From: Olaf Kirch <okir@suse.de>
Subject: NFS over TCP: random drop
Date: Tue, 16 Mar 2004 12:11:49 +0100
Sender: nfs-admin@lists.sourceforge.net
Message-ID: <20040316111149.GS25405@suse.de>
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To: nfs@lists.sourceforge.net
Errors-To: nfs-admin@lists.sourceforge.net

Hi,

I am looking into an NFS problem at one of our customers. They are
running NFS over TCP, and the client is some kind of mainframe OS.

What happens is this:

client:		initiates connections, 3way handshake completes
server:		closes connection (FIN)
client:		(at the same moment) sends data to server
server:		RST
client:		argh (reports EIO to application)

What seems to happen here is the random connection drop in svcsock.c,
where we randomly drop a connection if the total number of TCP connections
exceeds (nrthreads + 3) * 10.  "Randomly" here means either the oldest
connection, or the newest one (which happens to be the one we just
accepted).

The specific problem we have here is that the client doesn't grok TCP RSTs
generated by the server. It's arguably a client bug, but I'm nevertheless
thinking about a way to work around this

Possible solutions:

 -	always drop the oldest connection. The current strategy doesn't
 	help much against DoS anyway, because rather than dropping
	old connections for _every_ new one, we drop them for every
	two new connections. Big improvement :)

 -	Create a sysctl that allows to set a hard limit for active
 	TCP connections.

 -	increase the "random drop" threshold to something like
 	(nrthreads + 3) * 100.

Any comments?

Olaf
-- 
Olaf Kirch     |  Stop wasting entropy - start using predictable
okir@suse.de   |  tempfile names today!
---------------+ 


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