Subject: Timing out NS and sending data to the default router

From: "Wes Beebee (wbeebee)" <[email-masked]>
Date: Fri, 14 Dec 2007 14:38:13 -0500

I listened to the 6man WG on- and off-link presentation and responses
from the audio feed.
I have a comment relating to what Tony Hain said during the
presentation.
 
Inappropriately sending data to the default router is not catastrophic,
the data will be 
forwarded by the router to the appropriate destinations.  A redirect may
even be issued,
if supported by the router, to correct this problem.  However,
inappropriately sending out
an NS to resolve an on-link destination can be catastrophic if the
router does not respond
to the NS (and if others on the link are not physically connected to the
node sending the NS) -
as presented in the example of an aggregation router deployment in our
slides.
 
Tony's idea to time out an NS and send data to the default router seems
like a very useful
way to convert a catastrophic scenario into a perfectly acceptable one.
However, unfortunately
neither the currently existing IPv6 ND implementations nor the ND
specifications actually
follow/specify this behavior.  In fact, the SEND (RFC 3756) says in
section 4.2.5 (Bogus On-Link Prefix),
"If a sending host thinks the prefix is on-link, it will never send a
packet for that prefix to the router."
This is quite unfortunate, since now we have to be extremely careful to
understand EXACTLY when
a destination is on- or off-link.  On-link determination is no longer
just a performance optimization,
it's a basic data forwarding correctness issue. 
 
- Wes Beebee
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