path: root/doc/rfc/rfc8847.txt

Internet Engineering Task Force (IETF)                         R. Presta
Request for Comments: 8847                                   S P. Romano
Category: Experimental                              University of Napoli
ISSN: 2070-1721                                             January 2021


   Protocol for Controlling Multiple Streams for Telepresence (CLUE)

Abstract

   The Controlling Multiple Streams for Telepresence (CLUE) protocol is
   an application protocol conceived for the description and negotiation
   of a telepresence session.  The design of the CLUE protocol takes
   into account the requirements and the framework defined within the
   IETF CLUE Working Group.  A companion document, RFC 8848, delves into
   CLUE signaling details as well as the SIP / Session Description
   Protocol (SDP) session establishment phase.  CLUE messages flow over
   the CLUE data channel, based on reliable and ordered SCTP-over-DTLS
   transport.  ("SCTP" stands for "Stream Control Transmission
   Protocol".)  Message details, together with the behavior of CLUE
   Participants acting as Media Providers and/or Media Consumers, are
   herein discussed.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are candidates for any level of
   Internet Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8847.

Copyright Notice

   Copyright (c) 2021 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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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
   2.  Terminology
   3.  Conventions
   4.  Overview of the CLUE Protocol
   5.  Protocol Messages
     5.1.  'options'
     5.2.  'optionsResponse'
     5.3.  'advertisement'
     5.4.  'ack'
     5.5.  'configure'
     5.6.  'configureResponse'
     5.7.  Response Codes and Reason Strings
   6.  Protocol State Machines
     6.1.  Media Provider's State Machine
     6.2.  Media Consumer's State Machine
   7.  Versioning
   8.  Extensions
     8.1.  Extension Example
   9.  XML Schema
   10. Call Flow Example
     10.1.  CLUE Message No. 1: 'options'
     10.2.  CLUE Message No. 2: 'optionsResponse'
     10.3.  CLUE Message No. 3: 'advertisement'
     10.4.  CLUE Message No. 4: 'configure+ack'
     10.5.  CLUE Message No. 5: 'configureResponse'
     10.6.  CLUE Message No. 6: 'advertisement'
     10.7.  CLUE Message No. 7: 'ack'
     10.8.  CLUE Message No. 8: 'configure'
     10.9.  CLUE Message No. 9: 'configureResponse'
   11. Security Considerations
   12. IANA Considerations
     12.1.  URN Sub-Namespace Registration
     12.2.  XML Schema Registration
     12.3.  Media Type Registration for "application/clue+xml"
     12.4.  CLUE Protocol Registry
       12.4.1.  CLUE Message Types
       12.4.2.  CLUE Response Codes
   13. References
     13.1.  Normative References
     13.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   The Controlling Multiple Streams for Telepresence (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 as follows:

   1.  enabling a Media Provider (MP) to properly announce its current
       telepresence capabilities to a Media Consumer (MC) in terms of
       available media captures, groups of encodings, simultaneity
       constraints, and other information defined in [RFC8845].

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

   CLUE-capable endpoints are connected by means of the CLUE data
   channel -- an SCTP-over-DTLS channel that is opened and established
   as described in [RFC8848] and [RFC8850].  ("SCTP" stands for "Stream
   Control Transmission Protocol".)  CLUE protocol messages flowing over
   such a channel are detailed in this document, both syntactically and
   semantically.

   In Section 4, we provide a general overview of the CLUE protocol.
   CLUE protocol messages are detailed in Section 5.  The CLUE protocol
   state machines are introduced in Section 6.  Versioning and
   extensions are discussed in Sections 7 and 8, respectively.  The XML
   schema [W3C.REC-xml-20081126] defining the CLUE messages is provided
   in Section 9.

2.  Terminology

   This document refers to terminology that is also used in [RFC8845]
   and [RFC7262].  For convenience, we list those terms below.  The
   definition of "CLUE Participant", as also listed below, originates
   from this document.

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

   CLUE Participant (CP):  An entity able to use the CLUE protocol
      within a telepresence session.  It can be an endpoint or an MCU
      (Multipoint Control Unit) able to use the CLUE protocol.

   CLUE-capable device:  A device that (1) supports the CLUE data
      channel [RFC8850], the CLUE protocol, and the principles of CLUE
      negotiation and (2) seeks CLUE-enabled calls.

   Endpoint:  A CLUE-capable device that is the logical point of final
      termination through receiving, decoding, and rendering, and/or
      initiation through the capturing, encoding, and sending of media
      streams.  An endpoint consists of one or more physical devices
      that source and sink media streams, and exactly one participant
      (as described in [RFC4353]) that, in turn, includes exactly one
      user agent [RFC3261].  Endpoints can be anything from multiscreen/
      multicamera rooms to handheld devices.

   Multipoint Control Unit (MCU):  A CLUE-capable device that connects
      two or more endpoints together into one single multimedia
      conference [RFC7667].  An MCU includes a mixer (as defined in
      [RFC4353]), without the requirement per [RFC4353] to send media to
      each participant.

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

   Media Capture:  A source of media -- for example, from one or more
      Capture Devices or constructed from other Media streams.

   Media Consumer (MC):  A CP (i.e., an Endpoint or an MCU) able to
      receive Capture Encodings.

   Media Provider (MP):  A CP (i.e., an Endpoint or an MCU) able to send
      Capture Encodings.

   Stream:  A Capture Encoding sent from an MP to an MC via RTP
      [RFC3550].

3.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

4.  Overview of the CLUE Protocol

   The CLUE protocol is conceived to enable CLUE telepresence sessions.
   It is designed to address Session Description Protocol (SDP)
   limitations in terms of the description of some information about the
   multimedia streams that are involved in a real-time multimedia
   conference.  Indeed, by simply using SDP, it is not possible to
   convey information about the features of the flowing multimedia
   streams that are needed to enable a "being there" rendering
   experience.  Such information is contained in the CLUE framework
   document [RFC8845] and formally defined and described in the CLUE
   data model document [RFC8846].  The CLUE protocol represents the
   mechanism for the exchange of telepresence information between CPs.
   It mainly provides the messages to enable an MP to advertise its
   telepresence capabilities and to enable an MC to select the desired
   telepresence options.

   The CLUE protocol, as defined in this document and further described
   below, is a stateful client-server XML-based application protocol.
   CLUE protocol messages flow on a reliable and ordered SCTP-over-DTLS
   transport channel connecting two CPs.  Messages carry information
   taken from the XML-based CLUE data model [RFC8846].  Three main
   communication phases can be identified:

   Establishment of the CLUE data channel:
      In this phase, the CLUE data channel setup takes place.  If it
      completes successfully, the CPs are able to communicate and start
      the initiation phase.

   Negotiation of the CLUE protocol version and extensions
   (initiation phase):
      The CPs connected via the CLUE data channel agree on the protocol
      version and extensions to be used during the telepresence session.
      Special CLUE messages are used for such a task ('options' and
      'optionsResponse').  The negotiation of the version and extensions
      can be performed once during the CLUE session and only at this
      stage.  At the end of that basic negotiation, each CP starts its
      activity as a CLUE MP and/or CLUE MC.

   Description and negotiation of CLUE telepresence capabilities:
      In this phase, the MP-MC dialogues take place on the data channel
      by means of the CLUE protocol messages.

   As soon as the channel is ready, the CPs must agree on the protocol
   version and extensions to be used within the telepresence session.
   CLUE protocol version numbers are characterized by a major version
   number and a minor version number, both unsigned integers, separated
   by a dot.  While minor version numbers denote backward-compatible
   changes in the context of a given major version, different major
   version numbers generally indicate a lack of interoperability between
   the protocol implementations.  In order to correctly establish a CLUE
   dialogue, the involved CPs must have in common a major version number
   (see Section 7 for further details).  The subset of the extensions
   that are allowed within the CLUE session is also determined in the
   initiation phase.  It includes only the extensions that are supported
   by both parties.  A mechanism for the negotiation of the CLUE
   protocol version and extensions is part of the initiation phase.
   According to such a solution, the CP that is the CLUE Channel
   Initiator (CI) issues a proper CLUE message ('options') to the CP
   that is the Channel Receiver (CR), specifying the supported version
   and extensions.  The CR then answers by selecting the subset of the
   CI extensions that it is able to support and determines the protocol
   version to be used.

   After the negotiation phase is completed, CPs describe and agree on
   the media flows to be exchanged.  In many cases, CPs will seek to
   both transmit and receive media.  Hence, in a call between two CPs
   (e.g., CPs A and B), there would be two separate message exchange
   sequences, as follows:

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

   CLUE messages for the media session description and negotiation are
   designed by considering the MP side to be 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 its available media captures and encoding
   capabilities to the MC, as well as its simultaneity constraints,
   according to the information model defined in [RFC8845].  The CLUE
   message conveying the MP's multimedia offer is the 'advertisement'
   message.  Such a message leverages the XML data model definitions
   provided in [RFC8846].

   The MC selects the desired streams of the MP by using the 'configure'
   message, which makes reference to the information carried in the
   previously received 'advertisement'.

   Besides 'advertisement' and 'configure', other messages have been
   conceived in order to provide all needed mechanisms and operations.
   Such messages are detailed in the following sections.

5.  Protocol Messages

   CLUE protocol messages are textual XML-based messages that enable the
   configuration of the telepresence session.  The formal definition of
   such messages is provided in the XML schema in Section 9.  This
   section includes non-normative excerpts of the schema to aid in
   describing it.

   The XML definitions of the CLUE information provided in [RFC8846] are
   included within some CLUE protocol messages (namely the
   'advertisement' and 'configure' messages), in order to use the
   concepts defined in [RFC8845].

   The CLUE protocol messages are as follows:

   *  options

   *  optionsResponse

   *  advertisement

   *  ack

   *  configure

   *  configureResponse

   While the 'options' and 'optionsResponse' messages are exchanged in
   the initiation phase between the CPs, the other messages are involved
   in MP-MC dialogues.  Please note that the word "dialogue" as used in
   this document is not related to SIP's usage of the same term.  It
   refers to message exchange sequences between a CLUE MP and a Clue MC.

   Each CLUE message inherits a basic structure, as depicted in the
   following excerpt (Figure 1):

   <xs:complexType name="clueMessageType" abstract="true">
     <xs:sequence>
       <xs:element name="clueId" type="xs:string" minOccurs="0"/>
       <xs:element name="sequenceNr" type="xs:positiveInteger"/>
     </xs:sequence>
     <xs:attribute name="protocol" type="xs:string" fixed="CLUE"
           use="required"/>
     <xs:attribute name="v" type="versionType" use="required"/>
   </xs:complexType>

   <!-- VERSION TYPE -->
   <xs:simpleType name="versionType">
     <xs:restriction base="xs:string">
       <xs:pattern value="[1-9][0-9]*\.[0-9]+" />
     </xs:restriction>
   </xs:simpleType>

                   Figure 1: Structure of a CLUE Message

   The information contained in each CLUE message is as follows:

   clueId:  An optional XML element containing the identifier (in the
      form of a generic string) of the CP within the telepresence
      system.

   sequenceNr:  An XML element containing the local message sequence
      number.  The sender MUST increment the sequence number by one for
      each new message sent, and the receiver MUST remember the most
      recent sequence number received and send back a 402 error if it
      receives a message with an unexpected sequence number (e.g.,
      sequence number gap, repeated sequence number, sequence number too
      small).  The initial sequence number can be chosen randomly by
      each party.

   protocol:  A mandatory attribute set to "CLUE", identifying the
      protocol the messages refer to.

   v:  A mandatory attribute carrying the version of the protocol.  The
      content of the "v" attribute is composed of the major version
      number followed by a dot and then by the minor version number of
      the CLUE protocol in use.  The major number cannot be "0", and if
      it is more than one digit, it cannot start with a "0".  Allowed
      values of this kind are "1.3", "2.0", "20.44", etc.  This document
      describes version 1.0.

   Each CP is responsible for creating and updating up to three
   independent streams of sequence numbers in messages it sends: (i) one
   for the messages sent in the initiation phase, (ii) one for the
   messages sent as an MP (if it is acting as an MP), and (iii) one for
   the messages sent as an MC (if it is acting as an MC).

   In particular, CLUE response messages ('optionsResponse', 'ack',
   'configureResponse') derive from a base type, inheriting from the
   clueMessageType, which is defined as follows (Figure 2):

   <xs:complexType name="clueResponseType">
    <xs:complexContent>
     <xs:extension base="clueMessageType">
      <xs:sequence>
       <xs:element name="responseCode" type="responseCodeType"/>
       <xs:element name="reasonString" type="xs:string" minOccurs="0"/>
      </xs:sequence>
     </xs:extension>
    </xs:complexContent>
   </xs:complexType>

               Figure 2: Structure of CLUE Response Messages

   The elements <responseCode> and <reasonString> are populated as
   detailed in Section 5.7.

5.1.  'options'

   The 'options' message is sent by the CP that is the CI to the CP that
   is the CR as soon as the CLUE data channel is ready.  Besides the
   information envisioned in the basic structure, it specifies:

   <mediaProvider>:  A mandatory boolean field set to "true" if the CP
      is able to act as an MP.

   <mediaConsumer>:  A mandatory boolean field set to "true" if the CP
      is able to act as an MC.

   <supportedVersions>:  The list of supported versions.

   <supportedExtensions>:  The list of supported extensions.

   The XML schema of such a message is shown below (Figure 3):

   <!-- CLUE OPTIONS -->
   <xs:complexType name="optionsMessageType">
     <xs:complexContent>
       <xs:extension base="clueMessageType">
         <xs:sequence>
           <xs:element name="mediaProvider" type="xs:boolean" />
           <xs:element name="mediaConsumer" type="xs:boolean" />
           <xs:element name="supportedVersions" type="versionsListType"
                   minOccurs="0"/>
           <xs:element name="supportedExtensions"
                 type="extensionsListType"
                   minOccurs="0"/>
           <xs:any namespace="##other" processContents="lax"
                   minOccurs="0"/>
         </xs:sequence>
         <xs:anyAttribute namespace="##other" processContents="lax"/>
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>

   <!-- VERSIONS LIST TYPE -->
   <xs:complexType name="versionsListType">
     <xs:sequence>
       <xs:element name="version" type="versionType" minOccurs="1"
           maxOccurs="unbounded"/>
       <xs:any namespace="##other" processContents="lax" minOccurs="0"/>
     </xs:sequence>
     <xs:anyAttribute namespace="##other" processContents="lax"/>
   </xs:complexType>

   <!-- EXTENSIONS LIST TYPE -->
   <xs:complexType name="extensionsListType">
     <xs:sequence>
       <xs:element name="extension" type="extensionType" minOccurs="1"
           maxOccurs="unbounded"/>
       <xs:any namespace="##other" processContents="lax" minOccurs="0"/>
     </xs:sequence>
     <xs:anyAttribute namespace="##other" processContents="lax"/>
   </xs:complexType>

   <!-- EXTENSION TYPE -->
   <xs:complexType name="extensionType">
     <xs:sequence>
       <xs:element name="name" type="xs:string" />
       <xs:element name="schemaRef" type="xs:anyURI" />
       <xs:element name="version" type="versionType" />
       <xs:any namespace="##other" processContents="lax" minOccurs="0"/>
     </xs:sequence>
     <xs:anyAttribute namespace="##other" processContents="lax"/>
   </xs:complexType>

              Figure 3: Structure of a CLUE 'options' Message

   <supportedVersions> contains the list of versions that are supported
   by the CI, each one represented in a child <version> element.  The
   content of each <version> element is a string made of the major
   version number followed by a dot and then by the minor version number
   (e.g., 1.3 or 2.4).  Exactly one <version> element MUST be provided
   for each major version supported, containing the maximum minor
   version number of such a version, since all minor versions are
   backward compatible.  If no <supportedVersions> is carried within the
   'options' message, the CI supports only the version declared in the
   "v" attribute and all the versions having the same major version
   number and lower minor version number.  For example, if the "v"
   attribute has a value of "3.4" and there is no <supportedVersions>
   element in the 'options' message, it means the CI supports only major
   version 3 with all minor versions from 3.0 through 3.4.  If
   <supportedVersions> is provided, at least one <version> element MUST
   be included.  In this case, the "v" attribute SHOULD be set to the
   largest minor version of the smallest major version advertised in the
   <supportedVersions> list.  Indeed, the intention behind the "v"
   attribute is that some implementation that receives a version number
   in the "v" field with a major number higher than it understands is
   supposed to close the connection, since it runs a risk of
   misinterpreting the contents of messages.  The minor version is less
   useful in this context, since minor versions are defined to be both
   backward and forward compatible and the value can in any case be
   parsed out of the <version> list.  It is more useful to know the
   highest minor version supported than some random minor version, as it
   indicates the full feature set that is supported.

   The <supportedExtensions> element specifies the list of extensions
   supported by the CI.  If there is no <supportedExtensions> in the
   'options' message, the CI does not support anything other than what
   is envisioned in the versions it supports.  For each extension, an
   <extension> element is provided.  An extension is characterized by a
   name, an XML schema of reference where the extension is defined, and
   the version of the protocol that the extension refers to.

5.2.  'optionsResponse'

   The 'optionsResponse' (Figure 4) is sent by a CR to a CI as a reply
   to the 'options' message.  The 'optionsResponse' contains a mandatory
   response code and a reason string indicating the processing result of
   the 'options' message.  If the responseCode is between 200 and 299
   inclusive, the response MUST also include <mediaProvider>,
   <mediaConsumer>, <version>, and <commonExtensions> elements; it MAY
   include them for any other response code.  <mediaProvider> and
   <mediaConsumer> elements (which are of a boolean nature) are
   associated with the supported roles (in terms of the MP and the MC,
   respectively), similarly to what the CI does in the 'options'
   message.  The <version> element indicates the highest commonly
   supported version number.  The content of the <version> element MUST
   be a string made of the major version number followed by a dot and
   then by the minor version number (e.g., 1.3 or 2.4).  Finally, the
   commonly supported extensions are copied in the <commonExtensions>
   element.

   <!-- CLUE 'optionsResponse' -->
   <xs:complexType name="optionsResponseMessageType">
     <xs:complexContent>
       <xs:extension base="clueResponseType">
         <xs:sequence>
           <xs:element name="mediaProvider" type="xs:boolean"
                   minOccurs="0"/>
           <xs:element name="mediaConsumer" type="xs:boolean"
                   minOccurs="0"/>
           <xs:element name="version" type="versionType" minOccurs="0"/>
           <xs:element name="commonExtensions" type="extensionsListType"
                   minOccurs="0"/>
           <xs:any namespace="##other" processContents="lax"
                   minOccurs="0"/>
         </xs:sequence>
         <xs:anyAttribute namespace="##other" processContents="lax"/>
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>

          Figure 4: Structure of a CLUE 'optionsResponse' Message

   Upon reception of the 'optionsResponse', the version to be used is
   the one provided in the <version> element of the message.  The
   subsequent CLUE messages MUST use such a version number in the "v"
   attribute.  The allowed extensions in the CLUE dialogue are those
   indicated in the <commonExtensions> element of the 'optionsResponse'
   message.

5.3.  'advertisement'

   The 'advertisement' message is used by each MP to advertise the
   available media captures and related information to the corresponding
   MC.  The MP sends an 'advertisement' to the MC as soon as it is ready
   after the successful completion of the initiation phase, i.e., as
   soon as the CPs have agreed on the version and extensions of the CLUE
   protocol.  During a single CLUE session, an MP may send new
   'advertisement' messages to replace the previous advertisement if,
   for instance, its CLUE telepresence media capabilities change
   mid-call.  A new 'advertisement' completely replaces the previous
   'advertisement'.

   The 'advertisement' structure is defined in the schema excerpt below
   (Figure 5).  The 'advertisement' contains elements compliant with the
   CLUE data model that characterize the MP's telepresence offer.
   Namely, such elements are the list of

   *  media captures (<mediaCaptures>),

   *  encoding groups (<encodingGroups>),

   *  capture scenes (<captureScenes>),

   *  simultaneous sets (<simultaneousSets>),

   *  global views (<globalViews>), and

   *  represented participants (<people>).

   Each of them is fully described in the CLUE framework document
   [RFC8845] and formally defined in the CLUE data model document
   [RFC8846].

   <!-- CLUE ADVERTISEMENT MESSAGE TYPE -->
   <xs:complexType name="advertisementMessageType">
     <xs:complexContent>
       <xs:extension base="clueMessageType">
         <xs:sequence>
           <!-- mandatory -->
           <xs:element name="mediaCaptures"
                 type="dm:mediaCapturesType"/>
           <xs:element name="encodingGroups"
                 type="dm:encodingGroupsType"/>
           <xs:element name="captureScenes"
                 type="dm:captureScenesType"/>
           <!-- optional -->
           <xs:element name="simultaneousSets"
                 type="dm:simultaneousSetsType" minOccurs="0"/>
           <xs:element name="globalViews" type="dm:globalViewsType"
                 minOccurs="0"/>
           <xs:element name="people"
                 type="dm:peopleType" minOccurs="0"/>
           <xs:any namespace="##other" processContents="lax"
                 minOccurs="0"/>
         </xs:sequence>
         <xs:anyAttribute namespace="##other" processContents="lax"/>
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>

           Figure 5: Structure of a CLUE 'advertisement' Message

5.4.  'ack'

   The 'ack' message is sent by an MC to an MP to acknowledge an
   'advertisement' message.  As can be seen from the message schema
   provided in the following excerpt (Figure 6), the 'ack' contains a
   response code and may contain a reason string for describing the
   processing result of the 'advertisement'.  The <advSequenceNr>
   element carries the sequence number of the 'advertisement' message
   the 'ack' refers to.

   <!-- 'ack' MESSAGE TYPE -->
   <xs:complexType name="advAcknowledgementMessageType">
     <xs:complexContent>
       <xs:extension base="clueResponseType">
         <xs:sequence>
           <xs:element name="advSequenceNr" type="xs:positiveInteger"/>
           <xs:any namespace="##other" processContents="lax"
                   minOccurs="0"/>
         </xs:sequence>
         <xs:anyAttribute namespace="##other" processContents="lax"/>
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>

                Figure 6: Structure of a CLUE 'ack' Message

5.5.  'configure'

   The 'configure' message is sent from an MC to an MP to list the
   advertised captures the MC wants to receive.  The MC MUST send a
   'configure' after the reception of an 'advertisement', as well as
   each time it wants to request other captures that have been
   previously advertised by the MP.  The content of the 'configure'
   message is shown below (Figure 7).

   <!-- CLUE 'configure' MESSAGE TYPE -->
   <xs:complexType name="configureMessageType">
     <xs:complexContent>
       <xs:extension base="clueMessageType">
         <xs:sequence>
           <!-- mandatory fields -->
           <xs:element name="advSequenceNr" type="xs:positiveInteger"/>
           <xs:element name="ack" type="successResponseCodeType"
                   minOccurs="0"/>
           <xs:element name="captureEncodings"
                   type="dm:captureEncodingsType" minOccurs="0"/>
           <xs:any namespace="##other" processContents="lax"
                   minOccurs="0"/>
         </xs:sequence>
         <xs:anyAttribute namespace="##other" processContents="lax"/>
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>

             Figure 7: Structure of a CLUE 'configure' Message

   The <advSequenceNr> element contains the sequence number of the
   'advertisement' message the 'configure' refers to.

   The optional <ack> element, when present, contains a success response
   code, as defined in Section 5.7.  It indicates that the 'configure'
   message also acknowledges with success the referred advertisement
   ('configure+ack' message).  The <ack> element MUST NOT be present if
   an 'ack' message (associated with the advertisement carrying that
   specific sequence number) has already been sent back to the MP.

   The most important content of the 'configure' message is the list of
   capture encodings provided in the <captureEncodings> element (see
   [RFC8846] for the definition of <captureEncodings>).  Such an element
   contains a sequence of capture encodings, representing the streams to
   be instantiated.

5.6.  'configureResponse'

   The 'configureResponse' message is sent from the MP to the MC to
   communicate the processing result of requests carried in the
   previously received 'configure' message.  As shown in Figure 8, it
   contains a response code (and, optionally, a reason string)
   indicating either the success or failure (along with failure details)
   of the 'configure' request processing.  The <confSequenceNr> element
   that follows contains the sequence number of the 'configure' message
   the response refers to.  There is no partial execution of commands.
   As an example, if an MP is able to understand all the selected
   capture encodings except one, then the whole command fails and
   nothing is instantiated.

   <!-- 'configureResponse' MESSAGE TYPE -->
   <xs:complexType name="configureResponseMessageType">
     <xs:complexContent>
       <xs:extension base="clueResponseType">
         <xs:sequence>
           <xs:element name="confSequenceNr"
                 type="xs:positiveInteger" />
           <xs:any namespace="##other" processContents="lax"
                 minOccurs="0"/>
         </xs:sequence>
         <xs:anyAttribute namespace="##other" processContents="lax" />
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>

         Figure 8: Structure of a CLUE 'configureResponse' Message

5.7.  Response Codes and Reason Strings

   Response codes are defined as a sequence of three digits.  A well-
   defined meaning is associated with the first digit.  Response codes
   beginning with "2" are associated with successful responses.
   Response codes that do not begin with either "2" or "1" indicate an
   error response, i.e., that an error occurred while processing a CLUE
   request.  In particular, response codes beginning with "3" indicate
   problems with the XML content of the message ("Bad syntax", "Invalid
   value", etc.), while response codes beginning with "4" refer to
   problems related to CLUE protocol semantics ("Invalid sequencing",
   "Version not supported", etc.).  200, 300, and 400 codes are the most
   generic codes in their respective categories.  Further response codes
   can be defined either in future versions of the protocol or by
   leveraging the extension mechanism.  In both cases, the new response
   codes MUST be registered with IANA.  Such new response codes MUST NOT
   override the codes defined in this document, and they MUST respect
   the semantics of the first code digit.

   This document does not define response codes starting with "1", and
   such response codes are not allowed to appear in major version 1 of
   the CLUE protocol.  The range from 100 to 199 inclusive is reserved
   for future major versions of the protocol to define response codes
   for delayed or incomplete operations, if necessary.  Response codes
   starting with "5" through "9" are reserved for future major versions
   of the protocol to define new classes of responses and are not
   allowed in major version 1 of the CLUE protocol.  Response codes
   starting with "0" are not allowed.

   The response codes and reason strings defined for use with version 1
   of the CLUE protocol are listed in Table 1.  The "Description" text
   contained in the table can be sent in the <reasonString> element of a
   response message.  Implementations can (and are encouraged to)
   include descriptions of the error condition that are more specific,
   if possible.

   +==========+===============+========================================+
   | Response | Reason String |              Description               |
   | Code     |               |                                        |
   +==========+===============+========================================+
   | 200      | Success       | The request has been                   |
   |          |               | successfully processed.                |
   +----------+---------------+----------------------------------------+
   | 300      | Low-level     | A generic low-level request            |
   |          | request error | error has occurred.                    |
   +----------+---------------+----------------------------------------+
   | 301      | Bad syntax    | The XML syntax of the message          |
   |          |               | is not correct.                        |
   +----------+---------------+----------------------------------------+
   | 302      | Invalid value | The message contains an                |
   |          |               | invalid parameter value.               |
   +----------+---------------+----------------------------------------+
   | 303      | Conflicting   | The message contains values            |
   |          | values        | that cannot be used together.          |
   +----------+---------------+----------------------------------------+
   | 400      | Semantic      | The received CLUE protocol             |
   |          | errors        | message contains semantic              |
   |          |               | errors.                                |
   +----------+---------------+----------------------------------------+
   | 401      | Version not   | The protocol version used in           |
   |          | supported     | the message is not supported.          |
   +----------+---------------+----------------------------------------+
   | 402      | Invalid       | The received message contains          |
   |          | sequencing    | an unexpected sequence number          |
   |          |               | (e.g., sequence number gap,            |
   |          |               | repeated sequence number, or           |
   |          |               | sequence number outdated).             |
   +----------+---------------+----------------------------------------+
   | 403      | Invalid       | The clueId used in the                 |
   |          | identifier    | message is invalid or                  |
   |          |               | unknown.                               |
   +----------+---------------+----------------------------------------+
   | 404      | Advertisement | The sequence number of the             |
   |          | expired       | advertisement the 'configure'          |
   |          |               | message refers to is out of            |
   |          |               | date.                                  |
   +----------+---------------+----------------------------------------+
   | 405      | Subset choice | The subset choice is not               |
   |          | not allowed   | allowed for the specified              |
   |          |               | Multiple Content Capture.              |
   +----------+---------------+----------------------------------------+

                        Table 1: CLUE Response Codes

6.  Protocol State Machines

   The CLUE protocol is an application protocol used between two CPs in
   order to properly configure a multimedia telepresence session.  CLUE
   protocol messages flow over the CLUE data channel, an SCTP-over-DTLS
   channel established as depicted in [RFC8850].  We herein discuss the
   state machines associated with the CP (Figure 9), the MP role
   (Figure 10 in Section 6.1), and the MC role (Figure 11 in
   Section 6.2), respectively.  Endpoints often wish to both send and
   receive media, i.e., act as both an MP and an MC.  As such, there
   will often be two sets of messages flowing in opposite directions;
   the state machines of these two flows do not interact with each
   other.  Only the CLUE application logic is considered.  The
   interaction of CLUE protocol and SDP negotiations for the media
   streams exchanged is discussed in [RFC8848].

                               +----+
       +---------------------->|IDLE|<----------------------------+
       |                       +-+--+                             |
       |                         |                                |
       |                         |  start                         |
       |                         |  channel                       |
       |                         v                                |
       |  channel error /     +--------+                          |
       |  session ends        | CHANNEL|                          |
       +----------------------+ SETUP  |                          |
       |                      +--+-----+                          |
       |                         |                                |
       |                         |  channel                       |
       |                         |  established                   |
       |  channel error /        v                 OPTIONS phase  |
       |  session ends         +-------+           failure        |
       +-----------------------+OPTIONS+--------------------------+
       |                       +-+-----+
       |                         |
       |                         |  OPTIONS phase
       |                         |  success
       |                         v
       |   channel error /    +---------+
       |   session ends       | ACTIVE  |
       +----------------------+         |
                              | +----+  +------------------+
                              | | MP |  |    send/receive  |
                              | +----+  |    CLUE messages |
                              |         |<-----------------+
                              | +----+  |
                              | | MC |  |
                              | +----+  |
                              |         |
                              +---------+

                  Figure 9: CLUE Participant State Machine

   The main state machines focus on the behavior of the CP acting as a
   CLUE CI/CR.

   The initial state is the IDLE state.  When in the IDLE state, the
   CLUE data channel is not established and no CLUE-controlled media are
   exchanged between the two CLUE-capable devices in question (if there
   is an ongoing exchange of media streams, such media streams are not
   currently CLUE controlled).

   When the CLUE data channel is set up ("start channel"), the CP moves
   from the IDLE state to the CHANNEL SETUP state.

   If the CLUE data channel is successfully set up ("channel
   established"), the CP moves from the CHANNEL SETUP state to the
   OPTIONS state.  Otherwise, if a "channel error" occurs, it moves back
   to the IDLE state.  The same transition happens if the CLUE-enabled
   telepresence session ends ("session ends"), i.e., when an SDP
   negotiation for removing the CLUE data channel is performed.

   When in the OPTIONS state, the CP addresses the initiation phase
   where both parts agree on the version and extensions to be used in
   the subsequent CLUE message exchange phase.  If the CP is the CI, it
   sends an 'options' message and waits for the 'optionsResponse'
   message.  If the CP is the CR, it waits for the 'options' message
   and, as soon as it arrives, replies with the 'optionsResponse'
   message.  If the negotiation is successfully completed ("OPTIONS
   phase success"), the CP moves from the OPTIONS state to the ACTIVE
   state.  If the initiation phase fails ("OPTIONS phase failure"), the
   CP moves from the OPTIONS state to the IDLE state.  The initiation
   phase might fail for one of the following reasons:

   1.  The CI receives an 'optionsResponse' with an error response code.

   2.  The CI does not receive any 'optionsResponse', and a timeout
       error is raised.

   3.  The CR does not receive any 'options', and a timeout error is
       raised.

   When in the ACTIVE state, the CP starts the envisioned sub-state
   machines (i.e., the MP state machine and the MC state machine)
   according to the roles it plays in the telepresence sessions.  Such
   roles have been previously declared in the 'options' and
   'optionsResponse' messages involved in the initiation phase (see
   Sections 5.1 and 5.2 for details).  When in the ACTIVE state, the CP
   delegates the sending and processing of the CLUE messages to the
   appropriate MP/MC sub-state machines.  If the CP receives a further
   'options'/'optionsResponse' message, it MUST ignore the message and
   stay in the ACTIVE state.

6.1.  Media Provider's State Machine

   As soon as the sub-state machine of the MP (Figure 10) is activated,
   it is in the ADV state.  In the ADV state, the MP prepares the
   'advertisement' message reflecting its actual telepresence
   capabilities.

                             +-----+
               +------------>| ADV |<-------------------+
               |             +-+---+                    |
               |  advertisement|       NACK received    |
               |           sent|                        |
               |               v                        |
        changed|             +--------+                 |
   telepresence+-------------+WAIT FOR+-----------------+
       settings|  +----------+  ACK   |
               |  |configure +-+------+
               |  |  +ack      |
               |  |received    |ack received
               |  |            v
               |  |          +--------+
               +-------------+WAIT FOR|
               |  |          |  CONF  |
               |  |          +-+------+<-----------------------------+
               |  |            |                                     |
               |  |            |configure received                   |
               |  |            v                                     |
               |  +--------->+---------+ error configureResponse sent|
               +-------------+CONF     |-----------------------------+
               |  +--------->|RESPONSE +
               |  |          +---------+
               |  |              |
               |  |              |successful
               |  |              |configureResponse
               |  |              |sent
               |  |              |
               |  |              |
               |  |configure     |
               |  |received      v
               |  |          +-----------+
               |  +----------+ESTABLISHED|
               +-------------+-----------+

                 Figure 10: Media Provider's State Machine

   After the 'advertisement' has been sent ("advertisement sent"), the
   MP moves from the ADV state to the WAIT FOR ACK state.  If an 'ack'
   message with a successful response code arrives ("ack received"), the
   MP moves to the WAIT FOR CONF state.  If a NACK arrives (i.e., an
   'ack' message with an error response code), the MP moves back to the
   ADV state for preparation of a new 'advertisement'.  When in the WAIT
   FOR ACK state, if a 'configure' message with the <ack> element set to
   "200" arrives ("configure+ack received"), the MP goes directly to the
   CONF RESPONSE state.  'configure+ack' messages referring to out-of-
   date (i.e., having a sequence number less than the highest generated
   so far) advertisements MUST be ignored, i.e., they do not trigger any
   state transition.  If the telepresence settings of the MP change
   while in the WAIT FOR ACK state ("changed telepresence settings"),
   the MP switches from the WAIT FOR ACK state to the ADV state to
   create a new 'advertisement'.

   When in the WAIT FOR CONF state, the MP listens to the channel for a
   'configure' request coming from the MC.  When a 'configure' arrives
   ("configure received"), the MP switches to the CONF RESPONSE state.
   If the telepresence settings change in the meantime ("changed
   telepresence settings"), the MP moves from the WAIT FOR CONF state
   back to the ADV state to create the new 'advertisement' to be sent to
   the MC.

   The MP in the CONF RESPONSE state processes the received 'configure'
   in order to produce a 'configureResponse' message.  If the MP
   successfully processes the MC's configuration, then it sends a 200
   'configureResponse' ("successful configureResponse sent") and moves
   to the ESTABLISHED state.  If there are errors in the 'configure'
   processing, then the MP issues a 'configureResponse' carrying an
   error response code and goes back to the WAIT FOR CONF state to wait
   for a new configuration request.  Finally, if there are changes in
   the MP's telepresence settings ("changed telepresence settings"), the
   MP switches to the ADV state.

   The MP in the ESTABLISHED state has successfully negotiated the media
   streams with the MC by means of the CLUE messages.  If there are
   changes in the MP's telepresence settings ("changed telepresence
   settings"), the MP moves back to the ADV state.  In the ESTABLISHED
   state, the CLUE-controlled media streams of the session are those
   described in the last successfully processed 'configure' message.

   Messages not shown for a state do not cause the state to change.

6.2.  Media Consumer's State Machine

   As soon as the sub-state machine of the MC (Figure 11) is activated,
   it is in the WAIT FOR ADV state.  An MC in the WAIT FOR ADV state is
   waiting for an 'advertisement' coming from the MP.  If the
   'advertisement' arrives ("ADV received"), the MC moves to the ADV
   PROCESSING state.  Otherwise, the MC stays in the WAIT FOR ADV state.

                         +----------+
                         | WAIT FOR |
                         |   ADV    |
                         +----+-----+<--------+
                              |               |
                 advertisement|      NACK sent|
                      received|               |
                              v               |
                         +-----------+--------+
                         |  ADV      +
                         | PROCESSING|<-----------------------+
                         +-+-----+---+                        |
                           |     |                            |
            configure+ack  |     |  ack                       |
                   sent    |     |  sent                      |
                           |     v                            |
                           |  +-----+                         |
                           |  |CONF |  advertisement received |
     +----------------------->|     +-------------------------+
     |error                |  +--+--+                         |
     |configureResponse    |     |                            |
     |received             |     |configure                   |
     |                     |     |sent                        |
     |                     |     |                            |
     |                     v     v              advertisement |
     +------------------+---------------+            received |
             +--------->|   WAIT FOR    +---------------------+
             |          |  CONF RESPONSE+                     |
             |          +-------+-------+                     |
             |                  |                             |
             |                  |                             |
             |                  |successful                   |
             |                  |configureResponse            |
             |                  |received                     |
             |configure         v                             |
             |sent        +-----------+ advertisement received|
             +------------+ESTABLISHED+-----------------------+
                          +-----------+

                 Figure 11: Media Consumer's State Machine

   In the ADV PROCESSING state, the 'advertisement' is parsed by the MC.
   If the 'advertisement' is successfully processed, two scenarios are
   possible.  In the first case, the MC issues a successful 'ack'
   message to the MP ("ack sent") and moves to the CONF state.  This
   typically happens when the MC needs some more time to produce the
   'configure' message associated with the received 'advertisement'.  In
   the latter case, the MC is able to immediately prepare and send back
   to the MP a 'configure' message.  Such a message will have the <ack>
   element set to "200" ("configure+ack sent") and will allow the MC to
   move directly to the WAIT FOR CONF RESPONSE state.

   If the processing of the 'advertisement' is unsuccessful (bad syntax,
   missing XML elements, etc.), the MC sends a NACK message (i.e., an
   'ack' with an error response code) to the MP and, optionally, further
   describes the problem via a proper reason phrase.  In this scenario
   ("NACK sent"), the MC switches back to the WAIT FOR ADV state and
   waits for a new 'advertisement'.

   When in the CONF state, the MC prepares the 'configure' request to be
   issued to the MP on the basis of the previously acked
   'advertisement'.  When the 'configure' has been sent ("configure
   sent"), the MC moves to the WAIT FOR CONF RESPONSE state.  If a new
   'advertisement' arrives in the meantime ("advertisement received"),
   the MC goes back to the ADV PROCESSING state.

   In the WAIT FOR CONF RESPONSE state, the MC waits for the MP's
   response to the issued 'configure' or 'configure+ack'.  If a 200
   'configureResponse' message is received ("successful
   configureResponse received"), it means that the MP and the MC have
   successfully agreed on the media streams to be shared.  Then, the MC
   can move to the ESTABLISHED state.  On the other hand, if an error
   response is received ("error configureResponse received"), the MC
   moves back to the CONF state to prepare a new 'configure' request.
   If a new 'advertisement' is received in the WAIT FOR CONF RESPONSE
   state, the MC switches to the ADV PROCESSING state.

   When the MC is in the ESTABLISHED state, the telepresence session
   configuration has been set up at the CLUE application level according
   to the MC's preferences.  Both the MP and the MC have agreed on (and
   are aware of) the CLUE-controlled media streams to be exchanged
   within the call.  While in the ESTABLISHED state, the MC might decide
   to change something in the call settings; in this case, the MC then
   issues a new 'configure' ("configure sent") and moves to the WAIT FOR
   CONF RESPONSE state to wait for the new 'configureResponse'.  On the
   other hand, if the MC is in the ESTABLISHED state and a new
   'advertisement' ("advertisement received") arrives from the MP, it
   means that something has changed on the MP's side; the MC then moves
   to the ADV PROCESSING state.

   Messages not shown for a state do not cause the state to change.

7.  Versioning

   CLUE protocol messages are XML messages compliant to the CLUE
   protocol XML schema [RFC8846].  The version of the protocol
   corresponds to the version of the schema.  Both the client and the
   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.

   The client and server cannot communicate if they do not share exactly
   the same XML schema.  Such a schema is associated with the CLUE URN
   "urn:ietf:params:xml:ns:clue-protocol".  If all CLUE-enabled devices
   use that schema, there will be no interoperability problems due to
   schema issues.

   This document defines version 1.0 of the CLUE protocol schema, using
   XML schema version 1.0 [W3C.REC-xml-20081126].  The version usage is
   similar in philosophy to the Extensible Messaging and Presence
   Protocol (XMPP) [RFC6120].  A version number has major and minor
   components, each a non-negative integer.  Changes to the major
   version denote non-interoperable changes.  Changes to the minor
   version denote schema changes that are backward compatible by
   ignoring unknown XML elements or other backward-compatible changes.

   The minor versions of the XML schema MUST be backward compatible, not
   only in terms of the schema but 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 an MP is able to
   "speak", for example, version 1.5 of the protocol while the MC only
   understands version 1.4, the MP should have no problem in reverting
   the dialogue back to version 1.4 without exploiting 1.5 features and
   functionality.  Version 1.4 is the one to be spoken and has to appear
   in the "v" attribute of the subsequent CLUE messages.  In other
   words, in this example, the MP MUST use version 1.4.  That said, and
   in keeping with the general IETF protocol "robustness principle"
   stating that an implementation must be conservative in its sending
   behavior and liberal in its receiving behavior [RFC1122], CPs MUST
   ignore unknown elements or attributes that are not envisioned in the
   negotiated protocol version and related extensions.

8.  Extensions

   Although the standard version of the CLUE protocol XML schema is
   designed to thoroughly cope with the requirements emerging from the
   application domain, new needs might arise, and new extensions can
   then be designed.  Extensions specify information and behaviors that
   are not described in a certain version of the protocol and specified
   in the related RFC document.  Such information and behaviors can be
   optionally used in a CLUE dialogue and MUST be negotiated in the CLUE
   initiation phase.  They can relate to:

   1.  new information, to be carried in the existing messages.  For
       example, more fields may be added within an existing message.

   2.  new 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.

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

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

   2.  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 as follows:

   *  elementFormDefault="qualified"

   *  attributeFormDefault="unqualified"

   The new information must be qualified by namespaces other than
   "urn:ietf:params:xml:ns:clue-protocol" (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".  Note that there are also
   extensibility issues for the XML elements defined in such a schema.
   Those issues are overcome by using <any> and <anyAttribute>
   placeholders.  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.

   On the other hand (the second category of extensions), "extra" CLUE
   protocol messages, i.e., messages not envisioned in the latest
   standard version of the schema, might be needed.  In that case, the
   messages and the associated behavior should be defined in external
   documents that both communication parties must be aware of.

   As shown in Figure 12, the fields of the <extension> element (either
   new information or new messages) take the following values:

   *  a name.

   *  an external XML schema defining the XML information and/or the XML
      messages representing the extension.

   *  the major standard version of the protocol that the extension
      refers to.

     <xs:complexType name="extensionType">
       <xs:sequence>
         <xs:element name="name" type="xs:string" />
         <xs:element name="schemaRef" type="xs:anyURI"/>
         <xs:element name="version" type="versionType"/>
         <xs:any namespace="##other" processContents="lax"
               minOccurs="0"/>
       </xs:sequence>
       <xs:anyAttribute namespace="##other" processContents="lax"/>
     </xs:complexType>

                     Figure 12: The <extension> Element

   The above-described <extension> element is carried within the
   'options' and 'optionsResponse' messages to represent the extensions
   supported by both the CI and the CR.

   Extensions MUST be defined in a separate XML schema file and MUST be
   provided with a companion document describing their semantics and
   use.

8.1.  Extension Example

   An example of an extension might be a "new" capture attribute (i.e.,
   a capture attribute that is not envisioned in the current standard
   defining the CLUE data model in [RFC8846]) needed to further describe
   a video capture.

   The CLUE data model document [RFC8846] envisions the possibility of
   adding this kind of "extra" information in the description of a video
   capture.  For convenience, the XML definition of a video capture
   taken from [RFC8846] is shown in Figure 13 below.

   <!-- VIDEO CAPTURE TYPE -->
      <xs:complexType name="videoCaptureType">
       <xs:complexContent>
        <xs:extension base="tns:mediaCaptureType">
         <xs:sequence>
          <xs:any namespace="##other" processContents="lax"
                minOccurs="0"
          maxOccurs="unbounded"/>
         </xs:sequence>
         <xs:anyAttribute namespace="##other" processContents="lax"/>
        </xs:extension>
       </xs:complexContent>
      </xs:complexType>

             Figure 13: XML Definition of a CLUE Video Capture

   According to such a definition, a video capture might have, after the
   set of generic media capture attributes, a set of new attributes
   defined elsewhere, i.e., in an XML schema defining an extension.  The
   XML schema defining the extension might look like the following
   (Figure 14):

   <?xml version="1.0" encoding="UTF-8" ?>
   <xs:schema version="1.0"
     targetNamespace="https://example.extensions.com/myVideoExtensions"
     xmlns:xs="https://www.w3.org/2001/XMLSchema"
     xmlns="https://example.extensions.com/myVideoExtensions"
     elementFormDefault="qualified"
     attributeFormDefault="unqualified">

           <!--
                 This is the new element to be put in place of the <any>
                 element in the video capture definition
                 of the CLUE data model schema
           -->

           <xs:element name="myVideoExtension">
                   <xs:complexType>
                           <xs:sequence>
                                <xs:element ref="newVideoAttribute1"/>
                                <xs:element ref="newVideoAttribute2"/>
                           </xs:sequence>
                   </xs:complexType>
           </xs:element>

           <xs:element name="newVideoAttribute1" type="xs:string"/>

           <xs:element name = "newVideoAttribute2" type = "xs:boolean"/>
   </xs:schema>

                Figure 14: XML Schema Defining an Extension

   By using the extension above, a video capture can be further
   described in the advertisement using the <myVideoExtension> element
   containing two extra pieces of information (<newVideoAttribute1> and
   <newVideoAttribute2>), besides using the attributes envisioned for a
   generic media capture.  As stated in this document, both participants
   must be aware of the extension schema and related semantics to use
   such an extension and must negotiate it via the 'options' and
   'optionsResponse' messages.

9.  XML Schema

   The XML schema defining the CLUE messages is provided below
   (Figure 15).

   <?xml version="1.0" encoding="UTF-8"?>
   <xs:schema xmlns:xs="https://www.w3.org/2001/XMLSchema"
           xmlns="urn:ietf:params:xml:ns:clue-protocol"
           xmlns:dm="urn:ietf:params:xml:ns:clue-info"
           version="1.0"
           targetNamespace="urn:ietf:params:xml:ns:clue-protocol"
           elementFormDefault="qualified"
           attributeFormDefault="unqualified">
     <!-- Import data model schema -->
     <xs:import namespace="urn:ietf:params:xml:ns:clue-info"/>
     <!-- ELEMENT DEFINITIONS -->
     <xs:element name="options" type="optionsMessageType" />
     <xs:element name="optionsResponse"
           type="optionsResponseMessageType"/>
     <xs:element name="advertisement" type="advertisementMessageType"/>
     <xs:element name="ack" type="advAcknowledgementMessageType"/>
     <xs:element name="configure" type="configureMessageType"/>
     <xs:element name="configureResponse"
           type="configureResponseMessageType"/>
     <!-- CLUE MESSAGE TYPE -->
     <xs:complexType name="clueMessageType" abstract="true">
       <xs:sequence>
         <xs:element name="clueId" type="xs:string" minOccurs="0" />
         <xs:element name="sequenceNr" type="xs:positiveInteger" />
       </xs:sequence>
       <xs:attribute name="protocol" type="xs:string" fixed="CLUE"
           use="required" />
       <xs:attribute name="v" type="versionType" use="required" />
     </xs:complexType>
     <!-- CLUE RESPONSE TYPE -->
     <xs:complexType name="clueResponseType">
       <xs:complexContent>
         <xs:extension base="clueMessageType">
           <xs:sequence>
             <xs:element name="responseCode" type="responseCodeType" />
             <xs:element name="reasonString" type="xs:string"
                   minOccurs="0"/>
           </xs:sequence>
         </xs:extension>
       </xs:complexContent>
     </xs:complexType>
     <!-- VERSION TYPE -->
     <xs:simpleType name="versionType">
       <xs:restriction base="xs:string">
         <xs:pattern value="[1-9][0-9]*\.[0-9]+" />
       </xs:restriction>
     </xs:simpleType>
     <!-- RESPONSE CODE TYPE -->
     <xs:simpleType name="responseCodeType">
       <xs:restriction base="xs:integer">
         <xs:pattern value="[1-9][0-9][0-9]" />
       </xs:restriction>
     </xs:simpleType>
     <!-- SUCCESS RESPONSE CODE TYPE -->
     <xs:simpleType name="successResponseCodeType">
       <xs:restriction base="xs:integer">
         <xs:pattern value="2[0-9][0-9]" />
       </xs:restriction>
     </xs:simpleType>
     <!-- CLUE OPTIONS -->
     <xs:complexType name="optionsMessageType">
       <xs:complexContent>
         <xs:extension base="clueMessageType">
           <xs:sequence>
             <xs:element name="mediaProvider" type="xs:boolean"/>
             <xs:element name="mediaConsumer" type="xs:boolean"/>
             <xs:element name="supportedVersions"
                   type="versionsListType"
                   minOccurs="0" />
             <xs:element name="supportedExtensions"
                   type="extensionsListType" minOccurs="0"/>
             <xs:any namespace="##other" processContents="lax"
                   minOccurs="0"/>
           </xs:sequence>
           <xs:anyAttribute namespace="##other" processContents="lax"/>
         </xs:extension>
       </xs:complexContent>
     </xs:complexType>
     <!-- VERSIONS LIST TYPE -->
     <xs:complexType name="versionsListType">
       <xs:sequence>
         <xs:element name="version" type="versionType" minOccurs="1"
           maxOccurs="unbounded"/>
         <xs:any namespace="##other" processContents="lax"
           minOccurs="0"/>
       </xs:sequence>
       <xs:anyAttribute namespace="##other" processContents="lax" />
     </xs:complexType>
     <!-- EXTENSIONS LIST TYPE -->
     <xs:complexType name="extensionsListType">
       <xs:sequence>
         <xs:element name="extension" type="extensionType"
           minOccurs="1"
           maxOccurs="unbounded"/>
         <xs:any namespace="##other" processContents="lax"
           minOccurs="0"/>
       </xs:sequence>
       <xs:anyAttribute namespace="##other" processContents="lax" />
     </xs:complexType>
     <!-- EXTENSION TYPE -->
     <xs:complexType name="extensionType">
       <xs:sequence>
         <xs:element name="name" type="xs:string" />
         <xs:element name="schemaRef" type="xs:anyURI" />
         <xs:element name="version" type="versionType" />
         <xs:any namespace="##other" processContents="lax"
           minOccurs="0"/>
       </xs:sequence>
       <xs:anyAttribute namespace="##other" processContents="lax"/>
     </xs:complexType>
     <!-- CLUE 'optionsResponse' -->
     <xs:complexType name="optionsResponseMessageType">
       <xs:complexContent>
         <xs:extension base="clueResponseType">
           <xs:sequence>
             <xs:element name="mediaProvider" type="xs:boolean"
                   minOccurs="0"/>
             <xs:element name="mediaConsumer" type="xs:boolean"
                   minOccurs="0"/>
             <xs:element name="version" type="versionType"
                   minOccurs="0"/>
             <xs:element name="commonExtensions"
                   type="extensionsListType" minOccurs="0"/>
             <xs:any namespace="##other" processContents="lax"
                   minOccurs="0"/>
           </xs:sequence>
           <xs:anyAttribute namespace="##other" processContents="lax"/>
         </xs:extension>
       </xs:complexContent>
     </xs:complexType>
     <!-- CLUE ADVERTISEMENT MESSAGE TYPE -->
     <xs:complexType name="advertisementMessageType">
       <xs:complexContent>
         <xs:extension base="clueMessageType">
           <xs:sequence>
             <!-- mandatory -->
             <xs:element name="mediaCaptures"
                   type="dm:mediaCapturesType"/>
             <xs:element name="encodingGroups"
                   type="dm:encodingGroupsType"/>
             <xs:element name="captureScenes"
                   type="dm:captureScenesType"/>
             <!-- optional -->
             <xs:element name="simultaneousSets"
                   type="dm:simultaneousSetsType" minOccurs="0"/>
             <xs:element name="globalViews" type="dm:globalViewsType"
                   minOccurs="0"/>
             <xs:element name="people"
                   type="dm:peopleType" minOccurs="0"/>
             <xs:any namespace="##other" processContents="lax"
                   minOccurs="0"/>
           </xs:sequence>
           <xs:anyAttribute namespace="##other" processContents="lax"/>
         </xs:extension>
       </xs:complexContent>
     </xs:complexType>
     <!-- 'ack' MESSAGE TYPE -->
     <xs:complexType name="advAcknowledgementMessageType">
       <xs:complexContent>
         <xs:extension base="clueResponseType">
           <xs:sequence>
             <xs:element name="advSequenceNr"
                   type="xs:positiveInteger"/>
             <xs:any namespace="##other" processContents="lax"
                   minOccurs="0"/>
           </xs:sequence>
           <xs:anyAttribute namespace="##other" processContents="lax"/>
         </xs:extension>
       </xs:complexContent>
     </xs:complexType>
     <!-- CLUE 'configure' MESSAGE TYPE -->
     <xs:complexType name="configureMessageType">
       <xs:complexContent>
         <xs:extension base="clueMessageType">
           <xs:sequence>
             <!-- mandatory fields -->
             <xs:element name="advSequenceNr"
                   type="xs:positiveInteger"/>
             <xs:element name="ack" type="successResponseCodeType"
                   minOccurs="0"/>
             <xs:element name="captureEncodings"
                   type="dm:captureEncodingsType" minOccurs="0"/>
             <xs:any namespace="##other" processContents="lax"
                   minOccurs="0"/>
           </xs:sequence>
           <xs:anyAttribute namespace="##other" processContents="lax"/>
         </xs:extension>
       </xs:complexContent>
     </xs:complexType>
     <!-- 'configureResponse' MESSAGE TYPE -->
     <xs:complexType name="configureResponseMessageType">
       <xs:complexContent>
         <xs:extension base="clueResponseType">
           <xs:sequence>
             <xs:element name="confSequenceNr"
                   type="xs:positiveInteger"/>
             <xs:any namespace="##other" processContents="lax"
                   minOccurs="0"/>
           </xs:sequence>
           <xs:anyAttribute namespace="##other" processContents="lax"/>
         </xs:extension>
       </xs:complexContent>
     </xs:complexType>
   </xs:schema>

                  Figure 15: Schema Defining CLUE Messages

10.  Call Flow Example

   This section describes the CLUE protocol messages exchanged in the
   following call flow.  For simplicity, only one direction of media is
   shown, as the other direction is precisely symmetric.

                    +-----+               +-----+
                    |     |               |     |
                    | CP1 |               | CP2 |
                    |     |               |     |
                    +--+--+               +--+--+
                       |                     |
                       |    1. options       |
                       +-------------------->|
                       |                     |
                       |                     |
                       |2. optionsResponse   |
                       |<--------------------+
                       |                     |
                       |                     |
                       |3. advertisement     |
                       +-------------------->|
                       |                     |
                       |                     |
                       |4. configure+ack     |
                       |<--------------------+
                       |                     |
                       |                     |
                       |5. configureResponse |
                       +-------------------->|
                       |                     |
                       |                     |
                       |6. advertisement     |
                       +-------------------->|
                       |                     |
                       |                     |
                       |    7. ack           |
                       |<--------------------+
                       |                     |
                       |                     |
                       |8. configure         |
                       |<--------------------+
                       |                     |
                       |                     |
                       |9. configureResponse |
                       +-------------------->|
                       |                     |
                       |                     |
                       .                     .
                       .                     .
                       .                     .

   Two CPs, CP1 and CP2, have successfully set up the CLUE channel
   according to [RFC8850].  CP1 is the CI, and CP2 is the CR.

   *  The initiation phase starts (negotiation of the CLUE protocol
      version and extensions).  CP1, as the CI, sends to CP2 an
      'options' message specifying the supported versions and extensions
      (Section 10.1).  CP1 supports (i) version 1.4 with extensions E1,
      E2, and E3 and (ii) version 2.7 with extensions E4 and E5.
      Because of such capabilities, CP1 sends an 'options' message with
      the "v" attribute set to "1.4" and explicitly specifies all the
      supported versions and extensions in the corresponding fields of
      the 'options' message.  In the 'options' message, CP1 also
      specifies that it intends to act as both an MP and an MC.

   *  CP2 supports versions 3.0, 2.9, and 1.9 of the CLUE protocol, each
      version without any extensions.  Version 2.7 is the best common
      choice.  Given the received 'options' message, CP2 answers with an
      'optionsResponse' message in which it specifies only version 2.7
      as the agreed-upon version of the CLUE protocol to be used,
      leaving blank the extensions part of the message to say that no
      extensions will be used in the CLUE session (Section 10.2).  In
      the 'optionsResponse' message, CP2 also specifies that it intends
      to act as both an MP and an MC.

   After the initiation phase is completed, CP1 and CP2 start their
   activity as the MP and the MC, respectively.  For the sake of
   simplicity, the rest of the call flow focuses only on the dialogue
   between MP CP1 and MC CP2.

   *  CP1 advertises a telepresence configuration like the one described
      in [RFC8846], Section 27, where there are (i) three main video
      streams captured by three cameras, with the central camera capable
      of capturing a zoomed-out view of the overall telepresence room,
      (ii) a multicontent capture of the loudest segment of the room,
      and (iii) one audio capture for the audio of the whole room
      (Section 10.3).

   *  CP2 receives CP1's 'advertisement' message and, after processing
      it, sends back to CP1 a 'configure+ack' message in which it
      declares its interest in the multicontent capture and the audio
      capture only (Section 10.4).

   *  CP1 answers CP2's 'configure+ack' message with a
      'configureResponse' message that includes a 200 (Success) response
      code to accept all of CP2's requests (Section 10.5).

   *  To reflect the changes in its telepresence offer, CP1 issues a new
      'advertisement' message to CP2 (Section 10.6), this time also
      adding a composed capture made of a big picture representing the
      current speaker and two picture-in-picture boxes representing the
      previous speakers (see [RFC8846], Section 28 for more details
      regarding the telepresence description).

   *  CP2 acknowledges the second 'advertisement' message with an 'ack'
      message (Section 10.7).

   *  Later in the session, CP2 changes the requested media streams from
      CP1 by sending to CP1 a 'configure' message replacing the
      previously selected video streams with the new composed media
      streams advertised by CP1 (Section 10.8).  Media streams from the
      previous configuration continue to flow during the reconfiguration
      process.

   *  Finally, CP1 accepts CP2's latest request with a
      'configureResponse' message (Section 10.9).

   We would also like to point out that in the depicted flow three
   distinct sequence number spaces per sender are involved (two of which
   appear in the snippets, since we only show one direction of media).
   The discontinuity between the sequence number values in Sections 10.2
   and 10.3 is hence correct.

10.1.  CLUE Message No. 1: 'options'

   <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
   <options xmlns="urn:ietf:params:xml:ns:clue-protocol"
    xmlns:ns2="urn:ietf:params:xml:ns:clue-info"
    xmlns:ns3="urn:ietf:params:xml:ns:vcard-4.0"
    xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
    protocol="CLUE" v="1.4">
       <clueId>CP1</clueId>
       <sequenceNr>51</sequenceNr>
       <mediaProvider>true</mediaProvider>
       <mediaConsumer>true</mediaConsumer>
       <supportedVersions>
           <version>1.4</version>
           <version>2.7</version>
       </supportedVersions>
       <supportedExtensions>
           <extension>
                   <name>E1</name>
                   <schemaRef>URL_E1</schemaRef>
                   <version>1.4</version>
           </extension>
           <extension>
                   <name>E2</name>
                   <schemaRef>URL_E2</schemaRef>
                   <version>1.4</version>
           </extension>
           <extension>
                   <name>E3</name>
                   <schemaRef>URL_E3</schemaRef>
                   <version>1.4</version>
           </extension>
           <extension>
                   <name>E4</name>
                   <schemaRef>URL_E4</schemaRef>
                   <version>2.7</version>
           </extension>
           <extension>
                   <name>E5</name>
                   <schemaRef>URL_E5</schemaRef>
                   <version>2.7</version>
           </extension>
       </supportedExtensions>
   </options>

10.2.  CLUE Message No. 2: 'optionsResponse'

   <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
   <optionsResponse xmlns="urn:ietf:params:xml:ns:clue-protocol"
    xmlns:ns2="urn:ietf:params:xml:ns:clue-info"
    xmlns:ns3="urn:ietf:params:xml:ns:vcard-4.0"
    xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
    protocol="CLUE" v="1.4">
       <clueId>CP2</clueId>
       <sequenceNr>62</sequenceNr>
       <responseCode>200</responseCode>
       <reasonString>Success</reasonString>
       <mediaProvider>true</mediaProvider>
       <mediaConsumer>true</mediaConsumer>
       <version>2.7</version>
   </optionsResponse>

10.3.  CLUE Message No. 3: 'advertisement'

   <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
   <ns2:advertisement xmlns="urn:ietf:params:xml:ns:clue-info"
    xmlns:ns2="urn:ietf:params:xml:ns:clue-protocol"
    xmlns:ns3="urn:ietf:params:xml:ns:vcard-4.0"
    xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
    protocol="CLUE" v="2.7">
       <ns2:clueId>CP1</ns2:clueId>
       <ns2:sequenceNr>11</ns2:sequenceNr>
       <ns2:mediaCaptures>
         <mediaCapture
            xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
            xsi:type="audioCaptureType" captureID="AC0"
            mediaType="audio">
            <captureSceneIDREF>CS1</captureSceneIDREF>
               <spatialInformation>
                   <captureOrigin>
                      <capturePoint>
                         <x>0.0</x>
                         <y>0.0</y>
                         <z>10.0</z>
                    </capturePoint>
                    <lineOfCapturePoint>
                      <x>0.0</x>
                      <y>1.0</y>
                      <z>10.0</z>
                     </lineOfCapturePoint>
                   </captureOrigin>
                 </spatialInformation>
                <individual>true</individual>
                <encGroupIDREF>EG1</encGroupIDREF>
                <description lang="en">main audio from the room
                </description>
                <priority>1</priority>
                <lang>it</lang>
                <mobility>static</mobility>
                <view>room</view>
                <capturedPeople>
                    <personIDREF>alice</personIDREF>
                    <personIDREF>bob</personIDREF>
                    <personIDREF>ciccio</personIDREF>
                </capturedPeople>
            </mediaCapture>
            <mediaCapture
             xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
             xsi:type="videoCaptureType" captureID="VC0"
             mediaType="video">
                <captureSceneIDREF>CS1</captureSceneIDREF>
                <spatialInformation>
                    <captureOrigin>
                            <capturePoint>
                            <x>-2.0</x>
                            <y>0.0</y>
                            <z>10.0</z>
                        </capturePoint>
                    </captureOrigin>
                    <captureArea>
                            <bottomLeft>
                            <x>-3.0</x>
                            <y>20.0</y>
                            <z>9.0</z>
                            </bottomLeft>
                            <bottomRight>
                            <x>-1.0</x>
                            <y>20.0</y>
                            <z>9.0</z>
                            </bottomRight>
                            <topLeft>
                            <x>-3.0</x>
                            <y>20.0</y>
                            <z>11.0</z>
                            </topLeft>
                            <topRight>
                            <x>-1.0</x>
                            <y>20.0</y>
                            <z>11.0</z>
                            </topRight>
                    </captureArea>
                </spatialInformation>
                <individual>true</individual>
                <encGroupIDREF>EG0</encGroupIDREF>
                <description lang="en">left camera video capture
                </description>
                <priority>1</priority>
                <lang>it</lang>
                <mobility>static</mobility>
                <view>individual</view>
                <capturedPeople>
                    <personIDREF>ciccio</personIDREF>
                </capturedPeople>
            </mediaCapture>
            <mediaCapture
             xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
             xsi:type="videoCaptureType" captureID="VC1"
             mediaType="video">
                <captureSceneIDREF>CS1</captureSceneIDREF>
                <spatialInformation>
                    <captureOrigin>
                            <capturePoint>
                            <x>0.0</x>
                            <y>0.0</y>
                            <z>10.0</z>
                        </capturePoint>
                    </captureOrigin>
                    <captureArea>
                            <bottomLeft>
                            <x>-1.0</x>
                            <y>20.0</y>
                            <z>9.0</z>
                            </bottomLeft>
                            <bottomRight>
                            <x>1.0</x>
                            <y>20.0</y>
                            <z>9.0</z>
                            </bottomRight>
                            <topLeft>
                            <x>-1.0</x>
                            <y>20.0</y>
                            <z>11.0</z>
                            </topLeft>
                            <topRight>
                            <x>1.0</x>
                            <y>20.0</y>
                            <z>11.0</z>
                            </topRight>
                    </captureArea>
                </spatialInformation>
                <individual>true</individual>
                <encGroupIDREF>EG0</encGroupIDREF>
                <description lang="en">central camera video capture
                </description>
                <priority>1</priority>
                <lang>it</lang>
                <mobility>static</mobility>
                <view>individual</view>
                <capturedPeople>
                    <personIDREF>alice</personIDREF>
                </capturedPeople>
            </mediaCapture>
            <mediaCapture
             xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
             xsi:type="videoCaptureType" captureID="VC2"
             mediaType="video">
                <captureSceneIDREF>CS1</captureSceneIDREF>
                <spatialInformation>
                    <captureOrigin>
                            <capturePoint>
                            <x>2.0</x>
                            <y>0.0</y>
                            <z>10.0</z>
                        </capturePoint>
                    </captureOrigin>
                    <captureArea>
                            <bottomLeft>
                            <x>1.0</x>
                            <y>20.0</y>
                            <z>9.0</z>
                            </bottomLeft>
                            <bottomRight>
                            <x>3.0</x>
                            <y>20.0</y>
                            <z>9.0</z>
                            </bottomRight>
                            <topLeft>
                            <x>1.0</x>
                            <y>20.0</y>
                            <z>11.0</z>
                            </topLeft>
                            <topRight>
                            <x>3.0</x>
                            <y>20.0</y>
                            <z>11.0</z>
                            </topRight>
                    </captureArea>
                </spatialInformation>
                <individual>true</individual>
                <encGroupIDREF>EG0</encGroupIDREF>
                <description lang="en">right camera video capture
                </description>
                <priority>1</priority>
                <lang>it</lang>
                <mobility>static</mobility>
                <view>individual</view>
                <capturedPeople>
                    <personIDREF>bob</personIDREF>
                </capturedPeople>
            </mediaCapture>
            <mediaCapture
             xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
             xsi:type="videoCaptureType" captureID="VC3"
             mediaType="video">
                <captureSceneIDREF>CS1</captureSceneIDREF>
                <spatialInformation>
                    <captureArea>
                            <bottomLeft>
                            <x>-3.0</x>
                            <y>20.0</y>
                            <z>9.0</z>
                            </bottomLeft>
                            <bottomRight>
                            <x>3.0</x>
                            <y>20.0</y>
                            <z>9.0</z>
                            </bottomRight>
                            <topLeft>
                            <x>-3.0</x>
                            <y>20.0</y>
                            <z>11.0</z>
                            </topLeft>
                            <topRight>
                            <x>3.0</x>
                            <y>20.0</y>
                            <z>11.0</z>
                            </topRight>
                    </captureArea>
                </spatialInformation>
                <content>
                    <sceneViewIDREF>SE1</sceneViewIDREF>
                </content>
                <policy>SoundLevel:0</policy>
                <encGroupIDREF>EG0</encGroupIDREF>
                <description lang="en">loudest room segment
                </description>
                <priority>2</priority>
                <lang>it</lang>
                <mobility>static</mobility>
                <view>individual</view>
                </mediaCapture>
            <mediaCapture
             xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
             xsi:type="videoCaptureType" captureID="VC4"
             mediaType="video">
                <captureSceneIDREF>CS1</captureSceneIDREF>
                <spatialInformation>
                    <captureOrigin>
                            <capturePoint>
                            <x>0.0</x>
                            <y>0.0</y>
                            <z>10.0</z>
                        </capturePoint>
                    </captureOrigin>
                    <captureArea>
                            <bottomLeft>
                            <x>-3.0</x>
                            <y>20.0</y>
                            <z>7.0</z>
                            </bottomLeft>
                            <bottomRight>
                            <x>3.0</x>
                            <y>20.0</y>
                            <z>7.0</z>
                            </bottomRight>
                            <topLeft>
                            <x>-3.0</x>
                            <y>20.0</y>
                            <z>13.0</z>
                            </topLeft>
                            <topRight>
                            <x>3.0</x>
                            <y>20.0</y>
                            <z>13.0</z>
                            </topRight>
                    </captureArea>
                </spatialInformation>
                <individual>true</individual>
                <encGroupIDREF>EG0</encGroupIDREF>
                <description lang="en">zoomed-out view of all people in
                the room</description>
                <priority>2</priority>
                <lang>it</lang>
                <mobility>static</mobility>
                <view>room</view>
                <capturedPeople>
                    <personIDREF>alice</personIDREF>
                    <personIDREF>bob</personIDREF>
                    <personIDREF>ciccio</personIDREF>
                    </capturedPeople>
            </mediaCapture>
        </ns2:mediaCaptures>
        <ns2:encodingGroups>
            <encodingGroup encodingGroupID="EG0">
                <maxGroupBandwidth>600000</maxGroupBandwidth>
                <encodingIDList>
                    <encodingID>ENC1</encodingID>
                    <encodingID>ENC2</encodingID>
                    <encodingID>ENC3</encodingID>
                </encodingIDList>
            </encodingGroup>
            <encodingGroup encodingGroupID="EG1">
                <maxGroupBandwidth>300000</maxGroupBandwidth>
                <encodingIDList>
                    <encodingID>ENC4</encodingID>
                    <encodingID>ENC5</encodingID>
                </encodingIDList>
            </encodingGroup>
        </ns2:encodingGroups>
        <ns2:captureScenes>
            <captureScene scale="unknown" sceneID="CS1">
                <sceneViews>
                    <sceneView sceneViewID="SE1">
                        <mediaCaptureIDs>
                            <mediaCaptureIDREF>VC0</mediaCaptureIDREF>
                            <mediaCaptureIDREF>VC1</mediaCaptureIDREF>
                            <mediaCaptureIDREF>VC2</mediaCaptureIDREF>
                        </mediaCaptureIDs>
                    </sceneView>
                    <sceneView sceneViewID="SE2">
                        <mediaCaptureIDs>
                            <mediaCaptureIDREF>VC3</mediaCaptureIDREF>
                        </mediaCaptureIDs>
                    </sceneView>
                    <sceneView sceneViewID="SE3">
                        <mediaCaptureIDs>
                            <mediaCaptureIDREF>VC4</mediaCaptureIDREF>
                        </mediaCaptureIDs>
                    </sceneView>
                    <sceneView sceneViewID="SE4">
                        <mediaCaptureIDs>
                            <mediaCaptureIDREF>AC0</mediaCaptureIDREF>
                        </mediaCaptureIDs>
                    </sceneView>
                </sceneViews>
            </captureScene>
        </ns2:captureScenes>
        <ns2:simultaneousSets>
            <simultaneousSet setID="SS1">
                <mediaCaptureIDREF>VC3</mediaCaptureIDREF>
                <sceneViewIDREF>SE1</sceneViewIDREF>
            </simultaneousSet>
            <simultaneousSet setID="SS2">
                <mediaCaptureIDREF>VC0</mediaCaptureIDREF>
                <mediaCaptureIDREF>VC2</mediaCaptureIDREF>
                <mediaCaptureIDREF>VC4</mediaCaptureIDREF>
            </simultaneousSet>
        </ns2:simultaneousSets>
        <ns2:people>
            <person personID="bob">
                <personInfo>
                    <ns3:fn>
                      <ns3:text>Bob</ns3:text>
                    </ns3:fn>
                </personInfo>
                <personType>minute taker</personType>
            </person>
            <person personID="alice">
                <personInfo>
                    <ns3:fn>
                        <ns3:text>Alice</ns3:text>
                    </ns3:fn>
                </personInfo>
                <personType>presenter</personType>
            </person>
            <person personID="ciccio">
                <personInfo>
                    <ns3:fn>
                        <ns3:text>Ciccio</ns3:text>
                    </ns3:fn>
                </personInfo>
                <personType>chairman</personType>
                <personType>timekeeper</personType>
            </person>
        </ns2:people>
   </ns2:advertisement>

10.4.  CLUE Message No. 4: 'configure+ack'

   <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
   <ns2:configure xmlns="urn:ietf:params:xml:ns:clue-info"
    xmlns:ns2="urn:ietf:params:xml:ns:clue-protocol"
    xmlns:ns3="urn:ietf:params:xml:ns:vcard-4.0"
    xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
    protocol="CLUE" v="2.7">
       <ns2:clueId>CP2</ns2:clueId>
       <ns2:sequenceNr>22</ns2:sequenceNr>
       <ns2:advSequenceNr>11</ns2:advSequenceNr>
       <ns2:ack>200</ns2:ack>
       <ns2:captureEncodings>
           <captureEncoding ID="ce123">
              <captureID>AC0</captureID>
              <encodingID>ENC4</encodingID>
           </captureEncoding>
           <captureEncoding ID="ce223">
              <captureID>VC3</captureID>
              <encodingID>ENC1</encodingID>
              <configuredContent>
                 <sceneViewIDREF>SE1</sceneViewIDREF>
              </configuredContent>
          </captureEncoding>
       </ns2:captureEncodings>
   </ns2:configure>

10.5.  CLUE Message No. 5: 'configureResponse'

   <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
   <ns2:configureResponse xmlns="urn:ietf:params:xml:ns:clue-info"
    xmlns:ns2="urn:ietf:params:xml:ns:clue-protocol"
    xmlns:ns3="urn:ietf:params:xml:ns:vcard-4.0"
    xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
    protocol="CLUE" v="2.7">
       <ns2:clueId>CP1</ns2:clueId>
       <ns2:sequenceNr>12</ns2:sequenceNr>
       <ns2:responseCode>200</ns2:responseCode>
       <ns2:reasonString>Success</ns2:reasonString>
       <ns2:confSequenceNr>22</ns2:confSequenceNr>
   </ns2:configureResponse>

10.6.  CLUE Message No. 6: 'advertisement'

   <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
   <ns2:advertisement xmlns="urn:ietf:params:xml:ns:clue-info"
    xmlns:ns2="urn:ietf:params:xml:ns:clue-protocol"
    xmlns:ns3="urn:ietf:params:xml:ns:vcard-4.0"
    xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
    protocol="CLUE" v="2.7">
       <ns2:clueId>CP1</ns2:clueId>
       <ns2:sequenceNr>13</ns2:sequenceNr>
       <ns2:mediaCaptures>
            <mediaCapture
             xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
             xsi:type="audioCaptureType" captureID="AC0"
             mediaType="audio">
                <captureSceneIDREF>CS1</captureSceneIDREF>
                <spatialInformation>
                    <captureOrigin>
                            <capturePoint>
                            <x>0.0</x>
                            <y>0.0</y>
                            <z>10.0</z>
                        </capturePoint>
                        <lineOfCapturePoint>
                            <x>0.0</x>
                            <y>1.0</y>
                            <z>10.0</z>
                        </lineOfCapturePoint>
                    </captureOrigin>
                </spatialInformation>
                <individual>true</individual>
                <encGroupIDREF>EG1</encGroupIDREF>
                <description lang="en">main audio from the room
                </description>
                <priority>1</priority>
                <lang>it</lang>
                <mobility>static</mobility>
                <view>room</view>
                <capturedPeople>
                    <personIDREF>alice</personIDREF>
                    <personIDREF>bob</personIDREF>
                    <personIDREF>ciccio</personIDREF>
                </capturedPeople>
            </mediaCapture>
            <mediaCapture
             xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
             xsi:type="videoCaptureType" captureID="VC0"
             mediaType="video">
                <captureSceneIDREF>CS1</captureSceneIDREF>
                <spatialInformation>
                    <captureOrigin>
                            <capturePoint>
                            <x>0.5</x>
                            <y>1.0</y>
                            <z>0.5</z>
                        </capturePoint>
                        <lineOfCapturePoint>
                            <x>0.5</x>
                            <y>0.0</y>
                            <z>0.5</z>
                        </lineOfCapturePoint>
                    </captureOrigin>
                </spatialInformation>
                <individual>true</individual>
                <encGroupIDREF>EG0</encGroupIDREF>
                <description lang="en">left camera video capture
                </description>
                <priority>1</priority>
                <lang>it</lang>
                <mobility>static</mobility>
                <view>individual</view>
                <capturedPeople>
                    <personIDREF>ciccio</personIDREF>
                </capturedPeople>
            </mediaCapture>
            <mediaCapture
             xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
             xsi:type="videoCaptureType" captureID="VC1"
             mediaType="video">
                <captureSceneIDREF>CS1</captureSceneIDREF>
                <spatialInformation>
                    <captureOrigin>
                            <capturePoint>
                            <x>0.0</x>
                            <y>0.0</y>
                            <z>10.0</z>
                        </capturePoint>
                    </captureOrigin>
                    <captureArea>
                            <bottomLeft>
                            <x>-1.0</x>
                            <y>20.0</y>
                            <z>9.0</z>
                            </bottomLeft>
                            <bottomRight>
                            <x>1.0</x>
                            <y>20.0</y>
                            <z>9.0</z>
                            </bottomRight>
                            <topLeft>
                            <x>-1.0</x>
                            <y>20.0</y>
                            <z>11.0</z>
                            </topLeft>
                            <topRight>
                            <x>1.0</x>
                            <y>20.0</y>
                            <z>11.0</z>
                            </topRight>
                    </captureArea>
                </spatialInformation>
                <individual>true</individual>
                <encGroupIDREF>EG0</encGroupIDREF>
                <description lang="en">central camera video capture
                </description>
                <priority>1</priority>
                <lang>it</lang>
                <mobility>static</mobility>
                <view>individual</view>
                <capturedPeople>
                    <personIDREF>alice</personIDREF>
                </capturedPeople>
            </mediaCapture>
            <mediaCapture
             xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
             xsi:type="videoCaptureType" captureID="VC2"
             mediaType="video">
                <captureSceneIDREF>CS1</captureSceneIDREF>
                <spatialInformation>
                    <captureOrigin>
                            <capturePoint>
                            <x>2.0</x>
                            <y>0.0</y>
                            <z>10.0</z>
                        </capturePoint>
                    </captureOrigin>
                    <captureArea>
                            <bottomLeft>
                            <x>1.0</x>
                            <y>20.0</y>
                            <z>9.0</z>
                            </bottomLeft>
                            <bottomRight>
                            <x>3.0</x>
                            <y>20.0</y>
                            <z>9.0</z>
                            </bottomRight>
                            <topLeft>
                            <x>1.0</x>
                            <y>20.0</y>
                            <z>11.0</z>
                            </topLeft>
                            <topRight>
                            <x>3.0</x>
                            <y>20.0</y>
                            <z>11.0</z>
                            </topRight>
                    </captureArea>
                </spatialInformation>
                <individual>true</individual>
                <encGroupIDREF>EG0</encGroupIDREF>
                <description lang="en">right camera video capture
                </description>
                <priority>1</priority>
                <lang>it</lang>
                <mobility>static</mobility>
                <view>individual</view>
                <capturedPeople>
                    <personIDREF>bob</personIDREF>
                </capturedPeople>
            </mediaCapture>
            <mediaCapture
             xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
             xsi:type="videoCaptureType" captureID="VC3"
             mediaType="video">
                <captureSceneIDREF>CS1</captureSceneIDREF>
                <spatialInformation>
                    <captureArea>
                            <bottomLeft>
                            <x>-3.0</x>
                            <y>20.0</y>
                            <z>9.0</z>
                            </bottomLeft>
                            <bottomRight>
                            <x>3.0</x>
                            <y>20.0</y>
                            <z>9.0</z>
                            </bottomRight>
                            <topLeft>
                            <x>-3.0</x>
                            <y>20.0</y>
                            <z>11.0</z>
                            </topLeft>
                            <topRight>
                            <x>3.0</x>
                            <y>20.0</y>
                            <z>11.0</z>
                            </topRight>
                    </captureArea>
                </spatialInformation>
                <content>
                    <sceneViewIDREF>SE1</sceneViewIDREF>
                </content>
                <policy>SoundLevel:0</policy>
                <encGroupIDREF>EG0</encGroupIDREF>
                <description lang="en">loudest room segment
                </description>
                <priority>2</priority>
                <lang>it</lang>
                <mobility>static</mobility>
                <view>individual</view>
            </mediaCapture>
            <mediaCapture
             xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
             xsi:type="videoCaptureType" captureID="VC4"
             mediaType="video">
                <captureSceneIDREF>CS1</captureSceneIDREF>
                <spatialInformation>
                    <captureOrigin>
                            <capturePoint>
                            <x>0.0</x>
                            <y>0.0</y>
                            <z>10.0</z>
                        </capturePoint>
                    </captureOrigin>
                    <captureArea>
                            <bottomLeft>
                            <x>-3.0</x>
                            <y>20.0</y>
                            <z>7.0</z>
                            </bottomLeft>
                            <bottomRight>
                            <x>3.0</x>
                            <y>20.0</y>
                            <z>7.0</z>
                            </bottomRight>
                            <topLeft>
                            <x>-3.0</x>
                            <y>20.0</y>
                            <z>13.0</z>
                            </topLeft>
                            <topRight>
                            <x>3.0</x>
                            <y>20.0</y>
                            <z>13.0</z>
                            </topRight>
                    </captureArea>
                </spatialInformation>
                <individual>true</individual>
                <encGroupIDREF>EG0</encGroupIDREF>
                <description lang="en">
                  zoomed-out view of all people in the room
                </description>
                <priority>2</priority>
                <lang>it</lang>
                <mobility>static</mobility>
                <view>room</view>
                <capturedPeople>
                    <personIDREF>alice</personIDREF>
                    <personIDREF>bob</personIDREF>
                    <personIDREF>ciccio</personIDREF>
                </capturedPeople>
            </mediaCapture>
            <mediaCapture
             xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
             xsi:type="videoCaptureType" captureID="VC5"
             mediaType="video">
                <captureSceneIDREF>CS1</captureSceneIDREF>
                <spatialInformation>
                    <captureArea>
                            <bottomLeft>
                            <x>-3.0</x>
                            <y>20.0</y>
                            <z>9.0</z>
                            </bottomLeft>
                            <bottomRight>
                            <x>3.0</x>
                            <y>20.0</y>
                            <z>9.0</z>
                            </bottomRight>
                            <topLeft>
                            <x>-3.0</x>
                            <y>20.0</y>
                            <z>11.0</z>
                            </topLeft>
                            <topRight>
                            <x>3.0</x>
                            <y>20.0</y>
                            <z>11.0</z>
                            </topRight>
                    </captureArea>
                </spatialInformation>
                <content>
                    <sceneViewIDREF>SE1</sceneViewIDREF>
                </content>
                <policy>SoundLevel:1</policy>
                <description lang="en">penultimate loudest room segment
                </description>
                <lang>it</lang>
                <mobility>static</mobility>
                <view>individual</view>
            </mediaCapture>
            <mediaCapture
             xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
             xsi:type="videoCaptureType" captureID="VC6"
             mediaType="video">
                <captureSceneIDREF>CS1</captureSceneIDREF>
                <spatialInformation>
                    <captureArea>
                            <bottomLeft>
                            <x>-3.0</x>
                            <y>20.0</y>
                            <z>9.0</z>
                            </bottomLeft>
                            <bottomRight>
                            <x>3.0</x>
                            <y>20.0</y>
                            <z>9.0</z>
                            </bottomRight>
                            <topLeft>
                            <x>-3.0</x>
                            <y>20.0</y>
                            <z>11.0</z>
                            </topLeft>
                            <topRight>
                            <x>3.0</x>
                            <y>20.0</y>
                            <z>11.0</z>
                            </topRight>
                    </captureArea>
                </spatialInformation>
                <content>
                    <sceneViewIDREF>SE1</sceneViewIDREF>
                </content>
                <policy>SoundLevel:2</policy>
                <description lang="en">last but two loudest room segment
                </description>
                <lang>it</lang>
                <mobility>static</mobility>
                <view>individual</view>
            </mediaCapture>
            <mediaCapture
             xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
             xsi:type="videoCaptureType" captureID="VC7"
             mediaType="video">
                <captureSceneIDREF>CS1</captureSceneIDREF>
                <spatialInformation>
                    <captureArea>
                            <bottomLeft>
                            <x>-3.0</x>
                            <y>20.0</y>
                            <z>9.0</z>
                            </bottomLeft>
                            <bottomRight>
                            <x>3.0</x>
                            <y>20.0</y>
                            <z>9.0</z>
                            </bottomRight>
                            <topLeft>
                            <x>-3.0</x>
                            <y>20.0</y>
                            <z>11.0</z>
                            </topLeft>
                            <topRight>
                            <x>3.0</x>
                            <y>20.0</y>
                            <z>11.0</z>
                            </topRight>
                    </captureArea>
                </spatialInformation>
                <content>
                    <mediaCaptureIDREF>VC3</mediaCaptureIDREF>
                    <mediaCaptureIDREF>VC5</mediaCaptureIDREF>
                    <mediaCaptureIDREF>VC6</mediaCaptureIDREF>
                </content>
                <maxCaptures exactNumber="true">3</maxCaptures>
                <encGroupIDREF>EG0</encGroupIDREF>
                <description lang="en">big picture of the current
                speaker + pips about previous speakers</description>
                <priority>3</priority>
                <lang>it</lang>
                <mobility>static</mobility>
                <view>individual</view>
            </mediaCapture>
        </ns2:mediaCaptures>
        <ns2:encodingGroups>
            <encodingGroup encodingGroupID="EG0">
                <maxGroupBandwidth>600000</maxGroupBandwidth>
                <encodingIDList>
                    <encodingID>ENC1</encodingID>
                    <encodingID>ENC2</encodingID>
                    <encodingID>ENC3</encodingID>
                </encodingIDList>
            </encodingGroup>
            <encodingGroup encodingGroupID="EG1">
                <maxGroupBandwidth>300000</maxGroupBandwidth>
                <encodingIDList>
                    <encodingID>ENC4</encodingID>
                    <encodingID>ENC5</encodingID>
                </encodingIDList>
            </encodingGroup>
        </ns2:encodingGroups>
        <ns2:captureScenes>
            <captureScene scale="unknown" sceneID="CS1">
                <sceneViews>
                    <sceneView sceneViewID="SE1">
                        <description lang="en">participants' individual
                        videos</description>
                        <mediaCaptureIDs>
                            <mediaCaptureIDREF>VC0</mediaCaptureIDREF>
                            <mediaCaptureIDREF>VC1</mediaCaptureIDREF>
                            <mediaCaptureIDREF>VC2</mediaCaptureIDREF>
                        </mediaCaptureIDs>
                    </sceneView>
                    <sceneView sceneViewID="SE2">
                        <description lang="en">loudest segment of the
                        room</description>
                        <mediaCaptureIDs>
                            <mediaCaptureIDREF>VC3</mediaCaptureIDREF>
                        </mediaCaptureIDs>
                    </sceneView>
                    <sceneView sceneViewID="SE5">
                        <description lang="en">loudest segment of the
                        room + pips</description>
                        <mediaCaptureIDs>
                            <mediaCaptureIDREF>VC7</mediaCaptureIDREF>
                        </mediaCaptureIDs>
                    </sceneView>
                    <sceneView sceneViewID="SE4">
                        <description lang="en">room audio</description>
                        <mediaCaptureIDs>
                            <mediaCaptureIDREF>AC0</mediaCaptureIDREF>
                        </mediaCaptureIDs>
                    </sceneView>
                    <sceneView sceneViewID="SE3">
                        <description lang="en">room video</description>
                        <mediaCaptureIDs>
                            <mediaCaptureIDREF>VC4</mediaCaptureIDREF>
                        </mediaCaptureIDs>
                    </sceneView>
                </sceneViews>
            </captureScene>
        </ns2:captureScenes>
        <ns2:simultaneousSets>
            <simultaneousSet setID="SS1">
                <mediaCaptureIDREF>VC3</mediaCaptureIDREF>
                <mediaCaptureIDREF>VC7</mediaCaptureIDREF>
                <sceneViewIDREF>SE1</sceneViewIDREF>
            </simultaneousSet>
            <simultaneousSet setID="SS2">
                <mediaCaptureIDREF>VC0</mediaCaptureIDREF>
                <mediaCaptureIDREF>VC2</mediaCaptureIDREF>
                <mediaCaptureIDREF>VC4</mediaCaptureIDREF>
            </simultaneousSet>
        </ns2:simultaneousSets>
        <ns2:people>
            <person personID="bob">
                <personInfo>
                    <ns3:fn>
                        <ns3:text>Bob</ns3:text>
                    </ns3:fn>
                </personInfo>
                <personType>minute taker</personType>
            </person>
            <person personID="alice">
                <personInfo>
                    <ns3:fn>
                        <ns3:text>Alice</ns3:text>
                    </ns3:fn>
                </personInfo>
                <personType>presenter</personType>
            </person>
            <person personID="ciccio">
                <personInfo>
                    <ns3:fn>
                        <ns3:text>Ciccio</ns3:text>
                    </ns3:fn>
                </personInfo>
                <personType>chairman</personType>
                <personType>timekeeper</personType>
            </person>
        </ns2:people>
   </ns2:advertisement>

10.7.  CLUE Message No. 7: 'ack'

   <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
   <ack xmlns="urn:ietf:params:xml:ns:clue-protocol"
    xmlns:ns2="urn:ietf:params:xml:ns:clue-info"
    xmlns:ns3="urn:ietf:params:xml:ns:vcard-4.0"
    xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
    protocol="CLUE" v="2.7">
       <clueId>CP2</clueId>
       <sequenceNr>23</sequenceNr>
       <responseCode>200</responseCode>
       <reasonString>Success</reasonString>
       <advSequenceNr>13</advSequenceNr>
   </ack>

10.8.  CLUE Message No. 8: 'configure'

   <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
   <ns2:configure xmlns="urn:ietf:params:xml:ns:clue-info"
    xmlns:ns2="urn:ietf:params:xml:ns:clue-protocol"
    xmlns:ns3="urn:ietf:params:xml:ns:vcard-4.0"
    xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
    protocol="CLUE" v="2.7">
       <ns2:clueId>CP2</ns2:clueId>
       <ns2:sequenceNr>24</ns2:sequenceNr>
       <ns2:advSequenceNr>13</ns2:advSequenceNr>
       <ns2:captureEncodings>
                  <captureEncoding ID="ce123">
                           <captureID>AC0</captureID>
                           <encodingID>ENC4</encodingID>
                   </captureEncoding>
                   <captureEncoding ID="ce456">
                           <captureID>VC7</captureID>
                           <encodingID>ENC1</encodingID>
                           <configuredContent>
                                   <sceneViewIDREF>SE5</sceneViewIDREF>
                           </configuredContent>
                   </captureEncoding>
        </ns2:captureEncodings>
   </ns2:configure>

10.9.  CLUE Message No. 9: 'configureResponse'

   <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
   <ns2:configureResponse xmlns="urn:ietf:params:xml:ns:clue-info"
    xmlns:ns2="urn:ietf:params:xml:ns:clue-protocol"
    xmlns:ns3="urn:ietf:params:xml:ns:vcard-4.0"
    xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
    protocol="CLUE" v="2.7">
       <ns2:clueId>CP1</ns2:clueId>
       <ns2:sequenceNr>14</ns2:sequenceNr>
       <ns2:responseCode>200</ns2:responseCode>
       <ns2:reasonString>Success</ns2:reasonString>
       <ns2:confSequenceNr>24</ns2:confSequenceNr>
   </ns2:configureResponse>

11.  Security Considerations

   As a general consideration, we would like to point out that the CLUE
   framework (and related protocol) has been conceived from the outset
   by embracing the security-by-design paradigm.  As a result, a number
   of requirements have been identified and properly standardized as
   mandatory within the entire set of documents associated with the CLUE
   architecture.  Requirements include (i) the use of cryptography and
   authentication, (ii) protection of all sensitive fields, (iii) mutual
   authentication between CLUE endpoints, (iv) the presence of
   authorization mechanisms, and (v) the presence of native defense
   mechanisms against malicious activities such as eavesdropping,
   selective modification, deletion, and replay (and related
   combinations thereof).  Hence, security of the single components of
   the CLUE solution cannot be evaluated independently of the integrated
   view of the final architecture.

   The CLUE protocol is an application-level protocol allowing a Media
   Producer and an MC to negotiate a variegated set of parameters
   associated with the establishment of a telepresence session.  This
   unavoidably exposes a CLUE-enabled telepresence system to a number of
   potential threats, most of which are extensively discussed in the
   CLUE framework document [RFC8845].  The Security Considerations
   section of [RFC8845] actually discusses issues associated with the
   setup and management of a telepresence session in both (1) the basic
   case involving two CLUE endpoints acting as the MP and the MC,
   respectively and (2) the more advanced scenario envisaging the
   presence of an MCU.

   The CLUE framework document [RFC8845] also mentions that the
   information carried within CLUE protocol messages might contain
   sensitive data, which SHOULD hence be accessed only by authenticated
   endpoints.  Security issues associated with the CLUE data model
   schema are discussed in [RFC8846].

   There is extra information carried by the CLUE protocol that is not
   associated with the CLUE data model schema and that exposes
   information that might be of concern.  This information is primarily
   exchanged during the negotiation phase via the 'options' and
   'optionsResponse' messages.  In the CP state machine's OPTIONS state,
   both parties agree on the version and extensions to be used in the
   subsequent CLUE message exchange phase.  A malicious participant
   might either (1) try to retrieve a detailed footprint of a specific
   CLUE protocol implementation during this initial setup phase or
   (2) force the communicating party to use a version of the protocol
   that is outdated and that they know how to break.  Indeed, exposing
   all of the supported versions and extensions could conceivably leak
   information about the specific implementation of the protocol.  In
   theory, an implementation could choose not to announce all of the
   versions it supports if it wants to avoid such leakage, although this
   would come at the expense of interoperability.  With respect to the
   above considerations, it is noted that the OPTIONS state is only
   reached after the CLUE data channel has been successfully set up.
   This ensures that only authenticated parties can exchange 'options'
   messages and related 'optionsResponse' messages, and hence
   drastically reduces the attack surface that is exposed to malicious
   parties.

   The CLUE framework clearly states the requirement to protect CLUE
   protocol messages against threats deriving from the presence of a
   malicious agent capable of gaining access to the CLUE data channel.
   Such a requirement is met by the CLUE data channel solution described
   in [RFC8850], which ensures protection from both message recovery and
   message tampering.  With respect to this last point, any
   implementation of the CLUE protocol compliant with the CLUE
   specification MUST rely on the exchange of messages that flow on top
   of a reliable and ordered SCTP-over-DTLS transport channel connecting
   two CPs.

12.  IANA Considerations

   This document registers a new XML namespace, a new XML schema, and
   the media type for the schema.  This document also registers the
   "CLUE" Application Service tag and the "CLUE" Application Protocol
   tag and defines registries for the CLUE messages and response codes.

12.1.  URN Sub-Namespace Registration

   This section registers a new XML namespace,
   "urn:ietf:params:xml:ns:clue-protocol".

   URI:  urn:ietf:params:xml:ns:clue-protocol

   Registrant Contact:  IESG (iesg@ietf.org).

   XML:

   <CODE BEGINS>
    <?xml version="1.0"?>
    <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
      "https://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
    <html xmlns="https://www.w3.org/1999/xhtml" xml:lang="en">
      <head>
        <title>CLUE Messages</title>
      </head>
      <body>
        <h1>Namespace for CLUE Messages</h1>
        <h2>urn:ietf:params:xml:ns:clue-protocol</h2>
        <p>See <a href="https://www.rfc-editor.org/rfc/rfc8847.txt">
           RFC 8847</a>.</p>
      </body>
    </html>
   <CODE ENDS>

12.2.  XML Schema Registration

   This section registers an XML schema per the guidelines in [RFC3688].

   URI:  urn:ietf:params:xml:schema:clue-protocol

   Registrant Contact:  IESG (iesg@ietf.org).

   Schema:  The XML for this schema can be found in Section 9 of this
      document.

12.3.  Media Type Registration for "application/clue+xml"

   This section registers the "application/clue+xml" media type.

   To:  ietf-types@iana.org

   Subject:  Registration of media type "application/clue+xml"

   Media type name:  application

   Subtype name:  clue+xml

   Required parameters:  (none)

   Optional parameters:  charset.  Same as the charset parameter of
      "application/xml" as specified in [RFC7303], Section 4.2.

   Encoding considerations:  Same as the encoding considerations of
      "application/xml" as specified in [RFC7303], Section 4.2.

   Security considerations:  This content type is designed to carry
      protocol data related to telepresence session control.  Some of
      the data could be considered private.  This media type does not
      provide any protection; thus, other mechanisms, such as those
      described in Section 11 of this document, are required to protect
      the data.  This media type does not contain executable content.

   Interoperability considerations:  None.

   Published specification:  RFC 8847

   Applications that use this media type:  CLUE Participants.

   Additional Information:

      Magic Number(s):  (none)
      File extension(s):  .xml
      Macintosh File Type Code(s):  TEXT

   Person & email address to contact for further information:  Simon
      Pietro Romano (spromano@unina.it).

   Intended usage:  LIMITED USE

   Author/Change controller:  The IETF

   Other information:  This media type is a specialization of
      application/xml [RFC7303], and many of the considerations
      described there also apply to application/clue+xml.

12.4.  CLUE Protocol Registry

   Per this document, IANA has created new registries for CLUE messages
   and response codes.

12.4.1.  CLUE Message Types

   The following summarizes the registry for CLUE messages:

   Related Registry:  CLUE Message Types

   Defining RFC:  RFC 8847

   Registration/Assignment Procedures:  Following the policies outlined
      in [RFC8126], the IANA policy for assigning new values for the
      CLUE message types for the CLUE protocol is Specification
      Required.

   Registrant Contact:  IESG (iesg@ietf.org).

   The initial table of CLUE messages is populated using the CLUE
   messages described in Section 5 and defined in the XML schema in
   Section 9.

    +===================+=================================+===========+
    | Message           | Description                     | Reference |
    +===================+=================================+===========+
    | options           | Sent by the CI to the CR in the | RFC 8847  |
    |                   | initiation phase to specify the |           |
    |                   | roles played by the CI, the     |           |
    |                   | supported versions, and the     |           |
    |                   | supported extensions.           |           |
    +-------------------+---------------------------------+-----------+
    | optionsResponse   | Sent by the CI to the CR in     | RFC 8847  |
    |                   | reply to an 'options' message,  |           |
    |                   | to establish the version and    |           |
    |                   | extensions to be used in the    |           |
    |                   | subsequent exchange of CLUE     |           |
    |                   | messages.                       |           |
    +-------------------+---------------------------------+-----------+
    | advertisement     | Sent by the MP to the MC to     | RFC 8847  |
    |                   | specify the telepresence        |           |
    |                   | capabilities of the MP          |           |
    |                   | expressed according to the CLUE |           |
    |                   | framework.                      |           |
    +-------------------+---------------------------------+-----------+
    | ack               | Sent by the MC to the MP to     | RFC 8847  |
    |                   | acknowledge the reception of an |           |
    |                   | 'advertisement' message.        |           |
    +-------------------+---------------------------------+-----------+
    | configure         | Sent by the MC to the MP to     | RFC 8847  |
    |                   | specify the desired media       |           |
    |                   | captures among those specified  |           |
    |                   | in the 'advertisement'.         |           |
    +-------------------+---------------------------------+-----------+
    | configureResponse | Sent by the MP to the MC in     | RFC 8847  |
    |                   | reply to a 'configure' message  |           |
    |                   | to communicate whether or not   |           |
    |                   | the configuration request has   |           |
    |                   | been successfully processed.    |           |
    +-------------------+---------------------------------+-----------+

                Table 2: Initial IANA Table of CLUE Messages

12.4.2.  CLUE Response Codes

   The following summarizes the registry for CLUE response codes:

   Related Registry:  CLUE Response Codes

   Defining RFC:  RFC 8847

   Registration/Assignment Procedures:  Following the policies outlined
      in [RFC8126], the IANA policy for assigning new values for the
      response codes for CLUE is Specification Required.

   Registrant Contact:  IESG (iesg@ietf.org).

   The initial table of CLUE response codes is populated using the
   response codes defined in Section 5.7 as follows:

   +========+===============+==============================+===========+
   | Number | Default       | Description                  | Reference |
   |        | Reason String |                              |           |
   +========+===============+==============================+===========+
   | 200    | Success       | The request has been         | RFC 8847  |
   |        |               | successfully                 |           |
   |        |               | processed.                   |           |
   +--------+---------------+------------------------------+-----------+
   | 300    | Low-level     | A generic low-level          | RFC 8847  |
   |        | request error | request error has            |           |
   |        |               | occurred.                    |           |
   +--------+---------------+------------------------------+-----------+
   | 301    | Bad syntax    | The XML syntax of the        | RFC 8847  |
   |        |               | message is not               |           |
   |        |               | correct.                     |           |
   +--------+---------------+------------------------------+-----------+
   | 302    | Invalid value | The message contains         | RFC 8847  |
   |        |               | an invalid parameter         |           |
   |        |               | value.                       |           |
   +--------+---------------+------------------------------+-----------+
   | 303    | Conflicting   | The message contains         | RFC 8847  |
   |        | values        | values that cannot be        |           |
   |        |               | used together.               |           |
   +--------+---------------+------------------------------+-----------+
   | 400    | Semantic      | The received CLUE            | RFC 8847  |
   |        | errors        | protocol message             |           |
   |        |               | contains semantic            |           |
   |        |               | errors.                      |           |
   +--------+---------------+------------------------------+-----------+
   | 401    | Version not   | The protocol version         | RFC 8847  |
   |        | supported     | used in the message is       |           |
   |        |               | not supported.               |           |
   +--------+---------------+------------------------------+-----------+
   | 402    | Invalid       | The received message         | RFC 8847  |
   |        | sequencing    | contains an unexpected       |           |
   |        |               | sequence number (e.g.,       |           |
   |        |               | sequence number gap,         |           |
   |        |               | repeated sequence            |           |
   |        |               | number, or sequence          |           |
   |        |               | number outdated).            |           |
   +--------+---------------+------------------------------+-----------+
   | 403    | Invalid       | The clueId used in the       | RFC 8847  |
   |        | identifier    | message is invalid or        |           |
   |        |               | unknown.                     |           |
   +--------+---------------+------------------------------+-----------+
   | 404    | Advertisement | The sequence number of       | RFC 8847  |
   |        | expired       | the advertisement the        |           |
   |        |               | 'configure' message          |           |
   |        |               | refers to is out of          |           |
   |        |               | date.                        |           |
   +--------+---------------+------------------------------+-----------+
   | 405    | Subset choice | The subset choice is         | RFC 8847  |
   |        | not allowed   | not allowed for the          |           |
   |        |               | specified Multiple           |           |
   |        |               | Content Capture.             |           |
   +--------+---------------+------------------------------+-----------+

             Table 3: Initial IANA Table of CLUE Response Codes

13.  References

13.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <https://www.rfc-editor.org/info/rfc3550>.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004,
              <https://www.rfc-editor.org/info/rfc3688>.

   [RFC7303]  Thompson, H. and C. Lilley, "XML Media Types", RFC 7303,
              DOI 10.17487/RFC7303, July 2014,
              <https://www.rfc-editor.org/info/rfc7303>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8845]  Duckworth, M., Ed., Pepperell, A., and S. Wenger,
              "Framework for Telepresence Multi-Streams", RFC 8845,
              DOI 10.17487/RFC8845, January 2021,
              <https://www.rfc-editor.org/info/rfc8845>.

   [RFC8846]  Presta, R. and S P. Romano, "An XML Schema for the
              Controlling Multiple Streams for Telepresence (CLUE) Data
              Model", RFC 8846, DOI 10.17487/RFC8846, January 2021,
              <https://www.rfc-editor.org/info/rfc8846>.

   [RFC8848]  Hanton, R., Kyzivat, P., Xiao, L., and C. Groves, "Session
              Signaling for Controlling Multiple Streams for
              Telepresence (CLUE)", RFC 8848, DOI 10.17487/RFC8848,
              January 2021, <https://www.rfc-editor.org/info/rfc8848>.

   [RFC8850]  Holmberg, C., "Controlling Multiple Streams for
              Telepresence (CLUE) Protocol Data Channel", RFC 8850,
              DOI 10.17487/RFC8850, January 2021,
              <https://www.rfc-editor.org/info/rfc8850>.

   [W3C.REC-xml-20081126]
              Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
              Edition)", World Wide Web Consortium Recommendation REC-
              xml-20081126, November 2008,
              <https://www.w3.org/TR/2008/REC-xml-20081126>.

13.2.  Informative References

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <https://www.rfc-editor.org/info/rfc3261>.

   [RFC4353]  Rosenberg, J., "A Framework for Conferencing with the
              Session Initiation Protocol (SIP)", RFC 4353,
              DOI 10.17487/RFC4353, February 2006,
              <https://www.rfc-editor.org/info/rfc4353>.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
              March 2011, <https://www.rfc-editor.org/info/rfc6120>.

   [RFC7262]  Romanow, A., Botzko, S., and M. Barnes, "Requirements for
              Telepresence Multistreams", RFC 7262,
              DOI 10.17487/RFC7262, June 2014,
              <https://www.rfc-editor.org/info/rfc7262>.

   [RFC7667]  Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667,
              DOI 10.17487/RFC7667, November 2015,
              <https://www.rfc-editor.org/info/rfc7667>.

Acknowledgements

   The authors thank all the CLUErs for their precious feedback and
   support -- in particular, Paul Kyzivat, Christian Groves, and
   Scarlett Liuyan.

Authors' Addresses

   Roberta Presta
   University of Napoli
   Via Claudio 21
   80125 Napoli
   Italy

   Email: roberta.presta@unina.it


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

   Email: spromano@unina.it