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

[Lorenzo Colitti <[e-mail]>]

On Fri, Dec 6, 2019 at 5:01 PM <[e-mail]> wrote:

> *[Med] *Do you mean that clients that receive the signal to release IPv4
> configuration will make the users unhappy anyway because they will need to
> wait for the pref64 to properly work?
>
>
>
> Well, that's the intent of the pref64 RA option. That allows the pref64 to
> be communicated in the same packet as the rest of the configuration
> parameters.
>
> [Med] If there is no delay then, the signals (pref64 presence, v6-only
> presence) are redundant. No?
>

The pref64 option doesn't guarantee the absence of a delay. This DHCP
option tell hosts: this network can provide IPv6-only service with NAT, and
would prefer that hosts use it over IPv4. That gives the host a reason to
wait before it configures IPv4, or not to configure IPv4 at all, because it
knows that (unless there is an outage or misconfiguration), native IPv6
with NAT64 will arrive very soon. The pref64 option is there to ensure that
NAT64 arrives as soon as possible and doesn't require roundtrips to the DNS
forwarders (or even to the root and then to the IANA servers).

> Note that IPv6 has built-in delays in the protocol itself (DAD for the
> link-local address; various randomized delays) that make it very difficult
> to get anything done before at least 1-2 seconds have passed. Hosts that
> know that 70% of networks are IPv4-only will not want to wait 1-2 seconds
> to connect to those networks.
>
>
>
> [Med] In that case, what is the benefit for the host to indicate its
> ipv6-only mode support?
>
>
>
> The option improves the situation by allowing the host to wait for IPv6
> only if it is expected to exist.
>
> [Med] The host can “wait” without requiring the option. The key point is
> that an administrator sets “ipv6-mode” supported.
>

Which administrator? If you mean the host administrator: only very few
hosts have administrators that understand networking. Most hosts don't. And
it's silly to wait unconditionally if the host doesn't know if the network
provides IPv6-only service. Almost all networks (except for cellular
networks, which don't use DHCP) don't provide such a service today, and it
doesn't make sense to wait unconditionally on all networks just because a
very few networks support IPv6-only.