Do we need PCP?

Hongyu GaoYinzhi Cao

Outline

• Design Goal• Underlying Assumption• Design Detail• Evaluation• Deployment• Conclusion

Design Goal

• Do we need fairness?• PCP claims in the design goal:– YES!

• However, it later claims:– Fairness is less important than transfer time

• We do NOT agree!Fairness is important.

Underlying Assumption

• Packet can reach the receiver at the first time it is sent through out the whole paper

• Unrealistic!– Network level protocol does NOT provide reliable

service. Instead, it provides best-effort service.– Noisy channel, random error may cause packat

lost

Consequences

• Probe is not accurate

• Rate compensation is delayed

We want to see how packat lost have effect on PCP.

Design Detail

• We assume n users probe at the same time.1.Some of them may succeed at the same time.2.They assume bandwidth falsely.3.They all send packets at the assumed high

rate.4.Packat Lost / Oscillation

Evaluation

• Missing crucial information– How well does the congestion control algorithm

work when there is congestion in the network?– Will the performance degrade gracefully?– In the evaluation, there is NO congestion in the

network

Evaluation

• Let us look at the first graph of Figure 10.– They transmit 1000 package in 400ms.– Let’s do a simple math.Package size = 1500bytes(in common Ethernet)1500bytes*8b/bytes*1000/0.4s = 28.6Mb/sBut they set interarrival times to be 60% of system load.So 40Mb/s*0.6=24Mb/sHow can they achieve a transmission rates larger than

possible?

Deployment

• What do we gain if we switch to PCP?1.Slow Start to Quick Start

(But if transmission is small, do we care about time? If transmission is large, slow start is only a very small portion.)

2.Good Performance when links are mostly idle.

Deployment

• What do we lose if we use PCP?1. Possible Starvation when TCP exists(If no

modification to original PCP.) And no evidence show us that PCP has better performance if TCP exists(maybe worse).

2. No communications with TCP client (They claim they can, but not implemented. And they don’t know if there is problem.)

Conclusion

• Design Goal - Unreasonable• Underlying Assumption - Unrealistic• Design Detail - Flawed• Evaluation - Incomplete• Deployment - Unlikely

• We ask you a question.

Do you use PCP?