How does TCP connection terminate if one of the machine dies?

Question

If a TCP connection is established between two hosts (A & B), and lets say host A has sent 5 octets to host B, and then the host B crashes (due to unknown reason). The host

Answer 1

in your case, A FIN will be generated (by the surviving node) and connection will eventually migrate to CLOSED state. If you keep grep-ing for netstat output on the destination ip address, you will watch the migration from ESTABLISHED state to TIMED_WAIT and then finally disappear.

In your case, this will happen since TCP keeps a timer to get the ACK for the packet it has sent. This timer is not long enough so detection will happen pretty quickly.

However, if the machine B dies after A gets ACK and after that A doesn't send anything, then the above timer can't detect the same event, however another timer (calls idle timeout) will detect that condition and connection will close then. This timeout period is high by default. But normally this is not the case, machine A will try to send stuff in between and will detect the error condition in send path.

In short, TCP is smart enough to close the connection by itself (and let application know about it) except for one case (Idle timeout: which by default is very high).

cforfun