Hypertext Transfer Protocol
he Hypertext Transfer Protocol (HTTP) is a protocol (a set of rules that describe how information is exchanged on a network) that allows a web browser and a web server to "talk" to each other using the ISO Latin1 alphabet, which is ASCII with extensions for European languages.
HTTP is based on a request/response model. The client connects to the server and sends a request to the server. The request contains the following: request method, URI, and protocol version. The client then sends some header information. The server's response includes the return of the protocol version, status code, followed by a message that contains server information, and the requested data. The connection is then closed.
Requests
A request from a client to a server includes the following information:
Request method
A client can request information using a number of methods. The commonly used methods include the following:
Request header
The client can send header fields to the server. Most are optional. Some commonly used request headers are shown in Table A.1.
Request data
If the client has made a POST request, it can send data after the request header and a blank line. If the client sends a GET or HEAD request, there is no data to send; the client waits for the server's response.
Responses
The server's response includes the following:
Status code
When a client makes a request, one item the server sends back is a status code, which are three-digit numeric codes divided into four categories.
Some common status codes include the following:
200 -- OK; successful transmission. This is not an error.
302 -- Found. Redirection to a new URL.The original URL has moved. This is not an error; most browsers will get the new page.
304 -- Use a local copy. If a browser already has a page in its cache, and the page is requested again, some browsers (such as Netscape Navigator) relay to the web server the "last-modified" timestamp on the browser's cached copy. If the copy on the server is not newer than the browser's copy, the server returns a 304 code instead of returning the page, reducing unnecessary network traffic. This is not an error.
401 -- Unauthorized. The user requested a document but didn't provide a valid username or password.
403 -- Forbidden. Access to this URL is forbidden.
404 -- Not found. The document requested isn't on the server. This code can also be sent if the server has been told to protect the document by telling unauthorized people that it doesn't exist.
500 -- Server error. A server-related error occurred. The server administrator should check the server's error log to see what happened.
Response header
The response header contains information about the server and information about the document that will follow. Common response headers are shown in Table A.2.
Response data
The server sends a blank line after the last header field. The server then sends the document data. If the client requested only header information, the server closes the connection.