QOS Mecanism

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Nov 20th, 2008
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I need explications about the Qos mecanism (dscp field). There are class of services (D1, D2 and D3 ...)

There are D1 IN Profile and D1 OUT of Profle and ... D2 OUT of Profile ...D3 IN Profile ...

This is my question: Is D3 IN of Prolile most priorized than D2 OUT of Profile ?

And where is configured that ?

Thanks for your help

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Joseph W. Doherty Thu, 11/20/2008 - 06:27
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If you're asking about AF drop tiers, e.g. AFx1, AFx2, AFx3, they can indicate to queue management, when there's congestion, which packets should be dropped/discarded first if necessary. AFx3 should be dropped/discarded before AFx2 which should be dropped/discarded before AFx1.

In and out of profile usually concerns the same traffic being monitoring for bandwidth utilization vs. some contracted bandwidth expectation. For instance, FTP traffic under 1 Mbps might be considered in-profile and marked with AFx1. The same traffic, up to 2 Mbps, might be marked AFx2. Any of this traffic exceeding 2 Mbps might be marked AFx3.

However, you can use the drop tiers for other reasons. FTP traffic between production servers could be marked AFx1 but between test servers, could be marked AFx2, and perhaps FTP to/from the Internet marked AFx3.

Whether the marking were set to indicate in or out of some agreed bandwidth profile, or whether they indicate relative business importance, the marking indicates to queue management, if there's a need to drop/discard packets, the importance of traffic within the same AF class.

Since you can mix why packets are marked, we would always assume AFx3 should be dropped/discarded before AFx2 regardless of why they were marked such.

jine Thu, 11/20/2008 - 06:47
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Ok thanks for your reply.

But there are a point not clear ...

AF13 is most priorized than AF33 or not ? Is AF33 is dropped in first or no ?


Joseph W. Doherty Thu, 11/20/2008 - 07:00
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Between AF classes, i.e. AF1x vs. AF3x, my understanding of the AF model drops/discards for each is independent of other AF classes. Whether AF33 is dropped/discarded before AF13, or the converse, should depend on what is happening within the individual AF class. This assumes each AF class has its own bandwidth allowance, so drops/discards within the class should occur independently of other classes.

In practice I don't usually restrict an AF class to contracted bandwith if there's available bandwidth. This does allow AF classes to compete for bandwidth against each other, and building upon prior IP Precedence, we often consider higher AF classes, e.g. AF4x vs. AF1x, more important. If we do, it's likely we'll drop AF13 before AF43, or even AF11 before AF43, but drop/discard priority combinations, when not doing "pure" AF, really depends on your policies.


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