ADDRCONF Working Group                        Susan Thomson (Bellcore)
INTERNET-DRAFT                                          March 24, 1995

               IPv6 Stateless Address Autoconfiguration

Status of this Memo

   This document is a submission to the ADDRCONF Working Group of the
   Internet Engineering Task Force (IETF). Comments should be submitted
   to the mailing list.

   This document is an Internet Draft.  Internet Drafts are working
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   This document specifies stateless address autoconfiguration.  A host
   can form a link-local address autonomously based on information local
   to the host.  A host can form an inter-link scope address in two
   ways: either autonomously, based on prefixes advertised by routers,
   or by using the IPv6 version of the Dynamic Host Configuration
   Protocol(DHCPv6). All mechanisms rely on a host having a token that
   is unique at least per link.  This document specifies how a host
   forms addresses autonomously.  DHCPv6 is described elsewhere.

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   An IPv6 host may have multiple addresses per interface. The addresses
   can have one of three scopes:

   1.   a link-local address,

   2.   a site-local address, and

   3.   a global address.

   All three address scopes can be autoconfigured.  A host can autocon-
   figure a link-local address autonomously. A host can autoconfigure a
   site-local or global address only when a router or a DHCPv6 server is
   present on the link.

   There is only one way to form a link-local address. On initialization
   of an interface, a host forms such an address by concatenating a
   well-known link-local prefix[1] to a token that is unique per link.
   The definition of the tokens used are link-dependent.  For example,
   in the case of a host  attached to an link that uses IEEE 802
   addresses, the token is the IEEE 802 address of the interface.

   There are two ways to form a site-local or global address. In the
   first mechanism, a host forms an inter-link scope address by con-
   catenating a network prefix advertised in a Router Advertisement[2,3]
   to a token that is unique per link.  Like the link-local address, the
   token is defined to be link-layer dependent.  This mechanism for
   forming a site-local or global address is suitable for environments
   where no administrative control is desired. It is a simple protocol
   designed for a very specific purpose: to make stateless address con-
   figuration very straightforward to use and implement.

   The other mechanism available to hosts is to use the IPv6 version of
   the Dynamic Host Configuration Protocol (DHCPv6). DHCPv6 is a more
   complex protocol allowing for very flexible address assignment under
   the control of a system administrator. This protocol typically
   requires significant system management, including server and database

   The choice of mechanism to use in forming an inter-link scope address
   is advertised by a router, if present, and this choice is configur-
   able by a system administrator.

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   This document describes how a host forms a link-local address and one
   or more site-local or global addresses autonomously. It also speci-
   fies how a host determines which mechanism to use to form an inter-
   link scope address: the autonomous (stateless) approach or DHCPv6.
   Section 2 describes the router's role in address autoconfiguration
   and Section 3 the host's role.

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   The stateless address autoconfiguration mechanism relies on the
   router discovery mechanism specified in [2,3] for forming addresses
   with site-local or global scope.  If configured to do so, routers
   advertise prefix information in periodic Router Advertisements.  The
   prefixes are contained in Prefix-Information extensions of a Router
   Advertisement. Each Prefix-Information extension indicates whether
   the prefix can be used for autonomous address autoconfiguration and,
   if so, for how long the resulting address is valid. Note that the
   lifetime of the address is defined separately from that of the Router
   Advertisement itself (other information is advertised in the adver-
   tisement which has different lifetime requirements).  The extension
   also explicitly indicates to hosts whether DHCPv6 is required to be
   used since it is possible that system administrators would like to
   have hosts autoconfigure addresses autonomously, but also use DHCPv6
   to acquire other configuration information besides the address.

   Router Advertisement and Solicitation processing is specified in [2]
   and the message formats in [3].

   DISCUSSION: An alternative approach is to advertise address confi-
   guration information in a separate advertisement entirely. This would
   be somewhat cleaner since the lifetime of the advertisement would
   then be that of the information advertised. On the other hand, having
   two types of router advertisements would mean that prefix information
   is advertised redundantly, and in particular, would double traffic on
   initialisation and on router solicitations.

2.1.  Router Configuration Variables

   In addition to the configuration variables specified in [2,3],
   routers MUST also be configurable as follows.

   For each of the IPv6 unicast addresses per interface:

      Autonomous Flag

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         If and only if TRUE, the prefix length is to be advertised for
         the purposes of autonomous address configuration.

         Default: TRUE

   For each interface:

      Administered Flag

         If and only if TRUE, the host must autoconfigure other confi-
         guration information using DHCPv6. Only valid in extensions
         with the Autonomous Flag set to TRUE.

         Default: FALSE


         The time allowed between sending unsolicited Address Advertise-
         ments from the interface, in seconds. The value must not be
         less than Maximum_Advertisement_Interval of Router Advertise-

         Default: XX


         The value to be placed in the Lifetime field of the
         Prefix_Information extension sent from the interface in
         seconds. The value must not be less than
         Address_Advertisement_Interval. This value indicates how long
         an address formed from the prefix advertised is valid.  Only
         valid in extensions with the Autonomous flag set to TRUE.

         Default: 3 * Address_Advertisement_Interval

   All routers advertising a given prefix on a link MUST be configured
   to advertise the same autoconfiguration mode to hosts.

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2.2.  Processing

   A router MUST advertise address autoconfiguration information in a
   Prefix Information Extension of a Router Advertisement. The values of
   the Autonomous and Administered flags are advertised along with
   Address_Lifetime.  The address configuration information need not be
   advertised in each Router Advertisement. It must be sent (almost)
   periodically in a Router Advertisement at an interval of approxi-
   mately Address_Advertisement_Interval.

   Address  configuration information must also be sent in the first few
   Router Advertisements after startup or enabling of an interface (up
   to MAX_INITIAL_ADVERTISEMENTS) and in a Router Advertisement that is
   sent in response to a Router Solicitation.

   Address  configuration information may also be sent in a Router
   Advertisement due to actions taken by system management, in particu-
   lar, when the Address_Lifetime of a prefix is set to zero, e.g.
   because the link is to be renumbered. In this case, a Prefix-
   Information extension must be transmitted in a Router Advertisement
   advertising the appropriate address prefix with the Autonomous Flag
   set to TRUE and Address_Lifetime set to zero.

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3.1.  Host Configuration Variables

   A host maintains a list of addresses per interface. At a minimum, the
   list includes the link-local address, which the host can form auto-
   nomously whenever an interface is initialised. If a router is
   attached to the link or DHCPv6 server is available, the list may also
   include site-local or global addresses formed either from subnet pre-
   fixes advertised in Router Advertisements or by making requests using
   DHCPv6. Addresses may also be manually configured. Note there may be
   multiple site-local or global addresses autoconfigured per interface.

   A host must maintain a list of the following configurable variables
   per interface:


      A valid IPv6 unicast address for this interface

      Default: None

   Prefix Length

      The length of the prefix in bits. Prefix length semantics are
      specified in [2].

   A host must also allow the following variable to be configured per


      If and only if TRUE, the host must perform address autoconfigura-
      tion processing.

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      Default: TRUE

3.2.  Host Initialization Behavior

   A host must perform the following autoconfiguration procedure when-
   ever an interface needs to be initialised:

      When a host has no address for an interface with
      Perform_Auto_Config flag set to TRUE, e.g. when a host boots or
      when an interface is enabled after being disabled, the host forms
      an address of link-local scope.  Appendix A specifies how a host
      that is attached to a link that uses IEEE 802 addresses forms a
      link-local address.

      Before adding the link-local address as a valid address to the
      list of addresses for the interface, the host SHOULD verify that
      the address is indeed unique. The procedure for validating an
      address is described in Section X. A host SHOULD also validate any
      manually configured addresses this way too.

      The host solicits a Router Advertisement using one or more Router
      Solicitations, if no Router Advertisements have been heard in the
      interface. The procedure for sending Router Solicitations is
      specified in [2].

      If no Router Advertisement is heard after sending
      MAX_SOLICITATIONS and waiting Router_Solicitation_Interval after
      each as specified in Sending Router Solicitations in [2], the host
      must determine whether a DHCPv6 server is present and whether any
      configuration information needs to be acquired.  This is to cater
      for a routerless topology which has a DHCPv6 server. Once a router
      is added to the network, however, a host MUST use Router Adver-
      tisements to determine the autoconfiguration mode in use as
      described in the section on Router Advertisement Processing.

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3.3.  Router Advertisement Processing

   Router Advertisement processing is specified in [2] and the message
   format in [3].  In addition to this processing, the host MUST perform
   the following address configuration processing when a solicited or
   unsolicited Router Advertisement is received over an interface:

   For each Prefix-Information extension in the Router Advertisement:
   (The format of the Prefix-Information extension as amended by this
   draft for autoconfiguration purposes is specified in Appendix C):

      The host silently ignores the extension for the purposes of auto-
      configuration if the Perform_Auto_Config flag for the interface is

      Otherwise, the host checks the autoconfiguration mode bits.

      If only the Autonomous flag is set in the Prefix-Information
      extension, the host forms or verifies a site-local or global
      address as specified below.

      If both the Autonomous and Administered flags are set in the
      Prefix-Information extension, the host forms or verifies a site-
      local or global address as specified below and uses or continues
      using DHCPv6 for other autoconfiguration.

      Otherwise, the host silently ignores the extension for the pur-
      poses of autonomous autoconfiguration.

   If none of the prefixes advertised in extensions of the Router Adver-
   tisement have the Autonomous flag set, then the host uses or contin-
   ues using DHCPv6 for autoconfiguration.

   Note that the above procedure should continue to operate when a sys-
   tem administrator decides to change the autoconfiguration mode from
   the autonomous mode to DHCPv6, and vice versa. The host should keep
   track of the current autoconfiguration mode, so that it can detect
   when there is a change.  The system administrator must ensure that,
   during a changeover, a DHCPv6 server is configured to hand out
   addresses that are unique per link, particularly with respect to
   addresses that hosts have configured autonomously and which may not

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   yet be invalidated.  To avoid problems during a changeover, it is
   recommended that a DHCP server should use exactly the same algorithm
   to form a host address as that used in the autonomous mode when the
   prefix is the same. It is also important to ensure that a DHCPv6
   server is configured to hand out addresses only to those hosts that
   it should, since, if a DHCPv6 server is present on a link, hosts may
   request the server for addresses (even if the network is configured
   to be in autonomous mode) when Router Advertisements are not heard
   because the router is down.

   For each Prefix-Information extension received over an autoconfigur-
   able interface, the host updates the address list as follows when the
   Autonomous flag is set:

   a)   If the prefix advertised matches the prefix of an autoconfigured
        address already in the list, then set a timer to that of the
        lifetime specified in the extension.  Note there is no timer
        associated with a link-local address or manually configured

   b)   If the prefix advertised does not match the prefix of an address
        already in the list, then form an address using this network
        prefix. Appendix A specifies how to form an address for hosts
        that have IEEE 802 tokens. The extension is ignored if the pre-
        fix is not the right length for forming an address as specified
        in Appendix A.

        Add this address to the list with a timer set to that of the
        lifetime specified in the extension.

3.3.1.  Address Deprecation and Invalidation

   An address is valid until the timer expires.

   When the lifetime of an address expires, an address is said to be
   deprecated.  In general, a deprecated address should no longer be
   used in new communications, but may be used in existing communica-

   In particular, the internetworking layer should not select a

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   deprecated address as a source address in new communications. How-
   ever, a deprecated address should be allowed to be used as a source
   address if it is in use by the transport layer in existing communica-
   tions or it is explicitly requested by an application.

   The internetworking layer must continue to accept datagrams destined
   to a deprecated address. The transport layer may refuse to accept new
   communications requests to a deprecated address, however.

   In addition, a host may send a Remote Redirect[2,3] to correspondents
   when the source address used in communications is deprecated as long
   as the host has a valid alternative address. Also, a deprecated
   address should be removed from the Domain Name System (DNS). This may
   be done by the host itself, given a DNS dynamic update protocol and
   sufficient authority, or it may be done on the host's behalf.

   The time at which a deprecated address becomes invalid (removed from
   the list of addresses per interface) is dependent on the storage
   available for the address list and is thus implementation-dependent.
   A host should keep a deprecated address until it is no longer in use,
   i.e. it is no longer being used in current communications such as an
   existing TCP connection, and it is no longer stored or cached in the
   Domain Name System.  After this point, a deprecated address may be
   removed from the address list.

   If Router Advertisements stop being heard and all autoconfigured
   inter-link scope addresses become deprecated, then the host must
   determine whether a DHCPv6 server is available for address autoconfi-
   guration. The host follows the same procedure as described in the
   initialisation procedure in this case.

3.4.  Detecting Duplicate IPv6 Addresses

   One of the basic assumptions of the autoconfiguration schemes out-
   lined in this document is that the host token is at least unique per
   link. Tokens are defined to be link-layer dependent, and the token is
   the link layer address in most cases. In practice, two hosts on the
   same link may have the same link layer address because of an assign-
   ment error, in which case the resulting IPv6 addresses will not be
   unique. For this reason, it is prudent to check that the addresses
   are indeed unique.  In IPv6, it is only necessary to check that one
   of the autoconfigured addresses is unique since the same token is

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   used to form all addresses and the prefixes used to form the
   addresses are all unique (the autoconfiguration procedure should
   ensure this). It is recommended that the link-local address be the
   address checked since it is formed once and first, on initialisation.

   The procedures use General Solicitations and Advertisements specified
   in [2,3] as modified below.  To validate an address, the node sends a
   General Solicitation with the source and destination set to that of
   the address to be checked.  The node should specify an appropriate
   Media-Access extension.

   On receiving a General Solicitation with a source address that is the
   same as the destination address and apparently from itself, a host
   must respond with a General Advertisement. The General Advertisement
   is sent to the All-Nodes Multicast Address with intra-link scope.
   The Media-Access extension from the General Solicitation MUST NOT be

   After sending a General Solicitation, the node waits for a period of
   General_Solicitation_Interval. If a General Advertisement is not
   received in response to the General Solicitation within the interval,
   the address is considered to be validated. If a General Advertisement
   is received with a source address the same as the address being vali-
   dated, it must cease operation on that interface and indicate an
   appropriate error.

   Note that this mechanism is not completely reliable, and so it is
   possible that duplicate addresses will still exist. If a duplicate
   address is discovered, the host will need to be configured with a new
   token, or if this is not possible, the IPv6 addresses will need to be
   manually configured.

   DISCUSSION: There is a problem with a race condition when two (or
   more) nodes boot up at the same time. Both will send out a General
   Solicitation, receive no advertisement and assume all is well.  A fix
   may be to have a node process General Solicitations during the vali-
   dation stage and flag an error if it sees  more than one General Sol-
   icitation for an address it is in the process of validating.

   DISCUSSION: Should the solicitations be dithered? Note that randomis-
   ing based on the token (link-layer address) only does not help if the
   token is not unique.

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   To be completed.

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   The token for an interface on an IEEE 802 link or any link that uses
   IEEE 802 addressing, such as FDDI, is the 48-bit IEEE 802 address in
   canonical format, i.e. the Individual/Group  bit is the low-order bit
   of the furst byte.

   A host forms an IPv6 address per link by concatenating an 80-bit pre-
   fix with the IEEE 802 address as follows:

      |              80 bits                  |      48 bits           |
      |              prefix                   |    IEEE 802 address    |

   In the case of a link-local prefix, the prefix is well-defined[1].

   The prefixes of other types of addresses need to be configured.

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   As has been mentioned, one of the basic assumptions of the autoconfi-
   guration scheme outlined in this document is that the host token is
   at least unique per link, but that tokens may not always be unique,
   in practice.  A host should check that an address is unique using the
   scheme proposed in this document. Since this is not completely reli-
   able, system administrators may also use DNS to help detect when such
   a problem occurs since two different hosts will register the same
   IPv6 address.

   Duplicate IPv6 addresses may occur as a result of non-unique tokens
   in any particular network topology.  One particular scenario deserves
   further mention though. Consider a topology consisting of two links
   with singly-homed hosts attached to each, a multi-homed host attached
   to both and no router. In this case, because no router is present,
   hosts will form link-local addresses only on all interfaces.  It is
   imperative that hosts have interface tokens that are unique across
   both links. However, this may not be true for two reasons: the links
   may be of different types and thus the tokens used may not be unique.
   Also, the token may not be unique if it is defined to be a link layer
   address and the link-layer address is only defined to be unique per
   link as is true in some cases.  Strictly speaking, we require that
   host tokens are globally unique to ensure correct operation in these
   topologies.  In practice, link layer addresses are frequently glo-
   bally unique and so the uniqueness problems in this scenario should
   not be appreciably worse than in more traditional topologies as
   described above.  If two link-local scope addresses are detected to
   be the same in this scenario, there are two solutions: one is to make
   the multihomed host a router, the other is to manually configure the
   link-local address of an offending host.

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7.  APPENDIX C: Prefix-Information Extension

      |   Extension   |    Length     |C|A|M|      0    | Prefix Size |
      |              Lifetime                         |  Preference   |
      |                                                               |
      |                        Identifier                             |
      |                                                               |
      |                                                               |

      Extension        As in [3]

      Length           As in [3]

      C                As in [3]

      A                Autonomous Mode

                       Form an address using prefix of advertised

      M                Administered Mode

                       Use DHCPv6 to autoconfigure other information.

      Prefix Size      Number of bits of identifier defining the
                       routing prefix for this link

      Preference       As in [3]

      Identifier       One of IPv6 unicast addresses of this interface

      This extension is found in Router Advertisements.

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   [1]  R. Hinden, "Internet Protocol Version (IPv6) Specification",
        Internet Draft, March 1995, <draft-ietf-ipngwg-ipv6-addr-arch-

   [2]  W. A. Simpson, "IPv6 Neighbor Discovery -- Processing", Internet
        Draft, January 1995, <draft-simpson-ipv6-discov-process-02.txt>

   [3]  W. A. Simpson, "IPv6 Neighbor Discovery -- ICMP Message For-
        mats", Internet Draft, January 1995, <draft-simpson-ipv6-


The author 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 Bill Simpson.

Author's Addresses

   Susan Thomson
   445 South Street
   Morristown, NJ 07960

   Phone: +1 201-829-4514

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