[Cerowrt-devel] Link layer adaptation VDSL2 - Basis for 8 byte overhead?

Sebastian Moeller moeller0 at gmx.de
Tue Apr 28 11:32:32 EDT 2015

Hi Kevin,

On Apr 28, 2015, at 15:20 , Kevin Darbyshire-Bryant <kevin at darbyshire-bryant.me.uk> wrote:

> Hi,
> Enquiring mind here :-)  Reading http://www.bufferbloat.net/projects/cerowrt/wiki/Everything_you_wanted_to_know_about_Link_Layer_Adaptation and the linked to email discussion, I'm curious as to where the 8 byte overhead recommendation for VDSL2 comes from.  Is this assumption that PPPoE is being used?  

	Not exactly, for all we know using PPPoE and a VLAN on a VDSL2 link results in:
VDSL2 header:
VDSL (IEEE 802.3-2012 61.3 relevant for VDSL2): 2 Byte PPP + 6 Byte PPPoE + 4 Byte VLAN + 1 Byte Start of Frame (S), 1 Byte End of Frame (Ck), 2 Byte TC-CRC (PTM-FCS), = 16 Byte

Or in other words, 8 byte either just reflect PPPoE or the real VDSL2 headers plus a VLAN (I am still unsure what to do with the ethernet FCS).

But I think you are right that initially the 8 byte came as a recommendation jus to handle PPPoE overhead ;)

> Based on that assumption it raises further questions in my mind:
> My ISP supplier (Sky in the UK) provide straight ethernet over PTM, with DHCP to obtain a public IP address, so in theory no PPPoE overhead unlike other ISPs offering 'fibre' (ha!) broadband in the UK.  There appears to be a tagged VLAN on the WAN port, therefore I think the correct overhead in my case is 4 (VLAN) and for everyone else it should be 12 (VLAN + PPPoE)

	See above. BUT this might or might not be relevant; my ISP actually throttles my link to a speed below the VDSL2 link speed and accounts for 16 bytes overhead at the BRAS level, so ymmv… I had a nice way to figure out the per packet overhead on ATM links (actually only ATM links using AAL5, but that should be all of them ;) ), but for PTM I have no real idea...

> Please correct my assumptions :-)

	The other thing I am uncertain of is the VLAN tag, if your router terminates it will the kernel account for it or not? Not that I can test this currently as my modem terminates the VLAN “silently"

Best Regards & hope that helps

> Kevin
> -- 
> Thanks,
> Kevin at Darbyshire-Bryant.me.uk
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