Re: Running code (Was: I-D Action: draft-ietf-6man-ipv6only-flag-03.txt)

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

On 2018-10-31 10:03, Toerless Eckert wrote:
> Brian,
> 
> I think i would be happier with a stronger goal of the flag
> relying less or not at all on heuristics of hosts.
> 
> Give me a reason why dual-stack hosts should want to do IPv4
> on a subnet if there is no IPv4-only host and no router routing IPv4. 

The example I've used is a an IP-based smoke detector trying to connect
to an IP-based central fire alarm system. Because human safety is
involved it needs to try every possible type of connectivity in any case,
so giving up on IPv4 because the IPv6 routers say so is not the right
answer. IMHO, a perfect case for an RFC2119 SHOULD:
  "... there
   may exist valid reasons in particular circumstances to ignore a
   particular item, but the full implications must be understood and
   carefully weighed before choosing a different course."

      Brian



> 
> This seems like the most simple case to discuss about and agree
> what to do for it. TO me it seems that it would be great to
> recognize this case and automatically disable IPv4 stack/interface
> because of a flag from routers on all dual-stack hosts.
> 
> Cheers
>     Toerless
> 
> On Wed, Oct 31, 2018 at 08:43:40AM +1300, Brian E Carpenter wrote:
>> On 2018-10-31 02:47, Philip Homburg wrote:
>>>> So, here's the challenge which you seem to be holding back from,
>>>> write down the actual rules. No hand waving that "that's a good
>>>> clue", but actual rules that "if you see <some observable state>
>>>> then <do some specific action>".
>>
>> If by "rules" you mean "a typical heuristic", sure. But there will
>> be hosts that follow a different logic for their own good reasons.
>> That simply isn't the IETF's business.
>>
>> The draft is quite clear on this:
>>
>>    Dual stack hosts that have a good reason to use IPv4, for example for
>>    a specific IPv4 link-local service, can attempt to do so.  Therefore
>>    respect of the IPv6-Only flag is recommended, not mandatory, for
>>    hosts.
>>
>> On 2018-10-31 04:18, Toerless Eckert wrote:
>> ....
>>> IMHO:
>>>
>>> If the host sees the ipv6only flag from all IPv6 routers, it MUST NOT
>>> perform IPv4 DHCP.
>>
>> Who cares? Legacy hosts will try DHCP anyway. We don't want an updated
>> host to waste spectrum on this, but that's a SHOULD NOT, because such
>> traffic won't actually break anything.
>>
>>     Brian
>>
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