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The U.S. Department of
Defense (DoD) created the TCP/IP reference model because it wanted a
network that could survive any conditions, even a nuclear war. To
illustrate further, imagine a world at war, criss-crossed by different
kinds of connections - wires, microwaves, optical fibers, and satellite
links. Then imagine that you need information/data (in the form of packets)
to flow, regardless of the condition of any particular node or network on
the internetwork (which in this case may have been destroyed by the war).
The DoD wants its packets to get through every time, under any conditions,
from any one point to any other point. It was this very difficult design
problem that brought about the creation of the TCP/IP model, and which has
since become the standard on which the Internet has grown.
As you read about the
TCP/IP model layers, keep in mind the original intent of the Internet; it
will help explain why certain things are as they are. The TCP/IP model has
four layers: the application layer, the transport layer, the Internet
layer, and the network access layer. It is important to note that some
of the layers in the TCP/IP model have the same name as layers in the OSI
model. Do not confuse the layers of the two models, because the
application layer has different functions in each model.
Application Layer
The designers of TCP/IP felt that the higher level protocols should include
the session and presentation layer details. They simply created an
application layer that handles high-level protocols, issues of
representation, encoding, and dialog control. The TCP/IP combines all
application-related issues into one layer, and assures this data is properly
packaged for the next layer.
Transport Layer
The transport layer deals with the quality-of-service issues of
reliability, flow control, and error correction. One of its protocols, the
transmission control protocol (TCP), provides excellent and flexible ways
to create reliable, well-flowing, low-error network communications. TCP is
a connection-oriented protocol. It dialogues between source and destination
while packaging application layer information into units called segments.
Connection-oriented does not mean that a circuit exists between the
communicating computers (that would be circuit switching). It does mean
that Layer 4 segments travel back and forth between two hosts to
acknowledge the connection exists logically for some period. This is known
as packet switching.
Internet Layer
The purpose of the Internet layer is to send source packets from any
network on the internetwork and have them arrive at the destination
independent of the path and networks they took to get there. The specific
protocol that governs this layer is called the Internet protocol (IP). Best
path determination and packet switching occur at this layer. Think of it in
terms of the postal system. When you mail a letter, you do not know how it
gets there (there are various possible routes), but you do care that it
arrives.
Network Access Layer
The name of this layer is very broad and somewhat confusing. It is also
called the host-to-network layer. It is the layer that is concerned with
all of the issues that an IP packet requires to actually make a physical
link, and then to make another physical link. It includes the LAN and WAN
technology details, and all the details in the OSI physical and data link
layers.
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