What is the meaning of HTTP?

The standard Web transfer protocol is HTTP (HyperText Transfer Protocol). HTTP is constantly evolving. Several versions are in use and others are under development. The HTTP protocol consists of two fairly distinct items: the set of requests from browsers to servers and the set of responses going back the other way.

Although HTTP was designed for use in the Web, it has been intentionally made more general than necessary with an eye to future object-oriented applications. For this reason, the first word on the full request line is simply the name of the method (command) to be executed on the Web page. The built-in methods are listed:

The GET method requests the server to send the page. If the GET is followed by an If-Modified-Since header, the server only sends the data if it has been modified since the data supplied. Using this mechanism, a browser that is asked to display a cached page can conditionally ask for it from the server, giving the modification time associated with the page. If the cache page is still valid, the server just sends back a status line announcing that fact, thus eliminating overhead of transferring the page again .

The HEAD method just asks for the message header, without the actual page. This method can be used to get a page’s time of last modification, to collect information for indexing purposes, or just to test a URL for validity.

The PUT method is the reverse of GET: instead of reading the page, it writes the page. This method makes it possible to build a collection of Web pages on a remote server. The lines following the PUT might include Content-Type and authentication headers, to prove that the caller indeed has permission to perform the requested operation.
The POST bears a URL, but instead of replacing the existing data, the new data is “append” to it.

DELETE removes the page with authentication and permission play a major role here. There is no guarantee that DELETE succeeds, since even if the remote HTTP server is willing to delete the page, the underlying file may have a mode that forbids the HTTP server from modifying or removing it.

The LINK and UNLINK methods allow connections to be established between existing pages or other sources.


URLs

Each Web page is assigned a URL (Uniform Resource Locator) that effectively serves as the page’s worldwide name. URLs have three parts (use http://www.cityu.edu.hk/welcome.html as example): the protocol (http), the DNS name of the server on which the page is located (www.cityu.edu.hk), and the file name (welcome.html).

Copycat

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it is the short form of hypertext transmission protocol
it is one kind of protocol, the other examples are ftp, nnsp, pop...

【DEFINITION】
HTTP (Hypertext Transfer Protocol) is the set of rules for transferring files (text, graphic images, sound, video, and other multimedia files) on the World Wide Web. As soon as a Web user opens their Web browser, the user is indirectly making use of HTTP. HTTP is an application protocol that runs on top of the TCP/IP suite of protocols (the foundation protocols for the Internet).

HTTP concepts include (as the Hypertext part of the name implies) the idea that files can contain references to other files whose selection will elicit additional transfer requests. Any Web server machine contains, in addition to the Web page files it can serve, an HTTP daemon, a program that is designed to wait for HTTP requests and handle them when they arrive. Your Web browser is an HTTP client, sending requests to server machines. When the browser user enters file requests by either "opening" a Web file (typing in a Uniform Resource Locator or URL) or clicking on a hypertext link, the browser builds an HTTP request and sends it to the Internet Protocol address (IP address) indicated by the URL. The HTTP daemon in the destination server machine receives the request and sends back the requested file or files associated with the request. (A Web page often consists of more than one file.)

【METHODS】
HTTP defines eight methods (sometimes referred to as "verbs") indicating the desired action to be performed on the identified resource.

HEAD
Asks for the response identical to the one that would correspond to a GET request, but without the response body. This is useful for retrieving meta-information written in response headers, without having to transport the entire content.

GET
Requests a representation of the specified resource. By far the most common method used on the Web today.

POST
Submits data to be processed (e.g. from a HTML form) to the identified resource. The data is included in the body of the request.

PUT
Uploads a representation of the specified resource.

DELETE
Deletes the specified resource.

TRACE
Echoes back the received request, so that a client can see what intermediate servers are adding or changing in the request.

OPTIONS
Returns the HTTP methods that the server supports. This can be used to check the functionality of a web server.

CONNECT
For use with a proxy that can change to being an SSL tunnel.

HTTP servers are supposed to implement at least the GET and HEAD methods and, whenever possible, also the OPTIONS method.