On symmetric links, particularly PPP ones, one can use the LCP layer to do
echo requests to the first layer-3 device.  This can be used to measure RTT
and through some math, the bandwidth.

On assymetric links, my instinct is that if you can measure the downlink
speed through another mechanism, that one might be able to subtract, but I
can't think exactly how right now.
I'm thinking that one can observe the downlink speed by observing packet
arrival times/sizes for awhile --- the calculation might be too low if the
sender is congested otherwise, but the average should go up slowly.

At first, this means that subtracting the downlink bandwidth from the uplink
bandwidth will, I think, result in too high an uplink speed, which will
result in rate limiting to a too high value, which is bad.  

But, if there something wrong with my notion?

My other notion is that the LCP packets could be time stamped by the PPP(oE)
gateway, and this would solve the asymmetry.   This would take an IETF action
to make standard and a decade to get deployed, but it might be a clearly
measureable marketing win for ISPs.

-- 
Michael Richardson
-on the road-