Re: Fwd: New Version Notification for draft-nordmark-6man-impatient-nud-01.txt

[Philip Homburg <[e-mail]>]

In your letter dated Wed, 20 Jul 2011 10:17:51 -0700 you wrote:
>> A few remarks about this draft:
>> 1) It must be somewhere in RFC-4861, but it is not easy to find and it's
>>     probably best to help implementors here: if a NCE for a router transitio
>ns
>>     to UNREACHABLE state and there are still default routers in REACHABLE,
>>     STALE, DELAY, or PROBE state then delete all destination Cache entries
>>     for that router.
>
>The RFC in section 5.2 says to redo next-hop selection; it doesn't say 
>to delete destination cache entries anywhere AFAIK.
>That should be sufficient even with the introduction of the UNREACHABLE 
>state.

I don't think the algorithm in 5.2 works all that well. In RFC-4861, when
NUD has failed, the NCE is deleted. However, this does not actually
signal anything to Destination Cache entries. There is only some wording
that some things need to be done. Without any specification on how that is
triggered exactly.

Deleting, or otherwise marking as invalid the DCEs that correspond to the 
failing router is the easiest way to get the right result.

>> 2) Similar for redirect. If there is a Destination Cache entry and it result
>ed
>     from a redirect, then delete it. Also update the host state description
>>     for the destination cache to include a redirected flag.
>
>I don't understand why this draft requires adding a redirect flag in RFC 
>4861. While such a flag might be useful in an implementation, it seems 
>to be orthogonal to these proposed changes; whether the NCE becomes 
>deleted (as in 4861 today) or marked as UNREACHABLE (with these changes) 
>the same next-hop selection needs to be triggered. (Perhaps that should 
>be stated more explicitly in the draft.)

In general, you can't figure out if NCE entry is the result of a redirect.
You would have to examine the DCE to see if the destination is off-link.

The easiest way is to always delete DCEs that refer to the NCE. The
conceptual Destination Cache doesn't have to concept of entries that need
next-hop selection.

Now, this may not be the place to actually update all those parts of RFC-4861.
But giving some hints on how things can be implemented may help.

>We could do the latter my augmenting the above text to also say to send 
>ICMP destination unreachables (with code 3) when the packet is sent 
>using a NCE that is in UNREACHABLE state.

While dropping the packet or also forwarding it using the last known MAC?