



Network Working Group                                          R. Hanton
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                              P. Kyzivat
Expires: June 11, 2020
                                                                 L. Xiao
                                                                  Huawei
                                                               C. Groves
                                                        December 9, 2019


  Session Signaling for Controlling Multiple Streams for Telepresence
                                 (CLUE)
                      draft-ietf-clue-signaling-15

Abstract

   This document specifies how CLUE-specific signaling such as the CLUE
   protocol and the CLUE data channel are used in conjunction with each
   other and with existing signaling mechanisms such as SIP and SDP to
   produce a telepresence call.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 11, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Media Feature Tag Definition  . . . . . . . . . . . . . . . .   4
   4.  SDP Grouping Framework CLUE Extension Semantics . . . . . . .   4
     4.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  The CLUE data channel and the CLUE grouping semantic  . .   5
     4.3.  CLUE-controlled media and the CLUE grouping semantic  . .   5
     4.4.  SDP semantics for CLUE-controlled media . . . . . . . . .   5
       4.4.1.  Signaling CLUE Encodings  . . . . . . . . . . . . . .   6
         4.4.1.1.  Referencing Encodings in the CLUE protocol  . . .   6
       4.4.2.  Negotiating receipt of CLUE Capture Encodings in SDP    7
     4.5.  SDP Offer/Answer Procedures . . . . . . . . . . . . . . .   7
       4.5.1.  Generating the Initial Offer  . . . . . . . . . . . .   7
       4.5.2.  Generating the Answer . . . . . . . . . . . . . . . .   8
         4.5.2.1.  Negotiating use of CLUE and the CLUE data channel   8
         4.5.2.2.  Negotiating CLUE-controlled media . . . . . . . .   8
         4.5.2.3.  Negotiating non-CLUE controlled media . . . . . .   9
       4.5.3.  Processing the initial Offer/Answer negotiation . . .   9
         4.5.3.1.  Successful CLUE negotiation . . . . . . . . . . .   9
         4.5.3.2.  CLUE negotiation failure  . . . . . . . . . . . .  10
       4.5.4.  Modifying the session . . . . . . . . . . . . . . . .  10
         4.5.4.1.  Adding and removing CLUE-controlled media . . . .  10
         4.5.4.2.  Enabling CLUE mid-call  . . . . . . . . . . . . .  10
         4.5.4.3.  Disabling CLUE mid-call . . . . . . . . . . . . .  10
         4.5.4.4.  CLUE protocol failure mid-call  . . . . . . . . .  11
   5.  Interaction of CLUE protocol and SDP negotiations . . . . . .  11
     5.1.  Independence of SDP and CLUE negotiation  . . . . . . . .  12
     5.2.  Constraints on sending media  . . . . . . . . . . . . . .  13
     5.3.  Recommendations for operating with non-atomic operations   13
   6.  Interaction of CLUE protocol and RTP/RTCP CaptureID . . . . .  14
     6.1.  CaptureID reception during MCC redefinition . . . . . . .  14
   7.  Multiplexing of CLUE-controlled media using BUNDLE  . . . . .  15
     7.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  15
     7.2.  Usage of BUNDLE with CLUE . . . . . . . . . . . . . . . .  15
       7.2.1.  Generating the Initial Offer  . . . . . . . . . . . .  16
       7.2.2.  Multiplexing of the data channel and RTP media  . . .  16
   8.  Example: A call between two CLUE-capable Endpoints  . . . . .  16
   9.  Example: A call between a CLUE-capable and non-CLUE Endpoint   26
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  27
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
     11.1.  New SDP Grouping Framework Attribute . . . . . . . . . .  27



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     11.2.  New SIP Media Feature Tag  . . . . . . . . . . . . . . .  28
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  28
   13. Change History  . . . . . . . . . . . . . . . . . . . . . . .  29
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  39
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  39
     14.2.  Informative References . . . . . . . . . . . . . . . . .  41
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  42

1.  Introduction

   To enable devices to participate in a telepresence call, selecting
   the sources they wish to view, receiving those media sources and
   displaying them in an optimal fashion, CLUE (ControLling mUltiple
   streams for tElepresence) employs two principal and inter-related
   protocol negotiations.  SDP [RFC4566], conveyed via SIP [RFC3261], is
   used to negotiate the specific media capabilities that can be
   delivered to specific addresses on a device.  Meanwhile, CLUE
   protocol [I-D.ietf-clue-protocol] messages, transported via a CLUE
   data channel [I-D.ietf-clue-datachannel], are used to negotiate the
   Capture Sources available, their attributes and any constraints in
   their use.  They also allow the far end device to specify which
   Captures they wish to receive.  It is recommended that those
   documents be read prior to this one as this document assumes
   familiarity with those protocols and hence uses terminology from each
   with limited introduction.

   Beyond negotiating the CLUE channel, SDP is also used to negotiate
   the details of supported media streams and the maximum capability of
   each of those streams.  As the CLUE Framework
   [I-D.ietf-clue-framework] defines a manner in which the Media
   Provider expresses their maximum encoding group capabilities, SDP is
   also used to express the encoding limits for each potential Encoding.

   Backwards-compatibility is an important consideration of the
   protocol: it is vital that a CLUE-capable device contacting a device
   that does not support CLUE is able to fall back to a fully functional
   non-CLUE call.  The document also defines how a non-CLUE call may be
   upgraded to CLUE in mid-call, and similarly how CLUE functionality
   can be removed mid-call to return to a standard non-CLUE call.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.




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   This document uses terminology defined in the CLUE Framework
   [I-D.ietf-clue-framework].

   A few additional terms specific to this document are defined as
   follows:

   non-CLUE device:  A device that supports standard SIP and SDP, but
      either does not support CLUE, or that does but does not currently
      wish to invoke CLUE capabilities.

   CLUE-controlled media:  A media "m=" line that is under CLUE control;
      the Capture Source that provides the media on this "m=" line is
      negotiated in CLUE.  See Section 4 for details of how this control
      is signaled in SDP.  There is a corresponding "non-CLUE-
      controlled" media term.

3.  Media Feature Tag Definition

   The "sip.clue" media feature tag [RFC3840] indicates support for CLUE
   in SIP [RFC3261] calls.  A CLUE-capable device SHOULD include this
   media feature tag in its REGISTER requests and OPTION responses.  It
   SHOULD also include the media feature tag in INVITE and UPDATE
   [RFC3311] requests and responses.

   Presence of the media feature tag in the contact field of a request
   or response can be used to determine that the far end supports CLUE.

4.  SDP Grouping Framework CLUE Extension Semantics

4.1.  General

   This section defines a new SDP Grouping Framework [RFC5888] extension
   called 'CLUE'.

   The CLUE extension can be indicated using an SDP session-level
   'group' attribute.  Each SDP media "m=" line that is included in this
   group, using SDP media-level mid attributes, is CLUE-controlled, by a
   CLUE data channel also included in this CLUE group.

   Currently only support for a single CLUE group is specified; support
   for multiple CLUE groups in a single session is outside the scope of
   this document.  A device MUST NOT include more than one CLUE group in
   its SDP message unless it is following a specification that defines
   how multiple CLUE channels are signaled, and is either able to
   determine that the other side of the SDP exchange supports multiple
   CLUE channels, or is able to fail gracefully in the event it does
   not.




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4.2.  The CLUE data channel and the CLUE grouping semantic

   The CLUE data channel [I-D.ietf-clue-datachannel] is a bidirectional
   data channel [I-D.ietf-rtcweb-data-channel] used for the transport of
   CLUE messages, conveyed within an SCTP over DTLS connection.  This
   channel must be established before CLUE protocol messages can be
   exchanged and CLUE-controlled media can be sent.

   The data channel is negotiated over SDP as described in
   [I-D.ietf-mmusic-data-channel-sdpneg].  A CLUE-capable device wishing
   to negotiate CLUE MUST also include a CLUE group in their SDP offer
   or answer and include the "mid" of the "m=" line for the data channel
   in that group.  The CLUE group MUST include the "mid" of the "m="
   line for one (and only one) data channel.

   Presence of the data channel in the CLUE group in an SDP offer or
   answer also serves, along with the "sip.clue" media feature tag, as
   an indication that the device supports CLUE and wishes to upgrade the
   call to include CLUE-controlled media.  A CLUE-capable device SHOULD
   include a data channel "m=" line in offers and, when allowed by
   [RFC3264], answers.

4.3.  CLUE-controlled media and the CLUE grouping semantic

   CLUE-controlled media lines in an SDP are "m=" lines in which the
   content of the media streams to be sent is negotiated via the CLUE
   protocol [I-D.ietf-clue-protocol].  For an "m=" line to be CLUE-
   controlled, its "mid" value MUST be included in the CLUE group.
   CLUE-controlled media is controlled by the CLUE protocol as
   negotiated on the CLUE data channel with an "mid" included in the
   CLUE group.

   "m=" lines not specified as under CLUE control follow normal rules
   for media streams negotiated in SDP as defined in documents such as
   [RFC3264].

   The restrictions on CLUE-controlled media that are defined below
   always apply to "m=" lines in an SDP offer or answer, even if
   negotiation of the data channel in SDP failed due to lack of CLUE
   support by the remote device or for any other reason, or in an offer
   if the recipient does not include the "mid" of the corresponding "m="
   line in their CLUE group.

4.4.  SDP semantics for CLUE-controlled media







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4.4.1.  Signaling CLUE Encodings

   The CLUE Framework [I-D.ietf-clue-framework] defines the concept of
   "Encodings", which represent the sender's encode ability.  Each
   Encoding the Media Provider wishes to signal is signaled via an "m="
   line of the appropriate media type, which MUST be marked as sendonly
   with the "a=sendonly" attribute or as inactive with the "a=inactive"
   attribute.

   The encoder limits of active (eg, "a=sendonly") Encodings can then be
   expressed using existing SDP syntax.  For instance, for H.264 see
   Table 6 in [RFC6184] for a list of valid parameters for representing
   encoder sender stream limits.

   These Encodings are CLUE-controlled and hence MUST include an "mid"
   in the CLUE group as defined above.

   As well as the normal restrictions defined in [RFC3264] the stream
   MUST be treated as if the "m=" line direction attribute had been set
   to "a=inactive" until the Media Provider has received a valid CLUE
   'configure' message specifying the Capture to be used for this
   stream.  This means that RTP packets MUST NOT be sent until
   configuration is complete, while non-media packets such as STUN, RTCP
   and DTLS MUST be sent as per their relevant specifications if
   negotiated.

   Every "m=" line representing a CLUE Encoding MUST contain a "label"
   attribute as defined in [RFC4574].  This label is used to identify
   the Encoding by the sender in CLUE 'advertisement' messages and by
   the receiver in CLUE 'configure' messages.  Each label used for a
   CLUE-controlled "m=" line MUST be different from the label on all
   other "m=" lines in the CLUE group, unless an "m=" line represents a
   dependent stream related to another "m=" line (such as an FEC
   stream), in which case it MUST have the same label value as the "m="
   line on which it depends.

4.4.1.1.  Referencing Encodings in the CLUE protocol

   CLUE Encodings are defined in SDP, but can be referenced from CLUE
   protocol messages - this is how the protocol defines which Encodings
   are part of an Encoding Group (in 'advertisement' messages) and which
   Encoding with which to encode a specific Capture (in 'configure'
   messages).  The labels on the CLUE-controlled "m=" lines are the
   references that are used in the CLUE protocol.

   Each <encID> (in encodingIDList) in a CLUE 'advertisement' message
   SHOULD represent an Encoding defined in SDP; the specific Encoding
   referenced is a CLUE-controlled "m=" line in the most recent SDP



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   Offer/Answer message sent by the sender of the 'advertisement'
   message with a label value corresponding to the text content of the
   <encID>.  If the <encID> is not defined in SDP it MUST be one it
   anticipates sending in a subsequent SDP Offer/Answer exchange.

   Each <encodingID> (in captureEncodingType) in a CLUE 'configure'
   message MUST represent an Encoding defined in SDP; the specific
   Encoding referenced is a CLUE-controlled "m=" line in the most recent
   SDP Offer/Answer message received by the sender of the 'configure'
   message with a label value corresponding to the text content of the
   <encodingID>.

   Note that the non-atomic nature of SDP/CLUE protocol interaction may
   mean that there are temporary periods where an <encID>/<encodingID>
   in a CLUE message does not reference an SDP "m=" line, or where an
   Encoding represented in SDP is not referenced in a CLUE protocol
   message.  See Section 5 for specifics.

4.4.2.  Negotiating receipt of CLUE Capture Encodings in SDP

   A receiver who wishes to receive a CLUE stream via a specific
   Encoding requires an "a=recvonly" "m=" line that matches the
   "a=sendonly" Encoding.

   These "m=" lines are CLUE-controlled and hence MUST include their
   "mid" in the CLUE group.  They MAY include a "label" attribute, but
   this is not required by CLUE, as only label values associated with
   "a=sendonly" Encodings are referenced by CLUE protocol messages.

4.5.  SDP Offer/Answer Procedures

4.5.1.  Generating the Initial Offer

   A CLUE-capable device sending an initial SDP offer of a SIP session
   and wishing to negotiate CLUE will include an "m=" line for the data
   channel to convey the CLUE protocol, along with a CLUE group
   containing the "mid" of the data channel "m=" line.

   For interoperability with non-CLUE devices a CLUE-capable device
   sending an initial SDP offer SHOULD NOT include any "m=" line for
   CLUE-controlled media beyond the "m=" line for the CLUE data channel,
   and SHOULD include at least one non-CLUE-controlled media "m=" line.

   If the device has evidence that the receiver is also CLUE-capable,
   for instance due to receiving an initial INVITE with no SDP but
   including a "sip.clue" media feature tag, the above recommendation is
   waived, and the initial offer MAY contain "m=" lines for CLUE-
   controlled media.



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   With the same interoperability recommendations as for Encodings, the
   sender of the initial SDP offer MAY also include "a=recvonly" media
   lines to preallocate "m=" lines to receive media.  Alternatively, it
   MAY wait until CLUE protocol negotiation has completed before
   including these lines in a new offer/answer exchange - see Section 5
   for recommendations.

4.5.2.  Generating the Answer

4.5.2.1.  Negotiating use of CLUE and the CLUE data channel

   If the recipient of an initial offer is CLUE-capable, and the offer
   contains both an "m=" line for a data channel and a CLUE group
   containing the "mid" for that "m=" line, they SHOULD negotiate data
   channel support for an "m=" line, and include the "mid" of that "m="
   line in a corresponding CLUE group.

   A CLUE-capable recipient that receives an "m=" line for a data
   channel but no corresponding CLUE group containing the "mid" of that
   "m=" line MAY still include a corresponding data channel "m=" line if
   there are any other non-CLUE protocols it can convey over that
   channel, but MUST NOT negotiate use of the CLUE protocol on this
   channel.

4.5.2.2.  Negotiating CLUE-controlled media

   If the initial offer contained "a=recvonly" CLUE-controlled media
   lines the recipient SHOULD include corresponding "a=sendonly" CLUE-
   controlled media lines for accepted Encodings, up to the maximum
   number of Encodings it wishes to advertise.  As CLUE-controlled
   media, the "mid" of these "m=" lines MUST be included in the
   corresponding CLUE group.  The recipient MUST set the direction of
   the corresponding "m=" lines of any remaining "a=recvonly" CLUE-
   controlled media lines received in the offer to "a=inactive".

   If the initial offer contained "a=sendonly" CLUE-controlled media
   lines the recipient MAY include corresponding "a=recvonly" CLUE-
   controlled media lines, up to the maximum number of Capture Encodings
   it wishes to receive.  Alternatively, it MAY wait until CLUE protocol
   negotiation has completed before including these lines in a new
   offer/answer exchange - see Section 5 for recommendations.  The
   recipient MUST set the direction of the corresponding "m=" lines of
   any remaining "a=sendonly" CLUE-controlled media lines received in
   the offer to "a=inactive"







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4.5.2.3.  Negotiating non-CLUE controlled media

   A CLUE-controlled device implementation MAY prefer to render initial,
   single-stream audio and/or video for the user as rapidly as possible,
   transitioning to CLUE-controlled media once that has been negotiated.
   Alternatively, an implementation MAY wish to suppress initial media,
   only providing media once the final, CLUE-controlled streams have
   been negotiated.

   The receiver of the initial offer, if making the call CLUE-enabled
   with their SDP answer, can make their preference clear by their
   action in accepting or rejecting non-CLUE-controlled media lines.
   Rejecting these "m=" lines will ensure that no non-CLUE-controlled
   media flows before the CLUE-controlled media is negotiated.  In
   contrast, accepting one or more non-CLUE-controlled "m=" lines in
   this initial answer will enable initial media to flow.

   If the answerer chooses to send initial non-CLUE-controlled media in
   a CLUE-enabled call, Section 4.5.4.1 addresses the need to disable it
   once CLUE-controlled media is fully negotiated.

4.5.3.  Processing the initial Offer/Answer negotiation

   In the event that both offer and answer include a data channel "m="
   line with a mid value included in corresponding CLUE groups, CLUE has
   been successfully negotiated and the call is now CLUE-enabled.  If
   not then the call is not CLUE-enabled.

4.5.3.1.  Successful CLUE negotiation

   In the event of successful CLUE-enablement of the call, devices MUST
   now begin negotiation of the CLUE channel, see
   [I-D.ietf-clue-datachannel] for negotiation details.  If negotiation
   is successful, sending of CLUE protocol [I-D.ietf-clue-protocol]
   messages can begin.

   A CLUE-capable device MAY choose not to send RTP on the non-CLUE-
   controlled channels during the period in which control of the CLUE-
   controlled media lines is being negotiated (though RTCP MUST still be
   sent and received as normal).  However, a CLUE-capable device MUST
   still be prepared to receive media on non-CLUE-controlled media lines
   that have been successfully negotiated as defined in [RFC3264].

   If either side of the call wishes to add additional CLUE-controlled
   "m=" lines to send or receive CLUE-controlled media they MAY now send
   a SIP request with a new SDP offer following the normal rules of SDP
   offer/answer and any negotiated extensions.




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4.5.3.2.  CLUE negotiation failure

   In the event that the negotiation of CLUE fails and the call is not
   CLUE-enabled once the initial offer/answer negotiation completes then
   CLUE is not in use in the call.  The CLUE-capable devices MUST either
   revert to non-CLUE behaviour or terminate the call.

4.5.4.  Modifying the session

4.5.4.1.  Adding and removing CLUE-controlled media

   Subsequent offer/answer exchanges MAY add additional "m=" lines for
   CLUE-controlled media, or activate or deactivate existing "m=" lines
   per the standard SDP mechanisms.

   In most cases at least one additional exchange after the initial
   offer/answer exchange will be required before both sides have added
   all the Encodings and ability to receive Encodings that they desire.
   Devices MAY delay adding "a=recvonly" CLUE-controlled "m=" lines
   until after CLUE protocol negotiation completes - see Section 5 for
   recommendations.

   Once CLUE media has been successfully negotiated devices SHOULD
   ensure that non-CLUE-controlled media is deactivated by setting their
   ports to 0 in cases where it corresponds to the media type of CLUE-
   controlled media that has been successfully negotiated.  This
   deactivation may require an additional SDP exchange, or may be
   incorporated into one that is part of the CLUE negotiation.

4.5.4.2.  Enabling CLUE mid-call

   A CLUE-capable device that receives an initial SDP offer from a non-
   CLUE device SHOULD include a new data channel "m=" line and
   corresponding CLUE group in any subsequent offers it sends, to
   indicate that it is CLUE-capable.

   If, in an ongoing non-CLUE call, an SDP offer/answer exchange
   completes with both sides having included a data channel "m=" line in
   their SDP and with the "mid" for that channel in a corresponding CLUE
   group then the call is now CLUE-enabled; negotiation of the data
   channel and subsequently the CLUE protocol begins.

4.5.4.3.  Disabling CLUE mid-call

   If, during an ongoing CLUE-enabled call a device wishes to disable
   CLUE, it can do so by following the procedures for closing a data
   channel defined in Section 5.2.4 of
   [I-D.ietf-mmusic-data-channel-sdpneg]: sending a new SDP offer/answer



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   exchange and subsequent SCTP SSN reset for the CLUE channel.  It MUST
   also remove the CLUE group.  Without the CLUE group any "m=" lines
   that were previously CLUE-controlled no longer are; implementations
   MAY disable them by setting their ports to 0 or MAY continue to use
   them - in the latter case how they are used is outside the scope of
   this document.

   If a device follows the procedure above, or an SDP offer-answer
   negotiation completes in a fashion in which either the "m=" CLUE data
   channel line was not successfully negotiated, and/or one side did not
   include the data channel in the CLUE group then CLUE for this call is
   disabled.  In the event that this occurs, CLUE is no longer enabled.
   Any active "m=" lines still included in the CLUE group are no longer
   CLUE-controlled and the implementation MAY either disable them in a
   subsequent negotiation or continue to use them in some other fashion.
   If the data channel is still present but not included in the CLUE
   group semantic CLUE protocol messages MUST no longer be sent.

4.5.4.4.  CLUE protocol failure mid-call

   In contrast to the specific disablement of the use of CLUE described
   above, the CLUE channel may fail unexpectedly.  Two circumstances
   where this can occur are:

   o  The CLUE data channel terminates, either gracefully or
      ungracefully, without any corresponding SDP renegotiation.

   o  A channel error of the CLUE protocol causes it to return to the
      IDLE state as defined in Section 6. of [I-D.ietf-clue-protocol].

   In this circumstance implementations SHOULD continue to transmit and
   receive CLUE-controlled media on the basis of the last negotiated
   CLUE messages, until the CLUE protocol is re-established (in the
   event of a channel error) or disabled mid-call by an SDP exchange as
   defined in Section 4.5.4.3.  Implementations MAY choose to send such
   an SDP request to disable CLUE immediately or MAY continue on in a
   call-preservation mode.

5.  Interaction of CLUE protocol and SDP negotiations

   Information about media streams in CLUE is split between two message
   types: SDP, which defines media addresses and limits, and the CLUE
   channel, which defines properties of Capture Devices available, scene
   information and additional constraints.  As a result certain
   operations, such as advertising support for a new transmissible
   Capture with associated stream, cannot be performed atomically, as
   they require changes to both SDP and CLUE messaging.




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   This section defines how the negotiation of the two protocols
   interact, provides some recommendations on dealing with intermediate
   stages in non-atomic operations, and mandates additional constraints
   on when CLUE-configured media can be sent.

5.1.  Independence of SDP and CLUE negotiation

   To avoid the need to implement interlocking state machines with the
   potential to reach invalid states if messages were to be lost, or be
   rewritten en-route by middle boxes, the state machines in SDP and
   CLUE operate independently.  The state of the CLUE channel does not
   restrict when an implementation may send a new SDP offer or answer,
   and likewise the implementation's ability to send a new CLUE
   'advertisement' or 'configure' message is not restricted by the
   results of or the state of the most recent SDP negotiation (unless
   the SDP negotiation has removed the CLUE channel).

   The primary implication of this is that a device may receive an SDP
   Offer/Answer message with a CLUE Encoding for which it does not yet
   have Capture information, or receive a CLUE 'configure' message
   specifying a Capture Encoding for which the far end has not
   negotiated a media stream in SDP.

   CLUE messages contain an <encID> (in encodingIDList) or <encodingID>
   (in captureEncodingType), which is used to identify a specific
   encoding or captureEncoding in SDP; see
   [I-D.ietf-clue-data-model-schema] for specifics.  The non-atomic
   nature of CLUE negotiation means that a sender may wish to send a new
   CLUE 'advertisement' message before the corresponding SDP message.
   As such the sender of the CLUE message MAY include an <encID> which
   does not currently match a CLUE-controlled "m=" line label in SDP; A
   CLUE-capable implementation MUST NOT reject a CLUE protocol message
   solely because it contains <encID> elements that do not match a label
   in SDP.

   The current state of the CLUE participant or Media Provider/Consumer
   state machines do not affect compliance with any of the normative
   language of [RFC3264].  That is, they MUST NOT delay an ongoing SDP
   exchange as part of a SIP server or client transaction; an
   implementation MUST NOT delay an SDP exchange while waiting for CLUE
   negotiation to complete or for a 'configure' message to arrive.

   Similarly, a device in a CLUE-enabled call MUST NOT delay any
   mandatory state transitions in the CLUE Participant or Media
   Provider/Consumer state machines due to the presence or absence of an
   ongoing SDP exchange.





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   A device with the CLUE Participant state machine in the ACTIVE state
   MAY choose to delay moving from ESTABLISHED to ADV (Media Provider
   state machine) or from ESTABLISHED to WAIT FOR CONF RESPONSE (Media
   Consumer state machine) based on the SDP state.  See
   [I-D.ietf-clue-protocol] for CLUE state machine specifics.
   Similarly, a device MAY choose to delay initiating a new SDP exchange
   based on the state of their CLUE state machines.

5.2.  Constraints on sending media

   While SDP and CLUE message states do not impose constraints on each
   other, both impose constraints on the sending of media - CLUE-
   controlled media MUST NOT be sent unless it has been negotiated in
   both CLUE and SDP: an implementation MUST NOT send a specific CLUE
   Capture Encoding unless its most recent SDP exchange contains an
   active media channel for that Encoding AND it has received a CLUE
   'configure' message specifying a valid Capture for that Encoding.

5.3.  Recommendations for operating with non-atomic operations

   CLUE-capable devices MUST be able to handle states in which CLUE
   messages make reference to EncodingIDs that do not match the most
   recently received SDP, irrespective of the order in which SDP and
   CLUE messages are received.  While these mismatches will usually be
   transitory a device MUST be able to cope with such mismatches
   remaining indefinitely.  However, this document makes some
   recommendations on message ordering for these non-atomic transitions.

   CLUE-capable devices MUST ensure that any inconsistencies between SDP
   and CLUE signaling are temporary by sending updated SDP or CLUE
   messages as soon as the relevant state machines and other constraints
   permit.

   Generally, implementations that receive messages for which they have
   incomplete information will be most efficient if they wait until they
   have the corresponding information they lack before sending messages
   to make changes related to that information.  For example, an
   answerer that receives a new SDP offer with three new "a=sendonly"
   CLUE "m=" lines for which it has received no CLUE 'advertisement'
   message providing the corresponding capture information would
   typically inclue corresponding "a=inactive" lines in its answer, and
   only make a new SDP offer with "a=recvonly" when and if a new
   'advertisement' message arrives with Captures relevant to those
   Encodings.

   Because of the constraints of SDP offer/answer and because new SDP
   negotiations are generally more 'costly' than sending a new CLUE
   message, implementations needing to make changes to both channels



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   SHOULD prioritize sending the updated CLUE message over sending the
   new SDP message.  The aim is for the recipient to receive the CLUE
   changes before the SDP changes, allowing the recipient to send their
   SDP answers without incomplete information, reducing the number of
   new SDP offers required.

6.  Interaction of CLUE protocol and RTP/RTCP CaptureID

   The CLUE Framework [I-D.ietf-clue-framework] allows for Multiple
   Content Captures (MCCs): Captures which contain multiple source
   Captures, whether composited into a single stream or switched based
   on some metric.

   The Captures that contribute to these MCCs may or may not be defined
   in the 'advertisement' message.  If they are defined and the MCC is
   providing them in a switched format the recipient may wish to
   determine which originating source Capture is currently being
   provided, so that they can apply geometric corrections based on that
   Capture's geometry, or take some other action based on the original
   Capture information.

   To do this, [I-D.ietf-clue-rtp-mapping] allows for the CaptureID of
   the originating Capture to be conveyed via RTP or RTCP.  A Media
   Provider sending switched media for an MCC with defined originating
   sources MUST send the CaptureID in both RTP and RTCP, as described in
   the mapping document.

6.1.  CaptureID reception during MCC redefinition

   Because the RTP/RTCP CaptureID is delivered via a different channel
   to the 'advertisement' message in which in the contents of the MCC
   are defined there is an intrinsic race condition in cases in which
   the contents of an MCC are redefined.

   When a Media Provider redefines an MCC which involves CaptureIDs, the
   reception of the relevant CaptureIDs by the recipient will either
   lead or lag reception and processing of the new 'advertisement'
   message by the recipient.  As such, a Media Consumer MUST NOT be
   disrupted by any of the following in any CLUE-controlled media stream
   it is receiving, whether that stream is for a static Capture or for
   an MCC (as any static Capture may be redefined to an MCC in a later
   'advertisement' message):

   o  Receiving RTP or RTCP containing a CaptureID when the most
      recently processed 'advertisement' message means that none are
      expected.





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   o  Receiving RTP or RTCP without CaptureIDs when the most recently
      processed 'advertisement' message means that media CaptureIDs are
      expected.

   o  Receiving a CaptureID in RTP or RTCP for a Capture defined in the
      most recently processed 'advertisement' message, but which the
      same 'advertisement' message does not include in the MCC.

   o  Receiving a CaptureID in RTP or RTCP for a Capture not defined in
      the most recently processed 'advertisement' message.

7.  Multiplexing of CLUE-controlled media using BUNDLE

7.1.  Overview

   A CLUE call may involve sending and/or receiving significant numbers
   of media streams.  Conventionally, media streams are sent and
   received on unique ports.  However, each separate port used for this
   purpose may impose costs that a device wishes to avoid, such as the
   need to open that port on firewalls and NATs, the need to collect ICE
   candidates [RFC8445], etc.

   The BUNDLE [I-D.ietf-mmusic-sdp-bundle-negotiation] extension can be
   used to negotiate the multiplexing of multiple media lines onto a
   single 5-tuple for sending and receiving media, allowing devices in
   calls to another BUNDLE-supporting device to potentially avoid some
   of the above costs.

   While CLUE-capable devices MAY support the BUNDLE extension for this
   purpose supporting the extension is not mandatory for a device to be
   CLUE-compliant.

   A CLUE-capable device that supports BUNDLE SHOULD also support rtcp-
   mux [RFC5761].  However, a CLUE-capable device that supports rtcp-mux
   may or may not support BUNDLE.

7.2.  Usage of BUNDLE with CLUE

   This specification imposes no additional requirements or restrictions
   on the usage of BUNDLE when used with CLUE.  There is no restriction
   on combining CLUE-controlled media lines and non-CLUE-controlled
   media lines in the same BUNDLE group or in multiple such groups.
   However, there are several steps an implementation may wish to take
   to ameliorate the cost and time requirements of extra SDP offer/
   answer exchanges between CLUE and BUNDLE.






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7.2.1.  Generating the Initial Offer

   BUNDLE mandates that the initial SDP offer MUST use a unique address
   for each "m=" line with a non-zero port.  Because CLUE
   implementations generally will not include CLUE-controlled media
   lines with the exception of the data channel in the initial SDP
   offer, CLUE devices that support large numbers of streams can avoid
   ever having to open large numbers of ports if they successfully
   negotiate BUNDLE.

   An implementation that does include CLUE-controlled media lines in
   its initial SDP offer while also using BUNDLE must take care to avoid
   renderings its CLUE-controlled media lines unusable in the event the
   far end does not negotiate BUNDLE if it wishes to avoid the risk of
   additional SDP exchanges to resolve this issue.  This is best
   achieved by not sending any CLUE-controlled media lines in an initial
   offer with the 'bundle-only' attribute unless it has been established
   via some other channel that the recipient supports and is able to use
   BUNDLE.

7.2.2.  Multiplexing of the data channel and RTP media

   BUNDLE-supporting CLUE-capable devices MAY include the data channel
   in the same BUNDLE group as RTP media.  In this case the device MUST
   be able to demultiplex the various transports - see section 9.2 of
   the BUNDLE draft [I-D.ietf-mmusic-sdp-bundle-negotiation].  If the
   BUNDLE group includes other protocols than the data channel
   transported via DTLS the device MUST also be able to differentiate
   the various protocols.

8.  Example: A call between two CLUE-capable Endpoints

   This example illustrates a call between two CLUE-capable Endpoints.
   Alice, initiating the call, is a system with three cameras and three
   screens.  Bob, receiving the call, is a system with two cameras and
   two screens.  A call-flow diagram is presented, followed by a summary
   of each message.

   To manage the size of this section the SDP snippets only illustrate
   video "m=" lines.  SIP ACKs are not always discussed.  Note that
   BUNDLE is not in use.










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                 +----------+                      +-----------+
                 |  Alice   |                      |    Bob    |
                 |          |                      |           |
                 +----+-----+                      +-----+-----+
                      |                                  |
                      |                                  |
                      | SIP INVITE 1                     |
                      |--------------------------------->|
                      |                                  |
                      |                                  |
                      |                     SIP 200 OK 1 |
                      |<---------------------------------|
                      |                                  |
                      |                                  |
                      | SIP ACK 1                        |
                      |--------------------------------->|
                      |                                  |
                      |                                  |
                      |                                  |
                      |<########### MEDIA 1 ############>|
                      |   1 video A->B, 1 video B->A     |
                      |<################################>|
                      |                                  |
                      |                                  |
                      |                                  |
                      |<================================>|
                      |   CLUE DATA CHANNEL ESTABLISHED  |
                      |<================================>|
                      |                                  |
                      |                                  |
                      | CLUE OPTIONS                     |
                      |<*********************************|
                      |                                  |
                      |                                  |
                      |            CLUE OPTIONS RESPONSE |
                      |*********************************>|
                      |                                  |
                      |                                  |
                      | CLUE ADVERTISEMENT 1             |
                      |*********************************>|
                      |                                  |
                      |                                  |
                      |             CLUE ADVERTISEMENT 2 |
                      |<*********************************|
                      |                                  |
                      |                                  |
                      | CLUE ACK 1                       |
                      |<*********************************|



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                      |                                  |
                      |                                  |
                      |                       CLUE ACK 2 |
                      |*********************************>|
                      |                                  |
                      |                                  |
                      | SIP INVITE 2 (+3 sendonly)       |
                      |--------------------------------->|
                      |                                  |
                      |                                  |
                      |                 CLUE CONFIGURE 1 |
                      |<*********************************|
                      |                                  |
                      |                                  |
                      |       SIP 200 OK 2 (+2 recvonly) |
                      |<---------------------------------|
                      |                                  |
                      |                                  |
                      | CLUE CONFIGURE RESPONSE 1        |
                      |*********************************>|
                      |                                  |
                      |                                  |
                      | SIP ACK 2                        |
                      |--------------------------------->|
                      |                                  |
                      |                                  |
                      |                                  |
                      |<########### MEDIA 2 ############>|
                      |   2 video A->B, 1 video B->A     |
                      |<################################>|
                      |                                  |
                      |                                  |
                      |       SIP INVITE 3 (+2 sendonly) |
                      |<---------------------------------|
                      |                                  |
                      |                                  |
                      | CLUE CONFIGURE 2                 |
                      |*********************************>|
                      |                                  |
                      |                                  |
                      | SIP 200 OK 3 (+2 recvonly)       |
                      |--------------------------------->|
                      |                                  |
                      |                                  |
                      |        CLUE CONFIGURE RESPONSE 2 |
                      |<*********************************|
                      |                                  |
                      |                                  |



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                      |                        SIP ACK 3 |
                      |<---------------------------------|
                      |                                  |
                      |                                  |
                      |                                  |
                      |<########### MEDIA 3 ############>|
                      |   2 video A->B, 2 video B->A     |
                      |<################################>|
                      |                                  |
                      |                                  |
                      |                                  |
                      v                                  v


   In SIP INVITE 1, Alice sends Bob a SIP INVITE including in the SDP
   body the basic audio and video capabilities and the data channel as
   per [I-D.ietf-mmusic-sctp-sdp].  Alice also includes the "sip.clue"
   media feature tag in the INVITE.  A snippet of the SDP showing the
   grouping attribute and the video "m=" line are shown below.  Alice
   has included a "CLUE" group, and included the mid corresponding to a
   data channel in the group (3).  Note that Alice has chosen not to
   include any CLUE-controlled media in the initial offer - the mid
   value of the video line is not included in the "CLUE" group.


      ...
      a=group:CLUE 3
      ...
      m=video 6002 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
      a=sendrecv
      a=mid:2
      ...
      m=application 6100 UDP/DTLS/SCTP webrtc-datachannel
      a=setup:actpass
      a=sctp-port: 5000
      a=dcmap:2 subprotocol="CLUE";ordered=true
      a=mid:3


   Bob responds with a similar SDP in SIP 200 OK 1, which also has a
   "CLUE" group including the mid value of a data channel; due to their
   similarity no SDP snippet is shown here.  Bob wishes to receive
   initial media, and so includes corresponding non-CLUE-controlled
   audio and video lines.  Bob also includes the "sip.clue" media
   feature tag in the 200 OK.  Alice and Bob are each now able to send a
   single audio and video stream.  This is illustrated as MEDIA 1.



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   With the successful initial SDP Offer/Answer exchange complete Alice
   and Bob are also free to negotiate the CLUE data channel.  This is
   illustrated as CLUE DATA CHANNEL ESTABLISHED.

   Once the data channel is established CLUE protocol negotiation
   begins.  In this case Bob was the DTLS client (sending a=active in
   his SDP answer) and hence is the CLUE Channel Initiator and sends a
   CLUE OPTIONS message describing his version support.  On receiving
   that message Alice sends her corresponding CLUE OPTIONS RESPONSE.

   With the OPTIONS phase complete Alice now sends her CLUE
   'advertisement' message (CLUE ADVERTISEMENT 1).  She advertises three
   static Captures representing her three cameras.  She also includes
   switched Captures suitable for two- and one-screen systems.  All of
   these Captures are in a single Capture Scene, with suitable Capture
   Scene Views to tell Bob that he should either subscribe to the three
   static Captures, the two switched Captures or the one switched
   Capture.  Alice has no simultaneity constraints, so includes all six
   Captures in one simultaneous set.  Finally, Alice includes an
   Encoding Group with three Encoding IDs: "enc1", "enc2" and "enc3".
   These Encoding IDs aren't currently valid, but will match the next
   SDP offer she sends.

   Bob received CLUE ADVERTISEMENT 1 but does not yet send a 'configure'
   message, because he has not yet received Alice's Encoding
   information, so as yet he does not know if she will have sufficient
   resources to send him the two streams he ideally wants at a quality
   he is happy with.  Because Bob is not sending an immediate
   'configure' message with the "ack" element set he must send an
   explicit 'ack' message (CLUE ACK 1) to signal receipt of CLUE
   ADVERTISEMENT 1.

   Bob also sends his CLUE 'advertisement' message (CLUE ADVERTISEMENT
   2) - though the diagram shows that this occurs after Alice sends CLUE
   ADVERTISEMENT 1 Bob sends his 'advertisement' message independently
   and does not wait for CLUE ADVERTISEMENT 1 to arrive.  He advertises
   two static Captures representing his cameras.  He also includes a
   single composed Capture for single-screen systems, in which he will
   composite the two camera views into a single video stream.  All three
   Captures are in a single Capture Scene, with suitable Capture Scene
   Views to tell Alice that she should either subscribe to the two
   static Captures, or the single composed Capture.  Bob also has no
   simultaneity constraints, so includes all three Captures in one
   simultaneous set.  Bob also includes a single Encoding Group with two
   Encoding IDs: "foo" and "bar".






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   Similarly, Alice receives CLUE ADVERTISEMENT 2 but does not yet send
   a 'configure' message, because she has not yet received Bob's
   Encoding information, sending instead an 'ack' message (CLUE ACK 2).

   Both sides have now sent their CLUE 'advertisement' messages and an
   SDP exchange is required to negotiate Encodings.  For simplicity, in
   this case Alice is shown sending an INVITE with a new offer; in many
   implementations both sides might send an INVITE, which would be
   resolved by use of the 491 Request Pending resolution mechanism from
   [RFC3261].

   Alice now sends SIP INVITE 2.  She maintains the sendrecv audio,
   video and CLUE "m=" lines, and she adds three new sendonly "m=" lines
   to represent the three CLUE-controlled Encodings she can send.  Each
   of these "m=" lines has a label corresponding to one of the Encoding
   IDs from CLUE ADVERTISEMENT 1.  Each also has its mid added to the
   grouping attribute to show they are controlled by the CLUE data
   channel.  A snippet of the SDP showing the grouping attribute, data
   channel and the video "m=" lines are shown below:
































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      ...
      a=group:CLUE 3 4 5 6
      ...
      m=video 6002 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
      a=sendrecv
      a=mid:2
      ...
      m=application 6100 UDP/DTLS/SCTP webrtc-datachannel
      a=sctp-port: 5000
      a=dcmap:2 subprotocol="CLUE";ordered=true
      a=mid:3
      ...
      m=video 6004 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016
      a=sendonly
      a=mid:4
      a=label:enc1
      m=video 6006 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016
      a=sendonly
      a=mid:5
      a=label:enc2
      m=video 6008 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016
      a=sendonly
      a=mid:6
      a=label:enc3


   Bob now has all the information he needs to decide which streams to
   configure, allowing him to send both a CLUE 'configure' message and
   his SDP answer.  As such he now sends CLUE CONFIGURE 1.  This
   requests the pair of switched Captures that represent Alice's scene,
   and he configures them with encoder ids "enc1" and "enc2".

   Bob also sends his SDP answer as part of SIP 200 OK 2.  Alongside his
   original audio, video and CLUE "m=" lines he includes three
   additional "m=" lines corresponding to the three added by Alice; two
   active recvonly "m= "lines and an inactive "m=" line for the third.
   He adds their mid values to the grouping attribute to show they are
   controlled by the CLUE data channel.  A snippet of the SDP showing
   the grouping attribute and the video "m=" lines are shown below (mid
   100 represents the CLUE data channel, not shown):



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      ...
      a=group:CLUE 11 12 13 100
      ...
      m=video 58722 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
      a=sendrecv
      a=mid:10
      ...
      m=video 58724 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
      a=recvonly
      a=mid:11
      m=video 58726 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
      a=recvonly
      a=mid:12
      m=video 58728 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
      a=inactive
      a=mid:13


   Alice receives Bob's message CLUE CONFIGURE 1 and sends CLUE
   CONFIGURE RESPONSE 1 to ack its reception.  She does not yet send the
   Capture Encodings specified, because at this stage she hasn't
   processed Bob's answer SDP and so hasn't negotiated the ability for
   Bob to receive these streams.

   On receiving SIP 200 OK 2 from Bob Alice sends her SIP ACK (SIP ACK
   2).  She is now able to send the two streams of video Bob requested -
   this is illustrated as MEDIA 2.

   The constraints of offer/answer meant that Bob could not include his
   encoding information as new "m=" lines in SIP 200 OK 2.  As such Bob
   now sends SIP INVITE 3 to generate a new offer.  Along with all the
   streams from SIP 200 OK 2 Bob also includes two new sendonly streams.
   Each stream has a label corresponding to the Encoding IDs in his CLUE
   ADVERTISEMENT 2 message.  He also adds their mid values to the
   grouping attribute to show they are controlled by the CLUE data
   channel.  A snippet of the SDP showing the grouping attribute and the
   video "m=" lines are shown below (mid 100 represents the CLUE data
   channel, not shown):





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      ...
      a=group:CLUE 11 12 14 15 100
      ...
      m=video 58722 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
      a=sendrecv
      a=mid:10
      ...
      m=video 58724 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
      a=recvonly
      a=mid:11
      m=video 58726 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
      a=recvonly
      a=mid:12
      m=video 0 RTP/AVP 96
      a=mid:13
      m=video 58728 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016
      a=sendonly
      a=label:foo
      a=mid:14
      m=video 58730 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016
      a=sendonly
      a=label:bar
      a=mid:15


   Having received this, Alice now has all the information she needs to
   send her CLUE 'configure' message and her SDP answer.  In CLUE
   CONFIGURE 2 she requests the two static Captures from Bob, to be sent
   on Encodings "foo" and "bar".

   Alice also sends SIP 200 OK 3, matching two recvonly "m=" lines to
   Bob's new sendonly lines.  She includes their mid values in the
   grouping attribute to show they are controlled by the CLUE cdata
   hannel.  Alice also now deactivates the initial non-CLUE-controlled
   media, as bidirectional CLUE-controlled media is now available.  A
   snippet of the SDP showing the grouping attribute and the video "m="
   lines are shown below (mid 3 represents the data channel, not shown):




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      ...
      a=group:CLUE 3 4 5 7 8
      ...
      m=video 0 RTP/AVP 96
      a=mid:2
      ...
      m=video 6004 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016
      a=sendonly
      a=mid:4
      a=label:enc1
      m=video 6006 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016
      a=sendonly
      a=mid:5
      a=label:enc2
      m=video 0 RTP/AVP 96
      a=mid:6
      m=video 6010 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
      a=recvonly
      a=mid:7
      m=video 6012 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
      a=recvonly
      a=mid:8


   Bob receives Alice's message CLUE CONFIGURE 2 and sends CLUE
   CONFIGURE RESPONSE 2 to ack its reception.  Bob does not yet send the
   Capture Encodings specified, because he hasn't yet received and
   processed Alice's SDP answer and negotiated the ability to send these
   streams.

   Finally, on receiving SIP 200 OK 3 Bob is now able to send the two
   streams of video Alice requested - this is illustrated as MEDIA 3.

   Both sides of the call are now sending multiple video streams with
   their sources defined via CLUE negotiation.  As the call progresses
   either side can send new 'advertisement' or 'configure' message or
   new SDP offer/answers to add, remove or change what they have
   available or want to receive.





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9.  Example: A call between a CLUE-capable and non-CLUE Endpoint

   In this brief example Alice is a CLUE-capable Endpoint making a call
   to Bob, who is not CLUE-capable (i.e. is not able to use the CLUE
   protocol).



         +----------+                      +-----------+
         |  Alice   |                      |    Bob    |
         |          |                      |           |
         +----+-----+                      +-----+-----+
              |                                  |
              |                                  |
              | SIP INVITE 1                     |
              |--------------------------------->|
              |                                  |
              |                                  |
              |                         200 0K 1 |
              |<---------------------------------|
              |                                  |
              |                                  |
              | SIP ACK 1                        |
              |--------------------------------->|
              |                                  |
              |                                  |
              |                                  |
              |<########### MEDIA 1 ############>|
              |   1 video A->B, 1 video B->A     |
              |<################################>|
              |                                  |
              |                                  |
              |                                  |
              |                                  |
              v                                  v


   In SIP INVITE 1, Alice sends Bob a SIP INVITE including in the SDP
   body the basic audio and video capabilities and the data channel as
   per [I-D.ietf-mmusic-sctp-sdp].  Alice also includes the "sip.clue"
   media feature tag in the INVITE.  A snippet of the SDP showing the
   grouping attribute and the video "m=" line are shown below.  Alice
   has included a "CLUE" group, and included the mid corresponding to a
   data channel in the group (3).  Note that Alice has chosen not to
   include any CLUE-controlled media in the initial offer - the mid
   value of the video line is not included in the "CLUE" group.





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      ...
      a=group:CLUE 3
      ...
      m=video 6002 RTP/AVP 96
      a=rtpmap:96 H264/90000
      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
      a=sendrecv
      a=mid:2
      ...
      m=application 6100 UDP/DTLS/SCTP webrtc-datachannel
      a=sctp-port: 5000
      a=dcmap:2 subprotocol="CLUE";ordered=true
      a=mid:3


   Bob is not CLUE-capable, and hence does not recognize the "CLUE"
   semantic for grouping attribute, nor does he support the data
   channel.  IN SIP 200 OK 1 he responds with an answer with audio and
   video, but with the data channel zeroed.

   From the lack of a CLUE group Alice understands that Bob does not
   support CLUE, or does not wish to use it.  Both sides are now able to
   send a single audio and video stream to each other.  Alice at this
   point begins to send her fallback video: in this case likely a
   switched view from whichever camera shows the current loudest
   participant on her side.

10.  Acknowledgements

   Besides the authors, the team focusing on this draft consists of:
   Roni Even, Simon Pietro-Romano, Roberta Presta.

   Christian Groves, Jonathan Lennox and Adam Roach have contributed
   detailed comments and suggestions.

11.  IANA Considerations

11.1.  New SDP Grouping Framework Attribute

   This document registers the following semantics with IANA in the
   "Semantics for the "group" SDP Attribute" subregistry (under the
   "Session Description Protocol (SDP) Parameters" registry per
   [RFC5888]:








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   Semantics                              Token   Reference
   -------------------------------------  ------  ---------
   CLUE-controlled m-line                 CLUE    [this draft]


11.2.  New SIP Media Feature Tag

   This specification registers a new media feature tag in the SIP
   [RFC3261] tree per the procedures defined in [RFC2506] and [RFC3840].

   Media feature tag name: sip.clue

   ASN.1 Identifier: [to be assigned]

   Summary of the media feature indicated by this tag: This feature tag
   indicates that the device supports CLUE-controlled media.

   Values appropriate for use with this feature tag: Boolean.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms:

   This feature tag is most useful in a communications application for
   describing the capabilities of a device to use the CLUE control
   protocol to negotiate the use of multiple media streams.

   Related standards or documents: [this draft]

   Security Considerations: Security considerations for this media
   feature tag are discussed in Section 12 of [this draft].

   Name(s) & email address(es) of person(s) to contact for further
   information:

   o  Internet Engineering Steering Group: iesg@ietf.org

   Intended usage: COMMON

12.  Security Considerations

   CLUE makes use of a number of protocols and mechanisms, either
   defined by CLUE or long-standing.  The security considerations
   section of the CLUE Framework [I-D.ietf-clue-framework] addresses the
   need to secure these mechanisms by following the recommendations of
   the individual protocols.

   Beyond the need to secure the constituent protocols, the use of CLUE
   does impose additional security concerns.  One area of increased risk



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   involves the potential for a malicious party to subvert a CLUE-
   capable device to attack a third party by driving large volumes of
   media (particularly video) traffic at them by establishing a
   connection to the CLUE-capable device and directing the media to the
   victim.  While this is a risk for all media devices, a CLUE-capable
   device may allow the attacker to configure multiple media streams to
   be sent, significantly increasing the volume of traffic directed at
   the victim.

   This attack can be prevented by ensuring that the media recipient
   intends to receive the media packets.  As such all CLUE-capable
   devices MUST support key negotiation and receiver intent assurance
   via DTLS-SRTP [RFC5763] on CLUE-controlled RTP "m=" lines, and MUST
   use it or some other mechanism that provides receiver intent
   assurance.  All CLUE-controlled RTP "m" lines must be secured and
   implemented using mechanisms such as SRTP [RFC3711].  CLUE
   implementations MAY choose not to require the use of SRTP to secure
   legacy (non-CLUE-controlled) media for backwards compatibility with
   older SIP clients that are incapable of supporting it.

   CLUE also defines a new media feature tag that indicates CLUE
   support.  This tag may be present even in non-CLUE calls, which
   increases the metadata available about the sending device, which can
   help an attacker differentiate between multiple devices and help them
   identify otherwise anonymised users via the fingerprint of features
   their device supports.  To prevent this, SIP signaling used to set up
   CLUE sessions SHOULD always be encrypted using TLS [RFC5630].

   The CLUE protocol also carries additional information that could be
   used to help fingerprint a particular user or to identify the
   specific version of software being used.  CLUE Framework
   [I-D.ietf-clue-protocol] provides details of these issues and how to
   mitigate them.

13.  Change History

   Note to RFC Editor: please remove this section prior to publication

   -15:  Revision by Rob Hanton

      *  Clarified that using an 'EncID' defined in SDP in an CLUE
         ADVERTISEMENT message is only a SHOULD because of the inherent
         race conditions about the ordering of the SDP and CLUE message.
         In contrast, changed the use of 'EncID' in a CLUE CONFIGURE
         message to a MUST as that is defined by the far end and so
         there is no way for the sending of the CONFIGURE to anticipate
         it.




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      *  Updated the description of handling the failure of the CLUE
         channel to reflect the fact that the protocol state machine now
         returns to the IDLE state on failure rather than a specific
         termination state, which also means defining an allowance for
         the CLUE channel being recovered.

      *  Updated all instances of advertisment, configure and ack
         messages throughout to match the styling of the protocol
         document

      *  Security section updated to make DTLs-SRTP mandatory to use as
         well as support unless intent assurance is provided by some
         other mechanism per mailing list proposal (to resolve the
         concern from a previous IETF session of those wanting to use
         CLUE in a closed environment where intent assurance was
         provided by other prorietary mechanisms).

      *  Removed OID value for "sip.clue" media feature tag pending its
         actual assignment on registration, leaving a placeholder

      *  All lower-case uses of 'must', 'should' and 'may' reviewed and
         a few made normative

      *  Fixed various spelling mistakes, clarified grammar, and fixed a
         copy/paste error.

      *  Updated boilerplate to RFC 8174

      *  Some informative references moved to normative.

   -14:  Revision by Rob Hanton

      *  Reference to RFC5245 updated to RFC8445

      *  Updated my name to reflect surname change (Hansen to Hanton).

      *  Reviewed recent changes to clue protocol document and concluded
         that none affected this document

      *  Added recommendation that the SDP O/A spec and clue protocol be
         read prior to this document

      *  Several acronyms expanded at the point of initial use

      *  Some unnecessary normative language replaced with prose

   -13:  Revision by Rob Hansen




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      *  Added a section on handling failures of the protocol channel or
         data channel mid-call - instructions are that media must
         continue as if the clue channel were still established and
         unchanged until CLUE is disabled by either side via SDP
         exchange.

      *  Example in section on efficient operation with non-atomic
         transactions has had all normative language removed and is now
         entirely descriptive (normative language retained in the non-
         example portion).

      *  draft-ietf-clue-protocol-14 reviewed for relevant changes, and
         use of CLUE ACK and RESPONSE messages made consistent with that
         document (ADVERTISEMENT ACKNOWLEDGEMENT and CONFIGURE RESPONSE
         respectively).

      *  Order of authors revised to reflect updates since Jan 2014.

   -12:  Revision by Rob Hansen

      *  Title change to expand and elucidate our totally-not-contrived
         acronym

      *  Explicit reference to RFC3840 added when first mentioning media
         feature tags

      *  Have standardised references to Clue protocol messages to
         ADVERTISEMENT, CONFIGURE and ACK, in line with section 12.4.1.
         of the protocol document (though the protocol document also
         uses ADV and CONF).

      *  'MUST' in opening paragraph of 4.2 changed from normative
         'MUST' to logical 'must'

      *  Per his request, removed Cristian's company affiliation and
         changed his email address

      *  Clarified that an implementation that chooses not to send media
         during the initial negotiation process must still send RTCP as
         normal

      *  Rewrote the section on adding/remove clue m-lines after the
         initial exchange to make clear that this is just standard SDP.
         For non-clue controlled lines, recommended they are deactivated
         by zeroing the port when turning them off after clue is
         successfully negotiated.





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      *  Added guidance that an initial offer containing clue-controlled
         m-lines MUST NOT set them bundle-only unless they somehow know
         the far end actually supports BUNDLE

      *  Added section saying that CLUE devices that do BUNDLE SHOULD do
         rtcp-mux, but that the requirement doesn't exist in the other
         direction (eg, supporting rtcp-mux does not require or imply
         the need to implement BUNDLE)

      *  For clue-controlled m-lines where the sender included more
         encodings than the recipient wants, have standardised on using
         "a=inactive" to not receive RTP on them (previously had a mix
         of "a=inactive" or port 0, or in some cases did not specify).

      *  Page break added before the big ladder diagram in the example

      *  Have added a direction attribute to the SDP example in the data
         channel, and made explicit that Bob is the DTLS client and
         hence the CLUE Channel Initiator.

      *  Have removed all language that referenced the possibility of
         having multiple CLUE groups

      *  Removed names appearing in the authors list from the
         acknowledgements

      *  Changed the contact for the IANA registration to iesg@ietf.org

      *  Security section updated to clarify that DTLS-SRTP must be
         supported (as opposed to DTLS) and removed the reference to
         RFC7202.

      *  Other syntactic tweaks based on Paul and Adam's feedback

   -11:  Revision by Rob Hansen

      *  Some informative references added for SIP and SDP.

      *  'a=mid' lines added to example m-lines with port 0, per RFC5888
         section 6.

      *  Instace of 'must' changed to normative 'MUST', along with
         various minor clarifications and corrections.

      *  Abstract made standalone without citations, per RFC7322 section
         4.3.

      *  RFC editor note added to remove this section.



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   -10:  Revision by Rob Hansen

      *  Changes to draft-ietf-clue-protocol between 07 and 11 reviewed
         to ensure compatibility between documents has been maintained.

      *  Expanded the portion of the document related to fingerprinting
         with info on the CLUE channel as well as SIP.

   -09:  Revision by Rob Hansen

      *  A few minor spelling tweaks

      *  Made removing the CLUE group mandatory when disabling CLUE mid-
         call.  Made clear that any CLUE-controlled m-lines should be
         disabled or else how they're used is up to the implementation.

   -08:  Revision by Rob Hansen

      *  Spelling and grammar fixes from Paul and Christian gratefully
         adopted

      *  Expanded the section on disabling CLUE mid-call to make
         explicit the actions required to disable the CLUE channel
         gracefully, or to handle someone else doing the same.

      *  Made a number of fixes to the example call flow to better
         reflect the recommendations in the document.

   -07:  Revision by Rob Hansen

      *  Removed the entire 'Media line directionality' section as a
         discussion of the pros/cons of using bidirectional vs
         unidirectional schemes wasn't suitable for a finalised version.
         The unidirectionality requirement is covered normatively in an
         earlier section.

      *  BUNDLE no longer includes an address synchronisation step so
         the suggestion to wait until that done has been replaced with
         some general language about following any negotiated
         extensions.

      *  Added OPTIONS negotiation to the example flow, and revised the
         flow to ensure it matched protocol document.

      *  Section on not sending CLUE control media until CLUE
         negotiation completes narrowed to notify that only RTP should
         not be sent until negotiation completes and add RTCP to the




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         list of things that should be sent as normal, in line with
         a=inactive.

      *  Make explicit that m=recvonly lines don't need to have a label,
         as only m=sendonly lines are referenced by CLUE protocol
         messages.

      *  Fix formatting of IANA sections.  Improve syntax of feature tag
         section in line with Paul's suggestions.  Definition of feature
         tag narrowed to be multiple media lines *negotiated via CLUE
         protocol* rather than more generic 'multiple media lines'.

      *  General corrections to grammar, spelling and readability based
         on Christian, Paul and Mark; in many cases suggested text was
         gratefully accepted.

   -06:  Revision by Rob Hansen

      *  State machine interactions updated to match versions in -04 of
         protocol doc.

      *  Section on encoding updated to specify both encID and
         encodingID from data model doc.

      *  Removed the limitations on describing H264 encoding limits
         using SDP syntax as an open issue.

      *  Previous draft had SRTP and DTLS mandatory to implement and to
         use on CLUE- controlled m lines.  Current version has DTLS
         mandatory to implement, and 'security' mandatory to use but
         does not define what that security is.

      *  Terminology reference to framework doc reinforced.  All
         terminology that duplicates framework removed.  All text
         updated with capitalisation that matches framework document's
         terminology.

      *  SDP example syntax updated to match that of ietf-clue-
         datachannel and hence ietf-mmusic-data-channel-sdpneg.

   -05:  Revision by Rob Hansen

      *  SRTP/DTLS made mandatory for CLUE-controlled media lines.

      *  IANA consideration section added (text as proposed by Christian
         Groves).





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      *  Includes provision for dependent streams on seperate "m" lines
         having the same encID as their parent "m" line.

      *  References to putting CLUE-controlled media and data channels
         in more than one CLUE group removed, since the document no
         longer supports using more than one CLUE group.

      *  Section on CLUE controlled media restrictions still applying
         even if the call does not end up being CLUE enabled being
         rewritten to hopefully be clearer.

      *  Other minor syntax improvements.

   -04:  Revision by Rob Hansen

      *  Updated DTLS/SCTP channel syntax in examples to fix errors and
         match latest format defined in draft-ietf-mmusic-sctp-sdp-07.

      *  Clarified the behaviour if an SDP offer includes a CLUE-
         controlled "m" line and the answer accepts that "m" line but
         without CLUE control of that line.

      *  Added a new section on the sending and receiving of CaptureIDs
         in RTP and RTCP.  Includes a section on the necessity of the
         receiver coping with unexpected CaptureIDs (or the lack
         thereof) due to MCCs being redefined in new Advertisement
         messages.

      *  Added reminder on IANA section on registering grouping semantic
         and media feature tag, removed the less formal sections that
         did the same job.

      *  Fixed and clarified issues raised by Christian's document
         review.

      *  Added a number of security considerations.

   -03:  Revision by Rob Hansen

      *  Clarified text on not rejecting messages because they contain
         unknown encIDs.

      *  Removed normative language in section on accepting/rejecting
         non-CLUE-controlled media in the initial answer.

      *  Example SDP updated to include the data channel "m" lines.





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      *  Example call flow updated to show disablement of non-CLUE-
         controlled media once CLUE-controlled media is flowing.

   -02:  Revision by Rob Hansen

      *  Added section on not accepting non-CLUE-controlled "m" lines in
         the initial answer when CLUE is to be negotiated.

      *  Removed previous language attempting to describe media
         restrictions for CLUE-controlled "m" lines that had not been
         configured, and replaced it with much more accurate 'treat as
         "a=inactive" was set'.

      *  Made label element mandatory for CLUE-controlled media (was
         previously "SHOULD include", but there didn't seem a good
         reason for this - anyone wishing to include the "m" line but
         not immediately use it in CLUE can simply leave it out of the
         <encodingIDList>.)

      *  Added a section on the specifics of relating encodings in SDP
         to <encID> elements in the CLUE protocol, including the fact
         that both Advertisement and Configure messages reference the
         *encoding* (eg, in the Configure case the sender of the
         Configure message includes the labels of the recipient's "m"
         lines as their <encID> contents).

      *  Minor revisions to the section on complying with normative SDP/
         CLUEstate machine language to clarify that these were not new
         normative language, merely that existing normative language
         still applies.

      *  Removed appendices which previously contained information to be
         transferred to the protocol and data channel drafts.  Removed
         other text that discussed alternatives to the current approach.

      *  Cleaned up some 'todo' text.

   -01:  Revision by Rob Hansen

      *  Revised terminology - removed the term 'CLUE-enabled' device as
         insufficiently distinct from 'CLUE-capable' and instead added a
         term for 'CLUE-enabled' calls.

      *  Removed text forbidding RTCP and instead added text that ICE/
         DTLS negotiation for CLUE controlled media must be done as
         normal irrespective of CLUE negotiation.





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      *  Changed 'sip.telepresence' to 'sip.clue' and 'TELEPRESENCE'
         grouping semantic back to CLUE.

      *  Made it mandatory to have exactly one mid corresponding to a
         data channel in a CLUE group

      *  Forbade having multiple CLUE groups unless a specification for
         doing so is published.

      *  Refactored SDP-related text; previously the encoding
         information had been in the "initial offer" section despite the
         fact that we recommend that the initial offer doesn't actually
         include any encodings.  I moved the specifications of encodings
         and how they're received to an earlier, seperate section.

      *  Added text on how the state machines in CLUE and SDP are
         allowed to affect one another, and further recommendations on
         how a device should handle the sending of CLUE and SDP changes.

   -00:  Revision by Rob Hansen

      *  Submitted as -00 working group document

   draft-kyzivat-08:  Revisions by Rob Hansen

      *  Added media feature tag for CLUE support ('sip.telepresence')

      *  Changed grouping semantic from 'CLUE' to 'TELEPRESENCE'

      *  Restructured document to be more centred on the grouping
         semantic and its use with O/A

      *  Lots of additional text on usage of the grouping semantic

      *  Stricter definition of CLUE-controlled m lines and how they
         work

      *  Some additional text on defining what happens when CLUE
         supports is added or removed

      *  Added details on when to not send RTCP for CLUE-controlled "m"
         lines.

      *  Added a section on using BUNDLE with CLUE

      *  Updated data channel references to point at new WG document
         rather than indivual draft




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   draft-kyzivat-07:  Revisions by Rob Hansen

      *  Removed the text providing arguments for encoding limits being
         in SDP and Encoding Groups in the CLUE protocol in favor of the
         specifics of how to negotiate encodings in SDP

      *  Added normative language on the setting up of a CLUE call, and
         added sections on mid-call changes to the CLUE status.

      *  Added references to [I-D.ietf-clue-datachannel] where
         appropriate.

      *  Added some terminology for various types of CLUE and non-CLUE
         states of operation.

      *  Moved language related to topics that should be in
         [I-D.ietf-clue-datachannel] and [I-D.ietf-clue-protocol], but
         that has not yet been resolved in those documents, into an
         appendix.

   draft-kyzivat-06:  Revisions by Rob Hansen

      *  Removed CLUE message XML schema and details that are now in
         draft-presta-clue-protocol

      *  Encoding limits in SDP section updated to note that this has
         been investigated and discussed and is the current working
         assumption of the WG, though consensus has not been fully
         achieved.

      *  A section has also been added on the current mandation of
         unidirectional "m" lines.

      *  Updated CLUE messaging in example call flow to match draft-
         presta-clue-protocol-03

   draft-kyzivat-05:  Revisions by pkyzivat:

      *  Specified versioning model and mechanism.

      *  Added explicit response to all messages.

      *  Rearranged text to work with the above changes.  (Which
         rendered diff almost useless.)

   draft-kyzivat-04:  Revisions by Rob Hansen: ???

   draft-kyzivat-03:  Revisions by pkyzivat:



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      *  Added a syntax section with an XML schema for CLUE messages.
         This is a strawhorse, and is very incomplete, but it
         establishes a template for doing this based on elements defined
         in the data model.  (Thanks to Roberta for help with this!)

      *  Did some rewording to fit the syntax section in and reference
         it.

      *  Did some relatively minor restructuring of the document to make
         it flow better in a logical way.

   draft-kyzivat-02:  A bunch of revisions by pkyzivat:

      *  Moved roberta's call flows to a more appropriate place in the
         document.

      *  New section on versioning.

      *  New section on NAK.

      *  A couple of possible alternatives for message acknowledgment.

      *  Some discussion of when/how to signal changes in provider
         state.

      *  Some discussion about the handling of transport errors.

      *  Added a change history section.

      These were developed by Lennard Xiao, Christian Groves and Paul,
      so added Lennard and Christian as authors.

   draft-kyzivat-01:  Updated by roberta to include some sample call
      flows.

   draft-kyzivat-00:  Initial version by pkyzivat.  Established general
      outline for the document, and specified a few things thought to
      represent wg consensus.

14.  References

14.1.  Normative References

   [I-D.ietf-clue-data-model-schema]
              Presta, R. and S. Romano, "An XML Schema for the CLUE data
              model", draft-ietf-clue-data-model-schema-17 (work in
              progress), August 2016.




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   [I-D.ietf-clue-datachannel]
              Holmberg, C., "CLUE Protocol data channel", draft-ietf-
              clue-datachannel-18 (work in progress), April 2019.

   [I-D.ietf-clue-framework]
              Duckworth, M., Pepperell, A., and S. Wenger, "Framework
              for Telepresence Multi-Streams", draft-ietf-clue-
              framework-25 (work in progress), January 2016.

   [I-D.ietf-clue-protocol]
              Presta, R. and S. Romano, "Protocol for Controlling
              Multiple Streams for Telepresence (CLUE)", draft-ietf-
              clue-protocol-19 (work in progress), July 2019.

   [I-D.ietf-clue-rtp-mapping]
              Even, R. and J. Lennox, "Mapping RTP streams to CLUE Media
              Captures", draft-ietf-clue-rtp-mapping-14 (work in
              progress), February 2017.

   [I-D.ietf-mmusic-data-channel-sdpneg]
              Drage, K., Makaraju, M., Ejzak, R., Marcon, J., and R.
              Even, "SDP-based Data Channel Negotiation", draft-ietf-
              mmusic-data-channel-sdpneg-28 (work in progress), May
              2019.

   [I-D.ietf-mmusic-sctp-sdp]
              Holmberg, C., Shpount, R., Loreto, S., and G. Camarillo,
              "Session Description Protocol (SDP) Offer/Answer
              Procedures For Stream Control Transmission Protocol (SCTP)
              over Datagram Transport Layer Security (DTLS) Transport.",
              draft-ietf-mmusic-sctp-sdp-26 (work in progress), April
              2017.

   [I-D.ietf-mmusic-sdp-bundle-negotiation]
              Holmberg, C., Alvestrand, H., and C. Jennings,
              "Negotiating Media Multiplexing Using the Session
              Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
              negotiation-54 (work in progress), December 2018.

   [I-D.ietf-rtcweb-data-channel]
              Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data
              Channels", draft-ietf-rtcweb-data-channel-13 (work in
              progress), January 2015.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.



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   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,
              <https://www.rfc-editor.org/info/rfc3711>.

   [RFC3840]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
              "Indicating User Agent Capabilities in the Session
              Initiation Protocol (SIP)", RFC 3840,
              DOI 10.17487/RFC3840, August 2004,
              <https://www.rfc-editor.org/info/rfc3840>.

   [RFC4574]  Levin, O. and G. Camarillo, "The Session Description
              Protocol (SDP) Label Attribute", RFC 4574,
              DOI 10.17487/RFC4574, August 2006,
              <https://www.rfc-editor.org/info/rfc4574>.

   [RFC5763]  Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
              for Establishing a Secure Real-time Transport Protocol
              (SRTP) Security Context Using Datagram Transport Layer
              Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
              2010, <https://www.rfc-editor.org/info/rfc5763>.

   [RFC5888]  Camarillo, G. and H. Schulzrinne, "The Session Description
              Protocol (SDP) Grouping Framework", RFC 5888,
              DOI 10.17487/RFC5888, June 2010,
              <https://www.rfc-editor.org/info/rfc5888>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

14.2.  Informative References

   [RFC2506]  Holtman, K., Mutz, A., and T. Hardie, "Media Feature Tag
              Registration Procedure", BCP 31, RFC 2506,
              DOI 10.17487/RFC2506, March 1999,
              <https://www.rfc-editor.org/info/rfc2506>.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <https://www.rfc-editor.org/info/rfc3261>.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,
              <https://www.rfc-editor.org/info/rfc3264>.



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   [RFC3311]  Rosenberg, J., "The Session Initiation Protocol (SIP)
              UPDATE Method", RFC 3311, DOI 10.17487/RFC3311, October
              2002, <https://www.rfc-editor.org/info/rfc3311>.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <https://www.rfc-editor.org/info/rfc4566>.

   [RFC5630]  Audet, F., "The Use of the SIPS URI Scheme in the Session
              Initiation Protocol (SIP)", RFC 5630,
              DOI 10.17487/RFC5630, October 2009,
              <https://www.rfc-editor.org/info/rfc5630>.

   [RFC5761]  Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
              Control Packets on a Single Port", RFC 5761,
              DOI 10.17487/RFC5761, April 2010,
              <https://www.rfc-editor.org/info/rfc5761>.

   [RFC6184]  Wang, Y., Even, R., Kristensen, T., and R. Jesup, "RTP
              Payload Format for H.264 Video", RFC 6184,
              DOI 10.17487/RFC6184, May 2011,
              <https://www.rfc-editor.org/info/rfc6184>.

   [RFC8445]  Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
              Connectivity Establishment (ICE): A Protocol for Network
              Address Translator (NAT) Traversal", RFC 8445,
              DOI 10.17487/RFC8445, July 2018,
              <https://www.rfc-editor.org/info/rfc8445>.

Authors' Addresses

   Robert Hanton
   Cisco Systems

   Email: rohanse2@cisco.com


   Paul Kyzivat

   Email: pkyzivat@alum.mit.edu


   Lennard Xiao
   Huawei

   Email: lennard.xiao@huawei.com





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   Christian Groves

   Email: cngroves.std@gmail.com
















































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