[e2e] packet-pair probe implementation

[Rik Wade]

On Mon, 12 May 2003, Michael Welzl wrote:

> Isn't this discussion pointless unless we have an idea what exactly
> the "available bandwidth" may be?
> 
> I mean, the available bandwidth during what interval? Or an available
> bandwidth that will be available for interval x with probability y,
> according to prediction method z?
> 
> Sadly, it's more complicated than "bottleneck capacity with
> cross traffic".

I spent a great deal of time in simulation environments working with
packet pair and other methods of available bandwidth discovery. My 
opinion is that packet pair can be used as a "reasonably good estimation"
of the volume of data you can successfully transmit over a given path. 

In order to maximise "goodput" rather than "throughput", I used packet pair
as an estimation of the available capacity on the link. A token bucket flow
control element was then initiated to send at the estimated rate. The flow
from the token bucket was changed in response to variations in RTT. Therefore
after a small number of ACKs had been returned, I was able to quickly 
adjust the transmission rate in response to the current network status. This
change was proportional to the change in network conditions (increase/decrease
in RTT).

So like I say, I believe that packet pair is a good "estimator", bearing
in mind that the information is ~1/2 RTT old by the time you receive it.
Furthermore, as others have mentioned, it is slightly dependent on the
queueing algorithms configured on the path. However, I'm inclined to 
forget these as a major influencing factor from a Real World point of view.

--
rik