Internet Engineering Task Force (IETF) V. Pascual
Request for Comments: 8857
Category: Standards Track A. Román
ISSN: 2070-1721 Quobis
The WebSocket Protocol as a Transport for the Binary Floor Control
The WebSocket protocol enables two-way real-time communication
between clients and servers. This document specifies the use of
Binary Floor Control Protocol (BFCP) as a new WebSocket subprotocol
enabling a reliable transport mechanism between BFCP entities in new
Status of This Memo
This is an Internet Standards Track document.
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). Further information on
Internet Standards is available in 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/rfc8857
Copyright (c) 2021 IETF Trust and the persons identified as the
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) in effect on the date of
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Table of Contents 1.
The WebSocket Protocol 4.
The WebSocket BFCP Subprotocol 4.1.
BFCP Encoding 5.
Transport Reliability 6.
SDP Considerations 6.1.
Transport Negotiation 6.2.
SDP Media Attributes 7.
SDP Offer/Answer Procedures 7.1.
Example Usage of 'websocket-uri' SDP Attribute 8.
Security Considerations 10.
IANA Considerations 10.1.
Registration of the WebSocket BFCP Subprotocol 10.2.
Registration of the 'TCP/WS/BFCP' and 'TCP/WSS/BFCP' SDP
"proto" Values 11.
Normative References 11.2.
The WebSocket (WS) protocol [RFC6455
] enables two-way message
exchange between clients and servers on top of a persistent TCP
connection, optionally secured with Transport Layer Security (TLS)
]. The initial protocol handshake makes use of Hypertext
Transfer Protocol (HTTP) [RFC7230
] semantics, allowing the WebSocket
protocol to reuse existing HTTP infrastructure.
The Binary Floor Control Protocol (BFCP) is a protocol to coordinate
access to shared resources in a conference. It is defined in
] and is used between floor participants and floor control
servers, and between floor chairs (i.e., moderators) and floor
Modern web browsers include a WebSocket client stack complying with
the WebSocket API [WS-API] as specified by the W3C. It is expected
that other client applications (those running in personal computers
and devices such as smartphones) will also make a WebSocket client
stack available. This document extends the applicability of
] and [RFC8856
] to enable the usage of BFCP in these
The transport over which BFCP entities exchange messages depends on
how the clients obtain information to contact the floor control
server (e.g., using a Session Description Protocol (SDP) offer/answer
exchange per [RFC8856
] or the procedure described in RFC 5018
] defines two transports for BFCP: TCP and UDP.
This specification defines a new WebSocket subprotocol (as defined in
Section 1.9 of [RFC6455
]) for transporting BFCP messages between a
WebSocket client and server. This subprotocol provides a reliable
and boundary-preserving transport for BFCP when run on top of TCP.
Since WebSocket provides a reliable transport, the extensions defined
] for sending BFCP over unreliable transports are not
The key words "MUST
", "MUST NOT
", "SHALL NOT
", "SHOULD NOT
", "NOT RECOMMENDED
" in this document are to be interpreted as described in BCP
] when, and only when, they appear in all
capitals, as shown here.
BFCP WebSocket Client: Any BFCP entity capable of opening outbound
connections to WebSocket servers and communicating using the
WebSocket BFCP subprotocol as defined by this document.
BFCP WebSocket Server: Any BFCP entity capable of listening for
inbound connections from WebSocket clients and communicating
using the WebSocket BFCP subprotocol as defined by this
3. The WebSocket Protocol
The WebSocket protocol [RFC6455
] is a transport layer on top of TCP
(optionally secured with TLS [RFC8446
]) in which both client and
server exchange message units in both directions. The protocol
defines a connection handshake, WebSocket subprotocol and extensions
negotiation, a frame format for sending application and control data,
a masking mechanism, and status codes for indicating disconnection
The WebSocket connection handshake is based on HTTP [RFC7230
utilizes the HTTP GET method with an Upgrade header field. This is
sent by the client and then answered by the server (if the
negotiation succeeded) with an HTTP 101 status code. Once the
handshake is completed, the connection upgrades from HTTP to the
WebSocket protocol. This handshake procedure is designed to reuse
the existing HTTP infrastructure. During the connection handshake,
the client and server agree on the application protocol to use on top
of the WebSocket transport. Such an application protocol (also known
as a "WebSocket subprotocol") defines the format and semantics of the
messages exchanged by the endpoints. This could be a custom protocol
or a standardized one (as the WebSocket BFCP subprotocol defined in
this document). Once the HTTP 101 response is processed, both the
client and server reuse the underlying TCP connection for sending
WebSocket messages and control frames to each other. Unlike plain
HTTP, this connection is persistent and can be used for multiple
The WebSocket protocol defines message units to be used by
applications for the exchange of data, so it provides a message
boundary-preserving transport layer.
4. The WebSocket BFCP Subprotocol
The term WebSocket subprotocol refers to an application-level
protocol layered on top of a WebSocket connection. This document
specifies the WebSocket BFCP subprotocol for carrying BFCP messages
over a WebSocket connection.
The BFCP WebSocket client and BFCP WebSocket server negotiate usage
of the WebSocket BFCP subprotocol during the WebSocket handshake
procedure as defined in Section 1.3 of [RFC6455
]. The client MUST
include the value "bfcp" in the Sec-WebSocket-Protocol header field
in its handshake request. The 101 reply from the server MUST
"bfcp" in its corresponding Sec-WebSocket-Protocol header field.
Below is an example of a WebSocket handshake in which the client
requests the WebSocket BFCP subprotocol support from the server:
GET / HTTP/1.1
The handshake response from the server accepting the WebSocket BFCP
subprotocol would look as follows:
HTTP/1.1 101 Switching Protocols
Once the negotiation has been completed, the WebSocket connection is
established and can be used for the transport of BFCP messages.
4.2. BFCP Encoding
BFCP messages use a TLV (Type-Length-Value) binary encoding,
therefore BFCP WebSocket clients and BFCP WebSocket servers MUST
transported in unfragmented binary WebSocket frames (FIN: 1, opcode:
%x2) to exchange BFCP messages. The WebSocket frame data MUST
valid BFCP message, so the length of the payload of the WebSocket
be lower than the maximum size allowed (2^(16) +12 bytes)
for a BFCP message as described in [RFC8855
]. In addition, the
encoding rules for reliable protocols defined in [RFC8855
While this specification assumes that BFCP encoding is only TLV
binary, future documents may define other mechanisms, like JSON
serialization. If encoding changes, a new subprotocol identifier
would need to be selected.
Each BFCP message MUST
be carried within a single WebSocket message,
and a WebSocket message MUST NOT
contain more than one BFCP message.
5. Transport Reliability
The WebSocket protocol [RFC6455
] provides a reliable transport, and
therefore the BFCP WebSocket subprotocol defined by this document
also provides reliable BFCP transport. Thus, client and server
transactions using the WebSocket protocol for transport MUST
the procedures for reliable transports as defined in [RFC8855
BFCP WebSocket clients cannot receive incoming WebSocket connections
initiated by any other peer. This means that a BFCP WebSocket client MUST
actively initiate a connection towards a BFCP WebSocket server.
The BFCP server will have a globally routable address and thus does
not require ICE, as clients always initiate connections to it.
6. SDP Considerations
6.1. Transport Negotiation
Rules to generate an "m=" line for a BFCP stream are described in
], Section 4
New values are defined for the SDP "proto" field: 'TCP/WS/BFCP' and
'TCP/WS/BFCP' is used when BFCP runs on top of WS, which in turn
runs on top of TCP.
'TCP/WSS/BFCP' is used when BFCP runs on top of secure WebSocket
(WSS), which in turn runs on top of TLS and TCP.
The "port" field is set following the rules in Section 4
and Section 7.1
]. Depending on the value of the SDP 'setup'
attribute defined in [RFC4145
], the "port" field contains the port to
which the remote endpoint will direct BFCP messages, or it is
irrelevant (i.e., the endpoint will initiate the connection towards
the remote endpoint) and should be set to a value of '9', which is
the discard port. The 'connection' attribute and port MUST
the rules of [RFC4145
While this document recommends the use of secure WebSocket (i.e.,
TCP/WSS) for security reasons, TCP/WS is also permitted so as to
achieve maximum compatibility among clients.
6.2. SDP Media Attributes
] defines a new SDP attribute to indicate the connection
Uniform Resource Identifier (URI) for the WebSocket client. The SDP
attribute 'websocket-uri' defined in Section 3
used when BFCP runs on top of WS or WSS. When the 'websocket-uri'
attribute is present in the media section of the SDP, the procedures
mentioned in Section 4
7. SDP Offer/Answer Procedures
An endpoint (i.e., both the offerer and the answerer) MUST
SDP media description ("m=" line) for each BFCP-over-WebSocket media
stream and MUST
assign either a 'TCP/WSS/BFCP' or 'TCP/WS/BFCP' value
to the "proto" field of the "m=" line depending on whether the
endpoint wishes to use secure WebSocket or WebSocket. Furthermore,
the server side, which could be either the offerer or answerer, MUST
add a 'websocket-uri' attribute in the media section depending on
whether it wishes to use WebSocket or secure WebSocket. This new
follow the syntax defined in [RFC8124
the SDP offer/answer procedures defined in Section 4
be followed for the "m=" line associated with a BFCP-over-
WebSocket media stream.
7.2. Example Usage of 'websocket-uri' SDP Attribute
The following is an example of an "m=" line for a BFCP connection.
In this example, the offerer sends the SDP with the "proto" field
having a value of 'TCP/WSS/BFCP', indicating that the offerer wishes
to use secure WebSocket as a transport for the media stream, and the
"fmt" field having a value of '*' (for details on the "fmt" field,
see Section 4
m=application 9 TCP/WSS/BFCP *
m=audio 55000 RTP/AVP 0
m=video 55002 RTP/AVP 31
m=application 50000 TCP/WSS/BFCP *
m=audio 50002 RTP/AVP 0
m=video 50004 RTP/AVP 31
It is possible that an endpoint (e.g., a browser) sends an offerless
INVITE to the server. In such cases, the server will act as SDP
offerer. The server MUST
assign the SDP 'setup' attribute with a
value of 'passive'. The server MUST
have a 'websocket-uri' attribute
with a 'ws-URI' or 'wss-URI' value depending on whether the server
wishes to use WebSocket or secure WebSocket. This attribute MUST
follow the syntax defined in Section 3
]. For BFCP
application, the "proto" value in the "m=" line MUST
BFCP' if WebSocket is over TLS, else it MUST
8. Authentication Section 9
] states that BFCP clients and floor control
authenticate each other prior to accepting messages,
and RECOMMENDS that mutual TLS/DTLS authentication be used. However,
browser-based WebSocket clients have no control over the use of TLS
in the WebSocket API [WS-API], so it is RECOMMENDED
web-based methods for client and server authentication are used, as
When a BFCP WebSocket client connects to a BFCP WebSocket server, it SHOULD
use TCP/WSS as its transport. If the signaling or control
protocol traffic used to set up the conference is authenticated and
confidentiality and integrity protected, secure WebSocket (WSS) MUST
be used, and the floor control server MUST
authenticate the client.
The WebSocket client MUST
follow the procedures in [RFC7525
setting up TLS connection with the WebSocket server. The BFCP client
validates the server by means of verifying the server certificate.
This means the 'websocket-uri' value MUST
contain a hostname. The
verification process does not use "a=fingerprint".
A floor control server that receives a message over TCP/WS can
mandate the use of TCP/WSS by generating an Error message, as
described in Section 13.8 of [RFC8855
], with an error code with a
value of 9 (Use TLS).
Prior to sending BFCP requests, a BFCP WebSocket client connects to a
BFCP WebSocket server and performs the connection handshake. As
described in Section 4.1
, the handshake procedure involves an HTTP
GET method request from the client and a response from the server
including an HTTP 101 status code.
In order to authorize the WebSocket connection, the BFCP WebSocket
inspect any cookie header fields [RFC6265
] present in
the HTTP GET request. For many web applications, the value of such a
cookie is provided by the web server once the user has authenticated
themselves to the web server, which could be done by many existing
mechanisms. As an alternative method, the BFCP WebSocket server
could request HTTP authentication by replying to the client's GET
method request with an HTTP 401 status code. The WebSocket protocol
] covers this usage in Section 4.1
If the status code received from the server is not 101, the
WebSocket client stack handles the response per HTTP [RFC7230
procedures; in particular, the client might perform authentication
if it receives an 401 status code. The WebSocket clients are
vulnerable to the attacks of basic authentication (mentioned in Section 4
]) and digest authentication (mentioned in Section 5
]). To overcome some of these weaknesses,
WebSocket clients can use the HTTP Origin-Bound Authentication
(HOBA) mechanism mentioned in [RFC7486
], for example.
9. Security Considerations
Considerations from [RFC8855
], and [RFC5018
BFCP relies on lower-layer security mechanisms to provide replay and
integrity protection and confidentiality. It is RECOMMENDED
BFCP traffic transported over WebSocket be protected by using a
Secure WebSocket connection (using TLS [RFC8446
] over TCP). The
security considerations in [RFC6455
] apply for BFCP over WebSocket as
well. The security model here is a typical webserver-client model
where the client validates the server certificate and then connects
to the server. Section 8
describes the authentication procedures
between client and server.
When using BFCP over WebSocket, the security mechanisms defined in
] are not used. Instead, the application is required to
build and rely on the security mechanisms in [RFC6455
The rest of this section analyses the threats described in Section 14
] when WebSocket is used as a transport protocol for BFCP.
An attacker attempting to impersonate a floor control server is
avoided by having servers accept BFCP messages over WSS only. As
with any other web connection, the clients will verify the server's
certificate. The BFCP WebSocket client MUST
follow the procedures in
] (including hostname verification as per Section 6.1
]) while setting up a TLS connection with floor control
An attacker attempting to impersonate a floor control client is
avoided by having servers accept BFCP messages over WSS only. As
described in Section 10.5 of [RFC6455
] the floor control server can
use any client authentication mechanism and follow the steps in Section 8
of this document.
Attackers may attempt to modify messages exchanged by a client and a
floor control server. This can be prevented by having WSS between
client and server.
An attacker trying to replay the messages is prevented by having
floor control servers check that messages arriving over a given WSS
connection use an authorized user ID.
Attackers may eavesdrop on the network to get access to confidential
information between the floor control server and a client (e.g., why
a floor request was denied). In order to ensure that BFCP users are
getting the level of protection that they would get using BFCP
directly, applications need to have a way to control the WebSocket
libraries to use encryption algorithms specified in Section 7
]. Since the WebSocket API [WS-API] does not have a way to
allow an application to select the encryption algorithm to be used,
the protection level provided when WSS is used is limited to the
underlying TLS algorithm used by the WebSocket library.
10. IANA Considerations
10.1. Registration of the WebSocket BFCP Subprotocol
IANA has registered the WebSocket BFCP subprotocol under the
"WebSocket Subprotocol Name Registry" as follows:
Subprotocol Identifier: bfcp
Subprotocol Common Name: WebSocket Transport for BFCP (Binary Floor
Subprotocol Definition: RFC 8857
10.2. Registration of the 'TCP/WS/BFCP' and 'TCP/WSS/BFCP' SDP "proto"
This document defines two new values for the SDP "proto" subregistry
within the "Session Description Protocol (SDP) Parameters" registry.
The resulting entries are shown in Table 1:
| Value | Reference |
| TCP/WS/BFCP | RFC 8857
| TCP/WSS/BFCP | RFC 8857
Table 1: Values for the
SDP "proto" Field
11.1. Normative References
] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119
, March 1997,
] Yon, D. and G. Camarillo, "TCP-Based Media Transport in
the Session Description Protocol (SDP)", RFC 4145
, September 2005,
] Camarillo, G., "Connection Establishment in the Binary
Floor Control Protocol (BFCP)", RFC 5018
, September 2007,
] Fette, I. and A. Melnikov, "The WebSocket Protocol", RFC 6455
, DOI 10.17487/RFC6455
, December 2011,
] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525
, DOI 10.17487/RFC7525
] Ravindranath, R. and G. Salgueiro, "The Session
Description Protocol (SDP) WebSocket Connection URI
Attribute", RFC 8124
, DOI 10.17487/RFC8124
, March 2017,
] 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
] Camarillo, G., Drage, K., Kristensen, T., Ott, J., and C.
Eckel, "The Binary Floor Control Protocol (BFCP)", RFC 8855
, DOI 10.17487/RFC8855
, January 2021,
] Camarillo, G., Kristensen, T., and C. Holmberg, "Session
Description Protocol (SDP) Format for Binary Floor Control
Protocol (BFCP) Streams", RFC 8856
, DOI 10.17487/RFC8856
January 2021, <https://www.rfc-editor.org/info/rfc8856
11.2. Informative References
] Barth, A., "HTTP State Management Mechanism", RFC 6265
, April 2011,
] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230
, DOI 10.17487/RFC7230
, June 2014,
] Farrell, S., Hoffman, P., and M. Thomas, "HTTP Origin-
Bound Authentication (HOBA)", RFC 7486
, March 2015,
] Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
Digest Access Authentication", RFC 7616
, September 2015,
] Reschke, J., "The 'Basic' HTTP Authentication Scheme", RFC 7617
, DOI 10.17487/RFC7617
, September 2015,
] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446
, DOI 10.17487/RFC8446
, August 2018,
[WS-API] Hickson, I., Ed., "The WebSocket API", W3C Candidate
Recommendation, September 2012,
The authors want to thank Robert Welbourn from Acme Packet and Sergio
Garcia Murillo, who made significant contributions to the first draft
version of this document. This work benefited from the thorough
review and constructive comments of Charles Eckel, Christer Holmberg,
Paul Kyzivat, Dan Wing, and Alissa Cooper. Thanks to Bert Wijnen,
Robert Sparks, and Mirja Kühlewind for their reviews and comments on
Thanks to Spencer Dawkins, Ben Campbell, Kathleen Moriarty, Alexey
Melnikov, Jari Arkko, and Stephen Farrell for their feedback and
comments during IESG reviews.
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