To better understand the need for the three-handshake connection establishment, let us go through a scenario. Alice and Bob have no access to telephones or the Internet (think about the old days) to establish their next meeting at a place far from their homes.
a. Suppose that Alice sends a letter to Bob and defines the day and the time of their meeting. Can Alice go to the meeting place and be sure that Bob is there?
b. Suppose that Bob responds to Alice’s request with a letter and confirms the date and time. Can Bob go to the meeting place and be sure that Alice is there?
c. Suppose that Alice responds to Bob’s letter and confirms the same date and time. Can either one go to the meeting and be sure that the other person is there?