Re: Packet queueing / scheduling ?

From: Kasper Dupont (kasperd_at_daimi.au.dk)
Date: 12/02/04 

Date: Thu, 02 Dec 2004 20:49:28 +0100

Robert Redelmeier wrote:
>
> or UDP (data better never than late).

Not all UDP traffic is like that, but most of it
probably is. Some RR or RPC protocols will just
retransmit a few times in case of lost packets.
If we can make a roundtrip before the retransmit
timeout, late is better than never. If OTOH the
roundtrip is longer than the timeout, a drop may
reduce overall traffic assuming the second
roundtrip can make it before the timeout. Any
decent protocol will increase the timeout after
each try, which should mean a better probability
in the second attempt.

>
> A big problem is that apps are not aware of other running
> apps. It is the OS's job to arbitrate.

Absolutely.

>
> Probably true. I just hope most apps don't hammer
> the resends but have some timers.

The application have to implement a timer. Even
if there is no congestion, there will still be
a delay.

>
> You check the closest machine past the expected bottleneck.
> I'm on 1.5/256 ADSL and get very consistant 12ms ping times
> from my ISPs gateway machine.

Of course you are right about that. It is just so
difficult to tell which router it is. Those cable
modems can behave really strange sometimes. If a
traceroute show me, that the first two routers
respond, but the third is unreachable, I know the
solution is to reboot the cable modem. (Don't ask
me why it is so, if have learned to restart the
cable modem whenever there are unexplainable
symptoms). In fact though the cable modem have an
IP adress, but the cable modem is not gateway, the
gateway is a computer outside. In a traceroute
neither the IP address of the cable modem, nor the
IP address of the gateway show up. 

-- 
Kasper Dupont

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