Re: A common problem with SLAAC in "renumbering" scenarios

[Nick Hilliard <[e-mail]>]

Brian E Carpenter wrote on 06/02/2019 19:31:
> On 2019-02-06 23:15, Nick Hilliard wrote:
> In the general case we can't "ensure" that. Firstly it may be against
> business policy or privacy policy for some ISPs. Secondly even if it
> was the intended policy, there can always be failure scenarios. Thirdly,
> a host might be disconnected from one LAN and reconnected to another
> one without knowing it. So even if this is the recommended practice,
> no host can rely on it.

It's not ensuring that the customer gets the same lease all the time; 
it's ensuring that if a customer is disconnected where their previous 
lease is still active, and then they reconnect within the lifetime of 
that lease, that they can still use that lease for as long as it's active.

In other words - as some other people put it - it's about ensuring that 
ISP leases are honoured even if a customer disconnects and reconnects.

This is probably the least difficult option but requires a level of 
centralised provisioning, so will mean some push-back from service 
providers.  This option also has the smallest footprint.

>> 2) the intermediate device remembers the old lease, compares it with the
>> new lease and if they're different, invalidates the old prefix by
>> issuing an updated PIO,
> 
> Again, we can't rely in this, due to legacy equipment and the possibility
> of failures or reconnections.

I don't necessarily see why we can't rely on this to work in most cases.

>> 3) pushing the smarts down to the host layer and detecting / monitoring
>> whether any particular prefix is still valid
> 
> https://tools.ietf.org/html/rfc6059 ?
> 
> In any case, because neither of the preceding options is guaranteed,
> I think there is no choice: hosts must be able to recover from
> dead addresses.

This is potentially the boil-the-ocean option.  It's hard not to think: 
"perfect is the enemy of good enough".

Nick