TCP Windowsize 0 after FIN packet

Question

Is it okay if a machine sets the TCP windowsize to zero after receiving a FIN?

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.

192.168.1.1 192.168.1.6 TCP 3450 > 102 [FIN, ACK] Seq=48 Ack=50 Win=65486 Len=0
192.168.1.6 192.168.1.1 TCP [TCP ZeroWindow] 102 > 3450 [ACK] Seq=50 Ack=49 Win=0 Len=0
192.168.1.6 192.168.1.1 TCP 102 > 3450 [FIN, PSH, ACK] Seq=50 Ack=49 Win=0 Len=0
192.168.1.1 192.168.1.6 TCP 3450 > 102 [ACK] Seq=49 Ack=51 Win=65486 Len=0

BTW: .1 is a regular windows PC while.6 is a Siemens PLC. (S7-400)

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After some investigation it looks like a weird but valid way to end a TCP conversation.

Answer 1

I see nothing wrong with sending a zero window after a FIN ACK ... presumably 192.168.1.6 sent a FIN to 192.168.1.1 , so they are now closing the connection.

192.168.1.6 192.168.1.1 TCP [TCP ZeroWindow] 102 > 3450 [ACK] Seq=50 Ack=49 Win=0 Len=0

But immediately setting a PSH flag and sending no data (Len=0) right after that ACK , looks rather odd (but not technically wrong) to me...

192.168.1.6 192.168.1.1 TCP 102 > 3450 [FIN, PSH, ACK] Seq=50 Ack=49 Win=0 Len=0