[lwip-users] Potential problem with IP Fragmentation and TCP Retransmit

[Tom Hennen]

I've run into what may be a problem with IP Fragmentation when
combined with TCP Retransmit in lwip 1.1.1 (I'm not sure if this
problem exists in 1.2).

It seems that ip_frag (via copy_from_pbuf) modifies the pbuf given to
it.  During normal operation this is fine; it causes problems when TCP
tries to retransmit a segment that has already been fragmented.  In
that case the pbuf's len and payload pointers will be incorrect
(thanks to the first call to ip_frag) and this can cause errors during
the *second* call to ip_frag.

I'm not quite sure what the correct solution is.  Either a copy of the
pbuf chain could be made (minus the payloads) or copy_from_pbuf could
be modified so as not to change the state of the pbuf (or at least
restore it once it's done).

Thoughts?

Thanks,

Tom Hennen