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Chapter 13. IGMP: Internet Group Management Protocol

Chapter 13. IGMP: Internet Group Management Protocol

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Chapter13.IGMP:Internet

GroupManagementProtocol

Section13.1.Introduction

Section13.2.CodeIntroduction

Section13.3.igmpStructure

Section13.4.IGMPprotoswStructure

Section13.5.JoiningaGroup:

igmp_joingroupFunction







Section13.6.igmp_fasttimoFunction

Section13.7.InputProcessing:

igmp_inputFunction

Section13.8.LeavingaGroup:

igmp_leavegroupFunction

Section13.9.Summary



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TCP/IPIllustrated,Volume2:The

ImplementationByGaryR.Wright,

W.RichardStevens

TableofContents



Chapter13.IGMP:InternetGroup

ManagementProtocol



13.1Introduction

IGMPconveysgroupmembership

informationbetweenhostsandrouterson

alocalnetwork.Routersperiodically

multicastIGMPqueriestotheall-hosts

group.Hostsrespondtothequeriesby

multicastingIGMPreportmessages.The

IGMPspecificationappearsinRFC1112.

Chapter13ofVolume1describesthe

specificationofIGMPandprovidessome

examples.

Fromanarchitectureperspective,IGMPis

atransportprotocolaboveIP.Ithasa

protocolnumber(2)anditsmessagesare

carriedinIPdatagrams(aswithICMP).



IGMPusuallyisn'taccesseddirectlybya

processbut,aswithICMP,aprocesscan

sendandreceiveIGMPmessagesthrough

anIGMPsocket.Thisfeatureenables

multicastroutingdaemonstobe

implementedasuser-levelprocesses.

Figure13.1showstheoverallorganization

oftheIGMPprotocolinNet/3.



Figure13.1.SummaryofIGMP

processing.



ThekeytoIGMPprocessingisthe

collectionofin_multistructuresshownin







thecenterofFigure13.1.Anincoming

IGMPquerycausesigmp_inputtoinitialize

acountdowntimerforeachin_multi

structure.Thetimersareupdatedby

igmp_fasttimo,whichcalls

igmp_sendreportaseachtimerexpires.

WesawinChapter12thatip_setmoptions

callsigmp_joingroupwhenanewin_multi

structureiscreated.igmp_joingroupcalls

igmp_sendreporttoannouncethenew

groupandenablesthegroup'stimerto

scheduleasecondannouncementashort

timelater.igmp_sendreporttakescareof

formattinganIGMPmessageandpassing

ittoip_output.

OntheleftandrightofFigure13.1wesee

thatarawsocketcansendandreceive

IGMPmessagesdirectly.



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TCP/IPIllustrated,Volume2:The

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W.RichardStevens

TableofContents



Chapter13.IGMP:InternetGroup

ManagementProtocol



13.2CodeIntroduction

TheIGMPprotocolisimplementedinfour

fileslistedinFigure13.2.



Figure13.2.Filesdiscussedinthis

chapter.



GlobalVariables

Threenewglobalvariables,shownin



Figure13.3,areintroducedinthischapter.



Figure13.3.Globalvariablesintroduced

inthischapter.



Statistics

IGMPstatisticsaremaintainedinthe

igmpstatvariablesshowninFigure13.4.



Figure13.4.IGMPstatistics.



Figure13.5showssomesampleoutputof

thesestatistics,fromthenetstat-pigmp







commandonvangogh.cs.berkeley.edu.



Figure13.5.SampleIGMPstatistics.



FromFigure13.5wecantellthatvangogh

isattachedtoanetworkwhereIGMPis

beingused,butthatvangoghisnotjoining

anymulticastgroups,since

igps_snd_reportsis0.



SNMPVariables

ThereisnostandardSNMPMIBforIGMP,

but[McCloghrieandFarinacci1994a]

describesanexperimentalMIBforIGMP.



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W.RichardStevens

TableofContents



Chapter13.IGMP:InternetGroup

ManagementProtocol



13.3igmpStructure

AnIGMPmessageisonly8byteslong.

Figure13.6showstheigmpstructureused

byNet/3.



Figure13.6.igmpstructure.



43-44

A4-bitversioncodeanda4-bittypecode

arecontainedwithinigmp_type.Figure



13.7showsthestandardvalues.



Figure13.7.IGMPmessagetypes.



Onlyversion1messagesareusedby

Net/3.Multicastrouterssendtype1

(IGMP_HOST_MEMBERSHIP_QUERY)

messagestosolicitmembershipreports

fromhostsonthelocalnetwork.The

responsetoatype1IGMPmessageisa

type2

(IGMP_HOST_MEMBERSHIP_REPORT)

messagefromthehostsreportingtheir

multicastmembershipinformation.Type3

messagestransportmulticastrouting

informationbetweenrouters(Chapter14).

Ahostneverprocessestype3messages.

Theremainderofthischapterdiscusses

onlytype1and2messages.

45-46

igmp_codeisunusedinIGMPversion1,

andigmp_cksumisthefamiliarIP







checksumcomputedoverall8bytesofthe

IGMPmessage.

47-48

igmp_groupis0forqueries.Forreplies,it

containsthemulticastgroupbeing

reported.

Figure13.8showsthestructureofan

IGMPmessagerelativetoanIPdatagram.



Figure13.8.AnIGMPmessage

(igmp_omitted).



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