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INTERNET-DRAFT                                         J. Ott/C. Perkins
Expires: November 2003                                           TZI/ISI
                                                             15 May 2003

                            SDPng Transition

Status of this memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 10 of RFC2026.

   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-

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

   The list of current Internet-Drafts can be accessed at

   To view the list Internet-Draft Shadow Directories, see

   Distribution of this document is unlimited.

   This document is a product of the Multiparty Multimedia Session
   Control (MMUSIC) working group of the Internet Engineering Task
   Force.  Comments are solicited and should be addressed to the working
   group's mailing list at mmusic@ietf.org and/or the authors.


   The Session Description Protocol (SDP) is today widely used in the
   Internet to announce as well as negotiate multimedia sessions and
   exchange capabilities.  Having originally been designed for session
   announcements only, as opposed to announcements and capabilities
   negotiation announcements, native SDP lacks numerous features to be
   applicable in many session scenarios.  Numerous extensions have been
   developed to circumvent SDP's shortcomings -- but they have also
   repeatedly shown its inherent limitations.  A successor protocol --
   termed "SDPng" for the time being -- is developed to address the
   aforementioned needs of Internet applications in a more structured
   manner.  With the huge installed base of SDP-based applications, a
   migration path needs to be developed to move from SDP to SDPng over
   time.  This document outlines how this migration can be achieved: in
   general as well as for the various IETF control protocols that

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   potentially make use of SDP and SDPng.

1.  Introduction

   SDP is now widely used within the Internet community to describe
   media sessions and, in a limited fashion, system capabilities
   relating to (multi)media sessions, for a variety of application
   scenarios: session announcements, interactive session setup,
   capability assessment and remote control of media streams.  All but
   the first of these are rather different from what SDP was originally
   designed for -- but all of them share the idea of setting up and
   configuring media streams.  Over time, its wide range of uses has
   revealed numerous shortcomings -- most of which stem from the fact
   that SDP has been used for lack of a better alternative and its
   semantics have been re-interpreted to make it fit the respective
   scenarios' needs.  In many cases, workrounds (typically called
   "extensions") for those shortcomings could be found which are often
   rather cumbersome.  While this practice has extended SDP's lifetime
   and provided at least a suitable basis for numerous applications, in
   parallel, a successor protocol -- currently referred to as "SDPng"
   -- has been developed.

        It is worthwhile noting that the aforementioned applications'
        needs are sufficiently similar for a single description protocol
        to take care of them if it was designed for this purpose from
        the beginning.  As a lesson learned from SDP, any further
        expansion in scope should be avoided where no clear fit can be
        seen -- and specific (different) solutions should be developed

   The design of SDPng takes into account the requirements arising from
   the above application scenarios and puts particular emphasis on
   protocol extensibility and modularization of extensions, at the same
   time keeping the core description format simple.  SDPng uses a
   different (more expressive) syntax than SDP does and hence is not
   backward compatible at the syntax level.  Nevertheless, the concepts
   of SDPng take into account the migration issues from SDP to SDPng by
   providing straightforward mappings between the two formats where
   possible and try to maximize compatibility from a semantics

   The current revisions of SDP and SDPng are documented in

   o    draft-ietf-mmusic-sdp-new-12.txt [1] and

   o    draft-ietf-mmusic-sdpng-06.txt [9].

   For SDP, a number of additional features are defined in the following
   documents that need to be taken into account:

   o    the offer/answer scheme of interpreting and matching media
        session descriptions to negotiate media sessions to be used in

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        call or conference in a single round-trip (RFC 3264 [6]);

   o    full support for IPv6 as network layer protocol (RFC 3266 [2]);

   o    SDP extensions to allow selecting ATM virtual circuits as
        additional network protocol and specifying ATM-specific
        parameters (RFC 3108 [3]);

   o    a general extension to deal with connection-oriented transport
        protocols such as TCP [12];

   o    an extension to support SCTP as media transport protocol in
        addition to UDP and TCP [17];

   o    an SDP extension to explicitly specify RTCP port number to
        enable the necessary expressiveness for NAT traversal [8];

   o    a mechanism (media identification, "mid") for naming and
        grouping ("group") SDP media lines according to one or more
        criteria, e.g. for the purpose of lip-synchronization or for
        identifying media sessions carrying the same content (RFC 3388
        [4], [10]);

   o    the capability to indicate which media sessions shall be mapped
        into the same resource reservation context [13];

   o    an extension to allow expression of simultanous capabilities
        across media sessions and formats (RFC 3407 [5])

   o    attributes for passing parameters of a keying protocol (such as
        MIKEY) as part of a session description [11];

   o    support for conveying cryptographic parameters as part of a
        session description [15];

   o    a mechanism to explicitly specify the sources allowed to provide
        input to media sessions [16]; and

   o    a simple language to provide instructions to media mixers on
        which incoming media streams shall be combined to produce which
        outgoing ones (and possbily how they shall be combined) [14].

   This document outlines a migration path from SDP to SDPng, starting
   from a short overview of the current application scenarios.  In the
   next step, we highlight which design decisions taken for SDPng should
   simplify a smooth migration and describe how mappings between the two
   description formats can be performed at an abstract level.  We then
   address procedural issues for integrating SDP and SDPng into the
   various protocols relying on those media description formats.
   Finally, we summarize work items on the agenda for SDPng.

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2.  Application Scenarios

   The following session control protocols that make use of SDP have
   been standardized in the IETF so far:

   1.   SDP was originally developed to announce (Mbone-based)
        multimedia sessions via session directories using the Session
        Announcement Protocol (SAP) -- but other mechanisms for
        disseminating the session descriptions (such as HTTP, SMTP,
        NNTP, etc.) are conceivable as well.

        The major property of this application scenario is that the
        creator of the session description defines a (set of) fixed
        choice(s) for all media types in a conference and the conference
        partipants have no way to influence these.  If they support at
        least one of the codecs for a particular media type they can
        participate in this media session, otherwise they cannot.  There
        is no interaction between sender(s) and receiver(s) to negotiate
        the media stream codecs and parameters.

        This scenario is referred to as "announcement".

   2.   Another use of SDP is in conjunction with the Real-Time
        Streaming Protocol (RTSP).  In RTSP, SDP is used to convey
        descriptions of a media stream interactively requested to be
        played from a server (or recorded by a server).  SDP itself is
        not used for capability negotiation, not even for the addresses
        to be used; those are negotiated within RTSP and may override
        the addresses specified as part of SDP.

        This scenario is referred to as "retrieval".

   3.   With SIP, SDP is used to propose media stream configurations and
        choose out of these (i.e. enable a subset of these).  By
        proposing and accepting media stream configurations, endpoints
        use SDP to implicitly describe their capabilities and carry out
        a negotiation procedure on the media streams to use.

        In the context of SIP, specific meanings (including required
        extensions) have been defined for use of SDP with unicast
        addresses, for connection-oriented transports, and for certain
        media level attributes (such as the direction attribute send-
        only, receive-only, and inactive).

        Numerous extensions have been proposed to extend SDP to better
        suit SIP's needs.  Besides a description of the offer/answer
        model, these extensions particularly include the ability to
        describe simultaneous capabilites and to group media stream

        This scenario is referred to as "offer/answer".

   4.   SDP is used to convey the capability descriptions of a MEGACO

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        media gateway (MG) to its media gateway controller (MGC) as well
        as for the MGC to instruct the MG where to send media streams to
        and from where to receive media streams, including codec and
        parameter choice.

        For this purpose, SDP has been modified/extended to some degree
        to fit the MEGACO needs.

        This scenario is referred to as "gateway control".

   It should noted that the original SDP concept already provided an
   extension mechanism to cover other network types than IPv4 and IPv6;
   however, specific extensions have only been defined recently for ATM
   and are now under discussion for TDM.  Extensions to other transport
   (including radio interfaces or next generation wireless networks) as
   well as to new types of session descriptions (e.g. electronic program
   guides) are conceivable.

3.  Mapping SDP to SDPng

   On a transition path from SDP to SDPng, allowing for a somewhat
   straightforward mapping of (parts of) one description format onto the
   other is of crucial importance.  SDPng has been designed in way that
   allows many of the session description features of SDP to be easily
   mapped onto the SDPng format and vice versa -- except that SDPng is
   more expressive than SDP and hence information loss is not unlikely
   to occur when doing the reverse mapping.  The final mapping rules
   between SDP and SDPng to be drawn up shall ensure that when mapping
   SDP to SDPng and then back to SDP will produce an SDP that is
   functionally identical to the one originally fed into the mapping
   process.  Note that the use of a number of SDP extensions (FID,
   SIMCAP) may be implied in this mapping process, depending on the use
   of SDP.  The mapping rules will ensure that no information loss will
   occur when translating from SDP to SDPng.

   The SDPng design uses a structure of four sections: definitions,
   potential or actual configurations, constraints and session
   attributes.  Of these, the "Configurations" and "Session
   Attributes" sections map well onto the current SDP. The
   "Definitions" and "Constraints" sections provide additional
   structure which is not directly expressible in SDP.

   o    At the media description level, the Potential and Actual
        Configurations specified in the "Configurations" section maps
        well to media descriptions ("m=", possibly "c=", and
        associated attributes ("a=") lines).

   o    At the session description level, the SDP session parameters are
        largely reflected in the "Session Attributes" section of
        SDPng.  The attributes proven suitable for session announcements
        have been used as the basis when defining SDPng.

   In SDPng, media descriptions are explicitly tagged with identifiers

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   and thus are easily referenced for semantically grouping media
   streams (e.g. to describe alternative audio in different languages,
   media streams to be synchronized, or media streams to carry the same
   information simultaneously but with different encodings) -- as has
   been defined for SDP in a limited fashion by the "fid" attribute
   set.  SDPng allows even to more formally describe the syntax of
   individual or compound media streams in the "Session Attributes"
   section.  Furthermore, SDPng supports a superset of additional
   constraints that may be realized by the "simcap" extensions for SDP
   in the "Constraints" section.

   Additional address families such as ATM or TDM bearers, next
   generation wireless network bearers, DVB channels, etc. can be
   incorporated into SDPng by defining the appropriate extensions for
   the SDPng transports.

   Similarly, new codecs can be added by just defining new codec
   specifications or defining entire new classes of applications to be
   described as new content types ("codec") to be carried in a media
   session (including e.g. text, fax, slide presentations, shared
   editors, etc).  If necessary, sophisticated parameter structures can
   be supported (even though the authors believe that simplicity is key
   to interoperability here).  This is similar to, but more structured
   than, the definition of new codec attributes in MIME registrations
   for SDP.

        It is expected that the MIME namespace for codecs will be mapped
        into the SDPng namespace, and a consistent mapping from SDP
        "a=fmtp:" attributes to SDPng codec parameters will be

   By means of its conceptual differentiation into Potential and Actual
   Configurations, SDPng supports both indicating a system's
   capabilities (without specifying transport addresses) separately from
   the instantiation of a particular media stream as well as conveying
   capability descriptions and instantiation proposals at the same time
   -- thereby providing a good fit for all the above session control
   scenarios: the "announcement" and "retrieval" scenarios will just
   use rather fixed Actual Configurations.  The "offer/answer" model
   will use use Actual Configuration but use them to negotiate media
   strams in a two-way handshake but may in addition use Potential
   Configurations to indicate capabilities that shall not be used
   immediately.  The "gateway control" scenario will use both:
   Potential Configurations to describe an MG's capabilities and Actual
   Configurations for setting up media sessions at MGs as well as
   retrieving information about currently active media sessions.  This
   differentiation is not directly expressible in SDP, although various
   extensions can be used to overload SDP semantics to achieve at least
   part of this effect.

   Finally, while the short-term SDPng specification aims at supporting
   only the widespread offer/answer model for capability negotiation, a
   future extension will also allow for content-independent negotiation

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   of session parameters by defining collapsing/intersection rules.  In
   particular, SDPng will take the need for multicast-based distributed
   calculation of joint capabilities into account for those rules (but
   note that it is *not* intended as a generic format for describing
   conference state information).  Such functionality is not covered by
   current SDP -- but there is also no perceived urgent demand so that
   this sophisticated functional component of SDPng is left to a future
   protocol extension.  The base SDPng protocol will provide the
   necessary flexibility, however.

4.  Integration with Session Control Protocols

   This section outlines for each of the session control protocols
   described above how SDP and SDPng can be used in parallel and
   indicates how a suitable transition could be achieved.

4.1.  Session Announcement Protocol (SAP)

   There are two revisions of SAP specified, version 0 which is
   implemented in a number of experimental tools, and version 1 which is
   defined in RFC 2972.

   SAPv0:SAPv0 does not support a mechanism to identify the content type
        of a session announcement but implicitly assumes SDP.  Proper
        parsers will note that the contents of the SAPv0 message does
        not begin with a "v=" line and hence will ignore the entire
        announcement.  SDPng contents MAY be identified by a different
        character sequence in the beginning of the announcement body,
        but this is not recommended. Instead, SAPv1 should be used,
        since it contains an explicit payload identifier.

   SAPv1:In SAPv1, an explicit payload type field (containing a MIME
        type) is available and should be used to differentiate between
        SDP anf SDPng contents.  Two approaches are conceivable: Either
        multipart MIME message is used with two parts containing the
        same session descriptions -- one expressing it in SDP and the
        other in SDPng.  Alternatively, two alternate session
        announcements may be used (being properly distinguished by the
        SDP "o=" field and the SDPng equivalent).

        It is recommended that implementations recognize the MIME
        multipart/alternative type in SAPv1 announcements, allowing for
        a simple transition to SDPng.

   It should be noted that current session directory implementations
   only support SDP.  Nevertheless, using the SAP Message Identifier
   Hash and the source address, they should be able to perform session
   deletions and modifications properly -- even without understanding
   the format contained in the SAP message body.

   For the introduction of SDPng, session announcements should be made
   "bi-lingual", i.e. in SDP and SDPng.  If a SAP announcer for some
   reason knows that all its potential audience will support SDPng, the

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   SDP announcement should be omitted.

   It should be noted that, for IPv4-based multicast sessions, session
   directories still may rely on parsing the session specifications to
   avoid clashes in the multicast address space.  Introducing a new
   session description language will prevent older implementations from
   continuing this practice successfully -- assuming that only SDPng
   announcements are used and/or that old implementations do not support
   MIME multipart/alternative message bodies.  This use of SAP is
   deprecated, however.

4.2.  Real-Time Streaming Protocol (RTSP)

   RTSP uses SDP to provide presentation descriptions (with a
   presentation comprising one or more media sessions), typically
   communicated from the server to the client (for playing) and in the
   opposite direction for recording.  The presentation description may
   also include initialization data for the various media streams and
   URLs to be used for controlling the entire presentation as well as
   the individual media sessions.  Transport parameters -- such as IP
   addresses, port numbers, etc. -- are conveyed as part of RTSP header

   RTSP uses the Content-Type: header field to indicate the format of
   the enclosed entity.  This provides a straightforward means for
   distinguishing SDP and SDPng-based presentation descriptions.  In
   addition, the Accept: header should be used by the client to indicate
   which content types it supports.  If the client specifies both SDP
   and SDPng as acceptable, the server should provide only the SDPng-
   based presentation description.

   If the client does not indicate a particular Content-Type: the server
   can, theoretically, use MIME multipart bodies (e.g.
   "multipart/alternative") to convey both description types
   simultaneously.  However, it is generally not expected that today's
   RTSP clients and servers will be able handle multipart bodies.
   Hence, if no hint is provided by the client by means of the Accept:
   header, the server must provide only an SDP description.

   In general, it would be preferrable to have the servers migrate to
   always supporting both description formats, thus enabling the clients
   to choose.  The servers should indicate SDPng support by means of
   suitable header fields whenever possible.

   Finally, RTSP makes special provision to interwork with firewalls by
   including the crucial transport parameters in a separate RTSP header
   field _in_addition_ to the presentation description.  This practice,
   in principle, allows to change the presentation description format
   without having to worry about the operation of firewalls and similar

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4.3.  Session Initiation Protocol (SIP)

   The use of SDP with SIP follows the offer/anser model and is
   described in [6].  It is key to the (efficiency of the) offer/answer
   model that a complete capability exchange and media stream
   instantiation be carried out in one round-trip -- which is supported
   by SDP.  While SDPng allows to separate capability exchange from
   media sesssion instantiation, those two pieces are also easily
   integrated in a single step.

   SIP also uses a Content-Type: header to indicate the nature of data
   carried in its message body; and SIP explicitly calls for supporting
   MIME multipart message bodies.  While, again, the use of MIME
   multipart/alternative would in principle be possible (from a
   theoretical perspective), issues regarding the actual implementation
   of multipart/alternative in SIP entities have been raised.  As
   backward compatibility has to be achieved, a different approach is

   A SIP UAC MAY use an SDPng message body in a SIP INVITE (or other)
   message.  If the SIP UAS does not support SDPng, it will return a
   "415 Unsupported Media Type" response to the UAC and indicate
   acceptable content types in the Accept: header (probably including
   "application/sdp").  The SIP UAC must then retry INVITE (or other)
   message using the indicated session description language.  The SIP
   UAC should cache knowledge about which peers did not understand SDPng
   as session description formats for a limited amount of time (e.g.
   several days) so that extra round-trips for session setup are only
   incurred infrequently.  Whenever a peer has sent an SDPng description
   (or it is known from other means that the peer supports SDPng), this
   information should also be cached.

   The SIP Accept: header can be exploited to determine the capability
   of a peer to understand SDPng in addition (or instead) plain SDP.
   Methods such as OPTIONS MAY be used to determine a peer's support for
   SDPng.  However, a peer's capabilities may not be known when the
   first message is sent which may introduce an extra round-trip if
   including SDP and SDPng in the initial INVITE message is not an
   option.  Further approaches to make a UA's support for SDPng known
   ahead of time should be explored.

   A number of SDP extensions have been motivated by SIP-based
   applications and these need to be accommodated in SDPng as well.
   Features such as "simcap" and "FID" are inherently supported by
   SDPng; proper definitions for connection-oriented media need to be
   fully understood and then incorporated.  Key management attributes as
   defined in [11] need to be included (not just for SIP) and so may
   need to be general mechanisms to signal security capabilities [11]
   [15] and indicate their optional or mandatory use.  The same applies
   to quality of service parameters [13] (which are largely also
   motivated by SIP but are also useful with control protocols).

   Numerous extensions to SDP have been developed for the purpose of

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   supporting certain SIP requirements -- actually most of those listed
   in section 1. fall into this category.  The following paragraphs
   address how those are handled by and mapped to SDPng.

   IPv6 is natively supported by SDPng.  For other network protocols --
   such as ATM and TDM, which have only come up in the context of
   MEGACO, see below -- similar SDPng packages need to be defined that
   provide the same information as the corresponding SDP extensions.

   Support for connection-oriented media in general will be supported in
   SDPng using a similar parameterization.  Support for SCTP will be
   equivalent to the approach taken for SDP as the parameters are

   SDP's explicit RTCP port number parameter (that helps with NAT
   traversal) is inherently available in the RTP transport specification
   of SDPng.

   Media session identification is also provided by the SDPng spec by
   means of naming attributes in the potential as well as actual
   configurations.  The "Session Attributes" section of SDPng is meant
   to provide meta-information about the media sessions such as grouping
   of lip synchronization, indicating streams semantics, etc.  This
   section is also the place to express media "mixing" attributes as
   discussed in [14].  QoS parameterizations for SDPng are developed
   separately as package enhancements and are still under discussion.

   Simultaneous capabilities are dealt with by the Constraints section
   of SDPng where restrictions across several components as well as
   within a single component can be expressed.

   Security parameters have not yet been developed for the SDPng
   specification.  The intention is to define a separate security
   package (similar to codec and transport definitions).  Security
   parameters may be provided in the definition section for later
   reference from within the component specification or may be specified
   inline in a component.

   Indicating permissible sources for unicast and particularly multicast
   media sessions is already covered in the basic SDPng transport

   In summary, most of the newly developed SDP attributes and their
   usages have either been considered in the SDPng base specification
   and the transport packagaes or will be added additional attributes or
   as separate packages.

   Note that the above discussions are not just applicable to SIP but
   may be used in a broader scope (e.g. with RTSP or MEGACO).

4.4.  Media Gateway Control Protocol (MEGACOP)

   The MEGACO specification already supports two different encodings for

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   capability and media stream descriptions: a text-based variant based
   upon (a modified) SDP and a binary representation of the same
   information set.  MGCs are required to implement both encodings while
   MGs have the choice to pick either or both.  Differentiation between
   the protocol encoding variants is done using different port numbers:
   2944 for the text-based and 2945 for the binary encoding.

   Unfortunately, within the text-based encoding, there is no means to
   differentiate several description formats.  SDP messages are carried
   as an "octet string" without any type identifier.  Defining a third
   port number for this further differentiation does not seem to be
   appropriate, particularly since the message encoding is still a text

   The remaining means for distinction is that an SDP specification
   would start with a "v=0" line while an SDPng document would begin
   with a different character sequence.

        Note that MEGACOP also supports a binary encoding for SDP
        messages; current practice seems to favor the text encoding for
        SDP and hence we will not address a binary encoding for SDPng.

   Within the context of MEGACO, various extensions to SDP have been
   defined, addressing its use for capability description and also
   defining support for further network types (presently, ATM and TDM).
   Capability descriptions are inherently supported by SDPng.  To add
   support for further networks, the respective parameters need to be
   defined as network-specific SDNng packages.

5.  SDPng and Middleboxes

   Middleboxes (e.g. firewalls, NATs, NAPTs) are of significant
   importance for many deployment scenarios for SDP-based signaling
   protocols.  The SDP description typically carries addressing
   paramaters for media sessions which may be invalidated by
   middleboxes: by firewalls because they block packet destined at the
   respective addresses and by NA(P)Ts because they change the addresses
   that must actually be used.

   A number of approaches have been devised to deal with currently
   deployed and, eventually, future middleboxes: 1) co-locating a proxy
   for the respective signaling protocol(s) with a middlebox, 2) using
   extra protocols to determine the presence and the mode of operation
   of a NA(P)T (which does not work for firewalls), and 3) the
   definition of a control protocol for middleboxes.  While approach 1)
   is entirely up to the manufacturers of middleboxes, 2) and 3) are
   subject to IETF standardization in the MIDCOM WG.

   1)   Proxies that are incorporated in middleboxes to parse and (in
        case of NATs) possibly alter the contents of session
        descriptions exchanged in the signaling path will need to
        implement SDPng in the future.  For a (potentially long) interim
        period, both SDP and SDPng need to be supported by such devices.

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   2)   If entities involved in the respective signaling path use
        protocols such as STUN to determine the presence of a NAT and
        its mode of operation, it is up to these entities to include the
        correct addressing information in the SDP or SDPng session
        descriptions.  NATs continue to operate as before and do not
        require any changes because of a migration from SDP to SDPng.

   3)   If local signaling servers or other entities use a MIDCOM
        protocol to configure a firewall or NAT to allow certain media
        streams to pass through, again, no changes need to be made to
        MIDCOM-enabled firewalls.  The migration from SDP to SDPng is
        transparent to them; only the involved signaling component need
        to support -- but they would need to do so anyway.

6.  Directing the Evolution of SDP

   With the transition from SDP to SDPng, there is the question of the
   evolution of SDP, and legacy systems which use it.

   The SDP specification [1] is stable, and mostly corrects errors in
   the original specification, with the addition of very few new
   features.  The revision is expected to be published as a proposed
   standard RFC shortly, obsoleting RFC 2327.

   A number of extensions to SDP for use in offer/answer scenarios are
   also available. These include grouping [4] and capability negotiation
   [5], which have recently been approved as proposed standard RFCs,
   bringing minimal capability/alternative descriptions to SDP.

   Related is the SDP offer/answer model for SDP, published as RFC3264
   [6].  This defines the model used to complete the steps of a
   negotiation using SDP.  A similar mode of operation will be provided
   for SDPng for baseline operation with SIP (and possibly RTSP).

   All these are subsumed into SDPng, so there should be no further need
   for development in these areas; applications with requirements that
   are not met by these specifications should use SDPng.

   There have recently been proposals to add quality of service
   negotiation for SDP and, similarly, we expect other extensions to be
   proposed over time.  Due to the well-known limitations of SDP, we do
   not believe it appropriate to continue development of more elaborate
   extensions: for negotiation, for QoS, for security, and for other
   general-purpose or application-specific needs.

   The exceptions to this rule are clearly the addition of security
   features to SDP (which are required for many current SDP deployment
   scenarios) as well as minor extensions for media session attributes
   (e.g. indicating the use of joint vs. separate resource reservations
   as documented in [7]).

   Overall, new work should be done in the framework of SDPng where
   applications and their requirements for (new) expressiveness in end-

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   to-end exchanges to negotiate and configure media sessions will
   hopefully act as a driver for that process.

7.  IANA Considerations

   The transition from SDP to SDPng will require IANA to define new
   parameter registries, which will be created and populated as SDPng
   evolves. This memo does not, in itself, require any action by IANA.

8.  Security Considerations

   Since SDPng performs largely the same role as SDP+extensions, it is
   not expected that there will be significant new security
   considerations as a result of the transition. The security
   considerations section of [9] provides further details.

   During the transition process it is likely that dual descriptions
   will be common. There is a potential for inconsistancy between
   definitions, which may have unintended consequences if one part of
   the system, for example a middlebox, interprets the SDP format whilst
   another interprets the SDPng format definition.

9.  Author's Addresses

   Joerg Ott <jo@tzi.org>
   Universitaet Bremen
   MZH 5180
   Bibliothekstr. 1
   D-28359 Bremen

   Colin Perkins <csp@csperkins.org>
   USC Information Sciences Institute
   3811 North Fairfax Drive, Suite 200
   Arlington, VA 22203

10.  Normative References

   [1]  Mark Handley, Van Jacobson, Colin Perkins, "SDP: Session
        Description Protocol," Internet Draft draft-ietf-mmusic-sdp-
        new-11.txt, Work in Progress, November 2002.

   [2]  Sean Olson, Gonzalo Camarillo, Adam Roach, "Support for IPv6 in
        Session Description Protocol (SDP)", RFC 3266, June 2002.

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   [3]  3108 R. Kumar and M. Mostafa, "Conventions for the use of the
        Session Description Protocol (SDP) for ATM Bearer Connections,"
        RFC 3108, May 2001.

   [4]  Gonzalo Camarillo, Jan Holler, Goran AP Eriksson, Henning
        Schulzrinne, "Grouping of media lines in SDP," RFC 3388,
        December 2002.

   [5]  Flemming Andreasen, "SDP Simple Capability Declaration," RFC
        3407, October 2002.

   [6]  Jonathan Rosenberg and Henning Schulzrinne, "An Offer/Answer
        Model with SDP," RFC 3264, June 2002.

   [7]  G. Camarillo and A. Monrad, "Mapping of Media Streams to
        Resource Reservation Flows", RFC 3524, April 2003.

   [8]  Christian Huitema, "RTCP attribute in SDP," Internet Draft
        draft-ietf-mmusic-sdp4nat-03.txt, Work in Progress, September

10.1.  Informative References

   [9]  Dirk Kutscher, Joerg Ott, Carsten Bormann, "Session Description
        and Capability Negotiation", Internet Draft draft-ietf-mmusic-
        sdpng-06.txt, Work in Progress, March 2003.

   [10] Gonzalo Camarillo and Jonathan Rosenberg, "The Alternative
        Semantics for the Session Description Protocol Grouping
        Framework," Internet Draft draft-camarillo-mmusic-alt-00.txt,
        Work in Progress, February 2003.

   [11] Jari Arkko, Elisabetta Carrarra, Fredrik Lindholm, Mats Naslund,
        and Karl Norrman, "Key Management Extensions for SDP and
        RTSP," Internet Draft draft-ietf-mmusic-kmgmt-ext-06.txt, Work
        in Progress, February 2003.

   [12] David Yon, "Connection-Oriented Media Transport in SDP,"
        Internet Draft draft-ietf-mmusic-sdp-comedia-05.txt, Work in
        Progress, March 2003.

   [13] G. Camarillo (ed), W. Marshall (ed), J. Rosenberg, "Integration
        of Resource Management and SIP", Internet Draft draft-ietf-sip-
        manyfolks-resource-03.txt, Work in Progress, November 2001.

   [14] G. Camarillo, H. Schulzrinne, and E. Burger, "The source and
        sink attributes for the Session Description Protocol", Internet
        Draft draft-camarillo-mmusic-source-sink-00.txt, Work in
        Progress, September 2002.

   [15] Mark Baugher, "SDP Security Descriptions for Media Streams",
        Internet Draft draft-ietf-mmusic-sdescriptions-00.txt, Work in

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        Progress, February 2003.

   [16] B. Quinn and R. Finlayson, "SDP Source-Filters", Internet
        Draft draft-ietf-mmusic-sdp-srcfilter-04.txt, Work in Progress,
        April 2003.

   [17] Robert Fairlie-Cuninghame, "Guidelines for specifying SCTP-
        based media transport using SDP," Internet Draft draft-fairlie-
        mmusic-sdp-sctp-00.txt, Work in Progress, May 2001.

11.  Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implmentation may be prepared, copied, published and
   distributed, in whole or in part, without restriction of any kind,
   provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an

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