Routing in Mobile Ad hoc Networks

1
Routing in Mobile Ad hoc Networks

• Sumesh J. Philip
• CSE620 Fall 2004

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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

3
Mobile Ad hoc Network

• Collection of mobile nodes 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, multi-hop
  graphs
• Highly co-operative, each host is an independent
  router

4
Applications

• Ad hoc centric
• Conferences/meetings
• Search and Rescue
• Automated battlefields
• Data centric
• Collecting information in large, dynamic, energy
  constrained networks (sensors)
• Revenue centric
• Increasing coverage and capacity

5
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

6
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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Ad hoc Routing Protocols

• Proactive Protocols
• Table driven
• Continuously evaluate routes
• No latency in route discovery
• Large capacity to keep network information
  current
• A lot of routing information may never be used

• Reactive Protocols
• On Demand
• Route discovery by global search
• Bottleneck due to latency of route discovery
• May not be appropriate for real-time communication

8
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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• Each node i maintains a Distance table (iDjk),
  Routing table (Destination Identifier, Distance
  iDj , Predecessor Pj ,the successor Sj), link
  cost table (Cost, Update Period)
• Processing Updates and creating Route Table
• Update from k causes i to re-compute the
  distances of all paths with k as the predecessor
• For a destination j, a neighbor p is selected as
  the successor if p-gtj does not include i, and is
  the shortest path to j

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Operation
(0, J)
J
10
(2, K)
B
X
5
10
I
1
1
(2, K)
1
K
(1, K)
(?, K)
(11, B)
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Destination Sequenced Distance Vector (DSDV)

• Each Route is tagged with a sequence number
  originated by destination
• Hosts perform periodic triggered updates,
  issuing a new sequence number
• Sequence number indicates the freshness of a
  route
• Routes 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 8
• Any route through a hop with a broken link is
  also assigned a metric of 8
• 8 routes are assigned new sequence numbers by
  any host and immediately broadcast via a
  triggered update
• If a node has an equal/later sequence number with
  a finite metric for an 8 route, a route update
  is triggered

13
DSDV Operation
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Damping Fluctuations

• Routes preferred if later sequence numbers, or
  smaller metric for same sequence numbers
• Problem Table fluctuations if worse metrics are
  received first, causing a ripple of triggered
  updates
• Solution Use average settling time as a
  parameter before advertising routes
• Tantamount to using two tables, one for
  forwarding packets and another for advertising
  routes

15
Dynamic Source Routing (DSR)

• Each packet header contains a route, which is
  represented as a complete sequence of nodes
  between a source destination pair
• Protocol consists of two phases
• route discovery
• route maintenance
• Optimizations for efficiency
• Route cache
• Piggybacking
• Error handling

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DSR Route Discovery

• Source broadcasts route request (id, target)
• Intermediate node action
• Discard if id is in ltinitiator, request idgt or
  node is in route record
• Else append address in route record rebroadcast
• If node is the target, route record contains the
  full route to the 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 request and buffer route 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 for during route discovery
• Promiscuous mode to add more routes
• Use hop based delays for local congestion
• Must be careful to avoid loop formation
• Expanding ring search

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Optimizations

• Piggybacking
• Data piggybacked on route request Packet
• Problem route caching can cause piggybacked
  data to be discarded
• Improved Error Handling
• when network becomes partitioned, buffer packets
  and use exponential back-off for route discovery
• Listen to route replies promiscuously to remove
  entries
• Use negative information to ignore corrupt replies

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Ad-hoc On DemandDistance Vector (AODV)

• On demand protocol that uses sequence numbers
  (DSDV) to build loop free routes
• Key difference from DSR is that source route is
  no longer required
• Path discovery
• Reverse Path setup
• Forward path setup
• Table management and path maintenance
• Local connectivity management

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AODV Reverse path setup

• Counters Sequence number, Broadcast id
• Reverse Path
• Broadcast route request (RREQ) lt source_addr,
  source_sequence- , broadcast_id, dest_addr,
  dest_sequence_, hop_cnt gt
• RREQ uniquely identified by ltsource_addr ,
  broadcast_idgt
• Route reply (RREP) if neighbor is the target, or
  knows a higher dest_sequence_
• Otherwise setup a pointer to the neighbor from
  whom RREQ was received
• Maintain reverse path entries based on timeouts
  

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AODV Forward path setup

• RREQ arrives at a node that has current route to
  the destination ( larger/same sequence number)
• Unicast request reply (RREP)ltsource_addr,
  dest_addr, dest_sequence_, hop_cnt,lifetimegt to
  neighbor
• RREP travels back to the source along reverse
  path
• Each upstream node updates dest_sequence_, sets
  up a forward pointer to the neighbor who transmit
  the RREP

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AODV Operation
D
S
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Protocol Maintenance

• Route Table management
• Route request expiration timer purges reverse
  paths that do not lie on active route
• Active neighbor relays a packet within
  active_route_timeout
• Route cache timer purges inactive routes
• New routes preferred if higher destination
  sequence number or lower metric

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AODV Maintenance

• Path maintenance
• Upon link breakage, affected node propagates an
  unsolicited RREP ltdest_sequence_1, 8gt to all
  upstream nodes
• Source may restart route discovery process
• Local connectivity management
• Broadcasts used to update local connectivity
  information
• Inactive nodes in an active path required to send
  hello messages

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Temporally OrderedRouting Algorithm (TORA)

• Link reversal algorithm
• Destination oriented Directed Acyclic Graph (DAG)
• Full/Partial reversal of links
• Assigns a reference level (height) to each node
• Adjust reference level to restore routes on link
  failure
• Multiple routes to destination route optimality
  not important
• Query, Update, Clear packets used for creating,
  maintaining and erasing routes

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Creating Routes
A
B
QRY
E
C
D
G (DEST)
F
H
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Route Maintenance
UPD
A
B
UPD
E
C
UPD
D
G (DEST)
X
F
H
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Erasing Invalid Routes
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Performance Analysis

• Simulation Environment
• Network Simulator, 50 nodes in a 1500x300m
  rectangular flat grid
• Random waypoint mobility (Average 10 m/sec)
• Constant bit rate traffic (UDP)
• Address resolution ARP implementation in BSD
  Unix
• Medium Access Control IEEE 802.11
• Physical Layer model combines both free space
  and two ray ground reflection model
• Protocols studied DSDV(SQ), AODV-LL, DSR, TORA

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Performance Analysis

• Metrics
• Packet Delivery Ratio Ratio of number of
  packets generated by CBR sources to that received
  by CBR sinks at destination
• Routing Overhead number of routing packets
  sent each transmission counts as one
  transmission
• Path Optimality Difference between length of
  actual path took and the length of the shortest
  path

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Packet Delivery Ratio

• 95-100 in most cases for DSR, AODV
• Stale route entries in DSDV cause drops
• Short lived loops in TORA as part of link
  reversal
• All protocols perform well when there is low node
  mobility

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Routing Overhead (packets)

• Route caching and non-propagating RREQs in DSR
• TORA
• Sum of mobility dependant, independent overhead
  for TORA
• Congestive collapse
• Nearly constant for DSDV due to periodic updates

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Routing Overhead (Bytes)

• DSR more expensive than AODV except at high
  mobility
• Smaller packets in AODV, may be more expensive in
  terms of media access, power and network
  utilization

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Path Optimality

• DSDV, DSR use routes close to optimal
• TORA not designed to find shortest path
• TORA, AODV use paths close to optimum when node
  mobility is low

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Using Location Information

• Several solutions for locating wireless devices
• Location represented as latitude, longitude,
  altitude/x,y,z
• Outdoor environment
• GPS positioning, Cellular Network based
• Indoor environment
• RADAR, Cricket system
• Beacon algorithms for ad hoc networks
• Ad hoc Positioning System (APS)
• How to incorporate locations into routing ?

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Distance Routing EffectAlgorithm for Mobility
(DREAM)

• Proactively disseminate location information
• Distance Effect
• Closer nodes are updated more frequently
• age field in location update
• Mobility Effect
• rate of location update controlled by mobility
• No bandwidth wastage for no movement
• Routing policy
• If no entry for destination in table, flood
• Otherwise forward data to m neighbors in the
  direction of destination

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Example of Dream
How to determine ? ?
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Location Aided Routing (LAR)

• On Demand protocol used restricted flooding for
  locating destination
• Flooding is restricted to a request zone,
  defined by an expected zone
• A node forwards a route request only if it
  belongs to the request zone
• Tradeoff between latency of route determination
  and message overhead
• Resorts to flooding when prior information of
  destination is not available

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LAR Scheme 1

• Source calculates the expected zone, defines a
  request zone in the request packet and
  initiates route discovery
• Node I receiving the route request forwards the
  request if it falls inside the request zone,
  otherwise discards it
• When destination receives the request, replies
  with a route reply including current location,
  time and average speed
• Size of request zone is large at low and high
  node speeds

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LAR Scheme 2

• Source calculates the distance Dists to
  destination (xd, yd) and initiates route
  discovery with both parameters
• Node I calculates its distance Disti from (xd,
  yd) and forwards the request only if Distilt
  Dists d, otherwise discards the request
• Node I replaces Dists with Disti before
  forwarding the request
• Non zero d increases probability of route
  discovery

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LAR schemes
D(xd,yd)
D(xd,yd)
R v(t-t0)
N
I
N
I
J
J
S (xs,ys)
S (xs,ys)
Scheme 1
Scheme 2
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Zone Routing Protocol (ZRP)

• Proactive/reactive protocols have scalability
  issues for large networks
• Tables updates
• Flooding aspect
• Zone routing
• Zone (hop based) defined for each node
• Interior nodes, peripheral nodes
• Proactive topology maintenance within a zone
  (IARP)
• Reactive bordercast within zones (IERP)

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ZRP example
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Summary

• Introduced ad hoc networks and multi- hop
  relaying in wireless environment
• Mobility imposes considerable challenge in
  routing
• Rapidly dynamic topology
• Conventional routing protocols not designed to
  withstand such rapid changes
• Proactive vs. Reactive protocols
• Presented the tip of an iceberg literature is
  filled with routing protocols, performance
  studies etc.