HTTP protocol

1. Overview of HTTP protocol

　1. What is HTTP? Hypertext Transfer Protocol.
　　2. Describes the data standards on the client and server sides. This protocol is maintained and managed by W3C.
Version:
HTTP1.0: A network connection needs to be established every time a request is made
HTTP1.1: (Mainstream) Multiple requests are issued and multiple responses are obtained on one network connection. A few more heads.

　3. Clarity: The browser will automatically issue a request when it encounters the following tags

　　

　　
　　

2. Composition of HTTP protocol

　1. Request part

　　

　　2. Response part

　　

3. Detailed explanation of the request part

　1. Request line:
　　GET /app1/1.html HTTP/1.1
　　GET: request method.
　　Commonly used request methods: GET (default), POST, HEAD, OPTIONS, etc.
　　GET: Default
　　http://localhost:8080/app1/1.html?username=abc&password=123
　　The submitted data is displayed , relatively unsafe, the first line of the protocol has a length limit, <1kb.
　 (recommended when requesting data) POST: can be submitted through

　　username=abc&password=123
　　The data is in the request body, which is relatively safe, and there is no limit on the length.

　　/app1/1.html: The requested resource address. (URI)
URL: protocol + host: port + resource address
http://localhost:8080/app1/1.html

　　HTTP/1.1: The version of the protocol used by the client browser.

　 2. Request message headers: (take notes and remember commonly used headers)
　 Function: transfer additional information (secret instructions) to the server
　 Accept: inform the server of the data types (MIME types) that the client can accept
　 File System: distinguishes different files by their file extensions. txt jpeg
　 MIME type: large type/small type. txt--->text/plain html---->text/html js---->text/javascript (specific correspondence: Tomcatconfweb.xml)
　　Accept-Encoding: Inform the server that the client can accept it compression encoding. For example, gzip
　Accept-Language: Inform the server of the language supported by the client.
　Referer: Tell the server which page it came from.

　　

　 Function: Statistics of advertising effectiveness; Prevent hot links.
　Content-Type: Inform the server of the MIME type of the request body
　　Default type: application/x-www-form-urlencoded (the default value of the form enctype attribute)
　　Specific embodiment: username=abc&password=123
　　Other types: multipart/ form-data (used when uploading files)
　　If-Modified-Since: Inform the server of the resource currently being accessed and the last modification time of the file in the cache.
　　User-Agent: Inform the server of the type of browser
　　Content-Length: The data length of the request body
　　Cookie: (*****Important) Related to session management

　3. Request text:
　　For data to be submitted to the server, the input field in the form must have a name attribute value
　　Only for POST requests can there be text
　　username=abc&password=123

4. Detailed explanation of the response part

　　1. Detailed explanation of the response part
　　Response line:
　　　HTTP/1.1 200 OK
　　HTTP/1.1: Describe the protocol version used by the server
　　　200: Response status code
　　　1xx 2xx 3xx 4xx 5xx
　2xx: Processed
4xx: The request is available Error
　　5xx: Server error
　　Remember the commonly used response codes:
　　200: Everything is normal
　　302/307: Request for redirection
　　304: The resource on the server has not changed
　　404: The accessed address does not exist
　　50 0: server Terminal error
　　OK: Response? Description

　 2. Response header:
　　 Function: Additional information (secret command) passed from the server to the client
　　 Location: Inform the client of the address you are visiting.
　　 Implement request redirection with 302/307
　　Content-Encoding: Inform the client of the compression encoding (gzip) used in the response body
　　Content-Length: Inform the client of the length of the response body
　　Content-Type: Inform the client of the response body The MIME type of the text. Default text/html
　　Refresh: Notify the client to refresh regularly
　　Content-Disposition: Notify the client to open it by downloading
　　　Attachment; filename=23.jpg
　　Set-Cookie: (*****) Session related
Expires: -1 to control time

　　　　 Cache-Control: no-cache (1.1)
　　　 Pragma: no-cache (1.0)
　　　 The three headers are used together to tell the browser not to cache.

　　 3. Response text:
　　 The text content parsed by the browser is the same as right-clicking to view the source code.