On Monday 13 March 2006 19:34, Russell Stuart wrote: <snip> > My calculations in that email were wrong for PPPoA - as > someone else pointed out. This is how I calculated it for > PPPoA: > > PPP overhead = 2 > ATM AAL5 SAR overhead = 4 > ----- > 6 > > Those calculations are right as far as they go, but > unfortunately, that isn't the end of the story. On > outbound traffic, ie on traffic your box is transmitting, > the packet length reported by the kernel and thus used by > htb's rate calculations includes the layer 2 header. If > your layer 2 is Ethernet, then this header is 14 bytes > long. I had not allowed for this. For PPPoA this header > is stripped off by your ADSL modem before the packet is > transmitted over the wire. In effect that means the kernel > has added 14 bytes of overhead that doesn't exist. So > the real story for PPPoA is: > > PPP overhead = 2 > ATM AAL5 SAR overhead = 4 > less Ethernet header added by kernel = -14 > ---- > -8 > > So you need to give a negative overhead figure to "tc". > Unfortunately there is currently no way to do that. > > The same calculation for PPPoE is: > > PPP overhead = 2 > PPPoE overhead = 6 > Ethernet Header = 14 > Ethernet CRC = 4 > ATM AAL5 SAR = 8 > less Ethernet header added by kernel = -14 > ---- > 20 So, instead of PPPoA + VC/Mux: tc class add htb … overhead 10 atm PPPoA + VC/LLC: tc class add htb … overhead 18 atm PPPoE + VC/Mux: tc class add htb … overhead 34 atm PPPoE + VC/LLC: tc class add htb … overhead 42 atm we have? PPPoE + VC/Mux: tc class add htb … overhead 20 atm ? PPPoA + ?: tc class add htb … overhead ? atm ? What's the implication of a negative overhead value for PPPoA? Does that reduce the overhead per MTU such that it positively compensates for the standard 5 byte overhead per ATM cell for each packet? Obviously I don't follow. -- Jason Boxman http://edseek.com/ - Linux and FOSS stuff _______________________________________________ LARTC mailing list LARTC@xxxxxxxxxxxxxxx http://mailman.ds9a.nl/cgi-bin/mailman/listinfo/lartc
