Mobile Ad-hoc Network (MANET) Routing Algorithms─ Part 1

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Mobile Ad-hoc and Sensor Networks
Lesson 04
Mobile Ad-hoc Network (MANET)
Routing Algorithms─ Part 1

Ad-hoc networks deployment
• For routing, target detection, and service
discovery
• Routing protocols used for routing by
MANET nodes

Dynamic Source Routing (DSR) Protocol
• Deploys source routing
• Source routing means that each data
packet includes the routing-node
addresses also

Reactive protocol feature of DSR
• Reacts to the changes and dynamically
maintains only the routing addresses
from source to destination, which are the
active paths to a destination at a given
instant
• Performs unicast routing
• Unicast means routing packets to a
single destined address

DSR Nodes
• Each node caches the specified route to
destination during source routing of a
packet through that node
• This enables a node to provide route-
specification when a packet source
routes from that node

DSR Nodes
• Each node deletes the specified route to
destination during routing of error packet
in reverse path to the source in case of
observing a disconnection during forward
path to destination
• The deletion of link shown in a table or
cache is called link reversal

Phase 1 in DSR Protocol
• Source node initiates a route discovery
process
• It broadcasts the packets, each with a
header
• It then expects return of
acknowledgement from each destination
• The packets are called route request
(RREQ) packets

• DSR uses flooding (sends multiple
RREQs
• A header for each route request packet
has the unique request number and
source and destination addresses
• This enables identification of request at
each intermediate node in the request
and acknowledged packet(s)

• When the process starts, initially only the
source address is given in the header
• When the packet reaches a neighbour,
that is, any intermediate node, the node
adds its own address in the header if it is
able to successfully send the packet to
its next neighbour

• When the packet reaches the destined
address, its header therefore has all
addresses of the nodes in the path

Example
• Assume that node D is a source and G is
a destination and the path DEFG
is not known. In such a case the path
cannot be placed in the header.

Example of MANET

Source Route Discovery Process
• Source header from D puts the sequence
number q and source address D in the
packet header and sends the packet to
its next neighbour. When the packet
reaches E, its header is (q, D)
• Assume that no route error packet
bounced back from neighbour E

• The packet is then transmitted to F
• When the packet reaches F, its header is
(q, D, E). Assume that no route error
packet bounced back from neighbour F

• The packet is transmitted from F to G.
Assume that no route error packet
bounced back from neighbour G.
• When the packet reaches G, its header
is (q, D, E, F)

Domain Specific rules for consistency
• When packet reaches the destination, a
route reply (RREP) for the sequence
used in RREQ is generated
• On return path, the route cache builds up
at each intermediate node for
deployment at a later instant of phase 2

Phase 2 in DSR
• When any source node desires to send a
message, it first looks at its route cache
• If the required route is available in cache,
the source node puts all the addresses of
the nodes for the path to destination in
the header

Source routing addresses in DSR
• Assuming that there is a message from a
MANET node D in the network
Node Destination Cached Path
D A DCA
D B DCAB
D F DEF
D G DEFG

Ad-hoc On-demand Distance Vector
(AODV) Routing protocol
• A reactive protocol
• Reacts to the changes and maintains
only the active routes in the caches or
tables for a pre-specified expiration time
• Routes that are found that are available
at a given instant

AODV
• Performs unicast routing
• Distance vector means a set of distant
nodes, which defines the path to
destination

AODV
• DEFG is a distance vector for
source-destination pair D and G
• In AODV, a distance vector is provided
on demand during forwarding of a packet
to destination by a node in the path and
not by the route cache providing path
through the header in the source data
packet [phase 2]

AODV
• Every node keeps a next-hop routing
table, which contains the destinations to
which it currently has a route
• A routing table entry expires if it has not
been used or reactivated for a pre-
specified expiration time
• AODV adopts the destination sequence
number technique

AODV
• Does not deploy flooding (multiple
RREQs)
• Stores the next hop routing information
of the active routes in the routing caches
(tables) at each node
• Therefore, has small header size and
thus reduces the network traffic
overhead

Phase 1 in AODV Protocol
• A node uses hello messages to notify its
existence to its neighbours
• Therefore, the link status to the next hop in
an active route is continuously monitored

• When any node discovers a link
disconnection, it broadcasts a route error
(RERR) packet to its neighbors, who in
turn propagate the RERR packet towards
those nodes whose routes may be
affected by the disconnected link
• Then, the affected source can be informed

• Source node initiates a route discovery
process if no route is available in the
routing table
• It broadcasts the demand through the
RREQ packets

• Each RREQ has an ID and the addresses
of the source and destination in its header
• It expects return acknowledgement from
destination
• A node identifies the last observed
sequence number of the destination from
the ID

• Each RREQ starts with a small TTL (time
to live) value [Number of attempts]
• If the destination is not found during the
TTL, the TTL is increased in subsequent
RREQ packets
• The node also identifies sequence number
of source node

• Sequence numbers ensure loop-free and
up-to-date routes.
• Loop-free means free from bouncing of a
packet to a node after intermediate hops

• Each node rejects the RREQ which it had
observed before
• This reduces flooding which means it
reduces too many RREQs present in the
network at a given instant.

Route table in AODV
• Keep entries for a specified period and
each node maintains a cache
• The cache saves the received RREQs
• Only the RREQ of highest sequence
numbers are accepted and previous ones
are discarded

Route table in AODV
• The cache also saves the return path for
each RREQ source
• When a node having a route to the
destination or the destined node receives
the RREQ, it checks the destination
sequence number it currently knows and
the one specified in the RREQ

Route table in AODV
• RREP packet is created and forwarded
back to the source only if the destination
sequence number is equal to or greater
than the one specified in RREQ
• It guarantees the up-dation of routing
cache information

Summary
• DSR
• Deploys source routing
• Reacts to the changes and dynamically
maintains only the routing addresses
from source to destination, which are the
active paths to a destination at a given
instant
…

…Summary
• AODV reactive protocol
• Routes that are found that are available
at a given instant
• Every node keeps a next-hop routing
table
• A routing table entry expires if it has not
been used or reactivated for a pre-
specified expiration time

…Summary
• AODV adopts the destination sequence
number technique

End of Lesson 04
Mobile Ad-hoc Network (MANET) Routing
Algorithms─ Part 1

Mobile Ad-hoc Network (MANET) Routing Algorithms─ Part 1

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Mobile Ad-hoc Network (MANET) Routing Algorithms─ Part 1