TCP keepalive

http://tldp.org/HOWTO/TCP-Keepalive-HOWTO/overview.html

With PSH/RST packet, we can:
1. check peer is alive or not (if peer respond the packet)
2. wait peer to represent again when they disconnect for a short time (timeout)

In the next section we will distinguish between the two target tasks for keepalive:

Checking for dead peers
Preventing disconnection due to network inactivity

http://sharkfest.wireshark.org/sharkfest.10/B-2_Tompkins%20Analyzing%20TCPIP%20Networks%20with%20Wireshark.pdf

•A TCP keep-alive packet is simply an ACK with the sequence number set to one less than the current sequence number for the connection.
•A host receiving one of these ACKs will respond with an ACK for the current sequence number.
•Keep-Alives can be used to verify that the computer at the remote end of a connection is still available.
•TCP keep-Alives can be sent once every KeepAliveTime(defaults to 7,200,000 milliseconds or two hours), if no other data or higher level keep-aliveshave been carried over the TCP connection. If there is no response to a keep-alive, it is repeated once every KeepAliveIntervalseconds. KeepAliveIntervaldefaults to 1 second.