[no subject]

This is as much art as science.  My rules of thumb:

- High speed backbone interfaces, 1-3ms of buffer.
- Medium to high speed links inside of a single pop/site, 2-5ms of buffer.
- Low speed access/edge, 5-20ms of buffer.

I have rarely seen an application benefit from more than 20ms of buffer.
Remember, this is per hop.  If you take a 20 hop traceroute and each hop
that 20ms of buffer, you would be waiting 400ms if all the buffers were
full!  That's even if all 20 hops are in the same building, so the RTT
is 1ms.  Imagine how crappy a 1ms RTT would be with random 4/10ths of a
second pauses would be!

Now, here's where it gets non-intuitive.  Reducing the buffers will
_increase_ packet drops, which will make your customers _happier_.
It will also generally smooth out sawtooth patterns (caused by
congestion collapse syncronization, everyone fills the buffer at the
same time, backs off at the same time, etc).  So your links may go from
spiky between 90-100%, to flatlined at 100%, but your customers will be
happier.

Run the math the other way to see how many ms your current buffer size
allows the router to hold.

-- 
       Leo Bicknell - bicknell at ufp.org - CCIE 3440
        PGP keys at http://www.ufp.org/~bicknell/
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