Re: [lwip-users] tcp_close() and lwip still holds pcb

[Kieran Mansley]

On Fri, 2009-03-20 at 09:10 +0100, Reither, Robert (AU14) wrote:

> This means, lwip still holds the pcb for 2*TCP_MSL == 120s and than
> frees the pcb in tcp_slowtmr.
> 
> Is this the correct style, I mean the client has already terminated
> the connection, why does lwip still wait for some response ??

Yes, this is correct.  TCP is very cautious about delayed packets from
one connection interfering with a later connection, and so it holds one
end in TIME_WAIT to prevent the same port numbers being used for a
while.  This isn't lwIP specific - it's part of every TCP.  That said,
there have been lots of attempts to subvert this behaviour: windows for
example will use RSTs rather than a FIN to close a connection sometimes,
and there is an SO_REUSEADDR socket option that (if implemented - I
doubt it is in lwIP) allows you to reuse the ports that are in TIME_WAIT
in some circumstances.

Kieran