[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits] [IPR]

Versions: (draft-kyzivat-clue-signaling) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14

Network Working Group                                         P. Kyzivat
Internet-Draft
Intended status: Standards Track                                 L. Xiao
Expires: September 22, 2016                                    C. Groves
                                                                  Huawei
                                                               R. Hansen
                                                           Cisco Systems
                                                          March 21, 2016


                             CLUE Signaling
                      draft-ietf-clue-signaling-09

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 September 22, 2016.

Copyright Notice

   Copyright (c) 2016 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



Kyzivat, et al.        Expires September 22, 2016               [Page 1]


Internet-Draft               CLUE Signaling                   March 2016


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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Media Feature Tag Definition  . . . . . . . . . . . . . . . .   4
   4.  SDP Grouping Framework CLUE Extension Semantics . . . . . . .   4
     4.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  The CLUE data channel and the CLUE grouping semantic  . .   4
     4.3.  CLUE-controlled media and the CLUE grouping semantic  . .   5
     4.4.  SDP semantics for CLUE-controlled media . . . . . . . . .   5
       4.4.1.  Signaling CLUE Encodings  . . . . . . . . . . . . . .   5
         4.4.1.1.  Referencing Encodings in the CLUE protocol  . . .   6
       4.4.2.  Negotiating receipt of CLUE Capture Encodings in SDP    7
     4.5.  SDP Offer/Answer Procedures . . . . . . . . . . . . . . .   7
       4.5.1.  Generating the Initial Offer  . . . . . . . . . . . .   7
       4.5.2.  Generating the Answer . . . . . . . . . . . . . . . .   7
         4.5.2.1.  Negotiating use of CLUE and the CLUE data channel   7
         4.5.2.2.  Negotiating CLUE-controlled media . . . . . . . .   8
         4.5.2.3.  Negotiating non-CLUE controlled media . . . . . .   8
       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  . . . . . . . . . . . .   9
       4.5.4.  Modifying the session . . . . . . . . . . . . . . . .   9
         4.5.4.1.  Adding and removing CLUE-controlled media . . . .   9
         4.5.4.2.  Enabling CLUE mid-call  . . . . . . . . . . . . .  10
         4.5.4.3.  Disabling CLUE mid-call . . . . . . . . . . . . .  10
   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 . . . . . . .  13
   7.  Multiplexing of CLUE-controlled media using BUNDLE  . . . . .  14
     7.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.2.  Usage of BUNDLE with CLUE . . . . . . . . . . . . . . . .  14
       7.2.1.  Generating the Initial Offer  . . . . . . . . . . . .  15
       7.2.2.  Multiplexing of the data channel and RTP media  . . .  15
   8.  Example: A call between two CLUE-capable Endpoints  . . . . .  15
   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
     11.2.  New SIP Media Feature Tag  . . . . . . . . . . . . . . .  26



Kyzivat, et al.        Expires September 22, 2016               [Page 2]


Internet-Draft               CLUE Signaling                   March 2016


   12. Security Considerations . . . . . . . . . . . . . . . . . . .  26
   13. Change History  . . . . . . . . . . . . . . . . . . . . . . .  27
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  34
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  34
     14.2.  Informative References . . . . . . . . . . . . . . . . .  35
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  36

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 employs 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, CLUE protocol
   [I-D.ietf-clue-protocol] messages, transported via a CLUE data
   channel [I-D.ietf-clue-datachannel], are used to negotiate the
   Capture Sources available, their attributes and any constraints in
   their use.  They also allow the far end device to specify which
   Captures they wish to receive.

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

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

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "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:





Kyzivat, et al.        Expires September 22, 2016               [Page 3]


Internet-Draft               CLUE Signaling                   March 2016


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

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

3.  Media Feature Tag Definition

   The "sip.clue" media feature tag 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 [RFC5888] extension
   called 'CLUE'.

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

   Currently only support for a single CLUE group is specified; support
   for multiple CLUE groups in a single session is 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 signaled, and is either able to determine
   that the other side of the SDP exchange supports multiple CLUE
   channels, or is able to fail gracefully in the event it does not.

4.2.  The CLUE data channel and the CLUE grouping semantic

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




Kyzivat, et al.        Expires September 22, 2016               [Page 4]


Internet-Draft               CLUE Signaling                   March 2016


   The data channel is negotiated over SDP as described in
   [I-D.ietf-mmusic-data-channel-sdpneg].  A CLUE-capable device wishing
   to negotiate CLUE MUST also include a CLUE group in 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.  Signaling CLUE Encodings

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

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



Kyzivat, et al.        Expires September 22, 2016               [Page 5]


Internet-Draft               CLUE Signaling                   March 2016


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

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

   Every "m=" line representing a CLUE Encoding MUST contain a "label"
   attribute as defined in [RFC4574].  This label is used to identify
   the Encoding by the sender in CLUE Advertisement messages and by the
   receiver in CLUE Configure messages.  Each label used for a CLUE-
   controlled "m=" line MUST be different from the label on all other
   "m=" lines in the 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 depends.

4.4.1.1.  Referencing Encodings in the CLUE protocol

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

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




Kyzivat, et al.        Expires September 22, 2016               [Page 6]


Internet-Draft               CLUE Signaling                   March 2016


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.  They MAY include a "label" attribute,
   but this is not required by CLUE, as only label values associated
   with "a=sendonly" Encodings are referenced by CLUE protocol messages.

4.5.  SDP Offer/Answer Procedures

4.5.1.  Generating the Initial Offer

   A CLUE-capable device sending an initial SDP offer of a SIP session
   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.

   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.



Kyzivat, et al.        Expires September 22, 2016               [Page 7]


Internet-Draft               CLUE Signaling                   March 2016


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

4.5.2.2.  Negotiating CLUE-controlled media

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

   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.

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.








Kyzivat, et al.        Expires September 22, 2016               [Page 8]


Internet-Draft               CLUE Signaling                   March 2016


4.5.3.  Processing the initial Offer/Answer negotiation

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

4.5.3.1.  Successful CLUE negotiation

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

   A CLUE-capable device MAY choose not to send 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=" lines to send or receive CLUE-controlled media they MAY now send
   a SIP request with a new SDP offer following the normal rules of SDP
   offer/answer and any negotiated extensions.

4.5.3.2.  CLUE negotiation failure

   In the event that the negotiation of CLUE fails and the call is not
   CLUE-enabled once the initial offer/answer negotiation completes then
   CLUE is not in use in the call.  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 deactivate "m=" lines for
   CLUE-controlled media.




Kyzivat, et al.        Expires September 22, 2016               [Page 9]


Internet-Draft               CLUE Signaling                   March 2016


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

4.5.4.2.  Enabling CLUE mid-call

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

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

4.5.4.3.  Disabling CLUE mid-call

   If, during an ongoing CLUE-enabled call a device wishes to disable
   CLUE, it can do so by following the procedures for closing a data
   channel defined in Section 5.2.4 of
   [I-D.ietf-mmusic-data-channel-sdpneg]: sending a new SDP offer/answer
   exchange and subsequent SCTP SSN reset for the CLUE channel.  It MUST
   also remove the CLUE group.  Without the CLUE group any "m=" lines
   that were previously CLUE-controlled no longer are; implementations
   MAY disable them by setting their ports to 0 or may continue to use
   them - in the latter case how they are used is outside the scope of
   this document.

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

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



Kyzivat, et al.        Expires September 22, 2016              [Page 10]


Internet-Draft               CLUE Signaling                   March 2016


   in this circumstance.  These measures also apply if the CLUE data
   channel fails, or is closed/reset without a corresponding SDP
   exchange to disable the "m=" line.

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 intermediate
   stages in non-atomic operations, and mandates additional constraints
   on when CLUE-configured media can be sent.

5.1.  Independence of SDP and CLUE negotiation

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

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

   CLUE messages contain an <encID> (in encodingIDList) or <encodingID>
   (in captureEncodingType), which is used to identify a specific
   encoding or captureEncoding in SDP; see
   [I-D.ietf-clue-data-model-schema] for specifics.  The non-atomic
   nature of CLUE negotiation means that a sender may wish to send a new
   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 message solely
   because it contains <encID> elements that do not match a label in
   SDP.



Kyzivat, et al.        Expires September 22, 2016              [Page 11]


Internet-Draft               CLUE Signaling                   March 2016


   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 mismatches will usually be
   transitory a device MUST be able to cope with such mismatches
   remaining indefinitely.  However, this document makes some
   recommendations on message ordering for these non-atomic transitions.

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

   Generally, implementations that receive messages for which they have
   incomplete information SHOULD wait until they have the corresponding
   information they lack before sending messages to make changes related



Kyzivat, et al.        Expires September 22, 2016              [Page 12]


Internet-Draft               CLUE Signaling                   March 2016


   to that information.  For example, an answerer that receives a new
   SDP offer with three new "a=sendonly" CLUE "m=" lines for which it
   has received no CLUE Advertisement providing the corresponding
   capture information SHOULD include corresponding "a=inactive" lines
   in its answer, and SHOULD make a new SDP offer with "a=recvonly" when
   and if a new Advertisement arrives with Captures relevant to those
   Encodings.

   Because of the constraints of SDP offer/answer and because new SDP
   negotiations are generally more 'costly' than sending a new CLUE
   message, implementations needing to make changes to both channels
   SHOULD prioritize sending the updated CLUE message over sending the
   new SDP message.  The aim is for the recipient to receive the CLUE
   changes before the SDP changes, allowing the recipient to send their
   SDP answers without incomplete information, 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 contribute to these MCCs may or may not be defined
   in the Advertisement message.  If they are defined and the MCC is
   providing them in a switched format the recipient may wish to
   determine which originating source Capture is currently being
   provided, so that they can apply geometric corrections based on that
   Capture's geometry, or take some other action based on the original
   Capture information.

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

6.1.  CaptureID reception during MCC redefinition

   Because the RTP/RTCP CaptureID is delivered via a different channel
   to the Advertisement 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 Consumer MUST not be disrupted by any of



Kyzivat, et al.        Expires September 22, 2016              [Page 13]


Internet-Draft               CLUE Signaling                   March 2016


   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 separate port used for this
   purpose may impose costs that a device wishes to avoid, such as the
   need to open that port on firewalls and NATs, the need to collect ICE
   candidates [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.

7.2.  Usage of BUNDLE with CLUE

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




Kyzivat, et al.        Expires September 22, 2016              [Page 14]


Internet-Draft               CLUE Signaling                   March 2016


7.2.1.  Generating the Initial Offer

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

7.2.2.  Multiplexing of the data channel and RTP media

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

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

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

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



                 +----------+                      +-----------+
                 |  Alice   |                      |    Bob    |
                 |          |                      |           |
                 +----+-----+                      +-----+-----+
                      |                                  |
                      |                                  |
                      | SIP INVITE 1                     |
                      |--------------------------------->|
                      |                                  |
                      |                                  |
                      |                     SIP 200 OK 1 |
                      |<---------------------------------|
                      |                                  |
                      |                                  |



Kyzivat, et al.        Expires September 22, 2016              [Page 15]


Internet-Draft               CLUE Signaling                   March 2016


                      | SIP ACK 1                        |
                      |--------------------------------->|
                      |                                  |
                      |                                  |
                      |                                  |
                      |<########### MEDIA 1 ############>|
                      |   1 video A->B, 1 video B->A     |
                      |<################################>|
                      |                                  |
                      |                                  |
                      |                                  |
                      |<================================>|
                      |   CLUE DATA CHANNEL ESTABLISHED  |
                      |<================================>|
                      |                                  |
                      |                                  |
                      | CLUE OPTIONS                     |
                      |<*********************************|
                      |                                  |
                      |                                  |
                      |            CLUE OPTIONS RESPONSE |
                      |*********************************>|
                      |                                  |
                      |                                  |
                      | CLUE ADVERTISEMENT 1             |
                      |*********************************>|
                      |                                  |
                      |                                  |
                      |             CLUE ADVERTISEMENT 2 |
                      |<*********************************|
                      |                                  |
                      |                                  |
                      | CLUE ADVERTISEMENT ACK 1         |
                      |<*********************************|
                      |                                  |
                      |                                  |
                      |         CLUE ADVERTISEMENT ACK 2 |
                      |*********************************>|
                      |                                  |
                      |                                  |
                      | SIP INVITE 2 (+3 sendonly)       |
                      |--------------------------------->|
                      |                                  |
                      |                                  |
                      |                 CLUE CONFIGURE 1 |
                      |<*********************************|
                      |                                  |
                      |                                  |



Kyzivat, et al.        Expires September 22, 2016              [Page 16]


Internet-Draft               CLUE Signaling                   March 2016


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





Kyzivat, et al.        Expires September 22, 2016              [Page 17]


Internet-Draft               CLUE Signaling                   March 2016


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


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


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

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

   Once the data channel is established CLUE protocol negotiation
   begins.  In this case Bob is the Channel Initiator and sends a CLUE
   OPTIONS message describing his version support.  On receiving that
   message Alice sends her corresponding CLUE OPTIONS RESPONSE.

   With the OPTIONS phase complete Alice now sends her CLUE
   Advertisement (CLUE ADVERTISEMENT 1).  She advertises three static
   Captures representing her three cameras.  She also includes switched
   Captures suitable for two- and one-screen systems.  All of these
   Captures are in a single Capture Scene, with suitable Capture Scene



Kyzivat, et al.        Expires September 22, 2016              [Page 18]


Internet-Draft               CLUE Signaling                   March 2016


   Views to tell Bob that he should either subscribe to the three static
   Captures, the two switched Captures or the one switched Capture.
   Alice has no simultaneity constraints, so includes all six Captures
   in one simultaneous set.  Finally, Alice includes an Encoding Group
   with three Encoding IDs: "enc1", "enc2" and "enc3".  These Encoding
   IDs aren't currently valid, but will match the next SDP offer she
   sends.

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

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

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

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

   Alice now sends SIP INVITE 2.  She maintains the sendrecv audio,
   video and CLUE "m=" lines, and she adds three new sendonly "m=" lines
   to represent the three CLUE-controlled Encodings she can send.  Each
   of these "m=" lines has a label corresponding to one of the Encoding
   IDs from CLUE ADVERTISEMENT 1.  Each also has its mid added to the



Kyzivat, et al.        Expires September 22, 2016              [Page 19]


Internet-Draft               CLUE Signaling                   March 2016


   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:


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


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

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



Kyzivat, et al.        Expires September 22, 2016              [Page 20]


Internet-Draft               CLUE Signaling                   March 2016


   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):


      ...
      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


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

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

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



Kyzivat, et al.        Expires September 22, 2016              [Page 21]


Internet-Draft               CLUE Signaling                   March 2016


      ...
      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 her CLUE Configure message and her SDP answer.  In CLUE
   CONFIGURE 2 she requests the two static Captures from Bob, to be sent
   on Encodings "foo" and "bar".

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





Kyzivat, et al.        Expires September 22, 2016              [Page 22]


Internet-Draft               CLUE Signaling                   March 2016


      ...
      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


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

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

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







Kyzivat, et al.        Expires September 22, 2016              [Page 23]


Internet-Draft               CLUE Signaling                   March 2016


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





Kyzivat, et al.        Expires September 22, 2016              [Page 24]


Internet-Draft               CLUE Signaling                   March 2016


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


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

   From the lack of 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 Jonathan 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 per
   [RFC5888]:








Kyzivat, et al.        Expires September 22, 2016              [Page 25]


Internet-Draft               CLUE Signaling                   March 2016


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


11.2.  New SIP Media Feature Tag

   This specification registers a new media feature tag in the SIP
   [RFC3264] tree per the procedures defined in [RFC2506] and [RFC3840].

   Media feature tag name: sip.clue

   ASN.1 Identifier: 1.3.6.1.8.4.29

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

   Values appropriate for use with this feature tag: Boolean.

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

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

   Related standards or documents: [this draft]

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

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

   o  CLUE workgroup: clue@ietf.org

   o  CLUE chairs: clue-chairs@ietf.org

   Intended usage: COMMON

12.  Security Considerations

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




Kyzivat, et al.        Expires September 22, 2016              [Page 26]


Internet-Draft               CLUE Signaling                   March 2016


   Beyond the need to secure the constituent protocols, the use of CLUE
   does impose additional security concerns.  One area of increased risk
   involves the potential for a malicious party to subvert a CLUE-
   capable device to attack a third party by driving large volumes of
   media (particularly video) traffic at them by establishing a
   connection to the CLUE-capable device and directing the media to the
   victim.  While this is a risk for all media devices, a CLUE-capable
   device may allow the attacker to configure multiple media streams to
   be sent, significantly increasing the volume of traffic directed at
   the victim.

   This attack can be prevented by ensuring that the media recipient
   intends to receive the media packets.  As such all CLUE-capable
   devices MUST support key negotiation and receiver intent assurance
   via DTLS [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, this 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
   their device supports.  To prevent this, SIP signaling SHOULD always
   be encrypted using TLS [RFC5630].

13.  Change History

   -09:  Revision by Rob Hansen

      *  A few minor spelling tweaks

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

   -08:  Revision by Rob Hansen

      *  Spelling and grammar fixes from Paul and Christian gratefully
         adopted

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




Kyzivat, et al.        Expires September 22, 2016              [Page 27]


Internet-Draft               CLUE Signaling                   March 2016


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

   -07:  Revision by Rob Hansen

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

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

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

      *  Section on not sending CLUE control media until CLUE
         negotiation completes narrowed to notify that only RTP should
         not be sent until negotiation completes and add RTCP to the
         list of things that should be sent as normal, in line with
         a=inactive.

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

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

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

   -06:  Revision by Rob Hansen

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

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

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



Kyzivat, et al.        Expires September 22, 2016              [Page 28]


Internet-Draft               CLUE Signaling                   March 2016


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

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

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

   -05:  Revision by Rob Hansen

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

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

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

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

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

      *  Other minor syntax improvements.

   -04:  Revision by Rob Hansen

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

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

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




Kyzivat, et al.        Expires September 22, 2016              [Page 29]


Internet-Draft               CLUE Signaling                   March 2016


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

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

      *  Added a number of security considerations.

   -03:  Revision by Rob Hansen

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

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

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

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

   -02:  Revision by Rob Hansen

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

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

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

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

      *  Minor revisions to the section on complying with normative SDP/
         CLUEstate machine language to clarify that these were not new




Kyzivat, et al.        Expires September 22, 2016              [Page 30]


Internet-Draft               CLUE Signaling                   March 2016


         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.

      *  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'




Kyzivat, et al.        Expires September 22, 2016              [Page 31]


Internet-Draft               CLUE Signaling                   March 2016


      *  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.

      *  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.



Kyzivat, et al.        Expires September 22, 2016              [Page 32]


Internet-Draft               CLUE Signaling                   March 2016


      *  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:

      *  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.




Kyzivat, et al.        Expires September 22, 2016              [Page 33]


Internet-Draft               CLUE Signaling                   March 2016


      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-25 (work in progress), January 2016.

   [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-13 (work in
              progress), March 2016.

   [I-D.ietf-clue-protocol]
              Presta, R. and S. Romano, "CLUE protocol", draft-ietf-
              clue-protocol-06 (work in progress), October 2015.

   [I-D.ietf-clue-datachannel]
              Holmberg, C., "CLUE Protocol data channel", draft-ietf-
              clue-datachannel-12 (work in progress), March 2016.

   [I-D.ietf-clue-rtp-mapping]
              Even, R. and J. Lennox, "Mapping RTP streams to CLUE media
              captures", draft-ietf-clue-rtp-mapping-06 (work in
              progress), January 2016.

   [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-16 (work in progress), February 2016.

   [I-D.ietf-mmusic-data-channel-sdpneg]
              Drage, K., Makaraju, M., Stoetzer-Bradler, J., Ejzak, R.,
              and J. Marcon, "SDP-based Data Channel Negotiation",
              draft-ietf-mmusic-data-channel-sdpneg-08 (work in
              progress), February 2016.



Kyzivat, et al.        Expires September 22, 2016              [Page 34]


Internet-Draft               CLUE Signaling                   March 2016


   [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>.

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

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,
              <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>.

   [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>.



Kyzivat, et al.        Expires September 22, 2016              [Page 35]


Internet-Draft               CLUE Signaling                   March 2016


   [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>.

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

   [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-27 (work in progress), February 2016.

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

Authors' Addresses

   Paul Kyzivat

   Email: pkyzivat@alum.mit.edu


   Lennard Xiao
   Huawei

   Email: lennard.xiao@huawei.com



Kyzivat, et al.        Expires September 22, 2016              [Page 36]


Internet-Draft               CLUE Signaling                   March 2016


   Christian Groves
   Huawei

   Email: Christian.Groves@nteczone.com


   Robert Hansen
   Cisco Systems

   Email: rohanse2@cisco.com









































Kyzivat, et al.        Expires September 22, 2016              [Page 37]

Html markup produced by rfcmarkup 1.129b, available from https://tools.ietf.org/tools/rfcmarkup/