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

[Roger Cover]

Greetings List,

This is in regards to a question I posted some time ago...

I am now able to reproduce this error in my lab (in 6-10 hours of operation). I 
have attached a packet capture to refresh your memory about the problem (it 
occurs at the end). Unfortunately, the problem only occurs in a heavy-traffic 
environment. If you can point me to the relevant area in the source, I would be 
happy to add some custom debug messaging to help track the problem without 
getting swamped in messages.

Regards,
Roger
-----Original Message-----
From: address@hidden [mailto:address@hidden On Behalf Of Kieran Mansley
Sent: Friday, September 17, 2010 8:51 AM
To: Mailing list for lwIP users
Subject: Re: [lwip-users] Connection timeout (sometimes)

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  


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SYN_ACK_NO_SYN_trim.pcap
Description: SYN_ACK_NO_SYN_trim.pcap