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Versions: (draft-andreasen-mmusic-sdp-capability-negotiation) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 5939

MMUSIC Working Group                                       F. Andreasen
Internet-Draft                                            Cisco Systems
Intended Status: Proposed Standard                    February 13, 2007
Expires: August 2007


                        SDP Capability Negotiation
            draft-ietf-mmusic-sdp-capability-negotiation-02.txt


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   This Internet-Draft will expire on August 13, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

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



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   (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.

Table of Contents


   1. Introduction...................................................3
   2. Conventions used in this document..............................5
   3. SDP Capability Negotiation Solution............................6
      3.1. Solution Overview.........................................6
      3.2. Version and Extension Indication Attributes...............9
         3.2.1. Supported Capability Negotiation Extensions Attribute9
         3.2.2. Required Capability Negotiation Extension Attribute.10
      3.3. Capability Attributes....................................12
         3.3.1. Attribute Capability Attribute......................12
         3.3.2. Transport Protocol Capability Attribute.............13
      3.4. Configuration Attributes.................................15
         3.4.1. Potential Configuration Attribute...................15
         3.4.2. Actual Configuration Attribute......................18
      3.5. Offer/Answer Model Extensions............................20
         3.5.1. Generating the Initial Offer........................20
         3.5.2. Generating the Answer...............................21
         3.5.3. Offerer Processing of the Answer....................22
         3.5.4. Modifying the Session...............................22
      3.6. Interactions with ICE....................................23
      3.7. Processing Media before Answer...........................24
   4. Examples......................................................24
      4.1. Best-Effort Secure RTP...................................24
      4.2. Multiple Transport Protocols.............................27
      4.3. Session-Level MIKEY and Media Level Security Descriptions30
      4.4. Capability Negotiation with Interactive Connectivity
      Establishment.................................................30
   5. Security Considerations.......................................30
   6. IANA Considerations...........................................30
   7. To Do and Open Issues.........................................30
   8. Acknowledgments...............................................30


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   9. Change Log....................................................31
      9.1. draft-ietf-mmusic-sdp-capability-negotiation-02..........31
      9.2. draft-ietf-mmusic-sdp-capability-negotiation-01..........31
      9.3. draft-ietf-mmusic-sdp-capability-negotiation-00..........32
   10. References...................................................34
      10.1. Normative References....................................34
      10.2. Informative References..................................34
   Author's Addresses...............................................36
   Intellectual Property Statement..................................36
   Full.............................................................37
   Copyright Statement..............................................37
   Acknowledgment...................................................37

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, 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 for the session and its associated media
      stream components, supported by each side.


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   o  A set of 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 combinations of
      session parameters and media stream components to use and with
      what parameters.

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

   SDP by itself was designed to provide only one of these, 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 as well as 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


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   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 limitations 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 is the Session Initiation
   Protocol (SIP) [RFC3261].  SIP uses the offer/answer model (which is
   not specific to SIP) to negotiate sessions and hence the mechanism
   defined here defines the offer/answer procedures to use for the
   capability negotiation framework.

   The rest of the document is structured as follows. In Section 3. we
   present our SDP capability negotiation solution, which consists of
   new SDP attributes and associated offer/answer procedures. In Section
   4. we provide examples illustrating its use and in Section 5. we
   provide the security considerations.

2. 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].




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3. 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.

3.1. Solution Overview

   The solution consists of the following:

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

       o  A new attribute ("a=csup") that lists the supported 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 in order to do capability
          negotiation.

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

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

       o  A new attribute ("a=tcap") 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
          capabilities from the SDP in question. Multiple potential
          configurations have an explicitly indicated ordering
          associated with them. Extension capabilities can be defined
          and referenced 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 be included as
          well.




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   o  Extensions to the offer/answer model that allow for capabilities
      and potential configurations to be included in an offer.
      Capabilities can be provided at the session level or the media
      level. Potential configurations can be included at the media level
      only, where they constitute alternative 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=acfg") in the
      answer.  Capabilities 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 (SRTP)                |
                  |<---------------------------------|
                  |                                  |

   Alice's offer includes RTP and SRTP as alternatives. RTP is the
   default (actual configuration), but SRTP is the preferred one
   (potential configuration):

      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=creq: v0
      a=tcap:1 RTP/SAVP
      a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_32
         inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
      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 includes the option tag "v0" to indicate
   that the base framework defined here must be supported. The
   capabilities are provided by the "a=tcap" and "a=acap" attributes.
   The transport capabilities ("a=tcap") indicate that secure RTP under
   the AVP profile ("RTP/SAVP") is supported with an associated


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   transport capability handle of 1. The "acap" attribute provides an
   attribute capability with a handle of 1. The attribute capability is
   a "crypto" attribute, which provides the keying material for SRTP
   using SDP security descriptions [SDES]. The "a=pcfg" attribute
   provides the potential configuration included in the offer by
   reference to the capability parameters.  One alternative is provided;
   it has a configuration number of 1 and it consists of transport
   protocol capability 1 (i.e. the RTP/SAVP profile - secure RTP), and
   the attribute capability 1, i.e. the crypto attribute provided.
   Potential configurations are always preferred over actual
   configurations, and hence Alice is expressing a preference for using
   secure RTP.

   Bob receives the SDP offer from Alice. Bob supports SRTP and the SDP
   Capability Negotiation framework, and hence he accepts the
   (preferred) 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=acfg:1 t=1 a=1

   Bob includes the "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
   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. If Bob supported one or more extensions to the
   capability negotiation framework, he would have included those in the
   answer as well (in an "a=csup" attribute).

   Note that in this particular example, the answerer supported the
   capability negotiation extensions defined here, however had he not,
   the answerer would simply have ignored the new attributes and
   accepted the (actual configuration) 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



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      t=0 0
      m=audio 4567 RTP/AVP 0 18

3.2. Version and Extension Indication Attributes

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

3.2.1. 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 6.

   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
   generated the SDP. The attribute is defined as follows:

      a=csup: <option-tag-list>

   RFC 4566, Section 9, provides the ABNF for SDP attributes. The "csup"
   attribute adheres to the RFC 4566 "attribute" production, with an
   att-value defined as follows:

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

   Note that white-space is permitted before the option-tag-list. Also,
   implementers familiar with SIP should note that the above definition
   of COMMA differs from the one in [RFC3261].

   A special base option tag with a value of "v0" is defined for the
   basic SDP capability negotiation framework. Entities 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 the "v0" option tags and two hypothetical option tags, "foo" and
   "bar":




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      a=csup: v0
      a=csup: foo
      a=csup: bar
      a=csup: v0, 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 (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 at the session and/or media-level, unless those option tags
   are already provided in one or more "a=creq" attribute (see Section
   3.2.2. ) at the relevant levels. The base option tag MAY be included.

3.2.2. 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 6.

   The Required Capability Negotiation Extensions attribute ("a=creq")
   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>

   The "creq" attribute adheres to the RFC 4566 "attribute" production,
   with an att-value defined as follows:

      att-value         = *WSP option-tag-list

   where "option-tag-list" is defined in Section 3.2.1.  Note that
   white-space is permitted before the option-tag-list.

   The following examples illustrate the use of the "a=creq" attribute
   with the "v0" base option tag and two hypothetical option tags, "foo"
   and "bar":


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      a=creq: v0
      a=creq: foo
      a=creq: bar
      a=creq: v0, 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 (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).

   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 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 ("a=creq") or supported
     ("a=csup")  attributes from the offer, then the answerer is not
     required to include a required ("a=creq") attribute with any
     option-tags that may need to be supported (such as the base option
     tag - "v0").

   A recipient that receives an SDP and does not support one or more of
   the required 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 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, i.e. all the capability negotiation attributes should be
   ignored.  In that case, the entity SHOULD include a "csup" attribute
   listing the SDP capability negotiation extensions it actually
   supports.



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     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

3.3. Capability Attributes

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

3.3.1. Attribute Capability Attribute

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

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

   where <att-cap-num> is an integer between 1 and 2^31-1 (both
   included) used to number the attribute capability and <att-par> is an
   attribute ("a=") in its full  '<type>=<value>' form (see [SDP]).

   The "acap" attribute adheres to the RFC 4566 "attribute" production,
   with an att-value defined as follows:

      att-value   = *WSP att-cap-num 1*WSP att-par
      att-cap-num = 1*DIGIT ;defined in [RFC4234]
      att-par     = attribute  ;defined in RFC 4266

   Note that white-space is permitted before the att-cap-num. The "acap"
   attribute can be provided at the session level for session-level
   attributes and the media level for media-level attributes. The "acap"
   attribute MUST NOT be used to provide a media-level attribute at the
   session-level or vice versa.

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

     There is a need to be able to reference both session-level and
     media-level attributes in potential configurations at the media



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     level, and this provides for a simple solution to avoiding overlap
     between the references (handles) to each attribute capability.

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

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

      a=acap: 1 a=ptime:20

      a=acap: 2 a=ptime:30

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

      a=acap: 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 provides SRTP parameters by using MIKEY with the key-mgmt
   attribute [KMGMT]. The fourth provides SRTP parameters by use of
   security descriptions with the crypto attribute [SDES]. Note that the
   line-wrapping and new-lines in example three and four are provided
   for formatting reasons only - they are not permitted in actual SDP.

     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 "acap"
     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 "acap"
     attribute also is not automatically associated with any particular
     capabilities.

3.3.2. Transport Protocol Capability Attribute

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

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

   where <trpr-cap-num> is an integer between 1 and 2^31-1 (both
   included) used to number the transport address capability for later


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   reference, and <proto-list> is one or more <proto>, separated by
   white space, as defined in the SDP "m=" line.

   The "tcap" attribute adheres to the RFC 4566 "attribute" production,
   with an att-value defined as follows:

      att-value      = *WSP trpr-cap-num 1*WSP proto-list
      trpr-cap-num   = 1*DIGIT ;defined in [RFC4234]
      proto-list     = proto *(1*WSP proto) ; defined in RFC 4566

   Note that white-space is permitted before the trpr-cap-num. The
   "tcap" attribute can be provided at the session- and media-level.
   Each occurrence of the "tcap" 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 capability attributes,
   i.e., they form a separate name-space for transport protocol
   capabilities.

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

      a=tcap: 1 RTP/AVP
      a=tcap: 2 RTP/AVPF
      a=tcap: 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.

   Transport capabilities are inherently included in the "m=" line,
   however they still need to be specified explicitly in a "tcap"
   attribute, if they are to be used as a capability. This may seem
   redundant (and indeed it is from the offerer's point of view),
   however it is done to protect against middle-boxes that may modify
   "m=" lines while passing unknown attributes through. If an implicit
   capability were used instead (e.g. a reserved transport capability
   number could be used to refer to the transport protocol in the "m="
   line), and a middle-box were to modify the transport protocol in the
   "m=" line (e.g. to translate between plain RTP and secure RTP), then
   the potential configuration referencing that implicit transport
   capability may no longer be correct. With explicit capabilities, we
   avoid this pitfall, although the potential configuration preference
   (see Section 3.4.1. ) may not reflect that of the middle-box (which
   some may view as a feature).


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3.4. Configuration Attributes

3.4.1. Potential Configuration Attribute

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

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

   where <config-number> is an integer between 1 and 2^31-1 (both
   included).

   The "pcfg" attribute adheres to the RFC 4566 "attribute" production,
   with an att-value defined as follows:

      att-value      = *WSP config-number 1*WSP pot-cfg-list
      config-number  = 1*DIGIT ;defined in [RFC4234]
      pot-cfg-list   = pot-config *(1*WSP pot-config)
      pot-config     = pot-attribute-parameter-config /
                       pot-transport-protocol-config /
                       pot-extension-config

   The missing productions are defined below. Note that white-space is
   permitted before the config-number.

   The potential configuration attribute can be provided at the media-
   level only. The attribute includes a configuration number, which is
   an integer between 1 and 2^31-1 (both included). The configuration
   number MUST be unique within the media stream. The configuration
   number also indicates the relative preference of potential
   configurations; lower numbers are preferred over higher numbers.

   After the configuration number, one or more potential configuration
   parameters MUST be provided. This document defines potential
   attribute parameter configurations and potential transport protocol
   configurations.  Each of these MUST NOT be present more than once in
   a particular potential configuration attribute. Potential extension
   configurations can be included as well; unknown potential extension
   configurations MUST be ignored (if support is required, then the
   "a=creq" with a suitable option tag should be used). 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 potential configuration.





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   There can be multiple potential configurations provided within a
   media description. Each of these indicates not only a willingness,
   but in fact a desire to use the potential configuration.

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

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

   Each potential attribute parameter configuration list is a comma-
   separated list of attribute capability numbers where att-cap-num
   refers to attribute capability numbers defined above and hence MUST
   be between 1 and 2^31-1 (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 with the most preferred listed first.

   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 =
                           "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 2^31-1 (both included).
   Alternative potential transport protocol configurations are separated
   by a vertical bar ("|").  The alternatives are ordered by preference
   with the most preferred listed first. When transport protocol
   capabilities are not included in a potential configuration at the
   media level, the transport protocol information from the associated
   "m=" line will be used.

     In the presence of middle-boxes (the existence of which may not be
     known), care should be taken with assuming that the transport
     protocol in the "m=" line will not be modified by a middle-box. Use
     of an explicit capability will guard against the capability
     indications of that.




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

      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 2^31-1 (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.

     The "creq" attribute and its associated rules can be used to ensure
     that required extensions are supported in the first place.

   Potential configurations can be provided at the media level only,
   however it is possible to reference capabilities provided at either
   the session or media level. There are certain semantic rules and
   restrictions associated with this:

   A (media level) potential configuration in a given media description
   MUST NOT reference a media-level capability provided in a different
   media description; doing so invalidates that potential configuration.
   A potential configuration can however reference a session-level
   capability. The semantics of doing so (should that potential
   configuration be chosen), depends on the type of capability. In the
   case of transport capabilities, this has no particular implication.
   In the case of attribute capabilities however, it does. More
   specifically, the corresponding attribute value (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 with that value at the session-
   level in the first place. Note that individual media streams perform
   capability negotiation individually, and hence it is possible that
   another media stream (where the attribute was part of a potential
   configuration) chose a configuration without that session level
   attribute. The session-level attribute however remains "active" and
   hence applies to the entire session. It is up to the entity that
   generates the SDP to ensure that in such cases, the resulting active
   configuration SDP is still meaningful.



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   The session-level operation of extension capabilities is undefined:
   Consequently, if session-level extension capabilities are defined,
   they MUST specify the implication of making them part of an active
   configuration at the media level.

   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/AVPF 0 18
      a=creq: v0
      a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32
         inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
      a=tcap: 1 RTP/AVPF RTP/AVP
      a=tcap: 3 RTP/SAVP RTP/SAVPF
      a=pcfg:1 t=4|3 a=1
      a=pcfg:8 t=1|2

   We have two potential configurations listed here. The first one (and
   most preferred, since its configuration number is "1") indicates that
   either of the profiles RTP/SAVPF or RTP/SAVP (specified by the
   transport protocol capability numbers 4 and 3) can be supported with
   attribute capability 1 (the "crypto" attribute); RTP/SAVPF is
   preferred over RTP/SAVP since its capability number (4) is listed
   first in the preferred potential configuration. The second potential
   configuration indicates that the RTP/AVPF of RTP/AVP profile can be
   used, with RTP/AVPF being the preferred one. This non secure RTP
   alternative is the less preferred one since its configuration number
   is "8".

3.4.2. 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 configuration
   number and the attribute 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. Note that the
   configuration number and all capability numbers used are those from
   the offer; not the answer.



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

      a=acfg: <act-cfg-list>

   The "acfg" attribute adheres to the RFC 4566 "attribute" production,
   with an att-value defined as follows:

      att-value      = *WSP config-number 1*WSP act-cfg-list
                        ;config-number defined in Section 3.4.1.
      act-cfg-list   =  capability *(1*WSP capability)
      capability     =  act-attribute-parameter-config /
                           act-transport-protocol-config /
                           act-extension-config

      act-attribute-parameter-config =
               "a=" acap-cap-list   ; defined in Section 3.4.1.

      act-transport-protocol-config =
               "t=" trpr-cap-num    ; defined in Section 3.4.1.

      act-extension-config =
               ext-cap-name "=" ext-cap-list ; defined in Section 3.4.1.

   Note that white-space is permitted before the config-number. The
   actual configuration ("a=acfg") attribute can be provided at the
   media-level only. 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/SAVPF 0
      a=creq: 0
      a=acfg:1 t=4 a=1

   It indicates that the answerer used an offer consisting of potential
   configuration number 1 with transport protocol capability 4 from the
   offer (RTP/SAVPF) and attribute capability 1 (the "crypto"
   attribute).





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3.5. 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.

3.5.1. 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 required extensions attribute ("a-
      creq") that contains the option tag "v0". It must either be
      provided at the session-level or for each individual media stream.
      Option tags for any other required extensions MUST be included as
      well (in accordance with Section 3.2.2. )

   o  one or more attribute capability attributes (as defined in Section
      3.3.1. ) 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 3.3.2. ) 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 3.4. ) if alternative potential configurations are to be
      negotiated.

   o  one or more required capability negotiation extension attributes
      (as defined in Section 3.2.2. ), if the answerer is required to
      support one or more SDP capability negotiation extensions.

   The offerer SHOULD furthermore include the following:







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   o  one or more supported capability negotiation extension attributes
      ("a=csup" as defined in Section 3.2.1. ), if the offerer supports
      one or more SDP capability negotiation 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.

   The current actual configuration is included in the "m=" line (as
   defined by [RFC3264]). Per [RFC3264], once the offerer generates the
   offer, he must be prepared to receive incoming media in accordance
   with that offer. That rule applies here as well, but for the actual
   configurations only; media received by the offerer according to one
   of the potential configurations MAY be discarded, until the offerer
   receives an answer indicating what the actual configuration is. Once
   that answer is received, incoming media MUST be processed in
   accordance with the actual configuration indicated and the answer
   received.

3.5.2. 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 (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


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   used by the answerer. The actual configuration attribute in the
   answer MUST include information about the attribute capabilities,
   transport protocol parameters, and extension capabilities from the
   potential configuration that were used to generate the answer.

3.5.3.  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 attribute capabilities ("a=acap"),
   transport protocol capabilities ("a=tcap"), 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.

   If the answerer selected one of the potential configurations from the
   offer as the actual configuration, then the offerer SHOULD perform
   another offer/answer exchange, where the offer contains the selected
   potential configuration as the actual configuration, i.e. with the
   actual configuration used in the "m=" line and any other relevant
   attributes. This second offer/answer exchange will not modify the
   session anyway, however it will help intermediaries that look at the
   SDP, but do not understand the capability negotiation extensions, to
   understand the details of the negotiated media streams.

3.5.4. 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.

   If the answer indicates use of a potential configuration from the
   offer, then a second offer/answer exchange using that potential
   configuration as the actual configuration SHOULD be performed.




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3.6. Interactions with ICE

   Interactive Connectivity Establishment (ICE) [ICE] provides a
   mechanism for verifying connectivity between two endpoints by sending
   STUN messages directly between the media endpoints. The basic ICE
   specification [ICE] is defined to support UDP-based connectivity
   only, however it allows for extensions to support other transport
   protocols, such as TCP, which is being specified in [ICETCP]. ICE
   defines a new "a=candidate" attribute, which, among other things,
   indicates the possible transport protocol(s) to use and then
   associates a priority with each of them. The most preferred transport
   protocol that *successfully* verifies connectivity will end up being
   used.

   When using ICE, it is thus possible that the transport protocol that
   will be used differs from what is specified in the "m=" line.
   Furthermore, since both ICE and SDP Capability Negotiation may now
   specify alternative transport protocols, there is a potentially
   unintended interaction when using these together.

   We provide the following guidelines for addressing that.

      [EDITOR'S NOTE: This requires more work]

   There are two basic scenarios to consider here:

   1) A particular media stream can run over different transport
   protocols (e.g. UDP, TCP, or TCP/TLS), and the intent is simply to
   use the one that works (in the preference order specified).

   2) A particular media stream can run over different transport
   protocols (e.g. UDP, TCP, or TCP/TLS) and the intent is to have the
   negotiation process decide which one to use (e.g. T.38 over TCP or
   UDP).

   In scenario 1, there should be ICE "a=candidate" attributes for UDP,
   TCP, etc. but otherwise nothing special in the potential
   configuration attributes to indicate the desire to use different
   transport protocols (e.g. UDP, or TCP). The ICE procedures
   essentially cover the capability negotiation required (by having the
   answerer select something it supports and then use of trial and
   error).

   Scenario 2 does not require a need to support or use ICE. Instead, we
   simply use transport protocol capabilities and potential
   configuration attributes to indicate the desired outcome.



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   The scenarios may be combined, e.g. by offering potential
   configuration alternatives where some of them can support one
   transport protocol only (e.g. UDP), whereas others can support
   multiple transport protocols (e.g. UDP or TCP). In that case, the ICE
   candidate attributes should be defined as attribute capabilities and
   the relevant ones should then be included in the proper potential
   configurations (for example candidate attributes for UDP only for
   potential configurations that are restricted to UDP, whereas there
   could be candidate attributes for UDP, TCP, and TCP/TLS for potential
   configurations that can use all three).

3.7. Processing Media before Answer

   The offer/answer model requires an offerer to be able to receive
   media in accordance with the offer prior to receiving the answer.
   This property is retained with the SDP capability negotiation
   extensions defined here, but only when the actual configuration is
   selected by the answerer. If a potential configuration is chosen, it
   is permissible for the offerer to not process any media received
   before the answer is received. This however may lead to clipping.

   In the case of SIP, this issue could be solved easily by defining a
   precondition [RFC3312] for capability negotiation, however
   preconditions are viewed as complicated to implement and they add to
   overall session establishment delay by requiring an extra
   offer/answer exchange. An alternative is therefore desirable.

   The SDP capability negotiation framework does not define such an
   alternative, however extensions may do so. For example, one technique
   proposed for best-effort SRTP in [BESRTP] is to provide different RTP
   payload type mappings for different transport protocols used. The
   basic SDP capability negotiation framework defined here does not
   include the ability to do so, however extensions that enable that may
   be defined.

4. Examples

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

4.1. 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,


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   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 (SRTP)                |
                  |<---------------------------------|
                  |                                  |
                  | (3) Offer (SRTP)                 |
                  |--------------------------------->|
                  |                                  |
                  | (4) Answer (SRTP)                |
                  |<---------------------------------|
                  |                                  |


   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=creq: v0
      a=tcap:1 RTP/SAVP RTP/AVP
      a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_80
         inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
         FEC_ORDER=FEC_SRTP
      a=pcfg:1 t=1 a=1

   The "m=" line indicates that Alice is offering to use plain RTP with
   PCMU or G.729.  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=tcap" and "a=acap" attributes.  The "tcap" capability indicates
   that both Secure RTP and normal RTP are supported. The "acap"
   attribute provides a capability parameter with a handle of 1. The
   capability parameter is a "crypto" attribute, which provides the
   keying material for SRTP using SDP security descriptions [SDES]. The


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   "a=pcfg" attribute provides the potential configurations included in
   the offer by reference to the capabilities.  A single potential
   configuration with a configuration number of "1" is provided. It
   includes is transport protocol capability 1 (RTP/SAVP, i.e. secure
   RTP) together with the attribute capability 1, i.e. the crypto
   attribute provided.

   Bob receives the SDP offer from Alice. Bob supports SRTP and the 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=acfg:1 t=1 a=1

   Bob includes the "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
   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.

   When Alice receives Bob's answer, session negotiation has completed,
   however Alice nevertheless generates a new offer using the actual
   configuration. This is done purely to assist any middle-boxes that
   may reside between Alice and Bob but do not support the capability
   negotiation extensions (and hence may not understand the negotiation
   that just took place):

   Alice's updated offer includes only SRTP, and it is not using the SDP
   capability negotiation extensions (Alice could have included the
   capabilities as well is she wanted to):

      v=0
      o=- 25678 753850 IN IP4 128.96.41.1
      s=
      c=IN IP4 128.96.41.1
      t=0 0
      m=audio 3456 RTP/SAVP 0 18
      a=crypto:1 AES_CM_128_HMAC_SHA1_80



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         inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
         FEC_ORDER=FEC_SRTP

   The "m=" line now indicates that Alice is offering to use secure RTP
   with PCMU or G.729.  The "crypto" attribute, which provides the SRTP
   keying material, is included with the same value again.

   Bob receives the SDP offer from Alice, which he accepts, and then
   generates an answer to Alice:

      v=0
      o=- 24351 621815 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

   Bob includes the same crypto attribute as before, and the session
   proceeds without change. Although Bob did not include any
   capabilities in his answer, he could of course have done so if he
   wanted to.

   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 received in step 1 and
   accepted the offer to use normal RTP. In that case, the following
   answer would have been generated in step 2 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


4.2. Multiple Transport Protocols

   [EDITOR'S NOTE: Example to be updated - old copy below]

   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,


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   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 (SRTP)                |@@
                  |<---------------------------------|
                  |                                  |
                  | (3) Offer (SRTP)                 |
                  |--------------------------------->|
                  |                                  |
                  | (4) Answer (SRTP)                |
                  |<---------------------------------|
   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=creq: v0
      a=tcap:1 RTP/SAVP RTP/AVP
      a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_80
         inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
         FEC_ORDER=FEC_SRTP
      a=pcfg:5 t=1 a=1
      a=pcfg:10 t=2

   The "m=" line indicates that Alice is offering to use plain RTP with
   PCMU or G.729.  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=tcap" and "a=acap" attributes.  The capabilities indicate that
   both Secure RTP and normal RTP are supported. The "acap" 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


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   alternatives are provided; the first one with preference "5" (and
   hence the 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 1, i.e. the crypto attribute
   provided. The second one is using transport protocol capability 2.
   Note that we could have omitted the second potential configuration
   since it equals the actual configuration (which is always the least
   preferred configuration).

   Bob receives the SDP offer from Alice. Bob supports SRTP and the 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=csup: foo
      a=acfg:1 t=1 a=1

   Bob includes the "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
   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



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      m=audio 4567 RTP/AVP 0 18




4.3. Session-Level MIKEY and Media Level Security Descriptions

   [EDITOR'S NOTE: Example to be added]



4.4. Capability Negotiation with Interactive Connectivity Establishment

   [EDITOR'S NOTE: Example to be added]



5. Security Considerations

   TBD.

6. 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]



7. To Do and Open Issues

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

8. Acknowledgments

   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.




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

9. Change Log

9.1. draft-ietf-mmusic-sdp-capability-negotiation-02

   The following are the major changes compared to version -01:

   o  Potential configurations are no longer allowed at the session
      level

   o  Renamed capability attributes ("capar" to "acap" and "ctrpr" to
      "tcap")

   o  Changed name and semantics of the initial number (now called
      configuration number) in potential configuration attributes; must
      now be unique and can be used as a handle

   o  Actual configuration attribute now includes configuration number
      from the selected potential configuration attribute

   o  Added ABNF throughout

   o  Specified that answerer should include "a=csup" in case of
      unsupported required extensions in offer.

   o  Specified use of second offer/answer exchange when answerer
      selected a potential configuration

   o  Updated rules (and added restrictions) for referencing media- and
      session-level capabilities in potential configurations (at the
      media level)

   o  Added initial section on ICE interactions

   o  Added initial section on receiving media before answer

9.2. 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.




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   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.

9.3. 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 "tcap"
      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.


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















































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10. References

10.1. 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.

10.2. 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.





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   [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.





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   [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.

   [ICE]     J. Rosenberg, "Interactive Connectivity Establishment
             (ICE): A Methodology for Network Address Translator (NAT)
             Traversal for Offer/Answer Protocols", work in progress,
             January 2007.

   [ICETCP]  J. Rosenberg, "TCP Candidates with Interactive Connectivity
             Establishment (ICE)", work in progress, October 2006.



   [RFC3312] G. Camarillo, W. Marshall, and J. Rosenberg, "Integration
             of Resource Management and Session Initiatio Protocol
             (SIP)", RFC 3312, October 2002.



Author's Addresses

   Flemming Andreasen
   Cisco Systems
   Edison, NJ

   Email: fandreas@cisco.com


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Acknowledgment

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