1. Routing Protocols for Ad Hoc Mobile Wireless Network
Mingliu Zhang
3/2/04

2. Outline Review of routing algorithm for Internet IP Link-state
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)

3. 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)

4. Distance-vector Routing ExampleB C D F E G
Destination
Cost
NextHop B 1 C D  - E F G
Destination
Cost
NextHop B 1 C D 2 E F G
Initial routing table at A
Final routing table at A

5. 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 doesn’t 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

6. Slow Convergence Solution
Split Horizon-Don’t 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 don’t accept the new routes for a period of time

7. 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”

8. 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.

9. Link-State Routing ExampleB 5 3 A C 10 11 D 2 A
B/5
C/10 B
A/5
C/3
D/11 C
A/10
B/3
D/2 D
B/11
C/2
Link State Database in router D

10. Dijkstra’s Algorithm as Done by DC B 5 3 2 11 10 D B C
(11)
(2)
(5) A
(12)
(0) D B C
(0)
(11)
(2)
2. Place C in path, examine C’s LSP.Better path to B found
1. Place D in path, examine D’s LSPs (Link State Packets)

11. Dijkstra’s Algorithm as Done by D(cont.)5 3 2 11 10
(0) D C
(2) B
(5) A
(12)
(10)
(0) D C B
(5) A
(2)
(10)
3. Place B in path,examine B’s LSP. Better Path to A found
4. Place A in path,examine A’s LSP. No nodes left.Terminate.

12. 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

13. 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

14. 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

15. 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”.

16. 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

17. On-demand Routing(Reactive)
Find routes as needed
Cache information from other nodes’ requests
No static overhead
Slow start before transmitting data
AODV, DSR

18. 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

19. DSDV Routing [Charles 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

20. 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.

21. 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 destination’s address
The number of costs (hops) required to reach the destination
The sequence number of the information received,originally stamped by the destination.

22. 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”

23. 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

24. 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?)

25. Route Settling Time and SolutionC B D
MN collection 1
MN collection 2
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.
Exception:When broken link is found, broadcast immediately

26. 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

27. 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

28. 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)

29. 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

30. 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 RREQ’s hop count
Broadcast the RREQ to its neighbors

31. Route Discovery in AODV
Node responds to the RREQ (not necessary destination node)
Must have an unexpired entry for destination
The sequence # associated >= 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

32. Route Discovery in AODVB D
C responds to the RREQ with RREP, which includes
Its record of the destination’s 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

33. Forward Path Setup S E C A B D
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

34. 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

35. 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

36. Route Maintenance in AODVS 4 D 3’ 3 2 1
RERR2
RERR1 3’ 1 3 S 2 D 4

37. 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)

38. Dynamic Source Routing [Dave Johnson]
Internet drafts available on MANET webpage
Reactive (on demand)
Source routing
Based on link-state routing algorithm

39. 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

40. B A S E F H J C G I K Z Y M N L D
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

41. Broadcast transmission
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

42. 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

43. Route Discovery in DSR Y Z S E F B
[S,E,F] 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

44. 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

45. 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 that require a bidirectional frame exchange
Avoids the overhead of a possible second route discovery
Tests the discovered route
Advantage : support asymmetric link

46. 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
Route reply with its associated route record back to the source node

47. 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

48. 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”