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Re: detecting Socket disconnect

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On Wed, 17 Oct 2012 13:12:13 -0700, bob smith wrote:

> I have a java.net.Socket that I only write to, and I can't figure out
> how to tell if it's disconnected.
> 
> I call socket.isConnected(), but it still returns true after I broke the
> TCP connection.  I had telnetted in thru Windows telnet and closed the
> window.
> 
> Anyone know how to detect the disconnect?
>
The most you can do is enable S_KEEPALIVE by calling Socket.setKeepAlive
(). This causes the TCP/IP stack to send a periodic system message to 
check whether the connection is still there. However, this is fairly 
useless because the default keepalive frequency can be as slow as once 
every two hours (it says) and there's no Java method that can change that 
frequency. As a result TCP/IP doesn't notice a broken connection until 
you try to send a message over it. About the best you can do is arrange 
for the client to exchange an 'are you still there/yes I am' message pair 
with the server at intervals when the connection is idle.

This makes TCP/IP somewhat deficient for some tasks, e.g. running an ATM 
network since, unless each ATM polls the server fairly frequently its 
likely to say its in service until the punter sticks his card in and 
enters his PIN - that's when the first message is sent to the bank's 
server and so, without a fairly frequent 'Im alive, are you' poll, thats 
when the ATM discovers the line or server is dead and pisses off the 
punter by putting up its OUT OF ORDER display and giving his card back.

It explains why X.25, which instantly notices disconnects of endpoint 
failures and notified both endpoints of the failure, is/was so popular 
for ATM networks. Even the much older proprietary polled network 
protocols such as SNA and 3270 Bisync, will notice a network failure 
pretty much instantly: as soon as a poll goes unanswered the system knows 
something has broken, and these protocols polled their terminals several 
times a second.

HTH


-- 
martin@   | Martin Gregorie
gregorie. | Essex, UK
org       |

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Thread

detecting Socket disconnect bob smith <bob@coolfone.comze.com> - 2012-10-17 13:12 -0700
  Re: detecting Socket disconnect Arne Vajhoej <arne@vajhoej.dk> - 2012-10-17 16:45 -0400
  Re: detecting Socket disconnect Lew <lewbloch@gmail.com> - 2012-10-17 13:47 -0700
    Re: detecting Socket disconnect Arne Vajhoej <arne@vajhoej.dk> - 2012-10-17 17:19 -0400
  Re: detecting Socket disconnect Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-10-17 13:59 -0700
    Re: detecting Socket disconnect Arne Vajhoej <arne@vajhoej.dk> - 2012-10-17 17:24 -0400
      Re: detecting Socket disconnect Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-10-17 14:42 -0700
    Re: detecting Socket disconnect Steven Simpson <ss@domain.invalid> - 2012-10-18 09:31 +0100
      Re: detecting Socket disconnect Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-10-18 07:09 -0700
  Re: detecting Socket disconnect Martin Gregorie <martin@address-in-sig.invalid> - 2012-10-17 21:14 +0000
  Re: detecting Socket disconnect Roedy Green <see_website@mindprod.com.invalid> - 2012-10-17 15:52 -0700

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