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TCP
/ IP Model
The
historical and technical standard of the Internet is the TCP/IP
model. The U.S. Department of Defense (DoD) created the TCP/IP
reference model, because it wanted to design a network that
could survive any conditions, including a nuclear war. In a
world connected by different types of communication media such
as copper wires, microwaves, optical fibers and satellite links,
the DoD wanted transmission of packets every time and under
any conditions. This very difficult design problem brought about
the creation of the TCP/IP model.
The TCP/IP model has the following four layers:
• Application layer
• Transport layer
• Internet layer
• Network access layer
Although some of the layers in the TCP/IP model have the same
name as layers in the OSI model, the layers of the two models
do not correspond exactly. Most notably, the application layer
has different functions in each model.
The designers of TCP/IP felt that the application layer should
include the OSI session and presentation layer details. They
created an application layer that handles issues of representation,
encoding, and dialog control.
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 maintains a dialogue
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. It does mean that Layer 4 segments travel back and
forth between two hosts to acknowledge the connection exists
logically for some period.
The purpose of the Internet layer is to divide TCP segments
into packets and send them from any network. The packets arrive
at the destination network independent of the path 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.
The relationship between IP and TCP is an important one. IP
can be thought to point the way for the packets, while TCP provides
a reliable transport.
The Network Access layer is concerned with all of the components,
both physical and logical, that are required to make a physical
link. It includes the networking technology details, including
all the details in the OSI physical and data link layers.
A comparison of the OSI model and the TCP/IP models will point
out some similarities and differences.
Similarities include:
• Both have layers.
• Both have application layers, though they include very different
services.
• Both have comparable transport and network layers.
• Both models need to be known by networking professionals.
• Both assume packets are switched. This means that individual
packets may take different paths to reach the same destination.
This is contrasted with circuit-switched networks where all
the packets take the same path.
Differences include:
• TCP/IP combines the presentation and session layer issues
into its application layer.
• TCP/IP combines the OSI data link and physical layers into
the network access layer.
• TCP/IP appears simpler because it has fewer layers.
• TCP/IP protocols are the standards around which the Internet
developed, so the TCP/IP model gains credibility just because
of its protocols. In contrast, networks are not usually built
on the OSI protocol, even though the OSI model is used as a
guide.
Networking professionals differ in their opinions on which model
to use. Due to the nature of the industry it is necessary to
become familiar with both.
Additional
Resources on the Internet
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