Chapter 05 Ad Hoc Networks

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Chapter 05 Ad Hoc Networks. Outline. Introduction Unicast routing TCP on Mobile Ad Hoc Networks Selected security issues. 5.1 Mobile Ad Hoc Networks (MANET) Introduction and Generalities. 5.1.1 Mobile Ad Hoc Networks. Formed by wireless hosts which may be mobile
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Chapter 05 Ad HocNetworksOutline
  • Introduction
  • Unicast routing
  • TCP on Mobile Ad Hoc Networks
  • Selected security issues
  • 5.1 Mobile Ad Hoc Networks (MANET)Introduction and Generalities5.1.1 Mobile Ad Hoc Networks
  • Formed by wireless hosts which may be mobile
  • Without (necessarily) using a pre-existing infrastructure
  • Routes between nodes may potentially contain multiple hops
  • Mobile Ad Hoc Networks
  • May need to traverse multiple links to reach a destination
  • ABMobile Ad Hoc Networks (MANET)
  • Mobility causes route changes
  • AB5.1.2 Why Ad Hoc Networks ?
  • Ease of deployment
  • Speed of deployment
  • Decreased dependence on infrastructure
  • 5.1.3 Many Applications
  • Personal area networking
  • cell phone, laptop, ear phone, wrist watch
  • Military environments
  • soldiers, tanks, planes
  • Civilian environments
  • Mesh networks
  • taxi cab network
  • meeting rooms
  • sports stadiums
  • boats, small aircraft
  • Emergency operations
  • search-and-rescue
  • policing and fire fighting
  • 5.1.4 Many Variations
  • Fully Symmetric Environment
  • all nodes have identical capabilities andresponsibilities
  • 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)
  • 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
  • Many Variations
  • Mobility patterns may be different
  • people sitting at an airport lounge
  • New York taxi cabs
  • kids playing
  • military movements
  • personal area network
  • Mobility characteristics
  • speed
  • predictability
  • direction of movement
  • pattern of movement
  • uniformity (or lack thereof) of mobility characteristics among different nodes
  • 5.1.5 Challenges
  • Limited wireless transmission range
  • Broadcast nature of the wireless medium
  • 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)
  • 5.1.6 The Holy Grail
  • A one-size-fits-all solution
  • Perhaps using an adaptive/hybrid approach that can adapt to situation at hand
  • Difficult problem
  • Many solutions proposed trying to address a
  • sub-space of the problem domain5.1.7 Assumption
  • Unless stated otherwise, fully symmetric environment is assumed implicitly
  • all nodes have identical capabilities and responsibilities
  • 5.2 Unicast RoutinginMobile Ad Hoc Networks5.2.1IntroductionWhy is Routing in MANET different ?
  • Host mobility
  • link failure/repair due to mobility may have different characteristics 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 mobility
  • energy consumption
  • 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
  • Routing Protocols
  • Proactive protocols
  • Determine routes independent of traffic pattern
  • Traditional link-state and distance-vector routing protocols are proactive
  • Reactive protocols
  • Maintain routes only if needed
  • Hybrid protocols
  • Trade-Off
  • Latency of route discovery
  • Proactive protocols may have lower latency since routes are maintained 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 routes are determined only if needed
  • Proactive protocols can (but not necessarily) result in higher overhead due to continuous route updating
  • Which approach achieves a better trade-off depends on the traffic and mobility patterns
  • 5.2.2 Overview of Unicast Routing ProtocolsFlooding 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
  • Flooding for Data DeliveryYZSEFBCMLJAGHDKINRepresents a node that has received packet PRepresents that connected nodes are within each other’s transmission rangeFlooding for Data DeliveryYBroadcast transmissionZSEFBCMLJAGHDKINRepresents a node that receives packet P forthe first timeRepresents transmission of packet PFlooding for Data DeliveryYZSEFBCMLJAGHDKIN
  • Node H receives packet P from two neighbors:
  • potential for collision
  • Flooding for Data DeliveryYZSEFBCMLJAGHDKIN
  • Node C receives packet P from G and H, but does not forward
  • it again, because node C has already forwarded packet P once
  • Flooding for Data DeliveryYZSEFBCMLJAGHDKIN
  • Nodes J and K both broadcast packet P to node D
  • Since nodes J and K are hidden from each other, their
  • transmissions may collide
  • =>Packet P may not be delivered to node D at all,
  • despite the use of flooding
  • Flooding for Data DeliveryYZSEFBCMLJAGHDKIN
  • Node D does not forward packet P, because node D
  • is the intended destination of packet P
  • Flooding for Data DeliveryYZSEFBCMLJAGHDKIN
  • 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 destination D
  • also do not receive packet P (example: node N)
  • Flooding for Data DeliveryYZSEFBCMLJAGHDKIN
  • Flooding may deliver packets to too many nodes
  • (in the worst case, all nodes reachable from sender
  • may receive the packet)
  • Flooding for Data Delivery: Advantages
  • Simplicity
  • May be more efficient than other protocols when rate of information transmission is low enough that the overhead of explicit route discovery/maintenance incurred by other protocols is relatively higher
  • this scenario may occur, for instance, when nodes transmit small data packets relatively infrequently, and many topology changes occur between consecutive packet transmissions
  • Potentially higher reliability of data delivery
  • Because packets may be delivered to the destination on multiple paths
  • Flooding for Data Delivery: Disadvantages
  • Potentially, very high overhead
  • Data packets may be delivered to too many nodes who do not need to receive them
  • Potentially lower reliability of data delivery
  • Flooding uses broadcasting -- hard to implement reliable broadcast delivery without significantly increasing overhead
  • Broadcasting in IEEE 802.11 MAC is unreliable
  • In 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
  • Flooding of Control Packets
  • Many protocols perform (potentially limited) flooding of control packets, instead of data 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
  • 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 identifier when forwarding RREQ
  • Route Discovery in DSRYZSEFBCMLJAGHDKINRepresents a node that has received RREQ for D from SRoute Discovery in DSRYBroadcast transmissionZ[S]SEFBCMLJAGHDKINRepresents transmission of RREQ[X,Y] Represents list of identifiers appended to RREQRoute Discovery in DSRYZS[S,E]EFBCMLJAG[S,C]HDKIN
  • Node H receives packet RREQ from two neighbors:
  • potential for collision
  • Route Discovery in DSRYZSEF[S,E,F]BCMLJAGHDK[S,C,G]IN
  • Node C receives RREQ from G and H, but does not forward
  • it again, because node C has already forwarded RREQ once
  • Route Discovery in DSRYZSEF[S,E,F,J]BCMLJAGHDKIN[S,C,G,K]
  • Nodes J and K both broadcast RREQ to node D
  • Since nodes J and K are hidden from each other, their
  • transmissions may collide
  • Route Discovery in DSRYZSE[S,E,F,J,M]FBCMLJAGHDKIN
  • Node D does not forward RREQ, because node D
  • is the intended target of the route discovery
  • 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
  • Route Reply in DSRYZSRREP [S,E,F,J,D]EFBCMLJAGHDKINRepresents RREP control messageRoute Reply in DSR
  • Route Reply can be sent by reversing the route in Route Request (RREQ) only if links are guaranteed to be bi-directional
  • To ensure this, RREQ should be forwarded only if it received on 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 the Route 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)
  • 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 in a packet to determine to whom a packet should be forwarded
  • Data Delivery in DSRYZDATA [S,E,F,J,D]SEFBCMLJAGHDKINPacket header size grows with route lengthWhen to Perform a Route Discovery
  • When node S wants to send data to node D, but does not know a valid route node D
  • 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
  • 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
  • Use of Route Caching[S,E,F,J,D][E,F,J,D]SE[F,J,D],[F,E,S]FB[J,F,E,S]CMLJAG[C,S]HDK[G,C,S]INZ[P,Q,R] Represents cached route at a node (DSR maintains the cached routes in a tree format)Use of Route Caching:Can Speed up Route Discovery[S,E,F,J,D][E,F,J,D]SE[F,J,D],[F,E,S]FB[J,F,E,S]CML[G,C,S]JAG[C,S]HDK[K,G,C,S]INRREPRREQZWhen node Z sends a route requestfor node C, node K sends back a routereply [Z,K,G,C] to node Z using a locallycached routeUse of Route Caching:Can Reduce Propagation of Route RequestsY[S,E,F,J,D][E,F,J,D]SE[F,J,D],[F,E,S]FB[J,F,E,S]CML[G,C,S]JAG[C,S]HDK[K,G,C,S]INRREPRREQZAssume that there is no link between D and Z.Route Reply (RREP) from node K limits flooding of RREQ.In general, the reduction may be less dramatic.Route Error (RERR)YZRERR [J-D]SEFBCMLJAGHDKINJ 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 failsNodes hearing RERR update their route cache to remove link J-DRoute 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]
  • Dynamic Source Routing: Advantages
  • Routes maintained only between nodes who need to communicate
  • reduces overhead of route maintenance
  • Route caching can further reduce route discovery overhead
  • A single route discovery may yield many routes to the destination, due to intermediate nodes replying from local caches
  • 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
  • Dynamic Source Routing: Disadvantages
  • An intermediate node may send Route Reply using a stale cached route, thus polluting other caches
  • This problem can be eased if some mechanism to purge (potentially) invalid cached routes is incorporated.
  • For some proposals for cache invalidation, see [Hu00Mobicom]
  • Static timeouts
  • Adaptive timeouts based on link stability
  • 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]
  • 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
  • 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
  • Route Requests in AODVYZSEFBCMLJAGHDKINRepresents a node that has received RREQ for D from SRoute Requests in AODVYBroadcast transmissionZSEFBCMLJAGHDKINRepresents transmission of RREQRoute Requests in AODVYZSEFBCMLJAGHDKINRepresents links on Reverse PathReverse Path Setup in AODVYZSEFBCMLJAGHDKIN
  • Node C receives RREQ from G and H, but does not forward
  • it again, because node C has already forwarded RREQ once
  • Reverse Path Setup in AODVYZSEFBCMLJAGHDKINReverse Path Setup in AODVYZSEFBCMLJAGHDKIN
  • Node D does not forward RREQ, because node D
  • is the intended target of the RREQ
  • Route Reply in AODVYZSEFBCMLJAGHDKINRepresents links on path taken by RREP Route Reply in AODV
  • An intermediate node (not the destination) may also send a Route Reply (RREP) provided that it knows a more 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 DSR
  • A 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
  • Forward Path Setup in AODVYZSEFBCMLJAGHDKINForward links are setup when RREP travels alongthe reverse pathRepresents a link on the forward pathData Delivery in AODVYDATAZSEFBCMLJAGHDKINRouting table entries used to forward data packet.Route is not included in packet header.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 if not used for a active_route_timeout interval
  • if no data is being sent using a particular routing table entry, that entry will be deleted from the routing table (even if the route may actually still be valid)
  • 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_timeout interval 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
  • 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 number for D cached at node X
  • The incremented sequence number N 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
  • 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
  • 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
  • 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-D because RERR sent by C is lost
  • Now C performs a route discovery for D. Node A receives the 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 )
  • ABCDEWhy Sequence Numbers in AODV
  • Loop C-E-A-B-C
  • ABCDEOptimization: Expanding Rin
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