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Versions: 00 draft-ietf-mmusic-sdp-capability-negotiation-reqts

     MMUSIC Working Group                                       F. Andreasen
     Internet Draft                                            Cisco Systems
     Expires: June 2007                                    December 10, 2006
     
     
     
                             SDP Capability Negotiation:
                      Requirements and Review of Existing Work
     
     
           draft-andreasen-mmusic-sdp-capability-negotiation-reqts-00.txt
     
     
     Status of this Memo
     
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        This Internet-Draft will expire on June 10, 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
     
     
     
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        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...................................................2
        2. Requirements...................................................5
        3. Review of Existing Work........................................8
           3.1. Grouping of Media Lines...................................8
           3.2. Session Description Protocol (SDP) Simple Capability
           Declaration...................................................10
           3.3. Session Description and Capability Negotiation (SDPng)...10
           3.4. Multipart/alternative....................................12
           3.5. Sharing Ports Between "m=" Lines.........................13
           3.6. Opportunistic Encryption Using a Session Attribute.......14
           3.7. Best-Effort Secure Real-Time Transport Protocol..........15
           3.8. Opportunistic Encryption using Probing...................15
        4. Security Considerations.......................................16
        5. IANA Considerations...........................................16
        6. Acknowledgments...............................................16
        7. Change Log....................................................16
        8. References....................................................17
           8.1. Normative References.....................................17
           8.2. Informative References...................................17
        Author's Addresses...............................................19
        Intellectual Property Statement..................................19
        Disclaimer of Validity...........................................20
        Copyright Statement..............................................20
        Acknowledgment...................................................20
     
     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
     
     
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        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 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
        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 -
     
     
     
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        potential configurations are not supported. The answer however may
        reduce the set of actual configurations from the offer.
     
        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.
     
        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 are 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
     
     
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        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.
     
           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.
     
     
     
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        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. Ideally,
        the mechanism should be completely transparent to entities that do
        not support it, without the need for any further signaling.
     
        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
           configuration (as opposed to the actual configuration) until the
           answer has been received.
     
     
     
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        REQ-130: The mechanism MUST work in the presence of 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 MUST 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 MUST 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.
     
        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-310: The following requirements are considered enhancements as
        defined in REQ-300:
     
          * REQ-50 (alternative combinations of transport protocol and media
          type)
     
          * REQ-150 (valid combinations of media lines)
     
     
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          * REQ-160 (valid combinations of media formats between media
          streams)
     
     
     
          [Editor's Note: Should we have any security requirements - see e.g.
          Section 4.
     
     
     
        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 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:
     
     
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        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:
     
        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.]
     
     
     
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     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
        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
     
     
     
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        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.
     
        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:
     
     
     
     
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        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:
     
        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.jenning-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].
     
     
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        Use of multipart/alternative to solve the SDP capability negotiation
        problems however has several shortcomings:
     
        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 main advantages of this approach are:
     
        o  Compatible with non-SRTP-aware endpoints.
     
        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.
     
     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 keying technique.
     
        The main advantages of this approach are:
     
     
     
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        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
        possible 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.
     
          [Editor's note: This almost sounds like we should have some
          specific requirements around all of this].
     
     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
     
        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.
     
        [RFC2234] Crocker, D. and Overell, P.(Editors), "Augmented BNF for
                  Syntax Specifications: ABNF", RFC 2234, Internet Mail
                  Consortium and Demon Internet Ltd., November 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.
     
     
     
     
     
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        [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.
     
        [SDPCapNeg] Andreasen, F. "SDP Capability Negotiation", work in
                  progress, December 2006.
     
     
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        [MIKEY]   J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K.
                  Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
                  August 2004.
     
        [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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