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Network Working Group R. Draves
Request for Comments: 4191 D. Thaler
Category: Standards Track Microsoft
November 2005
Default Router Preferences and More-Specific Routes
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
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
determine if a destination address is on-link and uses 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.
(Section 5 gives specific examples of these scenarios.)
Draves & Thaler Standards Track [Page 1]
RFC 4191 Router Preferences and More-Specific Routes November 2005
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.
Note that since these procedures are applicable to hosts only, the
forwarding algorithm used by the routers (including hosts with
enabled IP forwarding) is not affected.
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 lifetimes
so it requires frequent message traffic with greater processing
overheads.
Draves & Thaler Standards Track [Page 2]
RFC 4191 Router Preferences and More-Specific Routes November 2005
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.
Draves & Thaler Standards Track [Page 3]
RFC 4191 Router Preferences and More-Specific Routes November 2005
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:
Prf (Default Router Preference)
2-bit signed integer. Indicates whether to prefer this
router over other default routers. If the 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:
Draves & Thaler Standards Track [Page 4]
RFC 4191 Router Preferences and More-Specific Routes November 2005
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 sending 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 24
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 the
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.
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RFC 4191 Router Preferences and More-Specific Routes November 2005
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 option, not both, so a
new option does not introduce duplication.
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 a 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].
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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
the 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 a 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 a lifetime of 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 a Medium preference and a
lifetime of 100 seconds. A type C host will have an entry in its
Routing Table for ::/0 -> router X, with a Low preference and a
lifetime of 200 seconds. During processing of the Router
Advertisement, a type C host MAY have a transient state, in which it
has an entry in its Routing Table for ::/0 -> router X with a Medium
preference and a lifetime of 100 seconds.
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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 that may not support
preferences.
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
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RFC 4191 Router Preferences and More-Specific Routes November 2005
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 time. 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 a Medium preference
2002::/16 -> router X with a Medium preference
2001:db8::/32-> router Y with a High preference
2001:db8::/32-> router Z with a 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 in which an 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
Draves & Thaler Standards Track [Page 9]
RFC 4191 Router Preferences and More-Specific Routes November 2005
prefix in the routing table installed by another source, such as a
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 an 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
the actions of system management. For instance, the lifetime or
preference of advertised routes may change, or new routes could be
added. 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.
Draves & Thaler Standards Track [Page 10]
RFC 4191 Router Preferences and More-Specific Routes November 2005
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
configure the settings that will work in an optimal fashion
regardless of 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 in which 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 and router Y need to be default routers.
To make type B hosts work well, router X should advertise itself with
a High default router preference. This will cause type B hosts to
prefer router X, minimizing the number of redirects.
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RFC 4191 Router Preferences and More-Specific Routes November 2005
To make type C hosts work well, router X should in addition advertise
the ::/0 route with a Low preference and the 2002::/16 route with a
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
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 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.
If the routers are configured appropriately, a multi-homed type B
host in this same situation would have stable Internet connectivity,
but would have no connectivity to the isolated test network.
If the routers are configured appropriately, a multi-homed type C
host in this same situation can correctly choose between routers X
and Y. 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 a
Low preference. Router X should advertise itself as a default router
with a Medium preference.
6. Security Considerations
A malicious node could send Router Advertisement messages, specifying
a 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.
Draves & Thaler Standards Track [Page 12]
RFC 4191 Router Preferences and More-Specific Routes November 2005
For example, it could fabricate Router Advertisement messages with a
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 RFC
2461, where a malicious node causes a prefix to appear as on-link
indefinitely, resulting in a 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 a 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 24 within
the numbering space for IPv6 Neighbor Discovery Option Formats.
8. Acknowledgements
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.
Draves & Thaler Standards Track [Page 13]
RFC 4191 Router Preferences and More-Specific Routes November 2005
[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.
[RFC3756] Nikander, P., 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 & Thaler Standards Track [Page 14]
RFC 4191 Router Preferences and More-Specific Routes November 2005
Full Copyright Statement
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Draves & Thaler Standards Track [Page 15]