1. A Fault Tolerance Protocol for Uploads: Design and Evaluation
Leslie Cheung*, Cheng-Fu Chou#, Leana Golubchik*, Yan Yang*
*Internet Multimedia Lab
Computer Science Department & IMSC
University of Southern California
#Department of Computer Science and Information Engineering 
National Taiwan University

2. Background and Motivation
Bistro: A scalable, secure, wide area upload architecture
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3. Background and Motivation
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4. Problem
When some intermediaries fail or are malicious, the original protocol does not perform well
Need to ask clients to retransmit lost data
Goals
Improve performance by reducing retransmissions
Reduce the amount of redundant data
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5. Outline of Fault Tolerance Protocol
Erasure Code: Let k be the number of data packets, an erasure code encoder adds (n-k) parity packets to make it a n packets file
Erasure codes assume that received packets are correct.
This assumption is invalid because data can be corrupted.
Solution: Use checksums to detect corrupted packets
Drop corrupted packets, and treat them as losses
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6. Outline of Fault Tolerance Protocol
Definition: Checksum groups
Generate a checksum of a group of packets
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7. Outline of Fault Tolerance Protocol
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8. Analytical Models Reliability Model Performance Model Cost Function
Packet Lost Independently with probability p
Metric (c1)
Probability of retransmissions
Performance Model
Performance at the first step
Size of the timestamp request messages
Metric (c2)
Number of checksums per data packet
Cost Function
Cost = w1 * c1 + w2 * c2 w1, w2: weights
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9. Numerical Results Vary different parameters in the cost function
Parameters of interest
(n-k): number of parity packets in FEC group
k: number of data packets in FEC group
Few large FEC groups vs many small FEC groups
Z: number of checksum groups in a FEC group
p: probability of losing a packet
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10. Numerical Results
Varying (n-k), number of parity packets per FEC group
Y = 5, k = 10, Z = 2,3,…, p = 0.01, w1 = 0.9, w2=0.1
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11. Numerical Results Varying k, number of data packets per FEC group
W = 100, n = 2k , Z = 2, p = 0.01, w1 = 0.9, w2=0.1
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12. Numerical Results Varying Z, number of checksum groups per FEC group
Y = 5, n = 20, k = 10, p = 0.01, w1 = 0.9, w2=0.1
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13. Numerical Results Varying p, probability of losing a packet
Y = 5, n = 20, k = 10, Z = 2, w1 = 0.9, w2=0.1
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14. Conclusions and Future Work
Fault tolerance is important in uploads
Our protocol is in the right direction
Future Work
How to set the parameters?
Striping reliability and performance
Data collection problem (not all packets are needed with erasure code)
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15. Ordering of FEC group & checksum group
Can we reverse the order of FEC group and checksum group?
No. We drop the all packets in a checksum group if the checksum check fails.
If we reverse the order, losing one packet would result in dropping all packets in a checksum group, which consist of a number of FEC groups.
Do not have any packets to recover the lost part.
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