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Versions: (draft-jinmei-ipv6-rfc2462bis) 00 01 02 03 04 05 06 07 08 RFC 4862

IETF IPv6 Working Group                                       S. Thomson
Internet-Draft                                                     Cisco
Expires: December 13, 2004                                     T. Narten
                                                                     IBM
                                                               T. Jinmei
                                                                 Toshiba
                                                              H. Soliman
                                                    Flarion Technologies
                                                           June 14, 2004


                IPv6 Stateless Address Autoconfiguration
                   draft-ietf-ipv6-rfc2462bis-01.txt

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on December 13, 2004.

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
   mechanism, the stateful mechanism, or both. This document defines the



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   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 . . . . . . . . . . 15
   5.4.4 Receiving Neighbor Advertisement Messages  . . . . . . . . . 16
   5.4.5 When Duplicate Address Detection Fails . . . . . . . . . . . 17
   5.5   Creation of Global Addresses . . . . . . . . . . . . . . . . 17
   5.5.1 Soliciting Router Advertisements . . . . . . . . . . . . . . 17
   5.5.2 Absence of Router Advertisements . . . . . . . . . . . . . . 17
   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 . . . . . . . . 21
   6.    SECURITY CONSIDERATIONS  . . . . . . . . . . . . . . . . . . 21
   7.    IANA CONSIDERATIONS  . . . . . . . . . . . . . . . . . . . . 22
   8.    Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 22
         Normative References . . . . . . . . . . . . . . . . . . . . 22
         Informative References . . . . . . . . . . . . . . . . . . . 23
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 23
   A.    LOOPBACK SUPPRESSION & DUPLICATE ADDRESS DETECTION . . . . . 24
   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. 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 RFC
   3315 [7] and RFC 3736 [8].

   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
   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
   will be used [8]. 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
   configuration information.



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   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 [5].

   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
   of the protocol, while Section 5 describes the protocol in detail.




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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 the IPv6 addressing architecture [4].

   solicited-node multicast address - a multicast address to which
      Neighbor Solicitation messages are sent. The algorithm for
      computing the address is given in RFC 2461 [5].



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   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.

   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.




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   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 [4]. 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., IPv6 over Ethernet [2]). Note that the address
      architecture [4] 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 RFC 2119 [3].

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 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



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      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"
   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



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   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 [5]. 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 [7] to obtain
   addresses. An "other stateful configuration (O)" flag indicates
   whether hosts can use stateful autoconfiguration [8] to obtain
   additional information (excluding addresses).

   The details of how a host may use the M flags, 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
   separate document. Similarly, the details of how a host may use the O
   flags, 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



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   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
   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



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   address will fail, should the address's valid lifetime expire before
   communication ends.  To avoid this scenario, higher layers should use
   a preferred address (assuming one of sufficient scope exists) to
   increase the likelihood that an address will remain valid for the
   duration of the communication.  It is up to system administrators to
   set appropriate prefix lifetimes in order to minimize the impact of
   failed communication when renumbering takes place.  The deprecation
   period should be long enough that most, if not all, communications
   are using the new address at the time an address becomes invalid.

   The IP layer is expected to provide a means for upper layers
   (including applications) to select the most appropriate source
   address given a particular destination and possibly other
   constraints.  An application may choose to select the source address
   itself before starting a new communication or may leave the address
   unspecified, in which case the upper networking layers will use the
   mechanism provided by the IP layer to choose a suitable address on
   the application's behalf.

   Detailed address selection rules are beyond the scope of this
   document.

5. PROTOCOL SPECIFICATION

   Autoconfiguration is performed on a per-interface basis on
   multicast-capable interfaces.  For multihomed hosts,
   autoconfiguration is performed independently on each interface.
   Autoconfiguration applies primarily to hosts, with two exceptions.
   Routers are expected to generate a link-local address using the
   procedure outlined below. In addition, routers perform Duplicate
   Address Detection on all addresses prior to assigning them to an
   interface.

5.1 Node Configuration Variables

   A node MUST allow the following autoconfiguration-related variable to
   be configured by system management for each multicast interface:

   DupAddrDetectTransmits

      The number of consecutive Neighbor Solicitation messages sent
      while performing Duplicate Address Detection on a tentative
      address. A value of zero indicates that Duplicate Address
      Detection is not performed on tentative addresses. A value of one
      indicates a single transmission with no follow up retransmissions.

      Default: 1, but may be overridden by a link-type specific value in
      the document that covers issues related to the transmission of IP



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      over a particular link type (e.g., IPv6 over Ethernet [2]).

      Autoconfiguration also assumes the presence of the variable
      RetransTimer as defined in RFC 2461 [5]. 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 [4] (of appropriate length) 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 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 [4] (see Section 2). These documents
   will carefully define the length so that link-local addresses can be



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   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:

   IP - Duplicate Address Detection MUST NOT be performed on anycast
      addresses.

   IP - 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 [10] [11]. 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
   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



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   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 to
   the tentative address containing 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 RFC 2461 [5]. A Neighbor Solicitation or Advertisement
   message that passes these validity checks is called a valid
   solicitation or valid advertisement, respectively.

5.4.2 Sending Neighbor Solicitation Messages

   Before sending a Neighbor Solicitation, an interface MUST join the
   all-nodes multicast address and the solicited-node multicast address
   of the tentative address. The former ensures that the node receives
   Neighbor Advertisements from other nodes already using the address;
   the latter ensures that two nodes attempting to use the same address
   simultaneously 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,



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   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 RFC
   2461 [5]. 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 MLD report message
   [12]. Since RFC 2710 [12] 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
   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



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   the receiving interface), the solicitation is processed as described
   in RFC 2461 [5].  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.


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



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   solicitation is processed as described in RFC 2461 [5]. 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
   (e.g., EUI-64), the interface SHOULD be disabled. 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 shutting down 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 duplicated link-local address is not formed
   from an interface identifier based on the hardware address, the
   interface MAY continue to be used.

5.5 Creation of Global Addresses

   Global addresses are formed by appending an interface identifier to a
   prefix of appropriate length. Prefixes are obtained from Prefix
   Information options contained in Router Advertisements. Creation of
   global addresses 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 RFC 2461 [5].

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.


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