At 05:28 PM 30/10/01 +1300, Bruce Hoult wrote:

>At 4:41 PM +1300 30/10/01, Nicholas Lee wrote:
>>On Tue, Oct 30, 2001 at 10:32:00AM +1300, Bruce Hoult wrote:
>>>
>>> - small isolated requests of less than maybe 3 - 4 KB appear to come
>>> down at full DSL line speed, NOT at 16 KB/sec. Small text web pages
>>> (e.g. www.theinquirer.net) come down at ShitThat'sFast speed -- no
>>> doubt from Paradise's cache. So do most usenet posts.
>>
>>
>>Interesting. Haven't really examined that closely myself.
>>
>>Since they use packet dropping to limit traffic the first couple packets
>>might get in before the shaping starts.
>
>Packet "dropping""?? Wouldn't that cause extra traffic from retries?
>Surely it's more like a queue with a fixed release rate? The queue
>would want to be at least as big as the TCP window size.

All forms of traffic shaping ultimately work by dropping packets, the
difference is in which packets they decide to drop, and when. Some
algorithms do a certain amount of buffering, but you can't just buffer
indefinately because the device doing the traffic shaping only has a
certain amount of memory available and could potentially be traffic shaping
thousands of different connections.

Just buffering and releasing packets at a certain rate isn't good enough
because large window sizes that are typically used mean that as much as
64KB of data (or more) could be sent by the sending end before it stops to
wait for replies from the other end to catch up, and thats far too much to
be buffered by a router doing traffic shaping. At the end of the day if the
data is comming in too fast you've just got to throw some away. (With TCP
it will be retried at a slower speed, with UDP it will be lost)

TCP is smart enough that when packets start going missing it throttles back
the sending rate and/or the window size until packets stop going missing,
and maintains the speed (more or less) at that optimal point. It tries to
creep the speed up slowly, if packets start going missing it slows down a
bit again.

If you have a throttled download (say 16KB/sec) and you suddenly remove the
throttling, it doesnt suddenly go faster, it can take up to 20 seconds or
more for the speed to climb back up, (to say 200KB/sec) as the endpoints
progressively try going faster and faster until they reach the new point of
equilibrium.

(This speed creep also happens when new connections are first created - so
called "slow-start" where they begin at a conservative speed, then
progressively try to go faster and faster until they reach the point where
packet loss begins)

Regards,
Simon

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Received on Thu Nov 1 09:36:23 2001