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IPng Working Group                                          Richard Draves
Internet Draft                                          Microsoft Research
Document: draft-ietf-ipngwg-default-addr-select-05.txt        June 4, 2001
Category: Standards Track

                   Default Address Selection for IPv6
Status of this Memo
   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026 [1].
   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as Internet-
   Drafts.
   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."
   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.
   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.
Abstract
   This document describes two algorithms, for source address selection
   and for destination address selection. The algorithms specify
   default behavior for all 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 framework, including an
   optional mechanism for allowing administrators to provide policy
   that can override the default behavior. In dual stack
   implementations, the framework allows the destination address
   selection algorithm to consider both IPv4 and IPv6 addresses -
   depending on the available source addresses, the algorithm might
   prefer IPv6 addresses over IPv4 addresses, or vice-versa.
   All IPv6 nodes, including both hosts and routers, must implement
   default address selection as defined in this specification.
1. Introduction
   The IPv6 addressing architecture [2] allows multiple unicast
   addresses to be assigned to interfaces. These addresses may have
   different reachability scopes (link-local, site-local, or global).
   These addresses may also be "preferred" or "deprecated" [3]. Privacy
   considerations have introduced the concepts of "public addresses"

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   and "temporary addresses" [4]. The mobility architecture introduces
   "home addresses" and "care-of addresses" [5]. In addition, multi-
   homing situations will result in more addresses per node. For
   example, a node may have multiple interfaces, some of them tunnels
   or virtual interfaces, or a site may 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 [6], 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.
   This document specifies source address selection and destination
   address selection separately, but using a common framework 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).
   Furthermore, this document suggests a preferred method, longest
   matching prefix, for choosing among otherwise equivalent addresses
   in the absence of better information.
   The framework also has 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.

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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 RFC 2119 [7].
2. Framework
   Our framework for address selection derives from the most common
   implementation architecture, which separates the choice of
   destination address from the choice of source address. Consequently,
   the framework specifies 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 [8] like
   getaddrinfo() 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 might use only the first address in the list, or it
   might loop over the list of addresses to find 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 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 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 framework to source
   address selection in an IPv4 network layer may be possible but this
   is not explored further here.
   Well-behaved applications should iterate through the list of
   addresses returned from getaddrinfo() until they find a working
   addresses.
   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, public addresses [4] are preferred over temporary
   addresses. In mobile situations [5], 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

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   over addresses that are solely a home address or solely a care-of
   address.
   The framework optionally allows for the possibility of
   administrative configuration of policy that can override the default
   behavior of the algorithms. The policy override takes the form of a
   configurable table that specifies precedence values and preferred
   source prefixes for destination prefixes. If an implementation is
   not configurable, or if an implementation has not been configured,
   then the default policy table specified in this document SHOULD be
   used.
2.1. 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), subnet-local (0x3), admin-local
   (0x4), site-local (0x5), organization-local (0x8), and global (0xE)
   scopes [9].
   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.
   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 [9].
2.2. IPv4 Addresses and IPv4-Mapped Addresses
   The destination address selection algorithm operates on both IPv6
   and IPv4 addresses. For this purpose, IPv4 addresses should be
   represented as IPv4-mapped addresses [2]. For example, to lookup the
   precedence or other attributes of an IPv4 address in the policy
   table, lookup the corresponding IPv4-mapped IPv6 address.
   IPv4 addresses are assigned scopes as follows. IPv4 auto-
   configuration addresses [6], which have the prefix 169.254/16, are
   assigned link-local scope. IPv4 private addresses [10], which have
   the prefixes 10/8, 172.16/12, and 192.168/16, are assigned site-
   local scope. IPv4 loopback addresses [11, section 4.2.2.11], which
   have the prefix 127/8, are assigned link-local scope (analogously to
   the treatment of the IPv6 loopback address [9, section 4]). Other
   IPv4 addresses are assigned global scope.

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   IPv4 addresses should be treated as having "preferred" configuration
   status.
2.3. IPv6 Addresses with Embedded IPv4 Addresses
   IPv4-compatible addresses [2] and 6to4 addresses [12] contain an
   embedded IPv4 address. For the purposes of this document, these
   addresses should be treated as having global scope.
   IPv4-compatible addresses should be treated as having "preferred"
   configuration status.
2.4. Loopback Address and Other Format Prefixes
   The loopback address should be treated as having link-local
   scope [9, section 4] and "preferred" configuration status.
   NSAP addresses and other addresses with as-yet-undefined format
   prefixes should be treated as having global scope and "preferred"
   configuration status. Later standards may supersede this treatment.
2.5. 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 Precedence(A) and a
   classification or label 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. 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:
          Prefix        Precedence Label
          ::1/128               50     0
          ::/0                  40     1
          2002::/16             30     2
          ::/96                 20     3
          ::ffff:0:0/96         10     4

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   One effect of the default policy table is to prefer using native
   source addresses with native destination addresses, 6to4 [12] source
   addresses with 6to4 destination addresses, and v4-compatible [2]
   source addresses with v4-compatible destination addresses. 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 scoped address prefixes 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.
2.6. Common Prefix Length
   We define the common prefix length CommonPrefixLen(A, B) of two
   addresses A and B as the length of the longest prefix (looking at
   the most significant, or leftmost, bits) that the two addresses have
   in common. It ranges from 0 to 128.
3. Candidate Source Addresses
   The source address selection algorithm uses the concept of a
   "candidate set" of potential source addresses for a given
   destination address. 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.
     Discussion: The Neighbor Discovery Redirect mechanism [13]
     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. Implementations that wish to support the use
     of global source addresses assigned to a loopback interface should
     behave as if the loopback interface originates and forwards the
     packet.
   In some cases the destination address may be qualified with a zone
   index or other information that will constrain the candidate set.
   For 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.

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     Discussion: The restriction for multicast destination addresses is
     necessary because currently-deployed multicast forwarding
     algorithms use Reverse Path Forwarding (RPF) checks.
   For site-local 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, anycast addresses, multicast addresses, and the
   unspecified address MUST NOT be included in a candidate set.
   If an 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 is an error. The specified source address may
   influence the candidate set, by affecting the choice of outgoing
   interface. 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.
   Discussion:
4. Source Address Selection
   The source address selection algorithm chooses a source address for
   use with a destination address D. It is specified here in terms of
   the pair-wise comparison of addresses SA and SB. The pair-wise
   comparison can be used to select an address from the set
   CandidateSource(D).
   This source address selection algorithm only applies to IPv6
   destination addresses, not IPv4 addresses.
   The pair-wise comparison consists of eight rules, which should be
   applied in order. If a rule chooses an address, then the remaining
   rules are not relevant and should be ignored. Subsequent rules act
   as tie-breakers for earlier rules. If the eight rules fail to choose
   an address, some unspecified tie-breaker should be used.
   Rule 1: Prefer same address.
   If SA = D, then choose SA. Similarly, if SB = D, then choose SB.
   Rule 2: Prefer appropriate scope.
   If Scope(SA) < Scope(SB): If Scope(SA) < Scope(D), then choose SB
   and otherwise choose SA.
   Similarly, if Scope(SB) < Scope(SA): If Scope(SB) < Scope(D), then
   choose SA and otherwise choose SB.

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   Rule 3: Avoid deprecated addresses.
   The addresses SA and SB have the same scope. If one of the source
   addresses is "preferred" and one of them is "deprecated", choose 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.
   An implementation may support a per-connection configuration
   mechanism (for example, a socket option) to reverse the sense of
   this preference and prefer care-of addresses over home addresses.
   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 6: Prefer matching label.
   If Label(SA) = Label(D) and Label(SB) <> Label(D), then choose SA.
   Similarly, if Label(SB) = Label(D) and Label(SA) <> Label(D), then
   choose SB.
   Rule 7: Prefer public addresses.
   If SA is a public address and SB is a temporary address, then prefer
   SA. Similarly, if SB is a public address and SA is a temporary
   address, then prefer SB.
   An implementation may support a per-connection configuration
   mechanism (for example, a socket option) to reverse the sense of
   this preference and prefer temporary addresses over public
   addresses.
   This rule avoids 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
   privacy considerations outweigh these application compatibility
   concerns MAY reverse the sense of this rule and by default prefer
   temporary addresses over public addresses.
   Rule 8: Use longest matching prefix.
   If CommonPrefixLen(SA, D) > CommonPrefixLen(SB, D), then choose SA.
   Similarly, if CommonPrefixLen(SB, D) > CommonPrefixLen(SA, D), then
   choose SB.
   Rule 8 may be superseded if the implementation has other means of
   choosing among source addresses. For example, if the implementation

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   somehow knows which source address will result in the "best"
   communications performance.
   Rule 2 (prefer appropriate scope) MUST be implemented and given high
   priority because it can affect interoperability.
5. 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 should 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 nine
   rules, which should be applied in order. If a rule determines a
   result, then the remaining rules are not relevant and should be
   ignored. Subsequent rules act as tie-breakers for earlier rules.
   Rule 1: Avoid unusable destinations.
   If there is no route to DB or the current next-hop neighbor for DB
   is known to be unreachable or if Source(DB) is undefined, then sort
   DA before DB. Similarly, if there is no route to DA or the current
   next-hop neighbor for DA is known to be unreachable or if Source(DA)
   is undefined, then sort DB before DA.
   For IPv6 destination addresses, the
   Rule 2: Prefer matching scope.
   If Scope(DA) = Scope(Source(DA)) and Scope(DB) <> Scope(Source(DB)),
   then sort DA before DB. Similarly, if Scope(DA) <> Scope(Source(DA))
   and Scope(DB) = Scope(Source(DB)), then sort DB before DA.
   Rule 3: Avoid deprecated addresses.
   If Source(DA) is deprecated and Source(DB) is not, then sort DB
   before DA. Similarly, if Source(DA) is not deprecated and Source(DB)
   is deprecated, then sort DA before DB.

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   Rule 4: Prefer home addresses.
   If Source(DA) is simultaneously a home address and care-of address
   and Source(DB) is not, then sort DA before DB. Similarly, if
   Source(DB) is simultaneously a home address and care-of address and
   Source(DA) is not, then sort DB before DA.
   If Source(DA) is just a home address and Source(DB) is just a care-
   of address, then sort DA before DB. Similarly, if Source(DA) is just
   a care-of address and Source(DB) is just a home address, then sort
   DB before DA.
   Rule 5: Prefer matching label.
   If Label(Source(DA)) = Label(DA) and Label(Source(DB)) <> Label(DB),
   then sort DA before DB. Similarly, if Label(Source(DA)) <> Label(DA)
   and Label(Source(DB)) = Label(DB), then sort DB before DA.
   Rule 6: Prefer higher precedence.
   If Precedence(DA) > Precedence(DB), then sort DA before DB.
   Similarly, if Precedence(DA) < Precedence(DB), then sort DB before
   DA.
   Rule 7: Prefer smaller scope.
   If Scope(DA) < Scope(DB), then sort DA before DB. Similarly, if
   Scope(DA) > Scope(DB), then sort DB before DA.
   Rule 8: Use longest matching prefix.
   If CommonPrefixLen(DA, Source(DA)) > CommonPrefixLen(DB,
   Source(DB)), then sort DA before DB. Similarly, if
   CommonPrefixLen(DA, Source(DA)) < CommonPrefixLen(DB, Source(DB)),
   then sort DB before DA.
   Rule 9: Otherwise, leave the order unchanged.
   Sort DA before DB.
   Rules 8 and 9 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.
6. 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 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.

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   Implementations may 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 route is advertising global prefix B. Then when sending
   via the first router, the host may prefer source addresses with
   prefix A and when sending via the second router, prefer source
   addresses with prefix B.
7. 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 IPv6 network layer with a list
   of destination addresses, sort the list in the network layer with
   full 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 resort 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 may 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 should 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 or source address assignments
   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 can detect if the derived
   state is potentially stale.
8. 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

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   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 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.
9. Examples
   This section contains a number of examples, first of default
   behavior and then demonstrating the utility of policy table
   configuration. These examples are provided for illustrative
   purposes; they should not be construed as normative.
9.1. Default Source Address Selection
   The source address selection rules, in conjunction with the default
   policy table, produce the following behavior:
   Destination: 2001::1
   Sources: 3ffe::1 vs fe80::1
   Result: 3ffe::1 (prefer appropriate scope)
   Destination: 2001::1
   Sources: fe80::1 vs fec0::1
   Result: fec0::1 (prefer appropriate scope)
   Destination: fec0::1
   Sources: fe80::1 vs 2001::1
   Result: 2001::1 (prefer appropriate scope)
   Destination: ff05::1
   Sources: fe80::1 vs fec0::1 vs 2001::1
   Result: fec0::1 (prefer appropriate scope)
   Destination: 2001::1
   Sources: 2001::1 (deprecated) vs 2002::1
   Result: 2001::1 (prefer same address)
   Destination: fec0::1
   Sources: fec0::2 (deprecated) vs 2001::1
   Result: fec0::2 (prefer appropriate scope)
   Destination: 2001::1
   Sources: 2001::2 vs 3ffe::2
   Result: 2001::2 (longest-matching-prefix)
   Destination: 2001::1
   Sources: 2001::2 (care-of address) vs 3ffe::2 (home address)
   Result: 3ffe::2 (prefer home address)

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   Destination: 2002:836b:2179::1
   Sources: 2002:836b:2179::d5e3:7953:13eb:22e8 (temporary) vs 2001::2
   Result: 2002:836b:2179::d5e3:7953:13eb:22e8 (prefer matching label)
   Destination: 2001::d5e3:0:0:1
   Sources: 2001::2 vs 2001::d5e3:7953:13eb:22e8 (temporary)
   Result: 2001::2 (prefer public address)
9.2. 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:
   Sources: 2001::2 or fe80::1 or 169.254.13.78
   Destinations: 2001::1 vs 131.107.65.121
   Result: 2001::1 (src 2001::2) then 131.107.65.121 (src
   169.254.13.78) (prefer matching scope)
   Sources: fe80::1 or 131.107.65.117
   Destinations: 2001::1 vs 131.107.65.121
   Result: 131.107.65.121 (src 131.107.65.117) then 2001::1 (src
   fe80::1) (prefer matching scope)
   Sources: 2001::2 or fe80::1 or 10.1.2.4
   Destinations: 2001::1 vs 10.1.2.3
   Result: 2001::1 (src 2001::2) then 10.1.2.3 (src 10.1.2.4) (prefer
   higher precedence)
   Sources: 2001::2 or fec0::2 or fe80::2
   Destinations: 2001::1 vs fec0::1 vs fe80::1
   Result: fe80::1 (src fe80::2) then fec0::1 (src fec0::2) then
   2001::1 (src 2001::2) (prefer smaller scope)
   Sources: 2001::2 (care-of address) or 3ffe::1 (home address) or
   fec0::2 (care-of address) or fe80::2 (care-of address)
   Destinations: 2001::1 vs fec0::1
   Result: 2001:1 (src 3ffe::1) then fec0::1 (src fec0::2) (prefer home
   address)
   Sources: 2001::2 or fec0::2 (deprecated) or fe80::2
   Destinations: 2001::1 vs fec0::1
   Result: 2001::1 (src 2001::2) then fec0::1 (src fec0::2) (avoid
   deprecated addresses)
   Sources: 2001::2 or 3f44::2 or fe80::2
   Destinations: 2001::1 vs 3ffe::1
   Result: 2001::1 (src 2001::2) then 3ffe::1 (src 3f44::2) (longest
   matching prefix)
   Sources: 2002:836b:4179::2 or fe80::2
   Destinations: 2002:836b:4179::1 vs 2001::1

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   Result: 2002:836b:4179::1 (src 2002:836b:4179::2) then 2001::1 (src
   2002:836b:4179::2) (prefer matching label)
   Sources: 2002:836b:4179::2 or 2001::2 or fe80::2
   Destinations: 2002:836b:4179::1 vs 2001::1
   Result: 2001::1 (src 2001::2) then 2002:836b:4179::1 (src
   2002:836b:4179::2) (prefer higher precedence)
9.3. Configuring Preference for IPv6 vs 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
          2002::/16             30     2
          ::/96                 20     3
          ::ffff:0:0/96        100     4

   This change to the default policy table produces the following
   behavior:
   Sources: 2001::2 or fe80::1 or 169.254.13.78
   Destinations: 2001::1 vs 131.107.65.121
   Unchanged Result: 2001::1 (src 2001::2) then 131.107.65.121 (src
   169.254.13.78) (prefer matching scope)
   Sources: fe80::1 or 131.107.65.117
   Destinations: 2001::1 vs 131.107.65.121
   Unchanged Result: 131.107.65.121 (src 131.107.65.117) then 2001::1
   (src fe80::1) (prefer matching scope)
   Sources: 2001::2 or fe80::1 or 10.1.2.4
   Destinations: 2001::1 vs 10.1.2.3
   New Result: 10.1.2.3 (src 10.1.2.4) then 2001::1 (src 2001::2)
   (prefer higher precedence)
9.4. Configuring Preference for Scoped Addresses
   The destination address selection rules give preference to
   destinations of smaller scope. For example, a site-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
   site-local destinations, and site-local destinations before link-
   local destinations:

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          Prefix        Precedence Label
          ::1/128               50     0
          ::/0                  40     1
          fec0::/10             37     1
          fe80::/10             33     1
          2002::/16             30     2
          ::/96                 20     3
          ::ffff:0:0/96         10     4

   This change to the default policy table produces the following
   behavior:
   Sources: 2001::2 or fec0::2 or fe80::2
   Destinations: 2001::1 vs fec0::1 vs fe80::1
   New Result: 2001::1 (src 2001::2) then fec0::1 (src fec0::2) then
   fe80::1 (src fe80::2) (prefer higher precedence)
   Sources: 2001::2 (deprecated) or fec0::2 or fe80::2
   Destinations: 2001::1 vs fec0::1
   Unchanged Result: fec0::1 (src fec0::2) then 2001::1 (src 2001::2)
   (avoid deprecated addresses)
9.5. 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:aaaa:aaaa::/48
   from the high-performance ISP and prefix 2007:0:aaaa::/48 from its
   normal ISP. Site B has prefix 2001:bbbb:bbbb::/48 from the high-
   performance ISP and prefix 2007:0:bbbb::/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:
   Sources: 2001:aaaa:aaaa::a or 2007:0:aaaa::a or fe80::a
   Destinations: 2001:bbbb:bbbb::b vs 2007:0:bbbb::b

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   Result: 2007:0:bbbb::b (src 2007:0:aaaa::a) then 2001:bbbb:bbbb::b
   (src 2001:aaaa:aaaa::a) (longest matching prefix)
   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.
   Sources: 2001:aaaa:aaaa::a or 2007:0:aaaa::a or fe80::a
   Destinations: 2001:cccc:cccc::c vs 2006:cccc:cccc::c
   Result: 2001:cccc:cccc::c (src 2001:aaaa:aaaa::a) then
   2006:cccc:cccc::c (src 2007:0:aaaa::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 may come back through
   the high-performance ISP. Again, this is not their desired behavior.
   This situation 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                         50     0
          2001:aaaa:aaaa::/48         45     5
          2001:bbbb:bbbb::/48         45     5
          ::/0                        40     1
          2002::/16                   30     2
          ::/96                       20     3
          ::ffff:0:0/96               10     4

   This policy table produces the following behavior:
   Sources: 2001:aaaa:aaaa::a or 2007:0:aaaa::a or fe80::a
   Destinations: 2001:bbbb:bbbb::b vs 2007:0:bbbb::b
   New Result: 2001:bbbb:bbbb::b (src 2001:aaaa:aaaa::a) then
   2007:0:bbbb::b (src 2007:0:aaaa::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.
   Sources: 2001:aaaa:aaaa::a or 2007:0:aaaa::a or fe80::a
   Destinations: 2001:cccc:cccc::c vs 2006:cccc:cccc::c
   New Result: 2006:cccc:cccc::c (src 2007:0:aaaa::a) then
   2001:cccc:cccc::c (src 2007:0:aaaa::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.

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References

   1  S. Bradner, "The Internet Standards Process -- Revision 3", BCP
      9, RFC 2026, October 1996.
   2  R. Hinden, S. Deering, "IP Version 6 Addressing Architecture",
      RFC 2373, July 1998.
   3  S. Thompson, T. Narten, "IPv6 Stateless Address Autoconfig-
      uration", RFC 2462 , December 1998.
   4  T. Narten, R. Draves, "Privacy Extensions for Stateless Address
      Autoconfiguration in IPv6", RFC 3041, January 2001.
   5  D. Johnson, C. Perkins, "Mobility Support in IPv6", draft-ietf-
      mobileip-ipv6-13.txt, November 2000.
   6  S. Cheshire, B. Aboba, "Dynamic Configuration of IPv4 Link-local
      Addresses", draft-ietf-zeroconf-ipv4-linklocal-02.txt, March
      2001.
   7  S. Bradner, "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.
   8  R. Gilligan, S. Thomson, J. Bound, W. Stevens, "Basic Socket
      Interface Extensions for IPv6", RFC 2553, March 1999.
   9  S. Deering et. al, "IP Version 6 Scoped Address Architecture",
      draft-ietf-ipngwg-scoping-arch-02.txt, March 2001.
   10 Y. Rekhter et. al, "Address Allocation for Private Internets",
      RFC 1918, February 1996.
   11 F. Baker, Editor, "Requirements for IP Version 4 Routers", RFC
      1812, June 1995.
   12 B. Carpenter, K. Moore, "Connection of IPv6 Domains via IPv4
      Clouds", RFC 3056, February 2001.
   13 T. Narten, E. Nordmark, and W. Simpson, "Neighbor Discovery for
      IP Version 6", RFC 2461, December 1998.
Acknowledgments
   The author would like to acknowledge the contributions of the IPng
   Working Group, particularly Marc Blanchet, Brian Carpenter, Matt
   Crawford, Steve Deering, Robert Elz, Jun-ichiro itojun Hagino, Tony
   Hain, M.T. Hollinger, JINMEI Tatuya, Erik Nordmark, Ken Powell,
   Markku Savela, Dave Thaler, Ole Troan, and Mauro Tortonesi. Please
   let the author know if you contributed to the development of this
   draft and are not mentioned here.

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Author's Address
   Richard Draves
   Microsoft Research
   One Microsoft Way
   Redmond, WA 98052
   Phone: 1-425-936-2268
   Email: richdr@microsoft.com
Revision History
Changes from draft-ietf-ipngwg-default-addr-select-04
   Clarified candidate set formation for routers.
   Added some explanatory discussion to the candidate set section.
   Replaced usages of scope id with zone index.
   Augmented the first destination-address selection rule, to avoid
   destination addresses for which the current next-hop neighbor is
   known to be unreachable.
Changes from draft-ietf-ipngwg-default-addr-select-03
   Reversed the treatment of temporary addresses, so that unless an
   application specifies otherwise public addresses are preferred over
   temporary addresses.
   Added text clarifying our expectation that applications should
   iterate through the list of possible destination addresses until
   finding a working address.
   Removed references to getipnodebyname().
Changes from draft-ietf-ipngwg-default-addr-select-02
   Changed scope treatment of IPv4-compatible and 6to4 addresses, so
   they are always considered to be global. Removed mention of IPX
   addresses.
   Changed home address rules to favor addresses that are
   simultaneously home and care-of addresses, over addresses that are
   just home addresses or just care-of addresses.
   Combined SrcLabel & DstLabel in the policy table into a single Label
   attribute.
   Added mention of the invalidation counter technique in the
   implementation section.

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Changes from draft-ietf-ipngwg-default-addr-select-01
   Added Examples section, demonstrating default behavior and some
   policy table configuration scenarios.
   Removed many uses of MUST. Remaining uses concern the candidate set
   of source addresses and the source address selection rule that
   prefers source addresses of appropriate scope.
   Simplified the default policy table. Reordered the source address
   selection rules to reduce the influence of policy labels. Added more
   destination address selection rules.
   Added scoping of v4-compatible and 6to4 addresses based on the
   embedded IPv4 address.
   Changed references to anonymous addresses to use the new term,
   temporary addresses.
   Clarified that a user-level implementation of destination address
   ordering, which does not have knowledge of the outgoing interface
   for each destination, may use a looser definition of the candidate
   set.
   Clarified that an implementation should prevent an application or
   upper-layer from choosing a source address that is not in the
   candidate set and not prevent an application or upper-layer from
   choosing a source address that is in the candidate set.
   Miscellaneous editorial changes, including adding some missing
   references.
Changes from draft-ietf-ipngwg-default-addr-select-00
   Changed the candidate set definition so that the strong host model
   is recommended but not required. Added a rule to source address
   selection to prefer addresses assigned to the outgoing interface.
   Simplified the destination address selection algorithm, by having it
   use source address selection as a subroutine.
   Added a rule to source address selection to handle anonymous/public
   addresses.
   Added a rule to source address selection to handle home/care-of
   addresses.
   Changed to allow destination address selection to sort both IPv6 and
   IPv4 addresses. Added entries in the default policy table for IPv4-
   mapped addresses.
   Changed default precedences, so v4-compatible addresses have lower
   precedence than 6to4 addresses.

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Changes from draft-draves-ipngwg-simple-srcaddr-01
   Added framework discussion.
   Added algorithm for destination address ordering.
   Added mechanism to allow the specification of administrative policy
   that can override the default behavior.
   Added section on routing interactions and TBD section on mobility
   interactions.
   Changed the candidate set definition for source address selection,
   so that only addresses assigned to the outgoing interface are
   allowed.
   Changed the loopback address treatment to link-local scope.
Changes from draft-draves-ipngwg-simple-srcaddr-00
   Minor wording changes because DHCPv6 also supports "preferred" and
   "deprecated" addresses.
   Specified treatment of other format prefixes; now they are
   considered global scope, "preferred" addresses.
   Reiterated that anycast and multicast addresses are not allowed as
   source addresses.
   Recommended that source addresses be taken from the outgoing
   interface. Required this for multicast destinations. Added analogous
   requirements for link-local and site-local destinations.
   Specified treatment of the loopback address.
   Changed the second selection rule so that if both candidate source
   addresses have scope greater or equal than the destination address
   and only of them is preferred, the preferred address is chosen.

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   Full Copyright Statement
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