6.964 pervasive computing grid: scalable ad hoc networking

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6.964 Pervasive Computing Grid: Scalable Ad Hoc Networking. 1 November 2001 Douglas S. J. De Couto Parallel and Distributed Operating Systems Group MIT Laboratory For Computer Science http://www.pdos.lcs.mit.edu/grid. Who are we?. Grid project in PDOS Professor: Robert Morris Students: - PowerPoint PPT Presentation


  • 6.964 Pervasive Computing Grid: Scalable Ad Hoc Networking

    1 November 2001 Douglas S. J. De Couto

    Parallel and Distributed Operating Systems GroupMIT Laboratory For Computer Sciencehttp://www.pdos.lcs.mit.edu/grid

  • Who are we?Grid project in PDOSProfessor: Robert MorrisStudents: Douglas De CoutoDan AguayoJinyang LiBen ChambersHu Imm Lee

  • OutlineMotivationClassic ad hoc protocolGeographic forwardingGrid location service (GLS)Location proxiesThe Grid network

  • So you want to build a pervasive network?AssumptionsWireless, packet-based, mobileBigger than just your living room (multihop)Todays approach: IEEE 802.11 base stationsSite survey, measure radio performanceChannel AllocationInter-base-station network (wiring?)

  • Base-station exampleB1B24Wired network213

  • Ad hoc: a better wayAd hoc means no infrastructure, no planningNormally implies wireless, mobile, multihopPlace devices (nodes) anywhereConstraint: devices should form connected networkIf not, add relay nodesCosts less!

  • Ad hoc exampler4213

  • Ad hoc scenariosTemporary, fast setupEmergenciesSocial eventsRooftop networksConnect neighborhoodsNo wires, trenches, etc.Developing communitiesAd hoc is cheaper, more incrementalAutomatic protocols no technicians needed

  • Other ad hoc benefitsBetter spectrum reuse (spatial)Better scalabilityPossibly better power

  • Ad hoc challengesHow do we find multihop routes?Is there enough network capacity?Does it use too much device power?Span: Chen et al., Mobicom 2001

  • Classic protocolDynamic Source Routing (DSR) Flooding route discovery finds source routes as neededAggressive caching helps performance

  • Why not use DSR?Protocol works well with about a hundred nodesSimulation results; vary with movement, data trafficProtocols scales poorlyPropagates topology information throughout networkOverhead grows too fast with network size, especially with mobility

  • DSR overheadNumber of nodesAvg. packets transmitted per node per second

  • Geographic forwarding (GF)Packets addressed to id,locationNext hop is chosen from neighbors to move packet geographically closer to destination locationRouting overhead constant as network size (nodes, area) growsABCDFCs radio rangeEG

  • ABCDEFAs nbrs:B, 1 hop (nh: B)C, 2 hops (nh: B)C, 2 hops (nh: B)Bs nbrs:A, 1 hop (nh: A)C, 1 hop (nh: C)D, 2 hops (nh: C) Local protocol is 2-hop distance vector A sends packets to F dcf > dbf but ddf < dbf and C is Bs next hop to D

    D, 2 hops (nh: C)GF With a Local Protocol

  • Geo. forwarding challengesHow do we find destination locations?How do nodes find their own locations?Location sensors not always practicalTopology problems (holes)General ad hoc problemsPower, capacity

  • AEHGBDFCJIKLEach node has a few servers that know its location.1. Node D sends location updates to its servers (B, H, K).2. Node J sends a query for D to one of Ds close servers.D?Grid Location Service (GLS) overview

  • Grid Node IdentifiersEach Grid node has a unique identifier.Identifiers are numbers.Perhaps a hash of the nodes IP address.Identifier X is the successor of Y if X is the smallest identifier greater than Y.

  • All nodes agree on the global origin of the grid hierarchyGLSs Spatial Hierarchy

  • 3 servers per node per level s is ns successor in that square. (Successor is the node with least ID greater than n )

  • Queries search for destinations successorsEach query step: visit ns successor at increasing level.nsssssssss3xs2s1

  • Geographic forwarding is less fragile than source routing. DSR queries use too much b/w with > 300 nodes.Fraction of data packets delivered successfullyGF + GLS performs wellBiggest network simulated:600 nodes, 2900x2900m(4-level grid hierarchy)

  • GLS propertiesSpreads load evenly over all nodesDegrades gracefully as nodes failQueries for nearby nodes stay localPer-node storage and communication costs grow slowly as the network size grows: O(log n), n nodesMore details: Li et al, Mobicom 2000

  • Geo. forwarding challengesHow do we find destination locations?How do nodes find their own locations?Location sensors not always practical

  • Location ProxiesNodes that know their location can act as location proxiesLocation proxies can communicate with each other using geographic forwarding and the local routing protocolNodes without location select proxies, and communicate through them using the local protocolNodes advertise proxy locations as their ownProxies not special besides knowing locations

  • Proxies Increase Delivery Rate

  • The Grid networkRed: 5th floorBlue: 6th floor> 20 relay nodesAbout 2 to 4 hops across each floor

  • Current Grid servicesIP routing, including Internet gatewayE.g. supports tracerouteGrid specific informationWho can my radio talk to?Who do I have routes to?

  • Grid services in progressLocation serviceWhere is node X?GeocastSend message m to every node in region RPosition estimation protocolI dont have a position sensorWhere am I?

  • Grid ApplicationsWhat is a Grid applicationUses unique Grid servicesUnder development: Grid chatRegular text + voice chatWhos nearby? (ask Grid)Whos at the student center? (ask Grid)

  • Grid detailsProtocol software implemented in the Click modular router Runs at userlevel, easy to interface to applicationsVery portableNodes:Mobile: iPaqs + 802.11 PCMCIA + LinuxRelay : small PCs + 802.11 PCI cards + OpenBSDGlobal distance vector (DV), or k-hop DV + GF

  • Grid SummaryGrid routing protocols are:Self-configuringEasy to deployScalable

    http://www.pdos.lcs.mit.edu/gridipkg: http://www.pdos.lcs.mit.edu/~decouto/grid-feedipkg install grid

    Talk about internet routing here why not usefule in ad hoc nets? dynamism!Simulation includes mobilityIncludes mobilityGeneral CS technique: add a level of indirection!!!!Proxies not special, for the sake of this protocol. But they can be, and are likely to be, for the sake of other constraints: power, storage.

    Proxies allow us to limit the scope of the local routing protocolOne sim per data pointExplain ``nbr: location ignorant nodes choose some loc aware nbr as their proxy, nothing special is done.Surprising that Basic does as well as it does! not that basic sucks (it should never work at all!)For proxy at 0.75 and 0.8, congestion causes 58% and 35% of drops. Otherwise, all drops are due to loss of route, no congestion drops wouldnt be anyway, routing overhead is fixed.

    The DSDV point sort of controls the slope of the blue proxy line. If DSDV gets better, so can proxy. With even slight infrastructure, proxy does much better than DSDV!

    DSDV only: 74% drops??????(60% never even have a route to the sender, perhaps more, if we treat broken route as no route)

    Why does proxy beat Nbr?GF more of the time because more nodes have a loc to advertise as destProxy technique in general increases local DV radius --- adaptively!

    Why isnt proxy perfect? running out of routing bandwidthAlmost every packet leaves the sender, so routes are breaking further along routes get longermore likely to break

    What kind of route drops? In local routing or in GF, i.e. no loc info for dest and no route, or have loc info but no best next hop and not in local route table? DONT have that data!

    In each sim, about 12,000 data packets sent (at 128 bytes each), and just over 12,000 route packets sent (at 1000 bytes each).Three 802.11 base stations on 5th floor


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