TCP/IP Network Interface Layer

The network interface layer defines the protocols and hardware required to deliver data across some physical network. The term network interface refers to the fact that this layer defines how to connect the host computer, which is not part of the network, to the network; it is the interface between the computer and the network. For instance, Ethernet is one example protocol at the TCP/IP network interface layer. Ethernet defines the required cabling, addressing, and protocols used to create an Ethernet LAN. Likewise, the connectors, cables, voltage levels, and protocols used to deliver data across WAN links are defined in a variety of other protocols that also fall into the network interface layer.

Just like every layer in any networking model, the TCP/IP network interface layer provides services to the layer above it in the model. The best way to understand the basics of the TCP/IP network interface layer is to examine the services that it provides to IP.

IP relies on the network interface layer to deliver IP packets across each physical network. IP understands the overall network topology, things such as which routers are connected to each other, which host computers are connected to which networks, and what the IP addressing scheme looks like. However, the IP protocol purposefully does not include the details about each of the underlying physical networks. Therefore, the Internet layer, as implemented by IP, uses the services of the network interface layer to deliver the packets over each physical network, respectively.

The network interface layer includes a large number of protocols. For instance, the network interface layer includes all the variations of Ethernet protocols and other LAN standards. This layer also includes the popular WAN standards, such as the Point-to-Point Protocol (PPP) and Frame Relay. The same familiar network is shown in Figure 2-5, with Ethernet and PPP used as the two network interface layer protocols.



To fully appreciate Figure 2-5, first think a little more deeply about how IP accomplishes its goal of delivering the packet from Bob to Larry. Bob wants to send the IP packet to Larry, but it must first do so by sending the packet to R2. Bob uses Ethernet to get the packet to R2. At R2, R2 strips the Ethernet header and trailer from the IP packet. To get the IP packet from R2 to R1, R2 does not need to use Ethernet–it instead needs to use the PPP serial link. To send the IP packet from R2 to R1, R2 needs to place a PPP header in front of the IP packet and a PPP trailer at the end. Similarly, after the packet is received by R1, R1 removes the PPP header and trailer because PPP’s job is to get the IP packet across the serial link. R1 then decides that it should forward the packet over the Ethernet to Larry. To do so, R1 adds a brand-new Ethernet header and trailer to the packet and forwards it to Larry.

In effect, IP uses the network interface layer protocols to deliver the IP packet to the next router or host, with each router repeating the process until the packet arrives at the destination. Each network interface protocol uses headers to encode the information needed to successfully deliver the data across the physical network, much like other layers use headers to achieve their goals.

CAUTION
Many people describe the network interface layer of the TCP/IP model as two layers, the data link layer and the physical layer. The reasons for the popularity of these alternate terms are explained in the section covering OSI because the terms originated with the OSI model.

In short, the TCP/IP Network Interface layer includes the protocols, cabling standards, headers and trailers that define how to send data across a wide variety of types of physical networks.