Internet Engineering Task Force S. Thomson
Internet-Draft Bellcore
Expires: April 19, 2004 T. Narten
IBM
T. Jinmei
Toshiba
H. Soliman
Flarion Technologies
October 20, 2003
IPv6 Stateless Address Autoconfiguration
draft-jinmei-ipv6-rfc2462bis-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at http://
www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on April 19, 2004.
Copyright Notice
Copyright (C) The Internet Society (2003). 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
Thomson, et al. Expires April 19, 2004 [Page 1]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
defines the process for generating a link-local address, the process
for generating site-local and global addresses via stateless address
autoconfiguration, and the Duplicate Address Detection procedure. The
details of autoconfiguration using the stateful protocol are
specified elsewhere.
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 . . . . . . . . . . . . . . 13
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 . . . . . . . . . . . 16
5.5 Creation of Global and Site-Local Addresses . . . . . . . . 16
5.5.1 Soliciting Router Advertisements . . . . . . . . . . . . . . 17
5.5.2 Absence of Router Advertisements . . . . . . . . . . . . . . 17
5.5.3 Router Advertisement Processing . . . . . . . . . . . . . . 17
5.5.4 Address Lifetime Expiry . . . . . . . . . . . . . . . . . . 19
5.6 Configuration Consistency . . . . . . . . . . . . . . . . . 19
6. SECURITY CONSIDERATIONS . . . . . . . . . . . . . . . . . . 20
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
Normative References . . . . . . . . . . . . . . . . . . . . 20
Informative References . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 21
A. LOOPBACK SUPPRESSION & DUPLICATE ADDRESS DETECTION . . . . . 21
B. CHANGES SINCE RFC 1971 . . . . . . . . . . . . . . . . . . . 23
C. CHANGE HISTORY . . . . . . . . . . . . . . . . . . . . . . . 23
D. OPEN ISSUES IN RFC 2462 . . . . . . . . . . . . . . . . . . 24
Intellectual Property and Copyright Statements . . . . . . . 26
Thomson, et al. Expires April 19, 2004 [Page 2]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
1. Introduction
This document specifies the steps a host takes in deciding how to
autoconfigure its interfaces in IP version 6. The autoconfiguration
process includes creating a link-local address and verifying its
uniqueness on a link, determining what information should be
autoconfigured (addresses, other information, or both), and in the
case of addresses, whether they should be obtained through the
stateless mechanism, the stateful mechanism, or both. This document
defines the process for generating a link-local address, the process
for generating site-local and global addresses via stateless address
autoconfiguration, and the Duplicate Address Detection procedure. The
details of autoconfiguration using the stateful protocol are
specified elsewhere.
IPv6 defines both a stateful and stateless address autoconfiguration
mechanism. Stateless autoconfiguration requires no manual
configuration of hosts, minimal (if any) configuration of routers,
and no additional servers. The stateless mechanism allows a host to
generate its own addresses using a combination of locally available
information and information advertised by routers. Routers advertise
prefixes that identify the subnet(s) associated with a link, while
hosts generate an "interface 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 RFC 2461 [5].
Thomson, et al. Expires April 19, 2004 [Page 3]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
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 insure that all configured addresses are likely to be unique on a
given link, nodes run a "duplicate address detection" algorithm on
addresses before assigning them to an interface. The Duplicate
Address Detection algorithm is performed on all addresses,
independent of whether they are obtained via stateless or stateful
autoconfiguration. This document defines the Duplicate Address
Detection algorithm.
The autoconfiguration process specified in this document applies only
to hosts and not routers. Since host autoconfiguration uses
information advertised by routers, routers will need to be configured
by some other means. However, it is expected that routers will
generate link-local addresses using the mechanism described in this
document. In addition, routers are expected to successfully pass the
Duplicate Address Detection procedure described in this document on
all addresses prior to assigning them to an interface.
Section 2 provides definitions for terminology used throughout this
document. Section 3 describes the design goals that lead to the
current autoconfiguration procedure. Section 4 provides an overview
of the protocol, while Section 5 describes the protocol in detail.
2. TERMINOLOGY
IP - Internet Protocol Version 6. The terms IPv4 and IPv6 are used
only in contexts where necessary to avoid ambiguity.
node - a device that implements IP.
Thomson, et al. Expires April 19, 2004 [Page 4]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
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 - link-layer addressa link-layer identifier for an
interface. Examples include IEEE 802 addresses for Ethernet links
and E.164 addresses for ISDN links.
Thomson, et al. Expires April 19, 2004 [Page 5]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
link-local address - an address having link-only scope that can be
used to reach neighboring nodes attached to the same link. All
interfaces have a link-local unicast address.
site-local address - an address having scope that is limited to the
local site.
global address - an address with unlimited scope.
communication - any packet exchange among nodes that requires that
the address of each node used in the exchange remain the same for
the duration of the packet exchange. Examples are a TCP
connection or a UDP request- response.
tentative address - an address whose uniqueness on a link is being
verified, prior to its assignment to an interface. A tentative
address is not considered assigned to an interface in the usual
sense. An interface discards received packets addressed to a
tentative address, but accepts Neighbor Discovery packets related
to Duplicate Address Detection for the tentative address.
preferred address - an address assigned to an interface whose use by
upper layer protocols is unrestricted. Preferred addresses may be
used as the source (or destination) address of packets sent from
(or to) the interface.
deprecated address - An address assigned to an interface whose use is
discouraged, but not forbidden. A deprecated address should no
longer be used as a source address in new communications, but
packets sent 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.
Thomson, et al. Expires April 19, 2004 [Page 6]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
preferred lifetime - the length of time that a valid address is
preferred (i.e., the time until deprecation). When the preferred
lifetime expires, the address becomes deprecated.
valid lifetime - the length of time an address remains in the valid
state (i.e., the time until invalidation). The valid lifetime must
be greater then or equal to the preferred lifetime. When the
valid lifetime expires, the address becomes invalid.
interface 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 the same as 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
Thomson, et al. Expires April 19, 2004 [Page 7]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
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
the presence of a stateful address configuration server. In order
to generate site-local or global addresses, hosts must determine
the prefixes that identify the subnets to which they attach.
Routers generate periodic Router Advertisements that include
options listing the set of active prefixes on a link.
o Address configuration should facilitate the graceful renumbering
of a site's machines. For example, a site may wish to renumber all
of its nodes when it switches to a new network service provider.
Renumbering is achieved through the leasing of addresses to
interfaces and the assignment of multiple addresses to the same
interface. Lease lifetimes provide the mechanism through which a
site phases out old prefixes. The assignment of multiple
addresses to an interface provides for a transition period during
which both a new address and the one being phased out work
simultaneously.
o System administrators need the ability to specify whether
stateless autoconfiguration, stateful autoconfiguration, or both
should be used. Router Advertisements include flags specifying
which mechanisms a host should use.
4. PROTOCOL OVERVIEW
This section provides an overview of the typical steps that take
place when an interface autoconfigures itself. Autoconfiguration is
performed only on multicast-capable links and begins when a
multicast-capable interface is enabled, e.g., during system startup.
Nodes (both hosts and routers) begin the autoconfiguration process by
generating a link-local address for the interface. A link-local
address is formed by appending the interface's 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
Thomson, et al. Expires April 19, 2004 [Page 8]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
system management.
If a node determines that its tentative link-local address is not
unique, autoconfiguration stops and manual configuration of the
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 it to the interface. At this point, the node has
IP-level connectivity with neighboring nodes. The remaining
autoconfiguration steps are performed only by hosts; the
(auto)configuration of routers is beyond the scope of this document.
The next phase of autoconfiguration involves obtaining a Router
Advertisement or determining that no routers are present. If routers
are present, they will send Router Advertisements that specify what
sort of autoconfiguration a host should do. If no routers are
present, stateful autoconfiguration should be invoked.
Routers send Router Advertisements periodically, but the delay
between successive advertisements will generally be longer than a
host performing autoconfiguration will want to wait RFC 2461 [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 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.
Thomson, et al. Expires April 19, 2004 [Page 9]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
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
advertisements.
For safety, all addresses must be tested for uniqueness prior to
their assignment to an interface. In the case of addresses created
through stateless autoconfig, however, the uniqueness of an address
is determined primarily by the portion of the address formed from an
interface 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
Thomson, et al. Expires April 19, 2004 [Page 10]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
period should be long enough that most, if not all, communications
are using the new address at the time an address becomes invalid.
The IP layer is expected to provide a means for upper layers
(including applications) to select the most appropriate source
address given a particular destination and possibly other
constraints. An application may choose to select the source address
itself before starting a new communication or may leave the address
unspecified, in which case the upper networking layers will use the
mechanism provided by the IP layer to choose a suitable address on
the application's behalf.
Detailed address selection rules are beyond the scope of this
document.
5. PROTOCOL SPECIFICATION
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),
Thomson, et al. Expires April 19, 2004 [Page 11]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
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 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 implicitely 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.
Thomson, et al. Expires April 19, 2004 [Page 12]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
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:
- Duplicate Address Detection MUST NOT be performed on anycast
addresses.
- 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
Thomson, et al. Expires April 19, 2004 [Page 13]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
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
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.
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 solicitation that passes these validity
checks is called a valid solicitation or valid advertisement.
Thomson, et al. Expires April 19, 2004 [Page 14]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
5.4.2 Sending Neighbor Solicitation Messages
Before sending a Neighbor Solicitation, an interface MUST join the
all-nodes multicast address and the solicited-node multicast address
of the tentative address. The former insures that the node receives
Neighbor Advertisements from other nodes already using the address;
the latter insures that two nodes attempting to use the same address
simultaneously detect each other's presence.
To check an address, a node sends DupAddrDetectTransmits Neighbor
Solicitations, each separated by RetransTimer milliseconds. The
solicitation's Target Address is set to the address being checked,
the IP source is set to the unspecified address and the IP
destination is set to the solicited-node multicast address of the
target address.
If the Neighbor Solicitation is the first message to be sent from an
interface after interface (re)initialization, the node should delay
sending the message 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. In order to improve the robustness of the
Duplicate Address Detection algorithm, an interface MUST receive and
process datagrams sent to the all-nodes multicast address or
solicited-node multicast address of the tentative address while
delaying transmission of the initial Neighbor Solicitation.
5.4.3 Receiving Neighbor Solicitation Messages
On receipt of a valid Neighbor Solicitation message on an interface,
node behavior depends on whether the target address is tentative or
not. If the target address is not tentative (i.e., it is assigned to
the receiving interface), the solicitation is processed 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
Thomson, et al. Expires April 19, 2004 [Page 15]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
of a duplicate address.
Implementor's Note: many interfaces provide a way for upper layers to
selectively enable and disable the looping back of multicast packets.
The details of how such a facility is implemented may prevent
Duplicate Address Detection from working correctly. See the Appendix
for further discussion.
The following tests identify conditions under which a tentative
address is not unique:
- If a Neighbor Solicitation for a tentative address is received
prior to having sent one, the tentative address is a duplicate.
This condition occurs when two nodes run Duplicate Address
Detection simultaneously, but transmit initial solicitations at
different times (e.g., by selecting different random delay values
before transmitting an initial solicitation).
- If the actual number of Neighbor Solicitations received exceeds
the number expected based on the loopback semantics (e.g., the
interface does not loopback packet, yet one or more solicitations
was received), the tentative address is a duplicate. This
condition occurs when two nodes run Duplicate Address Detection
simultaneously and transmit solicitations at roughly the same
time.
5.4.4 Receiving Neighbor Advertisement Messages
On receipt of a valid Neighbor Advertisement message on an interface,
node behavior depends on whether the target address is tentative or
matches a unicast or anycast address assigned to the interface. If
the target address is assigned to the receiving interface, the
solicitation is processed 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, the interface SHOULD be
disabled.
5.5 Creation of Global and Site-Local Addresses
Global and site-local addresses are formed by appending an interface
identifier to a prefix of appropriate length. Prefixes are obtained
Thomson, et al. Expires April 19, 2004 [Page 16]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
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, 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].
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
Thomson, et al. Expires April 19, 2004 [Page 17]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
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 |
+---------------------------------------+------------------------+
| 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 specific action to perform depends on the Valid
Lifetime in the received advertisement and the Lifetime associated
with the previously autoconfigured address (which we call
StoredLifetime in the discussion that follows):
1. If the received Lifetime is greater than 2 hours or greater
than StoredLifetime, update the stored Lifetime of the
corresponding address.
2. If the StoredLifetime is less than or equal to 2 hours and the
received Lifetime is less than or equal to StoredLifetime,
ignore the prefix, unless the Router Advertisement from which
this Prefix Information option was obtained has been
authenticated (e.g., via IPSec [1]). If the Router
Advertisement was authenticated, the StoredLifetime should be
set to the Lifetime in the received option.
3. Otherwise, reset the stored Lifetime in the corresponding
Thomson, et al. Expires April 19, 2004 [Page 18]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
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 Lifetimes could cause all of a node's addresses to expire
prematurely. The above rules insure that legitimate advertisements
(which are sent periodically) will "cancel" the short lifetimes
before they actually take effect.
5.5.4 Address Lifetime Expiry
A preferred address becomes deprecated when its preferred lifetime
expires. A deprecated address SHOULD continue to be used as a source
address in existing communications, but SHOULD NOT be used in new
communications if an alternate (non-deprecated) address is available
and has sufficient scope. IP and higher layers (e.g., TCP, UDP) MUST
continue to accept datagrams destined to a deprecated address since a
deprecated address is still a valid address for the interface. An
implementation MAY prevent any new communication from using a
deprecated address, but system management MUST have the ability to
disable such a facility, and the facility MUST be disabled by
default.
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.
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.
Thomson, et al. Expires April 19, 2004 [Page 19]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
6. SECURITY CONSIDERATIONS
Stateless address autoconfiguration allows a host to connect to a
network, configure an address and start communicating with other
nodes without ever registering or authenticating itself with the
local site. Although this allows unauthorized users to connect to
and use a network, the threat is inherently present in the Internet
architecture. Any node with a physical attachment to a network can
generate an address (using a variety of ad hoc techniques) that
provides connectivity.
The use of Duplicate Address Detection opens up the possibility of
denial of service attacks. Any node can respond to Neighbor
Solicitations for a tentative address, causing the other node to
reject 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 [1].
7. 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, 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", Internet Draft
draft-ietf-ipv6-addr-arch-v4-00.txt, October 2003.
[5] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for
IP Version 6 (IPv6)", RFC 2461, December 1998.
Informative References
Thomson, et al. Expires April 19, 2004 [Page 20]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
[6] Deering, S., "Host Extensions for IP Multicasting", RFC 1112,
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.
Authors' Addresses
Susan Thomson
Bellcore
445 South Street
Morristown, NJ 07960
USA
Phone: +1 201-829-4514
EMail: set@thumper.bellcore.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
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
Thomson, et al. Expires April 19, 2004 [Page 21]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
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. Unfortunately, use of that
criteria also results in the discarding of all packets sent by
another node using the same link-layer address. Duplicate Address
Detection will fail on interfaces that filter received packets in
this manner:
o If a node performing Duplicate Address Detection discards received
packets having the same source link-layer address as the receiving
interface, it will also discard packets from other nodes also
using the same link-layer address, including Neighbor
Advertisement and Neighbor Solicitation messages required to make
Duplicate Address Detection work correctly. This particular
problem can be avoided by temporarily disabling the software
suppression of loopbacks while a node performs Duplicate Address
Detection.
Thomson, et al. Expires April 19, 2004 [Page 22]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
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.
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.
Changes since RFC 2462 are:
o Fixed a typo in Section 2.
o Updated references and categorized them into normative and
informative ones.
o Built an issue list for RFC 2462.
Thomson, et al. Expires April 19, 2004 [Page 23]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
o There is dead code in the DoS prevention algorithm in Section
5.5.3
o Unclear text about a corner case in the inbound Neighbor
Advertisement processing.
o Unclear text about StoredLifetime.
o Source address selection issues with regards to deprecated
addresses. E.g., which one should be preferred as a source
address, between a deprecated address and a smaller-scope address?
o The semantics of "new communication" is not very clear; is a
passively opened TCP connection a new communication? What if an
application specifies a deprecated address as a source address?
o There was a question about the semantics where the on-link (L)
flag is 0 and the autonomous (A) flag is 1. (It is not clear if
this really needs to be addressed).
o There is conflict with the Multicast Listener Discovery
specification about random delay for the first packet. The address
autoconfiguration requires a random delay for a DAD packet if it
is the first packet from the node, but an MLD report packet should
usually be sent before the DAD packet.
o An implementation may want to use stable storage for
autoconfigured addresses.
o Many DAD related issues have been discussed, including if it is
okay to omit DAD in some environments or if DAD can be replaced
with DIID (duplicate interface ID detection).
o There is a possible denial of service attack not discussed: What
if a malicious node intentionally sends prefixes for other LANs?
o 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 RFC
this specification more explicitly reference the
Thomson, et al. Expires April 19, 2004 [Page 24]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
configuration-only version of DHCPv6 in the description of the
'O'flag?
o Whether a router (not a host) can autoconfigure itself using the
stateless autoconfiguration protocol may need to be discussed.
That includes:
* if a router can configure a global address by stateless
autoconfiguration
* if a router can configure a link-local address in a way
described in this document
* if a router can configure itself about "other" configuration
information
o Should this document define a 'not-yet-ready' status of an
autoconfigured address to help renumbering operation?
o The requirement about the interface failure upon DAD failure may
be too strong. Does it make sense to loosen it, e.g., allowing
automatic recovery?
o It is not very clear if this document always require a 64-bit
Interface ID.
o This document may need to be aligned with the SEND requirement
draft in its security consideration.
Thomson, et al. Expires April 19, 2004 [Page 25]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
Thomson, et al. Expires April 19, 2004 [Page 26]
Internet-Draft IPv6 Stateless Address Autoconfiguration October 2003
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Thomson, et al. Expires April 19, 2004 [Page 27]