TCP 3-WAY Handshake vs TCP Half-Open

• TCP 3-Way handshake

TCP is a connection-oriented protocol, a connection needs to be established before two devices can communicate. TCP uses a process called three-way handshake to negotiate the sequence and acknowledgment fields and start the session. Here is a graphical representation of the process

The three way handshake process consists of three steps:

a.     Host A initiates the connection by sending the TCP SYN packet to the destination host. The packet contains the random sequence number (e.g. 5432) which marks the beginning of the sequence numbers for data that the Host A will transmit.

b.     The Server receives the packet and responds with its own sequence number. The response also includes the acknowledgment number, which is Host A’s sequence number incremented by 1 (in our case, that would be 5433).

a.     Host A acknowledges the response of the Server by sending the acknowledgment number, which is the Server’s sequence number incremented by 1.

Post TCP 3 way handshake, the data transfer starts. After the data transmission process is finished, the connection will get terminated by sending FIN (FINISHED) flag which does graceful termination of connection.

• TCP Half Open connection

TCP connections are called Half Open connections when the third step of the 3-Way handshake sending final ACK to the server fails (as shown in below figure) or if one of the hosts closes the connection without acknowledging the other.

Half Open connection process is given below –

1.     Host initializes the request by sending SYN packet.

2.     Server replies to the client with SYN_ACK, and at this point server reserves some resource for the client and waits for the final ACK to arrive (Acknowledgment message).

3.     However, the client does not respond to the server with final Acknowledgment.

So an established connection is said to be "half-open" if one of the TCPs has closed or aborted the connection at its end without the knowledge of the other, or if the two ends of the connection have become desynchronized owing to a crash that resulted in loss of memory. Such connections will automatically become reset if an attempt is made to send data in either direction. However, half-open connections are expected to be unusual.

• F5 TCP-Half Open monitor

F5 also have TCP-Half Open monitor available under Local trafficàMonitors. Tcp_half_open monitor is most widely used for gateway monitoring when you just need to ensure the socket is responding to connection requests and desire the lowest overhead on the monitoring target.