title: Receiving ping pong on closed connection
body: <p>I am using nodejs and ws to create a websocket server. The server runs behind a nginx proxy. 
  I have written a c client using libwebsocket which connects to this client. This setup works well for couple of days but after that connection is reported closed  on nodejs server but client still continues to receive pong for each ping it sends.</p>
  
  <p>Nginx config is as followes</p>
  
  <p><code>proxy-connect-timeout:120
  proxy-read-timeout:3600
  proxy_send_timeout:3600</code></p>
  
  <p>in libwebsocket</p>
  
  <p><code>ka_time :  10s;
    ka_probes: 3;
    ka_interval: 120s;
  ws_ping_pong_interval: 120s</code></p>
  
  <p>Question 1) Does nginx send response to ping sent by client? 
  Question 2) What can be the config to avoid this?</p>