ADDRCONF Working Group                        Susan Thomson,  Bellcore
INTERNET-DRAFT                                      Thomas Narten, IBM
<draft-ietf-addrconf-ipv6-auto-04.txt>                 October 4,
<draft-ietf-addrconf-ipv6-auto-05.txt>                November 3, 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 addrconf@cisco.com mailing list.

   This document is an Internet Draft.  Internet Drafts are working
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   Distribution of this memo is unlimited.

   This Internet Draft expires April 4, May 3, 1996.

Abstract

   This document specifies how hosts autoconfigure addresses for their interfaces in
   IP version 6. In particular, the document describes the
   steps The autoconfiguration process includes creating a host takes in determining whether
   link-local address autoconfiguration and verifying its uniqueness on a link,
   determining what information should be used, autoconfigured (addresses,
   other information, or both), and if so, in the case of addresses, whether to use
   they should be obtained through the stateful stateless mechanism, the
   stateless mechanism stateful
   mechanism, or both.  This document also specifies stateless
   address autoconfiguration, and how nodes verify defines the uniqueness of an
   address before assigning it to an interface. process for generating
   a link-local address, the process for generating site-local and
   global addresses, and Duplicate Address Detection.  Stateful address
   autoconfiguration is specified elsewhere.

Contents

   Status of this Memo.......................................    1

   1.  INTRODUCTION..........................................    3

   2.  TERMINOLOGY...........................................    4
      2.1.  Requirements.....................................    7

   3.  DESIGN GOALS..........................................    8

   4.  PROTOCOL OVERVIEW.....................................    9
      4.1.  Site Renumbering.................................   10   11

   5.  PROTOCOL SPECIFICATION................................   11   12
      5.1.  Host  Node Configuration Variables.....................   11   12
      5.2.  Autoconfiguration-Related Variables..............   12
      5.3.  Creation of Link-Local Addresses.................   12
      5.4.  Verifying The Uniqueness Of An Address...........   13
      5.4.  Duplicate Address Detection......................   14
         5.4.1.  Message Validation..........................   14   15
         5.4.2.  Sending Neighbor Solicitation Messages......   14   15
         5.4.3.  Receiving Neighbor Solicitation and Messages....   16
         5.4.4.  Receiving Neighbor Advertisement Messages   15 Messages...   16
         5.4.5.  When Duplicate Address Detection Fails......   17
      5.5.  Creation of Global- and Site-Local Addresses.....   16   17
         5.5.1.  Sending  Soliciting Router Solicitations................   16 Advertisements............   17
         5.5.2.  Absence of Router Advertisements............   16   17
         5.5.3.  Router Advertisement Processing.............   16   17
         5.5.4.  Address Lifetime Expiry.....................   18   19
      5.6.  Configuration Consistency........................   18   19

   6.  OPEN ISSUES/TODO......................................   19   20

   7.  SECURITY CONSIDERATIONS...............................   20

   8.  REFERENCES............................................  APPENDIX: LOOPBACK SUPPRESSION AND DUPLICATE ADDRESS DETECTION   20

   9.  REFERENCES............................................   22

   AUTHORS' ADDRESSES........................................   20   22

   CHANGES SINCE PREVIOUS DOCUMENT...........................   24

1.  INTRODUCTION

This document specifies how hosts autoconfigure their 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 stateless stateful mechanism, or both.  This
document also specifies stateless address autoconfiguration. defines the process for generating a link-local address, the
process for generating site-local and global addresses, and Duplicate
Address Detection.  Stateful address autoconfiguration is specified
elsewhere.

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

In the stateful autoconfiguration model, hosts obtain interface
addresses and/or configuration information and parameters from a server.
Servers maintain a database that keeps track of which addresses have
been assigned to which hosts. In addition to
addresses, stateful servers can also supply configuration information
and parameters to a host. The stateful autoconfiguration mechanism 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 is described in
[DHCPv6].

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

IPv6 addresses are leased to an interface for a fixed (possibly
infinite) length of time. Each address has an associated lifetime that

indicates how long the address is bound to an interface. When a lifetime
expires, the binding (and address) becomes 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) gives preference to using a non-deprecated address, with should use
of the a preferred address when possible.  A deprecated
address restricted to should be used only by applications that have been using the deprecated address already it and
would have difficulty switching to another address without a service
disruption.

Finally, this document describes the algorithm a node employs to verify

To insure that an address it is about to assign to an interface is unique on the
link. The all configured addresses are unique, nodes run a
"duplicate address detection" algorithm is used on addresses before an
address assigning
them to an interface.  The Duplicate Address Detection algorithm is actually used,
performed on all addresses, independent of whether the address was 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 possible for expected that routers to use will generate link-local
addresses using the mechanism described in this document for generating a link-local address. All
nodes (not only hosts) should use document. In addition,
routers are expected to successfully pass the duplicate address detection
procedure. 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 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.

   autoconfigurable interface
               - an interface that has been configured by system
                 management to perform autoconfiguration.

   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.

   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 between 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. 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 Discover Discovery packets related to duplicate
                 address detection 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 (or destination) address of
                 packets sent from or to (or to) the interface.

   deprecated address
               - An address assigned to an interface whose use is
                 discouraged, but not forbidden.  A deprecated address
                 should no longer be used as a source address in new
                 communications, but packets sent to deprecated address
                 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
                 be able to
                 deliver packets sent to a valid address.

   invalid address
               - an address that is not assigned to any interface. A
                 valid address becomes invalid when its deprecation
                 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. 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.
                 state (i.e., the time until invalidation). The valid
                 lifetime must be greater then or equal to the preferred
                 lifetime.  When the valid lifetime expires, the address
                 becomes invalid.

   interface token
               - a link-dependent identifier for an interface that is
                 (at least) unique per link. Stateless address
                 autoconfiguration combines an interface token with a
                 prefix to form an address. An IEEE 802 hardware From address
                 is
                 autoconfiguration's perspective, an example of a possible interface token. token is
                 a bit string of known length.  The manner
                 in which exact length of an
                 interface token is created and its length the way it is link-specific, and created is described defined in
                 a separate link-specific document that covers issues
                 related to the specification
                 for the transmission of IP over a particular
                 link type (e.g., [IPv6-ETHER]).  In many cases, the
                 token will be the same as the interface's link-layer
                 address.

2.1.  Requirements

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

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

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

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

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

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

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 (e.g., (i.e.,
     an "interface token").  In the simplest case, an interface token
     consists of the link's hardware interface's link-layer address. An interface token
     can be combined with a prefix to form an address.

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

   o A large site with multiple networks and routers should not require
     the presence of a stateful address configuration server. To enable
     hosts In order
     to generate site-local or global addresses, Router
     Advertisements, hosts must determine
     the prefixes that identify the subnets to which are generated by routers, they attach.

     Routers generate periodic Router Advertisements that include
     options
     that list listing the set of active prefixes on a link.

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

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

4.  PROTOCOL OVERVIEW

This section describes provides an overview of the typical steps that take place
when an interface autoconfigures itself.  Autoconfiguration of a link-local
address normally takes place is performed
only on multicast-capable links and begins when an a multicast-capable
interface is (re)initialized, e.g.
at enabled, e.g., during system startup.  Autoconfiguration of global and site-local addresses  Nodes (both hosts
and
other parameters is done periodically, starting as soon as possible
after an interface has been initialised or enabled for
autoconfiguration.

A node starts routers) begin the autoconfiguration process by generating a link-local link-
local address for the interface. A link-local address is formed by
appending the interface's token to the well-known link-local prefix.

Before the link-local address can be assigned to an interface and used,
however, the a node attempts must attempt to verify that the this "tentative" address is
not already in use by another node on the link. The node Specifically, it sends out 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 machine
is required.  To simplify recovery in this case, it should be possible
for an administrator to supply an alternate interface token that
overrides the default token in such a way that the autoconfiguration
mechanism can then be applied using the new (presumably unique)
interface token.  Alternatively, link-local and other addresses will

need to be configured manually.

Once a node ascertains that the its tentative link-local address is unique,
it assigns it to the interface. At this point, the node has IP-level
connectivity with neighboring nodes via its link-local address.

To generate site-local 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 global addresses, determining that no routers are present. If routers are
present, they will send Router Advertisements that specify what sort of
autoconfiguration a host listens for should do.  If no routers are present, stateful
autoconfiguration should be invoked.

Routers send Router
Advertisements. Advertisements periodically, but the delay between
successive advertisements will generally be longer than a host
performing autoconfiguration will want to wait [DISCOVERY].  To obtain
an advertisement quickly, a host sends one or more Router Solicitations
to the all-routers multicast group. If no
Router Advertisement is received, the host assumes that stateful address
autoconfiguration is desired and invokes an appropriate protocol.  Router Advertisements contain two
flags indicating what type of stateful autoconfiguration (if any) should
be performed. A "managed address configuration" flag indicates whether
hosts should use stateful autoconfiguration to obtain addresses. An
"other stateful configuration" flag indicates whether hosts should use
stateful autoconfiguration to obtain additional information (excluding
addresses).

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

Routers advertise

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

For safety, all addresses obtained through autoconfiguration should must be tested for uniqueness.  In uniqueness prior to their
assignment to an interface.  In the case of addresses created through
stateless autoconfig, however, the uniqueness of an address is
determined primarily by the portion of the address formed from an

interface token.  Thus, if a node has already verified the uniqueness of
a link-local address, additional addresses created from the same
interface token need not be tested for uniqueness. individually. In contrast, all
addresses obtained manually or via stateful address autoconfiguration
should be tested for uniqueness individually. To accommodate sites that
believe the overhead of performing duplicate address detection Duplicate Address Detection outweighs
its benefits, the use of duplicate address detection 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 in hosts.  At present, the TCP/IP protocol suite
provides upper layer protocols such as
TCP provide no support for changing endpoint addresses while a TCP
connection is open. If an end-point address changes, becomes invalid, existing
connections break and all communication to the old invalid address fails.
Even when applications use UDP as a transport protocol, addresses must
generally remain the same during a packet exchange.

Distinguishing

Dividing valid addresses into preferred and deprecated categories
provides a way of indicating to upper layers that a valid address may
become invalid shortly, shortly and that future communication using the address
will fail, should the address's deprecation 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

Address autoconfiguration

Autoconfiguration is performed on a per-interface basis. basis on multicast-
capable interfaces.  For multihomed hosts, address 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.  Host  Node Configuration Variables

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

     AutoConfig     If set,

     DupAddrDetectTransmits

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

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

Autoconfiguration also assumes the presence of the variable RetransTimer
as defined in this document.

                    Default: TRUE

     DuplAddrDetect If set, [DISCOVERY]. For autoconfiguration purposes, RetransTimer
specifies the delay between consecutive Neighbor Solicitation
transmissions performed during Duplicate Address Detection (if
DupAddrDetectTransmits is greater than 1), as well as the time a node MUST use
waits  after sending the duplicate detection
                    procedure (Section 5.4) to verify addresses are
                    unique last Neighbor Solicitation before assigning them to an interface.

                    Default: TRUE ending the
Duplicate Address Detection process.

5.2.  Autoconfiguration-Related Variables

A host maintains a number of data structures and flags:

     ManagedFlag    Copied from flags related to
autoconfiguration. In the following, we present conceptual variables and

show how they are used to perform autoconfiguration. The specific
variables are used for demonstration purposes only, and an
implementation is not required to have them, so long as its external
behavior is consistent with that described in this document.

How routers (auto)configure their interfaces is beyond the scope of this
document.

   ManagedFlag      Copied from the Managed field of the most recently
                    received Router Advertisement message. The flag
                    indicates whether or not addresses are to be
                    configured using the stateful autoconfiguration
                    mechanism. It starts out in a FALSE state.

     OtherFlag

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

     AddressList    List

A host also maintains a list of addresses together with their associated
corresponding lifetimes. Addresses on the list can be obtained
                    through stateless or stateful The address
                    autoconfiguration, or some other external mechanism.
                    AddressList initially list contains no entries.

The values of these variables and flags changes over time as the
lifetimes of prefixes (and addresses) expire, new prefixes are learned,
etc.

If system management changes an interface's AutoConfig flag from TRUE to
FALSE, the value of ManagedFlag both autoconfigured
addresses and OtherFlag MUST be set to FALSE, with
any in-progress autoconfiguration activities interrupted as described
below in Section 5.5.3. those configured manually.

5.3.  Creation of Link-Local Addresses

A host node forms a link-local address whenever an interface is initialized
and the AutoConfig flag is TRUE. (Note that the AutoConfig flag may be
set independently of interface initialization. If the link-local address
has not yet been created when the AutoConfig is changed from FALSE to
TRUE, it is created at this time.) becomes enabled.
An interface is initialized 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 E8::0 FE80::0 [ADDR-ARCH] (of appropriate length) to the interface
token. If the interface token has a length of N bits, the interface
token replaces the right-most N zero bits of the link-local prefix.  If
the interface token is more than 118 bits in length, autoconfiguration
fails and manual configuration is required.

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

5.4.  Verifying The Uniqueness Of An  Duplicate Address Detection

Duplicate address detection is Address Detection MUST be performed on unicast addresses prior
to assigning them to an interface only if the
DuplAddrDetect configuration whose DupAddrDetectTransmits variable
is set to TRUE. greater than zero. Duplicate address detection is applied to an address once after an
address is created, but before assigning it to an interface, Address Detection takes place on all
unicast addresses, regardless of whether the address is they are obtained through
stateful, stateless or manual configuration.  All addresses  Each individual address
SHOULD be tested for uniqueness. However, when stateless address
autoconfiguration is used, address uniqueness is determined solely by
the interface token, assuming that subnet prefixes are assigned
correctly (i.e., if all of an interface's addresses are generated from
the same token, either all addresses or none of them will be
duplicates). Thus, for a set of addresses formed from the same interface
token, it is sufficient to check that one of the addresses link-local address generated
from the token is unique on the link. In such cases, one of those addresses the link-local
address MUST be
verified tested for uniqueness before any of the other addresses
can be assigned to an interface.
Normally, the link-local address would be tested, since it is the first
address to be formed.

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

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

It should also be noted that duplicate address detection will nearly

always need to Duplicate Address Detection must be
performed before prior to assigning an address is assigned to an interface in order to avoid problems that directly result from
prevent multiple nodes from using the same addresses. address simultaneously.  If a
node begins using an address resolution is done in parallel with
duplicate address detection, Duplicate Address
Detection, and the address is subsequently determined
to be in use by another node, node is already using the address, the node
performing duplicate address
detection may send packets containing Duplicate Address Detection will erroneously process traffic

intended for the tentative address that
interfere with other node, resulting in such possible negative
consequences as the proper functioning 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 other nodes, especially the
one already using tests indicate the address. 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 Neighbor Advertisement
message that does not specify pass the validity checks as specified in [DISCOVERY].
A solicitation that passes these validity checks is called a valid
solicitation or valid advertisement.

5.4.2.  Sending Neighbor Solicitation Messages

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

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

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

There should be a way for a node In order to determine whether a sending improve the robustness of the
Duplicate Address Detection algorithm, an interface loops back packets MUST receive and
process datagrams sent to a the all-nodes multicast address. Otherwise it
will not be possible for a node to determine whether a solicitation
received on an interface is from itself address or from another solicited-
node with a
duplicate address. This issue is discussed in more detail below. multicast address of the tentative address while delaying
transmission of the initial Neighbor Solicitation.

5.4.3.  Receiving Neighbor Solicitation and Advertisement 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, tentative (i.e., it is assigned to the
receiving interface), the solicitation is processed in the normal way
[DISCOVERY].  If the target address is tentative,
processing takes place as follows. There are two cases to consider.

If and the source address of the solicitation
is not the unspecified
address, a node unicast address, the solicitation's sender is performing address
resolution on the address.  The
node receiving target; the solicitation should be silently discard the message and ignored.
Otherwise, processing takes place as described below. In all cases, a
node MUST NOT return a response. Responding respond to address resolution requests a Neighbor Solicitation for a tentative address risks polluting the Neighbor Caches of other
nodes should the address already be in use by another node.
address.

If the source address of the Neighbor Solicitation is the unspecified
address, the solicitation is from a node performing duplicate address
detection. There are two cases to consider. First, Duplicate Address
Detection. If the solicitation may
have been sent by the receiving node (e.g., the packet was looped back).
Alternatively, another node (with the same hardware address and/or
interface token) is also attempting to use the address. In the first
case, the solicitation should be ignored. In the second case, from another node, the tentative
address is a duplicate and should not be used (by either node).

Determining whether a multicast solicitation was looped back to If the
sender or actually came from another node
solicitation is implementation-dependent.
If two interfaces happen to have from the same hardware link address, one
cannot distinguish node itself (because the two cases by comparing node loops back
multicast packets), the packet contents.
Instead, solicitation does not indicate the implementation must have a good understanding presence of the
interface's multicast loopback semantics. In particular: 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 Appendix XXX 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 a Neighbor Solicitation, one, the tentative address is a duplicate.
     This condition occurs when two nodes run Duplicate Address
     Detection simultaneously, but transmit initial solicitations at
     different times (e.g., by selecting different random delay values
     before transmitting an initial solicitation).

   - If a the actual number of Neighbor Solicitation has been sent, and an identical one is
     received, Solicitations received exceeds the tentative address is a duplicate if
     number expected based on the loopback semantics (e.g., the
     interface does not loopback multicast packets.

   - In all cases, if packet, yet one or more Neighbor Solicitation for the tentative
     address are received than have been sent, solicitations
     was received), the tentative address is a duplicate.

If This condition
     occurs when two nodes run Duplicate Address Detection
     simultaneously and transmit solicitations at roughly the same time.

5.4.4.  Receiving Neighbor Advertisement Messages

On receipt of a valid Neighbor Advertisement containing message on an interface,

node behavior depends on whether the target address is tentative or not.
If the target address is received
while performing duplicate not tentative, the solicitation is processed in
the normal way [DISCOVERY]. If the target address detection, is tentative, the node MUST disable
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 no such advertisement
is received within the time specified, the address is no longer

considered to be tentative and can be assigned to the interface.

If a duplicate link-local address is detected, the node does not respond to the
solicitation. Instead, it disables formed
from an interface token, the interface and logs a system
management error. SHOULD be disabled.

5.5.  Creation of Global- and Site-Local Addresses

5.5.1.  Sending Router Solicitations

Router Advertisements

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

5.5.1.  Soliciting Router Advertisements

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

5.5.2.  Absence of Router Advertisements

If a link has no routers, a host MUST attempt to use stateful
autoconfiguration to obtain addresses and other configuration
information. An implementation MAY provide a way to disable the
invocation of stateful autoconfiguration in this case, but the default
SHOULD be enabled.  From the perspective of autoconfiguration, a link
has no routers if no Router Advertisements are being received.  Router Advertisements can be absent
in two scenarios:

   - From the time autoconfiguration was last initiated, no Router
     Advertisements have been received at all, after having
sent a small number of Router Solicitations as described in [DISCOVERY].

   - At least one Router Advertisement was received, but enough time has
     elapsed since receipt of the last advertisement that a new one
     should have been received. Autoconfiguration does not attempt to
     detect this situation.

When a host determines that no routers are present on a link, it sets
the value of ManagedFlag and OtherFlag to TRUE, initiating stateful
autoconfiguration as described in Section 5.5.3 (if necessary).  If a
router subsequently begins sending Router Advertisements, the rules in
Section 5.5.3 insure that hosts process them in the proper way.

5.5.3.  Router Advertisement Processing

Autoconfiguration silently ignores Router Advertisement messages
received on interfaces in which the AutoConfig flag is set to FALSE.

On receipt of a valid Router Advertisement (as defined in [DISCOVERY]),
a host copies the value of the advertisement's Managed bit into
ManagedFlag. If the value of ManagedFlag changes from FALSE to TRUE, the
host should invoke the stateful address autoconfiguration protocol. protocol,

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

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

For each Prefix-Information option in the Router Advertisement:

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

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

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

 d) If the prefix advertised prefix matches the prefix of an autoconfigured
    address already in the list, then list of addresses associated with the interface, set
    the preferred timer to that of the option's preferred lifetime, and
    set the valid lifetime to that of the option's valid lifetime.

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

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

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

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

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

5.5.4.  Address Lifetime Expiry

A preferred address becomes deprecated when its preferred lifetime
expires.  A deprecated address SHOULD continue to be used as a source
address in existing communications, but SHOULD NOT be used in new
communications if a current an alternate (non-deprecated) address is available and it
has sufficient scope.  The IP layer MUST continue to accept datagrams
destined to a deprecated address since a deprecated address is still a
valid address for the interface.

An 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 valid destination address for the
interface in incoming communications. on a receiving interface.

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

5.6.  Configuration Consistency

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

have precedence over information learned earlier.

6.  OPEN ISSUES/TODO

  o Is duplicate address detection strong enough (we only send one NS).
     Constants OK? figure out how to do appendices in nroff

  o is configurability of DuplAddrDetect good enough? Note that:

       - One can't assume add wording that all nodes are on the net at any one time,
          so performing DAD just once or twice does not guarantee indicates that
          there won't be collisions later.

       - Turning DuplAddrDetect on/off is difficult in practice. It addrconf is a
          per-host (interface) flag, which means it must required to be turned off
          in each machine. If this is don't on
     by setting a kernel flag and
          then having everyone boot the same kernel, default?

  o Add wording suggesting DAD will be turned
          off and waiting for all nodes, not just a few.

       - it might RAs be nice to turn DuplAddrDetect on/off via RAs, but
          that means nodes will delay creating link-local addresses
          until they've received an RA or concluded that no routers are
          present. This is likely done in parallel.

7.  SECURITY CONSIDERATIONS

Stateless address autoconfiguration allows a host to delay the process longer than
          performing DAD. (Ouch.)

       - perhaps allow RSs connect to be sent out a
network, configure an address and start communicating with unspecified source
          address, in order to solicited RAs other nodes
without ever registering or authenticating itself with at "do/don't perform
          DAD"?

  o Possible Neighbor Discovery Changes

       -Should we allow RSs the local site.
Although this allows unauthorized users to be sent out with unspecified source
          address connect to allow DAD and use a network,
the RSs to be sent threat is inherently present in parallel,
          rather than sequentially. This would reduce the impact of DAD
          delay.

       Need Internet architecture. Any node
with a physical attachment to specify that a Router Solicitation is sent out when
          AutoConfig flag changes from FALSE network can generate an address (using a
variety of ad hoc techniques) that provides connectivity.

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

8.  APPENDIX: LOOPBACK SUPPRESSION AND DUPLICATE ADDRESS DETECTION

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

sent by another node; if one receives more solicitations than were sent,
the tentative address is a duplicate. However, the situation may not
always be this straightforward.

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

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

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

   o If a node that is already using a particular IP address discards
     received packets having the same link-layer source address as the
     interface, it will also discard Duplicate Address Detection-related
     Neighbor Solicitation messages sent by another node also using the
     same link-layer address.  Consequently, Duplicate Address Detection
     will fail, and the other node will configure a non-unique address.
     Since it is generally impossible to TRUE.

  o what know when another node is the correct language
     performing Duplicate Address Detection, this scenario can be
     avoided only if software suppression of loopback is permanently
     disabled.

Thus, to use perform Duplicate Address Detection correctly in talking about an "MAC
     address" used as the case where
two interfaces are using the same link-layer address, an interface token. Should we use "hardware
     address"?  "MAC" address? Something else?

  o ensure use implementation

must have a good understanding of "node" vs. "host" is right; autoconfig really applies
     to only hosts, but duplicate address detection wants to be more
     general. Also, link-local the interface's multicast loopback
semantics, and the interface cannot discard received packets simply
because the source link-layer address can apply to all nodes, not only
     hosts.

7.  SECURITY CONSIDERATIONS

   To be completed.

8. is the same as the interfaces.

9.  REFERENCES

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

   [IPv6-ETHER]
        M. Crawford. "A Method for the Transmission of IPv6 Packets over
        Ethernet Networks", Internet Draft.

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

   [ADDR-ARCH]
        R. Hinden and S. Deering, "Internet Protocol Version (IPv6)
        Addressing Architecture", Internet Draft, May 1995, draft-ietf-
        ipngwg-addr-arch-03.txt

   [DHCPv6]
        Internet Draft, Work in Progress.

   [DISCOVERY]
        T. Narten, E. Nordmark and W. A. Simpson, "Neighbor Discovery
        for IP Version 6 (IPv6)", Internet Draft, September 1995,
        <draft-ietf-ipngwg-discovery-02.txt>

Acknowledgements

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

AUTHORS' ADDRESSES

Susan Thomson                    Thomas Narten
Bellcore                         IBM Corporation
445 South Street                 P.O. Box 12195
Morristown, NJ 07960             Research Triangle Park, NC 27709-2195
USA                              USA

phone: +1 201-829-4514           phone: +1 919 254 7798
email: set@thumper.bellcore.com  email: narten@vnet.ibm.com

CHANGES SINCE PREVIOUS DOCUMENT

Changes since <draft-ietf-addrconf-ipv6-auto-04.txt> based on feedback
from the working group:

   o modified DAD text re loopback suppression and added appendix
     describing how DAD breaks if an interface discards all received
     packets having the same source link-layer address as the receiving
     interface. Added that DAD must be applied to link-layer address for
     stateless.

   o Numerous editorial/wordsmithing changes.

   o security section added

   o loosened requirement that interface be disabled if DAD fails. Now
     say address shouldn't be assigned to interface. However, if address
     was derived from interface token (e.g., link-layer address), then
     interface should be disabled since its effectively disabled in any
     case (no autoconfigured addresses can be formed on this interface.)

   o Use term "link-layer address" rather than "hardware address".

   o Corrected typo in definition of link-local prefix (E8 -> FE80).

   o Removed AutoConfig variable, left as implementation issue how user
     selects what type of autoconfig is desired, though default is
     enabled

   o Added DupAddrDetectTransmits variable specifying how many
     transmissions to perform as part of DAD (defaults to 1, may be 0),
     and specify that ND's RetransTimer as the retransmit timer between
     consecutive NSs.

   o defined interface token to be a bit string.

   o added text indicating  that autoconfiguration only applies to
     multicast-capable interfaces.

   o changed name of OtherFlag variable to OtherConfigFlag