[LARTC] Patch to allow for the ATM "cell tax"

[Jason Boxman]

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