1. 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.
---Willian Blake
Suppose you want to build a computer network, one that has the potential to grow to global proportions1 and to support applicatons as diverse2 as teleconferecing, video on demand, electronic commerce, distributed computing, and digital libraries. What available3 technologies would server as the underlying building blocks4, and what kind of software architecture would you design to integrate these building blocks into an effective communication service? Answering this question is the overriding5 goal of this book---- to describe the available building materials and then to show how they can be used to construct a network from the ground up.
假如你想要去打造一个计算机网络， 它必须拥有发展为全球性分布，并且支持多种多样的应用程序，比如电视会议设备，视频需求，电子商务， 分布式计算和数字图书馆。 什么样的可获得的技术用作底层建筑积木，你设计什么样的软件结构将能够整合这些建筑积木形成一个有效的通信服务。 回答这个问题是本书比较重要的目标----去描述可获得性的建筑积木，并且接着展示他们从地平线上怎样构建一个网络。
1. proportion // CET4 1.N-count 不可数名词 Formal 正式 A proportion of a group of or an amount is a part of it 部分；
A large proportion of the dolphins in that area will eventually die. 那个地区的很大一部分海豚最终都会死去 。
2. N-count 可数名词； 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 比例（某部分在总体中所占的）
The proportion of women in the profession had risen to 17.3%. 从事那个职业的女性已经增长到17.3%。
3. N-count 可数名词；The proportion of one amount to another is the relationship between the size of two amounts. 比值（一个量与另一个量）
Women's bodies tend to have a higher proportion of fat to water. 女性体内往往有更高的脂肪与水的比值。
4. N-plural 复数型名词 WRITTEN 书面 If you refer to the proportions of something, you are frfferring to its size, usually when this is extremely large. 大小（某物的）
In the tropics plants grow to huge proportions. 在热带，植物长得很高大。
5. Phrase 习语 If one thing increases or decreases in proportion to another thing, it increases or decreases to the same degree as that thing. 成比例地（与...）
The pressure in the cylinders would go up in proportion to the boiler pressure.
6. Phrase 习语 If something is small or large in proportion to something else, it is small or large when compared with that thing. 比（与...）
Children tend to have relatively larger heads than adults in proportion to the rest of their body.
与身体的其余部分相比，孩子头部所占比例往往比成人大。
7. Phrase 习语 If you say that somehting is out of all proportion to something else, you think that it is far greater or more serious than it should be. 完全不成比例（与...）
The punishment was out of all proportion to the crime. 惩罚与罪行完全不成比例。
2. diverse /diværs/ 1. adj 形容词 If a group of things is diverse, it is made up of a wide variety of things. 各式各样的
The building houses a wide and diverse variety of antiques. 这栋楼里摆放着大量各式各样的古董。
2. adj. Diverse people or things are very different from each other. 不同的
Albert Jones's new style will inevitably put him in touch with a much more diverse and perhaps younger audience. 艾伯特.琼斯的新风格无疑使他接触更多形形色色的、或许多更年轻的观众。
3. available /əveiləbəl/ CET 4 1. adj 形容词 If somehing you want or need is available, you can find it or obtain it. 可获得的
Since 1978， the amount of money available to buy books has fallen by 17%. 自从1978年以来， 可供买书的钱已经减少了17%。
The store has about 500 autographed copies of the book available for purchase. 这家书店有大约500本该书签名版本可供购买。
availablity /əv'eiləbiliti/ N-uncount 不可数名词 可获得性 ... the easy availability of guns. ... 获得枪支的容易性
ADJ. 形容词 Someone who is available is not busy and is therefore free to talk to you or to do a particular task. [v-link adj]有空的
Mr. Leach is on holiday and was not avaiable for comment. 利奇先生在休假，没空作评论。
4. block block CET4 /blɔk/ blocking, blocking
1. N-count 可数名词 A block of a substance is a large rectangular piece of it. 大块 ... a block of ice. 一大块冰
2.N-count 可数名词 A block of apartments or offices is a large building containing them. 大楼；
... a white-painted apartment block. 一幢漆白色的公寓楼。
3. N-count 可数名词 A block in a town or city is an area of land with streets on all its sides, or the area or distance between such streets. 街区
He walked around the block three times. 他绕着这个街区走了3圈。
She walked four blocks down High Street. 她沿着高街走了4个街区。
4. N-count 可数名词 blocks are wooden or plastic cubes, such as those used as toys by children. 积木。
5. VT 及物动词 To block a road, channel, or pip means to put an object across it or in it so that nothing can pass through it or along it. 堵塞；封锁；
Some students today blocked a highway that cuts through the center of the city. 一些学生封锁了穿过市中心的高速公路。。
6. VT 及物动词 If something blocks your view, it prevents you from seeing something because it is between you and that thing. 挡住
a row of spruce trees that blocked his view of the long north slope of the mountain. 挡住了他看向山北长坡视线的一排云杉。
7. VT 及物动词 If you block someone's way, you prevent them from going somewhere or entering a place by standing in fornt of them. 阻挡
I started to move around him, but he blocked my way. 我开始绕开他走，但他挡着我的路。
8. VT 及物动词 If you block something that is being arranged, you prevent it from being done. 阻止
For years the country has tried to block imports of various cheap foreign products。多年以来，这个国家试图阻止多种外国廉价产品的进口。
9. N-count 可数名词 A block of someting such as tickets or shares is a large quantity of them, especially when they are all sold at the same time and are in a particular sequence or order. 一 大叠， 一次性大量购买的入场券、股票等。
Those booking a block of seats get them at reduced rates. 那些一次预订大量座位的人可以享受折扣价。
5. override // CET6 + The verb is prounced /'əuvər'aid/ the noun is pronounced /'əuvəraid/
1. VT 及物动词： If one thing in a situation overrides other things, it is more important than they are 比... 更重要
The welfare of a child should always override the wishes of its parents. 孩子的幸福应该比其父母的心愿更重要。
Befor we can understand how to design a computer network, we should first agree on exactly what a computer netowrk is. At one time, the term network meant the set of serial lines used to attach8dumb6 terminals to mainframe computers. Other important networks include the voice telephone network and the cable TV networks include the voice telephone network and the cable TV network used to disseminate7 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 (terminals, hand receivers, and television sets).
在我们能够理解怎样去设计一个计算机网络， 我们首先应该很清楚地同意一个计算机网络是什么。 在某一时期，词汇“网络”意思是，一系列的串行集合，从被处理的无法交流的终端到巨型计算机。 另外一个重要的网络包括：语音电话网络和被用来传播视频信号的有线电视网络。 这些网络的共同点是：他们被用来专门处理一种类型的特殊数据（击键、语音和视频）和他们很可能连接到专用目的设备（终端、手持接收器、和电视机）。
6. dumb /dʌm/ CET
1. adj. 形容词 Someone who is dumb is completely unable to speak. 哑的 ... a young deaf and dumb man. ...一名年轻的聋哑人
2. adj. 形容词 LITERARY 文学性 If someone is dumb on a particular occasion, they cannot speak because they are angry, shocked, or surprised. [v-link adj] 说不出话的
We were all struck dumb, you mean that they are stupid or foolish. 我们一时个个哑口无言。
3. adj. 形容词 Informal 非正式 If you call a person dumb, you mean that they are stupid or foolish. 蠢的
The questions were set up to make her look dumb. 这些问题是安排好让她显得蠢的。
4. adj. 形容词 AM 美国英语 INFORMAL 非正式
If you say that something is dumb, you think that it is silly and annoying,愚蠢而恼人的
I came up with this dumb idea. 我想出了这个愚蠢而恼人的主意。
5. dumb down Phrasal verb 动词词组
If you dumb down something, you make it easier for people to understand, especially when this spoils it. 通俗化
This sounded like a case for dumbing down the magazine, which no one favored.这听起来像是一桩使杂志通俗化的事，没人赞成。
7. disseminate /disəmineit/ disminates disseminating disseminated CET6
1. VT 及物动词 Formal 正式
To disseminate information or knowledge means to distribute it so that it reaches many people or organizations. 散布；传播；
They disseminated anti-French propaganda. 他们散布了反法资料。
dis+semi+na+tion /dis/ N-uncount 不可数名词 散布；传播；
He actively promoted the dissemination of scientific ideas about matters such as morality. 他积极地推动诸如道德之类的科学观念的传播 。
8. attack CET4 // attacks, attacking, attacked
1. VT / VI 及物动词/ 不及物动词 To attack a person or place means to try to hurt or damage them using physical violence. 攻击
Fifty civilians in Masawa were killed when government planes attacked the town. 马萨瓦50名平民在政府飞机的袭击该市时丧生。
He bundled the old lady into her hallway and brutally attacked her. 他把老太太推进家门厅里，并且野蛮地袭击了她。
2. N-var 有变体名词 attack is also a noun. 攻击
a campaign of air attacks on strategic targets. ...对战略目标的空袭行动。
3. VT 及物动词 If you attack a person, belief, idea, or act, you criticize them strongly. 抨击
He publicly attacked the people who've been calling for secret ballot nominations. 他公开抨击了那些秘密投票提名的人。
4. N-var 有变体名词 attack is a noun. [usu with supp]抨击
The role of the state as prime mover in planning social change has been under attack. 国家作为规划社会变革发起者的角色已经遭到抨击。
5. VT 及物动词 If something such as a disease, a chemical, or an insect attacks something, it harms or spoils it. 侵害疾病、化学药品或昆虫。
The virus seems to have attacked his throat.病毒似乎已侵害到他的咽喉。
6. N-uncount不可数名词 attack is also a noun. [also N in pl]侵害
The virus can actually destroy those white blood cells, leaving the body wide open to attack from other infections. 该病毒实际上能破坏那些白血球， 使身体极易受到其他感染。
7. VT 及物动词 If you attack a job or problem, you start to deal with it in an energetic way 积极处理
Any attempt to attack the budget problem is going to have to in some way deal with those issues. 要想妥善处理预算问题，从某些方面来说必须先解决那些问题。
8. VT /VI 及物动词/不及物动词 In games such as soccer, when one team attacks the opponent's goal, they try to score a goal. 进攻， 在足球等比赛中。
Now the U.S. is controlling the ball and attacking the opponent's goal. 现在美国队在控球，正向对方的球门发起进攻。
The goal was just reward for their decision to attack constantly in the second half. 这粒进球正是他们决定在下半场连续进攻的回报
9.N-count 可数名词 attack is also a noun. 进攻
Lee was at hub of some incisive attacks in the second half. 李是下半场几次犀利进攻的主角。
10. N-count 可数名词 An attack of an illness is a short period in which you suffer badly from it. 发作疾病性
It had brought on an attack of asthma. 这引起了一次哮喘发作。
What distinguishes a computer network from these other types of networks? Probably the most important characteristic of a computer network is its generality9. Computer networks are built primarily from general-purpose programmable hardware, and they are not optimized from application like making 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 taking over the functions previously performed by single-use networks. 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.
计算机网络和那些其它网络不同点是什么？ 很可能，计算机网络的最重要特性是：它们的普通性。 计算机网络最初建造是为了专用目的的程序硬件，并且他们并没有被优化，从应用程序，例如打电话、发送电视信号。 代替的是， 他们能够传输许多不同类型 的数据， 并且他们支持一个更宽阔的、而且正在发展，范围更广的应用程序。 今天的计算机网络已经逐渐克服了先前单一用户网络执行的先前功能。 本章展示一些典型的计算机网络应用程 序， 并且讨论了，作为一个网络设计者希望支持诸如此类的应用程序，必须要注意的需求。
9. generality // n.概论；普遍性；一般性；通用性；
1. A statement or remark that concerns the main aspects of something rather than the details. 一般性；
2. The quality or state of being general; 普遍性；
3. a statement or principle that is true in most cases.通用性；
Once we understand the requirements, how do we proceed? Fortunately, we will not be building the first network. others, most notably11 the community12 of research responsible for the Internet, have gone before us. We will use the wealth13 of experience generated from the Internet to guide our design. This experience is embodied14 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.
一旦，我们理解了这些需求， 我们该怎样处理？ 幸运的，我们将来也不会建设第一个网络。 相反， 绝大多数，尤其是研究性社会团体对因特网负责，我们之前已经存在。我们也将要使用从因特网中提取的大量经验去指导和设计。 这种经验被包含在网络结构中，定义了可获得的硬件和软件部分，显示了他们怎样被排列组成一个完整的网络系统。
11. notably // 1. adv. 副词 You use notably to specify an important or typical example of something that you are talking about.【adv group /cl】 尤其
The divcore would be granted when more important problems, notably the fate of the chirldren, had been decided. 当更重要的问题，尤其是孩子们的命运被决定时，离婚就会获得批准。
2. adv. 副词 You can use notably to emphasize a particular quality that someone or something has [adv adj / adv] 明显地
Old established friends are notably absent, so it's a good opportunity to make new contacts. 老朋友们显然都已经不在身边， 所以这是结交新朋友的一个好机会。
12. community // CET 4 communities
1. N-sing-coll 单数型集体名词 The community is all the people who live in a particular area or place. 社区
He's well liked by people in the community. 他很受社区人们的喜爱。
2. N-count-coll 集体可数名词 A particular community is a group of people who are similar in some way. 团体
The police have not really done anything for the black community in particular. 警方并没有特别为黑人团体做什么。
3. N-uncount 不可数名词： community is friendship between different people or groups, and a sense of having something in common. 团体精神
Two of our greatest strengths are diversity and community. 我们的最大优点是多样性和团体精神。
13. wealth // CET 4 1. N-uncount 不可数名词 Wealth is the possession of a large amount of money, property, or other valuable things. You can also refer to a particular person's money or property as their wealth. 财富
Economic reform has brought relative wealth to peasant farmers. 经济改革使农民相对富裕起来。
2. N-sing 单数型名词 Formal 正式 If you say that someone or something has a wealth of good qualities or things, you are emphasizing that they have a very large number or amount of them.
大量的：
Their websites contain a wealth of information on the topic. 他们的网站上有大量与该主题相关的信息。
The city boasts a wealth of beautiful churches. 该城引以为荣的是有大量漂亮的教堂。
14. embody // CET6 embodies embodying embodied
1. VT 及物动词 To embody an idea or quality means to be a symbol or expression of that idea or quality 体现， 具体象征：
Jack Kennedy embodied all the hopes of the 1960s. 杰克.肯尼迪体现了20世纪60年代的全部希望。
For twenty-nine years, Checkpoint Charlie embodies the Cold War. 29 年来，查理检查站具体象征了冷战。
2. VT 及物动词 If something is embodied in particular thing, the second thing contains or consists of the first. 包含；收录：
The proposal has been embodied in a draft resolution. 这项提议已经被包含在一份决议草案中。
In addition to unnderstanding how networks are built, it is increasingly important to understand how they are operated or managed and how network applications are developed. Most of us now have computer networks in our homes, offices, and in some cases in our cars, so operating networks is no longer a matter only for a few specialists. And, with the proliferation15 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.
另外，对于理解网络是怎样建设的，对于理解网络应用程序怎样被开发和他们怎样被操作和管理逐渐显得更重要。 我们中绝大多数现在有计算机网络：在我们的家里、办公室，还有在我们的汽车里的一些东西里，因此操作网络不再仅仅是一些专家的事情。 并且，伴随着可程序化的激增，可接触式网络设备，例如，小灵通，这种更新中的大多数将开发网络应用程序比起在过去。 因此，我们需要考虑网络从多个角度：建设者、操作者、应用程序开发者。
15. proliferate /prəlifəreit/ CET 6+ proliferates proliferating proliferated
1. VI 不及物动词 Formal 正式： If things proliferate， they increase in number very quickly. 激增
Computerized databases are proliferating fast. 计算机化的数据库在迅速激增。
Pro+lif+era+tion /prəlifəreiʃən/ N-uncount 不可数名词 激增：
The proliferation of nuclear weapons. ... 核武器的激增。
To star 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 applications and different communities 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.
对于我们的开始， 在理解的道路上，怎样去建设、操作、并且编程一个网络， 本章要作四件事情。 第一件，它探索了需求：不同的应用程序和在网络上不同的交流人群。 第二，它包含网络架构的思想，对于本书的其它章节是基础。第三、它介绍了一些实现计算机网络的关键性元素。最后，它定义了关键性的度量，用它们来评估计算机网络的性能。
1.1 Applications
1.1 应用程序
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 the network. Internet users represent the largest class of people who interact with the Internet in some way, but there are several other important constituencies16. There is the group of people who create the applications--- a group that has greatly expanded17 in recent years as powerful programming platforms and new devices such as smartphones have created new opportunities to develop applications quickly and to bring them to a largest market. Then there are those who operate or manage networks--mostly a behind-the-scenes job, but a critical one and often a very complex one. With the prevalence18 of home networks, more and more people are also becoming, if only in a small way, network operators. Finally, there are those who design and build the devices and protocols that collectively make up the Internet. The final constituency is the traditional target of networking textbooks such as this one and will continue to be our main focus. However, throughout this book we will also consider the perspectives of application developers and network operators. Considering these perspectives will enable us to better understand the diverse requirements that a network must meet. Application developers will also be able to make applications that work better if they understand how the underlying technology works and interacts with the application. So before we start figuring out how to build a network, let's look more closely at the types of applications that today's networks support.
多数人知道因特网，通过它们的应用程序： 万维网， 伊妹儿， 在线社交网络，流语音和视频， 瞬时信息，文件共享，且说这几个例子。也就是说，我们接触因特网作为网络的用户。 因特网用户代表了最大阶层的人们， 他们在某些方面和因特网互动， 但是也有许多重要的团体。 有一组人创造应用程序--- 在最近几年，一组人非常快地扩展作为强大的编程平台， 并且新的设备例如，手机已经创造了新的机遇， 去快速开发应用程序，把他进带进一个更大的市场。 接着是操作和管理网络的那些人---绝大多是后台工作， 但是也是至关重要的一个并且经常是非常复杂的。 伴随着家庭网络的盛行，越来越多的人也变成了网络操作者，仅仅是以一种方式。 最后，还有一些设计和制造设备的人们， 以及被收集起来的协议组成因特网。 最终的团体是网络教科书的传统目标，例如这本书，并且将来继续作为我们的主要焦点。 无论如何，通过这本书，我们也将考虑从应用程序开发人员和网络操作人员的角度。 考虑这些方面也将促进我们更好地理解更多在网络中会遇到的需求。 应用程序开发人员也将使应用程序工作的更好，如果他们理解底层技术怎样工作，并且和应用程序交互操作。 因此， 我们开始描述清怎样构建一个网络前，让我们更关注今天网络所支持的应用程序类型。
16. constituency // CET6+ 1. N-count 可数名词 A particular constituency is a section of society that may give political support to a particular party or politician 选民阵营(会支持某政党或政客)
In lowa, farmers are a powerful political constituency. 在爱荷华州，农民是一个强大的政治选民阵营
2. N-count A constituency is an area for which someone is elected as the representative in a legislature or government. 选区
Voters in 17 constituencies are going back to the polls today. 17个选区的投票人今天会回到那些投票站的。
17. expand // CET expands, expanding, expanded
1.VT/ VI 及物动词/不及物动词 If something expands or is expanded, it becomes larger. 扩大；膨胀；
Engineers noticed that the pipes were not expanding as expected. 工程师注意到管子并未像预期的那样膨胀。
We have to expand the size of the image. 我们不得不扩大图像的尺寸。
2. VT/VI 及物动词/不及物动词 Business 商业
If something such as business, organization, or service expands, or if you expand it, it becomes bigger and includes more people, goods, or activities. 使... 发展；发展：
The popular ceramics industry expanded toward the middle of the 19th century. 大众陶瓷业在19世纪中叶得到了发展。
3. expand on expand upon Phrase verb 动词词组
If you expand on or expand upon something, you give more information or details about it when you write or talk about it. 更充分的讲述；进一步论述：
The president used today's speech to expand on remarks he made last month. 总统利用今天的演讲进一步阐述了他上个月的讲话内容。
18. prevalence/pr'əvələnt/ CET6 1. ADJ. 形容词 A condition, practice, or belief that is prevalent is common.盛行的；普遍存在的；
This condition is more prevalent in women than in men. 这种情况在女性中比在男性中更为普遍。
Somking is becoming increasingly prevalent among younger women. 吸烟在年轻女性中变得越来越盛行。
preva+lence N-uncount 不可数名词 盛行
... the prevalence of cocaine abuse in the 1980s. ... 20 世纪80年代可卡因滥用的盛行。
1.1.1 Classes of Applications
1.1.1 应用程序类型
The World Wide Web is the Internet application that catapulted the Internet from a somewhat20 obscure19 tool used mostly by scientists and engineers to the mainstream phenomenon that it is today. The web itself has become such a powerful platform that many people confuse21 it with the Internet (as in "the Internetwebs"), and it's a bit of a stretch22 to say that the web is a single application.
万维网是因特网的应用程序，它处理因特网从最初绝大多数科学家和工程师所使用的有点复杂的工具到今天的主流现象！ WEB本身变成了如此强大的平台，很多人把它和因特网混淆（比作因特网WEB）,
19. obscure/əbskyuə/ CET6 1. adj. If something or someone is obscure, they are unknown, or are known by only a few people. 鲜为人知的人
The origin of the custom is obscure. 该习俗的起源鲜为人知；
2. adj. Something that is obscure is different to understand or deal with, usually because it involves so many parts or details. 难处理的；复杂难懂的；
The contracts are written in obscure language. 那些合同是用复杂难懂的语言写的。
3. VT及物动词 If one thing obscures another, it prevents it from being seen or heard properly. 遮掩；
Trees obscured his vision; he couldn't see much of the square's southern half.树木遮住了他的视线，他不怎么看得见广场南半部。
4.VT 及物动词 To obscure something means to make it different to understand. 使... 难懂
... the jargon that frequently obscures educational writing. 常使教学行文变得复杂难懂的术语。
20. somewhat /sʌmwʌt/ CET4 1. adv副词 Formal 正式 You use somewhat to indicate that something is the case to a limited extent or degree. [adv with cl/group]稍微
He concluded that Oswald was somewhat abnormal. 他断定奥斯瓦德有点不正常。
He explained somewhat unconvincingly that the company was paying for everything. 他有点不令人相信地解释说公司正在支付一切费用。
21. confuse CET 4 /Kənfyuz/ CET 4 confuses confusing confused
1. VT 及物动词 If you confuse two things, you get them mixed up, so that you think one of them is the other one. 混淆
I always confuse my left with my right. 我总是混淆左右。
con+fu+sion /kənfyuʒən/ N-uncount 不可数名词 混淆
Use different colors of felt pen on your sketch to aviod confusion. 在草图上使用不同颜色的毡毛以免混淆。
VT及物动词
To confuse someone means to make it different for them to know exactly what is hapening or what to do. 使困惑
My words surprised and confused him. 我的话使他既惊讶又困惑。
VT及物动词
To confuse a situation means to make it complicated or difficult to understand. 使复杂化
To further confuse the issue, there is an enormous variation in the amount of sleep people feel happy with. 使这个问题进一步复杂化的是，人们感觉愉快的睡眠时间相差很大。
22. stretch // CET 4 stretches stretching stretched
1. VI 不及物动词 Something that stretches over an area or distance covers or exists in the whole of that area or distance. （not cont） 延伸
The procession stretched for several miles. 游行队伍延伸了数英里。
2. N-count 可数名词
A stretch of road, water, or land is a length or area of it. 一段
It's a very dangerous stretch of road. 这是一段非常危险的路。
3. VT/VI 及物动词/不及物动词
When you stretch, you put your arms or legs out straight and tighten your muscles. 伸直
He yawned and stretched. 他打了个哈欠，伸了伸腰。
Try stretching your legs and pulling your toes upwards.试着伸直双腿并向上拉拉脚趾。
4. N-count 可数名词 A stretch of time is a period of time. 一段时间
... after an 18-month stretch in the army. ... 在部队呆了18个月后。
5. N-count 可数名词 stretch is also a noun. 伸展
At the end of a workout spend time cooling down with some slow streches. 在健身末尾花时间一些缓慢的伸展来做缓和。
6. VT不及物动词 If something stretches from one time to another, it begins at the first time and ends at the second, which is longer than expected. 持续
... a working day that stretches from seven in the morning to eight at night. 从早上7点持续到晚上8点的一个工作日。
7. VI 不及物动词 If a group of things stretch from one type of thing to another, the group includes a wide range of things. 涉及
... a trading empire, with interests that stretched from chemicals to sugar. ... 一个兴趣范围从化学品到食糖的贸易帝国。
8. VT/VI 及物动词/不及物动词
When something soft or elastic stretches or is stretched, it becomes longer or bigger as well as thinner, usually because it is pulled. 有弹性
The cables are designed not to stretch. 这些缆绳设计成没有弹性。
9. VT/VI 及物动词/不及物动词
If you stretch an amount of something or if it stretches, you make it last longer than it usually would by being careful and not wasting any of it. 俭省
They're used to stretching their budgets. 他们习惯于精打细算。
10. VT 及物动词
If something stretches your money or resources, it uses them up so you have hardly enough for your needs. 耗尽钱或资源
The drought there is stretching resources. 那里的干旱正在耗尽财力物力。
11. VT 及物动词
If you say that a job or task stretches you, you mean that you like it because it makes you work hard and use all your energry and skills so that you do not become bored or achieve less than you should. 使充分发挥
I'm trying to move on and stretch myself with something different. 我正在试图换个工作，使自己在不同的事情上得到充分发挥。
12. Phrase 习语
If you say that something is not true or possible by any stretch of the imagination, you are emphasizing that it is completely untrue or absolutely impossible.无论如何想象
Her husband was not a womanizer by any stretch of the imagination. 她的丈夫无论如何也不会是个玩弄女性的人。
13. stretch out Phrasal verb 动词词组
If you stretch out or stretch yourself out, you lie with your legs and body in a straight line 平躺 使
The bathtub was too small to stretch out in. 这个浴缸太小了躺不下。
14. Phrase verb 动词词组
If you stretch out a part of your body, you hold it out straight. 伸出
He was about to stretch out his hand to grab me. 他正要伸手抓住我。
In its basic form, the Web presents an intuitively23 simple interface. Users view pages full of textual and graphical objects and click on objects that they want to learn more about, and a corresponding new page appears. Most people are also aware that just under the covers each selectable object on page is bound to an identifier for the next page or object to be viewed. This identifier called a Uniform Resource Locator (URL), provides a way of identifying all the possible objects that can be viewed from your web brower. For example,
http://www.cs.princeton.edu/~llp/index.html
is the URL for a page providing information about one of this book's author: the string http indicates that the Hypertext Transfer protocol (HTTP) should be used to download the page, www.cs.princeton.edu is the name of the machine that serves the page, and
/~llp/index.html
uniquely identifies Larry's home page at this site.
在它的基本格式中，WEB体现了一种直觉的、简单的结口。 用户浏览充满文本和图表目标的网页，并且点击他们想要了解更多的目标， 一个相应的新页面出现。 绝大多数人也意识到那，每一个可选目标的背后，存在一个标识，对于被浏览的下一个页面。 这个标识被称为“统一资源定位”， 提供了一种方法，标识所有可能的目标，在你的浏览器上可以看到的。例如：
http://www.cs.princeton.edu/~llp/index.html
是一个URL 对于本书作者提供了信息的页面： http 字符串暗示了超文本传输协议，被用来下载页面，http://www.cs.princeton.edu是服务器页面的机器名， 并且/~llp/index.html 在网页中唯一标识Larry 的页面。
23. intuitively /intuətiv/ 1. adj. 形容词 If you have an intuitive idea or feeling about something, you feel that it is true although you have no evidence or proof of it 直觉的
A positive pregnancy test soon confirmed her intuitive feelings. 阳性的孕检结果很快证实了她的直觉。
in+tui+tive+ly adv. 副词；凭直觉地
He seemed to know intuitively that I must be missing my mother. 他似乎凭直觉知道我一定是在思念我的母亲。
What most web users are not aware of, however, is that by clicking on just one such URL over a dozen messages may be exchanged over Internet, and many more than that if the web page is complicated with lots of embedded objects. This message exchange includes up to six messages to translate the server name (www.cs.princeton.edu) into its Internet Protocol (IP) address (128.112.136.35), three messages to set up a Transmission Control Protocol (TCP) connection between your browser and this server, four messages for your browser to send the HTTP "GET" require and the server to respond with the requested page (and for each side to acknowledge receipt of that message), and four messages to tear24down the TCP connection. Of course, this dose not include the millions of messages exchanged by Internet nodes throughout the day, just to let each other know that they exist and are ready to serve web pages, translate name to addresses, and forward messages toward their ultimate25 destination.
绝大多数用户并不关心那些， 无论如何，在因特网上仅仅需要敲击一个像这样的URL，许多信息交换， 许多比那更复杂的网页有许多嵌入式对象。 这种信息交换包括，六条以上的信息去把服务器名转换为IP地址（128.112.136.35）， 三条信息被用来搭建TCP连接，在浏览器和这台服务器之间，四条信息对于浏览器去发送HTTP的“get”请求， 服务器用请求页面作为响应。 四条信息撕碎TCP连接。 当然， 这并不包括，每天通过网络结点成千万条的信息， 仅仅是让彼此知道他们的存在，准备服务WEB页面，转换名字为IP地址，驱使信息向前到达最终目的地。
24. tear /tər/ 1. vt /vi 及物动词/不及物动词： If you tear paper, cloth, or another material, or if it tears, you pull it into two pieces or you pull it so that a hole appears in it.撕破纸、布、等撕开
I tore my coat on a nail. 我的外套在一个钉子上挂破了。
2. phrase verb 动词词组 tear up means the same as tear. 撕破
She tore the letter up. 她把那封信撕了。
Don't you dare tear up her ticket. 你竞敢撕坏她的票。
3. N-count 可数名词 A tear in paper, cloth, or another material is a hole that has been made in it. 破洞
I peered through a tear in the van's curtains. 我透过货车窗帘上的一个破洞偷偷窥视。
4. VT /VI 及物动词 /不及物动词
If you tear one of your muscles or ligaments, or if it tears, you injure it by accidentically moving it in the wrong way. 拉伤肌肉或韧带
He tore a muscle in his right thigh. 他拉伤了右大腿的一块肌肉。
If the muscle is stretched again it could even tear.这块肌肉再受到拉伸的话，它甚至可能会拉伤。
5.VT 及物动词 To tear something from somewhere means to remove it roughly and violently 撕掉，猛烈地
She tore the windscreen wipers from his car. 她猛地撕掉了他车上的雨刮器。
6. VI 不及物动词 If a person or animal tears at something, they pull it violently and try to break it into pieces. 撕扯
Female fans fought their way past bodyguards and tore at his clothes.
女性仰慕者拼命绕过保镖，撕扯他的衣服。
7. VI 不及物动词
If you tear somewhere, you run, drive, or move there quickly. 奔、冲
The door flew open and Mireanda tore into the room. 门猛地开了，米兰冲进了房间。
8. VT PASSIVE 被动及物动词
If you say that a place is torn by particular events, you means that unpleasant events which cause suffering andd division among people are happening there. 折磨
... a country has been torn by civil war and foreign invasion since its indenpendence. ... 一个独立以来饱受内战外扰之苦的国家。
10. tear apart phrasal verb 动词词组
If something tears people apart, it causes them to argue or to leave each other. 使...不和；人们...分裂。
Her pregnancy was tearing the family apart. 她的怀孕引起了家庭的不和。
11. Phrasal verb 动词短语 If something tears you apart , it makes you feel very upset, worried and unhappy. 使痛苦
Don't think it hasn't torn me apart to be away from you. 不要以为离开你，我并没有痛苦。
12. tear away Phrasal verb 动词词组
If you tear someone away from a place or activity, you force them to leave the place or stop doing the activity, even though they want to remain there or carry on 逼离开；某人迫使停止；
He finally tore himself away from the table long enough to pour me a drink. 他最终迫不得已离开了那张桌子片刻，给我倒了一杯饮料。
13. tear down phrasal verb 动词词组 If you tear something down, you destroy it or remove it completely. 拆除；拆毁
Angry Russians may have torn down the statue of Felix Dzerzhinsky. 愤怒的俄罗斯人可能已经拆除了菲利克斯. 捷尔任斯的雕像。
14. tear off phrasal verb 动词词组
If you tear off your clothes , you take them off in a rough and violent way. 胡乱脱掉；
Totally exhausted, he tore his clothes off and fell into bed. 由于疲惫不堪，他胡乱脱掉衣服就倒在了床上。
15. tear up Phrasal verb 动词词组
If something such as a road, railroad, or area of land is torn up, it is completely removed or destroyed. 拆除；拆毁
Dozens of miles of railroad track have been torn up. 数十英里的铁轨已被拆除。
tear // CET 4 tears
1. N-count 可数名词 tears are the drops of salty liquid that come out of your eyes when you are crying 眼泪
Her eyes filled with tears. 她的眼里噙满了泪水。
2. N-plural 复数型名词 You can use tears in expressions such as in tears, burst tears, and close to tears to indicate that someone is crying or is almost crying. 哭泣
He was in floods of tears on the phone.他在电话里大哭。
She burst into tears and ran from the kitchen.她突然哭了起来，跑出了厨房。
25. ultimate // CET4
1. ADJ. 形容词 You use ultimate to describe the final result or aim of a long series of events[adj +n ] 最终的
He said it is still possible to predict the ultimate outcome. 他说现在还不可能预测最终结果。
2. ADJ. 形容词 You use ultimate to describe the original source or cause of something. [adj n ]最终的；根本的；
Plants are the ultimate source of all foodstuffs. 植物是所有食物的最终来源。
3. adj. 形容词 You use ultimate to describe the most important or powerful thing of a particular kind. [adj +N ] 终极的， 指最重要的或最强大的；
My experience as player , coach and manager has prepared me for this ultimate challenge. 我做球员、教练和经理人的经历已经使我为这终极挑战作好了准备。
4.adj. 形容词 You use ultimate to describe the most extreme and unpleasant example of a particular thing. 【adj+n终极的；指最极端的、最恶劣的
Bringing back the death penalty would be the ultimate abuse of human rights. 恢复死刑是对人权的最极端恶劣的践踏。
Treachery was the ultimate sin. 背叛是弥天大罪。
5. Adj. 形容词 You use ultimate to describe the best possible example of a paricular thing. [adj+n]终极的；指最好的
Exprience the ultimate adventure! 去体验最有刺激的冒险吧！
6. phrase 习语 The ultimate in something is the best or most advanced example of it. 极致；最佳典范；
Ballet is the ultimate in human movement. 芭蕾舞是人类动作的至美。
This hotel is the ultimate in Luxury. 这家酒店为豪华之典范。
Another widespread application class of the Internet is the delivery of "streaming"26 audio and video. Services such as vido on demand and Internet radio27 use this technology. While we frequently start at a website to initiate28 a streaming session29, the delivery30 of audio and video has some important differences from fetching31 a simple web page of text and images. For example, you often don't want to download an entire video file--a process that might take minutes to hours -- before watching the first scene. Streaming audio and video implies a more timely transfer of messages from sender to receiver, and the receiver displays the video or plays the audio pretty much as it arrives.
另一个广 范围使用的因特网应用程序分配“流”语音和视频。 像按需视频 和因特网发射台使用主这种技术。 像按需视频和因特网发射的服务使用这种技术。 当我们很频繁的打开网址， 初始化一段流会议， 音频和视频的传送有许多不同点和抓取一些简单的网页文本和图像。 例如，你经常会下载一整个视频文件--一个程序也许会花费几分钟甚至几个小时---在首次观看荧屏时。 流视频和音频提供了一种更即时的信息传输从发送端到接收端， 接收者看视频或者播放语音更好和它接收到。
26. stream CET 4 // streams, streaming, streamed 1. N-count可数名词 A stream is a narrow river. 溪流
There was a small stream at the end of the garden. 这个花园的尽头有一条小溪。
2. N-count 可数名词 A stream of smoke, air, or liquid is a narrow moving mass of it 一股烟、气或液体
He breathed out a stream of cigrette smoke. 他吐出一缕香烟的烟雾。
3. N-count 可数名词 A stream of vehicle or people is a long moving line of them. 流（车或人）
There was a stream of traffic behind him. 他身后有一股车流。
4. N-count 可数名词 A stream of things is large number of them occurring one after another.
The discovery triggered a stream of readers' letters. 这个发现引来了一连串的读者来信。 ... a never-ending stream of jokes. 一个接连不断的笑话。
5. VI 不及物动词 If a liquid streams somewhere, it flows or comes out in large amounts. 流
Tears streams somewhere, it flows or comes out in large amounts.流
tears streamed down their faces. 泪水顺着他们的脸颊流下来。
6. VI 不及物动词 If your eyes were streaming, liquid is coming from them, for example because you have a cold. You can also say that your nose is streaming. 【usu cont】流眼泪；流鼻涕。
Her eyes were streaming now from the wind. 她的眼睛现在被风吹得直流泪。
7. VI 不及物动词 If people or vehicles stream somewhere, they move there quickly and in large number. 大批流动人或车辆
Refugees have been streaming into Travnik for months. 难民几个月来一直大量流入特拉弗尼克。
8. VI 不及物动词 When light streams into or out of a place , it shines strongly into or out of it.
Sunlight was streaming into the countryard. 阳光正照进那院子。
9. Phrase 习语
If something such as a new factory or a new system comes on stream or is brought on stream, it begins to operate or becomes available. 投入生产
As new mines come on stream , Chile's share of wold copper output will increase sharply. 随着新矿的投产，智 利在世界铜 产量中所占的分额大幅度增加。
27. radio /reidiəu/ CET4 radios, radioing, radioed
1. N-uncount 不可数名词 radio is the broadcasting of programs for the public to listen to, by sending out signals from a transmitter. 无线电广播
The last 12 months have been difficult ones for local radio. 最近的12个月对地方广播电台来说是艰难的。
2. N-sing 单数型名词 You can reffer to the programs broadcast by radio stations as the radio. 无线电广播节目
A lot of people listen to the radio in the morning. 很多人在早上收听广播节目。
3. N-COUNT 可数名词 A radio is the piece of equipment that you use in order to listen to radio to radio programs. 收音机
He sat down in the armchair and turned on the radio. 他在扶手椅上坐下来，打开了收音机。
4. N-uncount 不可数名词 radio is a system of sending sound over a distance by transmitting eletrical signals. 无线电通信系统。
They are in twice daily radio contact with the rebel leader. 他们一天两次通过无线电通信系统与反叛者领导人保持联系。
5. N-count 可数名词 A radio is a piece of equipment that is used for sending and receiving messages.无线电收发装置
Judge Bruce Laughland praised the courage of the young policeman, who managed to raise the alarm on his radio. 布鲁斯. 劳赫兰法官赞扬了那名年轻的警察，该警察设法通过自己的无线电收发装置发出了报警。
6. VT/VI及物动词/不及物动词
If you radio someone, you send a message to them by radio. 用无线电发送。
The officer radioed for advice. 那名军官发电请求指示。
28. initiating /iniʃieit/ CET6 initiates initiating initiated 1. vt 及物动词 If you initiate something, you start it or cause it to happen. 使开始；发起
They wanted to initiate someone into something, you introduce them to a particular skill or type of knowledge and teach them about it. 使初步了解；传授；
He initiated her into the study of other cultures. 他引导她进入对于其他文化的研究。
3. VT及物动词 If someone is initiated into something such as a religion, secret society, or social group, they become a member of it by taking part in special ceremonies. 使加入；常通过特殊仪式
In many societies, young people are formally initiated into their adult roles. 在很多社会， 年轻人要通过正式的仪式进入成年。
29. session/səʃən/ CET4 sessions 1. N-count 不可数名词： A session is a meeting of a court, legislature, or other official group. 开庭，会议；
After two late night seesions, the Security council has failed to reach agreement. 开过两次深夜会议之后，安理会仍未能达到协议。
The Arab League is meeting in emergency session today. 阿拉伯国家联盟今天正在举行紧急会议。
2. N-count 可数名词 A session is a period during which the meetings of a court, legislature, or other official group are regularly held. [also 'in' N] 开庭期；会期；
From September until December, Congress remained in session. 9月到12月国会一直处在开会期。
3. N-count 可数名词 A session of a particular activity is a period of that activity. 一场活动；
The two leaders emerged for a photo session. 两位领导人参加了合影。
30. delivery CET4 /dilivəri/ deliveries 1.N-var 有变体名词 delivery or a delivery is the bringing of letters, packages, or other goods to someone's house or to another plaace where they want them. 递送
Please allow 28 days for delivery. 请留出28天的递送时间。
The uprising is the threatening the delivery of humanitarian supplies of food and medicine. 这场暴乱正威胁到食物和药品等人道主义物资的运送。
2. N-var 有变体名词 delivery is the process of giving birth to a baby. 分娩
In the end, it was an easy delivery: a fine baby boy. 最终，分娩顺利---一个健康的男婴。
3. N-count 可数名词 A delivery of something is the goods that are delivered. 递送的货物。
I got a delivery person or service delivers things to a place. 今天早上，我收到了一批新鲜的鸡蛋。
4. adj. 形容词 A delivery persons or service delivers things to a place. [adj+n] 送货的
... a pizze delivery man. ... 一个送比萨饼的男子。
5. N-uncount 不可数名词 You talk about someone's delivery when you are referring to the way in which they give a speech or lecture. 演讲方式
His speeches were magnificently written but his delivery was hopeless. 他的演讲辞写的很好，但他的演讲却不可救药。
31. fetch/fətʃ/ CET 4 fetches, fetching, fetched
1. VT 及物动词 If you fetch something or someone, you go and get them from the place where they are. 去拿
Sylvia fetched a towel from the bathroom. 西尔娅从浴室拿了一条毛巾。
Fetch me a glass of water. 给我拿杯水。
2. VT 及物动词 If something fetches a particular sum of money, it is sold for that amount. 卖得
The painting is expected to fetch between two and three million dollar. 这幅画预计能卖到200至300万美元
Note that the difference between streaming applications and the more traditional delivery of a page of text or still image is that humans consume32 audio and video streams in a continuous manner, and discontinuity--- in the form of skipped33 sounds or stalled video--- is not acceptable. By constrast, a page of text can be delivered and read in bits and pieces. This difference affects how the network supports these different classes of applications.
32. consume// CET4 consumes, consuming, cconsumed 1. VT及物动词 Formal 正式 If you consume something, you eat or drink it. 吃喝
Martha would consume nearly a pound of cheese per day. 玛莎那时每天吃将近一磅奶酪。
2. VT及物动词 To consume an amount of fuel, energy, or time means to use it up 消耗
Some of the most efficient refrigerators consume 70 percent less electricity than traditional models. 一些能效高的冰箱比传统型号少消耗70%。
33. skip/skip/ CET6 Skips, skipping, skipped 1. VI 不及物动词 If you skip along, you move almost as if you are dancing, with a series of little jumps from one foot to the another. 蹦跳；蹦跳着走
They saw the man with a little girl skipping along behind him. 他们看见那个男人身后跟着一个蹦蹦跳跳的小女孩。
We went skipping down the street arm in arm. 我们挽着臂蹦蹦跳跳地沿着大街走着。
2. N-count 可数名词 skip is also noun. 蹦跳
The boxer gave a little skip as he came out of his corner. 那位拳手轻快地跳出了场角。
3. VT 及物动词 When someone skips rope, they jump up and down over a rop which they or two other people are holding at each end and turning around and around. 跳绳。
They skip rope and play catch, waiting for the bell. 他们又跳绳又是捉迷藏，等着铃声响。
skip+ping
N-uncount 不可数名词
跳绳
They did rope skipping and things like that. 我们跳绳或做类似的活动
VT 及物动词 If you skip something that you usually do or something that most people do, you decide not to do it. 不做 逃避
It is important not to skip meals. 重要的是， 不能不吃饭。
VT/VI 及物动词 /不及物动词
If you skip or skip over a part of something you are reading or a story you are telling, you miss it out or pass over it quickly and move on to something else. 匆匆翻阅，略过
You might want to skip the exercises in this chapter. 你可能想把这一章的练习跳过去。
VI 不及物动词
If you skip from one subject or activity to another, you move quickly from one to the other, although there is no obvious connection between them. 快速转换；无条理地；
She kept up a continuous chatter, skipping from one subject to the next. 她叽叽咕咕说个不停， 从一个话题跳到另一个话题。
N-count 可数名词 Brit 英国英语
A
A subtly different application class is real-time audio and video. These applications have considerably tighter timing constraints than streaming applications. When using a voice-over-IP application such as SkypeTM or a video conferencing application, the inter-action among the participants must be timely. When a person at one end gestures, then that action must be displayed at the other end as quickly as possible. When one person tries to interrupt another, the interrupted person needs to hear that as soon as possible1 and decide whether to allow the interruption or to keep talking over the interrupter.