title: HTTP response code when requested websocket subprotocol isn't supported/recognized
body: <p>When upgrading an HTTP connection to a websocket, one can provide one or more subprotocols in the optional HTTP header 'Sec-WebSocket-Protocol'.</p>
  
  <p>If the server accepts any of the subprotocols it responds with HTTP response code 101 ("HTTP/1.1 101 Switching Protocols") and includes the HTTP header 'Sec-WebSocket-Protocol' indicating the selected subprotocol.</p>
  
  <p>But how should the server correctly handle an unknown/unsupported subprotocol?</p>
  
  <p>Should this be done 'within' the HTTP connection -- by use of some HTTP response code?</p>
  
  <p>Or should the connection be upgraded to a websocket -- and immediately be closed by the server by sending a 'Close Frame' with some of the predefined websocket Status codes?</p>
  
  <p>What does the RFC6455 say? I cannot come to a conclusion.
  How does existing server implementations handle it?</p>
  
  <p>Regards
  /Per/</p>