17. Client Source Address and Interface Selection

This section preserves the RFC text for DHCPv6, including message exchanges, relay behavior, DUIDs, IA_NA, IA_TA, IA_PD, DHCP options, RKAP authentication, IANA registries, normative requirements, and appendix option-appearance matrices.

Original RFC Text

17.  Client Source Address and Interface Selection

   The client's behavior regarding interface selection is different,
   depending on the purpose of the configuration.

17.1.  Source Address and Interface Selection for Address Assignment

   When a client sends a DHCP message to the
   All_DHCP_Relay_Agents_and_Servers multicast address, it SHOULD send
   the message through the interface for which configuration information
   (including the addresses) is being requested.  However, the client
   MAY send the message through another interface if the interface for
   which configuration is being requested is a logical interface without
   direct link attachment or the client is certain that two interfaces
   are attached to the same link.

17.2.  Source Address and Interface Selection for Prefix Delegation

   Delegated prefixes are not associated with a particular interface in
   the same way as addresses are for address assignment as mentioned in
   Section 17.1 above.

   When a client sends a DHCP message for the purpose of prefix
   delegation, it SHOULD be sent on the interface associated with the
   upstream router (typically, connected to an ISP network); see
   [RFC7084].  The upstream interface is typically determined by
   configuration.  This rule applies even in the case where a separate
   IA_PD is used for each downstream interface.