In order to be able to communicate between devices
using a layered model, the protocols running at each
layer on the source host must be able to communicate
with their counterparts at the destination host. The data
that is being exchanged at each layer is called a protocol
data unit (PDU). Based on the layer processing the data,
the PDUs have different names:
At the application layer, the general term for the PDU is data.
At the transport layer, the PDU is called a segment.
At the Internet layer, the PDU is called a packet.
At the data link layer, the PDU is called a frame.Figure 16-3 shows the PDUs at each layer and the flow of
data from sender to receiver.
Figure 16-3 TCP/IP Reference Model PDUs at Each
Layer
As data moves down the layers at the sending host, it
gets processed at each layer. This process is called
encapsulation, and there is a corresponding process at
the destination host called de-encapsulation. The
encapsulation process consists in adding extra protocol
information at each layer so that layer-to-layer
communication can take place. Starting at the top, the
user data generated in the application layer gets passed
down to the transport layer for transmission. The