Why does TCP need a three-way handshake? Explain it to you in the most popular words

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TCP implementation principle and why three-way handshake is needed? Two-way handshake is not possible? Four-way handshake is not possible? Readers can read the detailed explanation of this blog with questions.

Ok, before explaining the reason, we still need to review the basic knowledge of TCP and the three-way handshake protocol:

1. What is TCP protocol?

TCP: Transmission Control Protocol is a connection-oriented, reliable, byte-stream-based transport layer protocol.

RFC 793 defines a TCP connection as follows:

Connections:

• The reliability and flow control mechanisms described above require that TCPs initialize and maintain certain status information for each data stream.
• The combination of this information, including sockets, sequence numbers, and window sizes, is called a connection.

The general meaning is that TCP connection is used to ensure reliability and flow control mechanism, including Socket, sequence number and window size.

The socket is composed of IP and port, the sequence number is used to solve the disorder problem, and the window size is used for flow control.

2. Characteristics of TCP protocol

• Connection-oriented: TCP is a protocol that connects the server and the client.
• Byte stream oriented: Data communication between TCP server and client is transmitted via byte stream data.
• Reliable: It means that the data transmission between the TCP server and the client is very stable. Even if the network is very poor, TCP can guarantee that the data will be transmitted to the recipient.

ps: The reliability of TCP transmission is due to the fact that TCP records the sending status of information. TCP will record which data is received and which data is not received. Then, in case of packet loss, that is, unsuccessful sending, TCP will resend the packet, so the reliability of TCP is guaranteed in this way.

3. TCP three-way handshake execution process

The TCP three-way handshake execution process is a very common question in interviews. Because this problem is also a very important foundation of computer science, it needs to be studied carefully.

Keyword description:

• SYN: Synchronize Sequence Numbers, synchronization sequence number
• ACK：Acknowledge Character, confirmation character
• SEQ：Sequence Number, sequence number

TCP three-way handshake execution process:

• First, both the server and the client are in the CLOSED state, then the server starts, listens to the port, and the state changes to the LISTEN state.
• In order to request resources, the client sends a connection and sends a synchronization sequence number SYN. ​​At this time, the client changes to the SYN-SEND state.
• After receiving the client request, the server sends SYN and ACK, and then the server state changes to SYN-RCVD state
• After the client receives the message, it sends ACK again, and then changes to the ESTABLISHED state. After the server receives the return message, the state also changes to the ESTABLISHED state.

4. Why does the TCP protocol require a three-way handshake?

OK, now that we know the basic working principle of TCP's three-way handshake, we can explain why TCP needs three-way handshakes? Why not just design it as a two-way handshake?

Reason: Avoid duplicate connections

In fact, the reason for the three-way handshake is pointed out in RFC 793 Transmission Control Protocol.

The principle reason for the three-way handshake is to prevent old duplicate connection initiations from causing confusion.

The main reason is to prevent old duplicate connections from causing connection confusion.

For example, in a complex network environment, the client may send multiple requests in succession. If only two handshakes are designed, the server can only receive requests and return request information, and it does not know whether the client's request is successful. These expired requests will cause confusion in the network connection.

Therefore, it is designed as a three-way handshake. After receiving the return message of SEQ+1 from the server, the client will know that this connection is a historical connection, so it will send a message to the server to tell the server.

Therefore, the purpose of designing TCP into a three-way handshake is to avoid repeated connections.

Then can it be designed as a four-way handshake? A five-way handshake? No?

The answer is yes, but in order to save resources, three handshakes can meet the actual situation, so there is no need to design four handshakes, five handshakes, etc.