AD HOC NETWORKS
Ian F. Akyildiz
Broadband & Wireless Networking Laboratory
School of Electrical and Computer Engineering
Georgia Institute of Technology
Tel: 404-894-5141; Fax: 404-894-7883
Email: ian@ece.gatech.edu
Web: http://www.ece.gatech.edu/research/labs/bwn
Ad Hoc Networks
 Infrastructure-less wireless networks
dynamically formed using only mobile
hosts (no routers)
 Network topology dynamic as all hosts
are mobile!
 Mobile hosts themselves double up as
routers!!
 Multi-hop paths …
 Highly resource constrained
 Extreme case of network mobility…
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Topologies
-Ad Hoc Networking
 An ad hoc network is a peer-to-peer network
(no centralized server) set up temporarily to
meet some immediate need.
 For example, a group of employees, each with a
laptop or palmtop computer may convene in a
conference room for a business or classroom
meeting.
 The employees link their computers in a
temporary network just for the duration of the
meeting.
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Topologies
-Ad Hoc Networking
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Topologies
-Ad Hoc Networking
Differences between
Ad-Hoc wireless LAN and a CLASSICAL wireless LAN
 The classical wireless LAN forms a stationary infrastructure
consisting of one or more cells with a control module for each
cell.
 Within a cell, there may be a number of stationary end systems.
 Nomadic stations can move from one cell to another.
 In contrast, there is no infrastructure for an ad-hoc network.
 Rather, a peer collection of stations within range of each other
may dynamically configure themselves into a temporary network.
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Illustration
A
B
C
D
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Wireless Ad Hoc Networks

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Disaster recovery
Battlefield
Smart office
Gaps in cellular infrastructure
Virtual Navigation (cities, buildings, areas, etc..)
Telemedicine (e.g., accident sites)
Tele-Geoprocessing Applications
Education via Internet
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Classes of Wireless Ad Hoc
Networks
 Three distinct classes
– Mobile Ad Hoc Networks (MANET)
 possibly highly mobile nodes
 power constrained
– Wireless Ad Hoc Sensor/Device Networks
 relatively immobile
 severely power constrained nodes
 large scale
– Wireless Ad Hoc Backbone Networks
 rapidly deployable wireless infrastructure
 largely immobile nodes
 Common attributes:
– Ad hoc deployment, no infrastructure
– Routes between S-D nodes may contain multiple hops
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Multihop Routing
 Traverse multiple links to reach a destination
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MANET
Mobility causes route changes
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Variations
 Fully symmetric vs. asymmetries in
– Transmission ranges
– Battery life
– Processing capability
– Speed, patterns, and predictability of movement
– Ability to act as multihop relay
– Ability to act as leaders of a cluster of nodes
 Coexistence with an infrastructure
 Variations in traffic characteristics
– Bit rate, timeliness
– Unicast/multicast/geocast
– Addressing (host, content, capability)
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Unicast Routing in MANET
 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
 Many protocols have been proposed
– Some have been invented specifically for MANET
– Others are adapted from older protocols for wired networks
 No single protocol works well in all environments
– some attempts made to develop adaptive protocols
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Types of Protocols
 Proactive Protocols
– Determine routes independent of traffic
pattern
– Traditional (link-state, distance-vector)
routing protocols are proactive
 Reactive Protocols
– Maintain routes only if needed
 Hybrid Protocols
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Traditional Routing Algorithms
 Distance Vector
– periodic exchange of messages with all physical neighbors that
contain information about who can be reached at what distance
– selection of the shortest path if several paths available
 Link State
– periodic notification of all routers about the current state of all
physical links
– router gets a complete picture of the network
 Example
– ARPA packet radio network (1973), DV-Routing
– every 7.5s exchange of routing tables including link quality
– updating of tables also by reception of packets
– routing problems solved with limited flooding
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Problems of Traditional Routing
Algorithms
 Dynamic of the topology
– frequent changes of connections, connection quality,
participants
 Limited performance of mobile systems
– periodic updates of routing tables need energy without
contributing to the transmission of user data, sleep modes
difficult to realize
– limited bandwidth of the system is reduced even more due to
the exchange of routing information
– links can be asymmetric, i.e., they can have a direction
dependent transmission quality
 Problem
– protocols have been designed for fixed networks with
infrequent changes and typically assume symmetric links
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Routing Protocols for Ad
Hoc Networks
 Proactive Routing Protocols
– DSDV (Destination Sequenced Distance Vector)
– LSR (Link State Routing)
 Reactive Routing Protocols
– DSR (Dynamic Source Routing)
– AODV (Ad-Hoc on-Demand Distance Vector)
 Hybrid Routing Protocols
– ZRP (Zone Routing Protocol)
– TORA
– CEDAR (Core Extraction Distributed Ad-hoc
Routing)
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Proactive Routing Protocols…
Unsuitable for such a dynamic network
For example, consider link-state
routing that sends out network-wide
floods for every link-state change …
Even in the absence of any existing
connections, considerable overhead
spent in maintaining “network state”
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Goals
Low overhead route computation
Ability to recover from frequent
failures at low-cost
Scalable (with respect to mobility
and number of hosts)
Robust
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Reactive (On-Demand) Protocols
Compute routes only when needed
Even if network state changes, any
re-computation done only when any
existing connections are affected
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Dynamic Source Routing
(DSR)
 Based on source routing
 On-demand
 Route computation performed on a perconnection basis
 Source, after route computation,
appends each packet with a source-route
 Intermediate hosts forward packet
based on source route
 TWO PHASES: ROUTE DISCOVERY &
ROUTE MAINTENANCE
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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 (bradcasts) Route Request
(RREQ) packet.
 RREQ packet contains DESTINATION ADDRESS,
SOURCE NODE ADDRESS and A UNIQUE
IDENTIFICATION NUMBER.
 Each node receiving the packet checks whether it
knows of a route to destination.
 If it does not, it appends/adds its own identifier
(address) to the route record and forwards the RREQ
packet.
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Dynamic Source Routing (DSR)
REMARK:
To limit the number of route requests, a node
only forwards the RREQ packet if the request
has not yet been seen by the node and if the
node’s address does not already appear in
the route record.
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Dynamic Source Routing I
 Split routing into discovering a path and
maintaining a path
 Discover a path
– only if a path for sending packets to a certain
destination is needed and no path is currently
available
 Maintaining a path
– only while the path is in use one has to make sure
that it can be used continuously
 No periodic updates needed!
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Dynamic Source Routing II
 Path discovery
– broadcast a packet with destination address and unique ID
– if a station receives a broadcast packet
 if the station is the receiver (i.e., has the correct destination
address) then return the packet to the sender (path was
collected in the packet)
 if the packet has already been received earlier (identified via
ID) then discard the packet
 otherwise, append own address and broadcast packet
– sender receives packet with the current path (address list)
 Optimizations
– limit broadcasting if maximum diameter of the network is known
– caching of address lists (i.e. paths) with help of passing packets
 stations can use the cached information for path discovery
(own paths or paths for other hosts)
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Dynamic Source Routing III
Maintaining paths
– after sending a packet
wait for a layer 2 acknowledgement (if
applicable)
listen into the medium to detect if other
stations forward the packet (if possible)
request an explicit acknowledgement
– if a station encounters problems it
can inform the sender of a packet
or look-up a new path locally
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Route Discovery in DSR
Y
Z
S
E
F
B
C
M
J
A
L
G
H
K
I
D
N
Represents a node that has received RREQ for D from S
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Route Discovery in DSR
Y
Broadcast transmission
[S]
S
E
F
B
C
L
G
H
K
I
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J
A
[X,Y]
Z
D
N
Represents transmission of RREQ
Represents list of identifiers appended to RREQ
27
Route Discovery in DSR
Y
Z
S
E
[S,E]
F
B
C
A
M
J
[S,C]
H
G
K
I
L
D
N
• Node H receives packet RREQ from two neighbors:
potential for collision
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Route Discovery in DSR
Y
Z
S
E
F
B
[S,E,F]
C
M
J
A
L
G
H
I
[S,C,G] K
D
N
• Node C receives RREQ from G and H, but does not forward
it again, because node C has already forwarded RREQ once
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Route Discovery in DSR
Y
Z
S
E
[S,E,F,J]
F
B
C
M
J
A
L
G
H
K
I
D
[S,C,G,K]
N
• Nodes J and K both broadcast RREQ to node D
• Since nodes J and K are hidden from each other, their
transmissions may collide
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Route Discovery in DSR
Y
Z
S
E
[S,E,F,J,M]
F
B
C
M
J
A
L
G
H
K
D
I
N
• Node D does not forward RREQ, because node D
is the intended target of the route discovery
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Route Discovery in DSR
 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
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Route Reply in DSR
Y
Z
S
E
RREP [S,E,F,J,D]
F
B
C
J
A
L
G
H
K
I
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D
N
Represents RREP control message
33
Route Reply in DSR
 Route Reply can be sent by reversing the route in Route
Request (RREQ) only if links are guaranteed to be bidirectional (SYMMETRICAL)
– 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)
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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 in DSR
Y
DATA [S,E,F,J,D]
S
Z
E
F
B
C
M
J
A
L
G
H
K
I
D
N
Packet header size grows with route length
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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
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Use of Route Caching
[S,E,F,J,D]
[E,F,J,D]
S
B
[F,J,D],[F,E,S]
E
F
C
M
J
[C,S]
A
[J,F,E,S]
L
G
H
[G,C,S]
D
K
I
N
Z
[P,Q,R] Represents cached route at a node
(DSR maintains the cached routes in a tree format)
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Use of Route Caching:
Can Speed up Route Discovery
[S,E,F,J,D]
[E,F,J,D]
S
[F,J,D],[F,E,S]
E
F
B
C
[G,C,S]
[C,S]
A
[J,F,E,S]
M
J
L
G
H
I
[K,G,C,S] K
D
RREP
N
RREQ
When node Z sends a route request
for node C, node K sends back a route
reply [Z,K,G,C] to node Z using a locally
cached route
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40
Use of Route Caching:
Can Reduce Propagation of Route Requests
[S,E,F,J,D]
Y
[E,F,J,D]
S
[F,J,D],[F,E,S]
E
F
B
C
[G,C,S]
[C,S]
A
[J,F,E,S]
M
J
L
G
H
I
D
[K,G,C,S] K
RREP
N
RREQ
Z
Route Reply (RREP) from node K limits flooding of RREQ.
In general, the reduction may be less dramatic.
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Route Error (RERR)
Y
RERR [J-D]
S
Z
E
F
B
C
M
J
A
L
G
H
K
I
D
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
Nodes hearing RERR update their route cache to remove link J-D
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Route Caching: Beware!
 Stale (obsolete) 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
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Dynamic Source Routing:
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
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Dynamic Source Routing:
Disadvantages
 Packet header size grows with route length
 Flood of route requests may potentially reach all nodes
 Care must be taken to avoid collisions between route requests
propagated by neighboring nodes
– insertion of random delays before forwarding RREQ
 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.
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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
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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 setup when Route Request is forwarded
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AODV
Ad Hoc On-demand Distance
Vector Routing
– Hop-by-hop routing as opposed to source routing
– On-demand
– When a source node wants to send a message to some
destination node and does not already have a valid
route to that destination, it initiates a path discovery
process to locate the destination (as in DSR case)
– It broadcasts the RREQ packet to its neighbors
– They then send it to their neighbors until the
destination or a node with “fresh enough cash
information” to the destination is located.
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AODV
Ad Hoc On-demand Distance
Vector Routing
– When RREQ propagates, routing tables are
updated at intermediate nodes (for route to
source of RREQ)
– When RREP is sent by destination, routing
tables updated at intermediate nodes (for
route to destination), and propagated back to
source
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AODV
Ad Hoc On-demand Distance
Vector Routing
– Each node maintains its own sequence number and a
broadcast ID.
– The broadcast ID is incremented for every RREQ the
node initiates and together with the node’s IP
address, uniquely identifies an RREQ.
– Along with its own sequence number and broadcast ID,
the source node includes in the RREQ the most recent
sequence number it has for the destination.
– Intermediate nodes can reply to the RREQ only if
they have a route to the destination whose
corresponding destination sequence number is greater
than or equal to that contained in the RREQ.
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AODV
Ad Hoc On-demand Distance
Vector Routing
– During the process of forwarding the RREQ,
intermediate nodes recording their route
tables the address of the neighbor from which
the first copy of the broadcast packet is
received establishing a reverse path.
– If more same RREQs are received later, they
are discarded.
– RREP packet is sent back to the neighbors and
the routing tables are accordingly updated.
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Route Requests in AODV
Y
Z
S
E
F
B
C
M
J
A
L
G
H
K
I
D
N
Represents a node that has received RREQ for D from S
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Route Requests in AODV
Y
Broadcast transmission
Z
S
E
F
B
C
M
J
A
L
G
H
K
I
D
N
Represents transmission of RREQ
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Route Requests in AODV
Y
Z
S
E
F
B
C
M
J
A
L
G
H
K
D
I
N
Represents links on Reverse Path
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Reverse Path Setup in AODV
Y
Z
S
E
F
B
C
M
J
A
L
G
H
K
I
D
N
• Node C receives RREQ from G and H, but does not forward
it again, because node C has already forwarded RREQ once
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Reverse Path Setup in AODV
Y
Z
S
E
F
B
C
J
A
L
G
H
K
I
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D
N
56
Reverse Path Setup in AODV
Y
Z
S
E
F
B
C
M
J
A
L
G
H
K
D
I
N
• Node D does not forward RREQ, because node D
is the intended target of the RREQ
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Route Reply in AODV
Y
Z
S
E
F
B
C
M
J
A
L
G
H
K
D
I
N
Represents links on path taken by RREP
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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
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Forward Path Setup in AODV
Y
Z
S
E
F
B
C
M
J
A
L
G
H
K
D
I
N
Forward links are setup when RREP travels along
the reverse path
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Represents a link on the forward path
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Data Delivery in AODV
Y
DATA
Z
S
E
F
B
C
M
J
A
L
G
H
K
I
D
N
Routing table entries used to forward data packet.
NOTE: Route is not included in packet header as in DSR.
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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 a
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)
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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
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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
 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
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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
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Why Sequence Numbers in AODV
 To avoid using old/broken routes
– To determine which route is newer
 To prevent formation of loops
A
B
C
D
E
– 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 )
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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
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Summary: AODV
 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
– DSR may maintain several routes for a single
destination
 Unused routes expire even if topology does not change
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Proactive Protocols
Most of the schemes discussed so
far are reactive
Proactive schemes based on
distance-vector and link-state
mechanisms have also been proposed
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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
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Destination-Sequenced
Distance-Vector (DSDV)
 Each node maintains a routing table which stores
– next hop towards each destination
– a cost metric for the path to each destination
– a destination sequence number that is created by the
destination itself
– Sequence numbers used to avoid formation of loops
 Each node periodically forwards the routing table to its
neighbors
– Each node increments and appends its sequence
number when sending its local routing table
– This sequence number will be attached to route
entries created for this node
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Destination-Sequenced DistanceVector (DSDV)
 Assume that node X receives routing
information from Y about a route to
node Z
X
Y
Z
 Let S(X) and S(Y) denote the destination
sequence number for node Z as stored at
node X, and as sent by node Y with its
routing table to node X, respectively
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Destination-Sequenced
Distance-Vector (DSDV)
 Node X takes the following steps:
X
Y
Z
– If S(X) > S(Y), then X ignores the routing
information received from Y
– If S(X) = S(Y), and cost of going through Y is smaller
than the route known to X, then X sets Y as the next
hop to Z
– If S(X) < S(Y), then X sets Y as the next hop to Z,
and S(X) is updated to equal S(Y)
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Comparison
Local
Low maintenance overhead
Low update overhead
High access overhead
Dynamic networks
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Link state
Low maintenance overhead
Low access overhead
High update overhead
Static networks
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Hybrid Protocols:
Zone Routing Protocol (ZRP)
 Zone routing protocol combines
– Proactive protocol: which pro-actively updates network
state and maintains route regardless of whether any
data traffic exists or not
– Reactive protocol: which only determines route to a
destination if there is some data to be sent to the
destination
 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
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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 too far away nodes. Route discovery is
similar to DSR with the exception that route requests are
propagated via peripheral nodes.
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Temporally-Ordered Routing
Algorithm (TORA)
 On demand routing.
 Designed for a dynamic environment.
 Each node keeps routing information about
adjacent nodes.
 A DAG (Directed Acyclic Graph) describes all
discovered paths to a node.
– When one link fails, its neighbors reverse
direction in the DAG, and utilize another
path.
– Timing of failures is important, so TORA
requires synchronized clocks (i.e. GPS)
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Temporally-Ordered Routing
Algorithm (TORA)
 Water flowing downhill toward to a destination node
through a network of tubes/pipes that models the
routing state of the real network.
 Tubes/pipes represent links between nodes in the
network, junctions of tubes represent the nodes, and
the water in the tubes represents the packets
flowing toward the destination.
 Each node has a height with respect to the
destination that is computed by the routing protocol.
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Temporally-Ordered Routing
Algorithm (TORA)
If a tube between nodes A and B
becomes blocked such that water can
no longer flow through it, the height of
A is set to a height greater than that
of any of is remaining neighbors, such
that water fill now flow back out of A
(and toward the other nodes that had
been routing packet to the destination
via A).
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Temporally-Ordered Routing
Algorithm (TORA)
TORA performs 3 basic functions:
 ROUTE CREATION
 ROUTE MAINTENANCE
 ROUTE ERASURE
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TORA– ROUTE CREATION &
MAINTENANCE
 For each node in the network a separate directed acyclic graph (DAG)
is maintained for each destination.
 When a node needs a route to a destination, it broadcasts a QUERY
packet containing the destination address.
 This packet travels through the network until it reaches either the
destination or an intermediate node which already has a path to that
destination.
 The recipient of the QUERY packet then broadcasts an UPDATE
packet listing its height with respect to the destination.
 Each node receiving this UPDATE packet sets is height to a value
greater than the height of the neighbor from which the UPDATE
packet has been received.
 When a node detects a network partition, it generates a CLEAR
packet that resets routing state and removes invalid routes from the
network.
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TORA– ROUTE CREATION &
MAINTENANCE
 During the route creation and maintenance phases, nodes
use the “height” metric to establish a DAG rooted at the
destination.
 Links are assigned a direction (upstream or downstream)
based on the relative height metric of neighboring nodes.
(See Figure)
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TORA– ROUTE CREATION &
MAINTENANCE
 For node mobility when the DAG route is broken, and
route maintenance is necessary to re-establish a DAG
rooted at the same destination.
 See Figure upon failure of the last downstream link, a
node generates a new reference level.
 Links are reversed to reflect the change.
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TORA
Destination
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TORA
Failure
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TORA
Reversal
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TORA
Reversal
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TORA
Reversal
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TORA
 TORA is partially proactive and partially reactive
 Reactive  route creation is initiated on demand.
 Proactive  Route maintenance is done proactively
such that multiple routing options are available in
case of link failures.
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So far ...
 All nodes had identical responsibilities
 Some schemes propose giving special
responsibilities to a subset of nodes
– Even if all nodes are physically identical
 Core-based schemes are examples of
such schemes
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Core-Extraction Distributed Ad
Hoc Routing (CEDAR)
 A subset of nodes in the network is identified as the core
 Each node in the network must be adjacent to at least one
node in the core
– Each node picks one core node as its dominator (or leader)
 Core is determined by periodic message exchanges between
each node and its neighbors
– attempt made to keep the number of nodes in the core
small
 Each core node determines paths to nearby core nodes by
means of a localized broadcast
– Each core node guaranteed to have a core node at <=3 hops
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CEDAR: Core Nodes
A
G
D
H
B
C
E
F
S
A core node
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J
K
Node E is the dominator
for nodes D, F and K
92
CEDAR
 Advantages
– Route discovery/maintenance duties
limited to a small number of core nodes
– Link state propagation a function of link
stability/quality
 Disadvantages
– Core nodes have to handle additional
traffic, associated with route discovery
and maintenance
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SUMMARY:
Characteristics of DSDV
Parameters
DSDV
Time Complexity
O(d)
Link Addition/Failure
Communication
O(x=N)
Complexity
Routing Philosophy
Flat
2
Number of Required Tables
N = # of Nodes in Network
d = Network diameter
h = Height of Routing Tree
x = # of affected Nodes
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SUMMARY:
Characteristics of On-Demand
Routing Protocols
Parameters
Time Complexity
Initialization
Time Complexity
Post Failure
AODV
O(2d)
DSR
O(2d)
TORA
O(2d)
O(2d)
O(2d)
or O(1)
O(2d)
N = # of Nodes in Network
d = Network diameter
x = # of affected Nodes
l = Diameter of affected segment
y = nods forming path for Reply packet
z = Diameter of path for Reply packet
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SUMMARY
Characteristics of On-Demand
Routing Protocols
Parameters
Comm. Complexity
Initialization
Comm. Complexity
Post Failure
AODV
O(2N)
DSR
O(2N)
TORA
O(2N)
O(2N)
O(2N)
O(2x)
N = # of Nodes in Network
d = Network diameter
x = # of affected Nodes
l = Diameter of affected segment
y = nodes forming path for Reply packet
z = Diameter of path for Reply packet
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SUMMARY
Characteristics of On-Demand
Routing Protocols
Parameters
Multicast Capability
Beaconing
Requirements
Multiple Route
Possibilities
Expiration Timers
Reconfiguration:
Erase Route, Notify
Source
Link Reversal; Route
Repair
Localized Broadcast
Query
IFA’2004
AODV
Yes
DSR
Yes
Yes
Yes
TORA
Optional
Yes
Yes
Yes
97
SUMMARY
Characteristics of On-Demand
Routing Protocols
Parameters
Routing Metric:
Shortest Path
Routing Metric:
Associativity
Routing Metric:
Stablility
AODV
Yes
Routing Metric:
Freshness
Routing Metric: Load
and Delay
Yes
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DSR
Yes
TORA
Yes
98
Implementation Issues
 Several implementations apparently exist (see IETF)
– only a few available publicly
 Where to implement ad hoc routing?
– Link layer, Network layer, Application layer
 Address assignment
– Restrict all nodes to be on the same subnet
– Nodes may be given random addresses
– How to assign addresses? DHCP for ad hoc network?
 Security
– How can I trust you to forward my packets without
tampering?
– How do I know you are what you claim to be ?
 Performance
– Can we make any guarantees on performance?
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References
 [1] D. B. Johnson, D. A. Maltz, Y.-C. Hu, “The Dynamic Source
Routing Protocol for Mobile Ad Hoc Networks (DSR)”, IETF
Draft, April 2003.
 [2] C. E. Perkins, E. M. Belding-Royer, Ian D. Chakeres, “Ad
hoc On-Demand Distance Vector (AODV) Routing”, IETF Draft,
January 2004.
 [3] Z. J. Haas, M. R. Pearlman, P. Samar, P., “The Zone
Routing Protocol (ZRP) for Ad Hoc Networks” IETF Internet
Draft, draft-ietf-manet-zone-zrp-04.txt, July 2002.
 [4] V. Park, S. Corson,”Temporally-Ordered Routing Algorithm
(TORA)”, IETF Draft, draft-ietf-manet-tora-spec-03.txt,
work in progress, June 2001.
 [5] P. Sinha, R. Sivakumar, and V. Bharghavan, "CEDAR: A
CoreExtraction Distributed Ad HocroutingAlgorithm," Proc. IEEE
INFOCOM '99, Mar. 1999.
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