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IETF RFC 6724
Last modified on Tuesday, September 11th, 2012
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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/RFC 6724.
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 [RFC 4291] 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" [RFC 4862].
Privacy considerations have introduced the concepts of "public
addresses" and "temporary addresses" [RFC 4941]. The mobility
architecture introduces "home addresses" and "care-of addresses"
[RFC 6275]. 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 [RFC 3927], 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
[RFC 2119].
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() [RFC 3493] 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 [RFC 4941] are preferred over
public addresses. In mobile situations [RFC 6275], 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) [RFC 4193] and 6to4 [RFC 3056] 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" [RFC 4862], 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
[RFC 4291]).
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 [RFC 4291]. 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 [RFC 4007].
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 [RFC 4291]. 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 [RFC 3927], 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 [RFC 1812]), which have the prefix 127/8, are assigned
link-local scope (analogously to the treatment of the IPv6 loopback
address (Section 4 of [RFC 4007])). Other IPv4 addresses (including
IPv4 private addresses [RFC 1918] and Shared Address Space addresses
[RFC 6598]) 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 [RFC 4291], IPv4-mapped [RFC 4291], IPv4-
converted [RFC 6145], IPv4-translatable [RFC 6145], and 6to4 addresses
[RFC 3056] 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 [RFC 4007]) 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 [RFC 6275]). 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 [RFC 4861]
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)
[RFC 6145], 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 [RFC 5014]). 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 [RFC 4861] and Section 3 of [RFC 4191]
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
[RFC 5014]). 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 [RFC 4941] 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 [RFC 4861] 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 [RFC 5969], the Intra-Site Automatic Tunnel
Addressing Protocol (ISATAP) [RFC 5214], and configured tunnels
[RFC 4213] 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 [RFC 5220] describe
address selection problems related to Unique Local Addresses (ULAs)
[RFC 4193]. 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
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 3056] Carpenter, B. and K. Moore, "Connection of IPv6
Domains via IPv4 Clouds", RFC 3056, February 2001.
[RFC 3879] Huitema, C. and B. Carpenter, "Deprecating Site Local
Addresses", RFC 3879, September 2004.
[RFC 4193] Hinden, R. and B. Haberman, "Unique Local IPv6
Unicast Addresses", RFC 4193, October 2005.
[RFC 4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC 4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
Network Address Translations (NATs)", RFC 4380,
February 2006.
[RFC 4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6
Stateless Address Autoconfiguration", RFC 4862,
September 2007.
[RFC 4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.
[RFC 6145] 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.
[RFC 1794] Brisco, T., "DNS Support for Load Balancing",
RFC 1794, April 1995.
[RFC 1812] Baker, F., "Requirements for IP Version 4 Routers",
RFC 1812, June 1995.
[RFC 1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot,
G., and E. Lear, "Address Allocation for Private
Internets", BCP 5, RFC 1918, February 1996.
[RFC 2827] 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.
[RFC 3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[RFC 3493] Gilligan, R., Thomson, S., Bound, J., McCann, J., and
W. Stevens, "Basic Socket Interface Extensions for
IPv6", RFC 3493, February 2003.
[RFC 3701] Fink, R. and R. Hinden, "6bone (IPv6 Testing Address
Allocation) Phaseout", RFC 3701, March 2004.
[RFC 3927] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
Configuration of IPv4 Link-Local Addresses",
RFC 3927, May 2005.
[RFC 4007] Deering, S., Haberman, B., Jinmei, T., Nordmark, E.,
and B. Zill, "IPv6 Scoped Address Architecture",
RFC 4007, March 2005.
[RFC 4191] Draves, R. and D. Thaler, "Default Router Preferences
and More-Specific Routes", RFC 4191, November 2005.
[RFC 4213] 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
[RFC 4861] Narten, T., Nordmark, E., Simpson, W., and H.
Soliman, "Neighbor Discovery for IP version 6
(IPv6)", RFC 4861, September 2007.
[RFC 5014] Nordmark, E., Chakrabarti, S., and J. Laganier, "IPv6
Socket API for Source Address Selection", RFC 5014,
September 2007.
[RFC 5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP)",
RFC 5214, March 2008.
[RFC 5220] 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.
[RFC 5969] Townsley, W. and O. Troan, "IPv6 Rapid Deployment on
IPv4 Infrastructures (6rd) -- Protocol
Specification", RFC 5969, August 2010.
[RFC 6275] Perkins, C., Johnson, D., and J. Arkko, "Mobility
Support in IPv6", RFC 6275, July 2011.
[RFC 6598] 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 [RFC 3484] 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 [RFC 4380] 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 [RFC 3879].
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 [RFC 4291] 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 [RFC 3701].
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 [RFC 1794] 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 [RFC 2827]
ingress filtering and egress filtering. Previously, RFC 3484 had
issues with multiple egress networks reached via the same
interface, as discussed in [RFC 5220].
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 [RFC 4291].
4. Changed mapping of RFC 1918 [RFC 1918] 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
RFC TOTAL SIZE: 74407 bytes
PUBLICATION DATE: Tuesday, September 11th, 2012
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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