ImageVerifierCode 换一换
格式:PPT , 页数:75 ,大小:3.55MB ,
文档编号:3562238      下载积分:28 文币
快捷下载
登录下载
邮箱/手机:
温馨提示:
系统将以此处填写的邮箱或者手机号生成账号和密码,方便再次下载。 如填写123,账号和密码都是123。
支付方式: 支付宝    微信支付   
验证码:   换一换

优惠套餐
 

温馨提示:若手机下载失败,请复制以下地址【https://www.163wenku.com/d-3562238.html】到电脑浏览器->登陆(账号密码均为手机号或邮箱;不要扫码登陆)->重新下载(不再收费)。

已注册用户请登录:
账号:
密码:
验证码:   换一换
  忘记密码?
三方登录: 微信登录  
下载须知

1: 试题类文档的标题没说有答案,则无答案;主观题也可能无答案。PPT的音视频可能无法播放。 请谨慎下单,一旦售出,概不退换。
2: 本站所有资源如无特殊说明,都需要本地电脑安装OFFICE2007和PDF阅读器。
3: 本文为用户(三亚风情)主动上传,所有收益归该用户。163文库仅提供信息存储空间,仅对用户上传内容的表现方式做保护处理,对上载内容本身不做任何修改或编辑。 若此文所含内容侵犯了您的版权或隐私,请立即通知163文库(点击联系客服),我们立即给予删除!。
4. 未经权益所有人同意不得将文件中的内容挪作商业或盈利用途。
5. 本站仅提供交流平台,并不能对任何下载内容负责。
6. 下载文件中如有侵权或不适当内容,请与我们联系,我们立即纠正。
7. 本站不保证下载资源的准确性、安全性和完整性, 同时也不承担用户因使用这些下载资源对自己和他人造成任何形式的伤害或损失。

版权提示 | 免责声明

1,本文(计算机网络第五版(英文版)(5)解析课件.ppt)为本站会员(三亚风情)主动上传,163文库仅提供信息存储空间,仅对用户上传内容的表现方式做保护处理,对上载内容本身不做任何修改或编辑。
2,用户下载本文档,所消耗的文币(积分)将全额增加到上传者的账号。
3, 若此文所含内容侵犯了您的版权或隐私,请立即通知163文库(发送邮件至3464097650@qq.com或直接QQ联系客服),我们立即给予删除!

计算机网络第五版(英文版)(5)解析课件.ppt

1、计算机网络 The Network LayerThe Network LayerTopicsNetwork Layer Design IssuesStore-and-Forward Packet SwitchingImplementation of Connectionless ServiceImplementation of Connection-Oriented ServiceComparison of Virtual-Circuit and Datagram SubnetsRouting AlgorithmsnCorrectnessnSimplicitynRobustnessnStabi

2、litynFairnessnOptimalityRouting AlgorithmsnShortest Path RoutingnFloodingnDistance Vector RoutingnLink State RoutingnHierarchical RoutingRouting Algorithm BasicsnThe routing algorithm is that part of the network layer software responsible for deciding which output line an incoming packet should be t

3、ransmitted on.nIf the subnet uses datagrams internally,this decision must be made anew for every arriving data packet since the best route may have changed since last time.nIf the subnet uses virtual circuits internally,routing decisions are made only when a new virtual circuit is being set up.Routi

4、ng Algorithm Basics(2)nAll optimal routes from station A to other stations in the network,jointly constitute a sink treenRouters have to collaborate to build the sink tree for each source station or destination station.Routing Algorithm Basics(3)nMetrics used for optimizationDistanceNumber of hopsTi

5、me delay nTwo major classes of routing algorithmsNonadaptive or Static:the routing table is computed in advance,off-line,and downloaded to the routers when the network is booted.Adaptive or Dynamic:change their routing decisions to reflect changes in the topology,and usually the traffic as well.Shor

6、test Path RoutingnIdea:build a graph of the subnet,with each node of the graph representing a router and each arc of the graph representing a communication line.To choose a route between a given pair of routers,the algorithm just finds the shortest path between them on the graph.nthe labels(weights)

7、on the arcs could be computed as a function of the distance,bandwidth,average traffic,communication cost,measured delay,and other factors.nShortest Path Criteria:sum of the weights all the way,or,the number of hops.Shortest Path RoutingDijkstra AlgorithmnBasic Idea:during each step,select a newly re

8、achable node at the lowest cost,and add the edge to that node,to the sink tree rooted at the source node built so far.FloodingnBasic idea:Forward an incoming packet across every outgoing line,except the one it came through.nBasic problem:how to avoid“drowning by packets”?Use a hop counter:after a pa

9、cket has been forwarded across N routers,it is discarded.Be sure to forward a packet only once(i.e.avoid directed cycles).Requires sequence numbers per source router.Flood selectively:only in the direction that makes sense.nFlooding makes sense only when robustness is needed,e.g.in military applicat

10、ionsDistance Vector RoutingBellman-Ford or Ford-Fulkerson algorithmnDistance Vector Routing Algorithms:each router maintains a table(i.e,a vector)giving the best known distance to each destination and which line to use to get there,which are updated by exchanging information with the neighbors perio

11、dically.nUpdating Process:Take a look at the costs that your direct neighbors are advertising to get a packet to the destination.Select the neighbor whose advertised cost,added with the cost to get to that neighbor,is the lowest.Advertise that new cost to the other neighbors.nIn DVR,there is the pro

12、blem of count-to-infinity in the presence of node crashes.DVR Examplen(a)A subnet.(b)Input from A,I,H,K,and the new routing table for JLink State RoutingnLink State Routing:broadcast info on the entire network topology to all routers,and let each of them calculate a sink tree to the other routers.nE

13、ach router must do the following:Discover its neighbors,learn their network address.Measure the delay or cost to each of its neighbors.Construct a packet telling all it has just learned.Send this packet to all other routers.Compute the shortest path to every other router.Measuring Line CostnJust sen

14、d an ECHO packet through each interface,and measure the round-trip delay.Thatll give you a reasonable estimate of the actual delay.nWhether to take the load into account when measuring the delay?Yes:the timer should be started when the ECHO packet is queued.You may redirect traffic in such a way tha

15、t the alternative route is unloaded.No:the timer should be started when the ECHO packet reaches the front of the queue.The shortest path you choose may be overloaded.Building Link State PacketsnThe packet starts with the identity of the sender,followed by a sequence number and age,and a list of dual

16、 items(neighbor,the delay to it).nThe hard part is when to build them,at regular intervals or when some significant event occurs?Practice shows that once an hour is often enough.Distributing the Link State PacketsnUse a flooding algorithm,and dam the flood through sequence numbers:all routers mainta

17、in a list of(source,seq number)-pairs.nTo safeguard against old data,down links,etc.an age is added to an LSP.The age is decremented once a second,and every time it is forwarded by a router.When the age hits zero,the LSP is discarded.nTo guard against errors on the router-router lines,all link state

18、 packets are acknowledged.Goods and Badsof LSRnGoodsGood consistency of each router informationQuick convergence for good and bad newsnBadsEach router need large memory to store the input link states of other routersThe computation time can be an issueHierarchical RoutingnProblem:No routing algorith

19、m discussed so far can scale:all of them require each router to know about all others=too demanding with respect to memory capacity and processing power.nGo for suboptimal routes by introducing hierarchical routing and regions,and separate algorithms for intra-region and inter-region routing.Example

20、:Hierarchical RoutingMulticasting routingnMOSPFnDVMRPnCBT Next discussion topicsnMulticasting routingnAd hoc routingnBroadcasting routingMobile hosts routingCongestion Control AlgorithmsnGeneral Principles of Congestion ControlnCongestion Prevention PoliciesnCongestion Control in Virtual-Circuit Sub

21、netsnCongestion Control in Datagram SubnetsnLoad SheddingnJitter ControlCongestion nCongestion:When too many packets are present,performance degradesnAt very high traffic,performance collapses completely and almost no packets are delivered.CongestionGeneral Principles of Congestion ControlnTwo group

22、s of solutions:nOpen loop:attempt to solve the problem by good design,in essence,to make sure it does not occur in the first place.nClosed loop:are based on the concept of a feedback loop.Monitor the system to detect when and where congestion occurs.Pass information to where action can be taken.Adju

23、st system operation to correct the problem.PolicyCongestion Control in Virtual-Circuit SubnetsHop-by-Hop Choke PacketsnWhen a router runs out of its resources,it sends a choke packet back to the source host,giving it the destination found in the packet.nWhen the source host gets the choke packet,it

24、will slow down the traffic sent to the specified destination.nHop-by-Hop:the choke packet affects every hop it passes through.n(a)A choke packet that affects only the source.n(b)Hop-by-Hop choke packet.Random early detectionnBy having routers drop packets before the situation has become hopeless(hen

25、ce the early in the name),the idea is that there is time for action to be taken before it is too late.nTo determine when to start discarding,routers maintain a running average of their queue lengths.nWhen the average queue length on some line exceeds a threshold,the link is said to be congestion and

26、 a small fraction of the packets are dropped at randomJitter ControlQuality of ServicenA stream of packets from a source to a destination is called a flow.nIn a connection-oriented network,all the packets belonging to a flow follow the same route;in a connectionless network,they may follow different

27、 routes.The needs of each flow can be characterized by four primary parameters:reliability,delay,jitter,and bandwidth.Together these determine the QoS(Quality of Service)the flow requires.Traffic ShapingnTraffic shaping is about regulating the average rate of data transmission.nThe goal is to allow

28、applications to transmit a wide variety of traffic that suits their needs,include some bursts.The Leaky Bucket AlgorithmnImagine a bucket with a small hole in the bottom.No matter the rate at which water enters the bucket,the outflow is at a constant rate,when there is any water in the bucket and ze

29、ro when the bucket is empty.*实现:-对定长分组的实现:以分组为单位;-对变长分组的实现:采用字节计数法。n漏桶算法(续):*举例:设计算机以25MB/s速率产生数据,路由器长时间的最佳工作速率不超过2MB/s。数据以每秒中有40ms,1MB的突发数据输入。为平稳输出,使用一个=2MB/s,容积C=1MB的漏桶。a图为漏桶的输入,b 图为漏桶的输出。The Token Bucket AlgorithmnThe leaky bucket algorithm enforces a rigid output pattern at the average rate,no m

30、atter how bursty the traffic is.For many applications,it is better to allow the output to speed up somewhat when large bursts arrive,so a more flexible algorithm is needed,preferably one that never loses data.One such algorithm is the token bucket algorithm.n通信量整形令牌桶算法:*实现:-采用分组计数法:设置令牌计数器,每隔T加1,每发送

31、一个分组减1,计数器为0时停止发送。-采用字节计数法:时钟每隔T令牌计数器加k字节,每发送一个分组减该分组长度。*对突发时间长度的限制:设突发时间长度S秒,令牌桶容量B字节,令牌到达率R字节/秒,最大速率M字节/秒,由B+RS=MS 得 S=B/(M-R)。n通信量整形令牌桶算法:*对突发长度的限制(续):例:容量C分别是250KB、500KB和750KB,=2MB,M=25MB/s。假定当1MB突发数据到达时,令牌桶已满。5.4.2 实现高QoS的技术(续)n资源预留:可能被预留资源的种类:-带宽 -缓冲区空间 -CPU时钟周期 Admission controlnAdmission con

32、trol:确定接收或拒绝一个流并非易事,原因:-由于有些应用可能知道带宽需求,但不知道缓冲区和CPU时钟周期的需求。-有些应用需要更高的容错能力。-有些应用希望讨价还价流的参数。Admission control flow specification(流说明):-能够准确描述规范化的流参数。-从源端到目的端沿途路由器可修改这些参数。举例:-令牌桶速率:长时间间隔的平均输入桶的每秒字节数。-令牌桶容量。-峰值数据速率:限制短暂时间间隔发送速率。-最小、最大分组长度:含头部。Packets scheduling n按比例路由:由于路由器一般不了解整个网络的流量,一种高QoS的方法是根据输出链路能力

33、按相等或比例划分,在多条路径上传输。nPackets scheduling:公平队列算法:*Nagle算法:-到达分组按分类或流进入各自队列;-轮流从各队列输出,一旦某队列空,立即从非空队列输出。Packets schedulingn分组调度(续):公平队列算法(FIFO)加权队列算法(WFQ)5.4.2 实现高QoS的技术(续)n分组调度(续):加权公平队列(WFQ):-对不同的队列设置优先级(即权重)。-按权重比例发送。5.4.2 实现高QoS的技术(续)n分组调度(续):令牌+WFQ提供可证明的最大队列延时:n条流的令牌桶+WFQ队列模型。b1r1bnrnw1wnInternetwork

34、ingnHow Networks DiffernHow Networks Can Be ConnectednConcatenated Virtual CircuitsnConnectionless InternetworkingnTunnelingnFragmentationConnecting NetworksHow Networks DifferHow Networks Can Be ConnectednRepeaters at the physical layer for boosting signals.nBridges to make the interconnection at t

35、he data link layer.nMultiprotocolrouters for forwarding,and possibly splitting up packets(bridges cant do the latter).nTransport gateways for interfacing between two transport connections.nApplication gateways,translate message semantics.Concatenated Virtual CircuitsConnectionless internetworkingnHa

36、ve the network layer offer only datagram services:unreliable,unordered packet flow.nMain problem:Addressing different networks use completely different addresses,so how do I address a host in an IP network when Ive only got SNA?nSolution:consider,using IP as a universal network protocolTunnelingnWe

37、can solve a lot of the internetworking problems when we can assume that the source and destination of the same type of network is connected by different network,we need only to tunnel packets through intermediate networks.FragmentationnFragmentation:Different networks may impose different maximum pa

38、cket sizes.This means that we may have to split a packet into smaller ones when forwarding it through a network whose maximum packet size is too smallnWhere to reassemble the fragments?Gateway:Transparent fragmentationThe destination host:Nontransparent fragmentationThe Network Layer in the Internet

39、The Internet Protocol(IP)nThe Internet is a collection of many networks connected together by a bunch of backbones.nThe glue that holds the whole Internet together is the network layer protocol,IP(Internet Protocol).The IPv4 HeaderThe IPv4 Header(2)AddressesIP Addresses(2)Subnets(1)nAll hosts on the

40、 same network must have the same network number.This may cause a single organization to acquire several classes of addresses for multiple LANs.nUse a single network address for the entire organization,and internally divide the host address space into a subnet address and a host idnTo implement subne

41、tting,the main router needs a subnet mask that indicate CIDR Classless InterDomainRoutingCIDR ExamplesCIDRAggregate EntriesnFor many routers outside 194.24.0.0,the only thing they see is that there are(at least)3 network addresses for which packets follow the same route.These entries can be aggregated into a single entry 194.24.0.0/19 with a single submaskof 19/13 1/0 bits.

侵权处理QQ:3464097650--上传资料QQ:3464097650

【声明】本站为“文档C2C交易模式”,即用户上传的文档直接卖给(下载)用户,本站只是网络空间服务平台,本站所有原创文档下载所得归上传人所有,如您发现上传作品侵犯了您的版权,请立刻联系我们并提供证据,我们将在3个工作日内予以改正。


163文库-Www.163Wenku.Com |网站地图|