Nokia

Barcelona Spain victor.pascual_avila@nokia.com

Quobis

Pol. Ind. A Granxa, Casa de Pedra O Porriño 36475 Spain anton.roman@quobis.com

Orange

42 rue des Coutures Caen 14000 France stephane.cazeaux@orange.com

Cisco Systems, Inc.

7200-12 Kit Creek Road Research Triangle Park NC 27709 US gsalguei@cisco.com

Cisco Systems, Inc.

Cessna Business Park Kadabeesanahalli Village, Varthur Hobli, Sarjapur-Marathahalli Outer Ring Road Bangalore Karnataka 560103 India rmohanr@cisco.com

IETF BFCPBIS Working Group BFCP WebSocket 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 scenarios.

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 .

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 () 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

• .  Introduction
• .  Terminology
  • .  Definitions
• .  The WebSocket Protocol
• .  The WebSocket BFCP Subprotocol
  • .  Handshake
  • .  BFCP Encoding
• .  Transport Reliability
• .  SDP Considerations
  • .  Transport Negotiation
  • .  SDP Media Attributes
• .  SDP Offer/Answer Procedures
  • .  General
  • .  Example Usage of 'websocket-uri' SDP Attribute
• .  Authentication
• .  Security Considerations
• . IANA Considerations
  • .  Registration of the WebSocket BFCP Subprotocol
  • .  Registration of the 'TCP/WS/BFCP' and 'TCP/WSS/BFCP' SDP "proto" Values
• . References
  • .  Normative References
  • .  Informative References
• Acknowledgements
• Authors' Addresses

Introduction The WebSocket (WS) protocol 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) 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 control servers. Modern web browsers include a WebSocket client stack complying with the WebSocket 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 to enable the usage of BFCP in these scenarios. 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 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 ) 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 in for sending BFCP over unreliable transports are not applicable.

Terminology 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 when, and only when, they appear in all capitals, as shown here.

Definitions

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 document.

The WebSocket Protocol The WebSocket protocol is a transport layer on top of TCP (optionally secured with TLS ) 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 causes. The WebSocket connection handshake is based on HTTP and 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 message exchanges. 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.

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.

Handshake The BFCP WebSocket client and BFCP WebSocket server negotiate usage of the WebSocket BFCP subprotocol during the WebSocket handshake procedure as defined in . 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 contain "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 Host: bfcp-ws.example.com Upgrade: websocket Connection: Upgrade Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ== Origin: http://www.example.com Sec-WebSocket-Protocol: bfcp Sec-WebSocket-Version: 13 The handshake response from the server accepting the WebSocket BFCP subprotocol would look as follows: HTTP/1.1 101 Switching Protocols Upgrade: websocket Connection: Upgrade Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo= Sec-WebSocket-Protocol: bfcp Once the negotiation has been completed, the WebSocket connection is established and can be used for the transport of BFCP messages.

BFCP Encoding BFCP messages use a TLV (Type-Length-Value) binary encoding, therefore BFCP WebSocket clients and BFCP WebSocket servers MUST be transported in unfragmented binary WebSocket frames (FIN: 1, opcode: %x2) to exchange BFCP messages. The WebSocket frame data MUST be a valid BFCP message, so the length of the payload of the WebSocket frame MUST be lower than the maximum size allowed (2¹⁶ +12 bytes) for a BFCP message as described in . In addition, the encoding rules for reliable protocols defined in MUST be followed. 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.

Transport Reliability The WebSocket protocol 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 follow the procedures for reliable transports as defined in and . 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.

SDP Considerations

Transport Negotiation Rules to generate an "m=" line for a BFCP stream are described in . New values are defined for the SDP "proto" field: 'TCP/WS/BFCP' and 'TCP/WSS/BFCP'.

• '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 and Section of . Depending on the value of the SDP 'setup' attribute defined in , 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 follow the rules of . 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.

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 MUST be 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 MUST be followed.

SDP Offer/Answer Procedures

General An endpoint (i.e., both the offerer and the answerer) MUST create an 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 attribute MUST follow the syntax defined in . Additionally, the SDP offer/answer procedures defined in MUST be followed for the "m=" line associated with a BFCP-over-WebSocket media stream.

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 ). Offer (browser): m=application 9 TCP/WSS/BFCP * a=setup:active a=connection:new a=floorctrl:c-only m=audio 55000 RTP/AVP 0 m=video 55002 RTP/AVP 31 Answer (server): m=application 50000 TCP/WSS/BFCP * a=setup:passive a=connection:new a=websocket-uri:wss://bfcp-ws.example.com?token=3170449312 a=floorctrl:s-only a=confid:4321 a=userid:1234 a=floorid:1 m-stream:10 a=floorid:2 m-stream:11 m=audio 50002 RTP/AVP 0 a=label:10 m=video 50004 RTP/AVP 31 a=label:11 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 . For BFCP application, the "proto" value in the "m=" line MUST be 'TCP/WSS/BFCP' if WebSocket is over TLS, else it MUST be 'TCP/WS/BFCP'.

Authentication states that BFCP clients and floor control servers SHOULD 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 , so it is RECOMMENDED that standard web-based methods for client and server authentication are used, as follows. 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 while 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 , 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 , 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 server SHOULD inspect any cookie header fields 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 :

• If the status code received from the server is not 101, the WebSocket client stack handles the response per HTTP 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 ) and digest authentication (mentioned in ). To overcome some of these weaknesses, WebSocket clients can use the HTTP Origin-Bound Authentication (HOBA) mechanism mentioned in , for example.

Security Considerations Considerations from , , and apply. BFCP relies on lower-layer security mechanisms to provide replay and integrity protection and confidentiality. It is RECOMMENDED that the BFCP traffic transported over WebSocket be protected by using a Secure WebSocket connection (using TLS over TCP). The security considerations in 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. 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 . The rest of this section analyses the threats described in 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 ) while setting up a TLS connection with floor control WebSocket server. An attacker attempting to impersonate a floor control client is avoided by having servers accept BFCP messages over WSS only. As described in the floor control server can use any client authentication mechanism and follow the steps in 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 . Since the WebSocket 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.

IANA Considerations

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 Control Protocol)

Subprotocol Definition:

RFC 8857

Registration of the 'TCP/WS/BFCP' and 'TCP/WSS/BFCP' SDP "proto" Values 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 : Values for the SDP "proto" Field

Value	Reference
TCP/WS/BFCP	RFC 8857
TCP/WSS/BFCP	RFC 8857

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document describes how to express media transport over TCP using the Session Description Protocol (SDP). It defines the SDP 'TCP' protocol identifier, the SDP 'setup' attribute, which describes the connection setup procedure, and the SDP 'connection' attribute, which handles connection reestablishment. [STANDARDS-TRACK] This document specifies how a Binary Floor Control Protocol (BFCP) client establishes a connection to a BFCP floor control server outside the context of an offer/answer exchange. Client and server authentication are based on Transport Layer Security (TLS). [STANDARDS-TRACK] The WebSocket Protocol enables two-way communication between a client running untrusted code in a controlled environment to a remote host that has opted-in to communications from that code. The security model used for this is the origin-based security model commonly used by web browsers. The protocol consists of an opening handshake followed by basic message framing, layered over TCP. The goal of this technology is to provide a mechanism for browser-based applications that need two-way communication with servers that does not rely on opening multiple HTTP connections (e.g., using XMLHttpRequest or <iframe>s and long polling). [STANDARDS-TRACK] Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) are widely used to protect data exchanged over application protocols such as HTTP, SMTP, IMAP, POP, SIP, and XMPP. Over the last few years, several serious attacks on TLS have emerged, including attacks on its most commonly used cipher suites and their modes of operation. This document provides recommendations for improving the security of deployed services that use TLS and DTLS. The recommendations are applicable to the majority of use cases. The WebSocket protocol enables bidirectional real-time communication between clients and servers in web-based applications. This document specifies extensions to Session Description Protocol (SDP) for application protocols using WebSocket as a transport. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References This document defines the HTTP Cookie and Set-Cookie header fields. These header fields can be used by HTTP servers to store state (called cookies) at HTTP user agents, letting the servers maintain a stateful session over the mostly stateless HTTP protocol. Although cookies have many historical infelicities that degrade their security and privacy, the Cookie and Set-Cookie header fields are widely used on the Internet. This document obsoletes RFC 2965. [STANDARDS-TRACK] The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document provides an overview of HTTP architecture and its associated terminology, defines the "http" and "https" Uniform Resource Identifier (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements, and describes related security concerns for implementations. HTTP Origin-Bound Authentication (HOBA) is a digital-signature-based design for an HTTP authentication method. The design can also be used in JavaScript-based authentication embedded in HTML. HOBA is an alternative to HTTP authentication schemes that require passwords and therefore avoids all problems related to passwords, such as leakage of server-side password databases. The Hypertext Transfer Protocol (HTTP) provides a simple challenge- response authentication mechanism that may be used by a server to challenge a client request and by a client to provide authentication information. This document defines the HTTP Digest Authentication scheme that can be used with the HTTP authentication mechanism. This document defines the "Basic" Hypertext Transfer Protocol (HTTP) authentication scheme, which transmits credentials as user-id/ password pairs, encoded using Base64. This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. W3C Candidate Recommendation, September 2012

Acknowledgements The authors want to thank from Acme Packet and , who made significant contributions to the first draft version of this document. This work benefited from the thorough review and constructive comments of , , , , and . Thanks to , , and for their reviews and comments on this document. Thanks to , , , , , and for their feedback and comments during IESG reviews.

Authors' Addresses Nokia

Barcelona Spain victor.pascual_avila@nokia.com

Quobis

Pol. Ind. A Granxa, Casa de Pedra O Porriño 36475 Spain anton.roman@quobis.com

Orange

42 rue des Coutures Caen 14000 France stephane.cazeaux@orange.com

Cisco Systems, Inc.

7200-12 Kit Creek Road Research Triangle Park NC 27709 US gsalguei@cisco.com

Cisco Systems, Inc.

Cessna Business Park Kadabeesanahalli Village, Varthur Hobli, Sarjapur-Marathahalli Outer Ring Road Bangalore Karnataka 560103 India rmohanr@cisco.com