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Re: UUNET Routing issues

  • From: Rafi Sadowsky
  • Date: Sat Oct 05 11:35:59 2002


## On 2002-10-04 23:50 +0200 Iljitsch van Beijnum typed:

IvB>
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 ?

IvB>
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

 >>>	 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)


   For a reality check you may want to have look at the links under
"Satellite links and performance" on
 <http://www.internet-2.org.il/documents.html>
(yes the docs are a bit dated but the principles aren't)

 >>> 20 ms worth of
IvB> buffer space with RED would keep those high-bandwidth applications in
IvB> check and allow a reasonable degree of interactive traffic. Maybe a
IvB> different buffer size would be better, but the 20 ms someone mentioned
IvB> seems as good a starting point as anything else.
IvB>
IvB>


-- 
	Rafi