Re: there should be a ULA prefix?? [was: A common problem with SLAAC in "renumbering" scenarios]

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

On 27-Feb-19 11:10, Mark Smith wrote:
> On Wed., 27 Feb. 2019, 06:36 Brian E Carpenter, <[e-mail]>
> wrote:
> 
>> Philip,
>>
>> On 26-Feb-19 23:33, Philip Homburg wrote:
>>>> So given that document is 12 years old, with that default copied from
>> one that
>>>> is 21 years old, is an update required?
>>>>
>>>> And if so, to what?
>>>
>>> I think this should be updated.
>>>
>>> A long time ago, the model was that you would get a prefix from your ISP
>>> and that was the only global prefix on the local network.
>>>
>>> So with short lifetimes, if the internet connection would go down for a
>>> relatively long period, there would be no global prefix anymore and hosts
>>> would have to resort to link local to communicate (which obviously fails
>>> if there are multiple subnets).
>>>
>>> Some time in the past, the thinking changed and now there should be a ULA
>>> prefix in addition to any global prefixes.
>>
>> Really? Where do you think that is stated?
>>
>> I happen to run my CPE with ULA enabled, but I'm not aware of any
>> recommendation to do so.
>>
> 
> RFC7084. Was also in its ancestor.

Not so. All it says is:

   ULA-1:  The IPv6 CE router SHOULD be capable of generating a ULA
           prefix [RFC4193].

Unless I've missed something, nowhere does it recommend that ULA
should be enabled by default, and even though I'm a strong advocate
of ULAs, it isn't obvious that they should be on by default as a
general recommendation.
 
> OpenWRT is enabling ULAs by default (although with infinite lifetimes which
> is something that should be changed).

And for homenets that is a reasonable default IMHO, and is advocated
by RFC7368. However, https://tools.ietf.org/html/rfc7788#section-6.5
doesn't make it mandatory.

> Back in 2010 Fritzboxes were doing ULAs too, although badly. They had all
> zeros random parts, and were trying to swap them in and out with the PD GUA
> addresses as the WAN link went up or down. (It seemed they didn't
> understand the fundamental idea that IPv6 supports multiaddressing.)

My 2013-vintage FritzBox does better than that.

   Brian> 
>>> So I think that with a ULA, it makes more sense for a CPE to limit
>> lifetimes
>>> to some multiple of the RA interval.
>>
>> Why? I don't expect my ULA prefix to change ever. Or do you mean the
>> lifetimes
>> for globally routeable prefixes?
> 
> 
>>    Brian
>>
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