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TCP/IP and the DoD Model
The DoD model is basically a condensed version of the OSI model—it’s composed of four,
instead of seven, layers:
- Process/Application layer
- Host-to-Host layer
- Internet layer
- Network Access layer
A vast array of protocols combine at the DoD model’s
Process/Application layer
to integrate
the various activities and duties spanning the focus of the OSI’s corresponding top three
layers (Application, Presentation, and Session). We’ll be looking closely at those protocols in
the next part of this chapter. The Process/Application layer defines protocols for node-to-node
application communication and also controls user-interface specifications.
The
Host-to-Host layer
parallels the functions of the OSI’s Transport layer, defining protocols
for setting up the level of transmission service for applications. It tackles issues such as
creating reliable end-to-end communication and ensuring the error-free delivery of data. It
handles packet sequencing and maintains data integrity.
The
Internet layer
corresponds to the OSI’s Network layer, designating the protocols relating
to the logical transmission of packets over the entire network. It takes care of the addressing
of hosts by giving them an IP (Internet Protocol) address, and it handles the routing of packets
among multiple networks.
At the bottom of the DoD model, the
Network Access layer
monitors the data exchange
between the host and the network. The equivalent of the Data Link and Physical layers of the
OSI model, the Network Access layer oversees hardware addressing and defines protocols for
the physical transmission of data.
The DoD and OSI models are alike in design and concept and have similar functions in
similar layers.
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