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Impact of New CC on Cross Traffic

Explore the impact of new congestion control on cross-traffic using varied approaches and scenarios, comparing throughput, fairness, and bandwidth allocation in congested links.

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Impact of New CC on Cross Traffic

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  1. Impact of New CC on Cross Traffic S. Floyd TCP Round Table November 2007

  2. The question: • How to assess the impact of new CC on cross-traffic (web-traffic, multimedia, other long transfers using different CC).

  3. Approaches from the literature: • Look at response functions. (Sending rate as a function of packet drop rate, for a single flow with fixed RTTs.) • Example: RFC 3649, HSTCP. • Compare aggregate throughput of TCP flows with aggregate throughput of NewCC flows. • X-axis: Number of web sessions. • Scenario includes reverse-path traffic. • Examples: RFC 4828, TFRC-SP, Figures 12-21. • The two aggregates being compared could include both long-lived and web traffic.

  4. Congested links with different ranges of RTTs. • Look at a range of congested links: • A local network (e.g., within a single institution), a transoceanic link, and a satellite link, as well as a congested link with a standard range of round-trip times. • Why? To evaluate proposals where the level of aggressiveness is a function of the RTT.

  5. Make sure there is a realistic range of connection sizes! • This includes medium-size flows slow-starting up to large windows, and then terminating. • This introduces typical cases of transient delay and packet drops.

  6. Make sure that flows have staggered start times: • So that some flows start out when the queue is already high from another flow slow-starting. • (E.g., for delay-based congestion control.)

  7. Some NewCC mechanisms need to be tested for fairness under arange of queue mechanisms. • E.g., RFC 4828, TFRC-SP. • The fairness is quite different for Drop-Tail queues in packets, Drop-Tail queues in bytes, AQM in packet mode, and AQM in byte mode.

  8. Bandwidth stolen from TCP: • Let there be two groups of flows, A and B. • Case 1: Groups A and B both use TCP. • Case 2: Group A uses TCP, group B uses NewCC. • Compare Group A’s fraction of bandwidth in Case 1 with Group A’s fraction of the bandwidth in Case 2. • The difference between the two fractions is the fraction of the bandwidth slolen from group A by the NewCC. • Example: RFC 3649, HSTCP.

  9. Approaches from the literature,for proposals for faster start-ups: • Plot drop rates for regular traffic when a fraction of the traffic is using NewCC. • X-axis: Number of web sessions. • Example: SAF07 paper for Quick-Start. • Plot flow completion times for regular traffic, with and without NewCC enabled for other traffic. • Example: SAF07 paper for Quick-Start.

  10. Approaches from the literature:slowly-responding CC.

  11. A benefit of testbeds: • In addition to queueing delay, testbeds might exhibit delay due to router CPU delay, delay at firewalls, etc. • This would be very interesting, particularly for evaluating delay-based congestion control.

  12. The impact on streaming media? • Measure the average queueing delay, and the average packet drop rates, with and without the NewCC. • Include streaming media, and look at the delay and packet drop rates experienced by those flows?

  13. Stress-testing fairness of delay-based congestion control? • ?

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