TCP Zero Window

Unanswered Question
Jun 10th, 2008
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I've noticed that on most of my FTP transfers I'll see "TCP Zero Window" messages throughout the transfer. In the capture (I'm capturing on the LAN side of the client before optimization) they all come from the Server and not the client. Sometimes there are very few and sometimes there is a Zero Window every 10 or 20 packets.

From my understanding with a setup like this:


There are three conversations going on, one between the client and the local WAE, one between the two WAE's and one between the Server and it's local WAE.

My question is where are the "TCP Zero Window" messages coming from?? Are they coming from the WAE or are they coming from the server back to the client. I've had the systems guys adjust the buffer settings on the server but I'm still seeing them.



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ebreniz Mon, 06/16/2008 - 13:21
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The reason could be that the application is not responding to read the data from the tcp buffers and the tcp on the receivers side is not sending any ACK's back to the sender. Even in this case the router should be able to ack the tcp zero window probes from the sender if it is receiving those packets. The ack timers seems to have expired sometime back and is not reset since by default delayed ack is turned on the router. Unless there is a new data packet to send from the router, the ACK is not piggybacked and hence is not sent to the remote. By disabling delayed ack using the hidden command

no ip tcp delayed-ack

Zach Seils Mon, 06/16/2008 - 23:42
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WAAS will provide back pressure end-to-end, so you'll need to take a simultaneous capture on both ends to determine where the zero window segment is being generated.




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