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IPng Working Group                                          Richard Draves
Internet Draft                                          Microsoft Research
Document: draft-ietf-ipngwg-default-addr-select-02.txt   November 24, 2000
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
   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() and getipnodebyname() 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()
   and getipnodebyname() 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.

   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 [4] are preferred over public
   addresses. In mobile situations [5], home addresses are preferred
   over care-of addresses.

   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

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   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 node-local
   (0x1), link-local (0x2), 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. Other IPv4
   addresses are assigned global scope.

   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 the scope of the embedded IPv4

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   address. For example, the IPv6 address ::169.254.3.18 has link-local
   scope and the address 2002:0a01:0203::1 has site-local 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
   and "preferred" configuration status.

   NSAP addresses, IPX addresses, and other addresses with as-yet-
   undefined format prefixes should be treated as having global scope
   and "preferred" configuration status. Later standards may supercede
   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 three values: a precedence value Precedence(A), a
   classification or label SrcLabel(A), and a second label DstLabel(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 labels SrcLabel(A) and DstLabel(A) allow 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 SrcLabel(S) = DstLabel(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 SrcLabel DstLabel
          ::1/128               50        0        0
          ::/0                  40        1        1
          2002::/16             30        2        2
          ::/96                 20        3        3
          ::ffff:0:0/96         10        4        4

   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


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   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 scope-id. If so, a prefix table entry only matches
   against an address during a lookup if the scope-id also matches the
   address's scope-id.

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 could forward the destination address to the outgoing
   interface.

   In some cases the destination address may be qualified with a scope-
   id 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.

   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. 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.


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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.

   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 a home address and SB is a care-of address, then prefer SA.
   Similarly, if SB is a home address and SA is 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) = MatchSrcLabel(D) and Label(SB) <> MatchSrcLabel(D),
   then choose SA. Similarly, if Label(SB) = MatchSrcLabel(D) and
   Label(SA) <> MatchSrcLabel(D), then choose SB.




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   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.
   An implementation may support a per-connection configuration
   mechanism (for example, a socket option) to reverse the sense of
   this preference and prefer public addresses over temporary
   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 superceded 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.

   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 if Source(DB) is undefined, then sort
   DA before DB. Similarly, if there is no route to DA or if Source(DA)
   is undefined, then sort DB before DA.


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   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.

   Rule 4: Prefer home addresses.
   If Source(DA) is a home address and Source(DB) is a care-of address,
   then sort DA before DB. Similarly, if Source(DA) is a care-of
   address and Source(DB) is a home address, then sort DB before DA.

   Rule 5: Prefer matching label.
   If SrcLabel(Source(DA)) = DstLabel(DA) and SrcLabel(Source(DB)) <>
   DstLabel(DB), then sort DA before DB. Similarly, if
   SrcLabel(Source(DA)) <> DstLabel(DA) and SrcLabel(Source(DB)) =
   DstLabel(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 superceded 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.



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   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.

   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 getipnodebyname() and 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 getipnodebyname() or
   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 getipnodebyname() or
   getaddrinfo(). To reduce overhead in this approach, the source
   address information can be cached, amortizing the overhead of
   retrieving it across multiple calls to getipnodebyname() and
   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.





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8. Security Considerations

   This document has no direct impact on Internet infrastructure
   security.

9. Examples

   This section contains a number of examples, first of default
   behavior and then demonstrating the utility of policy table
   configuration.

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)

   Destination: 2002:836b:2179::1
   Sources: 2002:836b:2179::2 vs 2001::d5e3:7953:13eb:22e8 (temporary)
   Result: 2002:836b:2179::2 (prefer matching label)



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   Destination: 2001::1
   Sources: 2001::2 vs 2001::d5e3:7953:13eb:22e8 (temporary)
   Result: 2001::d5e3:7953:13eb:22e8 (prefer temporary 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
   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

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   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 SrcLabel DstLabel
          ::1/128               50        0        0
          ::/0                  40        1        1
          2002::/16             30        2        2
          ::/96                 20        3        3
          ::ffff:0:0/96        100        4        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 SrcLabel DstLabel
          ::1/128               50        0        0
          ::/0                  40        1        1
          fec0::/10             37        1        1
          fe80::/10             33        1        1
          2002::/16             30        2        2
          ::/96                 20        3        3
          ::ffff:0:0/96         10        4        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 SrcLabel DstLabel
          ::1                         50        0        0
          2001:aaaa:aaaa::/48         45        5        5
          2001:bbbb:bbbb::/48         45        5        5
          ::/0                        40        1        1
          2002::/16                   30        2        2
          ::/96                       20        3        3
          ::ffff:0:0/96               10        4        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", draft-ietf-ipngwg-addrconf-privacy-
      01.txt, July 2000.

   5  D. Johnson, C. Perkins, "Mobility Support in IPv6", draft-ietf-
      mobileip-ipv6-12.txt, April 2000.

   6  R. Troll. "Automatically Choosing an IP Address in an Ad-Hoc IPv4
      Network", draft-ietf-dhc-ipv4-autoconfig-05.txt, March 2000.

   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, B. Haberman, B. Zill. "IP Version 6 Scoped Address
      Architecture", draft-ietf-ipngwg-scoping-arch-01.txt, March 2000.

   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", draft-ietf-ngtrans-6to4-07.txt, September 2000.

Acknowledgments

   The author would like to acknowledge the contributions of the IPng
   Working Group, particularly Steve Deering and Ken Powell. Please let
   the author know if you contributed to the development of this draft
   and are not mentioned here.

Author's Address

   Richard Draves
   Microsoft Research
   One Microsoft Way
   Redmond, WA 98052


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   Phone: 1-425-936-2268
   Email: richdr@microsoft.com

Revision History

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.



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   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.

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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