Re: [lwip-users] Connection timeout (sometimes)

[Kieran Mansley]

On Fri, 2010-09-17 at 08:00 -0700, Roger Cover wrote:
> Greetings List,
> 
> A customer of mine has encountered a problem. The problem only occurs rarely. 
> I have not seen it in my testing. The system is a few years old, so it was 
> built using lwIP 1.2. I need some information to help with my debugging 
> efforts. The symptoms are as follows:
> 
> Host computer sends a packet with the SYN flag set to open the connection.
> lwIP sends a packet back with ACK set, but no SYN.
> After a time, Host sends another SYN attempt.
> lwIP sends another packet back with ACK set, but no SYN.

This is very odd, and clearly wrong, but I've got no good ideas about
why it would happen.  To send an ACK in response to a SYN we must think
that the connection is not a listening one.

If we thought we'd sent a SYN-ACK already, that might explain it, as
receiving another SYN prompts us to try to retransmit it (and I wouldn't
be surprised if we ended up sending an ACK).

If we thought it was a socket in the TIME_WAIT state that would also be
a plausible explanation.  Any out-of-window SYN is ACKed.  This would
continue for a couple of minutes until the connection moved to CLOSED.

You could probably find out more by turning on the TCP_INPUT_DEBUG
messages, assuming it is easy to reproduce without a lot of traffic as
otherwise you'll be swamped with debug messages.

Kieran