spreadsheet          电子表格

client               客户

arrangement          结构

client-server model  客户－服务模型

client process       客户进程

server process       服务器进程

perform执行

communication medium 通信媒介

a common gripe around the water cooler这样带来的一个问题是？？

improved communication 新型通信

Airlines, bookstores, music vendors航空公司、书店、音乐零售商

electronic commerce 电子商务

virtual             虚拟的

cooperation         协同

videoconference     视频会议

Before we start to examine the technical issues in detail, it is worth devoting some time to pointing out why people are interested in computer networks and what they can be used for. After all, if nobody were interested in computer networks, few of them would be built. We will start with traditional uses at companies and for individuals and then move on to recent developments regarding mobile users and home networking.

1.1计算机网络的使用15.30

在我们详细探讨技术问题之前，有必要花点时间来解释，为什么人们对计算机网络有着浓厚的兴趣，计算机网络可以用来做些什么。毕竟，如果没有人对计算机网络感兴趣，这门学科也很难建立起来。首先，我们将介绍计算机网络在公司及对个人的传统用法；然后，介绍在移动用户及家庭网络方面近来的发展。

1.1.1 Business Applications

Many companies have a substantial number of computers. For example, a company may have separate computers to monitor production, keep track of inventories, and do the payroll. Initially, each of these computers may have worked in isolation from the others, but at some point, management may have decided to connect them to be able to extract and correlate information about the entire company.

1.1.1商业软件

许多公司拥有很大数量的计算机。例如，一家公司已经把计算机分配到控制生产，跟踪库存及计算薪资等不同的职能中。最初，每台计算机可能执行与其他计算机不同的功能，但是在某些方面，出于管理的需要，必须把他们连接起来从中提取对整个公司有用的信息，并使这些信息相互关联。

Put in slightly more general form, the issue here is resource sharing, and the goal is to make all programs, equipment, and especially data available to anyone on the network without regard to the physical location of the resource and the user. An obvious and widespread example is having a group of office workers share a common printer. None of the individuals really needs a private printer, and a high-volume networked printer is often cheaper, faster, and easier to maintain than a large collection of individual printers.

把它看作更为一般的形式，这里的问题即是如何实现资源共享，目标是使所有的程序，设备，特别是数据能为网络上所有人所用。资源共享的一个很明显且大家所熟知的例子是共享打印机，办公室里的所有员工共用一台打印机。单个人确实没必要用私人打印机。通常，一台大容量网络打印机比一大群的个人打印机价格上更便宜，速度更快，也更容易维护。

However, probably even more important than sharing physical resources such as printers, scanners, and CD burners, is sharing information. Every large and medium-sized company and many small companies are vitally dependent on computerized information. Most companies have customer records, inventories, accounts receivable, financial statements, tax information, and much more online. If all of its computers went down, a bank could not last more than five minutes. A modern manufacturing plant, with a computer-controlled assembly line, would not last even that long. Even a small travel agency or three-person law firm is now highly dependent on computer networks for allowing employees to access relevant information and documents instantly.

然而，比共享诸如打印机、扫描仪和CD播放器等设备资源更重要的也许是信息共享。每家大型和中型企业及许多小公司在很大程度上依赖于计算机化的信息。大部分公司有客户记录、库存状态、应收帐款、财务报表、税收信息及其他更多的联网信息。假如所有的计算机停止工作，银行不到五分钟就会倒闭；而即使是一家拥有计算机控制生产线的现代制造型工厂，也不会持续太长时间。如今，甚至一家小旅行社或者三个人的法律公司也高度依赖计算机网络，以使员工快速获得相关信息和文件。

For smaller companies, all the computers are likely to be in a single office or perhaps a single building, but for larger ones, the computers and employees may be scattered over dozens of offices and plants in many countries. Nevertheless, a sales person in New York might sometimes need access to a product inventory database in Singapore. In other words, the mere fact that a user happens to be 15,000 km away from his data should not prevent him from using the data as though they were local. This goal may be summarized by saying that it is an attempt to end the ''tyranny of geography.''

就一些小公司而言，所有的计算机可能放在一间办公室或者在一栋楼里；但对大公司来说，计算机和员工可能分散在许多国家的几十个办公室或工厂里。

与汽车、航空运输等其他行业相比，尽管计算机仍是朝阳产业，但是在短短的时间内却取得了惊人的进展。在头二十年的发展过程中，计算机系统高度集中化，通常在一个很大的房间内。一般这个房间有玻璃墙，透过玻璃墙，参观的人会被其中巨大奇异的电子设备所吸引。中等企业或是大学可能拥有一两台计算机，而大型的研究院最多几十台。在二十年里，让数百万的企业大规模生产比邮票还小的强大计算机的想法纯粹是科学幻想。（要在二十年内生产出大量同样功能但体积比邮票还小的计算机，在当时的人们看来纯属科学幻想）（虚拟！）

The merging of computers and communications has had a profound influence on the way computer systems are organized. The concept of the ''computer center'' as a room with a large computer to which users bring their work for processing is now totally obsolete. The old model of a single computer serving all of the organization's computational needs has been replaced by one in which a large number of separate but interconnected computers do the job. These systems are called computer networks. The design and organization of these networks are the subjects of this book.

计算机和通信的融合已经对计算机系统的组织方式产生了深远的影响。以前人们心中“计算机中心”的概念已完全过时了，即用户带着他们的任务到一间放大型计算机的房间内处理。那种由一台计算机为所有机构的计算需要提供服务的旧模式已经被大量独立而又相互联系的计算机所取代。这些系统就是计算机网络。而这些网络的设计和组织便是该书的主要内容。

Throughout the book we will use the term ''computer network'' to mean a collection of autonomous computers interconnected by a single technology. Two computers are said to be interconnected if they are able to exchange information. The connection need not be via a copper wire; fiber optics, microwaves, infrared, and communication satellites can also be used. Networks come in many sizes, shapes and forms, as we will see later. Although it may sound strange to some people, neither the Internet nor the World Wide Web is a computer network. By the end of this book, it should be clear why. The quick answer is: the Internet is not a single network but a network of networks and the Web is a distributed system that runs on top of the Internet.

书中，我们将用术语“计算机网络”来表示由一种技术相互联系起来的自主计算机的集合。两台计算机如果能相互交换信息，称作互联。这种连接不需要通过铜线；光纤，微波，红外线和通信卫星等都可以用来建立连接。后面我们可以看到，网络可以有不同的大小，形状，形式。尽管对很多人来说比较陌生，互联网和万维网都不是计算机网络（可能很多人对此会觉得很奇怪）。在本书的结尾部分，我们将详细阐述其原因。概言之（现在给出一个简单的答案），互联网不是单一的网络，而是许多网络联系在一起构成的一个网络；万维网是在互联网之上运行的分布式系统。

There is considerable confusion in the literature between a computer network and a distributed system. The key distinction 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 World Wide Web, in which everything looks like a document (Web page).

在一些文献中，计算机网络和分布式系统是比较容易混淆的概念。他们的主要区别在于，分布式系统中，相互独立的计算机组合对用户来说是一个单一的关联系统。（在一个分布式系统中，一组独立的计算机展现给用户的是一个统一的整体，就好像一个系统似的）通常，呈现给用户的只有一个模式或者范例。位于操作系统之上的一层软件称作中间件（middleware），负责实施该模式。分布式系统的一个众所周知的例子是万维网，即在万维网中，所有的内容看上去像文档（网页）一样。

In a computer network, this 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 operating systems, that is fully visible to the users. If a user wants to run a program on a remote machine, he [ ] has to log onto that machine and run it there.

[ ] ''He'' should be read as ''he or she'' throughout this book.

 在计算机网络中，这种关联、模式和软件都是不存在的。用户面对的是实实在在的机器，而不能通过系统使机器的样子和行为一致。如果机器有不同的硬件和操作系统，那么这些差异对用户来说是完全可见的。如果用户想运行一台远程计算机上的程序，他可以登录到那台机器并运行程序。（本书中，他可以看作“他”或“她”）

In effect, a distributed system is a software system built on top of a network. 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 hardware.

实际上，分布式系统是建立在网络之上的软件系统。该软件提供了高度的粘聚性和透明度。因此，网络与分布式系统之间的区别在于软件（特别是操作系统），而不是硬件。

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. Although this book primarily focuses on networks, many of the topics are also important in distributed systems. For more information about distributed systems, see (Tanenbaum and Van Steen, 2002).
尽管如此，分布式系统和计算机网络之间还是有很多重叠的地方。比如，他们都需要移动文档。差异在于谁发起的移动，是系统还是用户。虽然本书大部分在关注网络，但在分布式系统中，许多主题同样非常重要。关于分布式系统的更多信息，请参看 Tanenbaum and Van Steen, 2002 。

第一章 概述

Each of the past three centuries has been dominated by a single technology. The 18th century was the era of the great mechanical systems accompanying the Industrial Revolution. The 19th century was the age of the steam engine. During the 20th century, the key technology was information gathering, processing, and distribution. Among other developments, we saw the installation of worldwide telephone networks, the invention of radio and television, the birth and unprecedented growth of the computer industry, and the launching of communication satellites.

在过去的三个世纪中，每个世纪都有一种占主导地位的技术。18世纪伴随工业革命的到来，开辟了大机器系统的纪元。19世纪是蒸气机时代。20世纪（在20世纪的发展过程中），主要的技术是信息收集、加工及分类。在这个时代的其他一些发展中，我们看到世界范围内电话网络的安装，无线电广播及电视的发明，计算机产业的诞生及空前发展，还有通信卫星的发射。

As a result of rapid technological progress, these areas are rapidly converging and the differences between collecting, transporting, storing, and processing information are quickly disappearing. Organizations with hundreds of offices spread over a wide geographical area routinely expect to be able to examine the current status of even their most remote outpost at the push of a button. As our ability to gather, process, and distribute information grows, the demand for ever more sophisticated information processing grows even faster.
随着技术的快速发展（作为技术快速发展的一个直接结果是） ，这些领域迅速融合，信息采集、存贮及处理之间的差异很快消失（正在很快消失）。对于具有数百个办公室的大型组织来说，尽管这些办公室分布在广阔的地理区域中，但未来期望的情景是，工作人员只需单击一下按钮，就可查到最远处发布的状态随着我们信息收集、加工及分类能力的增长，这种对更为复杂的信息加工的永久需求得到更快的增长。