presente by:
Niraj Kumar

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Mobile Ad Hoc Networks
A collection of wireless mobile hosts forming a temporary network without the aid of any established infrastructure.
Significant Features:
Dynamic topology of interconnections
No administrator
Short transmission range- routes between nodes has one or more hops
Nodes act as routers or depend on others for routing
movement of nodes invalidates topology information
Applications of MANET
Useful where geographical or terrestrial constrains demand totally distributed network without fixed base station.
Military Battlefields
Disaster and Rescue Operations
Conferences
Peer to Peer Networks
Security Design Issues in MANET
Do not have any centrally administered secure routers.
Attackers from inside or outside can easily exploit the network.
Passive eavesdropping, data tampering, active interfering, leakage of secret information, etc.
Open peer-to-peer architecture.
Shared Wireless Medium.
Dynamic Topology.
Routing
Routing protocols in Mobile Adhoc Networks are majorly of two categories: 
- Proactive Protocols
- Reactive Protocols
Reactive Routing protocols are based on finding routes between two nodes, when it is required. This is different from traditional Proactive Routing Protocols in which nodes periodically sends messages to each other in order to maintain routes
Routing Overview
Network with nodes, edges
Goal: Devise scheme for transferring message from one node to another
Which path?
Who decides – source or intermediate nodes?
Which path?
Generally try to optimize something:
Shortest path (fewest hops)
Shortest time (lowest latency)
Shortest weighted path (utilize available bandwidth)
Etc…
Who determines route?
Two general approaches:
Source (“path”) routing
Source specifies entire route: places complete path to destination in message header: A – D – F – G
Intermediate nodes just forward to specified next hop: D would look at path in header, forward to F
Like airline travel – get complete set of tickets to final destination before departing…
Destination (“hop-by-hop”) routing
Source specifies only destination in message header: G
Intermediate nodes look at destination in header, consult internal tables to determine appropriate next hop
Like postal service – specify only the final destination on an envelope, and intermediate post offices select where to forward next…
Ad Hoc Routing
Every node participates in routing: no distinction between “routers” and “end nodes”
No external network setup: “self-configuring”
Especially useful when network topology is dynamic (frequent network changes – links break, nodes come and go)
Traditional AODV
Ad Hoc On Demand Distance Vector Routing Protocol
Reactive Protocol: discovers a route on demand.
Nodes do not have to maintain routing information.
Route Discovery
Route Maintenance
Hello messages:
used to determine local connectivity.
can reduce response time to routing requests.
can trigger updates when necessary.
AODV features
AODV uses a broadcast route discovery mechanism
AODV relies on dynamically establishing route table entries at intermediate nodes
Each ad-hoc node maintains a monotonically increasing sequence number counter which is used to supersede stale cached routes
The Ad-hoc On-Demand Distance Vector Algorithm
Pure on-demand route system
Node does not have to discover and maintain a route to another node until the two need to communicate
Nodes that do not lie on active paths neither maintain any routing information
AODV Protocol Activities
Route discovery
Undertaken whenever a node needs a “next hop” to forward a packet to a destination
Route maintenance
Used when page link breaks, rendering next hop unusable
Routing (easy!)
Route Discovery
Route Request:
Source broadcasts Route Request (RREQ) message for specified destination
Intermediate node:
Forwards (broadcasts) message toward destination
Creates next-hop entry for reverse path to source, to use when sending reply (assumes bidirectional link…)
Route Reply
Destination unicasts Route Reply (RREP) message to source
RREP contains sequence number, hop-count field (initialized to 0)
Will be sent along “reverse” path hops created by intermediate nodes which forwarded RREQ
Intermediate node:
Create next-hop entry for destination as RREP is received, forward along “reverse path” hop
Increment hop-count field in RREP and forward
Source:
If multiple replies, uses one with lowest hop count
Route Maintenance
Used when page link breakage occurs
Link breakage may be detected using link-layer ACKs, “passive ACKs”, DSR ACK request
Route Error message sent to source of message being forwarded when break detected
Intermediate nodes “eavesdrop”, adjust cached routes
Source deletes route; tries another if one cached, or issues new Route Request
Piggybacks Route Error on new Route Request to clear intermediate nodes’ route caches, prevent return of invalid route
Issues
Scalability
Discovery messages broadcast throughout network
Broadcast / Multicast
Use Route Request packets with data included
Duplicate rejection mechanisms prevent “storms”
Multicast treated as broadcast; no multicast-tree operation defined
Scalability issues
AODV – RREQ
RREQ packet contains: destination and source IP address, broadcast ID, source node’s sequence number and destination node’s sequence number.
Node 1 wants to send data packet to node 7. Node 6 knows a current route to node 7. Node 1 sends a RREQ packet to its neighbors.
Source_addr =1
dest_addr =7
broadcast_id = broadcast_id +1
source_sequence_# = source_sequence_# + 1
dest_sequence_# = last dest_sequence_# for node 7
Traditional AODV (RREQ)
Nodes 2 and 4 verify that this is a new RREQ (source_sequence_# is not stale) with respect to the reverse route to node 1.
Forward the RREQ, and increment hop_cnt in the RREQ packet.
RREQ reaches node 6 from node 4, which knows a route to 7.
Node 6 verify that the destination sequence number is less than or equal to the destination sequence number it has recorded for node 7.
Nodes 3 and 5 will forward the RREQ packet to node 6, but it recognizes the packets as duplicates.
Traditional AODV (RREP)
Node 6 has a route to destination. It sends a route reply RREP to the neighbor that sent the RREQ packet.
Intermediate nodes propagate RREP towards the source using cached reverse route entries.
Other RREP packets discarded unless, dest_seq_# is higher than the pervious, or same but hop_cnt is smaller.
Cached reverse routes timeout in nodes that do not see RREP packet.
Traditional AODV (RREP)
Node 6 sends RREP to node 4
Source_addr=1, dest_addr=7, dest_sequence_# = maximum (sequence no. stored for node 7, dest_sequence_# in RREQ), hop_cnt =1.
Node 4 finds out it is a new route reply and propagates the RREP packet to Node 1.
Problem Statement
In AODV, only one route is maintained per destination
DSR makes use of multiple paths
This is a major weakness of AODV
Whenever a path breaks, AODV has to perform a route discovery
The source broadcasts a route request packet
Increases contention, significant overhead
We want to avoid frequent route discoveries
Proposed Solutions
Basic Idea
Maintain multiple paths learned from a route discovery
When a path breaks, try to use an alternate path instead of initiating a new route discovery
Two approaches
Maintain multiple paths at the source (AOMDV)
Enhance local repair (ELRAODV)
AOMDV – Motivation
AOMDV set up multiple link-disjoint routes from source node to destination node.
AOMDV does not predict the page link failure between the nodes.
AOMDV improve performance in many aspect like packet delivery ratio, end-to-end delay and control overhead of the network.
ELRAODV – Motivation
Local repair generate more traffic to repair route.
The extra information of the local neighbors allow ELRAODV to repair a route by sending a unicast request instead of broadcast in original AODV.
Conclusions
Ad Hoc networks pose an interesting problem in networking with dynamic routing and highly insecure working environment
Need of Secure, Scalable, Reliable and Efficient algorithms for Key management and Routing