How to connect the network: Chapter 1 CSDN creation punch in

The user enters a... Into the browser URL The response returns , The life cycle of a network request .

The whole book is divided into six parts ：

1. Application layer client generation HTTP, Protocol stack delegated to the operating system
2. Protocol stack （TCP/IP modular ） Call the network card driver to generate an electrical signal
3. How does the network card reach the router used to access the Internet through the router
4. Internet internal relay transmission
5. arrive web The server passes the firewall first
6. web How does the server collect data

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The first chapter introduces , The client application parses URL The process of sending to the delegation protocol stack ：

1. The application layer client is based on url Protocol format extraction , Domain name or ip,port,URL, Request parameters and other information
2. according to HTTP The protocol generates a request message
3. Before sending a message , You need to know about each other's IP, entrust DNS The client parser uses the domain name to pass DNS Server acquisition IP
4. Use Socket Library delegation protocol stack （TCP/IP） Send a request

Generate HTTP Request information

Browser enter web address （URL）

URL Uniform Resource Locator Uniform resource locator

URL Format – Protocol type :// Form of agreement

 Use  US-ASCII
 foo://example.com:8042/over/there?name=ferret#nose
 \_/   \______________/\_________/ \_________/ \__/
  |           |            |            |        |
scheme     authority       path        query   fragment
  |   _____________________|__
 / \ /                        \
 urn:example:animal:ferret:nose

 use HTTP Agreement to access Web Server time 
http://user:[email protected]:80/dir/file1.htm
http://[ user name ]:[ password ]@web Server domain name :[ Port number ] File pathname 
​
ftp Upload and download files 
ftp://user:[email protected]:21/dir/file1.htm
ftp://[ user name ]:[ password ]@ftp Server domain name :[ Port number ] File pathname 
​
file://localhost/c:/path/file.zip
file://[ Computer name ] File pathname 

HTTP summary

Hypertext transfer protocol （HTTP） It's a kind of distributed , cooperative , Multimedia information system services , Application layer oriented protocols

HTTP Form of agreement

 The request message 
< Method >< Space ><URI>< Space ><HTTP edition >
< Field name >:< field value >
< Field name >:< field value >
< Field name >:< field value >
...
< Blank line >
< Message body >

 The response message 
<HTTP edition >< Space >< Status code >< Space >< Response phrase >
< Field name >:< field value >
< Field name >:< field value >
< Field name >:< field value >
...
< Blank line >
< Message body >

 Request header classification :  Universal head ( Applies to request headers   Response head )  Request header   Response head   Entity head 

 Response code  |  Response phrase 
1xx |  Inform the request processing progress and situation 
2xx |  success 
3xx |  Indicates that further operation is required 
4xx |  Client error 
5xx |  Server error 

 Only one request can be written URI, html  and   Pictures require separate requests 
WWW  working principle ,  Text data and embedded image data in web pages are stored independently | Separate the requested 

towards DNS Server query Web Server's IP Address

The previous step generated HTTP news , Before sending, you need to know the server of the other party IP.

DNS: Domain Name System Source server

IP Address

IP It consists of network number and host number , The dividing line between the network number and the host number is divided by the subnet mask .

ip View instructions
windows ipconfig
linux ifconfig

Illustrate with examples ：
ip: 10.11.12.13
Subnet mask : 255.255.255.0
network number ：10.11.12
Host number ：13

32 bit, 8 bit  A group of 

a. IP The representation of the address body 
10.11.12.13
IP Address 

b.  Subnet mask expression 
10.11.12.13/255.255.255.0
IP Address body / Subnet mask 

c.  The subnet mask is represented by the number of network number bits 
10.11.12.13/24
IP Address body / Subnet mask 

d.  Indicates the address of the subnet 
10.11.12.0/24
 All bits of the host number are  0  Represents the whole subnet 

d.  Indicates the address of the subnet 
10.11.12.255/24
 All bits of the host number are  1  The representative broadcasts to the subnet 

ip Address    10.11.12.255
 Subnet mask  255.255.255.0
 The subnet mask represents the boundary between the network number and the host number ,  Subnet mask 1 Part of the network number , 0 Some host numbers of 

 Number of network number bits : 24 == 255.255.255.0

Domain name and IP Address combination

 Human domain name ,  Easy to remember 
 Routers use IP,  Address length  32bit 4 byte ,  Network transmission efficiency is high ,  Good performance  

note:  If the server uses a virtual host ,  May not work  IP  visit 

How to query IP

1.  Ask about the latest  DNS The server ,  The domain name will correspond to  IP
2. DNS Server return  IP

 There will be one on the computer  DNS client   be called  DNS Parser 
 adopt  DNS  Inquire about  IP  It is called domain name resolution 
 The parser is actually a program ,  Included in the operating system  Socket In the library 

Socket library   Developed by the University of California, Berkeley BSD(UNIX operating system ) Medium C Language library ,  Other language references  Socket  Developed the corresponding network library 
Socket library   It is a collection of program components used to call network functions ,  Standard library of network development 

BSD (Berkeley Software Distribution, Berkeley Software Distribution )
https://baike.baidu.com/item/BSD/3794498?fr=aladdin

adopt DNS Parser towards DNS The server Send query

< Memory address > = gethostbyname(" domain name ");

Parser workflow

 When a web application calls the parser , The control flow of the program will be transferred to the inside of the parser .

1.  Application calls  DNS Parser 
2. DNS Parser   Generate  DNS Server query message , Call protocol stack （TCP/IP drive ）
3.  Protocol stack sends messages 
4.  Send a message to  DNS  The server .

HTTP  Message text , DNS  Messages are in binary 

Control process transfer ： Applications call other programs , The originally running program enters a suspended state , The called program starts running .

All over the world DNS The server Big relay

DNS The workflow of the server

1.  Receive query messages from clients 
2.  Return the response according to the message content 

Query message format

a)  domain name 
 The server ,  Mail server ([email protected] @ The following paragraph ) name 
b) Class
 Type always represents the Internet  IN
c)  Record type 
A:  The server 
MX(mail eXchange):  Mail server 

Response message format

 The record type is A   return   The server corresponds to IP
 The record type is MX  return   Priority and domain name corresponding to the mail server 

 priority ： When one email address corresponds to multiple email servers , The smaller the number, the higher the priority 

domain name	Class	Record type	The response data
www.baidu.com	IN	A	xxx.xxx.xxx.xxx
glasscom.com	IN	MX	10 mail.glasscom.com
mail.glasscom.com	IN	A	xxx.xxx.xxx.xxx

Domain name hierarchy

 Save according to the hierarchical structure of the domain name 
www.baidu.com  Period separation ,  The more right, the higher the level 

 Can direct Baidu " Domain name hierarchy "

Look for the corresponding DNS Server and get IP Address

1.  Register from bottom up 
2.  Top down query 

register

DNS The server A( management baidu.com)
DNS The server B( management com)

A register baidu.com as well as A Of ip To B
B register com as well as B Of ip To root server (.)

 The name of the root domain server IP Store in all DNS Server 
 The name of the root domain server IP The world is just 13 individual ,  Use multiple servers for one IP

Inquire about

 The client accesses the nearest DNS The server K, K Of ip On the client side TCP/IP There are 

 for example  www.baidu.com

K  Access root domain ,  return com Where the domain is located ip
K  visit com Domain ,  return baidu Where the domain is located ip
K  visit baidu Domain ,  return www Where the domain is located ip
K  visit www Domain ,  return www.lab.glasscom.com Where ip
 Last K return ip To the client 

DNS Server cache acceleration

 After querying the domain name  DNS The server will cache 
 If the domain name does not exist, it will be cached 
DNS  The cache information in the server will set a validity period . And will inform the client , The response result comes from the cache or the server that manages the domain name 

 cache ： Store the used data in a high-speed storage device close to the data used , In order to improve the speed of subsequent access .
CPU And memory   Between disk and memory   Network cache 

Delegate protocol stack to send message

Data sending and receiving operation

 Call... In order  Socket  Program components in the library , towards OS The internal protocol stack sends the delegation .

1.  Create socket （ Create socket phase ）
2.  Connect the pipe to the server socket （ Connection phase ）
3.  Manipulation data （ Communication stage ）
4.  Disconnect and delete socket （ Disconnection phase ）   It can be initiated by either side of the client server , Usually the server 

< Memory address > = gethostbyname("www.baidu.com");
< The descriptor > = socket(<ipv4>, < Flow mode >);
connect(< The descriptor >, < Server's IP Address and port number >, ...);
write(< The descriptor >, < send data >, < Send data length >);
< Length of received data > = read(< The descriptor >, < Receive buffer >, ...);
close(< The descriptor >);

 this 5 Each operation is an application delegate OS Protocol stack to execute .
 These delegate operations are done by calling Socket The program components in the library execute ,Socket The library acts as a bridge between applications and protocol stacks 

socket(<ipv4>, < Flow mode >);  A detailed explanation of parameters is attached (https://blog.csdn.net/lee244868149/article/details/43732025)

TCP It's a streaming protocol 
UDP Is a datagram mode protocol 

 In this paper  Socket/socket/ Socket 
Socket, Representation Library 
socket, Indicates the name of the program component 
 Socket , Represents the interface at both ends of the pipe , Abstract of the endpoint for two-way communication between application processes on different hosts in the network . The socket is connected to the application process , Lower network protocol stack , It's an interface for applications to communicate through network protocols , It is the interface between application program and network protocol stack .

Create socket

< The descriptor > = socket(<ipv4>, < Flow mode |bio|nio>)

 Applications 、 The operating system identifies sockets through socket descriptors 
 Through descriptors , The protocol stack can find the corresponding socket 

Connection phase

connect(< The descriptor >, < Server's IP Address >, < Server port number >, ...);
 The descriptor ： The protocol stack judges which socket to use to connect with the socket of the server according to the descriptor , And perform the connection 
 The server IP： Identify the server 
 Server port ： Identify the socket on the server 

 When creating a socket , The protocol stack will assign a port number （ You can also specify ）. Notify the server of this port number when connecting 
 call  connect  when , Perform the connect operation . After successful connection , The agreement will put the other party  IP PORT  And other information is saved in the socket 

The descriptor 、 The relationship between port number and socket

 The descriptor ： The mechanism that applications use to identify sockets 
IP+PORT： The mechanism between client and server to identify each other's socket 

IP Address is assigned to the network hardware installed in the device . If a device has multiple network hardware, there are multiple IP

Default port number

http 80
tomcat 8080
 E-mail  25

Communication stage ： The message

write(< The descriptor >, < send data >, < Send data length >);
< Length of received data > = read(< The descriptor >, < Receive buffer >, ...);

 adopt  socket  call  read write  Delegate the protocol stack to send and receive information 
 The receive buffer is located inside the application , Storing the message in the receive buffer is equivalent to forwarding it to the application 

Disconnection phase ： End of sending and receiving data

 The application receives data , The process of sending and receiving data is over . Next call  Socket  Of  close  Enter the disconnection phase .
 Finally, the socket and pipe are disconnected , The socket itself will be deleted .
 Depending on the type of application , Which side is the client and server first close. Some application clients execute first , Some servers execute first .

Disconnect process

HTTP Provisions of the agreement ： Disconnect the server first 
web After the server sends the response message , Actively perform disconnection , First call close disconnect 
web The client receives a disconnect signal , Enter the disconnection phase 
 Next , browser （ Applications ） call read receive data ,read Tell the browser （ Applications ） Sending and receiving data operation has ended , connection dropped 
 browser （ Applications ） After learning that , call close Enter the disconnection phase 

HTTP 1.1  Start connection multiplexing 
 The protocol stack is actually responsible for sending and receiving messages 、 network card 、 drive