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Module I
Routing in mobile ad hoc networks
Prof. JP Hubaux
Mobile Networks
http://mobnet.epfl.ch
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The classical solution for mobile networks
2nd generation (GSM, IS-41,) and 3rdgeneration (UMTS,)deployed soon
Huge, expensive fixed infrastructure
License for a share of the spectrum
Operational responsibility: network operators (telcos, ISPs)
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The new paradigm: ad hoc networks
Terminal and node merge
Everything is potentially mobile Initial applications: communication in the battlefield (Packet
Radio Networks, in the 70s)
The network is self-organizedwhen it is run by the usersthemselves
Similar trend at the application layer: peer-to-peer(e.g., Napster Gnutella)
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Application examples of ad hoc networks
Sensor networks
Hybrid cellular / ad hoc networks (multi-hop cellularnetworks)
Cars
Assisted driving (adaptive cruise control,)
Collision avoidance
Optimization of traffic flows
Crisis networks (e.g., rescue operations after majordisaster)
Military networks
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Upper bound for the throughput of ad hocnetworks
If we have:
- identical randomly located nodes
- each capable of transmitting bits/s
Then the throughput ( ) obtainable by each node
for a destination is
( )log
n
W
n
randomly chosenW
nn n
Ref: P. Gupta, P. Kumar, The Capacity of Wireless Networks
IEEE Transactions on Information Theory, March 2000
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Intuition behind the upper bound
N nodes (users)
O(N) users O(N) users
Cut set ~ N
O(N) transmissions from left to right
over
O( ) transmission links
mean
O( ) capacity per attempted transmission
N
1N
Ways to improve scalability:
Directional antennas
Locality of the traffic
Hybrid system
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Routing in ad hoc networks
Peculiarities
Node mobilityHigh rate of link failure Traditional routing approaches are not well suited
Assumptions
Multihop communication
Symmetric links (in most cases)Omnidirectional antennas (in most cases)All nodes have equal capabilities and responsibilities
Figures of merit
Latency of route discoveryOverhead (bandwidth, energy, processing power)
Security
Current status of research:
Many, many proposalsOptimal solution depends on deployment scenario: mobility
patterns, radio model, traffic characteristics,
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Brief reminder : Link-state protocols
Example: OSPF
May consume a lot of resources to update the routes
Techniques to alleviate the problem : limit thepropagation of information
Does not seem to be well suited to cope with mobility
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Distance vector routing (1/2)
0 1 5
1 0 1 3
5 1 0 7
3 7 0
A
B
D
C
1
3
1
5 7
A
B
C
D
A B C D
1 0 1 3
Distance
vector
Distance
vector of B
2 1 2 4
+ Distance from A to B =
Cost to dest.
via B
Take the min
0 1 2,B 4,B
(1 row stored in each node)
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Distance vector routing (2/2)
Even if the updates are asynchronous, the routingtables converge
The algorithm is often called Bellman-Ford
Problem: undesirable behaviour when links go upand down (e.g., count to infinity problem)
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Routing protocols for wireless ad hoc networks
Response time,
bandwidthEnergy
Mobile ad hoc networks Sensor networks
Proactive
protocols
Reactive
protocols
Destination-Sequenced
Distance-Vector (DSDV)
Optimized Link-
State Routing
(OLSR)
Ad Hoc On-Demand
Distance-Vector
(AODV)
Dynamic
Source
Routing
(DSR)
Geography-
based routing
Cluster-based
(or hierarchical)
routing
Geodesic
packetforwarding
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Dynamic source routing (DSR)
Reactive routing protocol
2 phases, operating both on demand:
Route discovery Used only when source S attempts to to send a packet to
destination D Based on flooding of Route Requests (RREQ)
Route maintenance makes S able to detect, while using a source route to D, if it can
no longer use its route (because a link along that route no
longer works)
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DSR: Route discovery (1)
E G
M
H
R
F
A
B
C
I
DS
K
N
L
P
J
Q
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DSR: Route discovery (2)
E G
M
H
R
F
A
B
C
I
DS
K
N
L
P
J
Q
(S)
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DSR: Route discovery (3)
E G
M
H
R
F
A
B
C
I
DS
K
N
L
P
J
Q
(S,A)
(S,E)
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DSR: Route discovery (4)
E G
M
H
R
F
A
B
C
I
DS
K
N
L
P
J
Q
(S,E,G)
(S,B,C)
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DSR: Route discovery (5)
E G
M
H
R
F
A
B
C
I
DS
K
N
L
P
J
Q
(S,E,G,J)
(S,A,F,H)
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DSR: Route discovery (6)
E G
M
H
R
F
A
B
C
I
DS
K
N
L
P
J
Q(S,A,F,H,K)
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DSR: Route discovery (7)
E G
M
H
R
F
A
B
C
I
DS
K
N
L
P
J
Q
(S,A,F,H,K,P)
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DSR: Route discovery (8)
E G
M
H
R
F
A
B
C
I
DS
K
N
L
P
J
Q
RREP(S,E,G,J,D)
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DSR: Route Discovery (9)
Route reply by reversing the route (as illustrated)works only if all the links along the route arebidirectional
If unidirectional links are allowed, then RREP may
need a route discovery from D to S Note: IEEE 802.11 assumes that links are
bidirectional
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DSR: Data delivery
E G
M
H
R
F
A
B
C
I
DS
K
N
L
P
J
Q
DATA(S,E,G,J,D)
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DSR: Route maintenance (1)
E G
M
H
R
F
A
B
C
I
DS
K
N
L
P
J
Q
DATA(S,E,G,J,D)
X
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DSR: Route maintenance (2)
E G
M
H
R
F
A
B
C
I
DS
K
N
L
P
J
Q
X
RERR(G-J)
When receiving the Route Error message (RERR),
S removes the broken link from its cache.
It then tries another route stored in its cache; if none,
it initializes a new route discovery
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DSR: Optimization of route discovery: routecaching
Principle: each node caches a new route it learns byany means
ExamplesWhen node S finds route (S, E, G, J, D) to D, it also learns
route (S, E, G) to node G
In the same way, node E learns the route to DSame phenomenon when transmitting route replies
Moreover, routes can be overheard by nodes in theneighbourhood
However, route caching has its downside: stalecaches can severely hamper the performance of thenetwork
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DSR: Strengths
Routes are set up and maintained only betweennodes who need to communicate
Route caching canfurther reduce the effort of routediscovery
A single route discovery may provide several routesto the destination
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DSR: Weaknesses
Route requests tend to flood the network andgenerally reach all the nodes of the network
Because of source routing, the packet header sizegrows with the route lengh
Risk of many collisions between route requests byneighboring nodes need for random delays beforeforwarding RREQ
Similar problem for the RREP (Route Reply stormproblem), in case links are not bidirectional
Note: Location-aided routingmay help reducing thenumber of useless control messages
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Ad Hoc On-Demand Distance VectorRouting (AODV)
As it is based on source routing, DSR includessource routes in data packet headers
Large packet headers in DSR risk of poorperformance if the number of hops is high
AODV uses a route discovery mechanism similar toDSR, but it maintains routing tables at the nodes
AODV agesthe routes and maintains a hop count
AODV assumes that all links are bi-directional
AODV R di ( )
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AODV : Route discovery (1)
E G
M
H
R
F
A
B
C
I
DS
K
N
L
P
J
Q
AODV R di (2)
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AODV : Route discovery (2)
E G
M
H
R
F
A
B
C
I
DS
K
N
L
P
J
Q
Note: if one of the intermediate nodes (e.g., A)
knows a route to D, it responds immediately to S: Route Request (RREQ)
AODV R di (3)
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AODV : Route discovery (3)
E G
M
H
R
F
A
B
C
I
DS
K
N
L
P
J
Q
: represents a link on the reverse path
AODV R t di (4)
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AODV : Route discovery (4)
E G
M
H
R
F
A
B
C
I
DS
K
N
L
P
J
Q
AODV R t di (5)
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AODV : Route discovery (5)
E G
M
H
R
F
A
B
C
I
DS
K
N
L
P
J
Q
AODV R t di (6)
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AODV : Route discovery (6)
M
D
K
L
P
J
E G
H
R
F
A
B
C
I
S
N
Q
AODV R t di (7)
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AODV : Route discovery (7)
M
D
K
L
P
J
E G
H
R
F
A
B
C
I
S
N
Q
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AODV : Route replyand setup of the forwardpath
M
D
K
L
P
J
E G
H
R
F
A
B
C
I
S
N
Q
: Link over which the RREP is transmitted
: Forward path
Route reply in AODV
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Route reply in AODV
In case it knows a path more recent than the onepreviously known to sender S, an intermediate nodemay also send a route reply (RREP)
The freshness of a path is assessed by means of
destination sequence numbers Both reverse and forward paths are purged at the
expiration of appropriately chosen timeout intervals
AODV : Data delivery
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AODV : Data delivery
M
D
K
L
P
J
E G
H
R
F
A
B
C
I
S
N
Q
Data
The route is not included in the packet header
AODV : Route maintenance (1)
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AODV : Route maintenance (1)
M
D
K
L
P
J
E G
H
R
F
A
B
C
I
S
N
Q
Data
X
AODV : Route maintenance (2)
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AODV : Route maintenance (2)
M
D
K
L
P
J
E G
H
R
F
A
B
C
I
S
N
Q
X
RERR(G-J)
When receiving the Route Error message (RERR),
S removes the broken link from its cache.
It then initializes a new route discovery.
AODV: Destination sequence numbers
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AODV: Destination sequence numbers
If the destination responds to RREP, it places itscurrent sequence number in the packet
If an intermediatenode responds, it places its recordof the destinations sequence number in the packet
Purpose of sequence numbers:
Avoid using stale information about routes
Avoid loops (no source routing!)
AODV : Avoiding the usage of stale routing
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AODV : Avoiding the usage of stale routingtables
S DA
B
DSN(D) = 5
: Forward path
S
D
A
B
DSN(D) = 5
DSN(D) = 8
1.2.
S
D
A
B
DSN(D) = 5
DSN(D) = 8
3.
RREQ
S
D
A
B
DSN(D) = 5
DSN(D) = 8
RREP
4.
AODV : Avoiding loops
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AODV : Avoiding loops
A B S D
C
: Forward path
Assume there is a route between A and D; link S-D breaks; assume A is not aware of this, e.g. because
RERR sent by S is lost
Assume now S wants to send to D. It performs a RREQ, which can be received by A via path S-C-A
Node A will reply since it knows a route to D via node B
This would result in a loop (S-C-A-B-S) The presence of sequence numbers will let S discover that the routing information from A is outdated
Principle: when S discovers that link S-D is broken, it increments its local value of DSN(D). In this way,
the new local value will be greater than the one stored by A.
X
AODV (unicast) : Conclusion
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AODV (unicast) : Conclusion
Nodes maintain routing information only for routesthat are in active use
Unused routes expire even when the topology doesnot change
Each node maintains at most one next-hop perdestination
Many comparisons with DSR (via simulation) havebeen performed no clear conclusion so far
Geodesic Packet Forwarding
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Geodesic Packet Forwarding
L. Blazevic, S. Giordano, J.-Y. Le Boudec (IP4)
AP -geographical anchor pointAGPF (Anchored Geodesic Packet Forwarding) - sourcerouting with anchors
S
AP1
D
AP2
S has anchored path {AP1,AP2}from S: packets are forwarded in direction of AP1from A: packets are forwarded in direction of AP2from B: packets are forwarded in direction of Ds position
from X : use of Terminode Local Routing (TLR)
A
B
X
TLR area of X
Other (Swiss) proposals
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Other (Swiss) proposals
Last Encounter Routing H. Dubois-Ferrire, M. Grossglauser, M. Vetterli (EPFL, IP1 & 7)
Principle: Nodes exchange information about their previousencounters
No explicit location service, no transmission overhead to toupdate the state
Ongoing work: prediction, based on declared mobility
Face routing
F. Kuhn, R. Wattenhofer, A. Zollinger (ETHZ, IP9)
Principle: exploit the geometric properties of the connectivitygraph
Worst-case optimal
Conclusion on routing
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Conclusion on routing
DSR and AODV are the mainstream proposals
Both have been extensively studied (by simulation)
No clear superiority of one wrt the other
Scalability is still an open issue
Other very promising proposals
References
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References
Ch. Perkins: Ad Hoc Networking, Addison Wesley,2001
www.mics.org: IP1, IP4, IP7 and IP9