Datasheet

“main” (Installation and Administration) — 2004/6/25 — 13:29 — page 416 — #442
i
i
i
i
i
i
i
i
Figure 21.1: Simplified Layer Model for TCP/IP
The diagram provides one or two examples for each layer. As you can see,
the layers are ordered according to abstraction levels. The lowest layer is
very close to the hardware. The uppermost layer, however, is almost a com-
plete abstraction from the hardware. Every layer has its own special func-
tion. The special functions of each layer are mostly implicit in their descrip-
tion. The bit transfer and security layers represent the physical network
used (such as ethernet).
While layer 1 deals with cable types, signal forms, signal codes, and
the like, layer 2 is responsible for accessing procedures (which host
may send data?) and error correction. Layer 1 is called the physical
layer. Layer 2 is called the data link layer.
Layer 3 is the network layer and is responsible for remote data transfer.
The network layer ensures that the data arrives at the correct remote
destination and can be delivered to it.
Layer 4, the transport layer, is responsible for application data. It en-
sures that data arrives in the correct order and is not lost. While the
data link layer is only there to make sure that the data as transmitted
is the correct one, the transport layer protects it from being lost.
Finally, layer 5 is the layer where data is processed by the application
itself.
416 21.1. TCP/IP — The Protocol Used by Linux