draft-ietf-ipv6-router-selection-07.txt   draft-ietf-ipv6-router-selection-08.txt 
IPng Working Group R. Draves This Internet-Draft, draft-ietf-ipv6-router-selection-07.txt, was published as a Proposed Standard, RFC 4191
Internet Draft D. Thaler (http://www.ietf.org/rfc/rfc4191.txt), on 2005-11-21.
Document: draft-ietf-ipv6-router-selection-07.txt Microsoft
January 17, 2005
Default Router Preferences and More-Specific Routes
Status of this Memo
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patent or other IPR claims of which I am aware have been disclosed,
or will be disclosed, and any of which I become aware will be
disclosed, in accordance with RFC 3668.
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Copyright Notice
Copyright (C) The Internet Society (2005). All Rights Reserved.
This document describes an optional extension to Router
Advertisement messages for communicating default router preferences
and more-specific routes from routers to hosts. This improves the
ability of hosts to pick an appropriate router, especially when the
host is multi-homed and the routers are on different links. The
preference values and specific routes advertised to hosts require
administrative configuration; they are not automatically derived
from routing tables.
1. Introduction
Neighbor Discovery [RFC2461] specifies a conceptual model for hosts
that includes a Default Router List and a Prefix List. Hosts send
Router Solicitation messages and receive Router Advertisement
messages from routers. Hosts populate their Default Router List and
Prefix List based on information in the Router Advertisement
messages. A conceptual sending algorithm uses the Prefix List to
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determine if a destination address is on-link and the Default Router
List to select a router for off-link destinations.
In some network topologies where the host has multiple routers on
its Default Router List, the choice of router for an off-link
destination is important. In some situations, one router may provide
much better performance than another for a destination. In other
situations, choosing the wrong router may result in a failure to
communicate. (A later section gives specific examples of these
This document describes an optional extension to Neighbor Discovery
Router Advertisement messages for communicating default router
preferences and more-specific routes from routers to hosts. This
improves the ability of hosts to pick an appropriate router for an
off-link destination.
Neighbor Discovery provides a Redirect message that routers can use
to correct a host's choice of router. A router can send a Redirect
message to a host, telling it to use a different router for a
specific destination. However, the Redirect functionality is limited
to a single link. A router on one link cannot redirect a host to a
router on another link. Hence, Redirect messages do not help multi-
homed (through multiple interfaces) hosts select an appropriate
Multi-homed hosts are an increasingly important scenario, especially
with IPv6. In addition to a wired network connection, like Ethernet,
hosts may have one or more wireless connections, like 802.11 or
Bluetooth. In addition to physical network connections, hosts may
have virtual or tunnel network connections. For example, in addition
to a direct connection to the public Internet, a host may have a
tunnel into a private corporate network. Some IPv6 transition
scenarios can add additional tunnels. For example, hosts may have
6to4 [RFC3056] or configured tunnel [RFC2893] network connections.
This document requires that the preference values and specific
routes advertised to hosts require explicit administrative
configuration. They are not automatically derived from routing
tables. In particular, the preference values are not routing metrics
and it is not recommended that routers "dump out" their entire
routing tables to hosts.
We use Router Advertisement messages, instead of some other protocol
like RIP [RFC2080], because Router Advertisements are an existing
standard, stable protocol for router-to-host communication.
Piggybacking this information on existing message traffic from
routers to hosts reduces network overhead. Neighbor Discovery shares
with Multicast Listener Discovery the property that they both define
host-to-router interactions, while shielding the host from having to
participate in more general router-to-router interactions. In
addition, RIP is unsuitable because it does not carry route
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lifetimes so it requires frequent message traffic with greater
processing overheads.
The mechanisms specified here are backwards-compatible, so that
hosts that do not implement them continue to function as well as
they did previously.
1.1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
this document are to be interpreted as described in [RFC2119].
2. Message Formats
2.1. Preference Values
Default router preferences and preferences for more-specific routes
are encoded the same way.
Preference values are encoded as a two bit signed integer, as
01 High
00 Medium (default)
11 Low
10 Reserved - MUST NOT be sent
Note that implementations can treat the value as a two-bit signed
Having just three values reinforces that they are not metrics and
more values do not appear to be necessary for reasonable scenarios.
2.2. Changes to Router Advertisement Message Format
The changes from Neighbor Discovery [RFC2461] section 4.2 and
[RFC3775] section 7.1 are as follows:
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
| Type | Code | Checksum |
| Cur Hop Limit |M|O|H|Prf|Resvd| Router Lifetime |
| Reachable Time |
| Retrans Timer |
| Options ...
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Prf (Default Router Preference)
2-bit signed integer. Indicates whether or not to prefer
this router over other default routers. If Router
Lifetime is zero, the preference value MUST be set to
(00) by the sender and MUST be ignored by the receiver.
If the Reserved (10) value is received, the receiver
MUST treat the value as if it were (00).
Resvd (Reserved)
A 3-bit unused field. It MUST be initialized to zero by
the sender and MUST be ignored by the receiver.
Possible Options:
Route Information
These options specify prefixes that are reachable via
the router.
Note that in addition to the preference value in the message header,
a Router Advertisement can also contain a Route Information Option
for ::/0, with a preference value and lifetime. Encoding a
preference value in the Router Advertisement header has some
1. It allows for a distinction between the "best router for the
default route" and the "router least likely to redirect common
traffic", as described below in section 5.1.
2. When the best router for the default route is also the router
least likely to redirect common traffic (which will be a common
case), encoding the preference value in the message header is more
efficient than having to send a separate option.
2.3. Route Information Option
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
| Type | Length | Prefix Length |Resvd|Prf|Resvd|
| Route Lifetime |
| Prefix (Variable Length) |
. .
. .
Type TBD
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Length 8-bit unsigned integer. The length of the option
(including the Type and Length fields) in units of
8 octets. The Length field is 1, 2, or 3 depending on
Prefix Length. If Prefix 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.
Prefix Length
8-bit unsigned integer. The number of leading bits in
the Prefix that are valid. The value ranges from 0 to
128. The Prefix field is 0, 8, or 16 octets depending on
Prf (Route Preference)
2-bit signed integer. The Route Preference indicates
whether to prefer the router associated with this prefix
over others, when multiple identical prefixes (for
different routers) have been received. If the Reserved
(10) value is received, the Route Information Option
MUST be ignored.
Resvd (Reserved)
Two 3-bit unused fields. They MUST be initialized to
zero by the sender and MUST be ignored by the receiver.
Route Lifetime
32-bit unsigned integer. The length of time in seconds
(relative to the time the packet is sent) that the
prefix is valid for route determination. A value of all
one bits (0xffffffff) represents infinity.
Prefix Variable-length field containing an IP address or a
prefix of an IP address. The Prefix Length field
contains the number of valid leading bits in the prefix.
The bits in the prefix after the prefix length (if any)
are reserved and MUST be initialized to zero by the
sender and ignored by the receiver.
Routers MUST NOT include two Route Information Options with the same
Prefix and Prefix Length in the same Router Advertisement.
There are several reasons for using a new Route Information Option,
instead of using flag bits to overload the existing Prefix
Information Option:
1. Prefixes will typically only show up in one or the other kind
of option, not both, so a new option does not introduce
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2. The Route Information Option is typically 16 octets while the
Prefix Information Option is 32 octets.
3. Using a new option may improve backwards-compatibility with
some host implementations.
3. Conceptual Model of a Host
There are three possible conceptual models for host implementation
of default router preferences and more-specific routes,
corresponding to different levels of support. We refer to these as
type A, type B, and type C.
3.1. Conceptual Data Structures for Hosts
Type A hosts ignore default router preferences and more-specific
routes. They use the conceptual data structures described in
Neighbor Discovery [RFC2461].
Type B hosts use a Default Router List augmented with preference
values, but ignore all Route Information Options. They use the
Default Router Preference value in the Router Advertisement header.
They ignore Route Information Options.
Type C hosts use a Routing Table instead of a Default Router List.
(The Routing Table may also subsume the Prefix List, but that is
beyond the scope of this document.) Entries in the Routing Table
have a prefix, prefix length, preference value, lifetime, and next-
hop router. Type C hosts use both the Default Router Preference
value in the Router Advertisement header and Route Information
When a type C host receives a Router Advertisement, it modifies its
Routing Table as follows. When processing a Router Advertisement, a
type C host first updates a ::/0 route based on the Router Lifetime
and Default Router Preference in the Router Advertisement message
header. Then as the host processes Route Information Options in the
Router Advertisement message body, it updates its routing table for
each such option. The Router Preference and Lifetime values in a
::/0 Route Information Option override the preference and lifetime
values in the Router Advertisement header. Updating each route is
done as follows. A route is located in the Routing Table based on
prefix, prefix length, and next-hop router. If the received route's
lifetime is zero, the route is removed from the Routing Table if
present. If a route's lifetime is non-zero, the route is added to
the Routing Table if not present and the route's lifetime and
preference is updated if the route is already present.
For example, suppose hosts receive a Router Advertisement from
router X with a Router Lifetime of 100 seconds and Default Router
Preference of Medium. The body of the Router Advertisement contains
a Route Information Option for ::/0 with a Route Lifetime of 200
seconds and a Route Preference of Low. After processing the Router
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Advertisement, a type A host will have an entry for router X in its
Default Router List with lifetime 100 seconds. If a type B host
receives the same Router Advertisement, it will have an entry for
router X in its Default Router List with Medium preference and
lifetime 100 seconds. A type C host will have an entry in its
Routing Table for ::/0 -> router X, with Low preference and lifetime
200 seconds. A type C host 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
Type A hosts use the conceptual sending algorithm described in
Neighbor Discovery [RFC2461].
When a type B host does next-hop determination and consults its
Default Router List, it primarily prefers reachable routers over
non-reachable routers and secondarily uses the router preference
values. If the host has no information about the router's
reachability then the host assumes the router is reachable.
When a type C host does next-hop determination and consults its
Routing Table for an off-link destination, it searches its routing
table to find the route with the longest prefix that matches the
destination, using route preference values as a tie-breaker if
multiple matching routes have the same prefix length. If the best
route points to a non-reachable router, this router is remembered
for the algorithm described in section 3.5 below, and the next best
route is consulted. This check is repeated until a matching route
is found that points to a reachable router, or no matching routes
remain. Again, if the host has no information about the router's
reachability then the host assumes the router is reachable.
If there are no routes matching the destination (i.e., no default
routes and no more-specific routes), then a type C host SHOULD
discard the packet and report a Destination Unreachable / No Route
To Destination error to the upper layer.
3.3. Destination Cache Management
When a type C host processes a Router Advertisement and updates its
conceptual Routing Table, it MUST invalidate or remove Destination
Cache Entries and redo next-hop determination for destinations
affected by the Routing Table changes.
3.4. Client Configurability
Type B and C hosts MAY be configurable with preference values that
override the values in Router Advertisements received. This is
especially useful for dealing with routers which may not support
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3.5. Router Reachability Probing
When a host avoids using any non-reachable router X and instead
sends a data packet to another router Y, and the host would have
used router X if router X were reachable, then the host SHOULD probe
each such router X's reachability by sending a single Neighbor
Solicitation to that router's address. 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
This requirement allows the host to discover when router X becomes
reachable and to start using router X at that point. Otherwise, the
host might not notice router X's reachability and continue to use
the less-desirable router Y until the next Router Advertisement is
sent by X. Note that the router may have been unreachable for
reasons other than being down (e.g., a switch in the middle being
down), so it may be up to 30 minutes (the maximum advertisement
period) before the next Router Advertisement would be sent.
For a type A host (following the algorithm in [RFC2461]), no probing
is needed since all routers are equally preferable. A type B or C
host, on the other hand, explicitly probes unreachable, preferable
routers to notice when they become reachable again.
3.6. Example
Suppose a type C host has four entries in its Routing Table:
::/0 -> router W with Medium preference
2002::/16 -> router X with Medium preference
2001:db8::/32-> router Y with High preference
2001:db8::/32-> router Z with Low preference
and the host is sending to 2001:db8::1, an off-link destination. If
all routers are reachable, then the host will choose router Y. If
router Y is not reachable, then router Z will be chosen and the
reachability of router Y will be probed. If routers Y and Z are not
reachable, then router W will be chosen and the reachability of
routers Y and Z will be probed. If routers W, Y, and Z are all not
reachable, then the host should use Y 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
Routers SHOULD NOT advertise preferences or routes by default. In
particular, they SHOULD NOT "dump out" their entire routing table to
Routers MAY have a configuration mode where announcement of a
specific prefix is dependent on a specific condition, such as
operational status of a link or presence of the same or another
prefix in the routing table installed by another source, such as a
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dynamic routing protocol. If done, router implementations SHOULD
make sure that announcement of prefixes to hosts is decoupled from
the routing table dynamics to prevent excessive load on hosts during
periods of routing instability. In particular, unstable routes
SHOULD NOT be announced to hosts until their stability has improved.
Routers SHOULD NOT send more than 17 Route Information Options in
Router Advertisements per link. This arbitrary bound is meant to
reinforce that relatively few and carefully selected routes should
be advertised to hosts.
The preference values (both Default Router Preferences and Route
Preferences) SHOULD NOT be routing metrics or automatically derived
from metrics: the preference values SHOULD be configured.
The information contained in Router Advertisements may change
through actions of system management. For instance, the lifetime or
preference of advertised routes may change, new routes could be
added, etc. In such cases, the router MAY transmit up to
MAX_INITIAL_RTR_ADVERTISEMENTS unsolicited advertisements, using the
same rules as in [RFC2461]. When ceasing to be an advertising
interface and sending Router Advertisements with a Router Lifetime
of zero, the Router Advertisement SHOULD also set the Route Lifetime
to zero in all Route Information Options.
4.1. Guidance to Administrators
The High and Low (non-default) preference values should only be used
when someone with knowledge of both routers and the network topology
configures them explicitly. For example, it could be a common
network administrator, or it could be a customer request to
different administrators managing the routers.
As one exception to this general rule, the administrator of a router
that does not have a connection to the Internet, or is connected
through a firewall that blocks general traffic, should configure the
router to advertise a Low Default Router Preference.
In addition, the administrator of a router should configure the
router to advertise a specific route for the site prefix of the
network(s) to which the router belongs. The administrator may also
configure the router to advertise specific routes for directly
connected subnets and any shorter prefixes for networks to which the
router belongs.
For example, if a home user sets up a tunnel into a firewalled
corporate network, the access router on the corporate network end of
the tunnel should advertise itself as a default router, but with a
Low preference. Furthermore the corporate router should advertise a
specific route for the corporate site prefix. The net result is that
destinations in the corporate network 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
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send Internet traffic into the corporate network or corporate
traffic into the Internet, leading to communication failure because
of the firewall.
It is worth noting that the network administrator setting up
preferences and/or more specific routes in Routing Advertisements
typically does not know which kind of nodes (Type A, B, and/or C)
will be connected to its links. This requires that the administrator
will need to configure the settings that will work in an optimal
fashion no matter which kinds of nodes will be attached. Two
examples of how to do so follow.
5. Examples
5.1. Best Router for ::/0 vs Router Least Likely to Redirect
The best router for the default route is the router with the best
route toward the wider Internet. The router least likely to
redirect traffic depends on the actual traffic usage. The two
concepts can be different when the majority of communication
actually needs to go through some other router.
For example, consider 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
site gateway.) Router Y is the best for ::/0. (It connects to the
native IPv6 Internet.) Router X forwards native IPv6 traffic to
router Y; router Y forwards 6to4 traffic to router X. If most
traffic from this site is sent to 2002:/16 destinations, then router
X is the one least likely to redirect.
To make type A hosts work well, both routers should advertise
themselves as default routers. In particular, if router Y goes down,
type A hosts should send traffic to router X to maintain 6to4
connectivity, so router X as well as router Y needs to be a default
To make type B hosts work well, router X should in addition
advertise itself with a High default router preference. This will
cause type B hosts to prefer router X, minimizing the number of
To make type C hosts work well, router X should in addition
advertise the ::/0 route with Low preference and the 2002::/16 route
with Medium preference. A type C host will end up with three routes
in its routing table: ::/0 -> router X (Low), ::/0 -> router Y
(Medium), 2002::/16 -> router X (Medium). It will send 6to4 traffic
to router X and other traffic to router Y. Type C hosts will not
cause any redirects.
Note that when type C hosts process the Router Advertisement from
router X, the Low preference for ::/0 overrides the High default
router preference. If the ::/0 specific route were not present, then
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a type C host would apply the High default router preference to its
::/0 route to router X.
5.2. Multi-Homed Host and Isolated Network
In another scenario, a multi-homed host is connected to the Internet
via router X on one link and to an isolated network via router Y on
another link. The multi-homed host might have a tunnel into a
firewalled corporate network, or it might be directly connected to
an isolated test network.
In this situation, a type A multi-homed host (which has no default
router preferences or more-specific routes) will have no way to
intelligently choose between the two routers X and Y on its Default
Router List. Users of the host will see unpredictable connectivity
failures, depending on the destination address and the choice of
A multi-homed type B hose in this same situation would have stable
Internet connectivity, if the routers are configured appropriately,
but would have no connectivity to the isolated test network.
A multi-homed type C host in this same situation can correctly
choose between routers X and Y, if the routers are configured
appropriately. For example, router Y on the isolated network should
advertise a Route Information Option for the isolated network
prefix. It might not advertise itself as a default router at all
(zero Router Lifetime), or it might advertise itself as a default
router with Low preference. Router X should advertise itself as a
default router with Medium preference.
6. Security Considerations
A malicious node could send Router Advertisement messages,
specifying High Default Router Preference or carrying specific
routes, with the effect of pulling traffic away from legitimate
routers. However, a malicious node could easily achieve this same
effect in other ways. For example, it could fabricate Router
Advertisement messages with zero Router Lifetime from the other
routers, causing hosts to stop using the other routes. By
advertising a specific prefix, this attack could be carried out in a
less noticeable way. However, this attack has no significant
incremental impact on Internet infrastructure security.
A malicious node could also include an infinite lifetime in a Route
Information Option causing the route to linger indefinitely. A
similar attack already exists with Prefix Information Options in
RFC2461, where a malicious node causes a prefix to appear as on-link
indefinitely, resulting in lack of connectivity if it is not. In
contrast, an infinite lifetime in a Route Information Option will
cause router reachability probing to continue indefinitely, but will
not result in lack of connectivity.
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Similarly, a malicious node could also try to overload hosts with a
large number of routes in Route Information Options, or with very
frequent Route Advertisements. Again this same attack already
exists with Prefix Information Options.
[RFC3756] provides more details on the trust models, and there is
work in progress in the SEND WG on securing router discovery
messages that will address these problems.
7. IANA Considerations
Section 2.3 defines a new Neighbor Discovery [RFC2461] option, the
Route Information Option, which has been assigned the value TBD
within the numbering space for IPv6 Neighbor Discovery Option
RFC EDITOR's NOTE (to be removed prior to publication): the IANA is
requested to assign a value for "TBD" in the IPv6 Neighbor Discovery
Option Formats. When the assignment has been made, the RFC Editor
is asked to replace "TBD" (above in this section, and in section
2.3) with the assigned value and to remove this note.
8. Acknowledgments
The authors would like to acknowledge the contributions of Balash
Akbari, Steve Deering, Robert Elz, Tony Hain, Bob Hinden, Christian
Huitema, JINMEI Tatuya, Erik Nordmark, Pekka Savola, Kresimir
Segaric, and Brian Zill. The packet diagrams are derived from
Neighbor Discovery [RFC2461].
9. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2461] Narten, T., Nordmark, E. and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461, December
[RFC3775] Johnson, D., Perkins, C. and J. Arkko, "Mobility Support
in IPv6", RFC 3775, June 2004.
10. Informative References
[RFC2080] Malkin, G. and R. Minnear, "RIPng for IPv6", RFC 2080,
January 1997.
[RFC2893] Gilligan, R. and E. Nordmark, "Transition Mechanisms for
IPv6 Hosts and Routers", RFC 2893, August 2000.
[RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains
via IPv4 Clouds", RFC 3056, February 2001.
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[RFC3756] Nikander, P., Ed., Kempf, J. and E. Nordmark, "IPv6
Neighbor Discovery (ND) Trust Models and Threats", RFC
3756, May 2004.
Authors' Addresses
Richard Draves
Microsoft Research
One Microsoft Way
Redmond, WA 98052
Phone: +1 425 706 2268
Email: richdr@microsoft.com
Dave Thaler
One Microsoft Way
Redmond, WA 98052
Phone: +1 425 703 8835
Email: dthaler@microsoft.com
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Full Copyright Statement
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