Ip multicast extensions for mobile internetworking

IPMulticastExtensionsforMobileInternetworking

ArupACHARYA,AjayBAKREandB.R.BADRINATH

DepartmentofComputerScience,RutgersUniversity,NewBrunswick,NJ08903,USA

{acharya/bakre/badri}@cs.rutgers.edu}

Thispaperdealswithmulticastinginaninternetworkwithmobilehosts,particularlywithregardtoMobile-IP[11]and

DistanceVectorMulticastRouting(DVMRP)[10,9]protocols.Whenthesourceofamulticastdatagramisamobilehost

(MH),thedatagrammaynotreachallgroupmemberstowhichthedatagramisaddressed,includingothermobilehosts.When

thesourceisastatichostandthemulticastgroupincludesmobilehosts,amobilegroupmembermayreceivedatagramsin

onecellbutnotinanother.Further,whenaMHentersacellwhichcontainsnoothermemberofthesamegroup,theMHwill

experienceadelaybeforeitstartsreceivingdatagramsaddressedtothatgroup.Mobilitybetweencampuses,whichresultin

aMHacquiringanadditionalunicastaddress,alsohasaneffectonmulticastrouting.WeproposeenhancementstoDVMRP

executedattheMobilitySupportRouters(MSR)thatensurecorrectforwardingofmulticastdatagramstoandfrommobile

hosts.Oursolutionsdonotrequireanychangeathostsandroutersunawareofmobility,i.e.themodificationsarelimitedto

MSRsandMHs.Wealsodescribeanimplementationincorporatingasubsetofourproposals.Lastly,weshowthatalternate

stylesofmulticastingormobilenetworking,viz.link-state(MOSPF[14])andIETFproposal[1],willfacesimilarproblems

andourproposedsolutionsarestillvalidintheircontext.

1Introduction

ThispaperexplorestheinteractionbetweenIP-

multicastingandhostmobility.Multicastingatthenet-

worklayerisanefficientmechanismfordeliveringcopies

ofthesamedatagramtomultipledestinations.Presently,

multicastroutingintheInternetisprimarilyimplemented

usingtheDistanceVectorMulticastRoutingProtocol

(DVMRP)[9,10]andformsthebasisofMBONE.MO-

SPF[14]isanothermulticastingprotocolbasedonlink-

stateunicastrouting.AnactiveareaofstudyisProtocol

IndependentMulticasting(PIM)[8]whichaimstore-

movedependenceofmulticastroutingonunderlyinguni-

castroutingprotocols.WewilldiscussIP-multicasting

inthispaperprimarilyinthecontextofDVMRPgiven

itspopularityandafreelyavailableimplementation,viz.

mrouted[16].

Theemergenceofportablecomputerscoupledwith

wirelessnetworkinghasmotivatedtheneedforprotocols

[4,5,11,13,15,17]thatallowmobilehoststointeroper-

atewithinexistingnetworks.Outoftheproposedproto-

cols,arguablythemostwidelyusedschemeisMobile-IP

fromColumbiaUniversity[11,12]andtheonlyonefor

which,apubliclyavailableimplementationexists.This

schemehasalsobeenusedasatestbedforstudyingthe

impactofmobilityontransport-layerprotocols[3,6].

Thispaperwillthereforefocusonextendingmulticast

protocolstomobilehostsassumingtheaddressingand

(unicast)routingmechanismsofColumbia’sMobile-IP.Multicastinginaninternetworkwithmobilehosts

leadstothefollowingproblems:

•whenthesourceofamulticastdatagramisamobile

host,thenacopyofthedatagrammaynotreach

allhosts(staticormobile)thataremembersofthe

multicastgroup

•aMHmayexperienceadelayinreceivingmulticast

datagramswhenitentersacellthathasnoothergroup

memberlocatedinthesamecell

•datagramsmulticastfromastaticsourcemaynotreach

somecellsdependingontheTTLvalueusedwiththe

multicastdatagram:thus,thesamemobilehost(MH)

mayreceivedatagramsfromthatsourceinsomecells

butnotinothers.

TherootcauseoftheseproblemsisthatwhileDVMRP

implicitlyassumesthatallhosts(interfaces)withacom-

monsubnetaddresshavelink-layerconnectivity,i.e.con-

nectedbyaphysicalwire,thisassumptionisnolonger

trueinthepresenceofmobilehosts.

Oursolutiontotheaboveproblemconsistsof“heal-

ingthepartition”amongsttheMSRsusingapre-defined

multicasttunnel.Thismulticasttunnelisusedtosimulate

thenecessarylink-layerconnectivityamongsttheMSRs.

Thisabstraction,alongwithappropriatemodificationsto

theInternetGroupMembershipProtocol(IGMP)[7],is

adequatetoensurecorrectroutingofmulticastdatagrams

frommobilehoststoallgroupmembers,andalsotoen-

surethataMHexperiencesnodelayinreceivingdata-

1gramsregardlessofitsmobilitywithinthecampus.This

solutionhasbeenimplementedforDVMRP/mroutedex-

ecutedbytheMSRs.Wealsoproposeenhancementsto

ensureallornonesemantics,i.e.amulticastdatagram

iseitherreceivedinallcellswithinacampusorinnone.

Lastly,weextendoursolutiontohandleinter-campus

moves.

Thoughourdiscussionofmulticastingto/frommo-

bilehostsisbasedonDVMRPandMobile-IP,theprob-

lemsandourproposedsolutionthereinarealsoapplicable

tootherstylesofmulticasting,viz.MOSPForPIM,and

mobilenetworking,viz.theIETFproposal[1].

Theproblemofreliablydeliveringmulticastmes-

sagestomobilehostsattheapplication-layerisdiscussed

in[2].Thisworkisnotconcernedwithroutingmulticast

messagestomobilehosts,butinsteadusesapplication-

layerprotocolstodeterminethesetofMSRsthatsupport

membersofamulticastgroupandtoensurethatamobile

hostdoesnotreceivemultiplecopiesofthesamemes-