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ADDRCONF Working Group                        Susan Thomson,  Bellcore
INTERNET-DRAFT                                      Thomas Narten, IBM
<draft-ietf-addrconf-ipv6-auto-06.txt>               November 15, 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
   documents of the Internet Engineering Task Force (IETF), its Areas,
   and its Working Groups. Note that other groups may also distribute
   working documents as Internet Drafts.

   Internet Drafts are draft documents valid for a maximum of six
   months. Internet Drafts may be updated, replaced, or obsoleted by
   other documents at any time.  It is not appropriate to use Internet
   Drafts as reference material or to cite them other than as a "working
   draft" or "work in progress."

   To learn the current status of any Internet Draft.  please check the
   1id-abstracts.txt listing contained in the Internet Drafts Shadow
   Directories on ds.internic.net, nic.nordu.net, ftp.nisc.sri.com or
   munnari.oz.au.

   Distribution of this memo is unlimited.

   This Internet Draft expires May 15, 1996.


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



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   specified elsewhere.


















































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Contents

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

   1.  INTRODUCTION.............................................    4

   2.  TERMINOLOGY..............................................    5
      2.1.  Requirements........................................    8

   3.  DESIGN GOALS.............................................    9

   4.  PROTOCOL OVERVIEW........................................   10
      4.1.  Site Renumbering....................................   12

   5.  PROTOCOL SPECIFICATION...................................   13
      5.1.  Node Configuration Variables........................   13
      5.2.  Autoconfiguration-Related Variables.................   14
      5.3.  Creation of Link-Local Addresses....................   14
      5.4.  Duplicate Address Detection.........................   15
         5.4.1.  Message Validation.............................   16
         5.4.2.  Sending Neighbor Solicitation Messages.........   16
         5.4.3.  Receiving Neighbor Solicitation Messages.......   17
         5.4.4.  Receiving Neighbor Advertisement Messages......   18
         5.4.5.  When Duplicate Address Detection Fails.........   18
      5.5.  Creation of Global- and Site-Local Addresses........   18
         5.5.1.  Soliciting Router Advertisements...............   18
         5.5.2.  Absence of Router Advertisements...............   19
         5.5.3.  Router Advertisement Processing................   19
         5.5.4.  Address Lifetime Expiry........................   20
      5.6.  Configuration Consistency...........................   21

   OPEN ISSUES/TODO.............................................   21

   SECURITY CONSIDERATIONS......................................   21

   REFERENCES...................................................   22

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

   APPENDIX: LOOPBACK SUPPRESSION & DUPLICATE ADDRESS DETECTION.   23

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









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1.  INTRODUCTION

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

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

In the stateful autoconfiguration model, hosts obtain interface
addresses and/or configuration information and parameters from a server.
Servers maintain a database that keeps track of which addresses have
been assigned to which hosts. The stateful autoconfiguration protocol
allows hosts to obtain addresses, other configuration information or
both from a server.  Stateless and stateful autoconfiguration complement
each other. For example, a host 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



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

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

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

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



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                 addressed to itself.

   host        - any node that is not a router.

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

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

   interface   - a node's attachment to a link.

   packet      - an IP header plus payload.

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

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

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

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



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   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.

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



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

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

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

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

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


2.1.  Requirements

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




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

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

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

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

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



3.  DESIGN GOALS

Stateless autoconfiguration is designed with the following goals in
mind:

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

   o Small sites consisting of a set of machines attached to a single
     link should not require the presence of a stateful server or router
     as a prerequisite for communicating.  Plug-and-play communication
     is achieved through the use of link-local addresses.  Link-local
     addresses have a well-known prefix that identifies the (single)



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     shared link to which a set of nodes attach. A host forms a link-
     local address by appending its interface token to the link-local
     prefix.

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

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

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



4.  PROTOCOL OVERVIEW

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

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



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

The next phase of autoconfiguration involves obtaining a Router
Advertisement or determining that no routers are present. If routers are
present, they will send Router Advertisements that specify what sort of
autoconfiguration a host should do.  If no routers are present, stateful
autoconfiguration should be invoked.

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

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

Because routers generate Router Advertisements periodically, hosts will
continually receive new advertisements. Hosts process the information



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contained in each advertisement as described above, adding to and
refreshing information received in previous advertisements.

For safety, all addresses must be tested for uniqueness prior to their
assignment to an interface.  In the case of addresses created through
stateless autoconfig, however, the uniqueness of an address is
determined primarily by the portion of the address formed from an
interface token.  Thus, if a node has already verified the uniqueness of
a link-local address, additional addresses created from the same
interface token need not be tested individually. In contrast, all
addresses obtained manually or via stateful address autoconfiguration
should be tested for uniqueness individually. To accommodate sites that
believe the overhead of performing Duplicate Address Detection outweighs
its benefits, the use of Duplicate Address Detection can be disabled
through the administrative setting of a per-interface configuration
flag.

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


4.1.  Site Renumbering

Address leasing facilitates site renumbering by providing a mechanism to
time-out addresses in hosts.  At present, upper layer protocols such as
TCP provide no support for changing endpoint addresses while a
connection is open. If an end-point address becomes invalid, existing
connections break and all communication to the invalid address fails.
Even when applications use UDP as a transport protocol, addresses must
generally remain the same during a packet exchange.

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




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

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



5.  PROTOCOL SPECIFICATION

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


5.1.  Node Configuration Variables

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

     DupAddrDetectTransmits

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

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

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



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waits  after sending the last Neighbor Solicitation before ending the
Duplicate Address Detection process.


5.2.  Autoconfiguration-Related Variables

A host maintains a number of data structures and flags related to
autoconfiguration. In the following, we present conceptual variables and
show how they are used to perform autoconfiguration. The specific
variables are used for demonstration purposes only, and an
implementation is not required to have them, so long as its external
behavior is consistent with that described in this document.

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


   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.

   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.

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


5.3.  Creation of Link-Local Addresses

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

   - 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.




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

A link-local address is formed by prepending the well-known link-local
prefix FE80::0 [ADDR-ARCH] (of appropriate length) to the interface
token. If the interface token has a length of N bits, the interface
token replaces the right-most N zero bits of the link-local prefix.  If
the interface token is more than 118 bits in length, autoconfiguration
fails and manual configuration is required.

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


5.4.  Duplicate Address Detection

Duplicate Address Detection MUST be performed on unicast addresses prior
to assigning them to an interface whose DupAddrDetectTransmits variable
is greater than zero. Duplicate Address Detection takes place on all
unicast addresses, regardless of whether they are obtained through
stateful, stateless or manual configuration. (Duplicate Address
Detection MUST NOT be performed on anycast addresses.) Each individual
unicast address SHOULD be tested for uniqueness. However, when stateless
address autoconfiguration is used, address uniqueness is determined
solely by the interface token, assuming that subnet prefixes are
assigned correctly (i.e., if all of an interface's addresses are
generated from the same token, either all addresses or none of them will
be duplicates). Thus, for a set of addresses formed from the same
interface token, it is sufficient to check that the link-local address
generated from the token is unique on the link. In such cases, the link-
local address MUST be tested for uniqueness before any of the other
addresses can be assigned to an interface.

The procedure for detecting duplicate addresses uses Neighbor
Solicitation and Advertisement messages as described below. If a
duplicate address is discovered during the procedure, the address cannot
be assigned to the interface. If the address is derived from an
interface token, a new token will need to be assigned to the interface,
or all IP addresses for the interface will need to be manually
configured.  Note that the method for detecting duplicates is not
completely reliable, and it is possible that duplicate addresses will
still exist (e.g., if the link was partitioned while Duplicate Address
Detection was performed).

An address on which the duplicate Address Detection Procedure is applied
is said to be tentative until the procedure has completed successfully.
A tentative address is not considered "assigned to an interface" in the
traditional sense. That is, the interface must accept Neighbor



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Solicitation and Advertisement messages containing the tentative address
in the Target Address field, but processes such packets differently from
those whose Target Address matches an address assigned to the interface.
Other packets addressed to the tentative address should be silently
discarded.

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

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


5.4.1.  Message Validation

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


5.4.2.  Sending Neighbor Solicitation Messages

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

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

If the Neighbor Solicitation is the first message to be sent from an
interface after interface (re)initialization, the node should delay



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sending the message by a random delay between 0 and
MAX_RTR_SOLICITATION_DELAY as specified in [DISCOVERY].  This serves to
alleviate congestion when many nodes start up on the link at the same
time, such as after a power failure, and may help to avoid race
conditions when more than one node is trying to solicit for the same
address at the same time. In order to improve the robustness of the
Duplicate Address Detection algorithm, an interface MUST receive and
process datagrams sent to the all-nodes multicast address or solicited-
node multicast address of the tentative address while delaying
transmission of the initial Neighbor Solicitation.




5.4.3.  Receiving Neighbor Solicitation Messages

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

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

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

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

   - If a Neighbor Solicitation for a tentative address is received
     prior to having sent one, the tentative address is a duplicate.
     This condition occurs when two nodes run Duplicate Address
     Detection simultaneously, but transmit initial solicitations at



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     different times (e.g., by selecting different random delay values
     before transmitting an initial solicitation).

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


5.4.4.  Receiving Neighbor Advertisement Messages

On receipt of a valid Neighbor Advertisement message on an interface,
node behavior depends on whether the target address is tentative or
matches a unicast or anycast address assigned to the interface.  If the
target address is assigned to the receiving interface, the solicitation
is processed in the normal way [DISCOVERY]. If the target address is
tentative, the tentative address is not unique.


5.4.5.  When Duplicate Address Detection Fails

A tentative address that is determined to be a duplicate as described
above, MUST NOT be assigned to an interface and the node SHOULD log a
system management error.  If the address is a link-local address formed
from an interface token, the interface SHOULD be disabled.


5.5.  Creation of Global- and Site-Local Addresses

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







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5.5.2.  Absence of Router Advertisements

If a link has no routers, a host MUST attempt to use stateful
autoconfiguration to obtain addresses and other configuration
information.  An implementation MAY provide a way to disable the
invocation of stateful autoconfiguration in this case, but the default
SHOULD be enabled.  From the perspective of autoconfiguration, a link
has no routers if no Router Advertisements are received after having
sent a small number of Router Solicitations as described in [DISCOVERY].


5.5.3.  Router Advertisement Processing

On receipt of a valid Router Advertisement (as defined in [DISCOVERY]),
a host copies the value of the advertisement's Managed bit into
ManagedFlag. If the value of ManagedFlag changes from FALSE to TRUE, the
host should invoke the stateful address autoconfiguration protocol,
requesting address information.  If the value of the ManagedFlag changes
from TRUE to FALSE, the host should terminate the stateful address
autoconfiguration protocol (i.e., stop requesting addresses and ignore
subsequent responses to in-progress transactions). If the value of the
flag stays unchanged, no special action takes place. In particular, a
host MUST NOT reinvoke stateful address configuration if it is already
participating in the stateful protocol as a result of an earlier
advertisement.

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

For each Prefix-Information option in the Router Advertisement:

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

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

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



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

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

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


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

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

    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 an alternate (non-deprecated) address is available and
has sufficient scope.  The IP layer MUST continue to accept datagrams
destined to a deprecated address since a deprecated address is still a
valid address for the interface. An implementation MAY prevent any new
communication from using a deprecated address, but system management
MUST have the ability to disable such a facility.

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




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


5.6.  Configuration Consistency

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


OPEN ISSUES/TODO


  o figure out how to do appendices in nroff

  o add wording that indicates that addrconf is required to be turned on
     by default?

  o Add wording suggesting DAD and waiting for RAs be done in parallel.


SECURITY CONSIDERATIONS


Stateless address autoconfiguration allows a host to connect to a
network, configure an address and start communicating with other nodes
without ever registering or authenticating itself with the local site.
Although this allows unauthorized users to connect to and use a network,
the threat is inherently present in the Internet architecture. Any node
with a physical attachment to a network can generate an address (using a
variety of ad hoc techniques) that provides connectivity.

The use of 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



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the address as a duplicate. This attack is similar to other attacks
involving the spoofing of Neighbor Discovery messages and can be
addressed by requiring that Neighbor Discovery packets be authenticated
[RFC1826].



REFERENCES



   [RFC1826]
        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






draft-ietf-addrconf-ipv6-auto-06.txt                           [Page 22]


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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




APPENDIX: LOOPBACK SUPPRESSION & 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



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

Thus, to perform Duplicate Address Detection correctly in the case where
two interfaces are using the same link-layer address, an implementation
must have a good understanding of the interface's multicast loopback
semantics, and the interface cannot discard received packets simply
because the source link-layer address is the same as the interfaces.


CHANGES SINCE PREVIOUS DOCUMENT


Changes since <draft-ietf-addrconf-ipv6-auto-06.txt> based on feedback
from the working group:

   o minor wordsmithing

   o made explicit that DAD could be turned off by system management.

   o Fixed references to link-type specific where they had been written
     as link-specific.

   o clarified DAD should not be performed on anycast addresses

   o allow an implementation to provide a switch that disables the use
     of deprecated addresses for any new communications.



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   o Fixed two typos where "deprecation lifetime" was used rather than
     "valid lifetime"

   o made abstract more accurate


     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





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