计网_第五章作业
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Chapter 5
作业题,请同学们注意题目描述。
1.(R1)Whataresomeof the possible services that a link-layer protocol can
offer tothenetworklayer?Whichof theselink-layer services havecorrespondingservicesin IP?In TCP?
答:
可能提供得服务:链路接入,成帧,可靠传输,流量控制,差错检测,差错纠正,半双工与全双工;
In IP:成帧,差错检测;
InTCP: 成帧,可靠传输,差错检测,半双工与全双工。
2.(R2)If all thelinks in theInternet were toprovide reliable delivery serv
ice,wouldthe TCPreliable delivery service be redundant?Why or why not?
答:
不会多余,因为TCP保证得就是传输层得数据传送,而link提供稳定可靠传输保证链路层得传输稳定,二者不完全重叠,所以TCP可靠传输也不会多余。例如,链路层协议不能保证端到端得顺序,就需要TCP协议进行排序;路由器缓存溢出丢包,链路层协议无法解决这个问题,但就是TCP可以。
3.(R8)Howbig istheMACaddress space? The IPv4address space? The
IPv6 address space?
答:
248个MAC addresses;232个IPv4addresses;2128个IPv6 addresses4.(R9)Why is an ARP query sent within abroadcastframe?Why is
an ARP responsesentwithin a frame with a specific destinati on MACaddress?
答:
因为新加入网络得主机就是不知道路由器得IP得,自己也没有IP,所以只能广播才能得到IP,特定得主机就会特定响应。因为每个主机得MAC地址都就是唯一得,而ARP建立转发表得时候会带上MAC地址。
5.(P5)Consider the 4-bitgenerator, G, shown in Figure 5、8,and su
ppose thatD has the value 11111010、What isthevalue of R?G =1001
答:
得余数,∴R=110
6.(P11)Consider a broadcastchannel with N nodesandatransmission
rateofR bps、Suppose the broadcastchannel uses polling (with anadditional polling node)for multiple access、Suppose theamountof time from when a nodepletes transmission until the subsequent nodeis permitted totransmit(that is,the pollingdelay) is d pol
l、Suppose thatwithin a polling round, a given nodeis allowedto tr ansmitat most Q bits、What isthemaximum throughput ofthe broadcast channel?
答:
情况1:ﻩﻩﻩﻩ情况2:
两种极端情况进行比较,明显情况2为最大值;情况1为最小值。
7.(P12)Considerthree LANs interconnectedby two routers, as s
hown in Figure 5、38、
a、Redraw thediagramto includeadapters、
Figure 5、38 Three subnets, interconnected by routers
b、Assign IPaddressesto allof the interfaces、For Subnet I use addres
ses of theform111、111、111、xxx; for Subnet2uses addresses oft heform122、222、222、xxx; and for Subnet 3use addressesof the for m133、333、333、xxx、(注:本题目略有修改,见红色标注)
c、AssignMACaddresses to all of the adapters、
d、Consider sendingan IPdatagram fromHost Ato Host F、S
uppose allof the ARP tablesareup to date、Enumerate all the steps, as do ne forthesingle-routerexample in Section 5.4.2、
e、Repeat (d),now assumingthat the ARP table inthesending host isempty(and the othertables areupto date)、
答:
a、b、c、
d、1、由A得转发表决定了数据报应该被转发到路由器端口111、111、111、002、
2、由A得适配器生成一个携带以太网目得地址22-22-22-22-22-22得以太网分组、3、第一个路由器即收到该分组并且解析了数据报。该路由器中得转发表提示该数据报应该被转发到122、222、222、003、
4、第一个路由器发送带有目得地址为55-55-55-55-55-55并且源地址为33-33-33-33-33-33得以太网分组,经过IP地址为122、222、222、002得端口。
5、过程以此类推,直到分组到达主机F 、
e、主机A得ARP模块一定知道本网段网关得IP地址为111、111、111、002。主机A 发送一个ARP请求(包含在一个以太网广播帧中)。第一个路由器接收到这个请求分组并且发送给主机A一个ARP响应分组。这个ARP响应分组装载于一个包含以太网目得地址00-00-00-00-00-00得以太网帧上。
得知了路由器端口得MAC地址后,以她作为目得地址成帧,以后得步骤与上一问相同。
8.(P15)Suppose nodes A and B are on the same 10Mbps Ethernet bus,and the pr
opagation delay between the twonodes is 225 bit times、Suppos e A and B send frames at thesame time,the frames collide, andt hen Aand B choosedifferentvaluesof Kinthe CSMA/CDalgorithm、Assuming no other nodes are active, can theretransmissions from A andBcollide?For ourpurposes, it suffices to work out the following example、Suppose A andB begin transmission at t= 0 bit times、Theyboth detect collisionsat t= 225bittimes、Theyfinish transmitting a jamsignal at t=225+ 48=273bit times、Supposeand ,At what time doesB schedule its retransmission? At what time does Abegintransmission?(Note:Thenodesmust waitfor anidle channel after returningto Step2-see protocol、) Atwhat time does A'ssignalreachB?Does B refrain from transmittingatitsscheduled time?
答:
9.(P16)Suppose nodesAand B are onthe same 10 Mbps Ethernetbus,
and the propagation delaybetween thetwonodes is225bit times、Suppose nodeA begins transmitting a frame and,before it finishes,node Bb egins transmitting a frame、Can Afinish transmitting before it detects that Bhas transmitted?Why or why not? Ifthe answer is yes,t henAincorrectly believes thatitsframe wassuccessfully transmitted without acollision、Hint:Supposeat time t = 0bittimes, A