rfc2462.txt

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Network Working Group                                         S. Thomson
Request for Comments: 2462                                      Bellcore
Obsoletes: 1971                                                T. Narten
Category: Standards Track                                            IBM
                                                           December 1998


                IPv6 Stateless Address Autoconfiguration

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (1998).  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 should be
   autoconfigured (addresses, other information, or both), and in the
   case of addresses, whether they should be obtained through the
   stateless mechanism, the stateful mechanism, or both.  This document
   defines the process for generating a link-local address, the process
   for generating site-local and global addresses via stateless address
   autoconfiguration, and the Duplicate Address Detection procedure. The
   details of autoconfiguration using the stateful protocol are
   specified elsewhere.

Table of Contents

   1.  INTRODUCTION.............................................    2
   2.  TERMINOLOGY..............................................    4
      2.1.  Requirements........................................    6
   3.  DESIGN GOALS.............................................    7
   4.  PROTOCOL OVERVIEW........................................    8
      4.1.  Site Renumbering....................................   10
   5.  PROTOCOL SPECIFICATION...................................   10
      5.1.  Node Configuration Variables........................   11
      5.2.  Autoconfiguration-Related Variables.................   11
      5.3.  Creation of Link-Local Addresses....................   12



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RFC 2462        IPv6 Stateless Address Autoconfiguration   December 1998


      5.4.  Duplicate Address Detection.........................   13
         5.4.1.  Message Validation.............................   14
         5.4.2.  Sending Neighbor Solicitation Messages.........   14
         5.4.3.  Receiving Neighbor Solicitation Messages.......   15
         5.4.4.  Receiving Neighbor Advertisement Messages......   16
         5.4.5.  When Duplicate Address Detection Fails.........   16
      5.5.  Creation of Global and Site-Local Addresses.........   16
         5.5.1.  Soliciting Router Advertisements...............   16
         5.5.2.  Absence of Router Advertisements...............   17
         5.5.3.  Router Advertisement Processing................   17
         5.5.4.  Address Lifetime Expiry........................   19
      5.6.  Configuration Consistency...........................   19
   6.  SECURITY CONSIDERATIONS..................................   20
   7.  References...............................................   20
   8.  Acknowledgements and Authors' Addresses..................   21
   9.  APPENDIX A: LOOPBACK SUPPRESSION & DUPLICATE ADDRESS
         DETECTION..............................................   22
   10. APPENDIX B: CHANGES SINCE RFC 1971.......................   24
   11. Full Copyright Statement.................................   25

1.  INTRODUCTION

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

   IPv6 defines both a stateful and stateless address autoconfiguration
   mechanism. Stateless autoconfiguration requires no manual
   configuration of hosts, minimal (if any) configuration of routers,
   and no additional servers.  The stateless mechanism allows a host to
   generate its own addresses using a combination of locally available
   information and information advertised by routers. Routers advertise
   prefixes that identify the subnet(s) associated with a link, while
   hosts generate an "interface 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.




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RFC 2462        IPv6 Stateless Address Autoconfiguration   December 1998


   In the stateful autoconfiguration model, hosts obtain interface
   addresses and/or configuration information and parameters from a
   server.  Servers maintain a database that keeps track of which
   addresses have been assigned to which hosts. The stateful
   autoconfiguration protocol allows hosts to obtain addresses, other
   configuration information or both from a server.  Stateless and
   stateful autoconfiguration complement each other. For example, a host
   can use stateless autoconfiguration to configure its own addresses,
   but use stateful autoconfiguration to obtain other information.
   Stateful autoconfiguration for IPv6 is the subject of future work
   [DHCPv6].

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

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

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






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RFC 2462        IPv6 Stateless Address Autoconfiguration   December 1998


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

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

2.  TERMINOLOGY

   IP - Internet Protocol Version 6.  The terms IPv4 and 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.



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RFC 2462        IPv6 Stateless Address Autoconfiguration   December 1998



   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 [ADDR-ARCH].

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

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

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

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

   global address - an address with unlimited scope.

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

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

   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.





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RFC 2462        IPv6 Stateless Address Autoconfiguration   December 1998


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

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

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

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

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

   interface identifier - a link-dependent identifier for an interface
        that is (at least) unique per link [ADDR-ARCH]. Stateless
        address autoconfiguration combines an interface identifier with
        a prefix to form an address. From address autoconfiguration's
        perspective, an interface identifier is a bit string of known
        length.  The exact length of an interface identifier and the way
        it is created is defined in a separate link-type specific
        document that covers issues related to the transmission of IP
        over a particular link type (e.g., [IPv6-ETHER]).  In many
        cases, the identifier will be the same as 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 [KEYWORDS].




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RFC 2462        IPv6 Stateless Address Autoconfiguration   December 1998


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