Updates on Backward Congestion Notification

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. 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 • Updates • Algorithm • Derivative to improve stability • Solicit Bit to accelerate recovery • AQM in rate limiter queues to reduce blocking • Simulations

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

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

5. Changes to Detection & Signaling

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

7. 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 QAQM (e.g., 10 pkts) • Fixed drop or mark probability P (e.g., 1%) • Two counters: • CTCP: Number of TCP packets in the RL queue • CUDP: Number of UDP packets in the RL queue • Drop or mark TCP packets with probability P when CTCP > QAQM • Drop UDP packets when CUDP > QAQM QAQM TCP UDP

8. ES6 Core Switch SJ DR2 ES1 ES2 ES3 ES4 ES5 SR2 SR1 STb1 STo1 STb2 STo2 STb3 STo3 STb4 STo4 DTb DTo DR1 Simulation Environment (1) TCP Bulk TCP On/Off Congestion TCP Ref1 TCP Ref2

9. 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 • 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) • 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. 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. Bulk & On/Off Application Throughput & Latency (Workload 1: TCP Only) Best Worst

12. Reference Applications Throughput & Latency (Workload 1: TCP Only) Best Worst

13. Buffer Utilization: No FC

14. Buffer Utilization: FC

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

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

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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