[Windump] Receive window size problem

[Geoff Shipton]

I am very new to this and feeling my way a bit, I work for a software 
company although I am not a software engineer.

I need to figure out why a TCP link we use is failing. The TCP link 
streams data from an instrument to a logging system continuously over a 
dedicated NIC pair.  The data volume is quite high although in general 
the Network usage rarely exceeds 10% (of a Gigabit link).

I have installed and run Windump successfully and I am able to see what 
happens immediately prior to the failure.

The failure always occurs eventually, but sometimes only after many 
hours. On every occasion I notice that even whiel the link is "up" the 
TCP receive window size varies  from the  default maximum  up and down  
sometimes dropping very low (104) and usually recovering.  But at some 
point the size drops to zero at which point the instrument stops sending 
and goes into "persist timer" then all I see is occasional (60 secs) 
requests from the instrument to the logging system; each time the logger 
responds with recieve window zero so the connection is effectively dead.

Simply going online and online at the logging end restores the connection.

The programmers I work with seem not have many ideas about this problem 
so any help is much appreciated.

Geoff