Fat Virtual Access Points

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Fat Virtual Access Points

• Srikanth Kandula

Kate Lin, Tural Badirkhanli and Dina Katabi
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State-of-the-art (802.11)
Connect to the AP with the highest RSSI
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In Homes, Hotspots
Problem 1
AP Uplink 2Mbps (DSL/Cable Modems)
Wireless Link 54Mbps (Theoretical Max)
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Uplink Bottleneck
Can Aggregate Bandwidth from nearby APs!
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At Work
Problem 2
20Mbps
Unnecessary congestion nearby APs are
idle Spread load Individual changes help
globally
15Mbps
4Mbps
Load Imbalance
Divide Total Bandwidth Among Users
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2 Problems, 1 Solution
State-of-the-art
Abstraction
Join closest AP
Join a Virtual AP, that is the sum of nearby APs

1. User can aggregate bandwidth from all APs
   
2. Compete for total ? balance load across APs

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Realize a fat virtual AP withonly client-side
changes
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Basic Operation
2Mbps
2Mbps
20Mbps
But, what about receive?
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Basic Operation
2Mbps
2Mbps
Drop
Power-Save Q
20Mbps
Pretend in power-save, so AP buffers when
disconnected (similar to Chandra et. al.
VirtualWiFi)
Divide Time and Data Across APs to get Fat
Virtual AP
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Realizing a Fat Virtual AP is Hard

• Sustain TCP flows through each AP
• Cannot lose packets yet Switch quickly
• Which APs to connect to and for how long?
• Some APs are more valuable than others
• How to divide traffic across the APs?

• FatVAP, an 802.11 driver design
• divides time across APs to maximize throughput
• is transparent to APs and remote ends

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FatVAP Overview
Channel 6
Channel 6
Channel 1
Channel 36

1. Scan for available APs
2. Compute a schedule to divide time across APs
3. Switch APs as per schedule
4. Spread traffic by pinning flows to APs

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How much time to spend at an AP?
Achievable Bandwidths end-to-end e, wireless w
Subsumes Wireless Link Quality, Contention at AP,
APs uplink capacity
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How to Divide Time Across APs?
AP Bandwidth (Mbps) AP1 AP2 AP3
End-to-end Achievable 5 4 3
Wireless Achievable 5 8 8
Usable Fraction 100 50 38
Optimal 7 Mbps
5 Mbps, 100 busy
Pick APs Greedily, on End-to-end rate
! more bang for the buck if wireless b/w is large
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How to Divide Time Across APs?
AP Bandwidth (Mbps) AP1 AP2 AP3 AP4 AP5 AP6
End-to-end Available 1 1 1 1 1 4.5
Wireless Available 5 5 5 5 5 4.5
Usable Fraction 20 20 20 20 20 100
5 Mbps, 100 busy
Pick APs Greedily, on End-to-end rate
Pick APs Greedily, on Wireless rate
! cost to switch is 5 ms ! cant linger too
long (100ms period)
Only 75 usable
No Greedy Solution!
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Say, fi is fraction of time at APi
Let s be switching time and D be the period
Value (Bandwidth)
Usefulness Constraint
Cost (Time)
Like Bin Packing, maximize value with bounded
cost!
(pseudo)-polynomial solution
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But, How to Estimate Bandwidths?

• Wireless Achievable

Naively send-rate of probe burst, APs
report load
Idea Use synchronous acks
Client TX Queue
AP Buffers
Time from head of tx queue to end of transmission
(ack)
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But, How to Estimate Bandwidths?

• Wireless Achievable

• End-to-end

Naively, send-rate of probe burst or APs
report load
t
Count bytes rcvd in a window
Idea Use synchronous acks
Client TX Queue
AP Buffers
Time from head of tx queue to end of transmission
(ack)
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But, How to Estimate Bandwidths?

• Wireless Achievable

• End-to-end

Naively, send-rate of probe burst or APs
report load
t
Count bytes rcvd in a window
Idea Use synchronous acks
May not receive data always
Idea only count back-to-back large packets!
Client TX Queue
AP Buffers
Time from head of tx queue to end of transmission
(ack)
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How to Spread Traffic Across APs?
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How to Spread Traffic Across APs?

• Put flows through all APs
• virtualize 802.11 state
• an IP for each interface
• toggle APs (and channels)
• By default, kernel sends all traffic to one AP

AP1
AP2
MIT 128.30.79.0/24
T-Mobile [phone]
Toggler
Hardware (Wireless Card)
802.11 State
AP1 State
AP2 State
Two Interfaces
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How to Spread Traffic Across APs?

• Put flows through all APs
• virtualize 802.11 state
• an IP for each interface
• toggle APs (and channels)
• By default, kernel sends all traffic to one AP
• Spread flows to APs
• Fast header re-writing

AP1
AP2
Hardware (Wireless Card)
Toggler
AP1 State
AP2 State
Two Interfaces
Spreader
Distribute load w/o changing APs and applications
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Switching Quickly Without Drops

• A Driver For Each Interface?
• warm-up cost on switch
• one instance soft-switch

AP1
AP2
One Driver
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Switching Quickly Without Drops

• A Driver For Each Interface?
• warm-up cost on switch
• one instance soft-switch
• 802.11 Control Packets
• Isolate Transitions

AP1
AP2
Hardware (Wireless Card)
AP1 State
AP2 State
Re-send AUTH
Send AUTH
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Switching Quickly Without Drops

• A Driver For Each Interface?
• warm-up cost on switch
• one instance soft-switch
• 802.11 Control Packets
• Isolate Transitions
• Pkts stuck in driver at switch
• Private Queues

AP1
AP2

1. Delay Switch till pkts drain
2. Drop Packets (madwifi)

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Switching Quickly Without Drops

• A Driver For Each Interface?
• warm-up cost on switch
• one instance soft-switch
• 802.11 Control Packets
• Isolate Transitions
• Pkts stuck in driver at switch
• Private Queues

AP1
AP2
Attach/Detach Queue Pointer Swap
Enables high-rate TCPs through multiple APs
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FatVAP Realizes a Fat Virtual AP

• Which APs to connect to and for how long?
• Estimate Bandwidths, Solve Optimization
• How to divide traffic across the APs?
• Virtualize, Pin Flows to APs, rewrite headers
• Switch quickly but without losing packets
• In-driver, Private Queues, Isolation

And, with only client-side changes
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Related Work

• VirtualWiFi (Microsoft Research)
• AP Selection (Intel Research, U Michigan)
• SyncScan (UCSD)
• MadWifi (open-source)
• Divide Time across APs to maximize throughput
• Sustain TCP flows through multiple APs
• Transparently spread traffic across APs

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Results
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Experimental Setup

• Compare FatVAP driver with unmodified MadWifi
• Scenarios
• Testbed built from Cisco, NetGear and MadWifi APs
• Residential deployments
• Commercial hotspots
• Traffic
• Long-lived TCP flows
• BitTorrent (Azureus client, Planetlab peers)
• Mimic Web Browsing (modified WebStone)

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Can FatVAP Aggregate Bandwidth?
6Mbps
22 Mbps
Aggregates end-to-end up to the wireless
bottleneck
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Can FatVAP Balance Load?
12Mbps
2Mbps
C1
C2
C3
C4
C5
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Can FatVAP Balance Load?
12Mbps
2Mbps
FatVAP
Unmodified MadWifi
5 4 3 2 1 0
4.4
3.8
3.5
3.3
3.1
2.9
2.8
2.7
Throughput (Mb/s)
.9
.9
C1
C2
C3
C4
C5
C1 C2 C3 C4 C5
C1 C2 C3 C4 C5
Simplifies Network Deployment!
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Can FatVAP Adapt to Changes?
5Mbps
15Mbps
Throughput (Mb/s)
Re-adjusts time_at_AP as necessary
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Contributions

• A new model for managed 802.11 LANs
• Aggregate uplink, Balance load
• First to realize a fat virtual AP
• Divide time and traffic across APs
• Transparent to APs, applications, servers