Re: [lwip-users] MSS Configuration Difficulty

[Kieran Mansley]

On Mon, 2011-02-14 at 07:30 -0700, address@hidden wrote:
> 
> We have been using lwip version 1.2

I would strongly suggest upgrading.  1.2 is years out of date and even
more unsupported than the current version (we're about to release
1.4.0).

> At this point, we are not using any TCP options, so we believe 1460
> would suffice. I would definitely appreciate any feedback on that
> assertion. 

1460 sounds good to me.

> In our case the p->tot_len is 3016, and the sum of
> the p->len is 1516.  Since the loop is traversing the packet chain
> until the tot_len is zero, this walks off the end of the chain.
> 
> What I'm unsure of is whether the IP fragmentation code can tolerate
> this misconfiguration of the MTU, or whether there is likely an error
> in the fragmentation code, or perhaps in one of our drivers. 

I'm not sure which is to blame either, but with mismatch between
p->tot_len and sum(p->len) is definitely a problem.  As I said before,
I'd strongly suggest upgrading to 1.4.0 as there's a chance this was an
lwIP bug that was already fixed (although it doesn't sounds familiar to
me).

> Any thoughts on this are greatly appreciated. Additionally, since we
> are executing in a real-time multi-threaded environment our
> investigation is focusing on timing and race condition scenarios. Feel
> free to chime in with observations and experience in those areas as
> well.   

You have to be particularly careful with lwIP in these environments and
ensure that only one thread is active in the lwIP core at once.  A
common way that failure to do this manifests itself is for lists of PCBs
or pbufs to become corrupted.  This therefore might be the cause of your
problem.

Kieran