计算机网络实验5Wireshark Lab IPWord下载.docx

计算机网络实验5Wireshark Lab IPWord下载.docx

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•IfyouareusingaWindowsplatform,startuppingplotterandenterthenameofatargetdestinationinthe“AddresstoTraceWindow.”Enter3inthe“#oftimestoTrace”field,soyoudon’tgathertoomuchdata.SelectthemenuitemEdit->

AdvancedOptions->

PacketOptionsandenteravalueof56inthePacketSizefieldandthenpressOK.ThenpresstheTracebutton.

•Next,sendasetofdatagramswithalongerlength,byselectingEdit->

PacketOptionsandenteravalueof2000inthePacketSizefieldandthenpressOK.ThenpresstheResumebutton.Finally,sendasetofdatagramswithalongerlength,byselectingEdit->

PacketOptionsandenteravalueof3500inthePacketSizefieldandthenpressOK.ThenpresstheResumebutton.

•StopWiresharktracing.

QUESTIONS

1.SelectthefirstICMPEchoRequestmessagesentbyyourcomputer,andexpandtheInternetProtocolpartofthepacketinthepacketdetailswindow.WhatistheIPaddressofyourcomputer?

TheIPaddressofmycomputeris172.18.40.131.

2.WithintheIPpacketheader,whatisthevalueintheupperlayerprotocolfield?

WithintheIPpacketheader,thevalueintheupperlayerprotocolfieldis1,whichmeansICMPprotocol.

3.HowmanybytesareintheIPheader?

HowmanybytesareinthepayloadoftheIPdatagram?

Explainhowyoudeterminedthenumberofpayloadbytes.

20bytesareintheIPheader.36bytesareinthepayloadoftheIPdatagram.Wecanseethatthetotallengthis56,andtheheaderis20bytes,sothepayloadbytesis56–20=36bytes.

4.HasthisIPdatagrambeenfragmented?

Explainhowyoudeterminedwhetherornotthedatagramhasbeenfragmented.

ThisIPdatagramhasn’tbeenfragmented.Becausethatboththeflagsandthefragmentoffsetare0,whichmeansthatithasn’tbeenfragmented.

5.WhichfieldsintheIPdatagramalwayschangefromonedatagramtothenextwithinthisseriesofICMPmessagessentbyyourcomputer?

Identification,Timetolive,Headerchecksum.

packet1

packet2

packet3

6.Whichfieldsstayconstant?

Whichofthefieldsmuststayconstant?

Whichfieldsmustchange?

Why?

Stayconstant:

Version,HeaderLength,DifferentiatedServices,UpperLayerProtocol,SourceIPandDestinationIP.

Muststayconstant:

Version,becausethatweareusingIPv4forallpackets

HeaderLength,becausethatthepacketsareICMPpackets.

DifferentiatedServices,becausethatallthepacketsareICMPpackets.

UpperLayerProtocol,becausethatallthepacketsareICMPpackets.

SourceIP,becausethatwearesendingpacketsfromthesamecomputer.

DestinationIP,becausethatwearesendingthepacketstothesameIPaddress.

Mustchange:

Identification,becausethateverypackethastohavedifferentIDs.

Timetolive:

becausethattracerouteincrementseachsubsequentpacket.

Headerchecksum:

becausethattheheaderchanges,thentheheaderchecksumchanges.

7.DescribethepatternyouseeinthevaluesintheIdentificationfieldoftheIPdatagram

EveryIPdatagramhasadifferentvalue,andtheIPIdentificationfieldincrementwitheachICMPecho（ping）request.Wecanseethatthevalueisadded1whenanewIPdatagramcomes.

packet1

packet2

packet3

8.WhatisthevalueintheIdentificationfieldandtheTTLfield?

ThevalueintheIdentificationfieldis62738.ThevalueintheTTLfieldis254.

9.DothesevaluesremainunchangedforalloftheICMPTTL-exceededrepliessenttoyourcomputerbythenearest（firsthop）router?

TheIdentificationfieldwillchangesforalltheICMPTTL-exceededrepliesbecausethattheIdentificationfieldisuniquevalueforeverypacket.

TheTTLfieldremainsunchangedbecausethatthefirsthoprouterdoesn’tchange,thentheTTLforthefirsthoprouterisalwaysthesame.

10.FindthefirstICMPEchoRequestmessagethatwassentbyyourcomputerafteryouchangedthePacketSizeinpingplottertobe2000.HasthatmessagebeenfragmentedacrossmorethanoneIPdatagram?

[Note:

ifyoufindyourpackethasnotbeenfragmented,youshoulddownloadthezipfilehttp:

//gaia.cs.umass.edu/wireshark-labs/wireshark-traces.zipandextracttheipethereal-trace-1packettrace.IfyourcomputerhasanEthernetinterface,apacketsizeof2000shouldcausefragmentation.]

ThatmessagehasbeenfragmentedacrossmorethanoneIPdatagram.

11.PrintoutthefirstfragmentofthefragmentedIPdatagram.WhatinformationintheIPheaderindicatesthatthedatagrambeenfragmented?

WhatinformationintheIPheaderindicateswhetherthisisthefirstfragmentversusalatterfragment?

HowlongisthisIPdatagram?