RFC2461 - Neighbor Discovery for IP Version 6 (IPv6)

Network Working Group T. Narten
Request for Comments: 2461 IBM
Obsoletes: 1970 E. Nordmark
Category: Standards Track Sun Microsystems
W. Simpson
Daydreamer
December 1998
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.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
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........................................ 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
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
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 RFC1970........................... 91
Full Copyright Statement..................................... 93
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 of the IPNGWG
working group and, in particular, (in alphabetical order) Ran
Atkinson, Jim Bound, Scott Bradner, Alex Conta, Stephen Deering,
Richard Draves, 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
function is described in [ADDR-ARCH]. The function is
chosen so that IP addresses which 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
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
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
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.
MUST NOT be included if the Source Address is the
unspecified address. Otherwise it SHOULD 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.
4.2. 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
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 choose 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
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. MUST NOT be
included when the source IP address is the
unspecified address. Otherwise, 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
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. This option MUST be
included on link layers that have addresses when
responding to multicast solicitations. When
responding to a unicast Neighbor Solicitation this
option SHOULD be included.
The option MUST be included for multicast
solicitations in order to avoid infinite Neighbor
Solicitation "recursion" when the peer node does
not have a cache entry to return a Neighbor
Advertisements message. When responding to unicast
solicitations, the option can be omitted since the
sender of the solicitation has the correct link-
layer address; otherwise it would not have be able
to send the unicast solicitation in the first
place. However, including the link-layer address in
this case adds little overhead and eliminates a
potential race condition where the sender deletes
the cached link-layer address prior to receiving a
response to a previous solicitation.
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
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 1280 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
(including the type and length fields) 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 (including the type and
length fields) 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 1280
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
the maximum MTU value that is 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