Routers with Very Small Buffers

1. Routers with Very Small Buffers Yashar Ganjali Stanford University Joint work with: Mihaela Enacescu, Ashish Goel, Nick McKeown, and Tim Roughgarden Presented by Arjumand Younus, 20093649

2. Outline (1/2) • Background and Problem Statement • Motivation • The Router Buffer Story • How Much Buffering Do We Need? • Single TCP Flow • Many TCP Flows • Buffer Size – Theory vs. Practice • Small Buffers Scenario Routers with Very Small Buffers, INFOCOM 2006

3. Outline (2/2) • Intuitive Explanation of O(log W) Buffer Size – Leaky Bucket • TCP Reno • Paced TCP • Simulations with O(log W) Buffers • Conclusion Routers with Very Small Buffers, INFOCOM 2006

4. Background and Problem Statement • Congestion Control • Buffering - first component of any congestion control solution. • Buffers ensure that link is utilized 100%. • The Problem: • How much buffering? – Is sparking much debates recently. Routers with Very Small Buffers, INFOCOM 2006

5. Motivation - Networks with Little or No Buffers (1/2) • Problem • Internet traffic is doubled every year • Disparity between traffic and router growth (space, power, cost) • Possible Solution • All-Optical Networking • Consequences • Large capacity  large traffic • Little or no buffers Routers with Very Small Buffers, INFOCOM 2006

6. Motivation - Why Does Buffer Size Matter? (2/2) • End to end latency: • Transmission delay • Propagation delay • Queuing delay • Buffers are costly. • 1/2 board space of routers • 1/3 power consumption • Small buffers: • On chip  higher density • Lower cost The only variable component of latency Routers with Very Small Buffers, INFOCOM 2006

7. # packets at 10Gb/s 1,000,000 10,000 20 Sawtooth Peak-to-trough Non-bursty arrivals Smoothing of many sawtooths Intuition & Proofs Simulated ManyTCP Flows Evidence The Story • Assume: Large number of desynchronized flows; 100% utilization • Assume: Large number of flows; <100% utilization Routers with Very Small Buffers, INFOCOM 2006

8. How Much Buffering Do We Need? • Universally applied rule-of-thumb: • A router needs a buffer size: • 2T is the two-way propagation delay (or just 250ms) • C is capacity of bottleneck link • Context • Mandated in backbone and edge routers. • Appears in RFPs and IETF architectural guidelines. • Usually referenced to Villamizar and Song: “High Performance TCP in ANSNET”, CCR, 1994. • Already known by inventors of TCP [Van Jacobson, 1988] • Has major consequences for router design Source Destination C 2T Routers with Very Small Buffers, INFOCOM 2006

9. Single TCP Flow Rule for adjusting W • If an ACK is received: W ← W+1/W • If a packet is lost: W ← W/2 Only Wpackets may be outstanding Routers with Very Small Buffers, INFOCOM 2006

10. Window size t Single TCP Flow Rule for adjusting W • If an ACK is received: W ← W+1/W • If a packet is lost: W ← W/2 Only Wpackets may be outstanding Source Dest Routers with Very Small Buffers, INFOCOM 2006

11. Probability Distribution Buffer Size Many TCP Flows B 0 Routers with Very Small Buffers, INFOCOM 2006

12. M/D/1 Poisson D B Smooth Traffic - Theory • Theory: For smooth traffic very small buffers are enough. • Poisson Traffic • Loss independent of link rate, • RTT, number of flows, etc. Can we make traffic look “Poisson-enough” when it arrives to the routers…? Routers with Very Small Buffers, INFOCOM 2006

13. Typical OC192 router linecard buffers over 1,000,000 packets Large Buffers - Practice Routers with Very Small Buffers, INFOCOM 2006

14. Small Buffers with Paced Injections • Assume: • Buffer Size > • Distance between consecutive packets of a single flow S > • Limited injection rate • Flows are not synchronized • Start times picked randomly and independently Window size t Routers with Very Small Buffers, INFOCOM 2006

15. Small Buffers – Realistic Scenario Assumptions: • Internet core is over-provisioned • Example: Load < 80% • There is spacing between packets of the same flow: • Natural: Slow access links • Artificial: Paced TCP Result: Traffic is very smooth, and loss rate is very low, independent of RTT, and number of flows. With a buffer size of about 20 packets we can gain high throughput. Routers with Very Small Buffers, INFOCOM 2006

16. Leaky Bucket – Paced vs. Reno Bucket drains with a constant rate. Load is 90% for both cases. Routers with Very Small Buffers, INFOCOM 2006

17. TCP Reno TCP Reno sends packets in a burst  High drop rate Routers with Very Small Buffers, INFOCOM 2006

18. Paced TCP Spacing packets  Much lower drop rate Routers with Very Small Buffers, INFOCOM 2006

19. TCP With Pacing Simulations with O(log W) Buffers Regular TCP Routers with Very Small Buffers, INFOCOM 2006

20. Simulations with O(log W) Buffers Routers with Very Small Buffers, INFOCOM 2006

21. Simulations with O(log W) Buffers Routers with Small Buffers

22. Conclusion • Very small buffers are OK if: • Sacrifice 10-20% throughput • Pacing: natural, or TCP modification • Major consequences for electronic routers: • Board space reduction • Power reduction • Increased density • Opens doors to all-optical networking. • Experimental validation is in progress. Routers with Very Small Buffers, INFOCOM 2006

23. Thank You!Questions? Routers with Very Small Buffers, INFOCOM 2006