Revealing the mysterious process behind the browser: analysis of the complete process from inputting the URL to page display

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Revealing the mysterious process behind the browser: analysis of the complete process from inputting the URL to page display

1 Introduction

In modern society, browsers have become the main way for people to access the Internet. Whether they are looking for information, online shopping or social entertainment, they are inseparable from the support of browsers. However, as users, we often only focus on the display of browser pages, but know very little about the workflow behind the browser. This article will delve into the complete process of a browser accessing a web page and reveal the mysterious process behind the browser.

2. Resolution from URL to IP address

2.1 The role and principle of DNS resolution

When we enter a URL into the browser, the browser first needs to convert the URL into the corresponding IP address so that it can establish a connection with the server. This process is called DNS resolution. DNS (Domain Name System) is a distributed database system that maps domain names to IP addresses.

The principle of DNS resolution is implemented through recursive query and iterative query. Recursive query means that the client initiates a query request to the local DNS server, and the local DNS server is responsible for querying the root DNS server level by level until the corresponding IP address is found. Iterative query means that the DNS server sends query requests to other DNS servers and continues querying according to the returned results until the corresponding IP address is found.

2.2 The role and optimization of DNS caching

In order to improve the efficiency of DNS resolution, browsers and operating systems will perform DNS caching. The DNS cache saves the resolved domain name and the corresponding IP address. The next time you access the same domain name again, you can obtain the IP address directly from the cache without further DNS resolution.

DNS cache can be divided into browser cache and operating system cache. Browser cache refers to the cache inside the browser, which is only valid for the current browser. The operating system cache refers to the cache inside the operating system and is valid for all applications.

2.3 Common DNS problems and solutions

During the DNS resolution process, common problems include DNS resolution failure, DNS hijacking, and DNS cache failure. When you encounter these problems, you can try the following solutions:

Check whether the network connection is normal and ensure normal access to the Internet.
Clear your browser cache and operating system cache to ensure you get the latest DNS resolution results.
Modify the DNS server address and use another reliable DNS server.
Use VPN and other methods to bypass DNS hijacking.

3. Establish a connection to the server

3.1 Basic principles of TCP/IP protocol

When communicating between the browser and the server, the TCP/IP protocol is used. TCP/IP protocol is a set of protocols used for Internet communication, which includes TCP (Transmission Control Protocol) and IP (Internet Protocol).

The TCP protocol is responsible for providing reliable data transmission. It divides the data into small data packets, and sorts and reassembles the data packets through sequence numbers to ensure the integrity and correctness of the data. The IP protocol is responsible for transmitting data packets from the source address to the destination address. It locates the data packets to the correct destination host through the IP address.

3.2 Process analysis of TCP three-way handshake and four-way wave

When establishing a TCP connection, a three-way handshake is required. The three-way handshake process is as follows:

The client sends a SYN (synchronization) packet to the server, indicating a request to establish a connection.
After receiving the SYN packet, the server sends a SYN+ACK (synchronization + confirmation) packet to the client to indicate acceptance of the connection request.
After receiving the SYN+ACK packet, the client sends an ACK (confirmation) packet to the server, indicating that the connection is successfully established.

When closing a TCP connection, four waves are required. The process of waving four times is as follows:

The client sends a FIN (end) packet to the server, indicating a request to close the connection.
After receiving the FIN packet, the server sends an ACK packet to the client to indicate that it accepts the shutdown request.
The server sends a FIN packet to the client, indicating that the server is ready to close the connection.
After receiving the FIN packet, the client sends an ACK packet to the server to close the connection.

3.3 Differences and usage scenarios of HTTP and HTTPS protocols

HTTP (Hypertext Transfer Protocol) and HTTPS (Hypertext Transfer Protocol Secure) are protocols used to communicate between browsers and servers. Their main difference is security.

The HTTP protocol is transmitted in clear text, and data is easily eavesdropped and tampered with during the transmission process, making it unsuitable for transmitting sensitive information. The HTTPS protocol uses the SSL/TLS protocol for encryption, and the data is encrypted during transmission to ensure data security.

Therefore, the HTTP protocol is suitable for general web browsing and data transmission, while the HTTPS protocol is suitable for websites that need to protect user privacy and sensitive information, such as bank websites, e-commerce websites, etc.

4. Send HTTP request

4.1 Structure and content of HTTP request message

The HTTP request message consists of three parts: request line, request header and request body.

The request line includes the request method, request URL, and HTTP protocol version. Common request methods include GET, POST, PUT, DELETE, etc., which are used to specify operations on resources. The request URL is the address of the resource to be accessed. The HTTP protocol version specifies the version number of the HTTP protocol used.

The request header includes a series of fields and values used to convey additional information about the request. Common request header fields include User-Agent (user agent), Accept (acceptable content type), Referer (reference page), etc.

The request body includes the data to be sent to the server, and is only included in methods such as POST.

4.2 Differences and usage scenarios between GET and POST requests

GET and POST are two commonly used request methods in the HTTP protocol.

The GET request is used to obtain resources, passing parameters through the URL, and the parameters will be displayed in the URL. GET requests are idempotent, that is, requesting the same URL multiple times will get the same result. The amount of data requested by GET is limited and is generally used to request a small amount of data.

POST requests are used to submit data, and the data is passed through the request body. POST requests can pass large amounts of data without any limit on the amount of data. POST requests are not idempotent, that is, requesting the same URL multiple times may result in different results.

Depending on the usage scenario, GET requests are suitable for data acquisition and query operations, while POST requests are suitable for data submission and modification operations.

4.3 Common fields and functions in request headers

The request header contains some common fields that are used to convey additional information about the request. The following are some common request header fields and their functions:

User-Agent: Identifies the type and version number of the browser. The server can return different content based on the User-Agent field.
Accept: Specify the content types that the client can accept, and the server can return corresponding content based on the Accept field.
Referer: Indicates which page the current request is redirected from, used for statistics and anti-leeching.
Cookie: used to transfer session information between the client and the server to maintain the user's login status.
Authorization: used to pass authentication information in the request, generally used for API requests.

5. The server processes the request

5.1 Analysis of the process of the server receiving the request

When the server receives the client's request, it will perform a series of processes. First, the server will parse the request message and extract the request method, URL, request header and other information. The server then finds the corresponding handler or resource based on the requested URL. Next, the server will execute the corresponding processing logic and generate response data. Finally, the server packages the response data into a response message and sends it to the client.

5.2 Server process and technology for processing requests

The process of the server processing the request can be divided into the following steps:

Parse the request message and extract the request method, URL, request header and other information.
Find the corresponding handler or resource based on the requested URL.
Execute corresponding processing logic and generate response data.
Pack the response data into a response message and send it to the client.

The server's technology for processing requests includes, but is not limited to:

Web server: such as Apache, Nginx, etc., used to receive and process HTTP requests.
Back-end programming languages: such as Java, Python, Node.js, etc., used to write handlers.
Database: such as MySQL, MongoDB, etc., used to store and obtain data.
Caching technology: such as Redis, Memcached, etc., used to improve data access speed.

5.3 Common server error status codes and their meanings

During the process of processing the request, the server may return some error status codes to indicate the processing result of the request. Common server error status codes and their meanings are as follows:

200 OK: The request was successful.
400 Bad Request: The request is incorrect and the server cannot understand the request.
401 Unauthorized: Unauthorized, authentication required.
403 Forbidden: Access is prohibited and the server refuses the request.
404 Not Found: Resource not found, the server cannot find the requested URL.
500 Internal Server Error: The server has an internal error and cannot complete the request.

6. Receive and render the page

6.1 Analysis of the process of receiving HTTP response messages

When the browser receives the response message from the server, it will perform a series of processing procedures. First, the browser parses the response message and extracts the response status code, response header, response body and other information. Then, the browser determines the processing result of the request based on the status code. Next, the browser determines how to handle the response body based on the content type in the response header. If it is an HTML page, the browser will render the page; if it is another type of file, the browser will perform corresponding processing according to the file type, such as downloading the file or displaying the image.

6.2 Differences, advantages and disadvantages between front-end rendering and back-end rendering

Front-end rendering and back-end rendering are two different ways of page display.

Front-end rendering refers to separating data and page structure, and dynamically generating page content through JavaScript on the browser side. The advantage of front-end rendering is that it can provide a better user experience, the page loads quickly, and dynamic interaction can be achieved. The disadvantage is that it requires higher browser performance and is less friendly to search engines.

Back-end rendering refers to combining data and page structure on the server side and then sending the entire page to the browser. The advantage of back-end rendering is that it has lower performance requirements on the browser and is more friendly to search engines. The disadvantage is that the page loads slowly and the user experience is poor.

According to actual needs and project characteristics, you can choose front-end rendering or back-end rendering, or combine the advantages of both for rendering.

6.3 Common techniques and strategies for front-end page optimization

To improve page loading speed and user experience, front-end developers can adopt the following common optimization tips and strategies:

Compress and merge resource files to reduce the number of HTTP requests.
Use browser caching to reduce duplicate requests.
Use CDN acceleration to deploy static resources to globally distributed CDN nodes.
Delay loading of images and scripts to improve first screen loading speed.
Use lazy loading technology to delay loading of content below the first screen.
Use image compression and optimization tools to reduce image size.
Use CSS Sprites technology to reduce HTTP requests for images.
Use font icons instead of images to reduce page load times.
Use asynchronous loading scripts or place scripts at the bottom of the page to avoid blocking page rendering.

7. Page display and resource loading

7.1 Process of HTML parsing and DOM tree construction

When the browser receives the HTML response body, it will perform the process of parsing and building a DOM tree. The HTML parser parses the HTML document into a DOM tree, which is a tree-like structure composed of a series of nodes.

The process of HTML parsing is divided into two stages: tokenization and construction. The tokenization phase decomposes the HTML document into a series of tags, and the construction phase builds a DOM tree based on these tags.

7.2 Process of CSS parsing and style calculation

During the process of parsing HTML, the browser also parses the CSS style sheet and applies the styles to elements in the DOM tree. The process of CSS parsing includes lexical analysis, syntax analysis and style calculation.

Lexical analysis breaks down CSS code into a series of tags. Syntax analysis generates a CSS rule tree based on these tags. Style calculation is based on the CSS rule tree and DOM tree to calculate the final style applied to each element.

7.3 JavaScript parsing and execution process

When the browser encounters JavaScript code, it will perform the process of parsing and executing it. A JavaScript parser parses JavaScript code into an abstract syntax tree (AST), and then converts the AST into executable bytecode or machine code.

The execution process of JavaScript is divided into two methods: interpreted execution and compiled execution. Interpretation execution is to interpret and execute JavaScript code line by line, generating and executing bytecode instantly. Compilation and execution is to compile JavaScript code into machine code and then directly execute the machine code.

7.4 Optimization methods for loading resources such as images, scripts and style sheets

In order to improve the loading speed and user experience of the page, the following optimization methods can be used:

Image loading optimization: use appropriate image formats, compress image sizes, use lazy loading or preload images.
Script loading optimization: Place scripts at the bottom of the page, use asynchronous loading scripts or lazy loading scripts.
Style sheet loading optimization: Place the style sheet at the head of the page, avoid using @import to introduce the style sheet, and use inline styles or inline style sheets.

8. Summary

This article deeply explores the complete process of browser accessing web pages, and analyzes the process from inputting URL to page display in detail. We learned about the principles and optimization of DNS resolution, the process and protocol of establishing a connection with the server, the structure and content of sending HTTP requests, the process and technology of server processing requests, the process and optimization methods of page display and resource loading, etc. By understanding these processes and technologies, we can better understand the mysterious process behind the browser, and carry out corresponding optimization strategies to improve the user's access experience.