On 9/27/2011 1:37 PM, tor-relays-request@lists.torproject.org wrote:

No idea what shaping algorithm Tor uses, nor any clue on recommended burst ratios under said algorithm. Anyone???.

I believe Tor uses a token bucket algorithm (https://secure.wikimedia.org/wikipedia/en/wiki/Token_bucket) for traffic shaping. Which works sort of like a fuel tank with a steady rate of fuel being replenished each second (RelayBandwidthRate) and a max capacity of RelayBandwidthBurst bytes. If the tank is full, up to RelayBandwidthBurst bytes can be consumed, but only RelayBandwidthRate bytes will be replenished each second. So there's a sort of reserve available of Burst - Rate bytes, and consumption > Rate bytes/second depletes the reserve, and consumption < Rate refills the reserve, up to the max capacity.

In general I think the higher the Burst you can allow, the better; it helps fast clients with small requests to zip through your relay quickly without clamping them to your overall Rate limit, and doesn't cost you anything since your long-term average is still enforced (bursts over your limit are balanced with slack periods under your limit). But I'm not clear on what the highest effective value would be. I could even see a point to having the Burst higher than your physical connection's actual limit, e.g. setting it to 10 times your connection's actual limit would allow bursts to max out your connection for 10 seconds (occasionally, to be balanced again by slack periods under your rate). But really, I doubt that values more than 2 or 3 times your Rate limit will actually matter, since Tor traffic doesn't seem to often come in those kinds of isolated high-multiple bursts (probably the natural averaging effect of having many clients routing dozens/hundreds? of circuits though your relay at once). A simple pragmatic test might be to run the Vidalia bandwidth meter for a day or two, and if the peak rate isn't hitting your Burst, it's high enough not to matter.

One interesting side effect of a high Burst might be its impact on the way your relay's bandwidth is measured by the official bandwidth scanners, though. If the bandwidth scanner's test download file is small enough (less than many second's worth of your overall Rate limit bandwidth), a high Burst might allow the test file to be downloaded much more quickly than with a low Burst, raising your official measured bandwidth rating (and drawing more traffic to your relay). Larger test download files would mitigate this, but increase the scanner's workload and time between each relay's re-scans. Just a thought, I don't know how much effect this actually has.