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... IPv6). Nodes (hosts and routers) use
Neighbor Discovery to determine the link-layer addresses ...
... Hosts also use Neighbor Discovery
to find neighboring routers that are willing to forward packets on
their behalf. Finally, nodes use the protocol to actively keep track
...
... neighbors are reachable and which are not, and to detect
changed link-layer addresses. When a router or the path to a router
fails, a host ...
... changed link-layer addresses. When a router or the path to a router
fails, a host actively searches for functioning alternates.
...
... forwarding packets (i.e., it is configured as a
router, not a host). For hosts, reachability ...
... query
messages on behalf of another node. A router acting
on behalf of a mobile node that has moved off-link ...
... that they are neighbors; by default, they
communicate through a router. Examples are large
(switched) public data networks such as SMDS ...
... used to reach neighbors. All interfaces on routers
MUST have a link-local address. Also, [ADDRCONF ...
... destinations that reside on-link from those only
reachable through a router.)
Parameter Discovery: How a node ...
... traffic for the destination should be sent. The next-
hop can be a router or the destination itself.
...
... neighbor is no longer reachable. For neighbors used as
routers, alternate default routers can be tried. For
both routers ...
... neighbors used as
routers, alternate default routers can be tried. For
both routers and hosts ...
... routers, alternate default routers can be tried. For
both routers and hosts, address resolution can be
...
... Neighbor Discovery defines five different ICMP packet types: A pair
of Router Solicitation and Router Advertisement messages, a pair of
Neighbor Solicitation ...
... ICMP packet types: A pair
of Router Solicitation and Router Advertisement messages, a pair of
Neighbor Solicitation and Neighbor ...
... interface becomes enabled, hosts may
send out Router Solicitations that request routers to
generate Router Advertisements ...
... hosts may
send out Router Solicitations that request routers to
generate Router Advertisements immediately rather than
...
... Router Solicitations that request routers to
generate Router Advertisements immediately rather than
at their next scheduled time.
...
... at their next scheduled time.
Router Advertisement: Routers advertise their presence together
with various link ...
...
Router Advertisement: Routers advertise their presence together
with various link and Internet ...
... link and Internet parameters either
periodically, or in response to a Router Solicitation
message. Router Advertisements contain prefixes ...
... periodically, or in response to a Router Solicitation
message. Router Advertisements contain prefixes that
are used for on-link ...
... link-layer address change.
Redirect: Used by routers to inform hosts of a better first hop
for a destination ...
... router periodically multicasts a
Router Advertisement packet announcing its availability. A host
receives Router Advertisements ...
... Router Advertisement packet announcing its availability. A host
receives Router Advertisements from all routers, building a list of
default routers ...
... host
receives Router Advertisements from all routers, building a list of
default routers. Routers ...
... Router Advertisements from all routers, building a list of
default routers. Routers generate Router Advertisements frequently
...
... routers, building a list of
default routers. Routers generate Router Advertisements frequently
enough that hosts ...
... default routers. Routers generate Router Advertisements frequently
enough that hosts will learn of their presence within a few minutes,
...
... hosts will learn of their presence within a few minutes,
but not frequently enough to rely on an absence of advertisements to
detect router failure; a separate Neighbor Unreachability Detection
algorithm ...
... link prefix. In such cases a router can send a Redirect informing
the sender that the destination ...
... Router Advertisements (and per-prefix flags) allow routers to inform
hosts how to perform Address Autoconfiguration ...
... hosts how to perform Address Autoconfiguration. For example, routers
can specify whether hosts should use stateful (DHCPv6 ...
... centralized administration of critical parameters that can be set on
routers and automatically propagated to all attached hosts.
...
...
Load balancing is handled by allowing routers to omit the
source link-layer address from Router Advertisement ...
... routers to omit the
source link-layer address from Router Advertisement packets,
thereby forcing neighbors to use Neighbor Solicitation ...
... Neighbor Solicitation
messages to learn link-layer addresses of routers. Returned
Neighbor Advertisement messages can then contain link-layer
addresses ...
...
Proxy advertisements - A router willing to accept packets on behalf
of a target address that is unable to respond to Neighbor ...
...
Router advertisements carry link-layer addresses; no additional
packet exchange is needed to resolve the router ...
... Router advertisements carry link-layer addresses; no additional
packet exchange is needed to resolve the router's link-layer
address.
...
...
Router advertisements enable Address Autoconfiguration.
...
... hosts learn all on-link prefixes from Router
Advertisements. However, routers may be configured to omit some
or all prefixes ...
... on-link prefixes from Router
Advertisements. However, routers may be configured to omit some
or all prefixes from Router Advertisements ...
... routers may be configured to omit some
or all prefixes from Router Advertisements. In such cases hosts
assume that destinations ...
... destinations are off-link and send traffic to routers.
A router can then issue redirects as appropriate.
...
... significantly improving the robustness of packet delivery in the
presence of failing routers, partially failing or partitioned
links and nodes ...
...
Unlike in IPv4 Router Discovery the Router Advertisement messages
do not contain a preference field. The preference field is not
...
... Unlike in IPv4 Router Discovery the Router Advertisement messages
do not contain a preference field. The preference field is not
needed to handle routers ...
... Router Advertisement messages
do not contain a preference field. The preference field is not
needed to handle routers of different "stability"; the Neighbor
Unreachability Detection will detect dead routers and switch ...
... needed to handle routers of different "stability"; the Neighbor
Unreachability Detection will detect dead routers and switch to a
working one.
...
...
The use of link-local addresses to uniquely identify routers (for
Router Advertisement and Redirect messages ...
... link-local addresses to uniquely identify routers (for
Router Advertisement and Redirect messages) makes it possible for
hosts ...
... Redirect messages) makes it possible for
hosts to maintain the router associations in the event of the site
renumbering to use new global prefixes.
...
... described in this document. Address resolution,
and the mechanism for delivering Router
Solicitations and Advertisements on NBMA links ...
... hosts support manual configuration of a list of
default routers, hosts can dynamically acquire the
link-layer addresses ...
... This specification does not address shared media
issues that only relate to routers, such as:
- How routers ...
... - How a router determines that it is the first-
hop router for a received packet.
The protocol is extensible (through the definition
...
... Router Solicitation Message Format ...
...
Hosts send Router Solicitations in order to prompt routers to
generate Router Advertisements ...
... Hosts send Router Solicitations in order to prompt routers to
generate Router Advertisements quickly.
...
... Router Advertisement Message Format ...
...
Routers send out Router Advertisement message periodically, or in
response to a Router Solicitation ...
...
Routers send out Router Advertisement message periodically, or in
response to a Router Solicitation.
...
... Routers send out Router Advertisement message periodically, or in
response to a Router Solicitation.
...
... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cur Hop Limit |M|O| Reserved | Router Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
... Destination Address
Typically the Source Address of an invoking Router
Solicitation or the all-nodes multicast address.
...
... IP
header for outgoing IP packets. A value of zero
means unspecified (by this router).
M 1-bit ...
... unsigned integer. The lifetime associated
with the default router in units of seconds. The
maximum value corresponds to 18.2 hours. A
Lifetime ...
... maximum value corresponds to 18.2 hours. A
Lifetime of 0 indicates that the router is not a
default router and SHOULD NOT appear on the default
router ...
... Lifetime of 0 indicates that the router is not a
default router and SHOULD NOT appear on the default
router list. The Router Lifetime ...
... router is not a
default router and SHOULD NOT appear on the default
router list. The Router Lifetime applies only to
...
... default router and SHOULD NOT appear on the default
router list. The Router Lifetime applies only to
the router ...
... Router Lifetime applies only to
the router's usefulness as a default router; it
does not apply to information contained in other
...
... Lifetime applies only to
the router's usefulness as a default router; it
does not apply to information contained in other
message fields ...
... algorithm (see Section 7.3). A value of
zero means unspecified (by this router).
Retrans Timer ...
... algorithm
(see Sections 7.2 and 7.3). A value of zero means
unspecified (by this router).
Possible options:
...
... link-layer address of the interface from which
the Router Advertisement is sent. Only used on
link layers that have addresses ...
... link layers that have addresses. A router MAY omit
this option in order to enable inbound load sharing
across multiple link-layer addresses ...
... and/or are used for address autoconfiguration. A
router SHOULD include all its on-link prefixes
...
... bit indicates that
the sender is a router. The R-bit is used by
Neighbor Unreachability Detection ...
... destination. Hosts can be redirected to a
better first-hop router but can also be informed by a redirect that
the destination is in fact a neighbor ...
... Address field. Otherwise the target is a better
first-hop router and the Target Address MUST be the
router ...
... sender of the packet. It
is used in the Neighbor Solicitation, Router
Solicitation, and Router Advertisement packets.
...
... is used in the Neighbor Solicitation, Router
Solicitation, and Router Advertisement packets.
The Target Link-Layer Address option ...
... sender and ignored by
the receiver. A router SHOULD NOT send a prefix
option for the link-local ...
...
The Prefix Information option appears in Router
Advertisement packets and MUST be silently ignored
for other messages.
...
... Description
The MTU option is used in Router Advertisement
messages to insure that all nodes on a link use the
...
... MTU on a per-neighbor basis. In
such cases, routers use the MTU option to specify
the maximum MTU ...
... This section describes a conceptual model of one possible data
structure organization that hosts (and to some extent routers) will
maintain in interacting with neighboring nodes. The described
...
... link-layer
address, a flag indicating whether the neighbor is
a router or a host (called IsRouter in this
document), a pointer to any queued packets waiting
...
... Prefix List entries
are created from information received in Router
Advertisements. Each entry has an associated
invalidation timer value (extracted from the
...
... prefix list with an infinite invalidation timer
regardless of whether routers are advertising a
prefix for it. Received Router Advertisements ...
... routers are advertising a
prefix for it. Received Router Advertisements
SHOULD NOT modify the invalidation timer for the
...
... Default Router List
- A list of routers to which packets may be sent.
Router list entries point to entries in the
...
... - A list of routers to which packets may be sent.
Router list entries point to entries in the
Neighbor Cache; the algorithm ...
... Neighbor Cache; the algorithm for selecting a
default router favors routers known to be reachable
over those whose reachability ...
... algorithm for selecting a
default router favors routers known to be reachable
over those whose reachability is suspect. Each
...
... entry also has an associated invalidation timer
value (extracted from Router Advertisements) used
to delete entries that are no longer advertised.
...
... critical that the Neighbor Cache entry for a router is shared by all
Destination Cache entries using that router ...
... router is shared by all
Destination Cache entries using that router in order to prevent
redundant Neighbor Unreachability Detection probes ...
... the Destination Cache, the Prefix List, and the Default Router List
to determine the IP address of the appropriate next hop ...
... destination address. Otherwise, the sender
selects a router from the Default Router List (following the rules
described in Section 6.3.6). If the Default Router ...
... sender
selects a router from the Default Router List (following the rules
described in Section 6.3.6). If the Default Router List is empty,
...
... router from the Default Router List (following the rules
described in Section 6.3.6). If the Default Router List is empty,
the sender assumes that the destination ...
... hop determination may need to be performed again. For example,
traffic through a failed router should be switched to a working
router. Likewise, it may be possible to reroute traffic ...
... traffic through a failed router should be switched to a working
router. Likewise, it may be possible to reroute traffic destined for
a mobile node ...
... interface of appropriate context.
For example, when responding to a Router Solicitation, the
corresponding Router Advertisement is sent out the interface ...
... For example, when responding to a Router Solicitation, the
corresponding Router Advertisement is sent out the interface on which
the solicitation was received.
...
...
A node should retain entries in the Default Router List and the
Prefix List until their lifetimes expire. However, a node ...
... node may
garbage collect entries prematurely if it is low on memory. If not
all routers are kept on the Default Router list, a node should retain
...
... garbage collect entries prematurely if it is low on memory. If not
all routers are kept on the Default Router list, a node should retain
at least two entries in the Default Router ...
... Default Router list, a node should retain
at least two entries in the Default Router List (and preferably more)
in order to maintain robust connectivity for off-link destinations ...
... caches that have become invalid. When removing an entry from the
Default Router List, however, any entries in the Destination Cache
that go through that router ...
... Default Router List, however, any entries in the Destination Cache
that go through that router must perform next-hop determination again
to select a new default router ...
... ROUTER AND PREFIX DISCOVERY ...
...
This section describes router and host behavior related to the Router
Discovery portion of Neighbor Discovery ...
... This section describes router and host behavior related to the Router
Discovery portion of Neighbor Discovery. Router Discovery is used to
...
... host behavior related to the Router
Discovery portion of Neighbor Discovery. Router Discovery is used to
locate neighboring routers as well as learn prefixes ...
... Neighbor Discovery. Router Discovery is used to
locate neighboring routers as well as learn prefixes and
configuration parameters ...
... IP addresses that reside on-link and can be reached directly
without going through a router. Routers send Router Advertisements
...
... on-link and can be reached directly
without going through a router. Routers send Router Advertisements
that indicate whether the sender ...
... without going through a router. Routers send Router Advertisements
that indicate whether the sender is willing to be a default router ...
... Router Advertisements
that indicate whether the sender is willing to be a default router.
Router Advertisements also contain Prefix ...
... sender is willing to be a default router.
Router Advertisements also contain Prefix Information options that
list the set of prefixes ...
... on-link determination, autoconfiguration information is piggybacked
on Router Discovery messages to reduce network traffic. Indeed, the
same prefixes ...
... Validation of Router Solicitation Messages ...
... A router MUST silently discard any received Router Solicitation
messages that do not satisfy all of the following validity checks:
...
... IP Hop Limit field has a value of 255, i.e., the packet
could not possibly have been forwarded by a router.
...
...
The contents of any defined options that are not specified to be used
with Router Solicitation messages MUST be ignored and the packet
processed as normal. The only defined option that may appear is the
Source Link-Layer Address option ...
... Validation of Router Advertisement Messages ...
... A node MUST silently discard any received Router Advertisement
messages that do not satisfy all of the following validity checks:
...
... IP Source Address is a link-local address. Routers must use
their link-local address as the source for Router Advertisement ...
... Routers must use
their link-local address as the source for Router Advertisement
and Redirect messages so that hosts ...
... IP Hop Limit field has a value of 255, i.e., the packet
could not possibly have been forwarded by a router.
...
...
The contents of any defined options that are not specified to be used
with Router Advertisement messages MUST be ignored and the packet
processed as normal. The only defined options that may appear are
the Source Link-Layer Address ...
... Router Specification ...
... Router Configuration Variables ...
...
A router MUST allow for the following conceptual variables to be
configured by system management. The specific variable names are
...
...
AdvSendAdvertisements
A flag indicating whether or not the router sends
periodic Router Advertisements and responds to
...
... A flag indicating whether or not the router sends
periodic Router Advertisements and responds to
Router Solicitations.
...
... periodic Router Advertisements and responds to
Router Solicitations.
Default: FALSE
...
... node will not accidentally start
acting as a router unless it is explicitly
configured by system management to send Router
Advertisements ...
... router unless it is explicitly
configured by system management to send Router
Advertisements.
MaxRtrAdvInterval
...
... MaxRtrAdvInterval
The maximum time allowed between sending
unsolicited multicast Router Advertisements from
the interface, in seconds. MUST be no less than 4
...
... MinRtrAdvInterval
The minimum time allowed between sending
unsolicited multicast Router Advertisements from
the interface, in seconds. MUST be no less than 3
...
... The TRUE/FALSE value to be placed in the "Managed
address configuration" flag field in the Router
Advertisement. See [ADDRCONF].
...
... AdvLinkMTU The value to be placed in MTU options sent by the
router. A value of zero indicates that no MTU
options are sent.
...
... AdvReachableTime
The value to be placed in the Reachable Time field
in the Router Advertisement messages sent by the
router. The value zero means unspecified (by this
...
... in the Router Advertisement messages sent by the
router. The value zero means unspecified (by this
router). MUST be no greater than 3,600,000
...
... router. The value zero means unspecified (by this
router). MUST be no greater than 3,600,000
milliseconds (1 hour).
...
... AdvRetransTimer The value to be placed in the Retrans Timer field
in the Router Advertisement messages sent by the
router. The value zero means unspecified (by this
...
... in the Router Advertisement messages sent by the
router. The value zero means unspecified (by this
router).
...
... default value to be placed in the Cur Hop Limit
field in the Router Advertisement messages sent by
the router. The value should be set to that
...
... field in the Router Advertisement messages sent by
the router. The value should be set to that
current diameter of the Internet ...
... diameter of the Internet. The value zero
means unspecified (by this router).
Default: The value specified in the "Assigned
...
...
AdvDefaultLifetime
The value to be placed in the Router Lifetime field
of Router Advertisements ...
... Router Lifetime field
of Router Advertisements sent from the interface,
in seconds. MUST be either zero or between
...
... in seconds. MUST be either zero or between
MaxRtrAdvInterval and 9000 seconds. A value of
zero indicates that the router is not to be used as
a default router.
...
... zero indicates that the router is not to be used as
a default router.
Default: 3 * MaxRtrAdvInterval
...
... prefixes to be placed in Prefix
Information options in Router Advertisement
messages sent from the interface.
...
...
The above variables contain information that is placed in outgoing
Router Advertisement messages. Hosts use the received information to
initialize a set of analogous variables that control their external
...
... CurHopLimit, RetransTimer, and ReachableTime) apply to all nodes
including routers. In practice, these variables may not actually be
present on routers, since their contents can be derived from the
...
... including routers. In practice, these variables may not actually be
present on routers, since their contents can be derived from the
variables described above. However, external router behavior MUST be
...
... present on routers, since their contents can be derived from the
variables described above. However, external router behavior MUST be
the same as host behavior with respect to these variables. In
...
... IP address
assigned to it and whose corresponding AdvSendAdvertisements flag is
TRUE. A router MUST NOT send Router Advertisements out any interface
...
... assigned to it and whose corresponding AdvSendAdvertisements flag is
TRUE. A router MUST NOT send Router Advertisements out any interface
that is not an advertising interface ...
... IP forwarding capability (i.e., changing the system
from being a host to being a router), when the interface's
AdvSendAdvertisements flag is TRUE.
...
... multicast address on an
advertising interface. Routers respond to Router Solicitations sent
to the all-routers ...
... advertising interface. Routers respond to Router Solicitations sent
to the all-routers address ...
... Routers respond to Router Solicitations sent
to the all-routers address and verify the consistency of Router
Advertisements ...
... all-routers address and verify the consistency of Router
Advertisements sent by neighboring routers.
...
... Router Advertisement Message Content ...
...
A router sends periodic as well as solicited Router Advertisements
out its advertising interfaces ...
...
A router sends periodic as well as solicited Router Advertisements
out its advertising interfaces. Outgoing Router Advertisements ...
... Router Advertisements
out its advertising interfaces. Outgoing Router Advertisements are
filled with the following values consistent with the message format
...
...
A router might want to send Router Advertisements without advertising
itself as a default router ...
...
A router might want to send Router Advertisements without advertising
itself as a default router. For instance, a router ...
... router might want to send Router Advertisements without advertising
itself as a default router. For instance, a router might advertise
prefixes ...
... Router Advertisements without advertising
itself as a default router. For instance, a router might advertise
prefixes for address autoconfiguration ...
... prefixes for address autoconfiguration while not wishing to forward
packets. Such a router sets the Router Lifetime field in outgoing
...
... address autoconfiguration while not wishing to forward
packets. Such a router sets the Router Lifetime field in outgoing
advertisements to zero.
...
...
A router MAY choose not to include some or all options when sending
unsolicited Router Advertisements. For example, if prefix ...
... A router MAY choose not to include some or all options when sending
unsolicited Router Advertisements. For example, if prefix lifetimes
are much longer than AdvDefaultLifetime, including them every few
...
... are much longer than AdvDefaultLifetime, including them every few
advertisements may be sufficient. However, when responding to a
Router Solicitation or while sending the first few initial
unsolicited advertisements, a router SHOULD include all options so
...
... Router Solicitation or while sending the first few initial
unsolicited advertisements, a router SHOULD include all options so
that all information (e.g., prefixes) is propagated quickly during
...
... Sending Unsolicited Router Advertisements ...
...
A host MUST NOT send Router Advertisement messages at any time.
...
...
Unsolicited Router Advertisements are not strictly periodic: the
interval between subsequent transmissions is randomized to reduce the
probability of synchronization ...
... probability of synchronization with the advertisements from other
routers on the same link [SYNC]. Each advertising interface ...
... timer SHOULD be set
to MAX_INITIAL_RTR_ADVERT_INTERVAL instead. Using a smaller interval
for the initial advertisements increases the likelihood of a router
being discovered quickly when it first becomes available, in the
presence of possible packet loss ...
...
The information contained in Router Advertisements may change through
actions of system management. For instance, the lifetime ...
... advertised prefixes may change, new prefixes could be added, a router
could cease to be a router (i.e., switch ...
... prefixes could be added, a router
could cease to be a router (i.e., switch from being a router to being
...
... could cease to be a router (i.e., switch from being a router to being
a host), etc. In such cases, the router ...
... router to being
a host), etc. In such cases, the router MAY transmit up to
MAX_INITIAL_RTR_ADVERTISEMENTS unsolicited advertisements, using the
same rules as when an interface ...
...
In such cases the router SHOULD transmit one or more (but not more
than MAX_FINAL_RTR_ADVERTISEMENTS) final multicast Router
Advertisements on the interface ...
... In such cases the router SHOULD transmit one or more (but not more
than MAX_FINAL_RTR_ADVERTISEMENTS) final multicast Router
Advertisements on the interface with a Router Lifetime ...
... than MAX_FINAL_RTR_ADVERTISEMENTS) final multicast Router
Advertisements on the interface with a Router Lifetime field of zero.
In the case of a router ...
... Router Lifetime field of zero.
In the case of a router becoming a host, the system SHOULD also
depart from the all-routers ...
... router becoming a host, the system SHOULD also
depart from the all-routers IP multicast group on all interfaces ...
... group on all interfaces on
which the router supports IP multicast (whether or not they had been
advertising interfaces ...
...
Note that system management may disable a router's IP forwarding
capability (i.e., changing the system from being a router ...
... router's IP forwarding
capability (i.e., changing the system from being a router to being a
host), a step that does not necessarily imply that the router ...
... router to being a
host), a step that does not necessarily imply that the router's
interfaces stop being advertising interfaces ...
... interfaces stop being advertising interfaces. In such cases,
subsequent Router Advertisements MUST set the Router Lifetime field
...
... interfaces. In such cases,
subsequent Router Advertisements MUST set the Router Lifetime field
to zero.
...
... Processing Router Solicitations ...
...
In addition to sending periodic, unsolicited advertisements, a router
sends advertisements in response to valid solicitations received on
...
... valid solicitations received on
an advertising interface. A router MAY choose to unicast the
response directly to the soliciting host ...
...
In all cases, Router Advertisements sent in response to a Router
Solicitation MUST be delayed by a random time between 0 and
MAX_RA ...
...
In all cases, Router Advertisements sent in response to a Router
Solicitation MUST be delayed by a random time between 0 and
MAX_RA_DELAY_TIME seconds. (If a single advertisement is sent in
...
... RA_DELAY_TIME seconds. (If a single advertisement is sent in
response to multiple solicitations, the delay is relative to the
first solicitation.) In addition, consecutive Router Advertisements
sent to the all-nodes multicast address ...
... Upon receipt of a Router Solicitation, compute a random delay
within the range 0 through MAX_RA ...
... range 0 through MAX_RA_DELAY_TIME. If the computed
value corresponds to a time later than the time the next multicast
Router Advertisement is scheduled to be sent, ignore the random
delay and send the advertisement at the already-scheduled time.
...
... If the router sent a multicast Router Advertisement (solicited or
unsolicited) within the last MIN_DELAY_BETWEEN_RAS seconds,
...
... If the router sent a multicast Router Advertisement (solicited or
unsolicited) within the last MIN_DELAY_BETWEEN_RAS seconds,
schedule the advertisement to be sent at a time corresponding to
...
... MIN_DELAY_BETWEEN_RAS plus the random value after the previous
advertisement was sent. This ensures that the multicast Router
Advertisements are rate limited.
...
... Otherwise, schedule the sending of a Router Advertisement at the
time given by the random value.
...
...
Note that a router is permitted to send multicast Router
Advertisements more frequently than indicated by the
MinRtrAdvInterval configuration variable ...
...
Note that a router is permitted to send multicast Router
Advertisements more frequently than indicated by the
MinRtrAdvInterval configuration variable so long as the more frequent
...
... MinRtrAdvInterval configuration variable so long as the more frequent
advertisements are responses to Router Solicitations. In all cases,
however, unsolicited multicast advertisements MUST NOT be sent more
...
... address MUST NOT update the router's Neighbor Cache; solicitations
with a proper source address ...
... update the Neighbor Cache as follows. If
the router already has a Neighbor Cache entry for the solicitation's
sender ...
... Neighbor Cache entry for the
solicitation's sender, the router creates one, installs the link-
...
... Router Advertisement Consistency ...
... Routers SHOULD inspect valid Router Advertisements sent by other
routers and verify that the routers ...
... valid Router Advertisements sent by other
routers and verify that the routers are advertising consistent
information on a link ...
... Router Advertisements sent by other
routers and verify that the routers are advertising consistent
information on a link. Detected inconsistencies indicate that one or
...
... information on a link. Detected inconsistencies indicate that one or
more routers might be misconfigured and SHOULD be logged to system or
network management. The minimum set of information to check
...
... time as specified in section 6.2.7 then the comparison of the
lifetimes can not compare the content of the fields in the Router
Advertisement but must instead compare the time at which the
prefix will become deprecated and invalidated, respectively. Due
...
... to link propagation delays and potentially poorly synchronized
clocks between the routers such comparison SHOULD allow some time
skew.
...
...
Note that it is not an error for different routers to advertise
different sets of prefixes. Also, some routers ...
... routers to advertise
different sets of prefixes. Also, some routers might leave some
fields as unspecified, i.e., with the value zero, while other routers
...
... prefixes. Also, some routers might leave some
fields as unspecified, i.e., with the value zero, while other routers
specify values. The logging of errors SHOULD be restricted to
conflicting information that causes hosts ...
...
Any other action on reception of Router Advertisement messages by a
router is beyond the scope of this document.
...
... Any other action on reception of Router Advertisement messages by a
router is beyond the scope of this document.
...
... source address to
identify the sender. If multiple packets from the same router
contain different source addresses, nodes ...
... source addresses, nodes will assume they come from
different routers, leading to undesirable behavior. For example, a
node will ignore Redirect messages ...
... node will ignore Redirect messages that are believed to have been
sent by a router other than the current first-hop router. Thus the
source address ...
... Redirect messages that are believed to have been
sent by a router other than the current first-hop router. Thus the
source address used in Router Advertisements ...
... router. Thus the
source address used in Router Advertisements sent by a particular
router must be identical to the target address ...
... source address used in Router Advertisements sent by a particular
router must be identical to the target address in a Redirect message
...
...
Using the link-local address to uniquely identify routers on the link
has the benefit that the address ...
... link
has the benefit that the address a router is known by should not
change when a site renumbers.
...
... interfaces,
it SHOULD inform hosts of this change. The router SHOULD multicast a
few Router Advertisements ...
... router SHOULD multicast a
few Router Advertisements from the old link-local address with the
Router ...
... Router Advertisements from the old link-local address with the
Router Lifetime field set to zero and also multicast ...
... Lifetime field set to zero and also multicast a few Router
Advertisements from the new link-local address. The overall effect
should be the same as if one interface ...
... These variables have default values that are overridden by
information received in Router Advertisement messages. The default
values are used when there is no router on the link ...
... information received in Router Advertisement messages. The default
values are used when there is no router on the link or when all
received Router Advertisements ...
... router on the link or when all
received Router Advertisements have left a particular value
unspecified.
...
... random value should be
calculated when BaseReachableTime changes (due to
Router Advertisements) or at least every few
hours even if no Router Advertisements are
...
... Router Advertisements) or at least every few
hours even if no Router Advertisements are
received.
...
... Processing Received Router Advertisements ...
...
When multiple routers are present, the information advertised
collectively by all routers may be a superset of the information
...
... When multiple routers are present, the information advertised
collectively by all routers may be a superset of the information
contained in a single Router Advertisement. Moreover, information
...
... collectively by all routers may be a superset of the information
contained in a single Router Advertisement. Moreover, information
may also be obtained through other dynamic means, such as stateful
autoconfiguration ...
... autoconfiguration. Hosts accept the union of all received
information; the receipt of a Router Advertisement MUST NOT
invalidate all information received in a previous advertisement or
from another source. However, when received information for a
...
... NOT interpret the unspecified value as meaning change back to the
default value that was in use before the first Router Advertisement
was received. This rule prevents hosts from continually changing an
...
... was received. This rule prevents hosts from continually changing an
internal variable when one router advertises a specific value, but
other routers advertise the unspecified value.
...
... internal variable when one router advertises a specific value, but
other routers advertise the unspecified value.
...
... If the address is not already present in the host's Default
Router List, and the advertisement's Router Lifetime is non-
...
... address is not already present in the host's Default
Router List, and the advertisement's Router Lifetime is non-
zero, create ...
... create a new entry in the list, and initialize its
invalidation timer value from the advertisement's Router
Lifetime field.
...
... If the address is already present in the host's Default Router
List as a result of a previously-received advertisement, reset
its invalidation timer ...
... List as a result of a previously-received advertisement, reset
its invalidation timer to the Router Lifetime value in the
newly-received advertisement.
...
... If the address is already present in the host's Default Router
List and the received Router Lifetime value ...
... host's Default Router
List and the received Router Lifetime value is zero, immediately
time-out the entry as specified in Section 6.3.5.
...
...
To limit the storage needed for the Default Router List, a host MAY
choose not to store all of the router ...
... Default Router List, a host MAY
choose not to store all of the router addresses discovered via
advertisements. However, a host ...
... addresses discovered via
advertisements. However, a host MUST retain at least two router
addresses and SHOULD retain more. Default router ...
... router
addresses and SHOULD retain more. Default router selections are made
whenever communication to a destination appears to be failing. Thus,
...
... whenever communication to a destination appears to be failing. Thus,
the more routers on the list, the more likely an alternative working
router can be found quickly (e.g., without having to wait for the
...
... the more routers on the list, the more likely an alternative working
router can be found quickly (e.g., without having to wait for the
next advertisement to arrive).
...
...
In most cases, the advertised Reachable Time value will be the same
in consecutive Router Advertisements and a host's BaseReachableTime
rarely changes. In such cases, an implementation SHOULD insure that
...
...
After extracting information from the fixed part of the Router
Advertisement message, the advertisement is scanned for valid
options. If the advertisement contains a Source Link-Layer Address
option ...
...
Cache entry for the router (creating an entry if necessary) and the
IsRouter flag in the Neighbor Cache entry MUST be set to TRUE. If no
...
... IsRouter flag is used by Neighbor Unreachability Detection to
determine when a router changes to being a host (i.e., no longer
capable of forwarding packets ...
... Neighbor Cache entry is created
for the router its reachability state MUST be set to STALE as
...
... on-link status of the address is to
forward the packet to a default router; the reception of a Prefix
Information option with the "on-link ...
... prefix list is not catastrophic (hosts would send packets
to a default router and receive a redirect rather than sending
packets directly to a neighbor) the Neighbor Discovery protocol ...
... of the prefixes by, e.g., passing a copy of each valid Router
Advertisement message to both an "on-link" and an "addrconf"
function. Each function can then operate independently on the
...
...
Whenever the Lifetime of an entry in the Default Router List expires,
that entry is discarded. When removing a router ...
... Default Router List expires,
that entry is discarded. When removing a router from the Default
Router list, the node MUST update ...
... that entry is discarded. When removing a router from the Default
Router list, the node MUST update the Destination Cache ...
... update the Destination Cache in such a way
that all entries using the router perform next-hop determination
again rather than continue sending traffic ...
... Default Router Selection ...
...
The algorithm for selecting a router depends in part on whether or
not a router is known to be reachable. The exact details of how a
...
... algorithm for selecting a router depends in part on whether or
not a router is known to be reachable. The exact details of how a
node keeps track of a neighbor ...
... state are covered in
Section 7.3. The algorithm for selecting a default router is invoked
during next-hop determination when no Destination Cache entry ...
... off-link destination or when communication through an existing
router appears to be failing. Under normal conditions, a router
would be selected the first time traffic ...
... destination or when communication through an existing
router appears to be failing. Under normal conditions, a router
would be selected the first time traffic is sent to a destination ...
... with subsequent traffic for that destination using the same router as
indicated in the Destination Cache modulo any changes to the
...
...
The policy for selecting routers from the Default Router List is as
follows:
...
...
The policy for selecting routers from the Default Router List is as
follows:
...
...
1) Routers that are reachable or probably reachable (i.e., in any
state other than INCOMPLETE) SHOULD be preferred over routers ...
... Routers that are reachable or probably reachable (i.e., in any
state other than INCOMPLETE) SHOULD be preferred over routers
whose reachability is unknown or suspect (i.e., in the
...
... state, or for which no Neighbor Cache entry exists).
An implementation may choose to always return the same router or
cycle through the router list in a round-robin ...
... An implementation may choose to always return the same router or
cycle through the router list in a round-robin fashion as long
as it always returns a reachable or a probably reachable router ...
... router list in a round-robin fashion as long
as it always returns a reachable or a probably reachable router
when one is available.
...
... when one is available.
2) When no routers on the list are known to be reachable or
probably reachable, routers SHOULD be selected in a round-robin ...
... 2) When no routers on the list are known to be reachable or
probably reachable, routers SHOULD be selected in a round-robin
fashion, so that subsequent requests for a default router ...
... routers SHOULD be selected in a round-robin
fashion, so that subsequent requests for a default router do not
return the same router until all other routers ...
... fashion, so that subsequent requests for a default router do not
return the same router until all other routers have been
selected.
...
... selected.
Cycling through the router list in this case ensures that all
available routers are actively probed by the Neighbor
Unreachability Detection ...
... Cycling through the router list in this case ensures that all
available routers are actively probed by the Neighbor
Unreachability Detection algorithm. A request for a default
router ...
... routers are actively probed by the Neighbor
Unreachability Detection algorithm. A request for a default
router is made in conjunction with the sending of a packet to a
router ...
... default
router is made in conjunction with the sending of a packet to a
router, and the selected router will be probed for reachability
...
... conjunction with the sending of a packet to a
router, and the selected router will be probed for reachability
as a side effect.
...
... as a side effect.
3) If the Default Router List is empty, assume that all
destinations are on-link ...
... Sending Router Solicitations ...
... interface becomes enabled, a host may be unwilling to wait
for the next unsolicited Router Advertisement to locate default
routers or learn prefixes ...
... for the next unsolicited Router Advertisement to locate default
routers or learn prefixes. To obtain Router Advertisements quickly,
...
... routers or learn prefixes. To obtain Router Advertisements quickly,
a host SHOULD transmit up to MAX_RTR_SOLICITATIONS Router
Solicitation ...
... Router Advertisements quickly,
a host SHOULD transmit up to MAX_RTR_SOLICITATIONS Router
Solicitation messages each separated by at least
RTR_SOLICITATION_INTERVAL seconds. Router Solicitations may be sent
...
... host SHOULD transmit up to MAX_RTR_SOLICITATIONS Router
Solicitation messages each separated by at least
RTR_SOLICITATION_INTERVAL seconds. Router Solicitations may be sent
after any of the following events:
...
...
A host sends Router Solicitations to the All-Routers multicast
address. The IP source address ...
... A host sends Router Solicitations to the All-Routers multicast
address. The IP source address is set to either one of the
...
... Duplicate Address Detection [ADDRCONF]) there is no need to delay
again before sending the first Router Solicitation message.
...
... Router Solicitation, and receives a valid
Router Advertisement with a non-zero Router Lifetime ...
... SHOULD send at least one solicitation in the case where an
advertisement is received prior to having sent a solicitation.
Unsolicited Router Advertisements may be incomplete (see Section
6.2.3); solicited advertisements are expected to contain complete
information.
...
... If a host sends MAX_RTR_SOLICITATIONS solicitations, and receives no
Router Advertisements after having waited MAX_RTR_SOLICITATION_DELAY
seconds after sending the last solicitation, the host concludes that
...
... seconds after sending the last solicitation, the host concludes that
there are no routers on the link for the purpose of [ADDRCONF].
...
... ADDRCONF].
However, the host continues to receive and process Router
Advertisements messages in the event that routers appear on the link.
...
... However, the host continues to receive and process Router
Advertisements messages in the event that routers appear on the link.
...
... IP Hop Limit field has a value of 255, i.e., the packet
could not possibly have been forwarded by a router.
...
... IP Hop Limit field has a value of 255, i.e., the packet
could not possibly have been forwarded by a router.
...
... to FALSE. This will be the case even if the Neighbor Solicitation is
sent by a router since the Neighbor Solicitation messages do not
contain an indication of whether or not the sender ...
... Neighbor Solicitation messages do not
contain an indication of whether or not the sender is a router. In
the event that the sender is a router ...
... router. In
the event that the sender is a router, subsequent Neighbor
Advertisement or Router Advertisement messages will set the correct
...
... sender is a router, subsequent Neighbor
Advertisement or Router Advertisement messages will set the correct
IsRouter value. If a Neighbor Cache entry already exists its
...
... Link-Layer option MUST be included in the advertisement.
Furthermore, if the node is a router, it MUST set the Router flag to
one; otherwise it MUST set the flag to zero.
...
... Furthermore, if the node is a router, it MUST set the Router flag to
one; otherwise it MUST set the flag to zero.
...
... It sets the IsRouter flag in the cache entry based on the Router
flag in the received advertisement.
...
... The IsRouter flag in the cache entry MUST be set based on the
Router flag in the received advertisement. In those cases where
the IsRouter flag changes from TRUE to FALSE as a result of this
update ...
... destinations
using that neighbor as a router as specified in Section 7.3.3.
This is needed to detect when a node that is used as a router ...
... router as specified in Section 7.3.3.
This is needed to detect when a node that is used as a router
stops forwarding packets due to being configured as a host ...
... Neighbor
Unreachability Detection algorithm. If the node is a router, it MUST
set the Router flag to one; otherwise it MUST set it to zero. The
...
... node is a router, it MUST
set the Router flag to one; otherwise it MUST set it to zero. The
Override flag MAY be set to either zero or one. In either case,
neighboring nodes ...
... Neighbor Advertisements indicate that it is
willing to accept packets not explicitly addressed to itself. For
example, a router might accept packets on behalf of a mobile node
that has moved off-link ...
... and neighboring nodes, including host-to-host, host-to-router, and
router-to-host communication. Neighbor Unreachability Detection ...
... host-to-host, host-to-router, and
router-to-host communication. Neighbor Unreachability Detection may
also be used between routers ...
... router-to-host communication. Neighbor Unreachability Detection may
also be used between routers, but is not required if an equivalent
mechanism is available, for example, as part of the routing
protocols.
...
... address resolution
should be performed again. If the neighbor is a router, however,
attempting to switch to another router ...
... router, however,
attempting to switch to another router would be appropriate. The
specific recovery that takes place is covered under next-hop
...
... off-link destinations,
forward progress implies that the first-hop router is reachable.
When available, this upper-layer information SHOULD be used.
...
...
In some cases (e.g., UDP-based protocols and routers forwarding
packets to hosts) such reachability ...
... Neighbor Solicitation.
Receipt of other Neighbor Discovery messages such as Router
Advertisements and Neighbor Advertisement with the Solicited flag set
to zero MUST NOT be treated as a reachability ...
... again. Invoking next-hop determination at this point insures that
alternate default routers are tried.
...
... receiving packets other than solicited Neighbor
Advertisements (i.e., Router Solicitations, Router Advertisements,
Redirects, and Neighbor ...
... Neighbor
Advertisements (i.e., Router Solicitations, Router Advertisements,
Redirects, and Neighbor Solicitations). These packets contain the
...
... router switches from being a
router to being a host (e.g., if its IP forwarding capability is
...
... turned off by system management), a node MUST compare the Router flag
field in all received Neighbor Advertisement messages with the
...
... update the Destination Cache as described in Section 6.3.5. Note
that a router may not be listed in the Default Router List, even
though a Destination Cache entry ...
... Destination Cache as described in Section 6.3.5. Note
that a router may not be listed in the Default Router List, even
though a Destination Cache entry is using it (e.g., a host ...
... redirected to it). In such cases, all Destination Cache entries that
reference the (former) router must perform next-hop determination
again before using the entry.
...
... routers to redirect a host to a better
first-hop router for a specific destination or to inform hosts that a
...
...
A router MUST be able to determine the link-local address for each of
its neighboring routers ...
... router MUST be able to determine the link-local address for each of
its neighboring routers in order to ensure that the target address in
a Redirect message ...
... a Redirect message identifies the neighbor router by its link-local
address. For static routing this requirement ...
... static routing this requirement implies that the next-
hop router's address should be specified using the link-local address
...
... address should be specified using the link-local address
of the router. For dynamic routing this requirement implies that all
...
... IPv6 routing protocols must somehow exchange the link-local addresses
of neighboring routers.
...
... IP Source Address is a link-local address. Routers must use
their link-local address as the source for Router Advertisement ...
... Routers must use
their link-local address as the source for Router Advertisement
and Redirect messages so that hosts ...
... IP Hop Limit field has a value of 255, i.e., the packet
could not possibly have been forwarded by a router.
...
... The IP source address of the Redirect is the same as the current
first-hop router for the specified ICMP Destination Address.
...
... Target Address is either a link-local address (when
redirected to a router) or the same as the ICMP Destination
Address ...
... Router Specification ...
... subject to rate limiting,
whenever it forwards a packet that is not explicitly addressed to
itself (i.e. a packet that is not source routed through the router)
in which:
...
... destination SHOULD be sent. If the target is a
router, that router's link-local address MUST be used. If the
...
...
A router MUST limit the rate at which Redirect messages are sent, in
order to limit the bandwidth ...
... however, one cannot reliably determine whether the Target Address is
a router. Consequently, newly created Neighbor Cache entries should
...
... cache entries should
leave the flag unchanged. If the Target is a router, subsequent
Neighbor Advertisement or Router Advertisement messages ...
... router, subsequent
Neighbor Advertisement or Router Advertisement messages will update
IsRouter accordingly.
...
... example, an implementation can choose to process the Prefix
Information option contained in a Router Advertisement message
in a user-space process while the link-layer address option in
...
... enough, it might not be necessary to include the Prefix
Information option in every Router Advertisement. In addition,
different routers might send different sets of options. Thus, a
...
... Information option in every Router Advertisement. In addition,
different routers might send different sets of options. Thus, a
receiver MUST NOT associate any action with the absence of an
...
...
If the number of included options in a Router Advertisement causes
the advertisement's size to exceed the link MTU ...
... the advertisement's size to exceed the link MTU, the router can send
multiple separate advertisements each containing a subset of the
options.
...
...
Router constants:
MAX_INITIAL_RTR_ADVERT_INTERVAL 16 seconds
...
... The Hop Limit field of all received packets is verified to contain
255, the maximum legal value. Because routers decrement the Hop
Limit on all packets they forward, received packets containing a Hop
Limit of 255 must have originated from a neighbor ...
... that can send packets with an arbitrary IP source address can both
advertise itself as a default router and also send "forged" Router
Advertisement messages that immediately time out all other default
routers ...
... IP source address can both
advertise itself as a default router and also send "forged" Router
Advertisement messages that immediately time out all other default
routers as well as all on-link ...
... default router and also send "forged" Router
Advertisement messages that immediately time out all other default
routers as well as all on-link prefixes. An intruder can achieve
...
... on-link prefixes. An intruder can achieve
this by sending out multiple Router Advertisements, one for each
legitimate router, with the source address ...
... this by sending out multiple Router Advertisements, one for each
legitimate router, with the source address set to the address of
...
... off-link
destinations, to go to the rogue router. That router can then
selectively examine, modify or drop all packets sent on the link ...
... destinations, to go to the rogue router. That router can then
selectively examine, modify or drop all packets sent on the link. The
...
... Neighbor Unreachability Detection will not detect such a black hole
as long as the rogue router politely answers the NUD probes with a
...
... trust model for redirects is the same as in IPv4. A redirect is
accepted only if received from the same router that is currently
being used for that destination. It is natural to trust ...
... this exposure is no worse than it was; the target host, once
subverted, could always act as a hidden router to forward traffic
elsewhere.
...
... The protocol contains no mechanism to determine which neighbors are
authorized to send a particular type of message (e.g., Router
Advertisements); any neighbor, presumably even in the presence of
authentication ...
... neighbor, presumably even in the presence of
authentication, can send Router Advertisement messages thereby being
able to cause denial of service. Furthermore, any neighbor ...
... link receiving the Router Advertisement messages in a timely manner.
However, a host might be turned off or be unreachable for an extended
...
... Lifetime until
that point in future. This "furthest into the future" time is simply
the maximum, over all Router Advertisements, of the time the
advertisement was sent plus the prefix's Lifetime ...
... robustness, in reality a host will be unable to communicate in the
absence of properly functioning routers. Such routers will be
sending Router Advertisements ...
... host will be unable to communicate in the
absence of properly functioning routers. Such routers will be
sending Router Advertisements that contain appropriate (and current)
...
... routers. Such routers will be
sending Router Advertisements that contain appropriate (and current)
prefixes. A host ...
... host connected to a network that has no functioning
routers is likely to have more serious problems than just a lack of a
valid prefix ...
... Deering, S., "ICMP Router Discovery Messages", RFC 1256prop, September 1991. ...
...
If a multihomed host receives Router Advertisements on all of its
interfaces, it will (probably) have learned on-link ...
... addresses residing on each link. When a packet must be sent through
a router, however, selecting the "wrong" router can result in a
suboptimal or non-functioning path. There are number of issues to
...
... link. When a packet must be sent through
a router, however, selecting the "wrong" router can result in a
suboptimal or non-functioning path. There are number of issues to
consider:
...
... In order for a router to send a redirect, it must determine that
the packet it is forwarding originates from a neighbor. The
...
... interface other than the interface from which it
was sent. In such cases, a router will not send redirects, and
suboptimal routing is likely. In order to be redirected, the
...
... destination, it will be unable to deliver the packet.
However, the destination may be reachable through a router on
one of the other interfaces. Neighbor Discovery ...
...
If a multihomed host fails to receive Router Advertisements on one or
more of its interfaces, it will not know (in the absence of
...
... host could choose to only send packets
out on the interfaces on which it has received Router
Advertisements. A key assumption made here, however, is that
routers on those other interfaces ...
... interfaces on which it has received Router
Advertisements. A key assumption made here, however, is that
routers on those other interfaces will be able to route packets
...
... register with an address resolution
server. This could for instance enable routers to ask hosts to
send them periodic unsolicited advertisements. Once again this
...
... hosts to
send them periodic unsolicited advertisements. Once again this
can be added using a new option sent in the Router Advertisements.
...
... reachability is part of normal operations. Such
procedures might allow hosts and routers to find usable paths on,
e.g., radio links.
...
... Neighbor Advertisement messages apply to either the source of
the packet (for Neighbor Solicitation, Router Solicitation, and
Router Advertisement messages) or the target address ...
... Neighbor Solicitation, Router Solicitation, and
Router Advertisement messages) or the target address (for Redirect
messages) as follows:
...
... The sender of a Router Solicitation is implicitly assumed to be a
host since there is no need for routers ...
... Router Solicitation is implicitly assumed to be a
host since there is no need for routers to send such messages.
...
... destination address in the packet being redirected, is implicitly
assumed to be a router. This is a natural assumption since that
node is expected to be able to forward the packets towards the
...
... destination, does not carry any host vs. router information. All
that is known is that the destination (i.e. target ...
... information the receipt of the message will cause the IsRouter flag
to be updated. But when there is no host vs. router information in
the ND message the receipt of the message MUST NOT cause a change to
...
... mischief when a node incorrectly thinks a host is a router, than the
other way around. In these cases a subsequent Neighbor Advertisement
...
... other way around. In these cases a subsequent Neighbor Advertisement
or Router Advertisement message will set the correct IsRouter value.
...