One document matched: draft-ietf-6lowpan-nd-21.xml
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<rfc category="std" ipr="trust200902" updates="4944" docName="draft-ietf-6lowpan-nd-21">
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<front>
<title abbrev="ND Optimization for 6LoWPAN">
Neighbor Discovery Optimization for Low Power and Lossy Networks (6LoWPAN)
</title>
<author initials="Z" surname="Shelby" fullname="Zach Shelby" role="editor">
<organization>
Sensinode
</organization>
<address>
<postal>
<street>Konekuja 2</street>
<city>Oulu</city>
<code>90620</code>
<country>FINLAND</country>
</postal>
<phone>+358407796297</phone>
<email>zach@sensinode.com</email>
</address>
</author>
<author initials="S" surname="Chakrabarti" fullname="Samita Chakrabarti">
<organization>Ericsson</organization>
<address>
<postal>
<street></street>
<city></city>
<country></country>
</postal>
<email>samita.chakrabarti@ericsson.com</email>
</address>
</author>
<author initials="E" surname="Nordmark" fullname="Erik Nordmark">
<organization>Cisco Systems</organization>
<address>
<postal>
<street></street>
<city></city>
<country></country>
</postal>
<email>nordmark@cisco.com</email>
</address>
</author>
<date year="2012"/>
<area>Internet</area>
<workgroup>6LoWPAN Working Group</workgroup>
<abstract>
<t> The IETF 6LoWPAN work defines IPv6 over Low-power Wireless Personal
Area Networks such as IEEE 802.15.4. This and other similar link
technologies have limited or no usage of multicast signaling due to
energy conservation. In addition, the wireless network may not strictly
follow the traditional concept of IP subnets and IP links. IPv6 Neighbor
Discovery was not designed for non-transitive wireless links, as its
reliance on the traditional IPv6 link concept and its heavy use of
multicast make it inefficient and sometimes impractical in a low-power
and lossy network. This document describes simple optimizations to IPv6
Neighbor Discovery, its addressing mechanisms and duplicate address
detection for Low-power Wireless Personal Area Networks and similar
networks. The document thus updates RFC 4944 to specify
the use of the optimizations defined here. </t>
</abstract>
</front>
<middle>
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<section anchor='introduction' title="Introduction">
<t> The IPv6-over-IEEE 802.15.4 <xref target="RFC4944"/> document
specifies how IPv6 is carried over an IEEE 802.15.4 network with
the help of an adaptation layer which sits between the MAC layer
and the IP network layer. A link in a Low-power Wireless Personal
Area Network (LoWPAN) is characterized as lossy, low-power, low
bit-rate, short range, with many nodes saving energy with long
sleep periods. Multicast as used in IPv6 Neighbor Discovery <xref
target="RFC4861"/> is not desirable in such a wireless low-power
and lossy network. Moreover, LoWPAN links are asymmetric and
non-transitive in nature. A LoWPAN is potentially composed of a
large number of overlapping radio ranges. Although a given radio
range has broadcast capabilities, the aggregation of these is a
complex Non-Broadcast MultiAccess (NBMA, <xref target="RFC2491"/>)
structure with generally no LoWPAN-wide multicast capabilities.
Link-local scope is in reality defined by reachability and radio
strength. Thus we can consider a LoWPAN to be made up of links
with undetermined connectivity properties as in <xref
target="RFC5889"/>, along with the corresponding address model
assumptions defined therein.</t>
<t>This specification introduces the following optimizations to
IPv6 Neighbor Discovery <xref target="RFC4861"/> specifically
aimed at low-power and lossy networks such as LoWPANs:
<list style="symbols">
<t>Host-initiated interactions to allow for sleeping hosts.</t>
<t>Elimination of multicast-based address resolution for hosts.</t>
<t>A host address registration feature using a new option in
unicast Neighbor Solicitation and Neighbor Advertisement
messages.</t>
<t>A new optional Neighbor Discovery option to distribute 6LoWPAN
header compression context to hosts.</t>
<t>Multihop distribution of prefix and 6LoWPAN header
compression context.
</t>
<t>Multihop duplicate address detection which uses
two new ICMPv6 message types.</t>
</list>
The two multihop items can be substituted by a
routing protocol mechanism if that is desired, see
<xref target="subst_features"/>.
</t>
<t>The document defines three new ICMPv6 message options: the
Address Registration,
Authoritative Border Router, and 6LoWPAN Context options. It also
defines two new ICMPv6 message types: the Duplicate Address
Request and Duplicate Address Confirmation.</t>
<section anchor='nd_problems'
title="The Shortcomings of IPv6 Neighbor Discovery">
<t>IPv6 Neighbor Discovery <xref target="RFC4861"/> provides
several important mechanisms used for Router Discovery, Address
Resolution, Duplicate Address Detection, Redirect, along with
Prefix and Parameter Discovery.</t>
<t>Following power-on and initialization of the network in IPv6
Ethernet networks, a node joins the solicited-node multicast
address on the interface and then performs Duplicate Address
Detection (DAD) for the acquired link-local address by sending a
solicited-node multicast message to the link. After that it sends
multicast messages to the all-router address to solicit router
advertisements. If the host receives a valid Router Advertisement
with the "A" flag, it autoconfigures the IPv6 address with the
advertised prefix in the Router Advertisement (RA) message.
Besides this, the IPv6 routers usually send router advertisements
periodically on the network. RAs are sent to the all-node
multicast address. Nodes send Neighbor Solicitation/Neighbor
Advertisement messages to resolve the IPv6 address of the
destination on the link. The Neighbor Solicitation messages used
for address resolution are multicast. The Duplicate Address
Detection procedure and the use of periodic Router Advertisement
messages assumes that the nodes are powered on and reachable most
of the time.</t>
<t>In Neighbor Discovery the routers find the hosts by assuming
that a subnet prefix maps to one broadcast domain, and then
multicast Neighbor Solicitation messages to find the host and its
link-layer address. Furthermore, the DAD use of multicast assumes
that all hosts that autoconfigure IPv6 addresses from the same
prefix can be reached using link-local multicast messages.</t>
<t>Note that the 'L' (on-link) bit in the Prefix Information
option can be set to zero in Neighbor Discovery, which makes the
host not use multicast Neighbor Solicitation (NS) messages for
address resolution of other hosts, but routers still use multicast
NS messages to find the hosts.</t>
<t>Due to the lossy nature of wireless communication and a changing
radio environment, the IPv6-link node-set may change due to
external physical factors. Thus the link is often unstable and the
nodes appear to be moving without necessarily moving physically.
</t>
<t> A LoWPAN can use two types of link-layer addresses; 16-bit
short addresses and 64-bit unique addresses as defined in <xref
target="RFC4944"/>. Moreover, the available link-layer payload
size is on the order of less than 100 bytes thus header
compression is very useful.</t>
<t>Considering the above characteristics in a LoWPAN, and the IPv6
Neighbor Discovery <xref target="RFC4861"/> protocol design,
some optimizations and extensions to Neighbor Discovery
are useful for the wide deployment of IPv6 over low-powered and
lossy networks (example: 6LoWPAN and other homogeneous low-power networks). </t>
</section>
<!--
<section anchor='mu_ro' title="Mesh-under and Route-over Concepts">
<t> In the 6LoWPAN context, often a link-layer mesh
mechanism is referred to as "mesh-under" while routing
packets using IP-layer addresses is referred to as "route-over". The
difference between mesh-under and route-over is similar to a
bridged-network versus IP-routing using Ethernet. In a mesh-under
network all nodes are on the same link which is served by one or
more routers, which we call 6LoWPAN Border Routers (6LBR). In a
route-over network, there are multiple overlapping links in the 6LoWPAN.
Unlike fixed IP links, these link's members may be changing due to the
nature of the low-power and lossy behavior of wireless technology.
Thus a route-over network is made up of a flexible set of links
interconnected by interior routers, which we call 6LoWPAN Routers
(6LR).</t>
<t> This specification is applicable to both mesh-under and
route-over networks. However, in route-over networks, we have two
types of routers - 6LBRs and 6LRs. 6LoWPAN Border Routers sit at the
boundary of the 6LoWPAN and the rest of the network while 6LoWPAN
Routers are inside the LoWPAN. 6LoWPAN Routers are assumed to be
running a routing protocol. </t>
<t> In a mesh-under configuration a 6LBR is acting as the IPv6
router where all the nodes in the LoWPAN are on the same link, thus
they are one IP hop away. No 6LoWPAN Routers exist in this
topology as forwarding is handled by a link-layer mesh
protocol. </t>
<t> In a route-over configuration, Neighbor Discovery operations
take place between hosts and 6LRs or 6LBRs. The 6LR nodes are able
to send and receive Router Advertisements, Router Solicitations as
well as forward and route IPv6 packets. Here packet forwarding
happens at the IP layer. </t>
<t>In both types of configurations, hosts do not take part in
routing and forwarding packets and they act as simple IPv6 hosts.</t>
</section>
-->
<section anchor='applicability' title="Applicability">
<t> In its Section 1, <xref target="RFC4861"/> foresees a document that
covers operating IP over a particular link type and defines an exception to
the otherwise general applicability of unmodified <xref target="RFC4861"/>.
The present specification improves the usage of IPv6 Neighbor Discovery for
LoWPANs in order to save energy and processing power of such nodes. The
document, thus updates <xref target="RFC4944"/> to specify the use of the
optimizations defined here. </t>
<t> The applicability of this specification is limited to LoWPANs
where all nodes on the subnet implement these optimizations in a
homogeneous way. Although it is noted that some of these
optimizations may be useful outside of 6LoWPAN, for example in
general IPv6 low-power and lossy networks and possibly even in
combination with <xref target="RFC4861"/>, the usage of such
combinations is out of scope of this document. </t>
<t> In this document, we specify a set of behaviors between hosts
and routers in LoWPANs. An implementation that adheres to this
document MUST implement those behaviors. The document also
specifies a set of behaviors (multihop prefix or context
dissemination, and separately multihop duplicate address detection)
which are needed in route-over configurations. An implementation of
this specification MUST support those pieces,
unless the implementation supports some alternative ("substitute")
from some some other specification.</t>
<t> The optimizations described in this document apply to different
topologies. They are most useful for route-over and mesh-under
configurations in Mesh topologies. However, Star topology
configurations will also benefit from the optimizations due to
reduced signaling, robust handling of the non-transitive link, and
header compression context information. </t>
</section>
<section anchor='goals' title="Goals and Assumptions">
<t>The document has the following main goals and assumptions.</t>
<t>Goals:
<list style="symbols">
<t>Optimize Neighbor Discovery with a mechanism that is minimal
yet sufficient for the operation in both mesh-under and
route-over configurations.</t>
<t>Minimize signaling by avoiding the use of multicast flooding
and reducing the use of link-scope multicast messages.</t>
<t>Optimize the interfaces between hosts and their default routers.</t>
<t>Support for sleeping hosts.</t>
<t>Disseminate context information to hosts as needed by 6LoWPAN
Header Compression <xref target="RFC6282"/>.</t>
<t>Disseminate context information and prefix information
from the border to all routers in a LoWPAN.</t>
<t>Multihop duplicate address detection mechanism suitable for
route-over LoWPANs.</t>
</list>
</t>
<t>Assumptions:
<list style="symbols">
<t>EUI-64 addresses are globally unique and the LoWPAN is homogeneous.</t>
<t>All nodes in the network have an EUI-64 interface identifier
in order to do address auto-configuration and detect duplicate
addresses.</t>
<t>The link layer technology is assumed to be low-power and
lossy, exhibiting undetermined connectivity, such as IEEE
802.15.4 <xref target="RFC4944"/>. However, the Address
Registration mechanism might be useful for other link layer
technologies.</t>
<t>A 6LoWPAN is configured to share one or more global IPv6
address prefixes to enable hosts to move between routers in the
LoWPAN without changing their IPv6 addresses.</t>
<t>When using the multihop DAD mechanism of <xref
target="multihop_dad"/> each 6LR registers with all
the 6LBRs available in the LoWPAN.</t>
<t>If IEEE 802.15.4 16-bit short addresses are used, then some
technique is used to ensure uniqueness of those link-layer
addresses. That could be done using DHCPv6, the Address
Registration Option based duplicate address detection
(specified in <xref target="multihop_dad"/>) or other
techniques outside of the scope of this document. </t>
<t>In order to preserve the uniqueness of addresses (see Section 5.4,
<xref target="RFC4862"/>) not derived
from an EUI-64, they must be either assigned or checked for
duplicates in the same way throughout the LoWPAN. This can be
done using DHCPv6 for assignment and/or using the duplicate
address detection mechanism specified in <xref
target="multihop_dad"/> (or any other protocols developed for
that purpose).</t>
<t>In order for 6LoWPAN Header Compression <xref target="RFC6282"/> to
operate correctly, the compression context must match for all
the hosts, 6LRs, and 6LBRs that can send, receive, or forward a
given packet. If <xref target='multihop_dist'/> is used to
distribute context information this implies that all the 6LBRs
must coordinate the context information they distribute within
a single LoWPAN.</t>
<t>This specification describes the
operation of ND within a single LoWPAN. The participation of a
node in multiple LoWPANs simultaneously may be possible, but is
out of scope of this document.</t>
<t> Since the LoWPAN shares its prefix(es) throughout the network,
mobility of nodes within the LoWPAN is transparent. Inter-LoWPAN
mobility is out-of-scope of this document. </t>
</list>
</t>
</section>
<section anchor='subst_features' title="Substitutable Features">
<t> This document defines the optimization of Neighbor Discovery messages
for the host-router interface and introduces two new mechanisms in a
Route-over topology. </t>
<t> Unless specified otherwise (in a document that defines a routing
protocol that is used in a 6LoWPAN) this document applies to networks with
any routing protocol. However, because the routing protocol may provide
good alternate mechanisms, this document defines certain features as
"substitutable", meaning they can be substituted by a routing protocol
specification that provides mechanisms achieving the same overall effect.
</t>
<t>
The features that are substitutable (individually or in a group):
<list style="symbols">
<t>Multihop distribution of prefix and 6LoWPAN header compression
context</t>
<t>Multihop duplicate address detection</t>
</list>
</t>
<t> Thus Multihop prefix distribution (ABRO option) and 6LoWPAN Context
Option (6CO, for distributing Header Compression Contexts) go
hand-in-hand. If substitution is intended for one of them, then both of
them MUST be substituted. </t>
<t>A guideline for feature implementation and deployment is provided
at the end of the document.</t>
</section>
</section>
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<section anchor='terminology' title="Terminology">
<t> The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
in this document are to be interpreted as described in <xref
target="RFC2119"/>. </t>
<t> This specification requires readers to be familiar with all the
terms and concepts that are discussed in <xref
target="RFC4861">"Neighbor Discovery for IP version 6"</xref> <xref
target="RFC4862">"IPv6 Stateless Address Autoconfiguration"</xref>,
<xref target="RFC4919">"IPv6 over Low-Power Wireless Personal Area
Networks (6LoWPANs): Overview, Assumptions, Problem Statement, and
Goals"</xref>, <xref target="RFC4944"> "Transmission of IPv6 Packets
over IEEE 802.15.4 Networks"</xref> and <xref target="RFC5889">"IP
Addressing Model in Ad Hoc Networks"</xref>. </t>
<t> This specification makes extensive use of the same terminology
defined in <xref target="RFC4861"/> unless otherwise defined below.
</t>
<t><list style="hanging">
<t hangText="6LoWPAN link:"><vspace /> A wireless link
determined by single IP hop reachability of neighboring nodes.
These are considered links with undetermined connectivity
properties as in <xref target="RFC5889"/>. </t>
<t hangText="6LoWPAN Node (6LN):"><vspace /> A 6LoWPAN Node is
any host or router participating in a LoWPAN. This term is used
when referring to situations in which either a host or router
can play the role described. </t>
<t hangText="6LoWPAN Router (6LR):"><vspace /> An intermediate
router in the LoWPAN that is able to send and receive Router
Advertisements, Router Solicitations as well as forward and
route IPv6 packets. 6LoWPAN routers are present only in
route-over topologies. </t>
<t hangText="6LoWPAN Border Router (6LBR):"><vspace /> A border
router located at the junction of separate 6LoWPAN networks or
between a 6LoWPAN network and another IP network. There may be
one or more 6LBRs at the 6LoWPAN network boundary. A 6LBR is the
responsible authority for IPv6 Prefix propagation for the
6LoWPAN network it is serving. An isolated LoWPAN also contains
a 6LBR in the network, which provides the prefix(es) for the
isolated network. </t>
<t hangText="Router:"><vspace/>Either a 6LR or a 6LBR. Note that
nothing in this document precludes a node being a router on some
interfaces and a host on other interfaces as allowed by <xref
target="RFC2460"/>.</t>
<t hangText="Mesh-under:"><vspace /> A topology where nodes are
connected to a 6LBR through a mesh using link-layer forwarding.
Thus in a mesh-under configuration all IPv6 hosts in a LoWPAN
are only one IP hop away from the 6LBR. This topology simulates
the typical IP-subnet topology with one router with multiple
nodes in the same subnet. </t>
<t hangText="Route-over:"><vspace /> A topology where hosts are
connected to the 6LBR through the use of intermediate layer-3
(IP) routing. Here hosts are typically multiple IP hops away
from a 6LBR. The route-over topology typically consists of a
6LBR, a set of 6LRs and hosts. </t>
<t hangText="Non-Transitive Link:"><vspace /> A link which
exhibits asymmetric reachability as defined in Section 2.2 of
<xref target="RFC4861"/>. </t>
<t hangText="IP-over-foo Document:"><vspace /> A specification
that covers operating IP over a particular link type, for
example <xref target="RFC4944"/> "Transmission of IPv6 Packets
over IEEE 802.15.4 Networks". </t>
<t hangText="Header Compression Context:"><vspace />
Address information shared across a LoWPAN and used by 6LoWPAN Header
Compression <xref target="RFC6282"/> to enable the elision of
information that would otherwise be sent repeatedly. In a "context", a
(potentially partial) address is associated with a Context Identifier,
which is then used in header compression as a shortcut for (parts of)
a source or destination address.</t>
<t hangText="Registration:"><vspace /> The process during which
a LoWPAN node sends an Neighbor Solicitation message with an
Address Registration option to a Router creating a Neighbor
Cache entry for the LoWPAN node with a specific timeout. Thus
for 6LoWPAN Routers the Neighbor Cache doesn't behave like a
cache. Instead it behaves as a registry of all the host
addresses that are attached to the Router. </t>
</list>
</t>
</section>
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<section anchor='overview' title="Protocol Overview">
<t>These Neighbor Discovery optimizations are applicable to both
mesh-under and route-over configurations. In a mesh-under
configuration only 6LoWPAN Border Routers and hosts exist; there are
no 6LoWPAN routers in mesh-under topologies.</t>
<t>The most important part of the optimizations is the evolved
host-to-router interaction that allows for sleeping nodes and avoids
using multicast Neighbor Discovery messages except for the case of a
host finding an initial set of default routers, and redoing such
determination when that set of routers have become unreachable. </t>
<t>The protocol also provides for header compression <xref
target="RFC6282"/> by carrying header compression information in a
new option in Router Advertisement messages.</t>
<t>In addition, there are separate mechanisms that
between 6LRs and 6LBRs to perform multihop Duplicate Address
Detection and distribution of the Prefix and compression Context
information from the 6LBRs to all the 6LRs, which in turn use normal
Neighbor Discovery mechanisms to convey this information to the
hosts.</t>
<t>The protocol is designed so that the host-to-router interaction
is not affected by the configuration of the 6LoWPAN; the
host-to-router interaction is the same in a mesh-under and
route-over configuration.</t>
<section anchor="Optimization" title="Extensions to RFC4861">
<t>This document specifies the following optimizations and
extensions to IPv6 Neighbor Discovery <xref target="RFC4861"/>:
<list style="symbols">
<t>Host initiated refresh of Router Advertisement information.
This removes the need for periodic or unsolicited Router
Advertisements from routers to hosts.</t>
<t>No Duplicate Address Detection (DAD) is performed if EUI-64
based IPv6 addresses are used (as these addresses are assumed to
be globally unique).</t>
<t>DAD is optional if DHCPv6 is used to assign addresses.</t>
<t>A New Address Registration mechanism using a new Address
Registration option between hosts and routers. This removes the
need for Routers to use multicast Neighbor Solicitations to find
hosts, and supports sleeping hosts. This also enables the same
IPv6 address prefix(es) to be used across a route-over 6LoWPAN.
It provides the host-to-router interface for Duplicate Address
Detection. </t>
<t>A new Router Advertisement option for Context
information used by 6LoWPAN header compression.</t>
<t>A new mechanism to perform Duplicate Address
Detection across a route-over 6LoWPAN using the new Duplicate
Address Request and Confirmation messages.</t>
<t>New mechanisms to distribute Prefixes and Context
information across a route-over network which uses a new
Authoritative Border Router option to control the flooding of
configuration changes.</t>
<t>A few new default protocol constants are introduced and some
existing Neighbor Discovery protocol constants are tuned.</t>
</list></t>
</section>
<section anchor="Addr-assign" title="Address Assignment">
<t>Hosts in a 6LoWPAN configure their IPv6 address as specified in
<xref target="RFC4861"/> and <xref target="RFC4862"/> based on the
information received in Router Advertisement messages. The use of
the M flag in this optimization is however more restrictive than in
<xref target="RFC4861"/>. When the M flag is set a host is assumed
to use DHCPv6 to assign any non-EUI-64 addresses. When the M flag is
not set, the nodes in the LoWPAN support duplicate address
detection, thus a host can then safely use the address registration
mechanism to check non-EUI-64 addresses for uniqueness.</t>
<t>6LRs MAY use the same mechanisms to configure their IPv6
addresses.</t>
<t>The 6LBRs are responsible for managing the prefix(es) assigned to
the 6LoWPAN, using manual configuration, DHCPv6 Prefix Delegation
<xref target="RFC3633"/>, or other mechanisms. In an isolated LoWPAN
a ULA <xref target="RFC4193"/> prefix SHOULD be generated by the
6LBR.</t>
</section>
<section anchor='overview_hr' title="Host-to-Router Interaction">
<t>A host sends Router Solicitation messages at startup and also when
Neighbor Unreachability Detection towards one of its default routers fails.
</t>
<t>Hosts receive Router Advertisement messages typically containing
the Authoritative Border Router option (ABRO) and may optionally
contain one or more 6LoWPAN Context options (6CO) in addition to the
existing Prefix Information options (PIO) as described in <xref
target="RFC4861"/>.</t>
<t>When a host has configured a non-link-local IPv6 address, it
registers that address with one or more of its default routers using
the Address Registration option (ARO) in an NS message. The host
chooses a lifetime of the registration and repeats the ARO option
periodically (before the lifetime runs out) to maintain the
registration. The lifetime should be chosen in such a way as to
maintain the registration even while a host is sleeping. Likewise,
mobile nodes that change their point of attachment often, should use
a suitably short lifetime. See <xref target="host_registration"/> for registration details and
<xref target="constants"/> for protocol constants. </t>
<t>The registration fails when an ARO option is returned to the host
with a non-zero Status. One reason may be that the router determines that the IPv6
address is already used by another host, that is, is used by a host
with a different EUI-64. This can be used to support non-EUI-64
based addresses such as temporary IPv6 addresses <xref
target="RFC4941"/> or addresses based on an Interface ID that is a
IEEE 802.15.4 16-bit short addresses. Failure can also occur if the
Neighbor Cache on that router is full.</t>
<t>The re-registration of an address can be combined with Neighbor
Unreachability Detection (NUD) of the router since both use unicast
Neighbor Solicitation messages. This makes things efficient when a
host wakes up to send a packet and both need to perform NUD to check
that the router is still reachable, and refresh its registration
with the router.</t>
<t>The response to an address registration might not be immediate
since in route-over configurations the 6LR might perform Duplicate
Address Detection against the 6LBR. A host retransmits the Address
Registration option until it is acknowledged by the receipt of a
Address Registration option.</t>
<t>As part of the optimizations, Address Resolution is not performed
by multicasting Neighbor Solicitation messages as in <xref
target="RFC4861"/>. Instead, the routers maintain Neighbor Cache
entries for all registered IPv6 addresses. If the address is not in
the Neighbor Cache in the router, then the address either doesn't
exist, or is assigned to a host attached to some other router in the
6LoWPAN, or is external to the 6LoWPAN. In a route-over
configuration the routing protocol is used to route such packets
toward the destination.</t>
</section>
<section anchor='overview_rr' title="Router-to-Router Interaction">
<t>The new router-to-router interaction is only for the route-over
configuration where 6LRs are present.
See also <xref target='subst_features'/>.</t>
<t>6LRs MUST act like a host during system startup and prefix
configuration by sending Router Solicitation messages and
autoconfiguring their IPv6 addresses unlike routers in <xref
target="RFC4861"/>.</t>
<t>When multihop prefix and context dissemination are used then the
6LRs store the ABRO, 6CO and Prefix Information received (directly
or indirectly) from the 6LBRs and redistribute this information in
the Router Advertisement they send to other 6LRs or send to hosts in
response to a Router Solicitations. There is a version number field
in the ABRO which is used to limit the flooding of updated
information between the 6LRs.</t>
<t>A 6LR can perform Duplicate Address Detection
against one or more 6LBRs using the new Duplicate Address Request
(DAR) and Confirmation (DAC) messages, which carry the information
from the Address Registration option. The DAR and DAC messages will
be forwarded between the 6LR and 6LBRs thus the <xref
target="RFC4861"/> rule for checking hop limit=255 does not apply to
the DAR and DAC messages. Those multihop DAD messages MUST NOT
modify any Neighbor Cache entries on the routers since we do not
have the security benefits provided by the hop limit=255 check.</t>
</section>
<section anchor='nce_management' title="Neighbor Cache Management">
<t>The use of explicit registrations with lifetimes plus the desire
to not multicast Neighbor Solicitation messages for hosts imply that
we manage the Neighbor Cache entries (NCE) slightly differently than
in <xref target="RFC4861"/>. This results in three different types
of NCEs and the types specify how those entries can be removed:
<list style='hanging' hangIndent='22'>
<t hangText="Garbage-collectible: "> Entries that are subject
to the normal rules in <xref target="RFC4861"/> that allow for
garbage collection when low on memory. </t>
<t hangText="Registered: "> Entries that have an explicit
registered lifetime and are kept until this lifetime expires
or they are explicitly unregistered. </t>
<t hangText="Tentative: "> Entries that are temporary with a
short lifetime, which typically get converted to Registered
entries.</t>
</list>
Note that the type of the NCE is orthogonal to the states specified
in <xref target="RFC4861"/>. </t>
<t>When a host interacts with a router by sending Router
Solicitations this results in a Tentative NCE. Once a router has
successfully had a node register with it, the result is a Registered NCE.
When Routers send RAs to hosts, and when routers receive
RA messages or receive multicast NS messages from other Routers, the
result is Garbage-collectible NCEs. There can only be one kind of
NCE for an IP address at a time.</t>
<t>Neighbor Cache entries on Routers can additionally be added or
deleted by a routing protocol used in the 6LoWPAN. This is useful if
the routing protocol carries the link-layer addresses of the
neighboring routers. Depending on the details of such routing
protocols such NCEs could be either Registered or
Garbage-collectible.</t>
</section>
</section>
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<section anchor='message_options'
title="New Neighbor Discovery Options and Messages">
<t>This section defines new Neighbor Discovery message options used
by this specification. The Address Registration Option is used by hosts,
whereas the Authoritative Border Router Option and 6LoWPAN Context
Option are used in the substitable router-to-router interaction.
This section also defines the new router-to-router
Duplicate Address Request and Confirmation messages.</t>
<section anchor="ARO" title="Address Registration Option">
<t>The routers need to know the set of host IP addresses that are
directly reachable and their corresponding link-layer addresses.
This needs to be maintained as the radio reachability changes. For
this purpose an Address Registration Option (ARO) is introduced,
which can be included in unicast Neighbor Solicitation (NS) messages
sent by hosts. Thus it can be included in the unicast NS messages
that a host sends as part of Neighbor Unreachability Detection to
determine that it can still reach a default router. The ARO is used
by the receiving router to reliably maintain its Neighbor Cache. The
same option is included in corresponding Neighbor Advertisement (NA)
messages with a Status field indicating the success or failure of
the registration. This option is always host initiated.</t>
<t>The information contained in the ARO is also included in the
multihop DAR and DAC messages used between 6LRs to 6LBRs, but the
option itself is not used in those messages.</t>
<t>The ARO is required for reliability and power saving. The
lifetime field provides flexibility to the host to register an
address which should be usable (continue to be advertised by the 6LR
in the routing protocol etc.) during its intended sleep schedule.</t>
<t>The sender of the NS also includes the EUI-64 <xref
target="EUI64"/> of the interface it is registering an address from.
This is used as a unique ID for the detection of duplicate
addresses. It is used to tell the difference between the same node
re-registering its address and a different node (with a different
EUI-64) registering an address that is already in use by someone
else. The EUI-64 is also used to deliver an NA carrying an error
Status code to the EUI-64 based link-local IPv6 address of the host
(see <xref target="errors"/>).</t>
<t>When the ARO is used by hosts an SLLA (Source Link-layer Address)
option <xref target="RFC4861"/> MUST be included and the address
that is to be registered MUST be the IPv6 source address of the
Neighbor Solicitation message.</t>
<figure>
<artwork><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length = 2 | Status | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Registration Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ EUI-64 +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>Fields:
<list style='hanging' hangIndent='15'>
<t hangText="Type:"> TBD1</t>
<t hangText="Length:"> 8-bit unsigned integer. The length of the option in
units of 8 bytes. Always 2.</t>
<t hangText="Status:"> 8-bit unsigned integer. Indicates the status of a
registration in the NA response. MUST be set to 0 in NS messages. See
below.</t>
<t hangText="Reserved:"> This field is unused. It MUST be
initialized to zero by the sender and MUST be ignored by the
receiver.</t>
<t hangText="Registration Lifetime:"> 16-bit unsigned integer. The
amount of time in a unit of 60 seconds that the router should
retain the Neighbor Cache entry for the sender of the NS that
includes this option. </t>
<t hangText="EUI-64:">64 bits. This field is used to uniquely
identify the interface of the registered address by including the
EUI-64 identifier <xref target="EUI64"/> assigned to it
unmodified.</t>
</list>
</t>
<t>The Status values used in Neighbor Advertisements are:</t>
<texttable anchor='status-codes'> <!--update status-codes-iana in sync-->
<ttcol align='center'> Status </ttcol>
<ttcol align='center'> Description</ttcol>
<c>0</c> <c>Success</c>
<c>1</c> <c>Duplicate Address</c>
<c>2</c> <c>Neighbor Cache Full</c>
<c>3-255</c> <c>Allocated using Standards Action <xref target="RFC5226"/></c>
</texttable>
</section>
<section anchor='s6CO' title="6LoWPAN Context Option">
<t> The optional 6LoWPAN Context Option (6CO) carries prefix
information for LoWPAN header compression, and is similar to the
Prefix Information Option of <xref target="RFC4861"/>. However, the
prefixes can be remote as well as local to the LoWPAN since header
compression potentially applies to all IPv6 addresses. This option
allows for the dissemination of multiple contexts identified by a
Context Identifier (CID) for use as specified in <xref
target="RFC6282"/>. A context may be a prefix of any length or an
address (/128), and up to 16 6LoWPAN Context options may be carried
in an Router Advertisement message. </t>
<figure anchor='f6co_format' title="6LoWPAN Context Option format">
<artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |Context Length | Res |C| CID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Valid Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Context Prefix .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
</figure>
<t><list style="hanging">
<t hangText="Type:">TBD2</t>
<t hangText="Length:"> 8-bit unsigned integer. The length of the
option (including the type and length fields) in units of 8 bytes.
May be 2 or 3 depending on the length of the Context Prefix
field.</t>
<t hangText="Context Length:"> 8-bit unsigned integer. The number
of leading bits in the Context Prefix field that are valid. The
value ranges from 0 to 128. If it is more than 64 then the Length
MUST be 3. </t>
<t hangText="C:"> 1-bit context compression flag. This flag
indicates if the context is valid for use in compression. A context
that is not valid MUST NOT be used for compression, but SHOULD be
used in decompression in case another compressor has not yet
received the updated context information. This flag is used to
manage the context lifecycle based on the recommendations in <xref
target="context_management"/>. </t>
<t hangText="CID:"> 4-bit Context Identifier for this prefix
information. CID is used by context based header compression
specified in <xref target="RFC6282"/>. The list of CIDs for a LoWPAN
is configured by on the 6LBR that originates the context information
for the 6LoWPAN. </t>
<t hangText="Res, Reserved:"> This field is unused. It MUST be
initialized to zero by the sender and MUST be ignored by the
receiver. </t>
<t hangText="Valid Lifetime:"> 16-bit unsigned integer. The length
of time in a unit of 60 seconds (relative to the time the packet is
received) that the context is valid for the purpose of header
compression or decompression. A value of all zero bits (0x0)
indicates that this context entry MUST be removed immediately. </t>
<t hangText="Context Prefix:"> The IPv6 prefix or address
corresponding to the Context ID (CID) field. The valid length of
this field is included in the Context Length field. This field is
padded with zeros in order to make the option a multiple of 8-bytes.
</t>
</list></t>
</section>
<section anchor='ABRO' title="Authoritative Border Router Option">
<t>The Authoritative Border Router Option (ABRO) is needed when
Router Advertisement (RA) messages are used to disseminate prefixes and
context information across a route-over topology. In this case 6LRs
receive Prefix Information options from other 6LRs. This implies that a
6LR can't just let the most recently received RA win. In order to be
able to reliably add and remove prefixes from the 6LoWPAN we need to
carry information from the authoritative 6LBR. This is done by
introducing a version number which the 6LBR sets and 6LRs propagate as
they propagate the prefix and context information with this
Authoritative Border Router Option. When there are multiple 6LBRs they
would have separate version number spaces. Thus this option needs to
carry the IP address of the 6LBR that originated that set of
information.</t>
<t>The Authoritative Border Router option MUST be included in all Router
Advertisement messages in the case when Router Advertisements are used
to propagate information between routers (as described in <xref
target="multihop_dad"/>).</t>
<figure>
<artwork><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length = 3 | Version Low |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version High | Valid Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ 6LBR Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>Fields:
<list style='hanging' hangIndent='15'>
<t hangText="Type:"> TBD3</t>
<t hangText="Length:"> 8-bit unsigned integer.
The length of the option in units of 8 bytes. Always 3.</t>
<t hangText="Version Low, Version High:"> Together, Version
Low and Version High are a 32-bit unsigned integer, where Version
Low is the least significant 16 bits and Version High is the most
significant 16 bits. The version number corresponding to this set
of information contained in the RA message. The authoritative 6LBR
originating the prefix increases this version number each time its
set of prefix or context information changes. </t>
<t hangText="Valid Lifetime:"> 16-bit unsigned integer. The
length of time in a unit of 60 seconds (relative to the time the
packet is received) that this set of border router information is
valid. A value of all zero bits (0x0) assumes a default value of
10,000 (~ one week). </t>
<t hangText="Reserved:"> This field is unused. It MUST be
initialized to zero by the sender and MUST be ignored by the
receiver.</t>
<t hangText="6LBR Address:"> IPv6 address of the 6LBR that is the
origin of the included version number.</t>
</list>
</t>
</section>
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<section anchor='DARDAC' title="Duplicate Address messages">
<t>For the multihop DAD exchanges between 6LR and 6LBR
specified in <xref target="multihop_dad"/> there are two new ICMPv6
message types called the Duplicate Address Request (DAR) and
Duplicate Address Confirmation (DAC). We avoid reusing the Neighbor
Solicitation and Neighbor Advertisement messages for this purpose
since these messages are not subject to the hop limit=255 check as
they are forwarded by intermediate 6LRs. The information contained
in the messages are otherwise the same as would be in a Neighbor
Solicitation carrying a Address Registration option, with the
message format inlining the fields that are in the ARO.</t>
<t>The DAR and DAC use the same message format with different ICMPv6
type values, and the Status field is only meaningful in the DAC
message.</t>
<figure>
<artwork><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status | Reserved | Registration Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ EUI-64 +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Registered Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>IP fields:
<list style='hanging' hangIndent='15'>
<t hangText="IPv6 source:"> A non link-local address of the sending
router.</t>
<t hangText="IPv6 destination:"> A non link-local address of the
sending router. In a DAC this is just the source from the DAR.</t>
<t hangText="Hop Limit:"> Set to MULTIHOP_HOPLIMIT on transmit. MUST
be ignored on receipt.</t>
</list></t>
<t>ICMP Fields:
<list style='hanging' hangIndent='15'>
<t hangText="Type:"> TBD4 for DAR and TBD5 for DAC</t>
<t hangText="Code:"> Set to zero on transmit. MUST be ignored on
receipt.</t>
<t hangText="Checksum:"> The ICMP checksum. See <xref
target="RFC4443"/>.</t>
<t hangText="Status:"> 8-bit unsigned integer. Indicates the status
of a registration in the DAC. MUST be set to 0 in DAR. See <xref
target="status-codes"/>.</t>
<t hangText="Reserved:"> This field is unused. It MUST be
initialized to zero by the sender and MUST be ignored by the
receiver.</t>
<t hangText="Registration Lifetime:"> 16-bit unsigned integer. The
amount of time in a unit of 60 seconds that the router should retain
the Neighbor Cache entry for the sender of the NS that includes this
option. A value of 0 indicates in an NS that the neighbor cache
entry should be removed.</t>
<t hangText="EUI-64:">64 bits. This field is used to uniquely
identify the interface of the registered address by including the
EUI-64 identifier <xref target="EUI64"/> assigned to it
unmodified.</t>
<t hangText="Registered Address:">128-bit field. Carries the host
address, which was contained in the IPv6 Source field in the NS that
contained the ARO option sent by the host.</t>
</list></t>
</section>
</section>
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<section anchor='host' title="Host Behavior">
<t>Hosts in a LoWPAN use the Address Registration option in the
Neighbor Solicitation messages they send as a way to maintain the
Neighbor Cache in the routers thereby removing the need for
multicast Neighbor Solicitations to do address resolution. Unlike in
<xref target="RFC4861"/> the hosts initiate updating the information
they receive in Router Advertisements by sending Router
Solicitations before the information expires. Finally, when Neighbor
Unreachability Detection indicates that one or all default routers
have become unreachable, then the host uses Router Solicitations to
find a new set of default routers. </t>
<section anchor="hFA" title="Forbidden Actions">
<t>A host MUST NOT multicast a Neighbor Solicitation message.</t>
</section>
<section anchor='host_interface' title="Interface Initialization">
<t>When the interface on a host is initialized it follows the
specification in <xref target="RFC4861"/>. A link-local address is
formed based on the EUI-64 identifier <xref target="EUI64"/>
assigned to the interface as per <xref target="RFC4944"/> or the
appropriate IP-over-foo document for the link, and then the host
sends Router Solicitation messages as described in <xref
target="RFC4861"/> Section 6.3.7.</t>
<t>There is no need to join the Solicited-Node multicast address
since nobody multicasts Neighbor Solicitations in this type of
network. A host MUST join the all-nodes multicast address.</t>
</section>
<section anchor='host_sending_rs' title="Sending a Router Solicitation">
<t>The Router Solicitation is formatted as specified in <xref
target="RFC4861"/> and sent to the IPv6 All-Routers multicast
address (see <xref target="RFC4861"/> Section 6.3.7 for details). An
SLLA option MUST be included to enable unicast Router Advertisements
in response. An unspecified source address MUST NOT be used in RS
messages.</t>
<t>If the link layer supports a way to send packets to some kind of
all-routers anycast link-layer address, then that MAY be used to
convey these packets to a router.</t>
<t>Since hosts do not depend on multicast Router Advertisements to discover
routers, the hosts need to intelligently retransmit Router Solicitations
whenever the default router list is empty, one of its default routers
becomes unreachable, or the lifetime of the prefixes and contexts in the
previous RA are about to expire. The RECOMMENDED retransmissions is to
initially send up to 3 (MAX_RTR_SOLICITATIONS) RS messages separated by at
least 10 seconds (RTR_SOLICITATION_INTERVAL) as specified in <xref
target="RFC4861"/>, and then switch to slower retransmissions. After the
initial retransmissions the host SHOULD do truncated binary exponential
backoff <xref target="ETHERNET"/> of the retransmission timer for each
subsequent retransmission, truncating the increase of the
retransmission timer at 60 seconds (MAX_RTR_SOLICITATION_INTERVAL). In all
cases the RS retransmissions are terminated when a RA is received. See <xref
target="constants"/> for protocol constants.</t>
</section>
<section anchor="ra_proc" title="Processing a Router Advertisement">
<t>The processing of Router Advertisements is as in <xref
target="RFC4861"/> with the addition of handling the 6LoWPAN Context
option and triggering address registration when a new address has
been configured. Furthermore, the SLLA option MUST be included in
the RA. Unlike in <xref target="RFC4861"/>, the maximum value of the
RA Router Lifetime field MAY be up to 0xFFFF (approximately 18
hours).</t>
<t>Should the host erroneously receive a Prefix Information option
with the 'L' (on-link) flag set, then that Prefix Information Option
(PIO) MUST be ignored.</t>
<section title="Address configuration">
<t>Address configuration follows <xref target="RFC4862"/>. For an
address not derived from an EUI-64, the M flag of the RA determines
how the address can be configured. If the M flag is set in the RA,
then DHCPv6 MUST be used to assign the address. If the M flag is not
set, then the address can be configured by any other means (and
duplicate detection is performed as part of the registration
process).</t>
<t>Once an address has been configured it will be registered by
unicasting a Neighbor Solicitation with the Address Registration
option to one or more routers.</t>
</section>
<section title="Storing Contexts">
<t>The host maintains a conceptual data structure for the context
information it receives from the routers, which is called the
Context Table. This includes the Context ID, the prefix (from the
Context Prefix field in the 6CO), the Compression bit, and the Valid
Lifetime. A Context Table entry that has the Compression bit clear
is used for decompression when receiving packets, but MUST NOT be
used for compression when sending packets.</t>
<t>When a 6CO option is received in a Router Advertisement it is
used to add or update the information in the Context Table. If the
Context ID field in the 6CO matches an existing Context Table entry,
then that entry is updated with the information in the 6CO. If the
Valid Lifetime field in the 6CO is zero, then the entry is
immediately deleted.</t>
<t>If there is no matching entry in the Context Table, and the Valid
Lifetime field is non-zero, then a new context is added to the
Context Table. The 6CO is used to update the created entry.</t>
<t>When the 6LBR changes the context information a host might not
immediately notice. And in the worst case a host might have stale
context information. For this reason 6LBRs use the recommendations
in <xref target="context_management"/> for carefully managing the
context lifecycle. Nodes should be careful about using header
compression in RA messages that include 6COs. </t>
</section>
<section title="Maintaining Prefix and Context Information">
<t>The prefix information is timed out as specified in <xref
target="RFC4861"/>. When the Valid Lifetime for a Context Table
entry expires the entry is placed in a receive-only mode, which is
the equivalent of receiving a 6CO for that context with C=0. The
entry is held in receive-only mode for a period of twice the Default
Router Lifetime, after which the entry is removed. </t>
<t>A host should inspect the various lifetimes to determine when it
should next initiate sending a Router Solicitation to ask for any
updates to the information. The lifetimes that matter are the
Default Router lifetime, the Valid Lifetime in the Prefix
Information options, and the Valid Lifetime in the 6CO. The host
SHOULD unicast one or more Router Solicitations to the router well
before the minimum of those lifetimes (across all the prefixes and
all the contexts) expire, and switch to multicast RS messages if
there is no response to the unicasts. The retransmission behavior
for the Router Solicitations is specified in <xref
target='host_sending_rs'/>.</t>
</section>
</section>
<section anchor='host_registration'
title="Registration and Neighbor Unreachability Detection">
<t>Hosts send Unicast Neighbor Solicitation (NS) messages to
register their IPv6 addresses, and also to do NUD to verify that
their default routers are still reachable. The registration is
performed by the host including an ARO in the Neighbor Solicitation
it sends. Even if the host doesn't have data to send, but is
expecting others to try to send packets to the host, the host needs
to maintain its Neighbor Cache entries in the routers. This is done
by sending NS messages with the ARO to the router well in advance of
the registration lifetime expiring. NS messages are retransmitted up
to MAX_UNICAST_SOLICIT times using a minimum timeout of
RETRANS_TIMER until the host receives an Neighbor Advertisement
message with an ARO option. </t>
<t>Hosts that receive Router Advertisement messages from multiple
default routers SHOULD attempt to register with more than one of
them in order to increase the robustness of the network.</t>
<t>Note that Neighbor Unreachability Detection probes can be
suppressed by Reachability Confirmations from transport protocols or
applications as specified in <xref target="RFC4861"/>.</t>
<t>When a host knows it will no longer use a router it is registered
to, it SHOULD de-register with the router by sending an NS with an
ARO containing a lifetime of 0. To handle the case when a host loses
connectivity with the default router involuntarily, the host SHOULD
use a suitably low registration lifetime. </t>
<section anchor='host_ns' title="Sending a Neighbor Solicitation">
<t>The host triggers sending Neighbor Solicitation (NS) messages
containing an ARO when a new address is configured, when it
discovers a new default router, or well before the Registration
Lifetime expires. Such an NS MUST include a Source Link-Layer
Address (SLLA) option, since the router needs to record the
link-layer address of the host. An unspecified source address MUST
NOT be used in NS messages.</t>
</section>
<section title="Processing a Neighbor Advertisement">
<t>A host handles Neighbor Advertisement messages as specified in
<xref target="RFC4861"/>, with added logic described in this section
for handling the Address Registration option.</t>
<t> In addition to the normal validation of a Neighbor Advertisement
and its options, the Address Registration option is verified as
follows (if present). If the Length field is not two, the option is
silently ignored. If the EUI-64 field does not match the EUI-64 of
the interface, the option is silently ignored. </t>
<t> If the status field is zero, then the address registration was
successful. The host saves the Registration Lifetime from the
Address Registration option for use to trigger a new NS well before
the lifetime expires. If the Status field is not equal to zero, the
address registration has failed. </t>
</section>
<section title="Recovering from Failures">
<t>The procedure for maintaining reachability information about a
neighbor is the same as in <xref target="RFC4861"/> Section 7.3 with
the exception that address resolution is not performed. </t>
<t>The address registration procedure may fail for two reasons: no
response to Neighbor Solicitations is received (NUD failure), or an
Address Registration option with a failure Status (Status > 0) is
received. In the case of NUD failure the entry for that router will
be removed thus address registration is no longer of importance.
When an Address Registration option with a non-zero Status field is
received this indicates that registration for that address has
failed. A failure Status of one indicates that a duplicate address
was detected and the procedure described in <xref target="RFC4862"/>
Section 5.4.5 is followed. The host MUST NOT use the address it
tried to register. If the host has valid registrations with other
routers, these MUST be removed by registering with each using a zero
ARO lifetime. </t>
<t>A Status code of two indicates that the Neighbor Cache of that
router is full. In this case the host SHOULD remove this router from
its default router list and attempt to register with another router.
If the host's default router list is empty, it needs to revert to
sending Router Solicitations as specified in <xref
target='host_sending_rs'/>.</t>
<t>Other failure codes may be defined in future documents.</t>
</section>
</section>
<section anchor="host-nexthop" title="Next-hop Determination">
<t> The IP address of the next-hop for a destination is determined
as follows. Destinations to the link-local prefix (FE80::) are
always sent on the link to that destination. It is assumed that
link-local addresses are formed as specified in <xref
target="host_interface"/> from the EUI-64, and address resolution is
not performed. Packets are sent to link local destinations by reversing the
procedure in Appendix A of <xref target="RFC4291"/>. </t>
<t> Multicast addresses are considered to be on-link and are
resolved as specified in <xref target="RFC4944"/> or the appropriate
IP-over-foo document. Note that <xref target="RFC4944"/> only
defines how to represent a multicast destination address in the
LoWPAN header. Support for multicast scopes larger than link-local
needs an appropriate multicast routing algorithm.</t>
<t>All other prefixes are assumed to be off-link <xref
target="RFC5889"/>. Anycast addresses are always considered to be
off-link. They are therefore sent to one of the routers in the
Default Router List. </t>
<t>A LoWPAN Node is not required to maintain a minimum of one buffer
per neighbor as specified in <xref target="RFC4861"/>, since packets
are never queued while waiting for address resolution. </t>
</section>
<section title="Address Resolution">
<t>The address registration mechanism and the SLLA option in Router
Advertisement messages provide sufficient a priori state in routers
and hosts to resolve an IPv6 address to its associated link-layer
address. As all prefixes, except the link-local prefix and multicast
addresses, are always assumed to be off-link, multicast-based
address resolution between neighbors is not needed. </t>
<t> Link-layer addresses for neighbors are stored in Neighbor Cache
entries <xref target="RFC4861"/>. In order to achieve LoWPAN
compression, most global addresses are formed using a link-layer
address. Thus a host can reduce memory usage by optimizing for
this case and only storing link-layer address information if it
differs from the link-layer address corresponding to the Interface
ID of the IPv6 address (i.e., differs in more than the
on-link/global bit being inverted). </t>
</section>
<section title="Sleeping">
<t>It is often advantageous for battery-powered hosts in LoWPANs to
keep a low duty cycle. The optimizations described in this document
enable hosts to sleep as described further in this section. Routers
may want to cache traffic destined to a host which is sleeping, but
such functionality is out of the scope of this document.</t>
<section title="Picking an Appropriate Registration Lifetime">
<t>As all Neighbor Discovery messages are initiated by the hosts, this
allows a host to sleep or otherwise be unreachable between NS/NA message
exchanges. The Address Registration option attached to NS messages indicates
to a router to keep the Neighbor Cache entry for that address valid for the
period in the Registration Lifetime field. A host should choose a sleep time
appropriate for its energy characteristics, and set a registration lifetime
larger than the sleep time to ensure the registration is renewed
successfully (considering e.g. clock drift and additional time for potential
retransmissions of the re-registration). External configuration of a host
should also consider the stability of the network (how quickly the topology
changes) when choosing its sleep time (and thus registration lifetime). A
dynamic network requires a shorter sleep time so that routers don't keep
invalid neighbor cache entries for nodes longer than necessary.</t>
</section>
<section anchor="be_wakeup" title="Behavior on Wakeup">
<t>When a host wakes up from a sleep period it SHOULD refresh its
current address registrations that will timeout before the next
wakeup. This is done by sending Neighbor Solicitation messages with
the Address Registration option as described in <xref
target="host_ns"/>. The host may also need to refresh its prefix and
context information by sending a new unicast Router Solicitation
(the maximum Router Lifetime is about 18 hours whereas the maximum
Registration lifetime is about 45.5 days). If after wakeup the host
(using NUD) determines that some or all previous default routers
have become unreachable, then the host will send multicast Router
Solicitations to discover new default router(s) and restart the
address registration process.</t>
<!-- Carsten: Is there an assumption that every RA will contain the entire context, i.e., up to 16 6COs? If not, one RS may not be enough - when has a
host sufficiently tried refreshing that info?
-->
</section>
</section>
</section>
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<section anchor='router' title="Router Behavior for 6LR and 6LBR">
<t>Both 6LRs and 6LBRs maintain the Neighbor Cache <xref
target="RFC4861"/> based on the Address Registration Options they
receive in Neighbor Advertisement messages from hosts, Neighbor
Discovery packets from other nodes, and potentially a routing
protocol used in the 6LoWPAN as outlined in <xref
target="nce_management"/>.</t>
<t>The routers SHOULD NOT garbage collect Registered Neighbor Cache
entries (see <xref target="overview_rr"/>) since they need to retain
them until the Registration Lifetime expires. Similarly, if Neighbor
Unreachability Detection on the router determines that the host is
UNREACHABLE (based on the logic in <xref target="RFC4861"/>), the
Neighbor Cache entry SHOULD NOT be deleted but be retained until the
Registration Lifetime expires. A renewed ARO should mark the cache
entry as STALE. Thus for 6LoWPAN Routers the Neighbor Cache doesn't
behave like a cache. Instead it behaves as a registry of all the
host addresses that are attached to the Router.</t>
<t>Routers MAY implement the Default Router Preferences <xref
target="RFC4191"/> and use that to indicate to the host whether the
router is a 6LBR or a 6LR. If this is implemented then 6LRs with no
route to a border router MUST set Prf to (11) for low preference,
other 6LRs MUST set Prf to (00) for normal preference, and 6LBRs
MUST set Prf to (01) for high preference.</t>
<section anchor="router-forbidden" title="Forbidden Actions">
<t>
Even if a router in a route-over topology can reach both a
host and another target, because
of radio propagation it generally cannot
know whether the host can directly reach the other target. Therefore it cannot
assume that redirect will actually work from one host to another.
Therefore it SHOULD NOT send Redirect messages. The only
potential exception to this "SHOULD NOT" is when the deployment/implementation
has a way to know how the host can reach the intended target. Hence it is RECOMMENDED that
the implementation by default does not send redirect messages but can be
configurable when the deployment calls for this. In contrast,
for mesh-under
topologies, the same considerations about Redirects apply as in 4861.
</t>
<t>A router MUST NOT set the 'L' (on-link) flag in the Prefix
Information options, since that might trigger hosts to send
multicast Neighbor Solicitations.</t>
</section>
<section title="Interface Initialization">
<t>The 6LBR router interface initialization behavior is the same as
in <xref target="RFC4861"/>. However, in dynamic configuration
scenario (see <xref target="multihop_dist"/>), a 6LR comes up as a
non-router and waits to receive the advertisement for configuring
its own interface address first before making its interfaces
advertising and turning into a router. </t>
</section>
<section anchor="rs_proc" title="Processing a Router Solicitation">
<t>A router processes Router Solicitation messages as specified in
<xref target="RFC4861"/>. The differences relate to the inclusion of
Authoritative Border Router options in the Router Advertisement (RA)
messages, and the exclusive use of unicast Router Advertisements. If
a 6LR has received an ABRO from a 6LBR, then it will include that
option unmodified in the Router Advertisement messages it sends.
And if the 6LR has received RAs, whether with the same prefixes and
context information or different, from a different 6LBR, then it
will need to keep those prefixes and context information separately
so that the RAs the 6LR sends will maintain the association between
the ABRO and the prefixes and context information. The router can
tell which 6LBR originated the prefixes and context information from
the 6LBR Address field in the ABRO. When a router has information
tied to multiple ABROs, a single RS will result in multiple RAs each
containing a different ABRO.</t>
<t>When the ABRO Valid Lifetime associated with a 6LBR times out, all
information related to that 6LBR MUST be removed. As an implementation note,
it is recommend that RAs are sent sufficiently more frequently than the ABRO
Valid Lifetime so that missing an RA does not result in removing all
information related to a 6LBR.</t>
<t>A Router Solicitation might be received from a host that has not
yet registered its address with the router. Thus the router MUST NOT
modify an existing Neighbor Cache entry based on the SLLA option
from the Router Solicitation. However, a router MAY create a
Tentative Neighbor Cache entry based on the SLLA option. Such a
Tentative Neighbor Cache entry SHOULD be timed out in
TENTATIVE_NCE_LIFETIME seconds unless a registration converts it
into a Registered NCE.</t>
<t>A 6LR or 6LBR MUST include a Source Link-layer address option in
the Router Advertisements it sends. That is required so that the
hosts will know the link-layer address of the router. Unlike in
<xref target="RFC4861"/>, the maximum value of the RA Router
Lifetime field MAY be up to 0xFFFF (approximately 18 hours).</t>
<t>Unlike <xref target="RFC4861"/> which suggests multicast Router
Advertisements, this specification improves the exchange by always
unicasting RAs in response to RSs. This is possible since the RS
always includes a SLLA option, which is used by the router to
unicast the RA. </t>
</section>
<section title="Periodic Router Advertisements">
<t>A router does not need to send any periodic Router Advertisement
messages since the hosts will solicit updated information by sending
Router Solicitations before the lifetimes expire.</t>
<t>However, if the routers use Router Advertisements to
distribute prefix and/or context information across a route-over
topology, that might require periodic Router Advertisement messages.
Such RAs are sent using the configurable MinRtrAdvInterval and
MaxRtrAdvInterval as per <xref target="RFC4861"/>.</t>
</section>
<section anchor='router_ns' title="Processing a Neighbor Solicitation">
<t>A router handles Neighbor Solicitation messages as specified in
<xref target="RFC4861"/>, with added logic described in this section
for handling the Address Registration option.</t>
<t>In addition to the normal validation of a Neighbor Solicitation
and its options, the Address Registration option is verified as
follows (if present). If the Length field is not two, or if the
Status field is not zero, then the Neighbor Solicitation is silently
ignored.</t>
<t>If the source address of the NS is the unspecified address, or if
no SLLA option is included, then any included ARO is ignored, that
is, the NS is processed as if it did not contain an ARO.</t>
<section anchor='duplicate' title="Checking for Duplicates">
<t>If the NS contains a valid ARO, then the router inspects its
Neighbor Cache on the arriving interface to see if it is a
duplicate. If there is no Neighbor Cache entry for the IPv6 source
address of the NS, then it isn't a duplicate. If there is such a
Neighbor Cache entry and the EUI-64 is the same, then it isn't a
duplicate either. Otherwise it is a duplicate address. Note that if
multihop DAD (<xref target="multihop_dad"/>) is used then the checks
are slightly different to take into account Tentative Neighbor Cache
entries. In the case it is a duplicate address then the router
responds with a unicast Neighbor Advertisement (NA) message with the
ARO Status field set to one (to indicate the address is a duplicate)
as described in <xref target="errors"/>. In this case there is no
modification to the Neighbor Cache. </t>
</section>
<section anchor='errors' title="Returning Address Registration Errors">
<t>Address registration errors are not sent back to the source
address of the NS due to a possible risk of L2 address collision.
Instead the NA is sent to the link-local IPv6 address with the IID
part derived from the EUI-64 field of the ARO as per <xref
target="RFC4944"/>. In particular, this means that the
universal/local bit needs to be inverted. The NA is formatted with a
copy of the ARO from the NS, but with the Status field set to
indicate the appropriate error. </t>
<t>The error is sent to the link-local address with the IID derived
from the EUI-64. Thus if the ARO was from and for a short address, the
L2 destination address for the NA with the ARO error will be the 64-bit
unique addresses.
</t>
</section>
<section anchor='router_cache' title="Updating the Neighbor Cache">
<t>If ARO did not result in a duplicate address being detected as
above, then if the Registration Lifetime is non-zero the router
creates (if it didn't exist) or updates (otherwise) a Neighbor Cache
entry for the IPv6 source address of the NS. If the Neighbor Cache
is full and a new entry needs to be created, then the router
responds with a unicast NA with the ARO Status field set to two (to
indicate the router's Neighbor Cache is full) as described in <xref
target="errors"/>. </t>
<t> The Registration Lifetime and the EUI-64 are recorded in the
Neighbor Cache entry. A unicast Neighbor Advertisement (NA) is then
sent in response to the NS. This NA SHOULD include a copy of the
ARO, with the Status field set to zero. A TLLA (Target Link-layer
Address) option <xref target="RFC4861"/> is not required in the NA,
since the host already knows the router's link-layer address from
Router Advertisements.</t>
<t>If the ARO contains a zero Registration Lifetime then any
existing Neighbor Cache entry for the IPv6 source address of the NS
MUST be deleted, and a NA sent as above.</t>
<t>Should the Registration Lifetime in a Neighbor Cache entry
expire, then the router MUST delete the cache entry.</t>
<t>The addition and removal of Registered Neighbor Cache entries
would result in notifying the routing protocol. </t>
<t>Note: If the substitutable multihop DAD (<xref
target="multihop_dad"/>) is used, then the updating of the Neighbor
Cache is slightly different due to Tentative NCEs.</t>
</section>
<section title="Next-hop Determination">
<t> In order to deliver a packet destined for a 6LN registered with
a router, next-hop determination is slightly different for routers
than hosts (see <xref target="host-nexthop"/>. The routing table is
checked to determine the next hop IP address. A registered Neighbor
Cache Entry (NCE) determines if the next hop IP-address is on-link.
It is the responsibility of the routing protocol of the router to
maintain on-link information about its registered neighbors.
Tentative NCEs MUST NOT be used to determine on-link status of the
registered nodes. </t>
</section>
<section anchor='router_resolution'
title="Address Resolution between Routers">
<t>There needs to be a mechanism somewhere for the routers to
discover each others' link-layer addresses. If the routing protocol
used between the routers provides this, then there is no need for
the routers to use the Address Registration option between each
other. Otherwise, the routers SHOULD use the ARO. When routers use ARO <!--OOO-->
to register with each other and the multihop DAD <xref
target="multihop_dad"/> is in use, then care must be taken to
ensure that there isn't a flood of ARO-carrying messages sent to the
6LBR as each router hears an ARO from their neighboring routers. The
details for this is out of scope of this document.</t>
<t>Routers MAY also use multicast Neighbor Solicitations as in
<xref target="RFC4861"/> to resolve each others link-layer
addresses. Thus Routers MAY multicast Neighbor Solicitations for
other routers, for example as a result of receiving some routing
protocol update. Routers MUST respond to multicast Neighbor
Solicitations. This implies that Routers MUST join the
Solicited-node multicast addresses as specified in <xref
target="RFC4861"/>.</t>
</section>
</section>
</section>
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<section anchor='border_router' title="Border Router Behavior">
<t>A 6LBR handles sending of Router Advertisements and processing of
Neighbor Solicitations from hosts as specified above in section
<xref target="router"/>. A 6LBR SHOULD always include an
Authoritative Border Router option in the Router Advertisements it
sends, listing itself as the 6LBR Address. That requires that the
6LBR maintain the version number in stable storage, and increases
the version number when some information in its Router
Advertisements change. The information whose change affects the
version are in the Prefix Information options (the prefixes or their
lifetimes) and in the 6CO option (the prefixes, Context IDs, or
lifetimes.)</t>
<t>In addition, a 6LBR is somehow configured with the prefix or
prefixes that are assigned to the LoWPAN, and advertises those in
Router Advertisements as in <xref target="RFC4861"/>. In
the case of route-over, those prefixes can be disseminated to all
the 6LRs using the technique in <xref target="multihop_dist"/>.
However, there might be mechanisms outside of the scope of this
document that can be used as a substitute for prefix
dissemination in the route-over topology (see <xref target="subst_features"/>).</t>
<t>If the 6LoWPAN uses Header Compression <xref target="RFC6282"/>
with context then the 6LBR needs to manage the context IDs, and
advertise those in Router Advertisements by including 6CO options in
its Router Advertisements so that directly attached hosts are
informed about the context IDs. Below we specify things to consider
when the 6LBR needs to add, remove, or change the context
information. In the case of route-over, the context
information is disseminated to all the 6LRs using the technique
in <xref target="substitutable"/> unless a different specification
provides a substitute for this multihop distribution.</t>
<section anchor='border_router_prefix' title="Prefix Determination">
<t>The prefix or prefixes used in a LoWPAN can be manually
configured, or can be acquired using DHCPv6 Prefix Delegation <xref
target="RFC3633"/>. For a LoWPAN that is isolated from the network,
either permanently or occasionally, the 6LBR can assign a ULA prefix
using <xref target="RFC4193"/>. The ULA prefix should be stored in
stable storage so that the same prefix is used after a failure of
the 6LBR. If the LoWPAN has multiple 6LBRs, then they should be
configured with the same set of prefixes. The set of prefixes are
included in the Router Advertisement messages as specified in <xref
target="RFC4861"/>. </t>
</section>
<section anchor='context_management'
title="Context Configuration and Management">
<t>If the LoWPAN uses Header Compression <xref target="RFC6282"/>
with context then the 6LBR must be configured with context <!--OOO-->
information and related context IDs. If the LoWPAN has multiple
6LBRs, then they MUST be configured with the same context
information and context IDs. For <xref target="RFC6282"/>,
maintaining consistency of context information is crucial for
ensuring packets will be decompressed correctly.</t>
<t>The context information carried in Router Advertisement (RA)
messages originate at 6LBRs and must be disseminated to all the
routers and hosts within the LoWPAN. RAs include one 6CO for each
context. </t>
<t> For the dissemination of context information using the 6CO, a
strict lifecycle SHOULD be used in order to ensure the context
information stays synchronized throughout the LoWPAN. New context
information SHOULD be introduced into the LoWPAN with C=0, to ensure
it is known by all nodes that may have to decompress based on this
context information. Only when it is reasonable to assume that this
information was successfully disseminated SHOULD an option with C=1
be sent, enabling the actual use of the context information for
compression. </t>
<t> Conversely, to avoid that nodes send packets making use of
previous values of contexts, resulting in ambiguity when receiving a
packet that uses a recently changed context, old values of a context
SHOULD be taken out of use for a while before new values are
assigned to this specific context. That is, in preparation for a
change of context information, its dissemination SHOULD continue for
at least MIN_CONTEXT_CHANGE_DELAY with C=0. Only when it is
reasonable to assume that the fact that the context is now invalid
was successfully disseminated, should the context ID be taken out of
dissemination or reused with a different Context Prefix field. In
the latter case, dissemination of the new value again SHOULD start
with C=0, as above. </t>
</section>
</section>
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<section anchor='substitutable' title="Substitutable Feature Behavior">
<t>Normally in a 6LoWPAN multihop network, the Router Advertisement
messages are used to
disseminate prefixes and context information to all the 6LRs in a
route-over topology. If all routers are configured to use a substitute
mechanism for such information distribution, any remaining use of the
6LoWPAN-ND mechanisms is governed by the substitute specification.
</t>
<t>There is also the option for a 6LR to perform multihop DAD (for
non-EUI-64 derived IPv6 addresses) against a 6LBR in a route-over
topology by using the DAR and DAC messages. This is substitutable because
there might be other ways to either allocate unique address, such as
DHCPv6 <xref target="RFC3315"/>, or other future mechanisms for
multihop DAD.
Again in this case, any remaining use of the
6LoWPAN-ND mechanisms is governed by the substitute specification.
</t>
<t>To be clear: Implementations MUST support the features
describes in <xref target="multihop_dist"/>
and <xref target="multihop_dad"/>, unless the implementation
supports some alternative ("substitute") from some
some other specification.</t>
<section anchor='multihop_dist'
title="Multihop Prefix and Context Distribution">
<t>The multihop distribution relies on Router Solicitation messages
and Router Advertisement (RA) messages sent between routers, and
using the ABRO version number to control the propagation of the
information (prefixes and context information) that is being sent in
the RAs.</t>
<t>This multihop distribution mechanism can handle arbitrary
information from an arbitrary number of 6LBRs. However, the
semantics of the context information requires that all the 6LNs use
the same information, whether they send, forward, or receive
compressed packets. Thus the manager of the 6LBRs need to somehow
ensure that the context information is in synchrony across the
6LBRs. This can be handled in different ways. One possible way to
ensure it is to treat the context and prefix information as
originating from some logical or virtual source, which in essence
means that it looks like the information is distributed from a
single source.</t>
<t>If a set of 6LBRs behave as a single one (using mechanisms out of
scope of this document) so that the prefixes and contexts and ABRO
version number will be the same from all the 6LBRs, then those 6LBRs
can pick a single IP address to use in the ABRO option.</t>
<section anchor="br_ra" title="6LBRs Sending Router Advertisements">
<t>6LBRs supporting multihop prefix and context distribution MUST
include an ABRO in each of its RAs. The ABRO Version Number field is
used to keep prefix and context information consistent throughout
the LoWPAN along with the guidelines in <xref
target="context_management"/>. Each time any information in the set
of PIO or 6CO options change, the ABRO Version is increased by one.
</t>
<t>This requires that the 6LBR maintain the PIO, 6CO, and ABRO
Version Number in stable storage, since an old version number will
be silently ignored by the 6LRs.</t>
</section>
<section anchor="r_rs_send" title="Routers Sending Router Solicitations">
<t>In a 6LoWPAN, unless substituted, multihop distribution is done
using Router Advertisement (RA)
messages. Thus on interface initialization a router (6LR) MUST send
Router Solicitation messages following the rules specified for
hosts in <xref
target="RFC4861"/>. That will cause the routers to respond with RA messages
which then can be used to initially seed the prefix and context information.
</t>
</section>
<section anchor="r_ra_proc"
title="Routers Processing Router Advertisements">
<t>If multihop distribution is not done using RA messages, then the
routers follow <xref target="RFC4861"/> which states that they
merely do some consistency checks and nothing in <xref
target="multihop_dist"/> applies. Otherwise the routers will check
and record the prefix and context information from the receive RAs,
and use that information as follows.</t>
<t>If a received RA does not contain a Authoritative Border Router
option, then the RA MUST be silently ignored.</t>
<t>The router uses the 6LBR Address field in the ABRO to check if it
has previously received information from the 6LBR. If it finds no
such information, then it just records the 6LBR Address, Version,
Valid Lifetime and the associated prefixes and context information.
If the 6LBR is previously known, then the Version number field MUST
be compared against the recorded version number for that 6LBR. If
the version number received in the packet is less than the stored
version number then the information in the RA is silently ignored.
Otherwise the recorded information and version number are
updated.</t>
</section>
<section anchor="store_info" title="Storing the Information">
<t>The router keeps state for each 6LBR that it sees with an ABRO.
This includes the Version number, the Valid Lifetime, and the
complete set of Prefix Information options and 6LoWPAN Context
options. The prefixes are timed out based on the Valid lifetime in
the Prefix Information Option. The Context Prefix is timed out based
on the Valid lifetime in the 6LoWPAN Context option.</t>
<t>While the prefixes and context information are stored in the
router their valid and preferred lifetimes are decremented as time
passes. This ensures that when the router is in turn later
advertising that information in the Router Advertisements it sends,
the 'expiry time' doesn't accidentally move further into the future.
For example, if a 6CO with a Valid lifetime of 10 minutes is
received at time T, and the router includes this in a RA it sends at
time T+5 minutes, the Valid lifetime in the 6CO it sends will be
only 5 minutes.</t>
</section>
<section anchor="mh_ra" title="Sending Router Advertisements">
<t>When multihop distribution is performed using RA messages, the
routers MUST ensure that the ABRO always stay together with the
prefixes and context information received with that ABRO. Thus if
the router has received prefix P1 with ABRO saying it is from one
6LBR, and prefix P2 from another 6LBR, then the router MUST NOT
include the two prefixes in the same RA message. Prefix P1 MUST be
in a RA that include a ABRO from the first 6LBR etc. Note that
multiple 6LBRs might advertise the same prefix and context
information, but they still need to be associated with the 6LBRs
that advertised them.</t>
<t>The routers periodically send Router Advertisements as in <xref
target="RFC4861"/>. This is for the benefit of the other routers
receiving the prefixes and context information. And the routers also
respond to Router Solicitations by unicasting RA messages. In both
cases the above constraint of keeping the ABRO together with 'its'
prefixes and context information apply.</t>
<t>When a router receives new information from a 6LBR, that is,
either it hears from a new 6LBR (a new 6LBR Address in the ABRO) or
the ABRO version number of an existing 6LBR has increased, then it
is useful to send out a few triggered updates. The recommendation is
to behave the same as when an interface has become an advertising
interface in <xref target="RFC4861"/>, that is, send up to three RA
messages. This ensures rapid propagation of new information to all
the 6LRs.</t>
</section>
</section>
<section anchor='multihop_dad' title="Multihop Duplicate Address Detection">
<t>The ARO can be used, in addition to registering an address in a
6LR, to have the 6LR verify that the address isn't used by some
other host known to the 6LR. However, that isn't sufficient in a
route-over topology (or in a LoWPAN with multiple 6LBRs) since some
host attached to another 6LR could be using the same address. There
might be different ways for the 6LRs to coordinate such Duplicate
Address Detection in the future, or addresses could be assigned
using a DHCPv6 server that verifies uniqueness as part of the
assignment. </t>
<t>This specification offers a substitutable simple technique for
6LRs and 6LBRs to perform Duplicate Address Detection that reuses
the information from Address Registration option in the DAR and DAC
messages. This technique is not needed when the Interface ID in the
address is based on an EUI-64, since those are assumed to be
globally unique. The technique assumes that the 6LRs either register
with all the 6LBRs, or that the network uses some out-of-scope
mechanism to keep the DAD tables in the 6LBRs synchronized.</t>
<t>The multihop DAD mechanism is used synchronously the first time
an address is registered with a particular 6LR. That is, the ARO
option is not returned to the host until multihop DAD has been
completed against the 6LBRs. For existing registrations in the 6LR
the multihop DAD needs to be repeated against the 6LBRs to ensure
that the entry for the address in the 6LBRs does not time out, but
that can be done asynchronously with the response to the hosts. For
instance, by tracking how much is left of the lifetime the 6LR
registered with the 6LBRs and re-registering with the 6LBR when this
lifetime is about to run out.</t>
<!-- Carsten: What is the lifetime that is sent back to the host then? The minimum of the one requested and the one still available from the previous
6LBR request? -->
<t>For the synchronous multihop DAD the 6LR performs some additional
checks to ensure that it has a Neighbor Cache entry it can use to
respond to the host when it receives a response from a 6LBR. This
consists of checking for an already existing (Tentative or
Registered) Neighbor Cache entry for the registered address with a
different EUI-64. If such a Registered NCE exists, then the 6LR
SHOULD respond that the address is a duplicate. If such a Tentative
NCE exists, then the 6LR SHOULD silently ignore the ARO thereby
relying on the host retransmitting the ARO. This is needed to handle
the case when multiple hosts try to register the same IPv6 address
at the same time. If no Neighbor Cache entry exists, then the 6LR
MUST create a Tentative Neighbor Cache entry with the EUI-64 and the
SLLA option. This entry will be used to send the response to the
host when the 6LBR responds positively.</t>
<t>When a 6LR receives a Neighbor Solicitation containing an Address
Registration option with a non-zero Registration Lifetime and it has
no existing Registered Neighbor Cache entry, then with this
mechanism the 6LR will invoke synchronous multihop DAD.</t>
<t>The 6LR will unicast a Duplicate Address Request message to one
or more 6LBRs, where the DAR contains the host's address in the
Registered Address field. The DAR will be forwarded by 6LRs until it
reaches the 6LBR, hence its IPv6 hop limit field will not be 255
when received by the 6LBR. The 6LBR will respond with a Duplicate
Address Confirmation message, which will have a hop limit less than
255 when it reaches the 6LR.</t>
<t>When the 6LR receives the DAC from the 6LBR, it will look for a
matching (same IP address and EUI-64) (Tentative or Registered)
Neighbor Cache entry. If no such entry is found then the DAC is
silently ignored. If an entry is found and the DAC had Status=0 then
the 6LR will mark the Tentative Neighbor Cache entry as Registered.
In all cases when an entry is found then the 6LR will respond to the
host with an NA, copying the Status and EUI-64 fields from the DAC
to an ARO option in the NA. In case the status is an error, then the
destination IP address of the NA is derived from the EUI-64 field of
the DAC.</t>
<t>A Tentative Neighbor Cache entry SHOULD be timed out
TENTATIVE_NCE_LIFETIME seconds after it was created in order to
allow for another host to attempt to register the IPv6 address.</t>
<section anchor='DAR-validation' title="Message Validation for DAR and DAC">
<t>A node MUST silently discard any received Duplicate Address
Request and Confirmation messages for which at least one of the following
validity checks is not satisfied:
<list style="symbols">
<t>If the message includes an IP Authentication Header, the message
authenticates correctly.</t>
<t>ICMP Checksum is valid.</t>
<t>ICMP Code is 0.</t>
<t>ICMP length (derived from the IP length) is 32 or more bytes.</t>
<t>The Registered Address is not a multicast address.</t>
<t>All included options have a length that is greater than zero.</t>
<t>The IP source address is not the unspecified address, nor a multicast
address.</t>
</list>
</t>
<t>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. </t>
<t>Note that due to the forwarding of the DAR and DAC messages
between the 6LR and 6LBR there is no hop limit check on receipt for
these ICMPv6 message types. </t>
</section>
<section title="Conceptual Data Structures">
<t>A 6LBR implementing multihop DAD needs to maintain
some state separate from the Neighbor Cache. We call this conceptual
data structure the DAD table. It is indexed by the IPv6 address -
the Registered Address in the DAR - and contains the EUI-64 and the
registration lifetime of the host that is using that address.</t>
</section>
<section title="6LR Sending a Duplicate Address Request">
<t>When a 6LR that implements multihop DAD receives an
NS from a host and subject to the above checks, the 6LR forms and
sends a DAR to at least one 6LBR. The DAR contains the following
information:
<list style="symbols">
<t>In the IPv6 source address, a global address of the 6LR.</t>
<t>In the IPv6 destination address, the address of the 6LBR.</t>
<t>In the IPv6 hop limit, MULTIHOP_HOPLIMIT.</t>
<t>The Status field MUST be set to zero</t>
<t>The EUI-64 and Registration lifetime are copied from the ARO
received from the host.</t>
<t>The Registered Address set to the IPv6 address of the host,
that is, the sender of the triggering NS.</t>
</list>
</t>
<t>When a 6LR receives an NS from a host with a zero Registration
Lifetime then, in addition to removing the Neighbor Cache entry for
the host as specified in <xref target="router"/>, an DAR is sent to
the 6LBRs as above.</t>
<t>A router MUST NOT modify the Neighbor Cache as a result of
receiving a Duplicate Address Request.</t>
</section>
<section title="6LBR Receiving a Duplicate Address Request">
<t>When a 6LBR that implements the substitutable multihop DAD receives an
DAR from a 6LR, it performs the message validation specified in
<xref target="DAR-validation"/>. If the DAR is valid the 6LBR
proceeds to look for the Registration Address in the DAD Table. If
an entry is found and the recorded EUI-64 is different than the
EUI-64 in the DAR, then it returns a DAC NA with the Status set to 1
('Duplicate Address'). Otherwise it returns a DAC with Status set to
zero and updates the lifetime.</t>
<t>If no entry is found in the DAD Table and the Registration
Lifetime is non-zero, then an entry is created and the EUI-64 and
Registered Address from the DAR are stored in that entry.</t>
<t>If an entry is found in the DAD Table, the EUI-64 matches, and
the Registration Lifetime is zero then the entry is deleted from the
table.</t>
<t>In both of the above cases the 6LBR forms an DAC with the
information copied from the DAR and the Status field is set to zero.
The DAC is sent back to the 6LR i.e., back to the source of the DAR.
The IPv6 hop limit is set to MULTIHOP_HOPLIMIT</t>
</section>
<section title="Processing a Duplicate Address Confirmation">
<t>When a 6LR implementing multihop DAD receives a
DAC message, then it first validates the message per <xref
target="DAR-validation"/>. For a valid DAC, if there is no Tentative
Neighbor Cache entry matching the Registered address and EUI-64,
then the DAC is silently ignored. Otherwise, the information in the
DAC and in the Tentative Neighbor Cache entry is used to form an NA
to send to the host. The Status code is copied from the DAC to the
ARO that is sent to the host. In case of the DAC indicates an error
(the Status is non-zero), the NA is returned to the host as
described in <xref target="errors"/> and the Tentative Neighbor
Cache entry for the Registered Address is removed. Otherwise it is
made into a Registered Neighbor Cache entry.</t>
<t>A router MUST NOT modify the Neighbor Cache as a result of
receiving a Duplicate Address Confirmation, unless there is a
Tentative Neighbor Cache entry matching the IPv6 address and
EUI-64.</t>
</section>
<section title="Recovering from Failures">
<t>If there is no response from a 6LBR after RETRANS_TIMER <xref
target="RFC4861"/> then the 6LR would retransmit the DAR to the 6LBR
up to MAX_UNICAST_SOLICIT <xref target="RFC4861"/> times. After this
the 6LR SHOULD respond to the host with an ARO Status of zero.</t>
</section>
</section>
</section>
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<section anchor='constants' title="Protocol Constants">
<t> This section defines the relevant protocol constants used in
this document based on a subset of <xref target="RFC4861"/>
constants. (*) indicates constants modified from <xref
target="RFC4861"/> and (+) indicates new constants. </t>
<t> Additional protocol constants are defined in <xref
target="message_options"/>. </t>
<t>6LBR Constants:
<list style="hanging" hangIndent="40">
<t hangText="MIN_CONTEXT_CHANGE_DELAY+">300 seconds</t>
</list>
</t>
<t>6LR Constants:
<list style="hanging" hangIndent="40">
<t hangText="MAX_RTR_ADVERTISEMENTS">3 transmissions</t>
<t hangText="MIN_DELAY_BETWEEN_RAS*">10 seconds</t>
<t hangText="MAX_RA_DELAY_TIME*">2 seconds</t>
<t hangText="TENTATIVE_NCE_LIFETIME+">20 seconds</t>
</list>
</t>
<t>Router Constants:
<list style="hanging" hangIndent="40">
<t hangText="MULTIHOP_HOPLIMIT+">64</t>
</list>
</t>
<t>Host Constants:
<list style="hanging" hangIndent="40">
<t hangText="RTR_SOLICITATION_INTERVAL*">10 seconds</t>
<t hangText="MAX_RTR_SOLICITATIONS">3 transmissions</t>
<t hangText="MAX_RTR_SOLICITATION_INTERVAL+">60 seconds</t>
</list>
</t>
</section>
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<section anchor='examples' title="Examples">
<section anchor='examples_msg' title="Message Examples">
<figure anchor='figmessrsra'
title="Basic Router Solicitation/Router Advertisement exchange between a node and 6LR or 6LBR">
<artwork><![CDATA[
STEP
6LN 6LR
| |
1. | ---------- Router Solicitation --------> |
| [SLLAO] |
| |
2. | <-------- Router Advertisement --------- |
| [PIO + 6CO + ABRO + SLLAO] |
]]></artwork>
</figure>
<figure anchor='figmessbasic'
title="Neighbor Discovery Address Registration ">
<artwork><![CDATA[
6LN 6LR
| |
1. | ------- NS with Address Registration ------> |
| [ARO + SLLAO] |
| |
2. | <----- NA with Address Registration -------- |
| [ARO with Status] |
]]></artwork>
</figure>
<figure anchor='figmessmdad'
title="Neighbor Discovery Address Registration with Multihop DAD">
<artwork><![CDATA[
6LN 6LR 6LBR
| | |
1. | --- NS with Address Reg --> | |
| [ARO + SLLAO] | |
| | |
2. | | ----------- DAR ----------> |
| | |
3. | | <---------- DAC ----------- |
| | |
4. | <-- NA with Address Reg --- | |
| [ARO with Status] |
]]></artwork>
</figure>
</section>
<section anchor='examples_boot' title="Host Bootstrapping Example">
<t>The following example describes the address bootstrapping
scenarios using the improved ND mechanisms specified in this
document. It is assumed that the 6LN first performs a sequence of
operations in order to get secure access at the link-layer of the
LoWPAN and obtain a key for link-layer security. The methods of how
to establish the link-layer security is out of scope of this
document. In this example an IEEE 802.15.4 6LN forms a 16-bit
short-address based IPv6 addresses without using DHCPv6 (i.e., the M
flag is not set in the Router Advertisements).</t>
<t>1. After obtaining link-level security, a 6LN assigns a
link-local IPv6 address to itself. A link-local IPv6 address is
configured based on the 6LN's EUI-64 link-layer address formed as
per <xref target="RFC4944"/>.</t>
<t>2. Next the 6LN determines one or more default routers in the
network by sending an RS to the all-routers multicast address with
the SLLA Option set to its EUI-64 link-local address. If the 6LN was
able to obtain the link-layer address of a router through its
link-layer operations then the 6LN may form a link-local destination
IPv6 address for the router and send it a unicast RS. The 6LR
responds with a unicast RA to the IP source using the SLLA option
from the RS (it may have created a tentative NCE). See <xref
target="figmessrsra"/>. </t>
<t>3. In order to communicate more than one IP hop away the 6LN
configures a global IPv6 address. In order to save overhead, this
6LN wishes to configure its IPv6 address based on a 16-bit short
address as per <xref target="RFC4944"/>. As the network is unmanaged
(M flag not set in RA), the 6LN randomly chooses a 16-bit link-layer
address and forms a tentative IPv6 address from it. </t>
<t>4. Next the 6LN registers that address with one or more of its
default routers by sending a unicast NS message with an ARO
containing its tentative global IPv6 address to register, the
registration lifetime and its EUI-64. An SLLA option is also
included with the link-layer address corresponding to the address
being registered. If a successful (status 0) NA message is received
the address can then be used and the 6LN assumes it has been
successfully checked for duplicates. If a duplicate address (status
1) NA message is received, the 6LN then removes the temporary IPv6
address and 16-bit link-layer address and goes back to step 3. If a
neighbor cache full (status 2) message is received, the 6LN attempts
to register with another default router, or if none, goes back to
step 2. See <xref target="figmessbasic"/>. Note that an NA message
returning an error would be sent back to the link-local EUI-64 based
IPv6 address of the 6LN instead of the 16-bit (duplicate) address.
</t>
<t>5. The 6LN now performs maintenance by sending a new NS address
registration before the lifetime expires. </t>
<t>If multihop DAD and multihop prefix and context distribution is
used, the effect of the 6LRs and hosts following the above
bootstrapping is a "wavefront" of 6LRs and host being configured
spreading from the 6LBRs. First the hosts and 6LRs that can directly
reach a 6LBR would receive one or more RAs and configure and
register their IPv6 addresses. Once that is done they would enable
the routing protocol and start sending out Router Advertisements.
That would result in a new set of 6LRs and hosts to receive
responses to their Router Solicitations, form and register their
addresses, etc. That repeats until all of the 6LRs and hosts have
been configured.</t>
<section anchor="examples_annotation" title="Host Bootstrapping Messages">
<t>This section brings specific message examples to the previous
bootstrapping process. When discussing messages, the following
notation is used:</t>
<t>LL64: Link-Local Address based on the EUI-64, which is also the
802.15.4 Long Address. </t>
<t>GP16: Global Address based on the 802.15.4 Short Address. This
address may not be unique. </t>
<t>GP64: Global addresses derived from the EUI-64 address as specified
in <xref target="RFC4944"/>.</t>
<t>MAC64: EUI-64 address used as the link-layer address.</t>
<t>MAC16: IEEE 802.15.4 16-bit short address.</t>
<t>Note that some implementations may use LL64 and GP16 style
addresses instead of LL64 and GP64. In the following, we will
show an example message flow as to how a node uses LL64 to register a
GP16 address for multihop DAD verification.</t>
<figure anchor='figmessexamples'
title="Detailed Message Address Examples">
<artwork><![CDATA[
6LN-----RS-------->6LR
Src= LL64 (6LN)
Dst= All-router-link-scope-multicast
SLLAO= MAC64 (6LN)
6LR------RA--------->6LN
Src= LL64 (6LR)
Dst= LL64 (6LN)
Note: Source address of RA must be a link-local
address (Section 4.2, RFC 4861).
6LN-------NS Reg------>6LR
Src= GP16 (6LN)
Dst= LL64 (6LR)
ARO
SLLAO= MAC16 (6LN)
6LR---------DAR----->6LBR
Src= GP64 or GP16 (6LR)
Dst= GP64 or GP16 (6LBR)
Registered Address= GP16 (6LN) and EUI-64 (6LN)
6LBR-------DAC--------->6LR
Src= GP64 or GP16 (6LBR)
Dst= GP64 or GP16 (6LR)
Copy of information from DAR
If Status is a Success:
6LR ---------NA-Reg------->6LN
Src= LL64 (6LR)
Dst= GP16 (6LN)
ARO with Status = 0
If Status is not a success:
6LR ---------NA-Reg-------->6LN
Src= LL64 (6LR)
Dst= LL64 (6LN) --> Derived from the EUI-64 of ARO
ARO with Status > 0
]]></artwork>
</figure>
</section>
</section>
<section anchor='examples_router' title="Router Interaction Example">
<t>In the Route-over topology, when a routing protocol is run across
6LRs the bootstrapping and neighbor cache management are handled a
little differently. The description in this paragraph provides only
a guideline for an implementation.</t>
<t>At the initialization of a 6LR, it may choose to bootstrap as a
host with the help of a parent 6LR if the substitutable multihop DAD is
performed with the 6LBR. The neighbor cache management of a router
and address resolution among the neighboring routers are described
in <xref target="router_cache"/> and <xref
target="router_resolution"/>, respectively. In this example, we
assume that the neighboring 6LoWPAN link is secure.</t>
<section title="Bootstrapping a Router">
<t>In this scenario, the bootstrapping 6LR, 'R1', is multiple hops
away from the 6LBR and surrounded by other 6LR neighbors. Initially
R1 behaves as a host. It sends multicast RS and receives an RA from
one or more neighboring 6LRs. R1 picks one 6LR as its temporary
default router and performs address resolution via this default
router. Note, if multihop DAD is not required (e.g. in a managed
network or using EUI-64 based addresses) then it does not need to
pick a temporary default router, however it may still want to send
the initial RS message if it wants to autoconfigure its address with
the global prefix disseminated by the 6LBR.</t>
<t>Based on the information received in the RAs, R1 updates its
cache with entries for all the neighboring 6LRs. Upon completion of
the address registration, the bootstrapping router deletes the
temporary entry of the default router and the routing protocol is
started.</t>
<t>Also note that R1 may refresh its multihop DAD registration
directly with the 6LBR (using the next hop neighboring 6LR
determined by the routing protocol for reaching the 6LBR).</t>
</section>
<section title="Updating the Neighbor Cache">
<t>In this example, there are three 6LRs, R1, R2, R3. Initially when
R2 boots it sees only R1, and accordingly R2 creates a neighbor
cache entry for R1. Now assume R2 receives a valid routing update
from router R3. R2 does not have any neighbor cache entry for R3. If
the implementation of R2 supports detecting link-layer address from
the routing information packets then it directly updates the its
neighbor cache using that link-layer information. If this is not
possible, then R2 should perform multicast NS with source set with
its link-local or global address depending on the scope of the
source IP-address received in the routing update packet. The target
address of the NS message is the source IPv6 address of the received
routing update packet. The format of the NS message is as described
in Section 4.3 of <xref target="RFC4861"/>.</t>
<t>More generally any 6LR that receives a valid route-update from a
neighboring router for which it does not have any neighbor cache
entry is required to update its neighbor cache as described
above.</t>
<t>The router (6LR and 6LBR) IP-addresses learned via Neighbor
Discovery are not redistributed to the routing protocol.</t>
</section>
</section>
</section>
<!-- **************************************************************** -->
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<section title="Security Considerations">
<t> The security considerations of IPv6 Neighbor Discovery <xref
target="RFC4861"/> and Address Autoconfiguration <xref target="RFC4862"/>
apply. Additional considerations can be found in <xref target="RFC3756"/>.
</t>
<t>There is a slight modification to those considerations due to the
fact that in this specification the M-flag in the Router Advertisements
disable the use of stateless address autoconfiguration for addresses not
derived from EUI-64. Thus a rogue router on the link can force the use
of only DHCP for short addresses, whereas in <xref target="RFC4861"/> and <xref
target="RFC4862"/> the rogue router could only cause the addition of DHCP and
not disable SLAAC for short addresses.</t>
<t> This specification assumes that the link layer is sufficiently
protected, for instance using MAC sublayer cryptography.
Thus, its threat model is no different from that of IPv6 Neighbor Discovery
<xref target="RFC4861"/>.
The threat model number 1 in section 3 of <xref target="RFC3756"/> applies here.
However, any future 6LoWPAN security
protocol that applies to Neighbor Discovery for 6LoWPAN protocol, is
out of scope of this document. </t>
<t>The multihop DAD mechanisms rely on DAR and DAC messages that are
forwarded by 6LRs, and as a result the hop_limit=255 check on the
receiver does not apply to those messages. This implies that any
node on the Internet could successfully send such messages. We avoid
any additional security issues due to this by requiring that the
routers never modify the Neighbor Cache entry due to such messages,
and that they discard them unless they are received on an interface
that has been explicitly configured to use these optimizations.</t>
<t>In some future deployments one might want to use SEcure Neighbor
Discovery <xref target="RFC3971"/> <xref target="RFC3972"/>. This is
possible with the Address Registration option as sent between hosts
and routers, since the address that is being registered is the IPv6
source address of the Neighbor Solicitation and SeND verifies the
IPv6 source address of the packet. Applying SeND to the
router-to-router communication in this document is out of scope.</t>
</section>
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<section anchor='iana' title="IANA Considerations">
<t> The document requires three new Neighbor Discovery option types
under the subregistry "IPv6 Neighbor Discovery Option Formats":
<list style="symbols">
<t>Address Registration Option (TBD1)</t>
<t>6LoWPAN Context Option (TBD2)</t>
<t>Authoritative Border Router Option (TBD3)</t>
</list>
</t>
<t> The document requires two new ICMPv6 types under the subregistry
"ICMPv6 type Numbers":
<list style="symbols">
<t>Duplicate Address Request (TBD4)</t>
<t>Duplicate Address Confirmation (TBD5)</t>
</list>
</t>
<t>This document also requests IANA to create a new sub-registry for
the Status values of the Address Registration Option, under the
ICMPv6 parameters registry.</t>
<t><list style='hanging'>
<t hangText='Address Registration Option Status Values registry:'></t>
<t>Possible values are 8-bit unsigned integers (0..255).</t>
<t>Registration procedure is "Standards Action" <xref
target="RFC5226"/>.</t>
<t>Initial allocation is as indicated in <xref
target="status-codes-iana"/>:</t>
</list></t>
<texttable anchor='status-codes-iana'> <!--update status-codes in sync-->
<ttcol align='center'> Status </ttcol>
<ttcol align='center'> Description</ttcol>
<c>0</c> <c>Success</c>
<c>1</c> <c>Duplicate Address</c>
<c>2</c> <c>Neighbor Cache Full</c>
<c>3-255</c> <c>Allocated using Standards Action <xref target="RFC5226"/></c>
</texttable>
</section>
<section title="Interaction with other Neighbor Discovery Extensions">
<t>There are two classes of Neighbor Discovery Extensions that have
different interaction with this specification.</t>
<t>One class are extensions to to the Duplicate Address Detection
mechanisms in <xref target="RFC4861"/> and <xref target="RFC4862"/>.
An example of this is Optimistic DAD <xref target="RFC4429"/>.
Such extensions does not apply when this specification is being used,
since it uses ARO for DAD (which is neither optimistic nor
pessimistic - always one roundtrip to the router to check DAD).</t>
<t>All other (non-DAD) Neighbor Discovery extensions, be it path
selection ones like Default Router Preferences <xref target="RFC4191"/>,
configuration ones like DNS config <xref target="RFC5006"/>, or
others like DNA <xref target="RFC6059"/>, are completely orthogonal
to this specification, and will work as is.</t>
</section>
<section title="Guideline for New Features">
<t> This section discusses a guideline of new protocol features defined in
this document. It also sets some expectations for implementation and
deployment of these features. This section is informative in nature: It
does not override the detailed specifications of the previous sections, but
summarizes them and presents them in a compact form that can be used as a
checklist. The checklist acts as a guideline to indicate the possible
importance of a feature in terms of a deployment as per information
available as of the writing of the document.
Note that in some cases the deployment is 'SHOULD' where the
implementation is a 'MUST'. This is due to the presence of substitutable features;
the deployment may use alternative methods for those.
Therefore, implementing a configuration knob is recommended for
the substitutable features.
The lists emphasize conciseness over completeness. </t>
<texttable anchor='guide1' title="Guideline for 6LoWPAN-ND features for hosts">
<ttcol align='left'>Section</ttcol>
<ttcol align='left'>Description</ttcol>
<ttcol align='left'>Deploy</ttcol>
<ttcol align='left'>Implement</ttcol>
<c><xref target="Optimization" format="counter"/></c><c>Host initiated RA</c><c>MUST</c><c>MUST</c>
<c><xref target="Addr-assign" format="counter"/></c><c>EUI-64 based IPv6-address</c><c>MUST</c><c>MUST</c>
<c> </c><c>16bit-MAC based address</c><c>MAY</c><c>SHOULD</c>
<c> </c><c>Other non-unique addresses</c><c>MAY</c><c>MAY</c>
<c><xref target="overview_hr" format="counter"/></c><c>Host Initiated RS</c><c>MUST</c><c>MUST</c>
<c> </c><c>ABRO Processing</c><c>SHOULD</c><c>MUST</c>
<c><xref target="ARO" format="counter"/></c><c>Registration with ARO</c><c>MUST</c><c>MUST</c>
<c>
<xref target="s6CO" format="counter"/>,
<xref target="ra_proc" format="counter"/></c><c>6LoWPAN Context Option</c><c>SHOULD</c><c>SHOULD</c>
<c><xref target="hFA" format="counter"/></c><c>Re-direct Message Acceptance</c><c>MUST NOT</c><c>MUST NOT</c>
<c> </c><c>Joining Solicited Node Multicast</c><c>N/A</c><c>N/A</c>
<c> </c><c>Joining all-node Multicast</c><c>MUST</c><c>MUST</c>
<c> </c><c>Using link-layer indication for NUD</c><c>MAY</c><c>MAY</c>
<c><xref target="host_registration" format="counter"/></c><c>6LoWPAN-ND NUD</c><c>MUST</c><c>MUST</c>
<c><xref target="be_wakeup" format="counter"/></c><c>Behavior on wake-up</c><c>SHOULD</c><c>SHOULD</c>
</texttable>
<texttable anchor='guide2' title="Guideline for 6LR features in 6LoWPAN-ND">
<ttcol align='left'>Section</ttcol>
<ttcol align='left'>Description</ttcol>
<ttcol align='left'>deploy</ttcol>
<ttcol align='left'>implement</ttcol>
<c><xref target="Optimization" format="counter"/></c><c>Periodic RA</c><c>SHOULD NOT</c><c>SHOULD NOT</c>
<c><xref target="Addr-assign" format="counter"/></c><c>Address assignment during Startup</c><c>SHOULD</c><c>MUST</c>
<c><xref target="overview_hr" format="counter"/></c><c>Supporting EUI-64 based MAC Hosts</c><c>MUST</c><c>MUST</c>
<c> </c><c>Supporting 16-bit MAC hosts</c><c>MAY</c><c>SHOULD</c>
<c>
<xref target="overview_rr" format="counter"/>,
<xref target="ABRO" format="counter"/>,
<xref target="r_ra_proc" format="counter"/>,
<xref target="store_info" format="counter"/></c><c>ABRO Processing/sending</c><c>SHOULD</c><c>MUST</c>
<c><xref target="multihop_dist" format="counter"/></c><c>Multihop Prefix storing and re-distribution</c><c>SHOULD</c><c>MUST</c>
<c><xref target="nce_management" format="counter"/></c><c>Tentative NCE</c><c>MUST</c><c>MUST</c>
<c><xref target="multihop_dad" format="counter"/></c><c>Multihop DAD</c><c>SHOULD</c><c>MUST</c>
<c>
<xref target="ARO" format="counter"/>,
<xref target="router_ns" format="counter"/>,
<xref target="duplicate" format="counter"/> -
<xref target="router_resolution" format="counter"/></c><c>ARO Support</c><c>MUST</c><c>MUST</c>
<c><xref target="s6CO" format="counter"/></c><c>6LoWPAN Context Option</c><c>SHOULD</c><c>SHOULD</c>
<c><xref target="rs_proc" format="counter"/></c><c>Process RS/ARO</c><c>MUST</c><c>MUST</c>
</texttable>
<texttable anchor='guide3' title="Guideline for 6LBR features in 6LoWPAN-ND">
<ttcol align='left'>Section</ttcol>
<ttcol align='left'>Description</ttcol>
<ttcol align='left'>deploy</ttcol>
<ttcol align='left'>implement</ttcol>
<c><xref target="Optimization" format="counter"/></c><c>Periodic RA</c><c>SHOULD NOT</c><c>SHOULD NOT</c>
<c><xref target="Addr-assign" format="counter"/></c><c>Address autoconf on Router interface</c><c>MUST NOT</c><c>MUST NOT</c>
<c><xref target="overview_hr" format="counter"/></c><c>EUI-64 MAC support on 6LoWPAN interface</c><c>MUST</c><c>MUST</c>
<c>
<xref target="multihop_dist" format="counter"/> -
<xref target="br_ra" format="counter"/>,
<xref target="mh_ra" format="counter"/></c><c>Multihop Prefix distribution</c><c>SHOULD</c><c>MUST</c>
<c><xref target="multihop_dad" format="counter"/></c><c>Multihop DAD</c><c>SHOULD</c><c>MUST</c>
</texttable>
</section>
<!--==================================================-->
<!-- SECTION: ACKNOWLEDGMENTS -->
<!--==================================================-->
<section title="Acknowledgments">
<t>The authors thank Pascal Thubert, Jonathan Hui, Carsten Bormann,
Richard Kelsey, Geoff Mulligan, Julien Abeille, Alexandru Petrescu,
Peter Siklosi, Pieter De Mil, Fred Baker, Anthony Schoofs, Phil
Roberts, Daniel Gavelle, Joseph Reddy, Robert Cragie, Mathilde
Durvy, Colin O'Flynn, Dario Tedeschi, Esko Dijk and Joakim Eriksson
for useful discussions and comments that have helped shaped and
improve this document.</t>
<t> Additionally, the authors would like to recognize Carsten
Bormann for the suggestions on the Context Prefix Option and
contribution to earlier version of the draft, Pascal Thubert for
contribution of the original registration idea and extensive
contributions to earlier versions of the draft, Jonathan Hui for
original ideas on prefix/context distribution and extensive
contributions to earlier versions of the draft, Colin O'Flynn for
useful Error-to suggestions and contributions to the Examples
section, Geoff Mulligan for suggesting the use of Address
Registration as part of existing IPv6 Neighbor Discovery messages,
and Mathilde Durvy for helping to clarify router interaction. </t>
</section>
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<section title="Changelog">
<t>Changes from -20 to -21:
<list>
<t>o Clarified the address an address registration error is sent to.</t>
<t>o Added a new section explaining the interaction with other Neighbor Discovery extensions.</t>
</list>
</t>
<t>Changes from -19 to -20:
<list>
<t>o Further clarification on substitutable features.</t>
<t>o Changed RFC 6282 to a normative reference.</t>
</list>
</t>
<t>Changes from -18 to -19:
<list>
<t>o Editorial improvements as a result of IESG comments (#135, #142).</t>
<t>o Extended ABRO with longer version number and valid lifetime, while maintaining backward compatibility (#141). </t>
<t>o Renamed optional features and described them as substitutable (#138).</t>
</list>
</t>
<t>Changes from -17 to -18:
<list>
<t>o Fixed nits related to IESG submission.</t>
</list>
</t>
<t>Changes from -16 to -17:
<list>
<t>o Removed unnecessary normative text from Assumptions.</t>
<t>o Clarified the next-hop determination of multicast addresses.
</t>
<t>o Editorial improvements from WGLC review.</t>
</list>
</t>
<t>Changes from -15 to -16:
<list>
<t>o Added an applicability section (#133) </t>
<t>o Updated document title to align with HC</t>
<t>o Minor editing as result of WGLC review (#134)</t>
</list>
</t>
<t>Changes from -14 to -15:
<list>
<t>o Changed use of redirect to SHOULD NOT for route-over and MAY for mesh-under. (#130) </t>
<t>o Changed the 16-bit lifetimes to a unit of 60 seconds (#131) </t>
<t>o Added text to Section 5.4.2 adding a receive-only state to context entries that timeout. (#132) </t>
</list>
</t>
<t>Changes from -13 to -14:
<list>
<t>o Introduced the new DAR and DAC ICMPv6 message types for
multihop DAD to avoid relying on the Length=4 checks for the ARO. This
simplifies implementing the hop limit check.</t>
<t>o Clarified the hop limit values for the multihop DAD messages
by introducing the MULTIHOP_HOPLIMIT constant set to 64.</t>
<t>o Clarified when a host should de-register from a router.</t>
<t>o Added a section on next-hop determination for routers.</t>
<t>o Removed the infinite lifetime from 6CO. </t>
<t>o Increased MIN_CONTEXT_CHANGE_DELAY to 300 seconds. </t>
</list>
</t>
<t>Changes from -12 to -13:
<list>
<t>o Error-to solution added for returning NA messages carrying an error ARO option to the link-local EUI-64 based IPv6 address of the host (#126). </t>
<t>o New examples added.</t>
</list>
</t>
<t>Changes from -11 to -12:
<list>
<t>o Version field of ABRO moved after Length for 32-bit alignment of the reserved space (#90).</t>
<t>o Several clarifications were made on router interaction, including a new section with router interaction examples (#91).</t>
<t>o Temporary Neighbor Cache Entry created upon host sending NS+ARO, and SLLAO removed from multihop DAD NS/NA messages (#87).</t>
</list>
</t>
<t>Changes from -10 to -11:
<list>
<t>o Reference to RFC1982 for version number comparison (#80)</t>
<t>o RA Router Lifetime field use clarified (#81)</t>
<t>o Make fields 16-bit rather than 32-bit where possible (#83)</t>
<t>o Unicast RA clarification (#84)</t>
<t>o Temporary ND option types (#85)</t>
<t>o SLLA/TLLA clarification (#86)</t>
<t>o GP16 as source address in initial NS clarification (#87)</t>
</list>
</t>
<t>Changes from -09 to -10:
<list>
<t>o Clarifications made to Section 8.2 (#66)</t>
<t>o Explained behavior of Neighbor Cache (#67)</t>
<t>o Clarified use of SLLAO in RS and NS messages (#68)</t>
<t>o Added new term 6LN (#69)</t>
<t>o Small clarification on 6CO flag (#70)</t>
<t>o Defined host behavior on ARO failure better (#72)</t>
<t>o Added bootstrapping example for a host (#73)</t>
<t>o Added new Neighbor Cache Full ARO error (#74)</t>
<t>o Added rule on the use of the M flag (#75)</t>
</list>
</t>
<t>Changes from -08 to -09:
<list>
<t>o Clean re-write of the draft (re-use of some introductory material)</t>
<t>o Merged in draft-chakrabarti-6lowpan-ipv6-nd-simple-00</t>
<t>o Changed address registration to an option piggybacked on NS/NA</t>
<t>o New Authoritative Border Router option</t>
<t>o New Address Registration Option</t>
<t>o Separated Prefix Information and Content Information</t>
<t>o Optional DAD to the edge</t>
</list>
</t>
<t>Changes from -07 to -08:
<list>
<t>o Removed Extended LoWPAN and Whiteboard related sections.</t>
<t>o Included reference to the autoconf addressing model.</t>
<t>o Added Optimistic Flag to 6AO.</t>
<t>o Added guidelines on routers performing DAD.</t>
<t>o Removed the NR/NC Advertising Interval.</t>
<t>o Added assumption of uniform IID formation and DAD throughout a LoWPAN.</t>
</list>
</t>
<t>Changes from -06 to -07:
<list>
<t>o Updated addressing and address resolution (#60).</t>
<t>o Changed the Address Option to 6LoWPAN Address Option, fixed S values (#61).</t>
<t>o Added support for classic RFC4861 RA Prefix Information messages to be processed (#62).</t>
<t>o Added a section on using 6LoWPAN-ND under a hard-wired RFC4861 stack (#63).</t>
<t>o Updated the NR/NC message with a new Router flag, combined the Code and Status fields into one byte, and added the capability to carry 6IOs (#64).</t>
<t>o Made co-existence with other ND mechanisms clear (#59).</t>
<t>o Added a new Protocol Specification section with all mechanisms specified there (#59).</t>
<t>o Removed dependencies and conflicts with RFC4861 wherever possible (#59).</t>
<t>o Some editorial cleanup.</t>
</list>
</t>
<t>Changes from -05 to -06:
<list>
<t>o Fixed the Prf codes (#52).</t>
<t>o Corrected the OIIO TID field to 8-bits. Changed the Nonce/OII order in both the OIIO and the NR/NC. (#53)</t>
<t>o Corrected an error in Table 1 (#54).</t>
<t>o Fixed asymmetric and a misplaced transient in the 6LoWPAN terminology section.</t>
<t>o Added Updates RFC4861 to header</t>
</list>
</t>
<t>Changes from -04 to -05:
<list>
<t>o Meaning of the RA's M-bit changed to original <xref target="RFC4861"/> meaning (#46).</t>
<t>o Terms "on-link" and "off-link" used in place of "on-link" and "off-link".</t>
<t>o Next-hop determination text simplified (#49).</t>
<t>o Neighbor cache and destination cache removed.</t>
<t>o IID to link-layer address requirement relaxed. </t>
<t>o NR/NC changes to enable on-link refresh with routers (#48).</t>
<t>o Modified 6LoWPAN Information Option (#47).</t>
<t>o Added a Protocol Constants section (#24)</t>
<t>o Added the NR processing table (#51)</t>
<t>o Considered the use of SeND on backbone NS/NA messages (#50)</t>
</list>
</t>
<t>Changes from -03 to -04:
<list>
<t>o Moved Ad-hoc LoWPAN operation to Section 7 and made ULA prefix generation a features useful also in Simple and Extended LoWPANs. (#41)</t>
<t>o Added a 32-bit Owner Nonce to the NR/NC messages and the Whiteboard, removed the TID history. (#39)</t>
<t>o Improved the duplicate OII detection algorithm using the Owner Nonce. (#39)</t>
<t>o Clarified the use of Source and Target link-layer options in NR/NC. (#43)</t>
<t>o Included text on the use of alternative methods to acquire addresses. (#38)</t>
<t>o Removed S=2 from Address Option (not needed). (#36)</t>
<t>o Added a section on router dissemination consistency. (#44)</t>
<t>o Small improvements and extensive editing. (#42, #37, #35)</t>
</list>
</t>
<t>Changes from -02 to -03:
<list>
<t>o Updated terminology, with RFC4861 non-transitive link model.</t>
<t>o 6LoWPAN and ND terminology separated.</t>
<t>o Protocol overview explains RFC4861 diff in detail.</t>
<t>o RR/RC is now Node Registration/Confirmation (NR/NC).</t>
<t>o Added NR failure codes.</t>
<t>o ER Metric now included in 6LoWPAN Summary Option for use in default router determination by hosts.</t>
<t>o Examples of host data structures, and the Whiteboard given.</t>
<t>o Whiteboard is supported by all Edge Routers for option simplicity.</t>
<t>o Edge Router Specification chapter re-structured, clarifying optional Extended LoWPAN operation.</t>
<t>o NS/NA now completely optional for nodes. No address resolution or NS/NA NUD required.</t>
<t>o link-local operation now compatible with oDAD (was broken).</t>
<t>o Exception to hop limit = 255 for NR/NC messages.</t>
<t>o Security considerations improved.</t>
<t>o ICMPv6 destination unreachable supported.</t>
</list>
</t>
<t>Changes from -01 to -02:
<list>
<t>o Fixed 16 != 0xff bug (ticket closed).</t>
<t>o Specified use of ULAs in ad-hoc LoWPAN section 9 (ticket closed).</t>
<t>o Terminology cleanup based on Alex's comments.</t>
<t>o General editing improvements.</t>
</list>
</t>
<t>Changes from -00 to -01:
<list>
<t>o Specified the duplicate owner interface identifier procedures. A TID lollipop algorithm was sufficient (nonce unnecessary).</t>
<t>o Defined fault tolerance using secondary bindings.</t>
<t>o Defined ad-hoc network operation.</t>
<t>o Removed the E flag from RA and the X flag from RR/RC.</t>
<t>o Completed message examples.</t>
<t>o Lots of improvements in text quality and consistency were made. </t>
</list>
</t>
</section>
</middle>
<back>
<references title='Normative References'>
&RFC2119;
&RFC5226;
&RFC2460;
&RFC2491;
&RFC4191;
&RFC4193;
&RFC4291;
&RFC4443;
&RFC4861;
&RFC4862;
&RFC4944;
&RFC6282;
<reference anchor='ETHERNET' target="http://standards.ieee.org/getieee802/download/802.3-2008_section1.pdf">
<front>
<title>Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific requirements - Part 3: Carrier sense multiple access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications</title>
<author>
<organization></organization>
</author>
<date month="December" year="2008" />
</front>
<seriesInfo name="IEEE" value="Std 802.3-2008" />
</reference>
</references>
<references title='Informative References'>
<reference anchor="EUI64" target="http://standards.ieee.org/regauth/oui/tutorials/EUI64.html">
<front>
<title>Guidelines for 64-bit Global Identifier (EUI-64) Registration Authority</title>
<author>
<organization>IEEE
</organization>
</author>
<date></date>
</front>
</reference>
&RFC3315;
&RFC3633;
&RFC3756;
&RFC3971;
&RFC3972;
&RFC4429;
&RFC4919;
&RFC4941;
&RFC5006;
&RFC5889;
&RFC6059;
</references>
</back>
</rfc>
<!-- LocalWords: Lossy LoWPAN IPv IEEE IP subnets lossy reachability
-->
<!-- LocalWords: optimizations LoWPANs multihop autoconfigures RAs
-->
<!-- LocalWords: subnet autoconfigure topologies unicast ICMPv LBR
-->
<!-- LocalWords: LBRs LRs EUI DHCPv Autoconfiguration ULA startup
-->
<!-- LocalWords: Unreachability ABRO PIO ARO NUD retransmits DAC NCE
-->
<!-- LocalWords: multicasting autoconfiguring NCEs SLLA lifecycle
-->
<!-- LocalWords: CIDs inlining ICMP multicasts anycast retransmit
-->
<!-- LocalWords: retransmissions backoff retransmission xFFFF COs
-->
<!-- LocalWords: unicasting unicasts retransmitted priori Wakeup Prf
-->
<!-- LocalWords: wakeup ABROs RSs IID TLLA synchrony retransmitting
-->
<!-- LocalWords: unmanaged wavefront misconfigured misconfiguration
-->
<!-- LocalWords: sublayer subregistry interoperability multicast
-->
<!-- LocalWords: Substitutable substitutable checksum IANA
-->
| PAFTECH AB 2003-2026 | 2026-04-23 16:29:57 |