A detailed explanation of the HTTP protocol

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1. Introduction to HTTP Protocol

• HTTP is the abbreviation of Hyper Text Transfer Protocol. It is a transmission protocol used to transfer hypertext from the World Wide Web (WWW) server to the local browser. All WWW files must comply with this standard.
• HTTP is a TCP/IP-based communication protocol for transmitting data (HTML files, image files, query results, etc.)
• HTTP is an object-oriented protocol belonging to the application layer. Due to its simple and fast approach, it is suitable for distributed hypermedia information systems. It consists of requests and responses and is a standard client-server model. HTTP is a stateless protocol
• The HTTP protocol works on a client-server architecture. The browser, as an HTTP client, sends all requests to the HTTP server, i.e., the web server, through a URL. The web server sends a response message to the client based on the received request.

2. Main Features of HTTP

• Supports client and server modes : HTTP works on the client-server architecture. The browser, as the client, sends requests to the server and web server through the URL. The web server sends response information to the client based on the received request.
• Simple and fast : When the client makes a request to the server, it only needs to send the request method and path. The request methods include post, get, etc. Each method specifies the type of connection between the client and the server. Since the HTTP protocol is simple, the HTTP service program is small in size and the communication speed is fast.
• Flexible : HTTP allows the transmission of any type of data objects
• Connectionless : Connectionless means that each connection is limited to processing only one request. After the server processes the client's request and receives the client's response, it disconnects. This method can save transmission time.
• Stateless : Stateless means that the protocol has no memory of transaction processing, which means that if the subsequent processing requires previous information, it must be retransmitted, which may increase the amount of data per connection. On the other hand, when the previous information is not needed, its response is faster.

3. HTTP request structure

HTTP request message consists of 3 parts (request line + request header + request body)

HTTP request structure:

3.1 Request Line

Request line : It consists of three fields: request field, URL field, and HTTP protocol version field, which are separated by spaces. For example: GET/material/index HTTP/1.1\r\nAccording to the HTTP standard, HTTP requests can use multiple request methods.

HTTP1.0 defines three request methods: GET, POST, and HEAD. HTTP1.1 adds five new request methods: OPTIONS, PUT, DELETE, TRACE, and CONNECT.

3.2 Request Header

Request header: HTTP client programs (such as browsers) must specify the request type (usually GET or POST) when sending requests to the server. If necessary, the client program can also choose to send other request headers. Most request headers are not required, but Content-Length is an exception. For POST requests, Content-Length must appear.

The request header informs the server about the client's request. Typical request headers are:

3.3 Blank lines

Its function is to tell the server that the request header information ends here.

3.4 Request Body

1. If the method is GET, this item is empty. (The data is in the URL address bar)
2. If the method is a post field, it usually contains the data to be submitted.

Examples:

4. HTTP response structure

HTTP response structure:

HTTP response message consists of three parts: (status line, response header, response body)

4.1 Response Line

Response line: describes the status of the response, which is generally composed of the protocol version, status code and its description, such as HTTP/1.1200OK\r\n, where the protocol version is HTTP/1.1 or HTTP/1.0, 200 is its status code, and OK is its description.

Five possible values:

Common status codes:

4.2 Response Header

Response header: Used to describe the basic information of the server and the data. Through the description of this data, the server can inform the client how to process the data it sends back later.

4.3 Response Body

The response body is the message body of the response, which contains the content of the response. It can contain HTML code, pictures, etc. The body consists of the data bytes transmitted in the HTTP message immediately following the header.

Examples:

5. Enter the URL in the browser and press Enter to request the process

1. DNS resolution : When our browser searches its own DNS cache first, that is, our hosts file. If the browser's own cache cannot find it, it will check the system's DNS cache. If it is found and has not expired, the search and resolution will stop here. If the local machine does not find the DNS cache, the browser will initiate a DNS system call, and will initiate a domain name resolution request to the locally configured preferred DNS server, and will eventually find the IP address corresponding to our domain name.

2. TCP resolution : After obtaining the IP address corresponding to the domain name, the User-Agent (usually the browser) will initiate a TCP connection request to the server's WEB program (commonly used are httpd, nginx, etc.) port 80 with a random port (1024 < port < 65535). After the connection request reaches the server, it enters the network card, then enters the TCP/IP protocol stack of the kernel, and finally reaches the WEB program, finally establishing a TCP/IP connection.

3. Send HTTP request : After establishing a TCP connection, initiate an HTTP request

4. The server processes the request and returns an HTTP message : The server accepts and processes the request and returns an HTTP response. The format of a response message is basically the same as the request message, consisting of a response line, a response header, a blank line, and an entity.

5. Browser parses and renders the page

6. Connection ended : TCP waves four times, port connected

6. The difference between Cookie and Session

6.1 Introduction to Cookies:

• It is a special message sent by the server to the client and stored in the client in the form of text.
• When the client requests again, the Cookie will be sent back
• After receiving the information, the server will parse the contents of the cookie and the client.

Cookie settings and sending process:

6.2 Session Introduction:

• Server-side mechanism, information stored on the server
• Parse client requests and manipulate session ids, saving status information as needed

6.3 Differences between Cookies and Sessions

• Cookie data is stored on the client's browser, and Session data is stored on the server.
• Session is more secure than Cookie
• If you want to reduce the burden on the server, you should use Cookies

VII. Conclusion

At this point, the HTTP protocol has been explained. The above is Xiao Nong’s understanding of HTTP.