Routing Protocols for Ad Hoc Mobile Wireless Network

1
Routing Protocols for Ad Hoc Mobile Wireless
Network

• Mingliu Zhang
• 3/2/04

2
Outline

• Review of routing algorithm for Internet IP
• Distance-vector
• Link-state
• Ad hoc protocol routing requirements
• Categorization of ad-hoc routing protocols
• Ad hoc protocols
• Destination-sequenced Distance-vector (DSDV)
• Ad-hoc On-demand Distance Vector (AODV)
• Dynamic Source Routing (DSR)

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Distance-Vector Routing Algorithm

• Distributed Bellman-Ford
• Each node constructs a one-dimensional array (a
  vector) containing the distance(costs) to all
  other nodes and next hop id
• Routers exchange their routing tables with
  immediate neighbors
• Information includes the distance and next hop id
• Typical exchange periods (30s-several minutes)

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Distance-vector Routing Example
Initial routing table at A
Final routing table at A
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Slow convergence
B
A
C
E
D
G
F

• Convergence-The process of getting consistent
  routing information to all the nodes
• A loses contact with E
• A doesnt know if there is another path to E
• In a table exchange it finds B has a route to E
  with distance 2
• A advertises that it has route to E with distance
  3
• B advertises that it has route to E with distance
  4 to A and C
• Cycle repeats-count to infinity problem
• Routing tables do not stabilize-slow convergence

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Slow Convergence Solution

• Split Horizon-Dont send those routes it learned
  from each neighbor back to that neighbor.
• Poison Reverse-Advertise routes with infinite
  distance
• Hold down-When a major change occurs advertise
  the change quickly,but dont accept the new
  routes for a period of time

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Routing Information Protocol(RIP)

• A quite popular routing protocol built on the
  distance-vector algorithm

RIP packet format
Table from Larry L. Peterson etc, Computer
Networks
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Link-State Routing Algorithm

• Routers broadcast their neighbor connections to
  all routers in the network
• Information mainly connectivity and cost of the
  link to each neighbor
• Every node has a complete map of the network.
• Dijkstra's Shortest Path Algorithm can be used to
  select the best route to the destination.
• No slow convergence problems.
• Somewhat larger capacity requirement.

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Link-State Routing Example
B
5
3

• A

C
10
11
2
D
Link State Database in router D
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Dijkstras Algorithm as Done by D
(0)
2. Place C in path, examine Cs LSP.Better path
to B found
1. Place D in path, examine Ds LSPs (Link State
Packets)
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Dijkstras Algorithm as Done by D(cont.)
(0)
(2)
(10)
4. Place A in path,examine As LSP. No nodes
left.Terminate.
3. Place B in path,examine Bs LSP. Better Path
to A found
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Link-State Routing Algorithm Summary

• Nice properties
• Stabilize quickly, generate not much traffic
• Loop free routing
• Respond rapidly to topology changes or node
  failures
• Downsides
• Higher memory requirement--the amount of
  information stored at each node can be quite
  large
• Scalability problem

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Open Shortest Path First Protocol(OSPF)

• Most widely used, based on link-state routing
  algorithm. Improvements added
• Introduces hierarchy into routing by allowing
  networks partitioned into areas
• Authentication of routing messages
• Allows load balancing

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Ad Hoc Protocol Routing Requirements

• Simple, reliable and efficient
• Distributed but lightweight in nature
• Quickly adapts to changes in topology
• Protocol reaction to topology changes should
  result in minimal control overhead
• Bandwidth efficient
• Mobility management involving user location and
  hand-off management

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Categorization of Ad-Hoc Routing Protocols
Figure from Elizabeth M. Royer, C-K Toh, A
Review of Current Routing Protocols for Ad-Hoc
Mobile Wireless Networks.
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Table Driven Based Routing (Proactive)

• Maintain table of all active links in network
• Calculate the shortest path from table
• Update table whenever nodes move
• Immediate tell if node is reachable
• Data can be sent immediately
• Very high overheads

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On-demand Routing(Reactive)

• Find routes as needed
• Cache information from other nodes requests
• No static overhead
• Slow start before transmitting data
• AODV, DSR

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Representative Ad Hoc Routing Protocols

• Destination-Sequenced Distance-Vector Routing
  (DSDV)- Proactive(table driven), next-hop
  routing, distance-vector
• Ad hoc On-demand Distance-Vector Routing
  (AODV)-reactive,next hop routing,distance-vector
• Dynamic Source Routing (DSR)-reactive (on
  demand),source routing,link state

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DSDV RoutingCharles Perkins Pravin Bhagwat

• Current in use on the MIT GRID projects
• Route advertisements
• Route table entry structure
• Responding to topology changes
• Route selection criteria
• Summary

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Route Advertisements in DSDV

• Each mobile node advertise its own route tables
  to its current neighbors
• Routing tables update periodically to adapt the
  dynamic change and maintain table consistency.

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Route Table Entry Structure in DSDV

• When advertisement, each mobile node contain its
  new sequence number and the following information
  for each new route
• The destinations address
• The number of costs (hops) required to reach the
  destination
• The sequence number of the information
  received,originally stamped by the destination.

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Example of Advertised Table in DSDV
MHi -address for the mobile node, MH4 is the node
advertising the route table update
Table from Charles E.Perkins, AD HOC Networking
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Responding to Topology Changes in DSDV

• Two types of packets defined for route updates
• full dump packets
• Carry all available routing information
• Size of multiple network protocol data units
  (NPDUs)
• Transmitted infrequently during period of
  occasional movement
• incremental packets
• Carry only information changed since last full
  dump
• Size of a NPDU
• Transmitted more frequently
• Additional table maintained to store the data
  from the incremental packets

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Route selection criteria

• Routes are preferred if the sequence numbers are
  newer
• If the sequence numbers are the same, the one
  with better metric is preferred
• Keep track of the settling time of routes-the
  weighted average time that routes to a
  destination will fluctuate before the route with
  best metric is received (Why?)

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Route Settling Time and Solution

• Problem
• A initiated a route update with a new sequence
  number
• Update from B arrives D 10 s before update from C
• Metric of update from C is better (less hops)
• Solution
• delay the broadcast of a routing by the length of
  the settling time.
• ExceptionWhen broken link is found, broadcast
  immediately

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Summary of DSDV Routing

• Essentially a modification to Bellman-Ford
  routing algorithm
• Using sequence number to guarantee loop-free
  paths
• Relies on periodic exchange of routing
  information.
• Inefficient due to periodic update transmissions
  even no changes in topology
• Overhead grows as O(n2) , limiting scalability

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AODV Properties

• Route discovery as needed and routes maintained
  as necessary
• Guarantee loop free through the use of sequence
  number
• Able to provide unicast, multicast
• Currently only use the symmetric links
• Nodes maintain only next-hop routing information

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Route Table Information in AODV

• The destination address
• The next hop IP address
• The destination sequence number
• A list of precursor nodes to reach the
  destination(for the purpose of route maintenance
  if link breaks)
• A lifetime for each route(expire the unused route)

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Route Discovery in AODV

• Source Node
• Initiate RREQ, packet contains
• Source node IP address and current sequence
• Destination IP address and last known sequence
• A broadcast ID
• Set a timer to wait for reply

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Route Discovery in AODV

• Intermediate nodes
• Check the unique identifier (Source IP address
  broadcast ID)of the RREQ
• If already seen, discards the packet
• If not,
• Set up a reverse route for the source node,
  associated with a lifetime
• Increase the RREQs hop count
• Broadcast the RREQ to its neighbors

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Route Discovery in AODV

• Node responds to the RREQ (not necessary
  destination node)
• Must have an unexpired entry for destination
• The sequence associated gt the sequence
  indicated in the RREQ
• Unicasts a RREP back to the source, using the
  node from which it received the RREQ as the next
  hop

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Route Discovery in AODV

• C responds to the RREQ with RREP, which includes
• Its record of the destinations sequence
• The hop count from C to D
• The lifetime of the route from C to D
• C unicasts the RREP to source node through A

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Forward Path Setup

• A receives RREP, sets up a forward path entry to
  D in its route table, includes
• IP address of D, IP address of the neighbor this
  RREP received (C)
• Its hop count to D
• The associated life time as contained in RREP

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Expanding Ring Search in AODV

• To reduce the impact of flooding
• Broadcast a RREQ with low time to live (TTL)
• If no response received, re-transmit request with
  increased TTL
• Fixed values for TTL start, increment and
  threshold
• Beyond threshold, the RREQ is broadcast across
  the entire network up to rreq-retries times

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Route Maintenance in AODV

• Route maintained as long as needed
• Source nodes moves during an active session,
  reinitiate route discovery
• Destination or intermediate node moves, Route
  Error(RERR) message sent

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Route Maintenance in AODV
3
1
3
S
2
D
4
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Summary of AODV Routing

• On demand, distance-vector routing protocol for
  highly mobile wireless nodes
• Support both unicast and multicast
• Good chance to scale to large node population
• (Reported results for
• AODV node population as great as 10,000 nodes
• Others of 100 to 200 nodes)

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Dynamic Source Routing Dave Johnson

• Internet drafts available on MANET webpage
• Reactive (on demand)
• Source routing
• Based on link-state routing algorithm

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Route Discovery in DSR

• Sender floods RREQ through the network
• Nodes forward RREQs after appending their names
• Destination node receives RREQ and unicasts a
  RREP back to sender node

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Route Discovery in DSR
Represents node that has received RREQ originated
from S to D
RREQ includes the source IP address, the
destination IP, a unique request ID
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Route Discovery in DSR
Y
Broadcast transmission
Z
S
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
Represents transmission of RREQ
X,Y Represents list of identifiers appended
to RREQ
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Route Discovery in DSR
Y
Z
S
S,E
E
F
B
C
M
L
J
A
G
S,C
H
D
K
I
N
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Route Discovery in DSR
Y
Z
S
E
F
S,E,F
B
C
M
L
J
A
G
H
D
K
S,C,G
I
N

• Node C receives RREQ from G and H, but does not
  forward
• it again, because node C has already forwarded
  RREQ once

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Route Discovery in DSR
Y
Z
S
E
F
S,E,F,J
B
C
M
L
J
A
G
H
D
K
I
N
S,C,G,K

• Nodes J and K both broadcast RREQ to node D

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Target Node Reaction

• Node D examines its route cache, if a route to S
  found, use it as source route for RREP.Otherwise,
• May perform its own route discovery to S,
  piggyback the RREP on its own RREQ to S
• Simply reverse the sequence of hops in the route
  record
• Preferred by IEEE 802.11 that require a
  bidirectional frame exchange
• Avoids the overhead of a possible second route
  discovery
• Tests the discovered route
• Advantage support asymmetric link

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Route Reply in DSR
Represents RREP control message
Route reply with its associated route record back
to the source node
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AODV vs. DSR

• DSR
• Potentially larger overhead
• Intended for moderate speed mobile nodes, source
  routing, not scalable to large networks
• No network topology changes, no overhead
• Support asymmetric link
• Allow nodes keep multiple routes to one
  destination in their cache, faster route recovery
• AODV
• Less control overhead
• Support multicast
• Require symmetric links between nodes
• Good chance of scalability

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References

• Romit Roy Choudhury and Nitin H. Vaidya, Impact
  of Directional Antennas on Ad Hoc Routing.
• Elizabeth M. Royer, C-K Toh, A Review of Current
  Routing Protocols for Ad-Hoc Mobile Wireless
  Networks.
• Charles E. Perkins, AD HOC Networking