Do you know all the HTTP protocols?

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1. HTTP protocol

HyperText Transfer Protocol (HTTP) .

Resources requested via HTTP or HTTPS are identified by Uniform Resource Identifiers (URIs). An HTTP client initiates a request and creates a TCP connection to a server-specified port (port 80 by default). The HTTP server listens for client requests on that port. Once a request is received, the server returns a status, such as "HTTP/1.1 200 OK", and the returned content, such as the requested file, error message, or other information.

2. URI and URL

▐ 2.1 URI

Uniform Resource Identifier, Every resource available on the Web, such as HTML documents, images, video clips, programs, etc., is located using a URI;

A URI generally consists of three parts:

1. Naming mechanism for accessing resources
2. The host name where the resource is stored
3. The name of the resource itself, represented by the path, with emphasis on the resource

▐ 2.2 URL

Uniform Resource Location, Uniform Resource Locator

URL is a string used to describe information resources on the Internet, mainly used in various WWW client programs and server programs. URL is a type of URI.

URL can be used to describe various information resources in a unified format, including files, server addresses and directories.

URLs generally consist of three parts

1. protocol
2. The IP address (or port number) of the host that can access the resource
3. The specific address of the host resource (directory plus file name)

▐ 2.3 URL composition

1. Protocol part : The protocol part is http:
2. Domain name part : The domain name part is for example "www.jianshu.com". Of course, the domain name can also be the IP address. The IP address has one less step to be resolved by the DNS server.
3. Port part : The domain name and port are separated by ":".
4. The port is not a required part of the URL. If the port is omitted, the default port number 80 will be used, so the actual request address is http://www.jianshu.com:80
5. Virtual directory part : From the first "/" after the domain name to the last "/", it is the virtual directory part. The virtual directory is not a required part of a URL.
6. File name part : refers to the path of the resource file accessed in the server.
7. Anchor part : From “#” to the end, it is the anchor part, which is not a required part.
8. Anyone who has worked with HTML knows that it is used to locate the sliding position of the page.
9. Parameter part : The part from "?" to "#" is the parameter part, also called the search part or query part. For example, ?page=1.

3. Types of Requests

The HTTP protocol defines eight methods or "actions" to indicate different ways of operating the resource specified by the Request-URI. Just like operating a database or file system, designing network requests is the same. URLs are used to locate network resources, creating PUT, DELETE, POST, and GET to correspond to add, delete, modify, and query operations. However, get and post are commonly used in actual applications, and other request methods can also be indirectly implemented through these two methods.

GET

Send a "show" message to the specified resource. The GET method is only used to obtain data, and the data itself should not be changed, that is, the data should not be operated or submitted.

POST

Submit data to a specified resource and request the server to process it (for example, submit parameters/forms, or upload files). The data may be manipulated and submitted to create resources.

PUT

Upload the latest content to the specified resource location.

DELETE

As the name suggests, it requests the server to delete the resource corresponding to the URI.

HEAD

Like the GET method, it sends a request to the server for a specified resource. However, the server will not send back the text of the resource. The advantage is that this method can be used to obtain "information about the resource" (meta-information or metadata) without transmitting the entire content.

TRACE

Echoes requests received by the server, mainly used for testing or diagnosis.

OPTIONS

This method allows the server to return all HTTP request methods supported by the resource. Substituting '*' for the resource name and sending an OPTIONS request to the web server can test whether the server function is working properly.

CONNECT

The HTTP/1.1 protocol reserves the proxy server that can change the connection to a pipeline mode. It is usually used for the connection of SSL encrypted servers (via non-encrypted HTTP proxy servers).

4. Differences between GET and POST

1. The parameter carrying position is different.

The data submitted by GET will be appended to the URL (that is, the data is placed in the request line) and will be displayed in the address bar. The value is passed by ?=, and multiple parameters are connected by &. The data submitted by POST is placed in the HTTP package body and is not displayed in the address bar.

2. The transmission data size limit is different.

Different browsers have restrictions on the length of URLs, so when submitting with GET, the transmitted data will be limited by the length of the URL. Since POST does not transmit values ​​through the URL, theoretically there is no limit on the data.

3. POST is more secure than GET because the browser may store the request address as a history record. For a GET request login, others can directly obtain your account and password from the URL, while POST cannot obtain parameters.

5. HTTP request information

The client sends an HTTP request to the service with the following format:

The request line consists of four parts: request line, request header (header, often used to store token), blank line and request data.

For example:

 GET /8669504-c2641e8e6eed5904.png HTTP/1.1
 Host www.jianshu.com
 User -Agent Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.106 Safari/537.36
 Accept image/webp,image/,/*;q=0.8
 Referer www.jianshu.com
 Accept-Encoding gzip, deflate, sdch
 Accept-Language zh-CN,zh;q=0.8
 name =Professional%20Ajax&publisher=lili

1. The first request line is used to describe the request type, the resource to be accessed, and the HTTP version to be used.
2. The second part, the request header between curly braces, is used to submit additional information to the server.
3. The third part, a blank line, is required after the request header.
4. The fourth part, the request data, is also called the body, and any other data can be added.

6. HTTP Response Information

The server accepts the request and sends back a response message which also consists of four parts:

Status line, message headers, a blank line, and the response body.

For example:

1. The first line includes the protocol version number and the return status code
2. The second line responds to the date and time
3. The third line is the response message header, Content-Type: specifies the MIME type HTML (text/html), and the encoding type is UTF-8
4. Response body, the text information returned by the server to the client.

7. HTTP Status Codes

The status code has three digits. The first digit indicates the current corresponding type. The types are:

• 1xx message - the request has been accepted by the server and continues to be processed
• 2xx Success - The request has been successfully received by the server (such as 200, 201)
• 3xx redirection - further action is required to complete this request
• 4xx request error - client error: the request contains a lexical error or cannot be executed (such as the 404 Not Found that makes you want to run away when you see it)
• 5xx Server Error - Server Error: The server encountered an error while processing a correct request (such as 500, which you want to find the backend as soon as you see it)

Common status codes are as follows:

• 200 OK : Client request successful
• 400 Bad Request : The client request has a syntax error and cannot be understood by the server.
• 401 Unauthorized : The request is unauthorized. This status code must be used with the WWW-Authenticate header field.
• 403 Forbidden : The server received the request but refused to provide service
• 500 Internal Server Error : An unexpected error occurred on the server
• 503 Server Unavailable : The server is currently unable to process the client's request and may return to normal after a period of time.

8. Summarize the complete process of an HTTP request

1. The client establishes a TCP connection with the server
2. Send HTTP request via TCP socket connection
3. The server parses the request, locates the requested resource, queries the resource and then returns HTML text data, which is read by the client.
4. Release TCP connection
5. The client browser parses the HTML content

Events that occur when the browser enters the website:

1. The browser requests the DNS server to resolve the IP address corresponding to the domain name in the URL;
2. After resolving the IP address, establish a TCP connection with the server based on the IP address and the default port 80;
3. The browser sends an HTTP request to read a file (the file corresponding to the part after the domain name in the URL). This request message is sent to the server as the data of the third message of the TCP three-way handshake.
4. The server responds to the browser request and sends the corresponding HTML text to the browser;
5. Release the TCP connection;
6. The browser takes the HTML text and displays the content;

The above is the complete summary of Http. This article is constantly improved and updated.