A Self-Managed Scheme for Free Citywide Wi-Fi
Elias C. Efstathiou and George C. Polyzos
Mobile Multimedia Laboratory
Department of Computer Science
Athens University of Economics and Business
WoWMoM-ACC, June 13, 2005
[email protected]
Outline

P2PWNC Overview and Motivation

Background (hotspot market, P2P, and incentives in P2P)

P2PWNC Design Principles

P2PWNC Design

The NWAY Decision Algorithm

Simulations

Protocol and Implementation

Summary and Conclusions
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The Peer-to-Peer Wireless Network
Confederation (P2PWNC)
A wireless LAN (WLAN) aggregation scheme
Unites WLANs in citywide [con]federations
 Requires no authorities
 Relies on reciprocity between peers

Motivation

Numerous WLANs, connected to the Internet,
are within the range of passersby
Manhattan WLANs, 2002
Skyhook Wireless Wi-Fi Positioning System (WPS)
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P2PWNC Motivation
Motivation (II)
Many WLANs are secured against outsiders
 Need incentives to keep them open

Motivation (III)

WLAN-enabled mobile phones are on the market
Motivation (IV)
Public WLAN operators mainly target “hotspots”
 Municipal wireless
still in its infancy

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Motorola
CN620
Nokia 9500
The Rules of P2PWNC
P2PWNC: An incentives-based P2P system
Teams provide WLAN access to each other
 Teams should provide in order to consume

Blue
team
White
team
Green
team
: WLAN access point
: team member
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WLAN view
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Team view
Background
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The Public Hotspot Market
From Gartner:
2001: 1200 public hotspots worldwide
 2003: 71 000 public hotspots worldwide
 2005: 23 500 WLANs in hotels worldwide

A subscription buys you (June 2005):
Sprint PCS: 19 000 hotspots worldwide
 Boingo Wireless: 17 400 hotspots worldwide
 T-Mobile HotSpot: 16 663 hotspots worldwide

Skyhook Wireless data (2005):

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50 000 WLANs in just 5 Massachusetts
cities and towns (Watertown, Brookline,
Roxbury, Newton, and Cambridge)
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P2P Systems
General term
Usually associated with file sharing systems
 Also includes:
• Grids (computation)
• (Mobile) ad hoc networks (packet forwarding)
• Distributed Hash Tables (scalable, fault-tolerant storage)
• eBay-like communities (electronically mediated communities
of providers and consumers)

Distinctive characteristics
Peers act as both providers and consumers of resources
 System relies on peer cooperation
 Free-riding will prevail if:
• there is a cost involved with providing resources
• there are no authorities that can punish or reward
• exclusion from consuming the shared resources is impossible

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Incentive Schemes for P2P
(and representative papers)
Micropayments
Digitally signed tokens used as payment
 Requires online bank to check for double spending (and to issue the credits)

Yang, Garcia-Molina, “PPay: Micropayments for P2P Systems,” ACM CCS’03
Tamperproof modules
Each peer maintains its own account balance
 Increase when providing, decrease when consuming

Buttyan, Hubaux, “Stimulating Cooperation in Self-Organizing MANETs,” ACM/Kluwer MONET 2003
Multiple account holders
Other peers maintain a peer’s account balance
 Use majority rule in case of disagreement

Visnumurthy, Chandrakumar, Sirer, “Karma: A Secure Economic Framework for P2P Resource
Sharing,” p2pecon’03
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P2PWNC Design Principles
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P2PWNC Design Principles
Why P2P?
A lot of underexploited WLANs out there set up by individuals
 Hotspot operators (the corresponding “centralized model”):
• operate only a small fraction of the WLANs out there
• further segregate WLANs by competing for venues among themselves

Micropayments, tamperproof modules, multiple account holders:
Why choose another incentive scheme?
Require central authority (micropayments)
 Are unrealistic (tamperproof modules)
 Assume peers want to maintain accounts for others and/or perform auditing
• by trying to encourage “account holding” we get back where we started

We need a simple incentive scheme that will
encourage participation and cooperation
even at the expense of accurate accounting
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N-way Exchanges
Adopt N-way exchanges as the incentive technique





A generalization of barter, which retains some of its simplicity
“Provide to those [who provided to those]* who provided to me”
A type of indirect reciprocity (sociology term)
Scales to larger populations, compared to direct-only exchanges
Does not require (central or distributed) authorities
A
B
C
D
Some variants of the basic N-way scheme:
Cox, Noble, “Samsara: Honor Among Thieves in P2P Storage,” SOSP’03
Ngan, Wallach, Druschel, “Enforcing Fair Sharing of P2P Resources, “ IPTPS’03
Anagnostakis, Greenwald, “Exchange-based Incentive Mechanisms for P2P File Sharing,” ICDCS’04
Feldman, Lai, Stoica, Chuang, “Robust Incentive Techniques for P2P Networks,” ACM EC’04
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P2PWNC Design
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System Entities
Team = Members + Access Points (APs)
Teams := P2PWNC peers
 Assume intra-team trust
 Team ID = (unique) PK-SK pair

PK: public key
SK: private key
Member certificate
Member ID = (unique) PK-SK pair
 Member certificate binds Member PK to Team PK

Receipt
Encodes P2PWNC transactions between teams
 Signed by consuming member
 Receipt weight: amount of bytes the AP forwarded

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Member PK
Team PK
Signed by Team SK
Team PK
Member cert
Timestamp
Weight
Signed by
Member SK
Receipt Generation
11:50am = t0 (member connects)
11:51am (P requests 1st receipt)
CONN
RREQ
C
C
P
P
CACK
RCPT
RCPT timestamp = t0
RCPT weight = w1
11:52am (P requests 2nd receipt)
11:53am (member has departed)
RREQ
C
RREQ
(timeout)
Receipt
Repository
P
RCPT
RCPT timestamp = t0
RCPT weight = w2 > w1
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P
RCPT
P stores last receipt
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The Receipt Graph
Directed weighted graph (with cycles)
W1
E
F
W2
W5
Graph security
W4
W3
B
W6
W9
W7
A
Free-riders and colluders can create
an arbitrary number of fake vertices
and edges
G
W14
They cannot create fake outgoing
edges starting from teams who are
outside the colluding group (they do
not have the relevant private keys)
I
W8
W13
W10
H
W11
D
C
W12
Vertices: team public keys
Edge weight: sum of weights of corresponding receipts
Edges point from the consuming team to the providing team
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Receipt Repository
Two options:
Centralized repository
• Requires a well-known server that all teams can agree on
• All receipts are visible by all teams
• Server drops oldest receipts when full
• Mostly used to gauge the effectiveness of decentralized repositories
• Could have some practical importance
 Decentralized repository
• Each team maintains its own private repository
• Fills it with receipts it receives during a WLAN transaction
• And with receipts it receives when gossiping

Gossiping algorithm:




Roaming members carry receipts from their team repositories
They present them to the teams they visit
With RSA-1024 keys, a receipt is about 650 bytes long
With ECC-160 keys, a receipt is about 150 bytes long
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Cooperation Strategies
Three cooperation strategies tested so far, each one:
Uses a different decision algorithm
• Input: the receipt graph
• Output: a decision of whether to provide service or not
 May use a different gossiping algorithm (in the decentralized case)
• Different ways to choose the receipts that roaming members present
 May use a different bootstrapping algorithm
• New teams need to provide before starting to consume
• For how long, and to whom?

Specific decision algorithms include:
NWAY (assumes unit weights on receipts)
 maxflow (borrowed from Feldman, Lai, Stoica, Chuang, “Robust Incentive Techniques for

P2P Networks,” ACM EC’04)

gmf (generalized maximum flow)
Progressively more robust against double-spending and collusion
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The NWAY Decision Algorithm
Searches for potential N-way exchanges
Red provides to Blue if there is a chain of receipts connecting Red to Blue
 Red then discards all receipts in the discovered chain

X
R: “B?” B
Y
Z
G
R
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Team PK
Team PK
Member cert
Member cert
Timestamp
Timestamp
Timestamp
Timestamp
Weight
Weight
Weight
Weight
Signed by
Member SK
Signed by
Member SK
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NWAY: Space Requirements
Each team maintains 4 receipt repositories
IR – Incoming Receipts
 OR – Outgoing Receipts
 RR – Random Receipts
 DR – Discarded Receipts
holding up to sIR, sOR, sRR, sDR entries
replacement rule: delete oldest receipt

R’s repositories
R
R
IR
OR
Hashes of DR
discarded
Each team has a Time Horizon (TH)


When DR overflows, TH holds the timestamp of the receipt that was just evicted
TH and DR allow ignoring all discarded receipts (at a cost…)
DR
(filled to capacity)
evictions
discardings
time
THOLD THNEW
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RR
NWAY Operation
C
is visiting
P
…
Step 1. Provider searches for chain in merged repositories
• Temporarily merge IRC, RRC, RRP, ORP
• Consumer should carry IRC, RRC: space requirements?
• Consumer information revelation: incentives?
…
…
C
Step 2. Provider discards receipts and updates TH
…
IRC
…
…
…
…
…
…
P
…
RRC+RRP
ORP
• First receipt can simply be deleted from ORP
• Provider may still want to send locally discarded receipts to its own roaming members
Step 3. Provider and consumer store new receipt
• Consumer sends it to home ORC: incentives?
Step 4. Receipt dissemination (as a side-effect)
• Provider updates RRP with receipts from IRC and RRC
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C
P
Evaluation Framework
Used to evaluate the performance of the 3 cooperation strategies against each
other and against 3 strategies that do not rely on feedback (ALLC, ALLD, RAND)
 Teams are randomly paired
• 1 round: every team gets one chance to consume, one to provide
• if a team decides to provide service, it loses c = 1 points
• if a team is provided service by another team, it gains b = 7 points


Peers may learn – evolve towards the highest-rated strategy from the ones available
• With probability proportional to the difference in rating
A strategy’s rating :=
the average of the running average score/round of its followers
• weighted according to how many rounds they have been using the strategy
We also simulate system growth

Start with two teams; a new team joins at the end of each round; teams never leave
NWAY parameters for the experiments that follow:
sIR = sRR = sDR = 100, sOR = 400
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NWAY against ALLC, ALLD
7
100
90
6
NWAY
80
ALLC
ALLD
70
% Population
Score/Round
5
4
3
60
50
40
30
2
20
1
10
0
0
1
51
101
151
201
251
1
Round #
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51
101
151
Round #
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201
251
NWAY against ALLC, ALLD, RAND
70
12
NWAY
ALLC
60
RAND
10
RAND
NWAY
ALLD
50
Failure Rate (%)
% Population
8
40
30
6
4
20
2
10
0
0
1
51
101
151
201
251
1
Round #
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51
101
151
Round #
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201
251
NWAY against Traitors
90
85
80
75
Traitor
70
NWAY
65
Failure Rate (%)
60
55
50
45
40
35
30
25
20
15
10
5
0
1
51
101
151
Round #
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201
251
Design Summary
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P2PWNC Implementation
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P2PWNC Protocol Messages (1/2)
CONN

Sent by the roaming member to the AP

Contains the member certificate (base64-encoded)
CACK

Positive or negative response to connection request, sent by the AP

If positive, contains timestamp of session and public key of the providing team
RREQ

Request to sign a new receipt for this session, sent by the AP

Contains the volume of traffic, as measured by the AP
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P2PWNC Protocol Messages (2/2)
RCPT

Receipt (base64-encoded)

Sent by members to APs and by APs to the repository
UPDT (only in distributed repository mode)
Request by member to home repository for an update with the latest receipts
(newer than a timestamp)
QUER and QRES (algorithm specific)
Communication between AP and home (or global) repository. The request
contains the public keys of the (prospective) consuming and providing teams. The
response contains a PROVIDE or DO_NOT_PROVIDE reply
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Linksys WRT54GS
Linux-based WLAN access point




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[email protected]
We implemented the P2PWNC protocol (AP side) on it
32 MB RAM, 8 MB Flash, 200 MHz CPU
Retails for less than $70
Cryptographic, maxflow performance comparable to 200 MHz PC
Can act as home repository (storing more than 10 000 receipts)
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Conclusion
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Final Points and Summary

Hurdle to P2PWNC deployment: ISP sharing prohibitions?

Centralized or decentralized deployments?

People living in the outskirts: will the notion of teams be enough to incorporate
them? (In all 3 decision algorithms, teams end up having to provide
approximately as much as they consume – how will this work within a team?)

We demonstrated a family of practical incentive techniques for WLAN sharing

Teams do not have to trust one another!

There are no hard service guarantees

More at http://mm.aueb.gr/research/p2pwnc/
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Thanks!
Elias C. Efstathiou
Mobile Multimedia Laboratory
Department of Computer Science
Athens University of Economics and Business
http://mm.aueb.gr/people/efstath/
[email protected]