perfsonar development work

[Verena Venus <>]

Hi Nicolas,

Maybe I can provide some answers. Note that these aren't the only ones, as 
these questions could be answered quite extensive and with regard to 
different aspects. So, what I am providing here is probably not exhaustive.

Am Tuesday 31 July 2007 17:13 schrieb Nicolas Simar:
> Hi,
>
> the question that is typically asked is
> 1) why do we need OWD?

.... instead of ping? The answer to this question would probably be: If you 
have an asymmetric path where one direction is significantly slower (or 
faster ;) ) than the other direction, you cannot see this with ping, because 
ping measures only the round trip time.
You need to measure both directions seperately to find out such behaviour.

> 2) why do we need a 1 ms accuracy?

Some effects on the network are very small but may have a noticable impact on 
an application. If the accuracy of your measurement is not high enough, you 
cannot see the correlation.

> 3) why wouldn't NTP be good enough?
>
This question surely is mistakeable... 
NTP is perfectly fine. It depends on the time _source_ how "good" it is.
If you are using a GPS clock as time source in your NTP daemon, you probably 
cannot get something better.
Synchronising your NTP to a far away time source over modem cable and half of 
the internet surely is quite bad.

So probably it's more the question of why not using NTP servers on the net 
rather than using a clock directly. And this depends on the delay between 
your system and the NTP server. Using more than one NTP server also helps to 
make your time "better". 

> I think we should have a web-page providing some answer to those
> questions. But what are the answer to those questions?
>
> Can you help me here?
>
> Cheers,

I hope this helps. To all: please comment (see "disclaimer" above).

Verena
-- 
Verena Venus  -  DFN Labor

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