1 / 48

Wireless TCP KAIST CS644 Advanced Topics in Networking

Wireless TCP KAIST CS644 Advanced Topics in Networking. Jeonghoon Mo <jhmo@icu.ac.kr> School of Engineering Information and Communications University. Overview. TCP Basics Challenges in Wireless Networks High Bit Error Rate Variable Delays Mobility Interaction with other Layers.

lonato
Download Presentation

Wireless TCP KAIST CS644 Advanced Topics in Networking

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Wireless TCP KAIST CS644Advanced Topics in Networking Jeonghoon Mo <jhmo@icu.ac.kr> School of Engineering Information and Communications University

  2. Overview • TCP Basics • Challenges in Wireless Networks • High Bit Error Rate • Variable Delays • Mobility • Interaction with other Layers

  3. TCP Basics • Transmission Control Protocol • A Protocol for Reliable Data Transmission • Two important roles • Congestion Control for efficient and fair bandwidth sharing • Addtive increase and multiplicative decrease (AIMD) of window • Fast recovery • Fast retransmit • Loss Recovery for Reliable Transmission • Timer-based Retransmission • RTO = mean (RTT) + 4 Std.Dev(RTT) • Cumulative ACK from receiver

  4. Challenges in Wireless Networks • High Bit Error Rate • Variable Delays • Mobility (Handoff) • Interaction with Lower Layer Protocol (MAC)

  5. High Bit Error Rate Challenge • Physical Property of Wireless Medium • Raleigh fading, Slow Fading, Interference • TCP Throughput •  1/sqrt(p) • Very poor performance with High Bit Error Rate

  6. Responses – High Bit Errors • Extensive Research • (599 results in IEEE Explorer) • Link Layer Retransmissions/ FEC • Differentiation of Wireless and Congestion Losses • Explicit Notification • End-to-End Differentiations • Use of Available Bandwidth Estimate

  7. TCP Host Link Layer Rxmt BS Link Layer Retransmission/FEC • Link Layer Solution • Little or No Modifications in the TCP stack • Cellular Networks uses ARQ/FEC. • TCP Snoop (95), I-TCP

  8. Link Layer Retransmission • Link Layer Retransmission helps improving performance. 16 Retrx Limit 128 Retrx Limit Source: Wireless TCP Performance with Link Layer FEC/ARQ (ICC 1999)

  9. Forward Error Correction • Westwood+ performs better than TCP with FEC. • Performance Improvement comes more from good ABE algorithm than FEC. Source: Adaptive End-to-End FEC for Improving TCP Performance over Wireless Links (ICC 2004)

  10. Wireless Loss vs. Congestion Loss • IDEAL TCP • CURRENT TCP (TCP Reno) • If (congestion) • Perform congestion recovery • If (loss from ERROR) • Perform loss recovery • If (congestion || loss from ERROR) • Perform congestion recovery and • loss recovery

  11. How to Differentiate them? • Explicit Notification • End-to-End Differentiation

  12. Explicit Notifications • Router or Access Point notifies TCP whether or not the loss is from Wireless Error. • Motivated from the ECN proposal • Usually, BS or AP needs some modifications • Deployment Issue • Next Generation Network will use feedback in the header. • Examples • Explicit Loss Notification (ELN) • Explicit Bad State Notification (ESBN) • XCP, TCP Jersey, FAST

  13. XCP XCP: An eXplicit Control Protocol Explicit feedback • Congestion Controller • Fairness Controller (Source: XCP Presentation)

  14. Round Trip Time Round Trip Time Congestion Window Congestion Window Feedback Feedback Congestion Header XCP Feedback = + 0.1 packet (Source: XCP Presentation)

  15. Round Trip Time Congestion Window Feedback = + 0.1 packet XCP Feedback = - 0.3 packet (Source: XCP Presentation)

  16. XCP Congestion Window = Congestion Window + Feedback XCP extends ECN and CSFQ Routers compute feedback without any per-flow state (Source: XCP Presentation)

  17. End-to-End Differentiation • To overcome deployment issue of ECN, use only available information to the end hosts. • Sender or Receiver tries to differentiate two different losses based on information such as • interarrival time of lost packets [biaz99] • use rtt, window size, loss pattern

  18. Some Ideas of E2E Differentiation • Vegas: use “w-xd” to measure congestion • Biaz: • Spike: • Jitter-based TCP (JTCP) • w· (queue_ratio) > k then congestion loss • queue_ratio = (interarrival – interdeparture)/ interdeparture

  19. Jitter-based TCP (Wu04) • Similar to Vegas. • However, uses jitter ratio measurement with the help from the receiver • If the expected queue ratio is larger than the threshold, the loss is from the congestion.

  20. Summary • ECN • Deployment Issue • Will be used in the Future • E2E Differentiation • Has Limitations in differentiating congestion losses from the wireless losses.

  21. Available Bandwidth Estimation • AIMD -> AIAD • adaptive decrease algorithm • After 3 DUPs, ssthresh =ABE() * RTTmin cwin = min (cwin, ssthresh) • After TIMEOUT ssthresh =ABE() * RTTmin cwin = 1 • TCP Westwood(01), TCP Jersey

  22. ABE Algorithm : arrival Time of the n-th ACK : acked amount by the n-th ACK : bandwidth estimate

  23. TCP Westwood Accuracy of ABE Throughput Gain source: TCP Westwood: Congestion Window Control Using Bandwidth Estimation

  24. TCP Jersey (04) • Similar to TCP Westwood in that it uses ABE technique • Their estimate method is a bit different • Uses Congestion Warning • simpler version of RED Jersey source: TCP-Jersey for Wireless IP Communications

  25. Decoupled TCP (Mo, Kang & Kim 04) • Motivations • How to differentiate has received attentions. • What to do with the differentiation has not. • Low performance of the newest protocols at high loss rate • TCP Westwood gets about 10% of whole bandwidth when the loss rate is 10%.

  26. Decoupled TCP • Two Ideas: • Decouple Loss Recovery from Congestion Control (NETBLT) • New ABE Algorithm

  27. New ABE Algorithm ABE Algorithm of DTCP ABE Algorithm of Westwood DTCP Westwood

  28. Decoupled TCP DTCP Reno, Sack Westwood

  29. Challenges in Wireless Networks • High Bit Error Rate • Variable Delays • Mobility (Handoff) • Interaction with Lower Layer Protocol (MAC)

  30. Variable Delays • TCP sets RTO = mean(RTT) + 4 std.dev(RTT) • High delay variation causes spurious TIMEOUT. • Spurious TIMEOUT causes unnecessary retransmission and rate reduction

  31. TCP-Eifel: Variable Delays • Eifel: A name of Mt. range in W. Germany • Use timestamp option • The sender timestamps packets and stores times of retransmitted packets. • The receiver echoes back the timestamp.

  32. Challenges in Wireless Networks • High Bit Error Rate • Variable Delays • Mobility (Handoff) • Interaction with Lower Layer Protocol (MAC)

  33. Mobility • As we move towards Ubiquitous paradigm, seamless service is considered more important. • Supporting mobile users with QoS has been an issue. • Impact of Mobility on TCP Performance • Mobile IP • Ad-hoc wireless Networks • Hand-off • Approaches to improve TCP performance • M-TCP • Freeze TCP • Use of Multicast

  34. Mobility Increase the Capacity of Ad-hoc Wireless Networks • Best paper award at INFOCOM 2001 • Showed that throughput can be increase via mobile relay nodes. • Can TCP exploit this results??? (Grossglauser and Tse, 01)

  35. TCP Throughput vs. Mobile Speed Decreasing function of speed Not Always source: Analysis of TCP Performance over Mobile Ad Hoc Networks

  36. Why this happens? • The TCP throughput decreases with movement because • after route failure, it takes time for TCP to recover • during which TCP fails to utilize bandwidth • Why not always, • sometimes after reroute, the new path can be better than old one. • Mobility increases the capacity of Ad-hoc Wireless networks [DTSE2001]

  37. BS BS Fixed Host (Sender) Probe res ZWP Connection ZWA MH MH MH MH Freeze-TCP (Goff, 00) Freeze transmission during hand-off ZWA: Zero Window Advertisement ZWP : Zero Window Probing (source: http://home.postech.ac.kr/~really97/Freeze.ppt)

  38. Challenges in Wireless Networks • High Bit Error Rate • Variable Delays • Mobility (Handoff) • Interaction with Lower Layer Protocol (MAC)

  39. Interaction with MAC • Downlink of GPRS networks : TDMA • Downlink of CDMA2000 HDR : PF scheduling • TCP performs AIMD cogestion control for fairness and Efficiency. • Is the AIMD control required?

  40. TCP Throughput vs. # of Hops in a 802.11 Network Not Always source: Analysis of TCP Performance over Mobile Ad Hoc Networks

  41. Challenges in Wireless Transport

  42. BACKUP

  43. window Cd+B Time AIMD Congestion Control Packet Loss Loss-basedCongestion detection: Increase the window size until packet loss. Additive Increase and Multiplicative Decrease (AIMD) of Window Size

  44. Fast Retransmit • RTO = mean(RTT) + 4 * std.dev(RTT) • The RTO value is too long. • Instead of waiting for the timer expiration, after 3 duplicate packets, the sender retransmit packets. • can cause spurious retransmission problem in ad-hoc networks

  45. Fast Recovery • Inflate congestion window temporarily to allow packet to be transmitted after a packet loss. • cwnd = ssthresh + number of dupacks • Why increase cwnd ? • the starting point of sliding window stops. • the sender cannot send until the lost packet is recovered due to window restriction • can cause link under utilization.

  46. Round Trip Time Measurement Adaptive Retransmission Time Out • Keep track of mean(RTT) and Deviation(RTT) • Retransmission Time Out (RTO) • RTO = mean(RTT) + 4 Deviation(RTT)

  47. TCP Implementations • TCP DCR [TAMU ] • Delayed Congestion Response • Similar to Snoop in that it uses link layer retransmission

  48. TCP-PR (Persistent Reordering) • does not use DUP as the signal of packet loss

More Related