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rfc:rfc1971

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

Thomson & Narten Standards Track [Page 1] RFC 1971 IPv6 Stateless Address Autoconfiguration August 1996

       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

Thomson & Narten Standards Track [Page 2] RFC 1971 IPv6 Stateless Address Autoconfiguration August 1996

 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.

Thomson & Narten Standards Track [Page 3] RFC 1971 IPv6 Stateless Address Autoconfiguration August 1996

 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

Thomson & Narten Standards Track [Page 4] RFC 1971 IPv6 Stateless Address Autoconfiguration August 1996

               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.

Thomson & Narten Standards Track [Page 5] RFC 1971 IPv6 Stateless Address Autoconfiguration August 1996

 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

Thomson & Narten Standards Track [Page 6] RFC 1971 IPv6 Stateless Address Autoconfiguration August 1996

               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.

Thomson & Narten Standards Track [Page 7] RFC 1971 IPv6 Stateless Address Autoconfiguration August 1996

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.

Thomson & Narten Standards Track [Page 8] RFC 1971 IPv6 Stateless Address Autoconfiguration August 1996

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

Thomson & Narten Standards Track [Page 9] RFC 1971 IPv6 Stateless Address Autoconfiguration August 1996

 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.

Thomson & Narten Standards Track [Page 10] RFC 1971 IPv6 Stateless Address Autoconfiguration August 1996

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.

Thomson & Narten Standards Track [Page 11] RFC 1971 IPv6 Stateless Address Autoconfiguration August 1996

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

Thomson & Narten Standards Track [Page 12] RFC 1971 IPv6 Stateless Address Autoconfiguration August 1996

                  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:
  1. The interface is initialized at system startup time.
  1. The interface is reinitialized after a temporary interface failure

or after being temporarily disabled by system management.

  1. The interface attaches to a link for the first time.
  1. 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.

Thomson & Narten Standards Track [Page 13] RFC 1971 IPv6 Stateless Address Autoconfiguration August 1996

 (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,

Thomson & Narten Standards Track [Page 14] RFC 1971 IPv6 Stateless Address Autoconfiguration August 1996

 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

Thomson & Narten Standards Track [Page 15] RFC 1971 IPv6 Stateless Address Autoconfiguration August 1996

 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:
  1. 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).
  1. 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

Thomson & Narten Standards Track [Page 16] RFC 1971 IPv6 Stateless Address Autoconfiguration August 1996

 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

Thomson & Narten Standards Track [Page 17] RFC 1971 IPv6 Stateless Address Autoconfiguration August 1996

 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.

Thomson & Narten Standards Track [Page 18] RFC 1971 IPv6 Stateless Address Autoconfiguration August 1996

  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

Thomson & Narten Standards Track [Page 19] RFC 1971 IPv6 Stateless Address Autoconfiguration August 1996

 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.

Thomson & Narten Standards Track [Page 20] RFC 1971 IPv6 Stateless Address Autoconfiguration August 1996

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

Thomson & Narten Standards Track [Page 21] RFC 1971 IPv6 Stateless Address Autoconfiguration August 1996

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

Thomson & Narten Standards Track [Page 22] RFC 1971 IPv6 Stateless Address Autoconfiguration August 1996

   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.

Thomson & Narten Standards Track [Page 23]

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