Re: [lwip-users] Significant overhead periodically during TCP receive transfers

[Bob McConnell]

Kieran Mansley wrote:
> On Wed, 2008-10-08 at 16:03 -0400, bill wrote:
> > > That truly is weird.  If you can get a packet capture from just before
> > > the failure, I'd be happy to take a look.
> > I don't know if the text version I sent helped, but I know a lot more now.
> >
> > What I see in the packet capture that causes the problem is that the PC
> > broadcasts an ARP "who has", 50uS later lwIP sends a response packet to the
> > PC. 4uS after that the PC sends 1460 bytes to lwIP, but the sequence number
> > is 13140 bytes *ahead* (9 * 1460) of the expect number.  It looks like the
> > PC has dropped 9 packets while it was doing its ARP query.  Unless WireShark
> > missed these packets (doubtful).
> >
> > If I change the PC's ARP table to make the lwIP device static, this problem
> > disappears!
>
> It sounds like you've got to the bottom of your problem, but why the ARP
> table should need refreshing when packets are being transmitted, and why
> the existing entry shouldn't be used while it's waiting to hear the
> answer is rather odd.  I wonder if you have two things with the same IP
> address on the network, and the PC is getting confused about which MAC
> address to use?

Most systems send periodic ARP requests, whether there is constant 
traffic over an open socket or just periodic keep-alive exchanges. Don't 
ask me why, but I have seen it in Wireshark traces from various versions 
of MS-Windows, Linux and FreeBSD. The only question here is why does the 
sender drop those nine extents? That sounds like a bug in their TCP/IP 
stack.

Bob McConnell
N2SPP