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rfc:rfc6871

Internet Engineering Task Force (IETF) R. Gilman Request for Comments: 6871 Independent Updates: 5939 R. Even Category: Standards Track Huawei Technologies ISSN: 2070-1721 F. Andreasen

                                                         Cisco Systems
                                                         February 2013
 Session Description Protocol (SDP) Media Capabilities Negotiation

Abstract

 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.  This documents extends the
 framework by defining media capabilities that can be used to
 negotiate media types and their associated parameters.
 This document updates the IANA Considerations of RFC 5939.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6871.

Gilman, et al. Standards Track [Page 1] RFC 6871 SDP Media Capabilities Negotiation February 2013

Copyright Notice

 Copyright (c) 2013 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Gilman, et al. Standards Track [Page 2] RFC 6871 SDP Media Capabilities Negotiation February 2013

Table of Contents

 1. Introduction ....................................................4
 2. Terminology .....................................................4
 3. SDP Media Capabilities ..........................................6
    3.1. Requirements ...............................................6
    3.2. Solution Overview ..........................................7
    3.3. New Capability Attributes .................................13
         3.3.1. The Media Format Capability Attributes .............13
         3.3.2. The Media Format Parameter Capability Attribute ....16
         3.3.3. The Media-Specific Capability Attribute ............19
         3.3.4. New Configuration Parameters .......................21
         3.3.5. The Latent Configuration Attribute .................23
         3.3.6. Enhanced Potential Configuration Attribute .........25
         3.3.7. Substitution of Media Payload Type Numbers
                in Capability ......................................29
         3.3.8. The Session Capability Attribute ...................30
    3.4. Offer/Answer Model Extensions .............................35
         3.4.1. Generating the Initial Offer .......................35
         3.4.2. Generating the Answer ..............................39
         3.4.3. Offerer Processing of the Answer ...................43
         3.4.4. Modifying the Session ..............................43
 4. Examples .......................................................44
    4.1. Alternative Codecs ........................................44
    4.2. Alternative Combinations of Codecs (Session
         Configurations) ...........................................47
    4.3. Latent Media Streams ......................................47
 5. IANA Considerations ............................................49
    5.1. New SDP Attributes ........................................49
    5.2. New SDP Capability Negotiation Option Tag .................50
    5.3. SDP Capability Negotiation Configuration
         Parameters Registry .......................................50
    5.4. SDP Capability Negotiation Configuration Parameter
         Registrations .............................................52
 6. Security Considerations ........................................52
 7. Acknowledgements ...............................................53
 8. References .....................................................54
    8.1. Normative References ......................................54
    8.2. Informative References ....................................54

Gilman, et al. Standards Track [Page 3] RFC 6871 SDP Media Capabilities Negotiation February 2013

1. Introduction

 "Session Description Protocol (SDP) Capability Negotiation" [RFC5939]
 provides a general framework for indicating and negotiating
 capabilities in SDP [RFC4566].  The base framework defines only
 capabilities for negotiating transport protocols and attributes.
 RFC 5939 [RFC5939] lists some of the issues with the current SDP
 capability negotiation process.  An additional real-life problem is
 to be able to offer one media stream (e.g., audio) but list the
 capability to support another media stream (e.g., video) without
 actually offering it concurrently.
 In this document, we extend the framework by defining media
 capabilities that can be used to indicate and negotiate media types
 and their associated format parameters.  This document also adds the
 ability to declare support for media streams, the use of which can be
 offered and negotiated later, and the ability to specify session
 configurations as combinations of media stream configurations.  The
 definitions of new attributes for media capability negotiation are
 chosen to make the translation from these attributes to
 "conventional" SDP [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 would be
 straightforward.
 This document updates RFC 5939 [RFC5939] by updating the IANA
 considerations.  All other extensions defined in this document are
 considered extensions above and beyond RFC 5939 [RFC5939].

2. Terminology

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [RFC2119] and
 indicate requirement levels for compliant implementations.
 Actual Configuration: An actual configuration specifies which
 combinations of SDP session parameters and media stream components
 can be used in the current offer/answer exchange and with what
 parameters.  Use of an actual configuration does not require any
 further negotiation in the offer/answer exchange.  See RFC 5939
 [RFC5939] for further details.

Gilman, et al. Standards Track [Page 4] RFC 6871 SDP Media Capabilities Negotiation February 2013

 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 negotiation document [RFC5939],
 or any other capability negotiation document.  In an offer SDP, the
 base configuration corresponds to the actual configuration as defined
 in RFC 5939 [RFC5939].
 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
 attributes.
 Media Format Capability: A media format, typically a media subtype
 such as PCMU, H263-1998, or T38, expressed in the form of a
 capability.
 Media Format Parameter Capability: A media format parameter ("a=fmtp"
 in conventional SDP) expressed in the form of a capability.  The
 media format parameter capability is associated with a media format
 capability.
 Media Capability: The combined set of capabilities associated with
 expressing a media format and its relevant parameters (e.g., media
 format parameters and media specific parameters).
 Potential Configuration: A potential configuration indicates which
 combinations of capabilities can be used for the session and its
 associated media stream components.  Potential configurations are not
 ready for use; however, they are offered for potential use in the
 current offer/answer exchange.  They provide an alternative that may
 be used instead of the actual configuration, subject to negotiation
 in the current offer/answer exchange.  See RFC 5939 [RFC5939] for
 further details.
 Latent Configuration: A latent configuration indicates which
 combinations of capabilities could be used in a future negotiation
 for the session and its associated media stream components.  Latent
 configurations are neither ready for use nor offered for actual or
 potential use in the current offer/answer exchange.  Latent
 configurations merely inform the other side of possible
 configurations supported by the entity.  Those latent configurations
 may be used to guide subsequent offer/answer exchanges, but they are
 not offered for use as part of the current offer/answer exchange.

Gilman, et al. Standards Track [Page 5] RFC 6871 SDP Media Capabilities Negotiation February 2013

3. SDP Media Capabilities

 The SDP capability negotiation [RFC5939] 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 RFC 5939 [RFC5939]; for example, they
 can be used only at the media level since they are media-level
 attributes.  RFC 5939 [RFC5939] does not provide a way to exchange
 media-level capabilities prior to the actual offer of the associated
 media stream.  This section provides an overview of extensions
 providing an SDP media capability negotiation solution offering more
 robust capabilities negotiation.  This is followed by definitions of
 new SDP attributes for the solution and its associated updated
 offer/answer procedures [RFC3264].

3.1. Requirements

 The capability negotiation extensions requirements considered herein
 are 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.  Ensure
          that each and every offered configuration can be easily
          translated into a corresponding SDP media block expressed
          with conventional SDP lines.
 REQ-04:  Ensure that 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.
 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.

Gilman, et al. Standards Track [Page 6] RFC 6871 SDP Media Capabilities Negotiation February 2013

 REQ-08:  Retain the extensibility of the base capability negotiation
          mechanism.
 REQ-09:  Provide the ability to specify acceptable combinations of
          media streams and media formats.  For example, offer a PCMU
          audio stream with an H264 video stream or a G729 audio
          stream with an H263 video stream.  This ability would give
          the offerer a means to limit processing requirements for
          simultaneous streams.  This would also permit an offer to
          include the choice of an audio/T38 stream or an image/T38
          stream, but not both.
 Other possible extensions have been discussed, but have not been
 treated in this document.  They may be considered in the future.
 Three such extensions are:
 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.
 FUT-03:  Provide capability negotiation attributes for all media-
          level SDP line types in the same manner as already done for
          the attribute type, with the exception of the media line
          type itself.  The media line type is handled in a special
          way to permit compact expression of media coding/format
          options.  The line types are bandwidth ("b="), information
          ("i="), connection data ("c="), and, possibly, the
          deprecated encryption key ("k=").

3.2. Solution Overview

 The solution consists of new capability attributes corresponding to
 conventional SDP line types, new parameters for the "pcfg", "acfg",
 and the new "lcfg" attributes extending the base attributes from RFC
 5939 [RFC5939], and a use of the "pcfg" attribute to return
 capability information in the SDP answer.
 Several new attributes are defined in a manner that can be related to
 the capabilities specified in a media line, and its corresponding
 "rtpmap" and "fmtp" attributes.

Gilman, et al. Standards Track [Page 7] RFC 6871 SDP Media Capabilities Negotiation February 2013

 o  A new attribute ("a=rmcap") defines RTP-based media format
    capabilities in the form of a media subtype (e.g., "PCMU"), and
    its encoding parameters (e.g., "/8000/2").  Each resulting media
    format type/subtype capability has an associated handle called a
    media capability number.  The encoding parameters are as specified
    for the "rtpmap" attribute defined in SDP [RFC4566], without the
    payload type number part.
 o  A new attribute ("a=omcap") defines other (non-RTP-based) media
    format capabilities in the form of a media subtype only (e.g.,
    "T38").  Each resulting media format type/subtype capability has
    an associated handle called a media capability number.
 o  A new attribute ("a=mfcap") specifies media format parameters
    associated with one or more media format capabilities.  The
    "mfcap" attribute is used primarily to associate the media format
    parameters normally carried in the "fmtp" attribute.  Note that
    media format parameters can be used with RTP and non-RTP-based
    media formats.
 o  A new attribute ("a=mscap") specifies media parameters associated
    with one or more media format capabilities.  The "mscap" attribute
    is used to associate capabilities with attributes other than
    "fmtp" or "rtpmap", for example, the "rtcp-fb" attribute defined
    in RFC 4585 [RFC4585].
 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 latent configurations for video even
    though no video is currently offered.  If the peer indicates
    support for one or more offered latent configurations, the
    corresponding media stream(s) may be added via a new offer/answer
    exchange.
 o  A new attribute ("a=sescap") is used to specify an acceptable
    combination of simultaneous media streams and their configurations
    as a list of potential and/or latent configurations.
 New parameters are defined for the potential configuration ("pcfg"),
 latent configuration ("lcfg"), and accepted configuration ("acfg")
 attributes to associate the new attributes with particular
 configurations.
 o  A new parameter type ("m=") is added to the potential
    configuration ("a=pcfg:") attribute and the actual configuration
    ("a=acfg:") attribute defined in RFC 5939 [RFC5939] and to the new
    latent configuration ("a=lcfg:") attribute.  This permits
    specification of media capabilities (including their associated

Gilman, et al. Standards Track [Page 8] RFC 6871 SDP Media Capabilities Negotiation February 2013

    parameters) and combinations thereof for the configuration.  For
    example, the "a=pcfg:" line might specify PCMU and telephone
    events [RFC4733] or G.729B and telephone events as acceptable
    configurations.  The "a=acfg:" line in the answer would specify
    the configuration chosen.
 o  A new parameter type ("pt=") is added to the potential
    configuration, actual configuration, and latent configuration
    attributes.  This parameter associates RTP payload type numbers
    with the referenced RTP-based media format capabilities and is
    appropriate only when the transport protocol uses RTP.
 o  A new parameter type ("mt=") is used to specify the media type for
    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
 configurations.
 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" and "mscap" capability attributes.  This reduces
    the necessity to use the deletion qualifier in the "a=pcfg"
    parameter in order to ignore "rtpmap", "fmtp", and certain other
    attributes in the base configuration.
 o  An argument concatenation rule is defined for "mfcap" attributes
    that refer to the same media capability number.  This makes it
    convenient to combine format options concisely by associating
    multiple mfcap lines with multiple media format capabilities.
 This document extends the base protocol extensions to the
 offer/answer model that allow for capabilities and potential
 configurations to be included in an offer.  Media capabilities
 constitute capabilities that can be used in potential and latent
 configurations.  Whereas potential configurations constitute
 alternative offers that may be accepted by the answerer instead of
 the actual configuration(s) included in the "m=" line(s) and
 associated parameters, latent configurations merely inform the other
 side of possible configurations supported by the entity.  Those
 latent configurations may be used to guide subsequent offer/answer
 exchanges, but they are not part of the current offer/answer
 exchange.

Gilman, et al. Standards Track [Page 9] RFC 6871 SDP Media Capabilities Negotiation February 2013

 The mechanism is illustrated by the offer/answer exchange below,
 where Alice sends an offer to Bob:
                 Alice                            Bob
                | (1) Offer (SRTP and RTP)         |
                |--------------------------------->|
                |                                  |
                | (2) Answer (RTP)                 |
                |<---------------------------------|
                |                                  |
 Alice's offer includes RTP and Secure Real-time Transport Protocol
 (SRTP) as alternatives.  RTP is the default, but SRTP is the
 preferred one (long lines are folded to fit the margins):
    v=0
    o=- 25678 753849 IN IP4 192.0.2.1
    s=
    c=IN IP4 192.0.2.1
    t=0 0
    a=creq:med-v0
    m=audio 3456 RTP/AVP 0 18
    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=rmcap:1,4 G729/8000/1
    a=rmcap:2 PCMU/8000/1
    a=rmcap: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 \
    inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
    a=pcfg:1 m=4,5|1,5 t=1 a=1 pt=1:100,4:101,5:102
    a=pcfg: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 RFC 5939 [RFC5939], with the option tag
 "med-v0", which indicates that the extension framework defined here
 must be supported.  The base-level capability negotiation support
 ("cap-v0" [RFC5939]) 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

Gilman, et al. Standards Track [Page 10] RFC 6871 SDP Media Capabilities Negotiation February 2013

 transport protocol (RTP/AVP in this case) and the default actual
 configuration.
 The "a=tcap:1" line, specified in the SDP capability negotiation base
 protocol [RFC5939], defines transport protocol capabilities, in this
 case Secure RTP (SAVP profile) as the first option and RTP (AVP
 profile) as the second option.
 The "a=rmcap:1,4" line defines two G.729 RTP-based media format
 capabilities, numbered 1 and 4, and their encoding rate.  The
 capabilities are of media type "audio" and subtype G729.  Note that
 the media subtype is explicitly specified here, rather than RTP
 payload type numbers.  This permits the assignment of payload type
 numbers in the media stream configuration specification.  In this
 example, two G.729 subtype capabilities are defined.  This permits
 the declaration of two sets of formatting parameters for G.729.
 The "a=rmcap:2" line defines a G.711 mu-law capability, numbered 2.
 The "a=rmcap:5" line defines an audio telephone-event capability,
 numbered 5.
 The "a=mfcap:1" line specifies the "fmtp" formatting parameters for
 capability 1 (offerer will not accept G.729 Annex B packets).
 The "a=mfcap:4" line specifies the "fmtp" formatting parameters for
 capability 4 (offerer will accept G.729 Annex B packets).
 The "a=mfcap:5" line specifies the "fmtp" formatting parameters for
 capability 5 (the dual-tone multi-frequency (DTMF) touchtones
 0-9,*,#).
 The "a=acap:1" line specified in the base protocol provides the
 "crypto" attribute that 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, attribute capabilities, and specified payload
 type number mappings.  Three explicit alternatives are provided; the
 lowest-numbered one is the preferred one.  The "a=pcfg:1 ..." line
 specifies media capabilities 4 and 5, i.e., G.729B and DTMF
 (including their associated media format parameters), or media
 capability 1 and 5, i.e., G.729 and DTMF (including their associated
 media format parameters).  Furthermore, it specifies transport
 protocol capability 1 (i.e., the RTP/SAVP profile - secure RTP), and
 the attribute capability 1, i.e., the "crypto" attribute provided.
 Last, it specifies a payload type number mapping for (RTP-based)

Gilman, et al. Standards Track [Page 11] RFC 6871 SDP Media Capabilities Negotiation February 2013

 media capabilities 1, 4, and 5, thereby permitting the offerer to
 distinguish between encrypted media and unencrypted media received
 prior to receipt of the answer.
 Use of unique payload type numbers in alternative configurations is
 not required; codecs such as Adaptive Multi-Rate Wideband (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 Security
 Descriptions (SDES) inline keying [RFC4568], the offerer will still
 need to receive the answer before being able to decrypt the stream.
 The second alternative ("a=pcfg:2 ...") specifies media capability 2,
 i.e., PCMU, under the RTP/SAVP profile, with the same SRTP key
 material.
 The third alternative ("a=pcfg:3 ...") offers G.729B unsecured; its
 only purpose in this example is to show a preference for G.729B over
 PCMU.
 Per RFC 5939 [RFC5939], the media line, with any qualifying
 attributes such as "fmtp" or "rtpmap", is itself considered a valid
 configuration (the current actual configuration); it has the lowest
 preference (per RFC 5939 [RFC5939]).
 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:
    v=0
    o=- 24351 621814 IN IP4 192.0.2.2
    s=
    c=IN IP4 192.0.2.2
    t=0 0
    a=csup:med-v0
    m=audio 4567 RTP/AVP 18
    a=rtpmap:18 G729/8000
    a=fmtp:18 annexb=yes
    a=acfg:3 m=4 t=2 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

Gilman, et al. Standards Track [Page 12] RFC 6871 SDP Media Capabilities Negotiation February 2013

 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
 negotiation.  The approach taken is to keep things similar to the
 existing media capabilities defined by the existing media
 descriptions ("m=" lines) and the associated "rtpmap" and "fmtp"
 attributes.  We use media subtypes and "media capability numbers" to
 link the relevant media capability parameters.  This permits the
 capabilities to be defined at the session level and be used for
 multiple streams, if desired.  For RTP-based media formats, payload
 types are then specified at the media level (see Section 3.3.4.2).
 A media capability merely indicates possible support for the media
 type and media format(s) and parameters in question.  In order to
 actually use a media capability in an offer/answer exchange, it MUST
 be referenced in a potential configuration.
 Media capabilities, i.e., the attributes associated with expressing
 media capability formats, parameters, etc., can be provided at the
 session level and/or the media level.  Media capabilities provided at
 the session level may be referenced in any "pcfg" or "lcfg" attribute
 at the media level (consistent with the media type), whereas media
 capabilities provided at the media level may be referenced only by
 the "pcfg" or "lcfg" attribute within that media stream.  In either
 case, the scope of the <med-cap-num> is the entire session
 description.  This enables each media capability to be uniquely
 referenced across the entire session description (e.g., in a
 potential configuration).

3.3.1. The Media Format Capability Attributes

 Media subtypes can be expressed as media format capabilities by use
 of the "a=rmcap" and "a=omcap" attributes.  The "a=rmcap" attribute
 MUST be used for RTP-based media, whereas the "a=omcap" attribute
 MUST be used for non-RTP-based (other) media formats.  The two
 attributes are defined as follows:

Gilman, et al. Standards Track [Page 13] RFC 6871 SDP Media Capabilities Negotiation February 2013

 a=rmcap:<media-cap-num-list> <encoding-name>/<clock-rate>
                              [/<encoding-parms>]
 a=omcap:<media-cap-num-list> <format-name>
 where <media-cap-num-list> is a (list of) media capability number(s)
 used to number a media format capability, the <encoding name> or
 <format-name> is the media subtype, e.g., H263-1998, PCMU, or T38,
 <clock rate> is the encoding rate, and <encoding parms> are the media
 encoding parameters for the media subtype.  All media format
 capabilities in the list are assigned to the same media type/subtype.
 Each occurrence of the "rmcap" and "omcap" attribute MUST use unique
 values in their <media-cap-num-list>; the media capability numbers
 are shared between the two attributes and the numbers MUST be unique
 across the entire SDP session.  In short, the "rmcap" and "omcap"
 attributes define media format capabilities and associate them with a
 media capability number in the same manner as the "rtpmap" attribute
 defines them and associates them with a payload type number.
 Additionally, the attributes allow multiple capability numbers to be
 defined for the media format in question by specifying a range of
 media capability numbers.  This permits the media format to be
 associated with different media parameters in different
 configurations.  When a range of capability numbers is specified, the
 first (leftmost) capability number MUST be strictly smaller than the
 second (rightmost), i.e., the range increases and covers at least two
 numbers.
 In ABNF [RFC5234], we have:
 media-capability-line = rtp-mcap / non-rtp-mcap
 rtp-mcap           = "a=rmcap:" media-cap-num-list
                         1*WSP encoding-name "/" clock-rate
                         ["/" encoding-parms]
 non-rtp-mcap       = "a=omcap:" media-cap-num-list 1*WSP format-name
 media-cap-num-list = media-cap-num-element
                      *("," media-cap-num-element)
 media-cap-num-element = media-cap-num
                              / media-cap-num-range
 media-cap-num-range = media-cap-num "-" media-cap-num
 media-cap-num      = NonZeroDigit *9(DIGIT)
 encoding-name      = token ;defined in RFC 4566
 clock-rate         = NonZeroDigit *9(DIGIT)
 encoding-parms     = token
 format-name        = token ;defined in RFC 4566
 NonZeroDigit       = %x31-39    ; 1-9

Gilman, et al. Standards Track [Page 14] RFC 6871 SDP Media Capabilities Negotiation February 2013

 The encoding-name, clock-rate, and encoding-params are as defined to
 appear in an "rtpmap" attribute for each media type/subtype.  Thus,
 it is easy to convert an "rmcap" attribute line into one or more
 "rtpmap" attribute lines, once a payload type number is assigned to a
 media-cap-num (see Section 3.3.5).
 The format-name is a media format description for non-RTP-based media
 as defined for the <fmt> part of the media description ("m=" line) in
 SDP [RFC4566].  In simple terms, it is the name of the media format,
 e.g., "t38".  This form can also be used in cases such as Binary
 Floor Control Protocol (BFCP) [RFC4585] where the fmt list in the
 "m=" line is effectively ignored (BFCP uses "*").
 The "rmcap" and "omcap" attributes can be provided at the session
 level and/or the media level.  There can be more than one "rmcap" and
 more than one "omcap" attribute at both the session and media levels
 (i.e., more than one of each at the session level and more than one
 of each in each media description).  Media capability numbers cannot
 include leading zeroes, and each media-cap-num MUST be unique within
 the entire SDP record; it is used to identify that media capability
 in potential, latent, and actual configurations, and in other
 attribute lines as explained below.  Note that the media-cap-num
 values are shared between the "rmcap" and "omcap" attributes; hence,
 the uniqueness requirement applies to the union of them.  When the
 media capabilities are used in a potential, latent, or actual
 configuration, the media formats referred by those configurations
 apply at the media level, irrespective of whether the media
 capabilities themselves were specified at the session or media level.
 In other words, the media capability applies to the specific media
 description associated with the configuration that invokes it.

Gilman, et al. Standards Track [Page 15] RFC 6871 SDP Media Capabilities Negotiation February 2013

 For example:
    v=0
    o=- 24351 621814 IN IP4 192.0.2.2
    s=
    c=IN IP4 192.0.2.2
    t=0 0
    a=rmcap:1 L16/8000/1
    a=rmcap:2 L16/16000/2
    a=rmcap:3 H263-1998/90000
    a=omcap:4 example
    m=audio 54320 RTP/AVP 0
    a=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
    a=tcap:1 TCP
    a=pcfg:11 m=4 t=1

3.3.2. The Media Format Parameter Capability Attribute

 This attribute is used to associate media format specific parameters
 with one or more media format capabilities.  The form of the
 attribute is
    a=mfcap:<media-caps> <list of parameters>
 where <media-caps> permits the list of parameters to be associated
 with one or more media format capabilities and the format parameters
 are specific to the type of media format.  The mfcap lines map to a
 single traditional SDP "fmtp" attribute line (one for each entry in
 <media-caps>) of the form
    a=fmtp:<fmt> <list of parameters>
 where <fmt> is the media format parameter 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 existing "acap" attribute MUST NOT be used to encode
 "fmtp" attributes.
 The "mfcap" attribute adheres to SDP [RFC4566] attribute production
 rules with
    media-format-parameter-capability =
           "a=mfcap:" media-cap-num-list 1*WSP fmt-specific-param-list
    fmt-specific-param-list = text ; defined in RFC 4566

Gilman, et al. Standards Track [Page 16] RFC 6871 SDP Media Capabilities Negotiation February 2013

 Note that media format parameters can be used with RTP-based and non-
 RTP-based media formats.

3.3.2.1. 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
 that are associated with the same media capability number.
 Therefore, different mfcap lines MAY include the same media-cap-num
 in their media-cap-num-list.  When a particular media capability is
 selected for processing, the parameters from each mfcap line that
 references the particular capability number in its media-cap-num-list
 are concatenated together via ";", in the order the "mfcap"
 attributes appear in the SDP record, to form the equivalent of a
 single "fmtp" attribute line.  This permits one to define a separate
 mfcap line for a single parameter and value that is to be applied to
 each media capability designated in the media-cap-num-list.  This
 provides a compact method to specify multiple combinations of format
 parameters when using codecs with multiple format options.  Note that
 order-dependent parameters SHOULD be placed in a single mfcap line to
 avoid possible problems with line rearrangement by a middlebox.
 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 that reference
 that media capability, the format-specific parameters are
 concatenated together and separated by ";" for construction of the
 corresponding format attribute ("a=fmtp").  The resulting format
 attribute will look something like the following (without line
 breaks):
      a=fmtp:<fmt> <fmt-specific-param-list1>;
                   <fmt-specific-param-list2>;
                   ...
 where <fmt> depends on the transport protocol in the manner defined
 in RFC 4566 [RFC4566].  SDP cannot assess the legality of the
 resulting parameter list in the "a=fmtp" line; the user must take
 care to ensure that legal parameter lists are generated.
 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.

Gilman, et al. Standards Track [Page 17] RFC 6871 SDP Media Capabilities Negotiation February 2013

 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=rmcap:1,2 G729/8000
    a=mfcap:2 annexb:no
 Media capability 1 supports G.729 with Annex B, whereas media
 capability 2 supports G.729 without Annex B.
 Example for H.263 video:
    a=rmcap:1 H263-1998/90000
    a=rmcap:2 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 Multi-Rate
 codec:
    a=rmcap:1-3 AMR/8000/1
    a=rmcap: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:
    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-set=0,2,4,7
 and AMR codec #4 with payload type number 99, depicted by the
 potential configuration:
    a=pcfg:4 m=4, pt=4:99

Gilman, et al. Standards Track [Page 18] RFC 6871 SDP Media Capabilities Negotiation February 2013

 is 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-set=0,3,5,6
 and so on for the other four combinations.  SDP could thus convert
 the media capabilities specifications into one or more alternative
 media stream specifications, one of which can be chosen for the
 answer.

3.3.3. The Media-Specific Capability Attribute

 Attributes and parameters associated with a media format are
 typically specified using the "rtpmap" and "fmtp" attributes in SDP,
 and the similar "rmcap" and "mfcap" attributes in SDP media
 capabilities.  Some SDP extensions define other attributes that need
 to be associated with media formats, for example, the "rtcp-fb"
 attribute defined in RFC 4585 [RFC4585].  Such media-specific
 attributes, beyond the "rtpmap" and "fmtp" attributes, may be
 associated with media capability numbers via a new media-specific
 attribute, "mscap", of the following form:
       a=mscap:<media caps star> <att field> <att value>
 where <media caps star> is a (list of) media capability number(s),
 <att field> is the attribute name, and <att value> is the value field
 for the named attribute.  Note that the media capability numbers
 refer to media format capabilities specified elsewhere in the SDP
 ("rmcap" and/or "omcap").  If a range of capability numbers is
 specified, the first (leftmost) capability number MUST be strictly
 smaller than the second (rightmost).  The media capability numbers
 may include a wildcard ("*"), which will be used instead of any
 payload type mappings in the resulting SDP (see, e.g., RFC 4585
 [RFC4585] and the example below).  In ABNF, we have:
        media-specific-capability = "a=mscap:"
                                     media-caps-star
                                     1*WSP att-field ; from RFC 4566
                                     1*WSP att-value ; from RFC 4566
        media-caps-star           =  media-cap-star-element
                                       *("," media-cap-star-element)
        media-cap-star-element    = (media-cap-num [wildcard])
                                  / (media-cap-num-range [wildcard])
        wildcard                  = "*"

Gilman, et al. Standards Track [Page 19] RFC 6871 SDP Media Capabilities Negotiation February 2013

 Given an association between a media capability and a payload type
 number as specified by the "pt=" parameters in a "pcfg" attribute
 line, a mscap line may be translated easily into a conventional SDP
 attribute line of the form:
    a=<att field>":"<fmt> <att value> ; <fmt> defined in SDP [RFC4566]
 A resulting attribute that is not a legal SDP attribute, as specified
 by RFC 4566, MUST be ignored by the receiver.
 If a media capability number (or range) contains a wildcard character
 at the end, any payload type mapping specified for that media-
 specific capability (or range of capabilities) will use the wildcard
 character in the resulting SDP instead of the payload type specified
 in the payload type mapping ("pt" parameter) in the configuration
 attribute.
 A single mscap line may refer to multiple media capabilities by use
 of a capability number range; this is equivalent to multiple mscap
 lines, each with the same attribute values (but different media
 capability numbers), one line per media capability.
 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 are
 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 RFC 5104 [RFC5104] (with the session level and audio media
 omitted).  If the offer contains a media block like the following
 (note the wildcard character),
    m=video 51372 RTP/AVP 98
    a=rtpmap:98 H263-1998/90000
    a=tcap:1 RTP/AVPF
    a=rmcap:1 H263-1998/90000
    a=mscap:1 rtcp-fb ccm tstr
    a=mscap:1 rtcp-fb ccm fir
    a=mscap:1* 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 the following

Gilman, et al. Standards Track [Page 20] RFC 6871 SDP Media Capabilities Negotiation February 2013

    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. New Configuration Parameters

 Along with the new attributes for media capabilities, new extension
 parameters are defined for use in the potential configuration, the
 actual configuration, and/or the new latent configuration defined in
 Section 3.3.5.

3.3.4.1. The Media Configuration Parameter (m=)

 The media configuration parameter is used to specify the media
 format(s) and related parameters for a potential, actual, or latent
 configuration.  Adhering to the ABNF for extension-config-list in RFC
 5939 [RFC5939] with
           ext-cap-name = "m"
           ext-cap-list = media-cap-num-list
                          [*(BAR media-cap-num-list)]
 we have
            media-config-list = ["+"] "m=" media-cap-num-list
                                           *(BAR media-cap-num-list)
                                 ;BAR is defined in RFC 5939
                                 ;media-cap-num-list is defined above
 Alternative media configurations are separated by a vertical bar
 ("|").  The alternatives are ordered by preference, most-preferred
 first.  When media capabilities are not included in a potential
 configuration at the media level, the media type and media format
 from the associated "m=" line will be used.  The use of the plus sign
 ("+") is described in RFC 5939.

3.3.4.2. The Payload Type Number Mapping Parameter (pt=)

 The payload type number mapping parameter is used to specify the
 payload type number to be associated with each RTP-based media format
 in a potential, actual, or latent configuration.  We define the
 payload type number mapping parameter, payload-number-config-list, in
 accordance with the extension-config-list format defined in RFC 5939
 [RFC5939].  In ABNF:

Gilman, et al. Standards Track [Page 21] RFC 6871 SDP Media Capabilities Negotiation February 2013

 payload-number-config-list = ["+"] "pt=" media-map-list
 media-map-list      = media-map *("," media-map)
 media-map           = media-cap-num ":" payload-type-number
                          ; media-cap-num is defined in Section 3.3.1
 payload-type-number = NonZeroDigit *2(DIGIT) ; RTP payload
                                              ; type number
 The example in Section 3.3.7 shows how the parameters from the rmcap
 line are mapped to payload type numbers from the "pcfg" "pt"
 parameter.  The use of the plus sign ("+") is described in RFC 5939
 [RFC5939].
 A latent configuration represents a future capability; hence, the
 "pt=" parameter is not directly meaningful in the "lcfg" attribute
 because no actual media session is being offered or accepted.  It is
 permitted in order to tie any payload type number parameters within
 attributes to the proper media format.  A primary example is the case
 of format parameters for the Redundant Audio Data (RED) [RFC2198]
 payload, which are payload type numbers.  Specific payload type
 numbers used in a latent configuration MAY be interpreted as
 suggestions to be used in any future offer based on the latent
 configuration, but they are not binding; the offerer and/or answerer
 may use any payload type numbers each deems appropriate.  The use of
 explicit payload type numbers for latent configurations can be
 avoided by use of the parameter substitution rule of Section 3.3.7.
 Future extensions are also permitted.  Note that leading zeroes are
 not permitted.

3.3.4.3. The Media Type Parameter

 When a latent configuration is specified (always at the media level),
 indicating the ability to support an additional media stream, it is
 necessary to specify the media type (audio, video, etc.)  as well as
 the format and transport type.  The media type parameter is defined
 in ABNF as
          media-type = ["+"] "mt=" media; media defined in RFC 4566
 At present, the media-type parameter is used only in the latent
 configuration attribute, and the use of the "+" prefix to specify
 that the entire attribute line is to be ignored if the mt= parameter
 is not understood is unnecessary.  However, if the media-type
 parameter is later added to an existing capability attribute such as
 "pcfg", then the "+" would be useful.  The media format(s) and
 transport type(s) are specified using the media configuration
 parameter ("+m=") defined above, and the transport parameter ("t=")
 defined in RFC 5939 [RFC5939], respectively.

Gilman, et al. Standards Track [Page 22] RFC 6871 SDP Media Capabilities Negotiation February 2013

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
 a session with an audio stream, and, at the same time, announce its
 ability to support a video stream as part of the same session.  The
 offerer can supply its video capabilities by offering one or more
 latent video configurations along with the media stream for audio;
 the responding party may indicate its ability and willingness to
 support such a video session by returning a corresponding latent
 configuration.
 Latent configurations returned in SDP answers MUST match offered
 latent configurations (or parameter subsets thereof).  Therefore, it
 is appropriate for the offering party to announce most, if not all,
 of its capabilities in the initial offer.  This choice has been made
 in order to keep the size of the answer more compact by not requiring
 acap, rmcap, tcap, etc.  lines in the answer.
 Latent configurations may be announced by use of the latent
 configuration attribute, which is defined in a manner very similar to
 the potential configuration attribute.  The latent configuration
 attribute combines the properties of a media line and a potential
 configuration.  A latent configuration MUST include a media type
 (mt=) and a transport protocol configuration parameter since the
 latent configuration is independent of any media line present.  In
 most cases, the media configuration (m=) parameter needs to be
 present as well (see Section 4 for examples).  The "lcfg" attribute
 is a media-level attribute.
    The "lcfg" attribute is defined as a media-level attribute since
    it specifies a possible future media stream.  However, the "lcfg"
    attribute is not necessarily related to the media description
    within which it is provided.  Session capability attributes
    ("a=sescap") may be used to indicate supported media stream
    configurations.
 Each media line in an SDP description represents an offered
 simultaneous media stream, whereas each latent configuration
 represents an additional stream that may be negotiated in a future
 offer/answer exchange.  Session capability attributes may be used to
 determine whether a latent configuration may be used to form an offer
 for an additional simultaneous stream or to reconfigure an existing
 stream in a subsequent offer/answer exchange.

Gilman, et al. Standards Track [Page 23] RFC 6871 SDP Media Capabilities Negotiation February 2013

 The latent configuration attribute is of the form:
      a=lcfg:<config-number> <latent-cfg-list>
 which adheres to the SDP [RFC4566] "attribute" production with
 att-field and att-value defined as:
    att-field  = "lcfg"
    att-value  = config-number 1*WSP lcfg-cfg-list
    config-number = NonZeroDigit *9(DIGIT)  ;DIGIT defined in RFC 5234
    lcfg-cfg-list = media-type 1*WSP pot-cfg-list
                                ; as defined in RFC 5939
                                ; and extended herein
 The media-type (mt=) parameter identifies the media type (audio,
 video, etc.)  to be associated with the latent media stream, and it
 MUST be present.  The pot-cfg-list MUST contain a transport-protocol-
 config-list (t=) parameter and a media-config-list (m=) parameter.
 The pot-cfg-list MUST NOT contain more than one instance of each type
 of parameter list.  As specified in RFC 5939 [RFC5939], the use of
 the "+" prefix with a parameter indicates that the entire
 configuration MUST be ignored if the parameter is not understood;
 otherwise, the parameter itself may be ignored.
 Media stream payload numbers are not assigned by a latent
 configuration.  Assignment will take place if and when the
 corresponding stream is actually offered via an "m=" line in a later
 exchange.  The payload-number-config-list is included as a parameter
 to the "lcfg" attribute in case it is necessary to tie payload
 numbers in attribute capabilities to specific media capabilities.
 If an "lcfg" attribute invokes an "acap" attribute that appears at
 the session level, then that attribute will be expected to appear at
 the session level of a subsequent offer when and if a corresponding
 media stream is offered.  Otherwise, "acap" attributes that appear at
 the media level represent media-level attributes.  Note, however,
 that "rmcap", omcap, "mfcap", "mscap", and "tcap" attributes may
 appear at the session level because they always result in media-level
 attributes or "m=" line parameters.
 The configuration numbers for latent configurations do not imply a
 preference; the offerer will imply a preference when actually
 offering potential configurations derived from latent configurations
 negotiated earlier.  Note, however, that the offerer of latent
 configurations MAY specify preferences for combinations of potential
 and latent configurations by use of the "sescap" attribute defined in
 Section 3.3.8.  For example, if an SDP offer contains, say, an audio

Gilman, et al. Standards Track [Page 24] RFC 6871 SDP Media Capabilities Negotiation February 2013

 stream with "pcfg:1", and two latent video configurations, "lcfg:2"
 and "lcfg:3", then a session with one audio stream and one video
 stream could be specified by including "a=sescap:1 1,2|3".  One audio
 stream and two video streams could be specified by including
 "a=sescap:2 1,2,3" in the offer.  In order to permit combinations of
 latent and potential configurations in session capabilities, latent
 configuration numbers MUST be different from those used for potential
 configurations.  This restriction is especially important if the
 offerer does not require cmed-v0 capability and the recipient of the
 offer doesn't support it.  If the "lcfg" attribute is not recognized,
 the capability attributes intended to be associated with it may be
 confused with those associated with a potential configuration of some
 other media stream.  Note also that leading zeroes are not permitted
 in configuration numbers.
 If a cryptographic attribute, such as the SDES "a=crypto:" attribute
 [RFC4568], is referenced by a latent configuration through an "acap"
 attribute, any keying material required in the conventional
 attribute, such as the SDES key/salt string, MUST be included in
 order to satisfy formatting rules for the attribute.  Since the
 keying material will be visible but not actually used at this stage
 (since it's a latent configuration), the value(s) of the keying
 material MUST NOT be a real value used for real exchange of media,
 and the receiver of the "lcfg" attribute MUST ignore the value(s).

3.3.6. Enhanced Potential Configuration Attribute

 The present work requires new extensions (parameters) for the "pcfg"
 attribute defined in the SDP capability negotiation base protocol
 [RFC5939].  The parameters and their definitions are "borrowed" from
 the definitions provided for the latent configuration attribute in
 Section 3.3.5.  The expanded ABNF definition of the "pcfg" attribute
 is
      a=pcfg: <config-number> [<pot-cfg-list>]
 where
      config-number = 1*DIGIT ;defined in [RFC5234]
      pot-cfg-list  = pot-config *(1*WSP pot-config)
      pot-config    =  attribute-config-list / ;def in [RFC5939]
           transport-protocol-config-list / ;defined in [RFC5939]
           extension-config-list / ;[RFC5939]
           media-config-list / ; Section 3.3.4.1
           payload-number-config-list ; Section 3.3.4.2
 Except for the extension-config-list, the pot-cfg-list MUST NOT
 contain more than one instance of each parameter list.

Gilman, et al. Standards Track [Page 25] RFC 6871 SDP Media Capabilities Negotiation February 2013

3.3.6.1. Returning Capabilities in the Answer

 Potential and/or latent configuration attributes may be returned
 within an answer SDP to indicate the ability of the answerer to
 support alternative configurations of the corresponding stream(s).
 For example, an offer may include multiple potential configurations
 for a media stream and/or latent configurations for additional
 streams.  The corresponding answer will indicate (via an "acfg"
 attribute) the configuration accepted and used to construct the base
 configuration for each active media stream in the reply, but the
 reply MAY also contain potential and/or latent configuration
 attributes, with parameters, to indicate which other offered
 configurations would be acceptable.  This information is useful if it
 becomes desirable to reconfigure a media stream, e.g., to reduce
 resource consumption.
 When potential and/or latent configurations are returned in an
 answer, all numbering MUST refer to the configuration and capability
 attribute numbering of the offer.  The offered capability attributes
 need not be returned in the answer.  The answer MAY include
 additional capability attributes and/or configurations (with distinct
 numbering).  The parameter values of any returned "pcfg" or "lcfg"
 attributes MUST be a subset of those included in the offered
 configurations and/or those added by the answerer; values MAY be
 omitted only if they were indicated as alternative sets, or optional,
 in the original offer.  The parameter set indicated in the returned
 "acfg" attribute need not be repeated in a returned "pcfg" attribute.
 The answerer MAY return more than one "pcfg" attribute with the same
 configuration number if it is necessary to describe selected
 combinations of optional or alternative parameters.
 Similarly, one or more session capability attributes ("a=sescap") MAY
 be returned to indicate which of the offered session capabilities
 is/are supportable by the answerer (see Section 3.3.8).
 Note that, although the answerer MAY return capabilities beyond those
 included by the offerer, these capabilities MUST NOT be used to form
 any base level media description in the answer.  For this reason, it
 is advisable for the offerer to include most, if not all, potential
 and latent configurations it can support in the initial offer, unless
 the size of the resulting SDP is a concern.  Either party MAY later
 announce additional capabilities by renegotiating the session in a
 second offer/answer exchange.

Gilman, et al. Standards Track [Page 26] RFC 6871 SDP Media Capabilities Negotiation February 2013

3.3.6.2. Payload Type Number Mapping

 When media format capabilities defined in "rmcap" attributes are used
 in potential configuration lines, the transport protocol uses RTP and
 it is necessary to assign payload type numbers.  In some cases, it is
 desirable to assign different payload type numbers to the same media
 format capability when used in different potential configurations.
 One example is when configurations for AVP and SAVP are offered: the
 offerer would like the answerer to use different payload type numbers
 for encrypted and unencrypted media, so the offerer can decide
 whether or not to render early media that arrives before the answer
 is received.
    For example, if use of AVP was selected by the answerer, then
    media received by the offerer is not encrypted; hence, it can be
    played out prior to receiving the answer.  Conversely, if SAVP was
    selected, cryptographic parameters and keying material present in
    the answer may be needed to decrypt received media.  If the offer
    configuration indicated that AVP media uses one set of payload
    types and SAVP a different set, then the offerer will know whether
    media received prior to the answer is encrypted or not by simply
    looking at the RTP payload type number in the received packet.
 This association of distinct payload type number(s) with different
 transport protocols requires a separate pcfg line for each protocol.
 Clearly, this technique cannot be used if the number of potential
 configurations exceeds the number of possible payload type numbers.

3.3.6.3. Processing of Media-Format-Related Conventional Attributes for

        Potential Configurations
 When media capabilities negotiation is employed, SDP records are
 likely to contain conventional attributes such as "rtpmap", "fmtp",
 and other media-format-related lines, as well as capability
 attributes such as "rmcap", omcap, "mfcap", and "mscap" that map into
 those conventional attributes when invoked by a potential
 configuration.  In such cases, it MAY be appropriate to employ the
 delete-attributes option [RFC5939] in the attribute configuration
 list parameter in order to avoid the generation of conflicting "fmtp"
 attributes for a particular configuration.  Any media-specific
 attributes in the media block that refer to media formats not used by
 the potential configuration MUST be ignored.

Gilman, et al. Standards Track [Page 27] RFC 6871 SDP Media Capabilities Negotiation February 2013

 For example:
    v=0
    o=- 25678 753849 IN IP4 192.0.2.1
    s=
    c=IN IP4 192.0.2.1
    t=0 0
    a=creq:med-v0
    m=audio 3456 RTP/AVP 0 18 100
    a=rtpmap:100 telephone-event
    a=fmtp:100 0-11
    a=rmcap:1 PCMU/8000
    a=rmcap:2 G729/8000
    a=rmcap:3 telephone-event/8000
    a=mfcap:3 0-15
    a=pcfg:1 m=2,3|1,3 a=-m pt=1:0,2:18,3:100
    a=pcfg:2
 In this example, PCMU is media capability 1, G729 is media capability
 2, and telephone-event is media capability 3.  The a=pcfg:1 line
 specifies that the preferred configuration is G.729 with extended
 DTMF events, second is G.711 mu-law with extended DTMF events, and
 the base media-level attributes are to be deleted.  Intermixing of
 G.729, G.711, and "commercial" DTMF events is least preferred (the
 base configuration provided by the "m=" line, which is, by default,
 the least preferred configuration).  The "rtpmap" and "fmtp"
 attributes of the base configuration are replaced by the "rmcap" and
 "mfcap" attributes when invoked by the proposed configuration.
 If the preferred configuration is selected, the SDP answer will look
 like the following
    v=0
    o=- 25678 753849 IN IP4 192.0.2.1
    s=
    c=IN IP4 192.0.2.1
    t=0 0
    a=csup:med-v0
    m=audio 3456 RTP/AVP 18 100
    a=rtpmap:100 telephone-event/8000
    a=fmtp:100 0-15
    a=acfg:1 m=2,3 pt=1:0,2:18,3:100

Gilman, et al. Standards Track [Page 28] RFC 6871 SDP Media Capabilities Negotiation February 2013

3.3.7. Substitution of Media Payload Type Numbers in Capability

      Attribute Parameters
 In some cases, for example, when an RFC 2198 [RFC2198] 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=rmcap:1 PCMU/8000
    a=rmcap: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
    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
 rule.  When an "mfcap", "mscap", or "acap" attribute is processed,
 its arguments will be scanned for a payload type number escape
 sequence of the following form (in ABNF):
    ptn-esc = "%m=" media-cap-num "%" ; defined in Section 3.3.1
 If the sequence is found, the sequence is replaced by the payload
 type number assigned to the media capability number, as specified by
 the "pt=" parameter in the selected potential configuration; only
 actual payload type numbers are supported -- wildcards are excluded.
 The sequence "%%" (null digit string) is replaced by a single percent
 sign and processing continues with the next character, if any.

Gilman, et al. Standards Track [Page 29] RFC 6871 SDP Media Capabilities Negotiation February 2013

 For example, the above offer sequence could have been written as
    m=audio 45678 RTP/AVP 0
    a=rtpmap:0 PCMU/8000
    a=rmcap:1 PCMU/8000
    a=rmcap:2 RED/8000
    a=mfcap:2 %m=1%/%m=1%
    a=pcfg:1 m=2,1 pt=2:98,1:0
 and the equivalent SDP is the same as above.

3.3.8. The Session Capability Attribute

 Potential and latent configurations enable offerers and answerers to
 express a wide range of alternative configurations for current and
 future negotiation.  However, in practice, it may not be possible to
 support all combinations of these configurations.
 The session capability attribute provides a means for the offerer
 and/or the answerer to specify combinations of specific media stream
 configurations that it is willing and able to support.  Each session
 capability in an offer or answer MAY be expressed as a list of
 required potential configurations, and MAY include a list of optional
 potential and/or latent configurations.
 The choices of session capabilities may be based on processing load,
 total bandwidth, or any other criteria of importance to the
 communicating parties.  If the answerer supports media capabilities
 negotiation, and session configurations are offered, it MUST accept
 one of the offered configurations, or it MUST refuse the session.
 Therefore, if the offer includes any session capabilities, it SHOULD
 include all the session capabilities the offerer is willing to
 support.
 The session capability attribute is a session-level attribute
 described by
     "a=sescap:" <session num> <list of configs>

Gilman, et al. Standards Track [Page 30] RFC 6871 SDP Media Capabilities Negotiation February 2013

 which corresponds to the standard value attribute definition with
         att-field        = "sescap"
         att-value        = session-num 1*WSP list-of-configs
                            [1*WSP optional-configs]
         session-num      = NonZeroDigit *9(DIGIT)  ; DIGIT defined
                                                    ; in RFC 5234
         list-of-configs  = alt-config *("," alt-config)
         optional-configs = "[" list-of-configs "]"
         alt-config       = config-number *("|" config-number)
 The session-num identifies the session: a lower-number session is
 preferred over a higher-number session, and leading zeroes are not
 permitted.  Each alt-config list specifies alternative media
 configurations within the session; preference is based on config-num
 as specified in RFC 5939 [RFC5939].  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 MUST be
 unique across the entire session; RFC 5939 [RFC5939] requires only
 that "pcfg" configuration numbers be unique within a media
 description.  Also, leading zeroes are not permitted.
 As an example, consider an endpoint that is capable of supporting an
 audio stream with either one H.264 video stream or two H.263 video
 streams with a floor control stream.  In the latter case, the second
 video stream is optional.  The SDP offer might look like the
 following (offering audio, an H.263 video streams, BFCP and another
 optional H.263 video stream) -- the empty lines are added for
 readability only (not part of valid SDP):
    v=0
    o=- 25678 753849 IN IP4 192.0.2.1
    s=
    c=IN IP4 192.0.2.1
    t=0 0
    a=creq:med-v0
    a=sescap:2 1,2,5,[3]
    a=sescap:1 1,4
    m=audio 54322 RTP/AVP 0
    a=rtpmap:0 PCMU/8000
    a=pcfg:1

Gilman, et al. Standards Track [Page 31] RFC 6871 SDP Media Capabilities Negotiation February 2013

    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=label:11
    a=pcfg:2
    a=rmcap: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)
    a=label:12
    a=pcfg:3
    m=application 33002 TCP/BFCP *
    a=setup:passive
    a=connection:new
    a=floorid:1 m-stream:11 12
    a=floor-control:s-only
    a=confid:4321
    a=userid:1234
    a=pcfg:5
 If the answerer understands MediaCapNeg, but cannot support the
 Binary Floor Control Protocol, then it would respond with (invalid
 empty lines in SDP included again for readability):
    v=0
    o=- 25678 753849 IN IP4 192.0.2.1
    s=
    c=IN IP4 192.0.2.22
    t=0 0
    a=csup:med-v0
    a=sescap:1 1,4
    m=audio 23456 RTP/AVP 0
    a=rtpmap:0 PCMU/8000
    a=acfg:1
    m=video 41234 RTP/AVP 104
    a=rtpmap:104 H264/90000
    a=fmtp:104 profile-level-id=42A01E; packetization-mode=2
    a=acfg:4 m=1 a=1 pt=1:104

Gilman, et al. Standards Track [Page 32] RFC 6871 SDP Media Capabilities Negotiation February 2013

    m=video 0 RTP/AVP 103
    a=acfg:3
    m=application 0 TCP/BFCP *
    a=acfg:5
 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 can include latent capabilities as well.  Here's
 a similar example in which the offerer wishes to initially establish
 an audio stream, and prefers to later establish two video streams
 with chair control.  If the answerer doesn't understand Media CapNeg,
 or cannot support the dual video streams or flow control, then it may
 support a single H.264 video stream.  Note that establishment of the
 most favored configuration will require two offer/answer exchanges.
    v=0
    o=- 25678 753849 IN IP4 192.0.2.1
    s=
    c=IN IP4 192.0.2.1
    t=0 0
    a=creq:med-v0
    a=sescap:1 1,3,4,5
    a=sescap:2 1,2
    a=sescap:3 1
    a=rmcap:1 H263-1998/90000
    a=mfcap:1 CIF=4;QCIF=2;F=1;K=1
    a=tcap:1 RTP/AVP TCP/BFCP
    m=audio 54322 RTP/AVP 0
    a=rtpmap:0 PCMU/8000
    a=pcfg:1
    m=video 22344 RTP/AVP 102
    a=rtpmap:102 H264/90000
    a=fmtp:102 profile-level-id=42A01E; packetization-mode=2
    a=label:11
    a=content:main
    a=pcfg:2
    a=lcfg:3 mt=video t=1 m=1 a=31,32
    a=acap:31 label:12
    a=acap:32 content:main
    a=lcfg:4 mt=video t=1 m=1 a=41,42
    a=acap:41 label:13
    a=acap:42 content:slides

Gilman, et al. Standards Track [Page 33] RFC 6871 SDP Media Capabilities Negotiation February 2013

    a=lcfg:5 mt=application m=51 t=51
    a=tcap:51 TCP/BFCP
    a=omcap:51 *
    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
 In this example, the default offer, as seen by endpoints that do not
 understand capabilities negotiation, proposes a PCMU audio stream and
 an H.264 video stream.  Note that the offered lcfg lines for the
 video streams don't carry "pt=" parameters because they're not needed
 (payload type numbers will be assigned in the offer/answer exchange
 that establishes the streams).  Note also that the three "rmcap",
 "mfcap", and "tcap" attributes used by "lcfg:3" and "lcfg:4" are
 included at the session level so they may be referenced by both
 latent configurations.  As per Section 3.3, the media attributes
 generated from the "rmcap", "mfcap", and "tcap" attributes are always
 media-level attributes.  If the answerer supports Media CapNeg, and
 supports the most desired configuration, it would return the
 following SDP:
    v=0
    o=- 25678 753849 IN IP4 192.0.2.1
    s=
    c=IN IP4 192.0.2.22
    t=0 0
    a=csup:med-v0
    a=sescap:1 1,3,4,5
    a=sescap:2 1,2
    a=sescap:3 1
    m=audio 23456 RTP/AVP 0
    a=rtpmap:0 PCMU/8000
    a=acfg:1
    m=video 0 RTP/AVP 102
    a=pcfg:2
    a=lcfg:3 mt=video t=1 m=1 a=31,32
    a=lcfg:4 mt=video t=1 m=1 a=41,42
    a=lcfg:5 mt=application t=2
 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.

Gilman, et al. Standards Track [Page 34] RFC 6871 SDP Media Capabilities Negotiation February 2013

3.4. Offer/Answer Model Extensions

 In this section, we define extensions to the offer/answer model
 defined in RFC 3264 [RFC3264] and RFC 5939 [RFC5939] to allow for
 media format and associated parameter capabilities, latent
 configurations, and acceptable combinations of media stream
 configurations to be used with the SDP capability negotiation
 framework.  Note that the procedures defined in this section extend
 the offer/answer procedures defined in RFC 5939 [RFC5939] Section 6;
 those procedures form a baseline set of capability negotiation
 offer/answer procedures that MUST be followed, subject to the
 extensions defined here.
 SDP capability negotiation [RFC5939] provides a relatively compact
 means to offer the equivalent of an ordered list of alternative
 configurations for offered media streams (as would be described by
 separate "m=" lines and associated attributes).  The attributes
 "acap", "mscap", "mfcap", "omcap", and "rmcap" are designed to map
 somewhat straightforwardly into equivalent "m=" lines and
 conventional attributes when invoked by a "pcfg", "lcfg", or "acfg"
 attribute with appropriate parameters.  The "a=pcfg:" lines, along
 with the "m=" line itself, represent offered media configurations.
 The "a=lcfg:" lines represent alternative capabilities for future
 use.

3.4.1. Generating the Initial Offer

 The media capabilities negotiation extensions defined in this
 document cover the following categories of features:
 o  Media format capabilities and associated parameters ("rmcap",
    "omcap", "mfcap", and "mscap" attributes)
 o  Potential configurations using those media format capabilities and
    associated parameters
 o  Latent media streams ("lcfg" attribute)
 o  Acceptable combinations of media stream configurations ("sescap"
    attribute).
 The high-level description of the operation is as follows:
 When an endpoint generates an initial offer and wants to use the
 functionality described in the current document, it SHOULD identify
 and define the media formats and associated parameters it can support
 via the "rmcap", "omcap", "mfcap", and "mscap" attributes.  The SDP
 media line(s) ("m=") should be made up with the actual configuration

Gilman, et al. Standards Track [Page 35] RFC 6871 SDP Media Capabilities Negotiation February 2013

 to be used if the other party does not understand capability
 negotiations (by default, this is the least preferred configuration).
 Typically, the media line configuration will contain the minimum
 acceptable configuration from the offerer's point of view.
 Preferred configurations for each media stream are identified
 following the media line.  The present offer may also include latent
 configuration ("lcfg") attributes, at the media level, describing
 media streams and/or configurations the offerer is not now offering
 but that 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.
 Lastly, if the offerer wishes to impose restrictions on the
 combinations of potential configurations to be used, it will include
 session capability ("sescap") attributes indicating those.
 If the offerer requires the answerer to understand the media
 capability extensions, the offerer MUST include a "creq" attribute
 containing the value "med-v0".  If media capability negotiation is
 required only for specific media descriptions, the "med-v0" value
 MUST be provided only in "creq" attributes within those media
 descriptions, as described in RFC 5939 [RFC5939].
 Below, we provide a more detailed description of how to construct the
 offer SDP.

3.4.1.1. Offer with Media Capabilities

 For each RTP-based media format the offerer wants to include as a
 media format capability, the offer MUST include an "rmcap" attribute
 for the media format as defined in Section 3.3.1.
 For each non-RTP-based media format the offer wants to include as a
 media format capability, the offer MUST include an "omcap" attribute
 for the media format as defined in Section 3.3.1.
 Since the media capability number space is shared between the "rmcap"
 and "omcap" attributes, each media capability number provided
 (including ranges) MUST be unique in the entire SDP.
 If an "fmtp" parameter value is needed for a media format (whether or
 not it is RTP based) in a media capability, then the offer MUST
 include one or more "mfcap" parameters with the relevant "fmtp"
 parameter values for that media format as defined in Section 3.3.2.
 When multiple "mfcap" parameters are provided for a given media
 capability, they MUST be provided in accordance with the
 concatenation rules in Section 3.3.2.1.

Gilman, et al. Standards Track [Page 36] RFC 6871 SDP Media Capabilities Negotiation February 2013

 For each of the media format capabilities above, the offer MAY
 include one or more "mscap" parameters with attributes needed for
 those specific media formats as defined in Section 3.3.3.  Such
 attributes will be instantiated at the media level; hence, session-
 level-only attributes MUST NOT be used in the "mscap" parameter.  The
 "mscap" parameter MUST NOT include an "rtpmap" or "fmtp" attribute
 ("rmcap" and "mfcap" are used instead).
 If the offerer wants to limit the relevance (and use) of a media
 format capability or parameter to a particular media stream, the
 media format capability or parameter MUST be provided within the
 corresponding media description.  Otherwise, the media format
 capabilities and parameters MUST be provided at the session level.
 Note, however, that the attribute or parameter embedded in these will
 always be instantiated at the media level.
    This is due to those parameters being effectively media-level
    parameters.  If session-level attributes are needed, the "acap"
    attribute defined in RFC 5939 [RFC5939] can be used; however, it
    does not provide for media-format-specific instantiation.
 Inclusion of the above does not constitute an offer to use the
 capabilities; a potential configuration is needed for that.  If the
 offerer wants to offer one or more of the media capabilities above,
 they MUST be included as part of a potential configuration ("pcfg")
 attribute as defined in Section 3.3.4.  Each potential configuration
 MUST include a config-number, and each config-number MUST be unique
 in the entire SDP (note that this differs from RFC 5939 [RFC5939],
 which only requires uniqueness within a media description).  Also,
 the config-number MUST NOT overlap with any config-number used by a
 latent configuration in the SDP.  As described in RFC 5939 [RFC5939],
 lower config-numbers indicate a higher preference; the ordering still
 applies within a given media description only though.
 For a media capability to be included in a potential configuration,
 there MUST be an "m=" parameter in the "pcfg" attribute referencing
 the media capability number in question.  When one or more media
 capabilities are included in an offered potential configuration
 ("pcfg"), they completely replace the list of media formats offered
 in the actual configuration ("m=" line).  Any attributes included for
 those formats remain in the SDP though (e.g., "rtpmap", "fmtp",
 etc.).  For non-RTP-based media formats, the format-name (from the
 "omcap" media capability) is simply added to the "m=" line as a media
 format (e.g., t38).  For RTP-based media, payload type mappings MUST
 be provided by use of the "pt" parameter in the potential
 configuration (see Section 3.3.4.2); payload type escaping may be
 used in "mfcap", "mscap", and "acap" attributes as defined in Section
 3.3.7.

Gilman, et al. Standards Track [Page 37] RFC 6871 SDP Media Capabilities Negotiation February 2013

 Note that the "mt" parameter MUST NOT be used with the "pcfg"
 attribute (since it is defined for the "lcfg" attribute only); the
 media type in a potential configuration cannot be changed from that
 of the encompassing media description.

3.4.1.2. Offer with Latent Configuration

 If the offerer wishes to offer one or more latent configurations for
 future use, the offer MUST include a latent configuration attribute
 ("lcfg") for each as defined in Section 3.3.6.
 Each "lcfg" attribute
 o  MUST be specified at the media level
 o  MUST include a config-number that is unique in the entire SDP
    (including for any potential configuration attributes).  Note that
    config-numbers in latent configurations do not indicate any
    preference order
 o  MUST include a media type ("mt")
 o  MUST reference a valid transport capability ("t")
 Each "lcfg" attribute MAY include additional capability references,
 which may refer to capabilities anywhere in the session description,
 subject to any restrictions normally associated with such
 capabilities.  For example, a media-level attribute capability must
 be present at the media level in some media description in the SDP.
 Note that this differs from the potential configuration attribute,
 which cannot validly refer to media-level capabilities in another
 media description (per RFC 5939 [RFC5939], Section 3.5.1).
    Potential configurations constitute an actual offer and may
    instantiate a referenced capability.  Latent configurations are
    not actual offers; hence, they cannot instantiate a referenced
    capability.  Therefore, it is safe for those to refer to
    capabilities in another media description.

3.4.1.3. Offer with Configuration Combination Restrictions

 If the offerer wants to indicate restrictions or preferences among
 combinations of potential and/or latent configurations, a session
 capability ("sescap") attribute MUST be provided at the session level
 for each such combination as described in Section 3.3.8.  Each
 "sescap" attribute MUST include a session-num that is unique in the

Gilman, et al. Standards Track [Page 38] RFC 6871 SDP Media Capabilities Negotiation February 2013

 entire SDP; the lower the session-num the more preferred that
 combination is.  Furthermore, "sescap" preference order takes
 precedence over any order specified in individual "pcfg" attributes.
    For example, if we have pcfg-1 and pcfg-2, and sescap-1 references
    pcfg-2, whereas sescap-2 references pcfg-1, then pcfg-2 will be
    the most preferred potential configuration.  Without the sescap,
    pcfg-1 would be the most preferred.

3.4.2. Generating the Answer

 When receiving an offer, the answerer MUST check the offer for "creq"
 attributes containing the value "med-v0"; answerers compliant with
 this specification will support this value in accordance with the
 procedures specified in RFC 5939 [RFC5939].
 The SDP MAY contain
 o  Media format capabilities and associated parameters ("rmcap",
    "omcap", "mfcap", and "mscap" attributes)
 o  Potential configurations using those media format capabilities and
    associated parameters
 o  Latent media streams ("lcfg" attribute)
 o  Acceptable combinations of media stream configurations ("sescap"
    attribute)
 The high-level informative description of the operation is as
 follows:
 When the answering party receives the offer, if it supports the
 required capability negotiation extensions, it should select the
 most-preferred configuration it can support for each media stream,
 and build its answer accordingly.  The configuration selected for
 each accepted media stream is placed into the answer as a media line
 with associated parameters and attributes.  If a proposed
 configuration is chosen for a given media stream, the answer must
 contain an actual configuration ("acfg") attribute for that media
 stream to indicate which offered "pcfg" attribute was used to build
 the answer.  The answer should also include any potential or latent
 configurations the answerer can support, especially any
 configurations compatible with other potential or latent
 configurations received in the offer.  The answerer should make note
 of those configurations it might wish to offer in the future.

Gilman, et al. Standards Track [Page 39] RFC 6871 SDP Media Capabilities Negotiation February 2013

 Below we provide a more detailed normative description of how the
 answerer processes the offer SDP and generates an answer SDP.

3.4.2.1. Processing Media Capabilities and Potential Configurations

 The answerer MUST first determine if it needs to perform media
 capability negotiation by examining the SDP for valid and preferred
 potential configuration attributes that include media configuration
 parameters (i.e., an "m" parameter in the "pcfg" attribute).
 Such a potential configuration is valid if
 1.  It is valid according to the rules defined in RFC 5939 [RFC5939].
 2.  It contains a config-number that is unique in the entire SDP and
     does not overlap with any latent configuration config-numbers.
 3.  All media format capabilities ("rmcap" or "omcap"), media format
     parameter capabilities ("mfcap"), and media-specific capabilities
     ("mscap") referenced by the potential configuration ("m"
     parameter) are valid themselves (as defined in Sections 3.3.1,
     3.3.2, and 3.3.3) and each of them is provided either at the
     session level or within this particular media description.
 4.  All RTP-based media format capabilities ("rmcap") have a
     corresponding payload type ("pt") parameter in the potential
     configuration that results in mapping to a valid payload type
     that is unique within the resulting SDP.
 5.  Any concatenation (see Section 3.3.2.1) and substitution (see
     Section 3.3.7) applied to any capability ("mfcap", "mscap", or
     "acap") referenced by this potential configuration results in a
     valid SDP.
 Note that since SDP does not interpret the value of "fmtp"
 parameters, any resulting "fmtp" parameter value will be considered
 valid.
 Secondly, the answerer MUST determine the order in which potential
 configurations are to be negotiated.  In the absence of any session
 capability ("sescap") attributes, this simply follows the rules of
 RFC 5939 [RFC5939], with a lower config-number within a media
 description being preferred over a higher one.  If a valid "sescap"
 attribute is present, the preference order provided in the "sescap"
 attribute MUST take precedence.  A "sescap" attribute is considered
 valid if

Gilman, et al. Standards Track [Page 40] RFC 6871 SDP Media Capabilities Negotiation February 2013

 1.  It adheres to the rules provided in Section 3.3.8.
 2.  All the configurations referenced by the "sescap" attribute are
     valid themselves (note that this can include the actual,
     potential, and latent configurations).
 The answerer MUST now process the offer for each media stream based
 on the most preferred valid potential configuration in accordance
 with the procedures specified in RFC 5939 [RFC5939], Section 3.6.2,
 and further extended below:
 o  If one or more media format capabilities are included in the
    potential configuration, then they replace all media formats
    provided in the "m=" line for that media description.  For non-
    RTP-based media formats ("omcap"), the format-name is added.  For
    RTP-based media formats ("rmcap"), the payload-type specified in
    the payload-type mapping ("pt") is added and a corresponding
    "rtpmap" attribute is added to the media description.
 o  If one or more media format parameter capabilities are included in
    the potential configuration, then the corresponding "fmtp"
    attributes are added to the media description.  Note that this
    inclusion is done indirectly via the media format capability.
 o  If one or more media-specific capabilities are included in the
    potential configuration, then the corresponding attributes are
    added to the media description.  Note that this inclusion is done
    indirectly via the media format capability.
 o  When checking to see if the answerer supports a given potential
    configuration that includes one or more media format capabilities,
    the answerer MUST support at least one of the media formats
    offered.  If he does not, the answerer MUST proceed to the next
    potential configuration based on the preference order that
    applies.
 o  If session capability ("sescap") preference ordering is included,
    then the potential configuration selection process MUST adhere to
    the ordering provided.  Note that this may involve coordinated
    selection of potential configurations between media descriptions.
    The answerer MUST accept one of the offered sescap combinations
    (i.e., all the required potential configurations specified) or it
    MUST reject the entire session.

Gilman, et al. Standards Track [Page 41] RFC 6871 SDP Media Capabilities Negotiation February 2013

 Once the answerer has selected a valid and supported offered
 potential configuration for all of the media streams (or has fallen
 back to the actual configuration plus any added session attributes),
 the answerer MUST generate a valid answer SDP as described in RFC
 5939 [RFC5939], Section 3.6.2, and further extended below:
 o  Additional answer capabilities and potential configurations MAY be
    returned in accordance with Section 3.3.6.1.  Capability numbers
    and configuration numbers for those MUST be distinct from the ones
    used in the offer SDP.
 o  Latent configuration processing and answer generation MUST be
    performed, as specified below.
 o  Session capability specification for the potential and latent
    configurations in the answer MAY be included (see Section 3.3.8).

3.4.2.2. Latent Configuration Processing

 The answerer MUST determine if it needs to perform any latent
 configuration processing by examining the SDP for valid latent
 configuration attributes ("lcfg").  An "lcfg" attribute is considered
 valid if:
 o  It adheres to the description in Section 3.3.5.
 o  It includes a config-number that is unique in the entire SDP and
    does not overlap with any potential configuration config-number.
 o  It includes a valid media type ("mt=").
 o  It references a valid transport capability ("t=").
 o  All other capabilities referenced by it are valid.
 For each such valid latent configuration in the offer, the answerer
 checks to see if it could support the latent configuration in a
 subsequent offer/answer exchange.  If so, it includes the latent
 configuration with the same configuration number in the answer,
 similar to the way potential configurations are processed and the
 selected one returned in an actual configuration attribute (see RFC
 5939 [RFC5939]).  If the answerer supports only a (non-mandatory)
 subset of the parameters offered in a latent configuration, the
 answer latent configuration will include only those parameters
 supported (similar to "acfg" processing).  Note that latent
 configurations do not constitute an actual offer at this point in
 time; they merely indicate additional configurations that could be
 supported.

Gilman, et al. Standards Track [Page 42] RFC 6871 SDP Media Capabilities Negotiation February 2013

 If a session capability ("sescap") attribute is included and it
 references a latent configuration, then the answerer processing of
 that latent configuration must be done within the constraints
 specified by that session capability.  That is, it must be possible
 to support it at the same time as any required (i.e., non-optional)
 potential configurations in the session capability.  The answerer may
 in turn add his own sescap indications in the answer as well.

3.4.3. Offerer Processing of the Answer

 The offerer MUST process the answer in accordance with Section 3.6.3
 of RFC 5939 [RFC5939] and the further explanation below.
 When the offerer processes the answer SDP based on a valid actual
 configuration attribute in the answer, and that valid configuration
 includes one or more media capabilities, the processing MUST
 furthermore be done as if the offer was sent using those media
 capabilities instead of the actual configuration.  In particular, the
 media formats in the "m=" line, and any associated payload type
 mappings ("rtpmap"), "fmtp" parameters ("mfcap"), and media-specific
 attributes ("mscap") MUST be used.  Note that this may involve use of
 concatenation and substitution rules (see Sections 3.3.2.1 and
 3.3.7).  The actual configuration attribute may also be used to infer
 the lack of acceptability of higher-preference configurations that
 were not chosen, subject to any constraints provided by a session
 capability ("sescap") attribute in the offer.  Note that the SDP
 capability negotiation base specification [RFC5939] requires the
 answerer to choose the highest-preference configuration it can
 support, subject to local policies.
 When the offerer receives the answer, it SHOULD furthermore make note
 of any capabilities and/or latent configurations included for future
 use, and any constraints on how those may be combined.

3.4.4. Modifying the Session

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

Gilman, et al. Standards Track [Page 43] RFC 6871 SDP Media Capabilities Negotiation February 2013

 In such a case, the new capabilities will replace the previously
 negotiated capabilities.  This may be useful if conditions change on
 the endpoint.

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 provides a choice of one of six variations of the
 Adaptive Multi-Rate codec.  In this example, the default
 configuration as specified by the media line is the same as the most
 preferred configuration.  Each configuration uses a different payload
 type number so the offerer can interpret early media.
    v=0
    o=- 25678 753849 IN IP4 192.0.2.1
    s=
    c=IN IP4 192.0.2.1
    t=0 0
    a=creq:med-v0
    m=audio 54322 RTP/AVP 96
    a=rtpmap:96 AMR-WB/16000/1
    a=fmtp:96 mode-change-capability=1; max-red=220; \
    mode-set=0,2,4,7
    a=rmcap:1,3,5 audio AMR-WB/16000/1
    a=rmcap:2,4,6 audio AMR/8000/1
    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
    a=pcfg:1 m=1 pt=1:96
    a=pcfg:2 m=2 pt=2:97
    a=pcfg:3 m=3 pt=3:98
    a=pcfg:4 m=4 pt=4:99
    a=pcfg:5 m=5 pt=5:100
    a=pcfg:6 m=6 pt=6:101
 In the above example, media capability 1 could have been excluded
 from the first "rmcap" declaration and from the corresponding "mfcap"
 attributes, and the "pcfg:1" attribute line could have been simply
 "pcfg:1".

Gilman, et al. Standards Track [Page 44] RFC 6871 SDP Media Capabilities Negotiation February 2013

 The next example offers a video stream with three options of H.264
 and four transports.  It also includes an audio stream with different
 audio qualities: four variations of AMR, or AC3.  The offer looks
 something like the following:
    v=0
    o=- 25678 753849 IN IP4 192.0.2.1
    s=An SDP Media NEG example
    c=IN IP4 192.0.2.1
    t=0 0
    a=creq:med-v0
    a=ice-pwd:speEc3QGZiNWpVLFJhQX
    m=video 49170 RTP/AVP 100
    c=IN IP4 192.0.2.56
    a=maxprate:1000
    a=rtcp:51540
    a=sendonly
    a=candidate 12345 1 UDP 9 192.0.2.56 49170 host
    a=candidate 23456 2 UDP 9 192.0.2.56 51540 host
    a=candidate 34567 1 UDP 7 198.51.100.1 41345 srflx raddr \
    192.0.2.56 rport 49170
    a=candidate 45678 2 UDP 7 198.51.100.1 52567 srflx raddr \
    192.0.2.56 rport 51540
    a=candidate 56789 1 UDP 3 192.0.2.100 49000 relay raddr \
    192.0.2.56 rport 49170
    a=candidate 67890 2 UDP 3 192.0.2.100 49001 relay raddr \
    192.0.2.56 rport 51540
    b=AS:10000
    b=TIAS:10000000
    b=RR:4000
    b=RS:3000
    a=rtpmap:100 H264/90000
    a=fmtp:100 profile-level-id=42A01E; packetization-mode=2; \
    sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==; \
    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=rmcap:1-3,7-9 H264/90000
    a=rmcap: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-req=64000; sprop-init-buf-time=102478; \
    deint-buf-cap=128000

Gilman, et al. Standards Track [Page 45] RFC 6871 SDP Media Capabilities Negotiation February 2013

    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
    a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_80 \
    inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|220|1:32
    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 192.0.2.56
    a=ptime:60
    a=maxptime:200
    a=rtcp:51534
    a=sendonly
    a=candidate 12345 1 UDP 9 192.0.2.56 49176 host
    a=candidate 23456 2 UDP 9 192.0.2.56 51534 host
    a=candidate 34567 1 UDP 7 198.51.100.1 41348 srflx \
    raddr 192.0.2.56 rport 49176
    a=candidate 45678 2 UDP 7 198.51.100.1 52569 srflx \
    raddr 192.0.2.56 rport 51534
    a=candidate 56789 1 UDP 3 192.0.2.100 49002 relay \
    raddr 192.0.2.56 rport 49176
    a=candidate 67890 2 UDP 3 192.0.2.100 49003 relay \
    raddr 192.0.2.56 rport 51534
    b=AS:512
    b=TIAS:512000
    b=RR:4000
    b=RS:3000
    a=maxprate:120
    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=rmcap:14 ac3/48000/6
    a=acap:23 crypto:1 AES_CM_128_HMAC_SHA1_80 \
    inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|220|1:32

Gilman, et al. Standards Track [Page 46] RFC 6871 SDP Media Capabilities Negotiation February 2013

    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
 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:
    v=0
    o=- 25678 753849 IN IP4 192.0.2.1
    s=
    c=IN IP4 192.0.2.1
    t=0 0
    a=creq:med-v0
    a=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=rmcap:1 PCMU/8000
    a=pcfg:1 m=1 pt=1:0
    a=pcfg:2
    m=video 54344 RTP/AVP 100
    a=rtpmap:100 H263-1998/90000
    a=rmcap:2 H264/90000
    a=mfcap:2 profile-level-id=42A01E; packetization-mode=2
    a=pcfg:3 m=2 pt=2:101
    a=pcfg:4
 Note that the preferred session configuration (and the default as
 well) is G.729B with H.263.  This overrides the individual media
 stream preferences that are PCMU and H.264 by the potential
 configuration numbering rule.

4.3. Latent Media Streams

 Consider a case in which the offerer can support either G.711 mu-law
 or G.729B, along with DTMF telephony events for the 12 common
 touchtone signals, but is willing to support simple G.711 mu-law

Gilman, et al. Standards Track [Page 47] RFC 6871 SDP Media Capabilities Negotiation February 2013

 audio as a last resort.  In addition, the offerer wishes to announce
 its ability to support video and Message Session Relay Protocol
 (MSRP) in the future, but does not wish to offer a video stream or an
 MSRP stream at present.  The offer might look like the following:
    v=0
    o=- 25678 753849 IN IP4 192.0.2.1
    s=
    c=IN IP4 192.0.2.1
    t=0 0
    a=creq:med-v0
    m=audio 23456 RTP/AVP 0
    a=rtpmap:0 PCMU/8000
    a=rmcap:1 PCMU/8000
    a=rmcap:2 G729/8000
    a=rmcap:3 telephone-event/8000
    a=mfcap:3 0-11
    a=pcfg:1 m=1,3|2,3 pt=1:0,2:18,3:100
    a=lcfg:2 mt=video t=1 m=10|11
    a=rmcap:10 H263-1998/90000
    a=rmcap:11 H264/90000
    a=tcap:1 RTP/AVP
    a=lcfg:3 mt=message t=2 m=20
    a=tcap:2 TCP/MSRP
    a=omcap:20 *
 The first "lcfg" attribute line ("lcfg:2") announces support for
 H.263 and H.264 video (H.263 preferred) for future negotiation.  The
 second "lcfg" attribute line ("lcfg:3") announces support for MSRP
 for future negotiation.  The "m=" line and the "rtpmap" attribute
 offer an audio stream and provide the lowest precedence configuration
 (PCMU without any DTMF encoding).  The rmcap lines define the RTP-
 based media format capabilities (PCMU, G729, telephone-event,
 H263-1998, and H264) and the omcap line defines the non-RTP-based
 media format capability (wildcard).  The "mfcap" attribute provides
 the format parameters for telephone-event, specifying the 12
 commercial DTMF 'digits'.  The "pcfg" attribute line defines the
 most-preferred media configuration as PCMU plus DTMF events and the
 next-most-preferred configuration as G.729B plus DTMF events.
 If the answerer is able to support all the potential configurations,
 and also support H.263 video (but not H.264), it would reply with an
 answer like the following:

Gilman, et al. Standards Track [Page 48] RFC 6871 SDP Media Capabilities Negotiation February 2013

    v=0
    o=- 24351 621814 IN IP4 192.0.2.2
    s=
    c=IN IP4 192.0.2.2
    t=0 0
    a=csup:med-v0
    m=audio 54322 RTP/AVP 0 100
    a=rtpmap:0 PCMU/8000
    a=rtpmap:100 telephone-event/8000
    a=fmtp:100 0-11
    a=acfg:1 m=1,3 pt=1:0,3:100
    a=pcfg:1 m=2,3 pt=2:18,3:100
    a=lcfg:2 mt=video t=1 m=10
 The "lcfg" attribute line announces the capability to support H.263
 video at a later time.  The media line and subsequent "rtpmap" and
 "fmtp" attribute lines present the selected configuration for the
 media stream.  The "acfg" attribute line identifies the potential
 configuration from which it was taken, and the "pcfg" attribute line
 announces the potential capability to support G.729 with DTMF events
 as well.  If, at some later time, congestion becomes a problem in the
 network, either party may, with expectation of success, offer a
 reconfiguration of the media stream to use G.729 in order to reduce
 packet sizes.

5. IANA Considerations

5.1. New SDP Attributes

 IANA has registered the following new SDP attributes:
    Attribute name: rmcap
    Long form name: RTP-based media format capability
    Type of attribute: session-level and media-level
    Subject to charset: no
    Purpose: associate RTP-based media capability number(s) with
    media subtype and encoding parameters
    Appropriate Values: see Section 3.3.1
    Contact name: Flemming Andreasen, fandres@cisco.com
    Attribute name: omcap
    Long form name: non-RTP-based media format capability
    Type of attribute: session-level and media-level
    Subject to charset: no
    Purpose: associate non-RTP-based media capability number(s) with
    media subtype and encoding parameters
    Appropriate Values: see Section 3.3.1
    Contact name: Flemming Andreasen, fandreas@cisco.com

Gilman, et al. Standards Track [Page 49] RFC 6871 SDP Media Capabilities Negotiation February 2013

    Attribute name: mfcap
    Long form name: media format parameter 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
    Contact name: Flemming Andreasen, fandreas@cisco.com
    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
    Contact name: Flemming Andreasen, fandreas@cisco.com
    Attribute name: lcfg
    Long form name: latent configuration
    Type of attribute: media-level
    Subject to charset: no
    Purpose: to announce supportable media streams
    without offering them for immediate use.
    Appropriate Values: see Section 3.3.5
    Contact name: Flemming Andreasen, fandreas@cisco.com
    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
    Contact name: Flemming Andreasen, fandreas@cisco.com

5.2. New SDP Capability Negotiation Option Tag

 IANA has added the new option tag "med-v0", defined in this document,
 to the "SDP Capability Negotiation Option Capability Tags" registry
 created for RFC 5939 [RFC5939].

5.3. SDP Capability Negotiation Configuration Parameters Registry

 IANA has changed the "SDP Capability Negotiation Potential
 Configuration Parameters" registry, currently registered and defined
 by RFC 5939 [RFC5939], as follows:

Gilman, et al. Standards Track [Page 50] RFC 6871 SDP Media Capabilities Negotiation February 2013

 The name of the registry should be "SDP Capability Negotiation
 Configuration Parameters Registry" and it should contain a table with
 the following column headings:
 o  Encoding Name: The syntactical value used for the capability
    negotiation configuration parameter, as defined in RFC 5939
    [RFC5939], Section 3.5.
 o  Descriptive Name: The name commonly used to refer to the
    capability negotiation configuration parameter.
 o  Potential Configuration Definition: A reference to the RFC that
    defines the configuration parameter in the context of a potential
    configuration attribute.  If the configuration parameter is not
    defined for potential configurations, the string "N/A" (Not
    Applicable) MUST be present instead.
 o  Actual Configuration Definition: A reference to the RFC that
    defines the configuration parameter in the context of an actual
    configuration attribute.  If the configuration parameter is not
    defined for actual configurations, the string "N/A" (Not
    Applicable) MUST be present instead.
 o  Latent Configuration Definition: A reference to the RFC that
    defines the configuration parameter in the context of a latent
    configuration attribute.  If the configuration parameter is not
    defined for latent configurations, the string "N/A" (Not
    Applicable) MUST be present instead.
 An IANA SDP Capability Negotiation Configuration registration MUST be
 documented in an RFC in accordance with the IETF Review policy
 [RFC5226].  Furthermore:
 o  The RFC MUST define the syntax and semantics of each new potential
    configuration parameter.
 o  The syntax MUST adhere to the syntax provided for extension
    configuration lists in RFC 5939 [RFC5939], Section 3.5.1, and the
    semantics MUST adhere to the semantics provided for extension
    configuration lists in RFC 5939 [RFC5939], Sections 3.5.1 and
    3.5.2.
 o  Configuration parameters that apply to latent configurations MUST
    furthermore adhere to the syntax provided in Section 3.3.5 and the
    semantics defined overall in this document.
 o  Associated with each registration MUST be the encoding name for
    the parameter as well as a short descriptive name for it.

Gilman, et al. Standards Track [Page 51] RFC 6871 SDP Media Capabilities Negotiation February 2013

 o  Each registration MUST specify if it applies to
  • Potential configurations
  • Actual configurations
  • Latent configurations

5.4. SDP Capability Negotiation Configuration Parameter Registrations

 IANA has registered the following capability negotiation
 configuration parameters:
    Encoding Name: a
    Descriptive Name: Attribute Configuration
    Potential Configuration Definition: [RFC5939]
    Actual Configuration Definition: [RFC5939]
    Latent Configuration Definition: [RFC6871]
    Encoding Name: t
    Descriptive Name: Transport Protocol Configuration
    Potential Configuration Definition: [RFC5939]
    Actual Configuration Definition: [RFC5939]
    Latent Configuration Definition: [RFC6871]
    Encoding Name: m
    Descriptive Name: Media Configuration
    Potential Configuration Definition: [RFC6871]
    Actual Configuration Definition: [RFC6871]
    Latent Configuration Definition: [RFC6871]
    Encoding Name: pt
    Descriptive Name: Payload Type Number Mapping
    Potential Configuration Definition: [RFC6871]
    Actual Configuration Definition: [RFC6871]
    Latent Configuration Definition: [RFC6871]
    Encoding Name: mt
    Descriptive Name: Media Type
    Potential Configuration Definition: N/A
    Actual Configuration Definition: N/A
    Latent Configuration Definition: [RFC6871]

6. Security Considerations

 The security considerations of RFC 5939 [RFC5939] apply for this
 document.

Gilman, et al. Standards Track [Page 52] RFC 6871 SDP Media Capabilities Negotiation February 2013

 In RFC 5939 [RFC5939], it was noted that negotiation of transport
 protocols (e.g., secure and non-secure) and negotiation of keying
 methods and material are potential security issues that warrant
 integrity protection to remedy.  Latent configuration support
 provides hints to the other side about capabilities supported for
 further offer/answer exchanges, including transport protocols and
 attribute capabilities, e.g., for keying methods.  If an attacker can
 remove or alter latent configuration information to suggest that only
 non-secure or less-secure alternatives are supported, then he may be
 able to force negotiation of a less secure session than would
 otherwise have occurred.  While the specific attack, as described
 here, differs from those described in RFC 5939 [RFC5939], the
 considerations and mitigation strategies are similar to those
 described in RFC 5939 [RFC5939].
 Another variation on the above attack involves the session capability
 ("sescap") attribute defined in this document.  The "sescap" enables
 a preference order to be specified for all the potential
 configurations, and that preference will take precedence over any
 preference indication provided in individual potential configuration
 attributes.  Consequently, an attacker that can insert or modify a
 "sescap" attribute may be able to force negotiation of an insecure or
 less secure alternative than would otherwise have occurred.  Again,
 the considerations and mitigation strategies are similar to those
 described in RFC 5939 [RFC5939].
 The addition of negotiable media formats and their associated
 parameters, defined in this specification can cause problems for
 middleboxes that attempt to control bandwidth utilization, media
 flows, and/or processing resource consumption as part of network
 policy, but that do not understand the media capability negotiation
 feature.  As for the initial SDP capability negotiation work
 [RFC5939], the SDP answer is formulated in such a way that it always
 carries the selected media encoding for every media stream selected.
 Pending an understanding of capabilities negotiation, the middlebox
 should examine the answer SDP to obtain the best picture of the media
 streams being established.  As always, middleboxes can best do their
 job if they fully understand media capabilities negotiation.

7. 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.

Gilman, et al. Standards Track [Page 53] RFC 6871 SDP Media Capabilities Negotiation February 2013

 We thank Ingemar Johansson and Magnus Westerlund for examples that
 stimulated this work, and for critical reading of the document.  We
 also thank Cullen Jennings, Christer Holmberg, and Miguel Garcia for
 their review of the document.

8. References

8.1. 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.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            May 2008.
 [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234, January 2008.
 [RFC5939]  Andreasen, F., "Session Description Protocol (SDP)
            Capability Negotiation", RFC 5939, September 2010.

8.2. Informative References

 [RFC2198]  Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
            Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse-
            Parisis, "RTP Payload for Redundant Audio Data", RFC 2198,
            September 1997.
 [RFC4568]  Andreasen, F., Baugher, M., and D. Wing, "Session
            Description Protocol (SDP) Security Descriptions for Media
            Streams", RFC 4568, July 2006.
 [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
            "Extended RTP Profile for Real-time Transport Control
            Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July
            2006.
 [RFC4733]  Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF
            Digits, Telephony Tones, and Telephony Signals", RFC 4733,
            December 2006.

Gilman, et al. Standards Track [Page 54] RFC 6871 SDP Media Capabilities Negotiation February 2013

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

Authors' Addresses

 Robert R Gilman
 Independent
 3243 W. 11th Ave. Dr.
 Broomfield, CO 80020
 USA
 EMail: bob_gilman@comcast.net
 Roni Even
 Huawei Technologies
 14 David Hamelech
 Tel Aviv  64953
 Israel
 EMail: roni.even@mail01.huawei.com
 Flemming Andreasen
 Cisco Systems
 Iselin, NJ
 USA
 EMail: fandreas@cisco.com

Gilman, et al. Standards Track [Page 55]

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