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[lwip-devel] [bug #24493] Dropped incoming packets cause a burst of dupl

From: Bill Auerbach
Subject: [lwip-devel] [bug #24493] Dropped incoming packets cause a burst of duplicate ACKs
Date: Tue, 21 Apr 2009 21:26:43 +0000
User-agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv: Gecko/2009032609 Firefox/3.0.8

Follow-up Comment #7, bug #24493 (project lwip):

Simon, I didn't capture at the right times to see the stall because I was
after the source of dropped packets, which were real, as they caused us to
have real-time issues.  The stall happened only a few times over dozens of
runs to find the pattern and cause of the dropped packets.  Once I figured out
how to stop the dropped packets, everything was (and has been) error-free.

Not recovering meant all transmissions stopped.  My guess was some queue in
lwIP was full and either could no longer accept packets or lwIP could no
longer transmit packets.  It had to be timing related because often it would
recover leaving us with our real-time issue (it took too much time for Windows
to recover from the packets it dropped and we would run out of data on the
lwIP receiving end).  This cause of dropped packets was different than normal
errors because Windows would just go away for a longer time than it would for
normal errors because it was a multiple packet drop. One or two packet drops
we can handle and maintain the real-time aspect of our application.

The duplicate ACKs are OK as Kieren mentioned.  My comment about the not
recovering was made because I thought TCP would always recover.  But because
the drops were on the Windows side, it very well could be the loss of
transmissions are also on the Windows side.

You could leave this open for 1.4.0 and I'll squirrel away some time to try
to duplicate and capture this.  It will take ~9:30S to run to duplicate the
packet drops as were occurring originally and who knows how long for the loss
of recovery.  Or I can try to simulate it and see if it fails the same way. 
If I can't duplicate it then it's a non-issue I guess.


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