Internet Engineering Task Force (IETF)                       C. HolmbergRequest for Comments: 8850                                      EricssonCategory: Experimental                                      January 2021ISSN: 2070-1721     Controlling Multiple Streams for Telepresence (CLUE) Protocol                              Data ChannelAbstract   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 Address1.  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 Channel3.1.  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 Considerations3.2.1.  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 Considerations3.3.1.  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 Channel4.  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 Considerations5.1.  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' Value6.  References6.1.  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: christer.holmberg@ericsson.com