1. T. Russell Hsing, FIEEE/FBCS(UK)/FSPIE
ICT Entrepreneurship for Telematics Services L2- Mobile Ad-Hoc Networks (MANETs)
February 6, 2013
T. Russell Hsing, FIEEE/FBCS(UK)/FSPIE

2. What Problems Do You See As Important to Address and Why?

3. What Are Mobile Ad-Hoc Networks?
No fixed network infrastructure
All nodes can be mobile and can be routers
Arbitrary and dynamically changing network topology

4. Why Are Mobile Ad-Hoc Networks Important?
Military operations
Vehicular networks and telematics applications
Emergency search and rescue operations
Scientific explorations
Meetings and conferences
………..

5. Key Technical Issues How to route in mobile ad-hoc networks?
How to secure an mobile ad-hoc network and communications over it?
High speed protocols process needed for some key applications (i.e. vehicular telematics, high speed railway (HSR))
How to conserve energy?
Nodes in mobile ad-hoc networks are likely to be energy-constrained

6. Military Mobile Ad-Hoc Network Illustration

7. What Makes Ad-Hoc Routing Difficult
Frequent node movements lead to frequent changes of network topology
Energy-constrained nodes
Large number of nodes
Fast moving nodes (e.g., airplanes, cars, HSR,etc)
Routing protocols designed for the Internet
were designed for relatively stable networks
and cannot handle the dynamic changes in ad-hoc networks

8. Desirable Characteristics of Ad-Hoc Routing
Distributed operations
Loop free
Scalable
Energy conservative
Rapid convergence
Security and privacy preserving

9. How is Routing Done in IP Networks?
Key methods
Link state routing
OSPF – Open Shortest Path First
Distance vector routing
RIP – Routing Information Protocol
BGP – Border Gateway Protocol
How to scale to large networks?
Hierarchical Routing
Divide network into domains
Different protocols for intra-domain routing (OSPF, RIP) and inter-domain routing (BGP)
Hierarchical IP addresses
Private addresses (NAT – Network Address Translation)

10. Using Existing IP Routing Protocols for Ad-Hoc Networks?
They are not designed to handle frequent changes in the network
Don’t scale well enough
But, some basic ideas can be used or enhanced to fit the ad-hoc network

11. A Fundamental Algorithm Behind Many Ad-Hoc Routing Protocols: Bellman-Ford Routing Algorithm

12. Bellman-Ford Routing Algorithm
A distance-vector routing algorithm
Initially designed in 1957
Currently used in many popular Internet routing protocols
ARPANET (Advanced Research Projects Agency Network, US DoD), the origin of the Internet
RIP (Routing Information Protocol)
BGP (Border Gateway Protocol)
NOVELL IPX
Foundation for many ad-hoc routing protocols

13. Bellman-Ford Routing Algorithm Key Characteristics
Distributed algorithm
Each node exchanges routing information with its directly attached neighbors and makes its own routing decisions
Iterative
Routing information exchange continues until no new information is exchanged between the neighborhood.
Asynchronous
Do not require all nodes to operate in lock step with each other

14. Bellman-Ford Routing Algorithm How It Works
Each router maintains a distance table, which is a one-dimensional array, hence the term “distance vector“
Distance table maintains the “distance” and the shortest path to each node in the network.
“Distance” can be hop-count distance or time it takes to transmit a packet to the destination
Referred to generally as "cost“
Assumption: Each node knows the cost of the link to each of its directly connected neighbors.
Destination Node
Distance (Cost of Path)
to Destination
Neighbor on shortest
path to Destination

15. Bellman-Ford Algorithm How It Works
Every node sends routing update messages to its directly connected neighbors containing its own distance table
Upon first joining the network or change of the distance table
Every node updates its distance table with cost and next hops learned from its neighbors
Repeat routing table exchange until no more new information between the neighbors.

16. Bellman-Ford Algorithm How It WorksX Z1 Y
Cost of direct link
C(X,Z)
Shortest distance from Z1 to Y: D(Z1,Y)
Shortest distance from X to Y via Z1 :
D(X,Y) = C(X,Z) + minZ {D(Z,Y)} Z2 Y
Node X knows C(X,Zj ) for every neighbor Zj
Node X can learn C(Zj,Y) from route updates from each neighbor Zj.
Node X can compute D(X,Y) based on C(Zj,Y) for every neighbor Zj, and C(X,Zj)

17. Bellman-Ford Algorithm Potential Problems: Counting to InfinityX 2 3
Converged
Stable
Condition A B C X
Routing update X 4 A 3 B X 5 B X 4 A X 5 B X 6 A

18. Bellman-Ford Algorithm Solving the Counting-to-Infinity Problem
Split Horizon
If you learn a route on an interface, do not send information about that route back out that interface.
Split Horizon eliminates routing loops with only two nodes. It is not effective in eliminating loops that involves more than two nodes
Make the infinity finite
Specify a maximum distance vector metric as infinity
Hold Down Timers or Hold down
Hold downs are ways to try to eliminate larger routing loops.
When a node eliminates a node from its routing table, it initiates a hold downs so no new entries can be accepted for that node for a specific period of time. This helps to keep that node from being a member of the loop.

19. Existing Ad-Hoc Routing Protocols
Pre-Computed (Table-Driven) Routing
Proactive routing where every node maintains up-to-date and consistent routes to every other node
On-Demand Routing
Reactive routing that discovers a route only when needed
Probabilistic/Opportunistic Routing
Take advantage of the randomness of the network to route packets to further increase scalability
Location-Added Routing
Use location of the destination to route the traffic

20. Pre-Computed Ad-Hoc Routing
Each node maintains routing tables to store up-to-date and consistent routes to each destination
Nodes respond to changes in network topology by propagating routing update messages throughout the network
Existing table-driven routing protocols differ primarily in how changes in network topology are propagated over the network
Flat routing
Requires each node to maintain a routing table containing all other nodes in the network
Hierarchical routing
Organizes nodes into a hierarchy of clusters

21. Sample Pre-Computed Routing Protocols
Flat table-driven routing protocols
Dynamic Destination-Sequenced Distance-Vector Routing Protocol (DSDV)
Wireless Routing Protocol (WRP)
Global State Routing (GSR)
Fisheye State Routing (FSR)
………
Hierarchical table-driven routing
Hierarchical State Routing (HSR)
Zone-based Hierarchical Link State Routing (ZLSR)
Dynamic Source Routing Protocol

22. Dynamic Destination-Sequenced Distance-Vector Routing Protocol (DSDV)
Based on Bellman-Ford Distance-Vector Routing Algorithm
Each node maintains a Routing Table, which contains
All available destinations
Number of hops to reach the destination
Sequence number assigned by the destination node used to avoid routing loops
Destination 1
Hop counts to Destination
Sequence Number 1
Destination 2
Hop counts to Destination
Sequence Number 2
Key difference from classical
Bellman-Ford protocol

23. Dynamic Destination-Sequenced Distance-Vector Routing Protocol (DSDV)
Each node sends route update packets to its neighbors periodically and also when their routing tables change significantly
Route update packet may carry the full routing table update or only an incremental update
Each route update packet contains
Address of known destinations
# of hops to each destination
A unique sequence number assigned by the originator for the route to the destination.
Route with the highest (i.e. most recent) sequence number is used.
If two routes have an identical sequence number, the route with the best metric (i.e. shortest route) is used

24. Dynamic Destination-Sequenced Distance-Vector Routing Protocol (DSDV)
Route used by X for destination Y (sequence #=1) X A B Y
Route Update (Y, Hop count to distance = 2, Seq. #=1)
Route used by X for destination Y (sequence #=2) X A Y
Route Update (Y, Hop count to distance = 1, Seq. #=2)

25. Wireless Routing Protocol (WRP) (1 / 7)
Another distance-vector routing protocol
Each node maintain 4 tables
Distance Table
Routing Table
Link-Cost Table
Message Retransmission List

26. Wireless Routing Protocol (WAP) (2 / 7) Distance Table
Distance to each destination via each neighbor
The downstream neighbor of each neighbor through which a path is realized
Node X remembers a 2-hop path toward destination
Destination X A B Y
Hop-Count Distance to Destination

27. Wireless Routing Protocol (WAP) (3 / 7) Routing Table
Distance to each destination node,
Predecessor node and successor node for each path
A tag to identify if the routing entry is a simple path, a loop, or invalid. A X B Y
Destination Y
Distance
= 2
Successor =A
Predecessor =B
Tag
Node X’s Routing Table

28. Wireless Routing protocol (WAP) (4 / 7) Using Predecessor Info to Avoid LoopsX A B C
Converged
Stable
Condition X 3 B X 2 C X A B C
Routing update
B knows from the
successor info it has
and the predecessor in
the route update
that the route
in this update
Is not valid X 3 B C
Destination
Next Hop
Distance
Predecessor

29. Wireless Routing Protocol (WAP) (5 / 7) Link Cost Table and Message Retransmission List
Cost of link to each neighboring node
The number of timeouts since an error-free message was received from that neighbor
Message Retransmission List (MRL) contains
Information to let a node know which of its neighbor has not acknowledged its update message and to retransmit update message to that neighbor.

30. Wireless Routing Protocol (WAP) (6 / 7) Routing Table Update
Node sends routing updates to its neighbors periodically, and upon route changes
Route may change when a neighbor moves away or when the node receives routing updates from its neighbors indicating new routes
If there is no change in routing table since last update, the node is required to send an idle Hello message to ensure connectivity
Nodes learn the existence of neighbors from receipt of ACKs and other messages
An route update message contains
A list of updates: destination, distance to destination, predecessor node of destination
A response list indicating which neighbors should acknowledge the update
Upon receiving an route update message, a node modifies its tables and looks for better paths using new information.

31. Wireless Routing Protocol (WAP) (7 / 7) Routing Table Update
Any new path is relayed back to the original nodes so that they can update their tables. The node updates its routing table if the new path is better than the existing path.
Upon receiving an ACK, the node updates its Message Retransmission List.
Node checks the consistency of the information reported on the predecessors by all its neighbors every time it detects a change in link of any of its neighbors. This helps eliminate loops and enable fast convergence.

32. Global State Routing (GSR)
A link-state routing protocol
Each node maintains 4 tables
Neighbor List: Contains all neighbor nodes
Topology Table: Contains the link state information as reported by the destination and the timestamp of the information for each destination
Next Hop Table: Contains the next hop toward each destination
Distance Table: Contains the shortest distance to each destination.

33. Fisheye State Routing (FSR)
FSR is an improvement of Global State Routing (GSR) to reduce the state update messages
Issues addressed
Update messages in GSR can be large and hence can waste a considerable amount of network bandwidth.
Solution in FSR
Each update message does not contain information about all nodes. Instead, it exchanges information about closer nodes more frequently than it does about farther nodes thus reducing the update message size.
So each node gets accurate information about neighbors and the detail and accuracy of information decreases as the distance from node increases.
Packets can be routed correctly because the route information becomes more and more accurate as the packet moves closer to the destination.

34. Hierarchical State Routing (HSR)
Network is divided into clusters of nodes
Each elects a cluster head as in a cluster-based routing algorithm
Cluster heads organize themselves into clusters and so on.
Nodes in a cluster broadcast their link information to each other
Each cluster head summarizes its cluster's information and sends it to neighboring cluster heads via gateways
A node at each level floods to its lower level the information that it obtains so the lower level has a hierarchical topology information.
Each node has a hierarchical address

35. Routing Hierarchy in HSR
Cluster
Head X
X.1
X.2
Virtual Link
Virtual
Nodes
Cluster
Head
Cluster
Head
Cluster
Head
X.2.2
Physical
Nodes
Physical Link
X.2.3
X.2.1
Cluster
Cluster
Head
Gateways

36. Zone-based Hierarchical Link State Routing Protocol (ZHLS) (1 / 2)
The network is divided into non-overlapping zones
No zone head is needed  better scalability
Uses 2 levels of topologies: node level and zone level
Use 2 types of Link State Packets (LSP)
Node LSP contains information on a node’s physical neighbors and is propagated within the zone
Zone LSP contains the zone information and is propagated globally so that each node has full node connectivity knowledge about the nodes in its zone and only zone connectivity information about other zones.

37. Zone-based Hierarchical Link State Routing Protocol (ZHLS) (2 / 2)
Nodes and zones are identified by Node IDs and Zone IDs
Packet routing
A packet is routed first based on the Zone ID to the destination zone.
The, within the destination zone, the packet is routed based on the destination’s Node ID

38. On-Demand Ad-Hoc Routing Protocols
Discovers a route only when needed, by flooding a query to all nodes
Each query packet remembers the intermediate nodes it traverses
Destination, or nodes that know how to reach the destination, sends the route information accumulated in the query packet back to the source
Routes are kept for a finite lifetime

39. Sample On-Demand Routing Protocols
Dynamic Source Routing Protocol (DSRP)
Ad hoc On-Demand Distance Vector (AODV) Routing
IETF RFC 3561
Cluster-based Routing Protocol
Temporarily Ordered Routing Protocol

40. Dynamic Source Routing Protocol
On-demand source routing protocol.
Source route: A packet carries in its header the entire route from source to destination
A node caches source routes that it knows and updates them when needed
A node can maintain multiple routes to the same destination
No periodic routing updates
Has two major phases
Route discovery and
Route maintenance.

41. Dynamic Source Routing Protocol Route Discovery Phase
When a source node wants to send a packet to a destination, it
Looks up its route cache to determine if it already knows a route to the destination.
If it knows an unexpired route to the destination, it uses this route to send the packet.
If is does not have an existing route, it initiates the route discovery process by broadcasting a route request packet
The route request packet contains
Addresses of the source and the destination, and
A unique ID.

42. Dynamic Source Routing Protocol Route Discovery Phase
Each intermediate node checks whether it knows a route to the destination.
If it does not, it appends its address to the route record of the packet and forwards the packet to its neighbors
If it knows a route to the destination, it sends a route reply message to the source along an unicast reverse path contained in the route request message
To limit the number of route requests propagated, a node processes the route request packet only if it has not already seen the packet and its own address is not present in the route record of the packet

43. Dynamic Source Routing Illustration
(1, 2) 2 5 1 4 7 3 6
Route Discovery
(1, 2, 5)
(1, 2, 5)
(1)
(1)
Source
Destination
(1, 4)
(1)
(1, 3, 6)
(1, 3)
Intermediate node caches
source route in the packet 2 5 1 4 7 3 6
Route Reply
Source
Destination
(1, 3, 6, 7)
(1, 3, 6, 7)
(1, 3, 6, 7)

44. Dynamic Source Routing Route Maintenance
Each node along a route is responsible for detecting failure of the part of the route that goes through it
Upon detecting a broken link, it informs the source
Source deletes any route in its cache that uses the broken link
Source may re-initiate the route discovery procedure to find a new route

45. Ad-Hoc On-Demand Distance Vector Routing (AODV)
Distance-vector routing used on an on-demand fashion
Node search for a route to a destination only when it needs to send packets to the destination and does not already have a route to the destination
Node keeps active routes in the same manner as in traditional routing tables: <destination, next hop>
An active route is one over which at least one packet has been forwarded over the past active timeout period
Two main operations
Route discovery
Route maintenance

46. AODV Route Discovery
To find a route to the destination, the source broadcasts a Route Request packet to all its neighbors, which in turn broadcast the Route Request to their neighbors until the Route Request reaches
The destination, or
An intermediate node that has an up-to-date route to the destination
To avoid route loops, a node discards a Route Request that it has already seen.
Route Request packets identified by sequence numbers

47. AODV Route Discovery
Destination or an intermediate node with a route to the destination unicasts a Route Reply packet back to the source along a reserve direction of the path traveled by the Route Request
To construct the reverse path, a node records the neighbor from which the first copy of each Route Request came
As the Route Reply travels back to the source, the nodes along the way enter the forward route into their routing tables, forming a forward route from the source to the destination

48. AODV Route Discovery
To ensure most recent route is used and to help avoid routing loop, AODV uses a Destination Sequence Number
When the destination sends back the Route Reply packet, it assigns a Destination Sequence Number to the route and insert the Destination Sequence Number in the Route Reply.
As the Route Reply travels towards the source, each node along the way records the Destination Sequence Number for the route in its routing table as illustrated below
Destination
IP Address
Destination
Sequence Number
Hop Count
Next Hop
Lifetime

49. AODV Route Discovery Illustration2 5 1 4 7 3 6
Route Discovery
Source
Destination
Intermediate node caches the
route to source 2 5 1 4 7 3 6
Route Reply
Source
Destination
Route establishment

50. AODV: Use of Destination Sequence Number
For route to Y
Source 1
Destination X A B Y
Source 2 Q
Destination Sequence
Number = 2
For route to Y
Destination A Y
Destination Sequence
Number = 2
For route to Y
Source 1
Destination
Route Request
(Dest. Seq. # =1) X A Y
Node A will use new route to Y
to respond to this Route Request

51. AODV Route Maintenance
A route may break when any node along the route moves away
If the source moves
The source can reinitiate route discovery to find a new route to the destination.
If an intermediate node moves
A neighbor of the moved node detects failure of link to the moved node and sends a link failure notification to its upstream neighbors, which in turn informs its upstream neighbors until the notification reaches the source
The source can reinitiate route discovery to find a new route to the destination

52. Cluster-Based Routing Protocol
Nodes are divided into clusters.
Algorithm for establishing clusters:
Upon powering up, node enters "undecided" state, starts a timer and broadcasts a Hello message
Cluster head responds to the Hello message with its own Hello message
Receipt of Hello from cluster head triggers the undecided node to transition its state into "member".
If the undecided node times out, then it makes itself the cluster-head if it has bi-directional link to some neighbor; otherwise it remains in undecided state and repeats the procedure again.
Cluster heads are changed as infrequently as possible.

53. Cluster-Based Routing Protocol
Each node maintains a neighbor table, containing
Status of the link to each neighbor and
State of the neighbor (cluster-head or member).
A cluster-head maintains information about
All members of its cluster and
A cluster adjacency table
Cluster adjacency table contains the following information about each neighboring cluster:
Gateway through which the cluster can be reached and
Cluster head of the cluster.

54. Cluster-Based Routing Protocol
When a source has to send packet to a destination, it sends a route request packet to its cluster head
Upon receiving the request, a cluster-head checks to see if the destination is in its cluster
If yes, it sends the request directly to the destination
Else, it sends it to all its adjacent cluster-heads.
The request contains the cluster heads’ addresses (i.e., source routing) so a cluster head discards a request packet that it has already seen.
When the destination receives the request packet, it replies with the route recorded in the request packet.
If the source does not receive a reply within a time period, it backs off exponentially before trying to re-send route request

55. Cluster-Based Routing Protocol
Route shortening technique
Upon receiving a source route packet, the node tries to find the farthest node in the route that is its neighbor (this could have happened due to a topology change) and sends the packet to that node thus reducing the route
While forwarding the packet if a node detects a broken link it sends back an error message to the source and then initiates local repair mechanism
Local repair: when a node finds the next hop is unreachable, it checks to see if the next hop can be reached through any of its neighbor or if the hop after next hop can be reached through any other neighbor. If any of the two works, the packet can be sent out over the repaired path.

56. Signal Stability-Based Adaptive Routing Protocol (SSR) (1 / 4)
On-demand routing that selects routes based on the radio signal strength between nodes, choosing "stronger" connectivity.
Uses two protocols
Dynamic Routing Protocol (DRP)
Static Routing Protocol (SRP).
Each node maintains
Signal Stability Table (SST) and
Routing Table (RT).

57. SSR (2 / 4)
Signal Stability Table (SST) stores the signal strength of neighboring nodes obtained by periodic beacons from the link layer of each neighboring node.
Signal strength is either recorded as a strong or weak.
All transmissions are received by DRP and processed.
After updating the appropriate table entries, the DRP passes the packet to the SRP.

58. SSR (3 / 4)
SRP passes the packet up the protocol stack if the node is the destination. Otherwise, it looks up the destination in the RT and forwards the packet.
If there is no entry for the destination in the RT, it initiates a route-search process to find a route by sending a route-search packet to neighbors from which it received strong signals
The destination chooses the first arriving route-search packet to send back as it is highly likely that this packet arrived over the shortest or least congested path.
The DRP reverses the selected route and sends a route-reply message back to the initiator of route-request.
The DRP of the nodes along the path update their RTs accordingly.

59. SSR (4 / 4)
Route-search packets arriving at the destination have necessarily arrived on the path of strongest signal stability because the packets arriving over a weak channel are dropped at intermediate nodes.
If the source times out before receiving a reply, it can re-transmit the packet and sets the PREF field in the packet header to indicate that weak channels can also be used
When an intermediate node detects a link failure, it sends an error message to the source indicating which channel has failed. The source then sends an erase message to notify all nodes of the broken link and initiates a new route-search process to find a new path to the destination.

60. What Problems Do You See As Important to Address and Why?

61. Thanks !