Mobile Ad-hoc Networks (MANET)

1
Mobile Ad-hoc Networks (MANET)

• Ad-hoc network
• A collection of wireless mobile hosts forming a
  temporary network without the aid of any
  established infrastructure or centralized
  administration.
• Significant differences to existing wired
  networks
• Wireless
• Self-starting
• No administrator
• Cannot assume that every computer is within
  communication range of every other computer
• Possibly quite dynamic topology of
  interconnections
• Traffic types unicast/multicast/anycast/geocast

2
Routing in MANET

• Routing assumptions for unicast traffic
• Flat topology assumption
• Proactive DSDV, TORA, WRP
• Reactive AODV, DSR, STAR
• Hierarchical topology assumption
• Clustering CBRP, PATM
• Geographic assumption
• Location aided routing LAR, GeoCast

3
Classification of Routing Protocols for MANETS
4
Desired Properties of Ad Hoc Routing Protocols

• Distributed
• Bandwidth efficient
• Reduce control traffic/overhead
• Battery efficient
• Fast route convergence
• Correct loop free
• Reduce overhead
• Unidirectional Link Support

5
Performance Metrics of Ad Hoc Routing Protocols

• Maximize
• end-to-end throughput
• delivery ratio
• Minimize
• Congestion (load-balancing)
• end-to-end delay
• packet loss
• shortest path/minimum hop (route length)
• overhead (bandwidth)
• energy consumption

6
Mobile Ad hoc Networks (MANET) vs. Sensor Networks
MANET SensorNet
applications meeting, group collaboration smart building, habitat monitoring
comm. address-centric comm. data centric comm.
topology peer-to-peer sensors ? base peer-to-peer
traffic random periodic, synchronous
platform laptops, PDAs motes more resource constrained
scale 10s to 100s gt1000 larger scale and more redundancy
mobility slow (meeting) fast (cars) focus on mobility slow (habitat) fast less focus on mobility so far
similarity No infrastructure, multi-hop, wireless networks No infrastructure, multi-hop, wireless networks
7
Address Centric Routing (AC)
Temperature Reading (source 2)
Temperature Reading (source 1)
Z
B
Give Me The Average Temperature? ( sink )
8
Data Centric Routing (DC)
Temperature Reading (source 2)
Temperature Reading (source 1)
Z
B
Give Me The Average Temperature? ( sink )
9
Dynamic Source Routing (DSR) Johnson96

• 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 using Route Request (RREQ)
• Each node appends own identifier when forwarding
  RREQ
• Promiscuous mode

10
Route Discovery in DSR
Y
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
Represents a node that has received RREQ for D
from S
11
Route Discovery in DSR
Y
Broadcast transmission
Z
S
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
Represents transmission of RREQ
X,Y Represents list of identifiers appended
to RREQ
12
Route Discovery in DSR
Y
Z
S
S,E
E
F
B
C
M
L
J
A
G
S,C
H
D
K
I
N

• Node H receives packet RREQ from two neighbors
• potential for collision

13
Route Discovery in DSR
Y
Z
S
E
F
S,E,F
B
C
M
L
J
A
G
H
D
K
S,C,G
I
N

• Node C receives RREQ from G and H, but does not
  forward
• it again, because node C has already forwarded
  RREQ once

14
Route Discovery in DSR
Y
Z
S
E
F
S,E,F,J
B
C
M
L
J
A
G
H
D
K
I
N
S,C,G,K

• Nodes J and K both broadcast RREQ to node D
• Caveat Since nodes J and K are hidden from each
  other, their
• transmissions may collide

15
Route Discovery in DSR

• Broadcast storm prevention
• Drop previously seen messages
• Loop prevention
• Host drops messages with its address in route
  record (like BGP)

16
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
• Not always the case, sometimes need new route
  request
• RREP includes the route from S to D on which RREQ
  was received by node D

17
Route Reply in DSR
Y
Z
S
RREP S,E,F,J,D
E
F
B
C
M
L
J
A
G
H
D
K
I
N
Represents RREP control message
18
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

19
Data Delivery in DSR
Y
Z
DATA S,E,F,J,D
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
Packet header size grows with route length
20
Data Delivery in DSR

• Send route error packet if next hop cannot be
  reached
• Delete route from the cache when receiving error
  packet
• Passive acknowledgement
• When node overhears next hop forwarding message.

21
DSR Optimization Route Caching

• Each node caches a new route it learns by any
  means
• Through Route Request (RREQ)
• When node K receives RREQ S,C,G destined for
  node D, node K learns route K,G,C,S to node S
• Through Route Reply (RREP)
• When node S finds RREP S,E,F,J,D to node D,
  node S also learns route S,E,F to node F
• When node F forwards RREP S,E,F,J,D, node F
  learns route F,J,D to node D

22
DSR Optimization Route Caching

• Through DATA packets source routes
• 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
• Problem Stale caches may increase overheads
• Splicing of cached routes
• Example know A,H,I overheard I,G,F

23
DSR Optimization Piggybacking

• Possible to piggyback route reply on new route
  requests
• Also small data
• TCP handshake
• Host must forward piggybacked data when replying
  to request with cached routes

24
DSR Optimization Error Handling

• Disconnected network leads to repeated route
  requests
• Addressed through exponential backoff
• Eavesdropping on route error packets
• Temporarily mark invalid route
• Other nodes may reply with invalid cached routes

25
Dynamic Source Routing Advantages

• Routes maintained only between nodes who need to
  communicate
• reduces overhead of route table maintenance
• Routing cache 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

26
Dynamic Source Routing Disadvantages

• Packet header size grows with route length due to
  source routing
• Flooding of route requests may potentially reach
  all nodes in the network
• Stale caches will lead to increased overhead

27
Distance-Vector routing

• Each node maintains a routing table containing
• Number of hops to each destination
• Next hop to reach each destination
• list of all destinations
• The succession of next hops leads to a
  destination
• Each node periodically broadcasts its current
  estimate of the shortest distance to each
  available destination to all of its neighbors
• Typical representative Distributed Bellman-Ford
  (DBF)

28
AODV (Ad Hoc On-Demand Distance Vector)

• AODV is based on the DSDV (Destination-Sequenced
  Distance Vector) algorithm
• Distance vector
• Different sequence numbers for each destination.
• Creation of routes on a demand basis traffic
  reactive
• Nodes that are not on a selected path do not
  maintain routing information or participate in
  routing table exchanges!
• Goal Minimize broadcast overhead and
  transmission latency

29
Route Sequence Numbers

• Unique counter for each destination
• Symbolizes the freshness of a route
• Source specifies the most recently known route
  during route establishment
• Updated occasionally
• Link failure
• Destination moves
• Intermediate nodes move

30
Route Requests from S to D in AODV
Y
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
Represents a node that has received RREQ for D
from S
31
Route Requests from S to D in AODV
Y
Broadcast transmission
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
Represents transmission of RREQ
32
Route Requests from S to D in AODV
Y
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
Represents links on Reverse Path
33
Reverse Path Setup from S to D in AODV
Y
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N

• Node C receives RREQ from G and H, but does not
  forward
• it again, because node C has already forwarded
  RREQ once

34
Reverse Path Setup from S to D in AODV
Y
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
35
Reverse Path Setup in AODV
Y

• Node D does not forward RREQ, because node D
• is the intended target of the RREQ

Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
36
Route Reply from D to S in AODV
Y
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
Represents links on path taken by RREP
37
Route Reply in AODV

• Intermediate node may also send a Route Reply
  (RREP) provided that it knows a more recent path
  than the one previously known to sender S
• Recent path means higher sequence number
• The likelihood that an intermediate node will
  send a RREP not as high as DSR
• An intermediate node which knows a route, but
  with a smaller sequence number, cannot send Route
  Reply

38
Forward Path Setup in AODV
Y
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
Forward links are setup when RREP travels
along the reverse path Represents a link on the
forward path
39
Data Delivery in AODV
Y
Routing table entries used to forward data
packet. Route is not included in packet header.
DATA
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
40
Local Link Maintenance

• Periodic hello messages broadcast to immediate
  neighbors
• Failing to receive hello messages indicates a
  link failure
• Link failure notifications sent to source nodes
• Sources rediscover new route to destination

41
AODV Key Advantages

• Partial routing tables are constructed
  reactively
• Entries are updated only when a node sends to an
  unreachable node
• No periodic global updates
• Node not on active paths maintain no routing
  entries
• ? Reduce packet overhead
• Routing table
• No source routing needed ? reduce bit overhead
• Route caching ? reduce establishment latency
• Sequence number ? override stale routes
• source based broadcast id? loop freedom
• Push link failure to relevant nodes
• ? Reduce establishment latency

42
AODV and DSR Disadvantages

• Common problems for both AODV and DSR
• Potential 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
• Random delays carrier sensing