1.1: Layers

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These three topics – LANs, IP and TCP – are often called layers; they constitute the Link layer, the Internetwork layer, and the Transport layer respectively. Together with the Application layer (the software you use), these form the “four-layer model” for networks. A layer, in this context, corresponds strongly to the idea of a programming interface or library, with the understanding that a given layer communicates directly only with the two layers immediately above and below it. An application hands off a chunk of data to the TCP library, which in turn makes calls to the IP library, which in turn calls the LAN layer for actual delivery. An application does not interact directly with the IP and LAN layers at all.

The LAN layer is in charge of actual delivery of packets, using LAN-layer-supplied addresses. It is often conceptually subdivided into the “physical layer” dealing with, eg, the analog electrical, optical or radio signaling mechanisms involved, and above that an abstracted “logical” LAN layer that describes all the digital – that is, non-analog – operations on packets (Section 2.1.4). The physical layer is generally of direct concern only to those designing LAN hardware; the kernel software interface to the LAN corresponds to the logical LAN layer.

This LAN physical/logical division gives us the Internet five-layer model. This is less a formal hierarchy as an ad hoc classification method. We will return to this below in 1.15 IETF and OSI, where we will also introduce two more rather obscure layers that complete the seven-layer model.