Re: UUNET Routing issues

From: Iljitsch van Beijnum (no email)
Date: Sat Oct 05 2002 - 12:29:38 EDT


On Sat, 5 Oct 2002, Rafi Sadowsky wrote:

> IvB> Obviously "some" packet loss and jitter are normal. But how much is
> IvB> normal? Even at a few tenths of a percent packet loss hurts TCP
> IvB> performance. The only way to keep jitter really low without dropping large
> IvB> numbers of packets is to severely overengineer the network. That costs
> IvB> money. So how much are customers prepared to pay to avoid jitter?

> There may be better ways to keep "reasonable" jitter but that depends on
> what is "really low" jitter - care to define numbers ?

I don't use applications that have jitter requirements, so I'm not in the
best position to comment on this. I'd say that with a line utilization of
50% or less, which leads to an average queue size of one packet or less,
jitter is "really low". If the level of jitter introduced here is too
high, then I don't think the application can successfully run over IP.

> IvB> In any case, delays of 1000 ms aren't within any accepted definition of
> IvB> "normal".

> Ever used a satellite link ?
> Practical RTT("normal" - end to end including the local loops at both
> sides) starts at about 600msec

So then a satellite link with a 1000 ms delay wouldn't be normal, would
it?

> >>> With these delays, high-bandwidth batch applications will
> IvB> monopolize the links and interactive traffic suffers.

> I'm assuming TCP since you didn't state otherwise
> TCP extensions for "fat pipes"(such as window scaling and SACK) disabled
> (as both sides of the TCP connection need to have them)

> IIRC the maximum TCP(theoretical)session BW under these conditions
> Is less than 1Mb/sec (for 600msec RTT)

Ok, so "1 Mbps batch applications" will monopolize the links and
interactive traffic suffers.








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