JINMEI Tatuya / ???? wrote:
I encountered one quick question when I reread the spec...the neighbor
discovery specification (1970, 2461, and 2461bis) specifies the
responding NA to an NS for DAD (identified by the source address being
::) must be sent to all-nodes multicast address:
If the source of the solicitation is the unspecified address, the
node MUST set the Solicited flag to zero and multicast the
advertisement to the all-nodes address.
(section 7.2.4 of 2461bis-05)
Does anyone know why this must be sent to the all-nodes address? I
mean, why can't this be a solicited-node multicast address for the
target address? Since the sender of the NS joins both the group
addresses according to the DAD specification, it seems to me that
either one should work fine. Is there any special reason for the
address selection, or is this just an arbitrary choice?
The reason as I recall it was that when the IPv6 address is based on an
EUI-64, a duplicate IPv6 address is an indication of a duplicate L2
address. When the L2 address is a duplicate then sending the response to
the *unicast* L2 address doesn't work.
We might not have thought of using the solicited-node multicast address
as an optimization at the time, but settled for the all-nodes.
(We're not likely to see a lot of these packets after all.)
AFAICT using the solicited-node MC address should work as well.
Erik
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