application layer

application layer

Naming in the Internet uses a hierarchical scheme called the domain name system (DNS). At the top level are the well-known generic domains, including com and edu as well as about 200 country domains. DNS is implemented as a distributed database system with servers all over the world. DNS holds records with IP addresses, mail exchanges, and other information. By querying a DNS server, a process can map an Internet domain name onto the IP address used to communicate with that domain.

E-mail is one of the two killer apps for the Internet. Everyone from small children to grandparents now use it. Most e-mail systems in the world use the mail system now defined in RFCs 2821 and 2822. Messages sent in this system use system ASCII headers to define message properties. Many kinds of content can be sent using MIME. Messages are sent using SMTP, which works by making a TCP connection from the source host to the destination host and directly delivering the e-mail over the TCP connection.

The other killer app for the Internet is the World Wide Web. The Web is a system for linking hypertext documents. Originally, each document was a page written in HTML with hyperlinks to other documents. Nowadays, XML is gradually starting to take over from HTML. Also, a large amount of content is dynamically generated, using server-side scripts (PHP, JSP, and ASP), as well as clientside scripts (notably JavaScript). A browser can display a document by establishing a TCP connection to its server, asking for the document, and then closing the connection. These request messages contain a variety of headers for providing additional information. Caching, replication, and content delivery networks are widely used to enhance Web performance.

The wireless Web is just getting started. The first systems are WAP and i-mode, each with small screens and limited bandwidth, but the next generation will be more powerful.

Multimedia is also a rising star in the networking firmament. It allows audio and video to be digitized and transported electronically for display. Audio requires less bandwidth, so it is further along. Streaming audio, Internet radio, and voice over IP are a reality now, with new applications coming along all the time. Video on demand is an up-and-coming area in which there is great interest. Finally, the MBone is an experimental, worldwide digital live television service sent over the Internet.