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Security and Cooperation in Wireless Networks
Tutorial at ACM MobiCom/MobiHoc 2007
Jean-Pierre Hubaux and Panos Papadimitratos{firstname.lastname}@epfl.ch
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Security and Cooperation in Wireless Networks
Part 1: New Wireless Networks andNew Challenges
Jean-Pierre [email protected]
Security and Cooperation in Wireless Networks
Thwarting Malicious and Selfish Behavior in the Age of Ubiquitous Computing
Levente Buttyan and Jean-Pierre Hubaux
With contributions from N. Ben Salem, M. Cagalj,S. Capkun, M. Felegyhazi, T. Holczer,
P. Papadimitratos, P. Schaffer, and M. Raya
http://secowinet.epfl.ch
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Security and Cooperation in Wireless Networks
1. Introduction2. Thwarting malicious behavior3. Thwarting selfish behavior
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The Internet : something went wrong
Network deploymentNetwork deployment
Observationof new misdeeds
(malicious or selfish)
Observationof new misdeeds
(malicious or selfish)
Install security patches(anti-virus, anti-spam, anti-spyware,
anti-phishing, firewalls,…)
Install security patches(anti-virus, anti-spam, anti-spyware,
anti-phishing, firewalls,…)
“The Internet is Broken”MIT Technology Review,Dec. 2005 – Jan. 2006
NSF FIND, GENI, etc.
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Where is this going ?
What if tomorrow’s wireless networks are even more unsafe than today’s Internet ?What if tomorrow’s wireless networks are even more unsafe than today’s Internet ?
The Economist, April 28, 2007MIT Technology Review,Dec. 2005 – Jan. 2006
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Upcoming wireless networks• New kinds of networks
– Personal communications• Small operators, community networks• Cellular operators in shared spectrum• Mesh networks• Hybrid ad hoc networks (also called “Multi-hop cellular networks”)• “Autonomous” ad hoc networks• Personal area networks
– Vehicular networks– Sensor and RFID networks– …
• New wireless communication technologies– Cognitive radios– MIMO– Ultra Wide Band– Directional antennas– …
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Upcoming wireless networks• New kinds of networks
– Personal communications• Small operators, community networks• Cellular operators in shared spectrum• Mesh networks• Hybrid ad hoc networks (also called “Multi-hop cellular networks”)• “Autonomous” ad hoc networks• Personal area networks
– Vehicular networks– Sensor and RFID networks– …
• New wireless communication technologies– Cognitive radios– MIMO– Ultra Wide Band– Directional antennas– …
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Community networks
• Centralized solution: FON, http://en.fon.com/
• Distributed solution:E. Pantelis, A. Frangoudis, and G. PolyzosStimulating Participation in Wireless Community NetworksINFOCOM 2006
• Incentive technique based on proof of contribution
Example: service reciprocation in community networks
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Mesh Networks: jamming
More on mesh networks: • IEEE Wireless Communications, Special Issue on Wireless Mesh Networking,
Vol. 13 No 2, April 2006
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Vehicular networks: why?
• Combat the awful side-effects of road traffic– In the EU, around 40’000 people die yearly on the roads;
more than 1.5 millions are injured– Traffic jams generate a tremendous waste of time and of fuel
• Most of these problems can be solved by providing appropriate information to the driver or to the vehicle
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Example of attack : Generate “intelligent collisions”
SLOW DOWN
The way is clear
For more information: http://ivc.epfl.chhttp://www.sevecom.org
• All carmakers are working on vehicular comm.• Vehicular networks will probably be the largestincarnation of mobile ad hoc networks
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Sensor networks
Vulnerabilities:• Theft reverse engineered and compromised, replicated • Limited capabilities risk of DoS attack, restriction on
cryptographic primitives to be used• Deployment can be random pre-configuration is difficult• Unattended some sensors can be maliciously moved around
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RFID• RFID = Radio-Frequency Identification
• RFID system elements– RFID tag + RFID reader + back-end database
• RFID tag = microchip + RF antenna– microchip stores data (few hundred bits)– Active tags
• have their own battery expensive– Passive tags
• powered up by the reader’s signal• reflect the RF signal of the reader modulated with stored data
RFID tagRFID reader
back-enddatabase
tagged object
detailedobject
information
readingsignal
IDID
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Trends and challenges in wireless networks
• From centralized to distributed to self-organized Security architectures must be redesigned
• Increasing programmability of the devicesincreasing risk of attacks and of greedy behavior
• Growing number of tiny, embbeded devices Growing vulnerability, new attacks
• From single-hopping to multi-hoppingIncreasing “security distance” between devices and
infrastructure, increased temptation for selfish behavior• Miniaturization of devices Limited capabilities• Pervasiveness Growing privacy concerns
… Yet, mobility and wireless can facilitate certain security mechanisms
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Reasons to trust organizations and individuals
• Moral values– Culture + education, fear of bad reputation
• Experience about a given party– Based on previous interactions
• Rule enforcement organization– Police or spectrum regulator
• Usual behavior– Based on statistical observation
• Rule enforcement mechanisms– Prevent malicious behavior (by appropriate security
mechanisms) and encourage cooperative behavior
}Will lose relevance
Scalability challenge
Can be misleading
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Upcoming networks vs. mechanisms
X X X X X X
X X X X
X
X
?
X
X
X X X X X X ?
X X X X X X X X
X X X X X X X X
X X X X X ? ? ?
X X X X X ? ?
X ? X X ?
Small operators, community networksCellular operators in shared spectrumMesh networks
Naming and addressing
Discouraging
greedy op.
Security
associa
tions
Securin
g neighbor disc
overy
Secure ro
uting
Privac
yEnforci
ng PKT FWing
Enforcing fa
ir MAC
Hybrid ad hoc networksSelf-organized ad hoc networks
Vehicular networks
Sensor networks
RFID networks
Upcomingwireless
networks
Ruleenforcement
mechanisms
Behavior
enforc.
Security Cooperation
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Security and Cooperation in Wireless Networks
1. Introduction2. Thwarting malice: security mechanisms
2.1 Naming and addressing2.2 Establishment of security associations2.3 Secure neighbor discovery2.4 Secure routing in multi-hop wireless networks2.5 Privacy protection2.6 Secure positioning
3. Thwarting selfishness: behavior enforcement3.0 Brief introduction to game theory3.1 Enforcing fair bandwidth sharing at the MAC layer 3.2 Enforcing packet forwarding3.3 Wireless operators in a shared spectrum3.4 Secure protocols for behavior enforcement
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2.1 Naming and addressing• Typical attacks:
– Sybil: the same node has multiple identities– Replication: the attacker captures a node and replicates it
several nodes share the same identity• Distributed protection technique in IPv6: Cryptographically Generated
Addresses (T. Aura, 2003; RFC 3972))
Public key
Hash function
Interface IDSubnet prefix
64 bits 64 bits
For higher security (hash function outputbeyond 64 bits), hashextension can be used
Parno, Perrig, and Gligor. Detection of node replication attacks in sensor networks. IEEE Symposium on Security and Privacy, 2005
IPv6 address
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2.2 Pairwise key establishment in sensor networks
1. Initialization
Keyreservoir(k keys)
m (<<k) keys in each sensor (“key ring of the node”)
2. Deployment
Do we have a common key?
Probability for any 2 nodes to have a common key:
)!2(!))!((1
2
mkkmkp−−
−=
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Probability for two sensors to have a common key
Eschenauer and Gligor, ACM CCS 2002See also:• Karlof, Sastry, Wagner: TinySec, Sensys 2004• Westhoff et al.: On Digital Signatures in Sensor Networks, ETT 2005
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2.3 Securing Neighbor Discovery:Thwarting Wormholes
• Routing protocols will choose routes that contain wormhole links– typically those routes appear to be shorter– Many of the routes (e.g., discovered by flooding based routing
protocols such as DSR and Ariadne) will go through the wormhole• The adversary can then monitor traffic or drop packets (DoS)
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Wormholes are not specific to ad hoc networks
access control system:gate equipped with contactless smart card reader
contactlesssmart card
contactlesssmart cardemulator
smart cardreader emulator
fastconnection
wormhole
user may befar away from the building
Hu, Perrig, and Johnson Packet leashes: a defense against wormhole attacks in wireless networksINFOCOM 2003
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2.4 Secure routing in wireless ad hoc networks
A
B
C
D
E
F
G
H
A *: [req,A,H; -] B, C, D, EB *: [req,A,H; B] AC *: [req,A,H; C] AD *: [req,A,H; D] A, E, GE *: [req,A,H; E] A, D, G, FF *: [req,A,H; E,F] E, G, HG *: [req,A,H; D,G] D, E, F, H
H A: [H,F,E,A; rep; E,F]
Exchange of messages in Dynamic Source Routing (DSR):
• Routing disruption attacks– routing loop– black hole / gray hole– partition– detour– wormhole
• Resource consumption attacks– injecting extra data packets in the network– injecting extra control packets in the network
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Operation of Ariadne illustrated
A *: [req, A, H, MACKAH, (), ()]E *: [req, A, H, h(E|MACKAH), (E), (MACKE,i)]F *: [req, A, H, h(F|h(E|MACKAH)), (E, F), (MACKE,i, MACKF,i)]
H F: [rep, H, A, (E, F), (MACKE,i, MACKF,i), MACKHA, ()]F E: [rep, H, A, (E, F), (MACKE,i, MACKF,i), MACKHA, (KF,i)]E A: [rep, H, A, (E, F), (MACKE,i, MACKF,i), MACKHA, (KF,i, KE,i)]
A
B
C
D
E
F
G
H
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Secure route discovery with the Secure Routing Protocol (SRP)
Route Reply (RREP): QID, T, V3, V2, V1, S, MAC(KS,T, QID, QSEQ, T, V3, V2, V1, S) (5) T → V3 : RREP; (6) V3 → V2 : RREP; (7) V2 → V1 : RREP; (8) V1 → S : RREP;
S V1 V3 V2 T
1 2 3 4
8 7 6 5
Route Request (RREQ): S, T, QSEQ, QID, MAC(KS,T, S, T, QSEQ, QID) (1) S broadcasts RREQ; (2) V1 broadcasts RREQ, V1; (3) V2 broadcasts RREQ, V1, V2; (4) V3 broadcasts RREQ, V1, V2, V3;
QSEQ: Query Sequence NumberQID : Query Identifier
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More on secure routing
Secure Data Communication
Secure Route Discovery
Hu, Perrig, and Johnson: Ariadne, Sept. 2002, SEAD, Jun. 2002
Zapata and Asokan: S-AODV, Sept. 2002
Papadimitratos and Haas: Secure Single Path (SSP) and Secure Multi-path (SMT) protocols, Jul./Sept. 2003, Feb. 2006
Papadimitratos and Haas: Secure Routing Protocol (SRP), Jan. 2002
Sangrizi, Dahill, Levine, Shields, and Royer: ARAN, Nov. 2002
All above proposals are difficult to assessG. Ács, L. Buttyán, and I. Vajda:
Provably Secure On-demand Source RoutingIEEE Transactions on Mobile Computing, Nov. 2006
Cross-layerattacks
Aad, Hubaux, Knightly: Jellyfish attacks, 2004
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2.5 Privacy: the case of RFID• RFID = Radio-Frequency Identification
• RFID system elements– RFID tag + RFID reader + back-end database
• RFID tag = microchip + RF antenna– microchip stores data (few hundred bits)– Active tags
• have their own battery expensive– Passive tags
• powered up by the reader’s signal• reflect the RF signal of the reader modulated with stored data
RFID tagRFID reader
back-enddatabase
tagged object
detailedobject
information
readingsignal
IDID
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RFID privacy problems• RFID tags respond to reader’s query automatically,
without authenticating the readerclandestine scanning of tags is a plausible threat
• Two particular problems:1. Inventorying: a reader can silently determine what objects
a person is carrying• books• medicaments • banknotes• underwear• …
2. Tracking: set of readers can determine where a given person is located• tags emit fixed unique identifiers• even if tag response is not unique it is possible
to track a set of particular tags
watch: Casio
book:Wireless Security
shoes: Nike
suitcase: Samsonite
jeans: Lee Cooper
Juels A., RFID Security and Privacy: A Research Survey, IEEE JSAC, Feb. 2006
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2.6 Secure positioning
m1
v2
v1
v1
vmc
- honest node - malicious node - compromised node
v3
m5
m3
m4
m2
c
c
WormholeNode displacement
a) b)
d) Dissemination of false position and distance information
c) Malicious distance enlargement
Node's actual positionNode's actual
distanceNode's measureddistance
Node's reportedposition
http://www.syssec.ethz.ch/research/spot
(not part of the book)
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Security and Cooperation in Wireless Ad Hoc Networks
1. Introduction2. Thwarting malice: security mechanisms
2.1 Naming and addressing2.2 Establishment of security associations2.3 Secure neighbor discovery2.4 Secure routing in multi-hop wireless networks2.5 Privacy protection2.6 Secure positioning
3. Thwarting selfishness: behavior enforcement3.0 Brief introduction to game theory3.1 Enforcing fair bandwidth sharing at the MAC layer 3.2 Enforcing packet forwarding3.3 Wireless operators in a shared spectrum3.4 Secure protocols for behavior enforcement
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3.0 Brief introduction to Game Theory
• Discipline aiming at modeling situations in which actors have to make decisions which have mutual, possibly conflicting, consequences
• Classical applications: economics, but also politics and biology
• Example: should a company invest in a new plant, or enter a new market, considering that the competitioncould make similar moves?
• Most widespread kind of game: non-cooperative(meaning that the players do not attempt to find an agreement about their possible moves)
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From a problem to a game
• Users controlling the devices are rational (or selfish): they try to maximize their benefit
• Game formulation: G = (P,S,U)– P: set of players– S: set of strategy functions– U: set of utility functions
• Strategic-form representation
• Reward for packet reaching the destination: 1• Cost of packet forwarding: c (0 < c << 1)
(1-c, 1-c) (-c, 1)(1, -c) (0, 0)
BlueGreen
Forward
Drop
Forward Drop
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Solving the Forwarder’s Dilemma (1/2)
' '( , ) ( , ), ,i i i i i i i i i iu s s u s s s S s S− − − −< ∀ ∈ ∀ ∈
iu U∈i is S− −∈
Strict dominance: strictly best strategy, for any strategy of the other player(s)
where: utility function of player istrategies of all players except player i
In Example 1, strategy Drop strictly dominates strategy Forward
(1-c, 1-c) (-c, 1)(1, -c) (0, 0)
BlueGreen
Forward
Drop
Forward Drop
Strategy strictly dominates ifis
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Solving the Forwarder’s Dilemma (2/2)
Solution by iterative strict dominance:
(1-c, 1-c) (-c, 1)(1, -c) (0, 0)
BlueGreen
Forward
Drop
Forward Drop
Drop strictly dominates ForwardDilemma
Forward would result in a better outcomeBUT }
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Nash equilibrium
Nash Equilibrium: no player can increase his utility by deviating unilaterally
(1-c, 1-c) (-c, 1)(1, -c) (0, 0)
BlueGreen
Forward
Drop
Forward Drop
The Forwarder’s Dilemma
(Drop, Drop) is the only Nash equilibrium of this game(Drop, Drop) is the only Nash equilibrium of this game
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Example 2: The Multiple Access game
Reward for successfultransmission: 1
Cost of transmission: c(0 < c << 1)
There is no strictly dominating strategy
(0, 0) (0, 1-c)(1-c, 0) (-c, -c)
BlueGreen
Quiet
Transmit
Quiet Transmit
There are two Nash equilibria
Time-division channel
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More on game theory
Properties of Nash equilibria to be investigated:• uniqueness• efficiency (Pareto-optimality)• emergence (dynamic games, agreements)
Promising area of application in wireless networks: cognitive radios
Pareto-optimalityA strategy profile is Pareto-optimal if the payoff of a player cannot be increased without decreasing the payoff of another player
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Security and Cooperation in Wireless Networks
1. Introduction2. Thwarting malice: security mechanisms
2.1 Naming and addressing2.2 Establishment of security associations2.3 Secure neighbor discovery2.4 Secure routing in multi-hop wireless networks2.5 Privacy protection2.6 Secure positioning
3. Thwarting selfishness: behavior enforcement3.0 Brief introduction to game theory3.1 Enforcing fair bandwidth sharing at the MAC layer 3.2 Enforcing packet forwarding3.3 Wireless operators in a shared spectrum3.4 Secure protocols for behavior enforcement
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3.1 Enforcing fair bandwidth sharing at the MAC layer
Well-behaved nodeCheater
The access point is trustedThe access point is trusted
• Kyasanur and Vaidya, DSN 2003• http://domino.epfl.ch• Cagalj et al., Infocom 2005 (game theory model for CSMA/CA ad hoc networks)
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3.2 Enforcing packet forwarding
• V. Srinivasan, P. Nuggehalli, C. Chiasserini, and R. Rao, Infocom 2003, IEEE TWC 2005
• M. Felegyhazi, JP Hubaux, and L. Buttyan, Personal Wireless Comm. Workshop 2003, IEEE TMC 2006
S1
S2
D1D2
Usually, the devices are assumed to be cooperative. But what if they are not, and there is no incentive to cooperate?
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Modeling packet forwarding as a game
time0time slot: 1 t
Strategy:cooperationlevel
pC(0) pC(1) pC(t)
Player: node
Payoff of node i: proportion of packets sent by node i reaching their destination
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3.3 Games between wireless operatorsMulti-domain sensor networks
• Typical cooperation: help in packet forwarding• Can cooperation emerge spontaneously in multi-domain sensor
networks based solely on the self-interest of the sensor operators?
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• Two CDMA operators: A and B• Adjust the pilot signals• Power control game (no power
cost):– players = operators– strategies = pilot powers – payoffs = attracted users (best
SINR)pilotpG
where: – pilot processing gain– pilot signal power of BS A– path loss between A and v– own-cell interference factor– other-to-own-cell interference factor– traffic signal power assigned to w
by BS A– set of users attached to BS A
0
pilotp A Avpilot
Av pilot pilotown other
G P dSINR
N W I I
α−⋅ ⋅=
⋅ + +
Signal-to-interference-plus-noise ratio
A
pilotown Av Aw
wI d Tας −
∈
⎛ ⎞= ⋅ ⎜ ⎟
⎝ ⎠∑M
B
pilotother Bv B Bw
wI d P Tαη −
∈
⎛ ⎞= ⋅ +⎜ ⎟
⎝ ⎠∑M
Own-cell interference
Other-to-own-cell interference
pilotpG
APAvd α−
ςη
AwT
AM
3.3 Border games of cellular operators (2/3)
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• Unique and Pareto-optimal Nash equilibrium• Higher pilot power than in the standard Ps = 2W• 10 users in total
Extended game with power costs = Prisoner’s Dilemma where:
U – fair payoff (half of the users)Δ – payoff difference by selfish behaviorC* - cost for higher pilot power
standard
Nash equilibrium
3.3 Border games of cellular operators (3/3)
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3.4 Secure protocols for behavior enforcement
V2
V1
V4
V3
DS 3
4
15
10
7
5
3
5
3
2
• Self-organized ad hoc network• Investigation of both routing and packet forwarding
S. Zhong, L. E. Li, Y. G. Liu, and Y. R. Yang.On designing incentive-compatible routing and forwarding protocols in wireless ad hoc networks – an integrated approach using game theoreticaland cryptographic techniquesMobicom 2005
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Who is malicious? Who is selfish?
There is no watertight boundary between malice and selfishnessBoth security and game theory approaches can be useful
There is no watertight boundary between malice and selfishnessBoth security and game theory approaches can be useful
Harm everyone: viruses,…
Selective harm: DoS,… Spammer
Cyber-gangster:phishing attacks,trojan horses,…
Big brother
Greedy operator
Selfish mobile station
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From discrete to continuousWarfare-inspired Manichaeism:
The more subtle case of commercial applications:
Bad guys (they)Attacker
Good guys (we)System (or country) to be defended
0 1
Undesirablebehavior
Desirablebehavior
0 1
• Security often needs incentives• Incentives usually must be secured
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Book structure (1/2)
X X X X X X
X X X X
X
X
?
X
X
X X X X X X ?
X X X X X X X X
X X X X X X X X
X X X X X ? ? ?
X X X X X ? ?
X ? X X ?
Small operators, community networksCellular operators in shared spectrumMesh networks
Naming and addressing
Discouraging
greedy op.
Security
associa
tions
Securin
g neighbor disc
overy
Secure ro
uting
Privac
yEnforci
ng PKT FWing
Enforcing fa
ir MAC
Hybrid ad hoc networksSelf-organized ad hoc networks
Vehicular networks
Sensor networks
RFID networks
Upcomingwireless
networks
Security and cooperation
mechanisms
Behavior
enforc.
Part I Part IIIPart II
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Book structure (2/2)
1. Existing networks
2. Upcoming networks
3. Trust
4. Naming and addressing
5. Security associations
6. Secure neighbor discovery
7. Secure routing
8. Privacy protection
9. Selfishness at MAC layer
10. Selfishness in PKT FWing
11. Operators in shared spectrum
12. Behavior enforcement
Appendix A:Security and crypto
Appendix B:Game theory
Security Cooperation
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Conclusion
• Upcoming wireless networks bring formidable challenges in terms of security and cooperation
• The proper treatment requires a thoroughunderstanding of upcoming wireless networks, of security, and of game theory
Slides available at http://secowinet.epfl.ch
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Security and Cooperation in Wireless Networks
Part 2: Thwarting Malicious Behavior
Panos [email protected]
© 2007 P. Papadimitratos
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Outline
• Topics– Security Association Establishment– Secure Neighbor Discovery– Secure Route Discovery– Secure Data Communication– Privacy Enhancing Technologies for Vehicular
Communication Systems
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Problem statement
• Establishing secure communication channels between devices
Alice
Carol
Bob
Dave
Alice-Bob secure channel
Carol-Dave secure channel
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Problem statement (cont’d)
• Security requirements– Authentication– Integrity– Confidentiality– Non-repudiation– …
Alice Bob
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Problem statement (cont’d)
• Security mechanisms– Message Authentication Codes (MACs)– Digital signatures– Encryption/decryption– Passwords– …
• Cryptography– Asymmetric key– Symmetric key
Alice Bob
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Problem statement (cont’d)
• Enable secure communication– Uni-directional– Bi-directional
• Issues to consider– Long- or short- term?– What fraction of the system nodes?– Is there a trusted third party?– …
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Public-key approach
• Pro: Any-to-any secure communication• Con: Need to bind public keys to identities
AliceIdentity: APublic key: KAPrivate key: kA
BobIdentity: BPublic key: KBPrivate key: kB
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Bob
(2) EA(sec-text,B), SigB(A,nA,EA(sec-text,B))
Public-key approach (cont’d)
Alice(1) nA, A, text, SigA(nA,A,text)
• Secure communication example– Message (1): signed with kA; nA is a nonce– Message (2): sec-text and B encrypted with KA; A, nA,
and ciphertext signed with kB
– Note: In practice, different keys are used for signing/verifying and encrypting/decrypting
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Public-key approach (cont’d)
• Certification Authority(CA)– Trusted Third Party– Known KCA
– CertCA: CA signature on the identity, public key, and other information (e.g., lifetime)
AliceIdentity: APublic key: KAPrivate key: kA
CertCA{A,KA}
BobIdentity: BPublic key: KBPrivate key: kB
CertCA{B,KB}
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Using a CA
• Largely independent of communication– Users obtain certificates over the wire-line
network– Certificates are installed at wireless devices
and the corresponding keys used to secure wireless communication
• Examples specific to wireless networks
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Using a CA (cont’d)
• Wireless local-area (e.g., campus-wide) networks– CA locally administered– IEEE 802.11 devices communicate securely with access
points
• Tactical networks– CA operated by the corresponding government
department– Keys and certificates installed at wireless-enabled
devices– Hierarchical network organization
• Vehicular Communication (VC) Systems• More detailed look next
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CA example: Vehicular networks (cont’d)
• Authorities– Hierarchical organization– ‘Forest’ with cross-certification
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Public key cryptography - Practical aspects
• There is no single trusted authority– Nodes belonging to different administrative
domains will in general be associated and execute security protocols
• PK cryptography is feasible even in low-end mobile platforms, but it is costly– Processing– Energy consumption– Delays– Transmission overhead
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Symmetric key establishment
• PK cryptography– Moderated use recommended– Examples:
• Session keys• Shared symmetric key establishment
• Key agreement– Both nodes contribute to the shared
symmetric key• Key transport
– One of the nodes ‘chooses’ the shared symmetric key
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Key agreement• Authenticated Diffie-Hellman protocol
– g publicly known parameter; G a multiplicative group – A selects a random number rA in G; it calculates – B selects a random number rB in G; it calculates
(1) tA, EB(A, nA)
ArAt g=
BrBt g=
K0= h(nA,nB)(2) tB, EA(nB), MAC(K0, tA, tB, B, A)
Bob
(3) MAC(K0, tA, tB, A, B)
Alice
( ) ( )A B A Br r r rAB B AK t g t= = =
H. Krawczyk, “SKEME: A versatile secure key exchange mechanism for Internet,” NDSS’96
75
(3) nB, B, SigA(nB, B)
(2) nB, A, nA, EA(B, KBA), SigB(nB, A, nA, EA(B, KBA))
Key transport
BobAlice
(1) nA, B, EB(A, KAB), SigA(nA, B, EB(A, KAB))
( , )AB AB BAK f K K=
X.509 three-pass key transport protocol
76
Hash chains• Cryptographic hash or one-way function
– h : {0,1}* {0,1}n
– Input: Arbitrary length – Output: Fixed length n
• Required properties– Collision resistance: it is computationally infeasible to
find two distinct inputs, x, y, which hash to a common value h(x)=h(y)
– Pre-image resistance: given a specific hash-value z, it is computationally infeasible to find an input x such that h(x)=z
– 2nd pre-image resistance: given x and h(x) it is computationally infeasible to find a second input y≠xsuch that h(y)=h(x)
– Low computational cost: given h and an input x, h(x) is easy to compute.
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Hash Chains (cont’d)
• Pick a random number r• Generate k elements by hashing r successively k
times
• H0 is the hash chain anchor• The remaining k-1 elements can be used for
authentication
( ) ( ) ( ) ( ) ( )( )( ) ( )1 3 2k kh r h rh r r h rh rh h−← ← ← ← = ←
0 1 3 2 1k kkH H H HH− −−← ← ← ←←
= = = = =
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Bootstrapping a hash chain
• Alice must ‘commit’ to the hash chain anchor• Each Bi node validates the commitment (signature) and
stores H0
• Alice can then utilize the hash chain elements
SigA(H0, A, text), H0, A, text, CertCA(KA, A)
Alice B1
B2
B3
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Using a hash chain
• Chain elements as authenticators, e.g., to transmit “yes” / “no”– “Yes” chain
– “No” chain0 1 3 2 1k kkH H H HH− −−← ← ← ←←
0 1 3 2 1k kkG G G GG− −−← ← ← ←←• Sender : ‘Reveal’ elements in this order
• Use Gi or Hi to authenticate a “no” or “yes”• Receiver: For the i – th message from Alice, verify that hi(Hi) =H0 or hi(Gi) =G0
R. Hauser, A. Przygienda, and G. Tsudik, “Reducing the Cost of Security in Link-State Routing,” NDSS’96
80
Using a hash chain (cont’d)
• Chain elements as symmetric keys
– Synchronized clocks at sender and receiver– Sender release keys (e.g., flooding them across the
network) at specific intervals– A posteriori validation at the receiver: reject messages
not generated sufficiently close to the release time
0 1 3 2 1k kkH H H HH− −−← ← ← ←←
Time Ti: : mi = A, texti, MAC(Hi, A, texti)Time Ti+j : Release Hi
S. Cheung, “An Efficient Message Authentication Scheme for Link State Routing,”Comp. Sec. App. Conf. ‘97A. Perrig et al., "Efficient and secure source authentication for multicast,“ NDSS '01
81
Recap: Public key enabled security
• Advantages– Any-to-any secure communication– Basis for bootstrapping symmetric key
primitives
• Disadvantages– Processing and communication overhead– Setting up a certification authority
• Comment– Methods discussed so far are rather ‘agnostic’
to the underlying network technology
82
What if no CA is available?
• Main challenge: Man-in-the-Middle attacks
BobAlice
Intended and perceived secure communication
MitM attacker
Actual communication
Actual communication
83
What if no CA is available? (cont’d)
• Can we leverage on characteristics of the network or the mobile application?
• Observation 1: Wireless, mobile devices are used by human beings, who can assist the security association establishment
• Observation 2: Wireless communication possible only within a very short range or within a line of sight can imply that no other device is present (caution!)
84
Leveraging on the users• Password-based key establishment
– : shared password–– q publicly known parameter– A, B select random numbers x, y respectively
(1) wA
(2) wB
BobAlice
( )2( ) modg h qπ π=π
modxAw g qπ=
modyBw g qπ=
modyAz w q=modx
Bz w q=Abort if z is not in range [2, q−2] Otherwise, z is a large shared secret
D. Jablon, “Extended Password Key Exchange Protocols Immune to Dictionary Attacks,” WET-ICE '97
85
Leveraging on the users (cont’d)
• Password-based key establishment– h : hash function– Once z is established, A and B prove to each other they
know the same z– A and B can then derive a session key from z
(3) oB
(4) oA
BobAlice
(03|| || || || )B A Bo h w w z gπ=(04|| || || || )A A Bo h w w z gπ=
3 (03|| || || || )A Bo h w w z gπ= 4 (04|| || || || )A Bo h w w z gπ=
Abort if or3Bo o≠ 4Ao o≠
86
Leveraging on the user (cont’d)
• The user verifies that the keys generated at the two devices are identical
• Visual and audible hashesPhrase 1Phrase 2
M. Goodrich, M. Sirivianos, J. Solis, G. Tsudik, and E. Uzun, “Loud And Clear Human-Verifiable Authentication Based on Audio", ICDCS'06
J. McCune, A. Perrig, and M. Reiter, "Seeing-is-Believing: Using Camera Phones for Human-Verifiable Authentiction." S&P'05
87
Leveraging on the wireless link
• ‘Off-line’ local channels– One example: infra-red
• D. Balfanz, D. Smetters, P. Stewart, and H. Wong, “Talking to strangers: Authentication in ad-hoc wireless networks,” NDSS’02
– Exchange information over the local channel that allows you to authenticate over the wireless radio channel
• Caution: System and protocol design must ensure that it is indeed impossible for the attacker to interfere actively with the communication over the local channel– For example, the attacker must be unable to act as an
‘invisible’ relay
88
Leveraging on the network
• Mobility– Users meeting each other, e.g., at a
conference, can set up symmetric keys or exchange public keys• Chapter 5 and S. Capkun, J-P. Hubaux, and L.
Buttyan, “Mobility helps security,” ACM Mobihoc’03
– More generally, a mobile device can be interested in obtaining public keys of other devices in proximity, e.g., within a few hops• Example later in secure routing
– Point of caution: communication pattern
89
Summary • One-to-one, one-to-many, one-to-all, any-to-any
secure communication• Need for protocols that allow dynamic
establishment of security associations – Public-key cryptography – Symmetric-key cryptography– Leveraging on the communication and computing
environment characteristics• Various communication patterns
– Duration, number of communicating devices, direction of communication
• Additional readings– Sensor network key distribution (Chapter 5)
91
Problem statement
• Node discovery– A node discovers other nodes it can directly
communicate with
AB
C
D
92
Problem statement (cont’d)
• R: nominal communication range• Caution: A, B are neighbors if and only if
they can communicate directly
A
B
RR
93
Problem statement (cont’d)
• B is neighbor of A if and only if it can receive directly from A
• Link (A,B) is up B is neighbor of A• Consider the case with different nominal
communication ranges, e.g., RA, RB; then (A,B) may be up while (B,A) is down
A
B
RR
94
Neighbor discovery
• Neighbor discovery is a building block for other system functionality– Communication– Access control– Physical access control
• Examples– First step before routing– Connection to an wireless LAN access point– Radio Frequency Identification (RFID) reader
controlling a door
95
Neighbor discovery (cont’d)
• Simple, widely used solution, but not secure• Easy to mislead B that A is its neighbor when
this is not the case
A B
“Hello, I’m A”
B: “A is my neighbor”;“A is added in myNeighbor List”
96
Attacking neighbor discovery
• Single adversary appears as multiple neighbors
M
“Hello, I’m A”
“Hello, I’m C”
“Hello, I’m Z” B: Neighbor List = {A, C, …, Z}
…
97
Securing neighbor discovery
• A first attempt– Authenticate “Hello” messages
• The adversary can record signed “Hello”messages and transmit (replay) them later
A B“Hello, I’m A”, SigA(“Hello, I’m A” ),CertCA(KA,A)
(1) Validate CertCA()(2) Validate SigA()(3) Add A to
neighbor iff (1), (2) are successful
98
(2) A, nA, nB, B, SigA(A, nA,nB, B), CertCA(KA,A)
Securing neighbor discovery (cont’d)
• A second attempt– Message authenticity and replay protection
• nA, nB are nonces– Bob essentially ‘challenges’ Alice to provide a ‘hello’
message
A B(1) nB, B
99
Attacking neighbor discovery (cont’d)
• “Relay” or “Wormhole” Attack– Simply relay any message, without any modification
A
“HelloB, I’m
A”“H
ello
B, I’
m A
”
B: Neighbor List = {A}
M“B: Anyone there?” “B
: Any
one
ther
e?”
100
Attacking neighbor discovery (cont’d)
• Long-range relay / wormhole– The attacker relays messages across large
distances
out-of-band or private channel
B: Neighbor List = {A}
“Hello, I’m A”
“Hello, I’m A” “Hello, I’m A”
B
M1 M2
A
101
Attack implications
• Network access control– The attacker ‘assists’ access – But it has control over the nodes’ communication
AP
AB
M1
obstacle
M2
102
Attack implications (cont’d)
A
B
M VU
DC
• Routing– The attacker creates a ‘link’ and ‘provides’ shortest routes– Attracted traffic is under the control of the adversary
103
Attack implications (cont’d)
• Physical access control– RFID based access control – Attacker close to the owner of the access-granting RFID tag; relays
signals from and to her accomplice, who obtains access
Illustration by M. Poturalski
Z. Kfir and A. Wool, “Picking virtual pockets using relay attacks on contact-less smartcard,” SECURECOMM ’05
104
Securing neighbor discovery (cont’d)
• A third attempt– Geographical packet leashes
• Nodes are aware of their location in a secure manner• Loosely synchronized clocks• Sender adds coordinates to each packet• Receiver checks if sender is within range
– Temporal packet leashes• Nodes have tightly synchronized clocks• Sender (A) adds a timestamp to each packet• Receiver (B) estimates its distance from the sender based on the
elapsed time, tprop = treceiveB – tsendA
• Dist(A,B) < ctprop– c is the speed of light– ‘Ignore’ the clock drift
Y-C. Hu, A. Perrig, and D. Johnson, “Packet Leashes: A Defense against Wormhole Attacks in Wireless Ad Hoc Networks,” Infocom’03
105
Attacking neighbor discovery (cont’d)
obstacleA B A B
M
• Observation: Physical proximity does not necessarily imply correct nodes are able to communicate directly
• No protocol using time-of-flight measurements candistinguish the two situations
M. Poturalski, P. Papadimitratos, and J-P. Hubaux, “Secure Neighbor Discovery: Is it Possible?” LCA-REPORT-2007-004, 2007
106
Securing neighbor discovery (cont’d)
• Location-aware nodes (securely) • Estimate neighbor distance in two ways
– Based on the time-of-flight (ToF)– Based on the location information (LOC)
• Compare the two distance estimates
B: Dist(LOCA, LOCB) = DistEstimate(tsendA, treceiveB)
B: “Add A toneighbor list”
A A, tsendA, LOCA, SigA(A, tsendA, LOCA) B
107
Securing neighbor discovery (cont’d)
• Secure Neighbor Discovery: exchange location information, and compare ToF and LOC based distance estimates
obstacle
A B
A, tsendA, LOCA, SigA(A, tsendA, LOCA)
A, tsendA, LOCA, SigA(A, tsendA, LOCA)
B: Dist(LOCA,LOCB) < DistEstimate(TSA,treceiveB)
B: “Do NOT add A to neighbor list”
M. Poturalski, P. Papadimitratos, and J-P. Hubaux, “Secure Neighbor Discovery: Is it Possible?” LCA-REPORT-2007-004, 2007
108
Summary
• Secure Neighbor Discovery– Solution for – Hard problem; solution is not easy to
implement in practice– Prerequisite for secure networking protocols
and system security– Additional reading
• Other methods, surveyed in [Poturalski-Papadimitratos-Hubaux] report: Using distance bounding, directional antennas, knowledge of topology, properties of the radio signal
• Centralized visual and statistical wormhole detection (Chapter 6)
112
Multi-hop routing (cont’d)
• Stage 0: neighbor discovery• Stage 1: route discovery
G
F
B
C E
D
A
H Route : Sequence of nodes (and edges); for simplicity: (A, G, E)
Sourcenode
Destinationnode
Intermediatenodes
113
Multi-hop routing (cont’d)
• Explicit route discovery– Fully, clearly expressed and readily observable
route returned by the routing protocol
G
F
B
C E
D
A
H
Route to E: (A, C, F, E)
Route to A: (E, F, C, A)
(Possibly)
114
Multi-hop routing (cont’d)
• Implicit route discovery– Distributed computation that returns a tuple of the
form (current node, relay node, destination node)
G
F
B
C E
D
A
H
Route to E: (A, C, E) Route to E:
(C, F, E)
Route to E: (F, E, E)
115
Multi-hop routing (cont’d)
• Basic route discovery– Explicit or implicit, providing only the structure of
the route
• Augmented route discovery– Need a function that assigns labels to links,
denoted as link metrics• For a link (V1,V2), metric m1,2
– Route metric: a function that is the aggregate of the route link metrics• For a route (V0, V1, …, Vn), route metric
g(m0,1, m1,2,…, mn-1,n)
116
Multi-hop routing (cont’d)
• Input: source, S, and destination, T, nodes• Output: an (S-T)-route (of n links) and
– The link labels (metrics) or– The route metric
,S T N∈ (Secure) Routing Protocol
Input
OutputAn (S,T)-route
and(i) Explicit:
(ii)Implicit:0,1 1,2 1,, , , n nm m m −…
0,1 1,2 1,( , , , )n ng m m m −…
117
Multi-hop routing (cont’d)
Source
Route Reply I
Destination
Route Reply II
Route Request
Route Error
• Example of route discovery– Reactive routing protocol
118
E
F
B
C H
G
A
D
RREP: “I am H”
RREQ: “A is looking for H”
Attacking route discovery (cont’d)
• Impersonation of the destination, for example, in any reactive routing protocol
119
Attacking route discovery (cont’d)
• Modification of the route links, for example, in DSR
E
F
B
C H
G
A
D
RREP :Route ={A, D, E, H}
RREP :Route ={A, D, H}
120
Attacking route discovery (cont’d)
• Abuse of the routing caching mechanism, for example, in DSR
E
F
B
C H
G
A
D
RREP :Route = {X, D, Y, H}
Route Cache:…
{A, D, Y}{A, D, Y, H}
…
121
Attacking route discovery
• Disrupting a link state routing protocol, for example, in OLSR
E
F
B
C H
G
A
D
Links(D, X)(D, H)(D, F)(D, Y)
122
Attacking route discovery (cont’d)
• Disrupting distance vector routing, for example, in AODV
E
F
B
C H
G
A
D
RREP: “Hop count = 3”
RREQ: “A is looking for H”
RREP: “Hop count = 2”
123
E
F
B
C H
G
A
D
Destination | Next Hop | DH | C | 2
Destination | Next Hop | D H | C | 2
Destination | Next Hop | DH | C | 2
D(C,H)=1
Attacking route discovery (cont’d)
• Disrupting distance vector routing, for example, in DSDV
124
Attacking route discovery (cont’d)
• Caution: none of the above-mentioned protocols (DSR, AODV, DSDV, OLSR) was designed with security in mind
• Many possible ways to attack the route discovery
• Outcome of attacks – Control communication
• Become part of utilized routes • Monopolize resources
– Disrupt communication• Degrade or deny
125
Secure route discovery requirements
• What do we need a secure routing protocol to do?
• Network model– Capture the system characteristics
• For example, dynamically changing topology
• Specification– Define the properties of any candidate secure
routing protocol independently of its functionality
126
Requirements• We are interested in protocols that discover routes with
the following two properties:
(1) Loop-freedom: an (S,T)-route is loop-free when it has no repetitions of nodes
(2) Freshness: an (S,T)-route is fresh with respect to a (t1,t2) interval if each of the route’s constituent links is up at some point during the (t1,t2)
• Loop-freedom and freshness are relevant for both explicit and implicit route discovery, and both basic and augmented protocols
P. Papadimitratos, Z.J. Haas, and J.-P. Hubaux, "How to Specify and How to Prove Correctness of Secure Routing Protocols for MANET," BroadNets’06
127
Secure Routing Protocol (SRP)
• Explicit basic route discovery• Observation
– It is hard to ‘know’ all nodes in the network, i.e., establish associations with all of them
– Often infeasible and very costly– Especially in ‘open’ networks
• SRP assumptions – Secure neighbor discovery– Hop-by-hop authentication of all control traffic– End nodes (source, destination) ‘know’ each other
• Can set up security associations
P. Papadimitratos and Z.J. Haas, "Secure Routing for Mobile Ad Hoc Networks," CNDS 2002
128
SRP (cont’d)
S V1 V3V2 T
Route Request (RREQ): S, T, QSEQ, QID, MAC(KS,T, S, T, QSEQ, QID)
1.S broadcasts RREQ;2.V1 broadcasts RREQ, {V1}; 3.V2 broadcasts RREQ, {V1, V2};4.V3 broadcasts RREQ, {V1, V2, V3};
1 2 3 4
129
SRP (cont’d)
Route Reply (RREP): QID, {T, V3, V2, V1, S},MAC(KS,T, QID, QSEQ, T, V3, V2, V1, S)
5. T → V3 : RREP;6. V3 → V2 : RREP;7. V2 → V1 : RREP;8. V1 → S : RREP;
S V1 V3V2 T
1 2 3 4
8 7 6 5
130
SRP (cont’d)
• Route requests verifiably reach destination– Intermediate node replies disabled– Aggressive caching of routing information disabled
• Route replies must trace back the paths traversed by route requests
• Intermediate nodes are not authenticated at the end nodes
• Dual route request identifier– QID: random, used by the intermediate nodes– QSEQ: sequence number, used by the destination– The adversary cannot launch a “sequence number”
attack
131
SRP (cont’d)
• Crucial to operate on top a secure neighbor discovery protocol
• Neighbor Lookup Protocol (NLP)– Secure neighbor discovery– Establish security associations between neighbors– Identify control traffic injected by each neighbor– Prevent attacks that misuse network addresses
• IP spoofing• Use of multiple identities• MAC spoofing
– DoS protection• Efficient mechanisms to discard spurious/
corrupted traffic at intermediate nodes– Replies relayed only if neighbors had previously
forwarded the corresponding request
132
SRP (cont’d)
• Routes discovered by SRP in the presence of independent adversaries are fresh – t1 is the point in time at which S transmitted a
RREQ for T, and t2 is the point at which S received the corresponding RREP
• In the presence of colluding adversaries SRP discovers ‘weakly fresh’ routes– A sequence of links, in general different than
those in the discovered route were up at some point in (t1,t2)
133
Secure Link State Protocol (SLSP)
• Secure Neighbor Discovery – Correct nodes discover only actual neighbors
• Periodic Link State Update (LSU) advertisements– Nodes distribute their discovered neighbors within an
extended neighborhood, the zone– LSUs are signed
• Link state accepted iff reported by both incident nodes
• Nodes distribute their public key throughout the zone
• SLSP can adjust its scope with different zone radii
P. Papadimitratos and Z.J. Haas, "Secure Link State Routing for Mobile Ad Hoc Networks," WSAAN’03
134
SLSP (cont’d)
}Zones
Neighbordiscovery
Key distribution,Link state updates
• SLSP can adjust its scope, with different zone radii
• It can operate locally, combined with another global route discovery, or network-wide
135
SLSP (cont’d)
• Keep the LSU propagation within the zone– Use a hash chain mechanism– zone_radius = XR=hR(x0)– hops_traversed = X1=h(x0), TTL = R-1 – After i hops (i=R-TTL), relay packet if:
• i < R, and• hR-i(hops_traversed) == zone_radius
– hops_traversed = H(hops_traversed)• Same idea can be applied in reactive
routing, to perform an expanding ring search
136
Authenticating intermediate nodes
• Source knows all nodes in the network• All nodes know any source and destination
node (especially in the case of reactive protocols)
• Overall, all nodes know all nodes, or equivalently have security associations established before any route discovery
• Hard to achieve, yet what if? For example, in small or closed networks
137
Ariadne
• Secures DSR, adding authentication of RREQ and RREP messages by each intermediate node that relays and modifies them
• All-to-all security associations• Use of different cryptographic primitives
– Signatures, Message Authentication Codes, and TESLA
Y.-C. Hu, A. Perrig, and D. Johnson,”Ariadne: A secure on-demand routing protocol for ad hoc networks,” Wireless Networks, 2005
138
Ariadne (cont’d)
• Operation across a route (S, F1, F2, D) with MACs• If TESLA is used, the delayed authentication (for
key disclosure) becomes part of the route discovery delay
Protocol operation as in Fig. 7.6 (p.202) of SeCoWiNet book
139
EndairA
• All-to-all security associations, digital signatures• Novelty: intermediate nodes sign only the RREP• Withstands provably attacks and reduces overhead
with respect to Ariadne
Protocol operation as in Fig. 7.8 (p.206) of SeCoWiNet book
G. Acs, L. Buttyan, and I. Vajda, “Provably secure on-demand source routing in mobile ad hoc networks,” TMC, 2006
140
Augmented Discovery: Requirement
• Let be the actual link metric for each link of a discovered (S,T)-route and
the actual route metric• The metric estimated (by a protocol) for link
(Vi,Vi+1) is mi,i+1
(3) Accuracy: an (S,T)-route is accurate with respect to a route metric g and a constant Δgood ≥ 0 if:
• Accuracy is relevant only to augmented, explicit or implicit, route discovery
, 1i il M+ ∈
0,1 1,( , ..., )n ng l l −
0,1 1, 0,1 1,| ( ,..., ) ( ,..., ) |n n n n goodg m m g l l− −− < Δ
141
Quality-of-Service Aware Discovery
• QoS-SRP: Secure QoS-aware routing• Nodes estimate metrics for their incident links
• For link (Vi ,Vi+1), node Vi calculates and Vi+1 calculates
• For some ε > 0,• ε is a protocol-selectable and metric-specific
threshold that allows for metric calculation inaccuracies
• is the maximum metric calculation error by a correct node
, 1ii im +
1, 1 , 1
i ii i i im m ε++ +− <
1, 1
ii im ++
0δ ≥
P. Papadimitratos and Z.J. Haas, "Secure Route Discovery for QoS-Aware Routing in Ad Hoc Networks," Sarnoff ‘05
142
QoS-SRP
S V1 V3V2 T
1 2 3 4
Route Request (RREQ): S, T, QSEQ, QID, MAC(KS,T, S, T, QSEQ, QID)
1. S broadcasts RREQ;2. V1 broadcasts RREQ, {V1}, { }; 3. V2 broadcasts RREQ, {V1,V2}, { };4. V3 broadcasts RREQ, {V1, V2, V3}, { };
1,1Sm
1 2,1 1,2,Sm m
1 2 3,1 1,2 2,3, ,Sm m m
143
QoS-SRP (cont’d)
S V1 V3V2 T
1 2 3 4
8 7 6 5
Route Reply (RREP):QID, {T, V3, V2, V1, S}, { }, MAC (KS,T, QSEQ, QID, T, V3, …, V1, S, )
5. T → V3 : RREP;6. V3 → V2 : RREP;7. V2 → V1 : RREP;8. V1 → S : RREP;
3 2 13, 2,3 1,2 ,1, , ,T
T Sm m m m1
3, 0,1, ...,TTm m
144
QoS-SRP (cont’d)
• Metric types
• ,
• If ,
can be written as
where
• ,
• ,
( )1
1 10,1 1, , 1
0,. ,
nn i
add n n i ii
g m m m−
+− +
=
=∑…
1, 1 0i
i im ++ > ( )
11 10,1 1, , 1
0
,. ,n
n in n i i
i
g m m m−
+− +
=
=∏…
( )10 ,1 1,, , n
add n ng m m −…
1 1, 1 , 1log( ), for 0 1i i
i i i im m i n+ ++ += ≤ ≤ −
( ) { }1 1max 0,1 1, , 10 1
,. , max n in n i ii n
g m m m +− +≤ ≤ −
=…mingoodΔ ( ) { }1 1
min 0,1 1, , 10 1,. , min n i
n n i ii ng m m m +
− +≤ ≤ −=…
maxgoodΔ
addgoodΔ
145
QoS-SRP (cont’d)
• Routes discovered by SRP in the presence of independent adversaries are accurate, with respect to (i) gadd and (ii) gmax and , and (iii) gminand , with n the number of route links, ε > 0 the maximum allowable difference between and , and the maximum error for a metric calculation by a correct node.
, 1ii im +
1, 1
ii im ++ 0δ ≥
2addgood n nε δΔ = +
maxgood nε δΔ = +
mingood nε δΔ = +
146
Attacking route discovery (cont’d)
• Adversary acting as a relay, ‘creating’ Byzantine links• Secure neighbor discovery and hop-by-hop
authentication can defeat this attack
E
F
B
C H
F
A
DRREQ: H, {A}
RREQ: H, {A} RREQ: H, {A, E}
RREP:Route={A, E, H}
147
Attacking Routing – Revisited (cont’d)
• Multiple Colluding Attackers– M1 and M3 are seemingly correct to their
neighbors, but they ‘omit’ protocol functionality when handling packets from M2
– Example: M2 relays RREQ and RREP packets without appearing in the route discovery
V’VS M1 M2 M3
T
148
Attacking Routing - Revisited
• Tunneling Attack– Two colluding attackers: M1, M2
– M1 encapsulates control traffic and forwards to M2 and vice versa
– Attackers seemingly follow the protocol with respect to their neighbors
ST
M1
M2P. Papadimitratos and Z.J. Haas, "Secure Routing for Mobile Ad Hoc Networks," CNDS 2002
149
Summary• Route discovery is vulnerable• Secure route discovery specification
– Loop freedom– Freshness– Accuracy
• Protocols relying on different trust assumptions• Securing basic and augmented route discovery
in open, dynamic networks• Colluding adversarial nodes can subvert any
route discovery protocol; ‘tunneling attack’• Additional reading
– Chapter 7: More secure routing protocols, including sensor network protocols
151
Problem statement
• Goal:– Reliable and low-delay data delivery in
the presence of attackers that disrupt the data communication
• Solution:– Detect and avoid compromised and
failing routes– Tolerate malicious and benign faults
• In general, hard to distinguish in highly dynamic networking environments
155
Data Communication (cont’d)
• How can an attacker be part of a route?– Make the route appear ‘preferable’ (shorter in hops,
delay, or any other metric)– Other routing protocol-specific attacks (e.g., ‘rushing’)– Do nothing that disrupts the secure route discovery
• Consider– An ideal secure routing protocol, ensuring loop-free,
fresh, and accurate routes against any possible attack– All nodes on the discovered route authenticated
• Still, the attacker can deny communication, dropping packets
• Worse even, the attacker can choose to hit when it hurts the most
156
Data Communication (cont’d)
• What is the impact of the adversary that ‘lies low’ and disrupts only the data communication?
Attacker Strength
Rel
iabi
lity
50% of the network nodes attacking
35% message delivery
100%
158
…
1
2
m-1
m
…3
1
2
n
n-2
n-3
Introduce redundancy
to the original message
=
Original message
Securing Data Communication (cont’d)
• Disperse data
159
…
1
3
n-2
n
…
3
1
2
n
n-2
n-3
Reconstruct message
if any m-out-of-n pieces are intact
=
Securing Data Communication (cont’d)
• Disperse data
160
G
F
B
C E
D
H
Sending n=3
E needs m=2
A
Received m pieces!
Securing Data Communication (cont’d)
• Transmit simultaneously across the routes
161
H G
F
B
C E
D
A
Route 1Route 2
Route 3
Tell A which pieces were
intact
• Get feedback
Securing Data Communication (cont’d)
162
Secure Routing OnlySecure Routing + Secure Data Communication
Attacker Strength
Rel
iabi
lity
50% of the network nodes are attacking
35% message delivery
93% message deliverywithout
retransmissions
Securing Data Communication (cont’d)
• Reliable and Real-Time Communication in Hostile Environments
163
RedundancyD
elay
1 3.5
1.2 s
0.4 s
Average delay for
100%message delivery
Redundancy
Rel
iabi
lity
1 3.5
82%
93%
Redundancy Message delivery without
retransmissions
Bandwidth For
Security
Securing Data Communication (cont’d)
164
Securing Data Communication (cont’d)
• Secure Message Transmission (SMT) protocol– Dispersion of the transmitted data– Simultaneous usage of multiple node-disjoint routes– Data integrity and origin authentication– End-to-end secure and robust feedback– Adaptation to the network conditions
• Secure Single Path (SSP) protocol– Discovery and utilization of a single route– End–to–end security and feedback
165
SMT Operation• The Active Path Set (APS)
– Maintain a (partial) view of the network topology– Construct a set of node disjoint routes (per destination)
– Routes remain in the APS until deemed non-operational
• Multi-path operation– Select the APS routes to transmit a dispersed message– Route selection attributes
• Path rating• Probability of path survival• Overall probability of successful message delivery
– Assign each message piece to one of the selected routes
166
SMT Operation (cont’d)
TimeSource
Destination
DispersedMessage
DispersedACK
Re-transmit
ACK
•Example: Transmission of a single message
timer timer
167
SMT Operation (cont’d)
• Secure and robust end-to-end feedback– Dispersed and returned over multiple routes – Informs on the successfully received pieces– Allows the correlation of successfully received
pieces with data routes– Provides “safe” information for the adaptation
of the protocol operation
168
SMT Operation (cont’d)
• Adapt to the network conditions– Detect non-operational routes– Switch to alternate (new) routes– Adapt the protocol configuration
• Number of routes• Transmission redundancy• Route selection• Additional route discovery
169
SMT Operation (cont’d)
• Path rating mechanism– Each route is associated to a rating
• Update rs for each transmission across the route• For each delivered piece, rs is increased by a
constant β• For each lost piece, rs is decreased by a constant α
• The route is discarded when its rating reaches rsthr
max
if a piece is lost
if a piece is received
max{ ( 1) , }, ( )
min{ ( 1) , },
thrs s
ss s
r i rr i
r i rαβ
⎧ − −= ⎨
− +⎩
max,thrs S Sr r r⎡ ⎤∈⎣ ⎦
170
SMT Operation (cont’d)
• Robustness to arbitrary attack patterns– Bounded fraction of data the adversary can
drop (Bandwidth Loss (BWL)) before the compromised route is detected
– Non-operational routes are promptly discarded– Route re-instatement after transient data loss
BWL βα β
≤+
P. Papadimitratos and Z.J. Haas, “Secure Message Transmission in Mobile Ad Hoc Networks,” ACM WiSe’03 P. Papadimitratos and Z.J. Haas. "Secure Message Transmission inMobile Ad Hoc Networks," Ad Hoc Networks, 2003
171
SMT Operation (cont’d)
• What is the appropriate choice for α,β ?– The attack pattern is not known in advance– The faster a non-operational route is
discarded the better – Not discarding a route after a transient packet
loss is preferable
• One criterion– Min-Max Regret
P. Papadimitratos and Z.J. Haas, " Secure Data Communication in Mobile Ad Hoc Networks," JSAC, 2006
174
SMT Operation (cont’d)
• Is the route rating sufficient to maintain reliable communication? What about mobility?
• The higher the route age is, the more likely it is to break • t : current route age of the i-th route in APS• pi(t ) : probability of survival of the route during a piece
transmission (delay d)• Estimate this from route lifetime samples (periods of time
from the discovery till the route removal from the APS
1
1
1 , if
ˆ ( ) , for such that:
1 , if
i j j
D
S t dS
S jp t j t dS
t dS
τ
τ τ
τ
+
−⎧ + <⎪⎪
−⎪= ≤ + <⎨⎪⎪ + >⎪⎩
175
SMT Operation (cont’d)
• Example of the estimated probability of path survival , based on collected data
• FA: Fraction of adversaries present in the network
176
SMT Operation (cont’d)
• Determine the appropriate message dispersion– To achieve the sought end-to-end reliability,
PGOAL, while minimizing • The transmission redundancy: PGOAL- rmin
• The number of utilized paths: PGOAL-Nmin
– To achieve a redundancy goal while maximizing the end-to-end reliability: rGOAL
177
Performance Evaluation
Nodes 50
Fraction of Adversaries
10%, 20%, 30%, 40%, or 50% of the network
Measurements 50 randomly seeded runs for each point
Security Bindings Single destination per source
Coverage Area 1000m-by-1000m
Simulated time 300 sec
Mobility Random waypoint; Pause times: 0, 20, 40, 60, 100, 150, 200, 250 seconds
Load 3, 7, 15, 20 CBR flows, Data payload: 512 BytesRates: 4, 10, 15, 20, 25, and 30 packets/sec
PHY/MAC 802.11, DCF, 2 and 5.5 Mbps, 300m
Tool OPNET
178
Performance Evaluation (cont’d)
• Secure Message Transmission (SMT) protocol
• Secure Single Path (SSP) protocol• Secure route discovery for both protocols
– Explicit, basic• Reactive, Proactive• SRP, SLSP
• Attack pattern– Full compliance with the route discovery– Discarding in–transit data packets
179
Performance Evaluation (cont’d)
SMT-LS: SMT with a Link State Protocol
Message Delivery Fraction Message Delay
184
Performance Evaluation (cont’d)
Transmission Redundancy
Average number of sent pieces (N) Average number of required pieces (M)
185
Performance Evaluation (cont’d)
Impact of mobility; SMT-RRD
Message Delivery Fraction Message Delay
186
Performance Evaluation (cont’d)
Impact of Load and interaction with TCP
Throughput – no flow control Throughput - SMT-RRD with TCP
187
Performance Evaluation (cont’d)
Impact of Load and SMT interaction with TCP
Message delay – no flow control Message delay - SMT-RRD with TCP
188
Summary
• Secure data communication is critical– Secure routing protocols are vulnerable– As long as attackers can place themselves on utilized
routes, they can degrade or deny communication– The only answer is to assess whether data are
delivered, and avoid non-operational routes
• Secure data communication is practical– Low-delay, low-jitter, and highly reliable; essentially,
real-time– Flexible– Low overhead– End-to-end– Effective against any data-dropping pattern
189
Privacy Enhancing Technologies for Vehicular Communication (VC) Systems
© 2007 P. Papadimitratos
190
VC System (cont’d)
• High rate broadcast communication• VANET-only (e.g., safety) and TCP/IP
communication
Safety Applications
IPv6
TCP / UDPC2C-CC Position Based
Routing
IEEE 802.11p MAC and PHY
C2C-CC MAC
Wave Short
Message Protocol (WSMP)
General Applications
IEEE 1609.4
191
Security for VC
• Focus: Communication• Main objectives
– Identity and Cryptographic Key Management– Privacy Enhancing Technologies (PET)– Secure Communication
• Requirements– Authentication, integrity, non-repudiation, access
control, confidentiality, availability– Privacy– Liability identification
P. Papadimitratos, V. Gligor, J.-P. Hubaux, “Securing Vehicular Communications – Assumptions, Requirements and Principles,”ESCAR 2006
192
Secure VC• Authorities
– Trusted entities – Issuing and managing identities and credentials
• Network nodes– Vehicles
• Public • Private
– Road-side units• Users
194
Secure VC (cont’d)
Graphic courtesy of DC
Wireless Communication
Module
Central Processing Module
Unique Identity
Credentials and
Cryptographic Keys
Abstract view of a vehicle in a
(secure) vehicular communications
system
Sensory Inputs Module
195
Secure VC (cont’d)
• Node Identity– Unique identity V– Integration of pre-VC and VC-specific
identifiers
• Node Keys – Public / private key pair KV, kV
• Node Credentials – Certificate CertX{KV,AV}– AV: attributes of node V
• Long-term identification
196
Secure VC (cont’d)
• Secure Communication– Single- and Multi-hop– Vehicle to vehicle – Vehicle to infrastructure
• Digital signatures more appropriate tool– Any-to-any communication; e.g., broadcast, geo-cast– High mobility
• Relatively simple networking protocols ‘shift’ the security focus to the application
197
Secure VC (cont’d)
• Secure Communication (cont’d)
Warning:Accident at (x,y,z)
!!
Payload
Location: (xV,yV,zV)
CertX{KV,AV}
Signature with kV
Vehicle V
Time: tV
Vehicle U
Warning:Accident at (x,y,z)
198
Problem statement
• Frequent, high-rate vehicle-to vehicle and vehicle-to-infrastructure communication– Periodic, triggered, dependent on network
characteristics (e.g., density)– Example: a vehicle transmits safety messages
every 100 to 300 milliseconds– Safety messages include vehicle-specific
information; e.g., its coordinates• Communication cannot be regulated or
controlled by the node/user – Safety messaging will be essentially an
‘always-on’ application
199
Problem statement (cont’d)
• Vehicle-originating wireless transmissions are particularly easy to eavesdrop– Data link very similar to a widely adopted
technology: IEEE 802.11p– Very large and increasing numbers of 802.11
access points already deployed– Road-side infrastructure deployed for other
services could be subverted into acting as an eavesdropper
• Linking messages to the transmitting vehicle and inferring private information about its passengers
201
Problem statement (cont’d)
• What are we after? – At least the same degree of privacy achieved nowadays,
before the advent of vehicular communications– Combination of strong security and privacy-enhancing
technologies– Ideally, anonymous and authentic communications, but:
• High processing and communication overhead• Often, messages from the same vehicle should be linkable
– Requirement: messages generated by a given vehicle can be linked at most over a protocol-selectable period of time
• The shorter this period, the harder to track a vehicle becomes
202
• Pseudonym: Remove all identifying information from certificate
• Equip vehicles with multiple pseudonyms – Alternate among pseudonyms over time (and space)– Sign message with the private key corresponding to
pseudonym– Append current pseudonym to signed message
PET for VC (cont’d)
PSNYM_1
PSNYM_2
PSNYM_3 PSNYM_1
PSNYM_2 PSNYM_1
PSNYM_2
PSNYM_3
203
PET for VC (cont’d)
• PET system setup
PSNYM_1, …, PSNYM_k
Authority XLong-term
Identification
Vehicle V
Authority APseudonym
Provider
Set of pseudonyms for V
204
PET for VC (cont’d)
• PET system setup (cont’d) • Multiple pseudonym providers
Organization 1 Organization 2 Organization n...
V-PNYM-1 V-PNYM-2 V-PNYM-n
...
Vehicle V
205
PET for VC (cont’d)
• Pseudonym format
• Supplying vehicles with pseudonyms– Sufficient in number– Periodic ‘refills’
PSNYM-Provider ID
Public Key Ki
PSNYM-Provider Signature
PSNYM Lifetime
PSNYM_1PSNYM_2
PSNYM_3PSNYM_4PSNYM_k1
PSNYM_1PSNYM_2
PSNYM_3PSNYM_4PSNYM_k2
PSNYM_1PSNYM_2
PSNYM_3PSNYM_4PSNYM_k3
Time
206
Secure VC Building Blocks (cont’d)
• Pseudonym Change MechanismPSNYM_1, …, PSNYM_k
Vehicle V
Pseudonym Selection Process
Inputs:
Vehicle Location
Vehicle Clock
Recipient(s) /
(Verifier(s))
Output:
Use PSNYM_i for period
[ti,ti+1]
Inputs:
Local (vehicle) and
Authority Privacy Policies
PSNYM_1, …, PSNYM_k
One pseudonym per day (?) One per transaction (?)
207
PET for VC (cont’d)
• Other vehicle network identifiers: e.g., IP and MAC addresses
• Change addresses along with pseudonyms• Maintain addresses only when necessary, but
encapsulate
PSNYM_i
APAAPB
APC
ServerS
IPA
PSNYM_j
IPB
IPS
PSNYM_k
IPc
IPS
208
PET for VC (cont’d)
• Credentials Management
Roadside Unit
‘Re-filling’ with or obtaining new
credentials
‘Re-filling’ with or obtaining new
credentials
Roadside Unit
Wire-lineConnections
209
PET for VC (cont’d)
• Pseudonym ResolutionPseudonymous CommunicationTranscript
“Vehicle V generated the transcript”
Authority O
210
PET for VC (cont’d)
• Challenge– Managing a pseudonymous authentication system is
cumbersome• Preload large numbers of pseudonyms or obtain them on-
the-fly• Costly computations at the side of the pseudonym provider• Costly wireless communication to obtain pseudonyms• Need reliable access to the pseudonym provider
• Solution– On-board generation of pseudonyms– G. Calandriello, P. Papadimitratos, A. Lloy, and J.-P.
Hubaux, "Efficient and Robust Pseudonymous Authentication in VANET," VANET 2007
211
Summary
• Security and privacy-enhancing mechanisms are a prerequisite for the VC systems deployment
• Securing VC systems is a complex yet ‘real’ problem that attracts the attention of the community
• Opportunity: Awareness and joint efforts in industry and academia
212
Acknowledgements
• Thanks to Marcin Poturalski and George Theodorakopoulos for their feedback on Part 2 of this tutorial
214
Security and Cooperation in Wireless Networks
Part 3: Thwarting Selfish Behavior
Jean-Pierre [email protected]
© 2007 Levente Buttyán and Jean-Pierre Hubaux
Security and Cooperation in Wireless Networks
http://secowinet.epfl.ch/
Appendix B: A tutorial on game theory for wireless networks
static games;dynamic games;repeated games;strict and weak dominance;Nash equilibrium;Pareto optimality;Subgame perfection;…
216Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Chapter outline
B.1 IntroductionB.2 Static gamesB.3 Dynamic gamesB.4 Repeated games
217Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Brief introduction to Game Theory
Discipline aiming at modeling situations in which actors have to make decisions which have mutual, possibly conflicting, consequencesClassical applications: economics, but also politics and biologyExample: should a company invest in a new plant, or enter a new market, considering that the competition may make similar moves?Most widespread kind of game: non-cooperative (meaning that the players do not attempt to find an agreement about their possible moves)
B.1 Introduction
218Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Classification of games
Non-cooperative Cooperative
Static Dynamic (repeated)
Strategic-form Extensive-form
Perfect information Imperfect information
Complete information Incomplete information
Cooperative
Imperfect information
Incomplete information
Perfect info: each player knows the identity of other players and, for eachof them, the payoff resulting of each strategy.
Complete info: each player can observe the action of each other player.
B.1 Introduction
219Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Cooperation in self-organized wireless networks
S1
S2
D1D2
Usually, the devices are assumed to be cooperative. But what if they are not?
B.1 Introduction
220Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Chapter outline
B.1 IntroductionB.2 Static gamesB.3 Dynamic gamesB.4 Repeated games
221Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Example 1: The Forwarder’s Dilemma
?Blue Green
?
B.2 Static games
222Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
From a problem to a game
users controlling the devices are rational = try to maximize their benefitgame formulation: G = (P,S,U)– P: set of players– S: set of strategy functions– U: set of utility functions
strategic-form representation
• Reward for packet reaching the destination: 1• Cost of packet forwarding: c (0 < c << 1)
(1-c, 1-c) (-c, 1)(1, -c) (0, 0)
BlueGreen
Forward
Drop
Forward Drop
B.2 Static games
223Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Solving the Forwarder’s Dilemma (1/2)
' '( , ) ( , ), ,i i i i i i i i i iu s s u s s s S s S− − − −< ∀ ∈ ∀ ∈
iu U∈i is S− −∈
where:
Strict dominance: strictly best strategy, for any strategy of the other player(s)
utility function of player i
strategies of all players except player i
In Example 1, strategy Drop strictly dominates strategy Forward
(1-c, 1-c) (-c, 1)(1, -c) (0, 0)
BlueGreen
Forward
Drop
Forward Drop
Strategy strictly dominates ifis
B.2 Static games
224Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Solving the Forwarder’s Dilemma (2/2)
Solution by iterative strict dominance:
(1-c, 1-c) (-c, 1)(1, -c) (0, 0)
BlueGreen
Forward
Drop
Forward Drop
Result: Tragedy of the commons ! (Hardin, 1968)
Drop strictly dominates ForwardDilemma
Forward would result in a better outcomeBUT }
B.2 Static games
225Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Example 2: The Joint Packet Forwarding Game
?Blue GreenSource Dest
?
No strictly dominated strategies !
• Reward for packet reaching the destination: 1• Cost of packet forwarding: c (0 < c << 1)
(1-c, 1-c) (-c, 0)(0, 0) (0, 0)
BlueGreen
ForwardDrop
Forward Drop
B.2 Static games
226Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Weak dominance
?Blue GreenSource Dest
?
(1-c, 1-c) (-c, 0)(0, 0) (0, 0)
'( , ) ( , ),i i i i i i i iu s s u s s s S− − − −≤ ∀ ∈
Weak dominance: strictly better strategy for at least one opponent strategy
with strict inequality for at least one s-i
Iterative weak dominanceBlue
Green
Drop
Forward
Forward
Drop
BUT
The result of the iterative weak dominance is not unique in general !
Strategy s’i is weakly dominated by strategy si if
B.2 Static games
227Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Nash equilibrium (1/2)
Nash Equilibrium: no player can increase its utility by deviating unilaterally
(1-c, 1-c) (-c, 1)(1, -c) (0, 0)
BlueGreen
Forward
Drop
Forward DropE1: The Forwarder’s Dilemma
E2: The Joint Packet Forwarding game (1-c, 1-c) (-c, 0)
(0, 0) (0, 0)
BlueGreen
Forward
Drop
Forward Drop
B.2 Static games
228Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Nash equilibrium (2/2)
* * *( , ) ( , ),i i i i i i i iu s s u s s s S− −≥ ∀ ∈
iu U∈i is S∈
where: utility function of player istrategy of player i
( ) arg max ( , )i i
i i i i is S
b s u s s− −∈
=
The best response of player i to the profile of strategies s-i is a strategy si such that:
Nash Equilibrium = Mutual best responses
Caution! Many games have more than one Nash equilibrium
Strategy profile s* constitutes a Nash equilibrium if, for each player i,
B.2 Static games
229Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Example 3: The Multiple Access game
Reward for successfultransmission: 1
Cost of transmission: c(0 < c << 1)
There is no strictly dominating strategy
(0, 0) (0, 1-c)(1-c, 0) (-c, -c)
BlueGreen
Quiet
Transmit
Quiet Transmit
There are two Nash equilibria
Time-division channel
B.2 Static games
230Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Mixed strategy Nash equilibrium
objectives– Blue: choose p to maximize ublue
– Green: choose q to maximize ugreen
(1 )(1 ) (1 )blueu p q c pqc p c q= − − − = − −(1 )greenu q c p= − −
1 , 1p c q c= − = −
is a Nash equilibrium
p: probability of transmit for Blueq: probability of transmit for Green
B.2 Static games
231Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Example 4: The Jamming game
transmitter:• reward for successfultransmission: 1• loss for jammed transmission: -1
jammer:• reward for successfuljamming: 1• loss for missed jamming: -1
two channels: C1 and C2
(-1, 1) (1, -1)(1, -1) (-1, 1)
There is no pure-strategy Nash equilibrium
BlueGreen
C2
C1
C2
C1
transmitter
jammer
1 1,2 2
p q= = is a Nash equilibrium
p: probability of transmit on C1 for Blueq: probability of transmit on C1 for Green
B.2 Static games
232Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Theorem by Nash, 1950
Theorem: Every finite strategic-form game has a mixed-strategy Nash equilibrium.
B.2 Static games
233Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Efficiency of Nash equilibria
E2: The Joint Packet Forwarding game
(1-c, 1-c) (-c, 0)(0, 0) (0, 0)
BlueGreen
Forward
Drop
Forward Drop
How to choose between several Nash equilibria ?Pareto-optimality: A strategy profile is Pareto-optimal if it is not possible to increase the payoff of any player without decreasing the payoff of another player.
B.2 Static games
234Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
How to study Nash equilibria ?
Properties of Nash equilibria to investigate:existenceuniquenessefficiency (Pareto-optimality)emergence (dynamic games, agreements)
B.2 Static games
235Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Chapter outline
B.1 IntroductionB.2 Static gamesB.3 Dynamic gamesB.4 Repeated games
236Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Extensive-form games
usually to model sequential decisionsgame represented by a treeExample 3 modified: the Sequential Multiple Access game:Blue plays first, then Green plays.
Green
BlueT Q
T Q T Q
(-c,-c) (1-c,0) (0,1-c) (0,0)
Reward for successfultransmission: 1
Cost of transmission: c(0 < c << 1)
Green
Time-division channel
B.3 Dynamic games
237Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Strategies in dynamic games
The strategy defines the moves for a player for every node in the game, even for those nodes that are not reached if the strategy is played.
Green
BlueT Q
T Q T Q
(1-c,0) (0,1-c) (0,0)
Green
(-c,-c)
strategies for Blue: T, Q
strategies for Green: TT, TQ, QT and QQ
If they have to decide independently: three Nash equilibria(T,QT), (T,QQ) and (Q,TT)
B.3 Dynamic games
238Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Backward induction
Solve the game by reducing from the final stageEliminates Nash equilibria that are increadible threats
Green
BlueT Q
T Q T Q
(1-c,0) (0,1-c) (0,0)
Green
(-c,-c)
incredible threat: (Q, TT)
B.3 Dynamic games
239Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Subgame perfection
Extends the notion of Nash equilibrium
Green
BlueT Q
T Q T Q
(1-c,0) (0,1-c) (0,0)
Green
Subgame perfect equilibria: (T, QT) and (T, QT) (-c,-c)
One-deviation property: A strategy si conforms to the one-deviation property if there does not exist any node of the tree, in which a playeri can gain by deviating from si and apply it otherwise.
Subgame perfect equilibrium: A strategy profile s constitutes a subgame perfect equilibrium if the one-deviation property holds for every strategy si in s.
Finding subgame perfect equilibria using backward induction
B.3 Dynamic games
240Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Chapter outline
B.1 IntroductionB.2 Static gamesB.3 Dynamic gamesB.4 Repeated games
241Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Repeated games
repeated interaction between the players (in stages)move: decision in one interactionstrategy: defines how to choose the next move, given the previous moveshistory: the ordered set of moves in previous stages– most prominent games are history-1 games (players consider only
the previous stage)
initial move: the first move with no historyfinite-horizon vs. infinite-horizon gamesstages denoted by t (or k)
B.4 Repeated games
242Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Utilities: Objectives in the repeated game
finite-horizon vs. infinite-horizon gamesmyopic vs. long-sighted repeated game
( )1i iu u t= +
( )0
T
i it
u u t=
=∑
( )0
i it
u u t∞
=
=∑
myopic:
long-sighted finite:
long-sighted infinite:
utility with discounting: ( )0
ti i
tu u t ω
∞
=
= ⋅∑0 1ω< ≤ is the discounting factor
B.4 Repeated games
243Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Strategies in the repeated game
usually, history-1 strategies, based on different inputs:
– others’ behavior:
– others’ and own behavior:
– utility:
( ) ( )1i i im t s m t−⎡ ⎤+ = ⎣ ⎦( ) ( ) ( )1 ,i i i im t s m t m t−⎡ ⎤+ = ⎣ ⎦
( ) ( )1i i im t s u t⎡ ⎤+ = ⎣ ⎦
Example strategies in the Forwarder’s Dilemma:
Blue (t) initial move
F D strategy name
F
F
D
F
F
AllC
F
F
D
D
Tit-For-Tat (TFT)
D AllD
FD Anti-TFT
Green (t+1)
B.4 Repeated games
244Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
The Repeated Forwarder’s Dilemma
(1-c, 1-c) (-c, 1)(1, -c) (0, 0)
BlueGreen
Drop
Forward
Forward Drop
?Blue Green
?
stage payoff
B.4 Repeated games
245Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Analysis of the Repeated Forwarder’s Dilemma (1/3)
Blue strategy Green strategy
AllD AllD
AllD TFT
AllD AllC
AllC AllC
AllC TFT
TFT TFT
infinite game with discounting: ( )0
ti i
tu u t ω
∞
=
= ⋅∑
Blue utility Green utility
0 0
1 -c
(1-c)/(1-ω) (1-c)/(1-ω)
1/(1-ω) -c/(1-ω)
(1-c)/(1-ω) (1-c)/(1-ω)
(1-c)/(1-ω) (1-c)/(1-ω)
B.4 Repeated games
246Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Analysis of the Repeated Forwarder’s Dilemma (2/3)
Blue strategy Green strategy
AllD AllD
AllD TFT
AllD AllC
AllC AllC
AllC TFT
TFT TFT
Blue utility Green utility
0 0
1 -c
(1-c)/(1-ω) (1-c)/(1-ω)
1/(1-ω) -c/(1-ω)
(1-c)/(1-ω) (1-c)/(1-ω)
(1-c)/(1-ω) (1-c)/(1-ω)
AllC receives a high payoff with itself and TFT, butAllD exploits AllCAllD performs poor with itselfTFT performs well with AllC and itself, andTFT retaliates the defection of AllD
TFT is the best strategy if ω is high !B.4 Repeated games
247Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Analysis of the Repeated Forwarder’s Dilemma (3/3)
Theorem: In the Repeated Forwarder’s Dilemma, if both players play AllD, it is a Nash equilibrium.
Theorem: In the Repeated Forwarder’s Dilemma, both players playing TFT is a Nash equilibrium as well.
Blue strategy Green strategy Blue utility Green utility
AllD
TFT
0 0AllD
TFT (1-c)/(1-ω) (1-c)/(1-ω)
The Nash equilibrium sBlue = TFT and sGreen = TFT is Pareto-optimal (but sBlue = AllD and sGreen = AllD is not) !
B.4 Repeated games
248Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Experiment: Tournament by Axelrod, 1984
any strategy can be submitted (history-X)strategies play the Repeated Prisoner’s Dilemma (Repeated Forwarder’s Dilemma) in pairsnumber of rounds is finite but unknown
TFT was the winnersecond round: TFT was the winner again
B.4 Repeated games
249Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Discussion on game theory
RationalityUtility function and costPricing and mechanism design (to promote desirable solutions)Infinite-horizon games and discountingReputationCooperative gamesImperfect / incomplete information
B.5 Discussion
250Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Who is malicious? Who is selfish?
Both security and game theory backgrounds are useful in many cases !! Both security and game theory backgrounds are useful in many cases !!
Harm everyone: viruses,…
Selective harm: DoS,… Spammer
Cyber-gangster:phishing attacks,trojan horses,…
Big brother
Greedy operator
Selfish mobile station
B.5 Discussion
251Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Conclusions
Game theory can help modeling greedy behavior in wireless networksDiscipline still in its infancyAlternative solutions– Ignore the problem– Build protocols in tamper-resistant hardware
© 2007 Levente Buttyán and Jean-Pierre Hubaux
Security and Cooperation in Wireless Networks
http://secowinet.epfl.ch/
Chapter 10: Selfishness in packet forwarding
253Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Introduction
the operation of multi-hop wireless networks requires the nodes to forward data packets on behalf of other nodeshowever, such cooperative behavior has no direct benefit for the forwarding node, and it consumes valuable resources (battery)hence, the nodes may tend to behave selfishly and deny cooperationif many nodes defect, then the operation of the entire network is jeopardized questions:– What are the conditions for the emergence of cooperation in packet
forwarding?– Can it emerge spontaneously or should it be stimulated by some
external mechanism?
254Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Modeling packet forwarding as a game
time0time slot:
1 t
Strategy:cooperationlevel
pC(0) pC(1) pC(t)
Players: nodes
Payoff (of node i): proportion of packets sent by node i reaching their destination
10.1 Game theoretic model of packet forwarding
255Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Cost function
( ) ( )trcrTtr jsf j,ˆ)(, τη ⋅⋅−=
Cost for forwarder fj :
where:Ts(r) – traffic sent by source s on route rc – unit cost of forwarding
Example :
( ) )()()(,ˆ},{
tptptptr CECEk
fC k⋅== ∏
∈
τ
( ) ( )ˆ, ( ) ,C A jr t T r c r tη τ= − ⋅ ⋅
A E C D
TA pE(t) pC(t)r (A→D):
10.1 Game theoretic model of packet forwarding
( ) )(,ˆ1
tptrj
kfj k∏
=
=τ
Normalized throughput at forwarder fj :
where:r – route on which fk is a forwardert – time slotfk – forwarders on route rpfk – cooperation level of forwarder fk
256Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Utility function
( ) )()()(, tptprTtr CEA ⋅⋅=τ
( ) )()(,1
tprTtrl
kfs k∏
=
⋅=τ
Experienced throughput :
where: s – sourcer – route on which s is a sourcet – time slotfk – forwarders for spfk – cooperation level of forwarder fk
A E C D
TA pE(t) pC(t)r (A→D):
Example :
Utility function :
10.1 Game theoretic model of packet forwarding
257Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Total payoff
( ) ( )( ) ( )( ) ( )
, ,i i
i i iq S t r F t
t u q t r tπ τ η∈ ∈
= +∑ ∑
The goal of each node is to maximize its total payoff over the game
Payoff = Utility - Cost
where: Si(t) – set of routes on which i is a sourceFi(t) – set of routes on which i is a forwarder
( ) t
tii t ωππ ⋅=∑
∞
=0max where: ω – discounting factor
t – time
time0time slot: 1 t
Payoff: πA(0) πA(1).ω πA(t).ωt
Example :
10.1 Game theoretic model of packet forwarding
258Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Representation of the nodes as players
Node i is playing against the rest of the network (represented by the box denoted by A-i )
yi
xi
A-i σi ))]1,(([)( )1( −∈−= tSrii itrtp τσStrategy function for node i:
where:τ (r,t) – experienced throughputSi – set of routes on which i is a source
10.1 Game theoretic model of packet forwarding
259Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Examples of strategies
1)( =ii yσ
iii xy =)(σ
0)( =ii yσ
StrategyFunctionInitial
cooperation level
AllD (always defect)
AllC (always cooperate)
TFT (Tit-For-Tat)
0
1
1
non-reactive strategies: the output of the strategy functionis independent of the input (example: AllD and AllC)
reactive strategies: the output of the strategy functiondepends on the input (example: TFT)
where yi stands for the input
iii yy =)(σ
10.1 Game theoretic model of packet forwarding
260Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Concept of dependency graph
dependency: the benefit of each source is dependent on the behavior of its forwarders
dependency loop
10.2 Meta-model
261Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Analytical Results (1/2)
0)( =IFσ
Theorem 1: If node i does not have any dependency loops, then its best strategy is AllD.
Theorem 2: If node i has only non-reactive dependency loops, then its best strategy is AllD.
Corollary 1: If every node plays AllD, it is a Nash-equilibrium. Corollary 1: If every node plays AllD, it is a Nash-equilibrium.
0)( =IEσ
node i
node playing a non-reactive strategy
other nodes
10.3 Analytical results
262Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Analytical results (2/2)
Corollary 2: If Theorem 3 holds for every node, it is a Nash-equilibrium.Corollary 2: If Theorem 3 holds for every node, it is a Nash-equilibrium.
Theorem 3 (simplified): Assuming that node i is a forwarder, its behavior will be cooperative only if it has a dependency loop with each of its sources
Theorem 3 (simplified): Assuming that node i is a forwarder, its behavior will be cooperative only if it has a dependency loop with each of its sources
Example in which Corollary 2 holds:
A B
C
A B
C
Network Dependency graph
10.3 Analytical results
263Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Classification of scenarios
D: Set of scenarios, in which every node playing AllD is a Nash equilibrium
C: Set of scenarios, in which a Nash equilibrium based on cooperation is not
excluded by Theorem 1
C2: Set of scenarios, in which cooperation is based on the conditions expressed in
Corollary 2
10.3 Analytical results
264Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Simulation settings
Number of nodes 100, 150, 200
Area type torus
Area size 1500x1500m, 1850x1850m, 2150x2150m
Radio range 200 m
Distribution of the nodes random uniform
Number of routes originating at each node
1-10
Route selection shortest path
Number of simulation runs 1000
10.4 Simulation results
265Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Simulation results
10.4 Simulation results
266Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Summary
Analytical results:– If everyone drops all packets, it is a Nash-equilibrium– In theory, given some conditions, a cooperative Nash-equilibrium
can exist ( i.e., each forwarder forwards all packets )
Simulation results: – In practice, the conditions for cooperative Nash-equilibria are very
restrictive : the likelihood that the conditions for cooperation hold for every node is extremely small
Consequences:– Cooperation cannot be taken for granted– Mechanisms that stimulate cooperation are necessary
• incentives based on virtual currency• reputation systems
10.5 Summary
© 2007 Levente Buttyán and Jean-Pierre Hubaux
Security and Cooperation in Wireless Networks
http://secowinet.epfl.ch/
Chapter 11: Wireless operators in shared spectrum
(multi-domain sensor networks)
border games in cellular networks
268Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Introduction
spectrum licenses do not regulate access over national bordersadjust pilot power to attract more users
Is there an incentive for operators to apply competitive pilot power control?
11.2 Border games in cellular networks
269Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
System model (1/2)
Network:cellular networks using CDMA– channels defined by orthogonal
codestwo operators: A and Bone base station eachpilot signal power control
Users:roaming usersusers uniformly distributedselect the best quality BSselection based signal-to-interference-plus-noise ratio (SINR)
11.2 Border games in cellular networks11.2.1 Model
270Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
System model (2/2)
0
pilotp i ivpilot
iv pilot pilotown other
G P gSINR
N I I⋅ ⋅
=⋅ + +W
i
pilotown iv iw
wI g Tς
∈
⎛ ⎞= ⋅ ⋅⎜ ⎟⎜ ⎟
⎝ ⎠∑M
i
pilotother jv j iw
j i wI g P Tη
≠ ∈
⎛ ⎞= ⋅ ⋅ +⎜ ⎟⎜ ⎟
⎝ ⎠∑ ∑
M
A Bv
PA
TAv
TAw
PB
TBw
0
trp iv ivtr
iv tr trown other
G T gSINR
N I I⋅ ⋅
=⋅ + +W
, i
pilotown iv i iw
w v wI g P Tς
≠ ∈
⎛ ⎞= ⋅ ⋅ +⎜ ⎟⎜ ⎟
⎝ ⎠∑
M
tr pilotother otherI I=
pilot signal SINR:
traffic signal SINR:
Pi – pilot power of i– processing gain for the pilot signalpilot
pG
ivg
0NW
– noise energy per symbol
ς
ivT
η
pilotownI
– channel gain between BS i and user v
– own-cell interference affecting the pilot signal
– own-cell interference factor– traffic power between BS i and user v
iM
– available bandwidth
– other-to-own-cell interference factor– set of users attached to BS i
11.2 Border games in cellular networks11.2.1 Model
271Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Game-theoretic model
Power Control Game, GPC– players → networks operators (BSs), A and B– strategy → pilot signal power, 0W < Pi < 10W, i = {A, B}– standard power, PS = 2W– payoff → profit, where is the expected income
serving user v– normalized payoff difference:
i
i vv
u θ∈
= ∑M
vθ
( ) ( )( )( )
max , ,
,i
S S Si i is
i S Si
u s P u P P
u P P
−Δ =
11.2 Border games in cellular networks11.2.2 Power control game
272Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Simulation settings
11.2 Border games in cellular networks11.2.2 Power control game
273Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Is there a game?
only A is strategic (B uses PB = PS)10 data users path loss exponent, α = 2
Δi
11.2 Border games in cellular networks11.2.2 Power control game
274Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
When both operators are strategic
10 data userspath loss exponent, α = 4
11.2 Border games in cellular networks11.2.2 Power control game
275Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Nash equilibria
10 data users 100 data users
11.2 Border games in cellular networks11.2.2 Power control game
276Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Efficiency (1/2)
10 data users
11.2 Border games in cellular networks11.2.2 Power control game
277Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Efficiency (2/2)
100 data users
11.2 Border games in cellular networks11.2.2 Power control game
278Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
convergence based on better-response dynamicsconvergence step: 2 W
Convergence to NE (1/2)
PA = 6.5 W
11.2 Border games in cellular networks11.2.3 Convergence to a Nash Equilibrium
279Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Convergence to NE (2/2)
convergence step: 0.1 W
11.2 Border games in cellular networks11.2.3 Convergence to a Nash Equilibrium
280Security and Cooperation in Wireless NetworksAppendix B: A tutorial on game theory for wireless networks
Conclusion on border games
not only individual nodes may exhibit selfish behavior, but operators can be selfish tooexample: adjusting pilot power to attract more users at national bordersthe problem can be modeled as a game between the operators– the game has an efficient Nash equilibrium– there’s a simple convergence algorithm that drives the system into
the Nash equilibrium
11.2 Border games in cellular networks