Ping's definition of mdev

Derek Burdick derek at burdick.cc
Fri Jun 17 00:27:44 MDT 2005


I found the answer.  It is the standard deviation.  mdev stands for mean 
deviation.

Derek Burdick wrote:

> I searched the man page and google but can not find a definition for 
> mdev, which is a returned statistic when ping exits.  Does anybody 
> know what it is measuring?  I have looked through the code and 
> determined mathematically what the algorithm does, but I don't see any 
> relevance to networking.  Does anybody have any insight?  I have 
> attached my comments on what the code is doing in case somebody knows 
> of a generic use for this algorithm.
>
> Here is the sample output
> 3460 packets transmitted, 3451 received, 0% packet loss, time 119983ms
> rtt min/avg/max/mdev = 303.108/305.426/347.859/2.979 ms, pipe 11
>
> Here is what I believe ping_common.c is doing to determine mdev:
>
> for each packet received and assuming tsum and tsum2 start at 0:
> tsum += triptime
> tsum2 += triptime * triptime
>
> Once ping finishes
> tsum /= nreceived + nrepeats // Get the average of the triptimes.  We 
> have 0 repeats so it has no effect here.
> tsum2 /= nreceived + nrepeats // Get the average of the squared 
> triptimes.  Once again we have 0 repeats
>
> tmdev = llsqrt(tsum2 - tsum * tsum); // Take the square root of the 
> difference of the average squared triptimes and the average triptime 
> squared.
>
> tmdev is then printed as printf("%ld.%03ld ms", tmdev /1000, 
> tmdev%1000);  //This is giving us the results in milliseconds
>
> If anybody has any insight or comments i'd love to hear them.
>
> Thanks,
>  Derek
>
> p.s.  This is not a class assignment.  Our sales team wants to sell IP 
> phone service to a client in India using the Internet for the 
> bandwidth.  I was looking for any numbers I could find to help me give 
> them a good idea of what to expect.
>
> I think I will add a Jitter calculation in to ping as that is usually 
> a great indication along with packet loss on what to expect with VOIP.
>
> Thanks again.
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