RFC 4861
Neighbor Discovery for IP version 6 (IPv6)
Pages: 97
Draft Standard
→ Errata
Obsoletes: 2461
Updated by: 5942 6980 7048 7527 7559 8028 8319 8425 9131 9685
Draft Standard
→ Errata
Obsoletes: 2461
Updated by: 5942 6980 7048 7527 7559 8028 8319 8425 9131 9685
Part 3 of 5 – Pages 38 to 58
6. Router and Prefix Discovery
This section describes router and host behavior related to the Router Discovery portion of Neighbor Discovery. Router Discovery is used to locate neighboring routers as well as learn prefixes and configuration parameters related to stateless address autoconfiguration. Prefix Discovery is the process through which hosts learn the ranges of IP addresses that reside on-link and can be reached directly without going through a router. Routers send Router Advertisements that indicate whether the sender is willing to be a default router. Router Advertisements also contain Prefix Information options that list the set of prefixes that identify on-link IP addresses. Stateless Address Autoconfiguration must also obtain subnet prefixes as part of configuring addresses. Although the prefixes used for address autoconfiguration are logically distinct from those used for on-link determination, autoconfiguration information is piggybacked on Router Discovery messages to reduce network traffic. Indeed, the same prefixes can be advertised for on-link determination and address autoconfiguration by specifying the appropriate flags in the Prefix Information options. See [ADDRCONF] for details on how autoconfiguration information is processed.
6.1. Message Validation
6.1.1. Validation of Router Solicitation Messages
Hosts MUST silently discard any received Router Solicitation Messages. A router MUST silently discard any received Router Solicitation messages that do not satisfy all of the following validity checks: - The IP Hop Limit field has a value of 255, i.e., the packet could not possibly have been forwarded by a router. - ICMP Checksum is valid. - ICMP Code is 0. - ICMP length (derived from the IP length) is 8 or more octets. - All included options have a length that is greater than zero. - If the IP source address is the unspecified address, there is no source link-layer address option in the message. The contents of the Reserved field, and of any unrecognized options, MUST be ignored. Future, backward-compatible changes to the protocol may specify the contents of the Reserved field or add new options; backward-incompatible changes may use different Code values. 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. A solicitation that passes the validity checks is called a "valid solicitation".6.1.2. 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 and Redirect messages so that hosts can uniquely identify routers.
- The IP Hop Limit field has a value of 255, i.e., the packet
could not possibly have been forwarded by a router.
- ICMP Checksum is valid.
- ICMP Code is 0.
- ICMP length (derived from the IP length) is 16 or more octets.
- All included options have a length that is greater than zero.
The contents of the Reserved field, and of any unrecognized options,
MUST be ignored. Future, backward-compatible changes to the protocol
may specify the contents of the Reserved field or add new options;
backward-incompatible changes may use different Code values.
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, Prefix Information and MTU options.
An advertisement that passes the validity checks is called a "valid
advertisement".
6.2. Router Specification
6.2.1. Router Configuration Variables
A router MUST allow for the following conceptual variables to be
configured by system management. The specific variable names are
used for demonstration purposes only, and an implementation is not
required to have them, so long as its external behavior is consistent
with that described in this document. Default values are specified
to simplify configuration in common cases.
The default values for some of the variables listed below may be
overridden by specific documents that describe how IPv6 operates over
different link layers. This rule simplifies the configuration of
Neighbor Discovery over link types with widely differing performance
characteristics.
For each interface:
IsRouter A flag indicating whether routing is enabled on
this interface. Enabling routing on the interface
would imply that a router can forward packets to or
from the interface.
Default: FALSE
AdvSendAdvertisements
A flag indicating whether or not the router sends
periodic Router Advertisements and responds to
Router Solicitations.
Default: FALSE
Note that AdvSendAdvertisements MUST be FALSE by
default so that a node will not accidentally start
acting as a router unless it is explicitly
configured by system management to send Router
Advertisements.
MaxRtrAdvInterval
The maximum time allowed between sending
unsolicited multicast Router Advertisements from
the interface, in seconds. MUST be no less than 4
seconds and no greater than 1800 seconds.
Default: 600 seconds
MinRtrAdvInterval
The minimum time allowed between sending
unsolicited multicast Router Advertisements from
the interface, in seconds. MUST be no less than 3
seconds and no greater than .75 *
MaxRtrAdvInterval.
Default: 0.33 * MaxRtrAdvInterval If
MaxRtrAdvInterval >= 9 seconds; otherwise, the
Default is MaxRtrAdvInterval.
AdvManagedFlag
The TRUE/FALSE value to be placed in the "Managed
address configuration" flag field in the Router
Advertisement. See [ADDRCONF].
Default: FALSE
AdvOtherConfigFlag
The TRUE/FALSE value to be placed in the "Other
configuration" flag field in the Router
Advertisement. See [ADDRCONF].
Default: FALSE
AdvLinkMTU The value to be placed in MTU options sent by the
router. A value of zero indicates that no MTU
options are sent.
Default: 0
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
router). MUST be no greater than 3,600,000
milliseconds (1 hour).
Default: 0
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
router).
Default: 0
AdvCurHopLimit
The 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 the current
diameter of the Internet. The value zero means
unspecified (by this router).
Default: The value specified in the "Assigned
Numbers" [ASSIGNED] that was in effect at the time
of implementation.
AdvDefaultLifetime
The value to be placed in the Router Lifetime field
of Router Advertisements sent from the interface,
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. These limits may be overridden
by specific documents that describe how IPv6
operates over different link layers. For instance,
in a point-to-point link the peers may have enough
information about the number and status of devices
at the other end so that advertisements are needed
less frequently.
Default: 3 * MaxRtrAdvInterval
AdvPrefixList
A list of prefixes to be placed in Prefix
Information options in Router Advertisement
messages sent from the interface.
Default: all prefixes that the router advertises
via routing protocols as being on-link for the
interface from which the advertisement is sent.
The link-local prefix SHOULD NOT be included in the
list of advertised prefixes.
Each prefix has an associated:
AdvValidLifetime
The value to be placed in the Valid
Lifetime in the Prefix Information option,
in seconds. The designated value of all
1's (0xffffffff) represents infinity.
Implementations MAY allow AdvValidLifetime
to be specified in two ways:
- a time that decrements in real time,
that is, one that will result in a
Lifetime of zero at the specified time
in the future, or
- a fixed time that stays the same in
consecutive advertisements.
Default: 2592000 seconds (30 days), fixed
(i.e., stays the same in consecutive
advertisements).
AdvOnLinkFlag
The value to be placed in the on-link flag
("L-bit") field in the Prefix Information
option.
Default: TRUE
Stateless address configuration [ADDRCONF] defines
additional information associated with each of the
prefixes:
AdvPreferredLifetime
The value to be placed in the Preferred
Lifetime in the Prefix Information option,
in seconds. The designated value of all
1's (0xffffffff) represents infinity. See
[ADDRCONF] for details on how this value is
used. Implementations MAY allow
AdvPreferredLifetime to be specified in two
ways:
- a time that decrements in real time,
that is, one that will result in a
Lifetime of zero at a specified time in
the future, or
- a fixed time that stays the same in
consecutive advertisements.
Default: 604800 seconds (7 days), fixed
(i.e., stays the same in consecutive
advertisements). This value MUST NOT be
larger than AdvValidLifetime.
AdvAutonomousFlag
The value to be placed in the Autonomous
Flag field in the Prefix Information
option. See [ADDRCONF].
Default: TRUE
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
behavior (see Section 6.3.2). Some of these host variables (e.g.,
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
variables described above. However, external router behavior MUST be the same as host behavior with respect to these variables. In particular, this includes the occasional randomization of the ReachableTime value as described in Section 6.3.2. Protocol constants are defined in Section 10.6.2.2. Becoming an Advertising Interface
The term "advertising interface" refers to any functioning and enabled interface that has at least one unicast IP address 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. An interface may become an advertising interface at times other than system startup. For example: - changing the AdvSendAdvertisements flag on an enabled interface from FALSE to TRUE, or - administratively enabling the interface, if it had been administratively disabled, and its AdvSendAdvertisements flag is TRUE, or - enabling IP forwarding capability (i.e., changing the system from being a host to being a router), when the interface's AdvSendAdvertisements flag is TRUE. A router MUST join the all-routers multicast address on an advertising interface. Routers respond to Router Solicitations sent to the all-routers address and verify the consistency of Router Advertisements sent by neighboring routers.6.2.3. Router Advertisement Message Content
A router sends periodic as well as solicited Router Advertisements out its advertising interfaces. Outgoing Router Advertisements are filled with the following values consistent with the message format given in Section 4.2: - In the Router Lifetime field: the interface's configured AdvDefaultLifetime. - In the M and O flags: the interface's configured AdvManagedFlag and AdvOtherConfigFlag, respectively.
- In the Cur Hop Limit field: the interface's configured
CurHopLimit.
- In the Reachable Time field: the interface's configured
AdvReachableTime.
- In the Retrans Timer field: the interface's configured
AdvRetransTimer.
- In the options:
o Source Link-Layer Address option: link-layer address of the
sending interface. This option MAY be omitted to
facilitate in-bound load balancing over replicated
interfaces.
o MTU option: the interface's configured AdvLinkMTU value if
the value is non-zero. If AdvLinkMTU is zero, the MTU
option is not sent.
o Prefix Information options: one Prefix Information option
for each prefix listed in AdvPrefixList with the option
fields set from the information in the AdvPrefixList entry
as follows:
- In the "on-link" flag: the entry's AdvOnLinkFlag.
- In the Valid Lifetime field: the entry's
AdvValidLifetime.
- In the "Autonomous address configuration" flag: the
entry's AdvAutonomousFlag.
- In the Preferred Lifetime field: the entry's
AdvPreferredLifetime.
A router might want to send Router Advertisements without advertising
itself as a default router. For instance, a router might advertise
prefixes for stateless 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 lifetimes
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 that all information (e.g., prefixes) is propagated quickly during system initialization. If including all options causes the size of an advertisement to exceed the link MTU, multiple advertisements can be sent, each containing a subset of the options.6.2.4. 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 with the advertisements from other routers on the same link [SYNC]. Each advertising interface has its own timer. Whenever a multicast advertisement is sent from an interface, the timer is reset to a uniformly distributed random value between the interface's configured MinRtrAdvInterval and MaxRtrAdvInterval; expiration of the timer causes the next advertisement to be sent and a new random value to be chosen. For the first few advertisements (up to MAX_INITIAL_RTR_ADVERTISEMENTS) sent from an interface when it becomes an advertising interface, if the randomly chosen interval is greater than MAX_INITIAL_RTR_ADVERT_INTERVAL, the 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 of advertised prefixes may change, new prefixes could be added, a router could cease to be a router (i.e., switch from being a 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 becomes an advertising interface.6.2.5. Ceasing To Be an Advertising Interface
An interface may cease to be an advertising interface, through actions of system management such as: - changing the AdvSendAdvertisements flag of an enabled interface from TRUE to FALSE, or - administratively disabling the interface, or
- shutting down the system.
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 field of zero.
In the case of a router becoming a host, the system SHOULD also
depart from the all-routers IP multicast group on all interfaces on
which the router supports IP multicast (whether or not they had been
advertising interfaces). In addition, the host MUST ensure that
subsequent Neighbor Advertisement messages sent from the interface
have the Router flag set to zero.
Note that system management may disable a 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's
interfaces stop being advertising interfaces. In such cases,
subsequent Router Advertisements MUST set the Router Lifetime field
to zero.
6.2.6. Processing Router Solicitations
A host MUST silently discard any received Router Solicitation
messages.
In addition to sending periodic, unsolicited advertisements, a router
sends advertisements in response to valid solicitations received on
an advertising interface. A router MAY choose to unicast the
response directly to the soliciting host's address (if the
solicitation's source address is not the unspecified address), but
the usual case is to multicast the response to the all-nodes group.
In the latter case, the interface's interval timer is reset to a new
random value, as if an unsolicited advertisement had just been sent
(see Section 6.2.4).
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
response to multiple solicitations, the delay is relative to the
first solicitation.) In addition, consecutive Router Advertisements
sent to the all-nodes multicast address MUST be rate limited to no
more than one advertisement every MIN_DELAY_BETWEEN_RAS seconds.
A router might process Router Solicitations as follows:
- Upon receipt of a Router Solicitation, compute a random delay
within the 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,
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 so long as the more frequent
advertisements are responses to Router Solicitations. In all cases,
however, unsolicited multicast advertisements MUST NOT be sent more
frequently than indicated by MinRtrAdvInterval.
Router Solicitations in which the Source Address is the unspecified
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, the solicitation contains a Source Link-Layer
Address option, and the received link-layer address differs from that
already in the cache, then the link-layer address SHOULD be updated
in the appropriate Neighbor Cache entry, and its reachability state
MUST also be set to STALE. If there is no existing Neighbor Cache
entry for the solicitation's sender, the router creates one, installs
the link- layer address and sets its reachability state to STALE as
specified in Section 7.3.3. If there is no existing Neighbor Cache
entry and no Source Link-Layer Address option was present in the
solicitation, the router may respond with either a multicast or a
unicast router advertisement. Whether or not a Source Link-Layer
Address option is provided, if a Neighbor Cache entry for the
solicitation's sender exists (or is created) the entry's IsRouter
flag MUST be set to FALSE.
6.2.7. Router Advertisement Consistency
Routers SHOULD inspect valid Router Advertisements sent by other routers and verify that the routers are advertising consistent 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 includes: - Cur Hop Limit values (except for the unspecified value of zero other inconsistencies SHOULD be logged to system network management). - Values of the M or O flags. - Reachable Time values (except for the unspecified value of zero). - Retrans Timer values (except for the unspecified value of zero). - Values in the MTU options. - Preferred and Valid Lifetimes for the same prefix. If AdvPreferredLifetime and/or AdvValidLifetime decrement in real time as specified in Section 6.2.1 then the comparison of the lifetimes cannot 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 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 to switch from one value to another with each received advertisement. Any other action on reception of Router Advertisement messages by a router is beyond the scope of this document.6.2.8. Link-local Address Change
The link-local address on a router should rarely change, if ever. Nodes receiving Neighbor Discovery messages use the source address to identify the sender. If multiple packets from the same router contain different source addresses, nodes will assume they come from different routers, leading to undesirable behavior. For example, a
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 used in Router Advertisements sent by a particular router must be identical to the target address in a Redirect message when redirecting to that router. Using the link-local address to uniquely identify routers on the link has the benefit that the address a router is known by should not change when a site renumbers. If a router changes the link-local address for one of its interfaces, it SHOULD inform hosts of this change. The router SHOULD multicast a few Router Advertisements from the old link-local address with the Router 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 ceases being an advertising interface, and a different one starts being an advertising interface.6.3. Host Specification
6.3.1. Host Configuration Variables
None.6.3.2. Host Variables
A host maintains certain Neighbor-Discovery-related variables in addition to the data structures defined in Section 5.1. The specific variable names are used for demonstration purposes only, and an implementation is not required to have them, so long as its external behavior is consistent with that described in this document. 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 or when all received Router Advertisements have left a particular value unspecified. The default values in this specification may be overridden by specific documents that describe how IP operates over different link layers. This rule allows Neighbor Discovery to operate over links with widely varying performance characteristics.
For each interface:
LinkMTU The MTU of the link.
Default: The valued defined in the specific
document that describes how IPv6 operates over
the particular link layer (e.g., [IPv6-ETHER]).
CurHopLimit The default hop limit to be used when sending IP
packets.
Default: The value specified in the "Assigned
Numbers" [ASSIGNED] that was in effect at the
time of implementation.
BaseReachableTime
A base value used for computing the random
ReachableTime value.
Default: REACHABLE_TIME milliseconds.
ReachableTime The time a neighbor is considered reachable after
receiving a reachability confirmation.
This value should be a uniformly distributed
random value between MIN_RANDOM_FACTOR and
MAX_RANDOM_FACTOR times BaseReachableTime
milliseconds. A new random value should be
calculated when BaseReachableTime changes (due to
Router Advertisements) or at least every few
hours even if no Router Advertisements are
received.
RetransTimer The time between retransmissions of Neighbor
Solicitation messages to a neighbor when
resolving the address or when probing the
reachability of a neighbor.
Default: RETRANS_TIMER milliseconds
6.3.3. Interface Initialization
The host joins the all-nodes multicast address on all multicast-
capable interfaces.
6.3.4. Processing Received Router Advertisements
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 may also be obtained through other dynamic means like DHCPv6. 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 specific parameter (e.g., Link MTU) or option (e.g., Lifetime on a specific Prefix) differs from information received earlier, and the parameter/option can only have one value, the most recently received information is considered authoritative. A Router Advertisement field (e.g., Cur Hop Limit, Reachable Time, and Retrans Timer) may contain a value denoting that it is unspecified. In such cases, the parameter should be ignored and the host should continue using whatever value it is already using. In particular, a host MUST 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 internal variable when one router advertises a specific value, but other routers advertise the unspecified value. On receipt of a valid Router Advertisement, a host extracts the source address of the packet and does the following: - If the address is not already present in the host's Default Router List, and the advertisement's Router Lifetime is non- zero, 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 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 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 addresses discovered via advertisements. However, a host MUST retain at least two router addresses and SHOULD retain more. Default router selections are made 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 next advertisement to arrive). If the received Cur Hop Limit value is non-zero, the host SHOULD set its CurHopLimit variable to the received value. If the received Reachable Time value is non-zero, the host SHOULD set its BaseReachableTime variable to the received value. If the new value differs from the previous value, the host SHOULD re-compute a new random ReachableTime value. ReachableTime is computed as a uniformly distributed random value between MIN_RANDOM_FACTOR and MAX_RANDOM_FACTOR times the BaseReachableTime. Using a random component eliminates the possibility that Neighbor Unreachability Detection messages will synchronize with each other. 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 ensure that a new random value gets re-computed at least once every few hours. The RetransTimer variable SHOULD be copied from the Retrans Timer field, if the received value is non-zero. 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, the link-layer address SHOULD be recorded in the Neighbor 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 Source Link-Layer Address is included, but a corresponding Neighbor Cache entry exists, its IsRouter flag MUST be set to TRUE. The 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). If a Neighbor Cache entry is created for the router, its reachability state MUST be set to STALE as specified in Section 7.3.3. If a cache entry already exists and is updated with a different link-layer address, the reachability state MUST also be set to STALE. If the MTU option is present, hosts SHOULD copy the option's value into LinkMTU so long as the value is greater than or equal to the minimum link MTU [IPv6] and does not exceed the maximum LinkMTU value specified in the link-type-specific document (e.g., [IPv6-ETHER]). Prefix Information options that have the "on-link" (L) flag set indicate a prefix identifying a range of addresses that should be considered on-link. Note, however, that a Prefix Information option
with the on-link flag set to zero conveys no information concerning on-link determination and MUST NOT be interpreted to mean that addresses covered by the prefix are off-link. The only way to cancel a previous on-link indication is to advertise that prefix with the L-bit set and the Lifetime set to zero. The default behavior (see Section 5.2) when sending a packet to an address for which no information is known about the 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" (L) flag set to zero does not change this behavior. The reasons for an address being treated as on-link is specified in the definition of "on-link" in Section 2.1. Prefixes with the on-link flag set to zero would normally have the autonomous flag set and be used by [ADDRCONF]. For each Prefix Information option with the on-link flag set, a host does the following: - If the prefix is the link-local prefix, silently ignore the Prefix Information option. - If the prefix is not already present in the Prefix List, and the Prefix Information option's Valid Lifetime field is non-zero, create a new entry for the prefix and initialize its invalidation timer to the Valid Lifetime value in the Prefix Information option. - If the prefix is already present in the host's Prefix List as the result of a previously received advertisement, reset its invalidation timer to the Valid Lifetime value in the Prefix Information option. If the new Lifetime value is zero, time-out the prefix immediately (see Section 6.3.5). - If the Prefix Information option's Valid Lifetime field is zero, and the prefix is not present in the host's Prefix List, silently ignore the option. Stateless address autoconfiguration [ADDRCONF] may in some circumstances use a larger Valid Lifetime of a prefix or ignore it completely in order to prevent a particular denial-of-service attack. However, since the effect of the same denial of service targeted at 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 does not impose such a check on the prefix lifetime values. Similarly, [ADDRCONF] may impose certain restrictions on the prefix length for address configuration purposes. Therefore, the prefix might be rejected by [ADDRCONF] implementation in the host. However, the
prefix length is still valid for on-link determination when combined
with other flags in the prefix option.
Note: Implementations can choose to process the on-link aspects of
the prefixes separately from the stateless address
autoconfiguration aspects 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 prefixes that have the appropriate flag set.
6.3.5. Timing out Prefixes and Default Routers
Whenever the invalidation timer expires for a Prefix List entry, that
entry is discarded. No existing Destination Cache entries need be
updated, however. Should a reachability problem arise with an
existing Neighbor Cache entry, Neighbor Unreachability Detection will
perform any needed recovery.
Whenever the Lifetime of an entry in the Default Router List expires,
that entry is discarded. When removing a router from the Default
Router list, the node MUST update the Destination Cache in such a way
that all entries using the router perform next-hop determination
again rather than continue sending traffic to the (deleted) router.
6.3.6. 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
node keeps track of a neighbor's reachability 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 exists
for an 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 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
Destination Cache caused by Redirect messages.
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
whose reachability is unknown or suspect (i.e., in the
INCOMPLETE state, or for which no Neighbor Cache entry exists).
Further implementation hints on default router selection when
multiple equivalent routers are available are discussed in
[LD-SHRE].
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 do not
return the same router until all other routers have been
selected.
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 is made in conjunction with the sending of a packet to a
router, and the selected router will be probed for reachability
as a side effect.
6.3.7. Sending Router Solicitations
When an interface becomes enabled, a host may be unwilling to wait
for the next unsolicited Router Advertisement to locate default
routers or learn prefixes. To obtain 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
after any of the following events:
- The interface is initialized at system startup time.
- The interface is reinitialized after a temporary interface
failure or after being temporarily disabled by system
management.
- The system changes from being a router to being a host, by
having its IP forwarding capability turned off by system
management.
- The host attaches to a link for the first time.
- The host re-attaches to a link after being detached for some
time.
A host sends Router Solicitations to the all-routers multicast
address. The IP source address is set to either one of the
interface's unicast addresses or the unspecified address. The Source
Link-Layer Address option SHOULD be set to the host's link-layer
address, if the IP source address is not the unspecified address.
Before a host sends an initial solicitation, it SHOULD delay the
transmission for a random amount of time between 0 and
MAX_RTR_SOLICITATION_DELAY. This serves to alleviate congestion when
many hosts start up on a link at the same time, such as might happen
after recovery from a power failure. If a host has already performed a random delay since the interface became (re)enabled (e.g., as part of Duplicate Address Detection [ADDRCONF]), there is no need to delay again before sending the first Router Solicitation message. In some cases, the random delay MAY be omitted if necessary. For instance, a mobile node, using [MIPv6], moving to a new link would need to discover such movement as soon as possible to minimize the amount of packet losses resulting from the change in its topological movement. Router Solicitations provide a useful tool for movement detection in Mobile IPv6 as they allow mobile nodes to determine movement to new links. Hence, if a mobile node received link-layer information indicating that movement might have taken place, it MAY send a Router Solicitation immediately, without random delays. The strength of such indications should be assessed by the mobile node's implementation depending on the level of certainty of the link-layer hints, and it is outside the scope of this specification. Note that using this mechanism inappropriately (e.g., based on weak or transient indications) may result in Router Solicitation storms. Furthermore, simultaneous mobility of a large number of mobile nodes that use this mechanism can result in a large number of solicitations sent simultaneously. Once the host sends a Router Solicitation, and receives a valid Router Advertisement with a non-zero Router Lifetime, the host MUST desist from sending additional solicitations on that interface, until the next time one of the above events occurs. Moreover, a host SHOULD send at least one solicitation in the case where an advertisement is received prior to having sent a solicitation. Responses to solicited advertisements may contain more information than unsolicited advertisements. 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 there are no routers on the link for the purpose of [ADDRCONF]. However, the host continues to receive and process Router Advertisements messages in the event that routers appear on the link.
(next page on part 4)