[c-nsp] TCP ZeroWindow

[Cheung, Rick]

	Hi, folks, this may be OT, but it goes through a pair of
3600s... :)

	I have an isolated network, with a Redhat 2.4 box on one end,
connected to a 3640 (12.3.15) with an x-over, bridging its FE0/0 with
the PPP Multilink (bundle of 8 PTP T1s), connected to another 3640 with
a unix type box, also with an x-over.

	The response times from Mu1 to Mu1 is around 50ms. However, file
transfers with a proprietary TCP based app between the two end stations
are terrible, at most 800kbps on the Mu1 interfaces. File transfers from
either direction suffer the same. Interfaces are clean of errors, and
CPU is less than 20%.

	A packet capture reveals that every so often, the Redhat box
sends ~20 packets with a packet length of 1514, but the window size of
0. This happens every 100 someodd packets. The window size increases to
~1888 after these 20 packets, then decreases steadily to 0, and the
process starts over again. Googling around, it seems a problem with
Redhat/Fedora.

	What's interesting is that the problem doesn't manifest itself
over the LAN, with latency lower than 10ms; its only over the WAN. The
nature of the error message reads: This effectively tells the sender to
stop sending because the receiver's buffer is full. Indicates a resource
issue on the receiver, as the application is not retrieving data from
the TCP buffer in a timely manner.

	I just want confirmation that its not the network, but the
Redhat.

	Also, does anyone know the fix?



Thanks,
Rick Cheung



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