MMUSIC K. Drage, Ed. Internet-Draft M. Makaraju Intended status: Standards Track Alcatel-Lucent Expires: August 18, 2014 R. Ejzak J. Marcon Unaffiliated February 14, 2014 SDP-based "SCTP over DTLS" data channel negotiation draft-ejzak-mmusic-data-channel-sdpneg-00 Abstract The Real-Time Communication in WEB-browsers (RTCWeb) working group is charged to provide protocols to support direct interactive rich communication using audio, video, and data between two peers' web- browsers. For the support of data communication, the RTCWeb working group has in particular defined the concept of bi-directional data channels over SCTP, where each data channel might be used to transport other protocols, called sub-protocols. Data channel setup can be done using either the in-band WebRTC Data Channel protocol or some external (in-band or out-of-band) negotiation. This document specifies how the SDP offer/answer exchange can be used to achieve such an external negotiation. Even though data channels are designed for RTCWeb use initially they may be used by other protocols like, but not limited to, the CLUE protocol. This document is intended to be used whereever data channels are used. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on August 18, 2014. Copyright Notice Drage, et al. Expires August 18, 2014 [Page 1] Internet-Draft SDP-based data channel negotiation February 2014 Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Data Channels . . . . . . . . . . . . . . . . . . . . . . . . 4 4.1. Stream identifier numbering . . . . . . . . . . . . . . . 5 4.2. Generic external negotiation . . . . . . . . . . . . . . . 5 4.2.1. Overview . . . . . . . . . . . . . . . . . . . . . . . 5 4.2.2. Opening a data channel . . . . . . . . . . . . . . . . 6 4.2.3. Closing a data channel . . . . . . . . . . . . . . . . 7 5. SDP-based external negotiation . . . . . . . . . . . . . . . . 7 5.1. SDP syntax . . . . . . . . . . . . . . . . . . . . . . . . 7 5.1.1. SDP attribute for data channel parameter negotiation . . . . . . . . . . . . . . . . . . . . . 7 5.1.1.1. stream parameter . . . . . . . . . . . . . . . . . 8 5.1.1.2. label parameter . . . . . . . . . . . . . . . . . 8 5.1.1.3. subprotocol parameter . . . . . . . . . . . . . . 8 5.1.1.4. max_retr parameter . . . . . . . . . . . . . . . . 9 5.1.1.5. max_time parameter . . . . . . . . . . . . . . . . 9 5.1.1.6. unordered parameter . . . . . . . . . . . . . . . 9 5.1.2. Sub-protocol specific attributes . . . . . . . . . . . 9 5.2. Procedures . . . . . . . . . . . . . . . . . . . . . . . . 10 5.2.1. Managing stream identifiers . . . . . . . . . . . . . 10 5.2.2. Opening a data channel . . . . . . . . . . . . . . . . 11 5.2.3. Closing a data channel . . . . . . . . . . . . . . . . 13 6. Security Considerations . . . . . . . . . . . . . . . . . . . 13 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 9.1. Normative References . . . . . . . . . . . . . . . . . . . 13 9.2. Informative References . . . . . . . . . . . . . . . . . . 14 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15 Drage, et al. Expires August 18, 2014 [Page 2] Internet-Draft SDP-based data channel negotiation February 2014 1. Introduction The RTCWeb working group has defined the concept of bi-directional data channels running on top of SCTP/DTLS. RTCWeb leaves it openfor other applications to use data channels and its in-band or out-of- band protocol for creating them. Each data channel consists of paired SCTP streams sharing the same SCTP Stream Identifier. Data channels are created by endpoint applications through the WebRTC API, or other users of data channel like CLUE, and can be used to transport proprietary or well-defined protocols, which in the latter case can be signaled by the data channel "sub-protocol" parameter, conceptually similar to the WebSocket "sub-protocol". However, apart from the "sub-protocol" value transmitted to the peer, RTCWeb leaves it open how endpoint applications can agree on how to instantiate a given sub-protocol on a data channel, and whether it is signaled in- band or out-of-band (or both). In particular, the SDP offer generated by the browser includes no channel-specific information. This document defines SDP-based out-of-band negotiation procedures to establish data channels for transport of well-defined sub-protocols. 2. Conventions 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]. 3. Terminology This document uses the following terms: Data channel: A bidirectional channel consisting of paired SCTP outbound and inbound streams. Data channel stack: An entity which, upon application request, runs data channel protocol to keep track of states, sending and receive data. If the application is browser based Javascript application then this stack resides in the browser. If the application is a native application then this stack resides in application and accessible to it via some sort of APIs. Data channel properties: fixed properties assigned to a data channel at the time of its creation. Some of these properties determine the way the data channel stack transmits data on this channel (e.g., stream identifier, reliability, order of delivery...) Drage, et al. Expires August 18, 2014 [Page 3] Internet-Draft SDP-based data channel negotiation February 2014 External negotiation: data channel negotiation based on out-of- band or in-band mechanisms other than the data channel protocol. In-band: transmission through the peer-to-peer SCTP association. In-band negotiation: data channel negotiation based on the in-band data channel protocol defined in [I-D.ietf-rtcweb-data-protocol]. Out-of-band: transmission through the application signaling path. Peer: From the perspective of one of the agents in a session, its peer is the other agent. Specifically, from the perspective of the SDP offerer, the peer is the SDP answerer. From the perspective of the SDP answerer, the peer is the SDP offerer. Stream identifier: the identifier of the outbound and inbound SCTP streams composing a data channel. 4. Data Channels This section summarizes how data channels work in general. Note that the references to 'browser' here is intentional as in this specific example the data channel user is a webrtc enabled browser. A WebRTC application creates a data channel via the Data Channel API, by providing a number of setup parameters (sub-protocol, label, reliability, order of delivery, priority). The application also specifies if it wants to make use of the in-band negotiation using the data protocol, or if the application intends to perform an "external negotiation" using some other in-band or out-of-band mechanism. In any case, the SDP offer generated by the browser is per [I-D.ietf-mmusic-sctp-sdp]. In brief, it contains one m-line for the SCTP association on top of which data channels will run, and one attribute per protocol assigned to the SCTP ports: OPEN ISSUE: The syntax in [I-D.ietf-mmusic-sctp-sdp] may change as that document progresses. In particular we expect "webrtc- datachannel" to become a more general term. m=application 54111 DTLS/SCTP 5000 5001 5002 c=IN IP4 79.97.215.79 a=sctpmap:5000 webrtc-datachannel a=sctpmap:5001 bfcp a=sctpmap:5002 t38 Drage, et al. Expires August 18, 2014 [Page 4] Internet-Draft SDP-based data channel negotiation February 2014 Note: A WebRTC browser will only create an sctpmap attribute for the webrtc-datachannel protocol, and will not create sctpmap attributes for other protocols such as bfcp or t38. This example shows the hypothetical power of the syntax to support multiplexing of SCTP associations for different protocols on the same DTLS connection. Note: This SDP syntax does not contain any channel-specific information. 4.1. Stream identifier numbering Independently from the requested type of negotiation, the application creating a data channel can either pass to the browser the stream identifier to assign to the data channel or else let the browser pick one identifier from the ones unused. To avoid glare situations, each endpoint can moreover own an exclusive set of stream identifiers, in which case an endpoint can only create a data channel with a stream identifier it owns. Which set of stream identifiers is owned by which endpoint is determined by convention or other means. For data channels negotiated in-band, one endpoint owns by convention the even stream identifiers, whereas the other owns the odd stream identifiers, as defined in [I-D.ietf-rtcweb-data-protocol]. For data channels externally negotiated, no convention is defined by default. 4.2. Generic external negotiation 4.2.1. Overview In-band negotiation only provides for negotiation of data channel transport parameters and does not provide for negotiation of sub- protocol specific parameters. External negotiation can be defined to allow negotiation of parameters beyond those handled by in-band negotiation, e.g., parameters specific to the sub-protocol instantiated on a particular data channel. See Section 5.1.2 for an example of such a parameter. The following procedures are common to all methods of external negotiation, whether in-band (communicated using proprietary means on an already established data channel) or out-of-band (using SDP or some other protocol associated with the signaling channel). Drage, et al. Expires August 18, 2014 [Page 5] Internet-Draft SDP-based data channel negotiation February 2014 4.2.2. Opening a data channel In the case of external negotiation, the endpoint application has the option to fully control the stream identifier assignments. However these assignments have to coexist with the assignments controlled by the data channel stack for the in-band negotiated data channels (if any). It is the responsibility of the application to ensure consistent assignment of stream identifiers. When the application requests the creation of a new data channel to be set up via external negotiation, the data channel stack creates the data channel locally without sending any DATA CHANNEL OPEN message in-band, and sets the data channel state to Connecting if the SCTP association is not yet established, or sets the data channel state to Open if the SCTP association is already established. The side which starts external negotiation creates data channel using underlying data channel stack API and the data channel is put into open state immediately (assuming ICE, SCTP procedures were already done). However, the application can't send data on this data channel until external negotiation is complete with the peer. This is because peer needs to be aware and accept the data channel via external negotiation. The peer after accepting the data channel offer can start sending data immediately. This implies that offerer may get data channel message before external negotiation is complete and the application should be ready to handle it. If the peer rejects the data channel part of the offer then it doesn't have to do anything as the data channel was not created using the stack. The offerer on the other hand need to close the data channel that was opened by invoking relevant data channel stack API. It is also worth noting that a data channel stack implementation may not provide any API to create and close data channels; instead the data channels are used on the fly as needed just by communicating via external means or by even having some local configuration/assumptions on both the peers. The application then externally negotiates the data channel properties and sub-protocol properties with the peer's application. [ASSUMPTION] The peer must then symmetrically create a data channel with these negotiated data channel properties. This is the only way for the peer's data channel stack to know which properties to apply when transmitting data on this channel. The data channel stack must allow data channel creation with any non-conflicting stream identifier so that both peers can create the data channel with the same stream identifier. Drage, et al. Expires August 18, 2014 [Page 6] Internet-Draft SDP-based data channel negotiation February 2014 In case the external negotiation is correlated with an SDP offer/ answer exchange that establishes the SCTP association, the SCTP initialization completion triggers a callback from the data channel stack to an application on both the ends to change the data channel state from Connecting to Open. The details of this interface is specific to the data channel user application. Browser based applications (could include hybrid apps) will use [http://www.w3.org/TR/webrtc/], while native applications use a compatible API, which is yet to be specified. See Section 5.2.2 for details on when the data channel stack can assume the data channel is open, and on when the application can assume the data channel is open. 4.2.3. Closing a data channel When the application requests the closing of an externally negotiated data channel, the data channel stack always performs an in-band SSN reset for this channel. Depending upon the method used for external negotiation and the sub- protocol associated with the data channel, the closing might in addition be signaled to the peer via external negotiation. 5. SDP-based external negotiation This section defines a method of external negotiation by which two clients can negotiate data channel-specific and sub-protocol-specific parameters, using the out-of-band SDP offer/answer exchange. 5.1. SDP syntax Two new SDP attributes are defined to support external negotiation of data channels. The first attribute provides for negotiation of channel-specific parameters. The second attribute provides for negotiation of sub-protocol-specific parameters. 5.1.1. SDP attribute for data channel parameter negotiation Associated with the m line that defines the SCTP association for data channels (defined in Section 4), each SDP offer and answer includes an attribute line that defines the data channel parameters for each data channel to be negotiated. Each attribute line specifies the following parameters for a data channel: Stream Identifier, sub- protocol, label, reliability, order of delivery, and priority. The following is an example of the attribute line for sub-protocol "BFCP" and stream id "2" : Drage, et al. Expires August 18, 2014 [Page 7] Internet-Draft SDP-based data channel negotiation February 2014 a=dcmap:5000 stream=2;label="channel 2"; \ subprotocol="BFCP";max_retr=3 This line MUST be replicated without changes in the SDP answer, if the answerer accepts the offered data channel. This line MUST be replicated without changes in any subsequent offer or answer, as long as the data channel is still opened at the time of offer or answer generation. Note: This attribute was defined in old version 03 of the following draft but was removed along with any support for SDP external negotiation in subsequent versions: [I-D.ietf-mmusic-sctp-sdp]. Note: This document does not provide a complete specification of how to negotiate the use of a data channel to transport BFCP. Procedures specific to each sub-protocol such as BFCP will be documented elsewhere. The use of BFCP is only an example of how the generic procedures described herein might apply to a specific sub-protocol. 5.1.1.1. stream parameter The 'stream' parameter indicates the actual stream identifier within the association used to form the channel. Stream is a mandatory parameter. stream-attr = "stream=" streamidentifier streamidentifier = 1*DIGIT 5.1.1.2. label parameter The optional 'label' parameter indicates the name of the channel. It represents a label that can be used to distinguish, in the context of the WebRTC API, an RTCDataChannel object from other RTCDataChannel objects. Label is a mandatory parameter. label-attr = "label=" labelstring labelstring = text text = byte-string 5.1.1.3. subprotocol parameter The 'subprotocol' parameter indicates which protocol the client expects to exchange via the channel. Subprotocol is a mandatory parameter. Drage, et al. Expires August 18, 2014 [Page 8] Internet-Draft SDP-based data channel negotiation February 2014 subprotocol-attr = "subprotocol=" labelstring labelstring = text text = byte-string [ACTION ITEM] The IANA registry to be used for the subprotocol parameter is still to be determined. It also needs to be determined what the relationship is to existing registries and how to reference already-existing protocols. 5.1.1.4. max_retr parameter The 'max_retr' parameter indicates the max times a user message will be retransmitted. The max_retr parameter is optional with default value unbounded. maxretr-attr = "maxretr=" maxretrvalue maxretrvalue = 1*DIGIT 5.1.1.5. max_time parameter A user messages will no longer be transmitted or retransmitted after a specified life-time given in milliseconds in the 'max_time' parameter. The max_time parameter is optional with default value unbounded. If the data channel is reliable then the SCTP association will be closed after 'maxtime' if data is still not transmitted successfully. This also means it impacts other streams. If the data channel is unreliable then the SCTP association stays up after 'maxtime' even if the data not is successfully transmitted. maxtime-attr = "maxtime=" maxtimevalue maxtimevalue = 1*DIGIT 5.1.1.6. unordered parameter The 'unordered' parameter indicates that DATA chunks in the channel MUST be dispatched to the upper layer by the receiver without any attempt to reorder. The unordered parameter is optional. If the unordered parameter is absent, the receiver is required to deliver DATA chunks to the upper layer in proper order. 5.1.2. Sub-protocol specific attributes In the SDP, each data channel declaration MAY also be followed by other SDP attributes specific to the sub-protocol in use. Each of these attributes is represented by one new attribute line, and it includes the contents of a media-level SDP attribute already defined for use with this (sub)protocol in another IETF specification. Sub- protocol-specific attributes might also be defined for exclusive use Drage, et al. Expires August 18, 2014 [Page 9] Internet-Draft SDP-based data channel negotiation February 2014 with data channel transport, but should use the same syntax described here for other sub-protocol-specific attributes. Each sub-protocol specific SDP attribute that would normally be used to negotiate the subprotocol using SDP is replaced with an attribute of the form "a=dcsa:sctp-port:stream-id original-attribute", where dcsa stands for "data channel sub-protocol attribute", sctp-port is the sctp port number assigned for data channel on the media line, stream-id is the sctp stream identifier assigned to this sub-protocol instance, and original-attribute represents the contents of the sub- protocol related attribute to be included. a=dcmap:5000 stream=2;label="channel 2"; \ subprotocol="MSRP";max_retr=3 a=dcsa:5000:2 accept-types:text/plain Thus in the example above, the original attribute line "a=accept- types:text/plain" is represented by the attribute line "a=dcsa:5000:2 accept-types:text/plain", which specifies that this instance of MSRP being transported on the sctp association using port number 5000 and the data channel with stream id 2 accepts plain text files. As opposed to the data channel setup parameters, these parameters are subject to offer/answer negotiation following the procedures defined in the sub-protocol specific documents. The same syntax applies to any other SDP attribute required for negotiation of this instance of the sub-protocol. Note: This document does not provide a complete specification of how to negotiate the use of a data channel to transport MSRP. Procedures specific to each sub-protocol such as MSRP will be documented elsewhere. The use of MSRP is only an example of how the generic procedures described herein might apply to a specific sub-protocol. 5.2. Procedures 5.2.1. Managing stream identifiers For the SDP-based external negotiation described in this document, the initial offerer based "SCTP over DTLS" owns by convention the even stream identifiers whereas the initial answerer owns the odd stream identifiers. This ownership is invariant for the whole lifetime of the signaling session, e.g. it does not change if the initial answerer sends a new offer to the initial offerer. [ACTION ITEM] This convention is different from the convention currently defined for in-band negotiation, where even/odd assignment Drage, et al. Expires August 18, 2014 [Page 10] Internet-Draft SDP-based data channel negotiation February 2014 is determined by DTLS role. Since DTLS role cannot be determined until after the initial SDP offer/answer is complete, this convention cannot be used for external negotiation. It might be appropriate to change the convention for stream identifier assignment for in-band negotiation for consistency with external negotiation. Otherwise it might be necessary to prohibit simultaneous use of in-band and external negotiation for data channels. 5.2.2. Opening a data channel The procedure for opening a data channel using external negotiation starts with the agent preparing to send an SDP offer. If a peer receives an SDP offer before getting to send a new SDP offer with data channels that are to be externally negotiated, or loses an SDP offer glare resolution procedure in this case, it must wait until the ongoing SDP offer/answer completes before resuming the external negotiation procedure. The agent that intends to send an SDP offer to create data channels through SDP-based external negotiation performs the following: o Creates data channels using stream identifiers from the owned set (see Section 5.2.1). o As described in Section 4.2.2, if the SCTP association is not yet established, then the newly created data channels are in the Connecting state, else if the SCTP association is already established, then the newly created data channels are in the Open state. o Generates a new SDP offer. In the case of the browser based applications browser generates the offer via createOffer() API call[JSEP reference here]. o Determines the list of stream identifiers assigned to data channels opened through external negotiation. o Completes the SDP offer with the dcmap and dcsa attributes needed for each externally-negotiated data channel, as described in Section 5.1. o Sends the SDP offer. The peer receiving such an SDP offer performs the following: o Applies the SDP offer. Note that the browser ignores data channel specific attributes in the SDP. Drage, et al. Expires August 18, 2014 [Page 11] Internet-Draft SDP-based data channel negotiation February 2014 o Analyzes the channel parameters and sub-protocol attributes to determine whether to accept each offered data channel. o For accepted data channels, creates peer instances for the data channels with the browser using the channel parameters described in the SDP offer. Note that the browser is asked to create data channels with stream identifiers not "owned" by the agent. o As described in Section 4.2.2, if the SCTP association is not yet established, then the newly created data channels are in the Connecting state, else if the SCTP association is already established, then the newly created data channels are in the Open state. o Generates an SDP answer. o Completes the SDP answer with the dcmap and dcsa attributes needed for each externally-negotiated data channel, as described in Section 5.1. o Sends the SDP answer. The agent receiving such an SDP answer performs the following: o Closes any created data channels (whether in Connecting or Open state) for which the expected dcmap and dcsa attributes are not present in the SDP answer. o Applies the SDP answer. Any data channels in Connecting state are transitioned to the Open state when the SCTP association is established. Each agent application MUST wait to send data until it has confirmation that the data channel at the peer is in the Open state. For webrtc, this is when both data channel stacks have channel parameters instantiated. This occurs: o At both peers when a data channel is created without an established SCTP association, as soon as the data channel stacks report that the data channel transitions to the Open state from the Connecting state. o At the agent receiving an SDP offer for which there is an established SCTP association, as soon as it creates an externally negotiated data channel in the Open state based on information signaled in the SDP offer. Drage, et al. Expires August 18, 2014 [Page 12] Internet-Draft SDP-based data channel negotiation February 2014 o At the agent sending an SDP offer to create a new externally negotiated data channel for which there is an established SCTP association, when it receives the SDP answer confirming acceptance of the data channel or when it begins to receive data on the data channel from the peer, whichever occurs first. 5.2.3. Closing a data channel When the application requests the closing of a data channel that was externally negotiated, the browser always performs an in-band SSN reset for this channel. It is specific to the sub-protocol whether this closing must in addition be signaled to the peer via a new SDP offer/answer exchange. The application must also close any data channel that was externally negotiated, for which the stream identifiers are not listed in an incoming SDP offer. 6. Security Considerations No security considerations are envisaged beyond those already documented in [RFC4566] 7. IANA Considerations To be completed. 8. Acknowledgments The authors wish to acknowledge the borrowing of ideas from other internet drafts by Salvatore Loreto, Gonzalo Camarillo, Peter Dunkley and Gavin Llewellyn, and to thank Paul Kyzivat, Jonathan Lennox, Uwe Rauschenbach and Keith Drage for their invaluable comments. 9. References 9.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. Drage, et al. Expires August 18, 2014 [Page 13] Internet-Draft SDP-based data channel negotiation February 2014 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002. [I-D.ietf-rtcweb-jsep] Uberti, J. and C. Jennings, "Javascript Session Establishment Protocol", draft-ietf-rtcweb-jsep-06 (work in progress), February 2014. [I-D.ietf-rtcweb-data-channel] Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data Channels", draft-ietf-rtcweb-data-channel-07 (work in progress), February 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)", draft-ietf-mmusic-sctp-sdp-06 (work in progress), February 2014. [WebRtcAPI] Bergkvist, A., Burnett, D., Narayanan, A., and C. Jennings, "WebRTC 1.0: Real-time Communication Between Browsers", World Wide Web Consortium WD WD-webrtc- 20120821, August 2012, . 9.2. Informative References [I-D.ietf-rtcweb-data-protocol] Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data Channel Establishment Protocol", draft-ietf-rtcweb-data-protocol-03 (work in progress), February 2014. [RFC4975] Campbell, B., Mahy, R., and C. Jennings, "The Message Session Relay Protocol (MSRP)", RFC 4975, September 2007. [RFC4976] Jennings, C., Mahy, R., and A. Roach, "Relay Extensions for the Message Sessions Relay Protocol (MSRP)", RFC 4976, September 2007. [RFC5547] Garcia-Martin, M., Isomaki, M., Camarillo, G., Loreto, S., and P. Kyzivat, "A Session Description Protocol (SDP) Offer/Answer Mechanism to Enable File Transfer", RFC 5547, May 2009. [RFC6135] Holmberg, C. and S. Blau, "An Alternative Connection Model Drage, et al. Expires August 18, 2014 [Page 14] Internet-Draft SDP-based data channel negotiation February 2014 for the Message Session Relay Protocol (MSRP)", RFC 6135, February 2011. [RFC6714] Holmberg, C., Blau, S., and E. Burger, "Connection Establishment for Media Anchoring (CEMA) for the Message Session Relay Protocol (MSRP)", RFC 6714, August 2012. Authors' Addresses Keith Drage (editor) Alcatel-Lucent Quadrant, Stonehill Green, Westlea Swindon UK Email: keith.drage@alcatel-lucent.com Raju Makaraju Alcatel-Lucent 2000 Lucent Lane Naperville, Illinois US Email: Raju.Makaraju@alcatel-lucent.com Richard Ejzak Unaffiliated Email: richard.ejzak@gmail.com Jerome Marcon Unaffiliated Drage, et al. Expires August 18, 2014 [Page 15]