IETF IPv6 Working Group S. Thomson
Internet-Draft Cisco
Expires: August 9, 2004 T. Narten
IBM
T. Jinmei
Toshiba
H. Soliman
Flarion Technologies
February 9, 2004
IPv6 Stateless Address Autoconfiguration
draft-ietf-ipv6-rfc2462bis-00.txt
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document specifies the steps a host takes in deciding how to
autoconfigure its interfaces in IP version 6. The autoconfiguration
process includes creating a link-local address and verifying its
uniqueness on a link, determining what information should be
autoconfigured (addresses, other information, or both), and in the
case of addresses, whether they should be obtained through the
stateless mechanism, the stateful mechanism, or both. This document
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defines the process for generating a link-local address, the process
for generating global addresses via stateless address
autoconfiguration, and the Duplicate Address Detection procedure. The
details of autoconfiguration using the stateful protocol are
specified elsewhere.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. TERMINOLOGY . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 Requirements . . . . . . . . . . . . . . . . . . . . . . . . 7
3. DESIGN GOALS . . . . . . . . . . . . . . . . . . . . . . . . 7
4. PROTOCOL OVERVIEW . . . . . . . . . . . . . . . . . . . . . 8
4.1 Site Renumbering . . . . . . . . . . . . . . . . . . . . . . 10
5. PROTOCOL SPECIFICATION . . . . . . . . . . . . . . . . . . . 11
5.1 Node Configuration Variables . . . . . . . . . . . . . . . . 11
5.2 Autoconfiguration-Related Variables . . . . . . . . . . . . 12
5.3 Creation of Link-Local Addresses . . . . . . . . . . . . . . 12
5.4 Duplicate Address Detection . . . . . . . . . . . . . . . . 13
5.4.1 Message Validation . . . . . . . . . . . . . . . . . . . . . 14
5.4.2 Sending Neighbor Solicitation Messages . . . . . . . . . . . 15
5.4.3 Receiving Neighbor Solicitation Messages . . . . . . . . . . 15
5.4.4 Receiving Neighbor Advertisement Messages . . . . . . . . . 16
5.4.5 When Duplicate Address Detection Fails . . . . . . . . . . . 17
5.5 Creation of Global Addresses . . . . . . . . . . . . . . . . 17
5.5.1 Soliciting Router Advertisements . . . . . . . . . . . . . . 17
5.5.2 Absence of Router Advertisements . . . . . . . . . . . . . . 17
5.5.3 Router Advertisement Processing . . . . . . . . . . . . . . 18
5.5.4 Address Lifetime Expiry . . . . . . . . . . . . . . . . . . 20
5.6 Configuration Consistency . . . . . . . . . . . . . . . . . 21
5.7 Retaining Configured Addresses for Stability . . . . . . . . 21
6. SECURITY CONSIDERATIONS . . . . . . . . . . . . . . . . . . 21
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 22
Normative References . . . . . . . . . . . . . . . . . . . . 22
Informative References . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 23
A. LOOPBACK SUPPRESSION & DUPLICATE ADDRESS DETECTION . . . . . 24
B. CHANGES SINCE RFC 1971 . . . . . . . . . . . . . . . . . . . 25
C. CHANGE HISTORY . . . . . . . . . . . . . . . . . . . . . . . 26
D. OPEN ISSUES . . . . . . . . . . . . . . . . . . . . . . . . 27
Intellectual Property and Copyright Statements . . . . . . . 28
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1. Introduction
This document specifies the steps a host takes in deciding how to
autoconfigure its interfaces in IP version 6. The autoconfiguration
process includes creating a link-local address and verifying its
uniqueness on a link, determining what information should be
autoconfigured (addresses, other information, or both), and in the
case of addresses, whether they should be obtained through the
stateless mechanism, the stateful mechanism, or both. This document
defines the process for generating a link-local address, the process
for generating global addresses via stateless address
autoconfiguration, and the Duplicate Address Detection procedure. The
details of autoconfiguration using the stateful protocol are
specified elsewhere.
IPv6 defines both a stateful and stateless address autoconfiguration
mechanism. Stateless autoconfiguration requires no manual
configuration of hosts, minimal (if any) configuration of routers,
and no additional servers. The stateless mechanism allows a host to
generate its own addresses using a combination of locally available
information and information advertised by routers. Routers advertise
prefixes that identify the subnet(s) associated with a link, while
hosts generate an "interface identifier" that uniquely identifies an
interface on a subnet. An address is formed by combining the two. In
the absence of routers, a host can only generate link-local
addresses. However, link-local addresses are sufficient for allowing
communication among nodes attached to the same link.
In the stateful autoconfiguration model, hosts obtain interface
addresses and/or configuration information and parameters from a
server. Servers maintain a database that keeps track of which
addresses have been assigned to which hosts. The stateful
autoconfiguration protocol allows hosts to obtain addresses, other
configuration information or both from a server. Stateless and
stateful autoconfiguration complement each other. For example, a host
can use stateless autoconfiguration to configure its own addresses,
but use stateful autoconfiguration to obtain other information.
Stateful autoconfiguration for IPv6 is the subject of DHCPv6 [7].
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 [5].
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IPv6 addresses are leased to an interface for a fixed (possibly
infinite) length of time. Each address has an associated lifetime
that indicates how long the address is bound to an interface. When a
lifetime expires, the binding (and address) become invalid and the
address may be reassigned to another interface elsewhere in the
Internet. To handle the expiration of address bindings gracefully, an
address goes through two distinct phases while assigned to an
interface. Initially, an address is "preferred", meaning that its use
in arbitrary communication is unrestricted. Later, an address becomes
"deprecated" in anticipation that its current interface binding will
become invalid. While in a deprecated state, the use of an address is
discouraged, but not strictly forbidden. New communication (e.g.,
the opening of a new TCP connection) should use a preferred address
when possible. A deprecated address should be used only by
applications that have been using it and would have difficulty
switching to another address without a service disruption.
To ensure that all configured addresses are likely to be unique on a
given link, nodes run a "duplicate address detection" algorithm on
addresses before assigning them to an interface. The Duplicate
Address Detection algorithm is performed on all addresses,
independent of whether they are obtained via stateless or stateful
autoconfiguration. This document defines the Duplicate Address
Detection algorithm.
The autoconfiguration process specified in this document applies only
to hosts and not routers. Since host autoconfiguration uses
information advertised by routers, routers will need to be configured
by some other means. However, it is expected that routers will
generate link-local addresses using the mechanism described in this
document. In addition, routers are expected to successfully pass the
Duplicate Address Detection procedure described in this document on
all addresses prior to assigning them to an interface.
Section 2 provides definitions for terminology used throughout this
document. Section 3 describes the design goals that lead to the
current autoconfiguration procedure. Section 4 provides an overview
of the protocol, while Section 5 describes the protocol in detail.
2. TERMINOLOGY
IP - Internet Protocol Version 6. The terms IPv4 and IPv6 are used
only in contexts where necessary to avoid ambiguity.
node - a device that implements IP.
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router - a node that forwards IP packets not explicitly addressed to
itself.
host - any node that is not a router.
upper layer - a protocol layer immediately above IP. Examples are
transport protocols such as TCP and UDP, control protocols such as
ICMP, routing protocols such as OSPF, and internet or lower-layer
protocols being "tunneled" over (i.e., encapsulated in) IP such as
IPX, AppleTalk, or IP itself.
link - a communication facility or medium over which nodes can
communicate at the link layer, i.e., the layer immediately below
IP. Examples are Ethernets (simple or bridged); PPP links; X.25,
Frame Relay, or ATM networks; and internet (or higher) layer
"tunnels", such as tunnels over IPv4 or IPv6 itself.
interface - a node's attachment to a link.
packet - an IP header plus payload.
address - an IP-layer identifier for an interface or a set of
interfaces.
unicast address - an identifier for a single interface. A packet sent
to a unicast address is delivered to the interface identified by
that address.
multicast address - an identifier for a set of interfaces (typically
belonging to different nodes). A packet sent to a multicast
address is delivered to all interfaces identified by that address.
anycast address - an identifier for a set of interfaces (typically
belonging to different nodes). A packet sent to an anycast
address is delivered to one of the interfaces identified by that
address (the "nearest" one, according to the routing protocol's
measure of distance). See the IPv6 addressing architecture [4].
solicited-node multicast address - a multicast address to which
Neighbor Solicitation messages are sent. The algorithm for
computing the address is given in RFC 2461 [5].
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.
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link-local address - an address having link-only scope that can be
used to reach neighboring nodes attached to the same link. All
interfaces have a link-local unicast address.
global address - an address with unlimited scope.
communication - any packet exchange among nodes that requires that
the address of each node used in the exchange remain the same for
the duration of the packet exchange. Examples are a TCP
connection or a UDP request-response.
tentative address - an address whose uniqueness on a link is being
verified, prior to its assignment to an interface. A tentative
address is not considered assigned to an interface in the usual
sense. An interface discards received packets addressed to a
tentative address, but accepts Neighbor Discovery packets related
to Duplicate Address Detection for the tentative address.
preferred address - an address assigned to an interface whose use by
upper layer protocols is unrestricted. Preferred addresses may be
used as the source (or destination) address of packets sent from
(or to) the interface.
deprecated address - An address assigned to an interface whose use is
discouraged, but not forbidden. A deprecated address should no
longer be used as a source address in new communications, but
packets sent from or to deprecated addresses are delivered as
expected. A deprecated address may continue to be used as a
source address in communications where switching to a preferred
address causes hardship to a specific upper-layer activity (e.g.,
an existing TCP connection).
valid address - a preferred or deprecated address. A valid address
may appear as the source or destination address of a packet, and
the internet routing system is expected to deliver packets sent to
a valid address to their intended recipients.
invalid address - an address that is not assigned to any interface. A
valid address becomes invalid when its valid lifetime expires.
Invalid addresses should not appear as the destination or source
address of a packet. In the former case, the internet routing
system will be unable to deliver the packet, in the later case the
recipient of the packet will be unable to respond to it.
preferred lifetime - the length of time that a valid address is
preferred (i.e., the time until deprecation). When the preferred
lifetime expires, the address becomes deprecated.
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valid lifetime - the length of time an address remains in the valid
state (i.e., the time until invalidation). The valid lifetime must
be greater then or equal to the preferred lifetime. When the
valid lifetime expires, the address becomes invalid.
interface identifier - a link-dependent identifier for an interface
that is (at least) unique per link [4]. Stateless address
autoconfiguration combines an interface identifier with a prefix
to form an address. From address autoconfiguration's perspective,
an interface identifier is a bit string of known length. The
exact length of an interface identifier and the way it is created
is defined in a separate link-type specific document that covers
issues related to the transmission of IP over a particular link
type (e.g., IPv6 over Ethernet [2]). In many cases, the identifier
will be derived from the interface's link-layer address.
2.1 Requirements
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in RFC 2119 [3].
3. DESIGN GOALS
Stateless autoconfiguration is designed with the following goals in
mind:
o Manual configuration of individual machines before connecting them
to the network should not be required. Consequently, a mechanism
is needed that allows a host to obtain or create unique addresses
for each of its interfaces. Address autoconfiguration assumes that
each interface can provide a unique identifier for that interface
(i.e., an "interface identifier"). In the simplest case, an
interface identifier consists of the interface's link-layer
address. An interface identifier 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 appending its interface identifier to the
link-local prefix.
o A large site with multiple networks and routers should not require
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the presence of a stateful address configuration server. In order
to generate global addresses, hosts must determine the prefixes
that identify the subnets to which they attach. Routers generate
periodic Router Advertisements that include options listing the
set of active prefixes on a link.
o Address configuration should facilitate the graceful renumbering
of a site's machines. For example, a site may wish to renumber all
of its nodes when it switches to a new network service provider.
Renumbering is achieved through the leasing of addresses to
interfaces and the assignment of multiple addresses to the same
interface. Lease lifetimes provide the mechanism through which a
site phases out old prefixes. The assignment of multiple
addresses to an interface provides for a transition period during
which both a new address and the one being phased out work
simultaneously.
o System administrators need the ability to specify whether
stateless autoconfiguration, stateful autoconfiguration, or both
should be used. Router Advertisements include flags specifying
which mechanisms a host should use.
4. PROTOCOL OVERVIEW
This section provides an overview of the typical steps that take
place when an interface autoconfigures itself. Autoconfiguration is
performed only on multicast-capable links and begins when a
multicast-capable interface is enabled, e.g., during system startup.
Nodes (both hosts and routers) begin the autoconfiguration process by
generating a link-local address for the interface. A link-local
address is formed by appending the interface's identifier to the
well-known link-local prefix.
Before the link-local address can be assigned to an interface and
used, however, a node must attempt to verify that this "tentative"
address is not already in use by another node on the link.
Specifically, it sends a Neighbor Solicitation message containing the
tentative address as the target. If another node is already using
that address, it will return a Neighbor Advertisement saying so. If
another node is also attempting to use the same address, it will send
a Neighbor Solicitation for the target as well. The exact number of
times the Neighbor Solicitation is (re)transmitted and the delay time
between consecutive solicitations is link-specific and may be set by
system management.
If a node determines that its tentative link-local address is not
unique, autoconfiguration stops and manual configuration of the
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interface is required. To simplify recovery in this case, it should
be possible for an administrator to supply an alternate interface
identifier that overrides the default identifier in such a way that
the autoconfiguration mechanism can then be applied using the new
(presumably unique) interface identifier. Alternatively, link-local
and other addresses will need to be configured manually.
Once a node ascertains that its tentative link-local address is
unique, it assigns the address to the interface. At this point, the
node has IP-level connectivity with neighboring nodes. The remaining
autoconfiguration steps are performed only by hosts; the
(auto)configuration of routers is beyond the scope of this document.
The next phase of autoconfiguration involves obtaining a Router
Advertisement or determining that no routers are present. If routers
are present, they will send Router Advertisements that specify what
sort of autoconfiguration a host should do. If no routers are
present, stateful autoconfiguration should be invoked.
Routers send Router Advertisements periodically, but the delay
between successive advertisements will generally be longer than a
host performing autoconfiguration will want to wait [5]. To obtain an
advertisement quickly, a host sends one or more Router Solicitations
to the all-routers multicast group. Router Advertisements contain
two flags indicating what type of stateful autoconfiguration (if any)
should be performed. A "managed address configuration" flag indicates
whether hosts should use stateful autoconfiguration to obtain
addresses. An "other stateful configuration" flag indicates whether
hosts should use stateful autoconfiguration to obtain additional
information (excluding addresses).
Router Advertisements also contain zero or more Prefix Information
options that contain information used by stateless address
autoconfiguration to generate global addresses. It should be noted
that the stateless and stateful address autoconfiguration fields in
Router Advertisements are processed independently of one another, and
a host may use both stateful and stateless address autoconfiguration
simultaneously. One Prefix Information option field, the "autonomous
address-configuration flag", indicates whether or not the option even
applies to stateless autoconfiguration. If it does, additional
option fields contain a subnet prefix together with lifetime values
indicating how long addresses created from the prefix remain
preferred and valid.
Because routers generate Router Advertisements periodically, hosts
will continually receive new advertisements. Hosts process the
information contained in each advertisement as described above,
adding to and refreshing information received in previous
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advertisements.
For safety, all addresses must be tested for uniqueness prior to
their assignment to an interface. In the case of addresses created
through stateless autoconfiguration, however, the uniqueness of an
address is determined primarily by the portion of the address formed
from an interface identifier. Thus, if a node has already verified
the uniqueness of a link-local address, additional addresses created
from the same interface identifier need not be tested individually.
In contrast, all addresses obtained manually or via stateful address
autoconfiguration should be tested for uniqueness individually. To
accommodate sites that believe the overhead of performing Duplicate
Address Detection outweighs its benefits, the use of Duplicate
Address Detection can be disabled through the administrative setting
of a per-interface configuration flag.
To speed the autoconfiguration process, a host may generate its
link-local address (and verify its uniqueness) in parallel with
waiting for a Router Advertisement. Because a router may delay
responding to a Router Solicitation for a few seconds, the total time
needed to complete autoconfiguration can be significantly longer if
the two steps are done serially.
4.1 Site Renumbering
Address leasing facilitates site renumbering by providing a mechanism
to time-out addresses assigned to interfaces in hosts. At present,
upper layer protocols such as TCP provide no support for changing
end-point addresses while a connection is open. If an end-point
address becomes invalid, existing connections break and all
communication to the invalid address fails. Even when applications
use UDP as a transport protocol, addresses must generally remain the
same during a packet exchange.
Dividing valid addresses into preferred and deprecated categories
provides a way of indicating to upper layers that a valid address may
become invalid shortly and that future communication using the
address will fail, should the address's valid lifetime expire before
communication ends. To avoid this scenario, higher layers should use
a preferred address (assuming one of sufficient scope exists) to
increase the likelihood that an address will remain valid for the
duration of the communication. It is up to system administrators to
set appropriate prefix lifetimes in order to minimize the impact of
failed communication when renumbering takes place. The deprecation
period should be long enough that most, if not all, communications
are using the new address at the time an address becomes invalid.
The IP layer is expected to provide a means for upper layers
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(including applications) to select the most appropriate source
address given a particular destination and possibly other
constraints. An application may choose to select the source address
itself before starting a new communication or may leave the address
unspecified, in which case the upper networking layers will use the
mechanism provided by the IP layer to choose a suitable address on
the application's behalf.
Detailed address selection rules are beyond the scope of this
document.
5. PROTOCOL SPECIFICATION
Autoconfiguration is performed on a per-interface basis on
multicast-capable interfaces. For multihomed hosts,
autoconfiguration is performed independently on each interface.
Autoconfiguration applies primarily to hosts, with two exceptions.
Routers are expected to generate a link-local address using the
procedure outlined below. In addition, routers perform Duplicate
Address Detection on all addresses prior to assigning them to an
interface.
5.1 Node Configuration Variables
A node MUST allow the following autoconfiguration-related variable to
be configured by system management for each multicast interface:
DupAddrDetectTransmits
The number of consecutive Neighbor Solicitation messages sent
while performing Duplicate Address Detection on a tentative
address. A value of zero indicates that Duplicate Address
Detection is not performed on tentative addresses. A value of one
indicates a single transmission with no follow up retransmissions.
Default: 1, but may be overridden by a link-type specific value in
the document that covers issues related to the transmission of IP
over a particular link type (e.g., IPv6 over Ethernet [2]).
Autoconfiguration also assumes the presence of the variable
RetransTimer as defined in RFC 2461 [5]. 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.
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5.2 Autoconfiguration-Related Variables
A host maintains a number of data structures and flags related to
autoconfiguration. In the following, we present conceptual variables
and show how they are used to perform autoconfiguration. The specific
variables are used for demonstration purposes only, and an
implementation is not required to have them, so long as its external
behavior is consistent with that described in this document.
Beyond the formation of a link-local address and using Duplicate
Address Detection, how routers (auto)configure their interfaces is
beyond the scope of this document.
Hosts maintain the following variables on a per-interface basis:
ManagedFlag
Copied from the M flag field (i.e., the "managed address
configuration" flag) of the most recently received Router
Advertisement message. The flag indicates whether or not addresses
are to be configured using the stateful autoconfiguration
mechanism. It starts out in a FALSE state.
OtherConfigFlag
Copied from the O flag field (i.e., the "other stateful
configuration" flag) of the most recently received Router
Advertisement message. The flag indicates whether or not
information other than addresses is to be obtained using the
stateful autoconfiguration mechanism. It starts out in a FALSE
state.
In addition, when the value of the ManagedFlag is TRUE, the value
of OtherConfigFlag is implicitly TRUE as well. It is not a valid
configuration for a host to use stateful address autoconfiguration
to request addresses only, without also accepting other
configuration information.
A host also maintains a list of addresses together with their
corresponding lifetimes. The address list contains both
autoconfigured addresses and those configured manually.
5.3 Creation of Link-Local Addresses
A node forms a link-local address whenever an interface becomes
enabled. An interface may become enabled after any of the following
events:
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- 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 FE80::0 [4] (of appropriate length) to the interface
identifier. If the interface identifier has a length of N bits, the
interface identifier replaces the right-most N zero bits of the
link-local prefix. If the interface identifier is more than 118 bits
in length, autoconfiguration fails and manual configuration is
required. Note that interface identifiers will typically be 64-bits
long and based on EUI-64 identifiers as described in [4].
5.4 Duplicate Address Detection
Duplicate Address Detection is performed on unicast addresses prior
to assigning them to an interface whose DupAddrDetectTransmits
variable is greater than zero. Duplicate Address Detection MUST take
place on all unicast addresses, regardless of whether they are
obtained through stateful, stateless or manual configuration, with
the exception of the following cases:
- Duplicate Address Detection MUST NOT be performed on anycast
addresses.
- Each individual unicast address SHOULD be tested for uniqueness.
However, when stateless address autoconfiguration is used, address
uniqueness is determined solely by the interface identifier,
assuming that subnet prefixes are assigned correctly (i.e., if all
of an interface's addresses are generated from the same
identifier, either all addresses or none of them will be
duplicates). Thus, for a set of addresses formed from the same
interface identifier, it is sufficient to check that the link-
local address generated from the identifier is unique on the link.
In such cases, the link-local address MUST be tested for
uniqueness, and if no duplicate address is detected, an
implementation MAY choose to skip Duplicate Address Detection for
additional addresses derived from the same interface identifier.
The procedure for detecting duplicate addresses uses Neighbor
Solicitation and Advertisement messages as described below. If a
duplicate address is discovered during the procedure, the address
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cannot be assigned to the interface. If the address is derived from
an interface identifier, a new identifier will need to be assigned to
the interface, or all IP addresses for the interface will need to be
manually configured. Note that the method for detecting duplicates
is not completely reliable, and it is possible that duplicate
addresses will still exist (e.g., if the link was partitioned while
Duplicate Address Detection was performed).
An address on which the Duplicate Address Detection Procedure is
applied is said to be tentative until the procedure has completed
successfully. A tentative address is not considered "assigned to an
interface" in the traditional sense. That is, the interface must
accept Neighbor 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. Note that the "other
packets" include Neighbor Solicitation and Advertisement messages to
the tentative address containing the tentative address in the Target
Address field. Such a case should not happen in normal operation,
though, since these messages are multicasted in the Duplicate Address
Detection Procedure.
It should also be noted that Duplicate Address Detection must be
performed prior to assigning an address to an interface in order to
prevent multiple nodes from using the same address simultaneously. If
a node begins using an address in parallel with Duplicate Address
Detection, and another node is already using the address, the node
performing Duplicate Address Detection will erroneously process
traffic intended for the other node, resulting in such possible
negative consequences as the resetting of open TCP connections.
The following subsections describe specific tests a node performs to
verify an address's uniqueness. An address is considered unique if
none of the tests indicate the presence of a duplicate address within
RetransTimer milliseconds after having sent DupAddrDetectTransmits
Neighbor Solicitations. Once an address is determined to be unique,
it may be assigned to an interface.
5.4.1 Message Validation
A node MUST silently discard any Neighbor Solicitation or
Advertisement message that does not pass the validity checks
specified in RFC 2461 [5]. A Neighbor Solicitation or Advertisement
message that passes these validity checks is called a valid
solicitation or valid advertisement, respectively.
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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 ensures that the node receives
Neighbor Advertisements from other nodes already using the address;
the latter ensures that two nodes attempting to use the same address
simultaneously detect each other's presence.
To check an address, a node sends DupAddrDetectTransmits Neighbor
Solicitations, each separated by RetransTimer milliseconds. The
solicitation's Target Address is set to the address being checked,
the IP source is set to the unspecified address and the IP
destination is set to the solicited-node multicast address of the
target address.
If the Neighbor Solicitation is going to be the first message to be
sent from an interface after interface (re)initialization, the node
should delay joining the solicited-node multicast address by a random
delay between 0 and MAX_RTR_SOLICITATION_DELAY as specified in RFC
2461 [5]. 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.
Note that the delay for joining the multicast address implicitly
means delaying transmission of the corresponding MLD report message
[9]. Since RFC 2710 [9] does not request a random delay to avoid race
conditions, just delaying Neighbor Solicitation would cause
congestion by the MLD report messages. The congestion would then
prevent MLD-snooping switches from working correctly, and, as a
result, prevent Duplicate Address Detection from working. The
requirement to include the delay for the MLD report in this case
avoids this scenario.
In order to improve the robustness of the Duplicate Address Detection
algorithm, an interface MUST receive and process datagrams sent to
the all-nodes multicast address or solicited-node multicast address
of the tentative address while the delaying period. This does not
necessarily conflict with the requirement that joining the multicast
group be delayed. In fact, in some cases it is possible for a node to
start listening to the group during the delay period before MLD
report transmission. It should be noted, however, that in some
link-layer environments, particularly with MLD-snooping switches, no
multicast reception will be available until the MLD report is sent.
5.4.3 Receiving Neighbor Solicitation Messages
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On receipt of a valid Neighbor Solicitation message on an interface,
node behavior depends on whether the target address is tentative or
not. If the target address is not tentative (i.e., it is assigned to
the receiving interface), the solicitation is processed as described
in RFC 2461 [5]. If the target address is tentative, and the source
address is a unicast address, the solicitation's sender is performing
address resolution on the target; the solicitation should be silently
ignored. Otherwise, processing takes place as described below. In
all cases, a node MUST NOT respond to a Neighbor Solicitation for a
tentative address.
If the source address of the Neighbor Solicitation is the unspecified
address, the solicitation is from a node performing Duplicate Address
Detection. If the solicitation is from another node, the tentative
address is a duplicate and should not be used (by either node). If
the solicitation is from the node itself (because the node loops back
multicast packets), the solicitation does not indicate the presence
of a duplicate address.
Implementor's Note: many interfaces provide a way for upper layers to
selectively enable and disable the looping back of multicast packets.
The details of how such a facility is implemented may prevent
Duplicate Address Detection from working correctly. See the Appendix
A for further discussion.
The following tests identify conditions under which a tentative
address is not unique:
- If a Neighbor Solicitation for a tentative address is received
prior to having sent one, the tentative address is a duplicate.
This condition occurs when two nodes run Duplicate Address
Detection simultaneously, but transmit initial solicitations at
different times (e.g., by selecting different random delay values
before joining the solicited-node multicast address and
transmitting an initial solicitation).
- If the actual number of Neighbor Solicitations received exceeds
the number expected based on the loopback semantics (e.g., the
interface does not loopback packet, yet one or more solicitations
was received), the tentative address is a duplicate. This
condition occurs when two nodes run Duplicate Address Detection
simultaneously and transmit solicitations at roughly the same
time.
5.4.4 Receiving Neighbor Advertisement Messages
On receipt of a valid Neighbor Advertisement message on an interface,
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node behavior depends on whether the target address is tentative or
matches a unicast or anycast address assigned to the interface. If
the target address is assigned to the receiving interface, the
solicitation is processed as described in RFC 2461 [5]. If the target
address is tentative, the tentative address is not unique.
5.4.5 When Duplicate Address Detection Fails
A tentative address that is determined to be a duplicate as described
above MUST NOT be assigned to an interface and the node SHOULD log a
system management error. If the address is a link-local address
formed from an interface identifier based on the hardware address
(e.g., EUI-64), the interface SHOULD be disabled. In this case, the
IP address duplication probably means duplicate hardware addresses
are in use, and trying to recover from it by configuring another IP
address will not result in a usable network. In fact, it probably
makes things worse by creating problems that are harder to diagnose
than just shutting down the interface; the user will see a partially
working network where some things work, and other things will not. On
the other hand, if the duplicated link-local address is not formed
from an interface identifier based on the hardware address, the
interface MAY continue to be used.
5.5 Creation of Global Addresses
Global addresses are formed by appending an interface identifier to a
prefix of appropriate length. Prefixes are obtained from Prefix
Information options contained in Router Advertisements. Creation of
global addresses and configuration of other parameters as described
in this section SHOULD be locally configurable. However, the
processing described below 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 RFC 2461 [5].
5.5.2 Absence of Router Advertisements
If a link has no routers, a host MUST attempt to use stateful
autoconfiguration to obtain addresses and other configuration
information. An implementation MAY provide a way to disable the
invocation of stateful autoconfiguration in this case, but the
default SHOULD be enabled. From the perspective of
autoconfiguration, a link has no routers if no Router Advertisements
are received after having sent a small number of Router Solicitations
as described in RFC 2461 [5].
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5.5.3 Router Advertisement Processing
On receipt of a valid Router Advertisement (as defined in RFC 2461
[5]), a host copies the value of the advertisement's M bit into
ManagedFlag. If the value of ManagedFlag changes from FALSE to TRUE,
and the host is not already running the stateful address
autoconfiguration protocol, the host should invoke the stateful
address autoconfiguration protocol, requesting both address
information and other information. If the value of the ManagedFlag
changes from TRUE to FALSE, the host should continue running the
stateful address autoconfiguration, i.e., the change in the value of
the ManagedFlag has no effect. 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 O flag field is processed in an analogous manner.
A host copies the value of the O flag into OtherConfigFlag. If the
value of OtherConfigFlag changes from FALSE to TRUE, the host should
invoke the stateful autoconfiguration protocol, requesting
information (excluding addresses if ManagedFlag is set to FALSE). If
the value of the OtherConfigFlag changes from TRUE to FALSE, the host
should continue running the stateful address autoconfiguration
protocol, i.e., the change in the value of OtherConfigFlag has no
effect. If the value of the flag stays unchanged, no special action
takes place. In particular, a host MUST NOT reinvoke stateful
configuration if it is already participating in the stateful protocol
as a result of an earlier advertisement.
For each Prefix-Information option in the Router Advertisement:
a) If the Autonomous flag is not set, silently ignore the Prefix
Information option.
b) If the prefix is the link-local prefix, silently ignore the
Prefix Information option.
c) If the preferred lifetime is greater than the valid lifetime,
silently ignore the Prefix Information option. A node MAY wish to
log a system management error in this case.
d) If the prefix advertised does not match the prefix of an address
already in the list, and the Valid Lifetime is not 0, form an
address (and add it to the list) by combining the advertised
prefix with the link's interface identifier as follows:
| 128 - N bits | N bits |
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+---------------------------------------+------------------------+
| link prefix | interface identifier |
+----------------------------------------------------------------+
e) If the advertised prefix matches the prefix of an autoconfigured
address (i.e., one obtained via stateless or stateful address
autoconfiguration) in the list of addresses associated with the
interface, the preferred lifetime of the address is reset to the
Preferred Lifetime in the received advertisement. The specific
action to perform for the valid lifetime of the address depends on
the Valid Lifetime in the received advertisement and the remaining
time to the valid lifetime expiration of the previously
autoconfigured address. We call the remaining time
"RemainingLifetime" in the following discussion:
1. If the received Valid Lifetime is greater than 2 hours or
greater than RemainingLifetime, set the valid lifetime of the
corresponding address to the advertised Valid Lifetime.
2. If RemainingLifetime is less than or equal to 2 hours, ignore
the Prefix Information option with regards to the valid
lifetime, unless the Router Advertisement from which this
option was obtained has been authenticated (e.g., via IP
security [1]). If the Router Advertisement was authenticated,
the valid lifetime of the corresponding address should be set
to the Valid Lifetime in the received option.
3. Otherwise, reset the valid lifetime of the corresponding
address to two hours.
The above rules address a specific denial of service attack in
which a bogus advertisement could contain prefixes with very small
Valid Lifetimes. Without the above rules, a single unauthenticated
advertisement containing bogus Prefix Information options with
short Valid Lifetimes could cause all of a node's addresses to
expire prematurely. The above rules ensure that legitimate
advertisements (which are sent periodically) will "cancel" the
short Valid Lifetimes before they actually take effect.
Note that the preferred lifetime of the corresponding address is
always reset to the Preferred Lifetime in the received Prefix
Information option, regardless of whether the valid lifetime is
also reset or ignored. The difference comes from the fact that the
possible attack for the preferred lifetime is relatively minor.
Additionally, it is even undesirable to ignore the preferred
lifetime when a valid administrator wants to deprecate a
particular address by sending a short preferred lifetime (and the
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valid lifetime is ignored by accident).
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 to
initiate new communications if an alternate (non-deprecated) address
of sufficient scope can easily be used instead.
Note that the feasibility of initiating new communication using a
non-deprecated address may be an application-specific decision, as
only the application may have knowledge about whether the (now)
deprecated address was (or still is) in use by the application. For
example, if an application explicitly specifies the protocol stack to
use a deprecated address as a source address, the protocol stack must
accept that; the application might request it because that IP address
is used for in higher-level communication and there might be a
requirement that the multiple connections in such a grouping use the
same pair of IP addresses.
IP and higher layers (e.g., TCP, UDP) MUST continue to accept and
process datagrams destined to a deprecated address as normal since a
deprecated address is still a valid address for the interface. In the
case of TCP, this means TCP SYN segments sent to a deprecated address
are responded to using the deprecated address as a source address in
the corresponding SYN-ACK (if the connection would otherwise be
allowed).
An implementation MAY prevent any new communication from using a
deprecated address, but system management MUST have the ability to
disable such a facility, and the facility MUST be disabled by
default.
Other subtle cases should also be noted about source address
selection. For example, the above description does not clarify which
address should be used between a deprecated, smaller-scope address
and a non-deprecated, enough scope address. The details of the
address selection including this case is described in RFC 3484 [8]
and beyond the scope of this document.
An address (and its association with an interface) becomes invalid
when its valid lifetime expires. An invalid address MUST NOT be used
as a source address in outgoing communications and MUST NOT be
recognized as a destination on a receiving interface.
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5.6 Configuration Consistency
It is possible for hosts to obtain address information using both
stateless and stateful protocols since both may be enabled at the
same time. It is also possible that the values of other
configuration parameters such as MTU size and hop limit will be
learned from both Router Advertisements and the stateful
autoconfiguration protocol. If the same configuration information is
provided by multiple sources, the value of this information should be
consistent. However, it is not considered a fatal error if
information received from multiple sources is inconsistent. Hosts
accept the union of all information received via the stateless and
stateful protocols. If inconsistent information is learned different
sources, the most recently obtained values always have precedence
over information learned earlier.
5.7 Retaining Configured Addresses for Stability
It is reasonable that implementations that have stable storage retain
their addresses and the preferred and valid lifetimes if the
addresses were acquired using stateless address autoconfiguration.
Assuming the lifetimes used are reasonable, this technique implies
that a temporary outage (less than the valid lifetime) of a router
will never result in the node losing its global address even if the
node were to reboot. This will particularly be useful in "zeroconf"
environments where nodes are configuring their addresses by stateless
address autoconfiguration but all communication is limited within a
single link. In such a case, the failure of a "router" (that provides
the prefix for address configuration) is not significant, but losing
the global addresses might be a pain; it is true that the node can
still use link-local addresses for communication within the link, but
the node may want to use global addresses when possible, especially
when the other nodes use global addresses.
When an implementation tries to reuse a retained address after
rebooting, it MUST first try to obtain Router Advertisements as
described in RFC 2461[5] and use the retained address only after
concluding there are no routers on the link. Additionally, the
implementation MUST run Duplicate Address Detection for the address
under the criteria described in Section 5.4, as though the address
were just configured by stateless address autoconfiguration. The
reason for this is because a different host may have started using
the address while the rebooting host cannot respond to Duplicate
Address Detection from the other host.
6. SECURITY CONSIDERATIONS
Stateless address autoconfiguration allows a host to connect to a
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network, configure an address and start communicating with other
nodes without ever registering or authenticating itself with the
local site. Although this allows unauthorized users to connect to
and use a network, the threat is inherently present in the Internet
architecture. Any node with a physical attachment to a network can
generate an address (using a variety of ad hoc techniques) that
provides connectivity.
The use of stateless address autoconfiguration and Duplicate Address
Detection opens up the possibility of several denial of service
attacks. For example, any node can respond to Neighbor Solicitations
for a tentative address, causing the other node to reject the address
as a duplicate. A separate document [10] discusses details about
these attacks. These attacks can be addressed by requiring that
Neighbor Discovery packets be authenticated [1]. However, it should
be noted that [10] points out the use of IP security is not always
feasible depending on network environments.
7. Acknowledgements
The authors would like to thank the members of both the IPNG (which
is now IPV6) and ADDRCONF working groups for their input. In
particular, thanks to Jim Bound, Steve Deering, Richard Draves, and
Erik Nordmark. Thanks also goes to John Gilmore for alerting the WG
of the "0 Lifetime Prefix Advertisement" denial of service attack
vulnerability; this document incorporates changes that address this
vulnerability.
Normative References
[1] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
November 1998.
[2] Crawford, M., "A Method for the Transmission of IPv6 Packets
over Ethernet Networks", RFC 2464, December 1998.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[4] Hinden, R. and S. Deering, "Internet Protocol Version (IPv6)
Addressing Architecture", RFC 3513, April 2003.
[5] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for
IP Version 6 (IPv6)", RFC 2461, December 1998.
Informative References
[6] Deering, S., "Host Extensions for IP Multicasting", RFC 1112,
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August 1989.
[7] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and M.
Carney, "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
[8] Draves, R., "Default Address Selection for Internet Protocol
version 6 (IPv6)", RFC 3484, February 2003.
[9] Deering, S., Fenner, W. and B. Haberman, "Multicast Listener
Discovery (MLD) for IPv6", RFC 2710, October 1999.
[10] Nikander, P., Kempf, J. and E. Nordmark, "IPv6 Neighbor
Discovery trust models and threats",
draft-ietf-send-psreq-04.txt (work in progress), October 2003.
[11] Park, S., Madanapalli, S. and O. Rao, "IPv6 DAD Consideration
for 802.11 Environment",
draft-park-ipv6-dad-problem-wlan-00.txt (work in progress),
July 2003.
Authors' Addresses
Susan Thomson
Cisco Systems
EMail: sethomso@cisco.com
Thomas Narten
IBM Corporation
P.O. Box 12195
Research Triangle Park, NC 27709-2195
USA
Phone: +1 919-254-7798
EMail: narten@us.ibm.com
Tatuya Jinmei
Corporate Research & Development Center, Toshiba Corporation
1 Komukai Toshiba-cho, Saiwai-ku
Kawasaki-shi, Kanagawa 212-8582
Japan
Phone: +81 44-549-2230
EMail: jinmei@isl.rdc.toshiba.co.jp
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Hesham Soliman
Flarion Technologies
EMail: H.Soliman@flarion.com
Appendix A. LOOPBACK SUPPRESSION & DUPLICATE ADDRESS DETECTION
Determining whether a received 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 identifier and link-layer
address, and they both send out packets with identical contents at
roughly the same time (e.g., Neighbor Solicitations for a tentative
address as part of Duplicate Address Detection messages). Although a
receiver will receive both packets, it cannot determine which packet
was looped back and which packet came from the other node by simply
comparing packet contents (i.e., the contents are identical). In this
particular case, it is not necessary to know precisely which packet
was looped back and which was sent by another node; if one receives
more solicitations than were sent, the tentative address is a
duplicate. However, the situation may not always be this
straightforward.
The IPv4 multicast specification [6] recommends that the service
interface provide a way for an upper-layer protocol to inhibit local
delivery of packets sent to a multicast group that the sending host
is a member of. Some applications know that there will be no other
group members on the same host, and suppressing loopback prevents
them from having to receive (and discard) the packets they themselves
send out. A straightforward way to implement this facility is to
disable loopback at the hardware level (if supported by the
hardware), with packets looped back (if requested) by software. On
interfaces in which the hardware itself suppresses loopbacks, a node
running Duplicate Address Detection simply counts the number of
Neighbor Solicitations received for a tentative address and compares
them with the number expected. If there is a mismatch, the tentative
address is a duplicate.
In those cases where the hardware cannot suppress loopbacks, however,
one possible software heuristic to filter out unwanted loopbacks is
to discard any received packet whose link-layer source address is the
same as the receiving interface's. There is even a link-layer
specification that requires to discard any such packets [11].
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:
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o If a node performing Duplicate Address Detection discards received
packets having the same source link-layer address as the receiving
interface, it will also discard packets from other nodes also
using the same link-layer address, including Neighbor
Advertisement and Neighbor Solicitation messages required to make
Duplicate Address Detection work correctly. This particular
problem can be avoided by temporarily disabling the software
suppression of loopbacks while a node performs Duplicate Address
Detection, if it is possible to disable the suppression.
o If a node that is already using a particular IP address discards
received packets having the same link-layer source address as the
interface, it will also discard Duplicate Address
Detection-related Neighbor Solicitation messages sent by another
node also using the same link-layer address. Consequently,
Duplicate Address Detection will fail, and the other node will
configure a non-unique address. Since it is generally impossible
to know when another node is performing Duplicate Address
Detection, this scenario can be avoided only if software
suppression of loopback is permanently disabled.
Thus, to perform Duplicate Address Detection correctly in the case
where two interfaces are using the same link-layer address, an
implementation must have a good understanding of the interface's
multicast loopback semantics, and the interface cannot discard
received packets simply because the source link-layer address is the
same as the interfaces. It should also be noted that a link-layer
specification can conflict with the condition necessary to make
Duplicate Address Detection work.
Appendix B. CHANGES SINCE RFC 1971
o Changed document to use term "interface identifier" rather than
"interface token" for consistency with other IPv6 documents.
o Clarified definition of deprecated address to make clear it is OK
to continue sending to or from deprecated addresses.
o Added rules to Section 5.5.3 Router Advertisement processing to
address potential denial-of-service attack when prefixes are
advertised with very short Lifetimes.
o Clarified wording in Section 5.5.4 to make clear that all upper
layer protocols must process (i.e., send and receive) packets sent
to deprecated addresses.
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Appendix C. CHANGE HISTORY
Changes since RFC 2462 are:
o Fixed a typo in Section 2.
o Updated references and categorized them into normative and
informative ones.
o Removed redundant code in denial of service protection in Section
5.5.3.
o Clarified that a unicasted NS or NA should be discarded while
performing Duplicate Address Detection.
o Replaced the word "StoredLifetime" with "RemainingLifetime" with a
precise definition to avoid confusion.
o Removed references to site-local and revise wording around the
keyword.
o Added a note about source address selection with regards to
deprecated vs insufficient-scope addresses, etc. Added a reference
to RFC 3484 for further details.
o Clarified what "new communication" means in Section 5.5.4.
o Added a new subsection (5.7) to mention the possibility to use
stable storage to retain configured addresses for stability.
o Revised the Security Considerations section with a refence to the
send requirement document and a note that the use of IP security
is not always feasible.
o Added a note with a reference in Appendix A about the case where a
link-layer filtering conflicts with a condition to make DAD work
correctly.
o Specified that a node performing Duplicate Address Detection delay
joining the solicited-node multicast group, not just delay sending
Neighbor Solicitations, explaining the detailed reason.
o Clarified the reason why the interface should be disabled after an
address duplicate is detected. Also clarified that the interface
may continue to be used if the interface identifier is not based
on the hardware address.
o Clarified that the preferred lifetime for an existing configured
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address is always reset to the advertised value by Router
Advertisement.
o Updated the description of interface identifier considering the
latest format.
Appendix D. OPEN ISSUES
Semi-Open Issues (resolutions were proposed, but they may need
further discussions):
o [2462bis issue 271] An implementation may want to use stable
storage for autoconfigured addresses.
o [2462bis issue 274] There is conflict with the Multicast Listener
Discovery specification about random delay for the first packet.
o [2462bis issue 278] Whether a router (not a host) can
autoconfigure itself using the stateless autoconfiguration
protocol may need to be discussed.
Open Issues (resolutions have not been proposed yet):
o [2462bis issue 275] Many DAD related issues have been discussed,
including whether it is okay to omit DAD in some environments or
whether DAD can be replaced with DIID (duplicate interface ID
detection).
o [2462bis issue 277] The semantics of the M/O flags is not very
clear.
1. the text needs to be updated to use RFC 2119 keywords
2. which keywords?
3. what is "the stateful configuration protocol"?
4. if the answer to the previous question is DHCPv6, should this
specification more explicitly reference the configuration-only
version of DHCPv6 in the description of the 'O'flag?
o [2462bis issue 281] It is not very clear whether this document
always require a 64-bit Interface ID.
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Internet-Draft IPv6 Stateless Address Autoconfiguration February 2004
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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