Module I Routing in mobile ad hoc networks

1
Module IRouting in mobile ad hoc networks
Mobile Networks

• Prof. JP Hubaux

http//mobnet.epfl.ch
2
The classical solution for mobile networks

• 2nd generation (GSM, IS-41,) and 3rd generation
  (UMTS,) deployed
• Huge, expensive fixed infrastructure
• License for a share of the spectrum
• Operational responsibility network operators
  (telcos, ISPs)

3
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-organized when it is run by
  the users themselves
• Similar trend at the application layer
  peer-to-peer (e.g., Napster ? Gnutella)

4
Application examples of ad hoc networks

• Sensor networks
• Hybrid cellular / ad hoc networks (multi-hop
  cellular networks)
• Cars
• Assisted driving (adaptive cruise control,)
• Collision avoidance
• Optimization of traffic flows
• Crisis networks (e.g., rescue operations after
  major disaster)
• Military networks

5
Upper bound for the throughput of ad hoc networks
Ref P. Gupta, P. Kumar, The Capacity of Wireless
Networks IEEE Transactions on Information Theory,
March 2000
6
Intuition behind the upper bound
N nodes (users)
O(N) transmissions from left to right over O(
) transmission links mean O( ) capacity per
attempted transmission

• Ways to improve scalability
• Directional antennas
• Locality of the traffic
• Hybrid system

O(N) users
O(N) users
Cut set
7
Routing in ad hoc networks

• Peculiarities
• Node mobility
• High 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 discovery
• Overhead (bandwidth, energy, processing power)
• Security
• Current status of research
• Many, many proposals
• Optimal solution depends on deployment scenario
  mobility patterns, radio model, traffic
  characteristics,

8
Brief reminder Link-state protocols

• Example OSPF
• May consume a lot of resources to update the
  routes
• Techniques to alleviate the problem limit the
  propagation of information
• Does not seem to be well suited to cope with
  mobility

9
Distance vector routing (1/2)
B
Distancevector
A
B
C
D
3
1
0 1 5 ?
1 0 1 3
5 1 0 7
? 3 7 0
A
1
A
D
5
7
B
C
C
D
(1 row stored in each node)
Distancevector of B
1 0 1 3
Take the min
Distance from A to B
Cost to dest.via B
2 1 2 4
0 1 2,B 4,B
10
Distance vector routing (2/2)

• Even if the updates are asynchronous, the routing
  tables converge
• The algorithm is often called Bellman-Ford
• Problem undesirable behaviour when links go up
  and down (e.g., count to infinity problem)

11
Routing protocols for wireless ad hoc networks
Sensor networks
Mobile ad hoc networks
Response time,bandwidth
Energy
Proactiveprotocols
Reactiveprotocols
DynamicSourceRouting(DSR)
Optimized Link-State Routing(OLSR)
Ad Hoc On-DemandDistance-Vector (AODV)
Destination-SequencedDistance-Vector (DSDV)
Geography-based routing
Cluster-based(or hierarchical)routing
Geodesic packetforwarding
12
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)

13
DSR Route discovery (1)
K
F
H
A
Q
E
P
G
D
S
J
B
M
R
I
L
C
N
14
DSR Route discovery (2)
K
F
H
A
Q
E
P
G
D
S
(S)
J
B
M
R
I
L
C
N
15
DSR Route discovery (3)
(S,A)
K
F
H
A
Q
(S,E)
E
P
G
D
S
J
B
M
R
I
L
C
N
16
DSR Route discovery (4)
K
F
H
A
Q
E
P
(S,E,G)
G
D
S
J
B
M
R
I
L
C
N
(S,B,C)
17
DSR Route discovery (5)
(S,A,F,H)
K
F
H
A
Q
E
P
(S,E,G,J)
G
D
S
J
B
M
R
I
L
C
N
18
DSR Route discovery (6)
K
F
H
(S,A,F,H,K)
A
Q
E
P
G
D
S
J
B
M
R
I
L
C
N
19
DSR Route discovery (7)
K
F
H
A
Q
E
P
G
D
S
J
(S,A,F,H,K,P)
B
M
R
I
L
C
N
20
DSR Route discovery (8)
K
F
H
A
Q
E
P
G
D
S
J
RREP(S,E,G,J,D)
B
M
R
I
L
C
N
21
DSR Route Discovery (9)

• Route reply by reversing the route (as
  illustrated) works only if all the links along
  the route are bidirectional
• 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

22
DSR Data delivery
K
F
H
A
Q
DATA(S,E,G,J,D)
E
P
G
D
S
J
B
M
R
I
L
C
N
23
DSR Route maintenance (1)
K
F
H
A
Q
DATA(S,E,G,J,D)
E
P
G
D
S
X
J
B
M
R
I
L
C
N
24
DSR Route maintenance (2)
K
F
H
A
Q
RERR(G-J)
E
P
G
D
S
X
J
B
M
R
I
L
C
N
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
25
DSR Optimization of route discovery route
caching

• Principle each node caches a new route it learns
  by any means
• Examples
• When 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 D
• Same phenomenon when transmitting route replies
• Moreover, routes can be overheard by nodes in the
  neighbourhood
• However, route caching has its downside stale
  caches can severely hamper the performance of the
  network

26
DSR Strengths

• Routes are set up and maintained only between
  nodes who need to communicate
• Route caching can further reduce the effort of
  route discovery
• A single route discovery may provide several
  routes to the destination

27
DSR Weaknesses

• Route requests tend to flood the network and
  generally reach all the nodes of the network
• Because of source routing, the packet header size
  grows with the route lengh
• Risk of many collisions between route requests
  by neighboring nodes ? need for random delays
  before forwarding RREQ
• Similar problem for the RREP (Route Reply storm
  problem), in case links are not bidirectional
• Note Location-aided routing may help reducing
  the number of useless control messages

28
Ad Hoc On-Demand Distance Vector Routing (AODV)

• As it is based on source routing, DSR includes
  source routes in data packet headers
• Large packet headers in DSR ? risk of poor
  performance if the number of hops is high
• AODV uses a route discovery mechanism similar to
  DSR, but it maintains routing tables at the nodes
• AODV ages the routes and maintains a hop count
• AODV assumes that all links are bi-directional

29
AODV Route discovery (1)
K
F
H
A
Q
E
P
G
D
S
J
B
M
R
I
L
C
N
30
AODV Route discovery (2)
K
F
H
A
Q
E
P
G
D
S
J
B
M
R
I
L
C
N
Note if one of the intermediate nodes (e.g.,
A)knows a route to D, it responds immediately to
S
Route Request (RREQ)
31
AODV Route discovery (3)
K
F
H
A
Q
E
P
G
D
S
J
B
M
R
I
L
C
N
represents a link on the reverse path
32
AODV Route discovery (4)
K
F
H
A
Q
E
P
G
D
S
J
B
M
R
I
L
C
N
33
AODV Route discovery (5)
K
F
H
A
Q
E
P
G
D
S
J
B
M
R
I
L
C
N
34
AODV Route discovery (6)
K
F
H
A
Q
P
E
D
G
S
J
B
M
R
I
L
C
N
35
AODV Route discovery (7)
K
F
H
A
Q
P
E
D
G
S
J
B
M
R
I
L
C
N
36
AODV Route reply and setup of the forward path
K
F
H
A
Q
P
E
D
G
S
J
B
M
R
I
L
C
N
Link over which the RREP is transmitted
Forward path
37
Route reply in AODV

• In case it knows a path more recent than the one
  previously known to sender S, an intermediate
  node may 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

38
AODV Data delivery
K
F
H
A
Q
Data
P
E
D
G
S
J
B
M
R
I
L
C
N
The route is not included in the packet header
39
AODV Route maintenance (1)
K
F
H
A
Q
Data
P
E
D
G
S
X
J
B
M
R
I
L
C
N
40
AODV Route maintenance (2)
K
F
H
A
Q
RERR(G-J)
P
E
D
G
S
X
J
B
M
R
I
L
C
N
When receiving the Route Error message (RERR), S
removes the broken link from its cache. It then
initializes a new route discovery.
41
AODV Destination sequence numbers

• If the destination responds to RREP, it places
  its current sequence number in the packet
• If an intermediate node responds, it places its
  record of the destinations sequence number in
  the packet
• Purpose of sequence numbers
• Avoid using stale information about routes
• Avoid loops (no source routing!)

42
AODV Avoiding the usage of stale routing tables



S
D
A
1.
S
A
2.
DSN(D) 5
DSN(D) 5
B
B

DSN(D) 8
Forward path
D


3.
4.
S
A
S
A
RREQ
DSN(D) 5
RREP
DSN(D) 5
B
B
DSN(D) 8
DSN(D) 8


D
D
43
AODV Avoiding loops
X
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.

44
AODV (unicast) Conclusion

• Nodes maintain routing information only for
  routes that are in active use
• Unused routes expire even when the topology does
  not change
• Each node maintains at most one next-hop per
  destination
• Many comparisons with DSR (via simulation) have
  been performed ? no clear conclusion so far

45
Geodesic Packet Forwarding L. Blazevic, S.
Giordano, J.-Y. Le Boudec (IP4)
AP -geographical anchor point AGPF (Anchored
Geodesic Packet Forwarding) - source routing with
anchors
B
AP2
A
S
D
X
TLR area of X
S has anchored path AP1,AP2 from S packets are
forwarded in direction of AP1 from A packets are
forwarded in direction of AP2 from B packets are
forwarded in direction of Ds position from X
use of Terminode Local Routing (TLR)
46
Other (Swiss) proposals

• Last Encounter Routing
• H. Dubois-Ferrière, M. Grossglauser, M. Vetterli
  (EPFL)
• Principle Nodes exchange information about their
  previous encounters
• No explicit location service, no transmission
  overhead to to update the state
• Ongoing work prediction, based on declared
  mobility
• Face routing
• F. Kuhn, R. Wattenhofer, A. Zollinger (ETHZ)
• Principle exploit the geometric properties of
  the connectivity graph
• Worst-case optimal

47
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

48
References

• Ch. Perkins Ad Hoc Networking, Addison Wesley,
  2001
• Rajaraman, R. 2002. Topology control and routing
  in ad hoc networks a survey. SIGACT News 33, 2
  (Jun. 2002), 60-73
• www.mics.org