互联网络联网的基本原理-5_路由(zslcn周生烈编译摘注评).doc

啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊 (fm.DocWiki CISCO / zslcn编译注)五Routing Basics 路由基本原理指导目录 Contents联网技术手册 指导目录 Part 1 Internetworking Basics 互联网络联网基本原理 Part 2 LAN Technologies 局域网技术 part 3 WAN Technologies 广域网技术 part 4 Internet Protocols 互联网协议 part 5 Bridging and Switching 桥接和交换 part 6 Routing 路由 part 7 Network Management 网络管理 part 8 Voice/Data Integration Technologies 声音/数据集成技术 part 9 Wireless Technologies 无线技术 part 10 Cable Access Technologies 有线接入技术 part 11 Dial-up Technology 拨号技术 part 12 Security Technologies 安全技术 part 13 Quality of Service Networking 联网服务质量 part 14 Network Caching Technologies 网络缓存技术 part 15 IBM Network Management IBM网络管理 part 16 Multiservice Access Technologies 多业务接入技术 /wiki/Routing_BasicsThis page was last modified on 17 December 2009, at 21:56五Routing Basics 路由基本原理From DocWikiJump to: navigation, searchThis article introduces the underlying concepts widely used in routing protocols. Topics summarized here include routing protocol components and algorithms. In addition, the role of routing protocols is briefly contrasted with the role of routed or network protocols. 本章介绍广泛用于路由协议的基本概念。在这里概述的论题包括路由协议的组成部分和算法。另外,将路由协议的作用与被路由或网络协议的作用作一简短的对比。在第VII部分的章节路由协议s,里,会更详细地讲述特殊的路由协议s,而使用路由协议s的网络协议,是在第VI部分,网络协议s,中讨论。Contentshide1 What Is Routing? 2 Routing Components 2.1 Path Determination 2.1.1 Figure: Destination/Next Hop Associations Determine the Datas Optimal Path 2.2 Switching 2.2.1 Figure: Numerous Routers May Come into Play During the Switching Process 3 Routing Algorithms 3.1 Design Goals 3.1.1 Figure: Slow Convergence and Routing Loops Can Hinder Progress 3.2 Algorithm Types 3.2.1 Static Versus Dynamic 3.2.2 Single-Path Versus Multipath 3.2.3 Flat Versus Hierarchical 3.2.4 Host-Intelligent Versus Router-Intelligent 3.2.5 Intradomain Versus Interdomain 3.2.6 Link-State Versus Distance Vector 3.2.7 Routing Metrics 4 Network Protocols 5 Review Questions 1. What Is Routing? 什么是路由？Routing is the act of moving information across an internetwork from a source to a destination. Along the way, at least one intermediate node typically is encountered. Routing is often contrasted with bridging, which might seem to accomplish precisely the same thing to the casual observer. The primary difference between the two is that bridging occurs at Layer 2 (the link layer) of the OSI reference model, whereas routing occurs at Layer 3 (the network layer). This distinction provides routing and bridging with different information to use in the process of moving information from source to destination, so the two functions accomplish their tasks in different ways. 路由就是一种作用,将信息从源经过互联网络移动到目的地。沿着这条通路,通常会遇到至少一个中间节点。路由通常与桥接相对照,在陌生的旁观者看来,它们精确地完成同一件事情。两者主要的不同在于,桥接发生在OSI参考模型第二层(链路层),而路由则发生在第三层(网络层)。这种不同提供给路由和桥接用不同的信息,来处理从源到目的信息移动的过程，所以这两种功能(路由和桥接)是用不同的方法来完成它们的任务的。The topic of routing has been covered in computer science literature for more than two decades, but routing achieved commercial popularity as late as the mid-1980s. The primary reason for this time lag is that networks in the 1970s were simple, homogeneous environments. Only relatively recently has large-scale internetworking become popular. 路由这个论题,出现在计算机科学文献里,已经有多于20年的时间,然而路由实现在商业上流行,则是迟至80年代中期。其原因在于70年代的网络s还是简单,同质的环境s。大规模互联网络联网的流行出现较晚。2. Routing Components 路由的组成部分Routing involves two basic activities: determining optimal routing paths and transporting information groups (typically called packets) through an internetwork. In the context of the routing process, the latter of these is referred to as packet switching. Although packet switching is relatively straightforward, path determination can be very complex. 路由涉及两种活动:确定优化路由通路s,和传输通过互联网络的信息组s(通常叫数据包s)。,在路由过程的上下文中,后一活动称为包交换。虽然包交换相对直观,通路确定却可能十分复杂。Path Determination 通路确定Routing protocols use metrics to evaluate what path will be the best for a packet to travel. A metric is a standard of measurement, such as path bandwidth, that is used by routing algorithms to determine the optimal path to a destination. To aid the process of path determination, routing algorithms initialize and maintain routing tables, which contain route information. Route information varies depending on the routing algorithm used. 路由协议使用度量来评价,这样使通路能最好地传输数据包。一次度量就是一次标准测量,如通路带宽,路由算法s使用它来确定到达一个目标的最佳路径。为了辅助路径确定的过程,路由算法s启动和维持包含路由信息的路由表s。路由信息的变化取决于所用的路由算法。Routing algorithms fill routing tables with a variety of information. Destination/next hop associations tell a router that a particular destination can be reached optimally by sending the packet to a particular router representing the next hop on the way to the final destination. When a router receives an incoming packet, it checks the destination address and attempts to associate this address with a next hop. 路由算法s将各种信息填入路由表s。目标/下一跳组合告诉路由器,一个具体的目标可以优化地到达,只要将数据包,沿着通向最终目标的通路,送到表示下一跳的具体路由器。当路由器接收到一个数据包时,它就检查目标地址,并在目标地址上结合下一跳相关地址。图5-1说明了一个例样目标/效益跳路由表。 Figure: Destination/Next Hop Associations Determine the Datas Optimal Path depicts a sample destination/next hop routing table. Figure: Destination/Next Hop Associations Determine the Datas Optimal PathRouting tables also can contain other information, such as data about the desirability of a path. Routers compare metrics to determine optimal routes, and these metrics differ depending on the design of the routing algorithm used. A variety of common metrics will be introduced and described later in this article. 路由表s也可以包含其他信息,如有关所希望通路的数据。路由器s比较度量值来确定优化由路,而这些度量值则取决于所使用的路由算法的设计,而有所不同。各种通用的度量s将被介绍,并在本章后面予以说明。Routers communicate with one another and maintain their routing tables through the transmission of a variety of messages. The routing update message is one such message that generally consists of all or a portion of a routing table. By analyzing routing updates from all other routers, a router can build a detailed picture of network topology. A link-state advertisement, another example of a message sent between routers, informs other routers of the state of the senders links. Link information also can be used to build a complete picture of network topology to enable routers to determine optimal routes to network destinations. 路由器s彼此通信,并通过传输各种消息来维持它们的路由表s。路由更新消息是一种这样的消息,它们构成了路由表的全部或部分。通过分析来自所有其它路由器的路由更新消息,一个路由器就可以建立一张详细的网络投票图。一个链路状态通告,路由器间发送消息的另一种例子,通知其它路由器,告知发送者链路s的状态。链路信息也可以用来建立一张完整的网络拓扑图,以使路由器s能确定由路到网络目标s的优化通路。Switching 交换Switching algorithms is relatively simple; it is the same for most routing protocols. In most cases, a host determines that it must send a packet to another host. Having acquired a routers address by some means, the source host sends a packet addressed specifically to a routers physical (Media Access Control MAC-layer) address, this time with the protocol (network layer) address of the destination host. 交换算法相对简单;对于大多数路由协议s都是一样的。在大多数的情况下,一个主机确定它必须将一个数据包传送给另外一个主机。通过某些方法,得到所需要的路由器地址,源主机就将一个带有目的主机协议(网络层)地址的数据包,针对寻址路由器的物理(MAC(介质访问控制)层)地址发送出去。As it examines the packets destination protocol address, the router determines that it either knows or does not know how to forward the packet to the next hop. If the router does not know how to forward the packet, it typically drops the packet. If the router knows how to forward the packet, however, it changes the destination physical address to that of the next hop and transmits the packet. 当路由器检查数据包的协议目的地址时,如果路由器不知道如何转发数据包,通常就丢弃它;如果知道,就改变目的物理地址到下一跳,并传送该数据包。The next hop may be the ultimate destination host. If not, the next hop is usually another router, which executes the same switching decision process. As the packet moves through the internetwork, its physical address changes, but its protocol address remains constant, as illustrated in Figure: Numerous Routers May Come into Play During the Switching Process. 这下一跳可能是最后目的主机。如果不是,下一跳通常是另一个路由器,它执行同样的交换决策过程。当数据包移动通过互联网络时,它的物理地址就改变了,但它的协议地址保持不变,如在图5-2中举例说明的那样。The preceding discussion describes switching between a source and a destination end system. The International Organization for Standardization (ISO) has developed a hierarchical terminology that is useful in describing this process. Using this terminology, network devices without the capability to forward packets between subnetworks are called end systems (ESs), whereas network devices with these capabilities are called intermediate systems (ISs). ISs are further divided into those that can communicate within routing domains (intradomain ISs) and those that communicate both within and between routing domains (interdomain ISs). A routing domain generally is considered a portion of an internetwork under common administrative authority that is regulated by a particular set of administrative guidelines. Routing domains are also called autonomous systems. With certain protocols, routing domains can be divided into routing areas, but intradomain routing protocols are still used for switching both within and between areas. 前面的讨论叙述了一个源和一个目的端系统间的交换。ISO已经开发了一类层次的术语,用来描述这个过程。按此类术语,一种网络设备,如果在子网间没有转发数据包的能力,该种设备就叫做ES(端系统);有此能力的,叫IS(中间系统)。ISs进一步分为域内ISs(在路由域内可以通信),和域间ISs(在路由域内和域间)可以通信。一个路由域通常认为是一个互联网络的一部分,该互联网络在公共的管理授权下,受一组特殊的管理原则指导。路由域也叫ASs(自治系统s)。在某些协议中,路由域s可以细分成路由区s,域内路由协议s仍可用于区内交换和区间交换。Figure: Numerous Routers May Come into Play During the Switching Process3. Routing Algorithms 路由算法Routing algorithms can be differentiated based on several key characteristics. First, the particular goals of the algorithm designer affect the operation of the resulting routing protocol. Second, various types of routing algorithms exist, and each algorithm has a different impact on network and router resources. Finally, routing algorithms use a variety of metrics that affect calculation of optimal routes. The following sections analyze these routing algorithm attributes. 路由算法s可以根据若干关键特征来区分。其一,算法设计者的具体目标s影响到最终路由协议的操作;其二,有各种类型的路由算法,每一种算法对于网络和路由资源有不同的影响;其三,路由算法使用各种影响优化由路计算的度量。下面各节分析这些路由算法的属性。 Design Goals 设计目标Routing algorithms often have one or more of the following design goals: 路由算法通常使用下列一个或多个设计目标： Optimality 优化性Simplicity and low overhead 简单性和低开销Robustness and stability 稳固性和稳定性Rapid convergence 快速收敛Flexibility 灵活性Optimality refers to the capability of the routing algorithm to select the best route, which depends on the metrics and metric weightings used to make the calculation. For example, one routing algorithm may use a number of hops and delays, but it may weigh delay more heavily in the calculation. Naturally, routing protocols must define their metric calculation algorithms strictly. 优化性是路由算法选择最佳由路的能力。这取决于用于计算的度量和度量加权。例如,一种路由算法可以使用若干跳和若干延迟,可以在计算中对延迟更重地加权。当然,路由协议s必须严格地定义它们的度量计算算法。Routing algorithms also are designed to be as simple as possible. In other words, the routing algorithm must offer its functionality efficiently, with a minimum of software and utilization overhead. Efficiency is particularly important when the software implementing the routing algorithm must run on a computer with limited physical resources. 路由算法也应该设计成尽可能地简单。换句话说,路由算法必须以最少的软件开销和使用开销,提供其有效的功能性。效率是特别重要的,如果路由算法必须采用采用软件实现,而计算机上的物理资源又很有限的话。Routing algorithms must be robust, which means that they should perform correctly in the face of unusual or unforeseen circumstances, such as hardware failures, high load conditions, and incorrect implementations. Because routers are located at network junction points, they can cause considerable problems when they fail. The best routing algorithms are often those that have withstood the test of time and that have proven stable under a variety of network conditions. 路由算法必须稳固,这意味着在面向不长用或想不到的环境中仍能准确地执行,如硬件失效,重载状况,和不正确地实现。由于路由器s位于网络连接点s,它们的失效可能引发可观的问题s。最好的路由算法通常容许有测试的时间,以及在各种网络状况下能保证其稳定性。In addition, routing algorithms must converge rapidly. Convergence is the process of agreement, by all routers, on optimal routes. When a network event causes routes to either go down or become available, routers distribute routing update messages that permeate networks, stimulating recalculation of optimal routes and eventually causing all routers to agree on these routes. Routing algorithms that converge slowly can cause routing loops or network outages. 另外,路由算法必须快速收敛。收敛是所有路由器,关于由路优化,都一致认可的过程。当一个网络事件引发由路关闭或成为可用时,路由器s发布路由更改消息遍布网络s,激发重新计算优化由路,并实际上引起所有路由器都要认可这些由路。收敛很慢的路由算法可能引起路由成环或网络断开。In the routing loop displayed in Figure: Slow Convergence and Routing Loops Can Hinder Progress, a packet arrives at Router 1 at time t1. Router 1 already has been updated and thus knows that the optimal route to the destination calls for Router 2 to be the next stop. Router 1 therefore forwards the packet to Router 2, but because this router has not yet been updated, it believes that the optimal next hop is Router 1. Router 2 therefore forwards the packet back to Router 1, and the packet continues to bounce back and forth between the two routers until Router 2 receives its routing update or until the packet has been switched the maximum number of times allowed. Figure: Slow Convergence and Routing Loops Can Hinder ProgressRouting algorithms should also be flexible, which means that they should quickly and accurately adapt to a variety of network circumstances. Assume, for example, that a network segment has gone down. As many routing algorithms become aware of the problem, they will quickly select the next-best path for all routes normally using that segment. Routing algorithms can be programmed to adapt to changes in network bandwidth, router queue size, and network delay, among other variables. 路由算法也应该是灵活的,意思是它们应当快而准确地适应各种网络环境s。例如,假定一个网络段已经断开了。当许多路由算法知道了问题,它们就迅速地从所有的由路中选择下一个最好的通路,以能正常地使用该网段。可以编程路由算法,以适应在网络带宽,路由器队列大小,和网络延迟,还有其它可变的因素等,而引起的变化。Algorithm Types 路由算法种类Routing algorithms can be classified by type. Key differentiators include these: Static versus dynamic Single-path versus multipath Flat versus hierarchical Host-intelligent versus router-intelligent Intradomain versus interdomain Link-state versus distance vector Static Versus Dynamic 对照静态与动态Static routing algorithms are hardly algorithms at all, but are table mappings established by the network administrator before the beginning of routing. These mappings do not change unless the network administrator alters them. Algorithms that use static routes are simple to design and work well in environments where network traffic is relatively predictable and where network design is relatively simple. 静态算法s毕竟是很难的算法s,是由网络管理员在路由开始前建立的表映射s。这些映射s不会变化,除非管理员改变它们。使用静态路由的算法,设计简单,在网络信流相对可预测,网络设计相对简单的的环境s里,工作得很好。Because static routing systems cannot react to network changes, they generally are considered unsuitable for todays large, constantly changing networks. Most of the dominant routing algorithms today are dynamic routing algorithms, which adjust to changing network circumstances by analyzing incoming routing update messages. If the message indicates that a network change has occurred, the routing software recalculates routes and sends out new routing update messages. These messages permeate the network, stimulating routers to rerun their algorithms and change their routing tables accordingly. 由于静态路由系统不能反映网络的变化,它们通常被认为不适合当今大型常变的网络s。如今大多数主流路由算法是动态路由算法s,它们通过分析收入的路由变更消息,调整变化的网络环境s。如果消息指示,一个网络的变化已经发生,路由软件就重新计算,并送出新的路由更改消息。这些消息遍布网络,激发路由器重新运行它们的算法s,并改变它们相应的路由表。,Dynamic routing algorithms can be supplemented with static routes where appropriate. A router of last resort (a router to which all unroutable packets are sent), for example, can be designated to act as a repository for all unroutable packets, ensuring that all messages are at least handled in some way. 动态路由算法可以被静态由路适当地补充。例如,一个最后求助的路由器(一个路由器,其所有不可路由的数据包都已被送出的路由器),可以被指定当作储藏所有不可路由的储藏室,以保证所有的消息,至少用某种方法,都能得到处理。Single-Path Versus Multipath 对照单通路与多通路Some sophisticated routing protocols support multiple paths to the same destination. Unlike single-path algorithms, these multipath algorithms permit traffic multiplexing over multiple lines. The advantages of multipath algorithms are obvious: They can provide substantially better throughput and reliability. This is generally called load sharing. 某些高级的路由协议支持指向同一信宿的多通路。不像单通路的算法,多通路算法在多条线路上执行信流复用。多通路算法的优点很明显:它们可以提供大得多的吞吐能力和可靠性。这通常称为负载共享。 Flat Versus Hierarchical 对照扁平与分层Some routing algorithms operate in a flat space, while others use routing hierarchies. In a flat routing system, the routers are peers of all others. In a hierarchical routing system, some routers form what amounts to a routing backbone. Packets from nonbackbone routers travel to the backbone routers, where they are sent through the backbone until they reach the general area of the destination. At this point, they travel from the last backbone router through one or more nonbackbone routers to the final destination. 某些路由算法操作在扁平空间,另一些则使用分层路由。在扁平路由系统中,所有路由器都是对等的。在分层路由系统中,一些路由器构成一个路由骨干网。数据包从非骨干路由器s传输到骨干路由器s,经由路由骨干网,直到它们到达目的地的一般区域。在那里,它们从离得最近的骨干路由器,经过一个或多个非骨干路由器,到达最终目的地。 Routing systems often designate logical groups of nodes, called domains, autonomous systems, or areas. In hierarchical systems, some routers in a domain can communicate with routers in other domains, while others can communicate only with routers within their domain. In very large networks, additional hierarchical levels may exist, with routers at the highest hierarchical level forming the routing backbone. 路由系统s通常将一组节点命名为域s,ASs(自治系统s),或区。在分层系统s中,某些在域中的路由器,可以与其它域的路由器通信;而其它的,就只能与它们域内的路由器通信。在非常大的网络s中,可以有附加的分层级s,其最高层的路由器s构成路由骨干网。The primary advantage of hierarchical routing is that it mimics the organization of most companies and therefore supports their traffic patterns well. Most networ