2.1 Introduction

From Figure 1.4 we see that the purpose of the link layer in the TCP/IP protocol suite is to send and receive (1) IP datagrams for the IP module, (2) ARP requests and replies for the ARP module, and (3) RARP requests and replies for the RARP module. TCP/IP supports many different link layers , depending on the type of networking hardware being used: Ethernet, token ring, FDDI (Fiber Distributed Data Interface), RS-232 serial lines, and the like.

In this chapter we'll look at some of the details involved in the Ethernet link layer, two specialized link layers for serial interfaces (SLIP and PPP), and the loopback driver that's part of most implementations . Ethernet and SLIP are the link layers used for most of the examples in the book. We also talk about the MTU (Maximum Transmission Unit), a characteristic of the link layer that we encounter numerous times in the remaining chapters. We also show some calculations of how to choose the MTU for a serial line.