[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

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                        March 4, 2007
Obsolotes: 3407
Expires: September 2007


                        SDP Capability Negotiation
            draft-ietf-mmusic-sdp-capability-negotiation-05.txt


Status of this Memo

   By submitting this Internet-Draft, each author represents that
   any applicable patent or other IPR claims of which he or she is
   aware have been or will be disclosed, and any of which he or she
   becomes aware will be disclosed, in accordance with Section 6 of
   BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

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

   The list of current Internet-Drafts can be accessed at
        http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
        http://www.ietf.org/shadow.html

   This Internet-Draft will expire on September 4, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   The Session Description Protocol (SDP) was intended for describing
   multimedia sessions for the purposes of session announcement, session
   invitation, and other forms of multimedia session initiation. SDP was
   not intended to provide capability indication or capability
   negotiation, however over the years, SDP has seen widespread adoption
   and as a result it has been gradually extended to provide limited
   support for these. SDP and its current extensions however do not have



Andreasen             Expires September 4, 2007                [Page 1]

Internet-Draft        SDP Capability Negotiation             March 2007


   the ability to negotiate one or more alternative transport protocols
   (e.g. RTP profiles) which makes it particularly difficult to deploy
   new RTP profiles such as secure RTP or RTP with RTCP-based feedback.
   The purpose of this document is to address that and other real-life
   limitations by extending SDP with capability negotiation parameters
   and associated offer/answer procedures to use those parameters in a
   backwards compatible manner.

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

Table of Contents


   1. Introduction...................................................3
   2. Conventions used in this document..............................6
   3. SDP Capability Negotiation Solution............................6
      3.1. Solution Overview.........................................6
      3.2. Relationship to RFC 3407..................................9
      3.3. Version and Extension Indication Attributes...............9
         3.3.1. Supported Capability Negotiation Extensions Attribute9
         3.3.2. Required Capability Negotiation Extension Attribute.11
      3.4. Capability Attributes....................................12
         3.4.1. Attribute Capability Attribute......................13
         3.4.2. Transport Protocol Capability Attribute.............14
         3.4.3. Extension Capability Attributes.....................16
      3.5. Configuration Attributes.................................16
         3.5.1. Potential Configuration Attribute...................16
         3.5.2. Actual Configuration Attribute......................22
      3.6. Offer/Answer Model Extensions............................24
         3.6.1. Generating the Initial Offer........................24
         3.6.2. Generating the Answer...............................27
            3.6.2.1. Example Views of Potential Configurations......31
         3.6.3. Offerer Processing of the Answer....................34
         3.6.4. Modifying the Session...............................35
      3.7. Interactions with ICE....................................35
      3.8. Processing Media before Answer...........................36
      3.9. Considerations for Specific Attribute Capabilities.......37
         3.9.1. The rtpmap and fmtp Attributes......................37
         3.9.2. Direction Attributes................................38
   4. Examples......................................................38
      4.1. Best-Effort Secure RTP...................................38
      4.2. Multiple Transport Protocols.............................41


Andreasen             Expires September 4, 2007                [Page 2]

Internet-Draft        SDP Capability Negotiation             March 2007


      4.3. Best-Effort SRTP with Session-Level MIKEY and Media Level
      Security Descriptions.........................................45
      4.4. SRTP with Session-Level MIKEY and Media Level Security
      Descriptions as Alternatives..................................49
   5. Security Considerations.......................................51
   6. IANA Considerations...........................................53
      6.1. New SDP Attributes.......................................53
      6.2. New SDP Capability Negotiation Option Tag Registry.......54
      6.3. New SDP Capability Negotiation Potential Configuration
      Parameter Registry............................................55
   7. To Do and Open Issues.........................................55
   8. Acknowledgments...............................................55
   9. Change Log....................................................56
      9.1. draft-ietf-mmusic-sdp-capability-negotiation-05..........56
      9.2. draft-ietf-mmusic-sdp-capability-negotiation-04..........57
      9.3. draft-ietf-mmusic-sdp-capability-negotiation-03..........57
      9.4. draft-ietf-mmusic-sdp-capability-negotiation-02..........57
      9.5. draft-ietf-mmusic-sdp-capability-negotiation-01..........58
      9.6. draft-ietf-mmusic-sdp-capability-negotiation-00..........59
   10. References...................................................60
      10.1. Normative References....................................60
      10.2. Informative References..................................60
   Author's Addresses...............................................62
   Intellectual Property Statement..................................63
   Full Copyright Statement.........................................63
   Acknowledgment...................................................63

1. Introduction

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

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



Andreasen             Expires September 4, 2007                [Page 3]

Internet-Draft        SDP Capability Negotiation             March 2007


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

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

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

   o  An actual configuration for the session and its associated media
      stream components, which specifies which combinations of session
      parameters and media stream components to use and with what
      parameters.

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

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

   Other relevant extensions have been defined. Simple capability
   declarations, which define how to provide a simple and limited set of
   capability descriptions in SDP was defined in RFC 3407.  Grouping of
   media lines, which defines how media lines in SDP can have other



Andreasen             Expires September 4, 2007                [Page 4]

Internet-Draft        SDP Capability Negotiation             March 2007


   semantics than the traditional "simultaneous media streams"
   semantics, was defined in RFC 3388, etc.

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

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

   As mentioned above, SDP is used by several protocols, and hence the
   mechanism should be usable by all of these.  One particularly
   important protocol for this problem is the Session Initiation
   Protocol (SIP) [RFC3261].  SIP uses the offer/answer model (which is
   not specific to SIP) to negotiate sessions and hence the mechanism


Andreasen             Expires September 4, 2007                [Page 5]

Internet-Draft        SDP Capability Negotiation             March 2007


   defined here defines the offer/answer procedures to use for the
   capability negotiation framework.

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

2. Conventions used in this document

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

3. SDP Capability Negotiation Solution

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

3.1. Solution Overview

   The solution consists of the following:

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

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

       o  A new attribute ("a=creq") that lists the extensions to the
          framework that are required to be supported by the entity
          receiving the SDP in order to do capability negotiation.

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

       o  A new attribute ("a=acap") that defines how to list an
          attribute name, either with or without an associated value, 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 attributes to negotiate configurations as follows:


Andreasen             Expires September 4, 2007                [Page 6]

Internet-Draft        SDP Capability Negotiation             March 2007


       o  A new attribute ("a=pcfg") that lists the potential
          configurations supported. This is done by reference to the
          capabilities from the SDP in question. Multiple potential
          configurations have an explicitly indicated ordering
          associated with them. Extension capabilities can be defined
          and referenced in the potential configurations.

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

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

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

                Alice                               Bob

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

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

      v=0
      o=- 25678 753849 IN IP4 192.0.2.1
      s=
      c=IN IP4 192.0.2.1
      t=0 0
      m=audio 53456 RTP/AVP 0 18



Andreasen             Expires September 4, 2007                [Page 7]

Internet-Draft        SDP Capability Negotiation             March 2007


      a=tcap:1 RTP/SAVP
      a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_32
         inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
      a=pcfg:1 t=1 a=1

   The "m=" line indicates that Alice is offering to use plain RTP with
   PCMU or G.729.  The capabilities are provided by the "a=tcap" and
   "a=acap" attributes. The transport capabilities ("a=tcap") indicate
   that secure RTP under the AVP profile ("RTP/SAVP") is supported with
   an associated transport capability handle of 1. The "acap" attribute
   provides an attribute capability with a handle of 1. The attribute
   capability is a "crypto" attribute, which provides the keying
   material for SRTP using SDP security descriptions [SDES]. The
   "a=pcfg" attribute provides the potential configuration included in
   the offer by reference to the capability parameters.  One alternative
   is provided; it has a configuration number of 1 and it consists of
   transport protocol capability 1 (i.e. the RTP/SAVP profile - secure
   RTP), and the attribute capability 1, i.e. the crypto attribute
   provided. Potential configurations are always preferred over 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:

      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 containing the potential
   configuration with transport protocol capability 1 and attribute
   capability 1 from the offer SDP (i.e. the RTP/SAVP profile using the
   keying material provided).  Bob also includes his keying material in
   a crypto attribute. If Bob supported one or more extensions to the
   capability negotiation framework, he would have included option tags
   for those in the answer as well (in an "a=csup" attribute).

   Note that in this particular example, the answerer supported the
   capability negotiation extensions defined here, however had he not,


Andreasen             Expires September 4, 2007                [Page 8]

Internet-Draft        SDP Capability Negotiation             March 2007


   the answerer would simply have ignored the new attributes and
   accepted the (actual configuration) offer to use normal RTP. In that
   case, the following answer would have been generated instead:

      v=0
      o=- 24351 621814 IN IP4 192.0.2.2
      s=
      c=IN IP4 192.0.2.2
      t=0 0
      m=audio 54568 RTP/AVP 0 18

3.2. 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 obsoletes RFC 3407 by defining new attributes for
   describing attribute capabilities and transport capabilities. It also
   defines procedures for actually using those capabilities as part of
   an offer/answer exchange. 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 however will not be able to
   use the RFC 3407 capability descriptions.

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.



Andreasen             Expires September 4, 2007                [Page 9]

Internet-Draft        SDP Capability Negotiation             March 2007


   Option-tags MUST be registered with IANA per the procedures defined
   in Section 6.

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

      a=csup: <option-tag-list>

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

      att-value         = option-tag-list
      option-tag-list   = option-tag *(COMMA option-tag)
      option-tag        = token    ; defined in [RFC4566]
      COMMA             = ","      ; defined in [RFC4234]

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

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

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

      a=csup:cap-v0

      a=csup:foo

      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 can be at most one "a=csup" attributes at
   the session-level and at most one at the media-level (one per media
   description in the latter case).


Andreasen             Expires September 4, 2007               [Page 10]

Internet-Draft        SDP Capability Negotiation             March 2007


   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. The base option tag MAY be included.

3.3.2. Required Capability Negotiation Extension Attribute

   The Required Capability Negotiation Extensions attribute ("a=creq")
   contains a comma-separated list of option tags (see Section 3.3.1. )
   identifying the SDP Capability negotiation extensions that MUST be
   supported by the entity receiving the SDP in order for that entity to
   properly process the SDP Capability Negotiation attributes and
   associated procedures. Support for the basic negotiation framework is
   implied by the presence of an "a=pcfg" attribute (see Section 3.5.1.
   ) and hence there is no need to include the "a=creq" attribute with
   the base option-tag ("cap-v0"). Still, it is allowed to do so.

   The attribute is defined as follows:

      a=creq: <option-tag-list>

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

      att-value   = option-tag-list

   The following examples illustrate the 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-stream
   in question only (required option tags provided at the session level
   apply as well). There can be at most one "a=creq" attribute at the
   session-level and at most one "a=creq" attribute at the media-level
   (one per media description in the latter case).


Andreasen             Expires September 4, 2007               [Page 11]

Internet-Draft        SDP Capability Negotiation             March 2007


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

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

   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
   capability negotiation MUST NOT be performed for the media stream in
   question.

   When an entity does not support one or more required SDP capability
   negotiation extensions, the entity SHOULD proceed as if the SDP
   capability negotiation attributes were not included in the first
   place, i.e. all the capability negotiation attributes should be
   ignored.  In that case, the entity SHOULD include a "csup" attribute
   listing the SDP capability negotiation extensions it actually
   supports.

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

3.4. Capability Attributes

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


Andreasen             Expires September 4, 2007               [Page 12]

Internet-Draft        SDP Capability Negotiation             March 2007


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 full  '<type>=<value>' form (see [RFC4566]).
   Support for a specific attribute (name) (without any particular
   values) can be indicated by providing only the '<type>' (i.e. the
   attribute name).

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

      att-value   = att-cap-num 1*WSP att-par
      att-cap-num = 1*DIGIT ;defined in [RFC4234]
      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 for
   session-level attributes and the media level for media-level
   attributes. The "acap" attribute MUST NOT be used to provide a media-
   level attribute at the session-level or vice versa.

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

     There is a need to be able to reference both session-level and
     media-level attributes in potential configurations at the media
     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 a=ptime:20

      a=acap:2 a=ptime:30



Andreasen             Expires September 4, 2007               [Page 13]

Internet-Draft        SDP Capability Negotiation             March 2007


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

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

      a=acap:5 a=crypto

      a=acap:6 a=key-mgmt

   The first two provide attribute values for the ptime attribute. The
   third provides SRTP parameters by using MIKEY with the key-mgmt
   attribute [KMGMT]. The fourth provides SRTP parameters by use of
   security descriptions with the crypto attribute [SDES]. Note that the
   line-wrapping and new-lines in example three and four are provided
   for formatting reasons only - they are not permitted in actual SDP.
   The 5th attribute capability merely indicates support for the
   "crypto" attribute (without any further information about particular
   values to use with it), and the 6th attribute capability merely
   indicates support for the "key-mgmt" attribute.

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

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


Andreasen             Expires September 4, 2007               [Page 14]

Internet-Draft        SDP Capability Negotiation             March 2007


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

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

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

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

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

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

   Transport capabilities are inherently included in the "m=" line,
   however they still need to be specified explicitly in a "tcap"
   attribute, if they are to be used as a capability.

     This may seem redundant (and indeed it is from the offerer's point
     of view), however it is done to protect against middle-boxes that
     may modify "m=" lines while passing unknown attributes through. If
     an implicit 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 a middle-box were to modify the
     transport protocol in the "m=" line (e.g. to translate between
     plain RTP and secure RTP), then the potential configuration
     referencing that implicit transport capability may no longer be
     correct. With explicit capabilities, we avoid this pitfall,
     although the potential configuration preference (see Section 3.5.1.
     ) may not reflect that of the middle-box (which some may view as a
     feature).




Andreasen             Expires September 4, 2007               [Page 15]

Internet-Draft        SDP Capability Negotiation             March 2007


3.4.3. Extension Capability Attributes

   The SDP Capability Negotiation framework allows for new 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.

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 "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 [RFC4234]
      pot-cfg-list   = pot-config *(1*WSP pot-config)
      pot-config     = pot-attribute-config-list /
                       pot-transport-protocol-config-list /
                       pot-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
   (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.



Andreasen             Expires September 4, 2007               [Page 16]

Internet-Draft        SDP Capability Negotiation             March 2007


   After the configuration number, one or more potential configuration
   lists MUST be provided. The potential configuration lists generally
   reference one or more capabilities, and those capabilities are
   (conceptually) used to construct a new internal version of the SDP by
   use of purely syntactic add, delete and replace operations on the
   original SDP (actual configuration), thereby generating a new
   potential configuration SDP that can be used by conventional SDP
   procedures if actually selected.

   This document defines potential attribute configuration lists and
   potential transport protocol configuration lists.  Each of these MUST
   NOT be present more than once in a particular potential configuration
   attribute. Potential extension configuration lists can be included as
   well; unknown potential extension configuration lists MUST be ignored
   (if support is required, then the "a=creq" attribute with suitable
   option tags should be used). There can be more than one potential
   extension configuration list, however each particular potential
   extension configuration list MUST NOT be present more than once in a
   given potential configuration attribute. Together, these potential
   configuration lists define a potential configuration.

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

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

      pot-attribute-config-list
                        = "a=" [delete-attributes ":"]
                              att-cap-inst-list *(BAR att-cap-inst-list)
      delete-attributes = DELETE ( "m"    ; media attributes
                              / "s"    ; session attributes
                              / "ms" ) ; media and session attributes
      att-cap-inst-list = att-cap-inst *(COMMA att-cap-inst)
      att-cap-inst      = [att-cap-operator] att-cap-num
      att-cap-operator  =  DELETE / REPLACE
      att-cap-num       = 1*DIGIT   ;defined in [RFC4234]
      BAR               = "|"
      DELETE            = "-"
      REPLACE           = "/"


   Note that white space is not permitted within this production.




Andreasen             Expires September 4, 2007               [Page 17]

Internet-Draft        SDP Capability Negotiation             March 2007


   Each potential attribute configuration list can optionally begin with
   instructions for how to handle attributes that are part of the actual
   configuration (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 instruction list comes next. It contains one
   or more alternative lists of attribute capability instructions. The
   lists are separated by a vertical bar ("|"), and each list contains
   one or more attribute capability instructions separated by commas
   (","). An attribute capability instruction is merely an attribute
   capability number that may optionally be prefixed by an attribute
   capability operator. Each attribute capability number (att-cap-num)
   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=-m:1,2,-3,/4|1,2,5

   where

   o  "a=-m:1,2,-3,/4|1,2,5" is the potential attribute configuration
      list

   o  "-m" is the delete-attributes

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

   o  "1" is an attribute capability instruction without any attribute
      capability operator. It is also an attribute capability number

   o  "-3" is an attribute capability instruction, where "-" is an
      attribute capability operator, and "3" is the attribute capability
      number.





Andreasen             Expires September 4, 2007               [Page 18]

Internet-Draft        SDP Capability Negotiation             March 2007


   o  "/4" is an attribute capability instruction, where "/" is an
      attribute capability operator, and "4" is the attribute capability
      number.

   By default, each referenced attribute capability will result in the
   corresponding attribute name and its associated value (contained
   inside the attribute capability) merely being added to the resulting
   potential configuration SDP. The attribute capability operators
   change this default behavior:

   o  For session-level attribute capabilities, the DELETE operator ("-
      ") will result in the deletion of all session-level occurrences of
      attributes with the same attribute-name (attribute values are
      ignored) as the attribute contained inside that attribute
      capability. The attribute contained inside the attribute
      capability will NOT be added to the resulting potential
      configuration SDP.

         For example, if delete was indicated for a session-level "key-
         mgmt" attribute capability ("a=acap:1 a=key-mgmt..."), all
         occurrences of "a=key-mgmt" at the session-level would be
         deleted.

   o  For media-level attribute capabilities, the DELETE operator ("-")
      will result in the deletion of all occurrences of attributes with
      the same attribute-name (attribute values are ignored) as the
      attribute contained inside that attribute capability, within this
      particular media description ("m=" line) only. The attribute
      contained inside the attribute capability will NOT be added to the
      resulting potential configuration SDP.

         For example, if delete was indicated for a media-level "crypto"
         attribute capability ("a=acap:1 a=crypto..."), all occurrences
         of "a=crypto" inside the media description ("m=" line) in
         question would be deleted.

   o  For session-level attribute capabilities, the REPLACE operator
      ("/") is similar to the DELETE operator, except that the attribute
      contained inside the attribute capability WILL be added to the
      resulting potential configuration SDP (at the session-level).

         For example, if replace was indicated for a session-level "key-
         mgmt" attribute capability ("a=acap:1 a=key-mgmt..."), all
         occurrences of "a=key-mgmt" at the session-level would be
         deleted, and the "key-mgmt" attribute name and its associated
         value from the attribute capability would be added to the
         resulting SDP for that configuration.


Andreasen             Expires September 4, 2007               [Page 19]

Internet-Draft        SDP Capability Negotiation             March 2007


   o  For media-level attribute capabilities, the REPLACE operator ("/")
      is similar to the DELETE operator, except that the attribute
      contained inside the attribute capability WILL be added to the
      resulting potential configuration SDP (within the media
      description in question).

         For example, if replace was indicated for a media-level
         "crypto" attribute capability ("a=acap:1 a=crypto..."), all
         occurrences of "a=crypto" inside the media description ("m="
         line) in question would be deleted, and the "crypto" attribute
         name and its associated value from the attribute capability
         would be added to the resulting SDP for that configuration.

   Alternative attribute capability instruction 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. Exactly one of the alternative lists MUST be selected in its
   entirety in order to use this potential configuration attribute.

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

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

   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 potential transport protocol capabilities are separated
   by a vertical bar ("|").  The alternatives are ordered by preference
   with the most preferred listed first. When there are no transport
   protocol capabilities included in a potential configuration at the
   media level, the transport protocol information from the associated
   "m=" line will be used. When included, exactly one of the
   alternatives MUST be selected in order to use this potential
   configuration attribute.

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



Andreasen             Expires September 4, 2007               [Page 20]

Internet-Draft        SDP Capability Negotiation             March 2007


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

      pot-extension-config-list  = ext-cap-name "="
                                    ext-cap-list
      ext-cap-name               = token     ; defined in [RFC4566]
      ext-cap-list               = 1*VCHAR      ; defined in [RFC4234]

   Note that white-space is not permitted within this production.

   The ext-cap-name refers to the type of 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.

     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:

   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 simply provided with that value at the session-
   level in the first place. Note that individual media streams perform
   capability negotiation individually, and hence it is possible that
   another media stream (where the attribute was part of a potential
   configuration) chose a configuration without that session level


Andreasen             Expires September 4, 2007               [Page 21]

Internet-Draft        SDP Capability Negotiation             March 2007


   attribute. The session-level attribute however remains "active" and
   hence applies to the entire resulting potential configuration SDP. It
   is up to the entity that generated the SDP with these capabilities
   and potential configuration attributes in the first place, to ensure,
   that in such cases, the resulting potential configuration SDP is
   still meaningful.

   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
      a=tcap:3 RTP/SAVP RTP/SAVPF
      a=pcfg:1 t=4|3 a=1
      a=pcfg:8 t=1|2

   We have two potential 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. The second
   potential configuration attribute indicates that the RTP/AVPF or
   RTP/AVP profile can be used, with RTP/AVPF being the preferred one.
   This non secure RTP alternative is the less preferred one since its
   configuration number is "8".

3.5.2. Actual Configuration Attribute

   The actual configuration attribute identifies which of the potential
   configurations from an offer SDP was selected and used as an actual
   configuration in an answer SDP.  This is done by including the
   configuration number and the configuration lists from the offer that


Andreasen             Expires September 4, 2007               [Page 22]

Internet-Draft        SDP Capability Negotiation             March 2007


   were actually selected and used by the answerer in his offer/answer
   procedure as follows:

   o  A selected potential attribute configuration MUST include the
      delete-attributes and the selected alternative att-cap-inst-list
      (i.e. containing both operators and 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 potential 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.

   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. The actual
     configuration attribute does not refer to any of those.

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

      a=acfg: <sel-cfg-list>

   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 ":"] att-cap-inst-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.


Andreasen             Expires September 4, 2007               [Page 23]

Internet-Draft        SDP Capability Negotiation             March 2007


   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"
   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 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
   extensions defined in this document MUST include the following in the
   offer:









Andreasen             Expires September 4, 2007               [Page 24]

Internet-Draft        SDP Capability Negotiation             March 2007


   o  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.

      Session-level attributes and associated values MUST be provided in
      attribute capabilities at the session-level only, whereas media-
      level attributes and associated values MUST be provided in
      attribute capabilities at the media-level only. Attributes that
      can be provided at either the session- or media-level can be
      represented as attribute capabilities at either the session- or
      media-level. Attribute capabilities for '<type>=<value>'
      attributes can include an attribute name only; such attribute
      capabilities MUST NOT be referenced by a potential configuration,
      unless the DELETE operator is used for it. If there is not a need
      to indicate any attributes as attribute capabilities, then there
      will not be any "a=acap" attributes either.

   o  One or more a transport protocol capability attributes ("a=tcap")
      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 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. If there is not a need to indicate
      any transport protocols as transport protocol capabilities, then
      there will not be any "a=tcap" attributes either.

   o  One or more extension capability attributes (as outlined in
      Section 3.4.3. ) for each extension capability that is referenced
      by a potential configuration.

   o  One or more potential configuration attributes ("a=pcfg") as
      defined in Section 3.5.1. within 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" attribute as follows:

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


Andreasen             Expires September 4, 2007               [Page 25]

Internet-Draft        SDP Capability Negotiation             March 2007


   o  For each media description that requires 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. ).

   The offerer SHOULD furthermore include the following:

   o  One or more supported capability negotiation extension attributes
      ("a=csup") as defined in Section 3.3.2. if the offerer supports
      one or more capability negotiation extensions 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=csup"
      attributes at the session-level indicate extensions supported for
      the entire session description whereas option tags provided in
      "a=csup" attributes 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 actually use them. Each potential configuration however
   constitutes an alternative offer that the offerer would like to use.
   The potential configurations may be used by the answerer to negotiate
   and establish the session.

   The offerer MUST include one or more potential configuration
   attributes ("a=pcfg") within 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 either at the session-level or within that particular media
   description; otherwise, the potential configuration is considered
   invalid.

   The current actual configuration is included in the "m=" line (as
   defined by [RFC3264]). Note that the actual configuration is by
   definition the least-preferred 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.



Andreasen             Expires September 4, 2007               [Page 26]

Internet-Draft        SDP Capability Negotiation             March 2007


   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).

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 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 MUST continue as per [RFC3264].
      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.



Andreasen             Expires September 4, 2007               [Page 27]

Internet-Draft        SDP Capability Negotiation             March 2007


   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 attempt to perform capability negotiation
   by using the most preferred potential configuration that is valid. A
   potential configuration is valid 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.

   3. All attribute capabilities referenced by the potential
      configuration are valid themselves (as defined in Section 3.4.1. )
      and each of them is furthermore provided either at the session-
      level or within this particular media description.

   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.

   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 offer SDP, with the
   following changes:

   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.





Andreasen             Expires September 4, 2007               [Page 28]

Internet-Draft        SDP Capability Negotiation             March 2007


   o  If attribute capabilities are present with a delete-attributes
      indication, then session-level attributes and/or media-level
      attributes for this media description MUST be deleted in
      accordance with the delete-attributes provided per the procedures
      in Section 3.5.1.

   o  If a session-level attribute capability is included, then the
      operation is as follows (see also Section 3.5.1. ):

       a. If the attribute capability number is not prefixed with an
          attribute capability operator, then the attribute (and its
          associated value, if any) 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.

       b. If the attribute capability number is prefixed with a DELETE
          operator in the potential configuration, then all session-
          level occurrences of an attribute with the same attribute-name
          (attribute values are ignored) as the attribute contained
          inside that attribute capability MUST be deleted from the
          original SDP as detailed in Section 3.5.1. .

       c. If the attribute capability number is prefixed with a REPLACE
          operator in the potential configuration, then the above DELETE
          operation MUST first be performed, and the attribute MUST then
          be added to the resulting SDP in the same order as specified
          above in a).

   o  If a media-level attribute capability is included, then the
      operation is as follows (see also Section 3.5.1. ):

       a. If the attribute capability number is not prefixed with an
          attribute capability operator, 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.






Andreasen             Expires September 4, 2007               [Page 29]

Internet-Draft        SDP Capability Negotiation             March 2007


       b. If the attribute capability number is prefixed with a DELETE
          operator in the potential configuration, then all media-level
          occurrences of an attribute with the same attribute-name
          (attribute values are ignored) as the attribute contained
          inside that attribute capability MUST be deleted from the
          original SDP in the media description in question as detailed
          in Section 3.5.1. .

       c. If the attribute capability number is prefixed with a REPLACE
          operator in the potential configuration, then the above DELETE
          operation MUST first be performed, and the attribute MUST then
          be added to the resulting SDP in the same order as specified
          above in a).

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

   Note that whereas a transport protocol from the potential
   configuration replaces the transport protocol in the actual
   configuration, an attribute capability from the potential
   configuration is instead added to the actual configuration by
   default. In some cases, this can result in having one or more
   meaningless attributes in the resulting SDP, or worse, ambiguous or
   potentially even illegal attributes. The delete-attributes for the
   session and/or media level attributes as well as the DELETE and
   REPLACE attribute capability operators MUST be used 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 the above operators to actually delete the
     crypto attribute.

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

   If the answerer is not able to support the most preferred valid
   potential configuration for the media description, the answerer MUST
   proceed to the second-most preferred valid potential configuration
   for the media description, etc. If the answerer is not able to


Andreasen             Expires September 4, 2007               [Page 30]

Internet-Draft        SDP Capability Negotiation             March 2007


   support any of the valid potential configurations, the answerer MUST
   process the offer per normal offer/answer rules, i.e. the actual
   configuration provided will be used as the least preferred
   alternative.

   Once the answerer has selected an offered configuration for the media
   stream, the answerer MUST generate a valid answer SDP based on the
   selected configuration as "seen" by the answerer. Furthermore, if the
   answerer selected one of the potential configurations in a media
   description, the answerer MUST include an actual configuration
   attribute within that media description that identifies the
   configuration number for that potential configuration as well as the
   actual parameters that were used from that potential configuration
   (if the potential configuration included alternatives, only the
   selected alternatives 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.

   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 offerer's originally provided actual configuration is contained
   in the media description's "m=" line (and associated parameters). The
   answerer can 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 start to send
   media to the offerer in accordance with the selected potential
   configuration, however the offerer MAY discard such media until the
   offerer receives the answer.

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


Andreasen             Expires September 4, 2007               [Page 31]

Internet-Draft        SDP Capability Negotiation             March 2007


      s=
      t=0 0
      c=IN IP4 lost.example.com
      a=tool:foo
      a=acap:1 a=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 a=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 a=crypto:1 AES_CM_128_HMAC_SHA1_80
         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


Andreasen             Expires September 4, 2007               [Page 32]

Internet-Draft        SDP Capability Negotiation             March 2007


   attribute capability 2 and 3 respectively (SDP security descriptions)
   for the audio and media 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
      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








Andreasen             Expires September 4, 2007               [Page 33]

Internet-Draft        SDP Capability Negotiation             March 2007


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
   capability negotiation.

   For each media description where the offerer included a potential
   configuration attribute, the offerer MUST first examine the 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, then the offerer MUST instead
   process the answer as follows:

   o  The actual configuration attribute specifies which of the
      potential configurations were used by the answerer to generate the
      answer. This includes all the capabilities from the potential
      configuration offered, i.e. the attribute capabilities and
      associated delete-attributes and operators, transport protocol
      capabilities, and any extension capability parameters included.

   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
      had contained the selected potential configuration as the actual
      configuration in the media description ("m=" line) and relevant
      attributes in the offer.

   If the offer/answer exchange was successful, and if the answerer
   selected one of the potential configurations from the offer as the
   actual configuration, then the offerer MAY perform another
   offer/answer exchange: The new offer should 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. This second offer/answer exchange will not modify the
   session in any way, however it will help intermediaries that look at
   the SDP, but do not understand or support the capability negotiation
   extensions, to understand the details of the media stream(s) that
   were actually negotiated. If it is known or suspected that one or
   more such intermediaries exist, then this second offer/answer SHOULD
   be performed (this is already done when using Interactive
   Connectivity Establishment [ICE]). 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.


Andreasen             Expires September 4, 2007               [Page 34]

Internet-Draft        SDP Capability Negotiation             March 2007


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.

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

   We provide the following guidelines for addressing that.

   There are two basic scenarios to consider here:

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

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

   In scenario 1, there should be ICE "a=candidate" attributes for UDP,


Andreasen             Expires September 4, 2007               [Page 35]

Internet-Draft        SDP Capability Negotiation             March 2007


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

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

   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
   actually be used for a particular configuration, yet the actual
   configuration may want to use all of them. 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, as well as the DELETE and REPLACE operators on
   attribute capabilities in a potential configuration can be used to
   ensure that ICE will not end up using a transport protocol that is
   not desired.

3.8. Processing Media before Answer

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

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

   The SDP capability negotiation framework does not define such an
   alternative, however extensions may do so. For example, one technique


Andreasen             Expires September 4, 2007               [Page 36]

Internet-Draft        SDP Capability Negotiation             March 2007


   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). The basic
   SDP capability negotiation framework defined here does not include
   the ability to do so, however extensions that enable that may be
   defined.

3.9. Considerations for Specific Attribute Capabilities

3.9.1. 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
   (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 is 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. They may still
   be useful as pure capabilities though (outside a potential
   configuration).

   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 and/or DELETE/REPLACE attribute capability operators 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


Andreasen             Expires September 4, 2007               [Page 37]

Internet-Draft        SDP Capability Negotiation             March 2007


   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 and/or DELETE/REPLACE attribute
   capability operators MUST be used to ensure that there is not
   multiple fmtp attributes for a given media format in a media
   description.

   Extensions to the core SDP capability negotiation framework of course
   may change the above behavior.

3.9.2. 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. Note that if used by a potential configuration, then
   the normal offer/answer procedures still apply. For example, if an
   offered potential configuration includes the "sendonly" direction
   attribute, it is selected as the actual configuration, then the
   answer must include a corresponding "recvonly" (or "inactive")
   attribute.

4. Examples

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

4.1. Best-Effort Secure RTP

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

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








Andreasen             Expires September 4, 2007               [Page 38]

Internet-Draft        SDP Capability Negotiation             March 2007


                Alice                               Bob

                  | (1) Offer (SRTP and RTP)         |
                  |--------------------------------->|
                  |                                  |
                  | (2) Answer (SRTP)                |
                  |<---------------------------------|
                  |                                  |
                  | (3) Offer (SRTP)                 |
                  |--------------------------------->|
                  |                                  |
                  | (4) Answer (SRTP)                |
                  |<---------------------------------|
                  |                                  |


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

      v=0
      o=- 25678 753849 IN IP4 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/AVP
      a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_80
         inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
         FEC_ORDER=FEC_SRTP
      a=pcfg:1 t=1 a=1

   The "m=" line indicates that Alice is offering to use plain RTP with
   PCMU or G.729.  The capabilities are provided by the "a=tcap" and
   "a=acap" attributes.  The "tcap" capability indicates that both
   Secure RTP and normal RTP are supported. The "acap" attribute
   provides 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 [SDES]. The "a=pcfg" attribute
   provides the potential configurations included in the offer by
   reference to the capabilities.  A single potential configuration with
   a configuration number of "1" is provided. It includes is transport
   protocol capability 1 (RTP/SAVP, i.e. secure RTP) together with the
   attribute capability 1, i.e. the crypto attribute provided.

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


Andreasen             Expires September 4, 2007               [Page 39]

Internet-Draft        SDP Capability Negotiation             March 2007


      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 containing the potential
   configuration with transport protocol capability 1 and attribute
   capability 1 from the offer SDP (i.e. the RTP/SAVP profile using the
   keying material provided).  Bob also includes his keying material in
   a crypto attribute.

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

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

      v=0
      o=- 25678 753850 IN IP4 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
         FEC_ORDER=FEC_SRTP

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

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

      v=0
      o=- 24351 621815 IN IP4 192.0.2.2


Andreasen             Expires September 4, 2007               [Page 40]

Internet-Draft        SDP Capability Negotiation             March 2007


      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 of course have done so if he
   wanted to.

   Note that in this particular example, the answerer supported the
   capability extensions defined here, however had he not, the answerer
   would simply have ignored the new attributes received in step 1 and
   accepted the offer to use normal RTP. In that case, the following
   answer would have been generated in step 2 instead:

      v=0
      o=- 24351 621814 IN IP4 192.0.2.2
      s=
      c=IN IP4 192.0.2.2
      t=0 0
      m=audio 54568 RTP/AVP 0 18


4.2. 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. As in the previous example, 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:












Andreasen             Expires September 4, 2007               [Page 41]

Internet-Draft        SDP Capability Negotiation             March 2007


                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 a=crypto:1 AES_CM_128_HMAC_SHA1_80
         inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
         FEC_ORDER=FEC_SRTP
      a=acap:2 a=rtcp-fb:0 nack
      a=pcfg:1 t=1 a=1,2
      a=pcfg:2 t=2 a=1
      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 [SDES]. 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


Andreasen             Expires September 4, 2007               [Page 42]

Internet-Draft        SDP Capability Negotiation             March 2007


   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).

   o  Potential configuration 2, which is the second most preferred
      potential configuration specifies use of transport protocol
      capability 2 (RTP/SAVP) and 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 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
      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 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 generates a new offer using the actual
   configuration. This is done purely to assist any middle-boxes that


Andreasen             Expires September 4, 2007               [Page 43]

Internet-Draft        SDP Capability Negotiation             March 2007


   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 RTP/AVPF, and it is not using the
   SDP capability negotiation extensions (Alice could have included the
   capabilities as well is she wanted to):

      v=0
      o=- 25678 753850 IN IP4 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

   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 of course have done so if he
   wanted to.

   Note that in this particular example, the answerer supported the
   capability extensions defined here, however had he not, the answerer
   would simply have ignored the new attributes received in step 1 and
   accepted the offer to use normal RTP. In that case, the following
   answer would have been generated in step 2 instead:

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


Andreasen             Expires September 4, 2007               [Page 44]

Internet-Draft        SDP Capability Negotiation             March 2007


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

4.3. 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 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:

             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 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 a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
      a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF
      m=audio 59000 RTP/AVP 98
      a=rtpmap:98 AMR/8000


Andreasen             Expires September 4, 2007               [Page 45]

Internet-Draft        SDP Capability Negotiation             March 2007


      a=acap:2 a=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 a=crypto:1 AES_CM_128_HMAC_SHA1_80
         inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
      a=acap:4 a=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). 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).

   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).

   o  The third configuration with configuration number 3 uses transport
      capability 3 (RTP/AVPF) and 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


Andreasen             Expires September 4, 2007               [Page 46]

Internet-Draft        SDP Capability Negotiation             March 2007


      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
   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 generates a new offer using the actual
   configuration. This is done purely to assist any middle-boxes that
   may reside between Alice and Bob but do not support the capability
   negotiation extensions (and hence may not understand the negotiation
   that just took place):

   Alice's updated offer includes only SRTP for the audio stream SRTP
   with RTCP-based feedback for the video stream, and it is not using
   the SDP capability negotiation extensions (Alice could have included
   the capabilities as well is she wanted to):

      v=0
      o=- 25678 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



Andreasen             Expires September 4, 2007               [Page 47]

Internet-Draft        SDP Capability Negotiation             March 2007


         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 now indicates that Alice is offering to use
   secure RTP with RTCP-based feedback with H.261. Each media stream
   includes a "crypto" attribute, which provides the SRTP keying
   material, 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 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 of course have done so if he
   wanted to.

   Note that in this particular example, the answerer supported the
   capability extensions defined here, however had he not, the answerer
   would simply have ignored the new attributes received in step 1 and
   accepted the offer to use normal RTP. In that case, the following
   answer would have been generated in step 2 instead:

      v=0
      o=- 24351 621814 IN IP4 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


Andreasen             Expires September 4, 2007               [Page 48]

Internet-Draft        SDP Capability Negotiation             March 2007


      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=
      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.

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

   The following example illustrates how to use the SDP Capability
   negotiation extensions 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:




Andreasen             Expires September 4, 2007               [Page 49]

Internet-Draft        SDP Capability Negotiation             March 2007


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

   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. Note the presence of
   attribute capability 1; it merely lists the "a=key-mgmt" attribute
   without any associated value. The potential configuration for the
   audio stream specifies that attribute capability 1 is to be deleted
   (i.e. any session-level "a=key-mgmt" attributes) and that 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 crypto attribute capability (3).

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


Andreasen             Expires September 4, 2007               [Page 50]

Internet-Draft        SDP Capability Negotiation             March 2007


   Security Descriptions and MIKEY. Since the potential configuration is
   more preferred than the actual configuration, he (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=-1,2
      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=-1,3

   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 attribute capability 1 deleted, and attribute capability 2
   included (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 attribute
   capability 1 deleted, and attribute capability 3 included.

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 Negotiations introduces additional security
   issues. Its use as a mechanism to enable alternative transport


Andreasen             Expires September 4, 2007               [Page 51]

Internet-Draft        SDP Capability Negotiation             March 2007


   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
   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 in
   turn 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 [SMIME] 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 actually 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


Andreasen             Expires September 4, 2007               [Page 52]

Internet-Draft        SDP Capability Negotiation             March 2007


   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 non-secure media stream and also does
   not support the capability negotiation framework, then negotiation of
   the media stream will fail. Alternatively, SDP security preconditions
   [sprecon] can be used. This will ensure that media is not flowing
   until session negotiation has completed and hence the selected
   configuration is known. Use of preconditions however requires both
   side 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.

   Additional security considerations apply to the answer SDP as well.
   The actual configuration attribute tells the offerer which potential
   configuration the answer was actually 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.

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.


Andreasen             Expires September 4, 2007               [Page 53]

Internet-Draft        SDP Capability Negotiation             March 2007


   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.

   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.

   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.

   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.

   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.

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 [RFC2434] Specification Required policy. The RFC MUST
   provide the name of the option tag, a syntax and a semantic


Andreasen             Expires September 4, 2007               [Page 54]

Internet-Draft        SDP Capability Negotiation             March 2007


   specification of any new SDP attributes and any extensions to the
   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
   document in an RFC in accordance with the [RFC2434] 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. To Do and Open Issues

   o  Add additional examples showing use of delete-attributes and the
      DELETE/REPLACE attribute capability operators.

8. Acknowledgments

   This document is heavily influenced by the discussions and work done
   by the SDP Capability Negotiation Design team. The following people
   in particular provided useful comments and suggestions to either the
   document itself or the overall direction of the solution defined in
   here: Francois Audet, John Elwell, Roni Even, Robert Gilman, Cullen
   Jennings, Matt Lepinski, Joerg Ott, Colin Perkins, Thomas Stach, and
   Dan Wing.





Andreasen             Expires September 4, 2007               [Page 55]

Internet-Draft        SDP Capability Negotiation             March 2007


9. Change Log

9.1. 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.

   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.


Andreasen             Expires September 4, 2007               [Page 56]

Internet-Draft        SDP Capability Negotiation             March 2007


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

9.2. 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.

   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.

9.3. 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

9.4. 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")




Andreasen             Expires September 4, 2007               [Page 57]

Internet-Draft        SDP Capability Negotiation             March 2007


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

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

   o  Added ABNF throughout

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

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

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

   o  Added initial section on ICE interactions

   o  Added initial section on receiving media before answer

9.5. 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.



Andreasen             Expires September 4, 2007               [Page 58]

Internet-Draft        SDP Capability Negotiation             March 2007


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

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

9.6. 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.















Andreasen             Expires September 4, 2007               [Page 59]

Internet-Draft        SDP Capability Negotiation             March 2007


10. References

10.1. Normative References

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

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

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

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

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

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

   [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
             IANA Considerations Section in RFCs", BCP 26, RFC 2434,
             October 1998.

10.2. Informative References

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

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

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

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





Andreasen             Expires September 4, 2007               [Page 60]

Internet-Draft        SDP Capability Negotiation             March 2007


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

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

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

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

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

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

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

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

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

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

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





Andreasen             Expires September 4, 2007               [Page 61]

Internet-Draft        SDP Capability Negotiation             March 2007


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

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

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

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

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



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

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

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

   [sprecon] Andreasen, F. and D. Wing, "Security Preconditions for
             Session Description Protocol Media Streams", Work in
             Progress, October 2006.



Author's Addresses

   Flemming Andreasen
   Cisco Systems
   Edison, NJ

   Email: fandreas@cisco.com



Andreasen             Expires September 4, 2007               [Page 62]

Internet-Draft        SDP Capability Negotiation             March 2007


Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that 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.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.

Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgment

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





Andreasen             Expires September 4, 2007               [Page 63]


Html markup produced by rfcmarkup 1.109, available from https://tools.ietf.org/tools/rfcmarkup/