DHCPv6 failover protocol?

[David W. Hankins]

On Thu, Mar 05, 2009 at 05:12:24PM -0500, Frank Sweetser wrote:
> While poring over RFC3315, I was curious how the validation section for 
> Renew and Rebind messages interacts with failover.  The RFC states that for 
> Renew messages:

I'm wondering if we're going to need actual "failover" with DHCPv6.

With DHCPv4, we needed a failover protocol so two servers could act
nearly like one because of the IPv4 address shortage.

With DHCPv6, for static leases you just configure the servers
consistently, but for dynamic leases you can just configure two (or
more!) huge pools, and clients will migrate (they get new addresses
/before/ their old ones expire, and are allowed to keep the old
addresses until the valid lifetime expires).

So we might need a failover protocol with IPv6, but I'm not convinced
yet.  I want to see how people build their systems.

It's also quite possible we will have something very different; a
stateless server algorithm so that (n+1) servers can answer
consistently without needing to update each other.

>    Servers MUST discard any received Renew message that meets any of the
>    following conditions:

Note that DHCPv4 had the same admonishment, the only reason the ISC
server never paid attention to the server-identifier is because it
wasn't sure what server identifier(s) it might have used until after
the client was being configured...it isn't expressly required for
failover (and actually makes our failover implementation weirder).

Of course we don't have the same excuse with the DHCPv6 server-id.

> So if you have two servers, A and B, and server A goes down, how will 
> server B react?  Is it permitted to start answering messages with the DUID 
> of A, or does it have to wait for the clients to give up and send out a new 
> Solicit message?

They Rebind long before they Solicit. :)

The server will reply with new addresses it can give the client.  The
client's old addresses are still valid (unless they are on the wrong
link, then the server includes them with zero lifetimes), but may not
be "preferred".

So in theory the client gradually migrates sessions between the
addresses.


Note that if both servers had the same host statement, this is the same
as failover, and they would both deliver the same address(es) still on
Rebind.

-- 
David W. Hankins	"If you don't do it right the first time,
Software Engineer		     you'll just have to do it again."
Internet Systems Consortium, Inc.		-- Jack T. Hankins
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