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ADDRCONF Working Group                        Susan Thomson,  Bellcore
INTERNET-DRAFT                                      Thomas Narten, IBM
<draft-ietf-addrconf-ipv6-auto-04.txt>                 October 4, 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 April 4, 1996.


Abstract

   This document specifies how hosts autoconfigure addresses for their
   interfaces in IP version 6. In particular, the document describes the
   steps a host takes in determining whether address autoconfiguration
   should be used, and if so, whether to use the stateful mechanism, the
   stateless mechanism or both. This document also specifies stateless
   address autoconfiguration, and how nodes verify the uniqueness of an
   address before assigning it to an interface.  Stateful address
   autoconfiguration is specified elsewhere.






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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

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

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

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

   8.  REFERENCES............................................   20

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















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1.  INTRODUCTION

This document specifies how hosts autoconfigure their interfaces in IP
version 6. The autoconfiguration process includes 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 mechanism, or both.  This
document also specifies stateless address autoconfiguration.  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 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
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 invalid and the address may



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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 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 use
of the deprecated address restricted to applications that have been
using the deprecated address already and would have difficulty switching
to another address without a service disruption.

Finally, this document describes the algorithm a node employs to verify
that an address it is about to assign to an interface is unique on the
link. The "duplicate address detection" algorithm is used before an
address is actually used, independent of whether the address was
obtained via stateless or stateful autoconfiguration.

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 routers to use the mechanism
described in this document for generating a link-local address. All
nodes (not only hosts) should use the duplicate address detection
procedure.

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



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                 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



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                 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 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 Discover 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 address
                 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.



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   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.
                 When the preferred lifetime expires, the address
                 becomes deprecated.

   valid lifetime
               - the length of time an address remains in the valid
                 state. 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 address
                 is an example of a possible interface token. The manner
                 in which an interface token is created and its length
                 is link-specific, and is described in the specification
                 for the particular link type (e.g., [IPv6-ETHER]).


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



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     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 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.,
     an "interface token").  In the simplest case, an interface token
     consists of the link's hardware 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 its
     interface token.

   o A large site with multiple networks and routers should not require
     the presence of a stateful address configuration server. To enable
     hosts to generate site-local or global addresses, Router
     Advertisements, which are generated by routers, include options
     that list the set of active prefixes on a link.




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   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 the typical steps that take place when an
interface autoconfigures itself. Autoconfiguration of a link-local
address normally takes place when an interface is (re)initialized, e.g.
at startup.  Autoconfiguration of global and site-local addresses and
other parameters is done periodically, starting as soon as possible
after an interface has been initialised or enabled for
autoconfiguration.

A node starts the autoconfiguration process by generating a link-local
address for the interface.  Before the address can be used, however, the
node attempts to verify that the "tentative" address is not already in
use by another node on the link. The node 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. If a node determines that its tentative link-local address is not
unique, autoconfiguration stops and manual configuration of the machine
is required.

Once a node ascertains that the tentative 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 or global addresses, a host listens for Router
Advertisements.  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.



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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 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 stateless address autoconfiguration
should be done. 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 Router Advertisements periodically. Hosts process the
information contained in each advertisement as described above.

For safety, all addresses obtained through autoconfiguration should be
tested for uniqueness.  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. In contrast, all
addresses obtained 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.


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 no support for changing endpoint addresses while a TCP
connection is open. If an end-point address changes, existing
connections break and all communication to the old address fails.  Even
when applications use UDP as a transport protocol, addresses must
generally remain the same during a packet exchange.

Distinguishing valid addresses into preferred and deprecated categories
provides a way of indicating to upper layers that a valid address may



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become invalid shortly, and 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 is performed on a per-interface basis.  For
multihomed hosts, address autoconfiguration is performed independently
on each interface.


5.1.  Host Configuration Variables

A host MUST allow the following variable to be configured for each
multicast interface:

     AutoConfig     If set, the host autoconfigures itself following the
                    procedure described in this document.

                    Default: TRUE


     DuplAddrDetect If set, the node MUST use the duplicate detection
                    procedure (Section 5.4) to verify addresses are
                    unique before assigning them to an interface.

                    Default: TRUE




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5.2.  Autoconfiguration-Related Variables

A host maintains a number of data structures and flags:


     ManagedFlag    Copied from the Managed field of the most recently
                    received Router Advertisement message. The flag
                    starts out in a FALSE state.

     OtherFlag      Copied from the Other field of the most recently
                    received Router Advertisement message.  The flag
                    starts out in a FALSE state.

     AddressList    List of addresses together with their associated
                    lifetimes. Addresses on the list can be obtained
                    through stateless or stateful address
                    autoconfiguration, or some other external mechanism.
                    AddressList initially 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 and OtherFlag MUST be set to FALSE, with
any in-progress autoconfiguration activities interrupted as described
below in Section 5.5.3.


5.3.  Creation of Link-Local Addresses


A host 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.) An interface is initialized after 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.

A link-local address is formed by prepending the well-known link-local
prefix E8::0 [ADDR-ARCH] (of appropriate length) to the interface token.



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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 Address

Duplicate address detection is performed on an interface only if the
DuplAddrDetect configuration variable is set to TRUE.

Duplicate address detection is applied to an address once after an
address is created, but before assigning it to an interface, regardless
of whether the address is obtained through stateful, stateless or manual
configuration.  All addresses 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 is
unique on the link. In such cases, one of those addresses MUST be
verified before any of the 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 Neighbor
Solicitation and Advertisement messages as described below. If a
duplicate address is discovered during the procedure, the interface will
need to be manually configured with a new token, 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 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



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always need to be performed before an address is assigned to an
interface to avoid problems that directly result from multiple nodes
using the same addresses. If address resolution is done in parallel with
duplicate address detection, and the address is subsequently determined
to be in use by another node, the node performing duplicate address
detection may send packets containing the tentative address that
interfere with the proper functioning of the other nodes, especially the
one already using the address.

5.4.1.  Message Validation

A node MUST silently discard any Neighbor Solicitation or Neighbor
Advertisement that does not specify 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 Neighbor Solicitation with a Target
Address set to the address being checked. The source of the solicitation
is set to the unspecified address and the 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 the
message by a random amount of time 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 to determine whether a sending
interface loops back packets sent to a multicast address. Otherwise it
will not be possible for a node to determine whether a solicitation
received on an interface is from itself or from another node with a
duplicate address. This issue is discussed in more detail below.






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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, 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 the source address of the solicitation is not the unspecified
address, a node is performing address resolution on the address.  The
node receiving the solicitation should silently discard the message and
MUST NOT return a response. Responding to address resolution requests
for a tentative address risks polluting the Neighbor Caches of other
nodes should the address already be in use by another node.

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, 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, the
tentative address is a duplicate and should not be used (by either
node).

Determining whether a multicast solicitation was looped back to the
sender or actually came from another node is implementation-dependent.
If two interfaces happen to have the same hardware link address, one
cannot distinguish the two cases by comparing the packet contents.
Instead, the implementation must have a good understanding of the
interface's multicast loopback semantics. In particular:

   - If a Neighbor Solicitation for a tentative address is received
     prior to having sent a Neighbor Solicitation, the tentative address
     is a duplicate.

   - If a Neighbor Solicitation has been sent, and an identical one is
     received, the tentative address is a duplicate if the interface
     does not loopback multicast packets.

   - In all cases, if more Neighbor Solicitation for the tentative
     address are received than have been sent, the tentative address is
     a duplicate.

If a Neighbor Advertisement containing the tentative address is received
while performing duplicate address detection, the node MUST disable that
interface and log a system management error.  If no such advertisement
is received within the time specified, the address is no longer



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

If a duplicate address is detected, the node does not respond to the
solicitation. Instead, it disables the interface and logs a system
management error.


5.5.  Creation of Global- and Site-Local Addresses

5.5.1.  Sending Router Solicitations

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 use stateful autoconfiguration to
obtain addresses and other configuration information.  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 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



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ManagedFlag. If the value of ManagedFlag changes from FALSE to TRUE, the
host should invoke the stateful address autoconfiguration protocol.  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. If the value of
OtherFlag changes from FALSE to TRUE, the host should invoke the
stateful autoconfiguration protocol.  If the value of the OtherFlag
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.

 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.

 d) If the prefix advertised matches the prefix of an autoconfigured
    address already in the list, then 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 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



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    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 (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 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.

Note that if a Prefix Information option is received with a preferred
lifetime of zero, the address with that prefix is immediately
deprecated. Similarly, if the advertised valid lifetime is zero, the
address with that prefix immediately becomes invalid.


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 both by a
router[DISCOVERY] and the stateful protocol.  If the same configuration
information is provided using multiple sources, then the value of this
information should be consistent. However, it is not an error if the
information is detected to be inconsistent: hosts accept the union of
all information received using the stateless and stateful protocols. If



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different sources configure the same information, then the parameters
are updated with the most recently advertised values.




6.  OPEN ISSUES/TODO

  o Is duplicate address detection strong enough (we only send one NS).
     Constants OK?

  o is configurability of DuplAddrDetect good enough? Note that:

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

       - Turning DuplAddrDetect on/off is difficult in practice. It is a
          per-host (interface) flag, which means it must be turned off
          in each machine. If this is don't by setting a kernel flag and
          then having everyone boot the same kernel, DAD will be turned
          off for all nodes, not just a few.

       - it might 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 to delay the process longer than
          performing DAD. (Ouch.)

       - perhaps allow RSs to be sent out with unspecified source
          address, in order to solicited RAs with at "do/don't perform
          DAD"?

  o Possible Neighbor Discovery Changes

       -Should we allow RSs to be sent out with unspecified source
          address to allow DAD and the RSs to be sent in parallel,
          rather than sequentially. This would reduce the impact of DAD
          delay.

       Need to specify that a Router Solicitation is sent out when
          AutoConfig flag changes from FALSE to TRUE.

  o what is the correct language to use in talking about an "MAC
     address" used as an interface token. Should we use "hardware
     address"?  "MAC" address? Something else?

  o ensure use of "node" vs. "host" is right; autoconfig really applies



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     to only hosts, but duplicate address detection wants to be more
     general. Also, link-local address can apply to all nodes, not only
     hosts.



7.  SECURITY CONSIDERATIONS

   To be completed.



8.  REFERENCES


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

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

   [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












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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











































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