| Network Working Group | P. Kyzivat |
| Internet-Draft | L. Xiao |
| Intended status: Standards Track | C. Groves |
| Expires: December 15, 2013 | Huawei |
| June 13, 2013 |
CLUE Signaling
draft-kyzivat-clue-signaling-03
This document specifies how signaling is conducted in the course of CLUE sessions. This includes how SIP/SDP signaling is applied to CLUE sessions as well as defining a CLUE-specific signaling protocol that complements SIP/SDP and supports negotiation of CLUE application level data.
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This document specifies how signaling is conducted in the course of CLUE sessions. This includes how SIP/SDP signaling is applied to CLUE sessions as well as defining a CLUE-specific signaling protocol that complements SIP/SDP and supports negotiation of CLUE application level data.
[Yes, this is a dup of the abstract for now. Eventually it should say more.]
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 CLUE Framework [I-D.ietf-clue-framework] mentions a CLUE-specific protocol for the exchange of ADVERTISEMENT and CONFIGURE messages, but gives little detail. The Data Model [I-D.presta-clue-data-model-schema] specifies a model and XML representation for CLUE-related data, but doesn't currently specify exactly what data belongs in each message, or how messages are sequenced. This document provides the detail missing from those documents.
There must be some provision for identifying incompatible protocol versions.
NOTE: We probably don't want to have incompatible versions. Typically changes will be introduced in a backward compatible way. But a time may come when this isn't possible, and we should be prepared for that. This is more likely to occur before an RFC is published. While it is probably unwise to deploy a product based on a draft, there will certainly be prototypes developed for testing, and those tests may lead to a need for incompatible change. So whatever the mechanism is, it should be applicable to changes that occur from draft to draft, as well as after an RFC has been published.
There must be some provision for dealing with optional-to-implement features in the specification, and/or for backward compatible extensions to the protocol. These are superficially different, but in practice they are more-or-less equivalent. To an implementation of the base protocol and some extensions, those extensions must be viewed as optional-to-implement features in peers.
One decision is whether extensions may be implemented mix-and-match, or whether there is a sequence of extensions, and one extension may only be supported if all the prior extensions have been supported.
Both version and options can be negotiated. Some mechanisms may work for both, while others are only appropriate for one or the other. Some possibilities:
The CLUE channel is reliable, so there is no need for acknowledgement to guarantee delivery. But there is still a need for application-to-application acknowledgement to report that the message has been received, parsed, and found to be of an acceptable format. One possibility is to introduce separate ACK and NAK messages. Another possibility is to add a confirmation element to each CLUE message, so that confirmation can be piggybacked on the basic messages. Some alternatives follow. [OTHER PROPOSALS WELCOME.]
The characteristics of this approach are:
The characteristics of this approach are:
The general format of every message is:
(The exact representation is TBD - by XML experts.)
There are loose ends to resolve here. In particular, how to acknowledge messages after NAKing one.
There needs to be a mechanism to report errors with other messages. The details of form, content, and usage still need to be specified, and need to be tuned to the details of the protocol. This could use distinct messages or be incorporated into the other messages. Errors this message must be able to report include:
Each message exchanged within a CLUE session could contain a complete description of the state it wishes to achieve. Or each message could describe just the changes that it wishes to make to the current state. Or the protocol could support both message forms. Which direction to pursue is TBD.
[Paul: while this does need to be decided, it is fundamentally just an optimization. IMO it does not have major impact on the other parts of this document, so I would prefer to continue deferring it until we are so far along with the remainder of the document that we can no longer defer it.]
There is a very basic introduction to this topic in section 4 (Overview) of the CLUE Framework [I-D.ietf-clue-framework]. After removing extraneous material it would look like:
+-----------+ +-----------+
| Endpoint1 | | Endpoint2 |
+----+------+ +-----+-----+
| |
| ADVERTISEMENT 1 |
|*********************************>|
| ADVERTISEMENT 2 |
|<*********************************|
| |
| CONFIGURE 1 |
|<*********************************|
| CONFIGURE 2 |
|*********************************>|
| |
But we need much more than this, to show multiple CONFIGUREs per ADVERTISEMENT, interleaving of ADVERTISEMENTs and CONFIGUREs in both directions, etc.
Message sequencing needs to be described at two levels:
Once a CLUE session has been established, ADVERTISEMENTs and CONFIGUREs exchanged, and media is flowing, a provider may experience a change in state that has an effect on what it wishes or is able to provide. In this case it may need to alter what it is sending and/or send a new ADVERTISEMENT. In some cases it will be necessary to alter what is being sent without first sending a new ADVERTISEMENT and waiting for a CONFIGURE conforming to it.
The following is a non-exhaustive list of situations and recommended actions:
If the Consumer for some reason looses the CLUE state information how does it ask for an Advertisement from the provider? There could be multiple possibilities. A error code approach? However error codes would typically be associated with a NACK so it may not be good for a Config message. Maybe send a message which means “send me a complete update”. An alternative may be to release the connection or just do new signaling to establish a new CLUE session.
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 Section 4.
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.
CLUE messages are encoded in XML. The Data Model [I-D.presta-clue-data-model-schema] defines many/most of the elements from which CLUE messages are composed. This document specifies an XML schema that contains an element definition for each CLUE message, with much of the content of those elements being drawn from the Data Model.
This message contains XML representations of captures, capture scenes, encoding groups, and simultaneous sets using the types defined for those in the Data Model [I-D.presta-clue-data-model-schema].
The XML definition for this is element <advertisement> in section Section 3.7
[[ Currently this does not contain any representation of encodings. It assumes those will be defined in SDP. ]]
This message optionally contains an XML representations of captureEncodings using the type defined in the Data Model [I-D.presta-clue-data-model-schema]. A configure message with no captureEncodings indicates that no captures are requested.
[[ It currently also contains a reference to the request number of the advertisement it is based upon. Whether this should be present, or if it should implicitly reference the most recently acknowledged advertisement is TBD. ]]
The XML definition for this is element <configure> in section Section 3.7
Need for, and details of, the ACK message are TBD.
The XML element <response> in section Section 3.7 could serve as the representation, either with no reason element, or a reason element with a special value.
Need for, and details of, the NACK message are TBD.
The XML element <response> in section Section 3.7 could serve to as the representation, with the reason element providing the details. Then the code value in the reason element should map to the errors in section Section 3.2.3.
[[ The following is a first cut at a schema for the actual messages in the clue protocol. It uses <encodingGroups> from the data model but not <encodings>. Rather, it assumes that encodings are described in SDP as m-lines with a text identifier, and that the identifier has the same value as the encodingIDs embedded in the <encodingGroups>. If we stick with this the data model should be adjusted to agree, but until then it should "work". The SDP encoding of the identifier is TBD. Candidates are 'a=label:ID' and 'a=mid:ID'. ]]
For now there only <advertisement> and <configure> are defined. More messages will be needed for acknowledgment.
<?xml version="1.0" encoding="UTF-8" ?>
<xs:schema
targetNamespace="urn:ietf:params:xml:ns:clue-message"
xmlns:tns="urn:ietf:params:xml:ns:clue-message"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:dm="urn:ietf:params:xml:ns:clue-info"
xmlns="urn:ietf:params:xml:ns:clue-message"
elementFormDefault="qualified"
attributeFormDefault="unqualified">
<!-- Import data model schema -->
<xs:import namespace="urn:ietf:params:xml:ns:clue-info"
schemaLocation="clue-data-model-04-wip.xsd"/>
<!-- ELEMENT DEFINITIONS -->
<xs:element name="advertisement" type="advertisementMessageType"/>
<xs:element name="configure" type="configureMessageType"/>
<xs:element name="response" type="responseMessageType"/>
<!-- CLUE MESSAGE TYPE -->
<xs:complexType name="clueMessageType" abstract="true">
<xs:sequence>
<!-- mandatory fields -->
<!-- TBS: version info -->
</xs:sequence>
</xs:complexType>
<!-- CLUE REQUEST MESSAGE TYPE -->
<xs:complexType name="clueRequestMessageType" abstract="true">
<xs:complexContent>
<xs:extension base="clueMessageType">
<xs:sequence>
<!-- mandatory fields -->
<xs:element name="requestNumber" type="xs:integer"/>
</xs:sequence>
</xs:extension>
</xs:complexContent>
</xs:complexType>
<!-- CLUE RESPONSE MESSAGE TYPE -->
<xs:complexType name="clueResponseMessageType">
<xs:complexContent>
<xs:extension base="clueMessageType">
<xs:sequence>
<!-- mandatory fields -->
<xs:element name="requestNumber" type="xs:integer"/>
<!-- optional fields -->
<xs:element name="reason" type="reasonType" minOccurs="0"/>
<xs:any namespace="##other"
processContents="lax" minOccurs="0"/>
</xs:sequence>
</xs:extension>
</xs:complexContent>
</xs:complexType>
<!-- CLUE ADVERTISEMENT MESSAGE TYPE -->
<xs:complexType name="advertisementMessageType">
<xs:complexContent>
<xs:extension base="clueRequestMessageType">
<xs:sequence>
<!-- mandatory fields -->
<xs:element name="mediaCaptures"
type="dm:mediaCapturesType"/>
<xs:element name="encodingGroups"
type="dm:encodingGroupsType"/>
<!-- The encodings are defined via identifiers in the SDP,
referenced in encodingGroups -->
<xs:element name="captureScenes"
type="dm:captureScenesType"/>
<!-- optional fields -->
<xs:element name="simultaneousSets"
type="dm:simultaneousSetsType" minOccurs="0"/>
<xs:any namespace="##other"
processContents="lax" minOccurs="0"/>
</xs:sequence>
</xs:extension>
</xs:complexContent>
</xs:complexType>
<!-- CLUE CONFIGURE MESSAGE TYPE -->
<xs:complexType name="configureMessageType">
<xs:complexContent>
<xs:extension base="clueRequestMessageType">
<xs:sequence>
<!-- mandatory fields -->
<xs:element name="advertisementNumber" type="xs:integer"/>
<!-- advertisementNumber is requestNumber
of the advertisement-->
<!-- optional fields -->
<xs:element name="captureEncodings"
type="dm:captureEncodingsType" minOccurs="0"/>
<xs:any namespace="##other"
processContents="lax" minOccurs="0"/>
</xs:sequence>
</xs:extension>
</xs:complexContent>
</xs:complexType>
<!-- REASON TYPE -->
<xs:complexType name="reasonType">
<xs:simpleContent>
<xs:extension base="xs:string">
<xs:attribute type="xs:short" name="code" use="required"/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:schema>
Message framing is provided by the SCTP transport protocol. Each CLUE message is carried in one SCTP message.
The CLUE channel is usually offered in the first SIP O/A exchange between two parties in an intended CLUE session. The offer of the CLUE channel is the indicator that this SIP session is proposing to establish a CLUE session.
(However it is also acceptable to start with a non-CLUE SIP session and upgrade it to a CLUE session later.)
The mechanism for negotiating a DTLS/SCTP connection is specified in [I-D.ietf-mmusic-sctp-sdp]. We need to specify how to select the specific pair of SCTP streams that comprise the CLUE channel.
Any specific usage/conventions required for coordination of SDP offers and answers with the CLUE messages should also be described here.
(We have a draft [I-D.even-clue-sdp-clue-relation] that can contribute to this.)
[[ This is a straw horse, based on a proposal in [I-D.hansen-clue-sdp-interaction]. It remains unclear if this approach will work well, but we'll try it out and see how it develops. ]]
Providers signal available encodings in SDP sent to the consumer, rather than in an Advertisement message. Each encoding is described by an SDP media section containing an identifier. Encoding groups contained in Advertisement messages reference encodings by including the SDP identifier. Configure messages also reference encodings via the identifier when selecting capture encodings.
An encoding referenced by an encoding group can only be used to send/receive media if SDP defining the corresponding identifier was defined in the most recent offer/answer exchange. However a consumer may configure a capture encoding using an advertised encoding that is not currently defined in SDP. In this case it can be used if/when the provider defines the label in a subsequent offer/answer exchange. This provides flexibility in coordinating CLUE messages and SDP, but all encodings referenced by an Advertisement SHOULD be specified as soon as possible. When the SDP definition of an encoding is not available, the consumer has insufficient information to decide whether to select it.
[[ Using this approach the description of an encoding has all, and only, the descriptive capability provided by SDP. Also, for now this assumes a single capture per-m-line and no m-line bundling. We will want to relax those assumptions later. ]]
This should include state machines and/or call flows. These will illustrate, and then provide normative rules for valid sequences of messages of both types. For instance this needs to show when SDP offers and answers must occur relative to an ADVERTISEMENT or CONFIGURE message that requires SDP changes.
[THIS IS A VERY IMPORTANT PART OF THIS DOCUMENT!]
[This text is taken from [I-D.hansen-clue-sdp-interaction].]
This draft proposes that CLUE messages and SDP messages should be independent: parameters in CLUE messages MAY exceed values negotiated in SDP, or may make reference to SDP contents not present in the most recent offer/answer exchange. Without this provision, SDP and CLUE messages become part of a single negotiation, and a change on either by either side may necessitate an exchange of the other message type. For instance, removing stream information from SDP might first necessitate sending a new CLUE message removing the references to this stream. The state machine required to ensure validity of negotiation will be complicated, and there will be a number of invalid states which must be avoided. This is further complicated by the fact that, even if both ends of a call obey the constraints to ensure validity, a middle box may choose to rewrite an SDP such that an invalid state is reached.
Making the two message types independent significantly reduces the complexity of the state machines required. And with the message flows independent there is no way for an invalid state to occur when the two negotiations contain contradictory information. A cost of this is that endpoints will now need to deal with the fact that CLUE messages may contain parameters exceeding those negotiated in SDP, or referencing SDP content that does not exist. However, this is analogous to an issue endpoints already deal with in SDP. For instance, the sum of bandwidth parameters for various m-lines can exceed the overall session bandwidth. Not only is this not invalid, but it can be desirable, as it allows the sender to prioritise streams. What can be sent for any device is simply the intersection of what is permitted by the most recent SDP offer/answer, and the outcome of the CLUE negotiation; implementations should ignore references to entities in the other negotiation that do no exist.
This does not mean that there will be no interaction between SDP and CLUE messaging - a device wishing to add a new stream may well need to update both their SDP and their CLUE negotiations. However, there is no fixed order in which this must be done and no requirement for them to be updated in a particular order or fashion; it is left to the implementation to renegotiate the channels as it sees fit. If updates to both negotiations are required for a new stream to be added, then the new stream will not be available until both renegotiations are complete - the completion of the first renegotiation will have no effect.
[[ NOTE: this material is now out of date with the rest of the document. ]]
This is the case where two CLUE-capable endpoints are willing to set up a CLUE telepresence session. In the following, a possible approach addressing the problem is illustrated.
+----------+ +-----------+
| EP1 | | EP2 |
| | | |
+----+-----+ +-----+-----+
| |
| |
| INVITE (BASIC SDP+COMEDIA) |
|--------------------------------->|
| |
| |
| 200 0K (BASIC SDP+COMEDIA)|
|<---------------------------------|
| |
| |
| ACK |
|--------------------------------->|
| |
| |
| |
|<################################>|
| ?? BASIC SDP MEDIA SESSION ?? |
|<################################>|
| |
| |
| CLUE CTRL CHANNEL SETUP |
|<================================>|
| ... |
|<================================>|
| CLUE CTRL CHANNEL ESTABLISHED |
|<================================>|
| |
| |
| ADVERTISEMENT 1 |
|*********************************>|
| |
| |
| ADVERTISEMENT 2 |
|<*********************************|
| |
| |
| |
| CONFIGURE 1 |
|<*********************************|
| |
| |
| CONFIGURE 2 |
|*********************************>|
| |
| |
| |
| REINVITE (UPDATED SDP) |
|--------------------------------->|
| |
| |
| 200 0K (UPDATED SDP)|
|<---------------------------------|
| |
| |
| ACK |
|--------------------------------->|
| |
|<################################>|
| UPDATED SDP MEDIA SESSION |
|<################################>|
| |
| |
| |
| |
| |
v v
First, endpoint EP1 sends to endpoint EP2 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].
After the successful SIP O/A phase, EP1 and EP2 are able to exchange audio and video streams ("BASIC AUDIO AND VIDEO"). [RP: Is this channel needed at this point of the call flow?] [RP: which streams are sent on this channel in this moment?]
Moreover, another effect of the above successful SIP O/A phase, is the opening of the control channel. After the setup phase, the channel is established and CLUE protocol messages can flow above it.
CLUE protocol messages have not been formally defined yet. However, up to now there is a common agreement on their names and their main purposes, that should be following.
CLUE protocol ADVERTISEMENT messages are used to better describe the media provider's available streams in order to make the media consumer able to reproduce them in a more realistic fashion, as it is the main purpose of a telepresence session. These messages are needed since it is not possible in an agile fashion to describe spatial information and several further metadata about media captures via SDP.
In this document it is assumed that ADVERTISEMENT messages contain the full description of the sender's telepresence room in terms of available media capture and encoding capabilities. [RP: open issue - the mapping between what is described in the advertisement messages and media streams exchanged in the eventual basic SDP session already established]
CLUE protocol CONFIGURE messages are used to let the media consumer indicate to the media provider which are the available streams it is interested in, so that the media provider can send to the media consumer what it desires.
In the following, it is considered one of the possible call flow that can lead to the desired session configuration.
EP1 sends the ADVERTISEMENT message to EP2 (ADVERTISEMENT 1), which replies with a CONFIGURE message (CONFIGURE 1). After receiving the CONFIGURE message, EP1 assumes the CLUE offer/answer negotiation it started is completed. EP1 then can issue a REINVITE to EP2 with an SDP body updated accordingly to the CLUE messages exchange.
Similarly, EP2 sends its ADVERTISEMENT to EP1 (ADVERTISEMENT 2), which replies with a CONFIGURE (CONFIGURE 2).
EP1 re-negotiates the media involved in the existent session via a SIP REINVITE message to EP2. The SDP body within the REINVITE message reflects the negotiation carried on by the CLUE message exchange. In the case represented in figure, EP2 builds the 200 OK response also according to the second CLUE O/A negotiation.
In this example, one of the two involved endpoint (EP2) is not CLUE-capable, i.e., it is not able to use the CLUE protocol.
+----------+ +-----------+
| EP1 | | EP2 |
| | | |
+----+-----+ +-----+-----+
| |
| |
| INVITE (BASIC SDP+COMEDIA) |
|--------------------------------->|
| |
| |
| 200 0K (BASIC SDP + *NO* COMEDIA)|
|<---------------------------------|
| |
| |
| ACK |
|--------------------------------->|
| |
| |
| |
|<################################>|
| ?? BASIC SDP MEDIA SESSION ?? |
|<################################>|
| |
| |
| |
| REINVITE (UPDATED SDP) |
|--------------------------------->|
| |
| |
| 200 0K (UPDATED SDP)|
|<---------------------------------|
| |
| |
| ACK |
|--------------------------------->|
| |
|<################################>|
| UPDATED SDP MEDIA SESSION |
|<################################>|
| |
| |
| |
| |
| |
v v
Endpoint EP1 sends to endpoint EP2 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, as envisioned by the COMEDIA approach ("COMEDIA") for DTLS/SCTP channel [I-D.ietf-mmusic-sctp-sdp].
Since EP2 is not CLUE-capable, it answers with a 200 OK in which basic audio and video capabilities are accepted while the opening of the CLUE channel is rejected.
From such a response, EP1 understands the peer is not CLUE-capable. In this example, EP1 re-negotiates the session according to a pre-determined bulk of media streams to be sent to non-CLUE-capable endpoints.
Any SDP extensions required to support CLUE signaling should 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.
This may just describe how the degenerate form of the general mechanisms work for legacy devices. Or it may describe special case handling that we mandate as part of CLUE. Or it may just discuss non-normative things for implementors should consider.
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-09, February 2013. |
| [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-02, February 2013. |
| [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-03, January 2013. |
| [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. |
| [RFC4353] | Rosenberg, J., "A Framework for Conferencing with the Session Initiation Protocol (SIP)", RFC 4353, February 2006. |
| [I-D.even-clue-sdp-clue-relation] | Even, R., "Signalling of CLUE and SDP offer/answer", Internet-Draft draft-even-clue-sdp-clue-relation-01, October 2012. |
| [I-D.even-clue-rtp-mapping] | Even, R. and J. Lennox, "Mapping RTP streams to CLUE media captures", Internet-Draft draft-even-clue-rtp-mapping-05, February 2013. |
| [I-D.hansen-clue-sdp-interaction] | Hansen, R., "SDP and CLUE message interactions", Internet-Draft draft-hansen-clue-sdp-interaction-00, February 2013. |