large RTT variation caused by link bandwidth oscillation

From: Gorry Fairhurst (gorry@erg.abdn.ac.uk)
Date: Sun Dec 09 2001 - 18:29:18 EST


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



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