外文翻譯--一個(gè)互聯(lián)網(wǎng)的內(nèi)部工作指南

1、<p>　　9900單詞，4.9萬英文字符，1.6萬漢字</p><p><b>　　外文翻譯</b></p><p><b>　　外文原文</b></p><p>　　An Insider’s Guide to the Internet </p><p>　　David D. Clark

2、</p><p>　　M.I.T. Computer Science and Artificial Intelligence Laboratory</p><p>　　Version 2.0 7/25/04</p><p>　　Almost everyone has heard of the Internet. We cruise the web, we watch

3、 the valuation of Internet companies on the stock market, and we read 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

4、 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 does. </p><p>　　Don’t forget—the Internet is not the World Wide Web

5、, 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 pa

6、ge. </p><p>　　1. Introduction to the Internet </p><p>　　The Internet is a communications facility designed to connect computers together so that they can exchange digital information. For this p

7、urpose, the Internet provides a basic communication service that conveys units of information, called packets, from a source computer attached to the Internet to one or more destination computers attached to the Internet

8、. Additionally, the Internet provides supporting services such as the naming of the attached computers. A number of high-level services or app</p><p>　　The Internet differs in important ways from the network

9、s 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

10、application. It may seem obvious that the telephone system 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.

11、 The </p><p>　　The Internet is not a specific communication “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 inten

12、tionally designed to allow as many technologies as possible to be exploited as part of the Internet, and to incorporate new technologies as they are invented. In the early days of the Internet, it was deployed using tech

13、nologies </p><p>　　(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 wire

14、less that are tailored specifically to the needs of the Internet—they were designed from the ground up to carry packets. </p><p>　　2. Separation of function </p><p>　　If the Internet is not a sp

15、ecific communications technology, nor for a specific purpose, what is it? Technically, its core is a very simple and minimal specification that describes its basic communication model. Figure 1 provides a framework that

16、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 des

17、ign goal of the Internet was to a</p><p>　　The 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

18、need to know the details of each technology, 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 det

19、ails of the applications and the details of the technologies are separated, so that each can evolve independently. </p><p>　　2 . 1 . The basic communication model of the Internet </p><p>　　The b

20、asic 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

21、system, and the sender identifies the 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 deli

22、very contract of the Internet is that the Internet will</p><p>　　This very indefinite and non-committal delivery contract has both benefit and risk. The benefit is that almost any underlying technology can i

23、mplement it. The risk of this vague 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 a

24、dequate in practice. As is discussed below, this simple service model does have limits, and it is being extended to deal with new objectiv</p><p>　　2 . 2 . Layering, not integration. </p><p>　　T

25、he 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 of the underlying tec

26、hnology. 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 carr

27、y telephone calls, make decisions that optimize that goal i</p><p>　　2 . 3 . Protocols </p><p>　　The word protocol is used to refer to the conventions and standards that define how each layer of

28、 the Internet operates. The Internet layer discussed above is specified in a document that defines the format of the packet headers, the control messages that can be sent, and so on. This set of definitions is called the

29、 Internet Protocol, or IP. </p><p>　　Different bodies have created the protocols that specify the different parts of the Internet. The Internet Engineering Task Force, an open working group that has grown up

30、 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.

31、Some protocols are defined by academic and industry consortia; for example the protocols that specify the Worl</p><p>　　3. Forwarding data—the Internet layer </p><p>　　3 . 1 . The packet model &

32、lt;/p><p>　　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 delivery informatio

33、n 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 Intern

34、et. (The most common version of the software that does this is called Transmission</p><p>　　The Internet consists of a series of communication links connected by relay points called routers. Figure 2 illustr

35、ates this conceptual representation. As figure 3 illustrates, the communication links that connect routers in the Internet can be of many sorts, as emphasized by the hourglass. They all share the basic function that they

36、 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 destinat</p><p>　　Typically, a router is a compute

37、r, either general purpose or specially designed for this role, running software and hardware that implements the forwarding functions. A high-performance router used in the interior of the Internet may be a very expensiv

38、e 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 routers

39、 perform the same basic comm</p><p>　　A 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 p

40、roceeds in a series of hops using different technologies (e.g. 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

41、in the Internet is called datagram delivery. While the post-office analogy is imperfect in a number of ways, it</p><p>　　3 . 2 . Details of packet processing. </p><p>　　This section discusses in

42、 more detail the packet forwarding process introduced in the previous section.</p><p>　　The information relevant to packet forwarding by the router is contained in a part of the packet headercalled the Inter

43、net header. Each separate piece of the header is called a field of the header. The importantfields in the Internet header are as follows:</p><p>　　Source address: the Internet address of the origin of the pa

44、cket.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 smaller packets to complete

45、 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,

46、</p><p>　　Processing in the router </p><p>　　The processing of the packet by each router along the route from source to destination proceeds as follows, each step closely related to the fields d

47、iscussed above. </p><p>　　1) 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 packet’s tur

48、n to be processed, the router proceeds as follows. </p><p>　　2) The router performs the checksum computation, and compares the resulting value with the value placed in the packet by the source. If the two va

49、lues 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 follows. </p><p>　

50、　3) The router 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 chang

51、ed to reflect this altered value. </p><p>　　4) The router reads the destination address from the packet, and consults a table (the forwarding table) to determine on which of the communications links attached

52、 to the router the packet should next be sent. The router places the packet on the transmission queue for that link. </p><p>　　5) When the packet reaches the head of the transmission queue, the router transm

53、its the packet across the associated communications link, towards either a next router, or towards the computer that is the final destination of the packet. </p><p>　　Processing in the source and destination

54、 computers </p><p>　　The source and destination computers are also concerned with the fields in the Internet header of the packet, but the operations are a little different. </p><p>　　The source

55、 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 Do

56、main Name System, below), and, using rules that have been specified, must select a suitable hop count to put in the packet. </p><p>　　The destination computer verifies the values in the header, including the

57、 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 field. </p><p>　　As discuss

58、ed above, packets carried across the Internet can be used for a number of purposes, and depending on the intended use, one or another intermediate level protocol will be used to further process the packet. The most commo

59、n protocol is Transmission Control Protocol, or TCP, discussed below; other examples include User Datagram Protocol, or UDP, and Real Time Protocol, or RTP. Depending on which protocol is being used, the packet must be h

60、anded off to one or another piece of software in t</p><p>　　Internet control messages </p><p>　　When some abnormal situation arises, a router along a path from a sender to a receiver may send a

61、packet with a control message back to the original sender of the packet. This can happen when the hop count goes to zero and the packet is discarded, and in certain other circumstances when an error occurs and a packet i

62、s lost. It is not the case that every lost packet generates a control message--the sender is supposed to use an error recovery mechanism such as the one in TCP, discussed below, to d</p><p>　　3 . 3 . Packet

63、headers and layers. </p><p>　　The Internet header is not the only sort of header information in the packet. The information in the packet header is organized into several parts, which correspond to the layer

64、s, or protocols, in the Internet design. First comes information that is used by the low-level technology connecting the routers together. The format of this will differ depending on what the technology is: local area ne

65、twork, telephone trunk, satellite link and so on. Next in the packet is the information at the Internet</p><p>　　4. TCP -- intermediate level services in the end-node </p><p>　　The delivery cont

66、ract of the Internet is very simple: the best effort service tries its best to deliver all the packets given it by the sender, but makes no guarantees—it may lose packets, duplicate them, deliver them out of order, and d

67、elay them unpredictably. Many applications find this service difficult to deal with, because there are so many kinds of errors to detect and correct. For this reason, the Internet protocols include a transport service th

68、at runs “on top of” the basic Internet serv</p><p>　　4 . 1 . Detailed operation of TCP </p><p>　　TCP is a rather more complex protocol than IP. This discussion describes the important functions,

69、 but of necessity omits some of the details. Normally, a full chapter or more of a textbook is required to discuss all of TCP. </p><p>　　When TCP is in use, the packet carries a TCP header, which has informa

70、tion relevant to the functions of TCP. The TCP header follows the Internet header in the packet, and the higher-level protocol field in the Internet header indicates that the next header in the packet is the TCP header.

71、The fields in the header are discussed in the context of the related function. </p><p>　　Loss detection and recovery: Packets may be lost inside the network, because the routing computation has temporarily f

72、ailed and the packet has been delivered to the wrong destination or routed aimlessly until the hop count is decremented to zero, or because the header has been damaged due to bit errors on a </p><p>　　commun

73、ication link, or because a processing or transmission queue in a router is full, and there is no room to hold the packet within one of the routers. TCP must detect that a packet is lost, and correct this failure. It does

74、 so as follows. </p><p>　　Conceptually each byte transmitted is assigned a sequence number that identifies it. In practice, since a packet can carry a number of bytes, only the sequence number of the first b

75、yte is explicitly carried in the sequence number field of the TCP header. When each packet is received by the destination end node, the TCP software looks at the sequence number, and computes whether the bytes in this pa

76、cket are the next in order to be delivered. If so, they are passed on. If not the packet is either </p><p>　　The TCP at the destination sends a message back to the TCP at the origin, indicating the highest s

77、equence number that has been received in order. This information is carried in the acknowledgement field in the TCP header in a packet being transmitted back from destination of the data towards the source. If the source

78、 does not receive the acknowledgment in reasonable time, it transmits the data again, and this repeats until either some copy of the packet finally makes it to the destination, or th</p><p>　　Flow and conges

79、tion control: The term flow control describes the mechanism that attempts to insure that the sender of data does not transmit faster then the destination can receive. The term congestion control describes the mechanism t

80、hat attempts to insure that the sender of data does not transmit faster than the routers in the network can process and transmit the packets. A router that receives packets faster than it can </p><p>　　trans

81、mit them along the next communication link must hold those packets temporarily. A router that is holding a number of packets for transmission is called congested. Congestion control is a critical aspect of the Internet;

82、since any attached end-node can in principle transmit at will, it is possible for more packets to arrive at a router than can be carried across the outgoing communications link. Both flow control and congestion control a

83、re implemented in TCP. </p><p>　　Packets flowing back from the destination of the data to the source carry, in addition to the acknowledgment field, the flow control field. This field conveys a count of the

84、number of bytes that the sender can send that have not been acknowledged. In other words, at any instant, there are some number of bytes that the sender has transmitted to the destination, but for which the acknowledgmen

85、t has not yet arrived back. The sender must limit the number of such bytes to the value noted in the flow </p><p>　　The implementation of congestion control is rather more complex. Apart from the flow contro

86、l limit passed back from the receiver, the sender maintains another estimate of the suitable sending limit called the "congestion limit". The congestion limit is never allowed to grow larger than the flow contr

87、ol limit from the receiver, but is often smaller. When the sending TCP starts to transmit packets to the receiving TCP, it makes an initial guess as to a suitable congestion limit. The initial guess </p><p>

88、　　Error detection: a transmission error on a data link can damage a bit in a packet. The Internet protocol specifies a checksum function that is used to detect if a bit in the Internet header is altered, but this only ap

89、plies to that header. TCP employs a similar checksum function to validate the TCP header as well as all the data bytes that follow it in the packet. The sending TCP computes the checksum and stores it in the packet; the

90、receiving TCP recomputes it and compares this value with the o</p><p>　　Reliable open and close: The Internet service was described as a datagram service. At that level, the sender transmits a packet to a de

91、stination address, and need not know whether that destination is prepared to receive it, or indeed whether it even exists. Most high-level services need to know that the receiver is there and functioning. For this reason

92、, before data is exchanged, the TCP software at the two ends of the communication exchange a sequence of packets with each other (containing no d</p><p>　　TCP can carry data in two directions at once. Each e

93、nd can send and receive at the same time, with separate flow and congestion limits in each direction. Packets carrying data in one direction can at the same time acknowledge data flowing in the other direction. </p>

94、;<p>　　Overall, the resulting behavior of TCP is as follows. When some software at the sending end gives TCP a sequence of bytes to transfer to the receiver, the sending TCP opens a connection and starts to break

95、these bytes into packets, attaching a TCP header to each and then handing them on to the Internet software for formatting and forwarding as described in section 3. TCP continues to transmit these packets so long as ackno

96、wledgments arrive. Based on the pattern of acknowledgments and losses, it </p><p>　　4 . 2 . Other intermediate protocols </p><p>　　TCP is the most commonly used protocol to enhance the basic com

97、munication service of the Internet, but it is not the only one. In some cases, the high-level application works best if it is built directly on the basic IP communication service. In this case, a simple interface to that

98、 service is used, called the User Datagram Protocol, or UDP. For real time services that are concerned more with delivery within a delay bound rather than completely reliable delivery, an alternative to TCP has been d<