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Versions: 02 03 04 05 06 07 08 draft-ietf-clue-signaling

Network Working Group                                         P. Kyzivat
Internet-Draft                                                   L. Xiao
Intended status: Standards Track                               C. Groves
Expires: October 13, 2014                                         Huawei
                                                               R. Hansen
                                                           Cisco Systems
                                                          April 11, 2014


                             CLUE Signaling
                    draft-kyzivat-clue-signaling-08

Abstract

   This document specifies how CLUE-specific signaling such as the CLUE
   protocol [I-D.presta-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 October 13, 2014.

Copyright Notice

   Copyright (c) 2014 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 . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Media Feature Tag Definition . . . . . . . . . . . . . . . . .  5
   4.  SDP Grouping Framework TELEPRESENCE Extension Semantics  . . .  5
     4.1.  General  . . . . . . . . . . . . . . . . . . . . . . . . .  5
     4.2.  The CLUE data channel and the TELEPRESENCE grouping
           semantic . . . . . . . . . . . . . . . . . . . . . . . . .  5
     4.3.  CLUE-controlled media and the TELEPRESENCE grouping
           semantic . . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.4.  SDP Offer/Answer Procedures  . . . . . . . . . . . . . . .  6
       4.4.1.  Generating the Initial Offer . . . . . . . . . . . . .  6
         4.4.1.1.  Signalling CLUE Encodings  . . . . . . . . . . . .  6
         4.4.1.2.  Receiving CLUE-controlled media  . . . . . . . . .  8
         4.4.1.3.  Interoperability with non-CLUE devices . . . . . .  8
       4.4.2.  Generating the Answer  . . . . . . . . . . . . . . . .  8
         4.4.2.1.  Negotiating use of CLUE and the CLUE data
                   channel  . . . . . . . . . . . . . . . . . . . . .  8
         4.4.2.2.  Negotiating receipt of CLUE capture encodings
                   in SDP . . . . . . . . . . . . . . . . . . . . . .  8
       4.4.3.  Processing the initial Offer/Answer negotiation  . . .  9
         4.4.3.1.  Successful CLUE negotiation  . . . . . . . . . . .  9
         4.4.3.2.  CLUE negotiation failure . . . . . . . . . . . . .  9
       4.4.4.  Modifying the session  . . . . . . . . . . . . . . . .  9
         4.4.4.1.  Enabling CLUE mid-call . . . . . . . . . . . . . .  9
         4.4.4.2.  Disabling CLUE mid-call  . . . . . . . . . . . . . 10
   5.  Interaction of CLUE protocol and SDP negotiations  . . . . . . 10
     5.1.  Independence of SDP and CLUE negotiation . . . . . . . . . 10
     5.2.  Recommendations for operating with non-atomic
           operations . . . . . . . . . . . . . . . . . . . . . . . . 11
     5.3.  Constraints on sending media . . . . . . . . . . . . . . . 11
   6.  Multiplexing of CLUE-controlled media using BUNDLE . . . . . . 12
     6.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . 12
     6.2.  Usage of BUNDLE with CLUE  . . . . . . . . . . . . . . . . 12
       6.2.1.  Generating the Initial Offer . . . . . . . . . . . . . 12
       6.2.2.  Bundle Address Synchronization . . . . . . . . . . . . 12
       6.2.3.  Multiplexing of the data channel and RTP media . . . . 13
   7.  Example: A call between two CLUE-capable endpoints . . . . . . 13
   8.  Example: A call between a CLUE-capable and non-CLUE
       endpoint . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
   9.  CLUE requirements on SDP O/A . . . . . . . . . . . . . . . . . 22
   10. SIP Signaling  . . . . . . . . . . . . . . . . . . . . . . . . 22



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   11. CLUE over RTCWEB . . . . . . . . . . . . . . . . . . . . . . . 22
   12. Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 23
   13. What else? . . . . . . . . . . . . . . . . . . . . . . . . . . 23
   14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
   15. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
   16. Security Considerations  . . . . . . . . . . . . . . . . . . . 23
   17. Change History . . . . . . . . . . . . . . . . . . . . . . . . 23
   18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
     18.1. Normative References . . . . . . . . . . . . . . . . . . . 25
     18.2. Informative References . . . . . . . . . . . . . . . . . . 26
   Appendix A.  CLUE Signalling and data channel concerns . . . . . . 27
     A.1.  Protocol Versioning and Options  . . . . . . . . . . . . . 27
       A.1.1.  Versioning Objectives  . . . . . . . . . . . . . . . . 27
       A.1.2.  Versioning Overview  . . . . . . . . . . . . . . . . . 28
       A.1.3.  Version Negotiation  . . . . . . . . . . . . . . . . . 30
       A.1.4.  Option Negotiation . . . . . . . . . . . . . . . . . . 31
       A.1.5.  Option Elements  . . . . . . . . . . . . . . . . . . . 31
         A.1.5.1.  <mediaProvider>  . . . . . . . . . . . . . . . . . 31
       A.1.6.  Version & option negotiation errors  . . . . . . . . . 32
       A.1.7.  Definition and Use of Version Numbers  . . . . . . . . 33
       A.1.8.  Version & Option Negotiation Examples  . . . . . . . . 34
         A.1.8.1.  Successful Negotiation - Multi-version . . . . . . 34
         A.1.8.2.  Successful Negotiation - Consumer-Only Endpoint  . 35
         A.1.8.3.  Successful Negotiation - Provider-Only Endpoint  . 36
         A.1.8.4.  Version Incompatibility  . . . . . . . . . . . . . 37
         A.1.8.5.  Option Incompatibility . . . . . . . . . . . . . . 38
         A.1.8.6.  Syntax Error . . . . . . . . . . . . . . . . . . . 39
     A.2.  Message Transport  . . . . . . . . . . . . . . . . . . . . 39
       A.2.1.  CLUE Channel Lifetime  . . . . . . . . . . . . . . . . 39
       A.2.2.  Channel Error Handling . . . . . . . . . . . . . . . . 40
     A.3.  Message Framing  . . . . . . . . . . . . . . . . . . . . . 40
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 40



















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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.presta-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 which 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.

   This document originally also defined the CLUE protocol itself.
   These details have mostly been split out into
   [I-D.presta-clue-protocol] and expanded, but at present some details
   remain in this document.


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 draws liberally from the terminology defined in the
   CLUE Framework [I-D.ietf-clue-framework].

   Other terms introduced here:







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   CLUE data channel:  A reliable, bidirectional, transport mechanism
      used to convey CLUE messages.  See [I-D.ietf-clue-datachannel] for
      more details..
   CLUE-capable device:  A device that supports the CLUE data channel
      [I-D.ietf-clue-datachannel], the CLUE protocol
      [I-D.presta-clue-protocol] and the principles of CLUE negotiation.
   CLUE-enabled device:  A CLUE-capable device that wishes to negotiate
      a CLUE data channel and send and/or receive CLUe-controlled media.
   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.  There is a corresponding "non-CLUE-
      controlled" media term.  See Section 4 for details of how this
      control is signalled in SDP


3.  Media Feature Tag Definition

   The "sip.telepresence" 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 TELEPRESENCE Extension Semantics

4.1.  General

   This section defines a new SDP Grouping Framework extension,
   TELEPRESENCE.

   The TELEPRESENCE 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 TELEPRESENCE group.

4.2.  The CLUE data channel and the TELEPRESENCE 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.



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   The data channel is a generic transport that is not specific to CLUE
   - if a device wishes to use the CLUE protocol on the data channel it
   MUST include a TELEPRESENCE group in the SDP and include the "mid" of
   the "m" line for the data channel in that group.  A TELEPRESENCE grup
   MUST NOT include the "mid"s for more than one data channel, and the
   data channel "mid" MUST NOT be included in more than one TELEPRESENCE
   group.

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

4.3.  CLUE-controlled media and the TELEPRESENCE 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.presta-clue-protocol].  For an "m" line to be CLUE-
   controlled, its "mid" value MUST be included in a TELEPRESENCE group.
   CLUE-controlled media line "mid"s MUST NOT be included in more than
   one TELEPRESENCE group.

   CLUE-controlled media is controlled by the CLUE protocol as
   negotiated on the CLUE data channel with an "mid" included in the
   TELEPRESENCE group.  If no data channel is included in the group the
   other "m" lines in the group are still considered CLUE-controlled and
   under all the restrictions of CLUE-controlled media specified in this
   document.

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

4.4.  SDP Offer/Answer Procedures

4.4.1.  Generating the Initial Offer

4.4.1.1.  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.




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   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 TELEPRESENCE group as defined above.

   As well as the normal restrictions defined in [RFC3264] media MUST
   NOT be sent on this stream until the media provider has received a
   valid CLUE CONFIGURE message specifying the capture to be used for
   this stream.  In the case of RTP media this includes corresponding
   RTCP packets.

   Every "m" line representing a CLUE encoding SHOULD 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.

4.4.1.1.1.  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.





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4.4.1.1.2.  Alternate encoding limit syntaxes

   Note that while the expressing of CLUE encoding limits in SDP has
   been discussed at some length by the working group and it has been
   agreed that this is the current, working assumption, formal consensus
   has not been agreed on this.  Alternatives include placing encoding
   limits in the CLUE ADVERTISEMENT message, or by using alternate SDP
   syntax, such as is suggested in [I-D.groves-clue-latent-config].

4.4.1.2.  Receiving CLUE-controlled media

   As well as including sendonly media lines to send CLUE-controlled
   media, the sender of the initial SDP offer MAY also include
   "a=recvonly" media lines to preallocate "m" lines to receive media;
   these are described in more detail in the next section.

4.4.1.3.  Interoperability with non-CLUE devices

   A CLUE-enabled 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-enabled,
   for instance due to receiving an initial INVITE with no SDP but
   including a 'sip.telepresence' media feature tag, the above
   recommendation is waived, and the initial offer MAY contain "m" lines
   for CLUE-controlled media.

4.4.2.  Generating the Answer

4.4.2.1.  Negotiating use of CLUE and the CLUE data channel

   If the recipient wishes to enable CLUE for the call, they MUST
   negotiate data channel support for an "m" line, and include the "mid"
   of that "m" line in a corresponding TELEPRESENCE group.

4.4.2.2.  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 an "mid"
   the corresponding TELEPRESENCE group corresponding to the encoding
   they wish to send.

   In the case of RTCP for RTP media or any other media type that



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   includes a bidirectional flow of packets for unidirectional media
   streams, such bidirectional packets MUST NOT be sent until the media
   consumer has received acknowledgement that the media provider has
   received a valid CLUE CONFIGURE message specifying the capture to be
   used for this stream.

4.4.3.  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 TELEPRESENCE groups
   CLUE has been successfully negotiated and the call is now CLUE-
   enabled, otherwise the call is not CLUE enabled.

4.4.3.1.  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.presta-clue-protocol]
   messages can begin.

   A CLUE-enabled 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-enabled
   device MUST still be prepared to receive media on non-CLUE-controlled
   media lines 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.  Note that if BUNDLE has been
   successfully negotiated and a Bundle Address Synchronization offer is
   required, the device to receive that offer SHOULD NOT generate a new
   SDP offer until it has received that BAS offer.

4.4.3.2.  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.

4.4.4.  Modifying the session

4.4.4.1.  Enabling CLUE mid-call

   A CLUE-enabled device that receives an initial SDP offer from a non-
   CLUE-enabled device SHOULD include a new data channel "m" line and
   corresponding TELEPRESENCE group in any subsequent offers it sends,



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   to indicate that it is CLUE-enabled.

   If, in an ongoing non-CLUE call, one or both sides of the call add
   the CLUE data channel "m" line to their SDP and places the "mid" for
   that channel in corresponding TELEPRESENCE groups then the call is
   now CLUE-enabled; negotiation of the data channel and subsequently
   the CLUE protocol begin.

4.4.4.2.  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 TELEPRESENCE 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 TELEPRESENCE group MUST stop.

   Note that this is distinct to cases where the CLUE data channel fails
   or an error occurs on the CLUE protocol; see
   [I-D.presta-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 complicated state machines with the potential to reach
   invalid states if messages were to be lost, or be rewritten en-route
   by middle boxes, the current proposal is that SDP and CLUE messages
   are independent.  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



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   of or the state of the most recent SDP negotiation.

   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.

   CLUE messages contain an EncodingID which is used to identify a
   specific encoding in SDP.  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 EncodingID which does not currently match an extant id
   in SDP.

5.2.  Recommendations for operating with non-atomic operations

   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.

5.3.  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 - 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.






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6.  Multiplexing of CLUE-controlled media using BUNDLE

6.1.  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.

   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.

6.2.  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.

6.2.1.  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.

6.2.2.  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



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   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.

6.2.3.  Multiplexing of the data channel and RTP media

   BUNDLE-supporting CLUE-enabled 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.


7.  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 only video is considered, and SDP
   snippets only illustrate video 'm' lines.  ACKs are not discussed.
   Note that BUNDLE is not in use.



                 +----------+                      +-----------+
                 |  Alice   |                      |    Bob    |
                 |          |                      |           |
                 +----+-----+                      +-----+-----+
                      |                                  |
                      |                                  |
                      | SIP INVITE 1 (BASIC SDP+COMEDIA) |
                      |--------------------------------->|
                      |                                  |
                      |                                  |
                      | SIP 200 OK 1 (BASIC SDP+COMEDIA) |
                      |<---------------------------------|
                      |                                  |
                      |                                  |
                      | SIP ACK 1                        |
                      |--------------------------------->|



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                      |                                  |
                      |                                  |
                      |                                  |
                      |<########### MEDIA 1 ############>|
                      |   1 video A->B, 1 video B->A     |
                      |<################################>|
                      |                                  |
                      |                                  |
                      |                                  |
                      |<================================>|
                      |   CLUE CTRL CHANNEL ESTABLISHED  |
                      |<================================>|
                      |                                  |
                      |                                  |
                      | CLUE ADVERTISEMENT 1             |
                      |*********************************>|
                      |                                  |
                      |                                  |
                      |             CLUE ADVERTISEMENT 2 |
                      |<*********************************|
                      |                                  |
                      |                                  |
                      | 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     |
                      |<################################>|
                      |                                  |
                      |                                  |



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                      |       SIP INVITE 3 (+2 sendonly) |
                      |<---------------------------------|
                      |                                  |
                      |                                  |
                      | CLUE CONFIGURE 2                 |
                      |*********************************>|
                      |                                  |
                      |                                  |
                      |                  CLUE RESPONSE 2 |
                      |<*********************************|
                      |                                  |
                      |                                  |
                      | SIP 200 OK 3 (+2 recvonly)       |
                      |--------------------------------->|
                      |                                  |
                      |                                  |
                      |                                  |
                      |                        SIP ACK 3 |
                      |<---------------------------------|
                      |                                  |
                      |                                  |
                      |                                  |
                      |<########### 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 ("BASIC SDP") 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 ("COMEDIA") 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 (mid 3
   represents the CLUE channel):


      ...
      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



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      a=mid:2


   Bob responds with a similar SDP (200 OK 1); due to their similiarity
   no SDP snippet is shown here.  Alice and Bob are each able to send a
   single audio and video stream (whether they choose to send this
   initial media before CLUE has been negotiated is implementation-
   dependent).  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
   CLUE negotiation can begin.  This is illustrated as CLUE 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 capture view
   or the one switched capture view.  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



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   and CLUE m-lines, and she adds three new sendonly m-lines to
   represents the maximum three 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 and the video m-lines are
   shown below (mid 3 represents the CLUE channel):


      ...
      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=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 ADVERTISMENT 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.



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   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
   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):


      ...
      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):








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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.  A snippet
   of the SDP showing the grouping attribute and the video m-lines are
   shown below (mid 3 represents the CLUE channel):



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      ...
      a=group:CLUE 3 4 5 7 8
      ...
      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=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 or new SDP
   negotiation to add, remove or change what they have available or want
   to receive.


8.  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., it is not able to use the CLUE



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   protocol.



         +----------+                      +-----------+
         |   EP1    |                      |    EP2    |
         |          |                      |           |
         +----+-----+                      +-----+-----+
              |                                  |
              |                                  |
              | SIP INVITE 1 (BASIC SDP+COMEDIA) |
              |--------------------------------->|
              |                                  |
              |                                  |
              | 200 0K 1 (BASIC SDP+*NO*COMEDIA) |
              |<---------------------------------|
              |                                  |
              |                                  |
              | 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 ("BASIC SDP") 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 ("COMEDIA") 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 (mid 3
   represents the CLUE channel):










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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


   Bob is not CLUE capable, and hence does not recognize the "CLUE"
   semantic for the grouping attribute, not does he support the CLUE
   channel.  He responds with an answer with audio and video, but with
   the CLUE channel zeroed.

   From the lack of the CLUE channel 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.


9.  CLUE requirements on SDP O/A

   The current proposal calls for a new "CLUE" semantic for the SDP
   Grouping Framework [RFC5888].

   Any other SDP extensions required to support CLUE signaling should
   also be specified here.  Then we will need to take action within
   MMUSIC to make those happen.  This section should be empty and
   removed before this document becomes an RFC.

   NOTE: The RTP mapping document [I-D.even-clue-rtp-mapping] is also
   likely to call for SDP extensions.  We will have to reconcile how to
   coordinate these two documents.


10.  SIP Signaling

   (Placeholder) This may be unremarkable.  If so we can drop it.


11.  CLUE over RTCWEB

   We may want to rule this out of scope for now.  But we should be
   thinking about this.




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12.  Open Issues

   Here are issues pertinent to signaling that need resolution.
   Resolution will probably result in changes somewhere in this
   document, but may also impact other documents.
   o  While the preference is to multiplex multiple capture encodings
      over a single RTP session, this will not always be desirable or
      possible.  The factors that prevent multiplexing may come from
      either the provider or the consumer.  So the extent of
      multiplexing must be negotiated.  The decision about how to
      multiplex affects the number and grouping of m-lines in the SDP.
      The endpoint of a CLUE session that sends an offer needs to know
      the mapping of capture encodings to m-lines for both sides.

      AFAIK this issue hasn't yet been considered at all.
   o  The current method for expressing encodings in SDP limits the
      parameters available when describing H264 encoder capabilities to
      those defined in Table 6 in [RFC6184]


13.  What else?


14.  Acknowledgements

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

   Christian Groves has contributed detailed comments and suggestions.

   The author list should be updated as people contribute substantial
   text to this document.


15.  IANA Considerations

   TBD


16.  Security Considerations

   TBD


17.  Change History






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   -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
   -07:  Revisions by Rob Hansen
      *  Removed the text providing arguments for encoding limits being
         in SDP and encoding groups in the CLUE protocol in favor of the
         specifics of how to negotiate encodings in SDP
      *  Added normative language on the setting up of a CLUE call, and
         added sections on mid-call changes to the CLUE status.
      *  Added references to [I-D.ietf-clue-datachannel] where
         appropriate.
      *  Added some terminology for various types of CLUE and non-CLUE
         states of operation.
      *  Moved language related to topics that should be in
         [I-D.ietf-clue-datachannel] and [I-D.presta-clue-protocol], but
         that has not yet been resolved in those documents, into an
         appendix.
   -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
   -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.)






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   -04:  Revisions by Rob Hansen: ???
   -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.
   -02:  A bunch of revisions by pkyzivat:
      *  Moved roberta's call flows to a more appropriate place in the
         document.
      *  New section on versioning.
      *  New section on NAK.
      *  A couple of possible alternatives for message acknowledgment.
      *  Some discussion of when/how to signal changes in provider
         state.
      *  Some discussion about the handling of transport errors.
      *  Added a change history section.
      These were developed by Lennard Xiao, Christian Groves and Paul,
      so added Lennard and Christian as authors.
   -01:  Updated by roberta to include some sample call flows.
   -00:  Initial version by pkyzivat.  Established general outline for
      the document, and specified a few things thought to represent wg
      consensus.


18.  References

18.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [I-D.ietf-clue-framework]
              Duckworth, M., Pepperell, A., and S. Wenger, "Framework
              for Telepresence Multi-Streams",
              draft-ietf-clue-framework-14 (work in progress),
              February 2014.

   [I-D.presta-clue-data-model-schema]
              Presta, R. and S. Romano, "An XML Schema for the CLUE data
              model", draft-presta-clue-data-model-schema-03 (work in
              progress), March 2013.

   [I-D.presta-clue-protocol]
              Presta, R. and S. Romano, "CLUE protocol",



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              draft-presta-clue-protocol-03 (work in progress),
              November 2013.

   [I-D.ietf-clue-datachannel]
              Holmberg, C., "CLUE Protocol Data Channel",
              draft-ietf-clue-datachannel-00 (work in progress),
              March 2014.

   [I-D.groves-clue-latent-config]
              Groves, C., Yang, W., and R. Even, "CLUE and latent
              configurations", draft-groves-clue-latent-config-00 (work
              in progress), January 2014.

   [I-D.ietf-mmusic-sctp-sdp]
              Loreto, S. and G. Camarillo, "Stream Control Transmission
              Protocol (SCTP)-Based Media Transport in the Session
              Description Protocol (SDP)", draft-ietf-mmusic-sctp-sdp-06
              (work in progress), February 2014.

   [I-D.tuexen-tsvwg-sctp-dtls-encaps]
              Jesup, R., Loreto, S., Stewart, R., and M. Tuexen, "DTLS
              Encapsulation of SCTP Packets for RTCWEB",
              draft-tuexen-tsvwg-sctp-dtls-encaps-01 (work in progress),
              July 2012.

   [RFC4574]  Levin, O. and G. Camarillo, "The Session Description
              Protocol (SDP) Label Attribute", RFC 4574, August 2006.

   [RFC5888]  Camarillo, G. and H. Schulzrinne, "The Session Description
              Protocol (SDP) Grouping Framework", RFC 5888, June 2010.

18.2.  Informative References

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              June 2002.

   [RFC3311]  Rosenberg, J., "The Session Initiation Protocol (SIP)
              UPDATE Method", RFC 3311, October 2002.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              April 2010.

   [RFC4353]  Rosenberg, J., "A Framework for Conferencing with the
              Session Initiation Protocol (SIP)", RFC 4353,
              February 2006.



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   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, March 2011.

   [RFC6184]  Wang, Y., Even, R., Kristensen, T., and R. Jesup, "RTP
              Payload Format for H.264 Video", RFC 6184, May 2011.

   [I-D.even-clue-sdp-clue-relation]
              Even, R., "Signalling of CLUE and SDP offer/answer",
              draft-even-clue-sdp-clue-relation-01 (work in progress),
              October 2012.

   [I-D.even-clue-rtp-mapping]
              Even, R. and J. Lennox, "Mapping RTP streams to CLUE media
              captures", draft-even-clue-rtp-mapping-05 (work in
              progress), February 2013.

   [I-D.hansen-clue-sdp-interaction]
              Hansen, R., "SDP and CLUE message interactions",
              draft-hansen-clue-sdp-interaction-01 (work in progress),
              February 2013.

   [I-D.ietf-mmusic-sdp-bundle-negotiation]
              Holmberg, C., Alvestrand, H., and C. Jennings,
              "Multiplexing Negotiation Using Session Description
              Protocol (SDP) Port Numbers",
              draft-ietf-mmusic-sdp-bundle-negotiation-06 (work in
              progress), April 2014.


Appendix A.  CLUE Signalling and data channel concerns

   [The specifics of the CLUE signaling protocol are in the process of
   being defined in [I-D.presta-clue-protocol], while the negotiation of
   the CLUE data channel is being defined in
   [I-D.ietf-clue-datachannel].  As such, considerable text originally
   in this section have been transitioned to these document.  The
   following text relates to issues that are no longer the focus of this
   document, but remain important and unresolved, and so have been
   preserved here.]

A.1.  Protocol Versioning and Options

A.1.1.  Versioning Objectives

   The CLUE versioning mechanism addresses the following needs:






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   o  Coverage:
      *  Versioning of basic behavior and options,
      *  CLUE message exchange,
      *  CLUE message exchange,
      *  coordinated use of SIP and SDP,
      *  required media behavior.
   o  Remain fixed for the duration of the CLUE channel
   o  Be extensible for configuration of new options.
   o  Be sufficient (with extensions) for all envisioned future
      versions.

A.1.2.  Versioning Overview

   An initial message exchange on the CLUE channel handles the
   negotiation of version and options.

   o  Dedicated message types are used for this negotiation.
   o  The negotiation is repeated if the CLUE channel is reestablished.

   The version usage is similar in philosophy to XMPP:

   o  See [RFC6120] section 4.7.5.
   o  A version has major and minor components.  (Each a non-negative
      integer.)
   o  Major version changes denote non-interoperable changes.
   o  Minor version changes denote schema changes that are backward
      compatible by ignoring unknown XML elements, or other backward
      compatible changes.
   o  If a common major version cannot be negotiated, then CLUE MUST NOT
      be used.
   o  The same message exchange also negotiates options.
   o  Each option is denoted by a unique XML element in the negotiation.

   Figure 1 shows the negotiation in simplified form:

















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                       | Supported       Supported |
                       |------------\ /------------|
                       |             X             |
                       |<-----------/ \----------->|
                       |                           |
                       | Required         Required |
                       |------------\ /------------|
                       |             X             |
                       |<-----------/ \----------->|
                       |                           |
                       |  Advertise/Configure/...  |
                       |<------------------------->|

              Figure 1: Basic Option Negotiation (simplified)

   Dedicated message types are used for the negotiation because:

   o  The protocol can then ensure that the negotiation is done first,
      and once.  Not changing mid-session means an endpoint can plan
      ahead, and predict what may be used and what might be received.
   o  This provides extensible framework for negotiating optional
      features.
   o  A full option negotiation can be completed before other messages
      are exchanged.

   Figure 2 and Figure 3 are simplified examples of the Supported and
   Required messages:

                 <supported>
                         <version major="1" minor="0">
                           <!- May repeat version if multiple
                          major versions supported.      ->
                    <!- Options follow ->
                    <mediaProvider/>
                         ...
                 </supported>

                 Figure 2: Supported Message (simplified)


                 <required>
                         <version major="1" minor="0">
                           <!- Requested options of peer follow ->
                    <!- Options follow ->
                    <mediaProvider/>
                         ...
                 </required>




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                  Figure 3: Required Message (simplified)

A.1.3.  Version Negotiation

   The Supported message includes one or more <version> elements, each
   denoting a major/minor version combination that the sender of the
   message is capable of supporting.

   The <version> element contains both a major and minor version.  Each
   is a non-negative integer.  Each <version> element in the message
   MUST contain a unique major version number, distinct from the major
   version number in all the other <version> elements in the message.
   The minor version in a <version> element denotes the largest minor
   version the sender supports for the corresponding major version.
   (Minor versions are always backwards compatible, so support for a
   minor version implies support for all smaller minor versions.)

   Each endpoint of the CLUE channel sends a Supported message, and
   receives the Supported message sent by the other end.  Then each end
   compares the versions sent and the versions received to determine the
   version to be used for this CLUE session.

   o  If there is no major version in common between the two ends,
      negotiation fails.
   o  The <version> elements from the two ends that have the largest
      matching major version are selected.
   o  After exchange each end determines compatible version numbers to
      be used for encoding and decoding messages, and other behavior in
      the CLUE session.
      *  The <version> elements from the two ends that have the largest
         matching major version are selected.
      *  The side that sent the smaller minor version chooses the one it
         sent.
      *  The side that sent the larger minor version may choose the
         minor version it received, or the one it sent, or any value
         between those two.
   o  Each end then sends a Required message with a single <version>
      element containing the major and minor versions it has chosen.

      [[Note: "required" is the wrong semantic for this.  Might want a
      better message name.]]
   o  Each end then behaves in accord with the specifications denoted by
      the version it chose.  This continues until the end of the CLUE
      session, or until changed as a result of another version
      negotiation when the CLUE channel is reestablished.

      [[Note: The version negotiation remains in effect even if the CLUE
      channel is lost.]]



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A.1.4.  Option Negotiation

   Option negotiation is used to agree upon which options will be
   available for use within the CLUE session.  (It does not say that
   these options must be used.)  This may be used for both standard and
   proprietary options.  (As used here, and option could be either a
   feature described as part of this specification that is optional to
   implement, or a feature defined in a separate specification that
   extends this one.)

   Each end includes, within the Supported message it sends, elements
   describing those options it is willing and able to use with this CLUE
   session.

   Each side, upon receiving a Supported message, selects from that
   message those option elements that it wishes the peer to use.  (If/
   when occasion for that use arises.)  It then includes those selected
   elements into the Required message that it sends.

   Within a received Supported message, unknown option elements MUST be
   ignored.  This includes elements that are of a known type that is not
   known to denote an option.

A.1.5.  Option Elements

   Each option is denoted, in the Supported and Required messages, by an
   XML element.  There are no special rules for these elements - they
   can be any XML element.  The attributes and body of the element may
   carry further information about the option.  The same element type is
   used to denote the option in the Supported message and the
   corresponding Required message, but the attributes and body may
   differ according to option-specific rules.  This may be used to
   negotiate aspects of a particular option.  The ordering of option
   elements is irrelevant within the Supported and Required messages,
   and need not be consistent in the two.

   Only one option element is defined in this document: <mediaProvider>.

A.1.5.1.  <mediaProvider>

   The <mediaProvider> element, when placed in a Supported message,
   indicates that the sender is willing and able to send ADVERTISEMENT
   messages and receive CONFIGURE messages.  When placed in a Required
   message, the <mediaProvider> element indicates that the sender is
   willing, able, and desirous of receiving ADVERTISEMENT messages and
   sending CONFIGURE messages.  If an endpoint does not receive
   <mediaProvider> in a Required message, it MUST NOT send ADVERTISEMENT
   messages.  For common cases <mediaProvider> should be supported and



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   required by both endpoints, to enable bidirectional exchange of
   media.  If not required by either end, the CLUE session is useless.
   This is an error condition, and SHOULD result in termination of the
   CLUE channel.

   The <mediaProvider> element has no defined attributes or body.

A.1.6.  Version & option negotiation errors

   The following are errors that may be detected and reported during
   version negotiation:

   o  Version incompatibility

      There is no common value between the major version numbers sent in
      a Supported message and those in the received Supported message.
   o  Option incompatibility

      This can occur if options supported by one endpoint are
      inconsistent with those supported by the other endpoint.  E.g.,
      The <mediaProvider> option is not specified by either endpoint.
      Options SHOULD be specified so as to make it difficult for this
      problem to occur.

      This error may also be used to indicate that insufficient options
      have been required among the two ends for a useful session to
      result.  This can occur with a feature that needs to be present on
      at least one end, but not on a specific end.  E.g., The
      <mediaProvider> option was Supported by at least one of the
      endpoints, but it was not Required by either.

      This may also be used to indicate that an option element in the
      Required message has attributes or body content that is
      syntactically correct, but in inconsistent with the rules for
      option negotiation specified for that particular element.  The
      definition of each option must specify the negotiation rules for
      that option.
   o  Unsupported option

      An option element type received in a Required message did not
      appear in the corresponding Supported element.

      (Unsupported options received in a Supported message do not
      trigger this error.  They are ignored.)

   These errors are reported using the normal message error reporting
   mechanism.




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   Other applicable error codes may also be returned in response to a
   Supported or Required message.

   Errors that occur at this stage result in negotiation failure.  When
   this occurs, CLUE cannot be used until the end of the SIP session, or
   until a new CLUE channel is negotiated and a subsequent version
   negotiation succeeds.  The SIP session may continue without CLUE
   features.

A.1.7.  Definition and Use of Version Numbers

   [[NOTE: THIS IS AWKWARD.  SUGGESTIONS FOR BETTER WAYS TO DEFINE THIS
   ARE WELCOME.]]

   This document defines CLUE version 1.0 (major=1, minor=0).  This
   denotes the normative behavior defined in this document and other
   documents upon which it normatively depends, including but is not
   limited to:

   o  the schema defined in [I-D.presta-clue-protocol];
   o  the schema defined in [clue-data-model];
   o  the protocol used to exchange CLUE messages;
   o  the protocol defined herein that defines valid sequence of CLUE
      messages;
   o  the specific rules defined herein for employing SIP, SDP, and RTP
      to realize the CLUE messages.

   Given two CLUE versions Vx and Vy, then Vx is backward compatible
   with Vy if and only if:

   o  All messages valid according to the schema of Vx are also valid
      according to the schemas of Vy
   o  All messages valid according to the schema of Vy can be made valid
      according to the schemas of Vx by deleting elements undefined in
      the schemas of Vx.

      [[NOTE: THIS PROBABLY NEEDS WORK!]]
   o  All normative behaviors defined for Vx are defined consistently
      for Vy.

      [[NOTE: SOME HAND WAVING HERE.]]

   Revisions, updates, to any of the documents denoted by Version 1.0
   MAY result in the definition of a new CLUE version.  If they do, then
   this document MUST be revised to define the new version.

   The CLUE version to be defined in a revision to this document MUST be
   determined as follows:



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   o  If the revision and the document being revised are mutually
      backward compatible (they are functionally equivalent), then the
      CLUE version MUST remain unchanged.
   o  Else if the revision is backward compatible with the document
      being revised, then the CLUE major version MUST remain unchanged,
      and the CLUE minor version MUST be increased by one (1).
   o  Else the CLUE major version must be increased by one (1), and the
      CLUE minor version set to zero (0).

   When a CLUE implementation sends a Supported message, it MUST include
   the CLUE versions it is willing and able to conform with.

A.1.8.  Version & Option Negotiation Examples

A.1.8.1.  Successful Negotiation - Multi-version


                       | Supported       Supported |
                       | Version 2.0               |
                       | Version 1.2   Version 1.1 |
                       | mediaProv       mediaProv |
                       |------------\ /------------|
                       |             X             |
                       |<-----------/ \----------->|
                       |                           |
                       | OK response   OK response |
                       |------------\ /------------|
                       |             X             |
                       |<-----------/ \----------->|
                       |                           |
                       | Required         Required |
                       | Version 1.2   Version 1.1 |
                       | mediaProv       mediaProv |
                       |------------\ /------------|
                       |             X             |
                       |<-----------/ \----------->|
                       |                           |
                       | OK response   OK response |
                       |------------\ /------------|
                       |             X             |
                       |<-----------/ \----------->|
                       |                           |
                       |         Advertise         |
                       |<------------------------->|
                       |                           |
                       |         Configure         |
                       |<------------------------->|




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   The endpoint on the left can support versions 1.2 and 2.0, and
   because of backward compatibility can support versions 1.0 and 1.1.
   The endpoint on the right supports only version 2.0.  Both endpoints
   with to both provide and consume media.  They each send a Supported
   message indicating what they support.

   The element on the left, upon receiving the Supported message,
   determines that it is permitted to use version 1.2 or 1.1, and
   decides to use 1.2.  It sends a Required message containing version
   1.2 and also includes the mediaProvider option element, because it
   wants its peer to provide media.

   The element on the right, upon receiving the Supported message,
   selects version 1.1 because it is the highest version in common to
   the two sides.  It sends a Required message containing version 1.1
   because that is the highest version in common.  It also includes the
   mediaProvider option element, because it wants its peer to provide
   media.

   Upon receiving the Required messages, both endpoints determine that
   they should send ADVERTISEMENTs.

   ADVERTISEMENT and CONFIGURE messages will flow in both directions.

A.1.8.2.  Successful Negotiation - Consumer-Only Endpoint


























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                       | Supported       Supported |
                       | Version 1.0   Version 1.0 |
                       | mediaProv       (no opts) |
                       |------------\ /------------|
                       |             X             |
                       |<-----------/ \----------->|
                       |                           |
                       | OK response   OK response |
                       |------------\ /------------|
                       |             X             |
                       |<-----------/ \----------->|
                       |                           |
                       | Required         Required |
                       | Version 1.0   Version 1.0 |
                       | (no opts)       mediaProv |
                       |------------\ /------------|
                       |             X             |
                       |<-----------/ \----------->|
                       |                           |
                       | OK response   OK response |
                       |------------\ /------------|
                       |             X             |
                       |<-----------/ \----------->|
                       |                           |
                       |  Advertise                |
                       |-------------------------->|
                       |                           |
                       |                 Configure |
                       |<--------------------------|

   The endpoint on the right consumes media, but doesn't provide any so
   it doesn't include the mediaProvider option element in the Supported
   message it sends.

   The element on the left would like to include a mediaProvider option
   element in the Requirements message it sends, but can't because it
   did not receive one in the Supported message it received.

   ADVERTISEMENT messages will only go from left to right, and CONFIGURE
   messages will only go from right to left.

A.1.8.3.  Successful Negotiation - Provider-Only Endpoint









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                       | Supported       Supported |
                       | Version 1.0   Version 1.0 |
                       | mediaProv       mediaProv |
                       |------------\ /------------|
                       |             X             |
                       |<-----------/ \----------->|
                       |                           |
                       | OK response   OK response |
                       |------------\ /------------|
                       |             X             |
                       |<-----------/ \----------->|
                       |                           |
                       | Required         Required |
                       | Version 1.0   Version 1.0 |
                       | (no opts)       mediaProv |
                       |------------\ /------------|
                       |             X             |
                       |<-----------/ \----------->|
                       |                           |
                       | OK response   OK response |
                       |------------\ /------------|
                       |             X             |
                       |<-----------/ \----------->|
                       |                           |
                       |  Advertise                |
                       |-------------------------->|
                       |                           |
                       |                 Configure |
                       |<--------------------------|

   The endpoint on the left provides media but does not consume any so
   it includes the mediaProvider option element in the Supported message
   it sends, but does't include the mediaProvider option element in the
   Required message it sends.

   ADVERTISEMENT messages will only go from left to right, and CONFIGURE
   messages will only go from right to left.

A.1.8.4.  Version Incompatibility












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                       | Supported       Supported |
                       | Version 1.2   Version 2.1 |
                       |------------\ /------------|
                       |             X             |
                       |<-----------/ \----------->|
                       |                           |
                       | Version           Version |
                       | Incompat.       Incompat. |
                       |------------\ /------------|
                       |             X             |
                       |<-----------/ \----------->|
                       |                           |
                       |     close clue channel    |
                       |<------------------------->|
                       |                           |
                       |     legacy mode or BYE    |
                       |<------------------------->|

   Upon receiving the Supported message, each endpoint discovers there
   is no major version in common, so CLUE usage is not possible.  Each
   sends an error response indicating this and then ceases CLUE usage.

A.1.8.5.  Option Incompatibility


                       | Supported       Supported |
                       | Version 1.0   Version 1.0 |
                       | mediaProv       mediaProv |
                       |------------\ /------------|
                       |             X             |
                       |<-----------/ \----------->|
                       |                           |
                       | Required         Required |
                       | (no opts)       (no opts) |
                       |------------\ /------------|
                       |             X             |
                       |<-----------/ \----------->|
                       |                           |
                       | Option             Option |
                       | Incompat.       Incompat. |
                       |------------\ /------------|
                       |             X             |
                       |<-----------/ \----------->|
                       |                           |
                       |     close clue channel    |
                       |<------------------------->|
                       |                           |
                       |     legacy mode or BYE    |



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                       |<------------------------->|

   Neither of the endpoints is willing to provide media.  It makes no
   sense to continue CLUE operation in this situation.  Each endpoint
   realizes this upon receiving the Supported message, sends an error
   response indicating this and then ceases CLUE usage.

A.1.8.6.  Syntax Error


                       | Supported          !@#$%^ |
                       |------------\ /------------|
                       |             X             |
                       |<-----------/ \----------->|
                       |                           |
                       | syntax error  OK response |
                       |------------\ /------------|
                       |             X             |
                       |<-----------/ \----------->|
                       |                           |
                       |     close clue channel    |
                       |-------------------------->|
                       |                           |
                       |     legacy mode or BYE    |
                       |<------------------------->|

A.2.  Message Transport

   CLUE messages are transported over a bidirectional CLUE channel.  In
   a two-party CLUE session, a CLUE channel connects the two endpoints.
   In a CLUE conference, each endpoint has a CLUE channel connecting it
   to an MCU.  (In conferences with cascaded mixers [RFC4353], two MCUs
   will be connected by a CLUE channel.)

A.2.1.  CLUE Channel Lifetime

   The transport mechanism used for CLUE messages is DTLS/SCTP as
   specified in [I-D.tuexen-tsvwg-sctp-dtls-encaps] and
   [I-D.ietf-mmusic-sctp-sdp].  A CLUE channel consists of one SCTP
   stream in each direction over a DTLS/SCTP session.  The mechanism for
   establishing the DTLS/SCTP session is described in
   [I-D.ietf-clue-datachannel].

   The CLUE channel will usually be offered during the initial SIP
   INVITE, and remain connected for the duration of the CLUE/SIP
   session.  However this need not be the case.  The CLUE channel may be
   established mid-session after desire and capability for CLUE have
   been determined, and the CLUE channel may be dropped mid-call if the



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   desire and/or capability to support it is lost.

   There may be cases when it becomes necessary to "reset" the CLUE
   channel.  This by be as a result of an error on the underlying SCTP
   association, a need to change the endpoint address of the SCTP
   association, loss of CLUE protocol state, or something else TBD.

   The precise mechanisms used to determine when a reset is required,
   and how to accomplish it and return to a well defined state are TBS.

A.2.2.  Channel Error Handling

   We will need to specify behavior in the face of transport errors that
   are so severe that they can't be managed via CLUE messaging within
   the CLUE channel.  Some errors of this sort are:
   o  Unable to establish the SCTP association after signaling it in
      SDP.
   o  CLUE channel setup rejected by peer.
   o  Error reported by transport while writing message to CLUE channel.
   o  Error reported by transport while reading message from CLUE
      channel.
   o  Timeout - overdue acknowledgement of a CLUE message.
      (Requirements for now soon a message must be responded to are
      TBD.)
   o  Application fault.  CLUE protocol state lost.
   The worst case is to drop the entire CLUE call.  Another possibility
   is to fall back to legacy compatibility mode.  Or perhaps a "reset"
   can be done on the protocol.  E.g. this might be accomplished by
   sending a new O/A and establishing a replacement SCTP association.
   Or a new CLUE channel might be established within the existing SCTP
   association.

A.3.  Message Framing

   Message framing is provided by the SCTP transport protocol.  Each
   CLUE message is carried in one SCTP message.


Authors' Addresses

   Paul Kyzivat
   Huawei

   Email: pkyzivat@alum.mit.edu







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   Lennard Xiao
   Huawei

   Email: lennard.xiao@huawei.com


   Christian Groves
   Huawei

   Email: Christian.Groves@nteczone.com


   Robert Hansen
   Cisco Systems

   Email: rohanse2@cisco.com



































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