equilibrium of heterogeneous protocols steven low cs, ee netlab.caltech.edu with a. tang, j. wang,...
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Equilibrium of Heterogeneous Protocols
Steven Low
CS, EEnetlab.CALTECH.edu
with A. Tang, J. Wang, ClatechM. Chiang, Princeton
Network model
F1
FN
G1
GL
R
RT
TCP Network AQM
x y
q p
))( ),(( )1(
))( ),(( )1(
tRxtpGtp
txtpRFtx T
Reno, Vegas
DT, RED, …
liRli link uses source if 1 IP routing
TCP-AQM:
Duality model
Equilibrium (x*,p*) primal-dual optimal:
F determines utility function U G determines complementary slackness
condition p* are Lagrange multipliers
Uniqueness of equilibrium x* is unique when U is strictly
concave p* is unique when R has full row rank
TCP-AQM:
Duality model
Equilibrium (x*,p*) primal-dual optimal:
F determines utility function U G determines complementary slackness
condition p* are Lagrange multipliers
The underlying concave program also leads to simple dynamic behavior
Duality model
Equilibrium (x*,p*) primal-dual optimal:
Vegas, FAST, STCP HSTCP (homogeneous
sources) Reno (homogeneous
sources) infinity XCP (single link
only)
(Mo & Walrand 00)
Congestion control
F1
FN
G1
GL
R
RT
TCP Network AQM
x y
q p
iililll
il
lliii
txRtpGtp
txtpRFtx
)( ),( )1(
)( ,)( )1(
same pricefor all sources
Heterogeneous protocols
F1
FN
G1
GL
R
RT
TCP Network AQM
x y
q p
)( ,)( )1(
)( ,)( )1(
txtpmRFtx
txtpRFtx
ji
ll
jlli
ji
ji
il
lliii Heterogeneousprices for
type j sources
Multiple equilibria: multiple constraint sets
eq 1 eq 2
Path 1 52M 13M
path 2 61M 13M
path 3 27M 93M
eq 1
eq 2
Tang, Wang, Hegde, Low, Telecom Systems, 2005
eq 1 eq 2
Path 1 52M 13M
path 2 61M 13M
path 3 27M 93M
eq 1
eq 2
Tang, Wang, Hegde, Low, Telecom Systems, 2005
eq 3 (unstable)
Multiple equilibria: multiple constraint sets
Multiple equilibria: single constraint sets
1 1
11x
21x
Smallest example for multiple equilibria Single constraint set but infinitely many
equilibria
J=1: prices are non-unique but rates are unique
J>1: prices and rates are both non-unique
Multi-protocol: J>1
Duality model no longer applies ! pl can no longer serve as
Lagrange multiplier
TCP-AQM equilibrium p:
TCP-AQM equilibrium p:
Multi-protocol: J>1
Need to re-examine all issues Equilibrium: exists? unique? efficient? fair? Dynamics: stable? limit cycle? chaotic? Practical networks: typical behavior? design
guidelines?
Summary: equilibrium structure
Uni-protocolUnique
bottleneck set Unique rates &
prices
Multi-protocol Non-unique
bottleneck sets Non-unique rates &
prices for each B.S.
always odd not all
stable uniqueness
conditions
TCP-AQM equilibrium p:
Multi-protocol: J>1
Simpler notation: equilibrium p iff
Multi-protocol: J>1
Linearized gradient projection algorithm:
Results: existence of equilibrium
Equilibrium p always exists despite lack of underlying utility maximization
Generally non-unique Network with unique bottleneck set
but uncountably many equilibria Network with non-unique bottleneck
sets each having unique equilibrium
Results: regular networks
Regular networks: all equilibria p are locally unique, i.e.
Results: regular networks
Regular networks: all equilibria p are locally unique
Theorem (Tang, Wang, Low, Chiang, Infocom 2005)
Almost all networks are regularRegular networks have finitely
many and odd number of equilibria (e.g. 1)
Proof: Sard’s Theorem and Index Theorem
Results: regular networks
Proof idea: Sard’s Theorem: critical value of
cont diff functions over open set has measure zero
Apply to y(p) = c on each bottleneck set regularity
Compact equilibrium set finiteness
Results: regular networks
Proof idea: Poincare-Hopf Index Theorem: if there exists a vector field s.t. dv/dp non-singular, then
Gradient projection algorithm defines such a vector field
Index theorem implies odd #equilibria
Results: global uniqueness
Theorem (Tang, Wang, Low, Chiang, Infocom 2005)
If all equilibria p all locally stable, then it is globally unique
Linearized gradient projection algorithm:
Proof idea: For all equilibrium p:
Results: global uniqueness
Theorem (Tang, Wang, Low, Chiang, Infocom 2005)
For J=1, equilibrium p is globally unique if R is full rank (Mo & Walrand ToN 2000)
For J>1, equilibrium p is globally unique if J(p) is `negative definite’ over a certain set
Results: global uniqueness
Theorem (Tang, Wang, Low, Chiang, Infocom 2005)
If price mapping functions mlj are
`similar’, then equilibrium p is globally unique
If price mapping functions mlj are linear
and link-independent, then equilibrium p is globally unique
Summary: equilibrium structure
Uni-protocolUnique
bottleneck set Unique rates &
prices
Multi-protocol Non-unique
bottleneck sets Non-unique rates &
prices for each B.S.
always odd not all
stable uniqueness
conditions
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