Subject: RE: globally unique site local addresses

From: "Brian Zill" <[email-masked]>
Date: Thu, 21 Nov 2002 11:45:13 -0800

We need to be careful here.  In our rush to eliminate the bad effects of
the ambiguity present in site-local addresses today, let's not forget
that there are some major plusses to existing site-local addresses that
are the result of this ambiguity:

1. They're free.
2. They can be (auto)configured without having
   to co-ordinate with some outside entity.
3. They cannot be externally routed (Some would
   consider this to be a minus as well).

I believe 1 and 2 can be solved (fairly) easily by other means.  The big
problem is number 3. The ambiguity is essential to preventing them from
ever being routed.  An IETF edict not to route some new form of
non-ambiguous addresses will be ignored by those who wish to route them.
This will likely lead to pressure to turn these new addresses into
yet-another type of global address with all the associated routing table
explosion inherent to non-aggregatable address space.  Another way to
look at it is that the current ambiguity in site-locals is a means for
protecting the global routing space.  If we do away with the ambiguity,
we need another method for keeping the global routing tables sane.

The only class of solution which I think will truly make everyone happy
is to come up with an *aggregatable* globally unique address space that
still has properties 1 and 2.  In other words, some sort of
provider-independent global address space.  Properties 1 and 2 are much
easier when the aggregatable property is not required.  The reason we're
in the state we are today is because this is a hard problem.

--Brian

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From: Bill Sommerfeld <[email-masked]>
Date: Thu, 21 Nov 2002 14:51:57 -0500

> ever being routed.  

so, i'm not conviced that we have this third property adequately
written down in the existing site-local case.

The ambiguity means that you can't count on them working, but because
of the likelihood of configuration error plus the likelihood that
many/most routers will forward site-locals by default, you can't count
on them reliably not working, either!

						- Bill

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From: Richard Nelson <[email-masked]>
Date: Fri, 22 Nov 2002 07:35:13 +0000

Brian Zill wrote:
>  An IETF edict not to route some new form of
I can understand that users of the non-aggregatable address space will
try and pressure their ISPs to route it, but they won't have any
contract with the upstream ISPs who _should_ be only too happy to filter
non-aggregatable addresses from the routing tables.  Do you think that
this + a really strong statements and specification up front has no
chance of working?

> in the state we are today is because this is a hard problem.

Well if you are connected then you should have enough PA addresses that
1. is not a problem and given SAA, configuration is not much more work
than configuring a few 
I
Pv4 addresses on to a NAT box so 2 isn't such a big deal either.  The
only real advantage is 3.  This solution really only has the advantage
of long term stability.  I think that inherent non routability would be
as much of a desirable feature of such addresses.

Richard.

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From: Brian E Carpenter <[email-masked]>
Date: Fri, 22 Nov 2002 01:09:21 +0100

> on them reliably not working, either!

In any case, if they can't be routed *even in a private network*
you can be sure they will end up being NATted (e.g. when two
companies merge, or when a company sets up a VPN to a business
partner). Conversely, if they are globally unique they *will*
be routed within private networks, using any old routing protocol.

From which I conclude that globally unique site locals will
vastly damage the market for v6 NAT, so let's figure them out
quickly.

   Brian