TCP Bulk Repeat

1. TCP Bulk Repeat CS218 Fall 2003 Students: Ricardo Oliveira, Joshua Choi, William So Tutor: Guang Yang 11/24/2003

2. Goal of our project • Implement TCPW-BR in freeBSD • Test this implementation in dummyNet • Correct possible flaws of initial design • Explore new loss discrimination algorithms (LDAs)

3. Motivation • Bit error rate of wireless links (~ 10 -6 to 10 -5) is substantially higher than wire-line links (~10 -12)  packet loss rate can be very high (2-10%) • Because errors occur in bursts, several packets can be affected in the same TCP window • Regular TCP (New Reno) retransmits one packet per RTT need multiple RTTs to recover from several losses • In pathological scenarios (i.e., all the pkts in a window are lost), TCP makes use of the Karn’s clamped retransmit backoff (doubling the RTO each time) • New Reno halves the congestion window when any loss occurs  bandwidth underutilized

4. Bulk Repeat Scheme • Bulk retransmit • retransmit several packets instead of one (Go-Back-N) • Fixed retransmit timeout • Avoid exponential backoff in presence of random losses • Intelligent window adjustment • avoid reducing cwnd in random loss events • Loss discrimination algorithm (LDA) • End-to-end scheme to differentiate random losses from congestion losses

5. Bulk Retransmit • When a lost is detected: • New Reno Retransmits one packet per RTT • Bulk Repeat retransmits the whole window

6. Bulk Retransmit Algorithm • When sender receives 3 dup ACKs or times out, it just retransmit the lost packet (like NewReno) • Bulk retransmit is only triggered by partial ACKs: • only if the LDA tells us the loss was random; otherwise retransmit as NewReno • Bulk retransmission means retransmitting all outstanding unacknowledged packets in the current window

7. Bulk Retransmit Issue • Compensate for inflated pipe - wait until effects of burst die down before transmitting new packets: • Transmit only after k ACKs have been received ( 0 ≤k≤ number of bulk retransmitted packets) • Maintain maximum of one bulk retransmission per RTT

8. Fixed Retransmit Timeout • Karn’s clamped retransmit backoff • Consecutive timeouts trigger doubling of RTO (up to a maximum of 64X) • Under high error rate, if packet or ACK is lost, bandwidth and time are wasted before timeout

9. Fixed Retransmit Timeout • Solution: • Use a fixed retransmit timeout • For error-induced losses (LDA), do not double retransmit timeout value

10. Intelligent Window Adjustment • High error rates keep ssthresh low • ssthresh should represent estimate of the pipe size • High error keeps ssthresh too conservative • TCP behavior • Sets cwnd to ssthresh on 3DUPACKs • Sets cwnd to 1 on timeout • Underutilization in non-congestion

11. Intelligent Window Adjustment (cont’d) • Solution: • If loss is due to random error, leave cwnd unchanged

12. Loss Discrimination Algorithm • High impact on Bulk Repeat performance: • Congestion  do TCP loss recovery • Random Error  Bulk Repeat • LDA inaccuracy can lead to aggravation of congestion or underutilization

13. LDA – Background • Two major types: • Network-layer assisted: ECN/RED, ELN, … • End-to-end: • Spike: uses ROTT (Relative one-way trip time) combined with hysteresis scheme; • ZigZag: uses the mean and standard deviation of ROTT; • Biaz scheme: assumes last hop bottleneck wireless link; uses pkt inter-arrival times; if (n+1)Tmin≤Ti<(n+2)Tmin then n missing pkts were lost due to wireless tx errors; • Rate Gap Threshold (RGT): uses TCPW ERE and compare to current rate to infer network congestion; Bulk Repeat uses end-to-end, namely Spike + RGT

14. LDA - Spike • Based on RTT measurement: • Bspikestart = RTTmin + α∙ (RTTmax - RTTmin) • Bspikeend = RTTmin + β∙ (RTTmax - RTTmin) • α>β; we use 0.4 and 0.05 respectively • If RTT goes above Bspikestart, TCP enters congestion mode • If RTT goes below Bspikeend, TCP exits congestion mode

15. LDA – RGT • Rate Gap Threshold: • Bulk Repeat uses TCPW ERE (Eligible Rate Estimate) • The sending rate is computed as cwnd/RTTmin • If the sending rate >> ERE then most likely the network is congested; otherwise loss is a random loss • Because Spike and RGT work well under different error rates, Bulk Repeat uses a combination of the two: • RGT is accurate under low error rate; but RGT can’t tell the difference between a heavy loss scenario and a congestion scenario (because it is based on rate estimation) • Spike is good under high error rate (because it’s based on RTT)

16. Simulations in ns-2 • 45Mbps wired + 11Mbps wireless; 70msec RTT; wireless link as shared bottleneck

17. Simulation Results • Throughput for different pkt error rates (NewReno, TCPW and TCPW-BR) • TCPW-BR performs better than the other TCP flavors in high error rate scenarios

18. Simulation Results - Fairness • Two TCPW-BR sharing the same bottleneck @ 5% error rate

19. Simulation Results - Friendliness • TCPW-BR and New Reno sharing the same bottleneck @ 5% error rate • New Reno gets its fair share as it was running alone

20. Experimental Setup • Last-hop wireless link with low delay • Took measures for several packet error loss rates in the wireless link • Sender was running freeBSD5.1 w/ TCPW-BR kernel • Receiver was running FreeBSD4.5 w/ New Reno kernel

21. Emulation/measurement tools • We used dummyNet to emulate the desire scenario • better than ns-2 (simulation) • not as perfect as a real testbed (like a satellite Internet link) • advantages: control over the bwd/delay/pkt error loss/queues • Iperf to generate TCP traffic from sender to receiver • simulates a client/server application • control the running time • generates CBR UDP traffic

22. Experimental Details • TCP socket size(advertised window) must be large enough both in receiver and sender; good heuristic: let it be twice the pipe size= 2*Bwd*RTT • We measured the throughput for each flavor of TCP • Each connection lasted about 1 minute

23. Experimental Results

24. Future Work • Test friendliness/fairness of our implementation • Test stability of protocol (check for deadlocks) • Change network scenario: wireless last-hop vs. wireless backbone • Different propagation delays • Tune the “inflated pipe” compensation scheme • Source code tweaking

25. References • Guang Yang, Ren Wang, Mario Gerla, M. Y. Sanadidi, “TCP with Bulk Repeat” • S. Cen, P. C. Cosman and G. M. Voelker, “End-to-end Differentiation of Congestion and Wireless losses”