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<rfc category="std" xmlns:xi="http://www.w3.org/2001/XInclude" category="exp" ipr="trust200902" docName='draft-ietf-clue-signaling-15'>

<!--56789012345678901234567890123456789012345678901234567890123456789-->  number="8848" docName="draft-ietf-clue-signaling-15" obsoletes="" updates="" submissionType="IETF" consensus="true" xml:lang="en" tocInclude="true" tocDepth="4" symRefs="true" sortRefs="false" version="3">
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<front>
    <title abbrev="CLUE Signaling">
      Session Signaling for Controlling Multiple Streams for Telepresence (CLUE)
    </title>
    <seriesInfo name="RFC" value="8848"/>
    <author initials="R." surname="Hanton" fullname="Robert Hanton">
      <organization>Cisco Systems</organization>
      <address>
        <email>rohanse2@cisco.com</email>
      </address>
    </author>
    <author initials="P." surname="Kyzivat" fullname="Paul Kyzivat">
      <address>
        <email>pkyzivat@alum.mit.edu</email>
      </address>
    </author>
<!--Note: per author, updated 'lennard.xiao@huawei.com' to 'lennard.xiao@outlook.com'-->
    <author initials="L." surname="Xiao" fullname="Lennard Xiao">
      <organization>Huawei</organization>
      <address>
         <email>lennard.xiao@huawei.com</email>
        <email>lennard.xiao@outlook.com</email>
      </address>
    </author>
    <author initials="C." surname="Groves" fullname="Christian Groves">
      <address>
        <email>cngroves.std@gmail.com</email>
      </address>
    </author>
    <date year="2019" /> month="July" year="2020"/>

<!-- [rfced] Please insert any keywords (beyond those that appear in
the title) for use on https://www.rfc-editor.org/search. -->

<keyword>example</keyword>

    <abstract>
      <t>
This document specifies how CLUE-specific signaling specific to Controlling Multiple Streams
for Telepresence (CLUE), such as the CLUE protocol and 2<sup>31</sup> the CLUE data channel are channel, is used in conjunction with each other and with existing signaling mechanisms mechanisms, such as SIP and SDP the Session Description Protocol (SDP), to produce a telepresence call.
      </t>
    </abstract>
  </front>
  <middle>
    <section title="Introduction"> numbered="true" toc="default">
      <name>Introduction</name>
      <t>

<!--[rfced] May we update 'selecting the sources they wish to view,
receiving those media sources and displaying them in an optimal
fashion' for clarity? Please let us know if the suggested text is
agreeable or if you prefer otherwise.

Original:
   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.

Perhaps:
   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.
-->
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, Controlling Multiple Streams for Telepresence (CLUE) employs
two principal and interrelated protocol negotiations.
SDP <xref target="RFC4566">SDP</xref>, target="RFC4566" format="default"/>, conveyed via
SIP <xref target="RFC3261">SIP</xref>, target="RFC3261" format="default"/>, is used to negotiate the specific media
capabilities that can be delivered to specific addresses on a device.
Meanwhile, CLUE protocol messages <xref target="I-D.ietf-clue-protocol">CLUE protocol</xref>
messages, target="RFC8847" format="default"/>, transported via a
<xref target="I-D.ietf-clue-datachannel">CLUE
CLUE data channel</xref>, channel <xref target="RFC8850" format="default"/>, are used to
negotiate the Capture Sources available, their attributes attributes, and any constraints
in their use. They also allow the far end 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.
      </t>
      <t>
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 <xref target="I-D.ietf-clue-framework">CLUE Framework</xref> target="RFC8845" format="default"/>
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.
      </t>
      <t>
Backwards-compatibility
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 mid-call and,
similarly, how CLUE functionality can be removed mid-call to return to a
standard non-CLUE call.
      </t>
    </section>
    <section title="Terminology"> numbered="true" toc="default">
      <name>Terminology</name>
        <t>
    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
"MAY", "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>", "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL
    NOT</bcp14>", "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>", "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
    "<bcp14>MAY</bcp14>", and "OPTIONAL" "<bcp14>OPTIONAL</bcp14>" in this document are to be interpreted as
    described in BCP 14 BCP&nbsp;14 <xref target="RFC2119"/> <xref target="RFC8174"/>
    when, and only when, they appear in all capitals, as shown here.
        </t>
      <t>
This document uses terminology defined in the CLUE Framework
<xref target="I-D.ietf-clue-framework">CLUE Framework</xref>. target="RFC8845" format="default"/>.
      </t>
      <t>
A few additional terms specific to this document are defined as follows:
      </t>
    <t>
      <list style='hanging'>
        <t hangText="non-CLUE device:">
      <dl newline="false" spacing="normal">
        <dt>non-CLUE device:</dt>
        <dd>
A device that supports standard SIP and SDP, SDP but either does not support CLUE, CLUE
or that does support CLUE but does not currently wish to invoke CLUE capabilities.
        </t>
        <t hangText="CLUE-controlled media:">
        </dd>
        <dt>CLUE-controlled media:</dt>
        <dd>
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
<xref target="sec.group" /> format="default"/> for details of on how this control is signaled in
SDP. There is a corresponding "non-CLUE-controlled" media term.
         </t>
      </list>
    </t>
         </dd>
      </dl>
    </section>
    <section title="Media anchor="sec.tag" numbered="true" toc="default">
      <name>Media Feature Tag Definition" anchor="sec.tag"> Definition</name>
      <t>
The "sip.clue" media feature tag <xref target="RFC3840"/> target="RFC3840" format="default"/> indicates
support for CLUE in SIP <xref target="RFC3261">SIP</xref> target="RFC3261" format="default"/> calls. A CLUE-capable
device SHOULD <bcp14>SHOULD</bcp14> include this media feature tag in its REGISTER requests and
OPTION responses. It SHOULD <bcp14>SHOULD</bcp14> also include the media feature tag in INVITE and
UPDATE <xref target="RFC3311" /> format="default"/> requests and responses.
      </t>
      <t>
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.
      </t>
    </section>
    <section title="SDP anchor="sec.group" numbered="true" toc="default">
      <name>SDP Grouping Framework CLUE Extension Semantics" anchor="sec.group"> Semantics</name>
      <section title="General"> numbered="true" toc="default">
        <name>General</name>
        <t>
This section defines a new SDP Grouping Framework <xref target="RFC5888" /> format="default"/>
extension called 'CLUE'.
        </t>
        <t>
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, CLUE controlled by a CLUE data
channel that is also included in this CLUE group.
        </t>
        <t>
Currently
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 <bcp14>MUST NOT</bcp14> include more than one CLUE group in its SDP
message unless it is following a specification that defines how multiple CLUE
channels are signaled, signaled and is either able to either determine that the other side of
the SDP exchange supports multiple CLUE channels, channels or is able to fail
gracefully in the event it does not.
        </t>
      </section>
      <section title="The numbered="true" toc="default">
        <name>The CLUE data channel Data Channel and the CLUE grouping semantic"> Grouping Semantic</name>
        <t>
The <xref target="I-D.ietf-clue-datachannel">CLUE CLUE data channel</xref> channel <xref target="RFC8850" format="default"/> is a
bidirectional data channel <xref target="I-D.ietf-rtcweb-data-channel">data channel</xref> target="RFC8831" format="default"/>
used for the transport of CLUE messages, conveyed within an SCTP a Stream Control Transmission Protocol (SCTP) over DTLS
connection. This channel must be established before CLUE protocol messages can
be exchanged and CLUE-controlled media can be sent.
        </t>
        <t>
The data channel is negotiated over SDP as described in
<xref target="I-D.ietf-mmusic-data-channel-sdpneg"/>. target="RFC8864" format="default"/>. A CLUE-capable
device wishing to negotiate CLUE MUST <bcp14>MUST</bcp14> 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 <bcp14>MUST</bcp14> include the "mid" of the "m=" line for one
(and only one) data channel.
        </t>
        <t>
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 <bcp14>SHOULD</bcp14> include a data channel "m=" line in offers
and, when allowed by <xref target="RFC3264" />, format="default"/>, answers.
        </t>
      </section>
      <section title="CLUE-controlled media numbered="true" toc="default">
        <name>CLUE-Controlled Media and the CLUE grouping semantic"> Grouping Semantic</name>
        <t>
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
<xref target="I-D.ietf-clue-protocol">CLUE protocol</xref>. target="RFC8847" format="default"/>. For an "m=" line
to be CLUE-controlled, CLUE controlled, its "mid" value MUST <bcp14>MUST</bcp14> be included in the CLUE group.
CLUE-controlled media is controlled by the CLUE protocol as negotiated on the
CLUE data channel with an a "mid" included in the CLUE group.
        </t>
        <t>
"m=" lines not specified as being under CLUE control follow normal rules for media
streams negotiated in SDP as defined in documents such as
<xref target="RFC3264" />. format="default"/>.
        </t>
        <t>
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.
        </t>
      </section>
      <section title="SDP semantics numbered="true" toc="default">
        <name>SDP Semantics for CLUE-controlled media"> CLUE-Controlled Media</name>
        <section title="Signaling anchor="sec.sdp_encodings" numbered="true" toc="default">
          <name>Signaling CLUE Encodings" anchor="sec.sdp_encodings"> Encodings</name>
          <t>
The CLUE Framework <xref target="I-D.ietf-clue-framework">CLUE Framework</xref> target="RFC8845" format="default"/> defines the
concept of "Encodings", which represent the sender's encode ability. Each
Encoding the Media Provider wishes to signal is signaled done so via an "m=" line of
the appropriate media type, which MUST <bcp14>MUST</bcp14> be marked as sendonly with the
"a=sendonly" attribute or as inactive with the "a=inactive" attribute.
          </t>
          <t>
The encoder limits of active (eg, (e.g., "a=sendonly") Encodings can then be
expressed using existing SDP syntax. For instance, for H.264 H.264, see Table 6 in
<xref target="RFC6184" /> sectionFormat="of" section="8.2.2"/> for a list of valid parameters for representing
encoder sender stream limits.
          </t>
          <t>
These Encodings are CLUE-controlled CLUE controlled and hence MUST <bcp14>MUST</bcp14> include an a "mid" in the
CLUE group as defined above.
          </t>
          <t>
As well as
In addition to the normal restrictions defined in <xref target="RFC3264" /> format="default"/>, the
stream MUST <bcp14>MUST</bcp14> 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 <bcp14>MUST NOT</bcp14> be sent until configuration is complete, while
non-media packets such as STUN, RTCP Session Traversal Utilities for NAT (STUN), the RTP Control Protocol (RTCP), and DTLS MUST <bcp14>MUST</bcp14> be sent as per their
relevant specifications specifications, if negotiated.
          </t>
          <t>
Every "m=" line representing a CLUE Encoding MUST <bcp14>MUST</bcp14> contain a "label" attribute
as defined in <xref target="RFC4574" />. format="default"/>. 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
<bcp14>MUST</bcp14> 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 a Forward Error Correction (FEC) stream), in which case it MUST <bcp14>MUST</bcp14> have the same label value as
the "m=" line on which it depends.
          </t>
          <section title="Referencing numbered="true" toc="default">
            <name>Referencing Encodings in the CLUE protocol"> Protocol</name>
            <t>
CLUE Encodings are defined in SDP, but can be referenced from CLUE protocol
messages - this is how
<!--[rfced] May we update the last part of this sentence to make it
more parallel?  Please clarify if the suggested text conveys the
intended meaning or if you prefer otherwise.

Original:
   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).

Perhaps:
   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
   Encodings are used to encode a specific Capture (in 'configure' messages).
-->
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.
            </t>
            <t>
Each &lt;encID&gt; (in encodingIDList) in a CLUE 'advertisement' message
SHOULD
<bcp14>SHOULD</bcp14> 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 &lt;encID&gt;. If the &lt;encID&gt;
is not defined in SDP SDP, it MUST <bcp14>MUST</bcp14> be one it anticipates sending in a subsequent
SDP Offer/Answer exchange.
            </t>
            <t>
Each &lt;encodingID&gt; (in captureEncodingType) in a CLUE 'configure' message
MUST
<bcp14>MUST</bcp14> 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 &lt;encodingID&gt;.
            </t>
            <t>
Note that the non-atomic nature of SDP/CLUE protocol interaction may mean that
there are temporary periods where an &lt;encID&gt;/&lt;encodingID&gt; 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 <xref target="sec.coordination" /> format="default"/> for specifics.
            </t>
          </section>
        </section>
        <section title="Negotiating receipt numbered="true" toc="default">
          <name>Negotiating Receipt of CLUE Capture Encodings in SDP"> SDP</name>
          <t>
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.
          </t>
          <t>
These "m=" lines are CLUE-controlled CLUE controlled and hence MUST <bcp14>MUST</bcp14> include their "mid" in the
CLUE group. They MAY <bcp14>MAY</bcp14> 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.
          </t>
        </section>
      </section>
      <section title="SDP numbered="true" toc="default">
        <name>SDP Offer/Answer Procedures"> Procedures</name>
        <section title="Generating numbered="true" toc="default">
          <name>Generating the Initial Offer"> Offer</name>
          <t>
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.
          </t>
          <t>
For interoperability with non-CLUE devices devices, a CLUE-capable device sending an
initial SDP offer SHOULD NOT <bcp14>SHOULD NOT</bcp14> include any "m=" line for CLUE-controlled media
beyond the "m=" line for the CLUE data channel, and SHOULD it <bcp14>SHOULD</bcp14> include at least
one non-CLUE-controlled media "m=" line.
          </t>
          <t>
If the device has evidence that the receiver is also CLUE-capable, CLUE capable, for
instance
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 <bcp14>MAY</bcp14> contain "m=" lines for CLUE-controlled media.
          </t>
          <t>
With the same interoperability recommendations as for Encodings, the sender of
the initial SDP offer MAY <bcp14>MAY</bcp14> also include "a=recvonly" media lines to
preallocate "m=" lines to receive media. Alternatively, it MAY <bcp14>MAY</bcp14> wait until CLUE
protocol negotiation has completed before including these lines in a new
offer/answer exchange - -- see <xref target="sec.coordination" /> format="default"/> for
recommendations.
          </t>
        </section>
        <section title="Generating numbered="true" toc="default">
          <name>Generating the Answer"> Answer</name>
          <section title="Negotiating use numbered="true" toc="default">
            <name>Negotiating Use of CLUE and the CLUE data channel"> Data Channel</name>
            <t>
If the recipient of an initial offer is CLUE-capable, 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 <bcp14>SHOULD</bcp14> negotiate data channel support for an "m=" line, line
and include the "mid" of that "m=" line in a corresponding CLUE group.
            </t>
            <t>
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 <bcp14>MAY</bcp14> 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 the use of the CLUE protocol <bcp14>MUST NOT</bcp14> be negotiated on this channel.
            </t>
          </section>
          <section title="Negotiating CLUE-controlled media"> numbered="true" toc="default">
            <name>Negotiating CLUE-Controlled Media</name>
            <t>
If the initial offer contained "a=recvonly" CLUE-controlled media lines lines, the
recipient SHOULD <bcp14>SHOULD</bcp14> 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
<bcp14>MUST</bcp14> be included in the corresponding CLUE group. The recipient MUST <bcp14>MUST</bcp14> set the
direction of the corresponding "m=" lines of any remaining "a=recvonly"
CLUE-controlled media lines received in the offer to "a=inactive".
            </t>
            <t>
If the initial offer contained "a=sendonly" CLUE-controlled media lines lines, the
recipient MAY <bcp14>MAY</bcp14> include corresponding "a=recvonly" CLUE-controlled media lines,
up to the maximum number of Capture Encodings it wishes to receive.
Alternatively, it MAY <bcp14>MAY</bcp14> wait until CLUE protocol negotiation has completed
before including these lines in a new offer/answer exchange - -- see
<xref target="sec.coordination" /> format="default"/> for recommendations. The recipient MUST <bcp14>MUST</bcp14> set
the direction of the corresponding "m=" lines of any remaining "a=sendonly"
CLUE-controlled media lines received in the offer to "a=inactive" "a=inactive".
            </t>
          </section>
          <section title="Negotiating non-CLUE controlled media"> numbered="true" toc="default">
            <name>Negotiating Non-CLUE-controlled Media</name>
            <t>
A CLUE-controlled device implementation MAY <bcp14>MAY</bcp14> 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 <bcp14>MAY</bcp14> wish to suppress initial media, only
providing media once the final, CLUE-controlled streams have been negotiated.
            </t>
            <t>
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.
            </t>
            <t>
If the answerer chooses to send initial non-CLUE-controlled media in a
CLUE-enabled call, <xref target="sec.clue-media" /> format="default"/> addresses the need to
disable it once the CLUE-controlled media is fully negotiated.
            </t>
          </section>
        </section>
        <section title="Processing numbered="true" toc="default">
          <name>Processing the initial Initial Offer/Answer negotiation"> Negotiation</name>
          <t>
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
negotiated, and the call is now CLUE-enabled. CLUE enabled. If not not, then the call is not
CLUE-enabled.
CLUE enabled.
          </t>
          <section title="Successful numbered="true" toc="default">
            <name>Successful CLUE negotiation"> Negotiation</name>
            <t>
In the event of successful CLUE-enablement CLUE enablement of the call, devices MUST <bcp14>MUST</bcp14> now begin
negotiation of the CLUE channel, channel; see
<xref target="I-D.ietf-clue-datachannel" /> target="RFC8850" format="default"/> for negotiation details. If
negotiation is successful, the sending of <xref target="I-D.ietf-clue-protocol"> CLUE protocol</xref> protocol messages <xref target="RFC8847" format="default"/> can begin.
            </t>
            <t>
A CLUE-capable device MAY <bcp14>MAY</bcp14> 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 <bcp14>MUST</bcp14> still be sent and received as normal).
However, a CLUE-capable device MUST <bcp14>MUST</bcp14> still be prepared to receive media on
non-CLUE-controlled media lines that have been successfully negotiated as
defined in <xref target="RFC3264" />. format="default"/>.
            </t>
            <t>
If either side of the call wishes to add additional CLUE-controlled "m=" lines
to send or receive CLUE-controlled media media, they MAY <bcp14>MAY</bcp14> now send a SIP request with
a new SDP offer following the normal rules of SDP offer/answer and any
negotiated extensions.
            </t>
          </section>
          <section title="CLUE negotiation failure"> numbered="true" toc="default">
            <name>CLUE Negotiation Failure</name>
            <t>
In the event that the negotiation of CLUE fails and the call is not
CLUE-enabled
CLUE enabled once the initial offer/answer negotiation completes completes, then CLUE is
not in use in the call. The CLUE-capable devices MUST <bcp14>MUST</bcp14> either revert to
non-CLUE behaviour behavior or terminate the call.
            </t>
          </section>
        </section>
        <section title="Modifying the session">
                <section title="Adding and removing CLUE-controlled media" anchor="sec.clue-media"> numbered="true" toc="default">
          <name>Modifying the Session</name>
          <section anchor="sec.clue-media" numbered="true" toc="default">
            <name>Adding and Removing CLUE-Controlled Media</name>
            <t>
Subsequent offer/answer exchanges MAY <bcp14>MAY</bcp14> add additional "m=" lines for
CLUE-controlled media, media or activate or deactivate existing "m=" lines per the
standard SDP mechanisms.
            </t>
            <t>
In most cases 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 <bcp14>MAY</bcp14> delay adding
"a=recvonly" CLUE-controlled "m=" lines until after CLUE protocol negotiation
completes - -- see <xref target="sec.coordination" /> format="default"/> for recommendations.
            </t>
            <t>
Once CLUE media has been successfully negotiated negotiated, devices SHOULD <bcp14>SHOULD</bcp14> 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,
exchange or may be incorporated into one that is part of the CLUE
negotiation.
            </t>
          </section>
          <section title="Enabling numbered="true" toc="default">
            <name>Enabling CLUE mid-call"> Mid-Call</name>
            <t>
A CLUE-capable device that receives an initial SDP offer from a non-CLUE
device SHOULD <bcp14>SHOULD</bcp14> include a new data channel "m=" line and corresponding CLUE
group in any subsequent offers it sends, to indicate that it is CLUE-capable. CLUE capable.
            </t>
            <t>
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 group, then the call is now
CLUE-enabled;
CLUE enabled; negotiation of the data channel and subsequently the CLUE
protocol begins.
            </t>
          </section>
          <section title="Disabling CLUE mid-call" anchor="sec.clue-disable">
                    <t> anchor="sec.clue-disable" numbered="true" toc="default">
            <name>Disabling CLUE Mid-Call</name>
            <t>

<!--[rfced] Please let us know how to update the following reference
as Section 5.2.4 does not exist in draft-ietf-mmusic-data-channel-sdpneg
(RFC-to-be 8864). Perhaps Section 6.6.1 (Closing a Data Channel) was
intended?

Original:
    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 <xref target="I-D.ietf-mmusic-data-channel-sdpneg"/>:
   [I-D.ietf-mmusic-data-channel-sdpneg]: sending a new SDP offer/answer
   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
-->
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
<xref target="RFC8864" sectionFormat="of" section="5.2.4"/>: sending
a new SDP offer/answer exchange and subsequent SCTP Stream Sequence Number (SSN) reset for the CLUE
channel. It <bcp14>MUST</bcp14> also remove the CLUE group. Without the CLUE group, any "m="
lines that were previously CLUE controlled no longer are; implementations <bcp14>MAY</bcp14>
disable them by setting their ports to 0 or MAY <bcp14>MAY</bcp14> continue to use them - -- in the
latter case case, how they are used is outside the scope of this document.
            </t>
            <t>
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, negotiated and/or one side did not include the data channel in
the CLUE group 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 CLUE controlled, and the implementation MAY <bcp14>MAY</bcp14> 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 semantic, CLUE protocol messages MUST <bcp14>MUST</bcp14> no longer be sent.
            </t>
          </section>
          <section title="CLUE protocol failure mid-call"> numbered="true" toc="default">
            <name>CLUE Protocol Failure Mid-Call</name>
            <t>
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:
                      <list style='symbols'>
                        <t>
            </t>
            <ul spacing="normal">
              <li>
The CLUE data channel terminates, either gracefully or ungracefully, without
any corresponding SDP renegotiation.
                        </t>
                        <t>
                        </li>
              <li>
A channel error of the CLUE protocol causes it to return to the IDLE state as
defined in Section 6. of <xref target="I-D.ietf-clue-protocol" />.
                        </t>
                      </list>
                    </t> target="RFC8847" sectionFormat="of" section="6"/>.
                        </li>
            </ul>
            <t>
In this circumstance circumstance, implementations SHOULD <bcp14>SHOULD</bcp14> 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
<xref target="sec.clue-disable" />. format="default"/>. Implementations MAY <bcp14>MAY</bcp14> choose to send such
an SDP request to disable CLUE immediately or MAY <bcp14>MAY</bcp14> continue on in a
call-preservation mode.
            </t>
          </section>
        </section>
      </section>
    </section>
    <section title="Interaction anchor="sec.coordination" numbered="true" toc="default">
      <name>Interaction of the CLUE protocol Protocol and SDP negotiations" anchor="sec.coordination"> Negotiations</name>
      <t>
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 information, and
additional constraints. As a result 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.
      </t>
      <t>
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.
      </t>
      <section title="Independence numbered="true" toc="default">
        <name>Independence of SDP and CLUE negotiation"> Negotiation</name>
        <t>
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 en route
by middle boxes, 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, and likewise answer; likewise, the implementation’s 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).
        </t>
        <t>
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, information or receive a CLUE 'configure' message specifying a
Capture Encoding for which the far end has not negotiated a media stream in
SDP.

        </t>
        <t>
CLUE messages contain an &lt;encID&gt; (in encodingIDList) or
&lt;encodingID&gt; (in captureEncodingType), which is used to identify a
specific encoding or captureEncoding in SDP; see
<xref target="I-D.ietf-clue-data-model-schema" /> target="RFC8846" format="default"/> 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
such, the sender of the CLUE message MAY <bcp14>MAY</bcp14> include an &lt;encID&gt; which that does
not currently match a CLUE-controlled "m=" line label in SDP; A a CLUE-capable
implementation MUST NOT <bcp14>MUST NOT</bcp14> reject a CLUE protocol message solely because it
contains &lt;encID&gt; elements that do not match a label in SDP.
        </t>
        <t>
The current state of the CLUE participant Participant or Media Provider/Consumer
state machines do does not affect compliance with any of the normative language of
<xref target="RFC3264" />. format="default"/>. That is, they MUST NOT <bcp14>MUST NOT</bcp14> delay an ongoing SDP
exchange as part of a SIP server or client transaction; an implementation MUST
NOT <bcp14>MUST
NOT</bcp14> delay an SDP exchange while waiting for CLUE negotiation to complete or
for a 'configure' message to arrive.
        </t>
        <t>
Similarly, a device in a CLUE-enabled call MUST NOT <bcp14>MUST NOT</bcp14> 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.
        </t>
        <t>
A device with the CLUE Participant state machine in the ACTIVE state
MAY
<bcp14>MAY</bcp14> 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
<xref target="I-D.ietf-clue-protocol"/> target="RFC8847" format="default"/> for CLUE state machine specifics.
Similarly, a device MAY <bcp14>MAY</bcp14> choose to delay initiating a new SDP exchange based on
the state of their CLUE state machines.
        </t>
      </section>
      <section title="Constraints numbered="true" toc="default">
        <name>Constraints on sending media"> Sending Media</name>
        <t>
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 <bcp14>MUST
NOT</bcp14> be sent unless it has been negotiated in both CLUE and SDP: an
implementation MUST NOT <bcp14>MUST NOT</bcp14> 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.
        </t>
      </section>
      <section title="Recommendations numbered="true" toc="default">
        <name>Recommendations for operating Operating with non-atomic operations"> Non-atomic Operations</name>
        <t>
CLUE-capable devices MUST <bcp14>MUST</bcp14> 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 transitory, a device MUST <bcp14>MUST</bcp14> be able to cope
with such mismatches remaining indefinitely. However, this document makes some
recommendations on message ordering for these non-atomic transitions.
        </t>
        <t>
CLUE-capable devices MUST <bcp14>MUST</bcp14> 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.
        </t>
        <t>
Generally, implementations that receive messages for which they have 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 inclue 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.
        </t>
        <t>
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 <bcp14>SHOULD</bcp14> 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, information and
reducing the number of new SDP offers required.
        </t>
      </section>
    </section>
    <section title="Interaction anchor="sec.capture-id" numbered="true" toc="default">
      <name>Interaction of the CLUE protocol Protocol and RTP/RTCP CaptureID" anchor="sec.capture-id"> CaptureID</name>
      <t>
The CLUE Framework <xref target="I-D.ietf-clue-framework">CLUE Framework</xref> target="RFC8845" format="default"/> allows for
Multiple Content Captures (MCCs): Captures which that contain multiple source
Captures, whether composited into a single stream or switched based on some
metric.
      </t>
      <t>
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 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, geometry or take some other action based
on the original Capture information.
      </t>
      <t>
To do this, <xref target="I-D.ietf-clue-rtp-mapping" /> target="RFC8849" format="default"/> 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
<bcp14>MUST</bcp14> send the CaptureID in both RTP and RTCP, as described
in the mapping document.
      </t>
      <section title="CaptureID reception numbered="true" toc="default">
        <name>CaptureID Reception during MCC redefinition"> Redefinition</name>
        <t>
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 defined, there
is an intrinsic race condition in cases in which where the contents of an MCC are
redefined.
        </t>
        <t>
When a Media Provider redefines an MCC which 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. recipient will either lead or lag.
As such, a Media Consumer MUST NOT <bcp14>MUST NOT</bcp14> 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):
           <list style='symbols'>
              <t>
        </t>
        <ul spacing="normal">
          <li>
Receiving RTP or RTCP containing a CaptureID when it's the most recently processed
'advertisement' message message, which means that none are expected.
              </t>
              <t>
              </li>
          <li>
Receiving RTP or RTCP without CaptureIDs when it's the most recently processed
'advertisement' message message, which means that media CaptureIDs are expected.
              </t>
              <t>
              </li>
          <li>
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.
              </t>
              <t>
              </li>
          <li>
Receiving a CaptureID in RTP or RTCP for a Capture not defined in the most
recently processed 'advertisement' message.
              </t>
           </list>
        </t>
              </li>
        </ul>
      </section>
    </section>
    <section title="Multiplexing anchor="sec-bundle" numbered="true" toc="default">
      <name>Multiplexing of CLUE-controlled media using BUNDLE" anchor="sec-bundle"> CLUE-Controlled Media Using BUNDLE</name>
      <section title="Overview"> numbered="true" toc="default">
        <name>Overview</name>
        <t>
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 <xref target="RFC8445">ICE candidates</xref>, target="RFC8445" format="default"/>, etc.
        </t>
        <t>
The <xref target="I-D.ietf-mmusic-sdp-bundle-negotiation">BUNDLE</xref> BUNDLE extension <xref target="RFC8843" format="default"/> 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.
        </t>
        <t>
While CLUE-capable devices MAY <bcp14>MAY</bcp14> support the BUNDLE extension for this purpose purpose,
supporting the extension is not mandatory for a device to be CLUE-compliant. CLUE compliant.
        </t>
        <t>
A CLUE-capable device that supports BUNDLE SHOULD <bcp14>SHOULD</bcp14> also support rtcp-mux
<xref target="RFC5761">rtcp-mux</xref>. target="RFC5761" format="default"/>. However, a CLUE-capable device that
supports rtcp-mux may or may not support BUNDLE.
        </t>
      </section>
      <section title="Usage numbered="true" toc="default">
        <name>Usage of BUNDLE with CLUE"> CLUE</name>
        <t>
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.
        </t>
        <section title="Generating numbered="true" toc="default">
          <name>Generating the Initial Offer"> Offer</name>
          <t>
BUNDLE mandates that the initial SDP offer MUST <bcp14>MUST</bcp14> use a unique address for each
"m=" line with a non-zero port. Because CLUE implementations generally will
not include CLUE-controlled media lines 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.
          </t>
          <t>
An implementation that does include CLUE-controlled media lines in its initial
SDP offer while also using BUNDLE must take care to avoid renderings 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.
          </t>
        </section>
        <section title="Multiplexing numbered="true" toc="default">
          <name>Multiplexing of the data channel Data Channel and RTP media"> Media</name>
          <t>
BUNDLE-supporting CLUE-capable devices MAY <bcp14>MAY</bcp14> include the data channel in the
same BUNDLE group as RTP media. In this case case, the device MUST <bcp14>MUST</bcp14> be able to
demultiplex the various transports - -- see section 9.2 of the
<xref target="I-D.ietf-mmusic-sdp-bundle-negotiation">BUNDLE draft</xref>. BUNDLE specification (<xref target="RFC8843" sectionFormat="of" section="9.2" />). If
the BUNDLE group includes other protocols other than the data channel transported
via DTLS DTLS, the device MUST <bcp14>MUST</bcp14> also be able to differentiate the various protocols.
          </t>
        </section>
      </section>
    </section>
    <section title="Example: anchor="sec-clueexample" numbered="true" toc="default">
      <name>Example: A call Call between two CLUE-capable Endpoints" anchor="sec-clueexample"> Two CLUE-Capable Endpoints</name>
      <t>
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.
      </t>
      <t>
To manage the size of this section section, the SDP snippets only illustrate video "m="
lines. SIP ACKs are not always discussed. Note that BUNDLE is not in use.
      </t>
      <t>
      <vspace blankLines="100" />
         <figure>
            <artwork>
            <![CDATA[
      <artwork name="" type="" align="left" alt=""><![CDATA[
              +----------+                      +-----------+
              |  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
            ]]>
            </artwork>
         </figure>
      </t> ]]></artwork>

      <t>
In SIP INVITE 1, Alice sends Bob a SIP INVITE including in the SDP body with the
basic audio and video capabilities and the data channel included in the SIP body as per
<xref target="I-D.ietf-mmusic-sctp-sdp"/>. target="RFC8841" format="default"/>. 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,
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.
         <figure>
            <artwork>
               <![CDATA[
      </t>
 <sourcecode type="sdp"><![CDATA[   ...
   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
               ]]>
            </artwork>
         </figure>
      </t>   ]]></sourcecode>
      <t>
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 similarity, no SDP
snippet is shown here. Bob wishes to receive initial media, media and so 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.
      </t>
      <t>
With the successful initial SDP Offer/Answer exchange complete complete, Alice and Bob
are also free to negotiate the CLUE data channel. This is illustrated as CLUE
DATA CHANNEL ESTABLISHED.
      </t>
      <t>
Once the data channel is established established, CLUE protocol negotiation begins. In this
case
case, Bob was the DTLS client (sending a=active "a=active" in his SDP answer) and hence is
the CLUE Channel Initiator and Initiator. He sends a CLUE OPTIONS message describing his
version support. On receiving that message message, Alice sends her corresponding CLUE
OPTIONS RESPONSE.
      </t>
      <t>
With the OPTIONS phase complete 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. systems with one or two screens. All of these Captures are in a single Capture
Scene, with suitable Capture Scene Views to that tell Bob that he should either
subscribe to the three static Captures, the two switched Captures Captures, or the one
switched Capture. Alice has no simultaneity constraints, so includes all six
Captures are included in one simultaneous set. Finally, Alice includes an Encoding Group
with three Encoding IDs: "enc1", "enc2" "enc2", and "enc3". These Encoding IDs aren't
currently valid, valid but will match the next SDP offer she sends.
      </t>
      <t>
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 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 set, he must
send an explicit 'ack' message (CLUE ACK 1) to signal receipt of CLUE
ADVERTISEMENT 1.
      </t>
      <t>
Bob also sends his CLUE 'advertisement' message (CLUE ADVERTISEMENT 2) - --
though the diagram shows that this occurs after Alice sends CLUE ADVERTISEMENT
1
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 that tell Alice that she should either subscribe to either the two
static Captures, 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".
      </t>
      <t>
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
information; instead, she sends an 'ack' message (CLUE ACK 2).
      </t>
      <t>
Both sides have now sent their CLUE 'advertisement' messages messages, and an SDP
exchange is required to negotiate Encodings. For simplicity, in this case case,
Alice is shown sending an INVITE with a new offer; in many implementations implementations,
both sides might send an INVITE, which would be resolved by use of the 491
Request Pending resolution mechanism from <xref target="RFC3261"/>. target="RFC3261" format="default"/>.
      </t>
      <t>
Alice now sends SIP INVITE 2. She maintains the sendrecv audio, video 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
channel, and the video "m=" lines are shown below:
         <figure>
            <artwork>
               <![CDATA[
      </t>
     <sourcecode type="sdp"><![CDATA[   ...
   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
               ]]>
            </artwork>
        </figure>
      </t>  ]]></sourcecode>
      <t>
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
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".
      </t>
      <t>
Bob also sends his SDP answer as part of SIP 200 OK 2. Alongside his original
audio, video video, and CLUE "m=" lines lines, he includes three additional "m=" lines
corresponding to the three added by Alice; 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):
         <figure>
            <artwork>
               <![CDATA[
      </t>
    <sourcecode type="sdp"><![CDATA[   ...
   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
               ]]>
            </artwork>
         </figure>
      </t>  ]]></sourcecode>
      <t>
Alice receives Bob's message CLUE CONFIGURE 1 message and sends CLUE CONFIGURE
RESPONSE 1 to ack acknowledge its reception. She does not yet send the Capture Encodings
specified, because at this stage stage, she hasn't processed Bob's answer SDP and so thus
hasn't negotiated the ability for Bob to receive these streams.
      </t>
      <t>
On receiving SIP 200 OK 2 from Bob 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.
      </t>
      <t>
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 such, Bob now sends SIP
INVITE 3 to generate a new offer. Along with all the streams from SIP 200 OK 2 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):
         <figure>
            <artwork>
               <![CDATA[
      </t>
     <sourcecode type="sdp"><![CDATA[   ...
   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
               ]]>
            </artwork>
         </figure>
      </t> ]]></sourcecode>
      <t>
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 2, she
requests the two static Captures from Bob, Bob to be sent on Encodings "foo" and
"bar".
      </t>
      <t>
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. data channel. Alice also now 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):
         <figure>
            <artwork>
               <![CDATA[
      </t>
    <sourcecode type="sdp"><![CDATA[   ...
   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
               ]]>
         </artwork>
      </figure>
      </t>  ]]></sourcecode>
      <t>
Bob receives Alice's message CLUE CONFIGURE 2 message and sends CLUE CONFIGURE
RESPONSE 2 to ack 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.
      </t>
      <t>
Finally, on receiving SIP 200 OK 3 3, Bob is now able to send the two streams of
video Alice requested - -- this is illustrated as MEDIA 3.
      </t>
      <t>
Both sides of the call are now sending multiple video streams with their
sources defined via CLUE negotiation. As the call progresses progresses, either side can
send a new 'advertisement' or 'configure' message or the new SDP offer/answers offers/answers to
add, remove remove, or change what they have available or want to receive.
      </t>
    </section>
    <section title="Example: anchor="sec-nonclueexample" numbered="true" toc="default">
      <name>Example: A call Call between a CLUE-capable CLUE-Capable and non-CLUE Endpoint" anchor="sec-nonclueexample"> Non-CLUE Endpoint</name>
      <t>
In this brief example example, Alice is a CLUE-capable Endpoint making a call to Bob,
who is not CLUE-capable (i.e. CLUE capable (i.e., is not able to use the CLUE protocol).
      </t>
         <figure>
            <artwork>
               <![CDATA[
      <artwork name="" type="" align="left" alt=""><![CDATA[
      +----------+                      +-----------+
      |  Alice   |                      |    Bob    |
      |          |                      |           |
      +----+-----+                      +-----+-----+
           |                                  |
           |                                  |
           | SIP INVITE 1                     |
           |--------------------------------->|
           |                                  |
           |                                  |
           |                         200 0K 1 |
           |<---------------------------------|
           |                                  |
           |                                  |
           | SIP ACK 1                        |
           |--------------------------------->|
           |                                  |
           |                                  |
           |                                  |
           |<########### MEDIA 1 ############>|
           |   1 video A->B, 1 video B->A     |
           |<################################>|
           |                                  |
           |                                  |
           |                                  |
           |                                  |
           v                                  v
               ]]>
            </artwork>
         </figure> ]]></artwork>
      <t>
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 in the SDP body as per
<xref target="I-D.ietf-mmusic-sctp-sdp"/>. target="RFC8841" format="default"/>. 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,
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.
         <figure>
            <artwork>
               <![CDATA[
      </t>
    <sourcecode type="sdp"><![CDATA[   ...
   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
               ]]>
            </artwork>
         </figure>
      </t>  ]]></sourcecode>
      <t>
Bob is not CLUE-capable, 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 1, he
responds with an answer with that includes audio and video, but with the data channel
zeroed.
      </t>
      <t>
From the lack of a CLUE group 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 At this point point, Alice begins to send her
fallback video: in this case case, it's likely a switched view from whichever camera
shows the current loudest participant on her side.
      </t>
    </section>

    <section title="Acknowledgements">
      <t>
Besides the authors, the team focusing on this draft consists of:
Roni Even,
Simon Pietro-Romano,
Roberta Presta.
      </t>
      <t>
Christian Groves, Jonathan Lennox and Adam Roach have contributed detailed
comments and suggestions.
      </t>
   </section>

   <section title="IANA Considerations">
      <section title="New numbered="true" toc="default">
      <name>IANA Considerations</name>
      <section numbered="true" toc="default">
        <name>New SDP Grouping Framework Attribute"> Attribute</name>
        <t>
This document registers the following semantics with IANA in the
"Semantics for the "group" 'group' SDP Attribute" subregistry (under the
"Session Description Protocol (SDP) Parameters" registry registry) per
<xref target="RFC5888"/>: target="RFC5888" format="default"/>:
        </t>
        <figure>
            <artwork>
            <![CDATA[
Semantics                              Token   Reference
-------------------------------------  ------  ---------
CLUE-controlled m-line                 CLUE    [this draft]
            ]]>
            </artwork>
         </figure>
      </section>
      <section title="New

<table anchor="IANA_table_1" align="left">
  <thead>
    <tr>
      <th align='center'>Semantics</th>
      <th align='center'>Token</th>
      <th align='center'>Mux Category</th>
      <th align='center'>Reference</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td align="center">CLUE-controlled m-line</td>
      <td align="left">CLUE</td>
      <td align="left">NORMAL</td>
      <td align="left">RFC 8848</td>
  </tr>
  </tbody>
</table>
      </section>
      <section numbered="true" toc="default">
        <name>New SIP Media Feature Tag"> Tag</name>
        <t>
This specification registers a new media feature tag in the SIP
<xref target="RFC3261">SIP</xref> target="RFC3261" format="default"/> tree per the procedures defined in
<xref target="RFC2506"/> target="RFC2506" format="default"/> and <xref target="RFC3840"/>. target="RFC3840" format="default"/>.
        </t>
         <t>
Media

<dl newline="false" spacing="normal">
<dt>Media feature tag name: sip.clue
         </t>
         <t>
ASN.1 Identifier: [to be assigned]
         </t>
         <t>
Summary name:</dt><dd>sip.clue</dd>

<dt>ASN.1 Identifier:</dt><dd>30</dd>

<dt>Summary of the media feature indicated by this tag: This tag:</dt><dd>This feature tag indicates
that the device supports CLUE-controlled media.
         </t>
         <t>
Values media.</dd>

<dt>Values appropriate for use with this feature tag: Boolean.
         </t>
         <t>
The tag:</dt><dd>Boolean.</dd>

<dt>The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation mechanisms:
         </t>
         <t>
This mechanisms:</dt>
<dd><t><br/>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.
         </t>
         <t>
Related streams.</t></dd>

<dt>Related standards or documents: [this draft]
         </t>
         <t>
Security Considerations: Security documents:</dt><dd>RFC 8848</dd>

<dt>Security Considerations:</dt><dd>Security considerations for this media
feature tag are discussed in <xref target="sec.security" /> format="default"/> of
[this draft].
         </t>
         <t>
Name(s)
RFC 8848.</dd>

<dt>Name(s) &amp; email address(es) of person(s) to contact for further
information:
         </t>
         <t><list style='symbols'>
           <t>
Internet
information:</dt><dd>Internet Engineering Steering Group: iesg@ietf.org
           </t>
         </list></t>
         <t>
Intended usage: COMMON
         </t> Group &lt;iesg@ietf.org&gt;</dd>

<dt>Intended usage:</dt><dd>COMMON</dd>
        </dl>
      </section>
    </section>
    <section title="Security Considerations" anchor="sec.security"> anchor="sec.security" numbered="true" toc="default">
      <name>Security Considerations</name>
      <t>
CLUE makes use of a number of protocols and mechanisms, either defined by CLUE
or long-standing. The security considerations Security Considerations section of the
CLUE Framework document <xref target="I-D.ietf-clue-framework">CLUE Framework</xref> target="RFC8845" format="default"/> addresses the
need to secure these mechanisms by following the recommendations of the
individual protocols.
      </t>
      <t>
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.
      </t>
      <t>
This attack can be prevented by ensuring that the media recipient intends to
receive the media packets. As such such, all CLUE-capable devices MUST <bcp14>MUST</bcp14> support key
negotiation and receiver intent assurance via DTLS / Secure Real-time Transport Protocol (SRTP) <xref target="RFC5763">DTLS-SRTP</xref> target="RFC5763" format="default"/> on CLUE-controlled RTP "m=" lines, and
MUST they
<bcp14>MUST</bcp14> 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 <xref target="RFC3711">SRTP</xref>. target="RFC3711" format="default"/>. CLUE implementations
MAY
<bcp14>MAY</bcp14> 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.
      </t>
      <t>
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 device; this 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 <bcp14>SHOULD</bcp14> always be encrypted using
TLS <xref target="RFC5630">TLS</xref>. target="RFC5630" format="default"/>.
      </t>
      <t>
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 <xref target="I-D.ietf-clue-protocol">CLUE Framework</xref> target="RFC8847" format="default"/> provides details
of
about these issues and how to mitigate them.
      </t>
    </section>

   <section title="Change History">
    <t>
    Note to RFC Editor: please remove this section prior to publication
    </t>
      <t><list style='hanging'>
        <t hangText="-15:"> Revision by Rob Hanton
        <list style='symbols'>
          <t>
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
  </middle>
  <back>

    <references>

<!-- [rfced] May we alphabetically sort the SDP and CLUE message. In contrast, changed references (using
sortRefs="true")?  (The instruction to sort the use of 'EncID' references was
absent in a CLUE CONFIGURE message to a MUST as that is defined by the far end and so there is no way submitted v2 XML file.) -->

      <name>References</name>
      <references>
        <name>Normative References</name>

<!--draft-ietf-clue-framework-25 is 8845 -->
<reference anchor='RFC8845' target='https://www.rfc-editor.org/info/rfc8845'>
<front>
<title>Framework for the sending of the CONFIGURE to anticipate it.
          </t>
          <t>
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.
          </t>
          <t>
Updated all instances of advertisment, configure and ack messages throughout to match the styling of the protocol document
          </t>
          <t>
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).
          </t>
          <t>
Removed OID value for "sip.clue" media feature tag pending its actual assignment on registration, leaving a placeholder
          </t>
          <t>
All lower-case uses of 'must', 'should' and 'may' reviewed and a few made normative
          </t>
          <t>
Fixed various spelling mistakes, clarified grammar, and fixed a copy/paste error.
          </t>
          <t>
Updated boilerplate to RFC 8174
          </t>
          <t>
Some informative references moved to normative.
          </t>
        </list>
        </t>
        <t hangText="-14:"> Revision by Rob Hanton
        <list style='symbols'>
          <t>
Reference to RFC5245 updated to RFC8445
          </t>
          <t>
Updated my name to reflect surname change (Hansen to Hanton).
          </t>
          <t>
Reviewed recent changes to clue protocol document and concluded that none
affected this document
          </t>
          <t>
Added recommendation that the SDP O/A spec and clue protocol be read prior to
this document
          </t>
          <t>
Several acronyms expanded at the point of initial use
          </t>
          <t>
Some unnecessary normative language replaced with prose
          </t>
        </list>
        </t>
        <t hangText="-13:"> Revision by Rob Hansen
        <list style='symbols'>
          <t>
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.
          </t>
          <t>
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).
          </t>
          <t>
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).
          </t>
          <t>
Order of authors revised to reflect updates since Jan 2014.
          </t>
        </list>
        </t>
        <t hangText="-12:"> Revision by Rob Hansen
        <list style='symbols'>
          <t>
Title change to expand and elucidate our totally-not-contrived acronym
          </t>
          <t>
Explicit reference to RFC3840 added when first mentioning media feature tags
          </t>
          <t>
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).
          </t>
          <t>
'MUST' in opening paragraph of 4.2 changed from normative 'MUST' to logical 'must'
          </t>
          <t>
Per his request, removed Cristian's company affiliation and changed his email address
          </t>
          <t>
Clarified that an implementation that chooses not to send media during the initial negotiation process must still send RTCP as normal
          </t>
          <t>
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.
          </t>
          <t>
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
          </t>
          <t>
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)
          </t>
          <t>
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).
          </t>
          <t>
Page break added before the big ladder diagram in the example
          </t>
          <t>
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.
          </t>
          <t>
Have removed all language that referenced the possibility of having multiple CLUE groups
          </t>
          <t>
Removed names appearing in the authors list from the acknowledgements
          </t>
          <t>
Changed the contact for the IANA registration to iesg@ietf.org
          </t>
          <t>
Security section updated to clarify that DTLS-SRTP must be supported (as opposed to DTLS) and removed the reference to RFC7202.
          </t>
          <t>
Other syntactic tweaks based on Paul and Adam's feedback
          </t>
        </list>
        </t>
        <t hangText="-11:"> Revision by Rob Hansen
        <list style='symbols'>
          <t>
Some informative references added for SIP and SDP.
          </t>
          <t>
'a=mid' lines added to example m-lines with port 0, per RFC5888 section 6.
          </t>
          <t>
Instace of 'must' changed to normative 'MUST', along with various minor
clarifications and corrections.
          </t>
          <t>
Abstract made standalone without citations, per RFC7322 section 4.3.
          </t>
          <t>
RFC editor note added to remove this section.
          </t>
        </list>
      </t>
        <t hangText="-10:"> Revision by Rob Hansen
        <list style='symbols'>
          <t>
Changes to draft-ietf-clue-protocol between 07 and 11 reviewed to ensure
compatibility between documents has been maintained.
          </t>
          <t>
Expanded the portion of the document related to fingerprinting with info on
the CLUE channel as well as SIP.
          </t>
        </list>
      </t>
        <t hangText="-09:"> Revision by Rob Hansen
        <list style='symbols'>
          <t>
A few minor spelling tweaks
          </t>
          <t>
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.
          </t>
        </list>
      </t>
        <t hangText="-08:"> Revision by Rob Hansen
        <list style='symbols'>
          <t>
Spelling and grammar fixes from Paul and Christian gratefully adopted
          </t>
          <t>
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.
          </t>
          <t>
Made a number of fixes to the example call flow to better reflect the
recommendations in the document.
          </t>
        </list>
      </t>
      <t hangText="-07:"> Revision by Rob Hansen
        <list style='symbols'>
          <t>
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.
          </t>
          <t>
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.
          </t>
          <t>
Added OPTIONS negotiation to the example flow, and revised the flow to ensure
it matched protocol document.
          </t>
          <t>
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 list of things that should be sent as normal, in
line with a=inactive.
          </t>
          <t>
Make explicit that m=recvonly lines don't need to have a label, as only
m=sendonly lines are referenced by CLUE protocol messages.
          </t>
          <t>
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'.
          </t>
          <t>
General corrections to grammar, spelling and readability based on Christian,
Paul and Mark; in many cases suggested text was gratefully accepted.
          </t>
        </list>
      </t>
      <t hangText="-06:"> Revision by Rob Hansen
        <list style='symbols'>
          <t>
State machine interactions updated to match versions in -04 of protocol doc.
          </t>
          <t>
Section on encoding updated to specify both encID and encodingID from data
model doc.
          </t>
          <t>
Removed the limitations on describing H264 encoding limits using SDP syntax
as an open issue.
          </t>
          <t>
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.
          </t>
          <t>
Terminology reference to framework doc reinforced. All terminology that
duplicates framework removed. All text updated with capitalisation that
matches framework document's terminology.
          </t>
          <t>
SDP example syntax updated to match that of ietf-clue-datachannel
and hence ietf-mmusic-data-channel-sdpneg.
          </t>
        </list>
      </t>
      <t hangText="-05:"> Revision by Rob Hansen
        <list style='symbols'>
          <t>
SRTP/DTLS made mandatory for CLUE-controlled media lines.
          </t>
          <t>
IANA consideration section added (text as proposed by Christian Groves).
          </t>
          <t>
Includes provision for dependent streams on seperate "m" lines having the same
encID as their parent "m" line.
          </t>
          <t>
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.
          </t>
          <t>
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.
          </t>
          <t>
Other minor syntax improvements.
          </t>
        </list>
      </t>
      <t hangText="-04:"> Revision by Rob Hansen
        <list style='symbols'>
          <t>
Updated DTLS/SCTP channel syntax in examples to fix errors and match latest
format defined in draft-ietf-mmusic-sctp-sdp-07.
          </t>
          <t>
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.
          </t>
          <t>
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.
          </t>
          <t>
Added reminder on IANA section on registering grouping semantic and media
feature tag, removed the less formal sections that did the same job.
          </t>
          <t>
Fixed and clarified issues raised by Christian's document review.
          </t>
          <t>
Added a number of security considerations.
          </t>
        </list>
      </t>
      <t hangText="-03:"> Revision by Rob Hansen
        <list style='symbols'>
          <t>
Clarified text on not rejecting messages because they contain unknown encIDs.
          </t>
          <t>
Removed normative language in section on accepting/rejecting
non-CLUE-controlled media in the initial answer.
          </t>
          <t>
Example SDP updated to include the data channel "m" lines.
          </t>
          <t>
Example call flow updated to show disablement of non-CLUE-controlled media
once CLUE-controlled media is flowing.
          </t>
        </list>
      </t>
        <t hangText="-02:"> Revision by Rob Hansen
          <list style='symbols'>
            <t>
Added section on not accepting non-CLUE-controlled "m" lines in the initial
answer when CLUE is to be negotiated.
            </t>
            <t>
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'.
            </t>
            <t>
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 &lt;encodingIDList&gt;.)
            </t>
            <t>
Added a section on the specifics of relating encodings in SDP to &lt;encID&gt;
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 &lt;encID&gt; contents).
            </t>
            <t>
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.
            </t>
            <t>
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.
            </t>
            <t>
Cleaned up some 'todo' text.
            </t>
          </list>
        </t>
        <t hangText="-01:"> Revision by Rob Hansen
          <list style='symbols'>
            <t>
Revised terminology - removed the term 'CLUE-enabled' device as insufficiently
distinct from 'CLUE-capable' and instead added a term for 'CLUE-enabled'
calls.
            </t>
            <t>
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.
            </t>
            <t>
Changed 'sip.telepresence' to 'sip.clue' and 'TELEPRESENCE' grouping semantic
back to CLUE.
            </t>
            <t>
Made it mandatory to have exactly one mid corresponding to a data channel in a
CLUE group
            </t>
            <t>
Forbade having multiple CLUE groups unless a specification for doing so is
published.
            </t>
            <t>
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.
            </t>
            <t>
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.
            </t>
          </list>
        </t>
            <t hangText="-00:"> Revision by Rob Hansen
              <list style='symbols'>
               <t>
Submitted as -00 working group document
               </t>
              </list>
             </t>
            <t hangText="draft-kyzivat-08:"> Revisions by Rob Hansen
              <list style='symbols'>
               <t>
Added media feature tag for CLUE support ('sip.telepresence')
               </t>
               <t>
Changed grouping semantic from 'CLUE' to 'TELEPRESENCE'
               </t>
               <t>
Restructured document to be more centred on the grouping semantic and its use
with O/A
               </t>
               <t>
Lots of additional text on usage of the grouping semantic
               </t>
               <t>
Stricter definition of CLUE-controlled m lines and how they work
               </t>
               <t>
Some additional text on defining what happens when CLUE supports is added or
removed
               </t>
               <t>
Added details on when to not send RTCP for CLUE-controlled "m" lines.
               </t>
               <t>
Added a section on using BUNDLE with CLUE
               </t>
                <t>
Updated data channel references to point at new WG document rather than
indivual draft
               </t>
              </list>
             </t>
       <t hangText="draft-kyzivat-07:"> Revisions by Rob Hansen
          <list style='symbols'>
           <t>
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
           </t>
           <t>
Added normative language on the setting up of a CLUE call, and added sections
on mid-call changes to the
CLUE status.
           </t>
           <t>
Added references to <xref target="I-D.ietf-clue-datachannel" Telepresence Multi-Streams</title>
<author initials='M' surname='Duckworth' fullname='Mark Duckworth' role='editor'>
    <organization /> where
appropriate.
           </t>
           <t>
Added some terminology for various types of CLUE and non-CLUE states of
operation.
           </t>
       <t>
Moved language related to topics that should be in
<xref target="I-D.ietf-clue-datachannel"
</author>
<author initials='A' surname='Pepperell' fullname='Andrew Pepperell'>
    <organization /> and
<xref target="I-D.ietf-clue-protocol" />, but that has not yet been resolved
in those documents, into
an appendix.
       </t>
          </list>
         </t>
         <t hangText="draft-kyzivat-06:"> Revisions by Rob Hansen
          <list style='symbols'>
           <t>
Removed CLUE message
</author>
<author initials='S' surname='Wenger' fullname='Stephan Wenger'>
    <organization />
</author>
<date month='July' year='2020' />
</front>
<seriesInfo name='RFC' value='8845' />
<seriesInfo name='DOI' value='10.17487/RFC8845' />
</reference>

<!--    &I-D.ietf-clue-data-model-schema; is 8846-->
<reference anchor="RFC8846" target="http://www.rfc-editor.org/info/rfc8846">
          <front>
            <title>An XML schema and details that are now in
draft-presta-clue-protocol
           </t>
           <t>
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.
           </t>
           <t>
A section has also been added on the current mandation of unidirectional
"m" lines.
           </t>
           <t>
Updated CLUE messaging in example call flow to match
draft-presta-clue-protocol-03
           </t>
          </list>
         </t>
         <t hangText="draft-kyzivat-05:"> Revisions by pkyzivat:
          <list style='symbols'>
            <t>
Specified versioning model and mechanism.
            </t>
            <t>
Added explicit response to all messages.
            </t>
            <t>
Rearranged text to work with Schema for the above changes.
(Which rendered diff almost useless.)
            </t>
          </list>
         </t>
         <t hangText="draft-kyzivat-04:"> Revisions by Rob Hansen: ???</t>
         <t hangText="draft-kyzivat-03:"> Revisions by pkyzivat:
          <list style='symbols'>
            <t>
Added a syntax section with an XML schema Controlling Multiple Streams for CLUE messages.
This is a strawhorse, and Telepresence (CLUE) Data Model</title>
            <author initials="R" surname="Presta" fullname="Roberta Presta">
              <organization/>
            </author>
            <author initials="S P." surname="Romano" fullname="Simon Romano">
              <organization/>
            </author>
            <date month="July" year="2020"/>
          </front>
            <seriesInfo name="DOI" value="10.17487/RFC8846"/>
            <seriesInfo name="RFC" value="8846"/>

</reference>

<!--draft-ietf-clue-protocol-19 is 8847 -->
<reference anchor='RFC8847' target='https://www.rfc-editor.org/info/rfc8847'>
<front>
<title>Protocol for Controlling Multiple Streams for Telepresence (CLUE)</title>
<author initials='R' surname='Presta' fullname='Roberta Presta'>
    <organization />
</author>
<author initials='S P' surname='Romano' fullname='Simon Pietro Romano'>
    <organization />
</author>
<date month='July' year='2020' />
</front>
<seriesInfo name='RFC' value='8847' />
<seriesInfo name='DOI' value='10.17487/RFC8847' />

</reference>

<!--draft-ietf-clue-datachannel-18 is very incomplete, but it establishes
a template 8850 -->
<reference anchor='RFC8850' target='https://www.rfc-editor.org/info/rfc8850'>
<front>
<title>Controlling Multiple Streams for doing this based on elements defined in the data model.
(Thanks Telepresence (CLUE) Protocol Data Channel</title>
<author initials='C' surname='Holmberg' fullname='Christer Holmberg'>
    <organization />
</author>
<date month='July' year='2020' />
</front>
<seriesInfo name='RFC' value='8850' />
<seriesInfo name='DOI' value='10.17487/RFC8850' />

</reference>

<!--draft-ietf-clue-rtp-mapping-14: 8849 -->
<reference anchor='RFC8849' target="https://www.rfc-editor.org/info/rfc8849">
<front>
<title>Mapping RTP Streams to Roberta Controlling Multiple Streams for help with this!)
            </t>
            <t>
Did some rewording to fit the syntax section Telepresence (CLUE) Media Captures</title>
<author initials='R' surname='Even' fullname='Roni Even'>
    <organization />
</author>
<author initials='J' surname='Lennox' fullname='Jonathan Lennox'>
    <organization />
</author>
<date month='July' year='2020' />
</front>
<seriesInfo name='RFC' value='8849' />
<seriesInfo name="DOI" value="10.17487/RFC8849"/>
</reference>

<!-- draft-ietf-mmusic-sctp-sdp -->
<reference anchor="RFC8841" target="https://www.rfc-editor.org/info/rfc8841">

  <front>
    <title>Session Description Protocol (SDP) Offer/Answer Procedures for
    Stream Control Transmission Protocol (SCTP) over Datagram Transport Layer
    Security (DTLS) Transport</title>

    <author initials="C." surname="Holmberg" fullname="Christer Holmberg">
      <organization />
    </author>

    <author initials="R." surname="Shpount" fullname="Roman Shpount">
      <organization />
    </author>

    <author initials="S." surname="Loreto" fullname="Salvatore Loreto">
      <organization />
    </author>

    <author initials="G." surname="Camarillo" fullname="Gonzalo Camarillo">
      <organization />
    </author>

    <date month="July" year="2020" />
  </front>
  <seriesInfo name="RFC" value="8841" />
  <seriesInfo name="DOI" value="10.17487/RFC8841"/>

</reference>

<!--draft-ietf-mmusic-data-channel-sdpneg-28; in and reference it.
            </t>
            <t>
Did some relatively minor restructuring REF - part of C238-->
<reference anchor='RFC8864' target="https://www.rfc-editor.org/info/rfc8864">
<front>
<title>Data Channel Negotiation Based on the document to make
it flow better in a logical way.
            </t>
          </list>

         </t>
         <t hangText="draft-kyzivat-02:"> A bunch of revisions by pkyzivat:
         <list style='symbols'>
            <t>
Moved roberta's call flows to a more appropriate place in Session Description Protocol (SDP)</title>
<author initials='K' surname='Drage' fullname='Keith Drage'>
    <organization />
</author>
<author initials='M' surname='Makaraju' fullname='Maridi Makaraju'>
    <organization />
</author>
<author initials='R' surname='Ejzak' fullname='Richard Ejzak'>
    <organization />
</author>
<author initials='J' surname='Marcon' fullname='Jerome Marcon'>
    <organization />
</author>
<author initials='R' surname='Even' fullname='Roni Even' role='editor'>
    <organization />
</author>
<date month='July' year='2020' />
</front>
<seriesInfo name='RFC' value='8864' />
<seriesInfo name="DOI" value="10.17487/RFC8864"/>
</reference>

<!-- draft-ietf-mmusic-sdp-bundle-negotiation (RFC 8843) -->
    <reference anchor="RFC8843" target="https://www.rfc-editor.org/info/rfc8843">
      <front>
        <title>Negotiating Media Multiplexing Using the document.
            </t>
            <t>
New section on versioning.
            </t>
            <t>
New section on NAK.
            </t>
            <t>
A couple of possible alternatives for message acknowledgment.
            </t>
            <t>
Some discussion of when/how to signal changes in provider state.
            </t> Session Description Protocol (SDP)</title>
        <author initials="C" surname="Holmberg" fullname="Christer Holmberg">
          <organization/>
        </author>
        <author initials="H" surname="Alvestrand" fullname="Harald Alvestrand">
          <organization/>
        </author>
        <author initials="C" surname="Jennings" fullname="Cullen Jennings">
          <organization/>
        </author>
        <date month="July" year="2020"/>
      </front>
        <seriesInfo name="RFC" value="8843"/>
        <seriesInfo name="DOI" value="10.17487/RFC8843"/>
    </reference>

<!-- draft-ietf-rtcweb-data-channel: 8831 -->
<reference anchor="RFC8831" target="https://www.rfc-editor.org/info/rfc8831">
<front>
<title>WebRTC Data Channels</title>
<author initials="R" surname="Jesup" fullname="Randell Jesup">
  <organization/>
</author>
<author initials="S" surname="Loreto" fullname="Salvatore Loreto">
  <organization/>
</author>
<author initials="M" surname="Tüxen" fullname="Michael Tüxen">
  <organization/>
</author>
<date month='July' year='2020'/>
</front>
<seriesInfo name="RFC" value="8831"/>
<seriesInfo name="DOI" value="10.17487/RFC8831"/>
</reference>

        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.3711.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.3840.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.4574.xml"/>
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        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.5888.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
      </references>
      <references>
        <name>Informative References</name>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2506.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.3261.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.3264.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.3311.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.4566.xml"/>
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        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.5761.xml"/>
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	    href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6184.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8445.xml"/>

      </references>
    </references>

   <section numbered="false" toc="default">
      <name>Acknowledgements</name>
      <t>
Some discussion about
Besides the handling of transport errors. authors, the team focusing on this document consists of:
<contact fullname="Roni Even"/>, <contact fullname="Simon Pietro Romano"/>, and <contact fullname="Roberta Presta"/>.
      </t>
      <t>
Added a change history section.
            </t>
         </list>
These were developed by Lennard Xiao, Christian Groves and Paul,
so added Lennard
<contact fullname="Christian Groves"/>, <contact fullname="Jonathan Lennox"/>, and <contact fullname="Adam Roach"/> have contributed detailed
comments and Christian as authors.

         </t>
         <t hangText="draft-kyzivat-01:">

Updated by roberta to include some sample call flows. suggestions.
      </t>
         <t hangText="draft-kyzivat-00:">

Initial version by pkyzivat. Established general outline for
    </section>

<!-- [rfced] Throughout the document, text, the following terminology appears to be used
inconsistently. Please review these occurrences and specified let us know if/how
they may be made consistent.

"CLUE" group vs. CLUE group

SDP Offer/Answer vs. SDP offer/answer

Encoding Group vs. encoding group

Encoding information vs. encoding information

*Note that there are 2 instances of 'encoding' (lowercase); should these be
captialized for consistency?
  - encoding limits
  - a few things thought to represent wg consensus.

         </t>
      </list></t>
   </section>

</middle>

<back>
   <references title="Normative References">
      <?rfc include="reference.I-D.ietf-clue-framework"?>
      <?rfc include="reference.I-D.ietf-clue-data-model-schema"?>
      <?rfc include="reference.I-D.ietf-clue-protocol"?>
      <?rfc include="reference.I-D.ietf-clue-datachannel"?>
      <?rfc include="reference.I-D.ietf-clue-rtp-mapping"?>
      <?rfc include="reference.I-D.ietf-rtcweb-data-channel"?>
      <?rfc include="reference.I-D.ietf-mmusic-sctp-sdp"?>
      <?rfc include="reference.I-D.ietf-mmusic-data-channel-sdpneg"?>
      <?rfc include="reference.I-D.ietf-mmusic-sdp-bundle-negotiation"?>
      <?rfc include="reference.RFC.2119"?>
      <?rfc include="reference.RFC.3711"?>
      <?rfc include="reference.RFC.3840"?>
      <?rfc include="reference.RFC.4574"?>
      <?rfc include="reference.RFC.5763"?>
      <?rfc include="reference.RFC.5888"?>
      <?rfc include="reference.RFC.8174"?>
   </references>
   <references title="Informative References">
      <?rfc include="reference.RFC.2506"?>
      <?rfc include="reference.RFC.3261"?>
      <?rfc include="reference.RFC.3264"?>
      <?rfc include="reference.RFC.3311"?>
      <?rfc include="reference.RFC.4566"?>
      <?rfc include="reference.RFC.8445"?>
      <?rfc include="reference.RFC.5630"?>
      <?rfc include="reference.RFC.5761"?>
      <?rfc include="reference.RFC.6184"?>
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-->

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</rfc>