Layer 4 – TCP and the three-way handshake

The Transmission Control Protocol (TCP) is one of the core protocols of the Internet Protocol (IP) suite and operates at the fourth layer of the OSI model. This layer is responsible for ensuring the reliable transmission of data over the network by providing flow control, error detection, and correction mechanisms.

In this article, we’ll explore the basics of TCP, including its key features and the three-way handshake process that establishes a connection between two hosts.

What is TCP?

TCP is a connection-oriented protocol that provides a reliable transmission of data over the network. Unlike the User Datagram Protocol (UDP), which operates at the same layer and is connectionless, TCP establishes a virtual connection between two hosts to ensure the reliable transmission of data.

TCP uses a combination of flow control and error detection mechanisms to guarantee the delivery of data. It divides data into segments, which are then sent over the network and reassembled at the receiving end. To avoid network congestion, TCP also implements flow control, which adjusts the rate at which data is sent over the network.

TCP Three-Way Handshake

One of the key features of TCP is the three-way handshake process that establishes a connection between two hosts. The three-way handshake process is initiated by the host that wants to initiate communication and consists of three steps:

  1. SYN (Synchronize)
  2. SYN-ACK (Synchronize-Acknowledge)
  3. ACK (Acknowledge)

The SYN message is sent by the host that wants to initiate communication to the recipient host. This message contains a sequence number that is used to synchronize the sequence numbers between the two hosts.

The recipient host then sends a SYN-ACK message, which acknowledges the receipt of the SYN message and contains its own sequence number.

Finally, the initiating host sends an ACK message to complete the three-way handshake process. This message acknowledges the receipt of the SYN-ACK message and confirms that the connection has been established.

Example of the Three-Way Handshake

Let’s consider a simple example of a three-way handshake process between two hosts, Host A and Host B.

  1. SYN: Host A wants to initiate communication with Host B, so it sends a SYN message to Host B with a sequence number of 1000.
  2. SYN-ACK: Host B receives the SYN message from Host A and sends a SYN-ACK message back to Host A with its own sequence number of 2000 and an acknowledgement of the receipt of the SYN message from Host A.
  3. ACK: Host A receives the SYN-ACK message from Host B and sends an ACK message back to Host B to confirm that the connection has been established. This message contains an acknowledgement of the receipt of the SYN-ACK message from Host B and confirms that the sequence numbers have been synchronized between the two hosts.

Why the Three-Way Handshake is Important

The three-way handshake process is important for several reasons:

  1. Establishing a Connection: The three-way handshake process is used to establish a connection between two hosts and is the first step in the transmission of data over the network.
  2. Synchronizing Sequence Numbers: The sequence numbers in the SYN and SYN-ACK messages are used to synchronize the sequence numbers between the two hosts. This is important for ensuring that the data is transmitted in the correct order and that no data is lost or duplicated during transmission.
  1. Flow Control: The three-way handshake process also helps to implement flow control by determining the rate at which data is sent over the network. By synchronizing the sequence numbers, the two hosts can regulate the flow of data to avoid network congestion.
  2. Error Detection and Correction: The three-way handshake process also helps to detect and correct errors in the transmission of data. If a message is lost or damaged during transmission, the recipient host will not receive the correct sequence number, and the transmission can be retried.

Conclusion

The Transmission Control Protocol (TCP) is a vital protocol in the Internet Protocol (IP) suite that provides reliable data transmission over the network. The three-way handshake process is one of the key features of TCP and plays a crucial role in establishing a connection between two hosts, synchronizing sequence numbers, implementing flow control, and detecting and correcting errors in the transmission of data.

In this article, we’ve explored the basics of TCP and the three-way handshake process, including an example of the process in action. Understanding TCP and the three-way handshake is important for anyone who works with computer networks and data transmission, as it forms the foundation for reliable data transmission over the network.

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