What is the TCP 3-time handshake process?

 

Now we mainly use TCP（ Transmission control protocol ） Data transmission in a connection oriented network . But have you ever thought about it , Why do we prefer TCP Not any other agreement ？

actually ,TCP It provides us with a safe and reliable connection link between the two devices . and , This only occurs during the establishment and closing of a connection between two devices TCP Medium 3 Only during the first handshake . seeing the name of a thing one thinks of its function , There are three steps to establishing and closing a connection . therefore , In this blog , We will understand TCP 3 The second handshake process and the different steps involved .

TCP 3 The handshake process

3-Way Handshake The process is TCP A set of defined steps that occur in , Used to create a secure and reliable communication link and close it . actually ,TCP Use before transferring data 3 The second handshake process establishes a connection between two devices . Once the connection is established , Data transmission between devices . Then you need to terminate the connection , This is also done by using 3 The second handshake process is completed . A secure and reliable connection is established to preserve the equipment CPU、 Buffer and bandwidth to communicate normally . therefore , These resources must be released by terminating the connection after data transmission . therefore , have access to TCP 3 The second handshake process establishes and terminates the connection in the network in a secure way .

Here is TCP Figure displaying the header .

TCP There are some elements in the header file for 3 The handshake process , They are ：

1. Serial number ： The serial number is a random 32 position （ The scope is 0 To (2^32 -1)） The number of , Assigned to the first bit of data . Usually , A serial number is used only once in a connection . For other data transfers in the same connection , Other random serial numbers can be used .
2. Confirmation no. ： It is the next serial number that the sending device expects from the sender . It is usually larger than the serial number received from the sender 1.
3. Window size ： The window size is the buffer size . It is the maximum amount of data that can be received in the buffer .
4. Maximum segment size ： It is the maximum size of each data segment that the connecting device can accept . Over this size , The device will not be able to receive data segments .
5. SYN sign ： SYN On behalf of the synchronization . It can be described as a request to establish a connection . If SYN by 1, It means that the device needs to establish a secure connection , Otherwise not .
6. ACK sign ： ACK The representative confirmed . Can be said to be SYN Response . If ACK by 1, The device has received SYN Message and confirm , Otherwise no .
7. FIN sign ： FIN The representative has completed . When the data transmission is complete , The device has to use FIN Flag to terminate the connection . If FIN by 1, Then the device will terminate the connection , Otherwise not .

Here's how to use 3 Figure displaying how to establish a connection during the second handshake .

 

Here are the USES TCP Medium 3 The three steps involved in establishing a connection in the second handshake process ：

1. The client sends to the server SYN： When the client wants to connect to the server . It will “SYN” Flag set to 1 And send the message to the server . This message has some additional information , Like serial number （ Any random 32 Digit number ）、 Here ACK Set to 0、 Window size and maximum segment size . for example , If the window size is 2000 position , The maximum segment size is 200 position , At most... Can be transmitted in the connection 10 Data segments （2000/200 = 10）.
2. The server replies to the client SYN and ACK： After the server receives the synchronization request from the client , By way of ACK Flag set to “1” Send a confirmation to the client .ACK The confirmation number of is one greater than the received serial number . for example , If the client sends a serial number = 1000 Of SYN, Then the server will send a confirmation number = 10001 Of ACK. Besides , Server will SYN Flag set to “1” And send it to the client , If the server also wants to establish a connection . Here for SYN The serial number of will be the same as that of the client SYN Different . The server also notifies the client of its window size and maximum segment size . When this is done , The client to server connection is established .
3. The client sends to the server ACK： client Received from the server SYN after , take ACK Flag set to '1', And greater than the server SYN Serial number confirmation number 1 Send to client . ad locum ,SYN The flag remains “0”. After completing this step , Now the connection from the server to the client is also established . Once the connection is established , The minimum of the maximum segment sizes of the sender and receiver will be considered for data transmission .

Here's how to use 3 Figure displaying how the secondary handshake process terminates the connection .

 

 

Here are the USES TCP Medium 3 The third handshake process involves three steps to terminate the connection ：

1. The client sends to the server FIN： When the client wants to terminate the connection . It will FIN Flag set to “1”, And use the random serial number to send the message to the server . here ,ACK Set to 0.
2. The server replies to the client FIN and ACK： After the server receives the termination request from the client , By way of ACK Flag set to “1” Send a confirmation to the client .ACK The confirmation number of is one greater than the received serial number . for example , If the client sends a serial number = 1000 Of FIN, Then the server will send a confirmation number = 10001 Of ACK. Besides , Server will FIN Flag set to “1” And send it to the client , If the server also wants to terminate the connection . Here for FIN The serial number of will be the same as that of the client FIN Different . After completing this step , The connection from the client to the server is terminated .
3. The client sends to the server ACK： client Received from the server FIN after , take ACK Flag set to '1', And with a larger than the server FIN Serial number confirmation number 1 Send to client . here ,FIN The flag remains “0”. After completing this step , The connection from the server to the client is now terminated .

This is about TCP Used in the protocol 3 The whole contents of the second handshake process and its steps .