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So what really makes the Internet work?
Why does the WWW work so well? How can we run so many
applications over the Internet at the same time? How do we know
that our data has been received? How does the data
actually know how to get to a certain destination? Well, it’s to
do with TCP , IP and routing protocol s . These three parts make the whole of the Internet work, and
work reliably. The IP part is responsible for getting the data packets
from the source to the destination (using the IP address ), the TCP part is then responsible for sending the data to the
required application program (using TCP sockets and sequence number
s), and the routing protocols are responsible for passing on
information about how to get to destinations (using
protocols such as RIP ). Isn’t it wonderful how a user can
run a few WWW browsers, a TELNET session, an FTP session, a video
conferencing session,
and it all works, seamlessly, even if there are multiple destinations.
Well it’s really Mr IP who’s sending the data to the right place,
and deciding if the data on the network is for them, and Dr TCP
who either tags all the outgoing data and clearly identifies the
virtual connection, or reorders and passes the received data to
the required application. But, Dr TCP is no egg-head who lives in
the realms of academia, he makes sure that all the data is properly
received, and makes sure that anything that he sends, he gets a
signed receipt for. If he doesn’t get a receipt, he’ll resend his
data. But, what if the place that he’s sending the data has blown-up,
or there’s a postal strike. Well Dr TCP has that side covered also,
he sends the data again, and then waits for a time-out period. If
no data is received, he gives up.
TCP and IP unite the world and allow everyone
in the world to communicate, no matter which computer they use,
which operating system they are running, which language
they speak, or which network they use. It fits onto virtually every
type of networking technology. It’ll work with Ethernet ,
ATM (although with some difficulty), FDDI ,
ISDN , Modems , RS-232 , blah, blah, blah.
So, whom should we thank for giving us these two great protocols.
Of course DARPA should be congratulated for conceiving
it, but the main award must go to the shy, but dependable workhorse
of the computing industry: UNIX . Through UNIX,
TCP, UDP and IP have been allowed to blossom, and show
their full potential.
UDP transmission
can be likened to sending electronic mail . In most electronic
mail packages the user can request that a receipt is sent back to
the originator when the electronic mail has been opened. This is
equivalent to TCP , where data is acknowledged
after a certain amount of data has been sent. If the user does not
receive a receipt for its electronic mail then it will send another
one, until it is receipted or until there is a reply. UDP is equivalent
to a user sending an electronic mail without asking for a receipt,
thus the originator has no idea if the data has been received, or
not.
TCP /IP is an excellent method for networked communications, as
IP provides the routing of the data , and TCP
allows acknowledgements for the data. Thus, the data can always
be guaranteed to be correct. Unfortunately there is an overhead
in the connection of the TCP socket, where the two communicating
stations must exchange parameters before the connection is made,
then they must maintain and acknowledge received TCP packets. UDP has the advantage that it is connectionless. So there is
no need for a connection to be made, and data is simply thrown in
the network, without the requirement for acknowledgements.
Thus UDP packets are much less reliable in their operation, and
a sending station cannot guarantee that the data is going to be
received. UDP is thus useful for remote data acquisition where data
can be simply transmitted without it being requested or without
a TCP/IP connection being made.
The concept
of ports and sockets is important in TCP /IP . Servers
wait and listen on a given port number . They only read packets
which have the correct port number. For example, a WWW server
listens for data on port 80, and an FTP server listens for port 21. Thus a properly set up communication
network requires a knowledge of the ports which are accessed. An
excellent method for virus writers and hackers to get
into a network is to install a program which responds to a given
port which the hacker uses to connect to. Once into the system they
can do a great deal of damage. Programming languages such as Java have built-in security to reduce this problem.
So why does
TCP have a PhD, and IP doesn’t. Well TCP
operates at a higher layer and allows the whole system to operate
reliably. It does an excellent job, whereas IP is a child that has
grown up too quickly for its own use. It’s excellent the way that
IP has created a world-wide addressing structure, but
it’s limited. Why is it that my IP address is 146.176.151.130,
while the address of a computer in the same street, that uses the
same Internet connection has the address of 138.154.33.100.
Well it’s because the IP address gives no indication about the location
of a node. Thus, we need complex routing protocol s ,
in which routers use to pass information about
the best way to get to a node. Some of these routing protocols,
like IP, have grown up too quickly, and have outgrown their usage.
The worst offender is RIP , which basically defines the number
of hops (the number of routers in the path to the destination) that
it takes to get to a destination. Unfortunately the maximum number
of hops is defined at 15, thus if a destination is more that 16,
the destination is not reachable. The other problem with RIP is
that it’s a bit lazy, and basically doesn’t try too hard. It doesn’t
want to know about the bandwidth of a connection, or
reliability, or its cost. Hop count is hardly very
taxing on computing the best way to get to a destination.
Just imagine if you were a car driver, and when you looked at a
map you choose the route which has the minimum number of junctions.
This could take you around country roads, or through congested roads.
Normally we would pick the route that allows the highest average
speed (the highest bandwidth), rather than the one with the minimum
number of junctions.
So the Internet we have is the Internet we have. TCP
, IP and
RIP are there, and they’re not going to be moved for
a long time. But the great thing about the Internet is that it’s
not going to go away either. It also allows for migration .
The key to this is found in IP which can exist on its own with any
other transport protocol above it, and it allows for different version
, which will allow the Internet to migrate away from the current
setup towards a more sensible structure (such as one based on area
codes). IP addresses are also very static . If you
move your computer from one network to another you must change your
IP address. This normally requires skilled operators to
make it work. Why can the network do it for you? In the next chapter,
let’s have a bit of fun, and look at how the Internet could look
in the future.
In this chapter, and Appendix C,
I have presented the two opposite ends of code development for TCP /IP
communications.
The C ++ code
is complex, but very powerful, and allows for a great deal of flexibility.
On the other hand, the Visual Basic code is simple to
implement but is difficult to implement for non-typical applications.
Thus, the code used tends to reflect the type of application. In many
cases Visual Basic gives an easy-to-implement package, with the required
functionality. I’ve seen many a student wilt at the prospect of implementing
a Microsoft Windows program in C++. ‘Where
do I start’, is always the first comment, and then ‘How do I do text
input’, and so on. Visual Basic, on the other hand, has matured into
an excellent development system which hides much of the complexity
of Microsoft Windows away from the developer. So, don’t worry about
computer language snobbery. Pick the best language to implement the
specification.
© W.
Buchanan, 2000
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