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Author Topic: BRAS IPv4 address  (Read 1784 times)

Weaver

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Re: BRAS IPv4 address
« Reply #15 on: November 30, 2018, 08:14:13 AM »

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I have read, including one from Germany, were not saying which generation of systems they referred to, and were just outdated it seems.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.
« Last Edit: November 30, 2018, 08:36:10 AM by Weaver »
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Weaver

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Re: BRAS IPv4 address
« Reply #16 on: November 30, 2018, 08:40:39 AM »

Could we at some point write consensus summaries for all the cases then, as I simply don’t have all of this vital stuff anywhere.

I didn’t have the stuff for 21CN upstream of the MSAN. I don’t understand anything much regarding the FTTC picture but I do have G.993.2 to read at least yet that is not remotely enough, I need all the stuff above it and the stuff upstream from the FTTC cab and further.
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Terrydaktal

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Re: BRAS IPv4 address
« Reply #17 on: November 30, 2018, 09:54:50 AM »

Right there are a few things I'd like to address actually. The main burning question for me right now is why the BTw converged network diagram features a 21cn bRAS, an IEA AND and MSPE, like, why are there so many bRASs on that path? shouldn't there just be a single one, what's the purpose of the IEA and 21cn bRAS? Isn't the MSPE the LAC here?. I guess I'm having trouble understanding some parts of that bottom right quarter of the diagram.

Secondly, the PPPoE issue is just terminology. PPPoE can be used to refer to the whole Ethernet frame with the PPPoE session inside of it. It's also known as PPPoEoE which implies that there is no ATM layer. PPPoEoA is that Ethernet frame with the PPPoE session inside it but sent over the ATM adaption layer, rather that RFC LLC layer, or let's just say ATM for short. Protocol stacks will however show 'PPPoE' as a separate encapsulation layer, which it is, check any wireshark capture.

Thirdly, something I didn't realise and which CarlT mentioned on another post is that the MSAN encapsulates the PPPoE session in L2TP (previously I believed that L2TP session begins at the bRAS). It appears that L2TP is only used between the MSAN and bRAS when the backhaul is 802.1ad Ethernet rather than ATM. Also, I understand the GEA Cablelink now that BTw purchases from BT openreach.

Fourthly, I'm still not sure how the BTw core is connected to tier 1 ISPs and in what exact manner
« Last Edit: November 30, 2018, 09:58:31 AM by Terrydaktal »
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Weaver

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Re: BRAS IPv4 address
« Reply #18 on: November 30, 2018, 11:50:56 AM »

The thing about PPPoE etc is just as you say, it can be one header, just the unique-to-PPPoE header, three headers PPP plus PPPoE plus Ethernet MAC, or protocol-in-context such as PPPoEoE ie ‘PPPoE on a LAN’, or PPPoEoA ‘PPPoE over ATM’ or ‘over ADSL’ and in these cases we could be referring to almost anything, even the whole protocol stack. It’s people such as me that promote these latter oEoX terms for disambiguation because it makes it clear which side of a modem were talking about. It is a bit messy and I am very likely one of the guilty ones using the term inconsistently. But for ADSL on the first mile there is one and only PPPoEoA protocol stack leaving aside the two variants of the RFC2684 shim layer in the middle. (The ‘LLC’ version of RFC2684 for PPPoEoA is the one that works on BT kit as I said before. Others may be able to help with other carriers. I can’t remember whether or not anyone has ever told me what the story is for PPPoEoA TT wholesale ADSL users. If so I forget.)

As for your third point. My ignorant guess based on no information was the same as you had though. I thought that on 21CN it was Ethernet from the DSLAM / MSAN (is it a grave sin to just always say DSLAM instead of MSAN, or not?) to the BRAS and L2TP started there. But if that were true it would have to be PPP over PPPoE over Ethernet from MSAN to BRAS as I don’t see how to send PPP in Ethernet frames straight, as it seems to have no defined exact method for doing it otherwise. (What ethertype would PPP naked in Ethernet frames have?) Anyway, exactly that was what I had always imagined. But CarlT corrects us and says that L2TP begins at the MSAN. As does Terry’s diagram. Does anyone have more references?

« Last Edit: November 30, 2018, 12:05:49 PM by Weaver »
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Weaver

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Re: BRAS IPv4 address
« Reply #19 on: November 30, 2018, 12:08:24 PM »

I recommend TR-101 - p 20 for the first mile, or what they call ‘the U point’ in that spec, that term is defined at the start of that document, where there is a reasonable diagram which could however use a bit more in the way of detail. On that page, b is PPPoEoA and d is PPPoA, the two you find with BT ADSL.
« Last Edit: November 30, 2018, 12:18:59 PM by Weaver »
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Weaver

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Re: BRAS IPv4 address
« Reply #20 on: November 30, 2018, 01:10:31 PM »

Now on p25 of TR101 we have protocol stacks for the ‘point V’, which is the link immediately upstream of the MSAN. This is one of the points that you and I are interested in.

So here’s the big question for this. Is 21CN TR-101? I have no idea. If not disregard all of this.

On p25 there’s no sign of L2TP, just Ethernet.

But who knows.

I wonder if we could look up the spec of some particular model of known MSAN hardware? It might say that it supports multiple options for upstream protocol stacks for all I know? Configurable per customer or something. And perhaps these docs like TR101 did not take off, or were adopted by some operators, or all. Or none.
« Last Edit: November 30, 2018, 01:16:13 PM by Weaver »
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CarlT

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Re: BRAS IPv4 address
« Reply #21 on: November 30, 2018, 05:51:05 PM »


Thirdly, something I didn't realise and which CarlT mentioned on another post is that the MSAN encapsulates the PPPoE session in L2TP (previously I believed that L2TP session begins at the bRAS). It appears that L2TP is only used between the MSAN and bRAS when the backhaul is 802.1ad Ethernet rather than ATM.

Did I say that the MSANs encapsulate PPP in L2TP or did I say that the MSE or BRAS does that?

https://www.btplc.com/SINet/sins/pdf/472v2p8.pdf
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Terrydaktal

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Re: BRAS IPv4 address
« Reply #22 on: November 30, 2018, 10:29:51 PM »

https://forum.kitz.co.uk/index.php/topic,21493.msg372055.html#msg372055

This was the post I was referring to. BTW, thanks for that SIN pdf, I've never seen that one before so I'll give that a read, it appears to have some nice diagrams on it. I recently found this https://www.btplc.com/SINet/sins/pdf/511v1p10.pdf which has a nice diagram on page 9.
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CarlT

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Re: BRAS IPv4 address
« Reply #23 on: November 30, 2018, 10:41:05 PM »

Ah okay. Too long ago for me to remember where that came from, think I caveated it by remarking it could be done in a couple of ways and it seems to do the second of the two and transport everything over IP to the BRAS. CVLAN inside SVLAN and VPLS to impersonate a layer 2 network.
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Terrydaktal

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Re: BRAS IPv4 address
« Reply #24 on: December 01, 2018, 01:09:19 AM »

I just found this. Interesting what it says about the IEA, also wonder what he means by 'AMSAN' https://prezi.com/goctwzwtmk_z/bt-work-experience/. EDIT: http://www.fujitsu.com/ie/solutions/telecommunications/access-aggregation/ (Information on AMSANs)
« Last Edit: December 01, 2018, 01:26:59 AM by Terrydaktal »
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Terrydaktal

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Re: BRAS IPv4 address
« Reply #25 on: December 01, 2018, 02:53:31 AM »

Weaver, with regards to the V interface for PPPoA modems, this would be my guess (note, PPP|PPPoE|Eth = PPPoE).

For 20CN customers between DSLAM and bRAS (MSiP)
    PPP|PPPoA|RFC2684|AAL5|ATM|SDH

For 21CN ADSL2+ / FTTC customers between MSAN and bRAS
    PPP|PPPoE|802.1ad|Eth|SDH
    I've also seen
    PPP|L2TP|UDP|IP|Eth|SDH
    I've also seen
    PPP|PPPoE|Eth|L2TP|UDP|IP|Eth|SDH
« Last Edit: December 01, 2018, 03:15:38 AM by Terrydaktal »
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Weaver

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Re: BRAS IPv4 address
« Reply #26 on: December 01, 2018, 06:11:26 AM »

Of your three guesses at the end, I had just told myself that it was the first one.

Sum-up/recap: Sorry, I got lost, and apologies if I misread CarlT, what is CarlT’s thinking?
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j0hn

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Re: BRAS IPv4 address
« Reply #27 on: December 01, 2018, 04:20:08 PM »

Small detail but confused why you call the 20CN equipment a DSLAM and FTTC equipment a MSAN.
It's quite the opposite.

The FTTC kit technically are MSAN's, but are configured as DSLAM's only.

There are not many 20CN exchanges left.

Quote
For 21CN ADSL2+ / FTTC customers

I would completely separate 21CN ADSL2+ and FTTC.
Actually I would separate all NGA products (FTTC/G.FAST/FTTP) from ADSL2+.

The post again assumes FTTC uses PPP. It does not necessarily do so.
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CarlT

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Re: BRAS IPv4 address
« Reply #28 on: December 01, 2018, 04:33:10 PM »

My thinking is that it doesn't really matter as things are going to be changing substantially in the not too distant anyway  :)
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Weaver

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Re: BRAS IPv4 address
« Reply #29 on: December 01, 2018, 05:14:02 PM »

j0hn is surely right that some ISPs do not use PPP, no ?
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