6. Destination Address Selection

The destination address selection algorithm takes a list of destination addresses and sorts the addresses to produce a new list. It is specified here in terms of the pair-wise comparison of addresses DA and DB, where DA appears before DB in the original list.

The algorithm sorts together both IPv6 and IPv4 addresses. To find the attributes of an IPv4 address in the policy table, the IPv4 address MUST be represented as an IPv4-mapped address.

We write Source(D) to indicate the selected source address for a destination D. For IPv6 addresses, the previous section specifies the source address selection algorithm. Source address selection for IPv4 addresses is not specified in this document.

We say that Source(D) is undefined if there is no source address available for destination D. For IPv6 addresses, this is only the case if CandidateSource(D) is the empty set.

The pair-wise comparison of destination addresses consists of ten rules, which MUST be applied in order. If a rule determines a result, then the remaining rules are not relevant and MUST be ignored. Subsequent rules act as tiebreakers for earlier rules. See the previous section for a lengthier description of how pair-wise comparison tiebreaker rules can be used to sort a list.

Rule 1: Avoid unusable destinations.
If DB is known to be unreachable or if Source(DB) is undefined, then prefer DA. Similarly, if DA is known to be unreachable or if Source(DA) is undefined, then prefer DB.

Discussion: An implementation might know that a particular destination is unreachable in several ways. For example, the destination might be reached through a network interface that is currently unplugged. For example, the implementation might retain information from Neighbor Unreachability Detection [RFC4861] for some period of time. In any case, the determination of unreachability for the purposes of this rule is implementation-dependent.

Rule 2: Prefer matching scope.
If Scope(DA) = Scope(Source(DA)) and Scope(DB) <> Scope(Source(DB)), then prefer DA. Similarly, if Scope(DA) <> Scope(Source(DA)) and Scope(DB) = Scope(Source(DB)), then prefer DB.

Rule 3: Avoid deprecated addresses.
If Source(DA) is deprecated and Source(DB) is not, then prefer DB. Similarly, if Source(DA) is not deprecated and Source(DB) is deprecated, then prefer DA.

Rule 4: Prefer home addresses.
If Source(DA) is simultaneously a home address and care-of address and Source(DB) is not, then prefer DA. Similarly, if Source(DB) is simultaneously a home address and care-of address and Source(DA) is not, then prefer DB.

If Source(DA) is just a home address and Source(DB) is just a care-of address, then prefer DA. Similarly, if Source(DA) is just a care-of address and Source(DB) is just a home address, then prefer DB.

Rule 5: Prefer matching label.
If Label(Source(DA)) = Label(DA) and Label(Source(DB)) <> Label(DB), then prefer DA. Similarly, if Label(Source(DA)) <> Label(DA) and Label(Source(DB)) = Label(DB), then prefer DB.

Rule 6: Prefer higher precedence.
If Precedence(DA) > Precedence(DB), then prefer DA. Similarly, if Precedence(DA) < Precedence(DB), then prefer DB.

Rule 7: Prefer native transport.
If DA is reached via an encapsulating transition mechanism (e.g., IPv6 in IPv4) and DB is not, then prefer DB. Similarly, if DB is reached via encapsulation and DA is not, then prefer DA.

Discussion: The IPv6 Rapid Deployment on IPv4 Infrastructures (6rd) Protocol [RFC5969], the Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) [RFC5214], and configured tunnels [RFC4213] are examples of encapsulating transition mechanisms for which the destination address does not have a specific prefix and hence can not be assigned a lower precedence in the policy table. An implementation MAY generalize this rule by using a concept of interface preference and giving virtual interfaces (like the IPv6-in-IPv4 encapsulating interfaces) a lower preference than native interfaces (like ethernet interfaces).

Rule 8: Prefer smaller scope.
If Scope(DA) < Scope(DB), then prefer DA. Similarly, if Scope(DA) > Scope(DB), then prefer DB.

Rule 9: Use longest matching prefix.
When DA and DB belong to the same address family (both are IPv6 or both are IPv4): If CommonPrefixLen(Source(DA), DA) > CommonPrefixLen(Source(DB), DB), then prefer DA. Similarly, if CommonPrefixLen(Source(DA), DA) < CommonPrefixLen(Source(DB), DB), then prefer DB.

Rule 10: Otherwise, leave the order unchanged.
If DA preceded DB in the original list, prefer DA. Otherwise, prefer DB.

Rules 9 and 10 MAY be superseded if the implementation has other means of sorting destination addresses. For example, if the implementation somehow knows which destination addresses will result in the "best" communications performance.