TCP WestwoodEnd-to-End Congestion Control for Wired/Wireless Networks                                            By       ...
TCP Westwood: Main features¤  Improves the performances of TCP Reno over leaky and    dynamically loaded canal as wireles...
TCP Westwood: How works?¤  TCPW works as TCP Reno but with smallest changes.¤  After 3 ACK losses   ¤  ssthresh = (BWE ...
TCP Westwood: How works?Image source: http://c3lab.poliba.it/index.php/Westwood
delivered to the destination.                                       equal to τ/2 is necessary. But, since the ACK         ...
delivered to the destination.                                       equal to τ/2 is necessary. But, since the ACK         ...
TCP Westwood: End-to-Endbandwidth measurement III
TCP Westwood: End-to-Endbandwidth measurement IV
TCP Westwood: performance with       lossy link & fair share¤  Avg. throughput versus    number of Reno connections    ov...
TCP Westwood: performance
TCP Westwood: performance
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TCP Westwood

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TCP Westwood

  1. 1. TCP WestwoodEnd-to-End Congestion Control for Wired/Wireless Networks By Guillem C. Mayol Adrian Viala Dídac Montero Mateu Seguí
  2. 2. TCP Westwood: Main features¤  Improves the performances of TCP Reno over leaky and dynamically loaded canal as wireless networks (small improve for wired network) ¤  TCPW in not very sensitive to random errors.¤  TCPW fully complies with end-to-end TCP design principle. ¤  Not requires inspection of TCP packets at intermediate nodes. ¤  BW estimated by continuously monitoring ACKs.¤  Friendliness with TCP variants, such as Reno. ¤  TCPW is a reactive process.¤  TCPW convergence to fair share.
  3. 3. TCP Westwood: How works?¤  TCPW works as TCP Reno but with smallest changes.¤  After 3 ACK losses ¤  ssthresh = (BWE * RTTmin)/seg_size; ¤  If (cwin > ssthresh) cwin = ssthresh;¤  Timeout expiration ¤  ssthresh = (BWE * RTTmin)/seg_size; (min 2) 3 ACK losses ¤  cwin = 1; Timeout
  4. 4. TCP Westwood: How works?Image source: http://c3lab.poliba.it/index.php/Westwood
  5. 5. delivered to the destination. equal to τ/2 is necessary. But, since the ACK We discuss the use of the information in (2) in section 2.3. chronous, the sampling frequency constraint c For now, let assume that an ACK is received at the source at anteed. Thus, to guarantee the Nyquist constr time tk , notifying that dk bytes have been received at the TCP lish that if a time τ/m (m 2) has elapsed receiver. We can measure the following sample bandwidth received ACK without receiving any new AC used by that connection as bk = dk / k , where k = tk −tk−1 ter assumes the reception of a virtual null sa and tk−1 is the time the previous ACK was received. The situation is shown in figure 1, where tk Since congestion occurs whenever the low-frequency input an ACK is received, tk+j are the arrival time TCP Westwood: End-to-End traffic rate exceeds the link capacity [15] we employ a low- samples, with tk+j +1 − tk+j = τ/m for j pass filter to average sampled measurements and to obtain the and bk+j = 0 for j = 0, n − 1 are th low-frequency components of the available bandwidth. No- ples. Then, bk+n = dk+n / k+n is the bandw bandwidth measurement I tice that this averaging is also useful in filtering out the noise tk+n . due to delayed acknowledgments. It is desirable that after a long time witho In our early design and experimentation, we used a filter because no new data were sent), the filter ac¤  BW estimated (BWE) by monitoring the TCP ACKs similar to the one used for RTT estimation in TCP. We de- vative fashion, progressively decreasing the b termined that such an exponential filter with constant coeffi- mation as time elapses without new samples. cients is not capable of efficiently filtering out high-frequency the operation of the TCPW filter when there is Transmitted Bytes components of the bandwidth measurements. We propose the sence of ACKs after a time t = tk . As can be dk dk bk = = ;BW Δ k tk − tk−1 Interarrival ACKs ACK received at source
  6. 6. delivered to the destination. equal to τ/2 is necessary. But, since the ACK We discuss the use of the information in (2) in section 2.3. chronous, the sampling frequency constraint c For now, let assume that an ACK is received at the source at anteed. Thus, to guarantee the Nyquist constr time tk , notifying that dk bytes have been received at the TCP lish that if a time τ/m (m 2) has elapsed receiver. We can measure the following sample bandwidth received ACK without receiving any new AC used by that connection as bk = dk / k , where k = tk −tk−1 ter assumes the reception of a virtual null sa and tk−1 is the time the previous ACK was received. The situation is shown in figure 1, where tk Since congestion occurs whenever the low-frequency input an ACK is received, tk+j are the arrival time TCP Westwood: End-to-End traffic rate exceeds the link capacity [15] we employ a low- samples, with tk+j +1 − tk+j = τ/m for j pass filter to average sampled measurements and to obtain the and bk+j = 0 for j = 0, n − 1 are th low-frequency components of the available bandwidth. No- ples. Then, bk+n = dk+n / k+n is the bandw bandwidth measurement II tice that this averaging is also useful in filtering out the noise tk+n . due to delayed acknowledgments. It is desirable that after a long time witho In our early design and experimentation, we used a filter because no new data were sent), the filter ac similar to the one used for RTT estimation in TCP. We de- vative fashion, progressively decreasing the b termined that such an exponential filter with constant coeffi- mation as time elapses without new samples.¤  TCPW uses a low pass filter to average sampled cients is not capable of efficiently filtering out high-frequency components of the bandwidth measurements. We propose the the operation of the TCPW filter when there is sence of ACKs after a time t = tk . As can be measurements and to obtain low-freq. components of the available bandwidth. BW (Actual) ACK received at source BWEˆ = α b + (1− α )( bk + bk−1 ) α k = 2τ − Δ kbk ˆ k k−1 k 2 2τ + Δ k Last BWE τ =1/cut-off freq. Interarrival ACKs¤  When interarrival ACKs increases (losses?) the most important values are the two most recent BW calculated, otherwise the Last BWE has more weight.
  7. 7. TCP Westwood: End-to-Endbandwidth measurement III
  8. 8. TCP Westwood: End-to-Endbandwidth measurement IV
  9. 9. TCP Westwood: performance with lossy link & fair share¤  Avg. throughput versus number of Reno connections over good and lossy link. "   Convergence toward fair bandwidth sharing when connection A started firstly
  10. 10. TCP Westwood: performance
  11. 11. TCP Westwood: performance
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