On 24/5/06 11:13 AM, "Mark Seward"
Interesting numbers. After a few weeks of bad performance on an Internet feed I look after I was drawn back to an issue I thought was long buried. This circuit was dropping 4-5% of its traffic at well below advertised capacity. The delivery method was ethernet over ATM/AAL5 (RFC1483). The usual suspects were rounded up and shot (duplex and error counters, purple switches, the cleaning lady).
The next question was overhead for cell tax and encapsulation. 12% seems to be the allowance by our supplier. IIRC this is the same as was used 10 years ago for ATM VCs delivered directly to routers (no ethernet involved); nanog has a few dusty posts about this number. It seems this is no longer enough. Perhaps average packet sizes have changed since then if 12% was ever a reasonable number.
In the end our supplier ramped overhead to about 20% and the pipe has performed nicely since. For an ethernet service, I don't understand why they don't simply provision the VC (VBR) as large as the customer bearer can handle and let the end devices do the shaping.
OK, I've tried out the effects of the various encapsulations, and get ATM Cell Tax Upstream: 18.35% (23.43% AAL5, 29.28% rfc1483/2225, 30.01% PPPoA, 29.68% UBS) Downstream: 14.84% (18.09% AAL5, 20.70% rfc1483/2225, 21.13% PPPoA, 20.55% UBS) The raw upstream is not terribly useful---it's just the ATM overhead. At a minimum you'll get AAL5 overhead. If you use rfc1483 or rfc2225 encapsulation you'll get some more. PPPoA has a different overhead, and for L2TP as per UBS, you'll have L2TP + PPP + RFC + AAL5. Note that this is just the cell tax. I.e., after you've padded your IP packet with L2TP + PPP + RFC, that's how much extra you need to allow for in your cell budget. I've heard off-list that our distribution matches at least one other organization. 12% seems to be the overhead you'd get for sending 1,500 byte packets in an ATM PVC. Basically, it is the minimum overhead you can ever achieve. -- Michael Newbery IP Architect TelstraClear Limited Tel: +64-4-920 3102 Mobile: +64-29-920 3102 Fax: +64-4-920 3361