Re: [dhcwg] RFC 8415 question: unicast Information-request

The client can unicast if the server tells it to, using the Server Unicast option. The reason for this is to make sure that all DHCP packets transit the same infrastructure: if a client unicasts a packet to the server, it can reach the server via IP routing rather than DHCP relay, and may miss some information that needs to be collected about the network on the way to the server.

> On Aug 29, 2020, at 5:55 AM, Felix Hamme <fhamme@united-internet.de> wrote:
> 
> Dear DHCPv6 experts,
> 
> 
> does RFC8415 permit Information-request messages with an IPv6 unicast
> destination address?
> 
> 
> Section 16 states:
> "A server MUST discard any Solicit, Confirm, Rebind, or
> Information-request messages it receives with a Layer 3 unicast
> destination address."
> (unicast Information-request forbidden)
> 
> Section 18.2 states:
> "If the client has a source address of sufficient scope that can be
> used by the server as a return address and the client has received a
> Server Unicast option (see Section 21.12) from the server, the client
> SHOULD unicast any Request, Renew, Release, and Decline messages to
> the server."
> (unicast Information-request not mentioned)
> 
> Section 18.4 states:
> "For example, unicast transmission is not allowed for Solicit, Confirm,
> and Rebind messages (see Sections 18.3.1, 18.3.3, and 18.3.5,
> respectively), even if the Server Unicast option (see Section 21.12) is
> configured. For Request, Renew, Information-request, Release, and
> Decline messages, it is allowed only if the Server Unicast option is
> configured."
> (unicast Information-request allowed if a Server Unicast option was
> received)
> 
> Appendix B ("informational") does not permit a Server Unicast option in
> a Reconfigure message.
> 
> 
> I am not sure how to interpret those statements in total.
> 
> When sending an Information-request message in response to a Reconfigure
> message, a client needs to include a Server Identifier option (see
> section 18.2.6) to address a specific server. It appears reasonable to
> me to permit a Server Unicast option in a Reconfigure message so that a
> client can respond with an Information-request message with an unicast
> destination address.
> 
> What if the Information-request message is sent after a
> Solicit-Advertise-Request-Reply message exchange, in which the server
> sent a Server Unicast option? May a client unicast Information-request
> messages in that case?
> 
> 
> Sincerely,
> Felix Hamme
> 
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