CLUE Signaling
Huaweipkyzivat@alum.mit.eduHuaweilennard.xiao@huawei.comHuaweiChristian.Groves@nteczone.comCisco Systemsrohanse2@cisco.com
This document specifies how CLUE-specific signaling such as the
CLUE protocol and the
CLUE data channel are used
with each other and with existing signaling mechanisms such as SIP and SDP to
produce a telepresence call.
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,
transported via a
CLUE data channel, is used to
negotiate the Capture Sources available, their attributes and any constraints
in their use, along with which Captures the far end provides a device wishes
to receive.
Beyond negotiating the CLUE channel, SDP is also used to negotiate the details
of supported media streams and the maximum capability of each of those
streams. As the CLUE Framework
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.
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 .
This document uses terminology defined in the
CLUE Framework.
A few additional terms specific to this document are defined as follows:
A device that supports standard SIP and SDP, but either does not support CLUE,
or that does but does not currently wish to invoke CLUE capabilities.
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 of how this control is signalled in
SDP. There is a corresponding "non-CLUE-controlled" media term.
The "sip.clue" media feature tag indicates support for CLUE. A CLUE-capable
device SHOULD include this media feature tag in its REGISTER requests and
OPTION responses. It SHOULD also include the media feature tag in INVITE and
UPDATE 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, CLUE.
The CLUE extension can be indicated using an SDP session-level
'group' attribute. Each SDP media "m" line that is included in this group,
using SDP media-level mid attributes, is CLUE-controlled, by a CLUE data
channel also included in this CLUE group.
Currently only support for a single CLUE group is specified; support for
multiple CLUE groups in a single session is beyond the scope of this document.
A device MUST NOT include more than one CLUE group in its SDP unless it is
following a specification that defines how multiple CLUE channels are
signalled, and is either able to determine that the other side of the SDP
exchange supports multiple CLUE channels, or is able to fail gracefully in the
event it does not.
The CLUE data channel is a
bidirectional SCTP over DTLS channel used for the transport of CLUE messages.
This channel must be established before CLUE protocol messages can be
exchanged and CLUE-controlled media can be sent.
The data channel is negotiated over SDP as described in the relevant
document. A CLUE-capable device wishing to negotiate CLUE MUST also include a
CLUE group in the SDP and include the "mid" of the "m" line for the data
channel in that group. A CLUE group MUST include the "mid" of the "m" line
for one (and only one) data channel.
Presence of the data channel in a CLUE group in an SDP offer or answer also
serves, along with the "sip.clue" media feature tag, as an indication that the
device supports CLUE and wishes to upgrade the call to include CLUE-controlled
media. A CLUE-capable device SHOULD include a data channel "m" line in offers
and, when allowed by , 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. For an "m" line
to be CLUE-controlled, its "mid" value MUST be included in a CLUE group.
CLUE-controlled media is controlled by the CLUE protocol as negotiated on the
CLUE data channel with an "mid" included in the CLUE group.
"m" lines not specified as under CLUE control follow normal rules for media
streams negotiated in SDP as defined in documents such as
.
The restrictions on CLUE-controlled media always apply to "m" lines in an SDP
offer or answer, even if negotiation of the data channel in SDP failed due to
lack of CLUE support by the remote device or for any other reason, or in an
offer if the recipient does not include the "mid" of the corresponding
"m" line in their CLUE group.
The 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
for a list of valid parameters for representing
encoder sender stream limits.
These Encodings are CLUE-controlled and hence MUST include an "mid" in a
CLUE group as defined above.
As well as 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
media packets MUST NOT be sent until configuration is complete, while
non-media packets such as STUN and DTLS MUST be sent as normal if negotiated.
Every "m" line representing a CLUE Encoding MUST contain a "label" attribute
as defined in . This label is used to identify the
Encoding by the sender in CLUE Advertisement messages and by the receiver in
CLUE Configure messages. Each label used for a CLUE-controlled "m" line MUST
be different from the label on all other "m" lines in the same CLUE group in
the SDP message, unless an "m" line represents a dependent stream related to
another "m" line (such as a FEC stream), in which case it MUST
have the same label value as the "m" line on which it is dependent.
CLUE Encodings are defined in SDP, but can be referenced from CLUE protocol
messages - this is how the protocol defines which Encodings are part of an
Encoding group (in Advertisement messages) and which Encoding with which to
encode a specific Capture (in Configure messages). The labels on the
CLUE-controlled "m" lines are the references that are used in the CLUE
protocol.
Each <encID> (in encodingIDListType) in a CLUE Advertisement message
SHOULD represent an Encoding defined in SDP; the specific Encoding referenced
is a CLUE-controlled "m" line in the most recent SDP sent by the sender of the
Advertisement message with a label value corresponding to the text content of
the <encID>.
Similarly, each <encodingID> (in captureEncodingType) in a CLUE
Configure message SHOULD represent an Encoding defined in SDP; the specific
Encoding referenced is a CLUE-controlled "m" line in the most recent SDP
received by the sender of the Configure message with a label value
corresponding to the text content of the <encodingID>.
Note that the non-atomic nature of SDP/CLUE protocol interaction may mean that
there are temporary periods where an <encID>/<encodingID> in a
CLUE message does not reference an SDP "m" line, or where an Encoding
represented in SDP is not referenced in a CLUE protocol message.
See for specifics.
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.
A receiver who wishes to receive a CLUE stream via a specific Encoding
requires an "a=recvonly" "m" line that matches the "a=sendonly" Encoding.
These "m" lines are CLUE-controlled and hence MUST include their "mid" in the
CLUE group corresponding to the CLUE group of the Encoding they wish to
receive.
A CLUE-capable device sending an initial SDP offer of a SIP session SHOULD
include an "m" line for the data channel to convey the CLUE protocol, along
with a CLUE group containing the "mid" of the data channel "m" line.
For interoperability with non-CLUE devices a CLUE-capable device sending an
initial SDP offer SHOULD NOT include any "m" line for CLUE-controlled media
beyond the "m" line for the CLUE data channel, and SHOULD include at least one
non-CLUE-controlled media "m" line.
If the device has evidence that the receiver is also CLUE-capable, for
instance due to receiving an initial INVITE with no SDP but including a
"sip.clue" media feature tag, the above recommendation is waived, and the
initial offer MAY contain "m" lines for CLUE-controlled media.
With the same interoperability recommendations as for Encodings, the sender of
the initial SDP offer MAY also include "a=recvonly" media lines to
preallocate "m" lines to receive media. Alternatively, it MAY wait until CLUE
protocol negotiation has completed before including these lines in a new
offer/answer exchange - see for
recommendations.
If the recipient is CLUE-capable and the initial offer contains both an "m"
line for a data channel and a CLUE group containing the "mid" for that "m"
line, they SHOULD negotiate data channel support for an "m" line, and include
the "mid" of that "m" line in a corresponding CLUE group.
A CLUE-capable recipient that receives an "m" line for a data channel but no
corresponding CLUE group containing the "mid" of that "m" line MAY still
include a corresponding data channel "m" line if there are any other non-CLUE
protocols it can convey over that channel, but MUST NOT negotiate use of the
CLUE protocol on this channel.
If the initial offer contained "a=recvonly" CLUE-controlled media lines the
recipient SHOULD include corresponding "a=sendonly" CLUE-controlled media
lines, up to the maximum number of Encodings it wishes to advertise. As
CLUE-controlled media, the "mid" of these "m" lines must be included in the
corresponding CLUE group.
If the initial offer contained "a=sendonly" CLUE-controlled media lines the
recipient MAY include corresponding "a=recvonly" CLUE-controlled media lines,
up to the maximum number of Capture Encodings it wishes to receive.
Alternatively, it MAY wait until CLUE protocol negotiation has completed
before including these lines in a new offer/answer exchange - see
for recommendations.
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.
In the event that both offer and answer include a data channel "m" line with a
mid value included in corresponding CLUE groups CLUE has been successfully
negotiated and the call is now CLUE-enabled, otherwise the call is not
CLUE-enabled.
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, sending of
CLUE protocol messages can begin.
A CLUE-capable device MAY choose not
to send media on the non-CLUE-controlled channels during the period in which
control of the CLUE-controlled media lines is being negotiated. However, a
CLUE-capable device MUST still be prepared to receive media on
non-CLUE-controlled media lines that have been successfully negotiated as
defined in .
If either side of the call wishes to add additional CLUE-controlled "m" line
to send or receive CLUE-controlled media they MAY now send a SIP request with
a new SDP offer. Note that if BUNDLE has been successfully negotiated and a
Bundle Address Synchronization offer is 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.
Subsequent offer/answer exchanges MAY add additional "m" lines for
CLUE-controlled media; in most cases at least one additional exchange will be
required before both sides have added all the Encodings and ability to receive
Encodings that they desire. Devices MAY delay adding "a=recvonly"
CLUE-controlled m-lines until after CLUE protocol negotiation completes - see
for recommendations.
Subsequent offer/answer exchanges MAY also deactive "m" lines for
CLUE-controlled media.
Once CLUE media has been successfully negotiated devices SHOULD ensure that
non-CLUE-controlled media is deactived in cases where it corresponds to
the media type of CLUE-controlled media that has been successfully negotiated.
This deactivate may require an additional SDP exchange, or may be incorporated
into one that is part of the CLUE negotiation.
A CLUE-capable device that receives an initial SDP offer from a non-CLUE
device SHOULD include a new data channel "m" line and corresponding CLUE group
in any subsequent offers it sends, to indicate that it is CLUE-capable.
If, in an ongoing non-CLUE call, an SDP offer/answer exchange completes with
both sides having included a data channel "m" line in their SDP and with the
"mid" for that channel in corresponding CLUE groups then the call is now
CLUE-enabled; negotiation of the data channel and subsequently the CLUE
protocol begin.
If, in an ongoing CLUE-enabled call, an SDP offer-answer negotiation completes
in a fashion in which either the CLUE data channel was not successfully
negotiated or one side did not include the data channel in a matching CLUE
group then CLUE for this channel is disabled. In the event that this occurs,
CLUE is no longer enabled and sending of all CLUE-controlled media associated
with the corresponding CLUE group MUST stop. If the data channel is still
present but not included in the CLUE group semantic CLUE protocol messages
MUST no longer be sent.
Note that this is distinct to cases where the CLUE data channel fails or an
error occurs on the CLUE protocol; see
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 the need to implement interlocking state machines with the potential
to reach invalid states if messages were to be lost, or be rewritten en-route
by middle boxes, the state machines in SDP and CLUE operate independently. The
state of the CLUE channel does not restrict when an implementation may send a
new SDP offer or answer, and likewise the implementation's ability to send a
new CLUE Advertisement or Configure message is not restricted by the results
of or the state of the most recent SDP negotiation (unless the SDP negotiation
has removed the CLUE channel).
The primary implication of this is that a device may receive an SDP with a
CLUE Encoding it does not yet have capture information for, or receive a CLUE
Configure message specifying a Capture Encoding for which the far end has not
negotiated a media stream in SDP.
CLUE messages contain an <encID> (in encodingIDListType) or
<encodingID> (in captureEncodingType), which is used to identify a
specific encoding or captureEncoding in SDP; see
for specifcs.
The non-atomic nature of CLUE negotiation means that a sender may wish to send
a new Advertisement before the corresponding SDP message. As such the sender
of the CLUE message MAY include an <encID> which does not currently
match a CLUE-controlled "m" line label in SDP; A CLUE-capable implementation
MUST NOT reject a CLUE protocol messages solely because it contains
<encID> elements that do not match an id in SDP.
The current state of the CLUE participant or Media Provider/Consumer
state machines do not affect compliance with any of the normative language of
. That is, they MUST NOT delay an ongoing SDP
exchange as part of a SIP server or client transaction; an implementation MUST
NOT delay an SDP exchange while waiting for CLUE negotiation to complete or
for a Configure message to arrive.
Similarly, a device in a CLUE-enabled call MUST NOT delay any mandatory state
transitions in the CLUE Participant or Media Provider/Consumer state machines
due to the presence or absence of an ongoing SDP exchange.
A device with the CLUE Participant state machine in the ACTIVE state
MAY choose not to move from ESTABLISHED to ADV (Media Provider
state machine) or from ESTABLISHED to WAIT FOR CONF RESPONSE (Media Consumer
state machine) based on the SDP state. See
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.
While SDP and CLUE message states do not impose constraints on each other,
both impose constraints on the sending of media - CLUE-controlled media MUST
NOT be sent unless it has been negotiated in both CLUE and SDP: an
implementation MUST NOT send a specific CLUE Capture Encoding unless its most
recent SDP exchange contains an active media channel for that Encoding AND
the far end has sent a CLUE Configure message specifying a valid Capture for
that Encoding.
CLUE-capable devices MUST be able to handle states in which CLUE messages make
reference to EncodingIDs that do not match the most recently received SDP,
irrespective of the order in which SDP and CLUE messages are received. While
these mis-matches will usually be transitory a device MUST be able to cope
with such mismatches remaining indefinitely. However, this document makes some
recommendations on message ordering for these non-atomic transitions.
CLUE-capable devices SHOULD ensure that any inconsistencies between SDP and
CLUE signalling are temporary by sending updated SDP or CLUE messages as soon
as the relevant state machines and other constraints permit.
Generally, implementations that receive messages for which they have
incomplete information SHOULD wait until they have the corresponding
information they lack before sending messages to make changes related to that
information. For instance, an implementation that receives a new SDP offer
with three new "a=sendonly" CLUE "m" lines that has not received the
corresponding CLUE Advertisement providing the capture information for those
streams SHOULD NOT include corresponding "a=recvonly" lines in its answer,
but instead should make a new SDP offer when and if a new Advertisement
arrives with Captures relevant to those Encodings.
Because of the constraints of offer/answer and because new SDP negotiations
are generally more 'costly' than sending a new CLUE message, implementations
needing to make changes to both channels SHOULD prioritize sending the updated
CLUE message over sending the new SDP message. The aim is for the recipient to
receive the CLUE changes before the SDP changes, allowing the recipient to
send their SDP answers without incomplete information, reducing the number of
new SDP offers required.
allows for Multiple Content Captures
MCCs): Captures which contain multiple source Captures, whether composited
into a single stream or switched based on some metric.
The Captures that constitute these MCCs may or may not be defined in the
Advertisement message. If they are defined and the MCC is providing them in a
switched format the recipient may wish to determine which originating source
Capture is currently being provided, so that they can apply geometric
corrections based on that Capture's geometry, or take some other action based
on the original Capture information.
To do this, allows for the
CaptureID of the originating Capture to be conveyed via RTP or RTCP. A Media
Provider sending switched media from an MCC with defined originating sources
MUST send the CaptureID in both RTP and RTCP, as described
in the mapping document.
Because the RTP/RTCP CaptureID is delivered via a different channel to the
Advertisement in which in the contents of the MCC are defined there is an
intrinsic race condition in cases in which the contents of an MCC are
redefined.
When a Media Provider redefines an MCC which involves CaptureIDs, the
reception of the relevant CaptureIDs by the recipient will either lead or lag
reception and processing of the new Advertisement by the recipient. As such,
a media recipient MUST not be disrupted by any of the following in any CLUE-
controlled media stream it is receiving, whether that stream is for a static
Capture or for an MCC (as any static Capture may be redefined to an MCC in a
later Advertisement):
Receiving RTP or RTCP containing a CaptureID when the most recently processed
Advertisement means that none are expected.
Receiving RTP or RTCP without CaptureIDs when the most recently processed
Advertisement means that media CaptureIDs are expected.
Receiving a CaptureID in RTP or RTCP for a Capture defined in the most
recently processed Advertisement, but which the same Advertisement does not
include in the MCC.
Receiving a CaptureID in RTP or RTCP for a Capture not defined in the most
recently processed Advertisement.
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, 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.
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-capable devices MAY include the data channel in the
same BUNDLE group as RTP media. In this case the device MUST be able to
demultiplex the various transports - see section 7.2 of the
BUNDLE draft. 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 SDP snippet only illustrate video 'm'
lines. ACKs are not discussed. Note that BUNDLE is not in use.
In INVITE 1, Alice sends Bob a SIP INVITE including in the SDP body the
basilar audio and video capabilities and the information needed for opening a
control channel to be used for CLUE protocol messages exchange, according to
what is envisioned in the COMEDIA approach for DTLS/SCTP channel
. A snippet of the SDP showing the
grouping attribute and the video m-line are shown below. Alice has included a
"CLUE" group, and included the mid corresponding to a data channel in the
group (3). Note that Alice has chosen not to include any CLUE-controlled media
in the initial offer - the mid value of the video line is not included in the
"CLUE" group.
Bob responds with a similar SDP (200 OK 1), which also has a "CLUE" group
including the mid value of a data channel; due to their similiarity no SDP
snippet is shown here. Bob wishes to receive initial media, and so includes
corresponding non-CLUE-controlled audio and video lines. Alice and Bob are
each now able to send a single audio and video stream. This is illustrated as
MEDIA 1.
With the successful initial O/A Alice and Bob are also free to negotiate the
CLUE channel. Once this is successfully established CLUE negotiation can
begin. This is illustrated as CLUE CTRL CHANNEL ESTABLISHED.
Alice now sends her CLUE Advertisement (ADVERTISEMENT 1). She advertises
three static Captures representing her three cameras. She also includes
switched Captures suitable for two- and one-screen systems. All of these
Captures are in a single Capture Scene, with suitable Capture Scene entries
to tell Bob that he should either subscribe to the three static Captures, the
two switched Captures or the one switched Capture. Alice has no
simultaneity constraints, so includes all six Captures in one simultaneous
set. Finally, Alice includes an Encoding Group with three Encoding IDs:
"enc1", "enc2" and "enc3". These Encoding IDs aren't currently valid, but will
match the next SDP offer she sends.
Bob received ADVERTISEMENT 1 but does not yet send a Configure message,
because he has not yet received Alice's Encoding information, so as yet he
does not know if she will have sufficient resources to send him the two
streams he ideally wants at a quality he is happy with.
Bob also sends his CLUE Advertisement (ADVERTISEMENT 2). He advertises two
static Captures representing his cameras. He also includes a single composed
Capture for single-screen systems, in which he will composite the two camera
views into a single video stream. All three Captures are in a single Capture
Scene, with suitable Capture Scene entries to tell Alice that she should
either subscribe to the two static Captures, or the single composed Capture.
Bob also has no simultaneity constraints, so includes all three Captures in
one simultaneous set. Bob also includes a single Encoding Group with two
Encoding IDs: "foo" and "bar".
Similarly, Alices receives ADVERTISEMENT 2 but does not yet send a Configure
message, because she has not yet received Bob's Encoding information.
Alice now sends INVITE 2. She maintains the sendrecv audio, video and CLUE
m-lines, and she adds three new sendonly m-lines to represents the three
CLUE-controlled Encodings she can send. Each of these m-lines has a label
corresponding to one of the Encoding IDs from ADVERTISEMENT 1. Each also has
its mid added to the grouping attribute to show they are controlled by the
CLUE channel. A snippet of the SDP showing the grouping attribute, data
channel and the video "m" lines are shown below:
Bob now has all the information he needs to decide which streams to configure.
As such he now sends CONFIGURE 1. This requests the pair of switched Captures
that represent Alice's scene, and he configures them with encoder ids "enc1"
and "enc2". This also serves as an ack for Alice's ADVERTISEMENT 1.
Alice receives Bob's message CONFIGURE 1 and sends RESPONSE 1 to ack its
receptions. She does not yet send the Capture Encodings specified, because at
this stage Bob hasn't negotiated the ability to receive these streams in SDP.
Bob now sends his SDP answer as part of 200 OK 2. Alongside his original
audio, video and CLUE m-lines he includes two active recvonly m-lines and a
zeroed m-line for the third. He adds their mid values to the grouping
attribute to show they are controlled by the CLUE channel. A snippet of the
SDP showing the grouping attribute and the video m-lines are shown below (mid
100 represents the CLUE channel, not shown):
On receiving 200 OK 2 from Bob Alice is now able to send the two streams of
video Bob requested - this is illustrated as MEDIA 2.
The constraints of offer/answer meant that Bob could not include his encoder
information as new m-lines in 200 OK 2. As such Bob now sends INVITE 3 to
generate a new offer. Along with all the streams from 200 OK 2 Bob also
includes two new sendonly streams. Each stream has a label corresponding to
the Encoding IDs in his ADVERTISEMENT 2 message. He also adds their mid values
to the grouping attribute to show they are controlled by the CLUE channel. A
snippet of the SDP showing the grouping attribute and the video m-lines are
shown below (mid 100 represents the CLUE channel, not shown):
Having received this Alice now has all the information she needs to send
CONFIGURE 2. She requests the two static Captures from Bob, to be sent on
Encodings "foo" and "bar".
Bob receives Alice's message CONFIGURE 2 and sends RESPONSE 2 to ack its
receptions. Bob does not yet send the Capture Encodings specified, because
Alice hasn't yet negotiated the ability to receive these streams in SDP.
Alice now sends 200 OK 3, matching two recvonly m-lines to Bob's new sendonly
lines. She includes their mid values in the grouping attribute to show they
are controlled by the CLUE channel. Alice also now deactivates the initial
non-CLUE-controlled media, as bidirectional CLUE-controlled media is now
available. A snippet of the SDP showing the grouping attribute and the video
m-lines are shown below (mid 3 represents the data channel, not shown):
Finally, on receiving 200 OK 3 Bob is now able to send the two streams of
video Alice requested - this is illustrated as MEDIA 3.
Both sides of the call are now sending multiple video streams with their
sources defined via CLUE negotiation. As the call progresses either side can
send new Advertisement or Configure message or new SDP negotiation to add, remove or
change what they have available or want to receive.
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).
In INVITE 1, Alice sends Bob a SIP INVITE including in the SDP body the
basilar audio and video capabilities and the information needed for opening a
control channel to be used for CLUE protocol messages exchange, according to
what is envisioned in the COMEDIA approach for a DTLS/SCTP channel
. A snippet of the SDP showing the
grouping attribute, data channel and the video m-line are shown below:
Bob is not CLUE-capable, and hence does not recognize the "CLUE" semantic for
grouping attribute, nor does he support the data channel. He responds with an
answer with audio and video, but with the data channel zeroed.
From the lack of the data channel and grouping framework Alice understands
that Bob does not support CLUE, or does not wish to use it. Both sides are now
able to send a single audio and video stream to each other. Alice at this
point begins to send her fallback video: in this case likely a switched view
from whichever camera shows the current loudest participant on her side.
The team focusing on this draft consists of:
Roni Even,
Rob Hansen,
Christer Holmberg,
Paul Kyzivat,
Simon Pietro-Romano,
Roberta Presta.
Christian Groves and Jonathon Lennox have contributed detailed comments and
suggestions.
This document registers the following semantics with IANA in the
"Semantics for the "group" SDP Attribute" subregistry (under the
"Session Description Protocol (SDP) Parameters" registry:
Semantics Token Reference
------------------------------------- ------ ---------
CLUE controlled m-line CLUE [this draft]
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: 1.3.6.1.8.4.26
Summary of the media feature indicated by this tag: This feature tag indicates
that the device supports CLUE controlled media.
Values appropriate for use with this feature tag: Boolean.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation mechanisms: This
feature tag is most useful in a communications application for
describing the capabilities of a device which uses multiple media streams.
CLUE makes use of a number of protocols and mechanism, either defined by CLUE
or long-standing. The security considerations section of the
CLUE Framework addresses the
need to secure these mechanisms by following the recommendations of the
individual protocols.
Beyond the need to secure the consistuent protocols, the use of CLUE does
impose additional security concerns. One area of increased risk involves the
potential for a malicious party to subvert a CLUE-capable device to attack a
third party by driving large volumes of media (particularly video) traffic at
them by establishing a connection to the CLUE-capable device and directing the
media to the victim. While this is a risk for all media devices, a
CLUE-capable device may allow the attacker to configure multiple media streams
to be sent, significantly increasing the volume of traffic directed at the
victim.
This attack can be prevented by ensuring that the media recipient intends to
receive the media packets. As such all CLUE-capable devices MUST support key
negotiation and receiver intent assurance via
DTLS on CLUE-controlled RTP "m" lines. All
CLUE-controlled RTP "m" lines must be secured and implemented using
mechanisms such as SRTP; no specific security
mechanisms are made mandatory to use due to the issues addressed in
. Due to the requirements of
backwards compatibility, these is not a mandatory requirement for
non-CLUE-controlled "m" lines.
CLUE also defines a new media feature tag that indicates CLUE support. This
tag may be present even in non-CLUE calls, which increases the metadata
available about the sending device, which can help an attacker differentiate
between multiple devices and help them identify otherwise anonymised users
via the fingerprint of features their device supports. To prevent this, SIP
signalling SHOULD always be encrypted using TLS.
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.
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.
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.
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.
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.
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.
Revision by Rob Hansen
Submitted as -00 working group document
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
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.
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
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.)
Revisions by Rob Hansen: ??? 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.
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.
Updated by roberta to include some sample call flows.
Initial version by pkyzivat. Established general outline for the document,
and specified a few things thought to represent wg consensus.