The last two network services, file and print services, make the network more convenient for users. Not long ago, disk drives and high-quality printers were relatively expensive, and diskless workstations were common. Today every system has a large hard drive and many have their own high-quality laser printers, but the demand for resource-sharing services is higher than ever.
File sharing is not the same as file transfer. It is not simply the ability to move a file from one system to another. A true file-sharing system does not require you to move entire files across the network. It allows files to be accessed at the record level so that it is possible for a client to read a record from a file located on a remote server, update that record, and write it back to the server - without moving the full file from the server to the client.
File sharing is transparent to the user and to the application software running on the user's system. Through file sharing, users and programs access files located on remote systems as if they were local files. In a perfect file-sharing environment, the user neither knows nor cares where files are actually stored.
File sharing didn't exist in the original TCP/IP protocol suite. It was added to support diskless workstations. Unlike a proprietary LAN where one vendor defines the official file-sharing protocol, TCP/IP is an open protocol suite and anyone can propose a new protocol. That's why there are three TCP/IP protocols for file sharing:
RFS was defined by AT&T for UNIX System V. It is offered on many UNIX systems, but rarely used.
AFS is a file-sharing system developed at Carnegie Mellon University. AFS has several performance enhancements that make it particularly well-suited for wide area network (WAN) use. AFS has evolved into Distributed File System (DFS). Despite its features, it is not the most widely used file sharing system.
NFS was defined by Sun Microsystems to support their diskless workstations. NFS is designed primarily for LAN applications and is implemented for all UNIX systems and many other operating systems.
You will probably use NFS, as it is the most widely used TCP/IP file-sharing protocol. For a detailed discussion, see Chapter 9.
A print server allows printers to be shared by everyone on the network. Printer sharing is not as important as file sharing, but it is a useful network service. The advantages of printer sharing are:
Fewer printers are needed, and less money is spent on printers and supplies.
Reduced maintenance. There are fewer machines to maintain, and fewer people spending time fiddling with printers.
Access to special printers. Very high-quality color printers and very high-speed printers are expensive and needed only occasionally. Sharing these printers makes the best use of expensive resources.
There are two techniques commonly used for sharing printers on a TCP/IP network. One technique is to use the network's file sharing services. The other approach is to use the traditional UNIX lpr command and an lpd server. Print server configuration is covered in Chapter 9.