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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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