Detailed explanation of HTTP protocol examples

HTTP is an application layer protocol, consisting of requests and responses, and is a standard client service model. HTTP is usually carried on the TCP protocol, and sometimes on the TLS or SSL protocol layer. At this time, it becomes commonly referred to as HTTPS. The default port number for HTTP is 80 and the port number for HTTPS is 443.

The disadvantage is that the server will not actively PUSH to the client

The application of the SPDY protocol requires a client browser and server Both terminals are supported at the same time. Optimizes the communication between the browser and the server, supports stream multiplexing, prioritizes requests, actively initiates requests, forces SSL secure transmission and other advanced features

4.1.2 How the HTTP protocol works

Browsing the web is the main application of the HTTP protocol. An HTTP operation is called a transaction

1 The client needs to establish a connection with the server, click a hyperlink, and the HTTP protocol The work begins.

2 After establishing the connection, the client sends a request to the server.

3 After receiving the request, the server gives the corresponding response information

4 The client receives the information returned by the server and displays it on the user's display screen, and then the client disconnects from the server

Some main concepts in HTTP protocol

1 Request

HTTP request consists of three parts: request header, request header, request body

Request line: Method Request-URL HTTP-Version CRLF

Method: Request method (GET, POST, HEAD, PUT, DELETE, TRACE, CONNECT, OPTIONS)

Request URL: A uniform resource identifier

HTTP Version: The requested HTTP protocol version

CRLF: Carriage return and line feed

2 Response

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

Status line: HTTP-Version Status-Code Reason-Phrase CRLF

HTTP Version: Server HTTP protocol version

Status-Code: Response status code sent back by the server

Reason Phrase: Text description of the status code

The status code consists of three digits and five possible values

1XX: Indication information--the request has been received , continue processing

2XX: Success--the request has been successfully received, understood and accepted

3XX: Redirect--further operations must be performed to complete the request

4XX: Client error--The request has a syntax error or the request cannot be fulfilled

5XX: Server error--The server failed to fulfill a legal request

Common status codes and status descriptions

200 OK: The client request is successful

400 bad Request: The client request has a syntax error and cannot be processed What the server understands

401 Unauthorize: The request is unauthorized, this status code must be used with the WWW Authenticate header

403 Forbidden: The server received the request, but refused to provide it Service

404 Not found: The requested resource does not exist, for example, the wrong URL was entered

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

3 Headers

HTTP message headers include ordinary headers, Request header, response header, entity header

1 There are a few header fields in the ordinary header that are used for all request and response messages, but are not used for the transmitted entity, only for the transmitted message ( Cache control, connection control)

2 The request header allows the client to pass additional information of the request to the server as well as the client's own information (UA header, Accept)

    3 The response header allows the server to pass additional response information that cannot be placed in the status line, as well as information about the server and information (Location) for next access to the resource identified by the Request URL

4 The entity header defines meta-information about the entity body and the resource identified by the request, such as whether there is an entity body

Several important headers:

Host The :header field specifies the Internet host and port number of the requested resource, and must indicate the location of the original server or gateway of the requested URL.

User Agent: UA for short, the content contains the user information that made the request. Usually contains the browser's information, mainly the name, version of the browser and the operating system used

Accept: Tells the server what file formats are acceptable. Usually this value is similar across browsers.

Cookie: There are two types: one is sent by the client to the server using the Cookie header, which can have multiple values; the other is sent by the server to the browser, with the header Set Cookie. There can only be one value, you need to specify domain, path

Cache Control: Specify the caching mechanism followed by the request and response. Setting Cache Control in a request message or response message does not modify the cache processing process in another message. Caching instructions when requesting include no cache, no store, max age, max state, no transform, must revalidate, proxy revalidate, max age

Referer: The header field allows the client to specify the source resource of the request URL Address, which allows the server to generate a fallback list for login and cache optimization. Referer is usually a parameter used by the traffic statistics system to record the visitor's address

Content Length: Content length

Content Range: The resource range of the response. The requested resource range can be marked in each request. When the connection is disconnected and reconnected, the client only requests the undownloaded part of the resource instead of re-requesting the entire resource to implement breakpoint resumption. This is the principle of Xunlei, which uses multi-threading to read the resources of silly girls on the Internet in sections, and finally merges them

Accept Encoding: Specifies the encoding method that can be received

Custom Headers: In http messages, you can also use some header fields that are not defined in the formal specifications in http1.1.

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