[smokeping-users] Packet loss calculations in Smokeping

[Chris Wilson]

Hi Vern,

On Mon, 12 Feb 2007, Vern.Dias at VerizonWireless.com wrote:

>  I have no control over the disclaimer. It is added automatically to all
>  outgoing mail by the companies mail gateway.  Legal will do what they are
>  required to do, I apologize in advance, but it's going to be on anything I
>  send.

You could ask them to do something about it. If nobody asks, then it won't 
happen.

>  There are never any NAN values that I have found in the actual latency
>  fields, regardless of the number of lost pings.  To me this means that any
>  attempt at having rrdtool calculate an accurate packet loss value is doomed
>  from the start. Looks to me like rrdtool is attempting to calculate all
>  latency values.

This is outside my understanding of rrdtool. Perhaps a Smokeping or rrdtool 
developer can help you more?

It's possible that smokeping interpolates the values itself in order to make it 
easier to draw the graphs.

Please remember that smokeping and rrdtool are _all_ about drawing pretty 
graphs, NOT about accurate storage and reporting. They are inaccurate by 
design.

>  Seems to me, that in the case of a packet loss situation within a single
>  poll, it is up to Smokeping to feed actual data from all 10 polls to
>  rrdtool, even if one or more or all of the polls are timeouts.

Yes, I expect that smokeping does have to interpolate some response values in 
order to "create" the missing values for the purpose of drawing the graph.

>  I'll leave the details of that to the developers of SmokePing, however I
>  would count the number of ping requests that are lost and use that value for
>  loss, rather than have rrdtool try to calculate it.

I think that it does, since clearly rrdtool does not have enough information to 
calculate the loss itself.

>  Having rrdtool
>  calculating averages to fill in the holes within a single rrd record
>  seems to me to be a very bad approach for calculating loss data
>  (although a good thing for interface traffic data).

I don't think it does quite that. I think that smokeping is doing the 
interpolation to ensure that the rrd database is full.

>  Since we are using Smokeping data for outage reporting, it is very important
>  to us the loss data be an accurate reflection of the loss on the network.

I believe that you cannot use rrdtool for this purpose, for which it was not 
designed and is evidently unsuitable. Nagios might serve you better; we use it 
for this purpose and it works well.

Cheers, Chris.
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