On Fri, 2004-10-22 at 13:57, neil gardner wrote:
Well, I've never met Bruce, and I do read Aardvark most days, but I think it would be good to get someone that really _knows_ what the impact of reducing the line speed on latency will be.
We have a few dslams here for testing but they're been used for real work right now.
Anyone _know_ what the effect will be?
We can estimate some of the effects. Back of envelope calculations (ok, I lied, I have machines that can do this sort of stuff). Time taken to send 1 bit at 8,000,000 bits/second: 125ns Time taken to send 1 bit at 288,000 bits/second : 3.47us Average SYN and SYN+ACK packet size: 60 bytes / 480 bits Average ACK (w/out data) : 52 bytes / 416 bits Time to send SYN/SYN+ACK @ 8Mbps : 60us Time to send SYN/SYN+ACK @ 288Kbps: 1.67ms Time to send ACK @ 8Mbps : 52us Time to send ACK @ 288Kbps: 1.44ms TCP 3-way handshake at 8Mbps : 172us TCP 3-way handshake at 288Kbps: 4.78ms TCP handshake increase: 4.61ms (or nearly 2800%) I'm ignoring ATM overhead here, so these figures are going to be somewhat higher in the real world, but it serves to give you an idea anyway. I used the 288Kbps figure as it was used earlier in this thread. I can understand that from Telecom's perspective, moving the ratelimiting out to the edges of their network a more scalable way to do things. That doesn't mean I have to like it though, I'm going to miss leeching from JSG :-) Regards, Nic. P.S. Disclaimer: I occasionally have brain farts; my maths may be wrong. -- Nic Bellamy <nic(a)bellamy.co.nz> Bellamy Consulting -- Software & Security -- http://www.bellamy.co.nz/ Phone: +64-6-377-4957 Fax: +64-6-377-0505 Mobile: +64-21-251-8954