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IETF RFC 5939
Session Description Protocol (SDP) Capability Negotiation
Last modified on Tuesday, September 14th, 2010
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Internet Engineering Task Force (IETF) F. Andreasen
Request for Comments: 5939 Cisco Systems
Category: Standards Track September 2010
ISSN: 2070-1721
Session Description Protocol (SDP) Capability Negotiation
 Abstract
The Session Description Protocol (SDP) was intended to describe
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 defined in RFC 3264. SDP does 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 security 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 document defines a general SDP Capability Negotiation framework.
It also specifies 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.
Andreasen Standards Track PAGE 1 
RFC 5939 SDP Capability Negotiation September 2010
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 5939.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Andreasen Standards Track PAGE 2
RFC 5939 SDP Capability Negotiation September 2010
Table of Contents
1. Introduction ....................................................4
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. Version and Extension Indication Attributes ...............14
3.4. Capability Attributes .....................................17
3.5. Configuration Attributes ..................................22
3.6. Offer/Answer Model Extensions .............................32
3.7. Interactions with ICE .....................................45
3.8. Interactions with SIP Option Tags .........................47
3.9. Processing Media before Answer ............................48
3.10. Indicating Bandwidth Usage ...............................49
3.11. Dealing with Large Number of Potential Configurations ....50
3.12. SDP Capability Negotiation and Intermediaries ............51
3.13. Considerations for Specific Attribute Capabilities .......52
3.14. Relationship to RFC 3407 .................................54
4. Examples .......................................................54
4.1. Multiple Transport Protocols ..............................54
4.2. DTLS-SRTP or SRTP with Media-Level Security Descriptions...58
4.3. Best-Effort SRTP with Session-Level MIKEY and Media-Level
Security Descriptions .....................................61
4.4. SRTP with Session-Level MIKEY and Media-Level Security
Descriptions as Alternatives ..............................66
5. Security Considerations ........................................69
6. IANA Considerations ............................................72
6.1. New SDP Attributes ........................................72
6.2. New SDP Capability Negotiation Option Tag Registry ........73
6.3. New SDP Capability Negotiation Potential
Configuration Parameter Registry ..........................74
7. Acknowledgments ................................................74
8. References .....................................................75
8.1. Normative References ......................................75
8.2. Informative References ....................................75
Andreasen Standards Track PAGE 3
RFC 5939 SDP Capability Negotiation September 2010
1. Introduction
The Session Description Protocol (SDP) was intended to describe
multimedia sessions for the purposes of session announcement, session
invitation, and other forms of multimedia session initiation. An SDP
session description 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.
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 [RFC 3264] defined the
offer/answer model, whereby an offerer constructs an offer SDP
session description that lists the media streams, codecs, and other
SDP parameters that the offerer is willing to use. This offer
session description is sent to the answerer, which chooses from among
the media streams, codecs and other session description parameters
provided, and generates an answer session description with his
parameters, based on that choice. The answer session description 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
indications by themselves do not imply a commitment to use the
capabilities in the session.
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RFC 5939 SDP Capability Negotiation September 2010
Capabilities can, for example, be that the "RTP/SAVP" profile is
supported, that the "PCMU" (Pulse Code Modulation mu-law) codec is
supported, or that the "crypto" attribute is supported with a
particular value.
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, that 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.
An 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
[RFC 3407] 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 5888 [RFC 5888], 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
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RFC 5939 SDP Capability Negotiation September 2010
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
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 and, as of the time of publication of this document,
work on SDPng has stopped. Existing real-time multimedia
communication protocols such as SIP, Real Time Streaming Protocol
(RTSP), Megaco, and Media Gateway Control Protocol (MGCP) continue to
use SDP. However, SDP does 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) [RFC 3711], RTP with RTCP-based feedback
[RFC 4585], etc. The 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
[RFC 3551], RTP/SAVP [RFC 3711], RTP/AVPF [RFC 4585], and RTP/SAVPF
[RFC 5124]. 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.
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RFC 5939 SDP Capability Negotiation September 2010
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
Protocol (SIP) [RFC 3261]. SIP uses the offer/answer model [RFC 3264]
(which is not specific to SIP) to negotiate sessions and hence the
mechanism defined here provides 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. 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 [RFC 2119].
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. We then define new SDP attributes
for the solution and provide 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 session description (which is done with the
offer/answer model defined in [RFC 3264]), but the actual
configuration SDP session description as well as one or more
alternative SDP session descriptions, i.e., potential configurations.
The answerer must choose either the actual configuration or one of
the potential configurations, and generate an answer SDP session
description based on that. The offerer may need to perform
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RFC 5939 SDP Capability Negotiation September 2010
processing on the answer, which depends on the offer that was chosen
(actual or potential configuration). The answerer therefore informs
the offerer which configuration the answerer chose. The process can
be viewed *conceptually* as follows:
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
| SDP o3 | chosen
| (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: the actual solution uses
a single SDP session description, which contains the actual
configuration (as with existing SDP session descriptions and the
offer/answer model defined in [RFC 3264]) and several new attributes
and associated procedures, that encode the capabilities and potential
configurations. A more accurate depiction of the actual offer SDP
session description is therefore as follows:
Andreasen Standards Track PAGE 8
RFC 5939 SDP Capability Negotiation September 2010
+--------------------+
| 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 session description 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 session description
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.
Andreasen Standards Track PAGE 9
RFC 5939 SDP Capability Negotiation September 2010
3.2. Solution Overview
The solution consists of the following:
o Two new SDP 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 session description in order to do capability
negotiation.
o Two new SDP 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 and its associated value (if any) 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 SDP attributes to negotiate configurations as follows:
o A new attribute ("a=pcfg") that lists potential configurations
supported. This is done by reference to the capabilities from
the SDP session description in question. Extension
capabilities can be defined and referenced in the potential
configurations. Alternative potential configurations have an
explicit ordering associated with them. Also, potential
configurations are by default preferred over the actual
configuration included in the "m=" line and its associated
parameters.
This preference order was chosen to provide maximum backwards
compatibility for the capability negotiation framework and the
possible values offered for a session. For example, an entity
that wants to establish a Secure RTP media stream but is
willing to accept a plain RTP media stream (assumed to be the
least common denominator for most endpoints), can offer plain
RTP in the actual configuration and use the capability
negotiation extensions to indicate the preference for Secure
RTP. Entities that do not support the capability negotiation
extensions or Secure RTP will then default to plain RTP.
Andreasen Standards Track PAGE 10
RFC 5939 SDP Capability Negotiation September 2010
o A new attribute ("a=acfg") to be used in an answer SDP session
description. The attribute identifies a potential
configuration from an offer SDP session description that was
used as an actual configuration to form the answer SDP session
description. Extension capabilities can be included as well.
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 only at the media
level, where they constitute alternative offers that may be
accepted by the answerer instead of the actual configuration(s)
included in the "m=" line(s) and associated parameters. The
mechanisms defined in this document enable potential
configurations to change the transport protocol, add new
attributes, as well as remove all existing attributes from the
actual configuration. 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) |
|<---------------------------------|
| |
| (3) Offer (SRTP) |
|--------------------------------->|
| |
| (4) Answer (SRTP) |
|<---------------------------------|
| |
Andreasen Standards Track PAGE 11
RFC 5939 SDP Capability Negotiation September 2010
Alice's offer includes RTP and SRTP as alternatives, where 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 crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
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 capability attribute ("a=tcap")
indicates 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 material for SRTP using SDP security descriptions [RFC 4568].
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 preferred
over the actual configuration included in the offer SDP session
description, and hence Alice is expressing a preference for using
Secure RTP.
Bob receives the SDP session description 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 session
description:
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
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RFC 5939 SDP Capability Negotiation September 2010
Bob includes the "a=acfg" attribute in the answer to inform Alice
that he based his answer on an offer using potential configuration 1
with transport protocol capability 1 and attribute capability 1 from
the offer SDP session description (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).
When Alice receives Bob's answer, session negotiation has completed;
however, Alice nevertheless generates a new offer using the
negotiated configuration as 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 if she wanted):
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
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 session description 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.
Andreasen Standards Track PAGE 13
RFC 5939 SDP Capability Negotiation September 2010
Note that in this particular example, the answerer supported the
capability negotiation extensions defined here. Had he not, he 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
3.3. Version and Extension Indication Attributes
In this section, we present the new attributes associated with
indicating the SDP Capability Negotiation extensions supported and
required.
3.3.1. 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 6.2.
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 session description. The attribute can be provided
at the session level and the media level, and it is defined as
follows:
a=csup: <option-tag-list>
RFC 4566, Section 9, provides the ABNF [RFC 5234] 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 [RFC 4566]
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
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RFC 5939 SDP Capability Negotiation September 2010
indicate support for the SDP Capability Negotiation framework
specified in this document. Please note that white space is not
allowed in this rule.
The following examples illustrate use of the "a=csup" attribute with
the "cap-v0" option tag 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 session description. When provided at the media level, it
applies only to the media description in question (option tags
provided at the session level apply as well). There MUST NOT be more
than one "a=csup" attribute at the session level and 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 session
description, 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 3.3.2) 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 3.5.1.
Use of the base option tag may still be useful in some scenarios,
e.g., when using SIP OPTIONS [RFC 3261] or generating an answer to an
offer that did not use the SDP Capability Negotiation framework.
3.3.2. Required Capability Negotiation Extensions Attribute
The Required Capability Negotiation Extensions attribute ("a=creq")
contains a comma-separated list of option tags (see Section 3.3.1)
specifying the SDP Capability Negotiation extensions that MUST be
supported by the entity receiving the SDP session description, in
order for that entity to properly process the SDP Capability
Negotiation attributes and associated procedures. There is no need
to include the base option tag ("cap-v0") with the "creq" attribute,
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since any entity that supports the "creq" attribute in the first
place also supports the base option tag. Still, it is permissible to
do so.
Such functionality may be important if a future version of the
Capability Negotiation framework were not backwards compatible.
The attribute can be provided at the session level and the media
level, and it 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 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 session description. When provided at the media level, it
applies only to the media description in question (required option
tags provided at the session level apply as well). There MUST NOT be
more than one "a=creq" attribute at the session level and one
"a=creq" attribute at the media level (one per media description in
the latter case).
When an entity generates an SDP session description and it requires
the recipient of that SDP session description to support one or more
SDP Capability Negotiation extensions (except for the base) at the
session or media level 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. If support for an extension is needed only in
one or more specific potential configurations, the potential
configuration provides a way to indicate that instead (see Section
3.5.1). Support for the basic negotiation framework is implied by
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the presence of an "a=pcfg" attribute (see Section 3.5.1) and hence
it is not required to include the "a=creq" attribute with the base
option tag ("cap-v0").
A recipient that receives an SDP session description 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; instead the recipient MUST proceed as if the SDP
Capability Negotiation attributes were not included in the first
place, i.e., the capability negotiation attributes are ignored. In
that case, if the SDP session description recipient is an SDP
answerer [RFC 3264], the recipient SHOULD include a "csup" attribute
in the resulting SDP session description answer 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
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.
3.4. Capability Attributes
In this section, we present the new attributes associated with
indicating the capabilities for use by the SDP Capability
Negotiation.
3.4.1. 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 "<attribute>" or "<attribute>:<value>" form,
i.e., excluding the "a=" part (see [RFC 4566]). The attribute can be
provided at the session level and the media level.
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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*10(DIGIT) ;defined in [RFC 5234]
att-par = attribute ;defined in [RFC 4566]
Note that white space is not permitted before the att-cap-num.
When the attribute capability contains a session-level attribute,
that "acap" attribute can only be provided at the session level.
Conversely, media-level attributes can be provided 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.
Implementations that conform only to the base framework MUST NOT
generate media-level attribute capabilities at the session level;
however, extensions may change this (see, e.g., [SDPMedCap] for one
such extension) and hence all implementations MUST still be prepared
to receive such capabilities (see Section 3.6.2 for processing
rules).
Each occurrence of the "acap" attribute in the entire session
description MUST use a different value of <att-cap-num>. Consecutive
numbering of the <att-cap-num> values is not required.
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 ptime:20
a=acap:2 ptime:30
a=acap:3 key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyONQ6gAA
AAAGEEoo2pee4hp2UaDX8ZE22YwKAAAPZG9uYWxkQGR1Y2suY29tAQAAAAAAAQAk0
JKpgaVkDaawi9whVBtBt0KZ14ymNuu62+Nv3ozPLygwK/GbAV9iemnGUIZ19fWQUO
SrzKTAv9zV
a=acap:4 crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
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The first two attribute capabilities provide attribute values for the
ptime attribute. The third provides SRTP parameters by using
Multimedia Internet KEYing (MIKEY) [RFC 3830] with the "key-mgmt"
attribute [RFC 4567]. The fourth provides SRTP parameters by use of
security descriptions with the "crypto" attribute [RFC 4568]. 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 session descriptions.
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 only attributes (the "cpar" attribute defined in RFC 3407
supports bandwidth information as well). The "acap" attribute
also is not automatically associated with any particular
capabilities. See Section 3.14 for the relationship to RFC 3407.
Attribute capabilities MUST NOT embed any capability negotiation
parameters. This restriction applies to all the capability
negotiation parameters defined in this document ("csup", "creq",
"acap", "tcap", "pcfg", and "acfg") as well as any capability
negotiation extensions defined. The following examples are thus
invalid attribute capabilities and MUST NOT be used:
a=acap:1 acap:2 foo:a ;Not allowed to embed "acap"
a=acap:2 a=pcfg:1 t=1 a=1 ;Not allowed to embed "pcfg"
The reason for this restriction is to avoid overly complex processing
rules resulting from the expansion of such capabilities into
potential configurations (see Section 3.6.2 for further details).
3.4.2. 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 attribute can be
provided at the session level and the media level.
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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*10(DIGIT) ;defined in [RFC 5234]
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 MUST NOT be more than one "a=tcap" attribute at
the session level and one at the media level (one per media
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. There MUST NOT be any capability
number overlap between different "tcap" attributes in the entire SDP
session description. 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. Consecutive numbering of the <trpr-cap-num>
values in different "tcap" attributes is not required.
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
a=tcap:5 UDP/TLS/RTP/SAVP
The first one provides a capability for the "RTP/AVP" profile defined
in [RFC 3551] and the second one provides a capability for the RTP
with RTCP-based feedback profile defined in [RFC 4585]. The third one
provides capabilities for the "RTP/SAVP" (transport capability number
3) and "RTP/SAVPF" profiles (transport protocol capability number 4).
The last one provides capabilities for "UDP/TLS/RTP/SAVP", i.e.,
DTLS-SRTP [RFC 5764] (transport capability number 5).
The "tcap" attribute by itself can only specify transport protocols
as defined by <proto> in [RFC 4566]; however, full specification of a
media stream requires further qualification of the transport protocol
by one or more media format descriptions, which themselves often
depend on the transport protocol. As an example, [RFC 3551] defines
the "RTP/AVP" transport for use with audio and video codecs (media
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formats), whereas [RFC 4145] defines the "TCP" transport, which, for
example, may be used to negotiate T.38 fax ("image/t38"), etc. In a
non-SDP context, some media formats could be viewed as transports
themselves (e.g., T.38); however, in the context of SDP and SDP
Capability Negotiation, they are not. If capability negotiation is
required for such media formats, they MUST all either be valid under
the transport protocol indicated in the "m=" line included for the
media stream description, or a suitable extension must be used, e.g.,
SDP Media Capabilities [SDPMedCap].
The ability to use a particular transport protocol is inherently
implied by including it in the "m=" line, regardless of whether or
not it is provided in a "tcap" attribute. However, if a potential
configuration needs to reference that transport protocol as a
capability, the transport protocol MUST be included explicitly in a
"tcap" attribute.
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., middleboxes) 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; however, the potential
configuration preference (see Section 3.5.1) may not reflect that
of the intermediary (which some may view as a feature).
Note that a transport protocol capability may be provided,
irrespective of whether or not it is referenced in a potential
configuration (just like any other capability).
3.4.3. Extension Capability Attributes
The SDP Capability Negotiation framework allows for new types of
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 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 Sections 3.5.1 and 3.5.2). The
definition of such extension capability attributes MUST also state
whether they can be applied at the session level, media level, or
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RFC 5939 SDP Capability Negotiation September 2010
both. Note that extensions can have option tags defined for them,
and option tags MUST be registered with the IANA in accordance with
the procedures specified in Section 6.2.
Extension capabilities SHOULD NOT embed any capability negotiation
parameters. This applies to all the capability negotiation
parameters defined in this document as well as any extensions
defined. The reason for this restriction is to avoid overly complex
processing rules resulting from the expansion of such capabilities
into potential configurations (see Section 3.6.2 for further
details). If an extension does not follow the above "SHOULD NOT"
recommendation, the extension MUST provide a careful analysis of why
such behavior is both necessary and safe.
3.5. Configuration Attributes
3.5.1. 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 attribute can be provided only at the media level.
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*10(DIGIT) ;defined in [RFC 5234]
pot-cfg-list = pot-config *(1*WSP pot-config)
pot-config = attribute-config-list /
transport-protocol-config-list /
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 only at the
media level 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 only media-level scope). The configuration number also
indicates the relative preference of potential configurations; lower
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numbers are preferred over higher numbers. Consecutive numbering of
the configuration numbers in different "pcfg" attributes in a media
description is not required.
A potential configuration list is normally provided after the
configuration number. When the potential configuration list is
omitted, the potential configuration equals the actual configuration.
The potential configuration list contains one or more of attribute,
transport, and extension configuration lists. A potential
configuration may for example include attribute capabilities and
transport capabilities, transport capabilities only, or some other
combination of capabilities. If transport capabilities are not
included in a potential configuration, the default transport for that
media stream is used.
The potential configuration lists generally reference one or more
capabilities (extension configuration lists MAY use a different
format). Those capabilities are (conceptually) used to construct a
new internal version of the SDP session description by use of purely
syntactic add and (possibly) delete operations on the original SDP
session description (actual configuration). This provides an
alternative potential configuration SDP session description that can
be used by conventional SDP and offer/answer procedures if selected.
This document defines attribute configuration lists and transport
protocol configuration lists. Each of these MUST NOT be present more
than once in a particular potential configuration attribute.
Attribute capabilities referenced by the attribute configuration list
(if included) are added to the actual configuration, whereas a
transport capability referenced by the transport protocol
configuration list (if included) replaces the default transport
protocol from the actual configuration. Extension configuration
lists can be included as well. There can be more than one extension
configuration list; however, each particular extension MUST NOT be
present more than once in a given "a=pcfg" attribute. Together, the
various configuration lists define a potential configuration.
There can be multiple potential configurations in a media
description. Each of these indicates not only a willingness, but in
fact a desire to use the potential configuration.
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The example SDP session description 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 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 transport protocol configuration
list that references transport capability 1 ("RTP/SAVP") and an
attribute configuration list that references attribute capability 1
("a=crypto:..."). Potential configuration 2 contains a transport
protocol configuration list that references transport capability 2
("RTP/SAVPF") and an attribute configuration list that references
attribute capability 1 ("a=crypto:...").
Attribute capabilities are used in a potential configuration by use
of the attribute-config-list parameter, which is defined by the
following ABNF:
attribute-config-list = "a=" delete-attributes
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*10(DIGIT) ;defined in [RFC 5234]
BAR = "|"
DELETE = "-"
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Note that white space is not permitted within the attribute-config-
list rule.
Each attribute configuration list can optionally begin with
instructions for how to handle attributes that are part of the actual
configuration SDP session description (i.e., the "a=" lines present
in the original SDP session description). By default, such
attributes will remain as part of the potential configuration in
question. However, if delete-attributes indicates "-m", then all
attribute lines within the media description in question will be
deleted in the resulting potential configuration SDP session
description (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 (if included). 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.
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 square 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 attribute configuration list
o "-m" indicates to delete all attributes from the media description
of the actual configuration
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RFC 5939 SDP Capability Negotiation September 2010
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, then 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 session description.
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 session description (e.g., an answerer receiving
this in an offer) to use this potential configuration, exactly one of
the alternative 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.
Transport protocol configuration lists are included in a potential
configuration by use of the transport-protocol-config-list parameter,
which is defined by the following ABNF:
transport-protocol-config-list =
"t=" trpr-cap-num *(BAR trpr-cap-num)
trpr-cap-num = 1*10(DIGIT) ; defined in [RFC 5234]
Note that white space is not permitted within this rule.
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 transport protocol capabilities are separated by a
vertical bar ("|"). The alternatives are ordered by preference with
the most preferred listed first. If there are no transport protocol
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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 session
description (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 capability negotiation implications of that.
Extension capabilities can be included in a potential configuration
as well by use of extension configuration lists. Extension
configuration lists MUST adhere to the following ABNF:
extension-config-list = ["+"] ext-cap-name "=" ext-cap-list
ext-cap-name = 1*(ALPHA / DIGIT)
ext-cap-list = 1*VCHAR ; defined in [RFC 5234]
Note that white space is not permitted within this rule.
The ext-cap-name refers to the name of the 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
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, unless they are prefixed
with the plus ("+") sign, which indicates that the extension is
mandatory and MUST be supported in order to use that potential
configuration.
The "creq" attribute and its associated rules can be used to
ensure that required extensions are supported in the first place.
Extension configuration lists define new potential configuration
parameters and hence they MUST be registered with IANA per the
procedures defined in Section 6.3.
Potential configuration attributes can be provided only at the media
level; 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:
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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 provided with that value at the session
level in the first place. As a result, 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 applies to
the entire resulting potential configuration SDP session description.
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, such examples seem to be rare (at least with
the SDP attributes that had been defined at time of publication of
this document).
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 [RFC 5888] 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 Forward Error Correction (FEC) grouping semantics
[RFC 4756] is one example where this in theory could cause
problems, however in practice, it is unclear that there is a
significant problem with the grouping semantics that had been
defined at time of publication of this document.
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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 SDP attributes defined at time of publication of
this document, it does not seem to be a significant problem, and
hence the base 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 session description SHOULD NOT assume that well-
defined operation will occur at the receiving entity. This implies
that mechanisms that might have such interactions cannot be used in
security critical contexts.
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 RTP/SAVP RTP/SAVPF
a=pcfg:1 t=4|3 a=1
a=pcfg:8 t=1|2
We have two potential configuration attributes listed here. The
first one (and most preferred, since its configuration number is "1")
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
although we have a single potential configuration attribute and
associated handle, we have two potential configurations.
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RFC 5939 SDP Capability Negotiation September 2010
The second potential configuration attribute indicates that the
RTP/AVPF or RTP/AVP profiles can be used, with RTP/AVPF being the
preferred one. This non-secure RTP alternative is the less preferred
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 session description above.
3.5.2. Actual Configuration Attribute
The actual configuration attribute identifies which of the potential
configurations from an offer SDP session description was selected and
used as the actual configuration to generate an answer SDP session
description. 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 attribute configuration MUST include the delete-
attributes and the known and supported parameters from the
selected alternative mo-att-cap-list (i.e., containing all
mandatory and all known and supported optional capability numbers
from the potential configuration). If delete-attributes were not
included in the potential configuration, they will of course not
be present here either.
o A selected 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.
o When a configuration list contains alternatives (separated by
"|"), the selected configuration only MUST be provided.
Note that the selected configuration number and all selected
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
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>]
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RFC 5939 SDP Capability Negotiation September 2010
where <config-number> is an integer between 1 and 2^31-1 (both
included) that refers to the selected potential configuration. The
attribute can be provided only at the media level.
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 3.5.1.
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 3.5.1.
sel-transport-protocol-config =
"t=" trpr-cap-num ; defined in Section 3.5.1.
sel-extension-config =
ext-cap-name "=" 1*VCHAR ; defined in Section 3.5.1.
Note that white space is not permitted before the config-number.
The actual configuration ("a=acfg") attribute can be provided only at
the media level. 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):
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.
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RFC 5939 SDP Capability Negotiation September 2010
3.6. Offer/Answer Model Extensions
In this section, we define extensions to the offer/answer model
defined in [RFC 3264] to allow for potential configurations to be
included in an offer, 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 procedures defined in the following subsections apply to both
unicast and multicast streams.
3.6.1. 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:
o Zero or more attribute capability attributes. There MUST be an
attribute capability attribute ("a=acap") as defined in Section
3.4.1 for each attribute name and associated value (if any) that
needs to be indicated as a capability in the offer. Attribute
capabilities may be included irrespective of whether or not they
are referenced by a potential configuration.
Session-level attributes and associated values MUST be provided in
attribute capabilities only at the session level, whereas media-
level attributes and associated values can be provided in
attribute capabilities at either the media level or session level.
Attributes that are allowed at either the session or media level
can be provided in attribute capabilities at either level.
o Zero or more transport protocol capability attributes. There MUST
be transport protocol capabilities as defined in Section 3.4.2
with values for each transport protocol that needs to be indicated
as a capability in the offer.
Transport protocol capabilities may be included irrespective of
whether or not they are referenced by a potential configuration.
Transport protocols that apply to multiple media descriptions
SHOULD be provided as transport protocol capabilities at the
session level whereas transport protocols that apply only to a
specific media description ("m=" line), SHOULD be provided as
transport protocol capabilities 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.
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RFC 5939 SDP Capability Negotiation September 2010
o Zero or more extension capability attributes. There MUST be one
or more extension capability attributes (as outlined in Section
3.4.3) for each extension capability that is referenced by a
potential configuration. Extension capability attributes that are
not referenced by a potential configuration can be provided as
well.
o Zero or more potential configuration attributes. There MUST be
one or more potential configuration attributes ("a=pcfg"), as
defined in Section 3.5.1, in each media description where
alternative potential configurations are to be negotiated. Each
potential configuration attribute MUST adhere to the rules
provided in Section 3.5.1 and the additional rules provided below.
If the offerer requires support for one or more extensions (besides
the base protocol defined here), then the offerer MUST include one or
more "a=creq" attributes as follows:
o If support for one or more capability negotiation extensions is
required 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 support for one or more
capability negotiation extensions not listed at the session level,
a single "creq" attribute containing all the required extensions
for that media description MUST be included within the media
description (in accordance with Section 3.3.2).
Note that extensions that only need to be supported by a particular
potential configuration can use the "mandatory" extension prefix
("+") within the potential configuration (see Section 3.5.1).
The offerer SHOULD furthermore include the following:
o A supported capability negotiation extension attribute ("a=csup")
at the session level and/or media level as defined in Section
3.3.2 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 only that particular
media description.
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RFC 5939 SDP Capability Negotiation September 2010
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") in 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
zero, one or more potential configuration lists, as described in
Section 3.5.1. Each potential configuration list MUST refer to
capabilities that are provided 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 not reference any session-
level attribute capabilities that contain media-level-only
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
3.5.1.
The current actual configuration is included in the "m=" line (as
defined by [RFC 3264]) and any associated parameters for the media
description (e.g., attribute ("a=") and bandwidth ("b=") lines).
Note that the actual configuration is by default the least-preferred
configuration, and hence the answerer will seek to negotiate use of
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 [RFC 3264], 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 only for the actual
configurations provided in the offer: Media received by the offerer
Andreasen Standards Track PAGE 34
RFC 5939 SDP Capability Negotiation September 2010
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 session description 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). In either case, please note that this document does not
place any requirements on the offerer to process and play media
before answer. For further details, please refer to Section 3.9.
3.6.2. 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
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 [RFC 3264].
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
Andreasen Standards Track PAGE 35
RFC 5939 SDP Capability Negotiation September 2010
provided for that particular media description. Instead, the
offer/answer procedures for that media description MUST continue
as per [RFC 3264] (SDP Capability Negotiation is still performed
for other media descriptions in the SDP session description).
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 perform capability negotiation by using
the most preferred potential configuration that is valid to the
answerer, subject to any local policies. A potential configuration
is valid to the answerer if:
1. It is in accordance with the syntax and semantics provided in
Section 3.5.1.
2. It contains a configuration number that is unique within that
media description.
3. All attribute capabilities referenced by the potential
configuration are valid themselves (as defined in Section 3.4.1)
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. Note that the answerer can only determine this for
attributes supported by the answerer. If an attribute is not
supported, it will simply be ignored by the answerer and hence
will not trigger an "invalid" potential configuration.
4. All transport protocol capabilities referenced by the potential
configuration are valid themselves (as defined in Section 3.4.2)
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
Andreasen Standards Track PAGE 36
RFC 5939 SDP Capability Negotiation September 2010
capabilities MUST be ignored, unless they are prefixed with the
plus ("+") sign, which indicates that the extension MUST be
supported in order to use that potential configuration. If the
extension is not supported, that potential configuration is not
valid to the answerer.
The most preferred valid potential configuration in a media
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 as follows. First, all capability negotiation
parameters from the offer SDP session description are removed,
thereby yielding an offer SDP session description with the actual
configuration as if SDP Capability Negotiation was not done in the
first place. Secondly, this actual configuration SDP session
description is modified as follows for each media stream offered,
based on the capability negotiation parameters included originally:
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") or media and session indication ("-ms"),
then all session-level attributes from the actual configuration
SDP session description MUST be deleted in the resulting potential
configuration SDP session description in accordance with the
procedures in Section 3.5.1. If attribute capabilities are
present with a delete-attributes media indication ("-m") or media
and session indication ("-ms"), then all attributes from the
actual configuration SDP session description 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 session description. All such added session-
level attributes MUST be listed before the session-level
attributes that were initially present in the SDP session
description. Furthermore, the added session-level attributes MUST
be added in the order they were provided in the potential
configuration (see also Section 3.5.1).
This allows for attributes with implicit preference ordering to
be added in the desired order; the "crypto" attribute [RFC 4568]
is one such example.
Andreasen Standards Track PAGE 37
RFC 5939 SDP Capability Negotiation September 2010
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 session description 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 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 3.5.1).
o If a supported extension capability is included, then it MUST be
processed in accordance with the rules provided for that
particular extension capability.
The above steps MUST be performed exactly once per potential
configuration, i.e., there MUST NOT be any recursive processing of
any additional capability negotiation parameters that may (illegally)
have been nested inside capabilities themselves.
As an example of this, consider the (illegal) attribute capability
a=acap:1 acap:2 foo:a
The resulting potential configuration SDP session description will,
after the above processing has been done, contain the attribute
capability
a=acap:2 foo:a
However, since we do not perform any recursive processing of
capability negotiation parameters, this second attribute capability
parameter will not be processed by the offer/answer procedure.
Instead, it will simply appear as a (useless) attribute in the SDP
session description that will be ignored by further processing.
Note that a transport protocol from the potential configuration
replaces the transport protocol in the actual configuration, but 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 session description, or worse, ambiguous or
potentially even illegal attributes. Use of delete-attributes for
the session- and/or media-level attributes MUST be done to avoid such
scenarios. Nevertheless, it is RECOMMENDED that implementations
ignore meaningless attributes that may result from potential
configurations.
Andreasen Standards Track PAGE 38
RFC 5939 SDP Capability Negotiation September 2010
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 delete-attributes to remove the
"crypto" attribute (but will then need to provide attribute
capabilities to reconstruct the SDP session description with the
necessary attributes deleted, e.g., rtpmaps).
Also note, that while it is permissible to include media-level
attribute capabilities at the session level, the base SDP Capability
Negotiation framework defined here does not define any procedures for
use of them, i.e., the answerer effectively ignores them.
Please refer to Section 3.6.2.1 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), the transport
protocol capability (if any) from the potential configuration, and
all mandatory extension capabilities from the potential configuration
(if any). If he does not, the answerer MUST proceed to the second
most preferred valid potential configuration for the media
description, etc.
o In the case of attribute capabilities, support implies that the
attribute name contained in the capability is supported and it can
(and will) be negotiated successfully in the offer/answer exchange
with the value provided. This does not necessarily imply that the
value provided is supported in its entirety. For example, the
"a=fmtp" parameter is often provided with one or more values in a
list, where the offerer and answerer negotiate use of some subset
of the values provided. Other attributes may include mandatory
and optional parts to their values; support for the mandatory part
is all that is required here.
A side effect of the above rule is that whenever an "fmtp" or
"rtpmap" parameter is provided as a mandatory attribute
capability, the corresponding media format (codec) must be
supported and use of it negotiated successfully. If this is
not the offerer's intent, the corresponding attribute
capabilities must be listed as optional instead.
Andreasen Standards Track PAGE 39
RFC 5939 SDP Capability Negotiation September 2010
o In the case of transport protocol capabilities, support implies
that the transport protocol contained in the capability is
supported and the transport protocol can (and will) be negotiated
successfully in the offer/answer exchange.
o In the case of extension capabilities, the extension MUST define
the rules for when the extension capability is considered
supported and those rules MUST be satisfied.
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 from another
media description in the offered SDP session description.
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 session description. 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
potential 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 virtual answer SDP session
description based on the selected potential configuration SDP session
description, as "seen" by the answerer using normal offer/answer
rules (see Section 3.6.2.1 for examples). The actual answer SDP
session description is formed from the virtual answer SDP session
description as follows: 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. The "a=acfg" attribute MUST identify 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, the selected
alternatives only 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. In the case
Andreasen Standards Track PAGE 40
RFC 5939 SDP Capability Negotiation September 2010
of attribute capabilities, only the known and supported capabilities
are included; unknown or unsupported attribute capabilities MUST NOT
be included.
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 only
particular media descriptions, then a supported capability
negotiation attribute with those option tags SHOULD be included
within each relevant media description. The required capability
negotiation attribute ("a=creq") MUST NOT be used in an answer.
The offerer's originally provided actual configuration is contained
in the offer 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 3.9. for additional
considerations and possible alternative solutions outside the base
SDP Capability Negotiation framework.
If the answerer selected a potential configuration instead of the
actual configuration, then it is RECOMMENDED that the answerer send
back an answer SDP session description 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 [RFC 3261] without any extensions,
this implies that if the offer was received in an INVITE message,
then the answer SDP session description should be provided in the
first non-100 provisional response sent back (per RFC 3261, the
answer would need to be repeated in the 200 response as well, unless
a relevant extension such as [RFC 3262] is being used).
Andreasen Standards Track PAGE 41
RFC 5939 SDP Capability Negotiation September 2010
3.6.2.1. Example Views of Potential Configurations
The following examples illustrate how the answerer may conceptually
"see" a potential configuration. Consider the following offered SDP
session description:
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=acap:1 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 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 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 session description 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
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RFC 5939 SDP Capability Negotiation September 2010
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:
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
Andreasen Standards Track PAGE 43
RFC 5939 SDP Capability Negotiation September 2010
3.6.3. 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
that media description for the presence of a valid actual
configuration attribute ("a=acfg"). An actual configuration
attribute is valid if:
o it refers to a potential configuration that was present in the
corresponding offer, and
o it contains the actual parameters that were used from that
potential configuration; if the potential configuration included
alternatives, the selected alternatives only MUST be included.
Note that the answer will include only parameters and attribute
capabilities that are known and supported by the answerer, as
described in Section 3.6.2.
If a valid actual configuration attribute is not present in a media
description, then the offerer MUST process the answer SDP session
description for that media stream per the normal offer/answer rules
defined in [RFC 3264]. However, if a valid 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 supported
attribute capabilities and the transport capabilities referenced
by the potential configuration selected, where the attribute
capabilities have any associated delete-attributes included.
Extension capabilities supported by the answerer are included as
well.
o The offerer MUST now process the answer in accordance with the
rules in [RFC 3264], except that it must be done as if the offer
consisted of the selected potential configuration instead of the
original 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 this derived answer is not a valid answer
to the potential configuration offer selected by the answerer, the
offerer MUST instead continue further processing as it would have
for a regular offer/answer exchange, where the answer received
does not adhere to the rules of [RFC 3264].
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RFC 5939 SDP Capability Negotiation September 2010
If the offer/answer exchange was successful, and if the answerer
selected one of the potential configurations from the offer as the
actual configuration, and the selected potential configuration
differs from the actual configuration in the offer (the "m=", "a=",
etc., lines), then the offerer SHOULD initiate another offer/answer
exchange. This second offer/answer exchange will not modify the
session in any way; however, it will help intermediaries (e.g.,
middleboxes), which look at the SDP session description but do not
support the capability negotiation extensions, understand the details
of the media stream(s) that were actually negotiated. This new offer
MUST 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, bandwidth parameters, etc.
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 [RFC 4566]). 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.
3.6.4. Modifying the Session
Capabilities and potential configurations may be included in
subsequent offers as defined in [RFC 3264], 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 3.6.3 for doing a
second offer/answer exchange using that potential configuration as
the actual configuration apply.
3.7. Interactions with ICE
Interactive Connectivity Establishment (ICE) [RFC 5245] provides a
mechanism for verifying connectivity between two endpoints by sending
Session Traversal Utilities for NAT (STUN) messages directly between
the media endpoints. The basic ICE specification [RFC 5245] is only
defined to support UDP-based connectivity; 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.
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RFC 5939 SDP Capability Negotiation September 2010
When using ICE, it is thus possible that the transport protocol that
will be used differs from what is specified in the "m=" line. Since
both ICE and SDP Capability Negotiation may 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:
1) A particular media stream can run over different transport
protocols (e.g., UDP, TCP, or TCP/TLS), and the intent is simply
to use the one that works (in the preference order specified).
2) A particular media stream can run over different transport
protocols (e.g., UDP, TCP, or TCP/TLS) and the intent is to have
the negotiation process decide which one to use (e.g., T.38 over
TCP or UDP).
In scenario 1, there should be ICE "a=candidate" attributes for UDP,
TCP, etc., but otherwise nothing special in the potential
configuration attributes to indicate the desire to use different
transport protocols (e.g., UDP, or TCP). The ICE procedures
essentially cover the capability negotiation required (by having the
answerer select something it supports and then use of trial and error
connectivity checks).
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 only one
transport protocol (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 be used for
a particular configuration, yet the actual configuration may want to
use all of the ICE candidates. 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 for a particular configuration.
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RFC 5939 SDP Capability Negotiation September 2010
SDP Capability Negotiation recommends use of a second offer/answer
exchange when the negotiated actual configuration was one of the
potential configurations from the offer (see Section 3.6.3).
Similarly, ICE requires use of a second offer/answer exchange if the
chosen candidate is not the same as the one in the m/c-line from the
offer. When ICE and capability negotiation are used at the same
time, the two secondary offer/answer exchanges SHOULD be combined to
a single one.
3.8. Interactions with SIP Option Tags
SIP [RFC 3261] 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 [RFC 4566] 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 [RFC 3312]. 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
such extensions may not produce the desired result. For example, if
one potential configuration requires SIP preconditions 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 [RFC 3312] 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.
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RFC 5939 SDP Capability Negotiation September 2010
3.9. Processing Media before Answer
The offer/answer model [RFC 3264] 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, the offerer may decide not to process any
media received before the answer is received. This may lead to
clipping. Consequently, the SDP Capability Negotiation framework
recommends sending back an answer SDP session description 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 [RFC 3312] for capability negotiation (or by using 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
[RFC 5245]. 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.
Please note that, the above considerations notwithstanding, this
document does not place any requirements on the offerer to process
and play media before answer; it merely provides recommendations for
how to ensure that media sent by the answerer and received by the
offerer prior to receiving the answer can in fact be rendered by the
offerer.
In some use cases, a three-way handshake is not needed. An example
is when the offerer does not need information from the answer, such
as keying material in the SDP session description, in order to
process incoming media. 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
Andreasen Standards Track PAGE 48
RFC 5939 SDP Capability Negotiation September 2010
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.
3.10. Indicating Bandwidth Usage
The amount of bandwidth used for a particular media stream depends on
the negotiated codecs, transport protocol and other parameters. For
example the use of Secure RTP [RFC 3711] with integrity protection
requires more bandwidth than plain RTP [RFC 3551]. SDP defines the
bandwidth ("b=") parameter to indicate the proposed bandwidth for the
session or media stream.
In SDP, as defined by [RFC 4566], each media description contains one
transport protocol and one or more codecs. When specifying the
proposed bandwidth, the worst case scenario must be taken into
account, i.e., use of the highest bandwidth codec provided, the
transport protocol indicated, and the worst case (bandwidth-wise)
parameters that can be negotiated (e.g., a 32-bit Hashed Message
Authentication Code (HMAC) or an 80-bit HMAC).
The base SDP Capability Negotiation framework does not provide a way
to negotiate bandwidth parameters. The issue thus remains; however,
it is potentially worse than with SDP per [RFC 4566], since it is
easier to negotiate additional codecs, and furthermore possible to
negotiate different transport protocols. The recommended approach
for addressing this is the same as for plain SDP; the worst case (now
including potential configurations) needs to be taken into account
when specifying the bandwidth parameters in the actual configuration.
This can make the bandwidth value less accurate than in SDP per
[RFC 4566] (due to potential greater variability in the potential
configuration bandwidth use). Extensions can be defined to address
this shortcoming.
Note, that when using RTP retransmission [RFC 4588] with the RTCP-
based feedback profile [RFC 4585] (RTP/AVPF), the retransmitted
packets are part of the media stream bandwidth when using
synchronization source (SSRC) multiplexing. If a feedback-based
protocol is offered as the actual configuration transport protocol, a
non-feedback-based protocol is offered as a potential configuration
transport protocol and ends up being used, the actual bandwidth usage
may be lower than the indicated bandwidth value in the offer (and
vice versa).
Andreasen Standards Track PAGE 49
RFC 5939 SDP Capability Negotiation September 2010
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 crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
FEC_ORDER=FEC_SRTP
a=acap:2 key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
a=acap:3 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 naive implementation that
first generates all the valid potential configuration SDP session
descriptions internally, could find itself being memory exhausted,
especially if it supports a large number of endpoints. Similarly, a
naive implementation that simply performs iterative trial-and-error
processing on each possible potential configuration SDP session
description (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. Potential configurations
with unsupported mandatory extension configurations in them can be
discarded as well.
Andreasen Standards Track PAGE 50
RFC 5939 SDP Capability Negotiation September 2010
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 needs to perform some kind of
processing on the SDP session descriptions 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-CSCFs) 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 session descriptions); however, they are a fact of
life in some deployment scenarios and hence deserve 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 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 recommending a second offer/answer
exchange when use of a potential configuration has been negotiated
(see Section 3.6.3). However, there are several limitations with
this approach. First of all, although the second offer/answer
exchange is RECOMMENDED, it 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 [RFC 3264]
leave the original offer/answer exchange in effect when a subsequent
one fails. Consider the following example:
1. Offerer generates an SDP session description 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).
Andreasen Standards Track PAGE 51
RFC 5939 SDP Capability Negotiation September 2010
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 session description.
3. The answerer chooses the high(er) bandwidth potential
configuration and generates an answer SDP session description
based on that.
4. The intermediary passes through the answer SDP session
description.
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 session description.
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 base SDP Capability Negotiation framework defines transport
capabilities and attribute capabilities. Media capabilities, which
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 base SDP Capability Negotiation framework as well.
Andreasen Standards Track PAGE 52
RFC 5939 SDP Capability Negotiation September 2010
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
codecs 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 3.5.1) 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 [RFC 4566]).
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 [RFC 4566]. The delete-attributes MUST be used to ensure that
there are not multiple "fmtp" attributes for a given media format in
a media description.
Extensions to the base 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
either the session level or the media level. In either case, it is
possible to define attribute capabilities for these direction
capabilities; if used by a potential configuration, 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.
Andreasen Standards Track PAGE 53
RFC 5939 SDP Capability Negotiation September 2010
3.14. Relationship to RFC 3407
RFC 3407 defines capability descriptions with limited abilities to
describe attributes, bandwidth parameters, transport protocols and
media formats. RFC 3407 does not define any negotiation procedures
for actually using those capability descriptions.
This document defines new attributes for describing attribute
capabilities and transport capabilities. It also defines procedures
for using those capabilities as part of an offer/answer exchange. In
contrast to RFC 3407, this document does not define bandwidth
parameters, and it also does not define how to express ranges of
values. Extensions to this document may be defined in order to fully
cover all the capabilities provided by RFC 3407 (for example, more
general media capabilities).
It is RECOMMENDED that implementations use the attributes and
procedures defined in this document instead of those defined in
[RFC 3407]. If capability description interoperability with legacy
RFC 3407 implementations is desired, implementations MAY include both
RFC 3407 capability descriptions and capabilities defined by this
document. The offer/answer negotiation procedures defined in this
document will not use the RFC 3407 capability descriptions.
4. Examples
In this section, we provide examples showing how to use the SDP
Capability Negotiation.
4.1. 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. The offerer uses the expected least-
common-denominator (plain RTP) as the actual configuration, and the
alternative transport protocols as the potential configurations.
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RFC 5939 SDP Capability Negotiation September 2010
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) 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
a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF
a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
FEC_ORDER=FEC_SRTP
a=acap:2 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 [RFC 4568]. The second "acap"
attribute provides an attribute capability with a handle of 2. The
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RFC 5939 SDP Capability Negotiation September 2010
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 session description 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:
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 session description (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.
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RFC 5939 SDP Capability Negotiation September 2010
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):
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 session description 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/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.
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:
Andreasen Standards Track PAGE 57
RFC 5939 SDP Capability Negotiation September 2010
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. DTLS-SRTP or SRTP with Media-Level Security Descriptions
The following example illustrates how to use the SDP Capability
Negotiation framework to negotiate use of SRTP using either SDP
security descriptions or DTLS-SRTP. The offerer (Alice) wants to
establish a Secure RTP audio stream but is willing to use plain RTP.
Alice prefers to use DTLS-SRTP as the key management protocol, but
supports SDP security descriptions as well (note that [RFC 5763]
contains additional DTLS-SRTP examples).
The example is illustrated by the offer/answer exchange below, where
Alice sends an offer to Bob:
Alice Bob
| (1) Offer (RTP/[S]AVP,SDES | DTLS-SRTP)|
|--------------------------------------->|
| |
|<--------- DTLS-SRTP handshake -------->|
| |
| (2) Answer (DTLS-SRTP) |
|<---------------------------------------|
| |
| (3) Offer (DTLS-SRTP) |
|--------------------------------------->|
| |
| (4) Answer (DTLS-SRTP) |
|<---------------------------------------|
| |
Alice's offer includes an audio stream that offers use of plain RTP
and Secure RTP as alternatives. For the Secure RTP stream, it can be
established using either DTLS-SRTP or SDP security descriptions:
Andreasen Standards Track PAGE 58
RFC 5939 SDP Capability Negotiation September 2010
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 setup:actpass
a=acap:2 fingerprint: SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
a=tcap:1 UDP/TLS/RTP/SAVP RTP/SAVP
m=audio 59000 RTP/AVP 98
a=rtpmap:98 AMR/8000
a=acap:3 crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=pcfg:1 t=1 a=1,2
a=pcfg:2 t=2 a=3
The first (and preferred) potential configuration for the audio
stream specifies use of transport capability 1 (UDP/TLS/RTP/SAVP),
i.e., DTLS-SRTP, and attribute capabilities 1 and 2 (active/passive
mode and certificate fingerprint), both of which must be supported to
choose this potential configuration. The second (and less preferred)
potential configuration specifies use of transport capability 2
(RTP/SAVP) and mandatory attribute capability 3, i.e., the SDP
security description.
Bob receives the SDP session description offer from Alice. Bob
supports DTLS-SRTP as preferred by Alice and Bob now initiates the
DTLS-SRTP handshake to establish the DTLS-SRTP session (see [RFC 5764]
for details).
Bob also sends back an answer to Alice as follows:
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
a=setup:active
a=fingerprint: SHA-1 \
FF:FF:FF:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
t=0 0
c=IN IP4 192.0.2.2
m=audio 54568 UDP/TLS/RTP/SAVP 98
a=rtpmap:98 AMR/8000
a=acfg:1 t=1 a=1,2
For the audio stream, Bob accepted the use of DTLS-SRTP, and hence
the profile in the "m=" line is "UDP/TLS/RTP/SAVP". Bob also
includes a "setup:active" attribute to indicate he is the active
Andreasen Standards Track PAGE 59
RFC 5939 SDP Capability Negotiation September 2010
endpoint for the DTLS-SRTP session as well as the fingerprint for
Bob's certificate. Bob's "acfg" attribute indicates that he chose
potential configuration 1 from Alice's offer.
When Alice receives Bob's answer, session negotiation has completed
(and Alice can verify the DTLS handshake using Bob's certificate
fingerprint in the answer); 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 DTLS-SRTP for the audio stream,
and it is not using the SDP Capability Negotiation framework (Alice
could have included the capabilities as well if she wanted):
v=0
o=- 25678 753850 IN IP4 192.0.2.1
s=
t=0 0
c=IN IP4 192.0.2.1
a=setup:actpass
a=fingerprint: SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
m=audio 59000 UDP/TLS/RTP/AVP 98
a=rtpmap:98 AMR/8000
The "m=" line for the audio stream now indicates that Alice is
offering to use DTLS-SRTP in active/passive mode using her
certificate fingerprint provided.
Bob receives the SDP session description 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=
a=setup:active
a=fingerprint: SHA-1 \
FF:FF:FF:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
t=0 0
c=IN IP4 192.0.2.2
m=audio 54568 UDP/TLS/RTP/SAVP 98
a=rtpmap:98 AMR/8000
a=acfg:1 t=1 a=1,2
Andreasen Standards Track PAGE 60
RFC 5939 SDP Capability Negotiation September 2010
Bob includes the same "setup:active" and fingerprint attributes 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.
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
Finally, if Bob had chosen to use SDP security descriptions instead
of DTLS-SRTP, the following answer would have been generated:
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:2 t=2 a=3
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
[RFC 3830] 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:
Andreasen Standards Track PAGE 61
RFC 5939 SDP Capability Negotiation September 2010
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 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 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 crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=acap:4 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
attribute capabilities is mandatory. There are three potential
configurations for the video stream.
Andreasen Standards Track PAGE 62
RFC 5939 SDP Capability Negotiation September 2010
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 session description 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
"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
Andreasen Standards Track PAGE 63
RFC 5939 SDP Capability Negotiation September 2010
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):
v=0
o=- 25678 753850 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.
Andreasen Standards Track PAGE 64
RFC 5939 SDP Capability Negotiation September 2010
Bob receives the SDP session description 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=
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.
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, the following answer would have been
generated:
Andreasen Standards Track PAGE 65
RFC 5939 SDP Capability Negotiation September 2010
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
t=0 0
c=IN IP4 192.0.2.2
a=key-mgmt:mikey AQEFgM0XflABAAAAAAAAAAAAAAYAyO...
m=audio 54568 RTP/SAVP 98
a=rtpmap:98 AMR/8000
a=acfg:1 t=2 a=1
m=video 55468 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 or not SDP
Capability Negotiation is being used). 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) |
|<---------------------------------------|
| |
Andreasen Standards Track PAGE 66
RFC 5939 SDP Capability Negotiation September 2010
Alice's offer includes an audio and a video stream. Both the audio
and the video stream offer use of Secure RTP:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
t=0 0
c=IN IP4 192.0.2.1
a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
m=audio 59000 RTP/SAVP 98
a=rtpmap:98 AMR/8000
a=acap:1 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 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 its own mandatory "crypto"
attribute capability (2). Note how the deletion of the session-level
attributes does not affect the media-level attributes.
Bob receives the SDP session description 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 session description 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:
Andreasen Standards Track PAGE 67
RFC 5939 SDP Capability Negotiation September 2010
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
t=0 0
c=IN IP4 192.0.2.2
m=audio 54568 RTP/SAVP 98
a=rtpmap:98 AMR/8000
a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32
a=acfg:1 a=-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 the
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 session description, when Bob
instead offers the "crypto" attribute as the actual configuration
keying mechanism and "key-mgmt" as the potential configuration:
Andreasen Standards Track PAGE 68
RFC 5939 SDP Capability Negotiation September 2010
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 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 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 crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=acap:4 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
from the actual configuration SDP session description, 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
session description.
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 [RFC 3264]. 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 [RFC 3261].
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) warrants
further security considerations. We discuss these issues below.
Andreasen Standards Track PAGE 69
RFC 5939 SDP Capability Negotiation September 2010
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 session description offer, he can thereby force the
negotiation of the weakest security mechanism that the offerer is
willing to accept. This may 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 session description 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
session description. 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 Secure/Multipurpose
Internet Mail Extensions (S/MIME) [RFC 5751] 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, is to use
SIP Identity [RFC 4474]; this requires the existence of an
authentication service (see [RFC 4474]). Although this mechanism
still requires a PKI, it only requires that servers (as opposed to
end-users) have third-party validatable certificates, which
significantly reduces the barrier to entry by ordinary users. 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 session description on a hop-by-hop basis. This
is less secure because SIP allows partially trusted intermediaries on
the signaling path, and such intermediaries processing the SIP
request at each hop would be able to perform a man-in-the-middle
attack by modifying the offered SDP session description. In simple
architectures where the two UA's proxies communicate directly, the
security provided by this method is roughly comparable to that
provided by the previously discussed signature-based mechanisms.
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) was selected by the answerer, until an answer indication
is received. This opens up a new security issue where an attacker
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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
[RFC 5027] 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
sides 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 session
description 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 3.9
presents other alternatives as well.
Additional security considerations apply to the answer SDP session
description as well. The actual configuration attribute tells the
offerer on which potential configuration the answer was based, 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 session description answer integrity problem
are the same as for the offer, i.e., use of end-to-end integrity
protection, SIP identity, or hop-by-hop protection. The mechanism to
use depends on the mechanisms supported by the offerer as well as the
acceptable security trade offs.
As described in Sections 3.1 and 3.11, SDP Capability Negotiation
conceptually allows an offerer to include many different offers in a
single SDP session description. This can 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,
unsupported or invalid mandatory attribute capabilities, or
unsupported mandatory extension configurations. The answerer should
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RFC 5939 SDP Capability Negotiation September 2010
also look out for potential configurations that are designed to pass
the above test, but nevertheless produce a large number of potential
configuration SDP session descriptions 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. At
the time of publication of this document, we do not know of such
an SDP attribute; however, this does not mean it does not exist,
or that it will not exist in the future. If such attributes are
found to exist, implementers should 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 or 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 has registered the following new SDP attributes:
Attribute name: csup
Long form name: Supported capability negotiation extensions
Type of attribute: Session-level and media-level
Subject to charset: No
Purpose: Option tags for supported SDP Capability
Negotiation extensions
Appropriate values: See Section 3.3.1 of RFC 5939
Contact name: Flemming Andreasen, fandreas@cisco.com
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 3.3.2 of RFC 5939
Contact name: Flemming Andreasen, fandreas@cisco.com
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RFC 5939 SDP Capability Negotiation September 2010
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 3.4.1 of RFC 5939
Contact name: Flemming Andreasen, fandreas@cisco.com
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 3.4.2 of RFC 5939
Contact name: Flemming Andreasen, fandreas@cisco.com
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 3.5.1 of RFC 5939
Contact name: Flemming Andreasen, fandreas@cisco.com
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 3.5.2 of RFC 5939
Contact name: Flemming Andreasen, fandreas@cisco.com
6.2. New SDP Capability Negotiation Option Tag Registry
The IANA has created 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 [RFC 5226] IETF
Review 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 configuration ("a=pcfg") and actual
configuration ("a=acfg") attributes provided in this document. If
the extension defines any new SDP attributes that are intended to be
capabilities for use by the capability negotiation framework (e.g.,
similar to "a=acap"), those capabilities MUST adhere to the
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guidelines provided in Section 3.4.3. Extensions to the potential
and actual configuration attributes MUST adhere to the syntax
provided in Sections 3.5.1 and 3.5.2.
The option tag "cap-v0" is defined in this document, and the IANA has
registered this option tag.
6.3. New SDP Capability Negotiation Potential Configuration Parameter
Registry
The IANA has created 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 [RFC 5226] IETF Review 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 3.5.1 and the semantics MUST adhere to the
semantics provided for extensions in Section 3.5.1 and 3.5.2.
Associated with each registration MUST be the encoding name for the
parameter as well as a short descriptive name for it.
The potential configuration parameters "a" for "attribute" and "t"
for "transport protocol" are defined in this document, and the IANA
has registered them.
7. Acknowledgments
The SDP Capability Negotiation solution defined in this document
draws on the overall capability negotiation framework that was
defined by [SDPng]. Also, the SDP Capability Negotiation solution 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
here: Francois Audet, John Elwell, Roni Even, Miguel Garcia, Robert
Gilman, Cullen Jennings, Jonathan Lennox, Matt Lepinski, Jean-
Francois Mule, Joerg Ott, Colin Perkins, Jonathan Rosenberg, Thomas
Stach, and Dan Wing.
General Area review comments were provided by Christian Vogt, and
Stephen Kent provided Security Directorate review comments. Eric
Rescorla provided textual input to the Security Considerations.
Alexey Melnikov, Robert Sparks, and Magnus Westerlund provided
several review comments as well.
Andreasen Standards Track PAGE 74
RFC 5939 SDP Capability Negotiation September 2010
8. References
8.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, June
2002.
[RFC 4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC 5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC 5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234, January
2008.
[RFC 5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245, April
2010.
8.2. Informative References
[RFC 3261] 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.
[RFC 3312] Camarillo, G., Ed., Marshall, W., Ed., and J. Rosenberg,
"Integration of Resource Management and Session Initiation
Protocol (SIP)", RFC 3312, October 2002.
[RFC 3262] Rosenberg, J. and H. Schulzrinne, "Reliability of
Provisional Responses in Session Initiation Protocol
(SIP)", RFC 3262, June 2002.
[RFC 3407] Andreasen, F., "Session Description Protocol (SDP) Simple
Capability Declaration", RFC 3407, October 2002.
[RFC 3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
July 2003.
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RFC 5939 SDP Capability Negotiation September 2010
[RFC 3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC 3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
August 2004.
[RFC 4145] Yon, D. and G. Camarillo, "TCP-Based Media Transport in
the Session Description Protocol (SDP)", RFC 4145,
September 2005.
[RFC 4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.
[RFC 4567] 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.
[RFC 4568] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol (SDP) Security Descriptions for Media
Streams", RFC 4568, July 2006.
[RFC 4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July
2006.
[RFC 4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
July 2006.
[RFC 4756] Li, A., "Forward Error Correction Grouping Semantics in
Session Description Protocol", RFC 4756, November 2006.
[RFC 5027] Andreasen, F. and D. Wing, "Security Preconditions for
Session Description Protocol (SDP) Media Streams", RFC
5027, October 2007.
[RFC 5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for
Real-time Transport Control Protocol (RTCP)-Based Feedback
(RTP/SAVPF)", RFC 5124, February 2008.
[RFC 5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
Andreasen Standards Track PAGE 76
RFC 5939 SDP Capability Negotiation September 2010
[RFC 5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
for Establishing a Secure Real-time Transport Protocol
(SRTP) Security Context Using Datagram Transport Layer
Security (DTLS)", RFC 5763, May 2010.
[RFC 5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764, May 2010.
[RFC 5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888, June 2010.
[BESRTP] Kaplan, H. and F. Audet, "Session Description Protocol
(SDP) Offer/Answer Negotiation For Best-Effort Secure
Real-Time Transport Protocol", Work in Progress, October
2006.
[ICETCP] Rosenberg, J., Keranen, A., Lowekamp, B., and A. Roach,
"TCP Candidates with Interactive Connectivity
Establishment (ICE)", Work in Progress, September 2010.
[SDPMedCap]
Gilman, R., Even, R., and F. Andreasen, "SDP media
capabilities Negotiation", Work in Progress, July 2010.
[SDPng] Kutscher, D., Ott, J., and C. Bormann, "Session
Description and Capability Negotiation", Work in Progress,
February 2005.
Author's Address
Flemming Andreasen
Cisco Systems
Iselin, NJ 08830
USA
EMail: fandreas@cisco.com
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Session Description Protocol (SDP) Capability Negotiation
RFC TOTAL SIZE: 188116 bytes
PUBLICATION DATE: Tuesday, September 14th, 2010
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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