Subject: RE: Redirect ICMP and TAHI test?

From: "Richard Draves" <[email-masked]>
Date: Mon, 19 Feb 2001 11:08:57 -0800

> > You're supposed to accept Redirects even if there is no Destination
> > Cache entry...
> 
> Where does it say so?

In RFC 2461:

8.3.  Host Specification

   A host receiving a valid redirect SHOULD update its Destination Cache
   accordingly so that subsequent traffic goes to the specified target.
   If no Destination Cache entry exists for the destination, an
   implementation SHOULD create such an entry.

Rich
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From: Erik Nordmark <[email-masked]>
Date: Tue, 20 Feb 2001 09:59:16 +0100 (CET)

>    implementation SHOULD create such an entry.

But the above is a SHOULD and the validity checks in section 8.1
say MUST discard when this fails:

      - The IP source address of the Redirect is the same as the current
        first-hop router for the specified ICMP Destination Address.

If there is only one default route pointing at the link
from which the redirect was received
(or for implementations with a richer routing table, 
the lookup of the destination address only finds one routing
table entry that points at that link), then the redirect can
be safely used to create a destination cache entry.

But when the lookup finds more than one possible next hops that might have
been used to send the original packet, it seems a bit dangerous to
accept the redirect.

   Erik