On Mon, 2006-03-13 at 13:09 -0500, Jason Boxman wrote:
> I finally patched my 2.6.15.5 kernel last night and use Stuart's userspace
> `tc` patch and I'm up and running. So far, things are working extremely
> well and exceeding my expectations. I only wish I actually knew my PPPoATM
> overhead, but my modem won't share. :) (I settled on the highest overhead
> number provided by Stuart in this thread, although I couldn't match them up
> with the numbers presented in your thesis' ATM overhead summary values so
> I'm confused.)
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
It was also pointed out that inbound, there is also
a overhead added to the packet size - a different one.
However the "igress" policing feature isn't commonly
used, so that doesn't matter. What does matter is
the IMQ interface. For that interface the original
figures I gave were correct - ie there is no header.
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