IPV6 planning

Karl Auer kauer at biplane.com.au
Sun Mar 6 01:08:55 UTC 2016

On Sun, 2016-03-06 at 01:57 +0200, Saku Ytti wrote:
> Technically speaking there is no reason not to support SLAAC on
> arbitrary size networks. I believe Cisco happily will autogenerate
> address for smaller subnets.

To support SLAAC with prefix lengths other than 64 you would have to
break numerous standards. RFC2464 is very clear on the matter, at least
for Ethernet interfaces, though RFC 4862 is carefully non-committal. 

Even if the router supports it, as far as I know a standards-conforming
host MUST ignore such prefixes for purposes of SLAAC on Ethernet.

As far as "technically speaking" goes, recall that the host generates
its own address; the only information supplied by the router is the
prefix and an A flag. There is no way for the router to tell hosts
*how* to generate the address. The hosts would have work out where in
the shorter (or longer) prefix the MAC address should go. If the prefix
is too short the hosts would have to work out which bits to discard,
and would have to work out which bit (if any) should be complemented to
indicate local vs global assignment. Might be a good idea to keep least
significant bits, to minimise impacts on solicited node multicast
addresses. Dunno what would happen to solicited node multicast if you
increased the prefix length beyond 104.

Temporary and privacy addresses might be easier - all you would need to
do would be randomise whatever bits were available to you, though as
prefixes got longer your protection would decrease.

There might be more to it than that - haven't really thought it

So to change the prefix length for SLAAC I think you would have to own
the whole ecosystem including host stacks. So while Cisco may support
it on router interfaces, I suspect it wouldn't actually *work* in
practice. But I have been wrong oh, so many times...

Regards, K.

Karl Auer (kauer at biplane.com.au)

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