Default Router Preferences and More-Specific Routes
draft-ietf-ipv6-router-selection-07

IPng Working Group                                             R. Draves 
Internet Draft                                                 D. Thaler 
Document: draft-ietf-ipv6-router-selection-07.txt              Microsoft 
January 17, 2005                                                         
 
          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 (2005).  All Rights Reserved. 

Abstract 

   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 
   scenarios.) 

   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 
   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 
   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", 
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in 
   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 
   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 
   integer. 

   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 ... 
   +-+-+-+-+-+-+-+-+-+-+-+- 

   Fields: 

  
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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. 

   Discussion: 

   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 
   advantages: 

     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)                    | 
   .                                                               . 
   .                                                               . 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 

   Fields: 

   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 
               Length. 

   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.  

   Discussion: 

   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 
     duplication. 

  
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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 
   Options. 

   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 
   preferences. 

  
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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 
   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 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 
   hosts.  

   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 
   router. 

   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 
   redirects. 

   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 
   router. 

   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 
   Formats. 

   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 
             1998. 

   [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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draft-ietf-ipv6-router-selection-07                   January 17, 2005 
 
 
   [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 
   Microsoft 
   One Microsoft Way 
   Redmond, WA 98052 
   Phone: +1 425 703 8835 
   Email: dthaler@microsoft.com 

  
Draves                    Expires April 2005                        13 

draft-ietf-ipv6-router-selection-07                   January 17, 2005 
 
 
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