If I'm the downstream router, I can't get a prefix, of course I issue warning
message. However, if I'm the one who still get an /64 and works fine as a leaf,
I won't issue an warning message for a fore-coming downstream router attached
to me.
So you have to implement a check and some sort of warning mechanism for
not getting a PD anyways in all your devices (as I suspect that all of
them could eventually be used as a "downstream" router). I don't think
that it's much more difficult to check whether the PD was for a /64. You
have to do that anyway, as your DHCP server won't be able to create a PD
from a /64 and issue a warning in any case. So actually no real extra
work needed there.
<non technical reason start>
The problem with having just the downstream router warning you though,
is that in the case of multihoming, and two prefixes being provided to
the homenet, one /64 from ISP1 and one /56 from ISP2, the downstream
router might get them over the same interface, and simply issue no
warning, as it's happy with the /56 from ISP2, and route everything over
that ISP, without the user ever knowing. If the upstream router issues a
warning about getting a /64 over an interface from an ISP, the user
knows there's something wrong and can fix things, and avoid unnecessary
high bills.
</non technical reason end/>
After the input of various posts I'd like to change the text in 3.4.1:
The home network needs to be adaptable to such ISP policies, and thus
make no assumptions about the stability of the prefix received from
an ISP, or the length of the prefix that may be offered. However, if
only a /64 is offered by the ISP, the homenet may be severely
constrained (with IPv6 not reaching all devices in the home, or use
of some form of IPv6 NAT being forced), or even unable to function.
While it may be possible to operate a DHCPv6-only network with
prefixes longer than /64, doing so would break SLAAC, and is thus not
recommended.
to the following text:
The home network needs to be adaptable to such ISP policies, and thus
make no assumptions about the stability of the prefix received from
an ISP, or the length of the prefix that may be offered. However, if
only a /64 is offered by the ISP, the homenet may be severely
constrained. Attempting to use subnet prefixes longer than /64
would break SLAAC, and is thus not recommended. Using ULA prefixes
internally with NPTv6 at the boundary would be possible, but is not
recommended for reasons given elsewhere. Reverting to bridging would
destroy subnetting, breaks multicast if bridged onto 802.11 wireless
networks and has serious limitations with regard to heterogeneous
link layer technologies and LLNs. For those reasons it is recommended
that DHCP-PD or OSPFv3 capable routers have the ability to issue a warning
upon receipt of a /64 if required to assign further prefixes within
the home network as described in Section 3.4.3.
Mat
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