Network Working Group P. Kyzivat
Internet-Draft L. Xiao
Intended status: Standards Track C. Groves
Expires: March 19, 2014 Huawei
R. Hansen
Cisco Systems
September 15, 2013
CLUE Signaling
draft-kyzivat-clue-signaling-05
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.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on March 19, 2014.
Copyright Notice
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document authors. All rights reserved.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. CLUE-Specific Signaling Protocol . . . . . . . . . . . . . . 4
3.1. CLUE Messages . . . . . . . . . . . . . . . . . . . . . . 4
3.1.1. Request Messages . . . . . . . . . . . . . . . . . . 4
3.1.2. Response Messages . . . . . . . . . . . . . . . . . . 5
3.1.3. CLUE Requests . . . . . . . . . . . . . . . . . . . . 5
3.1.3.1. ADVERTISEMENT Message . . . . . . . . . . . . . . 5
3.1.3.2. CONFIGURE Message . . . . . . . . . . . . . . . . 5
3.1.3.3. SUPPORTED Message . . . . . . . . . . . . . . . . 5
3.1.3.4. REQUIRED Message . . . . . . . . . . . . . . . . 5
3.1.4. CLUE Response Reasons . . . . . . . . . . . . . . . . 6
3.2. Message Sequencing . . . . . . . . . . . . . . . . . . . 7
3.2.1. Pairing of Requests and Responses . . . . . . . . . . 8
3.2.2. Version Negotiation Happens First . . . . . . . . . . 8
3.2.3. Provider and Consumer Roles . . . . . . . . . . . . . 8
3.2.4. Independent Sequencing of Provider and Consumer Roles 8
3.2.5. Signaling Changes in Provider State . . . . . . . . . 8
3.2.6. Signaling Changes in Consumer State . . . . . . . . . 9
3.2.7. Dangling Text [[TODO: FIX]] . . . . . . . . . . . . . 9
3.3. Protocol Versioning and Options . . . . . . . . . . . . . 10
3.3.1. Versioning Objectives . . . . . . . . . . . . . . . . 10
3.3.2. Versioning Overview . . . . . . . . . . . . . . . . . 11
3.3.3. Version Negotiation . . . . . . . . . . . . . . . . . 13
3.3.4. Option Negotiation . . . . . . . . . . . . . . . . . 14
3.3.5. Option Elements . . . . . . . . . . . . . . . . . . . 14
3.3.5.1. <mediaProvider> . . . . . . . . . . . . . . . . . 14
3.3.6. Version & option negotiation errors . . . . . . . . . 15
3.3.7. Definition and Use of Version Numbers . . . . . . . . 16
3.3.8. Version & Option Negotiation Examples . . . . . . . . 17
3.3.8.1. Successful Negotiation - Multi-version . . . . . 17
3.3.8.2. Successful Negotiation - Consumer-Only Endpoint . 18
3.3.8.3. Successful Negotiation - Provider-Only Endpoint . 19
3.3.8.4. Version Incompatibility . . . . . . . . . . . . . 20
3.3.8.5. Option Incompatibility . . . . . . . . . . . . . 21
3.3.8.6. Syntax Error . . . . . . . . . . . . . . . . . . 22
3.4. Message Syntax . . . . . . . . . . . . . . . . . . . . . 22
3.5. Message Transport . . . . . . . . . . . . . . . . . . . . 25
3.5.1. CLUE Channel Lifetime . . . . . . . . . . . . . . . . 26
3.5.2. Channel Error Handling . . . . . . . . . . . . . . . 26
3.6. Message Framing . . . . . . . . . . . . . . . . . . . . . 27
4. CLUE use of SDP O/A . . . . . . . . . . . . . . . . . . . . . 27
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4.1. Establishing the CLUE channel . . . . . . . . . . . . . . 27
4.2. Representing CLUE Encodings in SDP . . . . . . . . . . . 27
4.3. Representing CLUE Encoding Groups in SDP . . . . . . . . 28
4.4. Signaling CLUE control of "m" lines . . . . . . . . . . . 28
4.5. Ensuring interoperability with non-CLUE devices . . . . . 29
5. Interaction of CLUE and SDP negotiations . . . . . . . . . . 29
5.1. Independence of SDP and CLUE negotiation . . . . . . . . 30
5.2. Recommendations for operating with non-atomic operations 30
5.3. Constraints on sending media . . . . . . . . . . . . . . 31
6. Example: A call between two CLUE-capable endpoints . . . . . 31
7. Example: A call between a CLUE and non-CLUE-capable endpoint 38
8. CLUE requirements on SDP O/A . . . . . . . . . . . . . . . . 40
9. SIP Signaling . . . . . . . . . . . . . . . . . . . . . . . . 40
10. Interoperation with Legacy SIP Devices . . . . . . . . . . . 40
11. CLUE over RTCWEB . . . . . . . . . . . . . . . . . . . . . . 40
12. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 41
13. What else? . . . . . . . . . . . . . . . . . . . . . . . . . 41
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 41
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 41
16. Security Considerations . . . . . . . . . . . . . . . . . . . 41
17. Change History . . . . . . . . . . . . . . . . . . . . . . . 41
18. References . . . . . . . . . . . . . . . . . . . . . . . . . 43
18.1. Normative References . . . . . . . . . . . . . . . . . . 43
18.2. Informative References . . . . . . . . . . . . . . . . . 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44
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:
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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
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. CLUE Messages
CLUE messages have the following characteristics:
o They are encoded in XML, following the schema in section
Section 3.4].
o There are two kinds of messages - requests and responses. (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 also carry info describing the error.
3.1.1. Request Messages
A request message specifies an action the sender is requesting the
recipient to perform. There are a number of request types, each of
which describes a different action. Each carries parameters
qualifying the action.
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3.1.2. Response Messages
The recipient of a request message must send a response message that
acknowledges that the request has been received. The response
indicates if the request was processed successfully, and if not, then
the reason for failure. A special reason (OK) indicates that the
request was processes successfully.
3.1.3. CLUE Requests
3.1.3.1. 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 3.4.
[[ Currently this does not contain any representation of encodings.
It assumes those will be defined in SDP. ]]
3.1.3.2. 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 3.4
3.1.3.3. SUPPORTED Message
The Supported message describes the CLUE versions, and options,
supported by the sender. The details of how use this message are
presented in section Section 3.2.2.
3.1.3.4. REQUIRED Message
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The Required message describes the CLUE version, and options, that
have been negotiated by the sender and the receiver. The details of
how use this message are presented in section Section 3.2.2.
3.1.4. CLUE Response Reasons
The following reasons are defined:
OK: The message was successfully processed.
Syntax Error: Message has failed due to a syntax error detected at
the message level. The message does not conform to the schema.
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 incompatibility: There is no common value between the major
version numbers supported by the two endpoints of the CLUE
channel.
Option incompatibility: This can occur if options supported by one
endpoint are inconsistent with those supported by the other
endpoint. E.g., The <mediaProvider> option is not specified by
either endpoint. Options SHOULD be specified so as to make it
difficult for this problem to occur.
This error may also be used to indicate that insufficient options
have been required among the two ends for a useful session to
result. This can occur with a feature that needs to be present on
at least one end, but not on a specific end. E.g., The
<mediaProvider> option was Supported by at least one of the
endpoints, but it was not Required by either.
This may also be used to indicate that an option element in the
Required message has attributes or body content that is
syntactically correct, but in inconsistent with the rules for
option negotiation specified for that particular element. The
definition of each option must specify the negotiation rules for
that option.
Unsupported option: Unsupported option
An option element type received in a Required message did not
appear in the corresponding Supported element.
(This code is never received in response to a Supported message.)
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Unknown capture identity: The received Configure message contains an
unknown capture identity not previously declared by an
Advertisement. The message is ignored.
Invalid identity: The received Configure message contains an invalid
capture identity. For example a duplicated Capture scene identity
or some other semantically incorrect usage. The message has
ignored.
Invalid value: The received message contains an invalid parameter
value. The value is not according to the specification for the
containing element.
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.
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.
3.2. Message Sequencing
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[[NOTE: Should have some state machines formalizing this
sequencing.]]
3.2.1. Pairing of Requests and Responses
Each endpoint of a CLUE channel sends both requests and responses.
For each request sent, a prompt response is required. Each response
identifies the request to which it responds. After sending a
request, one or more requests may be received before a response is
received.
3.2.2. Version Negotiation Happens First
Upon establishment of the CLUE channel, version and option
negotiation, using Supported and Required requests, MUST complete
before any other requests are sent on the channel. Refer to section
Section 3.3 for details of this process.
3.2.3. Provider and Consumer Roles
Each CLUE endpoint has two roles that it potential performs: provider
and consumer. In common cases an endpoint can perform both roles.
Which roles are actually performed is determined during option
negotiation.
3.2.4. Independent Sequencing of Provider and Consumer Roles
In the provider role, an endpoint sends Advertisement messages and
receives Configure messages.
In the consumer role, an endpoint receives Advertisement messages and
sends Configure messages.
The messages (requests and responses) used by each role are exchanged
over the same CLUE channel, and may be interleaved in an arbitrary
manner, constrained only by the sequencing rules for each role.
3.2.5. 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.
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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.
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]
3.2.6. 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.
3.2.7. Dangling Text [[TODO: FIX]]
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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 |
|*********************************>|
| |
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.
3.3. Protocol Versioning and Options
3.3.1. Versioning Objectives
The CLUE versioning mechanism addresses the following needs:
o Coverage:
* Versioning of basic behavior and options,
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* CLUE message exchange,
* CLUE message exchange,
* coordinated use of SIP and SDP,
* required media behavior.
o Remain fixed for the duration of the CLUE channel
o Be extensible for configuration of new options.
o Be sufficient (with extensions) for all envisioned future
versions.
3.3.2. Versioning Overview
An initial message exchange on the CLUE channel handles the
negotiation of version and options.
o Dedicated message types are used for this negotiation.
o The negotiation is repeated if the CLUE channel is reestablished.
The version usage is similar in philosophy to XMPP:
o See [RFC6120] section 4.7.5.
o A version has major and minor components. (Each a non-negative
integer.)
o Major version changes denote non-interoperable changes.
o Minor version changes denote schema changes that are backward
compatible by ignoring unknown XML elements, or other backward
compatible changes.
o If a common major version cannot be negotiated, then CLUE MUST NOT
be used.
o The same message exchange also negotiates options.
o Each option is denoted by a unique XML element in the negotiation.
Figure 1 shows the negotiation in simplified form:
| Supported Supported |
|------------\ /------------|
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| X |
|<-----------/ \----------->|
| |
| Required Required |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| Advertise/Configure/... |
|<------------------------->|
Figure 1: Basic Option Negotiation (simplified)
Dedicated message types are used for the negotiation because:
o The protocol can then ensure that the negotiation is done first,
and once. Not changing mid-session means an endpoint can plan
ahead, and predict what may be used and what might be received.
o This provides extensible framework for negotiating optional
features.
o A full option negotiation can be completed before other messages
are exchanged.
Figure 2 and Figure 3 are simplified examples of the Supported and
Required messages:
<supported>
<version major="1" minor="0">
<!- May repeat version if multiple
major versions supported. ->
<!- Options follow ->
<mediaProvider/>
...
</supported>
Figure 2: Supported Message (simplified)
<required>
<version major="1" minor="0">
<!- Requested options of peer follow ->
<!- Options follow ->
<mediaProvider/>
...
</required>
Figure 3: Required Message (simplified)
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3.3.3. Version Negotiation
The Supported message includes one or more <version> elements, each
denoting a major/minor version combination that the sender of the
message is capable of supporting.
The <version> element contains both a major and minor version. Each
is a non-negative integer. Each <version> element in the message
MUST contain a unique major version number, distinct from the major
version number in all the other <version> elements in the message.
The minor version in a <version> element denotes the largest minor
version the sender supports for the corresponding major version.
(Minor versions are always backwards compatible, so support for a
minor version implies support for all smaller minor versions.)
Each endpoint of the CLUE channel sends a Supported message, and
receives the Supported message sent by the other end. Then each end
compares the versions sent and the versions received to determine the
version to be used for this CLUE session.
o If there is no major version in common between the two ends,
negotiation fails.
o The <version> elements from the two ends that have the largest
matching major version are selected.
o After exchange each end determines compatible version numbers to
be used for encoding and decoding messages, and other behavior in
the CLUE session.
* The <version> elements from the two ends that have the largest
matching major version are selected.
* The side that sent the smaller minor version chooses the one it
sent.
* The side that sent the larger minor version may choose the
minor version it received, or the one it sent, or any value
between those two.
o Each end then sends a Required message with a single <version>
element containing the major and minor versions it has chosen.
[[Note: "required" is the wrong semantic for this. Might want a
better message name.]]
o Each end then behaves in accord with the specifications denoted by
the version it chose. This continues until the end of the CLUE
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session, or until changed as a result of another version
negotiation when the CLUE channel is reestablished.
[[Note: The version negotiation remains in effect even if the CLUE
channel is lost.]]
3.3.4. Option Negotiation
Option negotiation is used to agree upon which options will be
available for use within the CLUE session. (It does not say that
these options must be used.) This may be used for both standard and
proprietary options. (As used here, and option could be either a
feature described as part of this specification that is optional to
implement, or a feature defined in a separate specification that
extends this one.)
Each end includes, within the Supported message it sends, elements
describing those options it is willing and able to use with this CLUE
session.
Each side, upon receiving a Supported message, selects from that
message those option elements that it wishes the peer to use. (If/
when occasion for that use arises.) It then includes those selected
elements into the Required message that it sends.
Within a received Supported message, unknown option elements MUST be
ignored. This includes elements that are of a known type that is not
known to denote an option.
3.3.5. Option Elements
Each option is denoted, in the Supported and Required messages, by an
XML element. There are no special rules for these elements - they
can be any XML element. The attributes and body of the element may
carry further information about the option. The same element type is
used to denote the option in the Supported message and the
corresponding Required message, but the attributes and body may
differ according to option-specific rules. This may be used to
negotiate aspects of a particular option. The ordering of option
elements is irrelevant within the Supported and Required messages,
and need not be consistent in the two.
Only one option element is defined in this document: <mediaProvider>.
3.3.5.1. <mediaProvider>
The <mediaProvider> element, when placed in a Supported message,
indicates that the sender is willing and able to send Advertisement
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messages and receive Configure messages. When placed in a Required
message, the <mediaProvider> element indicates that the sender is
willing, able, and desirous of receiving Advertisement messages and
sending Configure messages. If an endpoint does not receive
<mediaProvider> in a Required message, it MUST NOT send Advertisement
messages. For common cases <mediaProvider> should be supported and
required by both endpoints, to enable bidirectional exchange of
media. If not required by either end, the CLUE session is useless.
This is an error condition, and SHOULD result in termination of the
CLUE channel.
The <mediaProvider> element has no defined attributes or body.
3.3.6. Version & option negotiation errors
The following are errors that may be detected and reported during
version negotiation:
o Version incompatibility
There is no common value between the major version numbers sent in
a Supported message and those in the received Supported message.
o Option incompatibility
This can occur if options supported by one endpoint are
inconsistent with those supported by the other endpoint. E.g.,
The <mediaProvider> option is not specified by either endpoint.
Options SHOULD be specified so as to make it difficult for this
problem to occur.
This error may also be used to indicate that insufficient options
have been required among the two ends for a useful session to
result. This can occur with a feature that needs to be present on
at least one end, but not on a specific end. E.g., The
<mediaProvider> option was Supported by at least one of the
endpoints, but it was not Required by either.
This may also be used to indicate that an option element in the
Required message has attributes or body content that is
syntactically correct, but in inconsistent with the rules for
option negotiation specified for that particular element. The
definition of each option must specify the negotiation rules for
that option.
o Unsupported option
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An option element type received in a Required message did not
appear in the corresponding Supported element.
(Unsupported options received in a Supported message do not
trigger this error. They are ignored.)
These errors are reported using the normal message error reporting
mechanism.
Other applicable error codes may also be returned in response to a
Supported or Required message.
Errors that occur at this stage result in negotiation failure. When
this occurs, CLUE cannot be used until the end of the SIP session, or
until a new CLUE channel is negotiated and a subsequent version
negotiation succeeds. The SIP session may continue without CLUE
features.
3.3.7. Definition and Use of Version Numbers
[[NOTE: THIS IS AWKWARD. SUGGESTIONS FOR BETTER WAYS TO DEFINE THIS
ARE WELCOME.]]
This document defines CLUE version 1.0 (major=1, minor=0). This
denotes the normative behavior defined in this document and other
documents upon which it normatively depends, including but is not
limited to:
o the schema defined in Section 3.4 of this document;
o the schema defined in [clue-data-model];
o the protocol used to exchange CLUE messages;
o the protocol defined herein that defines valid sequence of CLUE
messages;
o the specific rules defined herein for employing SIP, SDP, and RTP
to realize the CLUE messages.
Given two CLUE versions Vx and Vy, then Vx is backward compatible
with Vy if and only if:
o All messages valid according to the schema of Vx are also valid
according to the schemas of Vy
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o All messages valid according to the schema of Vy can be made valid
according to the schemas of Vx by deleting elements undefined in
the schemas of Vx.
[[NOTE: THIS PROBABLY NEEDS WORK!]]
o All normative behaviors defined for Vx are defined consistently
for Vy.
[[NOTE: SOME HAND WAVING HERE.]]
Revisions, updates, to any of the documents denoted by Version 1.0
MAY result in the definition of a new CLUE version. If they do, then
this document MUST be revised to define the new version.
The CLUE version to be defined in a revision to this document MUST be
determined as follows:
o If the revision and the document being revised are mutually
backward compatible (they are functionally equivalent), then the
CLUE version MUST remain unchanged.
o Else if the revision is backward compatible with the document
being revised, then the CLUE major version MUST remain unchanged,
and the CLUE minor version MUST be increased by one (1).
o Else the CLUE major version must be increased by one (1), and the
CLUE minor version set to zero (0).
When a CLUE implementation sends a Supported message, it MUST include
the CLUE versions it is willing and able to conform with.
3.3.8. Version & Option Negotiation Examples
3.3.8.1. Successful Negotiation - Multi-version
| Supported Supported |
| Version 2.0 |
| Version 1.2 Version 1.1 |
| mediaProv mediaProv |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| OK response OK response |
|------------\ /------------|
| X |
|<-----------/ \----------->|
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| |
| Required Required |
| Version 1.2 Version 1.1 |
| mediaProv mediaProv |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| OK response OK response |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| Advertise |
|<------------------------->|
| |
| Configure |
|<------------------------->|
The endpoint on the left can support versions 1.2 and 2.0, and
because of backward compatibility can support versions 1.0 and 1.1.
The endpoint on the right supports only version 2.0. Both endpoints
with to both provide and consume media. They each send a Supported
message indicating what they support.
The element on the left, upon receiving the Supported message,
determines that it is permitted to use version 1.2 or 1.1, and
decides to use 1.2. It sends a Required message containing version
1.2 and also includes the mediaProvider option element, because it
wants its peer to provide media.
The element on the right, upon receiving the Supported message,
selects version 1.1 because it is the highest version in common to
the two sides. It sends a Required message containing version 1.1
because that is the highest version in common. It also includes the
mediaProvider option element, because it wants its peer to provide
media.
Upon receiving the Required messages, both endpoints determine that
they should send Advertisements.
Advertisement and Configure messages will flow in both directions.
3.3.8.2. Successful Negotiation - Consumer-Only Endpoint
| Supported Supported |
| Version 1.0 Version 1.0 |
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| mediaProv (no opts) |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| OK response OK response |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| Required Required |
| Version 1.0 Version 1.0 |
| (no opts) mediaProv |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| OK response OK response |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| Advertise |
|-------------------------->|
| |
| Configure |
|<--------------------------|
The endpoint on the right consumes media, but doesn't provide any so
it doesn't include the mediaProvider option element in the Supported
message it sends.
The element on the left would like to include a mediaProvider option
element in the Requirements message it sends, but can't because it
did not receive one in the Supported message it received.
Advertisement messages will only go from left to right, and Configure
messages will only go from right to left.
3.3.8.3. Successful Negotiation - Provider-Only Endpoint
| Supported Supported |
| Version 1.0 Version 1.0 |
| mediaProv mediaProv |
|------------\ /------------|
| X |
|<-----------/ \----------->|
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| |
| OK response OK response |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| Required Required |
| Version 1.0 Version 1.0 |
| (no opts) mediaProv |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| OK response OK response |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| Advertise |
|-------------------------->|
| |
| Configure |
|<--------------------------|
The endpoint on the left provides media but does not consume any so
it includes the mediaProvider option element in the Supported message
it sends, but does't include the mediaProvider option element in the
Required message it sends.
Advertisement messages will only go from left to right, and Configure
messages will only go from right to left.
3.3.8.4. Version Incompatibility
| Supported Supported |
| Version 1.2 Version 2.1 |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| Version Version |
| Incompat. Incompat. |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| close clue channel |
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|<------------------------->|
| |
| legacy mode or BYE |
|<------------------------->|
Upon receiving the Supported message, each endpoint discovers there
is no major version in common, so CLUE usage is not possible. Each
sends an error response indicating this and then ceases CLUE usage.
3.3.8.5. Option Incompatibility
| Supported Supported |
| Version 1.0 Version 1.0 |
| mediaProv mediaProv |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| Required Required |
| (no opts) (no opts) |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| Option Option |
| Incompat. Incompat. |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| close clue channel |
|<------------------------->|
| |
| legacy mode or BYE |
|<------------------------->|
Neither of the endpoints is willing to provide media. It makes no
sense to continue CLUE operation in this situation. Each endpoint
realizes this upon receiving the Supported message, sends an error
response indicating this and then ceases CLUE usage.
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3.3.8.6. Syntax Error
| Supported !@#$%^ |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| syntax error OK response |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| close clue channel |
|-------------------------->|
| |
| legacy mode or BYE |
|<------------------------->|
3.4. 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
proposed to be 'a=label:ID', though 'a=mid:ID' is another candidate.
]]
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"/>
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<!-- ELEMENT DEFINITIONS -->
<xs:element name="response" type="responseMessageType"/>
<xs:element name="advertisement" type="advertisementMessageType"/>
<xs:element name="configure" type="configureMessageType"/>
<xs:element name="supported" type="supportedMessageType"/>
<xs:element name="required" type="requiredMessageType"/>
<!-- CLUE MESSAGE TYPE -->
<xs:complexType name="clueMessageType" abstract="true">
<xs:sequence>
<!-- mandatory fields -->
<!-- TBS: version info -->
</xs:sequence>
</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"/>
<xs:element name="reason" type="reasonType" minOccurs="1"/>
<!-- optional fields -->
<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>
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<!-- 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"
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>
<!-- CLUE SUPPORTED MESSAGE TYPE -->
<xs:complexType name="supportedMessageType">
<xs:complexContent>
<xs:extension base="clueRequestMessageType">
<xs:sequence>
<!-- mandatory fields -->
<xs:element name="version"
type="versionType"
maxOccurs="unbounded"/>
<!-- optional fields -->
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<xs:element name="Options"
type="OptionsType" minOccurs="0"/>
<xs:any namespace="##other"
processContents="lax" minOccurs="0"/>
</xs:sequence>
</xs:extension>
</xs:complexContent>
</xs:complexType>
<!-- CLUE REQUIRED MESSAGE TYPE -->
<xs:complexType name="requiredMessageType">
<xs:complexContent>
<xs:extension base="clueRequestMessageType">
<xs:sequence>
<!-- mandatory fields -->
<xs:element name="version"
type="versionType"
maxOccurs="1"/>
<!-- optional fields -->
<xs:element name="Options"
type="OptionsType" minOccurs="0"/>
<xs:any namespace="##other"
processContents="lax" minOccurs="0"/>
</xs:sequence>
</xs:extension>
</xs:complexContent>
</xs:complexType>
<!-- OPTIONS TYPE -->
<xs:complexType name="optionsType">
<!-- each element represents one option -->
<xs:any processContents="lax" minOccurs="0"/>
</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>
3.5. Message Transport
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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
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.)
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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. Message Framing
Message framing is provided by the SCTP transport protocol. Each
CLUE message is carried in one SCTP message.
4. CLUE use of SDP O/A
4.1. Establishing the CLUE channel
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
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.
The presence of an active m-line for the CLUE channel in an SDP offer
is an indication that the offer that the sender is CLUE-capable and
hence can understand CLUE-specific syntax.
4.2. Representing CLUE Encodings in SDP
Many CLUE constructs have no good analog in SDP. Entities such as
'captures', which describe spatial and other properties of a capture
source such as a camera, are not tied directly to RTP streams, do not
have negotiated properties and would prove a significant challenge to
represent in SDP syntax (while also greatly increasing the size of
the SDP).
However, two entities defined in the CLUE Framework
[I-D.ietf-clue-framework] are a much closer fit for SDP: Encodings
and Encoding Groups. Both describe RTP media properties and
limitations, though unlike most SDP usage they describe the sender's
capabilities, not the receiver's. Representing encodings in CLUE
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splits media limitations across two protocols, and risks duplicated
and potentially contradictory information being sent in CLUE and SDP.
As such we are exploring representing this information in SDP, with
the decision to convey them in the CLUE messages only to be made if
the SDP approach proves impractical.
This draft presents an attempt to describe CLUE encodings in SDP. As
a decision has not yet been reached on how multiplexed RTP streams
are to be expressed in SDP, at this stage the draft does so without
multiplexing, using existing SDP attributes, with a seperate "m" line
and hence port per unidirectional RTP stream. This is done with the
understanding that when a decision is reached on new syntax for
multiplexing RTP streams in SDP the CLUE SDP signaling will be
modified to use it. Further, the framework document states that the
multiplexing of streams by an implementation is optional, and in the
case of a disaggregated system, with media streams going to different
addresses, may not be possible.
With the current scheme of using existing syntax, an encoding is
specified in SDP as a unicast "m" line, which MUST be marked as
sendonly with the "a=sendonly" attribute or as inactive with the
"a=inactive" attribute. The encoder capabilities of the stream are
defined here using existing syntax; for instance, for H.264 see Table
6 in [RFC6184] for a list of valid parameters for representing
encoder sender stream limits.
Every "m" line representing a CLUE encoding SHOULD contain a "label"
attribute as defined in [RFC4574]. This label is used to identify
the encoding by the sender in CLUE Advertisement messages and by the
receiver in CLUE Configure messages.
A receiver who wishes to receive a CLUE stream via this encoding
requires a matching "a=recvonly" "m" line. As well as the normal
restrictions defined in [RFC3264] media MUST NOT be sent on this
stream until the sender has received a valid CLUE Configure message
specifying the capture to be used for this stream.
4.3. Representing CLUE Encoding Groups in SDP
As per the previous section, there would be advantages to conveying
encoding group information in SDP. However, with current SDP syntax
there is no way to express the encoding group limits defined in the
Data Model [I-D.presta-clue-data-model-schema]. As such the current
draft keeps encoding groups as part of the Advertisement message for
the time being.
4.4. Signaling CLUE control of "m" lines
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In many cases an implementation may wish to mix media channels that
are under CLUE control with those that are not. It may want to
ensure that there are non-CLUE streams for purposes of
interoperability, or that can provide media from the start of the
call before CLUE negotiation completes, or because the implementation
wants CLUE-controlled video but traditional audio, or for any other
reasons.
Which "m" lines in an SDP body are under control of the CLUE channel
is signalled via the SDP Grouping Framework [RFC5888]. Devices that
wish to negotiate CLUE MUST support the grouping framework.
A new semantic for the "group" session-level attribute, "CLUE", is
used to signal which "m" lines are under the control of a CLUE
channel. As per the framework, all of the "m" lines of a session
description that uses "group" MUST be identified with a "mid"
attribute whether they are controlled by CLUE or not. The "mid" id
of any "m" lines controlled by a CLUE channel MUST be included in the
"CLUE" group attribute alongside the "mid" id of the CLUE channel
controlling them.
The CLUE group MUST NOT include more than one "m" line for a CLUE
channel. If a CLUE channel is part of the CLUE group attribute other
media "m" lines included in the group are under the control of that
CLUE channel; media MUST NOT be sent or received on these "m" lines
until the CLUE channel has been negotiated and negotiation has taken
place as defined in this document. If no CLUE channel is part of the
CLUE group attribute then media MUST NOT be sent or received on these
"m" lines.
"m" lines not specified as under CLUE control follow normal rules for
media streams negotiated in SDP as defined in documents such as
[RFC3264].
An SDP MAY include more than one group attribute with the "CLUE"
semantic. An "mid" id for a given "m" line MUST NOT be included in
more than one CLUE group.
4.5. Ensuring interoperability with non-CLUE devices
A CLUE-capable device sending an initial SDP offer SHOULD include an
"m" line for the CLUE channel, but SHOULD NOT include any other CLUE-
controlled "m" lines. Once each side of the call is aware that the
other side is CLUE-capable a new O/A exchange MAY be used to add
CLUE-controlled "m" lines.
5. Interaction of CLUE and SDP negotiations
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Information about media streams in CLUE is split between two message
types: SDP, which defines media addresses and limits, and the CLUE
channel, which defines properties of capture devices available, scene
information and additional constraints. As a result certain
operations, such as advertising support for a new transmissible
capture with associated stream, cannot be performed atomically, as
they require changes to both SDP and CLUE messaging.
This section defines how the negotiation of the two protocols
interact, provides some recommendations on dealing with intermediary
stages in non-atomic operations, and mandates additional constraints
on when CLUE-configured media can be sent.
5.1. Independence of SDP and CLUE negotiation
To avoid complicated state machines with the potential to reach
invalid states if messages were to be lost, or be rewritten en-route
by middle boxes, the current proposal is that SDP and CLUE messages
are independent. The state of the CLUE channel does not restrict
when an implementation may send a new SDP offer or answer, and
likewise the implementation's ability to send a new CLUE
Advertisement or Configure message is not restricted by the results
of or the state of the most recent SDP negotiation.
The primary implication of this is that a device may receive an SDP
with a CLUE encoding it does not yet have capture information for, or
receive a CLUE Configure message specifying a capture encoding for
which the far end has not negotiated a media stream in SDP.
CLUE messages contain an EncodingID which is used to identify a
specific encoding in SDP. The non-atomic nature of CLUE negotiation
means that a sender may wish to send a new Advertisement before the
corresponding SDP message. As such the sender of the CLUE message
MAY include an EncodingID which does not currently match an extant id
in SDP.
5.2. Recommendations for operating with non-atomic operations
Generally, implementations that receive messages for which they have
incomplete information SHOULD wait until they have the corresponding
information they lack before sending messages to make changes related
to that information. For instance, an implementation that receives a
new SDP offer with three new "a=sendonly" CLUE "m" lines that has not
received the corresponding CLUE Advertisement providing the capture
information for those streams SHOULD NOT include corresponding
"a=recvonly" lines in its answer, but instead should make a new SDP
offer when and if a new Advertisement arrives with captures relevant
to those encodings.
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Because of the constraints of offer/answer and because new SDP
negotiations are generally more 'costly' than sending a new CLUE
message, implementations needing to make changes to both channels
SHOULD prioritize sending the updated CLUE message over sending the
new SDP message. The aim is for the recipient to receive the CLUE
changes before the SDP changes, allowing the recipient to send their
SDP answers without incomplete information, reducing the number of
new SDP offers required.
5.3. Constraints on sending media
While SDP and CLUE message states do not impose constraints on each
other, both impose constraints on the sending of media - media MUST
NOT be sent unless it has been negotiated in both CLUE and SDP: an
implementation MUST NOT send a specific CLUE capture encoding unless
its most recent SDP exchange contains an active media channel for
that encoding AND the far end has sent a CLUE Configure message
specifying a valid capture for that encoding.
6. Example: A call between two CLUE-capable endpoints
This example illustrates a call between two CLUE-capable endpoints.
Alice, initiating the call, is a system with three cameras and three
screens. Bob, receiving the call, is a system with two cameras and
two screens. A call-flow diagram is presented, followed by an
summary of each message.
To manage the size of this section only video is considered, and SDP
snippets only illustrate video 'm' lines. ACKs are not discussed.
+----------+ +-----------+
| Alice | | Bob |
| | | |
+----+-----+ +-----+-----+
| |
| |
| INVITE 1 (BASIC SDP+COMEDIA) |
|--------------------------------->|
| |
| |
| 200 OK 2 (BASIC SDP+COMEDIA) |
|<---------------------------------|
| |
| |
| ACK 1 |
|--------------------------------->|
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| |
| |
| |
|<########### MEDIA 1 ############>|
| 1 video A->B, 1 video B->A |
|<################################>|
| |
| |
| |
|<================================>|
| CLUE CTRL CHANNEL ESTABLISHED |
|<================================>|
| |
| |
| ADVERTISEMENT 1 |
|*********************************>|
| |
| |
| ADVERTISEMENT 2 |
|<*********************************|
| |
| |
| INVITE 2 (+3 sendonly) |
|--------------------------------->|
| |
| |
| CONFIGURE 1 |
|<*********************************|
| |
| |
| 200 OK 2 (+2 recvonly) |
|<---------------------------------|
| |
| |
| ACK 2 |
|--------------------------------->|
| |
| |
| |
|<########### MEDIA 2 ############>|
| 2 video A->B, 1 video B->A |
|<################################>|
| |
| |
| INVITE 3 (+2 sendonly) |
|<---------------------------------|
| |
| |
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| CONFIGURE 3 |
|*********************************>|
| |
| |
| 200 OK 3 (+2 recvonly) |
|--------------------------------->|
| |
| |
| |
| ACK 3 |
|<---------------------------------|
| |
| |
| |
|<########### MEDIA 3 ############>|
| 2 video A->B, 2 video B->A |
|<################################>|
| |
| |
| |
v v
In INVITE 1, Alice sends Bob 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]. A snippet of the SDP showing the
grouping attribute and the video m-line are shown below (mid 3
represents the CLUE channel):
...
a=group:CLUE 3
...
m=video 6002 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
a=sendrecv
a=mid:2
Bob responds with a similar SDP (200 OK 1); due to their similiarity
no SDP snippet is shown here. Alice and Bob are each able to send a
single audio and video stream (whether they choose to send this
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initial media before CLUE has been negotiated is implementation-
dependent). This is illustrated as MEDIA 1.
With the successful initial O/A Alice and Bob are also free to
negotiate the CLUE channel. Once this is successfully established
CLUE negotiation can begin. This is illustrated as CLUE CHANNEL
ESTABLISHED.
Alice now sends her CLUE Advertisement (ADVERTISEMENT 1). She
advertises three static captures representing her three cameras. She
also includes switched captures suitable for two- and one-screen
systems. All of these captures are in a single capture scene, with
suitable capture scene entries to tell Bob that he should either
subscribe to the three static captures, the two switched capture view
or the one switched capture view. Alice has no simultaneity
constraints, so includes all six captures in one simultaneous set.
Finally, Alice includes an encoding group with three encoding IDs:
"enc1", "enc2" and "enc3". These encoding ids aren't currently
valid, but will match the next SDP offer she sends.
Bob received ADVERTISEMENT 1 but does not yet send a Configure
message, because he has not yet received Alice's encoding
information, so as yet he does not know if she will have sufficient
resources to send him the two streams he ideally wants at a quality
he is happy with.
Bob also sends his CLUE Advertisement (ADVERTISEMENT 2). He
advertises two static captures representing his cameras. He also
includes a single composed capture for single-screen systems, in
which he will composite the two camera views into a single video
stream. All three captures are in a single capture scene, with
suitable capture scene entries to tell Alice that she should either
subscribe to the two static captures, or the single composed capture.
Bob also has no simultaneity constraints, so includes all three
captures in one simultaneous set. Bob also includes a single
encoding group with two encoding IDs: "foo" and "bar".
Similarly, Alices receives ADVERTISEMENT 2 but does not yet send a
Configure message, because she has not yet received Bob's encoding
information.
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Alice now sends INVITE 2. She maintains the sendrecv audio, video
and CLUE m-lines, and she adds three new sendonly m-lines to
represents the maximum three encodings she can send. Each of these
m-lines has a label corresponding to one of the encoding ids from
ADVERTISEMENT 1. Each also has its mid added to the grouping
attribute to show they are controlled by the CLUE channel. A snippet
of the SDP showing the grouping attribute and the video m-lines are
shown below (mid 3 represents the CLUE channel):
...
a=group:CLUE 3 4 5 6
...
m=video 6002 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
a=sendrecv
a=mid:2
...
m=video 6004 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016
a=sendonly
a=mid:4
a=label:enc1
m=video 6006 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016
a=sendonly
a=mid:5
a=label:enc2
m=video 6008 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016
a=sendonly
a=mid:6
a=label:enc3
Bob now has all the information he needs to decide which streams to
configure. As such he now sends CONFIGURE 1. This requests the pair
of switched captures that represent Alice's scene, and he configures
them with encoder ids "enc1" and "enc2".
Alice receives Bob's message CONFIGURE 1 but does not yet send the
capture encodings specified, because at this stage Bob hasn't
negotiated the ability to receive these streams in SDP.
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Bob now sends his SDP answer as part of 200 OK 2. Alongside his
original audio, video and CLUE m-lines he includes two active
recvonly m-lines and a zeroed m-line for the third. He adds their
mid values to the grouping attribute to show they are controlled by
the CLUE channel. A snippet of the SDP showing the grouping
attribute and the video m-lines are shown below (mid 100 represents
the CLUE channel):
...
a=group:CLUE 11 12 100
...
m=video 58722 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
a=sendrecv
a=mid:10
...
m=video 58724 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
a=recvonly
a=mid:11
m=video 58726 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
a=recvonly
a=mid:12
m=video 0 RTP/AVP 96
On receiving 200 OK 2 from Bob Alice is now able to send the two
streams of video Bob requested - this is illustrated as MEDIA 2.
The constraints of offer/answer meant that Bob could not include his
encoder information as new m-lines in 200 OK 2. As such Bob now
sends INVITE 3 to generate a new offer. Along with all the streams
from 200 OK 2 Bob also includes two new sendonly streams. Each
stream has a label corresponding to the encoding ids in his
ADVERTISEMENT 2 message. He also adds their mid values to the
grouping attribute to show they are controlled by the CLUE channel.
A snippet of the SDP showing the grouping attribute and the video
m-lines are shown below (mid 100 represents the CLUE channel):
...
a=group:CLUE 11 12 13 14 100
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...
m=video 58722 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
a=sendrecv
a=mid:10
...
m=video 58724 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
a=recvonly
a=mid:11
m=video 58726 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
a=recvonly
a=mid:12
m=video 0 RTP/AVP 96
m=video 58728 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016
a=sendonly
a=label:foo
a=mid:13
m=video 58730 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016
a=sendonly
a=label:bar
a=mid:14
Having received this Alice now has all the information she needs to
send CONFIGURE 2. She requests the two static captures from Bob, to
be sent on encodings "foo" and "bar".
Bob receives Alice's message CONFIGURE 2 but does not yet send the
capture encodings specified, because Alice hasn't yet negotiated the
ability to receive these streams in SDP.
Alice now sends 200 OK 3, matching two recvonly m-lines to Bob's new
sendonly lines. She includes their mid values in the grouping
attribute to show they are controlled by the CLUE channel. A snippet
of the SDP showing the grouping attribute and the video m-lines are
shown below (mid 3 represents the CLUE channel):
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...
a=group:CLUE 3 4 5 7 8
...
m=video 6002 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
a=sendrecv
a=mid:2
...
m=video 6004 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016
a=sendonly
a=mid:4
a=label:enc1
m=video 6006 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016
a=sendonly
a=mid:5
a=label:enc2
m=video 0 RTP/AVP 96
m=video 6010 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
a=recvonly
a=mid:7
m=video 6012 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
a=recvonly
a=mid:8
Finally, on receiving 200 OK 3 Bob is now able to send the two
streams of video Alice requested - this is illustrated as MEDIA 3.
Both sides of the call are now sending multiple video streams with
their sources defined via CLUE negotiation. As the call progresses
either side can send new Advertisement or Configure or new SDP
negotiation to add, remove or change what they have available or want
to receive.
7. Example: A call between a CLUE and non-CLUE-capable endpoint
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In this brief example Alice is a CLUE-capable endpoint making a call
to Bob, who is not CLUE-capable, i.e., it is not able to use the CLUE
protocol.
+----------+ +-----------+
| EP1 | | EP2 |
| | | |
+----+-----+ +-----+-----+
| |
| |
| INVITE 1 (BASIC SDP+COMEDIA) |
|--------------------------------->|
| |
| |
| 200 0K 1 (BASIC SDP+*NO*COMEDIA) |
|<---------------------------------|
| |
| |
| ACK 1 |
|--------------------------------->|
| |
| |
| |
|<########### MEDIA 1 ############>|
| 1 video A->B, 1 video B->A |
|<################################>|
| |
| |
| |
| |
v v
In INVITE 1, Alice sends Bob 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]. A snippet of the SDP showing the
grouping attribute and the video m-line are shown below (mid 3
represents the CLUE channel):
...
a=group:CLUE 3
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...
m=video 6002 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
a=sendrecv
a=mid:2
Bob is not CLUE capable, and hence does not recognize the "CLUE"
semantic for the grouping attribute, not does he support the CLUE
channel. He responds with an answer with audio and video, but with
the CLUE channel zeroed.
From the lack of the CLUE channel Alice understands that Bob does not
support CLUE, or does not wish to use it. Both sides are now able to
send a single audio and video stream to each other. Alice at this
point begins to send her fallback video: in this case likely a
switched view from whichever camera shows the current loudest
participant on her side.
8. CLUE requirements on SDP O/A
The current proposal calls for a new "CLUE" semantic for the SDP
Grouping Framework [RFC5888].
Any other SDP extensions required to support CLUE signaling should
also 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.
9. SIP Signaling
(Placeholder) This may be unremarkable. If so we can drop it.
10. 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.
11. CLUE over RTCWEB
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We may want to rule this out of scope for now. But we should be
thinking about this.
12. 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.
o The current method for expressing encodings in SDP limits the
parameters available when describing H264 encoder capabilities to
those defined in Table 6 in [RFC6184]
13. What else?
14. 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.
15. IANA Considerations
TBD
16. Security Considerations
TBD
17. Change History
-05: Revisions by pkyzivat:
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* Specified versioning model and mechanism.
* Added explicit response to all messages.
* Rearranged text to work with the above changes. (Which
rendered diff almost useless.)
-04: Revisions by Rob Hansen: ???
-03: Revisions by pkyzivat:
* 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.
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18. References
18.1. 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-11 (work in progress), July 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-04
(work in progress), June 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.
[RFC4574] Levin, O. and G. Camarillo, "The Session Description
Protocol (SDP) Label Attribute", RFC 4574, August 2006.
[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888, June 2010.
18.2. Informative References
[RFC4353] Rosenberg, J., "A Framework for Conferencing with the
Session Initiation Protocol (SIP)", RFC 4353, February
2006.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, June
2002.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, March 2011.
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Internet-Draft CLUE Signaling September 2013
[RFC6184] Wang, Y., Even, R., Kristensen, T., and R. Jesup, "RTP
Payload Format for H.264 Video", RFC 6184, May 2011.
[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.
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
Robert Hansen
Cisco Systems
Email: rohanse2@cisco.com
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