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705 TCD AdHocNet Brief

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    Contents

    Introduction to Ad hoc networks

    Conventional routing drawback

    Table Driven (WRP, DSDV)

    On Demand (DSR, AODV, TORA) Performance Evaluation

    Location based routing (LAR, DREAM)

    Hybrid routing (ZRP)

    Summary

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    Mobile Ad hoc Network

    Collection of mobile nodes/hosts forming a network

    Hosts use wireless RF transceivers as network interface

    Omni directional (broadcast)

    Highly directional (point point)

    Combination

    Arbitrary movement and coverage pattern

    Connectivity in the form of random (w/o necessarily using

    a pre-existing infrastructure) multi-hop graphs

    Highly co-operative, each host is an independent router Routes between nodes may potentially contain multiple

    hops

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    4

    Mobile Ad Hoc Networks

    May need to traverse multiple links to reach a

    destination

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    5

    Mobile Ad Hoc Networks (MANET)

    Mobility causes route changes

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    6

    Why Ad Hoc Networks ?

    Ease of deployment

    Speed of deployment

    Decreased dependence on infrastructure

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    Applications

    Ad hoc centric

    Conferences/meetings

    Search and Rescue

    Automated battlefields

    Data centric

    Collecting information in large, dynamic, energy constrainednetworks (sensors)

    Revenue centric

    Increasing coverage and capacity

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    Constraints and Issues

    No centralized administration or standard support

    services

    Frequent and unpredictable network topology

    changes

    Routing and mobility management

    Channel access/bandwidth availability

    Hidden/Exposed station problem Lack of symmetrical links

    Power limitation

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    Conventional Routing Protocols ?

    Not designed for highly dynamic, low bandwidth

    networks

    Count-to-infinity problem and slow convergence for

    DV

    Loop formation during temporary node failures and

    network partitions

    Protocols that use flooding techniques (for e.g. LS)

    create excessive traffic and control overhead

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    11

    Many Variations

    Fully Symmetric Environment

    all nodes have identical capabilities and responsibilities

    Asymmetric Capabilities

    transmission ranges and radios may differ

    battery life at different nodes may differ

    processing capacity may be different at different nodes

    speed of movement

    Asymmetric Responsibilities

    only some nodes may route packets

    some nodes may act as leaders of nearby nodes (e.g.,cluster head)

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    Many Variations

    Traffic characteristics may differ in different ad hoc

    networks

    bit rate

    timeliness constraints

    reliability requirements

    unicast / multicast / geocast

    host-based addressing / content-based addressing /capability-based addressing

    May co-exist (and co-operate) with an infrastructure-

    based network

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    Challenges

    Limited wireless transmission range

    Broadcast nature of the wireless medium

    Hidden terminal problem (see next slide)

    Packet losses due to transmission errors

    Mobility-induced route changes

    Mobility-induced packet losses

    Battery constraints Potentially frequent network partitions

    Ease of snooping on wireless transmissions (security

    hazard)

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    Hidden Terminal Problem

    B CA

    Nodes A and C cannot hear each other

    Transmissions by nodes A and C can collide at node B

    Nodes A and C are hidden from each other

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    Unicast Routing

    in

    Mobile Ad Hoc Networks

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    Why is Routing in MANET different ?

    Host mobility

    link failure/repair due to mobility may have differentcharacteristics than those due to other causes

    Rate of link failure/repair may be high when nodes

    move fast

    New performance criteria may be used

    route stability despite mobilityenergy consumption

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    Unicast Routing Protocols

    Many protocols have been proposed

    Some have been invented specifically for MANET

    Others are adapted from previously proposed

    protocols for wired networks

    No single protocol works well in all environments

    some attempts made to develop adaptive protocols

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    Routing Protocols

    Proactive protocols

    Determine routes independent of traffic pattern

    Traditional link-state and distance-vector routing protocolsare proactive

    Reactive protocols

    Maintain routes only if needed

    Hybrid protocols

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    Ad hoc Routing Protocols

    Proactive Protocols

    Table driven

    Continuously evaluateroutes

    No latency in routediscovery

    Large capacity to keepnetwork information

    current

    A lot of routinginformation may never beused

    Reactive Protocols

    On Demand

    Route discovery byglobal search

    Bottleneck due to latencyof route discovery

    May not be appropriatefor real-time

    communication

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    Trade-Off

    Latency of route discovery

    Proactive protocols may have lower latency since routes aremaintained at all times

    Reactive protocols may have higher latency because a route

    from X to Y will be found only when X attempts to send to Y

    Overhead of route discovery/maintenance

    Reactive protocols may have lower overhead since routesare determined only if needed

    Proactive protocols can (but not necessarily) result in higheroverhead due to continuous route updating

    Which approach achieves a better trade-off depends

    on the traffic and mobility patterns

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    Overview ofUnicast Routing Protocols

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    Wireless Routing Protocol (WRP)

    Predecessor to destination (next to last hop) in the

    shortest path used

    Eliminates the Count-to-infinity problem and

    converges faster

    Neighbor connectivity via periodic Hello messages

    Update messages sent upon detecting a change in

    neighbor link

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    Cont.

    Each node imaintains a Distance table (iDjk), Routingtable (Destination Identifier, Distance iDj ,PredecessorPj ,the successorSj), link cost table(Cost, Update Period)

    Processing Updates and creating Route TableUpdate from kcauses i to re-compute the distances of all

    paths with kas the predecessor

    For a destinationj, a neighborp is selected as the successorifp->jdoes not include i, and is the shortest path to j

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    Operation

    J

    K

    IB

    (0, J)

    (2, K)

    (2, K)

    (1, K)

    X11

    10

    1

    5

    10

    (g, K)

    (10, B)

    (10, I)

    (11, B)

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    Destination Sequenced

    Distance Vector (DSDV)

    Each Route is tagged with a sequence numberoriginated by destination

    Hosts perform periodic & triggered updates, issuing anew sequence number

    Sequence number indicates the freshness of arouteRoutes with more recent sequence numbers are preferred

    for packet forwarding

    If same sequence number, one having smallest metric used

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    Topology changes

    Broken links assigned a metric of

    Any route through a hop with a broken link is also

    assigned a metric of

    routes are assigned new sequence numbers byany host and immediately broadcast via a triggered

    update

    If a node has an equal/later sequence number with a

    finite metric for an route, a route update is

    triggered

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    DSDV Operation

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    Damping Fluctuations

    Routes preferred if later sequence numbers, orsmaller metric for same sequence numbers

    Problem : Table fluctuations if worse metrics arereceived first, causing a ripple of triggered updates

    Solution : Use average settling time as a parameterbefore advertising routes

    Tantamount to using two tables, one for forwardingpackets and another for advertising routes

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    Flooding for Data Delivery

    Sender S broadcasts data packet P to all its

    neighbors

    Each node receiving P forwards P to its neighbors

    Sequence numbers used to avoid the possibility of

    forwarding the same packet more than once

    Packet P reaches destination D provided that D is

    reachable from sender S

    Node D does not forward the packet

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    Flooding for Data Delivery

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    Represents that connected nodes are within each

    others transmission range

    Z

    Y

    Represents a node that has received packetP

    M

    N

    L

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    Flooding for Data Delivery

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    Represents transmission of packetP

    Represents a node that receives packetP for

    the first time

    Z

    YBroadcast transmission

    M

    N

    L

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    Flooding for Data Delivery

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    Node C receives packet P from G and H, but does not forward

    it again, because node C has already forwarded packet P once

    Z

    Y

    M

    N

    L

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    Flooding for Data Delivery

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    Z

    Y

    M

    Nodes J and K both broadcast packetP to node D

    Since nodes J and K are hidden from each other, their

    transmissions may collide

    !" Packet P may not be delivered to nodeD at all,

    despite the use of flooding

    N

    L

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    Flooding for Data Delivery

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    Z

    Y

    Node Ddoes not forward packet P, because node D

    is the intended destination of packetP

    M

    N

    L

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    Flooding for Data Delivery

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    Flooding completed

    Nodes unreachable from S do not receive packet P (e.g., node Z)

    Nodes for which all paths from S go through the destinationD

    also do not receive packet P (example: node N)

    Z

    Y

    M

    N

    L

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    Flooding for Data Delivery

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    Flooding may deliver packets to too many nodes

    (in the worst case, all nodes reachable from sender

    may receive the packet)

    Z

    Y

    M

    N

    L

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    Flooding for Data Delivery:Advantages

    Simplicity

    May be more efficient than other protocols when rate

    of information transmission is low enough that theoverhead of explicit route discovery/maintenance

    incurred by other protocols is relatively higher

    this scenario may occur, for instance, when nodes transmitsmall data packets relatively infrequently, and many topology

    changes occurbetween consecutive packet transmissions

    Potentially higher reliability of data delivery

    Because packets may be delivered to the destination onmultiple paths

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    Flooding for Data Delivery: Disadvantages

    Potentially, very high overhead

    Data packets may be delivered to too many nodes who donot need to receive them

    Potentially lower reliability of data delivery

    Flooding uses broadcasting -- hard to implement reliablebroadcast delivery without significantly increasing overhead

    Broadcasting in IEEE 802.11 MAC is unreliableIn our example, nodes J and K may transmit to node D

    simultaneously, resulting in loss of the packet

    in this case, destination would not receive the packet at all

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    Flooding ofControl Packets

    Many protocols perform (potentially limited) flooding

    ofcontrol packets, instead ofdata packets

    The control packets are used to discover routes

    Discovered routes are subsequently used to send

    data packet(s)

    Overhead of control packet flooding is amortized over

    data packets transmitted between consecutive

    control packet floods

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    Dynamic Source Routing (DSR) [Johnson96]

    When node S wants to send a packet to node D, but

    does not know a route to D, node S initiates a route

    discovery

    Source node S floods Route Request (RREQ)

    Each node appends own identifierwhen forwardingRREQ

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    Dynamic Source Routing (DSR)

    Each packet header contains a route, which isrepresented as a complete sequence of nodesbetween a source destination pair

    Protocol consists of two phases

    route discoveryroute maintenance

    Optimizations for efficiencyRoute cachePiggybacking

    Error handling

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    DSR Route Discovery

    Source broadcasts route request(id, target)

    Intermediate node action

    Discard ifidis in or node is in routerecord

    Else append address in route record; rebroadcast

    If node is the target, route recordcontains the full route tothe target; return a route reply

    Use existing routes to source to send route reply;

    else piggyback

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    DSR Route Maintenance

    Use acknowledgements or a layer-2 scheme to

    detect broken links; inform sender via route error

    packet

    If no route to the source exists

    Use piggybacking

    Send out a route requestand bufferroute error

    Sender truncates all routes which use nodes

    mentioned in route error

    Initiate route discovery

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    Optimizations for efficiency

    Route Cache

    Use cached entries forduring route discovery

    Promiscuous mode toadd more routes

    Use hop based delaysfor local congestion

    Must be careful toavoid loop formation

    Expanding ring search

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    Optimizations

    PiggybackingData piggybacked on route requestPacketProblem : route caching can cause piggybacked data to be

    discarded

    Improved Error Handlingwhen network becomes partitioned, buffer packets and useexponential back-off for route discovery

    Listen to route replies promiscuously to remove entriesUse negative information to ignore corrupt replies

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    Route Discovery in DSR

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    Z

    Y

    Represents a node that has receivedRREQ forD from S

    M

    N

    L

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    Route Discovery in DSR

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    Represents transmission ofRREQ

    Z

    YBroadcast transmission

    M

    N

    L

    [S]

    [X,Y] Represents list of identifiers appended toRREQ

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    Route Discovery in DSR

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    Node H receives packet RREQ from two neighbors:

    potential for collision

    Z

    Y

    M

    N

    L

    [S,E]

    [S,C]

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    Route Discovery in DSR

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    Node C receives RREQ from G and H, but does not forward

    it again, because node C has already forwardedRREQ once

    Z

    Y

    M

    N

    L

    [S,C,G]

    [S,E,F]

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    Route Discovery in DSR

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    Z

    Y

    M

    Nodes J and K both broadcastRREQ to node D

    Since nodes J and K are hidden from each other, their

    transmissions may collide

    N

    L

    [S,C,G,K]

    [S,E,F,J]

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    Route Discovery in DSR

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    Z

    Y

    Node Ddoes not forwardRREQ, because node D

    is the intended target of the route discovery

    M

    N

    L

    [S,E,F,J,M]

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    Route Discovery in DSR

    Destination D on receiving the first RREQ, sends a

    Route Reply (RREP)

    RREP is sent on a route obtained by reversing the

    route appended to received RREQ

    RREP includes the route from S to D on which RREQ

    was received by node D

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    Route Reply in DSR

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    Z

    Y

    M

    N

    L

    RREP [S,E,F,J,D]

    Represents RREP control message

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    Route Reply in DSR

    Route Reply can be sent by reversing the route inRoute Request (RREQ) only if links are guaranteed

    to be bi-directional

    To ensure this, RREQ should be forwarded only if it receivedon a link that is known to be bi-directional

    If unidirectional (asymmetric) links are allowed, then

    RREP may need a route discovery for S from node D

    Unless node D already knows a route to node S

    If a route discovery is initiated by D for a route to S, then theRoute Reply is piggybacked on the Route Request from D.

    If IEEE 802.11 MAC is used to send data, then links

    have to be bi-directional (since Ack is used)

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    Dynamic Source Routing (DSR)

    Node S on receiving RREP, caches the route

    included in the RREP

    When node S sends a data packet to D, the entire

    route is included in the packet header

    hence the name source routing

    Intermediate nodes use the source route included ina packet to determine to whom a packet should be

    forwarded

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    Data Delivery in DSR

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    Z

    Y

    M

    N

    L

    DATA [S,E,F,J,D]

    Packet header size grows with route length

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    When to Perform a Route Discovery

    When node S wants to send data to node D, but does

    not know a valid route node D

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    DSR Optimization: Route Caching

    Each node caches a new route it learns by any

    means

    When node S finds route [S,E,F,J,D] to node D, node

    S also learns route [S,E,F] to node F

    When node K receives Route Request [S,C,G]

    destined for node, node K learns route [K,G,C,S] to

    node S

    When node F forwards Route Reply RREP

    [S,E,F,J,D], node F learns route [F,J,D] to node D When node E forwards Data [S,E,F,J,D] it learns

    route [E,F,J,D] to node D

    A node may also learn a route when it overhears

    Data packets

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    Use of Route Caching

    When node S learns that a route to node D is broken,

    it uses another route from its local cache, if such a

    route to D exists in its cache. Otherwise, node S

    initiates route discovery by sending a route request

    Node X on receiving a Route Request for some node

    D can send a Route Reply if node X knows a route to

    node D

    Use of route cache

    can speed up route discovery

    can reduce propagation of route requests

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    Use of Route Caching

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    [P,Q,R] Represents cached route at a node

    (DSR maintains the cached routes in a tree format)

    M

    N

    L

    [S,E,F,J,D][E,F,J,D]

    [C,S]

    [G,C,S]

    [F,J,D],[F,E,S]

    [J,F,E,S]

    Z

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    Use of Route Caching:

    Can Speed up Route Discovery

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    Z

    M

    N

    L

    [S,E,F,J,D][E,F,J,D]

    [C,S]

    [G,C,S]

    [F,J,D],[F,E,S]

    [J,F,E,S]

    RREQ

    When node Z sends a route request

    for node C, nodeK sends back a route

    reply [Z,K,G,C] to node Z using a locally

    cached route

    [K,G,C,S]RREP

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    Use of Route Caching:

    Can Reduce Propagation of Route Requests

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    Z

    Y

    M

    N

    L

    [S,E,F,J,D][E,F,J,D]

    [C,S]

    [G,C,S]

    [F,J,D],[F,E,S]

    [J,F,E,S]

    RREQ

    Assume that there is no link betweenD and Z.

    Route Reply (RREP) from node Klimits flooding ofRREQ.

    In general, the reduction may be less dramatic.

    [K,G,C,S]

    RREP

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    Route Error (RERR)

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    Z

    Y

    M

    N

    L

    RERR [J-D]

    J sends a route error to S along route J-F-E-S when its attempt to

    forward the data packet S (with route SEFJD) on J-D fails

    Nodes hearing RERR update their route cache to remove link J-D

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    Route Caching: Beware!

    Stale caches can adversely affect performance

    With passage of time and host mobility, cached

    routes may become invalid

    A sender host may try several stale routes (obtained

    from local cache, or replied from cache by other

    nodes), before finding a good route

    An illustration of the adverse impact on TCP will be

    discussed later in the tutorial [Holland99]

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    Dynamic Source Routing: Disadvantages

    Packet header size grows with route length due to

    source routing

    Flood of route requests may potentially reach all

    nodes in the network

    Care must be taken to avoid collisions between route

    requests propagated by neighboring nodes

    insertion of random delays before forwarding RREQ

    Increased contention if too many route replies come

    back due to nodes replying using their local cache

    Route Reply Storm problem

    Reply storm may be eased by preventing a node from

    sending RREP if it hears another RREP with a shorter route

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    Dynamic Source Routing: Disadvantages

    An intermediate node may send Route Reply using astale cached route, thus polluting other caches

    This problem can be eased if some mechanism topurge (potentially) invalid cached routes isincorporated.

    For some proposals for cache invalidation, see

    [Hu00Mobicom]Static timeoutsAdaptive timeouts based on link stability

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    Flooding of Control Packets

    How to reduce the scope of the route request flood ?

    LAR [Ko98Mobicom]

    Query localization [Castaneda99Mobicom]

    How to reduce redundant broadcasts ?

    The Broadcast Storm Problem [Ni99Mobicom]

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    Expected Zone in LAR

    X

    Y

    r

    X = last known location of nodeD, at time t0

    Y = location of nodeD at current

    time t1, unknown to node S

    r = (t1 - t0) * estimate ofDs speed

    Expected Zone

    R Z i LAR

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    Request Zone in LAR

    X

    Y

    r

    S

    Request Zone

    Network Space

    BA

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    LAR

    Only nodes within the request zone forward route

    requests

    Node A does not forward RREQ, but node B does (seeprevious slide)

    Request zone explicitly specified in the route request

    Each node must know its physical location to

    determine whether it is within the request zone

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    LAR

    Only nodes within the request zone forward route

    requests

    If route discovery using the smaller request zone fails

    to find a route, the sender initiates another route

    discovery (after a timeout) using a larger request

    zone

    the larger request zone may be the entire network

    Rest of route discovery protocol similar to DSR

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    LAR Variations:Adaptive Request Zone

    Each node may modify the request zone included in

    the forwarded request

    Modified request zone may be determined using

    more recent/accurate information, and may be

    smaller than the original request zone

    S

    B

    Request zone adapted by B

    Request zone defined by sender S

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    LAR Variations: Implicit Request Zone

    In the previous scheme, a route request explicitly

    specified a request zone

    Alternative approach:A node X forwards a route

    request received from Y if node X is deemed to be

    closer to the expected zone as compared to Y

    The motivation is to attempt to bring the route request

    physically closer to the destination node after each

    forwarding

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    Location-Aided Routing

    The basic proposal assumes that, initially, location

    information for node X becomes known to Y only

    during a route discovery

    This location information is used for a future route

    discoveryEach route discovery yields more updated information which

    is used for the next discovery

    Variations

    Location information can also be piggybacked on anymessage from Y to X

    Y may also proactively distribute its location

    information

    Similar to other protocols discussed later (e.g., DREAM,GLS)

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    Location Aided Routing (LAR)

    Advantages

    reduces the scope of route request flood

    reduces overhead of route discovery

    Disadvantages

    Nodes need to know their physical locations

    Does not take into account possible existence of

    obstructions for radio transmissions

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    Detour

    Routing Using Location Information

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    Distance Routing Effect Algorithm for Mobility

    (DREAM) [Basagni98Mobicom]

    Uses location and speed information (like LAR)

    DREAM uses flooding ofdata packets as the routing

    mechanism (unlike LAR)

    DREAM uses location information to limit the flood of datapackets to a small region

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    Distance Routing Effect Algorithm for Mobility

    (DREAM)

    S

    D

    Expected zone

    (in the LAR jargon)

    A

    Node A, on receiving the

    data packet, forwards it to

    its neighbors within the

    cone rooted at node A

    S sends data packetto all

    neighbors in the cone rooted

    at node S

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    Ad Hoc On-Demand Distance Vector Routing

    (AODV) [Perkins99Wmcsa]

    DSR includes source routes in packet headers

    Resulting large headers can sometimes degrade

    performance

    particularly when data contents of a packet are small

    AODV attempts to improve on DSR by maintaining

    routing tables at the nodes, so that data packets do

    not have to contain routes

    AODV retains the desirable feature of DSR that

    routes are maintained only between nodes which

    need to communicate

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    AODV

    Route Requests (RREQ) are forwarded in a manner

    similar to DSR

    When a node re-broadcasts a Route Request, it sets

    up a reverse path pointing towards the source

    AODV assumes symmetric (bi-directional) links

    When the intended destination receives a Route

    Request, it replies by sending a Route Reply

    Route Reply travels along the reverse path set-up

    when Route Request is forwarded

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    Route Requests in AODV

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    Z

    Y

    Represents a node that has receivedRREQ forD from S

    M

    N

    L

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    Reverse Path Setup in AODV

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    Node C receives RREQ from G and H, but does not forward

    it again, because node C has already forwardedRREQ once

    Z

    Y

    M

    N

    L

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    Reverse Path Setup in AODV

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    Z

    Y

    M

    N

    L

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    Reverse Path Setup in AODV

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    Z

    Y

    Node Ddoes not forwardRREQ, because node D

    is the intended target of the RREQ

    M

    N

    L

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    Route Reply in AODV

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    Z

    Y

    Represents links on path taken by RREP

    M

    N

    L

    Route Reply in AODV

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    Route Reply in AODV

    An intermediate node (not the destination) may also

    send a Route Reply (RREP) provided that it knows amore recent path than the one previously known to

    sender S

    To determine whether the path known to an

    intermediate node is more recent, destination

    sequence numbers are used

    The likelihood that an intermediate node will send a

    Route Reply when using AODV not as high as DSRA new Route Request by node S for a destination is

    assigned a higher destination sequence number. An

    intermediate node which knows a route, but with a smaller

    sequence number, cannot send Route Reply

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    Forward Path Setup in AODV

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    Z

    Y

    M

    N

    L

    Forward links are setup whenRREP travels along

    the reverse path

    Represents a link on the forward path

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    Data Delivery in AODV

    B

    A

    S E

    F

    H

    J

    D

    C

    G

    I

    K

    Z

    Y

    M

    N

    L

    Routing table entries used to forward data packet.

    Route is notincluded in packet header.

    DATA

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    Timeouts

    A routing table entry maintaining a reverse path is

    purged after a timeout interval

    timeout should be long enough to allow RREP to come back

    A routing table entry maintaining a forward path is

    purged ifnot usedfor a active_route_timeoutinterval

    if no is data being sent using a particular routing table entry,

    that entry will be deleted from the routing table (even if theroute may actually still be valid)

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    Link Failure Reporting

    A neighbor of node X is considered active for a

    routing table entry if the neighbor sent a packet within

    active_route_timeoutinterval which was forwarded

    using that entry

    When the next hop link in a routing table entry

    breaks, all active neighbors are informed

    Link failures are propagated by means of Route Error

    messages, which also update destination sequence

    numbers

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    Route Error

    When node X is unable to forward packet P (from

    node S to node D) on link (X,Y), it generates a RERR

    message

    Node X increments the destination sequence numberfor D cached at node X

    The incremented sequence numberN is included in

    the RERR

    When node S receives the RERR, it initiates a new

    route discovery for D using destination sequence

    number at least as large as N

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    Destination Sequence Number

    Continuing from the previous slide

    When node D receives the route request with

    destination sequence number N, node D will set its

    sequence number to N, unless it is already larger

    than N

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    Link Failure Detection

    Hello messages: Neighboring nodes periodically

    exchange hello message

    Absence of hello message is used as an indication of

    link failure

    Alternatively, failure to receive several MAC-level

    acknowledgement may be used as an indication of

    link failure

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    Why Sequence Numbers in AODV

    To avoid using old/broken routes

    To determine which route is newer

    To prevent formation of loops

    Assume that A does not know about failure of link C-Dbecause RERR sent by C is lost

    Now C performs a route discovery for D. Node A receivesthe RREQ (say, via path C-E-A)

    Node A will reply since A knows a route to D via node B

    Results in a loop (for instance, C-E-A-B-C )

    A B C D

    E

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    Why Sequence Numbers in AODV

    Loop C-E-A-B-C

    A B C D

    E

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    Optimization: Expanding Ring Search

    Route Requests are initially sent with small Time-to-

    Live (TTL) field, to limit their propagation

    DSR also includes a similar optimization

    If no Route Reply is received, then larger TTL tried

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    Summary: AODV

    Routes need not be included in packet headers

    Nodes maintain routing tables containing entries only

    for routes that are in active use

    At most one next-hop per destination maintained at

    each node

    DSR may maintain several routes for a single destination

    Unused routes expire even if topology does not

    change

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    So far ...

    All protocols discussed so far perform some form of

    flooding

    Now we will consider protocols which try to

    reduce/avoid such behavior

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    Link Reversal Algorithm [Gafni81]

    A FB

    C E G

    D

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    Link Reversal Algorithm

    Link (G,D) broke

    A FB

    C E G

    D

    Any node, other than the destination, that has no outgoing links

    reverses all its incoming links.

    Node G has no outgoing links

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    Link Reversal Algorithm

    A FB

    C E G

    D

    Now nodes E and F have no outgoing links

    Represents a

    link that was

    reversed recently

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    Link Reversal Algorithm

    A FB

    C E G

    D

    Now nodes B and G have no outgoing links

    Represents a

    link that was

    reversed recently

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    Link Reversal Algorithm

    A FB

    C E G

    D

    Now nodes A and F have no outgoing links

    Represents a

    link that was

    reversed recently

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    Link Reversal Algorithm

    A FB

    C E G

    D

    Now all nodes (other than destinationD) have an outgoing link

    Represents a

    link that was

    reversed recently

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    Link Reversal Algorithm

    A FB

    C E G

    D

    DAG has been restored with only the destination as a sink

    Li k R l Al i h

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    Link Reversal Algorithm

    Attempts to keep link reversals local to where the

    failure occurred

    But this is not guaranteed

    When the first packet is sent to a destination, the

    destination oriented DAG is constructed

    The initial construction does result in flooding ofcontrol packets

    Li k R l Al ith

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    Link Reversal Algorithm

    The previous algorithm is called a full reversal

    method since when a node reverses links, it reverses

    allits incoming links

    Partial reversal method [Gafni81]: A node reversesincoming links from only those neighbors who have

    not themselves reversed links previously

    If all neighbors have reversed links, then the node reversesall its incoming links

    Previously at node X means since the last linkreversaldone by node X

    P ti l R l M th d

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    Partial Reversal Method

    Link (G,D) broke

    A FB

    C E G

    D

    Node G has no outgoing links

    P ti l R l M th d

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    Partial Reversal Method

    A FB

    C E G

    D

    Now nodes E and F have no outgoing links

    Represents a

    link that was

    reversed recently

    Represents a

    node that hasreversed links

    P ti l R l M th d

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    Partial Reversal Method

    A FB

    C E G

    D

    Nodes E and F donotreverse links from node G

    Now node B has no outgoing links

    Represents a

    link that was

    reversed recently

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    P ti l R l M th d

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    Partial Reversal Method

    A FB

    C E G

    D

    Now all nodes (except destinationD) have outgoing links

    Represents a

    link that was

    reversed recently

    Partial Reversal Method

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    Partial Reversal Method

    A FB

    C E G

    D

    DAG has been restored with only the destination as a sink

    Link Reversal Methods: Advantages

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    Link Reversal Methods: Advantages

    Link reversal methods attempt to limit updates to

    routing tables at nodes in the vicinity of a broken link

    Partial reversal method tends to be better than full reversal

    method

    Each node may potentially have multiple routes to a

    destination

    Link Reversal Methods: Disadvantage

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    Link Reversal Methods: Disadvantage

    Need a mechanism to detect link failure

    hello messages may be used

    but hello messages can add to contention

    If network is partitioned, link reversals continue

    indefinitely

    Link Reversal in a Partitioned Network

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    Link Reversal in a Partitioned Network

    A FB

    C E G

    DThis DAG is fordestinationnode D

    Full Reversal in a Partitioned Network

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    Full Reversal in a Partitioned Network

    A FB

    C E G

    D

    A and G do not have outgoing links

    Full Reversal in a Partitioned Network

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    Full Reversal in a Partitioned Network

    A FB

    C E G

    D

    E and F do not have outgoing links

    Full Reversal in a Partitioned Network

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    Full Reversal in a Partitioned Network

    A FB

    C E G

    D

    B and G do not have outgoing links

    Full Reversal in a Partitioned Network

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    Full Reversal in a Partitioned Network

    A FB

    C E G

    D

    E and F do not have outgoing links

    Full Reversal in a Partitioned Network

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    Full Reversal in a Partitioned Network

    A FB

    C E G

    D

    In the partitiondisconnected from

    destination D, link

    reversals continue, until

    the partitions merge

    Need a mechanism to

    minimize this wasteful

    activity

    Similar scenario can

    occur with partial

    reversal method too

    Temporally-Ordered Routing Algorithm

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    (TORA) [Park97Infocom]

    TORA modifies the partial link reversal method to be

    able to detect partitions

    When a partition is detected, all nodes in the partition

    are informed, and link reversals in that partition cease

    Partition Detection in TORA

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    Partition Detection in TORA

    A

    B

    E

    D

    F

    C

    DAG for

    destination D

    Partition Detection in TORA

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    Partition Detection in TORA

    A

    B

    E

    D

    F

    C

    TORA uses a

    modified partialreversal method

    Node A has no outgoing links

    Partition Detection in TORA

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    Partition Detection in TORA

    A

    B

    E

    D

    F

    C

    TORA uses a

    modified partialreversal method

    Node B has no outgoing links

    Partition Detection in TORA

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    Partition Detection in TORA

    A

    B

    E

    D

    F

    C

    Node B has no outgoing links

    Partition Detection in TORA

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    Partition Detection in TORA

    A

    B

    E

    D

    F

    C

    Node C has no outgoing links -- all its neighbor have

    reversed links previously.

    Partition Detection in TORA

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    Partition Detection in TORA

    A

    B

    E

    D

    F

    C

    Nodes A and B receive the reflection from node C

    Node B now has no outgoing link

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    Partition Detection in TORA

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    Partition Detection in TORA

    A

    B

    E

    D

    F

    COn detecting a partition,

    node A sends a clear (CLR)message that purges all

    directed links in that

    partition

    TORA

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    TORA

    Improves on the partial link reversal method in

    [Gafni81] by detecting partitions and stopping non-

    productive link reversals

    Paths may not be shortest

    The DAG provides many hosts the ability to send

    packets to a given destinationBeneficial when many hosts want to communicate with a

    single destination

    TORA Design Decision

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    TORA Design Decision

    TORA performs link reversals as dictated by[Gafni81]

    However, when a link breaks, it looses its direction

    When a link is repaired, it may not be assigned adirection, unless some node has performed a routediscovery after the link brokeif no one wants to send packets to D anymore, eventually,

    the DAG for destination D may disappear

    TORA makes effort to maintain the DAG for D only ifsomeone needs route to DReactive behavior

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    All nodes had identical responsibilities

    Some schemes propose giving special

    responsibilities to a subset of nodesEven if all nodes are physically identical

    Core-based schemes are examples of such schemes

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    Core-Extraction Distributed Ad Hoc Routing

    (CEDAR) [Sivakumar99]

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    (CEDAR) [Sivakumar99]

    A subset of nodes in the network is identified as thecore

    Each node in the network must be adjacent to at leastone node in the core

    Each node picks one core node as its dominator(or leader)

    Core is determined by periodic message exchangesbetween each node and its neighbors

    attempt made to keep the number of nodes in the core small

    Each core node determines paths to nearby corenodes by means of a localized broadcastEach core node guaranteed to have a core node at

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    B

    A

    C E

    JS K

    D

    F

    H

    G

    A core node

    Node E is the dominator

    for nodes D, F and K

    Link State Propagation in CEDAR

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    p g

    The distance to which the state of a link is propagatedin the network is a function of

    whether the link is stable -- state of unstable links is notpropagated very far

    whether the link bandwidth is high or low -- only state of linkswith high bandwidth is propagated far

    Link state propagation occurs among core nodes

    Link state information includes dominators of link end-points

    Each core node knows the state of local links and

    stable high bandwidth links far away

    Route Discovery in CEDAR

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    When a node S wants to send packets to destination D

    Node S informs its dominator core node B

    Node B finds a route in the core network to the core

    node E which is the dominator for destination DThis is done by means of a DSR-like route discovery (butsomewhat optimized) process among the core nodes

    Core nodes on the above route then build a route

    from S to D using locally available link stateinformation

    Route from S to D may or may not include corenodes

    CEDAR: Core Maintenance

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    B

    A

    C E

    JS K

    D

    F

    H

    G

    A core node

    Link State at Core Nodes

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    B

    A

    C E

    JS K

    D

    F

    H

    G

    Links that node B is aware of

    CEDAR Route Discovery

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    B

    A

    C E

    JS K

    D

    F

    H

    G

    Partial route constructed by B

    Links that node C is aware of

    CEDAR Route Discovery

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    B

    A

    C E

    JS K

    D

    F

    H

    G

    Complete route -- last two hops determined by node C

    CEDAR

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    AdvantagesRoute discovery/maintenance duties limited to a small

    number of core nodes

    Link state propagation a function of link stability/quality

    Disadvantages

    Core nodes have to handle additional traffic, associated withroute discovery and maintenance

    Asymmetric Responsibilities:

    Cluster-Based Schemes

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    Cluster-Based Schemes

    Some cluster-based schemes have also beenproposed [Gerla95,Krishna97,Amis00]

    In some cluster-based schemes, a leader is elected

    for each clusterof node

    The leader often has some special responsibilities

    Different schemes may differ inhow clusters are determined

    the way cluster head (leader) is chosen

    duties assigned to the cluster head

    Proactive Protocols

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    Most of the schemes discussed so far are reactive

    Proactive schemes based on distance-vector and

    link-state mechanisms have also been proposed

    Link State Routing [Huitema95]

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    Each node periodically floods status of its links

    Each node re-broadcasts link state information

    received from its neighbor

    Each node keeps track of link state informationreceived from other nodes

    Each node uses above information to determine nexthop to each destination

    Optimized Link State Routing (OLSR)

    [Jacquet00ietf Jacquet99Inria]

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    [Jacquet00ietf,Jacquet99Inria]

    The overhead of flooding link state information isreduced by requiring fewer nodes to forward the

    information

    A broadcast from node X is only forwarded by itsmultipoint relays

    Multipoint relays of node X are its neighbors such

    that each two-hop neighbor of X is a one-hopneighbor of at least one multipoint relay of X

    Each node transmits its neighbor list in periodic beacons, sothat all nodes can know their 2-hop neighbors, in order to

    choose the multipoint relays

    Optimized Link State Routing (OLSR)

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    Nodes C and E are multipoint relays of node A

    A

    BF

    C

    D

    E H

    GK

    J

    Node that has broadcast state information from A

    Optimized Link State Routing (OLSR)

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    Nodes C and E forward information received from A

    A

    BF

    C

    D

    E H

    GK

    J

    Node that has broadcast state information from A

    Optimized Link State Routing (OLSR)

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    Nodes E and K are multipoint relays for node H Node K forwards information received from H

    E has already forwarded the same information once

    A

    BF

    C

    D

    E H

    GK

    J

    Node that has broadcast state information from A

    OLSR

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    OLSR floods information through the multipoint relays

    The flooded itself is fir links connecting nodes to

    respective multipoint relays

    Routes used by OLSR only include multipoint relays

    as intermediate nodes

    Destination-Sequenced Distance-Vector

    (DSDV) [Perkins94Sigcomm]

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    (DSDV) [Perkins94Sigcomm]

    Each node maintains a routing table which storesnext hop towards each destination

    a cost metric for the path to each destination

    a destination sequence number that is created by thedestination itself

    Sequence numbers used to avoid formation of loops

    Each node periodically forwards the routing table to

    its neighbors

    Each node increments and appends its sequence numberwhen sending its local routing table

    This sequence number will be attached to route entriescreated for this node

    Destination-Sequenced Distance-Vector

    (DSDV)

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    (DSDV)

    Assume that node X receives routing informationfrom Y about a route to node Z

    Let S(X) and S(Y) denote the destination sequence

    number for node Z as stored at node X, and as sent

    by node Y with its routing table to node X,

    respectively

    X Y Z

    Destination-Sequenced Distance-Vector

    (DSDV)

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    (DSDV)

    Node X takes the following steps:

    If S(X) > S(Y), then X ignores the routing informationreceived from Y

    If S(X) = S(Y), and cost of going through Y is smaller than

    the route known to X, then X sets Y as the next hop to Z

    If S(X) < S(Y), then X sets Y as the next hop to Z, and S(X)is updated to equal S(Y)

    X Y Z

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    Hybrid Protocols

    Zone Routing Protocol (ZRP) [Haas98]

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    Zone routing protocol combines

    Proactive protocol: which pro-actively updates

    network state and maintains route regardless of

    whether any data traffic exists or not

    Reactive protocol: which only determines route to a

    destination if there is some data to be sent to the

    destination

    ZRP

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    All nodes within hop distance at most dfrom a node

    X are said to be in the routing zone of node X

    All nodes at hop distance exactly dare said to beperipheral nodes of node Xs routing zone

    ZRP

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    Intra-zone routing: Pro-actively maintain state

    information for links within a short distance from any

    given node

    Routes to nodes within short distance are thus maintainedproactively (using, say, link state or distance vector protocol)

    Inter-zone routing: Use a route discovery protocol for

    determining routes to far away nodes. Route

    discovery is similar to DSR with the exception thatroute requests are propagated via peripheral nodes.

    ZRP: Example with

    Zone Radius = d = 2

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    SCA

    EF

    B

    D

    S performs routediscovery forD

    Denotes route request

    ZRP: Example with d = 2

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    SCA

    EF

    B

    D

    S performs routediscovery forD

    Denotes route reply

    E knows route from E to D,

    so route request need not be

    forwarded to D from E

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    LANMAR Routing to Nodes Within Scope

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    Assume that node C is within scope of node A

    Routing from A to C: Node A can determine next hop

    to node C using the available link state information

    A B

    C

    F

    H

    G

    E

    D

    LANMAR Routing to Nodes Outside Scope

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    Routing from node A to F which is outside As scope Let H be the landmark node for node F

    Node A somehow knows that H is the landmark for C

    Node A can determine next hop to node H using theavailable distance vector information

    A B

    C

    F

    H

    G

    E

    D

    LANMAR Routing to Nodes Outside Scope

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    Node D is within scope of node F

    Node D can determine next hop to node F using link

    state information

    The packet for F may never reach the landmark node

    H, even though initially node A sends it towards H

    A B

    C

    F

    H

    G

    E

    D

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    LANMAR scheme uses node identifiers as landmarks

    Anchored Geodesic Scheme [LeBoudec00] uses

    geographical regions as landmarks


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