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Versions: (draft-kyzivat-clue-signaling) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 Draft is active
In: IESG_Evaluation
Network Working Group                                         P. Kyzivat
Internet-Draft                                                   L. Xiao
Intended status: Standards Track                               C. Groves
Expires: February 6, 2016                                         Huawei
                                                               R. Hansen
                                                           Cisco Systems
                                                          August 5, 2015


                             CLUE Signaling
                      draft-ietf-clue-signaling-06

Abstract

   This document specifies how CLUE-specific signaling such as the CLUE
   protocol [I-D.ietf-clue-protocol] and the CLUE data channel
   [I-D.ietf-clue-datachannel] are used 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 http://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 February 6, 2016.

Copyright Notice

   Copyright (c) 2015 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
   (http://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



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   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  . .   4
     4.3.  CLUE-controlled media and the CLUE grouping semantic  . .   5
     4.4.  SDP semantics for CLUE-controlled media . . . . . . . . .   5
       4.4.1.  Signalling CLUE Encodings . . . . . . . . . . . . . .   5
         4.4.1.1.  Referencing Encodings in the CLUE protocol  . . .   6
         4.4.1.2.  Media line directionality . . . . . . . . . . . .   7
       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 . . . . . . . . . . . . .  11
   5.  Interaction of CLUE protocol and SDP negotiations . . . . . .  11
     5.1.  Independence of SDP and CLUE negotiation  . . . . . . . .  11
     5.2.  Constraints on sending media  . . . . . . . . . . . . . .  12
     5.3.  Recommendations for operating with non-atomic operations   12
   6.  Interaction of CLUE protocol and RTP/RTCP CaptureID . . . . .  13
     6.1.  CaptureID reception during MCC redefinition . . . . . . .  14
   7.  Multiplexing of CLUE-controlled media using BUNDLE  . . . . .  14
     7.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.2.  Usage of BUNDLE with CLUE . . . . . . . . . . . . . . . .  15
       7.2.1.  Generating the Initial Offer  . . . . . . . . . . . .  15
       7.2.2.  Bundle Address Synchronization  . . . . . . . . . . .  15
       7.2.3.  Multiplexing of the data channel and RTP media  . . .  15
   8.  Example: A call between two CLUE-capable Endpoints  . . . . .  16
   9.  Example: A call between a CLUE-capable and non-CLUE Endpoint   24
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  25
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  25
     11.1.  New SDP Grouping Framework Attribute . . . . . . . . . .  25



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     11.2.  New SIP Media Feature Tag  . . . . . . . . . . . . . . .  26
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  26
   13. Change History  . . . . . . . . . . . . . . . . . . . . . . .  27
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  32
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  32
     14.2.  Informative References . . . . . . . . . . . . . . . . .  33
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  34

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 involves two principal
   and inter-related protocol negotiations.  SDP, conveyed via SIP, is
   used to negotiate the specific media capabilities that can be
   delivered to specific addresses on a device.  Meanwhile, a CLUE
   protocol [I-D.ietf-clue-protocol], transported via a CLUE data
   channel [I-D.ietf-clue-datachannel], is used to negotiate the Capture
   Sources available, their attributes and any constraints in their use,
   along with which Captures the far end provides a device wishes to
   receive.

   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 capabilities, SDP is also
   used to express the encoding limits for each potential Encoding.

   Backwards-compatibility is an important consideration of the
   document: 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", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   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:




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   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 signalled in SDP.  There is a corresponding "non-CLUE-
      controlled" media term.

3.  Media Feature Tag Definition

   The "sip.clue" media feature tag indicates support for CLUE.  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 extension, 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 beyond the scope of
   this document.  A device MUST NOT include more than one CLUE group in
   its SDP unless it is following a specification that defines how
   multiple CLUE channels are signalled, 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.

4.2.  The CLUE data channel and the CLUE grouping semantic

   The CLUE data channel [I-D.ietf-clue-datachannel] is a bidirectional
   SCTP over DTLS channel used for the transport of CLUE messages.  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 the relevant
   document.  A CLUE-capable device wishing to negotiate CLUE MUST also



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   include a CLUE group in the SDP and include the "mid" of the "m" line
   for the data channel in that group.  A CLUE group MUST include the
   "mid" of the "m" line for one (and only one) data channel.

   Presence of the data channel in a 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 a 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 always apply to "m" lines
   in an SDP offer or answer, even if negotiation of the data channel in
   SDP failed due to lack of CLUE support by the remote device or for
   any other reason, or in an offer if the recipient does not include
   the "mid" of the corresponding "m" line in their CLUE group.

4.4.  SDP semantics for CLUE-controlled media

4.4.1.  Signalling 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 signalled 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 a CLUE group as defined above.



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   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 media packets MUST NOT be sent until configuration is
   complete, while non-media packets such as STUN and DTLS MUST be sent
   as normal 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 same CLUE group in the SDP message, unless an "m" line
   represents a dependent stream related to another "m" line (such as a
   FEC stream), in which case it MUST have the same label value as the
   "m" line on which it is dependent.

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 encodingIDListType) 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 sent
   by the sender of the Advertisement message with a label value
   corresponding to the text content of the <encID>.

   Similarly, each <encodingID> (in captureEncodingType) in a CLUE
   Configure message SHOULD represent an Encoding defined in SDP; the
   specific Encoding referenced is a CLUE-controlled "m" line in the
   most recent SDP 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.







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4.4.1.2.  Media line directionality

   Presently, this specification mandates that CLUE-controlled "m" lines
   must be unidirectional.  This is because setting "m" lines to
   "a=sendonly" allows the encoder limits to be expressed, whereas in
   other cases codec attributes express the receive capabilities of a
   media line.

   It is possible that in future versions of this draft or its successor
   this restriction will be relaxed.  If a device does not feel there is
   a benefit to expressing encode limitations, or if there are no
   meaningful codec-specific limitations to express (such as with many
   audio codecs) there are benefits to allowing bidirectional "m" lines.
   With bidirectional media lines recipients do not always need to
   create a new offer to add their own "m" lines to express their send
   capabilities; if they can produce an equal or lesser number of
   streams to send then they may not need additional "m" lines.

   However, at present the need to express encode limitations and the
   wish to simplify the offer/answer procedure means that for the time
   being only unidirectional media lines are allowed for CLUE-controlled
   media.  The highly asymmetric nature of CLUE means that the
   probability of the recipient of the initial offer needing to make
   their own offer to add additional "m" lines is significantly higher
   than it is for most other SIP call scenarios, in which there is a
   tendancy for both sides to have similar numbers of potential audio
   and video streams they can send.

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 corresponding to the CLUE group of the
   Encoding they wish to receive.

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
   SHOULD 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.





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   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.

   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 is CLUE-capable and the initial 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, 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.

   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.



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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,
   otherwise 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 media on the non-CLUE-
   controlled channels during the period in which control of the CLUE-
   controlled media lines is being negotiated.  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" line to send or receive CLUE-controlled media they MAY now send a
   SIP request with a new SDP offer.  Note that if BUNDLE has been
   successfully negotiated and a Bundle Address Synchronization offer is




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   required, the device to receive that offer SHOULD NOT generate a new
   SDP offer until it has received that BAS offer.

4.5.3.2.  CLUE negotiation failure

   In the event that the negotiation of CLUE fails and the call is not
   CLUE-enabled in the initial offer/answer then CLUE is not in use in
   the call, and 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; in most cases at least one additional 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.

   Subsequent offer/answer exchanges MAY also deactive "m" lines for
   CLUE-controlled media.

   Once CLUE media has been successfully negotiated devices SHOULD
   ensure that non-CLUE-controlled media is deactived in cases where it
   corresponds to the media type of CLUE-controlled media that has been
   successfully negotiated.  This deactivate 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 corresponding CLUE
   groups then the call is now CLUE-enabled; negotiation of the data
   channel and subsequently the CLUE protocol begin.








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4.5.4.3.  Disabling CLUE mid-call

   If, in an ongoing CLUE-enabled call, an SDP offer-answer negotiation
   completes in a fashion in which either the CLUE data channel was not
   successfully negotiated or one side did not include the data channel
   in a matching CLUE group then CLUE for this channel is disabled.  In
   the event that this occurs, CLUE is no longer enabled and sending of
   all CLUE-controlled media associated with the corresponding CLUE
   group MUST stop.  If the data channel is still present but not
   included in the CLUE group semantic CLUE protocol messages MUST no
   longer be sent.

   Note that this is distinct to cases where the CLUE data channel fails
   or an error occurs on the CLUE protocol; see [I-D.ietf-clue-protocol]
   for details of media and state preservation in this circumstance.

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.

   This section defines how the negotiation of the two protocols
   interact, provides some recommendations on dealing with intermediary
   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
   with a CLUE Encoding it does not yet have capture information for, or
   receive a CLUE Configure message specifying a Capture Encoding for
   which the far end has not negotiated a media stream in SDP.




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   CLUE messages contain an <encID> (in encodingIDListType) 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 specifcs.  The non-atomic
   nature of CLUE negotiation means that a sender may wish to send a new
   Advertisement 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 messages
   solely because it contains <encID> elements that do not match an id
   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.

   A device with the CLUE Participant state machine in the ACTIVE state
   MAY choose not to move 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 the far end has sent 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 mis-matches will usually be



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   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 SHOULD ensure that any inconsistencies between
   SDP and CLUE signalling 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 SHOULD wait until they have the corresponding
   information they lack before sending messages to make changes related
   to that information.  For instance, an implementation that receives a
   new SDP offer with three new "a=sendonly" CLUE "m" lines that has not
   received the corresponding CLUE Advertisement providing the capture
   information for those streams SHOULD NOT include corresponding
   "a=recvonly" lines in its answer, but instead should make a new SDP
   offer when and if a new Advertisement arrives with Captures relevant
   to those Encodings.

   Because of the constraints of 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, reducing the number of
   new SDP offers required.

6.  Interaction of CLUE protocol and RTP/RTCP CaptureID

   [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 constitute 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 from an MCC with defined originating
   sources MUST send the CaptureID in both RTP and RTCP, as described in
   the mapping document.



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6.1.  CaptureID reception during MCC redefinition

   Because the RTP/RTCP CaptureID is delivered via a different channel
   to the Advertisement 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 by the
   recipient.  As such, a media recipient 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):

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

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

   o  Receiving a CaptureID in RTP or RTCP for a Capture defined in the
      most recently processed Advertisement, but which the same
      Advertisement 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.

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 seperate 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 [RFC5245], 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.






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   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.

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
   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
   generarlly will not include CLUE-controlled media lines with the
   exception of the data channel CLUE devices that support large numbers
   of streams can avoid ever having to open large numbers of ports if
   they successfully negotiate BUNDLE.

7.2.2.  Bundle Address Synchronization

   When using BUNDLE the initial offerer may be mandated to send a
   Bundle Address Synchronisation offer.  If the initial offerer also
   followed the recommendation of not including CLUE-controlled media
   lines in their offer, they MAY choose to include them in this
   subsequent offer.  In this circumstance the BUNDLE specification
   recommends that the offerer does not "modify SDP parameters that
   could get the answerer to reject the BAS offer".  Including new CLUE-
   controlled media lines using codecs and other attributes used in
   existing media lines should not increase the chance of the answerer
   rejecting the BAS offer; implementations should consider carefully
   before including new codecs or other new SDP attributes in these
   CLUE-controlled media lines.

7.2.3.  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 7.2 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.




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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 an
   summary of each message.

   To manage the size of this section SDP snippet only illustrate video
   'm' lines.  ACKs are not 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 CTRL CHANNEL ESTABLISHED  |
                      |<================================>|
                      |                                  |
                      |                                  |
                      | CLUE ADVERTISEMENT 1             |
                      |*********************************>|
                      |                                  |
                      |                                  |
                      |             CLUE ADVERTISEMENT 2 |



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



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


   In INVITE 1, Alice sends Bob a SIP INVITE including in the SDP body
   the basilar audio and video capabilities and the information needed
   for opening a control channel to be used for CLUE protocol messages
   exchange, according to what is envisioned in the COMEDIA approach for
   DTLS/SCTP channel [I-D.ietf-mmusic-sctp-sdp].  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=sctp-port: 5000
      a=dcmap:2 subprotocol="CLUE";ordered=true
      a=mid:3


   Bob responds with a similar SDP (200 OK 1), which also has a "CLUE"
   group including the mid value of a data channel; due to their
   similiarity no SDP snippet is shown here.  Bob wishes to receive
   initial media, and so includes corresponding non-CLUE-controlled
   audio and video lines.  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 O/A Alice and Bob are also free to
   negotiate the CLUE channel.  Once this is successfully established



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   CLUE negotiation can begin.  This is illustrated as CLUE CTRL CHANNEL
   ESTABLISHED.

   Alice now sends her CLUE Advertisement (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 entries 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 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.

   Bob also sends his CLUE Advertisement (ADVERTISEMENT 2).  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 entries 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".

   Similarly, Alices receives ADVERTISEMENT 2 but does not yet send a
   Configure message, because she has not yet received Bob's Encoding
   information.

   Alice now sends INVITE 2.  She maintains the sendrecv audio, video
   and CLUE m-lines, and she adds three new sendonly m-lines to
   represents the three CLUE-controlled Encodings she can send.  Each of
   these m-lines has a label corresponding to one of the Encoding IDs
   from ADVERTISEMENT 1.  Each also has its mid added to the grouping
   attribute to show they are controlled by the CLUE 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.  As such he now sends CONFIGURE 1.  This requests the pair
   of switched Captures that represent Alice's scene, and he configures
   them with encoder ids "enc1" and "enc2".  This also serves as an ack
   for Alice's ADVERTISEMENT 1.

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

   Bob now sends his SDP answer as part of 200 OK 2.  Alongside his
   original audio, video and CLUE m-lines he includes two active
   recvonly m-lines and a zeroed m-line for the third.  He adds their



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   mid values to the grouping attribute to show they are controlled by
   the CLUE channel.  A snippet of the SDP showing the grouping
   attribute and the video m-lines are shown below (mid 100 represents
   the CLUE channel, not shown):


      ...
      a=group:CLUE 11 12 100
      ...
      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


   On receiving 200 OK 2 from Bob Alice 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
   encoder information as new m-lines in 200 OK 2.  As such Bob now
   sends INVITE 3 to generate a new offer.  Along with all the streams
   from 200 OK 2 Bob also includes two new sendonly streams.  Each
   stream has a label corresponding to the Encoding IDs in his
   ADVERTISEMENT 2 message.  He also adds their mid values to the
   grouping attribute to show they are controlled by the CLUE channel.
   A snippet of the SDP showing the grouping attribute and the video
   m-lines are shown below (mid 100 represents the CLUE channel, not
   shown):










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      ...
      a=group:CLUE 11 12 13 14 100
      ...
      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
      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:13
      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:14


   Having received this Alice now has all the information she needs to
   send CONFIGURE 2.  She requests the two static Captures from Bob, to
   be sent on Encodings "foo" and "bar".

   Bob receives Alice's message CONFIGURE 2 and sends RESPONSE 2 to ack
   its receptions.  Bob does not yet send the Capture Encodings
   specified, because Alice hasn't yet negotiated the ability to receive
   these streams in SDP.

   Alice now sends 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 channel.  Alice
   also now deactivates the initial non-CLUE-controlled media, as
   bidirectional CLUE-controlled media is now available.  A snippet of



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   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
      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


   Finally, on receiving 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 negotiation 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).



         +----------+                      +-----------+
         |   EP1    |                      |    EP2    |
         |          |                      |           |
         +----+-----+                      +-----+-----+
              |                                  |
              |                                  |
              | SIP INVITE 1                     |
              |--------------------------------->|
              |                                  |
              |                                  |
              |                         200 0K 1 |
              |<---------------------------------|
              |                                  |
              |                                  |
              | ACK 1                            |
              |--------------------------------->|
              |                                  |
              |                                  |
              |                                  |
              |<########### MEDIA 1 ############>|
              |   1 video A->B, 1 video B->A     |
              |<################################>|
              |                                  |
              |                                  |
              |                                  |
              |                                  |
              v                                  v


   In INVITE 1, Alice sends Bob a SIP INVITE including in the SDP body
   the basilar audio and video capabilities and the information needed
   for opening a control channel to be used for CLUE protocol messages
   exchange, according to what is envisioned in the COMEDIA approach for
   a DTLS/SCTP channel [I-D.ietf-mmusic-sctp-sdp].  A snippet of the SDP
   showing the grouping attribute, data channel and the video m-line are
   shown below:







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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.  He responds with an answer with audio and video, but with
   the data channel zeroed.

   From the lack of the data channel and grouping framework 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

   The team focusing on this draft consists of: Roni Even, Rob Hansen,
   Christer Holmberg, Paul Kyzivat, Simon Pietro-Romano, Roberta Presta.

   Christian Groves and Jonathon Lennox 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: Semantics
   Token Reference ------------------------------------- ------
   --------- CLUE controlled m-line CLUE [this draft]







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11.2.  New SIP Media Feature Tag

   This specification registers a new media feature tag in the SIP
   [RFC3264] tree per the procedures defined in [RFC2506] and [RFC3840].
   Media feature tag name: sip.clue ASN.1 Identifier: 1.3.6.1.8.4.26
   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 which uses multiple media streams.

12.  Security Considerations

   CLUE makes use of a number of protocols and mechanism, 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 consistuent 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 [RFC5763] on CLUE-controlled RTP "m" lines.  All CLUE-
   controlled RTP "m" lines must be secured and implemented using
   mechanisms such as SRTP [RFC3711]; no specific security mechanisms
   are made mandatory to use due to the issues addressed in [RFC7202].
   Due to the requirements of backwards compatibility, these is not a
   mandatory requirement for non-CLUE-controlled "m" lines.

   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




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   their device supports.  To prevent this, SIP signalling SHOULD always
   be encrypted using TLS [RFC5630].

13.  Change History

   Revision by Rob Hansen

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

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

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

   o  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.

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

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

   Revision by Rob Hansen

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

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

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

   o  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.

   o  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.

   o  Other minor syntax improvements.




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   Revision by Rob Hansen

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

   o  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.

   o  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.

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

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

   o  Added a number of security considerations.

   Revision by Rob Hansen

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

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

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

   o  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



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         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.

      *  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.






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      *  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

   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.




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      *  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:

      *  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:





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      *  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-framework]
              Duckworth, M., Pepperell, A., and S. Wenger, "Framework
              for Telepresence Multi-Streams", draft-ietf-clue-
              framework-22 (work in progress), April 2015.

   [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-10 (work in
              progress), June 2015.

   [I-D.ietf-clue-protocol]
              Presta, R. and S. Romano, "CLUE protocol", draft-ietf-
              clue-protocol-04 (work in progress), April 2015.

   [I-D.ietf-clue-datachannel]
              Holmberg, C., "CLUE Protocol data channel", draft-ietf-
              clue-datachannel-09 (work in progress), March 2015.




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   [I-D.ietf-clue-rtp-mapping]
              Even, R. and J. Lennox, "Mapping RTP streams to CLUE media
              captures", draft-ietf-clue-rtp-mapping-04 (work in
              progress), March 2015.

   [I-D.ietf-mmusic-sctp-sdp]
              Holmberg, C., Loreto, S., and G. Camarillo, "Stream
              Control Transmission Protocol (SCTP)-Based Media Transport
              in the Session Description Protocol (SDP)", draft-ietf-
              mmusic-sctp-sdp-14 (work in progress), March 2015.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://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,
              <http://www.rfc-editor.org/info/rfc3711>.

   [RFC4574]  Levin, O. and G. Camarillo, "The Session Description
              Protocol (SDP) Label Attribute", RFC 4574,
              DOI 10.17487/RFC4574, August 2006,
              <http://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, <http://www.rfc-editor.org/info/rfc5763>.

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,
              <http://www.rfc-editor.org/info/rfc2506>.

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

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




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   [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,
              <http://www.rfc-editor.org/info/rfc3840>.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              DOI 10.17487/RFC5245, April 2010,
              <http://www.rfc-editor.org/info/rfc5245>.

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

   [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,
              <http://www.rfc-editor.org/info/rfc6184>.

   [RFC7202]  Perkins, C. and M. Westerlund, "Securing the RTP
              Framework: Why RTP Does Not Mandate a Single Media
              Security Solution", RFC 7202, DOI 10.17487/RFC7202, April
              2014, <http://www.rfc-editor.org/info/rfc7202>.

   [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-23 (work in progress), July 2015.

Authors' Addresses

   Paul Kyzivat
   Huawei

   Email: pkyzivat@alum.mit.edu


   Lennard Xiao
   Huawei

   Email: lennard.xiao@huawei.com






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   Christian Groves
   Huawei

   Email: Christian.Groves@nteczone.com


   Robert Hansen
   Cisco Systems

   Email: rohanse2@cisco.com









































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