FreeBSD 7 TCP syncache fix: request for testers

Steve Kargl sgk at
Tue Jul 10 06:26:04 UTC 2007

On Tue, Jul 10, 2007 at 12:20:49AM -0500, Mike Silbersack wrote:
> I've found one of the causes of the network instability of FreeBSD 7; the 
> tcp syncache fails to retransmit SYN-ACK packets.  This causes interesting 
> problems when packet loss is experienced during connection setup.  The 
> symptoms that I have witnessed are twofold:
> 1.  If the third part of the 3WHS is lost, the client will believe that 
> the connection is in the ESTABLISHED state, while the server will still 
> have the connection in the syncache.
> 2.  Subsequently, the above syncache entry will stay stuck in the syncache 
> forever.  If you attempt to re-use that same 4-tuple, the syncache will 
> ack the new SYN with the old sequence number.
> Anyway, the attached patch simplifies the syncache structure a bit and 
> makes it retransmit properly.  I'd appreciate testing from anyone who has 
> experienced TCP problems with FreeBSD 7, as well as anyone who is pushing 
> significant traffic through FreeBSD 7.


Thanks for tracking this problem down.  I can't test this patch
for a few weeks due to deadlines.  I'll run some tests when I can.

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