alt Hacker News> Then there's IP fragmentation and PMTU that are a burning trash fire.
It's not significantly worse on v6 compared to v4. Yes, in theory, you can send v4 packets without DF and helpful routers will fragment for you. In practice, nobody wants that: end points don't like reassembling and may drop fragments; routers have limited cpu budget off the fast path and segment too big is off the fast path, so too big may be dropped rather than be fragmented and with DF, an ICMP may not always be sent, and some routers are configured in ways where they can't ever send an ICMP.
PMTUd blackholes suck just as much on v4 and v6. 6rd tunnels maybe make it a bit easier to hit if you advertise mtu 1500 and are really mtu 1480 because of a tunnel, but there's plenty of derpy networks out there for v4 as well.
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IPv4 allows fragmentation by the middleboxes, which in practice papers around a lot of PMTU issues.
The IPv6 failing was not taking advantage of the new protocol to properly engineer fragmentation handling. But wait, there's more! IPv6 also has braindead extension headers that require routers to do expensive pointer chasing, so packets with them are just dropped in the public Net. So we are stuck with the current mess without any way to fix it.
People are trying: https://datatracker.ietf.org/doc/rfc9268/ but it's futile. It's waaaay too late and too fundamental.
> but there's plenty of derpy networks out there for v4 as well.
God yes, I've helped so many users on PPPoE by telling them to set their MTU to something lower...