The IP address of the destination which is redirected
to the target.
Possible options:
Target link-layer address
The link-layer address for the target. It SHOULD be
included (if known). Note that on NBMA links, hosts
may rely on the presence of the Target Link-Layer
Address option in Redirect messages as the means for
determining the link-layer addresses of neighbors. In
such cases, the option MUST be included in Redirect
messages.
Redirected Header
As much as possible of the IP packet that triggered
the sending of the Redirect without making the
redirect packet exceed 576 octets.
4.6. Option Formats
Neighbor Discovery messages include zero or more options, some of
which may appear multiple times in the same message. All options are
of the form:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ... ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type 8-bit identifier of the type of option. The options
defined in this document are:
Option Name Type
Source Link-Layer Address 1
Target Link-Layer Address 2
Prefix Information 3
Redirected Header 4
MTU 5
Length 8-bit unsigned integer. The length of the option in
units of 8 octets. The value 0 is invalid. Nodes
MUST silently discard an ND packet that contains an
option with length zero.
4.6.1. Source/Target Link-layer Address
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Link-Layer Address ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type
1 for Source Link-layer Address
2 for Target Link-layer Address
Length The length of the option in units of 8 octets. For
example, the length for IEEE 802 addresses is 1
[IPv6-ETHER].
Link-Layer Address
The variable length link-layer address.
The content and format of this field (including byte
and bit ordering) is expected to be specified in
specific documents that describe how IPv6 operates
over different link layers. For instance, [IPv6-
ETHER].
Description
The Source Link-Layer Address option contains the
link-layer address of the sender of the packet. It is
used in the Neighbor Solicitation, Router
Solicitation, and Router Advertisement packets.
The Target Link-Layer Address option contains the
link-layer address of the target. It is used in
Neighbor Advertisement and Redirect packets.
These options MUST be silently ignored for other
Neighbor Discovery messages.
4.6.2. Prefix Information
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Prefix Length |L|A| Reserved1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Valid Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Preferred Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Prefix +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type 3
Length 4
Prefix Length 8-bit unsigned integer. The number of leading bits in
the Prefix that are valid. The value ranges from 0 to
128.
L 1-bit on-link flag. When set, indicates that this
prefix can be used for on-link determination. When
not set the advertisement makes no statement about
on-link or off-link properties of the prefix. For
instance, the prefix might be used for address
configuration with some of the addresses belonging to
the prefix being on-link and others being off-link.
A 1-bit autonomous address-configuration flag. When set
indicates that this prefix can be used for autonomous
address configuration as specified in [ADDRCONF].
Reserved1 6-bit unused field. It MUST be initialized to zero by
the sender and MUST be ignored by the receiver.
Valid Lifetime
32-bit unsigned integer. The length of time in
seconds (relative to the time the packet is sent) that
the prefix is valid for the purpose of on-link
determination. A value of all one bits (0xffffffff)
represents infinity. The Valid Lifetime is also used
by [ADDRCONF].
Preferred Lifetime
32-bit unsigned integer. The length of time in
seconds (relative to the time the packet is sent) that
addresses generated from the prefix via stateless
address autoconfiguration remain preferred [ADDRCONF].
A value of all one bits (0xffffffff) represents
infinity. See [ADDRCONF].
Reserved2 This field is unused. It MUST be initialized to zero
by the sender and MUST be ignored by the receiver.
Prefix An IP address or a prefix of an IP address. The
Prefix Length field contains the number of valid
leading bits in the prefix. The bits in the prefix
after the prefix length are reserved and MUST be
initialized to zero by the sender and ignored by the
receiver. A router SHOULD NOT send a prefix option
for the link-local prefix and a host SHOULD ignore
such a prefix option.
Description
The Prefix Information option provide hosts with on-
link prefixes and prefixes for Address
Autoconfiguration.
The Prefix Information option appears in Router
Advertisement packets and MUST be silently ignored for
other messages.
4.6.3. Redirected Header
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ IP header + data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type 4
Length The length of the option in units of 8 octets.
Reserved These fields are unused. They MUST be initialized to
zero by the sender and MUST be ignored by the
receiver.
IP header + data
The original packet truncated to ensure that the size
of the redirect message does not exceed 576 octets.
Description
The Redirected Header option is used in Redirect
messages and contains all or part of the packet that
is being redirected.
This option MUST be silently ignored for other
Neighbor Discovery messages.
4.6.4. MTU
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type 5
Length 1
Reserved This field is unused. It MUST be initialized to zero
by the sender and MUST be ignored by the receiver.
MTU 32-bit unsigned integer. The recommended MTU for the
link.
Description
The MTU option is used in Router Advertisement
messages to insure that all nodes on a link use the
same MTU value in those cases where the link MTU is
not well known.
This option MUST be silently ignored for other
Neighbor Discovery messages.
In configurations in which heterogeneous technologies
are bridged together, the maximum supported MTU may
differ from one segment to another. If the bridges do
not generate ICMP Packet Too Big messages,
communicating nodes will be unable to use Path MTU to
dynamically determine the appropriate MTU on a per-
neighbor basis. In such cases, routers use the MTU
option to specify an MTU value supported by all
segments.
5. CONCEPTUAL MODEL OF A HOST
This section describes a conceptual model of one possible data
structure organization that hosts (and to some extent routers) will
maintain in interacting with neighboring nodes. The described
organization is provided to facilitate the explanation of how the
Neighbor Discovery protocol should behave. This document does not
mandate that implementations adhere to this model as long as their
external behavior is consistent with that described in this document.
This model is only concerned with the aspects of host behavior
directly related to Neighbor Discovery. In particular, it does not
concern itself with such issues as source address selection or the
selecting of an outgoing interface on a multihomed host.
5.1. Conceptual Data Structures
Hosts will need to maintain the following pieces of information for
each interface:
Neighbor Cache
- A set of entries about individual neighbors to which
traffic has been sent recently. Entries are keyed on
the neighbor's on-link unicast IP address and contain
such information as its link-layer address, a flag
indicating whether the neighbor is a router or a host
(called IsRouter in this document), a pointer to any
queued packets waiting for address resolution to
complete, etc.
A Neighbor Cache entry also contains information used
by the Neighbor Unreachability Detection algorithm,
including the reachability state, the number of
unanswered probes, and the time the next Neighbor
Unreachability Detection event is scheduled to take
place.
Destination Cache
- A set of entries about destinations to which traffic
has been sent recently. The Destination Cache
includes both on-link and off-link destinations and
provides a level of indirection into the Neighbor
Cache; the Destination Cache maps a destination IP
address to the IP address of the next-hop neighbor.
This cache is updated with information learned from
Redirect messages. Implementations may find it
convenient to store additional information not
directly related to Neighbor Discovery in Destination
Cache entries, such as the Path MTU (PMTU) and round
trip timers maintained by transport protocols.
Prefix List - A list of the prefixes that define a set of addresses
that are on-link. Prefix List entries are created
from information received in Router Advertisements.
Each entry has an associated invalidation timer value
(extracted from the advertisement) used to expire
prefixes when they become invalid. A special
"infinity" timer value specifies that a prefix remains
valid forever, unless a new (finite) value is received
in a subsequent advertisement.
The link-local prefix is considered to be on the
prefix list with an infinite invalidation timer
regardless of whether routers are advertising a prefix
for it. Received Router Advertisements SHOULD NOT
modify the invalidation timer for the link-local
prefix.
Default Router List
- A list of routers to which packets may be sent.
Router list entries point to entries in the Neighbor
Cache; the algorithm for selecting a default router
favors routers known to be reachable over those whose
reachability is suspect. Each entry also has an
associated invalidation timer value (extracted from
Router Advertisements) used to delete entries that are
no longer advertised.
Note that the above conceptual data structures can be implemented
using a variety of techniques. One possible implementation is to use
a single longest-match routing table for all of the above data
structures. Regardless of the specific implementation, it is
critical that the Neighbor Cache entry for a router is shared by all
Destination Cache entries using that router in order to prevent
redundant Neighbor Unreachability Detection probes.
Note also that other protocols (e.g. IPv6 Mobility) might add
additional conceptual data structures. An implementation is at
liberty to implement such data structures in any way it pleases. For
example, an implementation could merge all conceptual data structures
into a single routing table.
The Neighbor Cache contains information maintained by the Neighbor
Unreachability Detection algorithm. A key piece of information is a
neighbor's reachability state, which is one of five possible values.
The following definitions are informal; precise definitions can be
found in Section 7.3.2.
INCOMPLETE Address resolution is in progress and the link-layer
address of the neighbor has not yet been determined.
REACHABLE Roughly speaking, the neighbor is known to have been
reachable recently (within tens of seconds ago).
STALE The neighbor is no longer known to be reachable but until
traffic is sent to the neighbor, no attempt should be
made to verify its reachability.
DELAY The neighbor is no longer known to be reachable, and
traffic has recently be sent to the neighbor. Rather
than probe the neighbor immediately, however, delay
sending probes for a short while in order to give upper
layer protocols a chance to provide reachability
confirmation.
PROBE The neighbor is no longer known to be reachable, and
unicast Neighbor Solicitation probes are being sent to
verify reachability.
5.2. Conceptual Sending Algorithm
When sending a packet to a destination, a node uses a combination of
the Destination Cache, the Prefix List, and the Default Router List
to determine the IP address of the appropriate next hop, an operation
known as "next-hop determination". Once the IP address of the next
hop is known, the Neighbor Cache is consulted for link-layer
information about that neighbor.
Next-hop determination for a given unicast destination operates as
follows. The sender performs a longest prefix match against the
Prefix List to determine whether the packet's destination is on- or
off-link. If the destination is on-link, the next-hop address is the
same as the packet's destination address. Otherwise, the sender
selects a router from the Default Router List (following the rules
described in Section 6.3.6). If the Default Router List is empty,
the sender assumes that the destination is on-link.
For efficiency reasons, next-hop determination is not performed on
every packet that is sent. Instead, the results of next-hop
determination computations are saved in the Destination Cache (which
also contains updates learned from Redirect messages). When the
sending node has a packet to send, it first examines the Destination
Cache. If no entry exists for the destination, next-hop
determination is invoked to create a Destination Cache entry.
Once the IP address of the next-hop node is known, the sender
examines the Neighbor Cache for link-layer information about that
neighbor. If no entry exists, the sender creates one, sets its state
to INCOMPLETE, initiates Address Resolution, and then queues the data
packet pending completion of address resolution. For multicast-
capable interfaces Address Resolution consists of sending a Neighbor
Solicitation message and waiting for a Neighbor Advertisement. When
a Neighbor Advertisement response is received, the link-layer
addresses is entered in the Neighbor Cache entry and the queued
packet is transmitted. The address resolution mechanism is described
in detail in Section 7.2.
For multicast packets the next-hop is always the (multicast)
destination address and is considered to be on-link. The procedure
for determining the link-layer address corresponding to a given IP
multicast address can be found in a separate document that covers
operating IP over a particular link type (e.g., [IPv6-ETHER]).
Each time a Neighbor Cache entry is accessed while transmitting a
unicast packet, the sender checks Neighbor Unreachability Detection
related information according to the Neighbor Unreachability
Detection algorithm (Section 7.3). This unreachability check might
result in the sender transmitting a unicast Neighbor Solicitation to
verify that the neighbor is still reachable.
Next-hop determination is done the first time traffic is sent to a
destination. As long as subsequent communication to that destination
proceeds successfully, the Destination Cache entry continues to be
used. If at some point communication ceases to proceed, as
determined by the Neighbor Unreachability Detection algorithm, next-
hop determination may need to be performed again. For example,
traffic through a failed router should be switched to a working
router. Likewise, it may be possible to reroute traffic destined for
a mobile node to a "mobility agent".
Note that when a node redoes next-hop determination there is no need
to discard the complete Destination Cache entry. In fact, it is
generally beneficial to retain such cached information as the PMTU
and round trip timer values that may also be kept in the Destination
Cache entry.
Routers and multihomed hosts have multiple interfaces. The remainder
of this document assumes that all sent and received Neighbor
Discovery messages refer to the interface of appropriate context.
For example, when responding to a Router Solicitation, the
corresponding Router Advertisement is sent out the interface on which
the solicitation was received.
5.3. Garbage Collection and Timeout Requirements
The conceptual data structures described above use different
mechanisms for discarding potentially stale or unused information.
From the perspective of correctness there is no need to periodically
purge Destination and Neighbor Cache entries. Although stale
information can potentially remain in the cache indefinitely, the
Neighbor Unreachability Detection algorithm ensures that stale
information is purged quickly if it is actually being used.
To limit the storage needed for the Destination and Neighbor Caches,
a node may need to garbage-collect old entries. However, care must
be taken to insure that sufficient space is always present to hold
the working set of active entries. A small cache may result in an
excessive number of Neighbor Discovery messages if entries are
discarded and rebuilt in quick succession. Any LRU-based policy that
only reclaims entries that have not been used in some time (e.g., ten
minutes or more) should be adequate for garbage-collecting unused
entries.
A node should retain entries in the Default Router List and the
Prefix List until their lifetimes expire. However, a node may
garbage collect entries prematurely if it is low on memory. If not
all routers are kept on the Default Router list, a node should retain
at least two entries in the Default Router List (and preferably more)
in order to maintain robust connectivity for off-link destinations.
When removing an entry from the Prefix List there is no need to purge
any entries from the Destination or Neighbor Caches. Neighbor
Unreachability Detection will efficiently purge any entries in these
caches that have become invalid. When removing an entry from the
Default Router List, however, any entries in the Destination Cache
that go through that router must perform next-hop determination again
to select a new default router.
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.
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.
Default: infinity.
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].
Default: 604800 seconds (7 days)
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.
When a router receives a Router Solicitation and the Source Address
is not the unspecified address, it records that the source of the
packet is a neighbor by creating or updating the Neighbor Cache
entry. If the solicitation contains a Source Link-Layer Address
option, and the router has a Neighbor Cache entry for the neighbor,
the link-layer address SHOULD be updated in the Neighbor Cache. If a
Neighbor Cache entry is created for the source 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. In either case the
entry's IsRouter flag SHOULD be set to false.
If the Source Address is the unspecified address the router MUST NOT
create or update the Neighbor Cache entry.
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.
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. 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 if the value 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