DHCPv6 failover protocol?

[Ted Lemon]

On Mar 5, 2009, at 6:43 PM, Chuck Anderson wrote:
> Does this mean long-lived TCP sessions will break when a different
> address is leased?  SHIM6 and/or SCTP could be solutions to this
> problem, but I don't think the world is as optimistic as I am about
> those.

The distinction with DHCPv6/IPv6 is that when you get an IP address,  
it has a preferred lifetime and a valid lifetime.   The DHCP client  
will go looking for a new address after half the preferred lifetime  
has expired, usually (it's a bit more complicated than that, but  
that's good enough to get the idea).

Suppose the DHCP server that issued the original address isn't  
answering.   Then at that point the client will get a new address from  
the other DHCP server.   But it still keeps the old address, and it  
can keep using the old address until the valid lifetime on that  
address expires.   So it's not like with DHCPv4 where when you get a  
lease on a new IP address, all your connections immediately break.

It's still the case that long-running connections will die when the  
valid lifetime expires, so if you have a connection that absolutely  
has to be up for a month, your valid lifetime had better be a month  
plus the renewal time.   Realistically, though, this probably means  
that your long-running connection will be broken by something other  
than a DHCPv6 outage.   One hopes that you don't have it set up so you  
lose a month's work if it breaks.