gnrc_ipv6_nib: include RIO with all subnets in downstream RA

[benpicco]

Contribution description

When RIOT is an upstream router to a subnet, but not the default route, we need to convey the information that this subnet is available through it to the downstream hosts.

Testing procedure

A riot board is connected to a Raspberry Pi over USB. The Raspberry Pi also has a WiFi uplink that constitutes it's default route.

The RIOT node is part of the fd00::/32 network, but by use of gnrc_ipv6_auto_subnets it's actually in the fd00:0:c000::/64 subnet.

To reach the fd00::1 host at the root of the upstream subnet, we have to inform the Raspberry Pi that the entire fd00::/32network is reachable through the RIOT node, not justfd00:0:c000::/64` - we can use the Route Information Option for that.

We don't want to include redundant RIOs in the RA, e.g. the networks fd00:0:8000::/33 and fd00:0:c000::/64 are both included in fd00::/32 (which is also reachable through our node) so we only have to include that one.

2023-05-23 14:54:46,023 - INFO # fd00::/32 via fe80::5884:f712:f280:5db9 dev #7
2023-05-23 14:54:46,025 - INFO # fd00:0:8000::/33 dev #7
2023-05-23 14:54:46,028 - INFO # fd00:0:c000::/64 dev #8
2023-05-23 14:54:46,032 - INFO # default via fe80::5884:f712:f280:5db9 dev #7

and on the Linux side

fd00:0:c000::/64 dev usb0 proto kernel metric 256 expires 80072sec pref medium
fd00:0:c000::/64 dev usb0 proto ra metric 303 pref medium
fd00::/32 via fe80::6481:20ff:fef9:b694 dev usb0 proto ra metric 1024 expires 4288820sec pref medium
fe80::/64 dev usb0 proto kernel metric 256 pref medium
default via fe80::6481:20ff:fef9:b694 dev usb0 proto ra metric 303 pref medium
default via fe80::6481:20ff:fef9:b694 dev usb0 proto ra metric 1024 expires 1606sec pref medium

Issues/PRs references

[fabian18]

Why should it be optional in final RAs but is always included in normal RAs?

[benpicco]

It's now also optional in normal RAs (and even more so as it's only needed when we are a) not the default router but have b) a route to a specific subnet we want to propagate).

[fabian18]

I'd say resolve the conflict and let's go.

[riot-ci]

Murdock results

✔️ PASSED

771e9e9 gnrc/ipv6_auto_subnets: remove superfluous newline

Success	Failures	Total	Runtime
6946	0	6946	10m:34s

Artifacts

• Documentation preview

[benpicco]

bors merge

[bors]

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• Murdock
• static-tests
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