WiFi-Reports:Improving Wireless Network Selection

Jeffrey Pang (CMU)with

Ben Greenstein (IRS)Michael Kaminsky (IRP)

Damon McCoy (U. Colorado)Srinivasan Seshan (CMU)

Problem: Commercial AP Selection

• tmobile

• attwifi (ap 1)

• attwifi (ap 2)

• seattlewifi

• linksys

• Free Public Wifi

• $3.99

• $9.99

• Free!

• Free!

• Which networks will run my

applications?

• Which ones have good performance?

• Quality

= ???

We often have many choices of wireless access points (APs),but little information about each

Jiwire.comHotspot database

Wifi-Reports

Wifi-ReportsDatabase

Bandwidth: 30 kbpsBlocked ports: Email

Bandwidth: 5 MbpsBlocked ports: None

community uploadsmeasurement reports

members downloadsummary statistics

Wifi-Reports

• tmobile

• attwifi (ap 1)

• attwifi (ap 2)

• seattlewifi

• linksys

• Free Public Wifi

• I need to use VoIP so this is the best network for me

Bandwidth: 300 kbpsBlocked ports: None

Bandwidth: 100 kbpsBlocked ports: None

Bandwidth: 300 kbpsBlocked ports: None

Doesn’t work!

Doesn’t work!

Bandwidth: 100 kbpsBlocked ports: Email, Skype

Bandwidth: 300 kbpsBlocked ports: None

Doesn’t work!

Wifi-Reports provides information about APperformance and application support

Doesn’t work!

Doesn’t work!

Bandwidth: 100 kbpsBlocked ports: None

Bandwidth: 300 kbpsBlocked ports: None

Bandwidth: 300 kbpsBlocked ports: None

Wifi-ReportsHotspot database

Bandwidth: 30 kbpsBlocked ports: Email

Bandwidth: 5 MbpsBlocked ports: None

Doesn’t work!

Research Challenges

Doesn’t work!

Doesn’t work!

Doesn’t work!

Doesn’t work!

Bandwidth: 300 kbpsBlocked ports: None • Location privacy

– Reports shouldn’t be linked, otherwise they can be used to track users

– But also need to limit fraud;e.g., 1 report per AP per user

– Solution: new ecash-like reporting protocol & robust summary functions

Research Challenges

Doesn’t work!

Doesn’t work!

Doesn’t work!

Doesn’t work!

Bandwidth: 300 kbpsBlocked ports: None

Bandwidth: 10 MbpsWorks great! (Really!)

• Location privacy– Reports shouldn’t be linked, otherwise

they can be used to track users– But also need to limit fraud;

e.g., 1 report per AP per user– Solution: new ecash-like reporting

protocol & robust summary functions

Research Challenges• Location privacy– Reports shouldn’t be linked, otherwise

they can be used to track users– But also need to limit fraud;

e.g., 1 report per AP per user– Solution: new ecash-like reporting

protocol & robust summary functions

• Location context– Performance dependent on location

with respect to AP– Wireless channel effects loss rate– Solution: estimate different loss regimes

w/ distributed measurements

Doesn’t work!

Doesn’t work!

Doesn’t work!

Doesn’t work!

Bandwidth: 300 kbpsBlocked ports: None

Bandwidth: 10 MbpsWorks great! (Really!)

100% loss 0% lossintermediate loss

WiFi-Reports Overview

WiFi-ReportsAccount Service

Independent Report Databases

Report on tmobile:Bandwidth: 1200 kbpsBlocked ports: None

Report on UBookstore Cafe:Bandwidth: 4000 kbpsBlocked ports: Email, Skype,…

• Users collect measurement reports when they use networks

• Reports are sent to databases for others to download

Questions and Challenges

• How useful would this system be in practice?– Do users have many choices of usable wireless networks?– Is there diversity in network performance/functionality?– Is performance stable enough to be predictive?– Are networks better for some applications but worse for others?– Do enough users use real networks to gather measurements?

• How do we limit “report fraud” and provide anonymity?– Not even the account service should be able to link a user’s reports to each other

(otherwise location privacy is violated)– But each user should only be able to report once per network

Part I: Measurement Study• We built a measurement tool to simulate

reports that users would collect– Bandwidth, latency, jitter, blocked ports,

number of other users, etc.• We measured all networks visible from:

– 13 hotspots near The Ave– ~7 days at different times of day

• Measurement procedure:– Sit near center of hotspot– Perform active spot measurements

(2-3 minutes per network)

shinka tea

tullys 1

starbucks 1

tullys 2

trabant

oasislounjin

yunnie bubble teasureshot

bookstore

cafeontheave

starbucks 2

cafesolstice

Our study is the first to examine pay networks and encrypted networks,

in addition to open networks

Results: Are there many APs?

Better

Results: Are there many APs?

Better

Results: Is there diversity?

Better

Results: Is there diversity?

Better

Results: Are measurements predictive?

Better

Results: Are there application trade-offs?

Better

Better

Better latency

Better bandwidth

Part II: Private and Accountable Reporting

• How do we limit “vote fraud” and provide anonymity?

• Requirements:– No one, even the account service, should be able to link a

user’s reports to each other (otherwise previous work says location privacy is violated)

– Each user should only be able to report once per network

Anonymizing Mix Network

Design SketchWiFi-Reports

Account Service

Report on tmobile:Bandwidth: 1200 kbpsBlocked ports: None

Independent Report Databases

tmobile

tmobileseattlewifi

CAFEONTHEAVE…

1. Client creates one token per AP2. Client blinds each token3. Account Service signs blinded tokens4. Client unblinds tokens…5. Client uses and measures an AP6. Client uses token to sign report7. Report is published via mix network

Report on tmobile:Bandwidth: 1200 kbpsBlocked ports: None

Report on tmobile:Bandwidth: 1200 kbpsBlocked ports: None

Report on tmobile:Bandwidth: 1200 kbpsBlocked ports: None

Design Sketch Details

• = random public key pair {Ktmobile, Ktmobile-1}

– Account Service signs the public key Ktmobile

– Private key Ktmobile-1 is used to sign reports can update reports

• Assumptions:– The account service only gives one identity to each user– Most users are honest

• Open issues:– Client must get all tokens signed before using them expensive

• If only get signed token after AP is used, use of the AP is revealed• Can obtain a subset of all tokens instead (e.g., all tokens for a city)

trade off between token signing overhead for more location privacy

Token Generation Time

• Time to generate tokens for all APs in one city(single threaded Xenon 3.4 Ghz server)

Resistance to Fraud

• CDF of Prediction accuracy (1 = most accurate)

Ongoing work

• Implemented Wifi-Reports service– Linux client exists– Currently implementing Android client.

Anyone want to help?

• MobiSys ‘09

Questions?

(Other summer projects)

Results: Is there diversity?

Better

Bloc

ked

port

= n

o m

easu

rem

ent

Results: Is there diversity?

Better

Bloc

ked

port

= n

o m

easu

rem

ent

Results: Are measurements predictive?

Better

Bloc

ked

port

= n

o m

easu

rem

ent