At 09:40 AM 1/11/01 +1300, Geoff Thornburrow (DSL HN) wrote:

>> >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.

Which is what I was discussing in the message that he replied to :-)

>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.

Yes, I think they use either a buffered or large windowed throttling
algorithm, which is what I was getting at in my original message. If they
are, I think its a good idea because it allows the overall download speed
to be controlled, but without ruining the interactive responsiveness.

Regards,
Simon

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Received on Thu Nov 1 10:01:48 2001