how to understand java remote socket is unreachable when remote socket's device is shutdowned or application force closed

I have an as3 serversocket, we are connecting to the this serversocket via java socket(an android app). The problem is that, when serversocket application force closed, i couldnt find the a way to understand that remote socket is closed or unreachable, except when trying to flush() a message to that socket throws broken pipe error. The reason that i want to solve this problem in another way is not to make server application busy with connection check messages ?

Answers


A TCP connection is not a continuous flow like a river. It's a sequence of segments sent at discrete times - thus is more like cars on a road. So there is no way to be notified if the road is broken, until a car tries to reach the other end, fails, and calls you back.

You should simply keep sending cars at reasonable intervals (30 seconds?), thus instructing your server to do nothing when it receives a NOOP message (short for No Operation). The client will be programmed to start sending NOOP when the connection is idle (no message sent or received for 30 seconds), and when the driver calls back that he can't reach his destination, you close the current socket and attempt to create another one.


There are 3 ways for TCP to understand that connection has been closed:

  1. One side got an error - it sends a Reset request to the other side to close the connection
  2. One side actually wants to close the connection so it sends a FIN message to the other side
  3. A time-out when trying to transmit something from one side to another. This time-out can only happen when a tcp-package is sent. After a connection has been made , tcp's packages are only sent on a data transmitting request (and connection closing request...).

So if one side disconnects suddenly, not in middle of data transmitting, there is no way of spotting a disconnection , but to send some package once a while.


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