Mobile Ad-hoc Networks: Issues and Challenges

1
Mobile Ad-hoc Networks Issues and Challenges

• Uyen Trang Nguyen
• Dept. of Computer Science Engineering
• York University (Toronto, Canada)

2
Wireless Networks Taxonomy
Wireless Networks
Infrastructure-based
Infrastructureless
Cellular Networks
Static Nodes
Mobile Nodes
Wireless LANs
Ad Hoc Networks
Mobile Adhoc Networks
Sensor Networks
3
MANETs Introduction

• No infrastructure (no base stations or access
  points)
• Mobile nodes
• Form a network in an ad-hoc manner
• Act both as hosts and routers
• Communicate using single or multi-hop wireless
  links
• Topology, locations, connectivity, transmission
  quality are variable.

4
Operations
X
D
D
Y
X
Z
S
S
5
Applications

• Civil
• Disaster recovery
• Taxi cabs
• Communications over water using floats
• Vehicular ad-hoc network
• Defense
• Battlefield communications
• Monitoring and planning

6
Challenges

• Unpredictable mobility
• Wireless channels error-prone media
• Low bandwidth channels
• Devices low power, limited resources
• Maintaining connectivity, states
• Security

7
Issues to Be Discussed

• Medium access control
• Transport layer issues
• Security
• Incentives for cooperation

8
MAC for MANETs Requirements

• To avoid interference among simultaneous
  transmissions
• But enable as many non-interfering transmission
  as possible
• Maintain fairness among transmissions
• No centralized coordinators fully distributed
  operations
• No clock synchronization asynchronous operations

9
Carrier Sensing in MANETs

• Problems
• Hidden terminal problem
• Exposed terminal problem
• Sensing range ? Transmission range
• Contention matters only at the receivers end

10
Hidden Terminal Problem
B
A
X
No carrier ? OK to transmit
11
Exposed Terminal Problem
B
A
X
Y
Presence of carrier ? holds off transmission
12
MACs Suitable for MANETs

• MACA Karn 1990
• Proposes to solve the hidden terminal problem by
  RTS/CTS dialog
• MACAW Bharghanvan 1994
• Increasing reliability by RTS/CTS/DATA/ACK dialog
• IEEE 802.11
• Distributed Coordination Function (DCF)
• Also use RTS/CTS/DATA/ACK dialog

13
RTS/CTS dialog (1)
Defer
RTS
Any node hearing this RTS will defer medium access
14
RTS/CTS dialog (2)
Defer
Defer
RTS
CTS
Any node hearing this CTS will defer medium access
15
RTS/CTS/DATA/ACK dialog
Defer
Defer
Data
ACK
16
IEEE 802.11 DCF

• Uses RTS/CTS exchange to avoid hidden terminal
  problem
• Any node overhearing a CTS cannot transmit for
  the duration of the transfer.
• Any node overhearing an RTS cannot transmit for
  the duration of the transfer (to avoid collision
  with ACK)
• Uses ACK to achieve reliability
• CSMA/CA
• Contention-based random access
• Collision detection not possible while
  transmitting

17
IEEE 802.11 DCF (cont.)

• Carrier sense in 802.11
• Physical carrier sense
• Virtual carrier sense using Network Allocation
  Vector (NAV)
• RTS/CTS specify duration of subsequent DATA/ACK
• NAV is updated based on overheard RTS/CTS
• Collision avoidance
• Nodes stay silent when carrier sensed busy
  (physical/virtual)
• Backoff intervals are used to reduce collision
  probability

18
Backoff Interval

• When channel is busy, choose a backoff interval
  in the range 0, cw.
• Count down the backoff interval when medium
  becomes idle.
• Count down is suspended if medium becomes busy
  again.
• When backoff interval reaches 0, transmit RTS.
• Binary exponential backoff in 802.11 DCF
• When a node fails to receive CTS, cw is doubled
  up (up to an upper bound).
• When a data transfer completes successfully, cw
  is reset to cwmin.

19
IEEE 802.11 CSMA/CA Example

• DIFS DCF inter-frame space SISF short
  inter-frame space

20
Disadvantages of IEEE 802.11 DCF

• High power consumption
• Hidden terminal problem not totally solved (e.g.,
  collision of RTS)
• Exposed terminal problem not solved
• Fairness problem among different transmitting
  nodes
• Only providing best-effort service

21
MAC for Multicast a Challenging Issue

• Multicast efficient info delivery from a source
  to a set of destinations simultaneously
• Uses 802.11 CSMA/CA
• Cannot use RTS/CTS exchange
• Currently there are no effective MAC protocols
  for multicast

22
Issues to Be Discussed

• Medium access control
• Transport layer issues
• Security
• Incentives for cooperation

23
TCP in Wired Networks

• Receiver sends ACKs for packets received
  correctly
• Sender times out on unacknowledged packets,
  retransmits
• Sender adjusts congestion window

24
TCP in MANETs

• TCP is designed for wired networks
• Low bit error rate
• Loss mainly caused by congestion
• Routes relatively fixed

• TCP in MANETs
• High bit error rate
• Unreliable wireless channels
• Route changes due to node mobility

25
Consequences

• TCP sender misinterprets losses as congestion
• retransmits unacknowledged segments
• Why retransmit when there is no route
• invokes congestion control
• enters slow start recovery
• Throughput is always low
• Why use TCP at all in such cases?
• For interactions with the Internet and seamless
  portability to applications using standard TCP
  (file transfer, email, browsers).

26
Approaches to Improving TCP

• Hide error losses from the sender
• Sender will not reduce congestion window
• Determine the cause of loss
• If due to errors, do not reduce window size
• Modifications are done at
• the sender only
• the receiver
• intermediate nodes only
• combinations of the above

27
ATCP in the TCP/IP Stack
Sender
Receiver
TCP
TCP
ATCP
IP
IP
Link layer
Link layer
28
ATCP Approach

• Uses network layer feedback from intermediates
  nodes for appropriate actions
• Different types of network feedback
• ICMP Destination Unreachable message indicates
  route break
• Stops transmission
• Waits until a new route is found and resumes trx
• ECN ACK with ECN flag indicates network
  congestion
• Invokes congestion control
• Retransmission time-out or 3 duplicate ACKs
• Retransmits unacknowledged segments without
  shrinking congestion window

29
Multicast Transport

• Reliable delivery
• Feedback ACK, NACK
• Congestion control
• Feedback loss rate, data rate
• Group-based
• not scalable
• feedback implosion
• exposure
• Tree-based
• scalable
• feedback implosion solved
• limited exposure

30
Multicast Transport in MANETs a Challenging
Issue

• Tree-based
• extremely difficult to establish a tree-based
  structure due to node mobility
• Group-based
• not scalable
• feedback suppression random timer, probabilistic
• multiple simultaneous transmissions potential
  collisions
• Calculation of round-trip-time difficult due to
• node mobility
• route break

31
Issues to Be Discussed

• Medium access control
• Transport layer issues
• Security
• Incentives for cooperation

32
Vulnerabilities of MANETs

• Wireless links ? jamming
• Broadcast nature ? eavesdropping
• Mobility, dynamics ? difficult to detect
  anomalies (e.g., bogus routes)
• No central authorities or infrastructures
• ? difficult key management
• Trade-offs between resource constraints and
  security

33
Potential Attacks

• Impersonation
• An attacker assumes identity and privileges of an
  authorized node
• Denial of service (e.g., jamming, flooding)
• Network layer attacks
• Blackhole
• a node falsely advertises good paths
• drops packets
• Wormhole 2 colluding attackers form a tunnel
• Byzantine creating routing loops, non-optimal
  paths
• Resource consumption attack generating
  unnecessary packets (false route requests,
  beacons)

34
Attack Countermeasures

• Defense against external attacks
• Authentication encryption (needs key and trust
  management)
• Defense against internal attacks
• Secure routing protocols (e.g. SEAD, ARAN, SAODV)
• Protect routing metrics by hashing
• Intrusion detection techniques
• CONFIDANT

35
Security Open Issues

• Key management
• Still relies on a central authority (or a group
  of trusted nodes)
• Consumes high power for processing
• Intrusion detection
• Can only detect a group of potential attackers
• Cannot single out the true attackers or
  compromised nodes
• High rate of false alarm due to unreliable
  wireless links and randomness of channel access
• No solutions to flooding/jamming attacks
• Incentives for nodes to cooperate

36
Issues to Be Discussed

• Medium access control
• Transport layer issues
• Security
• Incentives for cooperation

37
Incentives

• Rewards per-hop payment in every packet or in
  counters embedded in nodes
• may not always be effective
• Punishment
• punishing both selfish and malicious nodes
• Example CONFIDANT Buchegger 2002
• Based on the biological example in The Selfish
  Gene by Richard Dawkins (Oxford University
  Press, 1989 edition, 1976)

38
The Selfish Gene

• Reciprocal altruism is beneficial for every
  biological system when favors are granted
  simultaneously
• Example survival chances of birds grooming
  parasites off each others head.
• 3 types of birds
• Suckers always help others
• Cheats get help from but never help others
• Grudgers start out by helping every bird
• but bears a grudge against those that do not help
• and subsequently no longer groom their heads

39
CONFIDANT

• Every node has 4 components
• The monitor
• The trust manager
• The reputation system (node rating)
• The path manager

40
CONFIDANT Components

• The monitor neighborhood watch
• Listening to transmissions of next nodes
• Observing route protocol behaviors (no
  forwarding, unusual route attraction, unusually
  frequent route updates)
• Calling the reputation system when a bad behavior
  is detected
• The trust manager distributed and adaptive
• Using trust function to calculate trust levels
• Forwarding ALARM messages to warn others of
  malicious/selfish nodes
• Filtering ALARM messages based on trust level of
  the reporting node

41
CONFIDANT Components (cont.)

• The reputation system (node rating) based on
• Own experience greatest weight
• Observations smaller weight
• Reported experience (from ALARM messages) weight
  function according to trust level
• Rating lists are maintained locally, may be
  exchanged with friends
• The path manager
• Deleting paths containing malicious nodes
• Dealing with route requests from malicious nodes
  (e.g., ignore, alert the source)

42
Disadvantages of CONFIDANT

• Authentication is a prerequisite
• Requires efficient key management
• Implementing all the components
• requires high processing power and storage
• incurs lots of overheads

43
Summary

• MANETs
• Unpredictable mobility
• Low bandwidth channels
• Wireless channels error-prone media, vulnerable
  to attacks
• Devices low power, limited resources
• Issues
• Medium access control
• Transport layer issues
• Security
• Incentives for cooperation

44

• Thank you!