1. 1 Updates on Backward Congestion Notification Davide Bergamasco (davide@cisco.com) Cisco Systems, Inc. IEEE 802 Plenary Meeting San Francisco, USA July 20, 2005

2. 222 Agenda Previous presentation May 2005 IEEE 802.1 Interim Meeting in Berlin, Germany http://www.ieee802.org/1/files/public/docs2005/new- bergamasco-backward-congestion-notification-0505.pdf http://www.ieee802.org/1/files/public/docs2005/new- bergamasco-backward-congestion-notification-0505.pdf Updates Algorithm Derivative to improve stability Solicit Bit to accelerate recovery AQM in rate limiter queues to reduce blocking Simulations

3. 333 ----++++ Stop Generation of BCN Messages t Q Qeq Queue Stability ISSUE: Overshoots and undershoots accumulate over time SOLUTION: Signal only when Q > Q eq && dQ/dt > 0 Q < Q eq && dQ/dt < 0 Easy to implement in hardware: just an Up/Down counter Increment @ every enqueue Decrement @ every dequeue Reduces signaling rate by 50%!!

4. 444 Solicit Bit t BCN0 R Random Time BCN+2 R min R solicit Force Bit On BCN+2BCN+4 Force Bit Off BCN+1 ISSUE: When the rate is very low, recovery may take too long because of sampling. SOLUTION: Solicit Bit in RL tag if R < R solicit Solicit bit is set if R >= R solicit Solicit bit is cleared If possible, CP will generate a BCN+ for every frame with Solicit bit on, regardless of sampling

5. 555 Changes to Detection & Signaling

6. 666 Rate Limiter Queue AQM Tail Drop Flow Control ISSUE: Blocking @ RL queues due to buffer exhaustion SOLUTION: add an AQM mechanism to control buffer usage

7. 777 Rate Limiter Queue AQM Traditional AQM such as RED (mark/drop) don’t work well for RL queues: Buffer too small Very few flows Traffic statistics very different from Internet traffic A novel and very simple solution based on: Threshold on the RL queue Q AQM (e.g., 10 pkts) Fixed drop or mark probability P (e.g., 1%) Two counters: C TCP : Number of TCP packets in the RL queue C UDP : Number of UDP packets in the RL queue Drop or mark TCP packets with probability P when C TCP > Q AQM Drop UDP packets when C UDP > Q AQM TCP UDP Q AQM

8. 888 Simulation Environment (1) Congestion TCP Bulk TCP Ref1 TCP Ref2 TCP On/Off

9. 999 Simulation Environment (2) Short Range, High Speed DC Network Link Capacity = 10 Gbps Switch latency = 1  s Link Length = 100 m (.5  s propagation delay) Control loop delay ~ 3  s Workload 1) TCP only STb1-STb4: 3 parallel connections transferring 1 MB each continuosly STi1-STi4: 3 parallel connections transferring 1 MB then waiting 10 ms SR1: 1 connection transferring 10 KB (avg 16  s wait) SR2: 1 connection transferring 10 KB (1  s wait) 2) 80% TCP + 20% UDP STb1-STb4: same as above STi1-STi4: same as above SR1-SR2: same as above SU1-SU4: variable length bursts with average offered load of 2 Gbps

10. 10 Simulation Goals Study the performance of BCN with various congestion management techniques at the RL No Link-level Flow Control Link-level Flow Control Link-level Flow Control + RL simple AQM (drop/mark) Metrics: Throughput and Latency of TCP bulk and on/off connections Throughput and Latency of Reference Flows Bottleneck Link Utilization Buffer Utilization

11. 11 Bulk & On/Off Application Throughput & Latency (Workload 1: TCP Only) RL Congestion Management Mechanism Bulk TCP Throughput (Tps) Bulk TCP Latency (  s) On/Off TCP Throughput (Tps) On/Off TCP Latency (  s) Throughput on Bottleneck link (Gbps) No Flow Control 67.1715,22025.9227,8809.85 Flow Control63.0015,97032.9220,33710.00 Flow Control + RL AQM (drop) 61.8316,24933.4219,57010.00 Flow Control + RL AQM (mark) 59.1717,04336.6716,87310.00 WorstBest

12. 12 Reference Applications Throughput & Latency (Workload 1: TCP Only) RL Congestion Management Mechanism Ref1 TCP Throughput (Tps) Ref1 TCP Latency (  s) Ref2 TCP Throughput (Tps) Ref2 TCP Latency (  s) No Flow Control 6702132.83033431.97 Flow Control7108124.303103831.22 Flow Control + AQM (drop) 7210122.333130730.94 Flow Control + AQM (mark) 7419119.213136230.89 WorstBest

13. 13 Buffer Utilization: No FC

14. 14 Buffer Utilization: FC

15. 15 Buffer Utilization: FC + RL AQM (drop)

16. 16 Buffer Utilization: FC + RL AQM (mark)

17. 17 Summary & Next Steps A number of improvements have been made to BCN Derivative to improve stability Solicit Bit to speed up recovery AQM in RL queues to reduce blocking Future Steps Build a Prototype??? …

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