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Re: "6to4 damages the Internet" (was Re: draft-ietf-v6ops-6to4-to-historic (yet again))

2011-07-28 07:20:50
In your letter dated Thu, 28 Jul 2011 07:50:38 -0400 you wrote:
In general, all of a host's addresses (at least, those in the same
preference class in the address selection algorithm) need to work
equally well from everywhere.

But even that might not be sufficient.   Fred Baker has recently
been citing a different example of the same problem:  Imagine a
future where hosts on a network have both v6 and legacy v4 through
stateful NAT.  Because the v6 connection works well most of the
time, hosts tend to choose v6 destinations, and sites reduce the
capacity of their NATs over time.  Then the v6 connection fails,
and suddenly everything falls back to v4, and the connections
through NAT also fail for lack of sufficient capacity to handle
the load.

I'm sure some manager will just require this to work. But I would say that
this is just supposed to fail.

If you had in the past just an IPv4 connection and it went down, there would
be no Internet access. Same thing in the future. If your IPv6 connection goes
down you don't have an Internet conenction.

It would be different if the NAT was there just to connect to a very specific
collection of legacy IPv4 sites. But that can be solved easily but just
routing those IPv4 prefixes to the NAT. 

If you want the NAT to provide full redundancy, then it has to be scaled to
accept full load. Otherwise, no IPv6 just means no Internet. Very simple.

We want large scale deployment of IPv6 today not some time in the future
when we finally figured out address selection. And that means that today we
have to make sure that users don't end up with unreliable IPv6 connections.

If you make the constraint that nobody can use IPv6 at all until
the IPv6 connections work as well in all cases as the IPv4 connections,
that creates a huge barrier to the universal deployment of IPv6 -
which already has enough barriers as it is.

To some extent that is exactly the way it is. 

Suppose that a significant fraction of popular websites would have IPv6
addresses that don't actually work. Then essentially no ISP would enable IPv6
for their customers because their customers would suffer.

At the moment, most servers that do have IPv6 addresses seem to actually
support IPv6 so this doesn't seem to be a big problem.

But we don't know what the future might bring. If there would be a sudden
rush of servers that have PMTU problems, then we could have a very serious
problem. 

A less onerous constraint is that less reliable IPv6 connections
don't get used in preference to more reliable IPv4 connections.
We're lucky in the case of 6to4 in that it has a well-known prefix
that allows the traffic to be distinguished from other v6 traffic.
But it's still necessary to manage expectations about IPv4 as a
fallback path.

Happy Eyeballs can solve a lot of problems where a connection will fail
completely or have a high latency. But HE support is far from universal and
so far experience is limited to TCP.

But it doesn't do anything for PMTU problems. 

It also doesn't do anything for real-time streaming applications.


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