Why does TCP need three handshakes and four waves?

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This article is reprinted from the WeChat public account "JS Daily Question", the author is Huihui. Please contact the JS Daily Question public account to reprint this article.

1. Three-way handshake

Three-way handshake actually means that when establishing a TCP connection, the client and server need to send a total of 3 packets.

The main function is to confirm whether the receiving and sending capabilities of both parties are normal, and to specify their own initialization sequence numbers to prepare for subsequent reliable transmission.

The process is as follows:

• First handshake: The client sends a SYN message to the server and indicates the client's initialization sequence number ISN(c). At this time, the client is in the SYN_SENT state.
• Second handshake: After the server receives the SYN message from the client, it will respond with its own SYN message. In order to confirm the client's SYN, the client's ISN+1 is used as the ACK value. At this time, the server is in the SYN_RCVD state
• Third handshake: After the client receives the SYN message, it will send an ACK message with the value of the server's ISN+1. At this time, the client is in the ESTABLISHED state. After the server receives the ACK message, it is also in the ESTABLISHED state. At this time, the two parties have established a connection.

The purpose of each of the above handshakes is as follows:

• First handshake: The client sends a network packet, and the server receives it. In this way, the server can conclude that the client's sending capability and the server's receiving capability are normal.
• Second handshake: The server sends a packet, and the client receives it. In this way, the client can conclude that the receiving and sending capabilities of the server and the client are normal. However, at this time, the server cannot confirm whether the client's receiving capabilities are normal.
• The third handshake: The client sends a packet, and the server receives it. In this way, the server can conclude that the client's receiving and sending capabilities are normal, and the server's own sending and receiving capabilities are also normal.

Through the three-way handshake, it can be confirmed that the receiving and sending capabilities of both parties are normal. After that, normal communication can be carried out

Why not a two-way handshake?

If it is a two-way handshake, the sender can be sure that the information it sends can be received by the other party, and can also be sure that the packet sent by the other party can be received by itself, but the receiver can only be sure that it can receive the packet sent by the other party, but cannot be sure that the packet sent by itself can be received by the other party.

In addition, if there are two handshakes, the client may send multiple request messages due to network congestion and other reasons. The delayed requests will establish a connection with the server, wasting a lot of server resources.

Two or four waves

TCP terminates a connection, which requires four handshakes

The process is as follows:

• First wave: The client sends a FIN message, which specifies a sequence number. At this time, the client is in the FIN_WAIT1 state, stops sending data, and waits for confirmation from the server.
• Second wave: After receiving FIN, the server will send an ACK message, and use the client's sequence number value + 1 as the sequence number value of the ACK message, indicating that the client's message has been received. At this time, the server is in the CLOSE_WAIT state
• Third wave: If the server also wants to disconnect, it sends a FIN message and specifies a sequence number, just like the first wave of the client. At this time, the server is in the LAST_ACK state.
• Fourth wave: After receiving FIN, the client also sends an ACK message as a response, and uses the server's sequence number value + 1 as the sequence number value of its own ACK message. At this time, the client is in the TIME_WAIT state. It takes a while to ensure that the server receives its ACK message before entering the CLOSED state. After the server receives the ACK message, it is in the closed state.

Four waves reason

After receiving the Fin message from the client, the server will not close the connection immediately. Instead, it will send an ACK packet to inform the client that it has received the request to close the connection. Only when all the messages from the server have been sent will it send a FIN message to disconnect. Therefore, four handshakes are required.

Conclusion

A complete three-way handshake and four waves are shown in the following figure:

References

https://zhuanlan.zhihu.com/p/53374516

https://segmentfault.com/a/1190000020610336