RFC 8850

Controlling Multiple Streams for Telepresence (CLUE) Protocol Data Channel

📅 published January 2021, from ietf

RFC 8850 🎀 Controlling Multiple Streams for Telepresence (CLUE) Protocol Data Channel

okay bestie, title block first 💅

RFC number: 8850
Title: Controlling Multiple Streams for Telepresence (CLUE) Protocol Data Channel
Author: C. Holmberg, Ericsson
Stream: Internet Engineering Task Force (IETF)
Category: Experimental 🧪
Date: January 2021
ISSN: 2070-1721
Obsoletes / Updates: the title block doesn't list anything it obsoletes or updates 🙈

Abstract ✨

so this doc is about using the WebRTC data channel mechanism to make a data channel, called a Controlling Multiple Streams for Telepresence (CLUE) data channel 🦋. Its whole job is carrying CLUE protocol messages between two CLUE entities. iconic, right?

Status of This Memo 📜

this one is not an Internet Standards Track spec 😭. It's published so people can look at it, try it out in experimental implementations, and evaluate it.

This document defines an Experimental Protocol for the Internet community. It's a product of the Internet Engineering Task Force (IETF) and it represents the consensus of the IETF community. It got public review and was approved for publication by the Internet Engineering Steering Group (IESG). Not every document the IESG approves is a candidate for any level of Internet Standard, so check Section 2 of RFC 7841 if you want the details 🧐.

For the current status, any errata, and how to send feedback, go to https://www.rfc-editor.org/info/rfc8850 🔗.

Copyright Notice ⚖️

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please read these carefully, because they spell out your rights and restrictions for 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 they're provided without warranty as described in the Simplified BSD License.

Table of Contents 🗂️

1.  Introduction
2.  Conventions
3.  CLUE Data Channel
  3.1.  General
  3.2.  SCTP Considerations
    3.2.1.  General
    3.2.2.  SCTP Payload Protocol Identifier (PPID)
    3.2.3.  Reliability
    3.2.4.  Order
    3.2.5.  Stream Reset
    3.2.6.  SCTP Multihoming
    3.2.7.  Closing the CLUE Data Channel
  3.3.  SDP Considerations
    3.3.1.  General
    3.3.2.  SDP dcmap Attribute
    3.3.3.  SDP dcsa Attribute
    3.3.4.  Example
4.  Security Considerations
5.  IANA Considerations
  5.1.  Subprotocol Identifier "clue"
6.  References
  6.1.  Normative References
  6.2.  Informative References
Acknowledgements
Author's Address

1. Introduction 🌸

okay so this doc explains how to use the WebRTC data channel mechanism [RFC8831] to build a CLUE data channel. That's the pipe that carries CLUE protocol messages [RFC8847] between two CLUE entities. Literally the main character of this whole RFC 👑.

It also explains how to describe the SCTPoDTLS association [RFC8261] (we also call it "SCTP over DTLS" here) that's used to make the CLUE data channel. It does that with the Session Description Protocol (SDP) [RFC4566], and it covers the SDP-based "SCTP over DTLS" data channel negotiation mechanism [RFC8864] too 🫶. ("SCTP" stands for "Stream Control Transmission Protocol", fyi.) It includes the SCTP stuff specific to a CLUE data channel, the values for the SDP media description ("m=" line), and the SDP attributes that are specific to a CLUE data channel.

The details and procedures for the CLUE protocol, plus the SDP Offer/Answer procedures [RFC3264] for negotiating a CLUE data channel, are out of scope here 🙅‍♀️.

NOTE: The usage of the Data Channel Establishment Protocol (DCEP) [RFC8832] for establishing a CLUE data channel is outside the scope of this document.

2. Conventions 📖

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. (so only the caps versions count as the official keywords, bestie 💜)

Some terms, so we're all on the same page:

SCTPoDTLS association 🫧 Refers to an SCTP association carried over a DTLS connection [RFC8261].

WebRTC data channel 🌈 Refers to a pair of SCTP streams over an SCTPoDTLS association that's used to transport non-media data between two entities, as defined in [RFC8831].

CLUE data channel 🎀 Refers to a WebRTC data channel realization [RFC8831], with a specific set of SCTP characteristics, whose purpose is transporting CLUE protocol messages [RFC8847] between two CLUE entities.

CLUE entity 🧚 Refers to a SIP User Agent (UA) [RFC3261] that supports the CLUE data channel and the CLUE protocol.

CLUE session 🍓 Refers to a SIP session [RFC3261] between two SIP UAs, where a CLUE data channel associated with that SIP session has been established between the SIP UAs.

SCTP stream 🦋 Defined in [RFC4960] as a unidirectional logical channel established from one associated SCTP endpoint to another, within which all user messages are delivered in sequence except for those submitted to the unordered delivery service.

SCTP stream identifier 🔢 Defined in [RFC4960] as an unsigned integer. Identifies an SCTP stream.

3. CLUE Data Channel 💖

3.1. General

this section is all about making a CLUE data channel with the WebRTC data channel mechanism. It covers the set of SCTP characteristics specific to a CLUE data channel, the values of the "m=" line that describe the SCTPoDTLS association behind the WebRTC data channel, and how to use the SDP-based "SCTP over DTLS" data channel negotiation mechanism to create the CLUE data channel 🛠️.

As described in [RFC8831], the SCTP streams that make up a WebRTC data channel must be associated with the same SCTP association. On top of that, both SCTP streams in the WebRTC data channel must use the same SCTP stream identifier value. These rules also apply to a CLUE data channel 🔁.

Within a given CLUE session, a CLUE entity MUST use a single CLUE data channel for transport of all CLUE messages towards its peer. 🫶

3.2. SCTP Considerations

3.2.1. General

As described in [RFC8831], different SCTP options (for example, the ones about ordered delivery) can be used for a data channel. This section covers the SCTP options used for a CLUE data channel, and Section 3.3 explains how those SCTP options get signaled using SDP ✨.

3.2.2. SCTP Payload Protocol Identifier (PPID)

A CLUE entity MUST use the PPID value 51 when sending a CLUE message on a CLUE data channel 🔢.

NOTE: As described in [RFC8831], the PPID value 51 indicates that the SCTP message contains data encoded in UTF-8 format. The PPID value 51 does not indicate which application protocol the SCTP message is associated with -- only the format in which the data is encoded.

3.2.3. Reliability

The SCTP usage for the CLUE data channel ensures reliable transport of CLUE protocol messages [RFC8847]. no drops, periodt 💅.

[RFC8831] requires support for the partial reliability extension defined in [RFC3758] and the limited retransmission policy defined in [RFC7496]. But a CLUE entity MUST NOT use these extensions, because messages are required to always be sent reliably 🚫. And if a CLUE entity detects that its peer uses any of these extensions, it MUST terminate the session 🚨.

3.2.4. Order

A CLUE entity MUST use the ordered delivery SCTP service, as described in [RFC4960], for the CLUE data channel 📬.

3.2.5. Stream Reset

A CLUE entity MUST support the stream reset extension defined in [RFC6525] 🔄.

Per [RFC8831], the dynamic address reconfiguration extension parameter ('Supported Extensions Parameter') defined in [RFC5061] must be used to signal support for the stream reset extension defined in [RFC6525]. Other features defined in [RFC5061] MUST NOT be used for CLUE data channels 🙅‍♀️.

3.2.6. SCTP Multihoming

SCTP multihoming is not supported for SCTPoDTLS associations, so it can't be used for a CLUE data channel. no multihoming for this girl 💀.

3.2.7. Closing the CLUE Data Channel

As described in [RFC8831], to close a data channel, an entity sends an SCTP reset message [RFC6525] on its outgoing SCTP stream associated with the data channel. When the remote peer gets that reset message, it also sends a reset message on its outgoing SCTP stream for the data channel (unless it already sent one) 👋. The SCTPoDTLS association, and any other data channels on the same association, aren't affected by the SCTP reset messages 🌷.

3.3. SDP Considerations

3.3.1. General

This section explains how to (1) build the SDP media description ("m=" line) that describes the SCTPoDTLS association used to realize a CLUE data channel, and (2) use the SDP-based "SCTP over DTLS" data channel negotiation mechanism [RFC8864] to set up a CLUE data channel on that SCTPoDTLS association 📝.

NOTE: Protocols other than SDP for negotiating usage of an SCTPoDTLS association for realizing a CLUE data channel are outside the scope of this specification.

[RFC8848] describes the SDP Offer/Answer procedures for negotiating a CLUE session, including the CLUE-controlled media streams and the CLUE data channel 🤝.

3.3.1.1. SDP Media Description Fields

[RFC8841] defines how to set the values of an "m=" line that describes an SCTPoDTLS association. As RFC 8841 defines, for a CLUE data channel the values are set like this 👇:

  +===============+==========+============+======================+
  |     media     |   port   |   proto    |         fmt          |
  +===============+==========+============+======================+
  | "application" | UDP port | "UDP/DTLS/ | "webrtc-datachannel" |
  |               |  value   |   SCTP"    |                      |
  +---------------+----------+------------+----------------------+
  | "application" | TCP port | "TCP/DTLS/ | "webrtc-datachannel" |
  |               |  value   |   SCTP"    |                      |
  +---------------+----------+------------+----------------------+

                  Table 1: SDP "proto" Field Values

CLUE entities SHOULD NOT transport the SCTPoDTLS association used to realize the CLUE data channel over TCP (using the "TCP/DTLS/SCTP" proto value), unless it is known that UDP/DTLS/SCTP will not work. For example, that happens when the Interactive Connectivity Establishment (ICE) mechanism [RFC8445] is used and the ICE procedures determine that TCP transport is required 🧊.

3.3.1.2. SDP sctp-port Attribute

As defined in [RFC8841], the SDP sctp-port attribute value is set to the SCTP port of the SCTPoDTLS association. A CLUE entity can pick any valid SCTP port value [RFC8841] 🎯.

3.3.2. SDP dcmap Attribute 🎀

okay bestie, the SDP dcmap attribute [RFC8864] lives on the m= line that describes the SCTPoDTLS association carrying the WebRTC data channel, and its values get set like this 💖

+===========+=============+=============+=======+========+==========+
| stream-id | subprotocol |    label    |ordered|max-retr| max-time |
+===========+=============+=============+=======+========+==========+
|  Value of |    "CLUE"   | Application | "true"|  N/A   |   N/A    |
|  the SCTP |             |   specific  |       |        |          |
|stream used|             |             |       |        |          |
| to realize|             |             |       |        |          |
|  the CLUE |             |             |       |        |          |
|    data   |             |             |       |        |          |
|  channel  |             |             |       |        |          |
+-----------+-------------+-------------+-------+--------+----------+

Table 2: SDP dcmap Attribute Values

🚨 NOTE: CLUE entities are required to use ordered SCTP message delivery, with full reliability, according to the procedures in [RFC8864]. So the max-retr and max-time attribute parameters are not used when negotiating CLUE data channels. Those two are giving "not needed here" energy 💅

3.3.3. SDP dcsa Attribute ✨

The SDP dcsa attribute [RFC8864] is not used when establishing a CLUE data channel. Skip it, no cap 🙈

3.3.4. Example 🧚

Figure 1 is an SDP media description for a CLUE data channel. It's the whole vibe in three lines, so cute 🦋 Complete SDP examples can be found in [RFC8848].

        m=application 54111 UDP/DTLS/SCTP webrtc-datachannel
        a=sctp-port: 5000
        a=dcmap:2 subprotocol="CLUE";ordered=true

Figure 1: SDP Media Description for a CLUE Data Channel

4. Security Considerations 🔐

Okay this part is SERIOUS bestie 😭 This specification relies on the security properties of the WebRTC data channel described in [RFC8831], including reliance on DTLS. Since CLUE sessions are established using SIP/SDP, protecting the data channel against message modification and recovery requires the use of SIP authentication and authorization mechanisms described in [RFC3261] for session establishment prior to establishing the data channel. So lock the front door with SIP auth before the data channel even shows up, or the whole thing is giving vulnerable 💀🔥

5. IANA Considerations 🌈

5.1. Subprotocol Identifier "clue" 💜

This document adds the subprotocol identifier "clue" to the "WebSocket Subprotocol Name Registry", and the registration looks like this ✨

              +-------------------------+----------+
              | Subprotocol Identifier  | clue     |
              +-------------------------+----------+
              | Subprotocol Common Name | CLUE     |
              +-------------------------+----------+
              | Subprotocol Definition  | RFC 8850 |
              +-------------------------+----------+
              | Reference               | RFC 8850 |
              +-------------------------+----------+

Table 3: Registration of 'clue' Value

6. References 📚

6.1. Normative References 👑

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, https://www.rfc-editor.org/info/rfc2119.

[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, DOI 10.17487/RFC3261, June 2002, https://www.rfc-editor.org/info/rfc3261.

[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, DOI 10.17487/RFC3264, June 2002, https://www.rfc-editor.org/info/rfc3264.

[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, DOI 10.17487/RFC4566, July 2006, https://www.rfc-editor.org/info/rfc4566.

[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol", RFC 4960, DOI 10.17487/RFC4960, September 2007, https://www.rfc-editor.org/info/rfc4960.

[RFC5061] Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M. Kozuka, "Stream Control Transmission Protocol (SCTP) Dynamic Address Reconfiguration", RFC 5061, DOI 10.17487/RFC5061, September 2007, https://www.rfc-editor.org/info/rfc5061.

[RFC6525] Stewart, R., Tuexen, M., and P. Lei, "Stream Control Transmission Protocol (SCTP) Stream Reconfiguration", RFC 6525, DOI 10.17487/RFC6525, February 2012, https://www.rfc-editor.org/info/rfc6525.

[RFC7496] Tuexen, M., Seggelmann, R., Stewart, R., and S. Loreto, "Additional Policies for the Partially Reliable Stream Control Transmission Protocol Extension", RFC 7496, DOI 10.17487/RFC7496, April 2015, https://www.rfc-editor.org/info/rfc7496.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, https://www.rfc-editor.org/info/rfc8174.

[RFC8261] Tuexen, M., Stewart, R., Jesup, R., and S. Loreto, "Datagram Transport Layer Security (DTLS) Encapsulation of SCTP Packets", RFC 8261, DOI 10.17487/RFC8261, November 2017, https://www.rfc-editor.org/info/rfc8261.

[RFC8831] Jesup, R., Loreto, S., and M. Tüxen, "WebRTC Data Channels", RFC 8831, DOI 10.17487/RFC8831, January 2021, https://www.rfc-editor.org/info/rfc8831.

[RFC8841] Holmberg, C., Shpount, R., Loreto, S., and G. Camarillo, "Session Description Protocol (SDP) Offer/Answer Procedures for Stream Control Transmission Protocol (SCTP) over Datagram Transport Layer Security (DTLS) Transport", RFC 8841, DOI 10.17487/RFC8841, January 2021, https://www.rfc-editor.org/info/rfc8841.

[RFC8864] Drage, K., Makaraju, M., Ejzak, R., Marcon, J., and R. Even, Ed., "Negotiation Data Channels Using the Session Description Protocol (SDP)", RFC 8864, DOI 10.17487/RFC8864, January 2021, https://www.rfc-editor.org/info/rfc8864.

6.2. Informative References 🦋

[RFC3758] Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P. Conrad, "Stream Control Transmission Protocol (SCTP) Partial Reliability Extension", RFC 3758, DOI 10.17487/RFC3758, May 2004, https://www.rfc-editor.org/info/rfc3758.

[RFC8445] Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal", RFC 8445, DOI 10.17487/RFC8445, July 2018, https://www.rfc-editor.org/info/rfc8445.

[RFC8832] Jesup, R., Loreto, S., and M. Tüxen, "WebRTC Data Channel Establishment Protocol", RFC 8832, DOI 10.17487/RFC8832, January 2021, https://www.rfc-editor.org/info/rfc8832.

[RFC8847] Presta, R. and S P. Romano, "Protocol for Controlling Multiple Streams for Telepresence (CLUE)", RFC 8847, DOI 10.17487/RFC8847, January 2021, https://www.rfc-editor.org/info/rfc8847.

[RFC8848] Hanton, R., Kyzivat, P., Xiao, L., and C. Groves, "Session Signaling for Controlling Multiple Streams for Telepresence (CLUE)", RFC 8848, DOI 10.17487/RFC8848, January 2021, https://www.rfc-editor.org/info/rfc8848.

Acknowledgements 🫶

Thanks to Paul Kyzivat, Christian Groves, and Mark Duckworth for comments on this document. You're all iconic 🌸

Author's Address 🖋️

Christer Holmberg Ericsson Hirsalantie 11 FI-02420 Jorvas Finland

Email: christer.holmberg@ericsson.com

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