Illustrated network: the principle behind TCP three-time handshake, why can't two-time handshake?

Hello , This is the network technology alliance station .

TCP yes  Internet  One of the main protocols of the protocol suite , It lies between the application layer and the network layer , Used to provide reliable connection services , It's a connection oriented communication protocol , It helps to exchange messages between different devices through the network .

mention TCP, Then it must be mentioned TCP Three handshakes of , This is TCP Connect the core in the core , that TCP What is the process of three handshakes , What is the outstanding design ？

Today, Rego will show you a good explanation in the form of diagrams , Let's go straight to .

What is? TCP？

• English full name ：
  Transmission Control Protocol

• Chinese name ：
  Transmission control protocol

TCP It's a kind of

Connection oriented

The agreement , It can ensure the complete transmission of data to its destination ,TCP  First by using  TCP  Three way handshake with each host  TCP  port

Establish a conversation

, Then... With it

Data packets

In the form of data transmission , Each packet has one

Serial number

, When a packet is received at the destination ,TCP  Will be generated to the sending host

confirm

, If the packet in the sequence is not received , Then send the  TCP  After a certain time interval

Retransmission

Data packets .

TCP Three handshakes

TCP  During the establishment and closing of the connection between two devices  3  The handshake process , That is, establishing and closing connections requires three steps , Now let's have a detailed understanding of these three processes .

Three handshakes in life

Let's first illustrate the process of three handshakes with examples in life ：

Xiao Ming wants to call Xiaomei , But I'm not sure whether the other party is Xiaomei , So I will go through the following process ：

• Xiao Ming ： Hello , Is it Xiaomei ？【
  The first handshake
  】

• Xiaomei ： Yes , I'm Xiaomei .【
  The second handshake
  】

• Xiao Ming ： well , I know you are Xiaomei .【
  The third handshake
  】

After three handshakes , Xiao Ming can clearly confirm that the other party is Xiaomei , very

reliable

！

If there is only one handshake

, Then Xiao Ming finished asking “ Hello , Is it Xiaomei ？”, Xiao Ming is not sure whether the other party has received this greeting , Not to mention whether the other party is Xiaomei .

If only two handshakes

, Then Xiao Ming received Xiaomei's “ Yes , I'm Xiaomei .” After the reply , Xiaomei doesn't know whether Xiaoming has received her reply , So next, she was not sure whether to communicate with the person who asked her on the phone .

So three handshakes are just right .

Three professional handshakes

Now let's look at three professional handshakes .

Professional terms involved in three handshakes

• Server side
  ： The server is a physical computer dedicated to running services to meet the needs of other computers

• client
  ： The client is the computer hardware device or software that accesses the services provided by the server

• SYN
  ： Sync serial number （Synchronize Sequence Number）, This is the first packet from the client to the server , 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 .

• ACK
  ： confirm （Acknowledgement）, Can be said to be SYN Response , If  ACK  by  1, The device has received  SYN  Message and confirm , Otherwise no .

Three handshake steps

First step

： The client will  SYN  Flag set to  1  Send the message to the server .

The second step

： The server passes  ACK  Flag set to  1  To confirm the client request .

The third step

： The client receives synchronization from the server （SYN） after , Send confirmation to the server （ACK）.

Get... From the client （ACK） after , Establish a connection between the client and the server , Now data can be transferred between client and server .

More detailed process

• First step ：TCP The client sends... To the server  TCP SYN  Packets to start the connection , The packet contains a random serial number n, It represents the serial number of the data that the client should transmit
  Start
  .

• The second step ： The server receives the packet , And use its serial number （m） To respond , Its response also includes the confirmation number , That is, the serial number of the client plus  1, Here is n+1.

• The third step ： The client responds to the server by sending a confirmation number , The confirmation number is the serial number of the server plus  1, Here is m+1.

TCP Analysis of three handshakes

No amount of theory is in vain without practice , So the best way to verify our theory is to grab the bag and have a look TCP Three handshake process .

Pictured , host

172.16.16.128:2826

And host

212.58.226.142:80

Establish three handshake processes ：

• First step ： host
  172.16.16.128:2826
  Host computer
  212.58.226.142:80
  send out [SYN] Seq=0

• The second step ： host
  212.58.226.142:80
  Host computer
  172.16.16.128:2826
  send out [SYN,ACK] Seq=0,Ack=1

• The third step ： host
  172.16.16.128:2826
  Host computer
  212.58.226.142:80
  send out [ACK] Seq=1,Ack=1

This is the detailed three handshake message , I think at this moment , You can open your own wireshark Experience the bag grabbing tool to your heart's content TCP The magic of three handshakes .

TCP window

Everyone is looking at the wireshark When capturing screenshots , I must have noticed

Win=8192

Wording , This is TCP window .

The client sends a pile of data to the server , Then the server verifies the checksum of all packet levels and sends a ACK Data packets , It indicates that all data has been received correctly .

If you don't receive everything , You need to retransmit some or all of the data , Each device maintains a buffer for all data , In case you need to send it again , Received  ACK  Packets mean that the device can clear old data from the buffer .

TCP  window

Is receiving  ACK  The maximum number of bytes that can be sent before .

If the network is unreliable , It is best to TCP  Make the window smaller , such , If something goes wrong , There is no need to retransmit very large data .

TCP There is also a great design , That's it

The sliding window

, That is, the device can

dynamic

Change window size , Narrow the window in case of congestion , Enlarge the window when normal .

The other parameters

There are more than TCP  window , There are other parameters ：

• MSS
   (Maximum Segment Size), Maximum segment size .

• WS
   (Windows Scaling), Used to control the maximum  TCP  Receive window size .

TCP Why can't a second handshake establish a connection

We need to know Server  and  Client  Want to establish a connection , The following four conditions must be met ：

• The server needs to confirm that the server can receive packets from the client

• The client needs to confirm that the client can receive packets from the server

• The client needs to confirm that the server can receive packets from the client

• The server needs to confirm that the client can receive packets from the server

The four conditions seem very convoluted , In fact, it's just ：

• Xiaoming asked ：“ Hello , Excuse me, Shi Xiaomei ？”： After sending successfully , Xiaoming is sure that he can receive information from Xiaomei , The first condition is verified 【
  The server needs to confirm that the server can receive packets from the client
  】

• Xiaomeihui ：“ Yes , I'm Xiaomei ”： After sending successfully , Xiaomei is sure that she can receive information from Xiaoming , It can also confirm that Xiao Ming can receive information from her , Verified the second 、 The third condition 【
  The client needs to confirm that the client can receive packets from the server
  】【
  The client needs to confirm that the server can receive packets from the client
  】

• Xiaoming returns ：“ well , I know you are Xiaomei ”： After sending successfully , Xiaoming is sure that Xiaomei can receive information from him , Four conditions are verified 【
  The server needs to confirm that the client can receive packets from the server
  】

TCP  It is a two-way communication protocol , This means that either end should be able to send data reliably , So three handshakes are just right .

summary

TCP 3  This handshake is used to create a secure and reliable communication connection , This article combines examples in life , Vividly explained the process of three handshakes , The main contents are as follows ：

• What is? TCP？

• TCP Three handshakes

• Three handshakes in life

• Three professional handshakes

• TCP Analysis of three handshakes

• TCP window

• The other parameters

• TCP Why can't a second handshake establish a connection

• summary

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