MMUSIC Working Group F. Andreasen
Internet-Draft Cisco Systems
Intended Status: Proposed Standard July 8, 2007
Obsolotes: 3407
Expires: January 2008
SDP Capability Negotiation
draft-ietf-mmusic-sdp-capability-negotiation-06.txt
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Abstract
The Session Description Protocol (SDP) was intended for describing
multimedia sessions for the purposes of session announcement, session
invitation, and other forms of multimedia session initiation. SDP was
not intended to provide capability indication or capability
negotiation, however over the years, SDP has seen widespread adoption
and as a result it has been gradually extended to provide limited
support for these, notably in the form of the offer/answer model
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defined in RFC 3264. SDP and its current extensions however do not
define how to negotiate one or more alternative transport protocols
(e.g. RTP profiles) or attributes. This makes it difficult to deploy
new RTP profiles such as secure RTP or RTP with RTCP-based feedback,
negotiate use of different keying mechanisms, etc. It also presents
problems for some forms of media negotiation.
The purpose of this document is to address these shortcomings by
extending SDP with capability negotiation parameters and associated
offer/answer procedures to use those parameters in a backwards
compatible manner.
The solution provided in this document provides a general SDP
Capability Negotiation framework. It also defines specifically how to
provide attributes and transport protocols as capabilities and
negotiate them using the framework. Extensions for other types of
capabilities (e.g. media types and media formats) may be provided in
other documents.
Table of Contents
1. Introduction...................................................3
2. Conventions used in this document..............................7
3. SDP Capability Negotiation Solution............................7
3.1. SDP Capability Negotiation Model..........................7
3.2. Solution Overview........................................10
3.3. Relationship to RFC 3407.................................13
3.4. Version and Extension Indication Attributes..............13
3.4.1. Supported Capability Negotiation Extensions Attribute13
3.4.2. Required Capability Negotiation Extension Attribute.15
3.5. Capability Attributes....................................16
3.5.1. Attribute Capability Attribute......................16
3.5.2. Transport Protocol Capability Attribute.............18
3.5.3. Extension Capability Attributes.....................19
3.6. Configuration Attributes.................................20
3.6.1. Potential Configuration Attribute...................20
3.6.2. Actual Configuration Attribute......................27
3.7. Offer/Answer Model Extensions............................29
3.7.1. Generating the Initial Offer........................29
3.7.2. Generating the Answer...............................32
3.7.2.1. Example Views of Potential Configurations......37
3.7.3. Offerer Processing of the Answer....................39
3.7.4. Modifying the Session...............................41
3.8. Interactions with ICE....................................41
3.9. Interactions with SIP Option Tags........................42
3.10. Processing Media before Answer..........................43
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3.11. Dealing with Large Number of Potential Configurations...44
3.12. SDP Capability Negotiation and Intermediaries...........45
3.13. Considerations for Specific Attribute Capabilities......46
3.13.1. The rtpmap and fmtp Attributes.....................46
3.13.2. Direction Attributes...............................47
4. Examples......................................................48
4.1. Best-Effort Secure RTP...................................48
4.2. Multiple Transport Protocols.............................51
4.3. Best-Effort SRTP with Session-Level MIKEY and Media Level
Security Descriptions.........................................54
4.4. SRTP with Session-Level MIKEY and Media Level Security
Descriptions as Alternatives..................................59
5. Security Considerations.......................................62
6. IANA Considerations...........................................64
6.1. New SDP Attributes.......................................64
6.2. New SDP Capability Negotiation Option Tag Registry.......66
6.3. New SDP Capability Negotiation Potential Configuration
Parameter Registry............................................66
7. Acknowledgments...............................................66
8. Change Log....................................................67
8.1. draft-ietf-mmusic-sdp-capability-negotiation-06..........67
8.2. draft-ietf-mmusic-sdp-capability-negotiation-05..........68
8.3. draft-ietf-mmusic-sdp-capability-negotiation-04..........69
8.4. draft-ietf-mmusic-sdp-capability-negotiation-03..........70
8.5. draft-ietf-mmusic-sdp-capability-negotiation-02..........70
8.6. draft-ietf-mmusic-sdp-capability-negotiation-01..........71
8.7. draft-ietf-mmusic-sdp-capability-negotiation-00..........71
9. References....................................................73
9.1. Normative References.....................................73
9.2. Informative References...................................73
Author's Addresses...............................................76
Intellectual Property Statement..................................76
Full Copyright Statement.........................................76
Acknowledgment...................................................77
1. Introduction
The Session Description Protocol (SDP) was intended for describing
multimedia sessions for the purposes of session announcement, session
invitation, and other forms of multimedia session initiation. The SDP
contains one or more media stream descriptions with information such
as IP-address and port, type of media stream (e.g. audio or video),
transport protocol (possibly including profile information, e.g.
RTP/AVP or RTP/SAVP), media formats (e.g. codecs), and various other
session and media stream parameters that define the session.
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Simply providing media stream descriptions is sufficient for session
announcements for a broadcast application, where the media stream
parameters are fixed for all participants. When a participant wants
to join the session, he obtains the session announcement and uses the
media descriptions provided, e.g., joins a multicast group and
receives media packets in the encoding format specified. If the
media stream description is not supported by the participant, he is
unable to receive the media.
Such restrictions are not generally acceptable to multimedia session
invitations, where two or more entities attempt to establish a media
session that uses a set of media stream parameters acceptable to all
participants. First of all, each entity must inform the other of its
receive address, and secondly, the entities need to agree on the
media stream parameters to use for the session, e.g. transport
protocols and codecs. To solve this, RFC 3264 [RFC3264] defined the
offer/answer model, whereby an offerer constructs an offer SDP that
lists the media streams, codecs, and other SDP parameters that the
offerer is willing to use. This offer SDP is sent to the answerer,
which chooses from among the media streams, codecs and other SDP
parameters provided, and generates an answer SDP with his parameters,
based on that choice. The answer SDP is sent back to the offerer
thereby completing the session negotiation and enabling the
establishment of the negotiated media streams.
Taking a step back, we can make a distinction between the
capabilities supported by each participant, the way in which those
capabilities can be supported and the parameters that can actually be
used for the session. More generally, we can say that we have the
following:
o A set of capabilities for the session and its associated media
stream components, supported by each side. The capability
indication by itself does not imply a commitment to use the
capability in the session.
A capability can for example be that the "RTP/SAVP" profile is
supported, that the "PCMU" codec is supported, or that the
"crypto" attribute is supported with a particular value.
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o A set of potential configurations indicating which combinations of
those capabilities can be used for the session and its associated
media stream components. Potential configurations are not ready
for use. Instead, they provide an alternative that may be used,
subject to further negotiation.
A potential configuration can for example indicate that the "PCMU"
codec and the "RTP/SAVP" transport protocol are not only supported
(i.e. listed as capabilities), but they are offered for potential
use in the session.
o An actual configuration for the session and its associated media
stream components, which specifies which combinations of session
parameters and media stream components can be used currently and
with what parameters. Use of an actual configuration does not
require any further negotiation.
A actual configuration can for example be that the "PCMU" codec
and the "RTP/SAVP" transport protocol are offered for use
currently.
o A negotiation process that takes the set of actual and potential
configurations (combinations of capabilities) as input and
provides the negotiated actual configurations as output.
SDP by itself was designed to provide only one of these, namely
listing of the actual configurations, however over the years, use of
SDP has been extended beyond its original scope. Of particular
importance are the session negotiation semantics that were defined by
the offer/answer model in RFC 3264. In this model, both the offer and
the answer contain actual configurations; separate capabilities and
potential configurations are not supported.
Other relevant extensions have been defined as well. RFC 3407
[RFC3407] defined simple capability declarations, which extends SDP
with a simple and limited set of capability descriptions. Grouping
of media lines, which defines how media lines in SDP can have other
semantics than the traditional "simultaneous media streams"
semantics, was defined in RFC 3388 [RFC3388], etc.
Each of these extensions was designed to solve a specific limitation
of SDP. Since SDP had already been stretched beyond its original
intent, a more comprehensive capability declaration and negotiation
process was intentionally not defined. Instead, work on a "next
generation" of a protocol to provide session description and
capability negotiation was initiated [SDPng]. SDPng defined a
comprehensive capability negotiation framework and protocol that was
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not bound by existing SDP constraints. SDPng was not designed to be
backwards compatible with existing SDP and hence required both sides
to support it, with a graceful fallback to legacy operation when
needed. This combined with lack of ubiquitous multipart MIME support
in the protocols that would carry SDP or SDPng made it challenging to
migrate towards SDPng. In practice, SDPng has not gained traction but
rather remained as work in progress for an extended period of time.
Existing real-time multimedia communication protocols such as SIP,
RTSP, Megaco, and MGCP continue to use SDP. SDP and its current
extensions however do not address an increasingly important problem:
the ability to negotiate one or more alternative transport protocols
(e.g., RTP profiles) and associated parameters (e.g. SDP attributes).
This makes it difficult to deploy new RTP profiles such as secure RTP
(SRTP) [SRTP], RTP with RTCP-Based Feedback [AVPF], etc. This
particular problem is exacerbated by the fact that RTP profiles are
defined independently. When a new profile is defined and N other
profiles already exist, there is a potential need for defining N
additional profiles, since profiles cannot be combined automatically.
For 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
[SRTP], RTP/AVPF [AVPF], and RTP/SAVPF [SAVPF]. In addition to the
pressing profile negotiation problem, other important real-life
limitations have been found as well. Keying material and other
parameters for example need to be negotiated with some of the
transport protocols, but not others. Similarly, some media formats
and types of media streams need to negotiate a variety of different
parameters.
The purpose of this document is to define a mechanism that enables
SDP to provide limited support for indicating capabilities and their
associated potential configurations, and negotiate the use of those
potential configurations as actual configurations. It is not the
intent to provide a full-fledged capability indication and
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
limitations. More specifically, the solution provided in this
document provides a general SDP Capability Negotiation framework that
is backwards compatible with existing SDP. It also defines
specifically how to provide attributes and transport protocols as
capabilities and negotiate them using the framework. Extensions for
other types of capabilities (e.g. media types and formats) may be
provided in other documents.
As mentioned above, SDP is used by several protocols, and hence the
mechanism should be usable by all of these. One particularly
important protocol for this problem is the Session Initiation
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Protocol (SIP) [RFC3261]. SIP uses the offer/answer model [RFC3264]
(which is not specific to SIP) to negotiate sessions and hence the
mechanism defined here defines the offer/answer procedures to use for
the capability negotiation framework.
The rest of the document is structured as follows. In Section 3. we
present the SDP Capability Negotiation solution, which consists of
new SDP attributes and associated offer/answer procedures. In Section
4. we provide examples illustrating its use and in Section 5. we
provide the security considerations.
2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. SDP Capability Negotiation Solution
In this section we first present the conceptual model behind the SDP
capability negotiation framework, followed by an overview of the SDP
Capability Negotiation solution. This is followed by definitions of
new SDP attributes for the solution and its associated updated
offer/answer procedures.
3.1. SDP Capability Negotiation Model
Our model uses the concepts of
o Capabilities
o Potential Configurations
o Actual Configurations
o Negotiation Process
as defined in Section 1. Conceptually, we want to offer not just the
actual configuration SDP, as is done with the current offer/answer
model, but the actual configuration SDP as well as one or more
alternative SDPs, i.e. potential configurations. The answerer must
choose either the actual configuration, or one of the potential
configurations, and generate an answer SDP based on that. Since the
offerer may need to perform processing on the answer, which depends
on the offer that was chosen (actual or potential configuration), the
answerer informs the offerer of which configuration he chose. The
process can be viewed *conceptually* as follows:
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Offerer Answerer
======= ========
1) Generate offer with actual
configuration and alternative
potential configurations
2) Send offer with all configurations
+------------+
| SDP o1 |
| (actual |
| config |
| |-+ Offer
+------------+ | -----> 3) Process offered configurations
| SDP o2 | in order of preference indicated
| (potential | 4) Generate answer based on chosen
| config 1) |-+ configuration (e.g. o2), and inform
+------------+ | offerer which one was chosen
| SDP o3 |
| (potential |
| config 2) |-+
+------------+ |
| SDP ... |
: :
+------------+
| SDP a1 |
Answer | (actual |
<----- | config,o2)|
| |
5) Process answer based on +------------+
the configuration that was
chosen (o2), as indicated in
the answer
The above illustrates the conceptual model, however the actual
solution uses only a single SDP, which contains the actual
configuration (as with current SDP and the current offer/answer
model) enhanced with several new attributes and associated
procedures, that encode the capabilities and potential configurations
and negotiate which ones to use. A more accurate depiction of the
actual offer SDP is therefore as follows:
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+--------------------+
| SDP o1 |
| (actual |
| config |
| |
| +-------------+ |
| | capability 1| |
| | capability 2| |
| | ... | |
| +-------------+ | Offer
| | ----->
| +-------------+ |
| | potential | |
| | config 1 | |
| | potential | |
| | config 2 | |
| | ... | |
| +-------------+ |
| |
+--------------------+
The above structure is used for two reasons:
o Backwards compatibility: As noted above, support for multipart
MIME is not ubiquitous. By encoding both capabilities and
potential configurations in SDP attributes, we can represent
everything in a single SDP thereby avoiding any multipart MIME
support issues. Furthermore, since unknown SDP attributes are
ignored by the SDP recipient, we ensure that entities that do not
support the framework simply perform the regular RFC 3264
offer/answer procedures. This provides us with seamless backwards
compatibility.
o Message size efficiency: When we have multiple media streams,
each of which may potentially use two or more different transport
protocols with a variety of different associated parameters, the
number of potential configurations can be large. If each possible
alternative is represented as a complete SDP in an offer, we can
easily end up with large messages. By providing a more compact
encoding, we get more efficient message sizes.
In the next section, we describe the exact structure and specific SDP
parameters used to represent this.
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3.2. Solution Overview
The solution consists of the following:
o Two new attributes to support extensions to the framework itself
as follows:
o A new attribute ("a=csup") that lists the supported base
(optionally) and any supported extension options to the
framework.
o A new attribute ("a=creq") that lists the extensions to the
framework that are required to be supported by the entity
receiving the SDP in order to do capability negotiation.
o Two new attributes used to express capabilities as follows
(additional attributes can be defined as extensions):
o A new attribute ("a=acap") that defines how to list an
attribute name, either with or without an associated value, as
a capability.
o A new attribute ("a=tcap") that defines how to list transport
protocols (e.g. "RTP/AVP") as capabilities.
o Two new attributes to negotiate configurations as follows:
o A new attribute ("a=pcfg") that lists the potential
configurations supported. This is done by reference to the
capabilities from the SDP in question. Alternative potential
configurations have an explicit ordering associated with them.
Extension capabilities can be defined and referenced in the
potential configurations.
o A new attribute ("a=acfg") to be used in an answer SDP. The
attribute identifies a potential configuration from an offer
SDP which was used as an actual configuration to form the
answer SDP. Extension capabilities can be included as well.
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o Extensions to the offer/answer model that allow for capabilities
and potential configurations to be included in an offer.
Capabilities can be provided at the session level and the media
level. Potential configurations can be included at the media level
only, where they constitute alternative offers that may be
accepted by the answerer instead of the actual configuration(s)
included in the "m=" line(s). The answerer indicates which (if
any) of the potential configurations it used to form the answer by
including the actual configuration attribute ("a=acfg") in the
answer. Capabilities may be included in answers as well, where
they can aid in guiding a subsequent new offer.
The mechanism is illustrated by the offer/answer exchange below,
where Alice sends an offer to Bob:
Alice Bob
| (1) Offer (SRTP and RTP) |
|--------------------------------->|
| |
| (2) Answer (SRTP) |
|<---------------------------------|
| |
Alice's offer includes RTP and SRTP as alternatives. RTP is the
default (actual configuration), but SRTP is the preferred one
(potential configuration):
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
m=audio 53456 RTP/AVP 0 18
a=tcap:1 RTP/SAVP
a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=pcfg:1 t=1 a=1
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
"a=acap" attributes. The transport capabilities ("a=tcap") indicate
that secure RTP under the AVP profile ("RTP/SAVP") is supported with
an associated transport capability handle of 1. The "acap" attribute
provides an attribute capability with a handle of 1. The attribute
capability is a "crypto" attribute, which provides the keying
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material for SRTP using SDP security descriptions [SDES]. The
"a=pcfg" attribute provides the potential configuration included in
the offer by reference to the capability parameters. One alternative
is provided; it has a configuration number of 1 and it consists of
transport protocol capability 1 (i.e. the RTP/SAVP profile - secure
RTP), and the attribute capability 1, i.e. the crypto attribute
provided. Potential configurations are always preferred over the
actual configuration included in the offer SDP, and hence Alice is
expressing a preference for using secure RTP.
Bob receives the SDP offer from Alice. Bob supports SRTP and the SDP
Capability Negotiation framework, and hence he accepts the
(preferred) potential configuration for Secure RTP provided by Alice
and generates the following answer SDP:
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
t=0 0
m=audio 54568 RTP/SAVP 0 18
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4
a=acfg:1 t=1 a=1
Bob includes the "a=acfg" attribute in the answer to inform Alice
that he based his answer on an offer containing the potential
configuration with transport protocol capability 1 and attribute
capability 1 from the offer SDP (i.e. the RTP/SAVP profile using the
keying material provided). Bob also includes his keying material in
a "crypto" attribute. If Bob supported one or more extensions to the
capability negotiation framework, he would have included option tags
for those in the answer as well (in an "a=csup" attribute).
Note that in this particular example, the answerer supported the
capability negotiation extensions defined here, however had he not,
the answerer would simply have ignored the new attributes and
accepted the (actual configuration) offer to use normal RTP. In that
case, the following answer would have been generated instead:
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
t=0 0
m=audio 54568 RTP/AVP 0 18
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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
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
indicating the SDP Capability Negotiation extensions supported and
required.
3.4.1. Supported Capability Negotiation Extensions Attribute
The SDP Capability Negotiation solution allows for capability
negotiation extensions to be defined. Associated with each such
extension is an option tag that identifies the extension in question.
Option-tags MUST be registered with IANA per the procedures defined
in Section 6.
The Supported Capability Negotiation Extensions attribute ("a=csup")
contains a comma-separated list of option tags identifying the SDP
Capability Negotiation extensions supported by the entity that
generated the SDP. The attribute is defined as follows:
a=csup: <option-tag-list>
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RFC 4566, Section 9, provides the ABNF for SDP attributes. The "csup"
attribute adheres to the RFC 4566 "attribute" production, with an
att-value defined as follows:
att-value = option-tag-list
option-tag-list = option-tag *("," option-tag)
option-tag = token ; defined in [RFC4566]
A special base option tag with a value of "cap-v0" is defined for the
basic SDP Capability Negotiation framework defined in this document.
Entities can use this option tag with the "a=csup" attribute to
indicate support for the SDP Capability Negotiation framework
specified in this document.
The following examples illustrates the use of the "a=csup" attribute
with the "cap-v0" option tags and two hypothetical option tags, "foo"
and "bar" (note the lack of white space):
a=csup:cap-v0
a=csup:foo
a=csup:bar
a=csup:cap-v0,foo,bar
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
SDP. When provided at the media-level, it applies to the media
description in question only (option-tags provided at the session
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
description in the latter case).
Whenever an entity that supports one or more extensions to the SDP
Capability Negotiation framework generates an SDP, it SHOULD include
the "a=csup" attribute with the option tags for the extensions it
supports at the session and/or media-level, unless those option tags
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
OPTIONAL; support for the base framework can be inferred from
presence of the "a=pcfg" attribute defined in Section 3.6.1.
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
offer that did not use the SDP Capability Negotiation framework.
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3.4.2. Required Capability Negotiation Extension Attribute
The Required Capability Negotiation Extensions attribute ("a=creq")
contains a comma-separated list of option tags (see Section 3.4.1. )
specifying the SDP Capability Negotiation extensions that MUST be
supported by the entity receiving the SDP, in order for that entity
to properly process the SDP Capability Negotiation attributes and
associated procedures. Support for the basic negotiation framework is
implied by the presence of an "a=pcfg" attribute (see Section 3.6.1.
) and hence there is no need to include the "a=creq" attribute with
the base option-tag ("cap-v0"). Still, it is allowed to do so.
The attribute is defined as follows:
a=creq: <option-tag-list>
The "creq" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows:
att-value = option-tag-list
The following examples illustrate the use of the "a=creq" attribute
with the "cap-v0" base option tag and two hypothetical option tags,
"foo" and "bar" (note the lack of white space):
a=creq:cap-v0
a=creq:foo
a=creq:bar
a=creq:cap-v0,foo,bar
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
SDP. When provided at the media-level, it applies to the media
description in question only (required option tags provided at the
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
the media-level (one per media description in the latter case).
When an entity generates an SDP and it requires the recipient of that
SDP to support one or more SDP Capability Negotiation extensions
(except for the base), in order to properly process the SDP
Capability Negotiation, the "a=creq" attribute MUST be included with
option-tags that identify the required extensions at the session
and/or media level. Support for the basic negotiation framework is
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implied by the presence of an "a=pcfg" attribute (see Section 3.6.1.
) 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
the required extensions listed in a "creq" attribute, MUST NOT
perform the SDP Capability Negotiation defined in this document. For
non-supported extensions provided at the session-level, this implies
that SDP Capability Negotiation MUST NOT be performed at all. For
non-supported extensions at the media-level, this implies that SDP
Capability Negotiation MUST NOT be performed for the media stream in
question.
An entity that does not support the SDP Capability Negotiation
framework at all, will ignore these attributes (as well as the
other SDP Capability Negotiation attributes) and not perform any
SDP Capability Negotiation in the first place.
When an entity does not support one or more required SDP Capability
Negotiation extensions, the entity SHOULD proceed as if the SDP
Capability Negotiation attributes were not included in the first
place, i.e. all the capability negotiation attributes should be
ignored. In that case, the entity SHOULD include a "csup" attribute
listing the SDP Capability Negotiation extensions it actually
supports.
This ensures that introduction of the SDP Capability Negotiation
mechanism by itself does not lead to session failures.
3.5. Capability Attributes
In this section, we present the new attributes associated with
indicating the capabilities for use by the SDP Capability
Negotiation.
3.5.1. Attribute Capability Attribute
Attributes and their associated values can be expressed as
capabilities by use of a new attribute capability attribute
("a=acap"), which is defined as follows:
a=acap: <att-cap-num> <att-par>
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
attribute ("a=") in its full '<type>=<value>' form (see [RFC4566]).
Support for a specific attribute (name) (without any particular
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values) can be indicated by providing only the '<type>' (i.e. the
attribute name).
The "acap" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows:
att-value = att-cap-num 1*WSP att-par
att-cap-num = 1*DIGIT ;defined in [RFC4234]
att-par = attribute ;defined in RFC 4566
Note that white-space is not permitted before the att-cap-num.
The "acap" attribute can be provided at the session level only for
session-level attributes contained in the attribute capability,
whereas media level attributes can be provide in attribute
capabilities at either the media level or session-level. The base SDP
Capability Negotiation framework however only defines procedures for
use of media-level attribute capabilities at the media level
(extensions may define use at the session level).
Each occurrence of the "acap" attribute in the entire session
description MUST use a different value of <att-cap-num>.
There is a need to be able to reference both session-level and
media-level attributes in potential configurations at the media
level, and this provides for a simple solution to avoiding overlap
between the references (handles) to each attribute capability.
The <att-cap-num> values provided are independent of similar <cap-
num> values provided for other types of capabilities, i.e., they form
a separate name-space for attribute capabilities.
The following examples illustrate use of the "acap" attribute:
a=acap:1 a=ptime:20
a=acap:2 a=ptime:30
a=acap:3 a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyONQ6gAA
AAAGEEoo2pee4hp2UaDX8ZE22YwKAAAPZG9uYWxkQGR1Y2suY29tAQAAAAAAAQAk0
JKpgaVkDaawi9whVBtBt0KZ14ymNuu62+Nv3ozPLygwK/GbAV9iemnGUIZ19fWQUO
SrzKTAv9zV
a=acap:4 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=acap:5 a=crypto
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a=acap:6 a=key-mgmt
The first two attribute capabilities provide attribute values for the
ptime attribute. The third provides SRTP parameters by using MIKEY
with the key-mgmt attribute [KMGMT]. The fourth provides SRTP
parameters by use of security descriptions with the crypto attribute
[SDES]. Note that the line-wrapping and new-lines in example three
and four are provided for formatting reasons only - they are not
permitted in actual SDP. 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
the RFC 3407 "cpar" attribute and the above. There are however a
couple of important differences, notably that the "acap" attribute
contains a handle that enables referencing it and it furthermore
supports attributes only (the "cpar" attribute defined in RFC 3407
supports bandwidth information as well). The "acap" attribute also
is not automatically associated with any particular capabilities.
3.5.2. Transport Protocol Capability Attribute
Transport Protocols can be expressed as capabilities by use of a new
Transport Protocol Capability attribute ("a=tcap") defined as
follows:
a=tcap: <trpr-cap-num> <proto-list>
where <trpr-cap-num> is an integer between 1 and 2^31-1 (both
included) used to number the transport address capability for later
reference, and <proto-list> is one or more <proto>, separated by
white space, as defined in the SDP "m=" line.
The "tcap" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows:
att-value = trpr-cap-num 1*WSP proto-list
trpr-cap-num = 1*DIGIT ;defined in [RFC4234]
proto-list = proto *(1*WSP proto) ; defined in RFC 4566
Note that white-space is not permitted before the trpr-cap-num.
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
session-level and at most one at the media-level (one per media
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description in the latter case). Each occurrence of the "tcap"
attribute in the entire session description MUST use a different
value of <trpr-cap-num>. When multiple <proto> values are provided,
the first one is associated with the value <trpr-cap-num>, the second
one with the value one higher, etc. The <trpr-cap-num> values
provided are independent of similar <cap-num> values provided for
other capability attributes, i.e., they form a separate name-space
for transport protocol capabilities.
Below, we provide examples of the "a=tcap" attribute:
a=tcap:1 RTP/AVP
a=tcap:2 RTP/AVPF
a=tcap:3 RTP/SAVP RTP/SAVPF
The first one provides a capability for the "RTP/AVP" profile defined
in [RFC3551] and the second one provides a capability for the RTP
with RTCP-Based Feedback profile defined in [AVPF]. The third one
provides capabilities for the "RTP/SAVP" (transport capability number
3) and "RTP/SAVPF" profiles (transport protocol capability number 4).
Transport capabilities are inherently included in the "m=" line,
however they still need to be specified explicitly in a "tcap"
attribute, if they are to be used as a capability.
This may seem redundant (and indeed it is from the offerer's point
of view), however it is done to protect against intermediaries
(e.g. middle-boxes) that may modify "m=" lines while passing
unknown attributes through. If an implicit transport capability
were used instead (e.g. a reserved transport capability number
could be used to refer to the transport protocol in the "m=" line),
and an intermediary were to modify the transport protocol in the
"m=" line (e.g. to translate between plain RTP and secure RTP),
then the potential configuration referencing that implicit
transport capability may no longer be correct. With explicit
capabilities, we avoid this pitfall, although the potential
configuration preference (see Section 3.6.1. ) may not reflect that
of the intermediary (which some may view as a feature).
3.5.3. Extension Capability Attributes
The SDP Capability Negotiation framework allows for new capabilities
to be defined as extensions and used with the general capability
negotiation framework. The syntax and semantics of such new
capability attributes are not defined here, however in order to be
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used with potential configurations, they SHOULD allow for a numeric
handle to be associated with each capability. This handle can be used
as a reference within the potential and actual configuration
attributes (see Section 3.6.1. and 3.6.2. ). The definition of such
extension capability attributes MUST also state whether they can be
applied at the session-level, media-level, or both.
3.6. Configuration Attributes
3.6.1. Potential Configuration Attribute
Potential Configurations can be expressed by use of a new Potential
Configuration Attribute ("a=pcfg") defined as follows:
a=pcfg: <config-number> [<pot-cfg-list>]
where <config-number> is an integer between 1 and 2^31-1 (both
included).
The "pcfg" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows:
att-value = config-number [1*WSP pot-cfg-list]
config-number = 1*DIGIT ;defined in [RFC4234]
pot-cfg-list = pot-config *(1*WSP pot-config)
pot-config = pot-attribute-config-list /
pot-transport-protocol-config-list /
pot-extension-config-list
The missing productions are defined below. Note that white-space is
not permitted before the config-number.
The potential configuration attribute can be provided at the media-
level only and there can be multiple instances of it within a given
media description. The attribute includes a configuration number,
which is an integer between 1 and 2^31-1 (both included). The
configuration number MUST be unique within the media description
(i.e. it has media level scope only). The configuration number also
indicates the relative preference of potential configurations; lower
numbers are preferred over higher numbers.
After the configuration number, zero, one or more potential
configuration lists is provided. When the potential configuration
list is omitted, the potential configuration equals the actual
configuration. The potential configuration lists generally reference
one or more capabilities, and those capabilities are (conceptually)
used to construct a new internal version of the SDP by use of purely
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syntactic add and (possibly) delete operations on the original SDP
(actual configuration), thereby generating an alternative potential
configuration SDP that can be used by conventional SDP and
offer/answer procedures if actually selected.
This document defines potential attribute configuration lists and
potential transport protocol configuration lists. Each of these MUST
NOT be present more than once in a particular potential configuration
attribute. Potential extension configuration lists can be included as
well; unknown potential extension configuration lists MUST be ignored
(if support is required, then the "a=creq" attribute with suitable
option tags MUST be used). There can be more than one potential
extension configuration list, however each particular potential
extension configuration list MUST NOT be present more than once in a
given potential configuration attribute. Together, these potential
configuration lists define a potential configuration.
There can be multiple potential configurations provided within a
media description. Each of these indicates not only a willingness,
but in fact a desire to use the potential configuration.
The example SDP below contains two potential configurations:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
m=audio 53456 RTP/AVP 0 18
a=tcap:1 RTP/SAVP RTP/SAVPF
a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=pcfg:1 t=1 a=1
a=pcfg:2 t=2 a=1
Potential configuration 1 contains a potential transport protocol
configuration list that references transport capability 1
("RTP/SAVP") and a potential attribute configuration list that
references attribute capability 1 ("a=crypto:..."). Potential
configuration 2 contains a potential transport protocol configuration
list that references transport capability 2 ("RTP/SAVPF") and a
potential attribute configuration list that references attribute
capability 1 ("a=crypto:...").
Attribute capabilities are used in a potential configuration by use
of the pot-attribute-config-list parameter, which is defined by the
following ABNF:
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pot-attribute-config-list
= "a=" [delete-attributes ":"]
mo-att-cap-list *(BAR mo-att-cap-list)
delete-attributes = DELETE ( "m" ; media attributes
/ "s" ; session attributes
/ "ms" ) ; media and session attributes
mo-att-cap-list = mandatory-optional-att-cap-list |
mandatory-att-cap-list |
optional-att-cap-list
mandatory-optional-att-cap-list = mandatory-att-cap-list
"," optional-att-cap-list
mandatory-att-cap-list = att-cap-list
optional-att-cap-list = "[" att-cap-list "]"
att-cap-list = att-cap-num *("," att-cap-num)
att-cap-num = 1*DIGIT ;defined in [RFC4234]
BAR = "|"
DELETE = "-"
Note that white space is not permitted within this production.
Each potential attribute configuration list can optionally begin with
instructions for how to handle attributes that are part of the actual
configuration (i.e. the "a=" lines present in the original SDP). By
default, such attributes will remain as part of the configuration in
question. However, if delete-attributes indicates "-m", then all
attribute lines within the media description in question will be
deleted (i.e. all "a=" lines under the "m=" line in question). If
delete-attributes indicates "-s", then all attribute lines at the
session-level will be deleted (i.e. all "a=" lines before the first
"m=" line). If delete-attributes indicates "-ms", then all attribute
lines within this media description ("m=" line) and all attribute
lines at the session-level will be deleted.
The attribute capability list comes next. It contains one or more
alternative lists of attribute capabilities. The alternative
attribute capability lists are separated by a vertical bar ("|"), and
each list contains one or more attribute capabilities separated by
commas (","). The attribute capabilities are either mandatory or
optional. Mandatory attribute capabilities MUST be supported in order
to use the potential configuration, whereas optional attribute
capabilities MAY be supported in order to use the potential
configuration.
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Within each attribute capability list, all the mandatory attribute
capabilities (if any) are listed first, and all the optional
attribute capabilities (if any) are listed last. The optional
attribute capabilities are contained within a pair of angle brackets
("[" and "]"). Each attribute capability is merely an attribute
capability number (att-cap-num) that identifies a particular
attribute capability by referring to attribute capability numbers
defined above and hence MUST be between 1 and 2^31-1 (both included).
The following example illustrates the above:
a=pcfg:1 a=-m:1,2,[3,4]|1,7,[5]
where
o "a=-m:1,2,[3,4]|1,7,[5]" is the potential attribute configuration
list
o "-m" is the delete-attributes
o "1,2,[3,4]" and "1,7,[5]" are both attribute capability lists. The
two lists are alternatives, since they are separated by a vertical
bar above
o "1", "2" and "7" are mandatory 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
potential configuration(s), but there are actually two different
potential configurations (separated by a vertical bar). This is done
for message size efficiency reasons, which is especially important
when we add other types of capabilities to the potential
configuration. If there is a need to provide a unique handle for
each, separate "a=pcfg" attributes with different handles must be
used instead.
Each referenced attribute capability in the potential configuration
will result in the corresponding attribute name and its associated
value (contained inside the attribute capability) being added to the
resulting potential configuration SDP.
Alternative attribute capability lists are separated by a vertical
bar ("|"), the scope of which extends to the next alternative (i.e.
"," has higher precedence than "|"). The alternatives are ordered by
preference with the most preferred listed first. In order for a
recipient of the SDP (e.g. an answerer receiving this in an offer) to
use this potential configuration, exactly one of the alternative
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lists must be selected in its entirety. This requires that all
mandatory attribute capabilities referenced by the potential
configuration are supported with the attribute values provided.
Potential transport protocol configuration lists are included in a
potential configuration by use of the pot-transport-protocol-config-
list parameter, which is defined by the following ABNF:
pot-transport-protocol-config-list =
"t=" trpr-cap-num *(BAR trpr-cap-num)
trpr-cap-num = 1*DIGIT ; defined in [RFC4234]
Note that white-space is not permitted within this production.
The trpr-cap-num refers to transport protocol capability numbers
defined above and hence MUST be between 1 and 2^31-1 (both included).
Alternative potential transport protocol capabilities are separated
by a vertical bar ("|"). The alternatives are ordered by preference
with the most preferred listed first. When there are no transport
protocol capabilities included in a potential configuration at the
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
answerer receiving this in an offer) to use this potential
configuration, exactly one of the alternatives MUST be selected. This
requires that the transport protocol in question is supported.
In the presence of intermediaries (the existence of which may not
be known), care should be taken with assuming that the transport
protocol in the "m=" line will not be modified by an intermediary.
Use of an explicit transport protocol capability will guard against
any capability negotiation implications of that.
Extension capabilities can be included in a potential configuration
as well by use of potential extension configuration lists. Such
potential configuration extension lists MUST adhere to the following
ABNF:
pot-extension-config-list = ext-cap-name "="
ext-cap-list
ext-cap-name = token ; defined in [RFC4566]
ext-cap-list = 1*VCHAR ; defined in [RFC4234]
Note that white-space is not permitted within this production.
The ext-cap-name refers to the type of extension capability and the
ext-cap-list is here merely defined as a sequence of visible
characters. The actual extension supported MUST refine both of these
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further. For extension capabilities that merely need to be referenced
by a capability number, it is RECOMMENDED to follow a structure
similar to what has been specified above. Unsupported or unknown
potential extension configuration lists in a potential configuration
attribute MUST be ignored.
The "creq" attribute and its associated rules can be used to ensure
that required extensions are supported in the first place.
Potential configuration attributes can be provided at the media level
only, however it is possible to reference capabilities provided at
either the session or media level. There are certain semantic rules
and restrictions associated with this:
A (media level) potential configuration attribute in a given media
description MUST NOT reference a media-level capability provided in a
different media description; doing so invalidates that potential
configuration (note that a potential configuration attribute can
contain more than one potential configuration by use of
alternatives). A potential configuration attribute can however
reference a session-level capability. The semantics of doing so
depends on the type of capability. In the case of transport protocol
capabilities it has no particular implication. In the case of
attribute capabilities however, it does. More specifically, the
attribute name and value (provided within that attribute capability)
will be considered part of the resulting SDP for that particular
configuration at the *session* level. In other words, it will be as-
if that attribute was simply provided with that value at the session-
level in the first place. As a result of that, the base SDP
Capability Negotiation framework REQUIRES that potential
configurations do not reference any session-level attribute
capabilities that contain media-level attributes (since that would
place a media-level attribute at the session level). Extensions may
modify this behavior, as long as it is fully backwards compatible
with the base specification.
Individual media streams perform capability negotiation individually,
and hence it is possible that one media stream (where the attribute
was part of a potential configuration) chose a configuration without
a session level attribute that was chosen by another media stream.
The session-level attribute however remains "active" and hence
applies to the entire resulting potential configuration SDP. In
theory, this is problematic if one or more session-level attributes
either conflicts with or potentially interacts with another session-
level or media-level attribute in an undefined manner. In practice
however, such examples seem to be rare (at least with the currently
defined SDP attributes).
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A related set of problems can occur if we need coordination between
session-level attributes from multiple media streams in order for a
particular functionality to work. The grouping framework [RFC3388]
is an example of this. If we use the SDP Capability Negotiation
framework to select a session-level group attribute (provided as an
attribute capability), and we require two media descriptions to do
this consistently, we could have a problem. The FEC grouping
semantics [RFC4756] is one example where this in theory could cause
problems, however in practice, it is unclear that there is a
significant problem here with the currently defined grouping
semantics.
Resolving the above issues in general requires inter-media stream
constraints and synchronized potential configuration processing; this
would add considerable complexity to the overall solution. In
practice, with the currently defined SDP attributes, it does not seem
to be a significant problem, and hence the core SDP Capability
Negotiation solution does not provide a solution to this issue.
Instead, it is RECOMMENDED that use of session-level attributes in a
potential configuration is avoided when possible, and when not, that
such use is examined closely for any potential interaction issues. If
interaction is possible, the entity generating the SDP SHOULD NOT
assume that well-defined operation will occur at the receiving
entity.
The session-level operation of extension capabilities is undefined:
Consequently, each new session-level extension capability defined
MUST specify the implication of making it part of a configuration at
the media level.
Below, we provide an example of the "a=pcfg" attribute in a complete
media description in order to properly indicate the supporting
attributes:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
m=audio 53456 RTP/AVPF 0 18
a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=tcap:1 RTP/AVPF RTP/AVP
a=tcap:3 RTP/SAVP RTP/SAVPF
a=pcfg:1 t=4|3 a=1
a=pcfg:8 t=1|2
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We have two potential configuration attributes listed here. The 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
be supported with attribute capability 1 (the "crypto" attribute);
RTP/SAVPF is preferred over RTP/SAVP since its capability number (4)
is listed first in the preferred potential configuration. Note that
while we have only a single potential attribute and associated
handle, we have two potential configurations.
The second potential configuration attribute indicates that the
RTP/AVPF or RTP/AVP profile can be used, with RTP/AVPF being the
preferred one. This non secure RTP alternative is the less preferred
one since its configuration number is "8". Again, note that we have
two potential configurations here and hence a total of four potential
configurations in the SDP above.
3.6.2. Actual Configuration Attribute
The actual configuration attribute identifies which of the potential
configurations from an offer SDP was selected and used as the actual
configuration to generate an answer SDP. This is done by including
the configuration number and the configuration lists (if any) from
the offer that were selected and used by the answerer in his
offer/answer procedure as follows:
o A selected potential attribute configuration MUST include the
delete-attributes and the selected alternative mo-att-cap-list
(i.e. containing all mandatory and optional capability numbers
from the potential configuration, irrespective of whether the
optional ones were supported or not). If delete-attributes were
not included in the potential configuration, they will of course
not be present here either.
o A selected potential transport protocol configuration MUST include
the selected transport protocol capability number.
o A selected potential extension configuration MUST include the
selected extension configuration parameters as specified for that
particular extension.
Note that the selected configuration number and all selected
capability numbers used in the actual configuration attribute refer
to those from the offer; not the answer.
The answer may for example include capabilities as well to inform
the offerer of the answerers capabilities above and beyond the
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negotiated configuration. The actual configuration attribute does
not refer to any of those answer capabilities though.
The Actual Configuration Attribute ("a=acfg") is defined as follows:
a=acfg: <config-number> [<sel-cfg-list>]
where <config-number> is an integer between 1 and 2^31-1 (both
included).
The "acfg" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows:
att-value = config-number [1*WSP sel-cfg-list]
;config-number defined in Section 3.6.1.
sel-cfg-list = sel-cfg *(1*WSP sel-cfg)
sel-cfg = sel-attribute-config /
sel-transport-protocol-config /
sel-extension-config
sel-attribute-config =
"a=" [delete-attributes ":"] mo-att-cap-list
; defined in Section 3.6.1.
sel-transport-protocol-config =
"t=" trpr-cap-num ; defined in Section 3.6.1.
sel-extension-config =
ext-cap-name "=" 1*VCHAR ; defined in Section 3.6.1.
Note that white-space is not permitted before the config-number.
The actual configuration ("a=acfg") attribute can be provided at the
media-level only. There MUST NOT be more than one occurrence of an
actual configuration attribute within a given media description.
Below, we provide an example of the "a=acfg" attribute (building on
the previous example with the potential configuration attribute):
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v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
t=0 0
m=audio 54568 RTP/SAVPF 0
a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32
a=acfg:1 t=4 a=1
It indicates that the answerer used an offer consisting of potential
configuration number 1 with transport protocol capability 4 from the
offer (RTP/SAVPF) and attribute capability 1 (the "crypto"
attribute). The answerer includes his own "crypto" attribute as well.
3.7. Offer/Answer Model Extensions
In this section, we define extensions to the offer/answer model
defined in [RFC3264] to allow for potential configurations to be
included in an offer, where they constitute offers that may be
accepted by the answerer instead of the actual configuration(s)
included in the "m=" line(s).
The procedures defined in the following subsections apply to both
unicast and multicast streams.
3.7.1. Generating the Initial Offer
An offerer that wants to use the SDP Capability Negotiation defined
in this document MUST include the following in the offer:
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o An attribute capability attribute ("a=acap") as defined in Section
3.5.1. for each attribute name and associated value (if any) that
needs to be indicated as a capability in the offer.
Session-level attributes and associated values MUST be provided in
attribute capabilities at the session-level only, whereas media-
level attributes and associated values can be provided in
attribute capabilities at either the media-level or session-level.
Attributes that can be provided at either the session- or media-
level can be represented as attribute capabilities at either the
session- or media-level. Attribute capabilities for
'<type>=<value>' attributes MAY include an attribute name only,
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.
o One or more a transport protocol capability attributes ("a=tcap")
as defined in Section 3.5.2. with values for each transport
protocol that needs to be indicated as a capability in the offer.
Transport protocol capabilities that apply to multiple media
descriptions SHOULD be provided at the session-level whereas
transport protocol capabilities that apply to a specific media
description ("m=" line) only, SHOULD be provided within that
particular media description. In either case, there MUST NOT be
more than a single "a=tcap" attribute at the session-level and a
single "a=tcap" attribute in each media description. If there is
not a need to indicate any transport protocols as transport
protocol capabilities, then there will not be any "a=tcap"
attributes either.
o One or more extension capability attributes (as outlined in
Section 3.5.3. ) for each extension capability that is referenced
by a potential configuration.
o One or more potential configuration attributes ("a=pcfg") as
defined in Section 3.6.1. within each media description where
alternative potential configurations are to be negotiated. Each
potential configuration attribute MUST adhere to the rules
provided in Section 3.6.1. and the additional rules provided
below.
If the offerer requires support for more or extensions (besides the
base protocol defined here), then the offerer MUST include one or
more "a=creq" attribute as follows:
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o If one or more capability negotiation extensions are required to
be supported for the entire session description, then option tags
for those extensions MUST be included in a single session-level
"creq" attribute.
o For each media description that requires one or more capability
negotiation extensions not listed at the session-level, a single
"creq" attribute containing all the required extensions for that
media description MUST be included within the media description
(in accordance with Section 3.4.2. ).
The offerer SHOULD furthermore include the following:
o A supported capability negotiation extension attribute ("a=csup")
at the session-level and/or media-level as defined in Section
3.4.2. for each capability negotiation extension supported by the
offerer and not included in a corresponding "a=creq" attribute
(i.e. at the session-level or in the same media description).
Option tags provided in a "a=csup" attribute at the session-level
indicate extensions supported for the entire session description
whereas option tags provided in a "a=csup" attribute in a media
description indicate extensions supported for that particular
media description only.
Capabilities provided in an offer merely indicate what the offerer is
capable of doing. They do not constitute a commitment or even an
indication to use them. In contrast, each potential configuration
constitutes an alternative offer that the offerer would like to use.
The potential configurations MUST be used by the answerer to
negotiate and establish the session.
The offerer MUST include one or more potential configuration
attributes ("a=pcfg") within each media description where the offerer
wants to provide alternative offers (in the form of potential
configurations). Each potential configuration attribute in a given
media description MUST contain a unique configuration number and one
or more potential configuration lists, as described in Section 3.6.1.
Each potential configuration list MUST refer to capabilities that are
provided either at the session-level or within that particular media
description; otherwise, the potential configuration is considered
invalid. The base SDP Capability Negotiation framework REQUIRES that
potential configurations do not reference any session-level attribute
capabilities that contain media-level attributes, however extensions
may modify this behavior, as long as it is fully backwards compatible
with the base specification. Furthermore, it is RECOMMENDED that
potential configurations avoid use of session-level capabilities
whenever possible; refer to Section 3.6.1.
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The current actual configuration is included in the "m=" line (as
defined by [RFC3264]) and the various attribute ("a=") lines. Note
that the actual configuration is by definition the least-preferred
configuration, and hence the answerer will seek to negotiate use of
one of the potential configurations instead. If the offerer wishes a
different preference for the actual configuration, the offerer MUST
include a corresponding potential configuration with the relevant
configuration number (which indicates the relative preference between
potential configurations); this corresponding potential configuration
should simply duplicate the actual configuration.
This can either be done implicitly (by not referencing any
capabilities), or explicitly (by providing and using capabilities
for the transport protocol and all the attributes that are part of
the actual configuration). The latter may help detect
intermediaries that modify the actual configuration but are not SDP
Capability Negotiation aware.
Per [RFC3264], once the offerer generates the offer, he must be
prepared to receive incoming media in accordance with that offer.
That rule applies here as well, but for the actual configurations
provided in the offer only: Media received by the offerer according
to one of the potential configurations MAY be discarded, until the
offerer receives an answer indicating what the actual selected
configuration is. Once that answer is received, incoming media MUST
be processed in accordance with the actual selected configuration
indicated and the answer received (provided the offer/answer exchange
completed successfully).
The above rule assumes that the offerer can determine whether
incoming media adheres to the actual configuration offered or one of
the potential configurations instead; this may not always be the
case. If the offerer wants to ensure he does not play out any
garbage, the offerer SHOULD discard all media received before the
answer SDP is received. Conversely, if the offerer wants to avoid
clipping, he should attempt to play any incoming media as soon as it
is received (at the risk of playing out garbage). For further
details, please refer to Section 3.10.
3.7.2. Generating the Answer
When receiving an offer, the answerer MUST check for the presence of
a required capability negotiation extension attribute ("a=creq")
provided at the session level. If one is found, then capability
negotiation MUST be performed. If none is found, then the answerer
MUST check each offered media description for the presence of a
required capability negotiation extension attribute ("a=creq") and
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one or more potential configuration attributes ("a=pcfg"). Capability
negotiation MUST be performed for each media description where either
of those is present in accordance with the procedures described
below.
The answerer MUST first ensure that it supports any required
capability negotiation extensions:
o If a session-level "creq" attribute is provided, and it contains
an option-tag that the answerer does not support, then the
answerer MUST NOT use any of the potential configuration
attributes provided for any of the media descriptions. Instead,
the normal offer/answer procedures MUST continue as per [RFC3264].
Furthermore, the answerer MUST include a session-level supported
capability negotiation extensions attribute ("a=csup") with option
tags for the capability negotiation extensions supported by the
answerer.
o If a media-level "creq" attribute is provided, and it contains an
option tag that the answerer does not support, then the answerer
MUST NOT use any of the potential configuration attributes
provided for that particular media description. Instead, the
offer/answer procedures for that media description MUST continue
as per [RFC3264] (SDP Capability Negotiation is still performed
for other media descriptions in the SDP). Furthermore, the
answerer MUST include a supported capability negotiation
extensions attribute ("a=csup") in that media description with
option tags for the capability negotiation extensions supported by
the answerer for that media description.
Assuming all required capability negotiation extensions are
supported, the answerer now proceeds as follows.
For each media description where capability negotiation is to be
performed (i.e. all required capability negotiation extensions are
supported and at least one valid potential configuration attribute is
present), the answerer MUST attempt to perform capability negotiation
by using the most preferred potential configuration that is valid. A
potential configuration is valid if:
1. It is in accordance with the syntax and semantics provided in
Section 3.6.1.
2. It contains a configuration number that is unique within that
media description.
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3. All attribute capabilities referenced by the potential
configuration are valid themselves (as defined in Section 3.5.1. )
and each of them is provided either at the session-level or within
this particular media description. For session-level attribute
capabilities referenced, the attributes contained inside them MUST
NOT be media-level only attributes.
4. All transport protocol capabilities referenced by the potential
configuration are valid themselves (as defined in Section 3.5.2. )
and each of them is furthermore provided either at the session-
level or within this particular media description.
5. All extension capabilities referenced by the potential
configuration and supported by the answerer are valid themselves
(as defined by that particular extension) and each of them are
furthermore provided either at the session-level or within this
particular media description. Unknown or unsupported extension
capabilities MUST be ignored.
The most preferred valid potential configuration in a media
description is the valid potential configuration with the lowest
configuration number. The answerer MUST now process the offer for
that media stream based on the most preferred valid potential
configuration. Conceptually, this entails the answerer constructing
an (internal) offer that consists of the actual configuration offer
SDP, with the following changes for each media stream offered:
o If a transport protocol capability is included in the potential
configuration, then it replaces the transport protocol provided in
the "m=" line for that media description.
o If attribute capabilities are present with a delete-attributes
session indication ("-s"), then all session-level attributes from
the actual configuration SDP MUST be deleted in accordance with
the procedures in Section 3.6.1. If attribute capabilities are
present with a delete-attributes media indication ("-m"), then all
attributes from the actual configuration SDP inside this media
description MUST be deleted.
o If a session-level attribute capability is included, the attribute
(and its associated value, if any) contained in it MUST be added
to the resulting SDP. All such added session-level attributes MUST
be listed before the session-level attributes that were initially
present in the SDP. Furthermore, the added session-level
attributes MUST be added in the order they were provided in the
potential configuration (see also Section 3.6.1. ).
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This allows for attributes with implicit preference ordering to
be added in the desired order; the "crypto" attribute [SDES] is
one such example.
o If a media-level attribute capability is included, then the
attribute (and its associated value, if any) MUST be added to the
resulting SDP within the media description in question. All such
added media-level attributes MUST be listed before the media-level
attributes that were initially present in the SDP in the media
description in question. Furthermore, the added media-level
attributes MUST be added in the order they were provided in the
potential configuration (see also Section 3.6.1. ).
o If a supported extension capability is included, then it MUST be
processed in accordance with the rules provided for that
particular extension capability.
Note that whereas a transport protocol from the potential
configuration replaces the transport protocol in the actual
configuration, an attribute capability from the potential
configuration is simply added to the actual configuration. In some
cases, this can result in having one or more meaningless attributes
in the resulting potential configuration SDP, or worse, ambiguous or
potentially even illegal attributes. The delete-attributes for the
session and/or media level attributes MUST be used to avoid such
scenarios. Nevertheless, it is RECOMMENDED that implementations
ignore meaningless attributes that may result from potential
configurations.
For example, if the actual configuration was using Secure RTP and
included an "a=crypto" attribute for the SRTP keying material, then
use of a potential configuration that uses plain RTP would make the
"crypto" attribute meaningless. The answerer may or may not ignore
such a meaningless attribute. The offerer can here ensure correct
operation by using the delete-attributes to actually delete the
crypto attribute (but will then need to provide attribute
capabilities to reconstruct the SDP with the necessary attributes
deleted, e.g. rtpmaps).
Please refer to Section 3.7.2.1. for examples of how the answerer may
conceptually "see" the resulting offered alternative potential
configurations.
The answerer MUST check that he supports all mandatory attribute
capabilities from the potential configuration (if any) and the
transport protocol capability (if any) from the potential
configuration. If he does not, the answerer MUST proceed to the
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second-most preferred valid potential configuration for the media
description, etc.
If the answerer has exhausted all potential configurations for the
media description, without finding a valid one that is also
supported, then the answerer MUST process the offered media stream
based on the actual configuration plus any session-level attributes
added by a valid and supported potential configuration for another
media description in the offered SDP.
The above process describes potential configuration selection as a
per media stream process. Inter-media stream coordination of selected
potential configurations however is required in some cases. First of
all, session-level attributes added by a potential configuration for
one media description MUST NOT cause any problems for potential
configurations selected by other media descriptions in the offer SDP.
If the session-level attributes are mandatory, then those session-
level attributes MUST furthermore be supported by the session as a
whole (i.e. all the media descriptions if relevant). As mentioned
earlier, this adds additional complexity to the overall processing
and hence it is RECOMMENDED not to use session-level attribute
capabilities in potential configurations, unless absolutely
necessary.
Once the answerer has selected a valid and supported offered
configuration for all of the media streams (or has fallen back to the
actual configuration plus any added session attributes), the answerer
MUST generate a valid answer SDP based on the selected potential
configuration SDP, as "seen" by the answerer (see Section 3.7.2.1.
for examples). Furthermore, if the answerer selected one of the
potential configurations in a media description, the answerer MUST
include an actual configuration attribute ("a=acfg") within that
media description that identifies the configuration number for the
selected potential configuration as well as the actual parameters
that were used from that potential configuration (if the potential
configuration included alternatives, only the selected alternatives
must be included). Only the known and supported parameters will be
included. Unknown or unsupported parameters MUST NOT be included in
the actual configuration attribute.
This is not to be confused with mandatory and optional attribute
capabilities, where some of the optional attribute capabilities may
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.
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If the answerer supports one or more capability negotiation
extensions that were not included in a required capability
negotiation extensions attribute in the offer, then the answerer
SHOULD furthermore include a supported capability negotiation
attribute ("a=csup") at the session-level with option tags for the
extensions supported across media streams. Also, if the answerer
supports one or more capability negotiation extensions for particular
media descriptions only, then a supported capability negotiation
attribute with those option-tags SHOULD be included within each
relevant media description.
The offerer's originally provided actual configuration is contained
in the media description's "m=" line (and associated parameters). The
answerer MAY send media to the offerer in accordance with that actual
configuration as soon as it receives the offer, however it MUST NOT
send media based on that actual configuration if it selects an
alternative potential configuration. If the answerer selects one of
the potential configurations, then the answerer MAY immediately start
to send media to the offerer in accordance with the selected
potential configuration, however the offerer MAY discard such media
or play out garbage until the offerer receives the answer. Please
refer to section 3.10. for additional considerations and possible
alternative solutions outside the base SDP Capability Negotiation
framework.
If the offerer selected a potential configuration instead of the
actual configuration, then it is RECOMMENDED that the answerer sends
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
the case of SIP [RFC3261] without any extensions, this implies that
if the offer was received in an INVITE message, then the answer SDP
should be provided in the first non-100 provisional response sent
back (per RFC3261, the answer would need to be repeated in the 200
response as well, unless a relevant extension such as [RFC3262] is
being used).
3.7.2.1. Example Views of Potential Configurations
The following examples illustrate how the answerer may conceptually
"see" a potential configuration. Consider the following offered SDP:
v=0
o=alice 2891092738 2891092738 IN IP4 lost.example.com
s=
t=0 0
c=IN IP4 lost.example.com
a=tool:foo
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a=acap:1 a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
a=tcap:1 RTP/SAVP RTP/AVP
m=audio 59000 RTP/AVP 98
a=rtpmap:98 AMR/8000
a=acap:2 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=pcfg:1 t=1 a=1|2
m=video 52000 RTP/AVP 31
a=rtpmap:31 H261/90000
a=acap:3 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=pcfg:1 t=1 a=1|3
This particular SDP offers an audio stream and a video stream, each
of which can either use plain RTP (actual configuration) or secure
RTP (potential configuration). Furthermore, two different keying
mechanisms are offered, namely session-level Key Management
Extensions using MIKEY (attribute capability 1) and media-level SDP
Security Descriptions (attribute capabilities 2 and 3). There are
several potential configurations here, however, below we show the one
the answerer "sees" when using potential configuration 1 for both
audio and video, and furthermore using attribute capability 1 (MIKEY)
for both (we have removed all the capability negotiation attributes
for clarity):
v=0
o=alice 2891092738 2891092738 IN IP4 lost.example.com
s=
t=0 0
c=IN IP4 lost.example.com
a=tool:foo
a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
m=audio 59000 RTP/SAVP 98
a=rtpmap:98 AMR/8000
m=video 52000 RTP/SAVP 31
a=rtpmap:31 H261/90000
Note that the transport protocol in the media descriptions indicate
use of secure RTP.
Below, we show the offer the answerer "sees" when using potential
configuration 1 for both audio and video and furthermore using
attribute capability 2 and 3 respectively (SDP security descriptions)
for the audio and video stream - note the order in which the
resulting attributes are provided:
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v=0
o=alice 2891092738 2891092738 IN IP4 lost.example.com
s=
t=0 0
c=IN IP4 lost.example.com
a=tool:foo
m=audio 59000 RTP/SAVP 98
a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=rtpmap:98 AMR/8000
m=video 52000 RTP/SAVP 31
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=rtpmap:31 H261/90000
Again, note that the transport protocol in the media descriptions
indicate use of secure RTP.
And finally, we show the offer the answerer "sees" when using
potential configuration 1 with attribute capability 1 (MIKEY) for the
audio stream, and potential configuration 1 with attribute capability
3 (SDP security descriptions) for the video stream:
v=0
o=alice 2891092738 2891092738 IN IP4 lost.example.com
s=
t=0 0
c=IN IP4 lost.example.com
a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
a=tool:foo
m=audio 59000 RTP/SAVP 98
a=rtpmap:98 AMR/8000
m=video 52000 RTP/SAVP 31
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=rtpmap:31 H261/90000
3.7.3. Offerer Processing of the Answer
When the offerer attempted to use SDP Capability Negotiation in the
offer, the offerer MUST examine the answer for actual use of SDP
Capability Negotiation.
For each media description where the offerer included a potential
configuration attribute ("a=pcfg"), the offerer MUST first examine
the media description for the presence of an actual configuration
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attribute ("a=acfg"). If an actual configuration attribute is not
present in a media description, then the offerer MUST process the
answer SDP for that media stream per the normal offer/answer rules
defined in [RFC3264]. However, if one is found, the offerer MUST
instead process the answer as follows:
o The actual configuration attribute specifies which of the
potential configurations was used by the answerer to generate the
answer for this media stream. This includes all the capabilities
referenced by the potential configuration selected, i.e. the
attribute capabilities and any associated delete-attributes,
transport protocol capabilities, and any extension capability
parameters included.
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
consisted of the selected potential configuration instead of the
actual configuration, including any transport protocol changes in
the media ("m=") line(s), attributes added and deleted by the
potential configuration at the media and session level, and any
extensions used.
If the offer/answer exchange was successful, and if the answerer
selected one of the potential configurations from the offer as the
actual configuration, then the offerer MAY perform another
offer/answer exchange: This new offer SHOULD contain the selected
potential configuration as the actual configuration, i.e. with the
actual configuration used in the "m=" line and any other relevant
attributes and extensions. This second offer/answer exchange will not
modify the session in any way, however it will help intermediaries
(e.g. middleboxes) that look at the SDP, but do not understand or
support the capability negotiation extensions, to understand the
details of the media stream(s) that were actually negotiated. If it
is known or suspected that one or more such intermediaries exist,
then this second offer/answer SHOULD be performed (this is already
done when using Interactive Connectivity Establishment [ICE], and in
those cases, there will not be a need for a third offer/answer
exchange). Note that, per normal offer/answer rules, the second
offer/answer exchange still needs to update the version number in the
"o=" line ((<sess-version> in [RFC4566]). Attribute lines carrying
keying material SHOULD repeat the keys from the previous offer,
unless re-keying is necessary, e.g. due to a previously forked SIP
INVITE request. Please refer to Section 3.12. for additional
considerations related to intermediaries.
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3.7.4. Modifying the Session
Capabilities and potential configurations may be included in
subsequent offers as defined in [RFC3264], Section 8. The procedure
for doing so is similar to that described above with the answer
including an indication of the actual selected configuration used by
the answerer.
If the answer indicates use of a potential configuration from the
offer, then the guidelines provided in Section 3.7.3. for doing a
second offer/answer exchange using that potential configuration as
the actual configuration apply.
3.8. Interactions with ICE
Interactive Connectivity Establishment (ICE) [ICE] provides a
mechanism for verifying connectivity between two endpoints by sending
STUN messages directly between the media endpoints. The basic ICE
specification [ICE] is defined to support UDP-based connectivity
only, however it allows for extensions to support other transport
protocols, such as TCP, which is being specified in [ICETCP]. ICE
defines a new "a=candidate" attribute, which, among other things,
indicates the possible transport protocol(s) to use and then
associates a priority with each of them. The most preferred transport
protocol that *successfully* verifies connectivity will end up being
used.
When using ICE, it is thus possible that the transport protocol that
will be used differs from what is specified in the "m=" line.
Furthermore, since both ICE and SDP Capability Negotiation may now
specify alternative transport protocols, there is a potentially
unintended interaction when using these together.
We provide the following guidelines for addressing that.
There are two basic scenarios to consider here:
1) A particular media stream can run over different transport
protocols (e.g. UDP, TCP, or TCP/TLS), and the intent is simply to
use the one that works (in the preference order specified).
2) A particular media stream can run over different transport
protocols (e.g. UDP, TCP, or TCP/TLS) and the intent is to have the
negotiation process decide which one to use (e.g. T.38 over TCP or
UDP).
In scenario 1, there should be ICE "a=candidate" attributes for UDP,
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TCP, etc. but otherwise nothing special in the potential
configuration attributes to indicate the desire to use different
transport protocols (e.g. UDP, or TCP). The ICE procedures
essentially cover the capability negotiation required (by having the
answerer select something it supports and then use of trial and
error).
Scenario 2 does not require a need to support or use ICE. Instead, we
simply use transport protocol capabilities and potential
configuration attributes to indicate the desired outcome.
The scenarios may be combined, e.g. by offering potential
configuration alternatives where some of them can support one
transport protocol only (e.g. UDP), whereas others can support
multiple transport protocols (e.g. UDP or TCP). In that case, there
is a need for tight control over the ICE candidates that will
actually be used for a particular configuration, yet the actual
configuration may want to use all of them. In that case, the ICE
candidate attributes can be defined as attribute capabilities and the
relevant ones should then be included in the proper potential
configurations (for example candidate attributes for UDP only for
potential configurations that are restricted to UDP, whereas there
could be candidate attributes for UDP, TCP, and TCP/TLS for potential
configurations that can use all three). Furthermore, use of the
delete-attributes in a potential configuration can be used to ensure
that ICE will not end up using a transport protocol that is not
desired.
3.9. Interactions with SIP Option Tags
SIP [RFC3261] allows for SIP extensions to define a SIP option tag
that identifies the SIP extension. Support for one or more such
extensions can be indicated by use of the SIP Supported header, and
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
defined by the SDP Capability Negotiation framework are similar,
except that support for these two attributes by themselves cannot be
guaranteed (since they are specified as extensions to the SDP
specification [RFC4566] itself).
SIP extensions with associated option tags can introduce enhancements
to not only SIP, but also SDP. This is for example the case for SIP
preconditions defined in [RFC3312]. When using SDP Capability
Negotiation, some potential configurations may include certain SDP
extensions, whereas others may not. Since the purpose of the SDP
Capability Negotiation is to negotiate a session based on the
features supported by both sides, use of the SIP Require header for
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such extensions may not produce the desired result. For example, if
one potential configuration requires SIP precondition support,
another does not, and the answerer does not support preconditions,
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
potential configuration was included in the offer.
In general, this can be alleviated by use of mandatory and optional
attribute capabilities in a potential configuration. There are
however cases where permissible SDP values are tied to the use of the
SIP Require header. SIP preconditions [RFC3312] is one such example,
where preconditions with a "mandatory" strength-tag can only be used
when a SIP Require header with the SIP option tag "precondition" is
included. Future SIP extensions that may want to use the SDP
Capability Negotiation framework should avoid such coupling.
3.10. Processing Media before Answer
The offer/answer model requires an offerer to be able to receive
media in accordance with the offer prior to receiving the answer.
This property is retained with the SDP Capability Negotiation
extensions defined here, but only when the actual configuration is
selected by the answerer. If a potential configuration is chosen, it
is permissible for the offerer to not process any media received
before the answer is received. This however may lead to clipping.
Consequently, the SDP Capability Negotiation framework recommends
sending back an answer SDP as soon as possible.
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
precondition [RFC3312] for capability negotiation (or use an existing
precondition that is known to generate a second offer/answer exchange
before proceeding with the session), however preconditions are often
viewed as complicated to implement and they may add to overall
session establishment delay by requiring an extra offer/answer
exchange.
An alternative three-way handshake can be performed by use of ICE
[ICE]. When ICE is being used, and the answerer receives a STUN
Binding Request for any one of the accepted media streams from the
offerer, the answerer knows the offer has received his answer. At
that point, the answerer knows that the offerer will be able to
process incoming media according to the negotiated configuration and
hence he can start sending media without the risk of the offerer
either discarding it or playing garbage.
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In some use cases (notably, when the offerer does not need
information from the answer, such as keying material in the SDP, in
order to process incoming media), a three-way handshake is not
needed. The SDP Capability Negotiation framework does not define any
such solutions, however extensions may do so. For example, one
technique proposed for best-effort SRTP in [BESRTP] is to provide
different RTP payload type mappings for different transport protocols
used, outside of the actual configuration, while still allowing them
to be used by the answerer (exchange of keying material is still
needed, e.g. inband). The basic SDP Capability Negotiation framework
defined here does not include the ability to do so, however
extensions that enable that may be defined.
3.11. Dealing with Large Number of Potential Configurations
When using the SDP Capability Negotiation, it is easy to generate
offers that contain a large number of potential configurations. For
example, in the offer:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
m=audio 53456 RTP/AVP 0 18
a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF
a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
FEC_ORDER=FEC_SRTP
a=acap:2 a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
a=acap:3 a=rtcp-fb:0 nack
a=pcfg:1 t=1 a=1,3|2,3
a=pcfg:2 t=2 a=1|2
a=pcfg:3 t=3 a=3
we have 5 potential configurations on top of the actual configuration
for a single media stream. Adding an extension capability with just
two alternatives for each would double that number (to 10), and doing
the equivalent with two media streams would again double that number
(to 20). While it is easy (and inexpensive) for the offerer to
generate such offers, processing them at the answering side may not
be. Consequently, it is RECOMMENDED that offerers do not create
offers with unnecessarily large number of potential configurations in
them.
On the answering side, implementers MUST take care to avoid excessive
memory and CPU consumption. For example, a naïve implementation that
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first generates all the valid potential configuration SDPs
internally, could find itself being memory exhausted, especially if
it supports a large number of endpoints. Similarly, a naïve
implementation that simply performs iterative trial-and-error
processing on each possible potential configuration SDP (in the
preference order specified) could find itself being CPU constrained.
An alternative strategy is to prune the search space first by
discarding the set of offered potential configurations where the
transport protocol indicated (if any) is not supported, and/or one or
more mandatory attribute capabilities (if any) are either not
supported or not valid.
3.12. SDP Capability Negotiation and Intermediaries
An intermediary is here defined as an entity between a SIP user 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
session establishment to operate as intended. Examples of such
intermediaries include Session Border Controllers (SBCs) that may
perform media relaying, Proxy Call Session Control Functions (P-CSCF)
that may authorize use of a certain amount of network resources
(bandwidth), etc. The presence and design of such intermediaries may
not follow the "Internet" model or the SIP requirements for proxies
(which are not supposed to look in message bodies such as SDP),
however they are a fact of life in some deployment scenarios
currently and hence deserves consideration.
If the intermediary needs to understand the characteristics of the
media sessions being negotiated, e.g. the amount of bandwidth used or
the transport protocol negotiated, then use of the SDP Capability
Negotiation framework may impact them. For example, some
intermediaries are known to (currently) disallow answers where the
transport protocol differs from the one in the offer. Use of the SDP
Capability Negotiation framework in the presence of such
intermediaries could lead to session failures. Intermediaries that
need to authorize use of network resources based on the negotiated
media stream parameters are affected as well. If they inspect only
the offer, then they may authorize parameters assuming a different
transport protocol, codecs, etc. than what is actually being
negotiated. For these, and other, reasons it is RECOMMENDED that
implementers of intermediaries add support for the SDP Capability
Negotiation framework.
The SDP Capability Negotiation framework itself attempts to help out
these intermediaries as well, by optionally performing a second
offer/answer exchange when use of a potential configuration has been
negotiated (see Section 3.7.3. ). However, there are several
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limitations with this approach. First of all, the second offer/answer
exchange is not required and hence may not be performed. Secondly,
the intermediary may refuse the initial answer, e.g. due to perceived
transport protocol mismatch. Thirdly, the strategy is not foolproof,
since the offer/answer procedures [RFC3264] leave the original
offer/answer exchange in effect when a subsequent one fails; consider
the following example:
1. Offerer generates an SDP offer with the actual configuration
specifying a low bandwidth configuration (e.g. plain RTP) and a
potential configuration specifying a high(er) bandwidth
configuration (e.g. secure RTP with integrity).
2. An intermediary (e.g. an SBC or P-CSCF), that does not support SDP
Capability Negotiation, authorizes the session based on the actual
configuration it sees in the SDP.
3. The answerer chooses the high(er) bandwidth potential
configuration and generates an answer SDP based on that.
4. The intermediary passes through the answer SDP.
5. The offerer sees the accepted answer, and generates an updated
offer that contains the selected potential configuration as the
actual configuration. In other words, the high(er) bandwidth
configuration (which has already been negotiated successfully) is
now the actual configuration in the offer SDP.
6. The intermediary sees the new offer, however it does not authorize
the use of the high(er) bandwidth configuration, and consequently
generates a rejection message to the offerer.
7. The offerer receives the rejected offer.
After step 7, per RFC 3264, the offer/answer exchange that completed
in step 5 remains in effect, however the intermediary may not have
authorized the necessary network resources and hence the media stream
may experience quality issues. The solution to this problem is to
upgrade the intermediary to support the SDP Capability Negotiation
framework.
3.13. Considerations for Specific Attribute Capabilities
3.13.1. The rtpmap and fmtp Attributes
The core SDP Capability Negotiation framework defines transport
capabilities and attribute capabilities. Media capabilities, which
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can be used to describe media formats and their associated
parameters, are not defined in this document, however the "rtpmap"
and "fmtp" attributes can nevertheless be used as attribute
capabilities. Using such attribute capabilities in a potential
configuration requires a bit of care though.
The rtpmap parameter binds an RTP payload type to a media format
(e.g. codec). While it is possible to provide rtpmaps for payload
types not found in the corresponding "m=" line, such rtpmaps provide
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
applies to the core SDP Capability Negotiation framework as well:
Only the media formats (e.g. RTP payload types) provided in the "m="
line are actually offered; inclusion of rtpmap attributes with other
RTP payload types in a potential configuration does not change this
fact and hence they do not provide any useful information there. They
may still be useful as pure capabilities though (outside a potential
configuration) in order to inform a peer of additional codec
supported.
It is possible to provide an rtpmap attribute capability with a
payload type mapping to a different codec than a corresponding actual
configuration "rtpmap" attribute for the media description has. Such
practice is permissible as a way of indicating a capability. If that
capability is included in a potential configuration, then delete-
attributes (see Section 3.6.1. ) MUST be used to ensure that there is
not multiple rtpmap attributes for the 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
fmtp attribute capability for a media format not included in the "m="
line is useless in a potential configuration (but may be useful as a
capability by itself). An fmtp attribute capability in a potential
configuration for a media format that already has an fmtp attribute
in the actual configuration may lead to multiple fmtp format
parameters for that media format and that is not allowed by SDP
[RFC4566]. The delete-attributes MUST be used to ensure that there is
not multiple fmtp attributes for a given media format in a media
description.
Extensions to the core SDP Capability Negotiation framework may
change the above behavior.
3.13.2. Direction Attributes
SDP defines the "inactive", "sendonly", "recvonly", and "sendrecv"
direction attributes. The direction attributes can be applied at
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either the session-level or the media-level. In either case, it is
possible to define attribute capabilities for these direction
capabilities. Note that if used by a potential configuration, then
the normal offer/answer procedures still apply. For example, if an
offered potential configuration includes the "sendonly" direction
attribute, and it is selected as the actual configuration, then the
answer MUST include a corresponding "recvonly" (or "inactive")
attribute.
4. Examples
In this section, we provide examples showing how to use the SDP
Capability Negotiation.
4.1. Best-Effort Secure RTP
The following example illustrates how to use the SDP Capability
Negotiation extensions to support so-called Best-Effort Secure RTP.
In that scenario, the offerer supports both RTP and Secure RTP. If
the answerer does not support secure RTP (or the SDP Capability
Negotiation extensions), an RTP session will be established. However,
if the answerer supports Secure RTP and the SDP Capability
Negotiation extensions, a Secure RTP session will be established.
The best-effort Secure RTP negotiation is illustrated by the
offer/answer exchange below, where Alice sends an offer to Bob:
Alice Bob
| (1) Offer (SRTP and RTP) |
|--------------------------------->|
| |
| (2) Answer (SRTP) |
|<---------------------------------|
| |
| (3) Offer (SRTP) |
|--------------------------------->|
| |
| (4) Answer (SRTP) |
|<---------------------------------|
| |
Alice's offer includes RTP and SRTP as alternatives. RTP is the
default, but SRTP is the preferred one:
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v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
m=audio 53456 RTP/AVP 0 18
a=tcap:1 RTP/SAVP RTP/AVP
a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
FEC_ORDER=FEC_SRTP
a=pcfg:1 t=1 a=1
The "m=" line indicates that Alice is offering to use plain RTP with
PCMU or G.729. The capabilities are provided by the "a=tcap" and
"a=acap" attributes. The "tcap" capability indicates that both
Secure RTP and normal RTP are supported. The "acap" attribute
provides an attribute capability with a handle of 1. The capability
is a "crypto" attribute, which provides the keying material for SRTP
using SDP security descriptions [SDES]. The "a=pcfg" attribute
provides the potential configurations included in the offer by
reference to the capabilities. A single potential configuration with
a configuration number of "1" is provided. It includes the transport
protocol capability 1 (RTP/SAVP, i.e. secure RTP) together with the
attribute capability 1, i.e. the crypto attribute provided. Note
that attribute capability 1 is mandatory, and hence it must be
supported in order for the potential configuration to be used.
Bob receives the SDP offer from Alice. Bob supports SRTP and the SDP
Capability Negotiation framework, and hence he accepts the potential
configuration for Secure RTP provided by Alice:
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
t=0 0
m=audio 54568 RTP/SAVP 0 18
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4
a=acfg:1 t=1 a=1
Bob includes the "a=acfg" attribute in the answer to inform Alice
that he based his answer on an offer containing the potential
configuration with transport protocol capability 1 and attribute
capability 1 from the offer SDP (i.e. the RTP/SAVP profile using the
keying material provided). Bob also includes his keying material in
a crypto attribute.
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When Alice receives Bob's answer, session negotiation has completed,
however Alice nevertheless chooses to generate a new offer using the
actual configuration. This is done purely to assist any
intermediaries that may reside between Alice and Bob but do not
support the SDP Capability Negotiation framework (and hence may not
understand the negotiation that just took place):
Alice's updated offer includes only SRTP, and it is not using the SDP
Capability Negotiation framework (Alice could have included the
capabilities as well is she wanted to):
v=0
o=- 25678 753850 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
m=audio 53456 RTP/SAVP 0 18
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
FEC_ORDER=FEC_SRTP
The "m=" line now indicates that Alice is offering to use secure RTP
with PCMU or G.729. The "crypto" attribute, which provides the SRTP
keying material, is included with the same value again.
Bob receives the SDP offer from Alice, which he accepts, and then
generates an answer to Alice:
v=0
o=- 24351 621815 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
t=0 0
m=audio 54568 RTP/SAVP 0 18
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4
Bob includes the same crypto attribute as before, and the session
proceeds without change. Although Bob did not include any
capabilities in his answer, he could have done so if he wanted to.
Note that in this particular example, the answerer supported the SDP
Capability Negotiation framework, and hence the attributes and
procedures defined here, however had he not, the answerer would
simply have ignored the new attributes received in step 1 and
accepted the offer to use normal RTP. In that case, the following
answer would have been generated in step 2 instead:
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v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
t=0 0
m=audio 54568 RTP/AVP 0 18
4.2. Multiple Transport Protocols
The following example illustrates how to use the SDP Capability
Negotiation extensions to negotiate use of one out of several
possible transport protocols. As in the previous example, the offerer
uses the expected least-common-denominator (plain RTP) as the actual
configuration, and the alternative transport protocols as the
potential configurations.
The example is illustrated by the offer/answer exchange below, where
Alice sends an offer to Bob:
Alice Bob
| (1) Offer (RTP/[S]AVP[F]) |
|--------------------------------->|
| |
| (2) Answer (RTP/AVPF) |
|<---------------------------------|
| |
| (3) Offer (RTP/AVPF) |
|--------------------------------->|
| |
| (4) Answer (RTP/AVPF) |
|<---------------------------------|
| |
Alice's offer includes plain RTP (RTP/AVP), RTP with RTCP-based
feedback (RTP/AVPF), Secure RTP (RTP/SAVP), and Secure RTP with RTCP-
based feedback (RTP/SAVPF) and SRTP as alternatives. RTP is the
default, with RTP/SAVPF, RTP/SAVP, and RTP/AVPF as the alternatives
and preferred in the order listed:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
m=audio 53456 RTP/AVP 0 18
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a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF
a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
FEC_ORDER=FEC_SRTP
a=acap:2 a=rtcp-fb:0 nack
a=pcfg:1 t=1 a=1,[2]
a=pcfg:2 t=2 a=1
a=pcfg:3 t=3 a=[2]
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
"a=acap" attributes. The "tcap" capability indicates that Secure RTP
with RTCP-Based feedback (RTP/SAVPF), Secure RTP (RTP/SAVP), and RTP
with RTCP-Based feedback are supported. The first "acap" attribute
provides an attribute capability with a handle of 1. The capability
is a "crypto" attribute, which provides the keying material for SRTP
using SDP security descriptions [SDES]. The second "acap" attribute
provides an attribute capability with a handle of 2. The capability
is an "rtcp-fb" attribute, which is used by the RTCP-based feedback
profiles to indicate that payload type 0 (PCMU) supports feedback
type "nack". The "a=pcfg" attributes provide the potential
configurations included in the offer by reference to the
capabilities. There are three potential configurations:
o Potential configuration 1, which is the most preferred potential
configuration specifies use of transport protocol capability 1
(RTP/SAVPF) and attribute capabilities 1 (the "crypto" attribute)
and 2 (the "rtcp-fb" attribute). Support for the first one is
mandatory whereas support for the second one is optional.
o Potential configuration 2, which is the second most preferred
potential configuration specifies use of transport protocol
capability 2 (RTP/SAVP) and mandatory attribute capability 1 (the
"crypto" attribute).
o Potential configuration 3, which is the least preferred potential
configuration (but the second least preferred configuration
overall, since the actual configuration provided by the "m=" line
is always the least preferred configuration), specifies use of
transport protocol capability 3 (RTP/AVPF) and optional attribute
capability 2 (the "rtcp-fb" attribute).
Bob receives the SDP offer from Alice. Bob does not support any
secure RTP profiles, however he supports plain RTP and RTP with RTCP-
based feedback, as well as the SDP Capability Negotiation extensions,
and hence he accepts the potential configuration for RTP with RTCP-
based feedback provided by Alice:
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v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
t=0 0
m=audio 54568 RTP/AVPF 0 18
a=rtcp-fb:0 nack
a=acfg:1 t=3 a=[2]
Bob includes the "a=acfg" attribute in the answer to inform Alice
that he based his answer on an offer containing the potential
configuration with transport protocol capability 3 and optional
attribute capability 2 from the offer SDP (i.e. the RTP/AVPF profile
using the "rtcp-fb" value provided). Bob also includes an "rtcp-fb"
attribute with the value "nack" value for RTP payload type 0.
When Alice receives Bob's answer, session negotiation has completed,
however Alice nevertheless chooses to generate a new offer using the
actual configuration. This is done purely to assist any
intermediaries that may reside between Alice and Bob but do not
support the SDP Capability Negotiation framework (and hence may not
understand the negotiation that just took place):
Alice's updated offer includes only RTP/AVPF, and it is not using the
SDP Capability Negotiation framework (Alice could have included the
capabilities as well if she wanted to):
v=0
o=- 25678 753850 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
m=audio 53456 RTP/AVPF 0 18
a=rtcp-fb:0 nack
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
provides the feedback type "nack" for payload type 0 again (but as
part of the actual configuration).
Bob receives the SDP offer from Alice, which he accepts, and then
generates an answer to Alice:
v=0
o=- 24351 621815 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
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t=0 0
m=audio 54568 RTP/AVPF 0 18
a=rtcp-fb:0 nack
Bob includes the same "rtcp-fb" attribute as before, and the session
proceeds without change. Although Bob did not include any
capabilities in his answer, he could have done so if he wanted to.
Note that in this particular example, the answerer supported the SDP
Capability Negotiation framework and hence the attributes and
procedures defined here, however had he not, the answerer would
simply have ignored the new attributes received in step 1 and
accepted the offer to use normal RTP. In that case, the following
answer would have been generated in step 2 instead:
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
t=0 0
m=audio 54568 RTP/AVP 0 18
4.3. Best-Effort SRTP with Session-Level MIKEY and Media Level Security
Descriptions
The following example illustrates how to use the SDP Capability
Negotiation extensions to support so-called Best-Effort Secure RTP as
well as alternative keying mechanisms, more specifically MIKEY and
SDP Security Descriptions. The offerer (Alice) wants to establish an
audio and video session. Alice prefers to use session-level MIKEY as
the key management protocol, but supports SDP security descriptions
as well.
The example is illustrated by the offer/answer exchange below, where
Alice sends an offer to Bob:
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Alice Bob
| (1) Offer (RTP/[S]AVP[F], SDES|MIKEY) |
|--------------------------------------->|
| |
| (2) Answer (RTP/SAVP, SDES) |
|<---------------------------------------|
| |
| (3) Offer (RTP/SAVP, SDES) |
|--------------------------------------->|
| |
| (4) Answer (RTP/SAVP, SDES) |
|<---------------------------------------|
| |
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
video stream offers use of plain RTP, RTP with RTCP-based feedback,
Secure RTP, and Secure RTP with RTCP-based feedback as alternatives:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
t=0 0
c=IN IP4 192.0.2.1
a=acap:1 a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF
m=audio 59000 RTP/AVP 98
a=rtpmap:98 AMR/8000
a=acap:2 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=pcfg:1 t=2 a=1|2
m=video 52000 RTP/AVP 31
a=rtpmap:31 H261/90000
a=acap:3 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=acap:4 a=rtcp-fb:* nack
a=pcfg:1 t=1 a=1,4|3,4
a=pcfg:2 t=2 a=1|3
a=pcfg:3 t=3 a=4
The potential configuration for the audio stream specifies use of
transport capability 2 (RTP/SAVP) and either attribute capability 1
(session-level MIKEY as the keying mechanism) or 2 (SDP Security
Descriptions as the keying mechanism). Support for either of these
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attribute capabilities is mandatory. There are three potential
configurations for the video stream.
o The first configuration with configuration number 1 uses transport
capability 1 (RTP/SAVPF) with either attribute capabilities 1 and
4 (session-level MIKEY and the "rtcp-fb" attribute) or attribute
capabilities 3 and 4 (SDP security descriptions and the "rtcp-fb"
attribute). In this example, the offerer insists on not only the
keying mechanism being supported, but also that the "rtcp-fb"
attribute is supported with the value indicated. Consequently, all
the attribute capabilities are marked as mandatory in this
potential configuration.
o The second configuration with configuration number 2 uses
transport capability 2 (RTP/SAVP) and either attribute capability
1 (session-level MIKEY) or attribute capability 3 (SDP security
descriptions). Both attribute capabilities are mandatory in this
configuration.
o The third configuration with configuration number 3 uses transport
capability 3 (RTP/AVPF) and mandatory attribute capability 4 (the
"rtcp-fb" attribute).
Bob receives the SDP offer from Alice. Bob supports Secure RTP,
Secure RTP with RTCP-based feedback and the SDP Capability
Negotiation extensions. Bob also supports SDP Security Descriptions,
but not MIKEY, and hence he generates the following answer:
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
t=0 0
c=IN IP4 192.0.2.2
m=audio 54568 RTP/SAVP 98
a=rtpmap:98 AMR/8000
a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32
a=acfg:1 t=2 a=2
m=video 55468 RTP/SAVPF 31
a=rtpmap:31 H261/90000
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:AwWpVLFJhQX1cfHJSojd0RmdmcmVCspeEc3QGZiN|2^20|1:32
a=rtcp-fb:* nack
a=acfg:1 t=1 a=3,4
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
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"crypto" attribute with his own keying material, and an "acfg"
attribute identifying actual configuration 1 for the audio media
stream from the offer, using transport capability 2 (RTP/SAVP) and
attribute capability 2 (the crypto attribute from the offer). For 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
also includes a "crypto" attribute with his own keying material, and
an "acfg" attribute identifying actual configuration 1 for the video
stream from the offer, using transport capability 1 (RTP/SAVPF) and
attribute capabilities 3 (the crypto attribute from the offer) and 4
(the "rtcp-fb" attribute from the offer).
When Alice receives Bob's answer, session negotiation has completed,
however Alice nevertheless chooses to generate a new offer using the
actual configuration. This is done purely to assist any
intermediaries that may reside between Alice and Bob but do not
support the capability negotiation extensions (and hence may not
understand the negotiation that just took place):
Alice's updated offer includes only SRTP for the audio stream SRTP
with RTCP-based feedback for the video stream, and it is not using
the SDP Capability Negotiation framework (Alice could have included
the capabilities as well is she wanted to):
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
t=0 0
c=IN IP4 192.0.2.1
m=audio 59000 RTP/SAVP 98
a=rtpmap:98 AMR/8000
a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
m=video 52000 RTP/SAVPF 31
a=rtpmap:31 H261/90000
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=rtcp-fb:* nack
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
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
"crypto" attribute, which provides the SRTP keying material, with the
same value again.
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Bob receives the SDP offer from Alice, which he accepts, and then
generates an answer to Alice:
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
t=0 0
c=IN IP4 192.0.2.2
m=audio 54568 RTP/SAVP 98
a=rtpmap:98 AMR/8000
a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32
m=video 55468 RTP/SAVPF 31
a=rtpmap:31 H261/90000
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:AwWpVLFJhQX1cfHJSojd0RmdmcmVCspeEc3QGZiN|2^20|1:32
a=rtcp-fb:* nack
Bob includes the same crypto attribute as before, and the session
proceeds without change. Although Bob did not include any
capabilities in his answer, he could have done so if he wanted to.
Note that in this particular example, the answerer supported the
capability extensions defined here, however had he not, the answerer
would simply have ignored the new attributes received in step 1 and
accepted the offer to use normal RTP. In that case, the following
answer would have been generated in step 2 instead:
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
t=0 0
c=IN IP4 192.0.2.2
m=audio 54568 RTP/AVP 98
a=rtpmap:98 AMR/8000
m=video 55468 RTP/AVP 31
a=rtpmap:31 H261/90000
a=rtcp-fb:* nack
Finally, if Bob had chosen to use session-level MIKEY instead of SDP
security descriptions instead, the following answer would have been
generated:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
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t=0 0
c=IN IP4 192.0.2.1
a=key-mgmt:mikey AQEFgM0XflABAAAAAAAAAAAAAAYAyO...
m=audio 59000 RTP/AVP 98
a=rtpmap:98 AMR/8000
a=acfg:1 t=2 a=1
m=video 52000 RTP/SAVPF 31
a=rtpmap:31 H261/90000
a=rtcp-fb:* nack
a=acfg:1 t=1 a=1,4
It should be noted, that although Bob could have chosen session-level
MIKEY for one media stream, and SDP Security Descriptions for another
media stream, there are no well-defined offerer processing rules of
the resulting answer for this, and hence the offerer may incorrectly
assume use of MIKEY for both streams. To avoid this, if the answerer
chooses session-level MIKEY, then all secure RTP based media streams
SHOULD use MIKEY (this applies irrespective of whether SDP Capability
Negotiation is being used or not). Use of media-level MIKEY does not
have a similar constraint.
4.4. SRTP with Session-Level MIKEY and Media Level Security Descriptions
as Alternatives
The following example illustrates how to use the SDP Capability
Negotiation framework to negotiate use of either MIKEY or SDP
Security Descriptions, when one of them is included as part of the
actual configuration, and the other one is being selected. The
offerer (Alice) wants to establish an audio and video session. Alice
prefers to use session-level MIKEY as the key management protocol,
but supports SDP security descriptions as well.
The example is illustrated by the offer/answer exchange below, where
Alice sends an offer to Bob:
Alice Bob
| (1) Offer (RTP/[S]AVP[F], SDES|MIKEY) |
|--------------------------------------->|
| |
| (2) Answer (RTP/SAVP, SDES) |
|<---------------------------------------|
| |
Alice's offer includes an audio and a video stream. Both the audio
and the video stream offer use of secure RTP:
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v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
t=0 0
c=IN IP4 192.0.2.1
a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
m=audio 59000 RTP/SAVP 98
a=rtpmap:98 AMR/8000
a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=pcfg:1 a=-s:1
m=video 52000 RTP/SAVP 31
a=rtpmap:31 H261/90000
a=acap:2 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=pcfg:1 a=-s:2
Alice does not know whether Bob supports MIKEY or SDP Security
Descriptions. She could include attributes for both, however the
resulting procedures and potential interactions are not well-defined.
Instead, she places a session-level key-mgmt attribute for MIKEY in
the actual configuration with SDP security descriptions as an
alternative in the potential configuration. The potential
configuration for the audio stream specifies that all session level
attributes are to be deleted (i.e. the session-level "a=key-mgmt"
attribute) and that mandatory attribute capability 2 is to be used
(i.e. the crypto attribute). The potential configuration for the
video stream is similar, except it uses it's own mandatory crypto
attribute capability (2). Note how deletion of the session-level
attributes does not affect the media-level attributes.
Bob receives the SDP offer from Alice. Bob supports Secure RTP and
the SDP Capability Negotiation framework. Bob also supports both SDP
Security Descriptions and MIKEY. Since the potential configuration is
more preferred than the actual configuration, Bob (conceptually)
generates an internal potential configuration SDP that contains the
crypto attributes for the audio and video stream, but not the key-
mgmt attribute for MIKEY, thereby avoiding any ambiguity between the
two keying mechanisms. As a result, he generates the following
answer:
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
t=0 0
c=IN IP4 192.0.2.2
m=audio 54568 RTP/SAVP 98
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a=rtpmap:98 AMR/8000
a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32
a=acfg:1 a=-s:1
m=video 55468 RTP/SAVP 31
a=rtpmap:31 H261/90000
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:AwWpVLFJhQX1cfHJSojd0RmdmcmVCspeEc3QGZiN|2^20|1:32
a=acfg:1 a=-s:2
For the audio stream, Bob accepted the use of secure RTP using SDP
security descriptions. Bob therefore includes a "crypto" attribute
with his own keying material, and an "acfg" attribute identifying
actual configuration 1 for the audio media stream from the offer,
with the delete-attributes ("-s") and attribute capability 1 (the
crypto attribute from the offer). For the video stream, Bob also
accepted the use of secure RTP using SDP security descriptions. Bob
therefore includes a "crypto" attribute with his own keying material,
and an "acfg" attribute identifying actual configuration 1 for the
video stream from the offer, with the delete-attributes ("-s") and
attribute capability 2.
Below, we illustrate the offer SDP, when Bob instead offers the
"crypto" attribute as the actual configuration keying mechanism and
"key-mgmt" as the potential configuration:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
t=0 0
c=IN IP4 192.0.2.1
a=acap:1 a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
m=audio 59000 RTP/SAVP 98
a=rtpmap:98 AMR/8000
a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=acap:2 a=rtpmap:98 AMR/8000
a=pcfg:1 a=-m:1,2
m=video 52000 RTP/SAVP 31
a=rtpmap:31 H261/90000
a=acap:3 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=acap:4 a=rtpmap:31 H261/90000
a=pcfg:1 a=-m:1,4
Note how we this time need to perform delete-attributes at the media-
level instead of the session-level. When doing that, all attributes
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from the actual configuration SDP, including the rtpmaps provided,
are removed. Consequently, we had to include these rtpmaps as
capabilities as well, and then include them in the potential
configuration, thereby effectively recreating the original rtpmap
attributes in the resulting potential configuration SDP.
5. Security Considerations
The SDP Capability Negotiation Framework is defined to be used within
the context of the offer/answer model, and hence all the offer/answer
security considerations apply here as well. Similarly, the Session
Initiation Protocol (SIP) uses SDP and the offer/answer model, and
hence, when used in that context, the SIP security considerations
apply as well.
However, SDP Capability Negotiation introduces additional security
issues. Its use as a mechanism to enable alternative transport
protocol negotiation (secure and non-secure) as well as its ability
to negotiate use of more or less secure keying methods and material
warrant further security considerations. Also, the (continued)
support for receiving media before answer combined with negotiation
of alternative transport protocols (secure and non-secure) warrant
further security considerations. We discuss these issues below.
The SDP Capability Negotiation framework allows for an offered media
stream to both indicate and support various levels of security for
that media stream. Different levels of security can for example be
negotiated by use of alternative attribute capabilities each
indicating more or less secure keying methods as well as more or less
strong ciphers. Since the offerer indicates support for each of these
alternatives, he will presumably accept the answerer seemingly
selecting any of the offered alternatives. If an attacker can modify
the SDP offer, he can thereby force the negotiation of the weakest
security mechanism that the offerer is willing to accept. This may in
turn enable the attacker to compromise the security of the negotiated
media stream. Similarly, if the offerer wishes to negotiate use of a
secure media stream (e.g. secure RTP), but includes a non-secure
media stream (e.g. plain RTP) as a valid (but less preferred)
alternative, then an attacker that can modify the offered SDP will be
able to force the establishment of an insecure media stream. The
solution to both of these problems involves the use of integrity
protection over the SDP. Ideally, this integrity protection provides
end-to-end integrity protection in order to protect from any man-in-
the-middle attack; secure multiparts such as S/MIME [SMIME] provide
one such solution, however S/MIME requires use and availability of a
Public Key Infrastructure (PKI). A slightly less secure alternative
when using SIP, but generally much easier to deploy in practice
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(since it does not require a PKI), is to use SIP Identity [RFC4474];
this requires the existence of an authentication service (see
[RFC4474]). Yet another, and considerably less secure, alternative is
to use hop-by-hop security only, e.g. TLS or IPSec thereby ensuring
the integrity of the offered SDP on a hop-by-hop basis. Note however
that SIP proxies or other intermediaries processing the SIP request
at each hop are able to perform a man-in-the-middle attack by
modifying the offered SDP.
Per the normal offer/answer procedures, as soon as the offerer has
generated an offer, the offerer must be prepared to receive media in
accordance with that offer. The SDP Capability Negotiation preserves
that behavior for the actual configuration in the offer, however the
offerer has no way of knowing which configuration (actual or
potential) configuration was actually selected by the offerer, until
an answer indication is received. This opens up a new security issue
where an attacker may be able to interject media towards the offerer
until the answer is received. For example, the offerer may use plain
RTP as the actual configuration and secure RTP as an alternative
potential configuration. Even though the answerer selects secure RTP,
the offerer will not know that until he receives the answer, and
hence an attacker will be able to send media to the offerer
meanwhile. The easiest protection against such an attack is to not
offer use of the non-secure media stream in the actual configuration,
however that may in itself have undesirable side-effects: If the
answerer does not support the secure media stream and also does not
support the capability negotiation framework, then negotiation of the
media stream will fail. Alternatively, SDP security preconditions
[sprecon] can be used. This will ensure that media is not flowing
until session negotiation has completed and hence the selected
configuration is known. Use of preconditions however requires both
side to support them. If they don't, and use of them is required, the
session will fail. As a (limited) work around to this, it is
RECOMMENDED that SIP entities generate an answer SDP and send it to
the offerer as soon as possible, for example in a 183 Session
Progress message. This will limit the time during which an attacker
can send media to the offerer. Section 3.10. presents other
alternatives as well.
Additional security considerations apply to the answer SDP as well.
The actual configuration attribute tells the offerer which potential
configuration the answer was actually based on, and hence an attacker
that can either modify or remove the actual configuration attribute
in the answer can cause session failure as well as extend the time
window during which the offerer will accept incoming media that does
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-
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end integrity protection, SIP identity, or hop-by-hop protection. The
mechanism to use depends on the mechanisms supported by the offerer
as well as the acceptable security trade-offs.
As described in Section 3.1. , SDP Capability Negotiation
conceptually allows an offerer to include many different offers in a
single SDP. This can in turn cause the answerer to process a large
number of alternative potential offers, which can consume significant
memory and CPU resources. An attacker can use this amplification
feature to launch a denial of service attack against the answerer.
The answerer MUST protect itself from such attacks. As explained in
Section 3.11. , the answerer can help reduce the effects of such an
attack by first discarding all potential configurations that contain
unsupported transport protocols and/or unsupported or invalid
mandatory attribute capabilities. The answerer SHOULD also look out
for potential configurations that are designed to pass the above
test, but 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
valid session-level attribute that causes conflicts or otherwise
interferes with individual media description configurations.
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
future. If such attributes are found to exist, implementers should
explicitly protect against them.
A significant number of valid and supported potential configurations
may remain. However, since all of those contain only valid and
supported transport protocols and attributes, it is expected that
only a few of them will need to be processed on average. Still, the
answerer MUST ensure that it does not needlessly consume large
amounts of memory and CPU resources when processing those as well as
be prepared to handle the case where a large number of potential
configurations still need to be processed.
6. IANA Considerations
6.1. New SDP Attributes
The IANA is hereby requested to register the following new SDP
attributes as follows:
Attribute name: csup
Long form name: Supported capability negotiation extensions
Type of attribute: Session-level and media-level
Subject to charset: No
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Purpose: Option tags for supported SDP capability
negotiation extensions
Appropriate values: See Section 3.4.1.
Attribute name: creq
Long form name: Required capability negotiation extensions
Type of attribute: Session-level and media-level
Subject to charset: No
Purpose: Option tags for required SDP capability
negotiation extensions
Appropriate values: See Section 3.4.2.
Attribute name: acap
Long form name: Attribute capability
Type of attribute: Session-level and media-level
Subject to charset: No
Purpose: Attribute capability containing an attribute
name and associated value
Appropriate values: See Section 3.5.1.
Attribute name: tcap
Long form name: Transport Protocol Capability
Type of attribute: Session-level and media-level
Subject to charset: No
Purpose: Transport protocol capability listing one or
more transport protocols
Appropriate values: See Section 3.5.2.
Attribute name: pcfg
Long form name: Potential Configuration
Type of attribute: Media-level
Subject to charset: No
Purpose: Potential configuration for SDP capability
negotiation
Appropriate values: See Section 3.6.1.
Attribute name: acfg
Long form name: Actual configuration
Type of attribute: Media-level
Subject to charset: No
Purpose: Actual configuration for SDP capability
negotiation
Appropriate values: See Section 3.6.2.
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6.2. New SDP Capability Negotiation Option Tag Registry
The IANA is hereby requested to create a new SDP Capability
Negotiation Option Tag registry. An IANA SDP Capability Negotiation
option tag registration MUST be documented in an RFC in accordance
with the [RFC2434] Specification Required policy. The RFC MUST
provide the name of the option tag, a syntax and a semantic
specification of any new SDP attributes and any extensions to the
potential and actual configuration attributes provided in this
document. New SDP attributes that are intended to be capabilities for
use by the capability negotiation framework MUST adhere to the
guidelines provided in Section 3.5.3. Extensions to the potential and
actual configuration attributes MUST adhere to the syntax provided in
Section 3.6.1. and 3.6.2.
The option tag "cap-v0" is defined in this document and the IANA is
hereby requested to register this option tag.
6.3. New SDP Capability Negotiation Potential Configuration Parameter
Registry
The IANA is hereby requested to create a new SDP Capability
Negotiation Potential Configuration Parameter registry. An IANA SDP
Capability Negotiation potential configuration registration MUST be
documented in an RFC in accordance with the [RFC2434] Specification
Required policy. The RFC MUST define the syntax and semantics of each
new potential configuration parameter. The syntax MUST adhere to the
syntax provided for extensions in Section 3.6.1. and the semantics
MUST adhere to the semantics provided for extensions in Section
3.6.1. and 3.6.2. Associated with each registration MUST be the
encoding name for the parameter as well as a short descriptive name
for it.
The potential configuration parameters "a" for "attribute" and "t"
for "transport protocol" are defined in this document and the IANA is
hereby requested to register these.
7. Acknowledgments
This document is heavily influenced by the discussions and work done
by the SDP Capability Negotiation Design team. The following people
in particular provided useful comments and suggestions to either the
document itself or the overall direction of the solution defined in
here: Francois Audet, John Elwell, Roni Even, Robert Gilman, Cullen
Jennings, Jonathan Lennox, Matt Lepinski, Joerg Ott, Colin Perkins,
Jonathan Rosenberg, Thomas Stach, and Dan Wing.
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8. Change Log
8.1. draft-ietf-mmusic-sdp-capability-negotiation-06
o Added additional background text on terminology used, and a new
section on the negotiation model.
o Allowed for session-level attribute capabilities to contain media-
level only attributes, albeit the base framework does not define
(or allow) them to be used in a potential configuration
(extensions may change that)
o Disallowing multiple "a=tcap" attributes at the session-level
and/or on a per media description basis; at most one at the
session-level and per media description now.
o Changed the "a=pcfg" attribute to make a potential configuration
list optional in order to allow for the actual configuration to be
referenced.
o Removed the ability to delete and replace individual attributes
from the actual configuration SDP.
o Introduced the notion of mandatory and optional attribute
capabilities in a potential configuration and updated the "a=pcfg"
attribute and associated procedures accordingly.
o Specified that mandatory attribute capabilities and the transport
protocol (if any) from a potential configuration need to be
supported in order to select that potential configuration.
Offer/answer procedures updated accordingly as well.
o Noted potential interaction and synchronization issues with use of
session-level attributes and attribute capabilities and added
recommendation to avoid use of session-level attributes when
possible.
o Fixed error in "a=acfg" grammar (missing config-number) and
updated attribute definition in accordance with the "a=pcfg"
attribute changes.
o Updated text associated with processing media before answer to
allow for playing out garbage or discard until answer received.
Additional detail on alternative solutions provided as well.
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o Added recommendation to send back answer SDP as soon as possible,
when a potential configuration different from the actual
configuration has been chosen.
o Added new section on interactions with SIP option tags.
o Added new section on dealing with large number of potential
configurations.
o Added new section on SDP capability negotiation and
intermediaries.
o Updated examples in accordance with other changes and to
illustrate use of mandatory and optional attribute capabilities in
a potential configuration.
o Updated security considerations to address potential denial of
service attack caused by large number of potential configurations.
o Various editorial updates throughout.
8.2. draft-ietf-mmusic-sdp-capability-negotiation-05
o Allowed for '<type>=<value>' attributes to be listed as attribute
capabilities the attribute name only.
o Changed IP-address to conform to RFC 3330 guidelines.
o Added section on relationship to RFC 3407 and "Obsoletes: 3407" in
the front.
o Disallowed use of white space in a number of places for more
consistency with existing SDP practice
o Changed "csup" and "creq" attributes to not allow multiple
instances at the session-level and multiple instances per media
description (only one for each now)
o Changed to not require use of "creq" with base option tag ("cap-
v0").
o Relaxed restrictions on extension capabilities
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o Updated potential configuration attribute syntax and semantics. In
particular, potential configuration attributes can now replace and
delete various existing attributes in original SDP to better
control potential attribute interactions with the actual
configuration while preserving message size efficiency.
o Updated actual configuration attribute to align with the updates
to the potential configuration attributes.
o Updated offer/answer procedures to align with other changes.
o Changed recommendation for second offer/answer exchange to "MAY"
strength, unless for the cases where it is known or suspected that
it is needed.
o Updated ICE interactions to explain how the new attribute
delete/replace features can solve certain potential interactions.
o Updated rtpmap and fmtp section to allow potential configurations
to use remapped payload types in attribute capabilities for
rtpmaps and fmtp parameters.
o Added section on direction attributes.
o Added another example showing SRTP with session-level MIKEY and
SDP Security Descriptions using the attribute capability DELETE
operator.
8.3. draft-ietf-mmusic-sdp-capability-negotiation-04
The following are the major changes compared to version -03:
o Added explicit ordering rules for attributes added by potential
configurations.
o Noted that ICE interaction issues (ice-tcp specifically) may not
be as clear as originally thought.
o Added considerations on using rtpmap and fmtp attributes as
attribute capabilities.
o Added multiple transport protocol example.
o Added session-level MIKEY and media level security descriptions
example.
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8.4. draft-ietf-mmusic-sdp-capability-negotiation-03
The following are the major changes compared to version -02:
o Base option tag name changed from "v0" to "cap-v0".
o Added new section on extension capability attributes
o Firmed up offer/answer procedures.
o Added security considerations
o Added IANA considerations
8.5. draft-ietf-mmusic-sdp-capability-negotiation-02
The following are the major changes compared to version -01:
o Potential configurations are no longer allowed at the session
level
o Renamed capability attributes ("capar" to "acap" and "ctrpr" to
"tcap")
o Changed name and semantics of the initial number (now called
configuration number) in potential configuration attributes; must
now be unique and can be used as a handle
o Actual configuration attribute now includes configuration number
from the selected potential configuration attribute
o Added ABNF throughout
o Specified that answerer should include "a=csup" in case of
unsupported required extensions in offer.
o Specified use of second offer/answer exchange when answerer
selected a potential configuration
o Updated rules (and added restrictions) for referencing media- and
session-level capabilities in potential configurations (at the
media level)
o Added initial section on ICE interactions
o Added initial section on receiving media before answer
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8.6. draft-ietf-mmusic-sdp-capability-negotiation-01
The following are the major changes compared to version -00:
o Media capabilities are no longer considered a core capability and
hence have been removed. This leaves transport protocols and
attributes as the only capabilities defined by the core.
o Version attribute has been removed and an option tag to indicate
the actual version has been defined instead.
o Clarified rules for session-level and media level attributes
provided at either level as well how they can be used in potential
configurations.
o Potential configuration parameters no longer have implicit
ordering; an explicit preference indicator is now included.
o The parameter name for transport protocols in the potential and
actual configuration attributes have been changed "p" to "t".
o Clarified operator precedence within potential and actual
configuration attributes.
o Potential configurations at the session level now limited to
indicate latent capability configurations. Consequently, an actual
configuration attribute can no longer be provided at the session
level.
o Cleaned up capability and potential configuration terminology -
they are now two clearly different things.
8.7. draft-ietf-mmusic-sdp-capability-negotiation-00
Version 00 is the initial version. The solution provided in this
initial version is based on an earlier (individual submission)
version of [SDPCapNeg]. The following are the major changes compared
to that document:
o Solution no longer based on RFC 3407, but defines a set of similar
attributes (with some differences).
o Various minor changes to the previously defined attributes.
o Multiple transport capabilities can be included in a single "tcap"
attribute
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Internet-Draft SDP Capability Negotiation July 2007
o A version attribute is now included.
o Extensions to the framework are formally supported.
o Option tags and the ability to list supported and required
extensions are supported.
o A best-effort SRTP example use case has been added.
o Some terminology change throughout to more clearly indicate what
constitutes capabilities and what constitutes configurations.
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9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3264] Rosenberg, J., and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, June
2002.
[RFC3407] F. Andreasen, "Session Description Protocol (SDP) Simple
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
Specifications: ABNF", RFC 4234, October 2005.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
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
[RFC2046] Freed, N., and N. Borensteain, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
[RFC2327] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 2327, April 1998.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E. Schooler,
"SIP: Session Initiation Protocol", RFC 3261, June 2002.
[RFC3388] Camarillo, G., Eriksson, G., Holler, J., and H.
Schulzrinne, "Grouping of Media Lines in the Session
Description Protocol (SDP)", RFC 3388, December 2002.
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[RFC3551] Schulzrinne, H., and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", RFC 3551, July
2003.
[SRTP] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC3851] B. Ramsdell, "Secure/Multipurpose Internet Mail Extensions
(S/MIME) Version 3.1 Message Specification", RFC 3851, July
2004.
[RFC4091] Camarillo, G., and J. Rosenberg, The Alternative Network
Address Types (ANAT) Semantics for the Session Description
Protocol (SDP) Grouping Framework, RFC 4091, June 2005.
[AVPF] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for RTCP-Based Feedback (RTP/AVPF)",
Work in Progress, August 2004.
[I-D.jennings-sipping-multipart] Wing, D., and C. Jennings, "Session
Initiation Protocol (SIP) Offer/Answer with Multipart
Alternative", Work in Progress, March 2006.
[SAVPF] Ott, J., and E Carrara, "Extended Secure RTP Profile for
RTCP-based Feedback (RTP/SAVPF)", Work in Progress,
December 2005.
[SDES] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol Security Descriptions for Media
Streams", RFC 4568, July 2006.
[SDPng] Kutscher, D., Ott, J., and C. Bormann, "Session Description
and Capability Negotiation", Work in Progress, February
2005.
[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.
Carrara, "Key Management Extensions for Session Description
Protocol (SDP) and Real Time Streaming Protocol (RTSP)",
RFC 4567, July 2006.
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[SDPCapNegRqts] Andreasen, F. "SDP Capability Negotiation:
Requirementes and Review of Existing Work", work in
progress, December 2006.
[SDPCapNeg] Andreasen, F. "SDP Capability Negotiation", work in
progress, December 2006.
[MIKEY] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
August 2004.
[ICE] J. Rosenberg, "Interactive Connectivity Establishment
(ICE): A Methodology for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", work in progress,
January 2007.
[ICETCP] J. Rosenberg, "TCP Candidates with Interactive Connectivity
Establishment (ICE)", work in progress, October 2006.
[RFC3312] G. Camarillo, W. Marshall, and J. Rosenberg, "Integration
of Resource Management and Session Initiatio Protocol
(SIP)", RFC 3312, October 2002.
[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
Authenticated Identity Management in the Session Initiation
Protocol (SIP)", RFC 4474, August 2006.
[sprecon] Andreasen, F. and D. Wing, "Security Preconditions for
Session Description Protocol Media Streams", Work in
Progress, October 2006.
[RFC4756] A. Li, "Forward Error Correction Grouping Semantics in
Session Description Protocol", RFC 4756, November 2006.
[RFC3262] J. Rosenberg, and H. Schulzrinne, "Reliability of
Provisional Responses in Session Initiation Protocol
(SIP)", RFC 3262, June 2002.
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Author's Addresses
Flemming Andreasen
Cisco Systems
Edison, NJ
Email: fandreas@cisco.com
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