Block-switched Networks:A New Paradigm for Wireless T

ransport

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

• Link layer ARQ

How TCP works

• Link-layer ARQs/backoffs hurt TCP rate control

Hop Contribution

• A clean-slate design and implementation of a wireless transport protocol

• Using reliable per-hop block transfer as a building block

Hop Design

Reliable Block Transfer

• Structure of a block and Timeline of TCP vs Hop

Virtual Retransmission

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

Backpressure

• Limits #outstanding blocks per-flow at forwarder

Backpressure

• Limits #outstanding blocks per-flow at forwarder

Ack Withholding

• acknowledging only one BSYN packet

• withholding BACKs to other concurrent BSYN packet until outstanding block complete

• Mitigating impact of hidden terminals

Micro-block Prioritization

• Senders piggybacks small blocks to BSYN• Receivers prioritizes small block’s BACK• Low delay for small blocks

Evaluation

• A 20 node wireless mesh testbed

• Linux 2.6 kernel

• 802.11a/b/g Athero/MadWiFi card

• Spread around the CS building

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

Results

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

Graceful Degradation

• Sorted by TCP goodput

• Tried artificially dropping packets, examined goodput for different rates

Multi-Hop

• 100 random pairs - varying lengths

• TCP slightly inflated, OSLR only picks good links (Hop does well on lossy)

Hop Components

• Compare Hop against different versions of itself

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

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