Queue drops

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Jan 16th, 2008
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We are seeing continuous queue drop on one of the WAN link. As at the same time we see these queue drops, we are also seeing lots of activities for a particular PC. This PC is in a remote place and is heavily used.

Question, if we are not to get to change the PC is the nearest future what is the impact of these queue drop on the link?


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paolo bevilacqua Wed, 01/16/2008 - 05:07
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It makes you circuit congested, consequently all transfers are slowed down, in a way it is monopolizing the circuit. The simplest remedy is to enable "fair-queue" on the WAN interface, if that is not enough you may want to traffic-shapethat particular IP so it doesn't take more that a certain bandwidth.

Hope this helps, please rate post if it does!

Joseph W. Doherty Wed, 01/16/2008 - 17:46
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Your continuous queue drops might, or might not, be a problem. Besides the queue drops, does there appear to be network performance problems across the WAN link, especially intermittent?

The impact of queue drops depends on the type of traffic being dropped (e.g. UDP vs. TCP) and how it's being dropped (e.g. FIFO global tail, FQ tail, RED). With TCP, how packets are dropped also has somewhat different impact depending on what protocol and/or option "flavor" of TCP is being used (e.g. Tahoe, Reno, new-Reno, SACK, Compound TCP). There's also a question of whether you're trying to obtain best link utilization or minimize latency.

In other words, without analysis or what your traffic is and what you're trying to accomplish, one can't really provide a fully correct answer.

If you think you have a performance issue, Paolo's suggestion of using fair-queue (or fair-queue within CBWFQ), if not already active, is a great first step.

Paolo's other suggestion of traffic shaping the "problem PC" could be effective. Or, you could segregate its bandwidth consumption from other traffic using CBWFQ.


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