Re: ifconfig v2

[Gert Doering <gert%greenie.muc.de@localhost>]

Hi,

On Wed, Jun 12, 2013 at 11:08:12AM -0400, Mouse wrote:
> >> Why?  What harm are [noncontiguous netmasks] doing that outweighs
> >> (to you) breaking an occasionally useful (if little-used) facility
> >> that's been there pretty much since day one?
> 
> > No one can agree what they are supposed to mean.
> 
> Hm?  I thought they meant what contiguous netmasks do:
> "on-net" = "dst address & mask == local address & mask" (mutatis
> mutandis for "local address" for things like routes).  Who thinks they
> mean anything else?

How are the semantics when you have (partial) overlaps?  With CIDR prefixes,
"most specific wins", but what is "most specific" when matching two
possibly matching routes with a 255.0.255.0 and 255.0.0.255 mask?  This
is one of the problems I see.

The second drawback I can see is "data structures" - if you know netmasks
are contiguous, you can build more efficient route lookup structures for
that special case.

On the commercial side, Cisco stopped supporting non-continguous netmasks
about 10 years ago, mostly because they rewrote the FIB code (CEF) and the
new stuff was not written to handle it...

(I'm not a NetBSD kernel developer, I just read this list because there's
lots to learn - but my professional work is "routing in the Internet", so
these questions do show up occasionally)

gert
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Gert Doering - Munich, Germany                             
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