Re: [v6ops] AWS ipv6-only features

[Brian E Carpenter <[e-mail]>]

On 01-Dec-21 06:14, Owen DeLong wrote:
> 
> 
>> On Nov 30, 2021, at 01:47 , Philip Homburg <[e-mail]> wrote:
>>
>>>    On Nov 29, 2021, at 10:33 , Nick Buraglio <[1][e-mail]>
>>>    wrote:
>>>
>>>    "ULAs are preferred over GUAs, so when a host is presented with
>>>    both a ULA and GUA as possible ways to reach a destination, the
>>>    host will select the ULA. Once the ULA destination address is
>>>    chosen, the host will then choose its ULA as a source address
>>>    to reach the ULA destination. This preference of ULA addressing
>>>    over GUA addressing is the mechanism that provides internal
>>>    network connectivity independence from concurrent external
>>>    Internet connectivity."
>>>
>>>    Yep... The moral of the story is that GUA works as intended and
>>>    ULA is a bit of a mess.
>>
>> I'm a bit confused about this scenario.
>>
>> Typically a hosts gets addresses from DNS. So this suggests that people
>> create DNS RR sets that contains both GUAs and ULAs.
>>
>> Is this common practice somewhere? Do people expect that something sensible
>> will happen if you try that? Is it documented what should happen?
> 
> Probably more common in mDNS than DNS, but yes, something sensible SHOULD
> happen as documented in the RFCs:
> 
> AAAA record sorting in getaddrinfo() or getnameinfo() should return the ULA records
> before the GUA records in the linked list.

That should be an application programmer's choice, I think, because whether
ULAs should be preferred is an application-specific choice. In some cases,
a "happy eyeballs" heuristic might even be best. In any case, I don't see
how the IETF can design a one-size-fits-all solution.

Also, of course, there are choices like: does a printer need a GUA? Why
wouldn't a ULA be sufficient? The same goes for any internal-use-only
server.

Also a site could decide not only to use split-horizon DNS, but also
to use different DNS names for different addresses, e.g.
[e-mail] for the ULA and [e-mail]
for the GUA.

We abandoned draft-ietf-v6ops-ula-usage-recommendations and draft-ietf-v6ops-ula-usage-considerations. Maybe it's time to restart such an effort.

    Brian

> 
> As a result, an application that sensibly iterates through the list in order (as is expected
> behavior) should connect via GUA if possible (if not, it should rapidly receive an error
> and move on to the next item in the list, so no extraordinary processing or coding
> is required in the application).