>> none of the above document specify what IPv6 MTU to use for these
>> interfaces. (RFC3056 has a pointer to RFC2893, but RFC2893 has nothing
>> about IPv6 MTU)
>
>I see these words in RFC 2893 which seem to be relevant to this issue:
> The IPv6 layer in the encapsulating node can
> then view a tunnel as a link layer with an MTU equal to the IPv4 path
> MTU, minus the size of the encapsulating IPv4 header.
>
>This is suggesting that the interface MTU for such a tunnel is dynamic
>by being a function of the IPv4 path MTU across the tunnel.
>A result of such behavior is that the initial MTU for such a tunnel
>interface would be the interface MTUs for the underlying physical
>interface for the tunnel, minus 20 bytes for the IPv4 header.
>
>But I take it from your question that you'd like to see a fixed number for
>the MTU. Is this really necessary?
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. 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.
itojun
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