From: Dale Shaw (dale.shaw@gmail.com)
Date: Thu Feb 26 2009 - 04:32:58 ARST
Hi all,
Regarding 'shape adaptive' ..
I've read the docs on this and it seems pretty straight-forward, but
something I read in Wendell Odom's QoS book is a bit confusing.
When talking about the way adaptive shaping brings the actual shaping
rate back up to the configured shaping rate, he says:
"As a reminder, after 16 consecutive intervals with no received BECNs,
this router would start increasing the Shaping rate by 1/16 of the
original 96-kbps shaping rate teach Tc. To do so, CB Shaping actually
starts replenishing the token bucket with a little more each Tc,
specifically by (Bc + Be)/16 each Tc. As a result, the rate will
likely increase more slowly than it decreased in the presence of
BECNs."
It's the second sentence that's got me stuck. Does adaptive shaping
cause the actual shaping rate to go up by 1/16 of the original rate
each Tc, or by the amount specified in the formula (Bc + Be)/16 each
Tc? Because they are not necessarily the same.
Example:
Configured shaping rate: 4,000,000bps (4Mbps)
Tc: 25ms (default)
Bc: 100,000 bits (derived from Tc)
Be: 100,000 bits (same as Bc)
1/16 of 4,000,000 = 250,000, implying that the actual shaping rate
increased by 250Kbps every 25ms.
The second sentence implies that it would actually increase by 12,500
(12.5Kbps) every 25ms.
Which is it? Or am I just confused :-)
cheers,
Dale
Blogs and organic groups at http://www.ccie.net
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