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Re: [lwip-users] Closing TCP connections

From: Mark Lvov
Subject: Re: [lwip-users] Closing TCP connections
Date: Sun, 3 Aug 2014 16:21:07 +0400

This is exactly what I've done, only I have a class, each instance of which manages a connection to a particular endpoint (only, my "state" enum has ACTIVE_CLOSE and PASSIVE_CLOSE instead of CLOSING, since those are handled differently). The thing is, it does not really help much with the problem I've outlined in the earlier message. Even though I get the pointer to a particular instance of the class in the "arg" argument to the err callback, it is still impossible to know if it is the latest connection, that errored out, or if it is some other, older connection, that was also opened to the same endpoint, but was stuck in some "waiting" state, such as LAST_ACK. I hope, this makes sense.


On Sun, Aug 3, 2014 at 12:25 AM, M. Manca <address@hidden> wrote:
Il 02/08/2014 20:52, Mark Lvov ha scritto:

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.
This is one of the reason I wrote an "upper layer" to the raw functions to manage a tcp client connection.
The idea is simple: create a struct to embed every information needed to manage a particular connection so I can pass it as the void *arg
that is also present in the error callback.
I think you can understand my idea just reading the struct I made:

typedef enum
} TTcpRawSktState_e;

typedef struct
   TTcpRawSktState_e eState;
   err_t eError;
   struct pbuf *pPbuf;
   struct tcp_pcb *pPcb;
   struct ip_addr tServerIpAddr;
   uint16_t wServerPort;
   uint16_t wBindPort;
   char szDescription[8];
} TTcpRawSkt;

the description field isn't necessary, I used it to debug in the simplest way my code.
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.


On Fri, Aug 1, 2014 at 9:01 AM, Mark Lvov <address@hidden> wrote:
Well, shame on me!

I was actually using
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.


On Thu, Jul 31, 2014 at 10:01 PM, Sergio R. Caprile <address@hidden> wrote:
Read the wiki, and if it is not clear enough, I will change it




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Massimo Manca
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