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)
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
P1
P3
•Interference
•GW becomes a bottleneck
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Exploiting Locality through Caching
P1 and P3 perform on-path caching
P3
Path of the transfer: Alice, P1, P3, GW
P1
P2
P2 can perform opportunistic
caching
On-Path + Opportunistic Caching -> Ditto
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Ditto: Key Contributions
• Built an opportunistic caching system for WMNs
• Insights on opportunistic caching
– Is it feasible?
– Key factors
Evaluation on two testbeds
• 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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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
P3
1
2
1
P1
P2
2
Unlike P1, P2 cannot
ask for retransmissions
Successful overhearing of a
large file is unlikely
Main Challenge: Lossy overhearing
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More Overhearing Opportunities
P3
Path of the transfer: Alice, P1, P3,…
P1
P2
P2 may benefit from multi-hop
transfers
Reduces the problem of lossy overhearing
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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
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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)
Chunking
Cryptographic
Hash
Foo.txt
chunkID1
chunkID2
chunkID3
DOT Transfer
Receiver
Request – foo.txt
App
Sender
App
Response: chunk ids{A,B,C}
chunk ids
chunk ids
Chunk Request
DOT
DOT
Chunk Response
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An Example Ditto Transfer
Receiver
Sender
Same as DOT
App
App
chunk ids
chunk ids
Chunk request
request
request
DOT
DOT
Chunk response
response
Ditto Proxy
response
Ditto Proxy
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Ditto Proxy
Opportunistic
Caching
Separate TCP
connection on
each hop
Next-Hop based
on routing table
information
On-Path Caching
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Sniffer
Path of the transfer: Alice, P1, P3,…
P3
P1
P2 (Overhearing)
•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
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
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Emulab Wireless Testbed
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MAP Campus Testbed (Purdue Univ.)
Gateway
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Experimental Setup
Mode
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, 32KB
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Evaluation Scenarios
• Measuring Overhearing Effectiveness
Observer
P3
P1
Example
GW
Transfer
Receiver
Observer
Transfer
P2
Observer
Receiver
• Each observer reports the number of chunks
successfully reconstructed
• Each node becomes a receiver
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Reconstruction Efficiency
Around 30% of the observers
reconstruct at least 50% chunks
Around 60% of the
observers don’t
reconstruct
anything
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Reconstruction Efficiency
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
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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
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Throughput Improvement in Ditto
Median =
540 Kbps
Campus Testbed
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Throughput Improvement in Ditto
Median
= 1380
Kbps
Median =
540 Kbps
Campus Testbed
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Throughput Improvement in Ditto
Median
= 1380
Kbps
Median =
540 Kbps
Campus Testbed
Median
= 5370
Kbps
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Evaluation Summary
Metric/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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