Re: [lwip-users] TCP keep-alive not working on half-closed connections

[Joel Cunningham]

On 06/06/2018 02:13 PM, address@hidden wrote:
> > https://stackoverflow.com/questions/18110239/does-tcp-endpoint-that-has-sent-a-fin-still-send-keepalive
> > https://social.msdn.microsoft.com/Forums/en-US/d20e5e67-d23c-42a5-95c5-ba80f4263546/keepalive-packets-continue-being-sent-after-client-closes-server-socket-is-closewait-client
> >
> > Here's some findings on a FIN_WAIT_2 timeout:
> >
> > https://serverfault.com/questions/738300/why-are-connections-in-fin-wait2-state-not-closed-by-the-linux-kernel
> > https://www.akronohio.gov/manual/misc/fin_wait_2.html
> >
> > I don't believe a FIN_WAIT_2 timeout is implemented in LwIP.
>
> No, we don't have that. We always said it's OK for a connection to 
> stay in FIN_WAIT_2. However, it seems we must handle this differently 
> depending on wether the user has called "shutdown" or "close". Then we 
> can implement a timeout.

Agreed, if the application used shutdown() on only the TX pathway, we 
would not want to start a timeout until the RX pathway is also 
shutdown.  Maybe even further delay the timeout until the app is done 
with the socket as indicated by close().

> > I'm also thinking this issue may not be that common since tcp_alloc 
> > would reap the half-open connection eventually to prevent pcb exhaustion
>
> I don't think tcp_alloc will kill pcbs in FIN_WAIT_2 unless they have 
> a lower priority. A timeout on FIN_WAIT_2 *does* make sense!
You're right, I forgot the behavior was changed somewhat recently to not 
kill same priority anymore

Joel