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Stimulating Participation in Wireless Community Networks
Elias C. Efstathiou, Pantelis A. Frangoudis, George C. Polyzos
Mobile Multimedia LaboratoryDepartment of Computer Science
Athens University of Economics and Business
IEEE INFOCOM 2006, Barcelona, April 23-29, 2006
Connected to DSL and cable links Secured (WEP, WPA) Under-utilized
Motivation
Many private WLANs in cities
An opportunity? E.g. Skyhook Wireless WPS is a
positioning service that relies on detecting catalogued WLANs
Boston WLAN coverage © 2005 Skyhook Wireless Inc
Manhattan WLANs © 2002 Public Internet Project.org
Outline of Talk
1) The Peer-to-Peer Wireless Network Confederation
scheme for WLAN sharing
2) P2PWNC architecture and algorithms
3) Simulation-based evaluation of algorithms
4) P2PWNC protocol
5) P2PWNC implementation
What is P2PWNC ?
3) A public good created through private contributions
Wireless Internet (and VoIP) through third parties And what about freeriders?
2) A simple protocol for (selfish) WLAN phones and access points
Our implementation runs on Windows Mobile phones/PDAs and Linux access points
1) An incentive scheme for sharing our WLANs with others
Relies on direct and/or cyclical reciprocity Balances consumption and contribution
A B
A
C
B
Distinctive Characteristics
1) Complete decentralization, respect for privacy Open to all, without central authorities of any kind Free identities (aka cheap pseudonyms) No tamperproof modules
2) Selfishness Nobody cares about the good of the community Freeriding is rational, and it extends to accounting tasks No super peers, no P2P overlays
P2PWNCP2PWNC
?sophisticated freeridersAttacker model
centralizeddecentralizedArchitecture
WLAN sharingWLAN sharingGoal
versus
Caveats
1) ISP prohibitions on connection sharing
2) People living in the outskirts… what about them?
3) Designed for citywide everyday WLAN roaming, as opposed to global WLAN roaming
4) WLAN standards do not support fast handovers, yet
5) We do not defend against pure irrational malice (i.e. DoS attacks at various layers)
System Elements
1) Identities System identities are public-private key pairs Standard cryptographic assumptions apply
3) Receipts Generated each time WLAN service
is contributed and consumed Digitally signed by the consumer
consumer public keyconsumer public key
timestamptimestamp
signed using signed using consumer private keyconsumer private key
weightweight
provider public keyprovider public key
2) P2PWNC peers Each with a self-generated identity At least one WLAN access point per peer
(for contributing to the community) At least one WLAN client per peer
(for consuming from the community)
Receipt Graph
w14
A
B
C
G
H
FE
D
I
w1
w2
w3
w4
w5 w6
w7
w8
w9
w10
w11
w12
w13
1) A (logical) directed graph, with cycles
Vertices represent peer IDs Edges point from the consumer to
the provider (they represent debt) Edge weight is the sum of the
weights of the corresponding receipts
2) Graph security Freeriders and colluders can create
an arbitrary number of vertices and edges (Sybil/false trading attack)
They cannot create fake edges starting from peers who are notcolluding
3) Receipt repository Centralized version Distributed version Short-term history,
in both versions
Reciprocity Algorithm: Overviewmf(P→C) maxflow from P (Provider) to C (Consumer)
gmf(P→C) generalized maxflow from P to C: discount sub-flows, multiply them by 2(1—path_length)
quality of flow from P to C1C)mf(P
C)gmf(PC)qf(P
qfavg an average qf for the community, estimated
independently by each peer P, using:qfavg ← a × qfavg + (1–a) × qf(P→C), 0 < a < 1
r = r1 × r2
avg1 qf
C)qf(Pr
}1,P)mf(C
C)mf(Pmin{r2
benefit function
Gossiping and BootstrapAt one extreme, APs would only store graph edges pointing directly to themAt the other extreme, a central server would store everything
1) How do we circulate receipts through the system? Gossip
a) Copy “fresh” receipts from your WLAN AP to your phoneb) Present them to foreign APs before requesting servicec) Foreign APs merge these with their local receipt graph…verification overhead, time horizon, incentive compatibility
2) How to bootstrap a peer? Reciprocity Algorithm does not work without unconditional
cooperation in the beginning Define patience: the number of successful consumptions
after which a new peer starts to use the Reciprocity Algorithm
Evaluation FrameworkEvolutionary simulations Uniform peers (consumption rate, benefit and cost functions,
repository sizes, merging sizes, patience) Assume unit weight on receipts, corresponding to a “standard
WLAN session” Constant provider cost (per session), normalized to c = 1 Variable consumer benefit (bmin = 0, rth = 0.1, bmax = 6) Rounds: symmetric matches Strategy ratings Assume rational but myopic peers: “trial and error” Strategic mixtures, and learning and mutation Goal: the evolution of cooperation (average rating near
bmax – c) No congestion effects Also simulate system growth (up to n peers)
0.0
1.0
2.0
3.0
4.0
5.0
6.0
Str
ate
gy
Ra
ting
merge 0
merge 10
merge 100
merge 1000
0.0
1.0
2.0
3.0
4.0
5.0
6.0
Str
ate
gy
Ra
ting
100 200 300 400 500
Rounds
Merging and PatienceCommunity grows to n = 100 peers (at round 100) and private
receipt repositories contain 1000 receipts each
diminishing returns
patience=10
patience=100
0
10
20
30
40
50
60
70
80
90
100
po
pu
latio
n (
%)
ALLC
ALLD
RAND
RECI
0
10
20
30
40
50
60
70
80
90
100
po
pu
latio
n (
%)
100 200 300 400 500 600 700 800 900 1000 Rounds
Evolutionary LearningCommunity grows to n = 100 peers (by round 100) and private receipt repositories contain 1000 receipts each
Non-zero learning and mutation probabilities; merge 100
conditions favor freeriding mutations
set bmax/c = 3 (was: 6)
P2PWNC service not valuable enough, ALLD wins
0.0
1.0
2.0
3.0
4.0
5.0
6.0S
tra
teg
y R
atin
g
RECI average
attacker (alpha=0.25)
attacker (alpha=0.75)
Change ID to Erase Debt (i.e. r = r1)
avg1 qf
C)qf(Pr
r = r1 × r2
qfavg ← a × qfavg + (1–a) × qf(P→C), 0 < a < 1
1C)mf(P
C)gmf(PC)qf(P
B
CCi
Ci+1
A
P
hiding in qfavg’s variance,but still, worse off
}1,P)mf(C
C)mf(Pmin{r2
Community starts with n = 200 peers and private receipt repositories contain 2000 receipts each; merge 200
Simple text-based protocol over TCP/IP
The P2PWNC Protocol
C P
CONN
CACK
11:50am = t0 (consumer connects)
C P
RREQ
RCPT
11:51am (P requests 1st receipt)
RCPT timestamp = t0
RCPT weight = w1
C P
RREQ
RCPT
11:52am (P requests 2nd receipt)
RCPT timestamp = t0
RCPT weight = w2 > w1
P
RREQ
RCPT
11:53am (consumer has departed)
P stores last receipt
(timeout)
ReceiptRepository
P2PWNC Implementation
Open source: downloadable from http://mm.aueb.gr/research/P2PWNC
Linksys-centric all-in-one version
ECC and RSA digital signatures – tradeoffs
Guru-friendly modular version in preparation, for deployment in AWMN
P2PWNC client for Windows Mobile WLAN phones
Plus, VPN tunnels (PPTP and L2TP/IPsec), QoS module, VoIP proof-of-concept app.
Thanks!Thanks!
Elias C. Efstathiou
Mobile Multimedia LaboratoryDepartment of Computer Science
Athens University of Economics and [email protected]
http://mm.aueb.gr/research/P2PWNC
