Carnegie Mellon University Computer Science Department 1 Ditto: Eavesdropping for the Common Good in Multi-hop Wireless Networks Amar Phanishayee Fahad Dogar, Himabindu Pucha, Olatunji Ruwase, Dave Andersen Carnegie Mellon University CMU Speaking Skills Talk
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Carnegie Mellon University Computer Science Department Carnegie Mellon University Computer Science Department 1 Ditto : Eavesdropping for the Common Good.
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Carnegie Mellon UniversityComputer Science DepartmentCarnegie Mellon UniversityComputer Science Department 1
Carnegie Mellon UniversityComputer Science DepartmentCarnegie Mellon UniversityComputer Science Department 2
Wireless Networks
Wired Link
Wireless Router
WirelessLink
Cable Modem
Access-Point Based(Single Wireless Hop)
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No Infrastructure Cost Effective, Greater Coverage
Poor throughput
Wireless Networks
Multi-hop Wireless (Mesh) Networks
Gateway
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Hop Count Affects Throughput
• Pairwise transfers in a 28 node campus testbed• More hops slower transfers (low throughput)
Median3.2Mbps0.5Mbps
0.2Mbps
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Ditto: Key Contributions
Ditto improves throughput by reducing hop count to acquire commonly requested data Caching of overheard data
Also cache data on the path of a transfer Application independent caching
Ditto improves throughput (on 2 testbeds) Up to 7x better than on-path caching Up to 10x better than no caching
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Caching In Ditto Helps Speed Transfers
On-Path + Opportunistic Caching Ditto
Path of the transfer: Alice--A1--A2--Gateway A1 and A2 -- on-path caching
A3 -- opportunistic caching
A2
A3
GatewayA1
Request
Response
Cached copy
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Outline Overview
Why poor throughput in mesh networks?
Ditto’s Design
Evaluation
Future Work
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Transmit Range:Range in which router’s transmission can be fully decoded
Interference Range:Range in which router’s transmission can interfere but may not be fully decoded
Interference
Interference: Receivers in Interference Range cannot decode simultaneous transmissions
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Avoiding Interference: Don’t Transmit If You Know Others Are
Transmitting one at a time avoids interference
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GatewayA3A1 A2
• Subsequent Hops InterfereD1’s transmission from A2 to A3 stalled
• Cause for poor throughput in mesh networks
Subsequent Wireless Hops Interfere
D1D1
D2
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Mesh Networks Have Poor Throughput• Some links have a high loss rate• Subsequent hops interfere with each other• More hops slower transfers (low throughput)• Gateway is a bottleneck
A1A2
A3Gateway
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Outline Overview
Why poor throughput in mesh networks?
Ditto’s Design Opportunities to improve throughput How does Ditto utilize these opportunities?
Evaluation
Future Work
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Opportunities To Improve Throughput
Locality in access patterns Many clients from different parts of the network
request the same data over timee.g. Operating System Updates, Streaming video
Wireless routers can overhear other transfers The positive aspect of broadcast
Looks like a job for … Opportunistic Caching
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Challenge: Lossy Overhearing Wireless networks have high loss rates
Loss recovery: link layer retransmissions
Overhearing node also experiences losses Losses independent of that at Receiver Cannot ask for retransmissions
Successfully overhearing a large file is unlikely
1
1
22
A3
A1
A2
1 22A1
A2
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Chunk Based Transfers Lossy overhearing cannot overhear entire file
Smaller caching granularity Divide file into smaller chunks (8 – 32 KB) Use chunk as a unit of transfer
Ditto uses Data Oriented Transfer (DOT)1 for chunk based transfers
1 Tolia et al, An Architecture for Internet Data Transfer. NSDI 2006.