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rfc:rfc6724

Internet Engineering Task Force (IETF) D. Thaler, Ed. Request for Comments: 6724 Microsoft Obsoletes: 3484 R. Draves Category: Standards Track Microsoft Research ISSN: 2070-1721 A. Matsumoto

                                                                   NTT
                                                              T. Chown
                                             University of Southampton
                                                        September 2012
  Default Address Selection for Internet Protocol Version 6 (IPv6)

Abstract

 This document describes two algorithms, one for source address
 selection and one for destination address selection.  The algorithms
 specify default behavior for all Internet Protocol version 6 (IPv6)
 implementations.  They do not override choices made by applications
 or upper-layer protocols, nor do they preclude the development of
 more advanced mechanisms for address selection.  The two algorithms
 share a common context, including an optional mechanism for allowing
 administrators to provide policy that can override the default
 behavior.  In dual-stack implementations, the destination address
 selection algorithm can consider both IPv4 and IPv6 addresses --
 depending on the available source addresses, the algorithm might
 prefer IPv6 addresses over IPv4 addresses, or vice versa.
 Default address selection as defined in this specification applies to
 all IPv6 nodes, including both hosts and routers.  This document
 obsoletes RFC 3484.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6724.

Thaler, et al. Standards Track [Page 1] RFC 6724 Default Address Selection for IPv6 September 2012

Copyright Notice

 Copyright (c) 2012 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1. Introduction ....................................................3
    1.1. Conventions Used in This Document ..........................4
 2. Context in Which the Algorithms Operate .........................4
    2.1. Policy Table ...............................................6
    2.2. Common Prefix Length .......................................7
 3. Address Properties ..............................................7
    3.1. Scope Comparisons ..........................................8
    3.2. IPv4 Addresses and IPv4-Mapped Addresses ...................8
    3.3. Other IPv6 Addresses with Embedded IPv4 Addresses ..........9
    3.4. IPv6 Loopback Address and Other Format Prefixes ............9
    3.5. Mobility Addresses .........................................9
 4. Candidate Source Addresses .....................................10
 5. Source Address Selection .......................................11
 6. Destination Address Selection ..................................14
 7. Interactions with Routing ......................................16
 8. Implementation Considerations ..................................16
 9. Security Considerations ........................................17
 10. Examples ......................................................18
    10.1. Default Source Address Selection .........................18
    10.2. Default Destination Address Selection ....................19
    10.3. Configuring Preference for IPv6 or IPv4 ..................20
         10.3.1. Handling Broken IPv6 ..............................21
    10.4. Configuring Preference for Link-Local Addresses ..........21
    10.5. Configuring a Multi-Homed Site ...........................22
    10.6. Configuring ULA Preference ...............................24
    10.7. Configuring 6to4 Preference ..............................25
 11. References ....................................................26
    11.1. Normative References .....................................26
    11.2. Informative References ...................................27
 Appendix A.  Acknowledgements .....................................29
 Appendix B.  Changes since RFC 3484 ...............................29

Thaler, et al. Standards Track [Page 2] RFC 6724 Default Address Selection for IPv6 September 2012

1. Introduction

 The IPv6 addressing architecture [RFC4291] allows multiple unicast
 addresses to be assigned to interfaces.  These addresses might have
 different reachability scopes (link-local, site-local, or global).
 These addresses might also be "preferred" or "deprecated" [RFC4862].
 Privacy considerations have introduced the concepts of "public
 addresses" and "temporary addresses" [RFC4941].  The mobility
 architecture introduces "home addresses" and "care-of addresses"
 [RFC6275].  In addition, multi-homing situations will result in more
 addresses per node.  For example, a node might have multiple
 interfaces, some of them tunnels or virtual interfaces, or a site
 might have multiple ISP attachments with a global prefix per ISP.
 The end result is that IPv6 implementations will very often be faced
 with multiple possible source and destination addresses when
 initiating communication.  It is desirable to have default
 algorithms, common across all implementations, for selecting source
 and destination addresses so that developers and administrators can
 reason about and predict the behavior of their systems.
 Furthermore, dual- or hybrid-stack implementations, which support
 both IPv6 and IPv4, will very often need to choose between IPv6 and
 IPv4 when initiating communication, for example, when DNS name
 resolution yields both IPv6 and IPv4 addresses and the network
 protocol stack has available both IPv6 and IPv4 source addresses.  In
 such cases, a simple policy to always prefer IPv6 or always prefer
 IPv4 can produce poor behavior.  As one example, suppose a DNS name
 resolves to a global IPv6 address and a global IPv4 address.  If the
 node has assigned a global IPv6 address and a 169.254/16 auto-
 configured IPv4 address [RFC3927], then IPv6 is the best choice for
 communication.  But if the node has assigned only a link-local IPv6
 address and a global IPv4 address, then IPv4 is the best choice for
 communication.  The destination address selection algorithm solves
 this with a unified procedure for choosing among both IPv6 and IPv4
 addresses.
 The algorithms in this document are specified as a set of rules that
 define a partial ordering on the set of addresses that are available
 for use.  In the case of source address selection, a node typically
 has multiple addresses assigned to its interfaces, and the source
 address ordering rules in Section 5 define which address is the
 "best" one to use.  In the case of destination address selection, the
 DNS might return a set of addresses for a given name, and an
 application needs to decide which one to use first and in what order
 to try others if the first one is not reachable.  The destination
 address ordering rules in Section 6, when applied to the set of
 addresses returned by the DNS, provide such a recommended ordering.

Thaler, et al. Standards Track [Page 3] RFC 6724 Default Address Selection for IPv6 September 2012

 This document specifies source address selection and destination
 address selection separately but uses a common context so that
 together the two algorithms yield useful results.  The algorithms
 attempt to choose source and destination addresses of appropriate
 scope and configuration status ("preferred" or "deprecated" in the
 RFC 4862 sense).  Furthermore, this document suggests a preferred
 method, longest matching prefix, for choosing among otherwise
 equivalent addresses in the absence of better information.
 This document also specifies policy hooks to allow administrative
 override of the default behavior.  For example, using these hooks, an
 administrator can specify a preferred source prefix for use with a
 destination prefix or prefer destination addresses with one prefix
 over addresses with another prefix.  These hooks give an
 administrator flexibility in dealing with some multi-homing and
 transition scenarios, but they are certainly not a panacea.
 The selection rules specified in this document MUST NOT be construed
 to override an application or upper layer's explicit choice of a
 legal destination or source address.

1.1. Conventions Used in This Document

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in BCP 14, RFC 2119
 [RFC2119].

2. Context in Which the Algorithms Operate

 Our context for address selection derives from the most common
 implementation architecture, which separates the choice of
 destination address from the choice of source address.  Consequently,
 we have two separate algorithms for these tasks.  The algorithms are
 designed to work well together, and they share a mechanism for
 administrative policy override.
 In this implementation architecture, applications use APIs such as
 getaddrinfo() [RFC3493] that return a list of addresses to the
 application.  This list might contain both IPv6 and IPv4 addresses
 (sometimes represented as IPv4-mapped addresses).  The application
 then passes a destination address to the network stack with connect()
 or sendto().  The application would then typically try the first
 address in the list, looping over the list of addresses until it
 finds a working address.  In any case, the network layer is never in
 a situation where it needs to choose a destination address from
 several alternatives.  The application might also specify a source

Thaler, et al. Standards Track [Page 4] RFC 6724 Default Address Selection for IPv6 September 2012

 address with bind(), but often the source address is left
 unspecified.  Therefore, the network layer does often choose a source
 address from several alternatives.
 As a consequence, we intend that implementations of APIs such as
 getaddrinfo() will use the destination address selection algorithm
 specified here to sort the list of IPv6 and IPv4 addresses that they
 return.  Separately, the IPv6 network layer will use the source
 address selection algorithm when an application or upper layer has
 not specified a source address.  Application of this specification to
 source address selection in an IPv4 network layer might be possible,
 but this is not explored further here.
 Well-behaved applications SHOULD NOT simply use the first address
 returned from an API such as getaddrinfo() and then give up if it
 fails.  For many applications, it is appropriate to iterate through
 the list of addresses returned from getaddrinfo() until a working
 address is found.  For other applications, it might be appropriate to
 try multiple addresses in parallel (e.g., with some small delay in
 between) and use the first one to succeed.
 Although source and destination address selection is most typically
 done when initiating communication, a responder also must deal with
 address selection.  In many cases, this is trivially dealt with by an
 application using the source address of a received packet as the
 response destination and the destination address of the received
 packet as the response source.  Other cases, however, are handled
 like an initiator, such as when the request is multicast and hence
 source address selection must still occur when generating a response
 or when the request includes a list of the initiator's addresses from
 which to choose a destination.  Finally, a third application scenario
 is that of a listening application choosing on what local addresses
 to listen.  This third scenario is out of the scope of this document.
 The algorithms use several criteria in making their decisions.  The
 combined effect is to prefer destination/source address pairs for
 which the two addresses are of equal scope or type, prefer smaller
 scopes over larger scopes for the destination address, prefer non-
 deprecated source addresses, avoid the use of transitional addresses
 when native addresses are available, and all else being equal, prefer
 address pairs having the longest possible common prefix.  For source
 address selection, temporary addresses [RFC4941] are preferred over
 public addresses.  In mobile situations [RFC6275], home addresses are
 preferred over care-of addresses.  If an address is simultaneously a
 home address and a care-of address (indicating the mobile node is "at
 home" for that address), then the home/care-of address is preferred
 over addresses that are solely a home address or solely a care-of
 address.

Thaler, et al. Standards Track [Page 5] RFC 6724 Default Address Selection for IPv6 September 2012

 This specification optionally allows for the possibility of
 administrative configuration of policy (e.g., via manual
 configuration or a DHCP option such as that proposed in
 [ADDR-SEL-OPT]) that can override the default behavior of the
 algorithms.  The policy override consists of the following set of
 state, which SHOULD be configurable:
 o  Policy Table (Section 2.1): a table that specifies precedence
    values and preferred source prefixes for destination prefixes.
 o  Automatic Row Additions flag (Section 2.1): a flag that specifies
    whether the implementation is permitted to automatically add site-
    specific rows for certain types of addresses.
 o  Privacy Preference flag (Section 5): a flag that specifies whether
    temporary source addresses or stable source addresses are
    preferred by default when both types exist.

2.1. Policy Table

 The policy table is a longest-matching-prefix lookup table, much like
 a routing table.  Given an address A, a lookup in the policy table
 produces two values: a precedence value denoted Precedence(A) and a
 classification or label denoted Label(A).
 The precedence value Precedence(A) is used for sorting destination
 addresses.  If Precedence(A) > Precedence(B), we say that address A
 has higher precedence than address B, meaning that our algorithm will
 prefer to sort destination address A before destination address B.
 The label value Label(A) allows for policies that prefer a particular
 source address prefix for use with a destination address prefix.  The
 algorithms prefer to use a source address S with a destination
 address D if Label(S) = Label(D).
 IPv6 implementations SHOULD support configurable address selection
 via a mechanism at least as powerful as the policy tables defined
 here.  It is important that implementations provide a way to change
 the default policies as more experience is gained.  Sections 10.3
 through 10.7 provide examples of the kind of changes that might be
 needed.
 If an implementation is not configurable or has not been configured,
 then it SHOULD operate according to the algorithms specified here in
 conjunction with the following default policy table:

Thaler, et al. Standards Track [Page 6] RFC 6724 Default Address Selection for IPv6 September 2012

    Prefix        Precedence Label
    ::1/128               50     0
    ::/0                  40     1
    ::ffff:0:0/96         35     4
    2002::/16             30     2
    2001::/32              5     5
    fc00::/7               3    13
    ::/96                  1     3
    fec0::/10              1    11
    3ffe::/16              1    12
 An implementation MAY automatically add additional site-specific rows
 to the default table based on its configured addresses, such as for
 Unique Local Addresses (ULAs) [RFC4193] and 6to4 [RFC3056] addresses,
 for instance (see Sections 10.6 and 10.7 for examples).  Any such
 rows automatically added by the implementation as a result of address
 acquisition MUST NOT override a row for the same prefix configured
 via other means.  That is, rows can be added but never updated
 automatically.  An implementation SHOULD provide a means (the
 Automatic Row Additions flag) for an administrator to disable
 automatic row additions.
 As will become apparent later, one effect of the default policy table
 is to prefer using native source addresses with native destination
 addresses, 6to4 source addresses with 6to4 destination addresses,
 etc.  Another effect of the default policy table is to prefer
 communication using IPv6 addresses to communication using IPv4
 addresses, if matching source addresses are available.
 Policy table entries for address prefixes that are not of global
 scope MAY be qualified with an optional zone index.  If so, a prefix
 table entry only matches against an address during a lookup if the
 zone index also matches the address's zone index.

2.2. Common Prefix Length

 We define the common prefix length CommonPrefixLen(S, D) of a source
 address S and a destination address D as the length of the longest
 prefix (looking at the most significant, or leftmost, bits) that the
 two addresses have in common, up to the length of S's prefix (i.e.,
 the portion of the address not including the interface ID).  For
 example, CommonPrefixLen(fe80::1, fe80::2) is 64.

3. Address Properties

 In the rules given in later sections, addresses of different types
 (e.g., IPv4, IPv6, multicast, and unicast) are compared against each
 other.  Some of these address types have properties that aren't

Thaler, et al. Standards Track [Page 7] RFC 6724 Default Address Selection for IPv6 September 2012

 directly comparable to each other.  For example, IPv6 unicast
 addresses can be "preferred" or "deprecated" [RFC4862], while IPv4
 addresses have no such notion.  To compare such addresses using the
 ordering rules (e.g., to use "preferred" addresses in preference to
 "deprecated" addresses), the following mappings are defined.

3.1. Scope Comparisons

 Multicast destination addresses have a 4-bit scope field that
 controls the propagation of the multicast packet.  The IPv6
 addressing architecture defines scope field values for interface-
 local (0x1), link-local (0x2), admin-local (0x4), site-local (0x5),
 organization-local (0x8), and global (0xE) scopes (Section 2.7 of
 [RFC4291]).
 Use of the source address selection algorithm in the presence of
 multicast destination addresses requires the comparison of a unicast
 address scope with a multicast address scope.  We map unicast link-
 local to multicast link-local, unicast site-local to multicast site-
 local, and unicast global scope to multicast global scope.  For
 example, unicast site-local is equal to multicast site-local, which
 is smaller than multicast organization-local, which is smaller than
 unicast global, which is equal to multicast global.  (Note that IPv6
 site-local unicast addresses are deprecated [RFC4291].  However, some
 existing implementations and deployments may still use these
 addresses; they are therefore included in the procedures in this
 specification.  Also, note that ULAs are considered as global, not
 site-local, scope but are handled via the prefix policy table as
 discussed in Section 10.6.)
 We write Scope(A) to mean the scope of address A.  For example, if A
 is a link-local unicast address and B is a site-local multicast
 address, then Scope(A) < Scope(B).
 This mapping implicitly conflates unicast site boundaries and
 multicast site boundaries [RFC4007].

3.2. IPv4 Addresses and IPv4-Mapped Addresses

 The destination address selection algorithm operates on both IPv6 and
 IPv4 addresses.  For this purpose, IPv4 addresses MUST be represented
 as IPv4-mapped addresses [RFC4291].  For example, to look up the
 precedence or other attributes of an IPv4 address in the policy
 table, look up the corresponding IPv4-mapped IPv6 address.
 IPv4 addresses are assigned scopes as follows.  IPv4 auto-
 configuration addresses [RFC3927], which have the prefix 169.254/16,
 are assigned link-local scope.  IPv4 loopback addresses (Section

Thaler, et al. Standards Track [Page 8] RFC 6724 Default Address Selection for IPv6 September 2012

 4.2.2.11 of [RFC1812]), which have the prefix 127/8, are assigned
 link-local scope (analogously to the treatment of the IPv6 loopback
 address (Section 4 of [RFC4007])).  Other IPv4 addresses (including
 IPv4 private addresses [RFC1918] and Shared Address Space addresses
 [RFC6598]) are assigned global scope.
 IPv4 addresses MUST be treated as having "preferred" (in the RFC 4862
 sense) configuration status.

3.3. Other IPv6 Addresses with Embedded IPv4 Addresses

 IPv4-compatible addresses [RFC4291], IPv4-mapped [RFC4291], IPv4-
 converted [RFC6145], IPv4-translatable [RFC6145], and 6to4 addresses
 [RFC3056] contain an embedded IPv4 address.  For the purposes of this
 document, these addresses MUST be treated as having global scope.
 IPv4-compatible, IPv4-mapped, and IPv4-converted addresses MUST be
 treated as having "preferred" (in the RFC 4862 sense) configuration
 status.

3.4. IPv6 Loopback Address and Other Format Prefixes

 The loopback address MUST be treated as having link-local scope
 (Section 4 of [RFC4007]) and "preferred" (in the RFC 4862 sense)
 configuration status.
 NSAP addresses and other addresses with as-yet-undefined format
 prefixes MUST be treated as having global scope and "preferred" (in
 the RFC 4862) configuration status.  Later standards might supersede
 this treatment.

3.5. Mobility Addresses

 Some nodes might support mobility using the concepts of home address
 and care-of address (for example, see [RFC6275]).  Conceptually, a
 home address is an IP address assigned to a mobile node and used as
 the permanent address of the mobile node.  A care-of address is an IP
 address associated with a mobile node while visiting a foreign link.
 When a mobile node is on its home link, it might have an address that
 is simultaneously a home address and a care-of address.
 For the purposes of this document, it is sufficient to know whether
 one's own addresses are designated as home addresses or care-of
 addresses.  Whether an address ought to be designated a home address
 or care-of address is outside the scope of this document.

Thaler, et al. Standards Track [Page 9] RFC 6724 Default Address Selection for IPv6 September 2012

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.

Thaler, et al. Standards Track [Page 10] RFC 6724 Default Address Selection for IPv6 September 2012

 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.

5. Source Address Selection

 The source address selection algorithm produces as output a single
 source address for use with a given destination address.  This
 algorithm only applies to IPv6 destination addresses, not IPv4
 addresses.
 The algorithm is specified here in terms of a list of pair-wise
 comparison rules that (for a given destination address D) imposes a
 "greater than" ordering on the addresses in the candidate set
 CandidateSource(D).  The address at the front of the list after the
 algorithm completes is the one the algorithm selects.
 Note that conceptually, a sort of the candidate set is being
 performed, where a set of rules define the ordering among addresses.
 But because the output of the algorithm is a single source address,
 an implementation need not actually sort the set; it need only
 identify the "maximum" value that ends up at the front of the sorted
 list.
 The ordering of the addresses in the candidate set is defined by a
 list of eight pair-wise comparison rules, with each rule placing a
 "greater than", "less than", or "equal to" ordering on two source
 addresses with respect to each other (and that rule).  In the case
 that a given rule produces a tie, i.e., provides an "equal to" result
 for the two addresses, the remaining rules MUST be applied (in order)
 to just those addresses that are tied to break the tie.  Note that if
 a rule produces a single clear "winner" (or set of "winners" in the
 case of ties), those addresses not in the winning set can be
 discarded from further consideration, with subsequent rules applied
 only to the remaining addresses.  If the eight rules fail to choose a
 single address, the tiebreaker is implementation-specific.

Thaler, et al. Standards Track [Page 11] RFC 6724 Default Address Selection for IPv6 September 2012

 When comparing two addresses SA and SB from the candidate set, we say
 "prefer SA" to mean that SA is "greater than" SB, and similarly, we
 say "prefer SB" to mean that SA is "less than" SB.  If neither is
 stated to be preferred, this means that SA is "equal to" SB, and the
 remaining rules apply as noted above.
 Rule 1: Prefer same address.
 If SA = D, then prefer SA.  Similarly, if SB = D, then prefer SB.
 Rule 2: Prefer appropriate scope.
 If Scope(SA) < Scope(SB): If Scope(SA) < Scope(D), then prefer SB and
 otherwise prefer SA.  Similarly, if Scope(SB) < Scope(SA): If
 Scope(SB) < Scope(D), then prefer SA and otherwise prefer SB.
    Discussion: This rule must be given high priority because it can
    affect interoperability.
 Rule 3: Avoid deprecated addresses.
 If one of the two source addresses is "preferred" and one of them is
 "deprecated" (in the RFC 4862 sense), then prefer the one that is
 "preferred".
 Rule 4: Prefer home addresses.
 If SA is simultaneously a home address and care-of address and SB is
 not, then prefer SA.  Similarly, if SB is simultaneously a home
 address and care-of address and SA is not, then prefer SB.  If SA is
 just a home address and SB is just a care-of address, then prefer SA.
 Similarly, if SB is just a home address and SA is just a care-of
 address, then prefer SB.
 Implementations supporting home addresses MUST provide a mechanism
 allowing an application to reverse the sense of this preference and
 prefer care-of addresses over home addresses (e.g., via appropriate
 API extensions such as [RFC5014]).  Use of the mechanism MUST only
 affect the selection rules for the invoking application.
 Rule 5: Prefer outgoing interface.
 If SA is assigned to the interface that will be used to send to D and
 SB is assigned to a different interface, then prefer SA.  Similarly,
 if SB is assigned to the interface that will be used to send to D and
 SA is assigned to a different interface, then prefer SB.
 Rule 5.5: Prefer addresses in a prefix advertised by the next-hop.
 If SA or SA's prefix is assigned by the selected next-hop that will
 be used to send to D and SB or SB's prefix is assigned by a different
 next-hop, then prefer SA.  Similarly, if SB or SB's prefix is
 assigned by the next-hop that will be used to send to D and SA or
 SA's prefix is assigned by a different next-hop, then prefer SB.

Thaler, et al. Standards Track [Page 12] RFC 6724 Default Address Selection for IPv6 September 2012

    Discussion: An IPv6 implementation is not required to remember
    which next-hops advertised which prefixes.  The conceptual models
    of IPv6 hosts in Section 5 of [RFC4861] and Section 3 of [RFC4191]
    have no such requirement.  Hence, Rule 5.5 is only applicable to
    implementations that track this information.
 Rule 6: Prefer matching label.
 If Label(SA) = Label(D) and Label(SB) <> Label(D), then prefer SA.
 Similarly, if Label(SB) = Label(D) and Label(SA) <> Label(D), then
 prefer SB.
 Rule 7: Prefer temporary addresses.
 If SA is a temporary address and SB is a public address, then prefer
 SA.  Similarly, if SB is a temporary address and SA is a public
 address, then prefer SB.
 Implementations MUST provide a mechanism allowing an application to
 reverse the sense of this preference and prefer public addresses over
 temporary addresses (e.g., via appropriate API extensions such as
 [RFC5014]).  Use of the mechanism MUST only affect the selection
 rules for the invoking application.  This default is intended to
 address privacy concerns as discussed in [RFC4941] but introduces a
 risk of applications potentially failing due to the relatively short
 lifetime of temporary addresses or due to the possibility of the
 reverse lookup of a temporary address either failing or returning a
 randomized name.  Implementations for which application compatibility
 considerations outweigh these privacy concerns MAY reverse the sense
 of this rule and by default prefer public addresses over temporary
 addresses.  There SHOULD be an administrative option (the Privacy
 Preference flag) to change this preference, if the implementation
 supports temporary addresses.  If there is no such option, there MUST
 be an administrative option to disable temporary addresses.
 Rule 8: Use longest matching prefix.
 If CommonPrefixLen(SA, D) > CommonPrefixLen(SB, D), then prefer SA.
 Similarly, if CommonPrefixLen(SB, D) > CommonPrefixLen(SA, D), then
 prefer SB.
 Rule 8 MAY be superseded if the implementation has other means of
 choosing among source addresses.  For example, if the implementation
 somehow knows which source address will result in the "best"
 communications performance.

Thaler, et al. Standards Track [Page 13] RFC 6724 Default Address Selection for IPv6 September 2012

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.

Thaler, et al. Standards Track [Page 14] RFC 6724 Default Address Selection for IPv6 September 2012

 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.

Thaler, et al. Standards Track [Page 15] RFC 6724 Default Address Selection for IPv6 September 2012

 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.

7. Interactions with Routing

 This specification of source address selection assumes that routing
 (more precisely, selecting an outgoing interface on a node with
 multiple interfaces) is done before source address selection.
 However, implementations MAY use source address considerations as a
 tiebreaker when choosing among otherwise equivalent routes.
 For example, suppose a node has interfaces on two different links,
 with both links having a working default router.  Both of the
 interfaces have preferred (in the RFC 4862 sense) global addresses.
 When sending to a global destination address, if there's no routing
 reason to prefer one interface over the other, then an implementation
 MAY preferentially choose the outgoing interface that will allow it
 to use the source address that shares a longer common prefix with the
 destination.
 Implementations that support Rule 5.5 of source address selection
 (Section 5) also use the choice of router to influence the choice of
 source address.  For example, suppose a host is on a link with two
 routers.  One router is advertising a global prefix A and the other
 router is advertising global prefix B.  Then, when sending via the
 first router, the host might prefer source addresses with prefix A
 and when sending via the second router, prefer source addresses with
 prefix B.

8. Implementation Considerations

 The destination address selection algorithm needs information about
 potential source addresses.  One possible implementation strategy is
 for getaddrinfo() to call down to the network layer with a list of
 destination addresses, sort the list in the network layer with full

Thaler, et al. Standards Track [Page 16] RFC 6724 Default Address Selection for IPv6 September 2012

 current knowledge of available source addresses, and return the
 sorted list to getaddrinfo().  This is simple and gives the best
 results, but it introduces the overhead of another system call.  One
 way to reduce this overhead is to cache the sorted address list in
 the resolver, so that subsequent calls for the same name do not need
 to re-sort the list.
 Another implementation strategy is to call down to the network layer
 to retrieve source address information and then sort the list of
 addresses directly in the context of getaddrinfo().  To reduce
 overhead in this approach, the source address information can be
 cached, amortizing the overhead of retrieving it across multiple
 calls to getaddrinfo().  In this approach, the implementation might
 not have knowledge of the outgoing interface for each destination, so
 it MAY use a looser definition of the candidate set during
 destination address ordering.
 In any case, if the implementation uses cached and possibly stale
 information in its implementation of destination address selection or
 if the ordering of a cached list of destination addresses is possibly
 stale, then it MUST ensure that the destination address ordering
 returned to the application is no more than one second out of date.
 For example, an implementation might make a system call to check if
 any routing table entries, source address assignments, or prefix
 policy table entries that might affect these algorithms have changed.
 Another strategy is to use an invalidation counter that is
 incremented whenever any underlying state is changed.  By caching the
 current invalidation counter value with derived state and then later
 comparing against the current value, the implementation could detect
 if the derived state is potentially stale.

9. Security Considerations

 This document has no direct impact on Internet infrastructure
 security.
 Note that most source address selection algorithms, including the one
 specified in this document, expose a potential privacy concern.  An
 unfriendly node can infer correlations among a target node's
 addresses by probing the target node with request packets that force
 the target host to choose its source address for the reply packets
 (perhaps because the request packets are sent to an anycast or
 multicast address or perhaps because the upper-layer protocol chosen
 for the attack does not specify a particular source address for its
 reply packets).  By using different addresses for itself, the
 unfriendly node can cause the target node to expose the target's own
 addresses.  The source address selection default preference for
 temporary addresses helps mitigate this concern.

Thaler, et al. Standards Track [Page 17] RFC 6724 Default Address Selection for IPv6 September 2012

 Similarly, most source and destination address selection algorithms,
 including the one specified in this document, influence the choice of
 network path taken (as do routing algorithms that are orthogonal to,
 but used together with, such algorithms) and hence whether data might
 be sent over a path or network that might be more or less trusted
 than other paths or networks.  Administrators should consider the
 security impact of the rows they configure in the prefix policy
 table, just as they should consider the security impact of the
 interface metrics used in the routing algorithms.
 In addition, some address selection rules might be administratively
 configurable.  Care must be taken to make sure that all
 administrative options are secured against illicit modification, or
 else an attacker could redirect and/or block traffic.

10. Examples

 This section contains a number of examples, first showing default
 behavior and then demonstrating the utility of policy table
 configuration.  These examples are provided for illustrative
 purposes; they are not to be construed as normative.

10.1. Default Source Address Selection

 The source address selection rules, in conjunction with the default
 policy table, produce the following behavior:
 Destination: 2001:db8:1::1
 Candidate Source Addresses: 2001:db8:3::1 or fe80::1
 Result: 2001:db8::1 (prefer appropriate scope)
 Destination: ff05::1
 Candidate Source Addresses: 2001:db8:3::1 or fe80::1
 Result: 2001:db8:3::1 (prefer appropriate scope)
 Destination: 2001:db8:1::1
 Candidate Source Addresses: 2001:db8:1::1 (deprecated) or
 2001:db8:2::1
 Result: 2001:db8:1::1 (prefer same address)
 Destination: fe80::1
 Candidate Source Addresses: fe80::2 (deprecated) or 2001:db8:1::1
 Result: fe80::2 (prefer appropriate scope)
 Destination: 2001:db8:1::1
 Candidate Source Addresses: 2001:db8:1::2 or 2001:db8:3::2
 Result: 2001:db8:1:::2 (longest matching prefix)

Thaler, et al. Standards Track [Page 18] RFC 6724 Default Address Selection for IPv6 September 2012

 Destination: 2001:db8:1::1
 Candidate Source Addresses: 2001:db8:1::2 (care-of address) or 2001:
 db8:3::2 (home address)
 Result: 2001:db8:3::2 (prefer home address)
 Destination: 2002:c633:6401::1
 Candidate Source Addresses: 2002:c633:6401::d5e3:7953:13eb:22e8
 (temporary) or 2001:db8:1::2
 Result: 2002:c633:6401::d5e3:7953:13eb:22e8 (prefer matching label)
 Destination: 2001:db8:1::d5e3:0:0:1
 Candidate Source Addresses: 2001:db8:1::2 (public) or
 2001:db8:1::d5e3:7953:13eb:22e8 (temporary)
 Result: 2001:db8:1::d5e3:7953:13eb:22e8 (prefer temporary address)

10.2. Default Destination Address Selection

 The destination address selection rules, in conjunction with the
 default policy table and the source address selection rules, produce
 the following behavior:
 Candidate Source Addresses: 2001:db8:1::2 or fe80::1 or 169.254.13.78
 Destination Address List: 2001:db8:1::1 or 198.51.100.121
 Result: 2001:db8:1::1 (src 2001:db8:1::2) then 198.51.100.121 (src
 169.254.13.78) (prefer matching scope)
 Candidate Source Addresses: fe80::1 or 198.51.100.117
 Destination Address List: 2001:db8:1::1 or 198.51.100.121
 Result: 198.51.100.121 (src 198.51.100.117) then 2001:db8:1::1 (src
 fe80::1) (prefer matching scope)
 Candidate Source Addresses: 2001:db8:1::2 or fe80::1 or 10.1.2.4
 Destination Address List: 2001:db8:1::1 or 10.1.2.3
 Result: 2001:db8:1::1 (src 2001:db8:1::2) then 10.1.2.3 (src
 10.1.2.4) (prefer higher precedence)
 Candidate Source Addresses: 2001:db8:1::2 or fe80::2
 Destination Address List: 2001:db8:1::1 or fe80::1
 Result: fe80::1 (src fe80::2) then 2001:db8:1::1 (src 2001:db8:1::2)
 (prefer smaller scope)
 Candidate Source Addresses: 2001:db8:1::2 (care-of address) or 2001:
 db8:3::1 (home address) or fe80::2 (care-of address)
 Destination Address List: 2001:db8:1::1 or fe80::1
 Result: 2001:db8:1::1 (src 2001:db8:3::1) then fe80::1 (src fe80::2)
 (prefer home address)

Thaler, et al. Standards Track [Page 19] RFC 6724 Default Address Selection for IPv6 September 2012

 Candidate Source Addresses: 2001:db8:1::2 or fe80::2 (deprecated)
 Destination Address List: 2001:db8:1::1 or fe80::1
 Result: 2001:db8:1::1 (src 2001:db8:1::2) then fe80::1 (src fe80::2)
 (avoid deprecated addresses)
 Candidate Source Addresses: 2001:db8:1::2 or 2001:db8:3f44::2 or
 fe80::2
 Destination Address List: 2001:db8:1::1 or 2001:db8:3ffe::1
 Result: 2001:db8:1::1 (src 2001:db8:1::2) then 2001:db8:3ffe::1 (src
 2001:db8:3f44::2) (longest matching prefix)
 Candidate Source Addresses: 2002:c633:6401::2 or fe80::2
 Destination Address List: 2002:c633:6401::1 or 2001:db8:1::1
 Result: 2002:c633:6401::1 (src 2002:c633:6401::2) then 2001:db8:1::1
 (src 2002:c633:6401::2) (prefer matching label)
 Candidate Source Addresses: 2002:c633:6401::2 or 2001:db8:1::2 or
 fe80::2
 Destination Address List: 2002:c633:6401::1 or 2001:db8:1::1
 Result: 2001:db8:1::1 (src 2001:db8:1::2) then 2002:c633:6401::1 (src
 2002:c633:6401::2) (prefer higher precedence)

10.3. Configuring Preference for IPv6 or IPv4

 The default policy table gives IPv6 addresses higher precedence than
 IPv4 addresses.  This means that applications will use IPv6 in
 preference to IPv4 when the two are equally suitable.  An
 administrator can change the policy table to prefer IPv4 addresses by
 giving the ::ffff:0.0.0.0/96 prefix a higher precedence:
    Prefix        Precedence Label
    ::1/128               50     0
    ::/0                  40     1
    ::ffff:0:0/96        100     4
    2002::/16             30     2
    2001::/32              5     5
    fc00::/7               3    13
    ::/96                  1     3
    fec0::/10              1    11
    3ffe::/16              1    12
 This change to the default policy table produces the following
 behavior:
 Candidate Source Addresses: 2001:db8::2 or fe80::1 or 169.254.13.78
 Destination Address List: 2001:db8::1 or 198.51.100.121
 Unchanged Result: 2001:db8::1 (src 2001:db8::2) then 198.51.100.121
 (src 169.254.13.78) (prefer matching scope)

Thaler, et al. Standards Track [Page 20] RFC 6724 Default Address Selection for IPv6 September 2012

 Candidate Source Addresses: fe80::1 or 198.51.100.117
 Destination Address List: 2001:db8::1 or 198.51.100.121
 Unchanged Result: 198.51.100.121 (src 198.51.100.117) then
 2001:db8::1 (src fe80::1) (prefer matching scope)
 Candidate Source Addresses: 2001:db8::2 or fe80::1 or 10.1.2.4
 Destination Address List: 2001:db8::1 or 10.1.2.3
 New Result: 10.1.2.3 (src 10.1.2.4) then 2001:db8::1 (src
 2001:db8::2) (prefer higher precedence)

10.3.1. Handling Broken IPv6

 One problem in practice that has been recently observed occurs when a
 host has IPv4 connectivity to the Internet but has "broken" IPv6
 connectivity to the Internet in that it has a global IPv6 address but
 is disconnected from the IPv6 Internet.  Since the default policy
 table prefers IPv6, this can result in unwanted timeouts.
 This can be solved by configuring the table to prefer IPv4 as shown
 above.  An implementation that has some means to detect that it is
 not connected to the IPv6 Internet MAY do this automatically.  An
 implementation could instead treat it as part of its implementation
 of Rule 1 (avoid unusable destinations).

10.4. Configuring Preference for Link-Local Addresses

 The destination address selection rules give preference to
 destinations of smaller scope.  For example, a link-local destination
 will be sorted before a global scope destination when the two are
 otherwise equally suitable.  An administrator can change the policy
 table to reverse this preference and sort global destinations before
 link-local destinations:
    Prefix        Precedence Label
    ::1/128               50     0
    ::/0                  40     1
    ::ffff:0:0/96         35     4
    fe80::/10             33     1
    2002::/16             30     2
    2001::/32              5     5
    fc00::/7               3    13
    ::/96                  1     3
    fec0::/10              1    11
    3ffe::/16              1    12

Thaler, et al. Standards Track [Page 21] RFC 6724 Default Address Selection for IPv6 September 2012

 This change to the default policy table produces the following
 behavior:
 Candidate Source Addresses: 2001:db8::2 or fe80::2
 Destination Address List: 2001:db8::1 or fe80::1
 New Result: 2001:db8::1 (src 2001:db8::2) then fe80::1 (src fe80::2)
 (prefer higher precedence)
 Candidate Source Addresses: 2001:db8::2 (deprecated) or fe80::2
 Destination Address List: 2001:db8::1 or fe80::1
 Unchanged Result: fe80::1 (src fe80::2) then 2001:db8::1 (src 2001:
 db8::2) (avoid deprecated addresses)

10.5. Configuring a Multi-Homed Site

 Consider a site A that has a business-critical relationship with
 another site B.  To support their business needs, the two sites have
 contracted for service with a special high-performance ISP.  This is
 in addition to the normal Internet connection that both sites have
 with different ISPs.  The high-performance ISP is expensive, and the
 two sites wish to use it only for their business-critical traffic
 with each other.
 Each site has two global prefixes, one from the high-performance ISP
 and one from their normal ISP.  Site A has prefix 2001:db8:1aaa::/48
 from the high-performance ISP and prefix 2001:db8:70aa::/48 from its
 normal ISP.  Site B has prefix 2001:db8:1bbb::/48 from the high-
 performance ISP and prefix 2001:db8:70bb::/48 from its normal ISP.
 All hosts in both sites register two addresses in the DNS.
 The routing within both sites directs most traffic to the egress to
 the normal ISP, but the routing directs traffic sent to the other
 site's 2001 prefix to the egress to the high-performance ISP.  To
 prevent unintended use of their high-performance ISP connection, the
 two sites implement ingress filtering to discard traffic entering
 from the high-performance ISP that is not from the other site.
 The default policy table and address selection rules produce the
 following behavior:
 Candidate Source Addresses: 2001:db8:1aaa::a or 2001:db8:70aa::a or
 fe80::a
 Destination Address List: 2001:db8:1bbb::b or 2001:db8:70bb::b
 Result: 2001:db8:70bb::b (src 2001:db8:70aa::a) then 2001:db8:1bbb::b
 (src 2001:db8:1aaa::a) (longest matching prefix)

Thaler, et al. Standards Track [Page 22] RFC 6724 Default Address Selection for IPv6 September 2012

 In other words, when a host in site A initiates a connection to a
 host in site B, the traffic does not take advantage of their
 connections to the high-performance ISP.  This is not their desired
 behavior.
 Candidate Source Addresses: 2001:db8:1aaa::a or 2001:db8:70aa::a or
 fe80::a
 Destination Address List: 2001:db8:1ccc::c or 2001:db8:6ccc::c
 Result: 2001:db8:1ccc::c (src 2001:db8:1aaa::a) then 2001:db8:6ccc::c
 (src 2001:db8:70aa::a) (longest matching prefix)
 In other words, when a host in site A initiates a connection to a
 host in some other site C, the reverse traffic might come back
 through the high-performance ISP.  Again, this is not their desired
 behavior.
 This predicament demonstrates the limitations of the longest-
 matching-prefix heuristic in multi-homed situations.
 However, the administrators of sites A and B can achieve their
 desired behavior via policy table configuration.  For example, they
 can use the following policy table:
    Prefix        Precedence Label
    ::1/128               50     0
    2001:db8:1aaa::/48    43     6
    2001:db8:1bbb::/48    43     6
    ::/0                  40     1
    ::ffff:0:0/96         35     4
    2002::/16             30     2
    2001::/32              5     5
    fc00::/7               3    13
    ::/96                  1     3
    fec0::/10              1    11
    3ffe::/16              1    12
 This policy table produces the following behavior:
 Candidate Source Addresses: 2001:db8:1aaa::a or 2001:db8:70aa::a or
 fe80::a
 Destination Address List: 2001:db8:1bbb::b or 2001:db8:70bb::b
 New Result: 2001:db8:1bbb::b (src 2001:db8:1aaa::a) then 2001:db8:
 70bb::b (src 2001:db8:70aa::a) (prefer higher precedence)
 In other words, when a host in site A initiates a connection to a
 host in site B, the traffic uses the high-performance ISP as desired.

Thaler, et al. Standards Track [Page 23] RFC 6724 Default Address Selection for IPv6 September 2012

 Candidate Source Addresses: 2001:db8:1aaa::a or 2001:db8:70aa::a or
 fe80::a
 Destination Address List: 2001:db8:1ccc::c or 2001:db8:6ccc::c
 New Result: 2001:db8:6ccc::c (src 2001:db8:70aa::a) then 2001:db8:
 1ccc::c (src 2001:db8:70aa::a) (longest matching prefix)
 In other words, when a host in site A initiates a connection to a
 host in some other site C, the traffic uses the normal ISP as
 desired.

10.6. Configuring ULA Preference

 Sections 2.1.4, 2.2.2, and 2.2.3 of RFC 5220 [RFC5220] describe
 address selection problems related to Unique Local Addresses (ULAs)
 [RFC4193].  By default, global IPv6 destinations are preferred over
 ULA destinations, since an arbitrary ULA is not necessarily
 reachable:
 Candidate Source Addresses: 2001:db8:1::1 or fd11:1111:1111:1::1
 Destination Address List: 2001:db8:2::2 or fd22:2222:2222:2::2
 Result: 2001:db8:2::2 (src 2001:db8:1::1) then fd22:2222:2222:2::2
 (src fd11:1111:1111:1::1) (prefer higher precedence)
 However, a site-specific policy entry can be used to cause ULAs
 within a site to be preferred over global addresses as follows.
    Prefix        Precedence Label
    ::1/128               50     0
    fd11:1111:1111::/48   45    14
    ::/0                  40     1
    ::ffff:0:0/96         35     4
    2002::/16             30     2
    2001::/32              5     5
    fc00::/7               3    13
    ::/96                  1     3
    fec0::/10              1    11
    3ffe::/16              1    12
 Such a configuration would have the following effect:
 Candidate Source Addresses: 2001:db8:1::1 or fd11:1111:1111:1::1
 Destination Address List: 2001:db8:2::2 or fd22:2222:2222:2::2
 Unchanged Result: 2001:db8:2::2 (src 2001:db8:1::1) then fd22:2222:
 2222:2::2 (src fd11:1111:1111:1::1) (prefer higher precedence)

Thaler, et al. Standards Track [Page 24] RFC 6724 Default Address Selection for IPv6 September 2012

 Candidate Source Addresses: 2001:db8:1::1 or fd11:1111:1111:1::1
 Destination Address List: 2001:db8:2::2 or fd11:1111:1111:2::2
 New Result: fd11:1111:1111:2::2 (src fd11:1111:1111:1::1) then 2001:
 db8:2::2 (src 2001:db8:1::1) (prefer higher precedence)
 Since ULAs are defined to have a /48 site prefix, an implementation
 might choose to add such a row automatically on a machine with a ULA.
 It is also worth noting that ULAs are assigned global scope.  As
 such, the existence of one or more rows in the prefix policy table is
 important so that source address selection does not choose a ULA
 purely based on longest match:
 Candidate Source Addresses: 2001:db8:1::1 or fd11:1111:1111:1::1
 Destination Address List: ff00:1
 Result: 2001:db8:1::1 (prefer matching label)

10.7. Configuring 6to4 Preference

 By default, NATed IPv4 is preferred over 6to4-relayed connectivity:
 Candidate Source Addresses: 2002:c633:6401::2 or 10.1.2.3
 Destination Address List: 2001:db8:1::1 or 203.0.113.1
 Result: 203.0.113.1 (src 10.1.2.3) then 2001:db8:1::1 (src 2002:c633:
 6401::2) (prefer matching label)
 However, NATed IPv4 is now also preferred over 6to4-to-6to4
 connectivity by default.  Since a 6to4 prefix might be used natively
 within an organization, a site-specific policy entry can be used to
 cause native IPv6 communication (using a 6to4 prefix) to be preferred
 over NATed IPv4 as follows.
    Prefix        Precedence Label
    ::1/128               50     0
    2002:c633:6401::/48   45    14
    ::/0                  40     1
    ::ffff:0:0/96         35     4
    2002::/16             30     2
    2001::/32              5     5
    fc00::/7               3    13
    ::/96                  1     3
    fec0::/10              1    11
    3ffe::/16              1    12

Thaler, et al. Standards Track [Page 25] RFC 6724 Default Address Selection for IPv6 September 2012

 Such a configuration would have the following effect:
 Candidate Source Addresses: 2002:c633:6401:1::1 or 10.1.2.3
 Destination Address List: 2002:c633:6401:2::2 or 203.0.113.1
 New Result: 2002:c633:6401:2::2 (src 2002:c633:6401:1::1) then
 203.0.113.1 (sec 10.1.2.3) (prefer higher precedence)
 Since 6to4 addresses are defined to have a /48 site prefix, an
 implementation might choose to add such a row automatically on a
 machine with a native IPv6 address with a 6to4 prefix.

11. References

11.1. Normative References

 [RFC2119]       Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3056]       Carpenter, B. and K. Moore, "Connection of IPv6
                 Domains via IPv4 Clouds", RFC 3056, February 2001.
 [RFC3879]       Huitema, C. and B. Carpenter, "Deprecating Site Local
                 Addresses", RFC 3879, September 2004.
 [RFC4193]       Hinden, R. and B. Haberman, "Unique Local IPv6
                 Unicast Addresses", RFC 4193, October 2005.
 [RFC4291]       Hinden, R. and S. Deering, "IP Version 6 Addressing
                 Architecture", RFC 4291, February 2006.
 [RFC4380]       Huitema, C., "Teredo: Tunneling IPv6 over UDP through
                 Network Address Translations (NATs)", RFC 4380,
                 February 2006.
 [RFC4862]       Thomson, S., Narten, T., and T. Jinmei, "IPv6
                 Stateless Address Autoconfiguration", RFC 4862,
                 September 2007.
 [RFC4941]       Narten, T., Draves, R., and S. Krishnan, "Privacy
                 Extensions for Stateless Address Autoconfiguration in
                 IPv6", RFC 4941, September 2007.
 [RFC6145]       Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
                 Algorithm", RFC 6145, April 2011.

Thaler, et al. Standards Track [Page 26] RFC 6724 Default Address Selection for IPv6 September 2012

11.2. Informative References

 [ADDR-SEL-OPT]  Matsumoto, A., Fujisaki, T., Kato, J., and T. Chown,
                 "Distributing Address Selection Policy using DHCPv6",
                 Work in Progress, August 2012.
 [RFC1794]       Brisco, T., "DNS Support for Load Balancing",
                 RFC 1794, April 1995.
 [RFC1812]       Baker, F., "Requirements for IP Version 4 Routers",
                 RFC 1812, June 1995.
 [RFC1918]       Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot,
                 G., and E. Lear, "Address Allocation for Private
                 Internets", BCP 5, RFC 1918, February 1996.
 [RFC2827]       Ferguson, P. and D. Senie, "Network Ingress
                 Filtering: Defeating Denial of Service Attacks which
                 employ IP Source Address Spoofing", BCP 38, RFC 2827,
                 May 2000.
 [RFC3484]       Draves, R., "Default Address Selection for Internet
                 Protocol version 6 (IPv6)", RFC 3484, February 2003.
 [RFC3493]       Gilligan, R., Thomson, S., Bound, J., McCann, J., and
                 W. Stevens, "Basic Socket Interface Extensions for
                 IPv6", RFC 3493, February 2003.
 [RFC3701]       Fink, R. and R. Hinden, "6bone (IPv6 Testing Address
                 Allocation) Phaseout", RFC 3701, March 2004.
 [RFC3927]       Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
                 Configuration of IPv4 Link-Local Addresses",
                 RFC 3927, May 2005.
 [RFC4007]       Deering, S., Haberman, B., Jinmei, T., Nordmark, E.,
                 and B. Zill, "IPv6 Scoped Address Architecture",
                 RFC 4007, March 2005.
 [RFC4191]       Draves, R. and D. Thaler, "Default Router Preferences
                 and More-Specific Routes", RFC 4191, November 2005.
 [RFC4213]       Nordmark, E. and R. Gilligan, "Basic Transition
                 Mechanisms for IPv6 Hosts and Routers", RFC 4213,
                 October 2005.

Thaler, et al. Standards Track [Page 27] RFC 6724 Default Address Selection for IPv6 September 2012

 [RFC4861]       Narten, T., Nordmark, E., Simpson, W., and H.
                 Soliman, "Neighbor Discovery for IP version 6
                 (IPv6)", RFC 4861, September 2007.
 [RFC5014]       Nordmark, E., Chakrabarti, S., and J. Laganier, "IPv6
                 Socket API for Source Address Selection", RFC 5014,
                 September 2007.
 [RFC5214]       Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
                 Automatic Tunnel Addressing Protocol (ISATAP)",
                 RFC 5214, March 2008.
 [RFC5220]       Matsumoto, A., Fujisaki, T., Hiromi, R., and K.
                 Kanayama, "Problem Statement for Default Address
                 Selection in Multi-Prefix Environments: Operational
                 Issues of RFC 3484 Default Rules", RFC 5220,
                 July 2008.
 [RFC5969]       Townsley, W. and O. Troan, "IPv6 Rapid Deployment on
                 IPv4 Infrastructures (6rd) -- Protocol
                 Specification", RFC 5969, August 2010.
 [RFC6275]       Perkins, C., Johnson, D., and J. Arkko, "Mobility
                 Support in IPv6", RFC 6275, July 2011.
 [RFC6598]       Weil, J., Kuarsingh, V., Donley, C., Liljenstolpe,
                 C., and M. Azinger, "IANA-Reserved IPv4 Prefix for
                 Shared Address Space", BCP 153, RFC 6598, April 2012.

Thaler, et al. Standards Track [Page 28] RFC 6724 Default Address Selection for IPv6 September 2012

Appendix A. Acknowledgements

 RFC 3484 [RFC3484] acknowledged the contributions of the IPng Working
 Group, particularly Marc Blanchet, Brian Carpenter, Matt Crawford,
 Alain Durand, Steve Deering, Robert Elz, Jun-ichiro itojun Hagino,
 Tony Hain, M.T. Hollinger, JINMEI Tatuya, Thomas Narten, Erik
 Nordmark, Ken Powell, Markku Savela, Pekka Savola, Hesham Soliman,
 Dave Thaler, Mauro Tortonesi, Ole Troan, and Stig Venaas.  In
 addition, the anonymous IESG reviewers had many great comments and
 suggestions for clarification.
 This revision was heavily influenced by the work by Arifumi
 Matsumoto, Jun-ya Kato, and Tomohiro Fujisaki in a working document
 that made proposals for this revision to adopt, with input from Pekka
 Savola, Remi Denis-Courmont, Francois-Xavier Le Bail, and the 6man
 Working Group.  Dmitry Anipko, Mark Andrews, Ray Hunter, and Wes
 George also provided valuable feedback on this revision.

Appendix B. Changes since RFC 3484

 Some changes were made to the default policy table that were deemed
 to be universally useful and cause no harm in every reasonable
 network environment.  In doing so, care was taken to use the same
 preference and label values as in RFC 3484 whenever possible and for
 new rows to use label values less likely to collide with values that
 might already be in use in additional rows on some hosts.  These
 changes are:
 1.  Added the Teredo [RFC4380] prefix (2001::/32), with the
     preference and label values already widely used in popular
     implementations.
 2.  Added a row for ULAs (fc00::/7) below native IPv6 since they are
     not globally reachable, as discussed in Section 10.6.
 3.  Added a row for site-local addresses (fec0::/10) in order to
     depreference them, for consistency with the example in
     Section 10.3, since they are deprecated [RFC3879].
 4.  Depreferenced 6to4 (2002::/32) below native IPv4 since 6to4
     connectivity is less reliable today (and is expected to be phased
     out over time, rather than becoming more reliable).  It remains
     above Teredo since 6to4 is more efficient in terms of connection
     establishment time, bandwidth, and server load.
 5.  Depreferenced IPv4-Compatible addresses (::/96) since they are
     now deprecated [RFC4291] and not in common use.

Thaler, et al. Standards Track [Page 29] RFC 6724 Default Address Selection for IPv6 September 2012

 6.  Added a row for 6bone testing addresses (3ffe::/16) in order to
     depreference them as they have also been phased out [RFC3701].
 7.  Added optional ability for an implementation to add automatic
     rows to the table for site-specific ULA prefixes and site-
     specific native 6to4 prefixes.
 Similarly, some changes were made to the rules, as follows:
 1.  Changed the definition of CommonPrefixLen() to only compare bits
     up to the source address's prefix length.  The previous
     definition used the entire source address, rather than only its
     prefix.  As a result, when a source and destination addresses had
     the same prefix, common bits in the interface ID would previously
     result in overriding DNS load balancing [RFC1794] by forcing the
     destination address with the most bits in common to be always
     chosen.  The updated definition allows DNS load balancing to
     continue to be used as a tie breaker.
 2.  Added Rule 5.5 to allow choosing a source address from a prefix
     advertised by the chosen next-hop for a given destination.  This
     allows better connectivity in the presence of BCP 38 [RFC2827]
     ingress filtering and egress filtering.  Previously, RFC 3484 had
     issues with multiple egress networks reached via the same
     interface, as discussed in [RFC5220].
 3.  Removed restriction against anycast addresses in the candidate
     set of source addresses, since the restriction against using IPv6
     anycast addresses as source addresses was removed in Section 2.6
     of RFC 4291 [RFC4291].
 4.  Changed mapping of RFC 1918 [RFC1918] addresses to global scope
     in Section 3.2.  Previously, they were mapped to site-local
     scope.  However, experience has resulted in current
     implementations already using global scope instead.  When they
     were mapped to site-local, Destination Address Selection Rule 2
     (Prefer matching scope) would cause IPv6 to be preferred in
     scenarios such as that described in Section 10.7.  The change to
     global scope allows configurability via the prefix policy table.
 5.  Changed the default recommendation for Source Address Selection
     Rule 7 to prefer temporary addresses rather than public
     addresses, while providing an administrative override (in
     addition to the application-specific override that was already
     specified).  This change was made because of the increasing
     importance of privacy considerations, as well as the fact that
     widely deployed implementations have preferred temporary
     addresses for many years without major application issues.

Thaler, et al. Standards Track [Page 30] RFC 6724 Default Address Selection for IPv6 September 2012

 Finally, some editorial changes were made, including:
 1.  Changed global IP addresses in examples to use ranges reserved
     for documentation.
 2.  Added additional examples in Sections 10.6 and 10.7.
 3.  Added Section 10.3.1 on "broken" IPv6.
 4.  Updated references.

Thaler, et al. Standards Track [Page 31] RFC 6724 Default Address Selection for IPv6 September 2012

Authors' Addresses

 Dave Thaler (editor)
 Microsoft
 One Microsoft Way
 Redmond, WA  98052
 USA
 Phone: +1 425 703 8835
 EMail: dthaler@microsoft.com
 Richard Draves
 Microsoft Research
 One Microsoft Way
 Redmond, WA  98052
 USA
 Phone: +1 425 706 2268
 EMail: richdr@microsoft.com
 Arifumi Matsumoto
 NTT SI Lab
 Midori-Cho 3-9-11
 Musashino-shi, Tokyo  180-8585
 Japan
 Phone: +81 422 59 3334
 EMail: arifumi@nttv6.net
 Tim Chown
 University of Southampt on
 Southampton, Hampshire  SO17 1BJ
 United Kingdom
 EMail: tjc@ecs.soton.ac.uk

Thaler, et al. Standards Track [Page 32]

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