Internet DRAFT - draft-kyzivat-clue-signaling
draft-kyzivat-clue-signaling
Network Working Group P. Kyzivat
Internet-Draft L. Xiao
Intended status: Standards Track C. Groves
Expires: October 13, 2014 Huawei
R. Hansen
Cisco Systems
April 11, 2014
CLUE Signaling
draft-kyzivat-clue-signaling-08
Abstract
This document specifies how CLUE-specific signaling such as the CLUE
protocol [I-D.presta-clue-protocol] and the CLUE data channel
[I-D.ietf-clue-datachannel] are used with each other and with
existing signaling mechanisms such as SIP and SDP to produce a
telepresence call.
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 October 13, 2014.
Copyright Notice
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include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Media Feature Tag Definition . . . . . . . . . . . . . . . . . 5
4. SDP Grouping Framework TELEPRESENCE Extension Semantics . . . 5
4.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. The CLUE data channel and the TELEPRESENCE grouping
semantic . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.3. CLUE-controlled media and the TELEPRESENCE grouping
semantic . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.4. SDP Offer/Answer Procedures . . . . . . . . . . . . . . . 6
4.4.1. Generating the Initial Offer . . . . . . . . . . . . . 6
4.4.1.1. Signalling CLUE Encodings . . . . . . . . . . . . 6
4.4.1.2. Receiving CLUE-controlled media . . . . . . . . . 8
4.4.1.3. Interoperability with non-CLUE devices . . . . . . 8
4.4.2. Generating the Answer . . . . . . . . . . . . . . . . 8
4.4.2.1. Negotiating use of CLUE and the CLUE data
channel . . . . . . . . . . . . . . . . . . . . . 8
4.4.2.2. Negotiating receipt of CLUE capture encodings
in SDP . . . . . . . . . . . . . . . . . . . . . . 8
4.4.3. Processing the initial Offer/Answer negotiation . . . 9
4.4.3.1. Successful CLUE negotiation . . . . . . . . . . . 9
4.4.3.2. CLUE negotiation failure . . . . . . . . . . . . . 9
4.4.4. Modifying the session . . . . . . . . . . . . . . . . 9
4.4.4.1. Enabling CLUE mid-call . . . . . . . . . . . . . . 9
4.4.4.2. Disabling CLUE mid-call . . . . . . . . . . . . . 10
5. Interaction of CLUE protocol and SDP negotiations . . . . . . 10
5.1. Independence of SDP and CLUE negotiation . . . . . . . . . 10
5.2. Recommendations for operating with non-atomic
operations . . . . . . . . . . . . . . . . . . . . . . . . 11
5.3. Constraints on sending media . . . . . . . . . . . . . . . 11
6. Multiplexing of CLUE-controlled media using BUNDLE . . . . . . 12
6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.2. Usage of BUNDLE with CLUE . . . . . . . . . . . . . . . . 12
6.2.1. Generating the Initial Offer . . . . . . . . . . . . . 12
6.2.2. Bundle Address Synchronization . . . . . . . . . . . . 12
6.2.3. Multiplexing of the data channel and RTP media . . . . 13
7. Example: A call between two CLUE-capable endpoints . . . . . . 13
8. Example: A call between a CLUE-capable and non-CLUE
endpoint . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
9. CLUE requirements on SDP O/A . . . . . . . . . . . . . . . . . 22
10. SIP Signaling . . . . . . . . . . . . . . . . . . . . . . . . 22
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11. CLUE over RTCWEB . . . . . . . . . . . . . . . . . . . . . . . 22
12. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 23
13. What else? . . . . . . . . . . . . . . . . . . . . . . . . . . 23
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
16. Security Considerations . . . . . . . . . . . . . . . . . . . 23
17. Change History . . . . . . . . . . . . . . . . . . . . . . . . 23
18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
18.1. Normative References . . . . . . . . . . . . . . . . . . . 25
18.2. Informative References . . . . . . . . . . . . . . . . . . 26
Appendix A. CLUE Signalling and data channel concerns . . . . . . 27
A.1. Protocol Versioning and Options . . . . . . . . . . . . . 27
A.1.1. Versioning Objectives . . . . . . . . . . . . . . . . 27
A.1.2. Versioning Overview . . . . . . . . . . . . . . . . . 28
A.1.3. Version Negotiation . . . . . . . . . . . . . . . . . 30
A.1.4. Option Negotiation . . . . . . . . . . . . . . . . . . 31
A.1.5. Option Elements . . . . . . . . . . . . . . . . . . . 31
A.1.5.1. <mediaProvider> . . . . . . . . . . . . . . . . . 31
A.1.6. Version & option negotiation errors . . . . . . . . . 32
A.1.7. Definition and Use of Version Numbers . . . . . . . . 33
A.1.8. Version & Option Negotiation Examples . . . . . . . . 34
A.1.8.1. Successful Negotiation - Multi-version . . . . . . 34
A.1.8.2. Successful Negotiation - Consumer-Only Endpoint . 35
A.1.8.3. Successful Negotiation - Provider-Only Endpoint . 36
A.1.8.4. Version Incompatibility . . . . . . . . . . . . . 37
A.1.8.5. Option Incompatibility . . . . . . . . . . . . . . 38
A.1.8.6. Syntax Error . . . . . . . . . . . . . . . . . . . 39
A.2. Message Transport . . . . . . . . . . . . . . . . . . . . 39
A.2.1. CLUE Channel Lifetime . . . . . . . . . . . . . . . . 39
A.2.2. Channel Error Handling . . . . . . . . . . . . . . . . 40
A.3. Message Framing . . . . . . . . . . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 40
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1. Introduction
To enable devices to participate in a telepresence call, selecting
the sources they wish to view, receiving those media sources and
displaying them in an optimal fashion, CLUE involves two principal
and inter-related protocol negotiations. SDP, conveyed via SIP, is
used to negotiate the specific media capabilities that can be
delivered to specific addresses on a device. Meanwhile, a CLUE
protocol [I-D.presta-clue-protocol], transported via a CLUE data
channel [I-D.ietf-clue-datachannel], is used to negotiate the capture
sources available, their attributes and any constraints in their use,
along which which captures the far end provides a device wishes to
receive.
Beyond negotiating the CLUE channel, SDP is also used to negotiate
the details of supported media streams and the maximum capability of
each of those streams. As the CLUE Framework
[I-D.ietf-clue-framework] defines a manner in which the media
provider expresses their maximum encoding capabilities, SDP is also
used to express the encoding limits for each potential encoding.
Backwards-compatibility is an important consideration of the
document: it is vital that a CLUE-capable device contacting a device
that does not support CLUE is able to fall back to a fully functional
non-CLUE call. The document also defines how a non-CLUE call may be
upgraded to CLUE in mid-call, and similarly how CLUE functionality
can be removed mid-call to return to a standard non-CLUE call.
This document originally also defined the CLUE protocol itself.
These details have mostly been split out into
[I-D.presta-clue-protocol] and expanded, but at present some details
remain in this document.
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 data channel: A reliable, bidirectional, transport mechanism
used to convey CLUE messages. See [I-D.ietf-clue-datachannel] for
more details..
CLUE-capable device: A device that supports the CLUE data channel
[I-D.ietf-clue-datachannel], the CLUE protocol
[I-D.presta-clue-protocol] and the principles of CLUE negotiation.
CLUE-enabled device: A CLUE-capable device that wishes to negotiate
a CLUE data channel and send and/or receive CLUe-controlled media.
Non-CLUE device: A device that supports standard SIP and SDP, but
either does not support CLUE, or that does but does not currently
wish to invoke CLUE capabilities.
CLUE-controlled media: A media "m" line that is under CLUE control;
the capture source that provides the media on this "m" line is
negotiated in CLUE. There is a corresponding "non-CLUE-
controlled" media term. See Section 4 for details of how this
control is signalled in SDP
3. Media Feature Tag Definition
The "sip.telepresence" media feature tag indicates support for CLUE.
A CLUE-capable device SHOULD include this media feature tag in its
REGISTER requests and OPTION responses. It SHOULD also include the
media feature tag in INVITE and UPDATE [RFC3311] requests and
responses.
Presence of the media feature tag in the contact field of a request
or response can be used to determine that the far end supports CLUE.
4. SDP Grouping Framework TELEPRESENCE Extension Semantics
4.1. General
This section defines a new SDP Grouping Framework extension,
TELEPRESENCE.
The TELEPRESENCE extension can be indicated using an SDP session-
level 'group' attribute. Each SDP media "m" line that is included in
this group, using SDP media-level mid attributes, is CLUE-controlled,
by a CLUE data channel also included in this TELEPRESENCE group.
4.2. The CLUE data channel and the TELEPRESENCE grouping semantic
The CLUE data channel [I-D.ietf-clue-datachannel] is a bidirectional
SCTP over DTLS channel used for the transport of CLUE messages. This
channel must be established before CLUE protocol messages can be
exchanged and CLUE-controlled media can be sent.
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The data channel is a generic transport that is not specific to CLUE
- if a device wishes to use the CLUE protocol on the data channel it
MUST include a TELEPRESENCE group in the SDP and include the "mid" of
the "m" line for the data channel in that group. A TELEPRESENCE grup
MUST NOT include the "mid"s for more than one data channel, and the
data channel "mid" MUST NOT be included in more than one TELEPRESENCE
group.
Presence of the data channel in a CLUE group in an SDP offer or
answer also serves, along with the 'sip.telepresence' media feature
tag, as an indication that the device supports CLUE and wishes to
upgrade the call to include CLUE-controlled media. A CLUE-enabled
device SHOULD include a data channel "m" line in offers and, when
allowed by [RFC3264], answers.
4.3. CLUE-controlled media and the TELEPRESENCE grouping semantic
CLUE-controlled media lines in an SDP are "m" lines in which the
content of the media streams to be sent is negotiated via the CLUE
protocol [I-D.presta-clue-protocol]. For an "m" line to be CLUE-
controlled, its "mid" value MUST be included in a TELEPRESENCE group.
CLUE-controlled media line "mid"s MUST NOT be included in more than
one TELEPRESENCE group.
CLUE-controlled media is controlled by the CLUE protocol as
negotiated on the CLUE data channel with an "mid" included in the
TELEPRESENCE group. If no data channel is included in the group the
other "m" lines in the group are still considered CLUE-controlled and
under all the restrictions of CLUE-controlled media specified in this
document.
"m" lines not specified as under CLUE control follow normal rules for
media streams negotiated in SDP as defined in documents such as
[RFC3264].
4.4. SDP Offer/Answer Procedures
4.4.1. Generating the Initial Offer
4.4.1.1. Signalling CLUE Encodings
The CLUE Framework [I-D.ietf-clue-framework] defines the concept of
"encodings", which represent the sender's encode ability. Each
encoding the media provider wishes to signal is signalled via an "m"
line of the appropriate media type, which MUST be marked as sendonly
with the "a=sendonly" attribute or as inactive with the "a=inactive"
attribute.
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The encoder limits of active (eg, "a=sendonly") encodings can then be
expressed using existing SDP syntax. For instance, for H.264 see
Table 6 in [RFC6184] for a list of valid parameters for representing
encoder sender stream limits.
These encodings are CLUE-controlled and hence MUST include an "mid"
in a TELEPRESENCE group as defined above.
As well as the normal restrictions defined in [RFC3264] media MUST
NOT be sent on this stream until the media provider has received a
valid CLUE CONFIGURE message specifying the capture to be used for
this stream. In the case of RTP media this includes corresponding
RTCP packets.
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.
4.4.1.1.1. Media line directionality
Presently, this specification mandates that CLUE-controlled "m"-lines
must be unidirectional. This is because setting "m"-lines to
"a=sendonly" allows the encoder limits to be expressed, whereas in
other cases codec attributes express the receive capabilities of a
media line.
It is possible that in future versions of this draft or its successor
this restriction will be relaxed. If a device does not feel there is
a benefit to expressing encode limitations, or if there are no
meaningful codec-specific limitations to express (such as with many
audio codecs) there are benefits to allowing bidirectional "m"-lines.
With bidirectional media lines recipients do not always need to
create a new offer to add their own "m"-lines to express their send
capabilities; if they can produce an equal or lesser number of
streams to send then they may not need additional "m"-lines.
However, at present the need to express encode limitations and the
wish to simplify the offer/answer procedure means that for the time
being only unidirectional media lines are allowed for CLUE-controlled
media. The highly asymmetric nature of CLUE means that the
probability of the recipient of the initial offer needing to make
their own offer to add additional "m"-lines is significantly higher
than it is for most other SIP call scenarios, in which there is a
tendancy for both sides to have similar numbers of potential audio
and video streams they can send.
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4.4.1.1.2. Alternate encoding limit syntaxes
Note that while the expressing of CLUE encoding limits in SDP has
been discussed at some length by the working group and it has been
agreed that this is the current, working assumption, formal consensus
has not been agreed on this. Alternatives include placing encoding
limits in the CLUE ADVERTISEMENT message, or by using alternate SDP
syntax, such as is suggested in [I-D.groves-clue-latent-config].
4.4.1.2. Receiving CLUE-controlled media
As well as including sendonly media lines to send CLUE-controlled
media, the sender of the initial SDP offer MAY also include
"a=recvonly" media lines to preallocate "m" lines to receive media;
these are described in more detail in the next section.
4.4.1.3. Interoperability with non-CLUE devices
A CLUE-enabled device sending an initial SDP offer SHOULD NOT include
any "m" line for CLUE-controlled media beyond the "m" line for the
CLUE data channel, and SHOULD include at least one non-CLUE-
controlled media "m" line.
If the device has evidence that the receiver is also CLUE-enabled,
for instance due to receiving an initial INVITE with no SDP but
including a 'sip.telepresence' media feature tag, the above
recommendation is waived, and the initial offer MAY contain "m" lines
for CLUE-controlled media.
4.4.2. Generating the Answer
4.4.2.1. Negotiating use of CLUE and the CLUE data channel
If the recipient wishes to enable CLUE for the call, they MUST
negotiate data channel support for an "m" line, and include the "mid"
of that "m" line in a corresponding TELEPRESENCE group.
4.4.2.2. Negotiating receipt of CLUE capture encodings in SDP
A receiver who wishes to receive a CLUE stream via a specific
encoding requires an "a=recvonly" "m" line that matches the
"a=sendonly" encoding.
These "m" lines are CLUE-controlled and hence MUST include an "mid"
the corresponding TELEPRESENCE group corresponding to the encoding
they wish to send.
In the case of RTCP for RTP media or any other media type that
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includes a bidirectional flow of packets for unidirectional media
streams, such bidirectional packets MUST NOT be sent until the media
consumer has received acknowledgement that the media provider has
received a valid CLUE CONFIGURE message specifying the capture to be
used for this stream.
4.4.3. Processing the initial Offer/Answer negotiation
In the event that both offer and answer include a data channel "m"
line with a mid value included in corresponding TELEPRESENCE groups
CLUE has been successfully negotiated and the call is now CLUE-
enabled, otherwise the call is not CLUE enabled.
4.4.3.1. Successful CLUE negotiation
In the event of successful CLUE enablement of the call, devices MUST
now begin negotiation of the CLUE channel, see
[I-D.ietf-clue-datachannel] for negotiation details. If negotiation
is successful, sending of CLUE protocol [I-D.presta-clue-protocol]
messages can begin.
A CLUE-enabled device MAY choose not to send media on the non-CLUE-
controlled channels during the period in which control of the CLUE-
controlled media lines is being negotiated. However, a CLUE-enabled
device MUST still be prepared to receive media on non-CLUE-controlled
media lines as defined in [RFC3264].
If either side of the call wishes to add additional CLUE-controlled
"m" lines to send or receive CLUE-controlled media they MAY now send
a SIP request with a new SDP offer. Note that if BUNDLE has been
successfully negotiated and a Bundle Address Synchronization offer is
required, the device to receive that offer SHOULD NOT generate a new
SDP offer until it has received that BAS offer.
4.4.3.2. CLUE negotiation failure
In the event that the negotiation of CLUE fails and the call is not
CLUE enabled in the initial offer/answer then CLUE is not in use in
the call, and the CLUE-capable devices MUST either revert to non-CLUE
behaviour or terminate the call.
4.4.4. Modifying the session
4.4.4.1. Enabling CLUE mid-call
A CLUE-enabled device that receives an initial SDP offer from a non-
CLUE-enabled device SHOULD include a new data channel "m" line and
corresponding TELEPRESENCE group in any subsequent offers it sends,
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to indicate that it is CLUE-enabled.
If, in an ongoing non-CLUE call, one or both sides of the call add
the CLUE data channel "m" line to their SDP and places the "mid" for
that channel in corresponding TELEPRESENCE groups then the call is
now CLUE-enabled; negotiation of the data channel and subsequently
the CLUE protocol begin.
4.4.4.2. Disabling CLUE mid-call
If, in an ongoing CLUE-enabled call, an SDP offer-answer negotiation
completes in a fashion in which either the CLUE data channel was not
successfully negotiated or one side did not include the data channel
in a matching TELEPRESENCE group then CLUE for this channel is
disabled. In the event that this occurs, CLUE is no longer enabled
and sending of all CLUE-controlled media associated with the
corresponding TELEPRESENCE group MUST stop.
Note that this is distinct to cases where the CLUE data channel fails
or an error occurs on the CLUE protocol; see
[I-D.presta-clue-protocol] for details of media and state
preservation in this circumstance.
5. Interaction of CLUE protocol and SDP negotiations
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
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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.
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.
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6. Multiplexing of CLUE-controlled media using BUNDLE
6.1. Overview
A CLUE call may involve sending and/or receiving significant numbers
of media streams. Conventionally, media streams are sent and
received on unique ports. However, each seperate port used for this
purpose may impose costs that a device wishes to avoid, such as the
need to open that port on firewalls and NATs, the need to collect ICE
candidates [RFC5245], etc.
The BUNDLE [I-D.ietf-mmusic-sdp-bundle-negotiation] extension can be
used to negotiate the multiplexing of multiple media lines onto a
single 5-tuple for sending and receiving media, allowing devices in
calls to another BUNDLE-supporting device to potentially avoid some
of the above costs.
While CLUE-capable devices MAY support the BUNDLE extension for this
purpose supporting the extension is not mandatory for a device to be
CLUE-compliant.
6.2. Usage of BUNDLE with CLUE
This specification imposes no additional requirements or restrictions
on the usage of BUNDLE when used with CLUE. There is no restriction
on combining CLUE-controlled media lines and non-CLUE-controlled
media lines in the same BUNDLE group or in multiple such groups.
However, there are several steps an implementation may wish to
ameliorate the cost and time requirements of extra SDP offer/answer
exchanges between CLUE and BUNDLE.
6.2.1. Generating the Initial Offer
BUNDLE mandates that the initial SDP offer MUST use a unique address
for each m-line with a non-zero port. Because CLUE implementations
generarlly will not include CLUE-controlled media lines with the
exception of the data channel CLUE devices that support large numbers
of streams can avoid ever having to open large numbers of ports if
they successfully negotiate BUNDLE.
6.2.2. Bundle Address Synchronization
When using BUNDLE the initial offerer may be mandated to send a
Bundle Address Synchronisation offer. If the initial offerer also
followed the recommendation of not including CLUE-controlled media
lines in their offer, they MAY choose to include them in this
subsequent offer. In this circumstance the BUNDLE specification
recommends that the offerer does not "modify SDP parameters that
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could get the answerer to reject the BAS offer". Including new CLUE-
controlled media lines using codecs and other attributes used in
existing media lines should not increase the chance of the answerer
rejecting the BAS offer; implementations should consider carefully
before including new codecs or other new SDP attributes in these
CLUE-controlled media lines.
6.2.3. Multiplexing of the data channel and RTP media
BUNDLE-supporting CLUE-enabled devices MAY include the data channel
in the same BUNDLE group as RTP media. In this case the device MUST
be able to demultiplex the various transports - see section 7.2 of
the BUNDLE draft [I-D.ietf-mmusic-sdp-bundle-negotiation]. If the
BUNDLE group includes other protocols than the data channel
transported via DTLS the device MUST also be able to differentiate
the various protocols.
7. 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.
Note that BUNDLE is not in use.
+----------+ +-----------+
| Alice | | Bob |
| | | |
+----+-----+ +-----+-----+
| |
| |
| SIP INVITE 1 (BASIC SDP+COMEDIA) |
|--------------------------------->|
| |
| |
| SIP 200 OK 1 (BASIC SDP+COMEDIA) |
|<---------------------------------|
| |
| |
| SIP ACK 1 |
|--------------------------------->|
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| |
| |
| |
|<########### MEDIA 1 ############>|
| 1 video A->B, 1 video B->A |
|<################################>|
| |
| |
| |
|<================================>|
| CLUE CTRL CHANNEL ESTABLISHED |
|<================================>|
| |
| |
| CLUE ADVERTISEMENT 1 |
|*********************************>|
| |
| |
| CLUE ADVERTISEMENT 2 |
|<*********************************|
| |
| |
| SIP INVITE 2 (+3 sendonly) |
|--------------------------------->|
| |
| |
| CLUE CONFIGURE 1 |
|<*********************************|
| |
| |
| CLUE RESPONSE 1 |
|*********************************>|
| |
| |
| SIP 200 OK 2 (+2 recvonly) |
|<---------------------------------|
| |
| |
| SIP ACK 2 |
|--------------------------------->|
| |
| |
| |
|<########### MEDIA 2 ############>|
| 2 video A->B, 1 video B->A |
|<################################>|
| |
| |
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| SIP INVITE 3 (+2 sendonly) |
|<---------------------------------|
| |
| |
| CLUE CONFIGURE 2 |
|*********************************>|
| |
| |
| CLUE RESPONSE 2 |
|<*********************************|
| |
| |
| SIP 200 OK 3 (+2 recvonly) |
|--------------------------------->|
| |
| |
| |
| SIP 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
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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
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.
Alice now sends INVITE 2. She maintains the sendrecv audio, video
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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". This also serves as an ack
for Alice's ADVERTISMENT 1.
Alice receives Bob's message CONFIGURE 1 and sends RESPONSE 1 to ack
its receptions. She 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):
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...
a=group:CLUE 11 12 13 14 100
...
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
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 and sends RESPONSE 2 to ack
its receptions. Bob 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.
8. Example: A call between a CLUE-capable and non-CLUE endpoint
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
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protocol.
+----------+ +-----------+
| EP1 | | EP2 |
| | | |
+----+-----+ +-----+-----+
| |
| |
| SIP 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):
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...
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 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.
9. 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.
10. SIP Signaling
(Placeholder) This may be unremarkable. If so we can drop it.
11. CLUE over RTCWEB
We may want to rule this out of scope for now. But we should be
thinking about this.
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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
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-08: Revisions by Rob Hansen
* Added media feature tag for CLUE support ('sip.telepresence')
* Changed grouping semantic from 'CLUE' to 'TELEPRESENCE'
* Restructured document to be more centred on the grouping
semantic and its use with O/A
* Lots of additional text on usage of the grouping semantic
* Stricter definition of CLUE-controlled m lines and how they
work
* Some additional text on defining what happens when CLUE
supports is added or removed
* Added details on when to not send RTCP for CLUE-controlled "m"
lines.
* Added a section on using BUNDLE with CLUE
* Updated data channel references to point at new WG document
rather than indivual draft
-07: Revisions by Rob Hansen
* Removed the text providing arguments for encoding limits being
in SDP and encoding groups in the CLUE protocol in favor of the
specifics of how to negotiate encodings in SDP
* Added normative language on the setting up of a CLUE call, and
added sections on mid-call changes to the CLUE status.
* Added references to [I-D.ietf-clue-datachannel] where
appropriate.
* Added some terminology for various types of CLUE and non-CLUE
states of operation.
* Moved language related to topics that should be in
[I-D.ietf-clue-datachannel] and [I-D.presta-clue-protocol], but
that has not yet been resolved in those documents, into an
appendix.
-06: Revisions by Rob Hansen
* Removed CLUE message XML schema and details that are now in
draft-presta-clue-protocol
* Encoding limits in SDP section updated to note that this has
been investigated and discussed and is the current working
assumption of the WG, though consensus has not been fully
achieved.
* A section has also been added on the current mandation of
unidirectional "m"-lines.
* Updated CLUE messaging in example call flow to match
draft-presta-clue-protocol-03
-05: Revisions by pkyzivat:
* Specified versioning model and mechanism.
* Added explicit response to all messages.
* Rearranged text to work with the above changes. (Which
rendered diff almost useless.)
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-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.
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-14 (work in progress),
February 2014.
[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.presta-clue-protocol]
Presta, R. and S. Romano, "CLUE protocol",
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draft-presta-clue-protocol-03 (work in progress),
November 2013.
[I-D.ietf-clue-datachannel]
Holmberg, C., "CLUE Protocol Data Channel",
draft-ietf-clue-datachannel-00 (work in progress),
March 2014.
[I-D.groves-clue-latent-config]
Groves, C., Yang, W., and R. Even, "CLUE and latent
configurations", draft-groves-clue-latent-config-00 (work
in progress), January 2014.
[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-06
(work in progress), February 2014.
[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
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC3311] Rosenberg, J., "The Session Initiation Protocol (SIP)
UPDATE Method", RFC 3311, October 2002.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
April 2010.
[RFC4353] Rosenberg, J., "A Framework for Conferencing with the
Session Initiation Protocol (SIP)", RFC 4353,
February 2006.
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[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, March 2011.
[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.
[I-D.ietf-mmusic-sdp-bundle-negotiation]
Holmberg, C., Alvestrand, H., and C. Jennings,
"Multiplexing Negotiation Using Session Description
Protocol (SDP) Port Numbers",
draft-ietf-mmusic-sdp-bundle-negotiation-06 (work in
progress), April 2014.
Appendix A. CLUE Signalling and data channel concerns
[The specifics of the CLUE signaling protocol are in the process of
being defined in [I-D.presta-clue-protocol], while the negotiation of
the CLUE data channel is being defined in
[I-D.ietf-clue-datachannel]. As such, considerable text originally
in this section have been transitioned to these document. The
following text relates to issues that are no longer the focus of this
document, but remain important and unresolved, and so have been
preserved here.]
A.1. Protocol Versioning and Options
A.1.1. Versioning Objectives
The CLUE versioning mechanism addresses the following needs:
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o Coverage:
* Versioning of basic behavior and options,
* 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.
A.1.2. 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:
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| Supported Supported |
|------------\ /------------|
| 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>
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Figure 3: Required Message (simplified)
A.1.3. 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
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.]]
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A.1.4. 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.
A.1.5. 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>.
A.1.5.1. <mediaProvider>
The <mediaProvider> element, when placed in a Supported message,
indicates that the sender is willing and able to send ADVERTISEMENT
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
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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.
A.1.6. 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
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.
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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.
A.1.7. 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 [I-D.presta-clue-protocol];
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
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:
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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.
A.1.8. Version & Option Negotiation Examples
A.1.8.1. Successful Negotiation - Multi-version
| Supported Supported |
| Version 2.0 |
| Version 1.2 Version 1.1 |
| mediaProv mediaProv |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| OK response OK response |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| Required Required |
| Version 1.2 Version 1.1 |
| mediaProv mediaProv |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| OK response OK response |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| Advertise |
|<------------------------->|
| |
| Configure |
|<------------------------->|
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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.
A.1.8.2. Successful Negotiation - Consumer-Only Endpoint
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| Supported Supported |
| Version 1.0 Version 1.0 |
| 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.
A.1.8.3. Successful Negotiation - Provider-Only Endpoint
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| Supported Supported |
| Version 1.0 Version 1.0 |
| mediaProv mediaProv |
|------------\ /------------|
| 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 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.
A.1.8.4. Version Incompatibility
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| Supported Supported |
| Version 1.2 Version 2.1 |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| Version Version |
| Incompat. Incompat. |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| close clue channel |
|<------------------------->|
| |
| 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.
A.1.8.5. 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 |
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|<------------------------->|
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.
A.1.8.6. Syntax Error
| Supported !@#$%^ |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| syntax error OK response |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| close clue channel |
|-------------------------->|
| |
| legacy mode or BYE |
|<------------------------->|
A.2. 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.)
A.2.1. 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
[I-D.ietf-clue-datachannel].
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
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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.
A.2.2. 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.
A.3. Message Framing
Message framing is provided by the SCTP transport protocol. Each
CLUE message is carried in one SCTP message.
Authors' Addresses
Paul Kyzivat
Huawei
Email: pkyzivat@alum.mit.edu
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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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