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Re: [RRG] Consensus check: renumbering - missing dimension
On 26 aug 2008, at 0:35, Brian E Carpenter wrote:
stateless-autoconfig is entirely not sufficient for site admins to
use
in a 'renumbering' event.
I've tried it, and it works really well. The advantage here of
stateless autoconfig over DHCPv6 is that all the hosts get the same
information at the same time, so they're pretty much going to do the
same thing at the same time. With DHCPv6 address configuration hosts
will be asking for an address at random times. It would be possible
for the server to hand out lease times that synchronize the clients,
but that wouldn't work reliably because there is no control over when
the clients renew their leases. And it would be undesirable, because
it's a large amount of traffic at the same time. Worse, this is
multicast traffic, which is very bad for wireless networks. (Although
hacks should be possible to reduce this.) But the thing that DHCP
can't do is go out and tell all the clients about a change that
occurred between a lease and a renewal.
And of course all this complex stuff (maintaining timers and a fixed
address for the same host) requires a very solid DHCPv6 server that
keeps a lot of state. Presumably, this wouldn't be a problem in
enterprises, but you really don't want to have this as a requirement
in smaller installations.
There are many items passed out in DHCP
responses which are used by the end systems and not included in
stateless-autoconfig.
We now have RFC 5006 for DNS server addresses, which is the only thing
that clients really need, all the other stuff may or may not be
useful, but doesn't necessarily have to be autoconfigured.
And those so inclined can run stateless autoconfig for address
assignment and stateless DHCPv6 for "other information" but not
address assignment.
My memory is that back when stateless auto-config was conceived,
the main target was the "dentist's office" scenario, i.e.
basic Appletalk-like zeroconf sites.
Interestingly, IPv6 now has ALL the previously known address
configuration mechanisms (except old DECNET):
- like IPX: combine router advertised prefix with MAC address
- like Appletalk: combine router advertised prefix with random number
- like old IPv4: manual configuration
- like modern IPv4: DHCP
Unfortunately we still
have one hole in this area: no way to advertise a DNS server
address in RA messages. See RFC 4339.
RFC 5006.
Note also that the dichotomy between stateless autoconfig and central
management is an artificial one: you could centrally manage a DHCPv6
server that gives out prefixes to routers with prefix delegation, and
then these routers advertise those prefixes using stateless
autoconfig. I tried this three years ago with iffy DHCPv6
implementations and a Cisco router, and I was very impressed with how
well it worked. The router even copies the lease time in the RA
lifetimes so the change happens at the correct moment.
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