draft-ietf-mmusic-sdp-capability-negotiation-01.txt   draft-ietf-mmusic-sdp-capability-negotiation-02.txt 
MMUSIC Working Group F. Andreasen MMUSIC Working Group F. Andreasen
Internet Draft Cisco Systems Internet-Draft Cisco Systems
Expires: July 2007 January 28, 2007 Intended Status: Proposed Standard February 13, 2007
Expires: August 2007
SDP Capability Negotiation SDP Capability Negotiation
draft-ietf-mmusic-sdp-capability-negotiation-01.txt draft-ietf-mmusic-sdp-capability-negotiation-02.txt
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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
not intended to provide capability indication or capability not intended to provide capability indication or capability
negotiation, however over the years, SDP has seen widespread adoption negotiation, however over the years, SDP has seen widespread adoption
and as a result it has been gradually extended to provide limited and as a result it has been gradually extended to provide limited
support for these. SDP and its current extensions however do not have support for these. SDP and its current extensions however do not have
skipping to change at page 2, line 17 skipping to change at page 2, line 18
and associated offer/answer procedures to use those parameters in a and associated offer/answer procedures to use those parameters in a
backwards compatible manner. backwards compatible manner.
The solution provided in this document provides a general SDP The solution provided in this document provides a general SDP
capability negotiation framework. It also defines specifically how to capability negotiation framework. It also defines specifically how to
provide attributes and transport protocols as capabilities and provide attributes and transport protocols as capabilities and
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.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Table of Contents Table of Contents
1. Introduction...................................................3 1. Introduction...................................................3
2. SDP Capability Negotiation Solution............................5 2. Conventions used in this document..............................5
2.1. Solution Overview.........................................5 3. SDP Capability Negotiation Solution............................6
2.2. Version and Extension Indication Attributes...............8 3.1. Solution Overview.........................................6
2.2.1. Supported Capability Negotiation Extensions Attribute8 3.2. Version and Extension Indication Attributes...............9
2.2.2. Required Capability Negotiation Extension Attribute.10 3.2.1. Supported Capability Negotiation Extensions Attribute9
2.3. Capability Attributes....................................11 3.2.2. Required Capability Negotiation Extension Attribute.10
2.3.1. Attribute Capability Attribute......................11 3.3. Capability Attributes....................................12
2.3.2. Transport Protocol Capability Attribute.............13 3.3.1. Attribute Capability Attribute......................12
2.4. Configuration Attributes.................................13 3.3.2. Transport Protocol Capability Attribute.............13
2.4.1. Potential Configuration Attribute...................13 3.4. Configuration Attributes.................................15
2.4.2. Actual Configuration Attribute......................17 3.4.1. Potential Configuration Attribute...................15
2.5. Offer/Answer Model Extensions............................18 3.4.2. Actual Configuration Attribute......................18
2.5.1. Generating the Initial Offer........................18 3.5. Offer/Answer Model Extensions............................20
2.5.2. Generating the Answer...............................19 3.5.1. Generating the Initial Offer........................20
2.5.3. Offerer Processing of the Answer....................20 3.5.2. Generating the Answer...............................21
2.5.4. Modifying the Session...............................20 3.5.3. Offerer Processing of the Answer....................22
3. Examples......................................................21 3.5.4. Modifying the Session...............................22
3.1. Best-Effort Secure RTP...................................21 3.6. Interactions with ICE....................................23
4. Security Considerations.......................................23 3.7. Processing Media before Answer...........................24
5. IANA Considerations...........................................23 4. Examples......................................................24
6. To Do and Open Issues.........................................23 4.1. Best-Effort Secure RTP...................................24
7. Acknowledgments...............................................23 4.2. Multiple Transport Protocols.............................27
8. Change Log....................................................24 4.3. Session-Level MIKEY and Media Level Security Descriptions30
8.1. draft-ietf-mmusic-sdp-capability-negotiation-01..........24 4.4. Capability Negotiation with Interactive Connectivity
8.2. draft-ietf-mmusic-sdp-capability-negotiation-00..........24 Establishment.................................................30
9. References....................................................26 5. Security Considerations.......................................30
9.1. Normative References.....................................26 6. IANA Considerations...........................................30
9.2. Informative References...................................26 7. To Do and Open Issues.........................................30
Author's Addresses...............................................28 8. Acknowledgments...............................................30
Intellectual Property Statement..................................28 9. Change Log....................................................31
Disclaimer of Validity...........................................29 9.1. draft-ietf-mmusic-sdp-capability-negotiation-02..........31
Copyright Statement..............................................29 9.2. draft-ietf-mmusic-sdp-capability-negotiation-01..........31
Acknowledgment...................................................29 9.3. draft-ietf-mmusic-sdp-capability-negotiation-00..........32
10. References...................................................34
10.1. Normative References....................................34
10.2. Informative References..................................34
Author's Addresses...............................................36
Intellectual Property Statement..................................36
Full.............................................................37
Copyright Statement..............................................37
Acknowledgment...................................................37
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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following: following:
o A set of capabilities for the session and its associated media o A set of capabilities for the session and its associated media
stream components, supported by each side. stream components, supported by each side.
o A set of potential configurations indicating which of those o A set of potential configurations indicating which of those
capabilities can be used for the session and its associated media capabilities can be used for the session and its associated media
stream components. stream components.
o A set of actual configurations for the session and its associated o A set of actual configurations for the session and its associated
media stream components, which specifies which session parameters media stream components, which specifies which combinations of
to use and which media stream components to use and with what session parameters and media stream components to use and with
parameters. what parameters.
o A negotiation process that takes the set of potential o A negotiation process that takes the set of potential
configurations (lists of capabilities) as input and provides the configurations (combinations of capabilities) as input and
actual configurations as output. provides the actual configurations as output.
SDP by itself was designed to provide only one of these, namely the SDP by itself was designed to provide only one of these, namely the
actual configurations, however over the years, use of SDP has been actual configurations, however over the years, use of SDP has been
extended beyond its original scope. Session negotiation semantics extended beyond its original scope. Session negotiation semantics
were defined by the offer/answer model in RFC 3264. It defines how were defined by the offer/answer model in RFC 3264. It defines how
two entities, an offerer and an answerer, exchange session two entities, an offerer and an answerer, exchange session
descriptions to negotiate a session. The offerer can include one or descriptions to negotiate a session. The offerer can include one or
more media formats (codecs) per media stream, and the answerer then more media formats (codecs) per media stream, and the answerer then
selects one or more of those offered and returns them in an answer. selects one or more of those offered and returns them in an answer.
Both the offer and the answer contain actual configurations - Both the offer and the answer contain actual configurations;
capabilities and potential configurations are not supported. The capabilities and potential configurations are not supported. The
answer however may reduce the set of actual configurations from the answer however may reduce the set of actual configurations from the
offer. The answer may also extend the set of actual configurations offer as well as extend the set of actual configurations that can be
that can be used to receive media by the answerer. used to receive media by the answerer.
Other relevant extensions have been defined. Simple capability Other relevant extensions have been defined. Simple capability
declarations, which define how to provide a simple and limited set of declarations, which define how to provide a simple and limited set of
capability descriptions in SDP was defined in RFC 3407. Grouping of capability descriptions in SDP was defined in RFC 3407. Grouping of
media lines, which defines how media lines in SDP can have other media lines, which defines how media lines in SDP can have other
semantics than the traditional "simultaneous media streams" semantics than the traditional "simultaneous media streams"
semantics, was defined in RFC 3388, etc. semantics, was defined in RFC 3388, etc.
Each of these extensions was designed to solve a specific limitation Each of these extensions was designed to solve a specific limitation
of SDP. Since SDP had already been stretched beyond its original of SDP. Since SDP had already been stretched beyond its original
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deploy new RTP profiles such as secure RTP (SRTP) [SRTP], RTP with deploy new RTP profiles such as secure RTP (SRTP) [SRTP], RTP with
RTCP-Based Feedback [AVPF], etc. This particular problem is RTCP-Based Feedback [AVPF], etc. This particular problem is
exacerbated by the fact that RTP profiles are defined independently. exacerbated by the fact that RTP profiles are defined independently.
When a new profile is defined and N other profiles already exist, When a new profile is defined and N other profiles already exist,
there is a potential need for defining N additional profiles, since there is a potential need for defining N additional profiles, since
profiles cannot be combined automatically. For example, in order to profiles cannot be combined automatically. For example, in order to
support the plain and secure RTP version of RTP with and without support the plain and secure RTP version of RTP with and without
RTCP-based feedback, four separate profiles (and hence profile RTCP-based feedback, four separate profiles (and hence profile
definitions) are needed: RTP/AVP [RFC3551], RTP/SAVP [SRTP], RTP/AVPF definitions) are needed: RTP/AVP [RFC3551], RTP/SAVP [SRTP], RTP/AVPF
[AVPF], and RTP/SAVPF [SAVPF]. In addition to the pressing profile [AVPF], and RTP/SAVPF [SAVPF]. In addition to the pressing profile
negotiation problem, other important real-life constraints have been negotiation problem, other important real-life limitations have been
found as well. found as well.
The purpose of this document is to define a mechanism that enables The purpose of this document is to define a mechanism that enables
SDP to provide limited support for indicating capabilities and their SDP to provide limited support for indicating capabilities and their
associated potential configurations and negotiate the use of those associated potential configurations, and negotiate the use of those
potential configurations as actual configurations. It is not the potential configurations as actual configurations. It is not the
intent to provide a full-fledged capability indication and intent to provide a full-fledged capability indication and
negotiation mechanism along the lines of SDPng or ITU-T H.245. negotiation mechanism along the lines of SDPng or ITU-T H.245.
Instead, the focus is on addressing a set of well-known real-life Instead, the focus is on addressing a set of well-known real-life
limitations. More specifically, the solution provided in this limitations. More specifically, the solution provided in this
document provides a general SDP capability negotiation framework. It document provides a general SDP capability negotiation framework. It
also defines specifically how to provide attributes and transport also defines specifically how to provide attributes and transport
protocols as capabilities and negotiate them using the framework. protocols as capabilities and negotiate them using the framework.
Extensions for other types of capabilities (e.g. media types and Extensions for other types of capabilities (e.g. media types and
formats) may be provided in other documents. formats) may be provided in other documents.
As mentioned above, SDP is used by several protocols, and hence the As mentioned above, SDP is used by several protocols, and hence the
mechanism should be usable by all of these. One particularly mechanism should be usable by all of these. One particularly
important protocol for this problem however is the Session Initiation important protocol for this problem is the Session Initiation
Protocol (SIP) [RFC3261]. SIP uses the offer/answer model (which is Protocol (SIP) [RFC3261]. SIP uses the offer/answer model (which is
not specific to SIP) to negotiate sessions and hence any mechanism not specific to SIP) to negotiate sessions and hence the mechanism
must at least consider how it either interacts with offer/answer, or defined here defines the offer/answer procedures to use for the
how it should extend it. capability negotiation framework.
The rest of the document is structured as follows. In Section 2. we The rest of the document is structured as follows. In Section 3. we
present our SDP capability negotiation solution followed by examples present our SDP capability negotiation solution, which consists of
in Section 3. and security considerations in Section 4. new SDP attributes and associated offer/answer procedures. In Section
4. we provide examples illustrating its use and in Section 5. we
provide the security considerations.
2. SDP Capability Negotiation Solution 2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. SDP Capability Negotiation Solution
In this section we first provide an overview of the SDP Capability In this section we first provide an overview of the SDP Capability
negotiation solution. This is followed by definitions of new SDP negotiation solution. This is followed by definitions of new SDP
attributes for the solution and its associated updated offer/answer attributes for the solution and its associated updated offer/answer
procedures. procedures.
2.1. Solution Overview 3.1. Solution Overview
The solution consists of the following: The solution consists of the following:
o Two new attributes to support versioning and extensions to the o Two new attributes to support versioning and extensions to the
framework itself as follows: framework itself as follows:
o A new attribute ("a=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 base and or
extensions to the framework that are required to be supported extensions to the framework that are required to be supported
by the entity receiving the SDP. by the entity receiving the SDP in order to do capability
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=capar") that defines how to list attribute o A new attribute ("a=acap") that defines how to list attribute
parameter values ("a=" values) as capabilities. parameter values ("a=" values) as capabilities.
o A new attribute ("a=ctrpr") 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
associated with them. Extension capabilities can be defined associated with them. Extension capabilities can be defined
and included in the potential configurations. and referenced in the potential configurations.
o A new attribute ("a=acfg") to be used in an answer SDP. The o A new attribute ("a=acfg") to be used in an answer SDP. The
attribute identifies which of the potential configurations attribute identifies which of the potential configurations
from an offer SDP were used as actual configurations to form from an offer SDP were used as actual configurations to form
the answer SDP. Extension capabilities can included. the answer SDP. Extension capabilities can be included as
well.
o Extensions to the offer/answer model that allow for capabilities o Extensions to the offer/answer model that allow for capabilities
and potential configurations to be included in an offer. When and potential configurations to be included in an offer.
included at the session level, they constitute latent capabilities Capabilities can be provided at the session level or the media
that may be used to guide a subsequent offer. When included at the level. Potential configurations can be included at the media level
media level, they constitute offers that may be accepted by the only, where they constitute alternative offers that may be
answerer instead of the actual configuration(s) included in the accepted by the answerer instead of the actual configuration(s)
"m=" line(s). The answerer indicates which (if any) of the included in the "m=" line(s). The answerer indicates which (if
potential configurations it used to form the answer by including any) of the potential configurations it used to form the answer by
the actual configuration attribute ("a=acfg") in the answer. including the actual configuration attribute ("a=acfg") in the
Capabilities and potential configurations may be included in answer. Capabilities may be included in answers as well, where
answers as well, where they can aid in guiding a subsequent new they can aid in guiding a subsequent new offer.
offer.
The mechanism is illustrated by the offer/answer exchange below, The mechanism is illustrated by the offer/answer exchange below,
where Alice sends an offer to Bob: where Alice sends an offer to Bob:
Alice Bob Alice Bob
| (1) Offer (SRTP and RTP) | | (1) Offer (SRTP and RTP) |
|--------------------------------->| |--------------------------------->|
| | | |
| (2) Answer (RTP) | | (2) Answer (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 (actual configuration), but SRTP is the preferred one
(potential configuration):
v=0 v=0
o=- 25678 753849 IN IP4 128.96.41.1 o=- 25678 753849 IN IP4 128.96.41.1
s= s=
c=IN IP4 128.96.41.1 c=IN IP4 128.96.41.1
t=0 0 t=0 0
m=audio 3456 RTP/AVP 0 18 m=audio 3456 RTP/AVP 0 18
a=creq: v0 a=creq: v0
a=ctrpr:1 RTP/SAVP a=tcap:1 RTP/SAVP
a=capar: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 required base and extensions are provided by the
"a=creq" attribute, which indicates that the option tag "v0", which "a=creq" attribute, which includes the option tag "v0" to indicate
indicates the base framework defined here, must be supported. The that the base framework defined here must be supported. The
capabilities are provided by the "a=ctrpr" and "a=capar" attributes. capabilities are provided by the "a=tcap" and "a=acap" attributes.
The capabilities indicate that secure RTP under the AVP profile The transport capabilities ("a=tcap") indicate that secure RTP under
("RTP/SAVP") is supported with an associated transport capability the AVP profile ("RTP/SAVP") is supported with an associated
handle of 1. The "capar" attribute provides an attribute capability transport capability handle of 1. The "acap" attribute provides an
with a handle of 1. The attribute capability is a "crypto" attribute, attribute capability with a handle of 1. The attribute capability is
which provides the keying material for SRTP using SDP security a "crypto" attribute, which provides the keying material for SRTP
descriptions [SDES]. The "a=pcfg" attribute provides the potential using SDP security descriptions [SDES]. The "a=pcfg" attribute
configuration included in the offer by reference to the capability provides the potential configuration included in the offer by
parameters. One alternatives is provided; it has a preference of 1 reference to the capability parameters. One alternative is provided;
and it consists of transport protocol capability 1 (i.e. the RTP/SAVP it has a configuration number of 1 and it consists of transport
profile - secure RTP), and the attribute capability 1, i.e. the protocol capability 1 (i.e. the RTP/SAVP profile - secure RTP), and
crypto attribute provided. Potential configurations are always the attribute capability 1, i.e. the crypto attribute provided.
preferred over actual configurations, and hence Alice is expressing a Potential configurations are always preferred over actual
preference for using secure RTP. configurations, and hence Alice is 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 128.96.41.2
s= s=
c=IN IP4 128.96.41.2 c=IN IP4 128.96.41.2
t=0 0 t=0 0
m=audio 4567 RTP/SAVP 0 18 m=audio 4567 RTP/SAVP 0 18
a=crypto:1 AES_CM_128_HMAC_SHA1_80 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4 inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4
a=acfg: 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 those in the
answer as well. 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 offer to use normal RTP. In that case, the following accepted the (actual configuration) offer to use normal RTP. In that
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 128.96.41.2
s= s=
c=IN IP4 128.96.41.2 c=IN IP4 128.96.41.2
t=0 0 t=0 0
m=audio 4567 RTP/AVP 0 18 m=audio 4567 RTP/AVP 0 18
2.2. Version and Extension Indication Attributes 3.2. Version and Extension Indication Attributes
In this section, we present the new attributes associated with In this section, we present the new attributes associated with
indicating the SDP capability negotiation version and extensions indicating the SDP capability negotiation extensions supported and
supported and required. required.
2.2.1. Supported Capability Negotiation Extensions Attribute 3.2.1. 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 5. 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>
where <option-tag-list> is defined by the following ABNF: RFC 4566, Section 9, provides the ABNF for SDP attributes. The "csup"
attribute adheres to the RFC 4566 "attribute" production, with an
att-value defined as follows:
att-value = *WSP 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 [SDP]
COMMA = *WSP "," *WSP ; defined in [RFC4234] COMMA = *WSP "," *WSP ; defined in [RFC4234]
White-space is permitted before the <option-tag-list>. Note that white-space is permitted before the option-tag-list. Also,
implementers familiar with SIP should note that the above definition
Implementers familiar with SIP should note that the above of COMMA differs from the one in [RFC3261].
definition of COMMA differs from the one in [RFC3261].
A special base option tag with a value of "v0" is defined for the A special base option tag with a value of "v0" is defined for the
basic SDP capability negotiation framework specified in this basic SDP capability negotiation framework. Entities use this option
document. Entities can use this option tag with the "a=csup" tag with the "a=csup" attribute to indicate support for the SDP
attribute to indicate support for the SDP capability negotiation capability negotiation framework specified in this document.
framework specified in this document.
The following examples illustrates the use of the "a=csup" attribute The following examples illustrates the use of the "a=csup" attribute
with two hypothetical option tags, "foo" and "bar": with the "v0" option tags and two hypothetical option tags, "foo" and
"bar":
a=csup: v0
a=csup: foo a=csup: foo
a=csup: bar a=csup: bar
a=csup: foo, bar a=csup: 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-stream SDP. When provided at the media-level, it applies to the media-stream
in question only (option-tags provided at the session level apply as in question only (option-tags provided at the session level apply as
well). There can be one or more "a=csup" attributes at both the well). There can be one or more "a=csup" attributes at both the
session and media-level (one or more per media stream in the latter session and media-level (one or more per media stream in the latter
case). 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
2.2.2. ) at the relevant levels. The base option tag MAY be included. 3.2.2. ) at the relevant levels. The base option tag MAY be included.
2.2.2. Required Capability Negotiation Extension Attribute 3.2.2. Required Capability Negotiation Extension Attribute
The SDP Capability negotiation solution allows for capability The SDP Capability negotiation solution allows for capability
negotiation extensions to be defined. Associated with each such negotiation extensions to be defined. Associated with each such
extension is an option tag that identifies the extension in question. extension is an option tag that identifies the extension in question.
Option-tags MUST be registered with IANA per the procedures defined Option-tags MUST be registered with IANA per the procedures defined
in Section 5. in Section 6.
The Required Capability Negotiation Extensions attribute ("a=csup") The Required Capability Negotiation Extensions attribute ("a=creq")
contains a comma-separated list of option tags identifying the SDP contains a comma-separated list of option tags identifying the SDP
Capability negotiation extensions that MUST be supported by the Capability negotiation extensions that MUST be supported by the
entity receiving the SDP in order for that entity to properly process entity receiving the SDP in order for that entity to properly process
the SDP Capability negotiation. The attribute is defined as follows: the SDP Capability negotiation. The attribute is defined as follows:
a=creq: <option-tag-list> a=creq: <option-tag-list>
where <option-tag-list> is defined in Section 2.2.1. The "creq" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows:
White-space is permitted before the <option-tag-list>. att-value = *WSP 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 two hypothetical option tags, "foo" and "bar": with the "v0" base option tag and two hypothetical option tags, "foo"
and "bar":
a=creq: v0
a=creq: foo a=creq: foo
a=creq: bar a=creq: bar
a=creq: foo, bar a=creq: 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 one or more "a=creq" attributes at both
the session and media-level (one or more per media stream in the the session and media-level (one or more per media stream in the
latter case). 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 ("creq") or supported ("csup") was listed in either the required ("a=creq") or supported
attributes from the offer, then the answerer is not required to ("a=csup") attributes from the offer, then the answerer is not
include a required ("creq") attribute with any option-tags that may required to include a required ("a=creq") attribute with any
need to be supported (such as the base option tag - "v0"). option-tags that may need to be supported (such as the base option
tag - "v0").
A recipient that receives an SDP and does not support one or more of 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. ignored. In that case, the entity SHOULD include a "csup" attribute
listing the SDP capability negotiation extensions it actually
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 The above rules apply to the base option tag as well. Thus, entities
compliant to this specification MUST include a "creq" attribute (at compliant to this specification MUST include a "creq" attribute (at
least in an offer) that includes the option tag "v0" as illustrated least in an offer) that includes the option tag "v0" as illustrated
below: below:
a=creq: v0 a=creq: v0
2.3. Capability Attributes 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.
2.3.1. Attribute Capability Attribute 3.3.1. Attribute Capability Attribute
Attributes can be expressed as negotiable parameters by use of a new Attributes can be expressed as negotiable parameters by use of a new
attribute capability attribute ("a=capar"), which is defined as attribute capability attribute ("a=acap"), which is defined as
follows: follows:
a=capar: <att-cap-num> <att-par> a=acap: <att-cap-num> <att-par>
where <att-cap-num> is an integer between 1 and 2^32-1 (both where <att-cap-num> is an integer between 1 and 2^31-1 (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 [SDP]).
The "capar" attribute can be provided at the session level for The "acap" attribute adheres to the RFC 4566 "attribute" production,
session-level attributes and the media level for media-level with an att-value defined as follows:
attributes. The "capar" attribute MUST NOT be used to provide a
media-level attribute at the session-level or vice versa.
Each occurrence of the "capar" attribute in the entire session att-value = *WSP att-cap-num 1*WSP att-par
description MUST use a different value of <app-cap-num>. att-cap-num = 1*DIGIT ;defined in [RFC4234]
att-par = attribute ;defined in RFC 4266
Note that white-space is permitted before the att-cap-num. The "acap"
attribute can be provided at the session level for session-level
attributes and the media level for media-level 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
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 handle references. 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 attributes, i.e., they form a separate num> values provided for other capability attributes, i.e., they form
name-space for attribute parameter capabilities. a separate name-space for attribute capabilities.
The following examples illustrate use of the "capar" attribute: The following examples illustrate use of the "acap" attribute:
a=capar: 1 a=ptime:20 a=acap: 1 a=ptime:20
a=capar: 2 a=ptime:30 a=acap: 2 a=ptime:30
a=capar: 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=capar: 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
The first two provide attribute values for the ptime attribute. The The first two provide attribute values for the ptime attribute. The
third one 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 one provides SRTP parameters by use of attribute [KMGMT]. The fourth provides SRTP parameters by use of
security descriptions with the crypto attribute [SDES]. security descriptions with the crypto attribute [SDES]. Note that the
line-wrapping and new-lines in example three and four are provided
for formatting reasons only - they are not permitted in actual SDP.
Readers familiar with RFC 3407 may notice the similarity between Readers familiar with RFC 3407 may notice the similarity between
the RFC 3407 "cpar" attribute and the above. There are however a the RFC 3407 "cpar" attribute and the above. There are however a
couple of important differences, most notably that the "capar" couple of important differences, most notably that the "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 "capar" 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.
2.3.2. Transport Protocol Capability Attribute 3.3.2. Transport Protocol Capability Attribute
Transport Protocols can be expressed as capabilities by use of a new Transport Protocols can be expressed as capabilities by use of a new
Transport Protocol Capability attribute ("a=ctrpr") defined as Transport Protocol Capability attribute ("a=tcap") defined as
follows: follows:
a=ctrpr: <trpr-cap-num> <proto-list> a=tcap: <trpr-cap-num> <proto-list>
where <trpr-cap-num> is an integer between 1 and 2^32-1 (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 "ctrpr" attribute can be provided at the session- and media- The "tcap" attribute adheres to the RFC 4566 "attribute" production,
level. Each occurrence of the "ctrpr" attribute in the entire session with an att-value defined as follows:
att-value = *WSP trpr-cap-num 1*WSP proto-list
trpr-cap-num = 1*DIGIT ;defined in [RFC4234]
proto-list = proto *(1*WSP proto) ; defined in RFC 4566
Note that white-space is permitted before the trpr-cap-num. The
"tcap" attribute can be provided at the session- and media-level.
Each occurrence of the "tcap" attribute in the entire session
description MUST use a different value of <trpr-cap-num>. When description MUST use a different value of <trpr-cap-num>. When
multiple <proto> values are provided, the first one is associated multiple <proto> values are provided, the first one is associated
with the value <trpr-cap-num>, the second one with the value one with the value <trpr-cap-num>, the second one with the value one
higher, etc. The <trpr-cap-num> values provided are independent of higher, etc. The <trpr-cap-num> values provided are independent of
similar <cap-num> values provided for other attributes, i.e., they similar <cap-num> values provided for other capability attributes,
form a separate name-space for transport protocol capabilities. i.e., they form a separate name-space for transport protocol
capabilities.
Below, we provide examples of the "a=ctrpr" attribute: Below, we provide examples of the "a=tcap" attribute:
a=ctrpr: 1 RTP/AVP a=tcap: 1 RTP/AVP
a=ctrpr: 2 RTP/AVPF a=tcap: 2 RTP/AVPF
a=ctrpr: 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.
2.4. Configuration Attributes Transport capabilities are inherently included in the "m=" line,
however they still need to be specified explicitly in a "tcap"
attribute, if they are to be used as a capability. 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
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).
2.4.1. Potential Configuration Attribute 3.4. Configuration Attributes
3.4.1. Potential Configuration Attribute
Potential Configurations can be expressed by use of a new Potential Potential Configurations can be expressed by use of a new Potential
Configuration Attribute ("a=pcfg") defined as follows: Configuration Attribute ("a=pcfg") defined as follows:
a=pcfg: <preference> <pot-cfg-list> a=pcfg: <config-number> <pot-cfg-list>
where <preference> is an integer between 1 and 2^32-1 (both included) where <config-number> is an integer between 1 and 2^31-1 (both
and <pot-cfg-list> is defined as included).
The "pcfg" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows:
att-value = *WSP config-number 1*WSP pot-cfg-list
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-parameter-config /
pot-transport-protocol-config | pot-transport-protocol-config /
pot-extension-config pot-extension-config
The potential configuration attribute includes a preference The missing productions are defined below. Note that white-space is
indication (lowest number is most preferred) followed by one or more permitted before the config-number.
of potential attribute parameter configuration and transport protocol
configuration. Each of these MUST NOT be present more than once in a
particular potential configuration attribute. Potential extension
configurations can be included as well. There can be more than one
potential extension configuration, however each particular potential
extension configuration MUST NOT be present more than once in a given
potential configuration attribute. Together, these values define a
potential configuration.
There can be multiple potential configurations provided at the The potential configuration attribute can be provided at the media-
session-level as well as the media-level. The semantics for each of level only. The attribute includes a configuration number, which is
these levels differ. A potential configuration at the session level an integer between 1 and 2^31-1 (both included). The configuration
provides a set of latent capabilities. A latent capability is merely number MUST be unique within the media stream. The configuration
an indication that the potential configuration could be supported, number also indicates the relative preference of potential
however it does not represent a willingness to do so at the current configurations; lower numbers are preferred over higher numbers.
time. A potential configuration at the media level on the other hand
indicates not only a willingness, but in fact a desire to use the
potential configuration.
In the case of offer/answer, this implies that a potential After the configuration number, one or more potential configuration
configuration at the session level does not constitute an parameters MUST be provided. This document defines potential
alternative offer whereas it does at the media level. attribute parameter configurations and potential transport protocol
configurations. Each of these MUST NOT be present more than once in
a particular potential configuration attribute. Potential extension
configurations can be included as well; unknown potential extension
configurations MUST be ignored (if support is required, then the
"a=creq" with a suitable option tag should be used). There can be
more than one potential extension configuration, however each
particular potential extension configuration MUST NOT be present more
than once in a given potential configuration attribute. Together,
these values define a potential configuration.
Associated with each potential configuration is a preference There can be multiple potential configurations provided within a
indication, which is an integer between 1 and 2^32-1 (both included) media description. Each of these indicates not only a willingness,
to indicate the relative preference of potential configurations. The but in fact a desire to use the potential configuration.
scope of the preference (and in fact each occurrence of a potential
configuration attribute) is the session-level, when provided there,
or the particular media stream it is provided at.
Attribute capabilities are included in a potential configuration by Attribute capabilities are included in a potential configuration by
use of the pot-attribute-parameter-config parameter, which is defined use of the pot-attribute-parameter-config parameter, which is defined
by the following ABNF: by the following ABNF:
pot-attribute-parameter-config pot-attribute-parameter-config
= "a=" capar-cap-list *(BAR capar-cap-list) = "a=" acap-cap-list *(BAR acap-cap-list)
capar-cap-list = att-cap-num *(COMMA att-cap-num) acap-cap-list = att-cap-num *(COMMA att-cap-num)
att-cap-num = 1*DIGIT ;defined in [RFC4234] att-cap-num = 1*DIGIT ;defined in [RFC4234]
BAR = *WSP "|" *WSP ; defined in [RFC4234]
Each potential attribute parameter configuration list is a comma- Each potential attribute parameter configuration list is a comma-
separated list of attribute capability numbers where att-cap-num separated list of attribute capability numbers where att-cap-num
refers to attribute capability numbers defined above and hence MUST refers to attribute capability numbers defined above and hence MUST
be between 1 and 2^32-1 (both included). Alternative potential be between 1 and 2^31-1 (both included). Alternative potential
attribute parameter configurations are separated by a vertical bar attribute parameter configurations are separated by a vertical bar
("|"), the scope of which extends to the next alternative (i.e. "," ("|"), the scope of which extends to the next alternative (i.e. ","
has higher precedence than "|"). The alternatives are ordered by has higher precedence than "|"). The alternatives are ordered by
preference. preference with the most preferred listed first.
Transport protocol capabilities are included in a potential Transport protocol capabilities are included in a potential
configuration by use of the pot-transport-protocol-config parameter, configuration by use of the pot-transport-protocol-config parameter,
which is defined by the following ABNF: which is defined by the following ABNF:
pot-transport-protocol-config = pot-transport-protocol-config =
"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]
The trpr-cap-num refers to transport protocol capability numbers The trpr-cap-num refers to transport protocol capability numbers
defined above and hence MUST be between 1 and 2^32-1 (both included). defined above and hence MUST be between 1 and 2^31-1 (both included).
Alternative potential transport protocol configurations are separated Alternative potential transport protocol configurations are separated
by a vertical bar ("|"). The alternatives are ordered by preference. by a vertical bar ("|"). The alternatives are ordered by preference
When transport protocol capabilities are not included in a potential with the most preferred listed first. When transport protocol
configuration at the media level, the transport protocol information capabilities are not included in a potential configuration at the
from the associated "m=" line will be used. At the session-level, media level, the transport protocol information from the associated
lack of a transport protocol capability indication simply implies "m=" line will be used.
that no such information is provided.
In the presence of middle-boxes (the existence of which may not be
known), care should be taken with assuming that the transport
protocol in the "m=" line will not be modified by a middle-box. Use
of an explicit capability will guard against the capability
indications 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. Such extensions MUST adhere to the following ABNF:
pot-extension-config = ext-cap-name "=" pot-extension-config = ext-cap-name "="
ext-cap-list *(BAR ext-cap-list) ext-cap-list *(BAR ext-cap-list)
ext-cap-name = token ; defined in [SDP] ext-cap-name = token ; defined in [SDP]
ext-cap-list = ext-cap-num *(COMMA ext-cap-num) ext-cap-list = ext-cap-num *(COMMA ext-cap-num)
ext-cap-num = 1*DIGIT ; defined in [RFC4234] ext-cap-num = 1*DIGIT ; defined in [RFC4234]
The ext-cap-name refers to the type of extension capability and the The ext-cap-name refers to the type of extension capability and the
ext-cap-num refers to a capability number associated with that ext-cap-num refers to a capability number associated with that
particular type of extension capability. The number MUST be between particular type of extension capability. The number MUST be between
1 and 2^32-1 (both included). Alternative potential extension 1 and 2^31-1 (both included). Alternative potential extension
configurations for a particular extension are separated by a vertical configurations for a particular extension are separated by a vertical
bar ("|"),the scope of which extends to the next alternative (i.e. bar ("|"),the scope of which extends to the next alternative (i.e.
"," has higher precedence than "|"). Unsupported or unknown "," has higher precedence than "|"). Unsupported or unknown
potential extension configs MUST be ignored. potential extension configs 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 session level and the Potential configurations can be provided at the media level only,
media level and in either case, it is syntactically possible to however it is possible to reference capabilities provided at either
reference attribute capabilities provided at either the session or the session or media level. There are certain semantic rules and
the media level. There are however semantic rules and limitations restrictions associated with this:
associated with this: At the session-level, a potential configuration
MUST NOT reference any attribute capabilities provided at the media- A (media level) potential configuration in a given media description
level. The converse however is permitted, i.e. a media-level MUST NOT reference a media-level capability provided in a different
potential configuration can reference a session-level attribute media description; doing so invalidates that potential configuration.
A potential configuration can however reference a session-level
capability. The semantics of doing so (should that potential capability. The semantics of doing so (should that potential
configuration be chosen), is that the corresponding attribute configuration be chosen), depends on the type of capability. In the
(provided within that attribute capability) will be considered part case of transport capabilities, this has no particular implication.
of the active configuration at the *session* level. In other words, In the case of attribute capabilities however, it does. More
it will be as-if that attribute was simply provided at the session- specifically, the corresponding attribute value (provided within that
attribute capability) will be considered part of the active
configuration at the *session* level. In other words, it will be as-
if that attribute was simply provided 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 session. It is up to the entity that
generates the SDP to ensure that the resulting active configuration generates the SDP to ensure that in such cases, the resulting active
SDP is still meaningful. configuration SDP is still meaningful.
[EDITOR'S NOTE: There are too many subtle differences between The session-level operation of extension capabilities is undefined:
potential configurations at the session and media level. I'm Consequently, if session-level extension capabilities are defined,
inclined to have two similarly looking (but different) attributes they MUST specify the implication of making them part of an active
instead as that will make it more straightforward and intuitive. configuration at the media level.
That also leaves the door open to have more than latent
capabilities at the session level in case that is needed later]
Below, we provide an example of the "a=pcfg" attribute in a complete Below, we provide an example of the "a=pcfg" attribute in a complete
media description in order to properly indicate the supporting media description in order to properly indicate the supporting
attributes: attributes:
v=0 v=0
o=- 25678 753849 IN IP4 128.96.41.1 o=- 25678 753849 IN IP4 128.96.41.1
s= s=
c=IN IP4 128.96.41.1 c=IN IP4 128.96.41.1
t=0 0 t=0 0
m=audio 3456 RTP/AVPF 0 18 m=audio 3456 RTP/AVPF 0 18
a=creq: v0 a=creq: v0
a=capar: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=ctrpr: 1 RTP/AVPF RTP/AVP a=tcap: 1 RTP/AVPF RTP/AVP
a=ctrpr: 3 RTP/SAVPF RTP/SAVP a=tcap: 3 RTP/SAVP RTP/SAVPF
a=pcfg:1 t=3|4 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 configurations listed here. The first one (and
most preferred, since it's preference is "1") indicates that either most preferred, since its configuration number is "1") indicates that
of the profiles RTP/SAVPF or RTP/SAVP (specified by the transport either of the profiles RTP/SAVPF or RTP/SAVP (specified by the
protocol capability numbers 3 and 4) can be supported with attribute transport protocol capability numbers 4 and 3) can be supported with
capability 1 (the "crypto" attribute); RTP/SAVPF is preferred since attribute capability 1 (the "crypto" attribute); RTP/SAVPF is
it is listed first. The second potential configuration indicates that preferred over RTP/SAVP since its capability number (4) is listed
the RTP/AVPF of RTP/AVP profile can be used, with RTP/AVPF being the first in the preferred potential configuration. The second potential
preferred one. This non secure RTP alternative is the less preferred configuration indicates that the RTP/AVPF of RTP/AVP profile can be
one since it's preference is "8". used, with RTP/AVPF being the preferred one. This non secure RTP
alternative is the less preferred one since its configuration number
is "8".
2.4.2. Actual Configuration Attribute 3.4.2. Actual Configuration Attribute
The actual configuration attribute identifies which of the potential The actual configuration attribute identifies which of the potential
configurations from an offer SDP were used as actual configurations configurations from an offer SDP were used as actual configurations
in an answer SDP. This is done by reference to the attribute in an answer SDP. This is done by reference to the configuration
capabilities and transport protocol capabilities from the offer that number and the attribute capabilities and transport protocol
were actually used by the answerer in his offer/answer procedure. If capabilities from the offer that were actually used by the answerer
extension capabilities were used, those will be included by reference in his offer/answer procedure. If extension capabilities were used,
as well. those will be included by reference as well. Note that the
configuration number and all capability numbers used are those from
the offer; not the answer.
The Actual Configuration Attribute ("a=acfg") is defined as follows: The Actual Configuration Attribute ("a=acfg") is defined as follows:
a=acfg: <act-cfg-list> a=acfg: <act-cfg-list>
where <act-cfg-list> is defined as The "acfg" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows:
att-value = *WSP config-number 1*WSP act-cfg-list
;config-number defined in Section 3.4.1.
act-cfg-list = capability *(1*WSP capability) act-cfg-list = capability *(1*WSP capability)
capability = act-attribute-parameter-config /
capability = act-attribute-parameter-config | act-transport-protocol-config /
act-transport-protocol-config |
act-extension-config act-extension-config
act-attribute-parameter-config is defined by the following ABNF: act-attribute-parameter-config =
"a=" acap-cap-list ; defined in Section 3.4.1.
act-attribute-parameter-config = "a=" capar-cap-list
where capar-cap-list is as defined in Section 2.4.1.
act-transport-protocol-config is defined by the following ABNF:
act-transport-protocol-config = "t=" trpr-cap-num
where trpr-cap-num is as defined in Section 2.4.1.
trpr-cap-num = 1*3DIGIT ; defined in [RFC4234]
act-extension-config is defined by the following ABNF:
act-extension-config = ext-cap-name "=" ext-cap-list act-transport-protocol-config =
"t=" trpr-cap-num ; defined in Section 3.4.1.
where ext-cap-name and ext-cap-list are as defined in Section 2.4.1. act-extension-config =
ext-cap-name "=" ext-cap-list ; defined in Section 3.4.1.
The actual configuration ("a=acfg") attribute can be provided at the Note that white-space is permitted before the config-number. 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 128.96.41.2
s= s=
c=IN IP4 128.96.41.2 c=IN IP4 128.96.41.2
t=0 0 t=0 0
m=audio 4567 RTP/SAVPF 0 m=audio 4567 RTP/SAVPF 0
a=creq: 0 a=creq: 0
a=acfg: t=3 a=1 a=acfg:1 t=4 a=1
It indicates that the answerer used an offer consisting of transport It indicates that the answerer used an offer consisting of potential
protocol capability 2 from the offer (RTP/SAVPF) and attribute configuration number 1 with transport protocol capability 4 from the
capability 1 (the "crypto" attribute. offer (RTP/SAVPF) and attribute capability 1 (the "crypto"
attribute).
2.5. Offer/Answer Model Extensions 3.5. Offer/Answer Model Extensions
In this section, we define extensions to the offer/answer model In this section, we define extensions to the offer/answer model
defined in [RFC3264] to allow for potential configurations to be defined in [RFC3264] to allow for potential configurations to be
included in an offer, where they constitute offers that may be included in an offer, where they constitute offers that may be
accepted by the answerer instead of the actual configuration(s) accepted by the answerer instead of the actual configuration(s)
included in the "m=" line(s). included in the "m=" line(s).
[EDITOR'S NOTE: Multicast considerations have been omitted for [EDITOR'S NOTE: Multicast considerations have been omitted for
now.] now.]
TO DO: Elaborate and firm up offer/answer procedures. TO DO: Elaborate and firm up offer/answer procedures.
2.5.1. Generating the Initial Offer 3.5.1. Generating the Initial Offer
An offerer that wants to use the SDP capability negotiation An offerer that wants to use the SDP capability negotiation
extensions defined in this document MUST include the following in the extensions defined in this document MUST include the following in the
offer: offer:
o an SDP capability negotiation required extensions attribute ("a- o an SDP capability negotiation required extensions attribute ("a-
creq") that contains the option tag "v0". It must either be creq") that contains the option tag "v0". It must either be
provided at the session-level or for each individual media stream. provided at the session-level or for each individual media stream.
Option tags for any other required extensions MUST be included as Option tags for any other required extensions MUST be included as
well (in accordance with Section 2.2.2. ) well (in accordance with Section 3.2.2. )
o one or more attribute capability attributes (as defined in Section o one or more attribute capability attributes (as defined in Section
2.3.1. ) if alternative attribute parameter values are to be 3.3.1. ) if alternative attribute parameter values are to be
indicated as offerer capabilities or be negotiated. indicated as offerer capabilities or be negotiated.
o one or more transport protocol capability attributes (as defined o one or more transport protocol capability attributes (as defined
in Section 2.3.2. ) if alternative transport protocols are to be in Section 3.3.2. ) if alternative transport protocols are to be
to be indicated as offerer capabilities or be negotiated. to be indicated as offerer capabilities or be negotiated.
o one or more potential configuration attributes (as defined in o one or more potential configuration attributes (as defined in
Section 2.4. ) if alternative potential configurations are to be Section 3.4. ) if alternative potential configurations are to be
negotiated. negotiated.
o one or more required capability negotiation extension attributes o one or more required capability negotiation extension attributes
(as defined in Section 2.2.2. ), if the answerer is required to (as defined in Section 3.2.2. ), if the answerer is required to
support one or more SDP capability negotiation extensions. support one or more SDP capability negotiation extensions.
The offerer SHOULD furthermore include the following: The offerer SHOULD furthermore include the following:
o one or more supported capability negotiation extension attributes o one or more supported capability negotiation extension attributes
("a=csup" as defined in Section 2.2.1. ), if the offerer supports ("a=csup" as defined in Section 3.2.1. ), if the offerer supports
one or more SDP capability negotiation extensions that have not one or more SDP capability negotiation extensions that have not
been included in one or more "a=creq" attributes at the relevant been included in one or more "a=creq" attributes at the relevant
session and media level(s). session and media level(s).
The capabilities provided merely indicate what the offerer is capable The capabilities provided merely indicate what the offerer is capable
of doing. They do not constitute a commitment or even an indication of doing. They do not constitute a commitment or even an indication
to actually use them. This applies to potential configurations listed to actually use them. This applies to potential configurations listed
at the session level as well. Conversely, each of the potential at the session level as well. Conversely, each of the potential
configurations listed at the media level constitutes an alternative configurations listed at the media level constitutes an alternative
offer which may be used to negotiate and establish the session. offer which may be used to negotiate and establish the session.
The current actual configuration is included in the "m=" line (as The current actual configuration is included in the "m=" line (as
defined by [RFC3264]). defined by [RFC3264]). Per [RFC3264], once the offerer generates the
offer, he must be prepared to receive incoming media in accordance
with that offer. That rule applies here as well, but for the actual
configurations only; media received by the offerer according to one
of the potential configurations MAY be discarded, until the offerer
receives an answer indicating what the actual configuration is. Once
that answer is received, incoming media MUST be processed in
accordance with the actual configuration indicated and the answer
received.
2.5.2. Generating the Answer 3.5.2. Generating the Answer
When the answerer receives an offer with valid SDP capability When the answerer receives an offer with valid SDP capability
negotiation information in it and in particular with one or more negotiation information in it and in particular with one or more
valid potential configuration information attributes present, it may valid potential configuration information attributes present, it may
use any of the potential configurations as an alternative offer. A use any of the potential configurations as an alternative offer. A
potential configuration information attribute is valid if all of the potential configuration information attribute is valid if all of the
capabilities (attribute capabilities, transport protocol capabilities capabilities (attribute capabilities, transport protocol capabilities
and any extension capabilities) it references are present and valid and any extension capabilities) it references are present and valid
themselves. themselves.
skipping to change at page 20, line 25 skipping to change at page 22, line 9
configurations instead of the actual configuration, the answerer MUST configurations instead of the actual configuration, the answerer MUST
generate an answer as if the offer contained that potential generate an answer as if the offer contained that potential
configuration instead of the actual configuration included. The configuration instead of the actual configuration included. The
answerer MUST also include an actual configuration attribute in the answerer MUST also include an actual configuration attribute in the
answer that identifies the potential configuration from the offer answer that identifies the potential configuration from the offer
used by the answerer. The actual configuration attribute in the used by the answerer. The actual configuration attribute in the
answer MUST include information about the attribute capabilities, answer MUST include information about the attribute capabilities,
transport protocol parameters, and extension capabilities from the transport protocol parameters, and extension capabilities from the
potential configuration that were used to generate the answer. potential configuration that were used to generate the answer.
2.5.3. Offerer Processing of the Answer 3.5.3. Offerer Processing of the Answer
When the offerer included potential configurations for a media When the offerer included potential configurations for a media
stream, it MUST examine the answer for the presence of an actual stream, it MUST examine the answer for the presence of an actual
configuration attribute for each such media stream. If the attribute configuration attribute for each such media stream. If the attribute
is missing, offerer processing of the answer MUST proceed as defined is missing, offerer processing of the answer MUST proceed as defined
by [RFC3264]. If the attribute is present, processing continues as by [RFC3264]. If the attribute is present, processing continues as
follows: follows:
The actual configuration attribute specifies which of the potential The actual configuration attribute specifies which of the potential
configurations were used by the answerer to generate the answer. This configurations were used by the answerer to generate the answer. This
includes all the types of capabilities from the potential includes all the types of capabilities from the potential
configuration offered, i.e. the attribute capabilities ("a=capar"), configuration offered, i.e. the attribute capabilities ("a=acap"),
transport protocol capabilities ("a=ctrpr"), and any extension transport protocol capabilities ("a=tcap"), and any extension
capability parameters included. capability parameters included.
The offerer MUST now process the answer as if the offer had contained The offerer MUST now process the answer as if the offer had contained
the potential configuration as the actual configuration in the media the potential configuration as the actual configuration in the media
description ("m=" line) and relevant attributes in the offer. description ("m=" line) and relevant attributes in the offer.
2.5.4. Modifying the Session If the answerer selected one of the potential configurations from the
offer as the actual configuration, then the offerer SHOULD perform
another offer/answer exchange, where the offer contains the selected
potential configuration as the actual configuration, i.e. with the
actual configuration used in the "m=" line and any other relevant
attributes. This second offer/answer exchange will not modify the
session anyway, however it will help intermediaries that look at the
SDP, but do not understand the capability negotiation extensions, to
understand the details of the negotiated media streams.
3.5.4. Modifying the Session
Potential configurations may be included in subsequent offers as Potential configurations may be included in subsequent offers as
defined in [RFC3264, Section 8]. The procedure for doing so is defined in [RFC3264, Section 8]. The procedure for doing so is
similar to that described above with the answer including an similar to that described above with the answer including an
indication of the actual configuration used by the answerer. indication of the actual configuration used by the answerer.
3. Examples If the answer indicates use of a potential configuration from the
offer, then a second offer/answer exchange using that potential
configuration as the actual configuration SHOULD be performed.
3.6. Interactions with ICE
Interactive Connectivity Establishment (ICE) [ICE] provides a
mechanism for verifying connectivity between two endpoints by sending
STUN messages directly between the media endpoints. The basic ICE
specification [ICE] is defined to support UDP-based connectivity
only, however it allows for extensions to support other transport
protocols, such as TCP, which is being specified in [ICETCP]. ICE
defines a new "a=candidate" attribute, which, among other things,
indicates the possible transport protocol(s) to use and then
associates a priority with each of them. The most preferred transport
protocol that *successfully* verifies connectivity will end up being
used.
When using ICE, it is thus possible that the transport protocol that
will be used differs from what is specified in the "m=" line.
Furthermore, since both ICE and SDP Capability Negotiation may now
specify alternative transport protocols, there is a potentially
unintended interaction when using these together.
We provide the following guidelines for addressing that.
[EDITOR'S NOTE: This requires more work]
There are two basic scenarios to consider here:
1) A particular media stream can run over different transport
protocols (e.g. UDP, TCP, or TCP/TLS), and the intent is simply to
use the one that works (in the preference order specified).
2) A particular media stream can run over different transport
protocols (e.g. UDP, TCP, or TCP/TLS) and the intent is to have the
negotiation process decide which one to use (e.g. T.38 over TCP or
UDP).
In scenario 1, there should be ICE "a=candidate" attributes for UDP,
TCP, etc. but otherwise nothing special in the potential
configuration attributes to indicate the desire to use different
transport protocols (e.g. UDP, or TCP). The ICE procedures
essentially cover the capability negotiation required (by having the
answerer select something it supports and then use of trial and
error).
Scenario 2 does not require a need to support or use ICE. Instead, we
simply use transport protocol capabilities and potential
configuration attributes to indicate the desired outcome.
The scenarios may be combined, e.g. by offering potential
configuration alternatives where some of them can support one
transport protocol only (e.g. UDP), whereas others can support
multiple transport protocols (e.g. UDP or TCP). In that case, the ICE
candidate attributes should 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
potential configurations that are restricted to UDP, whereas there
could be candidate attributes for UDP, TCP, and TCP/TLS for potential
configurations that can use all three).
3.7. Processing Media before Answer
The offer/answer model requires an offerer to be able to receive
media in accordance with the offer prior to receiving the answer.
This property is retained with the SDP capability negotiation
extensions defined here, but only when the actual configuration is
selected by the answerer. If a potential configuration is chosen, it
is permissible for the offerer to not process any media received
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
precondition [RFC3312] for capability negotiation, however
preconditions are viewed as complicated to implement and they add to
overall session establishment delay by requiring an extra
offer/answer exchange. An alternative is therefore desirable.
The SDP capability negotiation framework does not define such an
alternative, however extensions may do so. For example, one technique
proposed for best-effort SRTP in [BESRTP] is to provide different RTP
payload type mappings for different transport protocols used. The
basic SDP capability negotiation framework defined here does not
include the ability to do so, however extensions that enable that may
be defined.
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.
3.1. 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
the answerer does not support secure RTP (or the SDP capability the answerer does not support secure RTP (or the SDP capability
negotiation extensions), an RTP session will be established. However, negotiation extensions), an RTP session will be established. However,
if the answerer supports Secure RTP and the SDP Capability if the answerer supports Secure RTP and the SDP Capability
Negotiation extensions, a Secure RTP session will be established. Negotiation extensions, a Secure RTP session will be established.
The best-effort Secure RTP negotiation is illustrated by the The best-effort Secure RTP negotiation is illustrated by the
offer/answer exchange below, where Alice sends an offer to Bob: offer/answer exchange below, where Alice sends an offer to Bob:
Alice Bob Alice Bob
| (1) Offer (SRTP and RTP) | | (1) Offer (SRTP and RTP) |
|--------------------------------->| |--------------------------------->|
| | | |
| (2) Answer (RTP) | | (2) Answer (SRTP) |
|<---------------------------------|
| |
| (3) Offer (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 128.96.41.1
s= s=
c=IN IP4 128.96.41.1 c=IN IP4 128.96.41.1
t=0 0 t=0 0
m=audio 3456 RTP/AVP 0 18 m=audio 3456 RTP/AVP 0 18
a=creq: v0 a=creq: v0
a=ctrpr:1 RTP/SAVP RTP/AVP a=tcap:1 RTP/SAVP RTP/AVP
a=capar: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
FEC_ORDER=FEC_SRTP
a=pcfg:1 t=1 a=1
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
defined here is required by including the "a=creq" attribute
containing the value "v0". The capabilities are provided by the
"a=tcap" and "a=acap" attributes. The "tcap" capability indicates
that both Secure RTP and normal RTP are supported. The "acap"
attribute provides a capability parameter with a handle of 1. The
capability parameter is a "crypto" attribute, which provides the
keying material for SRTP using SDP security descriptions [SDES]. The
"a=pcfg" attribute provides the potential configurations included in
the offer by reference to the capabilities. A single potential
configuration with a configuration number of "1" is provided. It
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
Capability Negotiation extensions, and hence he accepts the potential
configuration for Secure RTP provided by Alice:
v=0
o=- 24351 621814 IN IP4 128.96.41.2
s=
c=IN IP4 128.96.41.2
t=0 0
m=audio 4567 RTP/SAVP 0 18
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4
a=acfg:1 t=1 a=1
Bob includes the "a=acfg" attribute in the answer to inform Alice
that he based his answer on an offer containing the potential
configuration with transport protocol capability 1 and attribute
capability 1 from the offer SDP (i.e. the RTP/SAVP profile using the
keying material provided). Bob also includes his keying material in
a crypto attribute.
When Alice receives Bob's answer, session negotiation has completed,
however Alice nevertheless generates a new offer using the actual
configuration. This is done purely to assist any middle-boxes that
may reside between Alice and Bob but do not support the capability
negotiation extensions (and hence may not understand the negotiation
that just took place):
Alice's updated offer includes only SRTP, and it is not using the SDP
capability negotiation extensions (Alice could have included the
capabilities as well is she wanted to):
v=0
o=- 25678 753850 IN IP4 128.96.41.1
s=
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/SAVP 0 18
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
FEC_ORDER=FEC_SRTP
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
keying material, is included with the same value again.
Bob receives the SDP offer from Alice, which he accepts, and then
generates an answer to Alice:
v=0
o=- 24351 621815 IN IP4 128.96.41.2
s=
c=IN IP4 128.96.41.2
t=0 0
m=audio 4567 RTP/SAVP 0 18
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4
Bob includes the same crypto attribute as before, and the session
proceeds without change. Although Bob did not include any
capabilities in his answer, he could of course have done so if he
wanted to.
Note that in this particular example, the answerer supported the
capability extensions defined here, however had he not, the answerer
would simply have ignored the new attributes received in step 1 and
accepted the offer to use normal RTP. In that case, the following
answer would have been generated in step 2 instead:
v=0
o=- 24351 621814 IN IP4 128.96.41.2
s=
c=IN IP4 128.96.41.2
t=0 0
m=audio 4567 RTP/AVP 0 18
4.2. Multiple Transport Protocols
[EDITOR'S NOTE: Example to be updated - old copy below]
The following example illustrates how to use the SDP Capability
negotiation extensions to support so-called Best-Effort Secure RTP.
In that scenario, the offerer supports both RTP and Secure RTP. If
the answerer does not support secure RTP (or the SDP capability
negotiation extensions), an RTP session will be established. However,
if the answerer supports Secure RTP and the SDP Capability
Negotiation extensions, a Secure RTP session will be established.
The best-effort Secure RTP negotiation is illustrated by the
offer/answer exchange below, where Alice sends an offer to Bob:
Alice Bob
| (1) Offer (SRTP and RTP) |
|--------------------------------->|
| |
| (2) Answer (SRTP) |@@
|<---------------------------------|
| |
| (3) Offer (SRTP) |
|--------------------------------->|
| |
| (4) Answer (SRTP) |
|<---------------------------------|
Alice's offer includes RTP and SRTP as alternatives. RTP is the
default, but SRTP is the preferred one:
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/AVP 0 18
a=creq: v0
a=tcap:1 RTP/SAVP RTP/AVP
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:5 t=1 a=1 a=pcfg:5 t=1 a=1
a=pcfg:10 t=2 a=pcfg:10 t=2
The "m=" line indicates that Alice is offering to use plain RTP with The "m=" line indicates that Alice is offering to use plain RTP with
PCMU or G.729. Alice indicates that support for the base protocol PCMU or G.729. Alice indicates that support for the base protocol
defined here is required by including the "a=creq" attribute defined here is required by including the "a=creq" attribute
containing the value "v0". The capabilities are provided by the containing the value "v0". The capabilities are provided by the
"a=ctrpr" and "a=capar" attributes. The capabilities indicate that "a=tcap" and "a=acap" attributes. The capabilities indicate that
both Secure RTP and normal RTP are supported. The "capar" attribute both Secure RTP and normal RTP are supported. The "acap" attribute
provides a capability parameter with a handle of 1. The capability provides a capability parameter with a handle of 1. The capability
parameter is a "crypto" attribute in the capability set, which parameter is a "crypto" attribute in the capability set, which
provides the keying material for SRTP using SDP security descriptions provides the keying material for SRTP using SDP security descriptions
[SDES]. The "a=pcfg" attribute provides the potential configurations [SDES]. The "a=pcfg" attribute provides the potential configurations
included in the offer by reference to the capabilities. Two included in the offer by reference to the capabilities. Two
alternatives are provided; the first one with preference "5" (and alternatives are provided; the first one with preference "5" (and
hence the preferred one since the preference on the second one is hence the preferred one since the preference on the second one is
"10") is transport protocol capability 1 (RTP/SAVP, i.e. secure RTP) "10") is transport protocol capability 1 (RTP/SAVP, i.e. secure RTP)
together with the attribute capability 1, i.e. the crypto attribute together with the attribute capability 1, i.e. the crypto attribute
provided. The second one is using transport protocol capability 2. provided. The second one is using transport protocol capability 2.
skipping to change at page 22, line 34 skipping to change at page 29, line 26
v=0 v=0
o=- 24351 621814 IN IP4 128.96.41.2 o=- 24351 621814 IN IP4 128.96.41.2
s= s=
c=IN IP4 128.96.41.2 c=IN IP4 128.96.41.2
t=0 0 t=0 0
m=audio 4567 RTP/SAVP 0 18 m=audio 4567 RTP/SAVP 0 18
a=crypto:1 AES_CM_128_HMAC_SHA1_80 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4 inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4
a=csup: foo a=csup: foo
a=acfg: 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. Finally, Bob supports an SDP capability a crypto attribute. Finally, Bob supports an SDP capability
negotiation extension with the option tag "foo" and hence he includes negotiation extension with the option tag "foo" and hence he includes
the "a=csup" parameter containing value "foo" in the answer. the "a=csup" parameter containing value "foo" in the answer.
skipping to change at page 23, line 14 skipping to change at page 30, line 6
to use normal RTP. In that case, the following answer would have been to use normal RTP. In that case, the following answer would have been
generated instead: generated instead:
v=0 v=0
o=- 24351 621814 IN IP4 128.96.41.2 o=- 24351 621814 IN IP4 128.96.41.2
s= s=
c=IN IP4 128.96.41.2 c=IN IP4 128.96.41.2
t=0 0 t=0 0
m=audio 4567 RTP/AVP 0 18 m=audio 4567 RTP/AVP 0 18
4. Security Considerations 4.3. Session-Level MIKEY and Media Level Security Descriptions
[EDITOR'S NOTE: Example to be added]
4.4. Capability Negotiation with Interactive Connectivity Establishment
[EDITOR'S NOTE: Example to be added]
5. Security Considerations
TBD. TBD.
5. IANA Considerations 6. IANA Considerations
TBD. TBD.
[EDITOR'S NOTE: Need to define registry and procedures for option [EDITOR'S NOTE: Need to define registry and procedures for option
tags] tags]
[EIDTOR'S NOTE: Need to define registry and procedures for extension [EIDTOR'S NOTE: Need to define registry and procedures for extension
capabilities] capabilities]
6. To Do and Open Issues 7. To Do and Open Issues
o Capability descriptions, potential configurations and actual
configurations can be provided at both the session level and media
level. It needs to be decided what the relationship between the
session level and media level parameters are.
o Look for "EDITOR'S NOTE" throughout the document. o Look for "EDITOR'S NOTE" throughout the document.
7. Acknowledgments 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: Roni Even, Robert Gilman, Cullen Jennings, Matt Lepinski, Joerg here: Roni Even, Robert Gilman, Cullen Jennings, Matt Lepinski, Joerg
Ott, Colin Perkins, and Thomas Stach. Ott, Colin Perkins, and Thomas Stach.
Francois Audet and Dan Wing provided useful comments on earlier Francois Audet and Dan Wing provided useful comments on earlier
versions of this document. versions of this document.
8. Change Log 9. Change Log
8.1. draft-ietf-mmusic-sdp-capability-negotiation-01 9.1. draft-ietf-mmusic-sdp-capability-negotiation-02
The following are the major changes compared to version -01:
o Potential configurations are no longer allowed at the session
level
o Renamed capability attributes ("capar" to "acap" and "ctrpr" to
"tcap")
o Changed name and semantics of the initial number (now called
configuration number) in potential configuration attributes; must
now be unique and can be used as a handle
o Actual configuration attribute now includes configuration number
from the selected potential configuration attribute
o Added ABNF throughout
o Specified that answerer should include "a=csup" in case of
unsupported required extensions in offer.
o Specified use of second offer/answer exchange when answerer
selected a potential configuration
o Updated rules (and added restrictions) for referencing media- and
session-level capabilities in potential configurations (at the
media level)
o Added initial section on ICE interactions
o Added initial section on receiving media before answer
9.2. 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 24, line 39 skipping to change at page 32, line 29
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.
8.2. draft-ietf-mmusic-sdp-capability-negotiation-00 9.3. 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.
o Multiple transport capabilities can be included in a single o Multiple transport capabilities can be included in a single "tcap"
"ctrpr" attribute attribute
o A version attribute is now included. o A version attribute is now included.
o Extensions to the framework are formally supported. o Extensions to the framework are formally supported.
o Option tags and the ability to list supported and required o Option tags and the ability to list supported and required
extensions are supported. extensions are supported.
o A best-effort SRTP example use case has been added. o A best-effort SRTP example use case has been added.
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.
9. References 10. References
9.1. 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 [RFC2234] Crocker, D. and Overell, P.(Editors), "Augmented BNF for
Syntax Specifications: ABNF", RFC 2234, Internet Mail Syntax Specifications: ABNF", RFC 2234, Internet Mail
Consortium and Demon Internet Ltd., November 1997. 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
skipping to change at page 26, line 33 skipping to change at page 34, line 33
[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 [SDP] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006. Description Protocol", RFC 4566, July 2006.
9.2. 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,
November 1996. November 1996.
[RFC2327] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session [RFC2327] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 2327, April 1998. Description Protocol", RFC 2327, April 1998.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler,
skipping to change at page 28, line 16 skipping to change at page 36, line 16
Requirementes and Review of Existing Work", work in Requirementes and Review of Existing Work", work in
progress, December 2006. progress, December 2006.
[SDPCapNeg] Andreasen, F. "SDP Capability Negotiation", work in [SDPCapNeg] Andreasen, F. "SDP Capability Negotiation", work in
progress, December 2006. progress, December 2006.
[MIKEY] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K. [MIKEY] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830, Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
August 2004. August 2004.
[ICE] J. Rosenberg, "Interactive Connectivity Establishment
(ICE): A Methodology for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", work in progress,
January 2007.
[ICETCP] J. Rosenberg, "TCP Candidates with Interactive Connectivity
Establishment (ICE)", work in progress, October 2006.
[RFC3312] G. Camarillo, W. Marshall, and J. Rosenberg, "Integration
of Resource Management and Session Initiatio Protocol
(SIP)", RFC 3312, October 2002.
Author's Addresses Author's Addresses
Flemming Andreasen Flemming Andreasen
Cisco Systems Cisco Systems
Edison, NJ Edison, NJ
Email: fandreas@cisco.com Email: fandreas@cisco.com
Intellectual Property Statement Intellectual Property Statement
skipping to change at page 29, line 5 skipping to change at page 37, line 18
such proprietary rights by implementers or users of this such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr. http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at this standard. Please address the information to the IETF at
ietf-ipr@ietf.org. ietf-ipr@ietf.org.
Disclaimer of Validity Full Copyright Statement
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2007). Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors contained in BCP 78, and except as set forth therein, the authors
retain all their rights. retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgment Acknowledgment
Funding for the RFC Editor function is currently provided by the Funding for the RFC Editor function is currently provided by the
Internet Society. Internet Society.
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