Hypertext Transfer Protocol
Hypertext Transfer Protocol (HTTP) is a communications protocol. Its use for retrieving inter-linked text documents (hypertext) led to the establishment of the World Wide Web. HTTP is an application level protocol with lightness and speed necessary for distributed, collaborative, hypermedia information systems HTTP development was coordinated by the World Wide Web Consortium and the Internet Engineering Task Force (IETF), culminating in the publication of a series of Request for Comments (RFCs), most notably RFC 2616 (June 1999), which defines HTTP/1.1, the version of HTTP in common use.
HTTP is a request/response standard between a client and a server. A client is the end-user, the server is the web site. The client making a HTTP request—using a web browser, spider, or other end-user tool—is referred to as the user agent. The responding server—which stores or creates resources such as HTML files and images—is called the origin server. In between the user agent and origin server may be several intermediaries, such as proxies, gateways, and tunnels. HTTP is not constrained to using TCP/IP and its supporting layers, although this is its most popular application on the Internet. Indeed HTTP can be "implemented on top of any other protocol on the Internet, or on other networks. HTTP only presumes a reliable transport; any protocol that provides such guarantees can be used."
Typically, an HTTP client initiates a request. It establishes a Transmission Control Protocol (TCP) connection to a particular port on a host (port 80 by default; see List of TCP and UDP port numbers). An HTTP server listening on that port waits for the client to send a request message. Upon receiving the request, the server sends back a status line, such as "HTTP/1.1 200 OK", and a message of its own, the body of which is perhaps the requested resource, an error message, or some other information.
HTTP uses TCP and not UDP because much data must be sent for a webpage, and TCP provides transmission control, presents the data in order, and provides error correction. See the difference between TCP and UDP.
Resources
to be accessed by HTTP are identified using Uniform Resource Identifiers (URIs)
(or, more specifically, Uniform Resource Locators (URLs)) using
the http: or https URI schemes.
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