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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                        July 11, 2008
Expires: January 2009


                        SDP Capability Negotiation
           draft-ietf-mmusic-sdp-capability-negotiation-09.txt


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   This Internet-Draft will expire on January 11, 2009.

Copyright Notice

   Copyright (C) The IETF Trust (2008).

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, notably in the form of the


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   offer/answer model defined in RFC 3264. SDP does not define how to
   negotiate one or more alternative transport protocols (e.g. RTP
   profiles) or attributes. This makes it difficult to deploy new RTP
   profiles such as secure RTP or RTP with RTCP-based feedback,
   negotiate use of different security keying mechanisms, etc. It also
   presents problems for some forms of media negotiation.

   The purpose of this document is to address these shortcomings by
   extending SDP with capability negotiation parameters and associated
   offer/answer procedures to use those parameters in a backwards
   compatible manner.

   The document defines a general SDP Capability Negotiation framework.
   It also specifies 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 media formats)
   may be provided in other documents.

Table of Contents


   1. Introduction...................................................3
   2. Conventions used in this document..............................7
   3. SDP Capability Negotiation Solution............................7
      3.1. SDP Capability Negotiation Model..........................7
      3.2. Solution Overview........................................10
      3.3. Version and Extension Indication Attributes..............14
         3.3.1. Supported Capability Negotiation Extensions Attribute14
         3.3.2. Required Capability Negotiation Extensions Attribute15
      3.4. Capability Attributes....................................17
         3.4.1. Attribute Capability Attribute......................17
         3.4.2. Transport Protocol Capability Attribute.............19
         3.4.3. Extension Capability Attributes.....................21
      3.5. Configuration Attributes.................................21
         3.5.1. Potential Configuration Attribute...................21
         3.5.2. Actual Configuration Attribute......................29
      3.6. Offer/Answer Model Extensions............................31
         3.6.1. Generating the Initial Offer........................31
         3.6.2. Generating the Answer...............................34
            3.6.2.1. Example Views of Potential Configurations......40
         3.6.3. Offerer Processing of the Answer....................42
         3.6.4. Modifying the Session...............................43
      3.7. Interactions with ICE....................................44
      3.8. Interactions with SIP Option Tags........................45
      3.9. Processing Media before Answer...........................46
      3.10. Indicating Bandwidth Usage..............................47


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      3.11. Dealing with Large Number of Potential Configurations...48
      3.12. SDP Capability Negotiation and Intermediaries...........49
      3.13. Considerations for Specific Attribute Capabilities......51
         3.13.1. The rtpmap and fmtp Attributes.....................51
         3.13.2. Direction Attributes...............................52
      3.14. Relationship to RFC 3407................................52
   4. Examples......................................................52
      4.1. Multiple Transport Protocols.............................53
      4.2. Best-Effort SRTP with Session-Level MIKEY and Media Level
      Security Descriptions.........................................56
      4.3. SRTP with Session-Level MIKEY and Media Level Security
      Descriptions as Alternatives..................................61
   5. Security Considerations.......................................64
   6. IANA Considerations...........................................67
      6.1. New SDP Attributes.......................................67
      6.2. New SDP Capability Negotiation Option Tag Registry.......68
      6.3. New SDP Capability Negotiation Potential Configuration
      Parameter Registry............................................69
   7. Acknowledgments...............................................69
   8. Change Log....................................................70
      8.1. draft-ietf-mmusic-sdp-capability-negotiation-09..........70
      8.2. draft-ietf-mmusic-sdp-capability-negotiation-08..........70
      8.3. draft-ietf-mmusic-sdp-capability-negotiation-07..........70
      8.4. draft-ietf-mmusic-sdp-capability-negotiation-06..........71
      8.5. draft-ietf-mmusic-sdp-capability-negotiation-05..........72
      8.6. draft-ietf-mmusic-sdp-capability-negotiation-04..........73
      8.7. draft-ietf-mmusic-sdp-capability-negotiation-03..........74
      8.8. draft-ietf-mmusic-sdp-capability-negotiation-02..........74
      8.9. draft-ietf-mmusic-sdp-capability-negotiation-01..........75
      8.10. draft-ietf-mmusic-sdp-capability-negotiation-00.........76
   9. References....................................................77
      9.1. Normative References.....................................77
      9.2. Informative References...................................77
   Author's Addresses...............................................79
   Intellectual Property Statement..................................79
   Full Copyright Statement.........................................80
   Acknowledgment...................................................80

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


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   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. To solve this, RFC 3264 [RFC3264] defined the
   offer/answer model, whereby an offerer constructs an offer SDP that
   lists the media streams, codecs, and other SDP parameters that the
   offerer is willing to use. This offer SDP is sent to the answerer,
   which chooses from among the media streams, codecs and other SDP
   parameters provided, and generates an answer SDP with his
   parameters, based on that choice. The answer SDP is sent back to the
   offerer thereby completing the session negotiation and enabling the
   establishment of the negotiated media streams.

   Taking a step back, we can 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. The capability
      indications by themselves do not imply a commitment to use the
      capabilities in the session.

      Capabilities can for example be that the "RTP/SAVP" profile is
      supported, that the "PCMU" codec is supported, or that the
      "crypto" attribute is supported with a particular value.





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   o  A set of potential configurations indicating which combinations
      of those capabilities can be used for the session and its
      associated media stream components. Potential configurations are
      not ready for use. Instead, they provide an alternative that may
      be used, subject to further negotiation.

      A potential configuration can for example indicate that the
      "PCMU" codec and the "RTP/SAVP" transport protocol are not only
      supported (i.e. listed as capabilities), but they are offered for
      potential use in the session.

   o  An actual configuration for the session and its associated media
      stream components, that specifies which combinations of session
      parameters and media stream components can be used currently and
      with what parameters. Use of an actual configuration does not
      require any further negotiation.

      An actual configuration can for example be that the "PCMU" codec
      and the "RTP/SAVP" transport protocol are offered for use
      currently.

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

   SDP by itself was designed to provide only one of these, namely
   listing of the actual configurations, however over the years, use of
   SDP has been extended beyond its original scope.  Of particular
   importance are the session negotiation semantics that were defined
   by the offer/answer model in RFC 3264. In this model, both the offer
   and the answer contain actual configurations; separate capabilities
   and potential configurations are not supported.

   Other relevant extensions have been defined as well. RFC 3407
   [RFC3407] defined simple capability declarations, which extends SDP
   with a simple and limited set of capability descriptions.  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 [RFC3388], 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 defined a


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   comprehensive capability negotiation framework and protocol that was
   not bound by existing SDP constraints. SDPng was not designed to be
   backwards compatible with existing SDP and hence required both sides
   to support it, with a graceful fallback to legacy operation when
   needed. This, combined with lack of ubiquitous multipart MIME
   support in the protocols that would carry SDP or SDPng, made it
   challenging to migrate towards SDPng. In practice, SDPng has not
   gained traction and as of the time of publication of this document,
   work on SDPng has stopped.  Existing real-time multimedia
   communication protocols such as SIP, RTSP, Megaco, and MGCP continue
   to use SDP.  However, SDP does not address an increasingly important
   problem: the ability to negotiate one or more alternative transport
   protocols (e.g., RTP profiles) and associated parameters (e.g. SDP
   attributes).  This makes it difficult to deploy new RTP profiles
   such as secure RTP (SRTP) [RFC3711], RTP with RTCP-Based Feedback
   [RFC4585], etc.  The 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 [RFC3711], RTP/AVPF [RFC4585], and
   RTP/SAVPF [SAVPF].  In addition to the pressing profile negotiation
   problem, other important real-life limitations have been found as
   well. Keying material and other parameters for example need to be
   negotiated with some of the transport protocols, but not others.
   Similarly, some media formats and types of media streams need to
   negotiate a variety of different parameters.

   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
   that is backwards compatible with existing SDP. 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.




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   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 [RFC3264]
   (which is not specific to SIP) to negotiate sessions and hence the
   mechanism defined here provides 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 the 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].

3. SDP Capability Negotiation Solution

   In this section we first present the conceptual model behind the SDP
   capability negotiation framework followed by an overview of the SDP
   Capability Negotiation solution. We then define new SDP attributes
   for the solution and provide its associated updated offer/answer
   procedures.

3.1. SDP Capability Negotiation Model

   Our model uses the concepts of

   o  Capabilities

   o  Potential Configurations

   o  Actual Configurations

   o  Negotiation Process

   as defined in Section 1. Conceptually, we want to offer not just the
   actual configuration SDP (which is done with the offer/answer model
   defined in [RFC3264]), but the actual configuration SDP as well as
   one or more alternative SDPs, i.e. potential configurations. The
   answerer must choose either the actual configuration, or one of the
   potential configurations, and generate an answer SDP based on that.


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   The offerer may need to perform processing on the answer, which
   depends on the offer that was chosen (actual or potential
   configuration). The answerer therefore informs the offerer which
   configuration the answerer chose. The process can be viewed
   *conceptually* as follows:

        Offerer                           Answerer
        =======                           ========

   1) Generate offer with actual
      configuration and alternative
      potential configurations
   2) Send offer with all configurations

   +------------+
   | SDP o1     |
   | (actual    |
   |  config    |
   |            |-+      Offer
   +------------+ |      ----->   3) Process offered configurations
     | SDP o2     |                  in order of preference indicated
     | (potential |               4) Generate answer based on chosen
     |  config 1) |-+                configuration (e.g. o2), and
     +------------+ |                inform offerer which one was
       | SDP o3     |                chosen
       | (potential |
       |  config 2) |-+
       +------------+ |
         | SDP ...    |
         :            :

                                      +------------+
                                      | SDP a1     |
                        Answer        | (actual    |
                        <-----        |  config,o2)|
                                      |            |
   5) Process answer based on         +------------+
      the configuration that was
      chosen (o2), as indicated in
      the answer

   The above illustrates the conceptual model: The actual solution uses
   a single SDP, which contains the actual configuration (as with
   existing SDP and the offer/answer model defined in [RFC3264]) and
   several new attributes and associated procedures, that encode the



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   capabilities and potential configurations. A more accurate depiction
   of the actual offer SDP is therefore as follows:

          +--------------------+
          | SDP o1             |
          | (actual            |
          |  config            |
          |                    |
          | +-------------+    |
          | | capability 1|    |
          | | capability 2|    |
          | | ...         |    |
          | +-------------+    |   Offer
          |                    |   ----->
          | +-------------+    |
          | | potential   |    |
          | |   config 1  |    |
          | | potential   |    |
          | |   config 2  |    |
          | | ...         |    |
          | +-------------+    |
          |                    |
          +--------------------+

   The above structure is used for two reasons:

   o  Backwards compatibility:   As noted above, support for multipart
      MIME is not ubiquitous. By encoding both capabilities and
      potential configurations in SDP attributes, we can represent
      everything in a single SDP thereby avoiding any multipart MIME
      support issues. Furthermore, since unknown SDP attributes are
      ignored by the SDP recipient, we ensure that entities that do not
      support the framework simply perform the regular RFC 3264
      offer/answer procedures. This provides us with seamless backwards
      compatibility.

   o  Message size efficiency:   When we have multiple media streams,
      each of which may potentially use two or more different transport
      protocols with a variety of different associated parameters, the
      number of potential configurations can be large. If each possible
      alternative is represented as a complete SDP in an offer, we can
      easily end up with large messages. By providing a more compact
      encoding, we get more efficient message sizes.

   In the next section, we describe the exact structure and specific
   SDP parameters used to represent this.


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3.2. Solution Overview

   The solution consists of the following:

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

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

       o  A new attribute ("a=creq") that lists the 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 SDP 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 an
          attribute name and its associated value (if any) as a
          capability.

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

   o  Two new SDP attributes to negotiate configurations as follows:

       o  A new attribute ("a=pcfg") that lists potential
          configurations supported. This is done by reference to the
          capabilities from the SDP in question. Extension capabilities
          can be defined and referenced in the potential
          configurations. Alternative potential configurations have an
          explicit ordering associated with them. Also, potential
          configurations are preferred over the actual configuration
          included in the "m=" line and its associated parameters.

       o  A new attribute ("a=acfg") to be used in an answer SDP. The
          attribute identifies a potential configuration from an offer
          SDP which was used as an actual configuration 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 and 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) and associated parameters. 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)                |
                  |<---------------------------------|
                  |                                  |
                  | (3) Offer (SRTP)                 |
                  |--------------------------------->|
                  |                                  |
                  | (4) Answer (SRTP)                |
                  |<---------------------------------|
                  |                                  |


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

      v=0
      o=- 25678 753849 IN IP4 192.0.2.1
      s=
      c=IN IP4 192.0.2.1
      t=0 0
      m=audio 53456 RTP/AVP 0 18
      a=tcap:1 RTP/SAVP
      a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_80
         inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
      a=pcfg:1 t=1 a=1


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   The "m=" line indicates that Alice is offering to use plain RTP with
   PCMU or G.729.  The capabilities are provided by the "a=tcap" and
   "a=acap" attributes. The transport capability attribute ("a=tcap")
   indicates that secure RTP under the AVP profile ("RTP/SAVP") is
   supported with an associated 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
   [RFC4568]. 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 preferred over the actual configuration included
   in the offer SDP, 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
   and generates the following answer SDP:

      v=0
      o=- 24351 621814 IN IP4 192.0.2.2
      s=
      c=IN IP4 192.0.2.2
      t=0 0
      m=audio 54568 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 using potential configuration 1
   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 option tags for those
   in the answer as well (in an "a=csup" attribute).

   When Alice receives Bob's answer, session negotiation has completed,
   however Alice nevertheless generates a new offer using the
   negotiated configuration as the actual configuration. This is done
   purely to assist any intermediaries that may reside between Alice


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   and Bob but do not support the SDP Capability Negotiation framework,
   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 framework (Alice could have included the
   capabilities as well is she wanted to):

      v=0
      o=- 25678 753850 IN IP4 192.0.2.1
      s=
      c=IN IP4 192.0.2.1
      t=0 0
      m=audio 53456 RTP/SAVP 0 18
      a=crypto:1 AES_CM_128_HMAC_SHA1_80
         inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4

   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 192.0.2.2
      s=
      c=IN IP4 192.0.2.2
      t=0 0
      m=audio 54568 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 have done so if he wanted to.

   Note that in this particular example, the answerer supported the
   capability negotiation extensions defined here. Had he not, he 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 192.0.2.2
      s=
      c=IN IP4 192.0.2.2


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      t=0 0
      m=audio 54568 RTP/AVP 0 18

3.3. Version and Extension Indication Attributes

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

3.3.1. 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 can be provided at the session-
   level and the media-level, and it is defined as follows:

      a=csup: <option-tag-list>

   RFC 4566, Section 9, provides the ABNF [RFC5234] for SDP attributes.
   The "csup" attribute adheres to the RFC 4566 "attribute" production,
   with an att-value defined as follows:

      att-value         = option-tag-list
      option-tag-list   = option-tag *("," option-tag)
      option-tag        = token    ; defined in [RFC4566]

   A special base option tag with a value of "cap-v0" is defined for
   the basic SDP Capability Negotiation framework defined in this
   document. Entities can use this option tag with the "a=csup"
   attribute to indicate support for the SDP Capability Negotiation
   framework specified in this document.

   The following examples illustrate use of the "a=csup" attribute with
   the "cap-v0" option tag and two hypothetical option tags, "foo" and
   "bar" (note the lack of white space):

      a=csup:cap-v0

      a=csup:foo


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      a=csup:bar

      a=csup:cap-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
   description in question only (option-tags provided at the session
   level apply as well). There MUST NOT be more than one "a=csup"
   attribute at the session-level and one at the media-level (one per
   media description 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
   3.3.2. ) at the relevant levels. Inclusion of the base option tag is
   OPTIONAL; support for the base framework can be inferred from
   presence of the "a=pcfg" attribute defined in Section 3.5.1.

     Use of the base option tag may still be useful in some scenarios,
     e.g. when using SIP OPTIONS [RFC3261] or generating an answer to
     an offer that did not use the SDP Capability Negotiation
     framework.

3.3.2. Required Capability Negotiation Extensions Attribute

   The Required Capability Negotiation Extensions attribute ("a=creq")
   contains a comma-separated list of option tags (see Section 3.3.1. )
   specifying 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 attributes and
   associated procedures. There is no need to include the base option-
   tag ("cap-v0") with the "creq" attribute, since any entity that
   supports the "creq" attribute in the first place also supports the
   base option-tag. Still, it is permissible to do so.

     Such functionality may be important if a future version of the
     capability negotiation framework were not backwards compatible.

   The attribute can be provided at the session-level and the media-
   level, and it is defined as follows:

      a=creq: <option-tag-list>



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

      att-value   = option-tag-list

   The following examples illustrate use of the "a=creq" attribute with
   the "cap-v0" base option tag and two hypothetical option tags, "foo"
   and "bar" (note the lack of white space):

      a=creq:cap-v0

      a=creq:foo

      a=creq:bar

      a=creq:cap-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
   description in question only (required option tags provided at the
   session level apply as well). There MUST NOT be more than one
   "a=creq" attribute at the session-level and one "a=creq" attribute
   at the media-level (one per media description 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 (except for the base) at the session or media level 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. If
   support for an extension is needed only in one or more specific
   potential configurations, the potential configuration provides a way
   to indicate that instead (see Section 3.5.1. ). Support for the
   basic negotiation framework is implied by the presence of an
   "a=pcfg" attribute (see Section 3.5.1. ) and hence it is not
   required to include the "a=creq" attribute with the base option-tag
   ("cap-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



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   Capability Negotiation MUST NOT be performed for the media stream in
   question.

     An entity that does not support the SDP Capability Negotiation
     framework at all, will ignore these attributes (as well as the
     other SDP Capability Negotiation attributes) and not perform any
     SDP Capability Negotiation in the first place.

   When an SDP recipient does not support one or more required SDP
   Capability Negotiation extensions listed in the option tags, the
   recipient MUST 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,
   if the SDP recipient is an SDP answerer [RFC3264], the recipient
   SHOULD include a "csup" attribute in the resulting SDP answer
   listing the SDP Capability Negotiation extensions it actually
   supports.

     This ensures that introduction of the SDP Capability Negotiation
     mechanism by itself does not lead to session failures.

3.4. Capability Attributes

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

3.4.1. Attribute Capability Attribute

   Attributes and their associated values can be expressed as
   capabilities 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 "<attribute>" or <attribute>:<value>"
   form, i.e., excluding the "a=" part (see [RFC4566]). The attribute
   can be provided at the session-level and the media-level.

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





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      att-value   = att-cap-num 1*WSP att-par
      att-cap-num = 1*DIGIT ;defined in [RFC5234]
      att-par     = attribute  ;defined in RFC 4566

   Note that white space is not permitted before the att-cap-num.

   The "acap" attribute can be provided at the session level only when
   the attribute capability contains session-level attributes, whereas
   media level attributes can be provided in attribute capabilities at
   either the media level or session-level. The base SDP Capability
   Negotiation framework however only defines procedures for use of
   media-level attribute capabilities at the media level (extensions
   may define use at the session level).

   Each occurrence of the "acap" attribute in the entire session
   description MUST use a different value of <att-cap-num>.
   Consecutive numbering of the <att-cap-num> values is not required.

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

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

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

      a=acap:1 ptime:20

      a=acap:2 ptime:30

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

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

   The first two attribute capabilities provide attribute values for
   the ptime attribute. The third provides SRTP parameters by using
   MIKEY [RFC3830] with the key-mgmt attribute [RFC4567]. The fourth
   provides SRTP parameters by use of security descriptions with the
   crypto attribute [RFC4568]. Note that the line-wrapping and new-


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   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, 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.
     See Section 3.14. for the relationship to RFC 3407.

3.4.2. 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
   reference, and <proto-list> is one or more <proto>, separated by
   white space, as defined in the SDP "m=" line. The attribute can be
   provided at the session-level and the media-level.

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

      att-value      = trpr-cap-num 1*WSP proto-list
      trpr-cap-num   = 1*DIGIT ;defined in [RFC5234]
      proto-list     = proto *(1*WSP proto) ; defined in RFC 4566

   Note that white space is not permitted before the trpr-cap-num.

   The "tcap" attribute can be provided at the session-level and the
   media-level. There MUST NOT be more than one "a=tcap" attribute at
   the session-level and one at the media-level (one per media
   description in the latter case). 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. There MUST NOT be any
   capability number overlap between different "tcap" attributes in the
   entire SDP. The <trpr-cap-num> values provided are independent of
   similar <cap-num> values provided for other capability attributes,


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   i.e., they form a separate name-space for transport protocol
   capabilities. Consecutive numbering of the <trpr-cap-num> values in
   different "tcap" attributes is not required.

   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 [RFC4585]. The
   third one provides capabilities for the "RTP/SAVP" (transport
   capability number 3) and "RTP/SAVPF" profiles (transport protocol
   capability number 4).

   The ability to use a particular transport protocol is inherently
   implied by including it in the "m=" line, regardless of whether it
   is provided in a "tcap" attribute or not. However, if a potential
   configuration needs to reference that transport protocol as a
   capability, the transport protocol MUST be included explicitly in a
   "tcap" attribute.

     This may seem redundant (and indeed it is from the offerer's point
     of view), however it is done to protect against intermediaries
     (e.g. middle-boxes) that may modify "m=" lines while passing
     unknown attributes through. If an implicit transport 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 an intermediary 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; however, the potential
     configuration preference (see Section 3.5.1. ) may not reflect
     that of the intermediary (which some may view as a feature).

   Note that a transport protocol capability may be provided,
   irrespective of whether it is referenced in a potential
   configuration or not (just like any other capability).





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3.4.3. Extension Capability Attributes

   The SDP Capability Negotiation framework allows for new types of
   capabilities to be defined as extensions and used with the general
   capability negotiation framework. The syntax and semantics of such
   new capability attributes are not defined here, however in order to
   be used with potential configurations, they SHOULD allow for a
   numeric handle to be associated with each capability. This handle
   can be used as a reference within the potential and actual
   configuration attributes (see Section 3.5.1. and 3.5.2. ). The
   definition of such extension capability attributes MUST also state
   whether they can be applied at the session-level, media-level, or
   both. Note that extensions can have option tags defined for them,
   which can be registered with the IANA in accordance with the
   procedures specified in Section 6.2.

3.5. Configuration Attributes

3.5.1. 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 attribute can be provided at the media-level only.

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

      att-value      = config-number [1*WSP pot-cfg-list]
      config-number  = 1*DIGIT ;defined in [RFC5234]
      pot-cfg-list   = pot-config *(1*WSP pot-config)
      pot-config     = attribute-config-list /
                       transport-protocol-config-list /
                       extension-config-list

   The missing productions are defined below. Note that white space is
   not permitted before the config-number.

   The potential configuration attribute can be provided at the media-
   level only and there can be multiple instances of it within a given
   media description. 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 description


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   (i.e. it has media level scope only). The configuration number also
   indicates the relative preference of potential configurations; lower
   numbers are preferred over higher numbers. Consecutive numbering of
   the configuration numbers in different "pcfg" attributes in a media
   description is not required.

   A potential configuration list is normally provided after the
   configuration number. When the potential configuration list is
   omitted, the potential configuration equals the actual
   configuration. The potential configuration list contains one or more
   of attribute, transport and extension configuration lists. A
   potential configuration may for example include attribute
   capabilities and transport capabilities, transport capabilities
   only, or some other combination of capabilities.

   The configuration lists generally reference one or more capabilities
   (extension configuration lists MAY use a different format). Those
   capabilities are (conceptually) used to construct a new internal
   version of the SDP by use of purely syntactic add and (possibly)
   delete operations on the original SDP (actual configuration). This
   provides an alternative potential configuration SDP that can be used
   by conventional SDP and offer/answer procedures if selected.

   This document defines attribute configuration lists and transport
   protocol configuration lists.  Each of these MUST NOT be present
   more than once in a particular potential configuration attribute.
   Extension configuration lists can be included as well.  There can be
   more than one extension configuration list, however each particular
   extension MUST NOT be present more than once in a given "a=pcfg"
   attribute. Together, the various configuration lists define a
   potential configuration.

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

   The example SDP below contains two potential configurations:











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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
      m=audio 53456 RTP/AVP 0 18
      a=tcap:1 RTP/SAVP RTP/SAVPF
      a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32
         inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
      a=pcfg:1 t=1 a=1
      a=pcfg:2 t=2 a=1

   Potential configuration 1 contains a transport protocol
   configuration list that references transport capability 1
   ("RTP/SAVP") and an attribute configuration list that references
   attribute capability 1 ("a=crypto:..."). Potential configuration 2
   contains a transport protocol configuration list that references
   transport capability 2 ("RTP/SAVPF") and an attribute configuration
   list that references attribute capability 1 ("a=crypto:...").

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

      attribute-config-list
                        = "a=" [delete-attributes ":"]
                                 mo-att-cap-list *(BAR mo-att-cap-list)

      delete-attributes = DELETE ( "m"    ; media attributes
                              / "s"    ; session attributes
                              / "ms" ) ; media and session attributes

      mo-att-cap-list      = mandatory-optional-att-cap-list |
                                    mandatory-att-cap-list |
                                       optional-att-cap-list

      mandatory-optional-att-cap-list  = mandatory-att-cap-list
                                             "," optional-att-cap-list
      mandatory-att-cap-list           = att-cap-list
      optional-att-cap-list            = "[" att-cap-list "]"

      att-cap-list      = att-cap-num *("," att-cap-num)
      att-cap-num       = 1*DIGIT   ;defined in [RFC5234]
      BAR               = "|"
      DELETE            = "-"



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   Note that white space is not permitted within this production.

   Each attribute configuration list can optionally begin with
   instructions for how to handle attributes that are part of the
   actual configuration SDP (i.e., the "a=" lines present in the
   original SDP). By default, such attributes will remain as part of
   the configuration in question. However, if delete-attributes
   indicates "-m", then all attribute lines within the media
   description in question will be deleted (i.e., all "a=" lines under
   the "m=" line in question). If delete-attributes indicates "-s",
   then all attribute lines at the session-level will be deleted (i.e.,
   all "a=" lines before the first "m=" line). If delete-attributes
   indicates "-ms", then all attribute lines within this media
   description ("m=" line) and all attribute lines at the session-level
   will be deleted.

   The attribute capability list comes next. It contains one or more
   alternative lists of attribute capabilities. The alternative
   attribute capability lists are separated by a vertical bar ("|"),
   and each list contains one or more attribute capabilities separated
   by commas (","). The attribute capabilities are either mandatory or
   optional. Mandatory attribute capabilities MUST be supported in
   order to use the potential configuration, whereas optional attribute
   capabilities MAY be supported in order to use the potential
   configuration.

   Within each attribute capability list, all the mandatory attribute
   capabilities (if any) are listed first, and all the optional
   attribute capabilities (if any) are listed last. The optional
   attribute capabilities are contained within a pair of square
   brackets ("[" and "]"). Each attribute capability is merely an
   attribute capability number (att-cap-num) that identifies a
   particular attribute capability by referring to attribute capability
   numbers defined above and hence MUST be between 1 and 2^31-1 (both
   included). The following example illustrates the above:

      a=pcfg:1 a=-m:1,2,[3,4]|1,7,[5]

   where

   o  "a=-m:1,2,[3,4]|1,7,[5]" is the attribute configuration list

   o  "-m" indicates to delete all attributes from the media
      description of the actual configuration




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   o  "1,2,[3,4]" and "1,7,[5]" are both attribute capability lists.
      The two lists are alternatives, since they are separated by a
      vertical bar above

   o  "1", "2" and "7" are mandatory attribute capabilities

   o  "3", "4" and "5" are optional attribute capabilities

   Note that in the example above, we have a single handle ("1") for
   the potential configuration(s), but there are actually two different
   potential configurations (separated by a vertical bar). This is done
   for message size efficiency reasons, which is especially important
   when we add other types of capabilities to the potential
   configuration. If there is a need to provide a unique handle for
   each, then separate "a=pcfg" attributes with different handles MUST
   be used instead.

   Each referenced attribute capability in the potential configuration
   will result in the corresponding attribute name and its associated
   value (contained inside the attribute capability) being added to the
   resulting potential configuration SDP.

   Alternative attribute capability lists 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. In order for a
   recipient of the SDP (e.g., an answerer receiving this in an offer)
   to use this potential configuration, exactly one of the alternative
   lists MUST be selected in its entirety. This requires that all
   mandatory attribute capabilities referenced by the potential
   configuration are supported with the attribute values provided.

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

      transport-protocol-config-list =
                           "t=" trpr-cap-num *(BAR trpr-cap-num)
      trpr-cap-num        = 1*DIGIT   ; defined in [RFC5234]

   Note that white space is not permitted within this production.

   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 transport protocol capabilities are separated
   by a vertical bar ("|").  The alternatives are ordered by preference


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   with the most preferred listed first. If there are no transport
   protocol capabilities included in a potential configuration at the
   media level, the transport protocol information from the associated
   "m=" line MUST be used. In order for a recipient of the SDP (e.g.,
   an answerer receiving this in an offer) to use this potential
   configuration, exactly one of the alternatives MUST be selected.
   This requires that the transport protocol in question is supported.

     In the presence of intermediaries (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 an intermediary.
     Use of an explicit transport protocol capability will guard
     against capability negotiation implications of that.

   Extension capabilities can be included in a potential configuration
   as well by use of extension configuration lists. Such extension
   configuration lists MUST adhere to the following ABNF:

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

   Note that white space is not permitted within this production.

   The ext-cap-name refers to the name of the extension capability and
   the ext-cap-list is here merely defined as a sequence of visible
   characters. The actual extension supported MUST refine both of these
   further. For extension capabilities that merely need to be
   referenced by a capability number, it is RECOMMENDED to follow a
   structure similar to what has been specified above. Unsupported or
   unknown potential extension configuration lists in a potential
   configuration attribute MUST be ignored, unless they are prefixed
   with the plus ("+") sign, which indicates that the extension is
   mandatory and MUST be supported in order to use that potential
   configuration.

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

   Potential configuration attributes 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:




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   A (media level) potential configuration attribute in a given media
   description MUST NOT reference a media-level capability provided in
   a different media description; doing so invalidates that potential
   configuration (note that a potential configuration attribute can
   contain more than one potential configuration by use of
   alternatives). A potential configuration attribute can however
   reference a session-level capability. The semantics of doing so
   depends on the type of capability. In the case of transport protocol
   capabilities it has no particular implication. In the case of
   attribute capabilities however, it does. More specifically, the
   attribute name and value (provided within that attribute capability)
   will be considered part of the resulting SDP for that particular
   configuration at the *session* level. In other words, it will be as-
   if that attribute was provided with that value at the session-level
   in the first place. As a result, the base SDP Capability Negotiation
   framework REQUIRES that potential configurations do not reference
   any session-level attribute capabilities that contain media-level
   attributes (since that would place a media-level attribute at the
   session level). Extensions may modify this behavior, as long as it
   is fully backwards compatible with the base specification.

   Individual media streams perform capability negotiation
   individually, and hence it is possible that one media stream (where
   the attribute was part of a potential configuration) chose a
   configuration without a session level attribute that was chosen by
   another media stream. The session-level attribute however remains
   "active" and applies to the entire resulting potential configuration
   SDP. In theory, this is problematic if one or more session-level
   attributes either conflicts with or potentially interacts with
   another session-level or media-level attribute in an undefined
   manner. In practice, such examples seem to be rare (at least with
   the SDP attributes that had been defined at time of publication of
   this document).

     A related set of problems can occur if we need coordination
     between session-level attributes from multiple media streams in
     order for a particular functionality to work. The grouping
     framework [RFC3388] is an example of this. If we use the SDP
     Capability Negotiation framework to select a session-level group
     attribute (provided as an attribute capability), and we require
     two media descriptions to do this consistently, we could have a
     problem. The FEC grouping semantics [RFC4756] is one example where
     this in theory could cause problems, however in practice, it is
     unclear that there is a significant problem with the grouping
     semantics that had been defined at time of publication of this
     document.


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   Resolving the above issues in general requires inter-media stream
   constraints and synchronized potential configuration processing;
   this would add considerable complexity to the overall solution. In
   practice, with the SDP attributes defined at time of publication of
   this document, it does not seem to be a significant problem, and
   hence the core SDP Capability Negotiation solution does not provide
   a solution to this issue. Instead, it is RECOMMENDED that use of
   session-level attributes in a potential configuration is avoided
   when possible, and when not, that such use is examined closely for
   any potential interaction issues. If interaction is possible, the
   entity generating the SDP SHOULD NOT assume that well-defined
   operation will occur at the receiving entity.

   The session-level operation of extension capabilities is undefined:
   Consequently, each new session-level extension capability defined
   MUST specify the implication of making it part of a 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 192.0.2.1
      s=
      c=IN IP4 192.0.2.1
      t=0 0
      m=audio 53456 RTP/AVPF 0 18
      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 RTP/SAVP RTP/SAVPF
      a=pcfg:1 t=4|3 a=1
      a=pcfg:8 t=1|2

   We have two potential configuration attributes 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. Note that
   although we have a single potential configuration attribute and
   associated handle, we have two potential configurations.

   The second potential configuration attribute indicates that the
   RTP/AVPF or RTP/AVP profiles can be used, with RTP/AVPF being the


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   preferred one. This non secure RTP alternative is the less preferred
   one since its configuration number is "8". Again, note that we have
   two potential configurations here and hence a total of four
   potential configurations in the SDP above.

3.5.2. Actual Configuration Attribute

   The actual configuration attribute identifies which of the potential
   configurations from an offer SDP was selected and used as the actual
   configuration to generate an answer SDP.  This is done by including
   the configuration number and the configuration lists (if any) from
   the offer that were selected and used by the answerer in his
   offer/answer procedure as follows:

   o  A selected attribute configuration MUST include the delete-
      attributes and the known and supported parameters from the
      selected alternative mo-att-cap-list (i.e., containing all
      mandatory and all known and supported optional capability numbers
      from the potential configuration). If delete-attributes were not
      included in the potential configuration, they will of course not
      be present here either.

   o  A selected transport protocol configuration MUST include the
      selected transport protocol capability number.

   o  A selected potential extension configuration MUST include the
      selected extension configuration parameters as specified for that
      particular extension.

   o  When a configuration list contains alternatives (separated by
      "|"), the selected configuration only MUST be provided.

   Note that the selected configuration number and all selected
   capability numbers used in the actual configuration attribute refer
   to those from the offer; not the answer.

     The answer may for example include capabilities as well to inform
     the offerer of the answerers capabilities above and beyond the
     negotiated configuration. The actual configuration attribute does
     not refer to any of those answer capabilities though.

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

      a=acfg: <config-number> [<sel-cfg-list>]




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   where <config-number> is an integer between 1 and 2^31-1 (both
   included).  The attribute can be provided at the media-level only.

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

      att-value      = config-number [1*WSP sel-cfg-list]
                        ;config-number defined in Section 3.5.1.
      sel-cfg-list   = sel-cfg *(1*WSP sel-cfg)
      sel-cfg        = sel-attribute-config /
                           sel-transport-protocol-config /
                           sel-extension-config

      sel-attribute-config =
               "a=" [delete-attributes ":"] mo-att-cap-list
                                    ; defined in Section 3.5.1.

      sel-transport-protocol-config =
               "t=" trpr-cap-num    ; defined in Section 3.5.1.

      sel-extension-config =
               ext-cap-name "=" 1*VCHAR   ; defined in Section 3.5.1.

   Note that white space is not 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 192.0.2.2
      s=
      c=IN IP4 192.0.2.2
      t=0 0
      m=audio 54568 RTP/SAVPF 0
      a=crypto:1 AES_CM_128_HMAC_SHA1_32
         inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32
      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"



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   attribute). The answerer includes his own "crypto" attribute as
   well.

3.6. 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 alternative offers that
   may be accepted by the answerer instead of the actual
   configuration(s) included in the "m=" line(s).

   The procedures defined in the following subsections apply to both
   unicast and multicast streams.

3.6.1. Generating the Initial Offer

   An offerer that wants to use the SDP Capability Negotiation defined
   in this document MUST include the following in the offer:

   o  Zero or more attribute capability attributes. There MUST be an
      attribute capability attribute ("a=acap") as defined in Section
      3.4.1. for each attribute name and associated value (if any) that
      needs to be indicated as a capability in the offer. Attribute
      capabilities may be included irrespective of whether they are
      referenced by a potential configuration or not.

      Session-level attributes and associated values MUST be provided
      in attribute capabilities at the session-level only, whereas
      media-level attributes and associated values can be provided in
      attribute capabilities at either the media-level or session-
      level. Attributes that are allowed at either the session- or
      media-level can be provided in attribute capabilities at either
      level.















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   o  Zero or more transport protocol capability attributes. There MUST
      be transport protocol capabilities as defined in Section 3.4.2.
      with values for each transport protocol that needs to be
      indicated as a capability in the offer. Transport protocol
      capabilities may be included irrespective of whether they are
      referenced by a potential configuration or not.

      Transport protocol capabilities that apply to multiple media
      descriptions SHOULD be provided at the session-level whereas
      transport protocol capabilities that apply to a specific media
      description ("m=" line) only, SHOULD be provided within that
      particular media description. In either case, there MUST NOT be
      more than a single "a=tcap" attribute at the session-level and a
      single "a=tcap" attribute in each media description.

   o  Zero or more extension capability attributes. There MUST be one
      or more extension capability attributes (as outlined in Section
      3.4.3. ) for each extension capability that is referenced by a
      potential configuration. Extension capability attributes that are
      not referenced by a potential configuration can be provided as
      well.

   o  Zero or more potential configuration attributes. There MUST be
      one or more potential configuration attributes ("a=pcfg"), as
      defined in Section 3.5.1. , in each media description where
      alternative potential configurations are to be negotiated. Each
      potential configuration attribute MUST adhere to the rules
      provided in Section 3.5.1. and the additional rules provided
      below.

   If the offerer requires support for more or extensions (besides the
   base protocol defined here), then the offerer MUST include one or
   more "a=creq" attributes as follows:

   o  If support for one or more capability negotiation extensions is
      required for the entire session description, then option tags for
      those extensions MUST be included in a single session-level
      "creq" attribute.

   o  For each media description that requires support for one or more
      capability negotiation extensions not listed at the session-
      level, a single "creq" attribute containing all the required
      extensions for that media description MUST be included within the
      media description (in accordance with Section 3.3.2. ).




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   Note that extensions that only need to be supported by a particular
   potential configuration can use the "mandatory" extension prefix
   ("+") within the potential configuration (see Section 3.5.1. ).

   The offerer SHOULD furthermore include the following:

   o  A supported capability negotiation extension attribute ("a=csup")
      at the session-level and/or media-level as defined in Section
      3.3.2. for each capability negotiation extension supported by the
      offerer and not included in a corresponding "a=creq" attribute
      (i.e., at the session-level or in the same media description).
      Option tags provided in a "a=csup" attribute at the session-level
      indicate extensions supported for the entire session description,
      whereas option tags provided in a "a=csup" attribute in a media
      description indicate extensions supported for that particular
      media description only.

   Capabilities provided in an offer merely indicate what the offerer
   is capable of doing.  They do not constitute a commitment or even an
   indication to use them.  In contrast, each potential configuration
   constitutes an alternative offer that the offerer would like to use.
   The potential configurations MUST be used by the answerer to
   negotiate and establish the session.

   The offerer MUST include one or more potential configuration
   attributes ("a=pcfg") in each media description where the offerer
   wants to provide alternative offers (in the form of potential
   configurations). Each potential configuration attribute in a given
   media description MUST contain a unique configuration number and one
   or more potential configuration lists, as described in Section
   3.5.1.  Each potential configuration list MUST refer to capabilities
   that are provided at the session-level or within that particular
   media description; otherwise, the potential configuration is
   considered invalid. The base SDP Capability Negotiation framework
   REQUIRES that potential configurations do not reference any session-
   level attribute capabilities that contain media-level only
   attributes, however extensions may modify this behavior, as long as
   it is fully backwards compatible with the base specification.
   Furthermore, it is RECOMMENDED that potential configurations avoid
   use of session-level capabilities whenever possible; refer to
   Section 3.5.1.

   The current actual configuration is included in the "m=" line (as
   defined by [RFC3264]) and any associated parameters for the media
   description (e.g., attribute ("a=") and bandwidth ("b=") lines).
   Note that the actual configuration is by default the least-preferred


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   configuration, and hence the answerer will seek to negotiate use of
   one of the potential configurations instead. If the offerer wishes a
   different preference for the actual configuration, the offerer MUST
   include a corresponding potential configuration with the relevant
   configuration number (which indicates the relative preference
   between potential configurations); this corresponding potential
   configuration should simply duplicate the actual configuration.

     This can either be done implicitly (by not referencing any
     capabilities), or explicitly (by providing and using capabilities
     for the transport protocol and all the attributes that are part of
     the actual configuration). The latter may help detect
     intermediaries that modify the actual configuration but are not
     SDP Capability Negotiation aware.

   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
   provided in the offer 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 selected
   configuration is. Once that answer is received, incoming media MUST
   be processed in accordance with the actual selected configuration
   indicated and the answer received (provided the offer/answer
   exchange completed successfully).

   The above rule assumes that the offerer can determine whether
   incoming media adheres to the actual configuration offered or one of
   the potential configurations instead; this may not always be the
   case. If the offerer wants to ensure he does not play out any
   garbage, the offerer SHOULD discard all media received before the
   answer SDP is received. Conversely, if the offerer wants to avoid
   clipping, he should attempt to play any incoming media as soon as it
   is received (at the risk of playing out garbage). For further
   details, please refer to Section 3.9.

3.6.2. Generating the Answer

   When receiving an offer, the answerer MUST check for the presence of
   a required capability negotiation extension attribute ("a=creq")
   provided at the session level. If one is found, then capability
   negotiation MUST be performed. If none is found, then the answerer
   MUST check each offered media description for the presence of a
   required capability negotiation extension attribute ("a=creq") and
   one or more potential configuration attributes ("a=pcfg").
   Capability negotiation MUST be performed for each media description


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   where either of those is present in accordance with the procedures
   described below.

   The answerer MUST first ensure that it supports any required
   capability negotiation extensions:

   o  If a session-level "creq" attribute is provided, and it contains
      an option-tag that the answerer does not support, then the
      answerer MUST NOT use any of the potential configuration
      attributes provided for any of the media descriptions. Instead,
      the normal offer/answer procedures MUST continue as per
      [RFC3264]. Furthermore, the answerer MUST include a session-level
      supported capability negotiation extensions attribute ("a=csup")
      with option tags for the capability negotiation extensions
      supported by the answerer.

   o  If a media-level "creq" attribute is provided, and it contains an
      option tag that the answerer does not support, then the answerer
      MUST NOT use any of the potential configuration attributes
      provided for that particular media description. Instead, the
      offer/answer procedures for that media description MUST continue
      as per [RFC3264] (SDP Capability Negotiation is still performed
      for other media descriptions in the SDP).  Furthermore, the
      answerer MUST include a supported capability negotiation
      extensions attribute ("a=csup") in that media description with
      option tags for the capability negotiation extensions supported
      by the answerer for that media description.

   Assuming all required capability negotiation extensions are
   supported, the answerer now proceeds as follows.

   For each media description where capability negotiation is to be
   performed (i.e. all required capability negotiation extensions are
   supported and at least one valid potential configuration attribute
   is present), the answerer MUST perform capability negotiation by
   using the most preferred potential configuration that is valid to
   the answerer, subject to any local policies. A potential
   configuration is valid to the answerer if:

   1. It is in accordance with the syntax and semantics provided in
      Section 3.5.1.

   2. It contains a configuration number that is unique within that
      media description.




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   3. All attribute capabilities referenced by the potential
      configuration are valid themselves (as defined in Section 3.4.1.
      ) and each of them is provided either at the session-level or
      within this particular media description. For session-level
      attribute capabilities referenced, the attributes contained
      inside them MUST NOT be media-level only attributes.

   4. All transport protocol capabilities referenced by the potential
      configuration are valid themselves (as defined in Section 3.4.2.
      ) and each of them is furthermore provided either at the session-
      level or within this particular media description.

   5. All extension capabilities referenced by the potential
      configuration and supported by the answerer are valid themselves
      (as defined by that particular extension) and each of them are
      furthermore provided either at the session-level or within this
      particular media description. Unknown or unsupported extension
      capabilities MUST be ignored, unless they are prefixed with the
      plus ("+") sign, which indicates that the extension MUST be
      supported in order to use that potential configuration. If the
      extension is not supported, that potential configuration is not
      valid to the answerer.

   The most preferred valid potential configuration in a media
   description is the valid potential configuration with the lowest
   configuration number. The answerer MUST now process the offer for
   that media stream based on the most preferred valid potential
   configuration. Conceptually, this entails the answerer constructing
   an (internal) offer that consists of the actual configuration offer
   SDP, with the following changes for each media stream offered:

   o  If a transport protocol capability is included in the potential
      configuration, then it replaces the transport protocol provided
      in the "m=" line for that media description.

   o  If attribute capabilities are present with a delete-attributes
      session indication ("-s"), then all session-level attributes from
      the actual configuration SDP MUST be deleted in accordance with
      the procedures in Section 3.5.1. If attribute capabilities are
      present with a delete-attributes media indication ("-m"), then
      all attributes from the actual configuration SDP inside this
      media description MUST be deleted.






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   o  If a session-level attribute capability is included, the
      attribute (and its associated value, if any) contained in it MUST
      be added to the resulting SDP. All such added session-level
      attributes MUST be listed before the session-level attributes
      that were initially present in the SDP. Furthermore, the added
      session-level attributes MUST be added in the order they were
      provided in the potential configuration (see also Section 3.5.1.
      ).

         This allows for attributes with implicit preference ordering
         to be added in the desired order; the "crypto" attribute
         [RFC4568] is one such example.

   o  If a media-level attribute capability is included, then the
      attribute (and its associated value, if any) MUST be added to the
      resulting SDP within the media description in question. All such
      added media-level attributes MUST be listed before the media-
      level attributes that were initially present in the SDP in the
      media description in question. Furthermore, the added media-level
      attributes MUST be added in the order they were provided in the
      potential configuration (see also Section 3.5.1. ).

   o  If a supported extension capability is included, then it MUST be
      processed in accordance with the rules provided for that
      particular extension capability.

   Note that a transport protocol from the potential configuration
   replaces the transport protocol in the actual configuration, but an
   attribute capability from the potential configuration is simply
   added to the actual configuration. In some cases, this can result in
   having one or more meaningless attributes in the resulting potential
   configuration SDP, or worse, ambiguous or potentially even illegal
   attributes. Use of delete-attributes for the session and/or media
   level attributes MUST be done to avoid such scenarios. Nevertheless,
   it is RECOMMENDED that implementations ignore meaningless attributes
   that may result from potential configurations.

     For example, if the actual configuration was using Secure RTP and
     included an "a=crypto" attribute for the SRTP keying material,
     then use of a potential configuration that uses plain RTP would
     make the "crypto" attribute meaningless. The answerer may or may
     not ignore such a meaningless attribute. The offerer can here
     ensure correct operation by using delete-attributes to remove the
     crypto attribute (but will then need to provide attribute
     capabilities to reconstruct the SDP with the necessary attributes
     deleted, e.g. rtpmaps).


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   Please refer to Section 3.6.2.1. for examples of how the answerer
   may conceptually "see" the resulting offered alternative potential
   configurations.

   The answerer MUST check that he supports all mandatory attribute
   capabilities from the potential configuration (if any), the
   transport protocol capability (if any) from the potential
   configuration, and all mandatory extension capabilities from the
   potential configuration (if any). If he does not, the answerer MUST
   proceed to the second-most preferred valid potential configuration
   for the media description, etc.

   o  In the case of attribute capabilities, support implies that the
      attribute name contained in the capability is supported and it
      can (and will) be negotiated successfully in the offer/answer
      exchange with the value provided. This does not necessarily imply
      that the value provided is supported in its entirety. For
      example, the "a=fmtp" parameter is often provided with one or
      more values in a list, where the offerer and answerer negotiate
      use of some subset of the values provided. Other attributes may
      include mandatory and optional parts to their values; support for
      the mandatory part is all that is required here.

         A side-effect of the above rule is that whenever an "fmtp" or
         "rtpmap" parameter is provided as a mandatory attribute
         capability, the corresponding media format (codec) must be
         supported and use of it negotiated successfully. If this is
         not the offerer's intent, the corresponding attribute
         capabilities must be listed as optional instead.

   o  In the case of transport protocol capabilities, support implies
      that the transport protocol contained in the capability is
      supported and the transport protocol can (and will) be negotiated
      successfully in the offer/answer exchange.

   o  In the case of extension capabilities, the extension MUST define
      the rules for when the extension capability is considered
      supported and those rules MUST be satisfied.

   If the answerer has exhausted all potential configurations for the
   media description, without finding a valid one that is also
   supported, then the answerer MUST process the offered media stream
   based on the actual configuration plus any session-level attributes
   added by a valid and supported potential configuration from another
   media description in the offered SDP.



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   The above process describes potential configuration selection as a
   per media stream process. Inter-media stream coordination of
   selected potential configurations however is required in some cases.
   First of all, session-level attributes added by a potential
   configuration for one media description MUST NOT cause any problems
   for potential configurations selected by other media descriptions in
   the offer SDP. If the session-level attributes are mandatory, then
   those session-level attributes MUST furthermore be supported by the
   session as a whole (i.e., all the media descriptions if relevant).
   As mentioned earlier, this adds additional complexity to the overall
   processing and hence it is RECOMMENDED not to use session-level
   attribute capabilities in potential configurations, unless
   absolutely necessary.

   Once the answerer has selected a valid and supported offered
   potential configuration for all of the media streams (or has fallen
   back to the actual configuration plus any added session attributes),
   the answerer MUST generate a valid answer SDP based on the selected
   potential configuration SDP, as "seen" by the answerer (see Section
   3.6.2.1. for examples). Furthermore, if the answerer selected one of
   the potential configurations in a media description, the answerer
   MUST include an actual configuration attribute ("a=acfg") within
   that media description. The "a=acfg" attribute MUST identify the
   configuration number for the selected potential configuration as
   well as the actual parameters that were used from that potential
   configuration; if the potential configuration included alternatives,
   the selected alternatives only MUST be included. Only the known and
   supported parameters will be included. Unknown or unsupported
   parameters MUST NOT be included in the actual configuration
   attribute. In the case of attribute capabilities, only the known and
   supported capabilities are included; unknown or unsupported
   attribute capabilities MUST NOT be included.

   If the answerer supports one or more capability negotiation
   extensions that were not included in a required capability
   negotiation extensions attribute in the offer, then the answerer
   SHOULD furthermore include a supported capability negotiation
   attribute ("a=csup") at the session-level with option tags for the
   extensions supported across media streams. Also, if the answerer
   supports one or more capability negotiation extensions for
   particular media descriptions only, then a supported capability
   negotiation attribute with those option-tags SHOULD be included
   within each relevant media description. The required capability
   negotiation attribute ("a=creq") MUST NOT be used in an answer.




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   The offerer's originally provided actual configuration is contained
   in the offer media description's "m=" line (and associated
   parameters). The answerer MAY send media to the offerer in
   accordance with that actual configuration as soon as it receives the
   offer, however it MUST NOT send media based on that actual
   configuration if it selects an alternative potential configuration.
   If the answerer selects one of the potential configurations, then
   the answerer MAY immediately start to send media to the offerer in
   accordance with the selected potential configuration, however the
   offerer MAY discard such media or play out garbage until the offerer
   receives the answer. Please refer to section 3.9. for additional
   considerations and possible alternative solutions outside the base
   SDP Capability Negotiation framework.

   If the offerer selected a potential configuration instead of the
   actual configuration, then it is RECOMMENDED that the answerer sends
   back an answer SDP as soon as possible. This minimizes the risk of
   having media discarded or played out as garbage by the offerer. In
   the case of SIP [RFC3261] without any extensions, this implies that
   if the offer was received in an INVITE message, then the answer SDP
   should be provided in the first non-100 provisional response sent
   back (per RFC3261, the answer would need to be repeated in the 200
   response as well, unless a relevant extension such as [RFC3262] is
   being used).

3.6.2.1. Example Views of Potential Configurations

   The following examples illustrate how the answerer may conceptually
   "see" a potential configuration. Consider the following offered SDP:

      v=0
      o=alice 2891092738 2891092738 IN IP4 lost.example.com
      s=
      t=0 0
      c=IN IP4 lost.example.com
      a=tool:foo
      a=acap:1 key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
      a=tcap:1 RTP/SAVP RTP/AVP
      m=audio 59000 RTP/AVP 98
      a=rtpmap:98 AMR/8000
      a=acap:2 crypto:1 AES_CM_128_HMAC_SHA1_32
         inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
      a=pcfg:1 t=1 a=1|2
      m=video 52000 RTP/AVP 31
      a=rtpmap:31 H261/90000
      a=acap:3 crypto:1 AES_CM_128_HMAC_SHA1_80


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         inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
      a=pcfg:1 t=1 a=1|3


   This particular SDP offers an audio stream and a video stream, each
   of which can either use plain RTP (actual configuration) or secure
   RTP (potential configuration). Furthermore, two different keying
   mechanisms are offered, namely session-level Key Management
   Extensions using MIKEY (attribute capability 1) and media-level SDP
   Security Descriptions (attribute capabilities 2 and 3). There are
   several potential configurations here, however, below we show the
   one the answerer "sees" when using potential configuration 1 for
   both audio and video, and furthermore using attribute capability 1
   (MIKEY) for both (we have removed all the capability negotiation
   attributes for clarity):

      v=0
      o=alice 2891092738 2891092738 IN IP4 lost.example.com
      s=
      t=0 0
      c=IN IP4 lost.example.com
      a=tool:foo
      a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
      m=audio 59000 RTP/SAVP 98
      a=rtpmap:98 AMR/8000
      m=video 52000 RTP/SAVP 31
      a=rtpmap:31 H261/90000

   Note that the transport protocol in the media descriptions indicate
   use of secure RTP.

   Below, we show the offer the answerer "sees" when using potential
   configuration 1 for both audio and video and furthermore using
   attribute capability 2 and 3 respectively (SDP security
   descriptions) for the audio and video stream - note the order in
   which the resulting attributes are provided:

      v=0
      o=alice 2891092738 2891092738 IN IP4 lost.example.com
      s=
      t=0 0
      c=IN IP4 lost.example.com
      a=tool:foo
      m=audio 59000 RTP/SAVP 98
      a=crypto:1 AES_CM_128_HMAC_SHA1_32
         inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32


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      a=rtpmap:98 AMR/8000
      m=video 52000 RTP/SAVP 31
      a=crypto:1 AES_CM_128_HMAC_SHA1_80
         inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
         a=rtpmap:31 H261/90000

   Again, note that the transport protocol in the media descriptions
   indicate use of secure RTP.

   And finally, we show the offer the answerer "sees" when using
   potential configuration 1 with attribute capability 1 (MIKEY) for
   the audio stream, and potential configuration 1 with attribute
   capability 3 (SDP security descriptions) for the video stream:

      v=0
      o=alice 2891092738 2891092738 IN IP4 lost.example.com
      s=
      t=0 0
      c=IN IP4 lost.example.com
      a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
      a=tool:foo
      m=audio 59000 RTP/SAVP 98
      a=rtpmap:98 AMR/8000
      m=video 52000 RTP/SAVP 31
      a=crypto:1 AES_CM_128_HMAC_SHA1_80
         inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
      a=rtpmap:31 H261/90000

3.6.3.  Offerer Processing of the Answer

   When the offerer attempted to use SDP Capability Negotiation in the
   offer, the offerer MUST examine the answer for actual use of SDP
   Capability Negotiation.

   For each media description where the offerer included a potential
   configuration attribute ("a=pcfg"), the offerer MUST first examine
   that media description for the presence of an actual configuration
   attribute ("a=acfg"). If an actual configuration attribute is not
   present in a media description, then the offerer MUST process the
   answer SDP for that media stream per the normal offer/answer rules
   defined in [RFC3264]. However, if one is found, the offerer MUST
   instead process the answer as follows:






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   o  The actual configuration attribute specifies which of the
      potential configurations was used by the answerer to generate the
      answer for this media stream. This includes all the supported
      attribute capabilities and the transport capabilities referenced
      by the potential configuration selected, where the attribute
      capabilities have any associated delete-attributes included.
      Extension capabilities supported by the answerer are included as
      well.

   o  The offerer MUST now process the answer in accordance with the
      rules in [RFC3264], except that it must be done as if the offer
      consisted of the selected potential configuration instead of the
      original actual configuration, including any transport protocol
      changes in the media ("m=") line(s), attributes added and deleted
      by the potential configuration at the media and session level,
      and any extensions used.

   If the offer/answer exchange was successful, and if the answerer
   selected one of the potential configurations from the offer as the
   actual configuration, and the selected potential configuration
   differs from the actual configuration in the offer (the "m=", "a=",
   etc. lines), then the offerer SHOULD initiate another offer/answer
   exchange. This second offer/answer exchange will not modify the
   session in any way, however it will help intermediaries (e.g.
   middleboxes), that look at the SDP but do not support the capability
   negotiation extensions, understand the details of the media
   stream(s) that were actually negotiated. This new offer MUST contain
   the selected potential configuration as the actual configuration,
   i.e., with the actual configuration used in the "m=" line and any
   other relevant attributes, bandwidth parameters, etc.

   Note that, per normal offer/answer rules, the second offer/answer
   exchange still needs to update the version number in the "o=" line
   ((<sess-version> in [RFC4566]). Attribute lines carrying keying
   material SHOULD repeat the keys from the previous offer, unless re-
   keying is necessary, e.g. due to a previously forked SIP INVITE
   request. Please refer to Section 3.12. for additional considerations
   related to intermediaries.

3.6.4. Modifying the Session

   Capabilities and 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 selected configuration used by
   the answerer.


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   If the answer indicates use of a potential configuration from the
   offer, then the guidelines provided in Section 3.6.3. for doing a
   second offer/answer exchange using that potential configuration as
   the actual configuration apply.

3.7. 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. Since
   both ICE and SDP Capability Negotiation may specify alternative
   transport protocols, there is a potentially unintended interaction
   when using these together.

   We provide the following guidelines for addressing that.

   There are two basic scenarios to consider:

   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 connectivity checks).



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   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.

   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, there
   is a need for tight control over the ICE candidates that will be
   used for a particular configuration, yet the actual configuration
   may want to use all of the ICE candidates. In that case, the ICE
   candidate attributes can 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). Furthermore, use
   of the delete-attributes in a potential configuration can be used to
   ensure that ICE will not end up using a transport protocol that is
   not desired for a particular configuration.

   SDP Capability Negotiation recommends use of a second offer/answer
   exchange when the negotiated actual configuration was one of the
   potential configurations from the offer (see Section 3.6.3. ).
   Similarly, ICE requires use of a second offer/answer exchange if the
   chosen candidate is not the same as the one in the m/c-line from the
   offer. When ICE and capability negotiation are used at the same
   time, the two secondary offer/answer exchanges SHOULD be combined to
   a single one.

3.8. Interactions with SIP Option Tags

   SIP [RFC3261] allows for SIP extensions to define a SIP option tag
   that identifies the SIP extension. Support for one or more such
   extensions can be indicated by use of the SIP Supported header, and
   required support for one or more such extensions can be indicated by
   use of the SIP Require header. The "a=csup" and "a=creq" attributes
   defined by the SDP Capability Negotiation framework are similar,
   except that support for these two attributes by themselves cannot be
   guaranteed (since they are specified as extensions to the SDP
   specification [RFC4566] itself).

   SIP extensions with associated option tags can introduce
   enhancements to not only SIP, but also SDP. This is for example the
   case for SIP preconditions defined in [RFC3312]. When using SDP
   Capability Negotiation, some potential configurations may include


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   certain SDP extensions, whereas others may not. Since the purpose of
   the SDP Capability Negotiation is to negotiate a session based on
   the features supported by both sides, use of the SIP Require header
   for such extensions may not produce the desired result. For example,
   if one potential configuration requires SIP preconditions support,
   another does not, and the answerer does not support preconditions,
   then use of the SIP Require header for preconditions would result in
   a session failure, in spite of the fact that a valid and supported
   potential configuration was included in the offer.

   In general, this can be alleviated by use of mandatory and optional
   attribute capabilities in a potential configuration. There are
   however cases where permissible SDP values are tied to the use of
   the SIP Require header. SIP preconditions [RFC3312] is one such
   example, where preconditions with a "mandatory" strength-tag can
   only be used when a SIP Require header with the SIP option tag
   "precondition" is included. Future SIP extensions that may want to
   use the SDP Capability Negotiation framework should avoid such
   coupling.

3.9. 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 may lead to clipping.
   Consequently, the SDP Capability Negotiation framework recommends
   sending back an answer SDP as soon as possible.

   The issue can be resolved by introducing a three-way handshake. In
   the case of SIP, this can for example be done by defining a
   precondition [RFC3312] for capability negotiation (or use an
   existing precondition that is known to generate a second
   offer/answer exchange before proceeding with the session).  However,
   preconditions are often viewed as complicated to implement and they
   may add to overall session establishment delay by requiring an extra
   offer/answer exchange.

   An alternative three-way handshake can be performed by use of ICE
   [ICE]. When ICE is being used, and the answerer receives a STUN
   Binding Request for any one of the accepted media streams from the
   offerer, the answerer knows the offer has received his answer. At
   that point, the answerer knows that the offerer will be able to


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   process incoming media according to the negotiated configuration and
   hence he can start sending media without the risk of the offerer
   either discarding it or playing garbage.

   In some use cases a three-way handshake is not needed. An example is
   when the offerer does not need information from the answer, such as
   keying material in the SDP, in order to process incoming media. The
   SDP Capability Negotiation framework does not define any such
   solutions, 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,
   outside of the actual configuration, while still allowing them to be
   used by the answerer (exchange of keying material is still needed,
   e.g. inband). The basic SDP Capability Negotiation framework defined
   here does not include the ability to do so, however extensions that
   enable that may be defined.

3.10. Indicating Bandwidth Usage

   The amount of bandwidth used for a particular media stream depends
   on the negotiated codecs, transport protocol and other parameters.
   For example use of Secure RTP [RFC3711] with integrity protection
   requires more bandwidth than plain RTP [RFC3551]. SDP defines the
   bandwidth ("b=") parameter to indicate the proposed bandwidth for
   the session or media stream.

   In SDP as defined by [RFC4566], each media description contains one
   transport protocol and one or more codecs. When specifying the
   proposed bandwidth, the worst case scenario must be taken into
   account, i.e., use of the highest bandwidth codec provided, the
   transport protocol indicated, and the worst case (bandwidth-wise)
   parameters that can be negotiated (e.g., a 32-bit HMAC or an 80-bit
   HMAC).

   The core SDP capability negotiation framework does not provide a way
   to negotiate bandwidth parameters. The issue thus remains, however
   it is potentially worse than with SDP per [RFC4566], since it is
   easier to negotiate additional codecs, and furthermore possible to
   negotiate different transport protocols. The recommended approach
   for addressing this is the same as for plain SDP; the worst case
   (now including potential configurations) needs to be taken into
   account when specifying the bandwidth parameters in the actual
   configuration. This can make the bandwidth value less accurate than
   in SDP per [RFC4566] (due to potential greater variability in the
   potential configuration bandwidth use). Extensions can be defined to
   address this shortcoming. Also, the Transport Independent


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   Application Specific Maximum (TIAS) bandwidth type defined in
   [RFC3890] can be used to alleviate bandwidth variability concerns
   due to different transport protocols.

   Note, that when using RTP retransmission [RFC4588] with the RTCP-
   based feedback profile [RFC4585] (RTP/AVPF), the retransmitted
   packets are part of the media stream bandwidth when using SSRC-
   multiplexing.  If a feedback based protocol is offered as the actual
   configuration transport protocol, a non-feedback based protocol is
   offered as a potential configuration transport protocol and ends up
   being used, the actual bandwidth usage may be lower than the
   indicated bandwidth value in the offer (and vice versa).


3.11. Dealing with Large Number of Potential Configurations

   When using the SDP Capability Negotiation, it is easy to generate
   offers that contain a large number of potential configurations. For
   example, in the offer:

      v=0
      o=- 25678 753849 IN IP4 192.0.2.1
      s=
      c=IN IP4 192.0.2.1
      t=0 0
      m=audio 53456 RTP/AVP 0 18
      a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF
      a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_80
         inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
         FEC_ORDER=FEC_SRTP
      a=acap:2 key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
      a=acap:3 rtcp-fb:0 nack
      a=pcfg:1 t=1 a=1,3|2,3
      a=pcfg:2 t=2 a=1|2
      a=pcfg:3 t=3 a=3

   we have 5 potential configurations on top of the actual
   configuration for a single media stream. Adding an extension
   capability with just two alternatives for each would double that
   number (to 10), and doing the equivalent with two media streams
   would again double that number (to 20). While it is easy (and
   inexpensive) for the offerer to generate such offers, processing
   them at the answering side may not be. Consequently, it is
   RECOMMENDED that offerers do not create offers with unnecessarily
   large number of potential configurations in them.


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   On the answering side, implementers MUST take care to avoid
   excessive memory and CPU consumption. For example, a naive
   implementation that first generates all the valid potential
   configuration SDPs internally, could find itself being memory
   exhausted, especially if it supports a large number of endpoints.
   Similarly, a naive implementation that simply performs iterative
   trial-and-error processing on each possible potential configuration
   SDP (in the preference order specified) could find itself being CPU
   constrained. An alternative strategy is to prune the search space
   first by discarding the set of offered potential configurations
   where the transport protocol indicated (if any) is not supported,
   and/or one or more mandatory attribute capabilities (if any) are
   either not supported or not valid. Potential configurations with
   unsupported mandatory extension configurations in them can be
   discarded as well.

3.12. SDP Capability Negotiation and Intermediaries

   An intermediary is here defined as an entity between a SIP user
   agent A and a SIP user agent B, that need to perform some kind of
   processing on the SDP exchanged between A and B, in order for the
   session establishment to operate as intended. Examples of such
   intermediaries include Session Border Controllers (SBCs) that may
   perform media relaying, Proxy Call Session Control Functions (P-
   CSCF) that may authorize use of a certain amount of network
   resources (bandwidth), etc. The presence and design of such
   intermediaries may not follow the "Internet" model or the SIP
   requirements for proxies (which are not supposed to look in message
   bodies such as SDP), however they are a fact of life in some
   deployment scenarios and hence deserve consideration.

   If the intermediary needs to understand the characteristics of the
   media sessions being negotiated, e.g. the amount of bandwidth used
   or the transport protocol negotiated, then use of the SDP Capability
   Negotiation framework may impact them. For example, some
   intermediaries are known to disallow answers where the transport
   protocol differs from the one in the offer. Use of the SDP
   Capability Negotiation framework in the presence of such
   intermediaries could lead to session failures. Intermediaries that
   need to authorize use of network resources based on the negotiated
   media stream parameters are affected as well. If they inspect only
   the offer, then they may authorize parameters assuming a different
   transport protocol, codecs, etc. than what is actually being
   negotiated. For these, and other, reasons it is RECOMMENDED that
   implementers of intermediaries add support for the SDP Capability
   Negotiation framework.


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   The SDP Capability Negotiation framework itself attempts to help out
   these intermediaries as well, by optionally performing a second
   offer/answer exchange when use of a potential configuration has been
   negotiated (see Section 3.6.3. ). However, there are several
   limitations with this approach. First of all, the second
   offer/answer exchange is not required and hence may not be
   performed. Secondly, the intermediary may refuse the initial answer,
   e.g. due to perceived transport protocol mismatch. Thirdly, the
   strategy is not foolproof, since the offer/answer procedures
   [RFC3264] leave the original offer/answer exchange in effect when a
   subsequent one fails; consider the following example:

   1. Offerer generates an SDP offer with the actual configuration
      specifying a low bandwidth configuration (e.g. plain RTP) and a
      potential configuration specifying a high(er) bandwidth
      configuration (e.g. secure RTP with integrity).

   2. An intermediary (e.g. an SBC or P-CSCF), that does not support
      SDP Capability Negotiation, authorizes the session based on the
      actual configuration it sees in the SDP.

   3. The answerer chooses the high(er) bandwidth potential
      configuration and generates an answer SDP based on that.

   4. The intermediary passes through the answer SDP.

   5. The offerer sees the accepted answer, and generates an updated
      offer that contains the selected potential configuration as the
      actual configuration. In other words, the high(er) bandwidth
      configuration (which has already been negotiated successfully) is
      now the actual configuration in the offer SDP.

   6. The intermediary sees the new offer, however it does not
      authorize the use of the high(er) bandwidth configuration, and
      consequently generates a rejection message to the offerer.

   7. The offerer receives the rejected offer.

   After step 7, per RFC 3264, the offer/answer exchange that completed
   in step 5 remains in effect, however the intermediary may not have
   authorized the necessary network resources and hence the media
   stream may experience quality issues. The solution to this problem
   is to upgrade the intermediary to support the SDP Capability
   Negotiation framework.




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3.13. Considerations for Specific Attribute Capabilities

3.13.1. The rtpmap and fmtp Attributes

   The core SDP Capability Negotiation framework defines transport
   capabilities and attribute capabilities. Media capabilities, which
   can be used to describe media formats and their associated
   parameters, are not defined in this document, however the "rtpmap"
   and "fmtp" attributes can nevertheless be used as attribute
   capabilities. Using such attribute capabilities in a potential
   configuration requires a bit of care though.

   The rtpmap parameter binds an RTP payload type to a media format
   (e.g. codec). While it is possible to provide rtpmaps for payload
   types not found in the corresponding "m=" line, such rtpmaps provide
   no value in normal offer/answer exchanges, since only the payload
   types found in the "m=" line are part of the offer (or answer). This
   applies to the core SDP Capability Negotiation framework as well:
   Only the media formats (e.g. RTP payload types) provided in the "m="
   line are actually offered; inclusion of rtpmap attributes with other
   RTP payload types in a potential configuration does not change this
   fact and hence they do not provide any useful information there.
   They may still be useful as pure capabilities though (outside a
   potential configuration) in order to inform a peer of additional
   codecs supported.

   It is possible to provide an rtpmap attribute capability with a
   payload type mapping to a different codec than a corresponding
   actual configuration "rtpmap" attribute for the media description
   has. Such practice is permissible as a way of indicating a
   capability. If that capability is included in a potential
   configuration, then delete-attributes (see Section 3.5.1. ) MUST be
   used to ensure that there is not multiple rtpmap attributes for the
   same payload type in a given media description (which would not be
   allowed by SDP [RFC4566]).

   Similar considerations and rules apply to the "fmtp" attribute. An
   fmtp attribute capability for a media format not included in the
   "m=" line is useless in a potential configuration (but may be useful
   as a capability by itself). An fmtp attribute capability in a
   potential configuration for a media format that already has an fmtp
   attribute in the actual configuration may lead to multiple fmtp
   format parameters for that media format and that is not allowed by
   SDP [RFC4566]. The delete-attributes MUST be used to ensure that
   there is not multiple fmtp attributes for a given media format in a
   media description.


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   Extensions to the core SDP Capability Negotiation framework may
   change the above behavior.

3.13.2. Direction Attributes

   SDP defines the "inactive", "sendonly", "recvonly", and "sendrecv"
   direction attributes. The direction attributes can be applied at
   either the session-level or the media-level. In either case, it is
   possible to define attribute capabilities for these direction
   capabilities; if used by a potential configuration, the normal
   offer/answer procedures still apply. For example, if an offered
   potential configuration includes the "sendonly" direction attribute,
   and it is selected as the actual configuration, then the answer MUST
   include a corresponding "recvonly" (or "inactive") attribute.

3.14. Relationship to RFC 3407

   RFC 3407 defines capability descriptions with limited abilities to
   describe attributes, bandwidth parameters, transport protocols and
   media formats. RFC 3407 does not define any negotiation procedures
   for actually using those capability descriptions.

   This document defines new attributes for describing attribute
   capabilities and transport capabilities. It also defines procedures
   for using those capabilities as part of an offer/answer exchange. In
   contrast to RFC 3407, this document does not define bandwidth
   parameters, and it also does not define how to express ranges of
   values. Extensions to this document may be defined in order to fully
   cover all the capabilities provided by RFC 3407 (for example more
   general media capabilities).

   It is RECOMMENDED that implementations use the attributes and
   procedures defined in this document instead of those defined in
   [RFC3407]. If capability description interoperability with legacy
   RFC 3407 implementations is desired, implementations MAY include
   both RFC 3407 capability descriptions and capabilities defined by
   this document. The offer/answer negotiation procedures defined in
   this document will not use the RFC 3407 capability descriptions.

4. Examples

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





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4.1. Multiple Transport Protocols

   The following example illustrates how to use the SDP Capability
   Negotiation extensions to negotiate use of one out of several
   possible transport protocols. The offerer uses the expected least-
   common-denominator (plain RTP) as the actual configuration, and the
   alternative transport protocols as the potential configurations.

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

                Alice                               Bob

                  | (1) Offer (RTP/[S]AVP[F])        |
                  |--------------------------------->|
                  |                                  |
                  | (2) Answer (RTP/AVPF)            |
                  |<---------------------------------|
                  |                                  |
                  | (3) Offer (RTP/AVPF)             |
                  |--------------------------------->|
                  |                                  |
                  | (4) Answer (RTP/AVPF)            |
                  |<---------------------------------|
                  |                                  |


   Alice's offer includes plain RTP (RTP/AVP), RTP with RTCP-based
   feedback (RTP/AVPF), Secure RTP (RTP/SAVP), and Secure RTP with
   RTCP-based feedback (RTP/SAVPF) and SRTP as alternatives. RTP is the
   default, with RTP/SAVPF, RTP/SAVP, and RTP/AVPF as the alternatives
   and preferred in the order listed:

      v=0
      o=- 25678 753849 IN IP4 192.0.2.1
      s=
      c=IN IP4 192.0.2.1
      t=0 0
      m=audio 53456 RTP/AVP 0 18
      a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF
      a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_80
         inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
         FEC_ORDER=FEC_SRTP
      a=acap:2 rtcp-fb:0 nack
      a=pcfg:1 t=1 a=1,[2]
      a=pcfg:2 t=2 a=1


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      a=pcfg:3 t=3 a=[2]

   The "m=" line indicates that Alice is offering to use plain RTP with
   PCMU or G.729. The capabilities are provided by the "a=tcap" and
   "a=acap" attributes.  The "tcap" capability indicates that Secure
   RTP with RTCP-Based feedback (RTP/SAVPF), Secure RTP (RTP/SAVP), and
   RTP with RTCP-Based feedback are supported. The first "acap"
   attribute provides an attribute capability with a handle of 1. The
   capability is a "crypto" attribute, which provides the keying
   material for SRTP using SDP security descriptions [RFC4568]. The
   second "acap" attribute provides an attribute capability with a
   handle of 2. The capability is an "rtcp-fb" attribute, which is used
   by the RTCP-based feedback profiles to indicate that payload type 0
   (PCMU) supports feedback type "nack". The "a=pcfg" attributes
   provide the potential configurations included in the offer by
   reference to the capabilities. There are three potential
   configurations:

   o  Potential configuration 1, which is the most preferred potential
      configuration specifies use of transport protocol capability 1
      (RTP/SAVPF) and attribute capabilities 1 (the "crypto" attribute)
      and 2 (the "rtcp-fb" attribute). Support for the first one is
      mandatory whereas support for the second one is optional.

   o  Potential configuration 2, which is the second most preferred
      potential configuration specifies use of transport protocol
      capability 2 (RTP/SAVP) and mandatory attribute capability 1 (the
      "crypto" attribute).

   o  Potential configuration 3, which is the least preferred potential
      configuration (but the second least preferred configuration
      overall, since the actual configuration provided by the "m=" line
      is always the least preferred configuration), specifies use of
      transport protocol capability 3 (RTP/AVPF) and optional attribute
      capability 2 (the "rtcp-fb" attribute).

   Bob receives the SDP offer from Alice. Bob does not support any
   secure RTP profiles, however he supports plain RTP and RTP with
   RTCP-based feedback, as well as the SDP Capability Negotiation
   extensions, and hence he accepts the potential configuration for RTP
   with RTCP-based feedback provided by Alice:

      v=0
      o=- 24351 621814 IN IP4 192.0.2.2
      s=
      c=IN IP4 192.0.2.2


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      t=0 0
      m=audio 54568 RTP/AVPF 0 18
      a=rtcp-fb:0 nack
      a=acfg:1 t=3 a=[2]

   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 3 and optional
   attribute capability 2 from the offer SDP (i.e. the RTP/AVPF profile
   using the "rtcp-fb" value provided).  Bob also includes an "rtcp-fb"
   attribute with the value "nack" value for RTP payload type 0.

   When Alice receives Bob's answer, session negotiation has completed,
   however Alice nevertheless chooses to generate a new offer using the
   actual configuration. This is done purely to assist any
   intermediaries that may reside between Alice and Bob but do not
   support the SDP Capability Negotiation framework (and hence may not
   understand the negotiation that just took place):

   Alice's updated offer includes only RTP/AVPF, and it is not using
   the SDP Capability Negotiation framework (Alice could have included
   the capabilities as well if she wanted to):

      v=0
      o=- 25678 753850 IN IP4 192.0.2.1
      s=
      c=IN IP4 192.0.2.1
      t=0 0
      m=audio 53456 RTP/AVPF 0 18
      a=rtcp-fb:0 nack

   The "m=" line now indicates that Alice is offering to use RTP with
   RTCP-based feedback and using PCMU or G.729.  The "rtcp-fb"
   attribute provides the feedback type "nack" for payload type 0 again
   (but as part of the actual configuration).

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

      v=0
      o=- 24351 621815 IN IP4 192.0.2.2
      s=
      c=IN IP4 192.0.2.2
      t=0 0
      m=audio 54568 RTP/AVPF 0 18
      a=rtcp-fb:0 nack


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

   Note that in this particular example, the answerer supported the SDP
   Capability Negotiation framework and hence the attributes and
   procedures 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 192.0.2.2
      s=
      c=IN IP4 192.0.2.2
      t=0 0
      m=audio 54568 RTP/AVP 0 18

4.2. Best-Effort SRTP with Session-Level MIKEY and Media Level Security
   Descriptions

   The following example illustrates how to use the SDP Capability
   Negotiation extensions to support so-called Best-Effort Secure RTP
   as well as alternative keying mechanisms, more specifically MIKEY
   [RFC3830] and SDP Security Descriptions. The offerer (Alice) wants
   to establish an audio and video session. Alice prefers to use
   session-level MIKEY as the key management protocol, but supports SDP
   security descriptions as well.

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

















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             Alice                                     Bob

               | (1) Offer (RTP/[S]AVP[F], SDES|MIKEY)  |
               |--------------------------------------->|
               |                                        |
               | (2) Answer (RTP/SAVP, SDES)            |
               |<---------------------------------------|
               |                                        |
               | (3) Offer (RTP/SAVP, SDES)             |
               |--------------------------------------->|
               |                                        |
               | (4) Answer (RTP/SAVP, SDES)            |
               |<---------------------------------------|
               |                                        |


   Alice's offer includes an audio and a video stream. The audio stream
   offers use of plain RTP and secure RTP as alternatives, whereas the
   video stream offers use of plain RTP, RTP with RTCP-based feedback,
   Secure RTP, and Secure RTP with RTCP-based feedback as alternatives:

      v=0
      o=- 25678 753849 IN IP4 192.0.2.1
      s=
      t=0 0
      c=IN IP4 192.0.2.1
      a=acap:1 key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
      a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF
      m=audio 59000 RTP/AVP 98
      a=rtpmap:98 AMR/8000
      a=acap:2 crypto:1 AES_CM_128_HMAC_SHA1_32
         inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
      a=pcfg:1 t=2 a=1|2
      m=video 52000 RTP/AVP 31
      a=rtpmap:31 H261/90000
      a=acap:3 crypto:1 AES_CM_128_HMAC_SHA1_80
         inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
      a=acap:4 rtcp-fb:* nack
      a=pcfg:1 t=1 a=1,4|3,4
      a=pcfg:2 t=2 a=1|3
      a=pcfg:3 t=3 a=4

   The potential configuration for the audio stream specifies use of
   transport capability 2 (RTP/SAVP) and either attribute capability 1
   (session-level MIKEY as the keying mechanism) or 2 (SDP Security
   Descriptions as the keying mechanism). Support for either of these


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   attribute capabilities is mandatory. There are three potential
   configurations for the video stream.

   o  The first configuration with configuration number 1 uses
      transport capability 1 (RTP/SAVPF) with either attribute
      capabilities 1 and 4 (session-level MIKEY and the "rtcp-fb"
      attribute) or attribute capabilities 3 and 4 (SDP security
      descriptions and the "rtcp-fb" attribute). In this example, the
      offerer insists on not only the keying mechanism being supported,
      but also that the "rtcp-fb" attribute is supported with the value
      indicated. Consequently, all the attribute capabilities are
      marked as mandatory in this potential configuration.

   o  The second configuration with configuration number 2 uses
      transport capability 2 (RTP/SAVP) and either attribute capability
      1 (session-level MIKEY) or attribute capability 3 (SDP security
      descriptions). Both attribute capabilities are mandatory in this
      configuration.

   o  The third configuration with configuration number 3 uses
      transport capability 3 (RTP/AVPF) and mandatory attribute
      capability 4 (the "rtcp-fb" attribute).

   Bob receives the SDP offer from Alice. Bob supports Secure RTP,
   Secure RTP with RTCP-based feedback and the SDP Capability
   Negotiation extensions. Bob also supports SDP Security Descriptions,
   but not MIKEY, and hence he generates the following answer:

      v=0
      o=- 24351 621814 IN IP4 192.0.2.2
      s=
      t=0 0
      c=IN IP4 192.0.2.2
      m=audio 54568 RTP/SAVP 98
      a=rtpmap:98 AMR/8000
      a=crypto:1 AES_CM_128_HMAC_SHA1_32
         inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32
      a=acfg:1 t=2 a=2
      m=video 55468 RTP/SAVPF 31
      a=rtpmap:31 H261/90000
      a=crypto:1 AES_CM_128_HMAC_SHA1_80
         inline:AwWpVLFJhQX1cfHJSojd0RmdmcmVCspeEc3QGZiN|2^20|1:32
      a=rtcp-fb:* nack
      a=acfg:1 t=1 a=3,4

   For the audio stream, Bob accepted the use of secure RTP, and hence


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   the profile in the "m=" line is "RTP/SAVP". Bob also includes a
   "crypto" attribute with his own keying material, and an "acfg"
   attribute identifying actual configuration 1 for the audio media
   stream from the offer, using transport capability 2 (RTP/SAVP) and
   attribute capability 2 (the crypto attribute from the offer). For
   the video stream, Bob accepted the use of secure RTP with RTCP-based
   feedback, and hence the profile in the "m=" line is "RTP/SAVPF". Bob
   also includes a "crypto" attribute with his own keying material, and
   an "acfg" attribute identifying actual configuration 1 for the video
   stream from the offer, using transport capability 1 (RTP/SAVPF) and
   attribute capabilities 3 (the crypto attribute from the offer) and 4
   (the "rtcp-fb" attribute from the offer).

   When Alice receives Bob's answer, session negotiation has completed,
   however Alice nevertheless chooses to generate a new offer using the
   actual configuration. This is done purely to assist any
   intermediaries 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 for the audio stream SRTP
   with RTCP-based feedback for the video stream, and it is not using
   the SDP Capability Negotiation framework (Alice could have included
   the capabilities as well is she wanted to):

      v=0
      o=- 25678 753849 IN IP4 192.0.2.1
      s=
      t=0 0
      c=IN IP4 192.0.2.1
      m=audio 59000 RTP/SAVP 98
      a=rtpmap:98 AMR/8000
      a=crypto:1 AES_CM_128_HMAC_SHA1_32
         inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
      m=video 52000 RTP/SAVPF 31
      a=rtpmap:31 H261/90000
      a=crypto:1 AES_CM_128_HMAC_SHA1_80
         inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
      a=rtcp-fb:* nack

   The "m=" line for the audio stream now indicates that Alice is
   offering to use secure RTP with PCMU or G.729, whereas the "m=" line
   for the video stream indicates that Alice is offering to use secure
   RTP with RTCP-based feedback and H.261. Each media stream includes a
   "crypto" attribute, which provides the SRTP keying material, with
   the same value again.


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

      v=0
      o=- 24351 621814 IN IP4 192.0.2.2
      s=
      t=0 0
      c=IN IP4 192.0.2.2
      m=audio 54568 RTP/SAVP 98
      a=rtpmap:98 AMR/8000
      a=crypto:1 AES_CM_128_HMAC_SHA1_32
         inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32
      m=video 55468 RTP/SAVPF 31
      a=rtpmap:31 H261/90000
      a=crypto:1 AES_CM_128_HMAC_SHA1_80
         inline:AwWpVLFJhQX1cfHJSojd0RmdmcmVCspeEc3QGZiN|2^20|1:32
      a=rtcp-fb:* nack

   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 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 192.0.2.2
      s=
      t=0 0
      c=IN IP4 192.0.2.2
      m=audio 54568 RTP/AVP 98
      a=rtpmap:98 AMR/8000
      m=video 55468 RTP/AVP 31
      a=rtpmap:31 H261/90000
      a=rtcp-fb:* nack

   Finally, if Bob had chosen to use session-level MIKEY instead of SDP
   security descriptions instead, the following answer would have been
   generated:

      v=0
      o=- 25678 753849 IN IP4 192.0.2.1
      s=


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      t=0 0
      c=IN IP4 192.0.2.1
      a=key-mgmt:mikey AQEFgM0XflABAAAAAAAAAAAAAAYAyO...
      m=audio 59000 RTP/AVP 98
      a=rtpmap:98 AMR/8000
      a=acfg:1 t=2 a=1
      m=video 52000 RTP/SAVPF 31
      a=rtpmap:31 H261/90000
      a=rtcp-fb:* nack
      a=acfg:1 t=1 a=1,4

   It should be noted, that although Bob could have chosen session-
   level MIKEY for one media stream, and SDP Security Descriptions for
   another media stream, there are no well-defined offerer processing
   rules of the resulting answer for this, and hence the offerer may
   incorrectly assume use of MIKEY for both streams. To avoid this, if
   the answerer chooses session-level MIKEY, then all secure RTP based
   media streams SHOULD use MIKEY (this applies irrespective of whether
   SDP Capability Negotiation is being used or not). Use of media-level
   MIKEY does not have a similar constraint.

4.3. SRTP with Session-Level MIKEY and Media Level Security
   Descriptions as Alternatives

   The following example illustrates how to use the SDP Capability
   Negotiation framework to negotiate use of either MIKEY or SDP
   Security Descriptions, when one of them is included as part of the
   actual configuration, and the other one is being selected. The
   offerer (Alice) wants to establish an audio and video session. Alice
   prefers to use session-level MIKEY as the key management protocol,
   but supports SDP security descriptions as well.

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














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             Alice                                     Bob

               | (1) Offer (RTP/[S]AVP[F], SDES|MIKEY)  |
               |--------------------------------------->|
               |                                        |
               | (2) Answer (RTP/SAVP, SDES)            |
               |<---------------------------------------|
               |                                        |


   Alice's offer includes an audio and a video stream. Both the audio
   and the video stream offer use of secure RTP:

      v=0
      o=- 25678 753849 IN IP4 192.0.2.1
      s=
      t=0 0
      c=IN IP4 192.0.2.1
      a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
      m=audio 59000 RTP/SAVP 98
      a=rtpmap:98 AMR/8000
      a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32
         inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
      a=pcfg:1 a=-s:1
      m=video 52000 RTP/SAVP 31
      a=rtpmap:31 H261/90000
      a=acap:2 crypto:1 AES_CM_128_HMAC_SHA1_80
         inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
      a=pcfg:1 a=-s:2

   Alice does not know whether Bob supports MIKEY or SDP Security
   Descriptions. She could include attributes for both, however the
   resulting procedures and potential interactions are not well-
   defined. Instead, she places a session-level key-mgmt attribute for
   MIKEY in the actual configuration with SDP security descriptions as
   an alternative in the potential configuration. The potential
   configuration for the audio stream specifies that all session level
   attributes are to be deleted (i.e. the session-level "a=key-mgmt"
   attribute) and that mandatory attribute capability 2 is to be used
   (i.e. the crypto attribute). The potential configuration for the
   video stream is similar, except it uses it's own mandatory crypto
   attribute capability (2). Note how deletion of the session-level
   attributes does not affect the media-level attributes.

   Bob receives the SDP offer from Alice. Bob supports Secure RTP and
   the SDP Capability Negotiation framework. Bob also supports both SDP


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   Security Descriptions and MIKEY. Since the potential configuration
   is more preferred than the actual configuration, Bob (conceptually)
   generates an internal potential configuration SDP that contains the
   crypto attributes for the audio and video stream, but not the key-
   mgmt attribute for MIKEY, thereby avoiding any ambiguity between the
   two keying mechanisms. As a result, he generates the following
   answer:

      v=0
      o=- 24351 621814 IN IP4 192.0.2.2
      s=
      t=0 0
      c=IN IP4 192.0.2.2
      m=audio 54568 RTP/SAVP 98
      a=rtpmap:98 AMR/8000
      a=crypto:1 AES_CM_128_HMAC_SHA1_32
         inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32
      a=acfg:1 a=-s:1
      m=video 55468 RTP/SAVP 31
      a=rtpmap:31 H261/90000
      a=crypto:1 AES_CM_128_HMAC_SHA1_80
         inline:AwWpVLFJhQX1cfHJSojd0RmdmcmVCspeEc3QGZiN|2^20|1:32
      a=acfg:1 a=-s:2

   For the audio stream, Bob accepted the use of secure RTP using SDP
   security descriptions. Bob therefore includes a "crypto" attribute
   with his own keying material, and an "acfg" attribute identifying
   actual configuration 1 for the audio media stream from the offer,
   with the delete-attributes ("-s") and attribute capability 1 (the
   crypto attribute from the offer). For the video stream, Bob also
   accepted the use of secure RTP using SDP security descriptions. Bob
   therefore includes a "crypto" attribute with his own keying
   material, and an "acfg" attribute identifying actual configuration 1
   for the video stream from the offer, with the delete-attributes ("-
   s") and attribute capability 2.

   Below, we illustrate the offer SDP, when Bob instead offers the
   "crypto" attribute as the actual configuration keying mechanism and
   "key-mgmt" as the potential configuration:

      v=0
      o=- 25678 753849 IN IP4 192.0.2.1
      s=
      t=0 0
      c=IN IP4 192.0.2.1
      a=acap:1 key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...


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      m=audio 59000 RTP/SAVP 98
      a=rtpmap:98 AMR/8000
      a=crypto:1 AES_CM_128_HMAC_SHA1_32
         inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
      a=acap:2 rtpmap:98 AMR/8000
      a=pcfg:1 a=-m:1,2
      m=video 52000 RTP/SAVP 31
      a=rtpmap:31 H261/90000
      a=acap:3 crypto:1 AES_CM_128_HMAC_SHA1_80
         inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
      a=acap:4 rtpmap:31 H261/90000
      a=pcfg:1 a=-m:1,4

   Note how we this time need to perform delete-attributes at the
   media-level instead of the session-level. When doing that, all
   attributes from the actual configuration SDP, including the rtpmaps
   provided, are removed. Consequently, we had to include these rtpmaps
   as capabilities as well, and then include them in the potential
   configuration, thereby effectively recreating the original rtpmap
   attributes in the resulting potential configuration SDP.

5. Security Considerations

   The SDP Capability Negotiation Framework is defined to be used
   within the context of the offer/answer model, and hence all the
   offer/answer security considerations apply here as well. Similarly,
   the Session Initiation Protocol (SIP) uses SDP and the offer/answer
   model, and hence, when used in that context, the SIP security
   considerations apply as well.

   However, SDP Capability Negotiation introduces additional security
   issues. Its use as a mechanism to enable alternative transport
   protocol negotiation (secure and non-secure) as well as its ability
   to negotiate use of more or less secure keying methods and material
   warrant further security considerations. Also, the (continued)
   support for receiving media before answer combined with negotiation
   of alternative transport protocols (secure and non-secure) warrant
   further security considerations. We discuss these issues below.

   The SDP Capability Negotiation framework allows for an offered media
   stream to both indicate and support various levels of security for
   that media stream. Different levels of security can for example be
   negotiated by use of alternative attribute capabilities each
   indicating more or less secure keying methods as well as more or
   less strong ciphers. Since the offerer indicates support for each of
   these alternatives, he will presumably accept the answerer seemingly


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   selecting any of the offered alternatives. If an attacker can modify
   the SDP offer, he can thereby force the negotiation of the weakest
   security mechanism that the offerer is willing to accept. This may
   enable the attacker to compromise the security of the negotiated
   media stream. Similarly, if the offerer wishes to negotiate use of a
   secure media stream (e.g. secure RTP), but includes a non-secure
   media stream (e.g. plain RTP) as a valid (but less preferred)
   alternative, then an attacker that can modify the offered SDP will
   be able to force the establishment of an insecure media stream. The
   solution to both of these problems involves the use of integrity
   protection over the SDP. Ideally, this integrity protection provides
   end-to-end integrity protection in order to protect from any man-in-
   the-middle attack; secure multiparts such as S/MIME [RFC3851]
   provide one such solution, however S/MIME requires use and
   availability of a Public Key Infrastructure (PKI). A slightly less
   secure alternative when using SIP, but generally much easier to
   deploy in practice (since it does not require a PKI), is to use SIP
   Identity [RFC4474]; this requires the existence of an authentication
   service (see [RFC4474]). Yet another, and considerably less secure,
   alternative is to use hop-by-hop security only, e.g. TLS or IPSec
   thereby ensuring the integrity of the offered SDP on a hop-by-hop
   basis. Note however that SIP proxies or other intermediaries
   processing the SIP request at each hop are able to perform a man-in-
   the-middle attack by modifying the offered SDP.

   Per the normal offer/answer procedures, as soon as the offerer has
   generated an offer, the offerer must be prepared to receive media in
   accordance with that offer. The SDP Capability Negotiation preserves
   that behavior for the actual configuration in the offer, however the
   offerer has no way of knowing which configuration (actual or
   potential) configuration was selected by the offerer, until an
   answer indication is received. This opens up a new security issue
   where an attacker may be able to interject media towards the offerer
   until the answer is received. For example, the offerer may use plain
   RTP as the actual configuration and secure RTP as an alternative
   potential configuration. Even though the answerer selects secure
   RTP, the offerer will not know that until he receives the answer,
   and hence an attacker will be able to send media to the offerer
   meanwhile. The easiest protection against such an attack is to not
   offer use of the non-secure media stream in the actual
   configuration, however that may in itself have undesirable side-
   effects: If the answerer does not support the secure media stream
   and also does not support the capability negotiation framework, then
   negotiation of the media stream will fail. Alternatively, SDP
   security preconditions [RFC5027] can be used. This will ensure that
   media is not flowing until session negotiation has completed and


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   hence the selected configuration is known. Use of preconditions
   however requires both sides to support them. If they don't, and use
   of them is required, the session will fail. As a (limited) work
   around to this, it is RECOMMENDED that SIP entities generate an
   answer SDP and send it to the offerer as soon as possible, for
   example in a 183 Session Progress message. This will limit the time
   during which an attacker can send media to the offerer. Section 3.9.
   presents other alternatives as well.

   Additional security considerations apply to the answer SDP as well.
   The actual configuration attribute tells the offerer which potential
   configuration the answer was based on, and hence an attacker that
   can either modify or remove the actual configuration attribute in
   the answer can cause session failure as well as extend the time
   window during which the offerer will accept incoming media that does
   not conform to the actual answer. The solutions to this SDP answer
   integrity problem are the same as for the offer, i.e. use of end-to-
   end integrity protection, SIP identity, or hop-by-hop protection.
   The mechanism to use depends on the mechanisms supported by the
   offerer as well as the acceptable security trade-offs.

   As described in Section 3.1. , SDP Capability Negotiation
   conceptually allows an offerer to include many different offers in a
   single SDP. This can cause the answerer to process a large number of
   alternative potential offers, which can consume significant memory
   and CPU resources. An attacker can use this amplification feature to
   launch a denial of service attack against the answerer. The answerer
   MUST protect itself from such attacks. As explained in Section 3.10.
   , the answerer can help reduce the effects of such an attack by
   first discarding all potential configurations that contain
   unsupported transport protocols, unsupported or invalid mandatory
   attribute capabilities, or unsupported mandatory extension
   configurations. The answerer SHOULD also look out for potential
   configurations that are designed to pass the above test, but
   nevertheless produce a large number of potential configuration SDPs
   that cannot be supported.

     A possible way of achieving that is for an attacker to find a
     valid session-level attribute that causes conflicts or otherwise
     interferes with individual media description configurations. At
     time of publication of this document, we do not know of such an
     SDP attribute, however this does not mean it does not exist, or
     that it will not exist in the future. If such attributes are found
     to exist, implementers should explicitly protect against them.




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   A significant number of valid and supported potential configurations
   may remain. However, since all of those contain only valid and
   supported transport protocols and attributes, it is expected that
   only a few of them will need to be processed on average. Still, the
   answerer MUST ensure that it does not needlessly consume large
   amounts of memory or CPU resources when processing those as well as
   be prepared to handle the case where a large number of potential
   configurations still need to be processed.

6. IANA Considerations

6.1. New SDP Attributes

   The IANA is hereby requested to register the following new SDP
   attributes as follows:

   Attribute name:      csup
   Long form name:      Supported capability negotiation extensions
   Type of attribute:   Session-level and media-level
   Subject to charset:  No
   Purpose:             Option tags for supported SDP capability
                        negotiation extensions
   Appropriate values:  See Section 3.3.1. of RFCXXXX
                        -- Note to RFC editor:
                        -- replace RFCXXXX by this RFC number
   Contact name:        Flemming Andreasen, fandreas@cisco.com

   Attribute name:      creq
   Long form name:      Required capability negotiation extensions
   Type of attribute:   Session-level and media-level
   Subject to charset:  No
   Purpose:             Option tags for required SDP capability
                        negotiation extensions
   Appropriate values:  See Section 3.3.2. of RFCXXXX
                        -- Note to RFC editor:
                        -- replace RFCXXXX by this RFC number
   Contact name:        Flemming Andreasen, fandreas@cisco.com

   Attribute name:      acap
   Long form name:      Attribute capability
   Type of attribute:   Session-level and media-level
   Subject to charset:  No
   Purpose:             Attribute capability containing an attribute
                        name and associated value
   Appropriate values:  See Section 3.4.1. of RFCXXXX
                        -- Note to RFC editor:


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                        -- replace RFCXXXX by this RFC number
   Contact name:        Flemming Andreasen, fandreas@cisco.com

   Attribute name:      tcap
   Long form name:      Transport Protocol Capability
   Type of attribute:   Session-level and media-level
   Subject to charset:  No
   Purpose:             Transport protocol capability listing one or
                        more transport protocols
   Appropriate values:  See Section 3.4.2. of RFCXXXX
                        -- Note to RFC editor:
                        -- replace RFCXXXX by this RFC number
   Contact name:        Flemming Andreasen, fandreas@cisco.com
   Attribute name:      pcfg
   Long form name:      Potential Configuration
   Type of attribute:   Media-level
   Subject to charset:  No
   Purpose:             Potential configuration for SDP capability
                        negotiation
   Appropriate values:  See Section 3.5.1. of RFCXXXX
                        -- Note to RFC editor:
                        -- replace RFCXXXX by this RFC number
   Contact name:        Flemming Andreasen, fandreas@cisco.com

   Attribute name:      acfg
   Long form name:      Actual configuration
   Type of attribute:   Media-level
   Subject to charset:  No
   Purpose:             Actual configuration for SDP capability
                        negotiation
   Appropriate values:  See Section 3.5.2. of RFCXXXX
                        -- Note to RFC editor:
                        -- replace RFCXXXX by this RFC number
   Contact name:        Flemming Andreasen, fandreas@cisco.com



6.2. New SDP Capability Negotiation Option Tag Registry

   The IANA is hereby requested to create a new SDP Capability
   Negotiation Option Tag registry. An IANA SDP Capability Negotiation
   option tag registration MUST be documented in an RFC in accordance
   with the [RFC5226] Specification Required policy. The RFC MUST
   provide the name of the option tag, a syntax and a semantic
   specification of any new SDP attributes and any extensions to the


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   potential and actual configuration attributes provided in this
   document. New SDP attributes that are intended to be capabilities
   for use by the capability negotiation framework MUST adhere to the
   guidelines provided in Section 3.4.3. Extensions to the potential
   and actual configuration attributes MUST adhere to the syntax
   provided in Section 3.5.1. and 3.5.2.

   The option tag "cap-v0" is defined in this document and the IANA is
   hereby requested to register this option tag.

6.3. New SDP Capability Negotiation Potential Configuration Parameter
   Registry

   The IANA is hereby requested to create a new SDP Capability
   Negotiation Potential Configuration Parameter registry. An IANA SDP
   Capability Negotiation potential configuration registration MUST be
   documented in an RFC in accordance with the [RFC5226] Specification
   Required policy. The RFC MUST define the syntax and semantics of
   each new potential configuration parameter. The syntax MUST adhere
   to the syntax provided for extensions in Section 3.5.1. and the
   semantics MUST adhere to the semantics provided for extensions in
   Section 3.5.1. and 3.5.2. Associated with each registration MUST be
   the encoding name for the parameter as well as a short descriptive
   name for it.

   The potential configuration parameters "a" for "attribute" and "t"
   for "transport protocol" are defined in this document and the IANA
   is hereby requested to register these.

7. Acknowledgments

   The SDP Capability Negotiation solution defined in this document
   draws on the overall capability negotiation framework that was
   defined by [SDPng]. Also, the SDP Capability Negotiation solution 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: Francois Audet, John Elwell, Roni Even, Miguel Garcia, Robert
   Gilman, Cullen Jennings, Jonathan Lennox, Matt Lepinski, Jean-
   Francois Mule, Joerg Ott, Colin Perkins, Jonathan Rosenberg, Thomas
   Stach, and Dan Wing.






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8. Change Log

8.1. draft-ietf-mmusic-sdp-capability-negotiation-09

   Incorporated Working Group Chair review comments and a few additional
   comments as follows:

   o  Clarified that the "a=creq" attribute MUST NOT be used in an
      answer (Section 3.6.2. ).

   o  Various editorial changes throughout.

8.2. draft-ietf-mmusic-sdp-capability-negotiation-08

   Incorporated Working Group Last Call comments as follows:

   o  Added second offer/answer exchange to introductory example, fixed
      minor error in that example, and deleted similar example in the
      Examples Section.

   o  Fixed "type=value" semantic error in the attribute capability
      definition.

   o  Clarified that consecutive numbering of capabilities and
      potential configurations is not required.

   o  Fixed inconsistency for which parameters to include in the "acfg"
      attribute.

   o  Changed second offer/answer exchange from MAY to SHOULD strength.

   o  Clarified there should be a combined second offer/exchange when
      using ICE.

   o  Moved RFC 3407 to informative references.

   o  Various editorial clarifications.

8.3. draft-ietf-mmusic-sdp-capability-negotiation-07

   o  Removed the ability to have attribute capabilities provide
      attribute names without values, when those attributes otherwise
      require an associated value.

   o  Document no longer obsoletes RFC 3407 but instead recommends that
      it is being used instead of RFC 3407.


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   o  Added ability to specific that specific extensions in a potential
      configuration are mandatory.

   o  Changed ABNF for extension-config-list in potential
      configurations.

   o  Removed the redundant "a=" part of attribute capabilities.

   o  Clarified what it means to support an attribute capability in the
      offer/answer procedures.

   o  Changed "a=acap" attribute and offer/answer procedures to include
      only the known and supported attribute capabilities.

   o  Added new section on indicating bandwidth usage.

8.4. draft-ietf-mmusic-sdp-capability-negotiation-06

   o  Added additional background text on terminology used, and a new
      section on the negotiation model.

   o  Allowed for session-level attribute capabilities to contain
      media-level only attributes, albeit the base framework does not
      define (or allow) them to be used in a potential configuration
      (extensions may change that)

   o  Disallowing multiple "a=tcap" attributes at the session-level
      and/or on a per media description basis; at most one at the
      session-level and per media description now.

   o  Changed the "a=pcfg" attribute to make a potential configuration
      list optional in order to allow for the actual configuration to
      be referenced.

   o  Removed the ability to delete and replace individual attributes
      from the actual configuration SDP.

   o  Introduced the notion of mandatory and optional attribute
      capabilities in a potential configuration and updated the
      "a=pcfg" attribute and associated procedures accordingly.

   o  Specified that mandatory attribute capabilities and the transport
      protocol (if any) from a potential configuration need to be
      supported in order to select that potential configuration.
      Offer/answer procedures updated accordingly as well.



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   o  Noted potential interaction and synchronization issues with use
      of session-level attributes and attribute capabilities and added
      recommendation to avoid use of session-level attributes when
      possible.

   o  Fixed error in "a=acfg" grammar (missing config-number) and
      updated attribute definition in accordance with the "a=pcfg"
      attribute changes.

   o  Updated text associated with processing media before answer to
      allow for playing out garbage or discard until answer received.
      Additional detail on alternative solutions provided as well.

   o  Added recommendation to send back answer SDP as soon as possible,
      when a potential configuration different from the actual
      configuration has been chosen.

   o  Added new section on interactions with SIP option tags.

   o  Added new section on dealing with large number of potential
      configurations.

   o  Added new section on SDP capability negotiation and
      intermediaries.

   o  Updated examples in accordance with other changes and to
      illustrate use of mandatory and optional attribute capabilities
      in a potential configuration.

   o  Updated security considerations to address potential denial of
      service attack caused by large number of potential
      configurations.

   o  Various editorial updates throughout.

8.5. draft-ietf-mmusic-sdp-capability-negotiation-05

   o  Allowed for '<type>=<value>' attributes to be listed as attribute
      capabilities the attribute name only.

   o  Changed IP-address to conform to RFC 3330 guidelines.

   o  Added section on relationship to RFC 3407 and "Obsoletes: 3407"
      in the front.




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   o  Disallowed use of white space in a number of places for more
      consistency with existing SDP practice

   o  Changed "csup" and "creq" attributes to not allow multiple
      instances at the session-level and multiple instances per media
      description (only one for each now)

   o  Changed to not require use of "creq" with base option tag ("cap-
      v0").

   o  Relaxed restrictions on extension capabilities

   o  Updated potential configuration attribute syntax and semantics.
      In particular, potential configuration attributes can now replace
      and delete various existing attributes in original SDP to better
      control potential attribute interactions with the actual
      configuration while preserving message size efficiency.

   o  Updated actual configuration attribute to align with the updates
      to the potential configuration attributes.

   o  Updated offer/answer procedures to align with other changes.

   o  Changed recommendation for second offer/answer exchange to "MAY"
      strength, unless for the cases where it is known or suspected
      that it is needed.

   o  Updated ICE interactions to explain how the new attribute
      delete/replace features can solve certain potential interactions.

   o  Updated rtpmap and fmtp section to allow potential configurations
      to use remapped payload types in attribute capabilities for
      rtpmaps and fmtp parameters.

   o  Added section on direction attributes.

   o  Added another example showing SRTP with session-level MIKEY and
      SDP Security Descriptions using the attribute capability DELETE
      operator.

8.6. draft-ietf-mmusic-sdp-capability-negotiation-04

   The following are the major changes compared to version -03:

   o  Added explicit ordering rules for attributes added by potential
      configurations.


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   o  Noted that ICE interaction issues (ice-tcp specifically) may not
      be as clear as originally thought.

   o  Added considerations on using rtpmap and fmtp attributes as
      attribute capabilities.

   o  Added multiple transport protocol example.

   o  Added session-level MIKEY and media level security descriptions
      example.

8.7. draft-ietf-mmusic-sdp-capability-negotiation-03

   The following are the major changes compared to version -02:

   o  Base option tag name changed from "v0" to "cap-v0".

   o  Added new section on extension capability attributes

   o  Firmed up offer/answer procedures.

   o  Added security considerations

   o  Added IANA considerations

8.8. 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.


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   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

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

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

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

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

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

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

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

   o  Clarified operator precedence within potential and actual
      configuration attributes.

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

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






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8.10. 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.

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






















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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.

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

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

   [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
             IANA Considerations Section in RFCs", BCP 26, RFC 5226,
             May 2008.

9.2. Informative References

   [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.

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

   [RFC3262] J. Rosenberg, and H. Schulzrinne, "Reliability of
             Provisional Responses in Session Initiation Protocol
             (SIP)", RFC 3262, 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.

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

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


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

   [RFC3830] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K.
             Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
             August 2004.

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

   [RFC3890] M. Westerlund, "A Transport Independent Bandwidth Modifier
             for the Session Description Protocol (SDP).", RFC 3890,
             September 2004.

   [RFC4474] J. Peterson, and C. Jennings, "Enhancements for
             Authenticated Identity Management in the Session
             Initiation Protocol (SIP)", RFC 4474, August 2006.

   [RFC4567] 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.

   [RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session
             Description Protocol Security Descriptions for Media
             Streams", RFC 4568, July 2006.

   [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
             "Extended RTP Profile for Real-Time Transport Control
             Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July
             2006.

   [RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
             Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
             July 2006.

   [RFC4756] A. Li, "Forward Error Correction Grouping Semantics in
             Session Description Protocol", RFC 4756, November 2006.

   [RFC5027] Andreasen, F. and D. Wing, "Security Preconditions for
             Session Description Protocol Media Streams", RFC 5027,
             October 2007.




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   [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.

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

   [ICETCP]  J. Rosenberg, "TCP Candidates with Interactive
             Connectivity Establishment (ICE)", work in progress, July
             2007.

   [SAVPF]   Ott, J., and E Carrara, "Extended Secure RTP Profile for
             RTCP-based Feedback (RTP/SAVPF)", Work in Progress, May
             2007.

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

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

Author's Addresses

   Flemming Andreasen
   Cisco Systems
   Edison, NJ

   Email: fandreas@cisco.com


Intellectual Property Statement

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   it has made any independent effort to identify any such rights.
   Information on the procedures with respect to rights in RFC
   documents can be found in BCP 78 and BCP 79.




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   Copies of IPR disclosures made to the IETF Secretariat and any
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   attempt made to obtain a general license or permission for the use
   of such proprietary rights by implementers or users of this
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   The IETF invites any interested party to bring to its attention any
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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
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   This document and the information contained herein are provided on
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Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.














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