The Application Layer

Getting Wired Into the Internet: A Crash Course on FTP, Gopher, Web and More page 2

The Application Layer

The bulk of this article is concerned with the application layer, which is where all the fun takes place. Here “application” refers to a particular protocol built at the application layer of the TCP/IP suite. For any given application there will likely be dozens of implementations on each platform. Even under Windows there will likely be many different Windows Sockets-compatible applications to choose from, each with its own features and GUI interface. There are literally dozens of TCP/IP application protocols (see Figure 3).

**Figure 3** **Sampling of Common Application Layer Protocols in the TCP/IP Protocol Suite**
Protocol	Translation	Description
Archie		Catalog of the contents of 1000+ anonymous FTP servers disseminating the location of FTP retrievable files.
DHCP	Dynamic Host Configuration	Protocol Auto-configuration service that allows a machine to obtain an address without a priori knowledge at boot time.
DNS	Domain Name System	Distributed database that allows applications to map between computer names and IP addresses.
Finger		Returns information on users on a specific Internet machine.
FTP	File Transfer Protocol	Copies files from one Internet machine to another.
Gopher		Protocol for distributed document navigation and retrieval. The sum of Gopher resources is sometimes referred to as "gopherspace".
IRC	Internet Relay Chat	Real-time text-based conversation system.
MUD	Multi-User Dungeon	Multi-player adventure game known to consume massive network bandwidth and programmer cycles.
NFS	Network File System	Provides transparent file access across clients on the Internet.
PEM	Privacy Enhanced Mail	Protocol for electronic mail that provides for encryption and authentication using RSA and DES.
POP 2 and 3	Post Office Protocol	Protocol for the management of electronic mail using store and forward.
SMTP	Simple Mail Transfer Protocol	Governs the exchange of electronic mail between two message transfer agents (MTAs) on the Internet.
SNMP	Simple Network Management Protocol	Protocol for remote administration of TCP/IP machines.
Telnet	Telecommunications Network Protocol	Remote login between hosts running potentially different operating systems.
USENET	Uses NNTP	Huge collection of messages and newsgroups arranged in a Network News Transport Protocol hierarchical bulletin board system.
VERONICA	Very Easy Rodent-Oriented Netwide Index	Indexed search to gopherspace.
WAIS	Wide Area Information Server	Implementation of Z39.50 content indexing and information retrieval standard.
Whois	Who Is	Returns information on an Internet user voluntarily registered with InterNIC.
WWW (also Web or W3)	World Wide Web	Protocol for distributed hypertext document searching, navigation, and retrieval.

The proliferation of protocols is a by-product of the ease with which new protocols can be developed, disseminated, and established in the Internet community. Of course not all of these protocols are used in equal numbers. To give you some idea of how the current Internet traffic breaks down, let's look at some statistics.

Figure 4 (above) shows how each of the major TCP/IP application services contributes to the NSFNet traffic flow. NSFNet is one of the busiest networks in the Internet. It's the backbone connecting many of the educational and research facilities connected today. As you might suspect, file download using ftp accounts for the largest percentage of traffic. Although most of us use e-mail quite frequently, the average message size is small enough that networked mail does not account for a large percentage of the traffic. If you were to look at this graph in terms of user interactions, ignoring packet size, you'd see networked mail accounting for a larger percentage.

It's interesting just how much of the traffic is taken up by resolving names on the Internet. There is about a 6 percent overhead associated with name service lookups using the DNS, the distributed database of all of the Internet hosts. New applications like Web viewers and Gopher use DNS extensively (and sometimes poorly), clearly contributing to this statistic.

If you dive down another level and look at a specific month, March 1994, you can see how specific services are being used (see Figure 5, below). An average busy month in the world of the Internet, March 1994 saw over 14,024,028,116,050 bytes travel over just the NSFNet. These bytes were packaged up into 69,552,904,950 packets, giving an average packet size of about 202 bytes.

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