Tsung doen't respond with PONG

[n1architect]

Tsung doesn't respond with pong making all WebSocket connections close.

My config:

<?xml version="1.0"?>
<!DOCTYPE tsung SYSTEM "/usr/local/Cellar/tsung/1.7.0/share/tsung/tsung-1.0.dtd">
<tsung loglevel="debug" version="1.0">
  <clients>
           <client host="localhost" use_controller_vm="true" maxusers="60000"/>
  </clients>

  <servers>
    <server host="localhost" port="80" type="tcp"></server>
  </servers>

  <load>
    <arrivalphase phase="1" duration="100" unit="second">
      <users maxnumber="10" arrivalrate="1" unit="second" />
    </arrivalphase>
  </load>

  <options>
    <option name="ports_range" min="1025" max="65535"/>
  </options>

  <sessions>
    <session name="websocket" probability="100" type="ts_websocket">
             <request>
        <websocket type="connect" path="/app/097up9rt2wc0jbqhaagg"></websocket>
      </request>

      <request subst="true">
        <websocket type="message">{"event":"pusher:subscribe", "data": { "auth":"", "channel": "public-channel" }}</websocket>
      </request>

      <for var="i" from="1" to="1000" incr="1">
        <thinktime value="30"/>
      </for>
    </session>
  </sessions>
</tsung>