One document matched: draft-ietf-mmusic-sdp-media-capabilities-15.xml
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<rfc category="std" docName="draft-ietf-mmusic-sdp-media-capabilities-15"
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<front>
<title abbrev="SDP Media Capabilities Negotiation">Session Description Protocol (SDP) Media Capabilities
Negotiation</title>
<author fullname="Robert R Gilman " initials="R" surname="Gilman">
<organization>Independent</organization>
<address>
<postal>
<street>3243 W. 11th Ave. Dr.</street>
<city>Broomfield, CO 80020</city>
<country>USA</country>
</postal>
<email>bob_gilman@comcast.net</email>
</address>
</author>
<author fullname="Roni Even" initials="R" surname="Even">
<organization>Gesher Erove Ltd</organization>
<address>
<postal>
<street>14 David Hamelech</street>
<city>Tel Aviv</city>
<country>Israel</country>
<code>64953</code>
</postal>
<email>ron.even.tlv@gmail.com</email>
</address>
</author>
<author fullname="Flemming Andreasen " initials="F" surname="Andreasen">
<organization>Cisco Systems</organization>
<address>
<postal>
<street></street>
<city>Iselin, NJ</city>
<country>USA</country>
</postal>
<email>fandreas@cisco.com</email>
</address>
</author>
<date day="1" month="October" year="2012" />
<workgroup>MMUSIC</workgroup>
<abstract>
<t>Session Description Protocol (SDP) capability negotiation provides a
general framework for indicating and 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.</t>
<t>This document updates the IANA Considerations of RFC 5939.</t>
</abstract>
</front>
<middle>
<section anchor="intro" title="Introduction">
<t>Session Description Protocol (SDP) capability negotiation <xref
target="RFC5939"></xref> provides a general framework for indicating and
negotiating capabilities in SDP<xref target="RFC4566"></xref>. The base
framework defines only capabilities for negotiating transport protocols
and attributes.</t>
<t>The <xref target="RFC5939"></xref> 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 concurrently.</t>
<t>In this document, we extend the framework by defining media
capabilities that can be used to indicate and negotiate media types and
their associated format parameters. This document also adds the ability
to declare support for media streams, the use of which can be offered
and negotiated later, and the ability to specify session configurations
as combinations of media stream configurations. The definitions of new
attributes for media capability negotiation are chosen to make the
translation from these attributes to "conventional" SDP <xref
target="RFC4566"></xref> media attributes as straightforward as possible
in order to simplify implementation. This goal is intended to reduce
processing in two ways: each proposed configuration in an offer may be
easily translated into a conventional SDP media stream record for
processing by the receiver; and the construction of an answer based on a
selected proposed configuration is straightforward.</t>
<t>This document updates <xref target="RFC5939"></xref> by
updating the IANA Considerations. All other extensions defined
in this document are considered extensions above and beyond
<xref target="RFC5939"></xref>. </t>
</section>
<section anchor="terms" title="Terminology">
<t>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 <xref
target="RFC2119"></xref> and indicate requirement levels for compliant
RTP implementations.</t>
<t></t>
<t>"Actual Configuration": An actual configuration specifies which
combinations of SDP session parameters and media stream components can
be used in the current offer/answer exchange and with what parameters.
Use of an actual configuration does not require any further negotiation
in the offer/answer exchange. See <xref target="RFC5939"></xref> for further details. </t>
<t>"Base Attributes": Conventional SDP attributes appearing in the base
configuration of a media block.</t>
<t>"Base Configuration": The media configuration represented by a media
block exclusive of all the capability negotiation attributes defined in
this document, the base capability negotiation document<xref
target="RFC5939"></xref>, or any other capability negotiation document.
In an offer SDP, the base configuration corresponds to the actual
configuration as defined in <xref target="RFC5939"></xref>.</t>
<t>"Conventional Attribute": Any SDP attribute other than those defined
by the series of capability negotiation specifications.</t>
<t>"Conventional SDP": An SDP record devoid of capability negotiation
attributes.</t>
<t>"Media Format Capability": A media format, typically a media subtype such as
PCMU, H263-1998, or T38, expressed in the form of a capability.</t>
<t>"Media Format Parameter Capability": A media format parameter
("a=fmtp" in conventional SDP) expressed in the form of a
capability. The media format parameter capability is associated
with a media format capability.</t>
<t>"Media Capability": The combined set of capabilities
associated with expressing a media format and its relevant
parameters (e.g. media format parameters and media specific
parameters).</t>
<t>"Potential Configuration": A potential configuration indicates which
combinations of capabilities can be used for the session and its
associated media stream components. Potential configurations are not
ready for use, however they are offered for potential use in the current
offer/answer exchange. They provide an alternative that may be used
instead of the actual configuration, subject to negotiation in the
current offer/answer exchange. See <xref target="RFC5939"></xref> for
further details.</t>
<t>"Latent Configuration": A latent configuration indicates which
combinations of capabilities could be used in a future negotiation for
the session and its associated media stream components. Latent
configurations are neither ready for use, nor are they offered for
actual or potential use in the current offer/answer exchange. Latent
configurations merely inform the other side of possible configurations
supported by the entity. Those latent configurations may be used to
guide subsequent offer/answer exchanges, but they are not offered for
use as part of the current offer/answer exchange. </t>
</section>
<section anchor="sdpmedcap" title=" SDP Media Capabilities">
<t>The SDP capability negotiation <xref target="RFC5939"></xref>
discusses the use of any SDP <xref target="RFC4566"></xref> attribute
(a=) under the attribute capability "acap". The limitations of using
acap for fmtp and rtpmap in a potential configuration are described in
<xref target="RFC5939"></xref>; for example they can be used only at the
media level since they are media level attributes. The <xref
target="RFC5939"></xref> does not provide a way to exchange media-level
capabilities prior to the actual offer of the associated media stream.
This section provides an overview of extensions providing an 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 <xref
target="RFC3264"></xref></t>
<section anchor="reqts" title="Requirements">
<t>The capability negotiation extensions requirements considered
herein are as follows.</t>
<t><list style="hanging">
<t hangText="REQ-01: ">Support the specification of alternative
(combinations of) media formats (codecs) in a single media
block.</t>
</list></t>
<t><list style="hanging">
<t hangText="REQ-02: ">Support the specification of alternative
media format parameters for each media format.</t>
</list></t>
<t><list style="hanging">
<t hangText="REQ-03: ">Retain backward compatibility with
conventional SDP. Ensure that each and every offered configuration
can be easily translated into a corresponding SDP media block
expressed with conventional SDP lines.</t>
</list></t>
<t><list style="hanging">
<t hangText="REQ-04: ">Ensure the scheme operates within the
offer/answer model in such a way that media formats and parameters
can be agreed upon with a single exchange.</t>
</list></t>
<t><list style="hanging">
<t hangText="REQ-05: ">Provide the ability to express offers in
such a way that the offerer can receive media as soon as the offer
is sent. (Note that the offerer may not be able to render received
media prior to exchange of keying material.)</t>
</list></t>
<t><list style="hanging">
<t hangText="REQ-06: ">Provide the ability to offer latent media
configurations for future negotiation.</t>
</list></t>
<t><list style="hanging">
<t hangText="REQ-07: ">Provide reasonable efficiency in the
expression of alternative media formats and/or format parameters,
especially in those cases in which many combinations of options
are offered.</t>
</list></t>
<t><list style="hanging">
<t hangText="REQ-08: ">Retain the extensibility of the base
capability negotiation mechanism.</t>
</list></t>
<t><list style="hanging">
<t hangText="REQ-09: ">Provide the ability to specify acceptable
combinations of media streams and media formats. For example,
offer a PCMU audio stream with an H264 video stream, or a G729
audio stream with an H263 video stream. This ability would give
the offerer a means to limit processing requirements for
simultaneous streams. This would also permit an offer to include
the choice of an audio/T38 stream or an image/T38 stream, but not
both.</t>
</list></t>
<t>Other possible extensions have been discussed, but have not been
treated in this document. They may be considered in the future. Three
such extensions are:</t>
<t><list style="hanging">
<t hangText="FUT-01: ">Provide the ability to mix, or change,
media types within a single media block. Conventional SDP does not
support this capability explicitly; the usual technique is to
define a media subtype that represents the actual format within
the nominal media type. For example, T.38 FAX as an alternative to
audio/PCMU within an audio stream is identified as audio/T38; a
separate FAX stream would use image/T38.</t>
</list></t>
<t><list style="hanging">
<t hangText="FUT-02: ">Provide the ability to support multiple
transport protocols within an active media stream without
reconfiguration. This is not explicitly supported by conventional
SDP.</t>
</list></t>
<t><list style="hanging">
<t hangText="FUT-03: ">Provide capability negotiation attributes
for all media-level SDP line types in the same manner as already
done for the attribute type, with the exception of the media line
type itself. The media line type is handled in a special way to
permit compact expression of media coding/format options. The line
types are bandwidth ("b="), information ("i="), connection data
("c="), and, possibly, the deprecated encryption key ("k=").</t>
</list></t>
</section>
<section anchor="overview" title="Solution Overview">
<t>The solution consists of new capability attributes corresponding to
conventional SDP line types, new parameters for the pcfg, acfg, and
the new lcfg attributes extending the base attributes from <xref
target="RFC5939"></xref>, and a use of the pcfg attribute to return
capability information in the SDP answer.</t>
<t>Several new attributes are defined in a manner that can be related
to the capabilities specified in a media line, and its corresponding
rtpmap and fmtp attributes.</t>
<t><list style="symbols">
<t>A new attribute ("a=rmcap") defines RTP-based media
format capabilities in the form of a media subtype (e.g. "PCMU"), and its
encoding parameters (e.g. "/8000/2"). Each resulting media format
type/subtype capability has an associated handle called a media
capability number. The encoding parameters are as specified for
the rtpmap attribute defined in <xref target="RFC4566"></xref>,
without the payload type number part.</t>
<t>A new attribute ("a=omcap") defines other (non RTP-based)
media format capabilities in the form of a media subtype only (e.g.
"T38"). Each resulting media format type/subtype capability has an
associated handle called a media capability number.</t>
<t>A new attribute ("a=mfcap") specifies media format parameters
associated with one or more media format capabilities. The mfcap
attribute is used primarily to associate the media format parameters
normally carried in the fmtp attribute. Note that
media format parameters can be used with RTP and non-RTP based
media formats.</t>
<t>A new attribute ("a=mscap") that specifies media parameters
associated with one or more media format capabilities. The mscap
attribute is used to associate capabilities with attributes other
than fmtp or rtpmap, for example, the rtcp-fb attribute defined in
<xref target="RFC4585"></xref>.</t>
<t>A new attribute ("a=lcfg") specifies latent media stream
configurations when no corresponding media line ("m=") is offered.
An example is the offer of latent configurations for video even
though no video is currently offered. If the peer indicates
support for one or more offered latent configurations, the
corresponding media stream(s) may be added via a new offer/answer
exchange.</t>
<t>A new attribute ("a=sescap") is used to specify an acceptable
combination of simultaneous media streams and their configurations
as a list of potential and/or latent configurations.</t>
</list></t>
<t>New parameters are defined for the potential configuration (pcfg),
latent configuration (lcfg), and accepted configuration (acfg)
attributes to associate the new attributes with particular
configurations.</t>
<t><list style="symbols">
<t>A new parameter type ("m=") is added to the potential
configuration ("a=pcfg:") attribute and the actual configuration
("a=acfg:") attribute defined in <xref target="RFC5939"></xref>,
and to the new latent configuration ("a=lcfg:") attribute. This
permits specification of media capabilities (including their
associated parameters) and combinations thereof for the
configuration. For example, the "a=pcfg:" line might specify PCMU
and telephone events <xref target="RFC4733"></xref> or G.729B and
telephone events as acceptable configurations. The "a=acfg:" line
in the answer would specify the configuration chosen.</t>
<t>A new parameter type ("pt=") is added to the potential
configuration, actual configuration, and latent configuration
attributes. This parameter associates RTP payload type numbers
with the referenced RTP-based media format capabilities, and is
appropriate only when the transport protocol uses RTP.</t>
<t>A new parameter type ("mt=") is used to specify the media type
for latent configurations.</t>
</list></t>
<t>Special processing rules are defined for capability attribute
arguments in order to reduce the need to replicate
essentially-identical attribute lines for the base configuration and
potential configurations.</t>
<t><list style="symbols">
<t>A substitution rule is defined for any capability attribute to
permit the replacement of the (escaped) media capability number
with the media format identifier (e.g., the payload type number in
audio/video profiles).</t>
<t>Replacement rules are defined for the conventional SDP
equivalents of the mfcap and mscap capability attributes. This
reduces the necessity to use the deletion qualifier in the a=pcfg
parameter in order to ignore rtpmap, fmtp, and certain other
attributes in the base configuration.</t>
<t>An argument concatenation rule is defined for mfcap attributes
which refer to the same media capability number. This makes it
convenient to combine format options concisely by associating
multiple mfcap lines with multiple media format capabilities.</t>
</list></t>
<t>This 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) and associated
parameters, latent configurations merely inform the other side of
possible configurations supported by the entity. Those latent
configurations may be used to guide subsequent offer/answer exchanges,
but they are not part of the current offer/answer exchange.</t>
<t>The mechanism is illustrated by the offer/answer exchange below,
where Alice sends an offer to Bob:</t>
<figure>
<artwork><![CDATA[
Alice Bob
| (1) Offer (SRTP and RTP) |
|--------------------------------->|
| |
| (2) Answer (RTP) |
|<---------------------------------|
| |
]]></artwork>
</figure>
<t>Alice's offer includes RTP and SRTP as alternatives. RTP is the
default, but SRTP is the preferred one (long lines are folded to fit
the margins):</t>
<t><list hangIndent="10">
<t>v=0<vspace blankLines="0" /> o=- 25678 753849 IN IP4
192.0.2.1<vspace blankLines="0" /> s=<vspace
blankLines="0" /> c=IN IP4 192.0.2.1<vspace blankLines="0"
/> t=0 0<vspace blankLines="0" /> a=creq:med-v0<vspace
blankLines="0" /> m=audio 3456 RTP/AVP 0 18<vspace
blankLines="0" /> a=tcap:1 RTP/SAVP RTP/AVP<vspace
blankLines="0" /> a=rtpmap:0 PCMU/8000/1<vspace
blankLines="0" /> a=rtpmap:18 G729/8000/1<vspace
blankLines="0" /> a=fmtp:18 annexb=yes<vspace
blankLines="0" /> a=rmcap:1,4 G729/8000/1<vspace
blankLines="0" /> a=rmcap:2 PCMU/8000/1<vspace
blankLines="0" /> a=rmcap:5 telephone-event/8000<vspace
blankLines="0" /> a=mfcap:1 annexb=no<vspace
blankLines="0" /> a=mfcap:4 annexb=yes<vspace
blankLines="0" /> a=mfcap:5 0-11<vspace blankLines="0" />
a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32 \ <vspace
blankLines="0" />
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32<vspace
blankLines="0" /> a=pcfg:1 m=4,5|1,5 t=1 a=1
pt=1:100,4:101,5:102<vspace blankLines="0" /> a=pcfg:2 m=2
t=1 a=1 pt=2:103<vspace blankLines="0" /> a=pcfg:3 m=4 t=2
pt=4:18</t>
</list></t>
<t>The required base and extensions are provided by the "a=creq"
attribute defined in <xref target="RFC5939"></xref>, with the option
tag "med-v0", which indicates that the extension framework defined
here, must be supported. The base level capability negotiation support
("cap-v0" <xref target="RFC5939"></xref>) is implied since it is
required for the extensions.</t>
<t>The "m=" line indicates that Alice is offering to use plain RTP
with PCMU or G.729B. The media line implicitly defines the default
transport protocol (RTP/AVP in this case) and the default actual
configuration.</t>
<t>The "a=tcap:1" line, specified in the capability negotiation base
protocol <xref target="RFC5939"></xref>, defines transport protocol
capabilities, in this case Secure RTP (SAVP profile) as the first
option and RTP (AVP profile) as the second option.</t>
<t>The "a=rmcap:1,4" line defines two G.729 RTP-based media format
capabilities, numbered 1 and 4, and their encoding rate. The
capabilities are of media type "audio" and subtype G729. Note that the
media subtype is explicitly specified here, rather than RTP payload
type numbers. This permits the assignment of payload type numbers in
the media stream configuration specification. In this example, two
G.729 subtype capabilities are defined. This permits the declaration
of two sets of formatting parameters for G.729.</t>
<t>The "a=rmcap:2" line defines a G.711 mu-law capability, numbered
2.</t>
<t>The "a=rmcap:5" line defines an audio telephone-event capability,
numbered 5.</t>
<t>The "a=mfcap:1" line specifies the fmtp formatting parameters for
capability 1 (offerer will not accept G.729 Annex B packets).</t>
<t>The "a=mfcap:4" line specifies the fmtp formatting parameters for
capability 4 (offerer will accept G.729 Annex B packets).</t>
<t>The "a=mfcap:5" line specifies the fmtp formatting parameters for
capability 5 (the DTMF touchtones 0-9,*,#).</t>
<t>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.</t>
<t>The "a=pcfg:" attributes provide the potential
configurations included in the offer by reference to the media
capabilities, transport capabilities, attribute capabilities
and specified payload type number mappings. Three explicit
alternatives are provided; the lowest-numbered one is the
preferred one. The "a=pcfg:1 ..." line specifies media
capabilities 4 and 5, i.e., G.729B and DTMF (incl. their
associated media format parameters), or media capability 1 and
5, i.e., G.729 and DTMF (incl. their associated media format
parameters). Furthermore, it specifies transport protocol
capability 1 (i.e. the RTP/SAVP profile - secure RTP), and the
attribute capability 1, i.e. the crypto attribute
provided. Lastly, it specifies a payload type number mapping
for (RTP-based) media capabilities 1, 4, and 5, thereby
permitting the offerer to distinguish between encrypted media
and unencrypted media received prior to receipt of the
answer.</t>
<t>Use of unique payload type numbers in alternative configurations is
not required; codecs such as AMR-WB <xref target="RFC4867"></xref>
have the potential for 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 cannot be specified, then
the offerer will be obliged to wait for the SDP answer before
rendering received media. For SRTP using SDES inline keying <xref
target="RFC4568"></xref>, the offerer will still need to receive the
answer before being able to decrypt the stream.</t>
<t>The second alternative ("a=pcfg:2 ...") specifies media capability
2, i.e., PCMU, under the RTP/SAVP profile, with the same SRTP key
material.</t>
<t>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
PCMU.</t>
<t>Per <xref target="RFC5939"></xref>, the media line, with any
qualifying attributes such as fmtp or rtpmap, is itself considered a
valid configuration (the current actual configuration); it has the
lowest preference (per <xref target="RFC5939"></xref>).</t>
<t>Bob receives the SDP offer from Alice. Bob supports G.729B, PCMU,
and telephone events over RTP, but not SRTP, hence he accepts the
potential configuration 3 for RTP provided by Alice. Bob generates the
following answer:</t>
<t><list hangIndent="10">
<t>v=0<vspace blankLines="0" /> o=- 24351 621814 IN IP4
192.0.2.2<vspace blankLines="0" /> s=<vspace blankLines="0" />
c=IN IP4 192.0.2.2<vspace blankLines="0" /> t=0 0<vspace
blankLines="0" /> a=csup:med-v0<vspace blankLines="0" /> m=audio
4567 RTP/AVP 18<vspace blankLines="0" /> a=rtpmap:18
G729/8000<vspace blankLines="0" /> a=fmtp:18 annexb=yes<vspace
blankLines="0" /> a=acfg:3 m=4 t=2 pt=4:18</t>
</list></t>
<t>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
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.</t>
<t>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).</t>
</section>
<section anchor="newattrs" title="New Capability Attributes">
<t>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
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" to link the relevant
media capability parameters. This permits the capabilities to be
defined at the session level and be used for multiple streams, if
desired. For RTP-based media formats, payload types are then specified
at the media level (see <xref target="ptparm"></xref>).</t>
<t>A media capability merely indicates possible support for the media
type and media format(s) and parameters in question. In order to actually use a media
capability in an offer/answer exchange, it MUST be referenced in a
potential configuration.</t>
<t>Media capabilities, i.e. the attributes associated with
expressing media capability formats, parameters, etc., can be
provided at the session-level and/or the media-level. Media
capabilities provided at the session level may be referenced
in any pcfg or lcfg attribute at the media level (consistent
with the media type), whereas media capabilities provided at
the media level may be referenced only by the pcfg or lcfg
attribute within that media stream. 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).</t>
<section anchor="mcap" title="The Media Format Capability Attributes">
<t>Media subtypes can be expressed as media format capabilities by
use of the "a=rmcap" and "a=omcap" attributes. The "a=rmcap"
attribute MUST be used for RTP-based media whereas the "a=omcap"
attribute MUST be used for non-RTP-based (other) media formats. The
two attributes are defined as follows:</t>
<figure>
<artwork><![CDATA[
a=rmcap:<media-cap-num-list> <encoding-name>/<clock-rate>
[/<encoding-parms>]
a=omcap:<media-cap-num-list> <format-name>
]]></artwork>
</figure>
<t>where <media-cap-num-list> is a (list of) media capability
number(s) used to number a media format capability, the <encoding
name> or <format name> is the media subtype, e.g.,
H263-1998, PCMU, or T38, <clock rate> is the encoding rate,
and <encoding parms> are the media encoding parameters for the
media subtype. All media format capabilities in the list are
assigned to the same media type/subtype. Each occurrence of the
rmcap and omcap attribute MUST use unique values in their
<media-cap-num-list>; the media capability numbers are shared
between the two attributes and the numbers MUST be unique across the
entire SDP session. In short, the rmcap and omcap attributes define
media format capabilities and associate them with a media capability number
in the same manner as the rtpmap attribute defines them and
associates them with a payload type number. Additionally, the
attributes allow multiple capability numbers to be defined for the
media format in question. This permits the media format to be
associated with different media parameters in different
configurations.</t>
<t>In ABNF <xref target="RFC5234"></xref>, we have:</t>
<figure>
<artwork><![CDATA[
media-capability-line = rtp-mcap / non-rtp-mcap
rtp-mcap = "a=rmcap:" media-cap-num-list
1*WSP encoding-name "/" clock-rate
["/" encoding-parms]
non-rtp-mcap = "a=omcap:" media-cap-num-list 1*WSP format-name
media-cap-num-list = media-cap-num-element
*("," media-cap-num-element)
media-cap-num-element = media-cap-num
/ media-cap-num-range
media-cap-num-range = media-cap-num "-" media-cap-num
media-cap-num = 1*10(DIGIT)
encoding-name = token ;defined in RFC4566
clock-rate = 1*10(DIGIT)
encoding-parms = token
format-name = token ;defined in RFC4566
]]></artwork>
</figure>
<t>The encoding-name, clock-rate and encoding-params are as defined
to appear in an rtpmap attribute for each media type/subtype. Thus,
it is easy to convert an rmcap attribute line into one or more
rtpmap attribute lines, once a payload type number is assigned to a
media-cap-num (see <xref target="lcfg"></xref>).</t>
<t>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 <xref target="RFC4566"></xref>. In simple terms, it
is the name of the media format, e.g. "t38". This form can also be
used in cases such as BFCP <xref target="RFC4585"></xref> where the
fmt list in the m-line is effectively ignored (BFCP uses "*").</t>
<t>The "rmcap" and "omcap" attributes can be provided at the
session-level and/or the media-level. There can be more than one
rmcap and more than one omcap attribute at both the session and
media level (i.e., more than one of each at the session-level and
more than one of each in each media description). Each media-cap-num
MUST be unique within the entire SDP record; it is used to identify
that media capability in potential, latent and actual
configurations, and in other attribute lines as explained below.
Note that the media-cap-num values are shared between the rmcap and
omcap attributes, and hence the uniqueness requirement applies to
the union of them. When the media capabilities are used in a
potential, latent or actual configuration, the media formats
referred by those configurations apply at the media level,
irrespective of whether the media capabilities themselves were
specified at the session or media level. In other words, the media
capability applies to the specific media description associated with
the configuration which invokes it.</t>
<t>For example:</t>
<t><list hangIndent="5">
<t>v=0<vspace blankLines="0" />o=- 24351 621814 IN IP4
192.0.2.2<vspace blankLines="0" />s=<vspace
blankLines="0" />c=IN IP4 192.0.2.2<vspace blankLines="0" />t=0
0<vspace blankLines="0" />a=rmcap:1 L16/8000/1<vspace
blankLines="0" />a=rmcap:2 L16/16000/2<vspace
blankLines="0" />a=rmcap:3 H263-1998/90000<vspace
blankLines="0" />a=omcap:4 example<vspace
blankLines="0" />m=audio 54320 RTP/AVP 0<vspace
blankLines="0" />a=pcfg:1 m=1|2, pt=1:99,2:98<vspace
blankLines="0" />m=video 66544 RTP/AVP 100<vspace
blankLines="0" />a=rtpmap:100 H264/90000<vspace
blankLines="0" />a=pcfg:10 m=3 pt=3:101<vspace
blankLines="0" />a=tcap:1 TCP<vspace blankLines="0" />a=pcfg:11
m=4 t=1</t>
</list></t>
</section>
<section anchor="mfcap"
title="The Media Format Parameter Capability Attribute">
<t>This attribute is used to associate media format specific
parameters with one or more media format capabilities. The
form of the attribute is:</t>
<t><list hangIndent="5">
<t>a=mfcap:<media-caps> <list of parameters></t>
</list></t>
<t>where <media-caps> permits the list of parameters to be
associated with one or more media format capabilities and the format
parameters are specific to the type of media format. The mfcap lines
map to a single traditional SDP fmtp attribute line (one for each
entry in <media-caps>) of the form</t>
<t><list hangIndent="5">
<t>a=fmtp:<fmt> <list of parameters></t>
</list></t>
<t>where <fmt> is the media format parameter defined in RFC
4566 <xref target="RFC4566"></xref>, as appropriate for the
particular media stream. The mfcap attribute MUST be used to encode
attributes for media capabilities, which would conventionally appear
in an fmtp attribute. The existing acap attribute MUST NOT be used
to encode fmtp attributes.</t>
<t>The mfcap attribute adheres to <xref target="RFC4566"></xref>
attribute production rules with</t>
<figure>
<artwork><![CDATA[
media-format-parameter-capability =
"a=mfcap:" media-cap-num-list 1*WSP fmt-specific-param-list
fmt-specific-param-list = text ; defined in RFC4566
]]></artwork>
</figure>
<t>Note that media format parameters can be used with RTP-based and
non-RTP based media formats.</t>
<section anchor="mfcapcat"
title="Media Format Parameter Concatenation Rule">
<t>The appearance of media subtypes with a large number of
formatting options (e.g., AMR-WB <xref target="RFC4867"></xref>)
coupled with the restriction that only a single fmtp attribute can
appear per media format, suggests that it is useful to create a
combining rule for mfcap parameters which are associated with the
same media capability number. Therefore, different mfcap lines MAY
include the same media-cap-num in their media-cap-num-list. When a
particular media capability is selected for processing, the
parameters from each mfcap line which references the particular
capability number in its media-cap-num-list are concatenated
together via ";", in the order the mfcap attributes appear in the
SDP record, to form the equivalent of a single fmtp attribute
line. This permits one to define a separate mfcap line for a
single parameter and value that is to be applied to each media
capability designated in the media-cap-num-list. This provides a
compact method to specify multiple combinations of format
parameters when using codecs with multiple format options. Note
that order-dependent parameters SHOULD be placed in a single mfcap
line to avoid possible problems with line rearrangement by a
middlebox.</t>
<t>Format parameters are not parsed by SDP; their content is
specific to the media type/subtype. When format parameters for a
specific media capability are combined from multiple a=mfcap lines
which reference that media capability, the format-specific
parameters are concatenated together and separated by ";" for
construction of the corresponding format attribute (a=fmtp). The
resulting format attribute will look something like the following
(without line breaks):</t>
<t><figure>
<artwork><![CDATA[
a=fmtp:<fmt> <fmt-specific-param-list1>;
<fmt-specific-param-list2>;
...
]]></artwork>
</figure></t>
<t>where <fmt> depends on the transport protocol in the
manner defined in RFC4566. SDP cannot assess the legality of the
resulting parameter list in the "a=fmtp" line; the user must take
care to ensure that legal parameter lists are generated.</t>
<t>The "mfcap" attribute can be provided at the session-level and
the media-level. There can be more than one mfcap attribute at the
session or media level. The unique media-cap-num is used to
associate the parameters with a media capability.</t>
<t>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 defined by:</t>
<t><list hangIndent="5">
<t>a=rmcap:1,2 G729/8000<vspace blankLines="0" /> a=mfcap:2
annexb:no</t>
</list></t>
<t>Media capability 1 supports G.729 with Annex B, whereas
media capability 2 supports G.729 without Annex B.</t>
<t>Example for H.263 video:</t>
<t><list hangIndent="5">
<t>a=rmcap:1 H263-1998/90000<vspace blankLines="0" />
a=rmcap:2 H263-2000/90000<vspace blankLines="0" /> a=mfcap:1
CIF=4;QCIF=2;F=1;K=1<vspace blankLines="0" /> a=mfcap:2
profile=2;level=2.2</t>
</list></t>
<t>Finally, for six format combinations of the Adaptive MultiRate
codec:</t>
<t><list hangIndent="5">
<t>a=rmcap:1-3 AMR/8000/1<vspace blankLines="0" /> a=rmcap:4-6
AMR-WB/16000/1<vspace blankLines="0" /> a=mfcap:1,2,3,4
mode-change-capability=1<vspace blankLines="0" /> a=mfcap:5,6
mode-change-capability=2<vspace blankLines="0" />
a=mfcap:1,2,3,5 max-red=220<vspace blankLines="0" />
a=mfcap:3,4,5,6 octet-align=1<vspace blankLines="0" />
a=mfcap:1,3,5 mode-set=0,2,4,7<vspace blankLines="0" />
a=mfcap:2,4,6 mode-set=0,3,5,6</t>
</list></t>
<t>So that AMR codec #1, when specified in a pcfg attribute within
an audio stream block (and assigned payload type number 98) as
in</t>
<t><list hangIndent="5">
<t>a=pcfg:1 m=1 pt=1:98</t>
</list></t>
<t>is essentially equivalent to the following</t>
<t><list hangIndent="10">
<t>m=audio 49170 RTP/AVP 98<vspace blankLines="0" />
a=rtpmap:98 AMR/8000/1<vspace blankLines="0" /> a=fmtp:98
mode-change-capability=1; \ <vspace blankLines="0" />
max-red=220; mode-set=0,2,4,7</t>
</list></t>
<t>and AMR codec #4 with payload type number 99,depicted by the
potential configuration:</t>
<t><list hangIndent="10">
<t>a=pcfg:4 m=4, pt=4:99</t>
</list></t>
<t>is equivalent to the following:</t>
<t><list hangIndent="10">
<t>m=audio 49170 RTP/AVP 99<vspace blankLines="0" />
a=rtpmap:99 AMR-WB/16000/1<vspace blankLines="0" /> a=fmtp:99
mode-change-capability=1; octet-align=1; \ <vspace
blankLines="0" /> mode-set=0,3,5,6</t>
</list></t>
<t>and so on for the other four combinations. SDP could thus
convert the media capabilities specifications into one or more
alternative media stream specifications, one of which can be
chosen for the answer.</t>
</section>
</section>
<section anchor="mscap"
title="The Media-Specific Capability Attribute">
<t>Attributes and parameters associated with a media format are
typically specified using the "rtpmap" and "fmtp" attributes in SDP,
and the similar "rmcap" and "mfcap" attributes in SDP Media
Capabilities. Some SDP extensions define other attributes that need
to be associated with media formats, for example the "rtcp-fb"
attribute defined in <xref target="RFC4585"></xref>. Such
media-specific attributes, beyond the rtpmap and fmtp attributes,
may be associated with media capability numbers via a new
media-specific attribute, mscap, of the following form:</t>
<figure>
<artwork><![CDATA[
a=mscap:<media caps star> <att field> <att value>
]]></artwork>
</figure>
<t>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 for the named attribute. Note that the
media capability numbers refer to media format capabilities specified
elsewhere in the SDP ("rmcap" and/or "omcap"). The media capability
numbers may include a wildcard ("*"), which will be used instead of
any payload type mappings in the resulting SDP (see, e.g. <xref
target="RFC4585"></xref> and the example below). In ABNF, we
have:</t>
<figure>
<artwork><![CDATA[
media-specific-capability = "a=mscap:"
media-caps-star
1*WSP att-field ; from RFC4566
1*WSP att-value ; from RFC4566
media-caps-star = media-cap-star-element
*("," media-cap-star-element)
media-cap-star-element = (media-cap-num [wildcard])
/ (media-cap-num-range [wildcard])
wildcard = "*"
]]></artwork>
</figure>
<t>Given an association between a media capability and a payload
type number as specified by the pt= parameters in a pcfg attribute
line, a mscap line may be translated easily into a conventional SDP
attribute line of the form</t>
<t><list hangIndent="5">
<t>a=<att field>":"<fmt> <att value> ;
<fmt> defined in <xref target="RFC4566"></xref></t>
</list></t>
<t>A resulting attribute that is not a legal SDP attribute as
specified by RFC4566 MUST be ignored by the receiver.</t>
<t>If a media capability number (or range) contains a wildcard
character at the end, any payload type mapping specified for that
media specific capability (or range of capabilities) will use the
wildcard character in the resulting SDP instead of the payload type
specified in the payload type mapping ("pt" parameter) in the
configuration attribute.</t>
<t>A single mscap line may refer to multiple media capabilities;
this is equivalent to multiple mscap lines, each with the same
attribute values (but different media capability numbers), one line
per media capability.</t>
<t>Multiple mscap lines may refer to the same media capability, but,
unlike the mfcap attribute, no concatenation operation is defined.
Hence, multiple mscap lines applied to the same media capability is
equivalent to multiple lines of the specified attribute in a
conventional media record.</t>
<t>Here is an example with the rtcp-fb attribute, modified from an
example in <xref target="RFC5104"></xref> (with the session-level
and audio media omitted). If the offer contains a media block like
the following (note the wildcard character),</t>
<t><list hangIndent="10">
<t>m=video 51372 RTP/AVP 98<vspace blankLines="0" /> a=rtpmap:98
H263-1998/90000<vspace blankLines="0" /> a=tcap:1
RTP/AVPF<vspace blankLines="0" /> a=rmcap:1
H263-1998/90000<vspace blankLines="0" /> a=mscap:1 rtcp-fb ccm
tstr<vspace blankLines="0" /> a=mscap:1 rtcp-fb ccm fir<vspace
blankLines="0" /> a=mscap:1* rtcp-fb ccm tmmbr smaxpr=120<vspace
blankLines="0" /> a=pcfg:1 t=1 m=1 pt=1:98</t>
</list></t>
<t>and if the proposed configuration is chosen, then the equivalent
media block would look like</t>
<t><list hangIndent="10">
<t>m=video 51372 RTP/AVPF 98<vspace blankLines="0" />
a=rtpmap:98 H263-1998/90000<vspace blankLines="0" />
a=rtcp-fb:98 ccm tstr<vspace blankLines="0" /> a=rtcp-fb:98 ccm
fir<vspace blankLines="0" /> a=rtcp-fb:* ccm tmmbr
smaxpr=120</t>
</list></t>
</section>
<section anchor="configparms" title="New Configuration Parameters">
<t>Along with the new attributes for media capabilities, new
extension parameters are defined for use in the potential
configuration, the actual configuration, and/or the new latent
configuration defined in <xref target="lcfg"></xref>.</t>
<section anchor="mparm"
title="The Media Configuration Parameter (m=)">
<t>The media configuration parameter is used to specify the media
format(s) and related parameters for a potential, actual, or
latent configuration. Adhering to the ABNF for
extension-config-list in <xref target="RFC5939"></xref> with</t>
<figure>
<artwork><![CDATA[
ext-cap-name = "m"
ext-cap-list = media-cap-num-list
[*(BAR media-cap-num-list)]
]]></artwork>
</figure>
<t>we have</t>
<figure>
<artwork><![CDATA[
media-config-list = ["+"] "m=" media-cap-num-list
*(BAR media-cap-num-list)
;BAR is defined in RFC5939
;media-cap-num-list is defined above
]]></artwork>
</figure>
<t>Alternative media configurations are separated by a vertical
bar ("|"). The alternatives are ordered by preference,
most-preferred first. 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. The use
of the plus sign ("+") is described in RFC5939.</t>
</section>
<section anchor="ptparm"
title="The Payload Type Number Mapping Parameter (pt=)">
<t>The payload type number mapping parameter is used to
specify the payload type number to be associated with each
RTP-based media format in a potential, actual, or latent
configuration. We define the payload type number mapping
parameter, payload-number-config-list, in accordance with
the extension-config-list format defined in <xref
target="RFC5939"></xref>. In ABNF:</t>
<figure>
<artwork><![CDATA[
payload-number-config-list = ["+"] "pt=" media-map-list
media-map-list = media-map *("," media-map)
media-map = media-cap-num ":" payload-type-number
; media-cap-num is defined in 3.3.1
payload-type-number = 1*3(DIGIT) ; RTP payload type number
]]></artwork>
</figure>
<t>The example in <xref target="ptsubs"></xref> shows how the
parameters from the rmcap line are mapped to payload type numbers
from the pcfg "pt" parameter. The use of the plus sign ("+") is
described in <xref target="RFC5939"></xref>.</t>
<t>A latent configuration represents a future capability, hence
the pt= parameter is not directly meaningful in the lcfg attribute
because no actual media session is being offered or accepted; it
is permitted in order to tie any payload type number parameters
within attributes to the proper media format. A primary example is
the case of format parameters for the Redundant Audio Data (RED)
payload, which are payload type numbers. Specific payload type
numbers used in a latent configuration MAY be interpreted as
suggestions to be used in any future offer based on the latent
configuration, but they are not binding; the offerer and/or
answerer may use any payload type numbers each deems appropriate.
The use of explicit payload type numbers for latent configurations
can be avoided by use of the parameter substitution rule of <xref
target="ptsubs"></xref>. Future extensions are also permitted.</t>
</section>
<section anchor="mtparm" title="The Media Type Parameter">
<t>When a latent configuration is specified (always at the media
level), indicating the ability to support an additional media
stream, it is necessary to specify the media type (audio, video,
etc.) as well as the format and transport type. The media type
parameter is defined in ABNF as</t>
<figure>
<artwork><![CDATA[
media-type = ["+"] "mt=" media; media defined in RFC4566
]]></artwork>
</figure>
<t>At present, the media-type parameter is used only in the latent
configuration attribute, and the use of the "+" prefix to specify
that the entire attribute line is to be ignored if the mt=
parameter is not understood, is unnecessary. However, if the
media-type parameter is later added to an existing capability
attribute such as pcfg, then the "+" would be useful. The media
format(s) and transport type(s) are specified using the media
configuration parameter ("+m=") defined above, and the transport
parameter ("t=") defined in <xref target="RFC5939"></xref>,
respectively.</t>
</section>
</section>
<section anchor="lcfg" title="The Latent Configuration Attribute">
<t>One of the goals of this work is to permit the exchange of
supportable media configurations in addition to those offered or
accepted for immediate use. Such configurations are referred to as
"latent configurations". For example, a party may offer to establish
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
offerer can supply its video capabilities by offering one or more
latent video configurations along with the media stream for audio;
the responding party may indicate its ability and willingness to
support such a video session by returning a corresponding latent
configuration.</t>
<t>Latent configurations returned in SDP answers MUST match offered
latent configurations (or parameter subsets thereof). Therefore, it
is appropriate for the offering party to announce most, if not all,
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 acap, rmcap, tcap, etc. lines in the answer.</t>
<t>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. The latent configuration
attribute combines the properties of a media line and a potential
configuration. The media type (mt=) and the transport protocol(s)
(t=) MUST be specified since the latent configuration is independent
of any media line present. In most cases, the media configuration
(m=) parameter MUST be present as well (see <xref
target="examples"></xref> for examples). The lcfg attribute is a
media level attribute.</t>
<t><list>
<t>The lcfg attribute is defined as a media level attribute
since it specifies a possible future media stream. However the
lcfg attribute is not necessarily related to the media
description within which it is provided. Session capabilities
("sescap") may be used to indicate this.</t>
</list></t>
<t>Each media line in an SDP description represents an offered
simultaneous media stream, whereas each latent configuration
represents an additional stream which may be negotiated in a future
offer/answer exchange. Session capability attributes may be used to
determine whether a latent configuration may be used to form an
offer for an additional simultaneous stream or to reconfigure an
existing stream in a subsequent offer/answer exchange.</t>
<t>The latent configuration attribute is of the form:</t>
<figure>
<artwork><![CDATA[
a=lcfg:<config-number> <latent-cfg-list>
]]></artwork>
</figure>
<t>which adheres to the <xref target="RFC4566"></xref> "attribute"
production with att-field and att-value defined as:</t>
<figure>
<artwork><![CDATA[
att-field = "lcfg"
att-value = config-number 1*WSP lcfg-cfg-list
config-number = 1*10(DIGIT) ; defined in RFC5234
lcfg-cfg-list = media-type 1*WSP pot-cfg-list
; as defined in RFC5939
; and extended herein
]]></artwork>
</figure>
<t>The media-type (mt=) parameter identifies the media type (audio,
video, etc.) to be associated with the latent media stream, and MUST
be present. The pot-cfg-list MUST contain a
transport-protocol-config-list (t=) parameter and a
media-config-list (m=) parameter. The pot-cfg-list MUST NOT contain
more than one instance of each type of parameter list. As specified
in <xref target="RFC5939"></xref>, the use of the "+" prefix with a
parameter indicates that the entire configuration MUST be ignored if
the parameter is not understood; otherwise, the parameter itself may
be ignored.</t>
<t>Media stream payload numbers are not assigned by a latent
configuration. Assignment will take place if and when the
corresponding stream is actually offered via an m-line in a later
exchange. The payload-number-config-list is included as a parameter
to the lcfg attribute in case it is necessary to tie payload numbers
in attribute capabilities to specific media capabilities.</t>
<t>If an lcfg attribute invokes an acap attribute that appears at
the session level, then that attribute will be expected to appear at
the session level of a subsequent offer when and if a corresponding
media stream is offered. Otherwise, acap attributes which appear at
the media level represent media-level attributes. Note, however,
that rmcap, omcap, mfcap, mscap, and tcap attributes may appear at
the session level because they always result in media-level
attributes or m-line parameters.</t>
<t>The configuration numbers for latent configurations do not imply
a preference; the offerer will imply a preference when actually
offering potential configurations derived from latent configurations
negotiated earlier. Note however that the offerer of latent
configurations MAY specify preferences for combinations of potential
and latent configurations by use of the sescap attribute defined in
<xref target="sescap"></xref>. For example, if an SDP offer
contains, say, an audio stream with pcfg:1, and two latent video
configurations, lcfg:2, and lcfg:3, then a session with one audio
stream and one video stream could be specified by including
"a=sescap:1 1,2|3". One audio stream and two video streams could be
specified by including "a=sescap:2 1,2,3" in the offer. In order to
permit combinations of latent and potential configurations in
session capabilities, latent configuration numbers MUST be different
from those used for potential configurations. This restriction is
especially important if the offerer does not require cmed-v0
capability and the recipient of the offer doesn't support it. If the
lcfg attribute is not recognized, the capability attributes intended
to be associated with it may be confused with those associated with
a potential configuration of some other media stream.</t>
<t>If a cryptographic attribute, such as the SDES "a=crypto:"
attribute <xref target="RFC4568"></xref>, is referenced by a latent
configuration through an acap attribute, any keying material
required in the conventional attribute, such as the SDES key/salt
string, MUST be included in order to satisfy formatting rules for
the attribute. The actual value(s) of the keying material SHOULD be
meaningless, and the receiver of the lcfg attribute MUST ignore the
values.</t>
</section>
<section anchor="pcfg"
title="Enhanced Potential Configuration Attribute">
<t>The present work requires new extensions (parameters) for the
pcfg attribute defined in the base protocol <xref
target="RFC5939"></xref>. The parameters and their definitions are
"borrowed" from the definitions provided for the latent
configuration attribute in <xref target="lcfg"></xref>. The expanded
ABNF definition of the pcfg attribute is</t>
<t><figure>
<artwork><![CDATA[ a=pcfg: <config-number> [<pot-cfg-list>]
]]></artwork>
</figure></t>
<t>where</t>
<t><figure>
<artwork><![CDATA[ config-number = 1*DIGIT ;defined in [RFC5234]
pot-cfg-list = pot-config *(1*WSP pot-config)
pot-config = attribute-config-list / ;def in [RFC5939]
transport-protocol-config-list / ;defined in [RFC5939]
extension-config-list / ;[RFC5939]
media-config-list / ; Section 3.3.4.1
payload-number-config-list ; Section 3.3.4.2]]></artwork>
</figure></t>
<t>Except for the extension-config-list, the pot-cfg-list MUST NOT
contain more than one instance of each parameter list.</t>
<section anchor="answercaps"
title="Returning Capabilities in the Answer">
<t>Potential and/or latent configuration attributes may be
returned within an answer SDP to indicate the ability of the
answerer to support alternative configurations of the
corresponding stream(s). For example, an offer may include
multiple potential configurations for a media stream and/or latent
configurations for additional streams; the corresponding answer
will indicate (via an acfg attribute) the configuration accepted
and used to construct the base configuration for each active media
stream in the reply, but the reply MAY also contain potential
and/or latent configuration attributes, with parameters, to
indicate which other offered configurations would be acceptable.
This information is useful if it becomes desirable to reconfigure
a media stream, e.g., to reduce resource consumption.</t>
<t>When potential and/or latent configurations are returned in an
answer, all numbering MUST refer to the configuration and
capability attribute numbering of the offer. The offered
capability attributes need not be returned in the answer. The
answer MAY include additional capability attributes and/or
configurations (with distinct numbering). The parameter values of
any returned pcfg or lcfg attributes MUST be a subset of those
included in the offered configurations and/or those added by the
answerer; values MAY be omitted only if they were indicated as
alternative sets, or optional, in the original offer. The
parameter set indicated in the returned acfg attribute need not be
repeated in a returned pcfg attribute. The answerer MAY return
more than one pcfg attribute with the same configuration number if
it is necessary to describe selected combinations of optional or
alternative parameters.</t>
<t>Similarly, one or more session capability attributes (a=sescap)
MAY be returned to indicate which of the offered session
capabilities is/are supportable by the answerer (see <xref
target="sescap"></xref>.)</t>
<t>Note that, although the answerer MAY return capabilities beyond
those included by the offerer, these capabilities MUST NOT be used
to form any base level media description in the answer. For this
reason, it is advisable for the offerer to include most, if not
all, potential and latent configurations it can support in the
initial offer, unless the size of the resulting SDP is a concern.
Either party MAY later announce additional capabilities by
renegotiating the session in a second offer/answer exchange.</t>
</section>
<section anchor="ptmaping" title="Payload Type Number Mapping">
<t>When media format capabilities defined in rmcap attributes are used in
potential configuration lines, the transport protocol uses RTP and
it is necessary to assign payload type numbers. In some cases, it
is desirable to assign different payload type numbers to the same
media format capability when used in different potential configurations.
One example is when configurations for AVP and SAVP are offered:
the offerer would like the answerer to use different payload type
numbers for encrypted and unencrypted media, so the
offerer can decide whether or not to render early media which
arrives before the answer is received. </t>
<t><list>
<t>For example, if use of AVP was selected by the answerer,
then media received by the offerer is not encrypted and hence
can be played out prior to receiving the answer. Conversely,
if SAVP was selected, cryptographic parameters and keying
material present in the answer may be needed to decrypt
received media. If the offer configuration indicated that AVP
media uses one set of payload types and SAVP a different set,
then the offerer will know whether media received prior to the
answer is encrypted or not by simply looking at the RTP
payload type number in the received packet.</t>
</list></t>
<t>This association of distinct payload type number(s) with
different transport protocols requires a separate pcfg line for
each protocol. Clearly, this technique cannot be used if the
number of potential configurations exceeds the number of possible
payload type numbers.</t>
</section>
<section anchor="mediaattrs"
title="Processing of Media-Format-Related Conventional Attributes for Potential Configurations">
<t>When media capabilities negotiation is employed,
SDP records are likely to contain conventional attributes such as
rtpmap, fmtp, and other media-format-related lines, as well as
capability attributes such as rmcap, omcap, mfcap, and mscap which
map into those conventional attributes when invoked by a potential
configuration. In such cases, it MAY be appropriate to employ the
delete-attributes option <xref target="RFC5939"></xref> in the
attribute configuration list parameter in order to avoid the
generation of conflicting fmtp attributes for a particular
configuration. Any media-specific attributes in the media block
which refer to media formats not used by the potential
configuration MUST be ignored.</t>
<t>For example:</t>
<t><list hangIndent="10">
<t>v=0<vspace blankLines="0" /> o=- 25678 753849 IN IP4
192.0.2.1<vspace blankLines="0" /> s=<vspace blankLines="0" />
c=IN IP4 192.0.2.1<vspace blankLines="0" /> t=0 0<vspace
blankLines="0" /> a=creq:med-v0<vspace blankLines="0" />
m=audio 3456 RTP/AVP 0 18 100<vspace blankLines="0" />
a=rtpmap:100 telephone-events<vspace blankLines="0" />
a=fmtp:100 0-11<vspace blankLines="0" /> a=rmcap:1
PCMU/8000<vspace blankLines="0" /> a=rmcap:2 G729/8000<vspace
blankLines="0" /> a=rmcap:3 telephone-events/8000<vspace
blankLines="0" /> a=mfcap:3 0-15<vspace blankLines="0" />
a=pcfg:1 m=2,3|1,3 a=-m pt=1:0,2:18,3:100<vspace
blankLines="0" /> a=pcfg:2</t>
</list></t>
<t>In this example, PCMU is media capability 1, G729 is media
capability 2, and telephone-event is media capability 3. The
a=pcfg:1 line specifies that the preferred configuration is G.729
with extended dtmf events, second is G.711 mu-law with extended
dtmf events, and the base media-level attributes are to be
deleted. Intermixing of G.729, G.711, and "commercial" dtmf events
is least preferred (the base configuration provided by the "m="
line, which is, by default, the least preferred configuration).
The rtpmap and fmtp attributes of the base configuration are
replaced by the rmcap and mfcap attributes when invoked by the
proposed configuration.</t>
<t>If the preferred configuration is selected, the SDP answer will
look like</t>
<t><list hangIndent="10">
<t>v=0<vspace blankLines="0" /> o=- 25678 753849 IN IP4
192.0.2.1<vspace blankLines="0" /> s=<vspace blankLines="0" />
c=IN IP4 192.0.2.1<vspace blankLines="0" /> t=0 0<vspace
blankLines="0" /> a=csup:med-v0<vspace blankLines="0" />
m=audio 3456 RTP/AVP 18 100<vspace blankLines="0" />
a=rtpmap:100 telephone-events/8000<vspace blankLines="0" />
a=fmtp:100 0-15<vspace blankLines="0" /> a=acfg:1 m=2,3
pt=1:0,2:18,3:100</t>
</list></t>
</section>
</section>
<section anchor="ptsubs"
title="Substitution of Media Payload Type Numbers in Capability Attribute Parameters">
<t>In some cases, for example, when an RFC 2198 <xref
target="RFC2198"></xref> redundancy audio subtype (RED) capability
is defined in an mfcap attribute, the parameters to an attribute may
contain payload type numbers. Two options are available for
specifying such payload type numbers. They may be expressed
explicitly, in which case they are bound to actual payload types by
means of the payload type number parameter (pt=) in the appropriate
potential or latent configuration. For example, the following SDP
fragment defines a potential configuration with redundant G.711
mu-law:</t>
<t><list hangIndent="10">
<t>m=audio 45678 RTP/AVP 0<vspace blankLines="0" /> a=rtpmap:0
PCMU/8000<vspace blankLines="0" /> a=rmcap:1 PCMU/8000<vspace
blankLines="0" /> a=rmcap:2 RED/8000<vspace blankLines="0" />
a=mfcap:2 0/0<vspace blankLines="0" /> a=pcfg:1 m=2,1
pt=2:98,1:0</t>
</list></t>
<t>The potential configuration is then equivalent to</t>
<t><list hangIndent="10">
<t>m=audio 45678 RTP/AVP 98 0<vspace blankLines="0" />
a=rtpmap:0 PCMU/8000<vspace blankLines="0" /> a=rtpmap:98
RED/8000<vspace blankLines="0" /> a=fmtp:98 0/0</t>
</list></t>
<t>A more general mechanism is provided via the parameter
substitution rule. When an mfcap, mscap, or acap attribute is
processed, its arguments will be scanned for a payload type number
escape sequences of the following form (in ABNF):</t>
<t><list hangIndent="10">
<t>ptn-esc = "%m=" media-cap-num "%" ; defined in 3.3.1</t>
</list></t>
<t>If the sequence is found, the sequence is replaced by the payload
type number assigned to the media capability number, as specified by
the pt= parameter in the selected potential configuration; only
actual payload type numbers are supported - wildcards are excluded.
The sequence "%%" (null digit string) is replaced by a single
percent sign and processing continues with the next character, if
any.</t>
<t>For example, the above offer sequence could have been written
as</t>
<t><list hangIndent="10">
<t>m=audio 45678 RTP/AVP 0<vspace blankLines="0" /> a=rtpmap:0
PCMU/8000<vspace blankLines="0" /> a=rmcap:1 PCMU/8000<vspace
blankLines="0" /> a=rmcap:2 RED/8000<vspace blankLines="0" />
a=mfcap:2 %m=1%/%m=1%<vspace blankLines="0" /> a=pcfg:1 m=2,1
pt=2:98,1:0<vspace blankLines="0" /></t>
</list></t>
<t>and the equivalent SDP is the same as above.</t>
</section>
<section anchor="sescap" title="The Session Capability Attribute">
<t>Potential and latent configurations enable offerers and answerers
to express a wide range of alternative configurations for current
and future negotiation. However in practice, it may not be possible
to support all combinations of these configurations. </t>
<t>The session capability attribute provides a means for the offerer
and/or the answerer to specify combinations of specific media stream
configurations which it is willing and able to support. Each session
capability in an offer or answer MAY be expressed as a list of
required potential configurations, and MAY include a list of
optional potential and/or latent configurations.</t>
<t>The choices of session capabilities may be based on processing
load, total bandwidth, or any other criteria of importance to the
communicating parties. If the answerer supports media capabilities
negotiation, and session configurations are offered, it MUST accept
one of the offered configurations, or it MUST refuse the session.
Therefore, if the offer includes any session capabilities, it SHOULD
include all the session capabilities the offerer is willing to
support.</t>
<t>The session capability attribute is a session-level attribute
described by:</t>
<figure>
<artwork><![CDATA[
"a=sescap:" <session num> <list of configs>
]]></artwork>
</figure>
<t>which corresponds to the standard value attribute definition
with</t>
<figure>
<artwork><![CDATA[
att-field = "sescap"
att-value = session-num 1*WSP list-of-configs
[1*WSP optional-configs]
session-num = 1*10(DIGIT) ; defined in RFC5234
list-of-configs = alt-config *("," alt-config)
optional-configs = "[" list-of-configs "]"
alt-config = config-number *("|" config-number)
; config-number defined in RFC5939
]]></artwork>
</figure>
<t>The session-num identifies the session; a lower-number session is
preferred over a higher-numbered session. Each alt-config list
specifies alternative media configurations within the session;
preference is based on config-num as specified in <xref
target="RFC5939"></xref>. Note that the session preference order,
when present, takes precedence over the individual media stream
configuration preference order.</t>
<t>Use of session capability attributes requires that configuration
numbers assigned to potential and latent configurations MUST be
unique across the entire session; <xref target="RFC5939"></xref>
requires only that pcfg configuration numbers be unique within a
media description.</t>
<t>As an example, consider an endpoint that is capable of supporting
an audio stream with either one H.264 video stream or two H.263
video streams with a floor control stream. In the latter case, the
second video stream is optional. The SDP offer might look like the
following (offering audio, an H.263 video streams, BFCP and another
optional H.263 video stream)- the empty lines are added for
readability only (not part of valid SDP):</t>
<t><list hangIndent="10">
<t>v=0<vspace blankLines="0" /> o=- 25678 753849 IN IP4
192.0.2.1<vspace blankLines="0" /> s=<vspace blankLines="0" />
c=IN IP4 192.0.2.1<vspace blankLines="0" /> t=0 0<vspace
blankLines="0" /> a=creq:med-v0<vspace blankLines="0" />
a=sescap:2 1,2,5,[3]<vspace blankLines="0" /> a=sescap:1
1,4<vspace blankLines="0" /> <vspace blankLines="1" /> m=audio
54322 RTP/AVP 0<vspace blankLines="0" /> a=rtpmap:0
PCMU/8000<vspace blankLines="0" /> a=pcfg:1<vspace
blankLines="0" /> <vspace blankLines="1" /> m=video 22344
RTP/AVP 102<vspace blankLines="0" /> a=rtpmap:102
H263-1998/90000<vspace blankLines="0" /> a=fmtp:102
CIF=4;QCIF=2;F=1;K=1<vspace blankLines="0" /> i=main video
stream<vspace blankLines="0" /> a=label:11<vspace
blankLines="0" /> a=pcfg:2<vspace blankLines="0" /> a=rmcap:1
H264/90000<vspace blankLines="0" /> a=mfcap:1
profile-level-id=42A01E; packetization-mode=2<vspace
blankLines="0" /> a=acap:1 label:13<vspace blankLines="0" />
a=pcfg:4 m=1 a=1 pt=1:104<vspace blankLines="0" /> <vspace
blankLines="1" /> m=video 33444 RTP/AVP 103<vspace
blankLines="0" /> a=rtpmap:103 H263-1998/90000<vspace
blankLines="0" /> a=fmtp:103 CIF=4;QCIF=2;F=1;K=1<vspace
blankLines="0" /> i=secondary video (slides)<vspace
blankLines="0" /> a=label:12<vspace blankLines="0" />
a=pcfg:3<vspace blankLines="0" /> <vspace blankLines="1" />
m=application 33002 TCP/BFCP *<vspace blankLines="0" />
a=setup:passive<vspace blankLines="0" /> a=connection:new<vspace
blankLines="0" /> a=floorid:1 m-stream:11 12<vspace
blankLines="0" /> a=floor-control:s-only<vspace
blankLines="0" /> a=confid:4321<vspace blankLines="0" />
a=userid:1234<vspace blankLines="0" /> a=pcfg:5</t>
</list></t>
<t>If the answerer understands MediaCapNeg, but cannot support the
Binary Floor Control Protocol, then it would respond with (invalid
empty lines in SDP included again for readability):</t>
<t><list hangIndent="10">
<t>v=0<vspace blankLines="0" /> o=- 25678 753849 IN IP4
192.0.2.1<vspace blankLines="0" /> s=<vspace blankLines="0" />
c=IN IP4 192.0.2.22<vspace blankLines="0" /> t=0 0<vspace
blankLines="0" /> a=csup:med-v0<vspace blankLines="0" />
a=sescap:1 1,4<vspace blankLines="0" /> <vspace
blankLines="1" /> m=audio 23456 RTP/AVP 0<vspace
blankLines="0" /> a=rtpmap:0 PCMU/8000<vspace blankLines="0" />
a=acfg:1<vspace blankLines="0" /> <vspace blankLines="1" />
m=video 41234 RTP/AVP 104<vspace blankLines="0" /> a=rtpmap:104
H264/90000<vspace blankLines="0" /> a=fmtp:104
profile-level-id=42A01E; packetization-mode=2<vspace
blankLines="0" /> a=acfg:4 m=1 a=1 pt=1:104<vspace
blankLines="0" /> <vspace blankLines="1" /> m=video 0 RTP/AVP
103<vspace blankLines="0" /> a=acfg:3<vspace blankLines="0" />
<vspace blankLines="1" /> m=application 0 TCP/BFCP *<vspace
blankLines="0" /> a=acfg:5</t>
</list></t>
<t>An endpoint that doesn't support Media capabilities negotiation,
but does support H.263 video, would respond with one or two H.263
video streams. In the latter case, the answerer may issue a second
offer to reconfigure the session to one audio and one video channel
using H.264 or H.263.</t>
<t>Session capabilities can include latent capabilities as well.
Here's a similar example in which the offerer wishes to initially
establish an audio stream, and prefers to later establish two video
streams with chair control. If the answerer doesn't understand Media
CapNeg, or cannot support the dual video streams or flow control,
then it may support a single H.264 video stream. Note that
establishment of the most favored configuration will require two
offer/answer exchanges.</t>
<t><list hangIndent="10">
<t>v=0<vspace blankLines="0" /> o=- 25678 753849 IN IP4
192.0.2.1<vspace blankLines="0" /> s=<vspace blankLines="0" />
c=IN IP4 192.0.2.1<vspace blankLines="0" /> t=0 0<vspace
blankLines="0" /> a=creq:med-v0<vspace blankLines="0" />
a=sescap:1 1,3,4,5<vspace blankLines="0" /> a=sescap:2
1,2<vspace blankLines="0" /> a=sescap:3 1<vspace
blankLines="0" /> <vspace blankLines="0" /> a=rmcap:1
H263-1998/90000<vspace blankLines="0" /> a=mfcap:1
CIF=4;QCIF=2;F=1;K=1<vspace blankLines="0" /> a=tcap:1 RTP/AVP
TCP/BFCP<vspace blankLines="0" /> <vspace blankLines="0" />
m=audio 54322 RTP/AVP 0<vspace blankLines="0" /> a=rtpmap:0
PCMU/8000<vspace blankLines="0" /> a=pcfg:1<vspace
blankLines="0" /> <vspace blankLines="0" /> m=video 22344
RTP/AVP 102<vspace blankLines="0" /> a=rtpmap:102
H264/90000<vspace blankLines="0" /> a=fmtp:102
profile-level-id=42A01E; packetization-mode=2<vspace
blankLines="0" /> a=label:11<vspace blankLines="0" />
a=content:main<vspace blankLines="0" /> a=pcfg:2 <vspace
blankLines="0" /> a=lcfg:3 mt=video t=1 m=1 a=31,32<vspace
blankLines="0" /> a=acap:31 label:12<vspace blankLines="0" />
a=acap:32 content:main<vspace blankLines="0" /> <vspace
blankLines="0" /> a=lcfg:4 mt=video t=1 m=1 a=41,42<vspace
blankLines="0" /> a=acap:41 label:13<vspace blankLines="0" />
a=acap:42 content:slides<vspace blankLines="0" /> <vspace
blankLines="0" /> a=lcfg:5 mt=application m=51 t=51<vspace
blankLines="0" /> a=tcap:51 TCP/BFCP<vspace blankLines="0" />
a=omcap:51 * <vspace blankLines="0" /> a=acap:51
setup:passive<vspace blankLines="0" /> a=acap:52
connection:new<vspace blankLines="0" /> a=acap:53 floorid:1
m-stream:12 13<vspace blankLines="0" /> a=acap:54
floor-control:s-only<vspace blankLines="0" /> a=acap:55
confid:4321<vspace blankLines="0" /> a=acap:56
userid:1234<vspace blankLines="0" /></t>
</list></t>
<t>In this example, the default offer, as seen by endpoints which do
not understand capabilities negotiation, proposes a PCMU audio
stream and an H.264 video stream. Note that the offered lcfg lines
for the video streams don't carry pt= parameters because they're not
needed (payload type numbers will be assigned in the offer/answer
exchange that establishes the streams). Note also that the three
rmcap, mfcap, and tcap attributes used by lcfg:3 and lcfg:4 are
included at the session level so they may be referenced by both
latent configurations. As per <xref target="newattrs"></xref>, the
media attributes generated from the rmcap, mfcap, and tcap
attributes are always media-level attributes. If the answerer
supports Media CapNeg, and supports the most desired configuration,
it would return the following SDP:</t>
<t><list hangIndent="10">
<t>v=0<vspace blankLines="0" /> o=- 25678 753849 IN IP4
192.0.2.1<vspace blankLines="0" /> s=<vspace blankLines="0" />
c=IN IP4 192.0.2.22<vspace blankLines="0" /> t=0 0<vspace
blankLines="0" /> a=csup:med-v0<vspace blankLines="0" />
a=sescap:1 1,3,4,5<vspace blankLines="0" /> a=sescap:2
1,2<vspace blankLines="0" /> a=sescap:3 1<vspace
blankLines="0" /> <vspace blankLines="0" /> m=audio 23456
RTP/AVP 0<vspace blankLines="0" /> a=rtpmap:0 PCMU/8000<vspace
blankLines="0" /> a=acfg:1<vspace blankLines="0" /> <vspace
blankLines="0" /> m=video 0 RTP/AVP 102<vspace blankLines="0" />
a=pcfg:2 <vspace blankLines="0" /> a=lcfg:3 mt=video t=1 m=1
a=31,32<vspace blankLines="0" /> <vspace blankLines="0" />
a=lcfg:4 mt=video t=1 m=1 a=41,42 <vspace blankLines="0" />
<vspace blankLines="0" /> a=lcfg:5 mt=application t=2<vspace
blankLines="0" /></t>
</list></t>
<t>This exchange supports immediate establishment of an audio stream
for preliminary conversation. This exchange would presumably be
followed at the appropriate time with a "reconfiguration"
offer/answer exchange to add the video and chair control
streams.</t>
</section>
</section>
<section anchor="oaext" title="Offer/Answer Model Extensions">
<t>In this section, we define extensions to the offer/answer
model defined in RFC 3264 <xref target="RFC3264"></xref> and
RFC 5939 <xref target="RFC5939"></xref> to allow for media
format and associated parameter capabilities, latent
configurations and 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 <xref target="RFC5939"></xref>
Section 6; those procedures form a baseline set of capability
negotiation offer/answer procedures that MUST be followed,
subject to the extensions defined here.</t>
<t>The <xref target="RFC5939"></xref> provides a relatively compact
means to offer the equivalent of an ordered list of alternative
configurations for offered media streams (as would be described by
separate m= lines and associated attributes). The attributes acap,
mscap, mfcap, omcap and rmcap are designed to map somewhat straightforwardly
into equivalent m= lines and conventional attributes when invoked by a
pcfg, lcfg, or acfg attribute with appropriate parameters. The a=pcfg:
lines, along with the m= line itself, represent offered media
configurations. The a=lcfg: lines represent alternative capabilities
for future use.</t>
<section anchor="offergen" title="Generating the Initial Offer">
<t>The Media Capabilities negotiation extensions defined in this
document cover the following categories of features:</t>
<t><list style="symbols">
<t>Media Format Capabilities and associated parameters (rmcap, omcap,
mfcap, and mscap attributes)</t>
<t>Potential configurations using those media format capabilities and
associated parameters</t>
<t>Latent media streams (lcfg attribute)</t>
<t>Acceptable combinations of media stream configurations
(sescap attribute).</t>
</list> The high-level description of the operation is as
follows:</t>
<t>When an endpoint generates an initial offer and wants to use the
functionality described in the current document, it SHOULD identify
and define the media formats and associated parameters it can
support via the rmcap, omcap, mfcap and mscap attributes. The SDP
media line(s) ("m=") should be made up with the actual configuration
to be used if the other party does not understand capability
negotiations (by default, this is the least preferred
configuration). Typically, the media line configuration will contain
the minimum acceptable configuration from the offerer's point of
view.</t>
<t>Preferred configurations for each media stream are identified
following the media line. The present offer may also include latent
configuration (lcfg) attributes, at the media level, describing
media streams and/or configurations the offerer 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.</t>
<t>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.</t>
<t>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 <xref target="RFC5939"></xref>.</t>
<t>Below, we provide a more detailed description of how to construct
the offer SDP.</t>
<section title="Offer with Media Capabilities">
<t>For each RTP-based media format the offerer wants to include as
a media format capability, the offer MUST include an "rmcap" attribute
for the media format as defined in Section 3.3.1.</t>
<t>For each non RTP-based media format the offer wants to include
as a media format capability, the offer MUST include an "omcap" attribute
for the media format as defined in Section 3.3.1.</t>
<t>Since the media 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.</t>
<t>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.</t>
<t>For each of the media format 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).</t>
<t>If the offerer wants to limit the relevance (and use) of a
media format capability or parameter to a particular media stream, the
media format capability or parameter MUST be provided within the
corresponding media description. Otherwise, the media format 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.</t>
<t><list style="empty">
<t>This is due to those parameters being effectively
media-level parameters. If session-level attributes are
needed, the "acap" attribute defined in <xref target="RFC5939"></xref> can be used,
however it does not provide for media format-specific
instantiation.</t>
</list></t>
<t>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 <xref target="RFC5939"></xref>, 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 <xref target="RFC5939"></xref>, lower
config-numbers indicate a higher preference; the ordering still
applies within a given media description only though.</t>
<t>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 attributes 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.</t>
<t>Note that the "mt" parameter MUST NOT be used with the pcfg
attribute (since it is defined for the lcfg attribute only); the
media type in a potential configuration cannot be changed from
that of the encompassing media description.</t>
</section>
<section title="Offer with Latent Configuration">
<t>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.</t>
<t>Each lcfg attribute</t>
<t><list style="symbols">
<t>MUST be specified at the media level</t>
<t>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</t>
<t>MUST include a media type ("mt")</t>
<t>MUST reference a valid transport capability ("t")</t>
</list></t>
<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
<xref target="RFC5939"></xref>, Section 3.5.1).</t>
<t><list style="empty">
<t>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.</t>
</list></t>
</section>
<section title="Offer with Configuration Combination Restrictions">
<t>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.</t>
<t><list style="empty">
<t>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.</t>
</list></t>
</section>
</section>
<section anchor="answergen" title="Generating the Answer ">
<t>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 <xref target="RFC5939"></xref>.</t>
<t>The SDP MAY contain</t>
<t><list style="symbols">
<t>Media format capabilities and associated parameters (rmcap, omcap,
mfcap, and mscap attributes)</t>
<t>Potential configurations using those media format capabilities and
associated parameters</t>
<t>Latent media streams (lcfg attribute)</t>
<t>Acceptable combinations of media stream configurations
(sescap attribute)</t>
</list></t>
<t>The high-level informative description of the operation is as
follows:</t>
<t>When the answering party receives the offer and if it supports
the required capability negotiation extensions, it should select the
most-preferred configuration it can support for each media stream,
and build its answer accordingly. The configuration selected for
each accepted media stream is placed into the answer as a media line
with associated parameters and attributes. If a proposed
configuration is chosen for a given media stream, the answer must
contain an actual configuration (acfg) attribute for that media
stream to indicate which offered pcfg attribute was used to build
the answer. The answer should also include any potential or latent
configurations the answerer can support, especially any
configurations compatible with other potential or latent
configurations received in the offer. The answerer should make note
of those configurations it might wish to offer in the future.</t>
<t>Below we provide a more detailed normative description of how the
answerer processes the offer SDP and generates an answer SDP.</t>
<section title="Processing Media Capabilities and Potential Configurations">
<t>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).</t>
<t>Such a potential configuration is valid if:</t>
<t><list style="numbers">
<t>It is valid according to the rules defined in <xref target="RFC5939"></xref></t>
<t>It contains a config-number that is unique in the entire
SDP and does not overlap with any latent configuration
config-numbers</t>
<t>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.</t>
<t>All RTP-based media format capabilities (rmcap) have a
corresponding payload type ("pt") parameter in the potential
configuration that result in mapping to a valid payload type
that is unique within the resulting SDP.</t>
<t>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.</t>
</list></t>
<t>Note that, since SDP does not interpret the value of fmtp
parameters, any resulting fmtp parameter value will be considered
valid.</t>
<t>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 <xref target="RFC5939"></xref>, 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:</t>
<t><list style="numbers">
<t>It adheres to the rules provided in Section 3.3.8.</t>
<t>All the configurations referenced by the "sescap" attribute
are valid themselves (note that this can include the actual,
potential and latent configurations).</t>
</list></t>
<t>The answerer MUST now process the offer for each media stream
based on the most preferred valid potential configuration in
accordance with the procedures specified in <xref target="RFC5939"></xref>, Section
3.6.2, and further extended below:</t>
<t><list style="symbols">
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
</list></t>
<t>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 <xref target="RFC5939"></xref>, Section 3.6.2, and further extended
below:</t>
<t><list style="symbols">
<t>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.</t>
<t>Latent configuration processing and answer generation MUST
be performed, as specified below.</t>
<t>Session capability specification for the potential and
latent configurations in the answer MAY be included (see
Section 3.3.8).</t>
</list></t>
</section>
<section title="Latent Configuration Processing">
<t>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:</t>
<t><list style="symbols">
<t>It adheres to the description in Section 3.3.5.</t>
<t>It includes a config-number that is unique in the entire
SDP and does not overlap with any potential configuration
config-number</t>
<t>It includes a valid media type ("mt=")</t>
<t>It references a valid transport capability ("t=")</t>
<t>All other capabilities referenced by it are valid.</t>
</list></t>
<t>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 <xref target="RFC5939"></xref>). 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.</t>
<t>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.</t>
</section>
</section>
<section anchor="ansproc" title="Offerer Processing of the Answer ">
<t>The offerer MUST process the answer in accordance with <xref target="RFC5939"></xref>
Section 3.6.3, and further explained below.</t>
<t>When the offerer processes the answer SDP based on a valid actual
configuration attribute in the answer, and that valid configuration
includes one or more media capabilities, the processing MUST
furthermore be done as if the offer was sent using those media
capabilities instead of the actual configuration. In particular, the
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 <xref target="RFC5939"></xref> requires the answerer to choose the highest
preference configuration it can support, subject to local
policies.</t>
<t>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.</t>
</section>
<section anchor="sesmod" title="Modifying the Session">
<t>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 by changing the properties used on an existing stream, it
can do so via the mechanisms defined for offer/answer <xref
target="RFC3264"></xref>. If the initiating party has remembered the
codecs, potential configurations, latent configurations and session
capabilities provided 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, the initiator MAY
perform a new capabilities exchange as part of the reconfiguration.
In such a case, the new capabilities will replace the
previously-negotiated capabilities. This may be useful if conditions
change on the endpoint.</t>
</section>
</section>
</section>
<section anchor="examples" title="Examples">
<t>In this section, we provide examples showing how to use the Media
Capabilities with the SDP Capability Negotiation.</t>
<section anchor="altcodecs" title="Alternative Codecs">
<t>This example provides a choice of one of six variations of the
adaptive multirate codec. In this example, the default configuration
as specified by the media line is the same as the most preferred
configuration. Each configuration uses a different payload type number
so the offerer can interpret early media.</t>
<t><list hangIndent="10">
<t>v=0<vspace blankLines="0" /> o=- 25678 753849 IN IP4
192.0.2.1<vspace blankLines="0" /> s=<vspace blankLines="0" />
c=IN IP4 192.0.2.1<vspace blankLines="0" /> t=0 0<vspace
blankLines="0" /> a=creq:med-v0<vspace blankLines="0" /> m=audio
54322 RTP/AVP 96<vspace blankLines="0" /> a=rtpmap:96
AMR-WB/16000/1<vspace blankLines="0" /> a=fmtp:96
mode-change-capability=1; max-red=220; \ <vspace blankLines="0" />
mode-set=0,2,4,7<vspace blankLines="0" /> a=rmcap:1,3,5 audio
AMR-WB/16000/1<vspace blankLines="0" /> a=rmcap:2,4,6 audio
AMR/8000/1<vspace blankLines="0" /> a=mfcap:1,2,3,4
mode-change-capability=1<vspace blankLines="0" /> a=mfcap:5,6
mode-change-capability=2<vspace blankLines="0" /> a=mfcap:1,2,3,5
max-red=220<vspace blankLines="0" /> a=mfcap:3,4,5,6 octet-align=1
<vspace blankLines="0" /> a=mfcap:1,3,5 mode-set=0,2,4,7 <vspace
blankLines="0" /> a=mfcap:2,4,6 mode-set=0,3,5,6 <vspace
blankLines="0" /> a=pcfg:1 m=1 pt=1:96<vspace blankLines="0" />
a=pcfg:2 m=2 pt=2:97<vspace blankLines="0" /> a=pcfg:3 m=3
pt=3:98<vspace blankLines="0" /> a=pcfg:4 m=4 pt=4:99 <vspace
blankLines="0" /> a=pcfg:5 m=5 pt=5:100 <vspace blankLines="0" />
a=pcfg:6 m=6 pt=6:101</t>
</list></t>
<t>In the above example, media capability 1 could have been excluded
from the first rmcap declaration and from the corresponding mfcap
attributes, and the pcfg:1 attribute line could have been simply
"pcfg:1".</t>
<t>The next example offers a video stream with three options of H.264
and 4 transports. It also includes an audio stream with different
audio qualities: four variations of AMR, or AC3. The offer looks
something like:</t>
<t><list hangIndent="10">
<t>v=0<vspace blankLines="0" /> o=- 25678 753849 IN IP4
192.0.2.1<vspace blankLines="0" /> s=An SDP Media NEG
example<vspace blankLines="0" /> c=IN IP4 192.0.2.1<vspace
blankLines="0" /> t=0 0<vspace blankLines="0" />
a=creq:med-v0<vspace blankLines="0" />
a=ice-pwd:speEc3QGZiNWpVLFJhQX <vspace blankLines="0" /> m=video
49170 RTP/AVP 100 <vspace blankLines="0" /> c=IN IP4
192.0.2.56<vspace blankLines="0" /> a=maxprate:1000 <vspace
blankLines="0" /> a=rtcp:51540 <vspace blankLines="0" />
a=sendonly <vspace blankLines="0" /> a=candidate 12345 1 UDP 9
192.0.2.56 49170 host <vspace blankLines="0" /> a=candidate 23456
2 UDP 9 192.0.2.56 51540 host<vspace blankLines="0" /> a=candidate
34567 1 UDP 7 198.51.100.1 41345 srflx raddr \ <vspace
blankLines="0" /> 192.0.2.56 rport 49170<vspace blankLines="0" />
a=candidate 45678 2 UDP 7 198.51.100.1 52567 srflx raddr \ <vspace
blankLines="0" /> 192.0.2.56 rport 51540<vspace blankLines="0" />
a=candidate 56789 1 UDP 3 192.0.2.100 49000 relay raddr \ <vspace
blankLines="0" /> 192.0.2.56 rport 49170<vspace blankLines="0" />
a=candidate 67890 2 UDP 3 192.0.2.100 49001 relay raddr \ <vspace
blankLines="0" /> 192.0.2.56 rport 51540 <vspace blankLines="0" />
b=AS:10000 <vspace blankLines="0" /> b=TIAS:10000000 <vspace
blankLines="0" /> b=RR:4000 <vspace blankLines="0" />
b=RS:3000<vspace blankLines="0" /> a=rtpmap:100 H264/90000 <vspace
blankLines="0" /> a=fmtp:100 profile-level-id=42A01E;
packetization-mode=2; \ <vspace blankLines="0" />
sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==; \ <vspace
blankLines="0" /> sprop-interleaving-depth=45;
sprop-deint-buf-req=64000; \ <vspace blankLines="0" />
sprop-init-buf-time=102478; deint-buf-cap=128000 <vspace
blankLines="0" /> a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF<vspace
blankLines="0" /> a=rmcap:1-3,7-9 H264/90000<vspace
blankLines="0" /> a=rmcap:4-6 rtx/90000<vspace blankLines="0" />
a=mfcap:1-9 profile-level-id=42A01E<vspace blankLines="0" />
a=mfcap:1-9 aMljiA==<vspace blankLines="0" /> a=mfcap:1,4,7
packetization-mode=0<vspace blankLines="0" /> a=mfcap:2,5,8
packetization-mode=1<vspace blankLines="0" /> a=mfcap:3,6,9
packetization-mode=2<vspace blankLines="0" /> a=mfcap:1-9
sprop-parameter-sets=Z0IACpZTBYmI<vspace blankLines="0" />
a=mfcap:1,7 sprop-interleaving-depth=45; \ <vspace
blankLines="0" /> sprop-deint-buf-req=64000;
sprop-init-buf-time=102478; \ <vspace blankLines="0" />
deint-buf-cap=128000<vspace blankLines="0" /> a=mfcap:4
apt=100<vspace blankLines="0" /> a=mfcap:5 apt=99<vspace
blankLines="0" /> a=mfcap:6 apt=98<vspace blankLines="0" />
a=mfcap:4-6 rtx-time=3000<vspace blankLines="0" /> a=mscap:1-6
rtcp-fb nack<vspace blankLines="0" /> a=acap:1 crypto:1
AES_CM_128_HMAC_SHA1_80 \ <vspace blankLines="0" />
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|220|1:32 <vspace
blankLines="0" /> a=pcfg:1 t=1 m=1,4 a=1 pt=1:100,4:97<vspace
blankLines="0" /> a=pcfg:2 t=1 m=2,5 a=1 pt=2:99,4:96<vspace
blankLines="0" /> a=pcfg:3 t=1 m=3,6 a=1 pt=3:98,6:95<vspace
blankLines="0" /> a=pcfg:4 t=2 m=7 a=1 pt=7:100 <vspace
blankLines="0" /> a=pcfg:5 t=2 m=8 a=1 pt=8:99<vspace
blankLines="0" /> a=pcfg:6 t=2 m=9 a=1 pt=9:98<vspace
blankLines="0" /> a=pcfg:7 t=3 m=1,3 pt=1:100,4:97 <vspace
blankLines="0" /> a=pcfg:8 t=3 m=2,4 pt=2:99,4:96<vspace
blankLines="0" /> a=pcfg:9 t=3 m=3,6 pt=3:98,6:95<vspace
blankLines="0" /> m=audio 49176 RTP/AVP 101 100 99 98 <vspace
blankLines="0" /> c=IN IP4 192.0.2.56<vspace blankLines="0" />
a=ptime:60 <vspace blankLines="0" /> a=maxptime:200 <vspace
blankLines="0" /> a=rtcp:51534 <vspace blankLines="0" />
a=sendonly <vspace blankLines="0" /> a=candidate 12345 1 UDP 9
192.0.2.56 49176 host <vspace blankLines="0" /> a=candidate 23456
2 UDP 9 192.0.2.56 51534 host<vspace blankLines="0" /> a=candidate
34567 1 UDP 7 198.51.100.1 41348 srflx \ <vspace blankLines="0" />
raddr 192.0.2.56 rport 49176<vspace blankLines="0" /> a=candidate
45678 2 UDP 7 198.51.100.1 52569 srflx \ <vspace blankLines="0" />
raddr 192.0.2.56 rport 51534<vspace blankLines="0" /> a=candidate
56789 1 UDP 3 192.0.2.100 49002 relay \ <vspace blankLines="0" />
raddr 192.0.2.56 rport 49176<vspace blankLines="0" /> a=candidate
67890 2 UDP 3 192.0.2.100 49003 relay \ <vspace blankLines="0" />
raddr 192.0.2.56 rport 51534 <vspace blankLines="0" /> b=AS:512
<vspace blankLines="0" /> b=TIAS:512000 <vspace blankLines="0" />
b=RR:4000 <vspace blankLines="0" /> b=RS:3000 <vspace
blankLines="0" /> a=maxprate:120 <vspace blankLines="0" />
a=rtpmap:98 AMR-WB/16000 <vspace blankLines="0" /> a=fmtp:98
octet-align=1; mode-change-capability=2<vspace blankLines="0" />
a=rtpmap:99 AMR-WB/16000 <vspace blankLines="0" /> a=fmtp:99
octet-align=1; crc=1; mode-change-capability=2<vspace
blankLines="0" /> a=rtpmap:100 AMR-WB/16000/2 <vspace
blankLines="0" /> a=fmtp:100 octet-align=1; interleaving=30
<vspace blankLines="0" /> a=rtpmap:101 AMR-WB+/72000/2 <vspace
blankLines="0" /> a=fmtp:101 interleaving=50; int-delay=160000;
<vspace blankLines="0" /> a=rmcap:14 ac3/48000/6 <vspace
blankLines="0" /> a=acap:23 crypto:1 AES_CM_128_HMAC_SHA1_80 \
<vspace blankLines="0" />
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|220|1:32<vspace
blankLines="0" /> a=tcap:4 RTP/SAVP <vspace blankLines="0" />
a=pcfg:10 t=4 a=23 <vspace blankLines="0" /> a=pcfg:11 t=4 m=14
a=23 pt=14:102</t>
</list></t>
<t>This offer illustrates the advantage in compactness that arises if
one can avoid deleting the base configuration attributes and
recreating them in acap attributes for the potential
configurations.</t>
</section>
<section anchor="altcombos"
title="Alternative Combinations of Codecs (Session Configurations)">
<t>If an endpoint has limited signal processing capacity, it might be
capable of supporting, say, a G.711 mu-law audio stream in combination
with an H.264 video stream, or a G.729B audio stream in combination
with an H.263-1998 video stream. It might then issue an offer like the
following:</t>
<t><list hangIndent="10">
<t>v=0<vspace blankLines="0" /> o=- 25678 753849 IN IP4
192.0.2.1<vspace blankLines="0" /> s=<vspace blankLines="0" />
c=IN IP4 192.0.2.1<vspace blankLines="0" /> t=0 0<vspace
blankLines="0" /> a=creq:med-v0<vspace blankLines="0" />
a=sescap:1 2,4<vspace blankLines="0" /> a=sescap:2 1,3<vspace
blankLines="0" /> m=audio 54322 RTP/AVP 18<vspace
blankLines="0" /> a=rtpmap:18 G729/8000<vspace blankLines="0" />
a=fmtp:18 annexb=yes<vspace blankLines="0" /> a=rmcap:1
PCMU/8000<vspace blankLines="0" /> a=pcfg:1 m=1 pt=1:0<vspace
blankLines="0" /> a=pcfg:2<vspace blankLines="0" /> m=video 54344
RTP/AVP 100<vspace blankLines="0" /> a=rtpmap:100
H263-1998/90000<vspace blankLines="0" /> a=rmcap:2
H264/90000<vspace blankLines="0" /> a=mfcap:2
profile-level-id=42A01E; packetization-mode=2<vspace
blankLines="0" /> a=pcfg:3 m=2 pt=2:101<vspace blankLines="0" />
a=pcfg:4</t>
</list></t>
<t>Note that the preferred session configuration (and the default as
well) is G.729B with H.263. This overrides the individual media stream
preferences which are PCMU and H.264 by the potential configuration
numbering rule.</t>
</section>
<section anchor="latentstreams" title="Latent Media Streams">
<t>Consider a case in which the offerer can support either G.711
mu-law, or G.729B, along with DTMF telephony events for the 12 common
touchtone signals, but is willing to support simple G.711 mu-law audio
as a last resort. In addition, the offerer wishes to announce its
ability to support video and MSRP in the future, but does not wish to
offer a video stream or an MSRP stream at present. The offer might
look like the following:</t>
<t><list hangIndent="10">
<t>v=0<vspace blankLines="0" /> o=- 25678 753849 IN IP4
192.0.2.1<vspace blankLines="0" /> s=<vspace blankLines="0" />
c=IN IP4 192.0.2.1<vspace blankLines="0" /> t=0 0<vspace
blankLines="0" /> a=creq:med-v0<vspace blankLines="0" /> m=audio
23456 RTP/AVP 0<vspace blankLines="0" /> a=rtpmap:0
PCMU/8000<vspace blankLines="0" /> a=rmcap:1 PCMU/8000<vspace
blankLines="0" /> a=rmcap:2 G729/8000<vspace blankLines="0" />
a=rmcap:3 telephone-event/8000<vspace blankLines="0" /> a=mfcap:3
0-11<vspace blankLines="0" /> a=pcfg:1 m=1,3|2,3 pt=1:0,2:18,3:100
<vspace blankLines="0" /> a=lcfg:2 mt=video t=1 m=10|11<vspace
blankLines="0" /> a=rmcap:10 H263-1998/90000<vspace
blankLines="0" /> a=rmcap:11 H264/90000<vspace blankLines="0" />
a=tcap:1 RTP/AVP<vspace blankLines="0" /> a=lcfg:3 mt=message t=2
m=20 <vspace blankLines="0" />a=tcap:2 TCP/MSRP <vspace
blankLines="0" />a=omcap:20 *</t>
</list></t>
<t>The first lcfg attribute line ("lcfg:2") announces support for
H.263 and H.264 video (H.263 preferred) for future negotiation. The
second lcfg attribute line ("lcfg:3") announces support for MSRP for
future negotiation. The m-line and the rtpmap attribute offer an audio
stream and provide the lowest precedence configuration (PCMU without
any DTMF encoding). The rmcap lines define the RTP-based media
format capabilities (PCMU, G729, telephone-event, H263-1998 and H264) and the
omcap line defines the non-RTP based media format capability (wildcard). The
mfcap attribute provides the format parameters for telephone-events,
specifying the 12 commercial DTMF 'digits'. The pcfg attribute line
defines the most-preferred media configuration as PCMU plus DTMF
events and the next-most-preferred configuration as G.729B plus DTMF
events.</t>
<t>If the answerer is able to support all the potential
configurations, and also support H.263 video (but not H.264), it would
reply with an answer like:</t>
<t><list hangIndent="10">
<t>v=0<vspace blankLines="0" /> o=- 24351 621814 IN IP4
192.0.2.2<vspace blankLines="0" /> s=<vspace blankLines="0" />
c=IN IP4 192.0.2.2<vspace blankLines="0" /> t=0 0<vspace
blankLines="0" /> a=csup:med-v0<vspace blankLines="0" /> m=audio
54322 RTP/AVP 0 100<vspace blankLines="0" /> a=rtpmap:0
PCMU/8000<vspace blankLines="0" /> a=rtpmap:100
telephone-event/8000<vspace blankLines="0" /> a=fmtp:100
0-11<vspace blankLines="0" /> a=acfg:1 m=1,3 pt=1:0,3:100<vspace
blankLines="0" /> a=pcfg:1 m=2,3 pt=2:18,3:100<vspace
blankLines="0" /> a=lcfg:2 mt=video t=1 m=10<vspace
blankLines="0" /></t>
</list></t>
<t>The lcfg attribute line announces the capability to support H.263
video at a later time. The media line and subsequent rtpmap and fmtp
attribute lines present the selected configuration for the media
stream. The acfg attribute line identifies the potential configuration
from which it was taken, and the pcfg attribute line announces the
potential capability to support G.729 with DTMF events as well. If, at
some later time, congestion becomes a problem in the network, either
party may, with expectation of success, offer a reconfiguration of the
media stream to use G.729 in order to reduce packet sizes.</t>
</section>
</section>
<section anchor="iana" title="IANA Considerations">
<section anchor="ianaattrs" title="New SDP Attributes">
<t>The IANA is hereby requested to register the following new SDP
attributes:</t>
<t><list hangIndent="4">
<t>
Attribute name: rmcap<vspace blankLines="0" />
Long form name: RTP-based media format capability<vspace blankLines="0" />
Type of attribute: session-level and media-level<vspace blankLines="0" />
Subject to charset: no<vspace blankLines="0" />
Purpose: associate RTP-based media capability number(s) with<vspace blankLines="0" />
media subtype and encoding parameters<vspace blankLines="0" />
Appropriate Values: see <xref target="mcap"></xref> <vspace blankLines="0" />
Contact name: Flemming Andreasen, fandres@cisco.com</t>
</list></t>
<t><list hangIndent="4">
<t>Attribute name: omcap<vspace blankLines="0" />
Long form name: Non RTP-based media format capability<vspace blankLines="0" /> Type of
attribute: session-level and media-level<vspace blankLines="0" />
Subject to charset: no<vspace blankLines="0" /> Purpose: associate
non RTP-based media capability number(s) with<vspace
blankLines="0" /> media subtype and encoding parameters<vspace
blankLines="0" /> Appropriate Values: see <xref
target="mcap"></xref><vspace blankLines="0" />
Contact name: Flemming Andreasen, fandreas@cisco.com</t>
</list></t>
<t><list hangIndent="4">
<t>Attribute name: mfcap<vspace blankLines="0" /> Long form name:
media format parameter capability<vspace blankLines="0" /> Type of
attribute: session-level and media-level<vspace blankLines="0" />
Subject to charset: no<vspace blankLines="0" /> Purpose: associate
media format attributes and<vspace blankLines="0" /> parameters
with media format capabilities<vspace blankLines="0" />
Appropriate Values: see <xref target="mfcap"></xref><vspace blankLines="0" />
Contact name: Flemming Andreasen, fandreas@cisco.com</t>
</list></t>
<t><list hangIndent="4">
<t>Attribute name: mscap<vspace blankLines="0" /> Long form name:
media-specific capability<vspace blankLines="0" /> Type of
attribute: session-level and media-level<vspace blankLines="0" />
Subject to charset: no<vspace blankLines="0" /> Purpose: associate
media-specific attributes and<vspace blankLines="0" /> parameters
with media capabilities<vspace blankLines="0" /> Appropriate
Values: see <xref target="mscap"></xref><vspace blankLines="0" />
Contact name: Flemming Andreasen, fandreas@cisco.com</t>
</list></t>
<t><list hangIndent="4">
<t>Attribute name: lcfg<vspace blankLines="0" /> Long form name:
latent configuration<vspace blankLines="0" /> Type of attribute:
media-level<vspace blankLines="0" /> Subject to charset: no<vspace
blankLines="0" /> Purpose: to announce supportable media
streams<vspace blankLines="0" /> without offering them for
immediate use.<vspace blankLines="0" /> Appropriate Values: see
<xref target="lcfg"></xref><vspace blankLines="0" />
Contact name: Flemming Andreasen, fandreas@cisco.com</t>
</list></t>
<t><list hangIndent="4">
<t>Attribute name: sescap<vspace blankLines="0" /> Long form name:
session capability<vspace blankLines="0" /> Type of attribute:
session-level<vspace blankLines="0" /> Subject to charset:
no<vspace blankLines="0" /> Purpose: to specify and prioritize
acceptable<vspace blankLines="0" /> combinations of media stream
configurations.<vspace blankLines="0" /> Appropriate Values: see
<xref target="sescap"></xref><vspace blankLines="0" />
Contact name: Flemming Andreasen, fandreas@cisco.com</t>
</list></t>
</section>
<section anchor="ianaoption" title="New SDP Capability Negotiation Option Tag">
<t>The IANA is hereby requested to add the new option tag "med-v0",
defined in this document, to the SDP Capability Negotiation Option
Capability registry created for <xref target="RFC5939"></xref>.</t>
</section>
<section anchor="ianacfgregistry" title="SDP Capability Negotiation Configuration Parameters Registry">
<t>The IANA is hereby requested to change the "SDP Capability
Negotiation Potential Configuration Parameters" registry
currently registered and defined by <xref
target="RFC5939"></xref> as follows:</t>
<t>The name of the registry should be "SDP Capability
Negotiation Configuration Parameters Registry" and it should
contain a table with the following column headings:</t>
<t><list style="symbols">
<t>Encoding Name: The syntactical value used for the
capability negotiation configuration parameter, as defined
in <xref target="RFC5939"></xref>, Section 3.5.</t>
<t>Descriptive Name: The name commonly used to refer to the
capability negotiation configuration parameter.</t>
<t>Potential Configuration Definition: A reference to the
RFC that defines the configuration parameter in the context
of a potential configuration attribute. If the configuration
parameter is not defined for potential configurations, the
string "N/A" (Not Applicable) MUST be present instead.</t>
<t>Actual Configuration Definition: A reference to the RFC
that defines the configuration parameter in the context of
an actual configuration attribute. If the configuration
parameter is not defined for actual configurations, the
string "N/A" (Not Applicable) MUST be present instead. </t>
<t>Latent Configuration Definition: A reference to the RFC
that defines the configuration parameter in the context of a
latent configuration attribute. If the configuration
parameter is not defined for latent configurations, the
string "N/A" (Not Applicable) MUST be present instead.</t>
</list></t>
<t>An IANA SDP Capability Negotiation Configuration
registration MUST be documented in an RFC in accordance with
the <xref target="RFC5226"></xref> IETF Review
policy. Furthermore:</t>
<t><list style="symbols">
<t>The RFC MUST define the syntax and semantics of each new
potential configuration parameter.</t>
<t>The syntax MUST adhere to the syntax provided for
extension configuration lists in <xref target="RFC5939"></xref> Section 3.5.1
and the semantics MUST adhere to the semantics provided for
extension configuration lists in <xref target="RFC5939"></xref> Section 3.5.1
and 3.5.2. </t>
<t>Configuration parameters that apply to latent
configurations MUST furthermore adhere to the syntax
provided in <xref target="lcfg" /> and the semantics defined overall in this document.</t>
<t> Associated with each registration MUST be the encoding
name for the parameter as well as a short descriptive name
for it.</t>
<t>Each registration MUST specify if it applies to
<list style="symbols">
<t>Potential configurations</t>
<t>Actual configurations</t>
<t>Latent configurations</t>
</list>
</t>
</list></t>
</section>
<section anchor="ianacfgparmreg" title="SDP Capability Negotiation Configuration Parameter Registrations">
<t>The IANA is hereby requested to register the following
capability negotiation configuration parameters:</t>
<t><list hangIndent="4">
<t>
Encoding Name: a <vspace blankLines="0"/>
Descriptive Name: Attribute Configuration<vspace blankLines="0"/>
Potential Configuration Definition: <xref target="RFC5939" /> <vspace blankLines="0"/>
Actual Configuration Definition: <xref target="RFC5939"> </xref> <vspace blankLines="0"/>
Latent Configuration Definition: [Note to RFC Editor: This RFC]
</t>
</list></t>
<t><list hangIndent="4">
<t>
Encoding Name: t <vspace blankLines="0"/>
Descriptive Name: Transport Protocol Configuration<vspace blankLines="0"/>
Potential Configuration Definition: <xref target="RFC5939" /> <vspace blankLines="0"/>
Actual Configuration Definition: <xref target="RFC5939" /> <vspace blankLines="0"/>
Latent Configuration Definition: [Note to RFC Editor: This RFC]
</t>
</list></t>
<t><list hangIndent="4">
<t>
Encoding Name: m <vspace blankLines="0"/>
Descriptive Name: Media Configuration<vspace blankLines="0"/>
Potential Configuration Definition: [Note to RFC Editor: This RFC] <vspace blankLines="0"/>
Actual Configuration Definition: [Note to RFC Editor: This RFC] <vspace blankLines="0"/>
Latent Configuration Definition: [Note to RFC Editor: This RFC]
</t>
</list></t>
<t><list hangIndent="4">
<t>
Encoding Name: pt <vspace blankLines="0"/>
Descriptive Name: Payload Type Number Mapping<vspace blankLines="0"/>
Potential Configuration Definition: [Note to RFC Editor: This RFC] <vspace blankLines="0"/>
Actual Configuration Definition: [Note to RFC Editor: This RFC] <vspace blankLines="0"/>
Latent Configuration Definition: [Note to RFC Editor: This RFC]
</t>
</list></t>
<t><list hangIndent="4">
<t>
Encoding Name: mt <vspace blankLines="0"/>
Descriptive Name: Media Type <vspace blankLines="0"/>
Potential Configuration Definition: N/A <vspace blankLines="0"/>
Actual Configuration Definition: N/A <vspace blankLines="0"/>
Latent Configuration Definition: [Note to RFC Editor: This RFC]
</t>
</list></t>
</section>
</section>
<section anchor="security" title="Security Considerations">
<t>The security considerations of <xref target="RFC5939"></xref>
apply for this document.</t>
<t>In <xref target="RFC5939"></xref>, it was noted that negotiation of transport
protocols (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 <xref target="RFC5939"></xref>, the considerations and
mitigation strategies are similar to those described in
<xref target="RFC5939"></xref>.</t>
<t>Another variation on the above attack involves 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 <xref target="RFC5939"></xref>.</t>
<t>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 flows, and/or processing resource
consumption as part of network policy, but which do not
understand the media capability negotiation feature. As for
the initial CapNeg work <xref target="RFC5939"></xref>, the SDP answer is
formulated in such a way that it always carries the selected
media encoding for every media stream selected. Pending an
understanding of capabilities negotiation, the middlebox
should examine the answer SDP to obtain the best picture of
the media streams being established. As always, middleboxes
can best do their job if they fully understand media
capabilities negotiation.</t>
</section>
<section anchor="verchanges" title="Changes from previous versions">
<section title="Changes from version 14">
<t><list style="symbols">
<t>Updated IANA Considerations to fix configuration
parameter registry. Document now updates <xref
target="RFC5939"></xref> (IANA considerations only)</t>
<t>Minor ABNF updates to fix errors.</t>
<t>Editorial nit fixes to address protocol write-up review.</t>
</list></t>
</section>
<section title="Changes from version 13">
<t><list style="symbols">
<t>Various editorial clarifications and updates to address review
comments.</t>
</list></t>
</section>
<section title="Changes from version 12">
<t><list style="symbols">
<t>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.</t>
<t>Clarified that latent configuration attribute (lcfg) can only
be used at the media level and hence (technically) as part of a
media description</t>
<t>Rewrote offer/answer sections and expanded significantly on
offer/answer operation.</t>
<t>Updated security considerations</t>
<t>Various minor editorial clarifications and changes.</t>
</list></t>
</section>
<section title="Changes from version 11">
<t><list style="symbols">
<t>Corrected several statements implying lcfg was a session-level
attribute.</t>
<t>Added non-RTP based media format capabilities ("a=omcap") and
renamed "mcap" to "rmcap"</t>
</list></t>
</section>
<section title="Changes from version 10">
<t><list style="symbols">
<t>Defined the latent configuration attribute as a media-level
attribute because it specifies a possible future media stream.
Added text to clarify how to specify alternative configurations of
a single latent stream and/or multiple streams.</t>
<t>Improved the definition of the session capability attribute to
permit both required configurations and optional configurations -
latent configurations cannot be required because they have not yet
been offered.</t>
<t>Removed the special-case treatment of conflicts between
base-level fmtp attributes and fmtp attributes generated for a
configuration via invoked mcap and mfcap attributes.</t>
<t>Removed reference to bandwidth capability (bcap) attribute.</t>
<t>Changed various "must", etc., terms to normative terms ("MUST",
etc.) as appropriate, in <xref target="lcfg"></xref><xref
target="answercaps"></xref> <xref target="mediaattrs"></xref> and
<xref target="sescap"></xref></t>
<t>Attempted to clarify the substitution mechanism in <xref
target="ptsubs"></xref> and improve its uniqueness.</t>
<t>Made various editorial changes, including changing the title in
the header, and removing numbering from some SDP examples.</t>
</list></t>
</section>
<section title="Changes from version 09">
<t><list style="symbols">
<t>Additional corrections to latent media stream example in <xref
target="latentstreams"></xref></t>
<t>Fixed up attribute formatting examples and corresponding
ABNF.</t>
<t>Removed preference rule for latent configurations.</t>
<t>Various spelling and other editorial changes were made.</t>
<t>updated cross-references.</t>
</list></t>
</section>
<section title="Changes from version 08">
<t>The major change is in <xref target="latentstreams"></xref>, Latent
Media Streams, fixing the syntax of the answer. All the other changes
are editorial.</t>
</section>
<section title="Changes from version 04">
<t><list style="symbols">
<t>The definitions for bcap, ccap, icap, and kcap attributes have
been removed, and are to be defined in another document.</t>
<t>Corrected formatting of m= and p= configuration parameters to
conform to extension-config-list form defined in <xref
target="RFC5939"></xref></t>
<t>Reorganized definitions of new parameters to make them easier
to find in document.</t>
<t>Added ability to renegotiate capabilities when modifying the
session (<xref target="sesmod"></xref>).</t>
<t>Made various editorial changes, clarifications, and typo
corrections.</t>
</list></t>
</section>
<section title="Changes from version 03">
<t><list style="symbols">
<t>A new session capability attribute (sescap) has been added to
permit specification of acceptable media stream combinations.</t>
<t>Capability attribute definitions corresponding to the i, c, b,
and k SDP line types have been added for completeness.</t>
<t>Use of the pcfg: attribute in SDP answers has been included in
order to conveniently return information in the answer about
acceptable configurations in the media stream offer.</t>
<t>The use of the lcfg: attribute(s) in SDP answers has been
restricted to indicate just which latent configuration offers
would be acceptable to the answerer.</t>
<t>A suggestion for "naive" middleboxes has been added to the
Security Considerations.</t>
<t>Various editorial changes have been made.</t>
<t>Several errors/omissions have been corrected.</t>
<t>The description of the mscap attribute has been modified to
make it clear that it should not be used to generate undefined SDP
attributes, or to "extend" existing attributes.</t>
<t><ms-parameters> are made optional in the mscap attribute
definition.</t>
<t>"AMR" changed to "AMR-WB" in cases in which the sample rate is
16000.</t>
</list></t>
</section>
<section title="Changes from version 02">
<t>This version contains several detail changes intended to simplify
capability processing and mapping into conventional SDP media
blocks.</t>
<t><list style="symbols">
<t>The "mcap" attribute is enhanced to include the role of the
"ecap" attribute; the latter is eliminated.</t>
<t>The "fcap" attribute has been renamed "mfcap". New replacement
rules vis-a-vis fmtp attributes in the base media specification
have been added.</t>
<t>A new "mscap" attribute is defined to handle the problem of
attributes (other than rtpmap and fmtp) that are specific to a
particular payload type.</t>
<t>New rules for processing the mcap, mfcap, and mscap attributes,
and overriding standard rtpmap, fmtp, or other media-specific
attributes, are put forward to reduce the need to use the deletion
option in the a= parameter of the potential configuration (pcfg)
attribute.</t>
<t>A new parameter, "mt=" is added to the latent configuration
attribute (lcfg) to specify the media stream type (audio, video,
etc.) when the lcfg is declared at the session level.</t>
<t>The examples are expanded.</t>
<t>Numerous typos and misspellings have been corrected.</t>
</list></t>
</section>
<section title="Changes from version 01">
<t>The documents adds a new attribute for specifying bandwidth
capability and a parameter to list in the potential configuration.
Other changes are to align the document with the terminology and
attribute names from draft-ietf-mmusic-sdp-capability-negotiation-07.
The document also clarifies some previous open issues.</t>
</section>
<section title="Changes from version 00">
<t>The major changes include taking out the "mcap" and "cptmap"
parameter. The mapping of payload type is now in the "pt" parameter of
"pcfg". Media subtype need to explicitly defined in the "cmed"
attribute if referenced in the "pcfg"</t>
</section>
</section>
<section anchor="acks" title="Acknowledgements">
<t>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.</t>
<t>We thank Ingemar Johansson and Magnus Westerlund for examples that
stimulated this work, and for critical reading of the document. We also
thank Cullen Jennings, Christer Holmberg, and Miguel Garcia for their
review of the document.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.2119.xml"?>
<?rfc include="reference.RFC.3264.xml"?>
<?rfc include="reference.RFC.4566.xml"?>
<?rfc include="reference.RFC.5226.xml"?>
<?rfc include="reference.RFC.5234.xml"?>
<?rfc include="reference.RFC.5939.xml"?>
</references>
<references title="Informative References">
<?rfc include="reference.RFC.2198.xml"?>
<?rfc include="reference.RFC.4568.xml"?>
<?rfc include="reference.RFC.4585.xml"?>
<?rfc include="reference.RFC.4733.xml"?>
<?rfc include="reference.RFC.4867.xml"?>
<?rfc include="reference.RFC.5104.xml"?>
</references>
</back>
</rfc>
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