Internet DRAFT - draft-ietf-mmusic-sdp-media-capabilities
draft-ietf-mmusic-sdp-media-capabilities
MMUSIC R. Gilman
Internet-Draft Independent
Updates: 5939 (if approved) R. Even
Intended status: Standards Track Gesher Erove Ltd
Expires: July 8, 2013 F. Andreasen
Cisco Systems
January 4, 2013
Session Description Protocol (SDP) Media Capabilities Negotiation
draft-ietf-mmusic-sdp-media-capabilities-17
Abstract
Session Description Protocol (SDP) capability negotiation provides a
general framework for indicating and negotiating capabilities in SDP.
The base framework defines only capabilities for negotiating
transport protocols and attributes. In this document, we extend the
framework by defining media capabilities that can be used to
negotiate media types and their associated parameters.
This document updates the IANA Considerations of RFC 5939.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Task Force (IETF). Note that other groups may also distribute
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Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 8, 2013.
Copyright Notice
Copyright (c) 2013 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. SDP Media Capabilities . . . . . . . . . . . . . . . . . . . . 8
3.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. Solution Overview . . . . . . . . . . . . . . . . . . . . 9
3.3. New Capability Attributes . . . . . . . . . . . . . . . . 15
3.3.1. The Media Format Capability Attributes . . . . . . . . 15
3.3.2. The Media Format Parameter Capability Attribute . . . 18
3.3.3. The Media-Specific Capability Attribute . . . . . . . 20
3.3.4. New Configuration Parameters . . . . . . . . . . . . . 22
3.3.5. The Latent Configuration Attribute . . . . . . . . . . 24
3.3.6. Enhanced Potential Configuration Attribute . . . . . . 26
3.3.7. Substitution of Media Payload Type Numbers in
Capability Attribute Parameters . . . . . . . . . . . 30
3.3.8. The Session Capability Attribute . . . . . . . . . . . 31
3.4. Offer/Answer Model Extensions . . . . . . . . . . . . . . 35
3.4.1. Generating the Initial Offer . . . . . . . . . . . . . 36
3.4.2. Generating the Answer . . . . . . . . . . . . . . . . 39
3.4.3. Offerer Processing of the Answer . . . . . . . . . . . 43
3.4.4. Modifying the Session . . . . . . . . . . . . . . . . 44
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.1. Alternative Codecs . . . . . . . . . . . . . . . . . . . . 45
4.2. Alternative Combinations of Codecs (Session
Configurations) . . . . . . . . . . . . . . . . . . . . . 48
4.3. Latent Media Streams . . . . . . . . . . . . . . . . . . . 48
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 51
5.1. New SDP Attributes . . . . . . . . . . . . . . . . . . . . 51
5.2. New SDP Capability Negotiation Option Tag . . . . . . . . 52
5.3. SDP Capability Negotiation Configuration Parameters
Registry . . . . . . . . . . . . . . . . . . . . . . . . . 52
5.4. SDP Capability Negotiation Configuration Parameter
Registrations . . . . . . . . . . . . . . . . . . . . . . 53
6. Security Considerations . . . . . . . . . . . . . . . . . . . 55
7. Changes from previous versions . . . . . . . . . . . . . . . . 56
7.1. Changes from version 16 . . . . . . . . . . . . . . . . . 56
7.2. Changes from version 15 . . . . . . . . . . . . . . . . . 56
7.3. Changes from version 14 . . . . . . . . . . . . . . . . . 56
7.4. Changes from version 13 . . . . . . . . . . . . . . . . . 56
7.5. Changes from version 12 . . . . . . . . . . . . . . . . . 56
7.6. Changes from version 11 . . . . . . . . . . . . . . . . . 57
7.7. Changes from version 10 . . . . . . . . . . . . . . . . . 57
7.8. Changes from version 09 . . . . . . . . . . . . . . . . . 58
7.9. Changes from version 08 . . . . . . . . . . . . . . . . . 58
7.10. Changes from version 04 . . . . . . . . . . . . . . . . . 58
7.11. Changes from version 03 . . . . . . . . . . . . . . . . . 58
7.12. Changes from version 02 . . . . . . . . . . . . . . . . . 59
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7.13. Changes from version 01 . . . . . . . . . . . . . . . . . 60
7.14. Changes from version 00 . . . . . . . . . . . . . . . . . 60
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 61
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 62
9.1. Normative References . . . . . . . . . . . . . . . . . . . 62
9.2. Informative References . . . . . . . . . . . . . . . . . . 62
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 64
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1. Introduction
Session Description Protocol (SDP) capability negotiation [RFC5939]
provides a general framework for indicating and negotiating
capabilities in SDP [RFC4566]. The base framework defines only
capabilities for negotiating transport protocols and attributes.
RFC 5939 [RFC5939] lists some of the issues with the current SDP
capability negotiation process. An additional real life case is to
be able to offer one media stream (e.g. audio) but list the
capability to support another media stream (e.g. video) without
actually offering it concurrently.
In this document, we extend the framework by defining media
capabilities that can be used to indicate and negotiate media types
and their associated format parameters. This document also adds the
ability to declare support for media streams, the use of which can be
offered and negotiated later, and the ability to specify session
configurations as combinations of media stream configurations. The
definitions of new attributes for media capability negotiation are
chosen to make the translation from these attributes to
"conventional" SDP [RFC4566] media attributes as straightforward as
possible in order to simplify implementation. This goal is intended
to reduce processing in two ways: each proposed configuration in an
offer may be easily translated into a conventional SDP media stream
record for processing by the receiver; and the construction of an
answer based on a selected proposed configuration is straightforward.
This document updates RFC 5939 [RFC5939] by updating the IANA
Considerations. All other extensions defined in this document are
considered extensions above and beyond RFC 5939 [RFC5939].
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2. Terminology
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 [RFC2119] and
indicate requirement levels for compliant implementations.
"Actual Configuration": An actual configuration specifies which
combinations of SDP session parameters and media stream components
can be used in the current offer/answer exchange and with what
parameters. Use of an actual configuration does not require any
further negotiation in the offer/answer exchange. See RFC 5939
[RFC5939] for further details.
"Base Attributes": Conventional SDP attributes appearing in the base
configuration of a media block.
"Base Configuration": The media configuration represented by a media
block exclusive of all the capability negotiation attributes defined
in this document, the base capability negotiation document [RFC5939],
or any other capability negotiation document. In an offer SDP, the
base configuration corresponds to the actual configuration as defined
in RFC 5939 [RFC5939].
"Conventional Attribute": Any SDP attribute other than those defined
by the series of capability negotiation specifications.
"Conventional SDP": An SDP record devoid of capability negotiation
attributes.
"Media Format Capability": A media format, typically a media subtype
such as PCMU, H263-1998, or T38, expressed in the form of a
capability.
"Media Format Parameter Capability": A media format parameter
("a=fmtp" in conventional SDP) expressed in the form of a capability.
The media format parameter capability is associated with a media
format capability.
"Media Capability": The combined set of capabilities associated with
expressing a media format and its relevant parameters (e.g. media
format parameters and media specific parameters).
"Potential Configuration": A potential configuration indicates which
combinations of capabilities can be used for the session and its
associated media stream components. Potential configurations are not
ready for use, however they are offered for potential use in the
current offer/answer exchange. They provide an alternative that may
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be used instead of the actual configuration, subject to negotiation
in the current offer/answer exchange. See RFC 5939 [RFC5939] for
further details.
"Latent Configuration": A latent configuration indicates which
combinations of capabilities could be used in a future negotiation
for the session and its associated media stream components. Latent
configurations are neither ready for use, nor are they offered for
actual or potential use in the current offer/answer exchange. Latent
configurations merely inform the other side of possible
configurations supported by the entity. Those latent configurations
may be used to guide subsequent offer/answer exchanges, but they are
not offered for use as part of the current offer/answer exchange.
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3. SDP Media Capabilities
The SDP capability negotiation [RFC5939] discusses the use of any SDP
[RFC4566] attribute (a=) under the attribute capability "acap". The
limitations of using acap for fmtp and rtpmap in a potential
configuration are described in RFC 5939 [RFC5939]; for example they
can be used only at the media level since they are media level
attributes. RFC 5939 [RFC5939] does not provide a way to exchange
media-level capabilities prior to the actual offer of the associated
media stream. This section provides an overview of extensions
providing an SDP Media Capability negotiation solution offering more
robust capabilities negotiation. This is followed by definitions of
new SDP attributes for the solution and its associated updated offer/
answer procedures [RFC3264]
3.1. Requirements
The capability negotiation extensions requirements considered herein
are as follows.
REQ-01: Support the specification of alternative (combinations of)
media formats (codecs) in a single media block.
REQ-02: Support the specification of alternative media format
parameters for each media format.
REQ-03: Retain backward compatibility with conventional SDP.
Ensure that each and every offered configuration can be easily
translated into a corresponding SDP media block expressed with
conventional SDP lines.
REQ-04: Ensure the scheme operates within the offer/answer model in
such a way that media formats and parameters can be agreed upon
with a single exchange.
REQ-05: Provide the ability to express offers in such a way that
the offerer can receive media as soon as the offer is sent. (Note
that the offerer may not be able to render received media prior to
exchange of keying material.)
REQ-06: Provide the ability to offer latent media configurations
for future negotiation.
REQ-07: Provide reasonable efficiency in the expression of
alternative media formats and/or format parameters, especially in
those cases in which many combinations of options are offered.
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REQ-08: Retain the extensibility of the base capability negotiation
mechanism.
REQ-09: Provide the ability to specify acceptable combinations of
media streams and media formats. For example, offer a PCMU audio
stream with an H264 video stream, or a G729 audio stream with an
H263 video stream. This ability would give the offerer a means to
limit processing requirements for simultaneous streams. This
would also permit an offer to include the choice of an audio/T38
stream or an image/T38 stream, but not both.
Other possible extensions have been discussed, but have not been
treated in this document. They may be considered in the future.
Three such extensions are:
FUT-01: Provide the ability to mix, or change, media types within a
single media block. Conventional SDP does not support this
capability explicitly; the usual technique is to define a media
subtype that represents the actual format within the nominal media
type. For example, T.38 FAX as an alternative to audio/PCMU
within an audio stream is identified as audio/T38; a separate FAX
stream would use image/T38.
FUT-02: Provide the ability to support multiple transport protocols
within an active media stream without reconfiguration. This is
not explicitly supported by conventional SDP.
FUT-03: Provide capability negotiation attributes for all media-
level SDP line types in the same manner as already done for the
attribute type, with the exception of the media line type itself.
The media line type is handled in a special way to permit compact
expression of media coding/format options. The line types are
bandwidth ("b="), information ("i="), connection data ("c="), and,
possibly, the deprecated encryption key ("k=").
3.2. Solution Overview
The solution consists of new capability attributes corresponding to
conventional SDP line types, new parameters for the pcfg, acfg, and
the new lcfg attributes extending the base attributes from RFC 5939
[RFC5939], and a use of the pcfg attribute to return capability
information in the SDP answer.
Several new attributes are defined in a manner that can be related to
the capabilities specified in a media line, and its corresponding
rtpmap and fmtp attributes.
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o A new attribute ("a=rmcap") defines RTP-based media format
capabilities in the form of a media subtype (e.g. "PCMU"), and
its encoding parameters (e.g. "/8000/2"). Each resulting media
format type/subtype capability has an associated handle called a
media capability number. The encoding parameters are as specified
for the rtpmap attribute defined in SDP [RFC4566], without the
payload type number part.
o A new attribute ("a=omcap") defines other (non RTP-based) media
format capabilities in the form of a media subtype only (e.g.
"T38"). Each resulting media format type/subtype capability has
an associated handle called a media capability number.
o A new attribute ("a=mfcap") specifies media format parameters
associated with one or more media format capabilities. The mfcap
attribute is used primarily to associate the media format
parameters normally carried in the fmtp attribute. Note that
media format parameters can be used with RTP and non-RTP based
media formats.
o A new attribute ("a=mscap") that specifies media parameters
associated with one or more media format capabilities. The mscap
attribute is used to associate capabilities with attributes other
than fmtp or rtpmap, for example, the rtcp-fb attribute defined in
RFC 4585 [RFC4585].
o A new attribute ("a=lcfg") specifies latent media stream
configurations when no corresponding media line ("m=") is offered.
An example is the offer of latent configurations for video even
though no video is currently offered. If the peer indicates
support for one or more offered latent configurations, the
corresponding media stream(s) may be added via a new offer/answer
exchange.
o A new attribute ("a=sescap") is used to specify an acceptable
combination of simultaneous media streams and their configurations
as a list of potential and/or latent configurations.
New parameters are defined for the potential configuration (pcfg),
latent configuration (lcfg), and accepted configuration (acfg)
attributes to associate the new attributes with particular
configurations.
o A new parameter type ("m=") is added to the potential
configuration ("a=pcfg:") attribute and the actual configuration
("a=acfg:") attribute defined in RFC 5939 [RFC5939], and to the
new latent configuration ("a=lcfg:") attribute. This permits
specification of media capabilities (including their associated
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parameters) and combinations thereof for the configuration. For
example, the "a=pcfg:" line might specify PCMU and telephone
events [RFC4733] or G.729B and telephone events as acceptable
configurations. The "a=acfg:" line in the answer would specify
the configuration chosen.
o A new parameter type ("pt=") is added to the potential
configuration, actual configuration, and latent configuration
attributes. This parameter associates RTP payload type numbers
with the referenced RTP-based media format capabilities, and is
appropriate only when the transport protocol uses RTP.
o A new parameter type ("mt=") is used to specify the media type for
latent configurations.
Special processing rules are defined for capability attribute
arguments in order to reduce the need to replicate essentially-
identical attribute lines for the base configuration and potential
configurations.
o A substitution rule is defined for any capability attribute to
permit the replacement of the (escaped) media capability number
with the media format identifier (e.g., the payload type number in
audio/video profiles).
o Replacement rules are defined for the conventional SDP equivalents
of the mfcap and mscap capability attributes. This reduces the
necessity to use the deletion qualifier in the a=pcfg parameter in
order to ignore rtpmap, fmtp, and certain other attributes in the
base configuration.
o An argument concatenation rule is defined for mfcap attributes
which refer to the same media capability number. This makes it
convenient to combine format options concisely by associating
multiple mfcap lines with multiple media format capabilities.
This document extends the base protocol extensions to the offer/
answer model that allow for capabilities and potential configurations
to be included in an offer. Media capabilities constitute
capabilities that can be used in potential and latent configurations.
Whereas potential configurations 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, latent configurations merely inform the other side of
possible configurations supported by the entity. Those latent
configurations may be used to guide subsequent offer/answer
exchanges, but they are not part of the current offer/answer
exchange.
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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 (RTP) |
|<---------------------------------|
| |
Alice's offer includes RTP and SRTP as alternatives. RTP is the
default, but SRTP is the preferred one (long lines are folded to fit
the margins):
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
a=creq:med-v0
m=audio 3456 RTP/AVP 0 18
a=tcap:1 RTP/SAVP RTP/AVP
a=rtpmap:0 PCMU/8000/1
a=rtpmap:18 G729/8000/1
a=fmtp:18 annexb=yes
a=rmcap:1,4 G729/8000/1
a=rmcap:2 PCMU/8000/1
a=rmcap:5 telephone-event/8000
a=mfcap:1 annexb=no
a=mfcap:4 annexb=yes
a=mfcap:5 0-11
a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32 \
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=pcfg:1 m=4,5|1,5 t=1 a=1 pt=1:100,4:101,5:102
a=pcfg:2 m=2 t=1 a=1 pt=2:103
a=pcfg:3 m=4 t=2 pt=4:18
The required base and extensions are provided by the "a=creq"
attribute defined in RFC 5939 [RFC5939], with the option tag
"med-v0", which indicates that the extension framework defined here,
must be supported. The base level capability negotiation support
("cap-v0" [RFC5939]) is implied since it is required for the
extensions.
The "m=" line indicates that Alice is offering to use plain RTP with
PCMU or G.729B. The media line implicitly defines the default
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transport protocol (RTP/AVP in this case) and the default actual
configuration.
The "a=tcap:1" line, specified in the SDP Capability Negotiation base
protocol [RFC5939], defines transport protocol capabilities, in this
case Secure RTP (SAVP profile) as the first option and RTP (AVP
profile) as the second option.
The "a=rmcap:1,4" line defines two G.729 RTP-based media format
capabilities, numbered 1 and 4, and their encoding rate. The
capabilities are of media type "audio" and subtype G729. Note that
the media subtype is explicitly specified here, rather than RTP
payload type numbers. This permits the assignment of payload type
numbers in the media stream configuration specification. In this
example, two G.729 subtype capabilities are defined. This permits
the declaration of two sets of formatting parameters for G.729.
The "a=rmcap:2" line defines a G.711 mu-law capability, numbered 2.
The "a=rmcap:5" line defines an audio telephone-event capability,
numbered 5.
The "a=mfcap:1" line specifies the fmtp formatting parameters for
capability 1 (offerer will not accept G.729 Annex B packets).
The "a=mfcap:4" line specifies the fmtp formatting parameters for
capability 4 (offerer will accept G.729 Annex B packets).
The "a=mfcap:5" line specifies the fmtp formatting parameters for
capability 5 (the DTMF touchtones 0-9,*,#).
The "a=acap:1" line specified in the base protocol provides the
"crypto" attribute which provides the keying material for SRTP using
SDP security descriptions.
The "a=pcfg:" attributes provide the potential configurations
included in the offer by reference to the media capabilities,
transport capabilities, attribute capabilities and specified payload
type number mappings. Three explicit alternatives are provided; the
lowest-numbered one is the preferred one. The "a=pcfg:1 ..." line
specifies media capabilities 4 and 5, i.e., G.729B and DTMF (incl.
their associated media format parameters), or media capability 1 and
5, i.e., G.729 and DTMF (incl. their associated media format
parameters). Furthermore, it specifies transport protocol capability
1 (i.e. the RTP/SAVP profile - secure RTP), and the attribute
capability 1, i.e. the crypto attribute provided. Lastly, it
specifies a payload type number mapping for (RTP-based) media
capabilities 1, 4, and 5, thereby permitting the offerer to
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distinguish between encrypted media and unencrypted media received
prior to receipt of the answer.
Use of unique payload type numbers in alternative configurations is
not required; codecs such as AMR-WB [RFC4867] have the potential for
so many combinations of options that it may be impractical to define
unique payload type numbers for all supported combinations. If
unique payload type numbers cannot be specified, then the offerer
will be obliged to wait for the SDP answer before rendering received
media. For SRTP using SDES inline keying [RFC4568], the offerer will
still need to receive the answer before being able to decrypt the
stream.
The second alternative ("a=pcfg:2 ...") specifies media capability 2,
i.e., PCMU, under the RTP/SAVP profile, with the same SRTP key
material.
The third alternative ("a=pcfg:3 ...") offers G.729B unsecured; its
only purpose in this example is to show a preference for G.729B over
PCMU.
Per RFC 5939 [RFC5939], the media line, with any qualifying
attributes such as fmtp or rtpmap, is itself considered a valid
configuration (the current actual configuration); it has the lowest
preference (per RFC 5939 [RFC5939]).
Bob receives the SDP offer from Alice. Bob supports G.729B, PCMU,
and telephone events over RTP, but not SRTP, hence he accepts the
potential configuration 3 for RTP provided by Alice. Bob generates
the following answer:
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
t=0 0
a=csup:med-v0
m=audio 4567 RTP/AVP 18
a=rtpmap:18 G729/8000
a=fmtp:18 annexb=yes
a=acfg:3 m=4 t=2 pt=4:18
Bob includes the "a=csup" and "a=acfg" attributes in the answer to
inform Alice that he can support the med-v0 level of capability
negotiations. Note that in this particular example, the answerer
supported the capability extensions defined here, however had he not,
he would simply have processed the offer based on the offered PCMU
and G.729 codecs under the RTP/AVP profile only. Consequently, the
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answer would have omitted the "a=csup" attribute line and chosen one
or both of the PCMU and G.729 codecs instead. The answer carries the
accepted configuration in the "m=" line along with corresponding
rtpmap and/or fmtp parameters, as appropriate.
Note that per the base protocol, after the above, Alice MAY generate
a new offer with an actual configuration ("m=" line, etc.)
corresponding to the actual configuration referenced in Bob's answer
(not shown here).
3.3. New Capability Attributes
In this section, we present the new attributes associated with
indicating the media capabilities for use by the SDP Capability
negotiation. The approach taken is to keep things similar to the
existing media capabilities defined by the existing media
descriptions ("m=" lines) and the associated "rtpmap" and "fmtp"
attributes. We use media subtypes and "media capability numbers" to
link the relevant media capability parameters. This permits the
capabilities to be defined at the session level and be used for
multiple streams, if desired. For RTP-based media formats, payload
types are then specified at the media level (see Section 3.3.4.2).
A media capability merely indicates possible support for the media
type and media format(s) and parameters in question. In order to
actually use a media capability in an offer/answer exchange, it MUST
be referenced in a potential configuration.
Media capabilities, i.e. the attributes associated with expressing
media capability formats, parameters, etc., can be provided at the
session-level and/or the media-level. Media capabilities provided at
the session level may be referenced in any pcfg or lcfg attribute at
the media level (consistent with the media type), whereas media
capabilities provided at the media level may be referenced only by
the pcfg or lcfg attribute within that media stream. In either case,
the scope of the <med-cap-num> is the entire session description.
This enables each media capability to be uniquely referenced across
the entire session description (e.g. in a potential configuration).
3.3.1. The Media Format Capability Attributes
Media subtypes can be expressed as media format capabilities by use
of the "a=rmcap" and "a=omcap" attributes. The "a=rmcap" attribute
MUST be used for RTP-based media whereas the "a=omcap" attribute MUST
be used for non-RTP-based (other) media formats. The two attributes
are defined as follows:
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a=rmcap:<media-cap-num-list> <encoding-name>/<clock-rate>
[/<encoding-parms>]
a=omcap:<media-cap-num-list> <format-name>
where <media-cap-num-list> is a (list of) media capability number(s)
used to number a media format capability, the <encoding name> or
<format name> is the media subtype, e.g., H263-1998, PCMU, or T38,
<clock rate> is the encoding rate, and <encoding parms> are the media
encoding parameters for the media subtype. All media format
capabilities in the list are assigned to the same media type/subtype.
Each occurrence of the rmcap and omcap attribute MUST use unique
values in their <media-cap-num-list>; the media capability numbers
are shared between the two attributes and the numbers MUST be unique
across the entire SDP session. In short, the rmcap and omcap
attributes define media format capabilities and associate them with a
media capability number in the same manner as the rtpmap attribute
defines them and associates them with a payload type number.
Additionally, the attributes allow multiple capability numbers to be
defined for the media format in question by specifying a range of
media capability numbers. This permits the media format to be
associated with different media parameters in different
configurations. When a range of capability numbers is specified, the
first (leftmost) capability number MUST be strictly smaller than the
second (rightmost), i.e. the range increases and covers at least two
numbers.
In ABNF [RFC5234], we have:
media-capability-line = rtp-mcap / non-rtp-mcap
rtp-mcap = "a=rmcap:" media-cap-num-list
1*WSP encoding-name "/" clock-rate
["/" encoding-parms]
non-rtp-mcap = "a=omcap:" media-cap-num-list 1*WSP format-name
media-cap-num-list = media-cap-num-element
*("," media-cap-num-element)
media-cap-num-element = media-cap-num
/ media-cap-num-range
media-cap-num-range = media-cap-num "-" media-cap-num
media-cap-num = NonZeroDigit *9(DIGIT)
encoding-name = token ;defined in RFC4566
clock-rate = NonZeroDigit *9(DIGIT)
encoding-parms = token
format-name = token ;defined in RFC4566
NonZeroDigit = %x31-39 ; 1-9
The encoding-name, clock-rate and encoding-params are as defined to
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appear in an rtpmap attribute for each media type/subtype. Thus, it
is easy to convert an rmcap attribute line into one or more rtpmap
attribute lines, once a payload type number is assigned to a media-
cap-num (see Section 3.3.5).
The format-name is a media format description for non-RTP based media
as defined for the <fmt> part of the media description ("m=" line) in
SDP [RFC4566]. In simple terms, it is the name of the media format,
e.g. "t38". This form can also be used in cases such as BFCP
[RFC4585] where the fmt list in the m-line is effectively ignored
(BFCP uses "*").
The "rmcap" and "omcap" attributes can be provided at the session-
level and/or the media-level. There can be more than one rmcap and
more than one omcap attribute at both the session and media level
(i.e., more than one of each at the session-level and more than one
of each in each media description). Media capability numbers cannot
include leading zeroes, and each media-cap-num MUST be unique within
the entire SDP record; it is used to identify that media capability
in potential, latent and actual configurations, and in other
attribute lines as explained below. Note that the media-cap-num
values are shared between the rmcap and omcap attributes, and hence
the uniqueness requirement applies to the union of them. When the
media capabilities are used in a potential, latent or actual
configuration, the media formats referred by those configurations
apply at the media level, irrespective of whether the media
capabilities themselves were specified at the session or media level.
In other words, the media capability applies to the specific media
description associated with the configuration which invokes it.
For example:
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
t=0 0
a=rmcap:1 L16/8000/1
a=rmcap:2 L16/16000/2
a=rmcap:3 H263-1998/90000
a=omcap:4 example
m=audio 54320 RTP/AVP 0
a=pcfg:1 m=1|2, pt=1:99,2:98
m=video 66544 RTP/AVP 100
a=rtpmap:100 H264/90000
a=pcfg:10 m=3 pt=3:101
a=tcap:1 TCP
a=pcfg:11 m=4 t=1
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3.3.2. The Media Format Parameter Capability Attribute
This attribute is used to associate media format specific parameters
with one or more media format capabilities. The form of the
attribute is:
a=mfcap:<media-caps> <list of parameters>
where <media-caps> permits the list of parameters to be associated
with one or more media format capabilities and the format parameters
are specific to the type of media format. The mfcap lines map to a
single traditional SDP fmtp attribute line (one for each entry in
<media-caps>) of the form
a=fmtp:<fmt> <list of parameters>
where <fmt> is the media format parameter defined in RFC 4566
[RFC4566], as appropriate for the particular media stream. The mfcap
attribute MUST be used to encode attributes for media capabilities,
which would conventionally appear in an fmtp attribute. The existing
acap attribute MUST NOT be used to encode fmtp attributes.
The mfcap attribute adheres to SDP [RFC4566] attribute production
rules with
media-format-parameter-capability =
"a=mfcap:" media-cap-num-list 1*WSP fmt-specific-param-list
fmt-specific-param-list = text ; defined in RFC4566
Note that media format parameters can be used with RTP-based and non-
RTP based media formats.
3.3.2.1. Media Format Parameter Concatenation Rule
The appearance of media subtypes with a large number of formatting
options (e.g., AMR-WB [RFC4867]) coupled with the restriction that
only a single fmtp attribute can appear per media format, suggests
that it is useful to create a combining rule for mfcap parameters
which are associated with the same media capability number.
Therefore, different mfcap lines MAY include the same media-cap-num
in their media-cap-num-list. When a particular media capability is
selected for processing, the parameters from each mfcap line which
references the particular capability number in its media-cap-num-list
are concatenated together via ";", in the order the mfcap attributes
appear in the SDP record, to form the equivalent of a single fmtp
attribute line. This permits one to define a separate mfcap line for
a single parameter and value that is to be applied to each media
capability designated in the media-cap-num-list. This provides a
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compact method to specify multiple combinations of format parameters
when using codecs with multiple format options. Note that order-
dependent parameters SHOULD be placed in a single mfcap line to avoid
possible problems with line rearrangement by a middlebox.
Format parameters are not parsed by SDP; their content is specific to
the media type/subtype. When format parameters for a specific media
capability are combined from multiple a=mfcap lines which reference
that media capability, the format-specific parameters are
concatenated together and separated by ";" for construction of the
corresponding format attribute (a=fmtp). The resulting format
attribute will look something like the following (without line
breaks):
a=fmtp:<fmt> <fmt-specific-param-list1>;
<fmt-specific-param-list2>;
...
where <fmt> depends on the transport protocol in the manner defined
in RFC4566. SDP cannot assess the legality of the resulting
parameter list in the "a=fmtp" line; the user must take care to
ensure that legal parameter lists are generated.
The "mfcap" attribute can be provided at the session-level and the
media-level. There can be more than one mfcap attribute at the
session or media level. The unique media-cap-num is used to
associate the parameters with a media capability.
As a simple example, a G.729 capability is, by default, considered to
support comfort noise as defined by Annex B. Capabilities for G.729
with and without comfort noise support may thus be defined by:
a=rmcap:1,2 G729/8000
a=mfcap:2 annexb:no
Media capability 1 supports G.729 with Annex B, whereas media
capability 2 supports G.729 without Annex B.
Example for H.263 video:
a=rmcap:1 H263-1998/90000
a=rmcap:2 H263-2000/90000
a=mfcap:1 CIF=4;QCIF=2;F=1;K=1
a=mfcap:2 profile=2;level=2.2
Finally, for six format combinations of the Adaptive MultiRate codec:
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a=rmcap:1-3 AMR/8000/1
a=rmcap:4-6 AMR-WB/16000/1
a=mfcap:1,2,3,4 mode-change-capability=1
a=mfcap:5,6 mode-change-capability=2
a=mfcap:1,2,3,5 max-red=220
a=mfcap:3,4,5,6 octet-align=1
a=mfcap:1,3,5 mode-set=0,2,4,7
a=mfcap:2,4,6 mode-set=0,3,5,6
So that AMR codec #1, when specified in a pcfg attribute within an
audio stream block (and assigned payload type number 98) as in
a=pcfg:1 m=1 pt=1:98
is essentially equivalent to the following
m=audio 49170 RTP/AVP 98
a=rtpmap:98 AMR/8000/1
a=fmtp:98 mode-change-capability=1; \
max-red=220; mode-set=0,2,4,7
and AMR codec #4 with payload type number 99,depicted by the
potential configuration:
a=pcfg:4 m=4, pt=4:99
is equivalent to the following:
m=audio 49170 RTP/AVP 99
a=rtpmap:99 AMR-WB/16000/1
a=fmtp:99 mode-change-capability=1; octet-align=1; \
mode-set=0,3,5,6
and so on for the other four combinations. SDP could thus convert
the media capabilities specifications into one or more alternative
media stream specifications, one of which can be chosen for the
answer.
3.3.3. The Media-Specific Capability Attribute
Attributes and parameters associated with a media format are
typically specified using the "rtpmap" and "fmtp" attributes in SDP,
and the similar "rmcap" and "mfcap" attributes in SDP Media
Capabilities. Some SDP extensions define other attributes that need
to be associated with media formats, for example the "rtcp-fb"
attribute defined in RFC 4585 [RFC4585]. Such media-specific
attributes, beyond the rtpmap and fmtp attributes, may be associated
with media capability numbers via a new media-specific attribute,
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mscap, of the following form:
a=mscap:<media caps star> <att field> <att value>
where <media caps star> is a (list of) media capability number(s),
<att field> is the attribute name, and <att value> is the value field
for the named attribute. Note that the media capability numbers
refer to media format capabilities specified elsewhere in the SDP
("rmcap" and/or "omcap"). If a range of capability numbers is
specified, the first (leftmost) capability number MUST be strictly
smaller than the second (rightmost). The media capability numbers
may include a wildcard ("*"), which will be used instead of any
payload type mappings in the resulting SDP (see, e.g. RFC 4585
[RFC4585] and the example below). In ABNF, we have:
media-specific-capability = "a=mscap:"
media-caps-star
1*WSP att-field ; from RFC4566
1*WSP att-value ; from RFC4566
media-caps-star = media-cap-star-element
*("," media-cap-star-element)
media-cap-star-element = (media-cap-num [wildcard])
/ (media-cap-num-range [wildcard])
wildcard = "*"
Given an association between a media capability and a payload type
number as specified by the pt= parameters in a pcfg attribute line, a
mscap line may be translated easily into a conventional SDP attribute
line of the form
a=<att field>":"<fmt> <att value> ; <fmt> defined in SDP
[RFC4566]
A resulting attribute that is not a legal SDP attribute as specified
by RFC4566 MUST be ignored by the receiver.
If a media capability number (or range) contains a wildcard character
at the end, any payload type mapping specified for that media
specific capability (or range of capabilities) will use the wildcard
character in the resulting SDP instead of the payload type specified
in the payload type mapping ("pt" parameter) in the configuration
attribute.
A single mscap line may refer to multiple media capabilities by use
of a capability number range; this is equivalent to multiple mscap
lines, each with the same attribute values (but different media
capability numbers), one line per media capability.
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Multiple mscap lines may refer to the same media capability, but,
unlike the mfcap attribute, no concatenation operation is defined.
Hence, multiple mscap lines applied to the same media capability is
equivalent to multiple lines of the specified attribute in a
conventional media record.
Here is an example with the rtcp-fb attribute, modified from an
example in RFC 5104 [RFC5104] (with the session-level and audio media
omitted). If the offer contains a media block like the following
(note the wildcard character),
m=video 51372 RTP/AVP 98
a=rtpmap:98 H263-1998/90000
a=tcap:1 RTP/AVPF
a=rmcap:1 H263-1998/90000
a=mscap:1 rtcp-fb ccm tstr
a=mscap:1 rtcp-fb ccm fir
a=mscap:1* rtcp-fb ccm tmmbr smaxpr=120
a=pcfg:1 t=1 m=1 pt=1:98
and if the proposed configuration is chosen, then the equivalent
media block would look like
m=video 51372 RTP/AVPF 98
a=rtpmap:98 H263-1998/90000
a=rtcp-fb:98 ccm tstr
a=rtcp-fb:98 ccm fir
a=rtcp-fb:* ccm tmmbr smaxpr=120
3.3.4. New Configuration Parameters
Along with the new attributes for media capabilities, new extension
parameters are defined for use in the potential configuration, the
actual configuration, and/or the new latent configuration defined in
Section 3.3.5.
3.3.4.1. The Media Configuration Parameter (m=)
The media configuration parameter is used to specify the media
format(s) and related parameters for a potential, actual, or latent
configuration. Adhering to the ABNF for extension-config-list in RFC
5939 [RFC5939] with
ext-cap-name = "m"
ext-cap-list = media-cap-num-list
[*(BAR media-cap-num-list)]
we have
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media-config-list = ["+"] "m=" media-cap-num-list
*(BAR media-cap-num-list)
;BAR is defined in RFC5939
;media-cap-num-list is defined above
Alternative media configurations are separated by a vertical bar
("|"). The alternatives are ordered by preference, most-preferred
first. When media capabilities are not included in a potential
configuration at the media level, the media type and media format
from the associated "m=" line will be used. The use of the plus sign
("+") is described in RFC5939.
3.3.4.2. The Payload Type Number Mapping Parameter (pt=)
The payload type number mapping parameter is used to specify the
payload type number to be associated with each RTP-based media format
in a potential, actual, or latent configuration. We define the
payload type number mapping parameter, payload-number-config-list, in
accordance with the extension-config-list format defined in RFC 5939
[RFC5939]. In ABNF:
payload-number-config-list = ["+"] "pt=" media-map-list
media-map-list = media-map *("," media-map)
media-map = media-cap-num ":" payload-type-number
; media-cap-num is defined in 3.3.1
payload-type-number = NonZeroDigit *2(DIGIT) ; RTP payload
; type number
The example in Section 3.3.7 shows how the parameters from the rmcap
line are mapped to payload type numbers from the pcfg "pt" parameter.
The use of the plus sign ("+") is described in RFC 5939 [RFC5939].
A latent configuration represents a future capability, hence the pt=
parameter is not directly meaningful in the lcfg attribute because no
actual media session is being offered or accepted; it is permitted in
order to tie any payload type number parameters within attributes to
the proper media format. A primary example is the case of format
parameters for the Redundant Audio Data (RED) payload, which are
payload type numbers. Specific payload type numbers used in a latent
configuration MAY be interpreted as suggestions to be used in any
future offer based on the latent configuration, but they are not
binding; the offerer and/or answerer may use any payload type numbers
each deems appropriate. The use of explicit payload type numbers for
latent configurations can be avoided by use of the parameter
substitution rule of Section 3.3.7. Future extensions are also
permitted. Note that leading zeroes are not permitted.
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3.3.4.3. The Media Type Parameter
When a latent configuration is specified (always at the media level),
indicating the ability to support an additional media stream, it is
necessary to specify the media type (audio, video, etc.) as well as
the format and transport type. The media type parameter is defined
in ABNF as
media-type = ["+"] "mt=" media; media defined in RFC4566
At present, the media-type parameter is used only in the latent
configuration attribute, and the use of the "+" prefix to specify
that the entire attribute line is to be ignored if the mt= parameter
is not understood, is unnecessary. However, if the media-type
parameter is later added to an existing capability attribute such as
pcfg, then the "+" would be useful. The media format(s) and
transport type(s) are specified using the media configuration
parameter ("+m=") defined above, and the transport parameter ("t=")
defined in RFC 5939 [RFC5939], respectively.
3.3.5. The Latent Configuration Attribute
One of the goals of this work is to permit the exchange of
supportable media configurations in addition to those offered or
accepted for immediate use. Such configurations are referred to as
"latent configurations". For example, a party may offer to establish
a session with an audio stream, and, at the same time, announce its
ability to support a video stream as part of the same session. The
offerer can supply its video capabilities by offering one or more
latent video configurations along with the media stream for audio;
the responding party may indicate its ability and willingness to
support such a video session by returning a corresponding latent
configuration.
Latent configurations returned in SDP answers MUST match offered
latent configurations (or parameter subsets thereof). Therefore, it
is appropriate for the offering party to announce most, if not all,
of its capabilities in the initial offer. This choice has been made
in order to keep the size of the answer more compact by not requiring
acap, rmcap, tcap, etc. lines in the answer.
Latent configurations may be announced by use of the latent
configuration attribute, which is defined in a manner very similar to
the potential configuration attribute. The latent configuration
attribute combines the properties of a media line and a potential
configuration. A latent configuration MUST include a media type
(mt=) and a transport protocol configuration parameter since the
latent configuration is independent of any media line present. In
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most cases, the media configuration (m=) parameter needs to be
present as well (see Section 4 for examples). The lcfg attribute is
a media level attribute.
The lcfg attribute is defined as a media level attribute since it
specifies a possible future media stream. However the lcfg
attribute is not necessarily related to the media description
within which it is provided. Session capabilities ("sescap") may
be used to indicate this.
Each media line in an SDP description represents an offered
simultaneous media stream, whereas each latent configuration
represents an additional stream which may be negotiated in a future
offer/answer exchange. Session capability attributes may be used to
determine whether a latent configuration may be used to form an offer
for an additional simultaneous stream or to reconfigure an existing
stream in a subsequent offer/answer exchange.
The latent configuration attribute is of the form:
a=lcfg:<config-number> <latent-cfg-list>
which adheres to the SDP [RFC4566] "attribute" production with att-
field and att-value defined as:
att-field = "lcfg"
att-value = config-number 1*WSP lcfg-cfg-list
config-number = NonZeroDigit *9(DIGIT) ; DIGIT defined in RFC5234
lcfg-cfg-list = media-type 1*WSP pot-cfg-list
; as defined in RFC5939
; and extended herein
The media-type (mt=) parameter identifies the media type (audio,
video, etc.) to be associated with the latent media stream, and MUST
be present. The pot-cfg-list MUST contain a transport-protocol-
config-list (t=) parameter and a media-config-list (m=) parameter.
The pot-cfg-list MUST NOT contain more than one instance of each type
of parameter list. As specified in RFC 5939 [RFC5939], the use of
the "+" prefix with a parameter indicates that the entire
configuration MUST be ignored if the parameter is not understood;
otherwise, the parameter itself may be ignored.
Media stream payload numbers are not assigned by a latent
configuration. Assignment will take place if and when the
corresponding stream is actually offered via an m-line in a later
exchange. The payload-number-config-list is included as a parameter
to the lcfg attribute in case it is necessary to tie payload numbers
in attribute capabilities to specific media capabilities.
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If an lcfg attribute invokes an acap attribute that appears at the
session level, then that attribute will be expected to appear at the
session level of a subsequent offer when and if a corresponding media
stream is offered. Otherwise, acap attributes which appear at the
media level represent media-level attributes. Note, however, that
rmcap, omcap, mfcap, mscap, and tcap attributes may appear at the
session level because they always result in media-level attributes or
m-line parameters.
The configuration numbers for latent configurations do not imply a
preference; the offerer will imply a preference when actually
offering potential configurations derived from latent configurations
negotiated earlier. Note however that the offerer of latent
configurations MAY specify preferences for combinations of potential
and latent configurations by use of the sescap attribute defined in
Section 3.3.8. For example, if an SDP offer contains, say, an audio
stream with pcfg:1, and two latent video configurations, lcfg:2, and
lcfg:3, then a session with one audio stream and one video stream
could be specified by including "a=sescap:1 1,2|3". One audio stream
and two video streams could be specified by including "a=sescap:2
1,2,3" in the offer. In order to permit combinations of latent and
potential configurations in session capabilities, latent
configuration numbers MUST be different from those used for potential
configurations. This restriction is especially important if the
offerer does not require cmed-v0 capability and the recipient of the
offer doesn't support it. If the lcfg attribute is not recognized,
the capability attributes intended to be associated with it may be
confused with those associated with a potential configuration of some
other media stream. Note also that leading zeroes are not permitted
in configuration numbers.
If a cryptographic attribute, such as the SDES "a=crypto:" attribute
[RFC4568], is referenced by a latent configuration through an acap
attribute, any keying material required in the conventional
attribute, such as the SDES key/salt string, MUST be included in
order to satisfy formatting rules for the attribute. Since the
keying material will be visible but not actually used at this stage
(since it's a latent configuration), the value(s) of the keying
material MUST NOT be a real value used for real exchange of media,
and the receiver of the lcfg attribute MUST ignore the values.
3.3.6. Enhanced Potential Configuration Attribute
The present work requires new extensions (parameters) for the pcfg
attribute defined in the SDP Capability Negotiation base protocol
[RFC5939]. The parameters and their definitions are "borrowed" from
the definitions provided for the latent configuration attribute in
Section 3.3.5. The expanded ABNF definition of the pcfg attribute is
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a=pcfg: <config-number> [<pot-cfg-list>]
where
config-number = 1*DIGIT ;defined in [RFC5234]
pot-cfg-list = pot-config *(1*WSP pot-config)
pot-config = attribute-config-list / ;def in [RFC5939]
transport-protocol-config-list / ;defined in [RFC5939]
extension-config-list / ;[RFC5939]
media-config-list / ; Section 3.3.4.1
payload-number-config-list ; Section 3.3.4.2
Except for the extension-config-list, the pot-cfg-list MUST NOT
contain more than one instance of each parameter list.
3.3.6.1. Returning Capabilities in the Answer
Potential and/or latent configuration attributes may be returned
within an answer SDP to indicate the ability of the answerer to
support alternative configurations of the corresponding stream(s).
For example, an offer may include multiple potential configurations
for a media stream and/or latent configurations for additional
streams; the corresponding answer will indicate (via an acfg
attribute) the configuration accepted and used to construct the base
configuration for each active media stream in the reply, but the
reply MAY also contain potential and/or latent configuration
attributes, with parameters, to indicate which other offered
configurations would be acceptable. This information is useful if it
becomes desirable to reconfigure a media stream, e.g., to reduce
resource consumption.
When potential and/or latent configurations are returned in an
answer, all numbering MUST refer to the configuration and capability
attribute numbering of the offer. The offered capability attributes
need not be returned in the answer. The answer MAY include
additional capability attributes and/or configurations (with distinct
numbering). The parameter values of any returned pcfg or lcfg
attributes MUST be a subset of those included in the offered
configurations and/or those added by the answerer; values MAY be
omitted only if they were indicated as alternative sets, or optional,
in the original offer. The parameter set indicated in the returned
acfg attribute need not be repeated in a returned pcfg attribute.
The answerer MAY return more than one pcfg attribute with the same
configuration number if it is necessary to describe selected
combinations of optional or alternative parameters.
Similarly, one or more session capability attributes (a=sescap) MAY
be returned to indicate which of the offered session capabilities is/
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are supportable by the answerer (see Section 3.3.8.)
Note that, although the answerer MAY return capabilities beyond those
included by the offerer, these capabilities MUST NOT be used to form
any base level media description in the answer. For this reason, it
is advisable for the offerer to include most, if not all, potential
and latent configurations it can support in the initial offer, unless
the size of the resulting SDP is a concern. Either party MAY later
announce additional capabilities by renegotiating the session in a
second offer/answer exchange.
3.3.6.2. Payload Type Number Mapping
When media format capabilities defined in rmcap attributes are used
in potential configuration lines, the transport protocol uses RTP and
it is necessary to assign payload type numbers. In some cases, it is
desirable to assign different payload type numbers to the same media
format capability when used in different potential configurations.
One example is when configurations for AVP and SAVP are offered: the
offerer would like the answerer to use different payload type numbers
for encrypted and unencrypted media, so the offerer can decide
whether or not to render early media which arrives before the answer
is received.
For example, if use of AVP was selected by the answerer, then
media received by the offerer is not encrypted and hence can be
played out prior to receiving the answer. Conversely, if SAVP was
selected, cryptographic parameters and keying material present in
the answer may be needed to decrypt received media. If the offer
configuration indicated that AVP media uses one set of payload
types and SAVP a different set, then the offerer will know whether
media received prior to the answer is encrypted or not by simply
looking at the RTP payload type number in the received packet.
This association of distinct payload type number(s) with different
transport protocols requires a separate pcfg line for each protocol.
Clearly, this technique cannot be used if the number of potential
configurations exceeds the number of possible payload type numbers.
3.3.6.3. Processing of Media-Format-Related Conventional Attributes for
Potential Configurations
When media capabilities negotiation is employed, SDP records are
likely to contain conventional attributes such as rtpmap, fmtp, and
other media-format-related lines, as well as capability attributes
such as rmcap, omcap, mfcap, and mscap which map into those
conventional attributes when invoked by a potential configuration.
In such cases, it MAY be appropriate to employ the delete-attributes
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option [RFC5939] in the attribute configuration list parameter in
order to avoid the generation of conflicting fmtp attributes for a
particular configuration. Any media-specific attributes in the media
block which refer to media formats not used by the potential
configuration MUST be ignored.
For example:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
a=creq:med-v0
m=audio 3456 RTP/AVP 0 18 100
a=rtpmap:100 telephone-event
a=fmtp:100 0-11
a=rmcap:1 PCMU/8000
a=rmcap:2 G729/8000
a=rmcap:3 telephone-event/8000
a=mfcap:3 0-15
a=pcfg:1 m=2,3|1,3 a=-m pt=1:0,2:18,3:100
a=pcfg:2
In this example, PCMU is media capability 1, G729 is media capability
2, and telephone-event is media capability 3. The a=pcfg:1 line
specifies that the preferred configuration is G.729 with extended
dtmf events, second is G.711 mu-law with extended dtmf events, and
the base media-level attributes are to be deleted. Intermixing of
G.729, G.711, and "commercial" dtmf events is least preferred (the
base configuration provided by the "m=" line, which is, by default,
the least preferred configuration). The rtpmap and fmtp attributes
of the base configuration are replaced by the rmcap and mfcap
attributes when invoked by the proposed configuration.
If the preferred configuration is selected, the SDP answer will look
like
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
a=csup:med-v0
m=audio 3456 RTP/AVP 18 100
a=rtpmap:100 telephone-event/8000
a=fmtp:100 0-15
a=acfg:1 m=2,3 pt=1:0,2:18,3:100
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3.3.7. Substitution of Media Payload Type Numbers in Capability
Attribute Parameters
In some cases, for example, when an RFC 2198 [RFC2198] redundancy
audio subtype (RED) capability is defined in an mfcap attribute, the
parameters to an attribute may contain payload type numbers. Two
options are available for specifying such payload type numbers. They
may be expressed explicitly, in which case they are bound to actual
payload types by means of the payload type number parameter (pt=) in
the appropriate potential or latent configuration. For example, the
following SDP fragment defines a potential configuration with
redundant G.711 mu-law:
m=audio 45678 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=rmcap:1 PCMU/8000
a=rmcap:2 RED/8000
a=mfcap:2 0/0
a=pcfg:1 m=2,1 pt=2:98,1:0
The potential configuration is then equivalent to
m=audio 45678 RTP/AVP 98 0
a=rtpmap:0 PCMU/8000
a=rtpmap:98 RED/8000
a=fmtp:98 0/0
A more general mechanism is provided via the parameter substitution
rule. When an mfcap, mscap, or acap attribute is processed, its
arguments will be scanned for a payload type number escape sequences
of the following form (in ABNF):
ptn-esc = "%m=" media-cap-num "%" ; defined in 3.3.1
If the sequence is found, the sequence is replaced by the payload
type number assigned to the media capability number, as specified by
the pt= parameter in the selected potential configuration; only
actual payload type numbers are supported - wildcards are excluded.
The sequence "%%" (null digit string) is replaced by a single percent
sign and processing continues with the next character, if any.
For example, the above offer sequence could have been written as
m=audio 45678 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=rmcap:1 PCMU/8000
a=rmcap:2 RED/8000
a=mfcap:2 %m=1%/%m=1%
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a=pcfg:1 m=2,1 pt=2:98,1:0
and the equivalent SDP is the same as above.
3.3.8. The Session Capability Attribute
Potential and latent configurations enable offerers and answerers to
express a wide range of alternative configurations for current and
future negotiation. However in practice, it may not be possible to
support all combinations of these configurations.
The session capability attribute provides a means for the offerer
and/or the answerer to specify combinations of specific media stream
configurations which it is willing and able to support. Each session
capability in an offer or answer MAY be expressed as a list of
required potential configurations, and MAY include a list of optional
potential and/or latent configurations.
The choices of session capabilities may be based on processing load,
total bandwidth, or any other criteria of importance to the
communicating parties. If the answerer supports media capabilities
negotiation, and session configurations are offered, it MUST accept
one of the offered configurations, or it MUST refuse the session.
Therefore, if the offer includes any session capabilities, it SHOULD
include all the session capabilities the offerer is willing to
support.
The session capability attribute is a session-level attribute
described by:
"a=sescap:" <session num> <list of configs>
which corresponds to the standard value attribute definition with
att-field = "sescap"
att-value = session-num 1*WSP list-of-configs
[1*WSP optional-configs]
session-num = NonZeroDigit *9(DIGIT) ; DIGIT defined
; in RFC5234
list-of-configs = alt-config *("," alt-config)
optional-configs = "[" list-of-configs "]"
alt-config = config-number *("|" config-number)
The session-num identifies the session: a lower-number session is
preferred over a higher-number session, and leading zeroes are not
permitted. Each alt-config list specifies alternative media
configurations within the session; preference is based on config-num
as specified in RFC 5939 [RFC5939]. Note that the session preference
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order, when present, takes precedence over the individual media
stream configuration preference order.
Use of session capability attributes requires that configuration
numbers assigned to potential and latent configurations MUST be
unique across the entire session; RFC 5939 [RFC5939] requires only
that pcfg configuration numbers be unique within a media description.
Also, leading zeroes are not permitted.
As an example, consider an endpoint that is capable of supporting an
audio stream with either one H.264 video stream or two H.263 video
streams with a floor control stream. In the latter case, the second
video stream is optional. The SDP offer might look like the
following (offering audio, an H.263 video streams, BFCP and another
optional H.263 video stream)- the empty lines are added for
readability only (not part of valid SDP):
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
a=creq:med-v0
a=sescap:2 1,2,5,[3]
a=sescap:1 1,4
m=audio 54322 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=pcfg:1
m=video 22344 RTP/AVP 102
a=rtpmap:102 H263-1998/90000
a=fmtp:102 CIF=4;QCIF=2;F=1;K=1
i=main video stream
a=label:11
a=pcfg:2
a=rmcap:1 H264/90000
a=mfcap:1 profile-level-id=42A01E; packetization-mode=2
a=acap:1 label:13
a=pcfg:4 m=1 a=1 pt=1:104
m=video 33444 RTP/AVP 103
a=rtpmap:103 H263-1998/90000
a=fmtp:103 CIF=4;QCIF=2;F=1;K=1
i=secondary video (slides)
a=label:12
a=pcfg:3
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m=application 33002 TCP/BFCP *
a=setup:passive
a=connection:new
a=floorid:1 m-stream:11 12
a=floor-control:s-only
a=confid:4321
a=userid:1234
a=pcfg:5
If the answerer understands MediaCapNeg, but cannot support the
Binary Floor Control Protocol, then it would respond with (invalid
empty lines in SDP included again for readability):
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.22
t=0 0
a=csup:med-v0
a=sescap:1 1,4
m=audio 23456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=acfg:1
m=video 41234 RTP/AVP 104
a=rtpmap:104 H264/90000
a=fmtp:104 profile-level-id=42A01E; packetization-mode=2
a=acfg:4 m=1 a=1 pt=1:104
m=video 0 RTP/AVP 103
a=acfg:3
m=application 0 TCP/BFCP *
a=acfg:5
An endpoint that doesn't support Media capabilities negotiation, but
does support H.263 video, would respond with one or two H.263 video
streams. In the latter case, the answerer may issue a second offer
to reconfigure the session to one audio and one video channel using
H.264 or H.263.
Session capabilities can include latent capabilities as well. Here's
a similar example in which the offerer wishes to initially establish
an audio stream, and prefers to later establish two video streams
with chair control. If the answerer doesn't understand Media CapNeg,
or cannot support the dual video streams or flow control, then it may
support a single H.264 video stream. Note that establishment of the
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most favored configuration will require two offer/answer exchanges.
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
a=creq:med-v0
a=sescap:1 1,3,4,5
a=sescap:2 1,2
a=sescap:3 1
a=rmcap:1 H263-1998/90000
a=mfcap:1 CIF=4;QCIF=2;F=1;K=1
a=tcap:1 RTP/AVP TCP/BFCP
m=audio 54322 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=pcfg:1
m=video 22344 RTP/AVP 102
a=rtpmap:102 H264/90000
a=fmtp:102 profile-level-id=42A01E; packetization-mode=2
a=label:11
a=content:main
a=pcfg:2
a=lcfg:3 mt=video t=1 m=1 a=31,32
a=acap:31 label:12
a=acap:32 content:main
a=lcfg:4 mt=video t=1 m=1 a=41,42
a=acap:41 label:13
a=acap:42 content:slides
a=lcfg:5 mt=application m=51 t=51
a=tcap:51 TCP/BFCP
a=omcap:51 *
a=acap:51 setup:passive
a=acap:52 connection:new
a=acap:53 floorid:1 m-stream:12 13
a=acap:54 floor-control:s-only
a=acap:55 confid:4321
a=acap:56 userid:1234
In this example, the default offer, as seen by endpoints which do not
understand capabilities negotiation, proposes a PCMU audio stream and
an H.264 video stream. Note that the offered lcfg lines for the
video streams don't carry pt= parameters because they're not needed
(payload type numbers will be assigned in the offer/answer exchange
that establishes the streams). Note also that the three rmcap,
mfcap, and tcap attributes used by lcfg:3 and lcfg:4 are included at
the session level so they may be referenced by both latent
configurations. As per Section 3.3, the media attributes generated
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from the rmcap, mfcap, and tcap attributes are always media-level
attributes. If the answerer supports Media CapNeg, and supports the
most desired configuration, it would return the following SDP:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.22
t=0 0
a=csup:med-v0
a=sescap:1 1,3,4,5
a=sescap:2 1,2
a=sescap:3 1
m=audio 23456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=acfg:1
m=video 0 RTP/AVP 102
a=pcfg:2
a=lcfg:3 mt=video t=1 m=1 a=31,32
a=lcfg:4 mt=video t=1 m=1 a=41,42
a=lcfg:5 mt=application t=2
This exchange supports immediate establishment of an audio stream for
preliminary conversation. This exchange would presumably be followed
at the appropriate time with a "reconfiguration" offer/answer
exchange to add the video and chair control streams.
3.4. Offer/Answer Model Extensions
In this section, we define extensions to the offer/answer model
defined in RFC 3264 [RFC3264] and RFC 5939 [RFC5939] to allow for
media format and associated parameter capabilities, latent
configurations and acceptable combinations of media stream
configurations to be used with the SDP Capability Negotiation
framework. Note that the procedures defined in this section extend
the offer/answer procedures defined in RFC 5939 [RFC5939] Section 6;
those procedures form a baseline set of capability negotiation offer/
answer procedures that MUST be followed, subject to the extensions
defined here.
SDP Capability Negotiation [RFC5939] provides a relatively compact
means to offer the equivalent of an ordered list of alternative
configurations for offered media streams (as would be described by
separate m= lines and associated attributes). The attributes acap,
mscap, mfcap, omcap and rmcap are designed to map somewhat
straightforwardly into equivalent m= lines and conventional
attributes when invoked by a pcfg, lcfg, or acfg attribute with
appropriate parameters. The a=pcfg: lines, along with the m= line
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itself, represent offered media configurations. The a=lcfg: lines
represent alternative capabilities for future use.
3.4.1. Generating the Initial Offer
The Media Capabilities negotiation extensions defined in this
document cover the following categories of features:
o Media Format Capabilities and associated parameters (rmcap, omcap,
mfcap, and mscap attributes)
o Potential configurations using those media format capabilities and
associated parameters
o Latent media streams (lcfg attribute)
o Acceptable combinations of media stream configurations (sescap
attribute).
The high-level description of the operation is as follows:
When an endpoint generates an initial offer and wants to use the
functionality described in the current document, it SHOULD identify
and define the media formats and associated parameters it can support
via the rmcap, omcap, mfcap and mscap attributes. The SDP media
line(s) ("m=") should be made up with the actual configuration to be
used if the other party does not understand capability negotiations
(by default, this is the least preferred configuration). Typically,
the media line configuration will contain the minimum acceptable
configuration from the offerer's point of view.
Preferred configurations for each media stream are identified
following the media line. The present offer may also include latent
configuration (lcfg) attributes, at the media level, describing media
streams and/or configurations the offerer is not now offering, but
which it is willing to support in a future offer/answer exchange. A
simple example might be the inclusion of a latent video configuration
in an offer for an audio stream.
Lastly, if the offerer wishes to impose restrictions on the
combinations of potential configurations to be used, it will include
session capability (sescap) attributes indicating those.
If the offerer requires the answerer to understand the media
capability extensions, the offerer MUST include a creq attribute
containing the value "med-v0". If media capability negotiation is
required only for specific media descriptions, the "med-v0" value
MUST be provided only in creq attributes within those media
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descriptions, as described in RFC 5939 [RFC5939].
Below, we provide a more detailed description of how to construct the
offer SDP.
3.4.1.1. Offer with Media Capabilities
For each RTP-based media format the offerer wants to include as a
media format capability, the offer MUST include an "rmcap" attribute
for the media format as defined in Section 3.3.1.
For each non RTP-based media format the offer wants to include as a
media format capability, the offer MUST include an "omcap" attribute
for the media format as defined in Section 3.3.1.
Since the media capability number space is shared between the rmcap
and omcap attributes, each media capability number provided
(including ranges) MUST be unique in the entire SDP.
If an "fmtp" parameter value is needed for a media format (whether
RTP-based or not) in a media capability, then the offer MUST include
one or more "mfcap" parameters with the relevant fmtp parameter
values for that media format as defined in Section 3.3.2. When
multiple "mfcap" parameters are provided for a given media
capability, they MUST be provided in accordance with the
concatenation rules in Section 3.3.2.1.
For each of the media format capabilities above, the offer MAY
include one or more "mscap" parameters with attributes needed for
those specific media formats as defined in Section 3.3.3. Such
attributes will be instantiated at the media-level, and hence
session-level only attributes MUST NOT be used in the "mscap"
parameter. The "mscap" parameter MUST NOT include an "rtpmap" or
"fmtp" attribute (rmcap and mfcap are used instead).
If the offerer wants to limit the relevance (and use) of a media
format capability or parameter to a particular media stream, the
media format capability or parameter MUST be provided within the
corresponding media description. Otherwise, the media format
capabilities and parameters MUST be provided at the session level.
Note however, that the attribute or parameter embedded in these will
always be instantiated at the media-level.
This is due to those parameters being effectively media-level
parameters. If session-level attributes are needed, the "acap"
attribute defined in RFC 5939 [RFC5939] can be used, however it
does not provide for media format-specific instantiation.
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Inclusion of the above does not constitute an offer to use the
capabilities; a potential configuration is needed for that. If the
offerer wants to offer one or more of the media capabilities above,
they MUST be included as part of a potential configuration (pcfg)
attribute as defined in Section 3.3.4. Each potential configuration
MUST include a config-number, and each config-number MUST be unique
in the entire SDP (note that this differs from RFC 5939 [RFC5939],
which only requires uniqueness within a media description). Also,
the config-number MUST NOT overlap with any config-number used by a
latent configuration in the SDP. As described in RFC 5939 [RFC5939],
lower config-numbers indicate a higher preference; the ordering still
applies within a given media description only though.
For a media capability to be included in a potential configuration,
there MUST be an "m=" parameter in the pcfg attribute referencing the
media capability number in question. When one or more media
capabilities are included in an offered potential configuration
(pcfg), they completely replace the list of media formats offered in
the actual configuration (m= line). Any attributes included for
those formats remain in the SDP though (e.g., rtpmap, fmtp, etc.).
For non-RTP based media formats, the format-name (from the "omcap"
media capability) is simply added to the "m=" line as a media format
(e.g. t38). For RTP-based media, payload type mappings MUST be
provided by use of the "pt" parameter in the potential configuration
(see Section 3.3.4.2); payload type escaping may be used in mfcap,
mscap, and acap attributes as defined in Section 3.3.7.
Note that the "mt" parameter MUST NOT be used with the pcfg attribute
(since it is defined for the lcfg attribute only); the media type in
a potential configuration cannot be changed from that of the
encompassing media description.
3.4.1.2. Offer with Latent Configuration
If the offerer wishes to offer one or more latent configurations for
future use, the offer MUST include a latent configuration attribute
(lcfg) for each as defined in Section 3.3.5.
Each lcfg attribute
o MUST be specified at the media level
o MUST include a config-number that is unique in the entire SDP
(incl. for any potential configuration attributes). Note that
config-numbers in latent configurations do not indicate any
preference order
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o MUST include a media type ("mt")
o MUST reference a valid transport capability ("t")
Each lcfg attribute MAY include additional capability references,
which may refer to capabilities anywhere in the session description,
subject to any restrictions normally associated with such
capabilities. For example, a media-level attribute capability must
be present at the media-level in some media description in the SDP.
Note that this differs from the potential configuration attribute,
which cannot validly refer to media-level capabilities in another
media description (per RFC 5939 [RFC5939], Section 3.5.1).
Potential configurations constitute an actual offer and hence may
instantiate a referenced capability. Latent configurations are
not actual offers and hence cannot instantiate a referenced
capability; it is therefore safe for those to refer to
capabilities in another media description.
3.4.1.3. Offer with Configuration Combination Restrictions
If the offerer wants to indicate restrictions or preferences among
combinations of potential and/or latent configuration, a session
capability (sescap) attribute MUST be provided at the session-level
for each such combination as described in Section 3.3.8. Each sescap
attribute MUST include a session-num that is unique in the entire
SDP; the lower the session-num the more preferred that combination
is. Furthermore, sescap preference order takes precedence over any
order specified in individual pcfg attributes.
For example, if we have pcfg-1 and pcfg-2, and sescap-1 references
pcfg-2, whereas sescap-2 references pcfg-1, then pcfg-2 will be
the most preferred potential configuration. Without the sescap,
pcfg-1 would be the most preferred.
3.4.2. Generating the Answer
When receiving an offer, the answerer MUST check the offer for creq
attributes containing the value "med-v0"; answerers compliant with
this specification will support this value in accordance with the
procedures specified in RFC 5939 [RFC5939].
The SDP MAY contain
o Media format capabilities and associated parameters (rmcap, omcap,
mfcap, and mscap attributes)
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o Potential configurations using those media format capabilities and
associated parameters
o Latent media streams (lcfg attribute)
o Acceptable combinations of media stream configurations (sescap
attribute)
The high-level informative description of the operation is as
follows:
When the answering party receives the offer and if it supports the
required capability negotiation extensions, it should select the
most-preferred configuration it can support for each media stream,
and build its answer accordingly. The configuration selected for
each accepted media stream is placed into the answer as a media line
with associated parameters and attributes. If a proposed
configuration is chosen for a given media stream, the answer must
contain an actual configuration (acfg) attribute for that media
stream to indicate which offered pcfg attribute was used to build the
answer. The answer should also include any potential or latent
configurations the answerer can support, especially any
configurations compatible with other potential or latent
configurations received in the offer. The answerer should make note
of those configurations it might wish to offer in the future.
Below we provide a more detailed normative description of how the
answerer processes the offer SDP and generates an answer SDP.
3.4.2.1. Processing Media Capabilities and Potential Configurations
The answerer MUST first determine if it needs to perform media
capability negotiation by examining the SDP for valid and preferred
potential configuration attributes that include media configuration
parameters (i.e., an "m" parameter in the pcfg attribute).
Such a potential configuration is valid if:
1. It is valid according to the rules defined in RFC 5939 [RFC5939]
2. It contains a config-number that is unique in the entire SDP and
does not overlap with any latent configuration config-numbers
3. All media format capabilities (rmcap or omcap), media format
parameter capabilities (mfcap), and media-specific capabilities
(mscap) referenced by the potential configuration ("m" parameter)
are valid themselves (as defined in Section 3.3.1, 3.3.2, and
3.3.3) and each of them is provided either at the session level
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or within this particular media description.
4. All RTP-based media format capabilities (rmcap) have a
corresponding payload type ("pt") parameter in the potential
configuration that result in mapping to a valid payload type that
is unique within the resulting SDP.
5. Any concatenation (see Section 3.3.2.1) and substitution (see
Section 3.3.7) applied to any capability (mfcap, mscap, or acap)
referenced by this potential configuration results in a valid
SDP.
Note that, since SDP does not interpret the value of fmtp parameters,
any resulting fmtp parameter value will be considered valid.
Secondly, the answerer MUST determine the order in which potential
configurations are to be negotiated. In the absence of any Session
Capability ("sescap") attributes, this simply follows the rules of
RFC 5939 [RFC5939], with a lower config-number within a media
description being preferred over a higher one. If a valid "sescap"
attribute is present, the preference order provided in the "sescap"
attribute MUST take precedence. A "sescap" attribute is considered
valid if:
1. It adheres to the rules provided in Section 3.3.8.
2. All the configurations referenced by the "sescap" attribute are
valid themselves (note that this can include the actual,
potential and latent configurations).
The answerer MUST now process the offer for each media stream based
on the most preferred valid potential configuration in accordance
with the procedures specified in RFC 5939 [RFC5939], Section 3.6.2,
and further extended below:
o If one or more media format capabilities are included in the
potential configuration, then they replace all media formats
provided in the "m=" line for that media description. For non-RTP
based media formats (omcap), the format-name is added. For RTP-
based media formats (rmcap), the payload-type specified in the
payload-type mapping ("pt") is added and a corresponding "rtpmap"
attribute is added to the media description.
o If one or more media format parameter capabilities are included in
the potential configuration, then the corresponding "fmtp"
attributes are added to the media description. Note that this
inclusion is done indirectly via the media format capability.
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o If one or more media-specific capabilities are included in the
potential configuration, then the corresponding attributes are
added to the media description. Note that this inclusion is done
indirectly via the media format capability.
o When checking to see if the answerer supports a given potential
configuration that includes one or more media format capabilities,
the answerer MUST support at least one of the media formats
offered. If he does not, the answerer MUST proceed to the next
potential configuration based on the preference order that
applies.
o If Session Capability ("sescap") preference ordering is included,
then the potential configuration selection process MUST adhere to
the ordering provided. Note that this may involve coordinated
selection of potential configurations between media descriptions.
The answerer MUST accept one of the offered "sescap" combinations
(i.e. all the required potential configurations specified) or it
MUST reject the entire session.
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 answer SDP as described in RFC
5939 [RFC5939], Section 3.6.2, and further extended below:
o Additional answer capabilities and potential configurations MAY be
returned in accordance with Section 3.3.6.1. Capability numbers
and configuration numbers for those MUST be distinct from the ones
used in the offer SDP.
o Latent configuration processing and answer generation MUST be
performed, as specified below.
o Session capability specification for the potential and latent
configurations in the answer MAY be included (see Section 3.3.8).
3.4.2.2. Latent Configuration Processing
The answerer MUST determine if it needs to perform any latent
configuration processing by examining the SDP for valid latent
configuration attributes (lcfg). An lcfg attribute is considered
valid if:
o It adheres to the description in Section 3.3.5.
o It includes a config-number that is unique in the entire SDP and
does not overlap with any potential configuration config-number
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o It includes a valid media type ("mt=")
o It references a valid transport capability ("t=")
o All other capabilities referenced by it are valid.
For each such valid latent configuration in the offer, the answerer
checks to see if it could support the latent configuration in a
subsequent offer/answer exchange. If so, it includes the latent
configuration with the same configuration number in the answer,
similar to the way potential configurations are processed and the
selected one returned in an actual configuration attribute (see RFC
5939 [RFC5939]). If the answerer supports only a (non-mandatory)
subset of the parameters offered in a latent configuration, the
answer latent configuration will include only those parameters
supported (similar to "acfg" processing). Note that latent
configurations do not constitute an actual offer at this point in
time; they merely indicate additional configurations that could be
supported.
If a Session Capability ("sescap") attribute is included and it
references a latent configuration, then the answerer processing of
that latent configuration must be done within the constraints
specified by that Session Capability, i.e. it must be possible to
support it at the same time as any required (i.e. non-optional)
potential configurations in the session capability. The answerer may
in turn add his own "sescap" indications in the answer as well.
3.4.3. Offerer Processing of the Answer
The offerer MUST process the answer in accordance with RFC 5939
[RFC5939] Section 3.6.3, and further explained below.
When the offerer processes the answer SDP based on a valid actual
configuration attribute in the answer, and that valid configuration
includes one or more media capabilities, the processing MUST
furthermore be done as if the offer was sent using those media
capabilities instead of the actual configuration. In particular, the
media formats in the "m=" line, and any associated payload type
mappings (rtpmap), fmtp parameters (mfcap) and media-specific
attributes (mscap) MUST be used. Note that this may involve use of
concatenation and substitution rules (see Section 3.3.2.1 and 3.3.7).
The actual configuration attribute may also be used to infer the lack
of acceptability of higher-preference configurations that were not
chosen, subject to any constraints provided by a Session Capability
attribute ("sescap") in the offer. Note that the SDP Capability
Negotiation base specification [RFC5939] requires the answerer to
choose the highest preference configuration it can support, subject
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to local policies.
When the offerer receives the answer, it SHOULD furthermore make note
of any capabilities and/or latent configurations included for future
use, and any constraints on how those may be combined.
3.4.4. Modifying the Session
If, at a later time, one of the parties wishes to modify the
operating parameters of a session, e.g., by adding a new media
stream, or by changing the properties used on an existing stream, it
can do so via the mechanisms defined for offer/answer [RFC3264]. If
the initiating party has remembered the codecs, potential
configurations, latent configurations and session capabilities
provided by the other party in the earlier negotiation, it MAY use
this knowledge to maximize the likelihood of a successful
modification of the session. Alternatively, the initiator MAY
perform a new capabilities exchange as part of the reconfiguration.
In such a case, the new capabilities will replace the previously-
negotiated capabilities. This may be useful if conditions change on
the endpoint.
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4. Examples
In this section, we provide examples showing how to use the Media
Capabilities with the SDP Capability Negotiation.
4.1. Alternative Codecs
This example provides a choice of one of six variations of the
adaptive multirate codec. In this example, the default configuration
as specified by the media line is the same as the most preferred
configuration. Each configuration uses a different payload type
number so the offerer can interpret early media.
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
a=creq:med-v0
m=audio 54322 RTP/AVP 96
a=rtpmap:96 AMR-WB/16000/1
a=fmtp:96 mode-change-capability=1; max-red=220; \
mode-set=0,2,4,7
a=rmcap:1,3,5 audio AMR-WB/16000/1
a=rmcap:2,4,6 audio AMR/8000/1
a=mfcap:1,2,3,4 mode-change-capability=1
a=mfcap:5,6 mode-change-capability=2
a=mfcap:1,2,3,5 max-red=220
a=mfcap:3,4,5,6 octet-align=1
a=mfcap:1,3,5 mode-set=0,2,4,7
a=mfcap:2,4,6 mode-set=0,3,5,6
a=pcfg:1 m=1 pt=1:96
a=pcfg:2 m=2 pt=2:97
a=pcfg:3 m=3 pt=3:98
a=pcfg:4 m=4 pt=4:99
a=pcfg:5 m=5 pt=5:100
a=pcfg:6 m=6 pt=6:101
In the above example, media capability 1 could have been excluded
from the first rmcap declaration and from the corresponding mfcap
attributes, and the pcfg:1 attribute line could have been simply
"pcfg:1".
The next example offers a video stream with three options of H.264
and 4 transports. It also includes an audio stream with different
audio qualities: four variations of AMR, or AC3. The offer looks
something like:
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v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=An SDP Media NEG example
c=IN IP4 192.0.2.1
t=0 0
a=creq:med-v0
a=ice-pwd:speEc3QGZiNWpVLFJhQX
m=video 49170 RTP/AVP 100
c=IN IP4 192.0.2.56
a=maxprate:1000
a=rtcp:51540
a=sendonly
a=candidate 12345 1 UDP 9 192.0.2.56 49170 host
a=candidate 23456 2 UDP 9 192.0.2.56 51540 host
a=candidate 34567 1 UDP 7 198.51.100.1 41345 srflx raddr \
192.0.2.56 rport 49170
a=candidate 45678 2 UDP 7 198.51.100.1 52567 srflx raddr \
192.0.2.56 rport 51540
a=candidate 56789 1 UDP 3 192.0.2.100 49000 relay raddr \
192.0.2.56 rport 49170
a=candidate 67890 2 UDP 3 192.0.2.100 49001 relay raddr \
192.0.2.56 rport 51540
b=AS:10000
b=TIAS:10000000
b=RR:4000
b=RS:3000
a=rtpmap:100 H264/90000
a=fmtp:100 profile-level-id=42A01E; packetization-mode=2; \
sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==; \
sprop-interleaving-depth=45; sprop-deint-buf-req=64000; \
sprop-init-buf-time=102478; deint-buf-cap=128000
a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF
a=rmcap:1-3,7-9 H264/90000
a=rmcap:4-6 rtx/90000
a=mfcap:1-9 profile-level-id=42A01E
a=mfcap:1-9 aMljiA==
a=mfcap:1,4,7 packetization-mode=0
a=mfcap:2,5,8 packetization-mode=1
a=mfcap:3,6,9 packetization-mode=2
a=mfcap:1-9 sprop-parameter-sets=Z0IACpZTBYmI
a=mfcap:1,7 sprop-interleaving-depth=45; \
sprop-deint-buf-req=64000; sprop-init-buf-time=102478; \
deint-buf-cap=128000
a=mfcap:4 apt=100
a=mfcap:5 apt=99
a=mfcap:6 apt=98
a=mfcap:4-6 rtx-time=3000
a=mscap:1-6 rtcp-fb nack
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a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_80 \
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|220|1:32
a=pcfg:1 t=1 m=1,4 a=1 pt=1:100,4:97
a=pcfg:2 t=1 m=2,5 a=1 pt=2:99,4:96
a=pcfg:3 t=1 m=3,6 a=1 pt=3:98,6:95
a=pcfg:4 t=2 m=7 a=1 pt=7:100
a=pcfg:5 t=2 m=8 a=1 pt=8:99
a=pcfg:6 t=2 m=9 a=1 pt=9:98
a=pcfg:7 t=3 m=1,3 pt=1:100,4:97
a=pcfg:8 t=3 m=2,4 pt=2:99,4:96
a=pcfg:9 t=3 m=3,6 pt=3:98,6:95
m=audio 49176 RTP/AVP 101 100 99 98
c=IN IP4 192.0.2.56
a=ptime:60
a=maxptime:200
a=rtcp:51534
a=sendonly
a=candidate 12345 1 UDP 9 192.0.2.56 49176 host
a=candidate 23456 2 UDP 9 192.0.2.56 51534 host
a=candidate 34567 1 UDP 7 198.51.100.1 41348 srflx \
raddr 192.0.2.56 rport 49176
a=candidate 45678 2 UDP 7 198.51.100.1 52569 srflx \
raddr 192.0.2.56 rport 51534
a=candidate 56789 1 UDP 3 192.0.2.100 49002 relay \
raddr 192.0.2.56 rport 49176
a=candidate 67890 2 UDP 3 192.0.2.100 49003 relay \
raddr 192.0.2.56 rport 51534
b=AS:512
b=TIAS:512000
b=RR:4000
b=RS:3000
a=maxprate:120
a=rtpmap:98 AMR-WB/16000
a=fmtp:98 octet-align=1; mode-change-capability=2
a=rtpmap:99 AMR-WB/16000
a=fmtp:99 octet-align=1; crc=1; mode-change-capability=2
a=rtpmap:100 AMR-WB/16000/2
a=fmtp:100 octet-align=1; interleaving=30
a=rtpmap:101 AMR-WB+/72000/2
a=fmtp:101 interleaving=50; int-delay=160000;
a=rmcap:14 ac3/48000/6
a=acap:23 crypto:1 AES_CM_128_HMAC_SHA1_80 \
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|220|1:32
a=tcap:4 RTP/SAVP
a=pcfg:10 t=4 a=23
a=pcfg:11 t=4 m=14 a=23 pt=14:102
This offer illustrates the advantage in compactness that arises if
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one can avoid deleting the base configuration attributes and
recreating them in acap attributes for the potential configurations.
4.2. Alternative Combinations of Codecs (Session Configurations)
If an endpoint has limited signal processing capacity, it might be
capable of supporting, say, a G.711 mu-law audio stream in
combination with an H.264 video stream, or a G.729B audio stream in
combination with an H.263-1998 video stream. It might then issue an
offer like the following:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
a=creq:med-v0
a=sescap:1 2,4
a=sescap:2 1,3
m=audio 54322 RTP/AVP 18
a=rtpmap:18 G729/8000
a=fmtp:18 annexb=yes
a=rmcap:1 PCMU/8000
a=pcfg:1 m=1 pt=1:0
a=pcfg:2
m=video 54344 RTP/AVP 100
a=rtpmap:100 H263-1998/90000
a=rmcap:2 H264/90000
a=mfcap:2 profile-level-id=42A01E; packetization-mode=2
a=pcfg:3 m=2 pt=2:101
a=pcfg:4
Note that the preferred session configuration (and the default as
well) is G.729B with H.263. This overrides the individual media
stream preferences which are PCMU and H.264 by the potential
configuration numbering rule.
4.3. Latent Media Streams
Consider a case in which the offerer can support either G.711 mu-law,
or G.729B, along with DTMF telephony events for the 12 common
touchtone signals, but is willing to support simple G.711 mu-law
audio as a last resort. In addition, the offerer wishes to announce
its ability to support video and MSRP in the future, but does not
wish to offer a video stream or an MSRP stream at present. The offer
might look like the following:
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v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
a=creq:med-v0
m=audio 23456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=rmcap:1 PCMU/8000
a=rmcap:2 G729/8000
a=rmcap:3 telephone-event/8000
a=mfcap:3 0-11
a=pcfg:1 m=1,3|2,3 pt=1:0,2:18,3:100
a=lcfg:2 mt=video t=1 m=10|11
a=rmcap:10 H263-1998/90000
a=rmcap:11 H264/90000
a=tcap:1 RTP/AVP
a=lcfg:3 mt=message t=2 m=20
a=tcap:2 TCP/MSRP
a=omcap:20 *
The first lcfg attribute line ("lcfg:2") announces support for H.263
and H.264 video (H.263 preferred) for future negotiation. The second
lcfg attribute line ("lcfg:3") announces support for MSRP for future
negotiation. The m-line and the rtpmap attribute offer an audio
stream and provide the lowest precedence configuration (PCMU without
any DTMF encoding). The rmcap lines define the RTP-based media
format capabilities (PCMU, G729, telephone-event, H263-1998 and H264)
and the omcap line defines the non-RTP based media format capability
(wildcard). The mfcap attribute provides the format parameters for
telephone-event, specifying the 12 commercial DTMF 'digits'. The
pcfg attribute line defines the most-preferred media configuration as
PCMU plus DTMF events and the next-most-preferred configuration as
G.729B plus DTMF events.
If the answerer is able to support all the potential configurations,
and also support H.263 video (but not H.264), it would reply with an
answer like:
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
t=0 0
a=csup:med-v0
m=audio 54322 RTP/AVP 0 100
a=rtpmap:0 PCMU/8000
a=rtpmap:100 telephone-event/8000
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a=fmtp:100 0-11
a=acfg:1 m=1,3 pt=1:0,3:100
a=pcfg:1 m=2,3 pt=2:18,3:100
a=lcfg:2 mt=video t=1 m=10
The lcfg attribute line announces the capability to support H.263
video at a later time. The media line and subsequent rtpmap and fmtp
attribute lines present the selected configuration for the media
stream. The acfg attribute line identifies the potential
configuration from which it was taken, and the pcfg attribute line
announces the potential capability to support G.729 with DTMF events
as well. If, at some later time, congestion becomes a problem in the
network, either party may, with expectation of success, offer a
reconfiguration of the media stream to use G.729 in order to reduce
packet sizes.
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5. IANA Considerations
5.1. New SDP Attributes
The IANA is hereby requested to register the following new SDP
attributes:
Attribute name: rmcap
Long form name: RTP-based media format capability
Type of attribute: session-level and media-level
Subject to charset: no
Purpose: associate RTP-based media capability number(s) with
media subtype and encoding parameters
Appropriate Values: see Section 3.3.1
Contact name: Flemming Andreasen, fandres@cisco.com
Attribute name: omcap
Long form name: Non RTP-based media format capability
Type of attribute: session-level and media-level
Subject to charset: no
Purpose: associate non RTP-based media capability number(s) with
media subtype and encoding parameters
Appropriate Values: see Section 3.3.1
Contact name: Flemming Andreasen, fandreas@cisco.com
Attribute name: mfcap
Long form name: media format parameter capability
Type of attribute: session-level and media-level
Subject to charset: no
Purpose: associate media format attributes and
parameters with media format capabilities
Appropriate Values: see Section 3.3.2
Contact name: Flemming Andreasen, fandreas@cisco.com
Attribute name: mscap
Long form name: media-specific capability
Type of attribute: session-level and media-level
Subject to charset: no
Purpose: associate media-specific attributes and
parameters with media capabilities
Appropriate Values: see Section 3.3.3
Contact name: Flemming Andreasen, fandreas@cisco.com
Attribute name: lcfg
Long form name: latent configuration
Type of attribute: media-level
Subject to charset: no
Purpose: to announce supportable media streams
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without offering them for immediate use.
Appropriate Values: see Section 3.3.5
Contact name: Flemming Andreasen, fandreas@cisco.com
Attribute name: sescap
Long form name: session capability
Type of attribute: session-level
Subject to charset: no
Purpose: to specify and prioritize acceptable
combinations of media stream configurations.
Appropriate Values: see Section 3.3.8
Contact name: Flemming Andreasen, fandreas@cisco.com
5.2. New SDP Capability Negotiation Option Tag
The IANA is hereby requested to add the new option tag "med-v0",
defined in this document, to the SDP Capability Negotiation Option
Capability registry created for RFC 5939 [RFC5939].
5.3. SDP Capability Negotiation Configuration Parameters Registry
The IANA is hereby requested to change the "SDP Capability
Negotiation Potential Configuration Parameters" registry currently
registered and defined by RFC 5939 [RFC5939] as follows:
The name of the registry should be "SDP Capability Negotiation
Configuration Parameters Registry" and it should contain a table with
the following column headings:
o Encoding Name: The syntactical value used for the capability
negotiation configuration parameter, as defined in RFC 5939
[RFC5939], Section 3.5.
o Descriptive Name: The name commonly used to refer to the
capability negotiation configuration parameter.
o Potential Configuration Definition: A reference to the RFC that
defines the configuration parameter in the context of a potential
configuration attribute. If the configuration parameter is not
defined for potential configurations, the string "N/A" (Not
Applicable) MUST be present instead.
o Actual Configuration Definition: A reference to the RFC that
defines the configuration parameter in the context of an actual
configuration attribute. If the configuration parameter is not
defined for actual configurations, the string "N/A" (Not
Applicable) MUST be present instead.
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o Latent Configuration Definition: A reference to the RFC that
defines the configuration parameter in the context of a latent
configuration attribute. If the configuration parameter is not
defined for latent configurations, the string "N/A" (Not
Applicable) MUST be present instead.
An IANA SDP Capability Negotiation Configuration registration MUST be
documented in an RFC in accordance with the IETF Review policy
[RFC5226]. Furthermore:
o The RFC MUST define the syntax and semantics of each new potential
configuration parameter.
o The syntax MUST adhere to the syntax provided for extension
configuration lists in RFC 5939 [RFC5939] Section 3.5.1 and the
semantics MUST adhere to the semantics provided for extension
configuration lists in RFC 5939 [RFC5939] Section 3.5.1 and 3.5.2.
o Configuration parameters that apply to latent configurations MUST
furthermore adhere to the syntax provided in Section 3.3.5 and the
semantics defined overall in this document.
o Associated with each registration MUST be the encoding name for
the parameter as well as a short descriptive name for it.
o Each registration MUST specify if it applies to
* Potential configurations
* Actual configurations
* Latent configurations
5.4. SDP Capability Negotiation Configuration Parameter Registrations
The IANA is hereby requested to register the following capability
negotiation configuration parameters:
Encoding Name: a
Descriptive Name: Attribute Configuration
Potential Configuration Definition: [RFC5939]
Actual Configuration Definition: [RFC5939]
Latent Configuration Definition: [Note to RFC Editor: This RFC]
Encoding Name: t
Descriptive Name: Transport Protocol Configuration
Potential Configuration Definition: [RFC5939]
Actual Configuration Definition: [RFC5939]
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Latent Configuration Definition: [Note to RFC Editor: This RFC]
Encoding Name: m
Descriptive Name: Media Configuration
Potential Configuration Definition: [Note to RFC Editor: This
RFC]
Actual Configuration Definition: [Note to RFC Editor: This RFC]
Latent Configuration Definition: [Note to RFC Editor: This RFC]
Encoding Name: pt
Descriptive Name: Payload Type Number Mapping
Potential Configuration Definition: [Note to RFC Editor: This
RFC]
Actual Configuration Definition: [Note to RFC Editor: This RFC]
Latent Configuration Definition: [Note to RFC Editor: This RFC]
Encoding Name: mt
Descriptive Name: Media Type
Potential Configuration Definition: N/A
Actual Configuration Definition: N/A
Latent Configuration Definition: [Note to RFC Editor: This RFC]
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6. Security Considerations
The security considerations of RFC 5939 [RFC5939] apply for this
document.
In RFC 5939 [RFC5939], it was noted that negotiation of transport
protocols (e.g. secure and non-secure) and negotiation of keying
methods and material are potential security issues that warrant
integrity protection to remedy. Latent configuration support
provides hints to the other side about capabilities supported for
further offer/answer exchanges, including transport protocols and
attribute capabilities, e.g. for keying methods. If an attacker can
remove or alter latent configuration information to suggest that only
insecure or less secure alternatives are supported, then he may be
able to force negotiation of a less secure session than would
otherwise have occurred. While the specific attack as described here
differs from those described in RFC 5939 [RFC5939], the
considerations and mitigation strategies are similar to those
described in RFC 5939 [RFC5939].
Another variation on the above attack involves the Session Capability
("sescap") attribute defined in this document. The "sescap" enables
a preference order to be specified for all the potential
configurations, and that preference will take precedence over any
preference indication provided in individual potential configuration
attributes. Consequently, an attacker that can insert or modify a
"sescap" attribute may be able to force negotiation of an insecure or
less secure alternative than would otherwise have occurred. Again,
the considerations and mitigation strategies are similar to those
described in RFC 5939 [RFC5939].
The addition of negotiable media formats and their associated
parameters, defined in this specification can cause problems for
middleboxes which attempt to control bandwidth utilization, media
flows, and/or processing resource consumption as part of network
policy, but which do not understand the media capability negotiation
feature. As for the initial SDP Capability Negotiation work
[RFC5939], the SDP answer is formulated in such a way that it always
carries the selected media encoding for every media stream selected.
Pending an understanding of capabilities negotiation, the middlebox
should examine the answer SDP to obtain the best picture of the media
streams being established. As always, middleboxes can best do their
job if they fully understand media capabilities negotiation.
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7. Changes from previous versions
7.1. Changes from version 16
o Changed grammer covering numeric values to forbid use of leading
zeroes, and added text to that effect as well.
o Clarified that all numerical ranges must be strictly increasing
(from leftmost number to rightmost number).
o Now mandating against (rather than recommending against) using
real keying material in a crypto attribute when it is used for a
latent configuration only.
o Reworded Offer section description of config-number use in
potential configurations to make it clear that all config-numbers
must be unique.
7.2. Changes from version 15
o Fixed style of RFC references to be consistent in body of
document.
o Minor updates to address Gen-ART review comments.
7.3. Changes from version 14
o Updated IANA Considerations to fix configuration parameter
registry. Document now updates RFC 5939 [RFC5939] (IANA
considerations only)
o Minor ABNF updates to fix errors.
o Editorial nit fixes to address protocol write-up review.
7.4. Changes from version 13
o Various editorial clarifications and updates to address review
comments.
7.5. Changes from version 12
o Removed "dummy" form in the pcfg payload-type-number, since the
functionality is redundant with the non-RTP media capability
(omcap) and it was inconsistent with other RTP payload type
operation.
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o Clarified that latent configuration attribute (lcfg) can only be
used at the media level and hence (technically) as part of a media
description
o Rewrote offer/answer sections and expanded significantly on offer/
answer operation.
o Updated security considerations
o Various minor editorial clarifications and changes.
7.6. Changes from version 11
o Corrected several statements implying lcfg was a session-level
attribute.
o Added non-RTP based media format capabilities ("a=omcap") and
renamed "mcap" to "rmcap"
7.7. Changes from version 10
o Defined the latent configuration attribute as a media-level
attribute because it specifies a possible future media stream.
Added text to clarify how to specify alternative configurations of
a single latent stream and/or multiple streams.
o Improved the definition of the session capability attribute to
permit both required configurations and optional configurations -
latent configurations cannot be required because they have not yet
been offered.
o Removed the special-case treatment of conflicts between base-level
fmtp attributes and fmtp attributes generated for a configuration
via invoked mcap and mfcap attributes.
o Removed reference to bandwidth capability (bcap) attribute.
o Changed various "must", etc., terms to normative terms ("MUST",
etc.) as appropriate, in Section 3.3.5Section 3.3.6.1
Section 3.3.6.3 and Section 3.3.8
o Attempted to clarify the substitution mechanism in Section 3.3.7
and improve its uniqueness.
o Made various editorial changes, including changing the title in
the header, and removing numbering from some SDP examples.
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7.8. Changes from version 09
o Additional corrections to latent media stream example in
Section 4.3
o Fixed up attribute formatting examples and corresponding ABNF.
o Removed preference rule for latent configurations.
o Various spelling and other editorial changes were made.
o updated cross-references.
7.9. Changes from version 08
The major change is in Section 4.3, Latent Media Streams, fixing the
syntax of the answer. All the other changes are editorial.
7.10. Changes from version 04
o The definitions for bcap, ccap, icap, and kcap attributes have
been removed, and are to be defined in another document.
o Corrected formatting of m= and p= configuration parameters to
conform to extension-config-list form defined in RFC 5939
[RFC5939]
o Reorganized definitions of new parameters to make them easier to
find in document.
o Added ability to renegotiate capabilities when modifying the
session (Section 3.4.4).
o Made various editorial changes, clarifications, and typo
corrections.
7.11. Changes from version 03
o A new session capability attribute (sescap) has been added to
permit specification of acceptable media stream combinations.
o Capability attribute definitions corresponding to the i, c, b, and
k SDP line types have been added for completeness.
o Use of the pcfg: attribute in SDP answers has been included in
order to conveniently return information in the answer about
acceptable configurations in the media stream offer.
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o The use of the lcfg: attribute(s) in SDP answers has been
restricted to indicate just which latent configuration offers
would be acceptable to the answerer.
o A suggestion for "naive" middleboxes has been added to the
Security Considerations.
o Various editorial changes have been made.
o Several errors/omissions have been corrected.
o The description of the mscap attribute has been modified to make
it clear that it should not be used to generate undefined SDP
attributes, or to "extend" existing attributes.
o <ms-parameters> are made optional in the mscap attribute
definition.
o "AMR" changed to "AMR-WB" in cases in which the sample rate is
16000.
7.12. Changes from version 02
This version contains several detail changes intended to simplify
capability processing and mapping into conventional SDP media blocks.
o The "mcap" attribute is enhanced to include the role of the "ecap"
attribute; the latter is eliminated.
o The "fcap" attribute has been renamed "mfcap". New replacement
rules vis-a-vis fmtp attributes in the base media specification
have been added.
o A new "mscap" attribute is defined to handle the problem of
attributes (other than rtpmap and fmtp) that are specific to a
particular payload type.
o New rules for processing the mcap, mfcap, and mscap attributes,
and overriding standard rtpmap, fmtp, or other media-specific
attributes, are put forward to reduce the need to use the deletion
option in the a= parameter of the potential configuration (pcfg)
attribute.
o A new parameter, "mt=" is added to the latent configuration
attribute (lcfg) to specify the media stream type (audio, video,
etc.) when the lcfg is declared at the session level.
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o The examples are expanded.
o Numerous typos and misspellings have been corrected.
7.13. Changes from version 01
The documents adds a new attribute for specifying bandwidth
capability and a parameter to list in the potential configuration.
Other changes are to align the document with the terminology and
attribute names from draft-ietf-mmusic-sdp-capability-negotiation-07.
The document also clarifies some previous open issues.
7.14. Changes from version 00
The major changes include taking out the "mcap" and "cptmap"
parameter. The mapping of payload type is now in the "pt" parameter
of "pcfg". Media subtype need to explicitly defined in the "cmed"
attribute if referenced in the "pcfg"
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8. Acknowledgements
This document is heavily influenced by the discussions and work done
by the SDP Capability Negotiation Design team. The following people
in particular provided useful comments and suggestions to either the
document itself or the overall direction of the solution defined
herein: Cullen Jennings, Matt Lepinski, Joerg Ott, Colin Perkins, and
Thomas Stach.
We thank Ingemar Johansson and Magnus Westerlund for examples that
stimulated this work, and for critical reading of the document. We
also thank Cullen Jennings, Christer Holmberg, and Miguel Garcia for
their review of the document.
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9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5939] Andreasen, F., "Session Description Protocol (SDP)
Capability Negotiation", RFC 5939, September 2010.
9.2. Informative References
[RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse-
Parisis, "RTP Payload for Redundant Audio Data", RFC 2198,
September 1997.
[RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol (SDP) Security Descriptions for Media
Streams", RFC 4568, July 2006.
[RFC4585] 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.
[RFC4733] Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF
Digits, Telephony Tones, and Telephony Signals", RFC 4733,
December 2006.
[RFC4867] Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie,
"RTP Payload Format and File Storage Format for the
Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband
(AMR-WB) Audio Codecs", RFC 4867, April 2007.
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[RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
"Codec Control Messages in the RTP Audio-Visual Profile
with Feedback (AVPF)", RFC 5104, February 2008.
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Authors' Addresses
Robert R Gilman
Independent
3243 W. 11th Ave. Dr.
Broomfield, CO 80020
USA
Email: bob_gilman@comcast.net
Roni Even
Gesher Erove Ltd
14 David Hamelech
Tel Aviv 64953
Israel
Email: ron.even.tlv@gmail.com
Flemming Andreasen
Cisco Systems
Iselin, NJ
USA
Email: fandreas@cisco.com
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