[LARTC] Detecting stale TCP/IP

[LARTC] Detecting stale TCP/IP

[Ben Clewett]

Dear lartc,

I have a TCP/IP server connecting to a GPRS PDA.

Unfortuntatelly GPRS seems unstable, and for this reason or another, a 
interupt to the TCP/IP connection (like switching the clinet off 
suddenly) does not terminate the TCP/IP connection.  It can still be 
seen in 'netstat', and a test by the application shows it present.  It 
may time out after about 40 minutes.

My real problem is a call to 'write' in non-blocking mode returns 
success when the other end of the TCP/IP connection is not there.

Does any member know a reference to where I can test for the reply 
TCP/IP 'ACK' packet for this write call, and therefore timeout and 
terminate the connection?

Thanks in advance,

Ben Clewett.

_______________________________________________
LARTC mailing list / LARTC@mailman.ds9a.nl
http://mailman.ds9a.nl/mailman/listinfo/lartc HOWTO: http://lartc.org/

Re: [LARTC] Detecting stale TCP/IP

[Jasper Spaans]

On Tue, Jul 29, 2003 at 08:56:24AM +0000, Ben Clewett wrote:
> Dear lartc,
> 
> I have a TCP/IP server connecting to a GPRS PDA.
> 
> Unfortuntatelly GPRS seems unstable, and for this reason or another, a 
> interupt to the TCP/IP connection (like switching the clinet off 
> suddenly) does not terminate the TCP/IP connection.  It can still be 
> seen in 'netstat', and a test by the application shows it present.  It 
> may time out after about 40 minutes.
> 
> My real problem is a call to 'write' in non-blocking mode returns 
> success when the other end of the TCP/IP connection is not there.
> 
> Does any member know a reference to where I can test for the reply 
> TCP/IP 'ACK' packet for this write call, and therefore timeout and 
> terminate the connection?

You can try using the ioctl TIOCOUTQ, see socket(7). Although this manpage
states otherwise, at least on 2.4.21 and newer it returns the number of
unacked bytes.

Implement a select-loop, and see if the value of this ioctl changes. If not,
close the connection, and the kernel should handle the rest.


VrGr,
-- 
Jasper Spaans                 http://jsp.vs19.net/contact/

<=      Het bedrijf van de appelcomputer is niet      =>
<=    verantwoordelijk voor de omzettingsprecisie.    =>
_______________________________________________
LARTC mailing list / LARTC@mailman.ds9a.nl
http://mailman.ds9a.nl/mailman/listinfo/lartc HOWTO: http://lartc.org/

Re: [LARTC] Detecting stale TCP/IP

[Ben Clewett]

Jasper,

Thanks.  This works absolutelly perfectly!

I found that simply waiting for the TIOCOUTQ to fall to zero gives a 
perfect indication of data having been sent correctly, and if not, that 
the connection is stale.

I have put this into a:

while (test) sleep(1);

But this waists time.  With a ping of 300ms being common, a sleep of 
1000ms is way to much to check every send, so I can only check agregates 
of sending.  Would anybody know of a better method?

Ben

Jasper Spaans wrote:

> On Tue, Jul 29, 2003 at 08:56:24AM +0000, Ben Clewett wrote:
> 
>>Dear lartc,
>>
>>I have a TCP/IP server connecting to a GPRS PDA.
>>
>>Unfortuntatelly GPRS seems unstable, and for this reason or another, a 
>>interupt to the TCP/IP connection (like switching the clinet off 
>>suddenly) does not terminate the TCP/IP connection.  It can still be 
>>seen in 'netstat', and a test by the application shows it present.  It 
>>may time out after about 40 minutes.
>>
>>My real problem is a call to 'write' in non-blocking mode returns 
>>success when the other end of the TCP/IP connection is not there.
>>
>>Does any member know a reference to where I can test for the reply 
>>TCP/IP 'ACK' packet for this write call, and therefore timeout and 
>>terminate the connection?
> 
> 
> You can try using the ioctl TIOCOUTQ, see socket(7). Although this manpage
> states otherwise, at least on 2.4.21 and newer it returns the number of
> unacked bytes.
> 
> Implement a select-loop, and see if the value of this ioctl changes. If not,
> close the connection, and the kernel should handle the rest.
> 
> 
> VrGr,


_______________________________________________
LARTC mailing list / LARTC@mailman.ds9a.nl
http://mailman.ds9a.nl/mailman/listinfo/lartc HOWTO: http://lartc.org/