MMUSIC                                                         R. Gilman
Internet-Draft                                                      NDCI
Intended status: Standards Track                                 R. Even
Expires: January 15, 2009                                        Polycom
                                                            F. Andreasen
                                                           Cisco Systems
                                                           July 14, 2008

                   SDP media capabilities Negotiation

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   Copyright (C) The IETF Trust (2008).

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   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.  This
   extension is designed to map easily to existing and future SDP media

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  SDP Media Capabilities . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  6
     3.2.  Solution Overview  . . . . . . . . . . . . . . . . . . . .  7
     3.3.  New Capability Attributes  . . . . . . . . . . . . . . . . 12
       3.3.1.  The Media Encoding Capability Attribute  . . . . . . . 13
       3.3.2.  The Media Format Parameter Capability Attribute  . . . 14
       3.3.3.  The Media-Specific Capability Attribute  . . . . . . . 17
       3.3.4.  Additional Capability Attributes . . . . . . . . . . . 18
       3.3.5.  The Latent Configuration Attribute . . . . . . . . . . 20
       3.3.6.  Enhanced Potential Configuration Attribute . . . . . . 24
       3.3.7.  Substitution of Media Payload Type Numbers in
               Capability Attribute Parameters  . . . . . . . . . . . 27
       3.3.8.  The Session Capability Attribute . . . . . . . . . . . 28
     3.4.  Offer/Answer Model Extensions  . . . . . . . . . . . . . . 32
       3.4.1.  Generating the Initial Offer . . . . . . . . . . . . . 33
       3.4.2.  Generating the Answer  . . . . . . . . . . . . . . . . 33
       3.4.3.  Offerer Processing of the Answer . . . . . . . . . . . 34
       3.4.4.  Modifying the Session  . . . . . . . . . . . . . . . . 34
   4.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
     4.1.  Alternative Codecs . . . . . . . . . . . . . . . . . . . . 35
     4.2.  Alternative Combinations of Codecs (Session
           Configurations)  . . . . . . . . . . . . . . . . . . . . . 38
     4.3.  Latent Media Streams . . . . . . . . . . . . . . . . . . . 38
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 41
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 43
   7.  Changes from previous versions . . . . . . . . . . . . . . . . 44
     7.1.  Changes from version 03  . . . . . . . . . . . . . . . . . 44
     7.2.  Changes from version 02  . . . . . . . . . . . . . . . . . 44
     7.3.  Changes from version 01  . . . . . . . . . . . . . . . . . 45
     7.4.  Changes from version 00  . . . . . . . . . . . . . . . . . 45
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 46
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 47
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 47
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 47
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 49
   Intellectual Property and Copyright Statements . . . . . . . . . . 50

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1.  Introduction

   Session Description Protocol (SDP) capability negotiation [SDPCapNeg]
   provides a general framework for indicating and negotiating
   capabilities in SDP[RFC4566].  The base framework defines only
   capabilities for negotiating transport protocols and attributes.

   SDP Simple Capability Declaration (simcap) is defined in RFC 3407
   [RFC3407].  It defines a set of SDP attributes that enables a limited
   set of capabilities to be described at a session level or on a per
   media stream basis.  The capabilities include a simple form of media
   capabilities.  RFC 3407 defines capability declaration only.  Actual
   negotiation procedures taking advantage of such capabilities have not
   been defined.  The SDP capability negotiation framework defined in
   [SDPCapNeg] adds this required functionality, but does not define
   media capabilities.  This document updates RFC3407 and [SDPCapNeg]
   and new implementations SHOULD use the functionality defined in the
   current document to negotiate media capabilities.

   The [SDPCapNeg] document lists some of the issues with the current
   SDP capability negotiation process.  An additional real life case is
   to be able to offer one media stream (e.g. audio) but list the
   capability to support another media stream (e.g. video) without
   actually offering it currently.

   In this document, we extend the framework by defining media
   capabilities that can be used to indicate and negotiate media types
   and their associated stream parameters such as bandwidth.  This
   document also adds the ability to declare support for media streams,
   the use of which can be offered and negotiated later.  The
   definitions of new attributes for media capability negotiation are
   chosen to make the translation from these attributes to
   "conventional" SDP [RFC4566] 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.

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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC2119 [RFC2119] and
   indicate requirement levels for compliant RTP implementations.

   "Base Attributes": Conventional SDP attributes appearing in the base
   configuration of a media block.

   "Base Configuration": The media configuration represented by a media
   block exclusive of all the capability negotiation attributes defined
   in this document, the base capability document[SDPCapNeg], or any
   future capability negotiation document.

   "Conventional Attribute": Any SDP attribute other than those defined
   by the series of capability negotiation specifications.

   "Conventional SDP": An SDP record devoid of capability negotiation

   "Media Capability": A media encoding, typically a media subtype such
   as PCMU, H263-1998, or T38.

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3.   SDP Media Capabilities

   The SDP capability negotiation [SDPCapNeg] discusses the use of any
   SDP [RFC4566] attribute (a=) under the attribute capability "acap".
   The limitations of using acap for fmtp and rtpmap in a potential
   configuration are described in [SDPCapNeg]; for example they can be
   used only at the media level since they are media level attributes.
   The [SDPCapNeg] partially addresses the issue of bandwidth usage for
   different configurations but does not offer a way to negotiate
   different bandwidth for different codecs and profiles.  This section
   provides an overview of extensions providing SDP Media Capability and
   Bandwidth 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

3.1.  Requirements

   The capability negotiation extensions described herein are described
   as follows.

   REQ-01:   Support the specification of alternative (combinations of)
      media formats (codecs) in a single media block.

   REQ-02:   Support the specification of alternative media format
      parameters for each media format.

   REQ-03:   Retain backward compatibility with conventional SDP.
      Insure that each and every offered configuration can be easily
      translated into a corresponding SDP media block expressed with
      conventional SDP lines.

   REQ-04:   Insure 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.

   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.)

   REQ-06:   Provide the ability to offer latent media configurations
      for future negotiation.

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   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.

   REQ-08:   Retain the extensibility of the base capability negotiation

   REQ-09:   Provide the ability to specify acceptable combinations of
      media streams and encodings.  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.

   REQ-10:   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.

   Other possible extensions have been discussed, but have not been
   treated in this document.  They may be considered in the future.  Two
   such extensions are:

   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.

   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.

3.2.  Solution Overview

   The solution consists of new capability attributes corresponding to
   conventional SDP line types, new parameters for the pcfg attribute
   extending the base attributes from [SDPCapNeg], and a use of the pcfg
   attribute to return capability information in the SDP answer.

   Three 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.

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   o  A new media attribute ("a=mcap") defines media capabilities in the
      form of a 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 [RFC4566]

   o  A new attribute ("a=mfcap") specifies media format parameters
      associated with one or more media capabilities.  The mfcap
      attribute is used primarily to associate the formatting
      capabilities normally carried in the fmtp attribute.

   o  A new attribute ("a=mscap") that specifies media parameters
      associated with one or more media capabilities.  The mscap
      attribute is used to associate capabilities with attributes other
      than fmtp or rtpmap, for example, the rtcp-fb attribute.

   o  Several new capability attributes, corresponding to several SDP
      line types (e.g., the bandwidth type "b="}, are defined for use in
      capability negotiations.

   o  A new attribute ("a=lcfg") specifies latent media stream
      configurations when no corresponding media line ("m=") is offered.
      An example is the offer of a latent configuration for video even
      though no video is currently offered.  If both parties indicate
      support for one or more latent configurations, the corresponding
      media stream(s) may be added via a new offer/answer exchange.

   o  A new attribute ("a=sescap") is used to specify an acceptable
      combination of simultaneous media streams as a list of potential
      and/or latent configurations.

   New parameters are defined for the potential configuration (pcfg),
   latent configuration (lcfg), and accepted configuration (acfg)
   attributes to associate the new attributes with particular

   o  A new parameter type ("m=") is added to the potential
      configuration ("a=pcfg:") attribute and the actual configuration
      ("a=acfg:") attribute defined in [SDPCapNeg], 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 or G.729B and telephone events as acceptable
      configurations.  The "a=acfg:" line in the answer would specify
      the accepted choice.

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   o  A new parameter type ("pt=") is added to the potential
      configuration, actual configuration, and latent configuration
      attributes.  This parameter associates RTP payload types numbers
      with the referenced media capabilities, and is appropriate only
      when the transport protocol uses RTP.

   o  A new parameter type ("mt=") is used to specify the media type for
      latent configurations.

   o  New parameter types (e.g., "b=") are used to specify conventional
      SDP parameters in potential or latent configurations.

   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

   o  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).

   o  Replacement rules are defined for the conventional SDP equivalents
      of the mfcap, mscap, and bcap capability attributes.  This reduces
      the necessity to use the deletion qualifier in the pcfg a=
      parameter in order to ignore rtpmap, fmtp, and certain other
      attributes in the base configuration.

   o  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 capabilities.

   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

   The mechanism is illustrated by the offer/answer exchange below,
   where Alice sends an offer to Bob:

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                   Alice                               Bob
                  | (1) Offer (SRTP and RTP)         |
                  |                                  |
                  | (2) Answer (RTP)                 |
                  |                                  |

   Alice's offer includes RTP and SRTP as alternatives.  RTP is the
   default, but SRTP is the preferred one (long lines are folded to fit
   the margins):

             o=- 25678 753849 IN IP4
             c=IN IP4
             t=0 0
             m=audio 3456 RTP/AVP 0 18
             a=tcap:1 RTP/SAVP RTP/AVP
             a=rtpmap:0 PCMU/8000/1
             a=rtpmap:18 G729/8000/1
             a=fmtp:18 annexb=yes
             a=mcap:1,4 g729/8000/1
             a=mcap:2 PCMU/8000/1
             a=mcap:5 telephone-event/8000
             a=mfcap:1 annexb=no
             a=mfcap:4 annexb=yes
             a=mfcap:5 0-11
             a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32
             a=pcfg:1 m=4,5|1,5 t=1 a=1 pt=1:100,4:101,5:102
             a=pcfg:2 m=2 t=1 a=1 pt=2:103
             a=pcfg:3 m=4 t=2 pt=4:18

   The required base and extensions are provided by the "a=creq"
   attribute defined in [SDPCapNeg], with the option tag "med-v0", which
   indicates that the extension framework defined here, must be
   supported.  The Base level support is implied since it is required
   for the extensions.

   The "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

   The "a=tcap:1" line, specified in the base protocol, defines
   transport protocol capabilities, in this case Secure RTP (SAVP

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   profile) as the first option and RTP (AVP profile) as the second

   The "a=mcap:1,4" line defines two G.729 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.  In
   this example, two G.729 subtype capabilities are defined.  This
   permits the declaration of two sets of formatting parameters for

   The "a=mcap:2" line defines a G.711 mu-law capability, numbered 2.

   The "a=mcap:5" line defines an audio telephone-event capability,
   numbered 5.

   The "a=mfcap:1" line specifies the fmtp formatting parameters for
   capability 1 (no comfort noise packets).

   The "a=mfcap:4" line specifies the fmtp formatting parameters for
   capability 4 (G.729B).

   The "a=mfcap:5" line specifies the fmtp formatting parameters for
   capability 5 (the DTMF touchtones 0-9,*,#).

   The "a=acap:1" line specified in the base protocol provides the
   "crypto" attribute which provides the keying material for SRTP using
   SDP security descriptions.

   The "a=pcfg:" attributes provide the potential configurations
   included in the offer by reference to the media capabilities,
   transport capabilities, and associated payload type number mappings.
   Three explicit alternatives are provided; the first one, numbered 1
   is the preferred one.  It specifies media capabilities 4 and 5, i.e.,
   G.729B and DTMF, or media capability 1 and 5, i.e., G.729 and DTMF.
   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 media capabilities 1, 4, and 5, thereby permitting
   the offeror to distinguish between encrypted media and unencrypted
   media received prior to receipt of the answer.  Use of unique payload
   type numbers is not required; codecs such as AMR-WB [RFC4867] 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
   [RFC4568], the offeror will still need to receive the answer before

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   being able to decrypt the stream.

   The second alternative specifies media capability 2, i.e.  PCMU,
   under the RTP/SAVP profile, with the same SRTP key material.

   The third alternative offers G.729B unsecured; it's only purpose in
   this example is to show a preference for G.729B over PCMU.

   The media line, with any qualifying attributes such as fmtp or
   rtpmap, is itself considered a valid configuration; it is assumed to
   be the lowest preference.

   Bob receives the SDP offer from Alice.  Bob supports 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:

             o=- 24351 621814 IN IP4
             c=IN IP4
             t=0 0
             m=audio 4567 RTP/AVP 18
             a=rtpmap:18 G729/8000
             a=fmtp:18 annexb=yes
             a=acfg:3 m=4 pt=4:18

   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.

   Note that per the base protocol, after the above, Alice MAY generate
   a new offer with an actual configuration ("m=" line, etc.)
   corresponding to the actual configuration referenced in Bob's answer
   (not shown here).

3.3.  New Capability Attributes

   In this section, we present the new attributes associated with
   indicating the media capabilities for use by the SDP Capability

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   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"
   instead of payload type 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.  Payload
   types are then specified at the media level (see Section 3.3.1).

   A media capability merely indicates possible support for the media
   type and media format(s) in question.  In order to actually use a
   media capability in an offer/answer exchange, it must be referenced
   in a potential configuration.

   Media capabilities can be provided at the session-level and/or the
   media-level.  Media capabilities provided at the session level may be
   referenced in an lcfg attribute at the session level, or by any pcfg
   attribute at the media level (consistent with the media type),
   whereas media capabilities provided at the media level may be
   referenced by a pcfg or lcfg attribute within that media stream only.
   In either case, the scope of the <med-cap-num> is the entire session
   description.  This enables each media capability to be uniquely
   referenced across the entire session description (e.g. in a potential

3.3.1.  The Media Encoding Capability Attribute

   Media subtypes can be expressed as media encoding capabilities by use
   of the "a=mcap" attribute, which is defined as follows:

   a=mcap:<med-cap-num-list> <subtype>[/<encoding-parms>]
          <med-cap-num-list> = <med-cap-num> *[COMMA <med-cap-num>]

   where <med-cap-num>, or media capability number, is an integer
   between 1 and 2^31-1 (both included) used to number a media format
   capability, or a range of such integers and the <subtype> is the
   media subtype e.g.  H263-1998 or PCMU. and <encoding-parms> are the
   media encoding parameters for the <subtype>.  All media format
   capabilities in the list are assigned to the same media type/subtype.
   Each occurrence of the mcap attribute MUST use unique values in its
   <med-cap-num-list>; the media capability numbers must be unique
   across the entire SDP session.  In short, the mcap attribute defines
   media capabilities and associates them with a media capability number
   in the same manner as the rtpmap attribute defines them and
   associates them with a payload type number.

   In ABNF, we have:

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      media-capability-line = "a=mcap:" media-cap-num-list
                              1*WSP media-cap
                              ["/" clock-rate ["/" encoding-parms]]
      media-cap-num-list = media-cap-num *[COMMA media-cap-num]
      media-cap-num      = 1*DIGIT | media-cap-range
      media-cap-range    = 1*DIGIT "-" 1*DIGIT
      media-cap          = token ; Subtype name(PCMU, G729, etc.)
      clock-rate         = 1*DIGIT
      encoding-parms     = token

   The 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 mcap attribute line into one or more rtpmap attribute
   lines, once a payload type number is assigned to a media-cap-num (see
   section 3.3.5).

   The "mcap" attribute can be provided at the session-level and/or the
   media-level.  There can be more than one mcap attribute at the
   session or media level.  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.  When used in a potential, latent or actual
   configuration it is, in effect, a media level attribute regardless if
   it is 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.

   For example:

             a=mcap:1 L16/8000/1
             a=mcap:2 L16/16000/2
             a=mcap:3,4 H263-1998/90000
             m=audio 54320 RTP/AVP 0
             pcfg:1 m=1|2, pt=1:99,2:98
             m=video 66544 RTP/AVP 100
             a=rtpmap:100 H264/90000
             a=pcfg:10 m=3 pt=3:101

3.3.2.  The Media Format Parameter Capability Attribute

   This attribute is used to associate media-specific format parameters
   with one or more media capabilities.  The form of the attribute is:

             a=mfcap:<media-caps> <list of parameters>

   where <media-caps> permits the parameter(s) to be associated with one
   or more media capabilities and the format parameters are specific to

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   the type of codec.  The mfcap lines map to a single traditional SDP
   fmtp attribute line (one per <media-caps>) of the form

             a=fmtp:<fmt> <list of parameters>

   where <fmt> is the media format description defined in RFC 4566
   [RFC4566], 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 mfcap attribute adheres to RFC 4566[RFC4566] attribute production
   rules with

        media-format-capability = "a=mfcap:"<media-caps>

                        1*WSP <fmt-specific-param-list>

        media-caps = "*" ; wildcard: all media caps

                      / <media-cap-num-list> ; defined in 3.3.1

        fmt-specific-param-list = text ; defined in RFC 4566  Media Format Parameter Concatenation Rule

   The appearance of media subtypes with a large number of formatting
   options (e.g., AMR-WB [RFC4867]) 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 <med-cap-num>
   in their <med-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 med-cap-num-list
   are concatenated together via ";", in the order the mfcap lines
   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 med-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 MAY be placed in a single mfcap line to avoid
   possible problems with line rearrangement by a middlebox.

   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

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   concatenated together and separated by "; " for construction of the
   corresponding format attribute (a=fmtp):

        a= fmtp:<fmt> 1*WSP <fmt-specific-param-list> [1*[";" 1*WSP

                <fmt-specfic-param-list>] ;

   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
   insure that legal parameter lists are generated.

   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.

   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:

             a=mcap:1,2 audio G729/8000
             a=mfcap:2 annexb:no

   Media format capability 1 supports G.729 with Annex B, whereas media
   format capability 2 supports G.729 without Annex B.

   Example for H.263 video:

             a=mcap:1 video H263-1998/90000
             a=mcap:2 video H263-2000/90000
             a=mfcap:1 CIF=4;QCIF=2;F=1;K=1
             a=mfcap:2 profile=2;level=2.2

   Finally, for six format combinations of the Adaptive MultiRate codec:

             a=mcap:1-3 AMR/8000/1
             a=mcap:4-6 AMR-WB/16000/1
             a=mfcap:1,2,3,4 mode-change-capability=1
             a=mfcap:5,6 mode-change-capability=2
             a=mfcap:1,2,3,5 max-red=220
             a=mfcap:3,4,5,6 octet-align=1
             a=mfcap:1,3,5 mode-set=0,2,4,7
             a=mfcap:2,4,6 mode-set=0,3,5,6

   So that AMR codec #1, when specified in a pcfg attribute within an
   audio stream block (and assigned payload type number 98) as in

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             a=pcfg:1 m=1 pt=1:98

   is essentially equivalent to the following

             m=audio 49170 RTP/AVP 98
             a=rtpmap:98 AMR/8000/1
             a=fmtp:98 mode-change-capability=1; max-red=220; mode-

   and AMR codec #4 with payload type number 99, is essentially
   equivalent to the following:

             m=audio 49170 RTP/AVP 99
             a=rtpmap:99 AMR-WB/16000/1
             a=fmtp:99 mode-change-capability=1; octet-align=1; mode-

   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

3.3.3.  The Media-Specific Capability Attribute

   Media-specific attributes, beyond the rtpmap and fmtp attributes, may
   be associated with media capability numbers via a new media-specific
   attribute, mscap, as follows:

         media-specific-capability = "a=mscap:"
                                      <media-caps> ; defined in 3.3.2
                                      1*WSP <att-field> ; from [RFC4566]
                                      1*WSP <ms-parameters>

          ms-parameters = byte-string ; as defined per attribute.

   Given an association between a media capability and a payload type
   number as specified by the pt= parameters in an lcfg or pcfg
   attribute line, a mscap line may be translated easily into a
   conventional SDP attribute line of the form

        a=<att-field>":"<fmt> <ms-parameters> ; fmt defined in [RFC4566]

   The resulting attribute SHOULD be a legal SDP attribute, otherwise it
   MUST be ignored by the receiver.

   A single mscap line may refer to multiple media capabilities; this is
   equivalent to multiple mscap lines, each with the same attribute
   values, one line per media capability.

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   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.

   Here is an example with the rtcp-fb attribute, modified from an
   example in [RFC5104] (with the session-level and audio media
   omitted).  If the offer contains a media block like the following,

             m=video 51372 RTP/AVP 98
             a=rtpmap:98 H263-1998/90000
             a=tcap:1 RTP/AVPF
             a=mcap:1 H263-1998/90000
             a=mscap:1 rtcp-fb ccm tstr
             a=mscap:1 rtcp-fb ccm fir
             a=mscap:* rtcp-fb ccm tmmbr smaxpr=120
             a=pcfg:1 t=1 m=1 pt=1:98

   and if the proposed configuration is chosen, then the equivalent
   media block would look like

             m=video 51372 RTP/AVPF 98
             a=rtpmap:98 H263-1998/90000
             a=rtcp-fb:98 ccm tstr
             a=rtcp-fb:98 ccm fir
             a=rtcp-fb:* ccm tmmbr smaxpr=120

3.3.4.  Additional Capability Attributes

   SDP [RFC4566] provides a number of line types which may be useful in
   negotiation of media streams.  In particular, those line types that
   may be used at the media level, specifically media (m=), session
   information (i=), connection (c=), bandwidth (b=), encryption key
   (k=), and attribute (a=) may require a corresponding capability
   negotiation attribute.  The a= attribute line has been treated in
   [SDPCapNeg] to produce the acap attribute.  Consideration of the m=
   media line leads to the pcfg attribute defined in [SDPCapNeg] and the
   lcfg, mcap, mfcap, and mscap attributes defined in this document.  A
   CapNeg attribute for the connection line (c=) has been described in
   [I-D.garcia-mmusic-sdp-cs].  It seems better to define such
   attributes here, rather than have the definitions appear across a
   number of documents not directly related to capabilities negotiation.
   Without spending much effort to assess future utility, this section
   defines a number of capability attributes that may prove useful in
   the future.

   SDP types that can appear only at the session level, and only one

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   time, are not expected to be negotiable.  This applies to the
   protocol version ("v="), origin ("o="), session name ("s="), and URI
   ("u=") types.  Email ("e="), phone ("p="), and time/repeat/timezone
   fields ("t=", "r=", and "z=") may appear multiple times in a session
   description, but it's not clear why one might negotiate different
   email addresses or phone numbers for a particular session.  It might
   be useful to negotiate different times for a session (a broadcast of
   the CEOs speech, for example) depending on the video encoding to be
   used (e.g., H.263 or H.264), but this could just as well be
   accomplished by announcing two sessions, one for each encoding; in
   practice, a single session will likely offer multiple encodings at
   the same time.  For the present, we adopt the position that the
   session-level-only types need not be negotiable via CapNeg.  The
   following capability attributes are always to be interpreted as
   media-level fields when invoked by a potential (pcfg) or latent
   (lcfg) configuration.  Note that the SDP encryption key type (k=) is
   NOT RECOMMENDED in RFC 4566; use of the kcap attribute should be
   limited to cases in which no convenient alternative exists.  The
   encryption key field should be included only when/if the key is
   transported over a secure, trusted channel.

   The general form of the capability attributes is the same for the
   above SDP types.  Given the SDP line (from [RFC4566])


   we define the corresponding capability attribute as

             a=<type-cap-attr> ":" <type-attr-num> 1*WSP <value>


             <type-cap-attr> = <type> "cap"

             <type> = %x69 / ; "i"
                      %x63 / ; "c"
                      %x62 / ; "b"
                      %x6b / ; "k"
                      %x61 / ; "a"

             <type-attr-num> = 1*DIGIT ; integer, 1 to 2^31, inclusive

   where <value> is as defined specifically for each <type> in [RFC4566]
   and, possibly, extended in other RFCs.  The definitions above
   correspond to the acap definition in [SDPCapNeg] and the ccap
   definition proposed in [I-D.garcia-mmusic-sdp-cs].

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   As an example, consider the negotiation of bandwidth.  In some cases
   it is desirable to specify different bandwidth limits for different
   media configurations.  This may be done by use of the "a=bcap"
   attribute, which, from the above and [RFC4566], is defined as

             a=bcap:<bw-cap-num> <bwtype>:<bandwidth>

   where <bw-cap-num> is an integer between 1 and 2^31-1 (both included)
   used to identify the bandwidth capability, <bwtype> is the bandwidth
   type, and <bandwidth> is the bandwidth value, as defined for the b=
   line in RFC4566[RFC4566]

   The "bcap" attribute can appear at the session level, where it can be
   referenced by lcfg or pcfg attributes, or at the media level, where
   it can be referenced by pcfg attributes.  When invoked by a pcfg or
   lcfg attribute, the resulting bandwidth line (b=) is to be
   interpreted at the media-level for that configuration.  There can be
   more than one bcap attribute.  The unique bw-cap-num is used to
   identify it in potential configurations.  No provision has been made
   for negotiation of total session bandwidth capabilities.

   Bandwidth capabilities may be included in a potential configuration
   via the "b=" parameter (see below).  Any bandwidth capability
   included replaces any media-level bandwidth of the same type declared
   in a "b=" SDP line.

   The following example offers a preferred potential configuration for
   H.263 QCIF at 360 Kbit/sec and a second potential configuration for
   H.263 CIF at the offered 500 Kbit/sec

             m=video 49170 RTP/AVP 99
             a=rtpmap:99 H263-1998/90000
             a=fmtp:99 CIF=4; QCIF=2
             a=mcap:1,2 video H263-1998/90000
             a=mfcap:1 QCIF=2
             a=mfcap:2 CIF=4; QCIF=2;F=1;K=1
             a=bcap:1 TIAS:360000
             a=pcfg:1 m=1 b=1 pt=1:100
             a=pcfg:2 m=2 pt=2:101

3.3.5.  The Latent Configuration Attribute

   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

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   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 a latent video
   configuration 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.

   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.  [Editor's note: this
   choice has been made in order to keep the size of the answer more
   compact by not requiring acap, mcap, tcap, etc. lines in the answer.
   However, this restriction can be changed if it seems desirable.]

   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 media type (mt=) and the
   transport protocol(s) (t=) MUST be specified since there is no
   corresponding m-line for defaults.  In most cases, the media
   configuration (m=) parameter must be present as well (see section
   3.3.8 for examples).  The lcfg attribute is a session level
   attribute, and all capability attributes referenced by lcfg attribute
   parameters must appear at the session level in the SDP record.

             latent-media-config = "a=lcfg:" <config-number>
                                1*WSP "mt=" <media>   ;[RFC4566]
                                1*WSP transport-protocol-config-list
                                        ; defined in [SDPCapNeg]
                                [1*WSP media-config-list]
                                [1*WSP attribute-config-list]
                                           ; defined in [SDPCapNeg]
                                [1*WSP payload-number-config-list]
                                [1*WSP info-cap]
                                [1*WSP connection-config-list]
                                [1*WSP bandwidth-config-list]
                                [1*WSP key-config-list]
                                [1*(1*WSP extension-config-list)]
                                        ; defined in [SDPCapNeg]

   The mt= parameter identifies the media type (audio, video, etc.) to
   be associated with the latent media stream; the transport (t=) and
   attribute (a=) parameters are identical in format and meaning to
   those defined for the pcfg: attribute in [SDPCapNeg].  The media
   subtype(s) and format parameters to be associated with the stream are
   specified by the m= parameter.

   The transport parameter (t=) is identical in format and meaning as

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   defined for the pcfg attribute in [SDPCapNeg]

   We define the media configuration parameter, media-config-list, in
   accordance with the following format:

             m=<med-cap-list> *["|"<med-cap-list>]

   where <med-cap-list> is a comma-separated list of media capability
   numbers (media-cap-num) as defined by a=mcap: lines (section 3.3.1).
   In ABNF form (adhering to the ABNF for extension-config-list in

             media-config-list = "m=" med-cap-list *(BAR med-cap-list)
             ; BAR is defined in [SDPCapNeg]
             med-cap-list = med-cap-num *("," med-cap-num)
             med-cap-num = 1*DIGIT ; defined in SDP

   Each potential media configuration is a comma-separated list of media
   capability numbers where med-cap-num refers to media capability
   numbers defined explicitly by a=mcap attributes and hence MUST be
   between 1 and 2^31-1 (both included).  Alternative potential media
   configurations are separated by a vertical bar ("|").  The
   alternatives are ordered by preference.

   The attribute parameter (a=) is identical in format and meaning as
   defined for the pcfg attribute in [SDPCapNeg].

   We define the payload type number mapping parameter, pt-media-map, in
   accordance with the following format:

        payload-number-config-list = "pt=" med-map-list
        med-map-list = med-map *["," med-map]
        med-map = med-cap-num ":" payload-type-number

                 ; med-cap-num is defined in section 3.4.1

        payload-type-number = 1*DIGIT | "*" ; RTP payload type number

   The example in the section 3.3.7 shows how the parameters from the
   mcap line are mapped to payload type numbers from the pcfg "pt"
   parameter.  The "*" value is used in cases such as BFCP [RFC4583] in
   which the fmt list in the m-line is ignored.

   The session information parameter, info-cap, provides the capability
   number of a session information (icap) capability attribute.  In
   accordance with RFC 4566, which permits no more than one i-line per
   media stream, only one icap attribute may be specified.  The format
   of the parameter is defined as

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        info-cap = "i=" [delete] icap-num
        delete = "-:" ; delete media-level i= line, if present
        icap-num = 1*DIGIT ; cap-num from icap attribute

   The connection data parameter, connection-config-list, may be used to
   provide stream-specific connection data by reference to one or more
   ccap attributes in the SDP record.

        connection-config-list = "c=" [delete] full-ccap-list
        full-ccap-list = mandatory-ccap-list ["," optional-ccap-list]
        mandatory-ccap-list = ccap-list
        optional-ccap-list = "[" ccap-list "]"
        ccap-list = ccap-num [1*("," ccap-num)]
        ccap-num = 1*DIGIT ; cap-num from ccap attribute

   The bandwidth configuration parameter, bandwidth-config-list, may be
   used to provide bandwidth specifications by reference to bcap
   attributes described in section 3.3.4.

        bandwidth-config-list = "b=" [delete] full-bcap-list
        full-bcap-list = mandatory-bcap-list ["," optional-bcap-list]
        mandatory-bcap-list = bcap-list
        optional-bcap-list = "[" bcap-list "]"
        bcap-list = bcap-num *("," bcap-num)
        bcap-num = 1*DIGIT ; cap-num from bcap attribute

   Although use of the encryption key line is NOT RECOMMENDED by RFC
   4566, kcap attributes, defined in section 3.3.4, may be referenced
   through the k= parameter.

        key-config-list = "k=" [delete] full-kcap-list
        full-kcap-list = mandatory-kcap-list ["," optional-kcap-list]
        mandatory-kcap-list = kcap-list
        optional-kcap-list = "[" kcap-list "]"
        kcap-list = kcap-num *("," kcap-num)
        kcap-num = 1*DIGIT ; cap-num from kcap attribute

   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 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 offeror
   and/or answerer may use any payload type numbers each deems
   appropriate.  The use of explicit payload type numbers for latent

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   configurations can be avoided by use of the parameter substitution
   rule of section 3.3.7.  Future extensions are also permitted.

   Each latent configuration MUST be specified at the session level; it
   represents an additional media stream to those in the media block(s}
   of the offer or answer.  If an acap: attribute is declared at the
   session level for use in an lcfg line, it SHOULD NOT be used in a
   pcfg line at the media level unless it is to become a session-level
   attribute in the answer.

   The configuration numbers for latent configurations share the same
   preference rule as potential configurations: a lower-numbered
   configuration is preferred over a higher-numbered configuration.  If
   latent configurations are used in session capability (sescap=)
   attributes (section 3.3.8), the configuration numbers MUST be
   different from those used for pcfg attributes.

   If a cryptographic attribute, such as the SDES "a=crypto:" attribute
   [RFC4568], is referenced by a latent configuration through an acap
   attribute, any key 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 key material SHOULD be meaningless, and the receiver of the lcfg:
   attribute MUST ignore the values.

3.3.6.  Enhanced Potential Configuration Attribute

   The present work requires new extensions (parameters) for the pcfg:
   attribute defined in the base protocol [SDPCapNeg] The parameters and
   their definitions are "borrowed" from the definitions provided for
   the latent configuration attribute in section 3.3.5.  The expanded
   definition of the pcfg attribute is

        a=pcfg: <config-number> [<pot-cfg-list>]


        config-number = 1*DIGIT ;defined in [RFC5234]
        pot-cfg-list = pot-config *(1*WSP pot-config)
        pot-config = attribute-config-list /

                    ;defined in [SDPCapNeg]

        transport-protocol-config-list / ;defined in [SDPCapNeg]
        extension-config-list / ;[SDPCapNeg]
        media-config-list / ;sect. 3.3.5
        payload-number-config-list / ; 3.3.5
        info-cap / ;defined in section 3.3.5

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        connection-config-list / ; 3.3.5
        bandwidth-config-list / ; 3.3.5
        key-config-list ;section 3.3.5

   The pot-cfg-list MUST NOT contain more than one instance of each
   parameter list other than the extension-config-list.  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.  Returning Capabilities in the Answer

   Potential configuration attributes may be returned within the media
   block(s) of an answer SDP to indicate the ability of the answerer to
   to support alternative configurations of the corresponding stream(s).
   For example, an offer may include multiple potential configurations
   for a media stream; the corresponding answer will indicate (via an
   acfg attribute) which configuration is accepted, but it MAY also
   contain potential 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.

   When potential configurations are returned in an answer, all
   numbering MUST refer to the configuration and capability attribute
   numbering of the offer.  The referenced capability attributes MUST
   NOT be returned in the answer.  The parameter values of any returned
   pcfg attributes MUST be a subset of those included in the offered
   configurations; 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

   Note that the answerer is unable to return additional capabilities
   within an offered and answered media stream.  For this reason, it
   seems advisable for the offerer to include most, if not all,
   potential capabilities in the initial offer.  Additional capabilities
   MAY be announced via one or more latent configurations (representing
   a new media stream), or when renegotiating the session in a second
   offer/answer exchange.  Payload Type Number Mapping

   When media capabilities defined in mcap attributes are used in
   potential configuration lines, and the transport protocol uses RTP,

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   it is necessary to assign payload type numbers to them.  In some
   cases, it is desirable to assign different payload type numbers to
   the same media 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 that it
   (the offerer) can decide whether or not to render early media which
   arrives before the answer is received.  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.  Processing of Media-Format-Related Conventional Attributes for
          Potential Configurations

   In cases in which 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 mcap, mfcap, and mscap which map into those
   conventional attributes.

   When one or more media capabilities (a=mcap) are invoked in a
   potential configuration via m= arguments, each capability is
   associated with a payload type number by default or by a payload type
   number argument (pt=).  Special processing MUST be invoked on
   conventional attributes associated with that payload type number.  If
   the media capability is associated with one or more mfcap attributes,
   then any corresponding conventional fmtp attribute in the media block
   MUST be ignored for that configuration.  If no mfcap attributes are
   specified, then the fmtp attribute line within the media block with
   the matching payload type number, if any, will apply.  Conventional
   fmtp attributes with payload type numbers not referenced in the
   configuration MUST also be ignored.  Similarly, any other
   conventional media-specific attributes (e.g., rtcp-fb) in the media
   block with payload type number matching a mscap attribute will apply
   unless there is an applicable mscap attribute for the same attribute
   type (e.g., rtcp-fb), in which case all base level attributes of the
   same type and payload type number will be ignored.  Any media-
   specific attributes in the media block which refer to payload type
   numbers not used by the potential configuration will be ignored.
   These rules are intended to avoid the need to duplicate attributes
   and use the a=-m: form of invoking attributes in a potential
   configuration just to replace an rtpmap or fmtp attribute.

   For example:

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             o=- 25678 753849 IN IP4
             c=IN IP4
             t=0 0
             m=audio 3456 RTP/AVP 0 18 100
             a=rtpmap:100 telephone-events
             a=fmtp:100 0-11
             a=mcap:1 PCMU/8000
             a=mcap:2 g729/8000
             a=mcap:3 telephone-event/8000
             a=mfcap:3 0-15
             a=pcfg:1 m=2,3|1,3 pt=1:0,2:18,3:100

   In this example, PCMU is media capability 1, G729 is media capability
   2, and telephone-event is media capability 3.  The a=pcfg: line
   specifies that the preferred configuration is G.729 with extended
   dtmf events, second is G.711 mu-law with extended dtmf events.
   Intermixing of G.729, G.711, and "commercial" dtmf events is least
   preferred (the base configuration provided by the "m=" line, which is
   always the least preferred configuration).  The rtpmap and fmtp
   attributes of the base configuration are replaced by the mcap and
   mfcap attributes when invoked by the proposed configuration.

3.3.7.  Substitution of Media Payload Type Numbers in Capability
        Attribute Parameters

   In some cases, for example, when an RFC 2198 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:

             m=audio 45678 RTP/AVP 0
             a=rtpmap:0 PCMU/8000
             a=mcap:1 PCMU/8000
             a=mcap:2 RED/8000
             a=mfcap:2 0/0
             a=pcfg:1 m=2,1 pt=2:98,1:0

   The potential configuration is then equivalent to

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             m=audio 45678 RTP/AVP 98 0
             a=rtpmap:0 PCMU/8000
             a=rtpmap:98 RED/8000
             a=fmtp:98 0/0

   A more general mechanism is provided via the parameter substitution

   When an mfcap, mscap, or acap attribute is processed, its arguments
   will be scanned for sequences of the following form: "%" *DIGIT "%"
   If found, the digit string is interpreted as a media capability
   number and the sequence is replaced by the payload type number
   assigned to the media capability as specified by the pt= parameter in
   the selected potential configuration.  The sequence "%%" (null digit
   string) is replaced by a single percent sign and processing continues
   with the next character, if any.

   For example, the above sequence could have been written as

             m=audio 45678 RTP/AVP 0
             a=rtp map:0 PCMU/8000
             a=mcap:1 PCMU/8000
             a=mcap:2 RED/8000
             a=mfcap:2 %1%/%1%
             a=pcfg:1 m=2,1 pt=2:98,1:0

   and the equivalent SDP is the same as above.  This technique is
   especially useful for latent configurations, for which it may not
   otherwise be particularly useful to specify payload type numbers.

3.3.8.  The Session Capability Attribute

   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 is expressed as a list of potential and/or
   latent configurations; in an answer, the session capabilities refer
   to actual and/or latent media configurations.  The session capability
   attribute is described by:

           session-capability = "a=sescap:" <session-num>
                              1*WSP <list-of-configs>

           session-num     = 1*DIGIT  ; defined in [RFC5234]
           list-of-configs = <alt-config> *["," <alt-config>]
           alt-config = config-number *["|" config-number]
                         ; config-number defined in [SDPCapNeg]

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   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 [SDPCapNeg].  Note
   that the session preference order, when present, takes precedence
   over the individual media stream configuration preference order.

   Use of session capability attributes requires that configuration
   numbers assigned to potential and latent configurations be unique
   across the entire session; [SDPCapNeg] requires only that pcfg
   configuration numbers be unique within a media description.

   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.  The SDP offer might look like
   the following:

             o=- 25678 753849 IN IP4
             c=IN IP4
             t=0 0
             a=sescap:2 1,2,3,5
             a=sescap:1 1,4
             m=audio 54322 RTP/AVP 0
             a=rtpmap:0 PCMU/8000
             m=video 22344 RTP/AVP 102
             a=rtpmap:102 H263-1998/90000
             a=fmtp:102 CIF=4;QCIF=2;F=1;K=1
             i= main video stream
             a=mcap:1 H264/90000
             a=mfcap:1 profile-level-id=42A01E; packetization-mode=2
             a=acap:1 label:13
             a=pcfg:4 m=1 a=1 pt=1:104
             m=video 33444 RTP/AVP 103
             a=rtpmap:103 H263-1998/90000
             a=fmtp:103 CIF=4;QCIF=2;F=1;K=1
             i= secondary video (slides)
             m=application 33002 TCP/BFCP *
             a=floorid:1 m-stream:11 12

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   If the answerer understands MediaCapNeg, but cannot support the
   Binary Floor Control Protocol, then it would respond with:

             o=- 25678 753849 IN IP4
             c=IN IP4
             t=0 0
             a=sescap:1 1,4
             m=audio 23456 RTP/AVP 0
             a=rtpmap:0 PCMU/8000
             m=video 41234 RTP/AVP 104
             a=rtpmap:100 H264/90000
             a=fmtp:104 profile-level-id=42A01E; packetization-mode=2
             m=video 0 RTP/AVP 103
             m=application 0 TCP/BFCP *

   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.

   Session capabilities MAY 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.

             o=- 25678 753849 IN IP4
             c=IN IP4
             t=0 0

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             a=sescap:1 1,3,4,5
             a=sescap:2 1,2
             a=sescap:3 1
             a=mcap:1 H263-1998/90000
             a=mfcap:1 CIF=4;QCIF=2;F=1;K=1
             a=tcap:1 RTP/AVP TCP/BFCP
             a=acap:31 label:12
             a=acap:32 content:main
             a=icap:3 primary video channel
             a=lcfg:3 mt=video t=1 m=1 a=31,32 i=3
             a=acap:41 label:13
             a=acap:42 content:slides
             a=icap:4 secondary video channel (slides)
             a=lcfg:4 mt=video t=1 m=1 a=41,42 i=4
             a=tcap:5 TCP/BFCP
             a=acap:51 setup:passive
             a=acap:52 connection:new
             a=acap:53 floorid:1 m-stream:12 13
             a=acap:54 floor-control:s-only
             a=acap:55 confid:4321
             a=acap:56 userid:1234
             a=lcfg:5 mt=application t=2
             m=audio 54322 RTP/AVP 0
             i= voice
             a=rtpmap:0 PCMU/8000
             m=video 22344 RTP/AVP 102
             i= default video
             a=rtpmap:102 H264/90000
             a=fmtp:102 profile-level-id=42A01E; packetization-mode=2

   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.  The lcfg line for the BFCP connection
   doesn't even use a media-configuration (m=) parameter because the
   protocol is specified in the transport parameter.  If the answerer
   supports Media CapNeg, and supports the most desired configuration,
   it would return the following SDP:

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             o=- 25678 753849 IN IP4
             c=IN IP4
             t=0 0
             a=sescap:1 1,3,4,5
             a=sescap:2 1,2
             a=sescap:3 1
             a=lcfg:3 mt=video t=1 m=1 a=31,32 i=3
             a=lcfg:4 mt=video t=1 m=1 a=41,42 i=4
             a=lcfg:5 mt=application t=2
             m=audio 23456 RTP/AVP 0
             a=rtpmap:0 PCMU/8000
             m=video 0 RTP/AVP 102

   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.

   [Editors' note: We have considered a form of the lcfg: attribute that
   would permit an offerer to indicate that the configuration is
   available for immediate acceptance via an answer with a corresponding
   (new) m-line in the answer.  This would permit the establishment of
   the media streams to take place in one SDP exchange; at the same
   time, non-negotiating endpoints could be offered a simple
   configuration as in the above example.  Ultimately, when all
   endpoints understand the current specification, this would permit an
   offer with latent configurations only, and an answer with the m-lines
   for the accepted media streams.]

   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

3.4.  Offer/Answer Model Extensions

   In this section, we define extensions to the offer/answer model
   defined in RFC3264 [RFC3264] and [SDPCapNeg] to allow for media
   capabilities, bandwidth capabilities, and latent configurations to be

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   used with the SDP Capability Negotiation framework.

   The [SDPCapNeg] provides a relatively compact means to offer the
   equivalent of an ordered list of alternative media stream
   configurations (as would be described by separate m= lines and
   associated attributes).  The attributes acap, bcap, mscap, mfcap and
   mcap 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.

3.4.1.  Generating the Initial Offer

   When an endpoint generates an initial offer and wants to use the
   functionality described in the current document, it should identify
   and define the codecs it can support via mcap, mfcap and mscap
   attributes.  The SDP media line(s) should be made up with the
   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 capabilities.  The offer MUST include the
   level of capability negotiation extensions needed to support this
   functionality in a "creq" attribute.

   Preferred configurations for each media stream are identified
   following the media line.  The present offer may also include latent
   configuration (lcfg) attributes, at the session level, describing
   media streams and/or configurations the offeror is not now offering,
   but which it is willing to support in a future offer/answer exchange.
   A simple example might be the inclusion of a latent video
   configuration in an offer for an audio stream.

3.4.2.  Generating the Answer

   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, the answer must include the supported
   extension attribute and each media stream for which a proposed
   configuration was chosen must contain an actual configuration (acfg)
   attribute to indicate just which pcfg attribute was used to build the
   answer.  The answer should also include any latent configurations the
   answerer can support, especially any configurations compatible with

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   other proposed or latent configurations received in the offer.  The
   answerer should make note of those configurations it might wish to
   offer in the future.

3.4.3.  Offerer Processing of the Answer

   When the offeror receives the answer, it should make note of any
   capabilities and/or latent configurations for future use.  The media
   line(s) must be processed in the normal way to identify the media
   stream(s) accepted by the answer, if any.  The acfg attribute, if
   present, may be used to verify the proposed configuration used to
   form the answer, and to infer the lack of acceptability of higher-
   preference configurations that were not chosen.  Note that the base
   specification [SDPCapNeg] requires the answerer to choose the highest
   preference configuration it can support.

3.4.4.  Modifying the Session

   If, at a later time, one of the parties wishes to modify the
   operating parameters of a session, e.g., by adding a new media
   stream, or by changing the properties used on an existing stream, it
   may do so via the mechanisms defined for offer/answer[RFC3264].  If
   the initiating party has remembered the codecs, potential
   configurations, and latent configurations announced by the other
   party in the earlier negotiation, it may use this knowledge to
   maximize the likelihood of a successful modification of the session.
   Alternatively, it may perform a new capabilities exchange as part of
   the reconfiguration.

   "[Editors note: should a new offer/answer exchange replace all caps
   from previous exchange(s)?  This would permit an endpoint to remove
   caps it can no longer support.]

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4.  Examples

   In this section, we provide examples showing how to use the Media
   Capabilities with the SDP Capability Negotiation.

4.1.  Alternative Codecs

   This example provide 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 offeror can interpret early media.

             1. v=0
             2. o=- 25678 753849 IN IP4
             3. s=
             4. c=IN IP4
             5. t=0 0
             6. a=creq:med-v0
             7. m=audio 54322 RTP/AVP 96
             8. rtpmap:96 AMR-WB/16000/1
             9. a=fmtp:96 mode-change-capability=1; max-red=220; mode-
             10. a=macp:1,3,5 audio AMR-WB/16000/1
             11. a=mcap:2,4,6 audio AMR/8000/1
             12. a=mfcap:1,2,3,4 mode-change-capability=1
             13. a=mfcap:5,6 mode-change-capability=2
             14. a=mfcap:1,2,3,5 max-red=220
             15. a=mfcap:3,4,5,6 octet-align=1
             16. a=mfcap:1,3,5 mode-set=0,2,4,7
             17. a=mfcap:2,4,6 mode-set=0,3,5,6
             18. pcfg:1 m=1 pt=1:96
             19. pcfg:2 m=2 pt=2:97
             20. pcfg:3 m=3 pt=3:98
             21. pcfg:4 m=4 pt=4:99
             22. pcfg:5 m=5 pt=5:100
             23. pcfg:6 m=6 pt=6:101

   In the above example, media capability 1 could have been excluded
   from the mcap declaration in line 10 and from the mfcap attributes in
   lines 12, 14, and 16.  The pcfg line 18 could then have been simply

   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:

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             o=- 25678 753849 IN IP4
             s=An SDP Media NEG example
             c=IN IP4
             t=0 0
             m=video 49170 RTP/AVP 100
             c=IN IP4
             a=candidate 12345 1 UDP 9 49170 host
             a=candidate 23456 2 UDP 9 51540 host
             a=candidate 34567 1 UDP 7 41345 srflx raddr
    rport 49170
             a=candidate 45678 2 UDP 7 52567 srflx raddr
    rport 51540
             a=candidate 56789 1 UDP 3 49000 relay raddr
    rport 49170
             a=candidate 67890 2 UDP 3 49001 relay raddr
    rport 51540
             a=rtpmap:100 H264/90000
             a=fmtp:100 profile-level-id=42A01E; packetization-mode=2;
             sprop-interleaving-depth=45; sprop-deint-buf-req=64000;
             sprop-init-buf-time=102478; deint-buf-cap=128000
             a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF
             a=mcap:1-3,7-9 H264/90000
             a=mcap:4-6 rtx/90000
             a=mfcap:1-9 profile-level-id=42A01E
             a=mfcap:1-9 aMljiA==
             a=mfcap:1,4,7 packetization-mode=0
             a=mfcap:2,5,8 packetization-mode=1
             a=mfcap:3,6,9 packetization-mode=2
             a=mfcap:1-9 sprop-parameter-sets=Z0IACpZTBYmI
             a=mfcap:1,7 sprop-interleaving-depth=45; sprop-deint-buf-
             sprop-init-buf-time=102478; deint-buf-cap=128000
             a=mfcap:4 apt=100
             a=mfcap:5 apt=99
             a=mfcap:6 apt=98
             a=mfcap:4-6 rtx-time=3000
             a=mscap:1-6 rtcp-fb nack

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             a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_80
             a=pcfg:1 t=1 m=1,4 a=1 pt=1:100,4:97
             a=pcfg:2 t=1 m=2,5 a=1 pt=2:99,4:96
             a=pcfg:3 t=1 m=3,6 a=1 pt=3:98,6:95
             a=pcfg:4 t=2 m=7 a=1 pt=7:100
             a=pcfg:5 t=2 m=8 a=1 pt=8:99
             a=pcfg:6 t=2 m=9 a=1 pt=9:98
             a=pcfg:7 t=3 m=1,3 pt=1:100,4:97
             a=pcfg:8 t=3 m=2,4 pt=2:99,4:96
             a=pcfg:9 t=3 m=3,6 pt=3:98,6:95
             m=audio 49176 RTP/AVP 101 100 99 98
             c=IN IP4
             a=candidate 12345 1 UDP 9 49176 host
             a=candidate 23456 2 UDP 9 51534 host
             a=candidate 34567 1 UDP 7 41348 srflx raddr
    rport 49176
             a=candidate 45678 2 UDP 7 52569 srflx raddr
    rport 51534
             a=candidate 56789 1 UDP 3 49002 relay raddr
    rport 49176
             a=candidate 67890 2 UDP 3 49003 relay raddr
    rport 51534
             a=rtpmap:98 AMR-WB/16000
             a=fmtp:98 octet-align=1; mode-change-capability=2
             a=rtpmap:99 AMR-WB/16000
             a=fmtp:99 octet-align=1; crc=1; mode-change-capability=2
             a=rtpmap:100 AMR-WB/16000/2
             a=fmtp:100 octet-align=1; interleaving=30
             a=rtpmap:101 AMR-WB+/72000/2
             a=fmtp:101 interleaving=50; int-delay=160000;
             a=mcap:14 ac3/48000/6
             a=acap:23 crypto:1 AES_CM_128_HMAC_SHA1_80
             a=tcap:4 RTP/SAVP
             a=pcfg:10 t=4 a=23
             a=pcfg:11 t=4 m=14 a=23 pt=14:102

   This offer illustrates the advantage in compactness that arises if

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   one can avoid deleting the base configuration attributes and
   recreating them in acap attributes for the potential configurations.

4.2.  Alternative Combinations of Codecs (Session Configurations)

   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:

             o=- 25678 753849 IN IP4
             c=IN IP4
             t=0 0
             a=sescap:1 2,4
             a=sescap:2 1,3
             m=audio 54322 RTP/AVP 18
             a=rtpmap:18 G729/8000
             a=fmtp:18 annexb=yes
             a=mcap:1 PCMU/8000
             pcfg:1 m=1 pt=1:0
             m=video 54344 RTP/AVP 100
             a=rtpmap:100 H263-1998/90000
             a=mcap:2 H264/90000
             a=mfcap:2 profile-level-id=42A01E; packetization-mode=2
             a=pcfg:3 m=2 pt=2:101

   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.

4.3.  Latent Media Streams

   Consider a case in which the offeror 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 offeror wishes to announce
   its ability to support video in the future, but does not wish to
   offer a video stream at present.  The offer might look like the

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             1. v=0
             2. o=- 25678 753849 IN IP4
             3. s=
             4. c=IN IP4
             5. t=0 0
             6. a=creq:med-v0
             7. a=mcap:10 H263-1998/90000
             8. a=mcap:11 H264/90000
             9. a=lcfg:10 mt=video m=10|11
             10. m=audio 23456 RTP/AVP 0
             11. a=rtpmap:0 PCMU/8000
             12. a=mcap:1 PCMU/8000
             13. a=mcap:2 g729/8000
             14. a=mcap:3 telephone-event/8000
             15. a=mfcap:3 0-11
             16. a=pcfg:1 m=1,3|2,3 pt=1:0,2:18,3:100

   Lines 7-9 announce support for H.263 and H.264 video (H.263
   preferred) for future reference.  Lines 10 and 11 offer an audio
   stream and provide the lowest precedence configuration (PCMU without
   any DTMF encoding).  Lines 12-14 define the media capabilities to be
   offered: PCMU, G729, and telephone-event.  Line 15 provides the
   format parameters for telephone-events, specifying the 12 commercial
   DTMF 'digits'.  Line 16 defines the most-preferred media
   configuration as PCMU plus DTMF events and the next-most-preferred
   configuration as G.729B plus DTMF events.

   If the answerer is able to support all the potential configurations,
   and also support H.263 video (but not H.264), it would reply with an
   answer like:

             1. v=0
             2. o=- 24351 621814 IN IP4
             3. s=
             4. c=IN IP4
             5. t=0 0
             6. a=csup:med-v0
             7. a=mcap:10 H263-1998/90000
             8. a=lcfg:1 mt=video m=10
             9. m=audio 54322 RTP/AVP 0 100
             10. a=rtpmap:0 PCMU/8000
             11. a=rtpmap:100 telephone-event/8000
             12. a=fmtp:100 0-11
             13. a=acfg:1 m=1,3 pt=1:0,3:100
             14. a=mcap:1 G729/8000
             15. a=mcap:2 telephone-event/8000
             16. a=mfcap:2 0-11
             17. a=pcfg:2 m=1,2 pt=1:18,2:100

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   Lines 7 and 8 announce the capability to support H.263 video at a
   later time.  Lines 9-12 of the answer present the selected
   configuration for the media stream.  Line 13 identifies the potential
   configuration from which it was taken, and lines 14-17 announce 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 offer a reconfiguration of the media stream to use
   G.729 in order to reduce packet sizes.  Note that line 13 uses media
   capability numbering as provided in the original offer, whereas lines
   14-17 must use their own numbering.

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5.  IANA Considerations

   The IANA is hereby requested to register the following new SDP

             Attribute name: mcap
             Long form name: media capability
             Type of attribute: session-level and media-level
             Subject to charset: no
             Purpose: associate media capability number(s) with
             media subtype and encoding parameters
             Appropriate Values: see Section 3.3.1

             Attribute name: mfcap
             Long form name: media format capability
             Type of attribute: session-level and media-level
             Subject to charset: no
             Purpose: associate media format attributes and
             parameters with media format capabilities
             Appropriate Values: see Section 3.3.2

             Attribute name: mscap
             Long form name: media-specific capability
             Type of attribute: session-level and media-level
             Subject to charset: no
             Purpose: associate media-specific attributes and
             parameters with media capabilities
             Appropriate Values: see Section 3.3.3

             Attribute name: bcap
             Long form name: bandwidth capability
             Type of attribute: session-level or media-level
             Subject to charset: no
             Purpose: associate bandwidth limitations with potential
             or latent configurations.
             Appropriate Values: see Section 3.3.4

             Attribute name: icap
             Long form name: session information capability
             Type of attribute: media-level
             Subject to charset: no
             Purpose: associate session information with potential
             or latent configurations.
             Appropriate Values: see Section 3.3.4

             Attribute name: ccap
             Long form name: connection information capability
             Type of attribute: media-level

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             Subject to charset: no
             Purpose: associate connection information with potential
             or latent configurations.
             Appropriate Values: see Section 3.3.4

             Attribute name: kcap
             Long form name: encryption key capability
             Type of attribute: media-level
             Subject to charset: no
             Purpose: associate encryption key with potential
             or latent configurations.
             Appropriate Values: see Section 3.3.4

             Attribute name: lcfg
             Long form name: latent configuration
             Type of attribute: session-level
             Subject to charset: no
             Purpose: to announce supportable media configurations
             without offering them for immediate use.
             Appropriate Values: see Section 3.3.5

             Attribute name: sescap
             Long form name: session capability
             Type of attribute: session-level
             Subject to charset: no
             Purpose: to specify and prioritize acceptable
             combinations of media stream configurations.
             Appropriate Values: see Section 3.3.8

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6.  Security Considerations

   The security considertions of [SDPCapNeg] apply for this document.

   The addition of negotiable media encoding, bandwidth attributes, and
   connection data 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, the SDP answer is
   formulated in such a way that it always carries the selected media
   encoding and bandwidth parameters 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.

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7.  Changes from previous versions

7.1.  Changes from version 03

   o  A new session capability attribute (sescap) has been added to
      permit specification of acceptable media stream combinations.

   o  Capability attribute definitions corresponding to the i, c, b, and
      k SDP line types have been added for completeness.

   o  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.

   o  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.

   o  A suggestion for "naive" middleboxes has been added to the
      Security Considerations.

   o  Various editorial changes have been made.

   o  Several errors/omissions have been corrected.

   o  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.

   o  <ms-parameters> are made optional in the mscap attribute

   o  "AMR" changed to "AMR-WB" in cases in which the sample rate is

7.2.  Changes from version 02

   This version contains several detail changes intended to simplify
   capability processing and mapping into conventional SDP media blocks.

   o  The "mcap" attribute is enhanced to include the role of the "ecap"
      attribute; the latter is eliminated.

   o  The "fcap" attribute has been renamed "mfcap".  New replacement
      rules vis-a-vis fmtp attributes in the base media specification
      have been added.

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   o  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.

   o  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)

   o  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.

   o  The examples are expanded.

   o  Numerous typos and misspellings have been corrected.

7.3.  Changes from version 01

   The documents adds a new attribute for specifying bandwidth
   capability and a parametr to list in the potential configuration.
   Other changes are to align the document with the terminolgy and
   attribute names from draft-ietf-mmusic-sdp-capability-negotiation-07.
   The document also clarifies some previous open issues.

7.4.  Changes from version 00

   The major changes include taking out the "mcap" and "cptmap"
   parameter.  The mapping of payload type is now in the "pt" parameter
   of "pcfg".  Media subtype need to explictly definesd in the "cmed"
   attribute if referenced in the "pcfg"

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8.  Acknowledgements

   This document is heavily influenced by the discussions and work done
   by the SDP Capability Negotiation Design team.  The following people
   in particular provided useful comments and suggestions to either the
   document itself or the overall direction of the solution defined
   herein: Cullen Jennings, Matt Lepinski, Joerg Ott, Colin Perkins, and
   Thomas Stach.

   We thank Ingemar Johansson and Magnus Westerlund for examples that
   stimulated this work.

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9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              June 2002.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

              Andreasen, F., "SDP Capability Negotiation",
              draft-ietf-mmusic-sdp-capability-negotiation-08 (work in
              progress), December 2007.

9.2.  Informative References

              Garcia-Martin, M. and S. Veikkolainen, "Conventions for
              the Use of the Session Description Protocol (SDP) for
              Circuit-Switched Bearer Connections in the Public Switched
              Telephone Network (PSTN)", draft-garcia-mmusic-sdp-cs-01
              (work in progress), April 2008.

   [RFC3407]  Andreasen, F., "Session Description Protocol (SDP) Simple
              Capability Declaration", RFC 3407, October 2002.

   [RFC4568]  Andreasen, F., Baugher, M., and D. Wing, "Session
              Description Protocol (SDP) Security Descriptions for Media
              Streams", RFC 4568, July 2006.

   [RFC4583]  Camarillo, G., "Session Description Protocol (SDP) Format
              for Binary Floor Control Protocol (BFCP) Streams",
              RFC 4583, November 2006.

   [RFC4867]  Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie,
              "RTP Payload Format and File Storage Format for the
              Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband
              (AMR-WB) Audio Codecs", RFC 4867, April 2007.

   [RFC5104]  Wenger, S., Chandra, U., Westerlund, M., and B. Burman,

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              "Codec Control Messages in the RTP Audio-Visual Profile
              with Feedback (AVPF)", RFC 5104, February 2008.

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Authors' Addresses

   Robert R Gilman
   Broomfield, CO 80020


   Roni Even
   94 Derech Em Hamoshavot
   Petach Tikva  49130


   Flemming Andreasen
   Cisco Systems
   Edison, NJ


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Full Copyright Statement

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