Rate Limiting - CAR and burst

Unanswered Question
Aug 6th, 2008
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I've been asked to implement a 1Mb ethernet service with a 10MB burst.


As QoS is not my strong point I'm looking for advice on whether the following configuration is correct:


rate-limit input 1000000 10000000 10000000 conform-action transmit exceed-action drop

rate-limit output 1000000 10000000 10000000 conform-action transmit exceed-action drop


Also, if a burst is allowed up to 10MB, how long is the burst allowed for and what prevents the user from constantly bursting at that rate?


Any advice is appreciated


Regards


Steven

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Giuseppe Larosa Wed, 08/06/2008 - 12:33
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Hello Steven,


CAR works using a leaky bucket algorithm.


Traffic packets with their sizes are monitored.


It is a model where every second:

R/8 new credits are added to the normal bucket according to the bit rate R.

Every packet allowed consumes credits = its size in bytes.

So when a packet arrives and there are no credits for its size in the bucket it is discarded.

On the long time the allowed rate is the configured rate R.

here you have defined a normal burst size of 10MB and a second bucket of 10MB. the extended burst size.

The logic changes so that if the normal bucket is empty CAR looks at the second bucket to see if there are enough credits to send the packet.

Notice every second only R/8 credits are given to the buckets.

Here, if for some time traffic is low or at the beginning an effective burst of normal + extended is allowed.


Hope to help

Giuseppe



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