Re: Understanding IPv6-PD over PPPoE

On 16/06/2021 10:07, Andrei Borzenkov wrote:

The default and standard method to assign IPv6 address is SLAAC.
DHCPv6 is optional. You need to investigate why SLAAC address was not

As I said, I'm not 100% sure that the ISP is behaving correctly, although Bjørn indicated that what they are doing is fairly common.

That said, I'm not sure I've ever seen an ISP use SLAAC over PPP, so is this *really* in any way standard?

As far as I can see, there are 2 scenarios for connecting to an ISP:

1. A router, which has a PPP connection to the ISP and one or more LAN NICs.

In this case, the PPP NIC itself does not need a global address, since no one on the internet needs to address the router itself, only machines on the LAN side of the router. It might make sense to conserve addresses by not assigning an IP to the PPP NIC (although IPv6 addresses aren't exactly scarce!)

The router MAY be running some services (e.g. a caching DNS server, NTP client, etc.) which need to talk to services on the internet. They would need to use an IP address from one of the LAN NICs as the source address for their traffic.

The ISP therefore (probably) shouldn't do SLAAC or IA_NA, the ISP must delegate a prefix.

2. A non-routing device, which has a PPP connection to the ISP, but no LAN NICs.

In this case, the PPP NIC does need a global address - the machine needs a global address, and there's only the one NIC, so it should go on that.

The ISP therefore doesn't need to delegate a prefix, but needs to do SLAAC or IA_NA.

But it's not really reasonable to expect the ISP to know which situation you're in and adjust their RA / DHCPv6 responses appropriately. If the ISP has delegated a prefix, the client can surely make its own decision whether to assign an address to the PPP NIC.

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