Re: [v6ops] A common problem with SLAAC in "renumbering" scenarios

[Fernando Gont <[e-mail]>]

On 3/2/19 13:49, Mark Smith wrote:
[....]
>>> So a bug.
>>> What you are talking about is the case where the ISP breaks the contract. While it previously promised to delegate you a prefix until 20200301, all trace of that has gone.
>>
>> The CPE is the middle-man between the ISP and the LAN. No matter what
>> you may *expect* the CPE to do, the CPE is currently not actually
>> required to e.g. clean after the contract that the ISP broke (if you
>> assume/think there's such a thing), or even adjust prefix timers
>> according to the DHCPv6 lease times -- talk about under-specification of
>> the glue between e.g. DHCPv6-PD and SLAAC.
>>
>> Besides, the layer-8 contract between the user and the ISP may be that
>> you get dynamic prefixes. This means that whenever you request a lease,
>> you get a different prefix. You might say that if you don't do another
>> DHCPv6-PD request, you should be able to use the same prefix. But if you
>> do ask a new prefix, you might indeed get a new one -- and this is what
>> normally happens after reboots.
> 
> That's against the architecture and design of the Internet protocols.
> 
> A router reboot, anywhere along the path between communicating
> end-points, is supposed to have no more effect than a transient period
> of packet loss. Recovery is supposed to be via transport layer
> retransmission within the existing established connections.

If you change the addresses, all bets are off. Getting a different
prefix might mean, in theory, that you¡re attached to a different point
of the network topology.

In any case, the debate here is how to improve things when such
scenarios do happen.



>> The CPE should -- if possible -- be faithful to its LAN hosts, and
>> advertise if previous contracts between the CPE and the LAN hosts are
>> void. i.e., if the CPE does  not get leased the same prefix as before,
>> it shoudl notifiy its "clients". However, possibly for simplicity sake,
>> CPEs don't record what
>> information was previously advertised on the LAN -- they are not
>> required, so.... when they reboot, they may not not be in a position to
>> notify hosts accordingly.
>>
>> That's the environment hosts operate in -- no matter whether you or me
>> like it.
>>
>> In that environment, hosts can and should be smarter.
> 
> 
> Multipath transport layer protocols for the win. They're splitting
> identifier semantics off from IP addresses.

That's mostly irrelevant -- because the thing here is to get rid of the
invalid addresses which tends to be benefical for anything running on IP.



-- 
Fernando Gont
SI6 Networks
e-mail: [e-mail]
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