Internet Engineering Task Force (IETF) R. Presta

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
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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>


<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):


<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>


<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>


<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>


<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.


<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].


<xs:complexType name="advertisementMessageType">
<xs:complexContent>
<xs:extension base="clueMessageType">
<xs:sequence>

<xs:element name="mediaCaptures"
type="dm:mediaCapturesType"/>
<xs:element name="encodingGroups"
type="dm:encodingGroupsType"/>
<xs:element name="captureScenes"
type="dm:captureScenesType"/>

<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.


<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).


<xs:complexType name="configureMessageType">
<xs:complexContent>
<xs:extension base="clueMessageType">
<xs:sequence>

<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.


<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.


<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">



<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">

<xs:import namespace="urn:ietf:params:xml:ns:clue-info"/>

<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"/>

<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>

<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>

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

<xs:simpleType name="responseCodeType">
<xs:restriction base="xs:integer">
<xs:pattern value="[1-9][0-9][0-9]" />
</xs:restriction>
</xs:simpleType>

<xs:simpleType name="successResponseCodeType">
<xs:restriction base="xs:integer">
<xs:pattern value="2[0-9][0-9]" />
</xs:restriction>
</xs:simpleType>

<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>

<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>

<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>

<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>

<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>

<xs:complexType name="advertisementMessageType">
<xs:complexContent>
<xs:extension base="clueMessageType">
<xs:sequence>

<xs:element name="mediaCaptures"
type="dm:mediaCapturesType"/>
<xs:element name="encodingGroups"
type="dm:encodingGroupsType"/>
<xs:element name="captureScenes"
type="dm:captureScenesType"/>

<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>

<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>

<xs:complexType name="configureMessageType">
<xs:complexContent>
<xs:extension base="clueMessageType">
<xs:sequence>

<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>

<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 ([email protected]).

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 ([email protected]).

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: [email protected]

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 ([email protected]).

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 ([email protected]).

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 ([email protected]).

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: [email protected]

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

Email: [email protected]