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Versions: 00 01 02 03 04 05 06 RFC 1970
INTERNET-DRAFT Thomas Narten, IBM
September 15, 1995 Erik Nordmark, Sun Microsystems
W A Simpson, Daydreamer
Neighbor Discovery for IP Version 6 (IPv6)
<draft-ietf-ipngwg-discovery-02.txt>
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
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To learn the current status of any Internet-Draft, please check the
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(Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific
Rim).
Distribution of this memo is unlimited.
This Internet Draft expires March 15, 1996.
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.
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Contents
Status of this Memo.......................................... 1
1. INTRODUCTION............................................. 4
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. CONCEPTUAL MODEL OF A HOST............................... 16
4.1. Conceptual Data Structures.......................... 16
4.2. Conceptual Sending Algorithm........................ 18
4.3. Garbage Collection and Timeout Requirements......... 19
5. ROUTER AND PREFIX DISCOVERY.............................. 20
5.1. Message Formats..................................... 21
5.1.1. Router Solicitation Message Format............. 21
5.1.2. Router Advertisement Message Format............ 22
5.2. Router Specification................................ 24
5.2.1. Router Configuration Variables................. 24
5.2.2. Validation of Router Solicitation Messages..... 27
5.2.3. Router Behavior................................ 28
5.2.4. Router Advertisement Consistency............... 32
5.2.5. Link-local Address Change...................... 33
5.3. Host Specification.................................. 33
5.3.1. Host Configuration Variables................... 33
5.3.2. Host Variables................................. 34
5.3.3. Validation of Router Advertisement Messages.... 34
5.3.4. Host Behavior.................................. 35
6. ADDRESS RESOLUTION AND NEIGHBOR UNREACHABILITY DETECTION. 39
6.1. Message Formats..................................... 39
6.1.1. Neighbor Solicitation Message Format........... 39
6.1.2. Neighbor Advertisement Message Format.......... 42
6.2. Address Resolution.................................. 44
6.2.1. Node Specification............................. 44
6.2.2. Sending Neighbor Solicitations................. 44
6.2.3. Validation of Neighbor Solicitations........... 45
6.2.4. Receipt of Neighbor Solicitations.............. 46
6.2.5. Sending Solicited Neighbor Advertisements...... 46
6.2.6. Validation of Neighbor Advertisements.......... 47
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6.2.7. Receipt of Neighbor Advertisments.............. 47
6.2.8. Sending Unsolicited Neighbor Advertisements.... 48
6.2.9. Anycast Neighbor Advertisements................ 49
6.2.10. Proxy Neighbor Advertisements................. 50
6.3. Neighbor Unreachability Detection................... 50
6.3.1. Reachability Confirmation...................... 51
6.3.2. Node Behavior.................................. 52
7. REDIRECT FUNCTION........................................ 55
7.1. Redirect Message Format............................. 55
7.2. Router Specification................................ 57
7.3. Host Specification.................................. 58
7.3.1. Validation of Redirect Messages................ 58
7.3.2. Host Behavior.................................. 59
8. OPTIONS.................................................. 59
8.1. Source/Target Link-layer Address.................... 61
8.2. Prefix Information.................................. 62
8.3. Redirected Header................................... 64
8.4. MTU................................................. 64
9. MULTIHOMED HOSTS......................................... 65
10. PROTOCOL CONSTANTS...................................... 67
11. FUTURE EXTENSIONS....................................... 68
12. OPEN ISSUES............................................. 68
13. SECURITY CONSIDERATIONS................................. 68
REFERENCES................................................... 71
AUTHORS' ADDRESSES........................................... 72
CHANGES SINCE PREVIOUS DOCUMENT.............................. 73
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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.
This document is a revision of <draft-ietf-ipngwg-discovery-01.txt>
which was itself based on the protocol specified in the two documents:
<draft-simpson-ipv6-discov-formats-02.txt>, and
<draft-simpson-ipv6-discov-process-02.txt>
The authors would like to acknowledge the contributions the IPNGWG
working group an, in particular, (in alphabetical order) Ran Atkinson,
Jim Bound, Scott Bradner, Stephen Deering, Robert Hinden, Allison
Mankin, Dan McDonald, Charles Perkins, and Sue 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,
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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; and 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].
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
- a 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.
reachability
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- 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 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
potentially acts 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 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.
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2.2. Link Types
Different link layers have different properties. The ones of concern to
Neighbor Discovery are:
multicast - a link that supports some mechanism at the link
layer for sending packets to all (i.e. broadcast) or
a subset of all neighbors. Multicast/broadcast can
be provided by a variety of link layer mechanisms
such as the physical link layer itself (for example,
Ethernet), replicated unicast packets sent by the
link layer software, or multicast servers (such as
in ATM). Note that all point-to-point links are
multicast links.
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 any form of multicast or
broadcast (e.g., X.25).
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.
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
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properties.
2.3. Addresses
Neighbor Discovery makes use of a number of different addresses defined
in [ADDR-ARCH], including:
all-nodes multicast address
- the link-local scope address to reach all nodes.
FF02::1
all-routers multicast address
- the link-local scope address to reach all routers.
FF02::2
solicited-node multicast address
- a link-local scope multicast address that is computed
as a function of the solicited target's address. The
solicited-node multicast address is formed by taking
the low-order 32 bits of the target IP address and
appending those bits to the 96-bit prefix
FF02:0:0:0:0:1 to produce a multicast address within
the range FF02::1:0:0 to FF02::1:FFFF:FFFF. For
example, the solicited node multicast address
corresponding to the IP address 4037::01:800:200E:8C6C
is FF02::1:200E:8C6C. IP addresses that differ only in
the high-order bits, e.g. due to multiple high-order
prefixes associated with different providers, will map
to the same solicited-node address thereby reducing the
number of multicast addresses a node must join.
link-local address
- a unicast address having link-only scope that can be
used to reach neighbors. All interfaces MUST have a
link-local address. Routers MUST NOT forward packets
with a link-local source address. See [ADDR-ARCH].
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.
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2.4. Requirements
Throughout this document, the words that are used to define the
significance of the particular requirements are capitalized. These
words are:
MUST
This word or the adjective "REQUIRED" means that the item is an
absolute requirement of this specification.
MUST NOT
This phrase means the item is an absolute prohibition of this
specification.
SHOULD
This word or the adjective "RECOMMENDED" means that there may
exist valid reasons in particular circumstances to ignore this
item, but the full implications should be understood and the
case carefully weighed before choosing a different course.
SHOULD NOT
This phrase means that there may exist valid reasons in
particular circumstances when the listed behavior is acceptable
or even useful, but the full implications should be understood
and the case carefully weighted before implementing any behavior
described with this label.
MAY This word or the adjective "OPTIONAL" means that this item is
truly optional. One vendor may choose to include the item
because a particular marketplace requires it or because it
enhances the product, for example, another vendor may omit the
same item.
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
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reachable through a router.)
Parameter Discovery: How a node learns such link parameters as the
link MTU or such Internet parameters as the maximum 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 an explicit Router Solicitation
message. Router Advertisements contain prefixes that are
used for on-link determination and/or address
configuration, a Maximum Hop Limit value, etc.
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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 maximum hop 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
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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 IP address and link-layer address in the Neighbor
Solicitation.
Neighbor Solicitation messages can also be used to determine if more
than one node has been configured to use a particular 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 explicit 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 (but unreliably)
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
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Router Advertisement packets, thereby forcing neighbors to use
Neighbor Solicitation messages to learn the link-layer
addresses. 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 "Secondary" 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 Secondary 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
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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 and
avoids sending traffic to neighbors with which two-way connectivity
is absent.
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].
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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 is
available:
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 - All aspects of Neighbor Discovery are available.
non-broadcast multiple access (NBMA)
- The only Neighbor Discovery mechanisms available on
these links are Redirect handling and Neighbor
Unreachability Detection.
If hosts support manual configuration of a list of
default routers, the 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
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Unit) in order for multicast to work properly. When
multicasting, a sender has no way of knowing which
nodes will receive the packet, and cannot 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. 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 behavior is
consistent with the protocol specification.
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.
4.1. Conceptual Data Structures
Hosts will need to maintain the following pieces of information about an
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 IP address and contain such
information as its link-layer address, a flag
indicating whether the neighbor is a router or a host
(called "is_router" in this document), a pointer to
any queued packets waiting for Address Resolution to
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complete, etc.
A Neighbor Cache entry also contains information used
by the Neighbor Unreachability Detection algorithm.
This includes 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 for each destination 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.
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 delete prefixes that routers
stop advertising. A special "infinity" timer value
specifies that a prefix remains valid forever, unless
a new (finite) value is received in a subsequent
advertisement.
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.
However, in all cases it is important to not duplicate the conceptual
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Neighbor Cache entry for a router in order to prevent redundant Neighbor
Unreachability Detection probes.
The Neighbor Cache contains information maintained by the Neighbor
Unreachability Detection algorithm. A key piece of information is a
neighbor's reachability state, which is one of three possible values:
INCOMPLETE Address Resolution is in progress and the link-layer
address of the neighbor has not yet been determined.
REACHABLE Roughly speaking, the neighbor is known to have been
reachable recently (within tens of seconds ago).
PROBE The neighbor may be reachable, but the last explicit
reachability confirmation was received long enough ago
that verification is now actively sought.
4.2. Conceptual Sending Algorithm
When sending a packet, a node uses a combination of the Destination
Cache, the Prefix List, and the Default Router List to determine the IP
address of the appropriate next hop, an operation known as "next-hop
determination". Once the IP address of the next hop is known, the
Neighbor Cache is consulted for link-level information about that
neighbor.
Next-hop determination operates as follows for unicast packets. The
sender examines the Prefix List to determine whether the packet's
destination is on- or off-link. If the destination is on-link, the
next-hop address is the same as the packet's destination address. If
the destination is off-link, the sender selects a router from the
Default Router List (following the rules described in Section 5.3.4).
If the Default Router List is empty, the sender assumes that the
destination is on-link.
For multicast packets the next-hop is always the (multicast) destination
address.
For efficiency reasons, next-hop determination is not performed on every
packet that is sent. Instead, the results of next-hop determination
computations are saved in the Destination Cache. When the sending node
has a packet to send, it first examines the Destination Cache. If no
entry exists for the destination, next-hop determination is invoked to
create a Destination Cache entry.
Once the IP address of the next-hop node is known, the sender examines
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the Neighbor Cache for link-level information about that neighbor. If
no entry exists for a multicast destination, an entry is created using
the link specific mapping to a multicast link-layer address (see e.g.
[IPv6-ETHER]). If no entry exists for a unicast destination, the sender
creates a new one, sets its state to INCOMPLETE, sends a Neighbor
Solicitation message, and then queues the data packet pending completion
of Address Resolution. When a Neighbor Advertisement response is
received, the link-layer addresses is entered in the Neighbor Cache
entry and the queued packet is transmitted. The Address Resolution
mechanism is described in detail in Section 6.2.
Each time a Neighbor Cache entry is accessed while transmitting a
unicast packet, the sender checks Neighbor Unreachability Detection
related information according to the Neighbor Unreachability Detection
algorithm (Section 6.3), unless the upper-layer has indicated that such
checks are not needed. For instance, the Neighbor Discovery protocol
itself when sending packets should pass an indication to IP that the
packet should not trigger Neighbor Unreachability Detection. This
unreachability check might result in the sender transmitting a unicast
Neighbor Solicitation to verify that the neighbor is still reachable.
Next-hop determination is done the first time traffic is sent to a
destination. As long as subsequent communication to that destination
proceeds successfully, the Destination Cache entry continues to be used.
If at some point communication ceases to proceed, as determined by the
Neighbor Unreachability Detection algorithm, next-hop determination may
need to be performed again. For example, traffic through a failed
router should be switched to a working router. Likewise, it may be
possible to reroute traffic destined for a mobile node to a "mobility
agent".
Note that when a node redoes next-hop determination there is no need to
discard the complete Destination Cache entry. In fact, it is generally
beneficial to retain such cached information as the PMTU and round trip
timer values that may also be kept in the Destination Cache entry.
4.3. Garbage Collection and Timeout Requirements
The conceptual data structures described above use different mechanisms
for discarding potentially stale or unused information.
=46rom the perspective of correctness there is no need to periodically
purge Destination and Neighbor Cache entries. Although stale
information can potentially remain in the cache indefinitely, the
Neighbor Unreachability Detection algorithm ensures that stale
information is purged quickly if it is actually being used.
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To limit the storage needed for the Destination and Neighbor Caches, a
node may need to garbage-collect old entries. However, care must be
taken to insure that sufficient space is always present to hold the
working set of active entries. A small cache may result in an excessive
number of Neighbor Discovery messages if entries are discarded and
rebuilt in quick succession. Any LRU-based policy that only reclaims
entries that have not been used in some time (e.g., ten minutes or more)
should be adequate for garbage-collecting unused entries.
A node should retain entries in the Default Router List and the Prefix
List until their lifetimes expire. However, a node may garbage collect
entries prematurely if it is low on memory. If not all routers are kept
on the Default Router list, a node should retain at least two entries in
the Default Router List (and preferably more) in order to maintain
robust connectivity for off-link destinations.
When removing an entry from the Default Router List or the Prefix List
there is no need to purge any entries from the Destination or Neighbor
Caches. Neighbor Unreachability Detection will efficiently purge any
entries in these caches that have become invalid.
5. ROUTER AND PREFIX DISCOVERY
This section describes message formats, router behavior and host
behavior related to the Router Discovery portion of Neighbor Discovery.
Router Discovery is used to locate neighboring routers as well as learn
prefixes and configuration parameters related to address
autoconfiguration.
Prefix Discovery provides a mechanism through which hosts learn of
ranges of IP addresses that reside on-link and thus can be reached
directly without going through a router. Routers advertise a set of
prefixes that cover those IP addresses that are on-link. Prefix
discovery is logically separate from Router Discovery. In practice,
prefix information is included in options piggybacked on Router
Advertisement messages to reduce network traffic.
Address Autoconfiguration information is also logically separate from
Router Discovery. To reduce network traffic, however, autoconfiguration
information is piggybacked on Router Discovery messages. In fact, the
same prefixes can be advertised for on-link determination and address
autoconfiguration by specifying the appropriate flags in the Prefix
Information options. This document does not define how
autoconfiguration information is processed. See [ADDRCONF] for details.
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5.1. Message Formats
5.1.1. Router Solicitation Message Format
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
IP Fields:
Source Address
MUST be the link-local address assigned to the
interface from which this message is sent.
Destination Address
The all-routers link-local multicast address.
Hop Count 1
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.
Routing Header MUST NOT be sent.
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 ignored by the receiver.
Options:
Source link-layer address
The link-layer address for the sender. This option
SHOULD be included on link layers that have addresses
so that routers responding to the request can unicast
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a response without the need to first perform address
resolution.
Future versions of this protocol may define new option types.
Receivers MUST skip over and ignore any options they do not recognize
and continue processing the message.
5.1.2. Router Advertisement Message Format
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Max Hop Limit |M|O| 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
Either the Source Address of an invoking Router
Solicitation or the all-nodes link-local multicast
address.
Hop Count 1
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.
Routing Header MUST NOT be sent.
ICMP Fields:
Type 134
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Code 0
Checksum The ICMP checksum. See [ICMPv6].
Max Hop Limit 8-bit unsigned integer. The maximum hop limit that
the router suggests that hosts use when sending IP
packets. A value of zero means unspecified.
M 1-bit "Managed address configuration" flag. 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 configuration" flag. 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 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 does not apply to information
contained in any options in the message. 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
receiving some reachability confirmation. Used by the
Neighbor Unreachability Detection algorithm (see
Section 6.3). A value of zero means unspecified (by
the 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 6.2
and 6.3). A value of zero means unspecified (by the
router).
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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. 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 so that
multihomed hosts have complete prefix information
about on-link destinations for the links to which they
attach. If complete information is lacking, a
multihomed host may not be able to chose the correct
outgoing interface when sending traffic to its
neighbors.
Future versions of this protocol may define new option types.
Receivers MUST skip over and ignore any options they do not recognize
and continue processing the message.
5.2. Router Specification
5.2.1. Router Configuration Variables
A router MUST allow for the following variables to be configured by
system management; default values are specified so as to make it
unnecessary to configure any of these variables in many cases.
For each multicast interface:
AdvertiseDefault
A flag indicating whether or not the router should
advertise itself as a default router on the
interface.
Default: TRUE
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ManagedFlag The true/false value to be placed in the "Managed
address configuration" field in the Router
Advertisement. See [ADDRCONF].
Default: FALSE
OtherFlag The true/false value to be placed in the "Other
configuration" field in the Router Advertisement.
See [ADDRCONF].
Default: FALSE
LinkMTU The value to be placed in MTU options sent by the
router. If the value is set to zero no MTU options
are sent.
Default: 0
AdvReachableTime
The value to be placed in the Reachable Time field
in the Router Advertisement messages sent by the
router. The value zero means unspecified (by this
router). MUST be no greater than 3,600,000
milliseconds (1 hour).
Default: REACHABLE_TIME milliseconds
ReachableTime The time a neighbor is considered reachable after
receiving a reachability confirmation.
Default: If AdvReachableTime is non-zero (specified)
a uniformly-distributed random value between
MIN_RANDOM_FACTOR and MAX_RANDOM_FACTOR times
AdvReachableTime milliseconds. Otherwise, A
uniformly-distributed random value between
MIN_RANDOM_FACTOR and MAX_RANDOM_FACTOR times
REACHABLE_TIME milliseconds.
RetransTimer The value to be placed in the Retrans Timer field in
the Router Advertisement messages sent by the
router. The value zero means unspecified (by this
router).
Default: RETRANS_TIMER milliseconds
MaximumHopLimit
The value to be placed in the Max Hop Limit field in
the Router Advertisement messages sent by the
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router. The value zero means unspecified (by this
router).
Default: The value specified in the most recent
"Assigned Numbers" RFC [ASSIGNED].
MaxRtrAdvInterval
The maximum time allowed between sending unsolicited
multicast Router Advertisements from the interface,
in seconds. MUST be no less than 1 second and no
greater than 1800 seconds.
Default: 600 seconds
MinRtrAdvInterval
The minimum time allowed between sending unsolicited
multicast Router Advertisements from the interface,
in seconds. MUST be no less than 0.1 seconds and no
greater than .75 * MaxRtrAdvInterval.
Default: 0.33 * MaxRtrAdvInterval
RtrAdvLifetime
The value to be placed in the Router Lifetime field
of Router Advertisements sent from the interface, in
seconds. MUST be no less than MaxRtrAdvInterval and
no greater than 9000 seconds.
Note: if AdvertiseDefault is false, the value of
RtrAdvLifetime is irrelevant; a Lifetime value of 0
MUST be placed in outgoing Router Advertisements
messages so that hosts do not use the router as a
default router.
Default: 3 * MaxRtrAdvInterval
PrefixList
A list of prefixes to be placed in Prefix
Information options in Router Advertisement messages
sent from the interface.
Default: The PrefixList contains all prefixes that
the router advertises via routing protocols as being
on-link for the interface from which the
advertisement is sent.
Each prefix is associated with:
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InvalidationLifetime
The value to be placed in the Invalidation
Lifetime in the Prefix Information option,
in seconds. The designated value of all 1's
(0xffffffff) represents infinity.
Default: infinity.
OnLinkFlag
The value to be placed in the on-link flag
("L-bit") field in the Prefix Information
option.
Default: TRUE
Automatic address configuration [ADDRCONF] defines
additional information associated with each the
prefixes:
DeprecationLifetime
The value to be placed in the Deprecation
Lifetime in the Prefix Information option,
in seconds. The designated value of all 1's
(0xffffffff) represents infinity. See
[ADDRCONF].
Default: 604800 seconds (7 days)
AutonomousFlag
The value to be placed in the Autonomous
Flag field in the Prefix Information option.
See [ADDRCONF].
Default: TRUE
Protocol constants are defined in Section 10.
5.2.2. Validation of Router Solicitation Messages
A router MUST silently discard any received Router Solicitation messages
that do not satisfy all of the following validity checks:
- IP Source Address is a link-local address.
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- IP Destination Address is a link-local address or a multicast
address with link-local scope.
- IP Routing Header is not present.
- if the message includes an IP Authentication Header, the message
authenticates correctly.
- ICMP Checksum is valid.
- ICMP Code is 0.
- ICMP length (derived from the IP length) is 8 or more octets.
- all included options have a length that is greater than zero.
The contents of the Reserved field, and of any unrecognized options,
MUST be ignored. Future, backward-compatible changes to the protocol
may specify the contents of the Reserved field or add new options;
backward-incompatible changes may use different Code values.
A solicitation that passes the validity checks is called a "valid
solicitation".
Routers MUST also validate Router Advertisements as described in Section
5.3.3.
5.2.3. Router Behavior
A router MUST join the all-routers multicast address on all multicast
capable interfaces.
The term "advertising interface" refers to any functioning and enabled
interface that has at least one IP address assigned to it. From each
advertising interface, the router transmits periodic, multicast Router
Advertisements, containing the following values consistent with the
message format above:
- In the Router Lifetime field: the interface's configured
RtrAdvLifetime. If the router's AdvertiseDefault flag is set to
false, the Router Lifetime field MUST be set to 0.
- In the M and O flags: the interface's configured ManagedFlag and
OtherFlag, respectively. See [ADDRCONF].
- In the Max Hop Limit field: the interface's configured
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MaximumHopLimit.
- In the Reachable Time field: the interface's configured
AdvReachableTime.
- In the Retrans Timer field: the interface's configured
RetransTimer.
- In the options:
o Source Link-Layer Address option: link-layer address of the
sending interface. This option MAY be omitted to facilitate
in-bound load balancing over replicated interfaces.
o MTU option: the MTU value that all nodes should be using.
o Prefix Information options: one Prefix Information option for
each prefix listed in PrefixList with the option fields set
from the information in the PrefixList entry as follows:
- In the "on-link" flag: the entry's OnLinkFlag.
- In the Invalidation Lifetime field: the entry's
InvalidationLifetime.
- In the "Autonomous address configuration" flag: the
entry's AutonomousFlag.
- In the Deprecation Lifetime field: the entry's
DeprecationLifetime.
Router advertisements are not strictly periodic: the interval between
subsequent transmissions is randomized to reduce the probability of
synchronization with the advertisements from other routers on the same
link [SYNC]. Each advertising interface has its own timer. Whenever a
multicast advertisement is sent from an interface, that interface's
timer is reset to a uniformly-distributed random value between the
interface's configured MinRtrAdvInterval and MaxRtrAdvInterval;
expiration of the timer causes the next advertisement to be sent from
the interface, and a new random value to be chosen. (It is recommended
that routers include some unique value, such as one of their IP or
link-layer addresses, in the seed used to initialize their pseudo-random
number generators. Although the randomization range is configured in
units of seconds, the actual randomly-chosen values should not be in
units of whole seconds, but rather in units of the highest available
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timer resolution.)
For the first few advertisements sent from an interface (up to
MAX_INITIAL_RTR_ADVERTISEMENTS), if the randomly chosen interval is
greater than MAX_INITIAL_RTR_ADVERT_INTERVAL, the timer SHOULD be set to
MAX_INITIAL_RTR_ADVERT_INTERVAL instead. Using this smaller interval
for the initial advertisements increases the likelihood of a router
being discovered quickly when it first becomes available, in the
presence of possible packet loss.
In addition to the periodic, unsolicited advertisements, a router sends
advertisements in response to valid solicitations received on any of its
advertising interfaces. A router MAY choose to unicast the response
directly to the soliciting host's address, or multicast it to the all-
nodes address; in the latter case, the interface's interval timer is
reset to a new random value, as with unsolicited advertisements. A
unicast response MAY be delayed, and a multicast response MUST be
delayed, for a small random interval not greater than
MAX_RTR_RESPONSE_DELAY, in order to prevent synchronization with other
responding routers, and to allow multiple, closely-spaced solicitations
to be answered with a single multicast advertisement. A router that
chooses to delay responses behaves as follows:
- Upon receipt of a Router Solicitation, start a timer taken from a
random value within the range 0-MAX_RTR_RESPONSE_DELAY.
- When the timer expires, send out the Router Advertisement. If no
other Router Solicitation was received while waiting for the timer to
expire, unicast the advertisement. Otherwise, multicast the response
and reset the interface timer to a new random value, as is done when
multicasting an unsolicited response.
Note that a router is permitted to send multicast Router Advertisements
more frequently than indicated by the MinRtrAdvInterval configuration
variable if the additional advertisements are responses to explicit
solicitations. In all cases, however, unsolicited multicast
advertisements MUST NOT be sent more frequently than indicated by
MinRtrAdvInterval.
When a router receives a Router Solicitation it records that the source
of the packet is a neighbor. If the solicitation contains a Source
Link-Layer Address option, and the router has a Neighbor Cache entry for
the neighbor, the link-layer address SHOULD be updated in the Neighbor
Cache and the entry's "is_router" flag SHOULD be set to false. If a
Neighbor Cache entry is created for the source its reachability state
MUST be set to PROBE as specified in Section 6.3.2.
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It should be noted that an interface may become an advertising interface
at times other than system startup, as a result of recovery from an
interface failure or through actions of system management such as:
- enabling the interface, if it had been administratively disabled,
and its AdvertiseDefault flag is TRUE, or
- enabling IP forwarding capability (i.e., changing the system from
being a host to being a router), when the interface's
AdvertiseDefault flag is TRUE, or
- changing the AdvertiseDefault flag from FALSE to TRUE.
In such cases the router MUST commence transmission of periodic
advertisements on the new advertising interface, limiting the first few
advertisements to intervals no greater than
MAX_INITIAL_RTR_ADVERT_INTERVAL. In the case of a host becoming a
router, the system MUST also join the all-routers IP multicast group on
all interfaces on which the router supports IP multicast (whether or not
they are advertising interfaces).
An interface may also cease to be an advertising interface, through
actions of system management such as:
- administratively disabling the interface, or
- shutting down the system, or disabling the IP forwarding capability
(i.e., changing the system from being a router to being a host), or
- setting the AdvertiseDefault flag of the interface to FALSE.
In such cases the router SHOULD transmit a final multicast Router
Advertisement on the interface with a Router Lifetime field of zero. In
the case of a router becoming a host, the system MUST also depart from
the all-routers IP multicast group on all interfaces on which the router
supports IP multicast (whether or not they had been advertising
interfaces). In addition, the host MUST insure that subsequent Neighbor
Advertisement messages sent from the interface have the Router flag set
to zero.
The information advertised in Router Advertisements may change through
actions of system management. For instance, the lifetime of advertised
prefixes may change, the advertised MTU may change, etc. In such cases,
the router MAY transmit a few (no more than
MAX_INITIAL_RTR_ADVERTISEMENTS) Router Advertisements separated by an
interval of MAX_INITIAL_RTR_ADVERT_INTERVAL.
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A router might want to send Router Advertisements without advertising
itself as being a default router. For instance, a router might
advertise prefixes for address autoconfiguration while not wishing to
forward packets. Such a router MUST set the Router Lifetime field to
zero in its advertisements.
A router MAY choose not to include all Prefix Information options in
every Router Advertisement. For example, if prefix lifetimes are much
longer than RtrAdvLifetime, including them every few advertisements may
be sufficient. However, when responding to a Router Solicitation the
router SHOULD transmit all prefixes to allow hosts to quickly discover
the prefixes during system initialization.
5.2.4. Router Advertisement Consistency
Routers SHOULD inspect valid Router Advertisements sent by other routers
on the link and verify that the routers are advertising consistent
information. Detected inconsistencies indicate that one or more routers
might be misconfigured and SHOULD be logged to system or network
management. The minimum set of information that should be checked
includes:
- Max Hop Limit values (except for the unspecified value of zero).
- Values of the M or O flags.
- Reachable Time values (except for the unspecified value of zero).
- Retrans Timer values (except for the unspecified value of zero).
- Values in the MTU options.
- Invalidation Lifetimes for the same prefix.
- Deprecation Lifetimes for the same prefix.
Note that it is not an error for different routers to advertise
different sets of prefixes. Also, some routers might leave some fields
as unspecified i.e. with the value zero. The logging of errors SHOULD
be restricted to conflicting information that causes hosts to
continually switch from one value to another.
In addition, routers can optionally examine the source address of Router
Advertisements to determine which of a neighboring router's addresses is
its link-local address.
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Any other action on reception of Router Advertisement messages by a
router is beyond the scope of this document.
5.2.5. Link-local Address Change
The link-local address on a router SHOULD change infrequently. Nodes
receiving Neighbor Discovery messages use the source address to identify
the sender. If multiple packets from the same router contain different
source addresses, nodes will assume they come from different nodes,
leading to undesirable behavior. For example, a node will ignore
Redirect messages that are believed to have been sent by a router other
than the current first-hop router. Thus the source address used in
Router Advertisements must be identical to the target address in a
Redirect message when redirecting to the router.
Using the link-local address to uniquely identify routers on the link
has the benefit that the link-local address does not change when a site
renumbers.
If a router changes the link-local address for one of its interfaces, it
SHOULD inform hosts of this change. The router SHOULD multicast a few
Router Advertisements with Router Lifetime field set to zero for the old
link-local address and also multicast a few Router Advertisements for
the new link-local address. The exact procedures SHOULD be the same as
when an interface ceases being an advertising interface, and when an
interface becomes an advertising interface, respectively.
A router MUST be able to determine the link-local address for each of
its neighboring routers in order to ensure that the target address in a
Redirect message identifies the neighbor router by its link-local
address. This may require that routing protocols exchange link-local
addresses. Alternatively, routers could listen to Router Advertisements
messages to determine link-local addresses of neighboring routers.
However, doing so only works if all routers are sending out Router
Advertisements.
5.3. Host Specification
5.3.1. Host Configuration Variables
None.
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5.3.2. Host Variables
A host maintains certain Neighbor Discovery related variables in
addition to the data structures defined in Section 4.1. These variables
have default values that are overridden by information received in
Router Advertisement messages. The default values are used when there
is no router on the link, or when all received Router Advertisements
have left a particular value unspecified.
For each interface:
LinkMTU The MTU of the link.
Default: The valued defined in the specific document
that describe how IPv6 operates over the particular
link layer (e.g., [IPv6-ETHER]).
MaximumHopLimit
The maximum Hop Count to be used when sending IP
packets.
Default: The value specified in the most recent
"Assigned Numbers" RFC [ASSIGNED].
ReachableTime The time a neighbor is considered reachable after
receiving a reachability confirmation.
Default: A uniformly-distributed random value
between MIN_RANDOM_FACTOR and MAX_RANDOM_FACTOR
times REACHABLE_TIME milliseconds.
RetransTimer The time between retransmissions of Neighbor
Solicitation messages to a neighbor when resolving
the address or when probing the reachability of a
neighbor.
Default: RETRANS_TIMER milliseconds
5.3.3. Validation of Router Advertisement Messages
A node MUST silently discard any received Router Advertisement messages
that do not satisfy all of the following validity checks:
- IP Source Address is a link-local address.
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- IP Destination Address is a link-local address or a multicast
address with link-local scope.
- IP Routing Header is not present.
- if the message includes an IP Authentication Header, the message
authenticates correctly.
- ICMP Checksum is valid.
- ICMP Code is 0.
- ICMP length (derived from the IP length) is 16 or more octets.
- all included options have a length that is greater than zero.
The contents of the Reserved field, and of any unrecognized options,
MUST be ignored. Future, backward-compatible changes to the protocol
may specify the contents of the Reserved field or add new options;
backward-incompatible changes may use different Code values.
An advertisement that passes the validity checks is called a "valid
advertisement".
A host MUST silently discard any received Router Solicitation messages.
5.3.4. Host Behavior
The host joins the all-nodes multicast address on all multicast capable
interfaces.
A host MUST NOT send a Router Advertisement message at any time.
To process a valid Router Advertisement, a host extracts the source
address of the packet and does the following:
- If the address is not already present in the host's Default Router
List, and the advertisement's Router Lifetime is non-zero, create a
new entry in the list, and initialize its timer value from the
advertisement's Router Lifetime field.
- If the address is already present in the host's Default Router List
as a result of a previously-received advertisement, reset its timer
to the Router Lifetime value in the newly-received advertisement.
- If the address is already present in the host's Default Router List
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and the received Router Lifetime value is zero, time-out the entry
immediately and remove it from the Default Router list.
If the received Max Hop Limit value is non-zero the host SHOULD set its
MaximumHopLimit variable to the received value. Hosts use the last Max
Hop Limit value they have received; routers should be configured to
advertise identical values to avoid hosts switching between different
values.
The host SHOULD set its ReachableTime variable based on the Reachable
Time field, if the received value is non-zero. The value is computed as
a uniformly-distributed random value between MIN_RANDOM_FACTOR and
MAX_RANDOM_FACTOR times the value received in the Reachable Time field.
Reception of another Router Advertisement causes a new random value to
be chosen. This avoids any synchronization of Neighbor Unreachability
Detection messages.
The RetransTimer SHOULD be set to the Retrans Timer field, if the
received value is non-zero.
Hosts use the last Reachable Time and Retrans Timer values they have
received; routers should be configured to advertise identical values to
avoid having hosts switch between different values as they receive
advertisements from different routers.
After extracting information from the fixed part of the Router
Advertisement message, the advertisement MUST be scanned for valid
options. If the advertisement contains a source link-layer address
option the link-layer address MUST be recorded in the Neighbor Cache
entry for the router (creating an entry if necessary) and the
"is_router" flag in the Neighbor Cache entry MUST be set to true. The
"is_route" flag is used by Neighbor Unreachability Detection to
determine when a router changes to being a host (i.e. no longer capable
of forwarding packets). If a Neighbor Cache entry is created for the
router its reachability state MUST be set to PROBE as specified in
Section 6.3.2.
Received MTU options are handled as specified in Section 8.4.
For each Prefix Information option that has the "on-link" (L) flag set,
the host does the following:
- If the prefix is not already present in the Prefix List, create a
new entry for the prefix and initialize its invalidation timer to
the Invalidation Lifetime value in the Prefix Information option.
- If the prefix is already present in the host's Prefix List as the
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result of a previously-received advertisement, reset its
invalidation timer to the Invalidation Lifetime value in the Prefix
Information option. If the new Lifetime value is zero, time-out
the prefix immediately.
- If the received Invalidation Lifetime value is zero, and the prefix
is not present in the host's Prefix List, silently ignore the
option.
Note: Implementations can choose to process the on-link aspects of
the prefixes separately from the address autoconfiguration aspects of
the prefixes by e.g. passing a copy of each valid Router
Advertisement message to both an "on-link" and an "addrconf"
function. Each function can then operate independently on the
prefixes that have the appropriate flag set.
Whenever the invalidation timer expires for a Prefix List entry, that
entry is discarded. No existing Destination Cache entries are affected,
however.
Whenever a timer expires for an entry in the Default Router List, that
entry is discarded. Any entries in the Destination Cache going through
that router will continue to be used. Neighbor Unreachability Detection
will purge them if appropriate.
To limit the storage needed for the Default Router List, a host MAY
choose not to store all of the router addresses discovered via
advertisements. However, a host MUST retain at least two router
addresses and SHOULD retain more. Default router selections are made
whenever communication to a destination appears to be failing. Thus,
the more routers on the list, the more likely an alternative working
router can be found quickly (e.g., without having to wait for the next
advertisement to arrive).
The algorithm for selecting a router depends in part on whether or not a
router is known to be reachable. The exact details of how a node keeps
track of a neighbor's reachability state are covered in Section 6.3.
The algorithm for selecting a default router is invoked only when a
Destination Cache entry is incomplete or when communication through an
existing router appears to be failing. Under normal conditions, a
router would be selected the first time traffic is sent to a
destination, with subsequent traffic for that destination using the same
router as indicated in the Destination Cache. The policy for selecting
routers from the Default Router List is as follows:
1) Routers reachable or probably reachable (e.g., in the REACHABLE or
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PROBE state) MUST be preferred over routers whose reachability is
unknown or suspect. An implementation may choose to always return
the same router or cycle through the router list in a round-robin
fashion as long as it always returns a reachable or probably
reachable router when one is available.
2) When no routers on the list are known to be reachable or probably
reachable, routers SHOULD be selected in a round-robin fashion, so
that subsequent requests for a default router do not return the
same router until all other routers have been selected.
Cycling through the router list in this case ensures that all
available routers are actively probed by the Neighbor
Unreachability Detection algorithm. A request for a default router
is made in conjunction with the sending of a packet to a router,
and the selected router will be probed for reachability as a side
effect.
3) If the Default Router List is empty, assume that the destination is
on-link as specified in Section 4.2.
A host is permitted (but not required) to transmit up to
MAX_RTR_SOLICITATIONS Router Solicitation messages from any of its
multicast interfaces after any of the following events:
- The interface is initialized at system startup time.
- The interface is reinitialized after a temporary interface failure
or after being temporarily disabled by system management.
- The system changes from being a router to being a host, by having
its IP forwarding capability turned off by system management.
- The host is re-attached to a link after being detached for some
time.
The IP destination address of the solicitations is the all-routers
multicast address. The IP source address MUST be one of the interface's
addresses and MUST be a link-local address. The Source Link-Layer
Address option is set to the host's link-layer address.
If a host does choose to send a solicitation after one of the above
events, it SHOULD delay that transmission for a random amount of time
between 0 and MAX_RTR_SOLICITATION_DELAY. This serves to alleviate
congestion when many hosts start up on a link at the same time, such as
might happen after recovery from a power failure. (It is recommended
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that hosts include some unique value, such as one of their IP or link-
layer addresses, in the seed used to initialize their pseudo-random
number generators.) Although the randomization range is specified in
units of seconds, the actual randomly-chosen values should not be in
units of whole seconds, but rather in units of the highest available
timer resolution.
If a host has performed a random delay earlier during the system startup
(e.g. as part of Duplicate Address Detection [ADDRCONF]) there is no
need to randomly delay the first Router Solicitation message.
A host MAY also choose to further postpone its solicitations, subsequent
to one of the above events, until the first time it needs to use a
default router.
Upon receiving a valid advertisement with a non-zero Lifetime, the host
MUST desist from sending any solicitations on that interface (even if
none have been sent yet), until the next time one of the above events
occurs. The small number of retransmissions of a solicitation, which
are permitted if no such advertisement is received, SHOULD be sent at
intervals of RTR_SOLICITATION_INTERVAL seconds, without randomization.
6. ADDRESS RESOLUTION AND NEIGHBOR UNREACHABILITY DETECTION
This section describes the functions related to the Neighbor
Solicitation and Neighbor Advertisement messages and includes
descriptions of Address Resolution and the Neighbor Unreachability
Detection algorithm.
These messages are also used for Duplicate Address Detection as
specified by [ADDRCONF]. In particular, Duplicate Address Detection
sends Neighbor Solicitation messages using an unspecified source address
targeting its own address. This will generate a multicast Neighbor
Advertisement from any node(s) that have been configured with the same
address.
6.1. Message Formats
6.1.1. 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
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reachability of a neighbor.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Sender Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Target Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
IP Fields:
Source Address
MUST be either the link-local address assigned to the
interface from which this message is sent, or the
unspecified address. Use of the unspecified address
directs the target node to multicast the resultant
Neighbor Advertisement as required by Duplicate
Address Detection in [ADDRCONF].
Destination Address
Either the solicited-node link-local multicast address
corresponding to the target address, or the target
address. Packets unicast to the target address are
used to verify reachability.
Hop Count 1
Authentication Header
If a Security Association for the IP Authentication
Header exists between the sender and the destination
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address, then the sender SHOULD include this header.
Routing Header MUST NOT be sent.
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 ignored by the receiver.
Sender Address
An IP address assigned to the interface from which the
solicitation is sent. If the source address of the
data packet prompting the solicitation is the same of
one of the sending interface's addresses, that address
SHOULD be used. Doing so ensures that the receiver of
the solicitation places the data packet's source
address in its Neighbor Cache, eliminating the need
for address resolution in the likely case that reverse
traffic for that destination will follow.
Target Address
The IP address of the target of the solicitation. It
MUST NOT be a multicast address.
Options:
Source link-layer address
The link-layer address for the sender. MUST NOT be
included in unicast solicitations, in order to prevent
off-link senders from creating or modifying cached
link-layer addresses. For multicast solicitations
sent on link layers that have addresses it SHOULD be
included.
Future versions of this protocol may define new option types.
Receivers MUST skip over and ignore any options they do not recognize
and continue processing the message.
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6.1.2. Neighbor Advertisement Message Format
A node MUST send a Neighbor Advertisement in response to a Neighbor
Solicitation for a target IP address that matches an assigned address on
the receiving interface. A node MAY also send an unsolicited Neighbor
Advertisement if wishes to advertise that its link-layer address has
changed.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R|S|N| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Target 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 an invoking Neighbor
Solicitation or, if the source address in the
solicitation is the unspecified address, the all-nodes
link-local multicast address. For an unsolicited
advertisement the destination is typically the all-
nodes link-local multicast address.
Hop Count 1
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.
Routing Header MUST NOT be sent.
ICMP Fields:
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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. It MUST be
zero in a multicast advertisement and in an
unsolicited unicast advertisement.
N Secondary Advertisement flag. When set, the N-bit
indicates that the advertisement should only be used
if no other advertisement has been received i.e. the
advertisement will not update a cached link-layer
address. It SHOULD be set in solicited advertisements
for anycast addresses and in solicited proxy
advertisements. It SHOULD be zero in other solicited
advertisements and in unsolicited advertisements.
Reserved 29-bit unused field. It MUST be initialized to zero
by the sender and ignored by the receiver.
Target Address
The address from 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.
Options:
Target link-layer address
The link-layer address for the target. MUST be
included on link layers that have addresses.
Future versions of this protocol may define new option types.
Receivers MUST skip over and ignore any options they do not recognize
and continue processing the message.
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6.2. Address Resolution
Address Resolution provides the mechanism through which a node
determines the link-layer address of a neighbor. Address Resolution is
only used for destinations that are determined to be on-link and for
which the sender does not know the corresponding link-layer address.
Address resolution is never used for multicast destinations.
6.2.1. Node Specification
When a multicast-capable interface is initialized the node MUST join the
all-nodes multicast address on that interface, as well as the
solicited-node multicast address corresponding to each of the IP
addresses assigned to the interface.
The operation of automatic address configuration [ADDRCONF] may, over
time, change the set of addresses assigned to an interface; new
addresses might be added and old addresses might be removed. In such
case the node MUST join and leave the solicited-node multicast address
corresponding to the new and old addresses, respectively. Note that
multiple assigned addresses might correspond to the same solicited-node
multicast address; a node MUST NOT leave the solicited-node multicast
group until all assigned addresses corresponding to that multicast
address have been removed.
6.2.2. Sending Neighbor Solicitations
When a node has a unicast packet to send, but does not know the next-
hop's link-layer address, it performs address resolution by creating a
Neighbor Cache entry in the INCOMPLETE state and transmitting a Neighbor
Solicitation message targeted at the neighbor. The solicitation must be
sent to the solicited-node multicast address of the target address.
The sender SHOULD include its link-layer address (if it has one) in the
multicast solicitation as a Source Link-Layer Address option. If the
source address of the packet prompting the solicitation is the same as
one of the addresses assigned to the outgoing interface, that address
SHOULD be placed in the ICMP Sender Address of the outgoing
solicitation. Otherwise, the interface's link-local address should be
used. Using the prompting packet's source address when possible insures
that the recipient of the Neighbor Solicitation installs in its Neighbor
Cache the IP address that is highly likely to be used in subsequent
traffic belonging to the prompting packet's "connection".
While waiting for address resolution to complete, the sender MUST retain
packets waiting for address resolution to complete in a small queue.
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The queue MUST hold at least one packet, and MAY contain more. However,
the number of queued packets per neighbor SHOULD be limited to some
small value. When a queue overflows, the new arrival SHOULD replace the
oldest entry. Once address resolution completes, all queued packets
SHOULD be transmitted.
While awaiting a response, the sender MUST retransmit Neighbor
Solicitation messages approximately every RetransTimer milliseconds,
even in the absence of additional traffic to the neighbor.
Retransmissions MUST be rate-limited for each neighbor to at most one
solicitation every RetransTimer milliseconds.
If no advertisement is received after MAX_MULTICAST_SOLICIT
solicitations, address resolution has failed. The sender MUST return
ICMP destination unreachable indications with code 3 (Address
Unreachable) for each packet queued awaiting address resolution.
6.2.3. Validation of Neighbor Solicitations
A node MUST silently discard any received Neighbor Solicitation messages
that do not satisfy all of the following validity checks:
- IP Source Address is a link-local address or the unspecified
address.
- if the IP Destination Address is a multicast address, its scope is
link-local.
- IP Routing Header is not present.
- if the message includes an IP Authentication Header, the message
authenticates correctly.
- ICMP Checksum is valid.
- ICMP Code is 0.
- ICMP length (derived from the IP length) is 40 or more octets.
- Target Address is not a multicast address.
- if the Source Address is the unspecified address or the Destination
Address is a unicast address, there is no Source Link-layer Address
option.
- all included options have a length that is greater than zero.
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- the Target Address matches an address assigned to the receiving
interface.
The contents of the Reserved field, and of any unrecognized options,
MUST be ignored. Future, backward-compatible changes to the protocol
may specify the contents of the Reserved field or add new options;
backward-incompatible changes may use different Code values.
A Neighbor Solicitation that passes the validity checks is called a
"valid solicitation".
6.2.4. Receipt of Neighbor Solicitations
If and the Source Link-Layer Address option is present, the recipient
SHOULD update the Neighbor Cache entries for both the IP Source Address
and the ICMP Sender Address of the solicitation. In those cases where a
corresponding entry does not already exist, the node SHOULD create a new
one and set its reachability state to PROBE as specified in
Section 6.3.2. In all cases the source link-layer address option in the
received advertisement SHOULD replace any cached link-layer addresses.
A Neighbor Solicitation that is being used for Duplicate Address
Detection, i.e. with an unspecified source address, can not contain a
source link-layer address option thus it has no effect on the Neighbor
Cache.
6.2.5. Sending Solicited Neighbor Advertisements
A Neighbor Advertisement is sent in response to a valid Neighbor
Solicitation. The Target Address of the advertisement is copied from
the Target Address of the Solicitation. The Target Link-Layer Address
option SHOULD be included, using as its value the interface's link-layer
address. If the node is a router, it MUST set the Router flag to one;
otherwise it MUST set the flag to zero.
If the Target Address is either an anycast address or a unicast address
for which the node is providing proxy service, the Secondary
Advertisement flag SHOULD be set to one. Otherwise, it SHOULD be set
to 0. Proper setting of the Secondary Advertisement flag insures that
nodes give preference to "primary" advertisements, even when received
after "secondary" advertisements.
If the source of the solicitation is the unspecified address, the node
MUST set the Solicited flag to zero and multicast the advertisement to
the all-nodes address. Otherwise, the node MUST set the Solicited flag
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to one and unicast the advertisement to the link-local Source Address of
the solicitation.
6.2.6. Validation of Neighbor Advertisements
A node MUST silently discard any received Neighbor Advertisement
messages that do not satisfy all of the following validity checks:
- IP Source Address is a link-local address.
- IP Destination Address is a link-local address or a multicast
address with link-local scope.
- IP Routing Header is not present.
- if the message includes an IP Authentication Header, the message
authenticates correctly.
- ICMP Checksum is valid.
- ICMP Code is 0.
- ICMP length (derived from the IP length) is 24 or more octets.
- Target Address is not a multicast address.
- if the Destination Address is a multicast address the Solicited
flag is zero.
- all included options have a length that is greater than zero.
The contents of the Reserved field, and of any unrecognized options,
MUST be ignored. Future, backward-compatible changes to the protocol
may specify the contents of the Reserved field or add new options;
backward-incompatible changes may use different Code values.
A Neighbor Advertisements that passes the validity checks is called a
"valid advertisement".
6.2.7. Receipt of Neighbor Advertisments
When a valid Neighbor Advertisement is received (either solicited or
unsolicited), the Neighbor Cache is searched for the target's entry. If
no entry exists, the advertisement SHOULD be silently discarded. There
is no need to create an entry in this case, since the recipient has
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apparently not initiated any communication with the target.
Once the appropriate Neighbor Cache entry has been located, the specific
actions taken depend on the state of the Neighbor Cache entry. In
particular, if no link-layer address is cached for the target (e.g., it
is in the INCOMPLETE state), the first received advertisement would be
used. On the other hand, if we already have a cached link-layer
address, we can safely be more selective about what information is used
in received advertisements.
If the target's Neighbor Cache entry is in the INCOMPLETE state, the
advertisement is the first response to a solicitation. The receiving
node MUST record the link-layer address in the Neighbor Cache entry and
send any packets queued for the neighbor awaiting address resolution.
If the Solicited flag is set, the reachability state for the neighbor
MUST be set to REACHABLE; otherwise it MUST be set to PROBE. (A more
detailed explanation of reachability state is described in Section
6.3.2). The Secondary Advertisement flag is ignored if the entry is in
the INCOMPLETE state.
If the target's Neighbor Cache entry is in the REACHABLE or PROBE state,
the Secondary Advertisement flag is examined. If set, the entry's state
should be set to PROBE, and the packet SHOULD be silently discarded; no
other changes are made to the Neighbor Cache entry.
If the Secondary Advertisement flag is not set, the link-layer address
in the Target Link-Layer Address option should be copied into the
Neighbor Cache entry. Furthermore, if the Solicited flag is set, the
entry's state should be set to REACHABLE. Otherwise, the entry's state
should be set to PROBE.
Finally, the receiving node MUST examine the Router flag in the received
advertisement and update the "is_router" flag in the Neighbor Cache
entry to reflect whether the node is a host or router. In those cases
where the neighbor was previously used as a router, but the
advertisement's Router flag is now set to zero, the node MUST remove
that router from the Default Router List and update the Destination
Cache entries for all destinations using that neighbor as a router as
specified in Section 6.3.2.
6.2.8. Sending Unsolicited Neighbor Advertisements
In some cases a node may be able to determine that its link-layer
address has changed (e.g., hot-swap of an interface card) and may wish
to inform its neighbors of the new link-layer address quickly. In such
cases a node MAY send up to MAX_NEIGHBOR_ADVERTISEMENT unsolicited
Neighbor Advertisement messages to the all-nodes multicast address.
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These advertisements MUST be separated by at least
MIN_NEIGHBOR_ADVERT_INTERVAL seconds.
The Target Address field in the unsolicited advertisement is set to an
IP address of the interface, and the Target Link-Layer Address option is
filled with the new link-layer address. The Solicited flag MUST be set
to zero, in order to avoid confusing the Neighbor Unreachability
Detection algorithm. If the node is a router, it MUST set the Router
flag to one; otherwise it MUST set it to zero. The Secondary
Advertisement flag MAY be either set or cleared. In either case,
neighboring nodes will immediately change the state of their Neighbor
Cache entries for the Target Address to PROBE, prompting them to verify
the path for reachability. If the Secondary Advertisement is set,
neighboring nodes will install the new link-layer address in their
caches. Otherwise, they will ignore the new link-layer address,
choosing instead to probe the cached address instead.
A node that has multiple IP addresses assigned to an interface MAY
multicast a separate Neighbor Advertisement for each address. In such a
case the node SHOULD introduce a small delay between the sending of each
advertisement to reduce the probability of the advertisements being lost
due to congestion.
A proxy MAY multicast Neighbor Advertisements when its link-layer
address changes or when it is configured (by system management or other
mechanisms) to proxy for an address. If there are multiple nodes that
are providing proxy services for the same set of addresses the proxies
SHOULD provide a mechanism that prevents multiple proxies from
multicasting advertisements for any one address, in order to reduce the
risk of excessive multicast traffic.
Also, a node belonging to an anycast address MAY multicast unsolicited
Neighbor Advertisements for the anycast address when the node's link-
layer address changes.
Note that because unsolicited Neighbor Advertisements do not reliably
update caches in all nodes (the advertisements might not be received by
all nodes), they should only be viewed as a performance optimization to
quickly update the caches in most neighbors. The Neighbor
Unreachability Detection algorithm ensures that all nodes reliably
obtain the new link-layer address, though the delay may be slightly
longer.
6.2.9. Anycast Neighbor Advertisements
A node belonging to an anycast address MUST join the solicited-node
multicast address that corresponds to the anycast address.
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When a node responds to a Neighbor Solicitation for an anycast address,
it MUST respond with an Neighbor Advertisement that has the Secondary
Advertisement flag set to one. In addition, the sender should delay
sending a response for a random time between 0 and
MAX_ANYCAST_DELAY_TIME seconds.
Neighbor Unreachability Detection ensures that a node quickly detects
when the current binding for an anycast address becomes invalid.
6.2.10. Proxy Neighbor Advertisements
Under limited circumstances, a router MAY proxy for one or more other
nodes, that is, through Neighbor Advertisements indicate that it is
willing to accept packets not explicitly addressed to itself. For
example, a router might potentially accept packets on behalf of a mobile
node that has moved off-link. The mechanisms used by proxy are
identical to the mechanisms needed for anycast addresses.
A proxy MUST join the solicited-node multicast address(es) that
correspond to the IP address(es) assigned to the node for which it is
proxying.
All solicited proxy Neighbor Advertisement messages MUST have the
Secondary Advertisement flag set to one. This ensures that if the node
itself is present on the link its Neighbor Advertisement (with the
Secondary flag set to zero) will take precedence of any advertisement
received from a proxy. A proxy MAY send unsolicited advertisements with
the Secondary Advertisement flag set to zero as specified in Section
6.2.8, but doing so may cause the proxy advertisement to override a
valid entry created by the node itself.
Finally, when sending a proxy advertisement in response to a Neighbor
Solicitation, the sender should delay its response by a random time
between 0 and MAX_ANYCAST_DELAY_TIME seconds.
6.3. Neighbor Unreachability Detection
Communication to or through a neighbor may fail for numerous reasons at
any time, including hardware failure, hot-swap of an interface card,
etc. If the destination has failed, no recovery is possible and
communication fails. On the other hand, if it is the path that has
failed, recovery may be possible. Thus, a node actively tracks the
reachability "state" for the neighbors to which it is sending packets.
Neighbor Unreachability Detection is used for all paths between hosts
and neighboring nodes, including host-to-host, host-to-router, and
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router-to-host communication. Neighbor Unreachability Detection may
also be used between routers, but is not required if an equivalent
mechanism is available, for example, as part of the routing protocols.
The conceptual model allows an upper-layer to indicate to IP that
Neighbor Unreachability Detection is not needed for a packet being sent.
This is used by Neighbor Discovery to skip these checks when sending
Neighbor Discovery messages.
When a path to a neighbor appears to be failing, the specific recovery
procedure depends on how the neighbor is being used. For example, the
specific recovery procedure used when the neighbor is used as a router
differs from that used when the neighbor is the destination.
Neighbor Unreachability Detection is performed only for neighbors to
which unicast packets are sent; it is not used when sending to multicast
addresses.
6.3.1. Reachability Confirmation
A neighbor is considered reachable if the node has recently received a
confirmation that packets sent recently to the neighbor were received by
its IP layer. Positive confirmation can be gathered in two ways: hints
from upper layer protocols that indicate a connection is making "forward
progress", or receipt of a Neighbor Advertisement message that is a
response to an explicit Neighbor Solicitation probe.
A connection makes "forward progress" if the packets received from a
remote peer can only be arriving if recent packets sent to that peer are
actually reaching it. For example, receipt of a (new) acknowledgement
indicates that previously sent data reached the peer. Likewise, the
arrival of a new (non-duplicate) packet indicates that earlier
acknowledgements are being delivered to the remote peer. If packets are
reaching the peer, they must also be reaching the sender's next-hop
neighbor; thus "forward progress" is a confirmation that the next-hop
neighbor is reachable. For off-link destinations, forward progress
implies that the first-hop router is reachable. When available, this
upper-layer information SHOULD be used.
In some cases (e.g., UDP-based protocols and routers forwarding packets
to hosts) such reachability information may not be readily available
from upper-layer protocols. When no hints are available and a node is
sending packets to a neighbor, the node actively probes the neighbor
using unicast Neighbor Solicitation messages to verify that the forward
path is still working.
The receipt of a solicited Neighbor Advertisement that is a response to
a Neighbor Solicitation probe serves as reachability confirmation, since
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advertisements with the Solicited flag set to one are sent only in
response to a solicitation. Receipt of other Neighbor Discovery
messages such as Router Advertisements and Neighbor Advertisement with
the Solicited flag set to zero MUST NOT be treated as a reachability
confirmation. Receipt of such unsolicited messages only confirm the
one-way path from the neighbor to the recipient node. In contrast,
Neighbor Unreachability Detection requires that the forward path from
the sender to the neighbor be working. Note that an advertisement sent
in response to an explicit solicitation confirms that a path is working
in both directions; the solicitation reached the neighbor, prompting it
to generate an advertisement, and the advertisement reached the querying
node. However, from the perspective of Neighbor Unreachability
Detection, only the reachability of the forward path is of interest.
6.3.2. Node Behavior
Neighbor Unreachability Detection operates in parallel with the sending
of packets to a neighbor. While reasserting a neighbor's reachability,
a node continues sending packets to that neighbor using the cached
link-layer address.
A Neighbor Cache entry can be in one of three states:
INCOMPLETE Address resolution is being performed on the entry.
Specifically, a Neighbor Solicitation has been sent to
the solicited-node multicast address of the target, but
the corresponding Neighbor Advertisement has not yet been
received.
REACHABLE Positive confirmation was received within the last
ReachableTime milliseconds that the forward path to the
neighbor was functioning properly. While REACHABLE, no
special action takes place as packets are sent.
PROBE More than ReachableTime milliseconds have elapsed since the
last positive confirmation was received that the forward
path was functioning properly. Upon entering the PROBE
state, no Neighbor Solicitation is sent. However, a
timer is set to expire DELAY_FIRST_PROBE_TIME seconds
later, and a Neighbor Solicitation probe is sent if the
entry is still in a PROBE state when the timer expires.
Delaying the sending of the initial Neighbor Solicitation
gives the upper layers additional time to provide
reachability confirmation information. After the initial
delay, Neighbor Solicitations are retransmitted every
RetransTimer milliseconds until a reachability
confirmation is received.
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When an entry is created as a result of needing to perform address
resolution, a Neighbor Solicitation is sent to the solicited-node
multicast address of the target, a timer is started to expire
RETRANS_TIMER milliseconds later and the entry's state is set to
INCOMPLETE.
As specified in Section 6.2.2, when in the INCOMPLETE state, Neighbor
Solicitation messages are retransmitted every RETRANS_TIMER milliseconds
until a response is received. If no response is received within
RETRANS_TIMER milliseconds after sending MAX_MULTICAST_SOLICIT probes to
the solicited-node multicast address, address resolution fails. Upon
failure, ICMP destination unreachable indications with code 3 (Address
unreachable) are returned for any queued packets and the entry is
deleted. Note that deleting the entry implies that all destinations
using that neighbor must perform next-hop resolution again before
sending a subsequent packet. Thus, if the neighbor is a router, an
alternate router may be selected. Alternatively, a destination
previously thought to be on-link, may now only be reachable through a
router.
Unreachability detection changes a neighbor's state from REACHABLE to
PROBE only on-demand, as a side effect of sending a data packet to that
neighbor. If no traffic is sent to a neighbor, no probes are sent
either. Note that an entry may technically no longer be in a REACHABLE
state, but the condition need not be checked or acted upon until a
packet is sent to the neighbor.
The first time a Neighbor Cache entry is referenced and more than
ReachableTime milliseconds have passed since receipt of the last
reachability confirmation, its state changes to PROBE. However, no
Neighbor Solicitation probe is sent. Probing is deferred for an
additional DELAY_FIRST_PROBE_TIME seconds, an optimization that gives
the upper-layer protocol additional time to provide a reachability
confirmation in those cases where ReachableTime milliseconds have passed
since the last confirmation due to lack of recent traffic. Without this
optimization the opening of a TCP connection after a traffic lull would
initiate probes even though the subsequent three-way handshake would
provide a reachability confirmation almost immediately.
If no reachability confirmation is received within
DELAY_FIRST_PROBE_TIME seconds after entering the PROBE state, a unicast
Neighbor Solicitation message is sent to the neighbor using the cached
link-layer address. In addition, the sender starts a timer to
retransmit probe messages every RetransTimer milliseconds until the
desired solicitation is received. Subsequent probes are retransmitted
even if no additional packets are sent to the neighbor. If no response
is received after waiting RetransTimer milliseconds after sending the
MAX_UNICAST_SOLICIT solicitations, retransmissions cease and the entry
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SHOULD be deleted. Subsequent traffic to that neighbor recreates the
entry and performs address resolution again.
Note that all Neighbor Solicitations are rate-limited on a per-neighbor
basis. A node MUST NOT send Neighbor Solicitations to the same neighbor
more frequently than once every RetransTimer milliseconds.
A Neighbor Cache entry also enters the PROBE state when created as a
result of receiving packets other than solicited Neighbor Advertisements
(e.g., Router Solicitations, Router Advertisements, Redirects, and
Neighbor Solicitations). These packets contain the link-layer address
of either the sender or, in the case of Redirect, the redirection
target. However, receipt of these link-layer addresses does not confirm
reachability of the forward-direction path to that node. Placing a
newly created Neighbor Cache entry for which the link-layer address is
known in the PROBE state provides assurance that path failures are
detected quickly. As always, when entering the PROBE state, the first
probe is delayed for DELAY_FIRST_PROBE_TIME to give the upper layer some
time to provide a reachability confirmation thereby suppressing the
sending of a probe.
To detect the case where a router switches from being a router to being
a host (e.g., by having its IP forwarding capability turned off by
system management), a node MUST compare the Router flag field in all
received Neighbor Advertisement messages with the "is_router" flag
recorded in the Neighbor Cache entry. When a node detects that a
neighbor has changed from being a router to being a host, the node MUST
remove that router from the Default Router List and update the
Destination Cache so that all entries using that neighbor as a router
switch to another router. Note that a router may not be listed in the
Default Router List, even though a Destination Cache entry is using it
(e.g., the a host was redirected to it).
In some cases, link-specific information may indicate that a path to a
neighbor has failed (e.g., the resetting of a virtual circuit). In such
cases, link-specific information may be used to purge Neighbor Cache
entries before the Neighbor Unreachability Detection would do so.
However, link-specific information MUST NOT be used to confirm the
reachability of a neighbor; such information does not provide end-to-end
confirmation between neighboring IP layers.
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7. REDIRECT FUNCTION
This section describes the functions related to the sending and
processing of Redirect messages.
7.1. Redirect Message Format
A Redirect packet is sent from a router to a host to inform the host of
a better first-hop node on the path to a destination.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 Count 1
Authentication Header
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If a Security Association for the IP Authentication
Header exists between the sender and the destination
address, then the sender SHOULD include this header.
Routing Header MUST NOT be sent.
ICMP Fields:
Type 5
Code 0
Checksum The ICMP checksum. See [ICMPv6].
Reserved This field is unused. It MUST be initialized to zero
by the sender and ignored by the receiver.
Target Address An IP address of the node to which traffic for the
Destination SHOULD be sent. When the target is a
router, the Target Address MUST be the router's link-
local address so that hosts can uniquely identify
routers. When the target is the actual endpoint of
communication, the target address field MUST contain
the same value as the Destination Address field.
Destination Address
The IP address of the destination which is redirected
to the target.
Options:
Target link-layer address
The link-layer address for the target. It MUST be
included on non-broadcast links, since the host can
not use the multicast Neighbor Solicitation to resolve
the address. If known by the router, it SHOULD be
included on all link layers that have addresses.
Redirected Header
As much as possible of the IP packet that triggered
the sending of the Redirect without making the
redirect packet exceed 576 octets.
Future versions of this protocol may define new option types.
Receivers MUST skip over and ignore any options they do not recognize
and continue processing the message.
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7.2. Router Specification
A router SHOULD send a redirect message, subject to rate limiting,
whenever it forwards a packet in which:
- the Source Address field of the packet identifies a neighbor, and
- the router determines that a better first-hop node resides on the
same link as the sending node for the Destination Address of the
packet being forwarded, and
- the Destination Address of the packet is not a multicast address,
and
- the packet is not source routed through the router, i.e. the
destination address (when the packet was received by the router)
did not match one of the router's addresses. Other source routed
packets, not explicitly source routed through the router, can be
redirected.
The transmitted redirect packet contains, consistent with the above
message format:
- In the Target Address field: the address to which subsequent
packets for the destination SHOULD be sent. If the target is a
router, that router's link-local address MUST be used. If the
target is a host the target address field MUST be set to the same
value as the Destination Address field.
- In the Destination Address field: the destination address of the
invoking IP packet.
- In the options:
o Target Link-Layer Address option: link-layer address of the
target, if known.
o Redirected Header: as much of the forwarded packet as can fit
without the redirect packet exceeding 576 octets in size.
A router MUST limit the rate at which Redirect messages are sent, in
order to limit the bandwidth and processing costs incurred by the
Redirect messages when the source does not correctly respond to the
Redirects, or the source chooses to ignore unauthenticated Redirect
messages. More details on the rate-limiting of ICMP error messages can
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be found in [ICMPv6].
A router MUST NOT update its routing tables upon receipt of a Redirect.
7.3. Host Specification
7.3.1. Validation of Redirect Messages
A host MUST silently discard any received Redirect messages that do not
satisfy all of the following validity checks:
- IP Source Address is a link-local address.
- IP Routing Header is not present.
- if the message includes an IP Authentication Header, the message
authenticates correctly.
- ICMP Checksum is valid.
- ICMP Code is 0.
- ICMP length (derived from the IP length) is 40 or more octets.
- the IP source address of the Redirect is the same as the current
first-hop router for the specified destination.
- the Target Address of the redirect is not a multicast address.
- the Destination Address field in the redirect message does not
contain a multicast address.
- all included options have a length that is greater than zero.
The contents of the Reserved field, and of any unrecognized options MUST
be ignored. Future, backward-compatible changes to the protocol may
specify the contents of the Reserved field or add new options;
backward-incompatible changes may use different Code values.
A host MUST NOT consider a redirect invalid just because the Target
Address of the redirect is not covered under one of the link's prefixes.
That is, identical values in the Target and Destination Address fields
indicates that the target destination is on-link.
A redirect that passes the validity checks is called a "valid redirect".
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7.3.2. Host Behavior
A host receiving a valid redirect SHOULD update its routing information
accordingly. When a redirect is received, the host updates the
Destination Cache entry for the destination to use to the specified
target as the new next-hop. If no Destination Cache entry exists for
the destination, such an entry is created (placing it in the PROBE
state).
If the redirect contains a Target Link-Layer Address option the host
either creates or updates the Neighbor Cache entry for the target. The
link-layer address in the Neighbor Cache entry MUST be copied from the
Target Link-Layer Address option into the appropriate Neighbor Cache
entry. If a Neighbor Cache entry is created for the target its
reachability state MUST be set to PROBE as specified in Section 6.3.2.
In addition, if the Target Address is the same as the Destination
Address, the host MUST treat the destination as on-link and set the
"is_router" field in the corresponding Neighbor Cache entry to false.
Otherwise it MUST set to true.
A host MAY have a configuration switch that can be set to make it ignore
a Redirect message that does not have an IP Authentication header.
A host MUST NOT send Redirect messages.
8. OPTIONS
Options provide a mechanism for encoding variable length fields, fields
that may appear multiple times in the same packet, or information that
is optional and may not appear in all packets. Options can also be used
to add additional functionality to future versions of ND.
In order to ensure that future extensions properly coexist with current
implementations, all nodes MUST silently ignore any options they do not
recognize in received ND packets and continue processing the packet.
All options specified in this document MUST be recognized. A node MUST
NOT ignore valid options just because the ND message contains
unrecognized ones.
The current set of options is defined in such a way that receivers can
process multiple options in the same packet independently of each other.
In order to maintain these properties future options SHOULD follow the
simple rule:
The option MUST NOT depend on the presence or absence of any other
options. The semantics of an option should depend only on the
information in the fixed part of the ND packet and on the
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information contained in the option itself.
Adhering to the above rule has the following benefits:
1) Receivers can process options independently of one another. For
example, an implementation can choose to process the Prefix
Information option contained in a Router Advertisement message in a
user-space process while the link-layer address in the same message
is processed by routines in the kernel.
2) Should the number of options cause a packet to exceed a link's MTU,
multiple packets can carry subsets of the options without any
change in semantics.
3) Senders MAY send a subset of options in different packets. For
instance, if the prefix Invalidation Lifetime is high it might not
be necessary to include the Prefix Information option in every
Router Advertisement. In addition, different routers might send
different sets of options. Thus, a receiver MUST NOT associate any
action with the absence of an option in a particular packet. This
protocol specifies that receivers should only act on the expiration
of timers and on the information that is received in the packets.
When multiple options are present in a Neighbor Discovery packet, they
may appear in any order; receivers MUST be prepared to process them
independently of their order. There can also be multiple instances of
the same option in a message, for instance Prefix Information options.
The length of all options is a multiple of 8 octets, ensuring
appropriate alignment without any "pad" options. The fields in the
options, as well as the fields in ND packets, are defined to align them
on their natural boundaries (e.g. a 16-bit field is aligned on a 16-bit
boundary) with the exception of the 128-bit IP addresses/prefixes, which
are aligned on a 64-bit boundary.
The link-layer address field contains an uninterpreted octet string; it
is aligned on an 8-bit boundary.
All options are of the form:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ... ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Fields:
Type 8-bit identifier of the type of option. The options
defined in this document are:
Option Name Type
Source Link-Layer Address 1
Target Link-Layer Address 2
Prefix Information 3
Redirected Header 4
MTU 5
Length 8-bit unsigned integer. The length of the option in
units of 8 octets. The value 0 is invalid. Nodes
MUST silently discard an ND packet that contains an
option with length zero.
The size of an ND packet including the IP header is limited to the link
MTU (which is at least 576 octets). When adding options to an ND packet
a node MUST NOT exceed the link MTU.
The only ND packets that can potentially exceed the link MTU are Router
Advertisements and Redirects; the former due to a large number of Prefix
Information options and the latter due to the Redirected Header option.
If there are more Prefix Information options than can fit in a single
Router Advertisement packet the router MUST send multiple separate
advertisements that each contain a subset of the set of prefixes.
The amount of data to include in the Redirected Header option MUST be
limited so that the entire redirect packet does not exceed 576 octets.
8.1. Source/Target Link-layer Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Link-Layer Address ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type
1 for Source Link-layer Address
2 for Target Link-layer Address
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Length The length of the option in units of 8 octets. For
example, the length for IEEE 802 addresses is 1
[IPv6-ETHER].
Link-Layer Address
The variable length link-layer address.
The content and format of this field 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.
8.2. Prefix Information
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Prefix Length |L|A| Reserved1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Invalidation Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Deprecation Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Prefix +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type 3
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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.
A 1-bit autonomous address-configuration flag. When set
indicates that this prefix can used for autonomous
address configuration as specified in [ADDRCONF].
Reserved1 6-bit unused field. It MUST be initialized to zero by
the sender and ignored by the receiver.
Invalidation Lifetime
32-bit unsigned integer. The lifetime of the prefix
in seconds for the purpose of on-link determination.
A value of all one bits (0xffffffff) represents
infinity. This lifetime is also used by [ADDRCONF].
Deprecation Lifetime
32 bits reserved for autonomous address configuration.
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 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.
Description
The Prefix Information option is only used in Router
Advertisement packets. It provide hosts with on-link
prefixes and prefixes for Address Autoconfiguration.
Implementations can choose to process the on-link
aspects of the prefixes separately from the address
autoconfiguration aspects of the prefixes e.g. by
passing a copy of each valid Router Advertisement
message to both an "on-link" and an "addrconf"
function. Each function can then operate on the
prefixes that have the appropriate flag set.
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8.3. Redirected Header
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 ignored by the receiver.
IP header + data
The original packet truncated to ensure that the size
of the redirect message does not exceed 576 octets.
Description
The Redirected Header option MUST be included in
Redirect packets.
8.4. MTU
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type 5
Length 1
Reserved This field is unused. It MUST be initialized to zero
by the sender and ignored by the receiver.
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MTU 32-bit unsigned integer. The recommended MTU for the
link.
Description
The MTU option SHOULD be included in Router
Advertisement packets when the link has no well-known
MTU and it MAY be included on links with a well-known
MTU.
Hosts MUST handle this option by setting the LinkMTU
variable for the interface to the received value. If
the routers on the link are advertising different MTU
values this will result in hosts switching between the
different MTUs. Therefore, routers SHOULD verify the
consistency between the MTU they and other routers
advertise, logging a network management event when
contradictory advertisements are detected.
When a host or its interface is initialized the
LinkMTU of the interface SHOULD be set to the
predefined value for that type of link. If the host
receives no MTU option it MUST continue to use that
predefined value. The MTU option can be used by
routers to both increase and decrease the MTU.
In configurations in which heterogeneous technologies
are bridged together, the maximum supported MTU may
differ from one segment to another. If the bridges do
not generate ICMP Packet Too Big messages,
communicating nodes will be unable to use Path MTU to
dynamically determine the appropriate MTU on a per-
neighbor basis. In such cases, routers use the MTU
option to specify an MTU value supported by all
segments.
9. MULTIHOMED HOSTS
There are a number of complicating issues that arise when Neighbor
Discovery is used by hosts that have multiple interfaces. This section
does not attempt to define the proper operation of multihomed hosts with
regard to Neighbor Discovery. Rather, it identifies issues that require
further study. Implementors are encouraged to experiment with various
approaches to making Neighbor Discovery work on multihomed hosts and to
report their experiences.
If a multihomed host receives Router Advertisements on all of its
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interfaces, it will (probably) have learned on-link prefixes for the
addresses residing on each link. When a packet must be sent through a
router, however, selecting the "wrong" router can result in a suboptimal
or non-functioning path. There are number of issues to consider:
1) In order for a router to send a redirect, it must determine that
the packet it is forwarding originates from a neighbor. The
standard test for this case is to compare the source address of the
packet to the list of on-link prefixes associated with the
interface on which the packet was received. If the originating
host is multihomed, however, the source address it uses may belong
to an interface other than the interface from which it was sent.
In such cases, a router will not send redirects, and suboptimal
routing is likely. In order to be redirected, the sending host
must always send packets out the interface corresponding to the
outgoing packet's source address. Note that this issue never
arises with non-multihomed hosts; they only have one interface.
2) If the selected first-hop router does not have a route at all for
the destination, it will be unable to deliver the packet. However,
the destination may be reachable through a router on one of the
other interfaces. Neighbor Discovery does not address this
scenario; it does not arise in the non-multihomed case.
3) Even if the first-hop router does have a route for a destination,
there may be a better route via another interface. No mechanism
exists for the multihomed host to detect this situation.
If a multihomed host fails to receive Router Advertisements on one or
more of its interfaces, it will not know (in the absence of configured
information) which destinations are on-link on the affected
interface(s). This leads to a number of problems:
1) If no Router Advertisement is received on any interfaces, a
multihomed host will have no way of knowing which interface to send
packets out on, even for on-link destinations. Under similar
conditions in the non-multihomed host case, a node treats all
destinations as residing on-link, and communication proceeds. In
the multihomed case, however, additional information is needed to
select the proper outgoing interface. Alternatively, a node could
attempt to perform address resolution on all interfaces, a step
involving significant complexity that is not present in the non-
multihomed host case.
2) If Router Advertisements are received on some, but not all
interfaces, a multihomed host could choose to only send packets out
on the interfaces on which it has received Router Advertisements.
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A key assumption made here, however, is that routers on those other
interfaces will be able to route packets to the ultimate
destination, even when those destinations reside on the subnet to
which the sender connects, but has no on-link prefix information.
Should the assumption be false, communication would fail. Even if
the assumption holds, packets will traverse a sub-optimal path.
10. PROTOCOL CONSTANTS
Router constants:
MAX_INITIAL_RTR_ADVERT_INTERVAL 16 seconds
MAX_INITIAL_RTR_ADVERTISEMENTS 3 transmissions
MAX_RTR_RESPONSE_DELAY 6 seconds
Host constants:
MAX_RTR_SOLICITATION_DELAY 1 second
RTR_SOLICITATION_INTERVAL 3 seconds
MAX_RTR_SOLICITATIONS 3 transmissions
Node constants:
MAX_MULTICAST_SOLICIT 3 transmissions
MAX_UNICAST_SOLICIT 3 transmissions
MAX_ANYCAST_DELAY_TIME 1 second
MAX_NEIGHBOR_ADVERTISEMENT 3 transmissions
MIN_NEIGHBOR_ADVERT_INTERVAL 16 seconds
REACHABLE_TIME 30,000 milliseconds
RETRANS_TIMER 10,000 milliseconds
DELAY_FIRST_PROBE_TIME 5 seconds
MIN_RANDOM_FACTOR .5
MAX_RANDOM_FACTOR 1.5
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Additional protocol constants are defined with the message formats in
Section 5.1, 6.1, and 7.1.
All protocol constants are subject to change in future revisions of the
protocol.
11. FUTURE EXTENSIONS
Possible extensions for future study are:
o Using dynamic timers to be able to adapt to links with widely varying
delay. Measuring round trip times, however, requires acknowledgments
and sequence numbers in order to match received Neighbor
Advertisements with the actual Neighbor Solicitation that triggered
the advertisement. Implementors wishing to experiment with such a
facility could do so in a backwards-compatible way by defining a new
option carrying the necessary information. Nodes not understanding
the option would simply ignore it.
o Adding capabilities to facilitate the operation over links that
currently require hosts to register with an address resolution
server. This could for instance enable routers to ask hosts to send
them periodic unsolicited advertisements. Once again this can be
added using a new option sent in the Router Advertisements.
o Adding additional procedures for links where asymmetric and non-
transitive reachability is part of normal operations. Such
procedures might allow hosts and routers to find usable paths on,
e.g., radio links.
12. OPEN ISSUES
o Should the routers listed in Router Advertisements include a
precedence metric? What are the semantics of such metrics (e.g.,
"router preferences" vs. "default router preferences").
13. SECURITY CONSIDERATIONS
Neighbor Discovery is subject attacks that cause IP packets to flow to
unexpected places. Such attacks can be used to cause denial of service
but also allow nodes to intercept and optionally modify packets destined
for other nodes.
The protocol reduces the exposure to such threats in the absence of
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authentication by designing ND packets that modify neighbor state (e.g.
cached link-layer addresses) in such a way that routers cannot or will
not forward them. Limiting the scope of ND packets to a particular link
makes the protocol more robust against the accidental sending of ND
messages with a hop count larger than one. Specifically:
- the source address of all packets have link-local scope. Routers
MUST NOT forward such packets. See [ADDR-ARCH].
- with the exception of Redirects, the destination address in all ND
packets that can modify any state in the recipient node have link-
local scope; routers will be unable to forward them.
- packets containing a Routing Header are ignored upon receipt. If
Routing Headers were allowed, it would be possible to forward packets
through routers, even if the packet's ultimate destination has link-
local scope.
Note that the use of link-local destination address makes the checks for
link-local source address somewhat redundant for ND messages other than
Redirects. The Redirect message is the only message type sent to a
global unicast address that can modify the state in the receiving node.
Thus proper robustness for Redirect messages requires that routers not
forward packets with link-local source addresses.
The trust model for redirects is the same as in IPv4. A redirect is
accepted only if received from the same router that is currently being
used for that destination. It is natural to trust the routers on the
link. If a host has been redirected to another node (i.e. the
destination is on-link) there is no way to prevent the target from
issuing another redirect to some other destination. However, this
exposure is no worse than it was; the target host, once subverted, could
always act as a hidden router to forward traffic elsewhere.
The protocol contains no mechanism to determine which nodes are
authorized to send Router Advertisements; any node, presumably even in
the presence of authentication, can send Router Advertisement messages
thereby being able to cause denial of service. Furthermore, any node
can send proxy Neighbor Advertisements as well as unsolicited Neighbor
Advertisements as a potential denial of service attack.
Neighbor Discovery protocol packet exchanges can be authenticated using
the IP Authentication Header [IPv6-AUTH]. A node SHOULD include an
Authentication Header when sending Neighbor Discovery packets if a
security association for use with the IP Authentication Header exists
for the destination address. The security associations may have been
created through manual configuration or through the operation of some
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key management protocol.
Received Authentication Headers in Neighbor Discovery packets MUST be
verified for correctness and packets with incorrect authentication MUST
be ignored.
It SHOULD be possible for the system administrator to configure a node
to ignore any Neighbor Discovery messages that are not authenticated
using either the Authentication Header or Encapsulating Security
Payload. The configuration technique for this MUST be documented. Such
a switch SHOULD default to allowing unauthenticated messages.
Confidentiality issues are addressed by the IP Security Architecture and
the IP Encapsulating Security Payload documents [IPv6-SA, IPv6-ESP].
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REFERENCES
[ADDRCONF] S. Thomson, "IPv6 Address Autoconfiguration", Internet
Draft.
[ADDR-ARCH] S. Deering, R. Hinden, Editors, "IP Version 6 Addressing
Architecture", Internet Draft.
[ANYCST] C. Partridge, T. Mendez, and W. Milliken, "Host Anycasting
Service", RFC 1546, November 1993.
[ARP] D. Plummer, "An Ethernet Address Resolution Protocol", STD 37,
RFC 826, November 1982.
[HR-CL] R. Braden, Editor, "Requirements for Internet Hosts --
Communication Layers", STD 3, RFC 1122, October 1989.
[ICMPv4] J. Postel, "Internet Control Message Protocol", STD 5, RFC
792, September 1981.
[ICMPv6] A. Conta, and S. Deering, "ICMP for the Internet Protocol
Version 6 (IPv6)", Internet Draft.
[IPv6] S. Deering, R. Hinden, Editors, "Internet Protocol, Version 6
(IPv6) Specification", Internet Draft.
[IPv6-ETHER] M. Crawford. "A Method for the Transmission of IPv6
Packets over Ethernet Networks", Internet Draft.
[IPv6-SA] R. Atkinson. "Security Architecture for the Internet
Protocol". RFC 1825, August 1995.
[IPv6-AUTH] R. Atkinson. "IP Authentication Header", RFC 1826,
August 1995.
[IPv6-ESP] R. Atkinson. "IP Encapsulating Security Payload (ESP)",
RFC 1827, August 1995.
[RDISC] S. Deering, "ICMP Router Discovery Messages", RFC 1256,
September 1991.
[SH-MEDIA] R. Braden, J. Postel, Y. Rekhter, "Internet Architecture
Extensions for Shared Media", RFC 1620, May 1994.
[ASSIGNED] J. Reynolds, J. Postel, "ASSIGNED NUMBERS", RFC 1700,
October 1994.
draft-ietf-ipngwg-discovery-02.txt [Page 71]
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[SYNC] S. Floyd, V. Jacobsen, "The Synchronization of Periodic Routing
Messages", IEEE/ACM Transactions on Networking, April 1994.
ftp://ftp.ee.lbl.gov/papers/sync_94.ps.Z
AUTHORS' ADDRESSES
Erik Nordmark Thomas Narten
Sun Microsystems, Inc. IBM Corporation
2550 Garcia Ave P.O. Box 12195
Mt. View, CA 94041 Research Triangle Park, NC 27709-2195
USA USA
phone: +1 415 336 2788 phone: +1 919 254 7798
fax: +1 415 336 6015 fax: +1 919 254 4027
email: nordmark@sun.com email: narten@vnet.ibm.com
William Allen Simpson
Daydreamer
Computer Systems Consulting Services
1384 Fontaine
Madison Heights, Michigan 48071
USA
email: Bill.Simpson@um.cc.umich.edu
bsimpson@MorningStar.com
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CHANGES SINCE PREVIOUS DOCUMENT
There are several changes since the previous version documented in:
<draft-ietf-ipngwg-discovery-01.txt>
based on feedback from the working group:
o Link-local source address required for Neighbor Solicitation
and Neighbor Advertisement messages. This change implied
adding an ICMP Sender Address field to the Neighbor
Solicitation message and a Secondary Advertisement flag to
the Neighbor Advertisement message. This change improves
the robustness of the protocol - it is no longer possible
for off-link nodes to send ND messages to a link.
o Made the ReachableTime value random to avoid synchronizing
Neighbor Unreachability Detection messages when there is
more of less "constant" traffic (i.e. packets are sent with
spacing that is very short compared to the ReachableTime
value). Without such randomization the NUD probes from all
nodes on the link would be sent with almost the same spacing
which can result in synchronization. There is no need for
any additional randomization elsewhere in the protocol since
there is no long-term periodic behavior - at most 3 packets
are transmitted.
o Added definitions for MUST, SHOULD, and MAY.
o Made NUD and address resolution use the same retransmission
timer (which can be specified in the Router Advertisements).
Increased the default value of this timer from 3 seconds to
10 seconds.
o Restricted ReachableTime so that it can not be set to more
than 1 hour to prevent misconfiguration that would make ND
not detect e.g. changed link-layer addresses.
o Added text about the support for links with multiple MTUs
(e.g. bridged Ethernet and FDDI). With unmodified bridges
the routers must send MTU options containing the smaller
(smallest) MTU. If the bridges are made aware of IPv6 they
can participate in path MTU discovery (for unicast and
multicast) and send ICMP packet too big errors for IPv6
packets that cross the bridge.
o Added additional validatity checks for Router Solicitation
and Router Advertisement messages: the destination address
must be a link local address or a multicast address with
link-local scope.
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o Added validity check for all messages: no routing header is
allowed.
o Changed the use of the "designated address" term to using
"link-local address".
o Clarified authentication header text.
o Clarified how multicast packets are handled in the
conceptual model.
o Added text that ND messages are themselves not subject to
NUD probes. This avoids an observed problem where a NUD
NS/NA exchange would result in a subsequent NUD NS/NA
exchange of packets.
o Clarified that the neighbor cache entries generated by
unsolicited information (RS, RA, NA, Redirect) do still get
DELAY_FIRST_PROBE_TIME seconds before a probe is sent (in
order to benefit from upper-layer advise).
o Solicited Proxy/anycast advertisements are delayed 0-1
second to avoid creating a load on the network and/or
receiver.
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