Re: Limit bandwidth per-user (uid/gid)

[Rick Jones <rick.jones2@hp.com>]

> In your case I'd probably set the fq_codel target at 60ms or so, and interval
> pretty high as well (700ms RTTs? really?), and enable ecn in both directions

Well, if their satellite link is via a geosynchronous satellite, it is 
something like 22,236  miles above the equator [1].  One RTT that way 
will be 4x that distance.  However, since they are in Antarctica it will 
be a bit farther.  The diameter of the Earth is something like 7918 
miles (Google).  So, if we ass-u-me they are actually at the pole, we 
can add 1/2 that (3959 miles) to the 22,236 miles and call that one leg 
of a right triangle with the right angle at the center of the Earth, the 
other leg then is 3959 miles from the center of the Earth to the pole, 
and we can use good old Pythagoras to get the length of the hypotenuse, 
which will be the distance from the pole to the satellite.

Distance^2 = 3959^2 + (22236 + 3959)^2
Distance^2 = 15673681 + 686178025
Distance^2 = 701851706
Distance = 26492 miles

4x that is just shy of 105969 miles, which when divided by 186,000 miles 
per second yields 570 milliseconds.  Of course that assumes one is 
talking pole to pole which is unlikely, but it does get us a good chunk 
of the way to 700 milliseconds.

I think the initial posting said 512 kbit/s for the link speed, so if we 
further assume Ye Olde 64 Byte Ping that will be 64 + 20 + 14 bytes or 
784 bits.  At 512000 bits/s that would seem to be another 1.5 
milliseconds for each time it gets transmitted or 6.125 milliseconds all 
told taking us to 576.  That assumes 512 Kbit/s is after FEC and such 
and that this is all store-and-forward.  And probably assumes other 
things about the transmission but I've no idea what they would be.

If it is actually something like Iridium then it gets more complicated - 
there will be some average number of satellite to satellite hops on the 
way to/from the Iridium base station.

And of course there is whatever the RTT's are from the satellite base 
station to the actual destination(s).  I could see that easily adding 
another 100 milliseconds but that is just handwaving.

rick jones

[1] http://en.wikipedia.org/wiki/Geostationary_orbit

There are probably some papers out there on Iriduim performance one 
could find via a web search