MMUSIC Working Group                                       F. Andreasen
     Internet Draft                                            Cisco Systems
     Expires: July 2007                                     January 2, 28, 2007

                             SDP Capability Negotiation
                 draft-ietf-mmusic-sdp-capability-negotiation-00.txt
                 draft-ietf-mmusic-sdp-capability-negotiation-01.txt

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     Abstract

        The Session Description Protocol (SDP) was intended for describing
        multimedia sessions for the purposes of session announcement, session
        invitation, and other forms of multimedia session initiation. SDP was
        not intended to provide capability indication or capability
        negotiation, however over the years, SDP has seen widespread adoption
        and as a result it has been gradually extended to provide limited
        support for these. SDP and its current extensions however do not have
        the ability to negotiate one or more alternative transport protocols
        (e.g. RTP profiles) which makes it particularly difficult to deploy
        new RTP profiles such as secure RTP or RTP with RTCP-based feedback.

        The purpose of this document is to address that and other real-life
        limitations by extending SDP with capability negotiation parameters
        and associated offer/answer procedures to use those parameters in a
        backwards compatible manner.

        The solution provided in this document provides a general SDP
        capability negotiation framework. It also defines specifically how to
        provide attributes and transport protocols as capabilities and
        negotiate them using the framework. Extensions for other types of
        capabilities (e.g. media types and formats) may be provided in other
        documents.

     Conventions used in this document

        The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
        "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
        document are to be interpreted as described in [RFC2119].

     Table of Contents

        1. Introduction...................................................3
        2. SDP Capability Negotiation Solution............................5
           2.1. Solution Overview.........................................5
           2.2. Version and Extension Indication Attributes...............8
              2.2.1. SDP Capability Negotiation Version Attribute.........8
              2.2.2. Supported Capability Negotiation Extensions Attribute9
              2.2.3. Attribute8
              2.2.2. Required Capability Negotiation Extension Attribute.10
           2.3. Capability Attributes....................................11
              2.3.1. Media Type and Format Capability Attribute..........11
              2.3.2. Attribute Parameter Capability Attribute............14
              2.3.3. Attribute......................11
              2.3.2. Transport Protocol Capability Attribute.............15 Attribute.............13
           2.4. Configuration Attributes.................................16 Attributes.................................13
              2.4.1. Potential Configuration Attribute...................16 Attribute...................13
              2.4.2. Actual Configuration Attribute......................19 Attribute......................17
           2.5. Offer/Answer Model Extensions............................20 Extensions............................18
              2.5.1. Generating the Initial Offer........................21 Offer........................18
              2.5.2. Generating the Answer...............................22 Answer...............................19
              2.5.3. Offerer Processing of the Answer....................22 Answer....................20
              2.5.4. Modifying the Session...............................23 Session...............................20
        3. Examples......................................................23 Examples......................................................21
           3.1. Best-Effort Secure RTP...................................23 RTP...................................21
        4. Security Considerations.......................................25 Considerations.......................................23
        5. IANA Considerations...........................................25 Considerations...........................................23
        6. To Do and Open Issues.........................................26 Issues.........................................23
        7. Acknowledgments...............................................26 Acknowledgments...............................................23
        8. Change Log....................................................26 Log....................................................24
           8.1. draft-ietf-mmusic-sdp-capability-negotiation-00..........26 draft-ietf-mmusic-sdp-capability-negotiation-01..........24
           8.2. draft-ietf-mmusic-sdp-capability-negotiation-00..........24
        9. References....................................................27 References....................................................26
           9.1. Normative References.....................................27 References.....................................26
           9.2. Informative References...................................27 References...................................26
        Author's Addresses...............................................29 Addresses...............................................28
        Intellectual Property Statement..................................29 Statement..................................28
        Disclaimer of Validity...........................................30 Validity...........................................29
        Copyright Statement..............................................30
        Acknowledgment...................................................30 Statement..............................................29
        Acknowledgment...................................................29

     1. Introduction

        The Session Description Protocol (SDP) was intended for describing
        multimedia sessions for the purposes of session announcement, session
        invitation, and other forms of multimedia session initiation. The SDP
        contains one or more media stream descriptions with information such
        as IP-address and port, type of media stream (e.g. audio or video),
        transport protocol (possibly including profile information, e.g.
        RTP/AVP or RTP/SAVP), media formats (e.g. codecs), and various other
        session and media stream parameters that define the session.

        Simply providing media stream descriptions is sufficient for session
        announcements for a broadcast application, where the media stream
        parameters are fixed for all participants. When a participant wants
        to join the session, he obtains the session announcement and uses the
        media descriptions provided, e.g., joins a multicast group and
        receives media packets in the encoding format specified.  If the
        media stream description is not supported by the participant, he is
        unable to receive the media.

        Such restrictions are not generally acceptable to multimedia session
        invitations, where two or more entities attempt to establish a media
        session that uses a set of media stream parameters acceptable to all
        participants. First of all, each entity must inform the other of its
        receive address, and secondly, the entities need to agree on the
        media stream parameters to use for the session, e.g. transport
        protocols and codecs. We here make a distinction between the
        capabilities supported by each participant participant, the way in which those
        capabilities can be supported and the parameters that can actually be
        used for the session. More generally, we can say that we have the
        following:

        o  A set of capabilities and potential configurations of for the session and its associated media
           stream components, supported by each side.

        o  A set of actual potential configurations indicating which of those
           capabilities can be used for the session and its associated media
           stream components.

        o  A set of actual configurations for the session and its associated
           media stream components, which specifies which session parameters
           to use and which media stream components to use and with what
           parameters.

        o  A negotiation process that takes the set of potential
           configurations (capabilities) (lists of capabilities) as input and provides the
           actual configurations as output.

        SDP by itself was designed to provide only the second one of these, i.e., namely the
        actual configurations, however over the years, use of SDP has been
        extended beyond its original scope.  Session negotiation semantics
        were defined by the offer/answer model in RFC 3264.  It defines how
        two entities, an offerer and an answerer, exchange session
        descriptions to negotiate a session. The offerer can include one or
        more media formats (codecs) per media stream, and the answerer then
        selects one or more of those offered and returns them in an answer.
        Both the offer and the answer contain actual configurations -
        capabilities and potential configurations are not supported. The
        answer however may reduce the set of actual configurations from the
        offer. The answer may also extend the set of actual configurations
        that can be used to receive media by the answerer.

        Other relevant extensions have been defined. Simple capability
        declarations, which define how to provide a simple and limited set of
        capability descriptions in SDP was defined in RFC 3407.  Grouping of
        media lines, which defines how media lines in SDP can have other
        semantics than the traditional "simultaneous media streams"
        semantics, was defined in RFC 3388, etc.

        Each of these extensions was designed to solve a specific limitation
        of SDP.  Since SDP had already been stretched beyond its original
        intent, a more comprehensive capability declaration and negotiation
        process was intentionally not defined.  Instead, work on a "next
        generation" of a protocol to provide session description and
        capability negotiation was initiated [SDPng].  SDPng however has not
        gained traction and has remained as work in progress for an extended
        period of time.  Existing real-time multimedia communication
        protocols such as SIP, RTSP, Megaco, and MGCP continue to use SDP.
        SDP and its current extensions however do not address an increasingly
        important problem: the ability to negotiate one or more alternative
        transport protocols (e.g., RTP profiles).  This makes it difficult to
        deploy new RTP profiles such as secure RTP (SRTP) [SRTP], RTP with
        RTCP-Based Feedback [AVPF], etc.  This particular problem is
        exacerbated by the fact that RTP profiles are defined independently.
        When a new profile is defined and N other profiles already exist,
        there is a potential need for defining N additional profiles, since
        profiles cannot be combined automatically.  For example, in order to
        support the plain and secure RTP version of RTP with and without
        RTCP-based feedback, four separate profiles (and hence profile
        definitions) are needed: RTP/AVP [RFC3551], RTP/SAVP [SRTP], RTP/AVPF
        [AVPF], and RTP/SAVPF [SAVPF].  In addition to the pressing profile
        negotiation problem, other important real-life constraints have been
        found as well.

        The purpose of this document is to define a mechanism that enables
        SDP to provide limited support for indicating capabilities and their
        associated potential configurations and negotiate the use of those
        potential configurations as actual configurations.  It is not the
        intent to provide a full-fledged capability indication and
        negotiation mechanism along the lines of SDPng or ITU-T H.245.
        Instead, the focus is on addressing a set of well-known real-life
        limitations. More specifically, the solution provided in this
        document provides a general SDP capability negotiation framework. It
        also defines specifically how to provide attributes and transport
        protocols as capabilities and negotiate them using the framework.
        Extensions for other types of capabilities (e.g. media types and
        formats) may be provided in other documents.

        As mentioned above, SDP is used by several protocols, and hence the
        mechanism should be usable by all of these.  One particularly
        important protocol for this problem however is the Session Initiation
        Protocol (SIP) [RFC3261].  SIP uses the offer/answer model (which is
        not specific to SIP) to negotiate sessions and hence any mechanism
        must at least consider how it either interacts with offer/answer, or
        how it should extend it.

        The rest of the document is structured as follows. In Section 0we 2. we
        present our SDP capability negotiation solution followed by examples
        in Section 3. and security considerations in Section 4.

     2. SDP Capability Negotiation Solution

        In this section we first provide an overview of the SDP Capability
        negotiation solution. This is followed by definitions of new SDP
        attributes for the solution and its associated updated offer/answer
        procedures.

     2.1. Solution Overview

        The solution consists of the following:

        o  Three  Two new attributes to support versioning and extensions to the
           framework itself as follows:

            o  A new attribute ("a=cver") that lists the version of the SDP
               capability negotiation framework being used.

            o  A new attribute ("a=csup") that lists the supported extensions base and
               extension options to the framework.

            o  A new attribute ("a=creq") that lists the base and or
               extensions to the framework that are required to be supported
               by the entity receiving the SDP.

        o  Three  Two new attributes used to express capabilities as follows
           (additional attributes can be defined as extensions):

            o  A new attribute ("a=cmed") that defines how to list the media
               types and media formats supported as capabilities.

            o  A new attribute ("a=capar") that defines how to list the attribute
               parameter values ("a=" values) as capabilities.

            o  A new attribute ("a=ctrpr") that defines how to list transport
               protocols (e.g. "RTP/AVP") as capabilities.

        o  Two new attributes to negotiate configurations as follows:

            o  A new attribute ("a=pcfg") that lists the potential
               configurations supported. This is done by reference to the
               above
               capabilities from the SDP in question. The Multiple potential
               configurations are listed in order of preference. Extensions have an explicitly indicated ordering
               associated with them. Extension capabilities can be defined as well
               and included in the potential configurations.

            o  A new attribute ("a=acfg") to be used in an answer SDP. The
               attribute identifies which of the potential configurations
               from an offer SDP were used as actual configurations to form
               the answer SDP. Extension capabilities can included.

        o  Extensions to the offer/answer model that allow for capabilities
           and potential configurations to be included in an offer, where offer. When
           included at the session level, they constitute latent capabilities
           that may be used to guide a subsequent offer. When included at the
           media level, they constitute offers that may be accepted by the
           answerer instead of the actual configuration(s) included in the
           "m=" line(s). The answerer indicates which (if any) of the
           potential configurations it used to form the answer by including
           the actual configuration attribute ("a=cfg") ("a=acfg") in the answer.
           Capabilities and potential configurations may be included in
           answers as well, where they can aid in guiding a subsequent new
           offer.

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

                     Alice                               Bob

                       | (1) Offer (SRTP and RTP)         |
                       |--------------------------------->|
                       |                                  |
                       | (2) Answer (RTP)                 |
                       |<---------------------------------|
                       |                                  |

        Alice's offer includes RTP and SRTP as alternatives. RTP is the
        default, but SRTP is the preferred one:

           v=0
           o=- 25678 753849 IN IP4 128.96.41.1
           s=
           c=IN IP4 128.96.41.1
           t=0 0
           m=audio 3456 RTP/AVP 0 18
           a=cver:0
           a=cmed:1 audio RTP/AVP 0 18 96
           a=creq: v0
           a=ctrpr:1 RTP/SAVP
           a=capar:1 a=crypto:1 AES_CM_128_HMAC_SHA1_32
              inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
           a=capar:2 a=rtpmap:96 iLBC/8000
           a=pcfg: m=1,2|3 p=1 a=1,2
           a=pcfg: m=1,2|3 a=2
           a=pcfg:1 t=1 a=1

        The "m=" line indicates that Alice is offering to use plain RTP with
        PCMU or G.729.  The required base and extensions are provided by the
        "a=creq" attribute, which indicates that the option tag "v0", which
        indicates the base framework defined here, must be supported. The
        capabilities are provided by the "a=cver",
        "a=cmed", "a=ctrpr" and "a=capar" attributes.
        The capabilities indicate that PCMU, G.729 and iLBC are secure RTP under the AVP profile
        ("RTP/SAVP") is supported with either RTP or
        secure RTP. an associated transport capability
        handle of 1. The first "capar" attribute provides a an attribute capability
        parameter
        with a handle of 1. The attribute capability parameter is a "crypto"
        attribute in the capability set, attribute,
        which provides the keying material for SRTP using SDP security
        descriptions [SDES]. The second "capar"
        attribute provides the "rtpmap" for the dynamic payload type 96,
        which is mapped to the iLBC codec. The "a=pcfg" attribute provides the potential configurations
        configuration included in the offer by reference to the capability declarations.  Two
        parameters.  One alternatives are provided; the
        first one, and hence the preferred one is using media capabilities provided; it has a preference of 1
        and 2, i.e. PCMU and G.729, or media capability 3, i.e. iLBC.
        Furthermore, it consists of transport protocol capability 1 (i.e. the RTP/SAVP
        profile - secure RTP), and the attribute capability parameter 1, i.e. the
        crypto attribute provided, and the attribute capability parameter
        2, i.e. the rtpmap for iLBC is included. The second one is simply
        using media capabilities 1 and 2, i.e. PCMU provided. Potential configurations are always
        preferred over actual configurations, and G.729, or media
        capability 3, i.e. iLBC under the RTP/AVP profile as listed in the
        "m=" line. The "capar" parameter hence Alice is still needed to provide the
        rtpmap expressing a
        preference for iLBC. using secure RTP.

        Bob receives the SDP offer from Alice. Bob supports RTP, but not
        SRTP, SRTP and the SDP
        Capability Negotiation framework, and hence he accepts the
        (preferred) potential configuration for Secure RTP provided by Alice. Furthermore, Bob wants to use the iLBC codec and
        hence generates the following answer: Alice:

           v=0
           o=- 24351 621814 IN IP4 128.96.41.2
           s=
           c=IN IP4 128.96.41.2
           t=0 0
           m=audio 4567 RTP/AVP 96
           a=rtpmap:96 iLBC/8000
           a=cver: RTP/SAVP 0 18
           a=crypto:1 AES_CM_128_HMAC_SHA1_80
                 inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4
           a=acfg: m=3 a=2 t=1 a=1

        Bob includes the "a=cver" and "a=acfg" attribute in the answer to inform Alice
        that he based his answer on an offer containing the potential
        configuration with media transport protocol capability 3 1 and attribute
        capability 1 from the offer SDP (i.e. iLBC under the RTP/AVP profile) and the attribute capability
        parameter 2, i.e. RTP/SAVP profile using the associated rtpmap.  Note that in this
        particular example,
        keying material provided).  Bob also includes his keying material in
        a crypto attribute. If Bob supported one or more extensions to the
        capability negotiation framework, he would have included those in the
        answer as well.

        Note that in this particular example, the answerer supported the
        capability negotiation extensions defined here, however had he not, he
        the answerer would simply have processed the
        offer based on ignored the offered PCMU new attributes and G.729 codecs under
        accepted the RTP/AVP
        profile only. Consequently, offer to use normal RTP. In that case, the following
        answer would have omitted the
        "a=cver" and "a=acfg" attribute line and chosen one or both of the
        PCMU and G.729 codecs instead. been generated instead:

           v=0
           o=- 24351 621814 IN IP4 128.96.41.2
           s=
           c=IN IP4 128.96.41.2
           t=0 0
           m=audio 4567 RTP/AVP 0 18

     2.2. Version and Extension Indication Attributes

        In this section, we present the new attributes associated with
        indicating the SDP capability negotiation version and extensions
        supported and required.

     2.2.1. SDP Capability Negotiation Version Attribute

        The SDP Capability Negotiation Version attribute ("a=cver") lists the
        version of the SDP Capability Negotiation supported by the entity
        that generated the SDP. The attribute is defined as follows:

           a=cver: <version>

        where <version> is a non-zero positive integer. White space is
        permitted, but not required, before <version>. The value of <version>
        defined by this document is 0 as illustrated by the following
        example:

           a=cver: 0

        The SDP Capability Negotiation version attribute MUST be present in
        each SDP that uses the SDP Capability negotiation solution defined in
        this document. The attribute can be provided at either the session-
        or media-level, however there MUST NOT be more than one occurrence of
        it. Furthermore, the attribute SHOULD be the first of the SDP
        capability negotiation attributes provided.

     2.2.2. Supported Capability Negotiation Extensions Attribute

        The SDP Capability negotiation solution allows for capability
        negotiation extensions to be defined. Associated with each such
        extension is an option tag that identifies the extension in question.
        Option-tags MUST be registered with IANA per the procedures defined
        in Section 5.

        The Supported Capability Negotiation Extensions attribute ("a=csup")
        contains a comma-separated list of option tags identifying the SDP
        Capability negotiation extensions supported by the entity that that
        generated the SDP. The attribute is defined as follows:

           a=csup: <option-tag-list>

        where <option-tag-list> is defined by the following ABNF:

           option-tag-list   = option-tag *(COMMA option-tag)
           option-tag        = token    ; defined in [SDP]
           COMMA             = *WSP "," *WSP  ; defined in [RFC4234]

        White-space is permitted before the <option-tag-list>.

          Implementers familiar with SIP should note that the above
          definition of COMMA differs from the one in [RFC3261].

          [EDITOR'S NOTE: There's nothing specific to

        A special base option tag with a value of "v0" is defined for the
        basic SDP Capability
          Negotiation Solution for this parameter. Should consider
          generalizing and/or providing capability negotiation framework specified in a separate document.]

        The following examples illustrates the this
        document. Entities can use this option tag with the "a=csup"
        attribute to indicate support for the SDP capability negotiation
        framework specified in this document.

        The following examples illustrates the use of the "a=csup" attribute
        with two hypothetical option tags, "foo" and "bar":

           a=csup: foo
           a=csup: bar
           a=csup: foo, bar

        The "a=csup" attribute can be provided at the session and the media-
        level. When provided at the session-level, it applies to the entire
        SDP. When provided at the media-level, it applies to the media-stream
        in question only. only (option-tags provided at the session level apply as
        well). There can be one or more "a=csup" attributes at both the
        session and media-level (one or more per media stream in the latter
        case).

        Whenever an entity that supports one or more extensions to the SDP
        Capability Negotiation framework generates an SDP, it SHOULD include
        the "a=csup" attribute with the option tags for the extensions it
        supports.

     2.2.3.
        supports at the session and/or media-level, unless those option tags
        are already provided in one or more "a=creq" attribute (see Section
        2.2.2. ) at the relevant levels. The base option tag MAY be included.

     2.2.2. Required Capability Negotiation Extension Attribute

        The SDP Capability negotiation solution allows for capability
        negotiation extensions to be defined. Associated with each such
        extension is an option tag that identifies the extension in question.
        Option-tags MUST be registered with IANA per the procedures defined
        in Section 5.

        The Required Capability Negotiation Extensions attribute ("a=csup")
        contains a comma-separated list of option tags identifying the SDP
        Capability negotiation extensions that MUST be supported by the
        entity receiving the SDP in order for that entity to properly process
        the SDP Capability negotiation. The attribute is defined as follows:

           a=creq: <option-tag-list>

        where <option-tag-list> is defined in Section 2.2.2. 2.2.1.

        White-space is permitted before the <option-tag-list>.

          [EDITOR'S NOTE: There's nothing specific to the SDP Capability
          Negotiation Solution for this parameter. Should consider
          generalizing and/or providing in a separate document.]

        The following examples illustrates illustrate the use of the "a=creq" attribute
        with two hypothetical option tags, "foo" and "bar":

           a=creq: foo
           a=creq: bar
           a=creq: foo, bar

        The "a=creq" attribute can be provided at the session and the media-
        level. When provided at the session-level, it applies to the entire
        SDP. When provided at the media-level, it applies to the media-stream
        in question only. only (required option tags provided at the session level
        apply as well). There can be one or more "a=creq" attributes at both
        the session and media-level (one or more per media stream in the
        latter case).

        Whenever

        When an entity generates an SDP and it requires the recipient of that
        SDP to support one or more SDP capability negotiation extensions in
        order to properly process the SDP Capability negotiation, the
        "a=creq" attribute MUST be included with option-tags that identify
        the required extensions. extensions at the session and/or media level, unless it
        is already known that the receiving entity supports those option-tags
        at the relevant levels (in which case their inclusion is OPTIONAL).

          An example of this is when generating an answer to an offer. If the
          answerer supports the required option-tags from the offer, and the
          answerer does not require any additional option-tags beyond what
          was listed in either the required ("creq") or supported ("csup")
          attributes from the offer, then the answerer is not required to
          include a required ("creq") attribute with any option-tags that may
          need to be supported (such as the base option tag - "v0").

        A recipient that receives such an SDP and does not support one or more of
        the required extensions, extensions listed in a "creq" attribute, MUST NOT
        perform the SDP capability negotiation defined in this document. For
        non-supported extensions provided at the session-level, this implies
        that SDP capability negotiation MUST NOT be performed at all. For
        non-supported extensions at the media-level, this implies that SDP
        capability negotiation MSUT MUST NOT be performed for the media stream in
        question.

        When an entity does not support one or more required SDP capability
        negotiation extensions, the entity SHOULD proceed as if the SDP
        capability negotiation attributes were not included in the first
        place.
        place, i.e. all the capability negotiation attributes should be
        ignored.

          This ensures that introduction of the SDP capability negotiation
          mechanism does not introduce any new failure scenarios.

        The above rules apply to the base option tag as well. Thus, entities
        compliant to this specification MUST include a "creq" attribute (at
        least in an offer) that includes the option tag "v0" as illustrated
        below:

           a=creq: v0

     2.3. Capability Attributes

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

     2.3.1. Media Type and Format Attribute Capability Attribute

        Media types and media formats

        Attributes can be expressed as capabilities by use
        of the "a=cmed" attribute, which is defined as follows:

          a=cmed: <med-cap-num> <media> [<proto> <fmt list>]

        where <med-cap-num> is an integer between 1 and 2^32-1 (both
        included) used to number the media capabilities, and <media>,
        <proto>, and <fmt list> are defined as in the SDP "m=" line. The <fmt
        list> may contain multiple media formats. In that case, the media
        format capability number associated with the first one provided is
        the value of <med-cap-num>, the number associated with the second one
        is one higher, etc. Each occurrence of the attribute MUST use a
        different value of <med-cap-num>. Furthermore, when a "cmed"
        attribute indicates more than one media format, the capability
        numbers implied MUST NOT be used by any other "cmed" attribute in the
        session description (explicitly or implicitly).  When <proto> and
        <fmt list> are omitted, the media capability merely indicates support
        for the <media> type in question, without any details as to what kind
        of transport protocol and media formats are supported. This can for
        example be used to indicate support for additional types of media
        than those included as actual configurations in an offer or answer.

        A media capability merely indicates possible support for the media
        type and media format(s) in question. In order to actually use a
        media capability in an offer/answer exchange, it must be referenced
        in a potential configuration (see Section 2.4.1.

        Media capabilities can be provided at the session-level and the
        media-level. Media capabilities provided at the session level apply
        to the session description in general, whereas media capabilities
        provided at the media level apply to that media stream only.  In
        either case, the scope of the <med-cap-num> is the entire session
        description.  This enables each media capability to be referenced
        across the entire session description (e.g. in a potential
        configuration - see Section 2.4.1.

          [EDITOR'S NOTE: This is clear as mud. If a media capability applies
          to a media-stream only, then why can it still be referenced and
          hence used as capabilities in other media streams (by the "a=pcfg")
          attribute. The motivation is message size efficiency, but the means
          are not clean. Session versus media-level syntax and semantics need
          further consideration]

        The <proto> parameter indicates the default transport protocol
        associated with the media capability.  As described in [RFC4566], the
        value of the <proto> parameter guides the interpretation of the <fmt
        list>, which is why it is included here. Note that <proto> is also a
        capability that can be negotiated separately (see Section 2.3.3.

        The <fmt list> contains one or more media formats supported, the
        interpretation of which depends on the value of <proto>.  The rules
        that apply to "m=" lines (as defined in [SDP]) for interpretation of
        these apply here as well.  For RTP-based transports, this implies
        that the <fmt list> contains one or more RTP payload type numbers.
        When those payload type numbers are dynamic, SDP requires an
        "a=rtpmap" attribute to determine the actual codec.  In the case of
        SDP capability negotiation, such additional attribute parameters MUST
        be provided in conjunction with the media capability.  There are two
        different cases to consider for this:

        o  The media capability is provided at the session level:   In this
           case, the required parameters MUST be provided in one or more
           attribute parameter capabilities (see Section 2.3.2.  listed
           before the first "m=" line as well as before any other media
           capability attributes ("a=cmed").

        o  The media capability is provided at the media stream level: In
           this case, when the payload type numbers are part of the "m=" line
           itself, this is done by use of the "a=rtpmap" attribute as usual.
           In all other cases, the required parameters MUST be provided in
           one or more attribute parameter capabilities (see Section 2.3.2.
           within the media stream description (i.e. before the next "m="
           line).

          [EDITOR'S NOTE:    The above assumes that intermediaries will not
          reorder session-level attributes. It would be safer to explicitly
          link the two, but that will require yet another attribute. Also, it
          sends us down the path of building more complicated capabilities
          (made up of multiple parameters). Another issue here is that
          payload type numbers, which really have only media-level scope, are
          ill-suited to be used at the session-level or across multiple media
          streams (as is being done here). However, an alternative (and more
          proper) solution seems to involve significantly more work and
          deviations from the current SDP framework. This in turn makes it
          more difficult to automatically use new media types, formats,
          protocols, etc. defined elsewhere within this framework, and that
          is a major disadvantage. Another option is to forgo the session-
          level media capabilities as well as the ability to reference across
          media streams - it will make the solution less efficient though and
          difficult to express latent capabilities for media streams not
          included in the offer or answer.]

        The following example illustrates the first case above:

           v=0
           o=- 25678 753849 IN IP4 128.96.41.1
           s=
           c=IN IP4 128.96.41.1
           t=0 0
           a=cmed: 1 audio RTP/AVP 96
           a=capar: 1 a=rtpmap:96 G729/8000
           m=audio...

        The following example illustrates the second case above:

           v=0
           o=- 25678 753849 IN IP4 128.96.41.1
           s=
           c=IN IP4 128.96.41.1
           t=0 0
           m=audio 2345 RTP/AVP 96
           a=rtpmap:96 G729/8000
           a=cver:0
           a=cmed: 1 audio RTP/AVP 96 97
           a=capar: 1 a=rtpmap:97 iLBC/8000

          Note:  Readers familiar with RFC 3407 may notice the similarity
          between the "cmed" attribute defined above and the "cdsc" attribute
          defined in RFC 3407. There are however a couple of important
          differences, namely an increase in the capability numbering space
          as well as a relaxation of certain requirements found in RFC 3407.
          To simplify overall operation, the "cmed" parameter is limited to
          media-level operation only as well.

     2.3.2. Attribute Parameter Capability Attribute

        Attributes can be expressed as negotiable parameters negotiable parameters by use of a new
        attribute parameter capability attribute ("a=capar"), which is defined as
        follows:

           a=capar: <att-cap-num> <att-par>

        where <att-cap-num> is an integer between 1 and 2^32-1 (both
        included) used to number the attribute parameter capability and <att-
        par> <att-par> is an
        attribute ("a=") in its full  '<type>=<value>' form (see [SDP]).

        The "capar" attribute can be provided at the session level for
        session-level attributes and the media level. level for media-level
        attributes. The "capar" attribute MUST NOT be used to provide a
        media-level attribute at the session-level or vice versa.

        Each occurrence of the "capar" attribute in the entire session
        description MUST use a different value of <app-cap-num>.

          There is a need to be able to reference both session-level and
          media-level attributes in potential configurations at the media
          level, and this provides for a simple solution to avoiding overlap
          between the handle references.

        The <att-cap-num> values provided are independent of similar <cap-num> <cap-
        num> values provided for other attributes, i.e., they form a separate
        name-space for attribute parameter capabilities.

        Attribute parameter capabilities are generally used for two things.
        First of all, they may be necessary to interpret a media format
        capability (e.g. by including an rtpmap), or they may provide
        attribute value parameters that are referenced in potential
        configurations (see Section 2.4.1. )

        The following examples illustrate use of the "capar" attribute:

           a=capar: 1 a=ptime:20

           a=capar: 2 a=ptime:30

           a=capar: 3 a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyONQ6gAA
           AAAGEEoo2pee4hp2UaDX8ZE22YwKAAAPZG9uYWxkQGR1Y2suY29tAQAAAAAAAQAk0
           JKpgaVkDaawi9whVBtBt0KZ14ymNuu62+Nv3ozPLygwK/GbAV9iemnGUIZ19fWQUO
           SrzKTAv9zV

           a=capar: 4 a=crypto:1 AES_CM_128_HMAC_SHA1_32
                 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32

        The first two provide attribute values for the ptime attribute. The
        third one provides SRTP parameters by using MIKEY with the key-mgmt
        attribute [KMGMT]. The fourth one provides SRTP parameters by use of
        security descriptions with the crypto attribute [SDES].

          Readers familiar with RFC 3407 may notice the similarity between
          the RFC 3407 "cpar" attribute and the above. There are however a
          couple of important differences, most notably that the "capar"
          attribute contains a handle that enables referencing it and it
          furthermore supports attributes only (the "cpar" attribute defined
          in RFC 3407 supports bandwidth information as well). The "capar"
          attribute also is not automatically associated with any particular
          capabilities.

     2.3.3.

     2.3.2. Transport Protocol Capability Attribute

        Transport Protocols can be expressed as capabilities by use of a new
        Transport Protocol Capability attribute ("a=ctrpr") defined as
        follows:

           a=ctrpr: <trpr-cap-num> <proto-list>

        where <trpr-cap-num> is an integer between 1 and 255 2^32-1 (both
        included) used to number the transport address capability for later
        reference, and <proto-list> is one or more <proto>, separated by
        white space, as defined in the SDP "m=" line.

        The "ctrpr" attribute can be provided at the session- and media-
        level. Each occurrence of the "ctrpr" attribute in the entire session
        description MUST use a different value of <trpr-cap-num>.  When
        multiple <proto> values are provided, the first one is associated
        with the value <trpr-cap-num>, the second one with the value one
        higher, etc. The <trpr-cap-num> values provided are independent of
        similar <cap-num> values provided for other attributes, i.e., they
        form a separate name-space for transport protocol capabilities.

        Below, we provide examples of the "a=ctrpr" attribute:

           a=ctrpr: 1 RTP/AVP
           a=ctrpr: 2 RTP/AVPF
           a=ctrpr: 3 RTP/SAVP RTP/SAVPF

        The first one provides a capability for the "RTP/AVP" profile defined
        in [RFC3551] and the second one provides a capability for the RTP
        with RTCP-Based Feedback profile defined in [AVPF]. The third one
        provides capabilities for the "RTP/SAVP" and "RTP/SAVPF" profiles.

        Note that the "cmed" attribute provides a similar functionality by
        including <proto>, however having this as a separate capability
        indication can provide significant message size reduction when
        negotiating alternative profiles (of which there can be many). In
        particular, there is no need to repeat supported payload types. Also,
        use of this attribute combined with the potential configuration
        attribute (see Section 2.4. ) a capability for the RTP
        with RTCP-Based Feedback profile defined in [AVPF]. The third one
        provides capabilities for more expressive power. the "RTP/SAVP" and "RTP/SAVPF" profiles.

     2.4. Configuration Attributes

     2.4.1. Potential Configuration Attribute

        Potential Configurations can be expressed by use of a new Potential
        Configuration Attribute ("a=pcfg") defined as follows:

           a=pcfg: <preference> <pot-cfg-list>

        where <preference> is an integer between 1 and 2^32-1 (both included)
        and <pot-cfg-list> is defined as

           pot-cfg-list   =  pot-config *(1*WSP pot-config)
           pot-config     =  pot-media-config |  pot-attribute-parameter-config |
                             pot-transport-protocol-config |
                             pot-extension-config

        The potential configuration attribute includes a preference
        indication (lowest number is most preferred) followed by one or more sets
        of potential media configurations, attribute parameter configurations configuration and transport protocol configurations.
        configuration.  Each of these MUST NOT be present more than once in a
        particular potential configuration attribute. Potential extension
        configurations can be included as well.  There can be more than one
        potential extension configuration, however each particular potential
        extension configuration MUST NOT be present more than once in a given
        potential configuration attribute. Together, these values define a set of
        potential configurations. configuration.

        There can be one or more multiple potential configuration attributes configurations provided at the
        session-level as well as the media-level. The semantics for each media stream. The attributes
        are provided in order of preference.

          [EDITOR'S NOTE: We run into another issue with session-level media
          capabilities here. In the offer/answer model,
        these levels differ. A potential
          configurations configuration at the media-level constitute alternative offers, session level
        provides a set of latent capabilities. A latent capability is merely
        an indication that the potential configuration could be supported,
        however it does not represent a willingness to do so at the session-level, that would/should current
        time. A potential configuration at the media level on the other hand
        indicates not be only a willingness, but in fact a desire to use the
        potential configuration.

          In the case for
          media capabilities. Other parameters however may be used as
          alternative offers of offer/answer, this implies that a potential
          configuration at the session level (e.g. key-mgmt attributes does not constitute an
          alternative offer whereas it does at the session level)]

        pot-media-config is defined by the following ABNF:

           pot-media-config     = "m=" med-cap-list *(BAR med-cap-list)
           med-cap-list         = med-cap-num *(COMMA med-cap-num)
           med-cap-num          = 1*DIGIT   ; defined in [RFC4234]
           BAR                  = *WSP "|" *WSP  ; defined in [RFC4234]

        Each potential media level.

        Associated with each potential configuration is a comma-separated list of media
        capability numbers where med-cap-num refers to media capability
        numbers and hence MUST be preference
        indication, which is an integer between 1 and 2^32-1 (both included).
        Alternative included)
        to indicate the relative preference of potential media configurations are separated by a
        vertical bar ("|"). configurations. The alternatives are ordered by preference. When
        scope of the preference (and in fact each occurrence of a potential
        configuration attribute) is the session-level, when provided there,
        or the particular media stream it is provided at.

        Attribute capabilities are not included in a potential configuration at
        the media level, the media type and media format from by
        use of the associated
        "m=" line will be used. pot-attribute-parameter-config parameter, which is defined
        by the following ABNF:

           pot-attribute-parameter-config
                             = "a=" capar-cap-list *(BAR capar-cap-list)
           capar-cap-list    = att-cap-num *(COMMA att-cap-num)
           att-cap-num       = 1*DIGIT   ;defined in [RFC4234]

        Each potential attribute parameter configuration list is a comma-
        separated list of attribute capability parameter numbers where att-
        cap-num att-cap-num
        refers to attribute parameter capability numbers defined above and hence MUST
        be between 1 and 2^32-1 (both included). Alternative potential
        attribute parameter configurations are separated by a vertical bar ("|").
        ("|"), the scope of which extends to the next alternative (i.e. ","
        has higher precedence than "|"). The alternatives are ordered by
        preference.

        Transport protocol capabilities are included in a potential
        configuration by use of the pot-transport-protocol-config parameter,
        which is defined by the following ABNF:

           pot-transport-protocol-config =
                                "p="
                                "t=" trpr-cap-num *(BAR trpr-cap-num)
           trpr-cap-num        = 1*DIGIT   ; defined in [RFC4234]

        The trpr-cap-num refers to transport protocol capability numbers
        defined above and hence MUST be between 1 and 2^32-1 (both included).
        Alternative potential transport protocol configurations are separated
        by a vertical bar ("|").  The alternatives are ordered by preference.
        When transport protocol capabilities are not included in a potential
        configuration,
        configuration at the media level, the transport protocol information
        from an included
        potential media configuration the associated "m=" line will be used.  If a potential media
        configuration is not included,  At the session-level,
        lack of a transport protocol from the media
        description ("m=" line) will be used instead.

        pot-extension-config capability indication simply implies
        that no such information is defined by provided.

        Extension capabilities can be included in a potential configuration
        as well. Such extensions MUST adhere to the following ABNF:

           pot-extension-config=

           pot-extension-config = ext-cap-name "="
                                      ext-cap-list *(BAR ext-cap-list)
           ext-cap-name   = token     ; defined in [SDP]
           ext-cap-list   = ext-cap-num *(COMMA ext-cap-num)
           ext-cap-num    = 1*DIGIT   ; defined in [RFC4234]

        The ext-cap-name refers to the type of extension capability and the
        ext-cap-num refers to a capability number associated with that
        particular type of extension capability.  The number MUST be between
        1 and 2^32-1 (both included).  Alternative potential extension
        configurations for a particular extension are separated by a vertical
        bar ("|"). ("|"),the scope of which extends to the next alternative (i.e.
        "," has higher precedence than "|").  Unsupported or unknown
        potential extension configs MUST be ignored, unless an option tag showing the extension as being ignored.

          The "creq" attribute and its associated rules can be used to ensure
          that required was included (see Section 2.2.3. extensions are supported in the first place.

        Potential configurations can be provided at the session level and the
        media level and in either case, it is syntactically possible to
        reference attribute capabilities provided at either the session or
        the media level. There are however semantic rules and limitations
        associated with this: At the session-level, a potential configuration
        MUST NOT reference any attribute capabilities provided at the media-
        level. The converse however is permitted, i.e. a media-level
        potential configuration can reference a session-level attribute
        capability. The semantics of doing so (should that potential
        configuration ("a=pcfg") be chosen), is that the corresponding attribute can
        (provided within that attribute capability) will be considered part
        of the active configuration at the *session* level. In other words,
        it will be as-if that attribute was simply provided at the session session-
        level and in the media-level. Each occurrence of first place. Note that individual media streams perform
        capability negotiation individually, and hence it is possible that
        another media stream (where the attribute within a given media description ("m=" line) defines a set was part of a potential configurations
        configuration) chose a configuration without that can be used for session level
        attribute. The session-level attribute however remains "active" and
        hence applies to the entire session. It is up to the entity that media
        description.

          TO DO: Need
        generates the SDP to decide on relationship ensure that the resulting active configuration
        SDP is still meaningful.

          [EDITOR'S NOTE: There are too many subtle differences between session-level
          potential configurations at the session and
          media-level (how should conflicts, overlap, etc. be handled -
          simplicity media level. I'm
          inclined to have two similarly looking (but different) attributes
          instead as that will make it more straightforward and intuitive.
          That also leaves the door open to have more than latent
          capabilities at the possible expense of expressive power is
          preferable session level in the editor's opinion). case that is needed later]

        Below, we provide an example of the "a=pcfg" attribute in a complete
        media description in order to properly indicate the supporting
        attributes:

           v=0
           o=- 25678 753849 IN IP4 128.96.41.1
           s=
           c=IN IP4 128.96.41.1
           t=0 0
           m=audio 3456 RTP/SAVPF RTP/AVPF 0 18
           a=crypto:1
           a=creq: v0
           a=capar:1 crypto:1 AES_CM_128_HMAC_SHA1_32
              inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
           a=cver: 0
           a=cmed: 1 audio RTP/SAVP 0 4 18
           a=ctrpr: 1 RTP/AVP RTP/AVPF RTP/AVP
           a=ctrpr: 3 RTP/SAVP RTP/SAVPF
           a=pcfg: m=1|3 p=1|2|3|4
           a=pcfg: m=2 p=1 RTP/SAVP
           a=pcfg:1 t=3|4 a=1
           a=pcfg:8 t=1|2
        We have two potential configurations listed here. The first one (and
        most preferred, since it's preference is "1") indicates that PCMU (payload type number 0 referenced by media
        capability number 1) or G.729 (payload type number 18 referenced by
        media capability number 3) can be supported with either
        of the profiles RTP/AVP, RTP/AVPF, RTP/SAVP, or RTP/SAVPF or RTP/SAVP (specified by the transport
        protocol capability numbers 1, 2, 3 and 4). 4) can be supported with attribute
        capability 1 (the "crypto" attribute); RTP/SAVPF is preferred since
        it is listed first. The second potential configuration indicates that G.723 (payload type number 4
        referenced by media capability number 2)
        the RTP/AVPF of RTP/AVP profile can be supported used, with RTP/AVPF being the
        RTP/AVP profile only (transport protocol capability number 1).
        preferred one. This non secure RTP alternative is the less preferred
        one since it's preference is "8".

     2.4.2. Actual Configuration Attribute

        The actual configuration attribute identifies which of the potential
        configurations from an offer SDP were used as actual configurations
        in an answer SDP.  This is done by reference to the media
        capabilities, attribute parameter
        capabilities and transport protocol capabilities from the offer that
        were actually used by the answerer in his offer/answer procedure. If
        extension capabilities were used, those will be included by reference
        as well.

        The Actual Configuration Attribute ("a=acfg") is defined as follows:

           a=acfg: <act-cfg-list>

        where <act-cfg-list> is defined as

           act-cfg-list   =  capability *(1*WSP capability)

           capability        =  act-media-config |  act-attribute-parameter-config |
                                act-transport-protocol-config |
                                act-extension-config

        act-media-config is defined by the following ABNF:

           act-media-config      = "m=" med-cap-list

        where med-cap-list is as defined in Section 2.4.1.

        act-attribute-parameter-config is defined by the following ABNF:

           act-attribute-parameter-config = "a=" capar-cap-list

        where capar-cap-list is as defined in Section 2.4.1.

        act-transport-protocol-config is defined by the following ABNF:

           act-transport-protocol-config = "p=" "t=" trpr-cap-num

        where trpr-cap-num is as defined in Section 2.4.1.

           trpr-cap-num        = 1*3DIGIT   ; defined in [RFC4234]

        act-extension-config is defined by the following ABNF:

           act-extension-config = ext-cap-name "=" ext-cap-list

        where ext-cap-name and ext-cap-list are as defined in Section 2.4.1.

        The actual configuration ("a=acfg") attribute can be provided at the
        session-level and the media-level.
        media-level only. There MUST NOT be more than one occurrence of an
        actual configuration attribute at the session level,
        and there MUST NOT be more than one occurrence of an actual
        configuration attribute within a given media description.

        Below, we provide an example of the "a=acfg" attribute (building on
        the previous example with the potential configuration attribute):

           v=0
           o=- 24351 621814 IN IP4 128.96.41.2
           s=
           c=IN IP4 128.96.41.2
           t=0 0
           m=audio 4567 RTP/AVPF RTP/SAVPF 0
           a=cver:
           a=creq: 0
           a=acfg: m=1 p=2 t=3 a=1

        It indicates that the answerer used an offer consisting of media
        capability 1 from the offer (PCMU) and transport
        protocol capability 2 from the offer (RTP/AVPF). (RTP/SAVPF) and attribute
        capability 1 (the "crypto" attribute.

     2.5. Offer/Answer Model Extensions

        In this section, we define extensions to the offer/answer model
        defined in [RFC3264] to allow for potential configurations to be
        included in an offer, where they constitute offers that may be
        accepted by the answerer instead of the actual configuration(s)
        included in the "m=" line(s).

           [EDITOR'S NOTE: Multicast considerations have been omitted for
           now.]

           TO DO: Elaborate and firm up offer/answer procedures.

     2.5.1. Generating the Initial Offer

        An offerer that wants to use the SDP capability negotiation
        extensions defined in this document MUST include the following in the
        offer:

        o  an SDP capability negotiation version required extensions attribute with ("a-
           creq") that contains the version
           set to 0

        o  one option tag "v0". It must either be
           provided at the session-level or more media capabilities (as defined in Section 2.3.1. ), if
           alternative media types and for each individual media formats are to stream.
           Option tags for any other required extensions MUST be indicated included as
           offerer capabilities or be negotiated.
           well (in accordance with Section 2.2.2. )

        o  one or more attribute parameter capability attributes (as defined in Section 2.3.2.
           2.3.1. ) if alternative attribute parameter values are to be
           indicated as offerer capabilities or be negotiated.

        o  one or more transport protocol capability attributes (as defined
           in Section 2.3.3. 2.3.2. ) if alternative transport protocols are to be
           to be indicated as offerer capabilities or be negotiated.

        o  one or more potential configuration attributes (as defined in
           Section 2.4. ) if alternative potential configurations are to be
           negotiated.

        o  one or more required capability negotiation extension attributes
           (as defined in Section 2.2.3. 2.2.2. ), if the answerer is required to
           support one or more SDP capability negotiation extensions.

        The offerer SHOULD furthermore include the following:

        o  one or more supported capability negotiation extension attributes
           (as
           ("a=csup" as defined in Section 2.2.2. 2.2.1. ), if the offerer supports
           one or more SDP capability negotiation extensions. extensions that have not
           been included in one or more "a=creq" attributes at the relevant
           session and media level(s).

        The capabilities provided merely indicate what the offerer is capable
        of doing. They do not constitute a commitment or even an indication
        to actually use them. This applies to potential configurations listed
        at the session level as well. Conversely, each of the potential
        configurations listed at the media level constitutes an alternative
        offer which may be used to negotiate and establish the session.

          [EDITOR'S NOTE: This is only partially true for potential
          configurations listed at the session level. The only thing we want
          to offer up as alternative offers at the session level is
          attributes - not media types or media formats, which should be
          capabilities only at the session level]

        The current actual configuration is included in the "m=" line (as
        defined by [RFC3264]).

     2.5.2. Generating the Answer

        When the answerer receives an offer with valid SDP capability
        negotiation information in it and in particular with one or more
        valid potential configuration information attributes present, it may
        use any of the potential configurations as an alternative offer. A
        potential configuration information attribute is valid if all of the
        capabilities (media, attribute (attribute capabilities, transport protocol capabilities
        and any extension capabilities) it references are present and valid
        themselves.

        The actual configuration is contained in the media description's "m="
        line. The answerer can send media to the offerer in accordance with
        the actual configuration, however if it chooses to use one of the
        alternative potential configurations, media sent to the offerer may
        be discarded by the offerer until the answer is received.

        If the answerer chooses to accept one of the alternative potential
        configurations instead of the actual configuration, the answerer MUST
        generate an answer as if the offer contained that potential
        configuration instead of the actual configuration included. The
        answerer MUST also include an actual configuration attribute in the
        answer that identifies the potential configuration from the offer
        used by the answerer. The actual configuration attribute in the
        answer MUST include information about the media capabilities, attribute capability parameters, capabilities,
        transport protocol parameters, and extension capabilities from the
        potential configuration that were used to generate the answer.

     2.5.3.  Offerer Processing of the Answer

        When the offerer included potential configurations for a media
        stream, it MUST examine the answer for the presence of an actual
        configuration attribute for each such media stream.  If the attribute
        is missing, offerer processing of the answer MUST proceed as defined
        by [RFC3264]. If the attribute is present, processing continues as
        follows:

        The actual configuration attribute specifies which of the potential
        configurations were used by the answerer to generate the answer. This
        includes all the types of capabilities from the potential
        configuration offered, i.e. the media formats ("cmed" capabilities), attribute capability parameters ("capar"), capabilities ("a=capar"),
        transport protocol capabilities ("ctrpr"), ("a=ctrpr"), and any extension
        capability parameters included.

        The offerer MUST now process the answer as if the offer had contained
        the potential configuration as the actual configuration in the media
        description ("m=" line) and relevant attributes in the offer.

     2.5.4. Modifying the Session

        Potential configurations may be included in subsequent offers as
        defined in [RFC3264, Section 8].  The procedure for doing so is
        similar to that described above with the answer including an
        indication of the actual configuration used by the answerer.

     3. Examples

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

     3.1. Best-Effort Secure RTP

        The following example illustrates how to use the SDP Capability
        negotiation extensions to support so-called Best-Effort Secure RTP.
        In that scenario, the offerer supports both RTP and Secure RTP. If
        the answerer does not support secure RTP (or the SDP capability
        negotiation extensions), an RTP session will be established. However,
        if the answerer supports Secure RTP and the SDP Capability
        Negotiation extensions, a Secure RTP session will be established.

        The best-effort Secure RTP negotiation is illustrated by the
        offer/answer exchange below, where Alice sends an offer to Bob:

                     Alice                               Bob

                       | (1) Offer (SRTP and RTP)         |
                       |--------------------------------->|
                       |                                  |
                       | (2) Answer (RTP)                 |
                       |<---------------------------------|
                       |                                  |

        Alice's offer includes RTP and SRTP as alternatives. RTP is the
        default, but SRTP is the preferred one:

           v=0
           o=- 25678 753849 IN IP4 128.96.41.1
           s=
           c=IN IP4 128.96.41.1
           t=0 0
           m=audio 3456 RTP/AVP 0 18
           a=cver:0
           a=creq: v0
           a=ctrpr:1 RTP/SAVP RTP/AVP
           a=capar:1 a=crypto:1 AES_CM_128_HMAC_SHA1_80
              inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
              FEC_ORDER=FEC_SRTP
           a=pcfg: p=1
           a=pcfg:5 t=1 a=1
           a=pcfg: p=2
           a=pcfg:10 t=2

        The "m=" line indicates that Alice is offering to use plain RTP with
        PCMU or G.729.  The capability declaration  Alice indicates that support for the base protocol
        defined here is required by including the "a=creq" attribute
        containing the value "v0". The capabilities are provided by the
        "a=cver",
        "a=ctrpr" and "a=capar" attributes.  The capabilities indicate that
        both Secure RTP and normal RTP are supported. The "capar" attribute
        provides a capability parameter with a handle of 1. The capability
        parameter is a "crypto" attribute in the capability set, which
        provides the keying material for SRTP using SDP security descriptions
        [SDES]. The "a=pcfg" attribute provides the potential configurations
        included in the offer by reference to the capabilities.  Two
        alternatives are provided; the first one, and one with preference "5" (and
        hence the preferred preferred one since the preference on the second one is
        "10") is transport protocol capability 1 (RTP/SAVP, i.e. secure RTP)
        together with the attribute capability parameter 1, i.e. the crypto attribute
        provided. The second one is using transport protocol capability 2.  Since there are no media format capabilities
        included,
        Note that we could have omitted the media format parameters from second potential configuration
        since it equals the media description
        itself actual configuration (which is used. always the least
        preferred configuration).

        Bob receives the SDP offer from Alice. Bob supports SRTP and the SCP SDP
        Capability Negotiation extensions, and hence he accepts the potential
        configuration for Secure RTP provided by Alice:

           v=0
           o=- 24351 621814 IN IP4 128.96.41.2
           s=
           c=IN IP4 128.96.41.2
           t=0 0
           m=audio 4567 RTP/SAVP 0 18
           a=crypto:1 AES_CM_128_HMAC_SHA1_80
                 inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4
           a=cver: 0
           a=csup: foo
           a=acfg: p=1 t=1 a=1

        Bob includes the "a=cver" and "a=acfg" attribute in the answer to inform Alice
        that he based his answer on an offer containing the potential
        configuration with transport protocol capability 1 and attribute parameter
        capability 1 from the offer SDP (i.e. the RTP/SAVP profile using the
        keying material provided).  Bob also includes his keying material in
        a crypto attribute. Finally, Bob supports an SDP capability
        negotiation extension with the option tag "foo" and hence he includes
        the "a=csup" parameter containing value "foo" in the answer.

        Note that in this particular example, the answerer supported the
        capability extensions defined here, however had he not, the answerer
        would simply have ignored the new attributes and accepted the offer
        to use normal RTP. In that case, the following answer would have been
        generated instead:

           v=0
           o=- 24351 621814 IN IP4 128.96.41.2
           s=
           c=IN IP4 128.96.41.2
           t=0 0
           m=audio 4567 RTP/AVP 0 18

     4. Security Considerations

        TBD.

     5. IANA Considerations

        TBD.

        [EDITOR'S NOTE: Need to define registry and procedures for option
        tags]

        [EIDTOR'S NOTE: Need to define registry and procedures for extension
        capabilities]

     6. To Do and Open Issues

        o  Capability descriptions, potential configurations and actual
           configurations can be provided at both the session level and media
           level. It needs to be decided what the relationship between the
           session level and media level parameters are.

        o  Look for "EDITOR'S NOTE" throughout the document.

     7. Acknowledgments

        Thanks

        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 in
        here: Roni Even, Robert Gilman, Cullen Jennings, Matt Lepinski, Joerg
        Ott, Colin Perkins, and Thomas Stach.

        Francois Audet and Dan Wing for provided useful comments on earlier
        versions of this document.

     8. Change Log

     8.1. draft-ietf-mmusic-sdp-capability-negotiation-01

        The following are the major changes compared to version -00:

        o  Media capabilities are no longer considered a core capability and
           hence have been removed. This leaves transport protocols and
           attributes as the only capabilities defined by the core.

        o  Version attribute has been removed and an option tag to indicate
           the actual version has been defined instead.

        o  Clarified rules for session-level and media level attributes
           provided at either level as well how they can be used in potential
           configurations.

        o  Potential configuration parameters no longer have implicit
           ordering; an explicit preference indicator is now included.

        o  The parameter name for transport protocols in the potential and
           actual configuration attributes have been changed "p" to "t".

        o  Clarified operator precedence within potential and actual
           configuration attributes.

        o  Potential configurations at the session level now limited to
           indicate latent capability configurations. Consequently, an actual
           configuration attribute can no longer be provided at the session
           level.

        o  Cleaned up capability and potential configuration terminology -
           they are now two clearly different things.

     8.2. draft-ietf-mmusic-sdp-capability-negotiation-00

        Version 00 is the initial version. The solution provided in this
        initial version is based on an earlier (individual submission)
        version of [SDPCapNeg]. The following are the major changes compared
        to that document:

        o  Solution no longer based on RFC 3407, but defines a set of similar
           attributes (with some differences).

        o  Various minor changes to the previously defined attributes.

        o  Multiple transport capabilities can be included in a single
           "ctrpr" attribute

        o  A version attribute is now included.

        o  Extensions to the framework are formally supported.

        o  Option tags and the ability to list supported and required
           extensions are supported.

        o  A best-effort SRTP example use case has been added.

        o  Some terminology change throughout to more clearly indicate what
           constitutes capabilities and what constitutes configurations.

     9. References

     9.1. Normative References

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

        [RFC2234] Crocker, D. and Overell, P.(Editors), "Augmented BNF for
                  Syntax Specifications: ABNF", RFC 2234, Internet Mail
                  Consortium and Demon Internet Ltd., November 1997.

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

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

        [RFC3605] C. Huitema, "Real Time Control Protocol (RTCP) attribute in
                  Session Description Protocol (SDP)", RFC 3605, October
                  2003.

        [RFC4234] Crocker, D., and P. Overell, "Augmented BNF for Syntax
                  Specifications: ABNF", RFC 4234, October 2005.

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

     9.2. Informative References

        [RFC2046] Freed, N., and N. Borensteain, "Multipurpose Internet Mail
                  Extensions (MIME) Part Two: Media Types", RFC 2046,
                  November 1996.

        [RFC2327] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
                  Description Protocol", RFC 2327, April 1998.

        [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
                  A., Peterson, J., Sparks, R., Handley, M., and E. Schooler,
                  "SIP: Session Initiation Protocol", RFC 3261, June 2002.

        [RFC3388] Camarillo, G., Eriksson, G., Holler, J., and H.
                  Schulzrinne, "Grouping of Media Lines in the Session
                  Description Protocol (SDP)", RFC 3388, December 2002.

        [RFC3551] Schulzrinne, H., and S. Casner, "RTP Profile for Audio and
                  Video Conferences with Minimal Control", RFC 3551, July
                  2003.

        [SRTP]    Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
                  Norrman, "The Secure Real-time Transport Protocol (SRTP)",
                  RFC 3711, March 2004.

        [RFC3851] B. Ramsdell, "Secure/Multipurpose Internet Mail Extensions
                  (S/MIME) Version 3.1 Message Specification", RFC 3851, July
                  2004.

        [RFC4091] Camarillo, G., and J. Rosenberg, The Alternative Network
                  Address Types (ANAT) Semantics for the Session Description
                  Protocol (SDP) Grouping Framework, RFC 4091, June 2005.

        [AVPF]    Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
                  "Extended RTP Profile for RTCP-Based Feedback (RTP/AVPF)",
                  Work in Progress, August 2004.

        [I-D.jennings-sipping-multipart] Wing, D., and C. Jennings, "Session
                  Initiation Protocol (SIP) Offer/Answer with Multipart
                  Alternative", Work in Progress, March 2006.

        [SAVPF]   Ott, J., and E Carrara, "Extended Secure RTP Profile for
                  RTCP-based Feedback (RTP/SAVPF)", Work in Progress,
                  December 2005.

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

        [SDPng]   Kutscher, D., Ott, J., and C. Bormann, "Session Description
                  and Capability Negotiation", Work in Progress, February
                  2005.

        [BESRTP]  Kaplan, H., and F. Audet, "Session Description Protocol
                  (SDP) Offer/Answer Negotiation for Best-Effort Secure Real-
                  Time Transport Protocol, Work in progress, August 2006.

        [KMGMT]   Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.
                  Carrara, "Key Management Extensions for Session Description
                  Protocol (SDP) and Real Time Streaming Protocol (RTSP)",
                  RFC 4567, July 2006.

        [SDPCapNegRqts]   Andreasen, F. "SDP Capability Negotiation:
                  Requirementes and Review of Existing Work", work in
                  progress, December 2006.

        [SDPCapNeg] Andreasen, F. "SDP Capability Negotiation", work in
                  progress, December 2006.

        [MIKEY]   J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K.
                  Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
                  August 2004.

     Author's Addresses

        Flemming Andreasen
        Cisco Systems
        Edison, NJ

        Email: fandreas@cisco.com

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