Re: [lwip-users] Closing TCP connections

[Mark Lvov]

Hello,

It seems, I still have some unanswered questions with regard to
correct connection teardown. Let's consider the active close situation
(we are closing the connection). We've just called tcp_closed and are
waiting for the tcp_recv callback to be called with an empty pbuf.
But, as I understand, if the remote side sends RST or does not send
anything at all, the err handler will be called instead. The problem
is, one does now know which particular connection (pcb) the callback
is addressed to, because the callback function does not receive a pcb
as an argument.

Consider the scenario, when one needs to keep a connection open by
reconnecting to the remote side whenever the connection is closed
(either by the remote or the local side). It is all fine when the
remote side closes the connection - we just receive a NULL pbuf, after
that we can just call tcp_close on the "current" pcb, then immediately
ask for the new pcb and reconnect. If, on the other hand, *we* want to
close the connection (to reopen if afterwards), and call tcp_close, we
might have the err callback called and then we won't really know if it
means, that the current connection was terminated or some older
connection, that maybe stuck in LAST_ACK finally timed out.

Does this mean, that in such a situation, one has to keep only one err
callback active at a time? It seems, the best course of action is to
zero out the err callback on a pcb after calling on it tcp_close
successfully (actually, its more like this: zero out the callback,
then call tcp_close and if it fails reattach the callback, because we
will have to wait a bit before calling tcp_close again). But then,
once we call tcp_close, we have to start a timer (for, say, 5
seconds?) and if it runs out we consider the connection closed, remove
all callbacks from that pcb and ask for the new one.

Hopefully, I've managed to explain the problem I am facing. Sorry,
that it took such a long message.

Thanks,
Mark

On Fri, Aug 1, 2014 at 9:01 AM, Mark Lvov <address@hidden> wrote:
> Well, shame on me!
>
> I was actually using
> http://git.savannah.gnu.org/cgit/lwip.git/tree/doc/rawapi.txt?id=5b8b5d459e7dd890724515bbfad86c705234f9ec
> as a reference and it obviously lacks the details, that are present on
> the page you've linked. All my questions are answered by that page,
> thanks very much.
>
> Mark
>
> On Thu, Jul 31, 2014 at 10:01 PM, Sergio R. Caprile <address@hidden> wrote:
>> Counter-proposal:
>> Read the wiki, and if it is not clear enough, I will change it
>>
>> http://lwip.wikia.com/wiki/Raw/TCP
>>
>> Regards
>>
>> --
>>
>>
>> _______________________________________________
>> lwip-users mailing list
>> address@hidden
>> https://lists.nongnu.org/mailman/listinfo/lwip-users