Baseline Widely available
The HTTP Sec-WebSocket-Version request and response header is used in the WebSocket opening handshake to indicate the WebSocket protocol supported by the client, and the protocol versions supported by the server if it does not support the version specified in the request.
The header can only appear once in a request, and specifies the WebSocket version that web application is using. The current version of the protocol at time of writing is 13. The header is automatically added to requests by user agents when a WebSocket connection is established.
The server uses the version to determine if it can understand the protocol. If the server doesn't support the version, or any header in the handshake is not understood or has an incorrect value, the server should send a response with status 400 Bad Request and immediately close the socket. It should also include Sec-WebSocket-Version in the 400 response, listing the versions that it does support. The versions can be specified in individual headers, or as comma-separate values in a single header.
The header should not be sent in responses if the server understands the version specified by the client.
SyntaxRequest
Sec-WebSocket-Version: <version>
Response (on error only):
Sec-WebSocket-Version: <server-supported-versions>
<version>
The WebSocket protocol version the client wishes to use when communicating with the server. This number should be the most recent version possible listed in the IANA WebSocket Version Number Registry. The most recent final version of the WebSocket protocol is version 13.
<server-supported-versions>
On error, a comma-delineated list of the WebSocket protocol versions supported by the server. The header is not sent in responses if <version> is supported.
The version supported by the client is specified in the original WebSocket handshake request. For the current protocol, the version is "13", as shown below.
GET /chat HTTP/1.1
Host: example.com:8000
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Sec-WebSocket-Version: 13
If the server supports version 13 of the protocol, then Sec-WebSocket-Version will not appear in the response.
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