Terry, 20CN with PPPoEoA wasn’t quite as pictured because ATM used to extend further into the network according to our own kitz who has a full 20CN diagram. And PPPoA is far more common that PPPoEoA in the UK for ADSL users, and that of course is different.
That seems to be a plausible 21CN picture for all I know. So we are told in this thread by those who know that PPPoE and Ethernet no longer extend further upstream beyond the first mile, beyond the MSAN aka Posh DSLAM. I had just assumed that PPPoE extends nowadays all the way much further upstream to the BRAS, not having any clue, but explicit description of this kind of situation that I have read, including one from Germany, were not saying which generation of systems they referred to, and were just outdated it seems. There’s no reason to do such a thing. And Kitz told me that PPP gets terminated and possibly fiddled with initially, and a new PPP connection goes upstream from the BRAS all the way to the ISP, rather than one single unbroken PPP connection all the way from the user’s router to the ISP.
They have left out the RFC 2684 header. That is a bit of useless glue between Ethernet (which is below PPPoEoA) and AAL5 below. It is actually a set of several different alternative headers and protocols, two lots according to the different upper layer protocols above AAL5 and for each a set of further options that set the stupidity-level of how much extra bloat you want. There is only one sane choice given a free choice: the right answer has the least bloat and that’s what you always use unless the DSLAM you have won’t support it. RFC 2684 is a multiprotocol labelling thing, more possibly pointless bloat which contributes to a grand total of 32 bytes of total bloat in a typical PPPoEoA ADSL protocol stack. The Wikipedia article on PPPoE has a section on PPPoEoA header bloat and compares it with PPPoA (sanity). I hope that article might be useful, and I do hope it is accurate because I wrote the PPPoEoA section.
In any case the whole multiprotocol indication thing is completely wildly duplicated over and over again. There are protocol fields in Ethernet in the ‘ethertype’, PPP also handles protocol mux / demux very well rather more extensively, IP has the version field anyway which shows whether something is IPv4 or IPv6 (has anyone ever actually used that ? Try getting it wrong in the sense of putting correctly formatted packets of the wrong protocol in the wrong place? ) and there is something in the RFC2684 header too. Only ethertype and PPP are any use. RFC 1483 is the same thing, replaced by an update called RFC 2684 iirc so some modems mention ‘RFC 1483’ settings instead but there’s no difference at all.
Anyway, to be fully accurate you would say which one of the RFC 2684 variants is in use. To make things worse, in PPPoEoA the ‘ethernet’ header actually comes in one of two versions, in theory, a 14 byte one or an 18 byte one if you want to add yet more bloat and slowness. I have never tried experimenting so I don’t know if say BT ADSL 21CN or 20CN DSLAMs when using PPPoEoA can handle both types of Ethernet header. So the whole grad title would be ‘+ Ethernet (FCS|noFCS) + RFC 2684 ‘bridged’ LLC + AAL5 + ATM + ADSL’. And the ‘bridged’ means it is the set of RFC 2684 variants that is used for carrying Ethernet and so in this case where it is (always) Ethernet, the set of variants will always be the set called ‘bridged’. The ‘LLC’ option selects the specific RFC2684 header flavour within the allowed alternatives and with that we know exactly which section within the RFC2684 doc we are looking at and so exactly what form the header has.
So for ATM in ADSL this header in one sense cannot be forgotten, because this is where various modem settings live : ‘VC-MUX’ vs ‘LLC’ being one two-way choice and ‘FCS’ being a Boolean, on / off or present/absent. However in another sense you may well not need to bother about the RFC 2684 variants, because the modem’s defaults might well just be the sane choice (although I always check) anyway: Provided the DSLAM supports it, you never need to make use of the different possibilities, and can stick to one sane answer. The right answers for BT ADSL are always :- for PPPoA: “VC-MUX”; and for PPPoEoA: “LLC + no-FCS”. Never ever use anything else.
How did this post grow into such an uncontrolled monster, uncalled for and off-topic probably? What a fool I am.