it's extremely difficult to get them to cross the oceans to Europe
Almost certainly a host TCP tuning issue. Australia is a long way and the default Linux TCP buffers are for North America where resources are a lot closer than 200ms. Moments ago I got 120Mbps downloading a video, that probably reflects the sustained I/O throughput of their modest server.
*misconfigured* IPv6 address on this host
Assisted them to fix the subnet mask. Not using SLAC for that host is a good idea, as then they can swap faulty hardware without additional outage from waiting for DNS TTLs to expire.
This is why people can't adopt IPv6.
Misconfigurations aren't one of the major reasons for slow adoption. The lack of IPv6 technical knowledge is one of the lesser reasons, and it's probably this which was a root cause of the misconfiguration.
rarely responds with an ICMP "Address unreachable".
That is to misdiagnose the behaviour. If the host was misconfigured then the host couldn't send a ICMP which could reach you. Rather that ICMP came from the subnet's router. Routers rate limit ICMP generation to limit denial of service attacks against their control plane. This is only apparent to diagnostic programs, as other programs will heed the initial ICMP rather than keep pounding away (and if the program never received that ICMP then it is unlikely to receive a second ICMP the router may send).
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