Network Working Group T. Narten
Request for Comments: 4861 IBM
Obsoletes: 2461 E. Nordmark
Category: Standards Track Sun Microsystems
September 2007 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.
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.
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., Non-Broadcast
Multi-Access (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 are addressed in [IPv6-NBMA]. In addition, [IPv6-3GPP]
and[IPv6-CELL] discuss the use of this protocol over some cellular
links, which are examples of NBMA links.
IP - Internet Protocol Version 6. The terms IPv4 and IPv6
are used only in contexts where necessary to avoid
ICMP - Internet Control Message Protocol for the Internet
Protocol Version 6. The terms ICMPv4 and ICMPv6 are
used only in contexts where necessary to avoid
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-layer (or lower-layer) protocols being
"tunneled" over (i.e., encapsulated in) IP such as
Internetwork Packet Exchange (IPX), AppleTalk, or IP
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-layer (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
- 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.
- a link-layer identifier for an interface. Examples
include IEEE 802 addresses for Ethernet links.
on-link - an address that is assigned to an interface on a
specified link. A node considers an address to be on-
- it is covered by one of the link's prefixes (e.g.,
as indicated by the on-link flag in the Prefix
Information option), 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
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.
- 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
transmission unit over a link.
target - an address about which address resolution information
is sought, or an address that is the new first hop when
proxy - a node 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-
- 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
ensure that the granularity of the calculated random
component and the resolution of the timer used are both
high enough to ensure 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 identifier alone as the seed, since interface
identifiers will not always be unique. To reduce the
probability that duplicate interface identifiers 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 identifier. Additional information on
randomness and random number generation can be found in
2.2. Link Types
Different link layers have different properties. The ones of concern
to Neighbor Discovery are:
- 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 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
applications that need it (e.g., using multicast
servers). However, it is an issue for further study
whether ND should use such facilities or an
alternate mechanism that provides the equivalent
multicast capability for ND.
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 Switched
Multimegabit Data Service (SMDS) and Broadband
Integrated Services Digital Network (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.
- 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
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,
all-routers multicast address
- the link-local scope address to reach all routers,
solicited-node multicast address
- a link-local scope multicast address that is computed
as a function of the solicited target's address. The
function is described in [ADDR-ARCH]. The function is
chosen so that IP addresses that differ only in the
most significant bits, e.g., due to multiple prefixes
associated with different providers, will map to the
same solicited-node address thereby reducing the number
of multicast addresses a node must join at the link
- 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
- 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 stateless address autoconfiguration [ADDRCONF]).
The unspecified address has a value of 0:0:0:0:0:0:0:0.
Note that this specification does not strictly comply with the
consistency requirements in [ADDR-SEL] for the scopes of source and
destination addresses. It is possible in some cases for hosts to use
a source address of a larger scope than the destination address in
the IPv6 header.
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [KEYWORDS].
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
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 link parameters (such as the
link MTU) or Internet parameters (such as the hop limit
value) to place in outgoing packets.
Address Autoconfiguration: Introduces the mechanisms needed in
order to allow nodes to configure an address for an
interface in a stateless manner. Stateless address
autoconfiguration is specified in [ADDRCONF].
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
Duplicate Address Detection: How a node determines whether or not
an address it wishes to use is already in use by another
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 determining whether another address shares
the same link (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
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 DHCPv6 and/or autonomous
(stateless) address configuration.
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 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.
Neighbor Discovery allows a router to perform load balancing
for traffic addressed to itself 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, e.g.,
who issued the solicitation. This specification does not
define a mechanism that allows hosts to Load-balance incoming
packets. See [LD-SHRE].
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. A non-Override
advertisement is one that does not update or replace the
information sent by another advertisement. These
advertisements are discussed later in the context of Neighbor
advertisement messages. This invokes specific rules to
determine which of potentially multiple advertisements should
Proxy advertisements - A node willing to accept packets on behalf
of a target address that is unable to respond to Neighbor
Solicitations can issue non-Override Neighbor Advertisements.
Proxy advertisements are used by Mobile IPv6 Home Agents to
defend mobile nodes' addresses when they move 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 Address Resolution Protocol [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 the Hosts Requirements document
[HR-CL] does specify some possible algorithms for Dead Gateway
Detection (a subset of the problems Neighbor Unreachability Detection
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
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
Address resolution multicasts are "spread" over 16 million (2^24)
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
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
Neighbor Unreachability Detection is part of the base, which
significantly improves the robustness of packet delivery in the
presence of failing routers, partially failing or partitioned
links, or 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
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.
By setting the Hop Limit to 255, 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
generic IP-layer authentication and security mechanisms as
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.)
multicast - Neighbor Discovery operates over multicast capable
links 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 are
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
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
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 an
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
that all receivers can process (or Maximum Receive
- Neighbor Discovery detects the absence of
symmetric reachability; a node avoids paths to a
neighbor with which it does not have symmetric
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.
3.3. Securing Neighbor Discovery Messages
Neighbor Discovery messages are needed for various functions.
Several functions are designed to allow hosts to ascertain the
ownership of an address or the mapping between link-layer and IP-
layer addresses. Vulnerabilities related to Neighbor Discovery are
discussed in Section 11.1. A general solution for securing Neighbor
Discovery is outside the scope of this specification and is discussed
in [SEND]. However, Section 11.2 explains how and under which
constraints IPsec Authentication Header (AH) or Encapsulating
Security Payload (ESP) can be used to secure Neighbor Discovery.