What happens between entering the URL and displaying the page?

1. The world wide web and HTTP agreement

• web WWW It's a large-scale 、 An online information store / Data space （ written words 、 video 、 Audio ···）, It is a collection of countless main web sites and pages . These resources need a unified resource locator to identify .

• Uniform resource locator URL -> Unique identification -> resources （ written words 、 video 、 Audio ···）

• URL General form ：< agreement >://< host >:< port >/< route >

• Users get resources by clicking on hyperlinks , These resources are transmitted through hypertext protocol HTTP Send to user .

• The World Wide Web uses hypertext markup language HTML, It makes it easy for web page designers to link from one interface to another , And can be displayed on their own screen .

2. HTTP agreement （ Stateless ）

HTTP The protocol defines the browser （ Web client process ） How to WWW Server request WWW file , And how the server delivers documents to the browser .

How users browse the page ：

• Input URL
• Click hyperlink

Browser specific process ：

• Browser analysis URL

• Browser direction DNS Request parsing IP Address

• DNS It is concluded that IP Address （ See DNS System ）

• Browser and server setup TCP Connect

• The browser sends out the command to fetch files

• Server response

• Release TCP Connect

• Browser display （ The browser can download only the text part ）

• HTTP use TCP As a transport layer agreement , but HTTP The agreement itself is connectionless （ The two sides of the communication are exchanging HTTP There is no need to set up before sending a message HTTP Connect ）

3. DNS System

root ：.
Top-level domain name ：

• National top level domain cn,us,uk
• Universal top level domain com,net,org,gov
• Infrastructure domain name / Reverse domain name arpa
  Two level domain name ：
• Category domain name ac,com,edu,gov,net,org
• Domain name of Administrative Region Used in all provinces of China 、 Autonomous region 、 Municipalities directly under the central government bj,js
• Self registered domain name （ The only global ）
  Three level domain name ：
  pku.mail
  Four level domain name …

Domain name server ：

• Root domain server ：（ Manage all top-level domain names registered by the root server ）：
• Top-level domain server （ Manage all secondary domain names registered by the top-level domain name server ）：
• Domain name server （ Responsible for a zone domain name server ）：
• Local domain name server ： When a host sends DNS When querying a request , The query request message is sent to the local domain name server .

Domain name resolution process ：

• recursive query （ Give it to others for inquiry ）
  The host requesting the query first checks the local domain name server , The local domain name server checks the root domain name server , Root domain name server to check the top-level domain name server , The top-level domain name server will find the permission domain name server , After finding it, the original path is returned to the host requesting the query
• Combination of recursion and iteration （ Yes, there is. ）（ The first step is recursive query , Pass it to the local domain name server to query ）
  The host requesting the query first checks the local domain name server , Local domain name server Check the domain name server , The root domain name server told the local domain name server that the next step should be to check the top-level domain name server , The top-level domain name server told the local domain name server that the next step should be to check the permission domain name server , The authority domain name server returns the result to the local domain name server , The local domain name server then returns the result to the host requesting the query .