PCP: Efficient Endpoint Congestion Control

1. PCP: Efficient Endpoint Congestion Control Thomas Anderson, Andrew Collins, Arvind Krishnamurthy and John Zahorjan University of Washington NSDI, 2006 Presented by Aleksandar Kuzmanovic September 30, 2009

2. Overview • PCP -- Probe Control Protocol • Probe  Detect whether the network can currently support a test rate • End-to-end approach • Emulates network-based control  “Request and Set”

3. End Point Router Support Try and Backoff TCP, Vegas, RAP, Fast TCP, S-TCP High Speed TCP DecBit, ECN, RED, AQM Request and Set PCP ATM, XCP, WFQ, RCP Background

4. Design Goals Minimize transfer time Negligible packet loss & low queue variability Resources are fully allocated if there is sufficient demand Fairness Stable system even under high loads

5. Design Goals Minimize transfer time Common Case -- Most network paths are idle most of the time. • Most transfers are relatively short •  Startup efficiency is particularly important. • TCP congestion control was designed at a time when links were thin and usually fully utilized •  Efficiency loss of slow start is minimal

6. Design Goals Negligible packet loss & low queue variability • Packet loss: Queue overflow •  Can we prevent queues from overflow ? • Large queuing delays unnecessarily delay interactive response time and disrupt real-time traffic. •  Can we eliminate queues that might build up at routers?

7. Design Goals Minimize transfer time Negligible packet loss & low queue variability Resources are fully allocated if there is sufficient demand Fairness Stable system even under high loads • Goals of PCP: • Achieves rapid startup, small queues, and low loss rates, and that it does not compromise eventual efficiency, fairness and stability.

8. Application Examples • Moderate sized flows on idle links • Interactive applications • Applications demanding minimally variable response times • TCP managed networks perform poorly for these applications!

9. Goal 1. Minimize transfer time Direct Jump • Test a target rate by sending a short probe. • Given a successful test, senders immediately increase their base rate by the target rate of the probe. • Two important techniques: Probe control: how to vary the test rates? Using history: achieves constant startup time

10. Direct Jump Probe Control • Exponential increase and decrease • Start with a baseline rate: One maximum sized packet per round-trip. • Double the attempted rate increase after each successful probe. • Halve the attempted rate increase after each unsuccessful probe. Probe Rate Channel Capacity Probe Probe Time

11. Probes • Send packet train spaced at an interval to achieve desired rate -- Currently, five packets whose size could be varied • Check for queuing delays based on reception times

12. Direct Jump Using History Keep history informationabout the base rates previously used to each Internet address Set the initial probe rate based on previous base rate. Allows the end host to usually identify the optimal rate within two round trip times.

13. Goal 2. Negligible packet loss & low queue variability Rate compensation • Eliminate queues at routers: • Notice queue-buildups: • Reduce the sending rate by a factor of (Δout – Δin ) /Δout • Detect persistent queueing: • Reduce the sending rate by a factor of (max-delay – min-delay) / max-delay

14. Baseline Packets • Transmit the baseline packets in a paced manner (equally spaced) at the base rate. • Monitor the gap between baseline PCP packets • Δin -- gap used by the sender • Δout -- gap observed at the receiver • Monitor the one-way delays of baseline PCP packets • max-delay -- maximum one-way delay (maxdelay) observed in the previous round trip time • min-delay -- minimum observed one-way delay (will time out)

15. Probes • Send packet train spaced at an interval to achieve desired rate -- Currently, five packets whose size could be varied • Check for queuing delays based on reception times

16. Comparison of Baseline Packets & Probes Both are paced packets. Probes: short, high-rate bursts (sent at a test rate) Baseline packets: regular data traffic (sent at the base rate) Impact of a Probe is independent of its test rate. Easy to test aggressively without fear of disrupting existing connections. Probe Rate Channel Capacity Probe Probe Time

17. Evaluation • User-level implementation: • Response time improves by a factor of 2 over TCP • Better performance for long transfers as well • Simulation • Smaller response times, smaller queue sizes • PCP is compatible and benefits from fair queuing in the network

18. Conclusion • Emulating ‘request and set’ approach from the endpoints is possible • The key idea is to send short probes: • - impact of a probe is independent of its test rate • - easy to test aggressively • Achieves negligible packet loss & low queue variability • Superior start-up behavior • Effective over FQ routers