Chapter 1. Networking Fundamentals

1.1. Networking overview

Before discussing either NFS, or NIS, we'll provide a brief overview of network services.

NFS and NIS are high-level networking protocols, built on several lower-level protocols. In order to understand the way the high-level protocols function, you need to know how the underlying services work. The lower-level network protocols are quite complex, and several books have been written about them without even touching on NFS and NIS services. Therefore, this chapter contains only a brief outline of the network services used by NFS and NIS.

Network protocols are typically described in terms of a layered model, in which the protocols are "stacked" on top of each other. Data coming into a machine is passed from the lowest-level protocol up to the highest, and data sent to other hosts moves down the protocol stack. The layered model is a useful description because it allows network services to be defined in terms of their functions, rather than their specific implementations. New protocols can be substituted at lower levels without affecting the higher-level protocols, as long as these new protocols behave in the same manner as those that were replaced.

The standard model for networking protocols and distributed applications is the International Organization for Standardization (ISO) seven-layer model shown in Table 1-1.

Table 1-1. The ISO seven-layer model

Layer	Name	Physical Layer
7	Application	NFS and NIS
6	Presentation	XDR
5	Session	RPC
4	Transport	TCP or UDP
3	Network	IP
2	Data Link	Ethernet
1	Physical	CAT-5

Purists will note that the TCP/IP protocols do not precisely fit the specifications for the services in the ISO model. The functions performed by each layer, however, correspond very closely to the functions of each part of the TCP/IP protocol suite, and provide a good framework for visualizing how the various protocols fit together.

The lower levels have a well-defined job to do, and the higher levels rely on them to perform it independently of the particular medium or implementation. While TCP/IP most frequently is run over Ethernet, it can also be used with a synchronous serial line or fiber optic network. Different implementations of the first two network layers are used, but the higher-level protocols are unchanged. Consider an NFS server that uses all six lower protocol layers: it has no knowledge of the physical cabling connecting it to its clients. The server just worries about its NFS protocols and counts on the lower layers to do their job as well.

Throughout this book, the network stack or protocol stack refers to this layering of services. Layer or level will refer to one specific part of the stack and its relationship to its upper and lower neighbors. Understanding the basic structure of the network services on which NFS and NIS are built is essential for designing and configuring large networks, as well as debugging problems. A failure or overly tight constraint in a lower-level protocol affects the operation of all protocols above it. If the physical network cannot handle the load placed on it by all of the desktop workstations and servers, then NFS and NIS will not function properly. Even though NFS or NIS will appear "broken," the real issue is with a lower level in the network stack.

The following sections briefly describe the function of each layer and the mapping of NFS and NIS into them. Many books have been written about the ISO seven-layer model, TCP/IP, and Ethernet, so their treatment here is intentionally light. If you find this discussion of networking fundamentals too basic, feel free to skip over this chapter.