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Block-switched Networks: A New Paradigm for Wireless Transport

Block-switched Networks: A New Paradigm for Wireless Transport. Ming Li, Devesh Agrawal, Deepak Ganesan and Arun Venkataramani presented by zhen qin, marcel flores. Motivation. How TCP works. E2E rate control is error-prone. How TCP works. E2E retransmissions are wasteful. How TCP works.

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Block-switched Networks: A New Paradigm for Wireless Transport

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  1. Block-switched Networks:A New Paradigm for Wireless Transport Ming Li, Devesh Agrawal, Deepak Ganesan and Arun Venkataramani presented by zhen qin, marcel flores

  2. Motivation

  3. How TCP works • E2E rate control is error-prone

  4. How TCP works • E2E retransmissions are wasteful

  5. How TCP works • Link layer ARQ

  6. How TCP works • Link-layer ARQs/backoffs hurt TCP rate control

  7. Hop Contribution • A clean-slate design and implementation of a wireless transport protocol • Using reliable per-hop block transfer as a building block

  8. Hop Design

  9. Structure of a block and Timeline of TCP vs Hop Reliable Block Transfer

  10. Virtual Retransmission • Exploit caching at intermediate node • Hop routers store all packets they overhear • Transmit BSYN packet when block dropped

  11. Backpressure • Limits #outstanding blocks per-flow at forwarder

  12. Backpressure • Limits #outstanding blocks per-flow at forwarder

  13. Ack Withholding • acknowledging only one BSYN packet • withholding BACKs to other concurrent BSYN packet until outstanding block complete • Mitigating impact of hidden terminals

  14. Micro-block Prioritization • Senders piggybacks small blocks to BSYN • Receivers prioritizes small block’s BACK • Low delay for small blocks

  15. Evaluation • A 20 node wireless mesh testbed • Linux 2.6 kernel • 802.11a/b/g Athero/MadWiFi card • Spread around the CS building

  16. Comparisons • End-to-end • UDP • TCP with CUBIC congestion control • Hop-by-hop • Hop-by-hop TCP • TCP with backpressure • DTN2.5 • Always subtract TCP setup time

  17. Results • Single hop microbenchmarks • 100 Random links, transferred 10 MB file • Shows robust performance on poor links

  18. Graceful Degradation • Sorted by TCP goodput • Tried artificially dropping packets, examined goodput for different rates

  19. Multi-Hop • 100 random pairs - varying lengths • TCP slightly inflated, OSLR only picks good links (Hop does well on lossy)

  20. Hop Components • Compare Hop against different versions of itself

  21. Other Comparisons • High Load • WLAN access points • Small file transfers • Single hop transfer delay • Multi hop transfer delay • Robustness to partitions • Affect on VOIP • Network and link layer dynamics • 802.11g

  22. Conclusion for Hop vs. TCP • It looks like Hop would do well in this environment • Blocks seem effective as a paradigm • Do not claim TCP can’t be made better • Instead, have shown starting from the ground up has potential

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