Common Services

There are a number of services associated with TCP/IP and the Internet. The most commonly used service is electronic mail (e-mail), implemented by the Simple Mail Transfer Protocol (SMTP). Also, TELNET (terminal emulation), for remote terminal access, and FTP (file transfer protocol) are used widely. Beyond that, there are a number of services and protocols used for remote printing, remote file and disk sharing, management of distributed databases, and for information services. Following is a brief list of the most common services:

• SMTP - Simple Mail Transfer Protocol, used for sending and receiving electronic mail,
• TELNET - used for connecting to remote systems connected via the network, uses basic terminal emulation features,
• FTP - File Transfer Protocol, used to retrieve or store files on networked systems,
• DNS - Domain Name Service, used by TELNET, FTP, and other services for translating host names to IP addresses,
• information-based services, such as
  • gopher - a menu-oriented information browser and server that can provide a user-friendly interface to other information-based services,
  • WAIS - Wide Area Information Service, used for indexing and searching with databases of files, and
  • WWW/http - World Wide Web, a superset of FTP, gopher, WAIS, other information services, using the hypertext transfer protocol (http), with Mosaic being a popular WWW client,
• RPC-based services - Remote Procedure Call services, such as
  • NFS - Network File System, allows systems to share directories and disks, causes a remote directory or disk to appear to be local, and
  • NIS - Network Information Services, allows multiple systems to share databases, e.g., the password file, to permit centralized management,
• X Window System - a graphical windowing system and set of application libraries for use on workstations, and
• rlogin, rsh, and other ``r'' services - employs a concept of mutually trusting hosts, for executing commands on other systems without requiring a password.

Although TCP/IP can be used equally well in a local area or wide area networking environment, a common use is for file and printer sharing at the local area networking level and for electronic mail and remote terminal access at both the local and the wide area networking levels. Gopher and Mosaic are increasingly popular; both present problems to firewall designers as will be discussed in later sections.