Explain Write a brief note on TCP Connection Management Modeling with a finite state machine.

8. Explain Write a brief note on TCP Connection Management Modeling with a finite state
machine.

Answer:

1. TCP Connection management from server’s point of view:
2. The server does a LISTEN and settles down to see who turns up.
3. When a SYN comes in, the server acknowledges it and goes to the SYNRCVD state
4. When the servers SYN is itself acknowledged the 3-way handshake is complete and server goes to the ESTABLISHED state. Data transfer can now occur.
5. When the client has had enough, it does a close, which causes a FIN to arrive at the server [dashed box marked passive close].
6. The server is then signalled.
7. When it too, does a CLOSE, a FIN is sent to the client.
8. When the client’s acknowledgement shows up, the server releases the connection and deletes the connection record.

The finite state machine itself is shown in Fig. The common case of a client actively connecting to a passive server is shown with heavy lines—solid for the client, dotted for the server. The lightface lines are unusual event sequences Each line in Fig. is marked by an event/action pair. The event can either be a user-initiated system call (CONNECT, LISTEN, SEND, or CLOSE), a segment arrival (SYN, FIN, ACK, or RST), or, in one case, a timeout of twice the maximum packet lifetime. The action is the sending of a control segment (SYN, FIN, or RST) or nothing, indicated by —. Comments are shown in parentheses.

One can best understand the diagram by first following the path of a client (the heavy solid line), then later following the path of a server (the heavy dashed line). When an application program on the client machine issues a CONNECT request, the local TCP entity creates a connection record, marks it as being in the SYN SENT state, and shoots off a SYN segment. Note that many connections may be open (or being opened) at the same time on behalf of multiple applications, so the state is per connection and recorded in the connection record. When the SYN+ACK arrives, TCP sends the final ACK of the three-way handshake and switches into the ESTABLISHED state. Data can now be sent and received.

When an application is finished, it executes a CLOSE primitive, which causes the local TCP entity to send a FIN segment and wait for the corresponding ACK (dashed box marked ‘‘active close’’). When the ACK arrives, a transition is made to the state FIN WAIT 2 and one direction of the connection is closed. When the other side closes, too, a FIN comes in, which is acknowledged. Now both sides are closed, but TCP waits a time equal to twice the maximum packet lifetime to guarantee that all packets from the connection have died off, just in case the acknowledgement was lost. When the timer goes off, TCP deletes the connection record.

Now let us examine connection management from the server’s viewpoint. The server does a LISTEN and settles down to see who turns up. When a SYN is itself acknowledged, the three-way handshake is complete, and the server goes to the ESTABLISHED state. Data transfer can now occur. When the client is done transmitting its data, it does a CLOSE, which causes a FIN to arrive at the server (dashed box marked ‘‘passive close’’). The server is then signaled. When it, too, does a CLOSE, a FIN is sent to the client. When the client’s acknowledgement shows up, the server releases the connection and deletes the connection record.