In your letter dated Mon, 17 Oct 2011 10:25:19 -0500 you wrote:
>Appreciate the quick reply. Note BrianC already noted that 20 bits will
>not suffice by saying "It puts you into birthday-paradox territory on a
>LAN with a few hundred nodes.". His email is at the URL below.
>
>http://www.ietf.org/mail-archive/web/ipv6/current/msg14835.html
I'm not sure how you would get a birthday paradox here. Applying the birthday
paradox, you would get that if you have a network with a 1000 nodes, and all
nodes try to configure the exact same IPv6 address at the same time, then
there is a probability of about 0.5 that two nodes will mistakenly assume that
there is a loopback situation and all the others will detect a duplicate
address.
That's not a problem I would worry about.
>We can go for a 62 bits long nonce in the Nonce Option. Can't pick 64
>because the Nonce Option will then not end in an 8-byte boundary. See
>the details from RFC 3971 below.
>
> 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
> | Type | Length | Nonce ... |
> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
> | |
> . .
> . .
> | |
> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>
> Length
>
> The length of the option (including the Type, Length, and Nonce
> fields) in units of 8 octets.
>
> Nonce
>
> A field containing a random number selected by the sender of the
> solicitation message. The length of the random number MUST be at
> least 6 bytes. The length of the random number MUST be selected
> so that the length of the nonce option is a multiple of 8 octets.
So it looks like the shortest nonce is 6 octets or 48 bits and then the next
longer one is 14 octets or 112 bits.
SEND doesn't seem to say anything about the length of the nonce. I guess that
48 bits should be enough to be secure. I think 48 bits is more than enough
for DAD.
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