a stackelberg strategy for routing flow over time
DESCRIPTION
A Stackelberg Strategy for Routing Flow over Time. Umang Bhaskar , Lisa Fleischer Dartmouth College Elliot Anshelevich Rensselaer Polytechnic Institute. Traffic Routing. Road network. Data network. Traffic Routing. Safe zone. Evacuation planning. Traffic Routing. Routing Delays. - PowerPoint PPT PresentationTRANSCRIPT
A Stackelberg Strategy for Routing Flow over TimeUmang Bhaskar, Lisa Fleischer
Dartmouth CollegeElliot Anshelevich
Rensselaer Polytechnic Institute
2
Traffic Routing
• Data network• Road network
3
Traffic Routing
Safe zone
• Evacuation planning
4
Traffic Routing
5
Routing Delays
• Delay on edges increases with usage
delay
delay
6
Optimal Routing
• Delay on edges increases with usage
delay
delay
• Optimal routing minimizes f(delay)
7
Uncoordinated Routing
• But players choose route independently
delay
delay
8
Routing Games
• Each player picks route to minimize delay
• Delays depend on other players
delay
delay
9
Equilibria
• Equilibrium:- every player minimizes
delay w.r.t. others
• Studied in many contexts
- no player can unilaterally reduce delay
10
Price of Anarchy
• How does equilibrium compare to optimal routing?
• Price of Anarchy(PoA)
=f(delay) of worst equilibriaf(delay) of optimal routing
quality ofequilibria
measures system efficiency
11
Static Flows
s t
xe
• Static: time-invariant• Most routing games assume static flows
xe
xe
12
Flows over Time
s t
• Edges have fixed capacities and delays
• Flow on an edge may vary with time
ce, de
13
Flows over Time
M = 1000 bits
Total time: 12 seconds
d = 2 seconds
c = 100 bps
Bitsper
second
1 2 3 4 5 6 7 8 9 10 11 12
100
Arrival graph:
Time
Quickest flow: flow that gets to destination as soon as possible
A B
14
Temporal Routing Games
M = 1000 bitsd = 2 seconds
c = 100 bps
• Routing Games + Flows over Time = Temporal Routing Games
• Player minimizes time it reaches destination
A B
15
Temporal Routing GamesAssumptions:
• Players are infinitesimal
Model:
• Players are ordered at s• Each player picks a path from s to t• Minimizes the time it arrives at t
s t
M
Time
Flowrateat t
Arrival Graph:M
16
Temporal Routing Games
s tc = 2, d = 0 c = 1, d = 0
c = 1, d = 1
vTime
M
12
Flow rate at t
17
Temporal Routing Games
Time
s tc = 2, d = 0 c = 1, d = 0
c = 1, d = 1
v
Queue
M
12
Flow rate at t
18
Temporal Routing Games
Time
s tc = 2, d = 0 c = 1, d = 0
c = 1, d = 1
v
Queue
• Queues grow when inflow exceeds capacity on an edge• Queues are first in, first out (FIFO)• Queuing models used in modeling traffic [Vickrey ‘69, Yagar ‘71]
M
12
Flow rate at t
19
Temporal Routing Games
Time
s tc = 2, d = 0 c = 1, d = 0
c = 1, d = 1
vM
12
Flow rate at t
20
Temporal Routing Games
Time
s tc = 2, d = 0 c = 1, d = 0
c = 1, d = 1
vM
12
Flow rate at t
21
Temporal Routing Games
Time
s tc = 2, d = 0 c = 1, d = 0
c = 1, d = 1
vM
12
Flow rate at t
22
Temporal Routing Games
Time
s tc = 2, d = 0 c = 1, d = 0
c = 1, d = 1
vM
12
MFlow rate
at t
23
Temporal Routing Games
Time
s tc = 2, d = 0 c = 1, d = 0
c = 1, d = 1
v
Flow rate at t
Time
s tc = 2, d = 0 c = 1, d = 0
c = 1, d = 1
v
fp = 1
fp = 1
Quickest Flow:
M
M
M
M
12
12
Flow rate at t
24
Temporal Routing Games
• For single-source, single sink, equilibrium exists [Koch, Skutella, ’09; Cominetti, Correa, Larre ‘11]
our case
• How does equilibrium in these games compare with optimal?
• What is the PoA?
Flow rate at t
Time
s tc = 2, d = 0 c = 1, d = 0
c = 1, d = 1
vM
M
12
25
Objectives
|Flow| reaching t by time T
Time for M to reach t
Sum of times for M to reach t
Evacuation Price of Anarchy:
Time Price of Anarchy:
Total Delay Price of Anarchy:
Flow rateat t
Time T
M’ = ?
Maximum delay of a player?Flow rate
at t
Time
Flow rateat t
Time ?
Time
Flow rateat t
∫ = ?
M
26
Results
|Flow| reaching t by time T
Time for M to reach t
Flow rateat t
Time T
M’ = ?
Sum of times for M to reach t
Evacuation Price of Anarchy:
Time Price of Anarchy:
Total Delay Price of Anarchy:
Flow rateat t
Time
?
Flow rateat t
Time ?
Maximum delay of a player
Ω(log n) [KS ‘09]
Ω(n) [MLS ‘10]
≥ e/(e-1) [KS ‘09]
Flow rateat t
Time
∫ = ?
M
27
Results
|Flow| reaching t by time T
Time for M to reach t
Sum of times for M to reach t
Evacuation Price of Anarchy:
Time Price of Anarchy:
Total Delay Price of Anarchy:
Maximum delay of a player
Ω(log n) [KS ‘09]
≥ e/(e-1) [KS ‘09]
Ω(n) [MLS ‘10]
Our Results
= e/(e-1) †
= 2 e/(e-1) †
† can be enforced by reducing capacities
Flow rateat t
Time T
M’ = ?
Flow rateat t
Time
?
Flow rateat t
Time ?
Flow rateat t
Time
∫ = ?
M
28
Temporal Routing Games
- compute quickest flow (can compute in poly time [FF ’62])- reduce edge capacities to that used by quickest flow
Can enforce bound of e / (e-1) on Time PoAby reducing edge capacities
• Main Result:
• Procedure:
s tc = 2, d = 0 c = 1.5
c = 1, d = 1
vc = 1, d = 0
wfp = 1
fp = 1 , d = 0
?
M
29
Temporal Routing Games
- reduce edge capacities to that used by quickest flow
s tc = 2, d = 0 c = 1
c = 1, d = 1
vc = 1, d = 0
wfp = 1
fp = 1
• Main Result:
• Procedure:- compute quickest flow (can compute in poly time [FF ’62])
, d = 0
Can enforce bound of e / (e-1) on Time PoAby reducing edge capacities
?
M
30
Temporal Routing Games• Main Result:
- reduce edge capacities to that used by quickest flow
• Procedure:
If quickest flow saturates graph, thenTime PoA ≤ e / (e-1)
• Main Theorem:
- compute quickest flow (can compute in poly time [FF ’62])
Can enforce bound of e / (e-1) on Time PoAby reducing edge capacities
?
M
31
Temporal Routing Games
If quickest flow saturates graph, thenTime PoA ≤ e / (e-1)
Can enforce bound of e / (e-1) on Time PoAby reducing edge capacities
Theorem 1:
If quickest flow saturates graph, thenTotal Delay PoA ≤ 2 e / (e-1)
Can enforce bound of 2 e / (e-1) on Total Delay PoAby reducing edge capacities
Theorem 2:∫ = ?
M
?
32
Talk Outline
For this talk:
• Structure of equilibria
• Sketch weaker version of Theorem 1.
• Structure of Quickest flow
33
Quickest Flows
OPT:Flow rate
at t
Times t
c = 2, d = 0 c = 1, d = 0
c = 1, d = 1
v
• Quickest flow is static flow repeated over time [FF ‘62]
12
M
34
Quickest Flows
OPT:
Times t
c = 2, d = 0 c = 1, d = 0
c = 1, d = 1
v
• Quickest flow is static flow repeated over time [FF ‘62]
fp = 1
Flow rate at t
12
M
35
Quickest Flows
OPT:
Times t
c = 2, d = 0 c = 1, d = 0
c = 1, d = 1
v
• Quickest flow is static flow repeated over time [FF ‘62]
fp = 1
fp = 1
Flow rate at t
12
M
36
Phases in Equilibria
Time
s tc = 2, d = 0 c = 1, d = 0
c = 1, d = 1
v
QueueFlow rate
at t
12
M
37
Phases in Equilibria
Time
s tc = 2, d = 0 c = 1, d = 0
c = 1, d = 1
v
Flow rate at t
12
M
38
Phases in Equilibria
Time
s tc = 2, d = 0 c = 1, d = 0
c = 1, d = 1
v
Flow rate at t
12
M
39
• Equilibrium proceeds in “phases” [KS ‘09]
• Phase characterized by
• edges being used • edges with queue
• Within phase, flow rate at t is fixed
Phases in Equilibria
40
1-Phase Equilibrium
For one-phase equilibrium, Time PoA ≤ 2
For this talk:
• Phases in equilibria
• Sketch of proof
• Quickest flow
41
A Bound on EQ
EQ
Arrival Graphs:
Equilibrium
M M
s tc = 2, d = 0 c = 1, d = 0
c = 1, d = 1
vTime
ce = 1
Flow rate at t
42
A Bound on EQArrival Graphs:
Equilibrium
Quickest Flow
M M
Flow rate at t
s tc = 2, d = 0 c = 1, d = 0
c = 1, d = 1
v
fp = 1
fp = 1
s tc = 2, d = 0 c = 1, d = 0
c = 1, d = 1
vTime
Time
co = 2
OPT
EQ
ce = 1
Flow rate at t
43
A Bound on EQ
Time
Arrival Graphs:
Equilibrium
Quickest Flow
M M
Time
s tc = 2, d = 0 c = 1, d = 0
c = 1, d = 1
v
fp = 1
fp = 1
s tc = 2, d = 0 c = 1, d = 0
c = 1, d = 1
v
Claim 1: ce EQ ≤ co OPT
Flow rate at t
Flow rate at t
EQ ≤ OPT ce
co
co = 2
OPT
EQ
ce = 1
44
A Second Bound on EQ
Time
Arrival Graphs:
Equilibrium
s tc = 2, d = 0 c = 1, d = 0
c = 1, d = 1
v
Flow rate at t
EQ
ce = 1
45
A Second Bound on EQ
Time
Arrival Graphs:
Equilibrium
s tc = 2, d = 0 c = 1, d = 0
c = 1, d = 1
v
Flow rate at t
EQ
ce = 1
46
A Second Bound on EQ
Time
Arrival Graphs:
Equilibrium
s tc = 2, d = 0 c = 1, d = 0
c = 1, d = 1
vτ(θ) increasing
τ(θ) : minimum time taken to reach t at time θ
Flow rate at t
EQ
ce = 1
47
A Second Bound on EQ
Time
Arrival Graphs:
Equilibrium
s tc = 2, d = 0 c = 1, d = 0
c = 1, d = 1
v
Flow rate at t
τ(θ) increasing
τ(θ) : minimum time taken to reach t at time θ
Claim 2: τ(EQ) ≥ EQco
co - ce
EQ
ce = 1
48
A Second Bound on EQ
Time
Arrival Graphs:
Equilibrium
s tc = 2, d = 0 c = 1, d = 0
c = 1, d = 1
v
• But graph has path p of length ≤ OPT not being used (ce < co)• When τ(θ) ≥ OPT, this path p should be used (phase ends)
Flow rate at t
τ(θ) increasing
τ(θ) : minimum time taken to reach t at time θ
Claim 2: τ(EQ) ≥ EQco
co - ce
EQ
ce = 1
49
A Second Bound on EQ
Time
Arrival Graphs:
Equilibrium
s tc = 2, d = 0 c = 1, d = 0
c = 1, d = 1
v
Claim 3: τ(EQ) ≤ OPTco
co - ce
Flow rate at t
Claim 2: θ ≥ EQClaim 2: τ(EQ) ≥ EQco
co - ce
τ(θ) increasing
τ(θ) : minimum time taken to reach t at time θ
• But graph has path p of length ≤ OPT not being used (ce < co)• When τ(θ) ≥ OPT, this path p should be used (phase ends)
EQ
ce = 1
50
A Second Bound on EQ
Time
Arrival Graphs:
Equilibrium
s tc = 2, d = 0 c = 1, d = 0
c = 1, d = 1
v
Claim 4: EQ ≤ OPTco - ce
co
Flow rate at t
τ(θ) increasing
τ(θ) : minimum time taken to reach t at time θ
Claim 2: τ(EQ) ≥ EQco
co - ce Claim 3: τ(EQ) ≤ OPT+
EQ
ce = 1
51
Proof Sketch for 1-Phase
Claim 4: EQ ≤ OPTco - ce
co
EQ ≤ 2 OPT
+
(upper bounds equalwhen ce = co / 2)
Claim 1: EQ ≤ OPT ce
co
For one-phase equilibrium, Time PoA ≤ 2
52
1-Phase Examples
|Flow| reaching t by time T
Time for M to reach t
Flow rateat t
Time T
M’ = ?
Sum of times for M to reach t
Evacuation Price of Anarchy:
Time Price of Anarchy:
Total Delay Price of Anarchy:
Flow rateat t
Time
?
Flow rateat t
Time ?
Maximum delay of a player
Ω(log n) [KS ‘09]
Ω(n) [MLS ‘10]
≥ e/(e-1) [KS ‘09]
Flow rateat t
Time
∫ = ?
All 1-phase examples!
53
Summary
In paper: If quickest flow saturates graph, for arbitrary phases,
• Time PoA ≤ e/(e-1)
• Total Delay PoA ≤ 2 e/(e-1)
Thus, network administrator can enforce these bounds,by reducing edge capacities.
54
Open Questions1. Can PoA exceed e/(e-1) if capacity is not restricted?
3. What if players have imperfect information?
4. ...
2. PoA for multiple sources
Thanks for listening!
55
Optimal Routing
• Delay on edges increases with usage
delay
delay
56
Optimal Routing
• Optimal routing minimizes f(delay)• Delay on edges increases with usage
delay
delay
57
Uncoordinated Routing
• But players choose route independently
delay
delay
58
Routing Games
• Each player picks route to minimize delay• Delays depend on other players
delay
delay
59
Equilibrium
• Equilibrium: every player minimizes delay w.r.t. others- no player can unilaterally reduce delay
• Equilibria in routing games studied in many contexts
60
Equilibrium
• How does equilibrium compare to optimal routing?
• Price of Anarchy(PoA)
=f(delay) of worst equilibriaf(delay) of optimal routing
quality ofequilibria measures system efficiency
61
A 1-Phase Lower Bound on PoA
1-Phase example with PoA of e/(e-1) ~ 1.6 [KS ‘09]
s t
Flow rate at t
Time