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Versions: 01 02 03 04 05 06 07 rfc4191                                  
IPng Working Group                                          Richard Draves
Internet Draft                                          Microsoft Research
Document: draft-ietf-ipv6-router-selection-01.txt            March 1, 2002

          Default Router Preferences and More-Specific Routes

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026 [1].

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at


   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,
   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 [2] specifies a conceptual model for hosts that
   includes a Default Router List and a Prefix List. Hosts send Router
   Solicitation messages and receive from routers Router Advertisement
   messages. 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 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

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   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 hosts select an appropriate router.

   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
   6-over-4 [3] or configured tunnel [4] 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 [5], is that 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 is to
   unicast routing as Multicast Listener Discovery is to multicast
   routing. In both cases, a single simple protocol insulates the host
   from the variety of router-to-router protocols. In addition, RIP is
   unsuitable because it does not carry route 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.

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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 RFC-2119 [6].

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 in two bits, as follows:
        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 does not appear to be necessary for reasonable

2.2. Changes to Router Advertisement Message Format

   The changes from Neighbor Discovery [2] section 4.2 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 ...


   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, it MUST be initialized to zero by the
               sender and MUST be ignored by the receiver. If the
               Reserved (10) value is received, the receiver should
               treat the RA as having a zero Router Lifetime.

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   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 "best default router" and
     "best router for default", as described below.

     2. When the best default router is also the best router for
     default (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                             +
   |                                                               |
   +                                                               +
   |                                                               |


   Type        TBD

   Length      1, 2, or 3 depending on Prefix Length. If Prefix Length
               is greater than 64, then Length must be at least 3. If
               Prefix Length is greater than 0, then Length must be at
               least 2. If Prefix Length is zero, then Length may be 1.

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   Prefix Length
               8-bit unsigned integer. The number of leading bits in
               the Prefix that are valid. The value ranges from 0 to

   Prf (Route Preference)
               2-bit signed integer. Indicates whether or not to prefer
               this router for the prefix over others. 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      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 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

   Routers SHOULD NOT include in a Router Advertisement two Route
   Information Options with the same Prefix and Prefix Length. If a
   host processes a Router Advertisement carrying multiple Router
   Information Options with the same Prefix and Prefix Length, it MUST
   process one of the options (unspecified which one) and it MUST
   effectively ignore the rest. It MUST NOT retain some information
   (like preference) from one option and other information (like
   lifetime) from another option.


   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.

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     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
   host A, host B, and host C. Note that these are really classes of
   hosts, not individual hosts.

3.1. Conceptual Data Structures for Hosts

   Host A ignores default router preferences and more-specific routes.
   Host A uses the conceptual data structures described in Neighbor
   Discovery [2].

   Host B uses a Default Router List augmented with preference values.
   Host B does not have a routing table. Host B uses the Default Router
   Preference value in the Router Advertisement header. Host B ignores
   Route Information Options.

   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
   the scope of this document.) Entries in the Routing Table have a
   prefix, prefix length, preference value, lifetime, and next-hop
   router. Host C uses both the Default Router Preference value in the
   Router Advertisement header and Route Information Options.

   When host C receives a Router Advertisement, it modifies its Routing
   Table as follows. If a 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. A route is located in the Routing Table based on
   prefix, prefix length, and next-hop router. When processing a Router
   Advertisment, host C first updates a ::/0 route based on the Router
   Lifetime and Default Router Preference in the Router Advertisement
   message header. Then as host C 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.

   For example, suppose a host receives 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, 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 Advertisement, it will have an entry in its Default Router
   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,
   with Low preference and lifetime 200 seconds.

3.2. Conceptual Sending Algorithm for Hosts

   Host A uses the conceptual sending algorithm described in Neighbor
   Discovery [2].

   When host B does next-hop determination and consults its Default
   Router List, it first prefers reachable routers over non-reachable
   routers and second uses the router preference values. If all default
   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
   Table for an off-link destination, it first prefers reachable
   routers over non-reachable routers, second uses longest-matching-
   prefix, and third uses route preference values.

   If there are no reachable routers with routes matching the
   destination, then host C SHOULD round-robin among all routers with
   routes matching the destination regardless of preference value or
   prefix length.

   If there are no routes matching the destination, then if host C has
   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

   When a host processes a Router Advertisement and updates its
   conceptual routing table, it SHOULD invalidate or remove Destination
   Cache Entries and redo next-hop determination for destinations
   affected by the routing table changes. The host MAY implement this
   requirement by flushing its entire Destination Cache.

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3.4. Router Reachability Probing

   When a host avoids using a non-reachable router X and instead uses a
   reachable router Y, and the host would have used router X if router
   X were reachable, then the host SHOULD probe router XÆs reachability
   by sending a Neighbor Solicitation. 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
   reachable and to start using router X at that point. Otherwise, the
   host might not notice router XÆs reachability and continue to use
   the less-desirable router Y.

3.5. Host Load Sharing

   Sometimes a host has a choice of multiple "equivalent" routers for a
   destination. We say that two routers are equivalent for a
   destination if they have the same reachability, the same matching
   prefix length (if the host supports a Routing Table), and the same
   preference values (if the host supports preference values).

   When a host chooses from multiple equivalent routers, it SHOULD
   choose randomly.

   This has the effect of distributing load for new destinations among
   the equivalent routers. Note that traffic to a single destination
   will use a single router as long as the Destination Cache Entry for
   the destination is not deleted. Random selection, instead of round-
   robin, is used to avoid synchronization issues.

4. Router Configuration

   Routers should not advertise preferences or routes by default. In
   particular, they should not "dump out" their entire routing table to
   hosts. Routers MAY have a configuration mode where a filter is
   applied to their routing table to obtain the routes that are
   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 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, may configure the
   router to advertise a Low Default Router Preference.

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   An administrator of a router may configure the router to advertise
   specific routes for directly connected subnets and any shorter
   prefixes (eg, site, NLA, or TLA 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 can advertise itself as a default router, but with a Low
   preference. Furthermore the corporate router can 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.

   Routers SHOULD NOT send more than 17 Route Information Options in
   Router Advertisements per link.

5. Examples

5.1. Best Default Router vs Best Route for Default

   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
   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
   wider internet.

   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
   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. But most
   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
   best router for default.

   To make host A work well, both routers should advertise themselves
   as default routers. In particular, if router Y goes down host A
   should send traffic to router X to maintain 6to4 connectivity, so
   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
   itself with a High default router preference. This will cause host B
   to prefer router X, minimizing the number of redirects.

   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
   preference. Host C 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. Host C will not cause any redirects.

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   Note that when host C processes 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 host C
   would apply the High default router preference to its ::/0 route to
   router X.

5.2. Multi-Homed Host and Isolated Network

   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
   via router Y on another link. The multi-homed host might have a
   tunnel into a fire-walled corporate network, or it might be directly
   connected to an isolated test network.

   In this situation, a multi-homed host A (which has no default router
   preferences or more-specific routes) will have no way to 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 router.

   A multi-homed host C in this same situation can correctly choose
   between routers X and Y, if the routers are configured
   appropriately. For example, router X 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 Y 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. Hence, this
   document has no appreciable impact on Internet infrastructure


   1  S. Bradner, "The Internet Standards Process -- Revision 3", BCP
      9, RFC 2026, October 1996.

   2  T. Narten, E. Nordmark, W. Simpson. "Neighbor Discovery for IP
      Version 6 (IPv6)", RFC 2461, December 1998.

   3  B. Carpenter, K. Moore. "Connection of IPv6 Domains via IPv4
      Clouds", RFC 3056, February 2001.

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   4  R. Gilligan, E. Nordmark. "Transition Mechanisms for IPv6 Hosts
      and Routers", RFC 1933, April 1996.

   5  G. Malkin, R. Minnear. "RIPng for IPv6", RFC 2080 , January 1997.

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


   The author would like to acknowledge the contributions of Balash
   Akbari, Steve Deering, Robert Elz, Tony Hain, Bob Hinden, Christian
   Huitema, JINMEI Tatuya, Erik Nordmark, Dave Thaler, and Brian Zill.
   The packet diagrams are derived from Neighbor Discovery [2]. The
   description of host load sharing is derived from Bob Hinden's draft
   on the subject.

Author's Addresses

   Richard Draves
   Microsoft Research
   One Microsoft Way
   Redmond, WA 98052
   Phone: 1-425-706-2268
   Email: richdr@microsoft.com

Revision History

Changes from draft-ietf-ipngwg-router-selection-00

   Specified reachability probing of otherwise more-preferred but
   currently unreachable routers.

   Changed the requirement of Destination Cache invalidation, from MAY
   to SHOULD, but allowing flushing of the entire Destination Cache.

   Added a section specifying load sharing among equivalent routers.

Changes from draft-draves-ipngwg-router-selection-01

   Specified receiver processing when the Reserved preference value is

   Specified that routers SHOULD NOT send more than 17 Route
   Information Options.

   Added discussion of Destination Cache invalidation, allowing but not
   requiring it.

   Removed references to the fourth conceptual host model, host D.

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Changes from draft-draves-ipngwg-router-selection-00

   Made the option variable length. Must ignore prefix bits past prefix

   Added more allowable router configuration scenarios, weakening the
   requirement that one administrator must coordinate the configuration
   of all relevant routers.

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