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

On Wed, Mar 25, 2015 at 5:31 PM, Philip Homburg <philip.homburg at ripe.net> wrote:
> 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.

> 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.

One of the possible reasons might be that a home network does not have
IPv6-enabled uplink but
ULAs are used for internal connectivity. I actually did it at my place
at some point, when I wanted to have Ipv6-enalbed LAN but my ISP did
not provide me with v6 connectivity.

114 out of 350 does look like a lot (on the other hand a lot of probes
are hosted by network people who love to play with their home
networks; )

I do hope that this situation is not caused by ISPs using ULAs....
> 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.

What difference does it make?

> 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.

If ULA is configured in a CPE, then it should be announced in RAs. As long as it
does not break v6 for users (and it should not as there is not default
route), it's fine.

> 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?

But wait, if a probe has RFC1918 addresses only you do not mark it as
'no v4 connectivity', right?
If a probe has a address of a global scope (v4 or v6) but could not
reach the outside world it means the connectivity is broken. So IMHO
it makes slightly more sense to mark ULA-only probes as having broken
connectivity.
But, again, could you please clarify what is the difference between
two tags from practical perspective?

-- 
SY, Jen Linkova aka Furry