计算机网络实验5Wireshark Lab IP.docx
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计算机网络实验5Wireshark Lab IP.docx
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计算机网络实验5WiresharkLabIP
Lab5WiresharkLab:
IP
STEPS
•StartupWiresharkandbeginpacketcapture(Capture->Option)andthenpressOKontheWiresharkPacketCaptureOptionsscreen(we’llnotneedtoselectanyoptionshere).
•IfyouareusingaWindowsplatform,startuppingplotterandenterthenameofatargetdestinationinthe“AddresstoTraceWindow.”Enter3inthe“#oftimestoTrace”field,soyoudon’tgathertoomuchdata.SelectthemenuitemEdit->AdvancedOptions->PacketOptionsandenteravalueof56inthePacketSizefieldandthenpressOK.ThenpresstheTracebutton.
•Next,sendasetofdatagramswithalongerlength,byselectingEdit->AdvancedOptions->PacketOptionsandenteravalueof2000inthePacketSizefieldandthenpressOK.ThenpresstheResumebutton.Finally,sendasetofdatagramswithalongerlength,byselectingEdit->AdvancedOptions->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?
Why?
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?
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