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Network Working Group T.
Request for Comments: 2461
Obsoletes: 1970 E.
Category: Standards Track Sun
W.

December 1998

Neighbor Discovery for IP Version 6 (IPv6)

Status of this

This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is unlimited

Copyright

Copyright (C) The Internet Society (1998). All Rights Reserved

This document specifies the Neighbor Discovery protocol for
Version 6. IPv6 nodes on the same link use Neighbor Discovery
discover each other's presence, to determine each other's link-
addresses, to find routers and to maintain reachability
about the paths to active neighbors

Table of

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........................................ 9
3.1. Comparison with IPv4................................ 13
3.2. Supported Link Types................................ 15
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............................. 26

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RFC 2461 Neighbor Discovery for IPv6 December 1998

4.6. Option Formats...................................... 28
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............................................ 32
5. CONCEPTUAL MODEL OF A HOST............................... 33
5.1. Conceptual Data Structures.......................... 33
5.2. Conceptual Sending Algorithm........................ 35
5.3. Garbage Collection and Timeout Requirements......... 37
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.... 39
6.2. Router Specification................................ 40
6.2.1. Router Configuration Variables................. 40
6.2.2. Becoming An Advertising Interface.............. 44
6.2.3. Router Advertisement Message Content........... 44
6.2.4. Sending Unsolicited Router Advertisements...... 46
6.2.5. Ceasing To Be An Advertising Interface......... 46
6.2.6. Processing Router Solicitations................ 47
6.2.7. Router Advertisement Consistency............... 48
6.2.8. Link-local Address Change...................... 49
6.3. Host Specification.................................. 50
6.3.1. Host Configuration Variables................... 50
6.3.2. Host Variables................................. 50
6.3.3. Interface Initialization....................... 51
6.3.4. Processing Received Router Advertisements...... 51
6.3.5. Timing out Prefixes and Default Routers........ 54
6.3.6. Default Router Selection....................... 54
6.3.7. Sending Router Solicitations................... 55
7. ADDRESS RESOLUTION AND NEIGHBOR UNREACHABILITY DETECTION. 56
7.1. Message Validation.................................. 57
7.1.1. Validation of Neighbor Solicitations........... 57
7.1.2. Validation of Neighbor Advertisements.......... 58
7.2. Address Resolution.................................. 58
7.2.1. Interface Initialization....................... 59
7.2.2. Sending Neighbor Solicitations................. 59
7.2.3. Receipt of Neighbor Solicitations.............. 60
7.2.4. Sending Solicited Neighbor Advertisements...... 61
7.2.5. Receipt of Neighbor Advertisements............. 62
7.2.6. Sending Unsolicited Neighbor Advertisements.... 64
7.2.7. Anycast Neighbor Advertisements................ 65
7.2.8. Proxy Neighbor Advertisements.................. 65
7.3. Neighbor Unreachability Detection................... 66
7.3.1. Reachability Confirmation...................... 66
7.3.2. Neighbor Cache Entry States.................... 67
7.3.3. Node Behavior.................................. 68
8. REDIRECT FUNCTION........................................ 70

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RFC 2461 Neighbor Discovery for IPv6 December 1998

8.1. Validation of Redirect Messages..................... 71
8.2. Router Specification................................ 72
8.3. Host Specification.................................. 73
9. EXTENSIBILITY - OPTION PROCESSING........................ 74
10. PROTOCOL CONSTANTS...................................... 75
11. SECURITY CONSIDERATIONS................................. 76
12. RENUMBERING CONSIDERATIONS.............................. 78

References................................................... 80
Authors' Addresses........................................... 81
Appendix A: Multihomed Hosts................................. 82
Appendix B: Future Extensions................................ 84
Appendix C: State Machine for the Reachability State......... 85
Appendix D: Summary of ISROUTER Rules........................ 88
Appendix E: Implementation Issues............................ 89
Appendix E.1: Reachability confirmations................. 89
Appendix F: Changes since RFC 1970........................... 91
Full Copyright Statement..................................... 93

1.

This specification defines the Neighbor Discovery (ND) protocol
Internet Protocol Version 6 (IPv6). Nodes (hosts and routers)
Neighbor Discovery to determine the link-layer addresses
neighbors known to reside on attached links and to quickly
cached values that become invalid. Hosts also use Neighbor
to find neighboring routers that are willing to forward packets
their behalf. Finally, nodes use the protocol to actively keep
of which neighbors are reachable and which are not, and to
changed link-layer addresses. When a router or the path to a
fails, a host actively searches for functioning alternates

Unless specified otherwise (in a document that covers operating
over a particular link type) this document applies to all link types
However, because ND uses link-layer multicast for some of
services, it is possible that on some link types (e.g., NBMA links
alternative protocols or mechanisms to implement those services
be specified (in the appropriate document covering the operation
IP over a particular link type). The services described in
document that are not directly dependent on multicast, such
Redirects, Next-hop determination, Neighbor Unreachability Detection
etc., are expected to be provided as specified in this document.
details of how one uses ND on NBMA links is an area for
study

The authors would like to acknowledge the contributions of the
working group and, in particular, (in alphabetical order)
Atkinson, Jim Bound, Scott Bradner, Alex Conta, Stephen Deering

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RFC 2461 Neighbor Discovery for IPv6 December 1998

Richard Draves, Francis Dupont, Robert Elz, Robert Gilligan,
Hinden, Allison Mankin, Dan McDonald, Charles Perkins, Matt Thomas
and Susan Thomson

2.

2.1.

IP - Internet Protocol Version 6. The terms IPv4
IPv6 are used only in contexts where necessary to
ambiguity

ICMP - Internet Message Control Protocol for the
Protocol Version 6. The terms ICMPv4 and ICMPv6
used only in contexts where necessary to
ambiguity

node - a device that implements IP

router - a node that forwards IP packets not
addressed to itself

host - any node that is not a router

upper layer - a protocol layer immediately above IP. Examples
transport protocols such as TCP and UDP,
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
communicate at the link layer, i.e., the
immediately below IP. Examples are Ethernets (
or bridged), PPP links, X.25, Frame Relay, or
networks as well as internet (or higher)
"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
interfaces

anycast
- an identifier for a set of interfaces (
belonging to different nodes). A packet sent to

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RFC 2461 Neighbor Discovery for IPv6 December 1998

anycast address is delivered to one of the
identified by that address (the "nearest" one
according to the routing protocol's measure
distance). See [ADDR-ARCH].

Note that an anycast address is
indistinguishable from a unicast address. Thus,
sending packets to anycast addresses don't
know that an anycast address is being used.
the rest of this document, references to
addresses also apply to anycast addresses in
cases where the node is unaware that a unicast
is actually an anycast address

prefix - a bit string that consists of some number of
bits of an address

link-layer
- a link-layer identifier for an interface.
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
specified link. A node considers an address to be on
link if

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

- a neighboring router specifies the address
the target of a Redirect message,

- a Neighbor Advertisement message is received
the (target) address,

- any Neighbor Discovery message is received
the address

off-link - the opposite of "on-link"; an address that is
assigned to any interfaces on the specified link

longest prefix
- The process of determining which prefix (if any)
a set of prefixes covers a target address. A
address is covered by a prefix if all of the bits
the prefix match the left-most bits of the
address. When multiple prefixes cover an address
the longest prefix is the one that matches

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RFC 2461 Neighbor Discovery for IPv6 December 1998

- whether or not the one-way "forward" path to
neighbor is functioning properly. In particular
whether packets sent to a neighbor are reaching
IP layer on the neighboring machine and are
processed properly by the receiving IP layer.
neighboring routers, reachability means that
sent by a node's IP layer are delivered to
router's IP layer, and the router is
forwarding packets (i.e., it is configured as
router, not a host). For hosts, reachability
that packets sent by a node's IP layer are
to the neighbor host's IP layer

packet - an IP header plus payload

link MTU - the maximum transmission unit, i.e., maximum
size in octets, that can be conveyed in one
over a link

target - an address about which address
information is sought, or an address which is
new first-hop when being redirected

proxy - a router that responds to Neighbor Discovery
messages on behalf of another node. A router
on behalf of a mobile node that has moved off-
could potentially act as a proxy for the
node

ICMP destination unreachable
- an error indication returned to the original
of a packet that cannot be delivered for the
outlined in [ICMPv6]. If the error occurs on a
other than the node originating the packet, an
error message is generated. If the error occurs
the originating node, an implementation is
required to actually create and send an ICMP
packet to the source, as long as the upper-
sender is notified through an appropriate
(e.g., return value from a procedure call). Note
however, that an implementation may find
convenient in some cases to return errors to
sender by taking the offending packet, generating
ICMP error message, and then delivering it (locally
through the generic error handling routines

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RFC 2461 Neighbor Discovery for IPv6 December 1998

random
- when sending out messages, it is sometimes necessary
delay a transmission for a random amount of time
order to prevent multiple nodes from transmitting
exactly the same time, or to prevent long-
periodic transmissions from synchronizing with
other [SYNC]. When a random component is required,
node calculates the actual delay in such a way that
computed delay forms a uniformly-distributed
value that falls between the specified minimum
maximum delay times. The implementor must take care
insure that the granularity of the calculated
component and the resolution of the timer used are
high enough to insure that the probability of
nodes delaying the same amount of time is small

random delay
- If a pseudo-random number generator is used
calculating a random delay component, the
should be initialized with a unique seed prior to
used. Note that it is not sufficient to use
interface token alone as the seed, since
tokens will not always be unique. To reduce
probability that duplicate interface tokens cause
same seed to be used, the seed should be
from a variety of input sources (e.g.,
components) that are likely to be different even
identical "boxes". For example, the seed could
formed by combining the CPU's serial number with
interface token

2.2. Link

Different link layers have different properties. The ones of
to Neighbor Discovery are

multicast - a link that supports a native mechanism at
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.
point-to-point link is assumed to have
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
or broadcast (e.g., X.25, ATM, frame relay, etc.).

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RFC 2461 Neighbor Discovery for IPv6 December 1998

Note that all link types (including NBMA)
expected to provide multicast service for IP (e.g.,
using multicast servers), but it is an issue
further study whether ND should use such
or an alternate mechanism that provides
equivalent ND services

shared media - a link that allows direct communication among
number of nodes, but attached nodes are
in such a way that they do not have complete
information for all on-link destinations. That is
at the IP level, nodes on the same link may not
that they are neighbors; by default,
communicate through a router. Examples are
(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
describing how to run IP over the link layer

asymmetric
- a link where non-reflexive and/or non-
reachability is part of normal operation. (Non
reflexive reachability means packets from A reach
but packets from B don't reach A. Non-
reachability means packets from A reach B,
packets from B reach C, but packets from A don'
reach C.) Many radio links exhibit
properties

2.3.

Neighbor Discovery makes use of a number of different
defined in [ADDR-ARCH], including

all-nodes multicast
- the link-local scope address to reach all nodes
FF02::1

all-routers multicast
- the link-local scope address to reach all routers
FF02::2

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RFC 2461 Neighbor Discovery for IPv6 December 1998

solicited-node multicast
- a link-local scope multicast address that is
as a function of the solicited target's address.
function is described in [ADDR-ARCH]. The function
chosen so that IP addresses which differ only in
high-order bits, e.g., due to multiple high-
prefixes associated with different providers, will
to the same solicited-node address thereby reducing
number of multicast addresses a node must join

link-local
- a unicast address having link-only scope that can
used to reach neighbors. All interfaces on
MUST have a link-local address. Also, [ADDRCONF
requires that interfaces on hosts have a link-
address

unspecified
- a reserved address value that indicates the lack of
address (e.g., the address is unknown). It is
used as a destination address, but may be used as
source address if the sender does not (yet) know
own address (e.g., while verifying an address is
during address autoconfiguration [ADDRCONF]).
unspecified address has a value of 0:0:0:0:0:0:0:0.

2.4.

The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in
document, are to be interpreted as described in [KEYWORDS].

This document also makes use of internal conceptual variables
describe protocol behavior and external variables that
implementation must allow system administrators to change.
specific variable names, how their values change, and how
settings influence protocol behavior are provided to
protocol behavior. An implementation is not required to have them
the exact form described here, so long as its external behavior
consistent with that described in this document

3. PROTOCOL

This protocol solves a set of problems related to the
between nodes attached to the same link. It defines mechanisms
solving each of the following problems

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RFC 2461 Neighbor Discovery for IPv6 December 1998

Router Discovery: How hosts locate routers that reside on
attached link

Prefix Discovery: How hosts discover the set of address
that define which destinations are on-link for
attached link. (Nodes use prefixes to
destinations that reside on-link from those
reachable through a router.)

Parameter Discovery: How a node learns such link parameters as
link MTU or such Internet parameters as the hop
value to place in outgoing packets

Address Autoconfiguration: How nodes automatically configure
address for an interface

Address resolution: How nodes determine the link-layer address
an on-link destination (e.g., a neighbor) given only
destination's IP address

Next-hop determination: The algorithm for mapping an IP
address into the IP address of the neighbor to
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
neighbor is no longer reachable. For neighbors used
routers, alternate default routers can be tried.
both routers and hosts, address resolution can
performed again

Duplicate Address Detection: How a node determines that an
it wishes to use is not already in use by another node

Redirect: How a router informs a host of a better first-hop
to reach a particular destination

Neighbor Discovery defines five different ICMP packet types: A
of Router Solicitation and Router Advertisement messages, a pair
Neighbor Solicitation and Neighbor Advertisements messages, and
Redirect message. The messages serve the following purpose

Router Solicitation: When an interface becomes enabled, hosts
send out Router Solicitations that request routers
generate Router Advertisements immediately rather
at their next scheduled time

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RFC 2461 Neighbor Discovery for IPv6 December 1998

Router Advertisement: Routers advertise their presence
with various link and Internet parameters
periodically, or in response to a Router
message. Router Advertisements contain prefixes
are used for on-link determination and/or
configuration, a suggested hop limit value, etc

Neighbor Solicitation: Sent by a node to determine the link-
address of a neighbor, or to verify that a neighbor
still reachable via a cached link-layer address
Neighbor Solicitations are also used for
Address Detection

Neighbor Advertisement: A response to a Neighbor
message. A node may also send unsolicited
Advertisements to announce a link-layer address change

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

On multicast-capable links, each router periodically multicasts
Router Advertisement packet announcing its availability. A
receives Router Advertisements from all routers, building a list
default routers. Routers generate Router Advertisements
enough that hosts will learn of their presence within a few minutes
but not frequently enough to rely on an absence of advertisements
detect router failure; a separate Neighbor Unreachability
algorithm provides failure detection

Router Advertisements contain a list of prefixes used for on-
determination and/or autonomous address configuration;
associated with the prefixes specify the intended uses of
particular prefix. Hosts use the advertised on-link prefixes
build and maintain a list that is used in deciding when a packet'
destination is on-link or beyond a router. Note that a
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
the sender that the destination is a neighbor

Router Advertisements (and per-prefix flags) allow routers to
hosts how to perform Address Autoconfiguration. For example,
can specify whether hosts should use stateful (DHCPv6) and/
autonomous (stateless) address configuration. The exact
and usage of the address configuration-related information
specified in [ADDRCONF].

Router Advertisement messages also contain Internet parameters
as the hop limit that hosts should use in outgoing packets and

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RFC 2461 Neighbor Discovery for IPv6 December 1998

optionally, link parameters such as the link MTU. This
centralized administration of critical parameters that can be set
routers and automatically propagated to all attached hosts

Nodes accomplish address resolution by multicasting a
Solicitation that asks the target node to return its link-
address. Neighbor Solicitation messages are multicast to
solicited-node multicast address of the target address. The
returns its link-layer address in a unicast Neighbor
message. A single request-response pair of packets is sufficient
both the initiator and the target to resolve each other's link-
addresses; the initiator includes its link-layer address in
Neighbor Solicitation

Neighbor Solicitation messages can also be used to determine if
than one node has been assigned the same unicast address. The use
Neighbor Solicitation messages for Duplicate Address Detection
specified in [ADDRCONF].

Neighbor Unreachability Detection detects the failure of a
or the failure of the forward path to the neighbor. Doing
requires positive confirmation that packets sent to a neighbor
actually reaching that neighbor and being processed properly by
IP layer. Neighbor Unreachability Detection uses confirmation
two sources. When possible, upper-layer protocols provide a
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
confirmation is not forthcoming through such "hints", a node
unicast Neighbor Solicitation messages that solicit
Advertisements as reachability confirmation from the next hop.
reduce unnecessary network traffic, probe messages are only sent
neighbors to which the node is actively sending packets

In addition to addressing the above general problems,
Discovery also handles the following situations

Link-layer address change - A node that knows its link-
address has changed can multicast a few (unsolicited
Neighbor Advertisement packets to all nodes to quickly
cached link-layer addresses that have become invalid.
that the sending of unsolicited advertisements is
performance enhancement only (e.g., unreliable).
Neighbor Unreachability Detection algorithm ensures that
nodes will reliably discover the new address, though
delay may be somewhat longer

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RFC 2461 Neighbor Discovery for IPv6 December 1998

Inbound load balancing - Nodes with replicated interfaces may
to load balance the reception of incoming packets
multiple network interfaces on the same link. Such
have multiple link-layer addresses assigned to the
interface. For example, a single network driver
represent multiple network interface cards as a
logical interface having multiple link-layer addresses

Load balancing is handled by allowing routers to omit
source link-layer address from Router Advertisement packets
thereby forcing neighbors to use Neighbor
messages to learn link-layer addresses of routers.
Neighbor Advertisement messages can then contain link-
addresses that differ depending on who issued
solicitation

Anycast addresses - Anycast addresses identify one of a set
nodes providing an equivalent service, and multiple nodes
the same link may be configured to recognize the same
address. Neighbor Discovery handles anycasts by having
expect to receive multiple Neighbor Advertisements for
same target. All advertisements for anycast addresses
tagged as being non-Override advertisements. This
specific rules to determine which of potentially
advertisements should be used

Proxy advertisements - A router willing to accept packets on
of a target address that is unable to respond to
Solicitations can issue non-Override Neighbor Advertisements
There is currently no specified use of proxy, but
advertising could potentially be used to handle cases
mobile nodes that have moved off-link. However, it is
intended as a general mechanism to handle nodes that, e.g.,
do not implement this protocol

3.1. Comparison with IPv

The IPv6 Neighbor Discovery protocol corresponds to a combination
the IPv4 protocols ARP [ARP], ICMP Router Discovery [RDISC], and
Redirect [ICMPv4]. In IPv4 there is no generally agreed
protocol or mechanism for Neighbor Unreachability Detection,
Hosts Requirements [HR-CL] does specify some possible algorithms
Dead Gateway Detection (a subset of the problems
Unreachability Detection tackles).

The Neighbor Discovery protocol provides a multitude of
over the IPv4 set of protocols

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RFC 2461 Neighbor Discovery for IPv6 December 1998

Router Discovery is part of the base protocol set; there is
need for hosts to "snoop" the routing protocols

Router advertisements carry link-layer addresses; no
packet exchange is needed to resolve the router's link-
address

Router advertisements carry prefixes for a link; there is no
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
well-defined MTU

Address resolution multicasts are "spread" over 4 billion (2^32)
multicast addresses greatly reducing address resolution
interrupts on nodes other than the target. Moreover, non-IPv
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
redirect

Multiple prefixes can be associated with the same link.
default, hosts learn all on-link prefixes from
Advertisements. However, routers may be configured to omit
or all prefixes from Router Advertisements. In such cases
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
new next-hop is on-link. In IPv4, a host ignores
specifying a next-hop that is not on-link according to the link'
network mask. The IPv6 redirect mechanism is analogous to
XRedirect facility specified in [SH-MEDIA]. It is expected to
useful on non-broadcast and shared media links in which it
undesirable or not possible for nodes to know all prefixes
on-link destinations

Neighbor Unreachability Detection is part of the
significantly improving the robustness of packet delivery in
presence of failing routers, partially failing or
links and nodes that change their link-layer addresses.
instance, mobile nodes can move off-link without losing
connectivity due to stale ARP caches

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RFC 2461 Neighbor Discovery for IPv6 December 1998

Unlike ARP, Neighbor Discovery detects half-link failures (
Neighbor Unreachability Detection) and avoids sending traffic
neighbors with which two-way connectivity is absent

Unlike in IPv4 Router Discovery the Router Advertisement
do not contain a preference field. The preference field is
needed to handle routers of different "stability"; the
Unreachability Detection will detect dead routers and switch to
working one

The use of link-local addresses to uniquely identify routers (
Router Advertisement and Redirect messages) makes it possible
hosts to maintain the router associations in the event of the
renumbering to use new global prefixes

Using the Hop Limit equal to 255 trick Neighbor Discovery
immune to off-link senders that accidentally or intentionally
ND messages. In IPv4 off-link senders can send both
Redirects and Router Advertisement messages

Placing address resolution at the ICMP layer makes the
more media-independent than ARP and makes it possible to
standard IP authentication and security mechanisms as
[IPv6-AUTH, IPv6-ESP].

3.2. Supported Link

Neighbor Discovery supports links with different properties. In
presence of certain properties only a subset of the ND
mechanisms are fully specified in this document

point-to-point - Neighbor Discovery handles such links just
multicast links. (Multicast can be
provided on point to point links, and
can be assigned link-local addresses.)
Discovery should be implemented as described
this document

multicast - Neighbor Discovery should be implemented
described in this document

non-broadcast multiple access (NBMA
- Redirect, Neighbor Unreachability Detection
next-hop determination should be implemented
described in this document. Address resolution
and the mechanism for delivering
Solicitations and Advertisements on NBMA links
not specified in this document. Note that

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RFC 2461 Neighbor Discovery for IPv6 December 1998

hosts support manual configuration of a list
default routers, hosts can dynamically acquire
link-layer addresses for their neighbors
Redirect messages

shared media - The Redirect message is modeled after
XRedirect message in [SH-MEDIA] in order
simplify use of the protocol on shared
links

This specification does not address shared
issues that only relate to routers, such as

- How routers exchange reachability
on a shared media link

- How a router determines the link-layer
of a host, which it needs to send
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
of new options) so that other solutions might
possible in the future

variable MTU - Neighbor Discovery allows routers to specify a
for the link, which all nodes then use. All
on a link must use the same MTU (or
Receive Unit) in order for multicast to
properly. Otherwise when multicasting a sender
which can not know which nodes will receive
packet, could not determine a minimum packet
all receivers can process

asymmetric
- Neighbor Discovery detects the absence
symmetric reachability; a node avoids paths to
neighbor with which it does not have
connectivity

The Neighbor Unreachability Detection
typically identify such half-links and the
will refrain from using them

The protocol can presumably be extended in
future to find viable paths in environments

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RFC 2461 Neighbor Discovery for IPv6 December 1998

lack reflexive and transitive connectivity

4. MESSAGE

4.1. Router Solicitation Message

Hosts send Router Solicitations in order to prompt routers
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
An IP address assigned to the sending interface,
the unspecified address if no address is
to the sending interface

Destination
Typically the all-routers multicast address

Hop Limit 255

Authentication
If a Security Association for the IP
Header exists between the sender and
destination address, then the sender SHOULD
this header

ICMP Fields

Type 133

Code 0

Checksum The ICMP checksum. See [ICMPv6].

Reserved This field is unused. It MUST be initialized
zero by the sender and MUST be ignored by
receiver

Narten, et. al. Standards Track [Page 17]

RFC 2461 Neighbor Discovery for IPv6 December 1998

Valid Options

Source link-layer
The link-layer address of the sender, if known
MUST NOT be included if the Source Address is
unspecified address. Otherwise it SHOULD
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
and continue processing the message

4.2. Router Advertisement Message

Routers send out Router Advertisement message periodically, or
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 |M|O| Reserved | Router Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reachable Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Retrans Timer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-

IP Fields

Source
MUST be the link-local address assigned to
interface from which this message is sent

Destination
Typically the Source Address of an invoking
Solicitation or the all-nodes multicast address

Hop Limit 255

Authentication
If a Security Association for the IP
Header exists between the sender and
destination address, then the sender SHOULD
this header

Narten, et. al. Standards Track [Page 18]

RFC 2461 Neighbor Discovery for IPv6 December 1998

ICMP Fields

Type 134

Code 0

Checksum The ICMP checksum. See [ICMPv6].

Cur Hop Limit 8-bit unsigned integer. The default value
should be placed in the Hop Count field of the
header for outgoing IP packets. A value of
means unspecified (by this router).

M 1-bit "Managed address configuration" flag.
set, hosts use the administered (stateful)
for address autoconfiguration in addition to
addresses autoconfigured using stateless
autoconfiguration. The use of this flag
described in [ADDRCONF].

O 1-bit "Other stateful configuration" flag.
set, hosts use the administered (stateful)
for autoconfiguration of other (non-address
information. The use of this flag is described
[ADDRCONF].

Reserved A 6-bit unused field. It MUST be initialized
zero by the sender and MUST be ignored by
receiver

Router
16-bit unsigned integer. The lifetime
with the default router in units of seconds.
maximum value corresponds to 18.2 hours.
Lifetime of 0 indicates that the router is not
default router and SHOULD NOT appear on the
router list. The Router Lifetime applies only
the router's usefulness as a default router;
does not apply to information contained in
message fields or options. Options that need
limits for their information include their
lifetime fields

Reachable Time 32-bit unsigned integer. The time,
milliseconds, that a node assumes a neighbor
reachable after having received a
confirmation. Used by the Neighbor
Detection algorithm (see Section 7.3). A value

Narten, et. al. Standards Track [Page 19]

RFC 2461 Neighbor Discovery for IPv6 December 1998

zero means unspecified (by this router).

Retrans Timer 32-bit unsigned integer. The time,
milliseconds, between retransmitted
Solicitation messages. Used by address
and the Neighbor Unreachability Detection
(see Sections 7.2 and 7.3). A value of zero
unspecified (by this router).

Possible options

Source link-layer
The link-layer address of the interface from
the Router Advertisement is sent. Only used
link layers that have addresses. A router MAY
this option in order to enable inbound load
across multiple link-layer addresses

MTU SHOULD be sent on links that have a variable
(as specified in the document that describes how
run IP over the particular link type). MAY be
on other links

Prefix
These options specify the prefixes that are on-
and/or are used for address autoconfiguration.
router SHOULD include all its on-link
(except the link-local prefix) so that
hosts have complete prefix information about on
link destinations for the links to which
attach. If complete information is lacking,
multihomed host may not be able to choose
correct outgoing interface when sending traffic
its neighbors

Future versions of this protocol may define new option types
Receivers MUST silently ignore any options they do not
and continue processing the message

Narten, et. al. Standards Track [Page 20]

RFC 2461 Neighbor Discovery for IPv6 December 1998

4.3. Neighbor Solicitation Message

Nodes send Neighbor Solicitations to request the link-layer
of a target node while also providing their own link-layer address
the target. Neighbor Solicitations are multicast when the node
to resolve an address and unicast when the node seeks to verify
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
Either an address assigned to the interface
which this message is sent or (if Duplicate
Detection is in progress [ADDRCONF])
unspecified address

Destination
Either the solicited-node multicast
corresponding to the target address, or the
address

Hop Limit 255

Authentication
If a Security Association for the IP
Header exists between the sender and
destination address, then the sender SHOULD
this header

Narten, et. al. Standards Track [Page 21]

RFC 2461 Neighbor Discovery for IPv6 December 1998

ICMP Fields

Type 135

Code 0

Checksum The ICMP checksum. See [ICMPv6].

Reserved This field is unused. It MUST be initialized
zero by the sender and MUST be ignored by
receiver

Target
The IP address of the target of the solicitation
It MUST NOT be a multicast address

Possible options

Source link-layer
The link-layer address for the sender. MUST NOT
included when the source IP address is
unspecified address. Otherwise, on link
that have addresses this option MUST be included
multicast solicitations and SHOULD be included
unicast solicitations

Future versions of this protocol may define new option types
Receivers MUST silently ignore any options they do not
and continue processing the message

Narten, et. al. Standards Track [Page 22]

RFC 2461 Neighbor Discovery for IPv6 December 1998

4.4. Neighbor Advertisement Message

A node sends Neighbor Advertisements in response to
Solicitations and sends unsolicited Neighbor Advertisements in
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R|S|O| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Target Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-

IP Fields

Source
An address assigned to the interface from which
advertisement is sent

Destination
For solicited advertisements, the Source Address
an invoking Neighbor Solicitation or, if
solicitation's Source Address is the
address, the all-nodes multicast address

For unsolicited advertisements typically the all
nodes multicast address

Hop Limit 255

Authentication
If a Security Association for the IP
Header exists between the sender and
destination address, then the sender SHOULD
this header

Narten, et. al. Standards Track [Page 23]

RFC 2461 Neighbor Discovery for IPv6 December 1998

ICMP Fields

Type 136

Code 0

Checksum The ICMP checksum. See [ICMPv6].

R Router flag. When set, the R-bit indicates
the sender is a router. The R-bit is used
Neighbor Unreachability Detection to detect
router that changes to a host

S Solicited flag. When set, the S-bit indicates
the advertisement was sent in response to
Neighbor Solicitation from the Destination address
The S-bit is used as a reachability
for Neighbor Unreachability Detection. It MUST
be set in multicast advertisements or
unsolicited unicast advertisements

O Override flag. When set, the O-bit indicates
the advertisement should override an existing
entry and update the cached link-layer address
When it is not set the advertisement will
update a cached link-layer address though it
update an existing Neighbor Cache entry for
no link-layer address is known. It SHOULD NOT
set in solicited advertisements for
addresses and in solicited proxy advertisements
It SHOULD be set in other solicited
and in unsolicited advertisements

Reserved 29-bit unused field. It MUST be initialized
zero by the sender and MUST be ignored by
receiver

Target
For solicited advertisements, the Target
field in the Neighbor Solicitation message
prompted this advertisement. For an
advertisement, the address whose link-layer
has changed. The Target Address MUST NOT be
multicast address

Narten, et. al. Standards Track [Page 24]

RFC 2461 Neighbor Discovery for IPv6 December 1998

Possible options

Target link-layer
The link-layer address for the target, i.e.,
sender of the advertisement. This option MUST
included on link layers that have addresses
responding to multicast solicitations.
responding to a unicast Neighbor Solicitation
option SHOULD be included

The option MUST be included for
solicitations in order to avoid infinite
Solicitation "recursion" when the peer node
not have a cache entry to return a
Advertisements message. When responding to
solicitations, the option can be omitted since
sender of the solicitation has the correct link
layer address; otherwise it would not have be
to send the unicast solicitation in the
place. However, including the link-layer address
this case adds little overhead and eliminates
potential race condition where the sender
the cached link-layer address prior to receiving
response to a previous solicitation

Future versions of this protocol may define new option types
Receivers MUST silently ignore any options they do not
and continue processing the message

Narten, et. al. Standards Track [Page 25]

RFC 2461 Neighbor Discovery for IPv6 December 1998

4.5. Redirect Message

Routers send Redirect packets to inform a host of a better first-
node on the path to a destination. Hosts can be redirected to
better first-hop router but can also be informed by a redirect
the destination is in fact a neighbor. The latter is accomplished
setting the ICMP Target Address equal to the ICMP
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
MUST be the link-local address assigned to
interface from which this message is sent

Destination
The Source Address of the packet that triggered
redirect

Hop Limit 255

Narten, et. al. Standards Track [Page 26]

RFC 2461 Neighbor Discovery for IPv6 December 1998

Authentication
If a Security Association for the IP
Header exists between the sender and
destination address, then the sender SHOULD
this header

ICMP Fields

Type 137

Code 0

Checksum The ICMP checksum. See [ICMPv6].

Reserved This field is unused. It MUST be initialized
zero by the sender and MUST be ignored by
receiver

Target Address An IP address that is a better first hop to use
the ICMP Destination Address. When the target
the actual endpoint of communication, i.e.,
destination is a neighbor, the Target Address
MUST contain the same value as the ICMP
Address field. Otherwise the target is a
first-hop router and the Target Address MUST be
router's link-local address so that hosts
uniquely identify routers

Destination
The IP address of the destination which
redirected to the target

Possible options

Target link-layer
The link-layer address for the target. It
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
means for determining the link-layer addresses