[smokeping-users] Packet loss calculations in Smokeping

[Vern.Dias at VerizonWireless.com]

Sorry Tobi, not intending to imply that Smokeping is "fatally flawed".
I am simply trying to understand how the "loss" field is calculated and
how we have, for example, a 92.1% in the packet loss field of an rrd
record.

Thanks for jumping in. As the creator of Smokeping, I am confident that
you will provide an answer that I can pass on to those in the
corporation who are asking for this to be explained. 

Vern

-----Original Message-----
From: Tobias Oetiker [mailto:tobi at oetiker.ch] 
Sent: Monday, February 12, 2007 10:12 AM
To: Dias, Vern
Cc: smokeping-users at lists.oetiker.ch
Subject: Re: [smokeping-users] Packet loss calculations in Smokeping

Hi Vern,

Today Vern.Dias at VerizonWireless.com wrote:

> 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.

note that the first 3 fields in the RRD file are not round trips, but
the results of the analysis of the pings sent out in each round. We
store this data explicitly since it can not be rebuilt later from what
is stored in the rrd file:

0 - uptime - this is for measuring ip retention time with dynamic ips
             this is set to NaN unless you use this feature

1 - loss   - the number of lost packets

2 - median - the median round trip time

3-n - ping[1-n] - the ping latency

the roundtrip times are 'sorted'. if not all pings are successful the
successfull ones are positioned n the middle

> 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.

it does this ... but as you explained earlier, since the pings do not
arrive at 'step-time' it takes normally two losses in a row to manifest
them selfs in the log ...

regarding your 'analysis' ... don't you think something so fatally
flawed would have triggerd some discussion in all the years smokeping
has been around ? if you check the mailinglist archives you will see
that it has and eventually people understood how the data is handled,
and why it happens the way it does ...


cheers
tobi

--
Tobi Oetiker, OETIKER+PARTNER AG, Aarweg 15 CH-4600 Olten
http://it.oetiker.ch tobi at oetiker.ch ++41 62 213 9902


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