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Improving TCP over Wireless by Selectively Protecting Packet Transmissions

Explore improving TCP over wireless, challenges, proposed solutions, and a novel approach of selectively protecting critical packets for better QoS and energy efficiency.

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Improving TCP over Wireless by Selectively Protecting Packet Transmissions

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  1. Improving TCP over Wireless by Selectively Protecting Packet Transmissions Carla F. Chiasserini Michele Garetto Michela Meo Dipartimento di Elettronica Politecnico di Torino, Italy

  2. Presentation outline • Introduction to the problem • Solutions proposed in the literature • Our approach • Simulation results • Conclusions…

  3. Introduction: TCP over Wireless • Fundamental problem • TCP assumes all losses due to congestion • Wireless breaks this assumption: losses due to channel errors, handoffs • … and TCP unnecessarily reduces window, resulting in low throughput and high latency • Improving TCP over wireless is a must !

  4. Proposed schemes to improve the performance of TCP over wireless Wireless TCP (W-TCP) TCP HACK Adaptive Forward Error Correction (AFEC) Airmail Mobile TCP (M-TCP) CSDP Indirect-TCP Explicit Loss Notification (ELN) TCP-SACK FEC/ARQ protocols Wireless Explicit Congestion Notification (WECN) TCP Westwood Snoop protocol Delayed DUPACK’S Explicit Bad State Notification (EBSN) SMART protocol

  5. Classification of Schemes • End-to-End protocols • loss recovery handled by sender • make the sender realize some losses are due to bit-error, not congestion • Link-layer solutions • hide link-related losses from sender • since the problem is local, solve it locally (local retransmissions) • TCP sender may not be fully shielded • Split-connection approaches • Split each TCP connection into two separate TCP connections at the base station • isolate wired network from wireless network

  6. Link-layer Solutions Wired Internet Wireless link Fixed host Mobile host BS Standard TCP connection TCP TCP LL LL • Main techniques: • Forward Error Correction (FEC) • Automatic Repeat Request (ARQ)

  7. Link-layer Solutions • QoS / Energy trade-off : • Applying FEC coding to all of the packets allows high data transfer reliability but increases bandwidth and energy consumption (reduced battery life) • When channel conditions are good, ARQ alone obtains better performance using fewer resources ! • Hybrid FEC/ARQ solutions

  8. Our approach • A Link-Layer TCP-aware scheme • Combination of FEC and ARQ: • FECis applied only to TCP packets of “critical” importance to QoS • Convenient trade-off between energy saving and performance • End-to-End semantics is maintained

  9. Key Observations • “Critical” TCP packets: packets whose loss would force the sender to wait for a timeout (and reduce the window to one) • Timeouts severely affect the completion time of TCP connections • Since the radio channel is not congested, timeouts due to channel errors are useless and should be completely avoided !

  10. “Critical” TCP packets • The first 3 segments of a flow: • The transmission window is too small to allow the Fast Retransmit mechanism • The initial RTO is usually very large • The last 3 segments of a flows: • There are not enough duplicate ACKs to trigger the Fast Retransmit • Retrasmitted segments (by the TCP sender) : • If they get lost again, a timeout occurs and RTO is backed-off

  11. Simulation scenario Wired Internet • LL layer implemented into ns-2 simulator according to 3GPP specifications • Each TCP segment is divided into 3 LL data units (not protected) or 6 LL data units (protected using rate ½ FEC coding) • channel error model: 2-states Markov chain accounting for fading process (see Zorzi, Rao) 384 Kbps TCP NewReno sender Segment size = 1000 bytes TCP receiver BS 30 ms delay 10 Mbps

  12. Qos performance –10 packets flows 2.0 no fec selective total 1.5 Average completion time [s] 1.0 0.5 0.0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Fading margin, F [dB] Bad radio channel Good radio channel

  13. Energy consumption –10 packets flows 10.0 no fec selective 9.0 total 8.0 7.0 6.0 Av. # data units sent per segment 5.0 4.0 3.0 2.0 1.0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Fading margin, F [dB] Bad radio channel Good radio channel

  14. Conclusions… • Link-Layer approach to improve TCP over wireless: the best thing to do is a mixture of FEC/ARQ • The optimal combination of FEC/ARQ is difficult, because it depends on the channel conditions and on the desired Energy/QoS trade-off • We suggest a mechanism based on selective protection of “critical” packets, which is particularly effective for short-lived TCP flows (vast majority of Internet flows)

  15. Thank you !

  16. Qos performance –50 packets flows 7.0 no fec selective 6.0 total 5.0 4.0 Average completion time [s] 3.0 2.0 1.0 0.0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Fading margin, F [dB] Bad radio channel Good radio channel

  17. Energy consumption –50 packets flows 10.0 no fec selective 9.0 total 8.0 7.0 6.0 Av. # data units sent per segment 5.0 4.0 3.0 2.0 1.0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Fading margin, F [dB] Bad radio channel Good radio channel

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