互聯(lián)網(wǎng)絡(luò)聯(lián)網(wǎng)的基本原理-5_路由(zslcn周生烈編譯摘注評(píng)

1、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編譯注)五Routing Basics 路由基本原理指導(dǎo)目錄 Contents聯(lián)網(wǎng)技術(shù)手冊(cè) 指導(dǎo)目錄 Part 1 Internetworking Basics 互聯(lián)網(wǎng)絡(luò)聯(lián)網(wǎng)基本原理 Part 2 LAN Technologies 局域網(wǎng)技術(shù) part 3 WAN Technologies 廣域網(wǎng)技術(shù) part 4 Internet Protocols 互聯(lián)網(wǎng)協(xié)議 part 5 Bridging and Switching 橋接和交換 part 6 Routing 路由 part

2、 7 Network Management 網(wǎng)絡(luò)管理 part 8 Voice/Data Integration Technologies 聲音/數(shù)據(jù)集成技術(shù) part 9 Wireless Technologies 無線技術(shù) part 10 Cable Access Technologies 有線接入技術(shù) part 11 Dial-up Technology 撥號(hào)技術(shù) part 12 Security Technologies 安全技術(shù) part 13 Quality of Service Networking 聯(lián)網(wǎng)服務(wù)質(zhì)量 part 14 Network Caching Technolog

3、ies 網(wǎng)絡(luò)緩存技術(shù) part 15 IBM Network Management IBM網(wǎng)絡(luò)管理 part 16 Multiservice Access Technologies 多業(yè)務(wù)接入技術(shù) /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 c

4、oncepts 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. 本章介紹廣泛用于路由協(xié)議的基本概念。在這里概述的論題包括路由協(xié)議的組成部分和算法。另外,將路由協(xié)議的作用與被路由或網(wǎng)絡(luò)協(xié)議的作用作一簡(jiǎn)短的對(duì)比。

5、在第VII部分的章節(jié)路由協(xié)議s,里,會(huì)更詳細(xì)地講述特殊的路由協(xié)議s,而使用路由協(xié)議s的網(wǎng)絡(luò)協(xié)議,是在第VI部分,網(wǎng)絡(luò)協(xié)議s,中討論。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 Swi

6、tching 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 Intradoma

7、in 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

8、 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 ne

9、twork 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. 路由就是一種作用,將信息從源經(jīng)過互聯(lián)網(wǎng)絡(luò)移動(dòng)到目的地。沿著這條通路,通常會(huì)遇到至少一個(gè)中間節(jié)點(diǎn)。路由通常與橋接相對(duì)照,在陌生的旁觀者看來,它們精確地完成同

10、一件事情。兩者主要的不同在于,橋接發(fā)生在OSI參考模型第二層(鏈路層),而路由則發(fā)生在第三層(網(wǎng)絡(luò)層)。這種不同提供給路由和橋接用不同的信息,來處理從源到目的信息移動(dòng)的過程，所以這兩種功能(路由和橋接)是用不同的方法來完成它們的任務(wù)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 reaso

11、n for this time lag is that networks in the 1970s were simple, homogeneous environments. Only relatively recently has large-scale internetworking become popular. 路由這個(gè)論題,出現(xiàn)在計(jì)算機(jī)科學(xué)文獻(xiàn)里,已經(jīng)有多于20年的時(shí)間,然而路由實(shí)現(xiàn)在商業(yè)上流行,則是遲至80年代中期。其原因在于70年代的網(wǎng)絡(luò)s還是簡(jiǎn)單,同質(zhì)的環(huán)境s。大規(guī)?；ヂ?lián)網(wǎng)絡(luò)聯(lián)網(wǎng)的流行出現(xiàn)較晚。2. Routing Components 路由的組成部分Routing invo

12、lves 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,

13、 path determination can be very complex. 路由涉及兩種活動(dòng):確定優(yōu)化路由通路s,和傳輸通過互聯(lián)網(wǎng)絡(luò)的信息組s(通常叫數(shù)據(jù)包s)。,在路由過程的上下文中,后一活動(dòng)稱為包交換。雖然包交換相對(duì)直觀,通路確定卻可能十分復(fù)雜。Path Determination 通路確定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 band

14、width, 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. 路由協(xié)議使用度量來評(píng)

15、價(jià),這樣使通路能最好地傳輸數(shù)據(jù)包。一次度量就是一次標(biāo)準(zhǔn)測(cè)量,如通路帶寬,路由算法s使用它來確定到達(dá)一個(gè)目標(biāo)的最佳路徑。為了輔助路徑確定的過程,路由算法s啟動(dòng)和維持包含路由信息的路由表s。路由信息的變化取決于所用的路由算法。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 p

16、acket 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將各種信息填入路由表s。目標(biāo)/下一跳組合告訴路由器,一個(gè)具體的目標(biāo)可以優(yōu)化地到達(dá),只要將數(shù)據(jù)包,沿著通向最終目標(biāo)的通路,送到表示下一跳的具體

17、路由器。當(dāng)路由器接收到一個(gè)數(shù)據(jù)包時(shí),它就檢查目標(biāo)地址,并在目標(biāo)地址上結(jié)合下一跳相關(guān)地址。圖5-1說明了一個(gè)例樣目標(biāo)/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

18、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,如有關(guān)

19、所希望通路的數(shù)據(jù)。路由器s比較度量值來確定優(yōu)化由路,而這些度量值則取決于所使用的路由算法的設(shè)計(jì),而有所不同。各種通用的度量s將被介紹,并在本章后面予以說明。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 rout

20、ing 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 buil

21、d a complete picture of network topology to enable routers to determine optimal routes to network destinations. 路由器s彼此通信,并通過傳輸各種消息來維持它們的路由表s。路由更新消息是一種這樣的消息,它們構(gòu)成了路由表的全部或部分。通過分析來自所有其它路由器的路由更新消息,一個(gè)路由器就可以建立一張?jiān)敿?xì)的網(wǎng)絡(luò)投票圖。一個(gè)鏈路狀態(tài)通告,路由器間發(fā)送消息的另一種例子,通知其它路由器,告知發(fā)送者鏈路s的狀態(tài)。鏈路信息也可以用來建立一張完整的網(wǎng)絡(luò)拓?fù)鋱D,以使路由器s能確定由路到網(wǎng)絡(luò)目標(biāo)s的優(yōu)化通

22、路。Switching 交換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 physica

23、l (Media Access Control MAC-layer) address, this time with the protocol (network layer) address of the destination host. 交換算法相對(duì)簡(jiǎn)單;對(duì)于大多數(shù)路由協(xié)議s都是一樣的。在大多數(shù)的情況下,一個(gè)主機(jī)確定它必須將一個(gè)數(shù)據(jù)包傳送給另外一個(gè)主機(jī)。通過某些方法,得到所需要的路由器地址,源主機(jī)就將一個(gè)帶有目的主機(jī)協(xié)議(網(wǎng)絡(luò)層)地址的數(shù)據(jù)包,針對(duì)尋址路由器的物理(MAC(介質(zhì)訪問控制)層)地址發(fā)送出去。As it examines the packets destination prot

24、ocol 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 addre

25、ss to that of the next hop and transmits the packet. 當(dāng)路由器檢查數(shù)據(jù)包的協(xié)議目的地址時(shí),如果路由器不知道如何轉(zhuǎn)發(fā)數(shù)據(jù)包,通常就丟棄它;如果知道,就改變目的物理地址到下一跳,并傳送該數(shù)據(jù)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

26、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. 這下一跳可能是最后目的主機(jī)。如果不是,下一跳通常是另一個(gè)路由器,它執(zhí)行同樣的交換決策過程。當(dāng)數(shù)據(jù)包移動(dòng)通過互聯(lián)網(wǎng)絡(luò)時(shí),它的物理地址就改變了,但它的協(xié)議地址保持不變,如在圖5-2中舉例說明的那樣。The preceding discus

27、sion 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 betwe

28、en 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 (int

29、erdomain 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 in

30、to routing areas, but intradomain routing protocols are still used for switching both within and between areas. 前面的討論敘述了一個(gè)源和一個(gè)目的端系統(tǒng)間的交換。ISO已經(jīng)開發(fā)了一類層次的術(shù)語,用來描述這個(gè)過程。按此類術(shù)語,一種網(wǎng)絡(luò)設(shè)備,如果在子網(wǎng)間沒有轉(zhuǎn)發(fā)數(shù)據(jù)包的能力,該種設(shè)備就叫做ES(端系統(tǒng));有此能力的,叫IS(中間系統(tǒng))。ISs進(jìn)一步分為域內(nèi)ISs(在路由域內(nèi)可以通信),和域間ISs(在路由域內(nèi)和域間)可以通信。一個(gè)路由域通常認(rèn)為是一個(gè)互聯(lián)網(wǎng)絡(luò)的一部分,該互聯(lián)網(wǎng)絡(luò)在公共的管理

31、授權(quán)下,受一組特殊的管理原則指導(dǎo)。路由域也叫ASs(自治系統(tǒng)s)。在某些協(xié)議中,路由域s可以細(xì)分成路由區(qū)s,域內(nèi)路由協(xié)議s仍可用于區(qū)內(nèi)交換和區(qū)間交換。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 des

32、igner 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 se

33、ctions analyze these routing algorithm attributes. 路由算法s可以根據(jù)若干關(guān)鍵特征來區(qū)分。其一,算法設(shè)計(jì)者的具體目標(biāo)s影響到最終路由協(xié)議的操作;其二,有各種類型的路由算法,每一種算法對(duì)于網(wǎng)絡(luò)和路由資源有不同的影響;其三,路由算法使用各種影響優(yōu)化由路計(jì)算的度量。下面各節(jié)分析這些路由算法的屬性。 Design Goals 設(shè)計(jì)目標(biāo)Routing algorithms often have one or more of the following design goals: 路由算法通常使用下列一個(gè)或多個(gè)設(shè)計(jì)目標(biāo)： Optimality 優(yōu)化性Sim

34、plicity and low overhead 簡(jiǎn)單性和低開銷Robustness and stability 穩(wěn)固性和穩(wěn)定性Rapid convergence 快速收斂Flexibility 靈活性O(shè)ptimality 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 algori

35、thm 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. 優(yōu)化性是路由算法選擇最佳由路的能力。這取決于用于計(jì)算的度量和度量加權(quán)。例如,一種路由算法可以使用若干跳和若干延遲,可以在計(jì)算中對(duì)延遲更重地加權(quán)。當(dāng)然,路由協(xié)議s必須嚴(yán)格地定義它們的度量計(jì)算算法。Routing algorithms

36、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 li

37、mited physical resources. 路由算法也應(yīng)該設(shè)計(jì)成盡可能地簡(jiǎn)單。換句話說,路由算法必須以最少的軟件開銷和使用開銷,提供其有效的功能性。效率是特別重要的,如果路由算法必須采用采用軟件實(shí)現(xiàn),而計(jì)算機(jī)上的物理資源又很有限的話。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 conditio

38、ns, 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. 路由算法必須穩(wěn)固

39、,這意味著在面向不長(zhǎng)用或想不到的環(huán)境中仍能準(zhǔn)確地執(zhí)行,如硬件失效,重載狀況,和不正確地實(shí)現(xiàn)。由于路由器s位于網(wǎng)絡(luò)連接點(diǎn)s,它們的失效可能引發(fā)可觀的問題s。最好的路由算法通常容許有測(cè)試的時(shí)間,以及在各種網(wǎng)絡(luò)狀況下能保證其穩(wěn)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

40、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,路由算法必須快速收斂。收斂是所有路由器,關(guān)于由

41、路優(yōu)化,都一致認(rèn)可的過程。當(dāng)一個(gè)網(wǎng)絡(luò)事件引發(fā)由路關(guān)閉或成為可用時(shí),路由器s發(fā)布路由更改消息遍布網(wǎng)絡(luò)s,激發(fā)重新計(jì)算優(yōu)化由路,并實(shí)際上引起所有路由器都要認(rèn)可這些由路。收斂很慢的路由算法可能引起路由成環(huán)或網(wǎng)絡(luò)斷開。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

42、 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 c

43、ontinues 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 t

44、hey 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

45、 can be programmed to adapt to changes in network bandwidth, router queue size, and network delay, among other variables. 路由算法也應(yīng)該是靈活的,意思是它們應(yīng)當(dāng)快而準(zhǔn)確地適應(yīng)各種網(wǎng)絡(luò)環(huán)境s。例如,假定一個(gè)網(wǎng)絡(luò)段已經(jīng)斷開了。當(dāng)許多路由算法知道了問題,它們就迅速地從所有的由路中選擇下一個(gè)最好的通路,以能正常地使用該網(wǎng)段?？梢跃幊搪酚伤惴?以適應(yīng)在網(wǎng)絡(luò)帶寬,路由器隊(duì)列大小,和網(wǎng)絡(luò)延遲,還有其它可變的因素等,而引起的變化。Algorithm Types 路由算法種類Routing

46、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 對(duì)照靜態(tài)與動(dòng)態(tài)Static routing

47、 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 w

48、here network traffic is relatively predictable and where network design is relatively simple. 靜態(tài)算法s畢竟是很難的算法s,是由網(wǎng)絡(luò)管理員在路由開始前建立的表映射s。這些映射s不會(huì)變化,除非管理員改變它們。使用靜態(tài)路由的算法,設(shè)計(jì)簡(jiǎn)單,在網(wǎng)絡(luò)信流相對(duì)可預(yù)測(cè),網(wǎng)絡(luò)設(shè)計(jì)相對(duì)簡(jiǎn)單的的環(huán)境s里,工作得很好。Because static routing systems cannot react to network changes, they generally are considered unsuitabl

49、e 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 softw

50、are 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. 由于靜態(tài)路由系統(tǒng)不能反映網(wǎng)絡(luò)的變化,它們通常被認(rèn)為不適合當(dāng)今大型常變的網(wǎng)絡(luò)s。如今大多數(shù)主流路由算法是動(dòng)態(tài)路由算法s,它們通過分析收入的路由變更消息,調(diào)整變化的網(wǎng)絡(luò)環(huán)境s。如果消息指示,一個(gè)網(wǎng)絡(luò)的變化已經(jīng)發(fā)生,

51、路由軟件就重新計(jì)算,并送出新的路由更改消息。這些消息遍布網(wǎng)絡(luò),激發(fā)路由器重新運(yùn)行它們的算法s,并改變它們相應(yīng)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 packe

52、ts, ensuring that all messages are at least handled in some way. 動(dòng)態(tài)路由算法可以被靜態(tài)由路適當(dāng)?shù)匮a(bǔ)充。例如,一個(gè)最后求助的路由器(一個(gè)路由器,其所有不可路由的數(shù)據(jù)包都已被送出的路由器),可以被指定當(dāng)作儲(chǔ)藏所有不可路由的儲(chǔ)藏室,以保證所有的消息,至少用某種方法,都能得到處理。Single-Path Versus Multipath 對(duì)照單通路與多通路Some sophisticated routing protocols support multiple paths to the same destination. Unlike

53、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. 某些高級(jí)的路由協(xié)議支持指向同一信宿的多通路。不像單通路的算法,多通路算法在多條線路上執(zhí)

54、行信流復(fù)用。多通路算法的優(yōu)點(diǎn)很明顯:它們可以提供大得多的吞吐能力和可靠性。這通常稱為負(fù)載共享。 Flat Versus Hierarchical 對(duì)照扁平與分層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

55、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.

56、 某些路由算法操作在扁平空間,另一些則使用分層路由。在扁平路由系統(tǒng)中,所有路由器都是對(duì)等的。在分層路由系統(tǒng)中,一些路由器構(gòu)成一個(gè)路由骨干網(wǎng)。數(shù)據(jù)包從非骨干路由器s傳輸?shù)焦歉陕酚善鱯,經(jīng)由路由骨干網(wǎng),直到它們到達(dá)目的地的一般區(qū)域。在那里,它們從離得最近的骨干路由器,經(jīng)過一個(gè)或多個(gè)非骨干路由器,到達(dá)最終目的地。 Routing systems often designate logical groups of nodes, called domains, autonomous systems, or areas. In hierarchical systems, some routers in a

57、 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. 路由系統(tǒng)s通常將一組節(jié)點(diǎn)命名為域s,ASs(自治系統(tǒng)s),或區(qū)。在分層系統(tǒng)s中,某些在域中的路由器,可以與其它域的路由器通信;而其它的,就只能與它們域內(nèi)的路由器通信。在非常大的網(wǎng)絡(luò)s中,可以有附加的分層級(jí)s,其最高層的路由器s構(gòu)成路由骨干網(wǎng)。The primary advantage of hierarchical routing is that it mimics the organization of most companies and therefore supports their traffic patterns well. Most networ