MMUSIC R. Gilman
Internet-Draft Avaya, Inc.
Intended status: Standards Track R. Even, Ed.
Expires: May 18, 2008 Polycom
F. Andreasen
Cisco Systems
November 15, 2007
SDP media capabilities Negotiation
draft-ietf-mmusic-sdp-media-capabilities-02.txt
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Copyright (C) The IETF Trust (2007).
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Abstract
Session Description Protocol (SDP) capability negotiation provides a
general framework for negotiating capabilities in SDP. The base
framework defines only capabilities for negotiating transport
protocols and attributes. In this document, we extend the framework
by defining media capabilities that can be used to negotiate media
types and their associated parameters.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. SDP Media capabilities . . . . . . . . . . . . . . . . . . . . 6
3.1. Solution Overview . . . . . . . . . . . . . . . . . . . . 6
3.2. Capability Attributes . . . . . . . . . . . . . . . . . . 9
3.2.1. Media Type and Subtype Capability Attribute . . . . . 10
3.2.2. The Media Encoding Parameter Capability Attribute . . 11
3.2.3. The Media Format Parameter Capability Attribute . . . 12
3.2.4. The Bandwidth Capability Attribute . . . . . . . . . . 13
3.3. Extensions to the Potential Configuration Attribute . . . 15
3.3.1. The Media Capability Extension to the Potential
Configuration Attribute . . . . . . . . . . . . . . . 15
3.3.2. The Payload Type Mapping Extension to the
Potential Configuration Attribute . . . . . . . . . . 16
3.3.3. The Bandwidth Extension to the Potential
Configuration Attribute . . . . . . . . . . . . . . . 17
3.4. Extensions to the Actual Configuration Attribute . . . . . 17
3.5. The Latent Configuration Attribute . . . . . . . . . . . . 18
3.5.1. The crypto: Attribute in Latent Configurations . . . . 18
3.6. Offer/Answer Model Extensions . . . . . . . . . . . . . . 19
3.6.1. Generating the Initial Offer . . . . . . . . . . . . . 19
3.6.2. Generating the Answer . . . . . . . . . . . . . . . . 19
3.6.3. Offerer Processing of the Answer . . . . . . . . . . . 19
3.6.4. Modifying the Session . . . . . . . . . . . . . . . . 20
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.1. Alternative Codecs . . . . . . . . . . . . . . . . . . . . 21
4.2. Latent Media Streams . . . . . . . . . . . . . . . . . . . 21
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
6. Security Considerations . . . . . . . . . . . . . . . . . . . 23
7. Changes from previous versions . . . . . . . . . . . . . . . . 24
7.1. Changes from version 01 . . . . . . . . . . . . . . . . . 24
7.2. Changes from version 00 . . . . . . . . . . . . . . . . . 24
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
9.1. Normative References . . . . . . . . . . . . . . . . . . . 26
9.2. Informative References . . . . . . . . . . . . . . . . . . 26
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27
Intellectual Property and Copyright Statements . . . . . . . . . . 28
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1. Introduction
Session Description Protocol (SDP) capability negotiation [SDPCapNeg]
provides a general framework for negotiating capabilities in
SDP[RFC4566]. The base framework defines only capabilities for
negotiating transport protocols and attributes.
SDP Simple Capability Declaration (simcap) is defined in RFC 3407
[RFC3407]. It defines a set of SDP attributes that enables a limited
set of capabilities to be described at a session level or on a per
media stream basis. RFC 3407 defines capability declaration only.
Actual negotiation procedures taking advantage of such capabilities
have not been defined. The SDP capability negotiation framework adds
this required functionality. This document updates RFC3407 and new
implementation SHOULD use the functionality defined in the current
draft to negotiate media capabilities.
The [SDPCapNeg] document lists some of the issues with the current
SDP capability negotiation process. An additional real life case is
to be able to offer one media stream (e.g. audio) but list the
capability to support another media stream (e.g. video) without
actually offering it currently.
In this document, we extend the framework by defining media
capabilities that can be used to negotiate media types and their
associated parameters like bandwidth. This document also adds the
capability to declare support for media streams that can be offered
later rather than as a potential configuration for current use.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [RFC2119] and
indicate requirement levels for compliant RTP implementations.
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3. SDP Media capabilities
The SDP media capability negotiation [SDPCapNeg] discusses the use of
any SDP [RFC4566] attribute (a=) under the acap attribute. The
limitations of using acap for fmtp and rtpmap as potential
configuration are listed, for example they can be used only at the
media level since they are media level attributes. The [SDPCapNeg]
addresses the issue of bandwidth usage but does not offer a way to
negotiate different bandwidth for different codecs and profiles.
This section provides an overview of extensions providing SDP media
Capability negotiation solution offering more robust capabilities
negotiation. This is followed by definitions of new SDP attributes
for the solution and its associated updated offer/answer procedures
[RFC3264]
3.1. Solution Overview
The solution consist of the following new attributes extending the
base attributes from [SDPCapNeg].
Four attributes are used to make up media capabilities in a manner
that can be related to the capabilities specified in a media line,
its attributes and the bandwidth parameter lines.
o A new media attribute ("a=mcap") that lists media formats as
capabilities in the form a media type (e.g. "audio") and one or
more subtypes (e.g. "PCMU"), and associates a handle with each
subtype
o A new attribute ("a=ecap") that lists encoding parameter
capabilities associated with a one or more media format
capabilities.
o A new attribute ("a=fcap") that lists media format parameter
capabilities associated with a one or more media format
capabilities.
o A new attribute ("a=bcap") that is used to specify the bandwidth
parameter(s) for media capabilities.
o A new attribute ("a=lcfg") that specifies latent configurations
when no corresponding media line is offered. An example is a
latent configuration for video even though no video is currently
offered.
o A new parameter type ("m=") to the potential configuration
("a=pcfg:") attribute and the actual configuration ("a=acfg:")
attribute defined in [SDPCapNeg], which permits specification of
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media capabilities (including their associated parameters) and
combinations thereof for the configuration. For example, the
"a=pcfg:" line might specify PCMU and telephone events or G.729B
and telephone events as acceptable configurations. The "a=acfg:"
line in the answer would specify the accepted choice.
o A new parameter type ("pt=") to the potential configuration
("a=pcfg:") attribute which associates RTP payload types with the
referenced media capabilities.
o A new parameter type ("b=") used to specify bandwidth parameters
in a potential configuration.
The document extends the base protocol extensions to the offer/answer
model that allow for capabilities and potential configurations to be
included in an offer. Media capabilities constitute capabilities
that can be used in potential and latent configurations. Whereas
potential configurations constitute alternative offers that may be
accepted by the answerer instead of the actual configuration(s)
included in the "m=" line(s), 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.
The mechanism is illustrated by the offer/answer exchange below,
where Alice sends an offer to Bob:
Alice Bob
| (1) Offer (SRTP and RTP) |
|--------------------------------->|
| |
| (2) Answer (RTP) |
|<---------------------------------|
| |
Alice's offer includes RTP and SRTP as alternatives. RTP is the
default, but SRTP is the preferred one:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
a=creq:med-v0
a=mcap:1 audio g729 iLBC PCMU g729
a=ecap:2 8000
a=fcap:1 annexb:no
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a=fcap:2 mode=20
a=tcap:1 RTP/SAVP
m=audio 3456 RTP/AVP 0 18
a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=pcfg:1 m=1,3|4,3 t=1 a=1 pt=1:100,4:101,3:102
a=pcfg:2 m=2 pt=2:103
The required base and extensions are provided by the "a=creq"
attribute defined in [SDPCapNeg], with the option tag "med-v0", which
indicates that the extension framework defined here, must be
supported. The Base level support is implied since it is required
for the extensions.
The "a=mcap:1" line defines four audio media subtype capabilities ,
to be numbered consecutively starting with 1. Note that the media
subtypes specified in the m-line (PCMU and G729) are explicitly
specified here.
The "a=ecap:2" line specifies the clock rate and encoding parmeters
(see [RFC4566]) for capability 2, iLBC.
The "a=fcap:1" line specifies media format parameter capabilities for
codec 1 ( no Annex B for G.729).
The "a=fcap:2" line specifies a media format parameter for codec 2
(20ms packets for iLBC. [RFC3952]
The "a=tcap:1" line, specified in the base protocol, defines a
transport protocol capability, in this case Secure RTP.
The "m=" line indicates that Alice is offering to use plain RTP with
PCMU or G.729. The media line implicitly defines the default
transport protocol (RTP/AVP in this case) and the default actual
configuration.
The "a=acap:1" line ,specified in the base protocol provides the
"crypto" attribute which provides the keying material for SRTP using
SDP security descriptions.
The "a=pcfg:" attributes provide the potential configurations
included in the offer by reference to the media capabilities,
transport capabilities, and associated payload type mappings. Two
explicit alternatives are provided; the first one, numbered 1 is the
preferred one. It specifies media capabilities 1 and 3, i.e. G.729
and PCMU, or media capability 4 and 3, i.e., G.729B and PCMU.
Furthermore, it specifies transport protocol capability 1 (i.e. the
RTP/SAVP profile - secure RTP), and the attribute capability 1, i.e.
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the crypto attribute provided. Lastly, it specifies, a payload type
mapping for codecs 1, 3, and 4 thereby permitting the offerer to
distinguish between encrypted media and unencrypted media received
prior to receipt of the answer. For SRTP the offerer will still need
to receive the answer before being able to decrypt the stream.
The second alternative specifies media capability 2, i.e. iLBC, under
the default RTP/AVP profile. The media line, with any qualifying
attributes such as fmtp or rtpmap, is itself considered a valid
configuration; it is assumed to be the lowest preference.
Bob receives the SDP offer from Alice. Bob supports RTP, but not
SRTP, and hence he accepts the actual configuration for RTP provided
by Alice. Furthermore, Bob wants to use the iLBC codec and hence
generates the following answer:
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
t=0 0
a=csup:med-v0
m=audio 4567 RTP/AVP 103
a=rtpmap:103 iLBC/8000
a=fmtp:103 mode=20
a=acfg:2
Bob includes the "a=csup" and "a=acfg" attribute in the answer to
inform Alice that he can support the med-v0 level of capability
negotiations. Note that in this particular example, the answerer
supported the capability extensions defined here, however had he not,
he would simply have processed the offer based on the offered PCMU
and G.729 codecs under the RTP/AVP profile only. Consequently, the
answer would have omitted the "a=csup" attribute line and chosen one
or both of the PCMU and G.729 codecs instead. The answer carries the
accepted configuration in the m line along with corresponding rtpmap:
and/or fmtp: parameters, as appropriate.
Note that per the base protocol, after the above, Alice MAY generate
a new offer with an actual configuration ("m=" line, etc.)
corresponding to the actual configuration referenced in Bob's answer
(not shown here).
3.2. Capability Attributes
In this section, we present the new attributes associated with
indicating the media capabilities for use by the SDP Capability
negotiation. The approach taken is to keep things similar to the
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existing media capabilities defined by the existing media
descriptions ("m=" lines) and the associated "rtpmap" and "fmtp"
attributes. We use media subtypes and "media capability numbers"
instead of payload types to link the relevant media capability
parameters. This permits the capabilities to be defined at the
session level and used for multiple streams, if desired. Payload
types are then specified at the media level (see Section 3.2.2).
A media capability merely indicates possible support for the media
type and media format(s) in question. In order to actually use a
media capability in an offer/answer exchange, it must be referenced
in a potential configuration (see Section 2.3.1).
Media capabilities can be provided at the session-level and/or the
media-level. Media capabilities provided at the session level may be
referenced in an lcfg attribute at the session level, or by any pcfg
attribute at the media level, whereas media capabilities provided at
the media level may be referenced by a pcfg attribute within that
media stream only. In either case, the scope of the <med-cap-num> is
the entire session description. This enables each media capability
to be uniquely referenced across the entire session description (e.g.
in a potential configuration)
3.2.1. Media Type and Subtype Capability Attribute
Media types and subtypes can be expressed as media format
capabilities by use of the "a=mcap" attribute, which is defined as
follows:
a=mcap:<med-cap-num> <subtype>*[ <subtype>]
where <med-cap-num> is an integer between 1 and 2^31-1 (both
included) used to number the media format capabilities and the
<subtype> is the media subtype e.g. H263-1998, PCMU. The <med-cap-
num> is the media capability number associated with the first subtype
in the list, the number associated with the second subtype is one
higher, etc. Each occurrence of the attribute MUST use a different
value of <med-cap-num>. Furthermore, when an "mcap" attribute
indicates more than one media format, the capability numbers implied
MUST NOT be used by any other "mcap" attribute in the entire session
or media description (explicitly or implicitly).
In ABNF, we have:
media-capability-line = "a=mcap:" media-cap-num WSP media-type
1*WSP media-cap *(WSP media-cap)
media-cap-num = 1*DIGIT
media-cap = token ; Subtype name(PCMU, G729, etc.)
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The "mcap" attribute can be provided at the session-level and the
media-level. There can be more than one mcap attribute at the
session or media level. The unique media-cap-num is used to identify
it in potential configurations. When used in a potential
configuration it is a media level attribute regardless if it is
specified at the session or media level.
For example:
v=0
a=mcap:1 iLBC
a= mcap:2 H263-1998, H264
3.2.2. The Media Encoding Parameter Capability Attribute
Media format capabilities may require additional encoding parameters,
such as sample rate, to be precisely defined. The "a=ecap" encoding
attribute is defined as
a=ecap:<med-cap-num-list> <clock rate>[/<encoding parameters>]
where
med-cap-num-list = <med-cap-num> *[COMMA<med-cap-num>]
The clock rate and other encoding parameters are as defined for the
"a=rtpmap:" attribute defined in RFC 4566 [RFC4566].
The "ecap" attribute can be provided at the session-level and the
media-level. There can be more than one ecap attribute at the
session or media level. The unique media-cap-num is used to identify
it in potential configurations. When used in a potential
configuration it is a media level attribute regardless if it is
specified at the session or media level.
For example, a capability for low-bit-rate encoding at 8000 samples
per second could be specified by
a=mcap:1 iLBC
a=ecap:1 8000
The encoding becomes part of the media capability. Thus, if it is
desirable to specify the same subtype with, e.g., two different
encoding rates, then the subtype should be listed twice, and each
should be modified appropriately. For example:
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a=mcap:1 L16 L16
a=ecap:1 8000
a=ecap:2 16000/2
defines two low-data-rate codecs, codec 1 uses 8000 samples per
second, and codec 2 uses 16000 samples per second and 2 channels.
3.2.3. The Media Format Parameter Capability Attribute
This attribute is used to associate media format parameters with a
media capability. The form of the attribute is:
a=fcap:<med-cap-num-list> <list of format parameters>
where <med-cap-num-list> permits the format parameter(s) to be
associated with one or more media capabilities, and the format
parameters are specific to the type of codec(s), as described for the
fmtp: attribute defined in RFC 4566[RFC4566]. Unlike the fmtp
attribute, multiple fcap attributes may be associated with a
specified med-cap-num. This provides the ability to mix and match
format parameters when using codecs with multiple format options.
The fcap attribute adheres to RFC 4566 attribute production rules
with
media-format-capability-line = "a=fcap:" <att-value>
att-value = <med-cap-num-list> WSP <format-specific-parameter-
list>
med-cap-num-list = <med-cap-num> *["," <med-cap-num>] ; med-cap-
num is defined in Section 3.2.1
format-specific-parameter-list = <format-specfic-parameter>
*[";"<format-specfic-parameter>] ; format-specific-parameter-
list are defined per codec/capability.
The "fcap" attribute can be provided at the session-level and the
media-level. There can be more than one fcap attribute at the
session or media level. The unique media-cap-num is used to identify
it in potential configurations. When used in a potential
configuration it is a media level attribute regardless if it is
specified at the session or media level.
As a simple example, a G.729 capability is, by default, considered to
support comfort noise as defined by Annex B. Capabilities for G.729
with and without comfort noise support may thus be identified by:
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a=mcap:1 audio G729 G729
a=fcap:2 annexb:no
Example for H.263 video:
a=mcap:1 video H263-1998 H263-2000
a=fcap:1 CIF=4;QCIF=2;F=1;K=1
a=fcap:2 profile=2;level=2.2
Finally, for six format combinations of the Adaptive MultiRate codec:
a=mcap:1 audio AMR AMR AMR AMR AMR AMR
a=ecap:1,3,5 16000/1
a=ecap:2,4,6 8000/1
a=fcap:1,2,3,4 mode-change-capability=1
a=fcap:5,6 mode-change-capability=2
a=fcap:1,2,3,5 max-red=220
a=fcap:3,4,5,6 octet-align=1
a=fcap:1,3,5 mode-set=0,2,4,7
a=fcap:2,4,6 mode-set=0,3,5,6
So that AMR codec #1, when specified in a pcfg attribute (and
assigned payload type 98) as in
a=pcfg:1 m=1 pt=1:98
is essentially equivalent to the following
m=audio 49170 RTP/AVP 98
a=rtpmap:98 AMR/16000/1
a=fmtp:98 mode-change-capability=1; max-red=220; mode-
set=0,2,4,7
and AMR codec #4 with payload type 99, is essentially equivalent to
the following:
m=audio 49170 RTP/AVP 99
a=rtpmap:99 AMR/8000/1
a=fmtp:99 mode-change-capability=1; octet-align=1; mode-
set=0,3,5,6
and so on for the other four combinations.
3.2.4. The Bandwidth Capability Attribute
In some cases it is desirable to specify different bandwidth limits
for different media configurations. This may be done by use of the
"a=bcap" attribute, which is defined as follows:
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a=bcap:<bw-cap-num> <bwtype>:<bandwidth>
where <bw-cap-num> is an integer between 1 and 2^31-1 (both included)
used to identify the bandwidth capability, <bwtype> is the bandwidth
type, and <bandwidth> is the bandwidth value, as defined for the b=
line in RFC4566[RFC4566]
In ABNF, we have:
media-bandwidth-cap-line = "a=bcap:" bw-cap-num 1*WSP
bwtype ":" bandwidth
where
bw-cap-num = 1*DIGIT; the bandwidth "handle"
bwtype = token; as defined in RFC4566
bandwidth = 1*DIGIT; as defined in RFC4566
The "bcap" attribute can appear at the session level, where it can be
referenced by lcfg or pcfg attributes, or at the media level, where
it can be referenced by pcfg attributes. When invoked by a pcfg or
lcfg attribute, the resulting bandwidth line (b=) is to be
interpreted at the media-level. There can be more then one bcap
attribute. The unique bw-cap-num is used to identify it in potential
configurations.
Note: The session level bandwidth gives a maximum total values for
all the media in the session. Currently the authors suggest that
enforcing a potential limit can be done at the media level and by
using re-INVITE for the session level bandwidth.
Bandwidth capabilities may be included in a potential configuration
via the "b=" parameter (see below). Any bandwidth capability
included replaces any media-level bandwidth of the same type declared
in a "b=" SDP line.
The following example offers a preferred potential configuration for
H.263 QCIF at 360 Kbit/sec and a second potential configuration for
H.263 CIF at the offered 500 Kbit/sec
m=video 49170 RTP/AVP 99
b=TIAS:500000
a=rtpmap:99 H263-1998/90000
a=fmtp:99 CIF=4; QCIF=2
a=mcap:1 video H263-1998 H263-1998
a=fcap:1 QCIF=2
a=fcap:2 CIF=4; QCIF=2;F=1;K=1
a=bcap:1 TIAS:360000
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a=pcfg:1 m=1 b=1 pt:100
a=pcfg:2 m=2 pt:101
3.3. Extensions to the Potential Configuration Attribute
The extension protocol of capabilities negotiation requires three new
extensions for the pcfg: attribute defined in the base protocol. The
first extension permits the specification of media capabilities, or
combinations thereof; the second permits the assignment of payload
types to those capabilities when used in the specified configuration;
the third permits the specification of bandwidth limits for a media
stream.
3.3.1. The Media Capability Extension to the Potential Configuration
Attribute
The potential configuration attribute ("a=pcfg") as defined in SDP
capabilities negotiation, permits alternate attributes to be
associated with the media types defined in a media line. In this
document, we define an extension parameter for the specification of
media configurations in addition to the one specified on the media
line.
We define the media capability configuration parameter, pot-media-
config, in accordance with the following format:
m=<med-cap-list> *["|"<med-cap-list>]
where <med-cap-list> is a comma-separated list of media capability
numbers (media-cap-num) as defined by a=mcap: lines and media lines.
In ABNF form (adhering to the ABNF for pot-extension-config in
[SDPCapNeg]:
pot-media-config = "m=" med-cap-list *(BAR med-cap-list)
med-cap-list = med-cap-num *("," med-cap-num)
med-cap-num = 1*DIGIT ; defined in SDP
BAR = *WSP "|" *WSP
Each potential media configuration is a comma-separated list of media
capability numbers where med-cap-num refers to media capability
numbers defined explicitly by a=mcap attributes and hence MUST be
between 1 and 2^31-1 (both included). Alternative potential media
configurations are separated by a vertical bar ("|"). The
alternatives are ordered by preference. When media capabilities are
not included in a potential configuration at the media level, the
media type and media format from the associated "m=" line will be
used.
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For example:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
a=creq:med-v0
m=audio 3456 RTP/AVP 0 18 100
a=rtpmap:100 telephone-events
a=fmtp:100 0-15
a=mcap:1 audio PCMU g729 telephone-event
a=ecap:1,2,3 8000
a=fcap:3 0-15
a=pcfg:1 m=2,3|1,3 pt=1:0, 2:18, 3:100
In this example, G729 is media capability 2, PCMU is media capability
1, and events is media capability 3. The a=pcfg: line specifies that
the preferred configuration is G.729 with dtmf events, second is
G.711 mu-law with dtmf events. Intermixing of G.729, G.711, and dtmf
events is least preferred (the actual configuration provided by the
"m=" line, which is always the least preferred configuration).
3.3.2. The Payload Type Mapping Extension to the Potential
Configuration Attribute
When media capabilities defined in mcap: attributes are used in
potential configuration lines, it is necessary to assign payload
types to them. In some cases, it is desirable to assign different
payload types to media capabilities defined in the media line. One
example of the latter is when configurations for AVP and SAVP are
offered: the offerer would like the answerer to use different payload
types for encrypted and unencrypted media so that it (the offerer)
can decide whether or not to render early media which arrives before
the answer is received.
We define the media type mapping configuration parameter, pt-media-
map, in accordance with the following format:
pt-media-map = "pt=" med-map *("," med-map);
med-map = <med-cap-num> ":" <'payload-type> ;
med-cap-num is defined above
payload-type = 1*DIGIT ; RTP payload type
The example in the previous section shows how the parameters from the
mcap line are mapped to payload type in the pcfg "pt" parameter.
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3.3.3. The Bandwidth Extension to the Potential Configuration Attribute
A bandwidth parameter is added to the pcfg attribute in order to
provide the flexibility to specify different bandwidth limits for
different configurations.
The bandwidth parameter, bw-param, is defined in accordance with the
following format:
bw-param = "b=" bw-cap-list;
bw-cap-list = bw-cap-num *("," bw-cap-num);
bw-cap-num = 1*DIGIT; as defined above
Note that it is considered an error condition if the same bandwidth
type is referenced more than once in the bw-cap-list.
3.4. Extensions to the Actual Configuration Attribute
The Actual configuration attribute is specified in [SDPCapNeg]. The
actual configuration MUST list the potential configuration selected
by the answerer. This section adds extensions parameters enabling
the answerer to specify the potential configuration attributes
defined in this document and used in forming the answer.
We define actual configuration extensions based on the sel-extension-
config specified in [SDPCapNeg]. The new parameters are: parameter
act-media-config, act-med-map and act-bw-param in accordance with the
following ABNF:
act-media-config = "m=" med-cap-list ; defined in section
3.3.1
act-med-map = "pt=" med-map *("," med-map) ; defined in
section 3.3.2
act-bw-param = "b=" bw-cap-list ; defined in section 3.3.3
A response to the previous offer example in the above section might
be:
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
t=0 0
a=csup:med-v0
m=audio 5432 RTP/AVP 18 100
a=rtpmap:100 events
a=fmtp:100 0-15
a=acfg:1 m=2,3 pt=2:18,3:100
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Note that the capability numbers expressed in the acfg: attribute are
based on the offered capability numbering, not on those listed in the
answer, if any. The acfg attribute identifies to the offrer which
potential configuration and media encoding combination was selected
by the answerer.
3.5. The Latent Configuration Attribute
One of the goals of this work is to permit the exchange of media
configurations in addition to those offered for immediate use. Such
configurations are referred to as "latent configurations". For
example, a party may offer to establish an audio session, and, at the
same time, announce its ability to support a video session and supply
its video capabilities.
Latent configurations may be announced by use of the latent
configuration attribute, which is defined in a manner very similar to
the potential configuration attribute:
a=lcfg:<preference> ["m="<media-caps-list>] ["t="<transport>]
["a="<attributes>] ["b="<bw-cap-list>]
The m=, t=, b= and a= parameters are identical in format and meaning
to those defined for the pcfg: attribute. Note that the pt=
parameter is not permitted in the lcfg: attribute because no actual
media session is being offered or accepted.
Latent Configurations MUST be specified at the session level when
they represent an additional media stream to those in the offer or
answer. If an acap: attribute is declared at the session level for
use in a session-level lcfg line, it SHOULD NOT be used in a pcfg
line at the media level unless it is to become a session-level
attribute in the answer.
3.5.1. The crypto: Attribute in Latent Configurations
If the sdescriptions crypto: attribute is necessary as part of any
latent configuration which announces sdescriptions capabilities, it
presents a slight problem in that the rather long key/salt string is
useless and should be ignored. This problem is avoided if we exclude
the a= parameters from the lcfg: attribute. [Editor's note: Should
we define a new crypto: key-method, e.g. "latent", in which the key-
salt portion of key-info is empty? I think it may be sufficient to
just include the RTP/SAVP transport to indicate SRTP capability since
there will be a new offer when the latent configuration will be
offered.]
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3.6. Offer/Answer Model Extensions
In this section, we define extensions to the offer/answer model
defined in RFC3264 [RFC3264] to allow for media capabilities to be
used with the SDP Capability Negotiation framework.
3.6.1. Generating the Initial Offer
When an endpoint generates an initial offer and want to use the
functionality described in the current document, it should identify
and define the codecs it can support via mcap , ecap , and fcap
attributes. The SDP media line(s) should be made up with the default
(least preferred) configuration. Typically, the media line
configuration will contain the minimum acceptable capabilities. The
offer MUST include the required extensions needed to support this
functionality.
Preferred configurations for each media stream are identified
following the media line. The present offer may also include latent
configuration (lcfg) attributes, at the session level, describing
media streams and/or configurations the offeror is not now offering,
but which it is willing to support in a future offer/answer exchange.
A simple example might be the inclusion of a latent video
configuration in an offer for an audio stream.
3.6.2. Generating the Answer
When the answering party receives the offer and if it supports the
required extensions, it should select the most-preferred
configuration for each media stream, and build its answer
accordingly. Each selected media configuration is placed into the
answer as a media line with associated parameters, It must include
the supported extension attribute and an actual configuration (acfg)
attribute to indicate just which pcfg attribute was used to build the
answer. The answer should may also include any latent configurations
the answerer can support, especially any configurations compatible
with latent configurations received in the offer. The answerer
should make note of those configurations it might wish to offer in
the future.
3.6.3. Offerer Processing of the Answer
When the offerer receives the answer, it should make note of any
capabilities and/or latent configurations for future use. The media
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
present, may be used to verify the proposed configuration used to
form the answer, and to infer the lack of acceptability of higher-
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preference configurations that were not chosen. Note that the base
specification [SDPCapNeg] requires the answerer to choose the highest
preference configuration it can support.
3.6.4. Modifying the Session
If, at a later time, one of the parties wishes to modify the
operating parameters of a session, e.g., by adding a new media
stream, or changing a codec used on an existing stream, it may do so
via the mechanisms defined for offer/answer[RFC3264]. If the
initiating party has remembered the codecs, potential configurations,
and latent configurations announced by the other party in the earlier
negotiation, it may use this knowledge to maximize the likelihood of
a successful modification of the session. Alternatively, it may
perform a new capabilities exchange as part of the reconfiguration.
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4. Examples
In this section, we provide examples showing how to use the Media
Capabilities with the SDP Capability Negotiation.
4.1. Alternative Codecs
4.2. Latent Media Streams
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5. IANA Considerations
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6. Security Considerations
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7. Changes from previous versions
7.1. Changes from version 01
The documents adds a new attribute for specifying bandwidth
capability and a parametr to list in the potential configuration.
Other changes are to align the document with the terminolgy and
attribute names from draft-ietf-mmusic-sdp-capability-negotiation-07.
The document also clarifies some previous open issues.
7.2. Changes from version 00
The major changes include taking out the "mcap" and "cptmap"
parameter. The mapping of payload type is now in the "pt" parameter
of "pcfg". Media subtype need to explictly definesd in the "cmed"
attribute if referenced in the "pcfg"
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8. Acknowledgements
This document is heavily influenced by the discussions and work done
by the SDP Capability Negotiation Design team. The following people
in particular provided useful comments and suggestions to either the
document itself or the overall direction of the solution defined
herein: Cullen Jennings, Matt Lepinski, Joerg Ott, Colin Perkins, and
Thomas Stach.
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9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[SDPCapNeg]
Andreasen, F., "SDP Capability Negotiation",
draft-ietf-mmusic-sdp-capability-negotiation-07 (work in
progress), October 2007.
9.2. Informative References
[RFC3407] Andreasen, F., "Session Description Protocol (SDP) Simple
Capability Declaration", RFC 3407, October 2002.
[RFC3952] Duric, A. and S. Andersen, "Real-time Transport Protocol
(RTP) Payload Format for internet Low Bit Rate Codec
(iLBC) Speech", RFC 3952, December 2004.
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Authors' Addresses
Robert R Gilman
Avaya, Inc.
1300 West 120th Avenue
Westminster, CO 80234
USA
Email: rrg@avaya.com
Roni Even (editor)
Polycom
94 Derech Em Hamoshavot
Petach Tikva 49130
Israel
Email: roni.even@polycom.co.il
Flemming Andreasen
Cisco Systems
Edison, NJ
USA
Email: fandreas@cisco.com
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