> >So is that why like sometimes downloads are real fast at the
> begining, and
> >then drop down low...
>
> Maybe, but not necessarily, depending on how you're measuring
> the speed.
>
> If you're talking about downloading files using Internet
> Explorer, thats a
> bug - it starts receiving data from the download while you're
> selecting
> where to save the file with the Save as Dialogue box, but it
> doesnt count
> the time.

There is also the issue of Jetstart rate limiting. From what I can see
watching the traffic through my gateway, the 128kbps cap is applied over a
window of time, so that in that window your average download rate is
128kbps. As the window size approaches zero, the limiting approaches a true
bandwidth cap. I've noticed window sizes sometimes appear to be in the order
of a few seconds, giving an immediate burst of up to around 1Mbps before
levelling off.

The other way this may work is if rate limiting is done by queuing and
delaying outgoing IP packets when the rate goes over the cap. If your link
is idle and your web browser or whatever opens a lot of connections, then a
burst of TCP initiation packets go out, followed by a flood of data coming
back. Since the initial burst of outgoing packets did not exceed the rate
cap and the link was idle, they are not delayed at all. Only when the
incoming data exceeds 128kbps are the outgoing TCP ACK packets delayed, thus
reducing the flow of data that the remote host is sending.

Of course someone who actually knows how the system works may be able to
enlighten us...

Geoff

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Received on Thu Nov 1 09:40:58 2001