SDP and CLUE message interactions
draft-hansen-clue-sdp-interaction-00

CLUE WG                                                        R. Hansen
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                       February 18, 2013
Expires: August 22, 2013

                   SDP and CLUE message interactions
                  draft-hansen-clue-sdp-interaction-00

Abstract

   This document attempts to help resolve some of the complexities of
   interaction between SDP and CLUE messages in call flows by providing
   some strategies and some suggested syntax.

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   This Internet-Draft will expire on August 22, 2013.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Initial Assumptions . . . . . . . . . . . . . . . . . . . . . . 3
   4.  Interdependence of SDP and CLUE negotiation . . . . . . . . . . 4
   5.  The CLUE framework: dividing the information between SDP
       and CLUE messaging  . . . . . . . . . . . . . . . . . . . . . . 5
     5.1.  CLUE information principally in CLUE channel  . . . . . . . 5
     5.2.  Media encoding/decoding information in SDP, media
           content information in CLUE messaging . . . . . . . . . . . 6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     7.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 7

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1.  Introduction

   One issue that has repeatedly come up in the development of CLUE is
   the interconnected nature of many of the issues - making decisions in
   any one area requires that decisions are made in other areas.  One
   particularly problematic area has been that of producing call flows:
   many of the decisions that need to be made revolve around how offer/
   answer exchanges and CLUE messages will interact, but without a good
   understanding of what will be in SDP and what will be in CLUE these
   decisions have been difficult to make.

   In the hope of resolving some of these issues and allowing us to make
   more progress on the subject of call flows and CLUE signalling
   generally this draft addresses two issues that are hopefully not
   dependant on decisions in other areas, both aspects of the
   relationship between CLUE signalling and SDP.  Hopefully this draft
   will either provoke discussion, or document decisions that people
   feel have already been made but aren't reflected in writing.

2.  Terminology

   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 RFC 2119 [RFC2119] and
   indicate requirement levels for compliant implementations.

3.  Initial Assumptions

   This section enumerates a few assumptions based on previous
   discussion which are, at this stage, hopefully uncontroversial.

   CLUE information such as capture descriptions are unsuitable for SDP,
   and as such there will be a method for sending CLUE messages back and
   forth.  In a call scenario where both sides wish to send a receive
   this CLUE negotiation takes the form of two independent, uni-
   directional exchanges; on each exchange one device provides its send
   capabilities while the other side determines what it wishes to
   receive.

   This CLUE negotiation will never enable or require a call to exceed
   boundaries negotiated in SDP.  This most obviously applies to
   bandwidth, both for the total call and for negotiated sessions, but
   also means that codec-specific limitations such as the maximum number
   of H.264 macroblocks a second a receiver can process MUST be
   respected.

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4.  Interdependence of SDP and CLUE negotiation

   With separate negotiation of SDP and CLUE there is the question of
   how to deal with dependencies between these two channels.  Even
   without defining specific syntax for either channel or deciding which
   information fits where it is clear that there will be dependencies
   between the two negotiations.  These may be direct dependencies (eg,
   captures in a CLUE channel referencing the label of a specific m-line
   in SDP) or may be related information (eg, SDP provides a bandwidth
   limit for an m-line while the CLUE channel places per stream limits).
   There has been debate within the working group about how changes in
   one negotiation should affect the other, and such a decision will
   have significant effect on the design of call flows.

   This draft proposes that CLUE messages and SDP messages should be
   independent: parameters in CLUE messages MAY exceed values negotiated
   in SDP, or may make reference to SDP contents not present in the most
   recent offer/answer exchange.  Without this provision, SDP and CLUE
   messages become part of a single negotiation, and a change on either
   by either side may necessitate an exchange of the other message type.
   For instance, removing stream information from SDP might first
   necessitate sending a new CLUE message removing the references to
   this stream.  The state machine required to ensure validity of
   negotiation will be complicated, and it will be very possible to end
   up in an invalid state.  Finally, even if both ends of a call obey
   the constraints to ensure validity, a middle box may choose to
   rewrite an SDP such that the CLUE negotiation is no longer valid.

   Making the two message types independent significantly reduces the
   complexity of the state machines required.  And with the message
   flows independent there is no way for an invalid state to occur when
   the two negotiations contain contradictory information.  A cost of
   this is that endpoints will now need to deal with the fact that CLUE
   messages may contain parameters exceeding those negotiated in SDP, or
   referencing SDP content that does not exist.  However, this is
   analogous to an issue endpoints already deal with in SDP.  For
   instance, the sum of bandwidth parameters for various m-lines can
   exceed the overall session bandwidth.  Not only is this not invalid,
   but it can be desirable, as it allows the sender to prioritise
   streams.  What can be sent for any device is simply the intersection
   of what is permitted by the most recent SDP offer/answer, and the
   outcome of the CLUE negotiation; implementations should ignore
   references to entities in the other negotiation that do no exist.

   This does not mean that there will be no interaction between SDP and
   CLUE messaging - a device wishing to add a new stream may well need
   to update both their SDP and their CLUE negotiations.  However, there
   is no fixed order in which this must be done and no requirement for

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   them to be updated in a particular order or fashion; it is left to
   the implementation to renegotiate the channels as it sees fit.  If
   updates to both negotiations are required for a new stream to be
   added, then the new stream will not be available until both
   renegotiations are complete - the completion of the first
   renegotiation will have no effect.

5.  The CLUE framework: dividing the information between SDP and CLUE
    messaging

   The CLUE Framework [I-D.ietf-clue-framework] defines the information
   that will be needed to successfully negotiate a CLUE call, but does
   not define the mechanism by which this information is conveyed.  This
   section provides two options for dividing this information between
   SDP and CLUE signalling, without proposing explicit signalling for
   either channel (merely what information needs to be conveyed in
   each).

5.1.  CLUE information principally in CLUE channel

   One approach that has been a major part of CLUE discussions has been
   to make no significant additions to SDP, and continue to use it only
   for the negotiation of RTP sessions.  The sessions are then
   potentially subdivided into multiple streams using CLUE signalling.
   In this model standard SDP signalling provides the envelope within
   which CLUE negotiates the number and content of multiple streams.

   This method has a number of advantages - there is no need for
   additional SDP syntax, making interoperability with existing devices
   simple and concentrating new signalling in a single location (the
   CLUE negotiation).  There is also clear separation of
   responsibilities between SDP and CLUE: as normal SDP negotiates the
   specifics of the RTP sessions: address and ports, supported codecs,
   receive maxima and so on, while CLUE messaging then specifies how
   many streams are to be multiplexed on a port, details for
   demultiplexing, content of those streams, encoding limits and so on.
   The only necessary addition to the SDP would be a label [RFC4574]
   attribute per media line to allow CLUE messaging to identify them.

   Unfortunately, there are some downsides to this approach.  The
   primary one is that all multiplexing of streams is entirely dependant
   on the CLUE channel - as such this is not a method applicable to
   other applications.  Since other groups within the IETF have an
   interest in such multiplexing for reasons other than enabling
   telepresence scenarios they would have to invent other methods for
   negotiating similar multiplexing - both inefficient, and likely
   problematic when CLUE and some other solution involving

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   multistreaming are both used in the call scenario.

5.2.  Media encoding/decoding information in SDP, media content
      information in CLUE messaging

   An alternative approach is to divide the information in the CLUE
   Framework [I-D.ietf-clue-framework] CLUE framework into the
   information specific to encoding and decoding RTP streams, and the
   content of those streams.  On the advertising side this split is
   fairly natural: most of the information in the framework relates to
   the number, content, physical dimensions and simultaneity of the
   media captures available, information related to the contents of the
   media streams rather than the streams themselves.  In contrast, the
   encoder and encoder group information gives the limits on the media
   streams the sender can provide, with parameters such as bandwidth,
   max h.264 macroblocks per second and other parameters relevant to
   SDP.  These are defined as sender limitations rather than receiver
   ones and so are not directly analogous to existing SDP parameters,
   but are better suited to SDP than CLUE.

   When it comes to receiver selection the separation between parameters
   that logically should be in CLUE and should be in SDP is no longer so
   clean-cut, as the receiver must specify capture encodings, choosing
   both which captures they wish to receive and the media limitations on
   such streams.  The latter limitations are obviously suited to SDP,
   but information about captures is more relevant to the CLUE channel.
   The CLUE-specific information, however, is limited to simply
   selecting a capture for the stream.  While it might be simple to have
   that as a new parameter in SDP, this would make SDP messages
   dependant on CLUE.  If instead there was still to be a CLUE configure
   message that referenced a particular SDP stream via some identifier
   then all dependencies would be from CLUE to SDP; the SDP itself would
   be free of any references to information in the CLUE negotiation.

   The ability to describe the sender's encoder limitations for
   multiplexed streams along with the receiver's selection of those
   streams and the media limitations, SSRCs and other demultiplexing
   information are all requirements that are not specific to CLUE;
   having them in SDP means that a consistent mechanism can be used by
   CLUE as well as by other call scenarios wishing to support additional
   media streams in this fashion.  Capture information, in contrast, is
   CLUE-specific and as such is sensible to keep in the CLUE channel.
   The CLUE channel will also reference the SDP, linking captures to
   encoding capabilities and identifying which capture is desired for
   each stream.

   This split of information means that any change in capture
   information on the part of a sender does not necessitate an offer/

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   answer exchange of SDP if there is no corresponding change to the
   encoding capabilities of that sender - only a new CLUE advertisement
   is required.  There are no references in SDP to anything in the CLUE
   messaging, allowing middle boxes wishing to monitor or alter SDP to
   do so without necessarily needing to involve themselves in the CLUE
   channel - all dependencies of referencing are from the CLUE messaging
   to SDP.

   One remaining question with this approach is how senders should cope
   with completing an SDP negotiation for streams when they have not
   received the CLUE configure message specifying the content of those
   streams; the sender could be mandated to delay sending of those
   stream until it receives the CLUE configuration, or it could be left
   to specific implementations to decide.  In the latter case receivers
   that wanted to avoid receiving information they had not specified
   would have to ensure they sent their CLUE configure message first.

6.  Security Considerations

   This draft only addresses how best to split information between SDP
   and CLUE signalling and the interdependencies between these two
   methods of signalling, it does not define the signalling or
   information itself.  As such this draft should require no additional
   security considerations.

7.  References

7.1.  Normative References

   [I-D.ietf-clue-framework]
              Duckworth, M., Pepperell, A., and S. Wenger, "Framework
              for Telepresence Multi-Streams",
              draft-ietf-clue-framework-08 (work in progress),
              December 2012.

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

7.2.  Informative References

   [RFC4574]  Levin, O. and G. Camarillo, "The Session Description
              Protocol (SDP) Label Attribute", RFC 4574, August 2006.

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Author's Address

   Robert Hansen
   Cisco Systems
   San Jose, CA  95134
   USA

   Email: rohanse2@cisco.com

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