Managing NFS and NIS, 2nd Edition - Mike Eisler [10]
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.
Physical and data link layers
The physical and data link layers of the network protocol stack together define a machine's network interface. From a software perspective, the network interface defines how the Ethernet device driver gets packets from or to the network. The physical layer describes the way data is actually transmitted on the network medium. The data link layer defines how these streams of bits are put together into manageable chunks of data.
Ethernet is the best known implementation of the physical and data link layers. The Ethernet specification describes how bits are encoded on the cable and also how stations on the network detect the beginning and end of a transmission. We'll stick to Ethernet topics throughout this discussion, since it is the most popular network medium in networks using NFS and NIS.
Ethernet can be run over a variety of media, including thinnet, thicknet, unshielded twisted-pair (UTP) cables, and fiber optics. All Ethernet media are functionally equivalent — they differ only in terms of their convenience, cost of installation, and maintenance. Converters from one media to another operate at the physical layer, making a clean electrical connection between two different kinds of cable. Unless you have access to high-speed test equipment, the physical and data link layers are not that interesting when they are functioning normally. However, failures in them can have strange, intermittent effects on NFS and NIS operation. Some examples of these spectacular failures are given in Chapter 15.
Frames and network interfaces
The data link layer defines the format of data on the network. A series of bits, with a definite beginning and end, constitutes a network frame, commonly called a packet. A proper data link layer packet has checksum and network-specific addressing information in it so that each host on the network can recognize it as a valid (or invalid) frame and determine if the packet is addressed to it. The largest packet that can be sent through the data link layer defines the Maximum Transmission Unit, or MTU, of the network.
All hosts have at least one network interface, although any host connected to an Ethernet has at least two: the Ethernet interface and the loopback interface. The Ethernet interface handles the physical and logical connection to the outside world, while the loopback interface allows a host to send packets to itself. If a packet's destination is the local host, the data link layer chooses to "send" it via the loopback, rather than Ethernet, interface. The loopback device simply turns the packet around and queues it at