MobileComputingMANET2023 MobileComputingMANET2023.pptx

Mobile Computing
MANET
Mayank Pandey, MNNIT, Allahabad, India

MANET
• Formed by wireless hosts which may be mobile
• Without (necessarily) using a pre-existing
infrastructure
• Routes between nodes may potentially contain
multiple hops
07/11/2025 Mayank Pandey, MNNIT, Allahabad, India 2

MANET
• May need to traverse multiple links to reach a
destination
07/11/2025 3

MANET (Route Changes: Mobility)

Why Ad-hoc Networks
• Ease of deployment
• Speed of deployment
• Decreased dependence on infrastructure
07/11/2025 5

Many Applications

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Many Variations
• Fully Symmetric Environment
– all nodes have identical capabilities and responsibilities
• Asymmetric Capabilities
– transmission ranges and radios may differ
– battery life at different nodes may differ
– processing capacity may be different at different nodes
– speed of movement
• Asymmetric Responsibilities
– only some nodes may route packets
– some nodes may act as leaders of nearby nodes (e.g.
cluster head)

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Many Variations
• Traffic characteristics may differ in different ad hoc
networks
– bit rate
– timeliness constraints
– reliability requirements
– unicast / multicast / geocast
– host-based addressing / content-based addressing
/capability-based addressing
• May co-exist (and co-operate) with an infrastructure
based network

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Many Variations
• Mobility patterns may be different
– people sitting at an airport lounge
– taxi cabs
– kids playing
– military movements
– personal area network
• Mobility characteristics
– speed
– Predictability
• direction of movement
• pattern of movement
• uniformity (or lack) of mobility characteristics among
different nodes

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Challenges
• Limited wireless transmission range
• Broadcast nature of the wireless medium
• Hidden terminal problem
• Packet losses due to transmission errors
• Mobility-induced route changes
• Mobility-induced packet losses
• Battery constraints
• Potentially frequent network partitions
• Ease of snooping on wireless transmissions (security hazard)

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General Assumption
• Unless stated otherwise, fully symmetric
environment is assumed implicitly
– All nodes have identical capabilities and
responsibilities

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Why Routing in MANET different ?
• Host mobility
– link failure/repair due to mobility may have
different characteristics than those due to other
causes
• Rate of link failure/repair may be high when
nodes move fast
• New performance criteria may be used
– route stability despite mobility
– energy consumption

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Unicast Routing Protocols
• Many protocols have been proposed
• Some have been invented specifically for
MANET
• Others are adapted from previously proposed
protocols for wired networks
• No single protocol works well in all
environments
– some attempts made to develop adaptive
protocols

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Routing Protocols
• Proactive protocols
– Determine routes independent of traffic pattern
– Traditional link-state and distance-vector routing
protocols are proactive
• Reactive protocols
– Maintain routes only if needed
• Hybrid protocols

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Trade-Off
• Latency of route discovery
– Proactive protocols may have lower latency since routes are
maintained at all times
– Reactive protocols may have higher latency because a route from
X to Y will be found only when X attempts to send to Y
• Overhead of route discovery/maintenance
– Reactive protocols may have lower overhead since routes are
determined only if needed
– Proactive protocols can (but not necessarily) result in higher
overhead due to continuous route updating
• Which approach achieves a better trade-off depends on
the traffic and mobility patterns

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Flooding
• Sender S broadcasts data packet P to all its
neighbors
• Each node receiving P forwards P to its neighbors
• Sequence numbers used to avoid the possibility
of forwarding the same packet more than once
• Packet P reaches destination D provided that D is
reachable from sender S
• Node D does not forward the packet

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Flooding

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Flooding

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Flooding

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

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Flooding
• Nodes J and K both broadcast packet P to node D
– Since nodes J and K are hidden from each other, their transmissions may
collide
• Packet P may not be delivered to node D at all, despite the use of flooding

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Flooding
• Node D does not forward packet P, because node D is
the intended destination of packet P

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Flooding
• Flooding completed
• Nodes unreachable from S do not receive packet P (e.g., node Z)
• Nodes for which all paths from S go through the destination D also do not
receive packet P (example: node N)

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Flooding
• Flooding may deliver packets to too many nodes (in the worst
case, all nodes reachable from sender may receive the packet)

25
Flooding (Advantages)
• Simplicity
– May be more efficient than other protocols when rate of
information transmission is low enough that the overhead
of explicit route discovery/maintenance incurred by other
protocols is relatively higher
• this scenario may occur, for instance, when nodes transmit small
data packets relatively infrequently, and many topology changes
occur between consecutive packet transmissions
• Potentially higher reliability of data delivery
– Because packets may be delivered to the destination on
multiple paths

26
Flooding (Disadvantages)
• Potentially, very high overhead
– Data packets may be delivered to too many nodes who do not
need to receive them
• Potentially lower reliability of data delivery
– Flooding uses broadcasting -- hard to implement reliable
broadcast delivery without significantly increasing overhead
• Broadcasting in IEEE 802.11 MAC is unreliable
• In our example, nodes J and K may transmit to node D
simultaneously, resulting in loss of the packet
– in this case, destination would not receive the packet at all

27
Flooding of Control Packets
• Many protocols perform (potentially limited) flooding
of control packets, instead of data packets
• The control packets are used to discover routes
• Discovered routes are subsequently used to send
data packet(s)
• Overhead of control packet flooding is amortized
over data packets transmitted between consecutive
control packet floods

28
Dynamic Source Routing (DSR)
• When node S wants to send a packet to node D,
but does not know a route to D, node S initiates
a route discovery
• Source node S floods Route Request (RREQ)
• Each node appends own identifier when
forwarding RREQ

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Route Discovery (DSR)

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Mayank Pandey, MNNIT, Allahabad, India 33

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• Destination D on receiving the first RREQ,
sends a Route Reply (RREP)
• RREP is sent on a route obtained by reversing
the route appended to received RREQ
• RREP includes the route from S to D on which
RREQ was received by node D

36
Route Reply (DSR)

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Route Reply (DSR)
• Route Reply can be sent by reversing the route in Route
Request (RREQ) only if links are guaranteed to be bi-
directional
– To ensure this, RREQ should be forwarded only if it received on a link
that is known to be bi-directional
• If unidirectional (asymmetric) links are allowed, then RREP
may need a route discovery for S from node D
– Unless node D already knows a route to node S
– If a route discovery is initiated by D for a route to S, then the Route
Reply is piggybacked on the Route Request from D.
• If IEEE 802.11 MAC is used to send data, then links have to be
bi-directional (since Ack is used)

38
Dynamic Source Routing (DSR)
• Node S on receiving RREP, caches the route
included in the RREP
• When node S sends a data packet to D, the
entire route is included in the packet header
– hence the name source routing
• Intermediate nodes use the source route
included in a packet to determine to whom a
packet should be forwarded

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Data Delivery (DSR)
• Packet header size grows with route length
• When to Perform a Route Discovery
– When node S wants to send data to node D, but does not know a valid
route node D

40
DSR Optimization (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
• 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 learn a route when it overhears Data packets

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Use of Route Caching
• When node S learns that a route to node D is broken,
it uses another route from its local cache, if such a
route to D exists in its cache. Otherwise, node S
initiates route discovery by sending a route request
• Node X on receiving a Route Request for some node
D can send a Route Reply if node X knows a route to
node D
• Use of route cache
– can speed up route discovery
– can reduce propagation of route requests

42

Can Speed up Route Discovery

Can Reduce Propagation of Route Requests

Route Error (RERR)

Route caching: Cautions
• Stale caches can adversely affect performance
• With passage of time and host mobility,
cached routes may become invalid
• A sender host may try several stale routes
(obtained from local cache, or replied from
cache by other nodes), before finding a good
route

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

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
• Care must be taken to avoid collisions between
route requests propagated by neighboring
nodes
– insertion of random delays before forwarding RREQ

DSR (Disadvantages)
• Increased contention if too many route replies come
back due to nodes replying using their local cache
– Route Reply Storm problem
– Reply storm may be eased by preventing a node from
sending RREP if it hears another RREP with a shorter route
• An intermediate node may send Route Reply using a
stale cached route, thus polluting other caches
– This problem can be eased if some mechanism to purge
(potentially) invalid cached routes is incorporated.
• Static timeouts
• Adaptive timeouts based on link stability

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
• AODV retains the desirable feature of DSR that routes
are maintained only between nodes which need to
communicate

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
– AODV assumes symmetric (bi-directional) links
• When the intended destination receives a Route
Request, it replies by sending a Route Reply
• Route Reply travels along the reverse path set-up
when Route Request is forwarded

AODV (Properties)
• Route discovery cycle used for route finding
– Maintenance of active routes
– Sequence numbers used for loop prevention and as route
freshness criteria
• Whenever routes are not used, get expired they are
discarded
– Reduces stale routes
– Reduces need for route maintenance
– Minimizes number of active routes between an active
source and destination

AODV (Properties)
• AODV discovers routes as and when necessary
– Does not maintain routes from every node to
every other
– Routes are maintained just as long as necessary
• Every node maintains its monotonically
increasing sequence number -> increases
every time the node notices change in the
neighborhood topology

AODV (Properties)

AODV (Route Discovery)

Example

Route Reply AODV (Comments)

Route Discovery Contd.

Forward Path Setup

Receipt of Multiple RREP

AODV Data Delivery

Route Requests in AODV

Reverse Path Setup in AODV

Route Reply in AODV

Route Reply in AODV
• An intermediate node (not the destination) may also
send a Route Reply (RREP) provided that it knows a more
recent path than the one previously known to sender S
• To determine whether the path known to an
intermediate node is more recent, destination sequence
numbers are used
• The likelihood that an intermediate node will send a
Route Reply when using AODV not as high as DSR
– A new Route Request by node S for a destination is assigned
a higher destination sequence number. An intermediate node
which knows a route, but with a smaller sequence number,
cannot send Route Reply

Forward Path Setup in AODV

Data Delivery in AODV

Timeouts
• A routing table entry maintaining a reverse
path is purged after a timeout interval
– timeout should be long enough to allow RREP to
come back
• A routing table entry maintaining a forward path is
purged if not used for active_route_timeout interval
– if no data is being sent using a particular routing
table entry, that entry will be deleted from the
routing table (even if the route may actually still
be valid)

Link Failure Reporting
• A neighbor of node X is considered active for a
routing table entry if the neighbor sent a
packet within active_route_timeout interval
which was forwarded using that entry
• 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 messages, which also update
destination sequence numbers

Route Error
• When node X is unable to forward packet P (from
node S to node D) on link (X,Y), it generates a RERR
message
• Node X increments the destination sequence number
for D cached at node X
• The incremented sequence number N is included in
the RERR
• When node S receives the RERR, it initiates a new
route discovery for D using destination sequence
number at least as large as N

Destination Sequence Number
• Continuing from the previous slide …
– When node D receives the route request with
destination sequence number N, node D will set
its sequence number to N, unless it is already
larger than N

Link Failure Detection
• Hello messages: Neighboring nodes
periodically exchange hello message
• Absence of hello message is used as an
indication of link failure
• Alternatively, failure to receive several MAC-
level acknowledgement may be used as an
indication of link failure

Why Sequence Numbers in AODV
• To avoid using old/broken routes
– To determine which route is newer
• To prevent formation of loops
• 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 )

Optimization: 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

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
– Multi-path extensions can be designed
– DSR may maintain several routes for a single destination
• Unused routes expire even if topology does not change

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