[atlas] What to do with RIPE Atlas probes that have only a ULA as IPv6 address?

Dear community members,

The RIPE Atlas team has a question what to do with probes that have only
a Unique Local IPv6 Unicast Address (ULA) [RFC4193] as their IPv6
address. The question is whether to treat those probes as IPv6 capable
or not.

It is known that about 10% of the RIPE Atlas probes that have IPv6
addresses do not actually have IPv6 connectivity. This was documented by
Stéphane Bortzmeyer in his RIPE Labs article "How Many RIPE Atlas Probes
Believe They Have IPv6 (But Are Wrong)?"
(https://labs.ripe.net/Members/stephane_bortzmeyer/how-many-atlas-probes-believe-they-have-ipv6-but-are-wrong)

As a way of dealing with this problem, the RIPE Atlas system now tags
probes that have broken IPv6. Any probe that has an IPv6 address (other
than link local) but cannot reach the global internet is tagged as "IPv6
Doesn't Work" (see https://atlas.ripe.net/docs/probe-tags/)

At the moment, around 2800 probes are connected and have an IPv6
address. Of those probes, around 350 (12.5%) are tagged that IPv6
doesn't work. Of those 350 probes, 114 have the surprising condition
that the connect system call fails immediately with the error 'Network
is unreachable.'

Those 114 probes have two things in common, they have only a ULA address
and the do not have a default route. It is the lack of default route
that causes the connect system call to fail immediately.

This feature (ULA and no default route) is specified in RFC-7084 (IPv6
CE Router Requirements) requirement ULA-5 ("An IPv6 CE router MUST NOT
advertise itself as a default router with a Router Lifetime greater than
zero whenever all of its configured and delegated prefixes are ULA
prefixes.") The surprising thing is that for some probes this condition
persists for many months.

For the Atlas project, the question is how we should treat these probes.
Currently they are regarded as having broken IPv6 connectivity. However,
an alternative is to consider those probes as having no IPv6 at all.

Broader questions are: are CPEs doing the right thing here. Should a CPE
announce a ULA on the local LAN even if there hasn't been any IPv6
internet connectivity for a very long time? It is already complex enough
for normal users to understand that there is always a link local IPv6
address even if there is no IPv6 connectivity. Now we have to add ULA to
that group as well.

So the question to the community, should RIPE Atlas treat ULAs in the
same way as RFC-1918, addresses that should be ignored unless a valid
global address can be found elsewhere. Or should we keep the current
approach where ULAs are treated just like other global IPv6 addresses
and consider the probe host's network setup to be broken?

Philip