Traffic policing on 3550/3560 and TCP sawtooth

[george_daly]

Hi,

I have a scenario where I am deploying ports on 3550 and 3560 switches to customers with a 10Mb upload limit enforced through policing.

However when downloading from the server on a machine with a 100Mb connection there is a TCP sawtooth effect and average upload is 5 or 6Mb.

My police statement looks like:

police 10000000 1000000 exceed-action drop

I'd like a smoother way of handling the policing than just dropping excess packets as this means packets being retransmitted, TCP slow starts and the average bandwidth being 5-6Mb rather than close to 10Mb.

As I understand it 3550s don't support shaping. My solution needs to work on both 3550s and 3560s so I don't think shaping is an option.

Is my only option to increase the policed bandwidth over 10Mb in order to guarantee a 10Mb upload? I know that if I change the TCP window on the downloading client this would solve it but that is not an option.

Thanks for any replies,

George

[ariela]

Hi,

the "saw-tooth" effect is a well-known TCP behaviour:

http://www.cisco.com/web/about/ac123/ac147/archived_issues/ipj_9-2/gigabit_tcp.html

Yes, 3550 doesn't support shaping. So, "if the average rate of the saw tooth traffic is calculated, this rate is much lower than the policed rate. However, you can increase the burst in order to achieve better utilization. A good start is to set the burst equal to twice the amount of the traffic sent with the desired rate during Round-Trip Time (TCP RTT). If RTT is not known, you can double the value of the burst parameter."

Another point of view is shaping on Distribution Layer ... Let me know

For more infos, see:

http://www.cisco.com/en/US/products/hw/switches/ps646/products_tech_note09186a00800feff5.shtml

HTH

Andrea