On Thu, 2006-07-20 at 01:00 +0400, Alexey Kuznetsov wrote: > Hello! So you really do exist? I thought it was just rumour.
> Well, if fixed point arithmetics is not a problem. It shouldn't be. Any decimal number can be expressed as a fraction, eg: 0.00123 = 123/100000 Which can be calculated as a multiply and a divide. With MTU's up to 2048, it should be possible to do this with 99.9999% accuracy (ie 2048/2^23). With a bit more work in userspace (ie in tc), it can be be reduced to a multiply and a shift. > Plus, remember, the function is not R*size, it is at least > R*size+addend, to account for link overhead. Plus account for padding > of small packets. Plus, when policing it should deaccount already added > link headers, QoS counts only network payload. Yes, it is flexible - and has served us well up until now. It doesn't work well for ATM, but with a small bit of extra calculation in the kernel it could. However, it turns out that ATM is a special case. If ATM's cell payload was 58 bytes instead of 48 bytes (say), then it would not be possible to produce a RTAB that had small errors (eg < 10%) for smallish packet sizes (< 290 bytes). I seem to have trouble explaining why in a concise way that people understand, so I won't try here. So when Alan Cox said our ATM patch didn't solve the packetisation problem in general, he was right as our patch just built upon RTAB. Patrick's STAB proposal in general either for that matter, as it is just another implementation of RTAB with the same limitations. The only way I can think of to solve it in general is to move many more calculations into the kernel - as I proposed in a long winded answer to Patrick earlier in this thread. But doing so would get rid of the table implementation and the flexibility it has given us to date. For that reason I feel uncomfortable with it. The engineering decision becomes this - are there any other protocols like ATM out there that could justify such a change? (In my more cynical moments I think of it differently - has/is the world going to make a second engineering fuck up on the scale of ATM again? How on earth did anyone decide that pushing data packets over ATM, as happens in ADSL, was a good idea?) I know of no other such protocols. But then I don't have an encyclopedic knowledge of comms protocols, so that doesn't mean much. I suspect you know a good deal more about them than I do. What say you? _______________________________________________ LARTC mailing list LARTC@mailman.ds9a.nl http://mailman.ds9a.nl/cgi-bin/mailman/listinfo/lartc
