Jamming-Resistant MAC Protocol1 A Jamming-Resistant MAC Protocol for Single-Hop Wireless Networks Baruch Awerbuch (JHU) Andrea W. Richa (ASU) Christian Scheideler (Uni PB)

Jamming-Resistant MAC Protocol2 Wireless jamming ●blocking of the wireless channel due to interference, noise or collision at the receiver side wireless nodes X X X X

Jamming-Resistant MAC Protocol3 Adversarial physical layer jamming ●a jammer listens to the open medium and broadcasts in the same frequency band as the network –no special hardware required –can lead to significant disruption of communication at low cost for the jammer honest nodes jammer

Jamming-Resistant MAC Protocol4 Single-hop wireless network ●n reliable honest nodes and one jammer; all nodes within transmission range of each other and of the jammer jammer

Jamming-Resistant MAC Protocol5 Wireless communication model ●at each time step, a node may decide to transmit a packet (nodes continuously contend to send packets) ●a node may transmit or sense the channel at any time step (half-duplex) ●when sensing the channel a node v may –sense an idle channel –receive a packet –sense a busy channel v

Jamming-Resistant MAC Protocol6 Adaptive adversary ●knows protocol and entire history ●nodes cannot distinguish between adversarial jamming or a message collision –i.e., a node senses a busy channel in both cases ●(T,λ)-bounded adversary, 0 < λ < 1: in any time window of size w ≥ T, the adversary can jam ≤ λw time steps 0 1 … w steps jammed by adversary other steps

Jamming-Resistant MAC Protocol7 Constant-competitive protocol ●a protocol is called constant-competitive against a (T,λ)-bounded adversary if the nodes manage to perform successful transmission in at least a constant fraction of the non-jammed steps (w.h.p. or on expectation), for any sufficiently large number of steps successful transmissions steps jammed by adversary 0 1 … w other steps (idle channel, message collisions)

Jamming-Resistant MAC Protocol8 Our main contribution ●symmetric local-control MAC protocol that is constant-competitive against any (T,1-ε)-bounded adversary after Ω (T / ε) steps w.h.p., for any constant 0<ε<1 and any T. ●energy efficient: –converges to bounded amount of energy consumption due to message transmissions by nodes under continuous adversarial jamming (ε=0) ●fast recovery from any state ~

Jamming-Resistant MAC Protocol9 Pros and Cons Pros: ●no prior knowledge of global parameters –nodes do not know ε ●no IDs needed Cons: ●nodes know common rough estimate γ=O(1/(log T + loglog n)) –allow for superpolynomial change in n and polynomial change in T over time ●fair channel use is not guaranteed

Jamming-Resistant MAC Protocol10 Further contributions Leader election protocol: ●robust and efficient ●all nodes agree on a leader in O( T/ε ) steps w.h.p. Fair use of the wireless channel: ●share the channel fairly among all nodes (some nodes may dominate transmission probabilities in our MAC protocol) ●converges in O( n/ε ) steps w.h.p. ~

Jamming-Resistant MAC Protocol11 Traditional defenses ●spread spectrum: frequency hopping over a wide frequency band –hard for a jammer to detect the used frequency fast enough in order to jam it –Problem: commonly used wireless devices (e.g., ) have relatively narrow frequency bands ●random backoff: –adaptive adversary too powerful for MAC protocols based on random backoff or tournaments (including the standard MAC protocol of [BKLNRT’08])

Jamming-Resistant MAC Protocol12 Further related work ●MAC protocol in [GGN’06] would not be able to sustain constant-competitive ratio if adversary can jam more than ½ the time steps. –more general scenario (adversary can also introduce malicious messages) –nodes know n –not energy efficient ●reliable broadcast in grid [KBKV’06] –eventually terminates –honest nodes consume energy at much higher rates than adversary

Jamming-Resistant MAC Protocol13 Overview ●Jamming ●Model ●Our contributions ●Related work ●MAC protocol –Basic Approach –MAC protocol –Fast recovery –Energy Efficiency ●Leader Election & Fair Use of the Channel ●Future work

Jamming-Resistant MAC Protocol14 Simple idea ●each node v sends a message at current time step with probability p v ≤ p max, for constant 0 < p max << 1. p = ∑ p v (cumulative probability) q idle = probability the channel is idle q success = probability that only one node is transmitting (successful transmission) ●Claim. q idle. p ≤ q success ≤ (q idle. p)/ (1- p max ) if (number of times the channel is idle) = (number of successful transmissions) p = θ(1) ! (what we want!) ~

Jamming-Resistant MAC Protocol15 Basic approach ●a node v adapts p v based only on steps when an idle channel or a successful message transmission are observed, ignoring all other steps (including all the blocked steps when the adversary transmits!)! steps jammed by adversary idle steps successful transmissions steps where collision occurred but no jamming time

Jamming-Resistant MAC Protocol16 Basic approach ●a node v adapts p v based only on steps when an idle channel or a successful message transmission are observed, ignoring all other steps (including all the blocked steps when the adversary transmits!)! steps jammed by adversary idle steps successful transmissions steps where collision occurred but no jamming time

Jamming-Resistant MAC Protocol17 Naïve protocol Each time step: ●Node v sends a message with probability p v. If v does not send a message then –if the wireless channel is idle then p v = (1+ γ ) p v –if v received a message then p v = p v /(1+ γ) (Recall that γ = O(1/(log T + loglog n)). )

Jamming-Resistant MAC Protocol18 Problems ●Basic problem: Cumulative probability p could be too large. –all time steps blocked due to message collisions w.h.p. steps jammed by adversary idle steps successful transmissions steps where collision occurred but no jamming time

Jamming-Resistant MAC Protocol19 Problems ●Basic problem: Cumulative probability p could be too large. –all time steps blocked due to message collisions w.h.p. steps jammed by adversary idle steps successful transmissions steps where collision occurred but no jamming time

Jamming-Resistant MAC Protocol20 Problems ●Basic problem: Cumulative probability p could be too large. –all time steps blocked due to message collisions w.h.p. ●Idea: If more than T consecutive time steps without successful transmissions, then reduce probabilities, which results in fast recovery of p. ●Problem: Nodes do not know T. How to learn a good time window threshold? –It turns out that additive-increase additive-decrease is the right strategy!

Jamming-Resistant MAC Protocol21 MAC protocol ●each node v maintains –probability value p v, –time window threshold T v, and –counter c v ●Initially, T v = c v = 1 and p v = p max (< 1/24). ●synchronized time steps (for ease of explanation) ●Intuition: wait for an entire time window (according to current estimate T v ) until you can increase T v

Jamming-Resistant MAC Protocol22 MAC protocol In each step: ●node v sends a message with probability p v. If v decides not to send a message then –if v senses an idle channel, then p v = min{(1+ γ ) p v, p max } –if v successfully receives a message, then p v = p v /(1+ γ ) and T v = max{T v - 1, 1} ●c v = c v + 1. If c v > T v then –c v = 1 –if v did not receive a message successfully in the last T v steps then p v = p v /(1+ γ ) and T v = T v +1

Jamming-Resistant MAC Protocol23 Example: Low value of p ●p v = 1/n 2, T v = 3, c v = 1 v Wireless Channel (Idle) Sensing

Jamming-Resistant MAC Protocol24 Example: Low value of p ●p v = (1+ γ) /n 2, T v = 3, c v = 2 v Wireless Channel (Idle) Sensing

TAMU'08, Andrea Richa25 Example: Low value of p ●p v = (1+ γ) 2 /n 2, T v = 3, c v = 3 v Wireless Channel (Idle) Sensing 25Jamming-Resistant MAC Protocol

26 Example: Low value of p ●p v = (1+ γ) 3 /n 2, T v = 3, c v = 4 v p v = (1+ γ) 2 /n 2, T v = 4, c v =1 Wireless Channel (Jammed) Sensing

Jamming-Resistant MAC Protocol27 Example: Low value of p ●~ polylog (n) idle steps later: –p v = c/n, T v ≤ √T polylog (n) v Wireless Channel ~

Jamming-Resistant MAC Protocol28 Example: Large p ●p v = 1/c, T v = 2, c v = 1 v Wireless Channel Sending Message

Jamming-Resistant MAC Protocol29 Example: Large p ●p v = 1/c, T v = 2, c v = 2 v Wireless Channel (collision) Sensing

Jamming-Resistant MAC Protocol30 Example: Large p ●p v = 1/c, T v = 2, c v = 3 v Wireless Channel (Jammed) Sensing p v = 1/[c(1+ γ)], T v = 3, c v = 1

Jamming-Resistant MAC Protocol31 Example: Large p ●p v = 1/[c(1+ γ)], T v = 3, c v = 1 v Wireless Channel Sending Message

Jamming-Resistant MAC Protocol32 Example: Large p ●p v = 1/[c(1+ γ)], T v = 3, c v = 2 v Wireless Channel (Collision) Sensing

Jamming-Resistant MAC Protocol33 Example: Large p ●p v = 1/[c(1+ γ)], T v = 3, c v = 3 v Wireless Channel (Collision) Sensing

Jamming-Resistant MAC Protocol34 Example: Large p ●p v = 1/[c(1+ γ)], T v = 3, c v = 4 v Wireless Channel (Collision) Sensing p v = 1/[c(1+ γ) 2 ], T v = 4, c v = 1

Jamming-Resistant MAC Protocol35 MAC protocol In each step: ●node v sends a message with probability p v. If v decides not to send a message then –if v senses an idle channel, then p v = min{(1+ γ ) p v, p max } –if v successfully receives a message, then p v = p v /(1+ γ ) and T v = max{T v - 1, 1} ●c v = c v + 1. If c v > T v then –c v = 1 –if v did not receive a message successfully in the last T v steps then p v = p v /(1+ γ ) and T v = T v +1 Why only successful steps??

Counterexample Suppose that  v p v is very low. Repeat indefinitely: Jamming-Resistant MAC Protocol36 Channel jammed for T v steps Channel idle for one step pvpv    TvTv    Channel jammed for T v -1 steps no progress!

Jamming-Resistant MAC Protocol37 Our results ●Let N = max {T,n} ●Theorem. The MAC protocol is constant-competitive under any (T,1-ε)-bounded adversary if the protocol is executed for Ω(log N  max{T,log 3 N/(ε γ 2 )} / ε) steps w.h.p., for any constant 0<ε<1 and any T.

Jamming-Resistant MAC Protocol38 Proof sketch ●Show competitiveness for time frames of F = θ( (log N  max{T,log 3 N/(ε γ 2 )} / ε) many steps If we can show constant competitiveness for any such time frame of size F, the theorem follows ●Use induction over the number of sufficiently large time frames seen so far. We subdivide each frame: I I’ f = θ(max{T,log 3 N/(ε γ 2 )}) F = (log N / ε)  f

Jamming-Resistant MAC Protocol39 Proof sketch ●p > 1/(f 2 (1+γ) 2√f ) and T v < √F, in each subframe I’ w.h.p. ●p 1/12 within subframe I’ with moderate probability (so that adaptive adversarial jamming not successful) ●Constant throughput in I’ with moderate probability ●Over a logarithmic number of subframes, constant throughput in frame I of size F w.h.p.

TAMU'08, Andrea Richa40 Overview ●Jamming ●Model ●Our contributions ●Related work ●MAC protocol –Basic Approach –MAC protocol –Fast recovery –Energy Efficiency ●Leader Election & Fair Use of the Channel ●Future work 40Jamming-Resistant MAC Protocol

41 Fast recovery ●Our protocol quickly recovers from any (T v,c v,,p v )- values. ●Theorem. For any initial p 0 = ∑ p v and T 0 = max T v, it takes O([log (1+ γ) (1/ p 0 )]/ ε + T 0 2 ) w.h.p. until the MAC protocol satisfies again p ≥ 1/(f 2 (1+γ) 2√f ) and T v < √F for all v.

Jamming-Resistant MAC Protocol42 Proving fast recovery: p ●p 0 < 1/(f 2 (1+γ) 2√f ) ●we show that it takes roughly [log (1+ γ) (1/ p 0 )]/ ε steps to get down from p 0 to (p 0 ) 1/2 ; another [log (1+ γ) (1/ p 0 )]/ (2ε) steps to get to (p 0 ) 1/4 ; … roughly at most 2[log (1+ γ) (1/ p 0 )]/ ε until cumulative probability p ≥ 1/(f 2 (1+γ) 2√f )

Jamming-Resistant MAC Protocol43 Proving fast recovery: T ●Once cumulative probability p ≥ 1/(f 2 (1+γ) 2√f ), count number of steps until T v < √F for all v –repeated applications of similar inductive argument as MAC protocol’s, by repeatedly selecting appropriately geometric decreasing frame sizes (starting from 4 max T v ).

Jamming-Resistant MAC Protocol44 Energy efficiency ●Corollary. For any time frame of size F = Ω((log N  max{T,log 3 N/(ε γ 2 )} / ε), the total energy spent by all nodes together on sending out messages is bounded by O(F) whp. ●Total amount of energy spent proportional to number of successful transmissions.

TAMU'08, Andrea Richa45 Continuous jamming ●Moreover, under a more powerful adversary that can perform continuous jamming (after Ω(T) steps): ●Lemma. The total energy consumption (sending out messages) during an entire continuous jamming attack is O(√T), independent of the length of the attack. ●Exhaust adversary’s energy resources ~ ~ 45Jamming-Resistant MAC Protocol

46 Proving Lemma ●First we show that the total energy consumption is O(p 0. T 0 / γ + log N) whp, where p 0 =∑ p v and T 0 = max T v at the start of the attack –compute expected number transmissions out of each node given that p v decreases by 1/(1+ γ) every T 0 +1, then T 0 +2, then T 0 +3, … number of steps under continuous jamming –sum up expected values over all nodes and use Chernoff bounds ●Note that for an attack starting after Ω( T ) steps, p 0 =O(1) and T 0 = O(√F)= O(√T), whp. ~ ~

TAMU'08, Andrea Richa47 Overview ●Jamming ●Model ●Our contributions ●Related work ●MAC protocol –Basic Approach –MAC protocol –Fast recovery –Energy Efficiency ●Leader Election & Fair Use of the Channel ●Future work 47Jamming-Resistant MAC Protocol

48 Leader election ●all nodes agree on a leader in O( T/ε ) steps w.h.p. ●robust and efficient –tournament based leader election protocols are not robust against adversarial jamming ●Basic Idea: –each node will keep a counter for the number of successful transmissions received so far; –the node that receives the least amount of successful transmissions (and hence sent the largest amount of successful transmissions) will become the leader node ~

Jamming-Resistant MAC Protocol49 Leader election: Basic Ideas ●In addition to the triples (p v,T v,c v ) each node v maintains –counter s v for estimate on total number of successful transmissions so far –one of the states {unknown, leader, follower} ●Initially, all nodes are at unknown.

Jamming-Resistant MAC Protocol50 Leader election protocol ●Modify the MAC protocol slightly: … –if v successfully receives a message, then p v = p v /(1+ γ) and T v = max{T v - 1, 1} if v is still in the state unknown, then v checks if: (1) s v ≥ s w, then v becomes a follower (2) s v < s w, then v becomes a leader v sets s v := max{s v, s w }+1 …

Jamming-Resistant MAC Protocol51 Example: Leader election v s v = 0 s w = 0 Message, s v = 0 all other nodes w

Jamming-Resistant MAC Protocol52 Example: Leader election v s v = 0 s w = 0 Message, s v = 0 all other nodes w

Jamming-Resistant MAC Protocol53 Example: Leader election v s v = 0 all other nodes w s w = 0 Message, s v = 0

Jamming-Resistant MAC Protocol54 Example: Leader election v s v = 0 w s w = 0 s w ≥ s v, then w becomes a “follower” and s w = max{ s w, s v } + 1 = 1 s w = 1 follower Message, s v = 0

Jamming-Resistant MAC Protocol55 Example: Leader election v s v = 0 w s w = k+1 Message, s w = k+1 After v continues successfully transmitting for k more steps. Now the first node w ≠ v is transmitting a message to v, with s w = k+1.

Jamming-Resistant MAC Protocol56 Example: Leader election v s v = 0 w s w = k+1 Message, s w = k+1 After v continues successfully transmitting for k more steps. Now the first node w ≠ v is transmitting a message to v, with s w = k+1.

Jamming-Resistant MAC Protocol57 Example: Leader election v s v = 0 w s w = k+1 Message, s w = k+1 After v continues successfully transmitting for k more steps, until first node w ≠ v successfully transmits a message. Now s w = k+1, and s v = 0. s v < s w, then v becomes a “leader” leader s v = k+2

Jamming-Resistant MAC Protocol58 Our results: Leader election ●Let N = max {T,n} ●Theorem. Within O(log N  max{T,log 3 N/(ε γ 2 )} / ε) many steps, the leader election protocol reaches a state in which there is exactly one leader and the other nodes are followers, w.h.p. ●Proof: It takes O(log N  max{T,log 3 N/(ε γ 2 )} / ε) until two distinct nodes transmit a message.

Jamming-Resistant MAC Protocol59 Fairness ●share the channel fairly among nodes –MAC protocol can be rather unfair –some probabilities may eventually dominate others ●simple counter-based mechanism (as in the leader election protocol) would fail here…

Jamming-Resistant MAC Protocol60 Fairness: Basic ideas ●each node v also maintains an additional counter m v of the different nodes which had successful transmissions so far ●the protocol aims at setting p v = p max /2m v, which will eventually converge to p v = p max /2n (= θ(1/n)) ●converges in O( n/ε ) steps w.h.p.

Jamming-Resistant MAC Protocol61 Future work ●Can the MAC protocol be extended to multihop wireless networks? ●How can we adapt to node join and leave operations? ●Can the MAC protocol be modified so that no rough bound on n and T are required? ●stochastic/oblivious jammers: Simpler to handle? E.g., a constant gamma seems to work fine here. ●Other applications of the MAC protocol?

Jamming-Resistant MAC Protocol62 Questions?