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
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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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Draves Standards Track - Expires June 2001 19