title: how to activate the TCP Fast Open in linux
body: <p>I have 2 PCs linux (PC1: kernel 3.13.0-37 and PC2: kernel 3.11.0-12)</p>
  
  <pre><code>PC1-------PC2(TCP server port 4410)
  </code></pre>
  
  <p>From PC1, I m sending a tcp packet with TCP Fast Open (Fast Open Cookie Request)</p>
  
  <p><img src="https://i.stack.imgur.com/SAf8c.png" alt="enter image description here"></p>
  
  <p>I m expecting to get an answer from the server with TCP option (Fast Open Cookie: xxxxxxx) something like this:</p>
  
  <p><img src="https://i.stack.imgur.com/SeatT.png" alt="enter image description here"></p>
  
  <p>But I got a tcp packet without the TCP option (Fast Open Cookie: xxxxxxx).</p>
  
  <p>I m wondering if there is something to configure on my PC2 (linux) in order to activate the TCP Fastt Open option.</p>
  
  <p>For the TCP server, I m running a php script:</p>
  
  <pre><code>$sock = socket_create(AF_INET, SOCK_STREAM, 0);
  
  // Bind the socket to an address/port
  socket_bind($sock, "0.0.0.0", 4410) or die('Could not bind to address');
  for(;;) {
      // Start listening for connections
      socket_listen($sock);
      ...
  }
  </code></pre>