Internet Engineering Task Force (IETF)                       C. Holmberg
Request for Comments: 8850                                      Ericsson
Category: Experimental                                      January 2021
ISSN: 2070-1721


    Controlling Multiple Streams for Telepresence (CLUE) Protocol
                             Data Channel

Abstract

  This document defines how to use the WebRTC data channel mechanism to
  realize a data channel, referred to as a Controlling Multiple Streams
  for Telepresence (CLUE) data channel, for transporting CLUE protocol
  messages between two CLUE entities.

Status of This Memo

  This document is not an Internet Standards Track specification; it is
  published for examination, experimental implementation, and
  evaluation.

  This document defines an Experimental Protocol for the Internet
  community.  This document is a product of the Internet Engineering
  Task Force (IETF).  It represents the consensus of the IETF
  community.  It has received public review and has been approved for
  publication by the Internet Engineering Steering Group (IESG).  Not
  all documents approved by the IESG are candidates for any level of
  Internet Standard; see Section 2 of RFC 7841.

  Information about the current status of this document, any errata,
  and how to provide feedback on it may be obtained at
  https://www.rfc-editor.org/info/rfc8850.

Copyright Notice

  Copyright (c) 2021 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
  (https://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
  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

  This document defines how to use the WebRTC data channel mechanism
  [RFC8831] to realize a data channel, referred to as a Controlling
  Multiple Streams for Telepresence (CLUE) data channel, for
  transporting CLUE protocol messages [RFC8847] between two CLUE
  entities.

  This document also defines how to describe the SCTPoDTLS association
  [RFC8261] (also referred to as "SCTP over DTLS" in this document)
  used to realize the CLUE data channel using the Session Description
  Protocol (SDP) [RFC4566] and defines usage of the SDP-based "SCTP
  over DTLS" data channel negotiation mechanism [RFC8864].  ("SCTP"
  stands for "Stream Control Transmission Protocol".)  This includes
  SCTP considerations specific to a CLUE data channel, the SDP media
  description ("m=" line) values, and usage of SDP attributes specific
  to a CLUE data channel.

  Details and procedures associated with the CLUE protocol, and the SDP
  Offer/Answer procedures [RFC3264] for negotiating usage of a CLUE
  data channel, are outside the scope of this document.

     |  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.

  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 is 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, with the purpose of
     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 the SIP session, has been
     established between the SIP UAs.

  SCTP stream
     Defined in [RFC4960] as a unidirectional logical channel
     established from one to another associated SCTP endpoint, 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 describes the realization of a CLUE data channel, using
  the WebRTC data channel mechanism.  This includes a set of SCTP
  characteristics specific to a CLUE data channel, the values of the
  "m=" line describing the SCTPoDTLS association associated with the
  WebRTC data channel, and the usage of the SDP-based "SCTP over DTLS"
  data channel negotiation mechanism for creating the CLUE data
  channel.

  As described in [RFC8831], the SCTP streams realizing a WebRTC data
  channel must be associated with the same SCTP association.  In
  addition, both SCTP streams realizing 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 (e.g., regarding
  ordered delivery) can be used for a data channel.  This section
  describes the SCTP options used for a CLUE data channel.  Section 3.3
  describes how SCTP options are 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 usage of SCTP for the CLUE data channel ensures reliable
  transport of CLUE protocol messages [RFC8847].

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

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 the support of 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 and
  therefore cannot be used for a CLUE data channel.

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 receives the
  reset message, it also sends (unless already sent) a reset message on
  its outgoing SCTP stream associated with the data channel.  The
  SCTPoDTLS association, and other data channels established on the
  same association, are not affected by the SCTP reset messages.

3.3.  SDP Considerations

3.3.1.  General

  This section defines how to (1) construct the SDP media description
  ("m=" line) for describing the SCTPoDTLS association used to realize
  a CLUE data channel and (2) use the SDP-based "SCTP over DTLS" data
  channel negotiation mechanism [RFC8864] for establishing a CLUE data
  channel on the 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 describing an
  SCTPoDTLS association.  As defined in [RFC8841], for a CLUE data
  channel the values are set as follows:

    +===============+==========+============+======================+
    |     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 instance, 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 choose
  any valid SCTP port value [RFC8841].

3.3.2.  SDP dcmap Attribute

  The values of the SDP dcmap attribute [RFC8864], associated with the
  "m=" line describing the SCTPoDTLS association used to realize the
  WebRTC data channel, are set as follows:

  +===========+=============+=============+=======+========+==========+
  | 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: As CLUE entities are required to use ordered SCTP message
     |  delivery, with full reliability, according to the procedures in
     |  [RFC8864] the max-retr and max-time attribute parameters are
     |  not used when negotiating CLUE data channels.

3.3.3.  SDP dcsa Attribute

  The SDP dcsa attribute [RFC8864] is not used when establishing a CLUE
  data channel.

3.3.4.  Example

  The example in Figure 1 shows an SDP media description for a CLUE
  data channel.  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

  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.

5.  IANA Considerations

5.1.  Subprotocol Identifier "clue"

  This document adds the subprotocol identifier "clue" to the
  "WebSocket Subprotocol Name Registry" as follows:

                 +-------------------------+----------+
                 | 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.

Author's Address

  Christer Holmberg
  Ericsson
  Hirsalantie 11
  FI-02420 Jorvas
  Finland

  Email: [email protected]

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