IETF IPv6 Working Group S. Thomson
Internet-Draft Cisco
Expires: January 17, 2005 T. Narten
IBM
T. Jinmei
Toshiba
July 19, 2004
IPv6 Stateless Address Autoconfiguration
draft-ietf-ipv6-rfc2462bis-03.txt
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
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 can be
autoconfigured (addresses, other information, or both), and in the
case of addresses, whether they can be obtained through the stateless
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mechanism, the stateful mechanism, or both. This document defines
the process for generating a link-local address, the process for
generating global addresses via stateless address autoconfiguration,
and the Duplicate Address Detection procedure. The details of
autoconfiguration using the stateful protocol is specified in RFC
3315 and RFC 3736.
Table of Contents
1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. TERMINOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1 Requirements . . . . . . . . . . . . . . . . . . . . . . . 7
3. DESIGN GOALS . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. PROTOCOL OVERVIEW . . . . . . . . . . . . . . . . . . . . . . 8
4.1 Site Renumbering . . . . . . . . . . . . . . . . . . . . . 10
5. PROTOCOL SPECIFICATION . . . . . . . . . . . . . . . . . . . . 11
5.1 Node Configuration Variables . . . . . . . . . . . . . . . 11
5.2 Autoconfiguration-Related Structures . . . . . . . . . . . 12
5.3 Creation of Link-Local Addresses . . . . . . . . . . . . . 12
5.4 Duplicate Address Detection . . . . . . . . . . . . . . . 13
5.4.1 Message Validation . . . . . . . . . . . . . . . . . . 14
5.4.2 Sending Neighbor Solicitation Messages . . . . . . . . 14
5.4.3 Receiving Neighbor Solicitation Messages . . . . . . . 16
5.4.4 Receiving Neighbor Advertisement Messages . . . . . . 17
5.4.5 When Duplicate Address Detection Fails . . . . . . . . 17
5.5 Creation of Global Addresses . . . . . . . . . . . . . . . 17
5.5.1 Soliciting Router Advertisements . . . . . . . . . . . 18
5.5.2 Absence of Router Advertisements . . . . . . . . . . . 18
5.5.3 Router Advertisement Processing . . . . . . . . . . . 18
5.5.4 Address Lifetime Expiry . . . . . . . . . . . . . . . 20
5.6 Configuration Consistency . . . . . . . . . . . . . . . . 21
5.7 Retaining Configured Addresses for Stability . . . . . . . 22
6. SECURITY CONSIDERATIONS . . . . . . . . . . . . . . . . . . . 22
7. IANA CONSIDERATIONS . . . . . . . . . . . . . . . . . . . . . 22
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
9.1 Normative References . . . . . . . . . . . . . . . . . . . . 23
9.2 Informative References . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 24
A. LOOPBACK SUPPRESSION & DUPLICATE ADDRESS DETECTION . . . . . . 25
B. CHANGES SINCE RFC 1971 . . . . . . . . . . . . . . . . . . . . 26
C. CHANGE HISTORY . . . . . . . . . . . . . . . . . . . . . . . . 26
Intellectual Property and Copyright Statements . . . . . . . . 29
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1. INTRODUCTION
This document specifies the steps a host takes in deciding how to
autoconfigure its interfaces in IP version 6 (IPv6). The
autoconfiguration process includes creating a link-local address and
verifying its uniqueness on a link, determining what information can
be autoconfigured (addresses, other information, or both), and in the
case of addresses, whether they can 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 global addresses via stateless address autoconfiguration,
and the Duplicate Address Detection procedure. The details of
autoconfiguration using the stateful protocol is specified in
[RFC3315] and [RFC3736].
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 identifier" that uniquely identifies an
interface on a subnet. An address is formed by combining the two.
In the absence of routers, a host can only generate link-local
addresses. However, link-local addresses are sufficient for allowing
communication among nodes attached to the same link.
In the stateful autoconfiguration model, hosts obtain interface
addresses and/or configuration information and parameters from a
Dynamic Host Configuration Protocol (DHCPv6) 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.
To obtain other configuration information without configuring
addresses in the stateful autoconfiguration model, a subset of DHCPv6
[RFC3736] will be used. While the model is called "stateful" here in
order to highlight the contrast to the stateless protocol defined in
this document, the intended protocol is also defined to work in a
stateless fashion. This is based on a result, through operational
experiments, that all known "other" configuration information can be
managed by a stateless server, that is, a server that does not
maintain state of each client that the server provides with the
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configuration information.
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 is available through the setting of appropriate
fields in Router Advertisement messages [RFC2461].
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 ensure that all configured addresses are likely to be unique on a
given link, nodes run a "duplicate address detection" algorithm on
addresses before assigning them to an interface. The Duplicate
Address Detection algorithm is performed on all addresses,
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
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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 [RFC3513].
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solicited-node multicast address - a multicast address to which
Neighbor Solicitation messages are sent. The algorithm for
computing the address is given in [RFC3513].
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.
global address - an address with unlimited scope.
communication - any packet exchange among nodes that requires that
the address of each node used in the exchange remain the same for
the duration of the packet exchange. Examples are a TCP
connection or a UDP request-response.
tentative address - an address whose uniqueness on a link is being
verified, prior to its assignment to an interface. A tentative
address is not considered assigned to an interface in the usual
sense. An interface discards received packets addressed to a
tentative address, but accepts Neighbor Discovery packets related
to Duplicate Address Detection for the tentative address.
preferred address - an address assigned to an interface whose use by
upper layer protocols is unrestricted. Preferred addresses may be
used as the source (or destination) address of packets sent from
(or to) the interface.
deprecated address - An address assigned to an interface whose use is
discouraged, but not forbidden. A deprecated address should no
longer be used as a source address in new communications, but
packets sent from or to deprecated addresses are delivered as
expected. A deprecated address may continue to be used as a
source address in communications where switching to a preferred
address causes hardship to a specific upper-layer activity (e.g.,
an existing TCP connection).
valid address - a preferred or deprecated address. A valid address
may appear as the source or destination address of a packet, and
the internet routing system is expected to deliver packets sent to
a valid address to their intended recipients.
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invalid address - an address that is not assigned to any interface.
A valid address becomes invalid when its valid lifetime expires.
Invalid addresses should not appear as the destination or source
address of a packet. In the former case, the internet routing
system will be unable to deliver the packet, in the latter case
the recipient of the packet will be unable to respond to it.
preferred lifetime - the length of time that a valid address is
preferred (i.e., the time until deprecation). When the preferred
lifetime expires, the address becomes deprecated.
valid lifetime - the length of time an address remains in the valid
state (i.e., the time until invalidation). The valid lifetime
must be greater than or equal to the preferred lifetime. When the
valid lifetime expires, the address becomes invalid.
interface identifier - a link-dependent identifier for an interface
that is (at least) unique per link [RFC3513]. Stateless address
autoconfiguration combines an interface identifier with a prefix
to form an address. From address autoconfiguration's perspective,
an interface identifier is a bit string of known length. The
exact length of an interface identifier and the way it is created
is defined in a separate link-type specific document that covers
issues related to the transmission of IP over a particular link
type (e.g., [RFC2464]). Note that the address architecture
[RFC3513] also defines the length of the interface identifiers for
some set of addresses, but the two sets of definitions must be
consistent. In many cases, the identifier will be derived from
the interface's link-layer address.
2.1 Requirements
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [RFC2119].
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 identifier"). In the simplest
case, an interface identifier consists of the interface's
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link-layer address. An interface identifier can be combined with
a prefix to form an address.
o Small sites consisting of a set of machines attached to a single
link should not require the presence of a 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 identifier 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 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
are available. Router Advertisements include flags specifying
which mechanisms a host can 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 identifier 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"
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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
identifier that overrides the default identifier in such a way that
the autoconfiguration mechanism can then be applied using the new
(presumably unique) interface identifier. Alternatively, link-local
and other addresses will need to be configured manually.
Once a node ascertains that its tentative link-local address is
unique, it assigns the address to the interface. At this point, the
node has IP-level connectivity with neighboring nodes. The remaining
autoconfiguration steps are performed only by hosts; the
(auto)configuration of routers is beyond the scope of this document.
The next phase of autoconfiguration involves obtaining a Router
Advertisement or determining that no routers are present. If routers
are present, they will send Router Advertisements that specify what
sort of autoconfiguration a host can do. Note that stateful
autoconfiguration may still be available even if no routers are
present.
Routers send Router Advertisements periodically, but the delay
between successive advertisements will generally be longer than a
host performing autoconfiguration will want to wait [RFC2461]. 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) is available. A "managed address
configuration (M)" flag indicates whether hosts can use stateful
autoconfiguration [RFC3315] to obtain addresses. An "other stateful
configuration (O)" flag indicates whether hosts can use stateful
autoconfiguration [RFC3736] to obtain additional information
(excluding addresses).
The details of how a host may use the M flag, including any use of
the "on" and "off" transitions for this flag, to control the use of
the stateful protocol for address assignment will be described in a
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separate document. Similarly, the details of how a host may use the
O flag, including any use of the "on" and "off" transitions for this
flag, to control the use of the stateful protocol for getting other
configuration information will be described in a separate document.
Router Advertisements also contain zero or more Prefix Information
options that contain information used by stateless address
autoconfiguration to generate global addresses. It should be noted
that 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. The test should individually be
performed on all addresses obtained manually, via stateless address
autoconfiguration, or via stateful address autoconfiguration. 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
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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
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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., [RFC2464]).
Autoconfiguration also assumes the presence of the variable
RetransTimer as defined in [RFC2461]. 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 Structures
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.
A host maintains a list of addresses together with their
corresponding lifetimes. The address list contains both
autoconfigured addresses and those configured manually.
5.3 Creation of Link-Local Addresses
A node forms a link-local address whenever an interface becomes
enabled. An interface may become enabled after any of the following
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 [RFC3513] (of appropriate length not less than
10 bits) to the interface identifier. If the interface identifier
has a length of N bits, the interface identifier replaces the
right-most N zero bits of the link-local prefix. If the interface
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identifier is more than 118 bits in length, autoconfiguration fails
and manual configuration is required. The length of the interface
identifier is defined in a separate link-type specific document,
which should also be consistent with the address architecture
[RFC3513] (see Section 2). These documents will carefully define the
length so that link-local addresses can be autoconfigured on the
link.
A link-local address has an infinite preferred and valid lifetime; it
is never timed out.
5.4 Duplicate Address Detection
Duplicate Address Detection is performed on unicast addresses prior
to assigning them to an interface whose DupAddrDetectTransmits
variable is greater than zero. Duplicate Address Detection MUST take
place on all unicast addresses, regardless of whether they are
obtained through stateful, stateless or manual configuration, with
the exception of the following cases:
- Duplicate Address Detection MUST NOT be performed on anycast
addresses.
- Each individual unicast address SHOULD be tested for uniqueness.
Note that there are implementations deployed that only perform
Duplicate Address Detection for the link-local address and skip
the test for the global address using the same interface
identifier as that of the link-local address. Whereas this
document does not invalidate such implementations, this kind of
"optimization" is NOT RECOMMENDED, and new implementations MUST
NOT do that optimization. This optimization came from the
assumption that all of an interface's addresses are generated from
the same identifier. However, the assumption does actually not
stand; new types of addresses have been introduced where the
interface identifiers are not necessarily the same for all unicast
addresses on a single interface [RFC3041][I-D.ietf-send-cga].
Requiring to perform Duplicate Address Detection for all unicast
addresses will make the algorithm robust for the current and
future such special interface identifiers.
The procedure for detecting duplicate addresses uses Neighbor
Solicitation and Advertisement messages as described below. If a
duplicate address is discovered during the procedure, the address
cannot be assigned to the interface. If the address is derived from
an interface identifier, a new identifier will need to be assigned to
the interface, or all IP addresses for the interface will need to be
manually configured. Note that the method for detecting duplicates
is not completely reliable, and it is possible that duplicate
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addresses will still exist (e.g., if the link was partitioned while
Duplicate Address Detection was performed).
An address on which the Duplicate Address Detection procedure is
applied is said to be tentative until the procedure has completed
successfully. A tentative address is not considered "assigned to an
interface" in the traditional sense. That is, the interface must
accept Neighbor Solicitation and Advertisement messages containing
the tentative address in the Target Address field, but processes such
packets differently from those whose Target Address matches an
address assigned to the interface. Other packets addressed to the
tentative address should be silently discarded. Note that the "other
packets" include Neighbor Solicitation and Advertisement messages
which have the tentative (i.e., unicast) address as the IP
destination address and contain the tentative address in the Target
Address field. Such a case should not happen in normal operation,
though, since these messages are multicasted in the Duplicate Address
Detection procedure.
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 [RFC2461]. A Neighbor Solicitation or Advertisement
message that passes these validity checks is called a valid
solicitation or valid advertisement, respectively.
5.4.2 Sending Neighbor Solicitation Messages
Before sending a Neighbor Solicitation, an interface MUST join the
all-nodes multicast address and the solicited-node multicast address
of the tentative address. The former ensures that the node receives
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Neighbor Advertisements from other nodes already using the address;
the latter ensures 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 going to be the first message to be
sent from an interface after interface (re)initialization, the node
SHOULD delay joining the solicited-node multicast address by a random
delay between 0 and MAX_RTR_SOLICITATION_DELAY as specified in
[RFC2461]. This serves to alleviate congestion when many nodes start
up on the link at the same time, such as after a power failure, and
may help to avoid race conditions when more than one node is trying
to solicit for the same address at the same time.
Even if the Neighbor Solicitation is not going to be the first
message to be sent, the node SHOULD delay joining the solicited-node
multicast address by a random delay between 0 and
MAX_RTR_SOLICITATION_DELAY if the address being checked is configured
by a router advertisement message sent to a multicast address. The
delay will avoid similar congestion when multiple nodes are going to
configure addresses by receiving a same single multicast router
advertisement.
Note that the delay for joining the multicast address implicitly
means delaying transmission of the corresponding Multicast Listener
Discovery (MLD) report message [RFC2710]. Since [RFC2710] does not
request a random delay to avoid race conditions, just delaying
Neighbor Solicitation would cause congestion by the MLD report
messages. The congestion would then prevent MLD-snooping switches
from working correctly, and, as a result, prevent Duplicate Address
Detection from working. The requirement to include the delay for the
MLD report in this case avoids this scenario.
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 the delaying period. This does not
necessarily conflict with the requirement that joining the multicast
group be delayed. In fact, in some cases it is possible for a node
to start listening to the group during the delay period before MLD
report transmission. It should be noted, however, that in some
link-layer environments, particularly with MLD-snooping switches, no
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multicast reception will be available until the MLD report is sent.
5.4.3 Receiving Neighbor Solicitation Messages
On receipt of a valid Neighbor Solicitation message on an interface,
node behavior depends on whether 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 [RFC2461]. 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
A for further discussion.
The following tests identify conditions under which a tentative
address is not unique:
- If a Neighbor Solicitation for a tentative address is received
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 joining the solicited-node multicast address and
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.
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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 [RFC2461]. 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
identifier based on the hardware address which is supposed to be
uniquely assigned (e.g., EUI-64 for an Ethernet interface), IP
operation on the interface SHOULD be disabled. By disabling IP
operation, the node will then
- not send any IP packets from the interface
- silently drop any IP packets received on the interface
- not forward any IP packets to the interface (when acting as a
router or processing a packet with a Routing header)
In this case, the IP address duplication probably means duplicate
hardware addresses are in use, and trying to recover from it by
configuring another IP address will not result in a usable network.
In fact, it probably makes things worse by creating problems that are
harder to diagnose than just disabling network operation on the
interface; the user will see a partially working network where some
things work, and other things will not.
On the other hand, if the duplicate link-local address is not formed
from an interface identifier based on the hardware address which is
supposed to be uniquely assigned, IP operation on the interface MAY
be continued.
Note: as specified in Section 2, "IP" means "IPv6" in the above
description. While the background rationale about the hardware
address is independent of particular network protocols, the effect on
other protocols is beyond the scope of this document.
5.5 Creation of Global Addresses
Global addresses are formed by appending an interface identifier to a
prefix of appropriate length. Prefixes are obtained from Prefix
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Information options contained in Router Advertisements. Creation of
global 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 [RFC2461].
5.5.2 Absence of Router Advertisements
Even if a link has no routers, stateful autoconfiguration to obtain
addresses and other configuration information may still be available,
and hosts may want to use the mechanism. 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 [RFC2461].
Note that it is possible that there is no router on the link in this
sense but there is a node that has the ability to forward packets.
In this case, the forwarding node's address must be manually
configured in hosts to be able to send packets off-link, since the
only mechanism to configure the default router's address
automatically is the one using router advertisements.
5.5.3 Router Advertisement Processing
For each Prefix-Information option in the Router Advertisement:
a) If the Autonomous flag is not set, silently ignore the Prefix
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 prefix advertised is not equal to the prefix of an address
configured by stateless autoconfiguration already in the list of
addresses associated with the interface (where "equal" means the
two prefix lengths are the same and the first prefix-length bits
of the prefixes are identical), and the Valid Lifetime is not 0,
form an address (and add it to the list) by combining the
advertised prefix with the link's interface identifier as follows:
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| 128 - N bits | N bits |
+---------------------------------------+------------------------+
| link prefix | interface identifier |
+----------------------------------------------------------------+
If the sum of the prefix length and interface identifier length
does not equal 128 bits, the Prefix Information option MUST be
ignored. An implementation MAY wish to log a system management
error in this case. The length of the interface identifier is
defined in a separate link-type specific document, which should
also be consistent with the address architecture [RFC3513] (see
Section 2).
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 identifiers for that
link type. It should be noted, however, that this does not mean
the advertised prefix length is meaningless. In fact, the
advertised length has non trivial meaning for on-link
determination in [RFC2461] where the sum of the prefix length and
the interface identifier length may not be equal to 128. Thus, it
should be safe to validate the advertised prefix length here, in
order to detect and avoid a configuration error specifying an
invalid prefix length in the context of address autoconfiguration.
Note that a future revision of the address architecture [RFC3513]
and a future link-type specific document, which will still be
consistent with each other, could potentially allow for an
interface identifier of length other than the value defined in the
current documents. Thus, an implementation should not assume a
particular constant. Rather, it should expect any lengths of
interface identifiers.
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.
e) If the advertised prefix is equal to the prefix of an address
configured by stateless autoconfiguration in the list, the
preferred lifetime of the address is reset to the Preferred
Lifetime in the received advertisement. The specific action to
perform for the valid lifetime of the address depends on the Valid
Lifetime in the received advertisement and the remaining time to
the valid lifetime expiration of the previously autoconfigured
address. We call the remaining time "RemainingLifetime" in the
following discussion:
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1. If the received Valid Lifetime is greater than 2 hours or
greater than RemainingLifetime, set the valid lifetime of the
corresponding address to the advertised Valid Lifetime.
2. If RemainingLifetime is less than or equal to 2 hours, ignore
the Prefix Information option with regards to the valid
lifetime, unless the Router Advertisement from which this
option was obtained has been authenticated (e.g., via IP
security [RFC2402]). If the Router Advertisement was
authenticated, the valid lifetime of the corresponding address
should be set to the Valid Lifetime in the received option.
3. Otherwise, reset the valid lifetime of the corresponding
address to two hours.
The above rules address a specific denial of service attack in
which a bogus advertisement could contain prefixes with very small
Valid Lifetimes. Without the above rules, a single
unauthenticated advertisement containing bogus Prefix Information
options with short Valid Lifetimes could cause all of a node's
addresses to expire prematurely. The above rules ensure that
legitimate advertisements (which are sent periodically) will
"cancel" the short Valid Lifetimes before they actually take
effect.
Note that the preferred lifetime of the corresponding address is
always reset to the Preferred Lifetime in the received Prefix
Information option, regardless of whether the valid lifetime is
also reset or ignored. The difference comes from the fact that
the possible attack for the preferred lifetime is relatively
minor. Additionally, it is even undesirable to ignore the
preferred lifetime when a valid administrator wants to deprecate a
particular address by sending a short preferred lifetime (and the
valid lifetime is ignored by accident).
5.5.4 Address Lifetime Expiry
A preferred address becomes deprecated when its preferred lifetime
expires. A deprecated address SHOULD continue to be used as a source
address in existing communications, but SHOULD NOT be used to
initiate new communications if an alternate (non-deprecated) address
of sufficient scope can easily be used instead.
Note that the feasibility of initiating new communication using a
non-deprecated address may be an application-specific decision, as
only the application may have knowledge about whether the (now)
deprecated address was (or still is) in use by the application. For
example, if an application explicitly specifies the protocol stack to
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use a deprecated address as a source address, the protocol stack must
accept that; the application might request it because that IP address
is used for in higher-level communication and there might be a
requirement that the multiple connections in such a grouping use the
same pair of IP addresses.
IP and higher layers (e.g., TCP, UDP) MUST continue to accept and
process datagrams destined to a deprecated address as normal since a
deprecated address is still a valid address for the interface. In
the case of TCP, this means TCP SYN segments sent to a deprecated
address are responded to using the deprecated address as a source
address in the corresponding SYN-ACK (if the connection would
otherwise be allowed).
An implementation MAY prevent any new communication from using a
deprecated address, but system management MUST have the ability to
disable such a facility, and the facility MUST be disabled by
default.
Other subtle cases should also be noted about source address
selection. For example, the above description does not clarify which
address should be used between a deprecated, smaller-scope address
and a non-deprecated, enough scope address. The details of the
address selection including this case are described in [RFC3484] and
beyond the scope of this document.
An address (and its association with an interface) becomes invalid
when its valid lifetime expires. An invalid address MUST NOT be used
as a source address in outgoing communications and MUST NOT be
recognized as a destination on a receiving interface.
5.6 Configuration Consistency
It is possible for hosts to obtain address information using both
stateless 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 info
