Routing in Wireless and Adversarial Networks
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Routing in Wireless and Adversarial Networks
Christian Scheideler
Institut für Informatik
Technische Universität München
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Routing
• Path selection
• Scheduling
• Admission control
A
B
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Classical Routing Theory
Given a path collection with
• congestion C (max. number of paths over edge) and
• dilation D (max. length of a path)
find (near-)optimal schedule for packets.
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Classical Routing Theory
Leighton, Maggs, Rao 88:There is a schedule with O(C+D) runtime.Also for non-uniform edges [Feige & S 98]
Since then many randomized online protocols with runtime ~O(C+D) w.h.p.
Basic techniques: random delays or ranks
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Classical Routing Theory
Extensions faulty and wireless networks.
Adler & S 98:
• G=(V,E) with probabilities p:E ! [0,1]
• H=(V,E) with latencies l(e)=1/p(e)
• Valid routing schedule of length T for H can be simulated in G in time O(T log L + L log n), w.h.p.; L: max. latency
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Scheduling
Classical model: batch-like scheduling
More relevant models:
• Stochastic injection models(packets are continuously injected using Poisson distribution or Markov chains)
• Adversarial queueing theory(introduced by Borodin et al. 96)
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Adversarial Queueing Theory
Basic model:
• Static network G=(V,E)
• (w,)-bounded adversary continuously injects packets subject to the condition that for all edges e and all time intervals of length w, it injects at most w packets with paths containing e
• All packets have to be delivered (<=1)
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Adversarial Queueing Theory
Basic results:
• Universal stability and instability of various queueing disciplines (FIFO, SIS, LIS, NTO,…)
• Universal stability of networks
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Adversarial Queueing Theory
Networks with time-varying channels:
• Packet injections and edges under adversarial control
• Andrews and Zhang 04: Variant of NTO is universally stable in this model
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Adversarial Routing Theory
Paths are not given to system:
• Aiello, Kushilevitz, Ostrovsky, Rosen ’98:local load balancing techniques can be used to keep queues bounded
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Adversarial Routing Theory
Paths are not given to system:
• Awerbuch, Brinkmann & S ’03:local load balancing technique with bounded queues also handles admission,works even for adversarial networks
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Adversarial Routing Theory
Paths are not given to system:
• Awerbuch, Brinkmann & S ’03:load balancing technique with O(L/) times buffer space of OPT is (1+)-competitive w.r.t. throughput; L: max path length
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Path Selection
Problems: - packet-based paths: slow delivery - destination-based paths: congestion
Better: source-based path selection (MPLS: Multiprotocol Label Switching)
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Path Selection
Classical work: path selection strategies for specific networks (n£n-mesh)
A
B
x-y routing
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Path Selection
x-y routing: ~worst-case optimal congestion and dilation for permutation routing
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B
x-y routing
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Path Selection
x-y routing: far from optimal in general
s2
t1 x-y routing
t2
s1s3
t3
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Path Selection
Trick: use hierarchical randomized routing.
log n)-competitive for any problem
s2
t1
t2
s1s3
t3
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Oblivious Path Selection
Räcke 02: For any network with edge capacities, path collections for random path selection can be set up for every source-destination pair s.t. the expected congestion of routing any routing problem is O(log3 n)-competitive.
Best bound [HHR03]: ~O(log2 n)
1/21/4
1/4
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Oblivious Path Selection
Also works well for certain dynamic net-works for peer-to-peer systems.
Trick: continuous-discrete approach
• route in virtual space
• nodes partition virtual space among them
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Oblivious Path Selection
Does not work well for wireless, unknown or adversarial networks (e.g., unstructured P2P systems with adversarial presence)
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Adaptive Path Selection
Basic Idea: Garg & Könemann 98
Multicommodity flow problem: collection of commodities (source, dest., demand)
• Solution 1: use LP
• Solution 2: combinatorial approach (path packing using primal-dual approach)
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Garg-Könemann Framework
Problem: MCF (maximum concurrent flow problem), i.e., given commodities with demands di, find flows of value di for commodities s.t. maxe fe/ce minimized
Goal: find (1+)-approximate solution via path packing
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Garg-Könemann Framework
Initially, fei=0 for all commodities i and edges e
Algorithm runs in T=ln m/2 phases, routes a flow of di/T for each commodity i in each phase
A phase consists of several stepsIn each step, flows augmented simultaneously
subject to two constraints:• (1+)-shortest paths constraint, using edge
lengths le = mconge//ce with conge = fe/ce
• step-size constraint: le <= le(which implies fe <= 2 ce/ln m)
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Garg-Könemann Framework
Original Garg-Könemann approach:• Route commodities in round-robin fashion, one
commodity per step) #steps depends linearly on #commidities
Awerbuch, Khandekar and Rao 07:• Route commodities simultaneously in each step
using capacities cei = 2 fe
i/log m for comm i) multiplicative-increase strategy, faster conv
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Garg-Könemann Framework
Awerbuch, Khandekar and Rao 07:runtime O(L log3 m log k)L: max flow length, k: #commodities
• L small (hypercube): fast convergence
• L always boundable by expansion of net(flow shortening lemma [Kolman & S 02])
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Oblivious vs. Adaptive
Congestion for arbitrary routing problems in hypercubic networks:
• Oblivious path selection:(log n)-competitive, paths instantly, update of path system complicated
• Adaptive path selection:(1+)-competitive, paths in polylog comm rounds, continuous updates easy
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Adaptive Path Selection
Problem: previous approaches not stateless resp. self-stabilizing
Awerbuch and Khandekar 07:• Adaptive path selection strategy that only
needs to know current state• Fast convergence through greedy strategy
based on multiplicative increase, additive decrease
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Adversarial Path Selection
Scenario I: Adversaries part of network, but path along honest nodes available
A
B
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Adversarial Path Selection
Basic approach: A fixes a path from A to B.
Path does not work: A identifies bad edge.
A
B
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Adversarial Path Selection
Identification of bad edge: Acknowledgements via binary search
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Adversarial Path Selection
Maximum number of attempts: m (# edges)
Either successful or edge killed.
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Adversarial Path Selection
Improvement: use recommendations
If neighbor knows better, suggests a diff path
! collaborative learning
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B
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Adversarial Path Selection
Scenario II: All nodes adversarial.
Awerbuch and Kleinberg 04:
Learns best static path in hindsight
A
B
12
3 4
1
102
53
72
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Adversarial path selection
Model:• There is a set S of static strategies (paths)• Algorithm A interacts with adversary for T steps• In each step j, the adversary picks a cost
function cj:S ! IR and A picks a random strategy xj 2 S
• Only cost of chosen strategy revealed to A• The regret of the algorithm A is defined as
R(A) = E[j cj(xj) – minx 2 S j cj(x)]
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Adversarial Path Selection
Awerbuch and Kleinberg:
• Regret of O(T2/3 C m5/3) against oblivious adversaryC: maximum cost difference, m: #edges
• Regret of O(T2/3 C7/3 m1/3) against adaptive adversary
Regret does not depend on |S| !
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Adversarial Path Selection
Otto von Bismarck: Fools learn from experience; wise men learn from the experience of others.
Only collaborative learning result due to Awerbuch and Hayes 07, who study the dynamic regret for |S|=2: R(A) = avga E[j cj(xj) – j minx 2 S cj(x)]
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Adversarial Path Selection
Awerbuch and Hayes 07:
• N agents, n of which are honest
• In each round, agents make decisions in a fixed order, report the costs incurred
• Costs are either 0 or 1
• Dynamic regret: O(log N2 + T/n)log2 N: rounds to figure out whom to trustT/n: just one mistake per round
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Adversarial Path Selection
Scenario III: Network topology unknownbut position of destination known
• Geometric spanners (wireless networks)
• Navigable graphs (small world)pioneered by Kleinberg 96
How to design self-stabilizing processes?
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Adversarial Path Selection
Scenario IV: Network topology unknown and position of destination unknown! discovery via flooding
A
B
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Open Problems
• Scheduling: non-uniform problems
• Path selection: many open problems left
• Collaborative learning approaches particularly interesting
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Questions?