The Data Link Layer

After an overview of the interaction between IP and related link layer protocols, this chapter introduced the Ethernet and IEEE 802.3 protocols that are commonly used in corporate and home networks. Although address resolution is required to allow IP hosts to communicate over any link layer protocol, ARP is most readily understood as implemented for Ethernet; ARP as well as ARP messages were examined, as were ARP caches, and Proxy ARP and related address resolution protocols including RARP and InARP. Although the ATM protocols span both Layer 2 and Layer 3, in this chapter it is discussed about how Asynchronous Transfer Mode (ATM) and IP have traditionally interoperated, with IP treating ATM as a link layer protocol. We also examine the use of non-broadcast media with IP, particularly the mechanisms necessary to allow address resolution and multicast/broadcast under non-broadcast multiple access (NBMA) protocols. While Ethernet and ATM are both known network protocols, point to point protocol (PPP) is less obviously a link layer protocol: it does not even require nodes to be addressed, but that is only because addresses are superfluous when only two nodes are allowed on the medium — all outbound packets are destined to the "other" node, while all inbound packets are destined for "this" node. These three link layer protocols account for a significant portion, if not the majority, of all internet transmission; however, there are many other link layer protocols over which IP can be carried. This chapter concludes with a listing of current (as of mid-2002) specifications for IP over various link layers.