4. Candidate Source Addresses

The source address selection algorithm uses the concept of a "candidate set" of potential source addresses for a given destination address. The candidate set is the set of all addresses that could be used as a source address; the source address selection algorithm will pick an address out of that set. We write CandidateSource(A) to denote the candidate set for the address A.

It is RECOMMENDED that the candidate source addresses be the set of unicast addresses assigned to the interface that will be used to send to the destination (the "outgoing" interface). On routers, the candidate set MAY include unicast addresses assigned to any interface that forwards packets, subject to the restrictions described below. Implementations that wish to support the use of global source addresses assigned to a loopback interface MUST behave as if the loopback interface originates and forwards the packet.

Discussion: The Neighbor Discovery Redirect mechanism [RFC4861] requires that routers verify that the source address of a packet identifies a neighbor before generating a Redirect, so it is advantageous for hosts to choose source addresses assigned to the outgoing interface.

In some cases, the destination address might be qualified with a zone index or other information that will constrain the candidate set.

For all multicast and link-local destination addresses, the set of candidate source addresses MUST only include addresses assigned to interfaces belonging to the same link as the outgoing interface.

Discussion: The restriction for multicast destination addresses is necessary because currently deployed multicast forwarding algorithms use Reverse Path Forwarding (RPF) checks.

For site-local unicast destination addresses, the set of candidate source addresses MUST only include addresses assigned to interfaces belonging to the same site as the outgoing interface.

In any case, multicast addresses and the unspecified address MUST NOT be included in a candidate set.

On IPv6-only nodes that support Stateless IP/ICMP Translation (SIIT) [RFC6145], if the destination address is an IPv4-converted address, then the candidate set MUST contain only IPv4-translatable addresses.

If an application or upper layer specifies a source address, it may affect the choice of outgoing interface. Regardless, if the application or upper layer specifies a source address that is not in the candidate set for the destination, then the network layer MUST treat this as an error. If the application or upper layer specifies a source address that is in the candidate set for the destination, then the network layer MUST respect that choice. If the application or upper layer does not specify a source address, then the network layer uses the source address selection algorithm specified in the next section.