Hi Kevin, On Apr 28, 2015, at 15:20 , Kevin Darbyshire-Bryant <[email protected]> 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 Sebastian > > Kevin > > -- > Thanks, > > [email protected] > > > _______________________________________________ > Cerowrt-devel mailing list > [email protected] > https://lists.bufferbloat.net/listinfo/cerowrt-devel _______________________________________________ Cerowrt-devel mailing list [email protected] https://lists.bufferbloat.net/listinfo/cerowrt-devel
