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

IETF IPv6 Working Group                                       S. Thomson
Internet-Draft                                                     Cisco
Expires: February 9, 2005                                      T. Narten
                                                                     IBM
                                                               T. Jinmei
                                                                 Toshiba
                                                         August 11, 2004


                IPv6 Stateless Address Autoconfiguration
                   draft-ietf-ipv6-rfc2462bis-05.txt

Status of this Memo

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   or will be disclosed, and any of which I become aware will be
   disclosed, in accordance with RFC 3668.

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   This Internet-Draft will expire on February 9, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   This document specifies the steps a host takes in deciding how to
   autoconfigure its interfaces in IP version 6.  The autoconfiguration
   process includes creating a link-local address and verifying its
   uniqueness on a link, determining what information can be
   autoconfigured (addresses, other information, or both), and in the
   case of addresses, whether they can be obtained through the stateless



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   mechanism, the stateful mechanism, or both.  This document defines
   the process for generating a link-local address, the process for
   generating global addresses via stateless address autoconfiguration,
   and the Duplicate Address Detection procedure.  The details of
   autoconfiguration using the stateful protocol is specified in RFC
   3315 and RFC 3736.

Table of Contents

   1.  INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  TERMINOLOGY  . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1   Requirements . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  DESIGN GOALS . . . . . . . . . . . . . . . . . . . . . . . . .  7
   4.  PROTOCOL OVERVIEW  . . . . . . . . . . . . . . . . . . . . . .  8
     4.1   Site Renumbering . . . . . . . . . . . . . . . . . . . . . 10
   5.  PROTOCOL SPECIFICATION . . . . . . . . . . . . . . . . . . . . 11
     5.1   Node Configuration Variables . . . . . . . . . . . . . . . 11
     5.2   Autoconfiguration-Related Structures . . . . . . . . . . . 12
     5.3   Creation of Link-Local Addresses . . . . . . . . . . . . . 12
     5.4   Duplicate Address Detection  . . . . . . . . . . . . . . . 13
       5.4.1   Message Validation . . . . . . . . . . . . . . . . . . 14
       5.4.2   Sending Neighbor Solicitation Messages . . . . . . . . 14
       5.4.3   Receiving Neighbor Solicitation Messages . . . . . . . 16
       5.4.4   Receiving Neighbor Advertisement Messages  . . . . . . 17
       5.4.5   When Duplicate Address Detection Fails . . . . . . . . 17
     5.5   Creation of Global Addresses . . . . . . . . . . . . . . . 18
       5.5.1   Soliciting Router Advertisements . . . . . . . . . . . 18
       5.5.2   Absence of Router Advertisements . . . . . . . . . . . 18
       5.5.3   Router Advertisement Processing  . . . . . . . . . . . 18
       5.5.4   Address Lifetime Expiry  . . . . . . . . . . . . . . . 20
     5.6   Configuration Consistency  . . . . . . . . . . . . . . . . 21
     5.7   Retaining Configured Addresses for Stability . . . . . . . 22
   6.  SECURITY CONSIDERATIONS  . . . . . . . . . . . . . . . . . . . 22
   7.  IANA CONSIDERATIONS  . . . . . . . . . . . . . . . . . . . . . 23
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
   9.1   Normative References . . . . . . . . . . . . . . . . . . . . 23
   9.2   Informative References . . . . . . . . . . . . . . . . . . . 23
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 24
   A.  LOOPBACK SUPPRESSION & DUPLICATE ADDRESS DETECTION . . . . . . 25
   B.  CHANGES SINCE RFC 1971 . . . . . . . . . . . . . . . . . . . . 26
   C.  CHANGES SINCE RFC 2462 . . . . . . . . . . . . . . . . . . . . 27
   D.  CHANGE HISTORY . . . . . . . . . . . . . . . . . . . . . . . . 28
       Intellectual Property and Copyright Statements . . . . . . . . 31







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1.  INTRODUCTION

   This document specifies the steps a host takes in deciding how to
   autoconfigure its interfaces in IP version 6 (IPv6).  The
   autoconfiguration process includes creating a link-local address and
   verifying its uniqueness on a link, determining what information can
   be autoconfigured (addresses, other information, or both), and in the
   case of addresses, whether they can be obtained through the stateless
   mechanism, the stateful mechanism, or both.  This document defines
   the process for generating a link-local address, the process for
   generating global addresses via stateless address autoconfiguration,
   and the Duplicate Address Detection procedure.  The details of
   autoconfiguration using the stateful protocol is specified in
   [RFC3315] and [RFC3736].

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

   In the stateful autoconfiguration model, hosts obtain interface
   addresses and/or configuration information and parameters from a
   Dynamic Host Configuration Protocol (DHCPv6) server.  Servers
   maintain a database that keeps track of which addresses have been
   assigned to which hosts.  The stateful autoconfiguration protocol
   allows hosts to obtain addresses, other configuration information or
   both from a server.  Stateless and stateful autoconfiguration
   complement each other.  For example, a host can use stateless
   autoconfiguration to configure its own addresses, but use stateful
   autoconfiguration to obtain other information.

   To obtain other configuration information without configuring
   addresses in the stateful autoconfiguration model, a subset of DHCPv6
   [RFC3736] will be used.  While the model is called "stateful" here in
   order to highlight the contrast to the stateless protocol defined in
   this document, the intended protocol is also defined to work in a
   stateless fashion.  This is based on a result, through operational
   experiments, that all known "other" configuration information can be
   managed by a stateless server, that is, a server that does not
   maintain state of each client that the server provides with the



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   configuration information.

   The stateless approach is used when a site is not particularly
   concerned with the exact addresses hosts use, so long as they are
   unique and properly routable.  The stateful approach is used when a
   site requires tighter control over exact address assignments.  Both
   stateful and stateless address autoconfiguration may be used
   simultaneously.  The site administrator specifies which type of
   autoconfiguration is available through the setting of appropriate
   fields in Router Advertisement messages [RFC2461].

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

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

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

   Section 2 provides definitions for terminology used throughout this
   document.  Section 3 describes the design goals that lead to the
   current autoconfiguration procedure.  Section 4 provides an overview



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

2.  TERMINOLOGY

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

   node - a device that implements IP.

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

   host - any node that is not a router.

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

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

   interface - a node's attachment to a link.

   packet - an IP header plus payload.

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

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

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

   anycast address - an identifier for a set of interfaces (typically
      belonging to different nodes).  A packet sent to an anycast
      address is delivered to one of the interfaces identified by that
      address (the "nearest" one, according to the routing protocol's
      measure of distance).  See [RFC3513].





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   solicited-node multicast address - a multicast address to which
      Neighbor Solicitation messages are sent.  The algorithm for
      computing the address is given in [RFC3513].

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

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

   global address - an address with unlimited scope.

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

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

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

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

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







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

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

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

   interface identifier - a link-dependent identifier for an interface
      that is (at least) unique per link [RFC3513].  Stateless address
      autoconfiguration combines an interface identifier with a prefix
      to form an address.  From address autoconfiguration's perspective,
      an interface identifier is a bit string of known length.  The
      exact length of an interface identifier and the way it is created
      is defined in a separate link-type specific document that covers
      issues related to the transmission of IP over a particular link
      type (e.g., [RFC2464]).  Note that the address architecture
      [RFC3513] also defines the length of the interface identifiers for
      some set of addresses, but the two sets of definitions must be
      consistent.  In many cases, the identifier will be derived from
      the interface's link-layer address.

2.1  Requirements

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [RFC2119].

3.  DESIGN GOALS

   Stateless autoconfiguration is designed with the following goals in
   mind:

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



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      link-layer address.  An interface identifier can be combined with
      a prefix to form an address.

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

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

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

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

4.  PROTOCOL OVERVIEW

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

   Before the link-local address can be assigned to an interface and
   used, however, a node must attempt to verify that this "tentative"



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

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

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

   The next phase of autoconfiguration involves obtaining a Router
   Advertisement or determining that no routers are present.  If routers
   are present, they will send Router Advertisements that specify what
   sort of autoconfiguration a host can do.  Note that stateful
   autoconfiguration may still be available even if no routers are
   present.

   Routers send Router Advertisements periodically, but the delay
   between successive advertisements will generally be longer than a
   host performing autoconfiguration will want to wait [RFC2461].  To
   obtain an advertisement quickly, a host sends one or more Router
   Solicitations to the all-routers multicast group.  Router
   Advertisements contain two flags indicating what type of stateful
   autoconfiguration (if any) is available.  A "managed address
   configuration (M)" flag indicates whether hosts can use stateful
   autoconfiguration [RFC3315] to obtain addresses.  An "other stateful
   configuration (O)" flag indicates whether hosts can use stateful
   autoconfiguration [RFC3736] to obtain additional information
   (excluding addresses).

   The details of how a host may use the M flag, including any use of
   the "on" and "off" transitions for this flag, to control the use of
   the stateful protocol for address assignment will be described in a



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   separate document.  Similarly, the details of how a host may use the
   O flag, including any use of the "on" and "off" transitions for this
   flag, to control the use of the stateful protocol for getting other
   configuration information will be described in a separate document.

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

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

   For safety, all addresses must be tested for uniqueness prior to
   their assignment to an interface.  The test should individually be
   performed on all addresses obtained manually, via stateless address
   autoconfiguration, or via stateful address autoconfiguration.  To
   accommodate sites that believe the overhead of performing Duplicate
   Address Detection outweighs its benefits, the use of Duplicate
   Address Detection can be disabled through the administrative setting
   of a per-interface configuration flag.

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

4.1  Site Renumbering

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



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   use UDP as a transport protocol, addresses must generally remain the
   same during a packet exchange.

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

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

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

5.  PROTOCOL SPECIFICATION

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

5.1  Node Configuration Variables

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

   DupAddrDetectTransmits

      The number of consecutive Neighbor Solicitation messages sent
      while performing Duplicate Address Detection on a tentative



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      address.  A value of zero indicates that Duplicate Address
      Detection is not performed on tentative addresses.  A value of one
      indicates a single transmission with no follow up retransmissions.

      Default: 1, but may be overridden by a link-type specific value in
      the document that covers issues related to the transmission of IP
      over a particular link type (e.g., [RFC2464]).

      Autoconfiguration also assumes the presence of the variable
      RetransTimer as defined in [RFC2461].  For autoconfiguration
      purposes, RetransTimer specifies the delay between consecutive
      Neighbor Solicitation transmissions performed during Duplicate
      Address Detection (if DupAddrDetectTransmits is greater than 1),
      as well as the time a node waits after sending the last Neighbor
      Solicitation before ending the Duplicate Address Detection
      process.

5.2  Autoconfiguration-Related Structures

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

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

5.3  Creation of Link-Local Addresses

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

   -  The interface is initialized at system startup time.

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

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

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

   A link-local address is formed by prepending the well-known link-
   local prefix FE80::0 [RFC3513] (of appropriate length not less than
   10 bits) to the interface identifier.  If the interface identifier
   has a length of N bits, the interface identifier replaces the
   right-most N zero bits of the link-local prefix.  If the interface



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   identifier is more than 118 bits in length, autoconfiguration fails
   and manual configuration is required.  The length of the interface
   identifier is defined in a separate link-type specific document,
   which should also be consistent with the address architecture
   [RFC3513] (see Section 2).  These documents will carefully define the
   length so that link-local addresses can be autoconfigured on the
   link.

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

5.4  Duplicate Address Detection

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

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

   -  Each individual unicast address SHOULD be tested for uniqueness.
      Note that there are implementations deployed that only perform
      Duplicate Address Detection for the link-local address and skip
      the test for the global address using the same interface
      identifier as that of the link-local address.  Whereas this
      document does not invalidate such implementations, this kind of
      "optimization" is NOT RECOMMENDED, and new implementations MUST
      NOT do that optimization.  This optimization came from the
      assumption that all of an interface's addresses are generated from
      the same identifier.  However, the assumption does actually not
      stand; new types of addresses have been introduced where the
      interface identifiers are not necessarily the same for all unicast
      addresses on a single interface [RFC3041][I-D.ietf-send-cga].
      Requiring to perform Duplicate Address Detection for all unicast
      addresses will make the algorithm robust for the current and
      future such special interface identifiers.

   The procedure for detecting duplicate addresses uses Neighbor
   Solicitation and Advertisement messages as described below.  If a
   duplicate address is discovered during the procedure, the address
   cannot be assigned to the interface.  If the address is derived from
   an interface identifier, a new identifier will need to be assigned to
   the interface, or all IP addresses for the interface will need to be
   manually configured.  Note that the method for detecting duplicates
   is not completely reliable, and it is possible that duplicate



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   addresses will still exist (e.g., if the link was partitioned while
   Duplicate Address Detection was performed).

   An address on which the Duplicate Address Detection procedure is
   applied is said to be tentative until the procedure has completed
   successfully.  A tentative address is not considered "assigned to an
   interface" in the traditional sense.  That is, the interface must
   accept Neighbor Solicitation and Advertisement messages containing
   the tentative address in the Target Address field, but processes such
   packets differently from those whose Target Address matches an
   address assigned to the interface.  Other packets addressed to the
   tentative address should be silently discarded.  Note that the "other
   packets" include Neighbor Solicitation and Advertisement messages
   which have the tentative (i.e., unicast) address as the IP
   destination address and contain the tentative address in the Target
   Address field.  Such a case should not happen in normal operation,
   though, since these messages are multicasted in the Duplicate Address
   Detection procedure.

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

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

5.4.1  Message Validation

   A node MUST silently discard any Neighbor Solicitation or
   Advertisement message that does not pass the validity checks
   specified in [RFC2461].  A Neighbor Solicitation or Advertisement
   message that passes these validity checks is called a valid
   solicitation or valid advertisement, respectively.

5.4.2  Sending Neighbor Solicitation Messages

   Before sending a Neighbor Solicitation, an interface MUST join the
   all-nodes multicast address and the solicited-node multicast address
   of the tentative address.  The former ensures that the node receives



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   Neighbor Advertisements from other nodes already using the address;
   the latter ensures that two nodes attempting to use the same address
   simultaneously detect each other's presence.

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

   If the Neighbor Solicitation is going to be the first message to be
   sent from an interface after interface (re)initialization, the node
   SHOULD delay joining the solicited-node multicast address by a random
   delay between 0 and MAX_RTR_SOLICITATION_DELAY as specified in
   [RFC2461].  This serves to alleviate congestion when many nodes start
   up on the link at the same time, such as after a power failure, and
   may help to avoid race conditions when more than one node is trying
   to solicit for the same address at the same time.

   Even if the Neighbor Solicitation is not going to be the first
   message to be sent, the node SHOULD delay joining the solicited-node
   multicast address by a random delay between 0 and
   MAX_RTR_SOLICITATION_DELAY if the address being checked is configured
   by a router advertisement message sent to a multicast address.  The
   delay will avoid similar congestion when multiple nodes are going to
   configure addresses by receiving a same single multicast router
   advertisement.

   Note that the delay for joining the multicast address implicitly
   means delaying transmission of the corresponding Multicast Listener
   Discovery (MLD) report message [RFC2710].  Since [RFC2710] does not
   request a random delay to avoid race conditions, just delaying
   Neighbor Solicitation would cause congestion by the MLD report
   messages.  The congestion would then prevent MLD-snooping switches
   from working correctly, and, as a result, prevent Duplicate Address
   Detection from working.  The requirement to include the delay for the
   MLD report in this case avoids this scenario.  [RFC3590] also talks
   about some interaction issues between Duplicate Address Detection and
   MLD.

   In order to improve the robustness of the Duplicate Address Detection
   algorithm, an interface MUST receive and process datagrams sent to
   the all-nodes multicast address or solicited-node multicast address
   of the tentative address while the delaying period.  This does not
   necessarily conflict with the requirement that joining the multicast
   group be delayed.  In fact, in some cases it is possible for a node
   to start listening to the group during the delay period before MLD



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   report transmission.  It should be noted, however, that in some
   link-layer environments, particularly with MLD-snooping switches, no
   multicast reception will be available until the MLD report is sent.

5.4.3  Receiving Neighbor Solicitation Messages

   On receipt of a valid Neighbor Solicitation message on an interface,
   node behavior depends on whether the target address is tentative or
   not.  If the target address is not tentative (i.e., it is assigned to
   the receiving interface), the solicitation is processed as described
   in [RFC2461].  If the target address is tentative, and the source
   address is a unicast address, the solicitation's sender is performing
   address resolution on the target; the solicitation should be silently
   ignored.  Otherwise, processing takes place as described below.  In
   all cases, a node MUST NOT respond to a Neighbor Solicitation for a
   tentative address.

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

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

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

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

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



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      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 [RFC2461].  If the target
   address is tentative, the tentative address is not unique.

5.4.5  When Duplicate Address Detection Fails

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

   If the address is a link-local address formed from an interface
   identifier based on the hardware address which is supposed to be
   uniquely assigned (e.g., EUI-64 for an Ethernet interface), IP
   operation on the interface SHOULD be disabled.  By disabling IP
   operation, the node will then

   -  not send any IP packets from the interface
   -  silently drop any IP packets received on the interface
   -  not forward any IP packets to the interface (when acting as a
      router or processing a packet with a Routing header)

   In this case, the IP address duplication probably means duplicate
   hardware addresses are in use, and trying to recover from it by
   configuring another IP address will not result in a usable network.
   In fact, it probably makes things worse by creating problems that are
   harder to diagnose than just disabling network operation on the
   interface; the user will see a partially working network where some
   things work, and other things will not.

   On the other hand, if the duplicate link-local address is not formed
   from an interface identifier based on the hardware address which is
   supposed to be uniquely assigned, IP operation on the interface MAY
   be continued.

   Note: as specified in Section 2, "IP" means "IPv6" in the above
   description.  While the background rationale about hardware address
   is independent of particular network protocols, its effect on other
   protocols is beyond the scope of this document.






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5.5  Creation of Global Addresses

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

5.5.1  Soliciting Router Advertisements

   Router Advertisements are sent periodically to the all-nodes
   multicast address.  To obtain an advertisement quickly, a host sends
   out Router Solicitations as described in [RFC2461].

5.5.2  Absence of Router Advertisements

   Even if a link has no routers, stateful autoconfiguration to obtain
   addresses and other configuration information may still be available,
   and hosts may want to use the mechanism.  From the perspective of
   autoconfiguration, a link has no routers if no Router Advertisements
   are received after having sent a small number of Router Solicitations
   as described in [RFC2461].

   Note that it is possible that there is no router on the link in this
   sense but there is a node that has the ability to forward packets.
   In this case, the forwarding node's address must be manually
   configured in hosts to be able to send packets off-link, since the
   only mechanism to configure the default router's address
   automatically is the one using Router Advertisements.

5.5.3  Router Advertisement Processing

   For each Prefix-Information option in the Router Advertisement:

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

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

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







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    d) If the prefix advertised is not equal to the prefix of an address
      configured by stateless autoconfiguration already in the list of
      addresses associated with the interface (where "equal" means the
      two prefix lengths are the same and the first prefix-length bits
      of the prefixes are identical), and the Valid Lifetime is not 0,
      form an address (and add it to the list) by combining the
      advertised prefix with the link's interface identifier as follows:

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

      If the sum of the prefix length and interface identifier length
      does not equal 128 bits, the Prefix Information option MUST be
      ignored.  An implementation MAY wish to log a system management
      error in this case.  The length of the interface identifier is
      defined in a separate link-type specific document, which should
      also be consistent with the address architecture [RFC3513] (see
      Section 2).

      It is the responsibility of the system administrator to insure
      that the lengths of prefixes contained in Router Advertisements
      are consistent with the length of interface identifiers for that
      link type.  It should be noted, however, that this does not mean
      the advertised prefix length is meaningless.  In fact, the
      advertised length has non trivial meaning for on-link
      determination in [RFC2461] where the sum of the prefix length and
      the interface identifier length may not be equal to 128.  Thus, it
      should be safe to validate the advertised prefix length here, in
      order to detect and avoid a configuration error specifying an
      invalid prefix length in the context of address autoconfiguration.

      Note that a future revision of the address architecture [RFC3513]
      and a future link-type specific document, which will still be
      consistent with each other, could potentially allow for an
      interface identifier of length other than the value defined in the
      current documents.  Thus, an implementation should not assume a
      particular constant.  Rather, it should expect any lengths of
      interface identifiers.

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







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    e) If the advertised prefix is equal to the prefix of an address
      configured by stateless autoconfiguration in the list, the
      preferred lifetime of the address is reset to the Preferred
      Lifetime in the received advertisement.  The specific action to
      perform for the valid lifetime of the address depends on the Valid
      Lifetime in the received advertisement and the remaining time to
      the valid lifetime expiration of the previously autoconfigured
      address.  We call the remaining time "RemainingLifetime" in the
      following discussion:

      1.  If the received Valid Lifetime is greater than 2 hours or
          greater than RemainingLifetime, set the valid lifetime of the
          corresponding address to the advertised Valid Lifetime.

      2.  If RemainingLifetime is less than or equal to 2 hours, ignore
          the Prefix Information option with regards to the valid
          lifetime, unless the Router Advertisement from which this
          option was obtained has been authenticated (e.g., via IP
          security [RFC2402]).  If the Router Advertisement was
          authenticated, the valid lifetime of the corresponding address
          should be set to the Valid Lifetime in the received option.

      3.  Otherwise, reset the valid lifetime of the corresponding
          address to two hours.

      The above rules address a specific denial of service attack in
      which a bogus advertisement could contain prefixes with very small
      Valid Lifetimes.  Without the above rules, a single
      unauthenticated advertisement containing bogus Prefix Information
      options with short Valid Lifetimes could cause all of a node's
      addresses to expire prematurely.  The above rules ensure that
      legitimate advertisements (which are sent periodically) will
      "cancel" the short Valid Lifetimes before they actually take
      effect.

      Note that the preferred lifetime of the corresponding address is
      always reset to the Preferred Lifetime in the received Prefix
      Information option, regardless of whether the valid lifetime is
      also reset or ignored.  The difference comes from the fact that
      the possible attack for the preferred lifetime is relatively
      minor.  Additionally, it is even undesirable to ignore the
      preferred lifetime when a valid administrator wants to deprecate a
      particular address by sending a short preferred lifetime (and the
      valid lifetime is ignored by accident).

5.5.4  Address Lifetime Expiry

   A preferred address becomes deprecated when its preferred lifetime



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   expires.  A deprecated address SHOULD continue to be used as a source
   address in existing communications, but SHOULD NOT be used to
   initiate new communications if an alternate (non-deprecated) address
   of sufficient scope can easily be used instead.

   Note that the feasibility of initiating new communication using a
   non-deprecated address may be an application-specific decision, as
   only the application may have knowledge about whether the (now)
   deprecated address was (or still is) in use by the application.  For
   example, if an application explicitly specifies the protocol stack to
   use a deprecated address as a source address, the protocol stack must
   accept that; the application might request it because that IP address
   is used for in higher-level communication and there might be a
   requirement that the multiple connections in such a grouping use the
   same pair of IP addresses.

   IP and higher layers (e.g., TCP, UDP) MUST continue to accept and
   process datagrams destined to a deprecated address as normal since a
   deprecated address is still a valid address for the interface.  In
   the case of TCP, this means TCP SYN segments sent to a deprecated
   address are responded to using the deprecated address as a source
   address in the corresponding SYN-ACK (if the connection would
   otherwise be allowed).

   An implementation MAY prevent any new communication from using a
   deprecated address, but system management MUST have the ability to
   disable such a facility, and the facility MUST be disabled by
   default.

   Other subtle cases should also be noted about source address
   selection.  For example, the above description does not clarify which
   address should be used between a deprecated, smaller-scope address
   and a non-deprecated, enough scope address.  The details of the
   address selection including this case are described in [RFC3484] and
   beyond the scope of this document.

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

5.6  Configuration Consistency

   It is possible for hosts to obtain address information using both
   stateless and stateful protocols since both may be enabled at the
   same time.  It is also possible that the values of other
   configuration parameters such as MTU size and hop limit will be
   learned from both Router Advertisements and the stateful



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   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.

5.7  Retaining Configured Addresses for Stability

   An implementation that has stable storage may want to retain
   addresses in the storage when the addresses were acquired using
   stateless address autoconfiguration.  Assuming the lifetimes used are
   reasonable, this technique implies that a temporary outage (less than
   the valid lifetime) of a router will never result in the node losing
   its global address even if the node were to reboot.  When this
   technique is used, it should also be noted that the expiration times
   of the preferred and valid lifetimes must be retained, in order to
   prevent the use of an address after it has become deprecated or
   invalid.

   Further details on this kind of extension are beyond the scope of
   this document.

6.  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
   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 stateless address autoconfiguration and Duplicate Address
   Detection opens up the possibility of several denial of service
   attacks.  For example, any node can respond to Neighbor Solicitations
   for a tentative address, causing the other node to reject the address
   as a duplicate.  A separate document [RFC3756] discusses details
   about these attacks.  These attacks can be addressed by requiring
   that Neighbor Discovery packets be authenticated with IP security
   [RFC2402].  However, it should be noted that [RFC3756] points out the
   use of IP security is not always feasible depending on network
   environments.





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7.  IANA CONSIDERATIONS

   This document has no actions for IANA.

8.  Acknowledgements

   The authors would like to thank the members of both the IPNG (which
   is now IPV6) and ADDRCONF working groups for their input.  In
   particular, thanks to Jim Bound, Steve Deering, Richard Draves, and
   Erik Nordmark.  Thanks also goes to John Gilmore for alerting the WG
   of the "0 Lifetime Prefix Advertisement" denial of service attack
   vulnerability; this document incorporates changes that address this
   vulnerability.

   A number of people have contributed to identifying issues on a
   previous version of this document and to proposing resolutions to the
   issues, on which this version is based.  In addition to those listed
   above, the contributors include Jari Arkko, Brian E Carpenter,
   Gregory Daley, Ralph Droms, Jun-ichiro itojun Hagino, Christian
   Huitema, Suresh Krishnan, Soohong Daniel Park, Markku Savela, and
   Pekka Savola.

9.  References

9.1  Normative References

   [RFC2402]  Kent, S. and R. Atkinson, "IP Authentication Header", RFC
              2402, November 1998.

   [RFC2464]  Crawford, M., "Transmission of IPv6 Packets over Ethernet
              Networks", RFC 2464, December 1998.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3513]  Hinden, R. and S. Deering, "Internet Protocol Version 6
              (IPv6) Addressing Architecture", RFC 3513, April 2003.

   [RFC2461]  Narten, T., Nordmark, E. and W. Simpson, "Neighbor
              Discovery for IP Version 6 (IPv6)", RFC 2461, December
              1998.

9.2  Informative References

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and
              M. Carney, "Dynamic Host Configuration Protocol for IPv6
              (DHCPv6)", RFC 3315, July 2003.




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   [RFC3736]  Droms, R., "Stateless Dynamic Host Configuration Protocol
              (DHCP) Service for IPv6", RFC 3736, April 2004.

   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484, February 2003.

   [RFC3041]  Narten, T. and R. Draves, "Privacy Extensions for
              Stateless Address Autoconfiguration in IPv6", RFC 3041,
              January 2001.

   [I-D.ietf-send-cga]
              Aura, T., "Cryptographically Generated Addresses (CGA)",
              draft-ietf-send-cga-06 (work in progress), April 2004.

   [RFC2710]  Deering, S., Fenner, W. and B. Haberman, "Multicast
              Listener Discovery (MLD) for IPv6", RFC 2710, October
              1999.

   [RFC3590]  Haberman, B., "Source Address Selection for the Multicast
              Listener Discovery (MLD) Protocol", RFC 3590, September
              2003.

   [RFC3756]  Nikander, P., Kempf, J. and E. Nordmark, "IPv6 Neighbor
              Discovery (ND) Trust Models and Threats", RFC 3756, May
              2004.

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

   [IEEE802.11]
              IEEE, "Wireless LAN Medium Access Control (MAC) and
              Physical Layer (PHY) Specifications", ANSI/IEEE STd
              802.11, August 1999.


Authors' Addresses

   Susan Thomson
   Cisco Systems

   EMail: sethomso@cisco.com










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

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


   Tatuya Jinmei
   Corporate Research & Development Center, Toshiba Corporation
   1 Komukai Toshiba-cho, Saiwai-ku
   Kawasaki-shi, Kanagawa  212-8582
   Japan

   Phone: +81 44-549-2230
   EMail: jinmei@isl.rdc.toshiba.co.jp

Appendix A.  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 identifier 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



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   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.  There is even a link-layer
   specification that requires to discard any such packets [IEEE802.11].
   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 by temporarily disabling the software
      suppression of loopbacks while a node performs Duplicate Address
      Detection, if it is possible to disable the suppression.

   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 interface's.  It should also be noted that a link-layer
   specification can conflict with the condition necessary to make
   Duplicate Address Detection work.

Appendix B.  CHANGES SINCE RFC 1971

   o  Changed document to use term "interface identifier" rather than
      "interface token" for consistency with other IPv6 documents.



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   o  Clarified definition of deprecated address to make clear it is OK
      to continue sending to or from deprecated addresses.
   o  Added rules to Section 5.5.3 Router Advertisement processing to
      address potential denial-of-service attack when prefixes are
      advertised with very short Lifetimes.
   o  Clarified wording in Section 5.5.4 to make clear that all upper
      layer protocols must process (i.e., send and receive) packets sent
      to deprecated addresses.

Appendix C.  CHANGES SINCE RFC 2462

   Major changes that can affect existing implementations:

   o  Specified that a node performing Duplicate Address Detection delay
      joining the solicited-node multicast group, not just delay sending
      Neighbor Solicitations, explaining the detailed reason.
   o  Added a requirement for a random delay before sending Neighbor
      Solicitations for Duplicate Address Detection if the address being
      checked is configured by a multicasted Router Advertisements.
   o  Clarified that on failure of Duplicate Address Detection, IP
      network operation should be disabled and that the rule should
      apply when the hardware address is supposed to be unique.

   Major clarifications:

   o  Clarified how the length of interface identifiers should be
      determined, described the relationship with the prefix length
      advertised in Router Advertisements, and avoided using a
      particular length hard-coded in this document.
   o  Clarified that the "stateful" protocols for address configuration
      and other information configuration are RFC 3315 and RFC 3736,
      respectively.
   o  Clarified the semantics of the M and O flags based on the latest
      standard and operational status.  In particular, clarified that
      these flags show the availability of the stateful protocol instead
      of a trigger to invoke the stateful protocol.  ManagedFlag and
      OtherConfigFlag, which were implementation-internal variables,
      were removed accordingly.
   o  Recommended to perform Duplicate Address Detection for all unicast
      addresses more strongly, considering a variety of different
      interface identifiers, while keeping care of existing
      implementations.
   o  Clarified wording in Section 5.5.4 to make clear that a deprecated
      address specified by an application should be used for any
      communication.
   o  Revised the Security Considerations section with a reference to
      RFC 3756 and a note that the use of IP security is not always
      feasible.



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   o  Added a note when an implementation uses stable storage for
      autoconfigured addresses.

   Other miscellaneous clarifications:

   o  Removed references to site-local and revised wording around the
      keyword.
   o  Removed redundant code in denial of service protection in Section
      5.5.3.
   o  Clarified that a unicasted Neighbor Solicitation or Advertisement
      should be discarded while performing Duplicate Address Detection.

Appendix D.  CHANGE HISTORY

   [NOTE TO RFC EDITOR:  PLEASE REMOVE THIS SECTION UPON PUBLICATION.]

   Changes in draft-ietf-ipv6-rfc2462bis-00.txt since RFC 2462 are:

   o  Fixed a typo in Section 2.
   o  Updated references and categorized them into normative and
      informative ones.
   o  Removed redundant code in denial of service protection in Section
      5.5.3.
   o  Clarified that a unicasted NS or NA should be discarded while
      performing Duplicate Address Detection.
   o  Replaced the word "StoredLifetime" with "RemainingLifetime" with a
      precise definition to avoid confusion.
   o  Removed references to site-local and revised wording around the
      keyword.
   o  Added a note about source address selection with regards to
      deprecated vs insufficient-scope addresses, etc.  Added a
      reference to RFC 3484 for further details.
   o  Clarified what "new communication" means in Section 5.5.4.
   o  Added a new subsection (5.7) to mention the possibility to use
      stable storage to retain configured addresses for stability.
   o  Revised the Security Considerations section with a reference to
      RFC 3756 and a note that the use of IP security is not always
      feasible.
   o  Added a note with a reference in Appendix A about the case where a
      link-layer filtering conflicts with a condition to make Duplicate
      Address Detection work correctly.
   o  Specified that a node performing Duplicate Address Detection delay
      joining the solicited-node multicast group, not just delay sending
      Neighbor Solicitations, explaining the detailed reason.
   o  Clarified the reason why the interface should be disabled after an
      address duplicate is detected.  Also clarified that the interface
      may continue to be used if the interface identifier is not based
      on the hardware address.



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   o  Clarified that the preferred lifetime for an existing configured
      address is always reset to the advertised value by Router
      Advertisement.
   o  Updated the description of interface identifier considering the
      latest format.

   Changes since draft-ietf-ipv6-rfc2462bis-00.txt are:

   o  Clarified how the length of interface identifiers should be
      determined, described the relationship with the prefix length
      advertised in Router Advertisements, and avoided using a
      particular length hard-coded in this document.
   o  Added a note when an implementation uses stable storage for
      autoconfigured addresses.
   o  Resolved conflict with the Multicast Listener Discovery
      specification about random delay for the first packet from the
      host.
   o  Clarified the semantics of the M and O flags based on the latest
      standard and operational status.  In particular, clarified that
      these flags show the availability of the stateful protocol instead
      of a trigger to invoke the stateful protocol.  ManagedFlag and
      OtherConfigFlag, which were implementation-internal variables,
      were removed accordingly.
   o  Recommended to perform Duplicate Address Detection for all unicast
      addresses more strongly, considering a variety of different
      interface identifiers, while keeping care of existing
      implementations.
   o  Added a requirement for a random delay before sending Neighbor
      Solicitations for Duplicate Address Detection if the address being
      checked is configured by a multicasted Router Advertisements.
   o  Clarified that the prefix comparison in Section 5.5.3 is based on
      exact match.  Also clarified the comparison described in this
      document concentrates on addresses configured by the stateless
      mechanism.
   o  Revisited the author list.
   o  Added IANA Considerations Section.

   Changes since draft-ietf-ipv6-rfc2462bis-02.txt are:

   o  Updated the I-D / IPR boilerplate to the latest ones.  Applied
      other editorial changes to conform to I-D nits.
   o  Clarified that it is IP network operation that should be disabled
      on failure of Duplicate Address Detection, and that the rule
      should apply when the hardware address is supposed to be unique.
   o  Changed the reference on the algorithm of computing solicited-node
      multicast addresses to [RFC3513].
   o  Made the intent clearer in the clarification that unicasted NS or
      NA should be discarded during Duplicate Address Detection.



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   Changes since draft-ietf-ipv6-rfc2462bis-03.txt are:

   o  Added an informative reference to [RFC3590].
   o  Summarized major changes since RFC 2462 to prepare for
      publication.














































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