draft-ietf-mmusic-sdp-capability-negotiation-00.txt   draft-ietf-mmusic-sdp-capability-negotiation-01.txt 
MMUSIC Working Group F. Andreasen MMUSIC Working Group F. Andreasen
Internet Draft Cisco Systems Internet Draft Cisco Systems
Expires: July 2007 January 2, 2007 Expires: July 2007 January 28, 2007
SDP Capability Negotiation SDP Capability Negotiation
draft-ietf-mmusic-sdp-capability-negotiation-00.txt draft-ietf-mmusic-sdp-capability-negotiation-01.txt
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Abstract Abstract
The Session Description Protocol (SDP) was intended for describing The Session Description Protocol (SDP) was intended for describing
multimedia sessions for the purposes of session announcement, session multimedia sessions for the purposes of session announcement, session
invitation, and other forms of multimedia session initiation. SDP was invitation, and other forms of multimedia session initiation. SDP was
not intended to provide capability indication or capability not intended to provide capability indication or capability
negotiation, however over the years, SDP has seen widespread adoption negotiation, however over the years, SDP has seen widespread adoption
and as a result it has been gradually extended to provide limited and as a result it has been gradually extended to provide limited
support for these. SDP and its current extensions however do not have support for these. SDP and its current extensions however do not have
the ability to negotiate one or more alternative transport protocols the ability to negotiate one or more alternative transport protocols
(e.g. RTP profiles) which makes it particularly difficult to deploy (e.g. RTP profiles) which makes it particularly difficult to deploy
new RTP profiles such as secure RTP or RTP with RTCP-based feedback. new RTP profiles such as secure RTP or RTP with RTCP-based feedback.
The purpose of this document is to address that and other real-life The purpose of this document is to address that and other real-life
limitations by extending SDP with capability negotiation parameters limitations by extending SDP with capability negotiation parameters
and associated offer/answer procedures to use those parameters in a and associated offer/answer procedures to use those parameters in a
backwards compatible manner. backwards compatible manner.
The solution provided in this document provides a general SDP
capability negotiation framework. It also defines specifically how to
provide attributes and transport protocols as capabilities and
negotiate them using the framework. Extensions for other types of
capabilities (e.g. media types and formats) may be provided in other
documents.
Conventions used in this document Conventions used in this document
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]. document are to be interpreted as described in [RFC2119].
Table of Contents Table of Contents
1. Introduction...................................................3 1. Introduction...................................................3
2. SDP Capability Negotiation Solution............................5 2. SDP Capability Negotiation Solution............................5
2.1. Solution Overview.........................................5 2.1. Solution Overview.........................................5
2.2. Version and Extension Indication Attributes...............8 2.2. Version and Extension Indication Attributes...............8
2.2.1. SDP Capability Negotiation Version Attribute.........8 2.2.1. Supported Capability Negotiation Extensions Attribute8
2.2.2. Supported Capability Negotiation Extensions Attribute9 2.2.2. Required Capability Negotiation Extension Attribute.10
2.2.3. Required Capability Negotiation Extension Attribute.10
2.3. Capability Attributes....................................11 2.3. Capability Attributes....................................11
2.3.1. Media Type and Format Capability Attribute..........11 2.3.1. Attribute Capability Attribute......................11
2.3.2. Attribute Parameter Capability Attribute............14 2.3.2. Transport Protocol Capability Attribute.............13
2.3.3. Transport Protocol Capability Attribute.............15 2.4. Configuration Attributes.................................13
2.4. Configuration Attributes.................................16 2.4.1. Potential Configuration Attribute...................13
2.4.1. Potential Configuration Attribute...................16 2.4.2. Actual Configuration Attribute......................17
2.4.2. Actual Configuration Attribute......................19 2.5. Offer/Answer Model Extensions............................18
2.5. Offer/Answer Model Extensions............................20 2.5.1. Generating the Initial Offer........................18
2.5.1. Generating the Initial Offer........................21 2.5.2. Generating the Answer...............................19
2.5.2. Generating the Answer...............................22 2.5.3. Offerer Processing of the Answer....................20
2.5.3. Offerer Processing of the Answer....................22 2.5.4. Modifying the Session...............................20
2.5.4. Modifying the Session...............................23 3. Examples......................................................21
3. Examples......................................................23 3.1. Best-Effort Secure RTP...................................21
3.1. Best-Effort Secure RTP...................................23 4. Security Considerations.......................................23
4. Security Considerations.......................................25 5. IANA Considerations...........................................23
5. IANA Considerations...........................................25 6. To Do and Open Issues.........................................23
6. To Do and Open Issues.........................................26 7. Acknowledgments...............................................23
7. Acknowledgments...............................................26 8. Change Log....................................................24
8. Change Log....................................................26 8.1. draft-ietf-mmusic-sdp-capability-negotiation-01..........24
8.1. draft-ietf-mmusic-sdp-capability-negotiation-00..........26 8.2. draft-ietf-mmusic-sdp-capability-negotiation-00..........24
9. References....................................................27 9. References....................................................26
9.1. Normative References.....................................27 9.1. Normative References.....................................26
9.2. Informative References...................................27 9.2. Informative References...................................26
Author's Addresses...............................................29 Author's Addresses...............................................28
Intellectual Property Statement..................................29 Intellectual Property Statement..................................28
Disclaimer of Validity...........................................30 Disclaimer of Validity...........................................29
Copyright Statement..............................................30 Copyright Statement..............................................29
Acknowledgment...................................................30 Acknowledgment...................................................29
1. Introduction 1. Introduction
The Session Description Protocol (SDP) was intended for describing The Session Description Protocol (SDP) was intended for describing
multimedia sessions for the purposes of session announcement, session multimedia sessions for the purposes of session announcement, session
invitation, and other forms of multimedia session initiation. The SDP invitation, and other forms of multimedia session initiation. The SDP
contains one or more media stream descriptions with information such contains one or more media stream descriptions with information such
as IP-address and port, type of media stream (e.g. audio or video), as IP-address and port, type of media stream (e.g. audio or video),
transport protocol (possibly including profile information, e.g. transport protocol (possibly including profile information, e.g.
RTP/AVP or RTP/SAVP), media formats (e.g. codecs), and various other RTP/AVP or RTP/SAVP), media formats (e.g. codecs), and various other
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media stream description is not supported by the participant, he is media stream description is not supported by the participant, he is
unable to receive the media. unable to receive the media.
Such restrictions are not generally acceptable to multimedia session Such restrictions are not generally acceptable to multimedia session
invitations, where two or more entities attempt to establish a media invitations, where two or more entities attempt to establish a media
session that uses a set of media stream parameters acceptable to all session that uses a set of media stream parameters acceptable to all
participants. First of all, each entity must inform the other of its participants. First of all, each entity must inform the other of its
receive address, and secondly, the entities need to agree on the receive address, and secondly, the entities need to agree on the
media stream parameters to use for the session, e.g. transport media stream parameters to use for the session, e.g. transport
protocols and codecs. We here make a distinction between the protocols and codecs. We here make a distinction between the
capabilities supported by each participant and the parameters that capabilities supported by each participant, the way in which those
can actually be used for the session. More generally, we can say that capabilities can be supported and the parameters that can actually be
we have the following: used for the session. More generally, we can say that we have the
following:
o A set of capabilities and potential configurations of the media o A set of capabilities for the session and its associated media
stream components, supported by each side. stream components, supported by each side.
o A set of actual configurations of the media stream components, o A set of potential configurations indicating which of those
which specifies which media stream components to use and with what capabilities can be used for the session and its associated media
stream components.
o A set of actual configurations for the session and its associated
media stream components, which specifies which session parameters
to use and which media stream components to use and with what
parameters. parameters.
o A negotiation process that takes the set of potential o A negotiation process that takes the set of potential
configurations (capabilities) as input and provides the actual configurations (lists of capabilities) as input and provides the
configurations as output. actual configurations as output.
SDP by itself was designed to provide only the second of these, i.e., SDP by itself was designed to provide only one of these, namely the
the actual configurations, however over the years, use of SDP has actual configurations, however over the years, use of SDP has been
been extended beyond its original scope. Session negotiation extended beyond its original scope. Session negotiation semantics
semantics were defined by the offer/answer model in RFC 3264. It were defined by the offer/answer model in RFC 3264. It defines how
defines how two entities, an offerer and an answerer, exchange two entities, an offerer and an answerer, exchange session
session descriptions to negotiate a session. The offerer can include descriptions to negotiate a session. The offerer can include one or
one or more media formats (codecs) per media stream, and the answerer more media formats (codecs) per media stream, and the answerer then
then selects one or more of those offered and returns them in an selects one or more of those offered and returns them in an answer.
answer. Both the offer and the answer contain actual configurations - Both the offer and the answer contain actual configurations -
potential configurations are not supported. The answer however may capabilities and potential configurations are not supported. The
reduce the set of actual configurations from the offer. answer however may reduce the set of actual configurations from the
offer. The answer may also extend the set of actual configurations
that can be used to receive media by the answerer.
Other relevant extensions have been defined. Simple capability Other relevant extensions have been defined. Simple capability
declarations, which define how to provide a simple and limited set of declarations, which define how to provide a simple and limited set of
capability descriptions in SDP was defined in RFC 3407. Grouping of capability descriptions in SDP was defined in RFC 3407. Grouping of
media lines, which defines how media lines in SDP can have other media lines, which defines how media lines in SDP can have other
semantics than the traditional "simultaneous media streams" semantics than the traditional "simultaneous media streams"
semantics, was defined in RFC 3388, etc. semantics, was defined in RFC 3388, etc.
Each of these extensions was designed to solve a specific limitation Each of these extensions was designed to solve a specific limitation
of SDP. Since SDP had already been stretched beyond its original of SDP. Since SDP had already been stretched beyond its original
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negotiation problem, other important real-life constraints have been negotiation problem, other important real-life constraints have been
found as well. found as well.
The purpose of this document is to define a mechanism that enables The purpose of this document is to define a mechanism that enables
SDP to provide limited support for indicating capabilities and their SDP to provide limited support for indicating capabilities and their
associated potential configurations and negotiate the use of those associated potential configurations and negotiate the use of those
potential configurations as actual configurations. It is not the potential configurations as actual configurations. It is not the
intent to provide a full-fledged capability indication and intent to provide a full-fledged capability indication and
negotiation mechanism along the lines of SDPng or ITU-T H.245. negotiation mechanism along the lines of SDPng or ITU-T H.245.
Instead, the focus is on addressing a set of well-known real-life Instead, the focus is on addressing a set of well-known real-life
limitations. limitations. More specifically, the solution provided in this
document provides a general SDP capability negotiation framework. It
also defines specifically how to provide attributes and transport
protocols as capabilities and negotiate them using the framework.
Extensions for other types of capabilities (e.g. media types and
formats) may be provided in other documents.
As mentioned above, SDP is used by several protocols, and hence the As mentioned above, SDP is used by several protocols, and hence the
mechanism should be usable by all of these. One particularly mechanism should be usable by all of these. One particularly
important protocol for this problem however is the Session Initiation important protocol for this problem however is the Session Initiation
Protocol (SIP) [RFC3261]. SIP uses the offer/answer model (which is Protocol (SIP) [RFC3261]. SIP uses the offer/answer model (which is
not specific to SIP) to negotiate sessions and hence any mechanism not specific to SIP) to negotiate sessions and hence any mechanism
must at least consider how it either interacts with offer/answer, or must at least consider how it either interacts with offer/answer, or
how it should extend it. how it should extend it.
The rest of the document is structured as follows. In Section 0we The rest of the document is structured as follows. In Section 2. we
present our SDP capability negotiation solution followed by examples present our SDP capability negotiation solution followed by examples
in Section 3. and security considerations in Section 4. in Section 3. and security considerations in Section 4.
2. SDP Capability Negotiation Solution 2. SDP Capability Negotiation Solution
In this section we first provide an overview of the SDP Capability In this section we first provide an overview of the SDP Capability
negotiation solution. This is followed by definitions of new SDP negotiation solution. This is followed by definitions of new SDP
attributes for the solution and its associated updated offer/answer attributes for the solution and its associated updated offer/answer
procedures. procedures.
2.1. Solution Overview 2.1. Solution Overview
The solution consists of the following: The solution consists of the following:
o Three new attributes to support versioning and extensions to the o Two new attributes to support versioning and extensions to the
framework itself as follows: framework itself as follows:
o A new attribute ("a=cver") that lists the version of the SDP o A new attribute ("a=csup") that lists the supported base and
capability negotiation framework being used. extension options to the framework.
o A new attribute ("a=csup") that lists the supported extensions
to the framework.
o A new attribute ("a=creq") that lists the extensions to the o A new attribute ("a=creq") that lists the base and or
framework that are required to be supported by the entity extensions to the framework that are required to be supported
receiving the SDP. by the entity receiving the SDP.
o Three new attributes used to express capabilities as follows o Two new attributes used to express capabilities as follows
(additional attributes can be defined as extensions): (additional attributes can be defined as extensions):
o A new attribute ("a=cmed") that defines how to list the media o A new attribute ("a=capar") that defines how to list attribute
types and media formats supported as capabilities. parameter values ("a=" values) as capabilities.
o A new attribute ("a=capar") that defines how to list the
attribute parameter values as capabilities.
o A new attribute ("a=ctrpr") that defines how to list transport o A new attribute ("a=ctrpr") that defines how to list transport
protocols as capabilities. protocols (e.g. "RTP/AVP") as capabilities.
o Two new attributes to negotiate configurations as follows: o Two new attributes to negotiate configurations as follows:
o A new attribute ("a=pcfg") that lists the potential o A new attribute ("a=pcfg") that lists the potential
configurations supported. This is done by reference to the configurations supported. This is done by reference to the
above capabilities from the SDP in question. The potential capabilities from the SDP in question. Multiple potential
configurations are listed in order of preference. Extensions configurations have an explicitly indicated ordering
can be defined as well and included in the potential associated with them. Extension capabilities can be defined
configurations. and included in the potential configurations.
o A new attribute ("a=acfg") to be used in an answer SDP. The o A new attribute ("a=acfg") to be used in an answer SDP. The
attribute identifies which of the potential configurations attribute identifies which of the potential configurations
from an offer SDP were used as actual configurations to form from an offer SDP were used as actual configurations to form
the answer SDP. the answer SDP. Extension capabilities can included.
o Extensions to the offer/answer model that allow for capabilities o Extensions to the offer/answer model that allow for capabilities
and potential configurations to be included in an offer, where and potential configurations to be included in an offer. When
they constitute offers that may be accepted by the answerer included at the session level, they constitute latent capabilities
instead of the actual configuration(s) included in the "m=" that may be used to guide a subsequent offer. When included at the
line(s). The answerer indicates which (if any) of the potential media level, they constitute offers that may be accepted by the
configurations it used to form the answer by including the actual answerer instead of the actual configuration(s) included in the
configuration attribute ("a=cfg") in the answer. Capabilities and "m=" line(s). The answerer indicates which (if any) of the
potential configurations may be included in answers as well, where potential configurations it used to form the answer by including
they can aid in guiding a subsequent new offer. the actual configuration attribute ("a=acfg") in the answer.
Capabilities and potential configurations may be included in
answers as well, where they can aid 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) |
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Alice's offer includes RTP and SRTP as alternatives. RTP is the Alice's offer includes RTP and SRTP as alternatives. RTP is the
default, but SRTP is the preferred one: default, but SRTP is the preferred one:
v=0 v=0
o=- 25678 753849 IN IP4 128.96.41.1 o=- 25678 753849 IN IP4 128.96.41.1
s= s=
c=IN IP4 128.96.41.1 c=IN IP4 128.96.41.1
t=0 0 t=0 0
m=audio 3456 RTP/AVP 0 18 m=audio 3456 RTP/AVP 0 18
a=cver:0 a=creq: v0
a=cmed:1 audio RTP/AVP 0 18 96
a=ctrpr:1 RTP/SAVP a=ctrpr:1 RTP/SAVP
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=capar:2 a=rtpmap:96 iLBC/8000 a=pcfg:1 t=1 a=1
a=pcfg: m=1,2|3 p=1 a=1,2
a=pcfg: m=1,2|3 a=2
The "m=" line indicates that Alice is offering to use plain RTP with The "m=" line indicates that Alice is offering to use plain RTP with
PCMU or G.729. The capabilities are provided by the "a=cver", PCMU or G.729. The required base and extensions are provided by the
"a=cmed", "a=ctrpr" and "a=capar" attributes. The capabilities "a=creq" attribute, which indicates that the option tag "v0", which
indicate that PCMU, G.729 and iLBC are supported with either RTP or indicates the base framework defined here, must be supported. The
secure RTP. The first "capar" attribute provides a capability capabilities are provided by the "a=ctrpr" and "a=capar" attributes.
parameter with a handle of 1. The capability parameter is a "crypto" The capabilities indicate that secure RTP under the AVP profile
attribute in the capability set, which provides the keying material ("RTP/SAVP") is supported with an associated transport capability
for SRTP using SDP security descriptions [SDES]. The second "capar" handle of 1. The "capar" attribute provides an attribute capability
attribute provides the "rtpmap" for the dynamic payload type 96, with a handle of 1. The attribute capability is a "crypto" attribute,
which is mapped to the iLBC codec. The "a=pcfg" attribute provides which provides the keying material for SRTP using SDP security
the potential configurations included in the offer by reference to descriptions [SDES]. The "a=pcfg" attribute provides the potential
the capability declarations. Two alternatives are provided; the configuration included in the offer by reference to the capability
first one, and hence the preferred one is using media capabilities 1 parameters. One alternatives is provided; it has a preference of 1
and 2, i.e. PCMU and G.729, or media capability 3, i.e. iLBC. and it consists of transport protocol capability 1 (i.e. the RTP/SAVP
Furthermore, transport protocol capability 1 (i.e. the RTP/SAVP profile - secure RTP), and the attribute capability 1, i.e. the
profile - secure RTP), and the attribute capability parameter 1, i.e. crypto attribute provided. Potential configurations are always
the crypto attribute provided, and the attribute capability parameter preferred over actual configurations, and hence Alice is expressing a
2, i.e. the rtpmap for iLBC is included. The second one is simply preference for using secure RTP.
using media capabilities 1 and 2, i.e. PCMU and G.729, or media
capability 3, i.e. iLBC under the RTP/AVP profile as listed in the
"m=" line. The "capar" parameter is still needed to provide the
rtpmap for iLBC.
Bob receives the SDP offer from Alice. Bob supports RTP, but not Bob receives the SDP offer from Alice. Bob supports SRTP and the SDP
SRTP, and hence he accepts the potential configuration for RTP Capability Negotiation framework, and hence he accepts the
provided by Alice. Furthermore, Bob wants to use the iLBC codec and (preferred) potential configuration for Secure RTP provided by Alice:
hence generates the following answer:
v=0 v=0
o=- 24351 621814 IN IP4 128.96.41.2 o=- 24351 621814 IN IP4 128.96.41.2
s= s=
c=IN IP4 128.96.41.2 c=IN IP4 128.96.41.2
t=0 0 t=0 0
m=audio 4567 RTP/AVP 96 m=audio 4567 RTP/SAVP 0 18
a=rtpmap:96 iLBC/8000 a=crypto:1 AES_CM_128_HMAC_SHA1_80
a=cver: 0 inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4
a=acfg: m=3 a=2 a=acfg: t=1 a=1
Bob includes the "a=cver" and "a=acfg" attribute in the answer to Bob includes the "a=acfg" attribute in the answer to inform Alice
inform Alice that he based his answer on an offer containing the that he based his answer on an offer containing the potential
potential configuration with media capability 3 from the offer SDP configuration with transport protocol capability 1 and attribute
(i.e. iLBC under the RTP/AVP profile) and the attribute capability capability 1 from the offer SDP (i.e. the RTP/SAVP profile using the
parameter 2, i.e. the associated rtpmap. Note that in this keying material provided). Bob also includes his keying material in
particular example, the answerer supported the capability extensions a crypto attribute. If Bob supported one or more extensions to the
defined here, however had he not, he would simply have processed the capability negotiation framework, he would have included those in the
offer based on the offered PCMU and G.729 codecs under the RTP/AVP answer as well.
profile only. Consequently, the answer would have omitted the
"a=cver" and "a=acfg" attribute line and chosen one or both of the Note that in this particular example, the answerer supported the
PCMU and G.729 codecs instead. capability negotiation extensions defined here, however had he not,
the answerer would simply have ignored the new attributes and
accepted the offer to use normal RTP. In that case, the following
answer would have been generated instead:
v=0
o=- 24351 621814 IN IP4 128.96.41.2
s=
c=IN IP4 128.96.41.2
t=0 0
m=audio 4567 RTP/AVP 0 18
2.2. Version and Extension Indication Attributes 2.2. Version and Extension Indication Attributes
In this section, we present the new attributes associated with In this section, we present the new attributes associated with
indicating the SDP capability negotiation version and extensions indicating the SDP capability negotiation version and extensions
supported and required. supported and required.
2.2.1. SDP Capability Negotiation Version Attribute 2.2.1. Supported Capability Negotiation Extensions Attribute
The SDP Capability Negotiation Version attribute ("a=cver") lists the
version of the SDP Capability Negotiation supported by the entity
that generated the SDP. The attribute is defined as follows:
a=cver: <version>
where <version> is a non-zero positive integer. White space is
permitted, but not required, before <version>. The value of <version>
defined by this document is 0 as illustrated by the following
example:
a=cver: 0
The SDP Capability Negotiation version attribute MUST be present in
each SDP that uses the SDP Capability negotiation solution defined in
this document. The attribute can be provided at either the session-
or media-level, however there MUST NOT be more than one occurrence of
it. Furthermore, the attribute SHOULD be the first of the SDP
capability negotiation attributes provided.
2.2.2. Supported Capability Negotiation Extensions Attribute
The SDP Capability negotiation solution allows for capability The SDP Capability negotiation solution allows for capability
negotiation extensions to be defined. Associated with each such negotiation extensions to be defined. Associated with each such
extension is an option tag that identifies the extension in question. extension is an option tag that identifies the extension in question.
Option-tags MUST be registered with IANA per the procedures defined Option-tags MUST be registered with IANA per the procedures defined
in Section 5. in Section 5.
The Supported Capability Negotiation Extensions attribute ("a=csup") The Supported Capability Negotiation Extensions attribute ("a=csup")
contains a comma-separated list of option tags identifying the SDP contains a comma-separated list of option tags identifying the SDP
Capability negotiation extensions supported by the entity that that Capability negotiation extensions supported by the entity that
generated the SDP. The attribute is defined as follows: generated the SDP. The attribute is defined as follows:
a=csup: <option-tag-list> a=csup: <option-tag-list>
where <option-tag-list> is defined by the following ABNF: where <option-tag-list> is defined by the following ABNF:
option-tag-list = option-tag *(COMMA option-tag) option-tag-list = option-tag *(COMMA option-tag)
option-tag = token ; defined in [SDP] option-tag = token ; defined in [SDP]
COMMA = *WSP "," *WSP ; defined in [RFC4234] COMMA = *WSP "," *WSP ; defined in [RFC4234]
White-space is permitted before the <option-tag-list>. White-space is permitted before the <option-tag-list>.
Implementers familiar with SIP should note that the above Implementers familiar with SIP should note that the above
definition of COMMA differs from the one in [RFC3261]. definition of COMMA differs from the one in [RFC3261].
[EDITOR'S NOTE: There's nothing specific to the SDP Capability A special base option tag with a value of "v0" is defined for the
Negotiation Solution for this parameter. Should consider basic SDP capability negotiation framework specified in this
generalizing and/or providing in a separate document.] document. Entities can use this option tag with the "a=csup"
attribute to indicate support for the SDP capability negotiation
framework specified in this document.
The following examples illustrates the use of the "a=csup" attribute The following examples illustrates the use of the "a=csup" attribute
with two hypothetical option tags, "foo" and "bar": with two hypothetical option tags, "foo" and "bar":
a=csup: foo a=csup: foo
a=csup: bar a=csup: bar
a=csup: foo, bar a=csup: foo, bar
The "a=csup" attribute can be provided at the session and the media- The "a=csup" attribute can be provided at the session and the media-
level. When provided at the session-level, it applies to the entire level. When provided at the session-level, it applies to the entire
SDP. When provided at the media-level, it applies to the media-stream SDP. When provided at the media-level, it applies to the media-stream
in question only. There can be one or more "a=csup" attributes at in question only (option-tags provided at the session level apply as
both the session and media-level (one or more per media stream in the well). There can be one or more "a=csup" attributes at both the
latter case). session and media-level (one or more per media stream in the latter
case).
Whenever an entity that supports one or more extensions to the SDP Whenever an entity that supports one or more extensions to the SDP
Capability Negotiation framework generates an SDP, it SHOULD include Capability Negotiation framework generates an SDP, it SHOULD include
the "a=csup" attribute with the option tags for the extensions it the "a=csup" attribute with the option tags for the extensions it
supports. supports at the session and/or media-level, unless those option tags
are already provided in one or more "a=creq" attribute (see Section
2.2.2. ) at the relevant levels. The base option tag MAY be included.
2.2.3. Required Capability Negotiation Extension Attribute 2.2.2. Required Capability Negotiation Extension Attribute
The SDP Capability negotiation solution allows for capability The SDP Capability negotiation solution allows for capability
negotiation extensions to be defined. Associated with each such negotiation extensions to be defined. Associated with each such
extension is an option tag that identifies the extension in question. extension is an option tag that identifies the extension in question.
Option-tags MUST be registered with IANA per the procedures defined Option-tags MUST be registered with IANA per the procedures defined
in Section 5. in Section 5.
The Required Capability Negotiation Extensions attribute ("a=csup") The Required Capability Negotiation Extensions attribute ("a=csup")
contains a comma-separated list of option tags identifying the SDP contains a comma-separated list of option tags identifying the SDP
Capability negotiation extensions that MUST be supported by the Capability negotiation extensions that MUST be supported by the
entity receiving the SDP in order for that entity to properly process entity receiving the SDP in order for that entity to properly process
the SDP Capability negotiation. The attribute is defined as follows: the SDP Capability negotiation. The attribute is defined as follows:
a=creq: <option-tag-list> a=creq: <option-tag-list>
where <option-tag-list> is defined in Section 2.2.2. where <option-tag-list> is defined in Section 2.2.1.
White-space is permitted before the <option-tag-list>. White-space is permitted before the <option-tag-list>.
[EDITOR'S NOTE: There's nothing specific to the SDP Capability The following examples illustrate the use of the "a=creq" attribute
Negotiation Solution for this parameter. Should consider
generalizing and/or providing in a separate document.]
The following examples illustrates the use of the "a=creq" attribute
with two hypothetical option tags, "foo" and "bar": with two hypothetical option tags, "foo" and "bar":
a=creq: foo a=creq: foo
a=creq: bar a=creq: bar
a=creq: foo, bar a=creq: foo, bar
The "a=creq" attribute can be provided at the session and the media- The "a=creq" attribute can be provided at the session and the media-
level. When provided at the session-level, it applies to the entire level. When provided at the session-level, it applies to the entire
SDP. When provided at the media-level, it applies to the media-stream SDP. When provided at the media-level, it applies to the media-stream
in question only. There can be one or more "a=creq" attributes at in question only (required option tags provided at the session level
both the session and media-level (one or more per media stream in the apply as well). There can be one or more "a=creq" attributes at both
the session and media-level (one or more per media stream in the
latter case). latter case).
Whenever an entity generates an SDP and it requires the recipient of When an entity generates an SDP and it requires the recipient of that
that SDP to support one or more SDP capability negotiation extensions SDP to support one or more SDP capability negotiation extensions in
in order to properly process the SDP Capability negotiation, the order to properly process the SDP Capability negotiation, the
"a=creq" attribute MUST be included with option-tags that identify "a=creq" attribute MUST be included with option-tags that identify
the required extensions. the required extensions at the session and/or media level, unless it
is already known that the receiving entity supports those option-tags
at the relevant levels (in which case their inclusion is OPTIONAL).
A recipient that receives such an SDP and does not support one or An example of this is when generating an answer to an offer. If the
more of the required extensions, MUST NOT perform the SDP capability answerer supports the required option-tags from the offer, and the
negotiation defined in this document. For non-supported extensions answerer does not require any additional option-tags beyond what
provided at the session-level, this implies that SDP capability was listed in either the required ("creq") or supported ("csup")
negotiation MUST NOT be performed at all. For non-supported attributes from the offer, then the answerer is not required to
extensions at the media-level, this implies that SDP capability include a required ("creq") attribute with any option-tags that may
negotiation MSUT NOT be performed for the media stream in question. need to be supported (such as the base option tag - "v0").
A recipient that receives an SDP and does not support one or more of
the required extensions listed in a "creq" attribute, MUST NOT
perform the SDP capability negotiation defined in this document. For
non-supported extensions provided at the session-level, this implies
that SDP capability negotiation MUST NOT be performed at all. For
non-supported extensions at the media-level, this implies that SDP
capability negotiation MUST NOT be performed for the media stream in
question.
When an entity does not support one or more required SDP capability When an entity does not support one or more required SDP capability
negotiation extensions, the entity SHOULD proceed as if the SDP negotiation extensions, the entity SHOULD proceed as if the SDP
capability negotiation attributes were not included in the first capability negotiation attributes were not included in the first
place. place, i.e. all the capability negotiation attributes should be
ignored.
This ensures that introduction of the SDP capability negotiation This ensures that introduction of the SDP capability negotiation
mechanism does not introduce any new failure scenarios. mechanism does not introduce any new failure scenarios.
The above rules apply to the base option tag as well. Thus, entities
compliant to this specification MUST include a "creq" attribute (at
least in an offer) that includes the option tag "v0" as illustrated
below:
a=creq: v0
2.3. Capability Attributes 2.3. Capability Attributes
In this section, we present the new attributes associated with In this section, we present the new attributes associated with
indicating the capabilities for use by the SDP Capability indicating the capabilities for use by the SDP Capability
negotiation. negotiation.
2.3.1. Media Type and Format Capability Attribute 2.3.1. Attribute Capability Attribute
Media types and media formats can be expressed as capabilities by use
of the "a=cmed" attribute, which is defined as follows:
a=cmed: <med-cap-num> <media> [<proto> <fmt list>]
where <med-cap-num> is an integer between 1 and 2^32-1 (both
included) used to number the media capabilities, and <media>,
<proto>, and <fmt list> are defined as in the SDP "m=" line. The <fmt
list> may contain multiple media formats. In that case, the media
format capability number associated with the first one provided is
the value of <med-cap-num>, the number associated with the second one
is one higher, etc. Each occurrence of the attribute MUST use a
different value of <med-cap-num>. Furthermore, when a "cmed"
attribute indicates more than one media format, the capability
numbers implied MUST NOT be used by any other "cmed" attribute in the
session description (explicitly or implicitly). When <proto> and
<fmt list> are omitted, the media capability merely indicates support
for the <media> type in question, without any details as to what kind
of transport protocol and media formats are supported. This can for
example be used to indicate support for additional types of media
than those included as actual configurations in an offer or answer.
A media capability merely indicates possible support for the media
type and media format(s) in question. In order to actually use a
media capability in an offer/answer exchange, it must be referenced
in a potential configuration (see Section 2.4.1.
Media capabilities can be provided at the session-level and the
media-level. Media capabilities provided at the session level apply
to the session description in general, whereas media capabilities
provided at the media level apply to that media stream only. In
either case, the scope of the <med-cap-num> is the entire session
description. This enables each media capability to be referenced
across the entire session description (e.g. in a potential
configuration - see Section 2.4.1.
[EDITOR'S NOTE: This is clear as mud. If a media capability applies
to a media-stream only, then why can it still be referenced and
hence used as capabilities in other media streams (by the "a=pcfg")
attribute. The motivation is message size efficiency, but the means
are not clean. Session versus media-level syntax and semantics need
further consideration]
The <proto> parameter indicates the default transport protocol
associated with the media capability. As described in [RFC4566], the
value of the <proto> parameter guides the interpretation of the <fmt
list>, which is why it is included here. Note that <proto> is also a
capability that can be negotiated separately (see Section 2.3.3.
The <fmt list> contains one or more media formats supported, the
interpretation of which depends on the value of <proto>. The rules
that apply to "m=" lines (as defined in [SDP]) for interpretation of
these apply here as well. For RTP-based transports, this implies
that the <fmt list> contains one or more RTP payload type numbers.
When those payload type numbers are dynamic, SDP requires an
"a=rtpmap" attribute to determine the actual codec. In the case of
SDP capability negotiation, such additional attribute parameters MUST
be provided in conjunction with the media capability. There are two
different cases to consider for this:
o The media capability is provided at the session level: In this
case, the required parameters MUST be provided in one or more
attribute parameter capabilities (see Section 2.3.2. listed
before the first "m=" line as well as before any other media
capability attributes ("a=cmed").
o The media capability is provided at the media stream level: In
this case, when the payload type numbers are part of the "m=" line
itself, this is done by use of the "a=rtpmap" attribute as usual.
In all other cases, the required parameters MUST be provided in
one or more attribute parameter capabilities (see Section 2.3.2.
within the media stream description (i.e. before the next "m="
line).
[EDITOR'S NOTE: The above assumes that intermediaries will not
reorder session-level attributes. It would be safer to explicitly
link the two, but that will require yet another attribute. Also, it
sends us down the path of building more complicated capabilities
(made up of multiple parameters). Another issue here is that
payload type numbers, which really have only media-level scope, are
ill-suited to be used at the session-level or across multiple media
streams (as is being done here). However, an alternative (and more
proper) solution seems to involve significantly more work and
deviations from the current SDP framework. This in turn makes it
more difficult to automatically use new media types, formats,
protocols, etc. defined elsewhere within this framework, and that
is a major disadvantage. Another option is to forgo the session-
level media capabilities as well as the ability to reference across
media streams - it will make the solution less efficient though and
difficult to express latent capabilities for media streams not
included in the offer or answer.]
The following example illustrates the first case above:
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=
c=IN IP4 128.96.41.1
t=0 0
a=cmed: 1 audio RTP/AVP 96
a=capar: 1 a=rtpmap:96 G729/8000
m=audio...
The following example illustrates the second case above:
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=
c=IN IP4 128.96.41.1
t=0 0
m=audio 2345 RTP/AVP 96
a=rtpmap:96 G729/8000
a=cver:0
a=cmed: 1 audio RTP/AVP 96 97
a=capar: 1 a=rtpmap:97 iLBC/8000
Note: Readers familiar with RFC 3407 may notice the similarity
between the "cmed" attribute defined above and the "cdsc" attribute
defined in RFC 3407. There are however a couple of important
differences, namely an increase in the capability numbering space
as well as a relaxation of certain requirements found in RFC 3407.
To simplify overall operation, the "cmed" parameter is limited to
media-level operation only as well.
2.3.2. Attribute Parameter Capability Attribute
Attributes can be expressed as negotiable parameters by use of a new Attributes can be expressed as negotiable parameters by use of a new
attribute parameter capability attribute ("a=capar"), which is attribute capability attribute ("a=capar"), which is defined as
defined as follows: follows:
a=capar: <att-cap-num> <att-par> a=capar: <att-cap-num> <att-par>
where <att-cap-num> is an integer between 1 and 2^32-1 (both where <att-cap-num> is an integer between 1 and 2^32-1 (both
included) used to number the attribute parameter capability and <att- included) used to number the attribute capability and <att-par> is an
par> is an attribute ("a=") in its full '<type>=<value>' form (see attribute ("a=") in its full '<type>=<value>' form (see [SDP]).
[SDP]).
The "capar" attribute can be provided at the session level and the The "capar" attribute can be provided at the session level for
media level. Each occurrence of the attribute MUST use a different session-level attributes and the media level for media-level
value of <app-cap-num>. The <att-cap-num> values provided are attributes. The "capar" attribute MUST NOT be used to provide a
independent of similar <cap-num> values provided for other media-level attribute at the session-level or vice versa.
attributes, i.e., they form a separate name-space for attribute
parameter capabilities.
Attribute parameter capabilities are generally used for two things. Each occurrence of the "capar" attribute in the entire session
First of all, they may be necessary to interpret a media format description MUST use a different value of <app-cap-num>.
capability (e.g. by including an rtpmap), or they may provide
attribute value parameters that are referenced in potential There is a need to be able to reference both session-level and
configurations (see Section 2.4.1. ) media-level attributes in potential configurations at the media
level, and this provides for a simple solution to avoiding overlap
between the handle references.
The <att-cap-num> values provided are independent of similar <cap-
num> values provided for other attributes, i.e., they form a separate
name-space for attribute parameter capabilities.
The following examples illustrate use of the "capar" attribute: The following examples illustrate use of the "capar" attribute:
a=capar: 1 a=ptime:20 a=capar: 1 a=ptime:20
a=capar: 2 a=ptime:30 a=capar: 2 a=ptime:30
a=capar: 3 a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyONQ6gAA a=capar: 3 a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyONQ6gAA
AAAGEEoo2pee4hp2UaDX8ZE22YwKAAAPZG9uYWxkQGR1Y2suY29tAQAAAAAAAQAk0 AAAGEEoo2pee4hp2UaDX8ZE22YwKAAAPZG9uYWxkQGR1Y2suY29tAQAAAAAAAQAk0
JKpgaVkDaawi9whVBtBt0KZ14ymNuu62+Nv3ozPLygwK/GbAV9iemnGUIZ19fWQUO JKpgaVkDaawi9whVBtBt0KZ14ymNuu62+Nv3ozPLygwK/GbAV9iemnGUIZ19fWQUO
skipping to change at page 15, line 21 skipping to change at page 13, line 5
Readers familiar with RFC 3407 may notice the similarity between Readers familiar with RFC 3407 may notice the similarity between
the RFC 3407 "cpar" attribute and the above. There are however a the RFC 3407 "cpar" attribute and the above. There are however a
couple of important differences, most notably that the "capar" couple of important differences, most notably that the "capar"
attribute contains a handle that enables referencing it and it attribute contains a handle that enables referencing it and it
furthermore supports attributes only (the "cpar" attribute defined furthermore supports attributes only (the "cpar" attribute defined
in RFC 3407 supports bandwidth information as well). The "capar" in RFC 3407 supports bandwidth information as well). The "capar"
attribute also is not automatically associated with any particular attribute also is not automatically associated with any particular
capabilities. capabilities.
2.3.3. Transport Protocol Capability Attribute 2.3.2. Transport Protocol Capability Attribute
Transport Protocols can be expressed as capabilities by use of a new Transport Protocols can be expressed as capabilities by use of a new
Transport Protocol Capability attribute ("a=ctrpr") defined as Transport Protocol Capability attribute ("a=ctrpr") defined as
follows: follows:
a=ctrpr: <trpr-cap-num> <proto-list> a=ctrpr: <trpr-cap-num> <proto-list>
where <trpr-cap-num> is an integer between 1 and 255 (both included) where <trpr-cap-num> is an integer between 1 and 2^32-1 (both
used to number the transport address capability for later reference, included) used to number the transport address capability for later
and <proto-list> is one or more <proto>, separated by white space, as reference, and <proto-list> is one or more <proto>, separated by
defined in the SDP "m=" line. white space, as defined in the SDP "m=" line.
The "ctrpr" attribute can be provided at the session- and media- The "ctrpr" attribute can be provided at the session- and media-
level. Each occurrence of the attribute MUST use a different value of level. Each occurrence of the "ctrpr" attribute in the entire session
<trpr-cap-num>. When multiple <proto> values are provided, the first description MUST use a different value of <trpr-cap-num>. When
one is associated with the value <trpr-cap-num>, the second one with multiple <proto> values are provided, the first one is associated
the value one higher, etc. The <trpr-cap-num> values provided are with the value <trpr-cap-num>, the second one with the value one
independent of similar <cap-num> values provided for other higher, etc. The <trpr-cap-num> values provided are independent of
attributes, i.e., they form a separate name-space for transport similar <cap-num> values provided for other attributes, i.e., they
protocol capabilities. form a separate name-space for transport protocol capabilities.
Below, we provide examples of the "a=ctrpr" attribute: Below, we provide examples of the "a=ctrpr" attribute:
a=ctrpr: 1 RTP/AVP a=ctrpr: 1 RTP/AVP
a=ctrpr: 2 RTP/AVPF a=ctrpr: 2 RTP/AVPF
a=ctrpr: 3 RTP/SAVP RTP/SAVPF a=ctrpr: 3 RTP/SAVP RTP/SAVPF
The first one provides a capability for the "RTP/AVP" profile defined The first one provides a capability for the "RTP/AVP" profile defined
in [RFC3551] and the second one provides a capability for the RTP in [RFC3551] and the second one provides a capability for the RTP
with RTCP-Based Feedback profile defined in [AVPF]. The third one with RTCP-Based Feedback profile defined in [AVPF]. The third one
provides capabilities for the "RTP/SAVP" and "RTP/SAVPF" profiles. provides capabilities for the "RTP/SAVP" and "RTP/SAVPF" profiles.
Note that the "cmed" attribute provides a similar functionality by
including <proto>, however having this as a separate capability
indication can provide significant message size reduction when
negotiating alternative profiles (of which there can be many). In
particular, there is no need to repeat supported payload types. Also,
use of this attribute combined with the potential configuration
attribute (see Section 2.4. ) provides for more expressive power.
2.4. Configuration Attributes 2.4. Configuration Attributes
2.4.1. Potential Configuration Attribute 2.4.1. Potential Configuration Attribute
Potential Configurations can be expressed by use of a new Potential Potential Configurations can be expressed by use of a new Potential
Configuration Attribute ("a=pcfg") defined as follows: Configuration Attribute ("a=pcfg") defined as follows:
a=pcfg: <pot-cfg-list> a=pcfg: <preference> <pot-cfg-list>
where <pot-cfg-list> is defined as where <preference> is an integer between 1 and 2^32-1 (both included)
and <pot-cfg-list> is defined as
pot-cfg-list = pot-config *(1*WSP pot-config) pot-cfg-list = pot-config *(1*WSP pot-config)
pot-config = pot-attribute-parameter-config |
pot-config = pot-media-config |
pot-attribute-parameter-config |
pot-transport-protocol-config | pot-transport-protocol-config |
pot-extension-config pot-extension-config
The potential configuration attribute includes one or more sets of The potential configuration attribute includes a preference
potential media configurations, attribute parameter configurations indication (lowest number is most preferred) followed by one or more
and transport protocol configurations. Each of these MUST NOT be of potential attribute parameter configuration and transport protocol
present more than once in a particular potential configuration configuration. Each of these MUST NOT be present more than once in a
attribute. Potential extension configurations can be included as particular potential configuration attribute. Potential extension
well. There can be more than one potential extension configuration, configurations can be included as well. There can be more than one
however each particular potential extension configuration MUST NOT be potential extension configuration, however each particular potential
present more than once in a given potential configuration attribute. extension configuration MUST NOT be present more than once in a given
potential configuration attribute. Together, these values define a
Together, these values define a set of potential configurations. potential configuration.
There can be one or more potential configuration attributes provided
at the session-level as well as for each media stream. The attributes
are provided in order of preference.
[EDITOR'S NOTE: We run into another issue with session-level media
capabilities here. In the offer/answer model, potential
configurations at the media-level constitute alternative offers,
however at the session-level, that would/should not be the case for
media capabilities. Other parameters however may be used as
alternative offers at the session level (e.g. key-mgmt attributes
at the session level)]
pot-media-config is defined by the following ABNF: There can be multiple potential configurations provided at the
session-level as well as the media-level. The semantics for each of
these levels differ. A potential configuration at the session level
provides a set of latent capabilities. A latent capability is merely
an indication that the potential configuration could be supported,
however it does not represent a willingness to do so at the current
time. A potential configuration at the media level on the other hand
indicates not only a willingness, but in fact a desire to use the
potential configuration.
pot-media-config = "m=" med-cap-list *(BAR med-cap-list) In the case of offer/answer, this implies that a potential
med-cap-list = med-cap-num *(COMMA med-cap-num) configuration at the session level does not constitute an
med-cap-num = 1*DIGIT ; defined in [RFC4234] alternative offer whereas it does at the media level.
BAR = *WSP "|" *WSP ; defined in [RFC4234]
Each potential media configuration is a comma-separated list of media Associated with each potential configuration is a preference
capability numbers where med-cap-num refers to media capability indication, which is an integer between 1 and 2^32-1 (both included)
numbers and hence MUST be between 1 and 2^32-1 (both included). to indicate the relative preference of potential configurations. The
Alternative potential media configurations are separated by a scope of the preference (and in fact each occurrence of a potential
vertical bar ("|"). The alternatives are ordered by preference. When configuration attribute) is the session-level, when provided there,
media capabilities are not included in a potential configuration at or the particular media stream it is provided at.
the media level, the media type and media format from the associated
"m=" line will be used.
pot-attribute-parameter-config is defined by the following ABNF: Attribute capabilities are included in a potential configuration by
use of the pot-attribute-parameter-config parameter, which is defined
by the following ABNF:
pot-attribute-parameter-config pot-attribute-parameter-config
= "a=" capar-cap-list *(BAR capar-cap-list) = "a=" capar-cap-list *(BAR capar-cap-list)
capar-cap-list = att-cap-num *(COMMA att-cap-num) capar-cap-list = att-cap-num *(COMMA att-cap-num)
att-cap-num = 1*DIGIT ;defined in [RFC4234] att-cap-num = 1*DIGIT ;defined in [RFC4234]
Each potential attribute parameter configuration list is a comma- Each potential attribute parameter configuration list is a comma-
separated list of attribute capability parameter numbers where att- separated list of attribute capability numbers where att-cap-num
cap-num refers to attribute parameter capability numbers defined refers to attribute capability numbers defined above and hence MUST
above and hence MUST be between 1 and 2^32-1 (both included). be between 1 and 2^32-1 (both included). Alternative potential
Alternative potential attribute parameter configurations are attribute parameter configurations are separated by a vertical bar
separated by a vertical bar ("|"). The alternatives are ordered by ("|"), the scope of which extends to the next alternative (i.e. ","
has higher precedence than "|"). The alternatives are ordered by
preference. preference.
pot-transport-protocol-config is defined by the following ABNF: Transport protocol capabilities are included in a potential
configuration by use of the pot-transport-protocol-config parameter,
which is defined by the following ABNF:
pot-transport-protocol-config = pot-transport-protocol-config =
"p=" trpr-cap-num *(BAR trpr-cap-num) "t=" trpr-cap-num *(BAR trpr-cap-num)
trpr-cap-num = 1*DIGIT ; defined in [RFC4234] trpr-cap-num = 1*DIGIT ; defined in [RFC4234]
The trpr-cap-num refers to transport protocol capability numbers The trpr-cap-num refers to transport protocol capability numbers
defined above and hence MUST be between 1 and 2^32-1 (both included). defined above and hence MUST be between 1 and 2^32-1 (both included).
Alternative potential transport protocol configurations are separated Alternative potential transport protocol configurations are separated
by a vertical bar ("|"). The alternatives are ordered by preference. by a vertical bar ("|"). The alternatives are ordered by preference.
When transport protocol capabilities are not included in a potential When transport protocol capabilities are not included in a potential
configuration, the transport protocol information from an included configuration at the media level, the transport protocol information
potential media configuration will be used. If a potential media from the associated "m=" line will be used. At the session-level,
configuration is not included, the transport protocol from the media lack of a transport protocol capability indication simply implies
description ("m=" line) will be used instead. that no such information is provided.
pot-extension-config is defined by the following ABNF: Extension capabilities can be included in a potential configuration
as well. Such extensions MUST adhere to the following ABNF:
pot-extension-config= ext-cap-name "=" pot-extension-config= ext-cap-name "="
ext-cap-list *(BAR ext-cap-list) ext-cap-list *(BAR ext-cap-list)
ext-cap-name = token ; defined in [SDP] ext-cap-name = token ; defined in [SDP]
ext-cap-list = ext-cap-num *(COMMA ext-cap-num) ext-cap-list = ext-cap-num *(COMMA ext-cap-num)
ext-cap-num = 1*DIGIT ; defined in [RFC4234] ext-cap-num = 1*DIGIT ; defined in [RFC4234]
The ext-cap-name refers to the type of extension capability and the The ext-cap-name refers to the type of extension capability and the
ext-cap-num refers to a capability number associated with that ext-cap-num refers to a capability number associated with that
particular type of extension capability. The number MUST be between particular type of extension capability. The number MUST be between
1 and 2^32-1 (both included). Alternative potential extension 1 and 2^32-1 (both included). Alternative potential extension
configurations for a particular extension are separated by a vertical configurations for a particular extension are separated by a vertical
bar ("|"). Unsupported or unknown potential extension configs MUST bar ("|"),the scope of which extends to the next alternative (i.e.
be ignored, unless an option tag showing the extension as being "," has higher precedence than "|"). Unsupported or unknown
required was included (see Section 2.2.3. potential extension configs MUST be ignored.
The potential configuration ("a=pcfg") attribute can be provided at The "creq" attribute and its associated rules can be used to ensure
the session level and the media-level. Each occurrence of the that required extensions are supported in the first place.
attribute within a given media description ("m=" line) defines a set
of potential configurations that can be used for that media
description.
TO DO: Need to decide on relationship between session-level and Potential configurations can be provided at the session level and the
media-level (how should conflicts, overlap, etc. be handled - media level and in either case, it is syntactically possible to
simplicity at the possible expense of expressive power is reference attribute capabilities provided at either the session or
preferable in the editor's opinion). the media level. There are however semantic rules and limitations
associated with this: At the session-level, a potential configuration
MUST NOT reference any attribute capabilities provided at the media-
level. The converse however is permitted, i.e. a media-level
potential configuration can reference a session-level attribute
capability. The semantics of doing so (should that potential
configuration be chosen), is that the corresponding attribute
(provided within that attribute capability) will be considered part
of the active configuration at the *session* level. In other words,
it will be as-if that attribute was simply provided at the session-
level in the first place. Note that individual media streams perform
capability negotiation individually, and hence it is possible that
another media stream (where the attribute was part of a potential
configuration) chose a configuration without that session level
attribute. The session-level attribute however remains "active" and
hence applies to the entire session. It is up to the entity that
generates the SDP to ensure that the resulting active configuration
SDP is still meaningful.
[EDITOR'S NOTE: There are too many subtle differences between
potential configurations at the session and media level. I'm
inclined to have two similarly looking (but different) attributes
instead as that will make it more straightforward and intuitive.
That also leaves the door open to have more than latent
capabilities at the session level in case that is needed later]
Below, we provide an example of the "a=pcfg" attribute in a complete Below, we provide an example of the "a=pcfg" attribute in a complete
media description in order to properly indicate the supporting media description in order to properly indicate the supporting
attributes: attributes:
v=0 v=0
o=- 25678 753849 IN IP4 128.96.41.1 o=- 25678 753849 IN IP4 128.96.41.1
s= s=
c=IN IP4 128.96.41.1 c=IN IP4 128.96.41.1
t=0 0 t=0 0
m=audio 3456 RTP/SAVPF 0 18 m=audio 3456 RTP/AVPF 0 18
a=crypto:1 AES_CM_128_HMAC_SHA1_32 a=creq: v0
a=capar:1 crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=cver: 0 a=ctrpr: 1 RTP/AVPF RTP/AVP
a=cmed: 1 audio RTP/SAVP 0 4 18 a=ctrpr: 3 RTP/SAVPF RTP/SAVP
a=ctrpr: 1 RTP/AVP RTP/AVPF a=pcfg:1 t=3|4 a=1
a=ctrpr: 3 RTP/SAVP RTP/SAVPF a=pcfg:8 t=1|2
a=pcfg: m=1|3 p=1|2|3|4 We have two potential configurations listed here. The first one (and
a=pcfg: m=2 p=1 most preferred, since it's preference is "1") indicates that either
of the profiles RTP/SAVPF or RTP/SAVP (specified by the transport
We have two potential configurations listed here. The first one protocol capability numbers 3 and 4) can be supported with attribute
indicates that PCMU (payload type number 0 referenced by media capability 1 (the "crypto" attribute); RTP/SAVPF is preferred since
capability number 1) or G.729 (payload type number 18 referenced by it is listed first. The second potential configuration indicates that
media capability number 3) can be supported with either of the the RTP/AVPF of RTP/AVP profile can be used, with RTP/AVPF being the
profiles RTP/AVP, RTP/AVPF, RTP/SAVP, or RTP/SAVPF (specified by the preferred one. This non secure RTP alternative is the less preferred
transport protocol capability numbers 1, 2, 3 and 4). The second one since it's preference is "8".
potential configuration indicates that G.723 (payload type number 4
referenced by media capability number 2) can be supported with the
RTP/AVP profile only (transport protocol capability number 1).
2.4.2. Actual Configuration Attribute 2.4.2. Actual Configuration Attribute
The actual configuration attribute identifies which of the potential The actual configuration attribute identifies which of the potential
configurations from an offer SDP were used as actual configurations configurations from an offer SDP were used as actual configurations
in an answer SDP. This is done by reference to the media in an answer SDP. This is done by reference to the attribute
capabilities, attribute parameter capabilities and transport protocol capabilities and transport protocol capabilities from the offer that
capabilities from the offer that were actually used by the answerer were actually used by the answerer in his offer/answer procedure. If
in his offer/answer procedure. If extension capabilities were used, extension capabilities were used, those will be included by reference
those will be included by reference as well. as well.
The Actual Configuration Attribute ("a=acfg") is defined as follows: The Actual Configuration Attribute ("a=acfg") is defined as follows:
a=acfg: <act-cfg-list> a=acfg: <act-cfg-list>
where <act-cfg-list> is defined as where <act-cfg-list> is defined as
act-cfg-list = capability *(1*WSP capability) act-cfg-list = capability *(1*WSP capability)
capability = act-media-config | capability = act-attribute-parameter-config |
act-attribute-parameter-config |
act-transport-protocol-config | act-transport-protocol-config |
act-extension-config act-extension-config
act-media-config is defined by the following ABNF:
act-media-config = "m=" med-cap-list
where med-cap-list is as defined in Section 2.4.1.
act-attribute-parameter-config is defined by the following ABNF: act-attribute-parameter-config is defined by the following ABNF:
act-attribute-parameter-config = "a=" capar-cap-list act-attribute-parameter-config = "a=" capar-cap-list
where capar-cap-list is as defined in Section 2.4.1. where capar-cap-list is as defined in Section 2.4.1.
act-transport-protocol-config is defined by the following ABNF: act-transport-protocol-config is defined by the following ABNF:
act-transport-protocol-config = "p=" trpr-cap-num act-transport-protocol-config = "t=" trpr-cap-num
where trpr-cap-num is as defined in Section 2.4.1. where trpr-cap-num is as defined in Section 2.4.1.
trpr-cap-num = 1*3DIGIT ; defined in [RFC4234] trpr-cap-num = 1*3DIGIT ; defined in [RFC4234]
act-extension-config is defined by the following ABNF: act-extension-config is defined by the following ABNF:
act-extension-config = ext-cap-name "=" ext-cap-list act-extension-config = ext-cap-name "=" ext-cap-list
where ext-cap-name and ext-cap-list are as defined in Section 2.4.1. where ext-cap-name and ext-cap-list are as defined in Section 2.4.1.
The actual configuration ("a=acfg") attribute can be provided at the The actual configuration ("a=acfg") attribute can be provided at the
session-level and the media-level. There MUST NOT be more than one media-level only. There MUST NOT be more than one occurrence of an
occurrence of an actual configuration attribute at the session level, actual configuration attribute within a given media description.
and there MUST NOT be more than one occurrence of an actual
configuration attribute within a given media description.
Below, we provide an example of the "a=acfg" attribute (building on Below, we provide an example of the "a=acfg" attribute (building on
the previous example with the potential configuration attribute): the previous example with the potential configuration attribute):
v=0 v=0
o=- 24351 621814 IN IP4 128.96.41.2 o=- 24351 621814 IN IP4 128.96.41.2
s= s=
c=IN IP4 128.96.41.2 c=IN IP4 128.96.41.2
t=0 0 t=0 0
m=audio 4567 RTP/AVPF 0 m=audio 4567 RTP/SAVPF 0
a=cver: 0 a=creq: 0
a=acfg: m=1 p=2 a=acfg: t=3 a=1
It indicates that the answerer used an offer consisting of media It indicates that the answerer used an offer consisting of transport
capability 1 from the offer (PCMU) and transport protocol capability protocol capability 2 from the offer (RTP/SAVPF) and attribute
2 from the offer (RTP/AVPF). capability 1 (the "crypto" attribute.
2.5. Offer/Answer Model Extensions 2.5. 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 potential configurations to be defined in [RFC3264] to allow for potential configurations to be
included in an offer, where they constitute offers that may be included in an offer, where they constitute offers that may be
accepted by the answerer instead of the actual configuration(s) accepted by the answerer instead of the actual configuration(s)
included in the "m=" line(s). included in the "m=" line(s).
[EDITOR'S NOTE: Multicast considerations have been omitted for [EDITOR'S NOTE: Multicast considerations have been omitted for
now.] now.]
TO DO: Elaborate and firm up offer/answer procedures. TO DO: Elaborate and firm up offer/answer procedures.
2.5.1. Generating the Initial Offer 2.5.1. Generating the Initial Offer
An offerer that wants to use the SDP capability negotiation An offerer that wants to use the SDP capability negotiation
extensions defined in this document MUST include the following in the extensions defined in this document MUST include the following in the
offer: offer:
o an SDP capability negotiation version attribute with the version o an SDP capability negotiation required extensions attribute ("a-
set to 0 creq") that contains the option tag "v0". It must either be
provided at the session-level or for each individual media stream.
o one or more media capabilities (as defined in Section 2.3.1. ), if Option tags for any other required extensions MUST be included as
alternative media types and media formats are to be indicated as well (in accordance with Section 2.2.2. )
offerer capabilities or be negotiated.
o one or more attribute parameter capability attributes (as defined o one or more attribute capability attributes (as defined in Section
in Section 2.3.2. ) if alternative attribute parameter values are 2.3.1. ) if alternative attribute parameter values are to be
to be indicated as offerer capabilities or be negotiated. indicated as offerer capabilities or be negotiated.
o one or more transport protocol capability attributes (as defined o one or more transport protocol capability attributes (as defined
in Section 2.3.3. ) if alternative transport protocols are to be in Section 2.3.2. ) if alternative transport protocols are to be
to be indicated as offerer capabilities or be negotiated. to be indicated as offerer capabilities or be negotiated.
o one or more potential configuration attributes (as defined in o one or more potential configuration attributes (as defined in
Section 2.4. ) if alternative potential configurations are to be Section 2.4. ) if alternative potential configurations are to be
negotiated. negotiated.
o one or more required capability negotiation extension attributes o one or more required capability negotiation extension attributes
(as defined in Section 2.2.3. ), if the answerer is required to (as defined in Section 2.2.2. ), if the answerer is required to
support one or more SDP capability negotiation extensions. support one or more SDP capability negotiation extensions.
The offerer SHOULD furthermore include the following: The offerer SHOULD furthermore include the following:
o one or more supported capability negotiation extension attributes o one or more supported capability negotiation extension attributes
(as defined in Section 2.2.2. ), if the offerer supports one or ("a=csup" as defined in Section 2.2.1. ), if the offerer supports
more SDP capability negotiation extensions. one or more SDP capability negotiation extensions that have not
been included in one or more "a=creq" attributes at the relevant
session and media level(s).
The capabilities provided merely indicate what the offerer is capable The capabilities provided merely indicate what the offerer is capable
of doing. They do not constitute a commitment or even an indication of doing. They do not constitute a commitment or even an indication
to actually use them. Conversely, each of the potential to actually use them. This applies to potential configurations listed
configurations listed constitutes an alternative offer which may be at the session level as well. Conversely, each of the potential
used to negotiate and establish the session. configurations listed at the media level constitutes an alternative
offer which may be used to negotiate and establish the session.
[EDITOR'S NOTE: This is only partially true for potential
configurations listed at the session level. The only thing we want
to offer up as alternative offers at the session level is
attributes - not media types or media formats, which should be
capabilities only at the session level]
The current actual configuration is included in the "m=" line (as The current actual configuration is included in the "m=" line (as
defined by [RFC3264]). defined by [RFC3264]).
2.5.2. Generating the Answer 2.5.2. Generating the Answer
When the answerer receives an offer with valid SDP capability When the answerer receives an offer with valid SDP capability
negotiation information in it and in particular with one or more negotiation information in it and in particular with one or more
valid potential configuration information attributes present, it may valid potential configuration information attributes present, it may
use any of the potential configurations as an alternative offer. A use any of the potential configurations as an alternative offer. A
potential configuration information attribute is valid if all of the potential configuration information attribute is valid if all of the
capabilities (media, attribute capabilities, transport protocol and capabilities (attribute capabilities, transport protocol capabilities
any extension capabilities) it references are present and valid and any extension capabilities) it references are present and valid
themselves. themselves.
The actual configuration is contained in the media description's "m=" The actual configuration is contained in the media description's "m="
line. The answerer can send media to the offerer in accordance with line. The answerer can send media to the offerer in accordance with
the actual configuration, however if it chooses to use one of the the actual configuration, however if it chooses to use one of the
alternative potential configurations, media sent to the offerer may alternative potential configurations, media sent to the offerer may
be discarded by the offerer until the answer is received. be discarded by the offerer until the answer is received.
If the answerer chooses to accept one of the alternative potential If the answerer chooses to accept one of the alternative potential
configurations instead of the actual configuration, the answerer MUST configurations instead of the actual configuration, the answerer MUST
generate an answer as if the offer contained that potential generate an answer as if the offer contained that potential
configuration instead of the actual configuration included. The configuration instead of the actual configuration included. The
answerer MUST also include an actual configuration attribute in the answerer MUST also include an actual configuration attribute in the
answer that identifies the potential configuration from the offer answer that identifies the potential configuration from the offer
used by the answerer. The actual configuration attribute in the used by the answerer. The actual configuration attribute in the
answer MUST include information about the media capabilities, answer MUST include information about the attribute capabilities,
attribute capability parameters, transport protocol parameters, and transport protocol parameters, and extension capabilities from the
extension capabilities from the potential configuration that were potential configuration that were used to generate the answer.
used to generate the answer.
2.5.3. Offerer Processing of the Answer 2.5.3. Offerer Processing of the Answer
When the offerer included potential configurations for a media When the offerer included potential configurations for a media
stream, it MUST examine the answer for the presence of an actual stream, it MUST examine the answer for the presence of an actual
configuration attribute for each such media stream. If the attribute configuration attribute for each such media stream. If the attribute
is missing, offerer processing of the answer MUST proceed as defined is missing, offerer processing of the answer MUST proceed as defined
by [RFC3264]. If the attribute is present, processing continues as by [RFC3264]. If the attribute is present, processing continues as
follows: follows:
The actual configuration attribute specifies which of the potential The actual configuration attribute specifies which of the potential
configurations were used by the answerer to generate the answer. This configurations were used by the answerer to generate the answer. This
includes all the types of capabilities from the potential includes all the types of capabilities from the potential
configuration offered, i.e. the media formats ("cmed" capabilities), configuration offered, i.e. the attribute capabilities ("a=capar"),
attribute capability parameters ("capar"), transport protocol transport protocol capabilities ("a=ctrpr"), and any extension
capabilities ("ctrpr"), and any extension capability parameters capability parameters included.
included.
The offerer MUST now process the answer as if the offer had contained The offerer MUST now process the answer as if the offer had contained
the potential configuration as the actual configuration in the media the potential configuration as the actual configuration in the media
description ("m=" line) and relevant attributes in the offer. description ("m=" line) and relevant attributes in the offer.
2.5.4. Modifying the Session 2.5.4. Modifying the Session
Potential configurations may be included in subsequent offers as Potential configurations may be included in subsequent offers as
defined in [RFC3264, Section 8]. The procedure for doing so is defined in [RFC3264, Section 8]. The procedure for doing so is
similar to that described above with the answer including an similar to that described above with the answer including an
skipping to change at page 24, line 23 skipping to change at page 21, line 41
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 128.96.41.1 o=- 25678 753849 IN IP4 128.96.41.1
s= s=
c=IN IP4 128.96.41.1 c=IN IP4 128.96.41.1
t=0 0 t=0 0
m=audio 3456 RTP/AVP 0 18 m=audio 3456 RTP/AVP 0 18
a=cver:0 a=creq: v0
a=ctrpr:1 RTP/SAVP RTP/AVP a=ctrpr:1 RTP/SAVP RTP/AVP
a=capar:1 a=crypto:1 AES_CM_128_HMAC_SHA1_80 a=capar:1 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4 inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
FEC_ORDER=FEC_SRTP FEC_ORDER=FEC_SRTP
a=pcfg: p=1 a=1 a=pcfg:5 t=1 a=1
a=pcfg: p=2 a=pcfg:10 t=2
The "m=" line indicates that Alice is offering to use plain RTP with The "m=" line indicates that Alice is offering to use plain RTP with
PCMU or G.729. The capability declaration is provided by the PCMU or G.729. Alice indicates that support for the base protocol
"a=cver", "a=ctrpr" and "a=capar" attributes. The capabilities defined here is required by including the "a=creq" attribute
indicate that both Secure RTP and normal RTP are supported. The containing the value "v0". The capabilities are provided by the
"capar" attribute provides a capability parameter with a handle of 1. "a=ctrpr" and "a=capar" attributes. The capabilities indicate that
The capability parameter is a "crypto" attribute in the capability both Secure RTP and normal RTP are supported. The "capar" attribute
set, which provides the keying material for SRTP using SDP security provides a capability parameter with a handle of 1. The capability
descriptions [SDES]. The "a=pcfg" attribute provides the potential parameter is a "crypto" attribute in the capability set, which
configurations included in the offer by reference to the provides the keying material for SRTP using SDP security descriptions
capabilities. Two alternatives are provided; the first one, and [SDES]. The "a=pcfg" attribute provides the potential configurations
hence the preferred one is transport protocol capability 1 (RTP/SAVP, included in the offer by reference to the capabilities. Two
i.e. secure RTP) together with the attribute capability parameter 1, alternatives are provided; the first one with preference "5" (and
i.e. the crypto attribute provided. The second one is using transport hence the preferred one since the preference on the second one is
protocol capability 2. Since there are no media format capabilities "10") is transport protocol capability 1 (RTP/SAVP, i.e. secure RTP)
included, the media format parameters from the media description together with the attribute capability 1, i.e. the crypto attribute
itself is used. provided. The second one is using transport protocol capability 2.
Note that we could have omitted the second potential configuration
since it equals the actual configuration (which is always the least
preferred configuration).
Bob receives the SDP offer from Alice. Bob supports SRTP and the SCP Bob receives the SDP offer from Alice. Bob supports SRTP and the SDP
Capability Negotiation extensions, and hence he accepts the potential Capability Negotiation extensions, and hence he accepts the potential
configuration for Secure RTP provided by Alice: configuration for Secure RTP provided by Alice:
v=0 v=0
o=- 24351 621814 IN IP4 128.96.41.2 o=- 24351 621814 IN IP4 128.96.41.2
s= s=
c=IN IP4 128.96.41.2 c=IN IP4 128.96.41.2
t=0 0 t=0 0
m=audio 4567 RTP/SAVP 0 18 m=audio 4567 RTP/SAVP 0 18
a=crypto:1 AES_CM_128_HMAC_SHA1_80 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4 inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4
a=cver: 0 a=csup: foo
a=acfg: p=1 a=1 a=acfg: t=1 a=1
Bob includes the "a=cver" and "a=acfg" attribute in the answer to Bob includes the "a=acfg" attribute in the answer to inform Alice
inform Alice that he based his answer on an offer containing the that he based his answer on an offer containing the potential
potential configuration with transport protocol capability 1 and configuration with transport protocol capability 1 and attribute
attribute parameter capability 1 from the offer SDP (i.e. the capability 1 from the offer SDP (i.e. the RTP/SAVP profile using the
RTP/SAVP profile using the keying material provided). Bob also keying material provided). Bob also includes his keying material in
includes his keying material in a crypto attribute. a crypto attribute. Finally, Bob supports an SDP capability
negotiation extension with the option tag "foo" and hence he includes
the "a=csup" parameter containing value "foo" in the answer.
Note that in this particular example, the answerer supported the Note that in this particular example, the answerer supported the
capability extensions defined here, however had he not, the answerer capability extensions defined here, however had he not, the answerer
would simply have ignored the new attributes and accepted the offer would simply have ignored the new attributes and accepted the offer
to use normal RTP. In that case, the following answer would have been to use normal RTP. In that case, the following answer would have been
generated instead: generated instead:
v=0 v=0
o=- 24351 621814 IN IP4 128.96.41.2 o=- 24351 621814 IN IP4 128.96.41.2
s= s=
skipping to change at page 26, line 15 skipping to change at page 23, line 39
o Capability descriptions, potential configurations and actual o Capability descriptions, potential configurations and actual
configurations can be provided at both the session level and media configurations can be provided at both the session level and media
level. It needs to be decided what the relationship between the level. It needs to be decided what the relationship between the
session level and media level parameters are. session level and media level parameters are.
o Look for "EDITOR'S NOTE" throughout the document. o Look for "EDITOR'S NOTE" throughout the document.
7. Acknowledgments 7. Acknowledgments
Thanks to Francois Audet and Dan Wing for comments on earlier This document is heavily influenced by the discussions and work done
by the SDP Capability Negotiation Design team. The following people
in particular provided useful comments and suggestions to either the
document itself or the overall direction of the solution defined in
here: Roni Even, Robert Gilman, Cullen Jennings, Matt Lepinski, Joerg
Ott, Colin Perkins, and Thomas Stach.
Francois Audet and Dan Wing provided useful comments on earlier
versions of this document. versions of this document.
8. Change Log 8. Change Log
8.1. draft-ietf-mmusic-sdp-capability-negotiation-00 8.1. draft-ietf-mmusic-sdp-capability-negotiation-01
The following are the major changes compared to version -00:
o Media capabilities are no longer considered a core capability and
hence have been removed. This leaves transport protocols and
attributes as the only capabilities defined by the core.
o Version attribute has been removed and an option tag to indicate
the actual version has been defined instead.
o Clarified rules for session-level and media level attributes
provided at either level as well how they can be used in potential
configurations.
o Potential configuration parameters no longer have implicit
ordering; an explicit preference indicator is now included.
o The parameter name for transport protocols in the potential and
actual configuration attributes have been changed "p" to "t".
o Clarified operator precedence within potential and actual
configuration attributes.
o Potential configurations at the session level now limited to
indicate latent capability configurations. Consequently, an actual
configuration attribute can no longer be provided at the session
level.
o Cleaned up capability and potential configuration terminology -
they are now two clearly different things.
8.2. draft-ietf-mmusic-sdp-capability-negotiation-00
Version 00 is the initial version. The solution provided in this Version 00 is the initial version. The solution provided in this
initial version is based on an earlier (individual submission) initial version is based on an earlier (individual submission)
version of [SDPCapNeg]. The following are the major changes compared version of [SDPCapNeg]. The following are the major changes compared
to that document: to that document:
o Solution no longer based on RFC 3407, but defines a set of similar o Solution no longer based on RFC 3407, but defines a set of similar
attributes (with some differences). attributes (with some differences).
o Various minor changes to the previously defined attributes. o Various minor changes to the previously defined attributes.
 End of changes. 95 change blocks. 
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