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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 RFC 6871

MMUSIC                                                         R. Gilman
Internet-Draft                                               Avaya, Inc.
Intended status: Standards Track                            R. Even, Ed.
Expires: August 24, 2007                                         Polycom
                                                            F. Andreasen
                                                           Cisco Systems
                                                       February 20, 2007


                   SDP media capabilities Negotiation
            draft-ietf-mmusic-sdp-media-capabilities-01.txt

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Copyright Notice

   Copyright (C) The IETF Trust (2007).











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Abstract

   Session Description Protocol (SDP) capability negotiation provides a
   general framework for negotiating capabilities in SDP.  The base
   framework defines only capabilities for negotiating transport
   protocols and attributes.  In this document, we extend the framework
   by defining media capabilities that can be used to negotiate media
   types and their associated parameters.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  SDP Media capabilities . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Solution Overview  . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Capability Attributes  . . . . . . . . . . . . . . . . . .  8
       3.2.1.  Media Type and Subtype Capability Attribute  . . . . .  8
       3.2.2.  The Capability Encoding Parameters Attribute . . . . .  9
       3.2.3.  The Media Format Parameter Capability Attribute  . . . 10
     3.3.  Extensions to the Potential Configuration Attribute  . . . 11
       3.3.1.  The Media Capability Extension to the Potential
               Configuration Attribute  . . . . . . . . . . . . . . . 11
       3.3.2.  The Payload Type Mapping Extension to the
               Potential Configuration Attribute  . . . . . . . . . . 12
     3.4.  Extensions to the Actual Configuration Attribute . . . . . 12
     3.5.  The Latent Configuration Attribute . . . . . . . . . . . . 13
       3.5.1.  The crypto: Attribute in Latent Configurations . . . . 14
     3.6.  Offer/Answer Model Extensions  . . . . . . . . . . . . . . 14
       3.6.1.  Generating the Initial Offer . . . . . . . . . . . . . 14
       3.6.2.  Generating the Answer  . . . . . . . . . . . . . . . . 14
       3.6.3.  Offerer Processing of the Answer . . . . . . . . . . . 14
       3.6.4.  Modifying the Session  . . . . . . . . . . . . . . . . 14
   4.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     4.1.  Alternative Codecs . . . . . . . . . . . . . . . . . . . . 15
     4.2.  Latent Media Streams . . . . . . . . . . . . . . . . . . . 15
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   7.  Changes from version 00  . . . . . . . . . . . . . . . . . . . 18
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 20
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
   Intellectual Property and Copyright Statements . . . . . . . . . . 22






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1.  Introduction

   Session Description Protocol (SDP) capability negotiation [SDPCapNeg]
   provides a general framework for negotiating capabilities in
   SDP[RFC4566].  The base framework defines only capabilities for
   negotiating transport protocols and attributes.  In this document, we
   extend the framework by defining media capabilities that can be used
   to negotiate media types and their associated parameters.

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

































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2.  Terminology

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













































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3.   SDP Media capabilities

   In this section we first provide an overview of the SDP media
   Capability negotiation solution.  This is followed by definitions of
   new SDP attributes for the solution and its associated updated offer/
   answer procedures [RFC3264]

3.1.  Solution Overview

   The solution consist of the following new attributes extending the
   base attributes from [SDPCapNeg].

   Three attributes are used to make up media capabilities

   o  A new media attribute ("a=cmed") that lists media formats as
      capabilities in the form a media type (e.g. "audio") and one or
      more subtypes (e.g.  "PCMU"), and associates a handle with each
      subtype

   o  A new attribute ("a=cenc") that lists encoding parameter
      capabilities associated with a particular media format capability.

   o  A new attribute ("a=cfmt") that lists media format parameter
      capabilities associated with a particular media format capability.

   A new attribute ("a=lcfg") that specifies latent configurations when
   no corresponding media line is offered.  An example is a latent
   configuration for video even though no video is currently offered.

   A new parameter type ("m=") to the potential configuration
   ("a=pcfg:") attribute and the actual configuration ("a=acfg:")
   attribute defined in [SDPCapNeg], which permits specification of
   media capabilities (including their associated parameters) and
   combinations thereof for the configuration.  For example, the
   "a=pcfg:" line might specify PCUM and telephone events or G.729B and
   telephone events as acceptable configurations.  The "a=acfg:" line in
   the answer would specify the accepted choice.

   A new parameter type ("pt=") to the potential configuration
   ("a=pcfg:") attribute which associates RTP payload types with the
   referenced media capabilities.

   The document extends the base protocol extensions to the offer/answer
   model that allow for capabilities and potential configurations to be
   included in an offer.  Media capabilities constitute capabilities
   that can be used in potential and latent configurations.  Whereas
   potential configurations constitute alternative offers that may be
   accepted by the answerer instead of the actual configuration(s)



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   included in the "m=" line(s), latent configurations merely inform the
   other side of possible configurations supported by the entity.  Those
   latent configurations may be used to guide subsequent offer/answer
   exchanges.

   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 192.0.2.1
             s=
             c=IN IP4 192.0.2.1
             t=0 0
             a=creq:v1
             a=cmed:1 audio g729 iLBC PCMU g729
             a=cenc:2 8000
             a=cfmt:1 annexb:no
             a=ctrpr:1 RTP/SAVP
             m=audio 3456 RTP/AVP 0 18
             a=capar:1 a=crypto:1 AES_CM_128_HMAC_SHA1_32
             inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
             a=pcfg:1 m=1,3|4,3 t=1 a=1 pt=1:100,4:101,3:102
             a=pcfg:2 m=2 pt=2:103

   The required base and extensions are provided by the "a=creq"
   attribute defined in [SDPCapNeg], with the option tag "v1", which
   indicates that the extension framework defined here, must be
   supported.  The Base level support is implied since it is required
   for the extensions.

   The "a=cmed:1" line defines four audio media subtype capabilities ,
   to be numbered consecutively starting with 1.  Note that the media
   subtypes specified in the m-line (PCMU and G729) are explicitly
   specified here.

   The "a=cenc:2" line specifies the clock rate and encoding parmeters



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   (see [RFC4566]) for capability 2, iLBC.

   The "a=cfmt:1" line specifies media format parameter capabilities for
   codec 1 ( no Annex B for G.729).

   The "a=ctrpr:1" line, specified in the base protocol, defines a
   transport protocol capability, in this case Secure RTP.

   The "m=" line indicates that Alice is offering to use plain RTP with
   PCMU or G.729.  The media line implicitly defines the default
   transport protocol (RTP/AVP in this case) and the default actual
   configuration.

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

   The "a=pcfg:" attributes provide the potential configurations
   included in the offer by reference to the media capabilities,
   transport capabilities, and associated payload type mappings.  Two
   explicit alternatives are provided; the first one, numbered 1 is the
   preferred one.  It specifies media capabilities 1 and 3, i.e.  G.729
   and PCMU, or media capability 4 and 3, i.e., G.729B and PCMU.
   Furthermore, it specifies transport protocol capability 1 (i.e. the
   RTP/SAVP profile - secure RTP), and the attribute capability 1, i.e.
   the crypto attribute provided.  Lastly, it specifies, a payload type
   mapping for codecs 1, 3, and 4 thereby permitting the offerer to
   distinguish between encrypted media and unencrypted media received
   prior to receipt of the answer.  For SRTP the offerer will still need
   to receive the answer before being able to decrypt the stream.

   The second alternative specifies media capability 2, i.e. iLBC, under
   the default RTP/AVP profile .  The media line, with any qualifying
   attributes such as fmtp or rtpmap, is itself considered a valid
   configuration; it is assumed to be the lowest preference.

   Bob receives the SDP offer from Alice.  Bob supports RTP, but not
   SRTP, and hence he accepts the actual configuration for RTP provided
   by Alice.  Furthermore, Bob wants to use the iLBC codec and hence
   generates the following answer:

             v=0
             o=- 24351 621814 IN IP4 192.0.2.2
             s=
             c=IN IP4 192.0.2.2
             t=0 0
             a=csup:v1
             m=audio 4567 RTP/AVP 103



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             a=rtpmap:103 8000
             a=acfg:2

   Bob includes the "a=csup" and "a=acfg" attribute in the answer to
   inform Alice that he can support the v1 level of capability
   negotiations.  Note that in this particular example, the answerer
   supported the capability extensions defined here, however had he not,
   he would simply have processed the offer based on the offered PCMU
   and G.729 codecs under the RTP/AVP profile only.  Consequently, the
   answer would have omitted the "a=csup" attribute line and chosen one
   or both of the PCMU and G.729 codecs instead.  The answer carries the
   accepted configuration in the m line along with corresponding rtpmap:
   and/or fmtp: parameters, as appropriate.

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

3.2.  Capability Attributes

   In this section, we present the new attributes associated with
   indicating the media capabilities for use by the SDP Capability
   negotiation.  The approach taken is to keep things similar to the
   existing media capabilities defined by the existing media
   descriptions ("m=" lines) and the associated "rtpmap" and "fmtp"
   attributes, but using "media capability numbers" instead of payload
   types to link the relevant media capability parameters.

3.2.1.  Media Type and Subtype Capability Attribute

   Media types and subtypes can be expressed as media format
   capabilities by use of the "a=cmed" attribute, which is defined as
   follows:

   a=cmed:<med-cap-num> <type> <subtype>*[ <subtype>]

   where <med-cap-num> is an integer between 1 and 2^31-1 (both
   included) used to number the media format capabilities, <type> is a
   media type (e.g., audio or video), and the <subtype> is the media
   subtype e.g.  H263-1998, PCMU (Editors' note: can specify in cmed
   anything that can be specified in an m-line?).  The <med-cap-num> is
   the media capability number associated with the first subtype in the
   list, the number associated with the second subtype 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 entire session or media



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   description (explicitly or implicitly).  In ABNF, we have:

         media-capability-line = "a=cmed:" media-cap-num WSP media-type
                                    WSP media-cap *(WSP media-cap)
           media-cap-num      = 1*DIGIT
           media-type         = token ; Type name (audio, video, etc.)
           media-cap          = token ; Subtype  name(PCMU, G729, etc.)

   Media subtypes identified in "a=cmed" lines may be qualified via the
   attributes, "a=cenc" and "a=cfmt", in much the same way as media
   formats in "m=" lines payload types can have their clock rate and
   encoding parameters qualified by an "a=rtpmap" line and media format
   specific parameters can be provided by "a=fmtp" lines.

3.2.2.  The Capability Encoding Parameters Attribute

   Media format capabilities may require additional encoding parameters,
   such as sample rate, to be precisely defined.  The "a=cenc" encoding
   attribute is defined as

           a=cenc:<med-cap-num> <clock rate>[/<encoding parameters>]

   The clock rate and other encoding parameters are as defined for the
   "a=rtpmap:" attribute defined in RFC 4566 [RFC4566].  For example, a
   capability for low-bit-rate encoding at 8000 samples per second could
   be specified by

             a=cmed:1 iLBC
             a=cenc:1 8000

   The encoding becomes part of the media capability.  Thus, if it is
   desirable to specify the same subtype with, e.g., two different
   encoding rates, then the subtype should be listed twice, and each
   should be modified appropriately.  For example:

             a=cmed:1 L16 L16
             a=cenc:1 8000
             a=cenc:2 16000/2

   defines two low-data-rate codecs, codec 1 uses 8000 samples per
   second, and codec 2 uses 16000 samples per second and 2 channels.
   [EDITOR'S NOTE: I'm thinking that it might be better to put the
   encoding information in the cmed line pcfg "pt", along with the media
   subtype, and eliminate the cenc attribute.  This would make the above
   example look like:

   a=cmed:1 L16/8000 L16/16000/2




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   or

   a=pcfg:1 m=1,3|4,3 t=1 a=1 pt=1:100/8000, 4:101/16000/2, 3:102

   This can be done so long as the encoding attributes never contain
   whitespace.  Is this the case? or instead use semi colon as
   separator]

   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 2.3.1. ).

   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.)

3.2.3.  The Media Format Parameter Capability Attribute

   This attribute is used to associate media format parameters with a
   media capability.  The form of the attribute is:

           a=cfmt:<med-cap-num> <list of format parameters>

   where the format parameters are specific to the type of codec, as
   described for the fmtp: attribute defined in RFC 4566.  As an
   example, a G.729 capability is, by default, considered to support
   comfort noise as defined by Annex B.  Capabilities for G.729 with and
   without comfort noise support may thus be identified by:

             a=cmed:1 audio G729 G729
             a=cfmt:2 annexb:no

   Example for H.263 video:

             a=cmed:1 video H263-1998 H263-2000
             a=cfmt:1 CIF=4;QCIF=2;F=1;K=1
             a=cfmt:2 profile=2;level=2.2








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3.3.  Extensions to the Potential Configuration Attribute

   The extension protocol of capabilities negotiation requires two new
   extensions for the pcfg: attribute defined in the base protocol.  One
   extension permits the specification of media capabilities, or
   combinations thereof; the other permits the assignment of payload
   types to those capabilities when used in the specified configuration.

3.3.1.  The Media Capability Extension to the Potential Configuration
        Attribute

   The potential configuration attribute ("a=pcfg") as defined in SDP
   capabilities negotiation, permits alternate attributes to be
   associated with the media types defined in a media line.  In this
   extension (this document), we define an extension parameter for the
   specification of media configurations in addition to the one
   specified on the media line.

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

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

   where <med-cap-list> is a comma-separated list of media capability
   numbers (media-cap-num) as defined by a=cmed: lines and media lines.

   In ABNF form (adhering to the ABNF for pot-extension-config in
   [SDPCapNeg]:

           pot-media-config = "m=" med-cap-list *(BAR med-cap-list)
           med-cap-list        = med-cap-num *("," med-cap-num)
           med-cap-num      = 1*DIGIT      ; defined in SDP
           BAR                    = *WSP "|" *WSP

   Each potential media configuration is a comma-separated list of media
   capability numbers where med-cap-num refers to media capability
   numbers defined explicitly by a=cmed attributes or implicitly by the
   media line, and hence MUST be between 1 and 2^31-1 (both included).
   Alternative potential media configurations are separated by a
   vertical bar ("|").  The alternatives are ordered by preference.
   When media capabilities are not included in a potential configuration
   at the media level, the media type and media format from the
   associated "m=" line will be used.

   For example:






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             v=0
             o=- 25678 753849 IN IP4 192.0.2.1
             s=
             c=IN IP4 192.0.2.1
             t=0 0
             a=creq:v1
             a=cmed:1 audio PCMU g729 telephone-event
             m=audio 3456 RTP/AVP 0 18 100
             a=rtpmap:100 telephone-events
             a=fmtp:100 0-15
             a=pcfg:1 m=2,3|1,3 pt=1:0, 2:18, 3:100

   In this example, G729 is media capability 2, PCMU is media capability
   1, and events is media capability 3.  The a=pcfg: line specifies that
   the preferred configuration is G.729 with dtmf events, second is
   G.711 mu-law with dtmf events.  Intermixing of G.729, G.711, and dtmf
   events is least preferred (the actual configuration provided by the
   "m=" line, which is always the least preferred configuration).

3.3.2.  The Payload Type Mapping Extension to the Potential
        Configuration Attribute

   When media capabilities defined in cmed: attributes are used in
   potential configuration lines, it is necessary to assign payload
   types to them.  In some cases, it is desirable to assign different
   payload types to media capabilities defined in the media line.  One
   example of the latter is when configurations for AVP and SAVP are
   offered: the offerer would like the answerer to use different payload
   types for encrypted and unencrypted media so that it (the offerer)
   can decide whether or not to render early media which arrives before
   the answer is received.

   We define the media type mapping configuration parameter, pot-media-
   map, in accordance with the following format:

           pot-media-map = "pt=" med-map-num *("," *WSP med-map-num)
           med-map-num      = 1*DIGIT ":" 1*DIGIT      ;

   The example in the previous section shows how the parameters from the
   cmed line are mapped to payload type in the pcfg "pt" parameter.

3.4.  Extensions to the Actual Configuration Attribute

   We define an actual configuration extension parameter act-media-
   config in accordance with the following ABNF:

           a=acfg:<med-cap-num> *WSP "m=" <med-cap-list>




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   A response to the previous offer example in the above section might
   be:

             v=0
             o=- 24351 621814 IN IP4 192.0.2.2
             s=
             c=IN IP4 192.0.2.2
             t=0 0
             a=csup:v1
             m=audio 5432 RTP/AVP 18 100
             a=rtpmap:100 events
             a=fmtp:100 0-15
             a=acfg:1 m=2,3

   Note that the capability numbers expressed in the acfg: attribute are
   based on the offered capability numbering, not on those listed in the
   answer.  The acfg identify to the offrer which potetial configuration
   was selected by the answerer.

3.5.   The Latent Configuration Attribute

   One of the goals of this work is to permit the exchange of media
   configurations in addition to those offered for immediate use.  Such
   configurations are referred to as "latent configurations".  For
   example, a party may offer to establish an audio session, and, at the
   same time, announce its ability to support a video session.

   Latent configurations may be announced by use of the latent
   configuration attribute, which is defined in a manner very similar to
   the potential configuration attribute:

     a=lcfg:<preference> ["m="<media-caps>] ["t="<transport>]
                ["a="<attributes>]

   The m=, t= and a= parameters are identical in format and meaning to
   those defined for the pcfg: attribute.  Note that the pt= parameter
   is not permitted in the lcfg: attribute because no actual media
   session is being offered or accepted.

   Latent Configurations may be specified at the session level in offers
   and in answers.

   [Editor's note: Do you have a good example of an offer of an audio
   stream with a latent video stream?][Editor's note (rrg): What if, as
   Roni suggested, we exclude the a= parameters from the lcfg: lines?
   We can include the t= parameter as the 'cheapest' way to indicate
   SRTP.  That conveys most of the necessary info; the details can be
   firmed up in the actual offer when/if it's actually made.]



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3.5.1.   The crypto: Attribute in Latent Configurations

   If the sdescriptions crypto: attribute is necessary as part of any
   latent configuration which announces sdescriptions capabilities. then
   it presents a slight problem in that the rather long key/salt string
   is useless and should be ignored.  This problem is avoided if we
   exclude the a= parameters from the lcfg: attribute.  [Editor's note:
   should we define a new crypto: key-method, e.g. "latent", in which
   the key-salt portion of key-info is empty?  I think it may be
   sufficient to just include the RTP/SAVP transport to indicate SRTP
   capability]

3.6.  Offer/Answer Model Extensions

   In this section, we define extensions to the offer/answer model
   defined in RFC3264 to allow for media capabilities to be used with
   the SDP Capability Negotation framework.

3.6.1.  Generating the Initial Offer

3.6.2.  Generating the Answer

3.6.3.  Offerer Processing of the Answer

3.6.4.  Modifying the Session


























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4.  Examples

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

4.1.  Alternative Codecs

4.2.  Latent Media Streams











































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5.  IANA Considerations


















































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6.  Security Considerations


















































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7.  Changes from version 00

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













































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8.  Acknowledgements

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











































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9.  References

9.1.  Normative References

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

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

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

   [SDPCapNeg]
              Andreasen, F., "SDP Capability Negotiation",
              draft-ietf-mmusic-sdp-capability-negotiation-02 (work in
              progress), February 2007.

9.2.  Informative References

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




























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Authors' Addresses

   Robert R Gilman
   Avaya, Inc.
   1300 West 120th Avenue
   Westminster, CO 80234
   USA

   Email: rrg@avaya.com


   Roni Even (editor)
   Polycom
   94 Derech Em Hamoshavot
   Petach Tikva  49130
   Israel

   Email: roni.even@polycom.co.il


   Flemming Andreasen
   Cisco Systems
   Edison, NJ
   USA

   Email: fandreas@cisco.com

























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Full Copyright Statement

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