UDPWord格式文档下载.docx

UDPWord格式文档下载.docx

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Answerthesequestionsdirectlyfromwhatyouobserveinthepackettrace.）Namethesefields.

Thereare4fieldsintheUDPheader.

Theirnamesare:

Sourceport;

Destinationport;

Length;

Checksum.

2.Fromthepacketcontentfield,determinethelength（inbytes）ofeachoftheUDPheaderfields.

ThelengthofUDPdatagramis159bytes,thelengthofdatais151bytes.So,thelengthofUDPheaderis（159-151=）8bytes.

3.ThevalueintheLengthfieldisthelengthofwhat?

VerifyyourclaimwithyourcapturedUDPpacket.

ThevalueintheLengthfieldisthelengthofUDPdatagram.

4.WhatisthemaximumnumberofbytesthatcanbeincludedinaUDPpayload?

Thetotallengthis1500bytes.ThelengthofIPheaderis20bytes.ThelengthofUDPheaderis8bytes.So,Themaximumnumberofbytesis（1500-20-8=）1472bytes.

5.Whatisthelargestpossiblesourceportnumber?

ThelengthofUDPheaderis8bytes.Thereare4fields.Everyoneofthemarethesamelength.So,thesourceportis2bytes.

Thetotalnumberis（2^16=）65536.

Andtheboundaryofsourceportnumberis（0~65535）.

Thelargestpossiblesourceportnumberis65535.

6.WhatistheprotocolnumberforUDP?

Giveyouranswerinbothhexadecimalanddecimalnotation.（Toanswerthisquestion,you’llneedtolookintotheIPheader.）

TheprotocolnumberforUDPis17.

7.

Search“UDP”inGoogleanddeterminethefieldsoverwhichtheUDPchecksumiscalculated.

ThewaytocalculateUDPchecksum.

Thisprotocolprovidesaprocedureforapplicationprogramstosend

messagestootherprogramswithaminimumofprotocolmechanism.The

protocolistransactionoriented,anddeliveryandduplicateprotection

arenotguaranteed.Applicationsrequiringorderedreliabledeliveryof

streamsofdatashouldusetheTransmissionControlProtocol（TCP）[2].

Format

------

0781516232431

+--------+--------+--------+--------+

|Source|Destination|

|Port|Port|

|||

|Length|Checksum|

|

|dataoctets...

+----------------...

UserDatagramHeaderFormat

Fields

SourcePortisanoptionalfield,whenmeaningful,itindicatestheport

ofthesendingprocess,andmaybeassumedtobetheporttowhicha

replyshouldbeaddressedintheabsenceofanyotherinformation.If

notused,avalueofzeroisinserted.

Postel[page1]

28Aug1980

UserDatagramProtocolRFC768

DestinationPorthasameaningwithinthecontextofaparticular

internetdestinationaddress.

Lengthisthelengthinoctetsofthisuserdatagramincludingthis

headerandthedata.（Thismeanstheminimumvalueofthelengthis

eight.）

Checksumisthe16-bitone'

scomplementoftheone'

scomplementsumofa

pseudoheaderofinformationfromtheIPheader,theUDPheader,andthe

data,paddedwithzerooctetsattheend（ifnecessary）tomakea

multipleoftwooctets.

ThepseudoheaderconceptuallyprefixedtotheUDPheadercontainsthe

sourceaddress,thedestinationaddress,theprotocol,andtheUDP

length.Thisinformationgivesprotectionagainstmisrouteddatagrams.

ThischecksumprocedureisthesameasisusedinTCP.

+--------+--------+--------+--------+

|sourceaddress|

|destinationaddress|

|zero|protocol|UDPlength|

Ifthecomputedchecksumiszero,itistransmittedasallones（the

equivalentinone'

scomplementarithmetic）.Anallzerotransmitted

checksumvaluemeansthatthetransmittergeneratednochecksum（for

debuggingorforhigherlevelprotocolsthatdon'

tcare）.

8.ExamineapairofUDPpacketsinwhichthefirstpacketissentbyyourhostandthesecondpacketisareplytothefirstpacket.Describetherelationshipbetweentheportnumbersinthetwopackets.

TotheUDPpacketwhichmyhostsentandmyhostreceived,theirsourceportnumberanddestinationportnumberareeachother.Wecanseeitfromtheabove2images.

ExtraCredit

1.CaptureasmallUDPpacket.Manuallyverifythechecksuminthispacket.Showallworkandexplainallsteps.

以太网帧首部（14字节）：

01005e2217ea00247e0518370800

IP首部（20字节）：

4500003041f800000111e3a80a028829ea2217ea

UDP首部（8字节）：

fb43838a001cbab4

源端口号：

fb43目的端口号：

838a

UDP长度：

001c

伪首部字段：

32位源IP地址、32位目的IP地址、8位协议、16位UDP长度.

udp数据报和tcp报文段在为了计算检验和而设置了一个12字节长的伪首部。

由于udp数据报的长度可以为奇数字节，但是检验和算法是把若干个16bit字相加，因此可在填充字节字段填入0，可能增加的填充字节不被传送。

因此必须构造12字节的伪首部：

0a028829

ea2217ea

0011

001c

构造完毕之后，把udp检验和部分置0，然后对udp每个16bit进行二进制反码求和，

0a02+8829+ea22+17ea+0011+001c+fb43+838a+001c+0000+

1000+0000+…+4fbb+5ef3=454b

计算过程如下：

0a02+8829+ea22+17ea=9438

0011+001c+fb43+838a=7efb

001c+0000+1000+aed3+fa1e=b90e

Efff+fa4e+37fd+a80d=ca59

4fbb+5ef3=aeae

再把以上算的结果相加：

9438+7efb+b90e=cc42

ca59+aeae=7908

最后的结果为：

cc42+7908=454b

对454b求反即得：

CheckSum：

bab4

五、实验结论

1.UDP报文段头部8个字节，分为四个字段，分别为：

源端口号，目的端口号，长度和检验和。

2.UDP报文段的最大长度为1472个字节。