[lwip-users] Fragmented UDP package not triggering socket select

[Bram Peeters]

Hi, 

I am running into a strange problem where my application is waiting for data to 
arrive on and UDP socket via an (lwip_)select call, and it works fine if an 
ipv4 UDP packet with  828 bytes is send , but nothing happens when a fragmented 
(1480 + 43) packet arrives although the destination port/ip are the same. If 
afterwards another 828 byte package is send, it arrives without problems. 
This behavior is 100% reproducible, and I use a hub + wireshark to make sure 
the package really appears on the line connected to the device. 
At first sight the only relevant difference between the 2 scenarios is the 
package length and resulting fragmentation (though of course the contents are 
different too).
Reading the 2.0.3 release notes I saw that there was some fragmentation issue 
in the past, so I updated from 2.0.0 to  2.0.3 but the behavior remains the 
same.

I still need to dive into lwip to investigate what might be going on, but if 
this rings any bells let me know ! :)
Also hints to get started to debug how far the arriving fragemented package 
gets are welcome, it's been a while since I looked at lwip sources so ....

Thanks, 
Bram