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RFC1970 - Neighbor Discovery for IP Version 6 (IPv6)

王朝other·作者佚名  2008-05-31
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Network Working Group T. Narten

Request for Comments: 1970 IBM

Category: Standards Track E. Nordmark

Sun Microsystems

W. Simpson

Daydreamer

August 1996

Neighbor Discovery for IP Version 6 (IPv6)

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 Neighbor Discovery protocol for IP

Version 6. IPv6 nodes on the same link use Neighbor Discovery to

discover each other's presence, to determine each other's link-layer

addresses, to find routers and to maintain reachability information

about the paths to active neighbors.

Table of Contents

1. INTRODUCTION............................................. 3

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

2.1. General............................................. 4

2.2. Link Types.......................................... 7

2.3. Addresses........................................... 8

2.4. Requirements........................................ 9

3. PROTOCOL OVERVIEW........................................ 10

3.1. Comparison with IPv4................................ 14

3.2. Supported Link Types................................ 16

4. MESSAGE FORMATS.......................................... 17

4.1. Router Solicitation Message Format.................. 17

4.2. Router Advertisement Message Format................. 18

4.3. Neighbor Solicitation Message Format................ 21

4.4. Neighbor Advertisement Message Format............... 23

4.5. Redirect Message Format............................. 25

4.6. Option Formats...................................... 27

4.6.1. Source/Target Link-layer Address............... 28

4.6.2. Prefix Information............................. 29

4.6.3. Redirected Header.............................. 31

4.6.4. MTU............................................ 31

5. CONCEPTUAL MODEL OF A HOST............................... 32

5.1. Conceptual Data Structures.......................... 33

5.2. Conceptual Sending Algorithm........................ 35

5.3. Garbage Collection and Timeout Requirements......... 36

6. ROUTER AND PREFIX DISCOVERY.............................. 37

6.1. Message Validation.................................. 38

6.1.1. Validation of Router Solicitation Messages..... 38

6.1.2. Validation of Router Advertisement Messages.... 38

6.2. Router Specification................................ 39

6.2.1. Router Configuration Variables................. 39

6.2.2. Becoming An Advertising Interface.............. 43

6.2.3. Router Advertisement Message Content........... 43

6.2.4. Sending Unsolicited Router Advertisements...... 45

6.2.5. Ceasing To Be An Advertising Interface......... 45

6.2.6. Processing Router Solicitations................ 46

6.2.7. Router Advertisement Consistency............... 47

6.2.8. Link-local Address Change...................... 48

6.3. Host Specification.................................. 48

6.3.1. Host Configuration Variables................... 48

6.3.2. Host Variables................................. 48

6.3.3. Interface Initialization....................... 50

6.3.4. Processing Received Router Advertisements...... 50

6.3.5. Timing out Prefixes and Default Routers........ 52

6.3.6. Default Router Selection....................... 53

6.3.7. Sending Router Solicitations................... 54

7. ADDRESS RESOLUTION AND NEIGHBOR UNREACHABILITY DETECTION. 55

7.1. Message Validation.................................. 55

7.1.1. Validation of Neighbor Solicitations........... 55

7.1.2. Validation of Neighbor Advertisements.......... 56

7.2. Address Resolution.................................. 57

7.2.1. Interface Initialization....................... 57

7.2.2. Sending Neighbor Solicitations................. 57

7.2.3. Receipt of Neighbor Solicitations.............. 58

7.2.4. Sending Solicited Neighbor Advertisements...... 59

7.2.5. Receipt of Neighbor Advertisements............. 59

7.2.6. Sending Unsolicited Neighbor Advertisements.... 61

7.2.7. Anycast Neighbor Advertisements................ 62

7.2.8. Proxy Neighbor Advertisements.................. 62

7.3. Neighbor Unreachability Detection................... 63

7.3.1. Reachability Confirmation...................... 63

7.3.2. Neighbor Cache Entry States.................... 64

7.3.3. Node Behavior.................................. 66

8. REDIRECT FUNCTION........................................ 68

8.1. Validation of Redirect Messages..................... 68

8.2. Router Specification................................ 69

8.3. Host Specification.................................. 70

9. EXTENSIBILITY - OPTION PROCESSING........................ 71

10. PROTOCOL CONSTANTS...................................... 72

11. SECURITY CONSIDERATIONS................................. 73

REFERENCES................................................... 75

AUTHORS' ADDRESSES........................................... 76

APPENDIX A: MULTIHOMED HOSTS................................. 77

APPENDIX B: FUTURE EXTENSIONS................................ 78

APPENDIX C: STATE MACHINE FOR THE REACHABILITY STATE......... 78

APPENDIX D: IMPLEMENTATION ISSUES............................ 80

Appendix D.1: Reachability confirmations.................. 80

1. INTRODUCTION

This specification defines the Neighbor Discovery (ND) protocol for

Internet Protocol Version 6 (IPv6). Nodes (hosts and routers) use

Neighbor Discovery to determine the link-layer addresses for

neighbors known to reside on attached links and to quickly purge

cached values that become invalid. Hosts also use Neighbor Discovery

to find neighboring routers that are willing to forward packets on

their behalf. Finally, nodes use the protocol to actively keep track

of which neighbors are reachable and which are not, and to detect

changed link-layer addresses. When a router or the path to a router

fails, a host actively searches for functioning alternates.

Unless specified otherwise (in a document that covers operating IP

over a particular link type) this document applies to all link types.

However, because ND uses link-layer multicast for some of its

services, it is possible that on some link types (e.g., NBMA links)

alternative protocols or mechanisms to implement those services will

be specified (in the appropriate document covering the operation of

IP over a particular link type). The services described in this

document that are not directly dependent on multicast, such as

Redirects, Next-hop determination, Neighbor Unreachability Detection,

etc., are eXPected to be provided as specified in this document. The

details of how one uses ND on NBMA links is an area for further

study.

The authors would like to acknowledge the contributions the IPNGWG

working group and, in particular, (in alphabetical order) Ran

Atkinson, Jim Bound, Scott Bradner, Alex Conta, Stephen Deering,

Francis Dupont, Robert Elz, Robert Gilligan, Robert Hinden, Allison

Mankin, Dan McDonald, Charles Perkins, Matt Thomas, and Susan

Thomson.

2. TERMINOLOGY

2.1. General

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

are used only in contexts where necessary to avoid

ambiguity.

ICMP - Internet Message Control Protocol for the Internet

Protocol Version 6. The terms ICMPv4 and ICMPv6 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 as well as internet (or higher) layer

"tunnels", such as tunnels over IPv4 or IPv6 itself.

interface - a node's attachment to a link.

neighbors - nodes attached to the same link.

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

interfaces.

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].

Note that an anycast address is syntactically

indistinguishable from a unicast address. Thus, nodes

sending packets to anycast addresses don't generally

know that an anycast address is being used. Throughout

the rest of this document, references to unicast

addresses also apply to anycast addresses in those

cases where the node is unaware that a unicast address

is actually an anycast address.

prefix - a bit string that consists of some number of initial

bits of an address.

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.

on-link - an address that is assigned to an interface on a

specified link. A node considers an address to be on-

link if:

- it is covered by one of the link's prefixes, or

- a neighboring router specifies the address as the

target of a Redirect message, or

- a Neighbor Advertisement message is received for

the (target) address, or

- any Neighbor Discovery message is received from the

address.

off-link - the opposite of "on-link"; an address that is not

assigned to any interfaces on the specified link.

longest prefix match

- The process of determining which prefix (if any) in a

set of prefixes covers a target address. A target

address is covered by a prefix if all of the bits in

the prefix match the left-most bits of the target

address. When multiple prefixes cover an address, the

longest prefix is the one that matches.

reachability

- whether or not the one-way "forward" path to a neighbor

is functioning properly. In particular, whether

packets sent to a neighbor are reaching the IP layer on

the neighboring machine and are being processed

properly by the receiving IP layer. For neighboring

routers, reachability means that packets sent by a

node's IP layer are delivered to the router's IP layer,

and the router is indeed forwarding packets (i.e., it

is configured as a router, not a host). For hosts,

reachability means that packets sent by a node's IP

layer are delivered to the neighbor host's IP layer.

packet - an IP header plus payload.

link MTU - the maximum transmission unit, i.e., maximum packet

size in octets, that can be conveyed in one piece over

a link.

target - an address about which address resolution information

is sought, or an address which is the new first-hop

when being redirected.

proxy - a router that responds to Neighbor Discovery query

messages on behalf of another node. A router acting on

behalf of a mobile node that has moved off-link could

potentially act as a proxy for the mobile node.

ICMP destination unreachable indication

- an error indication returned to the original sender of

a packet that cannot be delivered for the reasons

outlined in [ICMPv6]. If the error occurs on a node

other than the node originating the packet, an ICMP

error message is generated. If the error occurs on the

originating node, an implementation is not required to

actually create and send an ICMP error packet to the

source, as long as the upper-layer sender is notified

through an appropriate mechanism (e.g., return value

from a procedure call). Note, however, that an

implementation may find it convenient in some cases to

return errors to the sender by taking the offending

packet, generating an ICMP error message, and then

delivering it (locally) through the generic error

handling routines.

random delay

- when sending out messages, it is sometimes necessary to

delay a transmission for a random amount of time in

order to prevent multiple nodes from transmitting at

exactly the same time, or to prevent long-range

periodic transmissions from synchronizing with each

other [SYNC]. When a random component is required, a

node calculates the actual delay in such a way that the

computed delay forms a uniformly-distributed random

value that falls between the specified minimum and

maximum delay times. The implementor must take care to

insure that the granularity of the calculated random

component and the resolution of the timer used are both

high enough to insure that the probability of multiple

nodes delaying the same amount of time is small.

random delay seed

- If a pseudo-random number generator is used in

calculating a random delay component, the generator

should be initialized with a unique seed prior to being

used. Note that it is not sufficient to use the

interface token alone as the seed, since interface

tokens will not always be unique. To reduce the

probability that duplicate interface tokens cause the

same seed to be used, the seed should be calculated

from a variety of input sources (e.g., machine

components) that are likely to be different even on

identical "boxes". For example, the seed could be

formed by combining the CPU's serial number with an

interface token.

2.2. Link Types

Different link layers have different properties. The ones of concern

to Neighbor Discovery are:

multicast - a link that supports a native mechanism at the link

layer for sending packets to all (i.e., broadcast)

or a subset of all neighbors.

point-to-point - a link that connects exactly two interfaces. A

point-to-point link is assumed to have multicast

capability and have a link-local address.

non-broadcast multi-Access (NBMA)

- a link to which more than two interfaces can attach,

but that does not support a native form of multicast

or broadcast (e.g., X.25, ATM, frame relay, etc.).

Note that all link types (including NBMA) are

expected to provide multicast service for IP (e.g.,

using multicast servers), but it is an issue for

further study whether ND should use such facilities

or an alternate mechanism that provides the

equivalent ND services.

shared media - a link that allows direct communication among a

number of nodes, but attached nodes are configured

in such a way that they do not have complete prefix

information for all on-link destinations. That is,

at the IP level, nodes on the same link may not know

that they are neighbors; by default, they

communicate through a router. Examples are large

(switched) public data networks such as SMDS and B-

ISDN. Also known as "large clouds". See [SH-

MEDIA].

variable MTU - a link that does not have a well-defined MTU (e.g.,

IEEE 802.5 token rings). Many links (e.g.,

Ethernet) have a standard MTU defined by the link-

layer protocol or by the specific document

describing how to run IP over the link layer.

asymmetric reachability

- a link where non-reflexive and/or non-transitive

reachability is part of normal operation. (Non-

reflexive reachability means packets from A reach B

but packets from B don't reach A. Non-transitive

reachability means packets from A reach B, and

packets from B reach C, but packets from A don't

reach C.) Many radio links exhibit these

properties.

2.3. Addresses

Neighbor Discovery makes use of a number of different addresses

defined in [ADDR-ARCH], including:

all-nodes multicast address

- the link-local scope address to reach all nodes.

FF02::1

all-routers multicast address

- the link-local scope address to reach all routers.

FF02::2

solicited-node multicast address

- a link-local scope multicast address that is computed

as a function of the solicited target's address. The

solicited-node multicast address is formed by taking

the low-order 32 bits of the target IP address and

appending those bits to the 96-bit prefix

FF02:0:0:0:0:1 to produce a multicast address within

the range FF02::1:0:0 to FF02::1:FFFF:FFFF. For

example, the solicited node multicast address

corresponding to the IP address 4037::01:800:200E:8C6C

is FF02::1:200E:8C6C. IP addresses that differ only in

the high-order bits, e.g., due to multiple high-order

prefixes associated with different providers, will map

to the same solicited-node address thereby reducing the

number of multicast addresses a node must join.

link-local address

- a unicast address having link-only scope that can be

used to reach neighbors. All interfaces on routers

MUST have a link-local address. Also, [ADDRCONF]

requires that interfaces on hosts have a link-local

address.

unspecified address

- a reserved address value that indicates the lack of an

address (e.g., the address is unknown). It is never

used as a destination address, but may be used as a

source address if the sender does not (yet) know its

own address (e.g., while verifying an address is unused

during address autoconfiguration [ADDRCONF]). The

unspecified address has a value of 0:0:0:0:0:0:0:0.

2.4. 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 weighted 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.

This document also makes use of internal conceptual variables to

describe protocol behavior and external variables that an

implementation must allow system administrators to change. The

specific variable names, how their values change, and how their

settings influence protocol behavior are provided to demonstrate

protocol behavior. An implementation is not required to have them in

the exact form described here, so long as its external behavior is

consistent with that described in this document.

3. PROTOCOL OVERVIEW

This protocol solves a set of problems related to the interaction

between nodes attached to the same link. It defines mechanisms for

solving each of the following problems:

Router Discovery: How hosts locate routers that reside on an

attached link.

Prefix Discovery: How hosts discover the set of address prefixes

that define which destinations are on-link for an

attached link. (Nodes use prefixes to distinguish

destinations that reside on-link from those only

reachable through a router.)

Parameter Discovery: How a node learns such link parameters as the

link MTU or such Internet parameters as the hop limit

value to place in outgoing packets.

Address Autoconfiguration: How nodes automatically configure an

address for an interface.

Address resolution: How nodes determine the link-layer address of an

on-link destination (e.g., a neighbor) given only the

destination's IP address.

Next-hop determination: The algorithm for mapping an IP destination

address into the IP address of the neighbor to which

traffic for the destination should be sent. The next-hop

can be a router or the destination itself.

Neighbor Unreachability Detection: How nodes determine that a

neighbor is no longer reachable. For neighbors used as

routers, alternate default routers can be tried. For

both routers and hosts, address resolution can be

performed again.

Duplicate Address Detection: How a node determines that an address

it wishes to use is not already in use by another node.

Redirect: How a router informs a host of a better first-hop node to

reach a particular destination.

Neighbor Discovery defines five different ICMP packet types: A pair

of Router Solicitation and Router Advertisement messages, a pair of

Neighbor Solicitation and Neighbor Advertisements messages, and a

Redirect message. The messages serve the following purpose:

Router Solicitation: When an interface becomes enabled, hosts may

send out Router Solicitations that request routers to

generate Router Advertisements immediately rather than at

their next scheduled time.

Router Advertisement: Routers advertise their presence together with

various link and Internet parameters either periodically,

or in response to a Router Solicitation message. Router

Advertisements contain prefixes that are used for on-link

determination and/or address configuration, a suggested

hop limit value, etc.

Neighbor Solicitation: Sent by a node to determine the link-layer

address of a neighbor, or to verify that a neighbor is

still reachable via a cached link-layer address.

Neighbor Solicitations are also used for Duplicate

Address Detection.

Neighbor Advertisement: A response to a Neighbor Solicitation

message. A node may also send unsolicited Neighbor

Advertisements to announce a link-layer address change.

Redirect: Used by routers to inform hosts of a better first hop for

a destination.

On multicast-capable links, each router periodically multicasts a

Router Advertisement packet announcing its availability. A host

receives Router Advertisements from all routers, building a list of

default routers. Routers generate Router Advertisements frequently

enough that hosts will learn of their presence within a few minutes,

but not frequently enough to rely on an absence of advertisements to

detect router failure; a separate Neighbor Unreachability Detection

algorithm provides failure detection.

Router Advertisements contain a list of prefixes used for on-link

determination and/or autonomous address configuration; flags

associated with the prefixes specify the intended uses of a

particular prefix. Hosts use the advertised on-link prefixes to

build and maintain a list that is used in deciding when a packet's

destination is on-link or beyond a router. Note that a destination

can be on-link even though it is not covered by any advertised on-

link prefix. In such cases a router can send a Redirect informing

the sender that the destination is a neighbor.

Router Advertisements (and per-prefix flags) allow routers to inform

hosts how to perform Address Autoconfiguration. For example, routers

can specify whether hosts should use stateful (DHCPv6) and/or

autonomous (stateless) address configuration. The exact semantics

and usage of the address configuration-related information is

specified in [ADDRCONF].

Router Advertisement messages also contain Internet parameters such

as the hop limit that hosts should use in outgoing packets and,

optionally, link parameters such as the link MTU. This facilitates

centralized administration of critical parameters that can be set on

routers and automatically propagated to all attached hosts.

Nodes accomplish address resolution by multicasting a Neighbor

Solicitation that asks the target node to return its link-layer

address. Neighbor Solicitation messages are multicast to the

solicited-node multicast address of the target address. The target

returns its link-layer address in a unicast Neighbor Advertisement

message. A single request-response pair of packets is sufficient for

both the initiator and the target to resolve each other's link-layer

addresses; the initiator includes its link-layer address in the

Neighbor Solicitation.

Neighbor Solicitation messages can also be used to determine if more

than one node has been assigned the same unicast address. The use of

Neighbor Solicitation messages for Duplicate Address Detection is

specified in [ADDRCONF].

Neighbor Unreachability Detection detects the failure of a neighbor

or the failure of the forward path to the neighbor. Doing so

requires positive confirmation that packets sent to a neighbor are

actually reaching that neighbor and being processed properly by its

IP layer. Neighbor Unreachability Detection uses confirmation from

two sources. When possible, upper-layer protocols provide a positive

confirmation that a connection is making "forward progress", that is,

previously sent data is known to have been delivered correctly (e.g.,

new acknowledgments were received recently). When positive

confirmation is not forthcoming through such "hints", a node sends

unicast Neighbor Solicitation messages that solicit Neighbor

Advertisements as reachability confirmation from the next hop. To

reduce unnecessary network traffic, probe messages are only sent to

neighbors to which the node is actively sending packets.

In addition to addressing the above general problems, Neighbor

Discovery also handles the following situations:

Link-layer address change - A node that knows its link-layer

address has changed can multicast a few (unsolicited) Neighbor

Advertisement packets to all nodes to quickly update cached

link-layer addresses that have become invalid. Note that the

sending of unsolicited advertisements is a performance

enhancement only (e.g., unreliable). The Neighbor

Unreachability Detection algorithm ensures that all nodes will

reliably discover the new address, though the delay may be

somewhat longer.

Inbound load balancing - Nodes with replicated interfaces may want

to load balance the reception of incoming packets across

multiple network interfaces on the same link. Such nodes have

multiple link-layer addresses assigned to the same interface.

For example, a single network driver could represent multiple

network interface cards as a single logical interface having

multiple link-layer addresses. Load balancing is handled by

allowing routers to omit the source link-layer address from

Router Advertisement packets, thereby forcing neighbors to use

Neighbor Solicitation messages to learn link-layer addresses

of routers. Returned Neighbor Advertisement messages can then

contain link-layer addresses that differ depending on who

issued the solicitation.

Anycast addresses - Anycast addresses identify one of a set of

nodes providing an equivalent service, and multiple nodes on

the same link may be configured to recognize the same Anycast

address. Neighbor Discovery handles anycasts by having nodes

expect to receive multiple Neighbor Advertisements for the

same target. All advertisements for anycast addresses are

tagged as being non-Override advertisements. This invokes

specific rules to determine which of potentially multiple

advertisements should be used.

Proxy advertisements - A router willing to accept packets on behalf

of a target address that is unable to respond to Neighbor

Solicitations can issue non-Override Neighbor Advertisements.

There is currently no specified use of proxy, but proxy

advertising could potentially be used to handle cases like

mobile nodes that have moved off-link. However, it is not

intended as a general mechanism to handle nodes that, e.g., do

not implement this protocol.

3.1. Comparison with IPv4

The IPv6 Neighbor Discovery protocol corresponds to a combination of

the IPv4 protocols ARP [ARP], ICMP Router Discovery [RDISC], and ICMP

Redirect [ICMPv4]. In IPv4 there is no generally agreed upon

protocol or mechanism for Neighbor Unreachability Detection, although

Hosts Requirements [HR-CL] does specify some possible algorithms for

Dead Gateway Detection (a subset of the problems Neighbor

Unreachability Detection tackles).

The Neighbor Discovery protocol provides a multitude of improvements

over the IPv4 set of protocols:

Router Discovery is part of the base protocol set; there is no need

for hosts to "snoop" the routing protocols.

Router advertisements carry link-layer addresses; no additional

packet exchange is needed to resolve the router's link-layer

address.

Router advertisements carry prefixes for a link; there is no need

to have a separate mechanism to configure the "netmask".

Router advertisements enable Address Autoconfiguration.

Routers can advertise an MTU for hosts to use on the link, ensuring

that all nodes use the same MTU value on links lacking a well-

defined MTU.

Address resolution multicasts are "spread" over 4 billion (2^32)

multicast addresses greatly reducing address resolution related

interrupts on nodes other than the target. Moreover, non-IPv6

machines should not be interrupted at all.

Redirects contain the link-layer address of the new first hop;

separate address resolution is not needed upon receiving a

redirect.

Multiple prefixes can be associated with the same link. By

default, hosts learn all on-link prefixes from Router

Advertisements. However, routers may be configured to omit some or

all prefixes from Router Advertisements. In such cases hosts

assume that destinations are off-link and send traffic to routers.

A router can then issue redirects as appropriate.

Unlike IPv4, the recipient of an IPv6 redirect assumes that the new

next-hop is on-link. In IPv4, a host ignores redirects specifying

a next-hop that is not on-link according to the link's network

mask. The IPv6 redirect mechanism is analogous to the XRedirect

facility specified in [SH-MEDIA]. It is expected to be useful on

non-broadcast and shared media links in which it is undesirable or

not possible for nodes to know all prefixes for on-link

destinations.

Neighbor Unreachability Detection is part of the base significantly

improving the robustness of packet delivery in the presence of

failing routers, partially failing or partitioned links and nodes

that change their link-layer addresses. For instance, mobile nodes

can move off-link without losing any connectivity due to stale ARP

caches.

Unlike ARP, Neighbor Discovery detects half-link failures (using

Neighbor Unreachability Detection) and avoids sending traffic to

neighbors with which two-way connectivity is absent.

Unlike in IPv4 Router Discovery the Router Advertisement messages

do not contain a preference field. The preference field is not

needed to handle routers of different "stability"; the Neighbor

Unreachability Detection will detect dead routers and switch to a

working one.

The use of link-local addresses to uniquely identify routers (for

Router Advertisement and Redirect messages) makes it possible for

hosts to maintain the router associations in the event of the site

renumbering to use new global prefixes.

Using the Hop Limit equal to 255 trick Neighbor Discovery is immune

to off-link senders that accidentally or intentionally send ND

messages. In IPv4 off-link senders can send both ICMP Redirects

and Router Advertisement messages.

Placing address resolution at the ICMP layer makes the protocol

more media-independent than ARP and makes it possible to use

standard IP authentication and security mechanisms as appropriate

[IPv6-AUTH, IPv6-ESP].

3.2. Supported Link Types

Neighbor Discovery supports links with different properties. In the

presence of certain properties only a subset of the ND protocol

mechanisms are fully specified in this document:

point-to-point - Neighbor Discovery handles such links just like

multicast links. (Multicast can be trivially

provided on point to point links, and interfaces can

be assigned link-local addresses.) Neighbor

Discovery should be implemented as described in this

document.

multicast - Neighbor Discovery should be implemented as

described in this document.

non-broadcast multiple access (NBMA)

- Redirect, Neighbor Unreachability Detection and

next-hop determination should be implemented as

described in this document. Address resolution, and

the mechanism for delivering Router Solicitations

and Advertisements on NBMA links is not specified in

this document. Note that if hosts support manual

configuration of a list of default routers, hosts

can dynamically acquire the link-layer addresses for

their neighbors from Redirect messages.

shared media - The Redirect message is modeled after the XRedirect

message in [SH-MEDIA] in order to simplify use of

the protocol on shared media links.

This specification does not address shared media

issues that only relate to routers, such as:

- How routers exchange reachability information on

a shared media link.

- How a router determines the link-layer address of

a host, which it needs to send redirect messages

to the host.

- How a router determines that it is the first-hop

router for a received packet.

The protocol is extensible (through the definition

of new options) so that other solutions might be

possible in the future.

variable MTU - Neighbor Discovery allows routers to specify a MTU

for the link, which all nodes then use. All nodes

on a link must use the same MTU (or Maximum Receive

Unit) in order for multicast to work properly.

Otherwise when multicasting a sender, which can not

know which nodes will receive the packet, could not

determine a minimum packet size all receivers can

process.

asymmetric reachability

- Neighbor Discovery detects the absence of symmetric

reachability; a node avoids paths to a neighbor with

which it does not have symmetric connectivity.

The Neighbor Unreachability Detection will typically

identify such half-links and the node will refrain

from using them.

The protocol can presumably be extended in the

future to find viable paths in environments that

lack reflexive and transitive connectivity.

4. MESSAGE FORMATS

4.1. Router Solicitation Message Format

Hosts send Router Solicitations in order to prompt routers to

generate Router Advertisements quickly.

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

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

Type Code Checksum

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

Reserved

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

Options ...

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

IP Fields:

Source Address

An IP address assigned to the sending interface, or

the unspecified address if no address is assigned to

the sending interface.

Destination Address

Typically the all-routers multicast address.

Hop Limit 255

Priority 15

Authentication Header

If a Security Association for the IP Authentication

Header exists between the sender and the destination

address, then the sender SHOULD include this header.

ICMP Fields:

Type 133

Code 0

Checksum The ICMP checksum. See [ICMPv6].

Reserved This field is unused. It MUST be initialized to zero

by the sender and MUST be ignored by the receiver.

Valid Options:

Source link-layer address

The link-layer address of the sender, if known.

Future versions of this protocol may define new option types.

Receivers MUST silently ignore any options they do not recognize and

continue processing the message.

4.2. Router Advertisement Message Format

Routers send out Router Advertisement message periodically, or in

response to a Router Solicitation.

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

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

Type Code Checksum

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

Cur Hop Limit MO Reserved Router Lifetime

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

Reachable Time

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

Retrans Timer

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

Options ...

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

IP Fields:

Source Address

MUST be the link-local address assigned to the

interface from which this message is sent.

Destination Address

Typically the Source Address of an invoking Router

Solicitation or the all-nodes multicast address.

Hop Limit 255

Priority 15

Authentication Header

If a Security Association for the IP Authentication

Header exists between the sender and the destination

address, then the sender SHOULD include this header.

ICMP Fields:

Type 134

Code 0

Checksum The ICMP checksum. See [ICMPv6].

Cur Hop Limit 8-bit unsigned integer. The default value that should

be placed in the Hop Count field of the IP header for

outgoing IP packets. A value of zero means

unspecified (by this router).

M 1-bit "Managed address configuration" flag. When set,

hosts use the administered (stateful) protocol for

address autoconfiguration in addition to any addresses

autoconfigured using stateless address

autoconfiguration. The use of this flag is described

in [ADDRCONF].

O 1-bit "Other stateful configuration" flag. When set,

hosts use the administered (stateful) protocol for

autoconfiguration of other (non-address) information.

The use of this flag is described in [ADDRCONF].

Reserved A 6-bit unused field. It MUST be initialized to zero

by the sender and MUST be ignored by the receiver.

Router Lifetime

16-bit unsigned integer. The lifetime associated with

the default router in units of seconds. The maximum

value corresponds to 18.2 hours. A Lifetime of 0

indicates that the router is not a default router and

SHOULD NOT appear on the default router list. The

Router Lifetime applies only to the router's

usefulness as a default router; it does not apply to

information contained in other message fields or

options. Options that need time limits for their

information include their own lifetime fields.

Reachable Time 32-bit unsigned integer. The time, in milliseconds,

that a node assumes a neighbor is reachable after

having received a reachability confirmation. Used by

the Neighbor Unreachability Detection algorithm (see

Section 7.3). A value of zero means unspecified (by

this router).

Retrans Timer 32-bit unsigned integer. The time, in milliseconds,

between retransmitted Neighbor Solicitation messages.

Used by address resolution and the Neighbor

Unreachability Detection algorithm (see Sections 7.2

and 7.3). A value of zero means unspecified (by this

router).

Possible options:

Source link-layer address

The link-layer address of the interface from which the

Router Advertisement is sent. Only used on link

layers that have addresses. A router MAY omit this

option in order to enable inbound load sharing across

multiple link-layer addresses.

MTU SHOULD be sent on links that have a variable MTU (as

specified in the document that describes how to run IP

over the particular link type). MAY be sent on other

links.

Prefix Information

These options specify the prefixes that are on-link

and/or are used for address autoconfiguration. A

router SHOULD include all its on-link prefixes (except

the link-local prefix) so that multihomed hosts have

complete prefix information about on-link destinations

for the links to which they attach. If complete

information is lacking, a multihomed host may not be

able to chose the correct outgoing interface when

sending traffic to its neighbors.

Future versions of this protocol may define new option types.

Receivers MUST silently ignore any options they do not recognize and

continue processing the message.

4.3. Neighbor Solicitation Message Format

Nodes send Neighbor Solicitations to request the link-layer address

of a target node while also providing their own link-layer address to

the target. Neighbor Solicitations are multicast when the node needs

to resolve an address and unicast when the node seeks to verify the

reachability of a neighbor.

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

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

Type Code Checksum

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

Reserved

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

+ +

+ Target Address +

+ +

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

Options ...

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

IP Fields:

Source Address

Either an address assigned to the interface from which

this message is sent or (if Duplicate Address

Detection is in progress [ADDRCONF]) the unspecified

address.

Destination Address

Either the solicited-node multicast address

corresponding to the target address, or the target

address.

Hop Limit 255

Priority 15

Authentication Header

If a Security Association for the IP Authentication

Header exists between the sender and the destination

address, then the sender SHOULD include this header.

ICMP Fields:

Type 135

Code 0

Checksum The ICMP checksum. See [ICMPv6].

Reserved This field is unused. It MUST be initialized to zero

by the sender and MUST be ignored by the receiver.

Target Address

The IP address of the target of the solicitation. It

MUST NOT be a multicast address.

Possible options:

Source link-layer address

The link-layer address for the sender. On link layers

that have addresses this option MUST be included in

multicast solicitations and SHOULD be included in

unicast solicitations.

Future versions of this protocol may define new option types.

Receivers MUST silently ignore any options they do not recognize and

continue processing the message.

4.4. Neighbor Advertisement Message Format

A node sends Neighbor Advertisements in response to Neighbor

Solicitations and sends unsolicited Neighbor Advertisements in order

to (unreliably) propagate new information quickly.

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

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

Type Code Checksum

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

RSO Reserved

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

+ +

+ Target Address +

+ +

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

Options ...

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

IP Fields:

Source Address

An address assigned to the interface from which the

advertisement is sent.

Destination Address

For solicited advertisements, the Source Address of an

invoking Neighbor Solicitation or, if the

solicitation's Source Address is the unspecified

address, the all-nodes multicast address.

For unsolicited advertisements typically the all-nodes

multicast address.

Hop Limit 255

Priority 15

Authentication Header

If a Security Association for the IP Authentication

Header exists between the sender and the destination

address, then the sender SHOULD include this header.

ICMP Fields:

Type 136

Code 0

Checksum The ICMP checksum. See [ICMPv6].

R Router flag. When set, the R-bit indicates that the

sender is a router. The R-bit is used by Neighbor

Unreachability Detection to detect a router that

changes to a host.

S Solicited flag. When set, the S-bit indicates that

the advertisement was sent in response to a Neighbor

Solicitation from the Destination address. The S-bit

is used as a reachability confirmation for Neighbor

Unreachability Detection. It MUST NOT be set in

multicast advertisements or in unsolicited unicast

advertisements.

O Override flag. When set, the O-bit indicates that the

advertisement should override an existing cache entry

and update the cached link-layer address. When it is

not set the advertisement will not update a cached

link-layer address though it will update an existing

Neighbor Cache entry for which no link-layer address

is known. It SHOULD NOT be set in solicited

advertisements for anycast addresses and in solicited

proxy advertisements. It SHOULD be set in other

solicited advertisements and in unsolicited

advertisements.

Reserved 29-bit unused field. It MUST be initialized to zero

by the sender and MUST be ignored by the receiver.

Target Address

For solicited advertisements, the Target Address field

in the Neighbor Solicitation message that prompted

this advertisement. For an unsolicited advertisement,

the address whose link-layer address has changed. The

Target Address MUST NOT be a multicast address.

Possible options:

Target link-layer address

The link-layer address for the target, i.e., the

sender of the advertisement. MUST be included on link

layers that have addresses.

Future versions of this protocol may define new option types.

Receivers MUST silently ignore any options they do not recognize and

continue processing the message.

4.5. Redirect Message Format

Routers send Redirect packets to inform a host of a better first-hop

node on the path to a destination. Hosts can be redirected to a

better first-hop router but can also be informed by a redirect that

the destination is in fact a neighbor. The latter is accomplished by

setting the ICMP Target Address equal to the ICMP Destination

Address.

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

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

Type Code Checksum

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

Reserved

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

+ +

+ Target Address +

+ +

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

+ +

+ Destination Address +

+ +

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

Options ...

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

IP Fields:

Source Address

MUST be the link-local address assigned to the

interface from which this message is sent.

Destination Address

The Source Address of the packet that triggered the

redirect.

Hop Limit 255

Priority 15

Authentication Header

If a Security Association for the IP Authentication

Header exists between the sender and the destination

address, then the sender SHOULD include this header.

ICMP Fields:

Type 137

Code 0

Checksum The ICMP checksum. See [ICMPv6].

Reserved This field is unused. It MUST be initialized to zero

by the sender and MUST be ignored by the receiver.

Target Address An IP address that is a better first hop to use for

the ICMP Destination Address. When the target is the

actual endpoint of communication, i.e., the

destination is a neighbor, the Target Address field

MUST contain the same value as the ICMP Destination

Address field. Otherwise the target is a better

first-hop router and the Target Address MUST be the

router's link-local address so that hosts can uniquely

identify routers.

Destination Address

The IP address of the destination which is redirected

to the target.

Possible options:

Target link-layer address

The link-layer address for the target. It SHOULD be

included (if known). Note that on NBMA links, hosts

may rely on the presence of the Target Link-Layer

Address option in Redirect messages as the means for

determining the link-layer addresses of neighbors. In

such cases, the option MUST be included in Redirect

messages.

Redirected Header

As much as possible of the IP packet that triggered

the sending of the Redirect without making the

redirect packet exceed 576 octets.

4.6. Option Formats

Neighbor Discovery messages include zero or more options, some of

which may appear multiple times in the same message. All options are

of the form:

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

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

Type Length ...

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

~ ... ~

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

Fields:

Type 8-bit identifier of the type of option. The options

defined in this document are:

Option Name Type

Source Link-Layer Address 1

Target Link-Layer Address 2

Prefix Information 3

Redirected Header 4

MTU 5

Length 8-bit unsigned integer. The length of the option in

units of 8 octets. The value 0 is invalid. Nodes

MUST silently discard an ND packet that contains an

option with length zero.

4.6.1. Source/Target Link-layer Address

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

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

Type Length Link-Layer Address ...

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

Fields:

Type

1 for Source Link-layer Address

2 for Target Link-layer Address

Length The length of the option in units of 8 octets. For

example, the length for IEEE 802 addresses is 1

[IPv6-ETHER].

Link-Layer Address

The variable length link-layer address.

The content and format of this field (including byte

and bit ordering) is expected to be specified in

specific documents that describe how IPv6 operates

over different link layers. For instance, [IPv6-

ETHER].

Description

The Source Link-Layer Address option contains the

link-layer address of the sender of the packet. It is

used in the Neighbor Solicitation, Router

Solicitation, and Router Advertisement packets.

The Target Link-Layer Address option contains the

link-layer address of the target. It is used in

Neighbor Advertisement and Redirect packets.

These options MUST be silently ignored for other

Neighbor Discovery messages.

4.6.2. Prefix Information

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

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

Type Length Prefix Length LA Reserved1

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

Valid Lifetime

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

Preferred Lifetime

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

Reserved2

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

+ +

+ Prefix +

+ +

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

Fields:

Type 3

Length 4

Prefix Length 8-bit unsigned integer. The number of leading bits in

the Prefix that are valid. The value ranges from 0 to

128.

L 1-bit on-link flag. When set, indicates that this

prefix can be used for on-link determination. When

not set the advertisement makes no statement about

on-link or off-link properties of the prefix. For

instance, the prefix might be used for address

configuration with some of the addresses belonging to

the prefix being on-link and others being off-link.

A 1-bit autonomous address-configuration flag. When set

indicates that this prefix can be used for autonomous

address configuration as specified in [ADDRCONF].

Reserved1 6-bit unused field. It MUST be initialized to zero by

the sender and MUST be ignored by the receiver.

Valid Lifetime

32-bit unsigned integer. The length of time in

seconds (relative to the time the packet is sent) that

the prefix is valid for the purpose of on-link

determination. A value of all one bits (0xffffffff)

represents infinity. The Valid Lifetime is also used

by [ADDRCONF].

Preferred Lifetime

32-bit unsigned integer. The length of time in

seconds (relative to the time the packet is sent) that

addresses generated from the prefix via stateless

address autoconfiguration remain preferred [ADDRCONF].

A value of all one bits (0xffffffff) represents

infinity. See [ADDRCONF].

Reserved2 This field is unused. It MUST be initialized to zero

by the sender and MUST be ignored by the receiver.

Prefix An IP address or a prefix of an IP address. The

Prefix Length field contains the number of valid

leading bits in the prefix. The bits in the prefix

after the prefix length are reserved and MUST be

initialized to zero by the sender and ignored by the

receiver. A router SHOULD NOT send a prefix option

for the link-local prefix and a host SHOULD ignore

such a prefix option.

Description

The Prefix Information option provide hosts with on-

link prefixes and prefixes for Address

Autoconfiguration.

The Prefix Information option appears in Router

Advertisement packets and MUST be silently ignored for

other messages.

4.6.3. Redirected Header

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

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

Type Length Reserved

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

Reserved

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

~ IP header + data ~

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

Fields:

Type 4

Length The length of the option in units of 8 octets.

Reserved These fields are unused. They MUST be initialized to

zero by the sender and MUST be ignored by the

receiver.

IP header + data

The original packet truncated to ensure that the size

of the redirect message does not exceed 576 octets.

Description

The Redirected Header option is used in Redirect

messages and contains all or part of the packet that

is being redirected.

This option MUST be silently ignored for other

Neighbor Discovery messages.

4.6.4. MTU

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

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

Type Length Reserved

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

MTU

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

Fields:

Type 5

Length 1

Reserved This field is unused. It MUST be initialized to zero

by the sender and MUST be ignored by the receiver.

MTU 32-bit unsigned integer. The recommended MTU for the

link.

Description

The MTU option is used in Router Advertisement

messages to insure that all nodes on a link use the

same MTU value in those cases where the link MTU is

not well known.

This option MUST be silently ignored for other

Neighbor Discovery messages.

In configurations in which heterogeneous technologies

are bridged together, the maximum supported MTU may

differ from one segment to another. If the bridges do

not generate ICMP Packet Too Big messages,

communicating nodes will be unable to use Path MTU to

dynamically determine the appropriate MTU on a per-

neighbor basis. In such cases, routers use the MTU

option to specify an MTU value supported by all

segments.

5. CONCEPTUAL MODEL OF A HOST

This section describes a conceptual model of one possible data

structure organization that hosts (and to some extent routers) will

maintain in interacting with neighboring nodes. The described

organization is provided to facilitate the explanation of how the

Neighbor Discovery protocol should behave. This document does not

mandate that implementations adhere to this model as long as their

external behavior is consistent with that described in this document.

This model is only concerned with the ASPects of host behavior

directly related to Neighbor Discovery. In particular, it does not

concern itself with such issues as source address selection or the

selecting of an outgoing interface on a multihomed host.

5.1. Conceptual Data Structures

Hosts will need to maintain the following pieces of information for

each interface:

Neighbor Cache

- A set of entries about individual neighbors to which

traffic has been sent recently. Entries are keyed on

the neighbor's on-link unicast IP address and contain

such information as its link-layer address, a flag

indicating whether the neighbor is a router or a host

(called IsRouter in this document), a pointer to any

queued packets waiting for address resolution to

complete, etc.

A Neighbor Cache entry also contains information used

by the Neighbor Unreachability Detection algorithm,

including the reachability state, the number of

unanswered probes, and the time the next Neighbor

Unreachability Detection event is scheduled to take

place.

Destination Cache

- A set of entries about destinations to which traffic

has been sent recently. The Destination Cache

includes both on-link and off-link destinations and

provides a level of indirection into the Neighbor

Cache; the Destination Cache maps a destination IP

address to the IP address of the next-hop neighbor.

This cache is updated with information learned from

Redirect messages. Implementations may find it

convenient to store additional information not

directly related to Neighbor Discovery in Destination

Cache entries, such as the Path MTU (PMTU) and round

trip timers maintained by transport protocols.

Prefix List - A list of the prefixes that define a set of addresses

that are on-link. Prefix List entries are created

from information received in Router Advertisements.

Each entry has an associated invalidation timer value

(extracted from the advertisement) used to expire

prefixes when they become invalid. A special

"infinity" timer value specifies that a prefix remains

valid forever, unless a new (finite) value is received

in a subsequent advertisement.

The link-local prefix is considered to be on the

prefix list with an infinite invalidation timer

regardless of whether routers are advertising a prefix

for it. Received Router Advertisements SHOULD NOT

modify the invalidation timer for the link-local

prefix.

Default Router List

- A list of routers to which packets may be sent.

Router list entries point to entries in the Neighbor

Cache; the algorithm for selecting a default router

favors routers known to be reachable over those whose

reachability is suspect. Each entry also has an

associated invalidation timer value (extracted from

Router Advertisements) used to delete entries that are

no longer advertised.

Note that the above conceptual data structures can be implemented

using a variety of techniques. One possible implementation is to use

a single longest-match routing table for all of the above data

structures. Regardless of the specific implementation, it is

critical that the Neighbor Cache entry for a router is shared by all

Destination Cache entries using that router in order to prevent

redundant Neighbor Unreachability Detection probes.

Note also that other protocols (e.g. IPv6 Mobility) might add

additional conceptual data structures. An implementation is at

liberty to implement such data structures in any way it pleases. For

example, an implementation could merge all conceptual data structures

into a single routing table.

The Neighbor Cache contains information maintained by the Neighbor

Unreachability Detection algorithm. A key piece of information is a

neighbor's reachability state, which is one of five possible values.

The following definitions are informal; precise definitions can be

found in Section 7.3.2.

INCOMPLETE Address resolution is in progress and the link-layer

address of the neighbor has not yet been determined.

REACHABLE Roughly speaking, the neighbor is known to have been

reachable recently (within tens of seconds ago).

STALE The neighbor is no longer known to be reachable but until

traffic is sent to the neighbor, no attempt should be

made to verify its reachability.

DELAY The neighbor is no longer known to be reachable, and

traffic has recently be sent to the neighbor. Rather

than probe the neighbor immediately, however, delay

sending probes for a short while in order to give upper

layer protocols a chance to provide reachability

confirmation.

PROBE The neighbor is no longer known to be reachable, and

unicast Neighbor Solicitation probes are being sent to

verify reachability.

5.2. Conceptual Sending Algorithm

When sending a packet to a destination, a node uses a combination of

the Destination Cache, the Prefix List, and the Default Router List

to determine the IP address of the appropriate next hop, an operation

known as "next-hop determination". Once the IP address of the next

hop is known, the Neighbor Cache is consulted for link-layer

information about that neighbor.

Next-hop determination for a given unicast destination operates as

follows. The sender performs a longest prefix match against the

Prefix List to determine whether the packet's destination is on- or

off-link. If the destination is on-link, the next-hop address is the

same as the packet's destination address. Otherwise, the sender

selects a router from the Default Router List (following the rules

described in Section 6.3.6). If the Default Router List is empty,

the sender assumes that the destination is on-link.

For efficiency reasons, next-hop determination is not performed on

every packet that is sent. Instead, the results of next-hop

determination computations are saved in the Destination Cache (which

also contains updates learned from Redirect messages). When the

sending node has a packet to send, it first examines the Destination

Cache. If no entry exists for the destination, next-hop

determination is invoked to create a Destination Cache entry.

Once the IP address of the next-hop node is known, the sender

examines the Neighbor Cache for link-layer information about that

neighbor. If no entry exists, the sender creates one, sets its state

to INCOMPLETE, initiates Address Resolution, and then queues the data

packet pending completion of address resolution. For multicast-

capable interfaces Address Resolution consists of sending a Neighbor

Solicitation message and waiting for a Neighbor Advertisement. When

a Neighbor Advertisement response is received, the link-layer

addresses is entered in the Neighbor Cache entry and the queued

packet is transmitted. The address resolution mechanism is described

in detail in Section 7.2.

For multicast packets the next-hop is always the (multicast)

destination address and is considered to be on-link. The procedure

for determining the link-layer address corresponding to a given IP

multicast address can be found in a separate document that covers

operating IP over a particular link type (e.g., [IPv6-ETHER]).

Each time a Neighbor Cache entry is accessed while transmitting a

unicast packet, the sender checks Neighbor Unreachability Detection

related information according to the Neighbor Unreachability

Detection algorithm (Section 7.3). This unreachability check might

result in the sender transmitting a unicast Neighbor Solicitation to

verify that the neighbor is still reachable.

Next-hop determination is done the first time traffic is sent to a

destination. As long as subsequent communication to that destination

proceeds successfully, the Destination Cache entry continues to be

used. If at some point communication ceases to proceed, as

determined by the Neighbor Unreachability Detection algorithm, next-

hop determination may need to be performed again. For example,

traffic through a failed router should be switched to a working

router. Likewise, it may be possible to reroute traffic destined for

a mobile node to a "mobility agent".

Note that when a node redoes next-hop determination there is no need

to discard the complete Destination Cache entry. In fact, it is

generally beneficial to retain such cached information as the PMTU

and round trip timer values that may also be kept in the Destination

Cache entry.

Routers and multihomed hosts have multiple interfaces. The remainder

of this document assumes that all sent and received Neighbor

Discovery messages refer to the interface of appropriate context.

For example, when responding to a Router Solicitation, the

corresponding Router Advertisement is sent out the interface on which

the solicitation was received.

5.3. Garbage Collection and Timeout Requirements

The conceptual data structures described above use different

mechanisms for discarding potentially stale or unused information.

From the perspective of correctness there is no need to periodically

purge Destination and Neighbor Cache entries. Although stale

information can potentially remain in the cache indefinitely, the

Neighbor Unreachability Detection algorithm ensures that stale

information is purged quickly if it is actually being used.

To limit the storage needed for the Destination and Neighbor Caches,

a node may need to garbage-collect old entries. However, care must

be taken to insure that sufficient space is always present to hold

the working set of active entries. A small cache may result in an

excessive number of Neighbor Discovery messages if entries are

discarded and rebuilt in quick succession. Any LRU-based policy that

only reclaims entries that have not been used in some time (e.g., ten

minutes or more) should be adequate for garbage-collecting unused

entries.

A node should retain entries in the Default Router List and the

Prefix List until their lifetimes expire. However, a node may

garbage collect entries prematurely if it is low on memory. If not

all routers are kept on the Default Router list, a node should retain

at least two entries in the Default Router List (and preferably more)

in order to maintain robust connectivity for off-link destinations.

When removing an entry from the Prefix List there is no need to purge

any entries from the Destination or Neighbor Caches. Neighbor

Unreachability Detection will efficiently purge any entries in these

caches that have become invalid. When removing an entry from the

Default Router List, however, any entries in the Destination Cache

that go through that router must perform next-hop determination again

to select a new default router.

6. ROUTER AND PREFIX DISCOVERY

This section describes router and host behavior related to the Router

Discovery portion of Neighbor Discovery. Router Discovery is used to

locate neighboring routers as well as learn prefixes and

configuration parameters related to address autoconfiguration.

Prefix Discovery is the process through which hosts learn the ranges

of IP addresses that reside on-link and can be reached directly

without going through a router. Routers send Router Advertisements

that indicate whether the sender is willing to be a default router.

Router Advertisements also contain Prefix Information options that

list the set of prefixes that identify on-link IP addresses.

Stateless Address Autoconfiguration must also oBTain subnet prefixes

as part of configuring addresses. Although the prefixes used for

address autoconfiguration are logically distinct from those used for

on-link determination, autoconfiguration information is piggybacked

on Router Discovery messages to reduce network traffic. Indeed, the

same prefixes can be advertised for on-link determination and address

autoconfiguration by specifying the appropriate flags in the Prefix

Information options. See [ADDRCONF] for details on how

autoconfiguration information is processed.

6.1. Message Validation

6.1.1. Validation of Router Solicitation Messages

Hosts MUST silently discard any received Router Solicitation

Messages.

A router MUST silently discard any received Router Solicitation

messages that do not satisfy all of the following validity checks:

- The IP Hop Limit field has a value of 255, i.e., the packet could

not possibly have been forwarded by a router.

- If the message includes an IP Authentication Header, the message

authenticates correctly.

- ICMP Checksum is valid.

- ICMP Code is 0.

- ICMP length (derived from the IP length) is 8 or more octets.

- All included options have a length that is greater than zero.

The contents of the Reserved field, and of any unrecognized options,

MUST be ignored. Future, backward-compatible changes to the protocol

may specify the contents of the Reserved field or add new options;

backward-incompatible changes may use different Code values.

The contents of any defined options that are not specified to be used

with Router Solicitation messages MUST be ignored and the packet

processed as normal. The only defined option that may appear is the

Source Link-Layer Address option.

A solicitation that passes the validity checks is called a "valid

solicitation".

6.1.2. Validation of Router Advertisement Messages

A node MUST silently discard any received Router Advertisement

messages that do not satisfy all of the following validity checks:

- IP Source Address is a link-local address. Routers must use their

link-local address as the source for Router Advertisement and

Redirect messages so that hosts can uniquely identify routers.

- The IP Hop Limit field has a value of 255, i.e., the packet could

not possibly have been forwarded by a router.

- If the message includes an IP Authentication Header, the message

authenticates correctly.

- ICMP Checksum is valid.

- ICMP Code is 0.

- ICMP length (derived from the IP length) is 16 or more octets.

- All included options have a length that is greater than zero.

The contents of the Reserved field, and of any unrecognized options,

MUST be ignored. Future, backward-compatible changes to the protocol

may specify the contents of the Reserved field or add new options;

backward-incompatible changes may use different Code values.

The contents of any defined options that are not specified to be used

with Router Advertisement messages MUST be ignored and the packet

processed as normal. The only defined options that may appear are

the Source Link-Layer Address, Prefix Information and MTU options.

An advertisement that passes the validity checks is called a "valid

advertisement".

6.2. Router Specification

6.2.1. Router Configuration Variables

A router MUST allow for the following conceptual variables to be

configured by system management. The specific variable names 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. Default values are specified

to simplify configuration in common cases.

The default values for some of the variables listed below may be

overridden by specific documents that describe how IPv6 operates over

different link layers. This rule simplifies the configuration of

Neighbor Discovery over link types with widely differing performance

characteristics.

For each multicast interface:

AdvSendAdvertisements

A flag indicating whether or not the router sends

periodic Router Advertisements and responds to

Router Solicitations.

Default: FALSE

Note that AdvSendAdvertisements MUST be false by

default so that a node will not accidentally start

acting as a router unless it is explicitly

configured by system management to send Router

Advertisements.

MaxRtrAdvInterval

The maximum time allowed between sending unsolicited

multicast Router Advertisements from the interface,

in seconds. MUST be no less than 4 seconds and no

greater than 1800 seconds.

Default: 600 seconds

MinRtrAdvInterval

The minimum time allowed between sending unsolicited

multicast Router Advertisements from the interface,

in seconds. MUST be no less than 3 seconds and no

greater than .75 * MaxRtrAdvInterval.

Default: 0.33 * MaxRtrAdvInterval

AdvManagedFlag

The true/false value to be placed in the "Managed

address configuration" flag field in the Router

Advertisement. See [ADDRCONF].

Default: FALSE

AdvOtherConfigFlag

The true/false value to be placed in the "Other

stateful configuration" flag field in the Router

Advertisement. See [ADDRCONF].

Default: FALSE

AdvLinkMTU The value to be placed in MTU options sent by the

router. A value of zero indicates that no MTU

options are sent.

Default: 0

AdvReachableTime

The value to be placed in the Reachable Time field

in the Router Advertisement messages sent by the

router. The value zero means unspecified (by this

router). MUST be no greater than 3,600,000

milliseconds (1 hour).

Default: 0

AdvRetransTimer

The value to be placed in the Retrans Timer field in

the Router Advertisement messages sent by the

router. The value zero means unspecified (by this

router).

Default: 0

AdvCurHopLimit

The default value to be placed in the Cur Hop Limit

field in the Router Advertisement messages sent by

the router. The value should be set to that current

diameter of the Internet. The value zero means

unspecified (by this router).

Default: The value specified in the "Assigned

Numbers" RFC[ASSIGNED] that was in effect at the

time of implementation.

AdvDefaultLifetime

The value to be placed in the Router Lifetime field

of Router Advertisements sent from the interface, in

seconds. MUST be either zero or between

MaxRtrAdvInterval and 9000 seconds. A value of zero

indicates that the router is not to be used as a

default router.

Default: 3 * MaxRtrAdvInterval

AdvPrefixList

A list of prefixes to be placed in Prefix

Information options in Router Advertisement messages

sent from the interface.

Default: all prefixes that the router advertises via

routing protocols as being on-link for the interface

from which the advertisement is sent. The link-

local prefix SHOULD NOT be included in the list of

advertised prefixes.

Each prefix has an associated:

AdvValidLifetime

The value to be placed in the Valid Lifetime

in the Prefix Information option, in

seconds. The designated value of all 1's

(0xffffffff) represents infinity.

Default: infinity.

AdvOnLinkFlag

The value to be placed in the on-link flag

("L-bit") field in the Prefix Information

option.

Default: TRUE

Automatic address configuration [ADDRCONF] defines

additional information associated with each the

prefixes:

AdvPreferredLifetime

The value to be placed in the Preferred

Lifetime in the Prefix Information option,

in seconds. The designated value of all 1's

(0xffffffff) represents infinity. See

[ADDRCONF].

Default: 604800 seconds (7 days)

AdvAutonomousFlag

The value to be placed in the Autonomous

Flag field in the Prefix Information option.

See [ADDRCONF].

Default: TRUE

The above variables contain information that is placed in outgoing

Router Advertisement messages. Hosts use the received information to

initialize a set of analogous variables that control their external

behavior (see Section 6.3.2). Some of these host variables (e.g.,

CurHopLimit, RetransTimer, and ReachableTime) apply to all nodes

including routers. In practice, these variables may not actually be

present on routers, since their contents can be derived from the

variables described above. However, external router behavior MUST be

the same as host behavior with respect to these variables. In

particular, this includes the occasional randomization of the

ReachableTime value as described in Section 6.3.2.

Protocol constants are defined in Section 10.

6.2.2. Becoming An Advertising Interface

The term "advertising interface" refers to any functioning and

enabled multicast interface that has at least one unicast IP address

assigned to it and whose corresponding AdvSendAdvertisements flag is

TRUE. A router MUST NOT send Router Advertisements out any interface

that is not an advertising interface.

An interface may become an advertising interface at times other than

system startup. For example:

- changing the AdvSendAdvertisements flag on an enabled interface

from FALSE to TRUE, or

- administratively enabling the interface, if it had been

administratively disabled, and its AdvSendAdvertisements flag is

TRUE, or

- enabling IP forwarding capability (i.e., changing the system from

being a host to being a router), when the interface's

AdvSendAdvertisements flag is TRUE.

A router MUST join the all-routers multicast address on an

advertising interface. Routers respond to Router Solicitations sent

to the all-routers address and verify the consistency of Router

Advertisements sent by neighboring routers.

6.2.3. Router Advertisement Message Content

A router sends periodic as well as solicited Router Advertisements

out its advertising interfaces. Outgoing Router Advertisements are

filled with the following values consistent with the message format

given in Section 4.2:

- In the Router Lifetime field: the interface's configured

AdvDefaultLifetime.

- In the M and O flags: the interface's configured AdvManagedFlag and

AdvOtherConfigFlag, respectively. See [ADDRCONF].

- In the Cur Hop Limit field: the interface's configured CurHopLimit.

- In the Reachable Time field: the interface's configured

AdvReachableTime.

- In the Retrans Timer field: the interface's configured

AdvRetransTimer.

- In the options:

o Source Link-Layer Address option: link-layer address of the

sending interface. This option MAY be omitted to facilitate

in-bound load balancing over replicated interfaces.

o MTU option: the interface's configured AdvLinkMTU value if the

value is non-zero. If AdvLinkMTU is zero the MTU option is

not sent.

o Prefix Information options: one Prefix Information option for

each prefix listed in AdvPrefixList with the option fields set

from the information in the AdvPrefixList entry as follows:

- In the "on-link" flag: the entry's AdvOnLinkFlag.

- In the Valid Lifetime field: the entry's

AdvValidLifetime.

- In the "Autonomous address configuration" flag: the

entry's AdvAutonomousFlag.

- In the Preferred Lifetime field: the entry's

AdvPreferredLifetime.

A router might want to send Router Advertisements without advertising

itself as a default router. For instance, a router might advertise

prefixes for address autoconfiguration while not wishing to forward

packets. Such a router sets the Router Lifetime field in outgoing

advertisements to zero.

A router MAY choose not to include some or all options when sending

unsolicited Router Advertisements. For example, if prefix lifetimes

are much longer than AdvDefaultLifetime, including them every few

advertisements may be sufficient. However, when responding to a

Router Solicitation or while sending the first few initial

unsolicited advertisements, a router SHOULD include all options so

that all information (e.g., prefixes) is propagated quickly during

system initialization.

If including all options causes the size of an advertisement to

exceed the link MTU, multiple advertisements can be sent, each

containing a subset of the options.

6.2.4. Sending Unsolicited Router Advertisements

A host MUST NOT send Router Advertisement messages at any time.

Unsolicited Router Advertisements are not strictly periodic: the

interval between subsequent transmissions is randomized to reduce the

probability of synchronization with the advertisements from other

routers on the same link [SYNC]. Each advertising interface has its

own timer. Whenever a multicast advertisement is sent from an

interface, the timer is reset to a uniformly-distributed random value

between the interface's configured MinRtrAdvInterval and

MaxRtrAdvInterval; expiration of the timer causes the next

advertisement to be sent and a new random value to be chosen.

For the first few advertisements (up to

MAX_INITIAL_RTR_ADVERTISEMENTS) sent from an interface when it

becomes an advertising interface, if the randomly chosen interval is

greater than MAX_INITIAL_RTR_ADVERT_INTERVAL, the timer SHOULD be set

to MAX_INITIAL_RTR_ADVERT_INTERVAL instead. Using a smaller interval

for the initial advertisements increases the likelihood of a router

being discovered quickly when it first becomes available, in the

presence of possible packet loss.

The information contained in Router Advertisements may change through

actions of system management. For instance, the lifetime of

advertised prefixes may change, new prefixes could be added, a router

could cease to be a router (i.e., switch from being a router to being

a host), etc. In such cases, the router MAY transmit up to

MAX_INITIAL_RTR_ADVERTISEMENTS unsolicited advertisements, using the

same rules as when an interface becomes an advertising interface.

6.2.5. Ceasing To Be An Advertising Interface

An interface may cease to be an advertising interface, through

actions of system management such as:

- changing the AdvSendAdvertisements flag of an enabled interface

from TRUE to FALSE, or

- administratively disabling the interface, or

- shutting down the system.

In such cases the router SHOULD transmit one or more (but not more

than MAX_FINAL_RTR_ADVERTISEMENTS) final multicast Router

Advertisements on the interface with a Router Lifetime field of zero.

In the case of a router becoming a host, the system SHOULD also

depart from the all-routers IP multicast group on all interfaces on

which the router supports IP multicast (whether or not they had been

advertising interfaces). In addition, the host MUST insure that

subsequent Neighbor Advertisement messages sent from the interface

have the Router flag set to zero.

Note that system management may disable a router's IP forwarding

capability (i.e., changing the system from being a router to being a

host), a step that does not necessarily imply that the router's

interfaces stop being advertising interfaces. In such cases,

subsequent Router Advertisements MUST set the Router Lifetime field

to zero.

6.2.6. Processing Router Solicitations

A host MUST silently discard any received Router Solicitation

messages.

In addition to sending periodic, unsolicited advertisements, a router

sends advertisements in response to valid solicitations received on

an advertising interface. A router MAY choose to unicast the

response directly to the soliciting host's address (if the

solicitation's source address is not the unspecified address), but

the usual case is to multicast the response to the all-nodes group.

In the latter case, the interface's interval timer is reset to a new

random value, as if an unsolicited advertisement had just been sent

(see Section 6.2.4).

In all cases, Router Advertisements sent in response to a Router

Solicitation MUST be delayed by a random time between 0 and

MAX_RA_DELAY_TIME seconds. (If a single advertisement is sent in

response to multiple solicitations, the delay is relative to the

first solicitation.) In addition, consecutive Router Advertisements

sent to the all-nodes multicast address MUST be rate limited to no

more than one advertisement every MIN_DELAY_BETWEEN_RAS seconds.

A router might process Router Solicitations as follows:

- Upon receipt of a Router Solicitation, compute a random delay within

the range 0 through MAX_RA_DELAY_TIME. If the computed value

corresponds to a time later than the time the next multicast Router

Advertisement is scheduled to be sent, ignore the random delay and

send the advertisement at the already-scheduled time.

- If the router sent a multicast Router Advertisement (solicited or

unsolicited) within the last MIN_DELAY_BETWEEN_RAS seconds, schedule

the advertisement to be sent at a time corresponding to

MIN_DELAY_BETWEEN_RAS plus the random value after the previous

advertisement was sent. This ensures that the multicast Router

Advertisements are rate limited.

- Otherwise, schedule the sending of a Router Advertisement at the time

given by the random value.

Note that a router is permitted to send multicast Router

Advertisements more frequently than indicated by the

MinRtrAdvInterval configuration variable so long as the more frequent

advertisements are responses to Router Solicitations. In all cases,

however, unsolicited multicast advertisements MUST NOT be sent more

frequently than indicated by MinRtrAdvInterval.

When a router receives a Router Solicitation and the Source Address

is not the unspecified address, it records that the source of the

packet is a neighbor by creating or updating the Neighbor Cache

entry. If the solicitation contains a Source Link-Layer Address

option, and the router has a Neighbor Cache entry for the neighbor,

the link-layer address SHOULD be updated in the Neighbor Cache. If a

Neighbor Cache entry is created for the source its reachability state

MUST be set to STALE as specified in Section 7.3.3. If a cache entry

already exists and is updated with a different link-layer address the

reachability state MUST also be set to STALE. In either case the

entry's IsRouter flag SHOULD be set to false.

If the Source Address is the unspecified address the router MUST NOT

create or update the Neighbor Cache entry.

6.2.7. Router Advertisement Consistency

Routers SHOULD inspect valid Router Advertisements sent by other

routers and verify that the routers are advertising consistent

information on a link. Detected inconsistencies indicate that one or

more routers might be misconfigured and SHOULD be logged to system or

network management. The minimum set of information to check

includes:

- Cur Hop Limit values (except for the unspecified value of zero).

- Values of the M or O flags.

- Reachable Time values (except for the unspecified value of zero).

- Retrans Timer values (except for the unspecified value of zero).

- Values in the MTU options.

- Preferred and Valid Lifetimes for the same prefix.

Note that it is not an error for different routers to advertise

different sets of prefixes. Also, some routers might leave some

fields as unspecified, i.e., with the value zero, while other routers

specify values. The logging of errors SHOULD be restricted to

conflicting information that causes hosts to switch from one value to

another with each received advertisement.

Any other action on reception of Router Advertisement messages by a

router is beyond the scope of this document.

6.2.8. Link-local Address Change

The link-local address on a router SHOULD change rarely, if ever.

Nodes receiving Neighbor Discovery messages use the source address to

identify the sender. If multiple packets from the same router

contain different source addresses, nodes will assume they come from

different routers, leading to undesirable behavior. For example, a

node will ignore Redirect messages that are believed to have been

sent by a router other than the current first-hop router. Thus the

source address used in Router Advertisements sent by a particular

router must be identical to the target address in a Redirect message

when redirecting to that router.

Using the link-local address to uniquely identify routers on the link

has the benefit that the address a router is known by should not

change when a site renumbers.

If a router changes the link-local address for one of its interfaces,

it SHOULD inform hosts of this change. The router SHOULD multicast a

few Router Advertisements from the old link-local address with the

Router Lifetime field set to zero and also multicast a few Router

Advertisements from the new link-local address. The overall effect

should be the same as if one interface ceases being an advertising

interface, and a different one starts being an advertising interface.

6.3. Host Specification

6.3.1. Host Configuration Variables

None.

6.3.2. Host Variables

A host maintains certain Neighbor Discovery related variables in

addition to the data structures defined in Section 5.1. The specific

variable names 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.

These variables have default values that are overridden by

information received in Router Advertisement messages. The default

values are used when there is no router on the link or when all

received Router Advertisements have left a particular value

unspecified.

The default values in this specification may be overridden by

specific documents that describe how IP operates over different link

layers. This rule allows Neighbor Discovery to operate over links

with widely varying performance characteristics.

For each interface:

LinkMTU The MTU of the link.

Default: The valued defined in the specific document

that describes how IPv6 operates over the particular

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

CurHopLimit The default hop limit to be used when sending

(unicast) IP packets.

Default: The value specified in the "Assigned

Numbers" RFC[ASSIGNED] that was in effect at the

time of implementation.

BaseReachableTime

A base value used for computing the random

ReachableTime value.

Default: REACHABLE_TIME milliseconds.

ReachableTime The time a neighbor is considered reachable after

receiving a reachability confirmation.

This value should be a uniformly-distributed random

value between MIN_RANDOM_FACTOR and

MAX_RANDOM_FACTOR times BaseReachableTime

milliseconds. A new random value should be

calculated when BaseReachableTime changes (due to

Router Advertisements) or at least every few hours

even if no Router Advertisements are received.

RetransTimer The time between retransmissions of Neighbor

Solicitation messages to a neighbor when resolving

the address or when probing the reachability of a

neighbor.

Default: RETRANS_TIMER milliseconds

6.3.3. Interface Initialization

The host joins the all-nodes multicast address on all multicast-

capable interfaces.

6.3.4. Processing Received Router Advertisements

When multiple routers are present, the information advertised

collectively by all routers may be a superset of the information

contained in a single Router Advertisement. Moreover, information

may also be obtained through other dynamic means, such as stateful

autoconfiguration. Hosts accept the union of all received

information; the receipt of a Router Advertisement MUST NOT

invalidate all information received in a previous advertisement or

from another source. However, when received information for a

specific parameter (e.g., Link MTU) or option (e.g., Lifetime on a

specific Prefix) differs from information received earlier, and the

parameter/option can only have one value, the most recently-received

information is considered authoritative.

Some Router Advertisement fields (e.g., Cur Hop Limit, Reachable Time

and Retrans Timer) may contain a value denoting unspecified. In such

cases, the parameter should be ignored and the host should continue

using whatever value it is already using. In particular, a host MUST

NOT interpret the unspecified value as meaning change back to the

default value that was in use before the first Router Advertisement

was received. This rule prevents hosts from continually changing an

internal variable when one router advertises a specific value, but

other routers advertise the unspecified value.

On receipt of a valid Router Advertisement, a host extracts the

source address of the packet and does the following:

- If the address is not already present in the host's Default Router

List, and the advertisement's Router Lifetime is non-zero, create a

new entry in the list, and initialize its invalidation timer value

from the advertisement's Router Lifetime field.

- If the address is already present in the host's Default Router List

as a result of a previously-received advertisement, reset its

invalidation timer to the Router Lifetime value in the newly-

received advertisement.

- If the address is already present in the host's Default Router List

and the received Router Lifetime value is zero, immediately time-

out the entry as specified in Section 6.3.5.

To limit the storage needed for the Default Router List, a host MAY

choose not to store all of the router addresses discovered via

advertisements. However, a host MUST retain at least two router

addresses and SHOULD retain more. Default router selections are made

whenever communication to a destination appears to be failing. Thus,

the more routers on the list, the more likely an alternative working

router can be found quickly (e.g., without having to wait for the

next advertisement to arrive).

If the received Cur Hop Limit value is non-zero the host SHOULD set

its CurHopLimit variable to the received value.

If the received Reachable Time value is non-zero the host SHOULD set

its BaseReachableTime variable to the received value. If the new

value differs from the previous value, the host SHOULD recompute a

new random ReachableTime value. ReachableTime is computed as a

uniformly-distributed random value between MIN_RANDOM_FACTOR and

MAX_RANDOM_FACTOR times the BaseReachableTime. Using a random

component eliminates the possibility Neighbor Unreachability

Detection messages synchronize with each other.

In most cases, the advertised Reachable Time value will be the same

in consecutive Router Advertisements and a host's BaseReachableTime

rarely changes. In such cases, an implementation SHOULD insure that

a new random value gets recomputed at least once every few hours.

The RetransTimer variable SHOULD be copied from the Retrans Timer

field, if the received value is non-zero.

After extracting information from the fixed part of the Router

Advertisement message, the advertisement is scanned for valid

options. If the advertisement contains a Source Link-Layer Address

option the link-layer address SHOULD be recorded in the Neighbor

Cache entry for the router (creating an entry if necessary) and the

IsRouter flag in the Neighbor Cache entry MUST be set to true. The

IsRouter flag is used by Neighbor Unreachability Detection to

determine when a router changes to being a host (i.e., no longer

capable of forwarding packets). If a Neighbor Cache entry is created

for the router its reachability state MUST be set to STALE as

specified in Section 7.3.3. If a cache entry already exists and is

updated with a different link-layer address the reachability state

MUST also be set to STALE.

If the MTU option is present, hosts SHOULD copy the option's value

into LinkMTU if the value does not exceed the default LinkMTU value

specified in the link type specific document (e.g., [IPv6-ETHER]).

Prefix Information options that have the "on-link" (L) flag set

indicate a prefix identifying a range of addresses that should be

considered on-link. Note, however, that a Prefix Information option

with the on-link flag set to zero conveys no information concerning

on-link determination and MUST NOT be interpreted to mean that

addresses covered by the prefix are off-link. The default behavior

(see Section 5.2) when no information is known about an address is to

send the packets to a default router and the reception of a Prefix

Information option with the "on-link " (L) flag set to zero does not

change this behavior. The reasons for an address being treated as

on-link is specified in the definition of "on-link" in Section 2.1.

Prefixes with the on-link flag set to zero would normally have the

autonomous flag set and be used by [ADDRCONF].

For each Prefix Information option with the on-link flag set, a host

does the following:

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

Information option.

- If the prefix is not already present in the Prefix List, and the

Prefix Information option's Valid Lifetime field is non-zero,

create a new entry for the prefix and initialize its invalidation

timer to the Valid Lifetime value in the Prefix Information option.

- If the prefix is already present in the host's Prefix List as the

result of a previously-received advertisement, reset its

invalidation timer to the Valid Lifetime value in the Prefix

Information option. If the new Lifetime value is zero, time-out

the prefix immediately (see Section 6.3.5).

- If the Prefix Information option's Valid Lifetime field is zero,

and the prefix is not present in the host's Prefix List, silently

ignore the option.

Note: Implementations can choose to process the on-link aspects of

the prefixes separately from the address autoconfiguration aspects of

the prefixes by, e.g., passing a copy of each valid Router

Advertisement message to both an "on-link" and an "addrconf"

function. Each function can then operate independently on the

prefixes that have the appropriate flag set.

6.3.5. Timing out Prefixes and Default Routers

Whenever the invalidation timer expires for a Prefix List entry, that

entry is discarded. No existing Destination Cache entries need be

updated, however. Should a reachability problem arise with an

existing Neighbor Cache entry, Neighbor Unreachability Detection will

perform any needed recovery.

Whenever the Lifetime of an entry in the Default Router List expires,

that entry is discarded. When removing a router from the Default

Router list, the node MUST update the Destination Cache in such a way

that all entries using the router perform next-hop determination

again rather than continue sending traffic to the (deleted) router.

6.3.6. Default Router Selection

The algorithm for selecting a router depends in part on whether or

not a router is known to be reachable. The exact details of how a

node keeps track of a neighbor's reachability state are covered in

Section 7.3. The algorithm for selecting a default router is invoked

during next-hop determination when no Destination Cache entry exists

for an off-link destination or when communication through an existing

router appears to be failing. Under normal conditions, a router

would be selected the first time traffic is sent to a destination,

with subsequent traffic for that destination using the same router as

indicated in the Destination Cache modulo any changes to the

Destination Cache caused by Redirect messages.

The policy for selecting routers from the Default Router List is as

follows:

1) Routers that are reachable or probably reachable (i.e., in any

state other than INCOMPLETE) SHOULD be preferred over routers whose

reachability is unknown or suspect (i.e., in the INCOMPLETE state,

or for which no Neighbor Cache entry exists). An implementation

may choose to always return the same router or cycle through the

router list in a round-robin fashion as long as it always returns a

reachable or a probably reachable router when one is available.

2) When no routers on the list are known to be reachable or probably

reachable, routers SHOULD be selected in a round-robin fashion, so

that subsequent requests for a default router do not return the

same router until all other routers have been selected.

Cycling through the router list in this case ensures that all

available routers are actively probed by the Neighbor

Unreachability Detection algorithm. A request for a default router

is made in conjunction with the sending of a packet to a router,

and the selected router will be probed for reachability as a side

effect.

3) If the Default Router List is empty, assume that all destinations

are on-link as specified in Section 5.2.

6.3.7. Sending Router Solicitations

When an interface becomes enabled, a host may be unwilling to wait

for the next unsolicited Router Advertisement to locate default

routers or learn prefixes. To obtain Router Advertisements quickly,

a host SHOULD transmit up to MAX_RTR_SOLICITATIONS Router

Solicitation messages each separated by at least

RTR_SOLICITATION_INTERVAL seconds. Router Solicitations may be sent

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 system changes from being a router to being a host, by having

its IP forwarding capability turned off by system management.

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

- The host re-attaches to a link after being detached for some time.

A host sends Router Solicitations to the all-routers multicast

address. The IP source address is set to either one of the

interface's unicast addresses or the unspecified address. The Source

Link-Layer Address option SHOULD be set to the host's link-layer

address, if the IP source address is a unicast address.

Before a host sends an initial solicitation, it SHOULD delay the

transmission for a random amount of time between 0 and

MAX_RTR_SOLICITATION_DELAY. This serves to alleviate congestion when

many hosts start up on a link at the same time, such as might happen

after recovery from a power failure. If a host has already performed

a random delay since the interface became (re)enabled (e.g., as part

of Duplicate Address Detection [ADDRCONF]) there is no need to delay

again before sending the first Router Solicitation message.

Once the host sends a Router Solicitation, and receives a valid

Router Advertisement with a non-zero Router Lifetime, the host MUST

desist from sending additional solicitations on that interface, until

the next time one of the above events occurs. Moreover, a host

SHOULD send at least one solicitation in the case where an

advertisement is received prior to having sent a solicitation.

Unsolicited Router Advertisements may be incomplete (see Section

6.2.3); solicited advertisements are expected to contain complete

information.

If a host sends MAX_RTR_SOLICITATIONS solicitations, and receives no

Router Advertisements after having waited MAX_RTR_SOLICITATION_DELAY

seconds after sending the last solicitation, the host concludes that

there are no routers on the link for the purpose of [ADDRCONF].

However, the host continues to receive and process Router

Advertisements messages in the event that routers appear on the link.

7. ADDRESS RESOLUTION AND NEIGHBOR UNREACHABILITY DETECTION

This section describes the functions related to Neighbor Solicitation

and Neighbor Advertisement messages and includes descriptions of

address resolution and the Neighbor Unreachability Detection

algorithm.

Neighbor Solicitation and Advertisement messages are also used for

Duplicate Address Detection as specified by [ADDRCONF]. In

particular, Duplicate Address Detection sends Neighbor Solicitation

messages with an unspecified source address targeting its own

"tentative" address. Such messages trigger nodes already using the

address to respond with a multicast Neighbor Advertisement indicating

that the address is in use.

7.1. Message Validation

7.1.1. Validation of Neighbor Solicitations

A node MUST silently discard any received Neighbor Solicitation

messages that do not satisfy all of the following validity checks:

- The IP Hop Limit field has a value of 255, i.e., the packet could

not possibly have been forwarded by a router.

- If the message includes an IP Authentication Header, the message

authenticates correctly.

- ICMP Checksum is valid.

- ICMP Code is 0.

- ICMP length (derived from the IP length) is 24 or more octets.

- Target Address is not a multicast address.

- All included options have a length that is greater than zero.

The contents of the Reserved field, and of any unrecognized options,

MUST be ignored. Future, backward-compatible changes to the protocol

may specify the contents of the Reserved field or add new options;

backward-incompatible changes may use different Code values.

The contents of any defined options that are not specified to be used

with Neighbor Solicitation messages MUST be ignored and the packet

processed as normal. The only defined option that may appear is the

Source Link-Layer Address option.

A Neighbor Solicitation that passes the validity checks is called a

"valid solicitation".

7.1.2. Validation of Neighbor Advertisements

A node MUST silently discard any received Neighbor Advertisement

messages that do not satisfy all of the following validity checks:

- The IP Hop Limit field has a value of 255, i.e., the packet could

not possibly have been forwarded by a router.

- If the message includes an IP Authentication Header, the message

authenticates correctly.

- ICMP Checksum is valid.

- ICMP Code is 0.

- ICMP length (derived from the IP length) is 24 or more octets.

- Target Address is not a multicast address.

- If the IP Destination Address is a multicast address the Solicited

flag is zero.

- All included options have a length that is greater than zero.

The contents of the Reserved field, and of any unrecognized options,

MUST be ignored. Future, backward-compatible changes to the protocol

may specify the contents of the Reserved field or add new options;

backward-incompatible changes may use different Code values.

The contents of any defined options that are not specified to be used

with Neighbor Advertisement messages MUST be ignored and the packet

processed as normal. The only defined option that may appear is the

Target Link-Layer Address option.

A Neighbor Advertisements that passes the validity checks is called a

"valid advertisement".

7.2. Address Resolution

Address resolution is the process through which a node determines the

link-layer address of a neighbor given only its IP address. Address

resolution is performed only on addresses that are determined to be

on-link and for which the sender does not know the corresponding

link-layer address. Address resolution is never performed on

multicast addresses.

7.2.1. Interface Initialization

When a multicast-capable interface becomes enabled the node MUST join

the all-nodes multicast address on that interface, as well as the

solicited-node multicast address corresponding to each of the IP

addresses assigned to the interface.

The set of addresses assigned to an interface may change over time.

New addresses might be added and old addresses might be removed

[ADDRCONF]. In such cases the node MUST join and leave the

solicited-node multicast address corresponding to the new and old

addresses, respectively. Note that multiple unicast addresses may

map into the same solicited-node multicast address; a node MUST NOT

leave the solicited-node multicast group until all assigned addresses

corresponding to that multicast address have been removed.

7.2.2. Sending Neighbor Solicitations

When a node has a unicast packet to send to a neighbor, but does not

know the neighbor's link-layer address, it performs address

resolution. For multicast-capable interfaces this entails creating a

Neighbor Cache entry in the INCOMPLETE state and transmitting a

Neighbor Solicitation message targeted at the neighbor. The

solicitation is sent to the solicited-node multicast address

corresponding to the target address.

If the source address of the packet prompting the solicitation is the

same as one of the addresses assigned to the outgoing interface, that

address SHOULD be placed in the IP Source Address of the outgoing

solicitation. Otherwise, any one of the addresses assigned to the

interface should be used. Using the prompting packet's source

address when possible insures that the recipient of the Neighbor

Solicitation installs in its Neighbor Cache the IP address that is

highly likely to be used in subsequent return traffic belonging to

the prompting packet's "connection".

If the solicitation is being sent to a solicited-node multicast

address, the sender MUST include its link-layer address (if it has

one) as a Source Link-Layer Address option. Otherwise, the sender

SHOULD include its link-layer address (if it has one) as a Source

Link-Layer Address option. Including the source link-layer address

in a multicast solicitation is required to give the target an address

to which it can send the Neighbor Advertisement.

While waiting for address resolution to complete, the sender MUST,

for each neighbor, retain a small queue of packets waiting for

address resolution to complete. The queue MUST hold at least one

packet, and MAY contain more. However, the number of queued packets

per neighbor SHOULD be limited to some small value. When a queue

overflows, the new arrival SHOULD replace the oldest entry. Once

address resolution completes, the node transmits any queued packets.

While awaiting a response, the sender SHOULD retransmit Neighbor

Solicitation messages approximately every RetransTimer milliseconds,

even in the absence of additional traffic to the neighbor.

Retransmissions MUST be rate-limited to at most one solicitation per

neighbor every RetransTimer milliseconds.

If no Neighbor Advertisement is received after MAX_MULTICAST_SOLICIT

solicitations, address resolution has failed. The sender MUST return

ICMP destination unreachable indications with code 3 (Address

Unreachable) for each packet queued awaiting address resolution.

7.2.3. Receipt of Neighbor Solicitations

A valid Neighbor Solicitation where the Target Address is not a

unicast or anycast address assigned to the receiving interface, and

the Target Address is not a "tentative" address on which Duplicate

Address Detection is being performed [ADDRCONF] MUST be silently

ignored. If the Target Address is tentative, the Neighbor

Solicitation should be processed as described in [ADDRCONF].

Upon receipt of a valid Neighbor Solicitation targeted at the node,

the recipient SHOULD update the Neighbor Cache entry for the IP

Source Address of the solicitation if the Source Address is not the

unspecified address. If an entry does not already exist, the node

SHOULD create a new one and set its reachability state to STALE as

specified in Section 7.3.3. If a cache entry already exists and is

updated with a different link-layer address its reachability state

MUST be set to STALE. If the solicitation contains a Source Link-

Layer Address option, the entry's cached link-layer address should be

replaced with the one in the solicitation.

If the Source Address is the unspecified address the node MUST NOT

create or update the Neighbor Cache entry.

After any updates to the Neighbor Cache, the node sends a Neighbor

Advertisement response as described in the next section.

7.2.4. Sending Solicited Neighbor Advertisements

A node sends a Neighbor Advertisement in response to a valid Neighbor

Solicitation targeting one of the node's assigned addresses. The

Target Address of the advertisement is copied from the Target Address

of the solicitation. If the solicitation's IP Destination Address is

a unicast or anycast address, the Target Link-Layer Address option

SHOULD NOT be included; the neighboring node's cached value must

already be current in order for the solicitation to have been

received. If the solicitation's IP Destination Address is a

solicited-node multicast address, the Target Link-Layer option MUST

be included in the advertisement. If the node is a router, it MUST

set the Router flag to one; otherwise it MUST set the flag to zero.

If the Target Address is either an anycast address or a unicast

address for which the node is providing proxy service, or the Target

Link-Layer Address option is not included in the outgoing

advertisement, the Override flag SHOULD be set to zero. Otherwise,

it SHOULD be set to one. Proper setting of the Override flag insures

that nodes give preference to non-proxy advertisements, even when

received after proxy advertisements, but that the first advertisement

for an anycast address "wins".

If the source of the solicitation is the unspecified address, the

node MUST set the Solicited flag to zero and multicast the

advertisement to the all-nodes address. Otherwise, the node MUST set

the Solicited flag to one and unicast the advertisement to the Source

Address of the solicitation.

If the Target Address is an anycast address the sender SHOULD delay

sending a response for a random time between 0 and

MAX_ANYCAST_DELAY_TIME seconds.

7.2.5. Receipt of Neighbor Advertisements

When a valid Neighbor Advertisement is received (either solicited or

unsolicited), the Neighbor Cache is searched for the target's entry.

If no entry exists, the advertisement SHOULD be silently discarded.

There is no need to create an entry in this case, since the recipient

has apparently not initiated any communication with the target.

Once the appropriate Neighbor Cache entry has been located, the

specific actions taken depend on the state of the Neighbor Cache

entry and the flags in the advertisement. If the entry is in an

INCOMPLETE state (i.e., no link-layer address is cached for the

target) the received advertisement updates the entry. If a cached

link-layer address is already present, however, a node might choose

to ignore the received advertisement and continue using the cached

link-layer address.

If the target's Neighbor Cache entry is in the INCOMPLETE state, the

receiving node records the link-layer address in the Neighbor Cache

entry and sends any packets queued for the neighbor awaiting address

resolution. If the Solicited flag is set, the reachability state for

the neighbor MUST be set to REACHABLE; otherwise it MUST be set to

STALE. (A more detailed explanation of reachability state is

described in Section 7.3.3). The Override flag is ignored if the

entry is in the INCOMPLETE state.

If the target's Neighbor Cache entry is in any state other than

INCOMPLETE when the advertisement is received, the advertisement's

Override flag's setting determines whether the Target Link-Layer

Address option (if present) replaces the cached address. If the

Override flag is set, the receiving node MUST install the link-layer

address in its cache; if the flag is zero, the receiving node MUST

NOT install the link-layer address in its cache. An advertisement's

sender sets the Override flag when it wants its Target Link-Layer

Address option to replace the cached value in Neighbor Cache entries,

regardless of their current contents.

If the target's Neighbor Cache entry is in any state other than

INCOMPLETE when the advertisement is received, the advertisement's

Solicited flag setting determines what the entry's new state should

be. If the Solicited flag is set, the entry's state MUST be set to

REACHABLE; if the flag is zero, the entry's state MUST be set to

STALE. An advertisement's Solicited flag should only be set if the

advertisement is a response to a Neighbor Solicitation. Because

Neighbor Unreachability Solicitations are sent to the cached link-

layer address, a receipt of a solicited advertisement indicates that

the forward path is working. Receipt of an unsolicited

advertisement, however, suggests that a neighbor has urgent

information to announce (e.g., a changed link-layer address).

Regardless of whether or not the new link-layer address is installed

in the cache, a node should verify the reachability of the path it is

currently using when it sends the next packet, so that it quickly

finds a working path if the existing path has failed (e.g., as would

be the case if the unsolicited Neighbor Advertisement is sent to

announce a link-layer address change).

In those cases where the cached link-layer address is updated, the

receiving node MUST examine the Router flag in the received

advertisement and update the IsRouter flag in the Neighbor Cache

entry to reflect whether the node is a host or router. In those

cases where the neighbor was previously used as a router, but the

advertisement's Router flag is now set to zero, the node MUST remove

that router from the Default Router List and update the Destination

Cache entries for all destinations using that neighbor as a router as

specified in Section 7.3.3.

7.2.6. Sending Unsolicited Neighbor Advertisements

In some cases a node may be able to determine that its link-layer

address has changed (e.g., hot-swap of an interface card) and may

wish to inform its neighbors of the new link-layer address quickly.

In such cases a node MAY send up to MAX_NEIGHBOR_ADVERTISEMENT

unsolicited Neighbor Advertisement messages to the all-nodes

multicast address. These advertisements MUST be separated by at

least RetransTimer seconds.

The Target Address field in the unsolicited advertisement is set to

an IP address of the interface, and the Target Link-Layer Address

option is filled with the new link-layer address. The Solicited flag

MUST be set to zero, in order to avoid confusing the Neighbor

Unreachability Detection algorithm. If the node is a router, it MUST

set the Router flag to one; otherwise it MUST set it to zero. The

Override flag MAY be set to either zero or one. In either case,

neighboring nodes will immediately change the state of their Neighbor

Cache entries for the Target Address to STALE, prompting them to

verify the path for reachability. If the Override flag is set to

one, neighboring nodes will install the new link-layer address in

their caches. Otherwise, they will ignore the new link-layer

address, choosing instead to probe the cached address.

A node that has multiple IP addresses assigned to an interface MAY

multicast a separate Neighbor Advertisement for each address. In

such a case the node SHOULD introduce a small delay between the

sending of each advertisement to reduce the probability of the

advertisements being lost due to congestion.

A proxy MAY multicast Neighbor Advertisements when its link-layer

address changes or when it is configured (by system management or

other mechanisms) to proxy for an address. If there are multiple

nodes that are providing proxy services for the same set of addresses

the proxies SHOULD provide a mechanism that prevents multiple proxies

from multicasting advertisements for any one address, in order to

reduce the risk of excessive multicast traffic.

Also, a node belonging to an anycast address MAY multicast

unsolicited Neighbor Advertisements for the anycast address when the

node's link- layer address changes.

Note that because unsolicited Neighbor Advertisements do not reliably

update caches in all nodes (the advertisements might not be received

by all nodes), they should only be viewed as a performance

optimization to quickly update the caches in most neighbors. The

Neighbor Unreachability Detection algorithm ensures that all nodes

obtain a reachable link-layer address, though the delay may be

slightly longer.

7.2.7. Anycast Neighbor Advertisements

From the perspective of Neighbor Discovery, anycast addresses are

treated just like unicast addresses in most cases. Because an

anycast address is syntactically the same as a unicast address, nodes

performing address resolution or Neighbor Unreachability Detection on

an anycast address treat it as if it were a unicast address. No

special processing takes place.

Nodes that have an anycast address assigned to an interface treat

them exactly the same as if they were unicast addresses with two

exceptions. First, Neighbor Advertisements sent in response to a

Neighbor Solicitation SHOULD be delayed by a random time between 0

and MAX_ANYCAST_DELAY_TIME to reduce the probability of network

congestion. Second, the Override flag in Neighbor Advertisements

SHOULD be set to 0, so that when multiple advertisements are

received, the first received advertisement is used rather than the

most recently received advertisement.

As with unicast addresses, Neighbor Unreachability Detection ensures

that a node quickly detects when the current binding for an anycast

address becomes invalid.

7.2.8. Proxy Neighbor Advertisements

Under limited circumstances, a router MAY proxy for one or more other

nodes, that is, through Neighbor Advertisements indicate that it is

willing to accept packets not explicitly addressed to itself. For

example, a router might accept packets on behalf of a mobile node

that has moved off-link. The mechanisms used by proxy are identical

to the mechanisms used with anycast addresses.

A proxy MUST join the solicited-node multicast address(es) that

correspond to the IP address(es) assigned to the node for which it is

proxying.

All solicited proxy Neighbor Advertisement messages MUST have the

Override flag set to zero. This ensures that if the node itself is

present on the link its Neighbor Advertisement (with the Override

flag set to one) will take precedence of any advertisement received

from a proxy. A proxy MAY send unsolicited advertisements with the

Override flag set to one as specified in Section 7.2.6, but doing so

may cause the proxy advertisement to override a valid entry created

by the node itself.

Finally, when sending a proxy advertisement in response to a Neighbor

Solicitation, the sender should delay its response by a random time

between 0 and MAX_ANYCAST_DELAY_TIME seconds.

7.3. Neighbor Unreachability Detection

Communication to or through a neighbor may fail for numerous reasons

at any time, including hardware failure, hot-swap of an interface

card, etc. If the destination has failed, no recovery is possible

and communication fails. On the other hand, if it is the path that

has failed, recovery may be possible. Thus, a node actively tracks

the reachability "state" for the neighbors to which it is sending

packets.

Neighbor Unreachability Detection is used for all paths between hosts

and neighboring nodes, including host-to-host, host-to-router, and

router-to-host communication. Neighbor Unreachability Detection may

also be used between routers, but is not required if an equivalent

mechanism is available, for example, as part of the routing

protocols.

When a path to a neighbor appears to be failing, the specific

recovery procedure depends on how the neighbor is being used. If the

neighbor is the ultimate destination, for example, address resolution

should be performed again. If the neighbor is a router, however,

attempting to switch to another router would be appropriate. The

specific recovery that takes place is covered under next-hop

determination; Neighbor Unreachability Detection signals the need for

next-hop determination by deleting a Neighbor Cache entry.

Neighbor Unreachability Detection is performed only for neighbors to

which unicast packets are sent; it is not used when sending to

multicast addresses.

7.3.1. Reachability Confirmation

A neighbor is considered reachable if the node has recently received

a confirmation that packets sent recently to the neighbor were

received by its IP layer. Positive confirmation can be gathered in

two ways: hints from upper layer protocols that indicate a connection

is making "forward progress", or receipt of a Neighbor Advertisement

message that is a response to a Neighbor Solicitation message.

A connection makes "forward progress" if the packets received from a

remote peer can only be arriving if recent packets sent to that peer

are actually reaching it. In TCP, for example, receipt of a (new)

acknowledgement indicates that previously sent data reached the peer.

Likewise, the arrival of new (non-duplicate) data indicates that

earlier acknowledgements are being delivered to the remote peer. If

packets are reaching the peer, they must also be reaching the

sender's next-hop neighbor; thus "forward progress" is a confirmation

that the next-hop neighbor is reachable. For off-link destinations,

forward progress implies that the first-hop router is reachable.

When available, this upper-layer information SHOULD be used.

In some cases (e.g., UDP-based protocols and routers forwarding

packets to hosts) such reachability information may not be readily

available from upper-layer protocols. When no hints are available

and a node is sending packets to a neighbor, the node actively probes

the neighbor using unicast Neighbor Solicitation messages to verify

that the forward path is still working.

The receipt of a solicited Neighbor Advertisement serves as

reachability confirmation, since advertisements with the Solicited

flag set to one are sent only in response to a Neighbor Solicitation.

Receipt of other Neighbor Discovery messages such as Router

Advertisements and Neighbor Advertisement with the Solicited flag set

to zero MUST NOT be treated as a reachability confirmation. Receipt

of unsolicited messages only confirm the one-way path from the sender

to the recipient node. In contrast, Neighbor Unreachability

Detection requires that a node keep track of the reachability of the

forward path to a neighbor from the its perspective, not the

neighbor's perspective. Note that receipt of a solicited

advertisement indicates that a path is working in both directions.

The solicitation must have reached the neighbor, prompting it to

generate an advertisement. Likewise, receipt of an advertisement

indicates that the path from the sender to the recipient is working.

However, the latter fact is known only to the recipient; the

advertisement's sender has no direct way of knowing that the

advertisement it sent actually reached a neighbor. From the

perspective of Neighbor Unreachability Detection, only the

reachability of the forward path is of interest.

7.3.2. Neighbor Cache Entry States

A Neighbor Cache entry can be in one of five states:

INCOMPLETE Address resolution is being performed on the entry.

Specifically, a Neighbor Solicitation has been sent to

the solicited-node multicast address of the target, but

the corresponding Neighbor Advertisement has not yet been

received.

REACHABLE Positive confirmation was received within the last

ReachableTime milliseconds that the forward path to the

neighbor was functioning properly. While REACHABLE, no

special action takes place as packets are sent.

STALE More than ReachableTime milliseconds have elapsed since

the last positive confirmation was received that the

forward path was functioning properly. While stale, no

action takes place until a packet is sent.

The STALE state is entered upon receiving an unsolicited

Neighbor Discovery message that updates the cached link-

layer address. Receipt of such a message does not

confirm reachability, and entering the STALE state

insures reachability is verified quickly if the entry is

actually being used. However, reachability is not

actually verified until the entry is actually used.

DELAY More than ReachableTime milliseconds have elapsed since

the last positive confirmation was received that the

forward path was functioning properly, and a packet was

sent within the last DELAY_FIRST_PROBE_TIME seconds. If

no reachability confirmation is received within

DELAY_FIRST_PROBE_TIME seconds of entering the DELAY

state, send a Neighbor Solicitation and change the state

to PROBE.

The DELAY state is an optimization that gives upper-layer

protocols additional time to provide reachability

confirmation in those cases where ReachableTime

milliseconds have passed since the last confirmation due

to lack of recent traffic. Without this optimization the

opening of a TCP connection after a traffic lull would

initiate probes even though the subsequent three-way

handshake would provide a reachability confirmation

almost immediately.

PROBE A reachability confirmation is actively sought by

retransmitting Neighbor Solicitations every RetransTimer

milliseconds until a reachability confirmation is

received.

7.3.3. Node Behavior

Neighbor Unreachability Detection operates in parallel with the

sending of packets to a neighbor. While reasserting a neighbor's

reachability, a node continues sending packets to that neighbor using

the cached link-layer address. If no traffic is sent to a neighbor,

no probes are sent.

When a node needs to perform address resolution on a neighboring

address, it creates an entry in the INCOMPLETE state and initiates

address resolution as specified in Section 7.2. If address

resolution fails, the entry SHOULD be deleted, so that subsequent

traffic to that neighbor invokes the next-hop determination procedure

again. Invoking next-hop determination at this point insures that

alternate default routers are tried.

When a reachability confirmation is received (either through upper-

layer advice or a solicited Neighbor Advertisement) an entry's state

changes to REACHABLE. The one exception is that upper-layer advice

has no effect on entries in the INCOMPLETE state (e.g., for which no

link-layer address is cached).

When ReachableTime milliseconds have passed since receipt of the last

reachability confirmation for a neighbor, the Neighbor Cache entry's

state changes from REACHABLE to STALE.

Note: An implementation may actually defer changing the state from

REACHABLE to STALE until a packet is sent to the neighbor, i.e.,

there need not be an explicit timeout event associated with the

expiration of ReachableTime.

The first time a node sends a packet to a neighbor whose entry is

STALE, the sender changes the state to DELAY and a sets a timer to

expire in DELAY_FIRST_PROBE_TIME seconds. If the entry is still in

the DELAY state when the timer expires, the entry's state changes to

PROBE. If reachability confirmation is received, the entry's state

changes to REACHABLE.

Upon entering the PROBE state, a node sends a unicast Neighbor

Solicitation message to the neighbor using the cached link-layer

address. While in the PROBE state, a node retransmits Neighbor

Solicitation messages every RetransTimer milliseconds until

reachability confirmation is obtained. Probes are retransmitted even

if no additional packets are sent to the neighbor. If no response is

received after waiting RetransTimer milliseconds after sending the

MAX_UNICAST_SOLICIT solicitations, retransmissions cease and the

entry SHOULD be deleted. Subsequent traffic to that neighbor will

recreate the entry and performs address resolution again.

Note that all Neighbor Solicitations are rate-limited on a per-

neighbor basis. A node MUST NOT send Neighbor Solicitations to the

same neighbor more frequently than once every RetransTimer

milliseconds.

A Neighbor Cache entry enters the STALE state when created as a

result of receiving packets other than solicited Neighbor

Advertisements (i.e., Router Solicitations, Router Advertisements,

Redirects, and Neighbor Solicitations). These packets contain the

link-layer address of either the sender or, in the case of Redirect,

the redirection target. However, receipt of these link-layer

addresses does not confirm reachability of the forward-direction path

to that node. Placing a newly created Neighbor Cache entry for which

the link-layer address is known in the STALE state provides assurance

that path failures are detected quickly. In addition, should a

cached link-layer address be modified due to receiving one of the

above messages the state SHOULD also be set to STALE to provide

prompt verification that the path to the new link-layer address is

working.

To properly detect the case where a router switches from being a

router to being a host (e.g., if its IP forwarding capability is

turned off by system management), a node MUST compare the Router flag

field in all received Neighbor Advertisement messages with the

IsRouter flag recorded in the Neighbor Cache entry. When a node

detects that a neighbor has changed from being a router to being a

host, the node MUST remove that router from the Default Router List

and update the Destination Cache as described in Section 6.3.5. Note

that a router may not be listed in the Default Router List, even

though a Destination Cache entry is using it (e.g., a host was

redirected to it). In such cases, all Destination Cache entries that

reference the (former) router must perform next-hop determination

again before using the entry.

In some cases, link-specific information may indicate that a path to

a neighbor has failed (e.g., the resetting of a virtual circuit). In

such cases, link-specific information may be used to purge Neighbor

Cache entries before the Neighbor Unreachability Detection would do

so. However, link-specific information MUST NOT be used to confirm

the reachability of a neighbor; such information does not provide

end-to-end confirmation between neighboring IP layers.

8. REDIRECT FUNCTION

This section describes the functions related to the sending and

processing of Redirect messages.

Redirect messages are sent by routers to redirect a host to a better

first-hop router for a specific destination or to inform hosts that a

destination is in fact a neighbor (i.e., on-link). The latter is

accomplished by having the ICMP Target Address be equal to the ICMP

Destination Address.

A router MUST be able to determine the link-local address for each of

its neighboring routers in order to ensure that the target address in

a Redirect message identifies the neighbor router by its link-local

address. For static routing this requirement implies that the next-

hop router's address should be specified using the link-local address

of the router. For dynamic routing this requirement implies that all

IPv6 routing protocols must somehow exchange the link-local addresses

of neighboring routers.

8.1. Validation of Redirect Messages

A host MUST silently discard any received Redirect message that does

not satisfy all of the following validity checks:

- IP Source Address is a link-local address. Routers must use their

link-local address as the source for Router Advertisement and

Redirect messages so that hosts can uniquely identify routers.

- The IP Hop Limit field has a value of 255, i.e., the packet could

not possibly have been forwarded by a router.

- If the message includes an IP Authentication Header, the message

authenticates correctly.

- ICMP Checksum is valid.

- ICMP Code is 0.

- ICMP length (derived from the IP length) is 40 or more octets.

- The IP source address of the Redirect is the same as the current

first-hop router for the specified ICMP Destination Address.

- The ICMP Destination Address field in the redirect message does not

contain a multicast address.

- The ICMP Target Address is either a link-local address (when

redirected to a router) or the same as the ICMP Destination Address

(when redirected to the on-link destination).

- All included options have a length that is greater than zero.

The contents of the Reserved field, and of any unrecognized options

MUST be ignored. Future, backward-compatible changes to the protocol

may specify the contents of the Reserved field or add new options;

backward-incompatible changes may use different Code values.

The contents of any defined options that are not specified to be used

with Redirect messages MUST be ignored and the packet processed as

normal. The only defined options that may appear are the Target

Link-Layer Address option and the Redirected Header option.

A host MUST NOT consider a redirect invalid just because the Target

Address of the redirect is not covered under one of the link's

prefixes. Part of the semantics of the Redirect message is that the

Target Address is on-link.

A redirect that passes the validity checks is called a "valid

redirect".

8.2. Router Specification

A router SHOULD send a redirect message, subject to rate limiting,

whenever it forwards a packet that is not explicitly addressed to

itself (i.e. a packet that is not source routed through the router)

in which:

- the Source Address field of the packet identifies a neighbor, and

- the router determines that a better first-hop node resides on the

same link as the sending node for the Destination Address of the

packet being forwarded, and

- the Destination Address of the packet is not a multicast address,

and

The transmitted redirect packet contains, consistent with the message

format given in Section 4.5:

- In the Target Address field: the address to which subsequent

packets for the destination SHOULD be sent. If the target is a

router, that router's link-local address MUST be used. If the

target is a host the target address field MUST be set to the same

value as the Destination Address field.

- In the Destination Address field: the destination address of the

invoking IP packet.

- In the options:

o Target Link-Layer Address option: link-layer address of the

target, if known.

o Redirected Header: as much of the forwarded packet as can fit

without the redirect packet exceeding 576 octets in size.

A router MUST limit the rate at which Redirect messages are sent, in

order to limit the bandwidth and processing costs incurred by the

Redirect messages when the source does not correctly respond to the

Redirects, or the source chooses to ignore unauthenticated Redirect

messages. More details on the rate-limiting of ICMP error messages

can be found in [ICMPv6].

A router MUST NOT update its routing tables upon receipt of a

Redirect.

8.3. Host Specification

A host receiving a valid redirect SHOULD update its Destination Cache

accordingly so that subsequent traffic goes to the specified target.

If no Destination Cache entry exists for the destination, an

implementation SHOULD create such an entry.

If the redirect contains a Target Link-Layer Address option the host

either creates or updates the Neighbor Cache entry for the target.

In both cases the cached link-layer address is copied from the Target

Link-Layer Address option. If a Neighbor Cache entry is created for

the target its reachability state MUST be set to STALE as specified

in Section 7.3.3. If a cache entry already existed and it is updated

with a different link-layer address its reachability state MUST also

be set to STALE.

In addition, if the Target Address is the same as the Destination

Address, the host MUST treat the destination as on-link and set the

IsRouter field in the corresponding Neighbor Cache entry to FALSE.

Otherwise it MUST set IsRouter to true.

Redirect messages apply to all flows that are being sent to a given

destination. That is, upon receipt of a Redirect for a Destination

Address, all Destination Cache entries to that address should be

updated to use the specified next-hop, regardless of the contents of

the Flow Label field that appears in the Redirected Header option.

A host MAY have a configuration switch that can be set to make it

ignore a Redirect message that does not have an IP Authentication

header.

A host MUST NOT send Redirect messages.

9. EXTENSIBILITY - OPTION PROCESSING

Options provide a mechanism for encoding variable length fields,

fields that may appear multiple times in the same packet, or

information that may not appear in all packets. Options can also be

used to add additional functionality to future versions of ND.

In order to ensure that future extensions properly coexist with

current implementations, all nodes MUST silently ignore any options

they do not recognize in received ND packets and continue processing

the packet. All options specified in this document MUST be

recognized. A node MUST NOT ignore valid options just because the ND

message contains unrecognized ones.

The current set of options is defined in such a way that receivers

can process multiple options in the same packet independently of each

other. In order to maintain these properties future options SHOULD

follow the simple rule:

The option MUST NOT depend on the presence or absence of any other

options. The semantics of an option should depend only on the

information in the fixed part of the ND packet and on the

information contained in the option itself.

Adhering to the above rule has the following benefits:

1) Receivers can process options independently of one another. For

example, an implementation can choose to process the Prefix

Information option contained in a Router Advertisement message in a

user-space process while the link-layer address option in the same

message is processed by routines in the kernel.

2) Should the number of options cause a packet to exceed a link's MTU,

multiple packets can carry subsets of the options without any

change in semantics.

3) Senders MAY send a subset of options in different packets. For

instance, if a prefix's Valid and Preferred Lifetime are high

enough, it might not be necessary to include the Prefix Information

option in every Router Advertisement. In addition, different

routers might send different sets of options. Thus, a receiver

MUST NOT associate any action with the absence of an option in a

particular packet. This protocol specifies that receivers should

only act on the expiration of timers and on the information that is

received in the packets.

Options in Neighbor Discovery packets can appear in any order;

receivers MUST be prepared to process them independently of their

order. There can also be multiple instances of the same option in a

message (e.g., Prefix Information options).

If the number of included options in a Router Advertisement causes

the advertisement's size to exceed the link MTU, the router can send

multiple separate advertisements each containing a subset of the

options.

The amount of data to include in the Redirected Header option MUST be

limited so that the entire redirect packet does not exceed 576

octets.

All options are a multiple of 8 octets of length, ensuring

appropriate alignment without any "pad" options. The fields in the

options (as well as the fields in ND packets) are defined to align on

their natural boundaries (e.g., a 16-bit field is aligned on a 16-bit

boundary) with the exception of the 128-bit IP addresses/prefixes,

which are aligned on a 64-bit boundary. The link-layer address field

contains an uninterpreted octet string; it is aligned on an 8-bit

boundary.

The size of an ND packet including the IP header is limited to the

link MTU (which is at least 576 octets). When adding options to an

ND packet a node MUST NOT exceed the link MTU.

Future versions of this protocol may define new option types.

Receivers MUST silently ignore any options they do not recognize and

continue processing the message.

10. PROTOCOL CONSTANTS

Router constants:

MAX_INITIAL_RTR_ADVERT_INTERVAL 16 seconds

MAX_INITIAL_RTR_ADVERTISEMENTS 3 transmissions

MAX_FINAL_RTR_ADVERTISEMENTS 3 transmissions

MIN_DELAY_BETWEEN_RAS 3 seconds

MAX_RA_DELAY_TIME .5 seconds

Host constants:

MAX_RTR_SOLICITATION_DELAY 1 second

RTR_SOLICITATION_INTERVAL 4 seconds

MAX_RTR_SOLICITATIONS 3 transmissions

Node constants:

MAX_MULTICAST_SOLICIT 3 transmissions

MAX_UNICAST_SOLICIT 3 transmissions

MAX_ANYCAST_DELAY_TIME 1 second

MAX_NEIGHBOR_ADVERTISEMENT 3 transmissions

REACHABLE_TIME 30,000 milliseconds

RETRANS_TIMER 1,000 milliseconds

DELAY_FIRST_PROBE_TIME 5 seconds

MIN_RANDOM_FACTOR .5

MAX_RANDOM_FACTOR 1.5

Additional protocol constants are defined with the message formats in

Section 4.

All protocol constants are subject to change in future revisions of

the protocol.

The constants in this specification may be overridden by specific

documents that describe how IPv6 operates over different link layers.

This rule allows Neighbor Discovery to operate over links with widely

varying performance characteristics.

11. SECURITY CONSIDERATIONS

Neighbor Discovery is subject to attacks that cause IP packets to

flow to unexpected places. Such attacks can be used to cause denial

of service but also allow nodes to intercept and optionally modify

packets destined for other nodes.

The protocol reduces the exposure to such threats in the absence of

authentication by ignoring ND packets received from off-link senders.

The Hop Limit field of all received packets is verified to contain

255, the maximum legal value. Because routers decrement the Hop

Limit on all packets they forward, received packets containing a Hop

Limit of 255 must have originated from a neighbor.

The trust model for redirects is the same as in IPv4. A redirect is

accepted only if received from the same router that is currently

being used for that destination. It is natural to trust the routers

on the link. If a host has been redirected to another node (i.e.,

the destination is on-link) there is no way to prevent the target

from issuing another redirect to some other destination. However,

this exposure is no worse than it was; the target host, once

subverted, could always act as a hidden router to forward traffic

elsewhere.

The protocol contains no mechanism to determine which neighbors are

authorized to send a particular type of message e.g. Router

Advertisements; any neighbor, presumably even in the presence of

authentication, can send Router Advertisement messages thereby being

able to cause denial of service. Furthermore, any neighbor can send

proxy Neighbor Advertisements as well as unsolicited Neighbor

Advertisements as a potential denial of service attack.

Neighbor Discovery protocol packet exchanges can be authenticated

using the IP Authentication Header [IPv6-AUTH]. A node SHOULD

include an Authentication Header when sending Neighbor Discovery

packets if a security association for use with the IP Authentication

Header exists for the destination address. The security associations

may have been created through manual configuration or through the

operation of some key management protocol.

Received Authentication Headers in Neighbor Discovery packets MUST be

verified for correctness and packets with incorrect authentication

MUST be ignored.

It SHOULD be possible for the system administrator to configure a

node to ignore any Neighbor Discovery messages that are not

authenticated using either the Authentication Header or Encapsulating

Security Payload. The configuration technique for this MUST be

documented. Such a switch SHOULD default to allowing unauthenticated

messages.

Confidentiality issues are addressed by the IP Security Architecture

and the IP Encapsulating Security Payload documents [IPv6-SA, IPv6-

ESP].

REFERENCES

[ADDRCONF] Thomson, S., and T. Narten, "IPv6 Address

Autoconfiguration", RFC1971, August 1996.

[ADDR-ARCH] Deering, S., and R. Hinden, Editors, "IP Version 6

Addressing Architecture", RFC1884, January 1996.

[ANYCST] Partridge, C., Mendez, T., and W. Milliken, "Host

Anycasting Service", RFC1546, November 1993.

[ARP] Plummer, D., "An Ethernet Address Resolution Protocol", STD

37, RFC826, November 1982.

[HR-CL] Braden, R., Editor, "Requirements for Internet Hosts --

Communication Layers", STD 3, RFC1122, October 1989.

[ICMPv4] Postel, J., "Internet Control Message Protocol", STD 5, RFC

792, September 1981.

[ICMPv6] Conta, A., and S. Deering, "Internet Control Message

Protocol (ICMPv6) for the Internet Protocol Version 6

(IPv6)", RFC1885, January 1996.

[IPv6] Deering, S., and R. Hinden, Editors, "Internet Protocol,

Version 6 (IPv6) Specification", RFC1883, January, 1996.

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

Packets over Ethernet Networks", RFC1972, August 1996.

[IPv6-SA] Atkinson, R., "Security Architecture for the Internet

Protocol", RFC1825, August 1995.

[IPv6-AUTH] Atkinson, R., "IP Authentication Header", RFC1826,

August 1995.

[IPv6-ESP] Atkinson, R., "IP Encapsulating Security Payload (ESP)",

RFC1827, August 1995.

[RDISC] Deering, S., "ICMP Router Discovery Messages", RFC1256,

September 1991.

[SH-MEDIA] Braden, R., Postel, J., and Y. Rekhter, "Internet

Architecture Extensions for Shared Media", RFC1620, May

1994.

[ASSIGNED] Reynolds, J., and J. Postel, "ASSIGNED NUMBERS", STD 2,

RFC1700, October 1994.

[SYNC] S. Floyd, V. Jacobsen, "The Synchronization of Periodic Routing

Messages", IEEE/ACM Transactions on Networking, April 1994.

FTP://ftp.ee.lbl.gov/papers/sync_94.ps.Z

AUTHORS' ADDRESSES

Erik Nordmark Thomas Narten

Sun Microsystems, Inc. IBM Corporation

2550 Garcia Ave P.O. Box 12195

Mt. View, CA 94041 Research Triangle Park, NC 27709-2195

USA USA

Phone: +1 415 786 5166 Phone: +1 919 254 7798

Fax: +1 415 786 5896 Fax: +1 919 254 4027

EMail: nordmark@sun.com EMail: narten@vnet.ibm.com

William Allen Simpson

Daydreamer

Computer Systems Consulting Services

1384 Fontaine

Madison Heights, Michigan 48071

USA

EMail: Bill.Simpson@um.cc.umich.edu

bsimpson@MorningStar.com

APPENDIX A: MULTIHOMED HOSTS

There are a number of complicating issues that arise when Neighbor

Discovery is used by hosts that have multiple interfaces. This

section does not attempt to define the proper operation of multihomed

hosts with regard to Neighbor Discovery. Rather, it identifies

issues that require further study. Implementors are encouraged to

experiment with various approaches to making Neighbor Discovery work

on multihomed hosts and to report their experiences.

If a multihomed host receives Router Advertisements on all of its

interfaces, it will (probably) have learned on-link prefixes for the

addresses residing on each link. When a packet must be sent through

a router, however, selecting the "wrong" router can result in a

suboptimal or non-functioning path. There are number of issues to

consider:

1) In order for a router to send a redirect, it must determine that

the packet it is forwarding originates from a neighbor. The

standard test for this case is to compare the source address of the

packet to the list of on-link prefixes associated with the

interface on which the packet was received. If the originating

host is multihomed, however, the source address it uses may belong

to an interface other than the interface from which it was sent.

In such cases, a router will not send redirects, and suboptimal

routing is likely. In order to be redirected, the sending host

must always send packets out the interface corresponding to the

outgoing packet's source address. Note that this issue never

arises with non-multihomed hosts; they only have one interface.

2) If the selected first-hop router does not have a route at all for

the destination, it will be unable to deliver the packet. However,

the destination may be reachable through a router on one of the

other interfaces. Neighbor Discovery does not address this

scenario; it does not arise in the non-multihomed case.

3) Even if the first-hop router does have a route for a destination,

there may be a better route via another interface. No mechanism

exists for the multihomed host to detect this situation.

If a multihomed host fails to receive Router Advertisements on one or

more of its interfaces, it will not know (in the absence of

configured information) which destinations are on-link on the

affected interface(s). This leads to a number of problems:

1) If no Router Advertisement is received on any interfaces, a

multihomed host will have no way of knowing which interface to send

packets out on, even for on-link destinations. Under similar

conditions in the non-multihomed host case, a node treats all

destinations as residing on-link, and communication proceeds. In

the multihomed case, however, additional information is needed to

select the proper outgoing interface. Alternatively, a node could

attempt to perform address resolution on all interfaces, a step

involving significant complexity that is not present in the non-

multihomed host case.

2) If Router Advertisements are received on some, but not all

interfaces, a multihomed host could choose to only send packets out

on the interfaces on which it has received Router Advertisements.

A key assumption made here, however, is that routers on those other

interfaces will be able to route packets to the ultimate

destination, even when those destinations reside on the subnet to

which the sender connects, but has no on-link prefix information.

Should the assumption be false, communication would fail. Even if

the assumption holds, packets will traverse a sub-optimal path.

APPENDIX B: FUTURE EXTENSIONS

Possible extensions for future study are:

o Using dynamic timers to be able to adapt to links with widely varying

delay. Measuring round trip times, however, requires acknowledgments

and sequence numbers in order to match received Neighbor

Advertisements with the actual Neighbor Solicitation that triggered

the advertisement. Implementors wishing to experiment with such a

facility could do so in a backwards-compatible way by defining a new

option carrying the necessary information. Nodes not understanding

the option would simply ignore it.

o Adding capabilities to facilitate the operation over links that

currently require hosts to register with an address resolution

server. This could for instance enable routers to ask hosts to send

them periodic unsolicited advertisements. Once again this can be

added using a new option sent in the Router Advertisements.

o Adding additional procedures for links where asymmetric and non-

transitive reachability is part of normal operations. Such

procedures might allow hosts and routers to find usable paths on,

e.g., radio links.

APPENDIX C: STATE MACHINE FOR THE REACHABILITY STATE

This appendix contains a summary of the rules specified in Sections

7.2 and 7.3. This document does not mandate that implementations

adhere to this model as long as their external behavior is consistent

with that described in this document.

When performing address resolution and Neighbor Unreachability

Detection the following state transitions apply using the conceptual

model:

State Event Action New state

- Packet to send. Create entry. INCOMPLETE

Send multicast NS.

Start retransmit timer

INCOMPLETE Retransmit timeout, Retransmit NS INCOMPLETE

less than N Start retransmit timer

retransmissions.

INCOMPLETE Retransmit timeout, Discard entry -

N or more Send ICMP error

retransmissions.

INCOMPLETE NA, Solicited=0, Record link-layer STALE

Override=any address. Send queued

packets.

INCOMPLETE NA, Solicited=1, Record link-layer REACHABLE

Override=any address. Send queued

packets.

!INCOMPLETE NA, Solicited=1, - REACHABLE

Override=0

!INCOMPLETE NA, Solicited=1, Record link-layer REACHABLE

Override=1 address.

!INCOMPLETE NA, Solicited=0, - STALE

Override=0

!INCOMPLETE NA, Solicited=0, Record link-layer STALE

Override=1 address.

!INCOMPLETE upper-layer reachability - REACHABLE

confirmation

REACHABLE timeout, more than - STALE

N seconds since

reachability confirm.

STALE Sending packet Start delay timer DELAY

DELAY Delay timeout Send unicast NS probe PROBE

Start retransmit timer

PROBE Retransmit timeout, Retransmit NS PROBE

less than N

retransmissions.

PROBE Retransmit timeout, Discard entry -

N or more

retransmissions.

The state transitions for receiving unsolicited information other

than Neighbor Advertisement messages apply to either the source of

the packet (for Neighbor Solicitation, Router Solicitation, and

Router Advertisement messages) or the target address (for Redirect

messages) as follows:

State Event Action New state

- NS, RS, RA, Redirect Create entry. STALE

INCOMPLETE NS, RS, RA, Redirect Record link-layer STALE

address. Send queued

packets.

!INCOMPLETE NS, RS, RA, Redirect Update link-layer STALE

Different link-layer address

address than cached.

!INCOMPLETE NS, RS, RA, Redirect - unchanged

Same link-layer

address as cached.

APPENDIX D: IMPLEMENTATION ISSUES

Appendix D.1: Reachability confirmations

Neighbor Unreachability Detection requires explicit confirmation that

a forward-path is functioning properly. To avoid the need for

Neighbor Solicitation probe messages, upper layer protocols should

provide such an indication when the cost of doing so is small.

Reliable connection-oriented protocols such as TCP are generally

aware when the forward-path is working. When TCP sends (or receives)

data, for instance, it updates its window sequence numbers, sets and

cancels retransmit timers, etc. Specific scenarios that usually

indicate a properly functioning forward-path include:

- Receipt of an acknowledgement that covers a sequence number (e.g.,

data) not previously acknowledged indicates that the forward path was

working at the time the data was sent.

- Completion of the initial three-way handshake is a special case of the

previous rule; although no data is sent during the handshake, the SYN

flags are counted as data from the sequence number perspective. This

applies to both the SYN+ACK for the active open the ACK of that

packet on the passively opening peer.

- Receipt of new data (i.e., data not previously received) indicates

that the forward-path was working at the time an acknowledgement was

sent that advanced the peer's send window that allowed the new data

to be sent.

To minimize the cost of communicating reachability information

between the TCP and IP layers, an implementation may wish to rate-

limit the reachability confirmations its sends IP. One possibility

is to process reachability only every few packets. For example, one

might update reachability information once per round trip time, if an

implementation only has one round trip timer per connection. For

those implementations that cache Destination Cache entries within

control blocks, it may be possible to update the Neighbor Cache entry

directly (i.e., without an expensive lookup) once the TCP packet has

been demultiplexed to its corresponding control block. For other

implementation it may be possible to piggyback the reachability

confirmation on the next packet submitted to IP assuming that the

implementation guards against the piggybacked confirmation becoming

stale when no packets are sent to IP for an extended period of time.

TCP must also guard against thinking "stale" information indicates

current reachability. For example, new data received 30 minutes

after a window has opened up does not constitute a confirmation that

the path is currently working. In merely indicates that 30 minutes

ago the window update reached the peer i.e. the path was working at

that point in time. An implementation must also take into account

TCP zero-window probes that are sent even if the path is broken and

the window update did not reach the peer.

For UDP based applications (RPC, DNS) it is relatively simple to make

the client send reachability confirmations when the response packet

is received. It is more difficult and in some cases impossible for

the server to generate such confirmations since there is no flow

control, i.e., the server can not determine whether a received

request indicates that a previous response reached the client.

Note that an implementation can not use negative upper-layer advise

as a replacement for the Neighbor Unreachability Detection algorithm.

Negative advise (e.g. from TCP when there are excessive

retransmissions) could serve as a hint that the forward path from the

sender of the data might not be working. But it would fail to detect

when the path from the receiver of the data is not functioning

causing, none of the acknowledgement packets to reach the

dgement

 
 
 
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