< draft-ietf-mmusic-sdp-media-capabilities-13.txt   draft-ietf-mmusic-sdp-media-capabilities-14.txt >
MMUSIC R. Gilman MMUSIC R. Gilman
Internet-Draft Independent Internet-Draft Independent
Intended status: Standards Track R. Even Intended status: Standards Track R. Even
Expires: September 13, 2012 Gesher Erove Ltd Expires: January 10, 2013 Gesher Erove Ltd
F. Andreasen F. Andreasen
Cisco Systems Cisco Systems
March 12, 2012 July 9, 2012
SDP Media Capabilities Negotiation SDP Media Capabilities Negotiation
draft-ietf-mmusic-sdp-media-capabilities-13 draft-ietf-mmusic-sdp-media-capabilities-14
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.
skipping to change at page 1, line 38 skipping to change at page 1, line 38
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 September 13, 2012. This Internet-Draft will expire on January 10, 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
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publication of this document. Please review these documents publication of this document. Please review these documents
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modifications of such material outside the IETF Standards Process. modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other it for publication as an RFC or to translate it into languages other
than English. than English.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. SDP Media Capabilities . . . . . . . . . . . . . . . . . . . . 6 3. SDP Media Capabilities . . . . . . . . . . . . . . . . . . . . 8
3.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 6 3.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. Solution Overview . . . . . . . . . . . . . . . . . . . . 7 3.2. Solution Overview . . . . . . . . . . . . . . . . . . . . 9
3.3. New Capability Attributes . . . . . . . . . . . . . . . . 13 3.3. New Capability Attributes . . . . . . . . . . . . . . . . 15
3.3.1. The Media Format Capability Attributes . . . . . . . . 13 3.3.1. The Media Format Capability Attributes . . . . . . . . 15
3.3.2. The Media Format Parameter Capability Attribute . . . 15 3.3.2. The Media Format Parameter Capability Attribute . . . 17
3.3.3. The Media-Specific Capability Attribute . . . . . . . 18 3.3.3. The Media-Specific Capability Attribute . . . . . . . 20
3.3.4. New Configuration Parameters . . . . . . . . . . . . . 20 3.3.4. New Configuration Parameters . . . . . . . . . . . . . 22
3.3.5. The Latent Configuration Attribute . . . . . . . . . . 21 3.3.5. The Latent Configuration Attribute . . . . . . . . . . 24
3.3.6. Enhanced Potential Configuration Attribute . . . . . . 24 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 . . . . . . . . . . . 27 Capability Attribute Parameters . . . . . . . . . . . 29
3.3.8. The Session Capability Attribute . . . . . . . . . . . 28 3.3.8. The Session Capability Attribute . . . . . . . . . . . 30
3.4. Offer/Answer Model Extensions . . . . . . . . . . . . . . 32 3.4. Offer/Answer Model Extensions . . . . . . . . . . . . . . 35
3.4.1. Generating the Initial Offer . . . . . . . . . . . . . 33 3.4.1. Generating the Initial Offer . . . . . . . . . . . . . 35
3.4.2. Generating the Answer . . . . . . . . . . . . . . . . 36 3.4.2. Generating the Answer . . . . . . . . . . . . . . . . 39
3.4.3. Offerer Processing of the Answer . . . . . . . . . . . 40 3.4.3. Offerer Processing of the Answer . . . . . . . . . . . 43
3.4.4. Modifying the Session . . . . . . . . . . . . . . . . 41 3.4.4. Modifying the Session . . . . . . . . . . . . . . . . 43
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.1. Alternative Codecs . . . . . . . . . . . . . . . . . . . . 42 4.1. Alternative Codecs . . . . . . . . . . . . . . . . . . . . 45
4.2. Alternative Combinations of Codecs (Session 4.2. Alternative Combinations of Codecs (Session
Configurations) . . . . . . . . . . . . . . . . . . . . . 45 Configurations) . . . . . . . . . . . . . . . . . . . . . 48
4.3. Latent Media Streams . . . . . . . . . . . . . . . . . . . 45 4.3. Latent Media Streams . . . . . . . . . . . . . . . . . . . 48
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 48 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 51
5.1. New SDP Attributes . . . . . . . . . . . . . . . . . . . . 48 5.1. New SDP Attributes . . . . . . . . . . . . . . . . . . . . 51
5.2. New SDP Option Tag . . . . . . . . . . . . . . . . . . . . 49 5.2. New SDP Option Tag . . . . . . . . . . . . . . . . . . . . 52
5.3. New SDP Capability Negotiation Parameters . . . . . . . . 49 5.3. New SDP Capability Negotiation Parameters . . . . . . . . 52
6. Security Considerations . . . . . . . . . . . . . . . . . . . 50 6. Security Considerations . . . . . . . . . . . . . . . . . . . 53
7. Changes from previous versions . . . . . . . . . . . . . . . . 51 7. Changes from previous versions . . . . . . . . . . . . . . . . 54
7.1. Changes from version 12 . . . . . . . . . . . . . . . . . 51 7.1. Changes from version 13 . . . . . . . . . . . . . . . . . 54
7.2. Changes from version 11 . . . . . . . . . . . . . . . . . 51 7.2. Changes from version 12 . . . . . . . . . . . . . . . . . 54
7.3. Changes from version 10 . . . . . . . . . . . . . . . . . 51 7.3. Changes from version 11 . . . . . . . . . . . . . . . . . 54
7.4. Changes from version 09 . . . . . . . . . . . . . . . . . 52 7.4. Changes from version 10 . . . . . . . . . . . . . . . . . 54
7.5. Changes from version 08 . . . . . . . . . . . . . . . . . 52 7.5. Changes from version 09 . . . . . . . . . . . . . . . . . 55
7.6. Changes from version 04 . . . . . . . . . . . . . . . . . 52 7.6. Changes from version 08 . . . . . . . . . . . . . . . . . 55
7.7. Changes from version 03 . . . . . . . . . . . . . . . . . 52 7.7. Changes from version 04 . . . . . . . . . . . . . . . . . 55
7.8. Changes from version 02 . . . . . . . . . . . . . . . . . 53 7.8. Changes from version 03 . . . . . . . . . . . . . . . . . 56
7.9. Changes from version 01 . . . . . . . . . . . . . . . . . 54 7.9. Changes from version 02 . . . . . . . . . . . . . . . . . 56
7.10. Changes from version 00 . . . . . . . . . . . . . . . . . 54 7.10. Changes from version 01 . . . . . . . . . . . . . . . . . 57
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 55 7.11. Changes from version 00 . . . . . . . . . . . . . . . . . 57
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 56 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 58
9.1. Normative References . . . . . . . . . . . . . . . . . . . 56 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 59
9.2. Informative References . . . . . . . . . . . . . . . . . . 56 9.1. Normative References . . . . . . . . . . . . . . . . . . . 59
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 57 9.2. Informative References . . . . . . . . . . . . . . . . . . 59
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 60
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 The [RFC5939] document 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
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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.
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 RFC2119 [RFC2119] and
indicate requirement levels for compliant RTP implementations. indicate requirement levels for compliant RTP implementations.
"Actual Configuration": An actual configuration specifies which
combinations of SDP session parameters and media stream components
can be used in the current offer/answer exchange and with what
parameters. Use of an actual configuration does not require any
further negotiation in the offer/answer exchange. See [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 [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 Capability": A media encoding, typically a media subtype such "Media Capability": A media format, typically a media subtype such as
as PCMU, H263-1998, or T38. PCMU, H263-1998, or T38. The media capability may also include media
format parameters and associated attributes.
"Potential Configuration": A potential configuration indicates which
combinations of capabilities can be used for the session and its
associated media stream components. Potential configurations are not
ready for use, however they are offered for potential use in the
current offer/answer exchange. They provide an alternative that may
be used instead of the actual configuration, subject to negotiation
in the current offer/answer exchange. See [RFC5939] for further
details.
"Latent Configuration": A latent configuration indicates which
combinations of capabilities could be used in a future negotiation
for the session and its associated media stream components. Latent
configurations are neither ready for use, nor are they offered for
actual or potential use in the current offer/answer exchange. Latent
configurations merely inform the other side of possible
configurations supported by the entity. Those latent configurations
may be used to guide subsequent offer/answer exchanges, but they are
not offered for use as part of the current offer/answer exchange.
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 [RFC5939]; for example they can be
used only at the media level since they are media level attributes. used only at the media level since they are media level attributes.
The [RFC5939] does not provide a way to exchange media-level The [RFC5939] does not provide a way to exchange media-level
capabilities prior to the actual offer of the associated media capabilities prior to the actual offer of the associated media
skipping to change at page 10, line 31 skipping to change at page 12, line 31
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
a=creq:med-v0 a=creq:med-v0
m=audio 3456 RTP/AVP 0 18 m=audio 3456 RTP/AVP 0 18
a=tcap:1 RTP/SAVP RTP/AVP a=tcap:1 RTP/SAVP RTP/AVP
a=rtpmap:0 PCMU/8000/1 a=rtpmap:0 PCMU/8000/1
a=rtpmap:18 G729/8000/1 a=rtpmap:18 G729/8000/1
a=fmtp:18 annexb=yes a=fmtp:18 annexb=yes
a=rmcap:1,4 g729/8000/1 a=rmcap:1,4 G729/8000/1
a=rmcap:2 PCMU/8000/1 a=rmcap:2 PCMU/8000/1
a=rmcap:5 telephone-event/8000 a=rmcap:5 telephone-event/8000
a=mfcap:1 annexb=no a=mfcap:1 annexb=no
a=mfcap:4 annexb=yes a=mfcap:4 annexb=yes
a=mfcap:5 0-11 a=mfcap:5 0-11
a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32 \ a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32 \
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=pcfg:1 m=4,5|1,5 t=1 a=1 pt=1:100,4:101,5:102 a=pcfg:1 m=4,5|1,5 t=1 a=1 pt=1:100,4:101,5:102
a=pcfg:2 m=2 t=1 a=1 pt=2:103 a=pcfg:2 m=2 t=1 a=1 pt=2:103
a=pcfg:3 m=4 t=2 pt=4:18 a=pcfg:3 m=4 t=2 pt=4:18
The required base and extensions are provided by the "a=creq" The required base and extensions are provided by the "a=creq"
attribute defined in [RFC5939], with the option tag "med-v0", which attribute defined in [RFC5939], with the option tag "med-v0", which
indicates that the extension framework defined here, must be indicates that the extension framework defined here, must be
supported. The Base level support is implied since it is required supported. The base level capability negotiation support ("cap-v0"
for the extensions. [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 base protocol, defines The "a=tcap:1" line, specified in the capability negotiation base
transport protocol capabilities, in this case Secure RTP (SAVP protocol [RFC5939], defines transport protocol capabilities, in this
profile) as the first option and RTP (AVP profile) as the second case Secure RTP (SAVP profile) as the first option and RTP (AVP
option. profile) as the second option.
The "a=rmcap:1,4" line defines two G.729 RTP-based media format The "a=rmcap:1,4" line defines two G.729 RTP-based media format
capabilities, numbered 1 and 4, and their encoding rate. The capabilities, numbered 1 and 4, and their encoding rate. The
capabilities are of media type "audio" and subtype G729. Note that capabilities are of media type "audio" and subtype G729. Note that
the media subtype is explicitly specified here, rather than RTP the media subtype is explicitly specified here, rather than RTP
payload type numbers. This permits the assignment of payload type payload type numbers. This permits the assignment of payload type
numbers in the media stream configuration specification. In this numbers in the media stream configuration specification. In this
example, two G.729 subtype capabilities are defined. This permits example, two G.729 subtype capabilities are defined. This permits
the declaration of two sets of formatting parameters for G.729. the declaration of two sets of formatting parameters for G.729.
skipping to change at page 12, line 23 skipping to change at page 14, line 23
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.
The media line, with any qualifying attributes such as fmtp or Per [RFC5939], the media line, with any qualifying attributes such as
rtpmap, is itself considered a valid configuration; it is assumed to fmtp or rtpmap, is itself considered a valid configuration (the
be the lowest preference. current actual configuration); it has the lowest preference (per
[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 15, line 6 skipping to change at page 17, line 7
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. [RFC4566]. In simple terms, it is the name of the media format, e.g.
"t38". This form can also be used in cases such as BFCP [RFC4585] "t38". This form can also be used in cases such as BFCP [RFC4585]
where the fmt list in the m-line is effectively ignored (BFCP uses 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 plus level and/or the media-level. There can be more than one rmcap and
one omcap attribute at the session or media level (i.e more than one more than one omcap attribute at both the session and media level
of each at the session-level and more than one of each in each media (i.e., more than one of each at the session-level and more than one
description). Each media-cap-num MUST be unique within the entire of each in each media description). Each media-cap-num MUST be
SDP record; it is used to identify that media capability in unique within the entire SDP record; it is used to identify that
potential, latent and actual configurations, and in other attribute media capability in potential, latent and actual configurations, and
lines as explained below. Note that the media-cap-num values are in other attribute lines as explained below. Note that the media-
shared between the rmcap and omcap attributes, and hence the cap-num values are shared between the rmcap and omcap attributes, and
uniqueness requirement applies to the union of them. When the media hence the uniqueness requirement applies to the union of them. When
capabilities are used in a potential, latent or actual configuration, the media capabilities are used in a potential, latent or actual
the media formats referred by those configurations apply at the media configuration, the media formats referred by those configurations
level, irrespective of whether the media capabilities themselves were apply at the media level, irrespective of whether the media
specified at the session or media level. In other words, the media capabilities themselves were specified at the session or media level.
capability applies to the specific media description associated with In other words, the media capability applies to the specific media
the configuration which invokes it. description associated with the configuration which invokes it.
For example: For example:
v=0 v=0
o=- 24351 621814 IN IP4 192.0.2.2 o=- 24351 621814 IN IP4 192.0.2.2
s= s=
c=IN IP4 192.0.2.2 c=IN IP4 192.0.2.2
t=0 0 t=0 0
a=rmcap:1 L16/8000/1 a=rmcap:1 L16/8000/1
a=rmcap:2 L16/16000/2 a=rmcap:2 L16/16000/2
a=rmcap:3,4 H263-1998/90000 a=rmcap:3 H263-1998/90000
a=omcap:4 example
m=audio 54320 RTP/AVP 0 m=audio 54320 RTP/AVP 0
a=pcfg:1 m=1|2, pt=1:99,2:98 a=pcfg:1 m=1|2, pt=1:99,2:98
m=video 66544 RTP/AVP 100 m=video 66544 RTP/AVP 100
a=rtpmap:100 H264/90000 a=rtpmap:100 H264/90000
a=pcfg:10 m=3 pt=3:101 a=pcfg:10 m=3 pt=3:101
a=tcap:1 TCP
a=pcfg:11 m=4 t=1
3.3.2. The Media Format Parameter Capability Attribute 3.3.2. The Media Format Parameter Capability Attribute
This attribute is used to associate media format specific format This attribute is used to associate media format specific format
parameters with one or more media capabilities. The form of the parameters with one or more media capabilities. The form of the
attribute is: attribute is:
a=mfcap:<media-caps> <list of parameters> a=mfcap:<media-caps> <list of parameters>
where <media-caps> permits the list of parameters to be associated where <media-caps> permits the list of parameters to be associated
skipping to change at page 17, line 23 skipping to change at page 19, line 26
The "mfcap" attribute can be provided at the session-level and the The "mfcap" attribute can be provided at the session-level and the
media-level. There can be more than one mfcap attribute at the media-level. There can be more than one mfcap attribute at the
session or media level. The unique media-cap-num is used to session or media level. The unique media-cap-num is used to
associate the parameters with a media capability. associate the parameters with a media capability.
As a simple example, a G.729 capability is, by default, considered to As a simple example, a G.729 capability is, by default, considered to
support comfort noise as defined by Annex B. Capabilities for G.729 support comfort noise as defined by Annex B. Capabilities for G.729
with and without comfort noise support may thus be defined by: with and without comfort noise support may thus be defined by:
a=rmcap:1,2 audio G729/8000 a=rmcap:1,2 G729/8000
a=mfcap:2 annexb:no a=mfcap:2 annexb:no
Media format capability 1 supports G.729 with Annex B, whereas media Media format capability 1 supports G.729 with Annex B, whereas media
format capability 2 supports G.729 without Annex B. format capability 2 supports G.729 without Annex B.
Example for H.263 video: Example for H.263 video:
a=rmcap:1 video H263-1998/90000 a=rmcap:1 H263-1998/90000
a=rmcap:2 video H263-2000/90000 a=rmcap:2 H263-2000/90000
a=mfcap:1 CIF=4;QCIF=2;F=1;K=1 a=mfcap:1 CIF=4;QCIF=2;F=1;K=1
a=mfcap:2 profile=2;level=2.2 a=mfcap:2 profile=2;level=2.2
Finally, for six format combinations of the Adaptive MultiRate codec: Finally, for six format combinations of the Adaptive MultiRate codec:
a=rmcap:1-3 AMR/8000/1 a=rmcap:1-3 AMR/8000/1
a=rmcap:4-6 AMR-WB/16000/1 a=rmcap:4-6 AMR-WB/16000/1
a=mfcap:1,2,3,4 mode-change-capability=1 a=mfcap:1,2,3,4 mode-change-capability=1
a=mfcap:5,6 mode-change-capability=2 a=mfcap:5,6 mode-change-capability=2
a=mfcap:1,2,3,5 max-red=220 a=mfcap:1,2,3,5 max-red=220
skipping to change at page 18, line 28 skipping to change at page 20, line 34
a=fmtp:99 mode-change-capability=1; octet-align=1; \ a=fmtp:99 mode-change-capability=1; octet-align=1; \
mode-set=0,3,5,6 mode-set=0,3,5,6
and so on for the other four combinations. SDP could thus convert and so on for the other four combinations. SDP could thus convert
the media capabilities specifications into one or more alternative the media capabilities specifications into one or more alternative
media stream specifications, one of which can be chosen for the media stream specifications, one of which can be chosen for the
answer. answer.
3.3.3. The Media-Specific Capability Attribute 3.3.3. The Media-Specific Capability Attribute
Media-specific attributes, beyond the rtpmap and fmtp attributes, may Attributes and parameters associated with a media format are
be associated with media capability numbers via a new media-specific typically specified using the "rtpmap" and "fmtp" attributes in SDP,
attribute, mscap, of the following form: and the similar "rmcap" and "mfcap" attributes in SDP Media
Capabilities. Some SDP extensions define other attributes that need
to be associated with media formats, for example the "rtcp-fb"
attribute defined in [RFC4585]. Such media-specific attributes,
beyond the rtpmap and fmtp attributes, may be associated with media
capability numbers via a new media-specific attribute, mscap, of the
following form:
a=mscap:<media caps star> <att field> <att value> a=mscap:<media caps star> <att field> <att value>
where <media caps star> is a (list of) media capability number(s), where <media caps star> is a (list of) media capability number(s),
<att field> is the attribute name, and <att value> is the value field <att field> is the attribute name, and <att value> is the value field
for the named attribute. Note that the media capability numbers for the named attribute. Note that the media capability numbers
refer to media capabilities specified elsewhere in the SDP ("rmcap" refer to media capabilities specified elsewhere in the SDP ("rmcap"
and/or "omcap"). The media capability numbers may include a wildcard and/or "omcap"). The media capability numbers may include a wildcard
("*"), which will be used instead of any payload type mappings. In ("*"), which will be used instead of any payload type mappings in the
ABNF, we have: resulting SDP (see, e.g. [RFC4585] and the example below). In ABNF,
we have:
media-specific-capability = "a=mscap:" media-specific-capability = "a=mscap:"
media-caps-star media-caps-star
1*WSP att-field ; from RFC4566 1*WSP att-field ; from 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 an lcfg or pcfg number as specified by the pt= parameters in a pcfg attribute line, a
attribute line, a mscap line may be translated easily into a mscap line may be translated easily into a conventional SDP attribute
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 [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
skipping to change at page 21, line 4 skipping to change at page 23, line 19
payload type number to be associated with each media type in a payload type number to be associated with each media type in a
potential, actual, or latent configuration. We define the payload potential, actual, or latent configuration. We define the payload
type number mapping parameter, payload-number-config-list, in 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
[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 desribed in RFC5939. The use of the plus sign ("+") is described in [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 22, line 28 skipping to change at page 24, line 43
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. The media type (mt=) and the transport protocol(s)
(t=) MUST be specified since the latent configuration is independent (t=) MUST be specified since the latent configuration is independent
of any media line present. In most cases, the media configuration of any media line present. In most cases, the media configuration
(m=) parameter MUST be present as well (see Section 4 for examples). (m=) parameter MUST be present as well (see Section 4 for examples).
The lcfg attribute is a media level attribute. The lcfg attribute is a media level attribute.
The lcfg attribute is defined as a media level attribute since it
specifies a possible future media stream. However the lcfg
attribute is not necessarily related to the media description
within which it is provided. Session capabilities ("sescap") may
be used to indicate this.
Each media line in an SDP description represents an offered 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:
skipping to change at page 25, line 4 skipping to change at page 27, line 25
reply MAY also contain potential and/or latent configuration reply MAY also contain potential and/or latent configuration
attributes, with parameters, to indicate which other offered attributes, with parameters, to indicate which other offered
configurations would be acceptable. This information is useful if it configurations would be acceptable. This information is useful if it
becomes desirable to reconfigure a media stream, e.g., to reduce becomes desirable to reconfigure a media stream, e.g., to reduce
resource consumption. resource consumption.
When potential and/or latent configurations are returned in an When potential and/or latent configurations are returned in an
answer, all numbering MUST refer to the configuration and capability answer, all numbering MUST refer to the configuration and capability
attribute numbering of the offer. The offered capability attributes attribute numbering of the offer. The offered capability attributes
need not be returned in the answer. The answer MAY include need not be returned in the answer. The answer MAY include
additional capability attributes and/or configuratons (with distinct additional capability attributes and/or configurations (with distinct
numbering). The parameter values of any returned pcfg or lcfg numbering). The parameter values of any returned pcfg or lcfg
attributes MUST be a subset of those included in the offered attributes MUST be a subset of those included in the offered
configurations and/or those added by the answerer; values may be configurations and/or those added by the answerer; values MAY be
omitted only if they were indicated as alternative sets, or optional, omitted only if they were indicated as alternative sets, or optional,
in the original offer. The parameter set indicated in the returned in the original offer. The parameter set indicated in the returned
acfg attribute need not be repeated in a returned pcfg attribute. acfg attribute need not be repeated in a returned pcfg attribute.
The answerer may return more than one pcfg attribute with the same The answerer MAY return more than one pcfg attribute with the same
configuration number if it is necessary to describe selected configuration number if it is necessary to describe selected
combinations of optional or alternative parameters. combinations of optional or alternative parameters.
Similarly, one or more session capability attributes (a=sescap) may Similarly, one or more session capability attributes (a=sescap) MAY
be returned to indicate which of the offered session capabilities is/ be returned to indicate which of the offered session capabilities is/
are supportable by the answerer (see Section 3.3.8.) are supportable by the answerer (see Section 3.3.8.)
Note that, although the answerer MAY return capabilities beyond those Note that, although the answerer MAY return capabilities beyond those
included by the offerer, these capabilities MUST NOT be used to form included by the offerer, these capabilities MUST NOT be used to form
any base level media description in the answer. For this reason, it any base level media description in the answer. For this reason, it
is advisable for the offerer to include most, if not all, potential is advisable for the offerer to include most, if not all, potential
and latent configurations it can support in the initial offer, unless and latent configurations it can support in the initial offer, unless
the size of the resulting SDP is a concern. Either party MAY later the size of the resulting SDP is a concern. Either party MAY later
announce additional capabilities by renegotiating the session in a announce additional capabilities by renegotiating the session in a
skipping to change at page 25, line 39 skipping to change at page 28, line 11
When media capabilities defined in rmcap attributes are used in When media capabilities defined in rmcap attributes are used in
potential configuration lines, the transport protocol uses RTP and it potential configuration lines, the transport protocol uses RTP and it
is necessary to assign payload type numbers. In some cases, it is is necessary to assign payload type numbers. In some cases, it is
desirable to assign different payload type numbers to the same media desirable to assign different payload type numbers to the same media
capability when used in different potential configurations. One capability when used in different potential configurations. One
example is when configurations for AVP and SAVP are offered: the example is when configurations for AVP and SAVP are offered: the
offerer would like the answerer to use different payload type numbers offerer would like the answerer to use different payload type numbers
for encrypted and unencrypted media so that it (the offerer) can for encrypted and unencrypted media so that it (the offerer) can
decide whether or not to render early media which arrives before the decide whether or not to render early media which arrives before the
answer is received. This association of distinct payload type answer is received.
number(s) with different transport protocols requires a separate pcfg
line for each protocol. Clearly, this technique cannot be used if For example, if use of AVP was selected by the answerer, then
the number of potential configurations exceeds the number of possible media received by the offerer is not encrypted and hence can be
payload type numbers. played out prior to receiving the answer. Conversely, if SAVP was
selected, cryptographic parameters and keying material present in
the answer may be needed to decrypt received media. If the offer
configuration indicated that AVP media uses one set of payload
types and SAVP a different set, then the offerer will know whether
media received prior to the answer is encrypted or not by simply
looking at the RTP payload type number in the received packet.
This association of distinct payload type number(s) with different
transport protocols requires a separate pcfg line for each protocol.
Clearly, this technique cannot be used if the number of potential
configurations exceeds the number of possible payload type numbers.
3.3.6.3. Processing of Media-Format-Related Conventional Attributes for 3.3.6.3. Processing of Media-Format-Related Conventional Attributes for
Potential Configurations Potential Configurations
In cases in which media capabilities negotiation is employed, SDP In cases in which media capabilities negotiation is employed, SDP
records are likely to contain conventional attributes such as rtpmap, records are likely to contain conventional attributes such as rtpmap,
fmtp, and other media-format-related lines, as well as capability fmtp, and other media-format-related lines, as well as capability
attributes such as rmcap, omcap, mfcap, and mscap which map into attributes such as rmcap, omcap, mfcap, and mscap which map into
those conventional attributes when invoked by a potential those conventional attributes when invoked by a potential
configuration. In such cases, it MAY be appropriate to employ the configuration. In such cases, it MAY be appropriate to employ the
skipping to change at page 26, line 24 skipping to change at page 29, line 7
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-events
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-events/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
skipping to change at page 26, line 50 skipping to change at page 29, line 33
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 6543 RTP/AVP 18 100 m=audio 3456 RTP/AVP 18 100
a=rtpmap:100 telephone-events/8000 a=rtpmap:100 telephone-events/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 redundancy audio subtype In some cases, for example, when an RFC 2198 [RFC2198] redundancy
(RED) capability is defined in an mfcap attribute, the parameters to audio subtype (RED) capability is defined in an mfcap attribute, the
an attribute may contain payload type numbers. Two options are parameters to an attribute may contain payload type numbers. Two
available for specifying such payload type numbers. They may be options are available for specifying such payload type numbers. They
expressed explicitly, in which case they are bound to actual payload may be expressed explicitly, in which case they are bound to actual
types by means of the payload type number parameter (pt=) in the payload types by means of the payload type number parameter (pt=) in
appropriate potential or latent configuration. For example, the the appropriate potential or latent configuration. For example, the
following SDP fragment defines a potential configuration with following SDP fragment defines a potential configuration with
redundant G.711 mu-law: redundant G.711 mu-law:
m=audio 45678 RTP/AVP 0 m=audio 45678 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
a=rmcap:1 PCMU/8000 a=rmcap:1 PCMU/8000
a=rmcap:2 RED/8000 a=rmcap:2 RED/8000
a=mfcap:2 0/0 a=mfcap:2 0/0
a=pcfg:1 m=2,1 pt=2:98,1:0 a=pcfg:1 m=2,1 pt=2:98,1:0
skipping to change at page 28, line 15 skipping to change at page 30, line 46
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
a=rmcap:1 PCMU/8000 a=rmcap:1 PCMU/8000
a=rmcap:2 RED/8000 a=rmcap:2 RED/8000
a=mfcap:2 %m=1%/%m=1% a=mfcap:2 %m=1%/%m=1%
a=pcfg:1 m=2,1 pt=2:98,1:0 a=pcfg:1 m=2,1 pt=2:98,1:0
and the equivalent SDP is the same as above. and the equivalent SDP is the same as above.
3.3.8. The Session Capability Attribute 3.3.8. The Session Capability Attribute
Potential and latent configurations enable offerers and answerers to
express a wide range of alternative configurations for current and
future negotiation. However in practice, it may not be possible to
support all combinations of these configurations.
The session capability attribute provides a means for the offerer The session capability attribute provides a means for the offerer
and/or the answerer to specify combinations of specific media stream and/or the answerer to specify combinations of specific media stream
configurations which it is willing and able to support. Each session configurations which it is willing and able to support. Each session
capability in an offer or answer MAY be expressed as a list of capability in an offer or answer MAY be expressed as a list of
required potential configurations, and MAY include a list of optional required potential configurations, and MAY include a list of optional
potential and/or latent configurations. potential and/or latent configurations.
The choices of session capabilities may be based on processing load, The choices of session capabilities may be based on processing load,
total bandwidth, or any other criteria of importance to the total bandwidth, or any other criteria of importance to the
communicating parties. If the answerer supports media capabilities communicating parties. If the answerer supports media capabilities
skipping to change at page 29, line 13 skipping to change at page 31, line 48
that the session preference order, when present, takes precedence that the session preference order, when present, takes precedence
over the individual media stream configuration preference order. 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; [RFC5939] requires only that pcfg
configuration numbers be unique within a media description. 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. The SDP offer might look like streams with a floor control stream. In the latter case, the second
the following (offering audio, two H.263 video streams and BFCP)- the video stream is optional. The SDP offer might look like the
empty lines are added for readability only (not part of valid SDP): following (offering audio, an H.263 video streams, BFCP and another
optional H.263 video stream)- the empty lines are added for
readability only (not part of valid SDP):
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
a=creq:med-v0 a=creq:med-v0
a=sescap:2 1,2,3,5 a=sescap:2 1,2,5,[3]
a=sescap:1 1,4 a=sescap:1 1,4
m=audio 54322 RTP/AVP 0 m=audio 54322 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
a=pcfg:1 a=pcfg:1
m=video 22344 RTP/AVP 102 m=video 22344 RTP/AVP 102
a=rtpmap:102 H263-1998/90000 a=rtpmap:102 H263-1998/90000
a=fmtp:102 CIF=4;QCIF=2;F=1;K=1 a=fmtp:102 CIF=4;QCIF=2;F=1;K=1
i=main video stream i=main video stream
skipping to change at page 30, line 26 skipping to change at page 33, line 18
c=IN IP4 192.0.2.22 c=IN IP4 192.0.2.22
t=0 0 t=0 0
a=csup:med-v0 a=csup:med-v0
a=sescap:1 1,4 a=sescap:1 1,4
m=audio 23456 RTP/AVP 0 m=audio 23456 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
a=acfg:1 a=acfg:1
m=video 41234 RTP/AVP 104 m=video 41234 RTP/AVP 104
a=rtpmap:100 H264/90000 a=rtpmap:104 H264/90000
a=fmtp:104 profile-level-id=42A01E; packetization-mode=2 a=fmtp:104 profile-level-id=42A01E; packetization-mode=2
a=acfg:4 m=1 a=1 pt=1:104 a=acfg:4 m=1 a=1 pt=1:104
m=video 0 RTP/AVP 103 m=video 0 RTP/AVP 103
a=acfg:3 a=acfg:3
m=application 0 TCP/BFCP * m=application 0 TCP/BFCP *
a=acfg:5 a=acfg:5
An endpoint that doesn't support Media capabilities negotiation, but An endpoint that doesn't support Media capabilities negotiation, but
skipping to change at page 31, line 19 skipping to change at page 34, line 7
t=0 0 t=0 0
a=creq:med-v0 a=creq:med-v0
a=sescap:1 1,3,4,5 a=sescap:1 1,3,4,5
a=sescap:2 1,2 a=sescap:2 1,2
a=sescap:3 1 a=sescap:3 1
a=rmcap:1 H263-1998/90000 a=rmcap:1 H263-1998/90000
a=mfcap:1 CIF=4;QCIF=2;F=1;K=1 a=mfcap:1 CIF=4;QCIF=2;F=1;K=1
a=tcap:1 RTP/AVP TCP/BFCP a=tcap:1 RTP/AVP TCP/BFCP
m=audio 54322 RTP/AVP 0 m=audio 54322 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
a=label:11
a=pcfg:1 a=pcfg:1
m=video 22344 RTP/AVP 102 m=video 22344 RTP/AVP 102
a=rtpmap:102 H264/90000 a=rtpmap:102 H264/90000
a=fmtp:102 profile-level-id=42A01E; packetization-mode=2 a=fmtp:102 profile-level-id=42A01E; packetization-mode=2
a=label:11 a=label:11
a=content:main a=content:main
a=pcfg:2 a=pcfg:2
a=lcfg:3 mt=video t=1 m=1 a=31,32 a=lcfg:3 mt=video t=1 m=1 a=31,32
a=acap:31 label:12 a=acap:31 label:12
a=acap:32 content:main a=acap:32 content:main
a=lcfg:4 mt=video t=1 m=1 a=41,42 a=lcfg:4 mt=video t=1 m=1 a=41,42
a=acap:41 label:13 a=acap:41 label:13
a=acap:42 content:slides a=acap:42 content:slides
a=lcfg:5 mt=application m=51 t=51 a=lcfg:5 mt=application m=51 t=51
a=tcap:51 TCP/BFCP a=tcap:51 TCP/BFCP
a=rmcap:51 * a=omcap:51 *
a=acap:51 setup:passive a=acap:51 setup:passive
a=acap:52 connection:new a=acap:52 connection:new
a=acap:53 floorid:1 m-stream:12 13 a=acap:53 floorid:1 m-stream:12 13
a=acap:54 floor-control:s-only a=acap:54 floor-control:s-only
a=acap:55 confid:4321 a=acap:55 confid:4321
a=acap:56 userid:1234 a=acap:56 userid:1234
In this example, the default offer, as seen by endpoints which do not In this example, the default offer, as seen by endpoints which do not
understand capabilities negotiation, proposes a PCMU audio stream and understand capabilities negotiation, proposes a PCMU audio stream and
an H.264 video stream. Note that the offered lcfg lines for the an H.264 video stream. Note that the offered lcfg lines for the
skipping to change at page 33, line 24 skipping to change at page 36, line 13
associated parameters associated parameters
o Latent media streams (lcfg attribute) o Latent media streams (lcfg attribute)
o Acceptable combinations of media stream configurations (sescap o Acceptable combinations of media stream configurations (sescap
attribute). attribute).
The high-level description of the operation is as follows: The high-level description of the operation is as follows:
When an endpoint generates an initial offer and wants to use the When an endpoint generates an initial offer and wants to use the
functionality described in the current document, it should identify functionality described in the current document, it SHOULD identify
and define the media formats and associated parameters it can support and define the media formats and associated parameters it can support
via the rmcap, omcap, mfcap and mscap attributes. The SDP media via the rmcap, omcap, mfcap and mscap attributes. The SDP media
line(s) ("m=") should be made up with the actual configuration to be line(s) ("m=") should be made up with the actual configuration to be
used if the other party does not understand capability negotiations used if the other party does not understand capability negotiations
(by default, this is the least preferred configuration). Typically, (by default, this is the least preferred configuration). Typically,
the media line configuration will contain the minimum acceptable the media line configuration will contain the minimum acceptable
configuration from the offerer's point of view. configuration from the offerer's point of view.
Preferred configurations for each media stream are identified Preferred configurations for each media stream are identified
following the media line. The present offer may also include latent following the media line. The present offer may also include latent
skipping to change at page 35, line 19 skipping to change at page 38, line 7
overlap with any config-number used by a latent configuration in the overlap with any config-number used by a latent configuration in the
SDP. As described in [RFC5939], lower config-numbers indicate a SDP. As described in [RFC5939], lower config-numbers indicate a
higher preference; the ordering still applies within a given media higher preference; the ordering still applies within a given media
description only though. 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 parameters 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
(e.g. t38). For RTP-based media, payload type mappings MUST be (e.g. t38). For RTP-based media, payload type mappings MUST be
provided by use of the "pt" parameter in the potential configuration provided by use of the "pt" parameter in the potential configuration
(see Section 3.3.4.2); payload type escaping may be used in mfcap, (see Section 3.3.4.2); payload type escaping may be used in mfcap,
mscap, and acap attributes as defined in Section 3.3.7. mscap, and acap attributes as defined in Section 3.3.7.
Note that the "mt" parameter MUST NOT be used with the pcfg Note that the "mt" parameter MUST NOT be used with the pcfg attribute
attribute; the media type in a potential configuration cannot be (since it is defined for the lcfg attribute only); the media type in
changed from that of the encompassing media description. a potential configuration cannot be changed from that of the
encompassing media description.
3.4.1.2. Offer with Latent Configuration 3.4.1.2. Offer with Latent Configuration
If the offerer wishes to offer one or more latent configurations for If the offerer wishes to offer one or more latent configurations for
future use, the offer MUST include a latent configuration attribute future use, the offer MUST include a latent configuration attribute
(lcfg) for each as defined in Section 3.3.5. (lcfg) for each as defined in Section 3.3.5.
Each lcfg attribute Each lcfg attribute
o MUST be specified at the media level o MUST be specified at the media level
skipping to change at page 36, line 41 skipping to change at page 39, line 30
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 [RFC5939].
The SDP may contain The SDP MAY contain
o Media capabilities and associated parameters (rmcap, omcap, mfcap, o Media capabilities and associated parameters (rmcap, omcap, mfcap,
and mscap attributes) and mscap attributes)
o Potential configurations using those media capabilities and o Potential configurations using those media capabilities and
associated parameters associated parameters
o Latent media streams (lcfg attribute) o Latent media streams (lcfg attribute)
o Acceptable combinations of media stream configurations (sescap o Acceptable combinations of media stream configurations (sescap
attribute) attribute)
The high-level description of the operation is as follows: The high-level informative description of the operation is as
follows:
When the answering party receives the offer and if it supports the When the answering party receives the offer and if it supports the
required capability negotiation extensions, it should select the required capability negotiation extensions, it should select the
most-preferred configuration it can support for each media stream, most-preferred configuration it can support for each media stream,
and build its answer accordingly. The configuration selected for and build its answer accordingly. The configuration selected for
each accepted media stream is placed into the answer as a media line each accepted media stream is placed into the answer as a media line
with associated parameters and attributes. If a proposed with associated parameters and attributes. If a proposed
configuration is chosen, the answer must include the supported configuration is chosen for a given media stream, the answer must
extension attribute and each media stream for which a proposed contain an actual configuration (acfg) attribute for that media
configuration was chosen must contain an actual configuration (acfg) stream to indicate which offered pcfg attribute was used to build the
attribute to indicate just which pcfg attribute was used to build the
answer. The answer should also include any potential or latent answer. The answer should also include any potential or latent
configurations the answerer can support, especially any configurations the answerer can support, especially any
configurations compatible with other potential or latent configurations compatible with other potential or latent
configurations received in the offer. The answerer should make note configurations received in the offer. The answerer should make note
of those configurations it might wish to offer in the future. of those configurations it might wish to offer in the future.
Below we provide a more detailed description of how the answerer Below we provide a more detailed normative description of how the
processes the offer SDP and generates an answer SDP. answerer processes the offer SDP and generates an answer SDP.
3.4.2.1. Processing Media Capabilities and Potential Configurations 3.4.2.1. Processing Media Capabilities and Potential Configurations
The answerer MUST first determine if it needs to perform media The answerer MUST first determine if it needs to perform media
capability negotiation by examining the SDP for valid and preferred capability negotiation by examining the SDP for valid and preferred
potential configuration attributes that include media configuration potential configuration attributes that include media configuration
parameters (i.e. an "m" parameter in the pcfg attribute). parameters (i.e. an "m" parameter in the pcfg attribute).
Such a potential configuration is valid if: Such a potential configuration is valid if:
skipping to change at page 38, line 28 skipping to change at page 41, line 16
being preferred over a higher one. If a valid "sescap" attribute is being preferred over a higher one. If a valid "sescap" attribute is
present, the preference order provided in the "sescap" attribute MUST present, the preference order provided in the "sescap" attribute MUST
take precedence. A "sescap" attribute is considered valid if: 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 each media stream based on The answerer MUST now process the offer for each media stream based
the most preferred valid potential configuration in accordance with on the most preferred valid potential configuration in accordance
the procedures specified in [RFC5939], Section 3.6.2, and further with the procedures specified in [RFC5939], Section 3.6.2, and
extended below: 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 41, line 10 skipping to change at page 43, line 44
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
may do so via the mechanisms defined for offer/answer [RFC3264]. If can do so via the mechanisms defined for offer/answer [RFC3264]. If
the initiating party has remembered the codecs, potential the initiating party has remembered the codecs, potential
configurations, latent configurations and session capabilities configurations, latent configurations and session capabilities
provided by the other party in the earlier negotiation, it may use provided by the other party in the earlier negotiation, it MAY use
this knowledge to maximize the likelihood of a successful this knowledge to maximize the likelihood of a successful
modification of the session. Alternatively, the initiator may modification of the session. Alternatively, the initiator MAY
perform a new capabilities exchange as part of the reconfiguration. perform a new capabilities exchange as part of the reconfiguration.
In such a case, the new capabilities will replace the previously- In such a case, the new capabilities will replace the previously-
negotiated capabilities. This may be useful if conditions change on negotiated capabilities. This may be useful if conditions change on
the endpoint. the endpoint.
4. Examples 4. Examples
In this section, we provide examples showing how to use the Media In this section, we provide examples showing how to use the Media
Capabilities with the SDP Capability Negotiation. Capabilities with the SDP Capability Negotiation.
4.1. Alternative Codecs 4.1. Alternative Codecs
This example provide a choice of one of six variations of the This example provides a choice of one of six variations of the
adaptive multirate codec. In this example, the default configuration adaptive multirate codec. In this example, the default configuration
as specified by the media line is the same as the most preferred as specified by the media line is the same as the most preferred
configuration. Each configuration uses a different payload type configuration. Each configuration uses a different payload type
number so the offerer can interpret early media. number so the offerer can interpret early media.
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
skipping to change at page 45, line 47 skipping to change at page 48, line 47
well) is G.729B with H.263. This overrides the individual media well) is G.729B with H.263. This overrides the individual media
stream preferences which are PCMU and H.264 by the potential stream preferences which are PCMU and H.264 by the potential
configuration numbering rule. configuration numbering rule.
4.3. Latent Media Streams 4.3. Latent Media Streams
Consider a case in which the offerer can support either G.711 mu-law, Consider a case in which the offerer can support either G.711 mu-law,
or G.729B, along with DTMF telephony events for the 12 common or G.729B, along with DTMF telephony events for the 12 common
touchtone signals, but is willing to support simple G.711 mu-law touchtone signals, but is willing to support simple G.711 mu-law
audio as a last resort. In addition, the offerer wishes to announce audio as a last resort. In addition, the offerer wishes to announce
its ability to support video in the future, but does not wish to its ability to support video and MSRP in the future, but does not
offer a video stream at present. The offer might look like the wish to offer a video stream or an MSRP stream at present. The offer
following: might look like the following:
v=0 v=0
o=- 25678 753849 IN IP4 192.0.2.1 o=- 25678 753849 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
a=creq:med-v0 a=creq:med-v0
m=audio 23456 RTP/AVP 0 m=audio 23456 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
a=rmcap:1 PCMU/8000 a=rmcap:1 PCMU/8000
a=rmcap:2 g729/8000 a=rmcap:2 G729/8000
a=rmcap:3 telephone-event/8000 a=rmcap:3 telephone-event/8000
a=mfcap:3 0-11 a=mfcap:3 0-11
a=pcfg:1 m=1,3|2,3 pt=1:0,2:18,3:100 a=pcfg:1 m=1,3|2,3 pt=1:0,2:18,3:100
a=lcfg:2 mt=video t=1 m=10|11 a=lcfg:2 mt=video t=1 m=10|11
a=rmcap:10 H263-1998/90000 a=rmcap:10 H263-1998/90000
a=rmcap:11 H264/90000 a=rmcap:11 H264/90000
a=tcap:1 RTP/AVP a=tcap:1 RTP/AVP
a=lcfg:3 mt=message t=2 m=20
a=tcap:2 TCP/MSRP
a=omcap:20 *
The lcfg attribute line announces support for H.263 and H.264 video The first lcfg attribute line ("lcfg:2") announces support for H.263
(H.263 preferred) for future reference. The m-line and the rtpmap and H.264 video (H.263 preferred) for future negotiation. The second
attribute offer an audio stream and provide the lowest precedence lcfg attribute line ("lcfg:3") announces support for MSRP for future
configuration (PCMU without any DTMF encoding). The rmcap lines negotiation. The m-line and the rtpmap attribute offer an audio
define the media capabilities (PCMU, G729, and telephone-event) to be stream and provide the lowest precedence configuration (PCMU without
offered in potential configurations. The mfcap attribute provides any DTMF encoding). The rmcap lines define the RTP-based media
the format parameters for telephone-events, specifying the 12 capabilities (PCMU, G729, telephone-event, H263-1998 and H264) and
commercial DTMF 'digits'. The pcfg attribute line defines the most- the omcap line defines the non-RTP based media capability (wildcard).
preferred media configuration as PCMU plus DTMF events and the next- The mfcap attribute provides the format parameters for telephone-
most-preferred configuration as G.729B plus DTMF events. events, specifying the 12 commercial DTMF 'digits'. The pcfg
attribute line defines the most-preferred media configuration as PCMU
plus DTMF events and the next-most-preferred configuration as G.729B
plus DTMF events.
If the answerer is able to support all the potential configurations, 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
s= s=
c=IN IP4 192.0.2.2 c=IN IP4 192.0.2.2
t=0 0 t=0 0
skipping to change at page 50, line 23 skipping to change at page 53, line 23
the other side about capabilities supported for further offer/answer the other side about capabilities supported for further offer/answer
exchanges, including transport protocols and attribute capabilities, exchanges, including transport protocols and attribute capabilities,
e.g. for keying methods. If an attacker can remove or alter latent e.g. for keying methods. If an attacker can remove or alter latent
configuration information to suggest that only insecure or less configuration information to suggest that only insecure or less
secure alternatives are supported, then he may be able to force secure alternatives are supported, then he may be able to force
negotiation of a less secure session than would otherwise have negotiation of a less secure session than would otherwise have
occurred. While the specific attack as described here differs from occurred. While the specific attack as described here differs from
those described in [RFC5939], the considerations and mitigation those described in [RFC5939], the considerations and mitigation
strategies are similar to those described in [RFC5939]. strategies are similar to those described in [RFC5939].
Another variation on the above attack involves the the Session Another variation on the above attack involves the Session Capability
Capability ("sescap") attribute defined in this document. The ("sescap") attribute defined in this document. The "sescap" enables
"sescap" enables a preference order to be specified for all the a preference order to be specified for all the potential
potential configurations, and that preference will take precedence configurations, and that preference will take precedence over any
over any preference indication provided in individual potential preference indication provided in individual potential configuration
configuration attributes. Consequently, an attacker that can insert attributes. Consequently, an attacker that can insert or modify a
or modify a "sescap" attribute may be able to force negotiation of an "sescap" attribute may be able to force negotiation of an insecure or
insecure or less secure alternative than would otherwise have less secure alternative than would otherwise have occurred. Again,
occurred. Again, the considerations and mitigation strategies are the considerations and mitigation strategies are similar to those
similar to those described in [RFC5939]. described in [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 CapNeg work [RFC5939], the SDP answer is
formulated in such a way that it always carries the selected media formulated in such a way that it always carries the selected media
encoding for every media stream selected. Pending an understanding encoding for every media stream selected. Pending an understanding
of capabilities negotiation, the middlebox should examine the answer of capabilities negotiation, the middlebox should examine the answer
SDP to obtain the best picture of the media streams being SDP to obtain the best picture of the media streams being
established. As always, middleboxes can best do their job if they established. As always, middleboxes can best do their job if they
fully understand media capabilities negotiation. fully understand media capabilities negotiation.
7. Changes from previous versions 7. Changes from previous versions
7.1. Changes from version 12 7.1. Changes from version 13
o Various editorial clarifications and updates to address review
comments.
7.2. 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.
7.2. Changes from version 11 7.3. 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"
7.3. Changes from version 10 7.4. 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 52, line 12 skipping to change at page 55, line 17
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.
7.4. Changes from version 09 7.5. 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 crossreferences. o updated cross-references.
7.5. Changes from version 08 7.6. 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.
7.6. Changes from version 04 7.7. 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 [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.
7.7. Changes from version 03 7.8. 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 53, line 33 skipping to change at page 56, line 38
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.
7.8. Changes from version 02 7.9. 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 54, line 14 skipping to change at page 57, line 19
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.
7.9. Changes from version 01 7.10. 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 parametr 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 terminolgy 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.
7.10. Changes from version 00 7.11. 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 explictly definesd 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
in particular provided useful comments and suggestions to either the in particular provided useful comments and suggestions to either the
document itself or the overall direction of the solution defined document itself or the overall direction of the solution defined
herein: Cullen Jennings, Matt Lepinski, Joerg Ott, Colin Perkins, and herein: Cullen Jennings, Matt Lepinski, Joerg Ott, Colin Perkins, and
Thomas Stach. Thomas Stach.
We thank Ingemar Johansson and Magnus Westerlund for examples that We thank Ingemar Johansson and Magnus Westerlund for examples that
stimulated this work, and for critical reading of the document. stimulated this work, and for critical reading of the document. We
also thank Cullen Jennings, Christer Holmberg, and Miguel Garcia for
their review of the document.
9. References 9. References
9.1. Normative References 9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, with Session Description Protocol (SDP)", RFC 3264,
skipping to change at page 56, line 27 skipping to change at page 59, line 27
Description Protocol", RFC 4566, July 2006. Description Protocol", RFC 4566, July 2006.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008. Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5939] Andreasen, F., "SDP Capability Negotiation", RFC 5939, [RFC5939] Andreasen, F., "SDP Capability Negotiation", RFC 5939,
September 2010. September 2010.
9.2. Informative References 9.2. Informative References
[RFC2198] "RTP Payload for Redundant Audio Data", September 1997.
[RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session [RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol (SDP) Security Descriptions for Media Description Protocol (SDP) Security Descriptions for Media
Streams", RFC 4568, July 2006. Streams", RFC 4568, July 2006.
[RFC4583] Camarillo, G., "Session Description Protocol (SDP) Format
for Binary Floor Control Protocol (BFCP) Streams",
RFC 4583, November 2006.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control "Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
July 2006. July 2006.
[RFC4733] Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF [RFC4733] Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF
Digits, Telephony Tones, and Telephony Signals", RFC 4733, Digits, Telephony Tones, and Telephony Signals", RFC 4733,
December 2006. December 2006.
[RFC4867] Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie, [RFC4867] Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie,
 End of changes. 72 change blocks. 
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