In message <199701280300.TAA09938@Pescadero.DSG.Stanford.EDU>Jonathan Stone wri
tes
>However,  I _think_ an 576-byte MTU may be required for IPv6,
>since 576-byte MTUs are allowed for multicast traffic. Fragmentation
>and reassembly is the responsibility of host-systems in IPv6; hosts
>should use path-MTU discovery and fragment accordingly, but
>there's currently no good way to negotiate a path MTU over a dynamic
>multicast tree....

Right you are.  Note that you can still get ICMP Packet Too Big 
messages from routers forwarding multicast packets (in IPv6) -- 
not that anyone has implemented multicast routing yet in IPv6. :-(
I understand several groups are working on it...

Has this gone sufficiently far afield yet?

Kevin