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Versions: 00 01

MMUSIC WG                                               M. Garcia-Martin
Internet-Draft                                                  Ericsson
Intended status: Standards Track                         S. Veikkolainen
Expires: September 9, 2011                                         Nokia
                                                               R. Gilman
                                                           March 8, 2011


Title and Bandwidth Capabilities Negotiation in the Session Description
                             Protocol (SDP)
                     draft-mmusic-sdp-icap-bcap-01

Abstract

   SDP has been extended with a capability negotiation mechanism
   framework that allows the endpoints to negotiate transport protocols
   and attributes.  This framework has been extended with a media
   capabilities negotiation mechanism that allows endpoints to negotiate
   additional media-related capabilities.  This negotiation is embedded
   into the widely-used SDP offer/answer procedures.

   This memo extends the SDP capability negotiation framework to allow
   endpoints to negotiate two additional SDP capabilities.  In
   particular, this memo provides a mechanism to negotiate titles ("i="
   line for each session or media) and bandwidth ("b=" line).

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 9, 2011.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  4
   3.  Protocol Description . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Extensions to SDP  . . . . . . . . . . . . . . . . . . . .  4
       3.1.1.  Bandwidth Capability . . . . . . . . . . . . . . . . .  6
       3.1.2.  Title Capability . . . . . . . . . . . . . . . . . . .  8
     3.2.  Session Level versus Media Level . . . . . . . . . . . . . 11
     3.3.  Offer/Answer model extensions  . . . . . . . . . . . . . . 11
       3.3.1.  Generating the Initial Offer . . . . . . . . . . . . . 12
       3.3.2.  Generating the Answer  . . . . . . . . . . . . . . . . 12
       3.3.3.  Offerer Processing of the Answer . . . . . . . . . . . 12
       3.3.4.  Modifying the Session  . . . . . . . . . . . . . . . . 12
   4.  Field Replacement Rules  . . . . . . . . . . . . . . . . . . . 12
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
     5.1.  New SDP Attributes . . . . . . . . . . . . . . . . . . . . 13
     5.2.  New Option Tags  . . . . . . . . . . . . . . . . . . . . . 14
     5.3.  New SDP Capability Negotiation Configuration Parameters  . 14
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 14
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 15
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15


























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1.  Introduction

   The Session Description Protocol (SDP) [RFC4566] is intended for
   describing multimedia sessions for the purposes of session
   announcement, session invitation, and other forms of multimedia
   session initiation.  SDP has been extended with a capability
   negotiation mechanism framework [RFC5939] which allows the endpoints
   to negotiate capabilities, such as support for Real-time Transport
   Protocol (RTP) [RFC3550] and Secure Real-time Transport Protocol
   (SRTP) [RFC3711].  The SDP media capabilities
   [I-D.ietf-mmusic-sdp-media-capabilities] provides negotiation
   capabilities to media lines as well.

   The capability negotiation is embedded into the widely used SDP
   offer/answer procedure [RFC3264].  This memo provides the means to
   negotiate further capabilities than those specified in the SDP
   capability negotiation mechanism framework [RFC5939] and the SDP
   media capabilities negotiation
   [I-D.ietf-mmusic-sdp-media-capabilities].  In particular, this memo
   provides a mechanism to negotiate session or media titles ("i=") and
   bandwidth ("b=").

   Since the two added capabilities are highly unconnected, it is not
   expected that implementations will support both at the same time.
   Instead, it is expected that applications will choose their needed
   capability for their specific purpose.  Due to this, we are writing
   the normative part pertaining to both capabilities in a self-
   contained section: Section 3.1.1 describes the bandwidth capability
   extension, and Section 3.1.2 describes the title capability
   extension.  Separate option tags are defined for both capabilities.


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 BCP 14, RFC 2119
   [RFC2119] and indicate requirement levels for compliant
   implementations.


3.  Protocol Description

3.1.  Extensions to SDP

   The SDP Capability Negotiation Framework [RFC5939] and the SDP media
   capabilities negotiation [I-D.ietf-mmusic-sdp-media-capabilities]
   specify attributes for negotiating SDP capabilities.  These documents



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   specify new attributes (e.g., 'acap', 'tcap', 'mcap') for achieving
   their purpose.  In this document we define two new additional
   capability attributes for SDP lines of the the general form:

      type=value

   for types "i" and "b".  The corresponding capability attributes are
   defined as "icap" for title capability, and "bcap" for bandwidth
   capability, respectively.

   From the sub-rules of "a=" line in SDP [RFC4566], SDP attributes are
   of the form:

   attribute          = (att-field ":" att-value) / att-field
   att-field          = token
   att-value          = byte-string

   Capability attributes use only the 'att-field:att-value' form.

   The new attributes may be referenced in potential configurations
   ("a=pcfg") or in latent configurations ("a=lcfg"), as productions
   conforming to the extension-config-list as defined in [RFC5939].

   extension-config-list = ["+"] ext-cap-name "=" ext-cap-list
   ext-cap-name          = 1*(ALPHA / DIGIT)
                          ; ALPHA and DIGIT defined in RFC5234
   ext-cap-list          = 1*VCHAR  ; VCHAR defined in RFC5234

   The optional "+" is used to indicate that the extension is mandatory
   and MUST be supported in order to use that potential configuration.

   The attributes may be referenced in actual configurations ("a=acfg")
   as productions conforming to the sel-extension-config defined in
   [RFC5939].

   sel-extension-config = ext-cap-name "=" 1*VCHAR

   The specific parameters are defined in the individual description of
   each capability, below.

   The "icap" and "bcap" capability attributes can be provided either at
   the session or media level.  According to the SDP Capability
   Negotiation [RFC5939], each extension capability must specify the
   implication of making it part of a configuration at the media level.

   According to SDP [RFC4566], "b=" and "i=" lines may appear either at
   session or media level.  In line with this, the "bcap" and "icap"
   capability attributes, when declared at session level, are to be



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   interpreted as-if that attribute was provided with that value at the
   session level.  The "bcap" and "icap" capability attributes declared
   at media level, are to be interpreted as-if that capability attribute
   was declared at the session level.

   For example, extending the example in
   [I-D.ietf-mmusic-sdp-media-capabilities] with "icap" and "bcap"
   capability attributes, we get the following SDP:

   v=0
   a=bcap:1 CT:200
   a=icap:1 Video conference
   m=audio 54320 RTP/AVP 0
   a=mcap:1 L16/8000/1
   a=mcap:2 L16/16000/2
   a=pcfg:1 m=1|2, pt=1:99,2:98
   m=video 66544 RTP/AVP 100
   a=mcap:3,4 H263-1998/90000
   a=rtpmap:100 H264/90000
   a=pcfg:10 m=3 pt=3:101 b=1 i=1

     Example SDP offer with bcap and icap defined at
          session level

   The above SDP defines one PCMU audio stream and one H.264 video
   stream.  It also defines two Media Format capabilities (numbered 1
   and 2), using L16 audio at 8 kbps and 16 kbps, respectively, as well
   as Media Format capabilities for H.263 video (numbered 3 and 4).  The
   Media Format capabilities all appear at the media level.  The example
   also contains a single bandwidth capability and a single title
   capability at session level.  According to the definition above, when
   the capabilities defined in "bcap", and "icap" attributes are
   referenced from the potential configuration, in the resulting SDP
   they are to be interpreted as session level attributes (but the Media
   Format capabilities are to be interpreted as media level attributes).

3.1.1.  Bandwidth Capability

   According to RFC 4566 [RFC4566] the bandwidth field denotes the
   proposed bandwidth to be used by the session or media.  In this memo,
   we specify the bandwidth capability attribute which can also appear
   either at session or media level.  The bandwidth field is specified
   in RFC 4566 [RFC4566] with the following syntax:

      b=<bwtype>:<bandwidth>

   where <bwtype> is an alphanumeric modifier giving the meaning of the
   <bandwidth> figure.




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   In this document, we define a new capability attribute: the bandwidth
   capability attribute "bcap".  This attribute lists bandwidth as
   capabilities according to the following definition:

      "a=bcap:" bw-cap-num 1*WSP bwtype ":" bandwidth CRLF

   where <bw-cap-num> is a unique integer between 1 and 2^31-1 (both
   included) user to number the bandwidth capability, and the other
   elements are as defined for the "b=" field in SDP [RFC4566].

   This format satisfies the general attribute production rules in SDP
   [RFC4566] according to the following Augmented Backus-Naur Form
   (ABNF) [RFC5234] syntax:

   att-field       = "bcap"
   att-value       = bw-cap-num 1*WSP bwtype ":" bandwidth
   bw-cap-num      = 1*10(DIGIT)   ; DIGIT defined in RFC5234

   Negotiation of bandwidth per media stream can be useful when
   negotiating media encoding capabilities with different bandwidths.

3.1.1.1.  Configuration Parameters

   The SDP capability negotiation framework [RFC5939] provides for the
   existence of the "pcfg" and "acfg" attributes.  The concept is
   extended by the SDP media capabilities negotiation
   [I-D.ietf-mmusic-sdp-media-capabilities] with an "lcfg" attribute
   that conveys latent configurations.

   Extensions to the "pcfg" and "lcfg" attributes are defined through
   <extension-config-list>, and extensions to the "acfg" attribute are
   defined through the <sel-extension-config> as defined in the SDP
   Capability Negotiation [RFC5939].

   In this document we extend the <extension-config-list> field to be
   able to convey lists of bandwidth capabilities in latent or potential
   configurations, according to the following Augmented Backus-Naur Form
   (ABNF) [RFC5234] syntax:

   extension-config-list  = bandwidth-config-list
   bandwidth-config-list  = ["+"] "b=" bw-cap-list *(BAR bw-cap-list)
                            ; BAR defined in RFC5939
   bw-cap-list            = bw-cap-num *("," bw-cap-num)
   bw-cap-num             = 1*10(DIGIT)   ; DIGIT defined in RFC5234

       Figure 1: Syntax of the bandwidth parameter in lcfg and pcfg
                                attributes




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   Each bandwidth capability configuration is a comma-separated list of
   bandwidth capability attribute numbers where 'bw-cap-num' refers to
   the bw-cap-num bandwidth capability numbers defined explicitly
   earlier in this document, and hence must be between 1 and 2^31-1
   (both included).  Alternative bandwidth configurations are separated
   by a vertical bar ("|").

   The above syntax is very flexible, allowing referencing to multiple
   "b=" lines per configuration, even for the same bwtype.  While the
   need for such definitions is not seen, we have not restricted this,
   as it is not restricted in SDP [RFC4566] either.

   The bandwidth parameter to the actual configuration attribute
   ("a=acfg") is formulated as a sel-extension-config with

      ext-cap-name = "b"

   hence

   sel-extension-config = sel-bandwidth-config
   sel-bandwidth-config = "b=" bw-cap-list  ; bw-cap-list as above.

      Figure 2: Syntax of the bandwidth parameter in acfg attributes

3.1.1.2.  Option tag

   The SDP Capability Negotiation Framework [RFC5939] allows for
   capability negotiation extensions to be defined.  Associated with
   each such extension is an option tag that identifies the extension in
   question.  Hereby, we define a new option tag "bcap-v0" that
   identifies support for the bandwidth capability.  The endpoints using
   the "bcap" capability attribute SHOULD add the option tag to other
   existing option tags present in the "csup" and "creq" attributes in
   SDP, according to the procedures defined in the SDP Capability
   Negotiation Framework [RFC5939].

3.1.2.  Title Capability

   SDP [RFC4566] provides for the existence of an information field
   expressed in the format of the "i=" line, which can appear either at
   the session level or at the media level.  An "i=" line that is
   present at the session level is known as the "session name", and its
   purpose is to convey a human-readable textual information about the
   session.

   The "i=" line in SDP can also appear at the media level, in which
   case it is used to provide human-readable information about the media
   stream to which it is related, e.g., it may indicate the purpose of



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   the media stream.  The "i=" line is not to be confused with the label
   attribute ("a=label:", [RFC4574]) which provides a machine-readable
   tag.  It is foreseen that applications declaring capabilities related
   to different configurations of a media stream may need to provide
   different identifying information for each of those configurations.
   That is, a party might offer alternative media configurations for a
   stream, each of which represents a different presentation of the same
   or similar information.  For example, an audio stream might offer
   English or Spanish configurations, or a video stream might offer a
   choice of video source such as speaker camera, group camera, or
   document viewer.  The title capability is needed to inform the
   answering user in order to select the proper choice, and the label is
   used to inform the offering machine which choice the answerer has
   selected.  Hence, there is value in defining a mechanism to provide
   titles of media streams as capabilities.

   According to SDP [RFC4566], the session information ("i=") line has
   the following syntax:

      "i="text

   where "text" represents a human-readable text indicating the purpose
   of the session or media stream.

   In this document we define a new capability attribute: the Title
   capability, "icap".  This attribute lists session or media titles as
   capabilities, according to the following definition:

      "a=icap:" title-cap-num 1*WSP text

   where <title-cap-num> is a unique integer between 1 and 2^31-1 (both
   included) user to number the unique ordinal identifier of the
   particular title capability and <text> is a human-readable text that
   indicates the purpose of the session or media stream it is supposed
   to characterize.

   As an example, one might use:

      a=icap:1 Document Camera

   to define a title capability number 1 to identify a particular source
   of a media stream.

   The title capability attribute satisfies the general attribute
   production rules in SDP [RFC4566] according to the following
   Augmented Backus-Naur Form (ABNF) [RFC5234] syntax:





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   att-field       = "icap"
   att-value       = title-cap-num 1*WSP text
                                    ; text defined in RFC4566
   title-cap-num    = 1*10(DIGIT)   ; DIGIT defined in RFC5234

3.1.2.1.  Configuration Parameters

   The SDP Capability Negotiation Framework [RFC5939] provides for the
   existence of the "pcfg" and "acfg" attributes.  The concept is
   extended by the SDP media capabilities negotiation
   [I-D.ietf-mmusic-sdp-media-capabilities] with an "lcfg" attribute
   that conveys latent configurations.

   In this document, we define an <title-config-list> parameter to be
   used to convey title capabilities in a potential or latent
   configuration.  This parameter is defined as an <extension-config-
   list> with the following associations:

      ext-cap-name = "i"

      ext-cap-list = title-cap-list

   This leads to the following definition for the title capability
   parameter:

   extension-config-list = title-config-list
   title-config-list    = ["+"] "i=" title-cap-list
   title-cap-list       = title-cap-num *(BAR title-cap-num)
                                      ; BAR defined in RFC5939
   title-cap-num        = 1*10(DIGIT) ; DIGIT defined in RFC5234

    Figure 3: Syntax of the title capability parameter in lcfg and pcfg
                                attributes

   Each potential capability configuration contains a single title
   capability attribute number where 'title-cap-num' is the title
   capability number defined explicitly earlier in this document, and
   hence must be between 1 and 2^31-1 (both included).  The title
   capability allows the expression of only a single capability in each
   alternative, since no more than a single title field is permitted per
   block.  Nevertheless, it is still allowed to express alternative
   potential title configurations separated by a vertical bar ("|").

   An endpoint includes a plus sign ("+") in this configuration
   attribute to mandate support for this extension.  An endpoint that
   receives this attribute prefixed with a plus sign and does not
   support this extension MUST treat that potential configuration as not
   valid.



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3.1.2.2.  Option Tag

   At present, it is difficult to envision a scenario in which the
   "icap" attribute must be supported or the offer must be rejected.  In
   most cases, if the icap attribute or its contents were to be ignored,
   an offered configuration could still be chosen based on other
   criteria such as configuration numbering.  However, one might imagine
   an SDP offer that contained English and Spanish potential
   configurations for an audio stream.  The session might be
   unintelligible if the choice is based on configuration numbering,
   rather than informed user selection.  Based on such considerations,
   it may well prove useful to announce the ability to use the icap
   attribute and its contents to select media configurations, or to
   inform the user about the selected configuration(s).  Therefore, we
   define a new option tag of "icap-v0" that identifies support for the
   title capability.  This option tag SHOULD be added to other existing
   option tags present in the "csup" and/or "creq" attributes in SDP,
   according to the procedures defined in the SDP Capability Negotiation
   Framework [RFC5939].  The discussion above suggests that "icap-v0"
   will typically appear in a "csup" attribute, but rarely in a "creq"
   attribute.

3.2.  Session Level versus Media Level

   The "icap" and "bcap" attributes can appear at the session level
   and/or at the media level.  Endpoints MUST interpret capabilities
   declared at session level as part of the session level in the
   resulting SDP for that particular configuration.  Endpoints MUST
   interpret capabilities declared at media level as part of the media
   level in the resulting SDP for that particular configuration.

   If an "icap" or "bcap" capability for the same bwtype is declared at
   both session and media level, the media level attribute overrides the
   value of the session level attribute.

   To avoid confusion, the <type-attr-num> for each "a=bcap" and
   "a=icap" line must be unique across all capability attributes of the
   same type within the entire session description.

3.3.  Offer/Answer model extensions

   In this section, we define extensions to the offer/answer model
   defined in SDP Offer/Answer Model [RFC3264] and extended in the SDP
   Capability Negotiation [RFC5939] to allow for bandwidth and title
   capabilities to be used with the SDP Capability Negotiation
   framework.





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3.3.1.  Generating the Initial Offer

   When an endpoint generates an initial offer and wants to use the
   functionality described in the current document, it first defines
   appropriate values for the bandwidth and title capability attributes
   according to rules defined in [RFC4566] for "b=" and "i=" lines.  The
   endpoint then MUST include the respective capability attributes and
   associated values in the SDP offer.  The preferred configurations for
   each media stream are identified following the media line in a "pcfg"
   attribute.  Bandwidth and title capabilities may also be referenced
   in latent configurations, defined in
   [I-D.ietf-mmusic-sdp-media-capabilities].

   The offer SHOULD include the level of capability negotiation
   extensions needed to support this functionality in a "creq"
   attribute.

3.3.2.  Generating the Answer

   When the answering party receives the offer, and if it supports the
   required capability negotiation extensions, it SHOULD select the most
   preferred configuration it can support for each media stream, and
   build the answer accordingly, as defined in Section 3.6.2 of the SDP
   Capability Negotiation [RFC5939].

3.3.3.  Offerer Processing of the Answer

   When the offerer receives the answer, it MUST process the media lines
   according to normal SDP processing rules to identify the media
   stream(s) accepted by the answer, if any.  The "acfg" attribute, if
   present, may be used to verify the proposed configuration used to
   form the answer, and to infer the lack of acceptability of higher-
   preference configurations that were not chosen.

3.3.4.  Modifying the Session

   If, at a later time, one of the parties wishes to modify the
   operating parameters of a session, e.g. by adding a new media stream,
   or by changing the properties used on an existing stream, it may do
   so via the mechanisms defined for SDP offer/answer [RFC3264].


4.  Field Replacement Rules

   To simplify the construction of SDP records, given the need to
   include fields within the media description in question for endpoints
   that do not support capabilities negotiation, we define some simple
   field-replacement rules for those fields invoked by potential or



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   latent configurations.  In particular, any "i=" line invoked by a
   configuration MUST replace the corresponding line, if present within
   the media description in question.  Any "b=" line invoked by a
   configuration MUST replace any "b=" of the same bandwidth type at the
   media level.


5.  IANA Considerations

5.1.  New SDP Attributes

   IANA is hereby requested to register new attributes in the "att-field
   (both session and media level)" of the "Session Description Protocol
   (SDP) Parameteres" registry, according to the following registration
   form:

      Attribute name: icap

      Long form name: Title Capability

      Type of attribute: Both media and session level

      Subject to charset: Yes

      Purpose: Negotiate human-readable information describing the
      session or media

      Appropriate values: See Section 3.1.2 of RFC XXXX

      [Note to the RFC Editor: Please replace the above RFC XXXX with
      the RFC number of this specification.

      Contact name: Miguel A. Garcia, Miguel.A.Garcia@ericsson.com

      Attribute name: bcap

      Long form name: Bandwidth Capability

      Type of attribute: Both media and session level

      Subject to charset: No

      Purpose: Negotiate session or media-level bandwidths

      Appropriate values: See Section 3.1.1 of RFC XXXX

      [Note to the RFC Editor: Please replace the above RFC XXXX with
      the RFC number of this specification.



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      Contact name: Miguel A. Garcia, Miguel.A.Garcia@ericsson.com

5.2.  New Option Tags

   IANA is hereby requested to add the new option tags "bcap-v0" and
   "icap-v0", defined herein, to the "SDP Capability Negotiation Option
   Tag subregistry" of the "Session Description Protocol (SDP)
   Parameters" registry.

5.3.  New SDP Capability Negotiation Configuration Parameters

   IANA is hereby requested to add the new parameter identifiers "i" for
   "title" and "b" for "bandwidth" to the "SDP Capability Negotiation
   Potential Configuration Parameters" subregistry of the "Session
   Description Protocol (SDP) Parameters" registry.  These parameters
   are permitted in 'lcfg', 'acfg', and 'pcfg' attributes.


6.  Security Considerations

   This document provides an extension on top of RFC 4566 [RFC4566], RFC
   3264 [RFC3264], SDP Capability Negotiation Framework [RFC5939], and
   SDP media capabilities negotiation
   [I-D.ietf-mmusic-sdp-media-capabilities].  As such, the security
   considerations of those documents apply.

   The bandwidth capability attribute may be used for reserving
   resources at endpoints and intermediaries which inspect the SDP.
   Modification of the bandwidth value by an attacker can lead to the
   network being underutilized (too high bandwidth value) or congested
   (too low bandwidth value).  In case it is essential to protect the
   bandwidth value, one of the security mechanisms proposed in [RFC5939]
   should be used.

   The "i=" line and thus the value carried in the title capability
   attribute is intended for human-readable description only.  It should
   not be parsed programmatically.


7.  Acknowledgments

   Thanks to Christer Holmberg, Alf Heidermark, and Ingemar Johansson
   for arguing for the existence of this document and early reviewing
   it.  Thanks to Flemming Andreasen for a detailed review and many
   improvement suggestions.


8.  References



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8.1.  Normative References

   [I-D.ietf-mmusic-sdp-media-capabilities]
              Gilman, R., Even, R., and F. Andreasen, "SDP Media
              Mapabilities Negotiation",
              draft-ietf-mmusic-sdp-media-capabilities-11 (work in
              progress), February 2011.

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

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

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

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

   [RFC5939]  Andreasen, F., "Session Description Protocol (SDP)
              Capability Negotiation", RFC 5939, September 2010.

8.2.  Informative References

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [RFC4574]  Levin, O. and G. Camarillo, "The Session Description
              Protocol (SDP) Label Attribute", RFC 4574, August 2006.















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Authors' Addresses

   Miguel A. Garcia-Martin
   Ericsson
   Calle Via de los Poblados 13
   Madrid,   28033
   Spain

   Phone: +34 91 339 1000
   Email: miguel.a.garcia@ericsson.com


   Simo Veikkolainen
   Nokia
   P.O. Box 407
   NOKIA GROUP, FI  00045
   Finland

   Phone: +358 50 486 4463
   Email: simo.veikkolainen@nokia.com


   Robert R. Gilman
   3243 W. 11th Ave. Dr.
   Broomfield, Colorado  80020
   U.S.A.

   Phone: +1 303 898 9780
   Email: bob_gilman@comcast.net






















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