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Protocol for Controlling Multiple Streams for Telepresence (CLUE)
University of NapoliVia Claudio 2180125NapoliItalyroberta.presta@unina.itUniversity of NapoliVia Claudio 2180125NapoliItalyspromano@unina.itCLUETelepresenceProtocolFramework
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.
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:
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 .
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 and
. ("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 , we provide a general overview of the
CLUE protocol.
CLUE protocol messages are detailed in .
The CLUE protocol state machines are introduced in
.
Versioning and extensions are discussed
in Sections and ,
respectively. The XML schema
defining the CLUE messages is provided in
.
TerminologyThis document refers to terminology that is also used in
and
.
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 .
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
, 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 )
that, in turn, includes exactly one user agent . 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 .
An MCU includes a mixer (as defined in ),
without the requirement per 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 .
ConventionsThe 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 when, and only when, they appear in all capitals,
as shown here.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
and formally defined and described in the CLUE data model document
.
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
.
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
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:
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.
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
.
The CLUE message conveying the MP's multimedia offer is the
'advertisement' message.
Such a message leverages the XML data model definitions provided in
.
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.
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 .
This section includes non-normative excerpts of the schema to aid in
describing it.
The XML definitions of the CLUE information provided in
are included
within some CLUE protocol messages
(namely the 'advertisement' and 'configure' messages), in
order to use the concepts defined in .
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 ():
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
():
Structure of CLUE Response Messages
]]>
The elements <responseCode> and <reasonString> are populated as detailed in
.
'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
():
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.
'optionsResponse'
The 'optionsResponse' ()
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.
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.
'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&nbhy;call. A new 'advertisement' completely
replaces the previous 'advertisement'.
The 'advertisement' structure is defined in the schema excerpt below
().
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
and formally defined in the CLUE
data model document .
Structure of a CLUE 'advertisement' Message]]>'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 (),
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.
Structure of a CLUE 'ack' Message]]>'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 ().
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 .
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 for the
definition of <captureEncodings>).
Such an element contains a sequence of capture encodings,
representing the streams to be instantiated.
'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 , 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.
Structure of a CLUE 'configureResponse' Message]]>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 .
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.
CLUE Response Codes
Response Code
Reason String
Description
200
Success
The request has been successfully processed.
300
Low-level request error
A generic low-level request 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 values
The message contains values that cannot be used together.
400
Semantic errors
The received CLUE protocol message contains semantic errors.
401
Version not supported
The protocol version used in the message is not supported.
402
Invalid sequencing
The received message contains an unexpected sequence number (e.g.,
sequence number gap, repeated sequence number, or sequence number
outdated).
403
Invalid identifier
The clueId used in the message is invalid or unknown.
404
Advertisement expired
The sequence number of the advertisement the 'configure' message refers to is out of date.
405
Subset choice not allowed
The subset choice is not allowed for the specified Multiple Content Capture.
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
.
We herein discuss the state machines associated with the
CP (),
the MP role ( in ), and the
MC role ( in ), 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 .
CLUE Participant State Machine|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 | |
| +----+ |
| |
+---------+]]>
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:
The CI receives an 'optionsResponse' with an error response code.
The CI does not receive any 'optionsResponse', and a timeout error
is raised.
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 and
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.
Media Provider's State Machine
As soon as the sub-state machine of the MP
() is activated, it is in
the ADV state.
In the ADV state, the MP prepares the 'advertisement' message
reflecting its actual telepresence capabilities.
Media Provider's State Machine| 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|
+-------------+-----------+]]>
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.Media Consumer's State Machine
As soon as the sub-state machine of the MC
() 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.
Media Consumer's State Machine| +-------------------------+
|error | +--+--+ |
|configureResponse | | |
|received | |configure |
| | |sent |
| | | |
| v v advertisement |
+------------------+---------------+ received |
+--------->| WAIT FOR +---------------------+
| | CONF RESPONSE+ |
| +-------+-------+ |
| | |
| | |
| |successful |
| |configureResponse |
| |received |
|configure v |
|sent +-----------+ advertisement received|
+------------+ESTABLISHED+-----------------------+
+-----------+]]>
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.Versioning
CLUE protocol messages are XML messages compliant to the CLUE protocol XML schema
.
The version of the protocol corresponds to the version of the schema.
Both 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 . The version usage is
similar in philosophy to the Extensible Messaging and Presence Protocol (XMPP) .
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 ,
CPs
MUST ignore unknown elements or attributes that are not envisioned
in the negotiated protocol version and related extensions.
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:
new information, to be carried in the existing messages.
For example, more fields may be added within an existing message.
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:
XML elements defined within the CLUE protocol XML schema itself
(protocol-specific information).
other XML elements compliant to the CLUE data model schema
(data model information).
When new protocol-specific information is needed somewhere in the protocol
messages, it can be added in place of the <any> elements and
<anyAttribute> elements envisioned by the protocol schema.
The policy currently defined in the protocol schema for handling
<any> and <anyAttribute> elements is 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 , 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.
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.
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
) needed to further
describe a video capture.
The CLUE data model document
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 is
shown in below.
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
():
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.
XML Schema
The XML schema defining the CLUE messages is provided below
().
Schema Defining CLUE Messages]]>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.
|
| |
| |
|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
.
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 ().
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
().
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
,
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 ().
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 ().
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
().
To reflect the changes in its telepresence offer, CP1 issues a new 'advertisement' message to CP2
(), 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 for
more details regarding the telepresence description).
CP2 acknowledges the second 'advertisement' message with an 'ack' message ().
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
().
Media streams from the previous configuration continue to
flow during the reconfiguration process.
Finally, CP1 accepts CP2's latest request with a 'configureResponse' message ().
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 and
is hence correct.
CLUE Message No. 1: 'options'CP151truetrue1.42.7E1URL_E11.4E2URL_E21.4E3URL_E31.4E4URL_E42.7E5URL_E52.7]]>CLUE Message No. 2: 'optionsResponse'CP262200Successtruetrue2.7]]>CLUE Message No. 3: 'advertisement'CP111CS10.00.010.00.01.010.0trueEG1main audio from the room
1itstaticroomalicebobciccioCS1-2.00.010.0-3.020.09.0-1.020.09.0-3.020.011.0-1.020.011.0trueEG0left camera video capture
1itstaticindividualciccioCS10.00.010.0-1.020.09.01.020.09.0-1.020.011.01.020.011.0trueEG0central camera video capture
1itstaticindividualaliceCS12.00.010.01.020.09.03.020.09.01.020.011.03.020.011.0trueEG0right camera video capture
1itstaticindividualbobCS1-3.020.09.03.020.09.0-3.020.011.03.020.011.0SE1SoundLevel:0EG0loudest room segment
2itstaticindividualCS10.00.010.0-3.020.07.03.020.07.0-3.020.013.03.020.013.0trueEG0zoomed-out view of all people in
the room2itstaticroomalicebobciccio600000ENC1ENC2ENC3300000ENC4ENC5VC0VC1VC2VC3VC4AC0VC3SE1VC0VC2VC4Bobminute takerAlicepresenterCicciochairmantimekeeper]]>CLUE Message No. 4: 'configure+ack'CP22211200AC0ENC4VC3ENC1SE1]]>CLUE Message No. 5: 'configureResponse'CP112200Success22]]>CLUE Message No. 6: 'advertisement'CP113CS10.00.010.00.01.010.0trueEG1main audio from the room
1itstaticroomalicebobciccioCS10.51.00.50.50.00.5trueEG0left camera video capture
1itstaticindividualciccioCS10.00.010.0-1.020.09.01.020.09.0-1.020.011.01.020.011.0trueEG0central camera video capture
1itstaticindividualaliceCS12.00.010.01.020.09.03.020.09.01.020.011.03.020.011.0trueEG0right camera video capture
1itstaticindividualbobCS1-3.020.09.03.020.09.0-3.020.011.03.020.011.0SE1SoundLevel:0EG0loudest room segment
2itstaticindividualCS10.00.010.0-3.020.07.03.020.07.0-3.020.013.03.020.013.0trueEG0
zoomed-out view of all people in the room
2itstaticroomalicebobciccioCS1-3.020.09.03.020.09.0-3.020.011.03.020.011.0SE1SoundLevel:1penultimate loudest room segment
itstaticindividualCS1-3.020.09.03.020.09.0-3.020.011.03.020.011.0SE1SoundLevel:2last but two loudest room segment
itstaticindividualCS1-3.020.09.03.020.09.0-3.020.011.03.020.011.0VC3VC5VC63EG0big picture of the current
speaker + pips about previous speakers3itstaticindividual600000ENC1ENC2ENC3300000ENC4ENC5participants' individual
videosVC0VC1VC2loudest segment of the
roomVC3loudest segment of the
room + pipsVC7room audioAC0room videoVC4VC3VC7SE1VC0VC2VC4Bobminute takerAlicepresenterCicciochairmantimekeeper]]>CLUE Message No. 7: 'ack'CP223200Success13]]>CLUE Message No. 8: 'configure'CP22413AC0ENC4VC7ENC1SE5]]>CLUE Message No. 9: 'configureResponse'CP114200Success24]]>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 .
The Security Considerations section of 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 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 .
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 , 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.
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.
URN Sub-Namespace Registration
This section registers a new XML namespace,
urn:ietf:params:xml:ns:clue-protocol.
]]>XML Schema Registration
This section registers an XML schema per the guidelines in .
URI:
urn:ietf:params:xml:schema:clue-protocol
Registrant Contact:
IESG (iesg@ietf.org).
Schema:
The XML for this schema can be found in
of this document.
Media Type Registration for "application/clue+xml"This section registers the
application/clue+xml media type.
To:
ietf-types@iana.org
Subject:
Registration of media type "application/clue+xml"
Media type name:
application
Subtype name:
clue+xml
Required parameters:
(none)
Optional parameters:
charset. Same as the charset parameter of "application&wj;/xml" as specified in
.
Encoding considerations:
Same as the encoding considerations of
"application/xml" as specified in
.
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
of this document, are required to protect the data. This media type does
not contain executable content.
Interoperability considerations:
None.
Published specification:
RFC 8847
Applications that use this media type:
CLUE Participants.
Additional Information:
Magic Number(s):
(none)
File extension(s):
.xml
Macintosh File Type Code(s):
TEXT
Person & email address to contact for further information:
Simon Pietro Romano (spromano@unina.it).
Intended usage:
LIMITED USE
Author/Change controller:
The IETF
Other information:
This media type is a specialization of
application/xml , and many of the considerations
described there also apply to application/clue+xml.
CLUE Protocol Registry
Per this document, IANA has created new registries for CLUE messages and response codes.
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 , the IANA policy for assigning new values for the
CLUE message types for the CLUE protocol is Specification Required.
Registrant Contact:
IESG (iesg@ietf.org).
The initial table of CLUE messages is populated using the
CLUE messages described in
and defined in the XML schema in .
Initial IANA Table of CLUE Messages
Message
Description
Reference
options
Sent by the CI to the CR in the initiation
phase to specify the roles played by the CI,
the supported versions, and the supported extensions.
RFC 8847
optionsResponse
Sent by the CI to the CR in reply to an
'options' message, to establish the
version and extensions to be used in the subsequent exchange of CLUE messages.
RFC 8847
advertisement
Sent by the MP to the MC to specify the telepresence capabilities
of the MP expressed according to the CLUE framework.
RFC 8847
ack
Sent by the MC to the MP to acknowledge the reception of
an 'advertisement' message.
RFC 8847
configure
Sent by the MC to the MP to specify the desired media captures
among those specified in the 'advertisement'.
RFC 8847
configureResponse
Sent by the MP to the MC in reply to a 'configure' message to
communicate whether or not the configuration request has been successfully
processed.
RFC 8847
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 , the IANA policy for assigning new values for the
response codes for CLUE is Specification Required.
Registrant Contact:
IESG (iesg@ietf.org).
The initial table of CLUE response codes is populated using the
response codes defined in
as follows:
Initial IANA Table of CLUE Response Codes
Number
Default Reason String
Description
Reference
200
Success
The request has been successfully processed.
RFC 8847
300
Low-level request error
A generic low-level request error has occurred.
RFC 8847
301
Bad syntax
The XML syntax of the message
is not correct.
RFC 8847
302
Invalid value
The message contains an
invalid parameter value.
RFC 8847
303
Conflicting values
The message contains values
that cannot be used together.
RFC 8847
400
Semantic errors
The received CLUE protocol message contains
semantic errors.
RFC 8847
401
Version not supported
The protocol version
used in the message
is not supported.
RFC 8847
402
Invalid sequencing
The received message contains an unexpected sequence number (e.g.,
sequence number gap, repeated sequence number, or sequence number
outdated).
RFC 8847
403
Invalid identifier
The clueId used in the
message is invalid
or unknown.
RFC 8847
404
Advertisement expired
The sequence number of the advertisement
the 'configure' message refers to is out
of date.
RFC 8847
405
Subset choice not allowed
The subset choice is not allowed
for the specified Multiple Content Capture.
RFC 8847
ReferencesNormative ReferencesFramework for Telepresence Multi-StreamsSession Signaling for Controlling Multiple Streams for Telepresence (CLUE)Controlling Multiple Streams for Telepresence (CLUE) Protocol Data ChannelAn XML Schema for the Controlling Multiple Streams for
Telepresence (CLUE) Data ModelExtensible Markup Language (XML) 1.0 (Fifth Edition)World Wide Web Consortium Recommendation REC-xml-20081126Informative ReferencesAcknowledgements
The authors thank all the CLUErs for their precious feedback and support -- in
particular, , , and .
