>> yes, it definitely needs to be a fixed number. in other words, IPv6
>> link MTU should not be computed based on IPv4 PMTU (which is dynamic).
>> PMTUD assumes that link MTUs are stable enough.
>I know of no such assumption.
>In fact PMTUD is designed deal with routing using different paths with
>different MTU over time (and even at the same time) by taking the minimum
>of the reported MTUs. Dealing with an interface which changes MTU
>falls out of that design for free.
i haven't seen links that changes MTU (*1) dynamically based on
dynamic changes from outside (*2). in this case (*1) is "IPv6 MTU
on top of tunnel" and (*2) is "IPv4 routing changes".
maybe my experience is limited, but anyway, i have never seen one.
>> by using IPv4 PMTU
>> as basis for IPv6 link MTU, IPv6 link MTU will be affected by IPv4
>> routing changes. it will have negative impact to IPv6 PMTUD.
>What exactly is the negative impact?
if link MTUs are not stable enough, there will be more ICMP too big
than we desire.
>Using a dyanmic MTU for the tunnels has the positive impact of being able
>to send larger, thereby fewer, packets when the path supports the larger MTU.
with the cost of complexity in tunnel endpoint implementations (needs
to maintain IPv4 path MTU and reflect it to IPv6 tunnel link MTU).
i would really like to know how many of existing implementations follow
this part of RFC2983.
itojun
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