Block-sw itched Netw orks: A New Paradigm for W ireless Transport - PowerPoint PPT Presentation

Block-sw itched Netw orks: A New Paradigm for W ireless Transport Ming Li, Devesh Agraw al, Deepak Ganesan, and Arun Venkataram ani University of Massachusetts Am herst U NIVERSITY OF U NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

W hat You Buy vs. W hat You Get TCP perform s poorly over w ireless links Advertised capacity: 1 .6 x 1 1 Mbps 2 x 4 0 x U U NIVERSITY OF NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

1 . E2 E Transport E2 E rate control is error-prone E2E retransmissions are wasteful E2E route disruptions cause unavailability E2E Feedback Loss Rate RTT Rate Rate Control Control Source Dest Congestion? Link loss? U NIVERSITY OF U NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

1 . E2 E Transport E2E rate control is error-prone E2 E retransm issions are w asteful E2E route disruptions cause unavailability E2 E Retransm issions P Source Dest Redundant Transmissions U NIVERSITY OF U NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

1 . E2 E Transport E2E rate control is error-prone E2E retransmissions are wasteful E2 E route disruptions cause unavailability Route disruptions due to mobility U U NIVERSITY OF NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

2 . Packet as Unit of Control Channel access Link layer ARQ Listen Backoff RTS/ CTS U U NIVERSITY OF NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

2 . Packet as Unit of Control Channel access Link layer ARQ Timeout Backoff Timeout Backoff U U NIVERSITY OF NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

3 . Com plex Cross-Layer I nteraction Link-layer ARQs/ backoffs hurt TCP rate control Rate Rate Control Control Highly Variable RTT Transport Link Link ARQ Link ARQ Link ARQ U NIVERSITY OF U NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

Hop : A Clean Slate Re-design Hop-by-Hop End-To-End Blocks Packets Minim alism Com plexity U U NIVERSITY OF NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

Hop Design Virtual Backpressure Retransmission Multi-hop Per-hop ACK Micro-block Withholding Prioritization Reliable Block Transfer U U NIVERSITY OF NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

Reliable Per-Hop Block Transfer Mechanism s Burst mode (TXOP) Block ACK based ARQ Benefits Amortizes control overhead B-SYN Request for B-ACK B-ACK Packet bitmap TXOP CSMA U NIVERSITY OF U NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

Hop Design Virtual Backpressure Retransmission Multi-hop Per-hop ACK Micro-block Withholding Prioritization Reliable Block Transfer U U NIVERSITY OF NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

Virtual Retransm ission ( VTX) Mechanism Leverages in-network caching Re-transmits blocks only when unavailable in cache Benefits Fewer transmissions Low overhead C Simple A B D B-SYN E B-ACK DATA U U NIVERSITY OF NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

Virtual Retransm ission ( VTX) Mechanism Leverages in-network caching Re-transmits blocks only when unavailable in cache Benefits Fewer transmissions Low overhead B-SYN VTX B-SYN with VTX flag set Simple E2E ACK A B D B-SYN E VTX Tim er VTX Tim er B-SYN VTX B-ACK U NIVERSITY OF U NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

Hop Design Virtual Backpressure Retransmission Multi-hop Per-hop ACK Micro-block Withholding Prioritization Reliable Block Transfer U U NIVERSITY OF NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

Backpressure Mechanism Limits # outstanding_blocks per-flow at forwarder Withhold B-ACK Slow A B C D E B-SYN Dest Source B-SYN B-SYN Limit of Outstanding Blocks= 2 U NIVERSITY OF U NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

Backpressure Mechanism Limits # outstanding blocks per-flow at forwarder Benefits Improves network utilization 2 0 Mbps D E Dest 2 0 Mbps 2 0 Mbps 1 0 Mbps A B C 2 0 Mbps Source 1 Mbps F G Dest Aggregate goodput without backpressure: 6 Mbps U NIVERSITY OF U NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

Backpressure Mechanism Limits # outstanding blocks per-flow at forwarder Benefits Improves network utilization 2 0 Mbps D E Dest 2 0 Mbps 2 0 Mbps 1 0 Mbps A B C 2 0 Mbps Source 1 Mbps G F Dest Limit of Outstanding Blocks= 1 Aggregate goodput with backpressure: 1 0 Mbps U NIVERSITY OF U NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

Hop Design Virtual Backpressure Retransmission Multi-hop Per-hop ACK Micro-block Withholding Prioritization Reliable Block Transfer RTS/ CTS is overly conservative and incurs high overhead. U U NIVERSITY OF NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

Ack W ithholding Mechanism : Receiver A C B withholds all but one B-ACK Benefit: B-SYN B-SYN Low overhead B-ACK W ithhold Less conservative B-ACK DATA Simple B-ACK DATA U NIVERSITY OF U NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

Hop Design Virtual Backpressure Retransmission Multi-hop Per-hop ACK Micro-block Withholding Prioritization Reliable Block Transfer U U NIVERSITY OF NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

Micro-block Prioritization Mechanism s Sender piggybacks small blocks to B-SYN Receiver prioritizes small block’s B-ACK Benefits Low delay for small blocks Sender Receiver Sender SSH FTP 6 4 B 1 MB B-SYN B-SYN DATA B-ACK U U NIVERSITY OF NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

Testbed 2 0 nodes on the 2 nd floor of UMass CS building Apple Mac Mini Dual Core 1.8GHz, 2GB RAM, Atheros 802.11 a/ b/ g card U U NIVERSITY OF NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

Single-flow Single-hop Perform ance 1 .2 x Tcp Hop 1 .6 x 2 8 x Hop achieves significant gains over TCP U U NIVERSITY OF NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

Single-flow Multi-hop Perform ance 1 .9 x Tcp Hop 2 .3 x 2 .7 x Hop achieves significant gains over TCP U U NIVERSITY OF NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

Graceful Degradation w ith Loss Em ulated link layer losses at the receiver Tcp Hop TCP drops to zero w ith m oderate losses U U NIVERSITY OF NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

Scalability to High Load 3 0 concurrent flow s Mean 2 x Goodput 2 0 x Tcp Hop 1 5 0 x Hop-by-hop TCP Hop achieves m assive gains over TCP and is m uch fairer U NIVERSITY OF U NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

Hop over W LAN AP Mean Median ( kbps) ( kbps) Hop 663 652 TCP 587 244 TCP+ RTS/ CTS 463 333 Hop im proves utilization over TCP+ RTS/ CTS U NIVERSITY OF U NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •

Low Delay for Sm all Transfers 4 nodes perform large transfers, 1 node perform s sm all transfer AP Transfer Size ( KB) Hop low ers delay across all file sizes U U NIVERSITY OF NIVERSITY OF M M ASSACHUSETTS ASSACHUSETTS , A , A MHERST • Department of Computer Science Department of Computer Science MHERST •