draft-ietf-ipv6-router-selection-01.txt   draft-ietf-ipv6-router-selection-02.txt 
IPng Working Group Richard Draves IPng Working Group R. Draves
Internet Draft Microsoft Research Internet Draft Microsoft Research
Document: draft-ietf-ipv6-router-selection-01.txt March 1, 2002 Document: draft-ietf-ipv6-router-selection-02.txt R. Hinden
Nokia
June 10, 2002
Default Router Preferences and More-Specific Routes Default Router Preferences, More-Specific Routes, and Load Sharing
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026 [1]. all provisions of Section 10 of RFC 2026 [1].
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
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material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt. http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
Abstract Abstract
This document describes an optional extension to Neighbor Discovery This document describes two changes to Neighbor Discovery. The first
Router Advertisement messages for communicating default router change is an optional extension to Router Advertisement messages for
preferences and more-specific routes from routers to hosts. This communicating default router preferences and more-specific routes
improves the ability of hosts to pick an appropriate router, from routers to hosts. This improves the ability of hosts to pick an
especially when the host is multi-homed and the routers are on appropriate router, especially when the host is multi-homed and the
different links. The preference values and specific routes routers are on different links. The preference values and specific
advertised to hosts require administrative configuration; they are routes advertised to hosts require administrative configuration;
not automatically derived from routing tables. they are not automatically derived from routing tables. The second
change is a mandatory modification of the conceptual sending
algorithm to support load-sharing among equivalent routers.
1. Introduction 1. Introduction
Neighbor Discovery [2] specifies a conceptual model for hosts that Neighbor Discovery [2] specifies a conceptual model for hosts that
includes a Default Router List and a Prefix List. Hosts send Router includes a Default Router List and a Prefix List. Hosts send Router
Solicitation messages and receive from routers Router Advertisement Solicitation messages and receive Router Advertisement messages from
messages. Hosts populate their Default Router List and Prefix List routers. Hosts populate their Default Router List and Prefix List
based on information in the Router Advertisement messages. A based on information in the Router Advertisement messages. A
conceptual sending algorithm uses the Prefix List to determine if a conceptual sending algorithm uses the Prefix List to determine if a
destination address is on-link and the Default Router List to select destination address is on-link and the Default Router List to select
a router for off-link destinations. a router for off-link destinations.
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In some network topologies where the host has multiple routers on In some network topologies where the host has multiple routers on
its Default Router List, the choice of router for an off-link its Default Router List, the choice of router for an off-link
destination is important. In some situations, one router may provide destination is important. In some situations, one router may provide
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much better performance than another for a destination. In other much better performance than another for a destination. In other
situations, choosing the wrong router may result in a failure to situations, choosing the wrong router may result in a failure to
communicate. (A later section gives specific examples of these communicate. (A later section gives specific examples of these
scenarios.) scenarios.)
This document describes an optional extension to Neighbor Discovery This document describes an optional extension to Neighbor Discovery
Router Advertisement messages for communicating default router Router Advertisement messages for communicating default router
preferences and more-specific routes from routers to hosts. This preferences and more-specific routes from routers to hosts. This
improves the ability of hosts to pick an appropriate router for an improves the ability of hosts to pick an appropriate router for an
off-link destination. off-link destination.
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6-over-4 [3] or configured tunnel [4] network connections. 6-over-4 [3] or configured tunnel [4] network connections.
This document requires that the preference values and specific This document requires that the preference values and specific
routes advertised to hosts require explicit administrative routes advertised to hosts require explicit administrative
configuration. They are not automatically derived from routing configuration. They are not automatically derived from routing
tables. In particular, the preference values are not routing metrics tables. In particular, the preference values are not routing metrics
and it is not recommended that routers "dump out" their entire and it is not recommended that routers "dump out" their entire
routing tables to hosts. routing tables to hosts.
We use Router Advertisement messages, instead of some other protocol We use Router Advertisement messages, instead of some other protocol
like RIP [5], is that Router Advertisements are an existing like RIP [5], because Router Advertisements are an existing
standard, stable protocol for router-to-host communication. standard, stable protocol for router-to-host communication.
Piggybacking this information on existing message traffic from Piggybacking this information on existing message traffic from
routers to hosts reduces network overhead. Neighbor Discovery is to routers to hosts reduces network overhead. Neighbor Discovery is to
unicast routing as Multicast Listener Discovery is to multicast unicast routing as Multicast Listener Discovery is to multicast
routing. In both cases, a single simple protocol insulates the host routing. In both cases, a single simple protocol insulates the host
from the variety of router-to-router protocols. In addition, RIP is from the variety of router-to-router protocols. In addition, RIP is
unsuitable because it does not carry route lifetimes so it requires unsuitable because it does not carry route lifetimes so it requires
frequent message traffic with greater processing overheads. frequent message traffic with greater processing overheads.
This document also describes a mandatory change in host behavior.
Neighbor DiscoveryĂs conceptual sending algorithm is modified to
require hosts to select randomly among equivalent routers. This
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distributes traffic to different destinations among the routers.
Traffic to a single destination continues to use a single router,
because of the Destination Cache.
The mechanisms specified here are backwards-compatible, so that The mechanisms specified here are backwards-compatible, so that
hosts that do not implement them continue to function as well as hosts that do not implement them continue to function as well as
they did previously. they did previously.
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1.1. Conventions used in this document 1.1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC-2119 [6]. this document are to be interpreted as described in RFC-2119 [6].
2. Message Formats 2. Message Formats
2.1. Preference Values 2.1. Preference Values
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reachable Time | | Reachable Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Retrans Timer | | Retrans Timer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ... | Options ...
+-+-+-+-+-+-+-+-+-+-+-+- +-+-+-+-+-+-+-+-+-+-+-+-
Fields: Fields:
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Prf (Default Router Preference) Prf (Default Router Preference)
2-bit signed integer. Indicates whether or not to prefer 2-bit signed integer. Indicates whether or not to prefer
this router over other default routers. If Router this router over other default routers. If Router
Lifetime is zero, it MUST be initialized to zero by the Lifetime is zero, it MUST be initialized to zero by the
sender and MUST be ignored by the receiver. If the sender and MUST be ignored by the receiver. If the
Reserved (10) value is received, the receiver should Reserved (10) value is received, the receiver should
treat the RA as having a zero Router Lifetime. treat the RA as having a zero Router Lifetime.
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Resvd (Reserved) Resvd (Reserved)
A 3-bit unused field. It MUST be initialized to zero by A 3-bit unused field. It MUST be initialized to zero by
the sender and MUST be ignored by the receiver. the sender and MUST be ignored by the receiver.
Possible Options: Possible Options:
Route Information Route Information
These options specify prefixes that are reachable via These options specify prefixes that are reachable via
the router. the router.
Discussion: Discussion:
Note that in addition to the preference value in the message header, Note that in addition to the preference value in the message header,
a Router Advertisement can also contain a Route Information Option a Router Advertisement can also contain a Route Information Option
for ::/0, with a preference value and lifetime. Encoding a for ::/0, with a preference value and lifetime. Encoding a
preference value in the Router Advertisement header has some preference value in the Router Advertisement header has some
advantages: advantages:
1. It allows for a distinction between "best default router" and 1. It allows for a distinction between "best default router" and
"best router for default", as described below. "best router for default", as described below in section 5.1.
2. When the best default router is also the best router for 2. When the best default router is also the best router for
default (which will be a common case), encoding the preference default (which will be a common case), encoding the preference
value in the message header is more efficient than having to send value in the message header is more efficient than having to send
a separate option. a separate option.
2.3. Route Information Option 2.3. Route Information Option
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
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+ + + +
| | | |
+ Prefix + + Prefix +
| | | |
+ + + +
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields: Fields:
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Type TBD Type TBD
Length 1, 2, or 3 depending on Prefix Length. If Prefix Length Length 8-bit unsigned integer. The length of the option
is greater than 64, then Length must be at least 3. If (including the Type and Length fields) in units of
Prefix Length is greater than 0, then Length must be at 8 octets.
least 2. If Prefix Length is zero, then Length may be 1.
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Prefix Length Prefix Length
8-bit unsigned integer. The number of leading bits in 8-bit unsigned integer. The number of leading bits in
the Prefix that are valid. The value ranges from 0 to the Prefix that are valid. The value ranges from 0 to
128. 128.
Prf (Route Preference) Prf (Route Preference)
2-bit signed integer. Indicates whether or not to prefer 2-bit signed integer. Indicates whether or not to prefer
this router for the prefix over others. If the Reserved this router for the prefix over others. If the Reserved
(10) value is received, the Route Information Option (10) value is received, the Route Information Option
MUST be ignored. MUST be ignored.
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Resvd (Reserved) Resvd (Reserved)
Two 3-bit unused fields. They MUST be initialized to Two 3-bit unused fields. They MUST be initialized to
zero by the sender and MUST be ignored by the receiver. zero by the sender and MUST be ignored by the receiver.
Route Lifetime Route Lifetime
32-bit unsigned integer. The length of time in seconds 32-bit unsigned integer. The length of time in seconds
(relative to the time the packet is sent) that the (relative to the time the packet is sent) that the
prefix is valid for route determination. A value of all prefix is valid for route determination. A value of all
one bits (0xffffffff) represents infinity. one bits (0xffffffff) represents infinity.
Prefix An IP address or a prefix of an IP address. The Prefix Prefix Variable-length field containing an IP address or a
Length field contains the number of valid leading bits prefix of an IP address. The Prefix Length field
in the prefix. The bits in the prefix after the prefix contains the number of valid leading bits in the prefix.
length are reserved and MUST be initialized to zero by The bits in the prefix after the prefix length (if any)
the sender and ignored by the receiver. are reserved and MUST be initialized to zero by the
sender and ignored by the receiver.
The Prefix field is 0, 8, or 16 octets depending on The Length field is 1, 2, or 3 depending on Prefix Length. If Prefix
Length. Length is greater than 64, then Length must be 3. If Prefix Length
is greater than 0, then Length must be 2 or 3. If Prefix Length is
zero, then Length must be 1, 2, or 3.
The Prefix field is 0, 8, or 16 octets depending on Length.
Routers SHOULD NOT include in a Router Advertisement two Route Routers SHOULD NOT include in a Router Advertisement two Route
Information Options with the same Prefix and Prefix Length. If a Information Options with the same Prefix and Prefix Length. If a
host processes a Router Advertisement carrying multiple Router host processes a Router Advertisement carrying multiple Router
Information Options with the same Prefix and Prefix Length, it MUST Information Options with the same Prefix and Prefix Length, it MUST
process one of the options (unspecified which one) and it MUST process one of the options (unspecified which one) and it MUST
effectively ignore the rest. It MUST NOT retain some information effectively ignore the rest. It MUST NOT retain some information
(like preference) from one option and other information (like (like preference) from one option and other information (like
lifetime) from another option. lifetime) from another option.
Discussion: Discussion:
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There are several reasons for using a new Route Information Option, There are several reasons for using a new Route Information Option,
instead of using flag bits to overload the existing Prefix instead of using flag bits to overload the existing Prefix
Information Option: Information Option:
1. Prefixes will typically only show up in one or the other kind 1. Prefixes will typically only show up in one or the other kind
of option, not both, so a new option does not introduce of option, not both, so a new option does not introduce
duplication. duplication.
2. The Route Information Option is typically 16 octets while the 2. The Route Information Option is typically 16 octets while the
Prefix Information Option is 32 octets. Prefix Information Option is 32 octets.
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3. Using a new option may improve backwards-compatibility with 3. Using a new option may improve backwards-compatibility with
some host implementations. some host implementations.
3. Conceptual Model of a Host 3. Conceptual Model of a Host
There are three possible conceptual models for host implementation There are three possible conceptual models for host implementation
of default router preferences and more-specific routes, of default router preferences and more-specific routes,
corresponding to different levels of support. We refer to these as corresponding to different levels of support. We refer to these as
host A, host B, and host C. Note that these are really classes of host A, host B, and host C. Note that these are really classes of
hosts, not individual hosts. hosts, not individual hosts.
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Route Information Options. Route Information Options.
Host C uses a Routing Table instead of a Default Router List. (The Host C uses a Routing Table instead of a Default Router List. (The
Routing Table may also subsume the Prefix List, but that is beyond Routing Table may also subsume the Prefix List, but that is beyond
the scope of this document.) Entries in the Routing Table have a the scope of this document.) Entries in the Routing Table have a
prefix, prefix length, preference value, lifetime, and next-hop prefix, prefix length, preference value, lifetime, and next-hop
router. Host C uses both the Default Router Preference value in the router. Host C uses both the Default Router Preference value in the
Router Advertisement header and Route Information Options. Router Advertisement header and Route Information Options.
When host C receives a Router Advertisement, it modifies its Routing When host C receives a Router Advertisement, it modifies its Routing
Table as follows. If a route's lifetime is zero, the route is Table as follows. If the received route's lifetime is zero, the
removed from the Routing Table if present. If a route's lifetime is route is removed from the Routing Table if present. If a route's
non-zero, the route is added to the Routing Table if not present and lifetime is non-zero, the route is added to the Routing Table if not
the route's lifetime and preference is updated if the route is present and the route's lifetime and preference is updated if the
already present. A route is located in the Routing Table based on route is already present. A route is located in the Routing Table
prefix, prefix length, and next-hop router. When processing a Router based on prefix, prefix length, and next-hop router. When processing
Advertisment, host C first updates a ::/0 route based on the Router a Router Advertisement, host C first updates a ::/0 route based on
Lifetime and Default Router Preference in the Router Advertisement the Router Lifetime and Default Router Preference in the Router
message header. Then as host C processes Route Information Options Advertisement message header. Then as host C processes Route
in the Router Advertisement message body, it updates its routing Information Options in the Router Advertisement message body, it
table for each such option. The Router Preference and Lifetime updates its routing table for each such option. The Router
values in a ::/0 Route Information Option override the preference Preference and Lifetime values in a ::/0 Route Information Option
and lifetime values in the Router Advertisement header.
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override the preference and lifetime values in the Router
Advertisement header.
For example, suppose a host receives a Router Advertisement from For example, suppose a host receives a Router Advertisement from
router X with a Router Lifetime of 100 seconds and Default Router router X with a Router Lifetime of 100 seconds and Default Router
Preference of Medium. The body of the Router Advertisement contains Preference of Medium. The body of the Router Advertisement contains
a Route Information Option for ::/0 with a Route Lifetime of 200 a Route Information Option for ::/0 with a Route Lifetime of 200
seconds and a Route Preference of Low. After processing the Router seconds and a Route Preference of Low. After processing the Router
Advertisement, host A will have an entry for router X in its Default Advertisement, host A will have an entry for router X in its Default
Router List with lifetime 100 seconds. If host B receives the same Router List with lifetime 100 seconds. If host B receives the same
Router Advertisement, it will have an entry in its Default Router Router Advertisement, it will have an entry in its Default Router
List for router X with Medium preference and lifetime 100 seconds. List for router X with Medium preference and lifetime 100 seconds.
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Host C will have an entry in its Routing Table for ::/0 -> router X, Host C will have an entry in its Routing Table for ::/0 -> router X,
with Low preference and lifetime 200 seconds. with Low preference and lifetime 200 seconds. Host C MAY have a
transient state, during processing of the Router Advertisement, in
which it has an entry in its Routing Table for ::/0 -> router X with
Medium preference and lifetime 100 seconds.
3.2. Conceptual Sending Algorithm for Hosts 3.2. Conceptual Sending Algorithm for Hosts
Host A uses the conceptual sending algorithm described in Neighbor Host A uses the conceptual sending algorithm described in Neighbor
Discovery [2]. Discovery [2], modified slightly to support load sharing as
described in section 3.5.
When host B does next-hop determination and consults its Default When host B does next-hop determination and consults its Default
Router List, it first prefers reachable routers over non-reachable Router List, it primarily prefers reachable routers over non-
routers and second uses the router preference values. If all default reachable routers and secondarily uses the router preference values.
routers are not reachable, then it SHOULD round-robin among them all
regardless of preference value.
When host C does next-hop determination and consults its Routing When host C does next-hop determination and consults its Routing
Table for an off-link destination, it first prefers reachable Table for an off-link destination, it first prefers reachable
routers over non-reachable routers, second uses longest-matching- routers over non-reachable routers, second uses longest-matching-
prefix, and third uses route preference values. prefix, and third uses route preference values.
If there are no reachable routers with routes matching the If there are no routes matching the destination (i.e., no default
destination, then host C SHOULD round-robin among all routers with routes and no more-specific routes), then if host C has a single
routes matching the destination regardless of preference value or interface then it SHOULD assume the destination is on-link to that
prefix length. interface. If host C has multiple interfaces then it SHOULD discard
the packet and report a Destination Unreachable / No Route To
If there are no routes matching the destination, then if host C has Destination error to the upper layer.
a single interface then it SHOULD assume the destination is on-link.
If host C has multiple interfaces then it SHOULD discard the packet
and report a Destination Unreachable / No Route To Destination error
to the upper layer.
For example: suppose host C has four entries in its Routing Table:
::/0 -> router W with Medium preference
2001::/16 -> router X with Medium preference
3ffe::/16 -> router Y with High preference
3ffe::/16 -> router Z with Low preference
and host C is sending to 3ffe::1, an off-link destination. If all
routers are reachable, then router Y will be chosen. If router Y is
not reachable, then router Z will be chosen. If routers Y and Z are
not reachable, then router W will be chosen. If routers W, Y, and Z
are all not reachable, then host C should round-robin among the
three routers. Router X will never be chosen because its prefix does
not match the destination.
3.3. Destination Cache Management 3.3. Destination Cache Management
When a host processes a Router Advertisement and updates its When host C processes a Router Advertisement and updates its
conceptual routing table, it SHOULD invalidate or remove Destination conceptual Routing Table, it SHOULD invalidate or remove Destination
Cache Entries and redo next-hop determination for destinations Cache Entries and redo next-hop determination for destinations
affected by the routing table changes. The host MAY implement this affected by the Routing Table changes. The host MAY implement this
requirement by flushing its entire Destination Cache. requirement by flushing its entire Destination Cache.
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3.4. Router Reachability Probing 3.4. Router Reachability Probing
When a host avoids using a non-reachable router X and instead uses a When a host avoids using a non-reachable router X and instead uses
reachable router Y, and the host would have used router X if router another router Y, and the host would have used router X if router X
X were reachable, then the host SHOULD probe router XĂs reachability were reachable, then the host SHOULD probe router X's reachability
by sending a Neighbor Solicitation. These probes MUST be rate-
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by sending a Neighbor Solicitation. A host MUST NOT probe a router's
reachability in the absence of useful traffic that the host would
have sent to the router if it were reachable. In any case, these
probes MUST be rate-limited to no more than one per minute per
router.
This requirement allows the host to discover when router X becomes This requirement allows the host to discover when router X becomes
reachable and to start using router X at that point. Otherwise, the reachable and to start using router X at that point. Otherwise, the
host might not notice router XĂs reachability and continue to use host might not notice router XĂs reachability and continue to use
the less-desirable router Y. the less-desirable router Y.
3.5. Host Load Sharing 3.5. Host Load Sharing
Sometimes a host has a choice of multiple "equivalent" routers for a Sometimes a host has a choice of multiple "equivalent" routers for a
destination. We say that two routers are equivalent for a destination. We say that two routers are equivalent for a
destination if they have the same reachability, the same matching destination if they have the same reachability, the same matching
prefix length (if the host supports a Routing Table), and the same prefix length (if the host supports a Routing Table), and the same
preference values (if the host supports preference values). preference values (if the host supports preference values).
When a host chooses from multiple equivalent routers, it SHOULD When a host chooses from multiple equivalent routers, it MUST choose
choose randomly. randomly.
This has the effect of distributing load for new destinations among This has the effect of distributing load for new destinations among
the equivalent routers. Note that traffic to a single destination the equivalent routers. Note that traffic to a single destination
will use a single router as long as the Destination Cache Entry for will use a single router as long as the Destination Cache Entry for
the destination is not deleted. Random selection, instead of round- the destination is not deleted. Random selection, instead of round-
robin, is used to avoid synchronization issues. robin, is used to avoid synchronization issues.
3.6. Example
For example: suppose host C has five entries in its Routing Table:
::/0 -> router W with Medium preference
2001::/16 -> router X with Medium preference
3ffe::/16 -> router Y1 with High preference
3ffe::/16 -> router Y2 with High preference
3ffe::/16 -> router Z with Low preference
and host C is sending to 3ffe::1, an off-link destination. If all
routers are reachable, then the host will choose randomly between
routers Y1 and Y2. If routers Y1 and Y2 are not reachable, then
router Z will be chosen and the reachability of routers Y1 and Y2
will be probed. If routers Y1, Y2, and Z are not reachable, then
router W will be chosen and the reachability of routers Y1, Y2, and
Z will be probed. If routers W, Y1, Y2, and Z are all not reachable,
then host C should round-robin among Y1 and Y2 while probing the
reachability of W and Z. Router X will never be chosen because its
prefix does not match the destination.
4. Router Configuration 4. Router Configuration
Routers should not advertise preferences or routes by default. In Routers should not advertise preferences or routes by default. In
particular, they should not "dump out" their entire routing table to particular, they should not "dump out" their entire routing table to
hosts. Routers MAY have a configuration mode where a filter is hosts. Routers MAY have a configuration mode where a filter is
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applied to their routing table to obtain the routes that are applied to their routing table to obtain the routes that are
advertised to hosts. advertised to hosts.
The preference values (both Default Router Preferences and Route The preference values (both Default Router Preferences and Route
Preferences) should not be routing metrics or automatically derived Preferences) should not be routing metrics or automatically derived
from metrics: the preference values should be configured. The High from metrics: the preference values should be configured. The High
and Low (non-default) preference values should only be used when and Low (non-default) preference values should only be used when
someone with knowledge of both routers and the network topology someone with knowledge of both routers and the network topology
configures them explicitly. For example, it could be a common configures them explicitly. For example, it could be a common
network administrator, or it could be a customer request to network administrator, or it could be a customer request to
different administrators managing the routers. different administrators managing the routers.
As one exception to this general rule, the administrator of a router As one exception to this general rule, the administrator of a router
that does not have a connection to the internet, or is connected that does not have a connection to the internet, or is connected
through a firewall that blocks general traffic, may configure the through a firewall that blocks general traffic, may configure the
router to advertise a Low Default Router Preference. router to advertise a Low Default Router Preference.
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An administrator of a router may configure the router to advertise An administrator of a router may configure the router to advertise
specific routes for directly connected subnets and any shorter specific routes for directly connected subnets and any shorter
prefixes (eg, site, NLA, or TLA prefixes) for networks to which the prefixes (eg, site, NLA, or TLA prefixes) for networks to which the
router belongs. router belongs.
For example, if a home user sets up a tunnel into a firewalled For example, if a home user sets up a tunnel into a firewalled
corporate network, the access router on the corporate network end of corporate network, the access router on the corporate network end of
the tunnel can advertise itself as a default router, but with a Low the tunnel can advertise itself as a default router, but with a Low
preference. Furthermore the corporate router can advertise a preference. Furthermore the corporate router can advertise a
specific route for the corporate site prefix. The net result is that specific route for the corporate site prefix. The net result is that
destinations in the corporate network will be reached via the destinations in the corporate network will be reached via the
tunnel, and general internet destinations will be reached via the tunnel, and general internet destinations will be reached via the
home ISP. Without these mechanisms, the home machine might choose to home ISP. Without these mechanisms, the home machine might choose to
send internet traffic into the corporate network or corporate send internet traffic into the corporate network or corporate
traffic into the internet, leading to communication failure because traffic into the internet, leading to communication failure because
of the firewall. of the firewall.
Routers SHOULD NOT send more than 17 Route Information Options in Routers SHOULD NOT send more than 17 Route Information Options in
Router Advertisements per link. Router Advertisements per link. This arbitrary bound is meant to
reinforce that relatively few and carefully selected routes should
be advertised to hosts.
5. Examples 5. Examples
5.1. Best Default Router vs Best Route for Default 5.1. Best Default Router vs Best Route for Default
The best default router is not quite the same thing as the best The best default router is not quite the same thing as the best
router for default. The best default router is the router that will router for default. The best default router is the router that will
generate the fewest number of redirects for the host's traffic. The generate the fewest number of redirects for the host's traffic. The
best router for default is the router with the best route toward the best router for default is the router with the best route toward the
wider internet. wider internet.
For example, suppose a situation where you have a link with two For example, suppose a situation where you have a link with two
routers X and Y. Router X is the best for 2002::/16. (It's your 6to4 routers X and Y. Router X is the best for 2002::/16. (It's your 6to4
site gateway.) Router Y is the best for ::/0. (It connects to the site gateway.) Router Y is the best for ::/0. (It connects to the
native IPv6 internet.) Router X forwards native IPv6 traffic to native IPv6 internet.) Router X forwards native IPv6 traffic to
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router Y; router Y forwards 6to4 traffic to router X. But most router Y; router Y forwards 6to4 traffic to router X. But most
traffic from this site is sent to 2002:/16 destinations. In this traffic from this site is sent to 2002:/16 destinations. In this
scenario, router X is the best default router and router Y is the scenario, router X is the best default router and router Y is the
best router for default. best router for default.
To make host A work well, both routers should advertise themselves To make host A work well, both routers should advertise themselves
as default routers. In particular, if router Y goes down host A as default routers. In particular, if router Y goes down host A
should send traffic to router X to maintain 6to4 connectivity, so should send traffic to router X to maintain 6to4 connectivity, so
router X as well as router Y needs to be a default router. router X as well as router Y needs to be a default router.
To make host B work well, router X should in addition advertise To make host B work well, router X should in addition advertise
itself with a High default router preference. This will cause host B itself with a High default router preference. This will cause host B
to prefer router X, minimizing the number of redirects. to prefer router X, minimizing the number of redirects.
To make host C work well, router X should in addition advertise the To make host C work well, router X should in addition advertise the
::/0 route with Low preference and the 2002::/16 route with Medium ::/0 route with Low preference and the 2002::/16 route with Medium
preference. Host C will end up with three routes in its routing preference. Host C will end up with three routes in its routing
table: ::/0 -> router X (Low), ::/0 -> router Y (Medium), 2002::/16 table: ::/0 -> router X (Low), ::/0 -> router Y (Medium), 2002::/16
-> router X (Medium). It will send 6to4 traffic to router X and -> router X (Medium). It will send 6to4 traffic to router X and
other traffic to router Y. Host C will not cause any redirects. other traffic to router Y. Host C will not cause any redirects.
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Note that when host C processes the Router Advertisement from router Note that when host C processes the Router Advertisement from router
X, the Low preference for ::/0 overrides the High default router X, the Low preference for ::/0 overrides the High default router
preference. If the ::/0 specific route were not present, then host C preference. If the ::/0 specific route were not present, then host C
would apply the High default router preference to its ::/0 route to would apply the High default router preference to its ::/0 route to
router X. router X.
5.2. Multi-Homed Host and Isolated Network 5.2. Multi-Homed Host and Isolated Network
Here's another scenario: a multi-homed host is connected to the Here's another scenario: a multi-homed host is connected to the
6bone/internet via router X on one link and to an isolated network 6bone/internet via router X on one link and to an isolated network
skipping to change at line 519 skipping to change at line 545
advertise a Route Information Option for the isolated network advertise a Route Information Option for the isolated network
prefix. It might not advertise itself as a default router at all prefix. It might not advertise itself as a default router at all
(zero Router Lifetime), or it might advertise itself as a default (zero Router Lifetime), or it might advertise itself as a default
router with Low preference. Router Y should advertise itself as a router with Low preference. Router Y should advertise itself as a
default router with Medium preference. default router with Medium preference.
6. Security Considerations 6. Security Considerations
A malicious node could send Router Advertisement messages, A malicious node could send Router Advertisement messages,
specifying High Default Router Preference or carrying specific specifying High Default Router Preference or carrying specific
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routes, with the effect of pulling traffic away from legitimate routes, with the effect of pulling traffic away from legitimate
routers. However, a malicious node could easily achieve this same routers. However, a malicious node could easily achieve this same
effect in other ways. For example, it could fabricate Router effect in other ways. For example, it could fabricate Router
Advertisement messages with zero Router Lifetime from the other Advertisement messages with zero Router Lifetime from the other
routers, causing hosts to stop using the other routes. Hence, this routers, causing hosts to stop using the other routes. Hence, this
document has no appreciable impact on Internet infrastructure document has no new appreciable impact on Internet infrastructure
security. security.
References References
1 S. Bradner, "The Internet Standards Process -- Revision 3", BCP 1 S. Bradner, "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996. 9, RFC 2026, October 1996.
2 T. Narten, E. Nordmark, W. Simpson. "Neighbor Discovery for IP 2 T. Narten, E. Nordmark, W. Simpson. "Neighbor Discovery for IP
Version 6 (IPv6)", RFC 2461, December 1998. Version 6 (IPv6)", RFC 2461, December 1998.
3 B. Carpenter, K. Moore. "Connection of IPv6 Domains via IPv4 3 B. Carpenter, K. Moore. "Connection of IPv6 Domains via IPv4
Clouds", RFC 3056, February 2001. Clouds", RFC 3056, February 2001.
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4 R. Gilligan, E. Nordmark. "Transition Mechanisms for IPv6 Hosts 4 R. Gilligan, E. Nordmark. "Transition Mechanisms for IPv6 Hosts
and Routers", RFC 1933, April 1996. and Routers", RFC 1933, April 1996.
5 G. Malkin, R. Minnear. "RIPng for IPv6", RFC 2080 , January 1997. 5 G. Malkin, R. Minnear. "RIPng for IPv6", RFC 2080 , January 1997.
6 S. Bradner, "Key words for use in RFCs to Indicate Requirement 6 S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
Acknowledgments Acknowledgments
The author would like to acknowledge the contributions of Balash The authors would like to acknowledge the contributions of Balash
Akbari, Steve Deering, Robert Elz, Tony Hain, Bob Hinden, Christian Akbari, Steve Deering, Robert Elz, Tony Hain, Christian Huitema,
Huitema, JINMEI Tatuya, Erik Nordmark, Dave Thaler, and Brian Zill. JINMEI Tatuya, Erik Nordmark, Pekka Savola, Dave Thaler, and Brian
The packet diagrams are derived from Neighbor Discovery [2]. The Zill. The packet diagrams are derived from Neighbor Discovery [2].
description of host load sharing is derived from Bob Hinden's draft The description of host load sharing is derived from Bob Hinden's
on the subject. draft on the subject.
Author's Addresses Author's Addresses
Richard Draves Richard Draves
Microsoft Research Microsoft Research
One Microsoft Way One Microsoft Way
Redmond, WA 98052 Redmond, WA 98052
Phone: 1-425-706-2268 Phone: +1 425 706 2268
Email: richdr@microsoft.com Email: richdr@microsoft.com
Robert M. Hinden
Nokia
313 Fairchild Drive
Mountain View, CA 94043
Phone: +1 650 625 2004
Email: hinden@iprg.nokia.com
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Revision History Revision History
Changes from draft-ietf-ipv6-router-selection-01
Added Bob Hinden as co-author.
Various clarifications and textual improvements.
Slightly simplified the specification of round-robining in next-hop
determination, relying on router-reachability probing in some cases.
Clarified that router reachability probing only happens when the
host is sending packets that would have gone to that router if it
were reachable.
Changed load sharing to a mandatory requirement and added supporting
text to the title, abstract, and introduction.
Changes from draft-ietf-ipngwg-router-selection-00 Changes from draft-ietf-ipngwg-router-selection-00
Specified reachability probing of otherwise more-preferred but Specified reachability probing of otherwise more-preferred but
currently unreachable routers. currently unreachable routers.
Changed the requirement of Destination Cache invalidation, from MAY Changed the requirement of Destination Cache invalidation, from MAY
to SHOULD, but allowing flushing of the entire Destination Cache. to SHOULD, but allowing flushing of the entire Destination Cache.
Added a section specifying load sharing among equivalent routers. Added a section specifying load sharing among equivalent routers.
skipping to change at line 590 skipping to change at line 642
seen. seen.
Specified that routers SHOULD NOT send more than 17 Route Specified that routers SHOULD NOT send more than 17 Route
Information Options. Information Options.
Added discussion of Destination Cache invalidation, allowing but not Added discussion of Destination Cache invalidation, allowing but not
requiring it. requiring it.
Removed references to the fourth conceptual host model, host D. Removed references to the fourth conceptual host model, host D.
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Changes from draft-draves-ipngwg-router-selection-00 Changes from draft-draves-ipngwg-router-selection-00
Made the option variable length. Must ignore prefix bits past prefix Made the option variable length. Must ignore prefix bits past prefix
length. length.
Added more allowable router configuration scenarios, weakening the Added more allowable router configuration scenarios, weakening the
requirement that one administrator must coordinate the configuration requirement that one administrator must coordinate the configuration
of all relevant routers. of all relevant routers.
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Full Copyright Statement Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved. Copyright (C) The Internet Society (2000). All Rights Reserved.
This document and translations of it may be copied and furnished to This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph kind, provided that the above copyright notice and this paragraph
are included on all such copies and derivative works. However, this are included on all such copies and derivative works. However, this
skipping to change at line 630 skipping to change at line 680
The limited permissions granted above are perpetual and will not be The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns. revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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 End of changes. 

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