Re: [v6ops] IPv6-Only Preferred DHCPv4 option

[Ted Lemon <[e-mail]>]

On Dec 6, 2019, at 6:45 AM, Bernie Volz (volz) <[e-mail]> wrote:
> which requires fewer changes on the DHCP server implementation

This is almost certainly not true.  In order for this to work, you’re going to have to have some special-purpose code to support it.  Better to do the clean solution than a hack that minimizes changes to the server at the expense of more complication in the client-server interaction.

> Existing servers can deploy this without ANY changes (provided you have a means to configure the server with whatever option number is finally assigned).

I think it’s okay to require changes to the server.   This is a significant new feature.

> Personally, I would NOT recommend breaking the semantics of a DHCPOFFER and always include an address because you never know how intermediate devices that may be "snooping" this traffic will handle this; for example, the intermediate device might drop what it feels is an invalid packet.

This is definitely not an issue, though, because the network operator is choosing to deploy this.  They can be responsible for ensuring correct behavior if there are any middleboxes intercepting the DHCPv4 traffic and doing stuff with it.   Expecting this to Just Work with no infrastructure changes seems unnecessary.

>> There is a reason why DHCP includes a parameter request list option.  If the client doesn’t ask for this newfangled option, it shouldn’t see it.  And so we can know for sure that the client supports this capability, because it asked for it.
> 
> Even if a DHCP server sent the option without it being requested, why would the client use it if it didn't ask for it?

If the DHCP server knows the client supports the feature, it can assume that when it sends the option, the client will implement the specified behavior.   If it doesn’t know for sure that the client supports the feature, then you have to deal with a much more complex set of possible behaviors.   Why add that complexity?

And that’s assuming that the client behaves correctly.  If you just send the new option, another possibility is that the client crashes when it doesn’t recognize the option, as we used to see with certain printers I the manufacturer of which I will not mention.   Or that it does some other random thing.  Why chance it?