Re: [netmod] 6021 ipv4-prefix

Juergen Schoenwaelder <j.schoenwaelder@jacobs-university.de> Wed, 01 May 2019 11:17 UTC

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Date: Wed, 1 May 2019 13:17:12 +0200
From: Juergen Schoenwaelder <j.schoenwaelder@jacobs-university.de>
To: Mikael Abrahamsson <swmike@swm.pp.se>
CC: "Rob Wilton (rwilton)" <rwilton@cisco.com>, "netmod@ietf.org" <netmod@ietf.org>
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Subject: Re: [netmod] 6021 ipv4-prefix
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On Wed, May 01, 2019 at 10:55:38AM +0200, Mikael Abrahamsson wrote:
> 
> So while you seem to think I am not reading your text, it seems to me you're
> not reading what I am saying either. You're not responding to the points I
> am trying to make anyway.
> 
> https://tools.ietf.org/html/rfc7950#section-9.1
> 
> This talks about *values*. If you drop bits in IPv6-prefix, then it's not
> the same *value* anymore.

I personally do take the standpoint that irrelevant bits do not matter
for the value of a prefix, i.e., 192.168.0.1/24 and 192.168.0.0/24 are
two different representations for the same prefix. You seem to take
the standpoint that 192.168.0.1/24 and 192.168.0.0/24 are different
prefixes since bits that are irrelevant do differ.

Apparently there are different views that people have concerning
prefixes. I think I have seen the following three alternatives:

a) non-prefix bits that are set to one are illegal in a prefix
b) non-prefix bits are irrelevant but they need to be preserved
c) non-prefix bits are irrelevant, ignore them and the canonical
   representation has non-prefix bits set to zero

The RFC 6991 definitions do c). If there is consensus that c) is
wrong, we need to deprecate the definitions and create new ones after
finding consensus on either b) or a).

System/kernel interfaces seem to show different behaviours. My Linux
box seems to do a), my MacOS box seems to do c). The system/kernel
interfaces likely all do the same if all non-prefix bits are zero,
i.e., when the system receives data in the canonical format.

Option b) seems to be the most expensive to implement (your server may
have to clear non-prefix bits before pushing the prefix into the
kernel and it needs to map data received from the kernel back to a
prefix that has unused bits preserved in the datastore). Hence, for me
the choice is between a) and c) and given that we have c) already
defined for years, my first preference is to just keep things as they
are.

/js

-- 
Juergen Schoenwaelder           Jacobs University Bremen gGmbH
Phone: +49 421 200 3587         Campus Ring 1 | 28759 Bremen | Germany
Fax:   +49 421 200 3103         <https://www.jacobs-university.de/>