Re: [Boost-users] Why cannot asio tcp socket server detect TCP disconnection when the client device lost power?

Subject: Re: [Boost-users] Why cannot asio tcp socket server detect TCP disconnection when the client device lost power?
From: Eric Prud'hommeaux (eric_at_[hidden])
Date: 2017-06-29 07:50:37

* Andreas Wehrmann via Boost-users <boost-users_at_[hidden]> [2017-06-29 09:18+0200]
> On 06/29/2017 09:01 AM, jupiter wrote:
> >Thanks for the response.
> >
> >The server program is calling boost::asio::async_read for waiting for
> >client's data, I am wondering why the async_read didn't fire an error when
> >the remote socket connection is gone. I thought that
> >boost::asio::async_read should detect lost connection. The fact is it does
> >in most of the situations when the client program is ended, but it cannot
> >detect when the client device suddenly lost power connection, I think that
> >might because TCP only send syn / ack during connection / disconnection,
> >the syn/ack would not be active after establishing the connection, correct
> >me if I am wrong here.
> >
> >You are right, if I have to use keep alive, I'll use it in user level which
> >could better control the time and use less bandwidth than the system keep
> >alive.
> >
> >Thank you.
> >
> >Kind regards,
> >
> >- j
> >
> >
> 
> The TCP connection is usually managed by your operating system (the TCP
> stack of it, to be precise).
> When your application suddenly ends or crashes, the operating system will
> clean up the TCP connections for you (i.e disconnect them)
> which is the reason why the other end sees an "immediate" disconnect in this
> case.
> Tearing down a connection involves specific signalling on the TCP layer
> (FIN,RST).
> 
> Now imagine your situation:
> You have two machines, and both have a TCP connection to each other.
> When the TCP connection is idle (i.e. nothing is sent between the two) there
> won't be any signalling on the TCP layer
> if TCP keep-alive is disabled and there is no application layer heartbeat
> mechanism.

Implied but not stated here: a TCP keep-alive would be sufficient to
notify a server about a client falling off the net, though you may
have other reasons to want your own heartbeat messages.

> You might already see the problem here: In this situation you won't ever
> detect whether the other machine is gone,
> if it suddenly disappeared (i.e. sudden power loss, or some kind of network
> problem) because:
> Disconnecting a TCP session or detecting presence always requires some kind
> of signalling.
> 
> So, you might already see that this is not a problem specific to asio but to
> TCP sockets in general.
> 
> Regards,
> Andreas
> 
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