Application Layer

Application Layer

Network applications are the raisons dí Ítre of a computer network - if we couldn't think of any useful applications, there wouldn't be any need to design networking protocols to support them. Over the past 40 years, various ingenious and wonderful network applications have been created. These applications contain the classic text-based applications that became famous in the 1970s and 1980s: text e-mail, remote access to computers, file transfers, newsgroups, and text chat. They include the killer application of the mid -1990s: the World Wide Web, containing Web surfing, search, and electronic commerce. They also contain the two killer applications introduced at the end of the millennium - instant messaging with buddy lists, and P2P file sharing. And they contain many successful audio and video applications, including Internet telephony, video sharing and streaming, Internet  radio, and IP television (IPTV). Furthermore, the increasing penetration of broadband residential access and the increasing ubiquity of wireless access are setting the stage for more new and interesting applications in the future.    

In this section we study the conceptual and implementation aspects of network applications. We begin by defining key application-layer concepts, including network services required by applications, clients and servers, processes, and transport-layer interfaces. We study several network applications in detail, including the Web, e-mail, DNS, peer-to-peer (P2P) file distribution, and P2P Internet telephony. We then cover network application development, over both TCP and UDP. Particularly, we study the socket API and walk through some simple client-server applications in Java.

The application layer is a particularly good place to start our study of protocols. It's familiar ground. We're acquainted with many of the applications that rely on the protocols we'll study. It will give us a good feel for what protocols are all about and will introduce us to many of the same issues that we'll see again when we study transport, network. and link layer protocols.


Tags

application layer, networking protocol, killer applications

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