Thus spake "Iljitsch van Beijnum" <iljitsch(_at_)muada(_dot_)com>
For any particular application and group of users, and in order to
switch over seamlessly, it is necessary that all servers become dual
stack, then clients can switch (without the need to run dual stack) and
after that the servers can drop IPv4.
That is one scenario; there are others.
And for peer-to-peer, _everyone_ is a server so _everyone_ has to
run dual stack before it's possible to drop IPv4.
Out of curiosity I sniffed my own P2P applications and found that over 50%
of the hosts my client tried to contact weren't available, presumably due to
NATs, firewalls, or other connectivity impediments. In spite of this
problem, the P2P networks work well enough nobody cares about the problem.
I conclude, therefore, that as long as a sizable minority of P2P users are
dual stack, the majority can safely speak only v4 or only v6.
S
Stephen Sprunk "God does not play dice." --Albert Einstein
CCIE #3723 "God is an inveterate gambler, and He throws the
K5SSS dice at every possible opportunity." --Stephen Hawking