draft-ietf-mmusic-sdp-media-capabilities-12.txt   draft-ietf-mmusic-sdp-media-capabilities-13.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: May 3, 2012 Gesher Erove Ltd Expires: September 13, 2012 Gesher Erove Ltd
F. Andreasen F. Andreasen
Cisco Systems Cisco Systems
October 31, 2011 March 12, 2012
SDP Media Mapabilities Negotiation SDP Media Capabilities Negotiation
draft-ietf-mmusic-sdp-media-capabilities-12 draft-ietf-mmusic-sdp-media-capabilities-13
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 May 3, 2012. This Internet-Draft will expire on September 13, 2012.
Copyright Notice Copyright Notice
Copyright (c) 2011 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.
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the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
skipping to change at page 3, line 23 skipping to change at page 3, line 23
3.3.1. The Media Format Capability Attributes . . . . . . . . 13 3.3.1. The Media Format Capability Attributes . . . . . . . . 13
3.3.2. The Media Format Parameter Capability Attribute . . . 15 3.3.2. The Media Format Parameter Capability Attribute . . . 15
3.3.3. The Media-Specific Capability Attribute . . . . . . . 18 3.3.3. The Media-Specific Capability Attribute . . . . . . . 18
3.3.4. New Configuration Parameters . . . . . . . . . . . . . 20 3.3.4. New Configuration Parameters . . . . . . . . . . . . . 20
3.3.5. The Latent Configuration Attribute . . . . . . . . . . 21 3.3.5. The Latent Configuration Attribute . . . . . . . . . . 21
3.3.6. Enhanced Potential Configuration Attribute . . . . . . 24 3.3.6. Enhanced Potential Configuration Attribute . . . . . . 24
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 . . . . . . . . . . . 27
3.3.8. The Session Capability Attribute . . . . . . . . . . . 28 3.3.8. The Session Capability Attribute . . . . . . . . . . . 28
3.4. Offer/Answer Model Extensions . . . . . . . . . . . . . . 32 3.4. Offer/Answer Model Extensions . . . . . . . . . . . . . . 32
3.4.1. Generating the Initial Offer . . . . . . . . . . . . . 32 3.4.1. Generating the Initial Offer . . . . . . . . . . . . . 33
3.4.2. Generating the Answer . . . . . . . . . . . . . . . . 33 3.4.2. Generating the Answer . . . . . . . . . . . . . . . . 36
3.4.3. Offerer Processing of the Answer . . . . . . . . . . . 33 3.4.3. Offerer Processing of the Answer . . . . . . . . . . . 40
3.4.4. Modifying the Session . . . . . . . . . . . . . . . . 34 3.4.4. Modifying the Session . . . . . . . . . . . . . . . . 41
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.1. Alternative Codecs . . . . . . . . . . . . . . . . . . . . 35 4.1. Alternative Codecs . . . . . . . . . . . . . . . . . . . . 42
4.2. Alternative Combinations of Codecs (Session 4.2. Alternative Combinations of Codecs (Session
Configurations) . . . . . . . . . . . . . . . . . . . . . 38 Configurations) . . . . . . . . . . . . . . . . . . . . . 45
4.3. Latent Media Streams . . . . . . . . . . . . . . . . . . . 38 4.3. Latent Media Streams . . . . . . . . . . . . . . . . . . . 45
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 41 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 48
5.1. New SDP Attributes . . . . . . . . . . . . . . . . . . . . 41 5.1. New SDP Attributes . . . . . . . . . . . . . . . . . . . . 48
5.2. New SDP Option Tag . . . . . . . . . . . . . . . . . . . . 42 5.2. New SDP Option Tag . . . . . . . . . . . . . . . . . . . . 49
5.3. New SDP Capability Negotiation Parameters . . . . . . . . 42 5.3. New SDP Capability Negotiation Parameters . . . . . . . . 49
6. Security Considerations . . . . . . . . . . . . . . . . . . . 43 6. Security Considerations . . . . . . . . . . . . . . . . . . . 50
7. Changes from previous versions . . . . . . . . . . . . . . . . 44 7. Changes from previous versions . . . . . . . . . . . . . . . . 51
7.1. Changes from version 11 . . . . . . . . . . . . . . . . . 44 7.1. Changes from version 12 . . . . . . . . . . . . . . . . . 51
7.2. Changes from version 10 . . . . . . . . . . . . . . . . . 44 7.2. Changes from version 11 . . . . . . . . . . . . . . . . . 51
7.3. Changes from version 09 . . . . . . . . . . . . . . . . . 44 7.3. Changes from version 10 . . . . . . . . . . . . . . . . . 51
7.4. Changes from version 08 . . . . . . . . . . . . . . . . . 45 7.4. Changes from version 09 . . . . . . . . . . . . . . . . . 52
7.5. Changes from version 04 . . . . . . . . . . . . . . . . . 45 7.5. Changes from version 08 . . . . . . . . . . . . . . . . . 52
7.6. Changes from version 03 . . . . . . . . . . . . . . . . . 45 7.6. Changes from version 04 . . . . . . . . . . . . . . . . . 52
7.7. Changes from version 02 . . . . . . . . . . . . . . . . . 46 7.7. Changes from version 03 . . . . . . . . . . . . . . . . . 52
7.8. Changes from version 01 . . . . . . . . . . . . . . . . . 46 7.8. Changes from version 02 . . . . . . . . . . . . . . . . . 53
7.9. Changes from version 00 . . . . . . . . . . . . . . . . . 47 7.9. Changes from version 01 . . . . . . . . . . . . . . . . . 54
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 48 7.10. Changes from version 00 . . . . . . . . . . . . . . . . . 54
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 49 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 55
9.1. Normative References . . . . . . . . . . . . . . . . . . . 49 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 56
9.2. Informative References . . . . . . . . . . . . . . . . . . 49 9.1. Normative References . . . . . . . . . . . . . . . . . . . 56
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 50 9.2. Informative References . . . . . . . . . . . . . . . . . . 56
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 57
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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not required; codecs such as AMR-WB [RFC4867] have the potential for not required; codecs such as AMR-WB [RFC4867] have the potential for
so many combinations of options that it may be impractical to define so many combinations of options that it may be impractical to define
unique payload type numbers for all supported combinations. If unique payload type numbers for all supported combinations. If
unique payload type numbers cannot be specified, then the offerer unique payload type numbers cannot be specified, then the offerer
will be obliged to wait for the SDP answer before rendering received will be obliged to wait for the SDP answer before rendering received
media. For SRTP using SDES inline keying [RFC4568], the offerer will media. For SRTP using SDES inline keying [RFC4568], the offerer will
still need to receive the answer before being able to decrypt the still need to receive the answer before being able to decrypt the
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 The media line, with any qualifying attributes such as fmtp or
rtpmap, is itself considered a valid configuration; it is assumed to rtpmap, is itself considered a valid configuration; it is assumed to
be the lowest preference. be the lowest preference.
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 19x2.0.2.2 c=IN IP4 192.0.2.2
t=0 0 t=0 0
a=csup:med-v0 a=csup:med-v0
m=audio 4567 RTP/AVP 18 m=audio 4567 RTP/AVP 18
a=rtpmap:18 G729/8000 a=rtpmap:18 G729/8000
a=fmtp:18 annexb=yes a=fmtp:18 annexb=yes
a=acfg:3 m=4 t=2 pt=4:18 a=acfg:3 m=4 t=2 pt=4:18
Bob includes the "a=csup" and "a=acfg" attributes in the answer to Bob includes the "a=csup" and "a=acfg" attributes in the answer to
inform Alice that he can support the med-v0 level of capability inform Alice that he can support the med-v0 level of capability
negotiations. Note that in this particular example, the answerer negotiations. Note that in this particular example, the answerer
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MUST be used for RTP-based media whereas the "a=omcap" attribute MUST MUST be used for RTP-based media whereas the "a=omcap" attribute MUST
be used for non-RTP-based (other) media formats. The two attributes be used for non-RTP-based (other) media formats. The two attributes
are defined as follows: are defined as follows:
a=rmcap:<media-cap-num-list> <encoding-name>/<clock-rate> a=rmcap:<media-cap-num-list> <encoding-name>/<clock-rate>
[/<encoding-parms>] [/<encoding-parms>]
a=omcap:<media-cap-num-list> <format-name> a=omcap:<media-cap-num-list> <format-name>
where <media-cap-num-list> is a (list of) media capability number(s) where <media-cap-num-list> is a (list of) media capability number(s)
used to number a media format capability, the <encoding name> is the used to number a media format capability, the <encoding name> or
media subtype e.g. H263-1998 or PCMU, <clock rate> is the encoding <format name> is the media subtype, e.g., H263-1998, PCMU, or T38,
rate, and <encoding parms> are the media encoding parameters for the <clock rate> is the encoding rate, and <encoding parms> are the media
media subtype;. All media format capabilities in the list are encoding parameters for the media subtype. All media format
assigned to the same media type/subtype. Each occurrence of the capabilities in the list are assigned to the same media type/subtype.
rmcap and omcap attribute MUST use unique values in their <media-cap- Each occurrence of the rmcap and omcap attribute MUST use unique
num-list>; the media capability numbers are shared between the two values in their <media-cap-num-list>; the media capability numbers
attributes and the numbers MUST be unique across the entire SDP are shared between the two attributes and the numbers MUST be unique
session. In short, the rmcap and omcap attributes define media across the entire SDP session. In short, the rmcap and omcap
capabilities and associates them with a media capability number in attributes define media capabilities and associate them with a media
the same manner as the rtpmap attribute defines them and associates capability number in the same manner as the rtpmap attribute defines
them with a payload type number. Additionally, the attributes allow them and associates them with a payload type number. Additionally,
multiple capability numbers to be defined for the media format in the attributes allow multiple capability numbers to be defined for
question. This permits the media format to be associated with the media format in question. This permits the media format to be
different media parameters in different configurations. associated with different media parameters in different
configurations.
In ABNF, we have: In ABNF, we have:
media-capability-line = rtp-mcap / non-rtp-mcap media-capability-line = rtp-mcap / non-rtp-mcap
rtp-mcap = "a=rmcap:" media-cap-num-list rtp-mcap = "a=rmcap:" media-cap-num-list
1*WSP encoding-name "/" clock-rate 1*WSP encoding-name "/" clock-rate
["/" encoding-parms] ["/" encoding-parms]
non-rtp-mcap = "a=omcap:" media-cap-num-list 1*WSP format-name non-rtp-mcap = "a=omcap:" media-cap-num-list 1*WSP format-name
media-cap-num-list = media-cap-num-element media-cap-num-list = media-cap-num-element
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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's the name of the media format, e.g. [RFC4566]. In simple terms, it is the name of the media format, e.g.
"t38". "t38". This form can also be used in cases such as BFCP [RFC4585]
where the fmt list in the m-line is effectively ignored (BFCP uses
"*").
The "rmcap" and "omcap" attributes can be provided at the session- 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 plus
one omcap attribute at the session or media level (i.e at most one of one omcap attribute at the session or media level (i.e more than one
each at the session-level and at most one of in each media of each at the session-level and more than one of each in each media
description). Each media-cap-num MUST be unique within the entire description). Each media-cap-num MUST be unique within the entire
SDP record; it is used to identify that media capability in SDP record; it is used to identify that media capability in
potential, latent and actual configurations, and in other attribute potential, latent and actual configurations, and in other attribute
lines as explained below. Note that the media-cap-num values are lines as explained below. Note that the media-cap-num values are
shared between the rmcap and omcap attributes, and hence the shared between the rmcap and omcap attributes, and hence the
uniqueness requirement applies to the union of them. When the media uniqueness requirement applies to the union of them. When the media
capabilities are used in a potential, latent or actual configuration, capabilities are used in a potential, latent or actual configuration,
the media formats referred by those configurations apply at the media the media formats referred by those configurations apply at the media
level, irrespective of whether the media capabilities themselves were level, irrespective of whether the media capabilities themselves were
specified at the session or media level. In other words, the media specified at the session or media level. In other words, the media
capability applies to the specific media description associated with capability applies to the specific media description associated with
the configuration which invokes it. the configuration which invokes it.
For example: For example:
v=0 v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
t=0 0
a=rmcap:1 L16/8000/1 a=rmcap: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:3,4 H263-1998/90000
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
3.3.2. The Media Format Parameter Capability Attribute 3.3.2. The Media Format Parameter Capability Attribute
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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 Media-specific attributes, beyond the rtpmap and fmtp attributes, may
be associated with media capability numbers via a new media-specific be associated with media capability numbers via a new media-specific
attribute, mscap, of the following form: attribute, mscap, of the following form:
a=mscap:<media caps star> <att field> <att value> a=mscap:<media caps star> <att field> <att value>
Where <media caps star> is a (list of) media capability number(s), where <media caps star> is a (list of) media capability number(s),
<att field> is the attribute name, and <att value> is the value field <att field> is the attribute name, and <att value> is the value field
for the named attribute. The media capability numbers may include a for the named attribute. Note that the media capability numbers
wildcard ("*"), which will be used instead of any payload type refer to media capabilities specified elsewhere in the SDP ("rmcap"
mappings. In ABNF, we have: and/or "omcap"). The media capability numbers may include a wildcard
("*"), which will be used instead of any payload type mappings. 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 = "*"
skipping to change at page 19, line 4 skipping to change at page 19, line 9
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 an lcfg or pcfg
attribute line, a mscap line may be translated easily into a attribute line, a mscap line may be translated easily into a
conventional SDP attribute line of the form conventional SDP attribute 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 will be use the wildcard character instead of the specific capability (or range of capabilities) will use the wildcard
payload type. character in the resulting SDP instead of the payload type specified
in the payload type mapping ("pt" parameter) in the configuration
attribute.
A single mscap line may refer to multiple media capabilities; this is A single mscap line may refer to multiple media capabilities; this is
equivalent to multiple mscap lines, each with the same attribute equivalent to multiple mscap lines, each with the same attribute
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
skipping to change at page 20, line 50 skipping to change at page 21, line 4
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
/ "*" ; dummy
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 desribed in RFC5939.
The "*" value for payload-type-number is used in cases such as BFCP
[RFC4583] in which the fmt list in the m-line is ignored.
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
binding; the offerer and/or answerer may use any payload type numbers binding; the offerer and/or answerer may use any payload type numbers
each deems appropriate. The use of explicit payload type numbers for each deems appropriate. The use of explicit payload type numbers for
latent configurations can be avoided by use of the parameter latent configurations can be avoided by use of the parameter
substitution rule of Section 3.3.7 . Future extensions are also substitution rule of Section 3.3.7. Future extensions are also
permitted. permitted.
3.3.4.3. The Media Type Parameter 3.3.4.3. The Media Type Parameter
When a latent configuration is specified (always at the media level), When a latent configuration is specified (always at the media level),
indicating the ability to support an additional media stream, it is indicating the ability to support an additional media stream, it is
necessary to specify the media type (audio, video, etc.) as well as necessary to specify the media type (audio, video, etc.) as well as
the format and transport type. The media type parameter is defined the format and transport type. The media type parameter is defined
in ABNF as in ABNF as
skipping to change at page 22, line 12 skipping to change at page 22, line 11
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
latent video configurations along with the media stream for audio; latent video configurations along with the media stream for audio;
the responding party may indicate its ability and willingness to the responding party may indicate its ability and willingness to
support such a video session by returning a corresponding latent support such a video session by returning a corresponding latent
configuration. configuration.
Latent configurations returned in SDP answers must match offered Latent configurations returned in SDP answers MUST match offered
latent configurations (or parameter subsets thereof). Therefore, it latent configurations (or parameter subsets thereof). Therefore, it
is appropriate for the offering party to announce most, if not all, is appropriate for the offering party to announce most, if not all,
of its capabilities in the initial offer. This choice has been made of its capabilities in the initial offer. This choice has been made
in order to keep the size of the answer more compact by not requiring in order to keep the size of the answer more compact by not requiring
acap, rmcap, tcap, etc. lines in the answer. acap, rmcap, tcap, etc. lines in the answer.
Latent configurations may be announced by use of the latent Latent configurations may be announced by use of the latent
configuration attribute, which is defined in a manner very similar to configuration attribute, which is defined in a manner very similar to
the potential configuration attribute. The latent configuration the potential configuration attribute. The latent configuration
attribute combines the properties of a media line and a potential attribute combines the properties of a media line and a potential
configuration. 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 and, like a media line, The lcfg attribute is a media level attribute.
it ends the session level of the session description if it appears
before any media line.
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 24, line 25 skipping to change at page 24, line 22
attribute defined in the base protocol [RFC5939]. The parameters and attribute defined in the base protocol [RFC5939]. The parameters and
their definitions are "borrowed" from the definitions provided for their definitions are "borrowed" from the definitions provided for
the latent configuration attribute in Section 3.3.5. The expanded the latent configuration attribute in Section 3.3.5. The expanded
ABNF definition of the pcfg attribute is 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.
3.3.6.1. Returning Capabilities in the Answer 3.3.6.1. Returning Capabilities in the Answer
Potential and/or latent configuration attributes may be returned Potential and/or latent configuration attributes may be returned
within an answer SDP to indicate the ability of the answerer to within an answer SDP to indicate the ability of the answerer to
support alternative configurations of the corresponding stream(s). support alternative configurations of the corresponding stream(s).
For example, an offer may include multiple potential configurations For example, an offer may include multiple potential configurations
skipping to change at page 25, line 9 skipping to change at page 25, line 7
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 configuratons (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 or those added by the answerer; values may be omitted configurations and/or those added by the answerer; values may be
only if they were indicated as alternative sets, or optional, in the omitted only if they were indicated as alternative sets, or optional,
original offer. The parameter set indicated in the returned acfg in the original offer. The parameter set indicated in the returned
attribute need not be repeated in a returned pcfg attribute. The acfg attribute need not be repeated in a returned pcfg attribute.
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
skipping to change at page 28, line 31 skipping to change at page 28, line 31
The choices of session capabilities may be based on processing load, The choices of session capabilities may be based on processing load,
total bandwidth, or any other criteria of importance to the total bandwidth, or any other criteria of importance to the
communicating parties. If the answerer supports media capabilities communicating parties. If the answerer supports media capabilities
negotiation, and session configurations are offered, it MUST accept negotiation, and session configurations are offered, it MUST accept
one of the offered configurations, or it MUST refuse the session. one of the offered configurations, or it MUST refuse the session.
Therefore, if the offer includes any session capabilities, it SHOULD Therefore, if the offer includes any session capabilities, it SHOULD
include all the session capabilities the offerer is willing to include all the session capabilities the offerer is willing to
support. support.
The session capability attribute is described by: The session capability attribute is a session-level attribute
described by:
"a=sescap:" <session num> <list of configs> "a=sescap:" <session num> <list of configs>
which corresponds to the standard value attribute definition with which corresponds to the standard value attribute definition with
att-field = "sescap" att-field = "sescap"
att-value = session-num 1*WSP list-of-configs att-value = session-num 1*WSP list-of-configs
[1*WSP optional-configs] [1*WSP optional-configs]
session-num = 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 [RFC5939]. Note
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
skipping to change at page 29, line 32 skipping to change at page 29, line 33
a=sescap:2 1,2,3,5 a=sescap:2 1,2,3,5
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
a=label:11 a=label:11
a=pcfg:2 a=pcfg:2
a=rmcap:1 H264/90000 a=rmcap:1 H264/90000
a=mfcap:1 profile-level-id=42A01E; packetization-mode=2 a=mfcap:1 profile-level-id=42A01E; packetization-mode=2
a=acap:1 label:13 a=acap:1 label:13
a=pcfg:4 m=1 a=1 pt=1:104 a=pcfg:4 m=1 a=1 pt=1:104
m=video 33444 RTP/AVP 103 m=video 33444 RTP/AVP 103
a=rtpmap:103 H263-1998/90000 a=rtpmap:103 H263-1998/90000
a=fmtp:103 CIF=4;QCIF=2;F=1;K=1 a=fmtp:103 CIF=4;QCIF=2;F=1;K=1
i= secondary video (slides) i=secondary video (slides)
a=label:12 a=label:12
a=pcfg:3 a=pcfg:3
m=application 33002 TCP/BFCP * m=application 33002 TCP/BFCP *
a=setup:passive a=setup:passive
a=connection:new a=connection:new
a=floorid:1 m-stream:11 12 a=floorid:1 m-stream:11 12
a=floor-control:s-only a=floor-control:s-only
a=confid:4321 a=confid:4321
a=userid:1234 a=userid:1234
skipping to change at page 31, line 22 skipping to change at page 31, line 27
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=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 i=3 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 i=4 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=rmcap: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
skipping to change at page 32, line 30 skipping to change at page 32, line 32
a=lcfg:4 mt=video t=1 m=1 a=41,42 a=lcfg:4 mt=video t=1 m=1 a=41,42
a=lcfg:5 mt=application t=2 a=lcfg:5 mt=application t=2
This exchange supports immediate establishment of an audio stream for This exchange supports immediate establishment of an audio stream for
preliminary conversation. This exchange would presumably be followed preliminary conversation. This exchange would presumably be followed
at the appropriate time with a "reconfiguration" offer/answer at the appropriate time with a "reconfiguration" offer/answer
exchange to add the video and chair control streams. exchange to add the video and chair control streams.
3.4. Offer/Answer Model Extensions 3.4. Offer/Answer Model Extensions
EDITOR'S NOTE: SECTION NEEDS MORE ELABORATE PROCEDURES
In this section, we define extensions to the offer/answer model In this section, we define extensions to the offer/answer model
defined in RFC3264 [RFC3264] and [RFC5939] to allow for media defined in RFC 3264 [RFC3264] and RFC 5939 [RFC5939] to allow for
capabilities, bandwidth capabilities, and latent configurations to be media capabilities and parameters, latent configurations and
used with the SDP Capability Negotiation framework. acceptable combinations of media stream configurations to be used
with the SDP Capability Negotiation framework. Note that the
procedures defined in this section extend the offer/answer procedures
defined in [RFC5939] Section 6; those procedures form a baseline set
of capability negotiation offer/answer procedures that MUST be
followed, subject to the extensions defined here.
The [RFC5939] provides a relatively compact means to offer the The [RFC5939] provides a relatively compact means to offer the
equivalent of an ordered list of alternative media stream equivalent of an ordered list of alternative configurations for
configurations (as would be described by separate m= lines and offered media streams (as would be described by separate m= lines and
associated attributes). The attributes acap, mscap, mfcap and rmcap associated attributes). The attributes acap, mscap, mfcap and rmcap
are designed to map somewhat straightforwardly into equivalent m= are designed to map somewhat straightforwardly into equivalent m=
lines and conventional attributes when invoked by a pcfg, lcfg, or lines and conventional attributes when invoked by a pcfg, lcfg, or
acfg attribute with appropriate parameters. The a=pcfg: lines, along acfg attribute with appropriate parameters. The a=pcfg: lines, along
with the m= line itself, represent offered media configurations. The with the m= line itself, represent offered media configurations. The
a=lcfg: lines represent alternative capabilities for future use. 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
document cover the following categories of features:
o Media Capabilities and associated parameters (rmcap, omcap, mfcap,
and mscap attributes)
o Potential configurations using those media capabilities and
associated parameters
o Latent media streams (lcfg attribute)
o Acceptable combinations of media stream configurations (sescap
attribute).
The high-level description of the operation is as follows:
When an endpoint generates an initial offer and wants to use the 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 codecs it can support via rmcap, mfcap and mscap and define the media formats and associated parameters it can support
attributes. The SDP media line(s) should be made up with the via the rmcap, omcap, mfcap and mscap attributes. The SDP media
configuration to be used if the other party does not understand line(s) ("m=") should be made up with the actual configuration to be
capability negotiations (by default, this is the least preferred used if the other party does not understand capability negotiations
configuration). Typically, the media line configuration will contain (by default, this is the least preferred configuration). Typically,
the minimum acceptable capabilities. The offer MUST include the the media line configuration will contain the minimum acceptable
level of capability negotiation extensions needed to support this configuration from the offerer's point of view.
functionality in a "creq" attribute.
Preferred configurations for each media stream are identified Preferred configurations for each media stream are identified
following the media line. The present offer may also include latent following the media line. The present offer may also include latent
configuration (lcfg) attributes, at the media level, describing media configuration (lcfg) attributes, at the media level, describing media
streams and/or configurations the offerer is not now offering, but streams and/or configurations the offerer is not now offering, but
which it is willing to support in a future offer/answer exchange. A which it is willing to support in a future offer/answer exchange. A
simple example might be the inclusion of a latent video configuration simple example might be the inclusion of a latent video configuration
in an offer for an audio stream. in an offer for an audio stream.
Lastly, if the offerer wishes to impose restrictions on the
combinations of potential configurations to be used, it will include
session capability (sescap) attributes indicating those.
If the offerer requires the answerer to understand the media
capability extensions, the offerer MUST include a creq attribute
containing the value "med-v0". If media capability negotiation is
required only for specific media descriptions, the "med-v0" value
MUST be provided only in creq attributes within those media
descriptions, as described in [RFC5939].
Below, we provide a more detailed description of how to construct the
offer SDP.
3.4.1.1. Offer with Media Capabilities
For each RTP-based media format the offerer wants to include as a
media capability, the offer MUST include an "rmcap" attribute for the
media format as defined in Section 3.3.1.
For each non RTP-based media format the offer wants to include as a
media capability, the offer MUST include an "omcap" attribute for the
media format as defined in Section 3.3.1.
Since the media format capability number space is shared between the
rmcap and omcap attributes, each media capability number provided
(including ranges) MUST be unique in the entire SDP.
If an "fmtp" parameter value is needed for a media format (whether
RTP-based or not) in a media capability, then the offer MUST include
one or more "mfcap" parameters with the relevant fmtp parameter
values for that media format as defined in Section 3.3.2. When
multiple "mfcap" parameters are provided for a given media
capability, they MUST be provided in accordance with the
concatenation rules in Section 3.3.2.1.
For each of the media capabilities above, the offer MAY include one
or more "mscap" parameters with attributes needed for those specific
media formats as defined in Section 3.3.3. Such attributes will be
instantiated at the media-level, and hence session-level only
attributes MUST NOT be used in the "mscap" parameter. The "mscap"
parameter MUST NOT include an "rtpmap" or "fmtp" attribute (rmcap and
mfcap are used instead).
If the offerer wants to limit the relevance (and use of) a media
capability or parameter to a particular media stream, the media
capability or parameter MUST be provided within the corresponding
media description. Otherwise, the media capabilities and parameters
MUST be provided at the session level. Note however, that the
attribute or parameter embedded in these will always be instantiated
at the media-level.
This is due to those parameters being effectively media-level
parameters. If session-level attributes are needed, the "acap"
attribute defined in [RFC5939] can be used, however it does not
provide for media format-specific instantiation.
Inclusion of the above does not constitute an offer to use the
capabilities; a potential configuration is needed for that. If the
offerer wants to offer one or more of the media capabilities above,
they MUST be included as part of a potential configuration (pcfg)
attribute as defined in Section 3.3.4. Each potential configuration
MUST include a config-number that is unique in the entire SDP (note
that this differs from [RFC5939], which only requires uniqueness
within a media description). Also, the config-number MUST NOT
overlap with any config-number used by a latent configuration in the
SDP. As described in [RFC5939], lower config-numbers indicate a
higher preference; the ordering still applies within a given media
description only though.
For a media capability to be included in a potential configuration,
there MUST be an "m=" parameter in the pcfg attribute referencing the
media capability number in question. When one or more media
capabilities are included in an offered potential configuration
(pcfg), they completely replace the list of media formats offered in
the actual configuration (m= line). Any parameters included for
those formats remain in the SDP though (e.g., rtpmap, fmtp, etc.).
For non-RTP based media formats, the format-name (from the "omcap"
media capability) is simply added to the "m=" line as a media format
(e.g. t38). For RTP-based media, payload type mappings MUST be
provided by use of the "pt" parameter in the potential configuration
(see Section 3.3.4.2); payload type escaping may be used in mfcap,
mscap, and acap attributes as defined in Section 3.3.7.
Note that the "mt" parameter MUST NOT be used with the pcfg
attribute; the media type in a potential configuration cannot be
changed from that of the encompassing media description.
3.4.1.2. Offer with Latent Configuration
If the offerer wishes to offer one or more latent configurations for
future use, the offer MUST include a latent configuration attribute
(lcfg) for each as defined in Section 3.3.5.
Each lcfg attribute
o MUST be specified at the media level
o MUST include a config-number that is unique in the entire SDP
(incl. for any potential configuration attributes). Note that
config-numbers in latent configurations do not indicate any
preference order
o MUST include a media type ("mt")
o MUST reference a valid transport capability ("t")
Each lcfg attribute MAY include additional capability references,
which may refer to capabilities anywhere in the session description,
subject to any restrictions normally associated with such
capabilities. For example, a media-level attribute capability must
be present at the media-level in some media description in the SDP.
Note that this differs from the potential configuration attribute,
which cannot validly refer to media-level capabilities in another
media description (per [RFC5939], Section 3.5.1).
Potential configurations constitute an actual offer and hence may
instantiate a referenced capability. Latent configurations are
not actual offers and hence cannot instantiate a referenced
capability; it is therefore safe for those to refer to
capabilities in another media description.
3.4.1.3. Offer with Configuration Combination Restrictions
If the offerer wants to indicate restrictions or preferences among
combinations of potential and/or latent configuration, a session
capability (sescap) attribute MUST be provided at the session-level
for each such combination as described in Section 3.3.8. Each sescap
attribute MUST include a session-num that is unique in the entire
SDP; the lower the session-num the more preferred that combination
is. Furthermore, sescap preference order takes precedence over any
order specified in individual pcfg attributes.
For example, if we have pcfg-1 and pcfg-2, and sescap-1 references
pcfg-2, whereas sescap-2 references pcfg-1, then pcfg-2 will be
the most preferred potential configuration. Without the sescap,
pcfg-1 would be the most preferred.
3.4.2. Generating the Answer 3.4.2. Generating the Answer
When receiving an offer, the answerer MUST check the offer for creq
attributes containing the value "med-v0"; answerers compliant with
this specification will support this value in accordance with the
procedures specified in [RFC5939].
The SDP may contain
o Media capabilities and associated parameters (rmcap, omcap, mfcap,
and mscap attributes)
o Potential configurations using those media capabilities and
associated parameters
o Latent media streams (lcfg attribute)
o Acceptable combinations of media stream configurations (sescap
attribute)
The high-level description of the operation is as follows:
When 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, the answer must include the supported
extension attribute and each media stream for which a proposed extension attribute and each media stream for which a proposed
configuration was chosen must contain an actual configuration (acfg) configuration was chosen must contain an actual configuration (acfg)
attribute to indicate just which 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
processes the offer SDP and generates an answer SDP.
3.4.2.1. Processing Media Capabilities and Potential Configurations
The answerer MUST first determine if it needs to perform media
capability negotiation by examining the SDP for valid and preferred
potential configuration attributes that include media configuration
parameters (i.e. an "m" parameter in the pcfg attribute).
Such a potential configuration is valid if:
1. It is valid according to the rules defined in [RFC5939]
2. It contains a config-number that is unique in the entire SDP and
does not overlap with any latent configuration config-numbers
3. All media format capabilities (rmcap or omcap), media format
parameter capabilities (mfcap), and media-specific capabilities
(mscap) referenced by the potential configuration ("m" parameter)
are valid themselves (as defined in Section 3.3.1, 3.3.2, and
3.3.3) and each of them is provided either at the session level
or within this particular media description.
4. All RTP-based media capabilities (rmcap) have a corresponding
payload type ("pt") parameter in the potential configuration that
results in mapping to a valid payload type that is unique within
the resulting SDP.
5. Any concatenation (see Section 3.3.2.1) and substitution (see
Section 3.3.7) applied to any capability (mfcap, mscap, or acap)
referenced by this potential configuration results in a valid
SDP.
Note that, since SDP does not interpret the value of fmtp parameters,
any resulting fmtp parameter value will be considered valid.
Secondly, the answerer MUST determine the order in which potential
configurations are to be negotiated. In the absence of any Session
Capability ("sescap") attributes, this simply follows the rules of
[RFC5939], with a lower config-number within a media description
being preferred over a higher one. If a valid "sescap" attribute is
present, the preference order provided in the "sescap" attribute MUST
take precedence. A "sescap" attribute is considered valid if:
1. It adheres to the rules provided in Section 3.3.8.
2. All the configurations referenced by the "sescap" attribute are
valid themselves (note that this can include the actual,
potential and latent configurations).
The answerer MUST now process the offer each media stream based on
the most preferred valid potential configuration in accordance with
the procedures specified in [RFC5939], Section 3.6.2, and further
extended below:
o If one or more media format capabilities are included in the
potential configuration, then they replace all media formats
provided in the "m=" line for that media description. For non-RTP
based media formats (omcap), the format-name is added. For RTP-
based media formats (rmcap), the payload-type specified in the
payload-type mapping ("pt") is added and a corresponding "rtpmap"
attribute is added to the media description.
o If one or more media format parameter capabilities are included in
the potential configuration, then the corresponding "fmtp"
attributes are added to the media description. Note that this
inclusion is done indirectly via the media format capability.
o If one or more media-specific capabilities are included in the
potential configuration, then the corresponding attributes are
added to the media description. Note that this inclusion is done
indirectly via the media format capability.
o When checking to see if the answerer supports a given potential
configuration that includes one or more media format capabilities,
the answerer MUST support at least one of the media formats
offered. If he does not, the answerer MUST proceed to the next
potential configuration based on the preference order that
applies.
o If Session Capability ("sescap") preference ordering is included,
then the potential configuration selection process MUST adhere to
the ordering provided. Note that this may involve coordinated
selection of potential configurations between media descriptions.
The answerer MUST accept one of the offered "sescap" combinations
(i.e. all the required potential configurations specified) or it
MUST reject the entire session.
Once the answerer has selected a valid and supported offered
potential configuration for all of the media streams (or has fallen
back to the actual configuration plus any added session attributes),
the answerer MUST generate a valid answer SDP as described in
[RFC5939], Section 3.6.2, and further extended below:
o Additional answer capabilities and potential configurations MAY be
returned in accordance with Section 3.3.6.1. Capability numbers
and configuration numbers for those MUST be distinct from the ones
used in the offer SDP.
o Latent configuration processing and answer generation MUST be
performed, as specified below.
o Session capability specification for the potential and latent
configurations in the answer MAY be included (see Section 3.3.8).
3.4.2.2. Latent Configuration Processing
The answerer MUST determine if it needs to perform any latent
configuration processing by examining the SDP for valid latent
configuration attributes (lcfg). An lcfg attribute is considered
valid if:
o It adheres to the description in Section 3.3.5.
o It includes a config-number that is unique in the entire SDP and
does not overlap with any potential configuration config-number
o It includes a valid media type ("mt=")
o It references a valid transport capability ("t=")
o All other capabilities referenced by it are valid.
For each such valid latent configuration in the offer, the answerer
checks to see if it could support the latent configuration in a
subsequent offer/answer exchange. If so, it includes the latent
configuration with the same configuration number in the answer,
similar to the way potential configurations are processed and the
selected one returned in an actual configuration attribute (see
[RFC5939]). If the answerer supports only a (non-mandatory) subset
of the parameters offered in a latent configuration, the answer
latent configuration will include only those parameters supported
(similar to "acfg" processing). Note that latent configurations do
not constitute an actual offer at this point in time; they merely
indicate additional configurations that could be supported.
If a Session Capability ("sescap") attribute is included and it
references a latent configuration, then the answerer processing of
that latent configuration must be done within the constraints
specified by that Session Capability, i.e. it must be possible to
support it at the same time as any required (i.e. non-optional)
potential configurations in the session capability. The answerer may
in turn add his own "sescap" indications in the answer as well.
3.4.3. Offerer Processing of the Answer 3.4.3. Offerer Processing of the Answer
When the offerer receives the answer, it should make note of any The offerer MUST process the answer in accordance with [RFC5939]
capabilities and/or latent configurations for future use. The media Section 3.6.3, and further explained below.
line(s) must be processed in the normal way to identify the media
stream(s) accepted by the answer, if any. The acfg attribute, if When the offerer processes the answer SDP based on a valid actual
present, may be used to verify the proposed configuration used to configuration attribute in the answer, and that valid configuration
form the answer, and to infer the lack of acceptability of higher- includes one or more media capabilities, the processing MUST
preference configurations that were not chosen. Note that the base furthermore be done as if the offer was sent using those media
specification [RFC5939] requires the answerer to choose the highest capabilities instead of the actual configuration. In particular, the
preference configuration it can support, subject to local policies. media formats in the "m=" line, and any associated payload type
mappings (rtpmap), fmtp parameters (mfcap) and media-specific
attributes (mscap) MUST be used. Note that this may involve use of
concatenation and substitution rules (see Section 3.3.2.1 and 3.3.7).
The actual configuration attribute may also be used to infer the lack
of acceptability of higher-preference configurations that were not
chosen, subject to any constraints provided by a Session Capability
attribute ("sescap") in the offer. Note that the base specification
[RFC5939] requires the answerer to choose the highest preference
configuration it can support, subject to local policies.
When the offerer receives the answer, it SHOULD furthermore make note
of any capabilities and/or latent configurations included for future
use, and any constraints on how those may be combined.
3.4.4. Modifying the Session 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 may 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, and latent configurations announced by the other configurations, latent configurations and session capabilities
party in the earlier negotiation, it may use this knowledge to provided by the other party in the earlier negotiation, it may use
maximize the likelihood of a successful modification of the session. this knowledge to maximize the likelihood of a successful
Alternatively, the initiator may perform a new capabilities exchange modification of the session. Alternatively, the initiator may
as part of the reconfiguration. In such a case, the new capabilities perform a new capabilities exchange as part of the reconfiguration.
will replace the previously-negotiated capabilities. This may be In such a case, the new capabilities will replace the previously-
useful if conditions change on the endpoint. negotiated capabilities. This may be useful if conditions change on
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 provide 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
skipping to change at page 35, line 25 skipping to change at page 42, line 25
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
a=creq:med-v0 a=creq:med-v0
m=audio 54322 RTP/AVP 96 m=audio 54322 RTP/AVP 96
rtpmap:96 AMR-WB/16000/1 a=rtpmap:96 AMR-WB/16000/1
a=fmtp:96 mode-change-capability=1; max-red=220; \ a=fmtp:96 mode-change-capability=1; max-red=220; \
mode-set=0,2,4,7 mode-set=0,2,4,7
a=rmcap:1,3,5 audio AMR-WB/16000/1 a=rmcap:1,3,5 audio AMR-WB/16000/1
a=rmcap:2,4,6 audio AMR/8000/1 a=rmcap:2,4,6 audio AMR/8000/1
a=mfcap:1,2,3,4 mode-change-capability=1 a=mfcap:1,2,3,4 mode-change-capability=1
a=mfcap:5,6 mode-change-capability=2 a=mfcap:5,6 mode-change-capability=2
a=mfcap:1,2,3,5 max-red=220 a=mfcap:1,2,3,5 max-red=220
a=mfcap:3,4,5,6 octet-align=1 a=mfcap:3,4,5,6 octet-align=1
a=mfcap:1,3,5 mode-set=0,2,4,7 a=mfcap:1,3,5 mode-set=0,2,4,7
a=mfcap:2,4,6 mode-set=0,3,5,6 a=mfcap:2,4,6 mode-set=0,3,5,6
skipping to change at page 39, line 17 skipping to change at page 46, line 17
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=lcfg:10 mt=video t=1 a=pcfg:1 m=1,3|2,3 pt=1:0,2:18,3:100
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
The lcfg attribute line announces support for H.263 and H.264 video The lcfg attribute line announces support for H.263 and H.264 video
(H.263 preferred) for future reference. The m-line and the rtpmap (H.263 preferred) for future reference. The m-line and the rtpmap
attribute offer an audio stream and provide the lowest precedence attribute offer an audio stream and provide the lowest precedence
configuration (PCMU without any DTMF encoding). The rmcap lines configuration (PCMU without any DTMF encoding). The rmcap lines
define the media capabilities (PCMU, G729, and telephone-event) to be define the media capabilities (PCMU, G729, and telephone-event) to be
offered in potential configurations. The mfcap attribute provides offered in potential configurations. The mfcap attribute provides
skipping to change at page 39, line 49 skipping to change at page 46, line 49
o=- 24351 621814 IN IP4 192.0.2.2 o=- 24351 621814 IN IP4 192.0.2.2
s= s=
c=IN IP4 192.0.2.2 c=IN IP4 192.0.2.2
t=0 0 t=0 0
a=csup:med-v0 a=csup:med-v0
m=audio 54322 RTP/AVP 0 100 m=audio 54322 RTP/AVP 0 100
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
a=rtpmap:100 telephone-event/8000 a=rtpmap:100 telephone-event/8000
a=fmtp:100 0-11 a=fmtp:100 0-11
a=acfg:1 m=1,3 pt=1:0,3:100 a=acfg:1 m=1,3 pt=1:0,3:100
a=pcfg:1 m=2,3 pt=2:18,3:100 a=lcfg:1 mt=video t=1 m=10 a=pcfg:1 m=2,3 pt=2:18,3:100
a=lcfg:2 mt=video t=1 m=10
The lcfg attribute line announces the capability to support H.263 The lcfg attribute line announces the capability to support H.263
video at a later time. The media line and subsequent rtpmap and fmtp video at a later time. The media line and subsequent rtpmap and fmtp
attribute lines present the selected configuration for the media attribute lines present the selected configuration for the media
stream. The acfg attribute line identifies the potential stream. The acfg attribute line identifies the potential
configuration from which it was taken, and the pcfg attribute line configuration from which it was taken, and the pcfg attribute line
announces the potential capability to support G.729 with DTMF events announces the potential capability to support G.729 with DTMF events
as well. If, at some later time, congestion becomes a problem in the as well. If, at some later time, congestion becomes a problem in the
network, either party may, with expectation of success, offer a network, either party may, with expectation of success, offer a
reconfiguration of the media stream to use G.729 in order to reduce reconfiguration of the media stream to use G.729 in order to reduce
skipping to change at page 43, line 7 skipping to change at page 50, line 7
Potential Configuration Parameter Registry established by [RFC5939] Potential Configuration Parameter Registry established by [RFC5939]
to become the SDP Capability Negotiation Configuration Parameter to become the SDP Capability Negotiation Configuration Parameter
Registry and to include parameters for the potential, actual and Registry and to include parameters for the potential, actual and
latent configuration attributes. The new parameters to be registered latent configuration attributes. The new parameters to be registered
are the "m" for "media", "pt" for "payload type number", and "mt" for are the "m" for "media", "pt" for "payload type number", and "mt" for
"media type" parameters. Note that the "mt" parameter is defined for "media type" parameters. Note that the "mt" parameter is defined for
use only in the latent configuration attribute. use only in the latent configuration attribute.
6. Security Considerations 6. Security Considerations
EDITOR'S NOTE: SECTION NEEDS TO BE EXPANDED
The security considerations of [RFC5939] apply for this document. The security considerations of [RFC5939] apply for this document.
The addition of negotiable media encoding, bandwidth attributes, and In [RFC5939], it was noted that negotiation of transport protocols
connection data in this specification can cause problems for (e.g. secure and non-secure) and negotiation of keying methods and
material are potential security issues that warrant integrity
protection to remedy. Latent configuration support provides hints to
the other side about capabilities supported for further offer/answer
exchanges, including transport protocols and attribute capabilities,
e.g. for keying methods. If an attacker can remove or alter latent
configuration information to suggest that only insecure or less
secure alternatives are supported, then he may be able to force
negotiation of a less secure session than would otherwise have
occurred. While the specific attack as described here differs from
those described in [RFC5939], the considerations and mitigation
strategies are similar to those described in [RFC5939].
Another variation on the above attack involves the the Session
Capability ("sescap") attribute defined in this document. The
"sescap" enables a preference order to be specified for all the
potential configurations, and that preference will take precedence
over any preference indication provided in individual potential
configuration attributes. Consequently, an attacker that can insert
or modify a "sescap" attribute may be able to force negotiation of an
insecure or less secure alternative than would otherwise have
occurred. Again, the considerations and mitigation strategies are
similar to those described in [RFC5939].
The addition of negotiable media formats and their associated
parameters, defined in this specification can cause problems for
middleboxes which attempt to control bandwidth utilization, media 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, 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 and bandwidth parameters for every media stream selected. encoding for every media stream selected. Pending an understanding
Pending an understanding of capabilities negotiation, the middlebox of capabilities negotiation, the middlebox should examine the answer
should examine the answer SDP to obtain the best picture of the media SDP to obtain the best picture of the media streams being
streams being established. established. As always, middleboxes can best do their job if they
fully understand media capabilities negotiation.
As always, middleboxes can best do their job if they fully understand
media capabilities negotiation.
7. Changes from previous versions 7. Changes from previous versions
7.1. Changes from version 11 7.1. Changes from version 12
o Removed "dummy" form in the pcfg payload-type-number, since the
functionality is redundant with the non-RTP media capability
(omcap) and it was inconsistent with other RTP payload type
operation.
o Clarified that latent configuration attribute (lcfg) can only be
used at the media level and hence (technically) as part of a media
description
o Rewrote offer/answer sections and expanded significantly on offer/
answer operation.
o Updated security considerations
o Various minor editorial clarifications and changes.
7.2. 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"
7.2. Changes from version 10 7.3. 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 44, line 43 skipping to change at page 52, line 12
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.
7.3. Changes from version 09 7.4. 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 crossreferences.
7.4. Changes from version 08 7.5. 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.
7.5. Changes from version 04 7.6. 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.
7.6. Changes from version 03 7.7. 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 46, line 17 skipping to change at page 53, line 33
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.
7.7. Changes from version 02 7.8. 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 46, line 47 skipping to change at page 54, line 14
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.
7.8. Changes from version 01 7.9. 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 parametr 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 terminolgy 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.
7.9. Changes from version 00 7.10. 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 explictly definesd 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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