Before requiring IPv6 support, it is necessary to revise obviously
broken parts of IPv6.
For example, ICMPv6 generated agaist multicast packets should be
forbidden or ICMPv6 implosions will occur. It will let ISPs filter
ICMPv6, including but not limited to, those against multicast,
which means PMTUD won't work.
Another example is lack of guaranteed value for payload size.
RFC791 specifies:
The number 576 is selected to allow a reasonable sized data block to
be transmitted in addition to the required header information. For
example, this size allows a data block of 512 octets plus 64 header
octets to fit in a datagram. The maximal internet header is 60
octets, and a typical internet header is 20 octets, allowing a
margin for headers of higher level protocols.
and DNS just send 512B messages (520B including UDP header,
which should be a mistake but is safe as no one use IPv4
options).
However, there is no such size specified with IPv6, because
arbitrarily lengthy header options may be inserted. Note that
some header options, such as mobility ones, are inserted by
IP layers without application control.
Thus, applications like DNS can not specify like RFC1035:
Messages carried by UDP are restricted to 512 bytes (not
counting the IP or UDP headers).
Masataka Ohta
PS
You have been warned.
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