Templin, Fred L wrote:
I am making a general statement that applies to all tunnels everywhere.
General statement? Even though you assume tunnel MTU 1500B and tunnel overhead 20B?
For those, specs say that all that is required for MRU is 1500 and not 1500+20.
That is a requirement for hosts with Ethernet interface, which is, by no means, general and has nothing to do with tunnels. For the general argument on tunnels, see, for example, RFC2473 "Generic Packet Tunneling in IPv6", where there is no requirement of 1500. Note that the RFC uses outer fragmentation: (b) if the original IPv6 packet is equal or smaller than the IPv6 minimum link MTU, the tunnel entry-point node encapsulates the original packet, and subsequently fragments the resulting IPv6 tunnel packet into IPv6 fragments that do not exceed the Path MTU to the tunnel exit-point. Masataka Ohta