This document defines a mechanism for running the WebSocket Protocol (RFC 6455) over a single stream of an HTTP/2 connection.
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/rfc8441.
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- 1. Introduction
- 2. Terminology
- 3. The SETTINGS_ENABLE_CONNECT_PROTOCOL SETTINGS Parameter
- 4. The Extended CONNECT Method
- 5. Using Extended CONNECT to Bootstrap the WebSocket Protocol
- 6. Design Considerations
- 7. About Intermediaries
- 8. Security Considerations
- 9. IANA Considerations
- 10. Normative References
- Author's Address
The Hypertext Transfer Protocol (HTTP) [RFC7230] provides compatible resource-level semantics across different versions, but it does not offer compatibility at the connection-management level. Other protocols that rely on connection-management details of HTTP, such as WebSockets, must be updated for new versions of HTTP.
The WebSocket Protocol [RFC6455] uses the HTTP/1.1 Upgrade mechanism (Section 6.7 of [RFC7230]) to transition a TCP connection from HTTP into a WebSocket connection. A different approach must be taken with HTTP/2 [RFC7540]. Due to its multiplexing nature, HTTP/2 does not allow connection-wide header fields or status codes, such as the Upgrade and Connection request-header fields or the 101 (Switching Protocols) response code. These are all required by the [RFC6455] opening handshake.
Being able to bootstrap WebSockets from HTTP/2 allows one TCP connection to be shared by both protocols and extends HTTP/2’s more efficient use of the network to WebSockets.
This document extends the HTTP CONNECT method, as specified for HTTP/2 in Section 8.3 of [RFC7540]. The extension allows the substitution of a new protocol name to connect to rather than the external host normally used by CONNECT. The result is a tunnel on a single HTTP/2 stream that can carry data for WebSockets (or any other protocol). The other streams on the connection may carry more extended CONNECT tunnels, traditional HTTP/2 data, or a mixture of both.
This tunneled stream will be multiplexed with other regular streams on the connection and enjoys the normal priority, cancellation, and flow-control features of HTTP/2.
Streams that successfully establish a WebSocket connection using a tunneled stream and the modifications to the opening handshake defined in this document then use the traditional WebSocket Protocol, treating the stream as if it were the TCP connection in that specification.
The new parameter name is SETTINGS_ENABLE_CONNECT_PROTOCOL. The value of the parameter MUST be 0 or 1.
Upon receipt of SETTINGS_ENABLE_CONNECT_PROTOCOL with a value of 1, a client MAY use the Extended CONNECT as defined in this document when creating new streams. Receipt of this parameter by a server does not have any impact.
A sender MUST NOT send a SETTINGS_ENABLE_CONNECT_PROTOCOL parameter with the value of 0 after previously sending a value of 1.
Using a SETTINGS parameter to opt into an otherwise incompatible protocol change is a use of “Extending HTTP/2” defined by Section 5.5 of [RFC7540]. Specifically, the addition a new pseudo-header field, “:protocol”, and the change in meaning of the :authority pseudo-header field in Section 4 require opt-in negotiation. If a client were to use the provisions of the extended CONNECT method defined in this document without first receiving a SETTINGS_ENABLE_CONNECT_PROTOCOL parameter, a non-supporting peer would detect a malformed request and generate a stream error (Section 188.8.131.52 of [RFC7540]).
- A new pseudo-header field :protocol MAY be included on request HEADERS indicating the desired protocol to be spoken on the tunnel created by CONNECT. The pseudo-header field is single valued and contains a value from the “Hypertext Transfer Protocol (HTTP) Upgrade Token Registry” located at <https://www.iana.org/assignments/http-upgrade-tokens/>
- On requests that contain the :protocol pseudo-header field, the :scheme and :path pseudo-header fields of the target URI (see Section 5) MUST also be included.
- On requests bearing the :protocol pseudo-header field, the :authority pseudo-header field is interpreted according to Section 184.108.40.206 of [RFC7540] instead of Section 8.3 of that document. In particular, the server MUST NOT create a tunnel to the host indicated by the :authority as it would with a CONNECT method request that was not modified by this extension.
Upon receiving a CONNECT request bearing the :protocol pseudo-header field, the server establishes a tunnel to another service of the protocol type indicated by the pseudo-header field. This service may or may not be co-located with the server.
The :protocol pseudo-header field MUST be included in the CONNECT request, and it MUST have a value of websocket to initiate a WebSocket connection on an HTTP/2 stream. Other HTTP request and response-header fields, such as those for manipulating cookies, may be included in the HEADERS with the CONNECT method as usual. This request replaces the GET-based request in [RFC6455] and is used to process the WebSockets opening handshake.
The scheme of the target URI (Section 5.1 of [RFC7230]) MUST be “https” for “wss”-schemed WebSockets and “http” for “ws”-schemed WebSockets. The remainder of the target URI is the same as the WebSocket URI. The WebSocket URI is still used for proxy autoconfiguration. The security requirements for the HTTP/2 connection used by this specification are established by [RFC7540] for https requests and [RFC8164] for http requests.
[RFC6455] requires the use of Connection and Upgrade header fields that are not part of HTTP/2. They MUST NOT be included in the CONNECT request defined here.
[RFC6455] requires the use of a Host header field that is also not part of HTTP/2. The Host information is conveyed as part of the :authority pseudo-header field, which is required on every HTTP/2 transaction.
Implementations using this extended CONNECT to bootstrap WebSockets do not do the processing of the Sec-WebSocket-Key and Sec-WebSocket-Accept header fields of [RFC6455] as that functionality has been superseded by the :protocol pseudo-header field.
The Origin [RFC6454], Sec-WebSocket-Version, Sec-WebSocket-Protocol, and Sec-WebSocket-Extensions header fields are used in the CONNECT request and response-header fields as defined in [RFC6455]. Note that HTTP/1 header field names were case insensitive, whereas HTTP/2 requires they be encoded as lowercase.
After successfully processing the opening handshake, the peers should proceed with the WebSocket Protocol [RFC6455] using the HTTP/2 stream from the CONNECT transaction as if it were the TCP connection referred to in [RFC6455]. The state of the WebSocket connection at this point is OPEN, as defined by [RFC6455], Section 4.1.
The HTTP/2 stream closure is also analogous to the TCP connection closure of [RFC6455]. Orderly TCP-level closures are represented as END_STREAM flags ([RFC7540], Section 6.1). RST exceptions are represented with the RST_STREAM frame ([RFC7540], Section 6.4) with the CANCEL error code ([RFC7540], Section 7).
[[ From Client ]] [[ From Server ]] SETTINGS SETTINGS_ENABLE_CONNECT_[..] = 1 HEADERS + END_HEADERS :method = CONNECT :protocol = websocket :scheme = https :path = /chat :authority = server.example.com sec-websocket-protocol = chat, superchat sec-websocket-extensions = permessage-deflate sec-websocket-version = 13 origin = http://www.example.com HEADERS + END_HEADERS :status = 200 sec-websocket-protocol = chat DATA WebSocket Data DATA + END_STREAM WebSocket Data DATA + END_STREAM WebSocket Data
A more native integration with HTTP/2 is certainly possible with larger additions to HTTP/2. This design was selected to minimize the solution complexity while still addressing the primary concern of running HTTP/2 and WebSockets concurrently.
This document does not change how WebSockets interacts with HTTP forward proxies. If a client wishing to speak WebSockets connects via HTTP/2 to an HTTP proxy, it should continue to use a traditional CONNECT (i.e., not with a :protocol pseudo-header field) to tunnel through that proxy to the WebSocket server via HTTP.
The resulting version of HTTP on that tunnel determines whether WebSockets is initiated directly or via a modified CONNECT request described in this document.
[RFC6455] ensures that non-WebSockets clients, especially XMLHttpRequest-based clients, cannot make a WebSocket connection. Its primary mechanism for doing that is the use of Sec-prefixed request-header fields that cannot be created by XMLHttpRequest-based clients. This specification addresses that concern in two ways:
- XMLHttpRequest also prohibits use of the CONNECT method in addition to Sec-prefixed request-header fields.
- The use of a pseudo-header field is something that is connection specific, and HTTP/2 never allows a pseudo-header to be created outside of the protocol stack.
- Initial Value:
- This document
This document registers an entry in the “HTTP Upgrade Tokens” registry that was established by [RFC7230].
- The Web Socket Protocol
- Expected Version Tokens:
- [RFC6455] [RFC8441]
10. Normative References
- 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>.
- Barth, A., “The Web Origin Concept”, RFC 6454, DOI 10.17487/RFC6454, December 2011, <https://www.rfc-editor.org/info/rfc6454>.
- Fette, I. and A. Melnikov, “The WebSocket Protocol”, RFC 6455, DOI 10.17487/RFC6455, December 2011, <https://www.rfc-editor.org/info/rfc6455>.
- 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, <https://www.rfc-editor.org/info/rfc7230>.
- Belshe, M., Peon, R., and M. Thomson, Ed., “Hypertext Transfer Protocol Version 2 (HTTP/2)”, RFC 7540, DOI 10.17487/RFC7540, May 2015, <https://www.rfc-editor.org/info/rfc7540>.
- Nottingham, M. and M. Thomson, “Opportunistic Security for HTTP/2”, RFC 8164, DOI 10.17487/RFC8164, May 2017, <https://www.rfc-editor.org/info/rfc8164>.
- 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>.
The 2017 HTTP Workshop had a very productive discussion that helped determine the key problem and acceptable level of solution complexity.