Internet Engineering Task Force (IETF)                         R. HantonRequest for Comments: 8848                                 Cisco SystemsCategory: Experimental                                        P. KyzivatISSN: 2070-1721                                                                                                                          L. Xiao                                                Beijing Chuangshiyoulian                                                               C. Groves                                                            January 2021  Session Signaling for Controlling Multiple Streams for Telepresence                                 (CLUE)Abstract   This document is about Controlling Multiple Streams for Telepresence   (CLUE) signaling.  It specifies how the CLUE protocol and the CLUE   data channel are used in conjunction with each other and with   existing signaling mechanisms, such as SIP and the Session   Description Protocol (SDP), to produce a telepresence call.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for examination, experimental implementation, and   evaluation.   This document defines an Experimental Protocol for the Internet   community.  This document is a product of the Internet Engineering   Task Force (IETF).  It represents the consensus of the IETF   community.  It has received public review and has been approved for   publication by the Internet Engineering Steering Group (IESG).  Not   all documents approved by the IESG are candidates for any level of   Internet Standard; see Section 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained at   https://www.rfc-editor.org/info/rfc8848.Copyright Notice   Copyright (c) 2021 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject to BCP 78 and the IETF Trust's Legal   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   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   2.  Terminology   3.  Media Feature Tag Definition   4.  SDP Grouping Framework CLUE Extension Semantics     4.1.  General     4.2.  The CLUE Data Channel and the CLUE Grouping Semantic     4.3.  CLUE-Controlled Media and the CLUE Grouping Semantic     4.4.  SDP Semantics for CLUE-Controlled Media       4.4.1.  Signaling CLUE Encodings         4.4.1.1.  Referencing Encodings in the CLUE Protocol       4.4.2.  Negotiating Receipt of CLUE Capture Encodings in SDP     4.5.  SDP Offer/Answer Procedures       4.5.1.  Generating the Initial Offer       4.5.2.  Generating the Answer         4.5.2.1.  Negotiating Use of CLUE and the CLUE Data Channel         4.5.2.2.  Negotiating CLUE-Controlled Media         4.5.2.3.  Negotiating Non-CLUE-controlled Media       4.5.3.  Processing the Initial Offer/Answer Negotiation         4.5.3.1.  Successful CLUE Negotiation         4.5.3.2.  CLUE Negotiation Failure       4.5.4.  Modifying the Session         4.5.4.1.  Adding and Removing CLUE-Controlled Media         4.5.4.2.  Enabling CLUE Mid-Call         4.5.4.3.  Disabling CLUE Mid-Call         4.5.4.4.  CLUE Protocol Failure Mid-Call   5.  Interaction of the CLUE Protocol and SDP Negotiations     5.1.  Independence of SDP and CLUE Negotiation     5.2.  Constraints on Sending Media     5.3.  Recommendations for Operating with Non-atomic Operations   6.  Interaction of the CLUE Protocol and RTP/RTCP CaptureID     6.1.  CaptureID Reception during MCC Redefinition   7.  Multiplexing of CLUE-Controlled Media Using BUNDLE     7.1.  Overview     7.2.  Usage of BUNDLE with CLUE       7.2.1.  Generating the Initial Offer       7.2.2.  Multiplexing of the Data Channel and RTP Media   8.  Example: A Call between Two CLUE-Capable Endpoints   9.  Example: A Call between a CLUE-Capable and Non-CLUE Endpoint   10. IANA Considerations     10.1.  New SDP Grouping Framework Attribute     10.2.  New SIP Media Feature Tag   11. Security Considerations   12. References     12.1.  Normative References     12.2.  Informative References   Acknowledgements   Authors' Addresses1.  Introduction   To enable devices to participate in a telepresence call, where they   select the sources they wish to view, receive those media sources,   and display them in an optimal fashion, Controlling Multiple Streams   for Telepresence (CLUE) employs two principal and interrelated   protocol negotiations.  SDP [RFC4566], conveyed via SIP [RFC3261], is   used to negotiate the specific media capabilities that can be   delivered to specific addresses on a device.  Meanwhile, CLUE   protocol messages [RFC8847], transported via a CLUE data channel   [RFC8850], are used to negotiate the Capture Sources available, their   attributes, and any constraints in their use.  They also allow the   far-end device to specify which Captures they wish to receive.  It is   recommended that those documents be read prior to this one as this   document assumes familiarity with those protocols and hence uses   terminology from each with limited introduction.   Beyond negotiating the CLUE channel, SDP is also used to negotiate   the details of supported media streams and the maximum capability of   each of those streams.  As the CLUE Framework [RFC8845] defines a   manner in which the Media Provider expresses their maximum Encoding   Group capabilities, SDP is also used to express the encoding limits   for each potential Encoding.   Backwards compatibility is an important consideration of the   protocol: it is vital that a CLUE-capable device contacting a device   that does not support CLUE is able to fall back to a fully functional   non-CLUE call.  The document also defines how a non-CLUE call may be   upgraded to CLUE mid-call and, similarly, how CLUE functionality can   be removed mid-call to return to a standard non-CLUE call.2.  Terminology   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and   "OPTIONAL" in this document are to be interpreted as described in   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all   capitals, as shown here.   This document uses terminology defined in the CLUE Framework   [RFC8845].   A few additional terms specific to this document are defined as   follows:   CLUE-controlled media:  A media "m=" line that is under CLUE control;      the Capture Source that provides the media on this "m=" line is      negotiated in CLUE.  See Section 4 for details on how this control      is signaled in SDP.  There is a corresponding "non-CLUE-      controlled" media term.   non-CLUE device:  A device that supports standard SIP and SDP but      either does not support CLUE or does support CLUE but does not      currently wish to invoke CLUE capabilities.   RTCP:  RTP Control Protocol.   SCTP:  Stream Control Transmission Protocol.   STUN:  Session Traversal Utilities for NAT.3.  Media Feature Tag Definition   The "sip.clue" media feature tag [RFC3840] indicates support for CLUE   in SIP [RFC3261] calls.  A CLUE-capable device SHOULD include this   media feature tag in its REGISTER requests and OPTION responses.  It   SHOULD also include the media feature tag in INVITE and UPDATE   [RFC3311] requests and responses.   Presence of the media feature tag in the contact field of a request   or response can be used to determine that the far end supports CLUE.4.  SDP Grouping Framework CLUE Extension Semantics4.1.  General   This section defines a new SDP Grouping Framework [RFC5888] extension   called 'CLUE'.   The CLUE extension can be indicated using an SDP session-level   'group' attribute.  Each SDP media "m=" line that is included in this   group, using SDP media-level mid attributes, is CLUE controlled by a   CLUE data channel that is also included in this CLUE group.   Currently, only support for a single CLUE group is specified; support   for multiple CLUE groups in a single session is outside the scope of   this document.  A device MUST NOT include more than one CLUE group in   its SDP message unless it is following a specification that defines   how multiple CLUE channels are signaled and is able to either   determine that the other side of the SDP exchange supports multiple   CLUE channels or fail gracefully in the event it does not.4.2.  The CLUE Data Channel and the CLUE Grouping Semantic   The CLUE data channel [RFC8850] is a bidirectional data channel   [RFC8831] used for the transport of CLUE messages, conveyed within an   SCTP over DTLS connection.  This channel must be established before   CLUE protocol messages can be exchanged and CLUE-controlled media can   be sent.   The data channel is negotiated over SDP as described in [RFC8864].  A   CLUE-capable device wishing to negotiate CLUE MUST also include a   CLUE group in their SDP Offer or Answer and include the "mid" of the   "m=" line for the data channel in that group.  The CLUE group MUST   include the "mid" of the "m=" line for one (and only one) data   channel.   Presence of the data channel in the CLUE group in an SDP Offer or   Answer also serves, along with the "sip.clue" media feature tag, as   an indication that the device supports CLUE and wishes to upgrade the   call to include CLUE-controlled media.  A CLUE-capable device SHOULD   include a data channel "m=" line in offers and, when allowed by   [RFC3264], answers.4.3.  CLUE-Controlled Media and the CLUE Grouping Semantic   CLUE-controlled media lines in an SDP are "m=" lines in which the   content of the media streams to be sent is negotiated via the CLUE   protocol [RFC8847].  For an "m=" line to be CLUE controlled, its   "mid" attribute value MUST be included in the CLUE group.  CLUE-   controlled media is controlled by the CLUE protocol as negotiated on   the CLUE data channel with a "mid" included in the CLUE group.   "m=" lines not specified as being under CLUE control follow normal   rules for media streams negotiated in SDP as defined in documents   such as [RFC3264].   The restrictions on CLUE-controlled media that are defined below   always apply to "m=" lines in an SDP Offer or Answer, even if   negotiation of the data channel in SDP failed due to lack of CLUE   support by the remote device or for any other reason, or in an offer   if the recipient does not include the "mid" of the corresponding "m="   line in their CLUE group.4.4.  SDP Semantics for CLUE-Controlled Media4.4.1.  Signaling CLUE Encodings   The CLUE Framework [RFC8845] defines the concept of "Encodings",   which represent the sender's encode ability.  Each Encoding the Media   Provider wishes to signal is done so via an "m=" line of the   appropriate media type, which MUST be marked as sendonly with the   "a=sendonly" attribute or as inactive with the "a=inactive"   attribute.   The encoder limits of active (e.g., "a=sendonly") Encodings can then   be expressed using existing SDP syntax.  For instance, for H.264, see   Table 6 in Section 8.2.2 of [RFC6184] for a list of valid parameters   for representing encoder sender stream limits.   These Encodings are CLUE controlled and hence MUST include a "mid" in   the CLUE group as defined above.   In addition to the normal restrictions defined in [RFC3264], the   stream MUST be treated as if the "m=" line direction attribute had   been set to "a=inactive" until the Media Provider has received a   valid CLUE 'configure' message specifying the Capture to be used for   this stream.  This means that RTP packets MUST NOT be sent until   configuration is complete, while non-media packets such as STUN,   RTCP, and DTLS MUST be sent as per their relevant specifications, if   negotiated.   Every "m=" line representing a CLUE Encoding MUST contain a "label"   attribute as defined in [RFC4574].  This label is used to identify   the Encoding by the sender in CLUE 'advertisement' messages and by   the receiver in CLUE 'configure' messages.  Each label used for a   CLUE-controlled "m=" line MUST be different from the label on all   other "m=" lines in the CLUE group, unless an "m=" line represents a   dependent stream related to another "m=" line (such as a Forward   Error Correction (FEC) stream), in which case it MUST have the same   label value as the "m=" line on which it depends.4.4.1.1.  Referencing Encodings in the CLUE Protocol   CLUE Encodings are defined in SDP but can be referenced from CLUE   protocol messages -- this is how the protocol defines which Encodings   are a part of an Encoding Group (in 'advertisement' messages) and   which Encoding is used to encode a specific Capture (in 'configure'   messages).  The labels on the CLUE-controlled "m=" lines are the   references that are used in the CLUE protocol.   Each <encID> (in encodingIDList) in a CLUE 'advertisement' message   SHOULD represent an Encoding defined in SDP; the specific Encoding   referenced is a CLUE-controlled "m=" line in the most recent SDP   Offer/Answer message sent by the sender of the 'advertisement'   message with a label value corresponding to the text content of the   <encID>.  If the <encID> is not defined in SDP, it MUST be one it   anticipates sending in a subsequent SDP Offer/Answer exchange.   Each <encodingID> (in captureEncodingType) in a CLUE 'configure'   message MUST represent an Encoding defined in SDP; the specific   Encoding referenced is a CLUE-controlled "m=" line in the most recent   SDP Offer/Answer message received by the sender of the 'configure'   message with a label value corresponding to the text content of the   <encodingID>.   Note that the non-atomic nature of SDP/CLUE protocol interaction may   mean that there are temporary periods where an <encID>/<encodingID>   in a CLUE message does not reference an SDP "m=" line, or where an   Encoding represented in SDP is not referenced in a CLUE protocol   message.  See Section 5 for specifics.4.4.2.  Negotiating Receipt of CLUE Capture Encodings in SDP   A receiver who wishes to receive a CLUE stream via a specific   Encoding requires an "a=recvonly" "m=" line that matches the   "a=sendonly" Encoding.   These "m=" lines are CLUE controlled and hence MUST include their   "mid" in the CLUE group.  They MAY include a "label" attribute, but   this is not required by CLUE, as only label values associated with   "a=sendonly" Encodings are referenced by CLUE protocol messages.4.5.  SDP Offer/Answer Procedures4.5.1.  Generating the Initial Offer   A CLUE-capable device sending an initial SDP Offer of a SIP session   and wishing to negotiate CLUE will include an "m=" line for the data   channel to convey the CLUE protocol, along with a CLUE group   containing the "mid" of the data channel "m=" line.   For interoperability with non-CLUE devices, a CLUE-capable device   sending an initial SDP Offer SHOULD NOT include any "m=" line for   CLUE-controlled media beyond the "m=" line for the CLUE data channel,   and it SHOULD include at least one non-CLUE-controlled media "m="   line.   If the device has evidence that the receiver is also CLUE capable,   for instance, due to receiving an initial INVITE with no SDP but   including a "sip.clue" media feature tag, the above recommendation is   waived, and the initial offer MAY contain "m=" lines for CLUE-   controlled media.   With the same interoperability recommendations as for Encodings, the   sender of the initial SDP Offer MAY also include "a=recvonly" media   lines to preallocate "m=" lines to receive media.  Alternatively, it   MAY wait until CLUE protocol negotiation has completed before   including these lines in a new offer/answer exchange -- see Section 5   for recommendations.4.5.2.  Generating the Answer4.5.2.1.  Negotiating Use of CLUE and the CLUE Data Channel   If the recipient of an initial offer is CLUE capable, and the offer   contains both an "m=" line for a data channel and a CLUE group   containing the "mid" for that "m=" line, they SHOULD negotiate data   channel support for an "m=" line and include the "mid" of that "m="   line in a corresponding CLUE group.   A CLUE-capable recipient that receives an "m=" line for a data   channel but no corresponding CLUE group containing the "mid" of that   "m=" line MAY still include a corresponding data channel "m=" line if   there are any other non-CLUE protocols it can convey over that   channel, but the use of the CLUE protocol MUST NOT be negotiated on   this channel.4.5.2.2.  Negotiating CLUE-Controlled Media   If the initial offer contained "a=recvonly" CLUE-controlled media   lines, the recipient SHOULD include corresponding "a=sendonly" CLUE-   controlled media lines for accepted Encodings, up to the maximum   number of Encodings it wishes to advertise.  As CLUE-controlled   media, the "mid" of these "m=" lines MUST be included in the   corresponding CLUE group.  The recipient MUST set the direction of   the corresponding "m=" lines of any remaining "a=recvonly" CLUE-   controlled media lines received in the offer to "a=inactive".   If the initial offer contained "a=sendonly" CLUE-controlled media   lines, the recipient MAY include corresponding "a=recvonly" CLUE-   controlled media lines, up to the maximum number of Capture Encodings   it wishes to receive.  Alternatively, it MAY wait until CLUE protocol   negotiation has completed before including these lines in a new   offer/answer exchange -- see Section 5 for recommendations.  The   recipient MUST set the direction of the corresponding "m=" lines of   any remaining "a=sendonly" CLUE-controlled media lines received in   the offer to "a=inactive".4.5.2.3.  Negotiating Non-CLUE-controlled Media   A CLUE-controlled device implementation MAY prefer to render initial,   single-stream audio and/or video for the user as rapidly as possible,   transitioning to CLUE-controlled media once that has been negotiated.   Alternatively, an implementation MAY wish to suppress initial media,   only providing media once the final, CLUE-controlled streams have   been negotiated.   The receiver of the initial offer, if making the call CLUE-enabled   with their SDP Answer, can make their preference clear by their   action in accepting or rejecting non-CLUE-controlled media lines.   Rejecting these "m=" lines will ensure that no non-CLUE-controlled   media flows before the CLUE-controlled media is negotiated.  In   contrast, accepting one or more non-CLUE-controlled "m=" lines in   this initial answer will enable initial media to flow.   If the answerer chooses to send initial non-CLUE-controlled media in   a CLUE-enabled call, Section 4.5.4.1 addresses the need to disable it   once the CLUE-controlled media is fully negotiated.4.5.3.  Processing the Initial Offer/Answer Negotiation   In the event that both the offer and answer include a data channel   "m=" line with a "mid" value included in corresponding CLUE groups,   CLUE has been successfully negotiated, and the call is now CLUE   enabled.  If not, then the call is not CLUE enabled.4.5.3.1.  Successful CLUE Negotiation   In the event of successful CLUE enablement of the call, devices MUST   now begin negotiation of the CLUE channel; see [RFC8850] for   negotiation details.  If negotiation is successful, the sending of   CLUE protocol messages [RFC8847] can begin.   A CLUE-capable device MAY choose not to send RTP on the non-CLUE-   controlled channels during the period in which control of the CLUE-   controlled media lines is being negotiated (though RTCP MUST still be   sent and received as normal).  However, a CLUE-capable device MUST   still be prepared to receive media on non-CLUE-controlled media lines   that have been successfully negotiated as defined in [RFC3264].   If either side of the call wishes to add additional CLUE-controlled   "m=" lines to send or receive CLUE-controlled media, they MAY now   send a SIP request with a new SDP Offer following the normal rules of   SDP Offer/Answer and any negotiated extensions.4.5.3.2.  CLUE Negotiation Failure   In the event that the negotiation of CLUE fails and the call is not   CLUE enabled once the initial offer/answer negotiation completes,   then CLUE is not in use in the call.  CLUE-capable devices MUST   either revert to non-CLUE behavior or terminate the call.4.5.4.  Modifying the Session4.5.4.1.  Adding and Removing CLUE-Controlled Media   Subsequent offer/answer exchanges MAY add additional "m=" lines for   CLUE-controlled media or activate or deactivate existing "m=" lines   per the standard SDP mechanisms.   In most cases, at least one additional exchange after the initial   offer/answer exchange will be required before both sides have added   all the Encodings and the ability to receive Encodings that they   desire.  Devices MAY delay adding "a=recvonly" CLUE-controlled "m="   lines until after CLUE protocol negotiation completes -- see   Section 5 for recommendations.   Once CLUE media has been successfully negotiated, devices SHOULD   ensure that non-CLUE-controlled media is deactivated by setting their   ports to 0 in cases where it corresponds to the media type of CLUE-   controlled media that has been successfully negotiated.  This   deactivation may require an additional SDP exchange or may be   incorporated into one that is part of the CLUE negotiation.4.5.4.2.  Enabling CLUE Mid-Call   A CLUE-capable device that receives an initial SDP Offer from a non-   CLUE device SHOULD include a new data channel "m=" line and   corresponding CLUE group in any subsequent offers it sends, to   indicate that it is CLUE capable.   If, in an ongoing non-CLUE call, an SDP Offer/Answer exchange   completes with both sides having included a data channel "m=" line in   their SDP and with the "mid" for that channel in a corresponding CLUE   group, then the call is now CLUE enabled; negotiation of the data   channel and subsequently the CLUE protocol begins.4.5.4.3.  Disabling CLUE Mid-Call   If, during an ongoing CLUE-enabled call, a device wishes to disable   CLUE, it can do so by following the procedures for closing a data   channel as defined in Section 6.6.1 of [RFC8864]: sending a new SDP   Offer/Answer exchange and subsequent SCTP Stream Sequence Number   (SSN) reset for the CLUE channel.  It MUST also remove the CLUE   group.  Without the CLUE group, any "m=" lines that were previously   CLUE controlled no longer are; implementations MAY disable them by   setting their ports to 0 or MAY continue to use them -- in the latter   case, how they are used is outside the scope of this document.   If a device follows the procedure above, or an SDP Offer/Answer   negotiation completes in a fashion in which either the "m=" CLUE data   channel line was not successfully negotiated and/or one side did not   include the data channel in the CLUE group, then CLUE for this call   is disabled.  In the event that this occurs, CLUE is no longer   enabled.  Any active "m=" lines still included in the CLUE group are   no longer CLUE controlled, and the implementation MAY either disable   them in a subsequent negotiation or continue to use them in some   other fashion.  If the data channel is still present but not included   in the CLUE group semantic, CLUE protocol messages MUST no longer be   sent.4.5.4.4.  CLUE Protocol Failure Mid-Call   In contrast to the specific disablement of the use of CLUE described   above, the CLUE channel may fail unexpectedly.  Two circumstances   where this can occur are:   *  The CLUE data channel terminates, either gracefully or      ungracefully, without any corresponding SDP renegotiation.   *  A channel error of the CLUE protocol causes it to return to the      IDLE state as defined in Section 6 of [RFC8847].   In this circumstance, implementations SHOULD continue to transmit and   receive CLUE-controlled media on the basis of the last negotiated   CLUE messages, until the CLUE protocol is re-established (in the   event of a channel error) or disabled mid-call by an SDP exchange as   defined in Section 4.5.4.3.  Implementations MAY choose to send such   an SDP request to disable CLUE immediately or MAY continue on in a   call-preservation mode.5.  Interaction of the CLUE Protocol and SDP Negotiations   Information about media streams in CLUE is split between two message   types: SDP, which defines media addresses and limits, and the CLUE   channel, which defines properties of Capture Devices available, scene   information, and additional constraints.  As a result, certain   operations, such as advertising support for a new transmissible   Capture with an associated stream, cannot be performed atomically, as   they require changes to both SDP and CLUE messaging.   This section defines how the negotiation of the two protocols   interact, provides some recommendations on dealing with intermediate   stages in non-atomic operations, and mandates additional constraints   on when CLUE-configured media can be sent.5.1.  Independence of SDP and CLUE Negotiation   To avoid the need to implement interlocking state machines with the   potential to reach invalid states if messages were to be lost, or be   rewritten en route by middleboxes, the state machines in SDP and CLUE   operate independently.  The state of the CLUE channel does not   restrict when an implementation may send a new SDP Offer or Answer;   likewise, the implementation's ability to send a new CLUE   'advertisement' or 'configure' message is not restricted by the   results of or the state of the most recent SDP negotiation (unless   the SDP negotiation has removed the CLUE channel).   The primary implication of this is that a device may receive an SDP   Offer/Answer message with a CLUE Encoding for which it does not yet   have Capture information or receive a CLUE 'configure' message   specifying a Capture Encoding for which the far end has not   negotiated a media stream in SDP.   CLUE messages contain an <encID> (in encodingIDList) or <encodingID>   (in captureEncodingType), which is used to identify a specific   Encoding or captureEncoding in SDP; see [RFC8846] for specifics.  The   non-atomic nature of CLUE negotiation means that a sender may wish to   send a new CLUE 'advertisement' message before the corresponding SDP   message.  As such, the sender of the CLUE message MAY include an   <encID> that does not currently match a CLUE-controlled "m=" line   label in SDP; a CLUE-capable implementation MUST NOT reject a CLUE   protocol message solely because it contains <encID> elements that do   not match a label in SDP.   The current state of the CLUE Participant or Media Provider/Consumer   state machines does not affect compliance with any of the normative   language of [RFC3264].  That is, they MUST NOT delay an ongoing SDP   exchange as part of a SIP server or client transaction; an   implementation MUST NOT delay an SDP exchange while waiting for CLUE   negotiation to complete or for a 'configure' message to arrive.   Similarly, a device in a CLUE-enabled call MUST NOT delay any   mandatory state transitions in the CLUE Participant or Media   Provider/Consumer state machines due to the presence or absence of an   ongoing SDP exchange.   A device with the CLUE Participant state machine in the ACTIVE state   MAY choose to delay moving from ESTABLISHED to ADV (Media Provider   state machine) or from ESTABLISHED to WAIT FOR CONF RESPONSE (Media   Consumer state machine) based on the SDP state.  See [RFC8847] for   CLUE state machine specifics.  Similarly, a device MAY choose to   delay initiating a new SDP exchange based on the state of their CLUE   state machines.5.2.  Constraints on Sending Media   While SDP and CLUE message states do not impose constraints on each   other, both impose constraints on the sending of media -- CLUE-   controlled media MUST NOT be sent unless it has been negotiated in   both CLUE and SDP: an implementation MUST NOT send a specific CLUE   Capture Encoding unless its most recent SDP exchange contains an   active media channel for that Encoding AND it has received a CLUE   'configure' message specifying a valid Capture for that Encoding.5.3.  Recommendations for Operating with Non-atomic Operations   CLUE-capable devices MUST be able to handle states in which CLUE   messages make reference to EncodingIDs that do not match the most   recently received SDP, irrespective of the order in which SDP and   CLUE messages are received.  While these mismatches will usually be   transitory, a device MUST be able to cope with such mismatches   remaining indefinitely.  However, this document makes some   recommendations on message ordering for these non-atomic transitions.   CLUE-capable devices MUST ensure that any inconsistencies between SDP   and CLUE signaling are temporary by sending updated SDP or CLUE   messages as soon as the relevant state machines and other constraints   permit.   Generally, implementations that receive messages with incomplete   information will be most efficient if they wait until they have the   corresponding information they lack before sending messages to make   changes related to that information.  For example, an answerer that   receives a new SDP Offer with three new "a=sendonly" CLUE "m=" lines   for which it has received no CLUE 'advertisement' message providing   the corresponding capture information would typically include   corresponding "a=inactive" lines in its answer, and it would only   make a new SDP Offer with "a=recvonly" when and if a new   'advertisement' message arrives with Captures relevant to those   Encodings.   Because of the constraints of SDP Offer/Answer and because new SDP   negotiations are generally more 'costly' than sending a new CLUE   message, implementations needing to make changes to both channels   SHOULD prioritize sending the updated CLUE message over sending the   new SDP message.  The aim is for the recipient to receive the CLUE   changes before the SDP changes, allowing the recipient to send their   SDP Answers without incomplete information and reducing the number of   new SDP Offers required.6.  Interaction of the CLUE Protocol and RTP/RTCP CaptureID   The CLUE Framework [RFC8845] allows for Multiple Content Captures   (MCCs): Captures that contain multiple source Captures, whether   composited into a single stream or switched based on some metric.   The Captures that contribute to these MCCs may or may not be defined   in the 'advertisement' message.  If they are defined and the MCC is   providing them in a switched format, the recipient may wish to   determine which originating source Capture is currently being   provided, so that they can apply geometric corrections based on that   Capture's geometry or take some other action based on the original   Capture information.   To do this, [RFC8849] allows for the CaptureID of the originating   Capture to be conveyed via RTP or RTCP.  A Media Provider sending   switched media for an MCC with defined originating sources MUST send   the CaptureID in both RTP and RTCP, as described in the mapping   document.6.1.  CaptureID Reception during MCC Redefinition   Because the RTP/RTCP CaptureID is delivered via a different channel   to the 'advertisement' message in which in the contents of the MCC   are defined, there is an intrinsic race condition in cases where the   contents of an MCC are redefined.   When a Media Provider redefines an MCC that involves CaptureIDs, the   reception of the relevant CaptureIDs by the recipient will either   lead or lag reception and the processing of the new 'advertisement'   message by the recipient.  As such, a Media Consumer MUST NOT be   disrupted by any of the following scenarios in any CLUE-controlled   media stream it is receiving, whether that stream is for a static   Capture or for an MCC (as any static Capture may be redefined to an   MCC in a later 'advertisement' message):   *  By receiving RTP or RTCP containing a CaptureID when the most      recently processed 'advertisement' message means that no media      CaptureIDs are expected.   *  By receiving RTP or RTCP without CaptureIDs when the most recently      processed 'advertisement' message means that media CaptureIDs are      expected.   *  By receiving a CaptureID in RTP or RTCP for a Capture defined in      the most recently processed 'advertisement' message, but which the      same 'advertisement' message does not include in the MCC.   *  By receiving a CaptureID in RTP or RTCP for a Capture not defined      in the most recently processed 'advertisement' message.7.  Multiplexing of CLUE-Controlled Media Using BUNDLE7.1.  Overview   A CLUE call may involve sending and/or receiving significant numbers   of media streams.  Conventionally, media streams are sent and   received on unique ports.  However, each separate port used for this   purpose may impose costs that a device wishes to avoid, such as the   need to open that port on firewalls and NATs, the need to collect   Interactive Connectivity Establishment (ICE) candidates [RFC8445],   etc.   The BUNDLE extension [RFC8843] can be used to negotiate the   multiplexing of multiple media lines onto a single 5-tuple for   sending and receiving media, allowing devices in calls to another   BUNDLE-supporting device to potentially avoid some of the above   costs.   While CLUE-capable devices MAY support the BUNDLE extension for this   purpose, supporting the extension is not mandatory for a device to be   CLUE compliant.   A CLUE-capable device that supports BUNDLE SHOULD also support rtcp-   mux [RFC5761].  However, a CLUE-capable device that supports rtcp-mux   may or may not support BUNDLE.7.2.  Usage of BUNDLE with CLUE   This specification imposes no additional requirements or restrictions   on the usage of BUNDLE when used with CLUE.  There is no restriction   on combining CLUE-controlled media lines and non-CLUE-controlled   media lines in the same BUNDLE group or in multiple such groups.   However, there are several steps an implementation may wish to take   to ameliorate the cost and time requirements of extra SDP Offer/   Answer exchanges between CLUE and BUNDLE.7.2.1.  Generating the Initial Offer   BUNDLE mandates that the initial SDP Offer MUST use a unique address   for each "m=" line with a non-zero port.  Because CLUE   implementations generally will not include CLUE-controlled media   lines, with the exception of the data channel in the initial SDP   Offer, CLUE devices that support large numbers of streams can avoid   ever having to open large numbers of ports if they successfully   negotiate BUNDLE.   An implementation that does include CLUE-controlled media lines in   its initial SDP Offer while also using BUNDLE must take care to avoid   rendering its CLUE-controlled media lines unusable in the event the   far end does not negotiate BUNDLE if it wishes to avoid the risk of   additional SDP exchanges to resolve this issue.  This is best   achieved by not sending any CLUE-controlled media lines in an initial   offer with the 'bundle-only' attribute unless it has been established   via some other channel that the recipient supports and is able to use   BUNDLE.7.2.2.  Multiplexing of the Data Channel and RTP Media   BUNDLE-supporting CLUE-capable devices MAY include the data channel   in the same BUNDLE group as RTP media.  In this case, the device MUST   be able to demultiplex the various transports -- see Section 9.2 of   the BUNDLE specification [RFC8843].  If the BUNDLE group includes   protocols other than the data channel transported via DTLS, the   device MUST also be able to differentiate the various protocols.8.  Example: A Call between Two CLUE-Capable Endpoints   This example illustrates a call between two CLUE-capable Endpoints.   Alice, initiating the call, is a system with three cameras and three   screens.  Bob, receiving the call, is a system with two cameras and   two screens.  A call-flow diagram is presented, followed by a summary   of each message.   To manage the size of this section, the SDP snippets only illustrate   video "m=" lines.  SIP ACKs are not always discussed.  Note that   BUNDLE is not in use.                 +----------+                      +-----------+                 |  Alice   |                      |    Bob    |                 |          |                      |           |                 +----+-----+                      +-----+-----+                      |                                  |                      |                                  |                      | SIP INVITE 1                     |                      |--------------------------------->|                      |                                  |                      |                                  |                      |                     SIP 200 OK 1 |                      |<---------------------------------|                      |                                  |                      |                                  |                      | SIP ACK 1                        |                      |--------------------------------->|                      |                                  |                      |                                  |                      |                                  |                      |<########### MEDIA 1 ############>|                      |   1 video A->B, 1 video B->A     |                      |<################################>|                      |                                  |                      |                                  |                      |                                  |                      |<================================>|                      |   CLUE DATA CHANNEL ESTABLISHED  |                      |<================================>|                      |                                  |                      |                                  |                      | CLUE OPTIONS                     |                      |<*********************************|                      |                                  |                      |                                  |                      |            CLUE OPTIONS RESPONSE |                      |*********************************>|                      |                                  |                      |                                  |                      | CLUE ADVERTISEMENT 1             |                      |*********************************>|                      |                                  |                      |                                  |                      |             CLUE ADVERTISEMENT 2 |                      |<*********************************|                      |                                  |                      |                                  |                      | CLUE ACK 1                       |                      |<*********************************|                      |                                  |                      |                                  |                      |                       CLUE ACK 2 |                      |*********************************>|                      |                                  |                      |                                  |                      | SIP INVITE 2 (+3 sendonly)       |                      |--------------------------------->|                      |                                  |                      |                                  |                      |                 CLUE CONFIGURE 1 |                      |<*********************************|                      |                                  |                      |                                  |                      |       SIP 200 OK 2 (+2 recvonly) |                      |<---------------------------------|                      |                                  |                      |                                  |                      | CLUE CONFIGURE RESPONSE 1        |                      |*********************************>|                      |                                  |                      |                                  |                      | SIP ACK 2                        |                      |--------------------------------->|                      |                                  |                      |                                  |                      |                                  |                      |<########### MEDIA 2 ############>|                      |   2 video A->B, 1 video B->A     |                      |<################################>|                      |                                  |                      |                                  |                      |       SIP INVITE 3 (+2 sendonly) |                      |<---------------------------------|                      |                                  |                      |                                  |                      | CLUE CONFIGURE 2                 |                      |*********************************>|                      |                                  |                      |                                  |                      | SIP 200 OK 3 (+2 recvonly)       |                      |--------------------------------->|                      |                                  |                      |                                  |                      |        CLUE CONFIGURE RESPONSE 2 |                      |<*********************************|                      |                                  |                      |                                  |                      |                        SIP ACK 3 |                      |<---------------------------------|                      |                                  |                      |                                  |                      |                                  |                      |<########### MEDIA 3 ############>|                      |   2 video A->B, 2 video B->A     |                      |<################################>|                      |                                  |                      |                                  |                      |                                  |                      v                                  v   In SIP INVITE 1, Alice sends Bob a SIP INVITE with the basic audio   and video capabilities and data channel included in the SIP body as   per [RFC8841].  Alice also includes the "sip.clue" media feature tag   in the INVITE.  A snippet of the SDP showing the grouping attribute   and the video "m=" line are shown below.  Alice has included a "CLUE"   group and the mid corresponding to a data channel in the group (3).   Note that Alice has chosen not to include any CLUE-controlled media   in the initial offer -- the "mid" value of the video line is not   included in the "CLUE" group.      ...      a=group:CLUE 3      ...      m=video 6002 RTP/AVP 96      a=rtpmap:96 H264/90000      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600      a=sendrecv      a=mid:2      ...      m=application 6100 UDP/DTLS/SCTP webrtc-datachannel      a=setup:actpass      a=sctp-port: 5000      a=dcmap:2 subprotocol="CLUE";ordered=true      a=mid:3   Bob responds with a similar SDP in SIP 200 OK 1, which also has a   "CLUE" group including the "mid" value of a data channel; due to   their similarity, no SDP snippet is shown here.  Bob wishes to   receive initial media and thus includes corresponding non-CLUE-   controlled audio and video lines.  Bob also includes the "sip.clue"   media feature tag in the 200 OK.  Alice and Bob are each now able to   send a single audio and video stream.  This is illustrated as MEDIA   1.   With the successful initial SDP Offer/Answer exchange complete, Alice   and Bob are also free to negotiate the CLUE data channel.  This is   illustrated as CLUE DATA CHANNEL ESTABLISHED.   Once the data channel is established, CLUE protocol negotiation   begins.  In this case, Bob was the DTLS client (sending "a=active" in   his SDP Answer) and hence is the CLUE Channel Initiator.  He sends a   CLUE OPTIONS message describing his version support.  On receiving   that message, Alice sends her corresponding CLUE OPTIONS RESPONSE.   With the OPTIONS phase complete, Alice now sends her CLUE   'advertisement' message (CLUE ADVERTISEMENT 1).  She advertises three   static Captures representing her three cameras.  She also includes   switched Captures suitable for systems with one or two screens.  All   of these Captures are in a single Capture Scene, with suitable   Capture Scene Views that tell Bob he should subscribe to the three   static Captures, the two switched Captures, or the one switched   Capture.  Alice has no simultaneity constraints, so all six Captures   are included in one simultaneous set.  Finally, Alice includes an   Encoding Group with three Encoding IDs: "enc1", "enc2", and "enc3".   These Encoding IDs aren't currently valid but will match the next SDP   Offer she sends.   Bob received CLUE ADVERTISEMENT 1 but does not yet send a 'configure'   message, because he has not yet received Alice's Encoding   information; thus, he does not know if she will have sufficient   resources in order to send him the two streams he ideally wants at a   quality he is happy with.  Because Bob is not sending an immediate   'configure' message with the "ack" element set, he must send an   explicit 'ack' message (CLUE ACK 1) to signal receipt of CLUE   ADVERTISEMENT 1.   Bob also sends his CLUE 'advertisement' message (CLUE ADVERTISEMENT   2) -- though the diagram shows that this occurs after Alice sends   CLUE ADVERTISEMENT 1, Bob sends his 'advertisement' message   independently and does not wait for CLUE ADVERTISEMENT 1 to arrive.   He advertises two static Captures representing his cameras.  He also   includes a single composed Capture for single-screen systems, in   which he will composite the two camera views into a single video   stream.  All three Captures are in a single Capture Scene, with   suitable Capture Scene Views that tell Alice she should subscribe to   either the two static Captures or the single composed Capture.  Bob   also has no simultaneity constraints, so he includes all three   Captures in one simultaneous set.  Bob also includes a single   Encoding Group with two Encoding IDs: "foo" and "bar".   Similarly, Alice receives CLUE ADVERTISEMENT 2 but does not yet send   a 'configure' message, because she has not yet received Bob's   Encoding information; instead, she sends an 'ack' message (CLUE ACK   2).   Both sides have now sent their CLUE 'advertisement' messages, and an   SDP exchange is required to negotiate Encodings.  For simplicity, in   this case, Alice is shown sending an INVITE with a new offer; in many   implementations, both sides might send an INVITE, which would be   resolved by use of the 491 Request Pending resolution mechanism from   [RFC3261].   Alice now sends SIP INVITE 2.  She maintains the sendrecv audio,   video, and CLUE "m=" lines, and she adds three new sendonly "m="   lines to represent the three CLUE-controlled Encodings she can send.   Each of these "m=" lines has a label corresponding to one of the   Encoding IDs from CLUE ADVERTISEMENT 1.  Each also has its mid added   to the grouping attribute to show they are controlled by the CLUE   data channel.  A snippet of the SDP showing the grouping attribute,   data channel, and video "m=" lines are shown below:      ...      a=group:CLUE 3 4 5 6      ...      m=video 6002 RTP/AVP 96      a=rtpmap:96 H264/90000      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600      a=sendrecv      a=mid:2      ...      m=application 6100 UDP/DTLS/SCTP webrtc-datachannel      a=sctp-port: 5000      a=dcmap:2 subprotocol="CLUE";ordered=true      a=mid:3      ...      m=video 6004 RTP/AVP 96      a=rtpmap:96 H264/90000      a=fmtp:96 profile-level-id=42e016      a=sendonly      a=mid:4      a=label:enc1      m=video 6006 RTP/AVP 96      a=rtpmap:96 H264/90000      a=fmtp:96 profile-level-id=42e016      a=sendonly      a=mid:5      a=label:enc2      m=video 6008 RTP/AVP 96      a=rtpmap:96 H264/90000      a=fmtp:96 profile-level-id=42e016      a=sendonly      a=mid:6      a=label:enc3   Bob now has all the information he needs to decide which streams to   configure, allowing him to send both a CLUE 'configure' message and   his SDP Answer.  As such, he now sends CLUE CONFIGURE 1.  This   requests the pair of switched Captures that represent Alice's scene,   and he configures them with encoder ids "enc1" and "enc2".   Bob also sends his SDP Answer as part of SIP 200 OK 2.  Alongside his   original audio, video, and CLUE "m=" lines, he includes three   additional "m=" lines corresponding to the three added by Alice: two   active recvonly "m= "lines and an inactive "m=" line for the third.   He adds their "mid" values to the grouping attribute to show they are   controlled by the CLUE data channel.  A snippet of the SDP showing   the grouping attribute and the video "m=" lines are shown below (mid   100 represents the CLUE data channel, which is not shown):      ...      a=group:CLUE 11 12 13 100      ...      m=video 58722 RTP/AVP 96      a=rtpmap:96 H264/90000      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600      a=sendrecv      a=mid:10      ...      m=video 58724 RTP/AVP 96      a=rtpmap:96 H264/90000      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600      a=recvonly      a=mid:11      m=video 58726 RTP/AVP 96      a=rtpmap:96 H264/90000      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600      a=recvonly      a=mid:12      m=video 58728 RTP/AVP 96      a=rtpmap:96 H264/90000      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600      a=inactive      a=mid:13   Alice receives Bob's CLUE CONFIGURE 1 message and sends CLUE   CONFIGURE RESPONSE 1 to acknowledge its reception.  She does not yet   send the Capture Encodings specified, because at this stage, she   hasn't processed Bob's answer SDP and thus hasn't negotiated the   ability for Bob to receive these streams.   On receiving SIP 200 OK 2 from Bob, Alice sends her SIP ACK (SIP ACK   2).  She is now able to send the two streams of video Bob requested   -- this is illustrated as MEDIA 2.   The constraints of offer/answer meant that Bob could not include his   Encoding information as new "m=" lines in SIP 200 OK 2.  As such, Bob   now sends SIP INVITE 3 to generate a new offer.  Along with all the   streams from SIP 200 OK 2, Bob also includes two new sendonly   streams.  Each stream has a label corresponding to the Encoding IDs   in his CLUE ADVERTISEMENT 2 message.  He also adds their "mid" values   to the grouping attribute to show they are controlled by the CLUE   data channel.  A snippet of the SDP showing the grouping attribute   and the video "m=" lines are shown below (mid 100 represents the CLUE   data channel, which is not shown):      ...      a=group:CLUE 11 12 14 15 100      ...      m=video 58722 RTP/AVP 96      a=rtpmap:96 H264/90000      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600      a=sendrecv      a=mid:10      ...      m=video 58724 RTP/AVP 96      a=rtpmap:96 H264/90000      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600      a=recvonly      a=mid:11      m=video 58726 RTP/AVP 96      a=rtpmap:96 H264/90000      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600      a=recvonly      a=mid:12      m=video 0 RTP/AVP 96      a=mid:13      m=video 58728 RTP/AVP 96      a=rtpmap:96 H264/90000      a=fmtp:96 profile-level-id=42e016      a=sendonly      a=label:foo      a=mid:14      m=video 58730 RTP/AVP 96      a=rtpmap:96 H264/90000      a=fmtp:96 profile-level-id=42e016      a=sendonly      a=label:bar      a=mid:15   Having received this, Alice now has all the information she needs to   send her CLUE 'configure' message and her SDP Answer.  In CLUE   CONFIGURE 2, she requests the two static Captures from Bob to be sent   on Encodings "foo" and "bar".   Alice also sends SIP 200 OK 3, matching two recvonly "m=" lines to   Bob's new sendonly lines.  She includes their "mid" values in the   grouping attribute to show they are controlled by the CLUE data   channel.  Alice then deactivates the initial non-CLUE-controlled   media, as bidirectional CLUE-controlled media is now available.  A   snippet of the SDP showing the grouping attribute and the video "m="   lines are shown below (mid 3 represents the data channel, not shown):      ...      a=group:CLUE 3 4 5 7 8      ...      m=video 0 RTP/AVP 96      a=mid:2      ...      m=video 6004 RTP/AVP 96      a=rtpmap:96 H264/90000      a=fmtp:96 profile-level-id=42e016      a=sendonly      a=mid:4      a=label:enc1      m=video 6006 RTP/AVP 96      a=rtpmap:96 H264/90000      a=fmtp:96 profile-level-id=42e016      a=sendonly      a=mid:5      a=label:enc2      m=video 0 RTP/AVP 96      a=mid:6      m=video 6010 RTP/AVP 96      a=rtpmap:96 H264/90000      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600      a=recvonly      a=mid:7      m=video 6012 RTP/AVP 96      a=rtpmap:96 H264/90000      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600      a=recvonly      a=mid:8   Bob receives Alice's CLUE CONFIGURE 2 message and sends CLUE   CONFIGURE RESPONSE 2 to acknowledge its reception.  Bob does not yet   send the Capture Encodings specified, because he hasn't yet received   and processed Alice's SDP Answer and negotiated the ability to send   these streams.   Finally, on receiving SIP 200 OK 3, Bob is now able to send the two   streams of video Alice requested -- this is illustrated as MEDIA 3.   Both sides of the call are now sending multiple video streams with   their sources defined via CLUE negotiation.  As the call progresses,   either side can send a new 'advertisement' or 'configure' message or   the new SDP Offers/Answers to add, remove, or change what they have   available or want to receive.9.  Example: A Call between a CLUE-Capable and Non-CLUE Endpoint   In this brief example, Alice is a CLUE-capable Endpoint making a call   to Bob, who is not CLUE capable (i.e., is not able to use the CLUE   protocol).         +----------+                      +-----------+         |  Alice   |                      |    Bob    |         |          |                      |           |         +----+-----+                      +-----+-----+              |                                  |              |                                  |              | SIP INVITE 1                     |              |--------------------------------->|              |                                  |              |                                  |              |                         200 0K 1 |              |<---------------------------------|              |                                  |              |                                  |              | SIP ACK 1                        |              |--------------------------------->|              |                                  |              |                                  |              |                                  |              |<########### MEDIA 1 ############>|              |   1 video A->B, 1 video B->A     |              |<################################>|              |                                  |              |                                  |              |                                  |              |                                  |              v                                  v   In SIP INVITE 1, Alice sends Bob a SIP INVITE including the basic   audio and video capabilities and data channel in the SDP body as per   [RFC8841].  Alice also includes the "sip.clue" media feature tag in   the INVITE.  A snippet of the SDP showing the grouping attribute and   the video "m=" line are shown below.  Alice has included a "CLUE"   group and the mid corresponding to a data channel in the group (3).   Note that Alice has chosen not to include any CLUE-controlled media   in the initial offer -- the "mid" value of the video line is not   included in the "CLUE" group.      ...      a=group:CLUE 3      ...      m=video 6002 RTP/AVP 96      a=rtpmap:96 H264/90000      a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600      a=sendrecv      a=mid:2      ...      m=application 6100 UDP/DTLS/SCTP webrtc-datachannel      a=sctp-port: 5000      a=dcmap:2 subprotocol="CLUE";ordered=true      a=mid:3   Bob is not CLUE capable and hence does not recognize the "CLUE"   semantic for the grouping attribute, nor does he support the data   channel.  IN SIP 200 OK 1, he responds with an answer that includes   audio and video, but with the data channel zeroed.   From the lack of a CLUE group, Alice understands that Bob does not   support CLUE, or does not wish to use it.  Both sides are now able to   send a single audio and video stream to each other.  At this point,   Alice begins to send her fallback video: in this case, it's likely a   switched view from whichever camera shows the current loudest   participant on her side.10.  IANA Considerations10.1.  New SDP Grouping Framework Attribute   This document registers the following semantics with IANA in the   "Semantics for the 'group' SDP Attribute" subregistry (under the   "Session Description Protocol (SDP) Parameters" registry) per   [RFC5888]:   +===========================+=======+==============+===========+   |         Semantics         | Token | Mux Category | Reference |   +===========================+=======+==============+===========+   | CLUE-controlled "m=" line | CLUE  | NORMAL       | RFC 8848  |   +---------------------------+-------+--------------+-----------+                               Table 110.2.  New SIP Media Feature Tag   This specification registers a new media feature tag in the SIP   [RFC3261] tree per the procedures defined in [RFC2506] and [RFC3840].   Media feature tag name:  sip.clue   ASN.1 Identifier:  30   Summary of the media feature indicated by this tag:  This feature tag      indicates that the device supports CLUE-controlled media.   Values appropriate for use with this feature tag:  Boolean.   The feature tag is intended primarily for use in the following   applications, protocols, services, or negotiation mechanisms:      This feature tag is most useful in a communications application      for describing the capabilities of a device to use the CLUE      control protocol to negotiate the use of multiple media streams.   Related standards or documents:  RFC 8848   Security Considerations:  Security considerations for this media      feature tag are discussed in Section 11 of RFC 8848.   Name(s) & email address(es) of person(s) to contact for further   information:  Internet Engineering Steering Group <iesg@ietf.org>   Intended usage:  COMMON11.  Security Considerations   CLUE makes use of a number of protocols and mechanisms, either   defined by CLUE or long-standing.  The Security Considerations   section of the CLUE Framework document [RFC8845] addresses the need   to secure these mechanisms by following the recommendations of the   individual protocols.   Beyond the need to secure the constituent protocols, the use of CLUE   does impose additional security concerns.  One area of increased risk   involves the potential for a malicious party to subvert a CLUE-   capable device to attack a third party by driving large volumes of   media (particularly video) traffic at them by establishing a   connection to the CLUE-capable device and directing the media to the   victim.  While this is a risk for all media devices, a CLUE-capable   device may allow the attacker to configure multiple media streams to   be sent, significantly increasing the volume of traffic directed at   the victim.   This attack can be prevented by ensuring that the media recipient   intends to receive the media packets.  As such, all CLUE-capable   devices MUST support key negotiation and receiver intent assurance   via DTLS / Secure Real-time Transport Protocol (SRTP) [RFC5763] on   CLUE-controlled RTP "m=" lines, and they MUST use it or some other   mechanism that provides receiver intent assurance.  All CLUE-   controlled RTP "m" lines must be secured and implemented using   mechanisms such as SRTP [RFC3711].  CLUE implementations MAY choose   not to require the use of SRTP to secure legacy (non-CLUE-controlled)   media for backwards compatibility with older SIP clients that are   incapable of supporting it.   CLUE also defines a new media feature tag that indicates CLUE   support.  This tag may be present even in non-CLUE calls, which   increases the metadata available about the sending device; this can   help an attacker differentiate between multiple devices and identify   otherwise anonymized users via the fingerprint of features their   device supports.  To prevent this, SIP signaling used to set up CLUE   sessions SHOULD always be encrypted using TLS [RFC5630].   The CLUE protocol also carries additional information that could be   used to help fingerprint a particular user or to identify the   specific version of software being used.  The CLUE Framework   [RFC8847] provides details about these issues and how to mitigate   them.12.  References12.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels", BCP 14, RFC 2119,              DOI 10.17487/RFC2119, March 1997,              <https://www.rfc-editor.org/info/rfc2119>.   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.              Norrman, "The Secure Real-time Transport Protocol (SRTP)",              RFC 3711, DOI 10.17487/RFC3711, March 2004,              <https://www.rfc-editor.org/info/rfc3711>.   [RFC3840]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat,              "Indicating User Agent Capabilities in the Session              Initiation Protocol (SIP)", RFC 3840,              DOI 10.17487/RFC3840, August 2004,              <https://www.rfc-editor.org/info/rfc3840>.   [RFC4574]  Levin, O. and G. Camarillo, "The Session Description              Protocol (SDP) Label Attribute", RFC 4574,              DOI 10.17487/RFC4574, August 2006,              <https://www.rfc-editor.org/info/rfc4574>.   [RFC5763]  Fischl, J., Tschofenig, H., and E. Rescorla, "Framework              for Establishing a Secure Real-time Transport Protocol              (SRTP) Security Context Using Datagram Transport Layer              Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May              2010, <https://www.rfc-editor.org/info/rfc5763>.   [RFC5888]  Camarillo, G. and H. Schulzrinne, "The Session Description              Protocol (SDP) Grouping Framework", RFC 5888,              DOI 10.17487/RFC5888, June 2010,              <https://www.rfc-editor.org/info/rfc5888>.   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,              May 2017, <https://www.rfc-editor.org/info/rfc8174>.   [RFC8831]  Jesup, R., Loreto, S., and M. Tüxen, "WebRTC Data              Channels", RFC 8831, DOI 10.17487/RFC8831, January 2021,              <https://www.rfc-editor.org/info/rfc8831>.   [RFC8841]  Holmberg, C., Shpount, R., Loreto, S., and G. Camarillo,              "Session Description Protocol (SDP) Offer/Answer              Procedures for Stream Control Transmission Protocol (SCTP)              over Datagram Transport Layer Security (DTLS) Transport",              RFC 8841, DOI 10.17487/RFC8841, January 2021,              <https://www.rfc-editor.org/info/rfc8841>.   [RFC8843]  Holmberg, C., Alvestrand, H., and C. Jennings,              "Negotiating Media Multiplexing Using the Session              Description Protocol (SDP)", RFC 8843,              DOI 10.17487/RFC8843, January 2021,              <https://www.rfc-editor.org/info/rfc8843>.   [RFC8845]  Duckworth, M., Ed., Pepperell, A., and S. Wenger,              "Framework for Telepresence Multi-Streams", RFC 8845,              DOI 10.17487/RFC8845, January 2021,              <https://www.rfc-editor.org/info/rfc8845>.   [RFC8846]  Presta, R. and S P. Romano, "An XML Schema for the              Controlling Multiple Streams for Telepresence (CLUE) Data              Model", RFC 8846, DOI 10.17487/RFC8846, January 2021,              <http://www.rfc-editor.org/info/rfc8846>.   [RFC8847]  Presta, R. and S P. Romano, "Protocol for Controlling              Multiple Streams for Telepresence (CLUE)", RFC 8847,              DOI 10.17487/RFC8847, January 2021,              <https://www.rfc-editor.org/info/rfc8847>.   [RFC8849]  Even, R. and J. Lennox, "Mapping RTP Streams to              Controlling Multiple Streams for Telepresence (CLUE) Media              Captures", RFC 8849, DOI 10.17487/RFC8849, January 2021,              <https://www.rfc-editor.org/info/rfc8849>.   [RFC8850]  Holmberg, C., "Controlling Multiple Streams for              Telepresence (CLUE) Protocol Data Channel", RFC 8850,              DOI 10.17487/RFC8850, January 2021,              <https://www.rfc-editor.org/info/rfc8850>.   [RFC8864]  Drage, K., Makaraju, M., Ejzak, R., Marcon, J., and R.              Even, Ed., "Negotiation Data Channels Using the Session              Description Protocol (SDP)", RFC 8864,              DOI 10.17487/RFC8864, January 2021,              <https://www.rfc-editor.org/info/rfc8864>.12.2.  Informative References   [RFC2506]  Holtman, K., Mutz, A., and T. Hardie, "Media Feature Tag              Registration Procedure", BCP 31, RFC 2506,              DOI 10.17487/RFC2506, March 1999,              <https://www.rfc-editor.org/info/rfc2506>.   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,              A., Peterson, J., Sparks, R., Handley, M., and E.              Schooler, "SIP: Session Initiation Protocol", RFC 3261,              DOI 10.17487/RFC3261, June 2002,              <https://www.rfc-editor.org/info/rfc3261>.   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model              with Session Description Protocol (SDP)", RFC 3264,              DOI 10.17487/RFC3264, June 2002,              <https://www.rfc-editor.org/info/rfc3264>.   [RFC3311]  Rosenberg, J., "The Session Initiation Protocol (SIP)              UPDATE Method", RFC 3311, DOI 10.17487/RFC3311, October              2002, <https://www.rfc-editor.org/info/rfc3311>.   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,              July 2006, <https://www.rfc-editor.org/info/rfc4566>.   [RFC5630]  Audet, F., "The Use of the SIPS URI Scheme in the Session              Initiation Protocol (SIP)", RFC 5630,              DOI 10.17487/RFC5630, October 2009,              <https://www.rfc-editor.org/info/rfc5630>.   [RFC5761]  Perkins, C. and M. Westerlund, "Multiplexing RTP Data and              Control Packets on a Single Port", RFC 5761,              DOI 10.17487/RFC5761, April 2010,              <https://www.rfc-editor.org/info/rfc5761>.   [RFC6184]  Wang, Y.-K., Even, R., Kristensen, T., and R. Jesup, "RTP              Payload Format for H.264 Video", RFC 6184,              DOI 10.17487/RFC6184, May 2011,              <https://www.rfc-editor.org/info/rfc6184>.   [RFC8445]  Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive              Connectivity Establishment (ICE): A Protocol for Network              Address Translator (NAT) Traversal", RFC 8445,              DOI 10.17487/RFC8445, July 2018,              <https://www.rfc-editor.org/info/rfc8445>.Acknowledgements   Besides the authors, the team focusing on this document consists of:   Roni Even, Simon Pietro Romano, and Roberta Presta.   Christian Groves, Jonathan Lennox, and Adam Roach have contributed   detailed comments and suggestions.Authors' Addresses   Robert Hanton   Cisco Systems   Email: rohanse2@cisco.com   Paul Kyzivat   Email: pkyzivat@alum.mit.edu   Lennard Xiao   Beijing Chuangshiyoulian   Email: lennard.xiao@outlook.com   Christian Groves   Email: cngroves.std@gmail.com