draft-ietf-addrconf-ipv6-auto-07.txt   rfc1971.txt 
ADDRCONF Working Group Susan Thomson, Bellcore Network Working Group S. Thomson
INTERNET-DRAFT Thomas Narten, IBM Request for Comments: 1971 Bellcore
<draft-ietf-addrconf-ipv6-auto-07.txt> December 13, 1995 Category: Standards Track T. Narten
IBM
August 1996
IPv6 Stateless Address Autoconfiguration IPv6 Stateless Address Autoconfiguration
Status of this Memo Status of this Memo
This document is a submission to the ADDRCONF Working Group of the This document specifies an Internet standards track protocol for the
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Abstract Abstract
This document specifies the steps a host takes in deciding how to This document specifies the steps a host takes in deciding how to
autoconfigure its interfaces in IP version 6. The autoconfiguration autoconfigure its interfaces in IP version 6. The autoconfiguration
process includes creating a link-local address and verifying its process includes creating a link-local address and verifying its
uniqueness on a link, determining what information should be uniqueness on a link, determining what information should be
autoconfigured (addresses, other information, or both), and in the autoconfigured (addresses, other information, or both), and in the
case of addresses, whether they should be obtained through the case of addresses, whether they should be obtained through the
stateless mechanism, the stateful mechanism, or both. This document stateless mechanism, the stateful mechanism, or both. This document
defines the process for generating a link-local address, the process defines the process for generating a link-local address, the process
for generating site-local and global addresses via stateless address for generating site-local and global addresses via stateless address
autoconfiguration, and the Duplicate Address Detection procedure. The autoconfiguration, and the Duplicate Address Detection procedure. The
details of autoconfiguration using the stateful protocol are details of autoconfiguration using the stateful protocol are
specified elsewhere. specified elsewhere.
Contents Table of Contents
Status of this Memo.......................................... 1
1. INTRODUCTION............................................. 4
2. TERMINOLOGY.............................................. 5
2.1. Requirements........................................ 8
3. DESIGN GOALS............................................. 9
4. PROTOCOL OVERVIEW........................................ 10
4.1. Site Renumbering.................................... 12
5. PROTOCOL SPECIFICATION................................... 13
5.1. Node Configuration Variables........................ 13
5.2. Autoconfiguration-Related Variables................. 14
5.3. Creation of Link-Local Addresses.................... 14
5.4. Duplicate Address Detection......................... 15
5.4.1. Message Validation............................. 16
5.4.2. Sending Neighbor Solicitation Messages......... 16
5.4.3. Receiving Neighbor Solicitation Messages....... 17
5.4.4. Receiving Neighbor Advertisement Messages...... 18
5.4.5. When Duplicate Address Detection Fails......... 18
5.5. Creation of Global and Site-Local Addresses......... 18
5.5.1. Soliciting Router Advertisements............... 18
5.5.2. Absence of Router Advertisements............... 19
5.5.3. Router Advertisement Processing................ 19
5.5.4. Address Lifetime Expiry........................ 20
5.6. Configuration Consistency........................... 21
SECURITY CONSIDERATIONS...................................... 21
REFERENCES................................................... 22
AUTHORS' ADDRESSES........................................... 22
APPENDIX: LOOPBACK SUPPRESSION & DUPLICATE ADDRESS DETECTION. 23 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 1. INTRODUCTION
This document specifies the steps a host takes in deciding how to This document specifies the steps a host takes in deciding how to
autoconfigure its interfaces in IP version 6. The autoconfiguration autoconfigure its interfaces in IP version 6. The autoconfiguration
process includes creating a link-local address and verifying its process includes creating a link-local address and verifying its
uniqueness on a link, determining what information should be uniqueness on a link, determining what information should be
autoconfigured (addresses, other information, or both), and in the case autoconfigured (addresses, other information, or both), and in the
of addresses, whether they should be obtained through the stateless case of addresses, whether they should be obtained through the
mechanism, the stateful mechanism, or both. This document defines the stateless mechanism, the stateful mechanism, or both. This document
process for generating a link-local address, the process for generating defines the process for generating a link-local address, the process
site-local and global addresses via stateless address autoconfiguration, for generating site-local and global addresses via stateless address
and the Duplicate Address Detection procedure. The details of autoconfiguration, and the Duplicate Address Detection procedure. The
autoconfiguration using the stateful protocol are specified elsewhere. details of autoconfiguration using the stateful protocol are
specified elsewhere.
IPv6 defines both a stateful and stateless address autoconfiguration IPv6 defines both a stateful and stateless address autoconfiguration
mechanism. Stateless autoconfiguration requires no manual configuration mechanism. Stateless autoconfiguration requires no manual
of hosts, minimal (if any) configuration of routers, and no additional configuration of hosts, minimal (if any) configuration of routers,
servers. The stateless mechanism allows a host to generate its own and no additional servers. The stateless mechanism allows a host to
addresses using a combination of locally available information and generate its own addresses using a combination of locally available
information advertised by routers. Routers advertise prefixes that information and information advertised by routers. Routers advertise
identify the subnet(s) associated with a link, while hosts generate an prefixes that identify the subnet(s) associated with a link, while
"interface token" that uniquely identifies an interface on a subnet. An hosts generate an "interface token" that uniquely identifies an
address is formed by combining the two. In the absence of routers, a interface on a subnet. An address is formed by combining the two. In
host can only generate link-local addresses. However, link-local the absence of routers, a host can only generate link-local
addresses are sufficient for allowing communication among nodes attached addresses. However, link-local addresses are sufficient for allowing
to the same link. communication among nodes attached to the same link.
In the stateful autoconfiguration model, hosts obtain interface In the stateful autoconfiguration model, hosts obtain interface
addresses and/or configuration information and parameters from a server. addresses and/or configuration information and parameters from a
Servers maintain a database that keeps track of which addresses have server. Servers maintain a database that keeps track of which
been assigned to which hosts. The stateful autoconfiguration protocol addresses have been assigned to which hosts. The stateful
allows hosts to obtain addresses, other configuration information or autoconfiguration protocol allows hosts to obtain addresses, other
both from a server. Stateless and stateful autoconfiguration complement configuration information or both from a server. Stateless and
each other. For example, a host can use stateless autoconfiguration to stateful autoconfiguration complement each other. For example, a host
configure its own addresses, but use stateful autoconfiguration to can use stateless autoconfiguration to configure its own addresses,
obtain other information. Stateful autoconfiguration is described in but use stateful autoconfiguration to obtain other information.
[DHCPv6]. Stateful autoconfiguration is described in [DHCPv6].
The stateless approach is used when a site is not particularly concerned The stateless approach is used when a site is not particularly
with the exact addresses hosts use, so long as they are unique and concerned with the exact addresses hosts use, so long as they are
properly routable. The stateful approach is used when a site requires unique and properly routable. The stateful approach is used when a
tighter control over exact address assignments. Both stateful and site requires tighter control over exact address assignments. Both
stateless address autoconfiguration may be used simultaneously. The stateful and stateless address autoconfiguration may be used
site administrator specifies which type of autoconfiguration to use simultaneously. The site administrator specifies which type of
through the setting of appropriate fields in Router Advertisement autoconfiguration to use through the setting of appropriate fields in
messages [DISCOVERY]. Router Advertisement messages [DISCOVERY].
IPv6 addresses are leased to an interface for a fixed (possibly IPv6 addresses are leased to an interface for a fixed (possibly
infinite) length of time. Each address has an associated lifetime that infinite) length of time. Each address has an associated lifetime
indicates how long the address is bound to an interface. When a lifetime that indicates how long the address is bound to an interface. When a
expires, the binding (and address) become invalid and the address may be lifetime expires, the binding (and address) become invalid and the
reassigned to another interface elsewhere in the Internet. To handle the address may be reassigned to another interface elsewhere in the
expiration of address bindings gracefully, an address goes through two Internet. To handle the expiration of address bindings gracefully, an
distinct phases while assigned to an interface. Initially, an address is address goes through two distinct phases while assigned to an
"preferred", meaning that its use in arbitrary communication is interface. Initially, an address is "preferred", meaning that its use
unrestricted. Later, an address becomes "deprecated" in anticipation in arbitrary communication is unrestricted. Later, an address becomes
that its current interface binding will become invalid. While in a "deprecated" in anticipation that its current interface binding will
deprecated state, the use of an address is discouraged, but not strictly become invalid. While in a deprecated state, the use of an address is
forbidden. New communication (e.g., the opening of a new TCP discouraged, but not strictly forbidden. New communication (e.g.,
connection) should use a preferred address when possible. A deprecated the opening of a new TCP connection) should use a preferred address
address should be used only by applications that have been using it and when possible. A deprecated address should be used only by
would have difficulty switching to another address without a service applications that have been using it and would have difficulty
disruption. switching to another address without a service disruption.
To insure that all configured addresses are likely to be unique on a To insure that all configured addresses are likely to be unique on a
given link, nodes run a "duplicate address detection" algorithm on given link, nodes run a "duplicate address detection" algorithm on
addresses before assigning them to an interface. The Duplicate Address addresses before assigning them to an interface. The Duplicate
Detection algorithm is performed on all addresses, independent of Address Detection algorithm is performed on all addresses,
whether they are obtained via stateless or stateful autoconfiguration. independent of whether they are obtained via stateless or stateful
This document defines the Duplicate Address Detection algorithm. autoconfiguration. This document defines the Duplicate Address
Detection algorithm.
The autoconfiguration process specified in this document applies only to The autoconfiguration process specified in this document applies only
hosts and not routers. Since host autoconfiguration uses information to hosts and not routers. Since host autoconfiguration uses
advertised by routers, routers will need to be configured by some other information advertised by routers, routers will need to be configured
means. However, it is expected that routers will generate link-local by some other means. However, it is expected that routers will
addresses using the mechanism described in this document. In addition, generate link-local addresses using the mechanism described in this
routers are expected to successfully pass the Duplicate Address document. In addition, routers are expected to successfully pass the
Detection procedure described in this document on all addresses prior to Duplicate Address Detection procedure described in this document on
assigning them to an interface. all addresses prior to assigning them to an interface.
Section 2 provides definitions for terminology used throughout this Section 2 provides definitions for terminology used throughout this
document. Section 3 describes the design goals that lead to the current document. Section 3 describes the design goals that lead to the
autoconfiguration procedure. Section 4 provides an overview of the current autoconfiguration procedure. Section 4 provides an overview
protocol, while Section 5 describes the protocol in detail. of the protocol, while Section 5 describes the protocol in detail.
2. TERMINOLOGY 2. TERMINOLOGY
IP - Internet Protocol Version 6. The terms IPv4 and IPv6 IP - Internet Protocol Version 6. The terms IPv4 and IPv6
are used only in contexts where necessary to avoid are used only in contexts where necessary to avoid
ambiguity. ambiguity.
node - a device that implements IP. node - a device that implements IP.
router - a node that forwards IP packets not explicitly router - a node that forwards IP packets not explicitly
skipping to change at page 8, line 48 skipping to change at page 7, line 16
a bit string of known length. The exact length of an a bit string of known length. The exact length of an
interface token and the way it is created is defined in interface token and the way it is created is defined in
a separate link-type specific document that covers a separate link-type specific document that covers
issues related to the transmission of IP over a issues related to the transmission of IP over a
particular link type (e.g., [IPv6-ETHER]). In many particular link type (e.g., [IPv6-ETHER]). In many
cases, the token will be the same as the interface's cases, the token will be the same as the interface's
link-layer address. link-layer address.
2.1. Requirements 2.1. Requirements
Throughout this document, the words that are used to define the Throughout this document, the words that are used to define the
significance of the particular requirements are capitalized. These significance of the particular requirements are capitalized. These
words are: words are:
MUST MUST
This word or the adjective "REQUIRED" means that the item is an This word or the adjective "REQUIRED" means that the item is an
absolute requirement of this specification. absolute requirement of this specification.
MUST NOT MUST NOT
This phrase means the item is an absolute prohibition of this This phrase means the item is an absolute prohibition of this
specification. specification.
SHOULD SHOULD
skipping to change at page 9, line 34 skipping to change at page 8, line 7
this label. this label.
MAY MAY
This word or the adjective "OPTIONAL" means that this item is truly This word or the adjective "OPTIONAL" means that this item is truly
optional. One vendor may choose to include the item because a optional. One vendor may choose to include the item because a
particular marketplace requires it or because it enhances the particular marketplace requires it or because it enhances the
product, for example, another vendor may omit the same item. product, for example, another vendor may omit the same item.
3. DESIGN GOALS 3. DESIGN GOALS
Stateless autoconfiguration is designed with the following goals in Stateless autoconfiguration is designed with the following goals in
mind: mind:
o Manual configuration of individual machines before connecting them o Manual configuration of individual machines before connecting them
to the network should not be required. Consequently, a mechanism is to the network should not be required. Consequently, a mechanism is
needed that allows a host to obtain or create unique addresses for needed that allows a host to obtain or create unique addresses for
each of its interfaces. Address autoconfiguration assumes that each each of its interfaces. Address autoconfiguration assumes that each
interface can provide a unique identifier for that interface (i.e., interface can provide a unique identifier for that interface (i.e.,
an "interface token"). In the simplest case, an interface token an "interface token"). In the simplest case, an interface token
consists of the interface's link-layer address. An interface token consists of the interface's link-layer address. An interface token
can be combined with a prefix to form an address. can be combined with a prefix to form an address.
skipping to change at page 10, line 32 skipping to change at page 9, line 7
to an interface provides for a transition period during which both to an interface provides for a transition period during which both
a new address and the one being phased out work simultaneously. a new address and the one being phased out work simultaneously.
o System administrators need the ability to specify whether stateless o System administrators need the ability to specify whether stateless
autoconfiguration, stateful autoconfiguration, or both should be autoconfiguration, stateful autoconfiguration, or both should be
used. Router Advertisements include flags specifying which used. Router Advertisements include flags specifying which
mechanisms a host should use. mechanisms a host should use.
4. PROTOCOL OVERVIEW 4. PROTOCOL OVERVIEW
This section provides an overview of the typical steps that take place This section provides an overview of the typical steps that take
when an interface autoconfigures itself. Autoconfiguration is performed place when an interface autoconfigures itself. Autoconfiguration is
only on multicast-capable links and begins when a multicast-capable performed only on multicast-capable links and begins when a
interface is enabled, e.g., during system startup. Nodes (both hosts multicast-capable interface is enabled, e.g., during system startup.
and routers) begin the autoconfiguration process by generating a link- Nodes (both hosts and routers) begin the autoconfiguration process by
local address for the interface. A link-local address is formed by generating a link-local address for the interface. A link-local
appending the interface's token to the well-known link-local prefix. 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, Before the link-local address can be assigned to an interface and
however, a node must attempt to verify that this "tentative" address is used, however, a node must attempt to verify that this "tentative"
not already in use by another node on the link. Specifically, it sends a address is not already in use by another node on the link.
Neighbor Solicitation message containing the tentative address as the Specifically, it sends a Neighbor Solicitation message containing the
target. If another node is already using that address, it will return a tentative address as the target. If another node is already using
Neighbor Advertisement saying so. If another node is also attempting to that address, it will return a Neighbor Advertisement saying so. If
use the same address, it will send a Neighbor Solicitation for the another node is also attempting to use the same address, it will send
target as well. The exact number of times the Neighbor Solicitation is a Neighbor Solicitation for the target as well. The exact number of
(re)transmitted and the delay time between consecutive solicitations is times the Neighbor Solicitation is (re)transmitted and the delay time
link-specific and may be set by system management. 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 If a node determines that its tentative link-local address is not
unique, autoconfiguration stops and manual configuration of the unique, autoconfiguration stops and manual configuration of the
interface is required. To simplify recovery in this case, it should be interface is required. To simplify recovery in this case, it should
possible for an administrator to supply an alternate interface token be possible for an administrator to supply an alternate interface
that overrides the default token in such a way that the token that overrides the default token in such a way that the
autoconfiguration mechanism can then be applied using the new autoconfiguration mechanism can then be applied using the new
(presumably unique) interface token. Alternatively, link-local and (presumably unique) interface token. Alternatively, link-local and
other addresses will need to be configured manually. other addresses will need to be configured manually.
Once a node ascertains that its tentative link-local address is unique, Once a node ascertains that its tentative link-local address is
it assigns it to the interface. At this point, the node has IP-level unique, it assigns it to the interface. At this point, the node has
connectivity with neighboring nodes. The remaining autoconfiguration IP-level connectivity with neighboring nodes. The remaining
steps are performed only by hosts; the (auto)configuration of routers is autoconfiguration steps are performed only by hosts; the
beyond the scope of this document. (auto)configuration of routers is beyond the scope of this document.
The next phase of autoconfiguration involves obtaining a Router The next phase of autoconfiguration involves obtaining a Router
Advertisement or determining that no routers are present. If routers are Advertisement or determining that no routers are present. If routers
present, they will send Router Advertisements that specify what sort of are present, they will send Router Advertisements that specify what
autoconfiguration a host should do. If no routers are present, stateful sort of autoconfiguration a host should do. If no routers are
autoconfiguration should be invoked. present, stateful autoconfiguration should be invoked.
Routers send Router Advertisements periodically, but the delay between Routers send Router Advertisements periodically, but the delay
successive advertisements will generally be longer than a host between successive advertisements will generally be longer than a
performing autoconfiguration will want to wait [DISCOVERY]. To obtain host performing autoconfiguration will want to wait [DISCOVERY]. To
an advertisement quickly, a host sends one or more Router Solicitations obtain an advertisement quickly, a host sends one or more Router
to the all-routers multicast group. Router Advertisements contain two Solicitations to the all-routers multicast group. Router
flags indicating what type of stateful autoconfiguration (if any) should Advertisements contain two flags indicating what type of stateful
be performed. A "managed address configuration" flag indicates whether autoconfiguration (if any) should be performed. A "managed address
hosts should use stateful autoconfiguration to obtain addresses. An configuration" flag indicates whether hosts should use stateful
"other stateful configuration" flag indicates whether hosts should use autoconfiguration to obtain addresses. An "other stateful
stateful autoconfiguration to obtain additional information (excluding configuration" flag indicates whether hosts should use stateful
addresses). autoconfiguration to obtain additional information (excluding
addresses).
Router Advertisements also contain zero or more Prefix Information Router Advertisements also contain zero or more Prefix Information
options that contain information used by stateless address options that contain information used by stateless address
autoconfiguration to generate site-local and global addresses. It autoconfiguration to generate site-local and global addresses. It
should be noted that the stateless and stateful address should be noted that the stateless and stateful address
autoconfiguration fields in Router Advertisements are processed autoconfiguration fields in Router Advertisements are processed
independently of one another, and a host may use both stateful and independently of one another, and a host may use both stateful and
stateless address autoconfiguration simultaneously. One Prefix stateless address autoconfiguration simultaneously. One Prefix
Information option field, the "autonomous address-configuration flag", Information option field, the "autonomous address-configuration
indicates whether or not the option even applies to stateless flag", indicates whether or not the option even applies to stateless
autoconfiguration. If it does, additional option fields contain a autoconfiguration. If it does, additional option fields contain a
subnet prefix together with lifetime values indicating how long subnet prefix together with lifetime values indicating how long
addresses created from the prefix remain preferred and valid. addresses created from the prefix remain preferred and valid.
Because routers generate Router Advertisements periodically, hosts will Because routers generate Router Advertisements periodically, hosts
continually receive new advertisements. Hosts process the information will continually receive new advertisements. Hosts process the
contained in each advertisement as described above, adding to and information contained in each advertisement as described above,
refreshing information received in previous advertisements. adding to and refreshing information received in previous
advertisements.
For safety, all addresses must be tested for uniqueness prior to their For safety, all addresses must be tested for uniqueness prior to
assignment to an interface. In the case of addresses created through their assignment to an interface. In the case of addresses created
stateless autoconfig, however, the uniqueness of an address is through stateless autoconfig, however, the uniqueness of an address
determined primarily by the portion of the address formed from an is determined primarily by the portion of the address formed from an
interface token. Thus, if a node has already verified the uniqueness of interface token. Thus, if a node has already verified the uniqueness
a link-local address, additional addresses created from the same of a link-local address, additional addresses created from the same
interface token need not be tested individually. In contrast, all interface token need not be tested individually. In contrast, all
addresses obtained manually or via stateful address autoconfiguration addresses obtained manually or via stateful address autoconfiguration
should be tested for uniqueness individually. To accommodate sites that should be tested for uniqueness individually. To accommodate sites
believe the overhead of performing Duplicate Address Detection outweighs that believe the overhead of performing Duplicate Address Detection
its benefits, the use of Duplicate Address Detection can be disabled outweighs its benefits, the use of Duplicate Address Detection can be
through the administrative setting of a per-interface configuration disabled through the administrative setting of a per-interface
flag. configuration flag.
To speed the autoconfiguration process, a host may generate its link- To speed the autoconfiguration process, a host may generate its
local address (and verify its uniqueness) in parallel with waiting for a link-local address (and verify its uniqueness) in parallel with
Router Advertisement. Because a router may delay responding to a Router waiting for a Router Advertisement. Because a router may delay
Solicitation for a few seconds, the total time needed to complete responding to a Router Solicitation for a few seconds, the total time
autoconfiguration can be significantly longer if the two steps are done needed to complete autoconfiguration can be significantly longer if
serially. the two steps are done serially.
4.1. Site Renumbering 4.1. Site Renumbering
Address leasing facilitates site renumbering by providing a mechanism to Address leasing facilitates site renumbering by providing a mechanism
time-out addresses assigned to interfaces in hosts. At present, upper to time-out addresses assigned to interfaces in hosts. At present,
layer protocols such as TCP provide no support for changing end-point upper layer protocols such as TCP provide no support for changing
addresses while a connection is open. If an end-point address becomes end-point addresses while a connection is open. If an end-point
invalid, existing connections break and all communication to the invalid address becomes invalid, existing connections break and all
address fails. Even when applications use UDP as a transport protocol, communication to the invalid address fails. Even when applications
addresses must generally remain the same during a packet exchange. use UDP as a transport protocol, addresses must generally remain the
same during a packet exchange.
Dividing valid addresses into preferred and deprecated categories Dividing valid addresses into preferred and deprecated categories
provides a way of indicating to upper layers that a valid address may provides a way of indicating to upper layers that a valid address may
become invalid shortly and that future communication using the address become invalid shortly and that future communication using the
will fail, should the address's valid lifetime expire before address will fail, should the address's valid lifetime expire before
communication ends. To avoid this scenario, higher layers should use a communication ends. To avoid this scenario, higher layers should use
preferred address (assuming one of sufficient scope exists) to increase a preferred address (assuming one of sufficient scope exists) to
the likelihood that an address will remain valid for the duration of the increase the likelihood that an address will remain valid for the
communication. It is up to system administrators to set appropriate duration of the communication. It is up to system administrators to
prefix lifetimes in order to minimize the impact of failed communication set appropriate prefix lifetimes in order to minimize the impact of
when renumbering takes place. The deprecation period should be long failed communication when renumbering takes place. The deprecation
enough that most, if not all, communications are using the new address period should be long enough that most, if not all, communications
at the time an address becomes invalid. 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 The IP layer is expected to provide a means for upper layers
applications) to select the most appropriate source address given a (including applications) to select the most appropriate source
particular destination and possibly other constraints. An application address given a particular destination and possibly other
may choose to select the source address itself before starting a new constraints. An application may choose to select the source address
communication or may leave the address unspecified, in which case the itself before starting a new communication or may leave the address
upper networking layers will use the mechanism provided by the IP layer unspecified, in which case the upper networking layers will use the
to choose a suitable address on the application's behalf. 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. Detailed address selection rules are beyond the scope of this
document.
5. PROTOCOL SPECIFICATION 5. PROTOCOL SPECIFICATION
Autoconfiguration is performed on a per-interface basis on multicast- Autoconfiguration is performed on a per-interface basis on
capable interfaces. For multihomed hosts, autoconfiguration is multicast-capable interfaces. For multihomed hosts,
performed independently on each interface. Autoconfiguration applies autoconfiguration is performed independently on each interface.
primarily to hosts, with two exceptions. Routers are expected to Autoconfiguration applies primarily to hosts, with two exceptions.
generate a link-local address using the procedure outlined below. In Routers are expected to generate a link-local address using the
addition, routers perform Duplicate Address Detection on all addresses procedure outlined below. In addition, routers perform Duplicate
prior to assigning them to an interface. Address Detection on all addresses prior to assigning them to an
interface.
5.1. Node Configuration Variables 5.1. Node Configuration Variables
A node MUST allow the following autoconfiguration-related variable to be A node MUST allow the following autoconfiguration-related variable to
configured by system management for each multicast interface: be configured by system management for each multicast interface:
DupAddrDetectTransmits DupAddrDetectTransmits
The number of consecutive Neighbor Solicitation The number of consecutive Neighbor Solicitation
messages sent while performing Duplicate Address messages sent while performing Duplicate Address
Detection on a tentative address. A value of zero Detection on a tentative address. A value of zero
indicates that Duplicate Address Detection is not indicates that Duplicate Address Detection is not
performed on tentative addresses. A value of one performed on tentative addresses. A value of one
indicates a single transmission with no follow up indicates a single transmission with no follow up
retransmissions. retransmissions.
Default: 1, but may be overridden by a link-type Default: 1, but may be overridden by a link-type
specific value in the document that covers issues specific value in the document that covers issues
related to the transmission of IP over a particular related to the transmission of IP over a particular
link type (e.g., [IPv6-ETHER]). link type (e.g., [IPv6-ETHER]).
Autoconfiguration also assumes the presence of the variable RetransTimer Autoconfiguration also assumes the presence of the variable
as defined in [DISCOVERY]. For autoconfiguration purposes, RetransTimer RetransTimer as defined in [DISCOVERY]. For autoconfiguration
specifies the delay between consecutive Neighbor Solicitation purposes, RetransTimer specifies the delay between consecutive
transmissions performed during Duplicate Address Detection (if Neighbor Solicitation transmissions performed during Duplicate
DupAddrDetectTransmits is greater than 1), as well as the time a node Address Detection (if DupAddrDetectTransmits is greater than 1), as
waits after sending the last Neighbor Solicitation before ending the well as the time a node waits after sending the last Neighbor
Duplicate Address Detection process. Solicitation before ending the Duplicate Address Detection process.
5.2. Autoconfiguration-Related Variables 5.2. Autoconfiguration-Related Variables
A host maintains a number of data structures and flags related to A host maintains a number of data structures and flags related to
autoconfiguration. In the following, we present conceptual variables and autoconfiguration. In the following, we present conceptual variables
show how they are used to perform autoconfiguration. The specific and show how they are used to perform autoconfiguration. The specific
variables are used for demonstration purposes only, and an variables are used for demonstration purposes only, and an
implementation is not required to have them, so long as its external implementation is not required to have them, so long as its external
behavior is consistent with that described in this document. behavior is consistent with that described in this document.
Beyond the formation of a link-local address and using Duplicate Address Beyond the formation of a link-local address and using Duplicate
Detection, how routers (auto)configure their interfaces is beyond the Address Detection, how routers (auto)configure their interfaces is
scope of this document. beyond the scope of this document.
Hosts maintain the following variables on a per-interface basis: Hosts maintain the following variables on a per-interface basis:
ManagedFlag Copied from the M flag field (i.e., the "managed ManagedFlag Copied from the M flag field (i.e., the "managed
address configuration" flag) of the most recently address configuration" flag) of the most recently
received Router Advertisement message. The flag received Router Advertisement message. The flag
indicates whether or not addresses are to be indicates whether or not addresses are to be
configured using the stateful autoconfiguration configured using the stateful autoconfiguration
mechanism. It starts out in a FALSE state. mechanism. It starts out in a FALSE state.
OtherConfigFlag Copied from the O flag field (i.e., the "other OtherConfigFlag Copied from the O flag field (i.e., the "other
stateful configuration" flag) of the most recently stateful configuration" flag) of the most recently
received Router Advertisement message. The flag received Router Advertisement message. The flag
indicates whether or not information other than indicates whether or not information other than
addresses are to be obtained using the stateful addresses are to be obtained using the stateful
autoconfiguration mechanism. It starts out in a autoconfiguration mechanism. It starts out in a
FALSE state. FALSE state.
A host also maintains a list of addresses together with their A host also maintains a list of addresses together with their
corresponding lifetimes. The address list contains both autoconfigured corresponding lifetimes. The address list contains both
addresses and those configured manually. autoconfigured addresses and those configured manually.
5.3. Creation of Link-Local Addresses 5.3. Creation of Link-Local Addresses
A node forms a link-local address whenever an interface becomes enabled. A node forms a link-local address whenever an interface becomes
An interface may become enabled after any of the following events: enabled. An interface may become enabled after any of the following
events:
- The interface is initialized at system startup time. - The interface is initialized at system startup time.
- The interface is reinitialized after a temporary interface failure - The interface is reinitialized after a temporary interface failure
or after being temporarily disabled by system management. or after being temporarily disabled by system management.
- The interface attaches to a link for the first time. - The interface attaches to a link for the first time.
- The interface becomes enabled by system management after having - The interface becomes enabled by system management after having
been administratively disabled. been administratively disabled.
A link-local address is formed by prepending the well-known link-local A link-local address is formed by prepending the well-known link-
prefix FE80::0 [ADDR-ARCH] (of appropriate length) to the interface local prefix FE80::0 [ADDR-ARCH] (of appropriate length) to the
token. If the interface token has a length of N bits, the interface interface token. If the interface token has a length of N bits, the
token replaces the right-most N zero bits of the link-local prefix. If interface token replaces the right-most N zero bits of the link-local
the interface token is more than 118 bits in length, autoconfiguration prefix. If the interface token is more than 118 bits in length,
fails and manual configuration is required. autoconfiguration fails and manual configuration is required.
A link-local address has an infinite preferred and valid lifetime; it is A link-local address has an infinite preferred and valid lifetime; it
never timed out. is never timed out.
5.4. Duplicate Address Detection 5.4. Duplicate Address Detection
Duplicate Address Detection MUST be performed on unicast addresses prior Duplicate Address Detection MUST be performed on unicast addresses
to assigning them to an interface whose DupAddrDetectTransmits variable prior to assigning them to an interface whose DupAddrDetectTransmits
is greater than zero. Duplicate Address Detection takes place on all variable is greater than zero. Duplicate Address Detection takes
unicast addresses, regardless of whether they are obtained through place on all unicast addresses, regardless of whether they are
stateful, stateless or manual configuration. (Duplicate Address obtained through stateful, stateless or manual configuration.
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 (Duplicate Address Detection MUST NOT be performed on anycast
Solicitation and Advertisement messages as described below. If a addresses.) Each individual unicast address SHOULD be tested for
duplicate address is discovered during the procedure, the address cannot uniqueness. However, when stateless address autoconfiguration is
be assigned to the interface. If the address is derived from an used, address uniqueness is determined solely by the interface token,
interface token, a new token will need to be assigned to the interface, assuming that subnet prefixes are assigned correctly (i.e., if all of
or all IP addresses for the interface will need to be manually an interface's addresses are generated from the same token, either
configured. Note that the method for detecting duplicates is not all addresses or none of them will be duplicates). Thus, for a set of
completely reliable, and it is possible that duplicate addresses will addresses formed from the same interface token, it is sufficient to
still exist (e.g., if the link was partitioned while Duplicate Address check that the link-local address generated from the token is unique
Detection was performed). 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.
An address on which the duplicate Address Detection Procedure is applied The procedure for detecting duplicate addresses uses Neighbor
is said to be tentative until the procedure has completed successfully. Solicitation and Advertisement messages as described below. If a
A tentative address is not considered "assigned to an interface" in the duplicate address is discovered during the procedure, the address
traditional sense. That is, the interface must accept Neighbor cannot be assigned to the interface. If the address is derived from
Solicitation and Advertisement messages containing the tentative address an interface token, a new token will need to be assigned to the
in the Target Address field, but processes such packets differently from interface, or all IP addresses for the interface will need to be
those whose Target Address matches an address assigned to the interface. manually configured. Note that the method for detecting duplicates
Other packets addressed to the tentative address should be silently is not completely reliable, and it is possible that duplicate
discarded. addresses will still exist (e.g., if the link was partitioned while
Duplicate Address Detection was performed).
It should also be noted that Duplicate Address Detection must be An address on which the duplicate Address Detection Procedure is
performed prior to assigning an address to an interface in order to applied is said to be tentative until the procedure has completed
prevent multiple nodes from using the same address simultaneously. If a successfully. A tentative address is not considered "assigned to an
node begins using an address in parallel with Duplicate Address interface" in the traditional sense. That is, the interface must
Detection, and another node is already using the address, the node accept Neighbor Solicitation and Advertisement messages containing
performing Duplicate Address Detection will erroneously process traffic the tentative address in the Target Address field, but processes such
intended for the other node, resulting in such possible negative packets differently from those whose Target Address matches an
consequences as the resetting of open TCP connections. address assigned to the interface. Other packets addressed to the
tentative address should be silently discarded.
The following subsections describe specific tests a node performs to It should also be noted that Duplicate Address Detection must be
verify an address's uniqueness. An address is considered unique if none performed prior to assigning an address to an interface in order to
of the tests indicate the presence of a duplicate address within prevent multiple nodes from using the same address simultaneously.
RetransTimer milliseconds after having sent DupAddrDetectTransmits If a node begins using an address in parallel with Duplicate Address
Neighbor Solicitations. Once an address is determined to be unique, it Detection, and another node is already using the address, the node
may be assigned to an interface. 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 5.4.1. Message Validation
A node MUST silently discard any Neighbor Solicitation or Advertisement A node MUST silently discard any Neighbor Solicitation or
message that does not pass the validity checks specified in [DISCOVERY]. Advertisement message that does not pass the validity checks
A solicitation that passes these validity checks is called a valid specified in [DISCOVERY]. A solicitation that passes these validity
solicitation or valid advertisement. checks is called a valid solicitation or valid advertisement.
5.4.2. Sending Neighbor Solicitation Messages 5.4.2. Sending Neighbor Solicitation Messages
Before sending a Neighbor Solicitation, an interface MUST join the all- Before sending a Neighbor Solicitation, an interface MUST join the
nodes multicast address and the solicited-node multicast address of the all-nodes multicast address and the solicited-node multicast address
tentative address. The former insures that the node receives Neighbor of the tentative address. The former insures that the node receives
Advertisements from other nodes already using the address; the latter Neighbor Advertisements from other nodes already using the address;
insures that two nodes attempting to use the same address simultaneously the latter insures that two nodes attempting to use the same address
detect each other's presence. simultaneously detect each other's presence.
To check an address, a node sends DupAddrDetectTransmits Neighbor To check an address, a node sends DupAddrDetectTransmits Neighbor
Solicitations, each separated by RetransTimer milliseconds. The Solicitations, each separated by RetransTimer milliseconds. The
solicitation's Target Address is set to the address being checked, the solicitation's Target Address is set to the address being checked,
IP source is set to the unspecified address and the IP destination is the IP source is set to the unspecified address and the IP
set to the solicited-node multicast address of the target address. 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 If the Neighbor Solicitation is the first message to be sent from an
interface after interface (re)initialization, the node should delay interface after interface (re)initialization, the node should delay
sending the message by a random delay between 0 and sending the message by a random delay between 0 and
MAX_RTR_SOLICITATION_DELAY as specified in [DISCOVERY]. This serves to MAX_RTR_SOLICITATION_DELAY as specified in [DISCOVERY]. This serves
alleviate congestion when many nodes start up on the link at the same to alleviate congestion when many nodes start up on the link at the
time, such as after a power failure, and may help to avoid race 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 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 address at the same time. In order to improve the robustness of the
Duplicate Address Detection algorithm, an interface MUST receive and Duplicate Address Detection algorithm, an interface MUST receive and
process datagrams sent to the all-nodes multicast address or solicited- process datagrams sent to the all-nodes multicast address or
node multicast address of the tentative address while delaying solicited-node multicast address of the tentative address while
transmission of the initial Neighbor Solicitation. delaying transmission of the initial Neighbor Solicitation.
5.4.3. Receiving Neighbor Solicitation Messages 5.4.3. Receiving Neighbor Solicitation Messages
On receipt of a valid Neighbor Solicitation message on an interface, On receipt of a valid Neighbor Solicitation message on an interface,
node behavior depends on whether the target address is tentative or not. node behavior depends on whether the target address is tentative or
If the target address is not tentative (i.e., it is assigned to the not. If the target address is not tentative (i.e., it is assigned to
receiving interface), the solicitation is processed as described in the receiving interface), the solicitation is processed as described
[DISCOVERY]. If the target address is tentative, and the source address in [DISCOVERY]. If the target address is tentative, and the source
is a unicast address, the solicitation's sender is performing address address is a unicast address, the solicitation's sender is performing
resolution on the target; the solicitation should be silently ignored. address resolution on the target; the solicitation should be silently
Otherwise, processing takes place as described below. In all cases, a ignored. Otherwise, processing takes place as described below. In
node MUST NOT respond to a Neighbor Solicitation for a tentative all cases, a node MUST NOT respond to a Neighbor Solicitation for a
address. tentative address.
If the source address of the Neighbor Solicitation is the unspecified If the source address of the Neighbor Solicitation is the unspecified
address, the solicitation is from a node performing Duplicate Address address, the solicitation is from a node performing Duplicate Address
Detection. If the solicitation is from another node, the tentative Detection. If the solicitation is from another node, the tentative
address is a duplicate and should not be used (by either node). If the address is a duplicate and should not be used (by either node). If
solicitation is from the node itself (because the node loops back the solicitation is from the node itself (because the node loops back
multicast packets), the solicitation does not indicate the presence of a multicast packets), the solicitation does not indicate the presence
duplicate address. of a duplicate address.
Implementor's Note: many interfaces provide a way for upper layers to Implementor's Note: many interfaces provide a way for upper layers to
selectively enable and disable the looping back of multicast packets. selectively enable and disable the looping back of multicast packets.
The details of how such a facility is implemented may prevent Duplicate The details of how such a facility is implemented may prevent
Address Detection from working correctly. See the Appendix for further Duplicate Address Detection from working correctly. See the Appendix
discussion. for further discussion.
The following tests identify conditions under which a tentative address The following tests identify conditions under which a tentative
is not unique: address is not unique:
- If a Neighbor Solicitation for a tentative address is received - If a Neighbor Solicitation for a tentative address is received
prior to having sent one, the tentative address is a duplicate. prior to having sent one, the tentative address is a duplicate.
This condition occurs when two nodes run Duplicate Address This condition occurs when two nodes run Duplicate Address
Detection simultaneously, but transmit initial solicitations at Detection simultaneously, but transmit initial solicitations at
different times (e.g., by selecting different random delay values different times (e.g., by selecting different random delay values
before transmitting an initial solicitation). before transmitting an initial solicitation).
- If the actual number of Neighbor Solicitations received exceeds the - If the actual number of Neighbor Solicitations received exceeds the
number expected based on the loopback semantics (e.g., the number expected based on the loopback semantics (e.g., the
interface does not loopback packet, yet one or more solicitations interface does not loopback packet, yet one or more solicitations
was received), the tentative address is a duplicate. This condition was received), the tentative address is a duplicate. This condition
occurs when two nodes run Duplicate Address Detection occurs when two nodes run Duplicate Address Detection
simultaneously and transmit solicitations at roughly the same time. simultaneously and transmit solicitations at roughly the same time.
5.4.4. Receiving Neighbor Advertisement Messages 5.4.4. Receiving Neighbor Advertisement Messages
On receipt of a valid Neighbor Advertisement message on an interface, On receipt of a valid Neighbor Advertisement message on an interface,
node behavior depends on whether the target address is tentative or node behavior depends on whether the target address is tentative or
matches a unicast or anycast address assigned to the interface. If the matches a unicast or anycast address assigned to the interface. If
target address is assigned to the receiving interface, the solicitation the target address is assigned to the receiving interface, the
is processed as described in [DISCOVERY]. If the target address is solicitation is processed as described in [DISCOVERY]. If the target
tentative, the tentative address is not unique. address is tentative, the tentative address is not unique.
5.4.5. When Duplicate Address Detection Fails 5.4.5. When Duplicate Address Detection Fails
A tentative address that is determined to be a duplicate as described 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 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 system management error. If the address is a link-local address
from an interface token, the interface SHOULD be disabled. formed from an interface token, the interface SHOULD be disabled.
5.5. Creation of Global and Site-Local Addresses 5.5. Creation of Global and Site-Local Addresses
Global and site-local addresses are formed by appending an interface Global and site-local addresses are formed by appending an interface
token to a prefix of appropriate length. Prefixes are obtained from token to a prefix of appropriate length. Prefixes are obtained from
Prefix Information options contained in Router Advertisements. Creation Prefix Information options contained in Router Advertisements.
of global and site-local addresses and configuration of other parameters Creation of global and site-local addresses and configuration of
as described in this section SHOULD be locally configurable. However, other parameters as described in this section SHOULD be locally
the processing described below MUST be enabled by default. configurable. However, the processing described below MUST be enabled
by default.
5.5.1. Soliciting Router Advertisements 5.5.1. Soliciting Router Advertisements
Router Advertisements are sent periodically to the all-nodes multicast Router Advertisements are sent periodically to the all-nodes
address. To obtain an advertisement quickly, a host sends out Router multicast address. To obtain an advertisement quickly, a host sends
Solicitations as described in [DISCOVERY]. out Router Solicitations as described in [DISCOVERY].
5.5.2. Absence of Router Advertisements 5.5.2. Absence of Router Advertisements
If a link has no routers, a host MUST attempt to use stateful If a link has no routers, a host MUST attempt to use stateful
autoconfiguration to obtain addresses and other configuration autoconfiguration to obtain addresses and other configuration
information. An implementation MAY provide a way to disable the information. An implementation MAY provide a way to disable the
invocation of stateful autoconfiguration in this case, but the default invocation of stateful autoconfiguration in this case, but the
SHOULD be enabled. From the perspective of autoconfiguration, a link default SHOULD be enabled. From the perspective of
has no routers if no Router Advertisements are received after having autoconfiguration, a link has no routers if no Router Advertisements
sent a small number of Router Solicitations as described in [DISCOVERY]. are received after having sent a small number of Router Solicitations
as described in [DISCOVERY].
5.5.3. Router Advertisement Processing 5.5.3. Router Advertisement Processing
On receipt of a valid Router Advertisement (as defined in [DISCOVERY]), On receipt of a valid Router Advertisement (as defined in
a host copies the value of the advertisement's M bit into ManagedFlag. [DISCOVERY]), a host copies the value of the advertisement's M bit
If the value of ManagedFlag changes from FALSE to TRUE, the host should into ManagedFlag. If the value of ManagedFlag changes from FALSE to
invoke the stateful address autoconfiguration protocol, requesting TRUE, the host should invoke the stateful address autoconfiguration
address information. If the value of the ManagedFlag changes from TRUE protocol, requesting address information. If the value of the
to FALSE, the host should terminate the stateful address ManagedFlag changes from TRUE to FALSE, the host should terminate the
autoconfiguration protocol (i.e., stop requesting addresses and ignore stateful address autoconfiguration protocol (i.e., stop requesting
subsequent responses to in-progress transactions). If the value of the addresses and ignore subsequent responses to in-progress
flag stays unchanged, no special action takes place. In particular, a transactions). If the value of the flag stays unchanged, no special
host MUST NOT reinvoke stateful address configuration if it is already action takes place. In particular, a host MUST NOT reinvoke stateful
participating in the stateful protocol as a result of an earlier address configuration if it is already participating in the stateful
advertisement. protocol as a result of an earlier advertisement.
An advertisement's O flag field is processed in an analogous manner. A An advertisement's O flag field is processed in an analogous manner.
host copies the value of the O flag into OtherConfigFlag. If the value A host copies the value of the O flag into OtherConfigFlag. If the
of OtherConfigFlag changes from FALSE to TRUE, the host should invoke value of OtherConfigFlag changes from FALSE to TRUE, the host should
the stateful autoconfiguration protocol, requesting information invoke the stateful autoconfiguration protocol, requesting
(excluding addresses). If the value of the OtherConfigFlag changes from information (excluding addresses). If the value of the
TRUE to FALSE, any activity related to stateful autoconfiguration for OtherConfigFlag changes from TRUE to FALSE, any activity related to
parameters other than addresses should be halted. If the value of the stateful autoconfiguration for parameters other than addresses should
flag stays unchanged, no special action takes place. In particular, a be halted. If the value of the flag stays unchanged, no special
host MUST NOT reinvoke stateful configuration if it is already action takes place. In particular, a host MUST NOT reinvoke stateful
participating in the stateful protocol as a result of an earlier configuration if it is already participating in the stateful protocol
advertisement. as a result of an earlier advertisement.
For each Prefix-Information option in the Router Advertisement: For each Prefix-Information option in the Router Advertisement:
a) If the Autonomous flag is not set, silently ignore the Prefix a) If the Autonomous flag is not set, silently ignore the Prefix
Information option. Information option.
b) If the prefix is the link-local prefix, silently ignore the Prefix b) If the prefix is the link-local prefix, silently ignore the Prefix
Information option. Information option.
c) If the preferred lifetime is greater than the valid lifetime, c) If the preferred lifetime is greater than the valid lifetime,
silently ignore the Prefix Information option. A node MAY wish to silently ignore the Prefix Information option. A node MAY wish to
log a system management error in this case. log a system management error in this case.
skipping to change at page 20, line 39 skipping to change at page 19, line 11
In those cases where a site requires the use of longer prefixes than In those cases where a site requires the use of longer prefixes than
can be accommodated by the interface token, stateful can be accommodated by the interface token, stateful
autoconfiguration can be used. autoconfiguration can be used.
If an address is formed successfully, the host adds it to the list If an address is formed successfully, the host adds it to the list
of addresses assigned to the interface, initializing its preferred of addresses assigned to the interface, initializing its preferred
and valid lifetime values from the Prefix Information option. and valid lifetime values from the Prefix Information option.
5.5.4. Address Lifetime Expiry 5.5.4. Address Lifetime Expiry
A preferred address becomes deprecated when its preferred lifetime A preferred address becomes deprecated when its preferred lifetime
expires. A deprecated address SHOULD continue to be used as a source expires. A deprecated address SHOULD continue to be used as a source
address in existing communications, but SHOULD NOT be used in new address in existing communications, but SHOULD NOT be used in new
communications if an alternate (non-deprecated) address is available and communications if an alternate (non-deprecated) address is available
has sufficient scope. The IP layer MUST continue to accept datagrams and has sufficient scope. The IP layer MUST continue to accept
destined to a deprecated address since a deprecated address is still a datagrams destined to a deprecated address since a deprecated address
valid address for the interface. An implementation MAY prevent any new is still a valid address for the interface. An implementation MAY
communication from using a deprecated address, but system management prevent any new communication from using a deprecated address, but
MUST have the ability to disable such a facility. system management MUST have the ability to disable such a facility.
An address (and its association with an interface) becomes invalid when An address (and its association with an interface) becomes invalid
its valid lifetime expires. An invalid address MUST NOT be used as a when its valid lifetime expires. An invalid address MUST NOT be used
source address in outgoing communications and MUST NOT be recognized as as a source address in outgoing communications and MUST NOT be
a destination on a receiving interface. recognized as a destination on a receiving interface.
Note that if a Prefix Information option is received with a preferred Note that if a Prefix Information option is received with a preferred
lifetime of zero, any addresses generated from that prefix are lifetime of zero, any addresses generated from that prefix are
immediately deprecated. Similarly, if both the advertised deprecated and immediately deprecated. Similarly, if both the advertised deprecated
valid lifetimes are zero, any addresses generated from that prefix and valid lifetimes are zero, any addresses generated from that
become invalid immediately. prefix become invalid immediately.
5.6. Configuration Consistency 5.6. Configuration Consistency
It is possible for hosts to obtain address information using both It is possible for hosts to obtain address information using both
stateless and stateful protocols since both may be enabled at the same stateless and stateful protocols since both may be enabled at the
time. It is also possible that the values of other configuration same time. It is also possible that the values of other
parameters such as MTU size and hop limit will be learned from both configuration parameters such as MTU size and hop limit will be
Router Advertisements and the stateful autoconfiguration protocol. If learned from both Router Advertisements and the stateful
the same configuration information is provided by multiple sources, the autoconfiguration protocol. If the same configuration information is
value of this information should be consistent. However, it is not provided by multiple sources, the value of this information should be
considered a fatal error if information received from multiple sources consistent. However, it is not considered a fatal error if
is inconsistent. Hosts accept the union of all information received via information received from multiple sources is inconsistent. Hosts
the stateless and stateful protocols. If inconsistent information is accept the union of all information received via the stateless and
learned from different sources, the most recently obtained values always stateful protocols. If inconsistent information is learned from
have precedence over information learned earlier. different sources, the most recently obtained values always have
precedence over information learned earlier.
SECURITY CONSIDERATIONS SECURITY CONSIDERATIONS
Stateless address autoconfiguration allows a host to connect to a Stateless address autoconfiguration allows a host to connect to a
network, configure an address and start communicating with other nodes network, configure an address and start communicating with other
without ever registering or authenticating itself with the local site. nodes without ever registering or authenticating itself with the
Although this allows unauthorized users to connect to and use a network, local site. Although this allows unauthorized users to connect to
the threat is inherently present in the Internet architecture. Any node and use a network, the threat is inherently present in the Internet
with a physical attachment to a network can generate an address (using a architecture. Any node with a physical attachment to a network can
variety of ad hoc techniques) that provides connectivity. generate an address (using a variety of ad hoc techniques) that
provides connectivity.
The use of Duplicate Address Detection opens up the possibility of The use of Duplicate Address Detection opens up the possibility of
denial of service attacks. Any node can respond to Neighbor denial of service attacks. Any node can respond to Neighbor
Solicitations for a tentative address, causing the other node to reject Solicitations for a tentative address, causing the other node to
the address as a duplicate. This attack is similar to other attacks reject the address as a duplicate. This attack is similar to other
involving the spoofing of Neighbor Discovery messages and can be attacks involving the spoofing of Neighbor Discovery messages and can
addressed by requiring that Neighbor Discovery packets be authenticated be addressed by requiring that Neighbor Discovery packets be
[RFC1826]. authenticated [RFC1826].
REFERENCES REFERENCES
[RFC1826] R. Atkinson. "IP Authentication Header", RFC 1826, August [RFC1826] Atkinson, R., "IP Authentication Header", RFC 1826, August
1995. 1995.
[IPv6-ETHER] M. Crawford. "A Method for the Transmission of IPv6 Packets [IPv6-ETHER] Crawford, M., "A Method for the Transmission of IPv6
over Ethernet Networks", Internet Draft. Packets over Ethernet Networks", RFC 1972, August 1996.
[RFC1112] S. Deering, "Host Extensions for IP Multicasting", RFC 1112, [RFC1112] Deering, S., "Host Extensions for IP Multicasting", STD 5,
August, 1989. RFC 1112, August 1989.
[ADDR-ARCH] R. Hinden and S. Deering, "Internet Protocol Version (IPv6) [ADDR-ARCH] Hinden, R., and S. Deering, "Internet Protocol Version
Addressing Architecture", Internet Draft, May 1995, draft- (IPv6) Addressing Architecture", RFC 1884, December 1995.
ietf-ipngwg-addr-arch-03.txt
[DHCPv6] Internet Draft, Work in Progress. [DHCPv6] Work in Progress.
[DISCOVERY] T. Narten, E. Nordmark and W. A. Simpson, "Neighbor [DISCOVERY] Narten, T., Nordmark, E., and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", Internet Draft, September Discovery for IP Version 6 (IPv6)", RFC 1970, August 1996.
1995, <draft-ietf-ipngwg-discovery-02.txt>
Acknowledgements Acknowledgements
The authors would like to thank the members of both the IPNG and The authors would like to thank the members of both the IPNG and
ADDRCONF working groups for their input. In particular, thanks to Jim ADDRCONF working groups for their input. In particular, thanks to Jim
Bound, Steve Deering, and Erik Nordmark. Bound, Steve Deering, and Erik Nordmark.
AUTHORS' ADDRESSES AUTHORS' ADDRESSES
Susan Thomson Thomas Narten Susan Thomson
Bellcore IBM Corporation Bellcore
445 South Street P.O. Box 12195 445 South Street
Morristown, NJ 07960 Research Triangle Park, NC 27709-2195 Morristown, NJ 07960
USA USA 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
phone: +1 201-829-4514 phone: +1 919 254 7798
email: set@thumper.bellcore.com email: narten@vnet.ibm.com
APPENDIX: LOOPBACK SUPPRESSION & DUPLICATE ADDRESS DETECTION APPENDIX: LOOPBACK SUPPRESSION & DUPLICATE ADDRESS DETECTION
Determining whether a received multicast solicitation was looped back to Determining whether a received multicast solicitation was looped back
the sender or actually came from another node is implementation- to the sender or actually came from another node is implementation-
dependent. A problematic case occurs when two interfaces attached to dependent. A problematic case occurs when two interfaces attached to
the same link happen to have the same token and link-layer address, and the same link happen to have the same token and link-layer address,
they both send out packets with identical contents at roughly the same and they both send out packets with identical contents at roughly the
time (e.g., Neighbor Solicitations for a tentative address as part of same time (e.g., Neighbor Solicitations for a tentative address as
Duplicate Address Detection messages). Although a receiver will receive part of Duplicate Address Detection messages). Although a receiver
both packets, it cannot determine which packet was looped back and which will receive both packets, it cannot determine which packet was
packet came from the other node by simply comparing packet contents looped back and which packet came from the other node by simply
(i.e., the contents are identical). In this particular case, it is not comparing packet contents (i.e., the contents are identical). In this
necessary to know precisely which packet was looped back and which was particular case, it is not necessary to know precisely which packet
sent by another node; if one receives more solicitations than were sent, was looped back and which was sent by another node; if one receives
the tentative address is a duplicate. However, the situation may not more solicitations than were sent, the tentative address is a
always be this straightforward. duplicate. However, the situation may not always be this
straightforward.
The IPv4 multicast specification [RFC1112] recommends that the service The IPv4 multicast specification [RFC1112] recommends that the
interface provide a way for an upper-layer protocol to inhibit local service interface provide a way for an upper-layer protocol to
delivery of packets sent to a multicast group that the sending host is a inhibit local delivery of packets sent to a multicast group that the
member of. Some applications know that there will be no other group sending host is a member of. Some applications know that there will
members on the same host, and suppressing loopback prevents them from be no other group members on the same host, and suppressing loopback
having to receive (and discard) the packets they themselves send out. A prevents them from having to receive (and discard) the packets they
straightforward way to implement this facility is to disable loopback at themselves send out. A straightforward way to implement this
the hardware level (if supported by the hardware), with packets looped facility is to disable loopback at the hardware level (if supported
back (if requested) by software. On interfaces in which the hardware by the hardware), with packets looped back (if requested) by
itself suppresses loopbacks, a node running Duplicate Address Detection software. On interfaces in which the hardware itself suppresses
simply counts the number of Neighbor Solicitations received for a loopbacks, a node running Duplicate Address Detection simply counts
tentative address and compares them with the number expected. If there the number of Neighbor Solicitations received for a tentative address
is a mismatch, the tentative address is a duplicate. 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, In those cases where the hardware cannot suppress loopbacks, however,
one possible software heuristic to filter out unwanted loopbacks is to one possible software heuristic to filter out unwanted loopbacks is
discard any received packet whose link-layer source address is the same to discard any received packet whose link-layer source address is the
as the receiving interface's. Unfortunately, use of that criteria also same as the receiving interface's. Unfortunately, use of that
results in the discarding of all packets sent by another node using the criteria also results in the discarding of all packets sent by
same link-layer address. Duplicate Address Detection will fail on another node using the same link-layer address. Duplicate Address
interfaces that filter received packets in this manner: Detection will fail on interfaces that filter received packets in
this manner:
o If a node performing Duplicate Address Detection discards received o If a node performing Duplicate Address Detection discards received
packets having the same source link-layer address as the receiving packets having the same source link-layer address as the receiving
interface, it will also discard packets from other nodes also using interface, it will also discard packets from other nodes also using
the same link-layer address, including Neighbor Advertisement and the same link-layer address, including Neighbor Advertisement and
Neighbor Solicitation messages required to make Duplicate Address Neighbor Solicitation messages required to make Duplicate Address
Detection work correctly. This particular problem can be avoided Detection work correctly. This particular problem can be avoided
by temporarily disabling the software suppression of loopbacks by temporarily disabling the software suppression of loopbacks
while a node performs Duplicate Address Detection. while a node performs Duplicate Address Detection.
skipping to change at page 24, line 16 skipping to change at page 23, line 18
received packets having the same link-layer source address as the received packets having the same link-layer source address as the
interface, it will also discard Duplicate Address Detection-related interface, it will also discard Duplicate Address Detection-related
Neighbor Solicitation messages sent by another node also using the Neighbor Solicitation messages sent by another node also using the
same link-layer address. Consequently, Duplicate Address Detection same link-layer address. Consequently, Duplicate Address Detection
will fail, and the other node will configure a non-unique address. will fail, and the other node will configure a non-unique address.
Since it is generally impossible to know when another node is Since it is generally impossible to know when another node is
performing Duplicate Address Detection, this scenario can be performing Duplicate Address Detection, this scenario can be
avoided only if software suppression of loopback is permanently avoided only if software suppression of loopback is permanently
disabled. disabled.
Thus, to perform Duplicate Address Detection correctly in the case where Thus, to perform Duplicate Address Detection correctly in the case
two interfaces are using the same link-layer address, an implementation where two interfaces are using the same link-layer address, an
must have a good understanding of the interface's multicast loopback implementation must have a good understanding of the interface's
semantics, and the interface cannot discard received packets simply multicast loopback semantics, and the interface cannot discard
because the source link-layer address is the same as the interfaces. received packets simply because the source link-layer address is the
same as the interfaces.
 End of changes. 78 change blocks. 
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