Session Signaling for Controlling Multiple Streams for Telepresence (CLUE)

Introduction To enable devices to participate in a telepresence call, by selecting the sources they wish to view, receiving those media sources, and displaying them in an optimal fashion, ControLling mUltiple streams for tElepresence (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 messages, transported via a CLUE data channel, are used to negotiate the Capture Sources available, their attributes, and any constraints in their use. They also allow the far-end device to specify which Captures they wish to receive. It is recommended that those documents be read prior to this one as this document assumes familiarity with those protocols and hence uses terminology from each with limited introduction. Beyond negotiating the CLUE channel, SDP is also used to negotiate the details of supported media streams and the maximum capability of each of those streams. As the CLUE Framework 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.

Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. This document uses terminology defined in the CLUE Framework. A few additional terms specific to this document are defined as follows:

non-CLUE device:: A device that supports standard SIP and SDP but either does not support CLUE or does support CLUE 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 for details on how this control is signaled in SDP. There is a corresponding "non-CLUE-controlled" media term.

Media Feature Tag Definition The "sip.clue" media feature tag indicates support for CLUE in SIP calls. A CLUE-capable device SHOULD include this media feature tag in its REGISTER requests and OPTION responses. It SHOULD also include the media feature tag in INVITE and UPDATE 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.

SDP Grouping Framework CLUE Extension Semantics

General This section defines a new SDP Grouping Framework extension called 'CLUE'. The CLUE extension can be indicated using an SDP session-level 'group' attribute. Each SDP media "m=" line that is included in this group, using SDP media-level mid attributes, is CLUE controlled by a CLUE data channel also included in this CLUE group. Currently, only support for a single CLUE group is specified; support for multiple CLUE groups in a single session is outside the scope of this document. A device MUST NOT include more than one CLUE group in its SDP message unless it is following a specification that defines how multiple CLUE channels are signaled and is able to either determine that the other side of the SDP exchange supports multiple CLUE channels or fail gracefully in the event it does not.

The CLUE Data Channel and the CLUE Grouping Semantic The CLUE data channel is a bidirectional data channel used for the transport of CLUE messages, conveyed within a Stream Control Transmission Protocol (SCTP) over DTLS connection. This channel must be established before CLUE protocol messages can be exchanged and CLUE-controlled media can be sent. The data channel is negotiated over SDP as described in . A CLUE-capable device wishing to negotiate CLUE MUST also include a CLUE group in their SDP offer or answer and include the "mid" of the "m=" line for the data channel in that group. The CLUE group MUST include the "mid" of the "m=" line for one (and only one) data channel. Presence of the data channel in the CLUE group in an SDP offer or answer also serves, along with the "sip.clue" media feature tag, as an indication that the device supports CLUE and wishes to upgrade the call to include CLUE-controlled media. A CLUE-capable device SHOULD include a data channel "m=" line in offers and, when allowed by , answers.

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. For an "m=" line to be CLUE controlled, its "mid" value MUST be included in the CLUE group. CLUE-controlled media is controlled by the CLUE protocol as negotiated on the CLUE data channel with a "mid" included in the CLUE group. "m=" lines not specified as being under CLUE control follow normal rules for media streams negotiated in SDP as defined in documents such as . The restrictions on CLUE-controlled media that are defined below always apply to "m=" lines in an SDP offer or answer, even if negotiation of the data channel in SDP failed due to lack of CLUE support by the remote device or for any other reason, or in an offer if the recipient does not include the "mid" of the corresponding "m=" line in their CLUE group.

SDP Semantics for CLUE-Controlled Media

Signaling CLUE Encodings The CLUE Framework defines the concept of "Encodings", which represents 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 (e.g., "a=sendonly") Encodings can then be expressed using existing SDP syntax. For instance, for H.264, see Table 6 in for a list of valid parameters for representing encoder sender stream limits. These Encodings are CLUE controlled and hence MUST include an "mid" in the CLUE group as defined above. In addition to the normal restrictions defined in , 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 Session Traversal Utilities for NAT (STUN), RTP Control Protocol (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 . This label is used to identify the Encoding by the sender in CLUE 'advertisement' messages and by the receiver in CLUE 'configure' messages. Each label used for a CLUE-controlled "m=" line MUST be different from the label on all other "m=" lines in the CLUE group, unless an "m=" line represents a dependent stream related to another "m=" line (such as a Forward Error Correction (FEC) stream), in which case it MUST have the same label value as the "m=" line on which it depends.

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 Offer/Answer message sent by the sender of the 'advertisement' message with a label value corresponding to the text content of the <encID>. If the <encID> is not defined in SDP, it MUST be one it anticipates sending in a subsequent SDP Offer/Answer exchange. Each <encodingID> (in captureEncodingType) in a CLUE 'configure' message MUST represent an Encoding defined in SDP; the specific Encoding referenced is a CLUE-controlled "m=" line in the most recent SDP Offer/Answer message received by the sender of the 'configure' message with a label value corresponding to the text content of the <encodingID>. Note that the non-atomic nature of SDP/CLUE protocol interaction may mean that there are temporary periods where an <encID>/<encodingID> in a CLUE message does not reference an SDP "m=" line, or where an Encoding represented in SDP is not referenced in a CLUE protocol message. See for specifics.

Negotiating Receipt of CLUE Capture Encodings in SDP A receiver who wishes to receive a CLUE stream via a specific Encoding requires an "a=recvonly" "m=" line that matches the "a=sendonly" Encoding. These "m=" lines are CLUE controlled and hence MUST include their "mid" in the CLUE group. They MAY include a "label" attribute, but this is not required by CLUE, as only label values associated with "a=sendonly" Encodings are referenced by CLUE protocol messages.

SDP Offer/Answer Procedures

Generating the Initial Offer A CLUE-capable device sending an initial SDP offer of a SIP session and wishing to negotiate CLUE will include an "m=" line for the data channel to convey the CLUE protocol, along with a CLUE group containing the "mid" of the data channel "m=" line. For interoperability with non-CLUE devices, a CLUE-capable device sending an initial SDP offer SHOULD NOT include any "m=" line for CLUE-controlled media beyond the "m=" line for the CLUE data channel, and it SHOULD include at least one non-CLUE-controlled media "m=" line. If the device has evidence that the receiver is also CLUE capable, for instance, due to receiving an initial INVITE with no SDP but including a "sip.clue" media feature tag, the above recommendation is waived, and the initial offer MAY contain "m=" lines for CLUE-controlled media. With the same interoperability recommendations as for Encodings, the sender of the initial SDP offer MAY also include "a=recvonly" media lines to preallocate "m=" lines to receive media. Alternatively, it MAY wait until CLUE protocol negotiation has completed before including these lines in a new offer/answer exchange -- see for recommendations.

Generating the Answer

Negotiating Use of CLUE and the CLUE Data Channel If the recipient of an initial offer is CLUE capable, and the offer contains both an "m=" line for a data channel and a CLUE group containing the "mid" for that "m=" line, they SHOULD negotiate data channel support for an "m=" line and include the "mid" of that "m=" line in a corresponding CLUE group. A CLUE-capable recipient that receives an "m=" line for a data channel but no corresponding CLUE group containing the "mid" of that "m=" line MAY still include a corresponding data channel "m=" line if there are any other non-CLUE protocols it can convey over that channel, but the use of the CLUE protocol MUST NOT be negotiated on this channel.

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

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, addresses the need to disable it once CLUE-controlled media is fully negotiated.

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.

Successful CLUE Negotiation In the event of successful CLUE enablement of the call, devices MUST now begin negotiation of the CLUE channel; see for negotiation details. If negotiation is successful, the sending of CLUE protocol messages can begin. A CLUE-capable device MAY choose not to send RTP on the non-CLUE-controlled channels during the period in which control of the CLUE-controlled media lines is being negotiated (though RTCP MUST still be sent and received as normal). However, a CLUE-capable device MUST still be prepared to receive media on non-CLUE-controlled media lines that have been successfully negotiated as defined in . 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.

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

Modifying the Session

Adding and Removing CLUE-Controlled Media Subsequent offer/answer exchanges MAY add additional "m=" lines for CLUE-controlled media or activate or deactivate existing "m=" lines per the standard SDP mechanisms. In most cases, at least one additional exchange after the initial offer/answer exchange will be required before both sides have added all the Encodings and the ability to receive Encodings that they desire. Devices MAY delay adding "a=recvonly" CLUE-controlled "m=" lines until after CLUE protocol negotiation completes -- see for recommendations. Once CLUE media has been successfully negotiated, devices SHOULD ensure that non-CLUE-controlled media is deactivated by setting their ports to 0 in cases where it corresponds to the media type of CLUE-controlled media that has been successfully negotiated. This deactivation may require an additional SDP exchange or may be incorporated into one that is part of the CLUE negotiation.

Enabling CLUE Mid Call A CLUE-capable device that receives an initial SDP offer from a non-CLUE device SHOULD include a new data channel "m=" line and corresponding CLUE group in any subsequent offers it sends, to indicate that it is CLUE capable. If, in an ongoing non-CLUE call, an SDP offer/answer exchange completes with both sides having included a data channel "m=" line in their SDP and with the "mid" for that channel in a corresponding CLUE group, then the call is now CLUE enabled; negotiation of the data channel and subsequently the CLUE protocol begins.

Disabling CLUE Mid Call If, during an ongoing CLUE-enabled call, a device wishes to disable CLUE, it can do so by following the procedures for closing a data channel as defined in : sending a new SDP offer/answer exchange and subsequent SCTP Stream Sequence Number (SSN) reset for the CLUE channel. It MUST also remove the CLUE group. Without the CLUE group, any "m=" lines that were previously CLUE controlled no longer are; implementations MAY disable them by setting their ports to 0 or MAY continue to use them -- in the latter case, how they are used is outside the scope of this document. If a device follows the procedure above, or an SDP offer-answer negotiation completes in a fashion in which either the "m=" CLUE data channel line was not successfully negotiated and/or one side did not include the data channel in the CLUE group, then CLUE for this call is disabled. In the event that this occurs, CLUE is no longer enabled. Any active "m=" lines still included in the CLUE group are no longer CLUE controlled, and the implementation MAY either disable them in a subsequent negotiation or continue to use them in some other fashion. If the data channel is still present but not included in the CLUE group semantic, CLUE protocol messages MUST no longer be sent.

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

The CLUE data channel terminates, either gracefully or ungracefully, without any corresponding SDP renegotiation.
A channel error of the CLUE protocol causes it to return to the IDLE state as defined in .

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

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.

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 enroute by middleboxes, the state machines in SDP and CLUE operate independently. The state of the CLUE channel does not restrict when an implementation may send a new SDP offer or answer; likewise, the implementation's ability to send a new CLUE 'advertisement' or 'configure' message is not restricted by the results of or the state of the most recent SDP negotiation (unless the SDP negotiation has removed the CLUE channel). The primary implication of this is that a device may receive an SDP Offer/Answer message with a CLUE Encoding for which it does not yet have Capture information or receive a CLUE 'configure' message specifying a Capture Encoding for which the far end has not negotiated a media stream in SDP. CLUE messages contain an <encID> (in encodingIDList) or <encodingID> (in captureEncodingType), which is used to identify a specific encoding or captureEncoding in SDP; see for specifics. The non-atomic nature of CLUE negotiation means that a sender may wish to send a new CLUE 'advertisement' message before the corresponding SDP message. As such, the sender of the CLUE message MAY include an <encID> that does not currently match a CLUE-controlled "m=" line label in SDP; a CLUE-capable implementation MUST NOT reject a CLUE protocol message solely because it contains <encID> elements that do not match a label in SDP. The current state of the CLUE Participant or Media Provider/Consumer state machines do not affect compliance with any of the normative language of . That is, they MUST NOT delay an ongoing SDP exchange as part of a SIP server or client transaction; an implementation MUST NOT delay an SDP exchange while waiting for CLUE negotiation to complete or for a 'configure' message to arrive. Similarly, a device in a CLUE-enabled call MUST NOT delay any mandatory state transitions in the CLUE Participant or Media Provider/Consumer state machines due to the presence or absence of an ongoing SDP exchange. A device with the CLUE Participant state machine in the ACTIVE state MAY choose to delay moving from ESTABLISHED to ADV (Media Provider state machine) or from ESTABLISHED to WAIT FOR CONF RESPONSE (Media Consumer state machine) based on the SDP state. See 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.

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

Recommendations for Operating with Non-atomic Operations CLUE-capable devices MUST be able to handle states in which CLUE messages make reference to EncodingIDs that do not match the most recently received SDP, irrespective of the order in which SDP and CLUE messages are received. While these mismatches will usually be transitory, a device MUST be able to cope with such mismatches remaining indefinitely. However, this document makes some recommendations on message ordering for these non-atomic transitions. CLUE-capable devices MUST ensure that any inconsistencies between SDP and CLUE signaling are temporary by sending updated SDP or CLUE messages as soon as the relevant state machines and other constraints permit. Generally, implementations that receive messages for which they have incomplete information will be most efficient if they wait until they have the corresponding information they lack before sending messages to make changes related to that information. For example, an answerer that receives a new SDP offer with three new "a=sendonly" CLUE "m=" lines for which it has received no CLUE 'advertisement' message providing the corresponding capture information would typically inclue corresponding "a=inactive" lines in its answer, and only make a new SDP offer with "a=recvonly" when and if a new 'advertisement' message arrives with Captures relevant to those Encodings. Because of the constraints of SDP offer/answer and because new SDP negotiations are generally more 'costly' than sending a new CLUE message, implementations needing to make changes to both channels SHOULD prioritize sending the updated CLUE message over sending the new SDP message. The aim is for the recipient to receive the CLUE changes before the SDP changes, allowing the recipient to send their SDP answers without incomplete information and reducing the number of new SDP offers required.

Interaction of the CLUE Protocol and RTP/RTCP CaptureID The CLUE Framework allows for Multiple Content Captures (MCCs): Captures that contain multiple source Captures, whether composited into a single stream or switched based on some metric. The Captures that contribute to these MCCs may or may not be defined in the 'advertisement' message. If they are defined and the MCC is providing them in a switched format, the recipient may wish to determine which originating source Capture is currently being provided, so that they can apply geometric corrections based on that Capture's geometry or take some other action based on the original Capture information. To do this, 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.

CaptureID Reception during MCC Redefinition Because the RTP/RTCP CaptureID is delivered via a different channel to the 'advertisement' message in which in the contents of the MCC are defined, there is an intrinsic race condition in cases where the contents of an MCC are redefined. When a Media Provider redefines an MCC that involves CaptureIDs, the reception of the relevant CaptureIDs by the recipient will either lead or lag reception and the processing of the new 'advertisement' message by the recipient. As such, a Media Consumer MUST NOT be disrupted by any of the following in any CLUE-controlled media stream it is receiving, whether that stream is for a static Capture or for an MCC (as any static Capture may be redefined to an MCC in a later 'advertisement' message):

Receiving RTP or RTCP containing a CaptureID when it's the most recently processed 'advertisement' message means that none are expected.
Receiving RTP or RTCP without CaptureIDs when it's the most recently processed 'advertisement' message means that media CaptureIDs are expected.
Receiving a CaptureID in RTP or RTCP for a Capture defined in the most recently processed 'advertisement' message, but which the same 'advertisement' message does not include in the MCC.
Receiving a CaptureID in RTP or RTCP for a Capture not defined in the most recently processed 'advertisement' message.

Multiplexing of CLUE-Controlled Media using BUNDLE

Overview A CLUE call may involve sending and/or receiving significant numbers of media streams. Conventionally, media streams are sent and received on unique ports. However, each separate port used for this purpose may impose costs that a device wishes to avoid, such as the need to open that port on firewalls and NATs, the need to collect Interactive Connectivity Establishment (ICE) candidates, etc. The BUNDLE extension can be used to negotiate the multiplexing of multiple media lines onto a single 5-tuple for sending and receiving media, allowing devices in calls to another BUNDLE-supporting device to potentially avoid some of the above costs. While CLUE-capable devices MAY support the BUNDLE extension for this purpose, supporting the extension is not mandatory for a device to be CLUE compliant. A CLUE-capable device that supports BUNDLE SHOULD also support rtcp-mux. However, a CLUE-capable device that supports rtcp-mux may or may not support BUNDLE.

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.

Generating the Initial Offer BUNDLE mandates that the initial SDP offer MUST use a unique address for each "m=" line with a non-zero port. Because CLUE implementations generally will not include CLUE-controlled media lines, with the exception of the data channel in the initial SDP offer, CLUE devices that support large numbers of streams can avoid ever having to open large numbers of ports if they successfully negotiate BUNDLE. An implementation that does include CLUE-controlled media lines in its initial SDP offer while also using BUNDLE must take care to avoid rendering its CLUE-controlled media lines unusable in the event the far end does not negotiate BUNDLE if it wishes to avoid the risk of additional SDP exchanges to resolve this issue. This is best achieved by not sending any CLUE-controlled media lines in an initial offer with the 'bundle-only' attribute unless it has been established via some other channel that the recipient supports and is able to use BUNDLE on.

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 the BUNDLE specification (). 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.

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 Acknowledgements (ACKs) are not always discussed. Note that BUNDLE is not in use. | | | | | | SIP 200 OK 1 | |<---------------------------------| | | | | | SIP ACK 1 | |--------------------------------->| | | | | | | |<########### MEDIA 1 ############>| | 1 video A->B, 1 video B->A | |<################################>| | | | | | | |<================================>| | CLUE DATA CHANNEL ESTABLISHED | |<================================>| | | | | | CLUE OPTIONS | |<*********************************| | | | | | CLUE OPTIONS RESPONSE | |*********************************>| | | | | | CLUE ADVERTISEMENT 1 | |*********************************>| | | | | | CLUE ADVERTISEMENT 2 | |<*********************************| | | | | | CLUE ACK 1 | |<*********************************| | | | | | CLUE ACK 2 | |*********************************>| | | | | | SIP INVITE 2 (+3 sendonly) | |--------------------------------->| | | | | | CLUE CONFIGURE 1 | |<*********************************| | | | | | SIP 200 OK 2 (+2 recvonly) | |<---------------------------------| | | | | | CLUE CONFIGURE RESPONSE 1 | |*********************************>| | | | | | SIP ACK 2 | |--------------------------------->| | | | | | | |<########### MEDIA 2 ############>| | 2 video A->B, 1 video B->A | |<################################>| | | | | | SIP INVITE 3 (+2 sendonly) | |<---------------------------------| | | | | | CLUE CONFIGURE 2 | |*********************************>| | | | | | SIP 200 OK 3 (+2 recvonly) | |--------------------------------->| | | | | | CLUE CONFIGURE RESPONSE 2 | |<*********************************| | | | | | SIP ACK 3 | |<---------------------------------| | | | | | | |<########### MEDIA 3 ############>| | 2 video A->B, 2 video B->A | |<################################>| | | | | | | v v ]]> In SIP INVITE 1, Alice sends Bob a SIP INVITE with the basic audio and video capabilities and data channel included in the SIP body as per . 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 (Media 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. Bob responds with a similar SDP in SIP 200 OK 1, which also has a "CLUE" group including the mid value of a data channel; due to their similarity, no SDP snippet is shown here. Bob wishes to receive initial media and thus includes corresponding non-CLUE-controlled audio and video lines. Bob also includes the "sip.clue" media feature tag in the 200 OK. Alice and Bob are each now able to send a single audio and video stream. This is illustrated as MEDIA 1. With the successful initial SDP Offer/Answer exchange complete, Alice and Bob are also free to negotiate the CLUE data channel. This is illustrated as CLUE DATA CHANNEL ESTABLISHED. Once the data channel is established, CLUE protocol negotiation begins. In this case, Bob was the DTLS client (sending "a=active" in his SDP answer) and hence is the CLUE Channel Initiator. He sends a CLUE OPTIONS message describing his version support. On receiving that message, Alice sends her corresponding CLUE OPTIONS RESPONSE. With the OPTIONS phase complete, Alice now sends her CLUE 'advertisement' message (CLUE ADVERTISEMENT 1). She advertises three static Captures representing her three cameras. She also includes switched Captures suitable for two- and one-screen systems. All of these Captures are in a single Capture Scene, with suitable Capture Scene Views that tell Bob he should subscribe to the three static Captures, the two switched Captures, or the one switched Capture. Alice has no simultaneity constraints, so all six Captures are included in one simultaneous set. Finally, Alice includes an Encoding Group with three Encoding IDs: "enc1", "enc2", and "enc3". These Encoding IDs aren't currently valid but will match the next SDP offer she sends. Bob received CLUE ADVERTISEMENT 1 but does not yet send a 'configure' message, because he has not yet received Alice's Encoding information, so as yet he does not know if she will have sufficient resources to send him the two streams he ideally wants at a quality he is happy with. Because Bob is not sending an immediate 'configure' message with the "ack" element set, he must send an explicit 'ack' message (CLUE ACK 1) to signal receipt of CLUE ADVERTISEMENT 1. Bob also sends his CLUE 'advertisement' message (CLUE ADVERTISEMENT 2) -- though the diagram shows that this occurs after Alice sends CLUE ADVERTISEMENT 1 Bob sends his 'advertisement' message independently and does not wait for CLUE ADVERTISEMENT 1 to arrive. He advertises two static Captures representing his cameras. He also includes a single composed Capture for single-screen systems, in which he will composite the two camera views into a single video stream. All three Captures are in a single Capture Scene, with suitable Capture Scene Views that tell Alice she should subscribe to either the two static Captures or the single composed Capture. Bob also has no simultaneity constraints, so he includes all three Captures in one simultaneous set. Bob also includes a single Encoding Group with two Encoding IDs: "foo" and "bar". Similarly, Alice receives CLUE ADVERTISEMENT 2 but does not yet send a 'configure' message, because she has not yet received Bob's Encoding information; instead, she sends instead an 'ack' message (CLUE ACK 2). Both sides have now sent their CLUE 'advertisement' messages and an SDP exchange is required to negotiate Encodings. For simplicity, in this case, Alice is shown sending an INVITE with a new offer; in many implementations, both sides might send an INVITE, which would be resolved by use of the 491 Request Pending resolution mechanism from . Alice now sends SIP INVITE 2. She maintains the sendrecv audio, video, and CLUE "m=" lines, and she adds three new sendonly "m=" lines to represent the three CLUE-controlled Encodings she can send. Each of these "m=" lines has a label corresponding to one of the Encoding IDs from CLUE ADVERTISEMENT 1. Each also has its mid added to the grouping attribute to show they are controlled by the CLUE data channel. A snippet of the SDP showing the grouping attribute, data channel, and video "m=" lines are shown below: Bob now has all the information he needs to decide which streams to configure, allowing him to send both a CLUE 'configure' message and his SDP answer. As such, he now sends CLUE CONFIGURE 1. This requests the pair of switched Captures that represent Alice's scene, and he configures them with encoder ids "enc1" and "enc2". Bob also sends his SDP answer as part of SIP 200 OK 2. Alongside his original audio, video, and CLUE "m=" lines, he includes three additional "m=" lines corresponding to the three added by Alice; two active recvonly "m= "lines and an inactive "m=" line for the third. He adds their mid values to the grouping attribute to show they are controlled by the CLUE data channel. A snippet of the SDP showing the grouping attribute and the video "m=" lines are shown below (mid 100 represents the CLUE data channel, not shown): Alice receives Bob's CLUE CONFIGURE 1 message and sends CLUE CONFIGURE RESPONSE 1 to acknowledge its reception. She does not yet send the Capture Encodings specified, because at this stage, she hasn't processed Bob's answer SDP and thus hasn't negotiated the ability for Bob to receive these streams. On receiving SIP 200 OK 2 from Bob, Alice sends her SIP ACK (SIP ACK 2). She is now able to send the two streams of video Bob requested -- this is illustrated as MEDIA 2. The constraints of offer/answer meant that Bob could not include his encoding information as new "m=" lines in SIP 200 OK 2. As such, Bob now sends SIP INVITE 3 to generate a new offer. Along with all the streams from SIP 200 OK 2, Bob also includes two new sendonly streams. Each stream has a label corresponding to the Encoding IDs in his CLUE ADVERTISEMENT 2 message. He also adds their mid values to the grouping attribute to show they are controlled by the CLUE data channel. A snippet of the SDP showing the grouping attribute and the video "m=" lines are shown below (mid 100 represents the CLUE data channel, not shown): Having received this, Alice now has all the information she needs to send her CLUE 'configure' message and her SDP answer. In CLUE CONFIGURE 2, she requests the two static Captures from Bob to be sent on Encodings "foo" and "bar". Alice also sends SIP 200 OK 3, matching two recvonly "m=" lines to Bob's new sendonly lines. She includes their mid values in the grouping attribute to show they are controlled by the CLUE data channel. Alice 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): Bob receives Alice's CLUE CONFIGURE 2 message and sends CLUE CONFIGURE RESPONSE 2 to acknowledge its reception. Bob does not yet send the Capture Encodings specified, because he hasn't yet received and processed Alice's SDP answer and negotiated the ability to send these streams. Finally, on receiving SIP 200 OK 3, Bob is now able to send the two streams of video Alice requested -- this is illustrated as MEDIA 3. Both sides of the call are now sending multiple video streams with their sources defined via CLUE negotiation. As the call progresses, either side can send a new 'advertisement' or 'configure' message or the new SDP offers/answers to add, remove, or change what they have available or want to receive.

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). | | | | | | 200 0K 1 | |<---------------------------------| | | | | | SIP ACK 1 | |--------------------------------->| | | | | | | |<########### MEDIA 1 ############>| | 1 video A->B, 1 video B->A | |<################################>| | | | | | | | | v v ]]> In SIP INVITE 1, Alice sends Bob a SIP INVITE including the basic audio and video capabilities and data channel in the SDP body as per . Alice also includes the "sip.clue" media feature tag in the INVITE. A snippet of the SDP showing the grouping attribute and the video "m=" line are shown below. Alice has included a "CLUE" group and the mid corresponding to a data channel in the group (3). Note that Alice has chosen not to include any CLUE-controlled media in the initial offer -- the mid value of the video line is not included in the "CLUE" group. Bob is not CLUE capable and hence does not recognize the "CLUE" semantic for the grouping attribute, nor does he support the data channel. IN SIP 200 OK 1, he responds with an answer that includes audio and video, but with the data channel zeroed. From the lack of a CLUE group, Alice understands that Bob does not support CLUE, or does not wish to use it. Both sides are now able to send a single audio and video stream to each other. At this point, Alice begins to send her fallback video: in this case, it's likely a switched view from whichever camera shows the current loudest participant on her side.

Acknowledgements Besides the authors, the team focusing on this document consists of: , , and . , , and have contributed detailed comments and suggestions.

IANA Considerations

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 :

Semantics	Token	Reference
CLUE-controlled m-line	CLUE	RFC XXXX

New SIP Media Feature Tag This specification registers a new media feature tag in the SIP tree per the procedures defined in and .

Media feature tag name:: sip.clue
ASN.1 Identifier:: [to be assigned]
Summary of the media feature indicated by this tag:: This feature tag indicates that the device supports CLUE-controlled media.
Values appropriate for use with this feature tag:: Boolean.
The feature tag is intended primarily for use in the following applications, protocols, services, or negotiation mechanisms:: This feature tag is most useful in a communications application for describing the capabilities of a device to use the CLUE control protocol to negotiate the use of multiple media streams.
Related standards or documents:: RFC XXXX
Security Considerations:: Security considerations for this media feature tag are discussed in of RFC XXXX.
Name(s) & email address(es) of person(s) to contact for further information:: Internet Engineering Steering Group, iesg@ietf.org
Intended usage:: COMMON

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 addresses the need to secure these mechanisms by following the recommendations of the individual protocols. Beyond the need to secure the constituent protocols, the use of CLUE does impose additional security concerns. One area of increased risk involves the potential for a malicious party to subvert a CLUE-capable device to attack a third party by driving large volumes of media (particularly video) traffic at them by establishing a connection to the CLUE-capable device and directing the media to the victim. While this is a risk for all media devices, a CLUE-capable device may allow the attacker to configure multiple media streams to be sent, significantly increasing the volume of traffic directed at the victim. This attack can be prevented by ensuring that the media recipient intends to receive the media packets. As such, all CLUE-capable devices MUST support key negotiation and receiver intent assurance via DTLS / Secure Real-time Transport Protocol (SRTP) on CLUE-controlled RTP "m=" lines, and they MUST use it or some other mechanism that provides receiver intent assurance. All CLUE-controlled RTP "m" lines must be secured and implemented using mechanisms such as SRTP. CLUE implementations MAY choose not to require the use of SRTP to secure legacy (non-CLUE-controlled) media for backwards compatibility with older SIP clients that are incapable of supporting it. CLUE also defines a new media feature tag that indicates CLUE support. This tag may be present even in non-CLUE calls, which increases the metadata available about the sending device and can help an attacker differentiate between multiple devices; it can also help them identify otherwise anonymised users via the fingerprint of features their device supports. To prevent this, SIP signaling used to set up CLUE sessions SHOULD always be encrypted using TLS. The CLUE protocol also carries additional information that could be used to help fingerprint a particular user or to identify the specific version of software being used. CLUE Framework provides details of these issues and how to mitigate them.

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

-15:

Revision by Rob Hanton

Clarified that using an 'EncID' defined in SDP in an CLUE ADVERTISEMENT message is only a SHOULD because of the inherent race conditions about the ordering of the SDP and CLUE message. In contrast, changed the use of 'EncID' in a CLUE CONFIGURE message to a MUST as that is defined by the far end and so there is no way for the sending of the CONFIGURE to anticipate it.
Updated the description of handling the failure of the CLUE channel to reflect the fact that the protocol state machine now returns to the IDLE state on failure rather than a specific termination state, which also means defining an allowance for the CLUE channel being recovered.
Updated all instances of advertisment, configure and ack messages throughout to match the styling of the protocol document
Security section updated to make DTLs-SRTP mandatory to use as well as support unless intent assurance is provided by some other mechanism per mailing list proposal (to resolve the concern from a previous IETF session of those wanting to use CLUE in a closed environment where intent assurance was provided by other prorietary mechanisms).
Removed OID value for "sip.clue" media feature tag pending its actual assignment on registration, leaving a placeholder
All lower-case uses of 'must', 'should' and 'may' reviewed and a few made normative
Fixed various spelling mistakes, clarified grammar, and fixed a copy/paste error.
Updated boilerplate to RFC 8174
Some informative references moved to normative.

-14:

Revision by Rob Hanton

Reference to RFC5245 updated to RFC8445
Updated my name to reflect surname change (Hansen to Hanton).
Reviewed recent changes to clue protocol document and concluded that none affected this document
Added recommendation that the SDP O/A spec and clue protocol be read prior to this document
Several acronyms expanded at the point of initial use
Some unnecessary normative language replaced with prose

-13:

Revision by Rob Hansen

Added a section on handling failures of the protocol channel or data channel mid-call - instructions are that media must continue as if the clue channel were still established and unchanged until CLUE is disabled by either side via SDP exchange.
Example in section on efficient operation with non-atomic transactions has had all normative language removed and is now entirely descriptive (normative language retained in the non-example portion).
draft-ietf-clue-protocol-14 reviewed for relevant changes, and use of CLUE ACK and RESPONSE messages made consistent with that document (ADVERTISEMENT ACKNOWLEDGEMENT and CONFIGURE RESPONSE respectively).
Order of authors revised to reflect updates since Jan 2014.

-12:

Revision by Rob Hansen

Title change to expand and elucidate our totally-not-contrived acronym
Explicit reference to RFC3840 added when first mentioning media feature tags
Have standardised references to Clue protocol messages to ADVERTISEMENT, CONFIGURE and ACK, in line with section 12.4.1. of the protocol document (though the protocol document also uses ADV and CONF).
'MUST' in opening paragraph of 4.2 changed from normative 'MUST' to logical 'must'
Per his request, removed Cristian's company affiliation and changed his email address
Clarified that an implementation that chooses not to send media during the initial negotiation process must still send RTCP as normal
Rewrote the section on adding/remove clue m-lines after the initial exchange to make clear that this is just standard SDP. For non-clue controlled lines, recommended they are deactivated by zeroing the port when turning them off after clue is successfully negotiated.
Added guidance that an initial offer containing clue-controlled m-lines MUST NOT set them bundle-only unless they somehow know the far end actually supports BUNDLE
Added section saying that CLUE devices that do BUNDLE SHOULD do rtcp-mux, but that the requirement doesn't exist in the other direction (eg, supporting rtcp-mux does not require or imply the need to implement BUNDLE)
For clue-controlled m-lines where the sender included more encodings than the recipient wants, have standardised on using "a=inactive" to not receive RTP on them (previously had a mix of "a=inactive" or port 0, or in some cases did not specify).
Page break added before the big ladder diagram in the example
Have added a direction attribute to the SDP example in the data channel, and made explicit that Bob is the DTLS client and hence the CLUE Channel Initiator.
Have removed all language that referenced the possibility of having multiple CLUE groups
Removed names appearing in the authors list from the acknowledgements
Changed the contact for the IANA registration to iesg@ietf.org
Security section updated to clarify that DTLS-SRTP must be supported (as opposed to DTLS) and removed the reference to RFC7202.
Other syntactic tweaks based on Paul and Adam's feedback

-11:

Revision by Rob Hansen

Some informative references added for SIP and SDP.
'a=mid' lines added to example m-lines with port 0, per RFC5888 section 6.
Instace of 'must' changed to normative 'MUST', along with various minor clarifications and corrections.
Abstract made standalone without citations, per RFC7322 section 4.3.
RFC editor note added to remove this section.

-10:

Revision by Rob Hansen

Changes to draft-ietf-clue-protocol between 07 and 11 reviewed to ensure compatibility between documents has been maintained.
Expanded the portion of the document related to fingerprinting with info on the CLUE channel as well as SIP.

-09:

Revision by Rob Hansen

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

-08:

Revision by Rob Hansen

Spelling and grammar fixes from Paul and Christian gratefully adopted
Expanded the section on disabling CLUE mid-call to make explicit the actions required to disable the CLUE channel gracefully, or to handle someone else doing the same.
Made a number of fixes to the example call flow to better reflect the recommendations in the document.

-07:

Revision by Rob Hansen

Removed the entire 'Media line directionality' section as a discussion of the pros/cons of using bidirectional vs unidirectional schemes wasn't suitable for a finalised version. The unidirectionality requirement is covered normatively in an earlier section.
BUNDLE no longer includes an address synchronisation step so the suggestion to wait until that done has been replaced with some general language about following any negotiated extensions.
Added OPTIONS negotiation to the example flow, and revised the flow to ensure it matched protocol document.
Section on not sending CLUE control media until CLUE negotiation completes narrowed to notify that only RTP should not be sent until negotiation completes and add RTCP to the list of things that should be sent as normal, in line with a=inactive.
Make explicit that m=recvonly lines don't need to have a label, as only m=sendonly lines are referenced by CLUE protocol messages.
Fix formatting of IANA sections. Improve syntax of feature tag section in line with Paul's suggestions. Definition of feature tag narrowed to be multiple media lines *negotiated via CLUE protocol* rather than more generic 'multiple media lines'.
General corrections to grammar, spelling and readability based on Christian, Paul and Mark; in many cases suggested text was gratefully accepted.

-06:

Revision by Rob Hansen

State machine interactions updated to match versions in -04 of protocol doc.
Section on encoding updated to specify both encID and encodingID from data model doc.
Removed the limitations on describing H264 encoding limits using SDP syntax as an open issue.
Previous draft had SRTP and DTLS mandatory to implement and to use on CLUE- controlled m lines. Current version has DTLS mandatory to implement, and 'security' mandatory to use but does not define what that security is.
Terminology reference to framework doc reinforced. All terminology that duplicates framework removed. All text updated with capitalisation that matches framework document's terminology.
SDP example syntax updated to match that of ietf-clue-datachannel and hence ietf-mmusic-data-channel-sdpneg.

-05:

Revision by Rob Hansen

SRTP/DTLS made mandatory for CLUE-controlled media lines.
IANA consideration section added (text as proposed by Christian Groves).
Includes provision for dependent streams on seperate "m" lines having the same encID as their parent "m" line.
References to putting CLUE-controlled media and data channels in more than one CLUE group removed, since the document no longer supports using more than one CLUE group.
Section on CLUE controlled media restrictions still applying even if the call does not end up being CLUE enabled being rewritten to hopefully be clearer.
Other minor syntax improvements.

-04:

Revision by Rob Hansen

Updated DTLS/SCTP channel syntax in examples to fix errors and match latest format defined in draft-ietf-mmusic-sctp-sdp-07.
Clarified the behaviour if an SDP offer includes a CLUE-controlled "m" line and the answer accepts that "m" line but without CLUE control of that line.
Added a new section on the sending and receiving of CaptureIDs in RTP and RTCP. Includes a section on the necessity of the receiver coping with unexpected CaptureIDs (or the lack thereof) due to MCCs being redefined in new Advertisement messages.
Added reminder on IANA section on registering grouping semantic and media feature tag, removed the less formal sections that did the same job.
Fixed and clarified issues raised by Christian's document review.
Added a number of security considerations.

-03:

Revision by Rob Hansen

Clarified text on not rejecting messages because they contain unknown encIDs.
Removed normative language in section on accepting/rejecting non-CLUE-controlled media in the initial answer.
Example SDP updated to include the data channel "m" lines.
Example call flow updated to show disablement of non-CLUE-controlled media once CLUE-controlled media is flowing.

-02:

Revision by Rob Hansen

Added section on not accepting non-CLUE-controlled "m" lines in the initial answer when CLUE is to be negotiated.
Removed previous language attempting to describe media restrictions for CLUE-controlled "m" lines that had not been configured, and replaced it with much more accurate 'treat as "a=inactive" was set'.
Made label element mandatory for CLUE-controlled media (was previously "SHOULD include", but there didn't seem a good reason for this - anyone wishing to include the "m" line but not immediately use it in CLUE can simply leave it out of the <encodingIDList>.)
Added a section on the specifics of relating encodings in SDP to <encID> elements in the CLUE protocol, including the fact that both Advertisement and Configure messages reference the *encoding* (eg, in the Configure case the sender of the Configure message includes the labels of the recipient's "m" lines as their <encID> contents).
Minor revisions to the section on complying with normative SDP/CLUEstate machine language to clarify that these were not new normative language, merely that existing normative language still applies.
Removed appendices which previously contained information to be transferred to the protocol and data channel drafts. Removed other text that discussed alternatives to the current approach.
Cleaned up some 'todo' text.

-01:

Revision by Rob Hansen

Revised terminology - removed the term 'CLUE-enabled' device as insufficiently distinct from 'CLUE-capable' and instead added a term for 'CLUE-enabled' calls.
Removed text forbidding RTCP and instead added text that ICE/DTLS negotiation for CLUE controlled media must be done as normal irrespective of CLUE negotiation.
Changed 'sip.telepresence' to 'sip.clue' and 'TELEPRESENCE' grouping semantic back to CLUE.
Made it mandatory to have exactly one mid corresponding to a data channel in a CLUE group
Forbade having multiple CLUE groups unless a specification for doing so is published.
Refactored SDP-related text; previously the encoding information had been in the "initial offer" section despite the fact that we recommend that the initial offer doesn't actually include any encodings. I moved the specifications of encodings and how they're received to an earlier, seperate section.
Added text on how the state machines in CLUE and SDP are allowed to affect one another, and further recommendations on how a device should handle the sending of CLUE and SDP changes.

-00:

Revision by Rob Hansen

Submitted as -00 working group document

draft-kyzivat-08:

Revisions by Rob Hansen

Added media feature tag for CLUE support ('sip.telepresence')
Changed grouping semantic from 'CLUE' to 'TELEPRESENCE'
Restructured document to be more centred on the grouping semantic and its use with O/A
Lots of additional text on usage of the grouping semantic
Stricter definition of CLUE-controlled m lines and how they work
Some additional text on defining what happens when CLUE supports is added or removed
Added details on when to not send RTCP for CLUE-controlled "m" lines.
Added a section on using BUNDLE with CLUE
Updated data channel references to point at new WG document rather than indivual draft

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 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 and , but that has not yet been resolved in those documents, into an appendix.

draft-kyzivat-06:

Revisions by Rob Hansen

Removed CLUE message XML schema and details that are now in draft-presta-clue-protocol
Encoding limits in SDP section updated to note that this has been investigated and discussed and is the current working assumption of the WG, though consensus has not been fully achieved.
A section has also been added on the current mandation of unidirectional "m" lines.
Updated CLUE messaging in example call flow to match draft-presta-clue-protocol-03

draft-kyzivat-05:

Revisions by pkyzivat:

Specified versioning model and mechanism.
Added explicit response to all messages.
Rearranged text to work with the above changes. (Which rendered diff almost useless.)

draft-kyzivat-04:

Revisions by Rob Hansen: ???

draft-kyzivat-03:

Revisions by pkyzivat:

Added a syntax section with an XML schema for CLUE messages. This is a strawhorse, and is very incomplete, but it establishes a template for doing this based on elements defined in the data model. (Thanks to Roberta for help with this!)
Did some rewording to fit the syntax section in and reference it.
Did some relatively minor restructuring of the document to make it flow better in a logical way.

draft-kyzivat-02:

A bunch of revisions by pkyzivat:

Moved roberta's call flows to a more appropriate place in the document.
New section on versioning.
New section on NAK.
A couple of possible alternatives for message acknowledgment.
Some discussion of when/how to signal changes in provider state.
Some discussion about the handling of transport errors.
Added a change history section.

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

draft-kyzivat-01:

Updated by roberta to include some sample call flows.

draft-kyzivat-00:

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