[phicoh]

1280 is a guarantee by the IPv6 standard. If you run a VPN over a link with 1280 MTU then the VPN protocol has to have a way of fragmenting the payload (which could be using fragmentation in IPv6, but application specific is likely to work better).

[yardstick]

Yes I mostly agree, the tricky part is what to do about packets marked as Don’t Fragment. Hopefully they are already well under the actual MTU, but if need be, should they be fragmented or dropped? (The answer is to fragment all except icmp imo, even if it’s against the wishes of the packet IP headers)

[phicoh]

Technically the packets don't get fragmented. It is the tunnel that fragments.

For example, in the bad old days, when there was still a lot of ATM, packets would routinely get transmitted in 48 byte pieces (with a 5 byte header). Nobody wondered whether this kind of fragmentation should honor the DF bit.

If I recall correctly, multi-link PPP could also split a packet in smaller pieces to transmit it over multiple links at the same time.

So for IPv6, anything that is 1280 octets or smaller needs to forwarded without triggering a packet too big ICMP. Otherwise the link violates the IPv6 standard.

[yardstick]

Good point, thanks for the insight!