RE: an example of optimistic DAD in action, with MIPv6

Greg -

> There's been some discussion recently as to the applicability 
> of the unsolicited NA, on non-predictive handovers.
> 
> It seems that unless there's already a neighbor cache entry 
> (there won't be, since we're non-predictive) no-one will 
> listen to the NA (SHOULD NOT, in RFC2461).

RFC-2461 says:
"When a valid Neighbor Advertisement is received (either solicited or
   unsolicited), the Neighbor Cache is searched for the target's entry.
   If no entry exists, the advertisement SHOULD be silently discarded.
   There is no need to create an entry if none exists, since the
   recipient has apparently not initiated any communication with the
   target."

The optimistic DAD draft says the following about router processing of
such an unsolicited NA:

"  It may be desirable for a Neighbour, for example the router, to
   rapidly establish communication with the newly configured ON. To do
   so, it must learn of the ON's arrival as soon as possible.  To avoid
   having to wait for Neighbour Discovery, the ON may wish to send
   unsolicited Neighbour Advertisements (with Override set
   appropriately), but for this to be effective the Neighbour must
   either:

   * be expecting the ON to arrive (eg: due to predictive mechanisms),
        and thus already have a NC entry for the peer, in state
        INCOMPLETE.

   * be willing to cache unsolicited NAs (for a short period of time),
        so that an entry will have been created with state STALE."

This seems like a reasonable router-specific extension to RFC-2461.

> Maybe it is sufficient to respond to NS's with NA, without 
> sending the unsolicited NAs (for non-predictive).

>From the optimistic DAD draft:

"   * (adds to 7.2.6)  The Optimistic node MAY send an unsolicited
        Neighbour Advertisement to All Nodes when it first configures an
        address. The Override flag on this advertisement MUST be set to
        0.

   * (adds to 7.2.6)  The Optimistic node SHOULD send an unsolicited NA
        to All Nodes when it completes DAD. The Override flag on this
        advertisement SHOULD be set to 1."

We implement the 1st requirement when we auto-configure CoAs (done
immediatly upon receiving the RA with the prefix information), and the
2nd requirement when we auto-configure the link-local scope address
(done after DAD has completed successfully). If for no other reason, it
is useful to see these unsolicited NAs appear in the protocol capture
for debugging purposes, but then also these unsolicited NAs could create
STALE Neighbor Cache entries in the router. If another node on the same
link is performing DAD on the exact same address(es) at the same time
(highly unlikely), then these unsolicited NAs could also help it detect
duplicates.

> Without the SLLAO, I think that the issues caused by using a 
> tentative address for RS are minimal or recoverable (I think 
> the new draft will cover this).
> 
> I think there are significant advantages to using a 
> link-local source address (since unicast RA responses 
> (+FastRA??) are available).

Yes, I agree with you, but currently it is unsafe to do so because you
would have to include the SLLAO, correct?

Thanks.
- Ed

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