Internet DRAFT - draft-ietf-clue-signaling
draft-ietf-clue-signaling
Network Working Group R. Hanton
Internet-Draft Cisco Systems
Intended status: Standards Track P. Kyzivat
Expires: June 11, 2020
L. Xiao
Huawei
C. Groves
December 9, 2019
Session Signaling for Controlling Multiple Streams for Telepresence
(CLUE)
draft-ietf-clue-signaling-15
Abstract
This document specifies how CLUE-specific signaling such as the CLUE
protocol and the CLUE data channel are used in conjunction with each
other and with existing signaling mechanisms such as SIP and SDP to
produce a telepresence call.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Media Feature Tag Definition . . . . . . . . . . . . . . . . 4
4. SDP Grouping Framework CLUE Extension Semantics . . . . . . . 4
4.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.2. The CLUE data channel and the CLUE grouping semantic . . 5
4.3. CLUE-controlled media and the CLUE grouping semantic . . 5
4.4. SDP semantics for CLUE-controlled media . . . . . . . . . 5
4.4.1. Signaling CLUE Encodings . . . . . . . . . . . . . . 6
4.4.1.1. Referencing Encodings in the CLUE protocol . . . 6
4.4.2. Negotiating receipt of CLUE Capture Encodings in SDP 7
4.5. SDP Offer/Answer Procedures . . . . . . . . . . . . . . . 7
4.5.1. Generating the Initial Offer . . . . . . . . . . . . 7
4.5.2. Generating the Answer . . . . . . . . . . . . . . . . 8
4.5.2.1. Negotiating use of CLUE and the CLUE data channel 8
4.5.2.2. Negotiating CLUE-controlled media . . . . . . . . 8
4.5.2.3. Negotiating non-CLUE controlled media . . . . . . 9
4.5.3. Processing the initial Offer/Answer negotiation . . . 9
4.5.3.1. Successful CLUE negotiation . . . . . . . . . . . 9
4.5.3.2. CLUE negotiation failure . . . . . . . . . . . . 10
4.5.4. Modifying the session . . . . . . . . . . . . . . . . 10
4.5.4.1. Adding and removing CLUE-controlled media . . . . 10
4.5.4.2. Enabling CLUE mid-call . . . . . . . . . . . . . 10
4.5.4.3. Disabling CLUE mid-call . . . . . . . . . . . . . 10
4.5.4.4. CLUE protocol failure mid-call . . . . . . . . . 11
5. Interaction of CLUE protocol and SDP negotiations . . . . . . 11
5.1. Independence of SDP and CLUE negotiation . . . . . . . . 12
5.2. Constraints on sending media . . . . . . . . . . . . . . 13
5.3. Recommendations for operating with non-atomic operations 13
6. Interaction of CLUE protocol and RTP/RTCP CaptureID . . . . . 14
6.1. CaptureID reception during MCC redefinition . . . . . . . 14
7. Multiplexing of CLUE-controlled media using BUNDLE . . . . . 15
7.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 15
7.2. Usage of BUNDLE with CLUE . . . . . . . . . . . . . . . . 15
7.2.1. Generating the Initial Offer . . . . . . . . . . . . 16
7.2.2. Multiplexing of the data channel and RTP media . . . 16
8. Example: A call between two CLUE-capable Endpoints . . . . . 16
9. Example: A call between a CLUE-capable and non-CLUE Endpoint 26
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 27
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
11.1. New SDP Grouping Framework Attribute . . . . . . . . . . 27
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11.2. New SIP Media Feature Tag . . . . . . . . . . . . . . . 28
12. Security Considerations . . . . . . . . . . . . . . . . . . . 28
13. Change History . . . . . . . . . . . . . . . . . . . . . . . 29
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 39
14.1. Normative References . . . . . . . . . . . . . . . . . . 39
14.2. Informative References . . . . . . . . . . . . . . . . . 41
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 42
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 (ControLling mUltiple
streams for tElepresence) employs two principal and inter-related
protocol negotiations. SDP [RFC4566], conveyed via SIP [RFC3261], is
used to negotiate the specific media capabilities that can be
delivered to specific addresses on a device. Meanwhile, CLUE
protocol [I-D.ietf-clue-protocol] messages, transported via a CLUE
data channel [I-D.ietf-clue-datachannel], are used to negotiate the
Capture Sources available, their attributes and any constraints in
their use. They also allow the far end device to specify which
Captures they wish to receive. It is recommended that those
documents be read prior to this one as this document assumes
familiarity with those protocols and hence uses terminology from each
with limited introduction.
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 group capabilities, SDP is
also used to express the encoding limits for each potential Encoding.
Backwards-compatibility is an important consideration of the
protocol: 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.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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This document uses terminology defined in the CLUE Framework
[I-D.ietf-clue-framework].
A few additional terms specific to this document are defined as
follows:
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. See Section 4 for details of how this control
is signaled in SDP. There is a corresponding "non-CLUE-
controlled" media term.
3. Media Feature Tag Definition
The "sip.clue" media feature tag [RFC3840] indicates support for CLUE
in SIP [RFC3261] calls. 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 CLUE Extension Semantics
4.1. General
This section defines a new SDP Grouping Framework [RFC5888] extension
called 'CLUE'.
The CLUE 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 CLUE group.
Currently only support for a single CLUE group is specified; support
for multiple CLUE groups in a single session is outside the scope of
this document. A device MUST NOT include more than one CLUE group in
its SDP message unless it is following a specification that defines
how multiple CLUE channels are signaled, and is either able to
determine that the other side of the SDP exchange supports multiple
CLUE channels, or is able to fail gracefully in the event it does
not.
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4.2. The CLUE data channel and the CLUE grouping semantic
The CLUE data channel [I-D.ietf-clue-datachannel] is a bidirectional
data channel [I-D.ietf-rtcweb-data-channel] used for the transport of
CLUE messages, conveyed within an SCTP over DTLS connection. This
channel must be established before CLUE protocol messages can be
exchanged and CLUE-controlled media can be sent.
The data channel is negotiated over SDP as described in
[I-D.ietf-mmusic-data-channel-sdpneg]. A CLUE-capable device wishing
to negotiate CLUE MUST also include a CLUE group in their SDP offer
or answer and include the "mid" of the "m=" line for the data channel
in that group. The CLUE group MUST include the "mid" of the "m="
line for one (and only one) data channel.
Presence of the data channel in the CLUE group in an SDP offer or
answer also serves, along with the "sip.clue" media feature tag, as
an indication that the device supports CLUE and wishes to upgrade the
call to include CLUE-controlled media. A CLUE-capable device SHOULD
include a data channel "m=" line in offers and, when allowed by
[RFC3264], answers.
4.3. CLUE-controlled media and the CLUE 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.ietf-clue-protocol]. For an "m=" line to be CLUE-
controlled, its "mid" value MUST be included in the CLUE group.
CLUE-controlled media is controlled by the CLUE protocol as
negotiated on the CLUE data channel with an "mid" included in the
CLUE group.
"m=" lines not specified as under CLUE control follow normal rules
for media streams negotiated in SDP as defined in documents such as
[RFC3264].
The restrictions on CLUE-controlled media that are defined below
always apply to "m=" lines in an SDP offer or answer, even if
negotiation of the data channel in SDP failed due to lack of CLUE
support by the remote device or for any other reason, or in an offer
if the recipient does not include the "mid" of the corresponding "m="
line in their CLUE group.
4.4. SDP semantics for CLUE-controlled media
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4.4.1. Signaling 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 signaled 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.
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 the CLUE group as defined above.
As well as the normal restrictions defined in [RFC3264] the stream
MUST be treated as if the "m=" line direction attribute had been set
to "a=inactive" until the Media Provider has received a valid CLUE
'configure' message specifying the Capture to be used for this
stream. This means that RTP packets MUST NOT be sent until
configuration is complete, while non-media packets such as STUN, RTCP
and DTLS MUST be sent as per their relevant specifications if
negotiated.
Every "m=" line representing a CLUE Encoding MUST 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. Each label used for a
CLUE-controlled "m=" line MUST be different from the label on all
other "m=" lines in the CLUE group, unless an "m=" line represents a
dependent stream related to another "m=" line (such as an FEC
stream), in which case it MUST have the same label value as the "m="
line on which it depends.
4.4.1.1. Referencing Encodings in the CLUE protocol
CLUE Encodings are defined in SDP, but can be referenced from CLUE
protocol messages - this is how the protocol defines which Encodings
are part of an Encoding Group (in 'advertisement' messages) and which
Encoding with which to encode a specific Capture (in 'configure'
messages). The labels on the CLUE-controlled "m=" lines are the
references that are used in the CLUE protocol.
Each <encID> (in encodingIDList) in a CLUE 'advertisement' message
SHOULD represent an Encoding defined in SDP; the specific Encoding
referenced is a CLUE-controlled "m=" line in the most recent SDP
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Offer/Answer message sent by the sender of the 'advertisement'
message with a label value corresponding to the text content of the
<encID>. If the <encID> is not defined in SDP it MUST be one it
anticipates sending in a subsequent SDP Offer/Answer exchange.
Each <encodingID> (in captureEncodingType) in a CLUE 'configure'
message MUST represent an Encoding defined in SDP; the specific
Encoding referenced is a CLUE-controlled "m=" line in the most recent
SDP Offer/Answer message received by the sender of the 'configure'
message with a label value corresponding to the text content of the
<encodingID>.
Note that the non-atomic nature of SDP/CLUE protocol interaction may
mean that there are temporary periods where an <encID>/<encodingID>
in a CLUE message does not reference an SDP "m=" line, or where an
Encoding represented in SDP is not referenced in a CLUE protocol
message. See Section 5 for specifics.
4.4.2. 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 their
"mid" in the CLUE group. They MAY include a "label" attribute, but
this is not required by CLUE, as only label values associated with
"a=sendonly" Encodings are referenced by CLUE protocol messages.
4.5. SDP Offer/Answer Procedures
4.5.1. Generating the Initial Offer
A CLUE-capable device sending an initial SDP offer of a SIP session
and wishing to negotiate CLUE will include an "m=" line for the data
channel to convey the CLUE protocol, along with a CLUE group
containing the "mid" of the data channel "m=" line.
For interoperability with non-CLUE devices a CLUE-capable 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-capable,
for instance due to receiving an initial INVITE with no SDP but
including a "sip.clue" media feature tag, the above recommendation is
waived, and the initial offer MAY contain "m=" lines for CLUE-
controlled media.
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With the same interoperability recommendations as for Encodings, the
sender of the initial SDP offer MAY also include "a=recvonly" media
lines to preallocate "m=" lines to receive media. Alternatively, it
MAY wait until CLUE protocol negotiation has completed before
including these lines in a new offer/answer exchange - see Section 5
for recommendations.
4.5.2. Generating the Answer
4.5.2.1. Negotiating use of CLUE and the CLUE data channel
If the recipient of an initial offer is CLUE-capable, and the offer
contains both an "m=" line for a data channel and a CLUE group
containing the "mid" for that "m=" line, they SHOULD negotiate data
channel support for an "m=" line, and include the "mid" of that "m="
line in a corresponding CLUE group.
A CLUE-capable recipient that receives an "m=" line for a data
channel but no corresponding CLUE group containing the "mid" of that
"m=" line MAY still include a corresponding data channel "m=" line if
there are any other non-CLUE protocols it can convey over that
channel, but MUST NOT negotiate use of the CLUE protocol on this
channel.
4.5.2.2. Negotiating CLUE-controlled media
If the initial offer contained "a=recvonly" CLUE-controlled media
lines the recipient SHOULD include corresponding "a=sendonly" CLUE-
controlled media lines for accepted Encodings, up to the maximum
number of Encodings it wishes to advertise. As CLUE-controlled
media, the "mid" of these "m=" lines MUST be included in the
corresponding CLUE group. The recipient MUST set the direction of
the corresponding "m=" lines of any remaining "a=recvonly" CLUE-
controlled media lines received in the offer to "a=inactive".
If the initial offer contained "a=sendonly" CLUE-controlled media
lines the recipient MAY include corresponding "a=recvonly" CLUE-
controlled media lines, up to the maximum number of Capture Encodings
it wishes to receive. Alternatively, it MAY wait until CLUE protocol
negotiation has completed before including these lines in a new
offer/answer exchange - see Section 5 for recommendations. The
recipient MUST set the direction of the corresponding "m=" lines of
any remaining "a=sendonly" CLUE-controlled media lines received in
the offer to "a=inactive"
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4.5.2.3. Negotiating non-CLUE controlled media
A CLUE-controlled device implementation MAY prefer to render initial,
single-stream audio and/or video for the user as rapidly as possible,
transitioning to CLUE-controlled media once that has been negotiated.
Alternatively, an implementation MAY wish to suppress initial media,
only providing media once the final, CLUE-controlled streams have
been negotiated.
The receiver of the initial offer, if making the call CLUE-enabled
with their SDP answer, can make their preference clear by their
action in accepting or rejecting non-CLUE-controlled media lines.
Rejecting these "m=" lines will ensure that no non-CLUE-controlled
media flows before the CLUE-controlled media is negotiated. In
contrast, accepting one or more non-CLUE-controlled "m=" lines in
this initial answer will enable initial media to flow.
If the answerer chooses to send initial non-CLUE-controlled media in
a CLUE-enabled call, Section 4.5.4.1 addresses the need to disable it
once CLUE-controlled media is fully negotiated.
4.5.3. 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 CLUE groups, CLUE has
been successfully negotiated and the call is now CLUE-enabled. If
not then the call is not CLUE-enabled.
4.5.3.1. 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.ietf-clue-protocol]
messages can begin.
A CLUE-capable device MAY choose not to send RTP on the non-CLUE-
controlled channels during the period in which control of the CLUE-
controlled media lines is being negotiated (though RTCP MUST still be
sent and received as normal). However, a CLUE-capable device MUST
still be prepared to receive media on non-CLUE-controlled media lines
that have been successfully negotiated 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 following the normal rules of SDP
offer/answer and any negotiated extensions.
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4.5.3.2. CLUE negotiation failure
In the event that the negotiation of CLUE fails and the call is not
CLUE-enabled once the initial offer/answer negotiation completes then
CLUE is not in use in the call. The CLUE-capable devices MUST either
revert to non-CLUE behaviour or terminate the call.
4.5.4. Modifying the session
4.5.4.1. Adding and removing CLUE-controlled media
Subsequent offer/answer exchanges MAY add additional "m=" lines for
CLUE-controlled media, or activate or deactivate existing "m=" lines
per the standard SDP mechanisms.
In most cases at least one additional exchange after the initial
offer/answer exchange will be required before both sides have added
all the Encodings and ability to receive Encodings that they desire.
Devices MAY delay adding "a=recvonly" CLUE-controlled "m=" lines
until after CLUE protocol negotiation completes - see Section 5 for
recommendations.
Once CLUE media has been successfully negotiated devices SHOULD
ensure that non-CLUE-controlled media is deactivated by setting their
ports to 0 in cases where it corresponds to the media type of CLUE-
controlled media that has been successfully negotiated. This
deactivation may require an additional SDP exchange, or may be
incorporated into one that is part of the CLUE negotiation.
4.5.4.2. Enabling CLUE mid-call
A CLUE-capable device that receives an initial SDP offer from a non-
CLUE device SHOULD include a new data channel "m=" line and
corresponding CLUE group in any subsequent offers it sends, to
indicate that it is CLUE-capable.
If, in an ongoing non-CLUE call, an SDP offer/answer exchange
completes with both sides having included a data channel "m=" line in
their SDP and with the "mid" for that channel in a corresponding CLUE
group then the call is now CLUE-enabled; negotiation of the data
channel and subsequently the CLUE protocol begins.
4.5.4.3. Disabling CLUE mid-call
If, during an ongoing CLUE-enabled call a device wishes to disable
CLUE, it can do so by following the procedures for closing a data
channel defined in Section 5.2.4 of
[I-D.ietf-mmusic-data-channel-sdpneg]: sending a new SDP offer/answer
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exchange and subsequent SCTP SSN reset for the CLUE channel. It MUST
also remove the CLUE group. Without the CLUE group any "m=" lines
that were previously CLUE-controlled no longer are; implementations
MAY disable them by setting their ports to 0 or MAY continue to use
them - in the latter case how they are used is outside the scope of
this document.
If a device follows the procedure above, or an SDP offer-answer
negotiation completes in a fashion in which either the "m=" CLUE data
channel line was not successfully negotiated, and/or one side did not
include the data channel in the CLUE group then CLUE for this call is
disabled. In the event that this occurs, CLUE is no longer enabled.
Any active "m=" lines still included in the CLUE group are no longer
CLUE-controlled and the implementation MAY either disable them in a
subsequent negotiation or continue to use them in some other fashion.
If the data channel is still present but not included in the CLUE
group semantic CLUE protocol messages MUST no longer be sent.
4.5.4.4. CLUE protocol failure mid-call
In contrast to the specific disablement of the use of CLUE described
above, the CLUE channel may fail unexpectedly. Two circumstances
where this can occur are:
o The CLUE data channel terminates, either gracefully or
ungracefully, without any corresponding SDP renegotiation.
o A channel error of the CLUE protocol causes it to return to the
IDLE state as defined in Section 6. of [I-D.ietf-clue-protocol].
In this circumstance implementations SHOULD continue to transmit and
receive CLUE-controlled media on the basis of the last negotiated
CLUE messages, until the CLUE protocol is re-established (in the
event of a channel error) or disabled mid-call by an SDP exchange as
defined in Section 4.5.4.3. Implementations MAY choose to send such
an SDP request to disable CLUE immediately or MAY continue on in a
call-preservation mode.
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.
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This section defines how the negotiation of the two protocols
interact, provides some recommendations on dealing with intermediate
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 the need to implement interlocking state machines with the
potential to reach invalid states if messages were to be lost, or be
rewritten en-route by middle boxes, the state machines in SDP and
CLUE operate independently. 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 (unless
the SDP negotiation has removed the CLUE channel).
The primary implication of this is that a device may receive an SDP
Offer/Answer message with a CLUE Encoding for which it does not yet
have Capture information, 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 <encID> (in encodingIDList) or <encodingID>
(in captureEncodingType), which is used to identify a specific
encoding or captureEncoding in SDP; see
[I-D.ietf-clue-data-model-schema] for specifics. The non-atomic
nature of CLUE negotiation means that a sender may wish to send a new
CLUE 'advertisement' message before the corresponding SDP message.
As such the sender of the CLUE message MAY include an <encID> which
does not currently match a CLUE-controlled "m=" line label in SDP; A
CLUE-capable implementation MUST NOT reject a CLUE protocol message
solely because it contains <encID> elements that do not match a label
in SDP.
The current state of the CLUE participant or Media Provider/Consumer
state machines do not affect compliance with any of the normative
language of [RFC3264]. That is, they MUST NOT delay an ongoing SDP
exchange as part of a SIP server or client transaction; an
implementation MUST NOT delay an SDP exchange while waiting for CLUE
negotiation to complete or for a 'configure' message to arrive.
Similarly, a device in a CLUE-enabled call MUST NOT delay any
mandatory state transitions in the CLUE Participant or Media
Provider/Consumer state machines due to the presence or absence of an
ongoing SDP exchange.
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A device with the CLUE Participant state machine in the ACTIVE state
MAY choose to delay moving from ESTABLISHED to ADV (Media Provider
state machine) or from ESTABLISHED to WAIT FOR CONF RESPONSE (Media
Consumer state machine) based on the SDP state. See
[I-D.ietf-clue-protocol] for CLUE state machine specifics.
Similarly, a device MAY choose to delay initiating a new SDP exchange
based on the state of their CLUE state machines.
5.2. 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 - CLUE-
controlled 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 it has received a CLUE
'configure' message specifying a valid Capture for that Encoding.
5.3. Recommendations for operating with non-atomic operations
CLUE-capable devices MUST be able to handle states in which CLUE
messages make reference to EncodingIDs that do not match the most
recently received SDP, irrespective of the order in which SDP and
CLUE messages are received. While these mismatches will usually be
transitory a device MUST be able to cope with such mismatches
remaining indefinitely. However, this document makes some
recommendations on message ordering for these non-atomic transitions.
CLUE-capable devices MUST ensure that any inconsistencies between SDP
and CLUE signaling are temporary by sending updated SDP or CLUE
messages as soon as the relevant state machines and other constraints
permit.
Generally, implementations that receive messages for which they have
incomplete information will be most efficient if they wait until they
have the corresponding information they lack before sending messages
to make changes related to that information. For example, an
answerer that receives a new SDP offer with three new "a=sendonly"
CLUE "m=" lines for which it has received no CLUE 'advertisement'
message providing the corresponding capture information would
typically inclue corresponding "a=inactive" lines in its answer, and
only make a new SDP offer with "a=recvonly" when and if a new
'advertisement' message arrives with Captures relevant to those
Encodings.
Because of the constraints of SDP 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
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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.
6. Interaction of CLUE protocol and RTP/RTCP CaptureID
The CLUE Framework [I-D.ietf-clue-framework] allows for Multiple
Content Captures (MCCs): Captures which contain multiple source
Captures, whether composited into a single stream or switched based
on some metric.
The Captures that contribute to these MCCs may or may not be defined
in the 'advertisement' message. If they are defined and the MCC is
providing them in a switched format the recipient may wish to
determine which originating source Capture is currently being
provided, so that they can apply geometric corrections based on that
Capture's geometry, or take some other action based on the original
Capture information.
To do this, [I-D.ietf-clue-rtp-mapping] allows for the CaptureID of
the originating Capture to be conveyed via RTP or RTCP. A Media
Provider sending switched media for an MCC with defined originating
sources MUST send the CaptureID in both RTP and RTCP, as described in
the mapping document.
6.1. CaptureID reception during MCC redefinition
Because the RTP/RTCP CaptureID is delivered via a different channel
to the 'advertisement' message in which in the contents of the MCC
are defined there is an intrinsic race condition in cases in which
the contents of an MCC are redefined.
When a Media Provider redefines an MCC which involves CaptureIDs, the
reception of the relevant CaptureIDs by the recipient will either
lead or lag reception and processing of the new 'advertisement'
message by the recipient. As such, a Media Consumer MUST NOT be
disrupted by any of the following in any CLUE-controlled media stream
it is receiving, whether that stream is for a static Capture or for
an MCC (as any static Capture may be redefined to an MCC in a later
'advertisement' message):
o Receiving RTP or RTCP containing a CaptureID when the most
recently processed 'advertisement' message means that none are
expected.
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o Receiving RTP or RTCP without CaptureIDs when the most recently
processed 'advertisement' message means that media CaptureIDs are
expected.
o Receiving a CaptureID in RTP or RTCP for a Capture defined in the
most recently processed 'advertisement' message, but which the
same 'advertisement' message does not include in the MCC.
o Receiving a CaptureID in RTP or RTCP for a Capture not defined in
the most recently processed 'advertisement' message.
7. Multiplexing of CLUE-controlled media using BUNDLE
7.1. 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 separate 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 [RFC8445], 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.
A CLUE-capable device that supports BUNDLE SHOULD also support rtcp-
mux [RFC5761]. However, a CLUE-capable device that supports rtcp-mux
may or may not support BUNDLE.
7.2. 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 take
to ameliorate the cost and time requirements of extra SDP offer/
answer exchanges between CLUE and BUNDLE.
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7.2.1. 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 generally will not include CLUE-controlled media
lines with the exception of the data channel in the initial SDP
offer, CLUE devices that support large numbers of streams can avoid
ever having to open large numbers of ports if they successfully
negotiate BUNDLE.
An implementation that does include CLUE-controlled media lines in
its initial SDP offer while also using BUNDLE must take care to avoid
renderings its CLUE-controlled media lines unusable in the event the
far end does not negotiate BUNDLE if it wishes to avoid the risk of
additional SDP exchanges to resolve this issue. This is best
achieved by not sending any CLUE-controlled media lines in an initial
offer with the 'bundle-only' attribute unless it has been established
via some other channel that the recipient supports and is able to use
BUNDLE.
7.2.2. Multiplexing of the data channel and RTP media
BUNDLE-supporting CLUE-capable 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 9.2 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.
8. 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 a summary
of each message.
To manage the size of this section the SDP snippets only illustrate
video "m=" lines. SIP ACKs are not always discussed. Note that
BUNDLE is not in use.
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+----------+ +-----------+
| Alice | | Bob |
| | | |
+----+-----+ +-----+-----+
| |
| |
| SIP INVITE 1 |
|--------------------------------->|
| |
| |
| SIP 200 OK 1 |
|<---------------------------------|
| |
| |
| SIP ACK 1 |
|--------------------------------->|
| |
| |
| |
|<########### MEDIA 1 ############>|
| 1 video A->B, 1 video B->A |
|<################################>|
| |
| |
| |
|<================================>|
| CLUE DATA CHANNEL ESTABLISHED |
|<================================>|
| |
| |
| CLUE OPTIONS |
|<*********************************|
| |
| |
| CLUE OPTIONS RESPONSE |
|*********************************>|
| |
| |
| CLUE ADVERTISEMENT 1 |
|*********************************>|
| |
| |
| CLUE ADVERTISEMENT 2 |
|<*********************************|
| |
| |
| CLUE ACK 1 |
|<*********************************|
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| |
| |
| CLUE ACK 2 |
|*********************************>|
| |
| |
| SIP INVITE 2 (+3 sendonly) |
|--------------------------------->|
| |
| |
| CLUE CONFIGURE 1 |
|<*********************************|
| |
| |
| SIP 200 OK 2 (+2 recvonly) |
|<---------------------------------|
| |
| |
| CLUE CONFIGURE RESPONSE 1 |
|*********************************>|
| |
| |
| SIP ACK 2 |
|--------------------------------->|
| |
| |
| |
|<########### MEDIA 2 ############>|
| 2 video A->B, 1 video B->A |
|<################################>|
| |
| |
| SIP INVITE 3 (+2 sendonly) |
|<---------------------------------|
| |
| |
| CLUE CONFIGURE 2 |
|*********************************>|
| |
| |
| SIP 200 OK 3 (+2 recvonly) |
|--------------------------------->|
| |
| |
| CLUE CONFIGURE RESPONSE 2 |
|<*********************************|
| |
| |
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| SIP ACK 3 |
|<---------------------------------|
| |
| |
| |
|<########### MEDIA 3 ############>|
| 2 video A->B, 2 video B->A |
|<################################>|
| |
| |
| |
v v
In SIP INVITE 1, Alice sends Bob a SIP INVITE including in the SDP
body the basic audio and video capabilities and the data channel as
per [I-D.ietf-mmusic-sctp-sdp]. Alice also includes the "sip.clue"
media feature tag in the INVITE. A snippet of the SDP showing the
grouping attribute and the video "m=" line are shown below. Alice
has included a "CLUE" group, and included the mid corresponding to a
data channel in the group (3). Note that Alice has chosen not to
include any CLUE-controlled media in the initial offer - the mid
value of the video line is not included in the "CLUE" group.
...
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
...
m=application 6100 UDP/DTLS/SCTP webrtc-datachannel
a=setup:actpass
a=sctp-port: 5000
a=dcmap:2 subprotocol="CLUE";ordered=true
a=mid:3
Bob responds with a similar SDP in SIP 200 OK 1, which also has a
"CLUE" group including the mid value of a data channel; due to their
similarity no SDP snippet is shown here. Bob wishes to receive
initial media, and so includes corresponding non-CLUE-controlled
audio and video lines. Bob also includes the "sip.clue" media
feature tag in the 200 OK. Alice and Bob are each now able to send a
single audio and video stream. This is illustrated as MEDIA 1.
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With the successful initial SDP Offer/Answer exchange complete Alice
and Bob are also free to negotiate the CLUE data channel. This is
illustrated as CLUE DATA CHANNEL ESTABLISHED.
Once the data channel is established CLUE protocol negotiation
begins. In this case Bob was the DTLS client (sending a=active in
his SDP answer) and hence is the CLUE Channel Initiator and sends a
CLUE OPTIONS message describing his version support. On receiving
that message Alice sends her corresponding CLUE OPTIONS RESPONSE.
With the OPTIONS phase complete Alice now sends her CLUE
'advertisement' message (CLUE 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 Views to tell Bob that he should either subscribe to the three
static Captures, the two switched Captures or the one switched
Capture. 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 CLUE 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. Because Bob is not sending an immediate
'configure' message with the "ack" element set he must send an
explicit 'ack' message (CLUE ACK 1) to signal receipt of CLUE
ADVERTISEMENT 1.
Bob also sends his CLUE 'advertisement' message (CLUE ADVERTISEMENT
2) - though the diagram shows that this occurs after Alice sends CLUE
ADVERTISEMENT 1 Bob sends his 'advertisement' message independently
and does not wait for CLUE ADVERTISEMENT 1 to arrive. 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
Views 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".
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Similarly, Alice receives CLUE ADVERTISEMENT 2 but does not yet send
a 'configure' message, because she has not yet received Bob's
Encoding information, sending instead an 'ack' message (CLUE ACK 2).
Both sides have now sent their CLUE 'advertisement' messages and an
SDP exchange is required to negotiate Encodings. For simplicity, in
this case Alice is shown sending an INVITE with a new offer; in many
implementations both sides might send an INVITE, which would be
resolved by use of the 491 Request Pending resolution mechanism from
[RFC3261].
Alice now sends SIP INVITE 2. She maintains the sendrecv audio,
video and CLUE "m=" lines, and she adds three new sendonly "m=" lines
to represent the three CLUE-controlled Encodings she can send. Each
of these "m=" lines has a label corresponding to one of the Encoding
IDs from CLUE ADVERTISEMENT 1. Each also has its mid added to the
grouping attribute to show they are controlled by the CLUE data
channel. A snippet of the SDP showing the grouping attribute, data
channel and the video "m=" lines are shown below:
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...
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=application 6100 UDP/DTLS/SCTP webrtc-datachannel
a=sctp-port: 5000
a=dcmap:2 subprotocol="CLUE";ordered=true
a=mid:3
...
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, allowing him to send both a CLUE 'configure' message and
his SDP answer. As such he now sends CLUE CONFIGURE 1. This
requests the pair of switched Captures that represent Alice's scene,
and he configures them with encoder ids "enc1" and "enc2".
Bob also sends his SDP answer as part of SIP 200 OK 2. Alongside his
original audio, video and CLUE "m=" lines he includes three
additional "m=" lines corresponding to the three added by Alice; two
active recvonly "m= "lines and an inactive "m=" line for the third.
He adds their mid values to the grouping attribute to show they are
controlled by the CLUE data channel. A snippet of the SDP showing
the grouping attribute and the video "m=" lines are shown below (mid
100 represents the CLUE data channel, not shown):
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...
a=group:CLUE 11 12 13 100
...
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 58728 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
a=inactive
a=mid:13
Alice receives Bob's message CLUE CONFIGURE 1 and sends CLUE
CONFIGURE RESPONSE 1 to ack its reception. She does not yet send the
Capture Encodings specified, because at this stage she hasn't
processed Bob's answer SDP and so hasn't negotiated the ability for
Bob to receive these streams.
On receiving SIP 200 OK 2 from Bob Alice sends her SIP ACK (SIP ACK
2). She 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
encoding information as new "m=" lines in SIP 200 OK 2. As such Bob
now sends SIP INVITE 3 to generate a new offer. Along with all the
streams from SIP 200 OK 2 Bob also includes two new sendonly streams.
Each stream has a label corresponding to the Encoding IDs in his CLUE
ADVERTISEMENT 2 message. He also adds their mid values to the
grouping attribute to show they are controlled by the CLUE data
channel. A snippet of the SDP showing the grouping attribute and the
video "m=" lines are shown below (mid 100 represents the CLUE data
channel, not shown):
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...
a=group:CLUE 11 12 14 15 100
...
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
a=mid:13
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:14
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:15
Having received this, Alice now has all the information she needs to
send her CLUE 'configure' message and her SDP answer. In CLUE
CONFIGURE 2 she requests the two static Captures from Bob, to be sent
on Encodings "foo" and "bar".
Alice also sends SIP 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 cdata
hannel. Alice also now deactivates the initial non-CLUE-controlled
media, as bidirectional CLUE-controlled media is now available. A
snippet of the SDP showing the grouping attribute and the video "m="
lines are shown below (mid 3 represents the data channel, not shown):
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...
a=group:CLUE 3 4 5 7 8
...
m=video 0 RTP/AVP 96
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
a=mid:6
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
Bob receives Alice's message CLUE CONFIGURE 2 and sends CLUE
CONFIGURE RESPONSE 2 to ack its reception. Bob does not yet send the
Capture Encodings specified, because he hasn't yet received and
processed Alice's SDP answer and negotiated the ability to send these
streams.
Finally, on receiving SIP 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' message or
new SDP offer/answers to add, remove or change what they have
available or want to receive.
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9. 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. is not able to use the CLUE
protocol).
+----------+ +-----------+
| Alice | | Bob |
| | | |
+----+-----+ +-----+-----+
| |
| |
| SIP INVITE 1 |
|--------------------------------->|
| |
| |
| 200 0K 1 |
|<---------------------------------|
| |
| |
| SIP ACK 1 |
|--------------------------------->|
| |
| |
| |
|<########### MEDIA 1 ############>|
| 1 video A->B, 1 video B->A |
|<################################>|
| |
| |
| |
| |
v v
In SIP INVITE 1, Alice sends Bob a SIP INVITE including in the SDP
body the basic audio and video capabilities and the data channel as
per [I-D.ietf-mmusic-sctp-sdp]. Alice also includes the "sip.clue"
media feature tag in the INVITE. A snippet of the SDP showing the
grouping attribute and the video "m=" line are shown below. Alice
has included a "CLUE" group, and included the mid corresponding to a
data channel in the group (3). Note that Alice has chosen not to
include any CLUE-controlled media in the initial offer - the mid
value of the video line is not included in the "CLUE" group.
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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
...
m=application 6100 UDP/DTLS/SCTP webrtc-datachannel
a=sctp-port: 5000
a=dcmap:2 subprotocol="CLUE";ordered=true
a=mid:3
Bob is not CLUE-capable, and hence does not recognize the "CLUE"
semantic for grouping attribute, nor does he support the data
channel. IN SIP 200 OK 1 he responds with an answer with audio and
video, but with the data channel zeroed.
From the lack of a CLUE group 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.
10. Acknowledgements
Besides the authors, the team focusing on this draft consists of:
Roni Even, Simon Pietro-Romano, Roberta Presta.
Christian Groves, Jonathan Lennox and Adam Roach have contributed
detailed comments and suggestions.
11. IANA Considerations
11.1. New SDP Grouping Framework Attribute
This document registers the following semantics with IANA in the
"Semantics for the "group" SDP Attribute" subregistry (under the
"Session Description Protocol (SDP) Parameters" registry per
[RFC5888]:
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Semantics Token Reference
------------------------------------- ------ ---------
CLUE-controlled m-line CLUE [this draft]
11.2. New SIP Media Feature Tag
This specification registers a new media feature tag in the SIP
[RFC3261] tree per the procedures defined in [RFC2506] and [RFC3840].
Media feature tag name: sip.clue
ASN.1 Identifier: [to be assigned]
Summary of the media feature indicated by this tag: This feature tag
indicates that the device supports CLUE-controlled media.
Values appropriate for use with this feature tag: Boolean.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation mechanisms:
This feature tag is most useful in a communications application for
describing the capabilities of a device to use the CLUE control
protocol to negotiate the use of multiple media streams.
Related standards or documents: [this draft]
Security Considerations: Security considerations for this media
feature tag are discussed in Section 12 of [this draft].
Name(s) & email address(es) of person(s) to contact for further
information:
o Internet Engineering Steering Group: iesg@ietf.org
Intended usage: COMMON
12. Security Considerations
CLUE makes use of a number of protocols and mechanisms, either
defined by CLUE or long-standing. The security considerations
section of the CLUE Framework [I-D.ietf-clue-framework] addresses the
need to secure these mechanisms by following the recommendations of
the individual protocols.
Beyond the need to secure the constituent protocols, the use of CLUE
does impose additional security concerns. One area of increased risk
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involves the potential for a malicious party to subvert a CLUE-
capable device to attack a third party by driving large volumes of
media (particularly video) traffic at them by establishing a
connection to the CLUE-capable device and directing the media to the
victim. While this is a risk for all media devices, a CLUE-capable
device may allow the attacker to configure multiple media streams to
be sent, significantly increasing the volume of traffic directed at
the victim.
This attack can be prevented by ensuring that the media recipient
intends to receive the media packets. As such all CLUE-capable
devices MUST support key negotiation and receiver intent assurance
via DTLS-SRTP [RFC5763] on CLUE-controlled RTP "m=" lines, and MUST
use it or some other mechanism that provides receiver intent
assurance. All CLUE-controlled RTP "m" lines must be secured and
implemented using mechanisms such as SRTP [RFC3711]. CLUE
implementations MAY choose not to require the use of SRTP to secure
legacy (non-CLUE-controlled) media for backwards compatibility with
older SIP clients that are incapable of supporting it.
CLUE also defines a new media feature tag that indicates CLUE
support. This tag may be present even in non-CLUE calls, which
increases the metadata available about the sending device, which can
help an attacker differentiate between multiple devices and help them
identify otherwise anonymised users via the fingerprint of features
their device supports. To prevent this, SIP signaling used to set up
CLUE sessions SHOULD always be encrypted using TLS [RFC5630].
The CLUE protocol also carries additional information that could be
used to help fingerprint a particular user or to identify the
specific version of software being used. CLUE Framework
[I-D.ietf-clue-protocol] provides details of these issues and how to
mitigate them.
13. Change History
Note to RFC Editor: please remove this section prior to publication
-15: Revision by Rob Hanton
* Clarified that using an 'EncID' defined in SDP in an CLUE
ADVERTISEMENT message is only a SHOULD because of the inherent
race conditions about the ordering of the SDP and CLUE message.
In contrast, changed the use of 'EncID' in a CLUE CONFIGURE
message to a MUST as that is defined by the far end and so
there is no way for the sending of the CONFIGURE to anticipate
it.
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* Updated the description of handling the failure of the CLUE
channel to reflect the fact that the protocol state machine now
returns to the IDLE state on failure rather than a specific
termination state, which also means defining an allowance for
the CLUE channel being recovered.
* Updated all instances of advertisment, configure and ack
messages throughout to match the styling of the protocol
document
* Security section updated to make DTLs-SRTP mandatory to use as
well as support unless intent assurance is provided by some
other mechanism per mailing list proposal (to resolve the
concern from a previous IETF session of those wanting to use
CLUE in a closed environment where intent assurance was
provided by other prorietary mechanisms).
* Removed OID value for "sip.clue" media feature tag pending its
actual assignment on registration, leaving a placeholder
* All lower-case uses of 'must', 'should' and 'may' reviewed and
a few made normative
* Fixed various spelling mistakes, clarified grammar, and fixed a
copy/paste error.
* Updated boilerplate to RFC 8174
* Some informative references moved to normative.
-14: Revision by Rob Hanton
* Reference to RFC5245 updated to RFC8445
* Updated my name to reflect surname change (Hansen to Hanton).
* Reviewed recent changes to clue protocol document and concluded
that none affected this document
* Added recommendation that the SDP O/A spec and clue protocol be
read prior to this document
* Several acronyms expanded at the point of initial use
* Some unnecessary normative language replaced with prose
-13: Revision by Rob Hansen
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* Added a section on handling failures of the protocol channel or
data channel mid-call - instructions are that media must
continue as if the clue channel were still established and
unchanged until CLUE is disabled by either side via SDP
exchange.
* Example in section on efficient operation with non-atomic
transactions has had all normative language removed and is now
entirely descriptive (normative language retained in the non-
example portion).
* draft-ietf-clue-protocol-14 reviewed for relevant changes, and
use of CLUE ACK and RESPONSE messages made consistent with that
document (ADVERTISEMENT ACKNOWLEDGEMENT and CONFIGURE RESPONSE
respectively).
* Order of authors revised to reflect updates since Jan 2014.
-12: Revision by Rob Hansen
* Title change to expand and elucidate our totally-not-contrived
acronym
* Explicit reference to RFC3840 added when first mentioning media
feature tags
* Have standardised references to Clue protocol messages to
ADVERTISEMENT, CONFIGURE and ACK, in line with section 12.4.1.
of the protocol document (though the protocol document also
uses ADV and CONF).
* 'MUST' in opening paragraph of 4.2 changed from normative
'MUST' to logical 'must'
* Per his request, removed Cristian's company affiliation and
changed his email address
* Clarified that an implementation that chooses not to send media
during the initial negotiation process must still send RTCP as
normal
* Rewrote the section on adding/remove clue m-lines after the
initial exchange to make clear that this is just standard SDP.
For non-clue controlled lines, recommended they are deactivated
by zeroing the port when turning them off after clue is
successfully negotiated.
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* Added guidance that an initial offer containing clue-controlled
m-lines MUST NOT set them bundle-only unless they somehow know
the far end actually supports BUNDLE
* Added section saying that CLUE devices that do BUNDLE SHOULD do
rtcp-mux, but that the requirement doesn't exist in the other
direction (eg, supporting rtcp-mux does not require or imply
the need to implement BUNDLE)
* For clue-controlled m-lines where the sender included more
encodings than the recipient wants, have standardised on using
"a=inactive" to not receive RTP on them (previously had a mix
of "a=inactive" or port 0, or in some cases did not specify).
* Page break added before the big ladder diagram in the example
* Have added a direction attribute to the SDP example in the data
channel, and made explicit that Bob is the DTLS client and
hence the CLUE Channel Initiator.
* Have removed all language that referenced the possibility of
having multiple CLUE groups
* Removed names appearing in the authors list from the
acknowledgements
* Changed the contact for the IANA registration to iesg@ietf.org
* Security section updated to clarify that DTLS-SRTP must be
supported (as opposed to DTLS) and removed the reference to
RFC7202.
* Other syntactic tweaks based on Paul and Adam's feedback
-11: Revision by Rob Hansen
* Some informative references added for SIP and SDP.
* 'a=mid' lines added to example m-lines with port 0, per RFC5888
section 6.
* Instace of 'must' changed to normative 'MUST', along with
various minor clarifications and corrections.
* Abstract made standalone without citations, per RFC7322 section
4.3.
* RFC editor note added to remove this section.
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-10: Revision by Rob Hansen
* Changes to draft-ietf-clue-protocol between 07 and 11 reviewed
to ensure compatibility between documents has been maintained.
* Expanded the portion of the document related to fingerprinting
with info on the CLUE channel as well as SIP.
-09: Revision by Rob Hansen
* A few minor spelling tweaks
* Made removing the CLUE group mandatory when disabling CLUE mid-
call. Made clear that any CLUE-controlled m-lines should be
disabled or else how they're used is up to the implementation.
-08: Revision by Rob Hansen
* Spelling and grammar fixes from Paul and Christian gratefully
adopted
* Expanded the section on disabling CLUE mid-call to make
explicit the actions required to disable the CLUE channel
gracefully, or to handle someone else doing the same.
* Made a number of fixes to the example call flow to better
reflect the recommendations in the document.
-07: Revision by Rob Hansen
* Removed the entire 'Media line directionality' section as a
discussion of the pros/cons of using bidirectional vs
unidirectional schemes wasn't suitable for a finalised version.
The unidirectionality requirement is covered normatively in an
earlier section.
* BUNDLE no longer includes an address synchronisation step so
the suggestion to wait until that done has been replaced with
some general language about following any negotiated
extensions.
* Added OPTIONS negotiation to the example flow, and revised the
flow to ensure it matched protocol document.
* Section on not sending CLUE control media until CLUE
negotiation completes narrowed to notify that only RTP should
not be sent until negotiation completes and add RTCP to the
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list of things that should be sent as normal, in line with
a=inactive.
* Make explicit that m=recvonly lines don't need to have a label,
as only m=sendonly lines are referenced by CLUE protocol
messages.
* Fix formatting of IANA sections. Improve syntax of feature tag
section in line with Paul's suggestions. Definition of feature
tag narrowed to be multiple media lines *negotiated via CLUE
protocol* rather than more generic 'multiple media lines'.
* General corrections to grammar, spelling and readability based
on Christian, Paul and Mark; in many cases suggested text was
gratefully accepted.
-06: Revision by Rob Hansen
* State machine interactions updated to match versions in -04 of
protocol doc.
* Section on encoding updated to specify both encID and
encodingID from data model doc.
* Removed the limitations on describing H264 encoding limits
using SDP syntax as an open issue.
* Previous draft had SRTP and DTLS mandatory to implement and to
use on CLUE- controlled m lines. Current version has DTLS
mandatory to implement, and 'security' mandatory to use but
does not define what that security is.
* Terminology reference to framework doc reinforced. All
terminology that duplicates framework removed. All text
updated with capitalisation that matches framework document's
terminology.
* SDP example syntax updated to match that of ietf-clue-
datachannel and hence ietf-mmusic-data-channel-sdpneg.
-05: Revision by Rob Hansen
* SRTP/DTLS made mandatory for CLUE-controlled media lines.
* IANA consideration section added (text as proposed by Christian
Groves).
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* Includes provision for dependent streams on seperate "m" lines
having the same encID as their parent "m" line.
* References to putting CLUE-controlled media and data channels
in more than one CLUE group removed, since the document no
longer supports using more than one CLUE group.
* Section on CLUE controlled media restrictions still applying
even if the call does not end up being CLUE enabled being
rewritten to hopefully be clearer.
* Other minor syntax improvements.
-04: Revision by Rob Hansen
* Updated DTLS/SCTP channel syntax in examples to fix errors and
match latest format defined in draft-ietf-mmusic-sctp-sdp-07.
* Clarified the behaviour if an SDP offer includes a CLUE-
controlled "m" line and the answer accepts that "m" line but
without CLUE control of that line.
* Added a new section on the sending and receiving of CaptureIDs
in RTP and RTCP. Includes a section on the necessity of the
receiver coping with unexpected CaptureIDs (or the lack
thereof) due to MCCs being redefined in new Advertisement
messages.
* Added reminder on IANA section on registering grouping semantic
and media feature tag, removed the less formal sections that
did the same job.
* Fixed and clarified issues raised by Christian's document
review.
* Added a number of security considerations.
-03: Revision by Rob Hansen
* Clarified text on not rejecting messages because they contain
unknown encIDs.
* Removed normative language in section on accepting/rejecting
non-CLUE-controlled media in the initial answer.
* Example SDP updated to include the data channel "m" lines.
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* Example call flow updated to show disablement of non-CLUE-
controlled media once CLUE-controlled media is flowing.
-02: Revision by Rob Hansen
* Added section on not accepting non-CLUE-controlled "m" lines in
the initial answer when CLUE is to be negotiated.
* Removed previous language attempting to describe media
restrictions for CLUE-controlled "m" lines that had not been
configured, and replaced it with much more accurate 'treat as
"a=inactive" was set'.
* Made label element mandatory for CLUE-controlled media (was
previously "SHOULD include", but there didn't seem a good
reason for this - anyone wishing to include the "m" line but
not immediately use it in CLUE can simply leave it out of the
<encodingIDList>.)
* Added a section on the specifics of relating encodings in SDP
to <encID> elements in the CLUE protocol, including the fact
that both Advertisement and Configure messages reference the
*encoding* (eg, in the Configure case the sender of the
Configure message includes the labels of the recipient's "m"
lines as their <encID> contents).
* Minor revisions to the section on complying with normative SDP/
CLUEstate machine language to clarify that these were not new
normative language, merely that existing normative language
still applies.
* Removed appendices which previously contained information to be
transferred to the protocol and data channel drafts. Removed
other text that discussed alternatives to the current approach.
* Cleaned up some 'todo' text.
-01: Revision by Rob Hansen
* Revised terminology - removed the term 'CLUE-enabled' device as
insufficiently distinct from 'CLUE-capable' and instead added a
term for 'CLUE-enabled' calls.
* Removed text forbidding RTCP and instead added text that ICE/
DTLS negotiation for CLUE controlled media must be done as
normal irrespective of CLUE negotiation.
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* Changed 'sip.telepresence' to 'sip.clue' and 'TELEPRESENCE'
grouping semantic back to CLUE.
* Made it mandatory to have exactly one mid corresponding to a
data channel in a CLUE group
* Forbade having multiple CLUE groups unless a specification for
doing so is published.
* Refactored SDP-related text; previously the encoding
information had been in the "initial offer" section despite the
fact that we recommend that the initial offer doesn't actually
include any encodings. I moved the specifications of encodings
and how they're received to an earlier, seperate section.
* Added text on how the state machines in CLUE and SDP are
allowed to affect one another, and further recommendations on
how a device should handle the sending of CLUE and SDP changes.
-00: Revision by Rob Hansen
* Submitted as -00 working group document
draft-kyzivat-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
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draft-kyzivat-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.ietf-clue-protocol], but
that has not yet been resolved in those documents, into an
appendix.
draft-kyzivat-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
draft-kyzivat-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.)
draft-kyzivat-04: Revisions by Rob Hansen: ???
draft-kyzivat-03: Revisions by pkyzivat:
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* 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.
draft-kyzivat-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.
draft-kyzivat-01: Updated by roberta to include some sample call
flows.
draft-kyzivat-00: Initial version by pkyzivat. Established general
outline for the document, and specified a few things thought to
represent wg consensus.
14. References
14.1. Normative References
[I-D.ietf-clue-data-model-schema]
Presta, R. and S. Romano, "An XML Schema for the CLUE data
model", draft-ietf-clue-data-model-schema-17 (work in
progress), August 2016.
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[I-D.ietf-clue-datachannel]
Holmberg, C., "CLUE Protocol data channel", draft-ietf-
clue-datachannel-18 (work in progress), April 2019.
[I-D.ietf-clue-framework]
Duckworth, M., Pepperell, A., and S. Wenger, "Framework
for Telepresence Multi-Streams", draft-ietf-clue-
framework-25 (work in progress), January 2016.
[I-D.ietf-clue-protocol]
Presta, R. and S. Romano, "Protocol for Controlling
Multiple Streams for Telepresence (CLUE)", draft-ietf-
clue-protocol-19 (work in progress), July 2019.
[I-D.ietf-clue-rtp-mapping]
Even, R. and J. Lennox, "Mapping RTP streams to CLUE Media
Captures", draft-ietf-clue-rtp-mapping-14 (work in
progress), February 2017.
[I-D.ietf-mmusic-data-channel-sdpneg]
Drage, K., Makaraju, M., Ejzak, R., Marcon, J., and R.
Even, "SDP-based Data Channel Negotiation", draft-ietf-
mmusic-data-channel-sdpneg-28 (work in progress), May
2019.
[I-D.ietf-mmusic-sctp-sdp]
Holmberg, C., Shpount, R., Loreto, S., and G. Camarillo,
"Session Description Protocol (SDP) Offer/Answer
Procedures For Stream Control Transmission Protocol (SCTP)
over Datagram Transport Layer Security (DTLS) Transport.",
draft-ietf-mmusic-sctp-sdp-26 (work in progress), April
2017.
[I-D.ietf-mmusic-sdp-bundle-negotiation]
Holmberg, C., Alvestrand, H., and C. Jennings,
"Negotiating Media Multiplexing Using the Session
Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
negotiation-54 (work in progress), December 2018.
[I-D.ietf-rtcweb-data-channel]
Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data
Channels", draft-ietf-rtcweb-data-channel-13 (work in
progress), January 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<https://www.rfc-editor.org/info/rfc3711>.
[RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
"Indicating User Agent Capabilities in the Session
Initiation Protocol (SIP)", RFC 3840,
DOI 10.17487/RFC3840, August 2004,
<https://www.rfc-editor.org/info/rfc3840>.
[RFC4574] Levin, O. and G. Camarillo, "The Session Description
Protocol (SDP) Label Attribute", RFC 4574,
DOI 10.17487/RFC4574, August 2006,
<https://www.rfc-editor.org/info/rfc4574>.
[RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
for Establishing a Secure Real-time Transport Protocol
(SRTP) Security Context Using Datagram Transport Layer
Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
2010, <https://www.rfc-editor.org/info/rfc5763>.
[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888,
DOI 10.17487/RFC5888, June 2010,
<https://www.rfc-editor.org/info/rfc5888>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
14.2. Informative References
[RFC2506] Holtman, K., Mutz, A., and T. Hardie, "Media Feature Tag
Registration Procedure", BCP 31, RFC 2506,
DOI 10.17487/RFC2506, March 1999,
<https://www.rfc-editor.org/info/rfc2506>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<https://www.rfc-editor.org/info/rfc3264>.
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[RFC3311] Rosenberg, J., "The Session Initiation Protocol (SIP)
UPDATE Method", RFC 3311, DOI 10.17487/RFC3311, October
2002, <https://www.rfc-editor.org/info/rfc3311>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <https://www.rfc-editor.org/info/rfc4566>.
[RFC5630] Audet, F., "The Use of the SIPS URI Scheme in the Session
Initiation Protocol (SIP)", RFC 5630,
DOI 10.17487/RFC5630, October 2009,
<https://www.rfc-editor.org/info/rfc5630>.
[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761,
DOI 10.17487/RFC5761, April 2010,
<https://www.rfc-editor.org/info/rfc5761>.
[RFC6184] Wang, Y., Even, R., Kristensen, T., and R. Jesup, "RTP
Payload Format for H.264 Video", RFC 6184,
DOI 10.17487/RFC6184, May 2011,
<https://www.rfc-editor.org/info/rfc6184>.
[RFC8445] Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
Connectivity Establishment (ICE): A Protocol for Network
Address Translator (NAT) Traversal", RFC 8445,
DOI 10.17487/RFC8445, July 2018,
<https://www.rfc-editor.org/info/rfc8445>.
Authors' Addresses
Robert Hanton
Cisco Systems
Email: rohanse2@cisco.com
Paul Kyzivat
Email: pkyzivat@alum.mit.edu
Lennard Xiao
Huawei
Email: lennard.xiao@huawei.com
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Christian Groves
Email: cngroves.std@gmail.com
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