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Ditto - A System for Opportunistic Caching in Multi-hop Mesh Networks Fahad Rafique Dogar Joint work with: Amar Phanishayee, Himabindu Pucha, Olatunji Ruwase, and Dave Andersen Carnegie Mellon University
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Wireless Mesh Networks (WMNs) 2 Cost Effective Greater Coverage Testbeds: RoofNet@MIT, MAP@Purdue, … Commercial: Meraki 100,000 users of San Francisco ‘Free the Net’ service
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Throughput Problem in WMNs 3 Interference GW becomes a bottleneck P1 P3
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Exploiting Locality through Caching 4 On-Path + Opportunistic Caching -> Ditto P1 P2 P3 Path of the transfer: Alice, P1, P3, GW P1 and P3 perform on-path caching P2 can perform opportunistic caching
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Ditto: Key Contributions Built an opportunistic caching system for WMNs Insights on opportunistic caching – Is it feasible? – Key factors Ditto’s throughput comparison with on-path and no caching scenarios – Up to 7x improvement over on-path caching – Up to 10x improvement over no caching 5 Evaluation on two testbeds
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Outline Challenge and Opportunity Ditto Design Evaluation Related Work 6
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Challenge for Opportunistic Caching Wireless networks experience high loss rates – Usually dealt with through link layer retransmissions Overhearing node also experiences losses 7 1 1 2 2 P1 P3 Unlike P1, P2 cannot ask for retransmissions Successful overhearing of a large file is unlikely P2 Main Challenge: Lossy overhearing
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More Overhearing Opportunities 8 Reduces the problem of lossy overhearing Path of the transfer: Alice, P1, P3,… P2 may benefit from multi-hop transfers P2 P1 P3
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Outline Challenge and Opportunity – Lossy Overhearing – Multiple Opportunities to Overhear Ditto Design – Chunk Based Transfers – Ditto Proxy – Sniffer Evaluation Related Work 9
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Chunk Based Transfers Motivation – Lossy overhearing -> smaller caching granularity Idea – Divide file into smaller chunks (8 – 32 KB) – Use chunk as a unit of transfer Ditto uses Data Oriented Transfer (DOT) 1 system for chunk based transfers 1 Tolia et al, An Architecture for Internet Data Transfer. NSDI 2006. An Architecture for Internet Data Transfer 10
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Data Oriented Transfer (DOT) 11 chunkID3 chunkID1 chunkID2 Foo.txtFoo.txt Cryptographic Hash App Request – foo.txt Response: chunk ids{A,B,C} DOT chunk ids Receiver Sender DOT chunk ids Chunk Request Chunk Response Chunking DOT Transfer
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An Example Ditto Transfer 12 App Same as DOT DOT chunk ids Chunk request DOT chunk ids Chunk response request response request response ReceiverSender Ditto Proxy
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13 Next-Hop based on routing table information Separate TCP connection on each hop On-Path Caching Opportunistic Caching
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Sniffer 14 P1 P2 (Overhearing) P3 Path of the transfer: Alice, P1, P3,… TCP stream identification through (Src IP, Src Port, Dst IP, Dst Port) Placement within the stream based on TCP sequence number Next Step: Inter-Stream Chunk Reassembly
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Inter-Stream Chunk Reassembly 15 Look for Ditto header Chunk Boundaries Exploits multiple overhearing opportunity
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Outline Challenges and Opportunities Ditto Design Evaluation – Testbeds – Experimental Setup – Key Results – Summary Related Work 16
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Emulab Wireless Testbed 17
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MAP Campus Testbed (Purdue Univ.) 18 Gateway
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Experimental Setup Mode802.11 b RateAuto Other Parameters Default RoutingStatic Cross TrafficNo Cache EvictionNo File Sizes1-5 MB Chunk Sizes8KB, 16KB, 32KB 19
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Evaluation Scenarios Measuring Overhearing Effectiveness P1 P2 GW P3 Each observer reports the number of chunks successfully reconstructed Each node becomes a receiver Transfer Receiver Observer 20 Example Observer Receiver Transfer
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Reconstruction Efficiency 21 Around 60% of the observers don’t reconstruct anything Around 30% of the observers reconstruct at least 50% chunks
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Reconstruction Efficiency 22 Around 50% observers are able to reconstruct at least 50% chunks
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23 Zooming In --- Campus Testbed
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25 Zooming In --- Campus Testbed
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26 Shield the gateway from becoming a bottleneck
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Throughput Evaluation Leaf Nodes request the same file from the gateway – e.g: software update on all nodes Different request patterns: – Sequential, staggered – Random order of receivers Schemes – Ditto’s comparison with On-Path and E2E 27
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Throughput Improvement in Ditto 28 Median = 540 Kbps Campus Testbed
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Throughput Improvement in Ditto 29 Median = 1380 Kbps Median = 540 Kbps Campus Testbed
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Throughput Improvement in Ditto 30 Median = 5370 Kbps Median = 1380 Kbps Campus Testbed Median = 540 Kbps
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Evaluation Summary Metric/FactorInsight ProximityNodes closer to gateway have a very high reconstruction efficiency Can shield the gateway from becoming a hotspot ThroughputOrders of magnitude improvement with Ditto Inter-Stream Reassembly Few multiple overhearing opportunities; 10% improvement where applicable Chunk Size8 - 32 KB chunk size provide good reconstruction efficiency with low overhead 31
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Related Work Hierarchical Caching [Fan98, Das07,..] – Caching more effective on lossy wireless links – Ditto’s overhearing feature is unique Packet Level Caching [Spring00, Afanasyev08] – Ditto is purely opportunistic – Ditto exploits similarity at inter-request timescale Making the best of broadcast [MORE, ExOR,..] – Largely orthogonal 32
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Conclusion Opportunistic caching works! – Key Ideas: Chunk based transfer, inter-stream chunk reconstruction – Feasibility established on two testbeds – Nodes closer to the gateway can shield it from becoming a bottleneck Significant benefit to end users – Up to 7x throughput improvement over on-path caching – Up to 10x throughput improvement over no caching 33
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Thank you! 34
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