Re: [v6ops] [dhcwg] IPv6-Only Preferred DHCPv4 option

Roy Marples <roy@marples.name> Thu, 05 December 2019 02:50 UTC

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To: Lorenzo Colitti <lorenzo@google.com>
Cc: Jen Linkova <furry13@gmail.com>, dhcwg@ietf.org, draft-link-dhc-v6only@ietf.org, V6 Ops List <v6ops@ietf.org>
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From: Roy Marples <roy@marples.name>
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Subject: Re: [v6ops] [dhcwg] IPv6-Only Preferred DHCPv4 option
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On 05/12/2019 02:27, Lorenzo Colitti wrote:
> On Thu, Dec 5, 2019 at 10:12 AM Roy Marples <roy@marples.name 
> <mailto:roy@marples.name>> wrote:
> 
>      > The draft covers that scenario. DHCPv4 is disabled for
>     V6ONLY_WAIT seconds.
> 
>     Is the wait seconds even needed?
>     Just set the T2 timer and treat it as leasing the unspecified address.
>     Less data on the wire, less churn in DHCP clients. V6ONLY_WAIT provides
>     nothing of value as I see it.
> 
> 
> I think there is value because the two are conceptually different.
> 
> The T2 timer is about the validity of the address that is offered

Wrong.
The T2 timer addresses the lease as a whole. At this point the address 
is just meta data, just like domain or dns servers.

, and
> specifies how often that address should be renewed if the client accepts 
> the offer. The V6ONLY_WAIT timer is about how long the client should 
> wait before asking again if it doesn't accept the offer.

Wrong again.
Or at least V6ONLY_WAIT is at the very least badly worded, but DHCP 
should not depend on any IPv6 settings - or vice versa. Is it too much 
to ask to keep protocol setup separate?

As we're not offering an address in this situation, how is V6ONLY_WAIT 
different from a timer source that cannot exist? As such, why invent a 
new timer source?

> I think these 
> two could be very different. For example, if IPv4 is a very scarce 
> resource and almost al clients are IPv6, the operator will want to use 
> short lease times (e.g., 1 hour, or shorter) to ensure that hosts that 
> disconnect from the network don't tie up scarce IPv4 addresses when 
> they're not connected. That same operator would want the V6ONLY_WAIT 
> timer to be large in order to avoid excessive load on the DHCP server 
> (and to conserve battery on the devices).
> 
> If there is consensus that this is useless we can remove it, but I don't 
> see a a strong reason to give up that extra flexibility. Is there?

I don't see any extra flexibility offered. You talk about lease times 
and yet say T1/T2 can't be used - why? All I see is more complication 
when things can be kept simple.

Roy