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Versions: 00 01 02 03 04 draft-ietf-clue-protocol

CLUE Working Group                                             R. Presta
Internet-Draft                                               S P. Romano
Intended status: Standards Track                    University of Napoli
Expires: May 9, 2014                                    November 5, 2013


                             CLUE protocol
                     draft-presta-clue-protocol-03

Abstract

   The CLUE protocol is an application protocol conceived for the
   description and negotiation of a CLUE telepresence session.  The
   design of the CLUE protocol takes into account the requirements and
   the framework defined, respectively, in [I-D.ietf-clue-framework] and
   [I-D.ietf-clue-telepresence-requirements].  The companion document
   [I-D.kyzivat-clue-signaling] delves into CLUE signaling details, as
   well as into the SIP/SDP session establishment phase.  We herein
   focus on the application level perspective.  Message details,
   together with the behavior of CLUE participants acting as Media
   Providers and/or Media Consumers, are discussed.

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 May 9, 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



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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.  Overview of the CLUE protocol  . . . . . . . . . . . . . . . .  4
     3.1.  ADVERTISEMENT  . . . . . . . . . . . . . . . . . . . . . .  7
     3.2.  CONFIGURE  . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.3.  RESPONSE . . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.4.  RE-ADV . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     3.5.  OPTIONS  . . . . . . . . . . . . . . . . . . . . . . . . . 12
   4.  Protocol state machines  . . . . . . . . . . . . . . . . . . . 12
   5.  CLUE Participant state machine . . . . . . . . . . . . . . . . 13
   6.  Media Consumer's state machine . . . . . . . . . . . . . . . . 15
   7.  Media Provider's state machine . . . . . . . . . . . . . . . . 17
   8.  About CLUE protocol XML schema versioning  . . . . . . . . . . 19
   9.  Extensibility issues . . . . . . . . . . . . . . . . . . . . . 20
     9.1.  Aspect 1 - new information within existing messages  . . . 20
     9.2.  Aspect 2 - new messages  . . . . . . . . . . . . . . . . . 21
   10. Managing protocol version negotiation and extensions: the
       OPTIONS request  . . . . . . . . . . . . . . . . . . . . . . . 22
     10.1. An example using OPTIONS . . . . . . . . . . . . . . . . . 24
   11. XML Schema of CLUE protocol messages . . . . . . . . . . . . . 25
   12. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
   13. Handling channel errors  . . . . . . . . . . . . . . . . . . . 29
   14. Diff with the -02 version  . . . . . . . . . . . . . . . . . . 29
   15. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 29
   16. Informative References . . . . . . . . . . . . . . . . . . . . 29


















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1.  Introduction

   The CLUE protocol is an application protocol used by two CLUE
   Participants to enhance the experience of a multimedia telepresence
   session.  The main goals of the CLUE protocol are:

   1.  enabling a MP to advertise its current telepresence capabilities
       to the MC in terms of available media captures, encodings, and
       simultaneity constraints;

   2.  enabling a MC to request the desired multimedia streams from the
       offering MP.

   CLUE Participants are connected by means of the CLUE signaling
   channel.  Such channel has been conceived as a DTLS/SCTP/UDP channel
   in [I-D.kyzivat-clue-signaling] and it is established as depicted in
   the same document.  CLUE protocol messages flow across such channel.

   While [I-D.kyzivat-clue-signaling] focuses on protocol signaling
   details and on its interaction with the SIP/SDP session establishment
   phase, we herein investigate the protocol in action.  We assume the
   DTLS/SCTP/UDP channel is established and define the behiavior of the
   CLUE Participants communicating on it.  We discuss how the CLUE
   dialogue between them can be exploited to successfully setup the
   telepresence session according to the principles and concepts pointed
   out in in [I-D.ietf-clue-framework].

   In Section 3 we provide an overview of the CLUE protocol and describe
   CLUE messages along with their features and functionality.  The CLUE
   participant state machine is introduced in Section 4.  Versioning,
   extensions and options management mechanisms are discussed in
   Section 8, Section 9 and Section 10, respectively.  The XML schema
   defining the CLUE messages is reported in Section 11.

2.  Terminology

   This document refers to the same terminology used in
   [I-D.ietf-clue-framework] and in
   [I-D.ietf-clue-telepresence-requirements].  We briefly recall herein
   some of the main terms used in the document.  We further introduce
   the definition of CLUE participant.

   CLUE Participant  An entity able to use the CLUE protocol within a
      telepresence session.  It can be either an endpoint or an MCU able
      to use the CLUE protocol.






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   Endpoint  The logical point of final termination through receiving,
      decoding and rendering, and/or initiation through capturing,
      encoding, and sending of media streams.  An endpoint consists of
      one or more physical devices which source and sink media streams,
      and exactly one [RFC4353] Participant (which, in turn, includes
      exactly one SIP User Agent).  Endpoints can be anything from
      multiscreen/multicamera room controllers to handheld devices.

   MCU  Multipoint Control Unit (MCU) - a device that connects two or
      more endpoints together into one single multimedia conference
      [RFC5117].  An MCU may include a Mixer [RFC4353].

   Media  Any data that, after suitable encoding, can be conveyed over
      RTP, including audio, video or timed text.

   Media Capture  A "Media Capture", or simply "Capture", is a source of
      Media.

   Capture Encoding  A specific encoding of a Media Capture, to be sent
      via RTP [RFC3550].

   Media Stream  The term "Media Stream", or simply "Stream", is used as
      a synonymous of Capture Encoding.

   Media Provider  A CLUE participant (i.e., an Endpoint or an MCU)
      capable to send Media Streams.

   Media Consumer  A CLUE participant (i.e., an Endpoint or an MCU)
      capable to receive Media Streams.

3.  Overview of the CLUE protocol

   The CLUE protocol has been conceived to enable telepresence sessions.
   It is designed in order to address SDP limitations in terms of the
   description of several qualitative parameters about the multimedia
   streams that are involved in a real-time multimedia conference
   session.  Indeed, by simply using SDP we are not able to convey all
   the information about the features of the flowing multimedia streams
   that is needed to enable a "being there" rendering experience.  Such
   information is being designed in the CLUE framework draft and
   formally defined and described in the CLUE data model draft.  The
   CLUE protocol represents the mechanism that enables the exchange of
   CLUE information between CLUE participants.

   The CLUE protocol, as defined in this document, is a stateful,
   client-server, XML-based application protocol.  CLUE protocol
   messages flow on a DTLS/SCTP/UDP channel connecting two CLUE
   Participants.  The main goals of the CLUE protocol are:



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   1.  enabling a MP to advertise its current telepresence capabilities
       to the MC in terms of available media captures, encodings, and
       simultaneity constraints;

   2.  enabling a MC to request the desired multimedia streams from the
       offering MP.

   Three main design layers can be identified:

   1.  Establishment of the CLUE channel.

   2.  Negotiation of the CLUE protocol version and extensions

   3.  Media session description and negotiation

   Signaling issues about the CLUE channel establishment are considered
   in [I-D.kyzivat-clue-signaling].  In particular, the CLUE channel is
   a DTLS/SCTP/UDP channel connecting two CLUE Participants.  While
   [I-D.kyzivat-clue-signaling] focuses on protocol signaling details
   and on its interaction with the SIP/SDP session establishment phase,
   we herein investigate the protocol in action at the CLUE application
   level.

   As soon as the channel is ready, the CLUE Participants must agree on
   the protocol version and extensions to be used within the
   telepresence session.  A mechanism for the negotiation of the CLUE
   protocol version and extensions is proposed in Section 10.  According
   to such solution, the CP which is the CLUE Channel Initiator (CI)
   issues a proper CLUE message (OPTIONS) to the CP which is the Channel
   Receiver (CR).  Such a message specifies the supported version and
   extensions.  The CR answers by selecting the subset of the CI's
   extensions that it is able to support and determines the protocol
   version to be used.

   After that negotiation phase is completed, CLUE Participants define
   the characteristics of the media streams to be exchanged in both
   directions.  Indeed, being A and B the considered CLUE Participants,
   it is possible to distinguish between two dialogues:

   1.  the one needed to describe and set up the media streams sent from
       A to B, i.e., the dialogue between A's Media Provider side and
       B's Media Consumer side

   2.  the one needed to describe and set up the media streams sent from
       B to A, i.e., the dialogue between B's Media Provider side and
       A's Media Consumer side

   CLUE messages for the media session description and negotiation are



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   designed by considering the MP side as the server side of the
   protocol, since it produces and provides media streams, and the MC
   side as the client side of the protocol, since it requests and
   receives media streams.  The messages that are exchanged to set up
   the telepresence media session are described by focusing on a single
   MP-MC dialogue.

   The MP first advertises the media captures and associated encodings
   to the MC, as well as possible simultaneity constraints.  The
   description of such telepresence features is made according to the
   information defined in the CLUE framework and data model
   ([I-D.ietf-clue-framework] and [I-D.ietf-clue-data-model-schema]).
   The CLUE message conveing the MP's multimedia offer is the
   ADVERTISEMENT message.  Such message leverages the XML definitions
   provided in [I-D.ietf-clue-data-model-schema] for the description of
   media captures, encodings, and simultaneity constraints features.

   The MC selects the desired streams coming from the MP by using the
   CONFIGURE message, which makes reference to the information carried
   in the ADVERTISEMENT previously received by the MP.

   In the following, a bird's-eye view of the CLUE protocol messages is
   provided.  For each message it is indicated who sends it, who
   receives it, a brief description of the information it carries, and
   how/when it is used.  Besides ADVERTISEMENT and CONFIGURE, new
   messages have been conceived in order to provide all the mechanisms
   and operations envisaged in [I-D.kyzivat-clue-signaling].

   o  ADVERTISEMENT (ADV)

   o  ACKNOWLEDGEMENT (ACK)

   o  CONFIGURE (CONF)

   o  CONFIGURE RESPONSE

   o  RE-ADV

   o  RE-ADV RESPONSE

   o  OPTIONS

   o  OPTIONS RESPONSE








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3.1.  ADVERTISEMENT



  +---------- ---+-----------------------------------------------------+
  |              |                                                     |
  | FROM         |                        MP                           |
  |              |                                                     |
  +--------------+-----------------------------------------------------+
  |              |                                                     |
  | TO           |                        MC                           |
  |              |                                                     |
  +--------------+-----------------------------------------------------+
  |              |                                                     |
  | TYPE         |                   Notification                      |
  |              |                                                     |
  +--------------+-----------------------------------------------------+
  |              |                                                     |
  | DESCRIPTION  |   This message is used by the MP to advertise the   |
  |              |   available media captures and related information  |
  |              |   to the MC.                                        |
  |              |   The ADV contains elements compliant with the      |
  |              |   CLUE data model and other information like the    |
  |              |   CLUE protocol version and a sequence number.      |
  |              |                                                     |
  +--------------+-----------------------------------------------------+
  |              |                                                     |
  | USAGE        |   The MP sends this message as soon as the          |
  |              |   CLUE channel is ready. The MP sends an ADV to the |
  |              |   MC each time there is a modification of the MP's  |
  |              |   telepresence capabilities.                        |
  |              |   The ADV message is also sent back to the MC       |
  |              |   when the MP receives a RE-ADV request.            |
  +--------------+-----------------------------------------------------+


   The ADV message is considered a notification since, during the
   session, it can be sent from the MP also on a per-event basis, i.e.
   when the CLUE capabilities of the MP change with respect to the last
   issued ADV.  It is still to be discussed if a "delta" mechanism for
   advertising only the changes with respect to the previous
   notification should be adopted.  Similar approaches have been
   proposed for partial notifications in centralized conferencing
   frameworks ([RFC6502]), leveraging the XML diff codification
   mechanism defined in [RFC5261].






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3.2.  CONFIGURE



  +--------------+-----------------------------------------------------+
  |              |                                                     |
  | FROM         |                        MC                           |
  |              |                                                     |
  +--------------+-----------------------------------------------------+
  |              |                                                     |
  | TO           |                        MP                           |
  |              |                                                     |
  +--------------+-----------------------------------------------------+
  |              |                                                     |
  | TYPE         |                     Request                         |
  |              |                                                     |
  +--------------+-----------------------------------------------------+
  |              |                                                     |
  | DESCRIPTION  |   This message allows a MC to ask for the           |
  |              |   desired (advertised) capture. It contains capture |
  |              |   encodings and other information like the CLUE     |
  |              |   protocol version and a sequence number.           |
  |              |                                                     |
  +--------------+-----------------------------------------------------+
  |              |                                                     |
  | USAGE        |   The MC can send a CONF after the reception of     |
  |              |   an ADV or each time it wants to request other     |
  |              |   advertised captures from the MP.                  |
  +--------------+-----------------------------------------------------+

3.3.  RESPONSE




















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  +--------------+-----------------------------------------------------+
  |              |                                                     |
  | FROM         |                        MP                           |
  |              |                                                     |
  +--------------+-----------------------------------------------------+
  |              |                                                     |
  | TO           |                        MC                           |
  |              |                                                     |
  +--------------+-----------------------------------------------------+
  |              |                                                     |
  | TYPE         |                      Response                       |
  |              |                                                     |
  +--------------+-----------------------------------------------------+
  |              |                                                     |
  | DESCRIPTION  |   This message allows a MP to answer a CONF         |
  |              |   message. Besides the protocol version and a       |
  |              |   sequence number, it contains a response code with |
  |              |   a response string indicating either the success   |
  |              |   or the failure (along with failure details) of    |
  |              |   a CONF request elaboration. Example response      |
  |              |   codes and strings are provided in the following   |
  |                              |   table.                                                                                        |
  |              |                                                     |
  +--------------+-----------------------------------------------------+
  |              |                                                     |
  | USAGE        |   The MP sends this message in response to a CONF   |
  |              |   message.                                          |
  +--------------+-----------------------------------------------------+


   Response codes can be designed by adhering to the HTTP semantics, as
   shown below.



















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   +-----------------+----------------------+--------------------------+
   |                 |                      |                          |
   |  Response code  |  Response string     |       Description        |
   |                 |                      |                          |
   +-----------------+----------------------+--------------------------+
   |                 |                      |                          |
   |   410           |  Bad syntax          |  The XML syntax of the   |
   |                 |                      |  CONF message is not     |
   |                 |                      |  correct.                |
   +-----------------+----------------------+--------------------------+
   |                 |                      |                          |
   |   411           |  Invalid value       |  The CONF message        |
   |                 |                      |  contains an invalid     |
   |                 |                      |  parameter value.        |
   +-----------------+----------------------+--------------------------+
   |                 |                      |                          |
   |   412           |  Invalid identifier  |  The identifier used for |
   |                 |                      |  requesting a capture is |
   |                 |                      |  not valid or unknown.   |
   +-----------------+----------------------+--------------------------+
   |                 |                      |                          |
   |   413           |  Conflicting values  |  The CONF message        |
   |                 |                      |  contains values that    |
   |                 |                      |  cannot be used together.|
   +-----------------+----------------------+--------------------------+
   |                 |                      |                          |
   |   420           |  Invalid sequencing  |  The sequence number of  |
   |                 |                      |  the CONF message is out |
   |                 |                      |  of date or corresponds  |
   |                 |                      |  to an obsoleted ADV.    |
   +-----------------+----------------------+--------------------------+
   |                 |                      |                          |
   |   510           | Version not supported|  The CLUE protocol       |
   |                 |                      |  version of the CONF     |
   |                 |                      |  message is not supported|
   |                 |                      |  by the MP.              |
   +-----------------+----------------------+--------------------------+
   |                 |                      |                          |
   |   511           | Option not supported |  The option requested in |
   |                 |                      |  the CONF message is not |
   |                 |                      |  supported by the MP.    |
   +-----------------+----------------------+--------------------------+



   ...  TBC.





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   +---------------+------------------------+
   |               |                        |
   | Response code | Description            |
   | family        |                        |
   +---------------+------------------------+
   |               |                        |
   |     1XX       | Temporary info         |
   |               |                        |
   +---------------+------------------------+
   |               |                        |
   |     2XX       | Success                |
   |               |                        |
   +---------------+------------------------+
   |               |                        |
   |     3XX       | Redirection            |
   |               |                        |
   +---------------+------------------------+
   |               |                        |
   |     4XX       | Client error           |
   |               |                        |
   +---------------+------------------------+
   |               |                        |
   |     5XX       | Server error           |
   |               |                        |
   +---------------+------------------------+


3.4.  RE-ADV























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  +---------- ---+-----------------------------------------------------+
  |              |                                                     |
  | FROM         |                        MC                           |
  |              |                                                     |
  +--------------+-----------------------------------------------------+
  |              |                                                     |
  | TO           |                        MP                           |
  |              |                                                     |
  +--------------+-----------------------------------------------------+
  |              |                                                     |
  | TYPE         |                      Request                        |
  |              |                                                     |
  +--------------+-----------------------------------------------------+
  |              |                                                     |
  | DESCRIPTION  |   This message allows a MC to request a MP to       |
  |              |   issue a new copy of the ADV. This message can     |
  |              |   contain a reason string indicating the motivation |
  |              |   for the request (e.g., refresh, missing elements  |
  |              |   in the received ADV, wrong syntax in the received |
  |              |   ADV, invalid capture area, invalid line of        |
  |              |   capture point, etc).                              |
  |              |                                                     |
  +--------------+-----------------------------------------------------+
  |              |                                                     |
  | USAGE        |   The MC sends this message to the MP when the      |
  |              |   timeout for the ADV is fired, or when the ADV is  |
  |              |   not compliant with the CLUE specifications (this  |
  |              |   can be useful for interoperability testing        |
  |              |   purposes)                                         |
  |              |                                                     |
  +--------------+-----------------------------------------------------+

3.5.  OPTIONS

   ToDo.  See Section 10.

4.  Protocol state machines

   The CLUE protocol is an application protocol used between a Media
   Provider (MP) and a Media Consumer (MC) in order to establish a
   multimedia telepresence session.  CLUE protocol messages flow upon a
   DTLS/SCTP channel established as depicted in
   [I-D.kyzivat-clue-signaling].  Over such a channel there are
   typically two CLUE streams between the channel terminations flowing
   in opposite directions.  In other words, typically, both channel
   terminations act simultaneously as a MP and as a MC.  We herein
   discuss the state machines associated, respectively, with the CLUE
   Participant, with the MC process and with the MP process.



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5.  CLUE Participant state machine

   The main state machines focus on describing the states of CLUE
   channel from a CLUE channel initiator/receiver perspective.  In the
   IDLE state, when the CP has established a CLUE channel, the main
   state moves to the ESTABLISHED state.  When in the ESTABLISHED state,
   if the CP is the Channel Initiator (CI), it sends an OPTIONS message
   for version negotiation; otherwise, if the CP is the Channel Receiver
   (CR), it listens to the channel for an OPTIONS message associated
   with version negotiation.  If an OPTIONS message is sent (or
   received), the CP moves to the NEGOTIATING state.  If the CP detects
   errors in the request message received, the main state goes back to
   the IDLE state.  When in the NEGOTIATING state, the CR prepares an
   OPTIONS response message while the CI listens to the channel for an
   OPTIONS response.  If an OPTIONS response message for version
   negotiation is sent (or received), the main state moves to the ACTIVE
   state.  If the CP detects errors in the response message received or
   receives an error response, it goes back to the IDLE state.  When the
   party enters the ACTIVE state, it creates two sub state machines: the
   MC state machine and the MP state machine.  When in the ACTIVE state,
   if the CP receives an OPTIONS message for version negotiation (or an
   OPTIONS response message for version negotiation), it must ignore the
   message and stay in the ACTIVE state.  When in the ACTIVE state, a CP
   which receives CLUE messages (including ADV, RE-ADV and CONF, as well
   as the corresponding response messages) dispatches them to the proper
   underlying sub state machine for further processing.  The same
   happens in case of changes in the telepresence settings.  The
   TERMINATED state is reachable from each of the aforementioned states
   whenever the session is canceled or released.  The IDLE state is
   reachable from each of the aforementioned states whenever the
   underlying channel is closed (only due to connection errors).




















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                               +-------------+<------------------------------+-------------+
                  +----------->+     IDLE    +<--------------------------+   +   TIME OUT  +
                  |            +------+------+<-----------+              |   +------+------+
                  |                   |                   |              |          |
                  |                   |                   |              |          |
              Connection            CLUE                  |              |          |
                error              channel                |              |          |
                  |           has been established        |              |          |
                  |                   |                   |              |          |
                  |                   V               Receive error      |          |
                  +------------+-------------+     (version mismatch,    |          |
            +------------------+ ESTABLISHED +      missing elements,..) |       time out
            |                  +------+------+            |              |          |
            |                         |                   |          Connection     |
            |                         |                   |             error       |
            |                  Send/Receive request       |              |          |
            |                  Version negotiation        |              |          |
            |                         |                   |              |          |
            |                         V                   |              |          |
            |                  +-------------+------------+              |          |
            |     +------------+ NEGOTIATING +---------------------------+          |
            |     |            +------+------+---------------------------|----------+
            |     |                   |                                  |
            |     |                   |                                  |
            |     |           Receive/Send response                 Connection
            |     |            Version negotiation                     error
         Session  |                   |                                  |
           end    |                   V                                  |
            |     |            +-------------+---------------------------+
            |     |            |   ACTIVE    +<-------------------+
            |     |            |  +-------+  |           Receive(request/response)
            |     |            |  |SUBIDLE|  |             Version negotiation
            |   Session        |  |MC     |  +--------------------+
            |    end           |  +-------+  |
            |     |            |  +-------+  +<-------------------+
            |     |            |  |SUBIDLE|  |       Send/Receive(ADV/CONF/RE-ADV/RESP)
            |     |            |  |MP     |  |         Change telepresence settings
            |     |            |  +-------+  |                    |
            |     |            +------+------+--------------------+
            |     |                   |
            |     |                   |
            |     |                Session
            |     |                  end
            |     |                   |
            |     |                   V
            |     |            +-------------+
            |     +----------->+ TERMINATED  +
            +----------------->+-------------+



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6.  Media Consumer's state machine

   An MC in the WAIT FOR ADV state is waiting for an ADV coming from the
   MP.  If the timeout expires ("timeout"), the MC switches to the
   TIMEOUT state.

   In the TIMEOUT state, if the number of trials is below the retry
   threshold, the MC sends a RE-ADV/refresh message to the MP ("send RE-
   ADV"), switching back to the WAIT FOR ADV.  Otherwise, the MC moves
   to the TERMINATED state.

   When the ADV has been received ("receive ADV"), the MC goes into the
   ADV RECEIVED state.  The ADV is then parsed.  If something goes wrong
   with the ADV (bad syntax, missing XML elements, etc.), the MC sends a
   NACK message to the MP specifying the encountered problem via a
   proper reason phrase.  In this way, the MC switches back to the WAIT
   FOR ADV state, waiting for a new copy of the ADV.  If the ADV is
   successfully processed, the MC issues an ACK message to the MP and
   moves to the ADV ACKED state.  When the CONF request is ready, the MC
   sends it and moves to the TRYING state.  Alternatively, if the ADV is
   successfully processed, and the CONF request is timely available, the
   MC can piggyback the ACK message within a CONF request and move from
   the ADV RECEIVED state directly to the TRYING state.

   While in the TRYING state, the MC is waiting for a CONF RESPONSE
   message (to the issued CONF) from the MP.  If the timeout expires
   ("timeout"), the MC moves to the TIMEOUT state and sends a RE-ADV in
   order to solicit a new ADV from the MP.  If a CONF RESPONSE with an
   error code is received ("receive 4xx, 5xx not supported"), then the
   MC moves back to the ADV RECEIVED state and produces a new CONF
   message to be sent to the MP.  If a successful RESPONSE arrives
   ("receive 200 OK"), the MC gets into the CONF COMPLETED state. state.

   When the MC is in the CONF COMPLETED state, it means that the
   telepresence session configuration has been set up according to the
   MC's preferences.  Both the MP and the MC have agreed on (and are
   aware of) the media streams to be exchanged within the call.  If the
   MC decides to change something in the call settings, it issues a new
   CONF ("send CONF") and moves back to the TRYING state.  If a new ADV
   arrives from the MP ("receive ADV"), it means that something has
   changed on the MP's side.  The MC then moves to the ADV-RCV state and
   prepares a new CONF taking into account the received updates.  When
   the underlying channel is closed, the MC moves into the TERMINATED
   state.

   The TERMINATED state is reachable from each of the aforementioned
   states whenever the underlying channel is closed.  The corresponding
   transitions have not been reported for the sake of simplicity.  This



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   termination condition is a temporary solution.




                                                                      +-----+
                     +---------------+-------------timeout------>+----+--+  |
                     | WAIT FOR ADV  |<----+                     |TIMEOUT|  |
                     +---------------+<----+--------send---------+-------+  |
                                |          |     RE-ADV(refresh)      ^     |
                                |          |                          |     |
                                |          |                          |     |
                             receive     send                         |     |
                               ADV       NACK                         |     |
           +---receive-------+  |       (missing elements,            |     |
           |   error RESP    |  |        invalid area,...)            |     |
           |                 v  v          |                          |     |
           +----------------+---------+----+      +--------+          |     |
          +---------------->|  ADV    |---send--->|  ADV   |      timeout   |
          |                 | RECEIVED|   ACK     | ACKED  |          |     |
          |     +---------->|         |           |        |          |     |
          |    recv  +----->+-----+---+<--recv----+----+---+          |     |
          |   error  |            |       ADV          |              |     |
          |   RESP   |          send                   |              |     |
      receive   +    |          CONF,              send|              |     |
        ADV     |    |         CONF+ACK            CONF|              |     |
          |     |    |            |                    |              |     |
          |     |  receive        v                    |              |     |
          |     |  ADV         +-----------+<----------+              |     |
          |     |    |         |           |+-------------------------+     |
          |     +----|--------+|  TRYING   |                                |
          +----------|---------|           |                                |
                 +---|---------+-----------+                                |
                 |   |          |          ^                                |
                 |   |          |          |                                |
                 |   |          |          |                                |
          receive|   |         receive    send                          retry
          error RESP,|          200 OK    CONF                        expires
          retry  |   |          |          |                                |
          expired|   |          |          |                                |
                 |   |          |          |                                |
                 |   |          v          |                                |
                 |   |       +---------+   |                                |
                 |   +-------| CONF    |   |                                |
                 |           |COMPLETED|---+                                |
                 |           +---------+                                    |
                 |              |                                           |
                 |              |                                           |



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                 |              |                                           |
                 |              |                                           |
                 |            connection                                    |
                 |             closed                                       |
                 |              |                                           |
                 |              |                                           |
                 |              |                                           |
                 |              |                                           |
                 |              |                                           |
                 |              v                                           |
                 |            +----------+<---------------------------------+
                 +----------->+TERMINATED|
                              +----------+


7.  Media Provider's state machine

   In the PREPARING ADV state, the MP is preparing the ADV message
   reflecting the actual telepresence capabilities.  After the ADV has
   been sent, the MP moves to the WAIT FOR ACK state.  If the ACK
   arrives, the MP moves to the WAIT FOR CONF state.  If a NACK arrives,
   it goes back to the PREPARING ADV state.

   When in the WAIT FOR ACK state, if a CONF or a CONF+ACK arrives, the
   MP switch to the CONF RECEIVED state directly.

   When in the WAIT FOR CONF state, the MP is listening to the channel
   for a CONF coming from the MC.  If a RE-ADV is received, the MP goes
   back to the IDLE state and issues an ADV again.  If telepresence
   settings change in the meanwhile, it moves back to the PREPARING ADV
   state and prepares a new ADV to be sent to the MC.  If a CONF
   arrives, the MP switches to the CONF RECEIVED state.  If nothing
   happens and the timeout expires, than the MC falls into the TIMEOUT
   state.

   In the TIMEOUT state, if the number of trials does not exceed the
   retry threshold, the MC comes back to the PREPARING ADV state for
   sending a new ADV.  Otherwise, it goes to the TERMINATED state.

   The MP in the CONF RECEIVED state is processing the received CONF in
   order to produce a CONF RESPONSE message.  If the MP is fine with the
   MC's configuration, then it sends back a 200 OK successful CONF
   RESPONSE and moves to the IN CALL state.  If there are errors duting
   CONF processing, then the MC returns a CONF RESPONSE carrying an
   error response code.  Finally, if there are changes in the
   telepresence settings, it goes back to the PREPARING ADV state to
   issue an updated ADV.




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   When in the CONF COMPLETED state, the MP has successfully configured
   the telepresence session according to the MC's specifications.  If a
   new CONF arrives, it switches to the CONF RECEIVED state to analyze
   the new request.  If a RE-ADV arrives, or some modifications are
   applied to the telepresence options, then it moves to the PREPARE-ADV
   state to issue the ADV.  When the channel is terminated, the MP falls
   into the TERMINATED state.

   The TERMINATED state is reachable from each of the aforementioned
   states whenever the underlying channel is closed.  The corresponding
   transitions have not been reported for the sake of simplicity.  This
   termination condition is a temporary solution.





                    +-----------+
                    |           |
                    | PREPARING |
 +----------------->|   ADV     |<--------------------------+
 |   +------------->|           |<-----------retry----------+---------+
 |   |       +----->|           |<--+         not           |         |
 |   |       |      +-----------+   |        expired        |         |
 |   |       |            |         |                       |         |
 |   |  change          send       receive                  |        ++------+
 |   |  telepresence     ADV        NACK                    |        |TIMEOUT|
 |   |  settings          |         |                       |        ++--+---+
 |   |       |            |         |                       |         ^  |
 |   |       |            v         |                       |         |  |
 |   |       |    +-------------+---+                       |         |  |
 |   |       +----+  WAIT FOR   +------------timeout--------+---------+  |
 |   |         +--+     ACK     |                           |            |
 change        |  +-------+-----+                           |            |
 telepresence  |          |                                 |            |
 settings      |        recv                                |            +
 +   +         |        ACK                                 +            |
 |   |         |          |                                 |            |
 |   |         |          v                                 |            |
 |   |         |     +-----------+                          |            |
 |   |      recv     | WAIT FOR  |                          |            |
 |   |       CONF,   |   CONF    |<-+                       |            |
 |   |    CONF+ACK   +----+------+  |                       |            |
 |   |         |          |         +                       |            |
 +   |         |        receive     CONF error,             +            |
 |   +         |         CONF       retry not expired,      |            +
 |   |         |          |         send error RESP         |            |
 |   |         |          |         |                       |            |



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 |   |         |          |         |                       |            |
 |   |         |          v         |                       |            |
 |   |         +--->+-----------+---+                       |            |
 +---+-------------+|   CONF    |                           |            |
     |       +----->| RECEIVED  |----CONF error,            |            |
     |       |      +-----+-----+    retry expired          |            |
     |       |            |          +                      |            |
     |       |            |          |                      |            |
     |       |            |          |                      |            |
 receive   receive      send         |                      |            |
 RE-ADV     CONF       200 OK        |                      |       retry|
     |       |            |          |                      |      expired
     |       |            |          |                      |            |
     |       |            |          |                      |            |
     |       |            v          |                      |            |
     |       |       +----------+    |   change             |            |
     |       +-------|  CONF    |----|---telepresence-------+            |
     +---------------| COMPLETED|    |   settings                        |
                     +----------+    |                                   |
                          |          |                                   |
                          |          |                                   |
                          |          |                                   |
                       connection    |                                   |
                        closed       |                                   |
                          |          |                                   |
                          |          |                                   |
                          v          |                                   +
                 +----------------+<-+                                   |
                 |   TERMINATED   |<-------------------------------------+
                 |                |
                 +----------------+


8.  About CLUE protocol XML schema versioning

   CLUE protocol messages are XML messages compliant to the CLUE
   protocol XML schema.  The version of the protocol corresponds to the
   version of the schema.  Both client and server have to test the
   compliance of the received messages with the XML schema of the CLUE
   protocol.  If the compliance is not verified, the message cannot be
   processed further.

   Obviously, client and server can not communicate if they do not share
   exactly the same XML schema.  Such a schema is the one included in
   the yet to come RFC, and associated with the CLUE URN
   "urn:ietf:params:xml:ns:clue-message".  If all CLUE-enabled devices
   use that schema there will be no interoperability problems due to
   schema issues.



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   The version of the XML schema contained in the standard document
   deriving from this draft will be 1.0.  The subsequent versions of the
   XML schema should be backward compatible, not only in terms of schema
   but also semantically and procedurally as well.  This means that they
   should define further features and functionality besides those
   defined in the previous versions, in an incremental way, without
   impacting the basic rules defined in the previous version of the
   schema.  In this way, if a MP is able to speak, e.g., version 5.0 of
   the protocol while the MC only understands version 4.0, the MP should
   have no problem in reverting the dialogue to version 4.0 without
   exploiting 5.0 features and functionality.

   It is expected that, before the CLUE protocol XML schema reaches a
   steady state, prototypes developed by different organizations will
   conduct interoperability testing.  In that case, in order to
   interoperate, they have to be compliant to the current version of the
   XML schema, i.e., the one copied in the most up-to-date version of
   the draft defining the CLUE protocol.  The versions of the non-
   standard XML schema will be numbered as 0.01, 0.02, and so on.
   During the standard development phase, the versions of the XML schema
   will probably not be backward compatible so it is left to prototype
   implementers the responsibility of keeping their products up to date.

   Even though strongly discouraged, if a future version of the protocol
   is designed which breaks the backward compatibility constraint, this
   aspect MUST be explicitly advertised in the corresponding new RFC
   document.  In such a case, it would be up to developers to update
   their systems accordingly.

9.  Extensibility issues

   Although the standard version of the CLUE protocol XML schema will be
   designed to thoroughly cope with the requirements emerging from the
   application domain, new needs might arise in the future.  Such needs
   may relate to two main aspects of the protocol:

      the information carried in the existing messages (for example, we
      may want to add more fields within an existing message);

      the meaning of the messages.  This is the case if there is no
      proper message for a certain task, so a brand new CLUE message
      needs to be defined.

9.1.  Aspect 1 - new information within existing messages

   CLUE messages are envelopes carrying two types of information:





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      XML elements defined within the CLUE protocol XML schema itself
      (protocol-specific information)

      other XML elements compliant to the CLUE data model schema (data
      model information)

   When new protocol-specific information is needed somewhere in the
   protocol messages, it can be added in place of the <any> elements and
   <anyAttribute> elements envisioned by the protocol schema.  The
   policy currently defined in the protocol schema for handling <any>
   and <anyAttribute> elements is:

      elementFormDefault="qualified"

      attributeFormDefault="unqualified"

   In that case, the new information must be qualified by namespaces
   other than "urn:ietf:params:xml:ns:clue-message" (the protocol URN)
   and "urn:ietf:params:xml:ns:clue-info" (the data model URN).
   Elements or attributes from unknown namespaces MUST be ignored.

   The other matter concerns data model information.  Data model
   information is defined by the XML schema associated with the URN
   "urn:ietf:params:xml:ns:clue-info".  Also for the XML elements
   defined in such a schema there are extensibility issues.  Those
   issues are overcome by using <any> and <anyAttribute> placeholders.
   Similarly to what said before, new information within data model
   elements can be added in place of <any> and <anyAttribute> schema
   elements, as long as they are properly namespace qualified.

9.2.  Aspect 2 - new messages

   New CLUE protocol messages, not envisioned in the standard version of
   the schema, are needed.  Also in that case we have three chances:

      writing down a new version of the protocol schema, with the new
      messages added after the existing ones.  The same considerations
      of the first option above hold here.

      putting all the new messages inside a brand new schema to be
      linked to a new URN that the most up to date telepresence system
      must be aware of.

      designing a wildcard envelope for future messages.  This is an
      approach used also within the CCMP protocol (Centralized
      Conferencing Manipulation Protocol, [RFC6503]).  In that case, a
      mechanism for the extension negotiation is also envisioned.




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10.  Managing protocol version negotiation and extensions: the OPTIONS
     request

   In this section we provide a mechanism for handling both protocol
   extension and version negotiation issues.

   We propose a new request message issued by the CI towards the CR as
   soon as the CLUE channel is istantiated: the OPTIONS message.  This
   message carries:

      the CLUE protocol version adopted by the CI

      the data model extensions supported by the CI

      the protocol extensions supported by the CI

   When the CR receives the OPTIONS message, it reads the CLUE protocol
   version of the CI (the highest protocol version of the CI).  If the
   CI's version is higher than the CR's one, then the CR responds to the
   CI by using in the OPTIONS RESPONSE message its own version.  The CI
   has to downgrade the CLUE dialogue to the version specified by the CR
   in the subsequent CLUE messages.  If CI's version is equal to or
   lower than CR's version, then the CR will use in the OPTIONS RESPONSE
   message the same version as the one in the OPTIONS message and all
   subsequent CLUE messages must carry that version number.  In the
   latter case, it is the CR who has to to downgrade the CLUE dialogue
   in order to be understood by the CI.

   A data model extension is a set of XML definitions related to the
   description of telepresence capabilities that is contained in an XML
   schema and which is different from the normative CLUE data model
   schema.  Such XML definitions can represent further entities not
   envisioned in the CLUE framework at the time of writing of the data
   model draft.  The entities defined in a data model extension can
   appear in place of the <any> and <anyAttribute> elements included in
   the data model document.  A data model extension is then represented
   by a reference to the defining XML schema.  The schema reference is
   represented by a URI defining the schema location.  [TBC] If a data
   model extension is supported by both a CR and a CI, this means that
   both are aware of the associated XML schema and of the meanings of
   the elements therein defined.

   A protocol extension is a set of XML definitions related to the CLUE
   protocol that is contained in an XML schema which is different from
   the normative CLUE protocol schema.  Such definitions can represent:
   (i) information to be carried within the existing messages in place
   of <any> and <anyAttribute> elements; (ii) new messages designed for
   the CLUE telepresence control.  Such XML definitions refer to



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   information not envisioned during the CLUE protocol design phase.  A
   protocol extension is then represented by a reference to the defining
   XML schema.  If a protocol extension is supported by both a CI and a
   CR, it means that both are aware of the associated XML schema and of
   the meanings of the elements defined within it.

   When the CR receives the CI's OPTIONS message, it selects the data
   model extensions and the protocol extensions that it is able to
   support, and then provides them into the OPTIONS RESPONSE message
   back to the CI.  Only the extensions included in the RESPONSE message
   can be used during the telepresence session.

   The XML schema definition of the OPTIONS message is provided in the
   following.



<!-- CLUE OPTIONS REQUEST -->
<xs:complexType name="optionsMessageType">
<xs:complexContent>
<xs:extension base="clueRequestMessageType">
<xs:sequence>
<!-- optional fields -->
<xs:element ref="options" minOccurs="0"/>
<xs:any namespace="##other"
processContents="lax" minOccurs="0"/>
</xs:sequence>
</xs:extension>
</xs:complexContent>
</xs:complexType>

<!-- CLUE OPTIONS -->
<xs:element name="options" type="optionsType"/>

<xs:complexType name="optionsType">
<xs:sequence>
<xs:element name="dm-exts" type="schemaRefList" minOccurs="0" maxOccurs="1"/>
<xs:element name="protocol-exts" type="schemaRefList" minOccurs="0"
            maxOccurs="1"/>
</xs:sequence>
</xs:complexType>

<!-- SCHEMA REF LIST TYPE -->
<xs:complexType name="schemaRefList">
<sequence>
<element name="schemaRef" type="xs:anyURI" maxOccurs="unbounded"/>
</sequence>
</xs:complexType>



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10.1.  An example using OPTIONS

   An example of OPTIONS dialogue is provided in the following.




   +------+               +------+
   |  CI  |               |  CR  |
   |      |               |      |
   +--+---+               +--+---+
      |                      |
      | OPTIONS 3.0          |
      | dm-ext: s1, s2, s3   |
      | pr-ext: s4, s5       |
      |<--------------------+|
      |                      |
      |        RESPONSE 1.0  |
      |        dm-ext: s1    |
      |        pr-ext: -     |
      |+-------------------->|
      |                      |
      |                      |
      |             ADV 1.0  |
      |+-------------------->|
      |                      |
      |  CONFIGURE+ACK 1.0   |
      |<--------------------+|
      |                      |
      |                      |
      v                      v


   When the CLUE channel is ready, the CI issues an OPTIONS request to
   the CR.  The CI uses the 3.0 version of the CLUE protocol, and
   supports schemas s1, s2, s3 as data model extensions and schemas s4,
   s5 as protocol extensions.

   The CR speaks the 1.0 version of the CLUE protocol and supports only
   the first data model extension among those indicated by the CI.  It
   then issues a v. 1.0 RESPONSE to the CI copying only the supported
   option.  The CI is able to understand that it can use only the 1.0
   version of the protocol and the s1 extension.

   After the negotiation phase is completed, both CP starts their MP and
   MC machines and the dialouge for the media sessions set up starts.
   An example of possible messaging flowing on the channel is
   represented by the ADV issued by the CI's MP towards the CR's MC and



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   the following CONF+ACK, both version 1.0.

11.  XML Schema of CLUE protocol messages

   In this section we paste the XML schema defining the ADVERTISEMENT,
   CONFIGURE and RESPONSE messages contained in
   [I-D.kyzivat-clue-signaling].  At the time of writing, 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>.  However, that
   assumption is still under discussion in the context of the CLUE-SDP
   coupling issues.

   *** TO BE UPDATED ***




<?xml version="1.0" encoding="UTF-8" ?>
<xs:schema
version="0.02"
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-01.xsd"/>

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

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




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<!-- 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 OPTIONS REQUEST -->
<xs:complexType name="optionsMessageType">
<xs:complexContent>
<xs:extension base="clueRequestMessageType">
<xs:sequence>
<!-- optional fields -->
<xs:element ref="options" minOccurs="0"/>
<xs:any namespace="##other"
processContents="lax" minOccurs="0"/>
</xs:sequence>
</xs:extension>
</xs:complexContent>
</xs:complexType>

<!-- CLUE OPTIONS -->
<xs:element name="options" type="optionsType"/>

<xs:complexType name="optionsType">
<xs:sequence>
<xs:element name="dm-exts" type="schemaRefList" minOccurs="0" maxOccurs="1"/>
<xs:element name="protocol-exts" type="schemaRefList" minOccurs="0"
            maxOccurs="1"/>
</xs:sequence>
</xs:complexType>

<!-- SCHEMA REF LIST TYPE -->
<xs:complexType name="schemaRefList">
<sequence>
<element name="schemaRef" type="xs:anyURI" maxOccurs="unbounded"/>
</sequence>
</xs:complexType>

<!-- CLUE NOTIFICATION MESSAGE TYPE -->
<xs:complexType name="clueNotificationMessageType" abstract="true">
<xs:complexContent>
<xs:extension base="clueMessageType">



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<xs:sequence>
<!-- mandatory fields -->
</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 -->
<xs:element name="requestNumber" type="xs:integer"/>
<xs:element name="reason" type="reasonType" minOccurs="1"/>
<!-- optional fields -->
<xs:any namespace="##other"
processContents="lax" minOccurs="0"/>
</xs:sequence>
</xs:extension>
</xs:complexContent>
</xs:complexType>

<!-- RESPONSE MESSAGE TYPE -->
<xs:complexType name="responseMessageType">
<xs:complexContent>
<xs:extension base="clueRequestMessageType">
<xs:sequence>
<!-- mandatory fields -->
<!-- TBD. -->
</xs:sequence>
</xs:extension>
</xs:complexContent>
</xs:complexType>


<!-- CLUE ADVERTISEMENT MESSAGE TYPE -->
<xs:complexType name="advertisementMessageType">
<xs:complexContent>
<xs:extension base="clueNotificationMessageType">
<xs:sequence>
<!-- mandatory fields -->
<xs:element name="advNumber" type="xs:unsignedInt"/>
<xs:element name="mediaCaptures"
type="dm:mediaCapturesType"/>
<xs:element name="encodingGroups"
type="dm:encodingGroupsType"/>



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<!-- 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="advNumber" type="xs:unsignedInt"/>
<!-- optional fields -->
<xs:element name="captureEncodings"
type="dm:captureEncodingsType" minOccurs="0"/>
<xs:any namespace="##other"
processContents="lax" minOccurs="0"/>
</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>








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12.  Examples

   TBD

13.  Handling channel errors

   TBD

14.  Diff with the -02 version

   1.  "Terminology" section added.

   2.  Introduced the concept of "CLUE Participant" - an Endpoint or a
       MCU able to use the CLUE protocol within a telepresence session.
       A CLUE Participant can act as a Media Provider and/or as a Media
       Consumer.

   3.  INtroduced the ACK/NACK mechanism for the ADVERTISEMENT.

   4.  MP and MC state machines have been updated.  The CP state machine
       has been added.

15.  Acknowledgments

   We would like to thank Liuyan Scarlett for her precious feedback on
   the protocol document, as well as for proposing the introduction of
   the concept of a main state machine including the MP and MC sub state
   machines.

16.  Informative References

   [I-D.ietf-clue-data-model-schema]          Presta, R. and S. Romano,
                                              "An XML Schema for the
                                              CLUE data model", draft-
                                              ietf-clue-data-model-
                                              schema-00 (work in
                                              progress), July 2013.

   [I-D.ietf-clue-framework]                  Duckworth, M., Pepperell,
                                              A., and S. Wenger,
                                              "Framework for
                                              Telepresence Multi-
                                              Streams", draft-ietf-clue-
                                              framework-12 (work in
                                              progress), October 2013.

   [I-D.ietf-clue-telepresence-requirements]  Romanow, A., Botzko, S.,
                                              and M. Barnes,



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                                              "Requirements for
                                              Telepresence Multi-
                                              Streams", draft-ietf-clue-
                                              telepresence-requirements-
                                              06 (work in progress),
                                              October 2013.

   [I-D.kyzivat-clue-signaling]               Kyzivat, P., Xiao, L.,
                                              Groves, C., and R. Hansen,
                                              "CLUE Signaling", draft-
                                              kyzivat-clue-signaling-05
                                              (work in progress),
                                              September 2013.

   [RFC3550]                                  Schulzrinne, H., Casner,
                                              S., Frederick, R., and V.
                                              Jacobson, "RTP: A
                                              Transport Protocol for
                                              Real-Time Applications",
                                              STD 64, RFC 3550,
                                              July 2003.

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

   [RFC5117]                                  Westerlund, M. and S.
                                              Wenger, "RTP Topologies",
                                              RFC 5117, January 2008.

   [RFC5261]                                  Urpalainen, J., "An
                                              Extensible Markup Language
                                              (XML) Patch Operations
                                              Framework Utilizing XML
                                              Path Language (XPath)
                                              Selectors", RFC 5261,
                                              September 2008.

   [RFC6502]                                  Camarillo, G., Srinivasan,
                                              S., Even, R., and J.
                                              Urpalainen, "Conference
                                              Event Package Data Format
                                              Extension for Centralized
                                              Conferencing (XCON)",
                                              RFC 6502, March 2012.



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   [RFC6503]                                  Barnes, M., Boulton, C.,
                                              Romano, S., and H.
                                              Schulzrinne, "Centralized
                                              Conferencing Manipulation
                                              Protocol", RFC 6503,
                                              March 2012.

Authors' Addresses

   Roberta Presta
   University of Napoli
   Via Claudio 21
   Napoli  80125
   Italy

   EMail: roberta.presta@unina.it


   Simon Pietro Romano
   University of Napoli
   Via Claudio 21
   Napoli  80125
   Italy

   EMail: spromano@unina.it


























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