IPv6 Network - Address Prefix and Router Lifetime Mismatch
Earlier, I mentioned the potential for shooting yourself in the foot because router advertisements and the prefixes they contain have independent lifetimes. This allows for four permutations:

• The RA lifetime is valid, and the prefix lifetime is valid: IPv6 works.
• The RA lifetime is invalid, and the prefix lifetime is invalid: IPv6 is disabled.
• The RA lifetime is valid, but the prefix lifetime is invalid: The system has an IPv6 default route but no global IPv6 address.
• The RA lifetime is invalid, but the prefix lifetime is valid: The system has a global IPv6 address but no IPv6 default route.

When a host has no global addresses but does have an IPv6 default route (case 3), it can’t reach the rest of the IPv6 Internet. Unfortunately, FreeBSD and MacOS hosts don’t know that: they try anyway, with long delays as a result. Only after trying all the remote destination’s IPv6 addresses and timing out, the system falls back on IPv4 (for applications that try more than one address). Linux, on the other hand, doesn’t install or ignores the IPv6 default route when there are no global IPv6 addresses present, so the timeout is immediate.

Windows XP does install the default route but magically manages to avoid lengthy timeouts anyway. On the other hand, Windows XP suffers timeouts when it has an IPv6 address but no default route (case 4). This is because Windows implements the on-link assumption: it will first do neighbor discovery on the local subnet for any IPv6 addresses. Only after neighbor discovery times out will Windows falls back on IPv4. FreeBSD and MacOS, however, don’t implement the on-link assumption, so they immediately notice that the IPv6 destination address is unreachable and fall back on IPv4, if an IPv4 address is available and the application cycles through all addresses. With Linux, the default route doesn’t seem to expire even though the timers eventually reach zero and lower. But addresses do expire and are removed when the lifetime for the associated prefix has timed out.

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