The HTTP protocol
The correct format for HTTP requests and responses depends on the version of the HTTP protocol (or HTTP specification) that is used by the client and by the server.
The versions of the HTTP protocol (or "HTTP versions") commonly used on the Internet are HTTP/1.0, which is an earlier protocol including fewer functions, and HTTP/1.1, which is a later protocol including more functions. The client and server might use different versions of the HTTP protocol. Both client and server must state the HTTP version of their request or response in the first line of their message.
Internet Society and IETF (Internet Engineering Task Force) Request for Comments documents (known as RFCs) provide the official definitions for the HTTP protocol:
In the HTTP response that is sent to a client, the status code, which is a 3-digit number, is accompanied by a reason phrase (also known as status text) that summarizes the meaning of the code. With the HTTP version of the response, these items are placed in the first line of the response, which is therefore known as the status line.