draft-ietf-mmusic-sdp-media-capabilities-17.txt   rfc6871.txt 
MMUSIC R. Gilman Internet Engineering Task Force (IETF) R. Gilman
Internet-Draft Independent Request for Comments: 6871 Independent
Updates: 5939 (if approved) R. Even Updates: 5939 R. Even
Intended status: Standards Track Gesher Erove Ltd Category: Standards Track Huawei Technologies
Expires: July 8, 2013 F. Andreasen ISSN: 2070-1721 F. Andreasen
Cisco Systems Cisco Systems
January 4, 2013 February 2013
Session Description Protocol (SDP) Media Capabilities Negotiation Session Description Protocol (SDP) Media Capabilities Negotiation
draft-ietf-mmusic-sdp-media-capabilities-17
Abstract Abstract
Session Description Protocol (SDP) capability negotiation provides a Session Description Protocol (SDP) capability negotiation provides a
general framework for indicating and negotiating capabilities in SDP. general framework for indicating and negotiating capabilities in SDP.
The base framework defines only capabilities for negotiating The base framework defines only capabilities for negotiating
transport protocols and attributes. In this document, we extend the transport protocols and attributes. This documents extends the
framework by defining media capabilities that can be used to framework by defining media capabilities that can be used to
negotiate media types and their associated parameters. negotiate media types and their associated parameters.
This document updates the IANA Considerations of RFC 5939. This document updates the IANA Considerations of RFC 5939.
Status of this Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering This is an Internet Standards Track document.
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
This Internet-Draft will expire on July 8, 2013. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6871.
Copyright Notice Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the Copyright (c) 2013 IETF Trust and the persons identified as the
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than English. than English.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 1. Introduction ....................................................4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 2. Terminology .....................................................4
3. SDP Media Capabilities . . . . . . . . . . . . . . . . . . . . 8 3. SDP Media Capabilities ..........................................6
3.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 8 3.1. Requirements ...............................................6
3.2. Solution Overview . . . . . . . . . . . . . . . . . . . . 9 3.2. Solution Overview ..........................................7
3.3. New Capability Attributes . . . . . . . . . . . . . . . . 15 3.3. New Capability Attributes .................................13
3.3.1. The Media Format Capability Attributes . . . . . . . . 15 3.3.1. The Media Format Capability Attributes .............13
3.3.2. The Media Format Parameter Capability Attribute . . . 18 3.3.2. The Media Format Parameter Capability Attribute ....16
3.3.3. The Media-Specific Capability Attribute . . . . . . . 20 3.3.3. The Media-Specific Capability Attribute ............19
3.3.4. New Configuration Parameters . . . . . . . . . . . . . 22 3.3.4. New Configuration Parameters .......................21
3.3.5. The Latent Configuration Attribute . . . . . . . . . . 24 3.3.5. The Latent Configuration Attribute .................23
3.3.6. Enhanced Potential Configuration Attribute . . . . . . 26 3.3.6. Enhanced Potential Configuration Attribute .........25
3.3.7. Substitution of Media Payload Type Numbers in 3.3.7. Substitution of Media Payload Type Numbers
Capability Attribute Parameters . . . . . . . . . . . 30 in Capability ......................................29
3.3.8. The Session Capability Attribute . . . . . . . . . . . 31 3.3.8. The Session Capability Attribute ...................30
3.4. Offer/Answer Model Extensions . . . . . . . . . . . . . . 35 3.4. Offer/Answer Model Extensions .............................35
3.4.1. Generating the Initial Offer . . . . . . . . . . . . . 36 3.4.1. Generating the Initial Offer .......................35
3.4.2. Generating the Answer . . . . . . . . . . . . . . . . 39 3.4.2. Generating the Answer ..............................39
3.4.3. Offerer Processing of the Answer . . . . . . . . . . . 43 3.4.3. Offerer Processing of the Answer ...................43
3.4.4. Modifying the Session . . . . . . . . . . . . . . . . 44 3.4.4. Modifying the Session ..............................43
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 4. Examples .......................................................44
4.1. Alternative Codecs . . . . . . . . . . . . . . . . . . . . 45 4.1. Alternative Codecs ........................................44
4.2. Alternative Combinations of Codecs (Session 4.2. Alternative Combinations of Codecs (Session
Configurations) . . . . . . . . . . . . . . . . . . . . . 48 Configurations) ...........................................47
4.3. Latent Media Streams . . . . . . . . . . . . . . . . . . . 48 4.3. Latent Media Streams ......................................47
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 51 5. IANA Considerations ............................................49
5.1. New SDP Attributes . . . . . . . . . . . . . . . . . . . . 51 5.1. New SDP Attributes ........................................49
5.2. New SDP Capability Negotiation Option Tag . . . . . . . . 52 5.2. New SDP Capability Negotiation Option Tag .................50
5.3. SDP Capability Negotiation Configuration Parameters 5.3. SDP Capability Negotiation Configuration
Registry . . . . . . . . . . . . . . . . . . . . . . . . . 52 Parameters Registry .......................................50
5.4. SDP Capability Negotiation Configuration Parameter 5.4. SDP Capability Negotiation Configuration Parameter
Registrations . . . . . . . . . . . . . . . . . . . . . . 53 Registrations .............................................52
6. Security Considerations . . . . . . . . . . . . . . . . . . . 55 6. Security Considerations ........................................52
7. Changes from previous versions . . . . . . . . . . . . . . . . 56 7. Acknowledgements ...............................................53
7.1. Changes from version 16 . . . . . . . . . . . . . . . . . 56 8. References .....................................................54
7.2. Changes from version 15 . . . . . . . . . . . . . . . . . 56 8.1. Normative References ......................................54
7.3. Changes from version 14 . . . . . . . . . . . . . . . . . 56 8.2. Informative References ....................................54
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
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
1. Introduction 1. Introduction
Session Description Protocol (SDP) capability negotiation [RFC5939] "Session Description Protocol (SDP) Capability Negotiation" [RFC5939]
provides a general framework for indicating and negotiating provides a general framework for indicating and negotiating
capabilities in SDP [RFC4566]. The base framework defines only capabilities in SDP [RFC4566]. The base framework defines only
capabilities for negotiating transport protocols and attributes. capabilities for negotiating transport protocols and attributes.
RFC 5939 [RFC5939] lists some of the issues with the current SDP RFC 5939 [RFC5939] lists some of the issues with the current SDP
capability negotiation process. An additional real life case is to capability negotiation process. An additional real-life problem is
be able to offer one media stream (e.g. audio) but list the to be able to offer one media stream (e.g., audio) but list the
capability to support another media stream (e.g. video) without capability to support another media stream (e.g., video) without
actually offering it concurrently. actually offering it concurrently.
In this document, we extend the framework by defining media In this document, we extend the framework by defining media
capabilities that can be used to indicate and negotiate media types capabilities that can be used to indicate and negotiate media types
and their associated format parameters. This document also adds the and their associated format parameters. This document also adds the
ability to declare support for media streams, the use of which can be ability to declare support for media streams, the use of which can be
offered and negotiated later, and the ability to specify session offered and negotiated later, and the ability to specify session
configurations as combinations of media stream configurations. The configurations as combinations of media stream configurations. The
definitions of new attributes for media capability negotiation are definitions of new attributes for media capability negotiation are
chosen to make the translation from these attributes to chosen to make the translation from these attributes to
"conventional" SDP [RFC4566] media attributes as straightforward as "conventional" SDP [RFC4566] media attributes as straightforward as
possible in order to simplify implementation. This goal is intended possible in order to simplify implementation. This goal is intended
to reduce processing in two ways: each proposed configuration in an to reduce processing in two ways: each proposed configuration in an
offer may be easily translated into a conventional SDP media stream offer may be easily translated into a conventional SDP media stream
record for processing by the receiver; and the construction of an record for processing by the receiver and the construction of an
answer based on a selected proposed configuration is straightforward. answer based on a selected proposed configuration would be
straightforward.
This document updates RFC 5939 [RFC5939] by updating the IANA This document updates RFC 5939 [RFC5939] by updating the IANA
Considerations. All other extensions defined in this document are considerations. All other extensions defined in this document are
considered extensions above and beyond RFC 5939 [RFC5939]. considered extensions above and beyond RFC 5939 [RFC5939].
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119] and document are to be interpreted as described in RFC 2119 [RFC2119] and
indicate requirement levels for compliant implementations. indicate requirement levels for compliant implementations.
"Actual Configuration": An actual configuration specifies which Actual Configuration: An actual configuration specifies which
combinations of SDP session parameters and media stream components combinations of SDP session parameters and media stream components
can be used in the current offer/answer exchange and with what can be used in the current offer/answer exchange and with what
parameters. Use of an actual configuration does not require any parameters. Use of an actual configuration does not require any
further negotiation in the offer/answer exchange. See RFC 5939 further negotiation in the offer/answer exchange. See RFC 5939
[RFC5939] for further details. [RFC5939] for further details.
"Base Attributes": Conventional SDP attributes appearing in the base Base Attributes: Conventional SDP attributes appearing in the base
configuration of a media block. configuration of a media block.
"Base Configuration": The media configuration represented by a media Base Configuration: The media configuration represented by a media
block exclusive of all the capability negotiation attributes defined block exclusive of all the capability negotiation attributes defined
in this document, the base capability negotiation document [RFC5939], in this document, the base capability negotiation document [RFC5939],
or any other capability negotiation document. In an offer SDP, the or any other capability negotiation document. In an offer SDP, the
base configuration corresponds to the actual configuration as defined base configuration corresponds to the actual configuration as defined
in RFC 5939 [RFC5939]. in RFC 5939 [RFC5939].
"Conventional Attribute": Any SDP attribute other than those defined Conventional Attribute: Any SDP attribute other than those defined by
by the series of capability negotiation specifications. the series of capability negotiation specifications.
"Conventional SDP": An SDP record devoid of capability negotiation Conventional SDP: An SDP record devoid of capability negotiation
attributes. attributes.
"Media Format Capability": A media format, typically a media subtype Media Format Capability: A media format, typically a media subtype
such as PCMU, H263-1998, or T38, expressed in the form of a such as PCMU, H263-1998, or T38, expressed in the form of a
capability. capability.
"Media Format Parameter Capability": A media format parameter Media Format Parameter Capability: A media format parameter ("a=fmtp"
("a=fmtp" in conventional SDP) expressed in the form of a capability. in conventional SDP) expressed in the form of a capability. The
The media format parameter capability is associated with a media media format parameter capability is associated with a media format
format capability. capability.
"Media Capability": The combined set of capabilities associated with Media Capability: The combined set of capabilities associated with
expressing a media format and its relevant parameters (e.g. media expressing a media format and its relevant parameters (e.g., media
format parameters and media specific parameters). format parameters and media specific parameters).
"Potential Configuration": A potential configuration indicates which Potential Configuration: A potential configuration indicates which
combinations of capabilities can be used for the session and its combinations of capabilities can be used for the session and its
associated media stream components. Potential configurations are not associated media stream components. Potential configurations are not
ready for use, however they are offered for potential use in the ready for use; however, they are offered for potential use in the
current offer/answer exchange. They provide an alternative that may current offer/answer exchange. They provide an alternative that may
be used instead of the actual configuration, subject to negotiation be used instead of the actual configuration, subject to negotiation
in the current offer/answer exchange. See RFC 5939 [RFC5939] for in the current offer/answer exchange. See RFC 5939 [RFC5939] for
further details. further details.
"Latent Configuration": A latent configuration indicates which Latent Configuration: A latent configuration indicates which
combinations of capabilities could be used in a future negotiation combinations of capabilities could be used in a future negotiation
for the session and its associated media stream components. Latent for the session and its associated media stream components. Latent
configurations are neither ready for use, nor are they offered for configurations are neither ready for use nor offered for actual or
actual or potential use in the current offer/answer exchange. Latent potential use in the current offer/answer exchange. Latent
configurations merely inform the other side of possible configurations merely inform the other side of possible
configurations supported by the entity. Those latent configurations configurations supported by the entity. Those latent configurations
may be used to guide subsequent offer/answer exchanges, but they are may be used to guide subsequent offer/answer exchanges, but they are
not offered for use as part of the current offer/answer exchange. not offered for use as part of the current offer/answer exchange.
3. SDP Media Capabilities 3. SDP Media Capabilities
The SDP capability negotiation [RFC5939] discusses the use of any SDP The SDP capability negotiation [RFC5939] discusses the use of any SDP
[RFC4566] attribute (a=) under the attribute capability "acap". The [RFC4566] attribute (a=) under the attribute capability "acap". The
limitations of using acap for fmtp and rtpmap in a potential limitations of using "acap" for "fmtp" and "rtpmap" in a potential
configuration are described in RFC 5939 [RFC5939]; for example they configuration are described in RFC 5939 [RFC5939]; for example, they
can be used only at the media level since they are media level can be used only at the media level since they are media-level
attributes. RFC 5939 [RFC5939] does not provide a way to exchange attributes. RFC 5939 [RFC5939] does not provide a way to exchange
media-level capabilities prior to the actual offer of the associated media-level capabilities prior to the actual offer of the associated
media stream. This section provides an overview of extensions media stream. This section provides an overview of extensions
providing an SDP Media Capability negotiation solution offering more providing an SDP media capability negotiation solution offering more
robust capabilities negotiation. This is followed by definitions of robust capabilities negotiation. This is followed by definitions of
new SDP attributes for the solution and its associated updated offer/ new SDP attributes for the solution and its associated updated
answer procedures [RFC3264] offer/answer procedures [RFC3264].
3.1. Requirements 3.1. Requirements
The capability negotiation extensions requirements considered herein The capability negotiation extensions requirements considered herein
are as follows. are as follows.
REQ-01: Support the specification of alternative (combinations of) REQ-01: Support the specification of alternative (combinations of)
media formats (codecs) in a single media block. media formats (codecs) in a single media block.
REQ-02: Support the specification of alternative media format REQ-02: Support the specification of alternative media format
parameters for each media format. parameters for each media format.
REQ-03: Retain backward compatibility with conventional SDP. REQ-03: Retain backward compatibility with conventional SDP. Ensure
Ensure that each and every offered configuration can be easily that each and every offered configuration can be easily
translated into a corresponding SDP media block expressed with translated into a corresponding SDP media block expressed
conventional SDP lines. with conventional SDP lines.
REQ-04: Ensure the scheme operates within the offer/answer model in REQ-04: Ensure that the scheme operates within the offer/answer
such a way that media formats and parameters can be agreed upon model in such a way that media formats and parameters can be
with a single exchange. agreed upon with a single exchange.
REQ-05: Provide the ability to express offers in such a way that REQ-05: Provide the ability to express offers in such a way that the
the offerer can receive media as soon as the offer is sent. (Note offerer can receive media as soon as the offer is sent.
that the offerer may not be able to render received media prior to (Note that the offerer may not be able to render received
exchange of keying material.) media prior to exchange of keying material.)
REQ-06: Provide the ability to offer latent media configurations REQ-06: Provide the ability to offer latent media configurations for
for future negotiation. future negotiation.
REQ-07: Provide reasonable efficiency in the expression of REQ-07: Provide reasonable efficiency in the expression of
alternative media formats and/or format parameters, especially in alternative media formats and/or format parameters,
those cases in which many combinations of options are offered. especially in those cases in which many combinations of
options are offered.
REQ-08: Retain the extensibility of the base capability negotiation REQ-08: Retain the extensibility of the base capability negotiation
mechanism. mechanism.
REQ-09: Provide the ability to specify acceptable combinations of REQ-09: Provide the ability to specify acceptable combinations of
media streams and media formats. For example, offer a PCMU audio media streams and media formats. For example, offer a PCMU
stream with an H264 video stream, or a G729 audio stream with an audio stream with an H264 video stream or a G729 audio
H263 video stream. This ability would give the offerer a means to stream with an H263 video stream. This ability would give
limit processing requirements for simultaneous streams. This the offerer a means to limit processing requirements for
would also permit an offer to include the choice of an audio/T38 simultaneous streams. This would also permit an offer to
stream or an image/T38 stream, but not both. 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 Other possible extensions have been discussed, but have not been
treated in this document. They may be considered in the future. treated in this document. They may be considered in the future.
Three such extensions are: Three such extensions are:
FUT-01: Provide the ability to mix, or change, media types within a FUT-01: Provide the ability to mix, or change, media types within a
single media block. Conventional SDP does not support this single media block. Conventional SDP does not support this
capability explicitly; the usual technique is to define a media capability explicitly; the usual technique is to define a
subtype that represents the actual format within the nominal media media subtype that represents the actual format within the
type. For example, T.38 FAX as an alternative to audio/PCMU nominal media type. For example, T.38 FAX as an alternative
within an audio stream is identified as audio/T38; a separate FAX to audio/PCMU within an audio stream is identified as
stream would use image/T38. audio/T38; a separate FAX stream would use image/T38.
FUT-02: Provide the ability to support multiple transport protocols FUT-02: Provide the ability to support multiple transport protocols
within an active media stream without reconfiguration. This is within an active media stream without reconfiguration. This
not explicitly supported by conventional SDP. is not explicitly supported by conventional SDP.
FUT-03: Provide capability negotiation attributes for all media- FUT-03: Provide capability negotiation attributes for all media-
level SDP line types in the same manner as already done for the level SDP line types in the same manner as already done for
attribute type, with the exception of the media line type itself. the attribute type, with the exception of the media line
The media line type is handled in a special way to permit compact type itself. The media line type is handled in a special
expression of media coding/format options. The line types are way to permit compact expression of media coding/format
bandwidth ("b="), information ("i="), connection data ("c="), and, options. The line types are bandwidth ("b="), information
possibly, the deprecated encryption key ("k="). ("i="), connection data ("c="), and, possibly, the
deprecated encryption key ("k=").
3.2. Solution Overview 3.2. Solution Overview
The solution consists of new capability attributes corresponding to The solution consists of new capability attributes corresponding to
conventional SDP line types, new parameters for the pcfg, acfg, and conventional SDP line types, new parameters for the "pcfg", "acfg",
the new lcfg attributes extending the base attributes from RFC 5939 and the new "lcfg" attributes extending the base attributes from RFC
[RFC5939], and a use of the pcfg attribute to return capability 5939 [RFC5939], and a use of the "pcfg" attribute to return
information in the SDP answer. capability information in the SDP answer.
Several new attributes are defined in a manner that can be related to Several new attributes are defined in a manner that can be related to
the capabilities specified in a media line, and its corresponding the capabilities specified in a media line, and its corresponding
rtpmap and fmtp attributes. "rtpmap" and "fmtp" attributes.
o A new attribute ("a=rmcap") defines RTP-based media format o A new attribute ("a=rmcap") defines RTP-based media format
capabilities in the form of a media subtype (e.g. "PCMU"), and capabilities in the form of a media subtype (e.g., "PCMU"), and
its encoding parameters (e.g. "/8000/2"). Each resulting media its encoding parameters (e.g., "/8000/2"). Each resulting media
format type/subtype capability has an associated handle called a format type/subtype capability has an associated handle called a
media capability number. The encoding parameters are as specified media capability number. The encoding parameters are as specified
for the rtpmap attribute defined in SDP [RFC4566], without the for the "rtpmap" attribute defined in SDP [RFC4566], without the
payload type number part. payload type number part.
o A new attribute ("a=omcap") defines other (non RTP-based) media o A new attribute ("a=omcap") defines other (non-RTP-based) media
format capabilities in the form of a media subtype only (e.g. format capabilities in the form of a media subtype only (e.g.,
"T38"). Each resulting media format type/subtype capability has "T38"). Each resulting media format type/subtype capability has
an associated handle called a media capability number. an associated handle called a media capability number.
o A new attribute ("a=mfcap") specifies media format parameters o A new attribute ("a=mfcap") specifies media format parameters
associated with one or more media format capabilities. The mfcap associated with one or more media format capabilities. The
attribute is used primarily to associate the media format "mfcap" attribute is used primarily to associate the media format
parameters normally carried in the fmtp attribute. Note that parameters normally carried in the "fmtp" attribute. Note that
media format parameters can be used with RTP and non-RTP based media format parameters can be used with RTP and non-RTP-based
media formats. media formats.
o A new attribute ("a=mscap") that specifies media parameters o A new attribute ("a=mscap") specifies media parameters associated
associated with one or more media format capabilities. The mscap with one or more media format capabilities. The "mscap" attribute
attribute is used to associate capabilities with attributes other is used to associate capabilities with attributes other than
than fmtp or rtpmap, for example, the rtcp-fb attribute defined in "fmtp" or "rtpmap", for example, the "rtcp-fb" attribute defined
RFC 4585 [RFC4585]. in RFC 4585 [RFC4585].
o A new attribute ("a=lcfg") specifies latent media stream o A new attribute ("a=lcfg") specifies latent media stream
configurations when no corresponding media line ("m=") is offered. configurations when no corresponding media line ("m=") is offered.
An example is the offer of latent configurations for video even An example is the offer of latent configurations for video even
though no video is currently offered. If the peer indicates though no video is currently offered. If the peer indicates
support for one or more offered latent configurations, the support for one or more offered latent configurations, the
corresponding media stream(s) may be added via a new offer/answer corresponding media stream(s) may be added via a new offer/answer
exchange. exchange.
o A new attribute ("a=sescap") is used to specify an acceptable o A new attribute ("a=sescap") is used to specify an acceptable
combination of simultaneous media streams and their configurations combination of simultaneous media streams and their configurations
as a list of potential and/or latent configurations. as a list of potential and/or latent configurations.
New parameters are defined for the potential configuration (pcfg), New parameters are defined for the potential configuration ("pcfg"),
latent configuration (lcfg), and accepted configuration (acfg) latent configuration ("lcfg"), and accepted configuration ("acfg")
attributes to associate the new attributes with particular attributes to associate the new attributes with particular
configurations. configurations.
o A new parameter type ("m=") is added to the potential o A new parameter type ("m=") is added to the potential
configuration ("a=pcfg:") attribute and the actual configuration configuration ("a=pcfg:") attribute and the actual configuration
("a=acfg:") attribute defined in RFC 5939 [RFC5939], and to the ("a=acfg:") attribute defined in RFC 5939 [RFC5939] and to the new
new latent configuration ("a=lcfg:") attribute. This permits latent configuration ("a=lcfg:") attribute. This permits
specification of media capabilities (including their associated specification of media capabilities (including their associated
parameters) and combinations thereof for the configuration. For parameters) and combinations thereof for the configuration. For
example, the "a=pcfg:" line might specify PCMU and telephone example, the "a=pcfg:" line might specify PCMU and telephone
events [RFC4733] or G.729B and telephone events as acceptable events [RFC4733] or G.729B and telephone events as acceptable
configurations. The "a=acfg:" line in the answer would specify configurations. The "a=acfg:" line in the answer would specify
the configuration chosen. the configuration chosen.
o A new parameter type ("pt=") is added to the potential o A new parameter type ("pt=") is added to the potential
configuration, actual configuration, and latent configuration configuration, actual configuration, and latent configuration
attributes. This parameter associates RTP payload type numbers attributes. This parameter associates RTP payload type numbers
with the referenced RTP-based media format capabilities, and is with the referenced RTP-based media format capabilities and is
appropriate only when the transport protocol uses RTP. appropriate only when the transport protocol uses RTP.
o A new parameter type ("mt=") is used to specify the media type for o A new parameter type ("mt=") is used to specify the media type for
latent configurations. latent configurations.
Special processing rules are defined for capability attribute Special processing rules are defined for capability attribute
arguments in order to reduce the need to replicate essentially- arguments in order to reduce the need to replicate essentially
identical attribute lines for the base configuration and potential identical attribute lines for the base configuration and potential
configurations. configurations.
o A substitution rule is defined for any capability attribute to o A substitution rule is defined for any capability attribute to
permit the replacement of the (escaped) media capability number permit the replacement of the (escaped) media capability number
with the media format identifier (e.g., the payload type number in with the media format identifier (e.g., the payload type number in
audio/video profiles). audio/video profiles).
o Replacement rules are defined for the conventional SDP equivalents o Replacement rules are defined for the conventional SDP equivalents
of the mfcap and mscap capability attributes. This reduces the of the "mfcap" and "mscap" capability attributes. This reduces
necessity to use the deletion qualifier in the a=pcfg parameter in the necessity to use the deletion qualifier in the "a=pcfg"
order to ignore rtpmap, fmtp, and certain other attributes in the parameter in order to ignore "rtpmap", "fmtp", and certain other
base configuration. attributes in the base configuration.
o An argument concatenation rule is defined for mfcap attributes o An argument concatenation rule is defined for "mfcap" attributes
which refer to the same media capability number. This makes it that refer to the same media capability number. This makes it
convenient to combine format options concisely by associating convenient to combine format options concisely by associating
multiple mfcap lines with multiple media format capabilities. multiple mfcap lines with multiple media format capabilities.
This document extends the base protocol extensions to the offer/ This document extends the base protocol extensions to the
answer model that allow for capabilities and potential configurations offer/answer model that allow for capabilities and potential
to be included in an offer. Media capabilities constitute configurations to be included in an offer. Media capabilities
capabilities that can be used in potential and latent configurations. constitute capabilities that can be used in potential and latent
Whereas potential configurations constitute alternative offers that configurations. Whereas potential configurations constitute
may be accepted by the answerer instead of the actual alternative offers that may be accepted by the answerer instead of
configuration(s) included in the "m=" line(s) and associated the actual configuration(s) included in the "m=" line(s) and
parameters, latent configurations merely inform the other side of associated parameters, latent configurations merely inform the other
possible configurations supported by the entity. Those latent side of possible configurations supported by the entity. Those
configurations may be used to guide subsequent offer/answer latent configurations may be used to guide subsequent offer/answer
exchanges, but they are not part of the current offer/answer exchanges, but they are not part of the current offer/answer
exchange. exchange.
The mechanism is illustrated by the offer/answer exchange below, The mechanism is illustrated by the offer/answer exchange below,
where Alice sends an offer to Bob: where Alice sends an offer to Bob:
Alice Bob Alice Bob
| (1) Offer (SRTP and RTP) | | (1) Offer (SRTP and RTP) |
|--------------------------------->| |--------------------------------->|
| | | |
| (2) Answer (RTP) | | (2) Answer (RTP) |
|<---------------------------------| |<---------------------------------|
| | | |
Alice's offer includes RTP and SRTP as alternatives. RTP is the Alice's offer includes RTP and Secure Real-time Transport Protocol
default, but SRTP is the preferred one (long lines are folded to fit (SRTP) as alternatives. RTP is the default, but SRTP is the
the margins): preferred one (long lines are folded to fit the margins):
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
a=creq:med-v0 a=creq:med-v0
m=audio 3456 RTP/AVP 0 18 m=audio 3456 RTP/AVP 0 18
a=tcap:1 RTP/SAVP RTP/AVP a=tcap:1 RTP/SAVP RTP/AVP
a=rtpmap:0 PCMU/8000/1 a=rtpmap:0 PCMU/8000/1
a=rtpmap:18 G729/8000/1 a=rtpmap:18 G729/8000/1
a=fmtp:18 annexb=yes a=fmtp:18 annexb=yes
a=rmcap:1,4 G729/8000/1 a=rmcap:1,4 G729/8000/1
a=rmcap:2 PCMU/8000/1 a=rmcap:2 PCMU/8000/1
a=rmcap:5 telephone-event/8000 a=rmcap:5 telephone-event/8000
a=mfcap:1 annexb=no a=mfcap:1 annexb=no
a=mfcap:4 annexb=yes a=mfcap:4 annexb=yes
a=mfcap:5 0-11 a=mfcap:5 0-11
a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32 \ a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32 \
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=pcfg:1 m=4,5|1,5 t=1 a=1 pt=1:100,4:101,5:102 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:2 m=2 t=1 a=1 pt=2:103
a=pcfg:3 m=4 t=2 pt=4:18 a=pcfg:3 m=4 t=2 pt=4:18
The required base and extensions are provided by the "a=creq" The required base and extensions are provided by the "a=creq"
attribute defined in RFC 5939 [RFC5939], with the option tag attribute defined in RFC 5939 [RFC5939], with the option tag
"med-v0", which indicates that the extension framework defined here, "med-v0", which indicates that the extension framework defined here
must be supported. The base level capability negotiation support must be supported. The base-level capability negotiation support
("cap-v0" [RFC5939]) is implied since it is required for the ("cap-v0" [RFC5939]) is implied since it is required for the
extensions. extensions.
The "m=" line indicates that Alice is offering to use plain RTP with The "m=" line indicates that Alice is offering to use plain RTP with
PCMU or G.729B. The media line implicitly defines the default PCMU or G.729B. The media line implicitly defines the default
transport protocol (RTP/AVP in this case) and the default actual transport protocol (RTP/AVP in this case) and the default actual
configuration. configuration.
The "a=tcap:1" line, specified in the SDP Capability Negotiation base The "a=tcap:1" line, specified in the SDP capability negotiation base
protocol [RFC5939], defines transport protocol capabilities, in this protocol [RFC5939], defines transport protocol capabilities, in this
case Secure RTP (SAVP profile) as the first option and RTP (AVP case Secure RTP (SAVP profile) as the first option and RTP (AVP
profile) as the second option. profile) as the second option.
The "a=rmcap:1,4" line defines two G.729 RTP-based media format 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, numbered 1 and 4, and their encoding rate. The
capabilities are of media type "audio" and subtype G729. Note that capabilities are of media type "audio" and subtype G729. Note that
the media subtype is explicitly specified here, rather than RTP the media subtype is explicitly specified here, rather than RTP
payload type numbers. This permits the assignment of payload type payload type numbers. This permits the assignment of payload type
numbers in the media stream configuration specification. In this numbers in the media stream configuration specification. In this
example, two G.729 subtype capabilities are defined. This permits example, two G.729 subtype capabilities are defined. This permits
the declaration of two sets of formatting parameters for G.729. 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:2" line defines a G.711 mu-law capability, numbered 2.
The "a=rmcap:5" line defines an audio telephone-event capability, The "a=rmcap:5" line defines an audio telephone-event capability,
numbered 5. numbered 5.
The "a=mfcap:1" line specifies the fmtp formatting parameters for The "a=mfcap:1" line specifies the "fmtp" formatting parameters for
capability 1 (offerer will not accept G.729 Annex B packets). capability 1 (offerer will not accept G.729 Annex B packets).
The "a=mfcap:4" line specifies the fmtp formatting parameters for The "a=mfcap:4" line specifies the "fmtp" formatting parameters for
capability 4 (offerer will accept G.729 Annex B packets). capability 4 (offerer will accept G.729 Annex B packets).
The "a=mfcap:5" line specifies the fmtp formatting parameters for The "a=mfcap:5" line specifies the "fmtp" formatting parameters for
capability 5 (the DTMF touchtones 0-9,*,#). capability 5 (the dual-tone multi-frequency (DTMF) touchtones
0-9,*,#).
The "a=acap:1" line specified in the base protocol provides the The "a=acap:1" line specified in the base protocol provides the
"crypto" attribute which provides the keying material for SRTP using "crypto" attribute that provides the keying material for SRTP using
SDP security descriptions. SDP security descriptions.
The "a=pcfg:" attributes provide the potential configurations The "a=pcfg:" attributes provide the potential configurations
included in the offer by reference to the media capabilities, included in the offer by reference to the media capabilities,
transport capabilities, attribute capabilities and specified payload transport capabilities, attribute capabilities, and specified payload
type number mappings. Three explicit alternatives are provided; the type number mappings. Three explicit alternatives are provided; the
lowest-numbered one is the preferred one. The "a=pcfg:1 ..." line 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. specifies media capabilities 4 and 5, i.e., G.729B and DTMF
their associated media format parameters), or media capability 1 and (including their associated media format parameters), or media
5, i.e., G.729 and DTMF (incl. their associated media format capability 1 and 5, i.e., G.729 and DTMF (including their associated
parameters). Furthermore, it specifies transport protocol capability media format parameters). Furthermore, it specifies transport
1 (i.e. the RTP/SAVP profile - secure RTP), and the attribute protocol capability 1 (i.e., the RTP/SAVP profile - secure RTP), and
capability 1, i.e. the crypto attribute provided. Lastly, it the attribute capability 1, i.e., the "crypto" attribute provided.
specifies a payload type number mapping for (RTP-based) media Last, it specifies a payload type number mapping for (RTP-based)
capabilities 1, 4, and 5, thereby permitting the offerer to media capabilities 1, 4, and 5, thereby permitting the offerer to
distinguish between encrypted media and unencrypted media received distinguish between encrypted media and unencrypted media received
prior to receipt of the answer. prior to receipt of the answer.
Use of unique payload type numbers in alternative configurations is Use of unique payload type numbers in alternative configurations is
not required; codecs such as AMR-WB [RFC4867] have the potential for not required; codecs such as Adaptive Multi-Rate Wideband (AMR-WB)
so many combinations of options that it may be impractical to define [RFC4867] have the potential for so many combinations of options that
unique payload type numbers for all supported combinations. If it may be impractical to define unique payload type numbers for all
unique payload type numbers cannot be specified, then the offerer supported combinations. If unique payload type numbers cannot be
will be obliged to wait for the SDP answer before rendering received specified, then the offerer will be obliged to wait for the SDP
media. For SRTP using SDES inline keying [RFC4568], the offerer will answer before rendering received media. For SRTP using Security
still need to receive the answer before being able to decrypt the Descriptions (SDES) inline keying [RFC4568], the offerer will still
stream. need to receive the answer before being able to decrypt the stream.
The second alternative ("a=pcfg:2 ...") specifies media capability 2, The second alternative ("a=pcfg:2 ...") specifies media capability 2,
i.e., PCMU, under the RTP/SAVP profile, with the same SRTP key i.e., PCMU, under the RTP/SAVP profile, with the same SRTP key
material. material.
The third alternative ("a=pcfg:3 ...") offers G.729B unsecured; its 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 only purpose in this example is to show a preference for G.729B over
PCMU. PCMU.
Per RFC 5939 [RFC5939], the media line, with any qualifying Per RFC 5939 [RFC5939], the media line, with any qualifying
attributes such as fmtp or rtpmap, is itself considered a valid attributes such as "fmtp" or "rtpmap", is itself considered a valid
configuration (the current actual configuration); it has the lowest configuration (the current actual configuration); it has the lowest
preference (per RFC 5939 [RFC5939]). preference (per RFC 5939 [RFC5939]).
Bob receives the SDP offer from Alice. Bob supports G.729B, PCMU, Bob receives the SDP offer from Alice. Bob supports G.729B, PCMU,
and telephone events over RTP, but not SRTP, hence he accepts the and telephone events over RTP, but not SRTP, hence he accepts the
potential configuration 3 for RTP provided by Alice. Bob generates potential configuration 3 for RTP provided by Alice. Bob generates
the following answer: the following answer:
v=0 v=0
o=- 24351 621814 IN IP4 192.0.2.2 o=- 24351 621814 IN IP4 192.0.2.2
s= s=
c=IN IP4 192.0.2.2 c=IN IP4 192.0.2.2
t=0 0 t=0 0
a=csup:med-v0 a=csup:med-v0
m=audio 4567 RTP/AVP 18 m=audio 4567 RTP/AVP 18
a=rtpmap:18 G729/8000 a=rtpmap:18 G729/8000
a=fmtp:18 annexb=yes a=fmtp:18 annexb=yes
a=acfg:3 m=4 t=2 pt=4:18 a=acfg:3 m=4 t=2 pt=4:18
Bob includes the "a=csup" and "a=acfg" attributes in the answer to 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 inform Alice that he can support the med-v0 level of capability
negotiations. Note that in this particular example, the answerer negotiations. Note that in this particular example, the answerer
supported the capability extensions defined here, however had he not, supported the capability extensions defined here; however, had he
he would simply have processed the offer based on the offered PCMU not, he would simply have processed the offer based on the offered
and G.729 codecs under the RTP/AVP profile only. Consequently, the PCMU and G.729 codecs under the RTP/AVP profile only. Consequently,
answer would have omitted the "a=csup" attribute line and chosen one the answer would have omitted the "a=csup" attribute line and chosen
or both of the PCMU and G.729 codecs instead. The answer carries the one or both of the PCMU and G.729 codecs instead. The answer carries
accepted configuration in the "m=" line along with corresponding the accepted configuration in the "m=" line along with corresponding
rtpmap and/or fmtp parameters, as appropriate. "rtpmap" and/or "fmtp" parameters, as appropriate.
Note that per the base protocol, after the above, Alice MAY generate Note that per the base protocol, after the above, Alice MAY generate
a new offer with an actual configuration ("m=" line, etc.) a new offer with an actual configuration ("m=" line, etc.)
corresponding to the actual configuration referenced in Bob's answer corresponding to the actual configuration referenced in Bob's answer
(not shown here). (not shown here).
3.3. New Capability Attributes 3.3. New Capability Attributes
In this section, we present the new attributes associated with In this section, we present the new attributes associated with
indicating the media capabilities for use by the SDP Capability indicating the media capabilities for use by the SDP capability
negotiation. The approach taken is to keep things similar to the negotiation. The approach taken is to keep things similar to the
existing media capabilities defined by the existing media existing media capabilities defined by the existing media
descriptions ("m=" lines) and the associated "rtpmap" and "fmtp" descriptions ("m=" lines) and the associated "rtpmap" and "fmtp"
attributes. We use media subtypes and "media capability numbers" to attributes. We use media subtypes and "media capability numbers" to
link the relevant media capability parameters. This permits the link the relevant media capability parameters. This permits the
capabilities to be defined at the session level and be used for capabilities to be defined at the session level and be used for
multiple streams, if desired. For RTP-based media formats, payload multiple streams, if desired. For RTP-based media formats, payload
types are then specified at the media level (see Section 3.3.4.2). types are then specified at the media level (see Section 3.3.4.2).
A media capability merely indicates possible support for the media A media capability merely indicates possible support for the media
type and media format(s) and parameters in question. In order to type and media format(s) and parameters in question. In order to
actually use a media capability in an offer/answer exchange, it MUST actually use a media capability in an offer/answer exchange, it MUST
be referenced in a potential configuration. be referenced in a potential configuration.
Media capabilities, i.e. the attributes associated with expressing Media capabilities, i.e., the attributes associated with expressing
media capability formats, parameters, etc., can be provided at the media capability formats, parameters, etc., can be provided at the
session-level and/or the media-level. Media capabilities provided at 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 session level may be referenced in any "pcfg" or "lcfg" attribute
the media level (consistent with the media type), whereas media at the media level (consistent with the media type), whereas media
capabilities provided at the media level may be referenced only by capabilities provided at the media level may be referenced only by
the pcfg or lcfg attribute within that media stream. In either case, the "pcfg" or "lcfg" attribute within that media stream. In either
the scope of the <med-cap-num> is the entire session description. case, the scope of the <med-cap-num> is the entire session
This enables each media capability to be uniquely referenced across description. This enables each media capability to be uniquely
the entire session description (e.g. in a potential configuration). referenced across the entire session description (e.g., in a
potential configuration).
3.3.1. The Media Format Capability Attributes 3.3.1. The Media Format Capability Attributes
Media subtypes can be expressed as media format capabilities by use Media subtypes can be expressed as media format capabilities by use
of the "a=rmcap" and "a=omcap" attributes. The "a=rmcap" attribute 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 MUST be used for RTP-based media, whereas the "a=omcap" attribute
be used for non-RTP-based (other) media formats. The two attributes MUST be used for non-RTP-based (other) media formats. The two
are defined as follows: attributes are defined as follows:
a=rmcap:<media-cap-num-list> <encoding-name>/<clock-rate> a=rmcap:<media-cap-num-list> <encoding-name>/<clock-rate>
[/<encoding-parms>] [/<encoding-parms>]
a=omcap:<media-cap-num-list> <format-name> a=omcap:<media-cap-num-list> <format-name>
where <media-cap-num-list> is a (list of) media capability number(s) where <media-cap-num-list> is a (list of) media capability number(s)
used to number a media format capability, the <encoding name> or used to number a media format capability, the <encoding name> or
<format name> is the media subtype, e.g., H263-1998, PCMU, or T38, <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 <clock rate> is the encoding rate, and <encoding parms> are the media
encoding parameters for the media subtype. All media format encoding parameters for the media subtype. All media format
capabilities in the list are assigned to the same media type/subtype. capabilities in the list are assigned to the same media type/subtype.
Each occurrence of the rmcap and omcap attribute MUST use unique Each occurrence of the "rmcap" and "omcap" attribute MUST use unique
values in their <media-cap-num-list>; the media capability numbers values in their <media-cap-num-list>; the media capability numbers
are shared between the two attributes and the numbers MUST be unique are shared between the two attributes and the numbers MUST be unique
across the entire SDP session. In short, the rmcap and omcap across the entire SDP session. In short, the "rmcap" and "omcap"
attributes define media format capabilities and associate them with a attributes define media format capabilities and associate them with a
media capability number in the same manner as the rtpmap attribute media capability number in the same manner as the "rtpmap" attribute
defines them and associates them with a payload type number. defines them and associates them with a payload type number.
Additionally, the attributes allow multiple capability numbers to be Additionally, the attributes allow multiple capability numbers to be
defined for the media format in question by specifying a range of defined for the media format in question by specifying a range of
media capability numbers. This permits the media format to be media capability numbers. This permits the media format to be
associated with different media parameters in different associated with different media parameters in different
configurations. When a range of capability numbers is specified, the configurations. When a range of capability numbers is specified, the
first (leftmost) capability number MUST be strictly smaller than the first (leftmost) capability number MUST be strictly smaller than the
second (rightmost), i.e. the range increases and covers at least two second (rightmost), i.e., the range increases and covers at least two
numbers. numbers.
In ABNF [RFC5234], we have: In ABNF [RFC5234], we have:
media-capability-line = rtp-mcap / non-rtp-mcap 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 rtp-mcap = "a=rmcap:" media-cap-num-list
appear in an rtpmap attribute for each media type/subtype. Thus, it 1*WSP encoding-name "/" clock-rate
is easy to convert an rmcap attribute line into one or more rtpmap ["/" encoding-parms]
attribute lines, once a payload type number is assigned to a media- non-rtp-mcap = "a=omcap:" media-cap-num-list 1*WSP format-name
cap-num (see Section 3.3.5). 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 RFC 4566
clock-rate = NonZeroDigit *9(DIGIT)
encoding-parms = token
format-name = token ;defined in RFC 4566
NonZeroDigit = %x31-39 ; 1-9
The encoding-name, clock-rate, and encoding-params are as defined to
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 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 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, 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 e.g., "t38". This form can also be used in cases such as Binary
[RFC4585] where the fmt list in the m-line is effectively ignored Floor Control Protocol (BFCP) [RFC4585] where the fmt list in the
(BFCP uses "*"). "m=" line is effectively ignored (BFCP uses "*").
The "rmcap" and "omcap" attributes can be provided at the session- 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 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 more than one "omcap" attribute at both the session and media levels
(i.e., more than one of each at the session-level and more than one (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 of each in each media description). Media capability numbers cannot
include leading zeroes, and each media-cap-num MUST be unique within include leading zeroes, and each media-cap-num MUST be unique within
the entire SDP record; it is used to identify that media capability the entire SDP record; it is used to identify that media capability
in potential, latent and actual configurations, and in other in potential, latent, and actual configurations, and in other
attribute lines as explained below. Note that the media-cap-num attribute lines as explained below. Note that the media-cap-num
values are shared between the rmcap and omcap attributes, and hence values are shared between the "rmcap" and "omcap" attributes; hence,
the uniqueness requirement applies to the union of them. When the the uniqueness requirement applies to the union of them. When the
media capabilities are used in a potential, latent or actual media capabilities are used in a potential, latent, or actual
configuration, the media formats referred by those configurations configuration, the media formats referred by those configurations
apply at the media level, irrespective of whether the media apply at the media level, irrespective of whether the media
capabilities themselves were specified at the session or media level. capabilities themselves were specified at the session or media level.
In other words, the media capability applies to the specific media In other words, the media capability applies to the specific media
description associated with the configuration which invokes it. description associated with the configuration that invokes it.
For example: For example:
v=0 v=0
o=- 24351 621814 IN IP4 192.0.2.2 o=- 24351 621814 IN IP4 192.0.2.2
s= s=
c=IN IP4 192.0.2.2 c=IN IP4 192.0.2.2
t=0 0 t=0 0
a=rmcap:1 L16/8000/1 a=rmcap:1 L16/8000/1
a=rmcap:2 L16/16000/2 a=rmcap:2 L16/16000/2
a=rmcap:3 H263-1998/90000 a=rmcap:3 H263-1998/90000
a=omcap:4 example a=omcap:4 example
m=audio 54320 RTP/AVP 0 m=audio 54320 RTP/AVP 0
a=pcfg:1 m=1|2, pt=1:99,2:98 a=pcfg:1 m=1|2, pt=1:99,2:98
m=video 66544 RTP/AVP 100 m=video 66544 RTP/AVP 100
a=rtpmap:100 H264/90000 a=rtpmap:100 H264/90000
a=pcfg:10 m=3 pt=3:101 a=pcfg:10 m=3 pt=3:101
a=tcap:1 TCP a=tcap:1 TCP
a=pcfg:11 m=4 t=1 a=pcfg:11 m=4 t=1
3.3.2. The Media Format Parameter Capability Attribute 3.3.2. The Media Format Parameter Capability Attribute
This attribute is used to associate media format specific parameters This attribute is used to associate media format specific parameters
with one or more media format capabilities. The form of the with one or more media format capabilities. The form of the
attribute is: attribute is
a=mfcap:<media-caps> <list of parameters> a=mfcap:<media-caps> <list of parameters>
where <media-caps> permits the list of parameters to be associated where <media-caps> permits the list of parameters to be associated
with one or more media format capabilities and the format parameters 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 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 single traditional SDP "fmtp" attribute line (one for each entry in
<media-caps>) of the form <media-caps>) of the form
a=fmtp:<fmt> <list of parameters> a=fmtp:<fmt> <list of parameters>
where <fmt> is the media format parameter defined in RFC 4566 where <fmt> is the media format parameter defined in RFC 4566
[RFC4566], as appropriate for the particular media stream. The mfcap [RFC4566], as appropriate for the particular media stream. The
attribute MUST be used to encode attributes for media capabilities, "mfcap" attribute MUST be used to encode attributes for media
which would conventionally appear in an fmtp attribute. The existing capabilities, which would conventionally appear in an "fmtp"
acap attribute MUST NOT be used to encode fmtp attributes. attribute. The existing "acap" attribute MUST NOT be used to encode
"fmtp" attributes.
The mfcap attribute adheres to SDP [RFC4566] attribute production The "mfcap" attribute adheres to SDP [RFC4566] attribute production
rules with rules with
media-format-parameter-capability = media-format-parameter-capability =
"a=mfcap:" media-cap-num-list 1*WSP fmt-specific-param-list "a=mfcap:" media-cap-num-list 1*WSP fmt-specific-param-list
fmt-specific-param-list = text ; defined in RFC4566 fmt-specific-param-list = text ; defined in RFC 4566
Note that media format parameters can be used with RTP-based and non- Note that media format parameters can be used with RTP-based and non-
RTP based media formats. RTP-based media formats.
3.3.2.1. Media Format Parameter Concatenation Rule 3.3.2.1. Media Format Parameter Concatenation Rule
The appearance of media subtypes with a large number of formatting The appearance of media subtypes with a large number of formatting
options (e.g., AMR-WB [RFC4867]) coupled with the restriction that options (e.g., AMR-WB [RFC4867]), coupled with the restriction that
only a single fmtp attribute can appear per media format, suggests only a single "fmtp" attribute can appear per media format, suggests
that it is useful to create a combining rule for mfcap parameters that it is useful to create a combining rule for "mfcap" parameters
which are associated with the same media capability number. that are associated with the same media capability number.
Therefore, different mfcap lines MAY include the same media-cap-num Therefore, different mfcap lines MAY include the same media-cap-num
in their media-cap-num-list. When a particular media capability is in their media-cap-num-list. When a particular media capability is
selected for processing, the parameters from each mfcap line which selected for processing, the parameters from each mfcap line that
references the particular capability number in its media-cap-num-list references the particular capability number in its media-cap-num-list
are concatenated together via ";", in the order the mfcap attributes are concatenated together via ";", in the order the "mfcap"
appear in the SDP record, to form the equivalent of a single fmtp attributes appear in the SDP record, to form the equivalent of a
attribute line. This permits one to define a separate mfcap line for single "fmtp" attribute line. This permits one to define a separate
a single parameter and value that is to be applied to each media mfcap line for a single parameter and value that is to be applied to
capability designated in the media-cap-num-list. This provides a each media capability designated in the media-cap-num-list. This
compact method to specify multiple combinations of format parameters provides a compact method to specify multiple combinations of format
when using codecs with multiple format options. Note that order- parameters when using codecs with multiple format options. Note that
dependent parameters SHOULD be placed in a single mfcap line to avoid order-dependent parameters SHOULD be placed in a single mfcap line to
possible problems with line rearrangement by a middlebox. avoid possible problems with line rearrangement by a middlebox.
Format parameters are not parsed by SDP; their content is specific to Format parameters are not parsed by SDP; their content is specific to
the media type/subtype. When format parameters for a specific media the media type/subtype. When format parameters for a specific media
capability are combined from multiple a=mfcap lines which reference capability are combined from multiple "a=mfcap" lines that reference
that media capability, the format-specific parameters are that media capability, the format-specific parameters are
concatenated together and separated by ";" for construction of the concatenated together and separated by ";" for construction of the
corresponding format attribute (a=fmtp). The resulting format corresponding format attribute ("a=fmtp"). The resulting format
attribute will look something like the following (without line attribute will look something like the following (without line
breaks): breaks):
a=fmtp:<fmt> <fmt-specific-param-list1>; a=fmtp:<fmt> <fmt-specific-param-list1>;
<fmt-specific-param-list2>; <fmt-specific-param-list2>;
... ...
where <fmt> depends on the transport protocol in the manner defined where <fmt> depends on the transport protocol in the manner defined
in RFC4566. SDP cannot assess the legality of the resulting in RFC 4566 [RFC4566]. SDP cannot assess the legality of the
parameter list in the "a=fmtp" line; the user must take care to resulting parameter list in the "a=fmtp" line; the user must take
ensure that legal parameter lists are generated. care to ensure that legal parameter lists are generated.
The "mfcap" attribute can be provided at the session-level and the The "mfcap" attribute can be provided at the session level and the
media-level. There can be more than one mfcap attribute at 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 session or media level. The unique media-cap-num is used to
associate the parameters with a media capability. associate the parameters with a media capability.
As a simple example, a G.729 capability is, by default, considered to 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 support comfort noise as defined by Annex B. Capabilities for G.729
with and without comfort noise support may thus be defined by: with and without comfort noise support may thus be defined by:
a=rmcap:1,2 G729/8000 a=rmcap:1,2 G729/8000
a=mfcap:2 annexb:no a=mfcap:2 annexb:no
Media capability 1 supports G.729 with Annex B, whereas media Media capability 1 supports G.729 with Annex B, whereas media
capability 2 supports G.729 without Annex B. capability 2 supports G.729 without Annex B.
Example for H.263 video: Example for H.263 video:
a=rmcap:1 H263-1998/90000 a=rmcap:1 H263-1998/90000
a=rmcap:2 H263-2000/90000 a=rmcap:2 H263-2000/90000
a=mfcap:1 CIF=4;QCIF=2;F=1;K=1 a=mfcap:1 CIF=4;QCIF=2;F=1;K=1
a=mfcap:2 profile=2;level=2.2 a=mfcap:2 profile=2;level=2.2
Finally, for six format combinations of the Adaptive MultiRate codec: Finally, for six format combinations of the Adaptive Multi-Rate
codec:
a=rmcap:1-3 AMR/8000/1 a=rmcap:1-3 AMR/8000/1
a=rmcap:4-6 AMR-WB/16000/1 a=rmcap:4-6 AMR-WB/16000/1
a=mfcap:1,2,3,4 mode-change-capability=1 a=mfcap:1,2,3,4 mode-change-capability=1
a=mfcap:5,6 mode-change-capability=2 a=mfcap:5,6 mode-change-capability=2
a=mfcap:1,2,3,5 max-red=220 a=mfcap:1,2,3,5 max-red=220
a=mfcap:3,4,5,6 octet-align=1 a=mfcap:3,4,5,6 octet-align=1
a=mfcap:1,3,5 mode-set=0,2,4,7 a=mfcap:1,3,5 mode-set=0,2,4,7
a=mfcap:2,4,6 mode-set=0,3,5,6 a=mfcap:2,4,6 mode-set=0,3,5,6
So that AMR codec #1, when specified in a pcfg attribute within an So that AMR codec #1, when specified in a "pcfg" attribute within an
audio stream block (and assigned payload type number 98) as in audio stream block (and assigned payload type number 98) as in:
a=pcfg:1 m=1 pt=1:98 a=pcfg:1 m=1 pt=1:98
is essentially equivalent to the following is essentially equivalent to the following:
m=audio 49170 RTP/AVP 98 m=audio 49170 RTP/AVP 98
a=rtpmap:98 AMR/8000/1 a=rtpmap:98 AMR/8000/1
a=fmtp:98 mode-change-capability=1; \ a=fmtp:98 mode-change-capability=1; \
max-red=220; mode-set=0,2,4,7 max-red=220; mode-set=0,2,4,7
and AMR codec #4 with payload type number 99,depicted by the and AMR codec #4 with payload type number 99, depicted by the
potential configuration: potential configuration:
a=pcfg:4 m=4, pt=4:99 a=pcfg:4 m=4, pt=4:99
is equivalent to the following: is equivalent to the following:
m=audio 49170 RTP/AVP 99 m=audio 49170 RTP/AVP 99
a=rtpmap:99 AMR-WB/16000/1 a=rtpmap:99 AMR-WB/16000/1
a=fmtp:99 mode-change-capability=1; octet-align=1; \ a=fmtp:99 mode-change-capability=1; octet-align=1; \
mode-set=0,3,5,6 mode-set=0,3,5,6
and so on for the other four combinations. SDP could thus convert and so on for the other four combinations. SDP could thus convert
the media capabilities specifications into one or more alternative the media capabilities specifications into one or more alternative
media stream specifications, one of which can be chosen for the media stream specifications, one of which can be chosen for the
answer. answer.
3.3.3. The Media-Specific Capability Attribute 3.3.3. The Media-Specific Capability Attribute
Attributes and parameters associated with a media format are Attributes and parameters associated with a media format are
typically specified using the "rtpmap" and "fmtp" attributes in SDP, typically specified using the "rtpmap" and "fmtp" attributes in SDP,
and the similar "rmcap" and "mfcap" attributes in SDP Media and the similar "rmcap" and "mfcap" attributes in SDP media
Capabilities. Some SDP extensions define other attributes that need capabilities. Some SDP extensions define other attributes that need
to be associated with media formats, for example the "rtcp-fb" to be associated with media formats, for example, the "rtcp-fb"
attribute defined in RFC 4585 [RFC4585]. Such media-specific attribute defined in RFC 4585 [RFC4585]. Such media-specific
attributes, beyond the rtpmap and fmtp attributes, may be associated attributes, beyond the "rtpmap" and "fmtp" attributes, may be
with media capability numbers via a new media-specific attribute, associated with media capability numbers via a new media-specific
mscap, of the following form: attribute, "mscap", of the following form:
a=mscap:<media caps star> <att field> <att value> a=mscap:<media caps star> <att field> <att value>
where <media caps star> is a (list of) media capability number(s), 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 <att field> is the attribute name, and <att value> is the value field
for the named attribute. Note that the media capability numbers for the named attribute. Note that the media capability numbers
refer to media format capabilities specified elsewhere in the SDP refer to media format capabilities specified elsewhere in the SDP
("rmcap" and/or "omcap"). If a range of capability numbers is ("rmcap" and/or "omcap"). If a range of capability numbers is
specified, the first (leftmost) capability number MUST be strictly specified, the first (leftmost) capability number MUST be strictly
smaller than the second (rightmost). The media capability numbers smaller than the second (rightmost). The media capability numbers
may include a wildcard ("*"), which will be used instead of any may include a wildcard ("*"), which will be used instead of any
payload type mappings in the resulting SDP (see, e.g. RFC 4585 payload type mappings in the resulting SDP (see, e.g., RFC 4585
[RFC4585] and the example below). In ABNF, we have: [RFC4585] and the example below). In ABNF, we have:
media-specific-capability = "a=mscap:" media-specific-capability = "a=mscap:"
media-caps-star media-caps-star
1*WSP att-field ; from RFC4566 1*WSP att-field ; from RFC 4566
1*WSP att-value ; from RFC4566 1*WSP att-value ; from RFC 4566
media-caps-star = media-cap-star-element media-caps-star = media-cap-star-element
*("," media-cap-star-element) *("," media-cap-star-element)
media-cap-star-element = (media-cap-num [wildcard]) media-cap-star-element = (media-cap-num [wildcard])
/ (media-cap-num-range [wildcard]) / (media-cap-num-range [wildcard])
wildcard = "*" wildcard = "*"
Given an association between a media capability and a payload type Given an association between a media capability and a payload type
number as specified by the pt= parameters in a pcfg attribute line, a number as specified by the "pt=" parameters in a "pcfg" attribute
mscap line may be translated easily into a conventional SDP attribute line, a mscap line may be translated easily into a conventional SDP
line of the form attribute line of the form:
a=<att field>":"<fmt> <att value> ; <fmt> defined in SDP a=<att field>":"<fmt> <att value> ; <fmt> defined in SDP [RFC4566]
[RFC4566]
A resulting attribute that is not a legal SDP attribute as specified A resulting attribute that is not a legal SDP attribute, as specified
by RFC4566 MUST be ignored by the receiver. by RFC 4566, MUST be ignored by the receiver.
If a media capability number (or range) contains a wildcard character If a media capability number (or range) contains a wildcard character
at the end, any payload type mapping specified for that media at the end, any payload type mapping specified for that media-
specific capability (or range of capabilities) will use the wildcard specific capability (or range of capabilities) will use the wildcard
character in the resulting SDP instead of the payload type specified character in the resulting SDP instead of the payload type specified
in the payload type mapping ("pt" parameter) in the configuration in the payload type mapping ("pt" parameter) in the configuration
attribute. attribute.
A single mscap line may refer to multiple media capabilities by use A single mscap line may refer to multiple media capabilities by use
of a capability number range; this is equivalent to multiple mscap of a capability number range; this is equivalent to multiple mscap
lines, each with the same attribute values (but different media lines, each with the same attribute values (but different media
capability numbers), one line per media capability. capability numbers), one line per media capability.
Multiple mscap lines may refer to the same media capability, but, Multiple mscap lines may refer to the same media capability, but,
unlike the mfcap attribute, no concatenation operation is defined. unlike the "mfcap" attribute, no concatenation operation is defined.
Hence, multiple mscap lines applied to the same media capability is Hence, multiple mscap lines applied to the same media capability are
equivalent to multiple lines of the specified attribute in a equivalent to multiple lines of the specified attribute in a
conventional media record. conventional media record.
Here is an example with the rtcp-fb attribute, modified from an Here is an example with the "rtcp-fb" attribute, modified from an
example in RFC 5104 [RFC5104] (with the session-level and audio media example in RFC 5104 [RFC5104] (with the session level and audio media
omitted). If the offer contains a media block like the following omitted). If the offer contains a media block like the following
(note the wildcard character), (note the wildcard character),
m=video 51372 RTP/AVP 98 m=video 51372 RTP/AVP 98
a=rtpmap:98 H263-1998/90000 a=rtpmap:98 H263-1998/90000
a=tcap:1 RTP/AVPF a=tcap:1 RTP/AVPF
a=rmcap:1 H263-1998/90000 a=rmcap:1 H263-1998/90000
a=mscap:1 rtcp-fb ccm tstr a=mscap:1 rtcp-fb ccm tstr
a=mscap:1 rtcp-fb ccm fir a=mscap:1 rtcp-fb ccm fir
a=mscap:1* rtcp-fb ccm tmmbr smaxpr=120 a=mscap:1* rtcp-fb ccm tmmbr smaxpr=120
a=pcfg:1 t=1 m=1 pt=1:98 a=pcfg:1 t=1 m=1 pt=1:98
and if the proposed configuration is chosen, then the equivalent and if the proposed configuration is chosen, then the equivalent
media block would look like media block would look like the following
m=video 51372 RTP/AVPF 98
m=video 51372 RTP/AVPF 98 a=rtpmap:98 H263-1998/90000
a=rtpmap:98 H263-1998/90000 a=rtcp-fb:98 ccm tstr
a=rtcp-fb:98 ccm tstr a=rtcp-fb:98 ccm fir
a=rtcp-fb:98 ccm fir a=rtcp-fb:* ccm tmmbr smaxpr=120
a=rtcp-fb:* ccm tmmbr smaxpr=120
3.3.4. New Configuration Parameters 3.3.4. New Configuration Parameters
Along with the new attributes for media capabilities, new extension Along with the new attributes for media capabilities, new extension
parameters are defined for use in the potential configuration, the parameters are defined for use in the potential configuration, the
actual configuration, and/or the new latent configuration defined in actual configuration, and/or the new latent configuration defined in
Section 3.3.5. Section 3.3.5.
3.3.4.1. The Media Configuration Parameter (m=) 3.3.4.1. The Media Configuration Parameter (m=)
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The media configuration parameter is used to specify the media The media configuration parameter is used to specify the media
format(s) and related parameters for a potential, actual, or latent format(s) and related parameters for a potential, actual, or latent
configuration. Adhering to the ABNF for extension-config-list in RFC configuration. Adhering to the ABNF for extension-config-list in RFC
5939 [RFC5939] with 5939 [RFC5939] with
ext-cap-name = "m" ext-cap-name = "m"
ext-cap-list = media-cap-num-list ext-cap-list = media-cap-num-list
[*(BAR media-cap-num-list)] [*(BAR media-cap-num-list)]
we have we have
media-config-list = ["+"] "m=" media-cap-num-list media-config-list = ["+"] "m=" media-cap-num-list
*(BAR media-cap-num-list) *(BAR media-cap-num-list)
;BAR is defined in RFC5939 ;BAR is defined in RFC 5939
;media-cap-num-list is defined above ;media-cap-num-list is defined above
Alternative media configurations are separated by a vertical bar Alternative media configurations are separated by a vertical bar
("|"). The alternatives are ordered by preference, most-preferred ("|"). The alternatives are ordered by preference, most-preferred
first. When media capabilities are not included in a potential first. When media capabilities are not included in a potential
configuration at the media level, the media type and media format 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 from the associated "m=" line will be used. The use of the plus sign
("+") is described in RFC5939. ("+") is described in RFC 5939.
3.3.4.2. The Payload Type Number Mapping Parameter (pt=) 3.3.4.2. The Payload Type Number Mapping Parameter (pt=)
The payload type number mapping parameter is used to specify the The payload type number mapping parameter is used to specify the
payload type number to be associated with each RTP-based media format payload type number to be associated with each RTP-based media format
in a potential, actual, or latent configuration. We define the in a potential, actual, or latent configuration. We define the
payload type number mapping parameter, payload-number-config-list, in payload type number mapping parameter, payload-number-config-list, in
accordance with the extension-config-list format defined in RFC 5939 accordance with the extension-config-list format defined in RFC 5939
[RFC5939]. In ABNF: [RFC5939]. In ABNF:
payload-number-config-list = ["+"] "pt=" media-map-list payload-number-config-list = ["+"] "pt=" media-map-list
media-map-list = media-map *("," media-map) media-map-list = media-map *("," media-map)
media-map = media-cap-num ":" payload-type-number media-map = media-cap-num ":" payload-type-number
; media-cap-num is defined in 3.3.1 ; media-cap-num is defined in Section 3.3.1
payload-type-number = NonZeroDigit *2(DIGIT) ; RTP payload payload-type-number = NonZeroDigit *2(DIGIT) ; RTP payload
; type number ; type number
The example in Section 3.3.7 shows how the parameters from the rmcap 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. line are mapped to payload type numbers from the "pcfg" "pt"
The use of the plus sign ("+") is described in RFC 5939 [RFC5939]. parameter. The use of the plus sign ("+") is described in RFC 5939
[RFC5939].
A latent configuration represents a future capability, hence the pt= A latent configuration represents a future capability; hence, the
parameter is not directly meaningful in the lcfg attribute because no "pt=" parameter is not directly meaningful in the "lcfg" attribute
actual media session is being offered or accepted; it is permitted in because no actual media session is being offered or accepted. It is
order to tie any payload type number parameters within attributes to permitted in order to tie any payload type number parameters within
the proper media format. A primary example is the case of format attributes to the proper media format. A primary example is the case
parameters for the Redundant Audio Data (RED) payload, which are of format parameters for the Redundant Audio Data (RED) [RFC2198]
payload type numbers. Specific payload type numbers used in a latent payload, which are payload type numbers. Specific payload type
configuration MAY be interpreted as suggestions to be used in any numbers used in a latent configuration MAY be interpreted as
future offer based on the latent configuration, but they are not suggestions to be used in any future offer based on the latent
binding; the offerer and/or answerer may use any payload type numbers configuration, but they are not binding; the offerer and/or answerer
each deems appropriate. The use of explicit payload type numbers for may use any payload type numbers each deems appropriate. The use of
latent configurations can be avoided by use of the parameter explicit payload type numbers for latent configurations can be
substitution rule of Section 3.3.7. Future extensions are also avoided by use of the parameter substitution rule of Section 3.3.7.
permitted. Note that leading zeroes are not permitted. Future extensions are also permitted. Note that leading zeroes are
not permitted.
3.3.4.3. The Media Type Parameter 3.3.4.3. The Media Type Parameter
When a latent configuration is specified (always at the media level), When a latent configuration is specified (always at the media level),
indicating the ability to support an additional media stream, it is indicating the ability to support an additional media stream, it is
necessary to specify the media type (audio, video, etc.) as well as necessary to specify the media type (audio, video, etc.) as well as
the format and transport type. The media type parameter is defined the format and transport type. The media type parameter is defined
in ABNF as in ABNF as
media-type = ["+"] "mt=" media; media defined in RFC4566 media-type = ["+"] "mt=" media; media defined in RFC 4566
At present, the media-type parameter is used only in the latent At present, the media-type parameter is used only in the latent
configuration attribute, and the use of the "+" prefix to specify configuration attribute, and the use of the "+" prefix to specify
that the entire attribute line is to be ignored if the mt= parameter that the entire attribute line is to be ignored if the mt= parameter
is not understood, is unnecessary. However, if the media-type is not understood is unnecessary. However, if the media-type
parameter is later added to an existing capability attribute such as parameter is later added to an existing capability attribute such as
pcfg, then the "+" would be useful. The media format(s) and "pcfg", then the "+" would be useful. The media format(s) and
transport type(s) are specified using the media configuration transport type(s) are specified using the media configuration
parameter ("+m=") defined above, and the transport parameter ("t=") parameter ("+m=") defined above, and the transport parameter ("t=")
defined in RFC 5939 [RFC5939], respectively. defined in RFC 5939 [RFC5939], respectively.
3.3.5. The Latent Configuration Attribute 3.3.5. The Latent Configuration Attribute
One of the goals of this work is to permit the exchange of One of the goals of this work is to permit the exchange of
supportable media configurations in addition to those offered or supportable media configurations in addition to those offered or
accepted for immediate use. Such configurations are referred to as accepted for immediate use. Such configurations are referred to as
"latent configurations". For example, a party may offer to establish "latent configurations". For example, a party may offer to establish
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latent video configurations along with the media stream for audio; latent video configurations along with the media stream for audio;
the responding party may indicate its ability and willingness to the responding party may indicate its ability and willingness to
support such a video session by returning a corresponding latent support such a video session by returning a corresponding latent
configuration. configuration.
Latent configurations returned in SDP answers MUST match offered Latent configurations returned in SDP answers MUST match offered
latent configurations (or parameter subsets thereof). Therefore, it latent configurations (or parameter subsets thereof). Therefore, it
is appropriate for the offering party to announce most, if not all, is appropriate for the offering party to announce most, if not all,
of its capabilities in the initial offer. This choice has been made 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 in order to keep the size of the answer more compact by not requiring
acap, rmcap, tcap, etc. lines in the answer. acap, rmcap, tcap, etc. lines in the answer.
Latent configurations may be announced by use of the latent Latent configurations may be announced by use of the latent
configuration attribute, which is defined in a manner very similar to configuration attribute, which is defined in a manner very similar to
the potential configuration attribute. The latent configuration the potential configuration attribute. The latent configuration
attribute combines the properties of a media line and a potential attribute combines the properties of a media line and a potential
configuration. A latent configuration MUST include a media type configuration. A latent configuration MUST include a media type
(mt=) and a transport protocol configuration parameter since the (mt=) and a transport protocol configuration parameter since the
latent configuration is independent of any media line present. In latent configuration is independent of any media line present. In
most cases, the media configuration (m=) parameter needs to be most cases, the media configuration (m=) parameter needs to be
present as well (see Section 4 for examples). The lcfg attribute is present as well (see Section 4 for examples). The "lcfg" attribute
a media level attribute. is a media-level attribute.
The lcfg attribute is defined as a media level attribute since it The "lcfg" attribute is defined as a media-level attribute since
specifies a possible future media stream. However the lcfg it specifies a possible future media stream. However, the "lcfg"
attribute is not necessarily related to the media description attribute is not necessarily related to the media description
within which it is provided. Session capabilities ("sescap") may within which it is provided. Session capability attributes
be used to indicate this. ("a=sescap") may be used to indicate supported media stream
configurations.
Each media line in an SDP description represents an offered Each media line in an SDP description represents an offered
simultaneous media stream, whereas each latent configuration simultaneous media stream, whereas each latent configuration
represents an additional stream which may be negotiated in a future represents an additional stream that may be negotiated in a future
offer/answer exchange. Session capability attributes may be used to offer/answer exchange. Session capability attributes may be used to
determine whether a latent configuration may be used to form an offer determine whether a latent configuration may be used to form an offer
for an additional simultaneous stream or to reconfigure an existing for an additional simultaneous stream or to reconfigure an existing
stream in a subsequent offer/answer exchange. stream in a subsequent offer/answer exchange.
The latent configuration attribute is of the form: The latent configuration attribute is of the form:
a=lcfg:<config-number> <latent-cfg-list> a=lcfg:<config-number> <latent-cfg-list>
which adheres to the SDP [RFC4566] "attribute" production with att- which adheres to the SDP [RFC4566] "attribute" production with
field and att-value defined as: att-field and att-value defined as:
att-field = "lcfg" att-field = "lcfg"
att-value = config-number 1*WSP lcfg-cfg-list att-value = config-number 1*WSP lcfg-cfg-list
config-number = NonZeroDigit *9(DIGIT) ; DIGIT defined in RFC5234 config-number = NonZeroDigit *9(DIGIT) ;DIGIT defined in RFC 5234
lcfg-cfg-list = media-type 1*WSP pot-cfg-list lcfg-cfg-list = media-type 1*WSP pot-cfg-list
; as defined in RFC5939 ; as defined in RFC 5939
; and extended herein ; and extended herein
The media-type (mt=) parameter identifies the media type (audio, The media-type (mt=) parameter identifies the media type (audio,
video, etc.) to be associated with the latent media stream, and MUST video, etc.) to be associated with the latent media stream, and it
be present. The pot-cfg-list MUST contain a transport-protocol- MUST be present. The pot-cfg-list MUST contain a transport-protocol-
config-list (t=) parameter and a media-config-list (m=) parameter. 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 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 of parameter list. As specified in RFC 5939 [RFC5939], the use of
the "+" prefix with a parameter indicates that the entire the "+" prefix with a parameter indicates that the entire
configuration MUST be ignored if the parameter is not understood; configuration MUST be ignored if the parameter is not understood;
otherwise, the parameter itself may be ignored. otherwise, the parameter itself may be ignored.
Media stream payload numbers are not assigned by a latent Media stream payload numbers are not assigned by a latent
configuration. Assignment will take place if and when the configuration. Assignment will take place if and when the
corresponding stream is actually offered via an m-line in a later corresponding stream is actually offered via an "m=" line in a later
exchange. The payload-number-config-list is included as a parameter exchange. The payload-number-config-list is included as a parameter
to the lcfg attribute in case it is necessary to tie payload numbers to the "lcfg" attribute in case it is necessary to tie payload
in attribute capabilities to specific media capabilities. numbers in attribute capabilities to specific media capabilities.
If an lcfg attribute invokes an acap attribute that appears at the If an "lcfg" attribute invokes an "acap" attribute that appears at
session level, then that attribute will be expected to appear at the the session level, then that attribute will be expected to appear at
session level of a subsequent offer when and if a corresponding media the session level of a subsequent offer when and if a corresponding
stream is offered. Otherwise, acap attributes which appear at the media stream is offered. Otherwise, "acap" attributes that appear at
media level represent media-level attributes. Note, however, that the media level represent media-level attributes. Note, however,
rmcap, omcap, mfcap, mscap, and tcap attributes may appear at the that "rmcap", omcap, "mfcap", "mscap", and "tcap" attributes may
session level because they always result in media-level attributes or appear at the session level because they always result in media-level
m-line parameters. attributes or "m=" line parameters.
The configuration numbers for latent configurations do not imply a The configuration numbers for latent configurations do not imply a
preference; the offerer will imply a preference when actually preference; the offerer will imply a preference when actually
offering potential configurations derived from latent configurations offering potential configurations derived from latent configurations
negotiated earlier. Note however that the offerer of latent negotiated earlier. Note, however, that the offerer of latent
configurations MAY specify preferences for combinations of potential configurations MAY specify preferences for combinations of potential
and latent configurations by use of the sescap attribute defined in 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 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 stream with "pcfg:1", and two latent video configurations, "lcfg:2"
lcfg:3, then a session with one audio stream and one video stream and "lcfg:3", then a session with one audio stream and one video
could be specified by including "a=sescap:1 1,2|3". One audio stream stream could be specified by including "a=sescap:1 1,2|3". One audio
and two video streams could be specified by including "a=sescap:2 stream and two video streams could be specified by including
1,2,3" in the offer. In order to permit combinations of latent and "a=sescap:2 1,2,3" in the offer. In order to permit combinations of
potential configurations in session capabilities, latent latent and potential configurations in session capabilities, latent
configuration numbers MUST be different from those used for potential configuration numbers MUST be different from those used for potential
configurations. This restriction is especially important if the configurations. This restriction is especially important if the
offerer does not require cmed-v0 capability and the recipient of 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, offer doesn't support it. If the "lcfg" attribute is not recognized,
the capability attributes intended to be associated with it may be the capability attributes intended to be associated with it may be
confused with those associated with a potential configuration of some confused with those associated with a potential configuration of some
other media stream. Note also that leading zeroes are not permitted other media stream. Note also that leading zeroes are not permitted
in configuration numbers. in configuration numbers.
If a cryptographic attribute, such as the SDES "a=crypto:" attribute If a cryptographic attribute, such as the SDES "a=crypto:" attribute
[RFC4568], is referenced by a latent configuration through an acap [RFC4568], is referenced by a latent configuration through an "acap"
attribute, any keying material required in the conventional attribute, any keying material required in the conventional
attribute, such as the SDES key/salt string, MUST be included in attribute, such as the SDES key/salt string, MUST be included in
order to satisfy formatting rules for the attribute. Since the order to satisfy formatting rules for the attribute. Since the
keying material will be visible but not actually used at this stage keying material will be visible but not actually used at this stage
(since it's a latent configuration), the value(s) of the keying (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, material MUST NOT be a real value used for real exchange of media,
and the receiver of the lcfg attribute MUST ignore the values. and the receiver of the "lcfg" attribute MUST ignore the value(s).
3.3.6. Enhanced Potential Configuration Attribute 3.3.6. Enhanced Potential Configuration Attribute
The present work requires new extensions (parameters) for the pcfg The present work requires new extensions (parameters) for the "pcfg"
attribute defined in the SDP Capability Negotiation base protocol attribute defined in the SDP capability negotiation base protocol
[RFC5939]. The parameters and their definitions are "borrowed" from [RFC5939]. The parameters and their definitions are "borrowed" from
the definitions provided for the latent configuration attribute in the definitions provided for the latent configuration attribute in
Section 3.3.5. The expanded ABNF definition of the pcfg attribute is Section 3.3.5. The expanded ABNF definition of the "pcfg" attribute
is
a=pcfg: <config-number> [<pot-cfg-list>] a=pcfg: <config-number> [<pot-cfg-list>]
where where
config-number = 1*DIGIT ;defined in [RFC5234] config-number = 1*DIGIT ;defined in [RFC5234]
pot-cfg-list = pot-config *(1*WSP pot-config) pot-cfg-list = pot-config *(1*WSP pot-config)
pot-config = attribute-config-list / ;def in [RFC5939] pot-config = attribute-config-list / ;def in [RFC5939]
transport-protocol-config-list / ;defined in [RFC5939] transport-protocol-config-list / ;defined in [RFC5939]
extension-config-list / ;[RFC5939] extension-config-list / ;[RFC5939]
media-config-list / ; Section 3.3.4.1 media-config-list / ; Section 3.3.4.1
skipping to change at page 27, line 26 skipping to change at page 26, line 12
Except for the extension-config-list, the pot-cfg-list MUST NOT Except for the extension-config-list, the pot-cfg-list MUST NOT
contain more than one instance of each parameter list. contain more than one instance of each parameter list.
3.3.6.1. Returning Capabilities in the Answer 3.3.6.1. Returning Capabilities in the Answer
Potential and/or latent configuration attributes may be returned Potential and/or latent configuration attributes may be returned
within an answer SDP to indicate the ability of the answerer to within an answer SDP to indicate the ability of the answerer to
support alternative configurations of the corresponding stream(s). support alternative configurations of the corresponding stream(s).
For example, an offer may include multiple potential configurations For example, an offer may include multiple potential configurations
for a media stream and/or latent configurations for additional for a media stream and/or latent configurations for additional
streams; the corresponding answer will indicate (via an acfg streams. The corresponding answer will indicate (via an "acfg"
attribute) the configuration accepted and used to construct the base attribute) the configuration accepted and used to construct the base
configuration for each active media stream in the reply, but the configuration for each active media stream in the reply, but the
reply MAY also contain potential and/or latent configuration reply MAY also contain potential and/or latent configuration
attributes, with parameters, to indicate which other offered attributes, with parameters, to indicate which other offered
configurations would be acceptable. This information is useful if it configurations would be acceptable. This information is useful if it
becomes desirable to reconfigure a media stream, e.g., to reduce becomes desirable to reconfigure a media stream, e.g., to reduce
resource consumption. resource consumption.
When potential and/or latent configurations are returned in an When potential and/or latent configurations are returned in an
answer, all numbering MUST refer to the configuration and capability answer, all numbering MUST refer to the configuration and capability
attribute numbering of the offer. The offered capability attributes attribute numbering of the offer. The offered capability attributes
need not be returned in the answer. The answer MAY include need not be returned in the answer. The answer MAY include
additional capability attributes and/or configurations (with distinct additional capability attributes and/or configurations (with distinct
numbering). The parameter values of any returned pcfg or lcfg numbering). The parameter values of any returned "pcfg" or "lcfg"
attributes MUST be a subset of those included in the offered attributes MUST be a subset of those included in the offered
configurations and/or those added by the answerer; values MAY be configurations and/or those added by the answerer; values MAY be
omitted only if they were indicated as alternative sets, or optional, omitted only if they were indicated as alternative sets, or optional,
in the original offer. The parameter set indicated in the returned in the original offer. The parameter set indicated in the returned
acfg attribute need not be repeated in a returned pcfg attribute. "acfg" attribute need not be repeated in a returned "pcfg" attribute.
The answerer MAY return more than one pcfg attribute with the same The answerer MAY return more than one "pcfg" attribute with the same
configuration number if it is necessary to describe selected configuration number if it is necessary to describe selected
combinations of optional or alternative parameters. combinations of optional or alternative parameters.
Similarly, one or more session capability attributes (a=sescap) MAY Similarly, one or more session capability attributes ("a=sescap") MAY
be returned to indicate which of the offered session capabilities is/ be returned to indicate which of the offered session capabilities
are supportable by the answerer (see Section 3.3.8.) is/are supportable by the answerer (see Section 3.3.8).
Note that, although the answerer MAY return capabilities beyond those Note that, although the answerer MAY return capabilities beyond those
included by the offerer, these capabilities MUST NOT be used to form included by the offerer, these capabilities MUST NOT be used to form
any base level media description in the answer. For this reason, it any base level media description in the answer. For this reason, it
is advisable for the offerer to include most, if not all, potential is advisable for the offerer to include most, if not all, potential
and latent configurations it can support in the initial offer, unless and latent configurations it can support in the initial offer, unless
the size of the resulting SDP is a concern. Either party MAY later the size of the resulting SDP is a concern. Either party MAY later
announce additional capabilities by renegotiating the session in a announce additional capabilities by renegotiating the session in a
second offer/answer exchange. second offer/answer exchange.
3.3.6.2. Payload Type Number Mapping 3.3.6.2. Payload Type Number Mapping
When media format capabilities defined in rmcap attributes are used When media format capabilities defined in "rmcap" attributes are used
in potential configuration lines, the transport protocol uses RTP and in potential configuration lines, the transport protocol uses RTP and
it is necessary to assign payload type numbers. In some cases, it is it is necessary to assign payload type numbers. In some cases, it is
desirable to assign different payload type numbers to the same media desirable to assign different payload type numbers to the same media
format capability when used in different potential configurations. format capability when used in different potential configurations.
One example is when configurations for AVP and SAVP are offered: the One example is when configurations for AVP and SAVP are offered: the
offerer would like the answerer to use different payload type numbers offerer would like the answerer to use different payload type numbers
for encrypted and unencrypted media, so the offerer can decide for encrypted and unencrypted media, so the offerer can decide
whether or not to render early media which arrives before the answer whether or not to render early media that arrives before the answer
is received. is received.
For example, if use of AVP was selected by the answerer, then For example, if use of AVP was selected by the answerer, then
media received by the offerer is not encrypted and hence can be media received by the offerer is not encrypted; hence, it can be
played out prior to receiving the answer. Conversely, if SAVP was played out prior to receiving the answer. Conversely, if SAVP was
selected, cryptographic parameters and keying material present in selected, cryptographic parameters and keying material present in
the answer may be needed to decrypt received media. If the offer the answer may be needed to decrypt received media. If the offer
configuration indicated that AVP media uses one set of payload configuration indicated that AVP media uses one set of payload
types and SAVP a different set, then the offerer will know whether types and SAVP a different set, then the offerer will know whether
media received prior to the answer is encrypted or not by simply media received prior to the answer is encrypted or not by simply
looking at the RTP payload type number in the received packet. looking at the RTP payload type number in the received packet.
This association of distinct payload type number(s) with different This association of distinct payload type number(s) with different
transport protocols requires a separate pcfg line for each protocol. transport protocols requires a separate pcfg line for each protocol.
Clearly, this technique cannot be used if the number of potential Clearly, this technique cannot be used if the number of potential
configurations exceeds the number of possible payload type numbers. configurations exceeds the number of possible payload type numbers.
3.3.6.3. Processing of Media-Format-Related Conventional Attributes for 3.3.6.3. Processing of Media-Format-Related Conventional Attributes for
Potential Configurations Potential Configurations
When media capabilities negotiation is employed, SDP records are When media capabilities negotiation is employed, SDP records are
likely to contain conventional attributes such as rtpmap, fmtp, and likely to contain conventional attributes such as "rtpmap", "fmtp",
other media-format-related lines, as well as capability attributes and other media-format-related lines, as well as capability
such as rmcap, omcap, mfcap, and mscap which map into those attributes such as "rmcap", omcap, "mfcap", and "mscap" that map into
conventional attributes when invoked by a potential configuration. those conventional attributes when invoked by a potential
In such cases, it MAY be appropriate to employ the delete-attributes configuration. In such cases, it MAY be appropriate to employ the
option [RFC5939] in the attribute configuration list parameter in delete-attributes option [RFC5939] in the attribute configuration
order to avoid the generation of conflicting fmtp attributes for a list parameter in order to avoid the generation of conflicting "fmtp"
particular configuration. Any media-specific attributes in the media attributes for a particular configuration. Any media-specific
block which refer to media formats not used by the potential attributes in the media block that refer to media formats not used by
configuration MUST be ignored. the potential configuration MUST be ignored.
For example: For example:
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
a=creq:med-v0 a=creq:med-v0
m=audio 3456 RTP/AVP 0 18 100 m=audio 3456 RTP/AVP 0 18 100
a=rtpmap:100 telephone-event a=rtpmap:100 telephone-event
a=fmtp:100 0-11 a=fmtp:100 0-11
a=rmcap:1 PCMU/8000 a=rmcap:1 PCMU/8000
a=rmcap:2 G729/8000 a=rmcap:2 G729/8000
a=rmcap:3 telephone-event/8000 a=rmcap:3 telephone-event/8000
a=mfcap:3 0-15 a=mfcap:3 0-15
a=pcfg:1 m=2,3|1,3 a=-m pt=1:0,2:18,3:100 a=pcfg:1 m=2,3|1,3 a=-m pt=1:0,2:18,3:100
a=pcfg:2 a=pcfg:2
In this example, PCMU is media capability 1, G729 is media capability 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 2, and telephone-event is media capability 3. The a=pcfg:1 line
specifies that the preferred configuration is G.729 with extended specifies that the preferred configuration is G.729 with extended
dtmf events, second is G.711 mu-law with extended dtmf events, and DTMF events, second is G.711 mu-law with extended DTMF events, and
the base media-level attributes are to be deleted. Intermixing of the base media-level attributes are to be deleted. Intermixing of
G.729, G.711, and "commercial" dtmf events is least preferred (the G.729, G.711, and "commercial" DTMF events is least preferred (the
base configuration provided by the "m=" line, which is, by default, base configuration provided by the "m=" line, which is, by default,
the least preferred configuration). The rtpmap and fmtp attributes the least preferred configuration). The "rtpmap" and "fmtp"
of the base configuration are replaced by the rmcap and mfcap attributes of the base configuration are replaced by the "rmcap" and
attributes when invoked by the proposed configuration. "mfcap" attributes when invoked by the proposed configuration.
If the preferred configuration is selected, the SDP answer will look If the preferred configuration is selected, the SDP answer will look
like like the following
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
a=csup:med-v0 a=csup:med-v0
m=audio 3456 RTP/AVP 18 100 m=audio 3456 RTP/AVP 18 100
a=rtpmap:100 telephone-event/8000 a=rtpmap:100 telephone-event/8000
a=fmtp:100 0-15 a=fmtp:100 0-15
a=acfg:1 m=2,3 pt=1:0,2:18,3:100 a=acfg:1 m=2,3 pt=1:0,2:18,3:100
3.3.7. Substitution of Media Payload Type Numbers in Capability 3.3.7. Substitution of Media Payload Type Numbers in Capability
Attribute Parameters Attribute Parameters
In some cases, for example, when an RFC 2198 [RFC2198] redundancy In some cases, for example, when an RFC 2198 [RFC2198] redundancy
audio subtype (RED) capability is defined in an mfcap attribute, the audio subtype (RED) capability is defined in an "mfcap" attribute,
parameters to an attribute may contain payload type numbers. Two the parameters to an attribute may contain payload type numbers. Two
options are available for specifying such payload type numbers. They options are available for specifying such payload type numbers. They
may be expressed explicitly, in which case they are bound to actual may be expressed explicitly, in which case they are bound to actual
payload types by means of the payload type number parameter (pt=) in payload types by means of the payload type number parameter (pt=) in
the appropriate potential or latent configuration. For example, the the appropriate potential or latent configuration. For example, the
following SDP fragment defines a potential configuration with following SDP fragment defines a potential configuration with
redundant G.711 mu-law: redundant G.711 mu-law
m=audio 45678 RTP/AVP 0 m=audio 45678 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
a=rmcap:1 PCMU/8000 a=rmcap:1 PCMU/8000
a=rmcap:2 RED/8000 a=rmcap:2 RED/8000
a=mfcap:2 0/0 a=mfcap:2 0/0
a=pcfg:1 m=2,1 pt=2:98,1:0 a=pcfg:1 m=2,1 pt=2:98,1:0
The potential configuration is then equivalent to The potential configuration is then equivalent to
m=audio 45678 RTP/AVP 98 0 m=audio 45678 RTP/AVP 98 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
a=rtpmap:98 RED/8000 a=rtpmap:98 RED/8000
a=fmtp:98 0/0 a=fmtp:98 0/0
A more general mechanism is provided via the parameter substitution A more general mechanism is provided via the parameter substitution
rule. When an mfcap, mscap, or acap attribute is processed, its rule. When an "mfcap", "mscap", or "acap" attribute is processed,
arguments will be scanned for a payload type number escape sequences its arguments will be scanned for a payload type number escape
of the following form (in ABNF): sequence of the following form (in ABNF):
ptn-esc = "%m=" media-cap-num "%" ; defined in 3.3.1 ptn-esc = "%m=" media-cap-num "%" ; defined in Section 3.3.1
If the sequence is found, the sequence is replaced by the payload If the sequence is found, the sequence is replaced by the payload
type number assigned to the media capability number, as specified by type number assigned to the media capability number, as specified by
the pt= parameter in the selected potential configuration; only the "pt=" parameter in the selected potential configuration; only
actual payload type numbers are supported - wildcards are excluded. actual payload type numbers are supported -- wildcards are excluded.
The sequence "%%" (null digit string) is replaced by a single percent The sequence "%%" (null digit string) is replaced by a single percent
sign and processing continues with the next character, if any. sign and processing continues with the next character, if any.
For example, the above offer sequence could have been written as For example, the above offer sequence could have been written as
m=audio 45678 RTP/AVP 0 m=audio 45678 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
a=rmcap:1 PCMU/8000 a=rmcap:1 PCMU/8000
a=rmcap:2 RED/8000 a=rmcap:2 RED/8000
a=mfcap:2 %m=1%/%m=1% a=mfcap:2 %m=1%/%m=1%
a=pcfg:1 m=2,1 pt=2:98,1:0 a=pcfg:1 m=2,1 pt=2:98,1:0
and the equivalent SDP is the same as above. and the equivalent SDP is the same as above.
3.3.8. The Session Capability Attribute 3.3.8. The Session Capability Attribute
Potential and latent configurations enable offerers and answerers to Potential and latent configurations enable offerers and answerers to
express a wide range of alternative configurations for current and express a wide range of alternative configurations for current and
future negotiation. However in practice, it may not be possible to future negotiation. However, in practice, it may not be possible to
support all combinations of these configurations. support all combinations of these configurations.
The session capability attribute provides a means for the offerer The session capability attribute provides a means for the offerer
and/or the answerer to specify combinations of specific media stream and/or the answerer to specify combinations of specific media stream
configurations which it is willing and able to support. Each session configurations that it is willing and able to support. Each session
capability in an offer or answer MAY be expressed as a list of capability in an offer or answer MAY be expressed as a list of
required potential configurations, and MAY include a list of optional required potential configurations, and MAY include a list of optional
potential and/or latent configurations. potential and/or latent configurations.
The choices of session capabilities may be based on processing load, The choices of session capabilities may be based on processing load,
total bandwidth, or any other criteria of importance to the total bandwidth, or any other criteria of importance to the
communicating parties. If the answerer supports media capabilities communicating parties. If the answerer supports media capabilities
negotiation, and session configurations are offered, it MUST accept negotiation, and session configurations are offered, it MUST accept
one of the offered configurations, or it MUST refuse the session. one of the offered configurations, or it MUST refuse the session.
Therefore, if the offer includes any session capabilities, it SHOULD Therefore, if the offer includes any session capabilities, it SHOULD
include all the session capabilities the offerer is willing to include all the session capabilities the offerer is willing to
support. support.
The session capability attribute is a session-level attribute The session capability attribute is a session-level attribute
described by: described by
"a=sescap:" <session num> <list of configs> "a=sescap:" <session num> <list of configs>
which corresponds to the standard value attribute definition with which corresponds to the standard value attribute definition with
att-field = "sescap" att-field = "sescap"
att-value = session-num 1*WSP list-of-configs att-value = session-num 1*WSP list-of-configs
[1*WSP optional-configs] [1*WSP optional-configs]
session-num = NonZeroDigit *9(DIGIT) ; DIGIT defined session-num = NonZeroDigit *9(DIGIT) ; DIGIT defined
; in RFC5234 ; in RFC 5234
list-of-configs = alt-config *("," alt-config) list-of-configs = alt-config *("," alt-config)
optional-configs = "[" list-of-configs "]" optional-configs = "[" list-of-configs "]"
alt-config = config-number *("|" config-number) alt-config = config-number *("|" config-number)
The session-num identifies the session: a lower-number session is The session-num identifies the session: a lower-number session is
preferred over a higher-number session, and leading zeroes are not preferred over a higher-number session, and leading zeroes are not
permitted. Each alt-config list specifies alternative media permitted. Each alt-config list specifies alternative media
configurations within the session; preference is based on config-num configurations within the session; preference is based on config-num
as specified in RFC 5939 [RFC5939]. Note that the session preference as specified in RFC 5939 [RFC5939]. Note that the session preference
order, when present, takes precedence over the individual media order, when present, takes precedence over the individual media
stream configuration preference order. stream configuration preference order.
Use of session capability attributes requires that configuration Use of session capability attributes requires that configuration
numbers assigned to potential and latent configurations MUST be numbers assigned to potential and latent configurations MUST be
unique across the entire session; RFC 5939 [RFC5939] requires only unique across the entire session; RFC 5939 [RFC5939] requires only
that pcfg configuration numbers be unique within a media description. that "pcfg" configuration numbers be unique within a media
Also, leading zeroes are not permitted. description. Also, leading zeroes are not permitted.
As an example, consider an endpoint that is capable of supporting an 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 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 streams with a floor control stream. In the latter case, the second
video stream is optional. The SDP offer might look like the video stream is optional. The SDP offer might look like the
following (offering audio, an H.263 video streams, BFCP and another following (offering audio, an H.263 video streams, BFCP and another
optional H.263 video stream)- the empty lines are added for optional H.263 video stream) -- the empty lines are added for
readability only (not part of valid SDP): readability only (not part of valid SDP):
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
a=creq:med-v0 a=creq:med-v0
a=sescap:2 1,2,5,[3] a=sescap:2 1,2,5,[3]
a=sescap:1 1,4 a=sescap:1 1,4
m=audio 54322 RTP/AVP 0 m=audio 54322 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
a=pcfg:1 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 22344 RTP/AVP 102 m=video 33444 RTP/AVP 103
a=rtpmap:102 H263-1998/90000 a=rtpmap:103 H263-1998/90000
a=fmtp:102 CIF=4;QCIF=2;F=1;K=1 a=fmtp:103 CIF=4;QCIF=2;F=1;K=1
i=main video stream i=secondary video (slides)
a=label:11 a=label:12
a=pcfg:2 a=pcfg:3
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 m=application 33002 TCP/BFCP *
a=rtpmap:103 H263-1998/90000 a=setup:passive
a=fmtp:103 CIF=4;QCIF=2;F=1;K=1 a=connection:new
i=secondary video (slides) a=floorid:1 m-stream:11 12
a=label:12 a=floor-control:s-only
a=pcfg:3 a=confid:4321
m=application 33002 TCP/BFCP * a=userid:1234
a=setup:passive a=pcfg:5
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 If the answerer understands MediaCapNeg, but cannot support the
Binary Floor Control Protocol, then it would respond with (invalid Binary Floor Control Protocol, then it would respond with (invalid
empty lines in SDP included again for readability): empty lines in SDP included again for readability):
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.22 c=IN IP4 192.0.2.22
t=0 0 t=0 0
a=csup:med-v0 a=csup:med-v0
a=sescap:1 1,4 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 m=audio 23456 RTP/AVP 0
a=rtpmap:104 H264/90000 a=rtpmap:0 PCMU/8000
a=fmtp:104 profile-level-id=42A01E; packetization-mode=2 a=acfg:1
a=acfg:4 m=1 a=1 pt=1:104
m=video 0 RTP/AVP 103 m=video 41234 RTP/AVP 104
a=acfg:3 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 * m=application 0 TCP/BFCP *
a=acfg:5 a=acfg:5
An endpoint that doesn't support Media capabilities negotiation, but An endpoint that doesn't support media capabilities negotiation, but
does support H.263 video, would respond with one or two H.263 video 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 streams. In the latter case, the answerer may issue a second offer
to reconfigure the session to one audio and one video channel using to reconfigure the session to one audio and one video channel using
H.264 or H.263. H.264 or H.263.
Session capabilities can include latent capabilities as well. Here's Session capabilities can include latent capabilities as well. Here's
a similar example in which the offerer wishes to initially establish a similar example in which the offerer wishes to initially establish
an audio stream, and prefers to later establish two video streams an audio stream, and prefers to later establish two video streams
with chair control. If the answerer doesn't understand Media CapNeg, with chair control. If the answerer doesn't understand Media CapNeg,
or cannot support the dual video streams or flow control, then it may 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 support a single H.264 video stream. Note that establishment of the
most favored configuration will require two offer/answer exchanges. most favored configuration will require two offer/answer exchanges.
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
a=creq:med-v0 a=creq:med-v0
a=sescap:1 1,3,4,5 a=sescap:1 1,3,4,5
a=sescap:2 1,2 a=sescap:2 1,2
a=sescap:3 1 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 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 that do not
understand capabilities negotiation, proposes a PCMU audio stream and understand capabilities negotiation, proposes a PCMU audio stream and
an H.264 video stream. Note that the offered lcfg lines for the 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 video streams don't carry "pt=" parameters because they're not needed
(payload type numbers will be assigned in the offer/answer exchange (payload type numbers will be assigned in the offer/answer exchange
that establishes the streams). Note also that the three rmcap, that establishes the streams). Note also that the three "rmcap",
mfcap, and tcap attributes used by lcfg:3 and lcfg:4 are included at "mfcap", and "tcap" attributes used by "lcfg:3" and "lcfg:4" are
the session level so they may be referenced by both latent included at the session level so they may be referenced by both
configurations. As per Section 3.3, the media attributes generated latent configurations. As per Section 3.3, the media attributes
from the rmcap, mfcap, and tcap attributes are always media-level generated from the "rmcap", "mfcap", and "tcap" attributes are always
attributes. If the answerer supports Media CapNeg, and supports the media-level attributes. If the answerer supports Media CapNeg, and
most desired configuration, it would return the following SDP: supports the most desired configuration, it would return the
following SDP:
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.22 c=IN IP4 192.0.2.22
t=0 0 t=0 0
a=csup:med-v0 a=csup:med-v0
a=sescap:1 1,3,4,5 a=sescap:1 1,3,4,5
a=sescap:2 1,2 a=sescap:2 1,2
a=sescap:3 1 a=sescap:3 1
m=audio 23456 RTP/AVP 0 m=audio 23456 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
a=acfg:1 a=acfg:1
m=video 0 RTP/AVP 102 m=video 0 RTP/AVP 102
a=pcfg:2 a=pcfg:2
a=lcfg:3 mt=video t=1 m=1 a=31,32 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:4 mt=video t=1 m=1 a=41,42
a=lcfg:5 mt=application t=2 a=lcfg:5 mt=application t=2
This exchange supports immediate establishment of an audio stream for This exchange supports immediate establishment of an audio stream for
preliminary conversation. This exchange would presumably be followed preliminary conversation. This exchange would presumably be followed
at the appropriate time with a "reconfiguration" offer/answer at the appropriate time with a "reconfiguration" offer/answer
exchange to add the video and chair control streams. exchange to add the video and chair control streams.
3.4. Offer/Answer Model Extensions 3.4. Offer/Answer Model Extensions
In this section, we define extensions to the offer/answer model In this section, we define extensions to the offer/answer model
defined in RFC 3264 [RFC3264] and RFC 5939 [RFC5939] to allow for defined in RFC 3264 [RFC3264] and RFC 5939 [RFC5939] to allow for
media format and associated parameter capabilities, latent media format and associated parameter capabilities, latent
configurations and acceptable combinations of media stream configurations, and acceptable combinations of media stream
configurations to be used with the SDP Capability Negotiation configurations to be used with the SDP capability negotiation
framework. Note that the procedures defined in this section extend framework. Note that the procedures defined in this section extend
the offer/answer procedures defined in RFC 5939 [RFC5939] Section 6; the offer/answer procedures defined in RFC 5939 [RFC5939] Section 6;
those procedures form a baseline set of capability negotiation offer/ those procedures form a baseline set of capability negotiation
answer procedures that MUST be followed, subject to the extensions offer/answer procedures that MUST be followed, subject to the
defined here. extensions defined here.
SDP Capability Negotiation [RFC5939] provides a relatively compact SDP capability negotiation [RFC5939] provides a relatively compact
means to offer the equivalent of an ordered list of alternative means to offer the equivalent of an ordered list of alternative
configurations for offered media streams (as would be described by configurations for offered media streams (as would be described by
separate m= lines and associated attributes). The attributes acap, separate "m=" lines and associated attributes). The attributes
mscap, mfcap, omcap and rmcap are designed to map somewhat "acap", "mscap", "mfcap", "omcap", and "rmcap" are designed to map
straightforwardly into equivalent m= lines and conventional somewhat straightforwardly into equivalent "m=" lines and
attributes when invoked by a pcfg, lcfg, or acfg attribute with conventional attributes when invoked by a "pcfg", "lcfg", or "acfg"
appropriate parameters. The a=pcfg: lines, along with the m= line attribute with appropriate parameters. The "a=pcfg:" lines, along
itself, represent offered media configurations. The a=lcfg: lines with the "m=" line itself, represent offered media configurations.
represent alternative capabilities for future use. The "a=lcfg:" lines represent alternative capabilities for future
use.
3.4.1. Generating the Initial Offer 3.4.1. Generating the Initial Offer
The Media Capabilities negotiation extensions defined in this The media capabilities negotiation extensions defined in this
document cover the following categories of features: document cover the following categories of features:
o Media Format Capabilities and associated parameters (rmcap, omcap, o Media format capabilities and associated parameters ("rmcap",
mfcap, and mscap attributes) "omcap", "mfcap", and "mscap" attributes)
o Potential configurations using those media format capabilities and o Potential configurations using those media format capabilities and
associated parameters associated parameters
o Latent media streams (lcfg attribute) o Latent media streams ("lcfg" attribute)
o Acceptable combinations of media stream configurations (sescap o Acceptable combinations of media stream configurations ("sescap"
attribute). attribute).
The high-level description of the operation is as follows: The high-level description of the operation is as follows:
When an endpoint generates an initial offer and wants to use the When an endpoint generates an initial offer and wants to use the
functionality described in the current document, it SHOULD identify functionality described in the current document, it SHOULD identify
and define the media formats and associated parameters it can support and define the media formats and associated parameters it can support
via the rmcap, omcap, mfcap and mscap attributes. The SDP media via the "rmcap", "omcap", "mfcap", and "mscap" attributes. The SDP
line(s) ("m=") should be made up with the actual configuration to be media line(s) ("m=") should be made up with the actual configuration
used if the other party does not understand capability negotiations to be used if the other party does not understand capability
(by default, this is the least preferred configuration). Typically, negotiations (by default, this is the least preferred configuration).
the media line configuration will contain the minimum acceptable Typically, the media line configuration will contain the minimum
configuration from the offerer's point of view. acceptable configuration from the offerer's point of view.
Preferred configurations for each media stream are identified Preferred configurations for each media stream are identified
following the media line. The present offer may also include latent following the media line. The present offer may also include latent
configuration (lcfg) attributes, at the media level, describing media configuration ("lcfg") attributes, at the media level, describing
streams and/or configurations the offerer is not now offering, but media streams and/or configurations the offerer is not now offering
which it is willing to support in a future offer/answer exchange. A but that it is willing to support in a future offer/answer exchange.
simple example might be the inclusion of a latent video configuration A simple example might be the inclusion of a latent video
in an offer for an audio stream. configuration in an offer for an audio stream.
Lastly, if the offerer wishes to impose restrictions on the Lastly, if the offerer wishes to impose restrictions on the
combinations of potential configurations to be used, it will include combinations of potential configurations to be used, it will include
session capability (sescap) attributes indicating those. session capability ("sescap") attributes indicating those.
If the offerer requires the answerer to understand the media If the offerer requires the answerer to understand the media
capability extensions, the offerer MUST include a creq attribute capability extensions, the offerer MUST include a "creq" attribute
containing the value "med-v0". If media capability negotiation is containing the value "med-v0". If media capability negotiation is
required only for specific media descriptions, the "med-v0" value required only for specific media descriptions, the "med-v0" value
MUST be provided only in creq attributes within those media MUST be provided only in "creq" attributes within those media
descriptions, as described in RFC 5939 [RFC5939]. descriptions, as described in RFC 5939 [RFC5939].
Below, we provide a more detailed description of how to construct the Below, we provide a more detailed description of how to construct the
offer SDP. offer SDP.
3.4.1.1. Offer with Media Capabilities 3.4.1.1. Offer with Media Capabilities
For each RTP-based media format the offerer wants to include as a For each RTP-based media format the offerer wants to include as a
media format capability, the offer MUST include an "rmcap" attribute media format capability, the offer MUST include an "rmcap" attribute
for the media format as defined in Section 3.3.1. 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 For each non-RTP-based media format the offer wants to include as a
media format capability, the offer MUST include an "omcap" attribute media format capability, the offer MUST include an "omcap" attribute
for the media format as defined in Section 3.3.1. for the media format as defined in Section 3.3.1.
Since the media capability number space is shared between the rmcap Since the media capability number space is shared between the "rmcap"
and omcap attributes, each media capability number provided and "omcap" attributes, each media capability number provided
(including ranges) MUST be unique in the entire SDP. (including ranges) MUST be unique in the entire SDP.
If an "fmtp" parameter value is needed for a media format (whether If an "fmtp" parameter value is needed for a media format (whether or
RTP-based or not) in a media capability, then the offer MUST include not it is RTP based) in a media capability, then the offer MUST
one or more "mfcap" parameters with the relevant fmtp parameter include one or more "mfcap" parameters with the relevant "fmtp"
values for that media format as defined in Section 3.3.2. When parameter values for that media format as defined in Section 3.3.2.
multiple "mfcap" parameters are provided for a given media When multiple "mfcap" parameters are provided for a given media
capability, they MUST be provided in accordance with the capability, they MUST be provided in accordance with the
concatenation rules in Section 3.3.2.1. concatenation rules in Section 3.3.2.1.
For each of the media format capabilities above, the offer MAY For each of the media format capabilities above, the offer MAY
include one or more "mscap" parameters with attributes needed for include one or more "mscap" parameters with attributes needed for
those specific media formats as defined in Section 3.3.3. Such those specific media formats as defined in Section 3.3.3. Such
attributes will be instantiated at the media-level, and hence attributes will be instantiated at the media level; hence, session-
session-level only attributes MUST NOT be used in the "mscap" level-only attributes MUST NOT be used in the "mscap" parameter. The
parameter. The "mscap" parameter MUST NOT include an "rtpmap" or "mscap" parameter MUST NOT include an "rtpmap" or "fmtp" attribute
"fmtp" attribute (rmcap and mfcap are used instead). ("rmcap" and "mfcap" are used instead).
If the offerer wants to limit the relevance (and use) of a media If the offerer wants to limit the relevance (and use) of a media
format capability or parameter to a particular media stream, the format capability or parameter to a particular media stream, the
media format capability or parameter MUST be provided within the media format capability or parameter MUST be provided within the
corresponding media description. Otherwise, the media format corresponding media description. Otherwise, the media format
capabilities and parameters MUST be provided at the session level. capabilities and parameters MUST be provided at the session level.
Note however, that the attribute or parameter embedded in these will Note, however, that the attribute or parameter embedded in these will
always be instantiated at the media-level. always be instantiated at the media level.
This is due to those parameters being effectively media-level This is due to those parameters being effectively media-level
parameters. If session-level attributes are needed, the "acap" parameters. If session-level attributes are needed, the "acap"
attribute defined in RFC 5939 [RFC5939] can be used, however it attribute defined in RFC 5939 [RFC5939] can be used; however, it
does not provide for media format-specific instantiation. does not provide for media-format-specific instantiation.
Inclusion of the above does not constitute an offer to use the Inclusion of the above does not constitute an offer to use the
capabilities; a potential configuration is needed for that. If the capabilities; a potential configuration is needed for that. If the
offerer wants to offer one or more of the media capabilities above, offerer wants to offer one or more of the media capabilities above,
they MUST be included as part of a potential configuration (pcfg) they MUST be included as part of a potential configuration ("pcfg")
attribute as defined in Section 3.3.4. Each potential configuration attribute as defined in Section 3.3.4. Each potential configuration
MUST include a config-number, and each config-number MUST be unique MUST include a config-number, and each config-number MUST be unique
in the entire SDP (note that this differs from RFC 5939 [RFC5939], in the entire SDP (note that this differs from RFC 5939 [RFC5939],
which only requires uniqueness within a media description). Also, which only requires uniqueness within a media description). Also,
the config-number MUST NOT overlap with any config-number used by a the config-number MUST NOT overlap with any config-number used by a
latent configuration in the SDP. As described in RFC 5939 [RFC5939], latent configuration in the SDP. As described in RFC 5939 [RFC5939],
lower config-numbers indicate a higher preference; the ordering still lower config-numbers indicate a higher preference; the ordering still
applies within a given media description only though. applies within a given media description only though.
For a media capability to be included in a potential configuration, For a media capability to be included in a potential configuration,
there MUST be an "m=" parameter in the pcfg attribute referencing the there MUST be an "m=" parameter in the "pcfg" attribute referencing
media capability number in question. When one or more media the media capability number in question. When one or more media
capabilities are included in an offered potential configuration capabilities are included in an offered potential configuration
(pcfg), they completely replace the list of media formats offered in ("pcfg"), they completely replace the list of media formats offered
the actual configuration (m= line). Any attributes included for in the actual configuration ("m=" line). Any attributes included for
those formats remain in the SDP though (e.g., rtpmap, fmtp, etc.). those formats remain in the SDP though (e.g., "rtpmap", "fmtp",
For non-RTP based media formats, the format-name (from the "omcap" etc.). For non-RTP-based media formats, the format-name (from the
media capability) is simply added to the "m=" line as a media format "omcap" media capability) is simply added to the "m=" line as a media
(e.g. t38). For RTP-based media, payload type mappings MUST be format (e.g., t38). For RTP-based media, payload type mappings MUST
provided by use of the "pt" parameter in the potential configuration be provided by use of the "pt" parameter in the potential
(see Section 3.3.4.2); payload type escaping may be used in mfcap, configuration (see Section 3.3.4.2); payload type escaping may be
mscap, and acap attributes as defined in Section 3.3.7. 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 Note that the "mt" parameter MUST NOT be used with the "pcfg"
(since it is defined for the lcfg attribute only); the media type in attribute (since it is defined for the "lcfg" attribute only); the
a potential configuration cannot be changed from that of the media type in a potential configuration cannot be changed from that
encompassing media description. of the encompassing media description.
3.4.1.2. Offer with Latent Configuration 3.4.1.2. Offer with Latent Configuration
If the offerer wishes to offer one or more latent configurations for If the offerer wishes to offer one or more latent configurations for
future use, the offer MUST include a latent configuration attribute future use, the offer MUST include a latent configuration attribute
(lcfg) for each as defined in Section 3.3.5. ("lcfg") for each as defined in Section 3.3.6.
Each lcfg attribute Each "lcfg" attribute
o MUST be specified at the media level o MUST be specified at the media level
o MUST include a config-number that is unique in the entire SDP o MUST include a config-number that is unique in the entire SDP
(incl. for any potential configuration attributes). Note that (including for any potential configuration attributes). Note that
config-numbers in latent configurations do not indicate any config-numbers in latent configurations do not indicate any
preference order preference order
o MUST include a media type ("mt") o MUST include a media type ("mt")
o MUST reference a valid transport capability ("t") o MUST reference a valid transport capability ("t")
Each lcfg attribute MAY include additional capability references, Each "lcfg" attribute MAY include additional capability references,
which may refer to capabilities anywhere in the session description, which may refer to capabilities anywhere in the session description,
subject to any restrictions normally associated with such subject to any restrictions normally associated with such
capabilities. For example, a media-level attribute capability must capabilities. For example, a media-level attribute capability must
be present at the media-level in some media description in the SDP. be present at the media level in some media description in the SDP.
Note that this differs from the potential configuration attribute, Note that this differs from the potential configuration attribute,
which cannot validly refer to media-level capabilities in another which cannot validly refer to media-level capabilities in another
media description (per RFC 5939 [RFC5939], Section 3.5.1). media description (per RFC 5939 [RFC5939], Section 3.5.1).
Potential configurations constitute an actual offer and hence may Potential configurations constitute an actual offer and may
instantiate a referenced capability. Latent configurations are instantiate a referenced capability. Latent configurations are
not actual offers and hence cannot instantiate a referenced not actual offers; hence, they cannot instantiate a referenced
capability; it is therefore safe for those to refer to capability. Therefore, it is safe for those to refer to
capabilities in another media description. capabilities in another media description.
3.4.1.3. Offer with Configuration Combination Restrictions 3.4.1.3. Offer with Configuration Combination Restrictions
If the offerer wants to indicate restrictions or preferences among If the offerer wants to indicate restrictions or preferences among
combinations of potential and/or latent configuration, a session combinations of potential and/or latent configurations, a session
capability (sescap) attribute MUST be provided at the session-level capability ("sescap") attribute MUST be provided at the session level
for each such combination as described in Section 3.3.8. Each sescap for each such combination as described in Section 3.3.8. Each
attribute MUST include a session-num that is unique in the entire "sescap" attribute MUST include a session-num that is unique in the
SDP; the lower the session-num the more preferred that combination entire SDP; the lower the session-num the more preferred that
is. Furthermore, sescap preference order takes precedence over any combination is. Furthermore, "sescap" preference order takes
order specified in individual pcfg attributes. precedence over any order specified in individual "pcfg" attributes.
For example, if we have pcfg-1 and pcfg-2, and sescap-1 references 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 pcfg-2, whereas sescap-2 references pcfg-1, then pcfg-2 will be
the most preferred potential configuration. Without the sescap, the most preferred potential configuration. Without the sescap,
pcfg-1 would be the most preferred. pcfg-1 would be the most preferred.
3.4.2. Generating the Answer 3.4.2. Generating the Answer
When receiving an offer, the answerer MUST check the offer for creq When receiving an offer, the answerer MUST check the offer for "creq"
attributes containing the value "med-v0"; answerers compliant with attributes containing the value "med-v0"; answerers compliant with
this specification will support this value in accordance with the this specification will support this value in accordance with the
procedures specified in RFC 5939 [RFC5939]. procedures specified in RFC 5939 [RFC5939].
The SDP MAY contain The SDP MAY contain
o Media format capabilities and associated parameters (rmcap, omcap, o Media format capabilities and associated parameters ("rmcap",
mfcap, and mscap attributes) "omcap", "mfcap", and "mscap" attributes)
o Potential configurations using those media format capabilities and o Potential configurations using those media format capabilities and
associated parameters associated parameters
o Latent media streams (lcfg attribute) o Latent media streams ("lcfg" attribute)
o Acceptable combinations of media stream configurations (sescap o Acceptable combinations of media stream configurations ("sescap"
attribute) attribute)
The high-level informative description of the operation is as The high-level informative description of the operation is as
follows: follows:
When the answering party receives the offer and if it supports the When the answering party receives the offer, if it supports the
required capability negotiation extensions, it should select the required capability negotiation extensions, it should select the
most-preferred configuration it can support for each media stream, most-preferred configuration it can support for each media stream,
and build its answer accordingly. The configuration selected for and build its answer accordingly. The configuration selected for
each accepted media stream is placed into the answer as a media line each accepted media stream is placed into the answer as a media line
with associated parameters and attributes. If a proposed with associated parameters and attributes. If a proposed
configuration is chosen for a given media stream, the answer must configuration is chosen for a given media stream, the answer must
contain an actual configuration (acfg) attribute for that media contain an actual configuration ("acfg") attribute for that media
stream to indicate which offered pcfg attribute was used to build the stream to indicate which offered "pcfg" attribute was used to build
answer. The answer should also include any potential or latent the answer. The answer should also include any potential or latent
configurations the answerer can support, especially any configurations the answerer can support, especially any
configurations compatible with other potential or latent configurations compatible with other potential or latent
configurations received in the offer. The answerer should make note configurations received in the offer. The answerer should make note
of those configurations it might wish to offer in the future. of those configurations it might wish to offer in the future.
Below we provide a more detailed normative description of how the Below we provide a more detailed normative description of how the
answerer processes the offer SDP and generates an answer SDP. answerer processes the offer SDP and generates an answer SDP.
3.4.2.1. Processing Media Capabilities and Potential Configurations 3.4.2.1. Processing Media Capabilities and Potential Configurations
The answerer MUST first determine if it needs to perform media The answerer MUST first determine if it needs to perform media
capability negotiation by examining the SDP for valid and preferred capability negotiation by examining the SDP for valid and preferred
potential configuration attributes that include media configuration potential configuration attributes that include media configuration
parameters (i.e., an "m" parameter in the pcfg attribute). parameters (i.e., an "m" parameter in the "pcfg" attribute).
Such a potential configuration is valid if: Such a potential configuration is valid if
1. It is valid according to the rules defined in RFC 5939 [RFC5939] 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 2. It contains a config-number that is unique in the entire SDP and
does not overlap with any latent configuration config-numbers does not overlap with any latent configuration config-numbers.
3. All media format capabilities (rmcap or omcap), media format 3. All media format capabilities ("rmcap" or "omcap"), media format
parameter capabilities (mfcap), and media-specific capabilities parameter capabilities ("mfcap"), and media-specific capabilities
(mscap) referenced by the potential configuration ("m" parameter) ("mscap") referenced by the potential configuration ("m"
are valid themselves (as defined in Section 3.3.1, 3.3.2, and parameter) are valid themselves (as defined in Sections 3.3.1,
3.3.3) and each of them is provided either at the session level 3.3.2, and 3.3.3) and each of them is provided either at the
or within this particular media description. session level or within this particular media description.
4. All RTP-based media format capabilities (rmcap) have a 4. All RTP-based media format capabilities ("rmcap") have a
corresponding payload type ("pt") parameter in the potential corresponding payload type ("pt") parameter in the potential
configuration that result in mapping to a valid payload type that configuration that results in mapping to a valid payload type
is unique within the resulting SDP. that is unique within the resulting SDP.
5. Any concatenation (see Section 3.3.2.1) and substitution (see 5. Any concatenation (see Section 3.3.2.1) and substitution (see
Section 3.3.7) applied to any capability (mfcap, mscap, or acap) Section 3.3.7) applied to any capability ("mfcap", "mscap", or
referenced by this potential configuration results in a valid "acap") referenced by this potential configuration results in a
SDP. valid SDP.
Note that, since SDP does not interpret the value of fmtp parameters, Note that since SDP does not interpret the value of "fmtp"
any resulting fmtp parameter value will be considered valid. parameters, any resulting "fmtp" parameter value will be considered
valid.
Secondly, the answerer MUST determine the order in which potential Secondly, the answerer MUST determine the order in which potential
configurations are to be negotiated. In the absence of any Session configurations are to be negotiated. In the absence of any session
Capability ("sescap") attributes, this simply follows the rules of capability ("sescap") attributes, this simply follows the rules of
RFC 5939 [RFC5939], with a lower config-number within a media RFC 5939 [RFC5939], with a lower config-number within a media
description being preferred over a higher one. If a valid "sescap" description being preferred over a higher one. If a valid "sescap"
attribute is present, the preference order provided in the "sescap" attribute is present, the preference order provided in the "sescap"
attribute MUST take precedence. A "sescap" attribute is considered attribute MUST take precedence. A "sescap" attribute is considered
valid if: valid if
1. It adheres to the rules provided in Section 3.3.8. 1. It adheres to the rules provided in Section 3.3.8.
2. All the configurations referenced by the "sescap" attribute are 2. All the configurations referenced by the "sescap" attribute are
valid themselves (note that this can include the actual, valid themselves (note that this can include the actual,
potential and latent configurations). potential, and latent configurations).
The answerer MUST now process the offer for each media stream based The answerer MUST now process the offer for each media stream based
on the most preferred valid potential configuration in accordance on the most preferred valid potential configuration in accordance
with the procedures specified in RFC 5939 [RFC5939], Section 3.6.2, with the procedures specified in RFC 5939 [RFC5939], Section 3.6.2,
and further extended below: and further extended below:
o If one or more media format capabilities are included in the o If one or more media format capabilities are included in the
potential configuration, then they replace all media formats potential configuration, then they replace all media formats
provided in the "m=" line for that media description. For non-RTP provided in the "m=" line for that media description. For non-
based media formats (omcap), the format-name is added. For RTP- RTP-based media formats ("omcap"), the format-name is added. For
based media formats (rmcap), the payload-type specified in the RTP-based media formats ("rmcap"), the payload-type specified in
payload-type mapping ("pt") is added and a corresponding "rtpmap" the payload-type mapping ("pt") is added and a corresponding
attribute is added to the media description. "rtpmap" attribute is added to the media description.
o If one or more media format parameter capabilities are included in o If one or more media format parameter capabilities are included in
the potential configuration, then the corresponding "fmtp" the potential configuration, then the corresponding "fmtp"
attributes are added to the media description. Note that this attributes are added to the media description. Note that this
inclusion is done indirectly via the media format capability. inclusion is done indirectly via the media format capability.
o If one or more media-specific capabilities are included in the o If one or more media-specific capabilities are included in the
potential configuration, then the corresponding attributes are potential configuration, then the corresponding attributes are
added to the media description. Note that this inclusion is done added to the media description. Note that this inclusion is done
indirectly via the media format capability. indirectly via the media format capability.
o When checking to see if the answerer supports a given potential o When checking to see if the answerer supports a given potential
configuration that includes one or more media format capabilities, configuration that includes one or more media format capabilities,
the answerer MUST support at least one of the media formats the answerer MUST support at least one of the media formats
offered. If he does not, the answerer MUST proceed to the next offered. If he does not, the answerer MUST proceed to the next
potential configuration based on the preference order that potential configuration based on the preference order that
applies. applies.
o If Session Capability ("sescap") preference ordering is included, o If session capability ("sescap") preference ordering is included,
then the potential configuration selection process MUST adhere to then the potential configuration selection process MUST adhere to
the ordering provided. Note that this may involve coordinated the ordering provided. Note that this may involve coordinated
selection of potential configurations between media descriptions. selection of potential configurations between media descriptions.
The answerer MUST accept one of the offered "sescap" combinations The answerer MUST accept one of the offered sescap combinations
(i.e. all the required potential configurations specified) or it (i.e., all the required potential configurations specified) or it
MUST reject the entire session. MUST reject the entire session.
Once the answerer has selected a valid and supported offered Once the answerer has selected a valid and supported offered
potential configuration for all of the media streams (or has fallen potential configuration for all of the media streams (or has fallen
back to the actual configuration plus any added session attributes), back to the actual configuration plus any added session attributes),
the answerer MUST generate a valid answer SDP as described in RFC the answerer MUST generate a valid answer SDP as described in RFC
5939 [RFC5939], Section 3.6.2, and further extended below: 5939 [RFC5939], Section 3.6.2, and further extended below:
o Additional answer capabilities and potential configurations MAY be o Additional answer capabilities and potential configurations MAY be
returned in accordance with Section 3.3.6.1. Capability numbers returned in accordance with Section 3.3.6.1. Capability numbers
skipping to change at page 42, line 46 skipping to change at page 42, line 26
o Latent configuration processing and answer generation MUST be o Latent configuration processing and answer generation MUST be
performed, as specified below. performed, as specified below.
o Session capability specification for the potential and latent o Session capability specification for the potential and latent
configurations in the answer MAY be included (see Section 3.3.8). configurations in the answer MAY be included (see Section 3.3.8).
3.4.2.2. Latent Configuration Processing 3.4.2.2. Latent Configuration Processing
The answerer MUST determine if it needs to perform any latent The answerer MUST determine if it needs to perform any latent
configuration processing by examining the SDP for valid latent configuration processing by examining the SDP for valid latent
configuration attributes (lcfg). An lcfg attribute is considered configuration attributes ("lcfg"). An "lcfg" attribute is considered
valid if: valid if:
o It adheres to the description in Section 3.3.5. 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 o It includes a config-number that is unique in the entire SDP and
does not overlap with any potential configuration config-number does not overlap with any potential configuration config-number.
o It includes a valid media type ("mt=") o It includes a valid media type ("mt=").
o It references a valid transport capability ("t=") o It references a valid transport capability ("t=").
o All other capabilities referenced by it are valid. o All other capabilities referenced by it are valid.
For each such valid latent configuration in the offer, the answerer For each such valid latent configuration in the offer, the answerer
checks to see if it could support the latent configuration in a checks to see if it could support the latent configuration in a
subsequent offer/answer exchange. If so, it includes the latent subsequent offer/answer exchange. If so, it includes the latent
configuration with the same configuration number in the answer, configuration with the same configuration number in the answer,
similar to the way potential configurations are processed and the similar to the way potential configurations are processed and the
selected one returned in an actual configuration attribute (see RFC selected one returned in an actual configuration attribute (see RFC
5939 [RFC5939]). If the answerer supports only a (non-mandatory) 5939 [RFC5939]). If the answerer supports only a (non-mandatory)
subset of the parameters offered in a latent configuration, the subset of the parameters offered in a latent configuration, the
answer latent configuration will include only those parameters answer latent configuration will include only those parameters
supported (similar to "acfg" processing). Note that latent supported (similar to "acfg" processing). Note that latent
configurations do not constitute an actual offer at this point in configurations do not constitute an actual offer at this point in
time; they merely indicate additional configurations that could be time; they merely indicate additional configurations that could be
supported. supported.
If a Session Capability ("sescap") attribute is included and it If a session capability ("sescap") attribute is included and it
references a latent configuration, then the answerer processing of references a latent configuration, then the answerer processing of
that latent configuration must be done within the constraints that latent configuration must be done within the constraints
specified by that Session Capability, i.e. it must be possible to specified by that session capability. That is, it must be possible
support it at the same time as any required (i.e. non-optional) to support it at the same time as any required (i.e., non-optional)
potential configurations in the session capability. The answerer may potential configurations in the session capability. The answerer may
in turn add his own "sescap" indications in the answer as well. in turn add his own sescap indications in the answer as well.
3.4.3. Offerer Processing of the Answer 3.4.3. Offerer Processing of the Answer
The offerer MUST process the answer in accordance with RFC 5939 The offerer MUST process the answer in accordance with Section 3.6.3
[RFC5939] Section 3.6.3, and further explained below. of RFC 5939 [RFC5939] and the further explanation below.
When the offerer processes the answer SDP based on a valid actual When the offerer processes the answer SDP based on a valid actual
configuration attribute in the answer, and that valid configuration configuration attribute in the answer, and that valid configuration
includes one or more media capabilities, the processing MUST includes one or more media capabilities, the processing MUST
furthermore be done as if the offer was sent using those media furthermore be done as if the offer was sent using those media
capabilities instead of the actual configuration. In particular, the capabilities instead of the actual configuration. In particular, the
media formats in the "m=" line, and any associated payload type media formats in the "m=" line, and any associated payload type
mappings (rtpmap), fmtp parameters (mfcap) and media-specific mappings ("rtpmap"), "fmtp" parameters ("mfcap"), and media-specific
attributes (mscap) MUST be used. Note that this may involve use of 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). concatenation and substitution rules (see Sections 3.3.2.1 and
The actual configuration attribute may also be used to infer the lack 3.3.7). The actual configuration attribute may also be used to infer
of acceptability of higher-preference configurations that were not the lack of acceptability of higher-preference configurations that
chosen, subject to any constraints provided by a Session Capability were not chosen, subject to any constraints provided by a session
attribute ("sescap") in the offer. Note that the SDP Capability capability ("sescap") attribute in the offer. Note that the SDP
Negotiation base specification [RFC5939] requires the answerer to capability negotiation base specification [RFC5939] requires the
choose the highest preference configuration it can support, subject answerer to choose the highest-preference configuration it can
to local policies. support, subject to local policies.
When the offerer receives the answer, it SHOULD furthermore make note When the offerer receives the answer, it SHOULD furthermore make note
of any capabilities and/or latent configurations included for future of any capabilities and/or latent configurations included for future
use, and any constraints on how those may be combined. use, and any constraints on how those may be combined.
3.4.4. Modifying the Session 3.4.4. Modifying the Session
If, at a later time, one of the parties wishes to modify the 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 operating parameters of a session, e.g., by adding a new media
stream, or by changing the properties used on an existing stream, it stream, or by changing the properties used on an existing stream, it
can do so via the mechanisms defined for offer/answer [RFC3264]. If can do so via the mechanisms defined for offer/answer [RFC3264]. If
the initiating party has remembered the codecs, potential the initiating party has remembered the codecs, potential
configurations, latent configurations and session capabilities configurations, latent configurations, and session capabilities
provided by the other party in the earlier negotiation, it MAY use provided by the other party in the earlier negotiation, it MAY use
this knowledge to maximize the likelihood of a successful this knowledge to maximize the likelihood of a successful
modification of the session. Alternatively, the initiator MAY modification of the session. Alternatively, the initiator MAY
perform a new capabilities exchange as part of the reconfiguration. perform a new capabilities exchange as part of the reconfiguration.
In such a case, the new capabilities will replace the previously-
In such a case, the new capabilities will replace the previously
negotiated capabilities. This may be useful if conditions change on negotiated capabilities. This may be useful if conditions change on
the endpoint. the endpoint.
4. Examples 4. Examples
In this section, we provide examples showing how to use the Media In this section, we provide examples showing how to use the media
Capabilities with the SDP Capability Negotiation. capabilities with the SDP capability negotiation.
4.1. Alternative Codecs 4.1. Alternative Codecs
This example provides a choice of one of six variations of the This example provides a choice of one of six variations of the
adaptive multirate codec. In this example, the default configuration Adaptive Multi-Rate codec. In this example, the default
as specified by the media line is the same as the most preferred configuration as specified by the media line is the same as the most
configuration. Each configuration uses a different payload type preferred configuration. Each configuration uses a different payload
number so the offerer can interpret early media. type number so the offerer can interpret early media.
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
a=creq:med-v0 a=creq:med-v0
m=audio 54322 RTP/AVP 96 m=audio 54322 RTP/AVP 96
a=rtpmap:96 AMR-WB/16000/1 a=rtpmap:96 AMR-WB/16000/1
a=fmtp:96 mode-change-capability=1; max-red=220; \ a=fmtp:96 mode-change-capability=1; max-red=220; \
mode-set=0,2,4,7 mode-set=0,2,4,7
a=rmcap:1,3,5 audio AMR-WB/16000/1 a=rmcap:1,3,5 audio AMR-WB/16000/1
a=rmcap:2,4,6 audio AMR/8000/1 a=rmcap:2,4,6 audio AMR/8000/1
a=mfcap:1,2,3,4 mode-change-capability=1 a=mfcap:1,2,3,4 mode-change-capability=1
a=mfcap:5,6 mode-change-capability=2 a=mfcap:5,6 mode-change-capability=2
a=mfcap:1,2,3,5 max-red=220 a=mfcap:1,2,3,5 max-red=220
a=mfcap:3,4,5,6 octet-align=1 a=mfcap:3,4,5,6 octet-align=1
a=mfcap:1,3,5 mode-set=0,2,4,7 a=mfcap:1,3,5 mode-set=0,2,4,7
a=mfcap:2,4,6 mode-set=0,3,5,6 a=mfcap:2,4,6 mode-set=0,3,5,6
a=pcfg:1 m=1 pt=1:96 a=pcfg:1 m=1 pt=1:96
a=pcfg:2 m=2 pt=2:97 a=pcfg:2 m=2 pt=2:97
a=pcfg:3 m=3 pt=3:98 a=pcfg:3 m=3 pt=3:98
a=pcfg:4 m=4 pt=4:99 a=pcfg:4 m=4 pt=4:99
a=pcfg:5 m=5 pt=5:100 a=pcfg:5 m=5 pt=5:100
a=pcfg:6 m=6 pt=6:101 a=pcfg:6 m=6 pt=6:101
In the above example, media capability 1 could have been excluded In the above example, media capability 1 could have been excluded
from the first rmcap declaration and from the corresponding mfcap from the first "rmcap" declaration and from the corresponding "mfcap"
attributes, and the pcfg:1 attribute line could have been simply attributes, and the "pcfg:1" attribute line could have been simply
"pcfg:1". "pcfg:1".
The next example offers a video stream with three options of H.264 The next example offers a video stream with three options of H.264
and 4 transports. It also includes an audio stream with different and four transports. It also includes an audio stream with different
audio qualities: four variations of AMR, or AC3. The offer looks audio qualities: four variations of AMR, or AC3. The offer looks
something like: something like the following:
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s=An SDP Media NEG example s=An SDP Media NEG example
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
a=creq:med-v0 a=creq:med-v0
a=ice-pwd:speEc3QGZiNWpVLFJhQX a=ice-pwd:speEc3QGZiNWpVLFJhQX
m=video 49170 RTP/AVP 100 m=video 49170 RTP/AVP 100
c=IN IP4 192.0.2.56 c=IN IP4 192.0.2.56
a=maxprate:1000 a=maxprate:1000
a=rtcp:51540 a=rtcp:51540
a=sendonly a=sendonly
a=candidate 12345 1 UDP 9 192.0.2.56 49170 host 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 23456 2 UDP 9 192.0.2.56 51540 host
a=candidate 34567 1 UDP 7 198.51.100.1 41345 srflx raddr \ a=candidate 34567 1 UDP 7 198.51.100.1 41345 srflx raddr \
192.0.2.56 rport 49170 192.0.2.56 rport 49170
a=candidate 45678 2 UDP 7 198.51.100.1 52567 srflx raddr \ a=candidate 45678 2 UDP 7 198.51.100.1 52567 srflx raddr \
192.0.2.56 rport 51540 192.0.2.56 rport 51540
a=candidate 56789 1 UDP 3 192.0.2.100 49000 relay raddr \ a=candidate 56789 1 UDP 3 192.0.2.100 49000 relay raddr \
192.0.2.56 rport 49170 192.0.2.56 rport 49170
a=candidate 67890 2 UDP 3 192.0.2.100 49001 relay raddr \ a=candidate 67890 2 UDP 3 192.0.2.100 49001 relay raddr \
192.0.2.56 rport 51540 192.0.2.56 rport 51540
b=AS:10000 b=AS:10000
b=TIAS:10000000 b=TIAS:10000000
b=RR:4000 b=RR:4000
b=RS:3000 b=RS:3000
a=rtpmap:100 H264/90000 a=rtpmap:100 H264/90000
a=fmtp:100 profile-level-id=42A01E; packetization-mode=2; \ a=fmtp:100 profile-level-id=42A01E; packetization-mode=2; \
sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==; \ sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==; \
sprop-interleaving-depth=45; sprop-deint-buf-req=64000; \ sprop-interleaving-depth=45; sprop-deint-buf-req=64000; \
sprop-init-buf-time=102478; deint-buf-cap=128000 sprop-init-buf-time=102478; deint-buf-cap=128000
a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF
a=rmcap:1-3,7-9 H264/90000 a=rmcap:1-3,7-9 H264/90000
a=rmcap:4-6 rtx/90000 a=rmcap:4-6 rtx/90000
a=mfcap:1-9 profile-level-id=42A01E a=mfcap:1-9 profile-level-id=42A01E
a=mfcap:1-9 aMljiA== a=mfcap:1-9 aMljiA==
a=mfcap:1,4,7 packetization-mode=0 a=mfcap:1,4,7 packetization-mode=0
a=mfcap:2,5,8 packetization-mode=1 a=mfcap:2,5,8 packetization-mode=1
a=mfcap:3,6,9 packetization-mode=2 a=mfcap:3,6,9 packetization-mode=2
a=mfcap:1-9 sprop-parameter-sets=Z0IACpZTBYmI a=mfcap:1-9 sprop-parameter-sets=Z0IACpZTBYmI
a=mfcap:1,7 sprop-interleaving-depth=45; \ a=mfcap:1,7 sprop-interleaving-depth=45; \
sprop-deint-buf-req=64000; sprop-init-buf-time=102478; \ sprop-deint-buf-req=64000; sprop-init-buf-time=102478; \
deint-buf-cap=128000 deint-buf-cap=128000
a=mfcap:4 apt=100 a=mfcap:4 apt=100
a=mfcap:5 apt=99 a=mfcap:5 apt=99
a=mfcap:6 apt=98 a=mfcap:6 apt=98
a=mfcap:4-6 rtx-time=3000 a=mfcap:4-6 rtx-time=3000
a=mscap:1-6 rtcp-fb nack a=mscap:1-6 rtcp-fb nack
a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_80 \ a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_80 \
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|220|1:32 inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|220|1:32
a=pcfg:1 t=1 m=1,4 a=1 pt=1:100,4:97 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: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: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:4 t=2 m=7 a=1 pt=7:100
a=pcfg:5 t=2 m=8 a=1 pt=8:99 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: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: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:8 t=3 m=2,4 pt=2:99,4:96
a=pcfg:9 t=3 m=3,6 pt=3:98,6:95 a=pcfg:9 t=3 m=3,6 pt=3:98,6:95
m=audio 49176 RTP/AVP 101 100 99 98 m=audio 49176 RTP/AVP 101 100 99 98
c=IN IP4 192.0.2.56 c=IN IP4 192.0.2.56
a=ptime:60 a=ptime:60
a=maxptime:200 a=maxptime:200
a=rtcp:51534 a=rtcp:51534
a=sendonly a=sendonly
a=candidate 12345 1 UDP 9 192.0.2.56 49176 host 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 23456 2 UDP 9 192.0.2.56 51534 host
a=candidate 34567 1 UDP 7 198.51.100.1 41348 srflx \ a=candidate 34567 1 UDP 7 198.51.100.1 41348 srflx \
raddr 192.0.2.56 rport 49176 raddr 192.0.2.56 rport 49176
a=candidate 45678 2 UDP 7 198.51.100.1 52569 srflx \ a=candidate 45678 2 UDP 7 198.51.100.1 52569 srflx \
raddr 192.0.2.56 rport 51534 raddr 192.0.2.56 rport 51534
a=candidate 56789 1 UDP 3 192.0.2.100 49002 relay \ a=candidate 56789 1 UDP 3 192.0.2.100 49002 relay \
raddr 192.0.2.56 rport 49176 raddr 192.0.2.56 rport 49176
a=candidate 67890 2 UDP 3 192.0.2.100 49003 relay \ a=candidate 67890 2 UDP 3 192.0.2.100 49003 relay \
raddr 192.0.2.56 rport 51534 raddr 192.0.2.56 rport 51534
b=AS:512 b=AS:512
b=TIAS:512000 b=TIAS:512000
b=RR:4000 b=RR:4000
b=RS:3000 b=RS:3000
a=maxprate:120 a=maxprate:120
a=rtpmap:98 AMR-WB/16000 a=rtpmap:98 AMR-WB/16000
a=fmtp:98 octet-align=1; mode-change-capability=2 a=fmtp:98 octet-align=1; mode-change-capability=2
a=rtpmap:99 AMR-WB/16000 a=rtpmap:99 AMR-WB/16000
a=fmtp:99 octet-align=1; crc=1; mode-change-capability=2 a=fmtp:99 octet-align=1; crc=1; mode-change-capability=2
a=rtpmap:100 AMR-WB/16000/2 a=rtpmap:100 AMR-WB/16000/2
a=fmtp:100 octet-align=1; interleaving=30 a=fmtp:100 octet-align=1; interleaving=30
a=rtpmap:101 AMR-WB+/72000/2 a=rtpmap:101 AMR-WB+/72000/2
a=fmtp:101 interleaving=50; int-delay=160000; a=fmtp:101 interleaving=50; int-delay=160000;
a=rmcap:14 ac3/48000/6 a=rmcap:14 ac3/48000/6
a=acap:23 crypto:1 AES_CM_128_HMAC_SHA1_80 \ a=acap:23 crypto:1 AES_CM_128_HMAC_SHA1_80 \
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|220|1:32 inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|220|1:32
a=tcap:4 RTP/SAVP a=tcap:4 RTP/SAVP
a=pcfg:10 t=4 a=23 a=pcfg:10 t=4 a=23
a=pcfg:11 t=4 m=14 a=23 pt=14:102 a=pcfg:11 t=4 m=14 a=23 pt=14:102
This offer illustrates the advantage in compactness that arises if This offer illustrates the advantage in compactness that arises if
one can avoid deleting the base configuration attributes and one can avoid deleting the base configuration attributes and
recreating them in acap attributes for the potential configurations. recreating them in "acap" attributes for the potential
configurations.
4.2. Alternative Combinations of Codecs (Session Configurations) 4.2. Alternative Combinations of Codecs (Session Configurations)
If an endpoint has limited signal processing capacity, it might be If an endpoint has limited signal processing capacity, it might be
capable of supporting, say, a G.711 mu-law audio stream in 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.264 video stream, or a G.729B audio stream in
combination with an H.263-1998 video stream. It might then issue an combination with an H.263-1998 video stream. It might then issue an
offer like the following: offer like the following:
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
a=creq:med-v0 a=creq:med-v0
a=sescap:1 2,4 a=sescap:1 2,4
a=sescap:2 1,3 a=sescap:2 1,3
m=audio 54322 RTP/AVP 18 m=audio 54322 RTP/AVP 18
a=rtpmap:18 G729/8000 a=rtpmap:18 G729/8000
a=fmtp:18 annexb=yes a=fmtp:18 annexb=yes
a=rmcap:1 PCMU/8000 a=rmcap:1 PCMU/8000
a=pcfg:1 m=1 pt=1:0 a=pcfg:1 m=1 pt=1:0
a=pcfg:2 a=pcfg:2
m=video 54344 RTP/AVP 100 m=video 54344 RTP/AVP 100
a=rtpmap:100 H263-1998/90000 a=rtpmap:100 H263-1998/90000
a=rmcap:2 H264/90000 a=rmcap:2 H264/90000
a=mfcap:2 profile-level-id=42A01E; packetization-mode=2 a=mfcap:2 profile-level-id=42A01E; packetization-mode=2
a=pcfg:3 m=2 pt=2:101 a=pcfg:3 m=2 pt=2:101
a=pcfg:4 a=pcfg:4
Note that the preferred session configuration (and the default as Note that the preferred session configuration (and the default as
well) is G.729B with H.263. This overrides the individual media well) is G.729B with H.263. This overrides the individual media
stream preferences which are PCMU and H.264 by the potential stream preferences that are PCMU and H.264 by the potential
configuration numbering rule. configuration numbering rule.
4.3. Latent Media Streams 4.3. Latent Media Streams
Consider a case in which the offerer can support either G.711 mu-law, 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 or G.729B, along with DTMF telephony events for the 12 common
touchtone signals, but is willing to support simple G.711 mu-law touchtone signals, but is willing to support simple G.711 mu-law
audio as a last resort. In addition, the offerer wishes to announce 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 its ability to support video and Message Session Relay Protocol
wish to offer a video stream or an MSRP stream at present. The offer (MSRP) in the future, but does not wish to offer a video stream or an
might look like the following: MSRP stream at present. The offer might look like the following:
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
a=creq:med-v0 a=creq:med-v0
m=audio 23456 RTP/AVP 0 m=audio 23456 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
a=rmcap:1 PCMU/8000 a=rmcap:1 PCMU/8000
a=rmcap:2 G729/8000 a=rmcap:2 G729/8000
a=rmcap:3 telephone-event/8000 a=rmcap:3 telephone-event/8000
a=mfcap:3 0-11 a=mfcap:3 0-11
a=pcfg:1 m=1,3|2,3 pt=1:0,2:18,3:100 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=lcfg:2 mt=video t=1 m=10|11
a=rmcap:10 H263-1998/90000 a=rmcap:10 H263-1998/90000
a=rmcap:11 H264/90000 a=rmcap:11 H264/90000
a=tcap:1 RTP/AVP a=tcap:1 RTP/AVP
a=lcfg:3 mt=message t=2 m=20 a=lcfg:3 mt=message t=2 m=20
a=tcap:2 TCP/MSRP a=tcap:2 TCP/MSRP
a=omcap:20 * a=omcap:20 *
The first lcfg attribute line ("lcfg:2") announces support for H.263 The first "lcfg" attribute line ("lcfg:2") announces support for
and H.264 video (H.263 preferred) for future negotiation. The second H.263 and H.264 video (H.263 preferred) for future negotiation. The
lcfg attribute line ("lcfg:3") announces support for MSRP for future second "lcfg" attribute line ("lcfg:3") announces support for MSRP
negotiation. The m-line and the rtpmap attribute offer an audio for future negotiation. The "m=" line and the "rtpmap" attribute
stream and provide the lowest precedence configuration (PCMU without offer an audio stream and provide the lowest precedence configuration
any DTMF encoding). The rmcap lines define the RTP-based media (PCMU without any DTMF encoding). The rmcap lines define the RTP-
format capabilities (PCMU, G729, telephone-event, H263-1998 and H264) based media format capabilities (PCMU, G729, telephone-event,
and the omcap line defines the non-RTP based media format capability H263-1998, and H264) and the omcap line defines the non-RTP-based
(wildcard). The mfcap attribute provides the format parameters for media format capability (wildcard). The "mfcap" attribute provides
telephone-event, specifying the 12 commercial DTMF 'digits'. The the format parameters for telephone-event, specifying the 12
pcfg attribute line defines the most-preferred media configuration as commercial DTMF 'digits'. The "pcfg" attribute line defines the
PCMU plus DTMF events and the next-most-preferred configuration as most-preferred media configuration as PCMU plus DTMF events and the
G.729B plus DTMF events. next-most-preferred configuration as G.729B plus DTMF events.
If the answerer is able to support all the potential configurations, 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 and also support H.263 video (but not H.264), it would reply with an
answer like: answer like the following:
v=0 v=0
o=- 24351 621814 IN IP4 192.0.2.2 o=- 24351 621814 IN IP4 192.0.2.2
s= s=
c=IN IP4 192.0.2.2 c=IN IP4 192.0.2.2
t=0 0 t=0 0
a=csup:med-v0 a=csup:med-v0
m=audio 54322 RTP/AVP 0 100 m=audio 54322 RTP/AVP 0 100
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
a=rtpmap:100 telephone-event/8000 a=rtpmap:100 telephone-event/8000
a=fmtp:100 0-11 a=fmtp:100 0-11
a=acfg:1 m=1,3 pt=1:0,3:100 a=acfg:1 m=1,3 pt=1:0,3:100
a=pcfg:1 m=2,3 pt=2:18,3:100 a=pcfg:1 m=2,3 pt=2:18,3:100
a=lcfg:2 mt=video t=1 m=10 a=lcfg:2 mt=video t=1 m=10
The lcfg attribute line announces the capability to support H.263 The "lcfg" attribute line announces the capability to support H.263
video at a later time. The media line and subsequent rtpmap and fmtp video at a later time. The media line and subsequent "rtpmap" and
attribute lines present the selected configuration for the media "fmtp" attribute lines present the selected configuration for the
stream. The acfg attribute line identifies the potential media stream. The "acfg" attribute line identifies the potential
configuration from which it was taken, and the pcfg attribute line configuration from which it was taken, and the "pcfg" attribute line
announces the potential capability to support G.729 with DTMF events announces the potential capability to support G.729 with DTMF events
as well. If, at some later time, congestion becomes a problem in the as well. If, at some later time, congestion becomes a problem in the
network, either party may, with expectation of success, offer a network, either party may, with expectation of success, offer a
reconfiguration of the media stream to use G.729 in order to reduce reconfiguration of the media stream to use G.729 in order to reduce
packet sizes. packet sizes.
5. IANA Considerations 5. IANA Considerations
5.1. New SDP Attributes 5.1. New SDP Attributes
The IANA is hereby requested to register the following new SDP IANA has registered the following new SDP attributes:
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 Attribute name: rmcap
Long form name: Non RTP-based media format capability Long form name: RTP-based media format capability
Type of attribute: session-level and media-level Type of attribute: session-level and media-level
Subject to charset: no Subject to charset: no
Purpose: associate non RTP-based media capability number(s) with Purpose: associate RTP-based media capability number(s) with
media subtype and encoding parameters media subtype and encoding parameters
Appropriate Values: see Section 3.3.1 Appropriate Values: see Section 3.3.1
Contact name: Flemming Andreasen, fandreas@cisco.com Contact name: Flemming Andreasen, fandres@cisco.com
Attribute name: mfcap Attribute name: omcap
Long form name: media format parameter capability Long form name: non-RTP-based media format capability
Type of attribute: session-level and media-level Type of attribute: session-level and media-level
Subject to charset: no Subject to charset: no
Purpose: associate media format attributes and Purpose: associate non-RTP-based media capability number(s) with
parameters with media format capabilities media subtype and encoding parameters
Appropriate Values: see Section 3.3.2 Appropriate Values: see Section 3.3.1
Contact name: Flemming Andreasen, fandreas@cisco.com 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 Attribute name: mscap
Long form name: media-specific capability Long form name: media-specific capability
Type of attribute: session-level and media-level Type of attribute: session-level and media-level
Subject to charset: no Subject to charset: no
Purpose: associate media-specific attributes and Purpose: associate media-specific attributes and
parameters with media capabilities parameters with media capabilities
Appropriate Values: see Section 3.3.3 Appropriate Values: see Section 3.3.3
Contact name: Flemming Andreasen, fandreas@cisco.com Contact name: Flemming Andreasen, fandreas@cisco.com
Attribute name: lcfg Attribute name: lcfg
Long form name: latent configuration Long form name: latent configuration
Type of attribute: media-level Type of attribute: media-level
Subject to charset: no Subject to charset: no
Purpose: to announce supportable media streams Purpose: to announce supportable media streams
without offering them for immediate use. without offering them for immediate use.
Appropriate Values: see Section 3.3.5 Appropriate Values: see Section 3.3.5
Contact name: Flemming Andreasen, fandreas@cisco.com Contact name: Flemming Andreasen, fandreas@cisco.com
Attribute name: sescap Attribute name: sescap
Long form name: session capability Long form name: session capability
Type of attribute: session-level Type of attribute: session-level
Subject to charset: no Subject to charset: no
Purpose: to specify and prioritize acceptable Purpose: to specify and prioritize acceptable
combinations of media stream configurations. combinations of media stream configurations.
Appropriate Values: see Section 3.3.8 Appropriate Values: see Section 3.3.8
Contact name: Flemming Andreasen, fandreas@cisco.com Contact name: Flemming Andreasen, fandreas@cisco.com
5.2. New SDP Capability Negotiation Option Tag 5.2. New SDP Capability Negotiation Option Tag
The IANA is hereby requested to add the new option tag "med-v0", IANA has added the new option tag "med-v0", defined in this document,
defined in this document, to the SDP Capability Negotiation Option to the "SDP Capability Negotiation Option Capability Tags" registry
Capability registry created for RFC 5939 [RFC5939]. created for RFC 5939 [RFC5939].
5.3. SDP Capability Negotiation Configuration Parameters Registry 5.3. SDP Capability Negotiation Configuration Parameters Registry
The IANA is hereby requested to change the "SDP Capability IANA has changed the "SDP Capability Negotiation Potential
Negotiation Potential Configuration Parameters" registry currently Configuration Parameters" registry, currently registered and defined
registered and defined by RFC 5939 [RFC5939] as follows: by RFC 5939 [RFC5939], as follows:
The name of the registry should be "SDP Capability Negotiation The name of the registry should be "SDP Capability Negotiation
Configuration Parameters Registry" and it should contain a table with Configuration Parameters Registry" and it should contain a table with
the following column headings: the following column headings:
o Encoding Name: The syntactical value used for the capability o Encoding Name: The syntactical value used for the capability
negotiation configuration parameter, as defined in RFC 5939 negotiation configuration parameter, as defined in RFC 5939
[RFC5939], Section 3.5. [RFC5939], Section 3.5.
o Descriptive Name: The name commonly used to refer to the o Descriptive Name: The name commonly used to refer to the
skipping to change at page 53, line 19 skipping to change at page 51, line 42
Applicable) MUST be present instead. Applicable) MUST be present instead.
An IANA SDP Capability Negotiation Configuration registration MUST be An IANA SDP Capability Negotiation Configuration registration MUST be
documented in an RFC in accordance with the IETF Review policy documented in an RFC in accordance with the IETF Review policy
[RFC5226]. Furthermore: [RFC5226]. Furthermore:
o The RFC MUST define the syntax and semantics of each new potential o The RFC MUST define the syntax and semantics of each new potential
configuration parameter. configuration parameter.
o The syntax MUST adhere to the syntax provided for extension o The syntax MUST adhere to the syntax provided for extension
configuration lists in RFC 5939 [RFC5939] Section 3.5.1 and the configuration lists in RFC 5939 [RFC5939], Section 3.5.1, and the
semantics MUST adhere to the semantics provided for extension semantics MUST adhere to the semantics provided for extension
configuration lists in RFC 5939 [RFC5939] Section 3.5.1 and 3.5.2. configuration lists in RFC 5939 [RFC5939], Sections 3.5.1 and
3.5.2.
o Configuration parameters that apply to latent configurations MUST o Configuration parameters that apply to latent configurations MUST
furthermore adhere to the syntax provided in Section 3.3.5 and the furthermore adhere to the syntax provided in Section 3.3.5 and the
semantics defined overall in this document. semantics defined overall in this document.
o Associated with each registration MUST be the encoding name for o Associated with each registration MUST be the encoding name for
the parameter as well as a short descriptive name for it. the parameter as well as a short descriptive name for it.
o Each registration MUST specify if it applies to o Each registration MUST specify if it applies to
* Potential configurations * Potential configurations
* Actual configurations * Actual configurations
* Latent configurations * Latent configurations
5.4. SDP Capability Negotiation Configuration Parameter Registrations 5.4. SDP Capability Negotiation Configuration Parameter Registrations
The IANA is hereby requested to register the following capability IANA has registered the following capability negotiation
negotiation configuration parameters: configuration parameters:
Encoding Name: a Encoding Name: a
Descriptive Name: Attribute Configuration Descriptive Name: Attribute Configuration
Potential Configuration Definition: [RFC5939] Potential Configuration Definition: [RFC5939]
Actual Configuration Definition: [RFC5939] Actual Configuration Definition: [RFC5939]
Latent Configuration Definition: [Note to RFC Editor: This RFC] Latent Configuration Definition: [RFC6871]
Encoding Name: t Encoding Name: t
Descriptive Name: Transport Protocol Configuration Descriptive Name: Transport Protocol Configuration
Potential Configuration Definition: [RFC5939] Potential Configuration Definition: [RFC5939]
Actual Configuration Definition: [RFC5939] Actual Configuration Definition: [RFC5939]
Latent Configuration Definition: [Note to RFC Editor: This RFC] Latent Configuration Definition: [RFC6871]
Encoding Name: m Encoding Name: m
Descriptive Name: Media Configuration Descriptive Name: Media Configuration
Potential Configuration Definition: [Note to RFC Editor: This Potential Configuration Definition: [RFC6871]
RFC] Actual Configuration Definition: [RFC6871]
Actual Configuration Definition: [Note to RFC Editor: This RFC] Latent Configuration Definition: [RFC6871]
Latent Configuration Definition: [Note to RFC Editor: This RFC]
Encoding Name: pt Encoding Name: pt
Descriptive Name: Payload Type Number Mapping Descriptive Name: Payload Type Number Mapping
Potential Configuration Definition: [Note to RFC Editor: This Potential Configuration Definition: [RFC6871]
RFC] Actual Configuration Definition: [RFC6871]
Actual Configuration Definition: [Note to RFC Editor: This RFC] Latent Configuration Definition: [RFC6871]
Latent Configuration Definition: [Note to RFC Editor: This RFC]
Encoding Name: mt Encoding Name: mt
Descriptive Name: Media Type Descriptive Name: Media Type
Potential Configuration Definition: N/A Potential Configuration Definition: N/A
Actual Configuration Definition: N/A Actual Configuration Definition: N/A
Latent Configuration Definition: [Note to RFC Editor: This RFC] Latent Configuration Definition: [RFC6871]
6. Security Considerations 6. Security Considerations
The security considerations of RFC 5939 [RFC5939] apply for this The security considerations of RFC 5939 [RFC5939] apply for this
document. document.
In RFC 5939 [RFC5939], it was noted that negotiation of transport In RFC 5939 [RFC5939], it was noted that negotiation of transport
protocols (e.g. secure and non-secure) and negotiation of keying protocols (e.g., secure and non-secure) and negotiation of keying
methods and material are potential security issues that warrant methods and material are potential security issues that warrant
integrity protection to remedy. Latent configuration support integrity protection to remedy. Latent configuration support
provides hints to the other side about capabilities supported for provides hints to the other side about capabilities supported for
further offer/answer exchanges, including transport protocols and further offer/answer exchanges, including transport protocols and
attribute capabilities, e.g. for keying methods. If an attacker can attribute capabilities, e.g., for keying methods. If an attacker can
remove or alter latent configuration information to suggest that only remove or alter latent configuration information to suggest that only
insecure or less secure alternatives are supported, then he may be non-secure or less-secure alternatives are supported, then he may be
able to force negotiation of a less secure session than would able to force negotiation of a less secure session than would
otherwise have occurred. While the specific attack as described here otherwise have occurred. While the specific attack, as described
differs from those described in RFC 5939 [RFC5939], the here, differs from those described in RFC 5939 [RFC5939], the
considerations and mitigation strategies are similar to those considerations and mitigation strategies are similar to those
described in RFC 5939 [RFC5939]. described in RFC 5939 [RFC5939].
Another variation on the above attack involves the Session Capability Another variation on the above attack involves the session capability
("sescap") attribute defined in this document. The "sescap" enables ("sescap") attribute defined in this document. The "sescap" enables
a preference order to be specified for all the potential a preference order to be specified for all the potential
configurations, and that preference will take precedence over any configurations, and that preference will take precedence over any
preference indication provided in individual potential configuration preference indication provided in individual potential configuration
attributes. Consequently, an attacker that can insert or modify a attributes. Consequently, an attacker that can insert or modify a
"sescap" attribute may be able to force negotiation of an insecure or "sescap" attribute may be able to force negotiation of an insecure or
less secure alternative than would otherwise have occurred. Again, less secure alternative than would otherwise have occurred. Again,
the considerations and mitigation strategies are similar to those the considerations and mitigation strategies are similar to those
described in RFC 5939 [RFC5939]. described in RFC 5939 [RFC5939].
The addition of negotiable media formats and their associated The addition of negotiable media formats and their associated
parameters, defined in this specification can cause problems for parameters, defined in this specification can cause problems for
middleboxes which attempt to control bandwidth utilization, media middleboxes that attempt to control bandwidth utilization, media
flows, and/or processing resource consumption as part of network flows, and/or processing resource consumption as part of network
policy, but which do not understand the media capability negotiation policy, but that do not understand the media capability negotiation
feature. As for the initial SDP Capability Negotiation work feature. As for the initial SDP capability negotiation work
[RFC5939], the SDP answer is formulated in such a way that it always [RFC5939], the SDP answer is formulated in such a way that it always
carries the selected media encoding for every media stream selected. carries the selected media encoding for every media stream selected.
Pending an understanding of capabilities negotiation, the middlebox Pending an understanding of capabilities negotiation, the middlebox
should examine the answer SDP to obtain the best picture of the media should examine the answer SDP to obtain the best picture of the media
streams being established. As always, middleboxes can best do their streams being established. As always, middleboxes can best do their
job if they fully understand media capabilities negotiation. job if they fully understand media capabilities negotiation.
7. Changes from previous versions 7. Acknowledgements
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.
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.
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.
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.
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"
8. Acknowledgements
This document is heavily influenced by the discussions and work done This document is heavily influenced by the discussions and work done
by the SDP Capability Negotiation Design team. The following people by the SDP Capability Negotiation design team. The following people
in particular provided useful comments and suggestions to either the in particular provided useful comments and suggestions to either the
document itself or the overall direction of the solution defined document itself or the overall direction of the solution defined
herein: Cullen Jennings, Matt Lepinski, Joerg Ott, Colin Perkins, and herein: Cullen Jennings, Matt Lepinski, Joerg Ott, Colin Perkins, and
Thomas Stach. Thomas Stach.
We thank Ingemar Johansson and Magnus Westerlund for examples that We thank Ingemar Johansson and Magnus Westerlund for examples that
stimulated this work, and for critical reading of the document. We stimulated this work, and for critical reading of the document. We
also thank Cullen Jennings, Christer Holmberg, and Miguel Garcia for also thank Cullen Jennings, Christer Holmberg, and Miguel Garcia for
their review of the document. their review of the document.
9. References 8. References
9.1. Normative References 8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, with Session Description Protocol (SDP)", RFC 3264, June
June 2002. 2002.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006. Description Protocol", RFC 4566, July 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226, IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008. May 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008. Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5939] Andreasen, F., "Session Description Protocol (SDP) [RFC5939] Andreasen, F., "Session Description Protocol (SDP)
Capability Negotiation", RFC 5939, September 2010. Capability Negotiation", RFC 5939, September 2010.
9.2. Informative References 8.2. Informative References
[RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., [RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse- Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse-
Parisis, "RTP Payload for Redundant Audio Data", RFC 2198, Parisis, "RTP Payload for Redundant Audio Data", RFC 2198,
September 1997. September 1997.
[RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session [RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol (SDP) Security Descriptions for Media Description Protocol (SDP) Security Descriptions for Media
Streams", RFC 4568, July 2006. Streams", RFC 4568, July 2006.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control "Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July
July 2006. 2006.
[RFC4733] Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF [RFC4733] Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF
Digits, Telephony Tones, and Telephony Signals", RFC 4733, Digits, Telephony Tones, and Telephony Signals", RFC 4733,
December 2006. December 2006.
[RFC4867] Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie, [RFC4867] Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie,
"RTP Payload Format and File Storage Format for the "RTP Payload Format and File Storage Format for the
Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband
(AMR-WB) Audio Codecs", RFC 4867, April 2007. (AMR-WB) Audio Codecs", RFC 4867, April 2007.
skipping to change at page 64, line 13 skipping to change at page 55, line 22
with Feedback (AVPF)", RFC 5104, February 2008. with Feedback (AVPF)", RFC 5104, February 2008.
Authors' Addresses Authors' Addresses
Robert R Gilman Robert R Gilman
Independent Independent
3243 W. 11th Ave. Dr. 3243 W. 11th Ave. Dr.
Broomfield, CO 80020 Broomfield, CO 80020
USA USA
Email: bob_gilman@comcast.net EMail: bob_gilman@comcast.net
Roni Even Roni Even
Gesher Erove Ltd Huawei Technologies
14 David Hamelech 14 David Hamelech
Tel Aviv 64953 Tel Aviv 64953
Israel Israel
Email: ron.even.tlv@gmail.com EMail: roni.even@mail01.huawei.com
Flemming Andreasen Flemming Andreasen
Cisco Systems Cisco Systems
Iselin, NJ Iselin, NJ
USA USA
Email: fandreas@cisco.com EMail: fandreas@cisco.com
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