Distributed Systems Ad hoc and sensor networks€¦ · Networking in mobile systems • Diﬃculty:...

DistributedSystems

AdhocandsensornetworksRikSarkar

UniversityofEdinburgh

Fall2018

Distributed Systems, Edinburgh, 2018

MobileandUbiquitouscompuEng

•  Devices(computers)arecarriedbypeople(mobile)–  Laptops,phones,watches…

•  Theyareeverywhere–  Carriedbypeople(mobile)–  Embeddedintheenvironment

•  Coffeemachines,cameras,sensorsforlightcontrol,elevators…

–  Producelargeamountsofdata•  Usage,sensing…


Ubiquitous•  Advantages:

–  Therearecomputerseverywhere–  Everythingis“smart”–  PotenEallyusecomputaEonsonthesetomakethemevensmarter

•  Challenges:–  Therearemorethingstogowrong–  Noteasytomakethingsworkwellcoherently–  ConsistentplaSormsformanagingubiquitousdevicesdonotexist(yet)

–  Devicesdonotinteroperateeasily


Mobile

•  Advantages:–  Thesamedeviceiscarriedbytheperson–easytogiveconsistentservice

–  InformaEonwhenever,wherevertheyneed–  Deviceshavesensors–potenEalforsensingtheenvironmentandadapEng

•  Disadvantages:–  ConnecEvityischallenge:dataiscostly;networkdoesnotworkthesameway;mobilityinterfereswithcomunicaEon

–  LimitedbaXery:can’tdotoomuchcommunicaEon–  Howtomakeuseofsensors,notsowellunderstood


ContextawarecompuEng

•  AdaptcomputaEonstothecircumstances– Timeofday–  Istheuserpresent?–  Isthephoneinhandorinpocket– Scanforwifionlywhenindoors– Turnoffringwhenincinema,mee-ng…– RecognizeacEvityandbringuprelevantinformaEon

– …


ContextawarecompuEng

•  AdaptcomputaEonstothecircumstances•  BasiccontextsareeasytoidenEfy,butitisnotalwaysclearhowtoadapt– Turndownvolumeatnight…butwhatifitisanimportantcall?

•  Manycontextsareveryhardtodetectreliably


OthercontextdetecEonexamples

•  Indoor/outdoor•  UsesoundtodetectuserinameeEng•  Detecttransportmode(walking,car,bus,tram..)– Usingaccelerometer

•  DetectpresenceofotherusersnearbyfromwifiacEvity


ContextdetecEon

•  Generallyhard•  Concernsaboutprivacy:youdonotwanttosendcontextinformaEontoaserver

•  PerhapsdistributedcomputaEoncanhelp– Usedatafrommanyphonestodetectcontext– Butagain,donotwanttosendalldatatoserver– Doasmuchofitaspossibleondevice–filter/processdataatsource


Networkinginmobilesystems

•  Difficulty:–  Thenetworkgraphchanges– Anodeisnotalwaysconnectedtothesamerouter

•  Examplesystem:Mobilead-hocnetworks– Ad-hoc:Unplanned– Devicessimplyconnecttonearbydevicesandroutepackets

– Alsoappliestosensornetworks


RouEnginadhocwirelessnetworks

•  Findroutebetweenpairsofnodeswishingtocommunicate.

•  ProacEveprotocols:maintainrouEngtablesateachnodethatisupdatedaschangesinthenetworktopologyaredetected.

–  Heavyoverheadwithhighnetworkdynamics(causedbylink/nodefailuresornodemovement).

–  NotpracEcalfornetworksthatchangefrequently


RouEnginadhocwirelessnetworks

•  ReacEveprotocols:routesareconstructedondemand.NoglobalrouEngtableismaintained.

•  Moreappropriatefornetworkswithhighrateofchanges

–  AdhocondemanddistancevectorrouEng(AODV)–  DynamicsourcerouEng(DSR)


DynamicSourceRouEng(DSR)

•  NodeSwantstosendamessagetonodeD•  SiniEatesaaroutediscovery•  Sfloodsthenetworkwithrouterequest(RREQ)message

•  Eachnodeappendsitsownidtothemessage


RouteDiscovery:RREQ


RouteDiscovery:RREQ


RouteDiscovery:RREQ


RouteDiscovery:RREQ


RouteDiscovery:RREQ


RouteDiscovery:RREQ


RouteDiscoveryinDSR

•  DesEnaEonDonreceivingthefirstRREQsendsaroutereply(RREP)

•  RREPissentonarouteobtainedbyreversingtherouteinreceivedRREQ


RouteDiscovery:RREQ


RouteDiscovery:RREQ


When

•  Whenalinkfails,anerrormessagewiththelinknameissentbacktoS.

•  Sdeletesanyrouteusingthatlinkandstartsdiscovery.


Routecaching

•  Whenanodereceivesorforwardsamessage,itlearnsroutestoallnodesonthepath

•  Advantage:–  SmaynotneedtosendRREQ–  IntermediatenodeonreceivingRREQ,canrespondwithcompleteroute

•  Disadvantage:–  Cachesmaybestale:StriesmanycachedroutesbeforestarEngadiscovery.Or,intermediatenodesreturnoutdatedinformaEon.


DSR:SummaryAdvantages:•  Routescomputedonlywhenneeded–goodforchanging

networks•  Cachingcanmakethingsefficient•  DoesnotcreateloopsDisadvantages•  EnEreroutemustbecontainedinmessage:canbelongfor

largenetworks•  FloodingcausescommunicaEontomanynodes•  Stalecachescanbeaproblem•  Notsuitablefornetworkswherechangesaretoofrequent


AdhocOn-DemandDistanceVectorRouEng(AODV)

•  MaintainsrouEngtablesatnodessothattherouteneednotbestoredinthemessage

•  NoCaches:OnlyonerouteperdesEnaEon


AODVRouteDiscovery

•  SourcefloodsthenetworkDistributed Systems, Edinburgh, 2018

AODVRouteDiscovery

•  Othernodescreateparentpointer•  AnodeforwardsaRREQonlyonce

Dst NxtHp Dist

S S 1

Dst NxtHp Dist

S S 1 Dst NxtHp Dist

S S 1


AODVRouteDiscovery

•  Othernodescreateparentpointer•  AnodeforwardsaRREQonlyonce

Dst NxtHp Dist

S S 1

Dst NxtHp Dist

S E 2


AODVRouteDiscovery

•  RREPisforwardedviareversepath

Dst NxtHp Dist

S E 2

Dst NxtHp Dist

S S 1

Dst NxtHp Dist

S F 3


AODVRouteDiscovery

•  RREPisforwardedviareversepath•  Createsaforwardpath

Dst NxtHp Dist

S E 2

D D 1

Dst NxtHp Dist

S S 1

D F 2

Dst NxtHp Dist

S F 3

D D 0

Dst NxtHp Dist

S S 0

D E 3


Routeexpiry

•  ApathexpiresifnotusedforacertainEme.•  IfanodeseesthatarouEngtableentryhasnotbeenusedbythisEme,itremovesthisentry

•  Evenifthepathitselfisvalid•  Goodfornetworkswithfrequentchanges•  BadforstaEcandstablenetworks


Cancreateloops

•  AssumeC->Dlinkhasfailed,butAdoesnotknowbecausetheERRmessagewaslost

•  CisnowtryingtofindpathtoD•  ArespondssinceAthinksithasapath•  Createsloop:C-E-A-B-C


SequencenumbersinAODV

•  IfAhasaroutetoD,Akeepsasequencenumber.

•  Aincrementsthisnumberperiodically:tellshowoldtheinformaEonis


Usingsequencenumbers

•  Rule:sequencenumbermustincreasealonganyroute


Sequencenumberruleavoidsloop

•  Adoesnotreply,sinceitssequenceno.islessthanthatofC


AODV

•  RouEngtables,messagedoesnotcontainroute

•  Freshroutespreferred•  Oldunusedroutesexpire•  StalerouteslessproblemaEc•  Needssequencenumberstopreventloops•  BeXerformoredynamic,changingenvironments


RouEnginadhocnetworks

•  ReacEveprotocols:routesareconstructedondemand.NoglobalrouEngtableismaintained.

•  Moreappropriatefornetworkswithhighrateofchanges

–  AdhocondemanddistancevectorrouEng(AODV)–  DynamicsourcerouEng(DSR)

•  Needflooding–  Inefficientinlargenetworks


GeographicalrouEng:UsinglocaEon

•  GeographicalrouEngusesanode’slocaEontodiscoverpathtothatnode.


x

y

Greedy Routing: Forward to the neighbor that is nearest to the destination

GeographicalrouEng

•  AssumpEons:–  NodesknowtheirowngeographicallocaEon–  Nodesknowtheir1-hopneighbors–  RouEngdesEnaEonsarespecified

geographically(alocaEon,orageographicalregion)

–  EachpacketcanholdasmallamountofrouEnginformaEon.


Sensornetwork

•  Sensorsenabledwithwireless– Cancommunicatewithnearbysensors– CommunicaEontoserverrelaEvelycostly

•  Lowpower,butlotsofdata– Notworthsendingeverythingtoserver

•  Tryusethedatadirectlyinsidethenetwork–  In-networkdistributedcompuEng


Problem:Howtofindtherelevantdata?

•  Atouristinaparkasks•  “Whereistheelephant?”•  Outofallthesensors/cameraswhichoneisclosetoanelephant?


DatacentricrouEng•  TradiEonalnetworkstrytoroutetoanIPaddress•  FindpathtothenodewithaparEcularID•  Butwhatifwetrytofinddata,notspecificnodes?

•  Akerall,deliveringdataistheulEmategoalofrouEngandnetworks

•  Datacentricstorage–  Storagedependsonthedata(elephant,giraffe,song…)

•  DatacentricrouEng(search)–  Routetothedata


DistributedDatabase

•  InformaEonProducer– Canbeanywhereinthenetwork– Maybemobile– Manyproducersmaygeneratedataofthesametype

•  UserorInformaEonConsumer– Canbeanywhere– Maybemany


DistributedDatabase:Challenges

•  Consumerdoesnotknowwheretheproduceris,andviceversa

•  Needtosearch:Mustbefast,efficient

Basicmethods(e.g.usingfloods):•  Push:Producerdisseminatesdata•  Pull:Consumerlooksforthedata•  Push-pull:Bothproducer,consumersearchforeach-other


Distributedhashtables

•  Useahashonthedata:h(song1.mp3)=node#26

•  Anyonethathassong1.mp3informsnode#26•  AnyonethatneedsSong1.mp3checkswithnode#26

•  UsedinpeertopeersystemslikeChord,pastryetc


GeographicHashTables•  Contentbasedhashgivescoordinates:–  h(lion)=(12,07)

•  Producersendsmsgto(12,07)bygeographicrouEngandstoresdata

•  Consumersendsmsgto(12,07)bygeographicrouEngandgetsdata


GHT

•  Whatifthereisnosensorat(12,07)?

•  Usethesensornearesttoit


Faulthandling•  Whatifhomenodeadies?•  Replicas(nodessurroundingahashlocaEon)haveaEmerthattriggersanewcheck

•  Anewnodebecomeshome


GHT

•  Advantages– Simple– Handlesloadbalancingandfaults

•  Disadvantages– NotdistancesensiEve:everyonehastogotohashnodeevenifproducerandconsumerareclose

–  Ifadataisqueriedorupdatedoken,thatnodehasalotoftraffic–boXleneck


RumorRouEng

•  Producer:Senddataalongacurveorrandomwalk,leavedataorpointersonnodes

•  Consumer:Routealonganothercurveorrandomwalk,hopetomeetdataorpointer


RumorrouEng

•  Eachnodemaintainsalistofevents•  Addseventsastheyhappen

•  Agents:Packetsthatcarryeventsinthenetwork– Aggregateeventsofeachnodetheypassthrough

•  Agentsmoveinrandomwalk.From1-hopneighborsselectonethathasnotbeenvisitedrecently


•  D


Mobile,Ad-hocandSensornetwork

•  Adifficultmodel–leastinfrastructure,lowpowernodes,communicaEon/computaEonexpensive

•  NotenErelyrealisEc•  However,itmakesleastnumberofassumptons

–  usefulasabasisfordevelopingdistributedprotocols/algorithms

– Whichcanthenbeenhancedusingavailableinfrastructureinspecificcases


Emergingtechnologies

•  IoT:localcomputaEonandcommunicaEonisimportant– Efficiency– Privacy

•  DevicetodevicecommunicaEon– Comingin5G– Ad-hocnetworkstosupplementcellnetworks


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Distributed Systems Ad hoc and sensor networks€¦ · Networking in mobile systems • Diﬃculty:...

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