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rfc2462.IPv6 Stateless Address Autoconfiguration

Network Working Group S. Thomson Request for Comments: 2462 Bellcore Obsoletes: 1971 T. Narten Category: Standards Track IBM December 1998 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 (6)

3. DESIGN GOALS (7)

4. PROTOCOL OVERVIEW (8)

4.1. Site Renumbering (10)

5. PROTOCOL SPECIFICATION (10)

5.1. Node Configuration Variables (11)

5.2. Autoconfiguration-Related Variables (11)

5.3. Creation of Link-Local Addresses (12)

Thomson & Narten Standards Track [Page 1]

5.4. Duplicate Address Detection (13)

5.4.1. Message Validation (14)

5.4.2. Sending Neighbor Solicitation Messages (14)

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 (16)

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. References (20)

8. Acknowledgements and Authors’ Addresses (21)

9. APPENDIX A: LOOPBACK SUPPRESSION & DUPLICATE ADDRESS

DETECTION (22)

10. APPENDIX B: CHANGES SINCE RFC 1971 (24)

11. Full Copyright Statement (25)

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

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.

Thomson & Narten Standards Track [Page 2]

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 future work

[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.

Thomson & Narten Standards Track [Page 3]

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 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.

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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.

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deprecated address - An address assigned to an interface whose use is discouraged, but not forbidden. A deprecated address should no longer be used as a source address in new communications, but

packets sent from or to deprecated addresses are delivered as

expected. A deprecated address may continue to be used as a

source address in communications where switching to a preferred address causes hardship to a specific upper-layer activity

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

valid address - a preferred or deprecated address. A valid address

may appear as the source or destination address of a packet, and the internet routing system is expected to deliver packets sent to a valid address to their intended recipients.

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

Invalid addresses should not appear as the destination or source address of a packet. In the former case, the internet routing

system will be unable to deliver the packet, in the later case

the recipient of the packet will be unable to respond to it.

preferred lifetime - the length of time that a valid address is

preferred (i.e., the time until deprecation). When the preferred lifetime expires, the address becomes deprecated.

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 [ADDR-ARCH]. 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-ETHER]). 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 [KEYWORDS].

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3. DESIGN GOALS

Stateless autoconfiguration is designed with the following goals in

mind:

o Manual configuration of individual machines before connecting

them to the network should not be required. Consequently, a

mechanism is needed that allows a host to obtain or create

unique addresses for each of its interfaces. Address

autoconfiguration assumes that each interface can provide a

unique identifier for that interface (i.e., an "interface

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

consists of the interface’s link-layer address. An interface

identifier can be combined with a prefix to form an address.

o Small sites consisting of a set of machines attached to a single link should not require the presence of a stateful server or

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

communication is achieved through the use of link-local

addresses. Link-local addresses have a well-known prefix that

identifies the (single) shared link to which a set of nodes

attach. A host forms a link-local address by appending its

interface identifier to the link-local prefix.

o A large site with multiple networks and routers should not

require 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.

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4. PROTOCOL OVERVIEW

This section provides an overview of the typical steps that take

place when an interface autoconfigures itself. Autoconfiguration is performed only on multicast-capable links and begins when a

multicast-capable interface is enabled, e.g., during system startup. Nodes (both hosts and routers) begin the autoconfiguration process by generating a link-local address for the interface. A link-local

address is formed by appending the interface’s identifier to the

well-known link-local prefix.

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

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

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

Specifically, it sends a Neighbor Solicitation message containing the tentative address as the target. If another node is already using

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

another node is also attempting to use the same address, it will send a Neighbor Solicitation for the target as well. The exact number of

times the Neighbor Solicitation is (re)transmitted and the delay time between consecutive solicitations is link-specific and may be set by system management.

If a node determines that its tentative link-local address is not

unique, autoconfiguration stops and manual configuration of the

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 [DISCOVERY]. To obtain an advertisement quickly, a host sends one or more Router Thomson & Narten Standards Track [Page 8]

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 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.

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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.

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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: Thomson & Narten Standards Track [Page 11]

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.

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.

Thomson & Narten Standards Track [Page 12]

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

local prefix FE80::0 [ADDR-ARCH] (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 [ADDR-ARCH].

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

out.

5.4. Duplicate Address Detection

Duplicate Address Detection is performed on unicast addresses prior

to assigning them to an interface whose DupAddrDetectTransmits

variable is greater than zero. Duplicate Address Detection MUST take place on all unicast addresses, regardless of whether they are

obtained through stateful, stateless or manual configuration, with

the exception of the following cases:

- Duplicate Address Detection MUST NOT be performed on anycast

addresses.

- Each individual unicast address SHOULD be tested for uniqueness. However, when stateless address autoconfiguration is used,

address uniqueness is determined solely by the interface

identifier, assuming that subnet prefixes are assigned correctly (i.e., if all of an interface’s addresses are generated from the same identifier, either all addresses or none of them will be

duplicates). Thus, for a set of addresses formed from the same

interface identifier, it is sufficient to check that the link-

local address generated from the identifier is unique on the

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

uniqueness, and if no duplicate address is detected, an

implementation MAY choose to skip Duplicate Address Detection

for additional addresses derived from the same interface

identifier.

The procedure for detecting duplicate addresses uses Neighbor

Solicitation and Advertisement messages as described below. If a

duplicate address is discovered during the procedure, the address

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

Thomson & Narten Standards Track [Page 13]

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, Thomson & Narten Standards Track [Page 14]

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:

Thomson & Narten Standards Track [Page 15]

- 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 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

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].

Thomson & Narten Standards Track [Page 16]

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, and the host is not already running the stateful address

autoconfiguration protocol, the host should invoke the stateful

address autoconfiguration protocol, requesting both address

information and other information. If the value of the ManagedFlag

changes from TRUE to FALSE, the host should continue running the

stateful address autoconfiguration, i.e., the change in the value of the ManagedFlag has no effect. If the value of the flag stays

unchanged, no special action takes place. In particular, a host MUST NOT reinvoke stateful address configuration if it is already

participating in the stateful protocol as a result of an earlier

advertisement.

An advertisement’s O flag field is processed in an analogous manner.

A host copies the value of the O flag into OtherConfigFlag. If the

value of OtherConfigFlag changes from FALSE to TRUE, the host should invoke the stateful autoconfiguration protocol, requesting

information (excluding addresses if ManagedFlag is set to FALSE). If the value of the OtherConfigFlag changes from TRUE to FALSE, the host should continue running the stateful address autoconfiguration

protocol, i.e., the change in the value of OtherConfigFlag has no

effect. If the value of the flag stays unchanged, no special action

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

configuration if it is already participating in the stateful protocol as a result of an earlier advertisement.

For each Prefix-Information option in the Router Advertisement:

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

Prefix Information

option.

Thomson & Narten Standards Track [Page 17]

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 |

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

If the sum of the prefix length and interface identifier 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 identifiers for that link type. Note that interface identifiers

will typically be 64-bits long and based on EUI-64 identifiers as described in [ADDR-ARCH].

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.

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 Thomson & Narten Standards Track [Page 18]

this Prefix Information option was obtained has been

authenticated (e.g., via IPSec [RFC2402]). If the Router

Advertisment was authenticated, the StoredLifetime should be

set to the Lifetime in the received option.

3) Otherwise, reset the stored Lifetime in the corresponding

address to two hours.

The above rules address a specific denial of service attack in

which a bogus advertisement could contain prefixes with very

small Valid Lifetimes. Without the above rules, a single

unauthenticated advertisement containing bogus Prefix Information options with short 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 Thomson & Narten Standards Track [Page 19]

stateful protocols. If inconsistent information is learned different sources, the most recently obtained values always have precedence

over information learned earlier.

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 [RFC2402].

7. References

[RFC2402] Kent, S. and R. Atkinson, "IP Authentication Header",

RFC 2402, November 1998.

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

IPv6 Packets over Ethernet Networks", RFC 2464,

December 1998.

[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate

Requirement Levels", BCP 14, RFC 2119, March

1997.

[RFC1112] Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC 1112, August

1989.

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

(IPv6) Addressing Architecture", RFC 2373, July 1998

[DHCPv6] Bound, J. and C. Perkins, "Dynamic Host Configuration

Protocol for IPv6 (DHCPv6)", Work in Progress.

Thomson & Narten Standards Track [Page 20]

神州租车宝沃免费一天活动给消费者带来用钱买得到的满意

神州租车宝沃免费一天活动给消费者带来用钱买得到的满意最近一向低调的宝沃可谓是“惊喜”不断，首先宝沃的“贵才能好，好才能贵”的刷屏广告，然后是听朋友讲到“免费开1天”的“神车租车版”的宝沃BX5，神州租车宝沃免费一天，真的很吸引人，今天来看看神州租车宝沃免费一天的试驾感受吧！提到宝沃，不得不提第一次把这个品牌带入大众视野的北汽福田。2014年，北汽福田以500万欧元将宝沃这个曾经与奔驰、宝马、奥迪齐称“BBBA”的德系豪华品牌收归旗下。虽然宝沃早在上世纪60年代就因经营不善而宣告破产，但不管怎么说，它曾经也是德系豪华品牌啊！于是经过近一年准备，在2015年的日内瓦车展上，福田宣布复活宝沃品牌。此举当时也被福田和全国媒体大量报道，犹如车界一大盛事。随后宝沃在2016年开始陆续在国内市场推出BX5、BX7、BX6和BXi7四款SUV车型。后面的事，大家也都看在眼里，沉寂、爆发，再沉寂，再到今年杨嵩入驻，开了一场被称为“史上最长发布会”的演讲，让宝沃品牌再次回到公众视野。不过也仅限于发布会，产品确实有些差强人意，仅有一款BX6和BX7的新能源车型推出，这两款，都知道不会走量，结果也确实如此。直到10月8日晚间，福田汽车发布《关于预挂牌转让北京宝沃汽车有限公司67%股权的议案》的公告。宝沃跌落神坛，却搭上另一个“神”——神州租车。如今神州租车宝沃免费一天的活动让人心动，下载神州租车App并登录后，选距离最近的租赁点及租赁时间后再选择需要租赁的车型，确认订单后到约定时间去指定租赁点取车即可。整个流程下来不到5分钟全部搞定，方便又快捷！芝麻信用700分，绑定支付宝后还免去了神州租车押金。神州租车宝沃免费一天正式开始。神州租车宝沃BX5的驾驶感也不错。这款车外观大气却不显得笨重，车内空间很充裕，座椅间距和尾箱空间都够用。车内配置也不错，车座、档杆都是真皮包裹，乘坐及握持感都相当不错，8英寸高清彩色中控触摸屏操作简单，极易上手。车子开起来动力足，有明显推背感，加速制动距离短，且SUV特质明显，驾驶视野较宽，方向盘转向精准，更重要的是车辆稳定性好，坑洼路段没有明显颠簸感，刹车也不突兀。免费的试驾，给消费者带来的却是用钱也买不到的对意向汽车最直接的感受，神州宝沃推出的深度试驾可以拥有一个小时的试驾时间，对于一款车的性能基本就摸清了。对于消费者来说，是个不错的福利~神州租车宝沃免费一天，其实是个双赢的策略，对于消费者来说，无疑，这会给消费者提供更直观更深入的车辆驾驶感受，而对于宝沃汽车而言，更是让消费者了解车辆性能的渠道。

宝沃汽车沉浮录

宝沃汽车沉浮录宝沃，这个曾与奔驰、宝马齐名的德系豪华品牌，1919年诞生于德国不莱梅，如果没有爽约这个汽车工业发展最快的半个世纪，它也一定可以轻松秀出一段近百年历史的厚重感。不过造化弄人，它没有摆脱盛极必衰的厄运，上世纪60年代宣布破产。不过好在现在“沉睡”半个世纪的宝沃如今又再起征程。在经过之前法兰克福、日内瓦车展的“辗转反侧”后，德国宝沃汽车首款产品宝沃BX7终于来到了中国内地。并在今年为期十天的北京车展上，BX7正式在中国上市。车展期间，宝沃展台也是吸引了很多人的驻足。这些人当中，有人可能是慕名而来，毕竟宝沃品牌在历史上也曾经是“叱咤风云”的“角色”，有人可能是抱着好奇而来，毕竟消失太久之后，冷不丁的重返舞台也算是吊足了大家的“胃口”。但不管怎样，明眼人一看，宝沃BX7在自己的外观上下足功夫，造型元素从宝沃的经典车型上演化成如今的造型元素，但还是能让人们一眼就能感受到不折不扣的日耳曼风格。在品牌塑造上，宝沃打出“触手可及的豪华”的品牌口号，抛出的是德国汽车工业 4.0智造这张牌。用德国宝沃汽车集团管理委员会主席及全球CEO华立新（Ulrich Walker）的话说，他

们是全球布局，以品牌价值重出江湖。对于这个“断档”半个世纪，如今渴望“复兴”的汽车品牌而言，怎样借助自身的品牌文化、血统、历史，而又能从头开始，是宝沃品牌当下不得不面对的现实。百年来，起起落落，不知道有多少汽车品牌折戟沉沙，复活的可谓寥寥。宝沃能够复活已实属不易，但怎样找准自身定位，走得更加精彩？重拾昔日的辉煌和地位？也并不那么简单。峥嵘历史：曾经的王者德国是汽车工业的故乡，同时，德国汽车工业也是全球汽车工业的开拓者与引领者，这里云集有世界顶级性能的汽车，有尖端的汽车技术，有高精尖的汽车产业工人。不仅如此，德国还是很多世界著名驰名品牌的诞生地，现在提起德国的豪华品牌，人们脑海中想必立刻“蹦出”脑际的便是德系三强――“ABB”。殊不知，在上个世纪三、四十年代，Borgward （宝沃）在德国的影响力并不比德系三强逊色，曾经许多人甚至把拥有一辆Borgward当作一种荣耀和成功的标志。Borgward这个已经拥有接近百年历史的汽车品牌，由卡尔?宝沃（Carl F.W. Borgward）1919年在德国不莱梅创建，拥有着跟梅赛德斯?奔驰、BMW宝马、AUDI奥迪一样纯正的德国血统。回顾Borgward汽车的发展史，之所以能取得如此瞩目的成就，离不开被誉为“汽车狂人”之称――卡尔?宝沃的强

宝沃汽车怎么样,车主提车150天后驾驶感受曝光

宝沃汽车怎么样，车主提车150天后驾驶感受曝光放眼当下，汽车行业已经成为炙手可热的高端市场，千姿百态的车型被厂商产出并推向大众。再加之当下车商都明白一个道理，不用心造车就面临失败，随着车型的不断更新迭代，消费者的心理预期也不断提高。注重改善自身生活水平的今天，汽车品牌只有找到自身的准确定位，就能顺藤摸瓜锁定核心客群。随着车主的年轻化，用户需求日益增多，一款汽车品牌若定位偏差就容易血本无归，汽车只有充分发挥特点才能在车界屹立不倒，正如宝沃BX7。恰巧遇到老司机车主王先生，刚刚提了一辆宝沃BX7，用车150天后，让我们听听他对于宝沃汽车怎么样给出的答案。关于宝沃汽车怎么样，王先生认为宝沃BX7外观大气不臃肿，稳重不张扬，让人一打眼就能相中。外观由名师设计，符合大部分人的审美观，是该车的一大亮点，达到BBA水平没地说，大气，耐看，没有审美疲劳感，尤其前脸格栅，其

他车都是横的，咱是竖的，立体感很强。前后灯看起来都比较修长，比较有拉伸感。最喜欢的就是日行灯和尾灯，棱角分明简约精致。轮毂的设计也是非常时尚，固特异的轮胎也是超赞的一个品牌。关于车内宝沃汽车怎么样，宝沃BX7内饰只能说中性，要是有别克昂科威的内饰，那这个车就太完美了。车内采用了鹰翼式环体座舱布局，有一种以“我”为中心的设计感觉，提高了整体的驾驶感受。内饰感觉比同等价位的要合算，比其他的国产车好太多了，车载互联网也用得不错。手感好，材质好，精致感，高大上。新车基本没啥气味，这是最值得称赞的，为了自己的身体健康着想，宝沃真是厚道。空间中规中矩，一家五口出游不会拥挤。如果加座位变成7人座，估计后排就拥挤了。储物空间也比较丰富，车内储物空间很实用，能放不少东西。后备箱超级大，拓展能力也比较强。驾驶感受方面宝沃汽车怎么样，宝沃BX7中控的操控性也是很强的，配置方面很好用。方向盘很小很轻盈，适合女人开，我考虑加个套。感觉挺厚重，底盘离地高度可能刚从轿子换过来，到哪儿都如履平地。底盘和档位还有悬挂做得都不错，轻轻松松就可以玩得很嗨的车。刹车有力，路感清晰，动力充沛，一般的功能按钮操作简单，功能实用。很舒适，我觉得应该给好评，其余车，别的不说，就车门跟铁皮样，车座硬的一匹。内饰的材料用料很好，再加上他的底盘和悬架的原因，给人的舒适度不错。对一些颠簸路面地过滤非常到位，基本也感受不到多余的弹跳感。七座车，最后一排座椅如果搬上来，侧躺一个人完全没问题。

德国宝沃SUV BX7报价

复兴路漫漫5问德国老牌豪华品牌宝沃 “路漫漫其修远兮，吾将上下而求索”，这句出自屈原《离骚》第97句的诗词原意是指：路迷糊又窄小兮，我要仔细分辨清，而用它来形容刚刚复活的宝沃品牌显得再合适不过了。作为消失半个世纪的德国老牌豪华品牌，Borgward（宝沃）如今要面对的竞争对手与1961年大为不同，各大品牌争相划分细分市场，唯恐失去先机。面对竞争如此激励的汽车市场，宝沃选择采用全球销量增速最快的SUV车型卷土重来，显然是经过深思熟虑的。

而在刚刚开幕的2015法兰克福车展上，宝沃也正式发布了品牌回归后推出的首款量产车型BX7。据悉，这款新车未来还将引进国内，并实现国产。既然与国内市场息息相关，那么宝沃到底是什么鬼？今天一起来了解一番吧。何为Borgward（宝沃）？

德国BORGWARD（宝沃）汽车集团是由Carl Friedrich Wilhelm Borgward 于1929年在不莱梅创建。在当时，宝沃是一家颇有实力的汽车厂商，像空气悬架和3挡自动变速箱都是宝沃车型率先采用的，品牌影响力甚至可以与宝马、奔驰齐名。到了20世纪50年代后期宝沃品牌达到了最鼎盛期，年销量突破100万台，仅次于当时的大众和欧宝。不过好景不长，由于种种原因，宝沃于1961年宣布破产，从此宝沃品牌就此消失在公众视野中。

在消失半个多世纪后的2015年3月，德国BORGWARD（宝沃）汽车集团在2015日内瓦车展上正式宣布宝沃品牌回归，而品牌旗下的首款量产车型便是刚刚在法兰克福车展上全球首发的BX7。小众品牌与中国有何关系？

目前，德国宝沃Borgward汽车已经确定与北汽福田达成战略合作关系，宝沃回归的首款车型BX7的国产项目将会落户北京密云工厂，同时中国方面正以福田为班底进行大量的人员招聘，办公地点将设在北京的望京地区。除了中国市场外，宝沃也将在包括印度和巴西在内的新兴市场销售。宝沃汽车集团表示，宝沃品牌将使用“资源链合，德国制造，互联直通”的全新运作模式。该模式是在德国以及其他新兴市场建设工厂，并且通过互联网及其他渠道，实现全球同步销售。品牌定位依旧瞄准奔驰、宝马？宝沃品牌宣布回归后，其首要难题便是自身定位。即便破产前宝沃曾与奔驰、宝马齐名，在德国汽车行业地位显著。但此后的半个世纪这个品牌也没有任