IPng Working Group                                               R. Draves
Internet Draft                                                   D. Thaler
Document: draft-ietf-ipv6-router-selection-06.txt                Microsoft
October 11, 2004

          Default Router Preferences and More-Specific Routes

Status of this Memo

   By submitting this Internet-Draft, I certify that any applicable
   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 (2004).  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.

   In addition, this document addresses the problem of tracking
   reachability of a hostÆs routers so that it does not try to use
   routers which it believes are unreachable.  Using end-to-end
   information is required to solve cases where the router advertises
   itself to hosts, but the path to the desired destination is broken
   at some other point.  This problem is outside the scope of this
   document and is left for future work.

   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:

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    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 ...


   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.

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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

   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.

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               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

     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

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   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
   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 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

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   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

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

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   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 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.

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   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
   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

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   (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
   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

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   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.

   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 EDITORÆs 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

   [RFC2461] Narten, T., Nordmark, E. and W. Simpson, "Neighbor
             Discovery for IP Version 6 (IPv6)", RFC 2461, December

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3775] Johnson, D., Perkins, C. and J. Arkko, "Mobility Support
             in IPv6", RFC 3775, June 2004.

10.     Informative References

   [RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains
             via IPv4 Clouds", RFC 3056, February 2001.

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   [RFC2983] Gilligan, R. and E. Nordmark, "Transition Mechanisms for
             IPv6 Hosts and Routers", RFC 2893, August 2000.

   [RFC2080] Malkin, G. and R. Minnear, "RIPng for IPv6", RFC 2080,
             January 1997.

   [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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Draves                    Expires April 2005                        14