From: Paresh Khatri (Paresh.Khatri@aapt.com.au)
Date: Sun Dec 04 2005 - 21:06:25 GMT-3
Hi all,
I'm just trying to re-inforce my understanding of how the different elements of QoS work with MQC and would appreciate any comments on the following:
Outbound Policing
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Why use egress policing instead of just ingress policing ?
To limit traffic of a particular class at an egress interface. If only ingress policers were used, there would be no way to limit traffic of that class at the ingress interface without setting inflexible hard limits on ingress. For example, say you want to limit DSCP AF41 traffic egressing out of an interface to 10Mbps. And say that you have two ingress interfaces, A and B, from which you are receiving AF41 traffic. If you use an egress policer on the egress interface to limit all such traffic to 10Mbps, you will never exceed that amount of traffic in the outbound direction. In addition, the sum total of all AF41 traffic received over A and B will be limited to 10Mbps. That means that you could support 10Mbps of this traffic from A if B is not offering any such traffic. However, if you use ingress policers on A and B and set hard limits of 5Mbps for each, the traffic ingressing each of these interfaces can never exceed 5Mbps. Therefore, the egress policing mechanism !
works better in this case.
When is egress policing applied ?
It would seem to make sense that egress policing is applied as packets are switched to the egress interface i.e. before these packets are placed in the queue for the class on that interface. It would not make sense to enqueue the packets before policing them since you would lose the inter-packet arrival time information for these packets once they are queued (and hence, would not be able to police them).
Comments appreciated.
Paresh.
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