Re: [lwip-users] Bind fails after previous server-initiated connection close

[Sergio R. Caprile]

According to major communications texts authors, due to parallel paths 
and some esoteric magic, some data might be in transit when you close a 
TCP connection. IIRC it is mandatory that it remains that way after closure.
There is a thread on this list where Simon answered (sort of) your question

If you need a faster response, you need to provide more pcbs, look at 
the httpd server, for example, which serves lots of short connections.