





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


                IPv6 Stateless Address Autoconfiguration

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

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

Table of Contents

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



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

1.  INTRODUCTION

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

   IPv6 defines both a stateful and stateless address autoconfiguration
   mechanism. Stateless autoconfiguration requires no manual
   configuration of hosts, minimal (if any) configuration of routers,
   and no additional servers.  The stateless mechanism allows a host to
   generate its own addresses using a combination of locally available
   information and information advertised by routers. Routers advertise
   prefixes that identify the subnet(s) associated with a link, while
   hosts generate an "interface token" that uniquely identifies an
   interface on a subnet. An address is formed by combining the two. In
   the absence of routers, a host can only generate link-local
   addresses. However, link-local addresses are sufficient for allowing
   communication among nodes attached to the same link.

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



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

   The stateless approach is used when a site is not particularly
   concerned with the exact addresses hosts use, so long as they are
   unique and properly routable. The stateful approach is used when a
   site requires tighter control over exact address assignments.  Both
   stateful and stateless address autoconfiguration may be used
   simultaneously.  The site administrator specifies which type of
   autoconfiguration to use through the setting of appropriate fields in
   Router Advertisement messages [DISCOVERY].

   IPv6 addresses are leased to an interface for a fixed (possibly
   infinite) length of time. Each address has an associated lifetime
   that indicates how long the address is bound to an interface. When a
   lifetime expires, the binding (and address) become invalid and the
   address may be reassigned to another interface elsewhere in the
   Internet. To handle the expiration of address bindings gracefully, an
   address goes through two distinct phases while assigned to an
   interface. Initially, an address is "preferred", meaning that its use
   in arbitrary communication is unrestricted. Later, an address becomes
   "deprecated" in anticipation that its current interface binding will
   become invalid. While in a deprecated state, the use of an address is
   discouraged, but not strictly forbidden.  New communication (e.g.,
   the opening of a new TCP connection) should use a preferred address
   when possible.  A deprecated address should be used only by
   applications that have been using it and would have difficulty
   switching to another address without a service disruption.

   To insure that all configured addresses are likely to be unique on a
   given link, nodes run a "duplicate address detection" algorithm on
   addresses before assigning them to an interface.  The Duplicate
   Address Detection algorithm is performed on all addresses,
   independent of whether they are obtained via stateless or stateful
   autoconfiguration.  This document defines the Duplicate Address
   Detection algorithm.

   The autoconfiguration process specified in this document applies only
   to hosts and not routers. Since host autoconfiguration uses
   information advertised by routers, routers will need to be configured
   by some other means. However, it is expected that routers will
   generate link-local addresses using the mechanism described in this
   document. In addition, routers are expected to successfully pass the
   Duplicate Address Detection procedure described in this document on
   all addresses prior to assigning them to an interface.





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   Section 2 provides definitions for terminology used throughout this
   document. Section 3 describes the design goals that lead to the
   current autoconfiguration procedure. Section 4 provides an overview
   of the protocol, while Section 5 describes the protocol in detail.

2.  TERMINOLOGY

   IP          - Internet Protocol Version 6.  The terms IPv4 and IPv6
                 are used only in contexts where necessary to avoid
                 ambiguity.

   node        - a device that implements IP.

   router      - a node that forwards IP packets not explicitly
                 addressed to itself.

   host        - any node that is not a router.

   upper layer - a protocol layer immediately above IP.  Examples are
                 transport protocols such as TCP and UDP, control
                 protocols such as ICMP, routing protocols such as OSPF,
                 and internet or lower-layer protocols being "tunneled"
                 over (i.e., encapsulated in) IP such as IPX, AppleTalk,
                 or IP itself.

   link        - a communication facility or medium over which nodes can
                 communicate at the link layer, i.e., the layer
                 immediately below IP.  Examples are Ethernets (simple
                 or bridged); PPP links; X.25, Frame Relay, or ATM
                 networks; and internet (or higher) layer "tunnels",
                 such as tunnels over IPv4 or IPv6 itself.

   interface   - a node's attachment to a link.

   packet      - an IP header plus payload.

   address     - an IP-layer identifier for an interface or a set of
                 interfaces.

   unicast address
               - an identifier for a single interface. A packet sent to
                 a unicast address is delivered to the interface
                 identified by that address.

   multicast address
               - an identifier for a set of interfaces (typically
                 belonging to different nodes). A packet sent to a
                 multicast address is delivered to all interfaces



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                 identified by that address.

   anycast address
               - an identifier for a set of interfaces (typically
                 belonging to different nodes).  A packet sent to an
                 anycast address is delivered to one of the interfaces
                 identified by that address (the "nearest" one,
                 according to the routing protocol's measure of
                 distance).  See [ADDR-ARCH].

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

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

   link-local address
               - an address having link-only scope that can be used to
                 reach neighboring nodes attached to the same link.  All
                 interfaces have a link-local unicast address.

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

   global address
               - an address with unlimited scope.

   communication
               - any packet exchange among nodes that requires that the
                 address of each node used in the exchange remain the
                 same for the duration of the packet exchange. Examples
                 are a TCP connection or a UDP request-response.

   tentative address
               - an address whose uniqueness on a link is being
                 verified, prior to its assignment to an interface.  A
                 tentative address is not considered assigned to an
                 interface in the usual sense. An interface discards
                 received packets addressed to a tentative address, but
                 accepts Neighbor Discovery packets related to Duplicate
                 Address Detection for the tentative address.





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   preferred address
               - an address assigned to an interface whose use by upper
                 layer protocols is unrestricted. Preferred addresses
                 may be used as the source (or destination) address of
                 packets sent from (or to) the interface.

   deprecated address
               - An address assigned to an interface whose use is
                 discouraged, but not forbidden.  A deprecated address
                 should no longer be used as a source address in new
                 communications, but packets sent to deprecated
                 addresses are delivered as expected.  A deprecated
                 address may continue to be used as a source address in
                 communications where switching to a preferred address
                 causes hardship to a specific upper-layer activity
                 (e.g., an existing TCP connection).

   valid address
               - a preferred or deprecated address. A valid address may
                 appear as the source or destination address of a
                 packet, and the internet routing system is expected to
                 deliver packets sent to a valid address.

   invalid address
               - an address that is not assigned to any interface. A
                 valid address becomes invalid when its valid lifetime
                 expires.  Invalid addresses should not appear as the
                 destination or source address of a packet. In the
                 former case, the internet routing system will be unable
                 to deliver the packet, in the later case the recipient
                 of the packet will be unable to respond to it.

   preferred lifetime
               - the length of time that a valid address is preferred
                 (i.e., the time until deprecation). When the preferred
                 lifetime expires, the address becomes deprecated.

   valid lifetime
               - the length of time an address remains in the valid
                 state (i.e., the time until invalidation). The valid
                 lifetime must be greater then or equal to the preferred
                 lifetime.  When the valid lifetime expires, the address
                 becomes invalid.

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



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                 prefix to form an address. From address
                 autoconfiguration's perspective, an interface token is
                 a bit string of known length.  The exact length of an
                 interface token and the way it is created is defined in
                 a separate link-type specific document that covers
                 issues related to the transmission of IP over a
                 particular link type (e.g., [IPv6-ETHER]).  In many
                 cases, the token will be the same as the interface's
                 link-layer address.

2.1.  Requirements

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

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

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

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

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

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









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3.  DESIGN GOALS

   Stateless autoconfiguration is designed with the following goals in
   mind:

   o Manual configuration of individual machines before connecting them
     to the network should not be required. Consequently, a mechanism is
     needed that allows a host to obtain or create unique addresses for
     each of its interfaces. Address autoconfiguration assumes that each
     interface can provide a unique identifier for that interface (i.e.,
     an "interface token").  In the simplest case, an interface token
     consists of the interface's link-layer address. An interface token
     can be combined with a prefix to form an address.

   o Small sites consisting of a set of machines attached to a single
     link should not require the presence of a stateful server or router
     as a prerequisite for communicating.  Plug-and-play communication
     is achieved through the use of link-local addresses.  Link-local
     addresses have a well-known prefix that identifies the (single)
     shared link to which a set of nodes attach. A host forms a link-
     local address by appending its interface token to the link-local
     prefix.

   o A large site with multiple networks and routers should not require
     the presence of a stateful address configuration server. In order
     to generate site-local or global addresses, hosts must determine
     the prefixes that identify the subnets to which they attach.
     Routers generate periodic Router Advertisements that include
     options listing the set of active prefixes on a link.

   o Address configuration should facilitate the graceful renumbering of
     a site's machines. For example, a site may wish to renumber all of
     its nodes when it switches to a new network service provider.
     Renumbering is achieved through the leasing of addresses to
     interfaces and the assignment of multiple addresses to the same
     interface.  Lease lifetimes provide the mechanism through which a
     site phases out old prefixes.  The assignment of multiple addresses
     to an interface provides for a transition period during which both
     a new address and the one being phased out work simultaneously.

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







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4.  PROTOCOL OVERVIEW

   This section provides an overview of the typical steps that take
   place when an interface autoconfigures itself.  Autoconfiguration is
   performed only on multicast-capable links and begins when a
   multicast-capable interface is enabled, e.g., during system startup.
   Nodes (both hosts and routers) begin the autoconfiguration process by
   generating a link-local address for the interface. A link-local
   address is formed by appending the interface's token to the well-
   known link-local prefix.

   Before the link-local address can be assigned to an interface and
   used, however, a node must attempt to verify that this "tentative"
   address is not already in use by another node on the link.
   Specifically, it sends a Neighbor Solicitation message containing the
   tentative address as the target. If another node is already using
   that address, it will return a Neighbor Advertisement saying so. If
   another node is also attempting to use the same address, it will send
   a Neighbor Solicitation for the target as well. The exact number of
   times the Neighbor Solicitation is (re)transmitted and the delay time
   between consecutive solicitations is link-specific and may be set by
   system management.

   If a node determines that its tentative link-local address is not
   unique, autoconfiguration stops and manual configuration of the
   interface is required.  To simplify recovery in this case, it should
   be possible for an administrator to supply an alternate interface
   token that overrides the default token in such a way that the
   autoconfiguration mechanism can then be applied using the new
   (presumably unique) interface token.  Alternatively, link-local and
   other addresses will need to be configured manually.

   Once a node ascertains that its tentative link-local address is
   unique, it assigns it to the interface. At this point, the node has
   IP-level connectivity with neighboring nodes.  The remaining
   autoconfiguration steps are performed only by hosts; the
   (auto)configuration of routers is beyond the scope of this document.

   The next phase of autoconfiguration involves obtaining a Router
   Advertisement or determining that no routers are present. If routers
   are present, they will send Router Advertisements that specify what
   sort of autoconfiguration a host should do.  If no routers are
   present, stateful autoconfiguration should be invoked.

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



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

   Router Advertisements also contain zero or more Prefix Information
   options that contain information used by stateless address
   autoconfiguration to generate site-local and global addresses.  It
   should be noted that the stateless and stateful address
   autoconfiguration fields in Router Advertisements are processed
   independently of one another, and a host may use both stateful and
   stateless address autoconfiguration simultaneously.  One Prefix
   Information option field, the "autonomous address-configuration
   flag", indicates whether or not the option even applies to stateless
   autoconfiguration.  If it does, additional option fields contain a
   subnet prefix together with lifetime values indicating how long
   addresses created from the prefix remain preferred and valid.

   Because routers generate Router Advertisements periodically, hosts
   will continually receive new advertisements. Hosts process the
   information contained in each advertisement as described above,
   adding to and refreshing information received in previous
   advertisements.

   For safety, all addresses must be tested for uniqueness prior to
   their assignment to an interface.  In the case of addresses created
   through stateless autoconfig, however, the uniqueness of an address
   is determined primarily by the portion of the address formed from an
   interface token.  Thus, if a node has already verified the uniqueness
   of a link-local address, additional addresses created from the same
   interface token need not be tested individually. In contrast, all
   addresses obtained manually or via stateful address autoconfiguration
   should be tested for uniqueness individually. To accommodate sites
   that believe the overhead of performing Duplicate Address Detection
   outweighs its benefits, the use of Duplicate Address Detection can be
   disabled through the administrative setting of a per-interface
   configuration flag.

   To speed the autoconfiguration process, a host may generate its
   link-local address (and verify its uniqueness) in parallel with
   waiting for a Router Advertisement. Because a router may delay
   responding to a Router Solicitation for a few seconds, the total time
   needed to complete autoconfiguration can be significantly longer if
   the two steps are done serially.



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4.1.  Site Renumbering

   Address leasing facilitates site renumbering by providing a mechanism
   to time-out addresses assigned to interfaces in hosts.  At present,
   upper layer protocols such as TCP provide no support for changing
   end-point addresses while a connection is open. If an end-point
   address becomes invalid, existing connections break and all
   communication to the invalid address fails.  Even when applications
   use UDP as a transport protocol, addresses must generally remain the
   same during a packet exchange.

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

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

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

5.  PROTOCOL SPECIFICATION

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





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5.1.  Node Configuration Variables

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

     DupAddrDetectTransmits

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

                    Default: 1, but may be overridden by a link-type
                    specific value in the document that covers issues
                    related to the transmission of IP over a particular
                    link type (e.g., [IPv6-ETHER]).

   Autoconfiguration also assumes the presence of the variable
   RetransTimer as defined in [DISCOVERY]. For autoconfiguration
   purposes, RetransTimer specifies the delay between consecutive
   Neighbor Solicitation transmissions performed during Duplicate
   Address Detection (if DupAddrDetectTransmits is greater than 1), as
   well as the time a node waits  after sending the last Neighbor
   Solicitation before ending the Duplicate Address Detection process.

5.2.  Autoconfiguration-Related Variables

   A host maintains a number of data structures and flags related to
   autoconfiguration. In the following, we present conceptual variables
   and show how they are used to perform autoconfiguration. The specific
   variables are used for demonstration purposes only, and an
   implementation is not required to have them, so long as its external
   behavior is consistent with that described in this document.

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

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


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



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

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

   A host also maintains a list of addresses together with their
   corresponding lifetimes. The address list contains both
   autoconfigured addresses and those configured manually.

5.3.  Creation of Link-Local Addresses

   A node forms a link-local address whenever an interface becomes
   enabled.  An interface may become enabled after any of the following
   events:

   - The interface is initialized at system startup time.

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

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

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

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

   A link-local address has an infinite preferred and valid lifetime; it
   is never timed out.

5.4.  Duplicate Address Detection

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



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

   The procedure for detecting duplicate addresses uses Neighbor
   Solicitation and Advertisement messages as described below. If a
   duplicate address is discovered during the procedure, the address
   cannot be assigned to the interface. If the address is derived from
   an interface token, a new token will need to be assigned to the
   interface, or all IP addresses for the interface will need to be
   manually configured.  Note that the method for detecting duplicates
   is not completely reliable, and it is possible that duplicate
   addresses will still exist (e.g., if the link was partitioned while
   Duplicate Address Detection was performed).

   An address on which the duplicate Address Detection Procedure is
   applied is said to be tentative until the procedure has completed
   successfully.  A tentative address is not considered "assigned to an
   interface" in the traditional sense. That is, the interface must
   accept Neighbor Solicitation and Advertisement messages containing
   the tentative address in the Target Address field, but processes such
   packets differently from those whose Target Address matches an
   address assigned to the interface.  Other packets addressed to the
   tentative address should be silently discarded.

   It should also be noted that Duplicate Address Detection must be
   performed prior to assigning an address to an interface in order to
   prevent multiple nodes from using the same address simultaneously.
   If a node begins using an address in parallel with Duplicate Address
   Detection, and another node is already using the address, the node
   performing Duplicate Address Detection will erroneously process
   traffic intended for the other node, resulting in such possible
   negative consequences as the resetting of open TCP connections.

   The following subsections describe specific tests a node performs to
   verify an address's uniqueness.  An address is considered unique if
   none of the tests indicate the presence of a duplicate address within
   RetransTimer milliseconds after having sent DupAddrDetectTransmits
   Neighbor Solicitations. Once an address is determined to be unique,



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   it may be assigned to an interface.

5.4.1.  Message Validation

   A node MUST silently discard any Neighbor Solicitation or
   Advertisement message that does not pass the validity checks
   specified in [DISCOVERY].  A solicitation that passes these validity
   checks is called a valid solicitation or valid advertisement.

5.4.2.  Sending Neighbor Solicitation Messages

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

   To check an address, a node sends DupAddrDetectTransmits Neighbor
   Solicitations, each separated by RetransTimer milliseconds. The
   solicitation's Target Address is set to the address being checked,
   the IP source is set to the unspecified address and the IP
   destination is set to the solicited-node multicast address of the
   target address.

   If the Neighbor Solicitation is the first message to be sent from an
   interface after interface (re)initialization, the node should delay
   sending the message by a random delay between 0 and
   MAX_RTR_SOLICITATION_DELAY as specified in [DISCOVERY].  This serves
   to alleviate congestion when many nodes start up on the link at the
   same time, such as after a power failure, and may help to avoid race
   conditions when more than one node is trying to solicit for the same
   address at the same time. In order to improve the robustness of the
   Duplicate Address Detection algorithm, an interface MUST receive and
   process datagrams sent to the all-nodes multicast address or
   solicited-node multicast address of the tentative address while
   delaying transmission of the initial Neighbor Solicitation.

5.4.3.  Receiving Neighbor Solicitation Messages

   On receipt of a valid Neighbor Solicitation message on an interface,
   node behavior depends on whether the target address is tentative or
   not.  If the target address is not tentative (i.e., it is assigned to
   the receiving interface), the solicitation is processed as described
   in [DISCOVERY].  If the target address is tentative, and the source
   address is a unicast address, the solicitation's sender is performing
   address resolution on the target; the solicitation should be silently
   ignored.  Otherwise, processing takes place as described below. In



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   all cases, a node MUST NOT respond to a Neighbor Solicitation for a
   tentative address.

   If the source address of the Neighbor Solicitation is the unspecified
   address, the solicitation is from a node performing Duplicate Address
   Detection. If the solicitation is from another node, the tentative
   address is a duplicate and should not be used (by either node). If
   the solicitation is from the node itself (because the node loops back
   multicast packets), the solicitation does not indicate the presence
   of a duplicate address.

   Implementor's Note: many interfaces provide a way for upper layers to
   selectively enable and disable the looping back of multicast packets.
   The details of how such a facility is implemented may prevent
   Duplicate Address Detection from working correctly.  See the Appendix
   for further discussion.

   The following tests identify conditions under which a tentative
   address is not unique:

   - If a Neighbor Solicitation for a tentative address is received
     prior to having sent one, the tentative address is a duplicate.
     This condition occurs when two nodes run Duplicate Address
     Detection simultaneously, but transmit initial solicitations at
     different times (e.g., by selecting different random delay values
     before transmitting an initial solicitation).

   - If the actual number of Neighbor Solicitations received exceeds the
     number expected based on the loopback semantics (e.g., the
     interface does not loopback packet, yet one or more solicitations
     was received), the tentative address is a duplicate. This condition
     occurs when two nodes run Duplicate Address Detection
     simultaneously and transmit solicitations at roughly the same time.

5.4.4.  Receiving Neighbor Advertisement Messages

   On receipt of a valid Neighbor Advertisement message on an interface,
   node behavior depends on whether the target address is tentative or
   matches a unicast or anycast address assigned to the interface.  If
   the target address is assigned to the receiving interface, the
   solicitation is processed as described in [DISCOVERY]. If the target
   address is tentative, the tentative address is not unique.

5.4.5.  When Duplicate Address Detection Fails

   A tentative address that is determined to be a duplicate as described
   above, MUST NOT be assigned to an interface and the node SHOULD log a
   system management error.  If the address is a link-local address



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   formed from an interface token, the interface SHOULD be disabled.

5.5.  Creation of Global and Site-Local Addresses

   Global and site-local addresses are formed by appending an interface
   token to a prefix of appropriate length. Prefixes are obtained from
   Prefix Information options contained in Router Advertisements.
   Creation of global and site-local addresses and configuration of
   other parameters as described in this section SHOULD be locally
   configurable. However, the processing described below MUST be enabled
   by default.

5.5.1.  Soliciting Router Advertisements

   Router Advertisements are sent periodically to the all-nodes
   multicast address. To obtain an advertisement quickly, a host sends
   out Router Solicitations as described in [DISCOVERY].

5.5.2.  Absence of Router Advertisements

   If a link has no routers, a host MUST attempt to use stateful
   autoconfiguration to obtain addresses and other configuration
   information. An implementation MAY provide a way to disable the
   invocation of stateful autoconfiguration in this case, but the
   default SHOULD be enabled.  From the perspective of
   autoconfiguration, a link has no routers if no Router Advertisements
   are received after having sent a small number of Router Solicitations
   as described in [DISCOVERY].

5.5.3.  Router Advertisement Processing

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

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



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

   For each Prefix-Information option in the Router Advertisement:

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

 b) If the prefix is the link-local prefix, silently ignore the Prefix
    Information option.

 c) If the preferred lifetime is greater than the valid lifetime,
    silently ignore the Prefix Information option. A node MAY wish to
    log a system management error in this case.

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

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

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


    If the sum of the prefix length and interface token length does not
    equal 128 bits, the Prefix Information option MUST be ignored. An
    implementation MAY wish to log a system management error in this
    case. It is the responsibility of the system administrator to insure
    that the lengths of prefixes contained in Router Advertisements are
    consistent with the length of interface tokens for that link type.

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




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    If an address is formed successfully, the host adds it to the list
    of addresses assigned to the interface, initializing its preferred
    and valid lifetime values from the Prefix Information option.

5.5.4.  Address Lifetime Expiry

   A preferred address becomes deprecated when its preferred lifetime
   expires.  A deprecated address SHOULD continue to be used as a source
   address in existing communications, but SHOULD NOT be used in new
   communications if an alternate (non-deprecated) address is available
   and has sufficient scope.  The IP layer MUST continue to accept
   datagrams destined to a deprecated address since a deprecated address
   is still a valid address for the interface. An implementation MAY
   prevent any new communication from using a deprecated address, but
   system management MUST have the ability to disable such a facility.

   An address (and its association with an interface) becomes invalid
   when its valid lifetime expires.  An invalid address MUST NOT be used
   as a source address in outgoing communications and MUST NOT be
   recognized as a destination on a receiving interface.

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

5.6.  Configuration Consistency

   It is possible for hosts to obtain address information using both
   stateless and stateful protocols since both may be enabled at the
   same time.  It is also possible that the values of other
   configuration parameters such as MTU size and hop limit will be
   learned from both Router Advertisements and the stateful
   autoconfiguration protocol.  If the same configuration information is
   provided by multiple sources, the value of this information should be
   consistent. However, it is not considered a fatal error if
   information received from multiple sources is inconsistent. Hosts
   accept the union of all information received via the stateless and
   stateful protocols. If inconsistent information is learned from
   different sources, the most recently obtained values always have
   precedence over information learned earlier.

SECURITY CONSIDERATIONS

   Stateless address autoconfiguration allows a host to connect to a
   network, configure an address and start communicating with other
   nodes without ever registering or authenticating itself with the



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   local site.  Although this allows unauthorized users to connect to
   and use a network, the threat is inherently present in the Internet
   architecture. Any node with a physical attachment to a network can
   generate an address (using a variety of ad hoc techniques) that
   provides connectivity.

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

REFERENCES

   [RFC1826] Atkinson, R., "IP Authentication Header", RFC 1826, August
             1995.

   [IPv6-ETHER] Crawford, M., "A Method for the Transmission of IPv6
             Packets over Ethernet Networks", RFC 1972, August 1996.

   [RFC1112] Deering, S., "Host Extensions for IP Multicasting", STD 5,
             RFC 1112, August 1989.

   [ADDR-ARCH] Hinden, R., and S. Deering, "Internet Protocol Version
             (IPv6) Addressing Architecture", RFC 1884, December 1995.

   [DHCPv6]  Work in Progress.

   [DISCOVERY] Narten, T., Nordmark, E., and W. Simpson, "Neighbor
             Discovery for IP Version 6 (IPv6)", RFC 1970, August 1996.

Acknowledgements

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













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AUTHORS' ADDRESSES

   Susan Thomson
   Bellcore
   445 South Street
   Morristown, NJ 07960
   USA

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


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

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































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APPENDIX: LOOPBACK SUPPRESSION & DUPLICATE ADDRESS DETECTION

   Determining whether a received multicast solicitation was looped back
   to the sender or actually came from another node is implementation-
   dependent.  A problematic case occurs when two interfaces attached to
   the same link happen to have the same token and link-layer address,
   and they both send out packets with identical contents at roughly the
   same time (e.g., Neighbor Solicitations for a tentative address as
   part of Duplicate Address Detection messages).  Although a receiver
   will receive both packets, it cannot determine which packet was
   looped back and which packet came from the other node by simply
   comparing packet contents (i.e., the contents are identical). In this
   particular case, it is not necessary to know precisely which packet
   was looped back and which was sent by another node; if one receives
   more solicitations than were sent, the tentative address is a
   duplicate. However, the situation may not always be this
   straightforward.

   The IPv4 multicast specification [RFC1112] recommends that the
   service interface provide a way for an upper-layer protocol to
   inhibit local delivery of packets sent to a multicast group that the
   sending host is a member of. Some applications know that there will
   be no other group members on the same host, and suppressing loopback
   prevents them from having to receive (and discard) the packets they
   themselves send out.  A straightforward way to implement this
   facility is to disable loopback at the hardware level (if supported
   by the hardware), with packets looped back (if requested) by
   software.  On interfaces in which the hardware itself suppresses
   loopbacks, a node running Duplicate Address Detection simply counts
   the number of Neighbor Solicitations received for a tentative address
   and compares them with the number expected. If there is a mismatch,
   the tentative address is a duplicate.

   In those cases where the hardware cannot suppress loopbacks, however,
   one possible software heuristic to filter out unwanted loopbacks is
   to discard any received packet whose link-layer source address is the
   same as the receiving interface's.  Unfortunately, use of that
   criteria also results in the discarding of all packets sent by
   another node using the same link-layer address. Duplicate Address
   Detection will fail on interfaces that filter received packets in
   this manner:

   o If a node performing Duplicate Address Detection discards received
     packets having the same source link-layer address as the receiving
     interface, it will also discard packets from other nodes also using
     the same link-layer address, including Neighbor Advertisement and
     Neighbor Solicitation messages required to make Duplicate Address
     Detection work correctly.  This particular problem can be avoided



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     by temporarily disabling the software suppression of loopbacks
     while a node performs Duplicate Address Detection.

   o If a node that is already using a particular IP address discards
     received packets having the same link-layer source address as the
     interface, it will also discard Duplicate Address Detection-related
     Neighbor Solicitation messages sent by another node also using the
     same link-layer address.  Consequently, Duplicate Address Detection
     will fail, and the other node will configure a non-unique address.
     Since it is generally impossible to know when another node is
     performing Duplicate Address Detection, this scenario can be
     avoided only if software suppression of loopback is permanently
     disabled.

   Thus, to perform Duplicate Address Detection correctly in the case
   where two interfaces are using the same link-layer address, an
   implementation must have a good understanding of the interface's
   multicast loopback semantics, and the interface cannot discard
   received packets simply because the source link-layer address is the
   same as the interfaces.































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