draft-ietf-mmusic-sdp-media-capabilities-15.txt   draft-ietf-mmusic-sdp-media-capabilities-16.txt 
MMUSIC R. Gilman MMUSIC R. Gilman
Internet-Draft Independent Internet-Draft Independent
Updates: 5939 (if approved) R. Even Updates: 5939 (if approved) R. Even
Intended status: Standards Track Gesher Erove Ltd Intended status: Standards Track Gesher Erove Ltd
Expires: April 4, 2013 F. Andreasen Expires: June 15, 2013 F. Andreasen
Cisco Systems Cisco Systems
October 1, 2012 December 12, 2012
Session Description Protocol (SDP) Media Capabilities Negotiation Session Description Protocol (SDP) Media Capabilities Negotiation
draft-ietf-mmusic-sdp-media-capabilities-15 draft-ietf-mmusic-sdp-media-capabilities-16
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. In this document, we extend 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.
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 4, 2013. This Internet-Draft will expire on June 15, 2013.
Copyright Notice Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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3.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 8 3.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. Solution Overview . . . . . . . . . . . . . . . . . . . . 9 3.2. Solution Overview . . . . . . . . . . . . . . . . . . . . 9
3.3. New Capability Attributes . . . . . . . . . . . . . . . . 15 3.3. New Capability Attributes . . . . . . . . . . . . . . . . 15
3.3.1. The Media Format Capability Attributes . . . . . . . . 15 3.3.1. The Media Format Capability Attributes . . . . . . . . 15
3.3.2. The Media Format Parameter Capability Attribute . . . 17 3.3.2. The Media Format Parameter Capability Attribute . . . 17
3.3.3. The Media-Specific Capability Attribute . . . . . . . 20 3.3.3. The Media-Specific Capability Attribute . . . . . . . 20
3.3.4. New Configuration Parameters . . . . . . . . . . . . . 22 3.3.4. New Configuration Parameters . . . . . . . . . . . . . 22
3.3.5. The Latent Configuration Attribute . . . . . . . . . . 24 3.3.5. The Latent Configuration Attribute . . . . . . . . . . 24
3.3.6. Enhanced Potential Configuration Attribute . . . . . . 26 3.3.6. Enhanced Potential Configuration Attribute . . . . . . 26
3.3.7. Substitution of Media Payload Type Numbers in 3.3.7. Substitution of Media Payload Type Numbers in
Capability Attribute Parameters . . . . . . . . . . . 29 Capability Attribute Parameters . . . . . . . . . . . 30
3.3.8. The Session Capability Attribute . . . . . . . . . . . 30 3.3.8. The Session Capability Attribute . . . . . . . . . . . 31
3.4. Offer/Answer Model Extensions . . . . . . . . . . . . . . 35 3.4. Offer/Answer Model Extensions . . . . . . . . . . . . . . 35
3.4.1. Generating the Initial Offer . . . . . . . . . . . . . 35 3.4.1. Generating the Initial Offer . . . . . . . . . . . . . 36
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 . . . . . . . . . . . . . . . . 43 3.4.4. Modifying the Session . . . . . . . . . . . . . . . . 44
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.1. Alternative Codecs . . . . . . . . . . . . . . . . . . . . 45 4.1. Alternative Codecs . . . . . . . . . . . . . . . . . . . . 45
4.2. Alternative Combinations of Codecs (Session 4.2. Alternative Combinations of Codecs (Session
Configurations) . . . . . . . . . . . . . . . . . . . . . 48 Configurations) . . . . . . . . . . . . . . . . . . . . . 48
4.3. Latent Media Streams . . . . . . . . . . . . . . . . . . . 48 4.3. Latent Media Streams . . . . . . . . . . . . . . . . . . . 48
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 51 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 51
5.1. New SDP Attributes . . . . . . . . . . . . . . . . . . . . 51 5.1. New SDP Attributes . . . . . . . . . . . . . . . . . . . . 51
5.2. New SDP Capability Negotiation Option Tag . . . . . . . . 52 5.2. New SDP Capability Negotiation Option Tag . . . . . . . . 52
5.3. SDP Capability Negotiation Configuration Parameters 5.3. SDP Capability Negotiation Configuration Parameters
Registry . . . . . . . . . . . . . . . . . . . . . . . . . 52 Registry . . . . . . . . . . . . . . . . . . . . . . . . . 52
5.4. SDP Capability Negotiation Configuration Parameter 5.4. SDP Capability Negotiation Configuration Parameter
Registrations . . . . . . . . . . . . . . . . . . . . . . 53 Registrations . . . . . . . . . . . . . . . . . . . . . . 53
6. Security Considerations . . . . . . . . . . . . . . . . . . . 55 6. Security Considerations . . . . . . . . . . . . . . . . . . . 55
7. Changes from previous versions . . . . . . . . . . . . . . . . 56 7. Changes from previous versions . . . . . . . . . . . . . . . . 56
7.1. Changes from version 14 . . . . . . . . . . . . . . . . . 56 7.1. Changes from version 15 . . . . . . . . . . . . . . . . . 56
7.2. Changes from version 13 . . . . . . . . . . . . . . . . . 56 7.2. Changes from version 14 . . . . . . . . . . . . . . . . . 56
7.3. Changes from version 12 . . . . . . . . . . . . . . . . . 56 7.3. Changes from version 13 . . . . . . . . . . . . . . . . . 56
7.4. Changes from version 11 . . . . . . . . . . . . . . . . . 56 7.4. Changes from version 12 . . . . . . . . . . . . . . . . . 56
7.5. Changes from version 10 . . . . . . . . . . . . . . . . . 56 7.5. Changes from version 11 . . . . . . . . . . . . . . . . . 56
7.6. Changes from version 09 . . . . . . . . . . . . . . . . . 57 7.6. Changes from version 10 . . . . . . . . . . . . . . . . . 57
7.7. Changes from version 08 . . . . . . . . . . . . . . . . . 57 7.7. Changes from version 09 . . . . . . . . . . . . . . . . . 57
7.8. Changes from version 04 . . . . . . . . . . . . . . . . . 57 7.8. Changes from version 08 . . . . . . . . . . . . . . . . . 58
7.9. Changes from version 03 . . . . . . . . . . . . . . . . . 58 7.9. Changes from version 04 . . . . . . . . . . . . . . . . . 58
7.10. Changes from version 02 . . . . . . . . . . . . . . . . . 58 7.10. Changes from version 03 . . . . . . . . . . . . . . . . . 58
7.11. Changes from version 01 . . . . . . . . . . . . . . . . . 59 7.11. Changes from version 02 . . . . . . . . . . . . . . . . . 59
7.12. Changes from version 00 . . . . . . . . . . . . . . . . . 59 7.12. Changes from version 01 . . . . . . . . . . . . . . . . . 59
7.13. Changes from version 00 . . . . . . . . . . . . . . . . . 60
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 60 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 61
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 61 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 62
9.1. Normative References . . . . . . . . . . . . . . . . . . . 61 9.1. Normative References . . . . . . . . . . . . . . . . . . . 62
9.2. Informative References . . . . . . . . . . . . . . . . . . 61 9.2. Informative References . . . . . . . . . . . . . . . . . . 62
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 63 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.
The [RFC5939] document 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 case is to
be able to offer one media stream (e.g. audio) but list the 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 is straightforward.
This document updates [RFC5939] by updating the IANA Considerations. This document updates RFC 5939 [RFC5939] by updating the IANA
All other extensions defined in this document are considered Considerations. All other extensions defined in this document are
extensions above and beyond [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 RFC2119 [RFC2119] and document are to be interpreted as described in RFC 2119 [RFC2119] and
indicate requirement levels for compliant RTP 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 [RFC5939] for further negotiation in the offer/answer exchange. See RFC 5939
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 [RFC5939]. in RFC 5939 [RFC5939].
"Conventional Attribute": Any SDP attribute other than those defined "Conventional Attribute": Any SDP attribute other than those defined
by the series of capability negotiation specifications. by 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.
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"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 [RFC5939] for further in the current offer/answer exchange. See RFC 5939 [RFC5939] for
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 are they offered for
actual or potential use in the current offer/answer exchange. Latent actual or 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 [RFC5939]; for example they can be configuration are described in RFC 5939 [RFC5939]; for example they
used only at the media level since they are media level attributes. can be used only at the media level since they are media level
The [RFC5939] does not provide a way to exchange media-level attributes. RFC 5939 [RFC5939] does not provide a way to exchange
capabilities prior to the actual offer of the associated media media-level capabilities prior to the actual offer of the associated
stream. This section provides an overview of extensions providing an media stream. This section provides an overview of extensions
SDP Media Capability negotiation solution offering more robust providing an SDP Media Capability negotiation solution offering more
capabilities negotiation. This is followed by definitions of new SDP robust capabilities negotiation. This is followed by definitions of
attributes for the solution and its associated updated offer/answer new SDP attributes for the solution and its associated updated offer/
procedures [RFC3264] 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
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attribute type, with the exception of the media line type itself. attribute type, with the exception of the media line type itself.
The media line type is handled in a special way to permit compact The media line type is handled in a special way to permit compact
expression of media coding/format options. The line types are expression of media coding/format options. The line types are
bandwidth ("b="), information ("i="), connection data ("c="), and, bandwidth ("b="), information ("i="), connection data ("c="), and,
possibly, the deprecated encryption key ("k="). 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, and
the new lcfg attributes extending the base attributes from [RFC5939], the new lcfg attributes extending the base attributes from RFC 5939
and a use of the pcfg attribute to return capability information in [RFC5939], and a use of the pcfg attribute to return capability
the SDP answer. 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 [RFC4566], without the payload for the rtpmap attribute defined in SDP [RFC4566], without the
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 mfcap
attribute is used primarily to associate the media format 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") that specifies media parameters
associated with one or more media format capabilities. The mscap associated with one or more media format capabilities. The mscap
attribute is used to associate capabilities with attributes other attribute is used to associate capabilities with attributes other
than fmtp or rtpmap, for example, the rtcp-fb attribute defined in than fmtp or rtpmap, for example, the rtcp-fb attribute defined in
[RFC4585]. 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 [RFC5939], and to the new latent ("a=acfg:") attribute defined in RFC 5939 [RFC5939], and to the
configuration ("a=lcfg:") attribute. This permits specification new latent configuration ("a=lcfg:") attribute. This permits
of media capabilities (including their associated parameters) and specification of media capabilities (including their associated
combinations thereof for the configuration. For example, the parameters) and combinations thereof for the configuration. For
"a=pcfg:" line might specify PCMU and telephone events [RFC4733] example, the "a=pcfg:" line might specify PCMU and telephone
or G.729B and telephone events as acceptable configurations. The events [RFC4733] or G.729B and telephone events as acceptable
"a=acfg:" line in the answer would specify the configuration configurations. The "a=acfg:" line in the answer would specify
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.
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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 [RFC5939], with the option tag "med-v0", which attribute defined in RFC 5939 [RFC5939], with the option tag
indicates that the extension framework defined here, must be "med-v0", which indicates that the extension framework defined here,
supported. The base level capability negotiation support ("cap-v0" must be supported. The base level capability negotiation support
[RFC5939]) is implied since it is required for the extensions. ("cap-v0" [RFC5939]) is implied since it is required for the
extensions.
The "m=" line indicates that Alice is offering to use plain RTP with 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 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
skipping to change at page 14, line 24 skipping to change at page 14, line 25
stream. 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 [RFC5939], the media line, with any qualifying attributes such as Per RFC 5939 [RFC5939], the media line, with any qualifying
fmtp or rtpmap, is itself considered a valid configuration (the attributes such as fmtp or rtpmap, is itself considered a valid
current actual configuration); it has the lowest preference (per configuration (the current actual configuration); it has the lowest
[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
skipping to change at page 17, line 7 skipping to change at page 17, line 7
format-name = token ;defined in RFC4566 format-name = token ;defined in RFC4566
The encoding-name, clock-rate and encoding-params are as defined to The encoding-name, clock-rate and encoding-params are as defined to
appear in an rtpmap attribute for each media type/subtype. Thus, it 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 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- attribute lines, once a payload type number is assigned to a media-
cap-num (see Section 3.3.5). 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
[RFC4566]. In simple terms, it is the name of the media format, e.g. SDP [RFC4566]. In simple terms, it is the name of the media format,
"t38". This form can also be used in cases such as BFCP [RFC4585] e.g. "t38". This form can also be used in cases such as BFCP
where the fmt list in the m-line is effectively ignored (BFCP uses [RFC4585] where the fmt list in the m-line is effectively ignored
"*"). (BFCP uses "*").
The "rmcap" and "omcap" attributes can be provided at the session- 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 level
(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). Each media-cap-num MUST be of each in each media description). Each media-cap-num MUST be
unique within the entire SDP record; it is used to identify that unique within the entire SDP record; it is used to identify that
media capability in potential, latent and actual configurations, and media capability in potential, latent and actual configurations, and
in other attribute lines as explained below. Note that the media- in other attribute lines as explained below. Note that the media-
cap-num values are shared between the rmcap and omcap attributes, and cap-num values are shared between the rmcap and omcap attributes, and
skipping to change at page 18, line 21 skipping to change at page 18, line 21
<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 mfcap
attribute MUST be used to encode attributes for media capabilities, attribute MUST be used to encode attributes for media capabilities,
which would conventionally appear in an fmtp attribute. The existing which would conventionally appear in an fmtp attribute. The existing
acap attribute MUST NOT be used to encode fmtp attributes. acap attribute MUST NOT be used to encode fmtp attributes.
The mfcap attribute adheres to [RFC4566] attribute production rules The mfcap attribute adheres to SDP [RFC4566] attribute production
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 RFC4566
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
skipping to change at page 20, line 43 skipping to change at page 20, line 43
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 [RFC4585]. Such media-specific attributes, attribute defined in RFC 4585 [RFC4585]. Such media-specific
beyond the rtpmap and fmtp attributes, may be associated with media attributes, beyond the rtpmap and fmtp attributes, may be associated
capability numbers via a new media-specific attribute, mscap, of the with media capability numbers via a new media-specific attribute,
following form: 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"). The media capability numbers may include a ("rmcap" and/or "omcap"). The media capability numbers may include a
wildcard ("*"), which will be used instead of any payload type wildcard ("*"), which will be used instead of any payload type
mappings in the resulting SDP (see, e.g. [RFC4585] and the example mappings in the resulting SDP (see, e.g. RFC 4585 [RFC4585] and the
below). In ABNF, we have: 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 RFC4566
1*WSP att-value ; from RFC4566 1*WSP att-value ; from RFC4566
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 line, a
mscap line may be translated easily into a conventional SDP attribute mscap line may be translated easily into a conventional SDP attribute
line of the form line of the form
a=<att field>":"<fmt> <att value> ; <fmt> defined in [RFC4566] a=<att field>":"<fmt> <att value> ; <fmt> defined in SDP
[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 RFC4566 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.
skipping to change at page 21, line 50 skipping to change at page 22, line 4
values (but different media capability numbers), one line per media values (but different media capability numbers), one line per media
capability. 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 is
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 [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
skipping to change at page 22, line 36 skipping to change at page 22, line 37
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=)
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 configuration. Adhering to the ABNF for extension-config-list in RFC
[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 RFC5939
skipping to change at page 23, line 15 skipping to change at page 23, line 16
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 RFC5939.
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 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 3.3.1
payload-type-number = 1*3(DIGIT) ; RTP payload type number payload-type-number = 1*3(DIGIT) ; RTP payload 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" parameter.
The use of the plus sign ("+") is described in [RFC5939]. 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 pt=
parameter is not directly meaningful in the lcfg attribute because no parameter is not directly meaningful in the lcfg attribute because no
actual media session is being offered or accepted; it is permitted in actual media session is being offered or accepted; it is permitted in
order to tie any payload type number parameters within attributes to order to tie any payload type number parameters within attributes to
the proper media format. A primary example is the case of format the proper media format. A primary example is the case of format
parameters for the Redundant Audio Data (RED) payload, which are parameters for the Redundant Audio Data (RED) payload, which are
payload type numbers. Specific payload type numbers used in a latent payload type numbers. Specific payload type numbers used in a latent
configuration MAY be interpreted as suggestions to be used in any configuration MAY be interpreted as suggestions to be used in any
future offer based on the latent configuration, but they are not future offer based on the latent configuration, but they are not
skipping to change at page 24, line 13 skipping to change at page 24, line 14
media-type = ["+"] "mt=" media; media defined in RFC4566 media-type = ["+"] "mt=" media; media defined in RFC4566
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 [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
a session with an audio stream, and, at the same time, announce its a session with an audio stream, and, at the same time, announce its
ability to support a video stream as part of the same session. The ability to support a video stream as part of the same session. The
offerer can supply its video capabilities by offering one or more offerer can supply its video capabilities by offering one or more
skipping to change at page 24, line 40 skipping to change at page 24, line 41
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. The media type (mt=) and the transport protocol(s) configuration. A latent configuration MUST include a media type
(t=) MUST be specified since the latent configuration is independent (mt=) and a transport protocol configuration parameter since the
of any media line present. In most cases, the media configuration latent configuration is independent of any media line present. In
(m=) parameter MUST be present as well (see Section 4 for examples). most cases, the media configuration (m=) parameter needs to be
The lcfg attribute is a media level attribute. present as well (see Section 4 for examples). The lcfg attribute is
a media level attribute.
The lcfg attribute is defined as a media level attribute since it The lcfg attribute is defined as a media level attribute since it
specifies a possible future media stream. However the lcfg 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 capabilities ("sescap") may
be used to indicate this. be used to indicate this.
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 which 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 [RFC4566] "attribute" production with att-field which adheres to the SDP [RFC4566] "attribute" production with att-
and att-value defined as: 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 = 1*10(DIGIT) ; defined in RFC5234 config-number = 1*10(DIGIT) ; defined in RFC5234
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 RFC5939
; 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 MUST
be present. The pot-cfg-list MUST contain a transport-protocol- 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 [RFC5939], the use of the "+" of parameter list. As specified in RFC 5939 [RFC5939], the use of
prefix with a parameter indicates that the entire configuration MUST the "+" prefix with a parameter indicates that the entire
be ignored if the parameter is not understood; otherwise, the configuration MUST be ignored if the parameter is not understood;
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 numbers
in attribute capabilities to specific media capabilities. 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 the
session level, then that attribute will be expected to appear at the session level, then that attribute will be expected to appear at the
skipping to change at page 26, line 28 skipping to change at page 26, line 31
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. other media stream.
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. The actual order to satisfy formatting rules for the attribute. Since the
value(s) of the keying material SHOULD be meaningless, and the keying material will be visible but not actually used at this stage
receiver of the lcfg attribute MUST ignore the values. (since it's a latent configuration), the value(s) of the keying
material SHOULD NOT be a real value used for real exchange of media,
and the receiver of the lcfg attribute MUST ignore the values.
3.3.6. Enhanced Potential Configuration Attribute 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 base protocol [RFC5939]. The parameters and attribute defined in the SDP Capability Negotiation base protocol
their definitions are "borrowed" from the definitions provided for [RFC5939]. The parameters and their definitions are "borrowed" from
the latent configuration attribute in Section 3.3.5. The expanded the definitions provided for the latent configuration attribute in
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
payload-number-config-list ; Section 3.3.4.2 payload-number-config-list ; Section 3.3.4.2
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.
skipping to change at page 29, line 8 skipping to change at page 29, line 15
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-events 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-events/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
skipping to change at page 29, line 38 skipping to change at page 29, line 45
If the preferred configuration is selected, the SDP answer will look If the preferred configuration is selected, the SDP answer will look
like like
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-events/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, the
parameters to an attribute may contain payload type numbers. Two 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
skipping to change at page 31, line 40 skipping to change at page 31, line 50
[1*WSP optional-configs] [1*WSP optional-configs]
session-num = 1*10(DIGIT) ; defined in RFC5234 session-num = 1*10(DIGIT) ; defined in RFC5234
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)
; config-number defined in RFC5939 ; config-number defined in RFC5939
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-numbered session. Each alt-config list preferred over a higher-numbered session. Each alt-config list
specifies alternative media configurations within the session; specifies alternative media configurations within the session;
preference is based on config-num as specified in [RFC5939]. Note preference is based on config-num as specified in RFC 5939 [RFC5939].
that the session preference order, when present, takes precedence Note that the session preference order, when present, takes
over the individual media stream configuration preference order. precedence over the individual media 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; [RFC5939] requires only that pcfg unique across the entire session; RFC 5939 [RFC5939] requires only
configuration numbers be unique within a media description. that pcfg configuration numbers be unique within a media description.
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
skipping to change at page 35, line 26 skipping to change at page 35, line 37
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 [RFC5939] Section 6; those the offer/answer procedures defined in RFC 5939 [RFC5939] Section 6;
procedures form a baseline set of capability negotiation offer/answer those procedures form a baseline set of capability negotiation offer/
procedures that MUST be followed, subject to the extensions defined answer procedures that MUST be followed, subject to the extensions
here. defined here.
The [RFC5939] provides a relatively compact means to offer the SDP Capability Negotiation [RFC5939] provides a relatively compact
equivalent of an ordered list of alternative configurations for means to offer the equivalent of an ordered list of alternative
offered media streams (as would be described by separate m= lines and configurations for offered media streams (as would be described by
associated attributes). The attributes acap, mscap, mfcap, omcap and separate m= lines and associated attributes). The attributes acap,
rmcap are designed to map somewhat straightforwardly into equivalent mscap, mfcap, omcap and rmcap are designed to map somewhat
m= lines and conventional attributes when invoked by a pcfg, lcfg, or straightforwardly into equivalent m= lines and conventional
acfg attribute with appropriate parameters. The a=pcfg: lines, along attributes when invoked by a pcfg, lcfg, or acfg attribute with
with the m= line itself, represent offered media configurations. The appropriate parameters. The a=pcfg: lines, along with the m= line
a=lcfg: lines represent alternative capabilities for future use. itself, represent offered media configurations. 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, omcap,
mfcap, and mscap attributes) mfcap, and mscap attributes)
o Potential configurations using those media format capabilities and o Potential configurations using those media format capabilities and
skipping to change at page 36, line 39 skipping to change at page 36, line 50
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 [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.
skipping to change at page 37, line 35 skipping to change at page 37, line 46
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 [RFC5939] can be used, however it does not attribute defined in RFC 5939 [RFC5939] can be used, however it
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 that is unique in the entire SDP (note MUST include a config-number that is unique in the entire SDP (note
that this differs from [RFC5939], which only requires uniqueness that this differs from RFC 5939 [RFC5939], which only requires
within a media description). Also, the config-number MUST NOT uniqueness within a media description). Also, the config-number MUST
overlap with any config-number used by a latent configuration in the NOT overlap with any config-number used by a latent configuration in
SDP. As described in [RFC5939], lower config-numbers indicate a the SDP. As described in RFC 5939 [RFC5939], lower config-numbers
higher preference; the ordering still applies within a given media indicate a higher preference; the ordering still applies within a
description only though. 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 the
media capability number in question. When one or more media 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 in
the actual configuration (m= line). Any attributes included for 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, etc.).
For non-RTP based media formats, the format-name (from the "omcap" For non-RTP based media formats, the format-name (from the "omcap"
media capability) is simply added to the "m=" line as a media format media capability) is simply added to the "m=" line as a media format
skipping to change at page 38, line 39 skipping to change at page 39, line 4
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 (incl. 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 [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 hence 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 and hence cannot instantiate a referenced
capability; it is therefore safe for those to refer to capability; it is therefore 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
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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 [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, omcap,
mfcap, and mscap attributes) 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)
skipping to change at page 40, line 25 skipping to change at page 40, line 37
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 [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" parameter)
are valid themselves (as defined in Section 3.3.1, 3.3.2, and are valid themselves (as defined in Section 3.3.1, 3.3.2, and
3.3.3) and each of them is provided either at the session level 3.3.3) and each of them is provided either at the session level
or within this particular media description. or within this particular media description.
skipping to change at page 41, line 5 skipping to change at page 41, line 17
Section 3.3.7) applied to any capability (mfcap, mscap, or acap) Section 3.3.7) applied to any capability (mfcap, mscap, or acap)
referenced by this potential configuration results in a valid referenced by this potential configuration results in a valid
SDP. SDP.
Note that, since SDP does not interpret the value of fmtp parameters, Note that, since SDP does not interpret the value of fmtp parameters,
any resulting fmtp parameter value will be considered valid. 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
[RFC5939], with a lower config-number within a media description RFC 5939 [RFC5939], with a lower config-number within a media
being preferred over a higher one. If a valid "sescap" attribute is description being preferred over a higher one. If a valid "sescap"
present, the preference order provided in the "sescap" attribute MUST attribute is present, the preference order provided in the "sescap"
take precedence. A "sescap" attribute is considered valid if: attribute MUST take precedence. A "sescap" attribute is considered
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 [RFC5939], Section 3.6.2, and with the procedures specified in RFC 5939 [RFC5939], Section 3.6.2,
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-RTP
based media formats (omcap), the format-name is added. For RTP- based media formats (omcap), the format-name is added. For RTP-
based media formats (rmcap), the payload-type specified in the based media formats (rmcap), the payload-type specified in the
payload-type mapping ("pt") is added and a corresponding "rtpmap" payload-type mapping ("pt") is added and a corresponding "rtpmap"
attribute is added to the media description. 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
skipping to change at page 42, line 9 skipping to change at page 42, line 23
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 the answerer MUST generate a valid answer SDP as described in RFC
[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
and configuration numbers for those MUST be distinct from the ones and configuration numbers for those MUST be distinct from the ones
used in the offer SDP. used in the offer SDP.
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
skipping to change at page 42, line 38 skipping to change at page 43, line 4
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 selected one returned in an actual configuration attribute (see RFC
[RFC5939]). If the answerer supports only a (non-mandatory) subset 5939 [RFC5939]). If the answerer supports only a (non-mandatory)
of the parameters offered in a latent configuration, the answer subset of the parameters offered in a latent configuration, the
latent configuration will include only those parameters supported answer latent configuration will include only those parameters
(similar to "acfg" processing). Note that latent configurations do supported (similar to "acfg" processing). Note that latent
not constitute an actual offer at this point in time; they merely configurations do not constitute an actual offer at this point in
indicate additional configurations that could be supported. time; they merely indicate additional configurations that could be
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, i.e. it must be possible to
support it at the same time as any required (i.e. non-optional) 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 [RFC5939] The offerer MUST process the answer in accordance with RFC 5939
Section 3.6.3, and further explained below. [RFC5939] Section 3.6.3, and further explained 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 Section 3.3.2.1 and 3.3.7).
The actual configuration attribute may also be used to infer the lack The actual configuration attribute may also be used to infer the lack
of acceptability of higher-preference configurations that were not of acceptability of higher-preference configurations that were not
chosen, subject to any constraints provided by a Session Capability chosen, subject to any constraints provided by a Session Capability
attribute ("sescap") in the offer. Note that the base specification attribute ("sescap") in the offer. Note that the SDP Capability
[RFC5939] requires the answerer to choose the highest preference Negotiation base specification [RFC5939] requires the answerer to
configuration it can support, subject to local policies. choose the highest preference configuration it can 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
skipping to change at page 49, line 35 skipping to change at page 49, line 35
The first lcfg attribute line ("lcfg:2") announces support for H.263 The first lcfg attribute line ("lcfg:2") announces support for H.263
and H.264 video (H.263 preferred) for future negotiation. The second and H.264 video (H.263 preferred) for future negotiation. The second
lcfg attribute line ("lcfg:3") announces support for MSRP for future lcfg attribute line ("lcfg:3") announces support for MSRP for future
negotiation. The m-line and the rtpmap attribute offer an audio negotiation. The m-line and the rtpmap attribute offer an audio
stream and provide the lowest precedence configuration (PCMU without stream and provide the lowest precedence configuration (PCMU without
any DTMF encoding). The rmcap lines define the RTP-based media any DTMF encoding). The rmcap lines define the RTP-based media
format capabilities (PCMU, G729, telephone-event, H263-1998 and H264) format capabilities (PCMU, G729, telephone-event, H263-1998 and H264)
and the omcap line defines the non-RTP based media format capability and the omcap line defines the non-RTP based media format capability
(wildcard). The mfcap attribute provides the format parameters for (wildcard). The mfcap attribute provides the format parameters for
telephone-events, specifying the 12 commercial DTMF 'digits'. The telephone-event, specifying the 12 commercial DTMF 'digits'. The
pcfg attribute line defines the most-preferred media configuration as pcfg attribute line defines the most-preferred media configuration as
PCMU plus DTMF events and the next-most-preferred configuration as PCMU plus DTMF events and the next-most-preferred configuration as
G.729B plus DTMF events. 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:
v=0 v=0
o=- 24351 621814 IN IP4 192.0.2.2 o=- 24351 621814 IN IP4 192.0.2.2
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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", The IANA is hereby requested to add the new option tag "med-v0",
defined in this document, to the SDP Capability Negotiation Option defined in this document, to the SDP Capability Negotiation Option
Capability registry created for [RFC5939]. Capability registry 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 The IANA is hereby requested to change the "SDP Capability
Negotiation Potential Configuration Parameters" registry currently Negotiation Potential Configuration Parameters" registry currently
registered and defined by [RFC5939] as follows: registered and defined 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 [RFC5939], negotiation configuration parameter, as defined in RFC 5939
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
capability negotiation configuration parameter. capability negotiation configuration parameter.
o Potential Configuration Definition: A reference to the RFC that o Potential Configuration Definition: A reference to the RFC that
defines the configuration parameter in the context of a potential defines the configuration parameter in the context of a potential
configuration attribute. If the configuration parameter is not configuration attribute. If the configuration parameter is not
defined for potential configurations, the string "N/A" (Not defined for potential configurations, the string "N/A" (Not
Applicable) MUST be present instead. Applicable) MUST be present instead.
skipping to change at page 53, line 12 skipping to change at page 53, line 12
defined for actual configurations, the string "N/A" (Not defined for actual configurations, the string "N/A" (Not
Applicable) MUST be present instead. Applicable) MUST be present instead.
o Latent Configuration Definition: A reference to the RFC that o Latent Configuration Definition: A reference to the RFC that
defines the configuration parameter in the context of a latent defines the configuration parameter in the context of a latent
configuration attribute. If the configuration parameter is not configuration attribute. If the configuration parameter is not
defined for latent configurations, the string "N/A" (Not defined for latent configurations, the string "N/A" (Not
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 [RFC5226] IETF Review documented in an RFC in accordance with the IETF Review policy
policy. 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 [RFC5939] Section 3.5.1 and the semantics configuration lists in RFC 5939 [RFC5939] Section 3.5.1 and the
MUST adhere to the semantics provided for extension configuration semantics MUST adhere to the semantics provided for extension
lists in [RFC5939] Section 3.5.1 and 3.5.2. configuration lists in RFC 5939 [RFC5939] Section 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
skipping to change at page 55, line 7 skipping to change at page 55, line 7
Latent Configuration Definition: [Note to RFC Editor: This RFC] 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: [Note to RFC Editor: This RFC]
6. Security Considerations 6. Security Considerations
The security considerations of [RFC5939] apply for this document. The security considerations of RFC 5939 [RFC5939] apply for this
document.
In [RFC5939], it was noted that negotiation of transport protocols In RFC 5939 [RFC5939], it was noted that negotiation of transport
(e.g. secure and non-secure) and negotiation of keying methods and protocols (e.g. secure and non-secure) and negotiation of keying
material are potential security issues that warrant integrity methods and material are potential security issues that warrant
protection to remedy. Latent configuration support provides hints to integrity protection to remedy. Latent configuration support
the other side about capabilities supported for further offer/answer provides hints to the other side about capabilities supported for
exchanges, including transport protocols and attribute capabilities, further offer/answer exchanges, including transport protocols and
e.g. for keying methods. If an attacker can remove or alter latent attribute capabilities, e.g. for keying methods. If an attacker can
configuration information to suggest that only insecure or less remove or alter latent configuration information to suggest that only
secure alternatives are supported, then he may be able to force insecure or less secure alternatives are supported, then he may be
negotiation of a less secure session than would otherwise have able to force negotiation of a less secure session than would
occurred. While the specific attack as described here differs from otherwise have occurred. While the specific attack as described here
those described in [RFC5939], the considerations and mitigation differs from those described in RFC 5939 [RFC5939], the
strategies are similar to those described in [RFC5939]. considerations and mitigation strategies are similar to those
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 [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 which 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 which do not understand the media capability negotiation
feature. As for the initial CapNeg work [RFC5939], the SDP answer is feature. As for the initial SDP Capability Negotiation work
formulated in such a way that it always carries the selected media [RFC5939], the SDP answer is formulated in such a way that it always
encoding for every media stream selected. Pending an understanding carries the selected media encoding for every media stream selected.
of capabilities negotiation, the middlebox should examine the answer Pending an understanding of capabilities negotiation, the middlebox
SDP to obtain the best picture of the media streams being should examine the answer SDP to obtain the best picture of the media
established. As always, middleboxes can best do their job if they streams being established. As always, middleboxes can best do their
fully understand media capabilities negotiation. job if they fully understand media capabilities negotiation.
7. Changes from previous versions 7. Changes from previous versions
7.1. Changes from version 14 7.1. 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.
7.2. Changes from version 14
o Updated IANA Considerations to fix configuration parameter o Updated IANA Considerations to fix configuration parameter
registry. Document now updates [RFC5939] (IANA considerations registry. Document now updates RFC 5939 [RFC5939] (IANA
only) considerations only)
o Minor ABNF updates to fix errors. o Minor ABNF updates to fix errors.
o Editorial nit fixes to address protocol write-up review. o Editorial nit fixes to address protocol write-up review.
7.2. Changes from version 13 7.3. Changes from version 13
o Various editorial clarifications and updates to address review o Various editorial clarifications and updates to address review
comments. comments.
7.3. Changes from version 12 7.4. Changes from version 12
o Removed "dummy" form in the pcfg payload-type-number, since the o Removed "dummy" form in the pcfg payload-type-number, since the
functionality is redundant with the non-RTP media capability functionality is redundant with the non-RTP media capability
(omcap) and it was inconsistent with other RTP payload type (omcap) and it was inconsistent with other RTP payload type
operation. operation.
o Clarified that latent configuration attribute (lcfg) can only be o Clarified that latent configuration attribute (lcfg) can only be
used at the media level and hence (technically) as part of a media used at the media level and hence (technically) as part of a media
description description
o Rewrote offer/answer sections and expanded significantly on offer/ o Rewrote offer/answer sections and expanded significantly on offer/
answer operation. answer operation.
o Updated security considerations o Updated security considerations
o Various minor editorial clarifications and changes. o Various minor editorial clarifications and changes.
7.4. Changes from version 11 7.5. Changes from version 11
o Corrected several statements implying lcfg was a session-level o Corrected several statements implying lcfg was a session-level
attribute. attribute.
o Added non-RTP based media format capabilities ("a=omcap") and o Added non-RTP based media format capabilities ("a=omcap") and
renamed "mcap" to "rmcap" renamed "mcap" to "rmcap"
7.5. Changes from version 10 7.6. Changes from version 10
o Defined the latent configuration attribute as a media-level o Defined the latent configuration attribute as a media-level
attribute because it specifies a possible future media stream. attribute because it specifies a possible future media stream.
Added text to clarify how to specify alternative configurations of Added text to clarify how to specify alternative configurations of
a single latent stream and/or multiple streams. a single latent stream and/or multiple streams.
o Improved the definition of the session capability attribute to o Improved the definition of the session capability attribute to
permit both required configurations and optional configurations - permit both required configurations and optional configurations -
latent configurations cannot be required because they have not yet latent configurations cannot be required because they have not yet
been offered. been offered.
skipping to change at page 57, line 27 skipping to change at page 57, line 36
o Changed various "must", etc., terms to normative terms ("MUST", o Changed various "must", etc., terms to normative terms ("MUST",
etc.) as appropriate, in Section 3.3.5Section 3.3.6.1 etc.) as appropriate, in Section 3.3.5Section 3.3.6.1
Section 3.3.6.3 and Section 3.3.8 Section 3.3.6.3 and Section 3.3.8
o Attempted to clarify the substitution mechanism in Section 3.3.7 o Attempted to clarify the substitution mechanism in Section 3.3.7
and improve its uniqueness. and improve its uniqueness.
o Made various editorial changes, including changing the title in o Made various editorial changes, including changing the title in
the header, and removing numbering from some SDP examples. the header, and removing numbering from some SDP examples.
7.6. Changes from version 09 7.7. Changes from version 09
o Additional corrections to latent media stream example in o Additional corrections to latent media stream example in
Section 4.3 Section 4.3
o Fixed up attribute formatting examples and corresponding ABNF. o Fixed up attribute formatting examples and corresponding ABNF.
o Removed preference rule for latent configurations. o Removed preference rule for latent configurations.
o Various spelling and other editorial changes were made. o Various spelling and other editorial changes were made.
o updated cross-references. o updated cross-references.
7.7. Changes from version 08 7.8. Changes from version 08
The major change is in Section 4.3, Latent Media Streams, fixing the The major change is in Section 4.3, Latent Media Streams, fixing the
syntax of the answer. All the other changes are editorial. syntax of the answer. All the other changes are editorial.
7.8. Changes from version 04 7.9. Changes from version 04
o The definitions for bcap, ccap, icap, and kcap attributes have o The definitions for bcap, ccap, icap, and kcap attributes have
been removed, and are to be defined in another document. been removed, and are to be defined in another document.
o Corrected formatting of m= and p= configuration parameters to o Corrected formatting of m= and p= configuration parameters to
conform to extension-config-list form defined in [RFC5939] conform to extension-config-list form defined in RFC 5939
[RFC5939]
o Reorganized definitions of new parameters to make them easier to o Reorganized definitions of new parameters to make them easier to
find in document. find in document.
o Added ability to renegotiate capabilities when modifying the o Added ability to renegotiate capabilities when modifying the
session (Section 3.4.4). session (Section 3.4.4).
o Made various editorial changes, clarifications, and typo o Made various editorial changes, clarifications, and typo
corrections. corrections.
7.9. Changes from version 03 7.10. Changes from version 03
o A new session capability attribute (sescap) has been added to o A new session capability attribute (sescap) has been added to
permit specification of acceptable media stream combinations. permit specification of acceptable media stream combinations.
o Capability attribute definitions corresponding to the i, c, b, and o Capability attribute definitions corresponding to the i, c, b, and
k SDP line types have been added for completeness. k SDP line types have been added for completeness.
o Use of the pcfg: attribute in SDP answers has been included in o Use of the pcfg: attribute in SDP answers has been included in
order to conveniently return information in the answer about order to conveniently return information in the answer about
acceptable configurations in the media stream offer. acceptable configurations in the media stream offer.
skipping to change at page 58, line 47 skipping to change at page 59, line 12
o The description of the mscap attribute has been modified to make o The description of the mscap attribute has been modified to make
it clear that it should not be used to generate undefined SDP it clear that it should not be used to generate undefined SDP
attributes, or to "extend" existing attributes. attributes, or to "extend" existing attributes.
o <ms-parameters> are made optional in the mscap attribute o <ms-parameters> are made optional in the mscap attribute
definition. definition.
o "AMR" changed to "AMR-WB" in cases in which the sample rate is o "AMR" changed to "AMR-WB" in cases in which the sample rate is
16000. 16000.
7.10. Changes from version 02 7.11. Changes from version 02
This version contains several detail changes intended to simplify This version contains several detail changes intended to simplify
capability processing and mapping into conventional SDP media blocks. capability processing and mapping into conventional SDP media blocks.
o The "mcap" attribute is enhanced to include the role of the "ecap" o The "mcap" attribute is enhanced to include the role of the "ecap"
attribute; the latter is eliminated. attribute; the latter is eliminated.
o The "fcap" attribute has been renamed "mfcap". New replacement o The "fcap" attribute has been renamed "mfcap". New replacement
rules vis-a-vis fmtp attributes in the base media specification rules vis-a-vis fmtp attributes in the base media specification
have been added. have been added.
skipping to change at page 59, line 30 skipping to change at page 59, line 42
attribute. attribute.
o A new parameter, "mt=" is added to the latent configuration o A new parameter, "mt=" is added to the latent configuration
attribute (lcfg) to specify the media stream type (audio, video, attribute (lcfg) to specify the media stream type (audio, video,
etc.) when the lcfg is declared at the session level. etc.) when the lcfg is declared at the session level.
o The examples are expanded. o The examples are expanded.
o Numerous typos and misspellings have been corrected. o Numerous typos and misspellings have been corrected.
7.11. Changes from version 01 7.12. Changes from version 01
The documents adds a new attribute for specifying bandwidth The documents adds a new attribute for specifying bandwidth
capability and a parameter to list in the potential configuration. capability and a parameter to list in the potential configuration.
Other changes are to align the document with the terminology and Other changes are to align the document with the terminology and
attribute names from draft-ietf-mmusic-sdp-capability-negotiation-07. attribute names from draft-ietf-mmusic-sdp-capability-negotiation-07.
The document also clarifies some previous open issues. The document also clarifies some previous open issues.
7.12. Changes from version 00 7.13. Changes from version 00
The major changes include taking out the "mcap" and "cptmap" The major changes include taking out the "mcap" and "cptmap"
parameter. The mapping of payload type is now in the "pt" parameter parameter. The mapping of payload type is now in the "pt" parameter
of "pcfg". Media subtype need to explicitly defined in the "cmed" of "pcfg". Media subtype need to explicitly defined in the "cmed"
attribute if referenced in the "pcfg" attribute if referenced in the "pcfg"
8. Acknowledgements 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
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