2895.D游程编码的设计及性能研究 外文翻译.doc

大学通达学院毕业设计(论文)外文资料翻译学院(系)： 学院 专业： 计算机科学与技术 学生姓名： 班级学号： 外文出处：Internetworking with TCP/IP voll: principles,protocols,and Architecture Fourth Edition附件：1.外文资料翻译译文；2.外文原文指导教师评语：指导教师签名：年月日Internet Introduction And ManagementInternet communication has become a fundamental part of life. The World Wid Web conditions information about such diverse subjects as atmospheric conditions, crop production, stock prices, and airline traffic. Groups establish electronic mailing lists shey can share information of common interest. Professional colleagues exchange business correspondence electronically, and relatives exchange personal greetings. Unfortunately, most network technologies are designed for a specific purpose. Each enterprise chooses hardware technology appropriate for specific communication needs and budget. More important, it is impossible to engineer a universal network from a single network technology because no single network suffices for all uses. Som groups need high-speed networks to connect computers in a single building. Low-cos technologies that fill the need cannot span large geographic distances. Other groups settle for a slower speed network that connects machines thousands of miles apart. For over two decades, a new technology has evolved that makes it possible to in terconnect many disparate physical networks and make them function as a coordinate unit. The technology, called internetworking, accommodates multiple, diverse underlying hardware technologies by providing a way to interconnect heterogeneous networksand a set of communication conventions that makes them interoperate. The internet technology hides the details of network hardware, and permits computers to communicate independent of their physical network connections. To appreciate internet technology, think of how it has changed business. In addition to high-speed communication among employees in the office environment, networking technologies provide instant feedback among the production side of the business, sales and marketing, and customers. As a result, the speed with which business can plan, implement, assess, and retool has increased; the change is dramatic.The term server applies to any program that offers a service that can be reached a network . A server accepts a request over the network, performs its service, and returns the result to the requester. For the simplest services, each request arrives in a IP datagram and the server returns a response in another datagram. An executing program becomes a client when it sends a request to a server and waits for a response. Because the client-server model is a convenient and natural extension of interprocess communication on a single machine, it is easy to build programs that use the model to interact. Servers can perform simple or complex tasks. For example, a time-of-day server merely returns the current time whenever a client sends the server a packet. A web server receives requests from a browser to fetch a copy of a Web page; the server obtains a copy of the file for the page and returns it to the browser. Usually, servers are implemented as application programst. The advantage of implementing servers as application programs is that they can execute on any computing system that supports TCP/IP communication. Thus, the server for a particular service can execute on a timesharing system along with other programs, or it can execute on a personal computer. Multiple servers can offer the same service, and can execute on the In fact, managers commonly replicate copies of same machine or on multiple machines. a given server onto physically independent machines to increase reliability or improve performance. If a computer's primary purpose is support of a particular server program, the term "server" may be applied to the computer as well as to the server program. Thus, one hears statements such as "machine A is our file server."Other groups around the world continue to connect to the Internet as well. The most significant change comes not from added network connections, however, but from additional traffic. As new users connect to the Internet and new applications appear, traffic patterns change. When physicists, chemists, and biologists began to use the Internet, they exchanged files of data collected from experiments. Files of scientific data were large compared to electronic mail messages. As the Internet became popular and users began to browse information using services like the World Wide Web, traffic patterns increased again. To accommodate growth in traffic,the capacity of the NSFNET backbone was increased three times.The final version, known as ANSNET after the company that supplied it, had a capacity approximately 840 times larger than the original.Since 1995,companies known as Internet Service Providers (ISPs) have each built their own backbone network, many of which have significantly more capacity than the last government-funded backbone. At the current time, it is difficult to foresee an end to the need for more capacity.Growth in demands for networking is not unexpected. The computer industry has enjoyed a continual demand for increased processing power and larger data storage for many years. Users have only begun to understand how to use networks. In the future we can expect continual increases in the demand for communications. Soon, for example, TCP/IP technologies will be used for telephone and video services.as well as data be needed to accom modate the growth services. Thus, higher-capacity communication technologies will be needed to accommodate the growth.Properties Of The InternetThe notion of universal service is important, but it alone does not capture all the ideas we have in mind for a unified internet because there can be many implementations of universal services. In our design, we want to hide the underlying internet architecture from the user. That is, we do not want to require users or application programs to understand the details of hardware interconnections to use the internet. We also do not want to mandate a network interconnection topology. In particular, adding a new network to the internet should not mean connecting to a centralized switching point, nor should it mean adding direct physical connections between the new network and all existing networks. We want to be able to send data across intermediate networks even though they are not directly connected to the source or destination computers. We want all computers in the internet to share a universal set of machine identifiers (which can be thought of as names or addresses). Our notion of a unified internet also includes the idea of network independence in the user interface. That is, we want the set of operations used to establish cornrnunication or to transfer data to remain independent of the underlying network technologies and the destination computer. Certainly, a user should not have to understand the network interconnection topology when creating or using application programs that communicate.Internet ArchitectureWe have seen how computers connect to individual networks. The question arises, "How are networks interconnected to form an internetwork?" The answer has two parts. Physically, two networks can only be connected by a computer that attaches to both of them. A physical attachment does not provide the interconnection we have in mind, however, because such a connection does not guarantee that the computer will cooperate with other machines that wish to communicate. To have a viable internet, we need special computers that are willing to transfer packets from one network to another. Computers that interconnect two networks and pass packets from one to the other are called internet gateways or internet routersf. From the user's point of view, the Internet appears to consist of a set of application programs that use the underlying network to carry out useful communication tasks. We use the tern1 interoperability to refer to the ability of diverse computing systems to cooperate in solving computational problems. Internet application programs exhibit a high degree of interoperability. Most users that access the Internet do so merely by running application programs without understanding the types of computers being accessed, the TCP/IP technology, the structure of the underlying internet, or even the path the data travels to its destination; they rely on the application programs and the underlying network software to handle such details. Only programmers who write network applica tion programs need to view a TCP/IP internet as a network and need to understand some of the technology. The most popular and widespread Internet application services include:l World Wide Web. The Web allows users to view documents that contain text and graphics, and to follow hypermedia links from one document to another. The Web grew to become the largest source of traffic on the global Internet between 1994 and 1995, and continues to dominate. Some service providers estimate that the Web now accounts for 80% of their Internet traffic. l Electronic mail (e-mail). Electronic mail allows a user to compose a memo and send a copy to individuals or groups. Another part of the mail application allows users to read memos that they have received. A recent innovation allows users to include "attachments" with a mail message that consist of arbitrary files. Electronic mail has been so successful that many Internet users depend on it for most correspondence. One reason for the popularity of Internet e-mail arises from a careful design: the protocol makes delivery reliable. Not only does the mail system on the sender's computer contact the mail system on the receiver's computer directly, but the protocol specifies that a message cannot be deleted by the sender until the receiver has successfully placed a copy on permanent storage. l File transfer. The file transfer application allows users to send or receive a copy of a data file. File transfer is one of the oldest, and still among the most heavily used application services in the Internet. Although small files can now be attached to an e-mail message, the file transfer service is still needed to handle arbitrarily large files. The system provides a way to check for authorized users, or even to prevent all access. Like mail, file transfer across a TCPAP internet is reliable because the two machines involved communicate directly, without relying on intermediate machines to make copies of the file along the way. l Remote login. Remote login allows a user sitting at one computer to connect to a remote machine and establish an interactive login session. The remote login makes it appear that a window on the user's screen connects directly to the remote machine by sending each keystroke from the user's keyboard to the remote machine and displaying each character the remote computer prints in the user's window. When the remote login session terminates, the application returns the user to the local system.Network-Level Internet ServicesA programmer who creates application programs that use TCP/IP protocols has an entirely different view of an internet than a user who merely executes applications like electronic mail.At the network level, an internet provides two broad types of service that all application programs use. While it is unimportant at this time to understand the details of these services, they cannot be omitted from any overview of TCP/IP: l Connectionless Packet Delivery Service. This service, explained in detail throughout the text, forms the basis for all other internet services. Connectionless delivery is an abstraction of the service that most packet-switching networks offer. It means simply that a TCPAP internet routes small messages from one computer to another based on address information camed in the message. Because the connectionless service routes each packet separately, it does not guarantee reliable, in-order delivery. Because it usually maps directly onto the underlying hardware, the connectionless service is extremely efficient. More important, having connectionless packet delivery as the basis for all internet services makes the TCP/IP protocols adaptable to a wide range of network hardware.l Reliable Stream Transport Service. Most applications need much more than packet delivery because they require the communication software to recover automatically from transmission errors, lost packets, or failures of intermediate switches along the path between sender and receiver. The reliable transport service handles such problems. It allows an application on one computer to establish a "connection" with an application on another computer, and then to send a large volume of data across the connection as if it were a permanent, direct hardware connection. Underneath, of course, the communication protocols divide the stream of data into small messages and send them, one at a time, waiting for the receiver to acknowledge reception. Many networks provide basic services similar to those outlined above, so one might wonder what distinguishes TCP/IP services from others. The primary distinuishing features are: l Network Technology Independence. Although TCP/IP is based on conventional packet switching technology, it is independent of any particular vendor's hardware. The global Internet includes a variety of network technologies ranging from networks designed to operate within a single building to those designed to span large distances.TCP/IP protocols define the unit of data transmission, called a datagram, and specify how to transmit datagrams on a particular network. l Universal Interconnection. A TCP/IP internet allows any pair of computers to which it attaches to communicate. Each computer is assigned an address that is universally recognized throughout the internet. Every datagram carries the addresses of its source and destination. Intermediate switching computers use the destination address to make routing decisions.l End-to-End Acknowledgements. The TCP/IP internet protocols provide acknowledgements between the original source and ultimate destination instead of between successive machines along the path, even if the source and destination do not connect to a common physical network. l Application Protocol Standarch. In addition to the basic transport-level services (like reliable stream connections), the TCP/IP protocols include standards for many common applications including electronic mail, file transfer, and remote login. Thus, when designing application programs that use TCP/IP, programmers ofte