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

[address@hidden]

On 06.06.2018 11:42, R. Diez wrote:
> [..]
> The trouble is, this changes the connection state (pcb->state) from
> ESTABLISHED to FIN_WAIT_1, which then turns quickly into FIN_WAIT_2.
> Afterwards, TCP keep-alive does not work anymore, because of this logic
> in tcp_slowtmr():
>
>     /* Check if KEEPALIVE should be sent */
>     if (ip_get_option(pcb, SOF_KEEPALIVE) &&
>        ((pcb->state == ESTABLISHED) ||
>         (pcb->state == CLOSE_WAIT))) {
>
> If I then pull the server's Ethernet cable while the connection is
> transferring data, the connection never times out on the client.
>
> I am no TCP/IP expert. Is there an error in my reasoning regarding
> half-closed connections? Can TCP keep-alive packets, whatever they are,
> still be sent and/or received on half-closed TCP connections?
Half-opened connections should be usable like you did it, yes. I'm not 
sure about keepalives though.
Technically, keepalives are just empty ACKs, so that should work.

However, I'm not sure wether tcp keepalive is meant to work on half-open 
connections.


Simon