计网第五次作业.docx

计网第五次作业.docx

《计网第五次作业.docx》由会员分享，可在线阅读，更多相关《计网第五次作业.docx（9页珍藏版）》请在冰豆网上搜索。

计网第五次作业

计网第五次作业

ReviewQuestions:

1.Whataresomeofthepossibleservicesthatalink-layerprotocolcanoffertothenetworklayer?

Whichoftheselink-layerserviceshavecorrespondingservicesinIP?

InTCP?

链路层协议提供给网络层的服务有哪些？

哪些是给IP的？

哪些是给TCP的？

Linkaccess,framing,reliabledeliverybetweenadjacentnodes,flowcontrol,errordetection,errorcorrection,half-duplexandfull-duplex.

InIP:

framing,errordetection.

InTCP:

framing,reliabledeliverybetweenadjacentnodes,errordetection,half-duplexandfull-duplex.

2.IfallthelinksintheInternetweretoprovidereliabledeliveryservice,wouldtheTCPreliabledeliveryserviceberedundant?

Whyorwhynot?

不会多余，因为TCP保证的是传输层的数据传送，而link提供稳定可靠传输保证链路层的传输稳定，二者不完全重叠，所以TCP可靠传输也不会多余。

3.InSection5.3,welistedfourdesirablecharacteristicsofabroadcastchannel.WhichofthesecharacteristicsdoesslottedALOHAhave?

Whichofthesecharacteristicsdoestokenpassinghave?

4.SupposetwonodesstarttotransmitatthesametimeapacketoflengthLoverabroadcastchannelofrateR.Denotethepropagationdelaybetweenthetwonodesasdprop.Willtherebeacollisionifdprop

Whyorwhynot?

因为L/R=包传递的时间，如果dprop

7.SupposenodesA,B,andCeachattachtothesamebroadcastLAN（throughtheiradapters）.IfAsendsthousandsofIPdatagramstoBwitheachencapsulatingframeaddressedtotheMACaddressofB,willC’sadapterprocesstheseframes?

Ifso,willC’sadapterpasstheIPdatagramsintheseframestothenetworklayerC?

HowwouldyouranswerschangeifAsendsframeswiththeMACbroadcastaddress?

不会，C会拆封帧从而读取报头的MAC，因为每一个host的MAC都唯一，C读取到数据报中的MAC和自己的不一样就不会继续拆封数据报，不会投递给C。

当使用LAN口广播地址的时候，C的适配器就会拆封帧，向C传递数据。

8.HowbigistheMACaddressspace?

TheIPv4addressspace?

TheIPv6addressspace?

2^48，2^32，2^128.

9.WhyisanARPquerysentwithinabroadcastframe?

WhyisanARPresponsesentwithinaframewithaspecificdestinationMACaddress?

因为新加入网络的主机是不知道路由器的IP的，自己也没有IP，所以只能广播才能得到IP。

因为每个主机的MAC地址都是唯一的，而ARP建立转发表的时候会带上MAC地址。

12.InCSMA/CD,afterthefifthcollision,whatistheprobabilitythatanodechoosesK=4?

TheresultK=4correspondstoadelayofhowmanysecondsona10MbpsEthernet?

可能，因为第五次K的取值范围是0-（2^5-1）即0-31。

Bittime=1bit/R=1bit/10Mbps=1msec，K=4，waittime=4*512*1msec=2048msec。

Problems:

1.Supposetheinformationcontentofapacketisthebitpattern1110110010001010andanevenparityschemeisbeingused.Whatwouldthevalueofthefieldcontainingtheparitybitsbeforthecaseofatwo-dimensionalparityscheme?

Youranswershouldbesuchthataminimum-lengthchecksumfieldisused.假设一个数据包的信息含量是XXX，使用偶校验方案。

采用二位奇偶校验方案的字段包含的奇偶校验位的字段的值是多少？

答案要使用最小长度校验。

10100

10100

10100

10111

00011

2.Supposetheinformationportionofapacket（DinFigure5.4）contains10bytesconsistingofthe8-bitunsignedbinaryASCIIrepresentationoftheintegers0through9.ComputetheInternetchecksumforthisdata.假设一个包的信息的一部分包括10bytes组成的8-bit无符号二进制码表示的整数0-9，计算该数据的网络校验。

算校验码先把0-9加起来

0000000000000001

0000001000000011

0000010000000101

0000011000000111

0000100000001001

0001010000011001

→→→→取反可得

校验码为1110101111100110

3.Considerthepreviousproblem,butinsteadofcontainingthebinaryofthenumbers0through9supposethese10bytescontain.ComputetheInternetchecksumforthisdata.

a.thebinaryrepresentationofthenumbers1through10.

0000000100000010

0000001100000100

0000010100000110

0000011100001000

0000100100010100

0001101100011110

→→→→取反可得

1110010011100001

b.theASCIIrepresentationofthelettersAthroughJ（uppercase）.

0100000101000010

0100001101000100

0100010101000110

0100011101001000

0100100101001010

0101100001011111

→→→→取反可得

1010011110100000

c.theASCIIrepresentationofthelettersathroughj（lowercase）.小写字母表示

0110000101100010

0110001101100100

0110010101100110

0110011101101000

0110100101101010

1111100111111101

→→→→取反可得

0000011000000010

6.Considerthepreviousproblem,butsupposethatDhasthevalue

a.10010001.

b.10100011.

c.01010101.

前一题题目：

Considerthe7-bitgenerator,G=10011,andsupposethatDhasthevalue1010101010.WhatisthevalueofR？

求余而已，记住不要做减法而是做与运算就好。

a.R=001

b.R=101

c.R=101

12.ConsiderthreeLANsinterconnectedbytworouters,asshowninFigure5.38.

a.Redrawthediagramtoincludeadapters.重新画图

b.AssignIPaddressestoalloftheinterfaces.ForSubnet1useaddressesoftheform111.111.111.xxx;forSubnet2usesaddressesoftheform122.122.122.xxx;andforSubnet3useaddressesoftheform133.133.133.xxx.所有的接口分配IP地址。

c.AssignMACaddressestoalloftheadapters.

a.b.c如图

e.Repeat（d）,nowassumingthattheARPtableinthesendinghostisempty（andtheothertablesareuptodate）.

发送方的ARP