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Fat Virtual Access Points
Srikanth Kandula
Kate Lin, Tural Badirkhanli and Dina Katabi
Connect to the AP with the highest RSSI
State-of-the-art (802.11):
Wireless Link ~ 54Mbps(Theoretical Max)
AP Uplink ~ 2Mbps(DSL/Cable Modems)
Can Aggregate Bandwidth from nearby APs!Can Aggregate Bandwidth from nearby APs!
Problem 1: In Homes, Hotspots…
2+2+2+…2+2+2+…
Uplink Bottleneck
Load Imbalance
Unnecessary congestion; nearby APs are idle
Spread load
Individual changes help globally
Problem 2:
Divide Total Bandwidth Among Users Divide Total Bandwidth Among Users
20Mbps
15Mbps
At Work …
4Mbps4Mbps 7Mbps7Mbps
Join closest AP Join a Virtual AP, that is the sum of nearby APs
State-of-the-art: Abstraction:
1. User can aggregate bandwidth from all APs
2. Compete for total balance load across APs
1. User can aggregate bandwidth from all APs
2. Compete for total balance load across APs
2 Problems, 1 Solution
Realize a “fat virtual AP” withonly client-side changes
Basic Operation
2Mbps
20Mbps
But, what about receive?
2Mbps
Basic Operation
Drop
Power-Save Q
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” Divide Time and Data Across APs to get “Fat Virtual AP”
2Mbps
20Mbps
2Mbps
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 design1. divides time across APs to maximize throughput
2. is transparent to APs and remote ends
FatVAP, an 802.11 driver design1. divides time across APs to maximize throughput
2. is transparent to APs and remote ends
FatVAP Overview
1. Scan for available APs2. Compute a schedule to divide time across APs3. Switch APs as per schedule4. Spread traffic by pinning flows to APs
Channel 1Channel 6 Channel 6
Channel 36
How much time to spend at an AP?
w
e
Useful fraction of time w
e≤
Achievable Bandwidths– end-to-end e, wireless w
Subsumes Wireless Link Quality, Contention at AP, APs uplink capacity
Subsumes Wireless Link Quality, Contention at AP, APs uplink capacity
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
Pick APs Greedily, on End-to-end rate
5 Mbps, 100% busy
! more bang for the buck if wireless b/w is large
Pick APs Greedily, on End-to-end ratePick APs Greedily, on Wireless rate
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
! cost to switch is ≈ 5 ms! can’t linger too long (100ms period)
No Greedy Solution!No Greedy Solution!
How to Divide Time Across APs?
Only 75% usable
Say, fi is fraction of time at APi
⎡ ⎤( )∑
∑
≤+
≤≤
DsfDf
w
efts
wf
ii
i
ii
iifi
0..
max
Usefulness Constraint
Value (Bandwidth)
Cost (Time)
Let s be switching time and D be the period
(pseudo)-polynomial solution(pseudo)-polynomial solution
Like Bin Packing, maximize value with bounded cost!
But, How to Estimate Bandwidths?Wireless Achievable
Client TX Queue
Time from head of tx queue to end of transmission (ack)
Naively– send-rate of probe burst, APs report load
AP Buffers
Idea: Use synchronous acks
w
But, How to Estimate Bandwidths?Wireless Achievable End-to-end
Client TX Queue
Time from head of tx queue to end of transmission (ack)
Naively, send-rate of probe burst or APs report load
AP Buffers
Idea: Use synchronous acks
e
tCount bytes rcvd in a window
But, How to Estimate Bandwidths?
Wireless Achievable End-to-end
Client TX Queue
Time from head of tx queue to end of transmission (ack)
Naively, send-rate of probe burst or APs report load
AP Buffers
Idea: Use synchronous acks
e
tCount bytes rcvd in a windowMay not receive data alwaysIdea: only count back-to-back large packets!
How to Spread Traffic Across APs?
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
T-Mobile 192.168.3.0/24
MIT 128.30.79.0/24
Hardware (Wireless Card)802.11 State
AP1 AP2
AP1 State AP2 State
Two Interfaces
Toggler
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
Hardware (Wireless Card)
AP1 AP2
AP1 State AP2 State
Two Interfaces
Toggler
Spreader
Distribute load w/o changing APs and applicationsDistribute load w/o changing APs and applications
Switching Quickly Without Drops
AP1 AP2A Driver For Each Interface?– warm-up cost on switch
– one instance + soft-switch
One Driver
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 AUTHSend AUTH
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)
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 Enables high-rate TCPs through multiple APs
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 changesAnd, with only client-side changes
Related Work
VirtualWiFi (Microsoft Research)AP Selection (Intel Research, U Michigan)SyncScan (UCSD)MadWifi (open-source)
o Divide Time across APs to maximize throughputo Sustain TCP flows through multiple APso Transparently spread traffic across APs
Results
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)
Can FatVAP Aggregate Bandwidth?
6Mbps
5.8
11.4
17
20.6 20.5
5.8 5.8 5.8 5.8 5.8
0
5
10
15
20
25
1 2 3 4 5
FatVAP Unmodified MadWifi
Thr
ough
put
(Mb/
s)
Number of APs Aggregates end-to-end up to the wireless bottleneck Aggregates end-to-end up to the wireless bottleneck
~22 Mbps
Can FatVAP Balance Load?
12Mbps2Mbps
C1 C2 C3 C4 C5
Can FatVAP Balance Load?
12Mbps2Mbps
Thr
ough
put
(Mb/
s)5
4
3
2
1
0
FatVAP
2.93.5
3.1 2.8 2.7
.9.9
4.4
3.33.8
Unmodified MadWifi
Simplifies Network Deployment!Simplifies Network Deployment!
C1 C2 C3 C4 C5
C1 C2 C3 C4 C5 C1 C2 C3 C4 C5
Can FatVAP Adapt to Changes?
Thr
ough
put
(Mb/
s)
5Mbps 15Mbps
Re-adjusts time@AP as necessary Re-adjusts time@AP as necessary
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