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QoS sensors show considerable PDV (Packet Delay Variation) readings, even through Local Loopback

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Using the QoS sensor we see readings of 20-40ms for Maximum und Minimum Packet Delay Variation while the average value is at 0ms.

We see these readings even when sending the sensor's UDP test packets through the local loopback to 127.0.0.1 (i.e. packets never leave the system nor do they touch the network).

On the local loopback we would not expect values above a few ms.

loopback packet-delay-variation pdv qos

Created on Jan 11, 2011 10:15:05 PM by  Dirk Paessler [Founder Paessler AG] (10,984) 3 5



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In a perfect world these values should stay between 0 ms and 2-5 ms.

But we have also seen PDV readings for UDP packets sent through the local loopback in the range of 5 ms to 30 ms (with an average PDV of 0ms).

This is usually the case on virtual systems (with high network/cpu load) and even more on systems that already have considerable network load. Both aspects cause delays for the packets (e.g. due to queueing) in the range of a few milliseconds which are then measured by the QoS sensor.

For probe devices with only marginal traffic we see PDV values below 2-3 ms.

Note: the PDV calculation was implemented according to RFC 3393: http://www.ietf.org/rfc/rfc3393.txt

Created on Jan 11, 2011 10:18:42 PM by  Dirk Paessler [Founder Paessler AG] (10,984) 3 5



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