ranveer chandra, jitendra padhye, lenin ravindranath, alec wolman

Ranveer Chandra, Jitendra Padhye, Lenin Ravindranath, Alec Wolman

Which AP is serving client C? At any given point in time, there should be only

one answer Responsibility for relaying downstream traffic Secondary purposes: PSM, capability

negotiation

“broadcast” applications: certain information is Inherently relevant to a particular location Largely independent of the receiving client▪ e.g. not confidential

Wi-Fi clients perform scans to discover nearby APs (disconnected and connected) Passive Scans▪ Stations switch channels, listening for

beacons Active Scans▪ Stations send probe requests, listen for probe

responses Networks Found

Connected

Low bandwidth communication protocol for IEEE 802.11 (Wi-Fi) networks

Construct a transport protocol by overloading beacons

Clients receive information from nearby APs Even when they are disconnected When connected to another AP

Primarily for “broadcast” applications Two way communication is also possible

Network SelectionLocalized Advertisements

Beacons overloaded with Pricing Information Number of active users Other network utilization Information (e.g. 802.11k )

Help end users or client software in making connection decisions

Handheld devices with Wi-Fi are proliferating Embed advertisements in beacons Clients receive ads regardless of their

connection status Implicitly location-sensitive “Push” model of ad delivery – improves privacy

model Can include dynamic information

e.g. 4 tables free, or 15 minute wait

Delivery of advertisements over the Internet has become a huge market Ads are targeted – Relevant to the user

Location-sensitive advertisements already becoming an important market

AdCenter APat CoffeeDay

WiFi Beacon

Within 250 m of CoffeeDay

CoffeeDay close to me..Hmm!

I love Hot Choc! Go to CoffeeDay

Coffee DayDrink of the day is Hot Chocolate

Client running our software

AP fragments each message and sends in successive beacons

Client application performs reassembly, and presents the information to users

Each fragment is of the form: <UniqueID>

- This identifies the specific message <SeqNo>

- Fragment number <More Flag>

- Boolean indicating whether there are more fragments in this message

<Content>- Arbitrary byte stream (usually text)

Three encoding strategies Overload the SSID Overload the BSSID Create a new Information Element

SSID is network name with 32 byte limit

Pros User level client software is enough, no driver mods

Cons Low Bandwidth Spams the user-interface (e.g. zero-config)

Networks Found

Connected

BSSID length: only 6 bytes per beacon All messages can use the same SSID, say

“BeaconTransport” Client assembles message from multiple beacons

Pros User level client software Avoids the spam problem

Cons Very low bandwidth

Each beacon carries a set of Information Elements Other management frames too (Probe Request, Probe

Response) 802.11 is extensible (sort-of) – Element IDs 32-255

“reserved” Maximum length of an Information Element: 253 bytes

Pros Moderate bandwidth Also avoids spam Feels less like an “ugly hack”

Cons Requires driver modifications on most clients

(except Vista)

When a fragment is lost, so is entire message FEC, or probes to request retransmissions

Throughput 100 ms is typical beacon interval, easy to

adjust Airtime utilization

Not necessary to send at lowest data rate Power management

We have built access point & client implementations on XP

Recently finished version of client for Windows Mobile

Future Test deployment (at MS? Or a local mall?) Implement bi-directional communication

(e.g. pub/sub for ads)

Questions?

Clients receive beacons (hence ads) even when they are not connected to any network

Clients receive beacons from other networks even when they are connected to a particular network

Client need not send any information to the Access Point

Can update Ad text to include dynamic information

Number of tickets left Daily specials in a restaurant Stock quotes

Starbucks: Get 10% off a Hot Chocolate

Step 1: Coupon Distribution Step 2: Coupon Validation

Present coupon

Discounted Coffee

User does not have to connect to the Internet!

Competitor’s AP sends fake coupons e.g. Tully’s AP sends invalid coupons for Starbucks

Solution: ASP generates public-private key pair for all registered stores Coupon is encrypted with store’s private key

▪ E[coupon + time of validity]store_private_key

▪ Time of validity prevents replay of outdated coupons

Store/ASP sends encrypted coupon to APs APs broadcasts encrypted coupons Clients validate coupon by decrypting with store’s public

key

User awareness: Require user input

AP tracking: Track which AP distributed the coupon

Limited coupons: Limit number of distributed coupons

AP not connected to Internet