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

Wireless Mesh Networks (WMNs)

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Cost Effective

Greater Coverage

•Testbeds: RoofNet@MIT, MAP@Purdue, …

•Commercial: Meraki•100,000 users of San Francisco ‘Free the Net’ service

Throughput Problem in WMNs

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•Interference

•GW becomes a bottleneck

P1P3

Exploiting Locality through Caching

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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

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

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Evaluation on two testbeds

Outline

• Challenge and Opportunity• Ditto Design• Evaluation• Related Work

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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

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1

1

2

2P1

P3

Unlike P1, P2 cannot ask for retransmissions

Successful overhearing of a large file is unlikelyP2

Main Challenge: Lossy overhearing

More Overhearing Opportunities

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Reduces the problem of lossy overhearing

Path of the transfer: Alice, P1, P3,…

P2 may benefit from multi-hop transfersP2

P1

P3

Outline

• Challenge and Opportunity– Lossy Overhearing– Multiple Opportunities to Overhear

• Ditto Design– Chunk Based Transfers– Ditto Proxy– Sniffer

• Evaluation• Related Work

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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.

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Data Oriented Transfer (DOT)

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chunkID3chunkID3

chunkID1chunkID1chunkID2chunkID2Foo.txtFoo.txtFoo.txtFoo.txt

Cryptographic Hash

App 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

An Example Ditto Transfer

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App App Same as DOT

DOT

chunk ids

Chunk request

DOT

chunk ids

Chunk response

request

response

request

response

Receiver Sender

Ditto Proxy Ditto Proxy

Ditto Proxy

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Next-Hop based on routing table

information

Separate TCP connection on

each hop

On-Path Caching

Opportunistic Caching

Sniffer

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P1

P2 (Overhearing)

P3Path 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

Inter-Stream Chunk Reassembly

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Look for Ditto header

Chunk Boundaries

Exploits multiple overhearing opportunity

Outline

• Challenges and Opportunities• Ditto Design• Evaluation– Testbeds– Experimental Setup– Key Results– Summary

• Related Work

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Emulab Wireless Testbed

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MAP Campus Testbed (Purdue Univ.)

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Gateway

Experimental SetupMode 802.11 b

Rate Auto

Other Parameters

Default

Routing Static

Cross Traffic No

Cache Eviction No

File Sizes 1-5 MB

Chunk Sizes 8KB, 16KB, 32KB19

Evaluation Scenarios

• Measuring Overhearing Effectiveness

P1P2

GWP3

• Each observer reports the number of chunks successfully reconstructed

• Each node becomes a receiver

Transfer

ReceiverObserver

Observer

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Example

Observer Receiver

Transfer

Reconstruction Efficiency

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Around 60% of the observers don’t

reconstruct anything

Around 30% of the observers reconstruct at least 50% chunks

Reconstruction Efficiency

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Around 50% observers are able to reconstruct at least 50% chunks

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Zooming In --- Campus Testbed

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Zooming In --- Campus Testbed

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Shield the gateway from becoming a

bottleneck

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

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Throughput Improvement in Ditto

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Median = 540 Kbps

Campus Testbed

Throughput Improvement in Ditto

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Median = 1380 Kbps

Median = 540 Kbps

Campus Testbed

Throughput Improvement in Ditto

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Median = 5370 Kbps

Median = 1380 Kbps

Campus Testbed

Median = 540 Kbps

Evaluation SummaryMetric/Factor Insight

Proximity Nodes closer to gateway have a very high reconstruction efficiency Can shield the gateway from becoming a hotspot

Throughput Orders of magnitude improvement with Ditto

Inter-Stream Reassembly

Few multiple overhearing opportunities; 10% improvement where applicable

Chunk Size 8 - 32 KB chunk size provide good reconstruction efficiency with low overhead

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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

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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

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Thank you!

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