Information Technology Reference
In-Depth Information
On top of the transport layer is the application layer. It contains all the
higher-level protocols. The early ones included virtual terminal (TELNET),
file transfer (FTP), and electronic mail (SMTP). Many other protocols have
been added to these over the years. Some important ones include the
Domain Name System (DNS), for mapping host names onto their network
addresses; HTTP, the protocol for fetching pages on the WWW; and RTP,
the protocol for delivering real-time media such as voice or movies.
OSI VERSUS TCP/IP REFERENCE MODEL
The OSI reference model was devised before the corresponding
protocols were invented. This ordering meant that the model
was not biased toward one particular set of protocols, a fact
that made it quite general. The downside of this ordering was that
the designers did not have much experience with the subject and did
not have a good idea of which functionality to put in which layer. For
example, the data link layer originally dealt only with point-to-point
networks. When broadcast networks came around, a new sublayer had
to be hacked into the model. Furthermore, no thought was given to
Internetworking. With TCP/IP, the reverse was true: the protocols came
first, and the model was really just a description of the existing proto-
cols. There was no problem with the protocols fitting the model. They
fit perfectly. The only trouble was that the model did not fit any other
protocol stacks. Consequently, it was not especially useful for describ-
ing other non-TCP/IP networks.
2.3 Internet
The origins of the Internet can be traced to the U.S. government support of
the ARPANET project. Computers in several U.S. universities were linked
via packet switching, and this allowed messages to be sent between the uni-
versities that were part of the network. The use of ARPANET was limited
initially to academia and to the U.S. military, and in the early years, there
was little interest from industrial companies.
However, by the mid-1980s, there were over 2000 hosts on the TCP/
IP-enabled network, and the ARPANET was becoming more heavily used
and congested. It was decided to shut down the network by the late 1980s,
and the National Science Foundation in the United States commenced work
on the NSFNET. This work commenced in the mid-1980s, and the network
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