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Versions: 02 03 04 05 06 07 08 draft-ietf-clue-signaling

Network Working Group                                         P. Kyzivat
Internet-Draft                                                   L. Xiao
Intended status: Standards Track                               C. Groves
Expires: January 16, 2014                                         Huawei
                                                               R. Hansen
                                                           Cisco Systems
                                                           July 15, 2013


                             CLUE Signaling
                    draft-kyzivat-clue-signaling-04

Abstract

   This document specifies how signaling is conducted in the course of
   CLUE sessions.  This includes how SIP/SDP signaling is applied to
   CLUE sessions as well as defining a CLUE-specific signaling protocol
   that complements SIP/SDP and supports negotiation of CLUE application
   level data.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on January 16, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must



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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  CLUE-Specific Signaling Protocol  . . . . . . . . . . . . . .   3
     3.1.  Protocol Versioning, Options & Extensions . . . . . . . .   4
       3.1.1.  Versioning  . . . . . . . . . . . . . . . . . . . . .   4
       3.1.2.  Options and/or Extensions . . . . . . . . . . . . . .   4
       3.1.3.  Negotiation . . . . . . . . . . . . . . . . . . . . .   4
       3.1.4.  Principles  . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Acknowledging Messages  . . . . . . . . . . . . . . . . .   5
       3.2.1.  Explicit Acknowledgment of Each Message . . . . . . .   5
       3.2.2.  Piggybacking ACK on Requests  . . . . . . . . . . . .   6
       3.2.3.  Reporting Message Errors  . . . . . . . . . . . . . .   7
     3.3.  Stand-alone messages or deltas? . . . . . . . . . . . . .   9
     3.4.  Message Sequencing  . . . . . . . . . . . . . . . . . . .   9
       3.4.1.  Signaling Changes in Provider State . . . . . . . . .  10
       3.4.2.  Signaling Changes in Consumer State . . . . . . . . .  11
     3.5.  Message Transport . . . . . . . . . . . . . . . . . . . .  11
       3.5.1.  CLUE Channel Lifetime . . . . . . . . . . . . . . . .  11
       3.5.2.  Channel Error Handling  . . . . . . . . . . . . . . .  12
     3.6.  CLUE Messages . . . . . . . . . . . . . . . . . . . . . .  12
       3.6.1.  ADVERTISEMENT Message . . . . . . . . . . . . . . . .  12
       3.6.2.  CONFIGURE Message . . . . . . . . . . . . . . . . . .  13
       3.6.3.  ACK Message . . . . . . . . . . . . . . . . . . . . .  13
       3.6.4.  NAK Message . . . . . . . . . . . . . . . . . . . . .  13
     3.7.  Message Syntax  . . . . . . . . . . . . . . . . . . . . .  13
     3.8.  Message Framing . . . . . . . . . . . . . . . . . . . . .  16
     3.9.  other . . . . . . . . . . . . . . . . . . . . . . . . . .  16
   4.  CLUE use of SDP O/A . . . . . . . . . . . . . . . . . . . . .  16
     4.1.  Establishing the CLUE channel . . . . . . . . . . . . . .  16
     4.2.  Representing CLUE Encodings in SDP  . . . . . . . . . . .  16
     4.3.  Representing CLUE Encoding Groups in SDP  . . . . . . . .  18
     4.4.  Signaling CLUE control of "m" lines . . . . . . . . . . .  18
     4.5.  Ensuring interoperability with non-CLUE devices . . . . .  19
   5.  Interaction of CLUE and SDP negotiations  . . . . . . . . . .  19
     5.1.  Independence of SDP and CLUE negotiation  . . . . . . . .  19
     5.2.  Recommendations for operating with non-atomic operations   19
     5.3.  Constraints on sending media  . . . . . . . . . . . . . .  20
   6.  Example: A call between two CLUE-capable endpoints  . . . . .  20
   7.  Example: A call between a CLUE and non-CLUE-capable endpoint   27
   8.  CLUE requirements on SDP O/A  . . . . . . . . . . . . . . . .  29
   9.  SIP Signaling . . . . . . . . . . . . . . . . . . . . . . . .  29
   10. Interoperation with Legacy SIP Devices  . . . . . . . . . . .  29



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   11. CLUE over RTCWEB  . . . . . . . . . . . . . . . . . . . . . .  29
   12. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . .  30
   13. What else?  . . . . . . . . . . . . . . . . . . . . . . . . .  30
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  30
   15. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  30
   16. Security Considerations . . . . . . . . . . . . . . . . . . .  30
   17. Change History  . . . . . . . . . . . . . . . . . . . . . . .  30
   18. References  . . . . . . . . . . . . . . . . . . . . . . . . .  31
     18.1.  Normative References . . . . . . . . . . . . . . . . . .  31
     18.2.  Informative References . . . . . . . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

1.  Introduction

   This document specifies how signaling is conducted in the course of
   CLUE sessions.  This includes how SIP/SDP signaling is applied to
   CLUE sessions as well as defining a CLUE-specific signaling protocol
   that complements SIP/SDP and supports negotiation of CLUE application
   level data.

   [Yes, this is a dup of the abstract for now.  Eventually it should
   say more.]

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 [RFC2119].

   This document draws liberally from the terminology defined in the
   CLUE Framework [I-D.ietf-clue-framework].

   Other terms introduced here:

   CLUE Channel:  A reliable, bidirectional, transport mechanism used to
      convey CLUE messages.  A CLUE channel consists of one SCTP stream
      in each direction over a DTLS/SCTP session.

3.  CLUE-Specific Signaling Protocol

   The CLUE Framework [I-D.ietf-clue-framework] mentions a CLUE-specific
   protocol for the exchange of ADVERTISEMENT and CONFIGURE messages,
   but gives little detail.  The Data Model
   [I-D.presta-clue-data-model-schema] specifies a model and XML
   representation for CLUE-related data, but doesn't currently specify
   exactly what data belongs in each message, or how messages are
   sequenced.  This document provides the detail missing from those
   documents.



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3.1.  Protocol Versioning, Options & Extensions

3.1.1.  Versioning

   There must be some provision for identifying incompatible protocol
   versions.

   NOTE: We probably don't want to have incompatible versions.
   Typically changes will be introduced in a backward compatible way.
   But a time may come when this isn't possible, and we should be
   prepared for that.  This is more likely to occur before an RFC is
   published.  While it is probably unwise to deploy a product based on
   a draft, there will certainly be prototypes developed for testing,
   and those tests may lead to a need for incompatible change.  So
   whatever the mechanism is, it should be applicable to changes that
   occur from draft to draft, as well as after an RFC has been
   published.

3.1.2.  Options and/or Extensions

   There must be some provision for dealing with optional-to-implement
   features in the specification, and/or for backward compatible
   extensions to the protocol.  These are superficially different, but
   in practice they are more-or-less equivalent.  To an implementation
   of the base protocol and some extensions, those extensions must be
   viewed as optional-to-implement features in peers.

   One decision is whether extensions may be implemented mix-and-match,
   or whether there is a sequence of extensions, and one extension may
   only be supported if all the prior extensions have been supported.

3.1.3.  Negotiation

   Both version and options can be negotiated.  Some mechanisms may work
   for both, while others are only appropriate for one or the other.
   Some possibilities:

   o  No negotiation at all.  Instead, unrecognized syntax in certain
      "extension points" is to be ignored.  If it is recognized, then a
      corresponding extension specification defines what to do.

   o  Negotiate via the SIP signaling.

   o  Negotiate as part of the O/A exchange that establishes the
      channel.  (E.g. it is likely that individual channels of the SCTP
      association will be specified in SDP with a specific sub-protocol
      type.  There could be a separate sub-protocol for each new
      version.)



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   o  Negotiate within the CLUE channel, via a special message exchange,
      before exchanging "normal" CLUE messages.

   o  Declare versioning in every CLUE message.  Define errors for
      unsupported versions and fallback to earlier versions.

3.1.4.  Principles

   o  CLUE SHOULD allow forwards and backwards compatibility through a
      version and extension mechanism.  Forward compatibility allows a
      version of a protocol to communicate effectively with and
      interwork with future versions of the protocol.  A version should
      not restrict the future protocol from providing extra
      capabilities.

   o  Whenever possible backwards compatibility should be maintained.
      Backward compatibility rules will be defined to ensure that
      endpoints implementing future versions of CLUE will be able to
      send protocol messages of the previous versions which will be
      understood and fully processed by the remote endpoint.

   o  Existing protocol elements should not be changed unless a protocol
      error needs to be corrected.

   o  The semantics of existing elements and values should not be
      changed.

   o  Established rules for formatting and encoding messages and
      elements should not be changed.

   o  When information elements are found to be obsolete they can be
      marked as not used.  However, the identifier for that information
      element will be marked as reserved.  In that way it cannot be used
      in future versions.

3.2.  Acknowledging Messages

   The CLUE channel is reliable, so there is no need for acknowledgement
   to guarantee delivery.  But there is still a need for application-to-
   application acknowledgement to report that the message has been
   received, parsed, and found to be of an acceptable format.  One
   possibility is to introduce separate ACK and NAK messages.  Another
   possibility is to add a confirmation element to each CLUE message, so
   that confirmation can be piggybacked on the basic messages.  Some
   alternatives follow.  [OTHER PROPOSALS WELCOME.]

3.2.1.  Explicit Acknowledgment of Each Message




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   The characteristics of this approach are:

   o  There are separate request and response messages.  (This is
      similar to SIP.)

   o  Every request message expects exactly one response message.

   o  Every request message carries a sequence number that identifies
      it.

   o  Each end of the connection assigns sequential sequence numbers to
      the requests it sends.

   o  Every response message carries the sequence number of the message
      to which it responds.

   o  Responses are to be sent promptly upon the receipt of a request.
      (Needs more detail.)

   o  Responses are either ACK or NAK.  NAK responses also carry info
      describing the error.

   o  Each CONFIGURE message is to be understood in the context of the
      most recent ACKed ADVERTISEMENT message.  A CONFIGURE message may
      be rejected if there is an outstanding ADVERTISEMENT for which no
      response has been received.  (Or it may be accepted if the
      advertiser is able to do so meaningfully.)

3.2.2.  Piggybacking ACK on Requests

   The characteristics of this approach are:

   o  Every message carries a sequence number that identifies it.

   o  Each end of the connection assigns sequential sequence numbers to
      the messages it sends.

   o  Every message carries the sequence number of the last message
      received and found valid.

   o  If a message is received and found invalid, then a NAK message is
      sent that refers to it and indicates what is wrong with it.

   o  If a valid message is received and a new message needs to be sent
      in response, then the responding message implicitly acknowledges
      the prior message.





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   o  If a valid message is received and there is no need to immediately
      send another message, then a NO-OP message is sent to acknowledge
      the received message.  But a NO-OP message is never sent in
      response to a NO-OP message.

   o  Each CONFIGURE message is to be understood in the context of the
      most recent *acknowledged* ADVERTISEMENT message.  A CONFIGURE
      message may be rejected if it doesn't acknowledge the most
      recently sent ADVERTISEMENT.  (Or it may be accepted if the
      advertiser is able to do so meaningfully.)

   The general format of every message is:

   o  sequence # of this message

   o  sequence # of most recently *received* and *valid* message

   o  message type (ADVERTISEMENT, CONFIG, NO-OP, NAK)

   o  body of the message, according to type

   (The exact representation is TBD - by XML experts.)

   There are loose ends to resolve here.  In particular, how to
   acknowledge messages after NAKing one.

3.2.3.  Reporting Message Errors

   There needs to be a mechanism to report errors with other messages.
   The details of form, content, and usage still need to be specified,
   and need to be tuned to the details of the protocol.  This could use
   distinct messages or be incorporated into the other messages.  Errors
   this message must be able to report include:

   Syntax error in message:  The message has been disregarded due to a
      syntax error detected at the message level.  The message does not
      conform to the productions of messages in [Protocol Document].
      Used when the message cannot be parsed.

   Sequencing Error:  Sequence number has already been used, or is
      greater than the expected number.  (Details of possible errors
      depend upon the specific sequence numbering mechanism.)

   Version not supported:  This indicates a lack of support for the
      protocol version indicated in the message header of the message.
      In the case of the version number being indicated in the message
      header, the message contents are disregarded.




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   Option not supported:  This indicates a lack of support for the
      protocol option the used in the message.  The message contents are
      disregarded.

   Unknown capture identity:  The received Configure message contains an
      unknown capture identity not previously declared by an
      Advertisement.  The message contents are disregarded.

   Invalid identity:  The received message contains an invalid capture
      identity.  For example a duplicated Capture scene identity or some
      other semantically incorrect usage.  The message contents are
      disregarded.

   Invalid value:  The received message contains an invalid parameter
      value.  The value is not according to the protocol definition in
      [protocol document] or according the extension documentation.

   Missing element:  The received message is missing an element.
      Certain parameters require multiple values, e.g. Point of capture
      requires X,Y,Z co-ordinates if one or more elements are missing
      this error code is used.

   Conflicting parameters or values:  The received message contains
      multiple values that may not be used together.

   Invalid capture area:  The received message defines a capture area
      that cannot be rendered in a sensible manner.  For example the
      capture area does not define a quadrilateral region.

   Invalid point of line of capture:  The indicated co-ordinate for the
      point on line of capture is invalid.  For example: does not lie
      between the point of capture and the area of capture or it is the
      same as the point of capture.

   Invalid capture scene entry:  The message contains an invalid capture
      scene entry.  For example the capture scene entry contains more
      than one media type.

   Invalid Simultaneous Set:  The simultaneous set contained in the
      message is invalid.  For example the simultaneous set refers to an
      undefined capture set or does not match the specified capture
      scene entries.

   Invalid Configuration:  The Configure message requests a
      configuration that the provider cannot support.






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   Invalid Advertisement reference:  The Configure message refers to an
      invalid Advertisement.  The message refers-to/depends-upon out-of-
      date ADVERTISEMENT message or provides an invalid reference.

3.3.  Stand-alone messages or deltas?

   Each message exchanged within a CLUE session could contain a complete
   description of the state it wishes to achieve.  Or each message could
   describe just the changes that it wishes to make to the current
   state.  Or the protocol could support both message forms.  Which
   direction to pursue is TBD.

   [Paul: while this does need to be decided, it is fundamentally just
   an optimization.  IMO it does not have major impact on the other
   parts of this document, so I would prefer to continue deferring it
   until we are so far along with the remainder of the document that we
   can no longer defer it.]

3.4.  Message Sequencing

   There is a very basic introduction to this topic in section 4
   (Overview) of the CLUE Framework [I-D.ietf-clue-framework].  After
   removing extraneous material it would look like:

              +-----------+                     +-----------+
              | Endpoint1 |                     | Endpoint2 |
              +----+------+                     +-----+-----+
                   |                                  |
                   | ADVERTISEMENT 1                  |
                   |*********************************>|
                   |                  ADVERTISEMENT 2 |
                   |<*********************************|
                   |                                  |
                   |                      CONFIGURE 1 |
                   |<*********************************|
                   | CONFIGURE 2                      |
                   |*********************************>|
                   |                                  |


   But we need much more than this, to show multiple CONFIGUREs per
   ADVERTISEMENT, interleaving of ADVERTISEMENTs and CONFIGUREs in both
   directions, etc.

   Message sequencing needs to be described at two levels:

   o  Basic sequencing of the CLUE messages themselves, without regard
      for the SIP/SDP signaling that may be going on at the same time.



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      This is useful to cover the basic concepts.  That should be
      covered in this section.  It provides context for understanding
      the more detailed treatment later.

      This could include some simple state machines.

   o  In reality there is a complex dependency between CLUE signaling
      and SDP Offer/Answer exchanges carried in SIP signaling.  So there
      is a need to describe the valid ways in which these two forms of
      signaling interact.  That is covered in Section 5.

3.4.1.  Signaling Changes in Provider State

   Once a CLUE session has been established, ADVERTISEMENTs and
   CONFIGUREs exchanged, and media is flowing, a provider may experience
   a change in state that has an effect on what it wishes or is able to
   provide.  In this case it may need to alter what it is sending and/or
   send a new ADVERTISEMENT.  In some cases it will be necessary to
   alter what is being sent without first sending a new ADVERTISEMENT
   and waiting for a CONFIGURE conforming to it.

   The following is a non-exhaustive list of situations and recommended
   actions:

   o  An advertised capture, that is not currently configured, is no
      longer available.

      To recover from this: Send a new ADVERTISEMENT that omits this
      capture.

   o  An advertised capture, that has been configured, is no longer
      available.

      To recover from this: (1) stop transmitting the configured
      encoding of this capture.  (2) Send a new ADVERTISEMENT that omits
      this capture.

   o  The provider loses some resource and must reduce the frame rate,
      frame size, or resolution of a capture encoding.

      If the reduced values still fall within the advertised values for
      the capture then the change may be made without any further
      signaling.

      If the change must be outside the range of what was advertised,
      then the provider must cease transmitting the capture encoding.
      It then must send a new ADVERTISEMENT reflecting what it is now
      capable of delivering.



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   o  New or changed scenes or scene geometry.  For instance, the
      addition of a new scene containing presentation captures.  Also,
      an MCU may make significant changes in what it advertises as new
      endpoints join a conference.

   o  [Add more]

3.4.2.  Signaling Changes in Consumer State

   If the Consumer for some reason looses the CLUE state information how
   does it ask for an Advertisement from the provider?  There could be
   multiple possibilities.  A error code approach?  However error codes
   would typically be associated with a NACK so it may not be good for a
   Config message.  Maybe send a message which means "send me a complete
   update".  An alternative may be to release the connection or just do
   new signaling to establish a new CLUE session.

3.5.  Message Transport

   CLUE messages are transported over a bidirectional CLUE channel.  In
   a two-party CLUE session, a CLUE channel connects the two endpoints.
   In a CLUE conference, each endpoint has a CLUE channel connecting it
   to an MCU.  (In conferences with cascaded mixers [RFC4353], two MCUs
   will be connected by a CLUE channel.)

3.5.1.  CLUE Channel Lifetime

   The transport mechanism used for CLUE messages is DTLS/SCTP as
   specified in [I-D.tuexen-tsvwg-sctp-dtls-encaps] and
   [I-D.ietf-mmusic-sctp-sdp].  A CLUE channel consists of one SCTP
   stream in each direction over a DTLS/SCTP session.  The mechanism for
   establishing the DTLS/SCTP session is described in Section 4.

   The CLUE channel will usually be offered during the initial SIP
   INVITE, and remain connected for the duration of the CLUE/SIP
   session.  However this need not be the case.  The CLUE channel may be
   established mid-session after desire and capability for CLUE have
   been determined, and the CLUE channel may be dropped mid-call if the
   desire and/or capability to support it is lost.

   There may be cases when it becomes necessary to "reset" the CLUE
   channel.  This by be as a result of an error on the underlying SCTP
   association, a need to change the endpoint address of the SCTP
   association, loss of CLUE protocol state, or something else TBD.

   The precise mechanisms used to determine when a reset is required,
   and how to accomplish it and return to a well defined state are TBS.




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3.5.2.  Channel Error Handling

   We will need to specify behavior in the face of transport errors that
   are so severe that they can't be managed via CLUE messaging within
   the CLUE channel.  Some errors of this sort are:

   o  Unable to establish the SCTP association after signaling it in
      SDP.

   o  CLUE channel setup rejected by peer.

   o  Error reported by transport while writing message to CLUE channel.

   o  Error reported by transport while reading message from CLUE
      channel.

   o  Timeout - overdue acknowledgement of a CLUE message.
      (Requirements for now soon a message must be responded to are
      TBD.)

   o  Application fault.  CLUE protocol state lost.

   The worst case is to drop the entire CLUE call.  Another possibility
   is to fall back to legacy compatibility mode.  Or perhaps a "reset"
   can be done on the protocol.  E.g. this might be accomplished by
   sending a new O/A and establishing a replacement SCTP association.
   Or a new CLUE channel might be established within the existing SCTP
   association.

3.6.  CLUE Messages

   CLUE messages are encoded in XML.  The Data Model
   [I-D.presta-clue-data-model-schema] defines many/most of the elements
   from which CLUE messages are composed.  This document specifies an
   XML schema that contains an element definition for each CLUE message,
   with much of the content of those elements being drawn from the Data
   Model.

3.6.1.  ADVERTISEMENT Message

   This message contains XML representations of captures, capture
   scenes, encoding groups, and simultaneous sets using the types
   defined for those in the Data Model
   [I-D.presta-clue-data-model-schema].

   The XML definition for this is element <advertisement> in section
   Section 3.7




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   [[ Currently this does not contain any representation of encodings.
   It assumes those will be defined in SDP. ]]

3.6.2.  CONFIGURE Message

   This message optionally contains an XML representations of
   captureEncodings using the type defined in the Data Model
   [I-D.presta-clue-data-model-schema].  A configure message with no
   captureEncodings indicates that no captures are requested.

   [[ It currently also contains a reference to the request number of
   the advertisement it is based upon.  Whether this should be present,
   or if it should implicitly reference the most recently acknowledged
   advertisement is TBD. ]]

   The XML definition for this is element <configure> in section
   Section 3.7

3.6.3.  ACK Message

   Need for, and details of, the ACK message are TBD.

   The XML element <response> in section Section 3.7 could serve as the
   representation, either with no reason element, or a reason element
   with a special value.

3.6.4.  NAK Message

   Need for, and details of, the NACK message are TBD.

   The XML element <response> in section Section 3.7 could serve to as
   the representation, with the reason element providing the details.
   Then the code value in the reason element should map to the errors in
   section Section 3.2.3.

3.7.  Message Syntax

   [[ The following is a first cut at a schema for the actual messages
   in the clue protocol.  It uses <encodingGroups> from the data model
   but not <encodings>.  Rather, it assumes that encodings are described
   in SDP as m-lines with a text identifier, and that the identifier has
   the same value as the encodingIDs embedded in the <encodingGroups>.
   If we stick with this the data model should be adjusted to agree, but
   until then it should "work".  The SDP encoding of the identifier is
   proposed to be 'a=label:ID', though 'a=mid:ID' is another candidate.
   ]]





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   For now there only <advertisement> and <configure> are defined.  More
   messages will be needed for acknowledgment.

   <?xml version="1.0" encoding="UTF-8" ?>
   <xs:schema
      targetNamespace="urn:ietf:params:xml:ns:clue-message"
      xmlns:tns="urn:ietf:params:xml:ns:clue-message"
      xmlns:xs="http://www.w3.org/2001/XMLSchema"
      xmlns:dm="urn:ietf:params:xml:ns:clue-info"
      xmlns="urn:ietf:params:xml:ns:clue-message"
      elementFormDefault="qualified"
      attributeFormDefault="unqualified">

   <!-- Import data model schema -->
      <xs:import namespace="urn:ietf:params:xml:ns:clue-info"
                 schemaLocation="clue-data-model-04-wip.xsd"/>

   <!-- ELEMENT DEFINITIONS -->
   <xs:element name="advertisement" type="advertisementMessageType"/>
   <xs:element name="configure" type="configureMessageType"/>
   <xs:element name="response" type="responseMessageType"/>

   <!-- CLUE MESSAGE TYPE -->
   <xs:complexType name="clueMessageType" abstract="true">
     <xs:sequence>
       <!-- mandatory fields -->
       <!-- TBS: version info -->
     </xs:sequence>
   </xs:complexType>

   <!-- CLUE REQUEST MESSAGE TYPE -->
   <xs:complexType name="clueRequestMessageType" abstract="true">
    <xs:complexContent>
     <xs:extension base="clueMessageType">
      <xs:sequence>
        <!-- mandatory fields -->
        <xs:element name="requestNumber" type="xs:integer"/>
      </xs:sequence>
     </xs:extension>
    </xs:complexContent>
   </xs:complexType>

   <!-- CLUE RESPONSE MESSAGE TYPE -->
   <xs:complexType name="clueResponseMessageType">
    <xs:complexContent>
     <xs:extension base="clueMessageType">
      <xs:sequence>
        <!-- mandatory fields -->



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        <xs:element name="requestNumber" type="xs:integer"/>
        <!-- optional fields -->
        <xs:element name="reason" type="reasonType" minOccurs="0"/>
        <xs:any namespace="##other"
            processContents="lax" minOccurs="0"/>
      </xs:sequence>
     </xs:extension>
    </xs:complexContent>
   </xs:complexType>

   <!-- CLUE ADVERTISEMENT MESSAGE TYPE -->
   <xs:complexType name="advertisementMessageType">
    <xs:complexContent>
     <xs:extension base="clueRequestMessageType">
      <xs:sequence>
        <!-- mandatory fields -->
        <xs:element name="mediaCaptures"
                    type="dm:mediaCapturesType"/>
        <xs:element name="encodingGroups"
                    type="dm:encodingGroupsType"/>
        <!-- The encodings are defined via identifiers in the SDP,
             referenced in encodingGroups -->
        <xs:element name="captureScenes"
                    type="dm:captureScenesType"/>
        <!-- optional fields -->
        <xs:element name="simultaneousSets"
                    type="dm:simultaneousSetsType" minOccurs="0"/>
        <xs:any namespace="##other"
                processContents="lax" minOccurs="0"/>
      </xs:sequence>
     </xs:extension>
    </xs:complexContent>
   </xs:complexType>

   <!-- CLUE CONFIGURE MESSAGE TYPE -->
   <xs:complexType name="configureMessageType">
    <xs:complexContent>
     <xs:extension base="clueRequestMessageType">
      <xs:sequence>
        <!-- mandatory fields -->
        <xs:element name="advertisementNumber" type="xs:integer"/>
        <!-- advertisementNumber is requestNumber
             of the advertisement-->
        <!-- optional fields -->
        <xs:element name="captureEncodings"
                    type="dm:captureEncodingsType" minOccurs="0"/>
        <xs:any namespace="##other"
                processContents="lax" minOccurs="0"/>



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      </xs:sequence>
     </xs:extension>
    </xs:complexContent>
   </xs:complexType>

   <!-- REASON TYPE -->
   <xs:complexType name="reasonType">
     <xs:simpleContent>
       <xs:extension base="xs:string">
         <xs:attribute type="xs:short" name="code" use="required"/>
       </xs:extension>
     </xs:simpleContent>
   </xs:complexType>

   </xs:schema>


3.8.  Message Framing

   Message framing is provided by the SCTP transport protocol.  Each
   CLUE message is carried in one SCTP message.

3.9.  other

4.  CLUE use of SDP O/A

4.1.  Establishing the CLUE channel

   The CLUE channel is usually offered in the first SIP O/A exchange
   between two parties in an intended CLUE session.  The offer of the
   CLUE channel is the indicator that this SIP session is proposing to
   establish a CLUE session.

   (However it is also acceptable to start with a non-CLUE SIP session
   and upgrade it to a CLUE session later.)

   The mechanism for negotiating a DTLS/SCTP connection is specified in
   [I-D.ietf-mmusic-sctp-sdp].  We need to specify how to select the
   specific pair of SCTP streams that comprise the CLUE channel.

   The presence of an active m-line for the CLUE channel in an SDP offer
   is an indication that the offer that the sender is CLUE-capable and
   hence can understand CLUE-specific syntax.

4.2.  Representing CLUE Encodings in SDP

   Many CLUE constructs have no good analog in SDP.  Entities such as
   'captures', which describe spatial and other properties of a capture



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   source such as a camera, are not tied directly to RTP streams, do not
   have negotiated properties and would prove a significant challenge to
   represent in SDP syntax (while also greatly increasing the size of
   the SDP).

   However, two entities defined in the CLUE Framework
   [I-D.ietf-clue-framework] are a much closer fit for SDP: Encodings
   and Encoding Groups.  Both describe RTP media properties and
   limitations, though unlike most SDP usage they describe the sender's
   capabilities, not the receiver's.  Representing encodings in CLUE
   splits media limitations across two protocols, and risks duplicated
   and potentially contradictory information being sent in CLUE and SDP.
   As such we are exploring representing this information in SDP, with
   the decision to convey them in the CLUE messages only to be made if
   the SDP approach proves impractical.

   This draft presents an attempt to describe CLUE encodings in SDP.  As
   a decision has not yet been reached on how multiplexed RTP streams
   are to be expressed in SDP, at this stage the draft does so without
   multiplexing, using existing SDP attributes, with a seperate "m" line
   and hence port per unidirectional RTP stream.  This is done with the
   understanding that when a decision is reached on new syntax for
   multiplexing RTP streams in SDP the CLUE SDP signaling will be
   modified to use it.  Further, the framework document states that the
   multiplexing of streams by an implementation is optional, and in the
   case of a disaggregated system, with media streams going to different
   addresses, may not be possible.

   With the current scheme of using existing syntax, an encoding is
   specified in SDP as a unicast "m" line, which MUST be marked as
   sendonly with the "a=sendonly" attribute or as inactive with the
   "a=inactive" attribute.  The encoder capabilities of the stream are
   defined here using existing syntax; for instance, for H.264 see Table
   6 in [RFC6184] for a list of valid parameters for representing
   encoder sender stream limits.

   Every "m" line representing a CLUE encoding SHOULD contain a "label"
   attribute as defined in [RFC4574].  This label is used to identify
   the encoding by the sender in CLUE Advertisement messages and by the
   receiver in CLUE Configure messages.

   A receiver who wishes to receive a CLUE stream via this encoding
   requires a matching "a=recvonly" "m" line.  As well as the normal
   restrictions defined in [RFC3264] media MUST NOT be sent on this
   stream until the sender has received a valid CLUE Configure message
   specifying the capture to be used for this stream.





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4.3.  Representing CLUE Encoding Groups in SDP

   As per the previous section, there would be advantages to conveying
   encoding group information in SDP.  However, with current SDP syntax
   there is no way to express the encoding group limits defined in the
   Data Model [I-D.presta-clue-data-model-schema].  As such the current
   draft keeps encoding groups as part of the Advertisement message for
   the time being.

4.4.  Signaling CLUE control of "m" lines

   In many cases an implementation may wish to mix media channels that
   are under CLUE control with those that are not.  It may want to
   ensure that there are non-CLUE streams for purposes of
   interoperability, or that can provide media from the start of the
   call before CLUE negotiation completes, or because the implementation
   wants CLUE-controlled video but traditional audio, or for any other
   reasons.

   Which "m" lines in an SDP body are under control of the CLUE channel
   is signalled via the SDP Grouping Framework [RFC5888].  Devices that
   wish to negotiate CLUE MUST support the grouping framework.

   A new semantic for the "group" session-level attribute, "CLUE", is
   used to signal which "m" lines are under the control of a CLUE
   channel.  As per the framework, all of the "m" lines of a session
   description that uses "group" MUST be identified with a "mid"
   attribute whether they are controlled by CLUE or not.  The "mid" id
   of any "m" lines controlled by a CLUE channel MUST be included in the
   "CLUE" group attribute alongside the "mid" id of the CLUE channel
   controlling them.

   The CLUE group MUST NOT include more than one "m" line for a CLUE
   channel.  If a CLUE channel is part of the CLUE group attribute other
   media "m" lines included in the group are under the control of that
   CLUE channel; media MUST NOT be sent or received on these "m" lines
   until the CLUE channel has been negotiated and negotiation has taken
   place as defined in this document.  If no CLUE channel is part of the
   CLUE group attribute then media MUST NOT be sent or received on these
   "m" lines.

   "m" lines not specified as under CLUE control follow normal rules for
   media streams negotiated in SDP as defined in documents such as
   [RFC3264].

   An SDP MAY include more than one group attribute with the "CLUE"
   semantic.  An "mid" id for a given "m" line MUST NOT be included in
   more than one CLUE group.



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4.5.  Ensuring interoperability with non-CLUE devices

   A CLUE-capable device sending an initial SDP offer SHOULD include an
   "m" line for the CLUE channel, but SHOULD NOT include any other CLUE-
   controlled "m" lines.  Once each side of the call is aware that the
   other side is CLUE-capable a new O/A exchange MAY be used to add
   CLUE-controlled "m" lines.

5.  Interaction of CLUE and SDP negotiations

   Information about media streams in CLUE is split between two message
   types: SDP, which defines media addresses and limits, and the CLUE
   channel, which defines properties of capture devices available, scene
   information and additional constraints.  As a result certain
   operations, such as advertising support for a new transmissible
   capture with associated stream, cannot be performed atomically, as
   they require changes to both SDP and CLUE messaging.

   This section defines how the negotiation of the two protocols
   interact, provides some recommendations on dealing with intermediary
   stages in non-atomic operations, and mandates additional constraints
   on when CLUE-configured media can be sent.

5.1.  Independence of SDP and CLUE negotiation

   To avoid complicated state machines with the potential to reach
   invalid states if messages were to be lost, or be rewritten en-route
   by middle boxes, the current proposal is that SDP and CLUE messages
   are independent.  The state of the CLUE channel does not restrict
   when an implementation may send a new SDP offer or answer, and
   likewise the implementation's ability to send a new CLUE
   Advertisement or Configure message is not restricted by the results
   of or the state of the most recent SDP negotiation.

   The primary implication of this is that a device may receive an SDP
   with a CLUE encoding it does not yet have capture information for, or
   receive a CLUE Configure message specifying a capture encoding for
   which the far end has not negotiated a media stream in SDP.

   CLUE messages contain an EncodingID which is used to identify a
   specific encoding in SDP.  The non-atomic nature of CLUE negotiation
   means that a sender may wish to send a new Advertisement before the
   corresponding SDP message.  As such the sender of the CLUE message
   MAY include an EncodingID which does not currently match an extant id
   in SDP.

5.2.  Recommendations for operating with non-atomic operations




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   Generally, implementations that receive messages for which they have
   incomplete information SHOULD wait until they have the corresponding
   information they lack before sending messages to make changes related
   to that information.  For instance, an implementation that receives a
   new SDP offer with three new "a=sendonly" CLUE "m" lines that has not
   received the corresponding CLUE Advertisement providing the capture
   information for those streams SHOULD NOT include corresponding
   "a=recvonly" lines in its answer, but instead should make a new SDP
   offer when and if a new Advertisement arrives with captures relevant
   to those encodings.

   Because of the constraints of offer/answer and because new SDP
   negotiations are generally more 'costly' than sending a new CLUE
   message, implementations needing to make changes to both channels
   SHOULD prioritize sending the updated CLUE message over sending the
   new SDP message.  The aim is for the recipient to receive the CLUE
   changes before the SDP changes, allowing the recipient to send their
   SDP answers without incomplete information, reducing the number of
   new SDP offers required.

5.3.  Constraints on sending media

   While SDP and CLUE message states do not impose constraints on each
   other, both impose constraints on the sending of media - media MUST
   NOT be sent unless it has been negotiated in both CLUE and SDP: an
   implementation MUST NOT send a specific CLUE capture encoding unless
   its most recent SDP exchange contains an active media channel for
   that encoding AND the far end has sent a CLUE Configure message
   specifying a valid capture for that encoding.

6.  Example: A call between two CLUE-capable endpoints

   This example illustrates a call between two CLUE-capable endpoints.
   Alice, initiating the call, is a system with three cameras and three
   screens.  Bob, receiving the call, is a system with two cameras and
   two screens.  A call-flow diagram is presented, followed by an
   summary of each message.

   To manage the size of this section only video is considered, and SDP
   snippets only illustrate video 'm' lines.  ACKs are not discussed.



                 +----------+                      +-----------+
                 |  Alice   |                      |    Bob    |
                 |          |                      |           |
                 +----+-----+                      +-----+-----+
                      |                                  |



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                      |                                  |
                      | INVITE 1 (BASIC SDP+COMEDIA)     |
                      |--------------------------------->|
                      |                                  |
                      |                                  |
                      |     200 OK 2 (BASIC SDP+COMEDIA) |
                      |<---------------------------------|
                      |                                  |
                      |                                  |
                      | ACK 1                            |
                      |--------------------------------->|
                      |                                  |
                      |                                  |
                      |                                  |
                      |<########### MEDIA 1 ############>|
                      |   1 video A->B, 1 video B->A     |
                      |<################################>|
                      |                                  |
                      |                                  |
                      |                                  |
                      |<================================>|
                      |   CLUE CTRL CHANNEL ESTABLISHED  |
                      |<================================>|
                      |                                  |
                      |                                  |
                      | ADVERTISEMENT 1                  |
                      |*********************************>|
                      |                                  |
                      |                                  |
                      |                  ADVERTISEMENT 2 |
                      |<*********************************|
                      |                                  |
                      |                                  |
                      | INVITE 2 (+3 sendonly)           |
                      |--------------------------------->|
                      |                                  |
                      |                                  |
                      |                      CONFIGURE 1 |
                      |<*********************************|
                      |                                  |
                      |                                  |
                      |           200 OK 2 (+2 recvonly) |
                      |<---------------------------------|
                      |                                  |
                      |                                  |
                      | ACK 2                            |
                      |--------------------------------->|
                      |                                  |



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                      |                                  |
                      |                                  |
                      |<########### MEDIA 2 ############>|
                      |   2 video A->B, 1 video B->A     |
                      |<################################>|
                      |                                  |
                      |                                  |
                      |           INVITE 3 (+2 sendonly) |
                      |<---------------------------------|
                      |                                  |
                      |                                  |
                      | CONFIGURE 3                      |
                      |*********************************>|
                      |                                  |
                      |                                  |
                      | 200 OK 3 (+2 recvonly)           |
                      |--------------------------------->|
                      |                                  |
                      |                                  |
                      |                                  |
                      |                            ACK 3 |
                      |<---------------------------------|
                      |                                  |
                      |                                  |
                      |                                  |
                      |<########### MEDIA 3 ############>|
                      |   2 video A->B, 2 video B->A     |
                      |<################################>|
                      |                                  |
                      |                                  |
                      |                                  |
                      v                                  v



   In INVITE 1, Alice sends Bob a SIP INVITE including in the SDP body
   the basilar audio and video capabilities ("BASIC SDP") and the
   information needed for opening a control channel to be used for CLUE
   protocol messages exchange, according to what is envisioned in the
   COMEDIA approach ("COMEDIA") for DTLS/SCTP channel
   [I-D.ietf-mmusic-sctp-sdp].  A snippet of the SDP showing the
   grouping attribute and the video m-line are shown below (mid 3
   represents the CLUE channel):


      ...
      a=group:CLUE 3
      ...



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      m=video 6002 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
      a=sendrecv
      a=mid:2



   Bob responds with a similar SDP (200 OK 1); due to their similiarity
   no SDP snippet is shown here.  Alice and Bob are each able to send a
   single audio and video stream (whether they choose to send this
   initial media before CLUE has been negotiated is implementation-
   dependent).  This is illustrated as MEDIA 1.

   With the successful initial O/A Alice and Bob are also free to
   negotiate the CLUE channel.  Once this is successfully established
   CLUE negotiation can begin.  This is illustrated as CLUE CHANNEL
   ESTABLISHED.

   Alice now sends her CLUE Advertisement (ADVERTISEMENT 1).  She
   advertises three static captures representing her three cameras.  She
   also includes switched captures suitable for two- and one-screen
   systems.  All of these captures are in a single capture scene, with
   suitable capture scene entries to tell Bob that he should either
   subscribe to the three static captures, the two switched capture view
   or the one switched capture view.  Alice has no simultaneity
   constraints, so includes all six captures in one simultaneous set.
   Finally, Alice includes an encoding group with three encoding IDs:
   "enc1", "enc2" and "enc3".  These encoding ids aren't currently
   valid, but will match the next SDP offer she sends.

   Bob received ADVERTISEMENT 1 but does not yet send a Configure
   message, because he has not yet received Alice's encoding
   information, so as yet he does not know if she will have sufficient
   resources to send him the two streams he ideally wants at a quality
   he is happy with.

   Bob also sends his CLUE Advertisement (ADVERTISEMENT 2).  He
   advertises two static captures representing his cameras.  He also
   includes a single composed capture for single-screen systems, in
   which he will composite the two camera views into a single video
   stream.  All three captures are in a single capture scene, with
   suitable capture scene entries to tell Alice that she should either
   subscribe to the two static captures, or the single composed capture.
   Bob also has no simultaneity constraints, so includes all three
   captures in one simultaneous set.  Bob also includes a single
   encoding group with two encoding IDs: "foo" and "bar".




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   Similarly, Alices receives ADVERTISEMENT 2 but does not yet send a
   Configure message, because she has not yet received Bob's encoding
   information.

   Alice now sends INVITE 2.  She maintains the sendrecv audio, video
   and CLUE m-lines, and she adds three new sendonly m-lines to
   represents the maximum three encodings she can send.  Each of these
   m-lines has a label corresponding to one of the encoding ids from
   ADVERTISEMENT 1.  Each also has its mid added to the grouping
   attribute to show they are controlled by the CLUE channel.  A snippet
   of the SDP showing the grouping attribute and the video m-lines are
   shown below (mid 3 represents the CLUE channel):


      ...
      a=group:CLUE 3 4 5 6
      ...
      m=video 6002 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
      a=sendrecv
      a=mid:2
      ...
      m=video 6004 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016
      a=sendonly
      a=mid:4
      a=label:enc1
      m=video 6006 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016
      a=sendonly
      a=mid:5
      a=label:enc2
      m=video 6008 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016
      a=sendonly
      a=mid:6
      a=label:enc3



   Bob now has all the information he needs to decide which streams to
   configure.  As such he now sends CONFIGURE 1.  This requests the pair
   of switched captures that represent Alice's scene, and he configures
   them with encoder ids "enc1" and "enc2".



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   Alice receives Bob's message CONFIGURE 1 but does not yet send the
   capture encodings specified, because at this stage Bob hasn't
   negotiated the ability to receive these streams in SDP.

   Bob now sends his SDP answer as part of 200 OK 2.  Alongside his
   original audio, video and CLUE m-lines he includes two active
   recvonly m-lines and a zeroed m-line for the third.  He adds their
   mid values to the grouping attribute to show they are controlled by
   the CLUE channel.  A snippet of the SDP showing the grouping
   attribute and the video m-lines are shown below (mid 100 represents
   the CLUE channel):


      ...
      a=group:CLUE 11 12 100
      ...
      m=video 58722 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
      a=sendrecv
      a=mid:10
      ...
      m=video 58724 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
      a=recvonly
      a=mid:11
      m=video 58726 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
      a=recvonly
      a=mid:12
      m=video 0 RTP/AVP 96



   On receiving 200 OK 2 from Bob Alice is now able to send the two
   streams of video Bob requested - this is illustrated as MEDIA 2.

   The constraints of offer/answer meant that Bob could not include his
   encoder information as new m-lines in 200 OK 2.  As such Bob now
   sends INVITE 3 to generate a new offer.  Along with all the streams
   from 200 OK 2 Bob also includes two new sendonly streams.  Each
   stream has a label corresponding to the encoding ids in his
   ADVERTISEMENT 2 message.  He also adds their mid values to the
   grouping attribute to show they are controlled by the CLUE channel.
   A snippet of the SDP showing the grouping attribute and the video
   m-lines are shown below (mid 100 represents the CLUE channel):



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      ...
      a=group:CLUE 11 12 13 14 100
      ...
      m=video 58722 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
      a=sendrecv
      a=mid:10
      ...
      m=video 58724 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
      a=recvonly
      a=mid:11
      m=video 58726 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
      a=recvonly
      a=mid:12
      m=video 0 RTP/AVP 96
      m=video 58728 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016
      a=sendonly
      a=label:foo
      a=mid:13
      m=video 58730 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016
      a=sendonly
      a=label:bar
      a=mid:14



   Having received this Alice now has all the information she needs to
   send CONFIGURE 2.  She requests the two static captures from Bob, to
   be sent on encodings "foo" and "bar".

   Bob receives Alice's message CONFIGURE 2 but does not yet send the
   capture encodings specified, because Alice hasn't yet negotiated the
   ability to receive these streams in SDP.

   Alice now sends 200 OK 3, matching two recvonly m-lines to Bob's new
   sendonly lines.  She includes their mid values in the grouping
   attribute to show they are controlled by the CLUE channel.  A snippet
   of the SDP showing the grouping attribute and the video m-lines are
   shown below (mid 3 represents the CLUE channel):



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      ...
      a=group:CLUE 3 4 5 7 8
      ...
      m=video 6002 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
      a=sendrecv
      a=mid:2
      ...
      m=video 6004 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016
      a=sendonly
      a=mid:4
      a=label:enc1
      m=video 6006 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016
      a=sendonly
      a=mid:5
      a=label:enc2
      m=video 0 RTP/AVP 96
      m=video 6010 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
      a=recvonly
      a=mid:7
      m=video 6012 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
      a=recvonly
      a=mid:8



   Finally, on receiving 200 OK 3 Bob is now able to send the two
   streams of video Alice requested - this is illustrated as MEDIA 3.

   Both sides of the call are now sending multiple video streams with
   their sources defined via CLUE negotiation.  As the call progresses
   either side can send new Advertisement or Configure or new SDP
   negotiation to add, remove or change what they have available or want
   to receive.

7.  Example: A call between a CLUE and non-CLUE-capable endpoint






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   In this brief example Alice is a CLUE-capable endpoint making a call
   to Bob, who is not CLUE-capable, i.e., it is not able to use the CLUE
   protocol.



         +----------+                      +-----------+
         |   EP1    |                      |    EP2    |
         |          |                      |           |
         +----+-----+                      +-----+-----+
              |                                  |
              |                                  |
              | INVITE 1 (BASIC SDP+COMEDIA)     |
              |--------------------------------->|
              |                                  |
              |                                  |
              | 200 0K 1 (BASIC SDP+*NO*COMEDIA) |
              |<---------------------------------|
              |                                  |
              |                                  |
              | ACK 1                            |
              |--------------------------------->|
              |                                  |
              |                                  |
              |                                  |
              |<########### MEDIA 1 ############>|
              |   1 video A->B, 1 video B->A     |
              |<################################>|
              |                                  |
              |                                  |
              |                                  |
              |                                  |
              v                                  v



   In INVITE 1, Alice sends Bob a SIP INVITE including in the SDP body
   the basilar audio and video capabilities ("BASIC SDP") and the
   information needed for opening a control channel to be used for CLUE
   protocol messages exchange, according to what is envisioned in the
   COMEDIA approach ("COMEDIA") for DTLS/SCTP channel
   [I-D.ietf-mmusic-sctp-sdp].  A snippet of the SDP showing the
   grouping attribute and the video m-line are shown below (mid 3
   represents the CLUE channel):


      ...
      a=group:CLUE 3



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      ...
      m=video 6002 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
      a=sendrecv
      a=mid:2



   Bob is not CLUE capable, and hence does not recognize the "CLUE"
   semantic for the grouping attribute, not does he support the CLUE
   channel.  He responds with an answer with audio and video, but with
   the CLUE channel zeroed.

   From the lack of the CLUE channel Alice understands that Bob does not
   support CLUE, or does not wish to use it.  Both sides are now able to
   send a single audio and video stream to each other.  Alice at this
   point begins to send her fallback video: in this case likely a
   switched view from whichever camera shows the current loudest
   participant on her side.

8.  CLUE requirements on SDP O/A

   The current proposal calls for a new "CLUE" semantic for the SDP
   Grouping Framework [RFC5888].

   Any other SDP extensions required to support CLUE signaling should
   also be specified here.  Then we will need to take action within
   MMUSIC to make those happen.  This section should be empty and
   removed before this document becomes an RFC.

   NOTE: The RTP mapping document [I-D.even-clue-rtp-mapping] is also
   likely to call for SDP extensions.  We will have to reconcile how to
   coordinate these two documents.

9.  SIP Signaling

   (Placeholder) This may be unremarkable.  If so we can drop it.

10.  Interoperation with Legacy SIP Devices

   This may just describe how the degenerate form of the general
   mechanisms work for legacy devices.  Or it may describe special case
   handling that we mandate as part of CLUE.  Or it may just discuss
   non-normative things for implementors should consider.

11.  CLUE over RTCWEB




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   We may want to rule this out of scope for now.  But we should be
   thinking about this.

12.  Open Issues

   Here are issues pertinent to signaling that need resolution.
   Resolution will probably result in changes somewhere in this
   document, but may also impact other documents.

   o  While the preference is to multiplex multiple capture encodings
      over a single RTP session, this will not always be desirable or
      possible.  The factors that prevent multiplexing may come from
      either the provider or the consumer.  So the extent of
      multiplexing must be negotiated.  The decision about how to
      multiplex affects the number and grouping of m-lines in the SDP.
      The endpoint of a CLUE session that sends an offer needs to know
      the mapping of capture encodings to m-lines for both sides.

      AFAIK this issue hasn't yet been considered at all.

   o  The current method for expressing encodings in SDP limits the
      parameters available when describing H264 encoder capabilities to
      those defined in Table 6 in [RFC6184]

13.  What else?

14.  Acknowledgements

   The team focusing on this draft consists of: Roni Even, Rob Hansen,
   Christer Holmberg, Paul Kyzivat, Simon Pietro-Romano, Roberta Presta.

   Christian Groves has contributed detailed comments and suggestions.

   The author list should be updated as people contribute substantial
   text to this document.

15.  IANA Considerations

   TBD

16.  Security Considerations

   TBD

17.  Change History

   -03:




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      *  Added a syntax section with an XML schema for CLUE messages.
         This is a strawhorse, and is very incomplete, but it
         establishes a template for doing this based on elements defined
         in the data model.  (Thanks to Roberta for help with this!)

      *  Did some rewording to fit the syntax section in and reference
         it.

      *  Did some relatively minor restructuring of the document to make
         it flow better in a logical way.

   -02:  A bunch of revisions by pkyzivat:

      *  Moved roberta's call flows to a more appropriate place in the
         document.

      *  New section on versioning.

      *  New section on NAK.

      *  A couple of possible alternatives for message acknowledgment.

      *  Some discussion of when/how to signal changes in provider
         state.

      *  Some discussion about the handling of transport errors.

      *  Added a change history section.

      These were developed by Lennard Xiao, Christian Groves and Paul,
      so added Lennard and Christian as authors.

   -01:  Updated by roberta to include some sample call flows.

   -00:  Initial version by pkyzivat.  Established general outline for
      the document, and specified a few things thought to represent wg
      consensus.

18.  References

18.1.  Normative References

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

   [I-D.ietf-clue-framework]





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              Duckworth, M., Pepperell, A., and S. Wenger, "Framework
              for Telepresence Multi-Streams", draft-ietf-clue-
              framework-11 (work in progress), July 2013.

   [I-D.presta-clue-data-model-schema]
              Presta, R. and S. Romano, "An XML Schema for the CLUE data
              model", draft-presta-clue-data-model-schema-03 (work in
              progress), March 2013.

   [I-D.ietf-mmusic-sctp-sdp]
              Loreto, S. and G. Camarillo, "Stream Control Transmission
              Protocol (SCTP)-Based Media Transport in the Session
              Description Protocol (SDP)", draft-ietf-mmusic-sctp-sdp-04
              (work in progress), June 2013.

   [I-D.tuexen-tsvwg-sctp-dtls-encaps]
              Jesup, R., Loreto, S., Stewart, R., and M. Tuexen, "DTLS
              Encapsulation of SCTP Packets for RTCWEB", draft-tuexen-
              tsvwg-sctp-dtls-encaps-01 (work in progress), July 2012.

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

   [RFC5888]  Camarillo, G. and H. Schulzrinne, "The Session Description
              Protocol (SDP) Grouping Framework", RFC 5888, June 2010.

18.2.  Informative References

   [RFC4353]  Rosenberg, J., "A Framework for Conferencing with the
              Session Initiation Protocol (SIP)", RFC 4353, February
              2006.

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

   [RFC6184]  Wang, Y., Even, R., Kristensen, T., and R. Jesup, "RTP
              Payload Format for H.264 Video", RFC 6184, May 2011.

   [I-D.even-clue-sdp-clue-relation]
              Even, R., "Signalling of CLUE and SDP offer/answer",
              draft-even-clue-sdp-clue-relation-01 (work in progress),
              October 2012.

   [I-D.even-clue-rtp-mapping]
              Even, R. and J. Lennox, "Mapping RTP streams to CLUE media
              captures", draft-even-clue-rtp-mapping-05 (work in
              progress), February 2013.



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   [I-D.hansen-clue-sdp-interaction]
              Hansen, R., "SDP and CLUE message interactions", draft-
              hansen-clue-sdp-interaction-01 (work in progress),
              February 2013.

Authors' Addresses

   Paul Kyzivat
   Huawei

   Email: pkyzivat@alum.mit.edu


   Lennard Xiao
   Huawei

   Email: lennard.xiao@huawei.com


   Christian Groves
   Huawei

   Email: Christian.Groves@nteczone.com


   Robert Hansen
   Cisco Systems

   Email: rohanse2@cisco.com






















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