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