Network Working Group S. Thomson
Request for Comments: 4862 Cisco
Obsoletes: 2462 T. Narten
Category: Standards Track IBM
September 2007 IPv6 Stateless Address Autoconfiguration
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
This document specifies the steps a host takes in deciding how to
autoconfigure its interfaces in IP version 6. The autoconfiguration
process includes generating a link-local address, generating global
addresses via stateless address autoconfiguration, and the Duplicate
Address Detection procedure to verify the uniqueness of the addresses
on a link.
This document specifies the steps a host takes in deciding how to
autoconfigure its interfaces in IP version 6 (IPv6). The
autoconfiguration process includes generating a link-local address,
generating global addresses via stateless address autoconfiguration,
and the Duplicate Address Detection procedure to verify the
uniqueness of the addresses on a link.
The IPv6 stateless autoconfiguration mechanism requires no manual
configuration of hosts, minimal (if any) configuration of routers,
and no additional servers. The stateless mechanism allows a host to
generate its own addresses using a combination of locally available
information and information advertised by routers. Routers advertise
prefixes that identify the subnet(s) associated with a link, while
hosts generate an "interface identifier" that uniquely identifies an
interface on a subnet. An address is formed by combining the two.
In the absence of routers, a host can only generate link-local
addresses. However, link-local addresses are sufficient for allowing
communication among nodes attached to the same link.
The stateless approach is used when a site is not particularly
concerned with the exact addresses hosts use, so long as they are
unique and properly routable. On the other hand, Dynamic Host
Configuration Protocol for IPv6 (DHCPv6) [RFC3315] is used when a
site requires tighter control over exact address assignments. Both
stateless address autoconfiguration and DHCPv6 may be used
IPv6 addresses are leased to an interface for a fixed (possibly
infinite) length of time. Each address has an associated lifetime
that indicates how long the address is bound to an interface. When a
lifetime expires, the binding (and address) become invalid and the
address may be reassigned to another interface elsewhere in the
Internet. To handle the expiration of address bindings gracefully,
an address goes through two distinct phases while assigned to an
interface. Initially, an address is "preferred", meaning that its
use in arbitrary communication is unrestricted. Later, an address
becomes "deprecated" in anticipation that its current interface
binding will become invalid. While an address is in a deprecated
state, its use is discouraged, but not strictly forbidden. New
communication (e.g., the opening of a new TCP connection) should use
a preferred address when possible. A deprecated address should be
used only by applications that have been using it and would have
difficulty switching to another address without a service disruption.
To ensure that all configured addresses are likely to be unique on a
given link, nodes run a "duplicate address detection" algorithm on
addresses before assigning them to an interface. The Duplicate
Address Detection algorithm is performed on all addresses,
independently of whether they are obtained via stateless
autoconfiguration or DHCPv6. This document defines the Duplicate
Address Detection algorithm.
The autoconfiguration process specified in this document applies only
to hosts and not routers. Since host autoconfiguration uses
information advertised by routers, routers will need to be configured
by some other means. However, it is expected that routers will
generate link-local addresses using the mechanism described in this
document. In addition, routers are expected to successfully pass the
Duplicate Address Detection procedure described in this document on
all addresses prior to assigning them to an interface.
Section 2 provides definitions for terminology used throughout this
document. Section 3 describes the design goals that lead to the
current autoconfiguration procedure. Section 4 provides an overview
of the protocol, while Section 5 describes the protocol in detail.
IP - Internet Protocol Version 6. The terms IPv4 and IPv6 are used
only in contexts where necessary to avoid ambiguity.
node - a device that implements IP.
router - a node that forwards IP packets not explicitly addressed to
host - any node that is not a router.
upper layer - a protocol layer immediately above IP. Examples are
transport protocols such as TCP and UDP, control protocols such as
ICMP, routing protocols such as OSPF, and Internet or lower-layer
protocols being "tunneled" over (i.e., encapsulated in) IP such as
IPX, AppleTalk, or IP itself.
link - a communication facility or medium over which nodes can
communicate at the link layer, i.e., the layer immediately below
IP. Examples are Ethernets (simple or bridged); PPP links; X.25,
Frame Relay, or ATM networks; and Internet (or higher) layer
"tunnels", such as tunnels over IPv4 or IPv6 itself. The protocol
described in this document will be used on all types of links
unless specified otherwise in the link-type-specific document
describing how to operate IP on the link in line with [RFC4861].
interface - a node's attachment to a link.
packet - an IP header plus payload.
address - an IP-layer identifier for an interface or a set of
unicast address - an identifier for a single interface. A packet
sent to a unicast address is delivered to the interface identified
by that address.
multicast address - an identifier for a set of interfaces (typically
belonging to different nodes). A packet sent to a multicast
address is delivered to all interfaces identified by that address.
anycast address - an identifier for a set of interfaces (typically
belonging to different nodes). A packet sent to an anycast
address is delivered to one of the interfaces identified by that
address (the "nearest" one, according to the routing protocol's
measure of distance). See [RFC4291].
solicited-node multicast address - a multicast address to which
Neighbor Solicitation messages are sent. The algorithm for
computing the address is given in [RFC4291].
link-layer address - a link-layer identifier for an interface.
Examples include IEEE 802 addresses for Ethernet links and E.164
addresses for Integrated Services Digital Network (ISDN) links.
link-local address - an address having link-only scope that can be
used to reach neighboring nodes attached to the same link. All
interfaces have a link-local unicast address.
global address - an address with unlimited scope.
communication - any packet exchange among nodes that requires that
the address of each node used in the exchange remain the same for
the duration of the packet exchange. Examples are a TCP
connection or a UDP request-response.
tentative address - an address whose uniqueness on a link is being
verified, prior to its assignment to an interface. A tentative
address is not considered assigned to an interface in the usual
sense. An interface discards received packets addressed to a
tentative address, but accepts Neighbor Discovery packets related
to Duplicate Address Detection for the tentative address.
preferred address - an address assigned to an interface whose use by
upper-layer protocols is unrestricted. Preferred addresses may be
used as the source (or destination) address of packets sent from
(or to) the interface.
deprecated address - An address assigned to an interface whose use
is discouraged, but not forbidden. A deprecated address should no
longer be used as a source address in new communications, but
packets sent from or to deprecated addresses are delivered as
expected. A deprecated address may continue to be used as a
source address in communications where switching to a preferred
address causes hardship to a specific upper-layer activity (e.g.,
an existing TCP connection).
valid address - a preferred or deprecated address. A valid address
may appear as the source or destination address of a packet, and
the Internet routing system is expected to deliver packets sent to
a valid address to their intended recipients.
invalid address - an address that is not assigned to any interface.
A valid address becomes invalid when its valid lifetime expires.
Invalid addresses should not appear as the destination or source
address of a packet. In the former case, the Internet routing
system will be unable to deliver the packet; in the latter case,
the recipient of the packet will be unable to respond to it.
preferred lifetime - the length of time that a valid address is
preferred (i.e., the time until deprecation). When the preferred
lifetime expires, the address becomes deprecated.
valid lifetime - the length of time an address remains in the valid
state (i.e., the time until invalidation). The valid lifetime
must be greater than or equal to the preferred lifetime. When the
valid lifetime expires, the address becomes invalid.
interface identifier - a link-dependent identifier for an interface
that is (at least) unique per link [RFC4291]. Stateless address
autoconfiguration combines an interface identifier with a prefix
to form an address. From address autoconfiguration's perspective,
an interface identifier is a bit string of known length. The
exact length of an interface identifier and the way it is created
is defined in a separate link-type specific document that covers
issues related to the transmission of IP over a particular link
type (e.g., [RFC2464]). Note that the address architecture
[RFC4291] also defines the length of the interface identifiers for
some set of addresses, but the two sets of definitions must be
consistent. In many cases, the identifier will be derived from
the interface's link-layer address.
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [RFC2119].
Note that this document intentionally limits the use of the keywords
to the protocol specification (Section 5).
3. Design Goals
Stateless autoconfiguration is designed with the following goals in
o Manual configuration of individual machines before connecting them
to the network should not be required. Consequently, a mechanism
is needed that allows a host to obtain or create unique addresses
for each of its interfaces. Address autoconfiguration assumes
that each interface can provide a unique identifier for that
interface (i.e., an "interface identifier"). In the simplest
case, an interface identifier consists of the interface's link-
layer address. An interface identifier can be combined with a
prefix to form an address.
o Small sites consisting of a set of machines attached to a single
link should not require the presence of a DHCPv6 server or router
as a prerequisite for communicating. Plug-and-play communication
is achieved through the use of link-local addresses. Link-local
addresses have a well-known prefix that identifies the (single)
shared link to which a set of nodes attach. A host forms a link-
local address by appending an interface identifier to the link-
o A large site with multiple networks and routers should not require
the presence of a DHCPv6 server for address configuration. In
order to generate global addresses, hosts must determine the
prefixes that identify the subnets to which they attach. Routers
generate periodic Router Advertisements that include options
listing the set of active prefixes on a link.
o Address configuration should facilitate the graceful renumbering
of a site's machines. For example, a site may wish to renumber
all of its nodes when it switches to a new network service
provider. Renumbering is achieved through the leasing of
addresses to interfaces and the assignment of multiple addresses
to the same interface. Lease lifetimes provide the mechanism
through which a site phases out old prefixes. The assignment of
multiple addresses to an interface provides for a transition
period during which both a new address and the one being phased
out work simultaneously.
4. Protocol Overview
This section provides an overview of the typical steps that take
place when an interface autoconfigures itself. Autoconfiguration is
performed only on multicast-capable links and begins when a
multicast-capable interface is enabled, e.g., during system startup.
Nodes (both hosts and routers) begin the autoconfiguration process by
generating a link-local address for the interface. A link-local
address is formed by appending an identifier of the interface to the
well-known link-local prefix [RFC4291].
Before the link-local address can be assigned to an interface and
used, however, a node must attempt to verify that this "tentative"
address is not already in use by another node on the link.
Specifically, it sends a Neighbor Solicitation message containing the
tentative address as the target. If another node is already using
that address, it will return a Neighbor Advertisement saying so. If
another node is also attempting to use the same address, it will send
a Neighbor Solicitation for the target as well. The exact number of
times the Neighbor Solicitation is (re)transmitted and the delay time
between consecutive solicitations is link-specific and may be set by
If a node determines that its tentative link-local address is not
unique, autoconfiguration stops and manual configuration of the
interface is required. To simplify recovery in this case, it should
be possible for an administrator to supply an alternate interface
identifier that overrides the default identifier in such a way that
the autoconfiguration mechanism can then be applied using the new
(presumably unique) interface identifier. Alternatively, link-local
and other addresses will need to be configured manually.
Once a node ascertains that its tentative link-local address is
unique, it assigns the address to the interface. At this point, the
node has IP-level connectivity with neighboring nodes. The remaining
autoconfiguration steps are performed only by hosts; the
(auto)configuration of routers is beyond the scope of this document.
The next phase of autoconfiguration involves obtaining a Router
Advertisement or determining that no routers are present. If routers
are present, they will send Router Advertisements that specify what
sort of autoconfiguration a host can do. Note that the DHCPv6
service for address configuration may still be available even if no
routers are present.
Routers send Router Advertisements periodically, but the delay
between successive advertisements will generally be longer than a
host performing autoconfiguration will want to wait [RFC4861]. To
obtain an advertisement quickly, a host sends one or more Router
Solicitations to the all-routers multicast group.
Router Advertisements also contain zero or more Prefix Information
options that contain information used by stateless address
autoconfiguration to generate global addresses. It should be noted
that a host may use both stateless address autoconfiguration and
DHCPv6 simultaneously. One Prefix Information option field, the
"autonomous address-configuration flag", indicates whether or not the
option even applies to stateless autoconfiguration. If it does,
additional option fields contain a subnet prefix, together with
lifetime values, indicating how long addresses created from the
prefix remain preferred and valid.
Because routers generate Router Advertisements periodically, hosts
will continually receive new advertisements. Hosts process the
information contained in each advertisement as described above,
adding to and refreshing information received in previous
By default, all addresses should be tested for uniqueness prior to
their assignment to an interface for safety. The test should
individually be performed on all addresses obtained manually, via
stateless address autoconfiguration, or via DHCPv6. To accommodate
sites that believe the overhead of performing Duplicate Address
Detection outweighs its benefits, the use of Duplicate Address
Detection can be disabled through the administrative setting of a
per-interface configuration flag.
To speed the autoconfiguration process, a host may generate its link-
local address (and verify its uniqueness) in parallel with waiting
for a Router Advertisement. Because a router may delay responding to
a Router Solicitation for a few seconds, the total time needed to
complete autoconfiguration can be significantly longer if the two
steps are done serially.
4.1. Site Renumbering
Address leasing facilitates site renumbering by providing a mechanism
to time-out addresses assigned to interfaces in hosts. At present,
upper-layer protocols such as TCP provide no support for changing
end-point addresses while a connection is open. If an end-point
address becomes invalid, existing connections break and all
communication to the invalid address fails. Even when applications
use UDP as a transport protocol, addresses must generally remain the
same during a packet exchange.
Dividing valid addresses into preferred and deprecated categories
provides a way of indicating to upper layers that a valid address may
become invalid shortly and that future communication using the
address will fail, should the address's valid lifetime expire before
communication ends. To avoid this scenario, higher layers should use
a preferred address (assuming one of sufficient scope exists) to
increase the likelihood that an address will remain valid for the
duration of the communication. It is up to system administrators to
set appropriate prefix lifetimes in order to minimize the impact of
failed communication when renumbering takes place. The deprecation
period should be long enough that most, if not all, communications
are using the new address at the time an address becomes invalid.
The IP layer is expected to provide a means for upper layers
(including applications) to select the most appropriate source
address given a particular destination and possibly other
constraints. An application may choose to select the source address
itself before starting a new communication or may leave the address
unspecified, in which case, the upper networking layers will use the
mechanism provided by the IP layer to choose a suitable address on
the application's behalf.
Detailed address selection rules are beyond the scope of this
document and are described in [RFC3484].
5. Protocol Specification
Autoconfiguration is performed on a per-interface basis on multicast-
capable interfaces. For multihomed hosts, autoconfiguration is
performed independently on each interface. Autoconfiguration applies
primarily to hosts, with two exceptions. Routers are expected to
generate a link-local address using the procedure outlined below. In
addition, routers perform Duplicate Address Detection on all
addresses prior to assigning them to an interface.
5.1. Node Configuration Variables
A node MUST allow the following autoconfiguration-related variable to
be configured by system management for each multicast-capable
DupAddrDetectTransmits The number of consecutive Neighbor
Solicitation messages sent while performing Duplicate Address
Detection on a tentative address. A value of zero indicates that
Duplicate Address Detection is not performed on tentative
addresses. A value of one indicates a single transmission with no
Default: 1, but may be overridden by a link-type specific value in
the document that covers issues related to the transmission of IP
over a particular link type (e.g., [RFC2464]).
Autoconfiguration also assumes the presence of the variable
RetransTimer as defined in [RFC4861]. For autoconfiguration
purposes, RetransTimer specifies the delay between consecutive
Neighbor Solicitation transmissions performed during Duplicate
Address Detection (if DupAddrDetectTransmits is greater than 1),
as well as the time a node waits after sending the last Neighbor
Solicitation before ending the Duplicate Address Detection
5.2. Autoconfiguration-Related Structures
Beyond the formation of a link-local address and use of Duplicate
Address Detection, how routers (auto)configure their interfaces is
beyond the scope of this document.
A host maintains a list of addresses together with their
corresponding lifetimes. The address list contains both
autoconfigured addresses and those configured manually.
5.3. Creation of Link-Local Addresses
A node forms a link-local address whenever an interface becomes
enabled. An interface may become enabled after any of the following
- 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 interface attaches to a link for the first time. This
includes the case where the attached link is dynamically changed
due to a change of the access point of wireless networks.
- The interface becomes enabled by system management after having
been administratively disabled.
A link-local address is formed by combining the well-known link-local
prefix FE80::0 [RFC4291] (of appropriate length) with an interface
identifier as follows:
1. The left-most 'prefix length' bits of the address are those of
the link-local prefix.
2. The bits in the address to the right of the link-local prefix are
set to all zeroes.
3. If the length of the interface identifier is N bits, the right-
most N bits of the address are replaced by the interface
If the sum of the link-local prefix length and N is larger than 128,
autoconfiguration fails and manual configuration is required. The
length of the interface identifier is defined in a separate link-
type-specific document, which should also be consistent with the
address architecture [RFC4291] (see Section 2). These documents will
carefully define the length so that link-local addresses can be
autoconfigured on the link.
A link-local address has an infinite preferred and valid lifetime; it
is never timed out.
5.4. Duplicate Address Detection
Duplicate Address Detection MUST be performed on all unicast
addresses prior to assigning them to an interface, regardless of
whether they are obtained through stateless autoconfiguration,
DHCPv6, or manual configuration, with the following exceptions:
- An interface whose DupAddrDetectTransmits variable is set to zero
does not perform Duplicate Address Detection.
- Duplicate Address Detection MUST NOT be performed on anycast
addresses (note that anycast addresses cannot syntactically be
distinguished from unicast addresses).
- Each individual unicast address SHOULD be tested for uniqueness.
Note that there are implementations deployed that only perform
Duplicate Address Detection for the link-local address and skip
the test for the global address that uses the same interface
identifier as that of the link-local address. Whereas this
document does not invalidate such implementations, this kind of
"optimization" is NOT RECOMMENDED, and new implementations MUST
NOT do that optimization. This optimization came from the
assumption that all of an interface's addresses are generated from
the same identifier. However, the assumption does actually not
stand; new types of addresses have been introduced where the
interface identifiers are not necessarily the same for all unicast
addresses on a single interface [RFC4941] [RFC3972]. Requiring
that Duplicate Address Detection be performed for all unicast
addresses will make the algorithm robust for the current and
future special interface identifiers.
The procedure for detecting duplicate addresses uses Neighbor
Solicitation and Advertisement messages as described below. If a
duplicate address is discovered during the procedure, the address
cannot be assigned to the interface. If the address is derived from
an interface identifier, a new identifier will need to be assigned to
the interface, or all IP addresses for the interface will need to be
manually configured. Note that the method for detecting duplicates
is not completely reliable, and it is possible that duplicate
addresses will still exist (e.g., if the link was partitioned while
Duplicate Address Detection was performed).
An address on which the Duplicate Address Detection procedure is
applied is said to be tentative until the procedure has completed
successfully. A tentative address is not considered "assigned to an
interface" in the traditional sense. That is, the interface must
accept Neighbor Solicitation and Advertisement messages containing
the tentative address in the Target Address field, but processes such
packets differently from those whose Target Address matches an
address assigned to the interface. Other packets addressed to the
tentative address should be silently discarded. Note that the "other
packets" include Neighbor Solicitation and Advertisement messages
that have the tentative (i.e., unicast) address as the IP destination
address and contain the tentative address in the Target Address
field. Such a case should not happen in normal operation, though,
since these messages are multicasted in the Duplicate Address
It should also be noted that Duplicate Address Detection must be
performed prior to assigning an address to an interface in order to
prevent multiple nodes from using the same address simultaneously.
If a node begins using an address in parallel with Duplicate Address
Detection, and another node is already using the address, the node
performing Duplicate Address Detection will erroneously process
traffic intended for the other node, resulting in such possible
negative consequences as the resetting of open TCP connections.
The following subsections describe specific tests a node performs to
verify an address's uniqueness. An address is considered unique if
none of the tests indicate the presence of a duplicate address within
RetransTimer milliseconds after having sent DupAddrDetectTransmits
Neighbor Solicitations. Once an address is determined to be unique,
it may be assigned to an interface.
5.4.1. Message Validation
A node MUST silently discard any Neighbor Solicitation or
Advertisement message that does not pass the validity checks
specified in [RFC4861]. A Neighbor Solicitation or Advertisement
message that passes these validity checks is called a valid
solicitation or valid advertisement, respectively.
5.4.2. Sending Neighbor Solicitation Messages
Before sending a Neighbor Solicitation, an interface MUST join the
all-nodes multicast address and the solicited-node multicast address
of the tentative address. The former ensures that the node receives
Neighbor Advertisements from other nodes already using the address;
the latter ensures that two nodes attempting to use the same address
simultaneously should detect each other's presence.
To check an address, a node sends DupAddrDetectTransmits Neighbor
Solicitations, each separated by RetransTimer milliseconds. The
solicitation's Target Address is set to the address being checked,
the IP source is set to the unspecified address, and the IP
destination is set to the solicited-node multicast address of the
If the Neighbor Solicitation is going to be the first message sent
from an interface after interface (re)initialization, the node SHOULD
delay joining the solicited-node multicast address by a random delay
between 0 and MAX_RTR_SOLICITATION_DELAY as specified in [RFC4861].
This serves to alleviate congestion when many nodes start up on the
link at the same time, such as after a power failure, and may help to
avoid race conditions when more than one node is trying to solicit
for the same address at the same time.
Even if the Neighbor Solicitation is not going to be the first
message sent, the node SHOULD delay joining the solicited-node
multicast address by a random delay between 0 and
MAX_RTR_SOLICITATION_DELAY if the address being checked is configured
by a router advertisement message sent to a multicast address. The
delay will avoid similar congestion when multiple nodes are going to
configure addresses by receiving the same single multicast router
Note that when a node joins a multicast address, it typically sends a
Multicast Listener Discovery (MLD) report message [RFC2710] [RFC3810]
for the multicast address. In the case of Duplicate Address
Detection, the MLD report message is required in order to inform MLD-
snooping switches, rather than routers, to forward multicast packets.
In the above description, the delay for joining the multicast address
thus means delaying transmission of the corresponding MLD report
message. Since the MLD specifications do not request a random delay
to avoid race conditions, just delaying Neighbor Solicitation would
cause congestion by the MLD report messages. The congestion would
then prevent the MLD-snooping switches from working correctly and, as
a result, prevent Duplicate Address Detection from working. The
requirement to include the delay for the MLD report in this case
avoids this scenario. [RFC3590] also talks about some interaction
issues between Duplicate Address Detection and MLD, and specifies
which source address should be used for the MLD report in this case.
In order to improve the robustness of the Duplicate Address Detection
algorithm, an interface MUST receive and process datagrams sent to
the all-nodes multicast address or solicited-node multicast address
of the tentative address during the delay period. This does not
necessarily conflict with the requirement that joining the multicast
group be delayed. In fact, in some cases it is possible for a node
to start listening to the group during the delay period before MLD
report transmission. It should be noted, however, that in some link-
layer environments, particularly with MLD-snooping switches, no
multicast reception will be available until the MLD report is sent.
5.4.3. Receiving Neighbor Solicitation Messages
On receipt of a valid Neighbor Solicitation message on an interface,
node behavior depends on whether or not the target address is
tentative. If the target address is not tentative (i.e., it is
assigned to the receiving interface), the solicitation is processed
as described in [RFC4861]. If the target address is tentative, and
the source address is a unicast address, the solicitation's sender is
performing address resolution on the target; the solicitation should
be silently ignored. Otherwise, processing takes place as described
below. In all cases, a node MUST NOT respond to a Neighbor
Solicitation for a tentative address.
If the source address of the Neighbor Solicitation is the unspecified
address, the solicitation is from a node performing Duplicate Address
Detection. If the solicitation is from another node, the tentative
address is a duplicate and should not be used (by either node). If
the solicitation is from the node itself (because the node loops back
multicast packets), the solicitation does not indicate the presence
of a duplicate address.
Implementer's Note: many interfaces provide a way for upper layers to
selectively enable and disable the looping back of multicast packets.
The details of how such a facility is implemented may prevent
Duplicate Address Detection from working correctly. See Appendix A
for further discussion.
The following tests identify conditions under which a tentative
address is not unique:
- If a Neighbor Solicitation for a tentative address is received
before one is sent, the tentative address is a duplicate. This
condition occurs when two nodes run Duplicate Address Detection
simultaneously, but transmit initial solicitations at different
times (e.g., by selecting different random delay values before
joining the solicited-node multicast address and transmitting an
- If the actual number of Neighbor Solicitations received exceeds
the number expected based on the loopback semantics (e.g., the
interface does not loop back the packet, yet one or more
solicitations was received), the tentative address is a duplicate.
This condition occurs when two nodes run Duplicate Address
Detection simultaneously and transmit solicitations at roughly the
5.4.4. Receiving Neighbor Advertisement Messages
On receipt of a valid Neighbor Advertisement message on an interface,
node behavior depends on whether the target address is tentative or
matches a unicast or anycast address assigned to the interface:
1. If the target address is tentative, the tentative address is not
2. If the target address matches a unicast address assigned to the
receiving interface, it would possibly indicate that the address
is a duplicate but it has not been detected by the Duplicate
Address Detection procedure (recall that Duplicate Address
Detection is not completely reliable). How to handle such a case
is beyond the scope of this document.
3. Otherwise, the advertisement is processed as described in
5.4.5. When Duplicate Address Detection Fails
A tentative address that is determined to be a duplicate as described
above MUST NOT be assigned to an interface, and the node SHOULD log a
system management error.
If the address is a link-local address formed from an interface
identifier based on the hardware address, which is supposed to be
uniquely assigned (e.g., EUI-64 for an Ethernet interface), IP
operation on the interface SHOULD be disabled. By disabling IP
operation, the node will then:
- not send any IP packets from the interface,
- silently drop any IP packets received on the interface, and
- not forward any IP packets to the interface (when acting as a
router or processing a packet with a Routing header).
In this case, the IP address duplication probably means duplicate
hardware addresses are in use, and trying to recover from it by
configuring another IP address will not result in a usable network.
In fact, it probably makes things worse by creating problems that are
harder to diagnose than just disabling network operation on the
interface; the user will see a partially working network where some
things work, and other things do not.
On the other hand, if the duplicate link-local address is not formed
from an interface identifier based on the hardware address, which is
supposed to be uniquely assigned, IP operation on the interface MAY
Note: as specified in Section 2, "IP" means "IPv6" in the above
description. While the background rationale about hardware address
is independent of particular network protocols, its effect on other
protocols is beyond the scope of this document.
5.5. Creation of Global Addresses
Global addresses are formed by appending an interface identifier to a
prefix of appropriate length. Prefixes are obtained from Prefix
Information options contained in Router Advertisements. Creation of
global addresses as described in this section SHOULD be locally
configurable. However, the processing described below MUST be
enabled by default.
5.5.1. Soliciting Router Advertisements
Router Advertisements are sent periodically to the all-nodes
multicast address. To obtain an advertisement quickly, a host sends
out Router Solicitations as described in [RFC4861].
5.5.2. Absence of Router Advertisements
Even if a link has no routers, the DHCPv6 service to obtain addresses
may still be available, and hosts may want to use the service. From
the perspective of autoconfiguration, a link has no routers if no
Router Advertisements are received after having sent a small number
of Router Solicitations as described in [RFC4861].
Note that it is possible that there is no router on the link in this
sense, but there is a node that has the ability to forward packets.
In this case, the forwarding node's address must be manually
configured in hosts to be able to send packets off-link, since the
only mechanism to configure the default router's address
automatically is the one using Router Advertisements.
5.5.3. Router Advertisement Processing
For each Prefix-Information option in the Router Advertisement:
a) If the Autonomous flag is not set, silently ignore the Prefix
b) If the prefix is the link-local prefix, silently ignore the
Prefix Information option.
c) If the preferred lifetime is greater than the valid lifetime,
silently ignore the Prefix Information option. A node MAY wish to
log a system management error in this case.
d) If the prefix advertised is not equal to the prefix of an
address configured by stateless autoconfiguration already in the
list of addresses associated with the interface (where "equal"
means the two prefix lengths are the same and the first prefix-
length bits of the prefixes are identical), and if the Valid
Lifetime is not 0, form an address (and add it to the list) by
combining the advertised prefix with an interface identifier of
the link as follows:
| 128 - N bits | N bits |
| link prefix | interface identifier |
If the sum of the prefix length and interface identifier length
does not equal 128 bits, the Prefix Information option MUST be
ignored. An implementation MAY wish to log a system management
error in this case. The length of the interface identifier is
defined in a separate link-type specific document, which should
also be consistent with the address architecture [RFC4291] (see
It is the responsibility of the system administrator to ensure
that the lengths of prefixes contained in Router Advertisements
are consistent with the length of interface identifiers for that
link type. It should be noted, however, that this does not mean
the advertised prefix length is meaningless. In fact, the
advertised length has non-trivial meaning for on-link
determination in [RFC4861] where the sum of the prefix length and
the interface identifier length may not be equal to 128. Thus, it
should be safe to validate the advertised prefix length here, in
order to detect and avoid a configuration error specifying an
invalid prefix length in the context of address autoconfiguration.
Note that a future revision of the address architecture [RFC4291]
and a future link-type-specific document, which will still be
consistent with each other, could potentially allow for an
interface identifier of length other than the value defined in the
current documents. Thus, an implementation should not assume a
particular constant. Rather, it should expect any lengths of
If an address is formed successfully and the address is not yet in
the list, the host adds it to the list of addresses assigned to
the interface, initializing its preferred and valid lifetime
values from the Prefix Information option. Note that the check
against the prefix performed at the beginning of this step cannot
always detect the address conflict in the list. It could be
possible that an address already in the list, configured either
manually or by DHCPv6, happens to be identical to the newly
created address, whereas such a case should be atypical.
e) If the advertised prefix is equal to the prefix of an address
configured by stateless autoconfiguration in the list, the
preferred lifetime of the address is reset to the Preferred
Lifetime in the received advertisement. The specific action to
perform for the valid lifetime of the address depends on the Valid
Lifetime in the received advertisement and the remaining time to
the valid lifetime expiration of the previously autoconfigured
address. We call the remaining time "RemainingLifetime" in the
1. If the received Valid Lifetime is greater than 2 hours or
greater than RemainingLifetime, set the valid lifetime of the
corresponding address to the advertised Valid Lifetime.
2. If RemainingLifetime is less than or equal to 2 hours, ignore
the Prefix Information option with regards to the valid
lifetime, unless the Router Advertisement from which this
option was obtained has been authenticated (e.g., via Secure
Neighbor Discovery [RFC3971]). If the Router Advertisement
was authenticated, the valid lifetime of the corresponding
address should be set to the Valid Lifetime in the received
3. Otherwise, reset the valid lifetime of the corresponding
address to 2 hours.
The above rules address a specific denial-of-service attack in
which a bogus advertisement could contain prefixes with very small
Valid Lifetimes. Without the above rules, a single
unauthenticated advertisement containing bogus Prefix Information
options with short Valid Lifetimes could cause all of a node's
addresses to expire prematurely. The above rules ensure that
legitimate advertisements (which are sent periodically) will
"cancel" the short Valid Lifetimes before they actually take
Note that the preferred lifetime of the corresponding address is
always reset to the Preferred Lifetime in the received Prefix
Information option, regardless of whether the valid lifetime is
also reset or ignored. The difference comes from the fact that
the possible attack for the preferred lifetime is relatively
minor. Additionally, it is even undesirable to ignore the
preferred lifetime when a valid administrator wants to deprecate a
particular address by sending a short preferred lifetime (and the
valid lifetime is ignored by accident).
5.5.4. Address Lifetime Expiry
A preferred address becomes deprecated when its preferred lifetime
expires. A deprecated address SHOULD continue to be used as a source
address in existing communications, but SHOULD NOT be used to
initiate new communications if an alternate (non-deprecated) address
of sufficient scope can easily be used instead.
Note that the feasibility of initiating new communication using a
non-deprecated address may be an application-specific decision, as
only the application may have knowledge about whether the (now)
deprecated address was (or still is) in use by the application. For
example, if an application explicitly specifies that the protocol
stack use a deprecated address as a source address, the protocol
stack must accept that; the application might request it because that
IP address is used in higher-level communication and there might be a
requirement that the multiple connections in such a grouping use the
same pair of IP addresses.
IP and higher layers (e.g., TCP, UDP) MUST continue to accept and
process datagrams destined to a deprecated address as normal since a
deprecated address is still a valid address for the interface. In
the case of TCP, this means TCP SYN segments sent to a deprecated
address are responded to using the deprecated address as a source
address in the corresponding SYN-ACK (if the connection would
otherwise be allowed).
An implementation MAY prevent any new communication from using a
deprecated address, but system management MUST have the ability to
disable such a facility, and the facility MUST be disabled by
Other subtle cases should also be noted about source address
selection. For example, the above description does not clarify which
address should be used between a deprecated, smaller-scope address
and a non-deprecated, sufficient scope address. The details of the
address selection including this case are described in [RFC3484] and
are beyond the scope of this document.
An address (and its association with an interface) becomes invalid
when its valid lifetime expires. An invalid address MUST NOT be used
as a source address in outgoing communications and MUST NOT be
recognized as a destination on a receiving interface.
5.6. Configuration Consistency
It is possible for hosts to obtain address information using both
stateless autoconfiguration and DHCPv6 since both may be enabled at
the same time. It is also possible that the values of other
configuration parameters, such as MTU size and hop limit, will be
learned from both Router Advertisements and DHCPv6. If the same
configuration information is provided by multiple sources, the value
of this information should be consistent. However, it is not
considered a fatal error if information received from multiple
sources is inconsistent. Hosts accept the union of all information
received via Neighbor Discovery and DHCPv6.
If inconsistent information is learned from different sources, an
implementation may want to give information learned securely
precedence over information learned without protection. For
instance, Section 8 of [RFC3971] discusses how to deal with
information learned through Secure Neighbor Discovery conflicting
with information learned through plain Neighbor Discovery. The same
discussion can apply to the preference between information learned
through plain Neighbor Discovery and information learned via secured
DHCPv6, and so on.
In any case, if there is no security difference, the most recently
obtained values SHOULD have precedence over information learned
5.7. Retaining Configured Addresses for Stability
An implementation that has stable storage may want to retain
addresses in the storage when the addresses were acquired using
stateless address autoconfiguration. Assuming the lifetimes used are
reasonable, this technique implies that a temporary outage (less than
the valid lifetime) of a router will never result in losing a global
address of the node even if the node were to reboot. When this
technique is used, it should also be noted that the expiration times
of the preferred and valid lifetimes must be retained, in order to
prevent the use of an address after it has become deprecated or
Further details on this kind of extension are beyond the scope of
6. Security Considerations
Stateless address autoconfiguration allows a host to connect to a
network, configure an address, and start communicating with other
nodes without ever registering or authenticating itself with the
local site. Although this allows unauthorized users to connect to
and use a network, the threat is inherently present in the Internet
architecture. Any node with a physical attachment to a network can
generate an address (using a variety of ad hoc techniques) that
The use of stateless address autoconfiguration and Duplicate Address
Detection opens up the possibility of several denial-of-service
attacks. For example, any node can respond to Neighbor Solicitations
for a tentative address, causing the other node to reject the address
as a duplicate. A separate document [RFC3756] discusses details
about these attacks, which can be addressed with the Secure Neighbor
Discovery protocol [RFC3971]. It should also be noted that [RFC3756]
points out that the use of IP security is not always feasible
depending on network environments.
Thomas Narten and Susan Thompson were the authors of RFCs 1971 and
2462. For this revision of the RFC, Tatuya Jinmei was the sole
The authors of RFC 2461 would like to thank the members of both the
IPNG (which is now IPV6) and ADDRCONF working groups for their input.
In particular, thanks to Jim Bound, Steve Deering, Richard Draves,
and Erik Nordmark. Thanks also goes to John Gilmore for alerting the
WG of the "0 Lifetime Prefix Advertisement" denial-of-service attack
vulnerability; this document incorporates changes that address this
A number of people have contributed to identifying issues with RFC
2461 and to proposing resolutions to the issues as reflected in this
version of the document. In addition to those listed above, the
contributors include Jari Arkko, James Carlson, Brian E. Carpenter,
Gregory Daley, Elwyn Davies, Ralph Droms, Jun-ichiro Itojun Hagino,
Christian Huitema, Suresh Krishnan, Soohong Daniel Park, Markku
Savela, Pekka Savola, Hemant Singh, Bernie Volz, Margaret Wasserman,
and Vlad Yasevich.
8.1. Normative References
[RFC2464] Crawford, M., "Transmission of IPv6 Packets over
Ethernet Networks", RFC 2464, December 1998.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
8.2. Informative References
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.
[RFC3972] Aura, T., "Cryptographically Generated Addresses
(CGA)", RFC 3972, March 2005.
[RFC2710] Deering, S., Fenner, W., and B. Haberman, "Multicast
Listener Discovery (MLD) for IPv6", RFC 2710,
[RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery
Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.
[RFC3590] Haberman, B., "Source Address Selection for the
Multicast Listener Discovery (MLD) Protocol", RFC 3590,
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander,
"SEcure Neighbor Discovery (SEND)", RFC 3971,
[RFC3756] Nikander, P., Kempf, J., and E. Nordmark, "IPv6
Neighbor Discovery (ND) Trust Models and Threats",
RFC 3756, May 2004.
[RFC1112] Deering, S., "Host extensions for IP multicasting",
STD 5, RFC 1112, August 1989.
[IEEE802.11] IEEE, "Wireless LAN Medium Access Control (MAC) and
Physical Layer (PHY) Specifications", ANSI/IEEE
STd 802.11, August 1999.
Appendix A. Loopback Suppression and Duplicate Address Detection
Determining whether a received multicast solicitation was looped back
to the sender or actually came from another node is implementation-
dependent. A problematic case occurs when two interfaces attached to
the same link happen to have the same identifier and link-layer
address, and they both send out packets with identical contents at
roughly the same time (e.g., Neighbor Solicitations for a tentative
address as part of Duplicate Address Detection messages). Although a
receiver will receive both packets, it cannot determine which packet
was looped back and which packet came from the other node simply by
comparing packet contents (i.e., the contents are identical). In
this particular case, it is not necessary to know precisely which
packet was looped back and which was sent by another node; if one
receives more solicitations than were sent, the tentative address is
a duplicate. However, the situation may not always be this
The IPv4 multicast specification [RFC1112] recommends that the
service interface provide a way for an upper-layer protocol to
inhibit local delivery of packets sent to a multicast group that the
sending host is a member of. Some applications know that there will
be no other group members on the same host, and suppressing loopback
prevents them from having to receive (and discard) the packets they
themselves send out. A straightforward way to implement this
facility is to disable loopback at the hardware level (if supported
by the hardware), with packets looped back (if requested) by
software. On interfaces in which the hardware itself suppresses
loopbacks, a node running Duplicate Address Detection simply counts
the number of Neighbor Solicitations received for a tentative address
and compares them with the number expected. If there is a mismatch,
the tentative address is a duplicate.
In those cases where the hardware cannot suppress loopbacks, however,
one possible software heuristic to filter out unwanted loopbacks is
to discard any received packet whose link-layer source address is the
same as the receiving interface's. There is even a link-layer
specification that requires that any such packets be discarded
[IEEE802.11]. Unfortunately, use of that criteria also results in
the discarding of all packets sent by another node using the same
link-layer address. Duplicate Address Detection will fail on
interfaces that filter received packets in this manner:
o If a node performing Duplicate Address Detection discards received
packets that have the same source link-layer address as the
receiving interface, it will also discard packets from other nodes
that also use the same link-layer address, including Neighbor
Advertisement and Neighbor Solicitation messages required to make
Duplicate Address Detection work correctly. This particular
problem can be avoided by temporarily disabling the software
suppression of loopbacks while a node performs Duplicate Address
Detection, if it is possible to disable the suppression.
o If a node that is already using a particular IP address discards
received packets that have the same link-layer source address as
the interface, it will also discard Duplicate Address Detection-
related Neighbor Solicitation messages sent by another node that
also use the same link-layer address. Consequently, Duplicate
Address Detection will fail, and the other node will configure a
non-unique address. Since it is generally impossible to know when
another node is performing Duplicate Address Detection, this
scenario can be avoided only if software suppression of loopback
is permanently disabled.
Thus, to perform Duplicate Address Detection correctly in the case
where two interfaces are using the same link-layer address, an
implementation must have a good understanding of the interface's
multicast loopback semantics, and the interface cannot discard
received packets simply because the source link-layer address is the
same as the interface's. It should also be noted that a link-layer
specification can conflict with the condition necessary to make
Duplicate Address Detection work.
Appendix B. Changes since RFC 1971
o Changed document to use term "interface identifier" rather than
"interface token" for consistency with other IPv6 documents.
o Clarified definition of deprecated address to make clear it is OK
to continue sending to or from deprecated addresses.
o Added rules to Section 5.5.3 Router Advertisement processing to
address potential denial-of-service attack when prefixes are
advertised with very short Lifetimes.
o Clarified wording in Section 5.5.4 to make clear that all upper
layer protocols must process (i.e., send and receive) packets sent
to deprecated addresses.
Appendix C. Changes since RFC 2462
Major changes that can affect existing implementations:
o Specified that a node performing Duplicate Address Detection delay
joining the solicited-node multicast group, not just delay sending
Neighbor Solicitations, explaining the detailed reason.
o Added a requirement for a random delay before sending Neighbor
Solicitations for Duplicate Address Detection if the address being
checked is configured by a multicasted Router Advertisements.
o Clarified that on failure of Duplicate Address Detection, IP
network operation should be disabled and that the rule should
apply when the hardware address is supposed to be unique.
o Clarified how the length of interface identifiers should be
determined, described the relationship with the prefix length
advertised in Router Advertisements, and avoided using a
particular length hard-coded in this document.
o Clarified the processing of received neighbor advertisements while
performing Duplicate Address Detection.
o Removed the text regarding the M and O flags, considering the
maturity of implementations and operational experiences.
ManagedFlag and OtherConfigFlag were removed accordingly. (Note
that this change does not mean the use of these flags is
o Avoided the wording of "stateful configuration", which is known to
be quite confusing, and simply used "DHCPv6" wherever appropriate.
o Recommended to perform Duplicate Address Detection for all unicast
addresses more strongly, considering a variety of different
interface identifiers, while keeping care of existing
o Clarified wording in Section 5.5.4 to make clear that a deprecated
address specified by an application can be used for any
o Clarified the prefix check described in Section 5.5.3 using more
appropriate terms and that the check is done against the prefixes
of addresses configured by stateless autoconfiguration.
o Changed the references to the IP security Authentication Header to
references to RFC 3971 (Secure Neighbor Discovery). Also revised
the Security Considerations section with a reference to RFC 3756.
o Added a note when an implementation uses stable storage for
o Added consideration about preference between inconsistent
information sets, one from a secured source and the other learned
Other miscellaneous clarifications:
o Removed references to site-local and revised wording around the
o Removed redundant code in denial-of-service protection in
o Clarified that a unicasted Neighbor Solicitation or Advertisement
should be discarded while performing Duplicate Address Detection.
o Noted in Section 5.3 that an interface can be considered as
becoming enabled when a wireless access point changes.
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