计算机网络课后习圈指导 陕西师范大学 得得路品 P16.Consider arouer buffer precedingan oubound link In this problem,you will use Litle'sformula a famous formula from queuing theory.Let N denote the average number of pac in the bulr plus the packet being transmitted.Leta denote the rate of packets arriving at the link Letd denote the average total delay (i.e.,the queuing delay plus the transmission delay)experienced by a packet.Little's formula is N-ad.Suppose that on average,the buffer contains 10 packets,and the average packet queuing delay is 10 msec.The link's transmission rate is 100 packets/sec Using Little's formula.what is thea uming there is no packet oss? 答：分组的总数量等于缓存的等特传输的队列分组和正在传输的那一个分组，所以 N=10+1=11。 因为N=ad,所以 10+1=ax(dqueue +dtrans) 1=a×001+100 解得a=550 packets/sec. P17.a Generalize Equation 1.2 in Section 1.4.3 for heterogeneous processing rates,transmission rates, b.Rep (a)but nov se that there is an a euing delay of dat each node 答：有Q个节点（包括源节点和Q-1 延，R9表示第q条链路的传输速率，分组由L个比特，则，第q个节点的传输速率为 d8as=L/R,用d品rop表示在第q条链路的传播速率，则端到端时延为： dend-te-mddirame b.用daueve表示在第q个节点的平均处理时延，则端到端时延为： demd-tonrdiramsro 0=1 P18.Perform a Traceroute between source and destination on the same continent at three different hours of the day. a.Find the average and standard deviation of the round-trip delays at each of the three hours. b.Find in the path the three hours.Did the paths change uring any of the hours? c.Try to identify the number of ISP networks that the Traceroute packets pass through from source to destination.Routers with similar names and/or similar IP addresses should be considered as part of the same ISP.In your experiments.do the largest delays occur at the peering
计算机网络课后习题指导 陕西师范大学 8 = / 1− = / 1 − = 1/ 1 − / = 1 − P16. Consider a router buffer preceding an outbound link. In this problem, you will use Little’s formula, a famous formula from queuing theory. Let N denote the average number of packets in the buffer plus the packet being transmitted. Let a denote the rate of packets arriving at the link. Let d denote the average total delay (i.e., the queuing delay plus the transmission delay) experienced by a packet. Little’s formula is N = a · d. Suppose that on average, the buffer contains 10 packets, and the average packet queuing delay is 10 msec. The link’s transmission rate is 100 packets/sec. Using Little’s formula, what is the average packet arrival rate, assuming there is no packet loss? 答：分组的总数量等于缓存的等待传输的队列分组和正在传输的那一个分组，所以： N=10+1=11。 因为 N = a · d，所以 10 + 1=a × + 11 = a × 0.01 + 1 100 解得a = 550packets/sec。 P17. a. Generalize Equation 1.2 in Section 1.4.3 for heterogeneous processing rates, transmission rates, and propagation delays. b. Repeat (a), but now also suppose that there is an average queuing delay of dqueue at each node. 答：a. 有 Q 个节点（包括源节点和 Q-1 个路由器节点），用d 表示第 q 个节点的处理时 延， 表示第 q 条链路的传输速率，分组由 L 个比特，则，第 q 个节点的传输速率为 = L/ ，用d 表示在第 q 条链路的传播速率，则端到端时延为： d = d + + d b. 用d 表示在第 q 个节点的平均处理时延，则端到端时延为： d = d + + d + d P18. Perform a Traceroute between source and destination on the same continent at three different hours of the day. a. Find the average and standard deviation of the round-trip delays at each of the three hours. b. Find the number of routers in the path at each of the three hours. Did the paths change during any of the hours? c. Try to identify the number of ISP networks that the Traceroute packets pass through from source to destination. Routers with similar names and/or similar IP addresses should be considered as part of the same ISP. In your experiments, do the largest delays occur at the peering
计算机网络课后习翘指导 陕西师范大学 interfaces between adjacent ISPs? d.Repeat the above fora and destination on different continents.Compare the and intor ainetTsub 答：在Lix你可以用如下命令 traceroute www.targethost.com 在Vindows环境你可以用如下命令： tracert www targethost com 不论在哪一种环境下做实验，你可以得到三组实验结果。在任意的三组实验中，你可以计 算平均往返时延的均值和方差。在不同时间段内 做实验并且评论是否有所变化， 以下是实验截图： 1.1 .be.s 从San Diego Super Computer Center到www.poly.ed的三次Traceroute a.在三个时间段内的平均往返时延分别是71.18ms,71.38ms和71.55ms,标准方差分别 为0.075ms.0.21ms.0.05ms 在这个例子中，在三个时段各自路由器数量均为12个，在这些时段中路径未发生变化 分组在源P地址和目的P地址之间经过了4个1SP网路 是的，在相邻的ISP 网络间对等接口处出现了最大的时延， 9
计算机网络课后习题指导 陕西师范大学 9 interfaces between adjacent ISPs? d. Repeat the above for a source and destination on different continents. Compare the intra-continent and inter-continent results. 答：在 Linux 你可以用如下命令： traceroute www.targethost.com 在 Windows 环境你可以用如下命令： tracert www.targethost.com 不论在哪一种环境下做实验，你可以得到三组实验结果。在任意的三组实验中，你可以计 算平均往返时延的均值和方差。在不同时间段内 做实验并且评论是否有所变化。 以下是实验截图： 从 San Diego Super Computer Center 到 www.poly.edu 的三次 Traceroute a. 在三个时间段内的平均往返时延分别是. ，. 和. ，标准方差分别 为. ，. ，. 。 b. 在这个例子中，在三个时段各自路由器数量均为 12 个，在这些时段中路径未发生变化。 c. Traceroute 分组在源 IP 地址和目的 IP 地址之间经过了 4 个 ISP 网络。是的，在相邻的 ISP 网络间对等接口处出现了最大的时延
计算机网络课后习指导 陕西师范大学 2. 111E14 1. 1语 从ww.stella-net.net(France)到ww.polv.ed(USA)的Traceroutes 在三个时间段内的平均往返时延分别是87.09ms,86.35ms和86.48ms,标准方差分别为 0.53m5,0.18ms,0.23m5。在这个例子中，在三个时段各自路由器数量均为11个，在 这些时段中路径未发生变化，Traceroute分组在源P地址和目的P地址之间经过了3个ISE 网络。是的， 在相邻的1SP网络间对等接口处出现了最大的时延。 P19.(a)Visit the sitewwracerouteor and perform traceroutes from twodifferent cities in France to the same destination host in the United States.How many links are the same in the two traceroutes? Is the transatlantic link the same? (b)Repeat (a)but this time Franceand another cityi Gemany (c)Pick a city in the United States,and perform traceroutes to two hosts,each in a different city in China.How many links are common in the two traceroutes?Do the two traceroutes diverge before reaching China? 实验结果如下：
计算机网络课后习题指导 陕西师范大学 10 从 www.stella-net.net (France) 到 www.poly.edu (USA)的 Traceroutes 在三个时间段内的平均往返时延分别是. ，. 和. ，标准方差分别为 . ，. ，. 。在这个例子中，在三个时段各自路由器数量均为 11 个，在 这些时段中路径未发生变化。Traceroute 分组在源 IP 地址和目的 IP 地址之间经过了 3 个 ISP 网络。是的，在相邻的 ISP 网络间对等接口处出现了最大的时延。 P19. (a) Visit the site www.traceroute.org and perform traceroutes from two different cities in France to the same destination host in the United States. How many links are the same in the two traceroutes? Is the transatlantic link the same? (b) Repeat (a) but this time choose one city in France and another city in Germany. (c) Pick a city in the United States, and perform traceroutes to two hosts, each in a different city in China. How many links are common in the two traceroutes? Do the two traceroutes diverge before reaching China? 实验结果如下：
计算机网络课后习指导 陕西师范大学 11201.a1.1r-t.5.31.12.25制0,5520.14 Caiaabitechernet7-1. ..(4.19 从法国一个城市到美国纽约市的Traceroute a.在这些从法国一个城市到美国纽约市的Traceroute的实验中包括大洋彼岸的链路在内 共有7条链路。 沿测8 5456 166 从法国一个城市到德国一个城市的Traceroute b.在这些从法国一个城市到德国一个城市的Traceroute的实验中一共有7条链路，没有大
计算机网络课后习题指导 陕西师范大学 11 从法国一个城市到美国纽约市的 Traceroute a. 在这些从法国一个城市到美国纽约市的 Traceroute 的实验中包括大洋彼岸的链路在内一 共有 7 条链路。 从法国一个城市到德国一个城市的 Traceroute b. 在这些从法国一个城市到德国一个城市的 Traceroute 的实验中一共有 7 条链路，没有大
计算机网络课后习题指导 陕西师范大学 洋彼岸的链路 oo. d-rtro c.r 美国的一台主机对中国两个不同城市的两台主机进行Traceroute c.在两个traceroute试验中，均是5条链路。这两个traceroute在抵达中国之前已分开， 0.Consider the Figure(b)that there re client-server p s rather than 1.the rates of the server and network link.Assume all other links have abundant capacity and that there is no other traffic in the network besides the traffic generated by the M client-server pairs.Derive a general expression for throughput in terms ofRR,and M. 答：吞肚量Throughput=minRR。R/M。 P21.Consider Figure 1.19(b).Now suppose that there are Mpaths between the server and the client. No two paths share any link.Path(=1....M)consists of Nlinks with transmission ratesR R张. Ry.If the server can only use one path to send data to the client.what is the maximum throughput that the server can achieve?If the server cn use all Mpaths to send data,what is the nachieve? 答：如果只能用一条路径，最大吞吐率为： max(min(R,R,…,R贴)，min(R经，R经，…，R肠)，…，min(R,R,…,R) 如果使用所有路径，最大吞吐率为：
计算机网络课后习题指导 陕西师范大学 12 洋彼岸的链路。 美国的一台主机对中国两个不同城市的两台主机进行 Traceroute c.在两个 traceroute 试验中，均是 5 条链路。这两个 traceroute 在抵达中国之前已分开。 P20. Consider the throughput example corresponding to Figure 1.20(b). Now suppose that there are M client-server pairs rather than 10. Denote Rs, Rc, and R for the rates of the server links, client links, and network link. Assume all other links have abundant capacity and that there is no other traffic in the network besides the traffic generated by the M client-server pairs. Derive a general expression for throughput in terms of Rs, Rc, R, and M. 答：吞吐量 Throughput = min{Rs, Rc, R/M}。 P21. Consider Figure 1.19(b). Now suppose that there are M paths between the server and the client. No two paths share any link. Path k (k = 1, . . ., M ) consists of N links with transmission rates Rk1, R2k, . . ., RNk. If the server can only use one path to send data to the client, what is the maximum throughput that the server can achieve? If the server can use all M paths to send data, what is the maximum throughput that the server can achieve? 答：如果只能用一条路径，最大吞吐率为： max( , , ⋯ , ), ( , , ⋯ , ), ⋯ , ( , , ⋯ , )， 如果使用所有路径，最大吞吐率为：