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Comparison between TCPWestwood and eXplicit Control Protocol (XCP)

Comparison between TCPWestwood and eXplicit Control Protocol (XCP). Jinsong Yang Shiva Navab CS218 Project - Fall 2003. Outline. Traditional TCP shortcomings How TCPW and XCP address those shortcomings XCP: eXplicit Control Protocol TCPW: TCP Westwood Simulation Results.

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Comparison between TCPWestwood and eXplicit Control Protocol (XCP)

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  1. Comparison between TCPWestwood and eXplicit Control Protocol (XCP) Jinsong Yang Shiva Navab CS218 Project - Fall 2003

  2. Outline • Traditional TCP shortcomings • How TCPW and XCP address those shortcomings • XCP: eXplicit Control Protocol • TCPW: TCP Westwood • Simulation Results

  3. Traditional TCP Shortcomings in High BW*Delay • Congestion Detection • Based on receiving ACK (congested or not) • No information on degree on congestion • Reaction to Random Loss • Throughput inversely proportional to RTT • Unfairness in different RTT • Reaching to the full link capacity in high BW • AIMD increase Cwind 1 per RTT • Short Flows can cause instability in high BW • Never exit slow start exponential increase

  4. Addressing TCP problemsCongestion Control Mechanism • TCPW • Rate Estimate based on ACK rate • Modification on the Sender • XCP • Based on INFORMATON on each ACK header • Modification on Sender, Receiver and Router

  5. TCP Westwood • Enhance congestion control via Eligible Rate Estimates (ERE) • Estimates are computed at the sender by sampling and exponential filtering methods • ERE determined from ACK arrival process statistics and info in ACKs regarding amounts of bytes delivered • ERE is used by sender to appropriately set cwnd and ssthresh after packet loss or during slow start

  6. TCPW Algorithm • When three duplicate ACKs are detected: • set ssthresh=ERE*RTTmin (instead of ssthresh=cwin/2 as in Reno) • if (cwin > ssthresh) set cwin=ssthresh • When a TIMEOUT expires: • set ssthresh=ERE*RTTmin (instead of ssthresh=cwnd/2 as in Reno) and cwin=1 Note: RTTmin = min round trip delay experienced by the connection and is an estimate of the propagation time over the path (roundtrip)

  7. eXplicit Control Protocol (XCP) • Senders express their setting (cwnd, RTT) to routers, and routers express changes required to senders • Exchange of information in packet header • Recognizes two types of requirements for Congestion Control: • Efficiency:Achieve high link utilization • Allocation:Allocate bandwidth according to desired criteria; e.g. fairness, QoS, etc.

  8. XCP Sender and Receiver • Sender • Receiver • Similar to TCP receiver (send back ACK) • But it copies the header of packet to ACK

  9. XCP Router • Approach: Decouple controls for efficiency and allocation • Control aggregate traffic to achieve efficient link utilization • Divide link bandwidth among connections to achieve desired criteria

  10. Efficiency Controller • Goal: Match aggregate input traffic to link capacity & drains the queue • Algorithm(MIMD): •  :Aggregate feedback (increase or decrease) •  increases with an increase in spare BW • decreases with an increase in the router queue size; i.e. • S: Spare Bandwidth & Q: Queue Size • d: Current router’s estimate of RTT

  11. Fairness Controller • Goal: Divide  among flows to converge to fairness criteria • Algorithm (AIMD): • If> 0 ⇒ Divide  equally between flows (regardless of current rate) • If < 0 ⇒ Divide  between flows in proportion to their current rates • If  = 0 ⇒ bandwidth Shuffling • Allocate & deallocate BW such that total traffic range doesn’t change y= input traffic in avg RTT

  12. Fairness Controller • Feedback field: • Positive Feedback: • Negative Feedback:

  13. Addressing TCP problemsReaction to Error Loss • TCP Reno • Halves cwind for each loss (error or overflow) • TCPW: • A small fraction of isolated “randomly” lost packets does not impact the ERE value in TCPW Thus, cwnd = ERE * RTTmin remains unchanged • XCP: • Distinguishes Random loss and recovers fast • Congestion drop will be preceded with a ACK (to tell the sender to decrease its cwind)

  14. Addressing TCP problemsReaching to Full Link Capacity • TCP Reno • AIMD- increasing 1 per RTT • TCPW • Doesn’t reduce cwind drastically catches up fast • XCP • Reaches the full capacity in several RTT based on the information about the spare bandwidth on the received ACK

  15. Pros of XCP • Stable for Bandwidth and delay • Uses AQM • Parameters independent of environment • Scalable for number of flows • No per flow state  keeps the state in the header • Almost NO Packet Drop • No slow start • Reaches to full capacity fast • Smaller queue size comparing to other queuing schemes

  16. Cons of XCP • Needs router participation • deployment might prove to be difficult • Malicious Sender can falsify the header and mess up the feedback calculation • Issue: Uses average RTT • Problem if RTT varies in a large range

  17. Simulation Results-NS2 • Topology • Bottleneck • single hop • Parameters • Bandwidth • Delay • Loss Rate • Number of Flows

  18. Throughput Comparison BW=20M Delay=10ms No Loss

  19. Impact of Capacity Single Flow Different BW Delay= 10ms

  20. Impact of Link Delay Different Delay BW= 20Mbps No Loss

  21. Different Loss Rate BW=20 Mbps Delay= 10ms

  22. Impact of Loss Rate Diff. Loss Rate BW= 20Mbps Delay= 10ms

  23. Impact of Number of Flows BW=10 Mbps Delay=20ms

  24. Impact of Web-Like Traffics BW 10 Mb, # of Short Flows 500, Start @ Random Time, Running for 1 sec ,Link Delay 45 ms 500/30=17 10M/18=0.55M XCP not friendly

  25. Fairness Study – No Loss TCPW XCP BW=100 Mbps Delay=20 ms

  26. Fairness Study – Different Delay TCPW XCP BW=20 Mbps d1=10, d2=50, d3=100ms

  27. References • [1] Katabi, D., M. Handley, C. Rohrs. Internet Congestion Control for Future High Bandwidth-Delay Product • [2] M. Gerla, M. Y. Sanadidi, R. Wang, A. Zanella, C. Casetti, S. Mascolo, "TCP Westwood: Congestion Window Control Using Bandwidth Estimation", In Proceedings of IEEE Globecom 2001, Volume: 3, pp 1698-1702, San Antonio, Texas, USA, November 25-29, 2001 • [3]Mascolo, S., C. Casetti, M. Geral, M. Y. Sanadidi, R. Wang. TCP Westwood: Bandwidth Estimation for Enhanced Transport over Wireless Links • [4] Ren Wang, Massimo Valla, M. Y. Sanadidi, and Mario Gerla, Adaptive Bandwidth Share Estimation in TCP Westwood, In Proc. IEEE Globecom 2002, Taipei, Taiwan, R.O.C., November 17-21, 2002 • [5]Claudio Casetti, Mario Gerla, Saverio Mascolo, M.Y. Sansadidi, and Ren Wang, TCP Westwood: End-to-End Congestion Control for Wired/Wireless Networks, In Wireless Networks Journal 8, 467-479, 2002 • More TCP Westwood papers on http://www.cs.ucla.edu/NRL/hpi/tcpw/ • [6] Network simulator ns-2. http://www.isi.edu/nsnam/ns • [7] Sally Floyd, HighSpeed TCP for Large Congestion Windows Internet draft draft-ietf-tsvwg-highspeed-01.txt, work in progress, August 2003. • [8] Red parameters http://www.icir.org/floyd/red.html#parametes

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