distributed systems mobile & iot compung · distributed systems mobile & iot compung rik...

DistributedSystems

Mobile&IoTCompu6ngRikSarkar

UniversityofEdinburgh

Fall2016

Distributed Systems, Edinburgh, 2016

MobileandUbiquitouscompu6ng

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

•  Theyareeverywhere–  Carriedbypeople(mobile)–  Embeddedintheenvironment

•  Coffeemachines,cameras,sensorsforlightcontrol,elevators…

–  Producelargeamountsofdata•  Usage,sensing…


Ubiquitous•  Advantages:

–  Therearecomputerseverywhere–  Everythingis“smart”–  Poten6allyusecomputa6onsonthesetomakethemevensmarter

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

–  Devicesdonotinteroperateeasily


Mobile

•  Advantages:–  Thesamedeviceiscarriedbytheperson–easytogiveconsistentservice

–  Informa6onwhenever,wherevertheyneed–  Deviceshavesensors–poten6alforsensingtheenvironmentandadap6ng

•  Disadvantages:–  Connec6vityischallenge:dataiscostly;networkdoesnotworkthesameway;mobilityinterfereswithcomunica6on

–  LimitedbaYery:can’tdotoomuchcommunica6on–  Howtomakeuseofsensors,notsowellunderstood


Contextawarecompu6ng

•  Adaptcomputa6onstothecircumstances– Timeofday–  Istheuserpresent?–  Isthephoneinhandorinpocket– Scanforwifionlywhenindoors– Turnoffringwhenincinema,mee-ng…– Recognizeac6vityandbringuprelevantinforma6on

– …


Contextawarecompu6ng

•  Adaptcomputa6onstothecircumstances•  Basiccontextsareeasytoiden6fy,butitisnotalwaysclearhowtoadapt– Turndownvolumeatnight…butwhatifitisanimportantcall?

•  Manycontextsareveryhardtodetectreliably


Example:IndoorvsOutdoor

•  Usesensorsonaphone,turnoffwifiscanningoutdoors

•  Lightlevelsaremuchhigheroutdoors–  Inday6meandifphoneisnotinpocket

•  Citystreetsarenoisier•  Cellularsignalstrengthsdropindoors

– Dependsonplace•  Temperature,magne6cfield…


Othercontextdetec6onexamples

•  Usesoundtodetectuserinamee6ng•  Detecttransportmode(walking,car,bus,tram..)– Usingaccelerometer

•  Detectpresenceofotherusersnearbyfromwifiac6vity


Contextdetec6on

•  Generallyhard•  Concernsaboutprivacy:youdonotwanttosendcontextinforma6ontoaserver

•  Perhapsdistributedcomputa6oncanhelp– Usedatafrommanyphonestodetectcontext– Butagain,donotwanttosendalldatatoserver– Doasmuchofitaspossibleondevice–filter/processdataatsource


Networkinginmobilesystems

•  Difficulty:–  Thenetworkgraphchanges– Anodeisnotalwaysconnectedtothesamerouter

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

– Alsoappliestosensornetworks


Rou6nginadhocwirelessnetworks

•  Findroutebetweenpairsofnodeswishingtocommunicate.

•  Proac6veprotocols:maintainrou6ngtablesateachnodethatisupdatedaschangesinthenetworktopologyaredetected.

–  Heavyoverheadwithhighnetworkdynamics(causedbylink/nodefailuresornodemovement).

–  Notprac6calfornetworksthatchangefrequently


Rou6nginadhocwirelessnetworks

•  Reac6veprotocols:routesareconstructedondemand.Noglobalrou6ngtableismaintained.

•  Moreappropriatefornetworkswithhighrateofchanges

–  Adhocondemanddistancevectorrou6ng(AODV)–  Dynamicsourcerou6ng(DSR)


DynamicSourceRou6ng(DSR)

•  NodeSwantstosendamessagetonodeD•  Sini6atesaaroutediscovery•  Sfloodsthenetworkwithrouterequest(RREQ)message

•  Eachnodeappendsitsownidtothemessage


RouteDiscovery:RREQ


RouteDiscoveryinDSR

•  Des6na6onDonreceivingthefirstRREQsendsaroutereply(RREP)

•  RREPissentonarouteobtainedbyreversingtherouteinreceivedRREQ


RouteDiscovery:RREQ


When

•  Whenalinkfails,anerrormessagewiththelinknameissentbacktoS.

•  Sdeletesanyrouteusingthatlinkandstartsdiscovery.


Routecaching

•  Whenanodereceivesorforwardsamessage,itlearnsroutestoallnodesonthepath

•  Advantage:–  SmaynotneedtosendRREQ–  IntermediatenodeonreceivingRREQ,canrespondwithcompleteroute

•  Disadvantage:–  Cachesmaybestale:Striesmanycachedroutesbeforestar6ngadiscovery.Or,intermediatenodesreturnoutdatedinforma6on.


DSR:SummaryAdvantages:•  Routescomputedonlywhenneeded–goodforchanging

networks•  Cachingcanmakethingsefficient•  DoesnotcreateloopsDisadvantages•  En6reroutemustbecontainedinmessage:canbelongfor

largenetworks•  Floodingcausescommunica6ontomanynodes•  Stalecachescanbeaproblem•  Notsuitablefornetworkswherechangesaretoofrequent


AdhocOn-DemandDistanceVectorRou6ng(AODV)

•  Maintainsrou6ngtablesatnodessothattherouteneednotbestoredinthemessage

•  NoCaches:Onlyonerouteperdes6na6on


AODVRouteDiscovery

•  SourcefloodsthenetworkDistributed Systems, Edinburgh, 2016

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

•  Apathexpiresifnotusedforacertain6me.•  Ifanodeseesthatarou6ngtableentryhasnotbeenusedbythis6me,itremovesthisentry

•  Evenifthepathitselfisvalid•  Goodfornetworkswithfrequentchanges•  Badforsta6candstablenetworks


Cancreateloops

•  AssumeC->Dlinkhasfailed,butAdoesnotknowbecausetheERRmessagewaslost

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


SequencenumbersinAODV

•  IfAhasaroutetoD,Akeepsasequencenumber.

•  Aincrementsthisnumberperiodically:tellshowoldtheinforma6onis


Usingsequencenumbers

•  Rule:sequencenumbermustincreasealonganyroute


Sequencenumberruleavoidsloop

•  Adoesnotreply,sinceitssequenceno.islessthanthatofC


AODV

•  Rou6ngtables,messagedoesnotcontainroute

•  Freshroutespreferred•  Oldunusedroutesexpire•  Stalerouteslessproblema6c•  Needssequencenumberstopreventloops•  BeYerformoredynamic,changingenvironments


Rou6nginadhocnetworks

•  Reac6veprotocols:routesareconstructedondemand.Noglobalrou6ngtableismaintained.

•  Moreappropriatefornetworkswithhighrateofchanges

–  Adhocondemanddistancevectorrou6ng(AODV)–  Dynamicsourcerou6ng(DSR)

•  Needflooding–  Inefficientinlargenetworks


Geographicalrou6ng:Usingloca6on

•  Geographicalrou6ngusesanode’sloca6ontodiscoverpathtothatnode.


x

y

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

Geographicalrou6ng

•  Assump6ons:–  Nodesknowtheirowngeographicalloca6on–  Nodesknowtheir1-hopneighbors–  Rou6ngdes6na6onsarespecified

geographically(aloca6on,orageographicalregion)

–  Eachpacketcanholdasmallamountofrou6nginforma6on.


Sensornetwork

•  Sensorsenabledwithwireless– Cancommunicatewithnearbysensors– Communica6ontoserverrela6velycostly

•  Lowpower,butlotsofdata– Notworthsendingeverythingtoserver

•  Tryusethedatadirectlyinsidethenetwork–  In-networkdistributedcompu6ng


Problem:Howtofindtherelevantdata?

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


Datacentricrou6ng•  Tradi6onalnetworkstrytoroutetoanIPaddress•  Findpathtothenodewithapar6cularID•  Butwhatifwetrytofinddata,notspecificnodes?

•  Alerall,deliveringdataistheul6mategoalofrou6ngandnetworks

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

•  Datacentricrou6ng(search)–  Routetothedata


DistributedDatabase

•  Informa6onProducer– Canbeanywhereinthenetwork– Maybemobile– Manyproducersmaygeneratedataofthesametype

•  UserorInforma6onConsumer– Canbeanywhere– Maybemany


DistributedDatabase:Challenges

•  Consumerdoesnotknowwheretheproduceris,andviceversa

•  Needtosearch:Mustbefast,efficient

Basicmethods:•  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)bygeographicrou6ngandstoresdata

•  Consumersendsmsgto(12,07)bygeographicrou6ngandgetsdata


GHT

•  Whatifthereisnosensorat(12,07)?

•  Usethesensornearesttoit


Faulthandling

•  Whatifhomenodeadies?•  Replicashavea6merthattriggersanewcheck•  Anewnodebecomeshome


GHT

•  Advantages– Simple– Handlesloadbalancingandfaults

•  Disadvantages– Notdistancesensi6ve:everyonehastogotohashnodeevenifproducerandconsumerareclose

–  Ifadataisqueriedorupdatedolen,thatnodehasalotoftraffic–boYleneck


RumorRou6ng

•  Producer:Senddataalongacurveorrandomwalk,leavedataorpointersonnodes

•  Consumer:Routealonganothercurveorrandomwalk,hopetomeetdataorpointer


Rumorrou6ng

•  Eachnodemaintainsalistofevents•  Addseventsastheyhappen

•  Agents:Packetsthatcarryeventsinthenetwork– Aggregateeventsofeachnodetheypassthrough

•  Agentsmoveinrandomwalk.From1-hopneighborsselectonethathasnotbeenvisitedrecently


•  D


Mobile,Ad-hocandSensornetwork

•  Adifficultmodel–leastinfrastructure,lowpowernodes,communica6on/computa6onexpensive

•  Noten6relyrealis6c•  However,itmakesleastnumberofassumptons

–  usefulasabasisfordevelopingdistributedprotocols/algorithms

– Whichcanthenbeenhancedusingavailableinfrastructureinspecificcases


distributed systems mobile & iot compung · distributed systems mobile & iot compung rik...

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