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Versions: (draft-andreasen-mmusic-sdp-capability-negotiation-reqts) 00 01

MMUSIC Working Group                                       F. Andreasen
Internet Draft                                            Cisco Systems
Expires: September 2007                                   March 4, 2007



                        SDP Capability Negotiation:
                 Requirements and Review of Existing Work


         draft-ietf-mmusic-sdp-capability-negotiation-reqts-01.txt


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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



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   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, for example, 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 and RTP with
   RTCP-based feedback. The purpose of this document is to identify a
   set of requirements for SDP Capability Negotiation and evaluate
   existing work in this area. The document does not provide any
   solutions to SDP Capability Negotiation

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].

Table of Contents


   1. Introduction...................................................3
   2. Requirements...................................................5
   3. Review of Existing Work.......................................10
      3.1. Grouping of Media Lines..................................10
      3.2. Session Description Protocol (SDP) Simple Capability
      Declaration...................................................11
      3.3. Session Description and Capability Negotiation (SDPng)...12
      3.4. Multipart/alternative....................................14
      3.5. Sharing Ports Between "m=" Lines.........................15
      3.6. Opportunistic Encryption Using a Session Attribute.......16
      3.7. Best-Effort Secure Real-Time Transport Protocol..........17
      3.8. Opportunistic Encryption using Probing...................18
   4. Security Considerations.......................................18
   5. IANA Considerations...........................................18
   6. Acknowledgments...............................................18
   7. Change Log....................................................18
      7.1. draft-ietf-mmusic-sdp-capability-negotiation-reqts-01.txt18
      7.2. draft-ietf-mmusic-sdp-capability-negotiation-reqts-00.txt19
      7.3. draft-andreasen-mmusic-sdp-capability-negotiation-reqts-
      00.txt........................................................19
   8. References....................................................20
      8.1. Normative References.....................................20
      8.2. Informative References...................................20
   Author's Addresses...............................................22
   Intellectual Property Statement..................................22
   Full Copyright Statement.........................................22


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   Acknowledgment...................................................23

1. Introduction

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

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

   Such restrictions are not generally acceptable to conversational
   multimedia session invitations, where two or more entities attempt to
   establish a media session using 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 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, or potential configurations of the media
      stream components, supported by each side.

   o  A set of actual configurations of the media stream components,
      which specifies which media stream components to use and with what
      parameters.

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

   SDP by itself was designed to provide only the second of these, i.e.,
   the actual configurations, however over the years, use of SDP has
   been extended beyond its original scope.  Session negotiation


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   semantics were defined by the offer/answer model in RFC 3264
   [RFC3264].  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 - potential configurations are not supported. The
   answer however may reduce the set of actual configurations from the
   offer.  The answerer may also include additional media formats in the
   answer, however those media formats can only be used in the answerers
   receive direction.  Such additional media formats would be actual
   configurations as well.

   Other relevant extensions have been defined. Simple capability
   declarations, which defines how to provide a simple and limited set
   of capability descriptions in SDP was defined in RFC 3407.  Grouping
   of media lines, which defines how media lines in SDP can have other
   semantics than the traditional "simultaneous media streams"
   semantics, was defined in RFC 3388, etc.

   Each of these extensions was designed to solve a specific limitation
   of SDP.  Since SDP had already been stretched beyond its original
   intent, a more comprehensive capability declaration and negotiation
   process was intentionally not defined.  Instead, work on a "next
   generation" of a protocol to provide session description and
   capability negotiation was initiated [SDPng].  SDPng however has not
   gained traction and has remained as work in progress for an extended
   period of time.  Existing real-time multimedia communication
   protocols such as SIP, RTSP, Megaco, and MGCP continue to use SDP.
   SDP and its current extensions however do not address an increasingly
   important problem: the ability to negotiate one or more alternative
   transport protocols (e.g., RTP profiles).  This makes it difficult to
   deploy new RTP profiles such as secure RTP (SRTP) [SRTP], RTP with
   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 constraints have been
   found as well.

   The purpose of this document is two-fold.



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   1. Identify a set of requirements for a SDP capability negotiation
   mechanism that will enable SDP to provide limited support for
   indicating potential configurations (capabilities) and negotiate the
   use of those potential configurations as actual configurations.

   2. Review relevant existing work in the area of SDP capability
   negotiation and see how it aligns with the proposed requirements.

   It should be noted, that it is not the intent to provide requirements
   for a full-fledged capability indication and negotiation mechanism
   along the lines of SDPng [SDPng] or ITU-T H.245 [H245]. Rather, the
   focus is on identifying requirements for addressing a set of well-
   known real-life limitations.

   The rest of the document is structured as follows. In Section 2, we
   provide a set of requirements for SDP capability negotiation, and in
   Section 3, we review relevant existing work in this area, how a
   solution based on that work might look, and the pros and cons
   associated with each. An actual solution to the proposed requirements
   will be provided separately.

2. Requirements

   REQ-10: It MUST be possible to indicate and negotiate alternative
   media formats on a per media stream basis.

      For example, many implementations support multiple codecs, but
      only one at a time.  Changes between codecs cannot be done on-
      the-fly, e.g. when receiving a simple RTP payload type change.

   REQ-20: It MUST be possible to indicate and negotiate alternative
   attribute values ("a=") on a per media stream basis.

      For example, T.38 [T38] defines new attributes that may need to
      be conveyed as part of a capability. Also, alternative SRTP
      keying mechanisms (e.g. [SDES] and [KMGMT]) may use SDP
      attributes to negotiate SRTP keying material.

   REQ-25: It MUST be possible to indicate and negotiate alternative
   attribute values ("a=") at the session level.

      For example, [KMGMT] may indicate alternative key management
      material for MIKEY [MIKEY] at the session level.

   REQ-30: It MUST be possible to indicate and negotiate alternative
   media format parameter values ("a=fmtp") per media format on a per
   media stream basis.


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      For example, a media format (codec) indicated as an alternative
      capability may include fmtp parameters.

   REQ-40: It MUST be possible to indicate and negotiate alternative
   transport protocols, e.g. different RTP profiles, on a per media
   stream basis.

      For example, "RTP/AVP" and "RTP/SAVP" may be alternatives.

   REQ-50: It MUST be possible to indicate and negotiate alternative
   transport protocol and media type combinations on a per media stream
   basis.

      For example, an entity may support a fax call using either T.38
      fax relay ("m=image <port> udptl t38") or PCMU ("m=audio <port>
      RTP/AVP 0").

   REQ-80: The mechanism MUST be backwards compatible for SIP endpoints;
   an entity that does not support the mechanism should behave as if the
   mechanism was not present in the signaling the first place.

      The above is referred to as "best-case" backwards compatibility.
      Another form of backwards compatibility could result in an error
      message indicating the extension is not supported, however this
      has undesirable side-effects for SIP (heterogeneous error
      response forking problem - HERFP) and leads to additional
      signaling. Another form of backwards compatibility would have an
      entity that does not support the extension behaving differently
      than it normally would (but not failing), possibly followed by
      additional signaling to remedy this.

   REQ-85: The mechanism SHOULD attempt to be as backwards compatible
   and friendly as reasonably possible for SIP middle-boxes. A SIP
   middle-box is here defined as a SIP entity between two SIP endpoints
   in a session, that may or may not have the media associated with the
   SIP session traverse through it. In either case, the middle box may
   have a need to understand the details of the media streams for the
   session.

      For example, a SIP middle-box may inspect the SDP exchanged to
      determine what Quality of Service to authorize for the associated
      media streams. While such middle boxes are not part of the SIP
      architecture or the Internet per se, they do exist in many real
      life scenarios, and we have a desire to provide a solution for
      those. A SIP middle-box such as the above, may be affected by a
      solution to the requirements provided above. For example, if the
      SDP exchanged appears to negotiate use of plain RTP, but in


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      reality, extensions are being used that result in Secure RTP with
      an added authentication tag being used, then the middle-box above
      may authorize less bandwidth than necessary. Similarly, if it
      tries to process media, unexpected results may occur. This is why
      such middle-boxes in theory should not pass SDP through them, if
      said SDP contains one or more attributes they do not understand
      or support. However, in practice, many SIP middle-boxes
      nevertheless do in an attempt to be as minimally invasive as
      possible. The alternative would be for a new extensions to either
      never make it through such middle-boxes or have SIP methods using
      them be rejected. The requirement provided here is to try and aid
      such middle-boxes under the assumption that they will be upgraded
      to support the extensions that will be defined, and hence the aid
      provided need not be optimal.

   REQ-90: The mechanism MUST either be backwards compatible for Megaco
   and MGCP or it MUST be possible to interwork it with Megaco and MGCP
   without any additional signaling between the MGC and its peer (e.g.
   another SIP UA as opposed to a media gateway).

      For example, if a media gateway controller (MGC) uses SIP to
      communicate with peers, and the MGC uses Megaco or MGCP to
      control a media gateway, it must be possible to translate between
      the mechanism and normal SDP. Avoiding interworking requirements
      in the MGC is desirable.

   REQ-100: The mechanism MUST work within the context of the
   offer/answer model [RFC3264]. Specifically, it MUST be possible to
   negotiate alternatives within a single offer/answer exchange.

   REQ-110: The offer/answer model requires the offerer to be able to
   receive media for any media streams listed as either "recvonly" or
   "sendrecv" in an offer, as soon as that offer is generated.  The
   mechanism MUST preserve this capability for all actual configurations
   included in an offer.

      Potential configurations do not have such a requirement.

   REQ-120: The mechanism MUST enable inclusion of potential
   configurations (alternative capabilities) in the offer - the answer
   would then indicate which, if any of these potential configurations
   were accepted. The offerer is not required to process media for a
   specific potential configuration until the offerer receives an answer
   showing that potential configuration was accepted.

      Note that this implies that it may not be possible for the
      offerer to process early media generated using a potential


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      configuration (as opposed to the actual configuration) until the
      answer has been received.

   REQ-121: The mechanism SHOULD enable inclusion of potential
   configurations for each offered media stream in the answer. The
   offerer may wish to use such potential configurations as input to
   generating additional offers subsequently.

   REQ-122: The mechanism SHOULD enable inclusion of additional media
   streams beyond those offered in the offer or accepted in the answer
   as latent potential configurations, provided this does not unduly
   complicate the mechanism. Latent potential configurations cannot be
   promoted to actual configurations in the current offer/answer
   exchange.

      For example, an offerer that offers only audio, but is also able
      to support video, may want to indicate this.  Similarly, an
      answerer that receives an offer for audio only, but is able to do
      video may want to indicate this. Such capabilities are purely
      information in the current offer/answer exchange - use of them
      cannot be negotiated until a new offer/exchange is performed.

   REQ-130: The mechanism MUST work as well as existing offer/answer
   procedures in the presence of SIP forking. The mechanism MUST NOT
   introduce any new failure scenarios for SIP forking.

   REQ-140: The mechanism SHOULD be reasonably efficient in terms of
   overall message size.

      This is a relative requirement to evaluate alternative solutions
      as opposed to an absolute and quantifiable requirement. Use of
      compression techniques can help reduce the size of text-based
      messages, however it is still considered important to try and
      keep the message size reasonably small.

   REQ-150: It SHOULD be possible to specify valid combinations of media
   lines.

      For example, an entity may be able to support audio and video or
      audio and IM, but not video and IM (or all three).

   REQ-160: It SHOULD be possible to specify valid combinations of media
   formats between media streams.

      For example, there may be constraints on which combinations of
      audio and video codecs can be supported.



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   REQ-170: The mechanism MUST be extensible and allow for new types of
   capabilities to be specified and used in potential and actual
   configurations.

      For example, the mechanism could be extended to negotiate unicast
      or multicast addresses as alternatives.

   REQ-300: The mechanism provided MUST be modular inasmuch as it can be
   divided into a required core set of functionality that all entities
   MUST support and an optional set of enhancements that entities MAY
   support. Entities that implement different sets of enhancements MUST
   be fully interoperable, albeit possibly with reduced functionality in
   terms of the actual negotiation performed.

      For example, not all entities may implement support for REQ-150
      and REQ-160

   REQ-301: The mechanism MUST provide a way for both the offerer and
   the answerer to indicate the current version(s) of the mechanism
   supported as well as the enhancements supported.

   REQ-302: The mechanism MUST provide a way for the offerer to indicate
   the version and enhancements that are used by the offerer and must be
   supported by the answerer (and vice versa), in order to correctly
   process the SDP capability negotiation in the offer (or answer).

      Note that this requirement applies to the SDP capability
      negotiation part of the SDP only - other SDP extensions are
      unaffected by this.

   REQ-310: The following requirements are considered enhancements as
   defined in REQ-300:

   o  REQ-10 (alternative media formats)

   o  REQ-30 (alternative fmtp parameters)

   o  REQ-50 (alternative combinations of transport protocol and media
      type)

   o  REQ-122 (additional media streams as latent configurations)

   o  REQ-150 (valid combinations of media lines)

   o  REQ-160 (valid combinations of media formats between media
      streams)



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      [Editor's Note: Need to consider layered codecs and how they may
     impact the requirements]

   Above, we presented the requirements for the capability negotiation
   mechanism. Below, we provide a set of features that were considered
   and then explicitly deemed to be out-of-scope:

   o  Support for negotiation of unicast and multicast addresses as
      alternatives. It was suggested as a requirement initially, but
      subsequent discussion led to its removal.

   o  Support for negotiation of IPv4 and IPv6 addresses as
      alternatives. It was suggested as a requirement initially, but
      subsequent discussion led to its removal.

   If necessary, it should be possible to define such capabilities in
   the form of extensions.

3. Review of Existing Work

   In this section, we provide an overview of existing relevant work
   that has either been completed or is work in progress.  For each
   item, we outline how/if it can be used to address the requirements
   provided and the pros and cons of doing so.

3.1. Grouping of Media Lines

   Grouping of Media Lines is defined in [RFC3388]. RFC 3388 defines a
   framework that enables two or more media lines to be grouped together
   for different purposes. Each media line is assigned an identifier and
   one or more group attributes then references two or more of those
   identifiers. Associated with each group attribute is a semantics
   indication. One semantic indication is the Alternative Network
   Address Types ("ANAT") [RFC4091] which allows for IPv4 and IPv6
   addresses to be specified as alternatives. The requirements presented
   above go beyond that, however a new semantic to simply indicate
   alternative media lines and associated negotiation rules could easily
   be defined.

   The main advantages of such an approach would be:

   o  Mechanism Reuse:  Several semantics have already been defined
      which increases the likelihood of an implementation supporting the
      framework.

   The disadvantages of such an approach would be:



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   o  Backwards Compatibility:   The mechanism is not transparently
      backwards compatible.  If an entity that does not support the
      mechanism receives it, the entity may incorrectly interpret the
      SDP as consisting of multiple media streams.  While RFC 3388
      defines procedures for recognizing and recover from this when
      using offer/answer, it can still lead to unintended behavior with
      endpoints that do not support the mechanism.

        In practice, it is not clear how much of an issue this is, at
        least for intelligent SIP endpoints. Most current
        implementations generally accept only one media stream of a
        given type (e.g. audio). Use of alternatives with different
        media stream types (e.g. a fax call using "audio" for voice-
        band data or "image" for T.38) makes it less clear though.
        Also, Media Gateway Controllers and Media Gateways that do not
        support grouping of media lines have been known to encounter
        problems.

   o  Semantics Combination Issues: Multiple semantics may be provided
      by use of grouping, however they may interact with each other
      unintentionally. For example, the "FID" semantics defined in RFC
      3388 forbids grouping of media lines with the same transport
      address, however that would be needed for alternative
      capabilities. Thus, using "FID" and alternative capabilities
      together would require special consideration.

   o  Some Combinatoric Explosion:  The mechanism is not ideal to
      indicate alternative capabilities for multiple parameters or media
      formats within a particular media stream. For example, alternative
      attribute values and media format parameters for several codecs
      would lead to combinatoric explosion.

      [Editor's note: In practice, it is not clear this is a huge issue
      though.]

   o  Message Size:  Each alternative requires full duplication of all
      the relevant media stream parameters.

      [Editor's note: In practice, it is not clear this is a huge issue
      though.]

3.2. Session Description Protocol (SDP) Simple Capability Declaration

   SDP Simple Capability Declaration (simcap) is defined in [RFC3407].
   It defines a set of SDP attributes that enables capabilities to be
   described at a session level or on a per media stream basis.  RFC
   3407 defines capability declaration only - actual negotiation


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   procedures taking advantage of such capabilities have not been
   defined. Such rules however could easily be defined - the negotiation
   part would extend the offer/answer model to examine alternative
   configurations (capabilities).  In conjunction with that, attributes
   to indicate the alternative configurations accepted would likely be
   needed as well.

   The main advantages of this approach are:

   o  Satisfies most of the requirements provided above.  In particular,
      by relying solely on SDP attributes, transparent backwards
      compatibility is always ensured.

   The disadvantages of this approach are:

   o  Offered Capabilities Hidden in Attributes:   An offer may be
      accepted by the answerer and a media stream established based on
      SDP parameters contained in SDP attributes not known to
      intermediaries. Such intermediaries may be back-to-back user
      agents, or proxies that need to inspect the SDP, e.g., to
      authorize Quality of Service, add transcoders, etc.

   o  Maximum of 255 alternative media formats per SDP:     RFC 3407
      currently allows a maximum of 255 alternative media formats
      (codecs) per SDP. This may be too restrictive.

3.3. Session Description and Capability Negotiation (SDPng)

   The Session Description and Capability Negotiation protocol [SDPng]
   was intended as a replacement for SDP [SDP].  SDPng includes a full
   capability indication and negotiation framework that would address
   the shortcomings of SDP and satisfy the requirements provided above.
   However, SDPng has not gained traction, in large part due to existing
   widespread adoption of SDP.  As a consequence, SDPng has remained as
   work in progress with limited progress for an extended period of
   time.

   SDPng consists of two things: an SDPng description, which is an XML
   document that describes the actual and/or potential configurations as
   well as an optional negotiation process (an offer/answer compliant
   process is included as part of SDPng). The SDPng description consists
   of up to five parts:

   o  Capabilities:     An optional list of capabilities (potential
      configurations) to be matched with the other parties'
      capabilities.



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   o  Definitions:      An optional set of definitions of commonly used
      parameters for later referencing.

   o  Configurations:   A mandatory description of the conference
      components, each of which can provide a list of alternative
      configurations.

   o  Constraints:      An optional set of constraints of combinations
      of configurations.  Constraints are not defined as part of the
      base SDPng specification.

   o  Session Information:    Optional meta information on the
      conferences and individual components.

   SDPng is application-agnostic with the base specification defining a
   basic XML schema supporting the above.  In order to actually use
   SDPng, application-specific packages are needed.  Packages define
   things such as media types, codecs and their configuration
   parameters, etc.  The base SDPng specification includes a couple of
   example packages to support audio, video, and RTP.

   One approach to extending SDP with capability indication and
   negotiation capabilities could be to adopt the mechanisms defined by
   SDPng that are necessary to satisfy the requirements provided above.
   Those areas could then be included within SDP itself, e.g. in the
   form of one or more SDP attributes ("a=") containing the actual SDPng
   description. The areas to consider here include:

   o  Capabilities:  This would be needed to describe alternative media
      formats and media format parameters.

   o  Configurations:   This would be needed to define alternative
      configurations

   The constraints and session information parts of SDPng would not be
   used.

   The main advantages of such an approach would be:

   o  SDPng was designed and intended to solve the general capability
      negotiation problems faced by SDP.  A considerable amount of work
      has already gone into it and it was originally targeted as the
      long-term direction (replacement for SDP).

   The disadvantages of such an approach would be:




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   o  Duplicate Encoding and Specification Work:   SDPng uses a
      different coding format than SDP and hence all SDP parameters
      (incl. codecs and transports) that need to be provided will need
      to have an equivalent SDPng definition.  There is currently no
      automatic process for translating all SDP parameters or values
      into corresponding SDPng parameters or values; many existing SDP
      parameters and values currently have no corresponding SDPng
      definition.

   o  SDPng is Work in Progress: SDPng is currently work in progress but
      has seen limited interest and progress for a while.  Adoption of a
      subset of its current definition may end up differing from the
      final specification.  Also, the current SDPng specification needs
      further clarification and semantic tightening in a number of areas
      that would be of relevance to this approach.

   o  Negotiation of Transport Parameters:   SDPng currently does not
      support negotiation of transport parameters as individual
      capabilities.  It is however still possible to negotiate different
      transport parameters by providing alternative configurations.

   o  Verbose Encoding and Large Message Size:  SDPng descriptions are
      XML documents, which are fairly verbose and result in descriptions
      that are substantially larger than existing SDP.

3.4. Multipart/alternative

   In [I-D.jennings-sipping-multipart], the use of multipart/alternative
   MIME is proposed as a way to support multiple alternative offers.
   Each alternative offer has an id associated with it by use of a new
   MIME header field called Content-Answering-CID. The answerer chooses
   one of the offers and performs normal offer/answer operation on that
   offer, and then sends back a single answer which includes the
   Content-Answering-CID value of the offer chosen.

   The main advantages of this approach are:

   o  It allows for use of alternative encodings of the offer, e.g. SDP
      and SDPng, as well as varying levels of confidentiality and
      integrity by use of S/MIME [RFC3851].

   Use of multipart/alternative to solve the SDP capability negotiation
   problems however has several shortcomings:






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   o  Backwards Compatibility:   Neither SIP nor RTSP mandate support
      for Multipart MIME. In the case of SIP, multipart/alternative is
      generally incompatible with existing SIP proxies, firewalls,
      Session Border Controllers, and endpoints.

   o  Heterogeneous Error Response Forking Problem (HERFP): When a SIP
      proxy forks a request to multiple Contacts, each of which generate
      a response, the proxy only forwards the "best" of these responses
      to the request originator.  If one or more of the Contacts do not
      support multipart/alternative, the request originator may never
      discover this.  Instead, only a Contact that supports
      multipart/alternative will be able to generate an answer that
      reaches the request originator.

   o  Combinatoric Explosions:   Use of multipart/alternative to convey
      alternatives on a per media stream basis or even per media format
      parameter basis quickly leads to combinatoric explosions.

   o  Message Size:  Each alternative requires full duplication of all
      the relevant SDP parameters (one complete SDP per alternative).

   It should be noted, that use of multipart/alternative has been
   discussed several times before and, in large part due to the problems
   mentioned above as well as the semantics defined for
   multipart/alternative [RFC2046], has met with opposition when it
   comes to addressing the above types of requirements.

3.5. Sharing Ports Between "m=" Lines

   SDP [SDP] does not state whether two "m=" lines can share the same
   transport address or not but rather leaves this explicitly undefined.
   It has been suggested that alternative capabilities for a media
   stream could be indicated by including multiple media stream
   descriptions sharing the same transport address (i.e. using the same
   port number in the "m=" line and sharing the same IP-address).

     Such practice was not defined in [RFC2327], however it was
     suggested in an Internet-Draft version of [SDP].  Following
     discussion of the potential problems it introduced, it was removed.

   The main advantages of this approach would be:








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   o  May not require any additional extensions to SDP - only additional
      semantics.

      [Editor's note: It is somewhat unclear how it would work without
      extensions if we allow for alternative attributes and media format
      parameters and the offerer needs to always know which ones were
      accepted]

   The disadvantages of this approach would be:

   o  Backwards Compatibility Issues:  Since sharing of transport
      addresses between multiple streams was never specified as part of
      SDP, backwards compatibility is likely to be an issue.  Some
      implementations may support it whereas others may not. The lack of
      an explicit signaling indication to indicate the desired operation
      may lead to ungraceful failure scenarios.  Offer/answer semantics
      would be unclear here as well.

   o  Some Combinatoric Explosion:  The mechanism is not ideal to
      indicate alternative capabilities for multiple parameters or media
      formats within a particular media stream. For example, alternative
      attribute values and media format parameters for several codecs
      would lead to combinatoric explosion.

   o  Message Size:  Each alternative requires full duplication of all
      the relevant media stream parameters.

      [Editor's note: In practice, it is not clear this is a huge issue
      though.]

3.6. Opportunistic Encryption Using a Session Attribute

   This approach was suggested to address the specific scenario of
   negotiating either RTP or SRTP. The endpoints signal their desire to
   do SRTP by listing RTP (RTP/AVP) as the transport protocol in the
   "m=" line in the offer together with an attribute ("a=") that
   indicates whether SRTP is supported or not. If the answerer supports
   SRTP and wants to use it, the answer then includes SRTP (RTP/SAVP) as
   the transport protocol in the "m=" line.

   The main advantages of this approach are:

   o  Compatible with non-SRTP-aware endpoints.

   The disadvantages of this approach are:




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   o  Does not allow the offerer to indicate alternatives other than
      SRTP (including vanilla RTP as an alternative to SRTP).

   o  Addresses only a small subset of the requirements provided above.

3.7. Best-Effort Secure Real-Time Transport Protocol

   This approach is documented in [BESRTP]. The approach is similar to
   the one described above, except it does not actually include any
   explicit signaling indication as to the transport protocols
   supported. Instead, support for the Secure RTP profile [SRTP] is
   inferred based on the presence of the crypto attribute defined in
   [SDES] and/or the key-mgmt attribute defined in [KMGMT]. The draft
   also proposes the use of separate payload types for codecs being used
   under different profiles as a way to enable the offerer to process
   early media packets (especially non-secure ones) prior to receiving
   an answer (which includes the selected profiles and, in some cases,
   information about SRTP keying material).

   The main advantages of this approach are:

   o  Compatible with non-SRTP-aware endpoints.

   o  Provides a (separable) solution to disambiguating secure from non-
      secure RTP packets before receiving an answer.

   The disadvantages of this approach are:

   o  Defines new semantics above and beyond those defined by RFC 3264,
      RFC 4567, and RFC 4568 without any explicit signaling in the offer
      to that effect. This in turn may lead to unintended side-effects.

         Without explicit signaling indication, it is questionable to
         infer that presence of e.g. a crypto parameter in the offer
         indeed indicates that the offer wants to use the mechanism
         defined by the proposal.  Furthermore, Section 5.1.2 of [SDES]
         defines generic operation where presence of a crypto attribute
         without e.g. SRTP as the offered transport protocol could
         result in the media stream being rejected.

   o  Does not allow the offerer to indicate alternatives other than the
      inferred SRTP (including vanilla RTP as an alternative to SRTP).

   o  Addresses only a small subset of the requirements provided above.





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3.8. Opportunistic Encryption using Probing

   This is another approach suggested to address the specific scenario
   of negotiating either RTP or SRTP. In this case, the endpoints first
   establish an RTP session using RTP (RTP/AVP). The endpoints send
   probe messages, over the media path, to determine if the remote
   endpoint supports their profile (e.g. RTP/SAVP) and keying technique.

   The main advantages of this approach are:

   o  Compatible with non-SRTP-aware endpoints.

   The disadvantages of this approach are:

   o  Addresses only a small subset of the requirements provided above.

4. Security Considerations

   One of the motivations for SDP capability negotiation is to enable
   best-effort SRTP negotiation, i.e. an offer/answer exchange offering
   both a secure and a non-secure version of RTP. The answerer in turn
   will select one of these. Such a negotiation where the offerer is
   willing to accept either a secure or insecure RTP profile, and
   possibly with more or less strong security algorithms as a result of
   the negotiation opens up for a range of possible security attacks. It
   is important that any solution for SDP capability negotiation
   properly addresses such security risks and/or notes any security
   threats inherent in the proposed solution.

5. IANA Considerations

   There are no IANA considerations in this document.

6. Acknowledgments

   Thanks to the MMUSIC WG for comments on the requirements in this
   document. Also, Francois Audet, Paul Jones and Dan Wing provided
   specific comments on a precursor to this document.

7. Change Log

7.1. draft-ietf-mmusic-sdp-capability-negotiation-reqts-01.txt

   o  Moved REQ-10, REQ-30, and REQ-122 from core to enhancements





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   o  Elaborated on note around middle-boxes passing unknown attributes
      through them, and the potential implication of doing so with
      respect to this framework.

7.2. draft-ietf-mmusic-sdp-capability-negotiation-reqts-00.txt

   Version -00 is based on draft-andreasen-mmusic-sdp-capability-
   negotiation-reqts-00.txt with the following changes:

   o  Noted that REQ-50 may have interactions with ICE

   o  Clarified requirements REQ-80, REQ-130.

   o  Added new requirements REQ-85, REQ-121, REQ-122, REQ-301, and REQ-
      302.

   o  Reduced requirements REQ-150 and REQ-160 to "SHOULD" strength.

   o  Minor updates to Section 3.7. and 3.8.



7.3. draft-andreasen-mmusic-sdp-capability-negotiation-reqts-00.txt

   Version -00 is the initial version. The requirements provided in this
   initial version were taken from an earlier version of [SDPCapNeg]
   with additional requirements added (from REQ-150 and up).

   The ability to indicate capabilities as either mandatory or optional
   is no longer explicitly out of scope (in order to support modularity
   and extensibility per the newly added requirements), and neither is
   the ability to indicate constraints on combinations of
   configurations.
















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8. References

8.1. 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.

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

8.2. Informative References

   [RFC2046] Freed, N., and N. Borenstein, "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.

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





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   [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.

   [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.


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   [T38]     ITU-T Recommendation T.38, "Procedures for real-time Group
             3 facsimile communication over IP networks", September
             2005.

   [H245]    ITU-T Recommendation H.245, "Control protocol for
             multimedia communication", May 2006.



Author's Addresses

   Flemming Andreasen
   Cisco Systems
   Edison, NJ

   Email: fandreas@cisco.com


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   This document is subject to the rights, licenses and restrictions
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