCMU"), and associates a handle with each
subtype
A new attribute ("a=cfmt") that lists potential media format o A new attribute ("a=cenc") that lists encoding parameter
capabilities capabilities associated with a particular media format capability.
A new media-level attribute ("a=mcap:") that assigns identifying o A new attribute ("a=cfmt") that lists media format parameter
numbers to media capabilities specified in the corresponding media capabilities associated with a particular media format capability.
("m=") line.
A new attribute ("a=lcfg") that specifies latent configurations when A new attribute ("a=lcfg") that specifies latent configurations when
no corresponding media line is offered. An example is a potential no corresponding media line is offered. An example is a latent
configuration for video even though no video is currently offered. configuration for video even though no video is currently offered.
A new parameter type ("m=") to the proposed-configuration ("a=pcfg:") A new parameter type ("m=") to the potential configuration
attribute and the accepted configuration ("a=acfg:") attribute, which ("a=pcfg:") attribute and the actual configuration ("a=acfg:")
permits specification of media capabilities and combinations thereof attribute defined in [SDPCapNeg], which permits specification of
for the configuration. For example, the "a=pcfg:" line might specify media capabilities (including their associated parameters) and
PCUM and telephony events or G.729B and telephony events as combinations thereof for the configuration. For example, the
acceptable configurations. The "a=acfg:" line in the answer would "a=pcfg:" line might specify PCUM and telephone events or G.729B and
specify the accepted choice. telephone events as acceptable configurations. The "a=acfg:" line in
the answer would specify the accepted choice.
A new parameter type ("pt=") to the proposed-configuration A new parameter type ("pt=") to the potential configuration
("a=pcfg:") attribute which permits specification of payload types ("a=pcfg:") attribute which associates RTP payload types with the
for the referenced media capabilities. referenced media capabilities.
The document extends the base protocol extensions to the offer/answer The document extends the base protocol extensions to the offer/answer
model that allow for capabilities and potential configurations to be model that allow for capabilities and potential configurations to be
included in an offer. When included at the session level, they included in an offer. Media capabilities constitute capabilities
constitute latent capabilities that may be used to guide a subsequent that can be used in potential and latent configurations. Whereas
offer. When included at the media level, they constitute offers that potential configurations constitute alternative offers that may be
may be accepted by the answerer instead of the actual accepted by the answerer instead of the actual configuration(s)
configuration(s) included in the "m=" line(s). The answerer replies included in the "m=" line(s), latent configurations merely inform the
by including the accepted configuration in the media line of the other side of possible configurations supported by the entity. Those
answer. Capabilities, latent configurations and potential latent configurations may be used to guide subsequent offer/answer
configurations may be included in answers as well, where they can aid exchanges.
in guiding a subsequent new offer.
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:
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1/24 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.1/24 c=IN IP4 192.0.2.1
t=0 0 t=0 0
a=creq:v1 a=creq:v1
a=cmed:1 audio g729 iLBC a=cmed:1 audio g729 iLBC PCMU g729
a=cenc:2 8000 a=cenc:2 8000
a=cfmt:1 annexb:no a=cfmt:1 annexb:no
a=ctrpr:1 RTP/SAVP a=ctrpr:1 RTP/SAVP
m=audio 3456 RTP/AVP 0 18 m=audio 3456 RTP/AVP 0 18
a=mcap:3
a=capar:1 a=crypto:1 AES_CM_128_HMAC_SHA1_32 a=capar:1 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=cptmap:1 1:100 2:101 3:102
a=pcfg:1 m=1,3|4,3 t=1 a=1 pt=1:100,4:101,3:102 a=pcfg:1 m=1,3|4,3 t=1 a=1 pt=1:100,4:101,3:102
a=pcfg:2 m=2 pt=2:103 a=pcfg:2 m=2 pt=2:103
The required base and extensions are provided by the "a=creq" The required base and extensions are provided by the "a=creq"
attribute, see base protocol, which indicates that the option tag attribute defined in [SDPCapNeg], with the option tag "v1", which
"v1", which indicates the extension framework defined here, must be indicates that the extension framework defined here, must be
supported. The Base level support is implied since it is required supported. The Base level support is implied since it is required
for the extensions. for the extensions.
The "a=cmed:1" line defines two audio media capabilities (mime The "a=cmed:1" line defines four audio media subtype capabilities ,
subtypes), to be numbered consecutively starting with 1. to be numbered consecutively starting with 1. Note that the media
subtypes specified in the m-line (PCMU and G729) are explicitly
specified here.
The "a=cenc:2" line specifies the encoding for capability 2, iLBC. The "a=cenc:2" line specifies the clock rate and encoding parmeters
(see [RFC4566]) for capability 2, iLBC.
The "a=cfmt:1" line specifies format parameters for codec 1 (no The "a=cfmt:1" line specifies media format parameter capabilities for
comfort noise for G.729). codec 1 ( no Annex B for G.729).
The "a=ctrpr:1" line, specified in the base protocol, defines a The "a=ctrpr:1" line, specified in the base protocol, defines a
transport profile, in this case using SRTP. transport protocol capability, in this case Secure RTP.
The "m=" line indicates that Alice is offering to use plain RTP with The "m=" line indicates that Alice is offering to use plain RTP with
PCMU or G.729B. The media line implicitly defines the default PCMU or G.729. The media line implicitly defines the default
transport profile (RTP/AVP in this case) and the default proposed transport protocol (RTP/AVP in this case) and the default actual
configuration. configuration.
The "a=mcap:3" line assigns numbers 3 and 4 to the media capabilities
in the m-line (PCMU and G.729B, respectively)
[Editors' note: we need some way to refer to the caps in the m= line
so that we don't have to repeat them in an a=cmed: line. I think we
can always refer to the current transport protocol from the m-line by
omission - it's always the default. Since we may want to refer to
the media caps outside the media session when/if we specify
constraints between media encodings, we might define a new media-
level attribute that lets us associate a unique set of numbers with
the media types in the m-line. For the sake of discussion, let's
call it "a=mcap:<n>", where <n> is the ordinal assigned to the first
payload or subtype in the m-line, and the subsequent payloads/
subtypes, if any, are assigned consecutive ordinals. This will
permit us to use those media caps in pcfg: attributes as well, and
it's constant under a reordering of the media lines and associated
attributes. Unique numbering will also let us refer to those caps
outside of the media session if we need to specify multimedia
constraints.]
The "a=capar:1" line ,specified in the base protocol provides the The "a=capar: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 capability included in the offer by reference to the media capabilities,
declarations. Two explicit alternatives are provided; the first one, transport capabilities, and associated payload type mappings. Two
numbered 1 is the preferred one, specifies media capabilities 1 and explicit alternatives are provided; the first one, numbered 1 is the
3, i.e. G.729 and PCMU, or media capability 4 and 3, i.e., G.729B preferred one. It specifies media capabilities 1 and 3, i.e. G.729
and PCMU. Furthermore, it specifies transport protocol capability 1 and PCMU, or media capability 4 and 3, i.e., G.729B and PCMU.
(i.e. the RTP/SAVP profile - secure RTP), and the attribute Furthermore, it specifies transport protocol capability 1 (i.e. the
capability parameter 1, i.e. the crypto attribute provided. Lastly, RTP/SAVP profile - secure RTP), and the attribute capability 1, i.e.
it specifies, a payload type mapping for codecs 1, 3, and 4 thereby the crypto attribute provided. Lastly, it specifies, a payload type
permitting the offerer to distinguish between encrypted media and mapping for codecs 1, 3, and 4 thereby permitting the offerer to
unencrypted media received prior to receipt of the answer. distinguish between encrypted media and unencrypted media received
prior to receipt of the answer. For SRTP the offerer will still need
to receive the answer before being able to decrypt the stream.
The second alternative specifies media capability 2, i.e. iLBC, under The second alternative specifies media capability 2, i.e. iLBC, under
the default RTP/AVP profile . The media line, with any qualifying the default RTP/AVP profile . The media line, with any qualifying
attributes such as fmtp or rtpmap, is itself considered a valid attributes such as fmtp or rtpmap, is itself considered a valid
configuration; it is assumed to be the lowest preference. configuration; it is assumed to be the lowest preference.
Bob receives the SDP offer from Alice. Bob supports RTP, but not Bob receives the SDP offer from Alice. Bob supports RTP, but not
SRTP, and hence he accepts the potential configuration for RTP SRTP, and hence he accepts the actual configuration for RTP provided
provided by Alice. Furthermore, Bob wants to use the iLBC codec and by Alice. Furthermore, Bob wants to use the iLBC codec and hence
hence generates the following answer: generates the following answer:
v=0 v=0
o=- 24351 621814 IN IP4 192.0.2.2/24 o=- 24351 621814 IN IP4 192.0.2.2
s= s=
c=IN IP4 192.0.2.2/24 c=IN IP4 192.0.2.2
t=0 0 t=0 0
a=csup:v1 a=csup:v1
m=audio 4567 RTP/AVP 103 m=audio 4567 RTP/AVP 103
a=rtpmap:103 8000 a=rtpmap:103 8000
a=acfg:2 a=acfg:2
Bob includes the "a=csup" and "a=acfg" attribute in the answer to Bob includes the "a=csup" and "a=acfg" attribute in the answer to
inform Alice that he can support the v1 level of capability inform Alice that he can support the v1 level of capability
negotiations. Note that in this particular example, the answerer negotiations. Note that in this particular example, the answerer
supported the capability extensions defined here, however had he not, supported the capability extensions defined here, however had he not,
he would simply have processed the offer based on the offered PCMU he would simply have processed the offer based on the offered PCMU
and G.729B codecs under the RTP/AVP profile only. Consequently, the and G.729 codecs under the RTP/AVP profile only. Consequently, the
answer would have omitted the "a=csup" attribute line and chosen one answer would have omitted the "a=csup" attribute line and chosen one
or both of the PCMU and G.729 codecs instead. The answer carries the or both of the PCMU and G.729 codecs instead. The answer carries the
accepted configuration in the m line along with corresponding rtpmap: accepted configuration in the m line along with corresponding rtpmap:
and/or fmtp: parameters, as appropriate. and/or fmtp: parameters, as appropriate.
Note that per the base protocol, after the above, Alice should
generate a new offer with an actual configuration ("m=" line, etc.)
corresponding to the actual configuration referenced in Bob's answer
(not shown here).
3.2. Capability Attributes 3.2. Capability Attributes
In this section, we present the new attributes associated with In this section, we present the new attributes associated with
indicating the media capabilities for use by the SDP Capability indicating the media capabilities for use by the SDP Capability
negotiation. The approach taken is to keep things similar to the negotiation. The approach taken is to keep things similar to the
existing media capabilities defined by the existing rtpmap: and fmtp: existing media capabilities defined by the existing media
descriptions ("m=" lines) and the associated "rtpmap" and "fmtp"
attributes, but using "media capability numbers" instead of payload attributes, but using "media capability numbers" instead of payload
types to identify each media capability. types to link the relevant media capability parameters.
3.2.1. Media Type and Subtype Capability Attribute 3.2.1. Media Type and Subtype Capability Attribute
Media types and subtypes can be expressed as capabilities by use of Media types and subtypes can be expressed as media format
the "a=cmed" attribute, which is defined as follows: capabilities by use of the "a=cmed" attribute, which is defined as
follows:
a=cmed:<med-cap-num> <type> <subtype>*[ <subtype>] a=cmed:<med-cap-num> <type> <subtype>*[ <subtype>]
where <med-cap-num> is an integer between 1 and 2^31-1 (both where <med-cap-num> is an integer between 1 and 2^31-1 (both
included) used to number the media capabilities, <type> is a media included) used to number the media format capabilities, <type> is a
type (e.g., audio or video), and the <subtype> is the media subtype media type (e.g., audio or video), and the <subtype> is the media
e.g. H263-1998, PMCU. The <med-cap-num> is the media capability subtype e.g. H263-1998, PCMU (Editors' note: can specify in cmed
number associated with the first subtype in the list, the number anything that can be specified in an m-line?). The <med-cap-num> is
associated with the second subtype is one higher, etc. Each the media capability number associated with the first subtype in the
occurrence of the attribute MUST use a different value of <med-cap- list, the number associated with the second subtype is one higher,
num>. Furthermore, when a "cmed" attribute indicates more than one etc. Each occurrence of the attribute MUST use a different value of
media format, the capability numbers implied MUST NOT be used by any <med-cap-num>. Furthermore, when a "cmed" attribute indicates more
other "cmed" attribute in the session or media description than one media format, the capability numbers implied MUST NOT be
(explicitly or implicitly). In ABNF, we have: used by any other "cmed" attribute in the entire session or media
description (explicitly or implicitly). In ABNF, we have:
media-capability-line = "a=cmed:" media-cap-num WSP media-type media-capability-line = "a=cmed:" media-cap-num WSP media-type
WSP media-cap *(WSP media-cap) WSP media-cap *(WSP media-cap)
media-cap-num = 1*DIGIT media-cap-num = 1*DIGIT
media-type = token ; MIME type (audio, video, etc.) media-type = token ; Type name (audio, video, etc.)
media-cap = token ; MIME subtype (PCMU, G729, etc.) media-cap = token ; Subtype name(PCMU, G729, etc.)
Media subtypes identified in "a=cmed" lines may be qualified via Media subtypes identified in "a=cmed" lines may be qualified via the
other attributes, "a=cenc" and "a=cfmt", in much the same way as attributes, "a=cenc" and "a=cfmt", in much the same way as media
payload types are qualified by "a=rtpmap" and "a=fmtp" lines. formats in "m=" lines payload types can have their clock rate and
encoding parameters qualified by an "a=rtpmap" line and media format
specific parameters can be provided by "a=fmtp" lines.
3.2.2. The Capability Encoding Parameters Attribute 3.2.2. The Capability Encoding Parameters Attribute
Capabilities defined in "a=cmed" lines may require additional Media format capabilities may require additional encoding parameters,
encoding parameters, such as sample rate, to be precisely defined. such as sample rate, to be precisely defined. The "a=cenc" encoding
The "a=cenc" encoding attribute is defined as attribute is defined as
a=cenc:<med-cap-num> <clock rate>[/<encoding parameters>] a=cenc:<med-cap-num> <clock rate>[/<encoding parameters>]
The clock rate and other encoding parameters are as defined for the The clock rate and other encoding parameters are as defined for the
"a=rtpmap:" line defined in RFC 4566. For example, a capability for "a=rtpmap:" attribute defined in RFC 4566 [RFC4566]. For example, a
low-bit-rate encoding at 8000 samples per second could be specified capability for low-bit-rate encoding at 8000 samples per second could
by be specified by
a=cmed:1 iLBC a=cmed:1 iLBC
a=cenc:1 8000 a=cenc:1 8000
The encoding becomes part of the media capability. Thus, if it is The encoding becomes part of the media capability. Thus, if it is
desirable to specify the same subtype with, e.g., two different desirable to specify the same subtype with, e.g., two different
encoding rates, then the subtype should be listed twice, and each encoding rates, then the subtype should be listed twice, and each
should be modified appropriately. For example: should be modified appropriately. For example:
a=cmed:1 L16 L16 a=cmed:1 L16 L16
a=cenc:1 8000 a=cenc:1 8000
a=cenc:2 16000/2 a=cenc:2 16000/2
defines two low-data-rate codecs, codec 1 uses 8000 samples per defines two low-data-rate codecs, codec 1 uses 8000 samples per
second, and codec 2 uses 16000 samples per second and 2 channels. second, and codec 2 uses 16000 samples per second and 2 channels.
[EDITOR'S NOTE: I'm thinking that it might be better to put the [EDITOR'S NOTE: I'm thinking that it might be better to put the
encoding information in the cmed line or cptmap, along with the MIME encoding information in the cmed line pcfg "pt", along with the media
subtype, and eliminate the cenc attribute. This would make the above subtype, and eliminate the cenc attribute. This would make the above
example look like: example look like:
a=cmed:1 L16/8000 L16/16000/2 a=cmed:1 L16/8000 L16/16000/2
or
a=pcfg:1 m=1,3|4,3 t=1 a=1 pt=1:100/8000, 4:101/16000/2, 3:102
This can be done so long as the encoding attributes never contain This can be done so long as the encoding attributes never contain
whitespace. Is this the case? or instead use semi colon as whitespace. Is this the case? or instead use semi colon as
separator] separator]
A media capability merely indicates possible support for the media A media capability merely indicates possible support for the media
type and media format(s) in question. In order to actually use a type and media format(s) in question. In order to actually use a
media capability in an offer/answer exchange, it must be referenced media capability in an offer/answer exchange, it must be referenced
in a potential configuration (see Section 2.3.1. ). in a potential configuration (see Section 2.3.1. ).
Media capabilities can be provided at the session-level and the Media capabilities can be provided at the session-level and the
media-level. Media capabilities provided at the session level apply media-level. Media capabilities provided at the session level apply
to the session description in general, whereas media capabilities to the session description in general, whereas media capabilities
provided at the media level apply to that media stream only. In provided at the media level apply to that media stream only. In
either case, the scope of the <med-cap-num> is the entire session either case, the scope of the <med-cap-num> is the entire session
description. This enables each media capability to be referenced description. This enables each media capability to be referenced
across the entire session description (e.g. in a potential across the entire session description (e.g. in a potential
configuration.) configuration.)
3.2.3. The Capability Format Parameters Attribute 3.2.3. The Media Format Parameter Capability Attribute
This attribute is used to associate format parameters with a media This attribute is used to associate media format parameters with a
capability. The form of the attribute is: media capability. The form of the attribute is:
a=cfmt:<med-cap-num> <list of format parameters> a=cfmt:<med-cap-num> <list of format parameters>
where the format parameters are specific to the type of codec, as where the format parameters are specific to the type of codec, as
described for the fmtp: attribute defined in RFC 4566. As an described for the fmtp: attribute defined in RFC 4566. As an
example, a G.729 capability is, by default, considered to support example, a G.729 capability is, by default, considered to support
comfort noise as defined by Annex B. Capabilities for G.729 with and comfort noise as defined by Annex B. Capabilities for G.729 with and
without comfort noise support may thus be identified by: without comfort noise support may thus be identified by:
a=cmed:1 audio G729 G729 a=cmed:1 audio G729 G729
a=cfmt:2 annexb:no a=cfmt:2 annexb:no
Example for H.263 video: Example for H.263 video:
a=cmed:1 video H263-1998 H263-2000 a=cmed:1 video H263-1998 H263-2000
a=cfmt:1 CIF=4;QCIF=2;F=1;K=1 a=cfmt:1 CIF=4;QCIF=2;F=1;K=1
a=cftp:2 profile=2;level=2.2 a=cfmt:2 profile=2;level=2.2
3.2.4. The Media Line Capabilities Attribute
This attribute is used to assign unique ordinals to the media
capabilities listed in the m= line. It is a media-level attribute,
but the numbers assigned must be unique across the session. The form
of the attribute is:
a=mcap:<med-cap-num>
where <media-cap-num> is the ordinal assigned to the first media
capability listed in the m-line; subsequent capabilities, if present,
are assigned consecutive numbers following the first. For example,
the following sequence defines three capabilies, 1 = GSM, 6 = PCMU,
and 7 = G.729. The m-line configuration is the preferred one, but
GSM support is offered as an alternative:
a=cmed:1 audio GSM
m=audio 4567 RTP/AVP 0 18
a=mcap:6
a=pcfg:1 m=6|7
a=pcfg:2 m=1
3.3. Extensions to the Potential Configuration Attribute 3.3. Extensions to the Potential Configuration Attribute
The extension protocol of capabilities negotiation requires two new The extension protocol of capabilities negotiation requires two new
extensions for the pcfg: attribute defined in the base protocol. One extensions for the pcfg: attribute defined in the base protocol. One
extension permits the specification of selected media capabilities, extension permits the specification of media capabilities, or
or combinations thereof; the other permits the assignment of payload combinations thereof; the other permits the assignment of payload
types to those capabilities when used in the specified configuration. types to those capabilities when used in the specified configuration.
3.3.1. The Media Capability Extension to the Potential Configuration 3.3.1. The Media Capability Extension to the Potential Configuration
Attribute Attribute
The potential configuration attribute ("a=pcfg") as defined in SDP The potential configuration attribute ("a=pcfg") as defined in SDP
capabilities negotiation, permits alternate attributes to be capabilities negotiation, permits alternate attributes to be
associated with the media types defined in a media line. In this associated with the media types defined in a media line. In this
extension (this document), we define an extension parameter for the extension (this document), we define an extension parameter for the
specification of media configurations in addition to the one specification of media configurations in addition to the one
specified on the media line. specified on the media line.
We define the media capability configuration parameter, pot-media- We define the media capability configuration parameter, pot-media-
config, in accordance with the following format: config, in accordance with the following format:
m=<med-cap-list> *["/"<med-cap-list>] m=<med-cap-list> *["|"<med-cap-list>]
where <med-cap-list> is a comma-separated list of media capability where <med-cap-list> is a comma-separated list of media capability
numbers (media-cap-num) as defined by a=cmed: lines and media lines. numbers (media-cap-num) as defined by a=cmed: lines and media lines.
In ABNF form, In ABNF form (adhering to the ABNF for pot-extension-config in
[SDPCapNeg]:
pot-media-config = "m=" med-cap-list *("/" med-cap-list) pot-media-config = "m=" med-cap-list *(BAR med-cap-list)
med-cap-list = med-cap-num *("," med-cap-num) med-cap-list = med-cap-num *("," med-cap-num)
med-cap-num = 1*DIGIT ; defined in SDP med-cap-num = 1*DIGIT ; defined in SDP
BAR = *WSP "|" *WSP
Each potential media configuration is a comma-separated list of media Each potential media configuration is a comma-separated list of media
capability numbers where med-cap-num refers to media capability capability numbers where med-cap-num refers to media capability
numbers defined explicitly by a=cmed attributes or implicitly by the numbers defined explicitly by a=cmed attributes or implicitly by the
media line, and hence MUST be between 1 and 2^31-1 (both included). media line, and hence MUST be between 1 and 2^31-1 (both included).
Alternative potential media configurations are separated by a forward Alternative potential media configurations are separated by a
slash ("/"). The alternatives are ordered by preference. When media vertical bar ("|"). The alternatives are ordered by preference.
capabilities are not included in a potential configuration at the When media capabilities are not included in a potential configuration
media level, the media type and media format from the associated "m=" at the media level, the media type and media format from the
line will be used. associated "m=" line will be used.
For example: For example:
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1/24 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.1/24 c=IN IP4 192.0.2.1
t=0 0 t=0 0
a=creq:v1 a=creq:v1
a=cmed:1 audio PCMU g729 telephone-event
m=audio 3456 RTP/AVP 0 18 100 m=audio 3456 RTP/AVP 0 18 100
a=rtpmap:100 events a=rtpmap:100 telephone-events
a=fmtp:100 0-15 a=fmtp:100 0-15
a=pcfg:1 m=2,3/1,3 a=pcfg:1 m=2,3|1,3 pt=1:0, 2:18, 3:100
In this example, G729 is media capability 2, PCMU is media capability In this example, G729 is media capability 2, PCMU is media capability
1, and events is media capability 3. The a=pcfg: line specifies that 1, and events is media capability 3. The a=pcfg: line specifies that
the preferred configuration is G.729B with dtmf events, second is the preferred configuration is G.729 with dtmf events, second is
G.711 mu-law with dtmf events. Intermixing of G.729, G.711, and dtmf G.711 mu-law with dtmf events. Intermixing of G.729, G.711, and dtmf
events is least preferred. events is least preferred (the actual configuration provided by the
"m=" line, which is always the least preferred configuration).
3.3.1.1. The Payload Type Mapping Extension to the Potential 3.3.2. The Payload Type Mapping Extension to the Potential
Configuration Attribute Configuration Attribute
When media capabilities defined in cmed: attributes are used in When media capabilities defined in cmed: attributes are used in
potential configuration lines, it is necessary to assign payload potential configuration lines, it is necessary to assign payload
types to them. In some cases, it is desirable to assign different types to them. In some cases, it is desirable to assign different
payload types to media capabilities defined in the media line. One payload types to media capabilities defined in the media line. One
example of the latter is when configurations for AVP and SAVP are example of the latter is when configurations for AVP and SAVP are
offered: the offerer would like the answerer to use different payload offered: the offerer would like the answerer to use different payload
types for encrypted and unencrypted media so that it (the offerer) types for encrypted and unencrypted media so that it (the offerer)
can decide whether or not to render early media which arrives before can decide whether or not to render early media which arrives before
the answer is received. the answer is received.
We define a payload-type-map attribute which define a (set of) We define the media type mapping configuration parameter, pot-media-
mappings from media capability number to payload type map, in accordance with the following format:
a = cptmap: 1*[<med-cap-num>:<payload-type>]
payload-type-map = 1*(WSP <med-cap-num)":"<payload-type>)
med-cap-num = 1*DIGIT ; from cmed: attribute
payload-type = 1*DIGIT ; RTP payload type
This is illustrated by extending an example from the version 0
document showing an offer of an audio session with or without SRTP.
The following specifies RTP/SAVP transport using payload type 98 for
PCMU and payload type 99 for G.729; RTP/AVP transport uses the
standard payload types from the media line.
v=0
o=- 25678 753849 IN IP4 192.0.2.1/24
s=
c=IN IP4 192.0.2.1/24
t=0 0
a=creq:v1
m=audio 3456 RTP/AVP 0 18
a=ctrpr:1 RTP/SAVP RTP/AVP
a=capar:1 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
FEC_ORDER=FEC_SRTP
a=cptmap:1 1:98 2:99
a=pcfg:5 t=1 a=1 pt=1
a=pcfg:10 t=2
[Editor's note: it seems to me that the cptmap: attribute is only pot-media-map = "pt=" med-map-num *("," *WSP med-map-num)
really useful within a media block, so perhaps we should drop it and med-map-num = 1*DIGIT ":" 1*DIGIT ;
just define the pt= parameter as a comma-separated list of PT
mappings like "pt=1:98,2:99" in the above example. Alternative is to
use the cenc=1:98/8000. What do you think?]
3.3.1.2. Extensions to the Actual Configuration Attribute The example in the previous section shows how the parameters from the
cmed line are mapped to payload type in the pcfg "pt" parameter.
[Editor's note: I'm thinking more and more that we don't need the 3.4. Extensions to the Actual Configuration Attribute
acfg: attribute at all, given the presence of the csup: attribute.
All the significant reply information is in the m=line and its
associated attributes. We should be able to infer that they came
from a pcfg: attribute or the default offer. What do you think?]
(will it be OK with offer answer)
We define an actual configuration extension parameter act-media- We define an actual configuration extension parameter act-media-
config in accordance with the following ABNF: config in accordance with the following ABNF:
act-media-config = "m=" med-cap-list a=acfg:<med-cap-num> *WSP "m=" <med-cap-list>
where med-cap-list is as defined in Section 2.3.1. Thus, a response A response to the previous offer example in the above section might
to the example offer in that section might be: be:
v=0 v=0
o=- 24351 621814 IN IP4 192.0.2.2/24 o=- 24351 621814 IN IP4 192.0.2.2
s= s=
c=IN IP4 192.0.2.2/24 c=IN IP4 192.0.2.2
t=0 0 t=0 0
a=csup:v1 a=csup:v1
m=audio 5432 RTP/AVP 18 100 m=audio 5432 RTP/AVP 18 100
a=rtpmap:100 events a=rtpmap:100 events
a=fmtp:100 0-15 a=fmtp:100 0-15
a=acfg:1 m=2,3 a=acfg:1 m=2,3
Note that the capability numbers expressed in the acfg: attribute are Note that the capability numbers expressed in the acfg: attribute are
based on the offered capability numbering, not on those listed in the based on the offered capability numbering, not on those listed in the
answer. In this particular example, it's somewhat redundant to answer. The acfg identify to the offrer which potetial configuration
include the acfg: line at all , but it is important in cases in which was selected by the answerer.
the payload type mappings are not the same in both directions.
3.3.1.3. The Latent Configuration Attribute 3.5. The Latent Configuration Attribute
One of the goals of this work is to permit the exchange of media One of the goals of this work is to permit the exchange of media
configurations in addition to those offered for immediate use. Such configurations in addition to those offered for immediate use. Such
configurations are referred to as "latent configurations". For configurations are referred to as "latent configurations". For
example, a party may offer to establish an audio session, and, at the example, a party may offer to establish an audio session, and, at the
same time, announce its ability to support a video session. same time, announce its ability to support a video session.
Latent configurations may be announced by use of the latent Latent configurations may be announced by use of the latent
configuration attribute, which is defined in a manner very similar to configuration attribute, which is defined in a manner very similar to
the potential configuration attribute: the potential configuration attribute:
skipping to change at page 15, line 6 skipping to change at page 14, line 5
Latent Configurations may be specified at the session level in offers Latent Configurations may be specified at the session level in offers
and in answers. and in answers.
[Editor's note: Do you have a good example of an offer of an audio [Editor's note: Do you have a good example of an offer of an audio
stream with a latent video stream?][Editor's note (rrg): What if, as stream with a latent video stream?][Editor's note (rrg): What if, as
Roni suggested, we exclude the a= parameters from the lcfg: lines? Roni suggested, we exclude the a= parameters from the lcfg: lines?
We can include the t= parameter as the 'cheapest' way to indicate We can include the t= parameter as the 'cheapest' way to indicate
SRTP. That conveys most of the necessary info; the details can be SRTP. That conveys most of the necessary info; the details can be
firmed up in the actual offer when/if it's actually made.] firmed up in the actual offer when/if it's actually made.]
3.3.1.3.1. The crypto: Attribute in Latent Configurations 3.5.1. The crypto: Attribute in Latent Configurations
If the sdescriptions crypto: attribute is necessary as part of any If the sdescriptions crypto: attribute is necessary as part of any
latent configuration which announces sdescriptions capabilities. then latent configuration which announces sdescriptions capabilities. then
it presents a slight problem in that the rather long key/salt string it presents a slight problem in that the rather long key/salt string
is useless and should be ignored. This problem is avoided if we is useless and should be ignored. This problem is avoided if we
exclude the a= parameters from the lcfg: attribute. [Editor's note: exclude the a= parameters from the lcfg: attribute. [Editor's note:
should we define a new crypto: key-method, e.g. "latent", in which should we define a new crypto: key-method, e.g. "latent", in which
the key-salt portion of key-info is empty? I think it may be the key-salt portion of key-info is empty? I think it may be
sufficient to just include the RTP/SAVP transport to indicate SRTP sufficient to just include the RTP/SAVP transport to indicate SRTP
capability] capability]
3.3.1.4. 2.6. Offer/Answer Model Extensions 3.6. Offer/Answer Model Extensions
In this section, we define extensions to the offer/answer model In this section, we define extensions to the offer/answer model
defined in RFC3264 to allow for media capabilities to be used with defined in RFC3264 to allow for media capabilities to be used with
the SDP Capability Negotation framework. the SDP Capability Negotation framework.
3.3.1.4.1. Generating the Initial Offer 3.6.1. Generating the Initial Offer
3.3.1.4.2. Generating the Answer 3.6.2. Generating the Answer
3.3.1.4.3. Offerer Processing of the Answer 3.6.3. Offerer Processing of the Answer
3.3.1.4.4. Modifying the Session 3.6.4. Modifying the Session
4. Examples 4. Examples
In this section, we provide examples showing how to use the Media In this section, we provide examples showing how to use the Media
Capabilities with the SDP Capability Negotiation. Capabilities with the SDP Capability Negotiation.
4.1. Alternative Codecs 4.1. Alternative Codecs
4.2. Latent Media Streams 4.2. Latent Media Streams
5. IANA Considerations 5. IANA Considerations
6. Security Considerations 6. Security Considerations
7. Acknowledgements 7. Changes from version 00
The major changes include taking out the "mcap" and "cptmap"
parameter. The mapping of payload type is now in the "pt" parameter
of "pcfg". Media subtype need to explictly definesd in the "cmed"
attribute if referenced in the "pcfg"
8. Acknowledgements
This document is heavily influenced by the discussions and work done This document is heavily influenced by the discussions and work done
by the SDP Capability Negotiation Design team. The following people by the SDP Capability Negotiation Design team. The following people
in particular provided useful comments and suggestions to either the in particular provided useful comments and suggestions to either the
document itself or the overall direction of the solution defined document itself or the overall direction of the solution defined
herein: Cullen Jennings, Matt Lepinski, Joerg Ott, Colin Perkins, and herein: Cullen Jennings, Matt Lepinski, Joerg Ott, Colin Perkins, and
Thomas Stach. Thomas Stach.
8. Normative References 9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, with Session Description Protocol (SDP)", RFC 3264,
June 2002. June 2002.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006. Description Protocol", RFC 4566, July 2006.
Authors' Addresses [SDPCapNeg]
Andreasen, F., "SDP Capability Negotiation",
draft-ietf-mmusic-sdp-capability-negotiation-02 (work in
progress), February 2007.
Flemming Andreasen 9.2. Informative References
Cisco Systems
Edison, NJ
USA
Email: fandreas@cisco.com [RFC3407] Andreasen, F., "Session Description Protocol (SDP) Simple
Capability Declaration", RFC 3407, October 2002.
Authors' Addresses
Robert R Gilman Robert R Gilman
Avaya, Inc. Avaya, Inc.
1300 West 120th Avenue 1300 West 120th Avenue
Westminster, CO 80234 Westminster, CO 80234
USA USA
Email: rrg@avaya.com Email: rrg@avaya.com
Roni Even (editor) Roni Even (editor)
Polycom Polycom
94 Derech Em Hamoshavot 94 Derech Em Hamoshavot
Petach Tikva 49130 Petach Tikva 49130
Israel Israel
Email: roni.even@polycom.co.il Email: roni.even@polycom.co.il
Flemming Andreasen
Cisco Systems
Edison, NJ
USA
Email: fandreas@cisco.com
Full Copyright Statement Full Copyright Statement
Copyright (C) The IETF Trust (2007). Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors contained in BCP 78, and except as set forth therein, the authors
retain all their rights. retain all their rights.
This document and the information contained herein are provided on an This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
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