一个互联网的内部工作指南外文翻译.doc

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1、一个互联网的内部工作指南外文翻译 毕业论文(设计)外文翻译外文原文An Insiders Guide to the Internet David D. Clark M.I.T. Computer Science and Artificial Intelligence LaboratoryVersion 2.0 7/25/04 Almost everyone has heard of the Internet. We cruise the web, we watch the valuation of Internet companies on the stock market, and we r

2、ead the pundits predictions about what will happen next. But not many people actually understand what it is and how it works. Take away the hype, and the basic operation of the Internet is rather simple. Here, in a few pages, is an overview of how it works inside, and why it works the way it doesDon

3、t forget?the Internet is not the World Wide Web, or e-mail. The Internet is what is “underneath” them, and makes them all happen. This paper describes what the Internet itself is, and also tells what actually happens, for example, when you click on a link in a Web page1. Introduction to the Internet

4、 The Internet is a communications facility designed to connect computers together so that they can exchange digital information. For this purpose, the Internet provides a basic communication service that conveys units of information, called packets, from a source computer attached to the Internet to

5、 one or more destination computers attached to the Internet. Additionally, the Internet provides supporting services such as the naming of the attached computers. A number of high-level services or applications have been designed and implemented making use of this basic communication service, includ

6、ing the World Wide Web, Internet e-mail, the Internet newsgroups, distribution of audio and video information, and file transfer and login between distant computers. The design of the Internet is such that new high-level services can be designed and deployed in the futureThe Internet differs in impo

7、rtant ways from the networks in other communications industries such as telephone, radio or television. In those industries, the communications infrastructure-wires, fibers, transmission towers and so on?has been put in place to serve a specific application. It may seem obvious that the telephone sy

8、stem was designed to carry telephone calls, but the Internet had no such clear purpose. To understand the role of the Internet, consider the personal computer, or PC. The PC was not designed for one application, such as word processing or spreadsheets, but is instead a general-purpose device, specia

9、lized to one use or another by the later addition of software. The Internet is a network designed to connect computers together, and shares this same design goal of generality. The Internet is a network designed to support a range of applications, depending on what software is loaded into the attach

10、ed computers, and what use that software makes of the Internet. Many communication patterns are possible: between pairs of computers, from a server to many clients, or among a group of co-operating computers. The Internet is designed to support all these modesThe Internet is not a specific communica

11、tion “technology”, such as fiber optics or radio. It makes use of these and other technologies in order to get packets from place to place. It was intentionally designed to allow as many technologies as possible to be exploited as part of the Internet, and to incorporate new technologies as they are

12、 invented. In the early days of the Internet, it was deployed using technologies e.g. telephone circuits originally designed and installed for other purposes. As the Internet has matured,we see the design of communication technologies such as Ethernet and 802.11 wireless that are tailored specifical

13、ly to the needs of the Internet?they were designed from the ground up to carry packets2. Separation of function If the Internet is not a specific communications technology, nor for a specific purpose, what is it? Technically, its core is a very simple and minimal specification that describes its bas

14、ic communication model. Figure 1 provides a framework that is helpful in understanding how the Internet is defined. At the top of the figure, there is a wide range of applications. At the bottom is a wide range of technologies for wide area and local area communications. The design goal of the Inter

15、net was to allow this wide range of applications to take advantage of all these technologiesThe heart of the Internet is the definition of a very simple service model between the applications and the technologies. The designer of each application does not need to know the details of each technology,

16、 but only this basic communication service. The designer of each technology must support this service, but need not know about the individual applications. In this way, the details of the applications and the details of the technologies are separated, so that each can evolve independently21The basic

17、 communication model of the Internet The basic service model for packet delivery is very simple. It contains two parts: the addresses and the delivery contract. To implement addressing, the Internet has numbers that identify end points, similar to the telephone system, and the sender identifies the

18、destination of a communication using these numbers. The delivery contract specifies what the sender can expect when it hands data over to the Internet for delivery. The original delivery contract of the Internet is that the Internet will do its best to deliver all the data given to it for carriage,

19、but makes no commitment as to data rate, delivery delay, or loss rates. This service is called the best effort delivery modelThis very indefinite and non-committal delivery contract has both benefit and risk. The benefit is that almost any underlying technology can implement it. The risk of this vag

20、ue contract is that applications cannot be successfully built on top of it. However, the demonstrated range of applications that have been deployed over the Internet suggests that it is adequate in practice. As is discussed below, this simple service model does have limits, and it is being extended

21、to deal with new objectives such as real time delivery of audio and video22Layering, not integrationThe design approach of the Internet is a common one in Computer Science: provide a simplified view of complex technology by hiding that technology underneath an interface that provides an abstraction

22、of the underlying technology. This approach is often called layering. In contrast, networks such as the telephone system are more integrated. In the telephone system, designers of the low level technology, knowing that the purpose is to carry telephone calls, make decisions that optimize that goal i

23、n all parts of the system. The Internet is not optimized to any one application; rather the goal is generality, flexibility and evolvability. Innovation can occur at the technology level independent of innovation at the application level, and this is one of the means to insure that the Internet can

24、evolve rapidly enough to keep pace with the rate of innovation in the computer industry23Protocols The word protocol is used to refer to the conventions and standards that define how each layer of the Internet operates. The Internet layer discussed above is specified in a document that defines the f

25、ormat of the packet headers, the control messages that can be sent, and so on. This set of definitions is called the Internet Protocol, or IPDifferent bodies have created the protocols that specify the different parts of the Internet. The Internet Engineering Task Force, an open working group that h

26、as grown up along with the Internet, created the Internet Protocol and the other protocols that define the basic communication service of the Internet. This group also developed the protocols for early applications such as e-mail. Some protocols are defined by academic and industry consortia; for ex

27、ample the protocols that specify the World Wide Web are mostly developed by the World Wide Web Consortium the W3C hosted at the Computer Science and Artificial Intelligence laboratory at MIT. These protocols, once developed, are then used as the basis of products that are sold to the various entitie

28、s involved in the deployment and operation of the Internet.3. Forwarding data?the Internet layer 31The packet model Data carried across the Internet is organized into packets, which are independent units of data, no more than some specified length 1000 to 2000 bytes is typical, complete with deliver

29、y information attached. An application program on a computer that needs to deliver data to another computer invokes software that breaks that data into some number of packets and transmits these packets one at a time into the Internet. The most common version of the software that does this is called

30、 Transmission Control Protocol, or TCP; it is discussed below. The Internet consists of a series of communication links connected by relay points called routers. Figure 2 illustrates this conceptual representation. As figure 3 illustrates, the communication links that connect routers in the Internet

31、 can be of many sorts, as emphasized by the hourglass. They all share the basic function that they can transport a packet from one router to another. At each router, the delivery information in the packet, called the header, is examined, and based on the destination address, a determination is made

32、as to where to send the packet next. This processing and forwarding of packets is the basic communication service of the InternetTypically, a router is a computer, either general purpose or specially designed for this role, running software and hardware that implements the forwarding functions. A hi

33、gh-performance router used in the interior of the Internet may be a very expensive and sophisticated device, while a router used in a small business or at other points near the edge of the network may be a small unit costing less than a hundred dollars. Whatever the price and performance, all router

34、s perform the same basic communication function of forwarding packetsA reasonable analogy to this process is the handling of mail by the post office or a commercial package handler. Every piece of mail carries a destination address, and proceeds in a series of hops using different technologies e.g.

35、truck, plane, or letter carrier. After each hop, the address is examined to determine the next hop to take. To emphasize this analogy, the delivery process in the Internet is called datagram delivery. While the post-office analogy is imperfect in a number of ways, it illustrates a number of other fe

36、atures of the Internet: the post office carries out other services to support the customer besides the simple transport of letters, and the transport of letter requires that they sometimes cross jurisdictional boundaries, in particular between countries32Details of packet processingThis section disc

37、usses in more detail the packet forwarding process introduced in the previous section.The information relevant to packet forwarding by the router is contained in a part of the packet headercalled the Internet header. Each separate piece of the header is called a field of the header. The importantfie

38、lds in the Internet header are as follows:Source address: the Internet address of the origin of the packet.Destination address: the Internet address of the destination of the packet.Length: the number of bytes in the packet.Fragmentation information: in some cases, a packet must be broken into small

39、er packets to complete itsprogress across the Internet. Several fields are concerned with this function, which is not discussed here.Header checksum: an error on the communications link might change the value of one of the bits in thepacket, in particular in the Internet header itself. This could al

40、ter important information such as the destination address. To detect this, a mathematical computation is performed by the source of the packet tocompute a checksum, which is a 16-bit value derived from all the other fields in the header. If any one ofthe bits in the header is modified, the checksum

41、computation will yield a different value with highprobability.Hop count: technically known as the time to live field. In rare cases, a packet may not proceed directlytowards the destination, but may get caught in a loop, where it could travel repeatedly among a series of routers. To detect this situ

42、ation, the packet carries an integer, which is decremented at each router. If this value is decremented to zero, the packet is discardedProcessing in the router The processing of the packet by each router along the route from source to destination proceeds as follows, each step closely related to th

43、e fields discussed above1 The packet is received by the router from one of the attached communications links, and stored in the memory of the router until it can be processed. When it is this packets turn to be processed, the router proceeds as follows2 The router performs the checksum computation,

44、and compares the resulting value with the value placed in the packet by the source. If the two values do not match, the router assumes that some bits in the Internet header of the packet have been damaged, and the packet is discarded. If the checksum is correct, the router proceeds as follows3 The r

45、outer reads the hop count in the packet, and subtracts one from it. If this leads to a result of zero, the packet is discarded. If not, this decremented value is put back in the packet, and the checksum is changed to reflect this altered value4 The router reads the destination address from the packe

46、t, and consults a table the forwarding table to determine on which of the communications links attached to the router the packet should next be sent. The router places the packet on the transmission queue for that link5 When the packet reaches the head of the transmission queue, the router transmits

47、 the packet across the associated communications link, towards either a next router, or towards the computer that is the final destination of the packetProcessing in the source and destination computers The source and destination computers are also concerned with the fields in the Internet header of

48、 the packet, but the operations are a little differentThe source computer creates the Internet header in the packet, filling in all the fields with the necessary values. The source must have determined the correct destination address to put in the packet see the discussion on the Domain Name System,

49、 below, and, using rules that have been specified, must select a suitable hop count to put in the packetThe destination computer verifies the values in the header, including the checksum and the source address. It then makes use of an additional field in the Internet header that is not relevant when the router forwards the packet: the next-level protocol fieldAs discussed above, packets carried across the Internet can be used for a number of purposes, and depending on the intended use, one or another intermediate l