💾 Archived View for gemini.theuse.net › texts › zines › The_Amateur_Computerist › Supplicament-4.txt captured on 2021-12-17 at 13:26:06.
-=-=-=-=-=-=-
[4] An Introduction to TCP/IP
by Jay Hauben
jrh29@columbia.edu
I. Introduction
The Internet as we know it in 1998, although vast, is still
a new and developing communications technology. It is based on a
number of ingenious engineering accomplishments. This article
will look at one of the most important, the Transmission Control
Protocol and Internet Protocol suite, known as TCP/IP.
Any quantitative description of the Internet includes the
number of networks interconnected (hence the name Internet from
internetworking), the number of computers among which electronic
data can be exchanged and ultimately the number of people who can
communicate with this vast computer and network resource and also
with each other. The elements that comprise the Internet are
computers and networks of computers. These being physical
entities, in order to perform reliably, require careful design
based on solid engineering principles. The Internet itself is
more than the sum of its elements. It too requires careful and
evolving design based on principles similar to those for
computers and networks and some unique to the Internet.
II The Internet
The Internet is the successful interconnecting of many
different networks to give the illusion of being one big computer
network. What the networks have in common is that they all use
packet switching technology(1). On the other hand, each of the
connected networks may have its own addressing mechanism, packet
size, speed etc. Any of the computers on the connected networks
no matter what its operating system or other characteristics can
communicate via the Internet if it has software implemented on it
that conforms to the set of protocols which resulted from open
research funded by the Advanced Research Projects Agency (ARPA)
of the United States Department of Defense in the late 1970s(2).
That set of protocols is built around the Internet Protocol (IP)
and the Transmission Control Protocol (TCP). Informally, the set
of protocols is called TCP/IP (pronounced by saying the names of
the letters T-C-P-I-P).
The Internet Protocol is the common agreement to have
software on every computer on the Internet add a bit of
additional information to each of packets that it sends out.
Without such software a computer can not be connected to the
Internet even if Internet traffic passes over the network that
the computer is attached to. A packet that has the additional
information required by IP is called an IP datagram. To each IP
datagram the computer adds its own network addressing
information. The whole package is called a network frame. It is
network frames containing IP datagrams rather than ordinary
packets that a computer must send onto its local packet
switching network in order to communicate with a computer on
another network via the Internet.
If the communication is between computers on the same
network the network information is enough to deliver the frame to
its intended destination computer. If the communication is
intended for a computer on a different network, the network
information directs the frame to the closest computer that serves
to connect the local network with a different network. Such a
special purpose computer is called a router (some times a
gateway). It is such routers that make internetworking possible.
The Internet is not a single giant network of computers. It
is hundreds of thousands of networks interconnected by routers. A
router is a high speed, electronic, digital computer very much
like all the other computers in use today. What makes a router
special is that it has all the hardware and connections necessary
to be able to connect to and communicate on two or more different
networks. It also has the software to create and interpret
network frames for each network it is attached to. In addition it
must have capabilities require by IP. It must have software that
can remove network information from the network frames that come
to it and read the IP information in the datagrams. Based on the
IP information it can add new network information to create a an
appropriate network frame and send it out on that different net
work. But how does it know where to send that the IP datagram?
The entire process of Internet communication requires that
each computer participating in the Internet has a unique digital
address. The unique addresses of the source and destination are
part of the IP information added to packets to make IP datagrams.
The unique number assigned to a computer is its Internet
Protocol or IP address. The IP address is a binary string of 32
digits. Therefore the Internet can provide communication among 2
to the 32nd power or about 4 billion 300 million computers (two
unique addresses for every three people in the world). Internet
addresses are written for example like 129.77.19.130. Each such
address has two parts, a network ID and a host ID. In this
example 129.77 (network ID) identifies that this computer is part
of a particular university network and 19.130 (the host ID)
identifies which particular computer it is.
A router's IP software examines the IP information to
determine the destination network from the net work ID of the
destination address. Then the software consults a routing table
to pick the next router to send the IP datagram to so that it
takes the "shortest" path. A path is short only if it is active
and it is not congested. Ingenious software programs called
routing daemons send and receive short messages among adjacent
routers characterizing the condition on each path. These messages
are analyzed and the routing table is continually up dated. In
this way IP datagrams pass from router to router over different
networks until they reach a router connected to their destination
network. That router puts network information into the network
frame that delivers the datagram to its destination computer. The
IP datagram is unchanged by this whole process. Each router has
put next router information along with the IP datagram into the
next network frame. When the IP datagram finally reaches its
destination it has no information how it got there and different
packets from the original source may have taken different paths
to get to the same destination.
IP as described above requires nothing of the interconnected
networks except that they are packet switching networks with IP
compliant routers. If a transmitting network uses a very small
frame size, the IP software can even fragment an IP datagram into
a few smaller ones to fit the network's frame size. It is this
minimum requirement by the Internet Protocol that makes it
possible for a great variety of networks to participate in the
Internet. But this minimum requirement also results in little or
no error detection. IP arranges for a best-effort process but
has no guarantee of reliability. The remainder of the TCP/IP set
of protocols adds a sufficient level of reliability to make the
Internet useful.
There are problems that IP does not solve. For example,
interspersed network frames from many computers can sometimes
arrive faster than a router can route them. A small backlog of
data can be stored on most routers but if too many frames keep
arriving some must be discarded. This possibility was antici-
pated. On most computers on the Internet except routers software
behaving according to the Transmission Control Protocol (TCP) is
installed. When IP datagrams arrive at the destination computer,
the TCP compliant software scans the IP information put into the
IP datagram at the source. From this information the software can
put packets, if they are all there, back together again. If there
are duplications the software will discard all but the first copy
of such packets to have arrived. But what if some IP datagrams
have been lost?
As a destination computer receives data, the TCP software
sends a short message back over the Internet to the original
source computer specifying what data has arrived. Such a message
is called an "acknowledgment". Every time TCP and IP software
send out data, TCP software starts a timer (sets a number and de
creases it periodically using the computer's internal clock) and
waits for an acknowledgment. If an acknowledgment arrives first,
the timer is canceled. If the timer expires before an
acknowledgment is received back the TCP software retransmits the
data. In this way missing data can usually be replaced at the
destination computer in a reasonable time. To achieve efficient
data transfer the timeout interval can not be preset. It needs to
be longer for more distant destinations and for times of greater
network congestion and shorter for closer destinations and times
of normal network traffic. TCP automatically adjusts the timeout
interval based on current delays and on the distance it
calculates according to the network address of destination. This
ability to dynamically adjust the timeout interval contributes
greatly to the success of the Internet.
Having been designed together and engineered to perform two
separate but related and needed tasks, TCP and IP complement each
other. IP makes possible the travel of packets over different
networks but it and thus the routers are not concerned with data
loss or data reassembly. The Internet is possible because so
little is required of the intervening networks. TCP makes the
Internet reliable by detecting and correcting duplications, out
of order arrival and data loss using an acknowledgment and time
out mechanism with dynamically adjusted timeout intervals.
III Conclusion
The Internet is a wonderful engineering achievement. Since
January 1, 1983, the cutoff date of the old ARPANET protocols,
TCP/IP technology has successfully dealt with tremendous
increases in usage and in the speed of connecting computers. This
is a testament to the success of the TCP/IP protocol design and
implementation process. Douglas Comer highlighted the features
of this process as follows:
* TCP/IP protocol software and the Internet were
designed by talented dedicated people.
* The Internet was a dream that inspired and challenged
the research team.
* Researchers were allowed to experiment, even
when there was no short-term economic payoff.
Indeed, Internet research often used new, innovative
technologies that were expensive compared to exist
ing technologies.
* Instead of dreaming about a system that solved all
problems, researchers built the Internet to operate
efficiently
* Researchers insisted that each part of the Internet
work well in practice before they adopted it as standard./
* Internet technology solves an important, practical
problem; the problem occurs whenever an organization has
multiple networks.
(from The Internet Book)
The high speed, electronic, digital, stored program
controlled computer and the TCP/IP Internet are major historic
breakthroughs in engineering technology. Every such breakthrough
in the past like the printing press, the steam engine, the
telephone, the airplane have had profound effects on human
society. The computer and the Internet have already begun to
have such effects and this promises to be just the beginning. In
the long run, despite the growing pains and dislocations every
great technological break through serves to make possible a more
fulfilling and comfortable life for more people. The computer and
the Internet have the potential to speed up this process although
it may take a hard fight for most people to experience any of the
improvement. We live however in a time of great invention and
great potential.
The TCP/IP Internet is a major historical achievement. It
provides human society with a new global communications
technology with great promise and potential. This Internet has
sustained unprecedented growth both in the number of its users
and the vol ume of messages it handles daily. In the 15 years
since the cutover from the NCP ARPANET to the TCP/IP Internet,
the Internet has proven itself founded on solid principles. But
there can be setbacks and false steps.
As proposals for further development of the Internet are
made, it would be proper to expect that they reaffirm and build
on the proven principles. But there is, for example, research
currently being under taken to "make IP more reliable." Since the
principle of minimal requirement on component networks is IP's
strength, such research if implemented would be a fundamental
change for the Internet. In exchange for reliability, IP has made
possible the interconnection of the most diverse of networks. To
require greater reliability at the IP level could be an imposition
of undue conformity on the component networks. That would
be a backwards step. When today's Internet is developed and
improved, the principles of TCP and IP will in all likelihood
play crucial roles in that development.
Bibliography
Comer, Douglas E. Internetworking with TCP/IP Vol I: Principles,
Protocols, and Architecture 2nd Edition. Englewood Cliffs, NJ.
Prentice Hall. 1991.
Comer, Douglas E. The Internet Book. Englewood Cliffs, NJ.
Prentice Hall. 1995.
Hauben, Michael and Ronda Hauben. Netizens: On the History and
Impact of Usenet and the Internet. Los Alamitos, CA. IEEE
Computer Society Press. 1997
Lynch, Daniel C. and Marshall T. Rose. Editors. Internet Systems
Handbook. Reading, MA. Addison-Wesley. 1993.
Stevens, W. Richard. TCP/IP Illustrated, Vol 1 Protocols.
Reading, MA. Addison-Wesley. 1994.
Strandh, Sigvard. The History of the Machine. New York. Dorset
Press. 1989 (Copyright 1979, AB NORDBOK, Gothenburg, Sweden).
---------------------
Notes:
1. See part IV of "The Computer and the Internet", the longer
version of this paper accessible at
http://www.ais.org/~jrh/paper.s98.html or by email
from the author at jrh@ais.org.
2. Ibid.