[lwip-users] TCP send() fails when other sockets perform retransmissions

[Daniel Pauli]

Hey there

I have a TCP client running on a Windows PC that communicates with a TCP 
server on a LWIP box. The client sends requests to the server in high 
frequency (polling) and receives responses of approx. 16 KB.

The problem: When a clients crashes (without being able to send FIN/RST 
because of e.g. temporary link down) and gets restarted, the server is 
unable to send responses over the new connection because send() blocks. 
This happens even though I'm using select() to determine if sockets are 
writable. I'm using a SO_SNDTIMEO of 3 seconds for tracking the problem 
down. Strangely, send() already returns after 1 second with EWOULDBLOCK.

I observed that LWIP performs TCP retransmissions on the socket from the 
crashed client. In my setting, these retransmissions are ignored by the 
PC (due to the Windows firewall).
The problem resolves after approx 25 min when LWIP stops performing 
these retransmissions. After that, send() works again for new connections.

Any hints are greatly appreciated :)

Regards
Daniel