https://tools.ietf.org/html/rfc6455#section-4.4

4.4. Supporting Multiple Versions of WebSocket Protocol

   This section provides some guidance on supporting multiple versions
   of the WebSocket Protocol in clients and servers.

   Using the WebSocket version advertisement capability (the
   |Sec-WebSocket-Version| header field), a client can initially request
   the version of the WebSocket Protocol that it prefers (which doesn‘t
   necessarily have to be the latest supported by the client).  If the
   server supports the requested version and the handshake message is
   otherwise valid, the server will accept that version.  If the server
   doesn‘t support the requested version, it MUST respond with a
   |Sec-WebSocket-Version| header field (or multiple
   |Sec-WebSocket-Version| header fields) containing all versions it is
   willing to use.  At this point, if the client supports one of the
   advertised versions, it can repeat the WebSocket handshake using a
   new version value.

   The following example demonstrates version negotiation described
   above:

      GET /chat HTTP/1.1
      Host: server.example.com
      Upgrade: websocket
      Connection: Upgrade
      ...
      Sec-WebSocket-Version: 25






Fette & Melnikov             Standards Track                   [Page 26]

 
RFC 6455                 The WebSocket Protocol            December 2011


   The response from the server might look as follows:

      HTTP/1.1 400 Bad Request
      ...
      Sec-WebSocket-Version: 13, 8, 7

   Note that the last response from the server might also look like:

      HTTP/1.1 400 Bad Request
      ...
      Sec-WebSocket-Version: 13
      Sec-WebSocket-Version: 8, 7

   The client now repeats the handshake that conforms to version 13:

      GET /chat HTTP/1.1
      Host: server.example.com
      Upgrade: websocket
      Connection: Upgrade
      ...
      Sec-WebSocket-Version: 13

Supporting Multiple Versions of WebSocket Protocol 支持多版本WebSocket协议