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 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. - If the message includes an IP Authentication Header, the message authenticates correctly. - 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. - If the message includes an IP Authentication Header, the message authenticates correctly. - 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 multicast interface: 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 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 stateful 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 that current diameter of the Internet. The value zero means unspecified (by this router).
Default: The value specified in the "Assigned Numbers" RFC [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. 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 MUST 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 Automatic address configuration [ADDRCONF] defines additional information associated with each 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 MUST 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). 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 multicast 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. See [ADDRCONF]. - 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 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 insure 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, 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. 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). - 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 6.2.7 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 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 change rarely, 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 (unicast) IP packets. Default: The value specified in the "Assigned Numbers" RFC [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, such as stateful 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 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. Some Router Advertisement fields (e.g., Cur Hop Limit, Reachable Time and Retrans Timer) may contain a value denoting 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 recompute 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 Neighbor Unreachability Detection messages 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 insure that a new random value gets recomputed 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 default 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 increase the 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. Note: Implementations can choose to process the on-link aspects of the prefixes separately from the 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). 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 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 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. 3) If the Default Router List is empty, assume that all destinations are on-link as specified in Section 5.2. 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. 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. 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 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. 7. ADDRESS RESOLUTION AND NEIGHBOR UNREACHABILITY DETECTION This section describes the functions related to Neighbor Solicitation and Neighbor Advertisement messages and includes descriptions of address resolution and the Neighbor Unreachability Detection algorithm.
Neighbor Solicitation and Advertisement messages are also used for Duplicate Address Detection as specified by [ADDRCONF]. In particular, Duplicate Address Detection sends Neighbor Solicitation messages with an unspecified source address targeting its own "tentative" address. Such messages trigger nodes already using the address to respond with a multicast Neighbor Advertisement indicating that the address is in use. 7.1. Message Validation 7.1.1. Validation of Neighbor Solicitations A node MUST silently discard any received Neighbor 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. - If the message includes an IP Authentication Header, the message authenticates correctly. - ICMP Checksum is valid. - ICMP Code is 0. - ICMP length (derived from the IP length) is 24 or more octets. - Target Address is not a multicast address. - All included options have a length that is greater than zero. - If the IP source address is the unspecified address, the IP destination address is a solicited-node multicast address. - 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 Neighbor 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 Neighbor Solicitation that passes the validity checks is called a "valid solicitation". 7.1.2. Validation of Neighbor Advertisements A node MUST silently discard any received Neighbor Advertisement 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. - If the message includes an IP Authentication Header, the message authenticates correctly. - ICMP Checksum is valid. - ICMP Code is 0. - ICMP length (derived from the IP length) is 24 or more octets. - Target Address is not a multicast address. - If the IP Destination Address is a multicast address the Solicited flag is zero. - 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 Neighbor Advertisement messages MUST be ignored and the packet processed as normal. The only defined option that may appear is the Target Link-Layer Address option. A Neighbor Advertisements that passes the validity checks is called a "valid advertisement". 7.2. Address Resolution Address resolution is the process through which a node determines the link-layer address of a neighbor given only its IP address. Address resolution is performed only on addresses that are determined to be on-link and for which the sender does not know the corresponding link-layer address. Address resolution is never performed on multicast addresses.
7.2.1. Interface Initialization When a multicast-capable interface becomes enabled the node MUST join the all-nodes multicast address on that interface, as well as the solicited-node multicast address corresponding to each of the IP addresses assigned to the interface. The set of addresses assigned to an interface may change over time. New addresses might be added and old addresses might be removed [ADDRCONF]. In such cases the node MUST join and leave the solicited-node multicast address corresponding to the new and old addresses, respectively. Note that multiple unicast addresses may map into the same solicited-node multicast address; a node MUST NOT leave the solicited-node multicast group until all assigned addresses corresponding to that multicast address have been removed. 7.2.2. Sending Neighbor Solicitations When a node has a unicast packet to send to a neighbor, but does not know the neighbor's link-layer address, it performs address resolution. For multicast-capable interfaces this entails creating a Neighbor Cache entry in the INCOMPLETE state and transmitting a Neighbor Solicitation message targeted at the neighbor. The solicitation is sent to the solicited-node multicast address corresponding to the target address. If the source address of the packet prompting the solicitation is the same as one of the addresses assigned to the outgoing interface, that address SHOULD be placed in the IP Source Address of the outgoing solicitation. Otherwise, any one of the addresses assigned to the interface should be used. Using the prompting packet's source address when possible insures that the recipient of the Neighbor Solicitation installs in its Neighbor Cache the IP address that is highly likely to be used in subsequent return traffic belonging to the prompting packet's "connection". If the solicitation is being sent to a solicited-node multicast address, the sender MUST include its link-layer address (if it has one) as a Source Link-Layer Address option. Otherwise, the sender SHOULD include its link-layer address (if it has one) as a Source Link-Layer Address option. Including the source link-layer address in a multicast solicitation is required to give the target an address to which it can send the Neighbor Advertisement. On unicast solicitations, an implementation MAY omit the Source Link-Layer Address option. The assumption here is that if the sender has a peer's link-layer address in its cache, there is a high probability that the peer will also have an entry in its cache for the sender. Consequently, it need not be sent.
While waiting for address resolution to complete, the sender MUST, for each neighbor, retain a small queue of packets waiting for address resolution to complete. The queue MUST hold at least one packet, and MAY contain more. However, the number of queued packets per neighbor SHOULD be limited to some small value. When a queue overflows, the new arrival SHOULD replace the oldest entry. Once address resolution completes, the node transmits any queued packets. While awaiting a response, the sender SHOULD retransmit Neighbor Solicitation messages approximately every RetransTimer milliseconds, even in the absence of additional traffic to the neighbor. Retransmissions MUST be rate-limited to at most one solicitation per neighbor every RetransTimer milliseconds. If no Neighbor Advertisement is received after MAX_MULTICAST_SOLICIT solicitations, address resolution has failed. The sender MUST return ICMP destination unreachable indications with code 3 (Address Unreachable) for each packet queued awaiting address resolution. 7.2.3. Receipt of Neighbor Solicitations A valid Neighbor Solicitation that does not meet any the following requirements MUST be silently discarded: - The Target Address is a "valid" unicast or anycast address assigned to the receiving interface [ADDRCONF], - The Target Address is a unicast address for which the node is offering proxy service, or - The Target Address is a "tentative" address on which Duplicate Address Detection is being performed [ADDRCONF]. If the Target Address is tentative, the Neighbor Solicitation should be processed as described in [ADDRCONF]. Otherwise, the following description applies. If the Source Address is not the unspecified address and, on link layers that have addresses, the solicitation includes a Source Link-Layer Address option, then the recipient SHOULD create or update the Neighbor Cache entry for the IP Source Address of the solicitation. If an entry does not already exist, the node SHOULD create a new one and set its reachability state to STALE as specified in Section 7.3.3. If an entry already exists, and the cached link-layer address differs from the one in the received Source Link-Layer option, the cached address should be replaced by the received address and the entry's reachability state MUST be set to STALE.
If a Neighbor Cache entry is created the IsRouter flag SHOULD be set 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 is a router. In the event that the 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 IsRouter flag MUST NOT be modified. If the Source Address is the unspecified address the node MUST NOT create or update the Neighbor Cache entry. After any updates to the Neighbor Cache, the node sends a Neighbor Advertisement response as described in the next section. 7.2.4. Sending Solicited Neighbor Advertisements A node sends a Neighbor Advertisement in response to a valid Neighbor Solicitation targeting one of the node's assigned addresses. The Target Address of the advertisement is copied from the Target Address of the solicitation. If the solicitation's IP Destination Address is not a multicast address, the Target Link-Layer Address option MAY be omitted; the neighboring node's cached value must already be current in order for the solicitation to have been received. If the solicitation's IP Destination Address is a multicast address, the Target 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. If the Target Address is either an anycast address or a unicast address for which the node is providing proxy service, or the Target Link-Layer Address option is not included, the Override flag SHOULD be set to zero. Otherwise, the Override flag SHOULD be set to one. Proper setting of the Override flag ensures that nodes give preference to non-proxy advertisements, even when received after proxy advertisements, and also ensures that the first advertisement for an anycast address "wins". If the source of the solicitation is the unspecified address, the node MUST set the Solicited flag to zero and multicast the advertisement to the all-nodes address. Otherwise, the node MUST set the Solicited flag to one and unicast the advertisement to the Source Address of the solicitation. If the Target Address is an anycast address the sender SHOULD delay sending a response for a random time between 0 and MAX_ANYCAST_DELAY_TIME seconds.
Because unicast Neighbor Solicitations are not required to include a Source Link-Layer Address, it is possible that a node sending a solicited Neighbor Advertisement does not have a corresponding link- layer address for its neighbor in its Neighbor Cache. In such situations, a node will first have to use Neighbor Discovery to determine the link-layer address of its neighbor (i.e, send out a multicast Neighbor Solicitation). 7.2.5. Receipt of Neighbor Advertisements When a valid Neighbor Advertisement is received (either solicited or unsolicited), the Neighbor Cache is searched for the target's entry. If no entry exists, the advertisement SHOULD be silently discarded. There is no need to create an entry if none exists, since the recipient has apparently not initiated any communication with the target. Once the appropriate Neighbor Cache entry has been located, the specific actions taken depend on the state of the Neighbor Cache entry, the flags in the advertisement and the actual link-layer address supplied. If the target's Neighbor Cache entry is in the INCOMPLETE state when the advertisement is received, one of two things happens. If the link layer has addresses and no Target Link-Layer address option is included, the receiving node SHOULD silently discard the received advertisement. Otherwise, the receiving node performs the following steps: - It records the link-layer address in the Neighbor Cache entry. - If the advertisement's Solicited flag is set, the state of the entry is set to REACHABLE, otherwise it is set to STALE. - It sets the IsRouter flag in the cache entry based on the Router flag in the received advertisement. - It sends any packets queued for the neighbor awaiting address resolution. Note that the Override flag is ignored if the entry is in the INCOMPLETE state. If the target's Neighbor Cache entry is in any state other than INCOMPLETE when the advertisement is received, processing becomes quite a bit more complex. If the Override flag is clear and the supplied link-layer address differs from that in the cache, then one of two actions takes place: if the state of the entry is REACHABLE,
set it to STALE, but do not update the entry in any other way; otherwise, the received advertisement should be ignored and MUST NOT update the cache. If the Override flag is set, both the Override flag is clear and the supplied link-layer address is the same as that in the cache, or no Target Link-layer address option was supplied, the received advertisement MUST update the Neighbor Cache entry as follows: - The link-layer address in the Target Link-Layer Address option MUST be inserted in the cache (if one is supplied and is different than the already recorded address). - If the Solicited flag is set, the state of the entry MUST be set to REACHABLE. If the Solicited flag is zero and the link-layer address was updated with a different address the state MUST be set to STALE. Otherwise, the entry's state remains unchanged. An advertisement's Solicited flag should only be set if the advertisement is a response to a Neighbor Solicitation. Because Neighbor Unreachability Detection Solicitations are sent to the cached link-layer address, receipt of a solicited advertisement indicates that the forward path is working. Receipt of an unsolicited advertisement, however, suggests that a neighbor has urgent information to announce (e.g., a changed link-layer address). If the urgent information indicates a change from what a node is currently using, the node should verify the reachability of the (new) path when it sends the next packet. There is no need to update the state for unsolicited advertisements that do not change the contents of the cache. - 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, the node MUST remove that router from the Default Router List and update the Destination Cache entries for all 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 stops forwarding packets due to being configured as a host. The above rules ensure that the cache is updated either when the Neighbor Advertisement takes precedence (i.e., the Override flag is set) or when the Neighbor Advertisement refers to the same link-layer address that is currently recorded in the cache. If none of the above apply, the advertisement prompts future Neighbor Unreachability Detection (if it is not already in progress) by changing the state in the cache entry.
7.2.6. Sending Unsolicited Neighbor Advertisements In some cases a node may be able to determine that its link-layer address has changed (e.g., hot-swap of an interface card) and may wish to inform its neighbors of the new link-layer address quickly. In such cases a node MAY send up to MAX_NEIGHBOR_ADVERTISEMENT unsolicited Neighbor Advertisement messages to the all-nodes multicast address. These advertisements MUST be separated by at least RetransTimer seconds. The Target Address field in the unsolicited advertisement is set to an IP address of the interface, and the Target Link-Layer Address option is filled with the new link-layer address. The Solicited flag MUST be set to zero, in order to avoid confusing the 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 Override flag MAY be set to either zero or one. In either case, neighboring nodes will immediately change the state of their Neighbor Cache entries for the Target Address to STALE, prompting them to verify the path for reachability. If the Override flag is set to one, neighboring nodes will install the new link-layer address in their caches. Otherwise, they will ignore the new link-layer address, choosing instead to probe the cached address. A node that has multiple IP addresses assigned to an interface MAY multicast a separate Neighbor Advertisement for each address. In such a case the node SHOULD introduce a small delay between the sending of each advertisement to reduce the probability of the advertisements being lost due to congestion. A proxy MAY multicast Neighbor Advertisements when its link-layer address changes or when it is configured (by system management or other mechanisms) to proxy for an address. If there are multiple nodes that are providing proxy services for the same set of addresses the proxies SHOULD provide a mechanism that prevents multiple proxies from multicasting advertisements for any one address, in order to reduce the risk of excessive multicast traffic. Also, a node belonging to an anycast address MAY multicast unsolicited Neighbor Advertisements for the anycast address when the node's link-layer address changes. Note that because unsolicited Neighbor Advertisements do not reliably update caches in all nodes (the advertisements might not be received by all nodes), they should only be viewed as a performance optimization to quickly update the caches in most neighbors. The Neighbor Unreachability Detection algorithm ensures that all nodes obtain a reachable link-layer address, though the delay may be
slightly longer. 7.2.7. Anycast Neighbor Advertisements From the perspective of Neighbor Discovery, anycast addresses are treated just like unicast addresses in most cases. Because an anycast address is syntactically the same as a unicast address, nodes performing address resolution or Neighbor Unreachability Detection on an anycast address treat it as if it were a unicast address. No special processing takes place. Nodes that have an anycast address assigned to an interface treat them exactly the same as if they were unicast addresses with two exceptions. First, Neighbor Advertisements sent in response to a Neighbor Solicitation SHOULD be delayed by a random time between 0 and MAX_ANYCAST_DELAY_TIME to reduce the probability of network congestion. Second, the Override flag in Neighbor Advertisements SHOULD be set to 0, so that when multiple advertisements are received, the first received advertisement is used rather than the most recently received advertisement. As with unicast addresses, Neighbor Unreachability Detection ensures that a node quickly detects when the current binding for an anycast address becomes invalid. 7.2.8. Proxy Neighbor Advertisements Under limited circumstances, a router MAY proxy for one or more other nodes, that is, through 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. The mechanisms used by proxy are identical to the mechanisms used with anycast addresses. A proxy MUST join the solicited-node multicast address(es) that correspond to the IP address(es) assigned to the node for which it is proxying. All solicited proxy Neighbor Advertisement messages MUST have the Override flag set to zero. This ensures that if the node itself is present on the link its Neighbor Advertisement (with the Override flag set to one) will take precedence of any advertisement received from a proxy. A proxy MAY send unsolicited advertisements with the Override flag set to one as specified in Section 7.2.6, but doing so may cause the proxy advertisement to override a valid entry created by the node itself.
Finally, when sending a proxy advertisement in response to a Neighbor Solicitation, the sender should delay its response by a random time between 0 and MAX_ANYCAST_DELAY_TIME seconds. 7.3. Neighbor Unreachability Detection Communication to or through a neighbor may fail for numerous reasons at any time, including hardware failure, hot-swap of an interface card, etc. If the destination has failed, no recovery is possible and communication fails. On the other hand, if it is the path that has failed, recovery may be possible. Thus, a node actively tracks the reachability "state" for the neighbors to which it is sending packets. Neighbor Unreachability Detection is used for all paths between hosts and neighboring nodes, including host-to-host, host-to-router, and 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. When a path to a neighbor appears to be failing, the specific recovery procedure depends on how the neighbor is being used. If the neighbor is the ultimate destination, for example, address resolution should be performed again. If the neighbor is a router, however, attempting to switch to another router would be appropriate. The specific recovery that takes place is covered under next-hop determination; Neighbor Unreachability Detection signals the need for next-hop determination by deleting a Neighbor Cache entry. Neighbor Unreachability Detection is performed only for neighbors to which unicast packets are sent; it is not used when sending to multicast addresses. 7.3.1. Reachability Confirmation A neighbor is considered reachable if the node has recently received a confirmation that packets sent recently to the neighbor were received by its IP layer. Positive confirmation can be gathered in two ways: hints from upper layer protocols that indicate a connection is making "forward progress", or receipt of a Neighbor Advertisement message that is a response to a Neighbor Solicitation message. A connection makes "forward progress" if the packets received from a remote peer can only be arriving if recent packets sent to that peer are actually reaching it. In TCP, for example, receipt of a (new) acknowledgement indicates that previously sent data reached the peer. Likewise, the arrival of new (non-duplicate) data indicates that
earlier acknowledgements are being delivered to the remote peer. If packets are reaching the peer, they must also be reaching the sender's next-hop neighbor; thus "forward progress" is a confirmation that the next-hop neighbor is reachable. For 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 information may not be readily available from upper-layer protocols. When no hints are available and a node is sending packets to a neighbor, the node actively probes the neighbor using unicast Neighbor Solicitation messages to verify that the forward path is still working. The receipt of a solicited Neighbor Advertisement serves as reachability confirmation, since advertisements with the Solicited flag set to one are sent only in response to a 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 confirmation. Receipt of unsolicited messages only confirm the one-way path from the sender to the recipient node. In contrast, Neighbor Unreachability Detection requires that a node keep track of the reachability of the forward path to a neighbor from the its perspective, not the neighbor's perspective. Note that receipt of a solicited advertisement indicates that a path is working in both directions. The solicitation must have reached the neighbor, prompting it to generate an advertisement. Likewise, receipt of an advertisement indicates that the path from the sender to the recipient is working. However, the latter fact is known only to the recipient; the advertisement's sender has no direct way of knowing that the advertisement it sent actually reached a neighbor. From the perspective of Neighbor Unreachability Detection, only the reachability of the forward path is of interest. 7.3.2. Neighbor Cache Entry States A Neighbor Cache entry can be in one of five states: INCOMPLETE Address resolution is being performed on the entry. Specifically, a Neighbor Solicitation has been sent to the solicited-node multicast address of the target, but the corresponding Neighbor Advertisement has not yet been received. REACHABLE Positive confirmation was received within the last ReachableTime milliseconds that the forward path to the neighbor was functioning properly. While
REACHABLE, no special action takes place as packets are sent. STALE More than ReachableTime milliseconds have elapsed since the last positive confirmation was received that the forward path was functioning properly. While stale, no action takes place until a packet is sent. The STALE state is entered upon receiving an unsolicited Neighbor Discovery message that updates the cached link-layer address. Receipt of such a message does not confirm reachability, and entering the STALE state insures reachability is verified quickly if the entry is actually being used. However, reachability is not actually verified until the entry is actually used. DELAY More than ReachableTime milliseconds have elapsed since the last positive confirmation was received that the forward path was functioning properly, and a packet was sent within the last DELAY_FIRST_PROBE_TIME seconds. If no reachability confirmation is received within DELAY_FIRST_PROBE_TIME seconds of entering the DELAY state, send a Neighbor Solicitation and change the state to PROBE. The DELAY state is an optimization that gives upper- layer protocols additional time to provide reachability confirmation in those cases where ReachableTime milliseconds have passed since the last confirmation due to lack of recent traffic. Without this optimization the opening of a TCP connection after a traffic lull would initiate probes even though the subsequent three-way handshake would provide a reachability confirmation almost immediately. PROBE A reachability confirmation is actively sought by retransmitting Neighbor Solicitations every RetransTimer milliseconds until a reachability confirmation is received. 7.3.3. Node Behavior Neighbor Unreachability Detection operates in parallel with the sending of packets to a neighbor. While reasserting a neighbor's reachability, a node continues sending packets to that neighbor using the cached link-layer address. If no traffic is sent to a neighbor, no probes are sent.
When a node needs to perform address resolution on a neighboring address, it creates an entry in the INCOMPLETE state and initiates address resolution as specified in Section 7.2. If address resolution fails, the entry SHOULD be deleted, so that subsequent traffic to that neighbor invokes the next-hop determination procedure again. Invoking next-hop determination at this point insures that alternate default routers are tried. When a reachability confirmation is received (either through upper- layer advice or a solicited Neighbor Advertisement) an entry's state changes to REACHABLE. The one exception is that upper-layer advice has no effect on entries in the INCOMPLETE state (e.g., for which no link-layer address is cached). When ReachableTime milliseconds have passed since receipt of the last reachability confirmation for a neighbor, the Neighbor Cache entry's state changes from REACHABLE to STALE. Note: An implementation may actually defer changing the state from REACHABLE to STALE until a packet is sent to the neighbor, i.e., there need not be an explicit timeout event associated with the expiration of ReachableTime. The first time a node sends a packet to a neighbor whose entry is STALE, the sender changes the state to DELAY and a sets a timer to expire in DELAY_FIRST_PROBE_TIME seconds. If the entry is still in the DELAY state when the timer expires, the entry's state changes to PROBE. If reachability confirmation is received, the entry's state changes to REACHABLE. Upon entering the PROBE state, a node sends a unicast Neighbor Solicitation message to the neighbor using the cached link-layer address. While in the PROBE state, a node retransmits Neighbor Solicitation messages every RetransTimer milliseconds until reachability confirmation is obtained. Probes are retransmitted even if no additional packets are sent to the neighbor. If no response is received after waiting RetransTimer milliseconds after sending the MAX_UNICAST_SOLICIT solicitations, retransmissions cease and the entry SHOULD be deleted. Subsequent traffic to that neighbor will recreate the entry and performs address resolution again. Note that all Neighbor Solicitations are rate-limited on a per- neighbor basis. A node MUST NOT send Neighbor Solicitations to the same neighbor more frequently than once every RetransTimer milliseconds.
A Neighbor Cache entry enters the STALE state when created as a result of receiving packets other than solicited Neighbor Advertisements (i.e., Router Solicitations, Router Advertisements, Redirects, and Neighbor Solicitations). These packets contain the link-layer address of either the sender or, in the case of Redirect, the redirection target. However, receipt of these link-layer addresses does not confirm reachability of the forward-direction path to that node. Placing a newly created Neighbor Cache entry for which the link-layer address is known in the STALE state provides assurance that path failures are detected quickly. In addition, should a cached link-layer address be modified due to receiving one of the above messages the state SHOULD also be set to STALE to provide prompt verification that the path to the new link-layer address is working. To properly detect the case where a 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 IsRouter flag recorded in the Neighbor Cache entry. When a node detects that a neighbor has changed from being a router to being a host, the node MUST remove that router from the Default Router List and 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 is using it (e.g., a host was 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. In some cases, link-specific information may indicate that a path to a neighbor has failed (e.g., the resetting of a virtual circuit). In such cases, link-specific information may be used to purge Neighbor Cache entries before the Neighbor Unreachability Detection would do so. However, link-specific information MUST NOT be used to confirm the reachability of a neighbor; such information does not provide end-to-end confirmation between neighboring IP layers.