The network interface layer, also called the link layer or the data-link layer, is the interface to the actual network hardware. This interface may or may not provide reliable delivery, and may be packet or stream oriented. In fact, TCP/IP does not specify any protocol here, but can use almost any network interface available, which illustrates the flexibility of the IP layer. Examples are IEEE 802.2, X.25 (which is reliable in itself), ATM, FDDI, and even SNA. Some physical networks and interfaces are discussed in Chapter 2, ôNetworkinterfacesö on page 29.

TCP/IP specifications do not describe or standardise any network layer protocols per se; they only standardise ways of accessing those protocols from the internetwork layer.