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     element SHOULD not be present at all if an ADV ACK message has been
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2	CLUE Working Group                                             R. Presta
3	Internet-Draft                                                 S. Romano
4	Intended status: Standards Track                    University of Napoli
5	Expires: December 21, 2014                                 June 19, 2014

7	                             CLUE protocol
8	                      draft-ietf-clue-protocol-00

10	Abstract

12	   The CLUE protocol is an application protocol conceived for the
13	   description and negotiation of a CLUE telepresence session.  The
14	   design of the CLUE protocol takes into account the requirements and
15	   the framework defined, respectively, in [I-D.ietf-clue-framework] and
16	   [I-D.ietf-clue-telepresence-requirements].  The companion document
17	   [I-D.kyzivat-clue-signaling] delves into CLUE signaling details, as
18	   well as on the SIP/SDP session establishment phase.  CLUE messages
19	   flow upon the CLUE data channel, based on reliable and ordered SCTP
20	   over DTLS transport, as described in [I-D.ietf-clue-datachannel].
21	   Message details, together with the behavior of CLUE Participants
22	   acting as Media Providers and/or Media Consumers, are herein
23	   discussed.

25	Status of This Memo

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

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

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

40	   This Internet-Draft will expire on December 21, 2014.

42	Copyright Notice

44	   Copyright (c) 2014 IETF Trust and the persons identified as the
45	   document authors.  All rights reserved.

47	   This document is subject to BCP 78 and the IETF Trust's Legal
48	   Provisions Relating to IETF Documents
49	   (http://trustee.ietf.org/license-info) in effect on the date of
50	   publication of this document.  Please review these documents
51	   carefully, as they describe your rights and restrictions with respect
52	   to this document.  Code Components extracted from this document must
53	   include Simplified BSD License text as described in Section 4.e of
54	   the Trust Legal Provisions and are provided without warranty as
55	   described in the Simplified BSD License.

57	Table of Contents

59	   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
60	   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
61	   3.  Overview of the CLUE protocol  . . . . . . . . . . . . . . . .  4
62	   4.  Protocol messages  . . . . . . . . . . . . . . . . . . . . . .  6
63	     4.1.  OPTIONS  . . . . . . . . . . . . . . . . . . . . . . . . .  8
64	     4.2.  OPTIONS RESPONSE . . . . . . . . . . . . . . . . . . . . . 10
65	     4.3.  ADVERTISEMENT  . . . . . . . . . . . . . . . . . . . . . . 11
66	     4.4.  ADVERTISEMENT ACKNOWLEDGEMENT  . . . . . . . . . . . . . . 12
67	     4.5.  CONFIGURE  . . . . . . . . . . . . . . . . . . . . . . . . 13
68	     4.6.  CONFIGURE RESPONSE . . . . . . . . . . . . . . . . . . . . 14
69	     4.7.  READV  . . . . . . . . . . . . . . . . . . . . . . . . . . 14
70	     4.8.  READV RESPONSE . . . . . . . . . . . . . . . . . . . . . . 15
71	     4.9.  Response codes and reason strings  . . . . . . . . . . . . 16
72	   5.  Protocol state machines  . . . . . . . . . . . . . . . . . . . 18
73	   6.  CLUE Participant's state machine . . . . . . . . . . . . . . . 18
74	     6.1.  Media Consumer's state machine . . . . . . . . . . . . . . 21
75	     6.2.  Media Provider's state machine . . . . . . . . . . . . . . 23
76	   7.  Versioning . . . . . . . . . . . . . . . . . . . . . . . . . . 25
77	   8.  Extensions and options . . . . . . . . . . . . . . . . . . . . 26
78	   9.  XML Schema . . . . . . . . . . . . . . . . . . . . . . . . . . 28
79	   10. Diff with draft-presta-clue-protocol-04  . . . . . . . . . . . 33
80	   11. Diff with draft-presta-clue-protocol-03  . . . . . . . . . . . 34
81	   12. Diff with draft-presta-clue-protocol-02  . . . . . . . . . . . 34
82	   13. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 34
83	   14. Informative References . . . . . . . . . . . . . . . . . . . . 34

85	1.  Introduction

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

91	   1.  enabling a MP to fully announce its current telepresence
92	       capabilities to a MC in terms of available media captures, groups
93	       of encodings, simultaneity constraints and other information
94	       envisioned in [I-D.ietf-clue-framework];

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

99	   CLUE-capable endpoints are connected by means of the CLUE data
100	   channel, an SCTP over DTLS channel which is opened and established as
101	   depicted respectively in [I-D.kyzivat-clue-signaling] and
102	   [I-D.kyzivat-clue-signaling].  CLUE protocol messages flowing upon
103	   such channel are detailed in the following, both syntactically and
104	   semantically.

106	   In Section 3 we provide a general overview of the CLUE protocol.
107	   CLUE protocol messages are detailed in Section 4 The CLUE Participant
108	   state machine is introduced in Section 5.  Versioning and extensions
109	   are discussed in Section 7 and Section 8, respectively.  The XML
110	   schema defining the CLUE messages is reported in Section 9.

112	2.  Terminology

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

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

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

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

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

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

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

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

148	   Media Provider  A CLUE Participant (i.e., an Endpoint or a MCU) able
149	      to send Media Streams.

151	   Media Consumer  A CLUE Participant (i.e., an Endpoint or a MCU) able
152	      to receive Media Streams.

154	3.  Overview of the CLUE protocol

156	   The CLUE protocol has been conceived to enable CLUE telepresence
157	   session.  It is designed in order to address SDP limitations in terms
158	   of the description of several information about the multimedia
159	   streams that are involved in a real-time multimedia conference.
160	   Indeed, by simply using SDP we are not able to convey the information
161	   about the features of the flowing multimedia streams that is needed
162	   to enable a "being there" rendering.  Such information is designed in
163	   the CLUE framework document and formally defined and described in the
164	   CLUE data model document.  The CLUE protocol represents the mechanism
165	   that enables the exchange of CLUE information between CLUE
166	   Participants.  It mainly provides the messages to enable a Media
167	   Provider to advertise its telepresence capabilities and to enable a
168	   Media Consumer to select the desired telepresence options.

170	   The CLUE protocol, as defined in the following, is a stateful,
171	   client-server, XML-based application protocol.  CLUE protocol
172	   messages flow on realiable and ordered SCTP over DTLS transport
173	   channel connecting two CLUE Participants.  Messages carries
174	   information taken from the XML-based CLUE data model
175	   ([I-D.ietf-clue-data-model-schema]).  Three main communication layers
176	   can be identified:

178	   1.  Establishment of the CLUE data channel: in this phase, the CLUE
179	       data channel setup takes place.  If it ends up successfully, the
180	       CPs are able to communicate and start the initiation phase.

182	   2.  Negotiation of the CLUE protocol version and options (initiation
183	       phase): the CPs connected via the CLUE data channel agree on the
184	       version and on the options to be used during the telepresence
185	       session.  Special CLUE messages are used for such a task.  At the
186	       end of that basic negotiation, each CP starts its activity as a
187	       CLUE MP and/or CLUE MC.

189	   3.  CLUE telepresence capabilities description and negotiation: in
190	       this phase, the MP-MC offer-answer dialogues take place on the
191	       data channel by means of the CLUE protocol messages.

193	   As soon as the channel is ready, the CLUE Participants must agree on
194	   the protocol version and extensions to be used within the
195	   telepresence session.  CLUE protocol version numbers are
196	   characterized by a major version number and a minor version number,
197	   both unsigned integer, separated by a dot.  While minor version
198	   numbers denote backword compatible changes in the context of a given
199	   major version, different major version numbers generally indicate a
200	   lack of interoperability between the protocol implementations.  In
201	   order to correctly establish a CLUE dialogue, the involved CPs MUST
202	   have in common a major version number (see Section 7 for further
203	   details).  The subset of the protocol options and extensions that are
204	   allowed within the CLUE session is also determined in the initiation
205	   phase, such subset being the one including only the options that are
206	   supported by both parties.  A mechanism for the negotiation of the
207	   CLUE protocol version and extensions is envisioned in the initiation
208	   phase.  According to such solution, the CP which is the CLUE Channel
209	   initiator (CI) issues a proper CLUE message (OPTIONS) to the CP which
210	   is the Channel Receiver (CR) specifying the supported version and
211	   extensions.  The CR then answers by selecting the subset of the CI
212	   extensions that it is able to support and determines the protocol
213	   version to be used.

215	   After that negotiation phase is completed, CLUE Participants describe
216	   and agree on the media flows to be exchanged.  Indeed, being CPs A
217	   and B both transmitting and receiving, it is possible to distinguish
218	   between two dialogues:

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

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

228	   CLUE messages for the media session description and negotiation is
229	   designed by considering the MP side as the server side of the
230	   protocol, since it produces and provides media streams, and the MC
231	   side as the client side of the protocol, since it requests and
232	   receives media streams.  The messages that are exchanged to set up
233	   the telepresence media session are described by focusing on a single
234	   MP-MC dialogue.

236	   The MP first advertises its available media captures and encoding
237	   capabilities to the MC, as well as its simultaneity constraints,
238	   according to the information model defined in
239	   [I-D.ietf-clue-framework].  The CLUE message conveing the MP's
240	   multimedia offer is the ADVERTISEMENT message.  Such message
241	   leverages the XML data model definitions provided in
242	   [I-D.ietf-clue-data-model-schema].

244	   The MC selects the desired streams of the MP by using the CONFIGURE
245	   message, which makes reference to the information carried in the
246	   previously received ADVERTISEMENT.

248	   Besides ADVERTISEMENT and CONFIGURE, other messages have been
249	   conceived in order to provide all the needed mechanisms and
250	   operations and will be detailed in the following sections.

252	4.  Protocol messages

254	   CLUE protocol messages are textual, XML-based messages that enable
255	   the configuration of the telepresence session.  The formal definition
256	   of such messages is provided in the XML Schema provided at the end of
257	   this document (Section 9).

259	   The XML definitions of the CLUE information provided in
260	   [I-D.ietf-clue-data-model-schema] are included within some CLUE
261	   protocol messages (namely the ADVERTISEMENT, the CONFIGURE, and the
262	   READV RESPONSE messages), in order to use the concept defined in
263	   [I-D.ietf-clue-framework].

265	   The CLUE protocol messages that have been defined up to now are the
266	   following:

268	   o  OPTIONS

270	   o  OPTIONS RESPONSE

272	   o  ADVERTISEMENT (ADV)

274	   o  ADVERTISEMENT ACKNOWLEDGE (ACK)
275	   o  CONFIGURE (CONF)

277	   o  CONFIGURE RESPONSE

279	   o  READV

281	   o  READV RESPONSE

283	   While the OPTIONS and OPTIONS RESPONSE messages are exchanged in the
284	   initiation phase between the CPs, the other messages are involved in
285	   MP-MC dialogues.

287	   Each CLUE message inherits a basic structure depicted in the
288	   following figure:

290	<!-- CLUE MESSAGE TYPE -->
291	<xs:complexType name="clueMessageType" abstract="true">
292	<xs:sequence>
293	<xs:element name="clueId" type="xs:string"/>
294	<xs:element name="sequenceNr" type="xs:unsignedInt"/>
295	</xs:sequence>
296	<xs:attribute name="protocol" type="xs:string" fixed="CLUE" use="required"/>
297	<xs:attribute name="v" type="xs:string" use="required"/>
298	</xs:complexType>

300	   The basic structure determines the mandatory information that is
301	   carried within each CLUE message.  Such an information is made by:

303	   o  clueId: an XML element containing the identifier of the CP within
304	      the telepresence system;

306	   o  sequenceNr: an XML element containing the local message sequence
307	      number;

309	   o  protocol: a mandatory attribute set to "CLUE" identifying the
310	      procotol the messages refer to;

312	   o  v: a mandatory attribute carrying the version of the protocol

314	   Each CP should manage uo to three streams of sequence numbers: (i)
315	   one for the messages exchanged in the initiation phase, (ii) one for
316	   the messages exchanged as MP, and (iii) one for the messages
317	   exchanged as MC.

319	4.1.  OPTIONS

321	   The OPTIONS message is sent by the CP which is the CI to the CP which
322	   is the CR as soon as the CLUE data channel is ready.  Besides the
323	   information envisioned in the basic structure, it specifies:

325	   o  mediaProvider: a mandatory boolean field set to "true" if the CP
326	      is able to act as a MP

328	   o  mediaConsumer: a mandatory boolean field set to "true" if the CP
329	      is able to act as a MC

331	   o  supportedVersions: the list of the supported versions

333	   o  supportedOptions: the list of the supported options

335	   The XML Schema of such a message is reported below:

337	<!-- CLUE OPTIONS -->
338	<xs:complexType name="optionsMessageType">
339	<xs:complexContent>
340	<xs:extension base="clueMessageType">
341	<xs:sequence>
342	<xs:element name="mediaProvider" type="xs:boolean"/>
343	<xs:element name="mediaConsumer" type="xs:boolean"/>
344	<xs:element name="supportedVersions" type="versionsListType" minOccurs="0"/>
345	<xs:element name="supportedOptions" type="optionsListType" minOccurs="0"/>
346	<xs:any namespace="##other" processContents="lax" minOccurs="0"/>
347	</xs:sequence>
348	<xs:anyAttribute namespace="##other" processContents="lax"/>
349	</xs:extension>
350	</xs:complexContent>
351	</xs:complexType>

353	<!-- VERSIONS LIST TYPE -->
354	<xs:complexType name="versionsListType">
355	<xs:sequence>
356	<xs:element name="version" type="xs:string" minOccurs="1"
357	 maxOccurs="unbounded"/>
358	<xs:any namespace="##other" processContents="lax" minOccurs="0"/>
359	</xs:sequence>
360	<xs:anyAttribute namespace="##other" processContents="lax"/>
361	</xs:complexType>

363	<!-- OPTIONS LIST TYPE -->
364	<xs:complexType name="optionsListType">
365	<xs:sequence>
366	<xs:element name="option" type="optionType" minOccurs="1"
367	 maxOccurs="unbounded"/>
368	<xs:any namespace="##other" processContents="lax" minOccurs="0"/>
369	</xs:sequence>
370	<xs:anyAttribute namespace="##other" processContents="lax"/>
371	</xs:complexType>

373	<!-- OPTION TYPE -->
374	<xs:complexType name="optionType">
375	<xs:sequence>
376	<xs:element name="name" type="xs:string" />
377	<xs:element name="schemaRef" type="xs:anyURI" minOccurs="0"/>
378	<xs:element name="version" type="xs:string" minOccurs="0"/>
379	<xs:any namespace="##other" processContents="lax" minOccurs="0"/>
380	</xs:sequence>
381	<xs:anyAttribute namespace="##other" processContents="lax"/>
382	</xs:complexType>
383	   <supportedVersions> contains the list of the versions that are
384	   supported by the CI.  Only one <version> element SHOULD be provided
385	   for each major version supported, containing the maximum minor
386	   version number of such a version, since all minor versions are
387	   backward compatible.  If no <supportedVersions> is carried whithin
388	   the OPTIONS message, the CI supports only the version declared in the
389	   "v" attribute.  For example, if the "v" attribute has a value of
390	   "3.4" and there is not a <supportedVersions> tag in the OPTIONS
391	   message, it means the CI supports only major version 3 with all the
392	   minor versions comprised between 3.0 the 3.4 included.  If a
393	   <supportedVersion> is provided, at least one <version> tag MUST be
394	   included.

396	   The <supportedOptions> element specifies the list of the options
397	   supported by the CI.  If there is no <supportedOptions> in the
398	   OPTIONS message, the CI does not support anything more than what is
399	   envisioned in the versions it supports.  For each option, an <option>
400	   element is provided.  An option is characterized by a name, an XML
401	   schema of reference where the option is defined, and the version of
402	   the protocol which the option refers to. [to be discussed: difference
403	   between options and extensions]

405	4.2.  OPTIONS RESPONSE

407	   The OPTIONS RESPONSE is sent by a CR to a CI as a reply to the
408	   OPTIONS message.  As depicted in the figure below, the OPTIONS
409	   RESPONSE contains mandatorily a response code and a response string
410	   indicating the processing result of the OPTIONS message.  Following,
411	   the CR attaches two boolean tags, <mediaProvider> and
412	   <mediaConsumer>, expressing the supported roles in terms of
413	   respectively MP and MC, similarly to what the CI does in the OPTIONS
414	   message.  Finally, the highest commonly supported version number is
415	   expressed in the <version> field and just the commonly supported
416	   options in the <commonOptions> field.

418	 <!-- CLUE OPTIONS RESPONSE (2 WAY) -->
419	 <xs:complexType name="optionsResponseMessageType">
420	 <xs:complexContent>
421	 <xs:extension base="clueMessageType">
422	 <xs:sequence>
423	 <xs:element name="responseCode" type="xs:short"/>
424	 <xs:element name="reasonString" type="xs:string"/>
425	 <xs:element name="mediaProvider" type="xs:boolean" minOccurs="0"/>
426	 <xs:element name="mediaConsumer" type="xs:boolean" minOccurs="0"/>
427	 <xs:element name="version" type="xs:string" minOccurs="0"/>
428	 <xs:element name="commonOptions" type="optionsListType" minOccurs="0"/>
429	 <xs:any namespace="##other"
430	 processContents="lax" minOccurs="0"/>
431	 </xs:sequence>
432	 <xs:anyAttribute namespace="##other" processContents="lax"/>
433	 </xs:extension>
434	 </xs:complexContent>
435	 </xs:complexType>

437	   After the reception of such message, the version to be used is
438	   determined by each part of the conversation.  Indeed, it is the one
439	   provided in the <version> tag of the OPTIONS RESPONSE message.  The
440	   following CLUE messages will use such a version number in the "v"
441	   attribute.  The allowed options in the CLUE dialogue will be those
442	   indicated in the <commonOptions> of the OPTIONS RESPONSE message.

444	4.3.  ADVERTISEMENT

446	   This message is used by the MP to advertise the available media
447	   captures and related information to the MC.  The MP sends to the MC
448	   an ADV as soon as it is ready after the successful completion of the
449	   initiation phase.  During the telepresence session, the ADV can be
450	   sent from the MP both periodically and on a per-event basis, i.e.,
451	   each time there are changes in the MP's CLUE telepresence
452	   capabilities.

454	   The ADV structure is defined in the picture below.  The ADV contains
455	   elements compliant with the CLUE data model that characterize the
456	   MP's telepresence offer.  Namely, such elements are: the list of the
457	   media captures (<mediaCaptures>), of the encoding groups
458	   (>encodingGroups>), of the capture scenes (>captureScenes>) and of
459	   the global capture entries (>globalCaptureEntries>), and the list of
460	   the represented participants (>participants>).  Each of them is fully
461	   described in the CLUE framework document and formally defined in the
462	   CLUE data model document.

464	<!-- CLUE ADVERTISEMENT MESSAGE TYPE -->
465	<xs:complexType name="advertisementMessageType">
466	<xs:complexContent>
467	<xs:extension base="clueMessageType">
468	<xs:sequence>
469	<!-- mandatory fields -->
470	<xs:element name="mediaCaptures" type="dm:mediaCapturesType"/>
471	<xs:element name="encodingGroups" type="dm:encodingGroupsType"/>
472	<xs:element name="captureScenes" type="dm:captureScenesType"/>
473	<xs:element name="simultaneousSets" type="dm:simultaneousSetsType"
474	 minOccurs="0"/>
475	<xs:element name="globalCaptureEntries" type="dm:globalCaptureEntriesType"
476	 minOccurs="0"/>
477	<xs:element name="participants" type="dm:participantsType" minOccurs="0"/>
478	<xs:any namespace="##other" processContents="lax" minOccurs="0"/>
479	</xs:sequence>
480	<xs:anyAttribute namespace="##other" processContents="lax"/>
481	</xs:extension>
482	</xs:complexContent>
483	</xs:complexType>

485	   [to be discussed: a "delta" mechanism for advertising only the
486	   changes with respect to the previous notification should be adopted.
487	   Similar approaches have been proposed for partial notifications in
488	   centralized conferencing frameworks ([RFC6502]), leveraging the XML
489	   diff codification mechanism defined in [RFC5261]].

491	4.4.  ADVERTISEMENT ACKNOWLEDGEMENT

493	   The ACK message is sent by a MC to a MP to acknowledge an ADV
494	   message.  As it can be seen from the message schema provided in the
495	   following, the ACK contains a response code and a reason string for
496	   describing the processing result of the ADV.  The <advSequenceNr>
497	   carries the sequence number of the ADV the ACK refers to.

499	   <!-- ADV ACK MESSAGE TYPE -->
500	   <xs:complexType name="advAcknowledgementMessageType">
501	   <xs:complexContent>
502	   <xs:extension base="clueMessageType">
503	   <xs:sequence>
504	   <xs:element name="responseCode" type="xs:short"/>
505	   <xs:element name="reasonString" type="xs:string"/>
506	   <xs:element name="advSequenceNr" type="xs:unsignedInt"/>
507	   <xs:any namespace="##other" processContents="lax" minOccurs="0"/>
508	   </xs:sequence>
509	   <xs:anyAttribute namespace="##other" processContents="lax"/>
510	   </xs:extension>
511	   </xs:complexContent>
512	   </xs:complexType>

514	4.5.  CONFIGURE

516	   The CLUE CONFIGURE message is sent from a MC to a MP to list the
517	   advertised captures the MC wants to receive.  The MC can send a CONF
518	   after the reception of an ADV or each time it wants to request other
519	   captures that have been previously advertised by the MP.  The content
520	   of the CONF message is shown below.

522	    <!-- CLUE CONFIGURE MESSAGE TYPE -->
523	   <xs:complexType name="configureMessageType">
524	   <xs:complexContent>
525	   <xs:extension base="clueMessageType">
526	   <xs:sequence>
527	   <!-- mandatory fields -->
528	   <xs:element name="advSequenceNr" type="xs:unsignedInt"/>
529	   <xs:element name="ack" type="xs:boolean" minOccurs="0" fixed="true"/>
530	   <xs:element name="captureEncodings" type="dm:captureEncodingsType"
531	    minOccurs="0"/>
532	   <xs:any namespace="##other" processContents="lax" minOccurs="0"/>
533	   </xs:sequence>
534	   <xs:anyAttribute namespace="##other" processContents="lax"/>
535	   </xs:extension>
536	   </xs:complexContent>
537	   </xs:complexType>

539	   In the >advSequenceNr< element is contained the sequence number of
540	   the ADVERTISEMENT or of the READV RESPONSE message the CONFIGURE
541	   refers to.

543	   The optional boolean <ack> element, set to "true", if present,
544	   indicates that the CONF message also acknowledge the referred
545	   advertisement, by applying in that way a piggibacking mechanism for
546	   simultaneously acknowledging and replying to the ADV message.  The
547	   <ack> element SHOULD not be present at all if an ADV ACK message has
548	   been already sent back to the MP and if the CONFIGURE refers to a
549	   READV RESPONSE message.

551	   The most important content of the CONFIGURE message is the list of
552	   the capture encodings provided in the <captureEncodings> element.
553	   Such an element is defined in the CLUE data model document and
554	   contains a sequence of capture encodings, representing the streams to
555	   be instantiated.

557	4.6.  CONFIGURE RESPONSE

559	   <!-- CONFIGURE RESPONSE MESSAGE TYPE -->
560	   <xs:complexType name="configureResponseMessageType">
561	   <xs:complexContent>
562	   <xs:extension base="clueMessageType">
563	   <xs:sequence>
564	   <xs:element name="responseCode" type="xs:short"/>
565	   <xs:element name="reasonString" type="xs:string"/>
566	   <xs:element name="confSequenceNr" type="xs:integer"/>
567	   <xs:any namespace="##other" processContents="lax" minOccurs="0"/>
568	   </xs:sequence>
569	   <xs:anyAttribute namespace="##other" processContents="lax"/>
570	   </xs:extension>
571	   </xs:complexContent>
572	   </xs:complexType>

574	   The CONF RESPONSE message is sent from the MP to the MC to
575	   communicate the processing result of requests carried in the
576	   previously received CONF message.  It contains a response code with a
577	   reason string indicating either the success or the failure (along
578	   with failure details) of a CONF request processing.  Following, the
579	   <confSequenceNr> field contains the number of the CONF message the
580	   response refers to.

582	4.7.  READV

584	   The READV message is a request the MC issues to the MP to retrieve an
585	   updated version of the MP's telepresence offer.  The content of the
586	   READV message is specified in the following.

588	   <!-- CLUE READV MESSAGE TYPE -->
589	   <xs:complexType name="readvMessageType">
590	   <xs:complexContent>
591	   <xs:extension base="clueMessageType">
592	   <xs:sequence>
593	   <xs:element name="lastReceivedAdv" type="xs:short"/>
594	   <xs:any namespace="##other" processContents="lax" minOccurs="0"/>
595	   </xs:sequence>
596	   <xs:anyAttribute namespace="##other" processContents="lax"/>
597	   </xs:extension>
598	   </xs:complexContent>
599	   </xs:complexType>

601	   The <lastReceivedAdv> element specifies the sequence number of the
602	   last ADVERTISEMENT or READV RESPONSE correctly received by the MC.

604	4.8.  READV RESPONSE

606	   The READV RESPONSE is sent by the MP to the MC to reply to a READV
607	   message.  As shown in the schema below, it contains, besides a
608	   response code and a reason string, all the information carried within
609	   an ADVERTISEMENT message (media captures, encoding groups, and so
610	   on).  If there are no updates with respect to the last telepresence
611	   offer successfully delivered to the MC (i.e, that having the sequence
612	   number specified in the <lastReceiveAdv> field of the READV message),
613	   the READV RESPONSE SHOULD carry only the response code with the
614	   reason string.

616	<!-- CLUE READV RESPONSE MESSAGE TYPE -->
617	<xs:complexType name="readvResponseMessageType">
618	<xs:complexContent>
619	<xs:extension base="clueMessageType">
620	<xs:sequence>
621	<xs:element name="responseCode" type="xs:short"/>
622	<xs:element name="reasonString" type="xs:string"/>
623	<xs:element name="readvSequenceNr" type="xs:string" minOccurs="0"/>
624	<xs:element name="mediaCaptures" type="dm:mediaCapturesType" minOccurs="0"/>
625	<xs:element name="encodingGroups" type="dm:encodingGroupsType" minOccurs="0"/>
626	<xs:element name="captureScenes" type="dm:captureScenesType" minOccurs="0"/>
627	<xs:element name="simultaneousSets" type="dm:simultaneousSetsType" minOccurs="0"/>
628	<xs:element name="globalCaptureEntries" type="dm:globalCaptureEntriesType" minOccurs="0"/>
629	<xs:element name="participants" type="dm:participantsType" minOccurs="0"/>
630	<xs:any namespace="##other"
631	 processContents="lax" minOccurs="0"/>
632	</xs:sequence>
633	</xs:extension>
634	</xs:complexContent>
635	</xs:complexType>

637	4.9.  Response codes and reason strings

639	   Examples of response codes and strings are provided in the following
640	   table.  Response codes can be designed by adhering to the HTTP
641	   semantics, as shown below.

643	   +-----------------+----------------------+--------------------------+
644	   |                 |                      |                          |
645	   |  Response code  |  Response string     |       Description        |
646	   |                 |                      |                          |
647	   +-----------------+----------------------+--------------------------+
648	   |                 |                      |                          |
649	   |   410           |  Bad syntax          |  The XML syntax of the   |
650	   |                 |                      |  CONF message is not     |
651	   |                 |                      |  correct.                |
652	   +-----------------+----------------------+--------------------------+
653	   |                 |                      |                          |
654	   |   411           |  Invalid value       |  The CONF message        |
655	   |                 |                      |  contains an invalid     |
656	   |                 |                      |  parameter value.        |
657	   +-----------------+----------------------+--------------------------+
658	   |                 |                      |                          |
659	   |   412           |  Invalid identifier  |  The identifier used for |
660	   |                 |                      |  requesting a capture is |
661	   |                 |                      |  not valid or unknown.   |
662	   +-----------------+----------------------+--------------------------+
663	   |                 |                      |                          |
664	   |   413           |  Conflicting values  |  The CONF message        |
665	   |                 |                      |  contains values that    |
666	   |                 |                      |  cannot be used together.|
667	   +-----------------+----------------------+--------------------------+
668	   |                 |                      |                          |
669	   |   420           |  Invalid sequencing  |  The sequence number of  |
670	   |                 |                      |  the CONF message is out |
671	   |                 |                      |  of date or corresponds  |
672	   |                 |                      |  to an obsoleted ADV.    |
673	   +-----------------+----------------------+--------------------------+
674	   |                 |                      |                          |
675	   |   510           | Version not supported|  The CLUE protocol       |
676	   |                 |                      |  version of the CONF     |
677	   |                 |                      |  message is not supported|
678	   |                 |                      |  by the MP.              |
679	   +-----------------+----------------------+--------------------------+
680	   |                 |                      |                          |
681	   |   511           | Option not supported |  The option requested in |
682	   |                 |                      |  the CONF message is not |
683	   |                 |                      |  supported by the MP.    |
684	   +-----------------+----------------------+--------------------------+

686	   ...  TBC.

688	   +---------------+------------------------+
689	   |               |                        |
690	   | Response code | Description            |
691	   | family        |                        |
692	   +---------------+------------------------+
693	   |               |                        |
694	   |     1XX       | Temporary info         |
695	   |               |                        |
696	   +---------------+------------------------+
697	   |               |                        |
698	   |     2XX       | Success                |
699	   |               |                        |
700	   +---------------+------------------------+
701	   |               |                        |
702	   |     3XX       | Redirection            |
703	   |               |                        |
704	   +---------------+------------------------+
705	   |               |                        |
706	   |     4XX       | Client error           |
707	   |               |                        |
708	   +---------------+------------------------+
709	   |               |                        |
710	   |     5XX       | Server error           |
711	   |               |                        |
712	   +---------------+------------------------+

714	5.  Protocol state machines

716	   The CLUE protocol is an application protocol used between two CPs in
717	   order to properly configure a multimedia telepresence session.  CLUE
718	   protocol messages flow upon the CLUE Data Channel, a DTLS/SCTP
719	   channel established as depicted in [I-D.kyzivat-clue-signaling].
720	   Over such a channel there are typically two CLUE streams between the
721	   channel terminations flowing in opposite directions.  In other words,
722	   typically, both channel terminations act simultaneously as a MP and
723	   as a MC.  We herein discuss the state machines associated,
724	   respectively, with the CLUE Participant, with MC process and with the
725	   MP process.

727	6.  CLUE Participant's state machine

729	   The main state machines focus on describing the states of CLUE
730	   channel from a CLUE channel initiator/receiver.  In the IDLE state,
731	   when the CP has established a CLUE channel, the main state moves to
732	   the ESTABLISHED state.  When in the ESTABLISHED state, if the CP is
733	   the Channel Initiator (CI), it prepares sending an OPTIONS message
734	   for version negotiation; otherwise, if the is the Channel Receiver
735	   (CR), it listens to the channel for an OPTIONS message for version
736	   negotiation.  If an OPTIONS message is sent or is received, the CP
737	   moves to the NEGOTIATING state.  If the CP checks some error in the
738	   request message received, the main state goes back to the IDLE state.
739	   [TODO: check this] When in the NEGOTIATING state, the CR prepares an
740	   OPTIONS RESPONSE message while the CI listens to the channel for an
741	   OPTIONS RESPONSE.  If an OPTIONS RESPONSE message for version
742	   negotiation is sent or is received, the main state moves to the
743	   ACTIVE state.  If the CI checks some error in the OPTIONS RESPONSE
744	   message received or receives an OPTIONS RESPONSE indicating an error,
745	   it goes back to the IDLE state.  When the CP enters in the ACTIVE
746	   state, it creates two sub state machines which are the MC state
747	   machine and the MP state machine, accordingly to the supported roles.
748	   When in the ACTIVE state, if the CP receives a further OPTIONS
749	   message for version negotiation or a further OPTIONS RESPONSE
750	   messages for version negotiation, it MUST ignore the messages and
751	   keep in the ACTIVE state.  When in the ACTIVE state, the CP delegates
752	   the sending and the processing of the CLUE messages the appropriate
753	   MP/MC sub-state machines.  The TERMINATED state is reachable from
754	   each of the aforementioned states whenever the session is canceled or
755	   released.  The IDLE state is reachable from each of the
756	   aforementioned states whenever the underlying channel is closed due
757	   to connection error.  [TODO: CLUE messages to cancel/release the
758	   session] [TODO: check the diagram]
759	                      +-------------+<------------------------------+---------+
760	         +----------->+     IDLE    +<--------------------------+   + TIMEOUT +
761	         |            +------+------+<-----------+              |   +------+--+
762	         |                   |                   |              |          |
763	         |                   |                   |              |          |
764	     Connection            CLUE                  |              |          |
765	       error              channel                |              |          |
766	         |           has been established        |              |          |
767	         |                   |                   |              |          |
768	         |                   V               Receive error      |          |
769	         +------------+-------------+     (version mismatch,    |          |
770	   +------------------+ ESTABLISHED +      missing elements,...)  |       time out
771	   |                  +------+------+            |              |          |
772	   |                         |                   |          Connection     |
773	   |                         |                   |             error       |
774	   |                  Send/Receive OPTIONS       |              |          |
775	   |                         |                   |              |          |
776	   |                         V                   |              |          |
777	   |                  +-------------+------------+              |          |
778	   |     +------------+ NEGOTIATING +---------------------------+          |
779	   |     |            +------+------+---------------------------|----------+
780	   |     |                   |                                  |
781	   |     |                   |                                  |
782	   |     |             Receive/Send                        Connection
783	   |     |            OPTIONS RESPONSE                       error
784	Session  |                   |                                  |
785	  ends   |                   V                                  |
786	   |     |            +-------------+---------------------------+
787	   |     |            |   ACTIVE    +<-------------------+
788	   |     |            |  +-------+  |           Receive OPTIONS/
789	   |     |            |  |SUBIDLE|  |             OPTIONS RESPONSE
790	   |   Session        |  |MC     |  +--------------------+
791	   |    ends          |  +-------+  |
792	   |     |            |  +-------+  +<-------------------+
793	   |     |            |  |SUBIDLE|  |       Send/Receive other CLUE messages
794	   |     |            |  |MP     |  |                    |
795	   |     |            |  +-------+  |                    |
796	   |     |            +------+------+--------------------+
797	   |     |                   |
798	   |     |                   |
799	   |     |                Session
800	   |     |                  ends
801	   |     |                   |
802	   |     |                   V
803	   |     |            +-------------+
804	   |     +----------->+ TERMINATED  +
805	   +----------------->+-------------+

807	6.1.  Media Consumer's state machine

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

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

818	   When the ADV has been received ("receive ADV"), the MC goes into the
819	   ADV RECEIVED state.  The ADV is then parsed.  If something goes wrong
820	   with the ADV (bad syntax, missing XML elements, etc.), the MC sends a
821	   NACK message (an ACK with an error response code) to the MP
822	   specifying the encountered problem via a proper reason phrase.  In
823	   this way, the MC switches back to the WAIT FOR ADV state, waiting for
824	   a new copy of the ADV.  If the ADV is successfully processed, the MC
825	   issues a successful ACK message to the MP and moves to the ADV ACKED
826	   state.  When the SDP information arrives, from the ADV RECEIVED or
827	   the ADV ACKED state the MC switches to the READY TO CONF state.  When
828	   the CONF request is ready, the MC sends it and moves to the TRYING
829	   state.  If the ADV has not been already sent, the MC can piggyback
830	   the ACK message within the CONF request.

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

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

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

858	                                                                      +-----+
859	                     +---------------+-------------timeout------>+----+--+  |
860	                     | WAIT FOR ADV  |<----+                     |TIMEOUT|  |
861	                     +---------------+<----+--------send---------+-------+  |
862	                                |          |     RE-ADV(refresh)      ^     |
863	                                |          |                          |     |
864	                                |          |                          |     |
865	                             receive     send                         |     |
866	                               ADV       NACK                         |     |
867	           +---receive-------+  |       (missing elements,            |     |
868	           |   error RESP    |  |        invalid area,...)            |     |
869	           |                 v  v          |                          |     |
870	           +----------------+---------+----+      +--------+          |     |
871	          +---------------->|  ADV    |---send--->|  ADV   |      timeout   |
872	          |                 | RECEIVED|   ACK     | ACKED  |          |     |
873	          |     +---------->|         |           |        |          |     |
874	          |    recv  +----->+-----+---+<--recv----+----+---+          |     |
875	          |   error  |            |       ADV          |              |     |
876	          |    CONF  |            |                    |              |     |
877	          |     |    |         SDP info            SDP info           |     |
878	          |     |    |         received             received          |     |
879	          |     |    |            |     +--------+     |              |     +
880	          +     |    +            +---->|READY TO|<----+              +     |
881	          |     |    |                  | CONF   |                    |     |
882	          |     +    |            +-----+--------+-----+              +     |
883	          |     |    |            |                    +              +     |
884	          |     |    +            |                    |              +     +
885	          +     +    |            +                    |              |     |
886	      receive   |    |           send              send|              |     |
887	        ADV     |    |         CONF+ACK            CONF|              |     |
888	          |     |    |            |                    |              |     |
889	          |     |  receive        v                    |              |     |
890	          |     |  ADV         +-----------+<----------+              |     |
891	          |     |    |         |           |+-------------------------+     |
892	          |     +----|--------+|  TRYING   |                                |
893	          +----------|---------|           |                                |
894	                 +---|---------+-----------+                                |
895	                 |   |          |          ^                                |
896	                 |   |          |          |                                |
897	                 |   |          |          |                                |
898	          receive|   |         receive    send                          retry
899	          error RESP,|          200 OK    CONF                        expires
900	          retry  |   |          |          |                                |
901	          expired|   |          |          |                                |
902	                 |   |          |          |                                |
903	                 |   |          v          |                                |
904	                 |   |       +---------+   |                                |
905	                 |   +-------| CONF    |   |                                |
906	                 |           |COMPLETED|---+                                |
907	                 |           +---------+                                    |
908	                 |              |                                           |
909	                 |              |                                           |
910	                 |              |                                           |
911	                 |              |                                           |
912	                 |            connection                                    |
913	                 |             closed                                       |
914	                 |              |                                           |
915	                 |              |                                           |
916	                 |              |                                           |
917	                 |              |                                           |
918	                 |              |                                           |
919	                 |              v                                           |
920	                 |            +----------+<---------------------------------+
921	                 +----------->+TERMINATED|
922	                              +----------+

924	6.2.  Media Provider's state machine

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

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

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

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

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

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

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

970	                    +-----------+
971	                    |           |
972	                    | PREPARING |
973	 +----------------->|   ADV     |<--------------------------+
974	 |   +------------->|           |<-----------retry----------+---------+
975	 |   |       +----->|           |<--+         not           |         |
976	 |   |       |      +-----------+   |        expired        |         |
977	 |   |       |            |         |                       |         |
978	 |   |  change          send       receive                  |        ++------+
979	 |   |  telepresence     ADV        NACK                    |        |TIMEOUT|
980	 |   |  settings          |         |                       |        ++--+---+
981	 |   |       |            |         |                       |         ^  |
982	 |   |       |            v         |                       |         |  |
983	 |   |       |    +-------------+---+                       |         |  |
984	 |   |       +----+  WAIT FOR   +------------timeout--------+---------+  |
985	 |   |         +--+     ACK     |                           |            |
986	 change        |  +-------+-----+                           |            |
987	 telepresence  |          |                                 |            |
988	 settings      |        recv                                |            +
989	 +   +         |        ACK                                 +            |
990	 |   |         |          |                                 |            |
991	 |   |         |          v                                 |            |
992	 |   |         |     +-----------+                          |            |
993	 |   |      recv     | WAIT FOR  |                          |            |
994	 |   |         |     |   CONF    |<-+                       |            |
995	 |   |    CONF+ACK   +----+------+  |                       |            |
996	 |   |         |          |         +                       |            |
997	 +   |         |        receive     CONF error,             +            |
998	 |   +         |         CONF       retry not expired,      |            +
999	 |   |         |          |         send error RESP         |            |
1000	 |   |         |          |         |                       |            |
1001	 |   |         |          |         |                       |            |
1002	 |   |         |          v         |                       |            |
1003	 |   |         +--->+-----------+---+                       |            |
1004	 +---+-------------+|   CONF    |                           |            |
1005	     |       +----->| RECEIVED  |----CONF error,            |            |
1006	     |       |      +-----+-----+    retry expired          |            |
1007	     |       |            |          +                      |            |
1008	     |       |            |          |                      |            |
1009	     |       |            |          |                      |            |
1010	 receive   receive      send         |                      |            |
1011	 RE-ADV     CONF       200 OK        |                      |       retry|
1012	     |       |            |          |                      |      expired
1013	     |       |            |          |                      |            |
1014	     |       |            |          |                      |            |
1015	     |       |            v          |                      |            |
1016	     |       |       +----------+    |   change             |            |
1017	     |       +-------|  CONF    |----|---telepresence-------+            |
1018	     +---------------| COMPLETED|    |   settings                        |
1019	                     +----------+    |                                   |
1020	                          |          |                                   |
1021	                          |          |                                   |
1022	                          |          |                                   |
1023	                       connection    |                                   |
1024	                        closed       |                                   |
1025	                          |          |                                   |
1026	                          |          |                                   |
1027	                          v          |                                   +
1028	                 +----------------+<-+                                   |
1029	                 |   TERMINATED   |<-------------------------------------+
1030	                 |                |
1031	                 +----------------+

1033	7.  Versioning

1035	   CLUE protocol messages are XML messages compliant to the CLUE
1036	   protocol XML schema.  The version of the protocol corresponds to the
1037	   version of the schema.  Both client and server have to test the
1038	   compliance of the received messages with the XML schema of the CLUE
1039	   protocol.  If the compliance is not verified, the message cannot be
1040	   processed further.

1042	   Obviously, client and server can not communicate if they do not share
1043	   exactly the same XML schema.  Such a schema is the one included in
1044	   the yet to come RFC, and associated with the CLUE URN
1045	   "urn:ietf:params:xml:ns:clue-message".  If all CLUE-enabled devices
1046	   use that schema there will be no interoperability problems due to
1047	   schema issues.

1049	   The version of the XML schema contained in the standard document
1050	   deriving from this draft will be 1.0.  The version usage is similar
1051	   in philosophy to XMPP (RFC6120).  A version number has major and
1052	   minor components, each a non-negative integer.  Major version changes
1053	   denote non-interoperable changes.  Minor version changes denote
1054	   schema changes that are backward compatible by ignoring unknown XML
1055	   elements, or other backward compatible changes.

1057	   The minor versions of the XML schema MUST be backward compatible, not
1058	   only in terms of schema but also semantically and procedurally as
1059	   well.  This means that they should define further features and
1060	   functionality besides those defined in the previous versions, in an
1061	   incremental way, without impacting the basic rules defined in the
1062	   previous version of the schema.  In this way, if a MP is able to
1063	   speak, e.g., version 1.5 of the protocol while the MC only
1064	   understands version 1.4, the MP should have no problem in reverting
1065	   the dialogue to version 1.4 without exploiting 1.5 features and
1066	   functionality.

1068	   It is expected that, before the CLUE protocol XML schema reaches a
1069	   steady state, prototypes developed by different organizations will
1070	   conduct interoperability testing.  In that case, in order to
1071	   interoperate, they have to be compliant to the current version of the
1072	   XML schema, i.e., the one copied in the most up-to-date version of
1073	   the draft defining the CLUE protocol.  The versions of the non-
1074	   standard XML schema will be numbered as 0.01, 0.02, and so on.
1075	   During the standard development phase, the versions of the XML schema
1076	   will probably not be backward compatible so it is left to prototype
1077	   implementers the responsibility of keeping their products up to date.

1079	8.  Extensions and options

1081	   Although the standard version of the CLUE protocol XML schema will be
1082	   designed to thoroughly cope with the requirements emerging from the
1083	   application domain, new needs might arise and extensions can be
1084	   designed.  Extensions specify information and behaviors that are not
1085	   described in a certain version of the protocol.  They can relate to:

1087	      the information carried in the existing messages (for example, we
1088	      may want to add more fields within an existing message);

1090	      the meaning of the messages.  This is the case if there is no
1091	      proper message for a certain task, so a brand new CLUE message
1092	      needs to be defined.

1094	   As to the first type of extensions, it is possible to distinguish
1095	   between protocol specific- and data model information.  Indeed, CLUE
1096	   messages are envelopes carrying both:

1098	      (i) XML elements defined within the CLUE protocol XML schema
1099	      itself (protocol-specific information)

1101	      (ii) other XML elements compliant to the CLUE data model schema
1102	      (data model information)

1104	   When new protocol-specific information is needed somewhere in the
1105	   protocol messages, it can be added in place of the <any> elements and
1106	   <anyAttribute> elements envisioned by the protocol schema.  The
1107	   policy currently defined in the protocol schema for handling <any>
1108	   and <anyAttribute> elements is:

1110	      elementFormDefault="qualified"

1112	      attributeFormDefault="unqualified"

1114	   In that case, the new information must be qualified by namespaces
1115	   other than "urn:ietf:params:xml:ns:clue-message" (the protocol URN)
1116	   and "urn:ietf:params:xml:ns:clue-info" (the data model URN).
1117	   Elements or attributes from unknown namespaces MUST be ignored.

1119	   The other matter concerns data model information.  Data model
1120	   information is defined by the XML schema associated with the URN
1121	   "urn:ietf:params:xml:ns:clue-info".  Also for the XML elements
1122	   defined in such a schema there are extensibility issues.  Those
1123	   issues are overcome by using <any> and <anyAttribute> placeholders.
1124	   Similarly to what said before, new information within data model
1125	   elements can be added in place of <any> and <anyAttribute> schema
1126	   elements, as long as they are properly namespace qualified.

1128	   On the other hand (second type of extensions), "extra" CLUE protocol
1129	   messages, i.e., messages not envisioned in the last standard version
1130	   of the schema, can be needed.  In that case, the messages and the
1131	   associated behavior should be defined in external documents that both
1132	   the communication parties must be aware of.

1134	   Both the types of extensions, i.e., the information and the protocol
1135	   extensions, can be characterized by:

1137	      a name;

1139	      an external XML Schema defining the XML information and/or the XML
1140	      messages representing the extension;

1142	      the standard version of the protocol the extension refers to.

1144	   For that reason, the extensions can be represented by means of the
1145	   <option> element as defined below, which is carried within the
1146	   OPTIONS and OPTIONS RESPONSE messages to represent the extensions
1147	   supported by the CI and by the CR.

1149	   <!-- OPTION TYPE -->
1150	   <xs:complexType name="optionType">
1151	   <xs:sequence>
1152	   <xs:element name="name" type="xs:string" />
1153	   <xs:element name="schemaRef" type="xs:anyURI" minOccurs="0"/>
1154	   <xs:element name="version" type="xs:string" minOccurs="0"/>
1155	   <xs:any namespace="##other" processContents="lax" minOccurs="0"/>
1156	   </xs:sequence>
1157	   <xs:anyAttribute namespace="##other" processContents="lax"/>
1158	   </xs:complexType>

1160	9.  XML Schema

1162	   In this section, the XML schema defining the CLUE messages is
1163	   provided.

1165	<?xml version="1.0" encoding="UTF-8" ?>
1166	<xs:schema
1167	version="0.02"
1168	targetNamespace="urn:ietf:params:xml:ns:clue-message"
1169	xmlns:tns="urn:ietf:params:xml:ns:clue-message"
1170	xmlns:xs="http://www.w3.org/2001/XMLSchema"
1171	xmlns:dm="urn:ietf:params:xml:ns:clue-info"
1172	xmlns="urn:ietf:params:xml:ns:clue-message"
1173	elementFormDefault="qualified"
1174	attributeFormDefault="unqualified">

1176	<!-- Import data model schema -->
1177	<xs:import namespace="urn:ietf:params:xml:ns:clue-info"
1178	schemaLocation="data-model-schema-05.xsd"/>

1180	<!-- ELEMENT DEFINITIONS -->
1181	<xs:element name="options" type="optionsMessageType"/>
1182	<xs:element name="optionsResponse" type="optionsResponseMessageType"/>
1183	<!--<xs:element name="optionsAck" type="optionsAcknowledgementMessageType"/>-->
1184	<xs:element name="advertisement" type="advertisementMessageType"/>
1185	<xs:element name="ack" type="advAcknowledgementMessageType"/>
1186	<xs:element name="configure" type="configureMessageType"/>
1187	<xs:element name="configureResponse" type="configureResponseMessageType"/>
1188	<xs:element name="readv" type="readvMessageType"/>
1189	<xs:element name="readvResponse" type="readvResponseMessageType"/>

1191	<!-- CLUE MESSAGE TYPE -->
1192	<xs:complexType name="clueMessageType" abstract="true">
1193	<xs:sequence>
1194	<xs:element name="clueId" type="xs:string"/>
1195	<xs:element name="sequenceNr" type="xs:unsignedInt"/>
1196	</xs:sequence>
1197	<xs:attribute name="protocol" type="xs:string" fixed="CLUE" use="required"/>
1198	<xs:attribute name="v" type="xs:string" use="required"/>
1199	</xs:complexType>

1201	<!-- CLUE OPTIONS -->
1202	<xs:complexType name="optionsMessageType">
1203	<xs:complexContent>
1204	<xs:extension base="clueMessageType">
1205	<xs:sequence>
1206	<xs:element name="mediaProvider" type="xs:boolean"/>
1207	<xs:element name="mediaConsumer" type="xs:boolean"/>
1208	<xs:element name="supportedVersions" type="versionsListType" minOccurs="0"/>
1209	<xs:element name="supportedOptions" type="optionsListType" minOccurs="0"/>
1210	<xs:any namespace="##other" processContents="lax" minOccurs="0"/>
1211	</xs:sequence>
1212	<xs:anyAttribute namespace="##other" processContents="lax"/>
1213	</xs:extension>
1214	</xs:complexContent>
1215	</xs:complexType>

1217	<!-- VERSIONS LIST TYPE -->
1218	<xs:complexType name="versionsListType">
1219	<xs:sequence>
1220	<xs:element name="version" type="xs:string" minOccurs="1"
1221	 maxOccurs="unbounded"/>
1222	<xs:any namespace="##other" processContents="lax" minOccurs="0"/>
1223	</xs:sequence>
1224	<xs:anyAttribute namespace="##other" processContents="lax"/>
1225	</xs:complexType>
1226	<!-- OPTIONS LIST TYPE -->
1227	<xs:complexType name="optionsListType">
1228	<xs:sequence>
1229	<xs:element name="option" type="optionType" minOccurs="1"
1230	 maxOccurs="unbounded"/>
1231	<xs:any namespace="##other" processContents="lax" minOccurs="0"/>
1232	</xs:sequence>
1233	<xs:anyAttribute namespace="##other" processContents="lax"/>
1234	</xs:complexType>

1236	<!-- OPTION TYPE -->
1237	<xs:complexType name="optionType">
1238	<xs:sequence>
1239	<xs:element name="name" type="xs:string" />
1240	<xs:element name="schemaRef" type="xs:anyURI" minOccurs="0"/>
1241	<xs:element name="version" type="xs:string" minOccurs="0"/>
1242	<xs:any namespace="##other" processContents="lax" minOccurs="0"/>
1243	</xs:sequence>
1244	<xs:anyAttribute namespace="##other" processContents="lax"/>
1245	</xs:complexType>

1247	<!-- CLUE OPTIONS RESPONSE (2 WAY) -->
1248	<xs:complexType name="optionsResponseMessageType">
1249	<xs:complexContent>
1250	<xs:extension base="clueMessageType">
1251	<xs:sequence>
1252	<xs:element name="responseCode" type="xs:short"/>
1253	<xs:element name="reasonString" type="xs:string"/>
1254	<xs:element name="mediaProvider" type="xs:boolean" minOccurs="0"/>
1255	<xs:element name="mediaConsumer" type="xs:boolean" minOccurs="0"/>
1256	<xs:element name="version" type="xs:string" minOccurs="0"/>
1257	<xs:element name="commonOptions" type="optionsListType" minOccurs="0"/>
1258	<xs:any namespace="##other"
1259	processContents="lax" minOccurs="0"/>
1260	</xs:sequence>
1261	<xs:anyAttribute namespace="##other" processContents="lax"/>
1262	</xs:extension>
1263	</xs:complexContent>
1264	</xs:complexType>

1266	<!-- CLUE OPTIONS RESPONSE (3 WAYS) -->
1267	<!-- <xs:complexType name="optionsResponseMessageType">
1268	<xs:complexContent>
1269	<xs:extension base="clueMessageType">
1270	<xs:sequence>
1271	<xs:element name="mediaProvider" type="xs:boolean"/>
1272	<xs:element name="mediaConsumer" type="xs:boolean"/>
1273	<xs:element name="supportedVersions" type="versionsListType"
1274	 minOccurs="0"/>
1275	<xs:element name="supportedOptions" type="optionsListType" minOccurs="0"/>
1276	<xs:any namespace="##other"
1277	processContents="lax" minOccurs="0"/>
1278	</xs:sequence>
1279	<xs:anyAttribute namespace="##other" processContents="lax"/>
1280	</xs:extension>
1281	</xs:complexContent>
1282	</xs:complexType>
1283	 -->

1285	<!-- CLUE OPTIONS ACK (3 WAYS)-->
1286	<!--
1287	<xs:complexType name="optionsAckMessageType">
1288	<xs:complexContent>
1289	<xs:extension base="clueMessageType">
1290	<xs:sequence>
1291	<xs:element name="responseCode" type="xs:short"/>
1292	<xs:element name="reasonString" type="xs:string"/>
1293	<xs:element name="version" type="xs:string" minOccurs="0"
1294	 maxOccurs="1"/>
1295	<xs:element name="commonOptions" type="supportedOptionsType" minOccurs="0"/>
1296	<xs:any namespace="##other"
1297	processContents="lax" minOccurs="0"/>
1298	</xs:sequence>
1299	<xs:anyAttribute namespace="##other" processContents="lax"/>
1300	</xs:extension>
1301	</xs:complexContent>
1302	</xs:complexType>
1303	 -->

1305	<!-- CLUE ADVERTISEMENT MESSAGE TYPE -->
1306	<xs:complexType name="advertisementMessageType">
1307	<xs:complexContent>
1308	<xs:extension base="clueMessageType">
1309	<xs:sequence>
1310	<!-- mandatory fields -->
1311	<xs:element name="mediaCaptures" type="dm:mediaCapturesType"/>
1312	<xs:element name="encodingGroups" type="dm:encodingGroupsType"/>
1313	<xs:element name="captureScenes" type="dm:captureScenesType"/>
1314	<xs:element name="simultaneousSets" type="dm:simultaneousSetsType"
1315	 minOccurs="0"/>
1316	<xs:element name="globalCaptureEntries" type="dm:globalCaptureEntriesType"
1317	 minOccurs="0"/>
1318	<xs:element name="participants" type="dm:participantsType" minOccurs="0"/>
1319	<xs:any namespace="##other" processContents="lax" minOccurs="0"/>
1320	</xs:sequence>
1321	<xs:anyAttribute namespace="##other" processContents="lax"/>
1322	</xs:extension>
1323	</xs:complexContent>
1324	</xs:complexType>

1326	<!-- ADV ACK MESSAGE TYPE -->
1327	<xs:complexType name="advAcknowledgementMessageType">
1328	<xs:complexContent>
1329	<xs:extension base="clueMessageType">
1330	<xs:sequence>
1331	<xs:element name="responseCode" type="xs:short"/>
1332	<xs:element name="reasonString" type="xs:string"/>
1333	<xs:element name="advSequenceNr" type="xs:unsignedInt"/>
1334	<xs:any namespace="##other" processContents="lax" minOccurs="0"/>
1335	</xs:sequence>
1336	<xs:anyAttribute namespace="##other" processContents="lax"/>
1337	</xs:extension>
1338	</xs:complexContent>
1339	</xs:complexType>

1341	<!-- CLUE CONFIGURE MESSAGE TYPE -->
1342	<xs:complexType name="configureMessageType">
1343	<xs:complexContent>
1344	<xs:extension base="clueMessageType">
1345	<xs:sequence>
1346	<!-- mandatory fields -->
1347	<xs:element name="advSequenceNr" type="xs:unsignedInt"/>
1348	<xs:element name="ack" type="xs:boolean" minOccurs="0" fixed="true"/>
1349	<xs:element name="captureEncodings" type="dm:captureEncodingsType"
1350	 minOccurs="0"/>
1351	<xs:any namespace="##other" processContents="lax" minOccurs="0"/>
1352	</xs:sequence>
1353	<xs:anyAttribute namespace="##other" processContents="lax"/>
1354	</xs:extension>
1355	</xs:complexContent>
1356	</xs:complexType>

1358	<!-- CONFIGURE RESPONSE MESSAGE TYPE -->
1359	<xs:complexType name="configureResponseMessageType">
1360	<xs:complexContent>
1361	<xs:extension base="clueMessageType">
1362	<xs:sequence>
1363	<xs:element name="responseCode" type="xs:short"/>
1364	<xs:element name="reasonString" type="xs:string"/>
1365	<xs:element name="confSequenceNr" type="xs:integer"/>
1366	<xs:any namespace="##other" processContents="lax" minOccurs="0"/>
1367	</xs:sequence>
1368	<xs:anyAttribute namespace="##other" processContents="lax"/>
1369	</xs:extension>
1370	</xs:complexContent>
1371	</xs:complexType>

1373	<!-- CLUE READV MESSAGE TYPE -->
1374	<xs:complexType name="readvMessageType">
1375	<xs:complexContent>
1376	<xs:extension base="clueMessageType">
1377	<xs:sequence>
1378	<xs:element name="lastReceivedAdv" type="xs:short"/>
1379	<xs:any namespace="##other" processContents="lax" minOccurs="0"/>
1380	</xs:sequence>
1381	<xs:anyAttribute namespace="##other" processContents="lax"/>
1382	</xs:extension>
1383	</xs:complexContent>
1384	</xs:complexType>

1386	<!-- CLUE READV RESPONSE MESSAGE TYPE -->
1387	<xs:complexType name="readvResponseMessageType">
1388	<xs:complexContent>
1389	<xs:extension base="clueMessageType">
1390	<xs:sequence>
1391	<xs:element name="responseCode" type="xs:short"/>
1392	<xs:element name="reasonString" type="xs:string"/>
1393	<xs:element name="readvSequenceNr" type="xs:string" minOccurs="0"/>
1394	<xs:element name="mediaCaptures" type="dm:mediaCapturesType" minOccurs="0"/>
1395	<xs:element name="encodingGroups" type="dm:encodingGroupsType" minOccurs="0"/>
1396	<xs:element name="captureScenes" type="dm:captureScenesType" minOccurs="0"/>
1397	<xs:element name="simultaneousSets" type="dm:simultaneousSetsType" minOccurs="0"/>
1398	<xs:element name="globalCaptureEntries" type="dm:globalCaptureEntriesType" minOccurs="0"/>
1399	<xs:element name="participants" type="dm:participantsType" minOccurs="0"/>
1400	<xs:any namespace="##other"
1401	 processContents="lax" minOccurs="0"/>
1402	</xs:sequence>
1403	</xs:extension>
1404	</xs:complexContent>
1405	</xs:complexType>

1407	</xs:schema>

1409	10.  Diff with draft-presta-clue-protocol-04

1411	   1.  The response code type error in the OPTIONS response (and in
1412	       other parts) has been corrected.

1414	11.  Diff with draft-presta-clue-protocol-03

1416	   1.  The XML Schema has been deeply revised and completed.

1418	   2.  The descriptions of the CLUE messages have been added.

1420	   3.  The distinction between major version numbers and minor version
1421	       numbers has been cut and pasted from
1422	       [I-D.kyzivat-clue-signaling].

1424	   4.  Besides the two way one, a three way mechanism for the options
1425	       negotiation has been proposed and provided to foster discussion.

1427	12.  Diff with draft-presta-clue-protocol-02

1429	   1.  "Terminology" section added.

1431	   2.  Introduced the concept of "CLUE Participant" - an Endpoint or a
1432	       MCU able to use the CLUE protocol within a telepresence session.
1433	       A CLUE Participant can act as a Media Provider and/or as a Media
1434	       Consumer.

1436	   3.  INtroduced the ACK/NACK mechanism for the ADVERTISEMENT.

1438	   4.  MP and MC state machines have been updated.  The CP state machine
1439	       has been added.

1441	13.  Acknowledgments

1443	   The authors thank all the CLUErs for their precious feedbacks and
1444	   support, in particular Paul Kyzivat, Christian Groves and Scarlett
1445	   Liuyan.

1447	14.  Informative References

1449	   [I-D.ietf-clue-data-model-schema]          Presta, R. and S. Romano,
1450	                                              "An XML Schema for the
1451	                                              CLUE data model", draft-
1452	                                              ietf-clue-data-model-
1453	                                              schema-04 (work in
1454	                                              progress), March 2014.

1456	   [I-D.ietf-clue-datachannel]                Holmberg, C., "CLUE
1457	                                              Protocol Data Channel", dr
1458	                                              aft-ietf-clue-datachannel-
1459	                                              00 (work in progress),
1460	                                              March 2014.

1462	   [I-D.ietf-clue-framework]                  Duckworth, M., Pepperell,
1463	                                              A., and S. Wenger,
1464	                                              "Framework for
1465	                                              Telepresence Multi-
1466	                                              Streams", draft-ietf-clue-
1467	                                              framework-15 (work in
1468	                                              progress), May 2014.

1470	   [I-D.ietf-clue-telepresence-requirements]  Romanow, A., Botzko, S.,
1471	                                              and M. Barnes,
1472	                                              "Requirements for
1473	                                              Telepresence Multi-
1474	                                              Streams", draft-ietf-clue-
1475	                                              telepresence-requirements-
1476	                                              07 (work in progress),
1477	                                              December 2013.

1479	   [I-D.kyzivat-clue-signaling]               Kyzivat, P., Xiao, L.,
1480	                                              Groves, C., and R. Hansen,
1481	                                              "CLUE Signaling", draft-
1482	                                              kyzivat-clue-signaling-08
1483	                                              (work in progress),
1484	                                              April 2014.

1486	   [RFC3550]                                  Schulzrinne, H., Casner,
1487	                                              S., Frederick, R., and V.
1488	                                              Jacobson, "RTP: A
1489	                                              Transport Protocol for
1490	                                              Real-Time Applications",
1491	                                              STD 64, RFC 3550,
1492	                                              July 2003.

1494	   [RFC4353]                                  Rosenberg, J., "A
1495	                                              Framework for Conferencing
1496	                                              with the Session
1497	                                              Initiation Protocol
1498	                                              (SIP)", RFC 4353,
1499	                                              February 2006.

1501	   [RFC5117]                                  Westerlund, M. and S.
1502	                                              Wenger, "RTP Topologies",
1503	                                              RFC 5117, January 2008.

1505	   [RFC5261]                                  Urpalainen, J., "An
1506	                                              Extensible Markup Language
1507	                                              (XML) Patch Operations
1508	                                              Framework Utilizing XML
1509	                                              Path Language (XPath)
1510	                                              Selectors", RFC 5261,
1511	                                              September 2008.

1513	   [RFC6502]                                  Camarillo, G., Srinivasan,
1514	                                              S., Even, R., and J.
1515	                                              Urpalainen, "Conference
1516	                                              Event Package Data Format
1517	                                              Extension for Centralized
1518	                                              Conferencing (XCON)",
1519	                                              RFC 6502, March 2012.

1521	   [RFC6503]                                  Barnes, M., Boulton, C.,
1522	                                              Romano, S., and H.
1523	                                              Schulzrinne, "Centralized
1524	                                              Conferencing Manipulation
1525	                                              Protocol", RFC 6503,
1526	                                              March 2012.

1528	Authors' Addresses

1530	   Roberta Presta
1531	   University of Napoli
1532	   Via Claudio 21
1533	   Napoli  80125
1534	   Italy

1536	   EMail: roberta.presta@unina.it

1538	   Simon Pietro Romano
1539	   University of Napoli
1540	   Via Claudio 21
1541	   Napoli  80125
1542	   Italy

1544	   EMail: spromano@unina.it