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RFC1971 - IPv6 Stateless Address Autoconfiguration

王朝other·作者佚名  2008-05-31
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Network Working Group S. Thomson

Request for Comments: 1971 Bellcore

Category: Standards Track T. Narten

IBM

August 1996

IPv6 Stateless Address Autoconfiguration

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

Abstract

This document specifies the steps a host takes in deciding how to

autoconfigure its interfaces in IP version 6. The autoconfiguration

process includes creating a link-local address and verifying its

uniqueness on a link, determining what information should be

autoconfigured (addresses, other information, or both), and in the

case of addresses, whether they should be oBTained through the

stateless mechanism, the stateful mechanism, or both. This document

defines the process for generating a link-local address, the process

for generating site-local and global addresses via stateless address

autoconfiguration, and the Duplicate Address Detection procedure. The

details of autoconfiguration using the stateful protocol are

specified elsewhere.

Table of Contents

1. INTRODUCTION............................................. 2

2. TERMINOLOGY.............................................. 4

2.1. Requirements........................................ 7

3. DESIGN GOALS............................................. 8

4. PROTOCOL OVERVIEW........................................ 9

4.1. Site Renumbering.................................... 11

5. PROTOCOL SPECIFICATION................................... 11

5.1. Node Configuration Variables........................ 12

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............................. 15

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......... 17

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

SECURITY CONSIDERATIONS...................................... 19

REFERENCES................................................... 20

AUTHORS' ADDRESSES........................................... 21

APPENDIX: LOOPBACK SUPPRESSION & DUPLICATE ADDRESS DETECTION. 22

1. INTRODUCTION

This document specifies the steps a host takes in deciding how to

autoconfigure its interfaces in IP version 6. The autoconfiguration

process includes creating a link-local address and verifying its

uniqueness on a link, determining what information should be

autoconfigured (addresses, other information, or both), and in the

case of addresses, whether they should be obtained through the

stateless mechanism, the stateful mechanism, or both. This document

defines the process for generating a link-local address, the process

for generating site-local and global addresses via stateless address

autoconfiguration, and the Duplicate Address Detection procedure. The

details of autoconfiguration using the stateful protocol are

specified elsewhere.

IPv6 defines both a stateful and stateless address autoconfiguration

mechanism. Stateless autoconfiguration requires no manual

configuration of hosts, minimal (if any) configuration of routers,

and no additional servers. The stateless mechanism allows a host to

generate its own addresses using a combination of locally available

information and information advertised by routers. Routers advertise

prefixes that identify the subnet(s) associated with a link, while

hosts generate an "interface token" that uniquely identifies an

interface on a subnet. An address is formed by combining the two. In

the absence of routers, a host can only generate link-local

addresses. However, link-local addresses are sufficient for allowing

communication among nodes attached to the same link.

In the stateful autoconfiguration model, hosts obtain interface

addresses and/or configuration information and parameters from a

server. Servers maintain a database that keeps track of which

addresses have been assigned to which hosts. The stateful

autoconfiguration protocol allows hosts to obtain addresses, other

configuration information or both from a server. Stateless and

stateful autoconfiguration complement each other. For example, a host

can use stateless autoconfiguration to configure its own addresses,

but use stateful autoconfiguration to obtain other information.

Stateful autoconfiguration is described in [DHCPv6].

The stateless approach is used when a site is not particularly

concerned with the exact addresses hosts use, so long as they are

unique and properly routable. The stateful approach is used when a

site requires tighter control over exact address assignments. Both

stateful and stateless address autoconfiguration may be used

simultaneously. The site administrator specifies which type of

autoconfiguration to use through the setting of appropriate fields in

Router Advertisement messages [DISCOVERY].

IPv6 addresses are leased to an interface for a fixed (possibly

infinite) length of time. Each address has an associated lifetime

that indicates how long the address is bound to an interface. When a

lifetime expires, the binding (and address) become invalid and the

address may be reassigned to another interface elsewhere in the

Internet. To handle the expiration of address bindings gracefully, an

address goes through two distinct phases while assigned to an

interface. Initially, an address is "preferred", meaning that its use

in arbitrary communication is unrestricted. Later, an address becomes

"deprecated" in anticipation that its current interface binding will

become invalid. While in a deprecated state, the use of an address is

discouraged, but not strictly forbidden. New communication (e.g.,

the opening of a new TCP connection) should use a preferred address

when possible. A deprecated address should be used only by

applications that have been using it and would have difficulty

switching to another address without a service disruption.

To insure that all configured addresses are likely to be unique on a

given link, nodes run a "duplicate address detection" algorithm on

addresses before assigning them to an interface. The Duplicate

Address Detection algorithm is performed on all addresses,

independent of whether they are obtained via stateless or stateful

autoconfiguration. This document defines the Duplicate Address

Detection algorithm.

The autoconfiguration process specified in this document applies only

to hosts and not routers. Since host autoconfiguration uses

information advertised by routers, routers will need to be configured

by some other means. However, it is expected that routers will

generate link-local addresses using the mechanism described in this

document. In addition, routers are expected to successfully pass the

Duplicate Address Detection procedure described in this document on

all addresses prior to assigning them to an interface.

Section 2 provides definitions for terminology used throughout this

document. Section 3 describes the design goals that lead to the

current autoconfiguration procedure. Section 4 provides an overview

of the protocol, while Section 5 describes the protocol in detail.

2. TERMINOLOGY

IP - Internet Protocol Version 6. The terms IPv4 and IPv6

are used only in contexts where necessary to avoid

ambiguity.

node - a device that implements IP.

router - a node that forwards IP packets not explicitly

addressed to itself.

host - any node that is not a router.

upper layer - a protocol layer immediately above IP. Examples are

transport protocols such as TCP and UDP, control

protocols such as ICMP, routing protocols such as OSPF,

and internet or lower-layer protocols being "tunneled"

over (i.e., encapsulated in) IP such as IPX, AppleTalk,

or IP itself.

link - a communication facility or medium over which nodes can

communicate at the link layer, i.e., the layer

immediately below IP. Examples are Ethernets (simple

or bridged); PPP links; X.25, Frame Relay, or ATM

networks; and internet (or higher) layer "tunnels",

such as tunnels over IPv4 or IPv6 itself.

interface - a node's attachment to a link.

packet - an IP header plus payload.

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

interfaces.

unicast address

- an identifier for a single interface. A packet sent to

a unicast address is delivered to the interface

identified by that address.

multicast address

- an identifier for a set of interfaces (typically

belonging to different nodes). A packet sent to a

multicast address is delivered to all interfaces

identified by that address.

anycast address

- an identifier for a set of interfaces (typically

belonging to different nodes). A packet sent to an

anycast address is delivered to one of the interfaces

identified by that address (the "nearest" one,

according to the routing protocol's measure of

distance). See [ADDR-ARCH].

solicited-node multicast address

- a multicast address to which Neighbor Solicitation

messages are sent. The algorithm for computing the

address is given in [DISCOVERY].

link-layer address

- a link-layer identifier for an interface. Examples

include IEEE 802 addresses for Ethernet links and E.164

addresses for ISDN links.

link-local address

- an address having link-only scope that can be used to

reach neighboring nodes attached to the same link. All

interfaces have a link-local unicast address.

site-local address

- an address having scope that is limited to the local

site.

global address

- an address with unlimited scope.

communication

- any packet exchange among nodes that requires that the

address of each node used in the exchange remain the

same for the duration of the packet exchange. Examples

are a TCP connection or a UDP request-response.

tentative address

- an address whose uniqueness on a link is being

verified, prior to its assignment to an interface. A

tentative address is not considered assigned to an

interface in the usual sense. An interface discards

received packets addressed to a tentative address, but

accepts Neighbor Discovery packets related to Duplicate

Address Detection for the tentative address.

preferred address

- an address assigned to an interface whose use by upper

layer protocols is unrestricted. Preferred addresses

may be used as the source (or destination) address of

packets sent from (or to) the interface.

deprecated address

- An address assigned to an interface whose use is

discouraged, but not forbidden. A deprecated address

should no longer be used as a source address in new

communications, but packets sent to deprecated

addresses are delivered as expected. A deprecated

address may continue to be used as a source address in

communications where switching to a preferred address

causes hardship to a specific upper-layer activity

(e.g., an existing TCP connection).

valid address

- a preferred or deprecated address. A valid address may

appear as the source or destination address of a

packet, and the internet routing system is expected to

deliver packets sent to a valid address.

invalid address

- an address that is not assigned to any interface. A

valid address becomes invalid when its valid lifetime

expires. Invalid addresses should not appear as the

destination or source address of a packet. In the

former case, the internet routing system will be unable

to deliver the packet, in the later case the recipient

of the packet will be unable to respond to it.

preferred lifetime

- the length of time that a valid address is preferred

(i.e., the time until deprecation). When the preferred

lifetime expires, the address becomes deprecated.

valid lifetime

- the length of time an address remains in the valid

state (i.e., the time until invalidation). The valid

lifetime must be greater then or equal to the preferred

lifetime. When the valid lifetime expires, the address

becomes invalid.

interface token

- a link-dependent identifier for an interface that is

(at least) unique per link. Stateless address

autoconfiguration combines an interface token with a

prefix to form an address. From address

autoconfiguration's perspective, an interface token is

a bit string of known length. The exact length of an

interface token and the way it is created is defined in

a separate link-type specific document that covers

issues related to the transmission of IP over a

particular link type (e.g., [IPv6-ETHER]). In many

cases, the token will be the same as the interface's

link-layer address.

2.1. Requirements

Throughout this document, the Words that are used to define the

significance of the particular requirements are capitalized. These

words are:

MUST

This word or the adjective "REQUIRED" means that the item is an

absolute requirement of this specification.

MUST NOT

This phrase means the item is an absolute prohibition of this

specification.

SHOULD

This word or the adjective "RECOMMENDED" means that there may exist

valid reasons in particular circumstances to ignore this item, but

the full implications should be understood and the case carefully

weighed before choosing a different course.

SHOULD NOT

This phrase means that there may exist valid reasons in particular

circumstances when the listed behavior is acceptable or even

useful, but the full implications should be understood and the case

carefully weighed before implementing any behavior described with

this label.

MAY

This word or the adjective "OPTIONAL" means that this item is truly

optional. One vendor may choose to include the item because a

particular marketplace requires it or because it enhances the

product, for example, another vendor may omit the same item.

3. DESIGN GOALS

Stateless autoconfiguration is designed with the following goals in

mind:

o Manual configuration of individual machines before connecting them

to the network should not be required. Consequently, a mechanism is

needed that allows a host to obtain or create unique addresses for

each of its interfaces. Address autoconfiguration assumes that each

interface can provide a unique identifier for that interface (i.e.,

an "interface token"). In the simplest case, an interface token

consists of the interface's link-layer address. An interface token

can be combined with a prefix to form an address.

o Small sites consisting of a set of machines attached to a single

link should not require the presence of a stateful server or router

as a prerequisite for communicating. Plug-and-play communication

is achieved through the use of link-local addresses. Link-local

addresses have a well-known prefix that identifies the (single)

shared link to which a set of nodes attach. A host forms a link-

local address by appending its interface token to the link-local

prefix.

o A large site with multiple networks and routers should not require

the presence of a stateful address configuration server. In order

to generate site-local or global addresses, hosts must determine

the prefixes that identify the subnets to which they attach.

Routers generate periodic Router Advertisements that include

options listing the set of active prefixes on a link.

o Address configuration should facilitate the graceful renumbering of

a site's machines. For example, a site may wish to renumber all of

its nodes when it switches to a new network service provider.

Renumbering is achieved through the leasing of addresses to

interfaces and the assignment of multiple addresses to the same

interface. Lease lifetimes provide the mechanism through which a

site phases out old prefixes. The assignment of multiple addresses

to an interface provides for a transition period during which both

a new address and the one being phased out work simultaneously.

o System administrators need the ability to specify whether stateless

autoconfiguration, stateful autoconfiguration, or both should be

used. Router Advertisements include flags specifying which

mechanisms a host should use.

4. PROTOCOL OVERVIEW

This section provides an overview of the typical steps that take

place when an interface autoconfigures itself. Autoconfiguration is

performed only on multicast-capable links and begins when a

multicast-capable interface is enabled, e.g., during system startup.

Nodes (both hosts and routers) begin the autoconfiguration process by

generating a link-local address for the interface. A link-local

address is formed by appending the interface's token to the well-

known link-local prefix.

Before the link-local address can be assigned to an interface and

used, however, a node must attempt to verify that this "tentative"

address is not already in use by another node on the link.

Specifically, it sends a Neighbor Solicitation message containing the

tentative address as the target. If another node is already using

that address, it will return a Neighbor Advertisement saying so. If

another node is also attempting to use the same address, it will send

a Neighbor Solicitation for the target as well. The exact number of

times the Neighbor Solicitation is (re)transmitted and the delay time

between consecutive solicitations is link-specific and may be set by

system management.

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

token that overrides the default token in such a way that the

autoconfiguration mechanism can then be applied using the new

(presumably unique) interface token. Alternatively, link-local and

other addresses will need to be configured manually.

Once a node ascertains that its tentative link-local address is

unique, it assigns it to the interface. At this point, the node has

IP-level connectivity with neighboring nodes. The remaining

autoconfiguration steps are performed only by hosts; the

(auto)configuration of routers is beyond the scope of this document.

The next phase of autoconfiguration involves obtaining a Router

Advertisement or determining that no routers are present. If routers

are present, they will send Router Advertisements that specify what

sort of autoconfiguration a host should do. If no routers are

present, stateful autoconfiguration should be invoked.

Routers send Router Advertisements periodically, but the delay

between successive advertisements will generally be longer than a

host performing autoconfiguration will want to wait [DISCOVERY]. To

obtain an advertisement quickly, a host sends one or more Router

Solicitations to the all-routers multicast group. Router

Advertisements contain two flags indicating what type of stateful

autoconfiguration (if any) should be performed. A "managed address

configuration" flag indicates whether hosts should use stateful

autoconfiguration to obtain addresses. An "other stateful

configuration" flag indicates whether hosts should use stateful

autoconfiguration to obtain additional information (excluding

addresses).

Router Advertisements also contain zero or more Prefix Information

options that contain information used by stateless address

autoconfiguration to generate site-local and global addresses. It

should be noted that the stateless and stateful address

autoconfiguration fields in Router Advertisements are processed

independently of one another, and a host may use both stateful and

stateless address autoconfiguration simultaneously. One Prefix

Information option field, the "autonomous address-configuration

flag", indicates whether or not the option even applies to stateless

autoconfiguration. If it does, additional option fields contain a

subnet prefix together with lifetime values indicating how long

addresses created from the prefix remain preferred and valid.

Because routers generate Router Advertisements periodically, hosts

will continually receive new advertisements. Hosts process the

information contained in each advertisement as described above,

adding to and refreshing information received in previous

advertisements.

For safety, all addresses must be tested for uniqueness prior to

their assignment to an interface. In the case of addresses created

through stateless autoconfig, however, the uniqueness of an address

is determined primarily by the portion of the address formed from an

interface token. Thus, if a node has already verified the uniqueness

of a link-local address, additional addresses created from the same

interface token need not be tested individually. In contrast, all

addresses obtained manually or via stateful address autoconfiguration

should be tested for uniqueness individually. To accommodate sites

that believe the overhead of performing Duplicate Address Detection

outweighs its benefits, the use of Duplicate Address Detection can be

disabled through the administrative setting of a per-interface

configuration flag.

To speed the autoconfiguration process, a host may generate its

link-local address (and verify its uniqueness) in parallel with

waiting for a Router Advertisement. Because a router may delay

responding to a Router Solicitation for a few seconds, the total time

needed to complete autoconfiguration can be significantly longer if

the two steps are done serially.

4.1. Site Renumbering

Address leasing facilitates site renumbering by providing a mechanism

to time-out addresses 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

(including applications) to select the most appropriate source

address given a particular destination and possibly other

constraints. An application may choose to select the source address

itself before starting a new communication or may leave the address

unspecified, in which case the upper networking layers will use the

mechanism provided by the IP layer to choose a suitable address on

the application's behalf.

Detailed address selection rules are beyond the scope of this

document.

5. PROTOCOL SPECIFICATION

Autoconfiguration is performed on a per-interface basis on

multicast-capable interfaces. For multihomed hosts,

autoconfiguration is performed independently on each interface.

Autoconfiguration applies primarily to hosts, with two exceptions.

Routers are expected to generate a link-local address using the

procedure outlined below. In addition, routers perform Duplicate

Address Detection on all addresses prior to assigning them to an

interface.

5.1. Node Configuration Variables

A node MUST allow the following autoconfiguration-related variable to

be configured by system management for each multicast interface:

DupAddrDetectTransmits

The number of consecutive Neighbor Solicitation

messages sent while performing Duplicate Address

Detection on a tentative address. A value of zero

indicates that Duplicate Address Detection is not

performed on tentative addresses. A value of one

indicates a single transmission with no follow up

retransmissions.

Default: 1, but may be overridden by a link-type

specific value in the document that covers issues

related to the transmission of IP over a particular

link type (e.g., [IPv6-ETHER]).

Autoconfiguration also assumes the presence of the variable

RetransTimer as defined in [DISCOVERY]. For autoconfiguration

purposes, RetransTimer specifies the delay between consecutive

Neighbor Solicitation transmissions performed during Duplicate

Address Detection (if DupAddrDetectTransmits is greater than 1), as

well as the time a node 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 are to be obtained using the stateful

autoconfiguration mechanism. It starts out in a

FALSE state.

A host also maintains a list of addresses together with their

corresponding lifetimes. The address list contains both

autoconfigured addresses and those configured manually.

5.3. Creation of Link-Local Addresses

A node forms a link-local address whenever an interface becomes

enabled. An interface may become enabled after any of the following

events:

- The interface is initialized at system startup time.

- The interface is reinitialized after a temporary interface failure

or after being temporarily disabled by system management.

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

- The interface becomes enabled by system management after having

been administratively disabled.

A link-local address is formed by prepending the well-known link-

local prefix FE80::0 [ADDR-ARCH] (of appropriate length) to the

interface token. If the interface token has a length of N bits, the

interface token replaces the right-most N zero bits of the link-local

prefix. If the interface token is more than 118 bits in length,

autoconfiguration fails and manual configuration is required.

A link-local address has an infinite preferred and valid lifetime; it

is never timed out.

5.4. Duplicate Address Detection

Duplicate Address Detection MUST be performed on unicast addresses

prior to assigning them to an interface whose DupAddrDetectTransmits

variable is greater than zero. Duplicate Address Detection takes

place on all unicast addresses, regardless of whether they are

obtained through stateful, stateless or manual configuration.

(Duplicate Address Detection MUST NOT be performed on anycast

addresses.) Each individual unicast address SHOULD be tested for

uniqueness. However, when stateless address autoconfiguration is

used, address uniqueness is determined solely by the interface token,

assuming that subnet prefixes are assigned correctly (i.e., if all of

an interface's addresses are generated from the same token, either

all addresses or none of them will be duplicates). Thus, for a set of

addresses formed from the same interface token, it is sufficient to

check that the link-local address generated from the token is unique

on the link. In such cases, the link-local address MUST be tested for

uniqueness before any of the other addresses formed from the token

can be assigned to an interface.

The procedure for detecting duplicate addresses uses Neighbor

Solicitation and Advertisement messages as described below. If a

duplicate address is discovered during the procedure, the address

cannot be assigned to the interface. If the address is derived from

an interface token, a new token will need to be assigned to the

interface, or all IP addresses for the interface will need to be

manually configured. Note that the method for detecting duplicates

is not completely reliable, and it is possible that duplicate

addresses will still exist (e.g., if the link was partitioned while

Duplicate Address Detection was performed).

An address on which the duplicate Address Detection Procedure is

applied is said to be tentative until the procedure has completed

successfully. A tentative address is not considered "assigned to an

interface" in the traditional sense. That is, the interface must

accept Neighbor Solicitation and Advertisement messages containing

the tentative address in the Target Address field, but processes such

packets differently from those whose Target Address matches an

address assigned to the interface. Other packets addressed to the

tentative address should be silently discarded.

It should also be noted that Duplicate Address Detection must be

performed prior to assigning an address to an interface in order to

prevent multiple nodes from using the same address simultaneously.

If a node begins using an address in parallel with Duplicate Address

Detection, and another node is already using the address, the node

performing Duplicate Address Detection will erroneously process

traffic intended for the other node, resulting in such possible

negative consequences as the resetting of open TCP connections.

The following subsections describe specific tests a node performs to

verify an address's uniqueness. An address is considered unique if

none of the tests indicate the presence of a duplicate address within

RetransTimer milliseconds after having sent DupAddrDetectTransmits

Neighbor Solicitations. Once an address is determined to be unique,

it may be assigned to an interface.

5.4.1. Message Validation

A node MUST silently discard any Neighbor Solicitation or

Advertisement message that does not pass the validity checks

specified in [DISCOVERY]. A solicitation that passes these validity

checks is called a valid solicitation or valid advertisement.

5.4.2. Sending Neighbor Solicitation Messages

Before sending a Neighbor Solicitation, an interface MUST join the

all-nodes multicast address and the solicited-node multicast address

of the tentative address. The former insures that the node receives

Neighbor Advertisements from other nodes already using the address;

the latter insures that two nodes attempting to use the same address

simultaneously detect each other's presence.

To check an address, a node sends DupAddrDetectTransmits Neighbor

Solicitations, each separated by RetransTimer milliseconds. The

solicitation's Target Address is set to the address being checked,

the IP source is set to the unspecified address and the IP

destination is set to the solicited-node multicast address of the

target address.

If the Neighbor Solicitation is the first message to be sent from an

interface after interface (re)initialization, the node should delay

sending the message by a random delay between 0 and

MAX_RTR_SOLICITATION_DELAY as specified in [DISCOVERY]. This serves

to alleviate congestion when many nodes start up on the link at the

same time, such as after a power failure, and may help to avoid race

conditions when more than one node is trying to solicit for the same

address at the same time. In order to improve the robustness of the

Duplicate Address Detection algorithm, an interface MUST receive and

process datagrams sent to the all-nodes multicast address or

solicited-node multicast address of the tentative address while

delaying transmission of the initial Neighbor Solicitation.

5.4.3. Receiving Neighbor Solicitation Messages

On receipt of a valid Neighbor Solicitation message on an interface,

node behavior depends on whether the target address is tentative or

not. If the target address is not tentative (i.e., it is assigned to

the receiving interface), the solicitation is processed as described

in [DISCOVERY]. If the target address is tentative, and the source

address is a unicast address, the solicitation's sender is performing

address resolution on the target; the solicitation should be silently

ignored. Otherwise, processing takes place as described below. In

all cases, a node MUST NOT respond to a Neighbor Solicitation for a

tentative address.

If the source address of the Neighbor Solicitation is the unspecified

address, the solicitation is from a node performing Duplicate Address

Detection. If the solicitation is from another node, the tentative

address is a duplicate and should not be used (by either node). If

the solicitation is from the node itself (because the node loops back

multicast packets), the solicitation does not indicate the presence

of a duplicate address.

Implementor's Note: many interfaces provide a way for upper layers to

selectively enable and disable the looping back of multicast packets.

The details of how such a facility is implemented may prevent

Duplicate Address Detection from working correctly. See the Appendix

for further discussion.

The following tests identify conditions under which a tentative

address is not unique:

- If a Neighbor Solicitation for a tentative address is received

prior to having sent one, the tentative address is a duplicate.

This condition occurs when two nodes run Duplicate Address

Detection simultaneously, but transmit initial solicitations at

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 [DISCOVERY]. If the target

address is tentative, the tentative address is not unique.

5.4.5. When Duplicate Address Detection Fails

A tentative address that is determined to be a duplicate as described

above, MUST NOT be assigned to an interface and the node SHOULD log a

system management error. If the address is a link-local address

formed from an interface token, the interface SHOULD be disabled.

5.5. Creation of Global and Site-Local Addresses

Global and site-local addresses are formed by appending an interface

token to a prefix of appropriate length. Prefixes are obtained from

Prefix Information options contained in Router Advertisements.

Creation of global and site-local addresses and configuration of

other parameters as described in this section SHOULD be locally

configurable. However, 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 [DISCOVERY].

5.5.2. Absence of Router Advertisements

If a link has no routers, a host MUST attempt to use stateful

autoconfiguration to obtain addresses and other configuration

information. An implementation MAY provide a way to disable the

invocation of stateful autoconfiguration in this case, but the

default SHOULD be enabled. From the perspective of

autoconfiguration, a link has no routers if no Router Advertisements

are received after having sent a small number of Router Solicitations

as described in [DISCOVERY].

5.5.3. Router Advertisement Processing

On receipt of a valid Router Advertisement (as defined in

[DISCOVERY]), a host copies the value of the advertisement's M bit

into ManagedFlag. If the value of ManagedFlag changes from FALSE to

TRUE, the host should invoke the stateful address autoconfiguration

protocol, requesting address information. If the value of the

ManagedFlag changes from TRUE to FALSE, the host should terminate the

stateful address autoconfiguration protocol (i.e., stop requesting

addresses and ignore subsequent responses to in-progress

transactions). If the value of the flag stays unchanged, no special

action takes place. In particular, a host MUST NOT reinvoke stateful

address configuration if it is already participating in the stateful

protocol as a result of an earlier advertisement.

An advertisement's 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 the value of the

OtherConfigFlag changes from TRUE to FALSE, any activity related to

stateful autoconfiguration for parameters other than addresses should

be halted. If the value of the flag stays unchanged, no special

action takes place. In particular, a host MUST NOT reinvoke stateful

configuration if it is already participating in the stateful protocol

as a result of an earlier advertisement.

For each Prefix-Information option in the Router Advertisement:

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

Information option.

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

Information option.

c) If the preferred lifetime is greater than the valid lifetime,

silently ignore the Prefix Information option. A node MAY wish to

log a system management error in this case.

d) If the advertised prefix matches the prefix of an autoconfigured

address (i.e., obtained via stateless or stateful address

autoconfiguration) in the list of addresses associated with the

interface, set the preferred timer to that of the option's preferred

lifetime, and set the valid lifetime to that of the option's valid

lifetime.

e) If the prefix advertised does not match the prefix of an address

already in the list, then form an address (and add it to the list)

by appending the interface token to the prefix as follows:

128 - N bits N bits

+---------------------------------------+------------------------+

link prefix interface token

+----------------------------------------------------------------+

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

equal 128 bits, the Prefix Information option MUST be ignored. An

implementation MAY wish to log a system management error in this

case. It is the responsibility of the system administrator to insure

that the lengths of prefixes contained in Router Advertisements are

consistent with the length of interface tokens for that link type.

In those cases where a site requires the use of longer prefixes than

can be accommodated by the interface token, stateful

autoconfiguration can be used.

If an address is formed successfully, the host adds it to the list

of addresses assigned to the interface, initializing its preferred

and valid lifetime values from the Prefix Information option.

5.5.4. Address Lifetime Expiry

A preferred address becomes deprecated when its preferred lifetime

expires. A deprecated address SHOULD continue to be used as a source

address in existing communications, but SHOULD NOT be used in new

communications if an alternate (non-deprecated) address is available

and has sufficient scope. The IP layer MUST continue to accept

datagrams destined to a deprecated address since a deprecated address

is still a valid address for the interface. An implementation MAY

prevent any new communication from using a deprecated address, but

system management MUST have the ability to disable such a facility.

An address (and its association with an interface) becomes invalid

when its valid lifetime expires. An invalid address MUST NOT be used

as a source address in outgoing communications and MUST NOT be

recognized as a destination on a receiving interface.

Note that if a Prefix Information option is received with a preferred

lifetime of zero, any addresses generated from that prefix are

immediately deprecated. Similarly, if both the advertised deprecated

and valid lifetimes are zero, any addresses generated from that

prefix become invalid immediately.

5.6. Configuration Consistency

It is possible for hosts to obtain address information using both

stateless and stateful protocols since both may be enabled at the

same time. It is also possible that the values of other

configuration parameters such as MTU size and hop limit will be

learned from both Router Advertisements and the stateful

autoconfiguration protocol. If the same configuration information is

provided by multiple sources, the value of this information should be

consistent. However, it is not considered a fatal error if

information received from multiple sources is inconsistent. Hosts

accept the union of all information received via the stateless and

stateful protocols. If inconsistent information is learned from

different sources, the most recently obtained values always have

precedence over information learned earlier.

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 [RFC1826].

REFERENCES

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

1995.

[IPv6-ETHER] Crawford, M., "A Method for the Transmission of IPv6

Packets over Ethernet Networks", RFC1972, August 1996.

[RFC1112] Deering, S., "Host Extensions for IP Multicasting", STD 5,

RFC1112, August 1989.

[ADDR-ARCH] Hinden, R., and S. Deering, "Internet Protocol Version

(IPv6) Addressing Architecture", RFC1884, December 1995.

[DHCPv6] Work in Progress.

[DISCOVERY] Narten, T., Nordmark, E., and W. Simpson, "Neighbor

Discovery for IP Version 6 (IPv6)", RFC1970, August 1996.

Acknowledgements

The authors would like to thank the members of both the IPNG and

ADDRCONF working groups for their input. In particular, thanks to Jim

Bound, Steve Deering, and Erik Nordmark.

AUTHORS' ADDRESSES

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@vnet.ibm.com

APPENDIX: 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 token and link-layer address,

and they both send out packets with identical contents at roughly the

same time (e.g., Neighbor Solicitations for a tentative address as

part of Duplicate Address Detection messages). Although a receiver

will receive both packets, it cannot determine which packet was

looped back and which packet came from the other node by simply

comparing packet contents (i.e., the contents are identical). In this

particular case, it is not necessary to know precisely which packet

was looped back and which was sent by another node; if one receives

more solicitations than were sent, the tentative address is a

duplicate. However, the situation may not always be this

straightforward.

The IPv4 multicast specification [RFC1112] recommends that the

service interface provide a way for an upper-layer protocol to

inhibit local delivery of packets sent to a multicast group that the

sending host is a member of. Some applications know that there will

be no other group members on the same host, and suppressing loopback

prevents them from having to receive (and discard) the packets they

themselves send out. A straightforward way to implement this

facility is to disable loopback at the hardware level (if supported

by the hardware), with packets looped back (if requested) by

software. On interfaces in which the hardware itself suppresses

loopbacks, a node running Duplicate Address Detection simply counts

the number of Neighbor Solicitations received for a tentative address

and compares them with the number expected. If there is a mismatch,

the tentative address is a duplicate.

In those cases where the hardware cannot suppress loopbacks, however,

one possible software heuristic to filter out unwanted loopbacks is

to discard any received packet whose link-layer source address is the

same as the receiving interface's. Unfortunately, use of that

criteria also results in the discarding of all packets sent by

another node using the same link-layer address. Duplicate Address

Detection will fail on interfaces that filter received packets in

this manner:

o If a node performing Duplicate Address Detection discards received

packets having the same source link-layer address as the receiving

interface, it will also discard packets from other nodes also using

the same link-layer address, including Neighbor Advertisement and

Neighbor Solicitation messages required to make Duplicate Address

Detection work correctly. This particular problem can be avoided

by temporarily disabling the software suppression of loopbacks

while a node performs Duplicate Address Detection.

o If a node that is already using a particular IP address discards

received packets having the same link-layer source address as the

interface, it will also discard Duplicate Address Detection-related

Neighbor Solicitation messages sent by another node also using the

same link-layer address. Consequently, Duplicate Address Detection

will fail, and the other node will configure a non-unique address.

Since it is generally impossible to know when another node is

performing Duplicate Address Detection, this scenario can be

avoided only if software suppression of loopback is permanently

disabled.

Thus, to perform Duplicate Address Detection correctly in the case

where two interfaces are using the same link-layer address, an

implementation must have a good understanding of the interface's

multicast loopback semantics, and the interface cannot discard

received packets simply because the source link-layer address is the

same as the interfaces.

 
 
 
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