| Network Working Group | P. Kyzivat |
| Internet-Draft | L. Xiao |
| Intended status: Standards Track | C. Groves |
| Expires: October 12, 2014 | Huawei |
| R. Hansen | |
| Cisco Systems | |
| April 10, 2014 |
CLUE Signaling
draft-kyzivat-clue-signaling-08
This document specifies how CLUE-specific signaling such as the CLUE protocol [I-D.presta-clue-protocol] and the CLUE data channel [I-D.ietf-clue-datachannel] are used with each other and with existing signaling mechanisms such as SIP and SDP to produce a telepresence call.
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To enable devices to participate in a telepresence call, selecting the sources they wish to view, receiving those media sources and displaying them in an optimal fashion, CLUE involves two principal and inter-related protocol negotiations. SDP, conveyed via SIP, is used to negotiate the specific media capabilities that can be delivered to specific addresses on a device. Meanwhile, a CLUE protocol [I-D.presta-clue-protocol], transported via a CLUE data channel [I-D.ietf-clue-datachannel], is used to negotiate the capture sources available, their attributes and any constraints in their use, along which which captures the far end provides a device wishes to receive.
Beyond negotiating the CLUE channel, SDP is also used to negotiate the details of supported media streams and the maximum capability of each of those streams. As the CLUE Framework [I-D.ietf-clue-framework] defines a manner in which the media provider expresses their maximum encoding capabilities, SDP is also used to express the encoding limits for each potential encoding.
Backwards-compatibility is an important consideration of the document: it is vital that a CLUE-capable device contacting a device that does not support CLUE is able to fall back to a fully functional non-CLUE call. The document also defines how a non-CLUE call may be upgraded to CLUE in mid-call, and similarly how CLUE functionality can be removed mid-call to return to a standard non-CLUE call.
This document originally also defined the CLUE protocol itself. These details have mostly been split out into [I-D.presta-clue-protocol] and expanded, but at present some details remain in this document.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
This document draws liberally from the terminology defined in the CLUE Framework [I-D.ietf-clue-framework].
Other terms introduced here:
The "sip.telepresence" media feature tag indicates support for CLUE. A CLUE-capable device SHOULD include this media feature tag in its REGISTER requests and OPTION responses. It SHOULD also include the media feature tag in INVITE and UPDATE [RFC3311] requests and responses.
Presence of the media feature tag in the contact field of a request or response can be used to determine that the far end supports CLUE.
This section defines a new SDP Grouping Framework extension, TELEPRESENCE.
The TELEPRESENCE extension can be indicated using an SDP session-level 'group' attribute. Each SDP media "m" line that is included in this group, using SDP media-level mid attributes, is CLUE-controlled, by a CLUE data channel also included in this TELEPRESENCE group.
The CLUE data channel [I-D.ietf-clue-datachannel] is a bidirectional SCTP over DTLS channel used for the transport of CLUE messages. This channel must be established before CLUE protocol messages can be exchanged and CLUE-controlled media can be sent.
The data channel is a generic transport that is not specific to CLUE - if a device wishes to use the CLUE protocol on the data channel it MUST include a TELEPRESENCE group in the SDP and include the "mid" of the "m" line for the data channel in that group. A TELEPRESENCE grup MUST NOT include the "mid"s for more than one data channel, and the data channel "mid" MUST NOT be included in more than one TELEPRESENCE group.
Presence of the data channel in a CLUE group in an SDP offer or answer also serves, along with the 'sip.telepresence' media feature tag, as an indication that the device supports CLUE and wishes to upgrade the call to include CLUE-controlled media. A CLUE-enabled device SHOULD include a data channel "m" line in offers and, when allowed by [RFC3264], answers.
CLUE-controlled media lines in an SDP are "m" lines in which the content of the media streams to be sent is negotiated via the CLUE protocol [I-D.presta-clue-protocol]. For an "m" line to be CLUE-controlled, its "mid" value MUST be included in a TELEPRESENCE group. CLUE-controlled media line "mid"s MUST NOT be included in more than one TELEPRESENCE group.
CLUE-controlled media is controlled by the CLUE protocol as negotiated on the CLUE data channel with an "mid" included in the TELEPRESENCE group. If no data channel is included in the group the other "m" lines in the group are still considered CLUE-controlled and under all the restrictions of CLUE-controlled media specified in this document.
"m" lines not specified as under CLUE control follow normal rules for media streams negotiated in SDP as defined in documents such as [RFC3264].
The CLUE Framework [I-D.ietf-clue-framework] defines the concept of "encodings", which represent the sender's encode ability. Each encoding the media provider wishes to signal is signalled via an "m" line of the appropriate media type, which MUST be marked as sendonly with the "a=sendonly" attribute or as inactive with the "a=inactive" attribute.
The encoder limits of active (eg, "a=sendonly") encodings can then be expressed using existing SDP syntax. For instance, for H.264 see Table 6 in [RFC6184] for a list of valid parameters for representing encoder sender stream limits.
These encodings are CLUE-controlled and hence MUST include an "mid" in a TELEPRESENCE group as defined above.
As well as the normal restrictions defined in [RFC3264] media MUST NOT be sent on this stream until the media provider has received a valid CLUE CONFIGURE message specifying the capture to be used for this stream. In the case of RTP media this includes corresponding RTCP packets.
Every "m" line representing a CLUE encoding SHOULD contain a "label" attribute as defined in [RFC4574]. This label is used to identify the encoding by the sender in CLUE ADVERTISEMENT messages and by the receiver in CLUE CONFIGURE messages.
Presently, this specification mandates that CLUE-controlled "m"-lines must be unidirectional. This is because setting "m"-lines to "a=sendonly" allows the encoder limits to be expressed, whereas in other cases codec attributes express the receive capabilities of a media line.
It is possible that in future versions of this draft or its successor this restriction will be relaxed. If a device does not feel there is a benefit to expressing encode limitations, or if there are no meaningful codec-specific limitations to express (such as with many audio codecs) there are benefits to allowing bidirectional "m"-lines. With bidirectional media lines recipients do not always need to create a new offer to add their own "m"-lines to express their send capabilities; if they can produce an equal or lesser number of streams to send then they may not need additional "m"-lines.
However, at present the need to express encode limitations and the wish to simplify the offer/answer procedure means that for the time being only unidirectional media lines are allowed for CLUE-controlled media. The highly asymmetric nature of CLUE means that the probability of the recipient of the initial offer needing to make their own offer to add additional "m"-lines is significantly higher than it is for most other SIP call scenarios, in which there is a tendancy for both sides to have similar numbers of potential audio and video streams they can send.
Note that while the expressing of CLUE encoding limits in SDP has been discussed at some length by the working group and it has been agreed that this is the current, working assumption, formal consensus has not been agreed on this. Alternatives include placing encoding limits in the CLUE ADVERTISEMENT message, or by using alternate SDP syntax, such as is suggested in [I-D.groves-clue-latent-config].
As well as including sendonly media lines to send CLUE-controlled media, the sender of the initial SDP offer MAY also include "a=recvonly" media lines to preallocate "m" lines to receive media; these are described in more detail in the next section.
A CLUE-enabled device sending an initial SDP offer SHOULD NOT include any "m" line for CLUE-controlled media beyond the "m" line for the CLUE data channel, and SHOULD include at least one non-CLUE-controlled media "m" line.
If the device has evidence that the receiver is also CLUE-enabled, for instance due to receiving an initial INVITE with no SDP but including a 'sip.telepresence' media feature tag, the above recommendation is waived, and the initial offer MAY contain "m" lines for CLUE-controlled media.
If the recipient wishes to enable CLUE for the call, they MUST negotiate data channel support for an "m" line, and include the "mid" of that "m" line in a corresponding TELEPRESENCE group.
A receiver who wishes to receive a CLUE stream via a specific encoding requires an "a=recvonly" "m" line that matches the "a=sendonly" encoding.
These "m" lines are CLUE-controlled and hence MUST include an "mid" the corresponding TELEPRESENCE group corresponding to the encoding they wish to send.
In the case of RTCP for RTP media or any other media type that includes a bidirectional flow of packets for unidirectional media streams, such bidirectional packets MUST NOT be sent until the media consumer has received acknowledgement that the media provider has received a valid CLUE CONFIGURE message specifying the capture to be used for this stream.
In the event that both offer and answer include a data channel "m" line with a mid value included in corresponding TELEPRESENCE groups CLUE has been successfully negotiated and the call is now CLUE-enabled, otherwise the call is not CLUE enabled.
In the event of successful CLUE enablement of the call, devices MUST now begin negotiation of the CLUE channel, see [I-D.ietf-clue-datachannel] for negotiation details. If negotiation is successful, sending of CLUE protocol [I-D.presta-clue-protocol] messages can begin.
A CLUE-enabled device MAY choose not to send media on the non-CLUE-controlled channels during the period in which control of the CLUE-controlled media lines is being negotiated. However, a CLUE-enabled device MUST still be prepared to receive media on non-CLUE-controlled media lines as defined in [RFC3264].
If either side of the call wishes to add additional CLUE-controlled "m" lines to send or receive CLUE-controlled media they MAY now send a SIP request with a new SDP offer. Note that if BUNDLE has been successfully negotiated and a Bundle Address Synchronization offer is required, the device to receive that offer SHOULD NOT generate a new SDP offer until it has received that BAS offer.
In the event that the negotiation of CLUE fails and the call is not CLUE enabled in the initial offer/answer then CLUE is not in use in the call, and the CLUE-capable devices MUST either revert to non-CLUE behaviour or terminate the call.
A CLUE-enabled device that receives an initial SDP offer from a non-CLUE-enabled device SHOULD include a new data channel "m" line and corresponding TELEPRESENCE group in any subsequent offers it sends, to indicate that it is CLUE-enabled.
If, in an ongoing non-CLUE call, one or both sides of the call add the CLUE data channel "m" line to their SDP and places the "mid" for that channel in corresponding TELEPRESENCE groups then the call is now CLUE-enabled; negotiation of the data channel and subsequently the CLUE protocol begin.
If, in an ongoing CLUE-enabled call, an SDP offer-answer negotiation completes in a fashion in which either the CLUE data channel was not successfully negotiated or one side did not include the data channel in a matching TELEPRESENCE group then CLUE for this channel is disabled. In the event that this occurs, CLUE is no longer enabled and sending of all CLUE-controlled media associated with the corresponding TELEPRESENCE group MUST stop.
Note that this is distinct to cases where the CLUE data channel fails or an error occurs on the CLUE protocol; see [I-D.presta-clue-protocol] for details of media and state preservation in this circumstance.
Information about media streams in CLUE is split between two message types: SDP, which defines media addresses and limits, and the CLUE channel, which defines properties of capture devices available, scene information and additional constraints. As a result certain operations, such as advertising support for a new transmissible capture with associated stream, cannot be performed atomically, as they require changes to both SDP and CLUE messaging.
This section defines how the negotiation of the two protocols interact, provides some recommendations on dealing with intermediary stages in non-atomic operations, and mandates additional constraints on when CLUE-configured media can be sent.
To avoid complicated state machines with the potential to reach invalid states if messages were to be lost, or be rewritten en-route by middle boxes, the current proposal is that SDP and CLUE messages are independent. The state of the CLUE channel does not restrict when an implementation may send a new SDP offer or answer, and likewise the implementation's ability to send a new CLUE ADVERTISEMENT or CONFIGURE message is not restricted by the results of or the state of the most recent SDP negotiation.
The primary implication of this is that a device may receive an SDP with a CLUE encoding it does not yet have capture information for, or receive a CLUE CONFIGURE message specifying a capture encoding for which the far end has not negotiated a media stream in SDP.
CLUE messages contain an EncodingID which is used to identify a specific encoding in SDP. The non-atomic nature of CLUE negotiation means that a sender may wish to send a new ADVERTISEMENT before the corresponding SDP message. As such the sender of the CLUE message MAY include an EncodingID which does not currently match an extant id in SDP.
Generally, implementations that receive messages for which they have incomplete information SHOULD wait until they have the corresponding information they lack before sending messages to make changes related to that information. For instance, an implementation that receives a new SDP offer with three new "a=sendonly" CLUE "m" lines that has not received the corresponding CLUE ADVERTISEMENT providing the capture information for those streams SHOULD NOT include corresponding "a=recvonly" lines in its answer, but instead should make a new SDP offer when and if a new ADVERTISEMENT arrives with captures relevant to those encodings.
Because of the constraints of offer/answer and because new SDP negotiations are generally more 'costly' than sending a new CLUE message, implementations needing to make changes to both channels SHOULD prioritize sending the updated CLUE message over sending the new SDP message. The aim is for the recipient to receive the CLUE changes before the SDP changes, allowing the recipient to send their SDP answers without incomplete information, reducing the number of new SDP offers required.
While SDP and CLUE message states do not impose constraints on each other, both impose constraints on the sending of media - media MUST NOT be sent unless it has been negotiated in both CLUE and SDP: an implementation MUST NOT send a specific CLUE capture encoding unless its most recent SDP exchange contains an active media channel for that encoding AND the far end has sent a CLUE CONFIGURE message specifying a valid capture for that encoding.
A CLUE call may involve sending and/or receiving significant numbers of media streams. Conventionally, media streams are sent and received on unique ports. However, each seperate port used for this purpose may impose costs that a device wishes to avoid, such as the need to open that port on firewalls and NATs, the need to collect ICE candidates [RFC5245], etc.
The BUNDLE [I-D.ietf-mmusic-sdp-bundle-negotiation] extension can be used to negotiate the multiplexing of multiple media lines onto a single 5-tuple for sending and receiving media, allowing devices in calls to another BUNDLE-supporting device to potentially avoid some of the above costs.
While CLUE-capable devices MAY support the BUNDLE extension for this purpose supporting the extension is not mandatory for a device to be CLUE-compliant.
This specification imposes no additional requirements or restrictions on the usage of BUNDLE when used with CLUE. There is no restriction on combining CLUE-controlled media lines and non-CLUE-controlled media lines in the same BUNDLE group or in multiple such groups. However, there are several steps an implementation may wish to ameliorate the cost and time requirements of extra SDP offer/answer exchanges between CLUE and BUNDLE.
BUNDLE mandates that the initial SDP offer MUST use a unique address for each m-line with a non-zero port. Because CLUE implementations generarlly will not include CLUE-controlled media lines with the exception of the data channel CLUE devices that support large numbers of streams can avoid ever having to open large numbers of ports if they successfully negotiate BUNDLE.
When using BUNDLE the initial offerer may be mandated to send a Bundle Address Synchronisation offer. If the initial offerer also followed the recommendation of not including CLUE-controlled media lines in their offer, they MAY choose to include them in this subsequent offer. In this circumstance the BUNDLE specification recommends that the offerer does not "modify SDP parameters that could get the answerer to reject the BAS offer". Including new CLUE-controlled media lines using codecs and other attributes used in existing media lines should not increase the chance of the answerer rejecting the BAS offer; implementations should consider carefully before including new codecs or other new SDP attributes in these CLUE-controlled media lines.
BUNDLE-supporting CLUE-enabled devices MAY include the data channel in the same BUNDLE group as RTP media. In this case the device MUST be able to demultiplex the various transports - see section 7.2 of the BUNDLE draft [I-D.ietf-mmusic-sdp-bundle-negotiation]. If the BUNDLE group includes other protocols than the data channel transported via DTLS the device MUST also be able to differentiate the various protocols.
This example illustrates a call between two CLUE-capable endpoints. Alice, initiating the call, is a system with three cameras and three screens. Bob, receiving the call, is a system with two cameras and two screens. A call-flow diagram is presented, followed by an summary of each message.
To manage the size of this section only video is considered, and SDP snippets only illustrate video 'm' lines. ACKs are not discussed. Note that BUNDLE is not in use.
+----------+ +-----------+
| Alice | | Bob |
| | | |
+----+-----+ +-----+-----+
| |
| |
| SIP INVITE 1 (BASIC SDP+COMEDIA) |
|--------------------------------->|
| |
| |
| SIP 200 OK 1 (BASIC SDP+COMEDIA) |
|<---------------------------------|
| |
| |
| SIP ACK 1 |
|--------------------------------->|
| |
| |
| |
|<########### MEDIA 1 ############>|
| 1 video A->B, 1 video B->A |
|<################################>|
| |
| |
| |
|<================================>|
| CLUE CTRL CHANNEL ESTABLISHED |
|<================================>|
| |
| |
| CLUE ADVERTISEMENT 1 |
|*********************************>|
| |
| |
| CLUE ADVERTISEMENT 2 |
|<*********************************|
| |
| |
| SIP INVITE 2 (+3 sendonly) |
|--------------------------------->|
| |
| |
| CLUE CONFIGURE 1 |
|<*********************************|
| |
| |
| CLUE RESPONSE 1 |
|*********************************>|
| |
| |
| SIP 200 OK 2 (+2 recvonly) |
|<---------------------------------|
| |
| |
| SIP ACK 2 |
|--------------------------------->|
| |
| |
| |
|<########### MEDIA 2 ############>|
| 2 video A->B, 1 video B->A |
|<################################>|
| |
| |
| SIP INVITE 3 (+2 sendonly) |
|<---------------------------------|
| |
| |
| CLUE CONFIGURE 2 |
|*********************************>|
| |
| |
| CLUE RESPONSE 2 |
|<*********************************|
| |
| |
| SIP 200 OK 3 (+2 recvonly) |
|--------------------------------->|
| |
| |
| |
| SIP ACK 3 |
|<---------------------------------|
| |
| |
| |
|<########### MEDIA 3 ############>|
| 2 video A->B, 2 video B->A |
|<################################>|
| |
| |
| |
v v
...
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
In INVITE 1, Alice sends Bob a SIP INVITE including in the SDP body the basilar audio and video capabilities ("BASIC SDP") and the information needed for opening a control channel to be used for CLUE protocol messages exchange, according to what is envisioned in the COMEDIA approach ("COMEDIA") for DTLS/SCTP channel [I-D.ietf-mmusic-sctp-sdp]. A snippet of the SDP showing the grouping attribute and the video m-line are shown below (mid 3 represents the CLUE channel):
Bob responds with a similar SDP (200 OK 1); due to their similiarity no SDP snippet is shown here. Alice and Bob are each able to send a single audio and video stream (whether they choose to send this initial media before CLUE has been negotiated is implementation-dependent). This is illustrated as MEDIA 1.
With the successful initial O/A Alice and Bob are also free to negotiate the CLUE channel. Once this is successfully established CLUE negotiation can begin. This is illustrated as CLUE CHANNEL ESTABLISHED.
Alice now sends her CLUE Advertisement (ADVERTISEMENT 1). She advertises three static captures representing her three cameras. She also includes switched captures suitable for two- and one-screen systems. All of these captures are in a single capture scene, with suitable capture scene entries to tell Bob that he should either subscribe to the three static captures, the two switched capture view or the one switched capture view. Alice has no simultaneity constraints, so includes all six captures in one simultaneous set. Finally, Alice includes an encoding group with three encoding IDs: "enc1", "enc2" and "enc3". These encoding ids aren't currently valid, but will match the next SDP offer she sends.
Bob received ADVERTISEMENT 1 but does not yet send a Configure message, because he has not yet received Alice's encoding information, so as yet he does not know if she will have sufficient resources to send him the two streams he ideally wants at a quality he is happy with.
Bob also sends his CLUE ADVERTISEMENT (ADVERTISEMENT 2). He advertises two static captures representing his cameras. He also includes a single composed capture for single-screen systems, in which he will composite the two camera views into a single video stream. All three captures are in a single capture scene, with suitable capture scene entries to tell Alice that she should either subscribe to the two static captures, or the single composed capture. Bob also has no simultaneity constraints, so includes all three captures in one simultaneous set. Bob also includes a single encoding group with two encoding IDs: "foo" and "bar".
Similarly, Alices receives ADVERTISEMENT 2 but does not yet send a CONFIGURE message, because she has not yet received Bob's encoding information.
...
a=group:CLUE 3 4 5 6
...
m=video 6002 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
a=sendrecv
a=mid:2
...
m=video 6004 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016
a=sendonly
a=mid:4
a=label:enc1
m=video 6006 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016
a=sendonly
a=mid:5
a=label:enc2
m=video 6008 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016
a=sendonly
a=mid:6
a=label:enc3
Alice now sends INVITE 2. She maintains the sendrecv audio, video and CLUE m-lines, and she adds three new sendonly m-lines to represents the maximum three encodings she can send. Each of these m-lines has a label corresponding to one of the encoding ids from ADVERTISEMENT 1. Each also has its mid added to the grouping attribute to show they are controlled by the CLUE channel. A snippet of the SDP showing the grouping attribute and the video m-lines are shown below (mid 3 represents the CLUE channel):
Bob now has all the information he needs to decide which streams to configure. As such he now sends CONFIGURE 1. This requests the pair of switched captures that represent Alice's scene, and he configures them with encoder ids "enc1" and "enc2". This also serves as an ack for Alice's ADVERTISMENT 1.
Alice receives Bob's message CONFIGURE 1 and sends RESPONSE 1 to ack its receptions. She does not yet send the capture encodings specified, because at this stage Bob hasn't negotiated the ability to receive these streams in SDP.
...
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
Bob now sends his SDP answer as part of 200 OK 2. Alongside his original audio, video and CLUE m-lines he includes two active recvonly m-lines and a zeroed m-line for the third. He adds their mid values to the grouping attribute to show they are controlled by the CLUE channel. A snippet of the SDP showing the grouping attribute and the video m-lines are shown below (mid 100 represents the CLUE channel):
On receiving 200 OK 2 from Bob Alice is now able to send the two streams of video Bob requested - this is illustrated as MEDIA 2.
...
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
The constraints of offer/answer meant that Bob could not include his encoder information as new m-lines in 200 OK 2. As such Bob now sends INVITE 3 to generate a new offer. Along with all the streams from 200 OK 2 Bob also includes two new sendonly streams. Each stream has a label corresponding to the encoding ids in his ADVERTISEMENT 2 message. He also adds their mid values to the grouping attribute to show they are controlled by the CLUE channel. A snippet of the SDP showing the grouping attribute and the video m-lines are shown below (mid 100 represents the CLUE channel):
Having received this Alice now has all the information she needs to send CONFIGURE 2. She requests the two static captures from Bob, to be sent on encodings "foo" and "bar".
Bob receives Alice's message CONFIGURE 2 and sends RESPONSE 2 to ack its receptions. Bob does not yet send the capture encodings specified, because Alice hasn't yet negotiated the ability to receive these streams in SDP.
...
a=group:CLUE 3 4 5 7 8
...
m=video 6002 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
a=sendrecv
a=mid:2
...
m=video 6004 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016
a=sendonly
a=mid:4
a=label:enc1
m=video 6006 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016
a=sendonly
a=mid:5
a=label:enc2
m=video 0 RTP/AVP 96
m=video 6010 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
a=recvonly
a=mid:7
m=video 6012 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
a=recvonly
a=mid:8
Alice now sends 200 OK 3, matching two recvonly m-lines to Bob's new sendonly lines. She includes their mid values in the grouping attribute to show they are controlled by the CLUE channel. A snippet of the SDP showing the grouping attribute and the video m-lines are shown below (mid 3 represents the CLUE channel):
Finally, on receiving 200 OK 3 Bob is now able to send the two streams of video Alice requested - this is illustrated as MEDIA 3.
Both sides of the call are now sending multiple video streams with their sources defined via CLUE negotiation. As the call progresses either side can send new ADVERTISEMENT or CONFIGURE or new SDP negotiation to add, remove or change what they have available or want to receive.
In this brief example Alice is a CLUE-capable endpoint making a call to Bob, who is not CLUE-capable, i.e., it is not able to use the CLUE protocol.
+----------+ +-----------+
| EP1 | | EP2 |
| | | |
+----+-----+ +-----+-----+
| |
| |
| SIP INVITE 1 (BASIC SDP+COMEDIA) |
|--------------------------------->|
| |
| |
| 200 0K 1 (BASIC SDP+*NO*COMEDIA) |
|<---------------------------------|
| |
| |
| ACK 1 |
|--------------------------------->|
| |
| |
| |
|<########### MEDIA 1 ############>|
| 1 video A->B, 1 video B->A |
|<################################>|
| |
| |
| |
| |
v v
...
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
In INVITE 1, Alice sends Bob a SIP INVITE including in the SDP body the basilar audio and video capabilities ("BASIC SDP") and the information needed for opening a control channel to be used for CLUE protocol messages exchange, according to what is envisioned in the COMEDIA approach ("COMEDIA") for DTLS/SCTP channel [I-D.ietf-mmusic-sctp-sdp]. A snippet of the SDP showing the grouping attribute and the video m-line are shown below (mid 3 represents the CLUE channel):
Bob is not CLUE capable, and hence does not recognize the "CLUE" semantic for the grouping attribute, not does he support the CLUE channel. He responds with an answer with audio and video, but with the CLUE channel zeroed.
From the lack of the CLUE channel Alice understands that Bob does not support CLUE, or does not wish to use it. Both sides are now able to send a single audio and video stream to each other. Alice at this point begins to send her fallback video: in this case likely a switched view from whichever camera shows the current loudest participant on her side.
The current proposal calls for a new "CLUE" semantic for the SDP Grouping Framework [RFC5888].
Any other SDP extensions required to support CLUE signaling should also be specified here. Then we will need to take action within MMUSIC to make those happen. This section should be empty and removed before this document becomes an RFC.
NOTE: The RTP mapping document [I-D.even-clue-rtp-mapping] is also likely to call for SDP extensions. We will have to reconcile how to coordinate these two documents.
(Placeholder) This may be unremarkable. If so we can drop it.
We may want to rule this out of scope for now. But we should be thinking about this.
Here are issues pertinent to signaling that need resolution. Resolution will probably result in changes somewhere in this document, but may also impact other documents.
The team focusing on this draft consists of: Roni Even, Rob Hansen, Christer Holmberg, Paul Kyzivat, Simon Pietro-Romano, Roberta Presta.
Christian Groves has contributed detailed comments and suggestions.
The author list should be updated as people contribute substantial text to this document.
TBD
TBD
These were developed by Lennard Xiao, Christian Groves and Paul, so added Lennard and Christian as authors.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [I-D.ietf-clue-framework] | Duckworth, M., Pepperell, A. and S. Wenger, "Framework for Telepresence Multi-Streams", Internet-Draft draft-ietf-clue-framework-14, February 2014. |
| [I-D.presta-clue-data-model-schema] | Presta, R. and S. Romano, "An XML Schema for the CLUE data model", Internet-Draft draft-presta-clue-data-model-schema-03, March 2013. |
| [I-D.presta-clue-protocol] | Presta, R. and S. Romano, "CLUE protocol", Internet-Draft draft-presta-clue-protocol-03, November 2013. |
| [I-D.ietf-clue-datachannel] | Holmberg, C., "CLUE Protocol Data Channel", Internet-Draft draft-ietf-clue-datachannel-00, March 2014. |
| [I-D.groves-clue-latent-config] | Groves, C., Yang, W. and R. Even, "CLUE and latent configurations", Internet-Draft draft-groves-clue-latent-config-00, January 2014. |
| [I-D.ietf-mmusic-sctp-sdp] | Loreto, S. and G. Camarillo, "Stream Control Transmission Protocol (SCTP)-Based Media Transport in the Session Description Protocol (SDP)", Internet-Draft draft-ietf-mmusic-sctp-sdp-06, February 2014. |
| [I-D.tuexen-tsvwg-sctp-dtls-encaps] | Jesup, R., Loreto, S., Stewart, R. and M. Tuexen, "DTLS Encapsulation of SCTP Packets for RTCWEB", Internet-Draft draft-tuexen-tsvwg-sctp-dtls-encaps-01, July 2012. |
| [RFC4574] | Levin, O. and G. Camarillo, "The Session Description Protocol (SDP) Label Attribute", RFC 4574, August 2006. |
| [RFC5888] | Camarillo, G. and H. Schulzrinne, "The Session Description Protocol (SDP) Grouping Framework", RFC 5888, June 2010. |
[The specifics of the CLUE signaling protocol are in the process of being defined in [I-D.presta-clue-protocol], while the negotiation of the CLUE data channel is being defined in [I-D.ietf-clue-datachannel]. As such, considerable text originally in this section have been transitioned to these document. The following text relates to issues that are no longer the focus of this document, but remain important and unresolved, and so have been preserved here.]
The CLUE versioning mechanism addresses the following needs:
An initial message exchange on the CLUE channel handles the negotiation of version and options.
The version usage is similar in philosophy to XMPP:
Figure 1 shows the negotiation in simplified form:
| Supported Supported |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| Required Required |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| Advertise/Configure/... |
|<------------------------->|
Figure 1: Basic Option Negotiation (simplified)
Dedicated message types are used for the negotiation because:
Figure 2 and Figure 3 are simplified examples of the Supported and Required messages:
<supported>
<version major=“1” minor=“0”>
<!– May repeat version if multiple
major versions supported. ->
<!- Options follow ->
<mediaProvider/>
...
</supported>
Figure 2: Supported Message (simplified)
<required>
<version major=“1” minor=“0”>
<!– Requested options of peer follow ->
<!- Options follow ->
<mediaProvider/>
...
</required>
Figure 3: Required Message (simplified)
The Supported message includes one or more <version> elements, each denoting a major/minor version combination that the sender of the message is capable of supporting.
The <version> element contains both a major and minor version. Each is a non-negative integer. Each <version> element in the message MUST contain a unique major version number, distinct from the major version number in all the other <version> elements in the message. The minor version in a <version> element denotes the largest minor version the sender supports for the corresponding major version. (Minor versions are always backwards compatible, so support for a minor version implies support for all smaller minor versions.)
Each endpoint of the CLUE channel sends a Supported message, and receives the Supported message sent by the other end. Then each end compares the versions sent and the versions received to determine the version to be used for this CLUE session.
Option negotiation is used to agree upon which options will be available for use within the CLUE session. (It does not say that these options must be used.) This may be used for both standard and proprietary options. (As used here, and option could be either a feature described as part of this specification that is optional to implement, or a feature defined in a separate specification that extends this one.)
Each end includes, within the Supported message it sends, elements describing those options it is willing and able to use with this CLUE session.
Each side, upon receiving a Supported message, selects from that message those option elements that it wishes the peer to use. (If/when occasion for that use arises.) It then includes those selected elements into the Required message that it sends.
Within a received Supported message, unknown option elements MUST be ignored. This includes elements that are of a known type that is not known to denote an option.
Each option is denoted, in the Supported and Required messages, by an XML element. There are no special rules for these elements – they can be any XML element. The attributes and body of the element may carry further information about the option. The same element type is used to denote the option in the Supported message and the corresponding Required message, but the attributes and body may differ according to option-specific rules. This may be used to negotiate aspects of a particular option. The ordering of option elements is irrelevant within the Supported and Required messages, and need not be consistent in the two.
Only one option element is defined in this document: <mediaProvider>.
The <mediaProvider> element, when placed in a Supported message, indicates that the sender is willing and able to send ADVERTISEMENT messages and receive CONFIGURE messages. When placed in a Required message, the <mediaProvider> element indicates that the sender is willing, able, and desirous of receiving ADVERTISEMENT messages and sending CONFIGURE messages. If an endpoint does not receive <mediaProvider> in a Required message, it MUST NOT send ADVERTISEMENT messages. For common cases <mediaProvider> should be supported and required by both endpoints, to enable bidirectional exchange of media. If not required by either end, the CLUE session is useless. This is an error condition, and SHOULD result in termination of the CLUE channel.
The <mediaProvider> element has no defined attributes or body.
The following are errors that may be detected and reported during version negotiation:
These errors are reported using the normal message error reporting mechanism.
Other applicable error codes may also be returned in response to a Supported or Required message.
Errors that occur at this stage result in negotiation failure. When this occurs, CLUE cannot be used until the end of the SIP session, or until a new CLUE channel is negotiated and a subsequent version negotiation succeeds. The SIP session may continue without CLUE features.
[[NOTE: THIS IS AWKWARD. SUGGESTIONS FOR BETTER WAYS TO DEFINE THIS ARE WELCOME.]]
This document defines CLUE version 1.0 (major=1, minor=0). This denotes the normative behavior defined in this document and other documents upon which it normatively depends, including but is not limited to:
Given two CLUE versions Vx and Vy, then Vx is backward compatible with Vy if and only if:
Revisions, updates, to any of the documents denoted by Version 1.0 MAY result in the definition of a new CLUE version. If they do, then this document MUST be revised to define the new version.
The CLUE version to be defined in a revision to this document MUST be determined as follows:
When a CLUE implementation sends a Supported message, it MUST include the CLUE versions it is willing and able to conform with.
| Supported Supported |
| Version 2.0 |
| Version 1.2 Version 1.1 |
| mediaProv mediaProv |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| OK response OK response |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| Required Required |
| Version 1.2 Version 1.1 |
| mediaProv mediaProv |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| OK response OK response |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| Advertise |
|<------------------------->|
| |
| Configure |
|<------------------------->|
The endpoint on the left can support versions 1.2 and 2.0, and because of backward compatibility can support versions 1.0 and 1.1. The endpoint on the right supports only version 2.0. Both endpoints with to both provide and consume media. They each send a Supported message indicating what they support.
The element on the left, upon receiving the Supported message, determines that it is permitted to use version 1.2 or 1.1, and decides to use 1.2. It sends a Required message containing version 1.2 and also includes the mediaProvider option element, because it wants its peer to provide media.
The element on the right, upon receiving the Supported message, selects version 1.1 because it is the highest version in common to the two sides. It sends a Required message containing version 1.1 because that is the highest version in common. It also includes the mediaProvider option element, because it wants its peer to provide media.
Upon receiving the Required messages, both endpoints determine that they should send ADVERTISEMENTs.
ADVERTISEMENT and CONFIGURE messages will flow in both directions.
| Supported Supported |
| Version 1.0 Version 1.0 |
| mediaProv (no opts) |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| OK response OK response |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| Required Required |
| Version 1.0 Version 1.0 |
| (no opts) mediaProv |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| OK response OK response |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| Advertise |
|-------------------------->|
| |
| Configure |
|<--------------------------|
The endpoint on the right consumes media, but doesn't provide any so it doesn't include the mediaProvider option element in the Supported message it sends.
The element on the left would like to include a mediaProvider option element in the Requirements message it sends, but can't because it did not receive one in the Supported message it received.
ADVERTISEMENT messages will only go from left to right, and CONFIGURE messages will only go from right to left.
| Supported Supported |
| Version 1.0 Version 1.0 |
| mediaProv mediaProv |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| OK response OK response |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| Required Required |
| Version 1.0 Version 1.0 |
| (no opts) mediaProv |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| OK response OK response |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| Advertise |
|-------------------------->|
| |
| Configure |
|<--------------------------|
The endpoint on the left provides media but does not consume any so it includes the mediaProvider option element in the Supported message it sends, but does't include the mediaProvider option element in the Required message it sends.
ADVERTISEMENT messages will only go from left to right, and CONFIGURE messages will only go from right to left.
| Supported Supported |
| Version 1.2 Version 2.1 |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| Version Version |
| Incompat. Incompat. |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| close clue channel |
|<------------------------->|
| |
| legacy mode or BYE |
|<------------------------->|
Upon receiving the Supported message, each endpoint discovers there is no major version in common, so CLUE usage is not possible. Each sends an error response indicating this and then ceases CLUE usage.
| Supported Supported |
| Version 1.0 Version 1.0 |
| mediaProv mediaProv |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| Required Required |
| (no opts) (no opts) |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| Option Option |
| Incompat. Incompat. |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| close clue channel |
|<------------------------->|
| |
| legacy mode or BYE |
|<------------------------->|
Neither of the endpoints is willing to provide media. It makes no sense to continue CLUE operation in this situation. Each endpoint realizes this upon receiving the Supported message, sends an error response indicating this and then ceases CLUE usage.
| Supported !@#$%^ |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| syntax error OK response |
|------------\ /------------|
| X |
|<-----------/ \----------->|
| |
| close clue channel |
|-------------------------->|
| |
| legacy mode or BYE |
|<------------------------->|
CLUE messages are transported over a bidirectional CLUE channel. In a two-party CLUE session, a CLUE channel connects the two endpoints. In a CLUE conference, each endpoint has a CLUE channel connecting it to an MCU. (In conferences with cascaded mixers [RFC4353], two MCUs will be connected by a CLUE channel.)
The transport mechanism used for CLUE messages is DTLS/SCTP as specified in [I-D.tuexen-tsvwg-sctp-dtls-encaps] and [I-D.ietf-mmusic-sctp-sdp]. A CLUE channel consists of one SCTP stream in each direction over a DTLS/SCTP session. The mechanism for establishing the DTLS/SCTP session is described in [I-D.ietf-clue-datachannel].
The CLUE channel will usually be offered during the initial SIP INVITE, and remain connected for the duration of the CLUE/SIP session. However this need not be the case. The CLUE channel may be established mid-session after desire and capability for CLUE have been determined, and the CLUE channel may be dropped mid-call if the desire and/or capability to support it is lost.
There may be cases when it becomes necessary to "reset" the CLUE channel. This by be as a result of an error on the underlying SCTP association, a need to change the endpoint address of the SCTP association, loss of CLUE protocol state, or something else TBD.
The precise mechanisms used to determine when a reset is required, and how to accomplish it and return to a well defined state are TBS.
We will need to specify behavior in the face of transport errors that are so severe that they can't be managed via CLUE messaging within the CLUE channel. Some errors of this sort are:
The worst case is to drop the entire CLUE call. Another possibility is to fall back to legacy compatibility mode. Or perhaps a "reset" can be done on the protocol. E.g. this might be accomplished by sending a new O/A and establishing a replacement SCTP association. Or a new CLUE channel might be established within the existing SCTP association.
Message framing is provided by the SCTP transport protocol. Each CLUE message is carried in one SCTP message.