Re: [lwip-users] Connections slowing down after some time

[Kieran Mansley]

On Fri, 2009-03-06 at 14:25 +0100, Steffen Wolfer wrote:
> On Fri, 2009-03-06 at 12:23 +0000, Kieran Mansley wrote:
> 
> > If you have a wireshark capture that is often useful to share so we can
> > see what's going on.
> 
> So here are two wireshark captures (each as a .pcap and a .txt file), showing 
> one of the very fast responses and one rather slow one. I also captured the 
> lwIP stats after each request.
> 192.168.0.10 is my computer, 192.168.0.23 the microcontroller running lwIP.

Looking at the traces there are two problems:

 1) Sometimes a SYN needs to be retransmitted
 2) Sometimes data segments are retransmitted only after a very large
gap

The first is explained by the stats: you're running out of TCP PCB
structures, which means when a new SYN arrives there's no PCB available
and we can't do anything with the connection.  This would be solved by
increasing the number of TCP PCBs in lwipopts.h

The second is rather odd as the time to wait before retransmitting is
down to the other end.  As an example see frame 60 in the
slow_reply.pcap trace.  This is a retransmission of frame 59, but it
waited 6 seconds, which is an unusually long time and isn't really the
fault of lwIP.  I'd expect it to wait around 200ms.  Are you running a
normal TCP/IP stack on the other end?

Kieran