perfsonar development work

["Jeff W. Boote" <>]

Hi Sven,

Sorry - just noticed this. I skipped it earlier thinking it was a JRA-1 issue 
only.

Sven Ubik wrote:
> 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.
>
> I think this describes what OWD measurement does, but not why we NEED 
> it. In my opinion importance of OWD is sometimes overstated, RTT is 
> usually enough to assess effect of delay on throughput. It is rather 
> hard to find an application that would really benefit from OWD instead 
> of RTT. I think that one use of OWD (or detection of assymetry in delay)

I'll give you 2 off the top of my head. I'm sure you also thought of these, but 
don't think OWD is really needed.

1) real-time remote device control
2) video/audio conferencing

These two applications probably only account for a small percentage of the 
packets on our networks, but I suspect they account for much higher percentage 
of the perceived value (and performance).

> could be in assessing whether router queues fill up, which a sort of TCP 
> implementations starting from Vegas unsuccessfully tried to do using RTT.

It is IPDV that is useful. But, you can't get that without OWD.

I would suggest that you have been able to get by just fine with RTT only 
because of two things:

1) End hardware for these applications has typically introduced more packet 
variation than the backbone networks.
2) Typical paths have been short enough that introducing a relatively large 
'jitter buffer' has not been too cumbersome.

I am not convinced these conditions will continue to hold true.

jeff