Subject: RFC 3041 clarification requested

From: Jari Arkko <[email-masked]>
Date: Fri, 14 Feb 2003 17:14:41 +0200

Hi,

I'm trying to understand what the following text means and
implies in Section 3.3 of RFC 3041:

   "Note: because multiple temporary addresses are generated from the
    same associated randomized interface identifier, there is little
    benefit in running DAD on every temporary address.  This document
    recommends that DAD be run on the first address generated from a
    given randomized identifier, but that DAD be skipped on all
    subsequent addresses generated from the same randomized interface
    identifier."

Does this refer to multiple addresses when the link has multiple
prefixes? Or when multiple temporary addresses are generated in
sequence? It says "addresses generated from the given randomized
identifier" so one might assume it means the multiple prefix case.
But on the other hand the randomized identifier is also fed as
history to the generation of new addresses, so it might mean the
sequence also.

Additionally, I'm wondering how DAD works with RFC 3041.
The scheme appears to rely on the order in which addresses
are generated. On a network with two prefixes A and B two
nodes might not generate and test the addresses in the same
order. For instance, node 1 could test A::<random> and node
2 could test B::<random> first. If the random values collide, the
collision apparently isn't detected and both nodes proceed
to use both A::<random> and B::<random>. Or did I miss
something?

Also, it wasn't clear to me whether link-local addresses are
generated for every new IID or not. If they are, RFC 2462
rules in Section 5.4 apply and the collision problem may
be solved that way. (Or does it -- where does it say that
"first" in 3041 refers to the link-local address?)

Jari

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From: "Nick 'Sharkey' Moore" <[email-masked]>
Date: Tue, 25 Feb 2003 14:13:08 +1100

>    subsequent addresses generated from the same randomized interface

G'day Jari,

	well, I'm not the author but I can hazard a guess that 
this is referrring to the bit of RFC2462 5.4 which says "[...]
an implementation MAY choose to skip Duplicate Address Detection
for additional addresses derived from the same interface
identifier".

> sequence?

The former, including the Link-Local Prefix.  In practise, a
node implementing 3041 would choose an II, create a Link-Local
Address (LLA) from that II, DAD for the LLA, and then just assume
that any other addresses formed from that II are fine.  Always
doing the LLA first gets around the ordering problem you
mention, too.

> "first" in 3041 refers to the link-local address?)

I can't find this mentioned explicitly in 3041, no.  Which
it probably should be.  I've been kicking some of these ideas
around myself, see draft-moore-ipv6-optimistic-dad-02.

cheers,
-----Nick