Subject: RE: globally unique site local addresses

From: "Michel Py" <[email-masked]>
Date: Sun, 24 Nov 2002 11:35:42 -0800

Pekka,

> Pekka Nikander wrote:
> "Good enough" ones are easy to generate without too
> much human intervention, for example, without any
> connection to the registry. OTOH, they are not
> necessarily unique, and therefore not "good enough"
> for some people.  IMHO, both types are needed.

Agreed.

> In any case, a modest suggestion:  Let's separate
> the GUPI prefix generation and registration processes,
> and make them sequential.

I have another suggestion: Let's split the FEC0::/10 space in two parts:
One for the unregistered "good-enough" and one for the registered truly
unique. By default, the "good enough" would be used and a random/hash
method would be used. But the network administrator would have a choice
of getting a truly unique registered prefix instead. This would likely
need to access some web page and pay a nominal fee.

If the address space is split, then we have a guarantee that the hash
process used for "good enough" will not grab addresses in the range that
is used for truly unique.

It is clear that the biggest the block for "good enough", the lesser
potential collisions. Therefore the truly unique addresses should use
only a modest portion of the FEC0::/10 block and leave the lion's share
to the hash algorithm.

Thoughts?
Michel.

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From: Pekka Nikander <[email-masked]>
Date: Sun, 24 Nov 2002 21:53:21 +0200

Michel,

> is used for truly unique.

Let me try to briefly analyse the differences between the methods,
the "mixed space"" and the "split space" methods:

   - In either method, you can generate a prefix and register
     it as a GUPI prefix belonging to you right away.

   - In the mixed space method, you may have to need a couple of
     prefixes before you get one registered.  OTOH, if you really
     need a registered prefix, you can combine generation and
     registration, and start configuring your routers only once
     you have got a prefix registered.  Thus, no big difference
     in practise.

   - In the mixed space method, you can generate a prefix now and
     try to register it later, with a fairly large probability
     of succeeding.  In the split space method, if you only later
     need a genuinely globally unique address and are not willing to
     pay for one now, you have to renumber.

   - In the split space method, you can tell directly from a prefix
     whether it is genuinely globally unique or not.  In the mixed
     space method you can query the registry whether the prefix has
     been registered, but even if it is, there may be also other sites
     that are using the prefix.  The probability of the existence
     of such sites is low, though.

   - In the split space method, you can be almost sure that nobody
     else is using your globally unique prefix, and if they do,
     they do it in error.  In the mixed space method, you cannot be
     sure that nobody else is using your prefix, but if they do,
     you know that they cannot register it.

Thus, from my point of view, the differences seem to boil down
into two issues:

   1. In the mixed space method, you can defer your registration and
      still have a fairly large probability of succeeding later in
      registration.

   2. In the split space method, you can have more confidence that
      no-one else is using your truly unique prefix.

I can't say which is better.

--Pekka Nikander

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From: Jari Arkko <[email-masked]>
Date: Sun, 24 Nov 2002 20:52:26 +0200

Pekka,

I still don't know if we really need the perfectly unique
site local addresses. The question is: if you need perfectly
unique addresses, why can't you use normal, global addresses?
There we already have an existing registration process and
standards. My thinking is that global addresses + statistically
unique site-locals would cover most of the need we have, and
it does not pay to construct complexity for the small amount
of folks who would need perfect uniqueness.

Of course, this is a judgement call. (You and me are perhaps
the wrong persons to argue about this; input from corporate
network managers and ISPs would be needed.)

Jari