Networking and Internetworking - Guide to Cloud Computing for Business and Technology Managers - page 41

Two computers are said to be networked if they are able to exchange infor-
mation. The connection need not be via a copper wire; fiber optics, micro-
waves, infrared, and communication satellites can also be used. Networks
come in many sizes, shapes, and forms, as we will see later. They are usu-
ally connected together to make larger networks, with the Internet being the
most well-known example of a network of networks.
Computer Network and a Distributed System: The key distinction between
them is that in a distributed system, a collection of independent computers
appears to its users as a single coherent system. Usually, it has a single model
or paradigm that it presents to the users. Often, a layer of software on top of
the operating system, called middleware, is responsible for implementing this
model. A well-known example of a distributed system is the WWW. It runs
on top of the Internet and presents a model in which everything looks like a
document (Web page). On the other hand, in a computer network, coherence,
model, and software are absent. Users are exposed to the actual machines,
without any attempt by the system to make the machines look and act in a
coherent way. If the machines have different hardware and different operat-
ing systems, that is fully visible to the users. If a user wants to run a program
on a remote machine, it entails logging onto that machine and run it there.
In effect, a distributed system is a software system built on top of a net-
work. The software gives it a high degree of cohesiveness and transparency.
Thus, the distinction between a network and a distributed system lies with
the software (especially the operating system), rather than with the hard-
ware. Nevertheless, there is considerable overlap between the two subjects.
For example, both distributed systems and computer networks need to move
files around. The difference lies in who invokes the movement, the system,
or the user.
2.2.1 Network Principles
2.2.1.1 Protocol
The term protocol is used to refer to a well-known set of rules and formats to
be used for communication between processes in order to perform a given
task. The definition of a protocol has two important parts to it:
1. A specification of the sequence of messages that must be exchanged
2. A specification of the format of the data in the messages
The existence of well-known protocols enables the separate software
components of distributed systems to be developed independently and
implemented in different programming languages on computers that may
have different order codes and data representations. A protocol is imple-
mented by a pair of software modules located in the sending and receiving