Subject: RE: one question...

From: "Christian Huitema" <[email-masked]>
Date: Mon, 25 Nov 2002 18:17:17 -0800

> > So I've been watching this debate about globally
> > ~unique site locals and I don't understand how the
> > end node knows whether a particular destination
> > address is in scope (reachable) or not. The old
> > way, it just matched it to its own scoped prefix
> > and was done with it. What I've been hearing is
> > some desire to be able to patch together other
> > sites (extranets)... how would a node know which
> > scope address to use in that case?
> 
> in general the only way for node A to determine whether node B
> is reachable is for A to send a packet to B.  if A gets a reply
> from B, B is reachable.  if A gets an ICMP message back, B
> is not reachable (for temporary or permanent reasons).  if A
> gets nothing back, either B is (temporarily) unreachable or
> B doesn't want to answer A.
> 
> but you'll never be able to determine this by looking at prefixes.

Actually, the "Default Address Selection for IPv6" draft includes
language of that nature. It would have to be amended to take into
account the GUPI proposal. Something about assuming reachability of GUPI
if on the same site, but not if on a remote site. And then, we would
have to add policy hooks to mention "connected sites."

-- Christian Huitema

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From: Keith Moore <[email-masked]>
Date: Tue, 26 Nov 2002 00:34:52 -0500

> language of that nature. 

well, there are lots of things wrong with that document.

and no, an app can't reasonably assume that B is reachable from A even 
if they're both within the same site.  and there are probably situations 
where a "non local" address (i.e. one with a shorter matching prefix) is 
preferable to a "local" one, because prefix length tells you nothing
about available bandwidth, reliability, delay, jitter, or policy.
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