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From: huitema@JERRY.INRIA.FR (Christian Huitema)
Newsgroups: comp.protocols.iso
Subject: Re: TP0-4 question (novice level)
Message-ID: <8910201419.AA11840@jerry>
Date: 20 Oct 89 14:19:15 GMT
References: <47107@bbn.COM>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
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Craig,

The outcome of "TELNET + TCP-IP" should be compared to either "X.29 +
X.25" or "VTP+PRES+SES+TP[0-4]", not to "TELNET+TP[0-4]". Both X.29 and
VTP+PRES implement character buffers, so that the size of the packets
will grow when the TP (or net) window based control becomes a
bottleneck. I know that this work in practice with X.28/X.29, but must
confess I never saw anyone using VTP...

In any case, you are right for the particular handling of timers which
is necessary with TCP-IP over X.25. You get strange patterns at low
load condition due to the window mechanism, which were however somewhat
hidden here because we use large windows + dont use end to end window
on X.25 (no ``D'' bit). We also use to get straight congestion patterns
if too many stations competed for the X.25 vc linking two subnets: if
the effective throughput dropped under 9.6k, the whole thing would fall
apart. That was with berkeley-4.2 implementations, and I suppose that
better time-out prediction algorithms would have helped, provided they
could converge on a window of less than 2Kbytes and a timer over 5 secs.

Chrsitian Huitema