CLUE Signaling
draft-kyzivat-clue-signaling-03
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
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|---|---|---|---|
| Authors | Paul Kyzivat , Lennard Xiao , Christian Groves | ||
| Last updated | 2013-06-13 | ||
| Replaced by | draft-ietf-clue-signaling, draft-ietf-clue-signaling, RFC 8848 | ||
| RFC stream | (None) | ||
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| Consensus boilerplate | Unknown | ||
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draft-kyzivat-clue-signaling-03
Network Working Group P. Kyzivat
Internet-Draft L. Xiao
Intended status: Standards Track C. Groves
Expires: December 15, 2013 Huawei
June 13, 2013
CLUE Signaling
draft-kyzivat-clue-signaling-03
Abstract
This document specifies how signaling is conducted in the course of
CLUE sessions. This includes how SIP/SDP signaling is applied to
CLUE sessions as well as defining a CLUE-specific signaling protocol
that complements SIP/SDP and supports negotiation of CLUE application
level data.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 15, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. CLUE-Specific Signaling Protocol . . . . . . . . . . . . . . 3
3.1. Protocol Versioning, Options & Extensions . . . . . . . . 4
3.1.1. Versioning . . . . . . . . . . . . . . . . . . . . . 4
3.1.2. Options and/or Extensions . . . . . . . . . . . . . . 4
3.1.3. Negotiation . . . . . . . . . . . . . . . . . . . . . 4
3.1.4. Principles . . . . . . . . . . . . . . . . . . . . . 5
3.2. Acknowledging Messages . . . . . . . . . . . . . . . . . 6
3.2.1. Explicit Acknowledgment of Each Message . . . . . . . 6
3.2.2. Piggybacking ACK on Requests . . . . . . . . . . . . 6
3.2.3. Reporting Message Errors . . . . . . . . . . . . . . 7
3.3. Stand-alone messages or deltas? . . . . . . . . . . . . . 9
3.4. Message Sequencing . . . . . . . . . . . . . . . . . . . 9
3.4.1. Signaling Changes in Provider State . . . . . . . . . 10
3.4.2. Signaling Changes in Consumer State . . . . . . . . . 11
3.5. Message Transport . . . . . . . . . . . . . . . . . . . . 11
3.5.1. CLUE Channel Lifetime . . . . . . . . . . . . . . . . 11
3.5.2. Channel Error Handling . . . . . . . . . . . . . . . 12
3.6. CLUE Messages . . . . . . . . . . . . . . . . . . . . . . 12
3.6.1. ADVERTISEMENT Message . . . . . . . . . . . . . . . . 13
3.6.2. CONFIGURE Message . . . . . . . . . . . . . . . . . . 13
3.6.3. ACK Message . . . . . . . . . . . . . . . . . . . . . 13
3.6.4. NAK Message . . . . . . . . . . . . . . . . . . . . . 14
3.7. Message Syntax . . . . . . . . . . . . . . . . . . . . . 14
3.8. Message Framing . . . . . . . . . . . . . . . . . . . . . 16
3.9. other . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4. CLUE use of SDP O/A . . . . . . . . . . . . . . . . . . . . . 16
4.1. Encodings represented in SDP . . . . . . . . . . . . . . 17
5. Coordination of CLUE protocol and SDP O/A . . . . . . . . . . 18
5.1. Independence of SDP and CLUE negotiation . . . . . . . . 18
5.2. Combined Protocol Use Cases . . . . . . . . . . . . . . . 19
5.2.1. Two CLUE-capable endpoints . . . . . . . . . . . . . 19
5.2.2. A case with a non-CLUE-capable endpoint . . . . . . . 22
6. CLUE requirements on SDP O/A . . . . . . . . . . . . . . . . 23
7. SIP Signaling . . . . . . . . . . . . . . . . . . . . . . . . 24
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8. Interoperation with Legacy SIP Devices . . . . . . . . . . . 24
9. CLUE over RTCWEB . . . . . . . . . . . . . . . . . . . . . . 24
10. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 24
11. What else? . . . . . . . . . . . . . . . . . . . . . . . . . 24
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 24
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
14. Security Considerations . . . . . . . . . . . . . . . . . . . 25
15. Change History . . . . . . . . . . . . . . . . . . . . . . . 25
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
16.1. Normative References . . . . . . . . . . . . . . . . . . 26
16.2. Informative References . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
1. Introduction
This document specifies how signaling is conducted in the course of
CLUE sessions. This includes how SIP/SDP signaling is applied to
CLUE sessions as well as defining a CLUE-specific signaling protocol
that complements SIP/SDP and supports negotiation of CLUE application
level data.
[Yes, this is a dup of the abstract for now. Eventually it should
say more.]
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This document draws liberally from the terminology defined in the
CLUE Framework [I-D.ietf-clue-framework].
Other terms introduced here:
CLUE Channel: A reliable, bidirectional, transport mechanism used to
convey CLUE messages. A CLUE channel consists of one SCTP stream
in each direction over a DTLS/SCTP session.
3. CLUE-Specific Signaling Protocol
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The CLUE Framework [I-D.ietf-clue-framework] mentions a CLUE-specific
protocol for the exchange of ADVERTISEMENT and CONFIGURE messages,
but gives little detail. The Data Model
[I-D.presta-clue-data-model-schema] specifies a model and XML
representation for CLUE-related data, but doesn't currently specify
exactly what data belongs in each message, or how messages are
sequenced. This document provides the detail missing from those
documents.
3.1. Protocol Versioning, Options & Extensions
3.1.1. Versioning
There must be some provision for identifying incompatible protocol
versions.
NOTE: We probably don't want to have incompatible versions.
Typically changes will be introduced in a backward compatible way.
But a time may come when this isn't possible, and we should be
prepared for that. This is more likely to occur before an RFC is
published. While it is probably unwise to deploy a product based on
a draft, there will certainly be prototypes developed for testing,
and those tests may lead to a need for incompatible change. So
whatever the mechanism is, it should be applicable to changes that
occur from draft to draft, as well as after an RFC has been
published.
3.1.2. Options and/or Extensions
There must be some provision for dealing with optional-to-implement
features in the specification, and/or for backward compatible
extensions to the protocol. These are superficially different, but
in practice they are more-or-less equivalent. To an implementation
of the base protocol and some extensions, those extensions must be
viewed as optional-to-implement features in peers.
One decision is whether extensions may be implemented mix-and-match,
or whether there is a sequence of extensions, and one extension may
only be supported if all the prior extensions have been supported.
3.1.3. Negotiation
Both version and options can be negotiated. Some mechanisms may work
for both, while others are only appropriate for one or the other.
Some possibilities:
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o No negotiation at all. Instead, unrecognized syntax in certain
"extension points" is to be ignored. If it is recognized, then a
corresponding extension specification defines what to do.
o Negotiate via the SIP signaling.
o Negotiate as part of the O/A exchange that establishes the
channel. (E.g. it is likely that individual channels of the SCTP
association will be specified in SDP with a specific sub-protocol
type. There could be a separate sub-protocol for each new
version.)
o Negotiate within the CLUE channel, via a special message exchange,
before exchanging "normal" CLUE messages.
o Declare versioning in every CLUE message. Define errors for
unsupported versions and fallback to earlier versions.
3.1.4. Principles
o CLUE SHOULD allow forwards and backwards compatibility through a
version and extension mechanism. Forward compatibility allows a
version of a protocol to communicate effectively with and
interwork with future versions of the protocol. A version should
not restrict the future protocol from providing extra
capabilities.
o Whenever possible backwards compatibility should be maintained.
Backward compatibility rules will be defined to ensure that
endpoints implementing future versions of CLUE will be able to
send protocol messages of the previous versions which will be
understood and fully processed by the remote endpoint.
o Existing protocol elements should not be changed unless a protocol
error needs to be corrected.
o The semantics of existing elements and values should not be
changed.
o Established rules for formatting and encoding messages and
elements should not be changed.
o When information elements are found to be obsolete they can be
marked as not used. However, the identifier for that information
element will be marked as reserved. In that way it cannot be used
in future versions.
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3.2. Acknowledging Messages
The CLUE channel is reliable, so there is no need for acknowledgement
to guarantee delivery. But there is still a need for application-to-
application acknowledgement to report that the message has been
received, parsed, and found to be of an acceptable format. One
possibility is to introduce separate ACK and NAK messages. Another
possibility is to add a confirmation element to each CLUE message, so
that confirmation can be piggybacked on the basic messages. Some
alternatives follow. [OTHER PROPOSALS WELCOME.]
3.2.1. Explicit Acknowledgment of Each Message
The characteristics of this approach are:
o There are separate request and response messages. (This is
similar to SIP.)
o Every request message expects exactly one response message.
o Every request message carries a sequence number that identifies
it.
o Each end of the connection assigns sequential sequence numbers to
the requests it sends.
o Every response message carries the sequence number of the message
to which it responds.
o Responses are to be sent promptly upon the receipt of a request.
(Needs more detail.)
o Responses are either ACK or NAK. NAK responses also carry info
describing the error.
o Each CONFIGURE message is to be understood in the context of the
most recent ACKed ADVERTISEMENT message. A CONFIGURE message may
be rejected if there is an outstanding ADVERTISEMENT for which no
response has been received. (Or it may be accepted if the
advertiser is able to do so meaningfully.)
3.2.2. Piggybacking ACK on Requests
The characteristics of this approach are:
o Every message carries a sequence number that identifies it.
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o Each end of the connection assigns sequential sequence numbers to
the messages it sends.
o Every message carries the sequence number of the last message
received and found valid.
o If a message is received and found invalid, then a NAK message is
sent that refers to it and indicates what is wrong with it.
o If a valid message is received and a new message needs to be sent
in response, then the responding message implicitly acknowledges
the prior message.
o If a valid message is received and there is no need to immediately
send another message, then a NO-OP message is sent to acknowledge
the received message. But a NO-OP message is never sent in
response to a NO-OP message.
o Each CONFIGURE message is to be understood in the context of the
most recent *acknowledged* ADVERTISEMENT message. A CONFIGURE
message may be rejected if it doesn't acknowledge the most
recently sent ADVERTISEMENT. (Or it may be accepted if the
advertiser is able to do so meaningfully.)
The general format of every message is:
o sequence # of this message
o sequence # of most recently *received* and *valid* message
o message type (ADVERTISEMENT, CONFIG, NO-OP, NAK)
o body of the message, according to type
(The exact representation is TBD - by XML experts.)
There are loose ends to resolve here. In particular, how to
acknowledge messages after NAKing one.
3.2.3. Reporting Message Errors
There needs to be a mechanism to report errors with other messages.
The details of form, content, and usage still need to be specified,
and need to be tuned to the details of the protocol. This could use
distinct messages or be incorporated into the other messages. Errors
this message must be able to report include:
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Syntax error in message: The message has been disregarded due to a
syntax error detected at the message level. The message does not
conform to the productions of messages in [Protocol Document].
Used when the message cannot be parsed.
Sequencing Error: Sequence number has already been used, or is
greater than the expected number. (Details of possible errors
depend upon the specific sequence numbering mechanism.)
Version not supported: This indicates a lack of support for the
protocol version indicated in the message header of the message.
In the case of the version number being indicated in the message
header, the message contents are disregarded.
Option not supported: This indicates a lack of support for the
protocol option the used in the message. The message contents are
disregarded.
Unknown capture identity: The received Configure message contains an
unknown capture identity not previously declared by an
Advertisement. The message contents are disregarded.
Invalid identity: The received message contains an invalid capture
identity. For example a duplicated Capture scene identity or some
other semantically incorrect usage. The message contents are
disregarded.
Invalid value: The received message contains an invalid parameter
value. The value is not according to the protocol definition in
[protocol document] or according the extension documentation.
Missing element: The received message is missing an element.
Certain parameters require multiple values, e.g. Point of capture
requires X,Y,Z co-ordinates if one or more elements are missing
this error code is used.
Conflicting parameters or values: The received message contains
multiple values that may not be used together.
Invalid capture area: The received message defines a capture area
that cannot be rendered in a sensible manner. For example the
capture area does not define a quadrilateral region.
Invalid point of line of capture: The indicated co-ordinate for the
point on line of capture is invalid. For example: does not lie
between the point of capture and the area of capture or it is the
same as the point of capture.
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Invalid capture scene entry: The message contains an invalid capture
scene entry. For example the capture scene entry contains more
than one media type.
Invalid Simultaneous Set: The simultaneous set contained in the
message is invalid. For example the simultaneous set refers to an
undefined capture set or does not match the specified capture
scene entries.
Invalid Configuration: The Configure message requests a
configuration that the provider cannot support.
Invalid Advertisement reference: The Configure message refers to an
invalid Advertisement. The message refers-to/depends-upon out-of-
date ADVERTISEMENT message or provides an invalid reference.
3.3. Stand-alone messages or deltas?
Each message exchanged within a CLUE session could contain a complete
description of the state it wishes to achieve. Or each message could
describe just the changes that it wishes to make to the current
state. Or the protocol could support both message forms. Which
direction to pursue is TBD.
[Paul: while this does need to be decided, it is fundamentally just
an optimization. IMO it does not have major impact on the other
parts of this document, so I would prefer to continue deferring it
until we are so far along with the remainder of the document that we
can no longer defer it.]
3.4. Message Sequencing
There is a very basic introduction to this topic in section 4
(Overview) of the CLUE Framework [I-D.ietf-clue-framework]. After
removing extraneous material it would look like:
+-----------+ +-----------+
| Endpoint1 | | Endpoint2 |
+----+------+ +-----+-----+
| |
| ADVERTISEMENT 1 |
|*********************************>|
| ADVERTISEMENT 2 |
|<*********************************|
| |
| CONFIGURE 1 |
|<*********************************|
| CONFIGURE 2 |
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|*********************************>|
| |
But we need much more than this, to show multiple CONFIGUREs per
ADVERTISEMENT, interleaving of ADVERTISEMENTs and CONFIGUREs in both
directions, etc.
Message sequencing needs to be described at two levels:
o Basic sequencing of the CLUE messages themselves, without regard
for the SIP/SDP signaling that may be going on at the same time.
This is useful to cover the basic concepts. That should be
covered in this section. It provides context for understanding
the more detailed treatment later.
This could include some simple state machines.
o In reality there is a complex dependency between CLUE signaling
and SDP Offer/Answer exchanges carried in SIP signaling. So there
is a need to describe the valid ways in which these two forms of
signaling interact. That is covered in Section 5.
3.4.1. Signaling Changes in Provider State
Once a CLUE session has been established, ADVERTISEMENTs and
CONFIGUREs exchanged, and media is flowing, a provider may experience
a change in state that has an effect on what it wishes or is able to
provide. In this case it may need to alter what it is sending and/or
send a new ADVERTISEMENT. In some cases it will be necessary to
alter what is being sent without first sending a new ADVERTISEMENT
and waiting for a CONFIGURE conforming to it.
The following is a non-exhaustive list of situations and recommended
actions:
o An advertised capture, that is not currently configured, is no
longer available.
To recover from this: Send a new ADVERTISEMENT that omits this
capture.
o An advertised capture, that has been configured, is no longer
available.
To recover from this: (1) stop transmitting the configured
encoding of this capture. (2) Send a new ADVERTISEMENT that omits
this capture.
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o The provider loses some resource and must reduce the frame rate,
frame size, or resolution of a capture encoding.
If the reduced values still fall within the advertised values for
the capture then the change may be made without any further
signaling.
If the change must be outside the range of what was advertised,
then the provider must cease transmitting the capture encoding.
It then must send a new ADVERTISEMENT reflecting what it is now
capable of delivering.
o New or changed scenes or scene geometry. For instance, the
addition of a new scene containing presentation captures. Also,
an MCU may make significant changes in what it advertises as new
endpoints join a conference.
o [Add more]
3.4.2. Signaling Changes in Consumer State
If the Consumer for some reason looses the CLUE state information how
does it ask for an Advertisement from the provider? There could be
multiple possibilities. A error code approach? However error codes
would typically be associated with a NACK so it may not be good for a
Config message. Maybe send a message which means "send me a complete
update". An alternative may be to release the connection or just do
new signaling to establish a new CLUE session.
3.5. Message Transport
CLUE messages are transported over a bidirectional CLUE channel. In
a two-party CLUE session, a CLUE channel connects the two endpoints.
In a CLUE conference, each endpoint has a CLUE channel connecting it
to an MCU. (In conferences with cascaded mixers [RFC4353], two MCUs
will be connected by a CLUE channel.)
3.5.1. CLUE Channel Lifetime
The transport mechanism used for CLUE messages is DTLS/SCTP as
specified in [I-D.tuexen-tsvwg-sctp-dtls-encaps] and
[I-D.ietf-mmusic-sctp-sdp]. A CLUE channel consists of one SCTP
stream in each direction over a DTLS/SCTP session. The mechanism for
establishing the DTLS/SCTP session is described in Section 4.
The CLUE channel will usually be offered during the initial SIP
INVITE, and remain connected for the duration of the CLUE/SIP
session. However this need not be the case. The CLUE channel may be
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established mid-session after desire and capability for CLUE have
been determined, and the CLUE channel may be dropped mid-call if the
desire and/or capability to support it is lost.
There may be cases when it becomes necessary to "reset" the CLUE
channel. This by be as a result of an error on the underlying SCTP
association, a need to change the endpoint address of the SCTP
association, loss of CLUE protocol state, or something else TBD.
The precise mechanisms used to determine when a reset is required,
and how to accomplish it and return to a well defined state are TBS.
3.5.2. Channel Error Handling
We will need to specify behavior in the face of transport errors that
are so severe that they can't be managed via CLUE messaging within
the CLUE channel. Some errors of this sort are:
o Unable to establish the SCTP association after signaling it in
SDP.
o CLUE channel setup rejected by peer.
o Error reported by transport while writing message to CLUE channel.
o Error reported by transport while reading message from CLUE
channel.
o Timeout - overdue acknowledgement of a CLUE message.
(Requirements for now soon a message must be responded to are
TBD.)
o Application fault. CLUE protocol state lost.
The worst case is to drop the entire CLUE call. Another possibility
is to fall back to legacy compatibility mode. Or perhaps a "reset"
can be done on the protocol. E.g. this might be accomplished by
sending a new O/A and establishing a replacement SCTP association.
Or a new CLUE channel might be established within the existing SCTP
association.
3.6. CLUE Messages
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CLUE messages are encoded in XML. The Data Model
[I-D.presta-clue-data-model-schema] defines many/most of the elements
from which CLUE messages are composed. This document specifies an
XML schema that contains an element definition for each CLUE message,
with much of the content of those elements being drawn from the Data
Model.
3.6.1. ADVERTISEMENT Message
This message contains XML representations of captures, capture
scenes, encoding groups, and simultaneous sets using the types
defined for those in the Data Model
[I-D.presta-clue-data-model-schema].
The XML definition for this is element <advertisement> in section
Section 3.7
[[ Currently this does not contain any representation of encodings.
It assumes those will be defined in SDP. ]]
3.6.2. CONFIGURE Message
This message optionally contains an XML representations of
captureEncodings using the type defined in the Data Model
[I-D.presta-clue-data-model-schema]. A configure message with no
captureEncodings indicates that no captures are requested.
[[ It currently also contains a reference to the request number of
the advertisement it is based upon. Whether this should be present,
or if it should implicitly reference the most recently acknowledged
advertisement is TBD. ]]
The XML definition for this is element <configure> in section
Section 3.7
3.6.3. ACK Message
Need for, and details of, the ACK message are TBD.
The XML element <response> in section Section 3.7 could serve as the
representation, either with no reason element, or a reason element
with a special value.
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3.6.4. NAK Message
Need for, and details of, the NACK message are TBD.
The XML element <response> in section Section 3.7 could serve to as
the representation, with the reason element providing the details.
Then the code value in the reason element should map to the errors in
section Section 3.2.3.
3.7. Message Syntax
[[ The following is a first cut at a schema for the actual messages
in the clue protocol. It uses <encodingGroups> from the data model
but not <encodings>. Rather, it assumes that encodings are described
in SDP as m-lines with a text identifier, and that the identifier has
the same value as the encodingIDs embedded in the <encodingGroups>.
If we stick with this the data model should be adjusted to agree, but
until then it should "work". The SDP encoding of the identifier is
TBD. Candidates are 'a=label:ID' and 'a=mid:ID'. ]]
For now there only <advertisement> and <configure> are defined. More
messages will be needed for acknowledgment.
<?xml version="1.0" encoding="UTF-8" ?>
<xs:schema
targetNamespace="urn:ietf:params:xml:ns:clue-message"
xmlns:tns="urn:ietf:params:xml:ns:clue-message"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:dm="urn:ietf:params:xml:ns:clue-info"
xmlns="urn:ietf:params:xml:ns:clue-message"
elementFormDefault="qualified"
attributeFormDefault="unqualified">
<!-- Import data model schema -->
<xs:import namespace="urn:ietf:params:xml:ns:clue-info"
schemaLocation="clue-data-model-04-wip.xsd"/>
<!-- ELEMENT DEFINITIONS -->
<xs:element name="advertisement" type="advertisementMessageType"/>
<xs:element name="configure" type="configureMessageType"/>
<xs:element name="response" type="responseMessageType"/>
<!-- CLUE MESSAGE TYPE -->
<xs:complexType name="clueMessageType" abstract="true">
<xs:sequence>
<!-- mandatory fields -->
<!-- TBS: version info -->
</xs:sequence>
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</xs:complexType>
<!-- CLUE REQUEST MESSAGE TYPE -->
<xs:complexType name="clueRequestMessageType" abstract="true">
<xs:complexContent>
<xs:extension base="clueMessageType">
<xs:sequence>
<!-- mandatory fields -->
<xs:element name="requestNumber" type="xs:integer"/>
</xs:sequence>
</xs:extension>
</xs:complexContent>
</xs:complexType>
<!-- CLUE RESPONSE MESSAGE TYPE -->
<xs:complexType name="clueResponseMessageType">
<xs:complexContent>
<xs:extension base="clueMessageType">
<xs:sequence>
<!-- mandatory fields -->
<xs:element name="requestNumber" type="xs:integer"/>
<!-- optional fields -->
<xs:element name="reason" type="reasonType" minOccurs="0"/>
<xs:any namespace="##other"
processContents="lax" minOccurs="0"/>
</xs:sequence>
</xs:extension>
</xs:complexContent>
</xs:complexType>
<!-- CLUE ADVERTISEMENT MESSAGE TYPE -->
<xs:complexType name="advertisementMessageType">
<xs:complexContent>
<xs:extension base="clueRequestMessageType">
<xs:sequence>
<!-- mandatory fields -->
<xs:element name="mediaCaptures"
type="dm:mediaCapturesType"/>
<xs:element name="encodingGroups"
type="dm:encodingGroupsType"/>
<!-- The encodings are defined via identifiers in the SDP,
referenced in encodingGroups -->
<xs:element name="captureScenes"
type="dm:captureScenesType"/>
<!-- optional fields -->
<xs:element name="simultaneousSets"
type="dm:simultaneousSetsType" minOccurs="0"/>
<xs:any namespace="##other"
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processContents="lax" minOccurs="0"/>
</xs:sequence>
</xs:extension>
</xs:complexContent>
</xs:complexType>
<!-- CLUE CONFIGURE MESSAGE TYPE -->
<xs:complexType name="configureMessageType">
<xs:complexContent>
<xs:extension base="clueRequestMessageType">
<xs:sequence>
<!-- mandatory fields -->
<xs:element name="advertisementNumber" type="xs:integer"/>
<!-- advertisementNumber is requestNumber
of the advertisement-->
<!-- optional fields -->
<xs:element name="captureEncodings"
type="dm:captureEncodingsType" minOccurs="0"/>
<xs:any namespace="##other"
processContents="lax" minOccurs="0"/>
</xs:sequence>
</xs:extension>
</xs:complexContent>
</xs:complexType>
<!-- REASON TYPE -->
<xs:complexType name="reasonType">
<xs:simpleContent>
<xs:extension base="xs:string">
<xs:attribute type="xs:short" name="code" use="required"/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:schema>
3.8. Message Framing
Message framing is provided by the SCTP transport protocol. Each
CLUE message is carried in one SCTP message.
3.9. other
4. CLUE use of SDP O/A
The CLUE channel is usually offered in the first SIP O/A exchange
between two parties in an intended CLUE session. The offer of the
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CLUE channel is the indicator that this SIP session is proposing to
establish a CLUE session.
(However it is also acceptable to start with a non-CLUE SIP session
and upgrade it to a CLUE session later.)
The mechanism for negotiating a DTLS/SCTP connection is specified in
[I-D.ietf-mmusic-sctp-sdp]. We need to specify how to select the
specific pair of SCTP streams that comprise the CLUE channel.
Any specific usage/conventions required for coordination of SDP
offers and answers with the CLUE messages should also be described
here.
(We have a draft [I-D.even-clue-sdp-clue-relation] that can
contribute to this.)
4.1. Encodings represented in SDP
[[ This is a straw horse, based on a proposal in
[I-D.hansen-clue-sdp-interaction]. It remains unclear if this
approach will work well, but we'll try it out and see how it
develops. ]]
Providers signal available encodings in SDP sent to the consumer,
rather than in an Advertisement message. Each encoding is described
by an SDP media section containing an identifier. Encoding groups
contained in Advertisement messages reference encodings by including
the SDP identifier. Configure messages also reference encodings via
the identifier when selecting capture encodings.
An encoding referenced by an encoding group can only be used to send/
receive media if SDP defining the corresponding identifier was
defined in the most recent offer/answer exchange. However a consumer
may configure a capture encoding using an advertised encoding that is
not currently defined in SDP. In this case it can be used if/when
the provider defines the label in a subsequent offer/answer exchange.
This provides flexibility in coordinating CLUE messages and SDP, but
all encodings referenced by an Advertisement SHOULD be specified as
soon as possible. When the SDP definition of an encoding is not
available, the consumer has insufficient information to decide
whether to select it.
[[ Using this approach the description of an encoding has all, and
only, the descriptive capability provided by SDP. Also, for now this
assumes a single capture per-m-line and no m-line bundling. We will
want to relax those assumptions later. ]]
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5. Coordination of CLUE protocol and SDP O/A
This should include state machines and/or call flows. These will
illustrate, and then provide normative rules for valid sequences of
messages of both types. For instance this needs to show when SDP
offers and answers must occur relative to an ADVERTISEMENT or
CONFIGURE message that requires SDP changes.
[THIS IS A VERY IMPORTANT PART OF THIS DOCUMENT!]
5.1. Independence of SDP and CLUE negotiation
[This text is taken from [I-D.hansen-clue-sdp-interaction].]
This draft proposes that CLUE messages and SDP messages should be
independent: parameters in CLUE messages MAY exceed values negotiated
in SDP, or may make reference to SDP contents not present in the most
recent offer/answer exchange. Without this provision, SDP and CLUE
messages become part of a single negotiation, and a change on either
by either side may necessitate an exchange of the other message type.
For instance, removing stream information from SDP might first
necessitate sending a new CLUE message removing the references to
this stream. The state machine required to ensure validity of
negotiation will be complicated, and there will be a number of
invalid states which must be avoided. This is further complicated by
the fact that, even if both ends of a call obey the constraints to
ensure validity, a middle box may choose to rewrite an SDP such that
an invalid state is reached.
Making the two message types independent significantly reduces the
complexity of the state machines required. And with the message
flows independent there is no way for an invalid state to occur when
the two negotiations contain contradictory information. A cost of
this is that endpoints will now need to deal with the fact that CLUE
messages may contain parameters exceeding those negotiated in SDP, or
referencing SDP content that does not exist. However, this is
analogous to an issue endpoints already deal with in SDP. For
instance, the sum of bandwidth parameters for various m-lines can
exceed the overall session bandwidth. Not only is this not invalid,
but it can be desirable, as it allows the sender to prioritise
streams. What can be sent for any device is simply the intersection
of what is permitted by the most recent SDP offer/answer, and the
outcome of the CLUE negotiation; implementations should ignore
references to entities in the other negotiation that do no exist.
This does not mean that there will be no interaction between SDP and
CLUE messaging - a device wishing to add a new stream may well need
to update both their SDP and their CLUE negotiations. However, there
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is no fixed order in which this must be done and no requirement for
them to be updated in a particular order or fashion; it is left to
the implementation to renegotiate the channels as it sees fit. If
updates to both negotiations are required for a new stream to be
added, then the new stream will not be available until both
renegotiations are complete - the completion of the first
renegotiation will have no effect.
5.2. Combined Protocol Use Cases
[[ NOTE: this material is now out of date with the rest of the
document. ]]
5.2.1. Two CLUE-capable endpoints
This is the case where two CLUE-capable endpoints are willing to set
up a CLUE telepresence session. In the following, a possible
approach addressing the problem is illustrated.
+----------+ +-----------+
| EP1 | | EP2 |
| | | |
+----+-----+ +-----+-----+
| |
| |
| INVITE (BASIC SDP+COMEDIA) |
|--------------------------------->|
| |
| |
| 200 0K (BASIC SDP+COMEDIA)|
|<---------------------------------|
| |
| |
| ACK |
|--------------------------------->|
| |
| |
| |
|<################################>|
| ?? BASIC SDP MEDIA SESSION ?? |
|<################################>|
| |
| |
| CLUE CTRL CHANNEL SETUP |
|<================================>|
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| ... |
|<================================>|
| CLUE CTRL CHANNEL ESTABLISHED |
|<================================>|
| |
| |
| ADVERTISEMENT 1 |
|*********************************>|
| |
| |
| ADVERTISEMENT 2 |
|<*********************************|
| |
| |
| |
| CONFIGURE 1 |
|<*********************************|
| |
| |
| CONFIGURE 2 |
|*********************************>|
| |
| |
| |
| REINVITE (UPDATED SDP) |
|--------------------------------->|
| |
| |
| 200 0K (UPDATED SDP)|
|<---------------------------------|
| |
| |
| ACK |
|--------------------------------->|
| |
|<################################>|
| UPDATED SDP MEDIA SESSION |
|<################################>|
| |
| |
| |
| |
| |
v v
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First, endpoint EP1 sends to endpoint EP2 a SIP INVITE including in
the SDP body the basilar audio and video capabilities ("BASIC SDP")
and the information needed for opening a control channel to be used
for CLUE protocol messages exchange, according to what is envisioned
in the COMEDIA approach ("COMEDIA") for DTLS/SCTP channel
[I-D.ietf-mmusic-sctp-sdp].
After the successful SIP O/A phase, EP1 and EP2 are able to exchange
audio and video streams ("BASIC AUDIO AND VIDEO"). [RP: Is this
channel needed at this point of the call flow?] [RP: which streams
are sent on this channel in this moment?]
Moreover, another effect of the above successful SIP O/A phase, is
the opening of the control channel. After the setup phase, the
channel is established and CLUE protocol messages can flow above it.
CLUE protocol messages have not been formally defined yet. However,
up to now there is a common agreement on their names and their main
purposes, that should be following.
CLUE protocol ADVERTISEMENT messages are used to better describe the
media provider's available streams in order to make the media
consumer able to reproduce them in a more realistic fashion, as it is
the main purpose of a telepresence session. These messages are
needed since it is not possible in an agile fashion to describe
spatial information and several further metadata about media captures
via SDP.
In this document it is assumed that ADVERTISEMENT messages contain
the full description of the sender's telepresence room in terms of
available media capture and encoding capabilities. [RP: open issue -
the mapping between what is described in the advertisement messages
and media streams exchanged in the eventual basic SDP session already
established]
CLUE protocol CONFIGURE messages are used to let the media consumer
indicate to the media provider which are the available streams it is
interested in, so that the media provider can send to the media
consumer what it desires.
In the following, it is considered one of the possible call flow that
can lead to the desired session configuration.
EP1 sends the ADVERTISEMENT message to EP2 (ADVERTISEMENT 1), which
replies with a CONFIGURE message (CONFIGURE 1). After receiving the
CONFIGURE message, EP1 assumes the CLUE offer/answer negotiation it
started is completed. EP1 then can issue a REINVITE to EP2 with an
SDP body updated accordingly to the CLUE messages exchange.
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Similarly, EP2 sends its ADVERTISEMENT to EP1 (ADVERTISEMENT 2),
which replies with a CONFIGURE (CONFIGURE 2).
EP1 re-negotiates the media involved in the existent session via a
SIP REINVITE message to EP2. The SDP body within the REINVITE
message reflects the negotiation carried on by the CLUE message
exchange. In the case represented in figure, EP2 builds the 200 OK
response also according to the second CLUE O/A negotiation.
5.2.2. A case with a non-CLUE-capable endpoint
In this example, one of the two involved endpoint (EP2) is not CLUE-
capable, i.e., it is not able to use the CLUE protocol.
+----------+ +-----------+
| EP1 | | EP2 |
| | | |
+----+-----+ +-----+-----+
| |
| |
| INVITE (BASIC SDP+COMEDIA) |
|--------------------------------->|
| |
| |
| 200 0K (BASIC SDP + *NO* COMEDIA)|
|<---------------------------------|
| |
| |
| ACK |
|--------------------------------->|
| |
| |
| |
|<################################>|
| ?? BASIC SDP MEDIA SESSION ?? |
|<################################>|
| |
| |
| |
| REINVITE (UPDATED SDP) |
|--------------------------------->|
| |
| |
| 200 0K (UPDATED SDP)|
|<---------------------------------|
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| |
| |
| ACK |
|--------------------------------->|
| |
|<################################>|
| UPDATED SDP MEDIA SESSION |
|<################################>|
| |
| |
| |
| |
| |
v v
Endpoint EP1 sends to endpoint EP2 a SIP INVITE including in the SDP
body the basilar audio and video capabilities ("BASIC SDP") and the
information needed for opening a control channel to be used for CLUE
protocol messages exchange, as envisioned by the COMEDIA approach
("COMEDIA") for DTLS/SCTP channel [I-D.ietf-mmusic-sctp-sdp].
Since EP2 is not CLUE-capable, it answers with a 200 OK in which
basic audio and video capabilities are accepted while the opening of
the CLUE channel is rejected.
From such a response, EP1 understands the peer is not CLUE-capable.
In this example, EP1 re-negotiates the session according to a pre-
determined bulk of media streams to be sent to non-CLUE-capable
endpoints.
6. CLUE requirements on SDP O/A
Any SDP extensions required to support CLUE signaling should be
specified here. Then we will need to take action within MMUSIC to
make those happen. This section should be empty and removed before
this document becomes an RFC.
NOTE: The RTP mapping document [I-D.even-clue-rtp-mapping] is also
likely to call for SDP extensions. We will have to reconcile how to
coordinate these two documents.
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7. SIP Signaling
(Placeholder) This may be unremarkable. If so we can drop it.
8. Interoperation with Legacy SIP Devices
This may just describe how the degenerate form of the general
mechanisms work for legacy devices. Or it may describe special case
handling that we mandate as part of CLUE. Or it may just discuss
non-normative things for implementors should consider.
9. CLUE over RTCWEB
We may want to rule this out of scope for now. But we should be
thinking about this.
10. Open Issues
Here are issues pertinent to signaling that need resolution.
Resolution will probably result in changes somewhere in this
document, but may also impact other documents.
o While the preference is to multiplex multiple capture encodings
over a single RTP session, this will not always be desirable or
possible. The factors that prevent multiplexing may come from
either the provider or the consumer. So the extent of
multiplexing must be negotiated. The decision about how to
multiplex affects the number and grouping of m-lines in the SDP.
The endpoint of a CLUE session that sends an offer needs to know
the mapping of capture encodings to m-lines for both sides.
AFAIK this issue hasn't yet been considered at all.
11. What else?
12. Acknowledgements
The team focusing on this draft consists of: Roni Even, Rob Hansen,
Christer Holmberg, Paul Kyzivat, Simon Pietro-Romano, Roberta Presta.
Christian Groves has contributed detailed comments and suggestions.
The author list should be updated as people contribute substantial
text to this document.
13. IANA Considerations
TBD
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14. Security Considerations
TBD
15. Change History
-03:
* Added a syntax section with an XML schema for CLUE messages.
This is a strawhorse, and is very incomplete, but it
establishes a template for doing this based on elements defined
in the data model. (Thanks to Roberta for help with this!)
* Did some rewording to fit the syntax section in and reference
it.
* Did some relatively minor restructuring of the document to make
it flow better in a logical way.
-02: A bunch of revisions by pkyzivat:
* Moved roberta's call flows to a more appropriate place in the
document.
* New section on versioning.
* New section on NAK.
* A couple of possible alternatives for message acknowledgment.
* Some discussion of when/how to signal changes in provider
state.
* Some discussion about the handling of transport errors.
* Added a change history section.
These were developed by Lennard Xiao, Christian Groves and Paul,
so added Lennard and Christian as authors.
-01: Updated by roberta to include some sample call flows.
-00: Initial version by pkyzivat. Established general outline for
the document, and specified a few things thought to represent wg
consensus.
16. References
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16.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[I-D.ietf-clue-framework]
Duckworth, M., Pepperell, A., and S. Wenger, "Framework
for Telepresence Multi-Streams", draft-ietf-clue-
framework-10 (work in progress), May 2013.
[I-D.presta-clue-data-model-schema]
Presta, R. and S. Romano, "An XML Schema for the CLUE data
model", draft-presta-clue-data-model-schema-03 (work in
progress), March 2013.
[I-D.ietf-mmusic-sctp-sdp]
Loreto, S. and G. Camarillo, "Stream Control Transmission
Protocol (SCTP)-Based Media Transport in the Session
Description Protocol (SDP)", draft-ietf-mmusic-sctp-sdp-03
(work in progress), January 2013.
[I-D.tuexen-tsvwg-sctp-dtls-encaps]
Jesup, R., Loreto, S., Stewart, R., and M. Tuexen, "DTLS
Encapsulation of SCTP Packets for RTCWEB", draft-tuexen-
tsvwg-sctp-dtls-encaps-01 (work in progress), July 2012.
16.2. Informative References
[RFC4353] Rosenberg, J., "A Framework for Conferencing with the
Session Initiation Protocol (SIP)", RFC 4353, February
2006.
[I-D.even-clue-sdp-clue-relation]
Even, R., "Signalling of CLUE and SDP offer/answer",
draft-even-clue-sdp-clue-relation-01 (work in progress),
October 2012.
[I-D.even-clue-rtp-mapping]
Even, R. and J. Lennox, "Mapping RTP streams to CLUE media
captures", draft-even-clue-rtp-mapping-05 (work in
progress), February 2013.
[I-D.hansen-clue-sdp-interaction]
Hansen, R., "SDP and CLUE message interactions", draft-
hansen-clue-sdp-interaction-01 (work in progress),
February 2013.
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Authors' Addresses
Paul Kyzivat
Huawei
Email: pkyzivat@alum.mit.edu
Lennard Xiao
Huawei
Email: lennard.xiao@huawei.com
Christian Groves
Huawei
Email: Christian.Groves@nteczone.com
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