draft-ietf-mmusic-sdp-capability-negotiation-04.txt   draft-ietf-mmusic-sdp-capability-negotiation-05.txt 
MMUSIC Working Group F. Andreasen MMUSIC Working Group F. Andreasen
Internet-Draft Cisco Systems Internet-Draft Cisco Systems
Intended Status: Proposed Standard February 25, 2007 Intended Status: Proposed Standard March 4, 2007
Expires: August 2007 Obsolotes: 3407
Expires: September 2007
SDP Capability Negotiation SDP Capability Negotiation
draft-ietf-mmusic-sdp-capability-negotiation-04.txt draft-ietf-mmusic-sdp-capability-negotiation-05.txt
Status of this Memo Status of this Memo
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aware have been or will be disclosed, and any of which he or she aware have been or will be disclosed, and any of which he or she
becomes aware will be disclosed, in accordance with Section 6 of becomes aware will be disclosed, in accordance with Section 6 of
BCP 79. BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on August 25, 2007. This Internet-Draft will expire on September 4, 2007.
Copyright Notice Copyright Notice
Copyright (C) The IETF Trust (2007). Copyright (C) The IETF Trust (2007).
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
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negotiate them using the framework. Extensions for other types of negotiate them using the framework. Extensions for other types of
capabilities (e.g. media types and formats) may be provided in other capabilities (e.g. media types and formats) may be provided in other
documents. documents.
Table of Contents Table of Contents
1. Introduction...................................................3 1. Introduction...................................................3
2. Conventions used in this document..............................6 2. Conventions used in this document..............................6
3. SDP Capability Negotiation Solution............................6 3. SDP Capability Negotiation Solution............................6
3.1. Solution Overview.........................................6 3.1. Solution Overview.........................................6
3.2. Version and Extension Indication Attributes...............9 3.2. Relationship to RFC 3407..................................9
3.2.1. Supported Capability Negotiation Extensions Attribute9 3.3. Version and Extension Indication Attributes...............9
3.2.2. Required Capability Negotiation Extension Attribute.10 3.3.1. Supported Capability Negotiation Extensions Attribute9
3.3. Capability Attributes....................................12 3.3.2. Required Capability Negotiation Extension Attribute.11
3.3.1. Attribute Capability Attribute......................12 3.4. Capability Attributes....................................12
3.3.2. Transport Protocol Capability Attribute.............13 3.4.1. Attribute Capability Attribute......................13
3.3.3. Extension Capability Attributes.....................15 3.4.2. Transport Protocol Capability Attribute.............14
3.4. Configuration Attributes.................................15 3.4.3. Extension Capability Attributes.....................16
3.4.1. Potential Configuration Attribute...................15 3.5. Configuration Attributes.................................16
3.4.2. Actual Configuration Attribute......................19 3.5.1. Potential Configuration Attribute...................16
3.5. Offer/Answer Model Extensions............................20 3.5.2. Actual Configuration Attribute......................22
3.5.1. Generating the Initial Offer........................20 3.6. Offer/Answer Model Extensions............................24
3.5.2. Generating the Answer...............................22 3.6.1. Generating the Initial Offer........................24
3.5.2.1. Example Views of Potential Configurations......26 3.6.2. Generating the Answer...............................27
3.5.3. Offerer Processing of the Answer....................28 3.6.2.1. Example Views of Potential Configurations......31
3.5.4. Modifying the Session...............................29 3.6.3. Offerer Processing of the Answer....................34
3.6. Interactions with ICE....................................29 3.6.4. Modifying the Session...............................35
3.7. Processing Media before Answer...........................31 3.7. Interactions with ICE....................................35
3.8. Attribute Capabilities Using rtpmap or fmtp..............31 3.8. Processing Media before Answer...........................36
4. Examples......................................................32 3.9. Considerations for Specific Attribute Capabilities.......37
4.1. Best-Effort Secure RTP...................................32 3.9.1. The rtpmap and fmtp Attributes......................37
4.2. Multiple Transport Protocols.............................35 3.9.2. Direction Attributes................................38
4.3. Session-Level MIKEY and Media Level Security Descriptions39 4. Examples......................................................38
4.4. Capability Negotiation with Interactive Connectivity 4.1. Best-Effort Secure RTP...................................38
Establishment.................................................43 4.2. Multiple Transport Protocols.............................41
5. Security Considerations.......................................43 4.3. Best-Effort SRTP with Session-Level MIKEY and Media Level
6. IANA Considerations...........................................45 Security Descriptions.........................................45
6.1. New SDP Attributes.......................................45 4.4. SRTP with Session-Level MIKEY and Media Level Security
6.2. New SDP Capability Negotiation Option Tag Registry.......47 Descriptions as Alternatives..................................49
5. Security Considerations.......................................51
6. IANA Considerations...........................................53
6.1. New SDP Attributes.......................................53
6.2. New SDP Capability Negotiation Option Tag Registry.......54
6.3. New SDP Capability Negotiation Potential Configuration 6.3. New SDP Capability Negotiation Potential Configuration
Parameter Registry............................................47 Parameter Registry............................................55
7. To Do and Open Issues.........................................47 7. To Do and Open Issues.........................................55
8. Acknowledgments...............................................47 8. Acknowledgments...............................................55
9. Change Log....................................................48 9. Change Log....................................................56
9.1. draft-ietf-mmusic-sdp-capability-negotiation-04..........48 9.1. draft-ietf-mmusic-sdp-capability-negotiation-05..........56
9.2. draft-ietf-mmusic-sdp-capability-negotiation-03..........48 9.2. draft-ietf-mmusic-sdp-capability-negotiation-04..........57
9.3. draft-ietf-mmusic-sdp-capability-negotiation-02..........48 9.3. draft-ietf-mmusic-sdp-capability-negotiation-03..........57
9.4. draft-ietf-mmusic-sdp-capability-negotiation-01..........49 9.4. draft-ietf-mmusic-sdp-capability-negotiation-02..........57
9.5. draft-ietf-mmusic-sdp-capability-negotiation-00..........50 9.5. draft-ietf-mmusic-sdp-capability-negotiation-01..........58
10. References...................................................51 9.6. draft-ietf-mmusic-sdp-capability-negotiation-00..........59
10.1. Normative References....................................51 10. References...................................................60
10.2. Informative References..................................51 10.1. Normative References....................................60
Author's Addresses...............................................54 10.2. Informative References..................................60
Intellectual Property Statement..................................54 Author's Addresses...............................................62
Full Copyright Statement.........................................54 Intellectual Property Statement..................................63
Acknowledgment...................................................55 Full Copyright Statement.........................................63
Acknowledgment...................................................63
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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3.1. Solution Overview 3.1. Solution Overview
The solution consists of the following: The solution consists of the following:
o Two new attributes to support extensions to the framework itself o Two new attributes to support extensions to the framework itself
as follows: as follows:
o A new attribute ("a=csup") that lists the supported base and o A new attribute ("a=csup") that lists the supported base and
extension options to the framework. extension options to the framework.
o A new attribute ("a=creq") that lists the base and or o A new attribute ("a=creq") that lists the extensions to the
extensions to the framework that are required to be supported framework that are required to be supported by the entity
by the entity receiving the SDP in order to do capability receiving the SDP in order to do capability negotiation.
negotiation.
o Two 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=acap") that defines how to list an o A new attribute ("a=acap") that defines how to list an
attribute name and its associated value as a capability. attribute name, either with or without an associated value, as
a capability.
o A new attribute ("a=tcap") that defines how to list transport o A new attribute ("a=tcap") that defines how to list transport
protocols (e.g. "RTP/AVP") 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
capabilities from the SDP in question. Multiple potential capabilities from the SDP in question. Multiple potential
configurations have an explicitly indicated ordering configurations have an explicitly indicated ordering
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| | | |
| (2) Answer (SRTP) | | (2) Answer (SRTP) |
|<---------------------------------| |<---------------------------------|
| | | |
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 (actual configuration), but SRTP is the preferred one default (actual configuration), but SRTP is the preferred one
(potential configuration): (potential configuration):
v=0 v=0
o=- 25678 753849 IN IP4 128.96.41.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 128.96.41.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
m=audio 3456 RTP/AVP 0 18 m=audio 53456 RTP/AVP 0 18
a=creq: cap-v0
a=tcap:1 RTP/SAVP a=tcap:1 RTP/SAVP
a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_32 a=acap: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=pcfg:1 t=1 a=1 a=pcfg:1 t=1 a=1
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 required base and extensions are provided by the PCMU or G.729. The capabilities are provided by the "a=tcap" and
"a=creq" attribute, which includes the option tag "cap-v0" to "a=acap" attributes. The transport capabilities ("a=tcap") indicate
indicate that the base framework defined here must be supported. The that secure RTP under the AVP profile ("RTP/SAVP") is supported with
capabilities are provided by the "a=tcap" and "a=acap" attributes. an associated transport capability handle of 1. The "acap" attribute
The transport capabilities ("a=tcap") indicate that secure RTP under provides an attribute capability with a handle of 1. The attribute
the AVP profile ("RTP/SAVP") is supported with an associated capability is a "crypto" attribute, which provides the keying
transport capability handle of 1. The "acap" attribute provides an material for SRTP using SDP security descriptions [SDES]. The
attribute capability with a handle of 1. The attribute capability is "a=pcfg" attribute provides the potential configuration included in
a "crypto" attribute, which provides the keying material for SRTP the offer by reference to the capability parameters. One alternative
using SDP security descriptions [SDES]. The "a=pcfg" attribute is provided; it has a configuration number of 1 and it consists of
provides the potential configuration included in the offer by transport protocol capability 1 (i.e. the RTP/SAVP profile - secure
reference to the capability parameters. One alternative is provided; RTP), and the attribute capability 1, i.e. the crypto attribute
it has a configuration number of 1 and it consists of transport provided. Potential configurations are always preferred over the
protocol capability 1 (i.e. the RTP/SAVP profile - secure RTP), and actual configuration included in the offer SDP, and hence Alice is
the attribute capability 1, i.e. the crypto attribute provided.
Potential configurations are always preferred over the actual
configuration included in the offer SDP, and hence Alice is
expressing a preference for using secure RTP. expressing a preference for using secure RTP.
Bob receives the SDP offer from Alice. Bob supports SRTP and the SDP Bob receives the SDP offer from Alice. Bob supports SRTP and the SDP
Capability Negotiation framework, and hence he accepts the Capability Negotiation framework, and hence he accepts the
(preferred) potential configuration for Secure RTP provided by Alice: (preferred) potential 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 192.0.2.2
s= s=
c=IN IP4 128.96.41.2 c=IN IP4 192.0.2.2
t=0 0 t=0 0
m=audio 4568 RTP/SAVP 0 18 m=audio 54568 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=acfg:1 t=1 a=1 a=acfg:1 t=1 a=1
Bob includes the "a=acfg" attribute in the answer to inform Alice Bob includes the "a=acfg" attribute in the answer to inform Alice
that he based his answer on an offer containing the potential that he based his answer on an offer containing the potential
configuration with transport protocol capability 1 and attribute configuration with transport protocol capability 1 and attribute
capability 1 from the offer SDP (i.e. the RTP/SAVP profile using the capability 1 from the offer SDP (i.e. the RTP/SAVP profile using the
keying material provided). Bob also includes his keying material in keying material provided). Bob also includes his keying material in
a crypto attribute. If Bob supported one or more extensions to the a crypto attribute. If Bob supported one or more extensions to the
capability negotiation framework, he would have included those in the capability negotiation framework, he would have included option tags
answer as well (in an "a=csup" attribute). for those in the answer as well (in an "a=csup" attribute).
Note that in this particular example, the answerer supported the Note that in this particular example, the answerer supported the
capability negotiation extensions defined here, however had he not, capability negotiation extensions defined here, however had he not,
the answerer would simply have ignored the new attributes and the answerer would simply have ignored the new attributes and
accepted the (actual configuration) offer to use normal RTP. In that accepted the (actual configuration) offer to use normal RTP. In that
case, the following answer would have been generated instead: case, the following answer would have been generated instead:
v=0 v=0
o=- 24351 621814 IN IP4 128.96.41.2 o=- 24351 621814 IN IP4 192.0.2.2
s= s=
c=IN IP4 128.96.41.2 c=IN IP4 192.0.2.2
t=0 0 t=0 0
m=audio 4568 RTP/AVP 0 18 m=audio 54568 RTP/AVP 0 18
3.2. Version and Extension Indication Attributes 3.2. Relationship to RFC 3407
RFC 3407 defines capability descriptions with limited abilities to
describe attributes, bandwidth parameters, transport protocols and
media formats. RFC 3407 does not define any negotiation procedures
for actually using those capability descriptions.
This document obsoletes RFC 3407 by defining new attributes for
describing attribute capabilities and transport capabilities. It also
defines procedures for actually using those capabilities as part of
an offer/answer exchange. Extensions to this document may be defined
in order to fully cover all the capabilities provided by RFC 3407
(for example more general media capabilities).
It is RECOMMENDED that implementations use the attributes and
procedures defined in this document instead of those defined in
[RFC3407].
If capability description interoperability with legacy RFC 3407
implementations is desired, implementations MAY include both RFC 3407
capability descriptions and capabilities defined by this document.
The offer/answer negotiation procedures however will not be able to
use the RFC 3407 capability descriptions.
3.3. 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 extensions supported and indicating the SDP capability negotiation extensions supported and
required. required.
3.2.1. Supported Capability Negotiation Extensions Attribute 3.3.1. 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 6. in Section 6.
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 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>
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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 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>
RFC 4566, Section 9, provides the ABNF for SDP attributes. The "csup" RFC 4566, Section 9, provides the ABNF for SDP attributes. The "csup"
attribute adheres to the RFC 4566 "attribute" production, with an attribute adheres to the RFC 4566 "attribute" production, with an
att-value defined as follows: att-value defined as follows:
att-value = *WSP option-tag-list att-value = option-tag-list
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 [RFC4566]
COMMA = *WSP "," *WSP ; defined in [RFC4234] COMMA = "," ; defined in [RFC4234]
Note that white-space is permitted before the option-tag-list. Also, Implementers familiar with the Session Initiation Protocol (SIP)
implementers familiar with SIP should note that the above definition should note that the above definition of COMMA differs from the one
of COMMA differs from the one in [RFC3261]. in [RFC3261].
A special base option tag with a value of "cap-v0" is defined for the A special base option tag with a value of "cap-v0" is defined for the
basic SDP capability negotiation framework. Entities use this option basic SDP capability negotiation framework. Entities can use this
tag with the "a=csup" attribute to indicate support for the SDP option tag with the "a=csup" attribute to indicate support for the
capability negotiation framework specified in this document. 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 the "cap-v0" option tags and two hypothetical option tags, "foo" with the "cap-v0" option tags and two hypothetical option tags, "foo"
and "bar": and "bar" (note the lack of white space):
a=csup: cap-v0 a=csup: cap-v0
a=csup: foo a=csup: foo
a=csup: bar a=csup: bar
a=csup: cap-v0, foo, bar a=csup: cap-v0, 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 SDP. When provided at the media-level, it applies to the media
description in question only (option-tags provided at the session description in question only (option-tags provided at the session
level apply as well). There can be one or more "a=csup" attributes at level apply as well). There can be at most one "a=csup" attributes at
both the session and media-level (one or more per media description the session-level and at most one at the media-level (one per media
in the latter case). description 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 at the session and/or media-level, unless those option tags supports at the session and/or media-level, unless those option tags
are already provided in one or more "a=creq" attribute (see Section are already provided in one or more "a=creq" attribute (see Section
3.2.2. ) at the relevant levels. The base option tag MAY be included. 3.3.2. ) at the relevant levels. The base option tag MAY be included.
3.2.2. Required Capability Negotiation Extension Attribute 3.3.2. Required Capability Negotiation Extension Attribute
The Required Capability Negotiation Extensions attribute ("a=creq") The Required Capability Negotiation Extensions attribute ("a=creq")
contains a comma-separated list of option tags (see Section 3.2.1. ) contains a comma-separated list of option tags (see Section 3.3.1. )
identifying the SDP Capability negotiation extensions that MUST be identifying the SDP Capability negotiation extensions that MUST be
supported by the entity receiving the SDP in order for that entity to supported by the entity receiving the SDP in order for that entity to
properly process the SDP Capability negotiation. The attribute is properly process the SDP Capability Negotiation attributes and
defined as follows: associated procedures. Support for the basic negotiation framework is
implied by the presence of an "a=pcfg" attribute (see Section 3.5.1.
) and hence there is no need to include the "a=creq" attribute with
the base option-tag ("cap-v0"). Still, it is allowed to do so.
The attribute is defined as follows:
a=creq: <option-tag-list> a=creq: <option-tag-list>
The "creq" attribute adheres to the RFC 4566 "attribute" production, The "creq" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows: with an att-value defined as follows:
att-value = *WSP option-tag-list att-value = option-tag-list
where "option-tag-list" is defined in Section 3.2.1. Note that
white-space is permitted before the option-tag-list.
The following examples illustrate the use of the "a=creq" attribute The following examples illustrate the use of the "a=creq" attribute
with the "cap-v0" base option tag and two hypothetical option tags, with the "cap-v0" base option tag and two hypothetical option tags,
"foo" and "bar": "foo" and "bar" (note the lack of white space):
a=creq: cap-v0 a=creq: cap-v0
a=creq: foo a=creq: foo
a=creq: bar a=creq: bar
a=creq: cap-v0, foo, bar a=creq: cap-v0, 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 (required option tags provided at the session level in question only (required option tags provided at the session level
apply as well). There can be one or more "a=creq" attributes at both apply as well). There can be at most one "a=creq" attribute at the
the session and media-level (one or more per media stream in the session-level and at most one "a=creq" attribute at the media-level
latter case). (one per media description in the latter case).
When an entity generates an SDP and it requires the recipient of that When an entity generates an SDP and it requires the recipient of that
SDP to support one or more SDP capability negotiation extensions in SDP to support one or more SDP capability negotiation extensions 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 at the session and/or media level, unless it the required extensions at the session and/or media level, unless it
is already known that the receiving entity supports those option-tags is already known that the receiving entity supports those option-tags
at the relevant levels (in which case their inclusion is OPTIONAL). at the relevant levels (in which case their inclusion is OPTIONAL).
An example of this is when generating an answer to an offer. If the An example of this is when generating an answer to an offer. If the
answerer supports the required option-tags from the offer, and the answerer supports the required option-tags from the offer, and the
answerer does not require any additional option-tags beyond what answerer does not require any additional option-tags beyond what
was listed in either the required ("a=creq") or supported was listed in either the required ("a=creq") or supported
("a=csup") attributes from the offer, then the answerer is not ("a=csup") attributes from the offer, then the answerer is not
required to include a required ("a=creq") attribute with any required to include a required ("a=creq") attribute with any
option-tags that may need to be supported (such as the base option option-tags that may need to be supported (such as the base option
tag - "cap-v0"). tag - "cap-v0").
Support for the basic negotiation framework is implied by the
presence of an "a=pcfg" attribute (see Section 3.5.1. ) and hence it
is not required to include the "a=creq" attribute with the base
option-tag ("cap-v0").
A recipient that receives an SDP and does not support one or more of A recipient that receives an SDP and does not support one or more of
the required extensions listed in a "creq" attribute, MUST NOT the required extensions listed in a "creq" attribute, MUST NOT
perform the SDP capability negotiation defined in this document. For perform the SDP capability negotiation defined in this document. For
non-supported extensions provided at the session-level, this implies non-supported extensions provided at the session-level, this implies
that SDP capability negotiation MUST NOT be performed at all. For that SDP capability negotiation MUST NOT be performed at all. For
non-supported extensions at the media-level, this implies that SDP non-supported extensions at the media-level, this implies that SDP
capability negotiation MUST NOT be performed for the media stream in capability negotiation MUST NOT be performed for the media stream in
question. 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, i.e. all the capability negotiation attributes should be place, i.e. all the capability negotiation attributes should be
ignored. In that case, the entity SHOULD include a "csup" attribute ignored. In that case, the entity SHOULD include a "csup" attribute
listing the SDP capability negotiation extensions it actually listing the SDP capability negotiation extensions it actually
supports. supports.
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 3.4. Capability Attributes
compliant to this specification MUST include a "creq" attribute (at
least in an offer) that includes the option tag "cap-v0" as
illustrated below:
a=creq: cap-v0
3.3. 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.
3.3.1. Attribute Capability Attribute 3.4.1. Attribute Capability Attribute
Attributes and their associated values can be expressed as Attributes and their associated values can be expressed as
capabilities by use of a new attribute capability attribute capabilities by use of a new attribute capability attribute
("a=acap"), which is defined as follows: ("a=acap"), which is defined as follows:
a=acap: <att-cap-num> <att-par> a=acap: <att-cap-num> <att-par>
where <att-cap-num> is an integer between 1 and 2^31-1 (both where <att-cap-num> is an integer between 1 and 2^31-1 (both
included) used to number the attribute capability and <att-par> is an included) used to number the attribute capability and <att-par> is an
attribute ("a=") in its full '<type>=<value>' form (see [SDP]). attribute ("a=") in its full '<type>=<value>' form (see [RFC4566]).
Support for a specific attribute (name) (without any particular
values) can be indicated by providing only the '<type>' (i.e. the
attribute name).
The "acap" attribute adheres to the RFC 4566 "attribute" production, The "acap" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows: with an att-value defined as follows:
att-value = *WSP att-cap-num 1*WSP att-par att-value = att-cap-num 1*WSP att-par
att-cap-num = 1*DIGIT ;defined in [RFC4234] att-cap-num = 1*DIGIT ;defined in [RFC4234]
att-par = attribute ;defined in RFC 4566 att-par = attribute ;defined in RFC 4566
Note that white-space is permitted before the att-cap-num. The "acap" Note that white-space is not permitted before the att-cap-num.
attribute can be provided at the session level for session-level
attributes and the media level for media-level attributes. The "acap" The "acap" attribute can be provided at the session level for
attribute MUST NOT be used to provide a media-level attribute at the session-level attributes and the media level for media-level
session-level or vice versa. attributes. The "acap" attribute MUST NOT be used to provide a media-
level attribute at the session-level or vice versa.
Each occurrence of the "acap" attribute in the entire session Each occurrence of the "acap" attribute in the entire session
description MUST use a different value of <att-cap-num>. description MUST use a different value of <att-cap-num>.
There is a need to be able to reference both session-level and There is a need to be able to reference both session-level and
media-level attributes in potential configurations at the media media-level attributes in potential configurations at the media
level, and this provides for a simple solution to avoiding overlap level, and this provides for a simple solution to avoiding overlap
between the references (handles) to each attribute capability. between the references (handles) to each attribute capability.
The <att-cap-num> values provided are independent of similar <cap- The <att-cap-num> values provided are independent of similar <cap-
num> values provided for other capability attributes, i.e., they form num> values provided for other types of capabilities, i.e., they form
a separate name-space for attribute capabilities. a separate name-space for attribute capabilities.
The following examples illustrate use of the "acap" attribute: The following examples illustrate use of the "acap" attribute:
a=acap: 1 a=ptime:20 a=acap: 1 a=ptime:20
a=acap: 2 a=ptime:30 a=acap: 2 a=ptime:30
a=acap: 3 a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyONQ6gAA a=acap: 3 a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyONQ6gAA
AAAGEEoo2pee4hp2UaDX8ZE22YwKAAAPZG9uYWxkQGR1Y2suY29tAQAAAAAAAQAk0 AAAGEEoo2pee4hp2UaDX8ZE22YwKAAAPZG9uYWxkQGR1Y2suY29tAQAAAAAAAQAk0
JKpgaVkDaawi9whVBtBt0KZ14ymNuu62+Nv3ozPLygwK/GbAV9iemnGUIZ19fWQUO JKpgaVkDaawi9whVBtBt0KZ14ymNuu62+Nv3ozPLygwK/GbAV9iemnGUIZ19fWQUO
SrzKTAv9zV SrzKTAv9zV
a=acap: 4 a=crypto:1 AES_CM_128_HMAC_SHA1_32 a=acap: 4 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=acap:5 a=crypto
a=acap:6 a=key-mgmt
The first two provide attribute values for the ptime attribute. The The first two provide attribute values for the ptime attribute. The
third provides SRTP parameters by using MIKEY with the key-mgmt third provides SRTP parameters by using MIKEY with the key-mgmt
attribute [KMGMT]. The fourth provides SRTP parameters by use of attribute [KMGMT]. The fourth provides SRTP parameters by use of
security descriptions with the crypto attribute [SDES]. Note that the security descriptions with the crypto attribute [SDES]. Note that the
line-wrapping and new-lines in example three and four are provided line-wrapping and new-lines in example three and four are provided
for formatting reasons only - they are not permitted in actual SDP. for formatting reasons only - they are not permitted in actual SDP.
The 5th attribute capability merely indicates support for the
"crypto" attribute (without any further information about particular
values to use with it), and the 6th attribute capability merely
indicates support for the "key-mgmt" attribute.
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 "acap" couple of important differences, most notably that the "acap"
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 "acap" in RFC 3407 supports bandwidth information as well). The "acap"
attribute also is not automatically associated with any particular attribute also is not automatically associated with any particular
capabilities. capabilities.
3.3.2. Transport Protocol Capability Attribute 3.4.2. 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=tcap") defined as Transport Protocol Capability attribute ("a=tcap") defined as
follows: follows:
a=tcap: <trpr-cap-num> <proto-list> a=tcap: <trpr-cap-num> <proto-list>
where <trpr-cap-num> is an integer between 1 and 2^31-1 (both where <trpr-cap-num> is an integer between 1 and 2^31-1 (both
included) used to number the transport address capability for later included) used to number the transport address capability for later
reference, and <proto-list> is one or more <proto>, separated by reference, and <proto-list> is one or more <proto>, separated by
white space, as defined in the SDP "m=" line. white space, as defined in the SDP "m=" line.
The "tcap" attribute adheres to the RFC 4566 "attribute" production, The "tcap" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows: with an att-value defined as follows:
att-value = *WSP trpr-cap-num 1*WSP proto-list att-value = trpr-cap-num 1*WSP proto-list
trpr-cap-num = 1*DIGIT ;defined in [RFC4234] trpr-cap-num = 1*DIGIT ;defined in [RFC4234]
proto-list = proto *(1*WSP proto) ; defined in RFC 4566 proto-list = proto *(1*WSP proto) ; defined in RFC 4566
Note that white-space is permitted before the trpr-cap-num. The Note that white-space is not permitted before the trpr-cap-num.
"tcap" attribute can be provided at the session- and media-level.
Each occurrence of the "tcap" attribute in the entire session The "tcap" attribute can be provided at the session- and media-level.
description MUST use a different value of <trpr-cap-num>. When There can be multiple "tcap" attributes at the session-level as well
multiple <proto> values are provided, the first one is associated as within each media description. Each occurrence of the "tcap"
with the value <trpr-cap-num>, the second one with the value one attribute in the entire session description MUST use a different
higher, etc. The <trpr-cap-num> values provided are independent of value of <trpr-cap-num>. When multiple <proto> values are provided,
similar <cap-num> values provided for other capability attributes, the first one is associated with the value <trpr-cap-num>, the second
i.e., they form a separate name-space for transport protocol one with the value one higher, etc. The <trpr-cap-num> values
capabilities. provided are independent of similar <cap-num> values provided for
other capability attributes, i.e., they form a separate name-space
for transport protocol capabilities.
Below, we provide examples of the "a=tcap" attribute: Below, we provide examples of the "a=tcap" attribute:
a=tcap: 1 RTP/AVP a=tcap: 1 RTP/AVP
a=tcap: 2 RTP/AVPF a=tcap: 2 RTP/AVPF
a=tcap: 3 RTP/SAVP RTP/SAVPF a=tcap: 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.
Transport capabilities are inherently included in the "m=" line, Transport capabilities are inherently included in the "m=" line,
however they still need to be specified explicitly in a "tcap" however they still need to be specified explicitly in a "tcap"
attribute, if they are to be used as a capability. This may seem attribute, if they are to be used as a capability.
redundant (and indeed it is from the offerer's point of view),
however it is done to protect against middle-boxes that may modify
"m=" lines while passing unknown attributes through. If an implicit
capability were used instead (e.g. a reserved transport capability
number could be used to refer to the transport protocol in the "m="
line), and a middle-box were to modify the transport protocol in the
"m=" line (e.g. to translate between plain RTP and secure RTP), then
the potential configuration referencing that implicit transport
capability may no longer be correct. With explicit capabilities, we
avoid this pitfall, although the potential configuration preference
(see Section 3.4.1. ) may not reflect that of the middle-box (which
some may view as a feature).
3.3.3. Extension Capability Attributes This may seem redundant (and indeed it is from the offerer's point
of view), however it is done to protect against middle-boxes that
may modify "m=" lines while passing unknown attributes through. If
an implicit transport capability were used instead (e.g. a reserved
transport capability number could be used to refer to the transport
protocol in the "m=" line), and a middle-box were to modify the
transport protocol in the "m=" line (e.g. to translate between
plain RTP and secure RTP), then the potential configuration
referencing that implicit transport capability may no longer be
correct. With explicit capabilities, we avoid this pitfall,
although the potential configuration preference (see Section 3.5.1.
) may not reflect that of the middle-box (which some may view as a
feature).
3.4.3. Extension Capability Attributes
The SDP Capability Negotiation framework allows for new capabilities The SDP Capability Negotiation framework allows for new capabilities
to be defined as extensions and used with the general capability to be defined as extensions and used with the general capability
negotiation framework. The syntax and semantics of such new negotiation framework. The syntax and semantics of such new
capability attributes are not defined here, however in order to be capability attributes are not defined here, however in order to be
used with potential configurations, they MUST allow for a numeric used with potential configurations, they SHOULD allow for a numeric
handle to be associated with each capability. This handle will be handle to be associated with each capability. This handle can be used
used as a reference within the potential and actual configuration as a reference within the potential and actual configuration
attributes (see Section 3.4.1. and 3.4.2. ). The definition of such attributes (see Section 3.5.1. and 3.5.2. ). The definition of such
extension capability attributes MUST also state whether they can be extension capability attributes MUST also state whether they can be
applied at the session-level, media-level, or both. applied at the session-level, media-level, or both.
3.4. Configuration Attributes 3.5. Configuration Attributes
3.4.1. Potential Configuration Attribute 3.5.1. 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: <config-number> <pot-cfg-list> a=pcfg: <config-number> <pot-cfg-list>
where <config-number> is an integer between 1 and 2^31-1 (both where <config-number> is an integer between 1 and 2^31-1 (both
included). included).
The "pcfg" attribute adheres to the RFC 4566 "attribute" production, The "pcfg" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows: with an att-value defined as follows:
att-value = *WSP config-number 1*WSP pot-cfg-list att-value = config-number 1*WSP pot-cfg-list
config-number = 1*DIGIT ;defined in [RFC4234] config-number = 1*DIGIT ;defined in [RFC4234]
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-attribute-config-list /
pot-transport-protocol-config / pot-transport-protocol-config-list /
pot-extension-config pot-extension-config-list
The missing productions are defined below. Note that white-space is The missing productions are defined below. Note that white-space is
permitted before the config-number. not permitted before the config-number.
The potential configuration attribute can be provided at the media- The potential configuration attribute can be provided at the media-
level only. The attribute includes a configuration number, which is level only and there can be multiple instances of it within a given
an integer between 1 and 2^31-1 (both included). The configuration media description. The attribute includes a configuration number,
number MUST be unique within the media stream. The configuration which is an integer between 1 and 2^31-1 (both included). The
number also indicates the relative preference of potential configuration number MUST be unique within the media description
configurations; lower numbers are preferred over higher numbers. (i.e. it has media level scope only). The configuration number also
indicates the relative preference of potential configurations; lower
numbers are preferred over higher numbers.
After the configuration number, one or more potential configuration After the configuration number, one or more potential configuration
parameters MUST be provided. This document defines potential lists MUST be provided. The potential configuration lists generally
attribute parameter configurations and potential transport protocol reference one or more capabilities, and those capabilities are
configurations. Each of these MUST NOT be present more than once in (conceptually) used to construct a new internal version of the SDP by
a particular potential configuration attribute. Potential extension use of purely syntactic add, delete and replace operations on the
configurations can be included as well; unknown potential extension original SDP (actual configuration), thereby generating a new
configurations MUST be ignored (if support is required, then the potential configuration SDP that can be used by conventional SDP
"a=creq" with a suitable option tag should be used). There can be procedures if actually selected.
more than one potential extension configuration, however each
particular potential extension configuration MUST NOT be present more This document defines potential attribute configuration lists and
than once in a given potential configuration attribute. Together, potential transport protocol configuration lists. Each of these MUST
these values define a potential configuration. NOT be present more than once in a particular potential configuration
attribute. Potential extension configuration lists can be included as
well; unknown potential extension configuration lists MUST be ignored
(if support is required, then the "a=creq" attribute with suitable
option tags should be used). There can be more than one potential
extension configuration list, however each particular potential
extension configuration list MUST NOT be present more than once in a
given potential configuration attribute. Together, these potential
configuration lists define a potential configuration.
There can be multiple potential configurations provided within a There can be multiple potential configurations provided within a
media description. Each of these indicates not only a willingness, media description. Each of these indicates not only a willingness,
but in fact a desire to use the potential configuration. but in fact a desire to use the potential configuration.
Attribute capabilities are included in a potential configuration by Attribute capabilities are used in a potential configuration by use
use of the pot-attribute-parameter-config parameter, which is defined of the pot-attribute-config-list parameter, which is defined by the
by the following ABNF: following ABNF:
pot-attribute-parameter-config pot-attribute-config-list
= "a=" acap-cap-list *(BAR acap-cap-list) = "a=" [delete-attributes ":"]
acap-cap-list = att-cap-num *(COMMA att-cap-num) att-cap-inst-list *(BAR att-cap-inst-list)
delete-attributes = DELETE ( "m" ; media attributes
/ "s" ; session attributes
/ "ms" ) ; media and session attributes
att-cap-inst-list = att-cap-inst *(COMMA att-cap-inst)
att-cap-inst = [att-cap-operator] att-cap-num
att-cap-operator = DELETE / REPLACE
att-cap-num = 1*DIGIT ;defined in [RFC4234] att-cap-num = 1*DIGIT ;defined in [RFC4234]
BAR = *WSP "|" *WSP ; defined in [RFC4234] BAR = "|"
DELETE = "-"
REPLACE = "/"
Each potential attribute parameter configuration list is a comma- Note that white space is not permitted within this production.
separated list of attribute capability numbers where att-cap-num
refers to attribute capability numbers defined above and hence MUST
be between 1 and 2^31-1 (both included). Alternative potential
attribute parameter configurations are separated by a vertical bar
("|"), the scope of which extends to the next alternative (i.e. ","
has higher precedence than "|"). The alternatives are ordered by
preference with the most preferred listed first.
Transport protocol capabilities are included in a potential Each potential attribute configuration list can optionally begin with
configuration by use of the pot-transport-protocol-config parameter, instructions for how to handle attributes that are part of the actual
which is defined by the following ABNF: configuration (i.e. the "a=" lines present in the original SDP). By
default, such attributes will remain as part of the configuration in
question. However, if delete-attributes indicates "-m", then all
attribute lines within the media description in question will be
deleted (i.e. all "a=" lines under the "m=" line in question). If
delete-attributes indicates "-s", then all attribute lines at the
session-level will be deleted (i.e. all "a=" lines before the first
"m=" line). If delete-attributes indicates "-ms", then all attribute
lines within this media description ("m=" line) and all attribute
lines at the session-level will be deleted.
pot-transport-protocol-config = The attribute capability instruction list comes next. It contains one
or more alternative lists of attribute capability instructions. The
lists are separated by a vertical bar ("|"), and each list contains
one or more attribute capability instructions separated by commas
(","). An attribute capability instruction is merely an attribute
capability number that may optionally be prefixed by an attribute
capability operator. Each attribute capability number (att-cap-num)
identifies a particular attribute capability by referring to
attribute capability numbers defined above and hence MUST be between
1 and 2^31-1 (both included). The following example illustrates the
above:
a=-m:1,2,-3,/4|1,2,5
where
o "a=-m:1,2,-3,/4|1,2,5" is the potential attribute configuration
list
o "-m" is the delete-attributes
o "1,2,-3,/4" and "1,2,5" are both attribute capability instruction
lists. The two lists are alternatives, since they are separated by
a vertical bar above
o "1" is an attribute capability instruction without any attribute
capability operator. It is also an attribute capability number
o "-3" is an attribute capability instruction, where "-" is an
attribute capability operator, and "3" is the attribute capability
number.
o "/4" is an attribute capability instruction, where "/" is an
attribute capability operator, and "4" is the attribute capability
number.
By default, each referenced attribute capability will result in the
corresponding attribute name and its associated value (contained
inside the attribute capability) merely being added to the resulting
potential configuration SDP. The attribute capability operators
change this default behavior:
o For session-level attribute capabilities, the DELETE operator ("-
") will result in the deletion of all session-level occurrences of
attributes with the same attribute-name (attribute values are
ignored) as the attribute contained inside that attribute
capability. The attribute contained inside the attribute
capability will NOT be added to the resulting potential
configuration SDP.
For example, if delete was indicated for a session-level "key-
mgmt" attribute capability ("a=acap:1 a=key-mgmt..."), all
occurrences of "a=key-mgmt" at the session-level would be
deleted.
o For media-level attribute capabilities, the DELETE operator ("-")
will result in the deletion of all occurrences of attributes with
the same attribute-name (attribute values are ignored) as the
attribute contained inside that attribute capability, within this
particular media description ("m=" line) only. The attribute
contained inside the attribute capability will NOT be added to the
resulting potential configuration SDP.
For example, if delete was indicated for a media-level "crypto"
attribute capability ("a=acap:1 a=crypto..."), all occurrences
of "a=crypto" inside the media description ("m=" line) in
question would be deleted.
o For session-level attribute capabilities, the REPLACE operator
("/") is similar to the DELETE operator, except that the attribute
contained inside the attribute capability WILL be added to the
resulting potential configuration SDP (at the session-level).
For example, if replace was indicated for a session-level "key-
mgmt" attribute capability ("a=acap:1 a=key-mgmt..."), all
occurrences of "a=key-mgmt" at the session-level would be
deleted, and the "key-mgmt" attribute name and its associated
value from the attribute capability would be added to the
resulting SDP for that configuration.
o For media-level attribute capabilities, the REPLACE operator ("/")
is similar to the DELETE operator, except that the attribute
contained inside the attribute capability WILL be added to the
resulting potential configuration SDP (within the media
description in question).
For example, if replace was indicated for a media-level
"crypto" attribute capability ("a=acap:1 a=crypto..."), all
occurrences of "a=crypto" inside the media description ("m="
line) in question would be deleted, and the "crypto" attribute
name and its associated value from the attribute capability
would be added to the resulting SDP for that configuration.
Alternative attribute capability instruction lists are separated by a
vertical bar ("|"), the scope of which extends to the next
alternative (i.e. "," has higher precedence than "|"). The
alternatives are ordered by preference with the most preferred listed
first. Exactly one of the alternative lists MUST be selected in its
entirety in order to use this potential configuration attribute.
Potential transport protocol configuration lists are included in a
potential configuration by use of the pot-transport-protocol-config-
list parameter, which is defined by the following ABNF:
pot-transport-protocol-config-list =
"t=" 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]
Note that white-space is not permitted within this production.
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 2^31-1 (both included). defined above and hence MUST be between 1 and 2^31-1 (both included).
Alternative potential transport protocol configurations are separated Alternative potential transport protocol capabilities are separated
by a vertical bar ("|"). The alternatives are ordered by preference by a vertical bar ("|"). The alternatives are ordered by preference
with the most preferred listed first. When transport protocol with the most preferred listed first. When there are no transport
capabilities are not included in a potential configuration at the protocol capabilities included in a potential configuration at the
media level, the transport protocol information from the associated media level, the transport protocol information from the associated
"m=" line will be used. "m=" line will be used. When included, exactly one of the
alternatives MUST be selected in order to use this potential
configuration attribute.
In the presence of middle-boxes (the existence of which may not be In the presence of middle-boxes (the existence of which may not be
known), care should be taken with assuming that the transport known), care should be taken with assuming that the transport
protocol in the "m=" line will not be modified by a middle-box. Use protocol in the "m=" line will not be modified by a middle-box. Use
of an explicit capability will guard against the capability of an explicit transport protocol capability will guard against any
indications of that. capability negotiation implications of that.
Extension capabilities can be included in a potential configuration Extension capabilities can be included in a potential configuration
as well. Such extensions MUST adhere to the following ABNF: as well by use of potential extension configuration lists. Such
potential configuration extension lists MUST adhere to the following
ABNF:
pot-extension-config = ext-cap-name "=" pot-extension-config-list = ext-cap-name "="
ext-cap-list *(BAR ext-cap-list) ext-cap-list
ext-cap-name = token ; defined in [SDP] ext-cap-name = token ; defined in [RFC4566]
ext-cap-list = ext-cap-num *(COMMA ext-cap-num) ext-cap-list = 1*VCHAR ; defined in [RFC4234]
ext-cap-num = 1*DIGIT ; defined in [RFC4234]
Note that white-space is not permitted within this production.
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-list is here merely defined as a sequence of visible
particular type of extension capability. The number MUST be between characters. The actual extension supported MUST refine both of these
1 and 2^31-1 (both included). Alternative potential extension further. For extension capabilities that merely need to be referenced
configurations for a particular extension are separated by a vertical by a capability number, it is RECOMMENDED to follow a structure
bar ("|"),the scope of which extends to the next alternative (i.e. similar to what has been specified above. Unsupported or unknown
"," has higher precedence than "|"). Unsupported or unknown potential extension configuration lists in a potential configuration
potential extension configs MUST be ignored. attribute MUST be ignored.
The "creq" attribute and its associated rules can be used to ensure The "creq" attribute and its associated rules can be used to ensure
that required extensions are supported in the first place. that required extensions are supported in the first place.
Potential configurations can be provided at the media level only, Potential configuration attributes can be provided at the media level
however it is possible to reference capabilities provided at either only, however it is possible to reference capabilities provided at
the session or media level. There are certain semantic rules and either the session or media level. There are certain semantic rules
restrictions associated with this: and restrictions associated with this:
A (media level) potential configuration in a given media description A (media level) potential configuration attribute in a given media
MUST NOT reference a media-level capability provided in a different description MUST NOT reference a media-level capability provided in a
media description; doing so invalidates that potential configuration. different media description; doing so invalidates that potential
A potential configuration can however reference a session-level configuration (note that a potential configuration attribute can
capability. The semantics of doing so (should that potential contain more than one potential configuration by use of
configuration be chosen), depends on the type of capability. In the alternatives). A potential configuration attribute can however
case of transport capabilities, this has no particular implication. reference a session-level capability. The semantics of doing so
In the case of attribute capabilities however, it does. More depends on the type of capability. In the case of transport protocol
specifically, the corresponding attribute value (provided within that capabilities it has no particular implication. In the case of
attribute capability) will be considered part of the active attribute capabilities however, it does. More specifically, the
attribute name and value (provided within that attribute capability)
will be considered part of the resulting SDP for that particular
configuration at the *session* level. In other words, it will be as- configuration at the *session* level. In other words, it will be as-
if that attribute was simply provided with that value at the session- if that attribute was simply provided with that value at the session-
level in the first place. Note that individual media streams perform level in the first place. Note that individual media streams perform
capability negotiation individually, and hence it is possible that capability negotiation individually, and hence it is possible that
another media stream (where the attribute was part of a potential another media stream (where the attribute was part of a potential
configuration) chose a configuration without that session level configuration) chose a configuration without that session level
attribute. The session-level attribute however remains "active" and attribute. The session-level attribute however remains "active" and
hence applies to the entire session. It is up to the entity that hence applies to the entire resulting potential configuration SDP. It
generates the SDP to ensure that in such cases, the resulting active is up to the entity that generated the SDP with these capabilities
configuration SDP is still meaningful. and potential configuration attributes in the first place, to ensure,
that in such cases, the resulting potential configuration SDP is
still meaningful.
The session-level operation of extension capabilities is undefined: The session-level operation of extension capabilities is undefined:
Consequently, if session-level extension capabilities are defined, Consequently, each new session-level extension capability defined
they MUST specify the implication of making them part of an active MUST specify the implication of making it part of a configuration at
configuration at the media level. the media level.
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 192.0.2.1
s= s=
c=IN IP4 128.96.41.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
m=audio 3456 RTP/AVPF 0 18 m=audio 53456 RTP/AVPF 0 18
a=creq: cap-v0
a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32 a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=tcap: 1 RTP/AVPF RTP/AVP a=tcap: 1 RTP/AVPF RTP/AVP
a=tcap: 3 RTP/SAVP RTP/SAVPF a=tcap: 3 RTP/SAVP RTP/SAVPF
a=pcfg:1 t=4|3 a=1 a=pcfg:1 t=4|3 a=1
a=pcfg:8 t=1|2 a=pcfg:8 t=1|2
We have two potential configurations listed here. The first one (and We have two potential configuration attributes listed here. The first
most preferred, since its configuration number is "1") indicates that one (and most preferred, since its configuration number is "1")
either of the profiles RTP/SAVPF or RTP/SAVP (specified by the indicates that either of the profiles RTP/SAVPF or RTP/SAVP
transport protocol capability numbers 4 and 3) can be supported with (specified by the transport protocol capability numbers 4 and 3) can
attribute capability 1 (the "crypto" attribute); RTP/SAVPF is be supported with attribute capability 1 (the "crypto" attribute);
preferred over RTP/SAVP since its capability number (4) is listed RTP/SAVPF is preferred over RTP/SAVP since its capability number (4)
first in the preferred potential configuration. The second potential is listed first in the preferred potential configuration. The second
configuration indicates that the RTP/AVPF of RTP/AVP profile can be potential configuration attribute indicates that the RTP/AVPF or
used, with RTP/AVPF being the preferred one. This non secure RTP RTP/AVP profile can be used, with RTP/AVPF being the preferred one.
alternative is the less preferred one since its configuration number This non secure RTP alternative is the less preferred one since its
is "8". configuration number is "8".
3.4.2. Actual Configuration Attribute 3.5.2. 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 an actual configuration configurations from an offer SDP was selected and used as an actual
in an answer SDP. This is done by reference to the configuration configuration in an answer SDP. This is done by including the
number and the attribute capabilities and transport protocol configuration number and the configuration lists from the offer that
capabilities from the offer that were actually used by the answerer were actually selected and used by the answerer in his offer/answer
in his offer/answer procedure. If extension capabilities were used, procedure as follows:
those will be included by reference as well. Note that the
configuration number and all capability numbers used are those from o A selected potential attribute configuration MUST include the
the offer; not the answer. delete-attributes and the selected alternative att-cap-inst-list
(i.e. containing both operators and capability numbers from the
potential configuration). If delete-attributes were not included
in the potential configuration, they will of course not be present
here either.
o A selected potential transport protocol configuration MUST include
the selected transport protocol capability number.
o A selected potential extension configuration MUST include the
selected extension configuration parameters as specified for that
particular extension.
Note that the selected configuration number and all selected
capability numbers used in the actual configuration attribute refer
to those from the offer; not the answer.
The answer may for example include capabilities as well. The actual
configuration attribute does not refer to any of those.
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: <sel-cfg-list>
The "acfg" attribute adheres to the RFC 4566 "attribute" production, The "acfg" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows: with an att-value defined as follows:
att-value = *WSP config-number 1*WSP act-cfg-list att-value = config-number 1*WSP sel-cfg-list
;config-number defined in Section 3.4.1. ;config-number defined in Section 3.5.1.
act-cfg-list = capability *(1*WSP capability) sel-cfg-list = sel-cfg *(1*WSP sel-cfg)
capability = act-attribute-parameter-config / sel-cfg = sel-attribute-config /
act-transport-protocol-config / sel-transport-protocol-config /
act-extension-config sel-extension-config
act-attribute-parameter-config = sel-attribute-config =
"a=" acap-cap-list ; defined in Section 3.4.1. "a=" [delete-attributes ":"] att-cap-inst-list
; defined in Section 3.5.1.
act-transport-protocol-config = sel-transport-protocol-config =
"t=" trpr-cap-num ; defined in Section 3.4.1. "t=" trpr-cap-num ; defined in Section 3.5.1.
act-extension-config = sel-extension-config =
ext-cap-name "=" ext-cap-list ; defined in Section 3.4.1. ext-cap-name "=" 1*VCHAR ; defined in Section 3.5.1.
Note that white-space is permitted before the config-number. The Note that white-space is not permitted before the config-number.
actual configuration ("a=acfg") attribute can be provided at the
The actual configuration ("a=acfg") attribute can be provided at the
media-level only. There MUST NOT be more than one occurrence of an media-level only. There MUST NOT be more than one occurrence of an
actual configuration attribute within a given media description. 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 192.0.2.2
s= s=
c=IN IP4 128.96.41.2 c=IN IP4 192.0.2.2
t=0 0 t=0 0
m=audio 4568 RTP/SAVPF 0 m=audio 54568 RTP/SAVPF 0
a=creq: 0 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32
a=acfg:1 t=4 a=1 a=acfg:1 t=4 a=1
It indicates that the answerer used an offer consisting of potential It indicates that the answerer used an offer consisting of potential
configuration number 1 with transport protocol capability 4 from the configuration number 1 with transport protocol capability 4 from the
offer (RTP/SAVPF) and attribute capability 1 (the "crypto" offer (RTP/SAVPF) and attribute capability 1 (the "crypto"
attribute). attribute). The answerer includes his own "crypto" attribute as well.
3.5. 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 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).
The procedures defined in the following subsections apply to both The procedures defined in the following subsections apply to both
unicast and multicast streams. unicast and multicast streams.
3.5.1. Generating the Initial Offer 3.6.1. 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 required extensions attribute
("a=creq") as defined in Section 3.2.2. that contains the option
tag "cap-v0".
This attribute SHOULD be provided at the session-level (if there
is only a single media stream, then it may make sense to include
it at the media-level). If one or more additional option tags are
required to be supported for the entire session description, then
option tags for those extensions MUST be included in the session-
level "creq" attribute. For each media description that requires
one or more capability negotiation extensions not listed at the
session-level, a "creq" attribute containing the required
extensions for that media description MUST be included within the
media description as well (in accordance with Section 3.2.2. ).
o An attribute capability attribute ("a=acap") as defined in Section o An attribute capability attribute ("a=acap") as defined in Section
3.3.1. for each attribute name and associated value that needs to 3.4.1. for each attribute name and associated value (if any) that
be indicated as a capability in the offer. needs to be indicated as a capability in the offer.
Session-level attributes and associated values MUST be provided in Session-level attributes and associated values MUST be provided in
attribute capabilities at the session-level only, whereas media- attribute capabilities at the session-level only, whereas media-
level attributes and associated values MUST be provided in level attributes and associated values MUST be provided in
attribute capabilities at the media-level only. Attributes that attribute capabilities at the media-level only. Attributes that
can be provided at either the session- or media-level can be can be provided at either the session- or media-level can be
represented as attribute capabilities at either the session- or represented as attribute capabilities at either the session- or
media-level. If there is not a need to indicate any attributes as media-level. Attribute capabilities for '<type>=<value>'
attribute capabilities, then there will not be any "a=acap" attributes can include an attribute name only; such attribute
attributes either. capabilities MUST NOT be referenced by a potential configuration,
unless the DELETE operator is used for it. If there is not a need
to indicate any attributes as attribute capabilities, then there
will not be any "a=acap" attributes either.
o One or more a transport protocol capability attributes ("a=tcap") o One or more a transport protocol capability attributes ("a=tcap")
as defined in Section 3.3.2. with values for each transport as defined in Section 3.4.2. with values for each transport
protocol that needs to be indicated as a capability in the offer. protocol that needs to be indicated as a capability in the offer.
Transport protocol capabilities that apply to multiple media Transport protocol capabilities that apply to multiple media
descriptions SHOULD be provided at the session-level whereas descriptions SHOULD be provided at the session-level whereas
transport protocol capabilities that apply to a specific media transport protocol capabilities that apply to a specific media
description ("m=" line) only, SHOULD be provided within that description ("m=" line) only, SHOULD be provided within that
particular media description. If there is not a need to indicate particular media description. If there is not a need to indicate
any transport protocols as transport protocol capabilities, then any transport protocols as transport protocol capabilities, then
there will not be any "a=tcap" attributes either. there will not be any "a=tcap" attributes either.
o One or more extension capability attributes (as outlined in o One or more extension capability attributes (as outlined in
Section 3.3.3. ) for each extension capability that is referenced Section 3.4.3. ) for each extension capability that is referenced
by a potential configuration. by a potential configuration.
o One or more potential configuration attributes ("a=pcfg") as o One or more potential configuration attributes ("a=pcfg") as
defined in Section 3.4.1. within each media description where defined in Section 3.5.1. within each media description where
alternative potential configurations are to be negotiated. Each alternative potential configurations are to be negotiated. Each
potential configuration attribute MUST adhere to the rules potential configuration attribute MUST adhere to the rules
provided in Section 3.4.1. and the additional rules provided provided in Section 3.5.1. and the additional rules provided
below. below.
If the offerer requires support for more or extensions (besides the
base protocol defined here), then the offerer MUST include one or
more "a=creq" attribute as follows:
o If one or more capability negotiation extensions are required to
be supported for the entire session description, then option tags
for those extensions MUST be included in a single session-level
"creq" attribute.
o For each media description that requires one or more capability
negotiation extensions not listed at the session-level, a single
"creq" attribute containing all the required extensions for that
media description MUST be included within the media description
(in accordance with Section 3.3.2. ).
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
("a=csup") as defined in Section 3.2.2. if the offerer supports ("a=csup") as defined in Section 3.3.2. if the offerer supports
one or more capability negotiation extensions not included in a one or more capability negotiation extensions not included in a
corresponding "a=creq" attribute (i.e. at the session-level or in corresponding "a=creq" attribute (i.e. at the session-level or in
the same media description). Option tags provided in "a=csup" the same media description). Option tags provided in "a=csup"
attributes at the session-level indicate extensions supported for attributes at the session-level indicate extensions supported for
the entire session description whereas option tags provided in the entire session description whereas option tags provided in
"a=csup" attributes in a media description indicate extensions "a=csup" attributes in a media description indicate extensions
supported for that particular media description only. supported for that particular media description only.
Capabilities provided in an offer merely indicate what the offerer is Capabilities provided in an offer merely indicate what the offerer is
capable of doing. They do not constitute a commitment or even an capable of doing. They do not constitute a commitment or even an
indication to actually use them. Each potential configuration however indication to actually use them. Each potential configuration however
constitutes an alternative offer that the offerer would like to use. constitutes an alternative offer that the offerer would like to use.
The potential configurations may be used by the answerer to negotiate The potential configurations may be used by the answerer to negotiate
and establish the session. and establish the session.
The offerer MUST include one or more potential configuration The offerer MUST include one or more potential configuration
attributes ("a=pcfg") within each media description where the offerer attributes ("a=pcfg") within each media description where the offerer
wants to provide alternative offers (in the form of potential wants to provide alternative offers (in the form of potential
configurations). Each potential configuration attribute in a given configurations). Each potential configuration attribute in a given
media description MUST contain a unique configuration number and one media description MUST contain a unique configuration number and one
or more potential configuration parameters, as described in Section or more potential configuration lists, as described in Section 3.5.1.
3.4.1. Each potential configuration parameter MUST refer to a Each potential configuration list MUST refer to capabilities that are
capability that is provided either at the session-level or within provided either at the session-level or within that particular media
that particular media description; otherwise, the potential description; otherwise, the potential configuration is considered
configuration is considered invalid. invalid.
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]). Note that the actual configuration is by defined by [RFC3264]). Note that the actual configuration is by
definition the least-preferred configuration, and hence the answerer definition the least-preferred configuration, and hence the answerer
will seek to negotiate use of one of the potential configurations will seek to negotiate use of one of the potential configurations
instead. If the offerer wishes a different preference for the actual instead. If the offerer wishes a different preference for the actual
configuration, the offerer MUST include a corresponding potential configuration, the offerer MUST include a corresponding potential
configuration with the relevant configuration number (which indicates configuration with the relevant configuration number (which indicates
the relative preference between potential configurations); this the relative preference between potential configurations); this
corresponding potential configuration should simply duplicate the corresponding potential configuration should simply duplicate the
actual configuration. actual configuration.
Per [RFC3264], once the offerer generates the offer, he must be Per [RFC3264], once the offerer generates the offer, he must be
prepared to receive incoming media in accordance with that offer. prepared to receive incoming media in accordance with that offer.
That rule applies here as well, but for the actual configurations That rule applies here as well, but for the actual configurations
provided in the offer only: Media received by the offerer according provided in the offer only: Media received by the offerer according
to one of the potential configurations MAY be discarded, until the to one of the potential configurations MAY be discarded, until the
offerer receives an answer indicating what the actual configuration offerer receives an answer indicating what the actual selected
is. Once that answer is received, incoming media MUST be processed in configuration is. Once that answer is received, incoming media MUST
accordance with the actual configuration indicated and the answer be processed in accordance with the actual selected configuration
received (provided the offer/answer exchange completed successfully). indicated and the answer received (provided the offer/answer exchange
completed successfully).
3.5.2. Generating the Answer 3.6.2. Generating the Answer
When receiving an offer, the answerer MUST check for the presence of When receiving an offer, the answerer MUST check for the presence of
a required capability negotiation extension attribute ("a=creq") a required capability negotiation extension attribute ("a=creq")
provided at the session level and containing the option tag "cap-v0". provided at the session level. If one is found, then capability
If one is found, then capability negotiation MUST be performed for negotiation MUST be performed. If none is found, then the answerer
each media description that contains a potential configuration MUST check each offered media description for the presence of a
attribute ("a=pcfg"). If none is found, then the answerer MUST check required capability negotiation extension attribute ("a=creq") and
each offered media description for a required capability negotiation one or more potential configuration attributes ("a=pcfg"). Capability
extension attribute ("a=creq") containing the option tag "cap-v0" and negotiation MUST be performed for each media description where either
one or more potential configuration attributes. Capability of those is present in accordance with the procedures described
negotiation MUST be performed for each such media description in below.
accordance with the procedures described below.
The answerer MUST first ensure that it supports any additional The answerer MUST first ensure that it supports any required
required capability negotiation extensions: capability negotiation extensions:
o If a session-level "creq" attribute is provided, and it contains o If a session-level "creq" attribute is provided, and it contains
an option-tag that the answerer does not support, then the an option-tag that the answerer does not support, then the
answerer MUST NOT use any of the potential configuration answerer MUST NOT use any of the potential configuration
attributes provided for any of the media descriptions. Instead, attributes provided for any of the media descriptions. Instead,
the normal offer/answer procedures MUST continue as per [RFC3264]. the normal offer/answer procedures MUST continue as per [RFC3264].
Furthermore, the answerer MUST include a session-level supported Furthermore, the answerer MUST include a session-level supported
capability negotiation extensions attribute ("a=csup") with option capability negotiation extensions attribute ("a=csup") with option
tags for the capability negotiation extensions supported by the tags for the capability negotiation extensions supported by the
answerer. answerer.
skipping to change at page 23, line 44 skipping to change at page 28, line 16
supported, the answerer now proceeds as follows. supported, the answerer now proceeds as follows.
For each media description where capability negotiation is to be For each media description where capability negotiation is to be
performed (i.e. all required capability negotiation extensions are performed (i.e. all required capability negotiation extensions are
supported and at least one valid potential configuration attribute is supported and at least one valid potential configuration attribute is
present), the answerer MUST attempt to perform capability negotiation present), the answerer MUST attempt to perform capability negotiation
by using the most preferred potential configuration that is valid. A by using the most preferred potential configuration that is valid. A
potential configuration is valid if: potential configuration is valid if:
1. It is in accordance with the syntax and semantics provided in 1. It is in accordance with the syntax and semantics provided in
Section 3.4.1. Section 3.5.1.
2. It contains a configuration number that is unique within that 2. It contains a configuration number that is unique within that
media description. media description.
3. All attribute capabilities referenced by the potential 3. All attribute capabilities referenced by the potential
configuration are valid themselves (as defined in Section 3.3.1. ) configuration are valid themselves (as defined in Section 3.4.1. )
and each of them are furthermore provided either at the session- and each of them is furthermore provided either at the session-
level or within this particular media description. level or within this particular media description.
4. All transport protocol capabilities referenced by the potential 4. All transport protocol capabilities referenced by the potential
configuration are valid themselves (as defined in Section 3.3.2. ) configuration are valid themselves (as defined in Section 3.4.2. )
and each of them are furthermore provided either at the session- and each of them is furthermore provided either at the session-
level or within this particular media description. level or within this particular media description.
5. All extension capabilities referenced by the potential 5. All extension capabilities referenced by the potential
configuration and supported by the answerer are valid themselves configuration and supported by the answerer are valid themselves
(as defined by that particular extension) and each of them are (as defined by that particular extension) and each of them are
furthermore provided either at the session-level or within this furthermore provided either at the session-level or within this
particular media description. Unknown or unsupported extension particular media description. Unknown or unsupported extension
capabilities MUST be ignored. capabilities MUST be ignored.
The most preferred valid potential configuration in a media The most preferred valid potential configuration in a media
skipping to change at page 24, line 34 skipping to change at page 29, line 5
configuration number. The answerer MUST now process the offer for configuration number. The answerer MUST now process the offer for
that media stream based on the most preferred valid potential that media stream based on the most preferred valid potential
configuration. Conceptually, this entails the answerer constructing configuration. Conceptually, this entails the answerer constructing
an (internal) offer that consists of the offer SDP, with the an (internal) offer that consists of the offer SDP, with the
following changes: following changes:
o If a transport protocol capability is included in the potential o If a transport protocol capability is included in the potential
configuration, then it replaces the transport protocol provided in configuration, then it replaces the transport protocol provided in
the "m=" line for that media description. the "m=" line for that media description.
o If a session-level attribute capability is included, then it is o If attribute capabilities are present with a delete-attributes
added to the list of session-level attributes for the session indication, then session-level attributes and/or media-level
description. The resulting SDP MUST have all such added session- attributes for this media description MUST be deleted in
level attributes listed before the session-level attributes that accordance with the delete-attributes provided per the procedures
were initially present in the SDP. Furthermore, the added session- in Section 3.5.1.
level attributes MUST be added in the order they were provided in
o If a session-level attribute capability is included, then the
operation is as follows (see also Section 3.5.1. ):
a. If the attribute capability number is not prefixed with an
attribute capability operator, then the attribute (and its
associated value, if any) MUST be added to the resulting SDP.
All such added session-level attributes MUST be listed before
the session-level attributes that were initially present in
the SDP. Furthermore, the added session-level attributes MUST
be added in the order they were provided in the potential
configuration.
b. If the attribute capability number is prefixed with a DELETE
operator in the potential configuration, then all session-
level occurrences of an attribute with the same attribute-name
(attribute values are ignored) as the attribute contained
inside that attribute capability MUST be deleted from the
original SDP as detailed in Section 3.5.1. .
c. If the attribute capability number is prefixed with a REPLACE
operator in the potential configuration, then the above DELETE
operation MUST first be performed, and the attribute MUST then
be added to the resulting SDP in the same order as specified
above in a).
o If a media-level attribute capability is included, then the
operation is as follows (see also Section 3.5.1. ):
a. If the attribute capability number is not prefixed with an
attribute capability operator, then the attribute (and its
associated value, if any) MUST be added to the resulting SDP
within the media description in question. All such added
media-level attributes MUST be listed before the media-level
attributes that were initially present in the SDP in the media
description in question. Furthermore, the added media-level
attributes MUST be added in the order they were provided in
the potential configuration. the potential configuration.
o If a media-level attribute capability is included, then it is b. If the attribute capability number is prefixed with a DELETE
added to the list of media-level attributes for that particular operator in the potential configuration, then all media-level
media description. The resulting SDP MUST have all such added occurrences of an attribute with the same attribute-name
media-level attributes listed before the media-level attributes (attribute values are ignored) as the attribute contained
that were initially present in the SDP for that media description. inside that attribute capability MUST be deleted from the
Furthermore, the added media-level attributes MUST be added in the original SDP in the media description in question as detailed
order they were provided in the potential configuration in Section 3.5.1. .
c. If the attribute capability number is prefixed with a REPLACE
operator in the potential configuration, then the above DELETE
operation MUST first be performed, and the attribute MUST then
be added to the resulting SDP in the same order as specified
above in a).
o If a supported extension capability is included, then it is o If a supported extension capability is included, then it is
processed in accordance with the rules provided for that processed in accordance with the rules provided for that
particular extension capability. particular extension capability.
Note that whereas a transport protocol from the potential Note that whereas a transport protocol from the potential
configuration replaces the transport protocol in the actual configuration replaces the transport protocol in the actual
configuration, an attribute capability from the potential configuration, an attribute capability from the potential
configuration is instead added to the actual configuration. In some configuration is instead added to the actual configuration by
cases, this may result in having one or more meaningless attributes default. In some cases, this can result in having one or more
from the actual configuration; such meaningless attributes SHOULD meaningless attributes in the resulting SDP, or worse, ambiguous or
simply be ignored. potentially even illegal attributes. The delete-attributes for the
session and/or media level attributes as well as the DELETE and
REPLACE attribute capability operators MUST be used to avoid such
scenarios. Nevertheless, it is RECOMMENDED that implementations
ignore meaningless attributes that may result from potential
configurations.
For example, if the actual configuration was using Secure RTP and For example, if the actual configuration was using Secure RTP and
included an "a=crypto" attribute for the SRTP keying material, included an "a=crypto" attribute for the SRTP keying material, then
then use of a potential configuration that uses plain RTP would use of a potential configuration that uses plain RTP would make the
make the "crypto" attribute meaningless. Rather than requiring "crypto" attribute meaningless. The answerer may or may not ignore
the actual configuration attributes to be present as attribute such a meaningless attribute. The offerer can here ensure correct
capabilities as well (which would increase the message size) and operation by using the above operators to actually delete the
then have the potential configuration completely replace the crypto attribute.
actual configuration, we instead make the use of attribute
capabilities additive to the session description. The answerer
may want to note (internally) which attributes came from the
potential configuration and which came from the actual
configuration in order to better resolve such issues.
Please refer to Section 3.5.2.1. for examples of how the answerer may Please refer to Section 3.6.2.1. for examples of how the answerer may
conceptually "see" the resulting offered alternative potential conceptually "see" the resulting offered alternative potential
configurations. configurations.
If the answerer is not able to support the most preferred valid If the answerer is not able to support the most preferred valid
potential configuration for the media description, the answerer MUST potential configuration for the media description, the answerer MUST
proceed to the second-most preferred valid potential configuration proceed to the second-most preferred valid potential configuration
for the media description, etc. If the answerer is not able to for the media description, etc. If the answerer is not able to
support any of the valid potential configurations, the answerer MUST support any of the valid potential configurations, the answerer MUST
process the offer per normal offer/answer rules, i.e. the actual process the offer per normal offer/answer rules, i.e. the actual
configuration provided will be used as the least preferred configuration provided will be used as the least preferred
skipping to change at page 26, line 23 skipping to change at page 31, line 37
extensions supported across media streams. Also, if the answerer extensions supported across media streams. Also, if the answerer
supports one or more capability negotiation extensions for particular supports one or more capability negotiation extensions for particular
media descriptions only, then a supported capability negotiation media descriptions only, then a supported capability negotiation
attribute with those option-tags SHOULD be included within each attribute with those option-tags SHOULD be included within each
relevant media description. relevant media description.
The offerer's originally provided actual configuration is contained The offerer's originally provided actual configuration is contained
in the media description's "m=" line (and associated parameters). The in the media description's "m=" line (and associated parameters). The
answerer can send media to the offerer in accordance with that actual answerer can send media to the offerer in accordance with that actual
configuration as soon as it receives the offer, however it MUST NOT configuration as soon as it receives the offer, however it MUST NOT
send media based on that actual configuration if it chooses an send media based on that actual configuration if it selects an
alternative potential configuration. If the answerer chooses one of alternative potential configuration. If the answerer selects one of
the potential configurations, then the answerer MAY start to send the potential configurations, then the answerer MAY start to send
media to the offerer in accordance with the selected potential media to the offerer in accordance with the selected potential
configuration, however the offerer MAY discard such media until the configuration, however the offerer MAY discard such media until the
offerer receives the answer. offerer receives the answer.
3.5.2.1. Example Views of Potential Configurations 3.6.2.1. Example Views of Potential Configurations
The following examples illustrate how the answerer may conceptually The following examples illustrate how the answerer may conceptually
"see" a potential configuration. Consider the following offered SDP: "see" a potential configuration. Consider the following offered SDP:
v=0 v=0
o=alice 2891092738 2891092738 IN IP4 lost.example.com o=alice 2891092738 2891092738 IN IP4 lost.example.com
s=Secret discussion s=
t=0 0 t=0 0
c=IN IP4 lost.example.com c=IN IP4 lost.example.com
a=tool:foo a=tool:foo
a=creq: cap-v0 a=acap:1 a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
a=acap:1 key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
a=tcap:1 RTP/SAVP RTP/AVP a=tcap:1 RTP/SAVP RTP/AVP
m=audio 39000 RTP/AVP 98 m=audio 59000 RTP/AVP 98
a=rtpmap:98 AMR/8000 a=rtpmap:98 AMR/8000
a=acap:2 a=crypto:1 AES_CM_128_HMAC_SHA1_32 a=acap:2 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=pcfg:1 t=1 a=1|2 a=pcfg:1 t=1 a=1|2
m=video 42000 RTP/AVP 31 m=video 52000 RTP/AVP 31
a=rtpmap:31 H261/90000 a=rtpmap:31 H261/90000
a=acap:3 a=crypto:1 AES_CM_128_HMAC_SHA1_80 a=acap:3 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32 inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=pcfg:1 t=1 a=1|3 a=pcfg:1 t=1 a=1|3
This particular SDP offers an audio stream and a video stream, each This particular SDP offers an audio stream and a video stream, each
of which can either use plain RTP (actual configuration) or secure of which can either use plain RTP (actual configuration) or secure
RTP (potential configuration). Furthermore, two different keying RTP (potential configuration). Furthermore, two different keying
mechanisms are offered, namely session-level Key Management mechanisms are offered, namely session-level Key Management
Extensions using MIKEY (attribute capability 1) and media-level SDP Extensions using MIKEY (attribute capability 1) and media-level SDP
Security Descriptions (attribute capabilities 2 and 3). There are Security Descriptions (attribute capabilities 2 and 3). There are
several alternative configurations here, however, below we show the several potential configurations here, however, below we show the one
one the answerer "sees" when using potential configuration 1 for both the answerer "sees" when using potential configuration 1 for both
audio and video, and furthermore using attribute capability 1 (MIKEY) audio and video, and furthermore using attribute capability 1 (MIKEY)
for both (we have removed all the capability negotiation attributes for both (we have removed all the capability negotiation attributes
for clarity): for clarity):
v=0 v=0
o=alice 2891092738 2891092738 IN IP4 lost.example.com o=alice 2891092738 2891092738 IN IP4 lost.example.com
s=Secret discussion s=
t=0 0 t=0 0
c=IN IP4 lost.example.com c=IN IP4 lost.example.com
a=tool:foo a=tool:foo
a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO... a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
m=audio 39000 RTP/SAVP 98 m=audio 59000 RTP/SAVP 98
a=rtpmap:98 AMR/8000 a=rtpmap:98 AMR/8000
m=video 42000 RTP/SAVP 31 m=video 52000 RTP/SAVP 31
a=rtpmap:31 H261/90000 a=rtpmap:31 H261/90000
Note that the transport protocol in the media descriptions indicate Note that the transport protocol in the media descriptions indicate
use of secure RTP. use of secure RTP.
Below, we show the offer the answerer "sees" when using potential Below, we show the offer the answerer "sees" when using potential
configuration 1 for both audio and video and furthermore using configuration 1 for both audio and video and furthermore using
attribute capability 2 and 3 respectively (SDP security descriptions) attribute capability 2 and 3 respectively (SDP security descriptions)
for the audio and media stream - note the order in which the for the audio and media stream - note the order in which the
resulting attributes are provided: resulting attributes are provided:
v=0 v=0
o=alice 2891092738 2891092738 IN IP4 lost.example.com o=alice 2891092738 2891092738 IN IP4 lost.example.com
s=Secret discussion s=
t=0 0 t=0 0
c=IN IP4 lost.example.com c=IN IP4 lost.example.com
a=tool:foo a=tool:foo
m=audio 39000 RTP/SAVP 98 m=audio 59000 RTP/SAVP 98
a=crypto:1 AES_CM_128_HMAC_SHA1_32 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=rtpmap:98 AMR/8000 a=rtpmap:98 AMR/8000
m=video 42000 RTP/SAVP 31 a=crypto:1 AES_CM_128_HMAC_SHA1_80 m=video 52000 RTP/SAVP 31
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32 inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=rtpmap:31 H261/90000 a=rtpmap:31 H261/90000
Again, note that the transport protocol in the media descriptions Again, note that the transport protocol in the media descriptions
indicate use of secure RTP. indicate use of secure RTP.
And finally, we show the offer the answerer "sees" when using And finally, we show the offer the answerer "sees" when using
potential configuration 1 with attribute capability 1 (MIKEY) for the potential configuration 1 with attribute capability 1 (MIKEY) for the
audio stream, and potential configuration 1 with attribute capability audio stream, and potential configuration 1 with attribute capability
3 (SDP security descriptions) for the video stream: 3 (SDP security descriptions) for the video stream:
v=0 v=0
o=alice 2891092738 2891092738 IN IP4 lost.example.com o=alice 2891092738 2891092738 IN IP4 lost.example.com
s=Secret discussion s=
t=0 0 t=0 0
c=IN IP4 lost.example.com c=IN IP4 lost.example.com
a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO... a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
a=tool:foo a=tool:foo
m=audio 39000 RTP/SAVP 98 m=audio 59000 RTP/SAVP 98
a=rtpmap:98 AMR/8000 a=rtpmap:98 AMR/8000
m=video 42000 RTP/SAVP 31 a=crypto:1 AES_CM_128_HMAC_SHA1_80 m=video 52000 RTP/SAVP 31
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32 inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=rtpmap:31 H261/90000 a=rtpmap:31 H261/90000
3.5.3. Offerer Processing of the Answer 3.6.3. Offerer Processing of the Answer
When the offerer attempted to use SDP Capability Negotiation in the When the offerer attempted to use SDP Capability Negotiation in the
offer, the offerer MUST examine the answer for actual use of offer, the offerer MUST examine the answer for actual use of
capability negotiation. capability negotiation.
For each media description where the offerer included a potential For each media description where the offerer included a potential
configuration attribute, the offerer MUST first examine the media configuration attribute, the offerer MUST first examine the media
description for the presence of an actual configuration attribute description for the presence of an actual configuration attribute
("a=acfg"). If an actual configuration attribute is not present in a ("a=acfg"). If an actual configuration attribute is not present in a
media description, then the offerer MUST process the answer SDP for media description, then the offerer MUST process the answer SDP for
that media stream per the normal offer/answer rules defined in that media stream per the normal offer/answer rules defined in
[RFC3264]. However, if one is found, then the offerer MUST instead [RFC3264]. However, if one is found, then the offerer MUST instead
process the answer as follows: process the answer as follows:
o The actual configuration attribute specifies which of the o The actual configuration attribute specifies which of the
potential configurations were used by the answerer to generate the potential configurations were used by the answerer to generate the
answer. This includes all the capabilities from the potential answer. This includes all the capabilities from the potential
configuration offered, i.e. the attribute capabilities ("a=acap"), configuration offered, i.e. the attribute capabilities and
transport protocol capabilities ("a=tcap"), and any extension associated delete-attributes and operators, transport protocol
capability parameters included. capabilities, and any extension capability parameters included.
o The offerer MUST now process the answer in accordance with the o The offerer MUST now process the answer in accordance with the
rules in [RFC3264], except that it must be done as if the offer rules in [RFC3264], except that it must be done as if the offer
had contained the potential configuration as the actual had contained the selected potential configuration as the actual
configuration in the media description ("m=" line) and relevant configuration in the media description ("m=" line) and relevant
attributes in the offer. attributes in the offer.
If the offer/answer exchange was successful, and if the answerer If the offer/answer exchange was successful, and if the answerer
selected one of the potential configurations from the offer as the selected one of the potential configurations from the offer as the
actual configuration, then the offerer SHOULD perform another actual configuration, then the offerer MAY perform another
offer/answer exchange: The new offer should contain the selected offer/answer exchange: The new offer should contain the selected
potential configuration as the actual configuration, i.e. with the potential configuration as the actual configuration, i.e. with the
actual configuration used in the "m=" line and any other relevant actual configuration used in the "m=" line and any other relevant
attributes. This second offer/answer exchange will not modify the attributes. This second offer/answer exchange will not modify the
session in any way, however it will help intermediaries that look at session in any way, however it will help intermediaries that look at
the SDP, but do not understand or support the capability negotiation the SDP, but do not understand or support the capability negotiation
extensions, to understand the details of the media stream(s) that extensions, to understand the details of the media stream(s) that
were actually negotiated. were actually negotiated. If it is known or suspected that one or
more such intermediaries exist, then this second offer/answer SHOULD
be performed (this is already done when using Interactive
Connectivity Establishment [ICE]). Note that, per normal offer/answer
rules, the second offer/answer exchange still needs to update the
version number in the "o=" line ((<sess-version> in [RFC4566]).
Attribute lines carrying keying material SHOULD repeat the keys from
the previous offer, unless re-keying is necessary, e.g. due to a
previously forked SIP INVITE request.
3.5.4. Modifying the Session 3.6.4. Modifying the Session
Capabilities and potential configurations may be included in Capabilities and potential configurations may be included in
subsequent offers as defined in [RFC3264, Section 8]. The procedure subsequent offers as defined in [RFC3264], Section 8. The procedure
for doing so is similar to that described above with the answer for doing so is similar to that described above with the answer
including an indication of the actual configuration used by the including an indication of the actual selected configuration used by
answerer. the answerer.
If the answer indicates use of a potential configuration from the If the answer indicates use of a potential configuration from the
offer, then a second offer/answer exchange using that potential offer, then the guidelines provided in Section 3.6.3. for doing a
configuration as the actual configuration SHOULD be performed. second offer/answer exchange using that potential configuration as
the actual configuration apply.
3.6. Interactions with ICE 3.7. Interactions with ICE
Interactive Connectivity Establishment (ICE) [ICE] provides a Interactive Connectivity Establishment (ICE) [ICE] provides a
mechanism for verifying connectivity between two endpoints by sending mechanism for verifying connectivity between two endpoints by sending
STUN messages directly between the media endpoints. The basic ICE STUN messages directly between the media endpoints. The basic ICE
specification [ICE] is defined to support UDP-based connectivity specification [ICE] is defined to support UDP-based connectivity
only, however it allows for extensions to support other transport only, however it allows for extensions to support other transport
protocols, such as TCP, which is being specified in [ICETCP]. ICE protocols, such as TCP, which is being specified in [ICETCP]. ICE
defines a new "a=candidate" attribute, which, among other things, defines a new "a=candidate" attribute, which, among other things,
indicates the possible transport protocol(s) to use and then indicates the possible transport protocol(s) to use and then
associates a priority with each of them. The most preferred transport associates a priority with each of them. The most preferred transport
skipping to change at page 30, line 42 skipping to change at page 36, line 18
answerer select something it supports and then use of trial and answerer select something it supports and then use of trial and
error). error).
Scenario 2 does not require a need to support or use ICE. Instead, we Scenario 2 does not require a need to support or use ICE. Instead, we
simply use transport protocol capabilities and potential simply use transport protocol capabilities and potential
configuration attributes to indicate the desired outcome. configuration attributes to indicate the desired outcome.
The scenarios may be combined, e.g. by offering potential The scenarios may be combined, e.g. by offering potential
configuration alternatives where some of them can support one configuration alternatives where some of them can support one
transport protocol only (e.g. UDP), whereas others can support transport protocol only (e.g. UDP), whereas others can support
multiple transport protocols (e.g. UDP or TCP). In that case, the ICE multiple transport protocols (e.g. UDP or TCP). In that case, there
candidate attributes should be defined as attribute capabilities and is a need for tight control over the ICE candidates that will
the relevant ones should then be included in the proper potential actually be used for a particular configuration, yet the actual
configuration may want to use all of them. In that case, the ICE
candidate attributes can be defined as attribute capabilities and the
relevant ones should then be included in the proper potential
configurations (for example candidate attributes for UDP only for configurations (for example candidate attributes for UDP only for
potential configurations that are restricted to UDP, whereas there potential configurations that are restricted to UDP, whereas there
could be candidate attributes for UDP, TCP, and TCP/TLS for potential could be candidate attributes for UDP, TCP, and TCP/TLS for potential
configurations that can use all three). configurations that can use all three). Furthermore, use of the
delete-attributes, as well as the DELETE and REPLACE operators on
[EDITOR'S NOTE: Not clear this is sufficient. It seems ICE is attribute capabilities in a potential configuration can be used to
essentially providing potential transport configurations when it ensure that ICE will not end up using a transport protocol that is
comes to use of UDP, TCP, or TLS as transport protocols. If that is not desired.
the case, then we will have a hard time expressing constraints on
some transport protocols in a potential configuration, but not the
actual configuration, at least with the current additive attribute
capability rules.]
3.7. Processing Media before Answer 3.8. Processing Media before Answer
The offer/answer model requires an offerer to be able to receive The offer/answer model requires an offerer to be able to receive
media in accordance with the offer prior to receiving the answer. media in accordance with the offer prior to receiving the answer.
This property is retained with the SDP capability negotiation This property is retained with the SDP capability negotiation
extensions defined here, but only when the actual configuration is extensions defined here, but only when the actual configuration is
selected by the answerer. If a potential configuration is chosen, it selected by the answerer. If a potential configuration is chosen, it
is permissible for the offerer to not process any media received is permissible for the offerer to not process any media received
before the answer is received. This however may lead to clipping. before the answer is received. This however may lead to clipping.
In the case of SIP, this issue could be solved easily by defining a In the case of SIP, this issue could be solved easily by defining a
precondition [RFC3312] for capability negotiation, however precondition [RFC3312] for capability negotiation, however
preconditions are viewed as complicated to implement and they add to preconditions are viewed as complicated to implement and they add to
overall session establishment delay by requiring an extra overall session establishment delay by requiring an extra
offer/answer exchange. An alternative is therefore desirable. offer/answer exchange. An alternative is therefore desirable.
The SDP capability negotiation framework does not define such an The SDP capability negotiation framework does not define such an
alternative, however extensions may do so. For example, one technique alternative, however extensions may do so. For example, one technique
proposed for best-effort SRTP in [BESRTP] is to provide different RTP proposed for best-effort SRTP in [BESRTP] is to provide different RTP
payload type mappings for different transport protocols used. The payload type mappings for different transport protocols used, outside
basic SDP capability negotiation framework defined here does not of the actual configuration, while still allowing them to be used by
include the ability to do so, however extensions that enable that may the answerer (exchange of keying material is still needed). The basic
be defined. SDP capability negotiation framework defined here does not include
the ability to do so, however extensions that enable that may be
defined.
3.8. Attribute Capabilities Using rtpmap or fmtp 3.9. Considerations for Specific Attribute Capabilities
3.9.1. The rtpmap and fmtp Attributes
The core SDP Capability Negotiation framework defines transport The core SDP Capability Negotiation framework defines transport
capabilities and attribute capabilities. Media capabilities, which capabilities and attribute capabilities. Media capabilities, which
can be used to describe media formats and their associated can be used to describe media formats and their associated
parameters, are not defined in this document, however the "rtpmap" parameters, are not defined in this document, however the "rtpmap"
and "fmtp" attributes can nevertheless be used as attribute and "fmtp" attributes can nevertheless be used as attribute
capabilities. Using such attribute capabilities in a potential capabilities. Using such attribute capabilities in a potential
configuration requires a bit of care though. configuration requires a bit of care though.
The rtpmap parameter binds an RTP payload type to a media format The rtpmap parameter binds an RTP payload type to a media format
skipping to change at page 32, line 16 skipping to change at page 37, line 40
media formats (e.g. RTP payload types) provided in the "m=" line are media formats (e.g. RTP payload types) provided in the "m=" line are
actually offered; inclusion of rtpmap attributes with other RTP actually offered; inclusion of rtpmap attributes with other RTP
payload types in a potential configuration does not change this fact payload types in a potential configuration does not change this fact
and hence they do not provide any useful information. They may still and hence they do not provide any useful information. They may still
be useful as pure capabilities though (outside a potential be useful as pure capabilities though (outside a potential
configuration). configuration).
It is possible to provide an rtpmap attribute capability with a It is possible to provide an rtpmap attribute capability with a
payload type mapping to a different codec than a corresponding actual payload type mapping to a different codec than a corresponding actual
configuration "rtpmap" attribute for the media description has. Such configuration "rtpmap" attribute for the media description has. Such
practice is permissible as a way of indicating a capability, however, practice is permissible as a way of indicating a capability. If that
if that capability is included in a potential configuration, the core capability is included in a potential configuration, then delete-
SDP capability negotiation framework does not provide a well-defined attributes and/or DELETE/REPLACE attribute capability operators MUST
outcome. Recall that attribute capabilities are additive to the media be used to ensure that there is not multiple rtpmap attributes for
description, and hence the result will be two different payload type the same payload type in a given media description, which would not
mappings for a single RTP payload type. This is not allowed by SDP be allowed by SDP [RFC4566].
[RFC4566].
Similar considerations and rules apply to the "fmtp" attribute. An Similar considerations and rules apply to the "fmtp" attribute. An
fmtp attribute capability for a media format not included in the "m=" fmtp attribute capability for a media format not included in the "m="
line is useless in a potential configuration. An fmtp attribute line is useless in a potential configuration (but may be useful as a
capability in a potential configuration for a media format that capability by itself) . An fmtp attribute capability in a potential
already has an fmtp attribute in the actual configuration may lead to configuration for a media format that already has an fmtp attribute
multiple fmtp format parameters for that media format. This is not in the actual configuration may lead to multiple fmtp format
allowed by SDP [RFC4566]. Still, use of it as a capability only may parameters for that media format and that is not allowed by SDP
be useful. [RFC4566]. The delete-attributes and/or DELETE/REPLACE attribute
capability operators MUST be used to ensure that there is not
multiple fmtp attributes for a given media format in a media
description.
Extensions to the core SDP capability negotiation framework of course Extensions to the core SDP capability negotiation framework of course
may change the above behavior. may change the above behavior.
3.9.2. Direction Attributes
SDP defines the "inactive", "sendonly", "recvonly", and "sendrecv"
direction attributes. The direction attributes can be applied at
either the session-level or the media-level. In either case, it is
possible to define attribute capabilities for these direction
capabilities. Note that if used by a potential configuration, then
the normal offer/answer procedures still apply. For example, if an
offered potential configuration includes the "sendonly" direction
attribute, it is selected as the actual configuration, then the
answer must include a corresponding "recvonly" (or "inactive")
attribute.
4. Examples 4. Examples
In this section, we provide examples showing how to use the SDP In this section, we provide examples showing how to use the SDP
Capability Negotiation. Capability Negotiation.
4.1. Best-Effort Secure RTP 4.1. Best-Effort Secure RTP
The following example illustrates how to use the SDP Capability The following example illustrates how to use the SDP Capability
negotiation extensions to support so-called Best-Effort Secure RTP. negotiation extensions to support so-called Best-Effort Secure RTP.
In that scenario, the offerer supports both RTP and Secure RTP. If In that scenario, the offerer supports both RTP and Secure RTP. If
skipping to change at page 33, line 27 skipping to change at page 39, line 24
|--------------------------------->| |--------------------------------->|
| | | |
| (4) Answer (SRTP) | | (4) Answer (SRTP) |
|<---------------------------------| |<---------------------------------|
| | | |
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 192.0.2.1
s= s=
c=IN IP4 128.96.41.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
m=audio 3456 RTP/AVP 0 18 m=audio 53456 RTP/AVP 0 18
a=creq: cap-v0
a=tcap:1 RTP/SAVP RTP/AVP a=tcap:1 RTP/SAVP RTP/AVP
a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_80 a=acap: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:1 t=1 a=1 a=pcfg:1 t=1 a=1
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. Alice indicates that support for the base protocol PCMU or G.729. The capabilities are provided by the "a=tcap" and
defined here is required by including the "a=creq" attribute "a=acap" attributes. The "tcap" capability indicates that both
containing the value "cap-v0". The capabilities are provided by the Secure RTP and normal RTP are supported. The "acap" attribute
"a=tcap" and "a=acap" attributes. The "tcap" capability indicates provides an attribute capability with a handle of 1. The capability
that both Secure RTP and normal RTP are supported. The "acap" is a "crypto" attribute, which provides the keying material for SRTP
attribute provides an attribute capability with a handle of 1. The using SDP security descriptions [SDES]. The "a=pcfg" attribute
capability is a "crypto" attribute, which provides the keying provides the potential configurations included in the offer by
material for SRTP using SDP security descriptions [SDES]. The reference to the capabilities. A single potential configuration with
"a=pcfg" attribute provides the potential configurations included in a configuration number of "1" is provided. It includes is transport
the offer by reference to the capabilities. A single potential protocol capability 1 (RTP/SAVP, i.e. secure RTP) together with the
configuration with a configuration number of "1" is provided. It attribute capability 1, i.e. the crypto attribute provided.
includes is transport protocol capability 1 (RTP/SAVP, i.e. secure
RTP) together with the attribute capability 1, i.e. the crypto
attribute provided.
Bob receives the SDP offer from Alice. Bob supports SRTP and the SDP 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 192.0.2.2
s= s=
c=IN IP4 128.96.41.2 c=IN IP4 192.0.2.2
t=0 0 t=0 0
m=audio 4568 RTP/SAVP 0 18 m=audio 54568 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=acfg:1 t=1 a=1 a=acfg:1 t=1 a=1
Bob includes the "a=acfg" attribute in the answer to inform Alice Bob includes the "a=acfg" attribute in the answer to inform Alice
that he based his answer on an offer containing the potential that he based his answer on an offer containing the potential
configuration with transport protocol capability 1 and attribute configuration with transport protocol capability 1 and attribute
capability 1 from the offer SDP (i.e. the RTP/SAVP profile using the capability 1 from the offer SDP (i.e. the RTP/SAVP profile using the
keying material provided). Bob also includes his keying material in keying material provided). Bob also includes his keying material in
a crypto attribute. a crypto attribute.
skipping to change at page 34, line 41 skipping to change at page 40, line 34
configuration. This is done purely to assist any middle-boxes that configuration. This is done purely to assist any middle-boxes that
may reside between Alice and Bob but do not support the capability may reside between Alice and Bob but do not support the capability
negotiation extensions (and hence may not understand the negotiation negotiation extensions (and hence may not understand the negotiation
that just took place): that just took place):
Alice's updated offer includes only SRTP, and it is not using the SDP Alice's updated offer includes only SRTP, and it is not using the SDP
capability negotiation extensions (Alice could have included the capability negotiation extensions (Alice could have included the
capabilities as well is she wanted to): capabilities as well is she wanted to):
v=0 v=0
o=- 25678 753850 IN IP4 128.96.41.1 o=- 25678 753850 IN IP4 192.0.2.1
s= s=
c=IN IP4 128.96.41.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
m=audio 3456 RTP/SAVP 0 18 m=audio 53456 RTP/SAVP 0 18
a=crypto:1 AES_CM_128_HMAC_SHA1_80 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
The "m=" line now indicates that Alice is offering to use secure RTP The "m=" line now indicates that Alice is offering to use secure RTP
with PCMU or G.729. The "crypto" attribute, which provides the SRTP with PCMU or G.729. The "crypto" attribute, which provides the SRTP
keying material, is included with the same value again. keying material, is included with the same value again.
Bob receives the SDP offer from Alice, which he accepts, and then Bob receives the SDP offer from Alice, which he accepts, and then
generates an answer to Alice: generates an answer to Alice:
v=0 v=0
o=- 24351 621815 IN IP4 128.96.41.2 o=- 24351 621815 IN IP4 192.0.2.2
s= s=
c=IN IP4 128.96.41.2 c=IN IP4 192.0.2.2
t=0 0 t=0 0
m=audio 4568 RTP/SAVP 0 18 m=audio 54568 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
Bob includes the same crypto attribute as before, and the session Bob includes the same crypto attribute as before, and the session
proceeds without change. Although Bob did not include any proceeds without change. Although Bob did not include any
capabilities in his answer, he could of course have done so if he capabilities in his answer, he could of course have done so if he
wanted to. wanted to.
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 received in step 1 and would simply have ignored the new attributes received in step 1 and
accepted the offer to use normal RTP. In that case, the following accepted the offer to use normal RTP. In that case, the following
answer would have been generated in step 2 instead: answer would have been generated in step 2 instead:
v=0 v=0
o=- 24351 621814 IN IP4 128.96.41.2 o=- 24351 621814 IN IP4 192.0.2.2
s= s=
c=IN IP4 128.96.41.2 c=IN IP4 192.0.2.2
t=0 0 t=0 0
m=audio 4568 RTP/AVP 0 18 m=audio 54568 RTP/AVP 0 18
4.2. Multiple Transport Protocols 4.2. Multiple Transport Protocols
The following example illustrates how to use the SDP Capability The following example illustrates how to use the SDP Capability
negotiation extensions to negotiate use of one out of several negotiation extensions to negotiate use of one out of several
possible transport protocols. As in the previous example, the offerer possible transport protocols. As in the previous example, the offerer
uses the expected least-common-denominator (plain RTP) as the actual uses the expected least-common-denominator (plain RTP) as the actual
configuration, and the alternative transport protocols as the configuration, and the alternative transport protocols as the
potential configurations. potential configurations.
skipping to change at page 36, line 27 skipping to change at page 42, line 27
|<---------------------------------| |<---------------------------------|
| | | |
Alice's offer includes plain RTP (RTP/AVP), RTP with RTCP-based Alice's offer includes plain RTP (RTP/AVP), RTP with RTCP-based
feedback (RTP/AVPF), Secure RTP (RTP/SAVP), and Secure RTP with RTCP- feedback (RTP/AVPF), Secure RTP (RTP/SAVP), and Secure RTP with RTCP-
based feedback (RTP/SAVPF) and SRTP as alternatives. RTP is the based feedback (RTP/SAVPF) and SRTP as alternatives. RTP is the
default, with RTP/SAVPF, RTP/SAVP, and RTP/AVPF as the alternatives default, with RTP/SAVPF, RTP/SAVP, and RTP/AVPF as the alternatives
and preferred in the order listed: and preferred in the order listed:
v=0 v=0
o=- 25678 753849 IN IP4 128.96.41.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 128.96.41.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
m=audio 3456 RTP/AVP 0 18 m=audio 53456 RTP/AVP 0 18
a=creq: cap-v0
a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF
a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_80 a=acap: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=acap:2 a=rtcp-fb:0 nack a=acap:2 a=rtcp-fb:0 nack
a=pcfg:1 t=1 a=1,2 a=pcfg:1 t=1 a=1,2
a=pcfg:2 t=2 a=1 a=pcfg:2 t=2 a=1
a=pcfg:3 t=3 a=2 a=pcfg:3 t=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. Alice indicates that support for the base protocol PCMU or G.729. The capabilities are provided by the "a=tcap" and
defined here is required by including the "a=creq" attribute "a=acap" attributes. The "tcap" capability indicates that Secure RTP
containing the value "cap-v0". The capabilities are provided by the with RTCP-Based feedback (RTP/SAVPF), Secure RTP (RTP/SAVP), and RTP
"a=tcap" and "a=acap" attributes. The "tcap" capability indicates with RTCP-Based feedback are supported. The first "acap" attribute
that Secure RTP with RTCP-Based feedback (RTP/SAVPF), Secure RTP provides an attribute capability with a handle of 1. The capability
(RTP/SAVP), and RTP with RTCP-Based feedback are supported. The first is a "crypto" attribute, which provides the keying material for SRTP
"acap" attribute provides an attribute capability with a handle of 1. using SDP security descriptions [SDES]. The second "acap" attribute
The capability is a "crypto" attribute, which provides the keying provides an attribute capability with a handle of 2. The capability
material for SRTP using SDP security descriptions [SDES]. The second is an "rtcp-fb" attribute, which is used by the RTCP-based feedback
"acap" attribute provides an attribute capability with a handle of 2. profiles to indicate that payload type 0 (PCMU) supports feedback
The capability is an "rtcp-fb" attribute, which is used by the RTCP- type "nack". The "a=pcfg" attributes provide the potential
based feedback profiles to indicate that payload type 0 (PCMU) configurations included in the offer by reference to the
supports feedback type "nack". The "a=pcfg" attributes provide the
potential configurations included in the offer by reference to the
capabilities. There are three potential configurations: capabilities. There are three potential configurations:
o Potential configuration 1, which is the most preferred potential o Potential configuration 1, which is the most preferred potential
configuration specifies use of transport protocol capability 1 configuration specifies use of transport protocol capability 1
(RTP/SAVPF) and attribute capabilities 1 (the "crypto" attribute) (RTP/SAVPF) and attribute capabilities 1 (the "crypto" attribute)
and 2 (the "rtcp-fb" attribute). and 2 (the "rtcp-fb" attribute).
o Potential configuration 2, which is the second most preferred o Potential configuration 2, which is the second most preferred
potential configuration specifies use of transport protocol potential configuration specifies use of transport protocol
capability 2 (RTP/SAVP) and attribute capability 1 (the "crypto" capability 2 (RTP/SAVP) and attribute capability 1 (the "crypto"
skipping to change at page 37, line 36 skipping to change at page 43, line 33
transport protocol capability 3 (RTP/AVPF) and attribute transport protocol capability 3 (RTP/AVPF) and attribute
capability 2 (the "rtcp-fb" attribute). capability 2 (the "rtcp-fb" attribute).
Bob receives the SDP offer from Alice. Bob does not support any Bob receives the SDP offer from Alice. Bob does not support any
secure RTP profiles, however he supports plain RTP and RTP with RTCP- secure RTP profiles, however he supports plain RTP and RTP with RTCP-
based feedback, as well as the SDP Capability Negotiation extensions, based feedback, as well as the SDP Capability Negotiation extensions,
and hence he accepts the potential configuration for RTP with RTCP- and hence he accepts the potential configuration for RTP with RTCP-
based feedback provided by Alice: based feedback provided by Alice:
v=0 v=0
o=- 24351 621814 IN IP4 128.96.41.2 o=- 24351 621814 IN IP4 192.0.2.2
s= s=
c=IN IP4 128.96.41.2 c=IN IP4 192.0.2.2
t=0 0 t=0 0
m=audio 4568 RTP/AVPF 0 18 m=audio 54568 RTP/AVPF 0 18
a=rtcp-fb:0 nack a=rtcp-fb:0 nack
a=acfg:1 t=3 a=2 a=acfg:1 t=3 a=2
Bob includes the "a=acfg" attribute in the answer to inform Alice Bob includes the "a=acfg" attribute in the answer to inform Alice
that he based his answer on an offer containing the potential that he based his answer on an offer containing the potential
configuration with transport protocol capability 3 and attribute configuration with transport protocol capability 3 and attribute
capability 2 from the offer SDP (i.e. the RTP/AVPF profile using the capability 2 from the offer SDP (i.e. the RTP/AVPF profile using the
"rtcp-fb" value provided). Bob also includes an "rtcp-fb" attribute "rtcp-fb" value provided). Bob also includes an "rtcp-fb" attribute
with the value "nack" value for RTP payload type 0. with the value "nack" value for RTP payload type 0.
skipping to change at page 38, line 17 skipping to change at page 44, line 13
configuration. This is done purely to assist any middle-boxes that configuration. This is done purely to assist any middle-boxes that
may reside between Alice and Bob but do not support the capability may reside between Alice and Bob but do not support the capability
negotiation extensions (and hence may not understand the negotiation negotiation extensions (and hence may not understand the negotiation
that just took place): that just took place):
Alice's updated offer includes only RTP/AVPF, and it is not using the Alice's updated offer includes only RTP/AVPF, and it is not using the
SDP capability negotiation extensions (Alice could have included the SDP capability negotiation extensions (Alice could have included the
capabilities as well is she wanted to): capabilities as well is she wanted to):
v=0 v=0
o=- 25678 753850 IN IP4 128.96.41.1 o=- 25678 753850 IN IP4 192.0.2.1
s= s=
c=IN IP4 128.96.41.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
m=audio 3456 RTP/AVPF 0 18 m=audio 53456 RTP/AVPF 0 18
a=rtcp-fb:0 nack a=rtcp-fb:0 nack
The "m=" line now indicates that Alice is offering to use RTP with The "m=" line now indicates that Alice is offering to use RTP with
RTCP-based feedback and using PCMU or G.729. The "rtcp-fb" attribute RTCP-based feedback and using PCMU or G.729. The "rtcp-fb" attribute
provides the feedback type "nack" for payload type 0 again (but as provides the feedback type "nack" for payload type 0 again (but as
part of the actual configuration). part of the actual configuration).
Bob receives the SDP offer from Alice, which he accepts, and then Bob receives the SDP offer from Alice, which he accepts, and then
generates an answer to Alice: generates an answer to Alice:
v=0 v=0
o=- 24351 621815 IN IP4 128.96.41.2 o=- 24351 621815 IN IP4 192.0.2.2
s= s=
c=IN IP4 128.96.41.2 c=IN IP4 192.0.2.2
t=0 0 t=0 0
m=audio 4568 RTP/AVPF 0 18 m=audio 54568 RTP/AVPF 0 18
a=rtcp-fb:0 nack a=rtcp-fb:0 nack
Bob includes the same "rtcp-fb" attribute as before, and the session Bob includes the same "rtcp-fb" attribute as before, and the session
proceeds without change. Although Bob did not include any proceeds without change. Although Bob did not include any
capabilities in his answer, he could of course have done so if he capabilities in his answer, he could of course have done so if he
wanted to. wanted to.
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 received in step 1 and would simply have ignored the new attributes received in step 1 and
accepted the offer to use normal RTP. In that case, the following accepted the offer to use normal RTP. In that case, the following
answer would have been generated in step 2 instead: answer would have been generated in step 2 instead:
v=0 v=0
o=- 24351 621814 IN IP4 128.96.41.2 o=- 24351 621814 IN IP4 192.0.2.2
s= s=
c=IN IP4 128.96.41.2 c=IN IP4 192.0.2.2
t=0 0 t=0 0
m=audio 4568 RTP/AVP 0 18 m=audio 54568 RTP/AVP 0 18
4.3. Session-Level MIKEY and Media Level Security Descriptions 4.3. Best-Effort SRTP with Session-Level MIKEY and Media Level Security
Descriptions
The following example illustrates how to use the SDP Capability The following example illustrates how to use the SDP Capability
negotiation extensions to support so-called Best-Effort Secure RTP as negotiation extensions to support so-called Best-Effort Secure RTP as
well as alternative keying mechanisms, more specifically MIKEY and well as alternative keying mechanisms, more specifically MIKEY and
SDP Security Descriptions. The offerer (Alice) wants to establish an SDP Security Descriptions. The offerer (Alice) wants to establish an
audio and video session. Alice prefers to use session-level MIKEY as audio and video session. Alice prefers to use session-level MIKEY as
the key management protocol, but supports SDP security descriptions the key management protocol, but supports SDP security descriptions
as well. as well.
The example is illustrated by the offer/answer exchange below, where The example is illustrated by the offer/answer exchange below, where
skipping to change at page 39, line 46 skipping to change at page 45, line 42
| (4) Answer (RTP/SAVP, SDES) | | (4) Answer (RTP/SAVP, SDES) |
|<---------------------------------------| |<---------------------------------------|
| | | |
Alice's offer includes an audio and a video stream. The audio stream Alice's offer includes an audio and a video stream. The audio stream
offers use of plain RTP and secure RTP as alternatives, whereas the offers use of plain RTP and secure RTP as alternatives, whereas the
video stream offers use plain RTP, RTP with RTCP-based feedback, video stream offers use plain RTP, RTP with RTCP-based feedback,
Secure RTP, and Secure RTP with RTCP-based feedback as alternatives: Secure RTP, and Secure RTP with RTCP-based feedback as alternatives:
v=0 v=0
o=- 25678 753849 IN IP4 128.96.41.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
t=0 0 t=0 0
c=IN IP4 128.96.41.1 c=IN IP4 192.0.2.1
a=creq: cap-v0 a=acap:1 a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
a=acap:1 key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF
m=audio 39000 RTP/AVP 98 m=audio 59000 RTP/AVP 98
a=rtpmap:98 AMR/8000 a=rtpmap:98 AMR/8000
a=acap:2 a=crypto:1 AES_CM_128_HMAC_SHA1_32 a=acap:2 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=pcfg:1 t=2 a=1|2 a=pcfg:1 t=2 a=1|2
m=video 42000 RTP/AVP 31 m=video 52000 RTP/AVP 31
a=rtpmap:31 H261/90000 a=rtpmap:31 H261/90000
a=acap:3 a=crypto:1 AES_CM_128_HMAC_SHA1_80 a=acap:3 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32 inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=acap:4 a=rtcp-fb:* nack a=acap:4 a=rtcp-fb:* nack
a=pcfg:1 t=1 a=1,4|3,4 a=pcfg:1 t=1 a=1,4|3,4
a=pcfg:2 t=2 a=1|3 a=pcfg:2 t=2 a=1|3
a=pcfg:3 t=3 a=4 a=pcfg:3 t=3 a=4
The potential configuration for the audio stream specifies use of The potential configuration for the audio stream specifies use of
transport capability 2 (RTP/SAVP) and either attribute capability 1 transport capability 2 (RTP/SAVP) and either attribute capability 1
skipping to change at page 40, line 47 skipping to change at page 46, line 43
o The third configuration with configuration number 3 uses transport o The third configuration with configuration number 3 uses transport
capability 3 (RTP/AVPF) and attribute capability 4 (the "rtcp-fb" capability 3 (RTP/AVPF) and attribute capability 4 (the "rtcp-fb"
attribute). attribute).
Bob receives the SDP offer from Alice. Bob supports Secure RTP, Bob receives the SDP offer from Alice. Bob supports Secure RTP,
Secure RTP with RTCP-based feedback and the SDP Capability Secure RTP with RTCP-based feedback and the SDP Capability
Negotiation extensions. Bob also supports SDP Security Descriptions, Negotiation extensions. Bob also supports SDP Security Descriptions,
but not MIKEY, and hence he generates the following answer: but not MIKEY, and hence he generates the following answer:
v=0 v=0
o=- 24351 621814 IN IP4 128.96.41.2 o=- 24351 621814 IN IP4 192.0.2.2
s= s=
t=0 0 t=0 0
c=IN IP4 128.96.41.2 c=IN IP4 192.0.2.2
m=audio 4568 RTP/SAVP 98 m=audio 54568 RTP/SAVP 98
a=rtpmap:98 AMR/8000 a=rtpmap:98 AMR/8000
a=crypto:1 AES_CM_128_HMAC_SHA1_32 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32 inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32
a=acfg:1 t=2 a=2 a=acfg:1 t=2 a=2
m=video 5468 RTP/SAVPF 31 m=video 55468 RTP/SAVPF 31
a=rtpmap:31 H261/90000 a=rtpmap:31 H261/90000
a=crypto:1 AES_CM_128_HMAC_SHA1_80 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:AwWpVLFJhQX1cfHJSojd0RmdmcmVCspeEc3QGZiN|2^20|1:32 inline:AwWpVLFJhQX1cfHJSojd0RmdmcmVCspeEc3QGZiN|2^20|1:32
a=rtcp-fb:* nack a=rtcp-fb:* nack
a=acfg:1 t=1 a=3,4 a=acfg:1 t=1 a=3,4
For the audio stream, Bob accepted the use of secure RTP, and hence For the audio stream, Bob accepted the use of secure RTP, and hence
the profile in the "m=" line is "RTP/SAVP". Bob also includes a the profile in the "m=" line is "RTP/SAVP". Bob also includes a
"crypto" attribute with his own keying material, and an "acfg" "crypto" attribute with his own keying material, and an "acfg"
attribute identifying actual configuration 1 for the audio media attribute identifying actual configuration 1 for the audio media
skipping to change at page 41, line 44 skipping to change at page 47, line 39
may reside between Alice and Bob but do not support the capability may reside between Alice and Bob but do not support the capability
negotiation extensions (and hence may not understand the negotiation negotiation extensions (and hence may not understand the negotiation
that just took place): that just took place):
Alice's updated offer includes only SRTP for the audio stream SRTP Alice's updated offer includes only SRTP for the audio stream SRTP
with RTCP-based feedback for the video stream, and it is not using with RTCP-based feedback for the video stream, and it is not using
the SDP capability negotiation extensions (Alice could have included the SDP capability negotiation extensions (Alice could have included
the capabilities as well is she wanted to): the capabilities as well is she wanted to):
v=0 v=0
o=- 25678 753849 IN IP4 128.96.41.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
t=0 0 t=0 0
c=IN IP4 128.96.41.1 c=IN IP4 192.0.2.1
m=audio 39000 RTP/SAVP 98 m=audio 59000 RTP/SAVP 98
a=rtpmap:98 AMR/8000 a=rtpmap:98 AMR/8000
a=crypto:1 AES_CM_128_HMAC_SHA1_32 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
m=video 42000 RTP/SAVPF 31 m=video 52000 RTP/SAVPF 31
a=rtpmap:31 H261/90000 a=rtpmap:31 H261/90000
a=crypto:1 AES_CM_128_HMAC_SHA1_80 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32 inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=rtcp-fb:* nack a=rtcp-fb:* nack
The "m=" line for the audio stream now indicates that Alice is The "m=" line for the audio stream now indicates that Alice is
offering to use secure RTP with PCMU or G.729, whereas the "m=" line offering to use secure RTP with PCMU or G.729, whereas the "m=" line
for the video stream now indicates that Alice is offering to use for the video stream now indicates that Alice is offering to use
secure RTP with RTCP-based feedback with H.261. Each media stream secure RTP with RTCP-based feedback with H.261. Each media stream
includes a "crypto" attribute, which provides the SRTP keying includes a "crypto" attribute, which provides the SRTP keying
material, with the same value again. material, with the same value again.
Bob receives the SDP offer from Alice, which he accepts, and then Bob receives the SDP offer from Alice, which he accepts, and then
generates an answer to Alice: generates an answer to Alice:
v=0 v=0
o=- 24351 621814 IN IP4 128.96.41.2 o=- 24351 621814 IN IP4 192.0.2.2
s= s=
t=0 0 t=0 0
c=IN IP4 128.96.41.2 c=IN IP4 192.0.2.2
m=audio 4568 RTP/SAVP 98 m=audio 54568 RTP/SAVP 98
a=rtpmap:98 AMR/8000 a=rtpmap:98 AMR/8000
a=crypto:1 AES_CM_128_HMAC_SHA1_32 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32 inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32
m=video 5468 RTP/SAVPF 31 m=video 55468 RTP/SAVPF 31
a=rtpmap:31 H261/90000 a=rtpmap:31 H261/90000
a=crypto:1 AES_CM_128_HMAC_SHA1_80 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:AwWpVLFJhQX1cfHJSojd0RmdmcmVCspeEc3QGZiN|2^20|1:32 inline:AwWpVLFJhQX1cfHJSojd0RmdmcmVCspeEc3QGZiN|2^20|1:32
a=rtcp-fb:* nack a=rtcp-fb:* nack
Bob includes the same crypto attribute as before, and the session Bob includes the same crypto attribute as before, and the session
proceeds without change. Although Bob did not include any proceeds without change. Although Bob did not include any
capabilities in his answer, he could of course have done so if he capabilities in his answer, he could of course have done so if he
wanted to. wanted to.
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 received in step 1 and would simply have ignored the new attributes received in step 1 and
accepted the offer to use normal RTP. In that case, the following accepted the offer to use normal RTP. In that case, the following
answer would have been generated in step 2 instead: answer would have been generated in step 2 instead:
v=0 v=0
o=- 24351 621814 IN IP4 128.96.41.2 o=- 24351 621814 IN IP4 192.0.2.2
s= s=
t=0 0 t=0 0
c=IN IP4 128.96.41.2 c=IN IP4 192.0.2.2
m=audio 4568 RTP/AVP 98 m=audio 54568 RTP/AVP 98
a=rtpmap:98 AMR/8000 a=rtpmap:98 AMR/8000
m=video 5468 RTP/AVP 31 m=video 55468 RTP/AVP 31
a=rtpmap:31 H261/90000 a=rtpmap:31 H261/90000
a=rtcp-fb:* nack a=rtcp-fb:* nack
Finally, if Bob had chosen to use session-level MIKEY instead of SDP Finally, if Bob had chosen to use session-level MIKEY instead of SDP
security descriptions instead, the following answer would have been security descriptions instead, the following answer would have been
generated: generated:
v=0 v=0
o=- 25678 753849 IN IP4 128.96.41.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
t=0 0 t=0 0
c=IN IP4 128.96.41.1 c=IN IP4 192.0.2.1
a=key-mgmt:mikey AQEFgM0XflABAAAAAAAAAAAAAAYAyO... a=key-mgmt:mikey AQEFgM0XflABAAAAAAAAAAAAAAYAyO...
m=audio 39000 RTP/AVP 98 m=audio 59000 RTP/AVP 98
a=rtpmap:98 AMR/8000 a=rtpmap:98 AMR/8000
a=acfg:1 t=2 a=1 a=acfg:1 t=2 a=1
m=video 42000 RTP/SAVPF 31 m=video 52000 RTP/SAVPF 31
a=rtpmap:31 H261/90000 a=rtpmap:31 H261/90000
a=rtcp-fb:* nack a=rtcp-fb:* nack
a=acfg:1 t=1 a=1,4 a=acfg:1 t=1 a=1,4
It should be noted, that although Bob could have chosen session-level It should be noted, that although Bob could have chosen session-level
MIKEY for one media stream, and SDP Security Descriptions for another MIKEY for one media stream, and SDP Security Descriptions for another
media stream, there are no well-defined offerer processing rules of media stream, there are no well-defined offerer processing rules of
the resulting answer for this, and hence the offerer may incorrectly the resulting answer for this, and hence the offerer may incorrectly
assume use of MIKEY for both streams. To avoid this, if the answerer assume use of MIKEY for both streams. To avoid this, if the answerer
chooses session-level MIKEY, then all secure RTP based media streams chooses session-level MIKEY, then all secure RTP based media streams
SHOULD use MIKEY (this applies irrespective of whether SDP capability SHOULD use MIKEY (this applies irrespective of whether SDP capability
negotiation is being used or not). Use of media-level MIKEY does not negotiation is being used or not). Use of media-level MIKEY does not
have a similar constraint. have a similar constraint.
4.4. Capability Negotiation with Interactive Connectivity Establishment 4.4. SRTP with Session-Level MIKEY and Media Level Security Descriptions
as Alternatives
[EDITOR'S NOTE: Example to be added] The following example illustrates how to use the SDP Capability
negotiation extensions to negotiate use of either MIKEY or SDP
Security Descriptions, when one of them is included as part of the
actual configuration, and the other one is being selected. The
offerer (Alice) wants to establish an audio and video session. Alice
prefers to use session-level MIKEY as the key management protocol,
but supports SDP security descriptions as well.
The example is illustrated by the offer/answer exchange below, where
Alice sends an offer to Bob:
Alice Bob
| (1) Offer (RTP/[S]AVP[F], SDES|MIKEY) |
|--------------------------------------->|
| |
| (2) Answer (RTP/SAVP, SDES) |
|<---------------------------------------|
| |
Alice's offer includes an audio and a video stream. Both the audio
and the video stream offer use of secure RTP:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
t=0 0
c=IN IP4 192.0.2.1
a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
a=acap:1 a=key-mgmt
m=audio 59000 RTP/SAVP 98
a=rtpmap:98 AMR/8000
a=acap:2 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=pcfg:1 a=-1,2
m=video 52000 RTP/SAVP 31
a=rtpmap:31 H261/90000
a=acap:3 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=pcfg:1 a=-1,3
Alice does not know whether Bob supports MIKEY or SDP Security
Descriptions. She could include attributes for both, however the
resulting procedures and potential interactions are not well-defined.
Instead, she places a session-level key-mgmt attribute for MIKEY in
the actual configuration with SDP security descriptions as an
alternative in the potential configuration. Note the presence of
attribute capability 1; it merely lists the "a=key-mgmt" attribute
without any associated value. The potential configuration for the
audio stream specifies that attribute capability 1 is to be deleted
(i.e. any session-level "a=key-mgmt" attributes) and that attribute
capability 2 is to be used (i.e. the crypto attribute). The potential
configuration for the video stream is similar, except it uses it's
own crypto attribute capability (3).
Bob receives the SDP offer from Alice. Bob supports Secure RTP and
the SDP Capability Negotiation extensions. Bob supports both SDP
Security Descriptions and MIKEY. Since the potential configuration is
more preferred than the actual configuration, he (conceptually)
generates an internal potential configuration SDP that contains the
crypto attributes for the audio and video stream, but not the key-
mgmt attribute for MIKEY, thereby avoiding any ambiguity between the
two keying mechanisms. As a result, he generates the following
answer:
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
t=0 0
c=IN IP4 192.0.2.2
m=audio 54568 RTP/SAVP 98
a=rtpmap:98 AMR/8000
a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32
a=acfg:1 a=-1,2
m=video 55468 RTP/SAVP 31
a=rtpmap:31 H261/90000
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:AwWpVLFJhQX1cfHJSojd0RmdmcmVCspeEc3QGZiN|2^20|1:32
a=acfg:1 a=-1,3
For the audio stream, Bob accepted the use of secure RTP using SDP
security descriptions. Bob therefore includes a "crypto" attribute
with his own keying material, and an "acfg" attribute identifying
actual configuration 1 for the audio media stream from the offer,
with attribute capability 1 deleted, and attribute capability 2
included (the crypto attribute from the offer). For the video stream,
Bob also accepted the use of secure RTP using SDP security
descriptions. Bob therefore includes a "crypto" attribute with his
own keying material, and an "acfg" attribute identifying actual
configuration 1 for the video stream from the offer, with attribute
capability 1 deleted, and attribute capability 3 included.
5. Security Considerations 5. Security Considerations
The SDP Capability Negotiation Framework is defined to be used within The SDP Capability Negotiation Framework is defined to be used within
the context of the offer/answer model, and hence all the offer/answer the context of the offer/answer model, and hence all the offer/answer
security considerations apply here as well. Similarly, the Session security considerations apply here as well. Similarly, the Session
Initiation Protocol (SIP) uses SDP and the offer/answer model, and Initiation Protocol (SIP) uses SDP and the offer/answer model, and
hence, when used in that context, the SIP security considerations hence, when used in that context, the SIP security considerations
apply as well. apply as well.
skipping to change at page 46, line 11 skipping to change at page 53, line 50
The IANA is hereby requested to register the following new SDP The IANA is hereby requested to register the following new SDP
attributes as follows: attributes as follows:
Attribute name: csup Attribute name: csup
Long form name: Supported capability negotiation extensions Long form name: Supported capability negotiation extensions
Type of attribute: Session-level and media-level Type of attribute: Session-level and media-level
Subject to charset: No Subject to charset: No
Purpose: Option tags for supported SDP capability Purpose: Option tags for supported SDP capability
negotiation extensions negotiation extensions
Appropriate values: See Section 3.2.1. Appropriate values: See Section 3.3.1.
Attribute name: creq Attribute name: creq
Long form name: Required capability negotiation extensions Long form name: Required capability negotiation extensions
Type of attribute: Session-level and media-level Type of attribute: Session-level and media-level
Subject to charset: No Subject to charset: No
Purpose: Option tags for required SDP capability Purpose: Option tags for required SDP capability
negotiation extensions negotiation extensions
Appropriate values: See Section 3.2.2. Appropriate values: See Section 3.3.2.
Attribute name: acap Attribute name: acap
Long form name: Attribute capability Long form name: Attribute capability
Type of attribute: Session-level and media-level Type of attribute: Session-level and media-level
Subject to charset: No Subject to charset: No
Purpose: Attribute capability containing an attribute Purpose: Attribute capability containing an attribute
name and associated value name and associated value
Appropriate values: See Section 3.3.1. Appropriate values: See Section 3.4.1.
Attribute name: tcap Attribute name: tcap
Long form name: Transport Protocol Capability Long form name: Transport Protocol Capability
Type of attribute: Session-level and media-level Type of attribute: Session-level and media-level
Subject to charset: No Subject to charset: No
Purpose: Transport protocol capability listing one or Purpose: Transport protocol capability listing one or
more transport protocols more transport protocols
Appropriate values: See Section 3.3.2. Appropriate values: See Section 3.4.2.
Attribute name: pcfg Attribute name: pcfg
Long form name: Potential Configuration Long form name: Potential Configuration
Type of attribute: Media-level Type of attribute: Media-level
Subject to charset: No Subject to charset: No
Purpose: Potential configuration for SDP capability Purpose: Potential configuration for SDP capability
negotiation negotiation
Appropriate values: See Section 3.4.1. Appropriate values: See Section 3.5.1.
Attribute name: acfg Attribute name: acfg
Long form name: Actual configuration Long form name: Actual configuration
Type of attribute: Media-level Type of attribute: Media-level
Subject to charset: No Subject to charset: No
Purpose: Actual configuration for SDP capability Purpose: Actual configuration for SDP capability
negotiation negotiation
Appropriate values: See Section 3.4.2. Appropriate values: See Section 3.5.2.
6.2. New SDP Capability Negotiation Option Tag Registry 6.2. New SDP Capability Negotiation Option Tag Registry
The IANA is hereby requested to create a new SDP Capability The IANA is hereby requested to create a new SDP Capability
Negotiation Option Tag registry. An IANA SDP capability negotiation Negotiation Option Tag registry. An IANA SDP capability negotiation
option tag registration MUST be documented in an RFC in accordance option tag registration MUST be documented in an RFC in accordance
with the [RFC2434] Specification Required policy. The RFC MUST with the [RFC2434] Specification Required policy. The RFC MUST
provide the name of the option tag, a syntax and a semantic provide the name of the option tag, a syntax and a semantic
specification of any new SDP attributes and any extensions to the specification of any new SDP attributes and any extensions to the
potential and actual configuration attributes provided in this potential and actual configuration attributes provided in this
document. New SDP attributes that are intended to be capabilities for document. New SDP attributes that are intended to be capabilities for
use by the capability negotiation framework MUST adhere to the use by the capability negotiation framework MUST adhere to the
guidelines provided in Section 3.3.3. Extensions to the potential and guidelines provided in Section 3.4.3. Extensions to the potential and
actual configuration attributes MUST adhere to the syntax provided in actual configuration attributes MUST adhere to the syntax provided in
Section 3.4.1. and 3.4.2. Section 3.5.1. and 3.5.2.
The option tag "cap-v0" is defined in this document and the IANA is The option tag "cap-v0" is defined in this document and the IANA is
hereby requested to register this option tag. hereby requested to register this option tag.
6.3. New SDP Capability Negotiation Potential Configuration Parameter 6.3. New SDP Capability Negotiation Potential Configuration Parameter
Registry Registry
The IANA is hereby requested to create a new SDP Capability The IANA is hereby requested to create a new SDP Capability
Negotiation Potential Configuration Parameter registry. An IANA SDP Negotiation Potential Configuration Parameter registry. An IANA SDP
Capability Negotiation potential configuration registration MUST be Capability Negotiation potential configuration registration MUST be
document in an RFC in accordance with the [RFC2434] Specification document in an RFC in accordance with the [RFC2434] Specification
Required policy. The RFC MUST define the syntax and semantics of each Required policy. The RFC MUST define the syntax and semantics of each
new potential configuration parameter. The syntax MUST adhere to the new potential configuration parameter. The syntax MUST adhere to the
syntax provided for extensions in Section 3.4.1. and the semantics syntax provided for extensions in Section 3.5.1. and the semantics
MUST adhere to the semantics provided for extensions in Section MUST adhere to the semantics provided for extensions in Section
3.4.1. and 3.4.2. Associated with each registration MUST be the 3.5.1. and 3.5.2. Associated with each registration MUST be the
encoding name for the parameter as well as a short descriptive name encoding name for the parameter as well as a short descriptive name
for it. for it.
The potential configuration parameters "a" for "attribute" and "t" The potential configuration parameters "a" for "attribute" and "t"
for "transport protocol" are defined in this document and the IANA is for "transport protocol" are defined in this document and the IANA is
hereby requested to register these. hereby requested to register these.
7. To Do and Open Issues 7. To Do and Open Issues
o Look for "EDITOR'S NOTE" throughout the document. o Add additional examples showing use of delete-attributes and the
DELETE/REPLACE attribute capability operators.
8. Acknowledgments 8. Acknowledgments
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 in document itself or the overall direction of the solution defined in
here: Francois Audet, John Elwell, Roni Even, Robert Gilman, Cullen here: Francois Audet, John Elwell, Roni Even, Robert Gilman, Cullen
Jennings, Matt Lepinski, Joerg Ott, Colin Perkins, Thomas Stach, and Jennings, Matt Lepinski, Joerg Ott, Colin Perkins, Thomas Stach, and
Dan Wing. Dan Wing.
9. Change Log 9. Change Log
9.1. draft-ietf-mmusic-sdp-capability-negotiation-04 9.1. draft-ietf-mmusic-sdp-capability-negotiation-05
o Allowed for '<type>=<value>' attributes to be listed as attribute
capabilities the attribute name only.
o Changed IP-address to conform to RFC 3330 guidelines.
o Added section on relationship to RFC 3407 and "Obsoletes: 3407" in
the front.
o Disallowed use of white space in a number of places for more
consistency with existing SDP practice
o Changed "csup" and "creq" attributes to not allow multiple
instances at the session-level and multiple instances per media
description (only one for each now)
o Changed to not require use of "creq" with base option tag ("cap-
v0").
o Relaxed restrictions on extension capabilities
o Updated potential configuration attribute syntax and semantics. In
particular, potential configuration attributes can now replace and
delete various existing attributes in original SDP to better
control potential attribute interactions with the actual
configuration while preserving message size efficiency.
o Updated actual configuration attribute to align with the updates
to the potential configuration attributes.
o Updated offer/answer procedures to align with other changes.
o Changed recommendation for second offer/answer exchange to "MAY"
strength, unless for the cases where it is known or suspected that
it is needed.
o Updated ICE interactions to explain how the new attribute
delete/replace features can solve certain potential interactions.
o Updated rtpmap and fmtp section to allow potential configurations
to use remapped payload types in attribute capabilities for
rtpmaps and fmtp parameters.
o Added section on direction attributes.
o Added another example showing SRTP with session-level MIKEY and
SDP Security Descriptions using the attribute capability DELETE
operator.
9.2. draft-ietf-mmusic-sdp-capability-negotiation-04
The following are the major changes compared to version -03: The following are the major changes compared to version -03:
o Added explicit ordering rules for attributes added by potential o Added explicit ordering rules for attributes added by potential
configurations. configurations.
o Noted that ICE interaction issues (ice-tcp specifically) may not o Noted that ICE interaction issues (ice-tcp specifically) may not
be as clear as originally thought. be as clear as originally thought.
o Added considerations on using rtpmap and fmtp attributes as o Added considerations on using rtpmap and fmtp attributes as
attribute capabilities. attribute capabilities.
o Added multiple transport protocol example. o Added multiple transport protocol example.
o Added session-level MIKEY and media level security descriptions o Added session-level MIKEY and media level security descriptions
example. example.
9.2. draft-ietf-mmusic-sdp-capability-negotiation-03 9.3. draft-ietf-mmusic-sdp-capability-negotiation-03
The following are the major changes compared to version -02: The following are the major changes compared to version -02:
o Base option tag name changed from "v0" to "cap-v0". o Base option tag name changed from "v0" to "cap-v0".
o Added new section on extension capability attributes o Added new section on extension capability attributes
o Firmed up offer/answer procedures. o Firmed up offer/answer procedures.
o Added security considerations o Added security considerations
o Added IANA considerations o Added IANA considerations
9.3. draft-ietf-mmusic-sdp-capability-negotiation-02 9.4. draft-ietf-mmusic-sdp-capability-negotiation-02
The following are the major changes compared to version -01: The following are the major changes compared to version -01:
o Potential configurations are no longer allowed at the session o Potential configurations are no longer allowed at the session
level level
o Renamed capability attributes ("capar" to "acap" and "ctrpr" to o Renamed capability attributes ("capar" to "acap" and "ctrpr" to
"tcap") "tcap")
o Changed name and semantics of the initial number (now called o Changed name and semantics of the initial number (now called
configuration number) in potential configuration attributes; must configuration number) in potential configuration attributes; must
skipping to change at page 49, line 27 skipping to change at page 58, line 27
selected a potential configuration selected a potential configuration
o Updated rules (and added restrictions) for referencing media- and o Updated rules (and added restrictions) for referencing media- and
session-level capabilities in potential configurations (at the session-level capabilities in potential configurations (at the
media level) media level)
o Added initial section on ICE interactions o Added initial section on ICE interactions
o Added initial section on receiving media before answer o Added initial section on receiving media before answer
9.4. draft-ietf-mmusic-sdp-capability-negotiation-01 9.5. draft-ietf-mmusic-sdp-capability-negotiation-01
The following are the major changes compared to version -00: The following are the major changes compared to version -00:
o Media capabilities are no longer considered a core capability and o Media capabilities are no longer considered a core capability and
hence have been removed. This leaves transport protocols and hence have been removed. This leaves transport protocols and
attributes as the only capabilities defined by the core. attributes as the only capabilities defined by the core.
o Version attribute has been removed and an option tag to indicate o Version attribute has been removed and an option tag to indicate
the actual version has been defined instead. the actual version has been defined instead.
skipping to change at page 50, line 13 skipping to change at page 59, line 13
configuration attributes. configuration attributes.
o Potential configurations at the session level now limited to o Potential configurations at the session level now limited to
indicate latent capability configurations. Consequently, an actual indicate latent capability configurations. Consequently, an actual
configuration attribute can no longer be provided at the session configuration attribute can no longer be provided at the session
level. level.
o Cleaned up capability and potential configuration terminology - o Cleaned up capability and potential configuration terminology -
they are now two clearly different things. they are now two clearly different things.
9.5. draft-ietf-mmusic-sdp-capability-negotiation-00 9.6. 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.
skipping to change at page 51, line 12 skipping to change at page 60, line 12
o Some terminology change throughout to more clearly indicate what o Some terminology change throughout to more clearly indicate what
constitutes capabilities and what constitutes configurations. constitutes capabilities and what constitutes configurations.
10. References 10. References
10.1. Normative References 10.1. 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.
[RFC2234] Crocker, D. and Overell, P.(Editors), "Augmented BNF for
Syntax Specifications: ABNF", RFC 2234, Internet Mail
Consortium and Demon Internet Ltd., November 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, June with Session Description Protocol (SDP)", RFC 3264, June
2002. 2002.
[RFC3407] F. Andreasen, "Session Description Protocol (SDP) Simple [RFC3407] F. Andreasen, "Session Description Protocol (SDP) Simple
Capability Declaration", RFC 3407, October 2002. Capability Declaration", RFC 3407, October 2002.
[RFC3605] C. Huitema, "Real Time Control Protocol (RTCP) attribute in [RFC3605] C. Huitema, "Real Time Control Protocol (RTCP) attribute in
Session Description Protocol (SDP)", RFC 3605, October Session Description Protocol (SDP)", RFC 3605, October
2003. 2003.
[RFC4234] Crocker, D., and P. Overell, "Augmented BNF for Syntax [RFC4234] Crocker, D., and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005. Specifications: ABNF", RFC 4234, October 2005.
[SDP] 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.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434, IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998. October 1998.
10.2. Informative References 10.2. Informative References
[RFC2046] Freed, N., and N. Borensteain, "Multipurpose Internet Mail [RFC2046] Freed, N., and N. Borensteain, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046, Extensions (MIME) Part Two: Media Types", RFC 2046,
 End of changes. 230 change blocks. 
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