draft-ietf-ipv6-router-selection-05.txt   draft-ietf-ipv6-router-selection-06.txt 
IPng Working Group R. Draves IPng Working Group R. Draves
Internet Draft D. Thaler Internet Draft D. Thaler
Document: draft-ietf-ipv6-router-selection-05.txt Microsoft Document: draft-ietf-ipv6-router-selection-06.txt Microsoft
August 10, 2004 October 11, 2004
Default Router Preferences and More-Specific Routes Default Router Preferences and More-Specific Routes
Status of this Memo Status of this Memo
By submitting this Internet-Draft, I certify that any applicable By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed, 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 or will be disclosed, and any of which I become aware will be
disclosed, in accordance with RFC 3668. disclosed, in accordance with RFC 3668.
skipping to change at line 56 skipping to change at line 56
1. Introduction 1. Introduction
Neighbor Discovery [RFC2461] specifies a conceptual model for hosts Neighbor Discovery [RFC2461] specifies a conceptual model for hosts
that includes a Default Router List and a Prefix List. Hosts send that includes a Default Router List and a Prefix List. Hosts send
Router Solicitation messages and receive Router Advertisement Router Solicitation messages and receive Router Advertisement
messages from routers. Hosts populate their Default Router List and messages from routers. Hosts populate their Default Router List and
Prefix List based on information in the Router Advertisement Prefix List based on information in the Router Advertisement
messages. A conceptual sending algorithm uses the Prefix List to 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 determine if a destination address is on-link and the Default Router
List to select a router for off-link destinations. List to select a router for off-link destinations.
In some network topologies where the host has multiple routers on In some network topologies where the host has multiple routers on
its Default Router List, the choice of router for an off-link its Default Router List, the choice of router for an off-link
destination is important. In some situations, one router may provide destination is important. In some situations, one router may provide
much better performance than another for a destination. In other much better performance than another for a destination. In other
situations, choosing the wrong router may result in a failure to situations, choosing the wrong router may result in a failure to
communicate. (A later section gives specific examples of these communicate. (A later section gives specific examples of these
scenarios.) scenarios.)
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We use Router Advertisement messages, instead of some other protocol We use Router Advertisement messages, instead of some other protocol
like RIP [RFC2080], because Router Advertisements are an existing like RIP [RFC2080], because Router Advertisements are an existing
standard, stable protocol for router-to-host communication. standard, stable protocol for router-to-host communication.
Piggybacking this information on existing message traffic from Piggybacking this information on existing message traffic from
routers to hosts reduces network overhead. Neighbor Discovery shares routers to hosts reduces network overhead. Neighbor Discovery shares
with Multicast Listener Discovery the property that they both define with Multicast Listener Discovery the property that they both define
host-to-router interactions, while shielding the host from having to host-to-router interactions, while shielding the host from having to
participate in more general router-to-router interactions. In participate in more general router-to-router interactions. In
addition, RIP is unsuitable because it does not carry route addition, RIP is unsuitable because it does not carry route
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lifetimes so it requires frequent message traffic with greater lifetimes so it requires frequent message traffic with greater
processing overheads. processing overheads.
In addition, this document addresses the problem of tracking
reachability of a hostĘs 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 The mechanisms specified here are backwards-compatible, so that
hosts that do not implement them continue to function as well as hosts that do not implement them continue to function as well as
they did previously. they did previously.
1.1. Conventions used in this document 1.1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in [RFC2119]. this document are to be interpreted as described in [RFC2119].
2. Message Formats 2. Message Formats
2.1. Preference Values 2.1. Preference Values
Default router preferences and preferences for more-specific routes Default router preferences and preferences for more-specific routes
are encoded the same way. are encoded the same way.
Preference values are encoded in two bits, as follows: Preference values are encoded as a two bit signed integer, as
follows:
01 High 01 High
00 Medium (default) 00 Medium (default)
11 Low 11 Low
10 Reserved - MUST NOT be sent 10 Reserved - MUST NOT be sent
Note that implementations can treat the value as a two-bit signed Note that implementations can treat the value as a two-bit signed
integer. integer.
Having just three values reinforces that they are not metrics and Having just three values reinforces that they are not metrics and
more values do not appear to be necessary for reasonable scenarios. more values do not appear to be necessary for reasonable scenarios.
2.2. Changes to Router Advertisement Message Format 2.2. Changes to Router Advertisement Message Format
The changes from Neighbor Discovery [RFC2461] section 4.2 and The changes from Neighbor Discovery [RFC2461] section 4.2 and
[RFC3775] section 7.1 are as follows: [RFC3775] section 7.1 are as follows:
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0 1 2 3 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 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 | | Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cur Hop Limit |M|O|H|Prf|Resvd| Router Lifetime | | Cur Hop Limit |M|O|H|Prf|Resvd| Router Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reachable Time | | Reachable Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Retrans Timer | | Retrans Timer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ... | Options ...
+-+-+-+-+-+-+-+-+-+-+-+- +-+-+-+-+-+-+-+-+-+-+-+-
Fields: Fields:
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Prf (Default Router Preference) Prf (Default Router Preference)
2-bit signed integer. Indicates whether or not to prefer 2-bit signed integer. Indicates whether or not to prefer
this router over other default routers. If Router this router over other default routers. If Router
Lifetime is zero, the preference value MUST be set to Lifetime is zero, the preference value MUST be set to
(00) by the sender and MUST be ignored by the receiver. (00) by the sender and MUST be ignored by the receiver.
If the Reserved (10) value is received, the receiver If the Reserved (10) value is received, the receiver
MUST treat the value as if it were (00). MUST treat the value as if it were (00).
Resvd (Reserved) Resvd (Reserved)
A 3-bit unused field. It MUST be initialized to zero by A 3-bit unused field. It MUST be initialized to zero by
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1. It allows for a distinction between the "best router for the 1. It allows for a distinction between the "best router for the
default route" and the "router least likely to redirect common default route" and the "router least likely to redirect common
traffic", as described below in section 5.1. traffic", as described below in section 5.1.
2. When the best router for the default route is also the router 2. When the best router for the default route is also the router
least likely to redirect common traffic (which will be a common least likely to redirect common traffic (which will be a common
case), encoding the preference value in the message header is more case), encoding the preference value in the message header is more
efficient than having to send a separate option. efficient than having to send a separate option.
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2.3. Route Information Option 2.3. Route Information Option
0 1 2 3 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 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| | Type | Length | Prefix Length |Resvd|Prf|Resvd|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Route Lifetime | | Route Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix (Variable Length) | | Prefix (Variable Length) |
. . . .
. . . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields: Fields:
Type TBD Type TBD
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Length 8-bit unsigned integer. The length of the option Length 8-bit unsigned integer. The length of the option
(including the Type and Length fields) in units of (including the Type and Length fields) in units of
8 octets. The Length field is 1, 2, or 3 depending on 8 octets. The Length field is 1, 2, or 3 depending on
Prefix Length. If Prefix Length is greater than 64, then Prefix Length. If Prefix Length is greater than 64, then
Length must be 3. If Prefix Length is greater than 0, 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 2 or 3. If Prefix Length is zero,
then Length must be 1, 2, or 3. then Length must be 1, 2, or 3.
Prefix Length Prefix Length
8-bit unsigned integer. The number of leading bits in 8-bit unsigned integer. The number of leading bits in
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Route Lifetime Route Lifetime
32-bit unsigned integer. The length of time in seconds 32-bit unsigned integer. The length of time in seconds
(relative to the time the packet is sent) that the (relative to the time the packet is sent) that the
prefix is valid for route determination. A value of all prefix is valid for route determination. A value of all
one bits (0xffffffff) represents infinity. one bits (0xffffffff) represents infinity.
Prefix Variable-length field containing an IP address or a Prefix Variable-length field containing an IP address or a
prefix of an IP address. The Prefix Length field prefix of an IP address. The Prefix Length field
contains the number of valid leading bits in the prefix. 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) The bits in the prefix after the prefix length (if any)
are reserved and MUST be initialized to zero by the are reserved and MUST be initialized to zero by the
sender and ignored by the receiver. sender and ignored by the receiver.
Routers MUST NOT include two Route Information Options with the same Routers MUST NOT include two Route Information Options with the same
Prefix and Prefix Length in the same Router Advertisement. Prefix and Prefix Length in the same Router Advertisement.
Discussion: Discussion:
There are several reasons for using a new Route Information Option, There are several reasons for using a new Route Information Option,
instead of using flag bits to overload the existing Prefix instead of using flag bits to overload the existing Prefix
Information Option: Information Option:
1. Prefixes will typically only show up in one or the other kind 1. Prefixes will typically only show up in one or the other kind
of option, not both, so a new option does not introduce of option, not both, so a new option does not introduce
duplication. duplication.
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2. The Route Information Option is typically 16 octets while the 2. The Route Information Option is typically 16 octets while the
Prefix Information Option is 32 octets. Prefix Information Option is 32 octets.
3. Using a new option may improve backwards-compatibility with 3. Using a new option may improve backwards-compatibility with
some host implementations. some host implementations.
3. Conceptual Model of a Host 3. Conceptual Model of a Host
There are three possible conceptual models for host implementation There are three possible conceptual models for host implementation
of default router preferences and more-specific routes, of default router preferences and more-specific routes,
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beyond the scope of this document.) Entries in the Routing Table beyond the scope of this document.) Entries in the Routing Table
have a prefix, prefix length, preference value, lifetime, and next- have a prefix, prefix length, preference value, lifetime, and next-
hop router. Type C hosts use both the Default Router Preference hop router. Type C hosts use both the Default Router Preference
value in the Router Advertisement header and Route Information value in the Router Advertisement header and Route Information
Options. Options.
When a type C host receives a Router Advertisement, it modifies its When a type C host receives a Router Advertisement, it modifies its
Routing Table as follows. When processing a Router Advertisement, a Routing Table as follows. When processing a Router Advertisement, a
type C host first updates a ::/0 route based on the Router Lifetime type C host first updates a ::/0 route based on the Router Lifetime
and Default Router Preference in the Router Advertisement message and Default Router Preference in the Router Advertisement message
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header. Then as the host processes Route Information Options in the header. Then as the host processes Route Information Options in the
Router Advertisement message body, it updates its routing table for Router Advertisement message body, it updates its routing table for
each such option. The Router Preference and Lifetime values in a each such option. The Router Preference and Lifetime values in a
::/0 Route Information Option override the preference and lifetime ::/0 Route Information Option override the preference and lifetime
values in the Router Advertisement header. Updating each route is values in the Router Advertisement header. Updating each route is
done as follows. If the received route's lifetime is zero, the done as follows. A route is located in the Routing Table based on
route is removed from the Routing Table if present. If a route's prefix, prefix length, and next-hop router. If the received route's
lifetime is non-zero, the route is added to the Routing Table if not lifetime is zero, the route is removed from the Routing Table if
present and the route's lifetime and preference is updated if the present. If a route's lifetime is non-zero, the route is added to
route is already present. A route is located in the Routing Table the Routing Table if not present and the route's lifetime and
based on prefix, prefix length, and next-hop router. preference is updated if the route is already present.
For example, suppose hosts receive a Router Advertisement from For example, suppose hosts receive a Router Advertisement from
router X with a Router Lifetime of 100 seconds and Default Router router X with a Router Lifetime of 100 seconds and Default Router
Preference of Medium. The body of the Router Advertisement contains Preference of Medium. The body of the Router Advertisement contains
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a Route Information Option for ::/0 with a Route Lifetime of 200 a Route Information Option for ::/0 with a Route Lifetime of 200
seconds and a Route Preference of Low. After processing the Router 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 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 Default Router List with lifetime 100 seconds. If a type B host
receives the same Router Advertisement, it will have an entry in its receives the same Router Advertisement, it will have an entry for
Default Router List for router X with Medium preference and lifetime router X in its Default Router List with Medium preference and
100 seconds. A type C host will have an entry in its Routing Table lifetime 100 seconds. A type C host will have an entry in its
for ::/0 -> router X, with Low preference and lifetime 200 seconds. Routing Table for ::/0 -> router X, with Low preference and lifetime
A type C host MAY have a transient state, during processing of the 200 seconds. A type C host MAY have a transient state, during
Router Advertisement, in which it has an entry in its Routing Table processing of the Router Advertisement, in which it has an entry in
for ::/0 -> router X with Medium preference and lifetime 100 its Routing Table for ::/0 -> router X with Medium preference and
seconds. lifetime 100 seconds.
3.2. Conceptual Sending Algorithm for Hosts 3.2. Conceptual Sending Algorithm for Hosts
Type A hosts use the conceptual sending algorithm described in Type A hosts use the conceptual sending algorithm described in
Neighbor Discovery [RFC2461]. Neighbor Discovery [RFC2461].
When a type B host does next-hop determination and consults its When a type B host does next-hop determination and consults its
Default Router List, it primarily prefers reachable routers over Default Router List, it primarily prefers reachable routers over
non-reachable routers and secondarily uses the router preference non-reachable routers and secondarily uses the router preference
values. If the host has no information about the router's values. If the host has no information about the router's
reachability then the host assumes the router is reachable. reachability then the host assumes the router is reachable.
When a type C host does next-hop determination and consults its When a type C host does next-hop determination and consults its
Routing Table for an off-link destination, it first prefers Routing Table for an off-link destination, it searches its routing
reachable routers over non-reachable routers, second uses longest- table to find the route with the longest prefix that matches the
matching-prefix, and third uses route preference values. Again, if destination, using route preference values as a tie-breaker if
the host has no information about the router's reachability then the multiple matching routes have the same prefix length. If the best
host assumes the router is reachable. 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 If there are no routes matching the destination (i.e., no default
routes and no more-specific routes), then a type C host SHOULD 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 discard the packet and report a Destination Unreachable / No Route
To Destination error to the upper layer. To Destination error to the upper layer.
3.3. Destination Cache Management 3.3. Destination Cache Management
When a type C host processes a Router Advertisement and updates its When a type C host processes a Router Advertisement and updates its
conceptual Routing Table, it MUST invalidate or remove Destination conceptual Routing Table, it MUST invalidate or remove Destination
Cache Entries and redo next-hop determination for destinations Cache Entries and redo next-hop determination for destinations
affected by the Routing Table changes. affected by the Routing Table changes.
3.4. Client Configurability 3.4. Client Configurability
Type B and C hosts MAY be configurable with preference values that Type B and C hosts MAY be configurable with preference values that
override the values in Router Advertisements received. This is override the values in Router Advertisements received. This is
especially useful for dealing with routers which may not support especially useful for dealing with routers which may not support
preferences. preferences.
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3.5. Router Reachability Probing 3.5. Router Reachability Probing
When a host avoids using any non-reachable router X and instead 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 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 used router X if router X were reachable, then the host SHOULD probe
each such router X's reachability by sending a single Neighbor each such router X's reachability by sending a single Neighbor
Solicitation to that router's address. A host MUST NOT probe a Solicitation to that router's address. A host MUST NOT probe a
router's reachability in the absence of useful traffic that the host 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, 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 these probes MUST be rate-limited to no more than one per minute per
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For a type A host (following the algorithm in [RFC2461]), no probing 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 is needed since all routers are equally preferable. A type B or C
host, on the other hand, explicitly probes unreachable, preferable host, on the other hand, explicitly probes unreachable, preferable
routers to notice when they become reachable again. routers to notice when they become reachable again.
3.6. Example 3.6. Example
Suppose a type C host has four entries in its Routing Table: Suppose a type C host has four entries in its Routing Table:
::/0 -> router W with Medium preference ::/0 -> router W with Medium preference
2001::/16 -> router X with Medium preference 2002::/16 -> router X with Medium preference
3ffe::/16 -> router Y with High preference 2001:db8::/32-> router Y with High preference
3ffe::/16 -> router Z with Low preference 2001:db8::/32-> router Z with Low preference
and the host is sending to 3ffe::1, an off-link destination. If all and the host is sending to 2001:db8::1, an off-link destination. If
routers are reachable, then the host will choose router Y. If router all routers are reachable, then the host will choose router Y. If
Y is not reachable, then router Z will be chosen and the 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 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 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 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 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 of W and Z. Router X will never be chosen because its prefix does
not match the destination. not match the destination.
4. Router Configuration 4. Router Configuration
Routers should not advertise preferences or routes by default. In Routers SHOULD NOT advertise preferences or routes by default. In
particular, they should not "dump out" their entire routing table to particular, they SHOULD NOT "dump out" their entire routing table to
hosts. Routers MAY have a configuration mode where a filter is hosts.
applied to their routing table to obtain the routes that are
advertised to hosts.
Routers SHOULD NOT send more than 17 Route Information Options in Routers SHOULD NOT send more than 17 Route Information Options in
Router Advertisements per link. This arbitrary bound is meant to Router Advertisements per link. This arbitrary bound is meant to
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reinforce that relatively few and carefully selected routes should reinforce that relatively few and carefully selected routes should
be advertised to hosts. be advertised to hosts.
The preference values (both Default Router Preferences and Route The preference values (both Default Router Preferences and Route
Preferences) should not be routing metrics or automatically derived Preferences) SHOULD NOT be routing metrics or automatically derived
from metrics: the preference values should be configured. from metrics: the preference values SHOULD be configured.
The information contained in Router Advertisements may change The information contained in Router Advertisements may change
through actions of system management. For instance, the lifetime or through actions of system management. For instance, the lifetime or
preference of advertised routes may change, new routes could be preference of advertised routes may change, new routes could be
added, etc. In such cases, the router MAY transmit up to added, etc. In such cases, the router MAY transmit up to
MAX_INITIAL_RTR_ADVERTISEMENTS unsolicited advertisements, using the MAX_INITIAL_RTR_ADVERTISEMENTS unsolicited advertisements, using the
same rules as in [RFC2461]. When ceasing to be an advertising same rules as in [RFC2461]. When ceasing to be an advertising
interface and sending Router Advertisements with a Router Lifetime interface and sending Router Advertisements with a Router Lifetime
of zero, the Router Advertisement SHOULD also set the Route Lifetime of zero, the Router Advertisement SHOULD also set the Route Lifetime
to zero in all Route Information Options. to zero in all Route Information Options.
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through a firewall that blocks general traffic, should configure the through a firewall that blocks general traffic, should configure the
router to advertise a Low Default Router Preference. router to advertise a Low Default Router Preference.
In addition, the administrator of a router should configure the In addition, the administrator of a router should configure the
router to advertise a specific route for the site prefix of the router to advertise a specific route for the site prefix of the
network(s) to which the router belongs. The administrator may also network(s) to which the router belongs. The administrator may also
configure the router to advertise specific routes for directly configure the router to advertise specific routes for directly
connected subnets and any shorter prefixes for networks to which the connected subnets and any shorter prefixes for networks to which the
router belongs. router belongs.
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For example, if a home user sets up a tunnel into a firewalled For example, if a home user sets up a tunnel into a firewalled
corporate network, the access router on the corporate network end of corporate network, the access router on the corporate network end of
the tunnel should advertise itself as a default router, but with a the tunnel should advertise itself as a default router, but with a
Low preference. Furthermore the corporate router should advertise a Low preference. Furthermore the corporate router should advertise a
specific route for the corporate site prefix. The net result is that specific route for the corporate site prefix. The net result is that
destinations in the corporate network will be reached via the destinations in the corporate network will be reached via the
tunnel, and general Internet destinations 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 home ISP. Without these mechanisms, the home machine might choose to
send Internet traffic into the corporate network or corporate send Internet traffic into the corporate network or corporate
traffic into the Internet, leading to communication failure because traffic into the Internet, leading to communication failure because
of the firewall. of the firewall.
It is worth noting that the network administrator setting up It is worth noting that the network administrator setting up
preferences and/or more specific routes in Routing Advertisements preferences and/or more specific routes in Routing Advertisements
typically does not know which kind of nodes (Type A, B, and/or C) 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 be connected to its links. This requires that the administrator
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will need to configure the settings that will work in an optimal will need to configure the settings that will work in an optimal
fashion no matter which kinds of nodes will be attached. fashion no matter which kinds of nodes will be attached. Two
examples of how to do so follow.
5. Examples 5. Examples
5.1. Best Router for ::/0 vs Router Least Likely to Redirect 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 The best router for the default route is the router with the best
route toward the wider Internet. The router least likely to route toward the wider Internet. The router least likely to
redirect traffic depends on the actual traffic usage. The two redirect traffic depends on the actual traffic usage. The two
concepts can be different when the majority of communication concepts can be different when the majority of communication
actually needs to go through some other router. actually needs to go through some other router.
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To make type B hosts work well, router X should in addition To make type B hosts work well, router X should in addition
advertise itself with a High default router preference. This will advertise itself with a High default router preference. This will
cause type B hosts to prefer router X, minimizing the number of cause type B hosts to prefer router X, minimizing the number of
redirects. redirects.
To make type C hosts work well, router X should in addition To make type C hosts work well, router X should in addition
advertise the ::/0 route with Low preference and the 2002::/16 route 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 with Medium preference. A type C host will end up with three routes
in its routing table: ::/0 -> router X (Low), ::/0 -> router Y 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 (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 to router X and other traffic to router Y. Type C hosts will not
cause any redirects. cause any redirects.
Note that when type C hosts process the Router Advertisement from Note that when type C hosts process the Router Advertisement from
router X, the Low preference for ::/0 overrides the High default router X, the Low preference for ::/0 overrides the High default
router preference. If the ::/0 specific route were not present, then router preference. If the ::/0 specific route were not present, then
a type C host would apply the High default router preference to its a type C host would apply the High default router preference to its
::/0 route to router X. ::/0 route to router X.
5.2. Multi-Homed Host and Isolated Network 5.2. Multi-Homed Host and Isolated Network
In another scenario, a multi-homed host is connected to the Internet 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 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 another link. The multi-homed host might have a tunnel into a
firewalled corporate network, or it might be directly connected to firewalled corporate network, or it might be directly connected to
an isolated test network. an isolated test network.
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In this situation, a type A multi-homed host (which has no default In this situation, a type A multi-homed host (which has no default
router preferences or more-specific routes) will have no way to router preferences or more-specific routes) will have no way to
intelligently choose between the two routers X and Y on its Default intelligently choose between the two routers X and Y on its Default
Router List. Users of the host will see unpredictable connectivity Router List. Users of the host will see unpredictable connectivity
failures, depending on the destination address and the choice of failures, depending on the destination address and the choice of
router. 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 A multi-homed type C host in this same situation can correctly
choose between routers X and Y, if the routers are configured choose between routers X and Y, if the routers are configured
appropriately. For example, router Y on the isolated network should appropriately. For example, router Y on the isolated network should
advertise a Route Information Option for the isolated network advertise a Route Information Option for the isolated network
prefix. It might not advertise itself as a default router at all prefix. It might not advertise itself as a default router at all
(zero Router Lifetime), or it might advertise itself as a default (zero Router Lifetime), or it might advertise itself as a default
router with Low preference. Router X should advertise itself as a router with Low preference. Router X should advertise itself as a
default router with Medium preference. default router with Medium preference.
6. Security Considerations 6. Security Considerations
skipping to change at line 572 skipping to change at line 591
routers. However, a malicious node could easily achieve this same routers. However, a malicious node could easily achieve this same
effect in other ways. For example, it could fabricate Router effect in other ways. For example, it could fabricate Router
Advertisement messages with zero Router Lifetime from the other Advertisement messages with zero Router Lifetime from the other
routers, causing hosts to stop using the other routes. By routers, causing hosts to stop using the other routes. By
advertising a specific prefix, this attack could be carried out in a advertising a specific prefix, this attack could be carried out in a
less noticeable way. However, this attack has no significant less noticeable way. However, this attack has no significant
incremental impact on Internet infrastructure security. incremental impact on Internet infrastructure security.
A malicious node could also include an infinite lifetime in a Route A malicious node could also include an infinite lifetime in a Route
Information Option causing the route to linger indefinitely. A Information Option causing the route to linger indefinitely. A
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similar attack already exists with Prefix Information Options in similar attack already exists with Prefix Information Options in
RFC2461, where a malicious node causes a prefix to appear as on-link RFC2461, where a malicious node causes a prefix to appear as on-link
indefinitely, resulting in lack of connectivity if it is not. In indefinitely, resulting in lack of connectivity if it is not. In
contrast, an infinite lifetime in a Route Information Option will contrast, an infinite lifetime in a Route Information Option will
cause router reachability probing to continue indefinitely, but will cause router reachability probing to continue indefinitely, but will
not result in lack of connectivity. 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 [RFC3756] provides more details on the trust models, and there is
work in progress in the SEND WG on securing router discovery work in progress in the SEND WG on securing router discovery
messages that will address these problems. messages that will address these problems.
7. IANA Considerations 7. IANA Considerations
Section 2.3 defines a new Neighbor Discovery [RFC2461] option, the Section 2.3 defines a new Neighbor Discovery [RFC2461] option, the
Route Information Option, which has been assigned the value TBD Route Information Option, which has been assigned the value TBD
within the numbering space for IPv6 Neighbor Discovery Option within the numbering space for IPv6 Neighbor Discovery Option
Formats. Formats.
RFC EDITORĘs NOTE (to be removed prior to publication): the IANA is RFC EDITORĘs NOTE (to be removed prior to publication): the IANA is
requested to assign a value for "TBD" in the IPv6 Neighbor Discovery requested to assign a value for "TBD" in the IPv6 Neighbor Discovery
Option Formats. When the assignment has been made, the RFC Editor Option Formats. When the assignment has been made, the RFC Editor
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is asked to replace "TBD" (above in this section, and in section is asked to replace "TBD" (above in this section, and in section
2.3) with the assigned value and to remove this note. 2.3) with the assigned value and to remove this note.
8. Acknowledgments 8. Acknowledgments
The authors would like to acknowledge the contributions of Balash The authors would like to acknowledge the contributions of Balash
Akbari, Steve Deering, Robert Elz, Tony Hain, Bob Hinden, Christian Akbari, Steve Deering, Robert Elz, Tony Hain, Bob Hinden, Christian
Huitema, JINMEI Tatuya, Erik Nordmark, Pekka Savola, Kresimir Huitema, JINMEI Tatuya, Erik Nordmark, Pekka Savola, Kresimir
Segaric, and Brian Zill. The packet diagrams are derived from Segaric, and Brian Zill. The packet diagrams are derived from
Neighbor Discovery [RFC2461]. Neighbor Discovery [RFC2461].
skipping to change at line 623 skipping to change at line 647
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3775] Johnson, D., Perkins, C. and J. Arkko, "Mobility Support [RFC3775] Johnson, D., Perkins, C. and J. Arkko, "Mobility Support
in IPv6", RFC 3775, June 2004. in IPv6", RFC 3775, June 2004.
10. Informative References 10. Informative References
[RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains [RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains
via IPv4 Clouds", RFC 3056, February 2001. via IPv4 Clouds", RFC 3056, February 2001.
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[RFC2983] Gilligan, R. and E. Nordmark, "Transition Mechanisms for [RFC2983] Gilligan, R. and E. Nordmark, "Transition Mechanisms for
IPv6 Hosts and Routers", RFC 2893, August 2000. IPv6 Hosts and Routers", RFC 2893, August 2000.
[RFC2080] Malkin, G. and R. Minnear, "RIPng for IPv6", RFC 2080, [RFC2080] Malkin, G. and R. Minnear, "RIPng for IPv6", RFC 2080,
January 1997. January 1997.
[RFC3756] Nikander, P., Ed., Kempf, J. and E. Nordmark, "IPv6 [RFC3756] Nikander, P., Ed., Kempf, J. and E. Nordmark, "IPv6
Neighbor Discovery (ND) Trust Models and Threats", RFC Neighbor Discovery (ND) Trust Models and Threats", RFC
3756, May 2004. 3756, May 2004.
skipping to change at line 649 skipping to change at line 674
Phone: +1 425 706 2268 Phone: +1 425 706 2268
Email: richdr@microsoft.com Email: richdr@microsoft.com
Dave Thaler Dave Thaler
Microsoft Microsoft
One Microsoft Way One Microsoft Way
Redmond, WA 98052 Redmond, WA 98052
Phone: +1 425 703 8835 Phone: +1 425 703 8835
Email: dthaler@microsoft.com Email: dthaler@microsoft.com
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Full Copyright Statement Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights. except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on This document and the information contained herein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
skipping to change at line 689 skipping to change at line 714
of such proprietary rights by implementers or users of this of such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository specification can be obtained from the IETF on-line IPR repository
at http://www.ietf.org/ipr. at http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf- this standard. Please address the information to the IETF at ietf-
ipr@ietf.org. ipr@ietf.org.
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 End of changes. 

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