1. Session 15 Mobile Adhoc Networks Prof. Sridhar Iyer IIT Bombay
IT 601: Mobile Computing
Session 15
Mobile Adhoc Networks
Prof. Sridhar Iyer
IIT Bombay
Session: 15
Prof. Sridhar Iyer

2. Multi-Hop wireless networks
May need to traverse multiple links to reach destination
Mobility causes route changes
Session: 15
Prof. Sridhar Iyer

3. Mobile Ad Hoc Networks (MANET)
Host movement frequent
Topology change frequent
No cellular infrastructure. Multi-hop wireless links.
Data must be routed via intermediate nodes. A B
Session: 15
Prof. Sridhar Iyer
Source: Vaidya

4. MANETs Do not need backbone infrastructure support Are easy to deploy
Useful when infrastructure is absent, destroyed or impractical
Infrastructure may not be present in a disaster area or war zone
Session: 15
Prof. Sridhar Iyer

5. Applications Military environments Emergency operations
soldiers, tanks, planes
Emergency operations
search-and-rescue
policing and fire fighting
Civilian environments
taxi cab network
meeting rooms
sports stadiums
Session: 15
Prof. Sridhar Iyer

6. MAC in MANET IEEE 802.11 DCF is most popular
Easy availability
Uses RTS-CTS to avoid hidden terminal problem
Uses ACK to achieve reliability
was designed for single-hop wireless
Does not do well for multi-hop ad hoc scenarios
Reduced throughput
Exposed terminal problem
Session: 15
Prof. Sridhar Iyer

7. Routing in MANET Mobile IP needs infrastructure MANET C
Home Agent/Foreign Agent in the fixed network
DNS, routing etc. are not designed for mobility
MANET
no default router available
“every” node also needs to be a router
Session: 15
Prof. Sridhar Iyer C

8. MANET routing protocols
Reactive protocols
Determine route if and when needed
Example: DSR (dynamic source routing)
Proactive protocols
Traditional distributed shortest-path protocols
Example: DSDV (destination sequenced distance vector)
Hybrid protocols
Adaptive; Combination of proactive and reactive
Example : ZRP (zone routing protocol)
Session: 15
Prof. Sridhar Iyer

9. Dynamic Source Routing (DSR)
Source S initiates a route discovery by flooding Route Request (RREQ)
Each node appends its own identifier when forwarding RREQ
Destination D on receiving the first RREQ, sends a Route Reply (RREP)
RREP sent on route obtained by reversing the route appended in RREQ
RREP includes the route from S to D, on which RREQ was received by D
S routes data using “source route” mechanism
Session: 15
Prof. Sridhar Iyer

10. Route Discovery in DSR Y Z S E F B C M L J A G H D K I N
Represents a node that has received RREQ for D from S
Session: 15
Prof. Sridhar Iyer
Source: Vaidya

11. 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
Session: 15
Prof. Sridhar Iyer

12. 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
Node H receives packet RREQ from two neighbors:
potential for collision
Session: 15
Prof. Sridhar Iyer

13. 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
Node C receives RREQ from G and H, but does not forward
it again, because node C has already forwarded RREQ once
Session: 15
Prof. Sridhar Iyer

14. 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
Since nodes J and K are hidden from each other, their
transmissions may collide
Session: 15
Prof. Sridhar Iyer

15. Route Discovery in DSR Y Z S E [S,E,F,J,M] F B C M L J A G H D K I N
Node D does not forward RREQ, because node D
is the intended target of the route discovery
Session: 15
Prof. Sridhar Iyer

16. Route Reply in DSR Y Z S RREP [S,E,F,J,D] E F B C M L J A G H D K I N
Represents RREP control message
Session: 15
Prof. Sridhar Iyer

17. Data Delivery in DSR Y Z DATA [S,E,F,J,D] S E F B C M L J A G H D K I
Packet header size grows with route length
Session: 15
Prof. Sridhar Iyer

18. Route Error (RERR) Y Z
RERR [J-D] S E F B C M L J A G H D K I N
J 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 fails
Session: 15
Prof. Sridhar Iyer

19. DSR: 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
Session: 15
Prof. Sridhar Iyer

20. Route caching
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 overhear Data to learn routes
Session: 15
Prof. Sridhar Iyer

21. Use of route caching S E F B C M L J A G H D K I N Z
[S,E,F,J,D]
[E,F,J,D] S E
[F,J,D],[F,E,S] F B
[J,F,E,S] C M L
[G,C,S] J A G
[C,S] H D K
[K,G,C,S] I N
RREP
RREQ Z
When Z sends a route request for C, node K sends back a route reply [Z,K,G,C] to Z using a locally cached route
Session: 15
Prof. Sridhar Iyer
Source: Vaidya

22. Route caching Uses: Problems:
Finding alternate routes in case original route breaks
Route reply from intermediate nodes
Problems:
Cached routes may become invalid over time and due to host mobility
Stale caches can adversely affect performance
Session: 15
Prof. Sridhar Iyer

23. DSR: 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
Session: 15
Prof. Sridhar Iyer

24. DSR: Disadvantages
Packet header size grows with route length due to source routing
Flood of route requests may potentially reach all nodes in the network
An intermediate node may send Route Reply using a stale cached route, thus polluting other caches
Session: 15
Prof. Sridhar Iyer

25. Ad Hoc On-Demand Distance Vector Routing (AODV)
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
Session: 15
Prof. Sridhar Iyer

26. 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
Route Reply (RREP) travels along the reverse path set-up when Route Request is forwarded
Session: 15
Prof. Sridhar Iyer

27. Route Requests in AODV Y Z S E F B C M L J A G H D K I N
Represents a node that has received RREQ for D from S
Session: 15
Prof. Sridhar Iyer
Source: Vaidya

28. Reverse Path Setup in AODVY Z S E F B C M L J A G H D K I N
Represents links on Reverse Path
Session: 15
Prof. Sridhar Iyer

29. Reverse Path Setup in AODVY Z S E F B C M L J A G H D K I N
Node D does not forward RREQ, because node D
is the intended target of the RREQ
Session: 15
Prof. Sridhar Iyer

30. Forward Path Setup in AODVY Z S E F B C M L J A G H D K I N
Forward links are setup when RREP travels along the reverse path
Represents a link on the forward path
Session: 15
Prof. Sridhar Iyer

31. Route Request and Route Reply
Route Request (RREQ) includes the last known sequence number for the destination
An intermediate node may also send a Route Reply (RREP) provided that it knows a more recent path than the one previously known to sender
Intermediate nodes that forward the RREP, also record the next hop to destination
Session: 15
Prof. Sridhar Iyer

32. AODV: Timeouts Neighboring nodes periodically exchange hello message
A routing table entry maintaining a reverse path is purged after a timeout interval
A routing table entry maintaining a forward path is purged if not used for a active_route_timeout interval
Session: 15
Prof. Sridhar Iyer

33. AODV: Link failure
Absence of hello message is used as an indication of link failure
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 (RERR) messages, which also update destination sequence numbers
Session: 15
Prof. Sridhar Iyer

34. AODV: Sequence numbers
To avoid using old/broken routes
To determine which route is newer
To prevent formation of loops A B C D E
Session: 15
Prof. Sridhar Iyer
Source: Vaidya

35. AODV: Sequence numbers
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 )
Session: 15
Prof. Sridhar Iyer

36. AODV: Expanding ring search
Route Requests are initially sent with small Time-to-Live (TTL) field, to limit their propagation
DSR also includes a similar optimization
If no Route Reply is received, then larger TTL tried
Session: 15
Prof. Sridhar Iyer

37. AODV: Summary Routes need not be included in packet headers
Nodes maintain routing tables containing entries only for routes that are in active use
At most one next-hop per destination maintained at each node
Sequence numbers are used to avoid old/broken routes and prevent routing loops
Session: 15
Prof. Sridhar Iyer

38. Protocol Trade-offs Reactive protocols
Lower overhead since routes are determined on demand
Significant delay in route determination
Employ flooding (global search)
Control traffic may be bursty
Session: 15
Prof. Sridhar Iyer

39. Protocol Trade-offs Proactive protocols
Based on traditional wired routing protocols
Always maintain routes
Little or no delay for route determination
Consume bandwidth to keep routes up-to-date
Maintain routes which may never be used
Session: 15
Prof. Sridhar Iyer

40. Link State Routing Each node periodically floods status of its links
Each node re-broadcasts link state information received from its neighbor
Each node keeps track of link state information received from other nodes
Each node uses above information to determine next hop to each destination
Session: 15
Prof. Sridhar Iyer

41. Optimized Link State Routing (OLSR)
The overhead of flooding link state information is reduced by requiring fewer nodes to forward the information
A broadcast from node X is only forwarded by its multipoint relays
Multipoint relays of node X are its neighbors such that each two-hop neighbor of X is a one-hop neighbor of at least one multipoint relay of X
Each node transmits its neighbor list in periodic beacons, so that all nodes can know their 2-hop neighbors, in order to choose the multipoint relays
Session: 15
Prof. Sridhar Iyer

42. Optimized Link State Routing (OLSR)
Nodes C and E are multipoint relays of node A
Nodes C and E forward information received from A B F J A E H C K G D
Node that has broadcast state information from A
Session: 15
Prof. Sridhar Iyer

43. Zone Routing Protocol (ZRP)
Intra-zone routing: Pro-actively maintain state information for links within a short distance from any given node
Routes to nodes within short distance are thus maintained proactively (using, say, link state or distance vector protocol)
Inter-zone routing: Use a route discovery protocol for determining routes to far away nodes. Route discovery is similar to DSR with the exception that route requests are propagated via peripheral nodes.
Session: 15
Prof. Sridhar Iyer

44. ZRP
All nodes within hop distance at most d from a node X are said to be in the routing zone of node X
All nodes at hop distance exactly d are said to be peripheral nodes of node X’s routing zone
Session: 15
Prof. Sridhar Iyer

45. ZRP example: Zone Radius = d = 2B
S performs route
discovery for D C A E S D F
E knows route from E to D,
so route request need not be
forwarded to D from E
Denotes route request
Session: 15
Prof. Sridhar Iyer

46. MANET variations Fully symmetric environment Asymmetric Capabilities
all nodes have identical capabilities and responsibilities
Asymmetric Capabilities
transmission ranges, battery life, processing capacity may differ at different nodes
Asymmetric Responsibilities
only some nodes may route packets
Session: 15
Prof. Sridhar Iyer

47. MANET variations
Mobility patterns may differ from one scenario to another
Mobility characteristics (speed, predictability) may be different for different applications
Traffic characteristics may differ
timeliness constraints
reliability requirements
Session: 15
Prof. Sridhar Iyer

48. MANET summary Routing is the most studied problem
Interplay of layers is being researched
Large number of simulation based expts
Small number of field trials
Very few reported deployments
Fertile area for imaginative applications
Session: 15
Prof. Sridhar Iyer