Subject: RE: /127 for P-t-P Considered Harmful

From: "Christian Huitema" <[email-masked]>
Date: Tue, 6 Nov 2001 08:18:06 -0800

/127 for a P-t-P link? This must be a bug! We need a /64!

Have people considered the privacy implications? The computer at the end
of the link may well want to use privacy addresses. Also, there is a
credible possibility that a computer is composed of multiple subsystems,
each with their own IPv6 address. In the aggregatable architecture, it
makes a lot of sense if "n" is in fact 64 for most links.

-- Christian Huitema
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From: Pekka Savola <[email-masked]>
Date: Tue, 6 Nov 2001 18:26:59 +0200 (EET)

> /127 for a P-t-P link? This must be a bug! We need a /64!

Tell that to ARIN et al which advocate(d) a single /64 as an IX
allocation.

Does "640kb should be enough for everyone." sound familiar... :-)
 
> makes a lot of sense if "n" is in fact 64 for most links.

I don't think it's being pushed that /127 be used (/64 should be
preferred), but sometimes a smaller allocation might make more sense, in
one way or another.  Under these circumstances, it is not often necessary
to worry about privacy.

-- 
Pekka Savola                 "Tell me of difficulties surmounted,
Netcore Oy                   not those you stumble over and fall"
Systems. Networks. Security.  -- Robert Jordan: A Crown of Swords

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From: Robert Elz <[email-masked]>
Date: Tue, 06 Nov 2001 23:33:44 +0700

Date:        Tue, 6 Nov 2001 08:18:06 -0800
    From:        "Christian Huitema" <[email-masked]>
    Message-ID:  <[email-masked]>

  | Have people considered the privacy implications? The computer at the end
  | of the link may well want to use privacy addresses. Also, there is a
  | credible possibility that a computer is composed of multiple subsystems,
  | each with their own IPv6 address. In the aggregatable architecture, it
  | makes a lot of sense if "n" is in fact 64 for most links.

Your message, and Matt's, are both missing the point.   The question
isn't whether using a /127 is the right way, or the only way, or
whether a subnet number needs to be allocated at all (Matt - one reason
is for connecting isolated systems, which have the P2P link as their
only network connection - they have no other addresses to use).

The issue Pekka raised, was assuming that a /127 is being used, how
are these particular anycast addresses to be allocated - and does that
mean that we should prohibit /127's or something.   It was certainly
a reasonable question.

Looking for ways to avoid the problem might be nice if you're faced with
deciding what to do in the field, but it isn't the correct approach when
what is being done is writing the specs.

I'll answer Charlie's message separately...

kre

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From: Erik Nordmark <[email-masked]>
Date: Wed, 7 Nov 2001 11:09:56 +0100 (CET)

> makes a lot of sense if "n" is in fact 64 for most links.

I can imagine router-router pt-pt links within a single domain
(i.e. not the link from the ISP to my house) could use different
prefix lengths than the other pt-pt links.
There shouldn't be any privacy considerations for the former.

But such router-router links might also not need any IPv6 addresses
(except the link local addresses assigned to the two attached interfaces).

  Erik

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From: Matt Crawford <[email-masked]>
Date: Wed, 07 Nov 2001 16:24:00 -0600

> only network connection - they have no other addresses to use).

I still don't see it.  That an argument against unnumbered links, but
it still doesn't imply any topological relation between the addresses
of the endpoints.  Kernel tables and routing protocols are both able
to cope with a p-p link from A::X to B::Y.  But, as I said, there may
be operational concerns of which I am innocent.
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From: Robert Elz <[email-masked]>
Date: Thu, 08 Nov 2001 17:59:59 +0700

Date:        Wed, 07 Nov 2001 16:24:00 -0600
    From:        Matt Crawford <[email-masked]>
    Message-ID:  <[email-masked]>

  | I still don't see it.  That an argument against unnumbered links, but
  | it still doesn't imply any topological relation between the addresses
  | of the endpoints.  Kernel tables and routing protocols are both able
  | to cope with a p-p link from A::X to B::Y.  But, as I said, there may
  | be operational concerns of which I am innocent.

Matt - this is way off the topic of the original question now, but if I'm
the end you're identifying as B::Y, and the p2p link is my only network
connection, from where do I get B ??

That is, consider the p2p link being a standalone workstation (PC if you
like...) with a ppp dialup to some provider.   It gets an address when
it connects, and that address comes from the provider's address block.
So, the p2p link will be numbered out of the A subnet space.  What length
prefixes are applied (anything from /128 down I guess) is what has been
being discussed.

kre