HTTP is a client terminal (user) and server (WEB) request and response criteria (TCP), by using a Web browser, web crawler or other tools, the client initiates a HTTP request to the specified port on the server, we call the client for the user agent.
Typically, a request is initiated by the HTTP client to create a TCP connection for the server specified port. The HTTP server listens to the clients request at that port. Upon receipt of the request, the server returns a state, such as HTTP/1.1 200 OK, and the returned content, such as requested files, error messages, or other information.
First of all the digital code represents the type of response current, respectively is:
1XX -request as the server receives, continue processing; 2XX-request success be brushed the reception, understanding and acceptance;
3XX -the need for follow-up operation to complete this request;
4XX -request contains lexical errors or not to be executed;
5XX - the server error occurred while processing a correct request.