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

[<[e-mail]>]

Re-,

Please see inline.

Cheers,
Med

De : Lorenzo Colitti [mailto:[e-mail]]
Envoyé : vendredi 6 décembre 2019 11:44
À : BOUCADAIR Mohamed TGI/OLN
Cc : Jen Linkova; Bjørn Mork; [e-mail]; V6 Ops List; [e-mail]
Objet : Re: [dhcwg] IPv6-Only Preferred DHCPv4 option

On Fri, Dec 6, 2019 at 5:01 PM <[e-mail]<mailto:[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.
[Med] Assuming the host is modified to behave in this mode.

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.
[Med] Still it will wait in that case.

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).
[Med] Yes. My point is that this information is explicit enough that a NAT64 service is available and that only the IPv6 prefix should be configured.

A host that is not supporting this mode won’t wait and will retrieve both ipv4/IPv6 configuration (if available).

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?
[Med] The one you have in the draft:

   A DHCP client SHOULD allow a device administrator to configure
                              ^^^^^^^^^^^^^^^^^^^^^^
   IPv6-only preferred mode either for a specific interface (to indicate
   that the device is IPv6-only capable if connected to a NAT64 network
   via that interface) or for all interfaces.

Perhaps, I misunderstood the intent of the draft. If you are referring to the network administrator that is able to touch that parameter on hosts on the LAN, that administrator can also disable dhcpv4 for IPv6-capable devices (on specific interfaces/networks). We don’t have a problem them.

Otherwise, I don’t see how a device can set automatically “IPv6-only preferred mode”. Do you?