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Backward Congestion Notification Version 2.0

Backward Congestion Notification Version 2.0. Davide Bergamasco ( davide@cisco.com ) Rong Pan (ropan@cisco.com) Cisco Systems, Inc. IEEE 802.1 Interim Meeting Garden Grove, CA (USA) September 22, 2005. Credits. Valentina Alaria (Cisco) Andrea Baldini (Cisco) Flavio Bonomi (Cisco)

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Backward Congestion Notification Version 2.0

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  1. Backward Congestion Notification Version 2.0 Davide Bergamasco (davide@cisco.com) Rong Pan (ropan@cisco.com) Cisco Systems, Inc. IEEE 802.1 Interim Meeting Garden Grove, CA (USA) September 22, 2005

  2. Credits • Valentina Alaria (Cisco) • Andrea Baldini (Cisco) • Flavio Bonomi (Cisco) • Manoj K. Wadekar (Intel)

  3. BCN v2.0 • Desire from Mick to see an analytical studyof BCN stability • BCN v2.0 improvements • Linear control loop allows analysis of stability • Simplified detection mechanism • Reduced signaling rate • Original BCN framework remains the same

  4. BCN Background

  5. Detection & Signaling

  6. Reaction

  7. Suggested BCN Message Format 0 15 31 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + DA = SA of sampled frame +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ SA = MAC Address of CP + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IEEE 802.1Q Tag or S-Tag | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | EtherType = BCN |Version| Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + CPID + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Qoff | Qdelta | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Timestamp | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | | First N bytes of sampled frame starting from DA | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | FCS | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  8. Suggested RLT Tag Format 0 3 7 15 31 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + DA of rate-limited frame +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ SA of rate-limited frame + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IEEE 802.1Q Tag or S-Tag of rate-limited frame | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | EtherType = RLT |Version| Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + CPID + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Timestamp |EtherType of rate limited frame| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + Payload of rate-limited frame + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | FCS | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  9. ES6 Core Switch SJ DR2 ES1 ES2 ES3 ES4 ES5 SR2 SR1 ST1 SU1 ST2 SU2 ST3 SU3 ST4 SU4 DT DU DR1 Simulation Environment (1) TCP Bulk UDP On/Off Congestion

  10. Simulation Environment (2) • Short Range, High Speed DC Network • Link Capacity = 10 Gbps • Switch latency = 1 s • Link Length = 100 m (0.5  s propagation delay) • Control loop • Delay ~ 3 s • Parameters • W = 2 • Gi = 4 • Gd = 1/64 • Ru = 8 Mbps • Workload • ST1-ST4: 10 parallel TCP connections transferring 1 MB each continuously • SU1-SU4: 64 KB bursts of UDP traffic starting at t = 10 ms

  11. BCNv1.0

  12. BCNv2.0 Faster Transient Response Higher Stability @ Steady State

  13. Simulation Environment (3) • Long Range, High Speed DC Network • Link Capacity = 10 Gbps • Switch latency = 1 s • Link Length = 20000 m (100  s propagation delay) • Control loop • Delay ~ 200 s • Parameters • W = 2 • Gi = 4 • Gd = 1/64 • Ru = 8 Mbps • Workload • ST1-ST4: 10 parallel TCP connections transferring 1 MB each continuously • SU1-SU4: 64 KB bursts of UDP traffic starting at t = 10 ms

  14. BCNv1.0

  15. BCNv2.0 Much higher stability @ steady state with larger loop delays

  16. Summary • BCN v2 has a number of advantages … • Can be studied analytically • Better protection of TCP flows in mixed TCP and UDP traffic scenarios • Detection algorithm independent of Switch implementation • Better Performance • Lower signaling frequency (from 10% to 1%) • Better stability • Increased tolerance to loop delays • … and one disadvantage • Slower convergence to fairness

  17. A Control-Theoretic Approach to BCNDesign and Analysis

  18. Notation N: Number of Flows C: Link Capacity : Round Trip Delay w: Weight of the Derivitive Pm: Sampling Probability Gi: Additive Increase Gain Gd: Multiplicative Decrease Gain

  19. Block Diagram of BCN Congestion Control C + + ∆R R q _ + + Gd _ Time Delay + Pm Gi

  20. Non-linear Differential Equations Link Control Source Control If Fb(t-) > 0 If Fb(t-) < 0

  21. Linearization Around Operating Point • Using feedback control to analyze local stability • Operating point: • R = C/N; • q’ = qeq – q = 0; • Linearization • Difficulty: depending on sgn(Fb(t-d)), the system responses are different • Luckily, a piecewise-linear function • Details are in the appendix

  22. add lead zero to compensate Block Diagram of BCN Feedback Control + R q + lose 90o margin Multiplicative Decrease: _ Fb Additive Increase: +

  23. zero:dq/dt The Effect Of Zero From Time Domain’s Eyes R q

  24. Choosing Parameters – an example • Network conditions (10G link) • N = 50 •  = 200us • Choose parameters such that the feedback loop is stable with a 35o margin • w = 4 • Gi = 2Mbps • Gd = 1/128 • Pm = 0.01

  25. With N = 50, delay = 200us, the system is stable • Phase margin translates into allowing extreme network conditions of N -> 1000 flows or  -> 1ms before oscillation Stability Result: lost 90o margin

  26. Simulation Result Shows A Stable System for N = 50; Delay = 200us

  27. Simulation Result Shows System is stable, but on the verge of oscillation: N = 50, Delay = 1ms

  28. When w = 1, a system with N = 50, delay = 200us already runs out of margin, on the verge of oscillation • w = 1, diminishing zero effect. System can’t cope with wide range of network conditions Change W = 4 -> 1

  29. Indeed System is stable, but on the verge of oscillation even for N = 50, Delay = 200us when w = 1.0

  30. Requests to 802.1 • Start a Task Force on Congestion Management • Use BCN as a Baseline Proposal

  31. Appendix

  32. Linearizing…

  33. Linearizing Additive Increase Function

  34. Linearizing Additive Increase Function

  35. Linearizing Multiplicative Decrease Function

  36. Linearizing Multiplicative Decrease Function

  37. Issue #1: Non-linearity 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

  38. Issue #2: Specific Detection Mechanism

  39. 39 39 39

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