Network Working Group S. Thomson
Request for Comments: 1971 Bellcore
Category: Standards Track T. Narten
IBM
August 1996

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.

Abstract

This document specifies the steps a host takes in deciding how to
autoconfigure its interfaces in IP version 6. The autoconfiguration
process includes creating a link-local address and verifying its
uniqueness on a link, determining what information should be
autoconfigured (addresses, other information, or both), and in the
case of addresses, whether they should be oBTained through the
stateless mechanism, the stateful mechanism, or both. This document
defines the process for generating a link-local address, the process
for generating site-local and global addresses via stateless address
autoconfiguration, and the Duplicate Address Detection procedure. The
details of autoconfiguration using the stateful protocol are
specified elsewhere.

Table of Contents

1. INTRODUCTION............................................. 2
2. TERMINOLOGY.............................................. 4
2.1. Requirements........................................ 7
3. DESIGN GOALS............................................. 8
4. PROTOCOL OVERVIEW........................................ 9
4.1. Site Renumbering.................................... 11
5. PROTOCOL SPECIFICATION................................... 11
5.1. Node Configuration Variables........................ 12
5.2. Autoconfiguration-Related Variables................. 12
5.3. Creation of Link-Local Addresses.................... 13
5.4. Duplicate Address Detection......................... 13
5.4.1. Message Validation............................. 15
5.4.2. Sending Neighbor Solicitation Messages......... 15
5.4.3. Receiving Neighbor Solicitation Messages....... 15

5.4.4. Receiving Neighbor Advertisement Messages...... 16
5.4.5. When Duplicate Address Detection Fails......... 16
5.5. Creation of Global and Site-Local Addresses......... 17
5.5.1. Soliciting Router Advertisements............... 17
5.5.2. Absence of Router Advertisements............... 17
5.5.3. Router Advertisement Processing................ 17
5.5.4. Address Lifetime EXPiry........................ 19
5.6. Configuration Consistency........................... 19
SECURITY CONSIDERATIONS...................................... 19
REFERENCES................................................... 20
AUTHORS' ADDRESSES........................................... 21
APPENDIX: LOOPBACK SUPPRESSION & DUPLICATE ADDRESS DETECTION. 22

1. INTRODUCTION

This document specifies the steps a host takes in deciding how to
autoconfigure its interfaces in IP version 6. The autoconfiguration
process includes creating a link-local address and verifying its
uniqueness on a link, determining what information should be
autoconfigured (addresses, other information, or both), and in the
case of addresses, whether they should be obtained through the
stateless mechanism, the stateful mechanism, or both. This document
defines the process for generating a link-local address, the process
for generating site-local and global addresses via stateless address
autoconfiguration, and the Duplicate Address Detection procedure. The
details of autoconfiguration using the stateful protocol are
specified elsewhere.

IPv6 defines both a stateful and stateless address autoconfiguration
mechanism. Stateless autoconfiguration 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 token" 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.

In the stateful autoconfiguration model, hosts obtain interface
addresses and/or configuration information and parameters from a
server. Servers maintain a database that keeps track of which
addresses have been assigned to which hosts. The stateful
autoconfiguration protocol allows hosts to obtain addresses, other
configuration information or both from a server. Stateless and
stateful autoconfiguration complement each other. For example, a host

can use stateless autoconfiguration to configure its own addresses,
but use stateful autoconfiguration to obtain other information.
Stateful autoconfiguration is described in [DHCPv6].

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. The stateful approach is used when a
site requires tighter control over exact address assignments. Both
stateful and stateless address autoconfiguration may be used
simultaneously. The site administrator specifies which type of
autoconfiguration to use through the setting of appropriate fields in
Router Advertisement messages [DISCOVERY].

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 in a deprecated state, the use of an address 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 insure 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,
independent of whether they are obtained via stateless or stateful
autoconfiguration. 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.

2. TERMINOLOGY

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 itself.

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.

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
interfaces.

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 [ADDR-ARCH].

solicited-node multicast address
- a multicast address to which Neighbor Solicitation
messages are sent. The algorithm for computing the
address is given in [DISCOVERY].

link-layer address
- a link-layer identifier for an interface. Examples
include IEEE 802 addresses for Ethernet links and E.164
addresses for 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.

site-local address
- an address having scope that is limited to the local
site.

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 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.

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 later 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 then or equal to the preferred
lifetime. When the valid lifetime expires, the address
becomes invalid.

interface token
- a link-dependent identifier for an interface that is
(at least) unique per link. Stateless address
autoconfiguration combines an interface token with a

prefix to form an address. From address
autoconfiguration's perspective, an interface token is
a bit string of known length. The exact length of an
interface token 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., [IPv6-ETHER]). In many
cases, the token will be the same as the interface's
link-layer address.

2.1. Requirements

Throughout this document, the Words that are used to define the
significance of the particular requirements are capitalized. These
words are:

MUST
This word or the adjective "REQUIRED" means that the item is an
absolute requirement of this specification.

MUST NOT
This phrase means the item is an absolute prohibition of this
specification.

SHOULD
This word or the adjective "RECOMMENDED" means that there may exist
valid reasons in particular circumstances to ignore this item, but
the full implications should be understood and the case carefully
weighed before choosing a different course.

SHOULD NOT
This phrase means that there may exist valid reasons in particular
circumstances when the listed behavior is acceptable or even
useful, but the full implications should be understood and the case
carefully weighed before implementing any behavior described with
this label.

MAY
This word or the adjective "OPTIONAL" means that this item is truly
optional. One vendor may choose to include the item because a
particular marketplace requires it or because it enhances the
product, for example, another vendor may omit the same item.

3. DESIGN GOALS

Stateless autoconfiguration is designed with the following goals in
mind:

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 token"). In the simplest case, an interface token
consists of the interface's link-layer address. An interface token
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 stateful 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 its interface token to the link-local
prefix.

o A large site with multiple networks and routers should not require
the presence of a stateful address configuration server. In order
to generate site-local or 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.

o System administrators need the ability to specify whether stateless
autoconfiguration, stateful autoconfiguration, or both should be
used. Router Advertisements include flags specifying which
mechanisms a host should use.

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 the interface's token to the well-
known link-local prefix.

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
system management.

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
token that overrides the default token in such a way that the
autoconfiguration mechanism can then be applied using the new
(presumably unique) interface token. 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 it 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 should do. If no routers are
present, stateful autoconfiguration should be invoked.

Routers send Router Advertisements periodically, but the delay
between successive advertisements will generally be longer than a
host performing autoconfiguration will want to wait [DISCOVERY]. To
obtain an advertisement quickly, a host sends one or more Router

Solicitations to the all-routers multicast group. Router
Advertisements contain two flags indicating what type of stateful
autoconfiguration (if any) should be performed. A "managed address
configuration" flag indicates whether hosts should use stateful
autoconfiguration to obtain addresses. An "other stateful
configuration" flag indicates whether hosts should use stateful
autoconfiguration to obtain additional information (excluding
addresses).

Router Advertisements also contain zero or more Prefix Information
options that contain information used by stateless address
autoconfiguration to generate site-local and global addresses. It
should be noted that the stateless and stateful address
autoconfiguration fields in Router Advertisements are processed
independently of one another, and a host may use both stateful and
stateless address autoconfiguration 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
advertisements.

For safety, all addresses must be tested for uniqueness prior to
their assignment to an interface. In the case of addresses created
through stateless autoconfig, however, the uniqueness of an address
is determined primarily by the portion of the address formed from an
interface token. Thus, if a node has already verified the uniqueness
of a link-local address, additional addresses created from the same
interface token need not be tested individually. In contrast, all
addresses obtained manually or via stateful address autoconfiguration
should be tested for uniqueness individually. 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.

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 interface:

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 follow up
retransmissions.

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., [IPv6-ETHER]).

Autoconfiguration also assumes the presence of the variable
RetransTimer as defined in [DISCOVERY]. 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 process.

5.2. Autoconfiguration-Related Variables

A host maintains a number of data structures and flags related to
autoconfiguration. In the following, we present conceptual variables
and show how they are used to perform autoconfiguration. The specific
variables 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.

Beyond the formation of a link-local address and using Duplicate
Address Detection, how routers (auto)configure their interfaces is
beyond the scope of this document.

Hosts maintain the following variables on a per-interface basis:

ManagedFlag Copied from the M flag field (i.e., the "managed
address configuration" flag) of the most recently
received Router Advertisement message. The flag
indicates whether or not addresses are to be

configured using the stateful autoconfiguration
mechanism. It starts out in a FALSE state.

OtherConfigFlag Copied from the O flag field (i.e., the "other
stateful configuration" flag) of the most recently
received Router Advertisement message. The flag
indicates whether or not information other than
addresses are to be obtained using the stateful
autoconfiguration mechanism. It starts out in a
FALSE state.

A host also 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
events:

- The interface is initialized at system startup time.

- The interface is reinitialized after a temporary interface failure
or after being temporarily disabled by system management.

- The interface attaches to a link for the first time.

- The interface becomes enabled by system management after having
been administratively disabled.

A link-local address is formed by prepending the well-known link-
local prefix FE80::0 [ADDR-ARCH] (of appropriate length) to the
interface token. If the interface token has a length of N bits, the
interface token replaces the right-most N zero bits of the link-local
prefix. If the interface token is more than 118 bits in length,
autoconfiguration fails and manual configuration is required.

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 unicast addresses
prior to assigning them to an interface whose DupAddrDetectTransmits
variable is greater than zero. Duplicate Address Detection takes
place on all unicast addresses, regardless of whether they are
obtained through stateful, stateless or manual configuration.

(Duplicate Address Detection MUST NOT be performed on anycast
addresses.) Each individual unicast address SHOULD be tested for
uniqueness. However, when stateless address autoconfiguration is
used, address uniqueness is determined solely by the interface token,
assuming that subnet prefixes are assigned correctly (i.e., if all of
an interface's addresses are generated from the same token, either
all addresses or none of them will be duplicates). Thus, for a set of
addresses formed from the same interface token, it is sufficient to
check that the link-local address generated from the token is unique
on the link. In such cases, the link-local address MUST be tested for
uniqueness before any of the other addresses formed from the token
can be assigned to an interface.

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 token, a new token 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.

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 [DISCOVERY]. A solicitation that passes these validity
checks is called a valid solicitation or valid advertisement.

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 insures that the node receives
Neighbor Advertisements from other nodes already using the address;
the latter insures that two nodes attempting to use the same address
simultaneously 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
target address.

If the Neighbor Solicitation is the first message to be sent from an
interface after interface (re)initialization, the node should delay
sending the message by a random delay between 0 and
MAX_RTR_SOLICITATION_DELAY as specified in [DISCOVERY]. 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. 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 while
delaying transmission of the initial Neighbor Solicitation.

5.4.3. Receiving Neighbor Solicitation Messages

On receipt of a valid Neighbor Solicitation message on an interface,
node behavior depends on whether the target address is tentative or
not. If the target address is not tentative (i.e., it is assigned to
the receiving interface), the solicitation is processed as described
in [DISCOVERY]. 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.

Implementor'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 the Appendix
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
prior to having sent one, 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 transmitting an initial solicitation).

- If the actual number of Neighbor Solicitations received exceeds the
number expected based on the loopback semantics (e.g., the
interface does not loopback 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 same time.

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. If
the target address is assigned to the receiving interface, the
solicitation is processed as described in [DISCOVERY]. If the target
address is tentative, the tentative address is not unique.

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 token, the interface SHOULD be disabled.

5.5. Creation of Global and Site-Local Addresses

Global and site-local addresses are formed by appending an interface
token to a prefix of appropriate length. Prefixes are obtained from
Prefix Information options contained in Router Advertisements.
Creation of global and site-local addresses and configuration of
other parameters 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 [DISCOVERY].

5.5.2. Absence of Router Advertisements

If a link has no routers, a host MUST attempt to use stateful
autoconfiguration to obtain addresses and other configuration
information. An implementation MAY provide a way to disable the
invocation of stateful autoconfiguration in this case, but the
default SHOULD be enabled. 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 [DISCOVERY].

5.5.3. Router Advertisement Processing

On receipt of a valid Router Advertisement (as defined in
[DISCOVERY]), a host copies the value of the advertisement's M bit
into ManagedFlag. If the value of ManagedFlag changes from FALSE to
TRUE, the host should invoke the stateful address autoconfiguration
protocol, requesting address information. If the value of the
ManagedFlag changes from TRUE to FALSE, the host should terminate the
stateful address autoconfiguration protocol (i.e., stop requesting
addresses and ignore subsequent responses to in-progress
transactions). If the value of the flag stays unchanged, no special
action takes place. In particular, a host MUST NOT reinvoke stateful
address configuration if it is already participating in the stateful
protocol as a result of an earlier advertisement.

An advertisement's O flag field is processed in an analogous manner.
A host copies the value of the O flag into OtherConfigFlag. If the
value of OtherConfigFlag changes from FALSE to TRUE, the host should
invoke the stateful autoconfiguration protocol, requesting

information (excluding addresses). If the value of the
OtherConfigFlag changes from TRUE to FALSE, any activity related to
stateful autoconfiguration for parameters other than addresses should
be halted. If the value of the flag stays unchanged, no special
action takes place. In particular, a host MUST NOT reinvoke stateful
configuration if it is already participating in the stateful protocol
as a result of an earlier advertisement.

For each Prefix-Information option in the Router Advertisement:

a) If the Autonomous flag is not set, silently ignore the Prefix
Information option.

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 advertised prefix matches the prefix of an autoconfigured
address (i.e., obtained via stateless or stateful address
autoconfiguration) in the list of addresses associated with the
interface, set the preferred timer to that of the option's preferred
lifetime, and set the valid lifetime to that of the option's valid
lifetime.

e) If the prefix advertised does not match the prefix of an address
already in the list, then form an address (and add it to the list)
by appending the interface token to the prefix as follows:

128 - N bits N bits
+---------------------------------------+------------------------+
link prefix interface token
+----------------------------------------------------------------+

If the sum of the prefix length and interface token 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. It is the responsibility of the system administrator to insure
that the lengths of prefixes contained in Router Advertisements are
consistent with the length of interface tokens for that link type.

In those cases where a site requires the use of longer prefixes than
can be accommodated by the interface token, stateful
autoconfiguration can be used.

If an address is formed successfully, 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.

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 in new
communications if an alternate (non-deprecated) address is available
and has sufficient scope. The IP layer MUST continue to accept
datagrams destined to a deprecated address since a deprecated address
is still a valid address for the interface. An implementation MAY
prevent any new communication from using a deprecated address, but
system management MUST have the ability to disable such a facility.

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.

Note that if a Prefix Information option is received with a preferred
lifetime of zero, any addresses generated from that prefix are
immediately deprecated. Similarly, if both the advertised deprecated
and valid lifetimes are zero, any addresses generated from that
prefix become invalid immediately.

5.6. Configuration Consistency

It is possible for hosts to obtain address information using both
stateless and stateful protocols 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 the stateful
autoconfiguration protocol. 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 the stateless and
stateful protocols. If inconsistent information is learned from
different sources, the most recently obtained values always have
precedence over information learned earlier.

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
provides connectivity.

The use of Duplicate Address Detection opens up the possibility of
denial of service attacks. Any node can respond to Neighbor
Solicitations for a tentative address, causing the other node to
reject the address as a duplicate. This attack is similar to other
attacks involving the spoofing of Neighbor Discovery messages and can
be addressed by requiring that Neighbor Discovery packets be
authenticated [RFC1826].

REFERENCES

[RFC1826] Atkinson, R., "IP Authentication Header", RFC1826, August
1995.

[IPv6-ETHER] Crawford, M., "A Method for the Transmission of IPv6
Packets over Ethernet Networks", RFC1972, August 1996.

[RFC1112] Deering, S., "Host Extensions for IP Multicasting", STD 5,
RFC1112, August 1989.

[ADDR-ARCH] Hinden, R., and S. Deering, "Internet Protocol Version
(IPv6) Addressing Architecture", RFC1884, December 1995.

[DHCPv6] Work in Progress.

[DISCOVERY] Narten, T., Nordmark, E., and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC1970, August 1996.

Acknowledgements

The authors would like to thank the members of both the IPNG and
ADDRCONF working groups for their input. In particular, thanks to Jim
Bound, Steve Deering, and Erik Nordmark.

AUTHORS' ADDRESSES

Susan Thomson
Bellcore
445 South Street
Morristown, NJ 07960
USA

Phone: +1 201-829-4514
EMail: set@thumper.bellcore.com

Thomas Narten
IBM Corporation
P.O. Box 12195
Research Triangle Park, NC 27709-2195
USA

Phone: +1 919 254 7798
EMail: narten@vnet.ibm.com

APPENDIX: LOOPBACK SUPPRESSION & 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 token 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 by simply
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
straightforward.

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. 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 having the same source link-layer address as the receiving
interface, it will also discard packets from other nodes also using
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.

o If a node that is already using a particular IP address discards
received packets having the same link-layer source address as the
interface, it will also discard Duplicate Address Detection-related
Neighbor Solicitation messages sent by another node also using 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 interfaces.