Foundation
I must create a system, or be enslav'd by another Man's ; I will not reason and compare: my business is to create.
--- William Blake
Suppose you want to build a computer networkone that has the potential to grow to global proportions and to support applications as dilverse as teleconferencing,
video on demand, electrinic commerce, distributed computer, and digital libraries.
假如你想去建设一个计算机网络， 那有可能成长为全球性的一部分，并且支持应用程序, 越来越多的电视会议，在线视频，电子商务，和数字图书馆。
potential // adj. possible but not yet actual. 可能性；
proportion/prəˈpɔːʃən/ (proportions)
1. [N-count] usu sing, usu N of n
A proportion of a group or an amount is a part of it (FORMAL). 一组或一个数目的proportion， 是其中的一部分。
A large proportion of the dolphins in that area will eventually die. 在那个地区的大部分dolphins最终将死亡。
2. [N-count] usu sing, usuw N of n
The proportion of one kind of person or thing in a group is the number of people or things of that kind compared to the total number of people or things in the group.
The proportion of women in profession had risen to 17.3%.
diverse// 1. [adj] If a group or range of things is diverse, it is made up of a wide variety of things
... shops selling a diverse range of gifts== varied
2. [adj.] diverse people or things are very different from each other.
Jones has a much more diverse and perhaps younger audience.
On demands: [Phrase] If something is available or happens on demand, you can have it or it happens whenever you want it or ask for it.如果一些事情是有用的，或者可能一些事情发生On demand, 你拥有它，或者它发生， 无论什么时候，你想问它，或者你想要它。
a national commitment to providing treatment on demand for drug abusers.一个国家性的承诺，提供药物滥用的治疗！
What available technologies would serve as the underlying building blocks, and what kind of software architecture would you design to integrate these building blocks into an effective communication service?
Answering this question is the overriding goal of this book-- to describle the avaible building materials and then to show how they can be used to construct a network from the ground up.
Before we can understand how to design a computer network, we should first agree on exactly what a computer network is?
At one time, the term network meant the set of serial lines used to attch dumb terminals to mainframe computers.
Other important networks include the voice telephone network and the cable TV network used to disseminate video signals.
The main things these networks have in common are that they are specialized to handle one particular kind of data (keystrokes, voice, or video) and they typically connect to special-purpose devices (terminal, hand receivers, and television sets).
What distinguishes a computer network from these other types of network?
Probably the most important characteristic of a computer network is its generality.
Computer networks are built primarily from general-purpose programmable hardware, are they are not optimized for a particular application like making phone calls or delivering television signals.
Instead, they are able to carry many different types of data, and they support a wide and ever growing, range of applications.
Today's computer networks are increasingly take over the functions previously performed by single-use network.
This chapter looks at some typical applications of computer networks and discusses the requirements that a network designer who wishes to support such applications must be aware of.
Once we understand the requirements, how do we proceed?
Fortunately, we will not be building the first network.
Others, most notably the community of researchers responsible for the Internet, have gone before us.
We will use the wealth of experience generated from the Internet to guide our design.
This experience is embodied in a network architecture that identifies the available hardware and software components and shows how they can be arranged to form a complete network system.
In addition to understanding how networks are built, it is increasingly important to understand how they are operated or managed and how network application are developed.
Most of us now have computer networks in our homes, offices, and in some cases in cars, so operating networks is no longer a matter only for a few specialists.
And, with the proliferation of programmable, network-attached devices such as smartphones, many more of this generation will develop networked applications than in the past.
So we need to consider networks from these multiple perspectives: builders, operators, application developers.
To start us on the road toward understanding how to build, operate, and program a network, this chapter does four things.
First, it explores the requirements that different communicaties of people place on the network.
Second, it introduces the idea of a network architecture, which lays the foundation for the rest of the book.
Third, it introduces some of the key elements in the implementation of computer networks.
Finally, it identifies the key metrics that are used to evaluate the performance of computer networks.
1.1 APPLICATIONS
Most people know the Internet through its applications: the World Wide Web, email, online social networking, streaming audio and video, instant messaging, file-sharing, to name just a few examples.
That is to say, we interact with the Internet as users of network.
Internet users represent the largest class of people who interact with the Internet in some way.