软件工程毕业论文外文文献—办公自动化系统

1、外文原文MODELING INTERNET TOPOLOGYAbstractThe topology of a network, or a group of networks such as the Internet, has a strong bearing on many management and performance issues. Good models of the topological structure of a network are essential for developing and analyzing internetworking technology. This article discusses how graph-based models can be used to represent the topology of large networks, particularly aspects of locality and hierarchy present in the Internet. Two implementations that g

2、enerate networks whose topology resembles that of typical internetworks are described, together with publicly available source code.1 IntroductionThe explosive growth of networking, and particularly of the Internet, has been accompanied by a wide range of internetworking problems related to routing, resource reservation, and administration. The study of algorithms and policies to address such problems often involves simulation or analysis using an abstraction or model of the actual network struc

3、ture. The reason for this is clear: networks that are large enough to be interesting are also expensive and difficult to control; therefore they are rarely available for experimental purposes. Moreover, it is generally more efficient to assess solutions using analysis or simulation - provided the model is a “good” abstraction of the real network. The topological structure of a network is typically modeled using a graph, with nodes representing switches or routers, and edges representing direct c

4、onnections (transmission links or networks) between switches or routers. Thus, the graph models paths -sequences of nodes-along which information flows between nodes in an internetwork. For example, a FDDI ring to which four IP routers are connected would be represented as a completely connected graph of four nodes. Hosts can also be represented as nodes; the typical host will be represented as a leaf connected to a single router node. Additional information about the network can be added to the

5、 topological structure by associating information with the nodes and edges. For example, nodes might be assigned numbers representing buffer capacity. An edge might have values of various types, including costs, such as the propagation delay on the link, and constraints, such as the bandwidth capacity of the link. The purpose of this article is to review the basic topological structure of the Internet, then present a modeling method designed to produce graphs that reflect the locality and hierar

6、chy present in the Internet. Two implementations of the method are available; each produces graphs according to the basic method. The differences between the implementations may be of importance in choosing an implementation for use. 1.1 Structure of the InternetHistorically, large networks such as the Public Switched Telephone Network have grown according to a topological design developed by some central authority or administration. In contrast, there is no central administration that controls

7、|or even keeps track of-the detailed topology of the Internet. Although its general shape may be influenced to some small degree by policies for assignment of IP addresses and government funding of interdomain exchange points, the Internet, for the most part, just grows. The technology used to route and forward packets is explicitly designed to operate in such an environment. Todays Internet can be viewed as a collection of interconnected routing domains. Each routing domain is a group of nodes

8、(routers, switches and hosts), under a single (technical) administration, that share routing information and policy. Each routing domain in the Internet can be classified as either a stub domain or a transit domain. A stub domain carries only traffic that originates or terminates in the domain. Transit domains do not have this restriction. The purpose of transit domains is to interconnect stub domains efficiently; without them, every pair of stub domains would need to be directly connected to ea

9、ch other. (See Figure 1.) Stub domains generally correspond to campus networks or other collections of interconnected LANs, while transit domains are almost always wide-or metropolitan-area networks (WANs or MANs). Figure 1-1. Example of Internet domain structureA transit domain consists of a set of backbone nodes. In a transit domain each backbone node may also connect to a number of stub domains, via gateway nodes in the stubs. Some backbone nodes also connect to other transit domains. Stub domains can be further classified as single-or multi-homed. Multi-homed stub domains have connections to more than one transit domain; single-homed stubs connect to only one transit domain. Some stubs domains may have links to other stubs. Transit domains may themselves be organized in hierarchies, e.g. MANs connect mainly to stubs domains and WANs. 1.2 Existing Topology ModelsOne of the most commonly used models for generating random networks algor

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