title: TCP Windowsize 0 after FIN packet
body: <p>Is it okay if a machine sets the TCP windowsize to zero after receiving a FIN?</p>
  
  <p>I've got the following packet dump from wireshark of the end of the connection and I'm just wondering if this is a valid way to end a connection or if something is wrong.</p>
  
  <pre><code>192.168.1.1 192.168.1.6 TCP 3450 &gt; 102 [FIN, ACK] Seq=48 Ack=50 Win=65486 Len=0
  192.168.1.6 192.168.1.1 TCP [TCP ZeroWindow] 102 &gt; 3450 [ACK] Seq=50 Ack=49 Win=0 Len=0
  192.168.1.6 192.168.1.1 TCP 102 &gt; 3450 [FIN, PSH, ACK] Seq=50 Ack=49 Win=0 Len=0
  192.168.1.1 192.168.1.6 TCP 3450 &gt; 102 [ACK] Seq=49 Ack=51 Win=65486 Len=0
  </code></pre>
  
  <p>BTW: .1 is a regular windows PC while .6 is a Siemens PLC. (S7-400)</p>
  
  <hr>
  
  <p>After some investigation it looks like a weird but valid way to end a TCP conversation. </p>