LTM :: Zero Window Server Side :: TCP Profiles

Reply from Barracuda:

Looks like this is pretty much ubiquitous on all devices that are under reasonably large load in the real world. All devices that I've checked that are within 60% or so of their max load show hundreds to thousands of these zero window packets being sent from the ESG to other mail servers. This is giving the SMTP service sufficient time to process the data lines as they are sent. This behavior goes all the way back to 6x firmware and is not limited to virtual boxes but applies to physical devices as well. Primary issue for this customer then was the low timeout setting on his load balancer for zerowindow issues. With that timeout set higher, we are now seeing very few to no connections dropped for this reason.

Solution:

I did the above on the server side, and left the client side default (20s).

Hopefully this helps somebody else!