A Self-Managed Scheme for Free Citywide Wi-Fi
Elias C. Efstathiou and George C. Polyzos
Mobile Multimedia LaboratoryDepartment of Computer Science
Athens University of Economics and Business
WoWMoM-ACC, June 13, 2005
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
The Peer-to-Peer Wireless Network Confederation (P2PWNC)
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Manhattan WLANs, 2002
Skyhook Wireless Wi-Fi Positioning System (WPS)
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
Nokia 9500Motorola CN620
Motivation (III) WLAN-enabled mobile phones are on the market
Motivation (IV) Public WLAN operators mainly target “hotspots” Municipal wireless
still in its infancy
Motivation (II) Many WLANs are secured against outsiders Need incentives to keep them open
P2PWNC Motivation
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P2PWNC: An incentives-based P2P system Teams provide WLAN access to each other Teams should provide in order to consume
WLAN viewWLAN view Team viewTeam view
: WLAN access point
: team member
Whiteteam
Greenteam
Blueteam
The Rules of P2PWNC
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From Gartner: 2001: 1200 public hotspots worldwide 2003: 71 000 public hotspots worldwide 2005: 23 500 WLANs in hotels worldwide
The Public Hotspot Market
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): 50 000 WLANs in just 5 Massachusetts
cities and towns (Watertown, Brookline, Roxbury, Newton, and Cambridge)
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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)
P2P Systems
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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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
Incentive Schemes for P2P(and representative papers)
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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
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
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
P2PWNC Design Principles
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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
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
N-way Exchanges
A B C D
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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
System Entities
Team = Members + Access Points (APs) Teams := P2PWNC peers Assume intra-team trust Team ID = (unique) PK-SK pair
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
Member PK
Team PK
Member cert
Timestamp
Team PK
Signed by Team SK
Signed by Member SK
Weight
PK: public key SK: private key
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Receipt Generation
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C P
CONN
CACK
11:50am = t0 (member 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 (member has departed)
P stores last receipt
(timeout)
ReceiptRepository
The Receipt Graph
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A
B
C
G
H
FE
D
I
Directed weighted graph (with cycles)
Vertices: team public keysEdge weight: sum of weights of corresponding receipts
Edges point from the consuming team to the providing team
W1
W2
W3
W4
W5 W6
W7
W8
W9
W10W11
W12
W13
W14
Graph security
Free-riders and colluders can create an arbitrary number of fake vertices and edges
They cannot create fake outgoing edges starting from teams who are outside the colluding group (they do not have the relevant private keys)
Receipt Repository
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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
Cooperation Strategies
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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
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
Team PK
Member cert
Signed by Member SK
R: “B?” B
G
R
Y
X
Z
Team PK
Member cert
Signed by Member SK
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Timestamp Timestamp
Weight Weight
Timestamp Timestamp
Weight Weight
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
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…)
timeTHOLD THNEW
discardingsevictions
DR(filled to capacity)
R’s repositories
Hashes ofdiscarded
DR
IR OR RR
R
R
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NWAY Operation
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?
Step 2. Provider discards receipts and updates TH• 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
PC
PCIRC RRC+RRP ORP
… …
…… ………
… …
… …
PC is visiting
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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
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ALLC
ALLD
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NWAY against ALLC, ALLD, RAND
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Round #
Failu
re R
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)
RAND
NWAY
NWAY against Traitors
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NWAY
P2PWNC Protocol Messages (1/2)
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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
P2PWNC Protocol Messages (2/2)
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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
Linux-based WLAN access point 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)
Linksys WRT54GS
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Final Points and Summary
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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/
Thanks!Thanks!
Elias C. Efstathiou
Mobile Multimedia Laboratory
Department of Computer Science
Athens University of Economics and Business
http://mm.aueb.gr/people/efstath/
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