netaddress: Simplify reachability logic

[dongcarl]

Our reachability logic was overly complicated, and most likely
incorrect. In particular, we would prefer advertising our IPv4 services
for peers on Toredo, which would most likely just result in us sending
packets to their Toredo provider.

The new logic is a bit simpler:

- Assume all peers can reach us through IPv4 (same as before)
- If peer is unknown or unroutable, assume they can reach through every
  network (same as before)
- IPv6 peers can reach through IPv6
- Onion peers can reach through Onion

---

What I need help on:

1. Is the "If peer is unknown or unroutable, assume they can reach through every network" rule reasonable? I suspect I'm misunderstanding the original intention.
2. Since the unknown and unroutable cases seem to follow the same logic, can we just make them one thing and eliminate the awkward re-purposing of NET_MAX?

[fanquake]

cc @EthanHeilman

[ariard]

1. On the interpretation of this rule, seems to me it's contrary to the first one, if we assume every peer can reach us through IPv4 we should announce an IPv4 address instead of a IPv6. A IPv6 host is likely to be able to reach a IPv4 peer, do we hold the reverse for true ?

2. I think so, see my comment.

[ariard]

Isn't the bias to favor IPv4 announcement over IPv6-tunneled lost with the change ? It may make connection relying on 6to4 routers less reliable..

[dongcarl]

Yes, I think we don't want to favor IPv4 over IPv6-tunneled, because it might be the case that the only connection they have is thru IPv6-tunneled.

[ariard]

Okay to be clear we do want to favor connection through IPv6-tunneled, this is behavior change. Maybe code should be better documented but I think commit description is right "IPv6 peers can reach through IPV6"

[DrahtBot]

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

Conflicts

No conflicts as of last run.

[glowang]

This cleanup looks very helpful! I organized the old reachability logic flow in this table (vertically listed are our networks), and it looks like the handling for Teredo is changed. The bolded grid values are now all changed to REACH_DEFAULT in the simplified logic. Is this the intended effect of

In particular, we would prefer advertising our IPv4 services
for peers on Toredo, which would most likely just result in us sending
packets to their Toredo provider.

?

	IPv4	IPv6	Onion	Teredo	Unknown	Unroutable
IPv4	REACH_IPV4	REACH_IPV4	REACH_IPV4	REACH_IPV4	REACH_IPV4	REACH_IPV4
IPv6	REACH_DEFAULT	REACH_IPV6 WEAK/STRONG	REACH_DEFAULT	REACH_IPV6_WEAK	REACH_IPV6_WEAK	REACH_IPV6_WEAK
Onion	REACH_DEFAULT	REACH_DEFAULT	REACH_PRIVATE	REACH_DEFAULT	REACH_PRIVATE	REACH_PRIVATE
Teredo	REACH_DEFAULT	REACH_TEREDO	REACH_DEFAULT	REACH_TEREDO	REACH_TEREDO	REACH_TEREDO

[glowang]

Could you provide some reasoning behind merging IPv6 WEAK and STRONG into one case?

[dongcarl]

Not going to work on this in the immediate future, closing