Abstract: TCP (Transmission Control Protocol) is a connection-based protocol that provides a reliable flow of data between two computers. It also provides a point-to-point channel for applications that require reliable communications. The Java programming language supports, through the java.net package, a suitable API for developing client/server applications that use this protocol for communication.

1.The principles of the TCP protocol

The TCP protocol requires the establishment of a session between the client and the server. A session essentially is a mechanism to get some data, so we are setting up a specialized communication session.

TCP behaves so similarly to a conversation on the phone.

• Pick up the phone
• Wait for the dial tone
• Dial the number
• Phone rings
• Pick up the phone
• Say hello
• Start the conversation
• Repeat if you haven't understood something

A three-way handshake (or TCP handshake) is a method used in a TCP/IP network to create a connection between a local host/client and server. It is a three-step method that requires both the client and server to exchange SYN and ACK (acknowledgment) packets before actual data communication begins.




This is how the TCP 3-way handshake works:

• A client node sends a SYN data packet over an IP network to a server on the same or an external network. The objective of this packet is to ask/infer if the server is open for new connections.
• The target server must have open ports that can accept and initiate new connections. When the server receives the SYN packet from the client node, it responds and returns a confirmation receipt – the ACK packet or SYN/ACK packet.
• The client node receives the SYN/ACK from the server and responds with an ACK packet.

The four-way disconnect is the method used in a TCP/IP network to close the connection between a client and a server.




This is how the TCP 4-way disconnect works:

• The client sends a FIN packet to the server and updates its state to FIN_WAIT_1
• The server receives the termination request from the client, responds with ACK and moves to CLOSE_WAIT
• The client receives the reply from the server and will go to FIN_WAIT_2
• The server is in CLOSE_WAIT and will follow up with FIN, which updates the state to LAST_ACK
• The client receives the termination request and replies with an ACK, which results in a TIME_WAIT state
• The server is finished and sets connection to CLOSED
• The client stays in TIME_WAIT for a maximum of 4 minutes before setting the connection to CLOSED

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Catalin Tudose
Java and Web Technologies Expert