Internet Engineering Task Force (IETF) R. Hanton
Request for Comments: 8848 Cisco Systems
Category: Experimental P. Kyzivat
ISSN: 2070-1721
L. Xiao
Beijing Chuangshiyoulian
C. Groves
January 2021
Session Signaling for Controlling Multiple Streams for Telepresence
(CLUE)
Abstract
This document is about Controlling Multiple Streams for Telepresence
(CLUE) signaling. It specifies how the CLUE protocol and the CLUE
data channel are used in conjunction with each other and with
existing signaling mechanisms, such as SIP and the Session
Description Protocol (SDP), to produce a telepresence call.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are candidates for any level of
Internet Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8848.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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described in the Simplified BSD License.
Table of Contents
1. Introduction
2. Terminology
3. Media Feature Tag Definition
4. SDP Grouping Framework CLUE Extension Semantics
4.1. General
4.2. The CLUE Data Channel and the CLUE Grouping Semantic
4.3. CLUE-Controlled Media and the CLUE Grouping Semantic
4.4. SDP Semantics for CLUE-Controlled Media
4.4.1. Signaling CLUE Encodings
4.4.1.1. Referencing Encodings in the CLUE Protocol
4.4.2. Negotiating Receipt of CLUE Capture Encodings in SDP
4.5. SDP Offer/Answer Procedures
4.5.1. Generating the Initial Offer
4.5.2. Generating the Answer
4.5.2.1. Negotiating Use of CLUE and the CLUE Data Channel
4.5.2.2. Negotiating CLUE-Controlled Media
4.5.2.3. Negotiating Non-CLUE-controlled Media
4.5.3. Processing the Initial Offer/Answer Negotiation
4.5.3.1. Successful CLUE Negotiation
4.5.3.2. CLUE Negotiation Failure
4.5.4. Modifying the Session
4.5.4.1. Adding and Removing CLUE-Controlled Media
4.5.4.2. Enabling CLUE Mid-Call
4.5.4.3. Disabling CLUE Mid-Call
4.5.4.4. CLUE Protocol Failure Mid-Call
5. Interaction of the CLUE Protocol and SDP Negotiations
5.1. Independence of SDP and CLUE Negotiation
5.2. Constraints on Sending Media
5.3. Recommendations for Operating with Non-atomic Operations
6. Interaction of the CLUE Protocol and RTP/RTCP CaptureID
6.1. CaptureID Reception during MCC Redefinition
7. Multiplexing of CLUE-Controlled Media Using BUNDLE
7.1. Overview
7.2. Usage of BUNDLE with CLUE
7.2.1. Generating the Initial Offer
7.2.2. Multiplexing of the Data Channel and RTP Media
8. Example: A Call between Two CLUE-Capable Endpoints
9. Example: A Call between a CLUE-Capable and Non-CLUE Endpoint
10. IANA Considerations
10.1. New SDP Grouping Framework Attribute
10.2. New SIP Media Feature Tag
11. Security Considerations
12. References
12.1. Normative References
12.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
To enable devices to participate in a telepresence call, where they
select the sources they wish to view, receive those media sources,
and display them in an optimal fashion, Controlling Multiple Streams
for Telepresence (CLUE) employs two principal and interrelated
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 messages [RFC8847], transported via a CLUE data channel
[RFC8850], 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 [RFC8845] 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 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.
This document uses terminology defined in the CLUE Framework
[RFC8845].
A few additional terms specific to this document are defined as
follows:
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 on how this control
is signaled in SDP. There is a corresponding "non-CLUE-
controlled" media term.
non-CLUE device: A device that supports standard SIP and SDP but
either does not support CLUE or does support CLUE but does not
currently wish to invoke CLUE capabilities.
RTCP: RTP Control Protocol.
SCTP: Stream Control Transmission Protocol.
STUN: Session Traversal Utilities for NAT.
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 that is 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 able to either
determine that the other side of the SDP exchange supports multiple
CLUE channels or fail gracefully in the event it does not.
4.2. The CLUE Data Channel and the CLUE Grouping Semantic
The CLUE data channel [RFC8850] is a bidirectional data channel
[RFC8831] 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 [RFC8864]. 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 [RFC8847]. For an "m=" line to be CLUE controlled, its
"mid" attribute 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 a "mid" included in the CLUE group.
"m=" lines not specified as being 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
4.4.1. Signaling CLUE Encodings
The CLUE Framework [RFC8845] defines the concept of "Encodings",
which represent the sender's encode ability. Each Encoding the Media
Provider wishes to signal is done so 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 (e.g., "a=sendonly") Encodings can then
be expressed using existing SDP syntax. For instance, for H.264, see
Table 6 in Section 8.2.2 of [RFC6184] for a list of valid parameters
for representing encoder sender stream limits.
These Encodings are CLUE controlled and hence MUST include a "mid" in
the CLUE group as defined above.
In addition to 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 a Forward
Error Correction (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 a part of an Encoding Group (in 'advertisement' messages) and
which Encoding is used 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
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 it 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.
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 the use of the CLUE protocol MUST NOT be negotiated 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".
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 the CLUE-controlled media is fully negotiated.
4.5.3. Processing the Initial Offer/Answer Negotiation
In the event that both the 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 [RFC8850] for
negotiation details. If negotiation is successful, the sending of
CLUE protocol messages [RFC8847] 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.
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. CLUE-capable devices MUST
either revert to non-CLUE behavior 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 the 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 as defined in Section 6.6.1 of [RFC8864]: sending a new SDP
Offer/Answer exchange and subsequent SCTP Stream Sequence Number
(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:
* The CLUE data channel terminates, either gracefully or
ungracefully, without any corresponding SDP renegotiation.
* A channel error of the CLUE protocol causes it to return to the
IDLE state as defined in Section 6 of [RFC8847].
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 the 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 an 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 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 middleboxes, 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;
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 [RFC8846] 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> that 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 does 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.
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 [RFC8847] 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 with 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 include
corresponding "a=inactive" lines in its answer, and it would 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
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 and reducing the number of
new SDP Offers required.
6. Interaction of the CLUE Protocol and RTP/RTCP CaptureID
The CLUE Framework [RFC8845] allows for Multiple Content Captures
(MCCs): Captures that 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, [RFC8849] 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 where the
contents of an MCC are redefined.
When a Media Provider redefines an MCC that involves CaptureIDs, the
reception of the relevant CaptureIDs by the recipient will either
lead or lag reception and the processing of the new 'advertisement'
message by the recipient. As such, a Media Consumer MUST NOT be
disrupted by any of the following scenarios 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):
* By receiving RTP or RTCP containing a CaptureID when the most
recently processed 'advertisement' message means that no media
CaptureIDs are expected.
* By receiving RTP or RTCP without CaptureIDs when the most recently
processed 'advertisement' message means that media CaptureIDs are
expected.
* By 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.
* By 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
Interactive Connectivity Establishment (ICE) candidates [RFC8445],
etc.
The BUNDLE extension [RFC8843] 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.
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
rendering 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 specification [RFC8843]. If the BUNDLE group includes
protocols other 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.
+----------+ +-----------+
| 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 |
|<*********************************|
| |
| |
| 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 |
|<*********************************|
| |
| |
| 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 with the basic audio
and video capabilities and data channel included in the SIP body as
per [RFC8841]. 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 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 thus 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.
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. He 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 systems with one or two screens. All
of these Captures are in a single Capture Scene, with suitable
Capture Scene Views that tell Bob he should subscribe to the three
static Captures, the two switched Captures, or the one switched
Capture. Alice has no simultaneity constraints, so all six Captures
are included 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; thus, he does not know if she will have sufficient
resources in order 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 that tell Alice she should subscribe to
either the two static Captures or the single composed Capture. Bob
also has no simultaneity constraints, so he includes all three
Captures in one simultaneous set. Bob also includes a single
Encoding Group with two Encoding IDs: "foo" and "bar".
Similarly, Alice receives CLUE ADVERTISEMENT 2 but does not yet send
a 'configure' message, because she has not yet received Bob's
Encoding information; instead, she sends 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 video "m=" lines are shown below:
...
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, which is not shown):
...
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 CLUE CONFIGURE 1 message and sends CLUE
CONFIGURE RESPONSE 1 to acknowledge its reception. She does not yet
send the Capture Encodings specified, because at this stage, she
hasn't processed Bob's answer SDP and thus 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, which is not shown):
...
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 data
channel. Alice then 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):
...
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 CLUE CONFIGURE 2 message and sends CLUE
CONFIGURE RESPONSE 2 to acknowledge 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 a new 'advertisement' or 'configure' message or
the new SDP Offers/Answers to add, remove, or change what they have
available or want to receive.
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 the basic
audio and video capabilities and data channel in the SDP body as per
[RFC8841]. 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 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=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 the grouping attribute, nor does he support the data
channel. IN SIP 200 OK 1, he responds with an answer that includes
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. At this point,
Alice begins to send her fallback video: in this case, it's likely a
switched view from whichever camera shows the current loudest
participant on her side.
10. IANA Considerations
10.1. 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]:
+===========================+=======+==============+===========+
| Semantics | Token | Mux Category | Reference |
+===========================+=======+==============+===========+
| CLUE-controlled "m=" line | CLUE | NORMAL | RFC 8848 |
+---------------------------+-------+--------------+-----------+
Table 1
10.2. 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: 30
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: RFC 8848
Security Considerations: Security considerations for this media
feature tag are discussed in Section 11 of RFC 8848.
Name(s) & email address(es) of person(s) to contact for further
information: Internet Engineering Steering Group <iesg@ietf.org>
Intended usage: COMMON
11. 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 document [RFC8845] 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
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 / Secure Real-time Transport Protocol (SRTP) [RFC5763] on
CLUE-controlled RTP "m=" lines, and they 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; this can
help an attacker differentiate between multiple devices and identify
otherwise anonymized 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. The CLUE Framework
[RFC8847] provides details about these issues and how to mitigate
them.
12. References
12.1. Normative References
[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>.
[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>.
[RFC8831] Jesup, R., Loreto, S., and M. Tüxen, "WebRTC Data
Channels", RFC 8831, DOI 10.17487/RFC8831, January 2021,
<https://www.rfc-editor.org/info/rfc8831>.
[RFC8841] 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",
RFC 8841, DOI 10.17487/RFC8841, January 2021,
<https://www.rfc-editor.org/info/rfc8841>.
[RFC8843] Holmberg, C., Alvestrand, H., and C. Jennings,
"Negotiating Media Multiplexing Using the Session
Description Protocol (SDP)", RFC 8843,
DOI 10.17487/RFC8843, January 2021,
<https://www.rfc-editor.org/info/rfc8843>.
[RFC8845] Duckworth, M., Ed., Pepperell, A., and S. Wenger,
"Framework for Telepresence Multi-Streams", RFC 8845,
DOI 10.17487/RFC8845, January 2021,
<https://www.rfc-editor.org/info/rfc8845>.
[RFC8846] Presta, R. and S P. Romano, "An XML Schema for the
Controlling Multiple Streams for Telepresence (CLUE) Data
Model", RFC 8846, DOI 10.17487/RFC8846, January 2021,
<http://www.rfc-editor.org/info/rfc8846>.
[RFC8847] Presta, R. and S P. Romano, "Protocol for Controlling
Multiple Streams for Telepresence (CLUE)", RFC 8847,
DOI 10.17487/RFC8847, January 2021,
<https://www.rfc-editor.org/info/rfc8847>.
[RFC8849] Even, R. and J. Lennox, "Mapping RTP Streams to
Controlling Multiple Streams for Telepresence (CLUE) Media
Captures", RFC 8849, DOI 10.17487/RFC8849, January 2021,
<https://www.rfc-editor.org/info/rfc8849>.
[RFC8850] Holmberg, C., "Controlling Multiple Streams for
Telepresence (CLUE) Protocol Data Channel", RFC 8850,
DOI 10.17487/RFC8850, January 2021,
<https://www.rfc-editor.org/info/rfc8850>.
[RFC8864] Drage, K., Makaraju, M., Ejzak, R., Marcon, J., and R.
Even, Ed., "Negotiation Data Channels Using the Session
Description Protocol (SDP)", RFC 8864,
DOI 10.17487/RFC8864, January 2021,
<https://www.rfc-editor.org/info/rfc8864>.
12.2. 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>.
[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.-K., 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>.
Acknowledgements
Besides the authors, the team focusing on this document consists of:
Roni Even, Simon Pietro Romano, and Roberta Presta.
Christian Groves, Jonathan Lennox, and Adam Roach have contributed
detailed comments and suggestions.
Authors' Addresses
Robert Hanton
Cisco Systems
Email: rohanse2@cisco.com
Paul Kyzivat
Email: pkyzivat@alum.mit.edu
Lennard Xiao
Beijing Chuangshiyoulian
Email: lennard.xiao@outlook.com
Christian Groves