diwakar yagyasen personal web site - mobile adhoc networks...
TRANSCRIPT
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Diwakar Yagyasen 1
ECS-087: Mobile Computing
Mobile Adhoc Networks and Routing in
MANETS
(most of the slides borrowed from Prof.
Sridhar Iyer)
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Index
• Mobile Ad Hoc Networks (MANET)
• MAC in MANET
• MANET routing protocols
• Dynamic Source Routing (DSR)
• DSDV (Destination-Sequenced DV)
• Ad Hoc On-Demand Distance Vector Routing (AODV)
• Temporally Ordered Routing Algorithm (TORA)
Diwakar Yagyasen 2
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Diwakar Yagyasen 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 A
B
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Diwakar Yagyasen 4
MANETS
• May need to traverse multiple links to reach destination
• Mobility causes route changes
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Diwakar Yagyasen 5
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
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Diwakar Yagyasen 6
Applications
• Military environments
– soldiers, tanks, planes
• Emergency operations
– search-and-rescue
– policing and fire fighting
• Civilian environments
– taxi cab network
– meeting rooms
– sports stadiums
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Diwakar Yagyasen 7
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
• 802.11 was designed for single-hop wireless
– Does not do well for multi-hop ad hoc scenarios
– Reduced throughput
– Exposed terminal problem
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Diwakar Yagyasen 8
Routing in MANET
• Mobile IP needs infrastructure
– 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
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Diwakar Yagyasen 9
Issues in Routing in MANET
• Mobility
– Topology highly dynamic due to movement of nodes
• Ongoing sessions suffer frequent path breaks
– Even though wired network protocol find alternate paths when
a path breaks, the convergence is slow
• Bandwidth constraint
– Limited bandwidth imposes constraint on routing protocols to
maintain topological information
• Due to frequent changes in topology the control
overhead of keeping the topology current could be
very high
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Diwakar Yagyasen 10
Issues in Routing in MANET
• Error prone shared broadcast radio channel
– Wireless links have time varying characteristics in
terms of link capacity and link error rate
– So routing protocol may need to interact with MAC
layer to find alternate routes through better quality
links
• Energy constraint
– Limited battery power requires that the nodes do not
spend too much resources on routing overhead
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Diwakar Yagyasen 11
Properties of good routing protocol in MANET
• Must be distributed
• Adaptive to frequent topology changes
• Must be localized, since global state maintenance
involves a huge state propagation control overhead
• Loop free and free from stale routes
• Convergence should be quick
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Diwakar Yagyasen 12
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)
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Diwakar Yagyasen 13
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
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Diwakar Yagyasen 14
Route Discovery in DSR
B
A
S E
F
H
J
D
C
G
I
K
Z
Y
Represents a node that has received RREQ for D from S
M
N
L
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Diwakar Yagyasen 15
Route Discovery in DSR
B
A
S E
F
H
J
D
C
G
I
K
Represents transmission of RREQ
Z
Y Broadcast transmission
M
N
L
[S]
[X,Y] Represents list of identifiers appended to RREQ
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Diwakar Yagyasen 16
Route Discovery in DSR
B
A
S E
F
H
J
D
C
G
I
K
• Node H receives packet RREQ from two neighbors:
potential for collision
Z
Y
M
N
L
[S,E]
[S,C]
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Diwakar Yagyasen 17
Route Discovery in DSR
B
A
S E
F
H
J
D
C
G
I
K
• Node C receives RREQ from G and H, but does not forward
it again, because node C has already forwarded RREQ once
Z
Y
M
N
L
[S,C,G]
[S,E,F]
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Diwakar Yagyasen 18
Route Discovery in DSR
B
A
S E
F
H
J
D
C
G
I
K
Z
Y
M
• Nodes J and K both broadcast RREQ to node D
• Since nodes J and K are hidden from each other, their
transmissions may collide
N
L
[S,C,G,K]
[S,E,F,J]
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Diwakar Yagyasen 19
Route Discovery in DSR
B
A
S E
F
H
J
D
C
G
I
K
Z
Y
• Node D does not forward RREQ, because node D
is the intended target of the route discovery
M
N
L
[S,E,F,J,M]
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Diwakar Yagyasen 20
Route Reply in DSR
B
A
S E
F
H
J
D
C
G
I
K
Z
Y
M
N
L
RREP [S,E,F,J,D]
Represents RREP control message
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Diwakar Yagyasen 21
Data Delivery in DSR
B
A
S E
F
H
J
D
C
G
I
K
Z
Y
M
N
L
DATA [S,E,F,J,D]
Packet header size grows with route length
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Diwakar Yagyasen 22
Route Error (RERR)
B
A
S E
F
H
J
D
C
G
I
K
Z
Y
M
N
L
RERR [J-D]
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 (an ACK mechanism has to be there in
packet forwarding)
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Diwakar Yagyasen 23
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
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Diwakar Yagyasen 24
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
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Diwakar Yagyasen 25
Use of route caching
B
A
S E
F
H
J
D
C
G
I
K
Z
M
N
L
[S,E,F,J,D] [E,F,J,D]
[C,S]
[G,C,S]
[F,J,D],[F,E,S]
[J,F,E,S]
RREQ
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
[K,G,C,S] RREP
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Diwakar Yagyasen 26
Route caching
• Uses:
– 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
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Diwakar Yagyasen 27
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
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Diwakar Yagyasen 28
DSR: Disadvantages
• Packet header size grows with route length due to
source routing
• Latency to discover a route before data can be sent
• 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
– Inconsistency during route reconstruction phase
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Diwakar Yagyasen 29
DSDV (Destination-Sequenced DV)
• Very similar to wireline DV protocol
• A table driven routing protocol
– Routes to all destinations are readily available
• Each mobile node advertises its own routing table to
each of its neighbors periodically
• When there is a significant new information (e.g. link
failed), a node immediately advertises its routing
table
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Diwakar Yagyasen 30
DSDV (Destination-Sequenced DV)
• Each entry of the advertised data contains – destination address
– number of hops required to reach the dst
– the seq number of the information received regarding that dst
• When a node receives new routing info – If it is newer than what is currently in the routing table
(comparing the seq number), then it replaces the current info
– The metric for routes received in the routing info is incremented by one
– Newly recorded routes are marked for immediate advertisement
– Routes which only got a more recent sequence number may be scheduled for advertisement at a later time
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Diwakar Yagyasen 31
DSDV
• When a link breaks (because of mobility) (may be
detected by layer-2 or inferred by layer-3 when no
broadcast is received from the neighbor for a while)
– infinity is assigned as metric to that link
– any route through that link is assigned infinity as
metric and a new seq number
• When a node receives infinity metric and it has an
equal or later seq number with finite metric, then it
triggers an update to propagate the new route
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Diwakar Yagyasen 32
Example DSDV
MH4
MH6
MH5
MH8
MH7
MH1
MH1
MH3
MH2
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Diwakar Yagyasen 33
Example DSDV
Destination Next hop Metric Sequence
number
MH1 MH2 2 S406_MH1
MH2 MH2 1 S128_MH2
MH3 MH2 2 S564_MH3
MH4 MH4 0 S710_MH4
MH5 MH6 2 S392_MH5
MH6 MH6 1 S076_MH6
MH7 MH6 2 S128_MH7
MH8 MH6 3 S050_MH8
Routing table at MH4
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Diwakar Yagyasen 34
Example DSDV
Destination Metric Sequence number
MH1 2 S406_MH1
MH2 1 S128_MH2
MH3 2 S564_MH3
MH4 0 S710_MH4
MH5 2 S392_MH5
MH6 1 S076_MH6
MH7 2 S128_MH7
MH8 3 S050_MH8
Advertisement from MH4
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Diwakar Yagyasen 35
Example DSDV
Destination Next hop Metric Sequence
number
MH1 MH6 3 S516_MH1
MH2 MH2 1 S128_MH2
MH3 MH2 2 S564_MH3
MH4 MH4 0 S710_MH4
MH5 MH6 2 S392_MH5
MH6 MH6 1 S076_MH6
MH7 MH6 2 S128_MH7
MH8 MH6 3 S050_MH8
Routing table at MH4 (after MH1 moves)
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Diwakar Yagyasen 36
Example DSDV
Destination Metric Sequence
number
MH4 0 S710_MH4
MH1 3 S516_MH1
MH2 1 S128_MH2
MH3 2 S564_MH3
MH5 2 S392_MH5
MH6 1 S076_MH6
MH7 2 S128_MH7
MH8 3 S050_MH8
Advertisement from MH4 (after MH1 moves)
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Diwakar Yagyasen 37
DSDV: Advantages
• Routes available to all destinations
– Less latency in route set up
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Diwakar Yagyasen 38
DSDV: Disadvantages
• Updates are propagated throughout the network
– Updates due to broken link (due to mobility) can lead
to heavy control traffic
– Even a small network with high mobility or large
network with low mobility can choke the network
• In order to get information about a particular
destination node, a node has to wait for a table
update msg initiated by the same destination node
– This delay would result in stale routing information
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Diwakar Yagyasen 39
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 intermediate nodes, so that data
packets do not have to contain routes
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Diwakar Yagyasen 40
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
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Diwakar Yagyasen 41
Route Requests in AODV
B
A
S E
F
H
J
D
C
G
I
K
Z
Y
Represents a node that has received RREQ for D from S
M
N
L
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Diwakar Yagyasen 42
Reverse Path Setup in AODV
B
A
S E
F
H
J
D
C
G
I
K
Represents links on Reverse Path
Z
Y
M
N
L
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Diwakar Yagyasen 43
Reverse Path Setup in AODV
B
A
S E
F
H
J
D
C
G
I
K
Z
Y
• Node D does not forward RREQ, because node D
is the intended target of the RREQ
M
N
L
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Diwakar Yagyasen 44
Forward Path Setup in AODV
B
A
S E
F
H
J
D
C
G
I
K
Z
Y
M
N
L
Forward links are setup when RREP travels
along the reverse path
Represents a link on the forward path
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Diwakar Yagyasen 45
Route Request and Route Reply • The RREQ request at the source contains
– src, dst ip address
– current seq number
– last known seq number
– broadcast id of the req (which is incremented every time the source node initiates a RREQ
– broadcast id and source id form a unique id for the RREQ msg
• used to identify duplicate RREQ msg
• When a node receives RREQ – drops the request if it has seen the req (by noting the unique broadcast id and
src id)
– otherwise it sets up a reverse route entry for the src node in the routing table (for forwarding RREP)
• contains src IP, seq number, IP addr of the neighbor from which the msg came
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Diwakar Yagyasen 46
Route Request and Route Reply • When a node receives RREP
– Sets up a forward path entry to the dst in its routing table
• this entry contains dst IP, nbr IP address from
where it received the RREP and the hop count, and
lifetime (contained in the RREP msg)
• To respond to a RREQ
– a node must have an unexpired entry for the destination in its
route table
– the seq number of the entry must be at least as great as
carried in the RREQ msg
• prevents loops
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Diwakar Yagyasen 47
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
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Diwakar Yagyasen 48
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
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Diwakar Yagyasen 49
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
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Diwakar Yagyasen 50
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 )
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Diwakar Yagyasen 51
AODV: Expanding ring search
• Each RREQ msg is broadcast to the entire network
– For a large network this could be detrimental
• To control the scope of broadcast, the src node
should use an expanding ring search technique
• 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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Diwakar Yagyasen 52
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
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Diwakar Yagyasen 53
Temporally Ordered Routing Algorithm (TORA)
• Source-initiated on-demand routing protocol
• Each node maintains its one hop local topology
• In case of topology change, control packets are
limited to small region
– This is an important property for MANET
• Basically uses a destination oriented directed acyclic
graph (DAG) using a Query/Update mechanism
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Diwakar Yagyasen 54
TORA
2
1
3
7
6
5
4
destination
source
H(7) < H(3) < H(2) < H(1)
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Diwakar Yagyasen 55
TORA
• When node 1 has data to send to destination 7, it originates
a Query packet (the packet carries address of destination)
• The Query packet is forwarded by intermediate nodes 2, 3,
4, 5 and 6 and reaches 7
• The node that terminates (in this case, 7) the Query packet,
replies with an Update packet containing its distance from
the destination (zero at the destination).
– Note that the Query packet need not always travel to the
destination
• Intermediate nodes may have a path to the
destination, so they can send Update packet
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Diwakar Yagyasen 56
TORA
• Each node that receives the Update packet sets its
distance (or Height) to a value higher than the
distance of the sender of the Update packet
• Thus, a set of directed links from the node which
originated the Query to the destination node 7 is
created.
• This forms a DAG
• Once source node 1 receives Update msg, it starts
sending data packets
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Diwakar Yagyasen 57
TORA
2
1
3
7
6
5
4
destination
source
H(5) > H(4) > H(1)
X
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Diwakar Yagyasen 58
TORA
• When node 5 discovers that its link to destination 7 is
broken, it changes its Height (or distance) value to a
value larger than its neighbors and originates a
Update msg.
• 4 receives this Update and reverses the link between
1 and 4 and forwards the Update msg (H(5) < H(4) <
H(1))
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Diwakar Yagyasen 59
TORA
• If link between 1 and 4 breaks
– 4 reverses link between itself and 5 and sends
Update msg to 5
– This conflicts with the earlier reversal: a partition can
be inferred by 5
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Diwakar Yagyasen 60
TORA
• Advantages
– Limits control packets to a small region when
topology changes: less overhead
• Disadvantage
– Local reconfiguration of paths could lead to non-
optimal routes
– Concurrent detection of partitions and subsequent
deletion of routes could lead to temporary oscillations
and transient loops.
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Diwakar Yagyasen 61
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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Diwakar Yagyasen 62
Optimized Link State Routing (OLSR) • A Proactive routing protocol
• Optimizes the link state protocol
– Reducing the number of links that are used for forwarding the link state advertisements
• 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
– 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
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Diwakar Yagyasen 63
Multi Point Relay (MPR) Set • 1. MPR(x) = φ
• 2. MPR(x) = {those nodes which belong to N1(x) and which are the only neighbors of nodes in N2(x)
• 3. While there exists some node in N2(x) which is not covered by MPR(x)
– a) For each node in N1(x) which is not in MPR(x), compute the maximum number of nodes that it covers among the uncovered nodes in the set N2(x).
– B) Add to MPR(x) the node belonging to N1(x) , for which this number is maximum
• Ni(x) = ith hop neighbor of x
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Diwakar Yagyasen 64
Optimized Link State Routing (OLSR) • Nodes C and E are multipoint relays of node A
• Nodes C and E forward information received from A
A
B F
C
D
E H
G K
J
Node that has broadcast state information from A
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Diwakar Yagyasen 65
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
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Diwakar Yagyasen 66
Protocol Trade-offs
• Reactive protocols
– Lower overhead since routes are determined on
demand
– routes carried in the data packets
– Significant delay in route determination
– Employ flooding (global search)
– Control traffic may be bursty
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Diwakar Yagyasen 67
Zone Routing Protocol (ZRP)
• Hybrid protocol
• 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.
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Diwakar Yagyasen 68
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
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Diwakar Yagyasen 69
ZRP
• Each node maintains the information about routes to all nodes within its routing zone by exchanging periodic route updates
• If source s and destination d are in the same zone, then the packet is directly delivered to the destination (the route is available in the routing database)
• Otherwise, s bordercasts (uses unicast routing to deliver packets directly to the border nodes) the RouteRequest packet to its peripheral nodes – If any peripheral node finds d in its routing zone, it sends
RouteReply back to s indicating the path.
– Otherwise, the node rebordercasts the RouteRequest packet to the peripheral nodes.
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Diwakar Yagyasen 70
ZRP example: Zone Radius = d = 2
S C A
E F
B
D
S performs route
discovery for D
Denotes route request
E knows route from E to D,
so route request need not be
forwarded to D from E
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Diwakar Yagyasen 71
ZRP
• Advantages
– Combines the best features of proactive and reactive
routing schemes
• Disadvantages
– When there are overlaps in the nodes’ routing zones,
there may be redundant RouteRequests sent out.
These need to be suppressed
– Choosing zone radius is quite tricky
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MANET variations
• Fully symmetric environment
– 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
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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
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MANET summary
• Routing is the most studied problem
• Cross-layer approach being researched
• Large number of simulation based experiments
• Small number of field trials
• Very few reported deployments
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References
• D. B. Johnson, D. A. Maltz, “Dynamic Source
Routing in Ad Hoc Wireless Networks”, Mobile
Computing, Kluwer Academic Publishers, vol. 353,
pp. 153-181, 1996.
• C. E. Perkins, E. M. Royer, “Ad Hoc On-Demand
Distance Vector Routing”, Proc. of IEEE Workshop
on Mobile Computing Systems and Applications,
1999, pp. 90-100, February 1999.