Re: [lwip-users] Options

[Kieran Mansley]

On Thu, 2010-06-10 at 19:48 +0100, Adam Fullerton wrote:

> It is like it won't send a TCP packet bigger than 566 bytes, but I can
> send UDP packets up to the MTU (1460 bytes of data). I am not
> expecting to see the IpFrag debug print statement but I do. Does any
> one know what configuration parameter I have got wrong?

It's odd that there should be a difference between TCP and UDP as the
lower layers should treat them identically.

Your IpFrag debug print statement doesn't look like anything to do with
IP fragmentation to me.  You're printing this out whenever you see a
chain of pbufs (i.e. pbuf->next != NULL).  Chains of pbufs can be used
in lots of circumstances within lwIP but normally (and probably in your
case) they're used when the packet being formed is larger than one pbuf
can hold.  I would guess that your pbuf size is something around 566
bytes, so to form a full MTU packet will need more than one pbuf.  If
you're ignoring subsequent pbufs in the chain, or treating them all as
separate packets, this would definitely cause the sort of problems that
you're seeing.

You need to either arrange for your hardware to send the set of pbufs in
the chain as a single packet (i.e. scatter-gather), configure your pbufs
larger to be able to hold a full MTU (potentially wasteful), or set your
MTU smaller.

Kieran