TCP three-way handshake: in-depth understanding and C# example implementation

In computer network communication, TCP (Transmission Control Protocol) plays a vital role as a connection-oriented, reliable, byte stream-based transport layer communication protocol. TCP uses a three-way handshake mechanism to ensure that a stable and reliable connection can be established between two communicating nodes. This article will explore the principle of TCP three-way handshake in depth and show how to implement this process in practical applications through C# sample code.

1. TCP three-way handshake principle
The TCP three-way handshake is the standard process for establishing a TCP connection in the TCP/IP protocol. It ensures that both parties of the data communication can synchronize the sequence number, laying the foundation for subsequent reliable data transmission. The three-way handshake process can be summarized into the following three steps:

First handshake: The client sends a SYN (synchronization sequence number) packet to the server. The packet does not contain application layer data, but only contains a SYN flag bit, which is used to synchronize the sequence number. At this time, the client enters the SYN_SENT state and waits for the server's confirmation.
Second handshake: After receiving the SYN packet segment from the client, the server will respond with its own SYN packet. The packet also does not contain application layer data, but contains two flag bits, SYN and ACK (confirmation sequence number validity). The ACK flag bit is used to confirm that the SYN packet segment from the client has been received, and the SYN flag bit indicates that the server wants to establish a connection. At this time, the server enters the SYN_RCVD state and waits for the client's confirmation.


The third handshake: After the client receives the SYN+ACK packet segment from the server, it will send an ACK packet segment to confirm that it has received the SYN packet from the server. At this point, both the client and the server have entered the ESTABLISHED state, indicating that the TCP connection has been successfully established and both parties can start transmitting data.
2. The purpose of the TCP three-way handshake
The main purpose of the TCP three-way handshake includes:

Synchronize the initial sequence numbers of both parties: Through the three-way handshake, the communicating parties can synchronize their initial sequence numbers, providing a reliable sequence number basis for subsequent data transmission.



Exchange TCP window size information: During the connection establishment process, both parties will exchange TCP window size information to effectively control the flow during data transmission.
Confirm the receiving and sending capabilities of both parties:


3. Implement TCP three-way handshake with C# sample code
Although the TCP three-way handshake is automatically completed in the underlying network protocol stack, we can simulate this process through C# code to deepen our understanding of the principle of TCP three-way handshake. The following is a simplified C# example that shows how to use the Socket class to simulate the connection establishment process between the TCP client and the server.

Server-side code



Note: The server code above does not actually simulate the TCP three-way handshake process directly, because the TCP three-way handshake is automatically completed by the network protocol stack at the bottom of the operating system. The example here is mainly to show how to use the C# Socket class to establish a TCP connection and send and receive data after the connection is established.

Client code

In this client example, we also did not directly simulate the TCP three-way handshake process. However, by sending data to the server and receiving a response, we simulated the data transmission process after the TCP connection was established.


4. In-depth understanding of TCP three-way handshake
Although the above C# example does not directly show the specific implementation of TCP three-way handshake, it helps us understand the basic process of TCP connection establishment and data transmission. In actual applications, TCP three-way handshake is automatically completed by the network protocol stack at the bottom of the operating system, without the need for manual intervention by programmers. However, understanding the principle of TCP three-way handshake is very important for developing high-performance and high-reliability network applications.



The TCP three-way handshake ensures that both parties in the data communication can synchronize the sequence numbers and confirm the receiving and sending capabilities of the other party. This mechanism is one of the cornerstones of the TCP protocol reliability and provides a solid foundation for subsequent data transmission.

V. Summary


This article explores the principle of TCP three-way handshake in depth, and combines C# sample code to show how to simulate the TCP connection establishment and data transmission process at the application layer. Although the sample code does not directly implement the specific steps of TCP three-way handshake, it helps us understand the basic process of TCP connection establishment and data transmission, as well as how to use the Socket class in C# to design network programs. I hope this article can help readers to have a deeper understanding of the principle of TCP three-way handshake and its implementation in practical applications.