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Typically (at least here in the UK), the authentication method on a DSL line is PPPoA. This makes sense, based on my understanding that DSL usually uses ATM mode, meaning ATM/AAL5 sits immediately above the low level DSL protocol in the stack. This begs the question, why do some ISPs support the use of PPPoE, and how is it possible over ATM? Is the PPPoE encapsulated within Ethernet frames first, and then passed to AAL5/ATM? Wouldn't this introduce additional overhead and complexity, and if so, why would you want to do it? Does PPPoE provide some advantage or additiional features that are desirable in certain circumstances?

TIA

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  • What problem, related to your own professional systems administration, are you trying to solve? If you're not sure why I'm asking that question, please read the "help" link and then consider editing your question to bring it into scope of what's intended for this site.
    – mfinni
    Jul 16, 2014 at 19:12
  • I'm familiar with the scope requirements. This is related to a real world problem, but the details are insignificant and including them would only detract from trying to make the question a useful reference for others by making it needlessly specific to my situation.
    – dbr
    Jul 16, 2014 at 19:47
  • I'm struggling to understand how this question was off-topic. The question related to a class of problems that I regularly encounter in my professional experience as an engineer working for a reseller/consultancy. Many of our smaller customers and branch office deployments make use of DSL, and this is where the question comes from.
    – dbr
    Apr 5, 2015 at 17:49
  • You're not going to be able to make an ISP change the way they deliver their DSL, so I'm wondering how an answer will actually help you solve a problem.
    – mfinni
    Apr 7, 2015 at 17:42

2 Answers 2

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why do some ISPs support the use of PPPoE

100% historical reasons. It's the way DSL was rolled out so that's the way it will stay. Why was it rolled out that way? At the time Telco's had dial-up Internet service using PPP, ATM networks, and Ethernet in their DCs. The cheapest way to integrate this new DSL-thing was to re-use as much of the existing systems as possible.

how is it possible over ATM

It's encapsulated, just like any other traffic.

Is the PPPoE encapsulated within Ethernet frames first, and then passed to AAL5/ATM?

Yep, just as bad as it sounds. You've got the payload in a PPP frame (2 bytes), in a PPPoE frame (6 bytes), in a Ethernet frame (18 bytes), in an AAL5 frame (10 bytes), in an ATM frame (7 bytes).

Wouldn't this introduce additional overhead and complexity

ATM packets are 60 bytes, so the overhead actually either gets absorbed by ATM cell padding (ie, the frame had 53 bytes of padding anyway, but now it's 53 bytes of "overhead") or it causes an extra ATM cell of 60 bytes on the wire.

As for the complexity, they already had systems that spoke almost all of these protocols, and the new PPPoE part is tiny compared to the rest.

Does PPPoE provide some advantage or additiional features that are desirable in certain circumstances?

Not anymore, but history has a way of biting technology in the butt 20-years after it was developed.

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    History? PPPoE was inferior from day one. Jul 16, 2014 at 19:22
  • Thanks Chris, very informative answer. I'm still slightly unclear on where the historical reason for PPPoE existing comes from. Was PPPoE used before PPPoA became the more popular of the two?
    – dbr
    Jul 17, 2014 at 17:41
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    Yes, PPPoA is a slight simplification of PPPoE, simply eliminating the Ethernet frame from the stack and using an ATM address in the PPPoA layer. The PPPoE protocol pretty much only recycles existing technology.
    – Chris S
    Jul 17, 2014 at 17:51
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One legitimate use of PPPoE may well be for people like me, who don't want their ADSL modem to "own" the public IPv4 address that their provider has assigned to them.

Instead, my ADSL modem is used solely as an ethernet/ATM frame translator, and the first Linux box inside that is the firewall device, which authenticates to the network via PPPoE and thereby owns the public IPv4 address (and controls the IPv6 /56, but that's another story).

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  • The DSL Modem could have been just a bridging device, allowing your computer "own" the IP just as it does now. No legacy-laden encapsulation protocol is necessary for that, and it's how Cable Modems generally work. The PPPoE standard was a shortsighted cost cutting by an industry too blinded by it's own dominance and lethargy to imagine a world dominated by high speed access, especially provided by anyone besides the CLECs.
    – Chris S
    Jul 17, 2014 at 1:48
  • As I understand it (which is fairly limited, I confess), DOCSIS doesn't have an authentication requirement in the same way that DSL does (multiple providers/username+password). If the modem is to act only as a bridge (as mine does), someone will surely still need to perform that step? Who and how, if not the next box in using PPPoE? I think my idea is not that PPPoE is in any way great, but that if they hadn't used an existing standard adapted for a new carrier, they'd've had to invent something wholly new, and quite possibly worse.
    – MadHatter
    Jul 17, 2014 at 5:58
  • DOCSIS uses a trusted CPE model. There's still pages around the net on how to hack your CM to override the speed settings or clone HFC MAC (allowing two modems to use the same service) - though it doesn't take a genius to figure out when someone has hacked their modem. The Telco's would never have imagined trusting CPE in the 80s when DSL was designed, so required centralized session authorization. You're absolutely right that given the Telco's mindset at the time they were either going to do PPPoE or something worse.
    – Chris S
    Jul 17, 2014 at 14:23
  • I don't think it has much to do with the telcos' mindset. You've accepted that DOCSIS has a different authentication model, and I agree with you that it's flawed. Given that DSL couldn't take the trusted CPE route - because the cable isn't tied to the provider - it had to use some kind of distributed authentication model. I still feel that overlaying on ppp was a reasonable decision at the time (better than inventing a new protocol from scratch). What else would you have liked them to do? I think you're right to dislike PPPoE, but wrong to blame the telcos for the mess.
    – MadHatter
    Jul 17, 2014 at 16:34
  • I'm not sure I follow. What do you mean that DSL couldn't take the trusted CPE route? Also the distributed authentication model?
    – Chris S
    Jul 17, 2014 at 17:49

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