Subject: large RTT variation caused by link bandwidth oscillation

From: Gorry Fairhurst <[email-masked]>
Date: Sun, 09 Dec 2001 23:29:18 +0000

The points raised about TCP behavior over links which
see a varying RTT, are classic symptoms of the chosen
"bandwidth on demand" allocation strategy. I remember
this effect with IP over X.25.  Similar lessons would
apply to narrow-band ISDN channels.

The tx interface builds a queue of excess traffic.
Extra capacity is allocated on a high speed channel.
Queue disappears. SRTT goes down, if low for several
path RTT. RTO becomes low. High speed link deallocated,
path RTT now higher than RTO.

So, is this a problem with the allocation strategy
of TCP?

Gorry Fairhurst