IPv6 /64 links (was Re: ipv6 book recommendations?)

Thu Jun 7 22:27:53 UTC 2012

On Thu, Jun 7, 2012 at 10:14 PM, Karl Auer <kauer at biplane.com.au> wrote:
> On Thu, 2012-06-07 at 21:07 +0000, Dave Hart wrote:
>> Bzzt.  With ARP, every IPv4 node on the link indicates each ARP packet
>> to the OS.  With ND, only those nodes sharing the same last 24 bits of
>> the IPv6 address indicate the packet up the stack.  The rest of the
>> IPv6 nodes filter the multicast in the NIC.
>
> Still not quite correct :-)
>
> The "filtering" is done by a MLD-aware switch, which will send multicast
> packets only to nodes that are listening to the appropriate multicast
> group. The filtering you describe is pretty much what ARP does - ALL
> nodes receive the packet, all but one ignore it. It depends on the
> platform whether the CPU that does the ignoring is just in the NIC or is
> in the node itself.

Karl, you seem to fail to understand how ethernet NICs are implemented
in the real world.  Ignoring the optional (but common) promiscuous
mode support and various offloading, IPv4 ARP is sent as ethernet
broadcast and the NIC hardware and driver is in no position to filter
-- it must be done by the IP stack.  In contrast, ND is sent as
ethernet multicast which are filtered by receivers in hardware.
Whether or not the switches are smart enough to filter is an
implementation decision that has no bearing on the requirement to
filter in the NIC hardware.

Cheers,
Dave Hart