draft-ietf-mmusic-sdp-capability-negotiation-06.txt   draft-ietf-mmusic-sdp-capability-negotiation-07.txt 
MMUSIC Working Group F. Andreasen MMUSIC Working Group F. Andreasen
Internet-Draft Cisco Systems Internet-Draft Cisco Systems
Intended Status: Proposed Standard July 8, 2007 Intended Status: Proposed Standard October 28, 2007
Obsolotes: 3407
Expires: January 2008
SDP Capability Negotiation SDP Capability Negotiation
draft-ietf-mmusic-sdp-capability-negotiation-06.txt draft-ietf-mmusic-sdp-capability-negotiation-07.txt
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Abstract Abstract
The Session Description Protocol (SDP) was intended for describing The Session Description Protocol (SDP) was intended for describing
multimedia sessions for the purposes of session announcement, session multimedia sessions for the purposes of session announcement,
invitation, and other forms of multimedia session initiation. SDP was session invitation, and other forms of multimedia session
not intended to provide capability indication or capability initiation. SDP was not intended to provide capability indication or
negotiation, however over the years, SDP has seen widespread adoption capability negotiation, however over the years, SDP has seen
and as a result it has been gradually extended to provide limited widespread adoption and as a result it has been gradually extended
support for these, notably in the form of the offer/answer model to provide limited support for these, notably in the form of the
defined in RFC 3264. SDP and its current extensions however do not offer/answer model defined in RFC 3264. SDP and its current
define how to negotiate one or more alternative transport protocols extensions do not define how to negotiate one or more alternative
(e.g. RTP profiles) or attributes. This makes it difficult to deploy transport protocols (e.g. RTP profiles) or attributes. This makes it
new RTP profiles such as secure RTP or RTP with RTCP-based feedback, difficult to deploy new RTP profiles such as secure RTP or RTP with
negotiate use of different keying mechanisms, etc. It also presents RTCP-based feedback, negotiate use of different security keying
problems for some forms of media negotiation. mechanisms, etc. It also presents problems for some forms of media
negotiation.
The purpose of this document is to address these shortcomings by The purpose of this document is to address these shortcomings by
extending SDP with capability negotiation parameters and associated extending SDP with capability negotiation parameters and associated
offer/answer procedures to use those parameters in a backwards offer/answer procedures to use those parameters in a backwards
compatible manner. compatible manner.
The solution provided in this document provides a general SDP The document defines a general SDP Capability Negotiation framework.
Capability Negotiation framework. It also defines specifically how to It also specifies how to provide attributes and transport protocols
provide attributes and transport protocols as capabilities and as capabilities and negotiate them using the framework. Extensions
negotiate them using the framework. Extensions for other types of for other types of capabilities (e.g. media types and media formats)
capabilities (e.g. media types and media formats) may be provided in may be provided in other documents.
other documents.
Table of Contents Table of Contents
1. Introduction...................................................3 1. Introduction...................................................3
2. Conventions used in this document..............................7 2. Conventions used in this document..............................7
3. SDP Capability Negotiation Solution............................7 3. SDP Capability Negotiation Solution............................7
3.1. SDP Capability Negotiation Model..........................7 3.1. SDP Capability Negotiation Model..........................7
3.2. Solution Overview........................................10 3.2. Solution Overview........................................10
3.3. Relationship to RFC 3407.................................13 3.3. Version and Extension Indication Attributes..............13
3.4. Version and Extension Indication Attributes..............13 3.3.1. Supported Capability Negotiation Extensions Attribute13
3.4.1. Supported Capability Negotiation Extensions Attribute13 3.3.2. Required Capability Negotiation Extensions Attribute14
3.4.2. Required Capability Negotiation Extension Attribute.15 3.4. Capability Attributes....................................16
3.5. Capability Attributes....................................16 3.4.1. Attribute Capability Attribute......................16
3.5.1. Attribute Capability Attribute......................16 3.4.2. Transport Protocol Capability Attribute.............18
3.5.2. Transport Protocol Capability Attribute.............18 3.4.3. Extension Capability Attributes.....................19
3.5.3. Extension Capability Attributes.....................19 3.5. Configuration Attributes.................................19
3.6. Configuration Attributes.................................20 3.5.1. Potential Configuration Attribute...................19
3.6.1. Potential Configuration Attribute...................20 3.5.2. Actual Configuration Attribute......................27
3.6.2. Actual Configuration Attribute......................27 3.6. Offer/Answer Model Extensions............................29
3.7. Offer/Answer Model Extensions............................29 3.6.1. Generating the Initial Offer........................29
3.7.1. Generating the Initial Offer........................29 3.6.2. Generating the Answer...............................32
3.7.2. Generating the Answer...............................32 3.6.2.1. Example Views of Potential Configurations......38
3.7.2.1. Example Views of Potential Configurations......37 3.6.3. Offerer Processing of the Answer....................40
3.7.3. Offerer Processing of the Answer....................39 3.6.4. Modifying the Session...............................41
3.7.4. Modifying the Session...............................41 3.7. Interactions with ICE....................................42
3.8. Interactions with ICE....................................41 3.8. Interactions with SIP Option Tags........................43
3.9. Interactions with SIP Option Tags........................42 3.9. Processing Media before Answer...........................44
3.10. Processing Media before Answer..........................43 3.10. Indicating Bandwidth Usage..............................45
3.11. Dealing with Large Number of Potential Configurations...44 3.11. Dealing with Large Number of Potential Configurations...46
3.12. SDP Capability Negotiation and Intermediaries...........45 3.12. SDP Capability Negotiation and Intermediaries...........47
3.13. Considerations for Specific Attribute Capabilities......46 3.13. Considerations for Specific Attribute Capabilities......48
3.13.1. The rtpmap and fmtp Attributes.....................46 3.13.1. The rtpmap and fmtp Attributes.....................48
3.13.2. Direction Attributes...............................47 3.13.2. Direction Attributes...............................49
4. Examples......................................................48 3.14. Relationship to RFC 3407................................50
4.1. Best-Effort Secure RTP...................................48 4. Examples......................................................50
4.2. Multiple Transport Protocols.............................51 4.1. Best-Effort Secure RTP...................................50
4.2. Multiple Transport Protocols.............................53
4.3. Best-Effort SRTP with Session-Level MIKEY and Media Level 4.3. Best-Effort SRTP with Session-Level MIKEY and Media Level
Security Descriptions.........................................54 Security Descriptions.........................................57
4.4. SRTP with Session-Level MIKEY and Media Level Security 4.4. SRTP with Session-Level MIKEY and Media Level Security
Descriptions as Alternatives..................................59 Descriptions as Alternatives..................................62
5. Security Considerations.......................................62 5. Security Considerations.......................................64
6. IANA Considerations...........................................64 6. IANA Considerations...........................................67
6.1. New SDP Attributes.......................................64 6.1. New SDP Attributes.......................................67
6.2. New SDP Capability Negotiation Option Tag Registry.......66 6.2. New SDP Capability Negotiation Option Tag Registry.......68
6.3. New SDP Capability Negotiation Potential Configuration 6.3. New SDP Capability Negotiation Potential Configuration
Parameter Registry............................................66 Parameter Registry............................................69
7. Acknowledgments...............................................66 7. Acknowledgments...............................................69
8. Change Log....................................................67 8. Change Log....................................................69
8.1. draft-ietf-mmusic-sdp-capability-negotiation-06..........67 8.1. draft-ietf-mmusic-sdp-capability-negotiation-07..........69
8.2. draft-ietf-mmusic-sdp-capability-negotiation-05..........68 8.2. draft-ietf-mmusic-sdp-capability-negotiation-06..........70
8.3. draft-ietf-mmusic-sdp-capability-negotiation-04..........69 8.3. draft-ietf-mmusic-sdp-capability-negotiation-05..........71
8.4. draft-ietf-mmusic-sdp-capability-negotiation-03..........70 8.4. draft-ietf-mmusic-sdp-capability-negotiation-04..........72
8.5. draft-ietf-mmusic-sdp-capability-negotiation-02..........70 8.5. draft-ietf-mmusic-sdp-capability-negotiation-03..........73
8.6. draft-ietf-mmusic-sdp-capability-negotiation-01..........71 8.6. draft-ietf-mmusic-sdp-capability-negotiation-02..........73
8.7. draft-ietf-mmusic-sdp-capability-negotiation-00..........71 8.7. draft-ietf-mmusic-sdp-capability-negotiation-01..........74
9. References....................................................73 8.8. draft-ietf-mmusic-sdp-capability-negotiation-00..........75
9.1. Normative References.....................................73 9. References....................................................76
9.2. Informative References...................................73 9.1. Normative References.....................................76
Author's Addresses...............................................76 9.2. Informative References...................................76
Intellectual Property Statement..................................76 Author's Addresses...............................................78
Full Copyright Statement.........................................76 Intellectual Property Statement..................................78
Acknowledgment...................................................77 Full Copyright Statement.........................................79
Acknowledgment...................................................79
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,
invitation, and other forms of multimedia session initiation. The SDP session invitation, and other forms of multimedia session
contains one or more media stream descriptions with information such initiation. The SDP contains one or more media stream descriptions
as IP-address and port, type of media stream (e.g. audio or video), with information such as IP-address and port, type of media stream
transport protocol (possibly including profile information, e.g. (e.g. audio or video), transport protocol (possibly including
RTP/AVP or RTP/SAVP), media formats (e.g. codecs), and various other profile information, e.g. RTP/AVP or RTP/SAVP), media formats (e.g.
session and media stream parameters that define the session. codecs), and various other session and media stream parameters that
define the session.
Simply providing media stream descriptions is sufficient for session Simply providing media stream descriptions is sufficient for session
announcements for a broadcast application, where the media stream announcements for a broadcast application, where the media stream
parameters are fixed for all participants. When a participant wants parameters are fixed for all participants. When a participant wants
to join the session, he obtains the session announcement and uses the to join the session, he obtains the session announcement and uses
media descriptions provided, e.g., joins a multicast group and the media descriptions provided, e.g., joins a multicast group and
receives media packets in the encoding format specified. If the receives media packets in the encoding format specified. If the
media stream description is not supported by the participant, he is media stream description is not supported by the participant, he is
unable to receive the media. unable to receive the media.
Such restrictions are not generally acceptable to multimedia session Such restrictions are not generally acceptable to multimedia session
invitations, where two or more entities attempt to establish a media invitations, where two or more entities attempt to establish a media
session that uses a set of media stream parameters acceptable to all session, that uses a set of media stream parameters acceptable to
participants. First of all, each entity must inform the other of its all participants. First of all, each entity must inform the other of
receive address, and secondly, the entities need to agree on the its receive address, and secondly, the entities need to agree on the
media stream parameters to use for the session, e.g. transport media stream parameters to use for the session, e.g. transport
protocols and codecs. To solve this, RFC 3264 [RFC3264] defined the protocols and codecs. To solve this, RFC 3264 [RFC3264] defined the
offer/answer model, whereby an offerer constructs an offer SDP that offer/answer model, whereby an offerer constructs an offer SDP that
lists the media streams, codecs, and other SDP parameters that the lists the media streams, codecs, and other SDP parameters that the
offerer is willing to use. This offer SDP is sent to the answerer, offerer is willing to use. This offer SDP is sent to the answerer,
which chooses from among the media streams, codecs and other SDP which chooses from among the media streams, codecs and other SDP
parameters provided, and generates an answer SDP with his parameters, parameters provided, and generates an answer SDP with his
based on that choice. The answer SDP is sent back to the offerer parameters, based on that choice. The answer SDP is sent back to the
thereby completing the session negotiation and enabling the offerer thereby completing the session negotiation and enabling the
establishment of the negotiated media streams. establishment of the negotiated media streams.
Taking a step back, we can make a distinction between the Taking a step back, we can make a distinction between the
capabilities supported by each participant, the way in which those capabilities supported by each participant, the way in which those
capabilities can be supported and the parameters that can actually be capabilities can be supported, and the parameters that can actually
used for the session. More generally, we can say that we have the be used for the session. More generally, we can say that we have the
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. The capability stream components, supported by each side. The capability
indication by itself does not imply a commitment to use the indications by themselves do not imply a commitment to use the
capability in the session. capabilities in the session.
A capability can for example be that the "RTP/SAVP" profile is Capabilities can for example be that the "RTP/SAVP" profile is
supported, that the "PCMU" codec is supported, or that the supported, that the "PCMU" codec is supported, or that the
"crypto" attribute is supported with a particular value. "crypto" attribute is supported with a particular value.
o A set of potential configurations indicating which combinations of o A set of potential configurations indicating which combinations
those capabilities can be used for the session and its associated of those capabilities can be used for the session and its
media stream components. Potential configurations are not ready associated media stream components. Potential configurations are
for use. Instead, they provide an alternative that may be used, not ready for use. Instead, they provide an alternative that may
subject to further negotiation. be used, subject to further negotiation.
A potential configuration can for example indicate that the "PCMU" A potential configuration can for example indicate that the
codec and the "RTP/SAVP" transport protocol are not only supported "PCMU" codec and the "RTP/SAVP" transport protocol are not only
(i.e. listed as capabilities), but they are offered for potential supported (i.e. listed as capabilities), but they are offered for
use in the session. potential use in the session.
o An actual configuration for the session and its associated media o An actual configuration for the session and its associated media
stream components, which specifies which combinations of session stream components, that specifies which combinations of session
parameters and media stream components can be used currently and parameters and media stream components can be used currently and
with what parameters. Use of an actual configuration does not with what parameters. Use of an actual configuration does not
require any further negotiation. require any further negotiation.
A actual configuration can for example be that the "PCMU" codec An actual configuration can for example be that the "PCMU" codec
and the "RTP/SAVP" transport protocol are offered for use and the "RTP/SAVP" transport protocol are offered for use
currently. currently.
o A negotiation process that takes the set of actual and potential o A negotiation process that takes the set of actual and potential
configurations (combinations of capabilities) as input and configurations (combinations of capabilities) as input and
provides the negotiated actual configurations as output. provides the negotiated actual configurations as output.
SDP by itself was designed to provide only one of these, namely SDP by itself was designed to provide only one of these, namely
listing of the actual configurations, however over the years, use of listing of the actual configurations, however over the years, use of
SDP has been extended beyond its original scope. Of particular SDP has been extended beyond its original scope. Of particular
importance are the session negotiation semantics that were defined by importance are the session negotiation semantics that were defined
the offer/answer model in RFC 3264. In this model, both the offer and by the offer/answer model in RFC 3264. In this model, both the offer
the answer contain actual configurations; separate capabilities and and the answer contain actual configurations; separate capabilities
potential configurations are not supported. and potential configurations are not supported.
Other relevant extensions have been defined as well. RFC 3407 Other relevant extensions have been defined as well. RFC 3407
[RFC3407] defined simple capability declarations, which extends SDP [RFC3407] defined simple capability declarations, which extends SDP
with a simple and limited set of capability descriptions. Grouping with a simple and limited set of capability descriptions. Grouping
of media lines, which defines how media lines in SDP can have other of 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 [RFC3388], etc. semantics, was defined in RFC 3388 [RFC3388], 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
intent, a more comprehensive capability declaration and negotiation intent, a more comprehensive capability declaration and negotiation
process was intentionally not defined. Instead, work on a "next process was intentionally not defined. Instead, work on a "next
generation" of a protocol to provide session description and generation" of a protocol to provide session description and
capability negotiation was initiated [SDPng]. SDPng defined a capability negotiation was initiated [SDPng]. SDPng defined a
comprehensive capability negotiation framework and protocol that was comprehensive capability negotiation framework and protocol that was
not bound by existing SDP constraints. SDPng was not designed to be not bound by existing SDP constraints. SDPng was not designed to be
backwards compatible with existing SDP and hence required both sides backwards compatible with existing SDP and hence required both sides
to support it, with a graceful fallback to legacy operation when to support it, with a graceful fallback to legacy operation when
needed. This combined with lack of ubiquitous multipart MIME support needed. This, combined with lack of ubiquitous multipart MIME
in the protocols that would carry SDP or SDPng made it challenging to support in the protocols that would carry SDP or SDPng, made it
migrate towards SDPng. In practice, SDPng has not gained traction but challenging to migrate towards SDPng. In practice, SDPng has not
rather remained as work in progress for an extended period of time. gained traction but rather remained as work in progress for an
Existing real-time multimedia communication protocols such as SIP, extended period of time. Existing real-time multimedia
RTSP, Megaco, and MGCP continue to use SDP. SDP and its current communication protocols such as SIP, RTSP, Megaco, and MGCP continue
extensions however do not address an increasingly important problem: to use SDP. However, SDP and its current extensions do not address
the ability to negotiate one or more alternative transport protocols an increasingly important problem: the ability to negotiate one or
(e.g., RTP profiles) and associated parameters (e.g. SDP attributes). more alternative transport protocols (e.g., RTP profiles) and
This makes it difficult to deploy new RTP profiles such as secure RTP associated parameters (e.g. SDP attributes). This makes it
(SRTP) [SRTP], RTP with RTCP-Based Feedback [AVPF], etc. This difficult to deploy new RTP profiles such as secure RTP (SRTP)
particular problem is exacerbated by the fact that RTP profiles are [RFC3711], RTP with RTCP-Based Feedback [RFC4585], etc. The problem
defined independently. When a new profile is defined and N other is exacerbated by the fact that RTP profiles are defined
profiles already exist, there is a potential need for defining N independently. When a new profile is defined and N other profiles
additional profiles, since profiles cannot be combined automatically. already exist, there is a potential need for defining N additional
For example, in order to support the plain and secure RTP version of profiles, since profiles cannot be combined automatically. For
RTP with and without RTCP-based feedback, four separate profiles (and example, in order to support the plain and secure RTP version of RTP
with and without RTCP-based feedback, four separate profiles (and
hence profile definitions) are needed: RTP/AVP [RFC3551], RTP/SAVP hence profile definitions) are needed: RTP/AVP [RFC3551], RTP/SAVP
[SRTP], RTP/AVPF [AVPF], and RTP/SAVPF [SAVPF]. In addition to the [RFC3711], RTP/AVPF [RFC4585], and RTP/SAVPF [SAVPF]. In addition
pressing profile negotiation problem, other important real-life to the pressing profile negotiation problem, other important real-
limitations have been found as well. Keying material and other life limitations have been found as well. Keying material and other
parameters for example need to be negotiated with some of the parameters for example need to be negotiated with some of the
transport protocols, but not others. Similarly, some media formats transport protocols, but not others. Similarly, some media formats
and types of media streams need to negotiate a variety of different and types of media streams need to negotiate a variety of different
parameters. parameters.
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 that document provides a general SDP Capability Negotiation framework
is backwards compatible with existing SDP. It also defines that is backwards compatible with existing SDP. It also defines
specifically how to provide attributes and transport protocols as specifically how to provide attributes and transport protocols as
capabilities and negotiate them using the framework. Extensions for capabilities and negotiate them using the framework. Extensions for
other types of capabilities (e.g. media types and formats) may be other types of capabilities (e.g. media types and formats) may be
provided in other documents. 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 is the Session Initiation important protocol for this problem is the Session Initiation
Protocol (SIP) [RFC3261]. SIP uses the offer/answer model [RFC3264] Protocol (SIP) [RFC3261]. SIP uses the offer/answer model [RFC3264]
(which is not specific to SIP) to negotiate sessions and hence the (which is not specific to SIP) to negotiate sessions and hence the
mechanism defined here defines the offer/answer procedures to use for mechanism defined here provides the offer/answer procedures to use
the capability negotiation framework. for the capability negotiation framework.
The rest of the document is structured as follows. In Section 3. we The rest of the document is structured as follows. In Section 3. we
present the SDP Capability Negotiation solution, which consists of present the SDP Capability Negotiation solution, which consists of
new SDP attributes and associated offer/answer procedures. In Section new SDP attributes and associated offer/answer procedures. In
4. we provide examples illustrating its use and in Section 5. we Section 4. we provide examples illustrating its use and in Section
provide the security considerations. 5. we provide the security considerations.
2. Conventions used in this document 2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
3. SDP Capability Negotiation Solution 3. SDP Capability Negotiation Solution
In this section we first present the conceptual model behind the SDP In this section we first present the conceptual model behind the SDP
capability negotiation framework, followed by an overview of the SDP capability negotiation framework followed by an overview of the SDP
Capability Negotiation solution. This is followed by definitions of Capability Negotiation solution. We then define new SDP attributes
new SDP attributes for the solution and its associated updated for the solution and provide its associated updated offer/answer
offer/answer procedures. procedures.
3.1. SDP Capability Negotiation Model 3.1. SDP Capability Negotiation Model
Our model uses the concepts of Our model uses the concepts of
o Capabilities o Capabilities
o Potential Configurations o Potential Configurations
o Actual Configurations o Actual Configurations
o Negotiation Process o Negotiation Process
as defined in Section 1. Conceptually, we want to offer not just the as defined in Section 1. Conceptually, we want to offer not just the
actual configuration SDP, as is done with the current offer/answer actual configuration SDP (which is done with the current
model, but the actual configuration SDP as well as one or more offer/answer model), but the actual configuration SDP as well as one
alternative SDPs, i.e. potential configurations. The answerer must or more alternative SDPs, i.e. potential configurations. The
choose either the actual configuration, or one of the potential answerer must choose either the actual configuration, or one of the
configurations, and generate an answer SDP based on that. Since the potential configurations, and generate an answer SDP based on that.
offerer may need to perform processing on the answer, which depends
on the offer that was chosen (actual or potential configuration), the The offerer may need to perform processing on the answer, which
answerer informs the offerer of which configuration he chose. The depends on the offer that was chosen (actual or potential
process can be viewed *conceptually* as follows: configuration). The answerer therefore informs the offerer which
configuration the answerer chose. The process can be viewed
*conceptually* as follows:
Offerer Answerer Offerer Answerer
======= ======== ======= ========
1) Generate offer with actual 1) Generate offer with actual
configuration and alternative configuration and alternative
potential configurations potential configurations
2) Send offer with all configurations 2) Send offer with all configurations
+------------+ +------------+
| SDP o1 | | SDP o1 |
| (actual | | (actual |
| config | | config |
| |-+ Offer | |-+ Offer
+------------+ | -----> 3) Process offered configurations +------------+ | -----> 3) Process offered configurations
| SDP o2 | in order of preference indicated | SDP o2 | in order of preference indicated
| (potential | 4) Generate answer based on chosen | (potential | 4) Generate answer based on chosen
| config 1) |-+ configuration (e.g. o2), and inform | config 1) |-+ configuration (e.g. o2), and
inform
+------------+ | offerer which one was chosen +------------+ | offerer which one was chosen
| SDP o3 | | SDP o3 |
| (potential | | (potential |
| config 2) |-+ | config 2) |-+
+------------+ | +------------+ |
| SDP ... | | SDP ... |
: : : :
+------------+ +------------+
| SDP a1 | | SDP a1 |
Answer | (actual | Answer | (actual |
<----- | config,o2)| <----- | config,o2)|
| | | |
5) Process answer based on +------------+ 5) Process answer based on +------------+
the configuration that was the configuration that was
chosen (o2), as indicated in chosen (o2), as indicated in
the answer the answer
The above illustrates the conceptual model, however the actual The above illustrates the conceptual model: The actual solution uses
solution uses only a single SDP, which contains the actual a single SDP, which contains the actual configuration (as with
configuration (as with current SDP and the current offer/answer current SDP and the current offer/answer model) and several new
model) enhanced with several new attributes and associated attributes and associated procedures, that encode the capabilities
procedures, that encode the capabilities and potential configurations and potential configurations. A more accurate depiction of the
and negotiate which ones to use. A more accurate depiction of the
actual offer SDP is therefore as follows: actual offer SDP is therefore as follows:
+--------------------+ +--------------------+
| SDP o1 | | SDP o1 |
| (actual | | (actual |
| config | | config |
| | | |
| +-------------+ | | +-------------+ |
| | capability 1| | | | capability 1| |
| | capability 2| | | | capability 2| |
skipping to change at page 9, line 46 skipping to change at page 10, line 5
compatibility. compatibility.
o Message size efficiency: When we have multiple media streams, o Message size efficiency: When we have multiple media streams,
each of which may potentially use two or more different transport each of which may potentially use two or more different transport
protocols with a variety of different associated parameters, the protocols with a variety of different associated parameters, the
number of potential configurations can be large. If each possible number of potential configurations can be large. If each possible
alternative is represented as a complete SDP in an offer, we can alternative is represented as a complete SDP in an offer, we can
easily end up with large messages. By providing a more compact easily end up with large messages. By providing a more compact
encoding, we get more efficient message sizes. encoding, we get more efficient message sizes.
In the next section, we describe the exact structure and specific SDP In the next section, we describe the exact structure and specific
parameters used to represent this. SDP parameters used to represent this.
3.2. Solution Overview 3.2. Solution Overview
The solution consists of the following: The solution consists of the following:
o Two new attributes to support extensions to the framework itself o Two new attributes to support extensions to the framework itself
as follows: as follows:
o A new attribute ("a=csup") that lists the supported base o A new attribute ("a=csup") that lists the supported base
(optionally) and any supported extension options to the (optionally) and any supported extension options to the
framework. framework.
o A new attribute ("a=creq") that lists the extensions to the o A new attribute ("a=creq") that lists the extensions to the
framework that are required to be supported by the entity framework that are required to be supported by the entity
receiving the SDP in order to do capability negotiation. 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=acap") that defines how to list an o A new attribute ("a=acap") that defines how to list an
attribute name, either with or without an associated value, as attribute name and its associated value (if any) as a
a capability. capability.
o A new attribute ("a=tcap") that defines how to list transport o A new attribute ("a=tcap") that defines how to list transport
protocols (e.g. "RTP/AVP") as capabilities. protocols (e.g. "RTP/AVP") as capabilities.
o Two new attributes to negotiate configurations as follows: o Two new attributes to negotiate configurations as follows:
o A new attribute ("a=pcfg") that lists the potential o A new attribute ("a=pcfg") that lists 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. Alternative potential capabilities from the SDP in question. Extension capabilities
configurations have an explicit ordering associated with them. can be defined and referenced in the potential
Extension capabilities can be defined and referenced in the configurations. Alternative potential configurations have an
potential configurations. explicit ordering associated with them.
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 a potential configuration from an offer attribute identifies a potential configuration from an offer
SDP which was used as an actual configuration to form the SDP which was used as an actual configuration to form the
answer SDP. Extension capabilities can be included as well. 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. and potential configurations to be included in an offer.
Capabilities can be provided at the session level and the media Capabilities can be provided at the session level and the media
level. Potential configurations can be included at the media level level. Potential configurations can be included at the media
only, where they constitute alternative offers that may be level only, where they constitute alternative 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). The answerer indicates which (if included in the "m=" line(s) and associated parameters. The
any) of the potential configurations it used to form the answer by answerer indicates which (if any) of the potential configurations
including the actual configuration attribute ("a=acfg") in the it used to form the answer by including the actual configuration
answer. Capabilities may be included in answers as well, where attribute ("a=acfg") in the answer. Capabilities may be included
they can aid in guiding a subsequent new offer. in answers as well, where they can aid in guiding a subsequent
new offer.
The mechanism is illustrated by the offer/answer exchange below, The mechanism is illustrated by the offer/answer exchange below,
where Alice sends an offer to Bob: where Alice sends an offer to Bob:
Alice Bob Alice Bob
| (1) Offer (SRTP and RTP) | | (1) Offer (SRTP and RTP) |
|--------------------------------->| |--------------------------------->|
| | | |
| (2) Answer (SRTP) | | (2) Answer (SRTP) |
skipping to change at page 11, line 39 skipping to change at page 11, line 40
Alice's offer includes RTP and SRTP as alternatives. RTP is the Alice's offer includes RTP and SRTP as alternatives. RTP is the
default (actual configuration), but SRTP is the preferred one default (actual configuration), but SRTP is the preferred one
(potential configuration): (potential configuration):
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
m=audio 53456 RTP/AVP 0 18 m=audio 53456 RTP/AVP 0 18
a=tcap:1 RTP/SAVP a=tcap:1 RTP/SAVP
a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_32 a=acap:1 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 capabilities are provided by the "a=tcap" and PCMU or G.729. The capabilities are provided by the "a=tcap" and
"a=acap" attributes. The transport capabilities ("a=tcap") indicate "a=acap" attributes. The transport capabilities ("a=tcap") indicate
that secure RTP under the AVP profile ("RTP/SAVP") is supported with that secure RTP under the AVP profile ("RTP/SAVP") is supported with
an associated transport capability handle of 1. The "acap" attribute an associated transport capability handle of 1. The "acap" attribute
provides an attribute capability with a handle of 1. The attribute provides an attribute capability with a handle of 1. The attribute
capability is a "crypto" attribute, which provides the keying capability is a "crypto" attribute, which provides the keying
material for SRTP using SDP security descriptions [SDES]. The material for SRTP using SDP security descriptions [RFC4568]. The
"a=pcfg" attribute provides the potential configuration included in "a=pcfg" attribute provides the potential configuration included in
the offer by reference to the capability parameters. One alternative the offer by reference to the capability parameters. One
is provided; it has a configuration number of 1 and it consists of alternative is provided; it has a configuration number of 1 and it
transport protocol capability 1 (i.e. the RTP/SAVP profile - secure consists of transport protocol capability 1 (i.e. the RTP/SAVP
RTP), and the attribute capability 1, i.e. the crypto attribute profile - secure RTP), and the attribute capability 1, i.e. the
provided. Potential configurations are always preferred over the crypto attribute provided. Potential configurations are preferred
actual configuration included in the offer SDP, and hence Alice is over the actual configuration included in the offer SDP, and hence
expressing a preference for using secure RTP. 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
and generates the following answer SDP: and generates the following answer SDP:
v=0 v=0
o=- 24351 621814 IN IP4 192.0.2.2 o=- 24351 621814 IN IP4 192.0.2.2
s= s=
c=IN IP4 192.0.2.2 c=IN IP4 192.0.2.2
skipping to change at page 12, line 39 skipping to change at page 12, line 40
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 option tags capability negotiation framework, he would have included option tags
for those in the answer as well (in an "a=csup" attribute). for those in the answer as well (in an "a=csup" attribute).
Note that in this particular example, the answerer supported the Note that in this particular example, the answerer supported the
capability negotiation extensions defined here, however had he not, capability negotiation extensions defined here. Had he not, he would
the answerer would simply have ignored the new attributes and simply have ignored the new attributes and accepted the (actual
accepted the (actual configuration) offer to use normal RTP. In that configuration) offer to use normal RTP. In that case, the following
case, the following answer would have been generated instead: answer would have been generated instead:
v=0 v=0
o=- 24351 621814 IN IP4 192.0.2.2 o=- 24351 621814 IN IP4 192.0.2.2
s= s=
c=IN IP4 192.0.2.2 c=IN IP4 192.0.2.2
t=0 0 t=0 0
m=audio 54568 RTP/AVP 0 18 m=audio 54568 RTP/AVP 0 18
3.3. Relationship to RFC 3407
RFC 3407 defines capability descriptions with limited abilities to
describe attributes, bandwidth parameters, transport protocols and
media formats. RFC 3407 does not define any negotiation procedures
for actually using those capability descriptions.
This document obsoletes RFC 3407 by defining new attributes for
describing attribute capabilities and transport capabilities. It also
defines procedures for actually using those capabilities as part of
an offer/answer exchange. Extensions to this document may be defined
in order to fully cover all the capabilities provided by RFC 3407
(for example more general media capabilities).
It is RECOMMENDED that implementations use the attributes and
procedures defined in this document instead of those defined in
[RFC3407].
If capability description interoperability with legacy RFC 3407 3.3. Version and Extension Indication Attributes
implementations is desired, implementations MAY include both RFC 3407
capability descriptions and capabilities defined by this document.
The offer/answer negotiation procedures however will not be able to
use the RFC 3407 capability descriptions.
3.4. Version and Extension Indication Attributes
In this section, we present the new attributes associated with In this section, we present the new attributes associated with
indicating the SDP Capability Negotiation extensions supported and indicating the SDP Capability Negotiation extensions supported and
required. required.
3.4.1. Supported Capability Negotiation Extensions Attribute 3.3.1. Supported Capability Negotiation Extensions Attribute
The SDP Capability Negotiation solution allows for capability The SDP Capability Negotiation solution allows for capability
negotiation extensions to be defined. Associated with each such negotiation extensions to be defined. Associated with each such
extension is an option tag that identifies the extension in question. extension is an option tag that identifies the extension in
Option-tags MUST be registered with IANA per the procedures defined question. Option-tags MUST be registered with IANA per the
in Section 6. procedures defined 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>
RFC 4566, Section 9, provides the ABNF for SDP attributes. The "csup"
attribute adheres to the RFC 4566 "attribute" production, with an RFC 4566, Section 9, provides the ABNF [RFC4234] for SDP attributes.
att-value defined as follows: The "csup" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows:
att-value = option-tag-list att-value = option-tag-list
option-tag-list = option-tag *("," option-tag) option-tag-list = option-tag *("," option-tag)
option-tag = token ; defined in [RFC4566] option-tag = token ; defined in [RFC4566]
A special base option tag with a value of "cap-v0" is defined for the A special base option tag with a value of "cap-v0" is defined for
basic SDP Capability Negotiation framework defined in this document. the basic SDP Capability Negotiation framework defined in this
Entities can use this option tag with the "a=csup" attribute to document. Entities can use this option tag with the "a=csup"
indicate support for the SDP Capability Negotiation framework attribute to indicate support for the SDP Capability Negotiation
specified in this document. framework specified in this document.
The following examples illustrates the use of the "a=csup" attribute The following examples illustrate use of the "a=csup" attribute with
with the "cap-v0" option tags and two hypothetical option tags, "foo" the "cap-v0" option tag and two hypothetical option tags, "foo" and
and "bar" (note the lack of white space): "bar" (note the lack of white space):
a=csup:cap-v0 a=csup:cap-v0
a=csup:foo a=csup:foo
a=csup:bar a=csup:bar
a=csup:cap-v0,foo,bar a=csup:cap-v0,foo,bar
The "a=csup" attribute can be provided at the session and the media- The "a=csup" attribute can be provided at the session and the media-
level. When provided at the session-level, it applies to the entire level. When provided at the session-level, it applies to the entire
SDP. When provided at the media-level, it applies to the media SDP. When provided at the media-level, it applies to the media
description in question only (option-tags provided at the session description in question only (option-tags provided at the session
level apply as well). There can be at most one "a=csup" attribute at level apply as well). There can be at most one "a=csup" attribute at
the session-level and at most one at the media-level (one per media the session-level and at most one at the media-level (one per media
skipping to change at page 14, line 43 skipping to change at page 14, line 21
description in question only (option-tags provided at the session description in question only (option-tags provided at the session
level apply as well). There can be at most one "a=csup" attribute at level apply as well). There can be at most one "a=csup" attribute at
the session-level and at most one at the media-level (one per media the session-level and at most one at the media-level (one per media
description in the latter case). description in the latter case).
Whenever an entity that supports one or more extensions to the SDP Whenever an entity that supports one or more extensions to the SDP
Capability Negotiation framework generates an SDP, it SHOULD include Capability Negotiation framework generates an SDP, it SHOULD include
the "a=csup" attribute with the option tags for the extensions it the "a=csup" attribute with the option tags for the extensions it
supports at the session and/or media-level, unless those option tags supports at the session and/or media-level, unless those option tags
are already provided in one or more "a=creq" attribute (see Section are already provided in one or more "a=creq" attribute (see Section
3.4.2. ) at the relevant levels. Inclusion of the base option tag is 3.3.2. ) at the relevant levels. Inclusion of the base option tag is
OPTIONAL; support for the base framework can be inferred from OPTIONAL; support for the base framework can be inferred from
presence of the "a=pcfg" attribute defined in Section 3.6.1. presence of the "a=pcfg" attribute defined in Section 3.5.1.
Use of the base option tag may still be useful in some scenarios, Use of the base option tag may still be useful in some scenarios,
e.g. when using SIP OPTIONS [RFC3261] or generating an answer to an e.g. when using SIP OPTIONS [RFC3261] or generating an answer to
offer that did not use the SDP Capability Negotiation framework. an offer that did not use the SDP Capability Negotiation
framework.
3.4.2. Required Capability Negotiation Extension Attribute 3.3.2. Required Capability Negotiation Extensions Attribute
The Required Capability Negotiation Extensions attribute ("a=creq") The Required Capability Negotiation Extensions attribute ("a=creq")
contains a comma-separated list of option tags (see Section 3.4.1. ) contains a comma-separated list of option tags (see Section 3.3.1. )
specifying the SDP Capability Negotiation extensions that MUST be specifying the SDP Capability Negotiation extensions that MUST be
supported by the entity receiving the SDP, in order for that entity supported by the entity receiving the SDP, in order for that entity
to properly process the SDP Capability Negotiation attributes and to properly process the SDP Capability Negotiation attributes and
associated procedures. Support for the basic negotiation framework is associated procedures. Support for the basic negotiation framework
implied by the presence of an "a=pcfg" attribute (see Section 3.6.1. is implied by the presence of an "a=pcfg" attribute (see Section
) and hence there is no need to include the "a=creq" attribute with 3.5.1. ) and hence there is no need to include the "a=creq"
the base option-tag ("cap-v0"). Still, it is allowed to do so. attribute with the base option-tag ("cap-v0"). Still, it is allowed
to do so.
The attribute is defined as follows: The attribute is defined as follows:
a=creq: <option-tag-list> a=creq: <option-tag-list>
The "creq" attribute adheres to the RFC 4566 "attribute" production, The "creq" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows: with an att-value defined as follows:
att-value = option-tag-list att-value = option-tag-list
The following examples illustrate the use of the "a=creq" attribute The following examples illustrate use of the "a=creq" attribute with
with the "cap-v0" base option tag and two hypothetical option tags, the "cap-v0" base option tag and two hypothetical option tags, "foo"
"foo" and "bar" (note the lack of white space): and "bar" (note the lack of white space):
a=creq:cap-v0 a=creq:cap-v0
a=creq:foo a=creq:foo
a=creq:bar a=creq:bar
a=creq:cap-v0,foo,bar a=creq:cap-v0,foo,bar
The "a=creq" attribute can be provided at the session and the media- The "a=creq" attribute can be provided at the session and the media-
level. When provided at the session-level, it applies to the entire level. When provided at the session-level, it applies to the entire
SDP. When provided at the media-level, it applies to the media SDP. When provided at the media-level, it applies to the media
description in question only (required option tags provided at the description in question only (required option tags provided at the
session level apply as well). There can be at most one "a=creq" session level apply as well). There can be at most one "a=creq"
attribute at the session-level and at most one "a=creq" attribute at attribute at the session-level and at most one "a=creq" attribute at
the media-level (one per media description in the latter case). the media-level (one per media description in the latter case).
When an entity generates an SDP and it requires the recipient of that When an entity generates an SDP and it requires the recipient of
SDP to support one or more SDP Capability Negotiation extensions that SDP to support one or more SDP Capability Negotiation
(except for the base), in order to properly process the SDP extensions (except for the base) at the session or media level in
Capability Negotiation, the "a=creq" attribute MUST be included with order to properly process the SDP Capability Negotiation, the
option-tags that identify the required extensions at the session "a=creq" attribute MUST be included with option-tags that identify
and/or media level. Support for the basic negotiation framework is the required extensions at the session and/or media level. If
implied by the presence of an "a=pcfg" attribute (see Section 3.6.1. support for an extension is needed only in one or more specific
) and hence it is not required to include the "a=creq" attribute with potential configurations, the potential configuration provides a way
the base option-tag ("cap-v0"). to indicate that instead (see Section 3.5.1. ). Support for the
basic negotiation framework is implied by the presence of an
"a=pcfg" attribute (see Section 3.5.1. ) and hence it is not
required to include the "a=creq" attribute with the base option-tag
("cap-v0").
A recipient that receives an SDP and does not support one or more of A recipient that receives an SDP and does not support one or more of
the required extensions listed in a "creq" attribute, MUST NOT the required extensions listed in a "creq" attribute, MUST NOT
perform the SDP Capability Negotiation defined in this document. For perform the SDP Capability Negotiation defined in this document. For
non-supported extensions provided at the session-level, this implies non-supported extensions provided at the session-level, this implies
that SDP Capability Negotiation MUST NOT be performed at all. For that SDP Capability Negotiation MUST NOT be performed at all. For
non-supported extensions at the media-level, this implies that SDP non-supported extensions at the media-level, this implies that SDP
Capability Negotiation MUST NOT be performed for the media stream in Capability Negotiation MUST NOT be performed for the media stream in
question. question.
An entity that does not support the SDP Capability Negotiation An entity that does not support the SDP Capability Negotiation
framework at all, will ignore these attributes (as well as the framework at all, will ignore these attributes (as well as the
other SDP Capability Negotiation attributes) and not perform any other SDP Capability Negotiation attributes) and not perform any
SDP Capability Negotiation in the first place. SDP Capability Negotiation in the first place.
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 listed in the option tags, the entity MUST
Capability Negotiation attributes were not included in the first proceed as if the SDP Capability Negotiation attributes were not
place, i.e. all the capability negotiation attributes should be included in the first place, i.e. all the capability negotiation
ignored. In that case, the entity SHOULD include a "csup" attribute attributes should be ignored. In that case, the entity SHOULD
listing the SDP Capability Negotiation extensions it actually include a "csup" attribute listing the SDP Capability Negotiation
supports. extensions it actually supports.
This ensures that introduction of the SDP Capability Negotiation This ensures that introduction of the SDP Capability Negotiation
mechanism by itself does not lead to session failures. mechanism by itself does not lead to session failures.
3.5. Capability Attributes 3.4. Capability Attributes
In this section, we present the new attributes associated with In this section, we present the new attributes associated with
indicating the capabilities for use by the SDP Capability indicating the capabilities for use by the SDP Capability
Negotiation. Negotiation.
3.5.1. Attribute Capability Attribute 3.4.1. Attribute Capability Attribute
Attributes and their associated values can be expressed as Attributes and their associated values can be expressed as
capabilities by use of a new attribute capability attribute capabilities by use of a new attribute capability attribute
("a=acap"), which is defined as follows: ("a=acap"), which is defined as follows:
a=acap: <att-cap-num> <att-par> a=acap: <att-cap-num> <att-par>
where <att-cap-num> is an integer between 1 and 2^31-1 (both where <att-cap-num> is an integer between 1 and 2^31-1 (both
included) used to number the attribute capability and <att-par> is an included) used to number the attribute capability and <att-par> is
attribute ("a=") in its full '<type>=<value>' form (see [RFC4566]). an attribute ("a=") in its full '<type>=<value>' form (see
Support for a specific attribute (name) (without any particular [RFC4566]).
values) can be indicated by providing only the '<type>' (i.e. the
attribute name).
The "acap" attribute adheres to the RFC 4566 "attribute" production, The "acap" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows: with an att-value defined as follows:
att-value = att-cap-num 1*WSP att-par att-value = att-cap-num 1*WSP att-par
att-cap-num = 1*DIGIT ;defined in [RFC4234] att-cap-num = 1*DIGIT ;defined in [RFC4234]
att-par = attribute ;defined in RFC 4566 att-par = attribute ;defined in RFC 4566
Note that white-space is not permitted before the att-cap-num. Note that white-space is not permitted before the att-cap-num.
The "acap" attribute can be provided at the session level only for The "acap" attribute can be provided at the session level only when
session-level attributes contained in the attribute capability, the attribute capability contains session-level attributes, whereas
whereas media level attributes can be provide in attribute media level attributes can be provided in attribute capabilities at
capabilities at either the media level or session-level. The base SDP either the media level or session-level. The base SDP Capability
Capability Negotiation framework however only defines procedures for Negotiation framework however only defines procedures for use of
use of media-level attribute capabilities at the media level media-level attribute capabilities at the media level (extensions
(extensions may define use at the session level). may define use at the session level).
Each occurrence of the "acap" attribute in the entire session Each occurrence of the "acap" attribute in the entire session
description MUST use a different value of <att-cap-num>. description MUST use a different value of <att-cap-num>.
There is a need to be able to reference both session-level and There is a need to be able to reference both session-level and
media-level attributes in potential configurations at the media media-level attributes in potential configurations at the media
level, and this provides for a simple solution to avoiding overlap level, and this provides for a simple solution to avoiding overlap
between the references (handles) to each attribute capability. between the references (handles) to each attribute capability.
The <att-cap-num> values provided are independent of similar <cap- The <att-cap-num> values provided are independent of similar <cap-
num> values provided for other types of capabilities, i.e., they form num> values provided for other types of capabilities, i.e., they
a separate name-space for attribute capabilities. form a separate name-space for attribute capabilities.
The following examples illustrate use of the "acap" attribute: The following examples illustrate use of the "acap" attribute:
a=acap:1 a=ptime:20 a=acap:1 ptime:20
a=acap:2 a=ptime:30 a=acap:2 ptime:30
a=acap:3 a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyONQ6gAA a=acap:3 key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyONQ6gAA
AAAGEEoo2pee4hp2UaDX8ZE22YwKAAAPZG9uYWxkQGR1Y2suY29tAQAAAAAAAQAk0 AAAGEEoo2pee4hp2UaDX8ZE22YwKAAAPZG9uYWxkQGR1Y2suY29tAQAAAAAAAQAk0
JKpgaVkDaawi9whVBtBt0KZ14ymNuu62+Nv3ozPLygwK/GbAV9iemnGUIZ19fWQUO JKpgaVkDaawi9whVBtBt0KZ14ymNuu62+Nv3ozPLygwK/GbAV9iemnGUIZ19fWQUO
SrzKTAv9zV SrzKTAv9zV
a=acap:4 a=crypto:1 AES_CM_128_HMAC_SHA1_32 a=acap:4 crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=acap:5 a=crypto The first two attribute capabilities provide attribute values for
a=acap:6 a=key-mgmt the ptime attribute. The third provides SRTP parameters by using
MIKEY [RFC3830] with the key-mgmt attribute [RFC4567]. The fourth
The first two attribute capabilities provide attribute values for the provides SRTP parameters by use of security descriptions with the
ptime attribute. The third provides SRTP parameters by using MIKEY crypto attribute [RFC4568]. Note that the line-wrapping and new-
with the key-mgmt attribute [KMGMT]. The fourth provides SRTP lines in example three and four are provided for formatting reasons
parameters by use of security descriptions with the crypto attribute only - they are not permitted in actual SDP.
[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. The 5th attribute capability merely
indicates support for the "crypto" attribute (without any further
information about particular values to use with it), and the 6th
attribute capability merely indicates support for the "key-mgmt"
attribute.
Readers familiar with RFC 3407 may notice the similarity between Readers familiar with RFC 3407 may notice the similarity between
the RFC 3407 "cpar" attribute and the above. There are however a the RFC 3407 "cpar" attribute and the above. There are however a
couple of important differences, notably that the "acap" attribute couple of important differences, notably that the "acap" attribute
contains a handle that enables referencing it and it furthermore contains a handle that enables referencing it and it furthermore
supports attributes only (the "cpar" attribute defined in RFC 3407 supports attributes only (the "cpar" attribute defined in RFC 3407
supports bandwidth information as well). The "acap" attribute also supports bandwidth information as well). The "acap" attribute also
is not automatically associated with any particular capabilities. is not automatically associated with any particular capabilities.
See Section 3.14. for the relationship to RFC 3407.
3.5.2. Transport Protocol Capability Attribute 3.4.2. Transport Protocol Capability Attribute
Transport Protocols can be expressed as capabilities by use of a new Transport Protocols can be expressed as capabilities by use of a new
Transport Protocol Capability attribute ("a=tcap") defined as Transport Protocol Capability attribute ("a=tcap") defined as
follows: follows:
a=tcap: <trpr-cap-num> <proto-list> a=tcap: <trpr-cap-num> <proto-list>
where <trpr-cap-num> is an integer between 1 and 2^31-1 (both where <trpr-cap-num> is an integer between 1 and 2^31-1 (both
included) used to number the transport address capability for later included) used to number the transport address capability for later
reference, and <proto-list> is one or more <proto>, separated by reference, and <proto-list> is one or more <proto>, separated by
skipping to change at page 19, line 7 skipping to change at page 18, line 35
proto-list = proto *(1*WSP proto) ; defined in RFC 4566 proto-list = proto *(1*WSP proto) ; defined in RFC 4566
Note that white-space is not permitted before the trpr-cap-num. Note that white-space is not permitted before the trpr-cap-num.
The "tcap" attribute can be provided at the session-level and the The "tcap" attribute can be provided at the session-level and the
media-level. There can be at most one "a=tcap" attribute at the media-level. There can be at most one "a=tcap" attribute at the
session-level and at most one at the media-level (one per media session-level and at most one at the media-level (one per media
description in the latter case). Each occurrence of the "tcap" description in the latter case). Each occurrence of the "tcap"
attribute in the entire session description MUST use a different attribute in the entire session description MUST use a different
value of <trpr-cap-num>. When multiple <proto> values are provided, value of <trpr-cap-num>. When multiple <proto> values are provided,
the first one is associated with the value <trpr-cap-num>, the second the first one is associated with the value <trpr-cap-num>, the
one with the value one higher, etc. The <trpr-cap-num> values second one with the value one higher, etc. There MUST NOT be any
provided are independent of similar <cap-num> values provided for capability number overlap between different "tcap" attributes in the
other capability attributes, i.e., they form a separate name-space entire SDP. The <trpr-cap-num> values provided are independent of
for transport protocol capabilities. similar <cap-num> values provided for other capability attributes,
i.e., they form a separate name-space for transport protocol
capabilities.
Below, we provide examples of the "a=tcap" attribute: Below, we provide examples of the "a=tcap" attribute:
a=tcap:1 RTP/AVP a=tcap:1 RTP/AVP
a=tcap:2 RTP/AVPF a=tcap:2 RTP/AVPF
a=tcap:3 RTP/SAVP RTP/SAVPF a=tcap:3 RTP/SAVP RTP/SAVPF
The first one provides a capability for the "RTP/AVP" profile
The first one provides a capability for the "RTP/AVP" profile defined defined in [RFC3551] and the second one provides a capability for
in [RFC3551] and the second one provides a capability for the RTP the RTP with RTCP-Based Feedback profile defined in [RFC4585]. The
with RTCP-Based Feedback profile defined in [AVPF]. The third one third one provides capabilities for the "RTP/SAVP" (transport
provides capabilities for the "RTP/SAVP" (transport capability number capability number 3) and "RTP/SAVPF" profiles (transport protocol
3) and "RTP/SAVPF" profiles (transport protocol capability number 4). capability number 4).
Transport capabilities are inherently included in the "m=" line, Transport capabilities are inherently included in the "m=" line,
however they still need to be specified explicitly in a "tcap" however they still need to be specified explicitly in a "tcap"
attribute, if they are to be used as a capability. attribute if they are to be used as a capability.
This may seem redundant (and indeed it is from the offerer's point This may seem redundant (and indeed it is from the offerer's point
of view), however it is done to protect against intermediaries of view), however it is done to protect against intermediaries
(e.g. middle-boxes) that may modify "m=" lines while passing (e.g. middle-boxes) that may modify "m=" lines while passing
unknown attributes through. If an implicit transport capability unknown attributes through. If an implicit transport capability
were used instead (e.g. a reserved transport capability number were used instead (e.g. a reserved transport capability number
could be used to refer to the transport protocol in the "m=" line), could be used to refer to the transport protocol in the "m="
and an intermediary were to modify the transport protocol in the line), and an intermediary were to modify the transport protocol
"m=" line (e.g. to translate between plain RTP and secure RTP), in the "m=" line (e.g. to translate between plain RTP and secure
then the potential configuration referencing that implicit RTP), then the potential configuration referencing that implicit
transport capability may no longer be correct. With explicit transport capability may no longer be correct. With explicit
capabilities, we avoid this pitfall, although the potential capabilities, we avoid this pitfall; however, the potential
configuration preference (see Section 3.6.1. ) may not reflect that configuration preference (see Section 3.5.1. ) may not reflect
of the intermediary (which some may view as a feature). that of the intermediary (which some may view as a feature).
3.5.3. Extension Capability Attributes 3.4.3. Extension Capability Attributes
The SDP Capability Negotiation framework allows for new capabilities The SDP Capability Negotiation framework allows for new types of
to be defined as extensions and used with the general capability capabilities to be defined as extensions and used with the general
negotiation framework. The syntax and semantics of such new capability negotiation framework. The syntax and semantics of such
capability attributes are not defined here, however in order to be new capability attributes are not defined here, however in order to
used with potential configurations, they SHOULD allow for a numeric be used with potential configurations, they SHOULD allow for a
handle to be associated with each capability. This handle can be used numeric handle to be associated with each capability. This handle
as a reference within the potential and actual configuration can be used as a reference within the potential and actual
attributes (see Section 3.6.1. and 3.6.2. ). The definition of such configuration attributes (see Section 3.5.1. and 3.5.2. ). The
extension capability attributes MUST also state whether they can be definition of such extension capability attributes MUST also state
applied at the session-level, media-level, or both. whether they can be applied at the session-level, media-level, or
both.
3.6. Configuration Attributes 3.5. Configuration Attributes
3.6.1. Potential Configuration Attribute 3.5.1. Potential Configuration Attribute
Potential Configurations can be expressed by use of a new Potential Potential Configurations can be expressed by use of a new Potential
Configuration Attribute ("a=pcfg") defined as follows: Configuration Attribute ("a=pcfg") defined as follows:
a=pcfg: <config-number> [<pot-cfg-list>] a=pcfg: <config-number> [<pot-cfg-list>]
where <config-number> is an integer between 1 and 2^31-1 (both where <config-number> is an integer between 1 and 2^31-1 (both
included). included).
The "pcfg" attribute adheres to the RFC 4566 "attribute" production, The "pcfg" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows: with an att-value defined as follows:
att-value = config-number [1*WSP pot-cfg-list] att-value = config-number [1*WSP pot-cfg-list]
config-number = 1*DIGIT ;defined in [RFC4234] config-number = 1*DIGIT ;defined in [RFC4234]
pot-cfg-list = pot-config *(1*WSP pot-config) pot-cfg-list = pot-config *(1*WSP pot-config)
pot-config = pot-attribute-config-list / pot-config = attribute-config-list /
pot-transport-protocol-config-list / transport-protocol-config-list /
pot-extension-config-list extension-config-list
The missing productions are defined below. Note that white-space is The missing productions are defined below. Note that white-space is
not permitted before the config-number. not permitted before the config-number.
The potential configuration attribute can be provided at the media- The potential configuration attribute can be provided at the media-
level only and there can be multiple instances of it within a given level only and there can be multiple instances of it within a given
media description. The attribute includes a configuration number, media description. The attribute includes a configuration number,
which is an integer between 1 and 2^31-1 (both included). The which is an integer between 1 and 2^31-1 (both included). The
configuration number MUST be unique within the media description configuration number MUST be unique within the media description
(i.e. it has media level scope only). The configuration number also (i.e. it has media level scope only). The configuration number also
indicates the relative preference of potential configurations; lower indicates the relative preference of potential configurations; lower
numbers are preferred over higher numbers. numbers are preferred over higher numbers.
After the configuration number, zero, one or more potential A potential configuration list is normally provided after the
configuration lists is provided. When the potential configuration configuration number. When the potential configuration list is
list is omitted, the potential configuration equals the actual omitted, the potential configuration equals the actual
configuration. The potential configuration lists generally reference configuration. The potential configuration list contains one or more
one or more capabilities, and those capabilities are (conceptually) of attribute, transport and extension configuration lists. The
used to construct a new internal version of the SDP by use of purely configuration lists generally reference one or more capabilities
syntactic add and (possibly) delete operations on the original SDP (extension configuration lists MAY use a different format). Those
(actual configuration), thereby generating an alternative potential capabilities are (conceptually) used to construct a new internal
configuration SDP that can be used by conventional SDP and version of the SDP by use of purely syntactic add and (possibly)
offer/answer procedures if actually selected. delete operations on the original SDP (actual configuration). This
provides an alternative potential configuration SDP that can be used
by conventional SDP and offer/answer procedures if selected.
This document defines potential attribute configuration lists and This document defines attribute configuration lists and transport
potential transport protocol configuration lists. Each of these MUST protocol configuration lists. Each of these MUST NOT be present
NOT be present more than once in a particular potential configuration more than once in a particular potential configuration attribute.
attribute. Potential extension configuration lists can be included as Extension configuration lists can be included as well. There can be
well; unknown potential extension configuration lists MUST be ignored more than one extension configuration list, however each particular
(if support is required, then the "a=creq" attribute with suitable extension MUST NOT be present more than once in a given "a=pcfg"
option tags MUST be used). There can be more than one potential attribute. Together, the various configuration lists define a
extension configuration list, however each particular potential potential configuration.
extension configuration list MUST NOT be present more than once in a
given potential configuration attribute. Together, these potential
configuration lists define a potential configuration.
There can be multiple potential configurations provided within a There can be multiple potential configurations in a media
media description. Each of these indicates not only a willingness, description. Each of these indicates not only a willingness, but in
but in fact a desire to use the potential configuration. fact a desire to use the potential configuration.
The example SDP below contains two potential configurations: The example SDP below contains two potential configurations:
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
m=audio 53456 RTP/AVP 0 18 m=audio 53456 RTP/AVP 0 18
a=tcap:1 RTP/SAVP RTP/SAVPF a=tcap:1 RTP/SAVP RTP/SAVPF
a=acap:1 a=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=pcfg:1 t=1 a=1 a=pcfg:1 t=1 a=1
a=pcfg:2 t=2 a=1 a=pcfg:2 t=2 a=1
Potential configuration 1 contains a potential transport protocol Potential configuration 1 contains a transport protocol
configuration list that references transport capability 1 configuration list that references transport capability 1
("RTP/SAVP") and a potential attribute configuration list that ("RTP/SAVP") and an attribute configuration list that references
references attribute capability 1 ("a=crypto:..."). Potential attribute capability 1 ("a=crypto:..."). Potential configuration 2
configuration 2 contains a potential transport protocol configuration contains a transport protocol configuration list that references
list that references transport capability 2 ("RTP/SAVPF") and a transport capability 2 ("RTP/SAVPF") and an attribute configuration
potential attribute configuration list that references attribute list that references attribute capability 1 ("a=crypto:...").
capability 1 ("a=crypto:...").
Attribute capabilities are used in a potential configuration by use Attribute capabilities are used in a potential configuration by use
of the pot-attribute-config-list parameter, which is defined by the of the attribute-config-list parameter, which is defined by the
following ABNF: following ABNF:
pot-attribute-config-list attribute-config-list
= "a=" [delete-attributes ":"] = "a=" [delete-attributes ":"]
mo-att-cap-list *(BAR mo-att-cap-list) mo-att-cap-list *(BAR mo-att-cap-list)
delete-attributes = DELETE ( "m" ; media attributes delete-attributes = DELETE ( "m" ; media attributes
/ "s" ; session attributes / "s" ; session attributes
/ "ms" ) ; media and session attributes / "ms" ) ; media and session attributes
mo-att-cap-list = mandatory-optional-att-cap-list | mo-att-cap-list = mandatory-optional-att-cap-list |
mandatory-att-cap-list | mandatory-att-cap-list |
optional-att-cap-list optional-att-cap-list
skipping to change at page 22, line 29 skipping to change at page 22, line 16
mandatory-att-cap-list = att-cap-list mandatory-att-cap-list = att-cap-list
optional-att-cap-list = "[" att-cap-list "]" optional-att-cap-list = "[" att-cap-list "]"
att-cap-list = att-cap-num *("," att-cap-num) att-cap-list = att-cap-num *("," att-cap-num)
att-cap-num = 1*DIGIT ;defined in [RFC4234] att-cap-num = 1*DIGIT ;defined in [RFC4234]
BAR = "|" BAR = "|"
DELETE = "-" DELETE = "-"
Note that white space is not permitted within this production. Note that white space is not permitted within this production.
Each potential attribute configuration list can optionally begin with Each attribute configuration list can optionally begin with
instructions for how to handle attributes that are part of the actual instructions for how to handle attributes that are part of the
configuration (i.e. the "a=" lines present in the original SDP). By actual configuration SDP (i.e., the "a=" lines present in the
default, such attributes will remain as part of the configuration in original SDP). By default, such attributes will remain as part of
question. However, if delete-attributes indicates "-m", then all the configuration in question. However, if delete-attributes
attribute lines within the media description in question will be indicates "-m", then all attribute lines within the media
deleted (i.e. all "a=" lines under the "m=" line in question). If description in question will be deleted (i.e., all "a=" lines under
delete-attributes indicates "-s", then all attribute lines at the the "m=" line in question). If delete-attributes indicates "-s",
session-level will be deleted (i.e. all "a=" lines before the first then all attribute lines at the session-level will be deleted (i.e.,
"m=" line). If delete-attributes indicates "-ms", then all attribute all "a=" lines before the first "m=" line). If delete-attributes
lines within this media description ("m=" line) and all attribute indicates "-ms", then all attribute lines within this media
lines at the session-level will be deleted. description ("m=" line) and all attribute lines at the session-level
will be deleted.
The attribute capability list comes next. It contains one or more The attribute capability list comes next. It contains one or more
alternative lists of attribute capabilities. The alternative alternative lists of attribute capabilities. The alternative
attribute capability lists are separated by a vertical bar ("|"), and attribute capability lists are separated by a vertical bar ("|"),
each list contains one or more attribute capabilities separated by and each list contains one or more attribute capabilities separated
commas (","). The attribute capabilities are either mandatory or by commas (","). The attribute capabilities are either mandatory or
optional. Mandatory attribute capabilities MUST be supported in order optional. Mandatory attribute capabilities MUST be supported in
to use the potential configuration, whereas optional attribute order to use the potential configuration, whereas optional attribute
capabilities MAY be supported in order to use the potential capabilities MAY be supported in order to use the potential
configuration. configuration.
Within each attribute capability list, all the mandatory attribute Within each attribute capability list, all the mandatory attribute
capabilities (if any) are listed first, and all the optional capabilities (if any) are listed first, and all the optional
attribute capabilities (if any) are listed last. The optional attribute capabilities (if any) are listed last. The optional
attribute capabilities are contained within a pair of angle brackets attribute capabilities are contained within a pair of angle brackets
("[" and "]"). Each attribute capability is merely an attribute ("[" and "]"). Each attribute capability is merely an attribute
capability number (att-cap-num) that identifies a particular capability number (att-cap-num) that identifies a particular
attribute capability by referring to attribute capability numbers attribute capability by referring to attribute capability numbers
defined above and hence MUST be between 1 and 2^31-1 (both included). defined above and hence MUST be between 1 and 2^31-1 (both
The following example illustrates the above: included). The following example illustrates the above:
a=pcfg:1 a=-m:1,2,[3,4]|1,7,[5] a=pcfg:1 a=-m:1,2,[3,4]|1,7,[5]
where where
o "a=-m:1,2,[3,4]|1,7,[5]" is the potential attribute configuration o "a=-m:1,2,[3,4]|1,7,[5]" is the attribute configuration list
list
o "-m" is the delete-attributes o "-m" is the delete-attributes
o "1,2,[3,4]" and "1,7,[5]" are both attribute capability lists. The o "1,2,[3,4]" and "1,7,[5]" are both attribute capability lists.
two lists are alternatives, since they are separated by a vertical The two lists are alternatives, since they are separated by a
bar above vertical bar above
o "1", "2" and "7" are mandatory attribute capabilities o "1", "2" and "7" are mandatory attribute capabilities
o "3", "4" and "5" are optional attribute capabilities o "3", "4" and "5" are optional attribute capabilities
Note that in the example above, we have a single handle ("1") for the Note that in the example above, we have a single handle ("1") for
potential configuration(s), but there are actually two different the potential configuration(s), but there are actually two different
potential configurations (separated by a vertical bar). This is done potential configurations (separated by a vertical bar). This is done
for message size efficiency reasons, which is especially important for message size efficiency reasons, which is especially important
when we add other types of capabilities to the potential when we add other types of capabilities to the potential
configuration. If there is a need to provide a unique handle for configuration. If there is a need to provide a unique handle for
each, separate "a=pcfg" attributes with different handles must be each, then separate "a=pcfg" attributes with different handles MUST
used instead. be used instead.
Each referenced attribute capability in the potential configuration Each referenced attribute capability in the potential configuration
will result in the corresponding attribute name and its associated will result in the corresponding attribute name and its associated
value (contained inside the attribute capability) being added to the value (contained inside the attribute capability) being added to the
resulting potential configuration SDP. resulting potential configuration SDP.
Alternative attribute capability lists are separated by a vertical Alternative attribute capability lists 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 "|"). The alternatives are ordered by "," has higher precedence than "|"). The alternatives are ordered by
preference with the most preferred listed first. In order for a preference with the most preferred listed first. In order for a
recipient of the SDP (e.g. an answerer receiving this in an offer) to recipient of the SDP (e.g., an answerer receiving this in an offer)
use this potential configuration, exactly one of the alternative to use this potential configuration, exactly one of the alternative
lists must be selected in its entirety. This requires that all lists MUST be selected in its entirety. This requires that all
mandatory attribute capabilities referenced by the potential mandatory attribute capabilities referenced by the potential
configuration are supported with the attribute values provided. configuration are supported with the attribute values provided.
Potential transport protocol configuration lists are included in a Transport protocol configuration lists are included in a potential
potential configuration by use of the pot-transport-protocol-config- configuration by use of the transport-protocol-config-list
list parameter, which is defined by the following ABNF: parameter, which is defined by the following ABNF:
pot-transport-protocol-config-list = transport-protocol-config-list =
"t=" trpr-cap-num *(BAR trpr-cap-num) "t=" trpr-cap-num *(BAR trpr-cap-num)
trpr-cap-num = 1*DIGIT ; defined in [RFC4234] trpr-cap-num = 1*DIGIT ; defined in [RFC4234]
Note that white-space is not permitted within this production. Note that white-space is not permitted within this production.
The trpr-cap-num refers to transport protocol capability numbers The trpr-cap-num refers to transport protocol capability numbers
defined above and hence MUST be between 1 and 2^31-1 (both included). defined above and hence MUST be between 1 and 2^31-1 (both
Alternative potential transport protocol capabilities are separated included). Alternative transport protocol capabilities are separated
by a vertical bar ("|"). The alternatives are ordered by preference by a vertical bar ("|"). The alternatives are ordered by preference
with the most preferred listed first. When there are no transport with the most preferred listed first. If there are no transport
protocol capabilities included in a potential configuration at the protocol capabilities included in a potential configuration at the
media level, the transport protocol information from the associated media level, the transport protocol information from the associated
"m=" line MUST be used. In order for a recipient of the SDP (e.g. an "m=" line MUST be used. In order for a recipient of the SDP (e.g.,
answerer receiving this in an offer) to use this potential an answerer receiving this in an offer) to use this potential
configuration, exactly one of the alternatives MUST be selected. This configuration, exactly one of the alternatives MUST be selected.
requires that the transport protocol in question is supported. This requires that the transport protocol in question is supported.
In the presence of intermediaries (the existence of which may not In the presence of intermediaries (the existence of which may not
be known), care should be taken with assuming that the transport be known), care should be taken with assuming that the transport
protocol in the "m=" line will not be modified by an intermediary. protocol in the "m=" line will not be modified by an intermediary.
Use of an explicit transport protocol capability will guard against Use of an explicit transport protocol capability will guard
any capability negotiation implications of that. against capability negotiation implications of that.
Extension capabilities can be included in a potential configuration Extension capabilities can be included in a potential configuration
as well by use of potential extension configuration lists. Such as well by use of extension configuration lists. Such extension
potential configuration extension lists MUST adhere to the following configuration lists MUST adhere to the following ABNF:
ABNF:
pot-extension-config-list = ext-cap-name "=" extension-config-list= ["+"] ext-cap-name "="
ext-cap-list ext-cap-list
ext-cap-name = token ; defined in [RFC4566] ext-cap-name = 1*(ALPHA / DIGIT)
ext-cap-list = 1*VCHAR ; defined in [RFC4234] ext-cap-list = 1*VCHAR ; defined in [RFC4234]
Note that white-space is not permitted within this production. Note that white-space is not permitted within this production.
The ext-cap-name refers to the type of extension capability and the The ext-cap-name refers to the name of the extension capability and
ext-cap-list is here merely defined as a sequence of visible the ext-cap-list is here merely defined as a sequence of visible
characters. The actual extension supported MUST refine both of these characters. The actual extension supported MUST refine both of these
further. For extension capabilities that merely need to be referenced further. For extension capabilities that merely need to be
by a capability number, it is RECOMMENDED to follow a structure referenced by a capability number, it is RECOMMENDED to follow a
similar to what has been specified above. Unsupported or unknown structure similar to what has been specified above. Unsupported or
potential extension configuration lists in a potential configuration unknown potential extension configuration lists in a potential
attribute MUST be ignored. configuration attribute MUST be ignored, unless they are prefixed
with the plus ("+") sign, which indicates that the extension is
mandatory and MUST be supported in order to use that potential
configuration.
The "creq" attribute and its associated rules can be used to ensure The "creq" attribute and its associated rules can be used to
that required extensions are supported in the first place. ensure that required extensions are supported in the first place.
Potential configuration attributes can be provided at the media level Potential configuration attributes can be provided at the media
only, however it is possible to reference capabilities provided at level only, however it is possible to reference capabilities
either the session or media level. There are certain semantic rules provided at either the session or media level. There are certain
and restrictions associated with this: semantic rules and restrictions associated with this:
A (media level) potential configuration attribute in a given media A (media level) potential configuration attribute in a given media
description MUST NOT reference a media-level capability provided in a description MUST NOT reference a media-level capability provided in
different media description; doing so invalidates that potential a different media description; doing so invalidates that potential
configuration (note that a potential configuration attribute can configuration (note that a potential configuration attribute can
contain more than one potential configuration by use of contain more than one potential configuration by use of
alternatives). A potential configuration attribute can however alternatives). A potential configuration attribute can however
reference a session-level capability. The semantics of doing so reference a session-level capability. The semantics of doing so
depends on the type of capability. In the case of transport protocol depends on the type of capability. In the case of transport protocol
capabilities it has no particular implication. In the case of capabilities it has no particular implication. In the case of
attribute capabilities however, it does. More specifically, the attribute capabilities however, it does. More specifically, the
attribute name and value (provided within that attribute capability) attribute name and value (provided within that attribute capability)
will be considered part of the resulting SDP for that particular will be considered part of the resulting SDP for that particular
configuration at the *session* level. In other words, it will be as- configuration at the *session* level. In other words, it will be as-
if that attribute was simply provided with that value at the session- if that attribute was provided with that value at the session-level
level in the first place. As a result of that, the base SDP in the first place. As a result, the base SDP Capability Negotiation
Capability Negotiation framework REQUIRES that potential framework REQUIRES that potential configurations do not reference
configurations do not reference any session-level attribute any session-level attribute capabilities that contain media-level
capabilities that contain media-level attributes (since that would attributes (since that would place a media-level attribute at the
place a media-level attribute at the session level). Extensions may session level). Extensions may modify this behavior, as long as it
modify this behavior, as long as it is fully backwards compatible is fully backwards compatible with the base specification.
with the base specification.
Individual media streams perform capability negotiation individually, Individual media streams perform capability negotiation
and hence it is possible that one media stream (where the attribute individually, and hence it is possible that one media stream (where
was part of a potential configuration) chose a configuration without the attribute was part of a potential configuration) chose a
a session level attribute that was chosen by another media stream. configuration without a session level attribute that was chosen by
The session-level attribute however remains "active" and hence another media stream. The session-level attribute however remains
applies to the entire resulting potential configuration SDP. In "active" and applies to the entire resulting potential configuration
theory, this is problematic if one or more session-level attributes SDP. In theory, this is problematic if one or more session-level
either conflicts with or potentially interacts with another session- attributes either conflicts with or potentially interacts with
level or media-level attribute in an undefined manner. In practice another session-level or media-level attribute in an undefined
however, such examples seem to be rare (at least with the currently manner. In practice, such examples seem to be rare (at least with
defined SDP attributes). the currently defined SDP attributes).
A related set of problems can occur if we need coordination between A related set of problems can occur if we need coordination
session-level attributes from multiple media streams in order for a between session-level attributes from multiple media streams in
particular functionality to work. The grouping framework [RFC3388] order for a particular functionality to work. The grouping
is an example of this. If we use the SDP Capability Negotiation framework [RFC3388] is an example of this. If we use the SDP
framework to select a session-level group attribute (provided as an Capability Negotiation framework to select a session-level group
attribute capability), and we require two media descriptions to do attribute (provided as an attribute capability), and we require
this consistently, we could have a problem. The FEC grouping two media descriptions to do this consistently, we could have a
semantics [RFC4756] is one example where this in theory could cause problem. The FEC grouping semantics [RFC4756] is one example where
problems, however in practice, it is unclear that there is a this in theory could cause problems, however in practice, it is
significant problem here with the currently defined grouping unclear that there is a significant problem with the currently
semantics. defined grouping semantics.
Resolving the above issues in general requires inter-media stream Resolving the above issues in general requires inter-media stream
constraints and synchronized potential configuration processing; this constraints and synchronized potential configuration processing;
would add considerable complexity to the overall solution. In this would add considerable complexity to the overall solution. In
practice, with the currently defined SDP attributes, it does not seem practice, with the currently defined SDP attributes, it does not
to be a significant problem, and hence the core SDP Capability seem to be a significant problem, and hence the core SDP Capability
Negotiation solution does not provide a solution to this issue. Negotiation solution does not provide a solution to this issue.
Instead, it is RECOMMENDED that use of session-level attributes in a Instead, it is RECOMMENDED that use of session-level attributes in a
potential configuration is avoided when possible, and when not, that potential configuration is avoided when possible, and when not, that
such use is examined closely for any potential interaction issues. If such use is examined closely for any potential interaction issues.
interaction is possible, the entity generating the SDP SHOULD NOT If interaction is possible, the entity generating the SDP SHOULD NOT
assume that well-defined operation will occur at the receiving assume that well-defined operation will occur at the receiving
entity. entity.
The session-level operation of extension capabilities is undefined: The session-level operation of extension capabilities is undefined:
Consequently, each new session-level extension capability defined Consequently, each new session-level extension capability defined
MUST specify the implication of making it part of a configuration at MUST specify the implication of making it part of a configuration at
the media level. the media level.
Below, we provide an example of the "a=pcfg" attribute in a complete Below, we provide an example of the "a=pcfg" attribute in a complete
media description in order to properly indicate the supporting media description in order to properly indicate the supporting
skipping to change at page 27, line 4 skipping to change at page 26, line 45
s= s=
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
m=audio 53456 RTP/AVPF 0 18 m=audio 53456 RTP/AVPF 0 18
a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32 a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=tcap:1 RTP/AVPF RTP/AVP a=tcap:1 RTP/AVPF RTP/AVP
a=tcap:3 RTP/SAVP RTP/SAVPF a=tcap:3 RTP/SAVP RTP/SAVPF
a=pcfg:1 t=4|3 a=1 a=pcfg:1 t=4|3 a=1
a=pcfg:8 t=1|2 a=pcfg:8 t=1|2
We have two potential configuration attributes listed here. The first
one (and most preferred, since its configuration number is "1") We have two potential configuration attributes listed here. The
indicates that either of the profiles RTP/SAVPF or RTP/SAVP first one (and most preferred, since its configuration number is
"1") indicates that either of the profiles RTP/SAVPF or RTP/SAVP
(specified by the transport protocol capability numbers 4 and 3) can (specified by the transport protocol capability numbers 4 and 3) can
be supported with attribute capability 1 (the "crypto" attribute); be supported with attribute capability 1 (the "crypto" attribute);
RTP/SAVPF is preferred over RTP/SAVP since its capability number (4) RTP/SAVPF is preferred over RTP/SAVP since its capability number (4)
is listed first in the preferred potential configuration. Note that is listed first in the preferred potential configuration. Note that
while we have only a single potential attribute and associated although we have a single potential configuration attribute and
handle, we have two potential configurations. associated handle, we have two potential configurations.
The second potential configuration attribute indicates that the The second potential configuration attribute indicates that the
RTP/AVPF or RTP/AVP profile can be used, with RTP/AVPF being the RTP/AVPF or RTP/AVP profiles can be used, with RTP/AVPF being the
preferred one. This non secure RTP alternative is the less preferred preferred one. This non secure RTP alternative is the less preferred
one since its configuration number is "8". Again, note that we have one since its configuration number is "8". Again, note that we have
two potential configurations here and hence a total of four potential two potential configurations here and hence a total of four
configurations in the SDP above. potential configurations in the SDP above.
3.6.2. Actual Configuration Attribute 3.5.2. Actual Configuration Attribute
The actual configuration attribute identifies which of the potential The actual configuration attribute identifies which of the potential
configurations from an offer SDP was selected and used as the actual configurations from an offer SDP was selected and used as the actual
configuration to generate an answer SDP. This is done by including configuration to generate an answer SDP. This is done by including
the configuration number and the configuration lists (if any) from the configuration number and the configuration lists (if any) from
the offer that were selected and used by the answerer in his the offer that were selected and used by the answerer in his
offer/answer procedure as follows: offer/answer procedure as follows:
o A selected potential attribute configuration MUST include the o A selected attribute configuration MUST include the delete-
delete-attributes and the selected alternative mo-att-cap-list attributes and the selected alternative mo-att-cap-list (i.e.,
(i.e. containing all mandatory and optional capability numbers containing all mandatory and optional capability numbers from the
from the potential configuration, irrespective of whether the potential configuration, irrespective of whether the optional
optional ones were supported or not). If delete-attributes were ones were supported or not). If delete-attributes were not
not included in the potential configuration, they will of course included in the potential configuration, they will of course not
not be present here either. be present here either.
o A selected potential transport protocol configuration MUST include o A selected transport protocol configuration MUST include the
the selected transport protocol capability number. selected transport protocol capability number.
o A selected potential extension configuration MUST include the o A selected potential extension configuration MUST include the
selected extension configuration parameters as specified for that selected extension configuration parameters as specified for that
particular extension. particular extension.
o When a configuration list contains alternatives (separated by
"|"), the selected configuration only MUST be provided.
Note that the selected configuration number and all selected Note that the selected configuration number and all selected
capability numbers used in the actual configuration attribute refer capability numbers used in the actual configuration attribute refer
to those from the offer; not the answer. to those from the offer; not the answer.
The answer may for example include capabilities as well to inform The answer may for example include capabilities as well to inform
the offerer of the answerers capabilities above and beyond the the offerer of the answerers capabilities above and beyond the
negotiated configuration. The actual configuration attribute does negotiated configuration. The actual configuration attribute does
not refer to any of those answer capabilities though. not refer to any of those answer capabilities though.
The Actual Configuration Attribute ("a=acfg") is defined as follows: The Actual Configuration Attribute ("a=acfg") is defined as follows:
a=acfg: <config-number> [<sel-cfg-list>] a=acfg: <config-number> [<sel-cfg-list>]
where <config-number> is an integer between 1 and 2^31-1 (both where <config-number> is an integer between 1 and 2^31-1 (both
included). included).
The "acfg" attribute adheres to the RFC 4566 "attribute" production, The "acfg" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows: with an att-value defined as follows:
att-value = config-number [1*WSP sel-cfg-list] att-value = config-number [1*WSP sel-cfg-list]
;config-number defined in Section 3.6.1. ;config-number defined in Section 3.5.1.
sel-cfg-list = sel-cfg *(1*WSP sel-cfg) sel-cfg-list = sel-cfg *(1*WSP sel-cfg)
sel-cfg = sel-attribute-config / sel-cfg = sel-attribute-config /
sel-transport-protocol-config / sel-transport-protocol-config /
sel-extension-config sel-extension-config
sel-attribute-config = sel-attribute-config =
"a=" [delete-attributes ":"] mo-att-cap-list "a=" [delete-attributes ":"] mo-att-cap-list
; defined in Section 3.6.1. ; defined in Section 3.5.1.
sel-transport-protocol-config = sel-transport-protocol-config =
"t=" trpr-cap-num ; defined in Section 3.6.1. "t=" trpr-cap-num ; defined in Section 3.5.1.
sel-extension-config = sel-extension-config =
ext-cap-name "=" 1*VCHAR ; defined in Section 3.6.1. ext-cap-name "=" 1*VCHAR ; defined in Section 3.5.1.
Note that white-space is not permitted before the config-number. Note that white-space is not permitted before the config-number.
The actual configuration ("a=acfg") attribute can be provided at the The actual configuration ("a=acfg") attribute can be provided at the
media-level only. There MUST NOT be more than one occurrence of an media-level only. There MUST NOT be more than one occurrence of an
actual configuration attribute within a given media description. actual configuration attribute within a given media description.
Below, we provide an example of the "a=acfg" attribute (building on Below, we provide an example of the "a=acfg" attribute (building on
the previous example with the potential configuration attribute): the previous example with the potential configuration attribute):
skipping to change at page 29, line 18 skipping to change at page 29, line 18
c=IN IP4 192.0.2.2 c=IN IP4 192.0.2.2
t=0 0 t=0 0
m=audio 54568 RTP/SAVPF 0 m=audio 54568 RTP/SAVPF 0
a=crypto:1 AES_CM_128_HMAC_SHA1_32 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32 inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32
a=acfg:1 t=4 a=1 a=acfg:1 t=4 a=1
It indicates that the answerer used an offer consisting of potential It indicates that the answerer used an offer consisting of potential
configuration number 1 with transport protocol capability 4 from the configuration number 1 with transport protocol capability 4 from the
offer (RTP/SAVPF) and attribute capability 1 (the "crypto" offer (RTP/SAVPF) and attribute capability 1 (the "crypto"
attribute). The answerer includes his own "crypto" attribute as well. attribute). The answerer includes his own "crypto" attribute as
well.
3.7. Offer/Answer Model Extensions 3.6. Offer/Answer Model Extensions
In this section, we define extensions to the offer/answer model In this section, we define extensions to the offer/answer model
defined in [RFC3264] to allow for potential configurations to be defined in [RFC3264] to allow for potential configurations to be
included in an offer, where they constitute offers that may be included in an offer, where they constitute alternative offers that
accepted by the answerer instead of the actual configuration(s) may be accepted by the answerer instead of the actual
included in the "m=" line(s). configuration(s) included in the "m=" line(s).
The procedures defined in the following subsections apply to both The procedures defined in the following subsections apply to both
unicast and multicast streams. unicast and multicast streams.
3.7.1. Generating the Initial Offer 3.6.1. Generating the Initial Offer
An offerer that wants to use the SDP Capability Negotiation defined An offerer that wants to use the SDP Capability Negotiation defined
in this document MUST include the following in the offer: in this document MUST include the following in the offer:
o An attribute capability attribute ("a=acap") as defined in Section o An attribute capability attribute ("a=acap") as defined in
3.5.1. for each attribute name and associated value (if any) that Section 3.4.1. for each attribute name and associated value (if
needs to be indicated as a capability in the offer. any) that needs to be indicated as a capability in the offer.
Session-level attributes and associated values MUST be provided in Session-level attributes and associated values MUST be provided
attribute capabilities at the session-level only, whereas media- in attribute capabilities at the session-level only, whereas
level attributes and associated values can be provided in media-level attributes and associated values can be provided in
attribute capabilities at either the media-level or session-level. attribute capabilities at either the media-level or session-
Attributes that can be provided at either the session- or media- level. Attributes that are allowed at either the session- or
level can be represented as attribute capabilities at either the media-level can be provided in attribute capabilities at either
session- or media-level. Attribute capabilities for level. If there is no need to indicate any attributes as
'<type>=<value>' attributes MAY include an attribute name only, attribute capabilities, then there will not be any "a=acap"
thereby merely indicating support for the attribute type; such
attribute capabilities MUST NOT be referenced by a potential
configuration. If there is not a need to indicate any attributes
as attribute capabilities, then there will not be any "a=acap"
attributes either. attributes either.
o One or more a transport protocol capability attributes ("a=tcap") o One or more transport protocol capability attributes ("a=tcap")
as defined in Section 3.5.2. with values for each transport as defined in Section 3.4.2. with values for each transport
protocol that needs to be indicated as a capability in the offer. protocol that needs to be indicated as a capability in the offer.
Transport protocol capabilities that apply to multiple media Transport protocol capabilities that apply to multiple media
descriptions SHOULD be provided at the session-level whereas descriptions SHOULD be provided at the session-level whereas
transport protocol capabilities that apply to a specific media transport protocol capabilities that apply to a specific media
description ("m=" line) only, SHOULD be provided within that description ("m=" line) only, SHOULD be provided within that
particular media description. In either case, there MUST NOT be particular media description. In either case, there MUST NOT be
more than a single "a=tcap" attribute at the session-level and a more than a single "a=tcap" attribute at the session-level and a
single "a=tcap" attribute in each media description. If there is single "a=tcap" attribute in each media description. If there is
not a need to indicate any transport protocols as transport no need to indicate any transport protocols as transport protocol
protocol capabilities, then there will not be any "a=tcap" capabilities, then there will not be any "a=tcap" attributes
attributes either. either.
o One or more extension capability attributes (as outlined in o One or more extension capability attributes (as outlined in
Section 3.5.3. ) for each extension capability that is referenced Section 3.4.3. ) for each extension capability that is referenced
by a potential configuration. by a potential configuration. Extension capability attributes
that are not referenced by a potential configuration MAY be
provided as well.
o One or more potential configuration attributes ("a=pcfg") as o One or more potential configuration attributes ("a=pcfg"), as
defined in Section 3.6.1. within each media description where defined in Section 3.5.1. , in each media description where
alternative potential configurations are to be negotiated. Each alternative potential configurations are to be negotiated. Each
potential configuration attribute MUST adhere to the rules potential configuration attribute MUST adhere to the rules
provided in Section 3.6.1. and the additional rules provided provided in Section 3.5.1. and the additional rules provided
below. below.
If the offerer requires support for more or extensions (besides the If the offerer requires support for more or extensions (besides the
base protocol defined here), then the offerer MUST include one or base protocol defined here), then the offerer MUST include one or
more "a=creq" attribute as follows: more "a=creq" attribute as follows:
o If one or more capability negotiation extensions are required to o If support for one or more capability negotiation extensions is
be supported for the entire session description, then option tags required for the entire session description, then option tags for
for those extensions MUST be included in a single session-level those extensions MUST be included in a single session-level
"creq" attribute. "creq" attribute.
o For each media description that requires one or more capability o For each media description that requires support for one or more
negotiation extensions not listed at the session-level, a single capability negotiation extensions not listed at the session-
"creq" attribute containing all the required extensions for that level, a single "creq" attribute containing all the required
media description MUST be included within the media description extensions for that media description MUST be included within the
(in accordance with Section 3.4.2. ). media description (in accordance with Section 3.3.2. ).
Note that extensions that only need to be supported by a particular
potential configuration can use the "mandatory" extension prefix
("+") within the potential configuration (see Section 3.5.1. ).
The offerer SHOULD furthermore include the following: The offerer SHOULD furthermore include the following:
o A supported capability negotiation extension attribute ("a=csup") o A supported capability negotiation extension attribute ("a=csup")
at the session-level and/or media-level as defined in Section at the session-level and/or media-level as defined in Section
3.4.2. for each capability negotiation extension supported by the 3.3.2. for each capability negotiation extension supported by the
offerer and not included in a corresponding "a=creq" attribute offerer and not included in a corresponding "a=creq" attribute
(i.e. at the session-level or in the same media description). (i.e., at the session-level or in the same media description).
Option tags provided in a "a=csup" attribute at the session-level Option tags provided in a "a=csup" attribute at the session-level
indicate extensions supported for the entire session description indicate extensions supported for the entire session description,
whereas option tags provided in a "a=csup" attribute in a media whereas option tags provided in a "a=csup" attribute in a media
description indicate extensions supported for that particular description indicate extensions supported for that particular
media description only. media description only.
Capabilities provided in an offer merely indicate what the offerer is Capabilities provided in an offer merely indicate what the offerer
capable of doing. They do not constitute a commitment or even an is capable of doing. They do not constitute a commitment or even an
indication to use them. In contrast, each potential configuration indication to use them. In contrast, each potential configuration
constitutes an alternative offer that the offerer would like to use. constitutes an alternative offer that the offerer would like to use.
The potential configurations MUST be used by the answerer to The potential configurations MUST be used by the answerer to
negotiate and establish the session. negotiate and establish the session.
The offerer MUST include one or more potential configuration The offerer MUST include one or more potential configuration
attributes ("a=pcfg") within each media description where the offerer attributes ("a=pcfg") in each media description where the offerer
wants to provide alternative offers (in the form of potential wants to provide alternative offers (in the form of potential
configurations). Each potential configuration attribute in a given configurations). Each potential configuration attribute in a given
media description MUST contain a unique configuration number and one media description MUST contain a unique configuration number and one
or more potential configuration lists, as described in Section 3.6.1. or more potential configuration lists, as described in Section
Each potential configuration list MUST refer to capabilities that are 3.5.1. Each potential configuration list MUST refer to capabilities
provided either at the session-level or within that particular media that are provided at the session-level or within that particular
description; otherwise, the potential configuration is considered media description; otherwise, the potential configuration is
invalid. The base SDP Capability Negotiation framework REQUIRES that considered invalid. The base SDP Capability Negotiation framework
potential configurations do not reference any session-level attribute REQUIRES that potential configurations do not reference any session-
capabilities that contain media-level attributes, however extensions level attribute capabilities that contain media-level only
may modify this behavior, as long as it is fully backwards compatible attributes, however extensions may modify this behavior, as long as
with the base specification. Furthermore, it is RECOMMENDED that it is fully backwards compatible with the base specification.
potential configurations avoid use of session-level capabilities Furthermore, it is RECOMMENDED that potential configurations avoid
whenever possible; refer to Section 3.6.1. use of session-level capabilities whenever possible; refer to
Section 3.5.1.
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]) and the various attribute ("a=") lines. Note defined by [RFC3264]) and any associated parameters for the media
that the actual configuration is by definition the least-preferred description (e.g., attribute ("a=") and bandwidth ("b=") lines).
Note that the actual configuration is by default the least-preferred
configuration, and hence the answerer will seek to negotiate use of configuration, and hence the answerer will seek to negotiate use of
one of the potential configurations instead. If the offerer wishes a one of the potential configurations instead. If the offerer wishes a
different preference for the actual configuration, the offerer MUST different preference for the actual configuration, the offerer MUST
include a corresponding potential configuration with the relevant include a corresponding potential configuration with the relevant
configuration number (which indicates the relative preference between configuration number (which indicates the relative preference
potential configurations); this corresponding potential configuration between potential configurations); this corresponding potential
should simply duplicate the actual configuration. configuration should simply duplicate the actual configuration.
This can either be done implicitly (by not referencing any This can either be done implicitly (by not referencing any
capabilities), or explicitly (by providing and using capabilities capabilities), or explicitly (by providing and using capabilities
for the transport protocol and all the attributes that are part of for the transport protocol and all the attributes that are part of
the actual configuration). The latter may help detect the actual configuration). The latter may help detect
intermediaries that modify the actual configuration but are not SDP intermediaries that modify the actual configuration but are not
Capability Negotiation aware. SDP Capability Negotiation aware.
Per [RFC3264], once the offerer generates the offer, he must be Per [RFC3264], once the offerer generates the offer, he must be
prepared to receive incoming media in accordance with that offer. prepared to receive incoming media in accordance with that offer.
That rule applies here as well, but for the actual configurations That rule applies here as well, but for the actual configurations
provided in the offer only: Media received by the offerer according provided in the offer only: Media received by the offerer according
to one of the potential configurations MAY be discarded, until the to one of the potential configurations MAY be discarded, until the
offerer receives an answer indicating what the actual selected offerer receives an answer indicating what the actual selected
configuration is. Once that answer is received, incoming media MUST configuration is. Once that answer is received, incoming media MUST
be processed in accordance with the actual selected configuration be processed in accordance with the actual selected configuration
indicated and the answer received (provided the offer/answer exchange indicated and the answer received (provided the offer/answer
completed successfully). exchange completed successfully).
The above rule assumes that the offerer can determine whether The above rule assumes that the offerer can determine whether
incoming media adheres to the actual configuration offered or one of incoming media adheres to the actual configuration offered or one of
the potential configurations instead; this may not always be the the potential configurations instead; this may not always be the
case. If the offerer wants to ensure he does not play out any case. If the offerer wants to ensure he does not play out any
garbage, the offerer SHOULD discard all media received before the garbage, the offerer SHOULD discard all media received before the
answer SDP is received. Conversely, if the offerer wants to avoid answer SDP is received. Conversely, if the offerer wants to avoid
clipping, he should attempt to play any incoming media as soon as it clipping, he should attempt to play any incoming media as soon as it
is received (at the risk of playing out garbage). For further is received (at the risk of playing out garbage). For further
details, please refer to Section 3.10. details, please refer to Section 3.9.
3.7.2. Generating the Answer 3.6.2. Generating the Answer
When receiving an offer, the answerer MUST check for the presence of When receiving an offer, the answerer MUST check for the presence of
a required capability negotiation extension attribute ("a=creq") a required capability negotiation extension attribute ("a=creq")
provided at the session level. If one is found, then capability provided at the session level. If one is found, then capability
negotiation MUST be performed. If none is found, then the answerer negotiation MUST be performed. If none is found, then the answerer
MUST check each offered media description for the presence of a MUST check each offered media description for the presence of a
required capability negotiation extension attribute ("a=creq") and required capability negotiation extension attribute ("a=creq") and
one or more potential configuration attributes ("a=pcfg"). Capability one or more potential configuration attributes ("a=pcfg").
negotiation MUST be performed for each media description where either Capability negotiation MUST be performed for each media description
of those is present in accordance with the procedures described where either of those is present in accordance with the procedures
below. described below.
The answerer MUST first ensure that it supports any required The answerer MUST first ensure that it supports any required
capability negotiation extensions: capability negotiation extensions:
o If a session-level "creq" attribute is provided, and it contains o If a session-level "creq" attribute is provided, and it contains
an option-tag that the answerer does not support, then the an option-tag that the answerer does not support, then the
answerer MUST NOT use any of the potential configuration answerer MUST NOT use any of the potential configuration
attributes provided for any of the media descriptions. Instead, attributes provided for any of the media descriptions. Instead,
the normal offer/answer procedures MUST continue as per [RFC3264]. the normal offer/answer procedures MUST continue as per
Furthermore, the answerer MUST include a session-level supported [RFC3264]. Furthermore, the answerer MUST include a session-level
capability negotiation extensions attribute ("a=csup") with option supported capability negotiation extensions attribute ("a=csup")
tags for the capability negotiation extensions supported by the with option tags for the capability negotiation extensions
answerer. supported by the answerer.
o If a media-level "creq" attribute is provided, and it contains an o If a media-level "creq" attribute is provided, and it contains an
option tag that the answerer does not support, then the answerer option tag that the answerer does not support, then the answerer
MUST NOT use any of the potential configuration attributes MUST NOT use any of the potential configuration attributes
provided for that particular media description. Instead, the provided for that particular media description. Instead, the
offer/answer procedures for that media description MUST continue offer/answer procedures for that media description MUST continue
as per [RFC3264] (SDP Capability Negotiation is still performed as per [RFC3264] (SDP Capability Negotiation is still performed
for other media descriptions in the SDP). Furthermore, the for other media descriptions in the SDP). Furthermore, the
answerer MUST include a supported capability negotiation answerer MUST include a supported capability negotiation
extensions attribute ("a=csup") in that media description with extensions attribute ("a=csup") in that media description with
option tags for the capability negotiation extensions supported by option tags for the capability negotiation extensions supported
the answerer for that media description. by the answerer for that media description.
Assuming all required capability negotiation extensions are Assuming all required capability negotiation extensions are
supported, the answerer now proceeds as follows. supported, the answerer now proceeds as follows.
For each media description where capability negotiation is to be For each media description where capability negotiation is to be
performed (i.e. all required capability negotiation extensions are performed (i.e. all required capability negotiation extensions are
supported and at least one valid potential configuration attribute is supported and at least one valid potential configuration attribute
present), the answerer MUST attempt to perform capability negotiation is present), the answerer MUST attempt to perform capability
by using the most preferred potential configuration that is valid. A negotiation by using the most preferred potential configuration that
potential configuration is valid if: is valid to the answerer. A potential configuration is valid to the
answerer if:
1. It is in accordance with the syntax and semantics provided in 1. It is in accordance with the syntax and semantics provided in
Section 3.6.1. Section 3.5.1.
2. It contains a configuration number that is unique within that 2. It contains a configuration number that is unique within that
media description. media description.
3. All attribute capabilities referenced by the potential 3. All attribute capabilities referenced by the potential
configuration are valid themselves (as defined in Section 3.5.1. ) configuration are valid themselves (as defined in Section 3.4.1.
and each of them is provided either at the session-level or within ) and each of them is provided either at the session-level or
this particular media description. For session-level attribute within this particular media description. For session-level
capabilities referenced, the attributes contained inside them MUST attribute capabilities referenced, the attributes contained
NOT be media-level only attributes. inside them MUST NOT be media-level only attributes.
4. All transport protocol capabilities referenced by the potential 4. All transport protocol capabilities referenced by the potential
configuration are valid themselves (as defined in Section 3.5.2. ) configuration are valid themselves (as defined in Section 3.4.2.
and each of them is furthermore provided either at the session- ) and each of them is furthermore provided either at the session-
level or within this particular media description. level or within this particular media description.
5. All extension capabilities referenced by the potential 5. All extension capabilities referenced by the potential
configuration and supported by the answerer are valid themselves configuration and supported by the answerer are valid themselves
(as defined by that particular extension) and each of them are (as defined by that particular extension) and each of them are
furthermore provided either at the session-level or within this furthermore provided either at the session-level or within this
particular media description. Unknown or unsupported extension particular media description. Unknown or unsupported extension
capabilities MUST be ignored. capabilities MUST be ignored, unless they are prefixed with the
plus ("+") sign, which indicates that the extension MUST be
supported in order to use that potential configuration. If the
extension is not supported, that potential configuration is not
valid to the answerer.
The most preferred valid potential configuration in a media The most preferred valid potential configuration in a media
description is the valid potential configuration with the lowest description is the valid potential configuration with the lowest
configuration number. The answerer MUST now process the offer for configuration number. The answerer MUST now process the offer for
that media stream based on the most preferred valid potential that media stream based on the most preferred valid potential
configuration. Conceptually, this entails the answerer constructing configuration. Conceptually, this entails the answerer constructing
an (internal) offer that consists of the actual configuration offer an (internal) offer that consists of the actual configuration offer
SDP, with the following changes for each media stream offered: SDP, with the following changes for each media stream offered:
o If a transport protocol capability is included in the potential o If a transport protocol capability is included in the potential
configuration, then it replaces the transport protocol provided in configuration, then it replaces the transport protocol provided
the "m=" line for that media description. in the "m=" line for that media description.
o If attribute capabilities are present with a delete-attributes o If attribute capabilities are present with a delete-attributes
session indication ("-s"), then all session-level attributes from session indication ("-s"), then all session-level attributes from
the actual configuration SDP MUST be deleted in accordance with the actual configuration SDP MUST be deleted in accordance with
the procedures in Section 3.6.1. If attribute capabilities are the procedures in Section 3.5.1. If attribute capabilities are
present with a delete-attributes media indication ("-m"), then all present with a delete-attributes media indication ("-m"), then
attributes from the actual configuration SDP inside this media all attributes from the actual configuration SDP inside this
description MUST be deleted. media description MUST be deleted.
o If a session-level attribute capability is included, the attribute o If a session-level attribute capability is included, the
(and its associated value, if any) contained in it MUST be added attribute (and its associated value, if any) contained in it MUST
to the resulting SDP. All such added session-level attributes MUST be added to the resulting SDP. All such added session-level
be listed before the session-level attributes that were initially attributes MUST be listed before the session-level attributes
present in the SDP. Furthermore, the added session-level that were initially present in the SDP. Furthermore, the added
attributes MUST be added in the order they were provided in the session-level attributes MUST be added in the order they were
potential configuration (see also Section 3.6.1. ). provided in the potential configuration (see also Section 3.5.1.
).
This allows for attributes with implicit preference ordering to This allows for attributes with implicit preference ordering
be added in the desired order; the "crypto" attribute [SDES] is to be added in the desired order; the "crypto" attribute
one such example. [RFC4568] is one such example.
o If a media-level attribute capability is included, then the o If a media-level attribute capability is included, then the
attribute (and its associated value, if any) MUST be added to the attribute (and its associated value, if any) MUST be added to the
resulting SDP within the media description in question. All such resulting SDP within the media description in question. All such
added media-level attributes MUST be listed before the media-level added media-level attributes MUST be listed before the media-
attributes that were initially present in the SDP in the media level attributes that were initially present in the SDP in the
description in question. Furthermore, the added media-level media description in question. Furthermore, the added media-level
attributes MUST be added in the order they were provided in the attributes MUST be added in the order they were provided in the
potential configuration (see also Section 3.6.1. ). potential configuration (see also Section 3.5.1. ).
o If a supported extension capability is included, then it MUST be o If a supported extension capability is included, then it MUST be
processed in accordance with the rules provided for that processed in accordance with the rules provided for that
particular extension capability. particular extension capability.
Note that whereas a transport protocol from the potential Note that a transport protocol from the potential configuration
configuration replaces the transport protocol in the actual replaces the transport protocol in the actual configuration, but an
configuration, an attribute capability from the potential attribute capability from the potential configuration is simply
configuration is simply added to the actual configuration. In some added to the actual configuration. In some cases, this can result in
cases, this can result in having one or more meaningless attributes having one or more meaningless attributes in the resulting potential
in the resulting potential configuration SDP, or worse, ambiguous or configuration SDP, or worse, ambiguous or potentially even illegal
potentially even illegal attributes. The delete-attributes for the attributes. Use of delete-attributes for the session and/or media
session and/or media level attributes MUST be used to avoid such level attributes MUST be done to avoid such scenarios. Nevertheless,
scenarios. Nevertheless, it is RECOMMENDED that implementations it is RECOMMENDED that implementations ignore meaningless attributes
ignore meaningless attributes that may result from potential that may result from potential configurations.
configurations.
For example, if the actual configuration was using Secure RTP and For example, if the actual configuration was using Secure RTP and
included an "a=crypto" attribute for the SRTP keying material, then included an "a=crypto" attribute for the SRTP keying material,
use of a potential configuration that uses plain RTP would make the then use of a potential configuration that uses plain RTP would
"crypto" attribute meaningless. The answerer may or may not ignore make the "crypto" attribute meaningless. The answerer may or may
such a meaningless attribute. The offerer can here ensure correct not ignore such a meaningless attribute. The offerer can here
operation by using the delete-attributes to actually delete the ensure correct operation by using delete-attributes to remove the
crypto attribute (but will then need to provide attribute crypto attribute (but will then need to provide attribute
capabilities to reconstruct the SDP with the necessary attributes capabilities to reconstruct the SDP with the necessary attributes
deleted, e.g. rtpmaps). deleted, e.g. rtpmaps).
Please refer to Section 3.7.2.1. for examples of how the answerer may Please refer to Section 3.6.2.1. for examples of how the answerer
conceptually "see" the resulting offered alternative potential may conceptually "see" the resulting offered alternative potential
configurations. configurations.
The answerer MUST check that he supports all mandatory attribute The answerer MUST check that he supports all mandatory attribute
capabilities from the potential configuration (if any) and the capabilities from the potential configuration (if any), the
transport protocol capability (if any) from the potential transport protocol capability (if any) from the potential
configuration. If he does not, the answerer MUST proceed to the configuration, and all mandatory extension capabilities from the
potential configuration (if any) in accordance with the rules
provided for these. If he does not, the answerer MUST proceed to the
second-most preferred valid potential configuration for the media second-most preferred valid potential configuration for the media
description, etc. description, etc. In the case of attribute capabilities, support
implies that the attribute name contained in the capability is
supported and it can (and will) be used successfully in the
negotiation process with the value provided. This does not
necessarily imply that the value provided is supported in its
entirety. For example, the "a=fmtp" parameter is often provided with
one or more values in a list, where the offerer and answerer
negotiate use of some subset of the values provided. Other
attributes may include mandatory and optional parts to their values;
support for the mandatory part is all that is required here.
A side-effect of the above rule is that whenever an "fmtp" or
"rtpmap" parameter is provided as a mandatory attribute
capability, the corresponding media format (codec) must be
supported and use of it negotiated successfully. If this is not
the offerer's intent, the corresponding attribute capabilities
must be listed as optional instead.
If the answerer has exhausted all potential configurations for the If the answerer has exhausted all potential configurations for the
media description, without finding a valid one that is also media description, without finding a valid one that is also
supported, then the answerer MUST process the offered media stream supported, then the answerer MUST process the offered media stream
based on the actual configuration plus any session-level attributes based on the actual configuration plus any session-level attributes
added by a valid and supported potential configuration for another added by a valid and supported potential configuration from another
media description in the offered SDP. media description in the offered SDP.
The above process describes potential configuration selection as a The above process describes potential configuration selection as a
per media stream process. Inter-media stream coordination of selected per media stream process. Inter-media stream coordination of
potential configurations however is required in some cases. First of selected potential configurations however is required in some cases.
all, session-level attributes added by a potential configuration for First of all, session-level attributes added by a potential
one media description MUST NOT cause any problems for potential configuration for one media description MUST NOT cause any problems
configurations selected by other media descriptions in the offer SDP. for potential configurations selected by other media descriptions in
If the session-level attributes are mandatory, then those session- the offer SDP. If the session-level attributes are mandatory, then
level attributes MUST furthermore be supported by the session as a those session-level attributes MUST furthermore be supported by the
whole (i.e. all the media descriptions if relevant). As mentioned session as a whole (i.e., all the media descriptions if relevant).
earlier, this adds additional complexity to the overall processing As mentioned earlier, this adds additional complexity to the overall
and hence it is RECOMMENDED not to use session-level attribute processing and hence it is RECOMMENDED not to use session-level
capabilities in potential configurations, unless absolutely attribute capabilities in potential configurations, unless
necessary. absolutely necessary.
Once the answerer has selected a valid and supported offered Once the answerer has selected a valid and supported offered
configuration for all of the media streams (or has fallen back to the potential configuration for all of the media streams (or has fallen
actual configuration plus any added session attributes), the answerer back to the actual configuration plus any added session attributes),
MUST generate a valid answer SDP based on the selected potential the answerer MUST generate a valid answer SDP based on the selected
configuration SDP, as "seen" by the answerer (see Section 3.7.2.1. potential configuration SDP, as "seen" by the answerer (see Section
for examples). Furthermore, if the answerer selected one of the 3.6.2.1. for examples). Furthermore, if the answerer selected one of
potential configurations in a media description, the answerer MUST the potential configurations in a media description, the answerer
include an actual configuration attribute ("a=acfg") within that MUST include an actual configuration attribute ("a=acfg") within
media description that identifies the configuration number for the that media description. The "a=acfg" attribute MUST identify the
selected potential configuration as well as the actual parameters configuration number for the selected potential configuration as
that were used from that potential configuration (if the potential well as the actual parameters that were used from that potential
configuration included alternatives, only the selected alternatives configuration; if the potential configuration included alternatives,
must be included). Only the known and supported parameters will be the selected alternatives only MUST be included. Only the known and
included. Unknown or unsupported parameters MUST NOT be included in supported parameters will be included. Unknown or unsupported
the actual configuration attribute. parameters MUST NOT be included in the actual configuration
attribute. In the case of attribute capabilities, only the known and
This is not to be confused with mandatory and optional attribute supported capabilities are included; unknown or unsupported
capabilities, where some of the optional attribute capabilities may attribute capabilities MUST NOT be included.
refer to unknown or unsupported attributes. The entire list of
attribute capabilities still need to be included thereby enabling
the offerer to determine which alternative potential configuration
was chosen by the answerer.
If the answerer supports one or more capability negotiation If the answerer supports one or more capability negotiation
extensions that were not included in a required capability extensions that were not included in a required capability
negotiation extensions attribute in the offer, then the answerer negotiation extensions attribute in the offer, then the answerer
SHOULD furthermore include a supported capability negotiation SHOULD furthermore include a supported capability negotiation
attribute ("a=csup") at the session-level with option tags for the attribute ("a=csup") at the session-level with option tags for the
extensions supported across media streams. Also, if the answerer extensions supported across media streams. Also, if the answerer
supports one or more capability negotiation extensions for particular supports one or more capability negotiation extensions for
media descriptions only, then a supported capability negotiation particular media descriptions only, then a supported capability
attribute with those option-tags SHOULD be included within each negotiation attribute with those option-tags SHOULD be included
relevant media description. within each relevant media description.
The offerer's originally provided actual configuration is contained The offerer's originally provided actual configuration is contained
in the media description's "m=" line (and associated parameters). The in the offer media description's "m=" line (and associated
answerer MAY send media to the offerer in accordance with that actual parameters). The answerer MAY send media to the offerer in
configuration as soon as it receives the offer, however it MUST NOT accordance with that actual configuration as soon as it receives the
send media based on that actual configuration if it selects an offer, however it MUST NOT send media based on that actual
alternative potential configuration. If the answerer selects one of configuration if it selects an alternative potential configuration.
the potential configurations, then the answerer MAY immediately start If the answerer selects one of the potential configurations, then
to send media to the offerer in accordance with the selected the answerer MAY immediately start to send media to the offerer in
potential configuration, however the offerer MAY discard such media accordance with the selected potential configuration, however the
or play out garbage until the offerer receives the answer. Please offerer MAY discard such media or play out garbage until the offerer
refer to section 3.10. for additional considerations and possible receives the answer. Please refer to section 3.9. for additional
alternative solutions outside the base SDP Capability Negotiation considerations and possible alternative solutions outside the base
framework. SDP Capability Negotiation framework.
If the offerer selected a potential configuration instead of the If the offerer selected a potential configuration instead of the
actual configuration, then it is RECOMMENDED that the answerer sends actual configuration, then it is RECOMMENDED that the answerer sends
back an answer SDP as soon as possible. This minimizes the risk of back an answer SDP as soon as possible. This minimizes the risk of
having media discarded or played out as garbage by the offerer. In having media discarded or played out as garbage by the offerer. In
the case of SIP [RFC3261] without any extensions, this implies that the case of SIP [RFC3261] without any extensions, this implies that
if the offer was received in an INVITE message, then the answer SDP if the offer was received in an INVITE message, then the answer SDP
should be provided in the first non-100 provisional response sent should be provided in the first non-100 provisional response sent
back (per RFC3261, the answer would need to be repeated in the 200 back (per RFC3261, the answer would need to be repeated in the 200
response as well, unless a relevant extension such as [RFC3262] is response as well, unless a relevant extension such as [RFC3262] is
being used). being used).
3.7.2.1. Example Views of Potential Configurations 3.6.2.1. Example Views of Potential Configurations
The following examples illustrate how the answerer may conceptually The following examples illustrate how the answerer may conceptually
"see" a potential configuration. Consider the following offered SDP: "see" a potential configuration. Consider the following offered SDP:
v=0 v=0
o=alice 2891092738 2891092738 IN IP4 lost.example.com o=alice 2891092738 2891092738 IN IP4 lost.example.com
s= s=
t=0 0 t=0 0
c=IN IP4 lost.example.com c=IN IP4 lost.example.com
a=tool:foo a=tool:foo
a=acap:1 a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO... a=acap:1 key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
a=tcap:1 RTP/SAVP RTP/AVP a=tcap:1 RTP/SAVP RTP/AVP
m=audio 59000 RTP/AVP 98 m=audio 59000 RTP/AVP 98
a=rtpmap:98 AMR/8000 a=rtpmap:98 AMR/8000
a=acap:2 a=crypto:1 AES_CM_128_HMAC_SHA1_32 a=acap:2 crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=pcfg:1 t=1 a=1|2 a=pcfg:1 t=1 a=1|2
m=video 52000 RTP/AVP 31 m=video 52000 RTP/AVP 31
a=rtpmap:31 H261/90000 a=rtpmap:31 H261/90000
a=acap:3 a=crypto:1 AES_CM_128_HMAC_SHA1_80 a=acap:3 crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32 inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=pcfg:1 t=1 a=1|3 a=pcfg:1 t=1 a=1|3
This particular SDP offers an audio stream and a video stream, each This particular SDP offers an audio stream and a video stream, each
of which can either use plain RTP (actual configuration) or secure of which can either use plain RTP (actual configuration) or secure
RTP (potential configuration). Furthermore, two different keying RTP (potential configuration). Furthermore, two different keying
mechanisms are offered, namely session-level Key Management mechanisms are offered, namely session-level Key Management
Extensions using MIKEY (attribute capability 1) and media-level SDP Extensions using MIKEY (attribute capability 1) and media-level SDP
Security Descriptions (attribute capabilities 2 and 3). There are Security Descriptions (attribute capabilities 2 and 3). There are
several potential configurations here, however, below we show the one several potential configurations here, however, below we show the
the answerer "sees" when using potential configuration 1 for both one the answerer "sees" when using potential configuration 1 for
audio and video, and furthermore using attribute capability 1 (MIKEY) both audio and video, and furthermore using attribute capability 1
for both (we have removed all the capability negotiation attributes (MIKEY) for both (we have removed all the capability negotiation
for clarity): attributes for clarity):
v=0 v=0
o=alice 2891092738 2891092738 IN IP4 lost.example.com o=alice 2891092738 2891092738 IN IP4 lost.example.com
s= s=
t=0 0 t=0 0
c=IN IP4 lost.example.com c=IN IP4 lost.example.com
a=tool:foo a=tool:foo
a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO... a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
m=audio 59000 RTP/SAVP 98 m=audio 59000 RTP/SAVP 98
a=rtpmap:98 AMR/8000 a=rtpmap:98 AMR/8000
m=video 52000 RTP/SAVP 31 m=video 52000 RTP/SAVP 31
a=rtpmap:31 H261/90000 a=rtpmap:31 H261/90000
Note that the transport protocol in the media descriptions indicate Note that the transport protocol in the media descriptions indicate
use of secure RTP. use of secure RTP.
Below, we show the offer the answerer "sees" when using potential Below, we show the offer the answerer "sees" when using potential
configuration 1 for both audio and video and furthermore using configuration 1 for both audio and video and furthermore using
attribute capability 2 and 3 respectively (SDP security descriptions) attribute capability 2 and 3 respectively (SDP security
for the audio and video stream - note the order in which the descriptions) for the audio and video stream - note the order in
resulting attributes are provided: which the resulting attributes are provided:
v=0 v=0
o=alice 2891092738 2891092738 IN IP4 lost.example.com o=alice 2891092738 2891092738 IN IP4 lost.example.com
s= s=
t=0 0 t=0 0
c=IN IP4 lost.example.com c=IN IP4 lost.example.com
a=tool:foo a=tool:foo
m=audio 59000 RTP/SAVP 98 m=audio 59000 RTP/SAVP 98
a=crypto:1 AES_CM_128_HMAC_SHA1_32 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=rtpmap:98 AMR/8000 a=rtpmap:98 AMR/8000
m=video 52000 RTP/SAVP 31 m=video 52000 RTP/SAVP 31
a=crypto:1 AES_CM_128_HMAC_SHA1_80 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32 inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=rtpmap:31 H261/90000 a=rtpmap:31 H261/90000
Again, note that the transport protocol in the media descriptions Again, note that the transport protocol in the media descriptions
indicate use of secure RTP. indicate use of secure RTP.
And finally, we show the offer the answerer "sees" when using And finally, we show the offer the answerer "sees" when using
potential configuration 1 with attribute capability 1 (MIKEY) for the potential configuration 1 with attribute capability 1 (MIKEY) for
audio stream, and potential configuration 1 with attribute capability the audio stream, and potential configuration 1 with attribute
3 (SDP security descriptions) for the video stream: capability 3 (SDP security descriptions) for the video stream:
v=0 v=0
o=alice 2891092738 2891092738 IN IP4 lost.example.com o=alice 2891092738 2891092738 IN IP4 lost.example.com
s= s=
t=0 0 t=0 0
c=IN IP4 lost.example.com c=IN IP4 lost.example.com
a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO... a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
a=tool:foo a=tool:foo
m=audio 59000 RTP/SAVP 98 m=audio 59000 RTP/SAVP 98
a=rtpmap:98 AMR/8000 a=rtpmap:98 AMR/8000
m=video 52000 RTP/SAVP 31 m=video 52000 RTP/SAVP 31
a=crypto:1 AES_CM_128_HMAC_SHA1_80 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32 inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=rtpmap:31 H261/90000 a=rtpmap:31 H261/90000
3.7.3. Offerer Processing of the Answer 3.6.3. Offerer Processing of the Answer
When the offerer attempted to use SDP Capability Negotiation in the When the offerer attempted to use SDP Capability Negotiation in the
offer, the offerer MUST examine the answer for actual use of SDP offer, the offerer MUST examine the answer for actual use of SDP
Capability Negotiation. Capability Negotiation.
For each media description where the offerer included a potential For each media description where the offerer included a potential
configuration attribute ("a=pcfg"), the offerer MUST first examine configuration attribute ("a=pcfg"), the offerer MUST first examine
the media description for the presence of an actual configuration that media description for the presence of an actual configuration
attribute ("a=acfg"). If an actual configuration attribute is not attribute ("a=acfg"). If an actual configuration attribute is not
present in a media description, then the offerer MUST process the present in a media description, then the offerer MUST process the
answer SDP for that media stream per the normal offer/answer rules answer SDP for that media stream per the normal offer/answer rules
defined in [RFC3264]. However, if one is found, the offerer MUST defined in [RFC3264]. However, if one is found, the offerer MUST
instead process the answer as follows: instead process the answer as follows:
o The actual configuration attribute specifies which of the o The actual configuration attribute specifies which of the
potential configurations was used by the answerer to generate the potential configurations was used by the answerer to generate the
answer for this media stream. This includes all the capabilities answer for this media stream. This includes all the supported
referenced by the potential configuration selected, i.e. the attribute capabilities and the transport capabilities referenced
attribute capabilities and any associated delete-attributes, by the potential configuration selected, where the attribute
transport protocol capabilities, and any extension capability capabilities have any associated delete-attributes included.
parameters included. Extension capabilities supported by the answerer are included as
well.
o The offerer MUST now process the answer in accordance with the o The offerer MUST now process the answer in accordance with the
rules in [RFC3264], except that it must be done as if the offer rules in [RFC3264], except that it must be done as if the offer
consisted of the selected potential configuration instead of the consisted of the selected potential configuration instead of the
actual configuration, including any transport protocol changes in original actual configuration, including any transport protocol
the media ("m=") line(s), attributes added and deleted by the changes in the media ("m=") line(s), attributes added and deleted
potential configuration at the media and session level, and any by the potential configuration at the media and session level,
extensions used. and any extensions used.
If the offer/answer exchange was successful, and if the answerer If the offer/answer exchange was successful, and if the answerer
selected one of the potential configurations from the offer as the selected one of the potential configurations from the offer as the
actual configuration, then the offerer MAY perform another actual configuration, then the offerer MAY perform another
offer/answer exchange: This new offer SHOULD contain the selected offer/answer exchange: This new offer SHOULD contain the selected
potential configuration as the actual configuration, i.e. with the potential configuration as the actual configuration, i.e., with the
actual configuration used in the "m=" line and any other relevant actual configuration used in the "m=" line and any other relevant
attributes and extensions. This second offer/answer exchange will not attributes and extensions. This second offer/answer exchange will
modify the session in any way, however it will help intermediaries not modify the session in any way, however it will help
(e.g. middleboxes) that look at the SDP, but do not understand or intermediaries (e.g. middleboxes) that look at the SDP, but do not
support the capability negotiation extensions, to understand the understand or support the capability negotiation extensions, to
details of the media stream(s) that were actually negotiated. If it understand the details of the media stream(s) that were actually
is known or suspected that one or more such intermediaries exist, negotiated. If it is known or suspected that one or more such
then this second offer/answer SHOULD be performed (this is already intermediaries exist, then this second offer/answer SHOULD be
done when using Interactive Connectivity Establishment [ICE], and in performed (this is already done when using Interactive Connectivity
those cases, there will not be a need for a third offer/answer Establishment [ICE], and in those cases, there will not be a need
exchange). Note that, per normal offer/answer rules, the second for a third offer/answer exchange). Note that, per normal
offer/answer exchange still needs to update the version number in the offer/answer rules, the second offer/answer exchange still needs to
"o=" line ((<sess-version> in [RFC4566]). Attribute lines carrying update the version number in the "o=" line ((<sess-version> in
keying material SHOULD repeat the keys from the previous offer, [RFC4566]). Attribute lines carrying keying material SHOULD repeat
unless re-keying is necessary, e.g. due to a previously forked SIP the keys from the previous offer, unless re-keying is necessary,
INVITE request. Please refer to Section 3.12. for additional e.g. due to a previously forked SIP INVITE request. Please refer to
considerations related to intermediaries. Section 3.12. for additional considerations related to
intermediaries.
3.7.4. Modifying the Session 3.6.4. Modifying the Session
Capabilities and potential configurations may be included in Capabilities and potential configurations may be included in
subsequent offers as defined in [RFC3264], Section 8. The procedure subsequent offers as defined in [RFC3264], Section 8. The procedure
for doing so is similar to that described above with the answer for doing so is similar to that described above with the answer
including an indication of the actual selected configuration used by including an indication of the actual selected configuration used by
the answerer. the answerer.
If the answer indicates use of a potential configuration from the If the answer indicates use of a potential configuration from the
offer, then the guidelines provided in Section 3.7.3. for doing a offer, then the guidelines provided in Section 3.6.3. for doing a
second offer/answer exchange using that potential configuration as second offer/answer exchange using that potential configuration as
the actual configuration apply. the actual configuration apply.
3.8. Interactions with ICE 3.7. Interactions with ICE
Interactive Connectivity Establishment (ICE) [ICE] provides a Interactive Connectivity Establishment (ICE) [ICE] provides a
mechanism for verifying connectivity between two endpoints by sending mechanism for verifying connectivity between two endpoints by
STUN messages directly between the media endpoints. The basic ICE sending STUN messages directly between the media endpoints. The
specification [ICE] is defined to support UDP-based connectivity basic ICE specification [ICE] is defined to support UDP-based
only, however it allows for extensions to support other transport connectivity only, however it allows for extensions to support other
protocols, such as TCP, which is being specified in [ICETCP]. ICE transport protocols, such as TCP, which is being specified in
defines a new "a=candidate" attribute, which, among other things, [ICETCP]. ICE defines a new "a=candidate" attribute, which, among
indicates the possible transport protocol(s) to use and then other things, indicates the possible transport protocol(s) to use
associates a priority with each of them. The most preferred transport and then associates a priority with each of them. The most preferred
protocol that *successfully* verifies connectivity will end up being transport protocol that *successfully* verifies connectivity will
used. end up being used.
When using ICE, it is thus possible that the transport protocol that When using ICE, it is thus possible that the transport protocol that
will be used differs from what is specified in the "m=" line. will be used differs from what is specified in the "m=" line. Since
Furthermore, since both ICE and SDP Capability Negotiation may now both ICE and SDP Capability Negotiation may specify alternative
specify alternative transport protocols, there is a potentially transport protocols, there is a potentially unintended interaction
unintended interaction when using these together. when using these together.
We provide the following guidelines for addressing that. We provide the following guidelines for addressing that.
There are two basic scenarios to consider here: There are two basic scenarios to consider:
1) A particular media stream can run over different transport 1) A particular media stream can run over different transport
protocols (e.g. UDP, TCP, or TCP/TLS), and the intent is simply to protocols (e.g. UDP, TCP, or TCP/TLS), and the intent is simply to
use the one that works (in the preference order specified). use the one that works (in the preference order specified).
2) A particular media stream can run over different transport 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 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 negotiation process decide which one to use (e.g. T.38 over TCP or
UDP). UDP).
In scenario 1, there should be ICE "a=candidate" attributes for UDP, In scenario 1, there should be ICE "a=candidate" attributes for UDP,
TCP, etc. but otherwise nothing special in the potential TCP, etc. but otherwise nothing special in the potential
configuration attributes to indicate the desire to use different configuration attributes to indicate the desire to use different
transport protocols (e.g. UDP, or TCP). The ICE procedures transport protocols (e.g. UDP, or TCP). The ICE procedures
essentially cover the capability negotiation required (by having the essentially cover the capability negotiation required (by having the
answerer select something it supports and then use of trial and answerer select something it supports and then use of trial and
error). error connectivity checks).
Scenario 2 does not require a need to support or use ICE. Instead, we Scenario 2 does not require a need to support or use ICE. Instead,
simply use transport protocol capabilities and potential we simply use transport protocol capabilities and potential
configuration attributes to indicate the desired outcome. configuration attributes to indicate the desired outcome.
The scenarios may be combined, e.g. by offering potential The scenarios may be combined, e.g. by offering potential
configuration alternatives where some of them can support one configuration alternatives where some of them can support one
transport protocol only (e.g. UDP), whereas others can support transport protocol only (e.g. UDP), whereas others can support
multiple transport protocols (e.g. UDP or TCP). In that case, there multiple transport protocols (e.g. UDP or TCP). In that case, there
is a need for tight control over the ICE candidates that will is a need for tight control over the ICE candidates that will be
actually be used for a particular configuration, yet the actual used for a particular configuration, yet the actual configuration
configuration may want to use all of them. In that case, the ICE may want to use all of the ICE candidates. In that case, the ICE
candidate attributes can be defined as attribute capabilities and the candidate attributes can be defined as attribute capabilities and
relevant ones should then be included in the proper potential the relevant ones should then be included in the proper potential
configurations (for example candidate attributes for UDP only for configurations (for example candidate attributes for UDP only for
potential configurations that are restricted to UDP, whereas there potential configurations that are restricted to UDP, whereas there
could be candidate attributes for UDP, TCP, and TCP/TLS for potential could be candidate attributes for UDP, TCP, and TCP/TLS for
configurations that can use all three). Furthermore, use of the potential configurations that can use all three). Furthermore, use
delete-attributes in a potential configuration can be used to ensure of the delete-attributes in a potential configuration can be used to
that ICE will not end up using a transport protocol that is not ensure that ICE will not end up using a transport protocol that is
desired. not desired for a particular configuration.
3.9. Interactions with SIP Option Tags 3.8. Interactions with SIP Option Tags
SIP [RFC3261] allows for SIP extensions to define a SIP option tag SIP [RFC3261] allows for SIP extensions to define a SIP option tag
that identifies the SIP extension. Support for one or more such that identifies the SIP extension. Support for one or more such
extensions can be indicated by use of the SIP Supported header, and extensions can be indicated by use of the SIP Supported header, and
required support for one or more such extensions can be indicated by required support for one or more such extensions can be indicated by
use of the SIP Require header. The "a=csup" and "a=creq" attributes use of the SIP Require header. The "a=csup" and "a=creq" attributes
defined by the SDP Capability Negotiation framework are similar, defined by the SDP Capability Negotiation framework are similar,
except that support for these two attributes by themselves cannot be except that support for these two attributes by themselves cannot be
guaranteed (since they are specified as extensions to the SDP guaranteed (since they are specified as extensions to the SDP
specification [RFC4566] itself). specification [RFC4566] itself).
SIP extensions with associated option tags can introduce enhancements SIP extensions with associated option tags can introduce
to not only SIP, but also SDP. This is for example the case for SIP enhancements to not only SIP, but also SDP. This is for example the
preconditions defined in [RFC3312]. When using SDP Capability case for SIP preconditions defined in [RFC3312]. When using SDP
Negotiation, some potential configurations may include certain SDP Capability Negotiation, some potential configurations may include
extensions, whereas others may not. Since the purpose of the SDP certain SDP extensions, whereas others may not. Since the purpose of
Capability Negotiation is to negotiate a session based on the the SDP Capability Negotiation is to negotiate a session based on
features supported by both sides, use of the SIP Require header for the features supported by both sides, use of the SIP Require header
such extensions may not produce the desired result. For example, if for such extensions may not produce the desired result. For example,
one potential configuration requires SIP precondition support, if one potential configuration requires SIP preconditions support,
another does not, and the answerer does not support preconditions, another does not, and the answerer does not support preconditions,
then use of the SIP Require header for preconditions would result in then use of the SIP Require header for preconditions would result in
a session failure, in spite of the fact that a valid and supported a session failure, in spite of the fact that a valid and supported
potential configuration was included in the offer. potential configuration was included in the offer.
In general, this can be alleviated by use of mandatory and optional In general, this can be alleviated by use of mandatory and optional
attribute capabilities in a potential configuration. There are attribute capabilities in a potential configuration. There are
however cases where permissible SDP values are tied to the use of the however cases where permissible SDP values are tied to the use of
SIP Require header. SIP preconditions [RFC3312] is one such example, the SIP Require header. SIP preconditions [RFC3312] is one such
where preconditions with a "mandatory" strength-tag can only be used example, where preconditions with a "mandatory" strength-tag can
when a SIP Require header with the SIP option tag "precondition" is only be used when a SIP Require header with the SIP option tag
included. Future SIP extensions that may want to use the SDP "precondition" is included. Future SIP extensions that may want to
Capability Negotiation framework should avoid such coupling. use the SDP Capability Negotiation framework should avoid such
coupling.
3.10. Processing Media before Answer 3.9. Processing Media before Answer
The offer/answer model requires an offerer to be able to receive The offer/answer model requires an offerer to be able to receive
media in accordance with the offer prior to receiving the answer. media in accordance with the offer prior to receiving the answer.
This property is retained with the SDP Capability Negotiation This property is retained with the SDP Capability Negotiation
extensions defined here, but only when the actual configuration is extensions defined here, but only when the actual configuration is
selected by the answerer. If a potential configuration is chosen, it selected by the answerer. If a potential configuration is chosen, it
is permissible for the offerer to not process any media received is permissible for the offerer to not process any media received
before the answer is received. This however may lead to clipping. before the answer is received. This may lead to clipping.
Consequently, the SDP Capability Negotiation framework recommends Consequently, the SDP Capability Negotiation framework recommends
sending back an answer SDP as soon as possible. sending back an answer SDP as soon as possible.
The issue can be resolved by introducing a three-way handshake. In The issue can be resolved by introducing a three-way handshake. In
the case of SIP, this can for example be done by defining a the case of SIP, this can for example be done by defining a
precondition [RFC3312] for capability negotiation (or use an existing precondition [RFC3312] for capability negotiation (or use an
precondition that is known to generate a second offer/answer exchange existing precondition that is known to generate a second
before proceeding with the session), however preconditions are often offer/answer exchange before proceeding with the session). However,
viewed as complicated to implement and they may add to overall preconditions are often viewed as complicated to implement and they
session establishment delay by requiring an extra offer/answer may add to overall session establishment delay by requiring an extra
exchange. offer/answer exchange.
An alternative three-way handshake can be performed by use of ICE An alternative three-way handshake can be performed by use of ICE
[ICE]. When ICE is being used, and the answerer receives a STUN [ICE]. When ICE is being used, and the answerer receives a STUN
Binding Request for any one of the accepted media streams from the Binding Request for any one of the accepted media streams from the
offerer, the answerer knows the offer has received his answer. At offerer, the answerer knows the offer has received his answer. At
that point, the answerer knows that the offerer will be able to that point, the answerer knows that the offerer will be able to
process incoming media according to the negotiated configuration and process incoming media according to the negotiated configuration and
hence he can start sending media without the risk of the offerer hence he can start sending media without the risk of the offerer
either discarding it or playing garbage. either discarding it or playing garbage.
In some use cases (notably, when the offerer does not need In some use cases a three-way handshake is not needed. An example is
information from the answer, such as keying material in the SDP, in when the offerer does not need information from the answer, such as
order to process incoming media), a three-way handshake is not keying material in the SDP, in order to process incoming media. The
needed. The SDP Capability Negotiation framework does not define any SDP Capability Negotiation framework does not define any such
such solutions, however extensions may do so. For example, one solutions, however extensions may do so. For example, one technique
technique proposed for best-effort SRTP in [BESRTP] is to provide proposed for best-effort SRTP in [BESRTP] is to provide different
different RTP payload type mappings for different transport protocols RTP payload type mappings for different transport protocols used,
used, outside of the actual configuration, while still allowing them outside of the actual configuration, while still allowing them to be
to be used by the answerer (exchange of keying material is still used by the answerer (exchange of keying material is still needed,
needed, e.g. inband). The basic SDP Capability Negotiation framework e.g. inband). The basic SDP Capability Negotiation framework defined
defined here does not include the ability to do so, however here does not include the ability to do so, however extensions that
extensions that enable that may be defined. enable that may be defined.
3.10. Indicating Bandwidth Usage
The amount of bandwidth to use for a particular media stream depends
on the codecs, transport protocol and other parameters being used.
For example use of Secure RTP [RFC3711] with integrity protection
requires more bandwidth than plain RTP [RFC3551]. SDP defines the
bandwidth ("b=") parameter to indicate the proposed bandwidth for
the session or media stream,.
In current SDP, each media description contains one transport
protocol and one or more codecs. When specifying the proposed
bandwidth, the worst case scenario must be taken into account, i.e.,
use of the highest bandwidth codec provided, the transport protocol
indicated, and the worst case (bandwidth-wise) parameters that can
be negotiated (e.g., a 32-bit HMAC or an 80-bit HMAC).
The core SDP capability negotiation framework does not provide a way
to negotiate bandwidth parameters. The issue thus remains, however
it is potentially worse than with current SDP, since it is easier to
negotiate additional codecs, and furthermore possible to negotiate
different transport protocols. The recommended approach for
addressing this is the same as for plain SDP; the worst case (now
including potential configurations) needs to be taken into account
when specifying the bandwidth parameters in the actual
configuration. This can make the bandwidth value less accurate than
in current SDP (due to potential greater variability in the
potential configuration bandwidth use). Extensions can be defined to
address this shortcoming. Also, the Transport Independent
Application Specific Maximum (TIAS) bandwidth type defined in
[RFC3890] can be used to alleviate bandwidth variability concerns
due to different transport protocols.
Note, that when using RTP retransmission [RFC4588] with the RTCP-
based feedback profile [RFC4585] (RTP/AVPF), the retransmitted
packets are part of the media stream bandwidth when using SSRC-
multiplexing. If a non-feedback based protocol is offered as an
alternative transport protocol, it is possible that the bandwidth
indication should have been lower.
3.11. Dealing with Large Number of Potential Configurations 3.11. Dealing with Large Number of Potential Configurations
When using the SDP Capability Negotiation, it is easy to generate When using the SDP Capability Negotiation, it is easy to generate
offers that contain a large number of potential configurations. For offers that contain a large number of potential configurations. For
example, in the offer: example, in the offer:
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
m=audio 53456 RTP/AVP 0 18 m=audio 53456 RTP/AVP 0 18
a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF
a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_80 a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4 inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
FEC_ORDER=FEC_SRTP FEC_ORDER=FEC_SRTP
a=acap:2 a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO... a=acap:2 key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
a=acap:3 a=rtcp-fb:0 nack a=acap:3 rtcp-fb:0 nack
a=pcfg:1 t=1 a=1,3|2,3 a=pcfg:1 t=1 a=1,3|2,3
a=pcfg:2 t=2 a=1|2 a=pcfg:2 t=2 a=1|2
a=pcfg:3 t=3 a=3 a=pcfg:3 t=3 a=3
we have 5 potential configurations on top of the actual configuration we have 5 potential configurations on top of the actual
for a single media stream. Adding an extension capability with just configuration for a single media stream. Adding an extension
two alternatives for each would double that number (to 10), and doing capability with just two alternatives for each would double that
the equivalent with two media streams would again double that number number (to 10), and doing the equivalent with two media streams
(to 20). While it is easy (and inexpensive) for the offerer to would again double that number (to 20). While it is easy (and
generate such offers, processing them at the answering side may not inexpensive) for the offerer to generate such offers, processing
be. Consequently, it is RECOMMENDED that offerers do not create them at the answering side may not be. Consequently, it is
offers with unnecessarily large number of potential configurations in RECOMMENDED that offerers do not create offers with unnecessarily
them. large number of potential configurations in them.
On the answering side, implementers MUST take care to avoid excessive On the answering side, implementers MUST take care to avoid
memory and CPU consumption. For example, a naļve implementation that excessive memory and CPU consumption. For example, a naļ¶„
first generates all the valid potential configuration SDPs implementation that first generates all the valid potential
internally, could find itself being memory exhausted, especially if configuration SDPs internally, could find itself being memory
it supports a large number of endpoints. Similarly, a naļve exhausted, especially if it supports a large number of endpoints.
implementation that simply performs iterative trial-and-error Similarly, a naļ¶„ implementation that simply performs iterative
processing on each possible potential configuration SDP (in the trial-and-error processing on each possible potential configuration
preference order specified) could find itself being CPU constrained. SDP (in the preference order specified) could find itself being CPU
An alternative strategy is to prune the search space first by constrained. An alternative strategy is to prune the search space
discarding the set of offered potential configurations where the first by discarding the set of offered potential configurations
transport protocol indicated (if any) is not supported, and/or one or where the transport protocol indicated (if any) is not supported,
more mandatory attribute capabilities (if any) are either not and/or one or more mandatory attribute capabilities (if any) are
supported or not valid. either not supported or not valid. Potential configurations with
unsupported mandatory extension configurations in them can be
discarded as well.
3.12. SDP Capability Negotiation and Intermediaries 3.12. SDP Capability Negotiation and Intermediaries
An intermediary is here defined as an entity between a SIP user agent An intermediary is here defined as an entity between a SIP user
A and a SIP user agent B, that need to perform some kind of agent A and a SIP user agent B, that need to perform some kind of
processing on the SDP exchanged between A and B in order for the processing on the SDP exchanged between A and B, in order for the
session establishment to operate as intended. Examples of such session establishment to operate as intended. Examples of such
intermediaries include Session Border Controllers (SBCs) that may intermediaries include Session Border Controllers (SBCs) that may
perform media relaying, Proxy Call Session Control Functions (P-CSCF) perform media relaying, Proxy Call Session Control Functions (P-
that may authorize use of a certain amount of network resources CSCF) that may authorize use of a certain amount of network
(bandwidth), etc. The presence and design of such intermediaries may resources (bandwidth), etc. The presence and design of such
not follow the "Internet" model or the SIP requirements for proxies intermediaries may not follow the "Internet" model or the SIP
(which are not supposed to look in message bodies such as SDP), requirements for proxies (which are not supposed to look in message
however they are a fact of life in some deployment scenarios bodies such as SDP), however they are a fact of life in some
currently and hence deserves consideration. deployment scenarios currently and hence deserve consideration.
If the intermediary needs to understand the characteristics of the If the intermediary needs to understand the characteristics of the
media sessions being negotiated, e.g. the amount of bandwidth used or media sessions being negotiated, e.g. the amount of bandwidth used
the transport protocol negotiated, then use of the SDP Capability or the transport protocol negotiated, then use of the SDP Capability
Negotiation framework may impact them. For example, some Negotiation framework may impact them. For example, some
intermediaries are known to (currently) disallow answers where the intermediaries are known to disallow answers where the transport
transport protocol differs from the one in the offer. Use of the SDP protocol differs from the one in the offer. Use of the SDP
Capability Negotiation framework in the presence of such Capability Negotiation framework in the presence of such
intermediaries could lead to session failures. Intermediaries that intermediaries could lead to session failures. Intermediaries that
need to authorize use of network resources based on the negotiated need to authorize use of network resources based on the negotiated
media stream parameters are affected as well. If they inspect only media stream parameters are affected as well. If they inspect only
the offer, then they may authorize parameters assuming a different the offer, then they may authorize parameters assuming a different
transport protocol, codecs, etc. than what is actually being transport protocol, codecs, etc. than what is actually being
negotiated. For these, and other, reasons it is RECOMMENDED that negotiated. For these, and other, reasons it is RECOMMENDED that
implementers of intermediaries add support for the SDP Capability implementers of intermediaries add support for the SDP Capability
Negotiation framework. Negotiation framework.
The SDP Capability Negotiation framework itself attempts to help out The SDP Capability Negotiation framework itself attempts to help out
these intermediaries as well, by optionally performing a second these intermediaries as well, by optionally performing a second
offer/answer exchange when use of a potential configuration has been offer/answer exchange when use of a potential configuration has been
negotiated (see Section 3.7.3. ). However, there are several negotiated (see Section 3.6.3. ). However, there are several
limitations with this approach. First of all, the second offer/answer limitations with this approach. First of all, the second
exchange is not required and hence may not be performed. Secondly, offer/answer exchange is not required and hence may not be
the intermediary may refuse the initial answer, e.g. due to perceived performed. Secondly, the intermediary may refuse the initial answer,
transport protocol mismatch. Thirdly, the strategy is not foolproof, e.g. due to perceived transport protocol mismatch. Thirdly, the
since the offer/answer procedures [RFC3264] leave the original strategy is not foolproof, since the offer/answer procedures
offer/answer exchange in effect when a subsequent one fails; consider [RFC3264] leave the original offer/answer exchange in effect when a
the following example: subsequent one fails; consider the following example:
1. Offerer generates an SDP offer with the actual configuration 1. Offerer generates an SDP offer with the actual configuration
specifying a low bandwidth configuration (e.g. plain RTP) and a specifying a low bandwidth configuration (e.g. plain RTP) and a
potential configuration specifying a high(er) bandwidth potential configuration specifying a high(er) bandwidth
configuration (e.g. secure RTP with integrity). configuration (e.g. secure RTP with integrity).
2. An intermediary (e.g. an SBC or P-CSCF), that does not support SDP 2. An intermediary (e.g. an SBC or P-CSCF), that does not support
Capability Negotiation, authorizes the session based on the actual SDP Capability Negotiation, authorizes the session based on the
configuration it sees in the SDP. actual configuration it sees in the SDP.
3. The answerer chooses the high(er) bandwidth potential 3. The answerer chooses the high(er) bandwidth potential
configuration and generates an answer SDP based on that. configuration and generates an answer SDP based on that.
4. The intermediary passes through the answer SDP. 4. The intermediary passes through the answer SDP.
5. The offerer sees the accepted answer, and generates an updated 5. The offerer sees the accepted answer, and generates an updated
offer that contains the selected potential configuration as the offer that contains the selected potential configuration as the
actual configuration. In other words, the high(er) bandwidth actual configuration. In other words, the high(er) bandwidth
configuration (which has already been negotiated successfully) is configuration (which has already been negotiated successfully) is
now the actual configuration in the offer SDP. now the actual configuration in the offer SDP.
6. The intermediary sees the new offer, however it does not authorize 6. The intermediary sees the new offer, however it does not
the use of the high(er) bandwidth configuration, and consequently authorize the use of the high(er) bandwidth configuration, and
generates a rejection message to the offerer. consequently generates a rejection message to the offerer.
7. The offerer receives the rejected offer. 7. The offerer receives the rejected offer.
After step 7, per RFC 3264, the offer/answer exchange that completed After step 7, per RFC 3264, the offer/answer exchange that completed
in step 5 remains in effect, however the intermediary may not have in step 5 remains in effect, however the intermediary may not have
authorized the necessary network resources and hence the media stream authorized the necessary network resources and hence the media
may experience quality issues. The solution to this problem is to stream may experience quality issues. The solution to this problem
upgrade the intermediary to support the SDP Capability Negotiation is to upgrade the intermediary to support the SDP Capability
framework. Negotiation framework.
3.13. Considerations for Specific Attribute Capabilities 3.13. Considerations for Specific Attribute Capabilities
3.13.1. The rtpmap and fmtp Attributes 3.13.1. The rtpmap and fmtp Attributes
The core SDP Capability Negotiation framework defines transport The core SDP Capability Negotiation framework defines transport
capabilities and attribute capabilities. Media capabilities, which capabilities and attribute capabilities. Media capabilities, which
can be used to describe media formats and their associated can be used to describe media formats and their associated
parameters, are not defined in this document, however the "rtpmap" parameters, are not defined in this document, however the "rtpmap"
and "fmtp" attributes can nevertheless be used as attribute and "fmtp" attributes can nevertheless be used as attribute
skipping to change at page 47, line 19 skipping to change at page 49, line 14
The rtpmap parameter binds an RTP payload type to a media format The rtpmap parameter binds an RTP payload type to a media format
(e.g. codec). While it is possible to provide rtpmaps for payload (e.g. codec). While it is possible to provide rtpmaps for payload
types not found in the corresponding "m=" line, such rtpmaps provide types not found in the corresponding "m=" line, such rtpmaps provide
no value in normal offer/answer exchanges, since only the payload no value in normal offer/answer exchanges, since only the payload
types found in the "m=" line are part of the offer (or answer). This types found in the "m=" line are part of the offer (or answer). This
applies to the core SDP Capability Negotiation framework as well: applies to the core SDP Capability Negotiation framework as well:
Only the media formats (e.g. RTP payload types) provided in the "m=" Only the media formats (e.g. RTP payload types) provided in the "m="
line are actually offered; inclusion of rtpmap attributes with other line are actually offered; inclusion of rtpmap attributes with other
RTP payload types in a potential configuration does not change this RTP payload types in a potential configuration does not change this
fact and hence they do not provide any useful information there. They fact and hence they do not provide any useful information there.
may still be useful as pure capabilities though (outside a potential They may still be useful as pure capabilities though (outside a
configuration) in order to inform a peer of additional codec potential configuration) in order to inform a peer of additional
supported. codecs supported.
It is possible to provide an rtpmap attribute capability with a It is possible to provide an rtpmap attribute capability with a
payload type mapping to a different codec than a corresponding actual payload type mapping to a different codec than a corresponding
configuration "rtpmap" attribute for the media description has. Such actual configuration "rtpmap" attribute for the media description
practice is permissible as a way of indicating a capability. If that has. Such practice is permissible as a way of indicating a
capability is included in a potential configuration, then delete- capability. If that capability is included in a potential
attributes (see Section 3.6.1. ) MUST be used to ensure that there is configuration, then delete-attributes (see Section 3.5.1. ) MUST be
not multiple rtpmap attributes for the same payload type in a given used to ensure that there is not multiple rtpmap attributes for the
media description (which would not be allowed by SDP [RFC4566]). same payload type in a given media description (which would not be
allowed by SDP [RFC4566]).
Similar considerations and rules apply to the "fmtp" attribute. An Similar considerations and rules apply to the "fmtp" attribute. An
fmtp attribute capability for a media format not included in the "m=" fmtp attribute capability for a media format not included in the
line is useless in a potential configuration (but may be useful as a "m=" line is useless in a potential configuration (but may be useful
capability by itself). An fmtp attribute capability in a potential as a capability by itself). An fmtp attribute capability in a
configuration for a media format that already has an fmtp attribute potential configuration for a media format that already has an fmtp
in the actual configuration may lead to multiple fmtp format attribute in the actual configuration may lead to multiple fmtp
parameters for that media format and that is not allowed by SDP format parameters for that media format and that is not allowed by
[RFC4566]. The delete-attributes MUST be used to ensure that there is SDP [RFC4566]. The delete-attributes MUST be used to ensure that
not multiple fmtp attributes for a given media format in a media there is not multiple fmtp attributes for a given media format in a
description. media description.
Extensions to the core SDP Capability Negotiation framework may Extensions to the core SDP Capability Negotiation framework may
change the above behavior. change the above behavior.
3.13.2. Direction Attributes 3.13.2. Direction Attributes
SDP defines the "inactive", "sendonly", "recvonly", and "sendrecv" SDP defines the "inactive", "sendonly", "recvonly", and "sendrecv"
direction attributes. The direction attributes can be applied at direction attributes. The direction attributes can be applied at
either the session-level or the media-level. In either case, it is either the session-level or the media-level. In either case, it is
possible to define attribute capabilities for these direction possible to define attribute capabilities for these direction
capabilities. Note that if used by a potential configuration, then capabilities; if used by a potential configuration, the normal
the normal offer/answer procedures still apply. For example, if an offer/answer procedures still apply. For example, if an offered
offered potential configuration includes the "sendonly" direction potential configuration includes the "sendonly" direction attribute,
attribute, and it is selected as the actual configuration, then the and it is selected as the actual configuration, then the answer MUST
answer MUST include a corresponding "recvonly" (or "inactive") include a corresponding "recvonly" (or "inactive") attribute.
attribute.
3.14. Relationship to RFC 3407
RFC 3407 defines capability descriptions with limited abilities to
describe attributes, bandwidth parameters, transport protocols and
media formats. RFC 3407 does not define any negotiation procedures
for actually using those capability descriptions.
This document defines new attributes for describing attribute
capabilities and transport capabilities. It also defines procedures
for using those capabilities as part of an offer/answer exchange. In
contrast to RFC 3407, this document does not define bandwidth
parameters, and it also does not define how to express ranges of
values. Extensions to this document may be defined in order to fully
cover all the capabilities provided by RFC 3407 (for example more
general media capabilities).
It is RECOMMENDED that implementations use the attributes and
procedures defined in this document instead of those defined in
[RFC3407]. If capability description interoperability with legacy
RFC 3407 implementations is desired, implementations MAY include
both RFC 3407 capability descriptions and capabilities defined by
this document. The offer/answer negotiation procedures defined in
this document will not use the RFC 3407 capability descriptions.
4. Examples 4. Examples
In this section, we provide examples showing how to use the SDP In this section, we provide examples showing how to use the SDP
Capability Negotiation. Capability Negotiation.
4.1. Best-Effort Secure RTP 4.1. Best-Effort Secure RTP
The following example illustrates how to use the SDP Capability The following example illustrates how to use the SDP Capability
Negotiation extensions to support so-called Best-Effort Secure RTP. Negotiation extensions to support so-called Best-Effort Secure RTP.
In that scenario, the offerer supports both RTP and Secure RTP. If In that scenario, the offerer supports both RTP and Secure RTP. If
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.
if the answerer supports Secure RTP and the SDP Capability However, 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) |
|--------------------------------->| |--------------------------------->|
| | | |
skipping to change at page 49, line 12 skipping to change at page 51, line 30
Alice's offer includes RTP and SRTP as alternatives. RTP is the Alice's offer includes RTP and SRTP as alternatives. RTP is the
default, but SRTP is the preferred one: default, but SRTP is the preferred one:
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
m=audio 53456 RTP/AVP 0 18 m=audio 53456 RTP/AVP 0 18
a=tcap:1 RTP/SAVP RTP/AVP a=tcap:1 RTP/SAVP RTP/AVP
a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_80 a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4 inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
FEC_ORDER=FEC_SRTP FEC_ORDER=FEC_SRTP
a=pcfg:1 t=1 a=1 a=pcfg:1 t=1 a=1
The "m=" line indicates that Alice is offering to use plain RTP with The "m=" line indicates that Alice is offering to use plain RTP with
PCMU or G.729. The capabilities are provided by the "a=tcap" and PCMU or G.729. The capabilities are provided by the "a=tcap" and
"a=acap" attributes. The "tcap" capability indicates that both "a=acap" attributes. The "tcap" capability indicates that both
Secure RTP and normal RTP are supported. The "acap" attribute Secure RTP and normal RTP are supported. The "acap" attribute
provides an attribute capability with a handle of 1. The capability provides an attribute capability with a handle of 1. The capability
is a "crypto" attribute, which provides the keying material for SRTP is a "crypto" attribute, which provides the keying material for SRTP
using SDP security descriptions [SDES]. The "a=pcfg" attribute using SDP security descriptions [RFC4568]. The "a=pcfg" attribute
provides the potential configurations included in the offer by provides the potential configurations included in the offer by
reference to the capabilities. A single potential configuration with reference to the capabilities. A single potential configuration
a configuration number of "1" is provided. It includes the transport with a configuration number of "1" is provided. It includes the
protocol capability 1 (RTP/SAVP, i.e. secure RTP) together with the transport protocol capability 1 (RTP/SAVP, i.e. secure RTP) together
attribute capability 1, i.e. the crypto attribute provided. Note with the attribute capability 1, i.e. the crypto attribute provided.
that attribute capability 1 is mandatory, and hence it must be Note that attribute capability 1 is mandatory, and hence it must be
supported in order for the potential configuration to be used. supported in order for the potential configuration to be used.
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 potential Capability Negotiation framework, and hence he accepts the potential
configuration for Secure RTP provided by Alice: configuration for Secure RTP provided by Alice:
v=0 v=0
o=- 24351 621814 IN IP4 192.0.2.2 o=- 24351 621814 IN IP4 192.0.2.2
s= s=
c=IN IP4 192.0.2.2 c=IN IP4 192.0.2.2
skipping to change at page 50, line 12 skipping to change at page 52, line 33
keying material provided). Bob also includes his keying material in keying material provided). Bob also includes his keying material in
a crypto attribute. a crypto attribute.
When Alice receives Bob's answer, session negotiation has completed, When Alice receives Bob's answer, session negotiation has completed,
however Alice nevertheless chooses to generate a new offer using the however Alice nevertheless chooses to generate a new offer using the
actual configuration. This is done purely to assist any actual configuration. This is done purely to assist any
intermediaries that may reside between Alice and Bob but do not intermediaries that may reside between Alice and Bob but do not
support the SDP Capability Negotiation framework (and hence may not support the SDP Capability Negotiation framework (and hence may not
understand the negotiation that just took place): understand the negotiation that just took place):
Alice's updated offer includes only SRTP, and it is not using the SDP Alice's updated offer includes only SRTP, and it is not using the
Capability Negotiation framework (Alice could have included the SDP Capability Negotiation framework (Alice could have included the
capabilities as well is she wanted to): capabilities as well is she wanted to):
v=0 v=0
o=- 25678 753850 IN IP4 192.0.2.1 o=- 25678 753850 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
m=audio 53456 RTP/SAVP 0 18 m=audio 53456 RTP/SAVP 0 18
a=crypto:1 AES_CM_128_HMAC_SHA1_80 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4 inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
skipping to change at page 51, line 16 skipping to change at page 53, line 39
o=- 24351 621814 IN IP4 192.0.2.2 o=- 24351 621814 IN IP4 192.0.2.2
s= s=
c=IN IP4 192.0.2.2 c=IN IP4 192.0.2.2
t=0 0 t=0 0
m=audio 54568 RTP/AVP 0 18 m=audio 54568 RTP/AVP 0 18
4.2. Multiple Transport Protocols 4.2. Multiple Transport Protocols
The following example illustrates how to use the SDP Capability The following example illustrates how to use the SDP Capability
Negotiation extensions to negotiate use of one out of several Negotiation extensions to negotiate use of one out of several
possible transport protocols. As in the previous example, the offerer possible transport protocols. As in the previous example, the
uses the expected least-common-denominator (plain RTP) as the actual offerer uses the expected least-common-denominator (plain RTP) as
configuration, and the alternative transport protocols as the the actual configuration, and the alternative transport protocols as
potential configurations. the potential configurations.
The example is illustrated by the offer/answer exchange below, where The example is illustrated by the offer/answer exchange below, where
Alice sends an offer to Bob: Alice sends an offer to Bob:
Alice Bob Alice Bob
| (1) Offer (RTP/[S]AVP[F]) | | (1) Offer (RTP/[S]AVP[F]) |
|--------------------------------->| |--------------------------------->|
| | | |
| (2) Answer (RTP/AVPF) | | (2) Answer (RTP/AVPF) |
|<---------------------------------| |<---------------------------------|
| | | |
| (3) Offer (RTP/AVPF) | | (3) Offer (RTP/AVPF) |
|--------------------------------->| |--------------------------------->|
| | | |
| (4) Answer (RTP/AVPF) | | (4) Answer (RTP/AVPF) |
|<---------------------------------| |<---------------------------------|
| | | |
Alice's offer includes plain RTP (RTP/AVP), RTP with RTCP-based Alice's offer includes plain RTP (RTP/AVP), RTP with RTCP-based
feedback (RTP/AVPF), Secure RTP (RTP/SAVP), and Secure RTP with RTCP- feedback (RTP/AVPF), Secure RTP (RTP/SAVP), and Secure RTP with
based feedback (RTP/SAVPF) and SRTP as alternatives. RTP is the RTCP-based feedback (RTP/SAVPF) and SRTP as alternatives. RTP is the
default, with RTP/SAVPF, RTP/SAVP, and RTP/AVPF as the alternatives default, with RTP/SAVPF, RTP/SAVP, and RTP/AVPF as the alternatives
and preferred in the order listed: and preferred in the order listed:
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
m=audio 53456 RTP/AVP 0 18 m=audio 53456 RTP/AVP 0 18
a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF
a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_80 a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4 inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
FEC_ORDER=FEC_SRTP FEC_ORDER=FEC_SRTP
a=acap:2 a=rtcp-fb:0 nack a=acap:2 rtcp-fb:0 nack
a=pcfg:1 t=1 a=1,[2] a=pcfg:1 t=1 a=1,[2]
a=pcfg:2 t=2 a=1 a=pcfg:2 t=2 a=1
a=pcfg:3 t=3 a=[2] a=pcfg:3 t=3 a=[2]
The "m=" line indicates that Alice is offering to use plain RTP with The "m=" line indicates that Alice is offering to use plain RTP with
PCMU or G.729. The capabilities are provided by the "a=tcap" and PCMU or G.729. The capabilities are provided by the "a=tcap" and
"a=acap" attributes. The "tcap" capability indicates that Secure RTP "a=acap" attributes. The "tcap" capability indicates that Secure
with RTCP-Based feedback (RTP/SAVPF), Secure RTP (RTP/SAVP), and RTP RTP with RTCP-Based feedback (RTP/SAVPF), Secure RTP (RTP/SAVP), and
with RTCP-Based feedback are supported. The first "acap" attribute RTP with RTCP-Based feedback are supported. The first "acap"
provides an attribute capability with a handle of 1. The capability attribute provides an attribute capability with a handle of 1. The
is a "crypto" attribute, which provides the keying material for SRTP capability is a "crypto" attribute, which provides the keying
using SDP security descriptions [SDES]. The second "acap" attribute material for SRTP using SDP security descriptions [RFC4568]. The
provides an attribute capability with a handle of 2. The capability second "acap" attribute provides an attribute capability with a
is an "rtcp-fb" attribute, which is used by the RTCP-based feedback handle of 2. The capability is an "rtcp-fb" attribute, which is used
profiles to indicate that payload type 0 (PCMU) supports feedback by the RTCP-based feedback profiles to indicate that payload type 0
type "nack". The "a=pcfg" attributes provide the potential (PCMU) supports feedback type "nack". The "a=pcfg" attributes
configurations included in the offer by reference to the provide the potential configurations included in the offer by
capabilities. There are three potential configurations: reference to the capabilities. There are three potential
configurations:
o Potential configuration 1, which is the most preferred potential o Potential configuration 1, which is the most preferred potential
configuration specifies use of transport protocol capability 1 configuration specifies use of transport protocol capability 1
(RTP/SAVPF) and attribute capabilities 1 (the "crypto" attribute) (RTP/SAVPF) and attribute capabilities 1 (the "crypto" attribute)
and 2 (the "rtcp-fb" attribute). Support for the first one is and 2 (the "rtcp-fb" attribute). Support for the first one is
mandatory whereas support for the second one is optional. mandatory whereas support for the second one is optional.
o Potential configuration 2, which is the second most preferred o Potential configuration 2, which is the second most preferred
potential configuration specifies use of transport protocol potential configuration specifies use of transport protocol
capability 2 (RTP/SAVP) and mandatory attribute capability 1 (the capability 2 (RTP/SAVP) and mandatory attribute capability 1 (the
"crypto" attribute). "crypto" attribute).
o Potential configuration 3, which is the least preferred potential o Potential configuration 3, which is the least preferred potential
configuration (but the second least preferred configuration configuration (but the second least preferred configuration
overall, since the actual configuration provided by the "m=" line overall, since the actual configuration provided by the "m=" line
is always the least preferred configuration), specifies use of is always the least preferred configuration), specifies use of
transport protocol capability 3 (RTP/AVPF) and optional attribute transport protocol capability 3 (RTP/AVPF) and optional attribute
capability 2 (the "rtcp-fb" attribute). capability 2 (the "rtcp-fb" attribute).
Bob receives the SDP offer from Alice. Bob does not support any Bob receives the SDP offer from Alice. Bob does not support any
secure RTP profiles, however he supports plain RTP and RTP with RTCP- secure RTP profiles, however he supports plain RTP and RTP with
based feedback, as well as the SDP Capability Negotiation extensions, RTCP-based feedback, as well as the SDP Capability Negotiation
and hence he accepts the potential configuration for RTP with RTCP- extensions, and hence he accepts the potential configuration for RTP
based feedback provided by Alice: with RTCP-based feedback provided by Alice:
v=0 v=0
o=- 24351 621814 IN IP4 192.0.2.2 o=- 24351 621814 IN IP4 192.0.2.2
s= s=
c=IN IP4 192.0.2.2 c=IN IP4 192.0.2.2
t=0 0 t=0 0
m=audio 54568 RTP/AVPF 0 18 m=audio 54568 RTP/AVPF 0 18
a=rtcp-fb:0 nack a=rtcp-fb:0 nack
a=acfg:1 t=3 a=[2] a=acfg:1 t=3 a=[2]
skipping to change at page 53, line 28 skipping to change at page 56, line 12
using the "rtcp-fb" value provided). Bob also includes an "rtcp-fb" using the "rtcp-fb" value provided). Bob also includes an "rtcp-fb"
attribute with the value "nack" value for RTP payload type 0. attribute with the value "nack" value for RTP payload type 0.
When Alice receives Bob's answer, session negotiation has completed, When Alice receives Bob's answer, session negotiation has completed,
however Alice nevertheless chooses to generate a new offer using the however Alice nevertheless chooses to generate a new offer using the
actual configuration. This is done purely to assist any actual configuration. This is done purely to assist any
intermediaries that may reside between Alice and Bob but do not intermediaries that may reside between Alice and Bob but do not
support the SDP Capability Negotiation framework (and hence may not support the SDP Capability Negotiation framework (and hence may not
understand the negotiation that just took place): understand the negotiation that just took place):
Alice's updated offer includes only RTP/AVPF, and it is not using the Alice's updated offer includes only RTP/AVPF, and it is not using
SDP Capability Negotiation framework (Alice could have included the the SDP Capability Negotiation framework (Alice could have included
capabilities as well if she wanted to): the capabilities as well if she wanted to):
v=0 v=0
o=- 25678 753850 IN IP4 192.0.2.1 o=- 25678 753850 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
m=audio 53456 RTP/AVPF 0 18 m=audio 53456 RTP/AVPF 0 18
a=rtcp-fb:0 nack a=rtcp-fb:0 nack
The "m=" line now indicates that Alice is offering to use RTP with The "m=" line now indicates that Alice is offering to use RTP with
RTCP-based feedback and using PCMU or G.729. The "rtcp-fb" attribute RTCP-based feedback and using PCMU or G.729. The "rtcp-fb"
provides the feedback type "nack" for payload type 0 again (but as attribute provides the feedback type "nack" for payload type 0 again
part of the actual configuration). (but as part of the actual configuration).
Bob receives the SDP offer from Alice, which he accepts, and then Bob receives the SDP offer from Alice, which he accepts, and then
generates an answer to Alice: generates an answer to Alice:
v=0 v=0
o=- 24351 621815 IN IP4 192.0.2.2 o=- 24351 621815 IN IP4 192.0.2.2
s= s=
c=IN IP4 192.0.2.2 c=IN IP4 192.0.2.2
t=0 0 t=0 0
m=audio 54568 RTP/AVPF 0 18 m=audio 54568 RTP/AVPF 0 18
skipping to change at page 54, line 30 skipping to change at page 57, line 16
o=- 24351 621814 IN IP4 192.0.2.2 o=- 24351 621814 IN IP4 192.0.2.2
s= s=
c=IN IP4 192.0.2.2 c=IN IP4 192.0.2.2
t=0 0 t=0 0
m=audio 54568 RTP/AVP 0 18 m=audio 54568 RTP/AVP 0 18
4.3. Best-Effort SRTP with Session-Level MIKEY and Media Level Security 4.3. Best-Effort SRTP with Session-Level MIKEY and Media Level Security
Descriptions Descriptions
The following example illustrates how to use the SDP Capability The following example illustrates how to use the SDP Capability
Negotiation extensions to support so-called Best-Effort Secure RTP as Negotiation extensions to support so-called Best-Effort Secure RTP
well as alternative keying mechanisms, more specifically MIKEY and as well as alternative keying mechanisms, more specifically MIKEY
SDP Security Descriptions. The offerer (Alice) wants to establish an [RFC3830] and SDP Security Descriptions. The offerer (Alice) wants
audio and video session. Alice prefers to use session-level MIKEY as to establish an audio and video session. Alice prefers to use
the key management protocol, but supports SDP security descriptions session-level MIKEY as the key management protocol, but supports SDP
as well. security descriptions as well.
The example is illustrated by the offer/answer exchange below, where The example is illustrated by the offer/answer exchange below, where
Alice sends an offer to Bob: Alice sends an offer to Bob:
Alice Bob Alice Bob
| (1) Offer (RTP/[S]AVP[F], SDES|MIKEY) | | (1) Offer (RTP/[S]AVP[F], SDES|MIKEY) |
|--------------------------------------->| |--------------------------------------->|
| | | |
| (2) Answer (RTP/SAVP, SDES) | | (2) Answer (RTP/SAVP, SDES) |
skipping to change at page 55, line 30 skipping to change at page 58, line 5
Alice's offer includes an audio and a video stream. The audio stream Alice's offer includes an audio and a video stream. The audio stream
offers use of plain RTP and secure RTP as alternatives, whereas the offers use of plain RTP and secure RTP as alternatives, whereas the
video stream offers use of plain RTP, RTP with RTCP-based feedback, video stream offers use of plain RTP, RTP with RTCP-based feedback,
Secure RTP, and Secure RTP with RTCP-based feedback as alternatives: Secure RTP, and Secure RTP with RTCP-based feedback as alternatives:
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
t=0 0 t=0 0
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
a=acap:1 a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO... a=acap:1 key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF
m=audio 59000 RTP/AVP 98 m=audio 59000 RTP/AVP 98
a=rtpmap:98 AMR/8000 a=rtpmap:98 AMR/8000
a=acap:2 a=crypto:1 AES_CM_128_HMAC_SHA1_32 a=acap:2 crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=pcfg:1 t=2 a=1|2 a=pcfg:1 t=2 a=1|2
m=video 52000 RTP/AVP 31 m=video 52000 RTP/AVP 31
a=rtpmap:31 H261/90000 a=rtpmap:31 H261/90000
a=acap:3 a=crypto:1 AES_CM_128_HMAC_SHA1_80 a=acap:3 crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32 inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=acap:4 a=rtcp-fb:* nack a=acap:4 rtcp-fb:* nack
a=pcfg:1 t=1 a=1,4|3,4 a=pcfg:1 t=1 a=1,4|3,4
a=pcfg:2 t=2 a=1|3 a=pcfg:2 t=2 a=1|3
a=pcfg:3 t=3 a=4 a=pcfg:3 t=3 a=4
The potential configuration for the audio stream specifies use of The potential configuration for the audio stream specifies use of
transport capability 2 (RTP/SAVP) and either attribute capability 1 transport capability 2 (RTP/SAVP) and either attribute capability 1
(session-level MIKEY as the keying mechanism) or 2 (SDP Security (session-level MIKEY as the keying mechanism) or 2 (SDP Security
Descriptions as the keying mechanism). Support for either of these Descriptions as the keying mechanism). Support for either of these
attribute capabilities is mandatory. There are three potential attribute capabilities is mandatory. There are three potential
configurations for the video stream. configurations for the video stream.
o The first configuration with configuration number 1 uses transport o The first configuration with configuration number 1 uses
capability 1 (RTP/SAVPF) with either attribute capabilities 1 and transport capability 1 (RTP/SAVPF) with either attribute
4 (session-level MIKEY and the "rtcp-fb" attribute) or attribute capabilities 1 and 4 (session-level MIKEY and the "rtcp-fb"
capabilities 3 and 4 (SDP security descriptions and the "rtcp-fb" attribute) or attribute capabilities 3 and 4 (SDP security
attribute). In this example, the offerer insists on not only the descriptions and the "rtcp-fb" attribute). In this example, the
keying mechanism being supported, but also that the "rtcp-fb" offerer insists on not only the keying mechanism being supported,
attribute is supported with the value indicated. Consequently, all but also that the "rtcp-fb" attribute is supported with the value
the attribute capabilities are marked as mandatory in this indicated. Consequently, all the attribute capabilities are
potential configuration. marked as mandatory in this potential configuration.
o The second configuration with configuration number 2 uses o The second configuration with configuration number 2 uses
transport capability 2 (RTP/SAVP) and either attribute capability transport capability 2 (RTP/SAVP) and either attribute capability
1 (session-level MIKEY) or attribute capability 3 (SDP security 1 (session-level MIKEY) or attribute capability 3 (SDP security
descriptions). Both attribute capabilities are mandatory in this descriptions). Both attribute capabilities are mandatory in this
configuration. configuration.
o The third configuration with configuration number 3 uses transport o The third configuration with configuration number 3 uses
capability 3 (RTP/AVPF) and mandatory attribute capability 4 (the transport capability 3 (RTP/AVPF) and mandatory attribute
"rtcp-fb" attribute). capability 4 (the "rtcp-fb" attribute).
Bob receives the SDP offer from Alice. Bob supports Secure RTP, Bob receives the SDP offer from Alice. Bob supports Secure RTP,
Secure RTP with RTCP-based feedback and the SDP Capability Secure RTP with RTCP-based feedback and the SDP Capability
Negotiation extensions. Bob also supports SDP Security Descriptions, Negotiation extensions. Bob also supports SDP Security Descriptions,
but not MIKEY, and hence he generates the following answer: but not MIKEY, and hence he generates the following answer:
v=0 v=0
o=- 24351 621814 IN IP4 192.0.2.2 o=- 24351 621814 IN IP4 192.0.2.2
s= s=
t=0 0 t=0 0
skipping to change at page 57, line 7 skipping to change at page 59, line 29
a=crypto:1 AES_CM_128_HMAC_SHA1_80 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:AwWpVLFJhQX1cfHJSojd0RmdmcmVCspeEc3QGZiN|2^20|1:32 inline:AwWpVLFJhQX1cfHJSojd0RmdmcmVCspeEc3QGZiN|2^20|1:32
a=rtcp-fb:* nack a=rtcp-fb:* nack
a=acfg:1 t=1 a=3,4 a=acfg:1 t=1 a=3,4
For the audio stream, Bob accepted the use of secure RTP, and hence For the audio stream, Bob accepted the use of secure RTP, and hence
the profile in the "m=" line is "RTP/SAVP". Bob also includes a the profile in the "m=" line is "RTP/SAVP". Bob also includes a
"crypto" attribute with his own keying material, and an "acfg" "crypto" attribute with his own keying material, and an "acfg"
attribute identifying actual configuration 1 for the audio media attribute identifying actual configuration 1 for the audio media
stream from the offer, using transport capability 2 (RTP/SAVP) and stream from the offer, using transport capability 2 (RTP/SAVP) and
attribute capability 2 (the crypto attribute from the offer). For the attribute capability 2 (the crypto attribute from the offer). For
video stream, Bob accepted the use of secure RTP with RTCP-based the video stream, Bob accepted the use of secure RTP with RTCP-based
feedback, and hence the profile in the "m=" line is "RTP/SAVPF". Bob feedback, and hence the profile in the "m=" line is "RTP/SAVPF". Bob
also includes a "crypto" attribute with his own keying material, and also includes a "crypto" attribute with his own keying material, and
an "acfg" attribute identifying actual configuration 1 for the video an "acfg" attribute identifying actual configuration 1 for the video
stream from the offer, using transport capability 1 (RTP/SAVPF) and stream from the offer, using transport capability 1 (RTP/SAVPF) and
attribute capabilities 3 (the crypto attribute from the offer) and 4 attribute capabilities 3 (the crypto attribute from the offer) and 4
(the "rtcp-fb" attribute from the offer). (the "rtcp-fb" attribute from the offer).
When Alice receives Bob's answer, session negotiation has completed, When Alice receives Bob's answer, session negotiation has completed,
however Alice nevertheless chooses to generate a new offer using the however Alice nevertheless chooses to generate a new offer using the
actual configuration. This is done purely to assist any actual configuration. This is done purely to assist any
skipping to change at page 57, line 47 skipping to change at page 60, line 24
m=video 52000 RTP/SAVPF 31 m=video 52000 RTP/SAVPF 31
a=rtpmap:31 H261/90000 a=rtpmap:31 H261/90000
a=crypto:1 AES_CM_128_HMAC_SHA1_80 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32 inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=rtcp-fb:* nack a=rtcp-fb:* nack
The "m=" line for the audio stream now indicates that Alice is The "m=" line for the audio stream now indicates that Alice is
offering to use secure RTP with PCMU or G.729, whereas the "m=" line offering to use secure RTP with PCMU or G.729, whereas the "m=" line
for the video stream indicates that Alice is offering to use secure for the video stream indicates that Alice is offering to use secure
RTP with RTCP-based feedback and H.261. Each media stream includes a RTP with RTCP-based feedback and H.261. Each media stream includes a
"crypto" attribute, which provides the SRTP keying material, with the "crypto" attribute, which provides the SRTP keying material, with
same value again. the same value again.
Bob receives the SDP offer from Alice, which he accepts, and then Bob receives the SDP offer from Alice, which he accepts, and then
generates an answer to Alice: generates an answer to Alice:
v=0 v=0
o=- 24351 621814 IN IP4 192.0.2.2 o=- 24351 621814 IN IP4 192.0.2.2
s= s=
t=0 0 t=0 0
c=IN IP4 192.0.2.2 c=IN IP4 192.0.2.2
m=audio 54568 RTP/SAVP 98 m=audio 54568 RTP/SAVP 98
skipping to change at page 59, line 15 skipping to change at page 61, line 37
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
a=key-mgmt:mikey AQEFgM0XflABAAAAAAAAAAAAAAYAyO... a=key-mgmt:mikey AQEFgM0XflABAAAAAAAAAAAAAAYAyO...
m=audio 59000 RTP/AVP 98 m=audio 59000 RTP/AVP 98
a=rtpmap:98 AMR/8000 a=rtpmap:98 AMR/8000
a=acfg:1 t=2 a=1 a=acfg:1 t=2 a=1
m=video 52000 RTP/SAVPF 31 m=video 52000 RTP/SAVPF 31
a=rtpmap:31 H261/90000 a=rtpmap:31 H261/90000
a=rtcp-fb:* nack a=rtcp-fb:* nack
a=acfg:1 t=1 a=1,4 a=acfg:1 t=1 a=1,4
It should be noted, that although Bob could have chosen session-level It should be noted, that although Bob could have chosen session-
MIKEY for one media stream, and SDP Security Descriptions for another level MIKEY for one media stream, and SDP Security Descriptions for
media stream, there are no well-defined offerer processing rules of another media stream, there are no well-defined offerer processing
the resulting answer for this, and hence the offerer may incorrectly rules of the resulting answer for this, and hence the offerer may
assume use of MIKEY for both streams. To avoid this, if the answerer incorrectly assume use of MIKEY for both streams. To avoid this, if
chooses session-level MIKEY, then all secure RTP based media streams the answerer chooses session-level MIKEY, then all secure RTP based
SHOULD use MIKEY (this applies irrespective of whether SDP Capability media streams SHOULD use MIKEY (this applies irrespective of whether
Negotiation is being used or not). Use of media-level MIKEY does not SDP Capability Negotiation is being used or not). Use of media-level
have a similar constraint. MIKEY does not have a similar constraint.
4.4. SRTP with Session-Level MIKEY and Media Level Security Descriptions 4.4. SRTP with Session-Level MIKEY and Media Level Security
as Alternatives Descriptions as Alternatives
The following example illustrates how to use the SDP Capability The following example illustrates how to use the SDP Capability
Negotiation framework to negotiate use of either MIKEY or SDP Negotiation framework to negotiate use of either MIKEY or SDP
Security Descriptions, when one of them is included as part of the Security Descriptions, when one of them is included as part of the
actual configuration, and the other one is being selected. The actual configuration, and the other one is being selected. The
offerer (Alice) wants to establish an audio and video session. Alice offerer (Alice) wants to establish an audio and video session. Alice
prefers to use session-level MIKEY as the key management protocol, prefers to use session-level MIKEY as the key management protocol,
but supports SDP security descriptions as well. but supports SDP security descriptions as well.
The example is illustrated by the offer/answer exchange below, where The example is illustrated by the offer/answer exchange below, where
skipping to change at page 60, line 13 skipping to change at page 62, line 39
and the video stream offer use of secure RTP: and the video stream offer use of secure RTP:
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
t=0 0 t=0 0
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO... a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
m=audio 59000 RTP/SAVP 98 m=audio 59000 RTP/SAVP 98
a=rtpmap:98 AMR/8000 a=rtpmap:98 AMR/8000
a=acap:1 a=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=pcfg:1 a=-s:1 a=pcfg:1 a=-s:1
m=video 52000 RTP/SAVP 31 m=video 52000 RTP/SAVP 31
a=rtpmap:31 H261/90000 a=rtpmap:31 H261/90000
a=acap:2 a=crypto:1 AES_CM_128_HMAC_SHA1_80 a=acap:2 crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32 inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=pcfg:1 a=-s:2 a=pcfg:1 a=-s:2
Alice does not know whether Bob supports MIKEY or SDP Security Alice does not know whether Bob supports MIKEY or SDP Security
Descriptions. She could include attributes for both, however the Descriptions. She could include attributes for both, however the
resulting procedures and potential interactions are not well-defined. resulting procedures and potential interactions are not well-
Instead, she places a session-level key-mgmt attribute for MIKEY in defined. Instead, she places a session-level key-mgmt attribute for
the actual configuration with SDP security descriptions as an MIKEY in the actual configuration with SDP security descriptions as
alternative in the potential configuration. The potential an alternative in the potential configuration. The potential
configuration for the audio stream specifies that all session level configuration for the audio stream specifies that all session level
attributes are to be deleted (i.e. the session-level "a=key-mgmt" attributes are to be deleted (i.e. the session-level "a=key-mgmt"
attribute) and that mandatory attribute capability 2 is to be used attribute) and that mandatory attribute capability 2 is to be used
(i.e. the crypto attribute). The potential configuration for the (i.e. the crypto attribute). The potential configuration for the
video stream is similar, except it uses it's own mandatory crypto video stream is similar, except it uses it's own mandatory crypto
attribute capability (2). Note how deletion of the session-level attribute capability (2). Note how deletion of the session-level
attributes does not affect the media-level attributes. attributes does not affect the media-level attributes.
Bob receives the SDP offer from Alice. Bob supports Secure RTP and Bob receives the SDP offer from Alice. Bob supports Secure RTP and
the SDP Capability Negotiation framework. Bob also supports both SDP the SDP Capability Negotiation framework. Bob also supports both SDP
Security Descriptions and MIKEY. Since the potential configuration is Security Descriptions and MIKEY. Since the potential configuration
more preferred than the actual configuration, Bob (conceptually) is more preferred than the actual configuration, Bob (conceptually)
generates an internal potential configuration SDP that contains the generates an internal potential configuration SDP that contains the
crypto attributes for the audio and video stream, but not the key- crypto attributes for the audio and video stream, but not the key-
mgmt attribute for MIKEY, thereby avoiding any ambiguity between the mgmt attribute for MIKEY, thereby avoiding any ambiguity between the
two keying mechanisms. As a result, he generates the following two keying mechanisms. As a result, he generates the following
answer: answer:
v=0 v=0
o=- 24351 621814 IN IP4 192.0.2.2 o=- 24351 621814 IN IP4 192.0.2.2
s= s=
t=0 0 t=0 0
skipping to change at page 61, line 21 skipping to change at page 63, line 49
inline:AwWpVLFJhQX1cfHJSojd0RmdmcmVCspeEc3QGZiN|2^20|1:32 inline:AwWpVLFJhQX1cfHJSojd0RmdmcmVCspeEc3QGZiN|2^20|1:32
a=acfg:1 a=-s:2 a=acfg:1 a=-s:2
For the audio stream, Bob accepted the use of secure RTP using SDP For the audio stream, Bob accepted the use of secure RTP using SDP
security descriptions. Bob therefore includes a "crypto" attribute security descriptions. Bob therefore includes a "crypto" attribute
with his own keying material, and an "acfg" attribute identifying with his own keying material, and an "acfg" attribute identifying
actual configuration 1 for the audio media stream from the offer, actual configuration 1 for the audio media stream from the offer,
with the delete-attributes ("-s") and attribute capability 1 (the with the delete-attributes ("-s") and attribute capability 1 (the
crypto attribute from the offer). For the video stream, Bob also crypto attribute from the offer). For the video stream, Bob also
accepted the use of secure RTP using SDP security descriptions. Bob accepted the use of secure RTP using SDP security descriptions. Bob
therefore includes a "crypto" attribute with his own keying material, therefore includes a "crypto" attribute with his own keying
and an "acfg" attribute identifying actual configuration 1 for the material, and an "acfg" attribute identifying actual configuration 1
video stream from the offer, with the delete-attributes ("-s") and for the video stream from the offer, with the delete-attributes ("-
attribute capability 2. s") and attribute capability 2.
Below, we illustrate the offer SDP, when Bob instead offers the Below, we illustrate the offer SDP, when Bob instead offers the
"crypto" attribute as the actual configuration keying mechanism and "crypto" attribute as the actual configuration keying mechanism and
"key-mgmt" as the potential configuration: "key-mgmt" as the potential configuration:
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
t=0 0 t=0 0
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
a=acap:1 a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO... a=acap:1 key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
m=audio 59000 RTP/SAVP 98 m=audio 59000 RTP/SAVP 98
a=rtpmap:98 AMR/8000 a=rtpmap:98 AMR/8000
a=crypto:1 AES_CM_128_HMAC_SHA1_32 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=acap:2 a=rtpmap:98 AMR/8000 a=acap:2 rtpmap:98 AMR/8000
a=pcfg:1 a=-m:1,2 a=pcfg:1 a=-m:1,2
m=video 52000 RTP/SAVP 31 m=video 52000 RTP/SAVP 31
a=rtpmap:31 H261/90000 a=rtpmap:31 H261/90000
a=acap:3 a=crypto:1 AES_CM_128_HMAC_SHA1_80 a=acap:3 crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32 inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=acap:4 a=rtpmap:31 H261/90000 a=acap:4 rtpmap:31 H261/90000
a=pcfg:1 a=-m:1,4 a=pcfg:1 a=-m:1,4
Note how we this time need to perform delete-attributes at the media- Note how we this time need to perform delete-attributes at the
level instead of the session-level. When doing that, all attributes media-level instead of the session-level. When doing that, all
from the actual configuration SDP, including the rtpmaps provided, attributes from the actual configuration SDP, including the rtpmaps
are removed. Consequently, we had to include these rtpmaps as provided, are removed. Consequently, we had to include these rtpmaps
capabilities as well, and then include them in the potential as capabilities as well, and then include them in the potential
configuration, thereby effectively recreating the original rtpmap configuration, thereby effectively recreating the original rtpmap
attributes in the resulting potential configuration SDP. attributes in the resulting potential configuration SDP.
5. Security Considerations 5. Security Considerations
The SDP Capability Negotiation Framework is defined to be used within The SDP Capability Negotiation Framework is defined to be used
the context of the offer/answer model, and hence all the offer/answer within the context of the offer/answer model, and hence all the
security considerations apply here as well. Similarly, the Session offer/answer security considerations apply here as well. Similarly,
Initiation Protocol (SIP) uses SDP and the offer/answer model, and the Session Initiation Protocol (SIP) uses SDP and the offer/answer
hence, when used in that context, the SIP security considerations model, and hence, when used in that context, the SIP security
apply as well. considerations apply as well.
However, SDP Capability Negotiation introduces additional security However, SDP Capability Negotiation introduces additional security
issues. Its use as a mechanism to enable alternative transport issues. Its use as a mechanism to enable alternative transport
protocol negotiation (secure and non-secure) as well as its ability protocol negotiation (secure and non-secure) as well as its ability
to negotiate use of more or less secure keying methods and material to negotiate use of more or less secure keying methods and material
warrant further security considerations. Also, the (continued) warrant further security considerations. Also, the (continued)
support for receiving media before answer combined with negotiation support for receiving media before answer combined with negotiation
of alternative transport protocols (secure and non-secure) warrant of alternative transport protocols (secure and non-secure) warrant
further security considerations. We discuss these issues below. further security considerations. We discuss these issues below.
The SDP Capability Negotiation framework allows for an offered media The SDP Capability Negotiation framework allows for an offered media
stream to both indicate and support various levels of security for stream to both indicate and support various levels of security for
that media stream. Different levels of security can for example be that media stream. Different levels of security can for example be
negotiated by use of alternative attribute capabilities each negotiated by use of alternative attribute capabilities each
indicating more or less secure keying methods as well as more or less indicating more or less secure keying methods as well as more or
strong ciphers. Since the offerer indicates support for each of these less strong ciphers. Since the offerer indicates support for each of
alternatives, he will presumably accept the answerer seemingly these alternatives, he will presumably accept the answerer seemingly
selecting any of the offered alternatives. If an attacker can modify selecting any of the offered alternatives. If an attacker can modify
the SDP offer, he can thereby force the negotiation of the weakest the SDP offer, he can thereby force the negotiation of the weakest
security mechanism that the offerer is willing to accept. This may in security mechanism that the offerer is willing to accept. This may
turn enable the attacker to compromise the security of the negotiated enable the attacker to compromise the security of the negotiated
media stream. Similarly, if the offerer wishes to negotiate use of a media stream. Similarly, if the offerer wishes to negotiate use of a
secure media stream (e.g. secure RTP), but includes a non-secure secure media stream (e.g. secure RTP), but includes a non-secure
media stream (e.g. plain RTP) as a valid (but less preferred) media stream (e.g. plain RTP) as a valid (but less preferred)
alternative, then an attacker that can modify the offered SDP will be alternative, then an attacker that can modify the offered SDP will
able to force the establishment of an insecure media stream. The be able to force the establishment of an insecure media stream. The
solution to both of these problems involves the use of integrity solution to both of these problems involves the use of integrity
protection over the SDP. Ideally, this integrity protection provides protection over the SDP. Ideally, this integrity protection provides
end-to-end integrity protection in order to protect from any man-in- end-to-end integrity protection in order to protect from any man-in-
the-middle attack; secure multiparts such as S/MIME [SMIME] provide the-middle attack; secure multiparts such as S/MIME [RFC3851]
one such solution, however S/MIME requires use and availability of a provide one such solution, however S/MIME requires use and
Public Key Infrastructure (PKI). A slightly less secure alternative availability of a Public Key Infrastructure (PKI). A slightly less
when using SIP, but generally much easier to deploy in practice secure alternative when using SIP, but generally much easier to
(since it does not require a PKI), is to use SIP Identity [RFC4474]; deploy in practice (since it does not require a PKI), is to use SIP
this requires the existence of an authentication service (see Identity [RFC4474]; this requires the existence of an authentication
[RFC4474]). Yet another, and considerably less secure, alternative is service (see [RFC4474]). Yet another, and considerably less secure,
to use hop-by-hop security only, e.g. TLS or IPSec thereby ensuring alternative is to use hop-by-hop security only, e.g. TLS or IPSec
the integrity of the offered SDP on a hop-by-hop basis. Note however thereby ensuring the integrity of the offered SDP on a hop-by-hop
that SIP proxies or other intermediaries processing the SIP request basis. Note however that SIP proxies or other intermediaries
at each hop are able to perform a man-in-the-middle attack by processing the SIP request at each hop are able to perform a man-in-
modifying the offered SDP. the-middle attack by modifying the offered SDP.
Per the normal offer/answer procedures, as soon as the offerer has Per the normal offer/answer procedures, as soon as the offerer has
generated an offer, the offerer must be prepared to receive media in generated an offer, the offerer must be prepared to receive media in
accordance with that offer. The SDP Capability Negotiation preserves accordance with that offer. The SDP Capability Negotiation preserves
that behavior for the actual configuration in the offer, however the that behavior for the actual configuration in the offer, however the
offerer has no way of knowing which configuration (actual or offerer has no way of knowing which configuration (actual or
potential) configuration was actually selected by the offerer, until potential) configuration was selected by the offerer, until an
an answer indication is received. This opens up a new security issue answer indication is received. This opens up a new security issue
where an attacker may be able to interject media towards the offerer where an attacker may be able to interject media towards the offerer
until the answer is received. For example, the offerer may use plain until the answer is received. For example, the offerer may use plain
RTP as the actual configuration and secure RTP as an alternative RTP as the actual configuration and secure RTP as an alternative
potential configuration. Even though the answerer selects secure RTP, potential configuration. Even though the answerer selects secure
the offerer will not know that until he receives the answer, and RTP, the offerer will not know that until he receives the answer,
hence an attacker will be able to send media to the offerer and hence an attacker will be able to send media to the offerer
meanwhile. The easiest protection against such an attack is to not meanwhile. The easiest protection against such an attack is to not
offer use of the non-secure media stream in the actual configuration, offer use of the non-secure media stream in the actual
however that may in itself have undesirable side-effects: If the configuration, however that may in itself have undesirable side-
answerer does not support the secure media stream and also does not effects: If the answerer does not support the secure media stream
support the capability negotiation framework, then negotiation of the and also does not support the capability negotiation framework, then
media stream will fail. Alternatively, SDP security preconditions negotiation of the media stream will fail. Alternatively, SDP
[sprecon] can be used. This will ensure that media is not flowing security preconditions [RFC5027] can be used. This will ensure that
until session negotiation has completed and hence the selected media is not flowing until session negotiation has completed and
configuration is known. Use of preconditions however requires both hence the selected configuration is known. Use of preconditions
side to support them. If they don't, and use of them is required, the however requires both sides to support them. If they don't, and use
session will fail. As a (limited) work around to this, it is of them is required, the session will fail. As a (limited) work
RECOMMENDED that SIP entities generate an answer SDP and send it to around to this, it is RECOMMENDED that SIP entities generate an
the offerer as soon as possible, for example in a 183 Session answer SDP and send it to the offerer as soon as possible, for
Progress message. This will limit the time during which an attacker example in a 183 Session Progress message. This will limit the time
can send media to the offerer. Section 3.10. presents other during which an attacker can send media to the offerer. Section 3.9.
alternatives as well. presents other alternatives as well.
Additional security considerations apply to the answer SDP as well. Additional security considerations apply to the answer SDP as well.
The actual configuration attribute tells the offerer which potential The actual configuration attribute tells the offerer which potential
configuration the answer was actually based on, and hence an attacker configuration the answer was based on, and hence an attacker that
that can either modify or remove the actual configuration attribute can either modify or remove the actual configuration attribute in
in the answer can cause session failure as well as extend the time the answer can cause session failure as well as extend the time
window during which the offerer will accept incoming media that does window during which the offerer will accept incoming media that does
not conform to the actual answer. The solutions to this SDP answer not conform to the actual answer. The solutions to this SDP answer
integrity problem are the same as for the offer, i.e. use of end-to- integrity problem are the same as for the offer, i.e. use of end-to-
end integrity protection, SIP identity, or hop-by-hop protection. The end integrity protection, SIP identity, or hop-by-hop protection.
mechanism to use depends on the mechanisms supported by the offerer The mechanism to use depends on the mechanisms supported by the
as well as the acceptable security trade-offs. offerer as well as the acceptable security trade-offs.
As described in Section 3.1. , SDP Capability Negotiation As described in Section 3.1. , SDP Capability Negotiation
conceptually allows an offerer to include many different offers in a conceptually allows an offerer to include many different offers in a
single SDP. This can in turn cause the answerer to process a large single SDP. This can cause the answerer to process a large number of
number of alternative potential offers, which can consume significant alternative potential offers, which can consume significant memory
memory and CPU resources. An attacker can use this amplification and CPU resources. An attacker can use this amplification feature to
feature to launch a denial of service attack against the answerer. launch a denial of service attack against the answerer. The answerer
The answerer MUST protect itself from such attacks. As explained in MUST protect itself from such attacks. As explained in Section 3.10.
Section 3.11. , the answerer can help reduce the effects of such an , the answerer can help reduce the effects of such an attack by
attack by first discarding all potential configurations that contain first discarding all potential configurations that contain
unsupported transport protocols and/or unsupported or invalid unsupported transport protocols, unsupported or invalid mandatory
mandatory attribute capabilities. The answerer SHOULD also look out attribute capabilities, or unsupported mandatory extension
for potential configurations that are designed to pass the above configurations. The answerer SHOULD also look out for potential
test, but nevertheless produce a large number of potential configurations that are designed to pass the above test, but
configuration SDPs that cannot be supported. nevertheless produce a large number of potential configuration SDPs
that cannot be supported.
A possible way of achieving that is for an attacker to find a A possible way of achieving that is for an attacker to find a
valid session-level attribute that causes conflicts or otherwise valid session-level attribute that causes conflicts or otherwise
interferes with individual media description configurations. interferes with individual media description configurations.
Currently, we do not know of such an SDP attribute, however this Currently, we do not know of such an SDP attribute, however this
does not mean it doesn't exist, or that it will not exist in the does not mean it does not exist, or that it will not exist in the
future. If such attributes are found to exist, implementers should future. If such attributes are found to exist, implementers should
explicitly protect against them. explicitly protect against them.
A significant number of valid and supported potential configurations A significant number of valid and supported potential configurations
may remain. However, since all of those contain only valid and may remain. However, since all of those contain only valid and
supported transport protocols and attributes, it is expected that supported transport protocols and attributes, it is expected that
only a few of them will need to be processed on average. Still, the only a few of them will need to be processed on average. Still, the
answerer MUST ensure that it does not needlessly consume large answerer MUST ensure that it does not needlessly consume large
amounts of memory and CPU resources when processing those as well as amounts of memory or CPU resources when processing those as well as
be prepared to handle the case where a large number of potential be prepared to handle the case where a large number of potential
configurations still need to be processed. configurations still need to be processed.
6. IANA Considerations 6. IANA Considerations
6.1. New SDP Attributes 6.1. New SDP Attributes
The IANA is hereby requested to register the following new SDP The IANA is hereby requested to register the following new SDP
attributes as follows: attributes as follows:
Attribute name: csup Attribute name: csup
Long form name: Supported capability negotiation extensions Long form name: Supported capability negotiation extensions
Type of attribute: Session-level and media-level Type of attribute: Session-level and media-level
Subject to charset: No Subject to charset: No
Purpose: Option tags for supported SDP capability Purpose: Option tags for supported SDP capability
negotiation extensions negotiation extensions
Appropriate values: See Section 3.4.1. Appropriate values: See Section 3.3.1.
Attribute name: creq Attribute name: creq
Long form name: Required capability negotiation extensions Long form name: Required capability negotiation extensions
Type of attribute: Session-level and media-level Type of attribute: Session-level and media-level
Subject to charset: No Subject to charset: No
Purpose: Option tags for required SDP capability Purpose: Option tags for required SDP capability
negotiation extensions negotiation extensions
Appropriate values: See Section 3.4.2. Appropriate values: See Section 3.3.2.
Attribute name: acap Attribute name: acap
Long form name: Attribute capability Long form name: Attribute capability
Type of attribute: Session-level and media-level Type of attribute: Session-level and media-level
Subject to charset: No Subject to charset: No
Purpose: Attribute capability containing an attribute Purpose: Attribute capability containing an attribute
name and associated value name and associated value
Appropriate values: See Section 3.5.1. Appropriate values: See Section 3.4.1.
Attribute name: tcap Attribute name: tcap
Long form name: Transport Protocol Capability Long form name: Transport Protocol Capability
Type of attribute: Session-level and media-level Type of attribute: Session-level and media-level
Subject to charset: No Subject to charset: No
Purpose: Transport protocol capability listing one or Purpose: Transport protocol capability listing one or
more transport protocols more transport protocols
Appropriate values: See Section 3.5.2. Appropriate values: See Section 3.4.2.
Attribute name: pcfg Attribute name: pcfg
Long form name: Potential Configuration Long form name: Potential Configuration
Type of attribute: Media-level Type of attribute: Media-level
Subject to charset: No Subject to charset: No
Purpose: Potential configuration for SDP capability Purpose: Potential configuration for SDP capability
negotiation negotiation
Appropriate values: See Section 3.6.1. Appropriate values: See Section 3.5.1.
Attribute name: acfg Attribute name: acfg
Long form name: Actual configuration Long form name: Actual configuration
Type of attribute: Media-level Type of attribute: Media-level
Subject to charset: No Subject to charset: No
Purpose: Actual configuration for SDP capability Purpose: Actual configuration for SDP capability
negotiation negotiation
Appropriate values: See Section 3.6.2. Appropriate values: See Section 3.5.2.
6.2. New SDP Capability Negotiation Option Tag Registry 6.2. New SDP Capability Negotiation Option Tag Registry
The IANA is hereby requested to create a new SDP Capability The IANA is hereby requested to create a new SDP Capability
Negotiation Option Tag registry. An IANA SDP Capability Negotiation Negotiation Option Tag registry. An IANA SDP Capability Negotiation
option tag registration MUST be documented in an RFC in accordance option tag registration MUST be documented in an RFC in accordance
with the [RFC2434] Specification Required policy. The RFC MUST with the [RFC2434] Specification Required policy. The RFC MUST
provide the name of the option tag, a syntax and a semantic provide the name of the option tag, a syntax and a semantic
specification of any new SDP attributes and any extensions to the specification of any new SDP attributes and any extensions to the
potential and actual configuration attributes provided in this potential and actual configuration attributes provided in this
document. New SDP attributes that are intended to be capabilities for document. New SDP attributes that are intended to be capabilities
use by the capability negotiation framework MUST adhere to the for use by the capability negotiation framework MUST adhere to the
guidelines provided in Section 3.5.3. Extensions to the potential and guidelines provided in Section 3.4.3. Extensions to the potential
actual configuration attributes MUST adhere to the syntax provided in and actual configuration attributes MUST adhere to the syntax
Section 3.6.1. and 3.6.2. provided in Section 3.5.1. and 3.5.2.
The option tag "cap-v0" is defined in this document and the IANA is The option tag "cap-v0" is defined in this document and the IANA is
hereby requested to register this option tag. hereby requested to register this option tag.
6.3. New SDP Capability Negotiation Potential Configuration Parameter 6.3. New SDP Capability Negotiation Potential Configuration Parameter
Registry Registry
The IANA is hereby requested to create a new SDP Capability The IANA is hereby requested to create a new SDP Capability
Negotiation Potential Configuration Parameter registry. An IANA SDP Negotiation Potential Configuration Parameter registry. An IANA SDP
Capability Negotiation potential configuration registration MUST be Capability Negotiation potential configuration registration MUST be
documented in an RFC in accordance with the [RFC2434] Specification documented in an RFC in accordance with the [RFC2434] Specification
Required policy. The RFC MUST define the syntax and semantics of each Required policy. The RFC MUST define the syntax and semantics of
new potential configuration parameter. The syntax MUST adhere to the each new potential configuration parameter. The syntax MUST adhere
syntax provided for extensions in Section 3.6.1. and the semantics to the syntax provided for extensions in Section 3.5.1. and the
MUST adhere to the semantics provided for extensions in Section semantics MUST adhere to the semantics provided for extensions in
3.6.1. and 3.6.2. Associated with each registration MUST be the Section 3.5.1. and 3.5.2. Associated with each registration MUST be
encoding name for the parameter as well as a short descriptive name the encoding name for the parameter as well as a short descriptive
for it. name for it.
The potential configuration parameters "a" for "attribute" and "t" The potential configuration parameters "a" for "attribute" and "t"
for "transport protocol" are defined in this document and the IANA is for "transport protocol" are defined in this document and the IANA
hereby requested to register these. is hereby requested to register these.
7. Acknowledgments 7. Acknowledgments
This document is heavily influenced by the discussions and work done This document is heavily influenced by the discussions and work done
by the SDP Capability Negotiation Design team. The following people by the SDP Capability Negotiation Design team. The following people
in particular provided useful comments and suggestions to either the in particular provided useful comments and suggestions to either the
document itself or the overall direction of the solution defined in document itself or the overall direction of the solution defined in
here: Francois Audet, John Elwell, Roni Even, Robert Gilman, Cullen here: Francois Audet, John Elwell, Roni Even, Robert Gilman, Cullen
Jennings, Jonathan Lennox, Matt Lepinski, Joerg Ott, Colin Perkins, Jennings, Jonathan Lennox, Matt Lepinski, Joerg Ott, Colin Perkins,
Jonathan Rosenberg, Thomas Stach, and Dan Wing. Jonathan Rosenberg, Thomas Stach, and Dan Wing.
8. Change Log 8. Change Log
8.1. draft-ietf-mmusic-sdp-capability-negotiation-06 8.1. draft-ietf-mmusic-sdp-capability-negotiation-07
o Removed the ability to have attribute capabilities provide
attribute names without values, when those attributes otherwise
require an associated value.
o Document no longer obsoletes RFC 3407 but instead recommends that
it is being used instead of RFC 3407.
o Added ability to specific that specific extensions in a potential
configuration are mandatory.
o Changed ABNF for extension-config-list in potential
configurations.
o Removed the redundant "a=" part of attribute capabilities.
o Clarified what it means to support an attribute capability in the
offer/answer procedures.
o Changed "a=acap" attribute and offer/answer procedures to include
only the known and supported attribute capabilities.
o Added new section on indicating bandwidth usage.
8.2. draft-ietf-mmusic-sdp-capability-negotiation-06
o Added additional background text on terminology used, and a new o Added additional background text on terminology used, and a new
section on the negotiation model. section on the negotiation model.
o Allowed for session-level attribute capabilities to contain media- o Allowed for session-level attribute capabilities to contain
level only attributes, albeit the base framework does not define media-level only attributes, albeit the base framework does not
(or allow) them to be used in a potential configuration define (or allow) them to be used in a potential configuration
(extensions may change that) (extensions may change that)
o Disallowing multiple "a=tcap" attributes at the session-level o Disallowing multiple "a=tcap" attributes at the session-level
and/or on a per media description basis; at most one at the and/or on a per media description basis; at most one at the
session-level and per media description now. session-level and per media description now.
o Changed the "a=pcfg" attribute to make a potential configuration o Changed the "a=pcfg" attribute to make a potential configuration
list optional in order to allow for the actual configuration to be list optional in order to allow for the actual configuration to
referenced. be referenced.
o Removed the ability to delete and replace individual attributes o Removed the ability to delete and replace individual attributes
from the actual configuration SDP. from the actual configuration SDP.
o Introduced the notion of mandatory and optional attribute o Introduced the notion of mandatory and optional attribute
capabilities in a potential configuration and updated the "a=pcfg" capabilities in a potential configuration and updated the
attribute and associated procedures accordingly. "a=pcfg" attribute and associated procedures accordingly.
o Specified that mandatory attribute capabilities and the transport o Specified that mandatory attribute capabilities and the transport
protocol (if any) from a potential configuration need to be protocol (if any) from a potential configuration need to be
supported in order to select that potential configuration. supported in order to select that potential configuration.
Offer/answer procedures updated accordingly as well. Offer/answer procedures updated accordingly as well.
o Noted potential interaction and synchronization issues with use of o Noted potential interaction and synchronization issues with use
session-level attributes and attribute capabilities and added of session-level attributes and attribute capabilities and added
recommendation to avoid use of session-level attributes when recommendation to avoid use of session-level attributes when
possible. possible.
o Fixed error in "a=acfg" grammar (missing config-number) and o Fixed error in "a=acfg" grammar (missing config-number) and
updated attribute definition in accordance with the "a=pcfg" updated attribute definition in accordance with the "a=pcfg"
attribute changes. attribute changes.
o Updated text associated with processing media before answer to o Updated text associated with processing media before answer to
allow for playing out garbage or discard until answer received. allow for playing out garbage or discard until answer received.
Additional detail on alternative solutions provided as well. Additional detail on alternative solutions provided as well.
skipping to change at page 68, line 18 skipping to change at page 71, line 31
o Added new section on interactions with SIP option tags. o Added new section on interactions with SIP option tags.
o Added new section on dealing with large number of potential o Added new section on dealing with large number of potential
configurations. configurations.
o Added new section on SDP capability negotiation and o Added new section on SDP capability negotiation and
intermediaries. intermediaries.
o Updated examples in accordance with other changes and to o Updated examples in accordance with other changes and to
illustrate use of mandatory and optional attribute capabilities in illustrate use of mandatory and optional attribute capabilities
a potential configuration. in a potential configuration.
o Updated security considerations to address potential denial of o Updated security considerations to address potential denial of
service attack caused by large number of potential configurations. service attack caused by large number of potential
configurations.
o Various editorial updates throughout. o Various editorial updates throughout.
8.2. draft-ietf-mmusic-sdp-capability-negotiation-05 8.3. draft-ietf-mmusic-sdp-capability-negotiation-05
o Allowed for '<type>=<value>' attributes to be listed as attribute o Allowed for '<type>=<value>' attributes to be listed as attribute
capabilities the attribute name only. capabilities the attribute name only.
o Changed IP-address to conform to RFC 3330 guidelines. o Changed IP-address to conform to RFC 3330 guidelines.
o Added section on relationship to RFC 3407 and "Obsoletes: 3407" in o Added section on relationship to RFC 3407 and "Obsoletes: 3407"
the front. in the front.
o Disallowed use of white space in a number of places for more o Disallowed use of white space in a number of places for more
consistency with existing SDP practice consistency with existing SDP practice
o Changed "csup" and "creq" attributes to not allow multiple o Changed "csup" and "creq" attributes to not allow multiple
instances at the session-level and multiple instances per media instances at the session-level and multiple instances per media
description (only one for each now) description (only one for each now)
o Changed to not require use of "creq" with base option tag ("cap- o Changed to not require use of "creq" with base option tag ("cap-
v0"). v0").
o Relaxed restrictions on extension capabilities o Relaxed restrictions on extension capabilities
o Updated potential configuration attribute syntax and semantics. In
particular, potential configuration attributes can now replace and o Updated potential configuration attribute syntax and semantics.
delete various existing attributes in original SDP to better In particular, potential configuration attributes can now replace
and delete various existing attributes in original SDP to better
control potential attribute interactions with the actual control potential attribute interactions with the actual
configuration while preserving message size efficiency. configuration while preserving message size efficiency.
o Updated actual configuration attribute to align with the updates o Updated actual configuration attribute to align with the updates
to the potential configuration attributes. to the potential configuration attributes.
o Updated offer/answer procedures to align with other changes. o Updated offer/answer procedures to align with other changes.
o Changed recommendation for second offer/answer exchange to "MAY" o Changed recommendation for second offer/answer exchange to "MAY"
strength, unless for the cases where it is known or suspected that strength, unless for the cases where it is known or suspected
it is needed. that it is needed.
o Updated ICE interactions to explain how the new attribute o Updated ICE interactions to explain how the new attribute
delete/replace features can solve certain potential interactions. delete/replace features can solve certain potential interactions.
o Updated rtpmap and fmtp section to allow potential configurations o Updated rtpmap and fmtp section to allow potential configurations
to use remapped payload types in attribute capabilities for to use remapped payload types in attribute capabilities for
rtpmaps and fmtp parameters. rtpmaps and fmtp parameters.
o Added section on direction attributes. o Added section on direction attributes.
o Added another example showing SRTP with session-level MIKEY and o Added another example showing SRTP with session-level MIKEY and
SDP Security Descriptions using the attribute capability DELETE SDP Security Descriptions using the attribute capability DELETE
operator. operator.
8.3. draft-ietf-mmusic-sdp-capability-negotiation-04 8.4. draft-ietf-mmusic-sdp-capability-negotiation-04
The following are the major changes compared to version -03: The following are the major changes compared to version -03:
o Added explicit ordering rules for attributes added by potential o Added explicit ordering rules for attributes added by potential
configurations. configurations.
o Noted that ICE interaction issues (ice-tcp specifically) may not o Noted that ICE interaction issues (ice-tcp specifically) may not
be as clear as originally thought. be as clear as originally thought.
o Added considerations on using rtpmap and fmtp attributes as o Added considerations on using rtpmap and fmtp attributes as
attribute capabilities. attribute capabilities.
o Added multiple transport protocol example. o Added multiple transport protocol example.
o Added session-level MIKEY and media level security descriptions o Added session-level MIKEY and media level security descriptions
example. example.
8.4. draft-ietf-mmusic-sdp-capability-negotiation-03 8.5. draft-ietf-mmusic-sdp-capability-negotiation-03
The following are the major changes compared to version -02: The following are the major changes compared to version -02:
o Base option tag name changed from "v0" to "cap-v0". o Base option tag name changed from "v0" to "cap-v0".
o Added new section on extension capability attributes o Added new section on extension capability attributes
o Firmed up offer/answer procedures. o Firmed up offer/answer procedures.
o Added security considerations o Added security considerations
o Added IANA considerations o Added IANA considerations
8.5. draft-ietf-mmusic-sdp-capability-negotiation-02 8.6. draft-ietf-mmusic-sdp-capability-negotiation-02
The following are the major changes compared to version -01: The following are the major changes compared to version -01:
o Potential configurations are no longer allowed at the session o Potential configurations are no longer allowed at the session
level level
o Renamed capability attributes ("capar" to "acap" and "ctrpr" to o Renamed capability attributes ("capar" to "acap" and "ctrpr" to
"tcap") "tcap")
o Changed name and semantics of the initial number (now called o Changed name and semantics of the initial number (now called
skipping to change at page 71, line 4 skipping to change at page 74, line 15
o Specified use of second offer/answer exchange when answerer o Specified use of second offer/answer exchange when answerer
selected a potential configuration selected a potential configuration
o Updated rules (and added restrictions) for referencing media- and o Updated rules (and added restrictions) for referencing media- and
session-level capabilities in potential configurations (at the session-level capabilities in potential configurations (at the
media level) media level)
o Added initial section on ICE interactions o Added initial section on ICE interactions
o Added initial section on receiving media before answer o Added initial section on receiving media before answer
8.6. draft-ietf-mmusic-sdp-capability-negotiation-01
8.7. 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.
o Clarified rules for session-level and media level attributes o Clarified rules for session-level and media level attributes
provided at either level as well how they can be used in potential provided at either level as well how they can be used in
configurations. potential configurations.
o Potential configuration parameters no longer have implicit o Potential configuration parameters no longer have implicit
ordering; an explicit preference indicator is now included. ordering; an explicit preference indicator is now included.
o The parameter name for transport protocols in the potential and o The parameter name for transport protocols in the potential and
actual configuration attributes have been changed "p" to "t". actual configuration attributes have been changed "p" to "t".
o Clarified operator precedence within potential and actual o Clarified operator precedence within potential and actual
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
configuration attribute can no longer be provided at the session actual configuration attribute can no longer be provided at the
level. session 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.
8.7. draft-ietf-mmusic-sdp-capability-negotiation-00 8.8. 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
attributes (with some differences). similar 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 "tcap" o Multiple transport capabilities can be included in a single
attribute "tcap" 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 9. References
9.1. Normative References 9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC3264] Rosenberg, J., and H. Schulzrinne, "An Offer/Answer Model [RFC3264] Rosenberg, J., and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, June with Session Description Protocol (SDP)", RFC 3264, June
2002. 2002.
[RFC3407] F. Andreasen, "Session Description Protocol (SDP) Simple [RFC3407] F. Andreasen, "Session Description Protocol (SDP) Simple
Capability Declaration", RFC 3407, October 2002. Capability Declaration", RFC 3407, October 2002.
[RFC3605] C. Huitema, "Real Time Control Protocol (RTCP) attribute in
Session Description Protocol (SDP)", RFC 3605, October
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.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006. Description Protocol", RFC 4566, July 2006.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
9.2. Informative References 9.2. Informative References
[RFC2046] Freed, N., and N. Borensteain, "Multipurpose Internet Mail [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
Extensions (MIME) Part Two: Media Types", RFC 2046, A., Peterson, J., Sparks, R., Handley, M., and E.
November 1996. Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC2327] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session [RFC3312] G. Camarillo, W. Marshall, and J. Rosenberg, "Integration
Description Protocol", RFC 2327, April 1998. of Resource Management and Session Initiatio Protocol
(SIP)", RFC 3312, October 2002.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, [RFC3262] J. Rosenberg, and H. Schulzrinne, "Reliability of
A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, Provisional Responses in Session Initiation Protocol
"SIP: Session Initiation Protocol", RFC 3261, June 2002. (SIP)", RFC 3262, June 2002.
[RFC3388] Camarillo, G., Eriksson, G., Holler, J., and H. [RFC3388] Camarillo, G., Eriksson, G., Holler, J., and H.
Schulzrinne, "Grouping of Media Lines in the Session Schulzrinne, "Grouping of Media Lines in the Session
Description Protocol (SDP)", RFC 3388, December 2002. Description Protocol (SDP)", RFC 3388, December 2002.
[RFC3551] Schulzrinne, H., and S. Casner, "RTP Profile for Audio and [RFC3551] Schulzrinne, H., and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", RFC 3551, July Video Conferences with Minimal Control", RFC 3551, July
2003. 2003.
[SRTP] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)", Norrman, "The Secure Real-time Transport Protocol
RFC 3711, March 2004. (SRTP).", RFC 3711, March 2004.
[RFC3851] B. Ramsdell, "Secure/Multipurpose Internet Mail Extensions [RFC3830] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K.
(S/MIME) Version 3.1 Message Specification", RFC 3851, July Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
2004. August 2004.
[RFC4091] Camarillo, G., and J. Rosenberg, The Alternative Network [RFC3851] B. Ramsdell, "Secure/Multipurpose Internet Mail Extensions
Address Types (ANAT) Semantics for the Session Description (S/MIME) Version 3.1 Message Specification", RFC 3851,
Protocol (SDP) Grouping Framework, RFC 4091, June 2005. July 2004.
[AVPF] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, [RFC3890] M. Westerlund, "A Transport Independent Bandwidth Modifier
"Extended RTP Profile for RTCP-Based Feedback (RTP/AVPF)", for the Session Description Protocol (SDP).", RFC 3890,
Work in Progress, August 2004. September 2004.
[I-D.jennings-sipping-multipart] Wing, D., and C. Jennings, "Session [RFC4474] J. Peterson, and C. Jennings, "Enhancements for
Initiation Protocol (SIP) Offer/Answer with Multipart Authenticated Identity Management in the Session
Alternative", Work in Progress, March 2006. Initiation Protocol (SIP)", RFC 4474, August 2006.
[SAVPF] Ott, J., and E Carrara, "Extended Secure RTP Profile for [RFC4567] Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.
RTCP-based Feedback (RTP/SAVPF)", Work in Progress, Carrara, "Key Management Extensions for Session
December 2005. Description Protocol (SDP) and Real Time Streaming
Protocol (RTSP)", RFC 4567, July 2006.
[SDES] Andreasen, F., Baugher, M., and D. Wing, "Session [RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol Security Descriptions for Media Description Protocol Security Descriptions for Media
Streams", RFC 4568, July 2006. Streams", RFC 4568, July 2006.
[SDPng] Kutscher, D., Ott, J., and C. Bormann, "Session Description [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
and Capability Negotiation", Work in Progress, February "Extended RTP Profile for Real-Time Transport Control
2005. Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July
2006.
[BESRTP] Kaplan, H., and F. Audet, "Session Description Protocol
(SDP) Offer/Answer Negotiation for Best-Effort Secure Real-
Time Transport Protocol, Work in progress, August 2006.
[KMGMT] Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E. [RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Carrara, "Key Management Extensions for Session Description Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
Protocol (SDP) and Real Time Streaming Protocol (RTSP)", July 2006.
RFC 4567, July 2006.
[SDPCapNegRqts] Andreasen, F. "SDP Capability Negotiation: [RFC4756] A. Li, "Forward Error Correction Grouping Semantics in
Requirementes and Review of Existing Work", work in Session Description Protocol", RFC 4756, November 2006.
progress, December 2006.
[SDPCapNeg] Andreasen, F. "SDP Capability Negotiation", work in [RFC5027] Andreasen, F. and D. Wing, "Security Preconditions for
progress, December 2006. Session Description Protocol Media Streams", RFC 5027,
October 2007.
[MIKEY] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K. [BESRTP] Kaplan, H., and F. Audet, "Session Description Protocol
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830, (SDP) Offer/Answer Negotiation for Best-Effort Secure
August 2004. Real-Time Transport Protocol, Work in progress, August
2006.
[ICE] J. Rosenberg, "Interactive Connectivity Establishment [ICE] J. Rosenberg, "Interactive Connectivity Establishment
(ICE): A Methodology for Network Address Translator (NAT) (ICE): A Methodology for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", work in progress, Traversal for Offer/Answer Protocols", work in progress,
January 2007. September 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.
[SMIME] B. Ramsdell, "Secure/Multipurpose Internet Mail Extensions
(S/MIME) Version 3.1 Message Specification", RFC 3851, July
2004.
[RFC4474] J. Peterson, and C. Jennings, "Enhancements for [ICETCP] J. Rosenberg, "TCP Candidates with Interactive
Authenticated Identity Management in the Session Initiation Connectivity Establishment (ICE)", work in progress, July
Protocol (SIP)", RFC 4474, August 2006. 2007.
[sprecon] Andreasen, F. and D. Wing, "Security Preconditions for [SAVPF] Ott, J., and E Carrara, "Extended Secure RTP Profile for
Session Description Protocol Media Streams", Work in RTCP-based Feedback (RTP/SAVPF)", Work in Progress, May
Progress, October 2006. 2007.
[RFC4756] A. Li, "Forward Error Correction Grouping Semantics in [SDPCapNeg] Andreasen, F. "SDP Capability Negotiation", work in
Session Description Protocol", RFC 4756, November 2006. progress, December 2006.
[RFC3262] J. Rosenberg, and H. Schulzrinne, "Reliability of [SDPng] Kutscher, D., Ott, J., and C. Bormann, "Session
Provisional Responses in Session Initiation Protocol Description and Capability Negotiation", Work in Progress,
(SIP)", RFC 3262, June 2002. February 2005.
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
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Acknowledgment Acknowledgment
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