[e2e] What are the shared resources? Re: Codel and Wireless
detlef.bosau at web.de
Mon Dec 23 13:02:24 PST 2013
Am 23.12.2013 01:30, schrieb Andrew Mcgregor:
> Look up all the research on 'effective bandwidth' and 'grade of service'
Frankly spoken, I was upset by your post.
What is "effective bandwidth"? Some other average - from what you derive
short term forecasts from long term observations.
This is no way a resource.
Perhaps, it was a bit courageous to put in question the "end to end
principle" and VJCC that frankly.
Perhaps, it was stupid.
However, the more I think about it, VJCC appears to me like a very
effective approach to work around congestion collapses and
retransmission collapses. However, as soon as we leave a qualitative
understanding of VJCC and try to develop a quantitative understanding,
the whole thing appears to me as a justification ex post.
As far as I see, VJCC was developed within a certain context: TCP as
designed in RFC 793 does not care for resources. As a result, we had
several congestion collapses, or retransmission collapses respectively.
VJCC attempted to work around these collapses: When a TCP flow suffered
from packet loss, this was taken as an indication for congestion and the
TCP flow was slowed down. (Actually, nothing was slowed down, on the one
hand the sending window was halved, the relation between window and rate
is a non trivial one, on the other hand, the RTO was doubled. Both
lowered the load offered to the network by the sender.)
All the quantitative interpretations of VJCC, rationales with Little's
law, "Bandwidth Delay Product". all the TCP models where, yes it sounds
harsh, in a sense hand waving.
A posteriori interpretations of an extremely useful work around - which
however never attempted to do any resource management.
Admittedly, I would appreciate talking about this issue with colleagues
here in Germany because the issue is extremely touchy and I would
appreciate the opportunity of talking about these issues in my mother
tongue. Unfortunately, I don't know anybody who is interested in this
kind of discussion.
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