WDIFF

draft-ietf-addrconf-ipv6-auto-04.txt --> draft-ietf-addrconf-ipv6-auto-05.txt

ADDRCONF Working Group Susan Thomson, Bellcore
INTERNET-DRAFT Thomas Narten, IBM
<draft-ietf-addrconf-ipv6-auto-04.txt> October 4,
<draft-ietf-addrconf-ipv6-auto-05.txt> November 3, 1995

IPv6 Stateless Address Autoconfiguration

Status of this Memo

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

This document is an Internet Draft. Internet Drafts are working
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
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To learn the current status of any Internet Draft. please check the
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Distribution of this memo is unlimited.

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

Abstract

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

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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 11

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

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

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

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

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

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

draft-ietf-addrconf-ipv6-auto-04.txt 22

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

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1. INTRODUCTION

This document specifies how hosts autoconfigure their interfaces in IP
version 6. The autoconfiguration process includes creating a link-local
address and verifying its uniqueness on a link, determining what
information should be autoconfigured (addresses, other information, or
both), and in the case of addresses, whether they should be obtained
through the stateless mechanism, the stateless stateful mechanism, or both. This
document also specifies stateless address autoconfiguration. defines the process for generating a link-local address, the
process for generating site-local and global addresses, and Duplicate
Address Detection. Stateful address autoconfiguration is specified
elsewhere.

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

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

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

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

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indicates how long the address is bound to an interface. When a lifetime
expires, the binding (and address) becomes become invalid and the address may

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reassigned to another interface elsewhere in the Internet. To handle the
expiration of address bindings gracefully, an address goes through two
distinct phases while assigned to an interface. Initially, an address is
"preferred", meaning that its use in arbitrary communication is
unrestricted. Later, an address becomes "deprecated" in anticipation
that its current interface binding will become invalid. While in a
deprecated state, the use of an address is discouraged, but not strictly
forbidden. New communication (e.g., the opening of a new TCP
connection) gives preference to using a non-deprecated address, with should use
of the a preferred address when possible. A deprecated
address restricted to should be used only by applications that have been using the deprecated address already it and
would have difficulty switching to another address without a service
disruption.

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

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

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

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

2. TERMINOLOGY

IP - Internet Protocol Version 6. The terms IPv4 and IPv6
are used only in contexts where necessary to avoid
ambiguity.

node - a device that implements IP.

router - a node that forwards IP packets not explicitly
addressed to itself.

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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

draft-ietf-addrconf-ipv6-auto-04.txt
interfaces have a link-local unicast address.

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

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

preferred address
- an address assigned to an interface whose use by upper
layer protocols is unrestricted. Preferred addresses
may be used as the source or destination (or destination) address of
packets sent from or to (or to) the interface.

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

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

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invalid address
- an address that is not assigned to any interface. A
valid address becomes invalid when its deprecation

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lifetime expires. Invalid addresses should not appear
as the destination or source address of a packet. In
the former case, the internet routing system will be
unable to deliver the packet, in the later case the
recipient of the packet will be unable to respond to
it.

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

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

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

2.1. Requirements

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

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

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

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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 weighed before implementing any behavior described with
this label.

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

3. DESIGN GOALS

Stateless autoconfiguration is designed with the following goals in
mind:

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

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

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

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

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

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

If a node determines that its tentative link-local address is not
unique, autoconfiguration stops and manual configuration of the machine
is required. To simplify recovery in this case, it should be possible
for an administrator to supply an alternate interface token that
overrides the default token in such a way that the autoconfiguration
mechanism can then be applied using the new (presumably unique)
interface token. Alternatively, link-local and other addresses will

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need to be configured manually.

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

To generate site-local or global addresses, 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 host listens for Router
Advertisements. To obtain an advertisement quickly,
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. If no
Router Advertisement is received, the host assumes that stateful address
autoconfiguration is desired and invokes an appropriate protocol.

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INTERNET-DRAFT IPv6 Stateless Address Autoconfiguration October 1995 Router Advertisements contain two
flags indicating what type of stateful autoconfiguration (if any) should
be performed. A "managed address configuration" flag indicates whether
hosts should use stateful autoconfiguration to obtain addresses. An
"other stateful configuration" flag indicates whether hosts should use
stateful autoconfiguration to obtain additional information (excluding
addresses).

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

Routers advertise

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

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

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interface token. Thus, if a node has already verified the uniqueness of
a link-local address, additional addresses created from the same
interface token need not be tested for uniqueness. individually. In contrast, all
addresses obtained manually or via stateful address autoconfiguration
should be tested for uniqueness individually. To accommodate sites that
believe the overhead of performing duplicate address detection Duplicate Address Detection outweighs
its benefits, the use of duplicate address detection Duplicate Address Detection can be disabled
through the administrative setting of a per-interface configuration
flag.

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

4.1. Site Renumbering

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

Distinguishing

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

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

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

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

5. PROTOCOL SPECIFICATION

Address autoconfiguration

Autoconfiguration is performed on a per-interface basis. basis on multicast-
capable interfaces. For multihomed hosts, address autoconfiguration is
performed independently on each interface.

5.1. Host Configuration Variables

A host MUST allow the following variable Autoconfiguration applies
primarily 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 hosts, with two exceptions. Routers are expected to
generate a link-local address using the duplicate detection procedure (Section 5.4) to verify outlined below. In
addition, routers perform Duplicate Address Detection on all addresses are
unique before
prior to assigning them to an interface.

Default: TRUE

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5.1. Node Configuration Variables

A node MUST allow the following autoconfiguration-related variable to be
configured 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-specific
value in the document that covers issues related to
the transmission of IP over a particular link type
(e.g., [IPv6-ETHER]).

Autoconfiguration October 1995 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
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: flags related to
autoconfiguration. In the following, we present conceptual variables and

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

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

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

OtherFlag

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

AddressList List

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

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

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

5.3. Creation of Link-Local Addresses

A host node forms a link-local address whenever an interface is initialized
and the AutoConfig flag is TRUE. (Note that the AutoConfig flag may be
set independently of interface initialization. If the link-local address
has not yet been created when the AutoConfig is changed from FALSE to
TRUE, it is created at this time.) becomes enabled.
An interface is initialized may become enabled after any of the following events:

- The interface is initialized at system startup time.

- The interface is reinitialized after a temporary interface failure
or after being temporarily disabled by system management.

- The interface attaches to a link for the first time.

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

A link-local address is formed by prepending the well-known link-local
prefix E8::0 FE80::0 [ADDR-ARCH] (of appropriate length) to the interface
token.

draft-ietf-addrconf-ipv6-auto-04.txt [Page 12]

INTERNET-DRAFT IPv6 Stateless Address Autoconfiguration October 1995 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.

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INTERNET-DRAFT IPv6 Stateless Address Autoconfiguration November 1995

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

5.4. Verifying The Uniqueness Of An Duplicate Address Detection

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

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

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

It should also be noted that duplicate address detection will nearly

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INTERNET-DRAFT IPv6 Stateless Duplicate Address Autoconfiguration October 1995

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

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INTERNET-DRAFT IPv6 Stateless Address Autoconfiguration November 1995

intended for the tentative address that
interfere with other node, resulting in such possible negative
consequences as the proper functioning resetting of open TCP connections.

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

5.4.1. Message Validation

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

5.4.2. Sending Neighbor Solicitation Messages

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

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

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

There should be a way for a node In order to determine whether a sending improve the robustness of the
Duplicate Address Detection algorithm, an interface loops back packets MUST receive and
process datagrams sent to a the all-nodes multicast address. Otherwise it
will not be possible for a address or solicited-
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.

draft-ietf-addrconf-ipv6-auto-04.txt multicast address of the tentative address while delaying
transmission of the initial Neighbor Solicitation.

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

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

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

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

- If a Neighbor Solicitation for
duplicate address.

Implementor's Note: many interfaces provide a tentative address is received
prior way for upper layers to having sent a Neighbor Solicitation,
selectively enable and disable the tentative address
is a duplicate.

- If looping back of multicast packets.
The details of how such a Neighbor Solicitation has been sent, and an identical one facility is
received, the implemented may prevent Duplicate
Address Detection from working correctly. See Appendix XXX for further
discussion.

The following tests identify conditions under which a tentative address
is a duplicate if the interface
does not loopback multicast packets. unique:

- In all cases, if more If a Neighbor Solicitation for the a tentative address are is received than have been sent,
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
different times (e.g., by selecting different random delay values
before transmitting an initial solicitation).

- If a Neighbor Advertisement containing the tentative address is actual number of Neighbor Solicitations received
while performing duplicate address detection, exceeds the node MUST disable that
interface and log a system management error. If no such advertisement
is received within
number expected based on the time specified, loopback semantics (e.g., the
interface does not loopback packet, yet one or more solicitations
was received), the tentative address is no longer

draft-ietf-addrconf-ipv6-auto-04.txt 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,

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INTERNET-DRAFT IPv6 Stateless Address Autoconfiguration October November 1995

considered to be

node behavior depends on whether the target address is tentative and can be assigned to or not.
If the interface. target address is not tentative, the solicitation is processed in
the normal way [DISCOVERY]. If a duplicate the target address is detected, tentative, the node does
tentative address is not respond unique.

5.4.5. When Duplicate Address Detection Fails

A tentative address that is determined to the
solicitation. Instead, it disables the be a duplicate as described
above, MUST NOT be assigned to an interface and logs 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

5.5.1. Sending Router Solicitations

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

5.5.1. Soliciting Router Advertisements

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

5.5.2. Absence of Router Advertisements

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

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

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

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

5.5.3. Router Advertisement Processing

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

On receipt of a valid Router Advertisement (as defined in [DISCOVERY]),
a host copies the value of the advertisement's Managed bit into

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INTERNET-DRAFT IPv6 Stateless Address Autoconfiguration October 1995
ManagedFlag. If the value of ManagedFlag changes from FALSE to TRUE, the
host should invoke the stateful address autoconfiguration protocol. protocol,

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INTERNET-DRAFT IPv6 Stateless Address Autoconfiguration November 1995

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

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

For each Prefix-Information option in the Router Advertisement:

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

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

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

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

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

If the sum of the prefix length and interface token length does not

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INTERNET-DRAFT IPv6 Stateless Address Autoconfiguration October 1995
equal 128 bits, the Prefix Information option MUST be ignored. An
implementation MAY wish to log a system management error in this

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INTERNET-DRAFT IPv6 Stateless Address Autoconfiguration November 1995

case. It is the responsibility of the system administrator to insure
that the lengths of prefixes contained in Router Advertisements are
consistent with the length of interface tokens for that link type.

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

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

5.5.4. Address Lifetime Expiry

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

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

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

5.6. Configuration Consistency

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

draft-ietf-addrconf-ipv6-auto-04.txt inconsistent information is
learned from different sources, the most recently obtained values always

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INTERNET-DRAFT IPv6 Stateless Address Autoconfiguration October November 1995

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

have precedence over information learned earlier.

6. OPEN ISSUES/TODO

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

o is configurability of DuplAddrDetect good enough? Note that:

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

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

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

- it might RAs be nice done in parallel.

7. SECURITY CONSIDERATIONS

Stateless address autoconfiguration allows a host to turn DuplAddrDetect on/off via RAs, but
that means nodes will delay creating link-local addresses
until they've received connect to a
network, configure an RA address and start communicating with other nodes
without ever registering or concluded that no routers are
present. This is likely to delay authenticating itself with the process longer than
performing DAD. (Ouch.)

- perhaps allow RSs local site.
Although this allows unauthorized users to be sent out with unspecified source
address, in order connect to solicited RAs and use a network,
the threat is inherently present in the Internet architecture. Any node
with at "do/don't perform
DAD"?

o Possible Neighbor Discovery Changes

-Should we allow RSs a physical attachment to be sent 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
the address as a duplicate. This attack is similar to other attacks
involving the spoofing of Neighbor Discovery messages and can be
addressed by requiring that Neighbor Discovery packets be authenticated
[RFC1826].

8. APPENDIX: LOOPBACK SUPPRESSION AND DUPLICATE ADDRESS DETECTION

Determining whether a multicast solicitation was looped back to the
sender or actually came from another node is implementation-dependent.
A problematic case occurs when two interfaces attached to the same link
happen to have the same token and link-layer address, and they both send
out packets with unspecified source identical contents at roughly the same time (e.g.,
Neighbor Solicitations for a tentative address to allow DAD 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 RSs other node by simply comparing packet contents
(i.e., the contents are identical). In this particular case, it is not
necessary to be know precisely which packet was looped back and which was

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INTERNET-DRAFT IPv6 Stateless Address Autoconfiguration November 1995

sent in parallel,
rather by another node; if one receives more solicitations than sequentially. This would reduce were sent,
the impact 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 DAD
delay.

Need packets sent to specify a multicast group that the sending host is a Router Solicitation
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 sent 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 when
AutoConfig flag changes unwanted loopbacks is to
discard any received packet whose link-layer source address is the same
as the receiving interface's. Unfortunately, use of that criteria also
results in the discarding of all packets sent by another node using the
same link-layer address. Duplicate Address Detection will fail on
interfaces that filter received packets in this manner:

o If a node performing Duplicate Address Detection discards received
packets having the same source link-layer address as the receiving
interface, it will also discard packets from FALSE other nodes also using
the same link-layer address, including Neighbor Advertisement and
Neighbor Solicitation messages required to TRUE. 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 what 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 the correct language permanently
disabled.

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

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

draft-ietf-addrconf-ipv6-auto-04.txt implementation

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INTERNET-DRAFT IPv6 Stateless Address Autoconfiguration October November 1995

to only hosts, but duplicate address detection wants to be more
general. Also, link-local

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 can apply to all nodes, not only
hosts.

7. SECURITY CONSIDERATIONS

To be completed.

8. is the same as the interfaces.

9. REFERENCES

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

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

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

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

[DHCPv6]
Internet Draft, Work in Progress.

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

Acknowledgements

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

AUTHORS' ADDRESSES

draft-ietf-addrconf-ipv6-auto-04.txt

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INTERNET-DRAFT IPv6 Stateless Address Autoconfiguration October November 1995

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

draft-ietf-addrconf-ipv6-auto-04.txt

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CHANGES SINCE PREVIOUS DOCUMENT

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

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

o Numerous editorial/wordsmithing changes.

o security section added

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

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

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

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

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

o defined interface token to be a bit string.

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

o changed name of OtherFlag variable to OtherConfigFlag

draft-ietf-addrconf-ipv6-auto-05.txt [Page 24]

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