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Internet Engineering Task Force (IETF) Z. Shelby, Ed.
Request for Comments: 6775 Sensinode
Updates: 4944 S. Chakrabarti
Category: Standards Track Ericsson
ISSN: 2070-1721 E. Nordmark
Cisco Systems
C. Bormann
Universitaet Bremen TZI
November 2012
Neighbor Discovery Optimization for IPv6 over Low-Power Wireless
Personal Area Networks (6LoWPANs)
Abstract
The IETF work in IPv6 over Low-power Wireless Personal Area Network
(6LoWPAN) defines 6LoWPANs 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.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6775.
Shelby, et al. Standards Track [Page 1]
RFC 6775 ND Optimization for 6LoWPANs November 2012
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................4
1.1. The Shortcomings of IPv6 Neighbor Discovery ................5
1.2. Applicability ..............................................6
1.3. Goals and Assumptions ......................................7
1.4. Substitutable Features .....................................8
2. Terminology .....................................................9
3. Protocol Overview ..............................................11
3.1. Extensions to RFC 4861 ....................................11
3.2. Address Assignment ........................................12
3.3. Host-to-Router Interaction ................................13
3.4. Router-to-Router Interaction ..............................14
3.5. Neighbor Cache Management .................................14
4. New Neighbor Discovery Options and Messages ....................15
4.1. Address Registration Option ...............................15
4.2. 6LoWPAN Context Option ....................................17
4.3. Authoritative Border Router Option ........................19
4.4. Duplicate Address Messages ................................20
5. Host Behavior ..................................................22
5.1. Forbidden Actions .........................................22
5.2. Interface Initialization ..................................22
5.3. Sending a Router Solicitation .............................23
5.4. Processing a Router Advertisement .........................23
5.4.1. Address Configuration ..............................23
5.4.2. Storing Contexts ...................................24
5.4.3. Maintaining Prefix and Context Information .........24
5.5. Registration and Neighbor Unreachability Detection ........25
5.5.1. Sending a Neighbor Solicitation ....................25
5.5.2. Processing a Neighbor Advertisement ................25
5.5.3. Recovering from Failures ...........................26
5.6. Next-Hop Determination ....................................26
5.7. Address Resolution ........................................27
Shelby, et al. Standards Track [Page 2]
RFC 6775 ND Optimization for 6LoWPANs November 2012
5.8. Sleeping ..................................................27
5.8.1. Picking an Appropriate Registration Lifetime .......27
5.8.2. Behavior on Wakeup .................................28
6. Router Behavior for 6LRs and 6LBRs .............................28
6.1. Forbidden Actions .........................................28
6.2. Interface Initialization ..................................29
6.3. Processing a Router Solicitation ..........................29
6.4. Periodic Router Advertisements ............................30
6.5. Processing a Neighbor Solicitation ........................30
6.5.1. Checking for Duplicates ............................30
6.5.2. Returning Address Registration Errors ..............31
6.5.3. Updating the Neighbor Cache ........................31
6.5.4. Next-Hop Determination .............................32
6.5.5. Address Resolution between Routers .................32
7. Border Router Behavior .........................................32
7.1. Prefix Determination ......................................33
7.2. Context Configuration and Management ......................33
8. Substitutable Feature Behavior .................................34
8.1. Multihop Prefix and Context Distribution ..................34
8.1.1. 6LBRs Sending Router Advertisements ................35
8.1.2. Routers Sending Router Solicitations ...............35
8.1.3. Routers Processing Router Advertisements ...........35
8.1.4. Storing the Information ............................36
8.1.5. Sending Router Advertisements ......................36
8.2. Multihop Duplicate Address Detection ......................37
8.2.1. Message Validation for DAR and DAC .................38
8.2.2. Conceptual Data Structures .........................39
8.2.3. 6LR Sending a Duplicate Address Request ............39
8.2.4. 6LBR Receiving a Duplicate Address Request .........39
8.2.5. Processing a Duplicate Address Confirmation ........40
8.2.6. Recovering from Failures ...........................40
9. Protocol Constants .............................................41
10. Examples ......................................................42
10.1. Message Examples .........................................42
10.2. Host Bootstrapping Example ...............................43
10.2.1. Host Bootstrapping Messages .......................45
10.3. Router Interaction Example ...............................46
10.3.1. Bootstrapping a Router ............................46
10.3.2. Updating the Neighbor Cache .......................47
11. Security Considerations .......................................47
12. IANA Considerations ...........................................48
13. Interaction with Other Neighbor Discovery Extensions ..........49
14. Guidelines for New Features ...................................49
15. Acknowledgments ...............................................52
16. References ....................................................52
16.1. Normative References .....................................52
16.2. Informative References ...................................53
Shelby, et al. Standards Track [Page 3]
RFC 6775 ND Optimization for 6LoWPANs November 2012
1. Introduction
The IPv6-over-IEEE 802.15.4 [RFC4944] document specifies how IPv6 is
carried over an IEEE 802.15.4 network with the help of an adaptation
layer that sits between the Media Access Control (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 (ND) [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 Multiple Access
(NBMA) [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 [RFC5889],
along with the corresponding address model assumptions defined
therein.
This specification introduces the following optimizations to IPv6
Neighbor Discovery [RFC4861] specifically aimed at low-power and
lossy networks such as LoWPANs:
o Host-initiated interactions to allow for sleeping hosts.
o Elimination of multicast-based address resolution for hosts.
o A host address registration feature using a new option in unicast
Neighbor Solicitation (NS) and Neighbor Advertisement (NA)
messages.
o A new Neighbor Discovery option to distribute 6LoWPAN header
compression context to hosts.
o Multihop distribution of prefix and 6LoWPAN header compression
context.
o Multihop Duplicate Address Detection (DAD), which uses two new
ICMPv6 message types.
The two multihop items can be substituted by a routing protocol
mechanism if that is desired; see Section 1.4.
Shelby, et al. Standards Track [Page 4]
RFC 6775 ND Optimization for 6LoWPANs November 2012
The document defines three new ICMPv6 message options: the Address
Registration Option (ARO), the Authoritative Border Router Option
(ABRO), and the 6LoWPAN Context Option (6CO). It also defines two
new ICMPv6 message types: the Duplicate Address Request (DAR) and the
Duplicate Address Confirmation (DAC).
1.1. The Shortcomings of IPv6 Neighbor Discovery
IPv6 Neighbor Discovery [RFC4861] provides several important
mechanisms used for router discovery, address resolution, Duplicate
Address Detection, and Redirect messages, along with prefix and
parameter discovery.
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-routers multicast address to solicit Router Advertisements (RAs).
If the host receives a valid RA with the A (autonomous address
configuration) flag, it autoconfigures the IPv6 address with the
advertised prefix in the RA message. Besides this, the IPv6 routers
usually send RAs periodically on the network. RAs are sent to the
all-nodes 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
assume that the nodes are powered on and reachable most of the time.
In Neighbor Discovery, the routers find the hosts by assuming that a
subnet prefix maps to one broadcast domain, and then they 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.
Note that the L (on-link) bit in the Prefix Information Option (PIO)
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.
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.
Shelby, et al. Standards Track [Page 5]
RFC 6775 ND Optimization for 6LoWPANs November 2012
A LoWPAN can use two types of link-layer addresses: 16-bit short
addresses and 64-bit unique addresses as defined in [RFC4944].
Moreover, the available link-layer payload size is on the order of
less than 100 bytes; thus, header compression is very useful.
Considering the above characteristics in a LoWPAN, and the IPv6
Neighbor Discovery [RFC4861] protocol design, some optimizations and
extensions to Neighbor Discovery are useful for the wide deployment
of IPv6 over low-power and lossy networks (example: 6LoWPAN and other
homogeneous low-power networks).
1.2. Applicability
In its Section 1, [RFC4861] foresees a document that covers operating
IP over a particular link type and defines an exception to the
otherwise general applicability of unmodified [RFC4861]. The present
specification improves the usage of IPv6 Neighbor Discovery for
LoWPANs in order to save energy and processing power of such nodes.
This document thus updates [RFC4944] to specify the use of the
optimizations defined here.
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 6LoWPANs, for example, in
general IPv6 low-power and lossy networks and possibly even in
combination with [RFC4861], the usage of such combinations is out of
scope of this document.
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) that are needed in route-over
configurations. An implementation of this specification MUST support
those pieces, unless the implementation supports some alternative
("substitute") from some other specification.
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.
Shelby, et al. Standards Track [Page 6]
RFC 6775 ND Optimization for 6LoWPANs November 2012
1.3. Goals and Assumptions
The document has the following main goals and assumptions.
Goals:
o Optimize Neighbor Discovery with a mechanism that is minimal yet
sufficient for the operation in both mesh-under and route-over
configurations.
o Minimize signaling by avoiding the use of multicast flooding and
reducing the use of link-scope multicast messages.
o Optimize the interfaces between hosts and their default routers.
o Provide support for sleeping hosts.
o Disseminate context information to hosts as needed by 6LoWPAN
header compression [RFC6282].
o Disseminate context information and prefix information from the
border to all routers in a LoWPAN.
o Provide a multihop Duplicate Address Detection mechanism suitable
for route-over LoWPANs.
Assumptions:
o 64-bit Extended Unique Identifier (EUI-64) [EUI64] addresses are
globally unique, and the LoWPAN is homogeneous.
o All nodes in the network have an EUI-64 Interface ID in order to
do address autoconfiguration and detect duplicate addresses.
o The link-layer technology is assumed to be low-power and lossy,
exhibiting undetermined connectivity, such as IEEE 802.15.4
[RFC4944]. However, the address registration mechanism might be
useful for other link-layer technologies.
o 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.
o When using the multihop DAD mechanism (Section 8.2), each 6LoWPAN
Router (6LR) registers with all the 6LoWPAN Border Routers (6LBRs)
available in the LoWPAN.
Shelby, et al. Standards Track [Page 7]
RFC 6775 ND Optimization for 6LoWPANs November 2012
o If IEEE 802.15.4 16-bit short addresses are used, then some
technique is used to ensure the uniqueness of those link-layer
addresses. That could be done using DHCPv6, Address Registration
Option-based Duplicate Address Detection (specified in
Section 8.2), or other techniques outside of the scope of this
document.
o In order to preserve the uniqueness of addresses (see Section 5.4
of [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 Section 8.2
(or any other protocols developed for that purpose).
o In order for 6LoWPAN header compression [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 Section 8.1 is used to distribute context information, this
implies that all the 6LBRs must coordinate the context information
they distribute within a single LoWPAN.
o 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.
o 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.
1.4. Substitutable Features
This document defines the optimization of Neighbor Discovery messages
for the host-router interface and introduces two new mechanisms in a
route-over topology.
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.
Shelby, et al. Standards Track [Page 8]
RFC 6775 ND Optimization for 6LoWPANs November 2012
The features that are substitutable (individually or in a group):
o Multihop distribution of prefix and 6LoWPAN header compression
context
o Multihop Duplicate Address Detection
Thus, multihop prefix distribution (the ABRO) and the 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.
Guidelines for feature implementation and deployment are provided in
Section 14.
2. Terminology
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 [RFC2119].
This specification requires readers to be familiar with all the terms
and concepts that are discussed in "Neighbor Discovery for IP
version 6 (IPv6)" [RFC4861], "IPv6 Stateless Address
Autoconfiguration" [RFC4862], "IPv6 over Low-Power Wireless Personal
Area Networks (6LoWPANs): Overview, Assumptions, Problem Statement,
and Goals" [RFC4919], "Transmission of IPv6 Packets over IEEE
802.15.4 Networks" [RFC4944], and "IP Addressing Model in Ad Hoc
Networks" [RFC5889].
This specification makes extensive use of the same terminology
defined in [RFC4861], unless otherwise defined below.
6LoWPAN link:
A wireless link determined by single IP hop reachability of
neighboring nodes. These are considered links with undetermined
connectivity properties as in [RFC5889].
6LoWPAN Node (6LN):
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.
6LoWPAN Router (6LR):
An intermediate router in the LoWPAN that is able to send and
receive Router Advertisements (RAs) and Router Solicitations (RSs)
as well as forward and route IPv6 packets. 6LoWPAN routers are
present only in route-over topologies.
Shelby, et al. Standards Track [Page 9]
RFC 6775 ND Optimization for 6LoWPANs November 2012
6LoWPAN Border Router (6LBR):
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.
Router:
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 [RFC2460].
Mesh-under:
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.
Route-over:
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.
Non-transitive link:
A link that exhibits asymmetric reachability as defined in
Section 2.2 of [RFC4861].
IP-over-foo document:
A specification that covers operating IP over a particular link
type, for example, [RFC4944] "Transmission of IPv6 Packets over
IEEE 802.15.4 Networks".
Header compression context:
Address information shared across a LoWPAN and used by 6LoWPAN
header compression [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 (CID), which is then used in header compression as a
shortcut for (parts of) a source or destination address.
Shelby, et al. Standards Track [Page 10]
RFC 6775 ND Optimization for 6LoWPANs November 2012
Registration:
The process during which a LoWPAN node sends a Neighbor
Solicitation message with an Address Registration Option to a
router creating a Neighbor Cache Entry (NCE) 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.
3. Protocol Overview
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.
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.
The protocol also provides for header compression [RFC6282] by
carrying header compression information in a new option in Router
Advertisement messages.
In addition, there are separate mechanisms that can be used 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.
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.
3.1. Extensions to RFC 4861
This document specifies the following optimizations and extensions to
IPv6 Neighbor Discovery [RFC4861]:
o Host-initiated refresh of Router Advertisement information. This
removes the need for periodic or unsolicited Router Advertisements
from routers to hosts.
o No Duplicate Address Detection (DAD) is performed if EUI-64-based
IPv6 addresses are used (as these addresses are assumed to be
globally unique).
Shelby, et al. Standards Track [Page 11]
RFC 6775 ND Optimization for 6LoWPANs November 2012
o DAD is optional if DHCPv6 is used to assign addresses.
o 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.
o A new Router Advertisement option, the 6LoWPAN Context Option, for
context information used by 6LoWPAN header compression.
o A new mechanism to perform Duplicate Address Detection across a
route-over 6LoWPAN using the new Duplicate Address Request and
Duplicate Address Confirmation messages.
o New mechanisms to distribute prefixes and context information
across a route-over network that uses a new Authoritative Border
Router Option to control the flooding of configuration changes.
o A few new default protocol constants are introduced, and some
existing Neighbor Discovery protocol constants are tuned.
3.2. Address Assignment
Hosts in a 6LoWPAN configure their IPv6 addresses as specified in
[RFC4861] and [RFC4862] based on the information received in Router
Advertisement messages. The use of the M (managed address
configuration) flag in this optimization is, however, more
restrictive than in [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.
6LRs MAY use the same mechanisms to configure their IPv6 addresses.
The 6LBRs are responsible for managing the prefix(es) assigned to the
6LoWPAN, using manual configuration, DHCPv6 Prefix Delegation
[RFC3633], or other mechanisms. In an isolated LoWPAN, a Unique
Local Address (ULA) [RFC4193] prefix SHOULD be generated by the 6LBR.
Shelby, et al. Standards Track [Page 12]
RFC 6775 ND Optimization for 6LoWPANs November 2012
3.3. Host-to-Router Interaction
A host sends Router Solicitation messages at startup and also when
the Neighbor Unreachability Detection (NUD) of one of its default
routers fails.
Hosts receive Router Advertisement messages typically containing the
Authoritative Border Router Option (ABRO) and may optionally contain
one or more 6LoWPAN Context Options (6COs) in addition to the
existing Prefix Information Options (PIOs) as described in [RFC4861].
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
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 often change their point of attachment should use a
suitably short lifetime. See Section 5.5 for registration details
and Section 9 for protocol constants.
The registration fails when an ARO 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, i.e., 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 [RFC4941]
or addresses based on an Interface ID that is an IEEE 802.15.4 16-bit
short address. Failure can also occur if the Neighbor Cache on that
router is full.
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 needs to both perform NUD to check
that the router is still reachable and refresh its registration with
the router.
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 an
Address Registration Option.
As part of the optimizations, address resolution is not performed by
multicasting Neighbor Solicitation messages as in [RFC4861].
Instead, the routers maintain Neighbor Cache Entries for all
registered IPv6 addresses. If the address is not in the Neighbor
Shelby, et al. Standards Track [Page 13]
RFC 6775 ND Optimization for 6LoWPANs November 2012
Cache in the router, then the address either doesn't exist, 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.
3.4. Router-to-Router Interaction
The new router-to-router interaction is only for the route-over
configuration where 6LRs are present. See also Section 1.4.
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 [RFC4861].
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 Solicitation. There is a Version Number field
in the ABRO (see Section 4.3), which is used to limit the flooding of
updated information between the 6LRs.
A 6LR can perform Duplicate Address Detection against one or more
6LBRs using the new Duplicate Address Request (DAR) and Duplicate
Address 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 [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.
3.5. Neighbor Cache Management
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 (NCEs) slightly differently than in
[RFC4861]. This results in three different types of NCEs, and the
types specify how those entries can be removed:
Garbage-collectible: Entries that are subject to the normal rules in
[RFC4861] that allow for garbage collection
when low on memory.
Registered: Entries that have an explicit registered
lifetime and are kept until this lifetime
expires or they are explicitly unregistered.
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Tentative: Entries that are temporary with a short
lifetime, which typically get converted to
Registered entries.
Note that the type of the NCE is orthogonal to the states specified
in [RFC4861].
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.
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.
4. New Neighbor Discovery Options and Messages
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 substitutable router-to-router
interaction. This section also defines the new router-to-router
Duplicate Address Request and Duplicate Address Confirmation
messages.
4.1. Address Registration Option
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 NS messages sent by hosts. Thus, it
can be included in the unicast NS messages that a host sends as part
of NUD 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 NA messages with
a Status field indicating the success or failure of the registration.
This option is always host initiated.
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The information contained in the ARO is also included in the multihop
DAR and DAC messages used between 6LRs and 6LBRs, but the option
itself is not used in those messages.
The ARO is required for reliability and power saving. The lifetime
field provides flexibility to the host to register an address that
should be usable (continue to be advertised by the 6LR in the routing
protocol, etc.) during its intended sleep schedule.
The sender of the NS also includes the EUI-64 [EUI64] of the
interface from which it is registering an address. 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 Section 6.5.2).
When the ARO is used by hosts, an SLLAO (Source Link-Layer Address
Option) [RFC4861] MUST be included, and the address that is to be
registered MUST be the IPv6 source address of the NS message.
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 +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type: 33
Length: 8-bit unsigned integer. The length of the
option in units of 8 bytes. Always 2.
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.
Reserved: This field is unused. It MUST be initialized
to zero by the sender and MUST be ignored by
the receiver.
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Registration Lifetime: 16-bit unsigned integer. The amount of time
in units of 60 seconds that the router should
retain the NCE for the sender of the NS that
includes this option.
EUI-64: 64 bits. This field is used to uniquely
identify the interface of the Registered
Address by including the EUI-64 identifier
[EUI64] assigned to it unmodified.
The Status values used in NAs are:
+--------+--------------------------------------------+
| Status | Description |
+--------+--------------------------------------------+
| 0 | Success |
| 1 | Duplicate Address |
| 2 | Neighbor Cache Full |
| 3-255 | Allocated using Standards Action [RFC5226] |
+--------+--------------------------------------------+
Table 1
4.2. 6LoWPAN Context Option
The 6LoWPAN Context Option (6CO) carries prefix information for
LoWPAN header compression and is similar to the PIO of [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 CID for use as specified in [RFC6282]. A context may
be a prefix of any length or an address (/128), and up to 16 6COs may
be carried in an RA message.
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 .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: 6LoWPAN Context Option Format
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Type: 34
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.
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.
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 life cycle based on the
recommendations in Section 7.2.
CID: 4-bit Context Identifier for this prefix
information. The CID is used by context-based
header compression as specified in [RFC6282]. The
list of CIDs for a LoWPAN is configured on the 6LBR
that originates the context information for the
6LoWPAN.
Res, Reserved: This field is unused. It MUST be initialized to
zero by the sender and MUST be ignored by the
receiver.
Valid Lifetime: 16-bit unsigned integer. The length of time in
units 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.
Context Prefix: The IPv6 prefix or address corresponding to the 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.
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4.3. Authoritative Border Router Option
The Authoritative Border Router Option (ABRO) is needed when RA
messages are used to disseminate prefixes and context information
across a route-over topology. In this case, 6LRs receive PIOs 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
that the 6LBR sets and that 6LRs propagate as they propagate the
prefix and context information with this ABRO. 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.
The ABRO MUST be included in all RA messages in the case when RAs are
used to propagate information between routers (as described in
Section 8.2).
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 +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type: 35
Length: 8-bit unsigned integer. The length of
the option in units of 8 bytes.
Always 3.
Version Low, Version High: Together, Version Low and Version High
constitute the Version Number field, a
32-bit unsigned integer where Version Low
is the least significant 16 bits and
Version High is the most significant
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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.
Valid Lifetime: 16-bit unsigned integer. The length of
time in units 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).
Reserved: This field is unused. It MUST be
initialized to zero by the sender and
MUST be ignored by the receiver.
6LBR Address: IPv6 address of the 6LBR that is the
origin of the included version number.
4.4. Duplicate Address Messages
For the multihop DAD exchanges between a 6LR and 6LBR as specified in
Section 8.2, there are two new ICMPv6 message types called the
Duplicate Address Request (DAR) and the Duplicate Address
Confirmation (DAC). We avoid reusing the NS and NA 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 is otherwise the same as would be in an NS
carrying an ARO, with the message format inlining the fields that are
in the ARO.
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.
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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 +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP fields:
IPv6 Source: A non-link-local address of the sending
router.
IPv6 Destination: In a DAR, a non-link-local address of a 6LBR.
In a DAC, this is just the source from the
DAR.
Hop Limit: Set to MULTIHOP_HOPLIMIT on transmit. MUST
be ignored on receipt.
ICMP Fields:
Type: 157 for the DAR and 158 for the DAC.
Code: Set to zero on transmit. MUST be ignored on
receipt.
Checksum: The ICMP checksum. See [RFC4443].
Status: 8-bit unsigned integer. Indicates the status
of a registration in the DAC. MUST be set to
0 in the DAR. See Table 1.
Reserved: This field is unused. It MUST be initialized
to zero by the sender and MUST be ignored by
the receiver.
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Registration Lifetime: 16-bit unsigned integer. The amount of time
in units of 60 seconds that the 6LBR should
retain the DAD table entry (Section 8.2.2)
for the Registered Address. A value of 0
indicates in a DAR that the DAD table entry
should be removed.
EUI-64: 64 bits. This field is used to uniquely
identify the interface of the Registered
Address by including the EUI-64 identifier
[EUI64] assigned to it unmodified.
Registered Address: 128-bit field. Carries the host address that
was contained in the IPv6 Source field in the
NS that contained the ARO sent by the host.
5. Host Behavior
Hosts in a LoWPAN use the ARO in the NS messages they send as a way
to maintain the Neighbor Cache in the routers, thereby removing the
need for multicast NSs to do address resolution. Unlike in
[RFC4861], the hosts initiate updating the information they receive
in RAs by sending RSs before the information expires. Finally, when
NUD indicates that one or all default routers have become
unreachable, then the host uses RSs to find a new set of default
routers.
5.1. Forbidden Actions
A host MUST NOT multicast an NS message.
5.2. Interface Initialization
When the interface on a host is initialized, it follows the
specification in [RFC4861]. A link-local address is formed based on
the EUI-64 identifier [EUI64] assigned to the interface as per
[RFC4944] or the appropriate IP-over-foo document for the link, and
then the host sends RS messages as described in [RFC4861]
Section 6.3.7.
There is no need to join the solicited-node multicast address, since
nobody multicasts NSs in this type of network. A host MUST join the
all-nodes multicast address.
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5.3. Sending a Router Solicitation
The RS is formatted as specified in [RFC4861] and sent to the IPv6
all-routers multicast address (see [RFC4861] Section 6.3.7 for
details). An SLLAO MUST be included to enable unicast RAs in
response. An unspecified source address MUST NOT be used in RS
messages.
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.
Since hosts do not depend on multicast RAs to discover routers, the
hosts need to intelligently retransmit RSs 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 is
about to expire. The RECOMMENDED rate of 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
[RFC4861], and then switch to slower retransmissions. After the
initial retransmissions, the host SHOULD do truncated binary
exponential backoff [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 an RA is
received. See Section 9 for protocol constants.
5.4. Processing a Router Advertisement
The processing of RAs is as in [RFC4861], with the addition of
handling the 6CO and triggering address registration when a new
address has been configured. Furthermore, the SLLAO MUST be included
in the RA. Unlike in [RFC4861], the maximum value of the RA Router
Lifetime field MAY be up to 0xFFFF (approximately 18 hours).
Should the host erroneously receive a PIO with the L (on-link) flag
set, then that PIO MUST be ignored.
5.4.1. Address Configuration
Address configuration follows [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).
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Once an address has been configured, it will be registered by
unicasting an NS with an ARO to one or more routers.
5.4.2. Storing Contexts
The host maintains a conceptual data structure for the context
information it receives from the routers. This structure is called
the context table. It includes the CID, 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.
When a 6CO is received in an RA, it is used to add or update the
information in the context table. If the CID 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.
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.
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 Section 7.2 for carefully managing the context life cycle. Nodes
should be careful about using header compression in RA messages that
include 6COs.
5.4.3. Maintaining Prefix and Context Information
The prefix information is timed out as specified in [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.
A host should inspect the various lifetimes to determine when it
should next initiate sending an RS to ask for any updates to the
information. The lifetimes that matter are the default Router
Lifetime, the Valid Lifetime in the PIOs, and the Valid Lifetime in
the 6CO. The host SHOULD unicast one or more RSs to the router well
before the shortest of those lifetimes (across all the prefixes and
all the contexts) expires and then switch to multicast RS messages if
there is no response to the unicasts. The retransmission behavior
for the RSs is specified in Section 5.3.
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5.5. Registration and Neighbor Unreachability Detection
Hosts send unicast 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 NS 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 NCEs in the routers. This is done by sending
NS messages with an 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 NA message with an ARO.
Hosts that receive RA messages from multiple default routers SHOULD
attempt to register with more than one of them in order to increase
the robustness of the network.
Note that NUD probes can be suppressed by reachability confirmations
from transport protocols or applications as specified in [RFC4861].
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.
5.5.1. Sending a Neighbor Solicitation
The host triggers sending 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 an SLLAO, since the router needs to record the link-layer
address of the host. An unspecified source address MUST NOT be used
in NS messages.
5.5.2. Processing a Neighbor Advertisement
A host handles NA messages as specified in [RFC4861], with added
logic described in this section for handling the ARO.
In addition to the normal validation of an NA and its options, the
ARO (if present) is verified as follows. 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.
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If the Status field is zero, then the address registration was
successful. The host saves the Registration Lifetime from the ARO
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.
5.5.3. Recovering from Failures
The procedure for maintaining reachability information about a
neighbor is the same as in [RFC4861] Section 7.3, with the exception
that address resolution is not performed.
The address registration procedure may fail for two reasons: no
response to NSs is received (NUD failure), or an ARO 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 ARO 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 [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.
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 RSs as specified in Section 5.3.
Other failure codes may be defined in future documents.
5.6. Next-Hop Determination
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 Section 5.2 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 [RFC4291].
Multicast addresses are considered to be on-link and are resolved as
specified in [RFC4944] or the appropriate IP-over-foo document. Note
that [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.
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All other prefixes are assumed to be off-link [RFC5889]. Anycast
addresses are always considered to be off-link. They are therefore
sent to one of the routers in the default router list.
A LoWPAN node is not required to maintain a minimum of one buffer per
neighbor as specified in [RFC4861], since packets are never queued
while waiting for address resolution.
5.7. Address Resolution
The address registration mechanism and the SLLAO in RA 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.
Link-layer addresses for neighbors are stored in NCEs [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).
5.8. Sleeping
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 further described in this section. Routers
may want to cache traffic destined to a host that is sleeping, but
such functionality is out of the scope of this document.
5.8.1. Picking an Appropriate Registration Lifetime
As all ND messages are initiated by the hosts, this allows a host to
sleep or otherwise be unreachable between NS/NA message exchanges.
The ARO attached to NS messages indicates to a router to keep the NCE
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 that the registration is renewed successfully
(considering, for example, 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
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time (and thus Registration Lifetime). A dynamic network requires a
shorter sleep time so that routers don't keep invalid NCEs for nodes
longer than necessary.
5.8.2. Behavior on Wakeup
When a host wakes up from a sleep period, it SHOULD refresh its
current address registrations that will time out before the next
wakeup. This is done by sending NS messages with an ARO as described
in Section 5.5.1. The host may also need to refresh its prefix and
context information by sending a new unicast RS (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 RSs to discover new default
router(s) and restart the address registration process.
6. Router Behavior for 6LRs and 6LBRs
Both 6LRs and 6LBRs maintain the Neighbor Cache [RFC4861] based on
the AROs they receive in NA messages from hosts, ND packets from
other nodes, and, potentially, a routing protocol used in the 6LoWPAN
as outlined in Section 3.5.
The routers SHOULD NOT garbage-collect Registered NCEs (see
Section 3.4), since they need to retain them until the Registration
Lifetime expires. Similarly, if NUD on the router determines that
the host is UNREACHABLE (based on the logic in [RFC4861]), the NCE
SHOULD NOT be deleted but rather 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.
Routers MAY implement the Default Router Preference (Prf) extension
[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.
6.1. Forbidden Actions
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
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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 [RFC4861].
A router MUST NOT set the L (on-link) flag in the PIOs, since that
might trigger hosts to send multicast NSs.
6.2. Interface Initialization
The 6LBR router interface initialization behavior is the same as in
[RFC4861]. However, in a dynamic configuration scenario (see
Section 8.1), a 6LR comes up as a non-router and waits to receive the
advertisement for configuring its own interface address first, before
setting its interfaces to be advertising interfaces and turning into
a router.
6.3. Processing a Router Solicitation
A router processes RS messages as specified in [RFC4861]. The
differences relate to the inclusion of ABROs in the RA messages and
the exclusive use of unicast RAs. If a 6LR has received an ABRO from
a 6LBR, then it will include that option unmodified in the RA
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 that
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.
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 recommended 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.
An RS might be received from a host that has not yet registered its
address with the router. Thus, the router MUST NOT modify an
existing NCE based on the SLLAO from the RS. However, a router MAY
create a Tentative NCE based on the SLLAO. Such a Tentative NCE
SHOULD be timed out in TENTATIVE_NCE_LIFETIME seconds, unless a
registration converts it into a Registered NCE.
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A 6LR or 6LBR MUST include an SLLAO in the RAs it sends; this is
required so that the hosts will know the link-layer address of the
router. Unlike in [RFC4861], the maximum value of the RA Router
Lifetime field MAY be up to 0xFFFF (approximately 18 hours).
Unlike [RFC4861], which suggests multicast RAs, this specification
improves the exchange by always unicasting RAs in response to RSs.
This is possible, since the RS always includes an SLLAO, which is
used by the router to unicast the RA.
6.4. Periodic Router Advertisements
A router does not need to send any periodic RA messages, since the
hosts will solicit updated information by sending RSs before the
lifetimes expire.
However, if the routers use RAs to distribute prefix and/or context
information across a route-over topology, that might require periodic
RA messages. Such RAs are sent using the configurable
MinRtrAdvInterval and MaxRtrAdvInterval as per [RFC4861].
6.5. Processing a Neighbor Solicitation
A router handles NS messages as specified in [RFC4861], with added
logic described in this section for handling the ARO.
In addition to the normal validation of an NS and its options, the
ARO is verified as follows (if present). If the Length field is not
two, or if the Status field is not zero, then the NS is silently
ignored.
If the source address of the NS is the unspecified address, or if no
SLLAO is included, then any included ARO is ignored, that is, the NS
is processed as if it did not contain an ARO.
6.5.1. Checking for Duplicates
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. It
isn't a duplicate if (1) there is no NCE for the IPv6 source address
of the NS or (2) there is such an NCE and the EUI-64 is the same.
Otherwise, it is a duplicate address. Note that if multihop DAD
(Section 8.2) is used, then the checks are slightly different, to
take into account Tentative NCEs. In the case where it is a
duplicate address, then the router responds with a unicast NA message
with the ARO Status field set to one (to indicate that the address is
a duplicate) as described in Section 6.5.2. In this case, there is
no modification to the Neighbor Cache.
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6.5.2. Returning Address Registration Errors
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 Interface ID
part derived from the EUI-64 field of the ARO as per [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.
The error is sent to the link-local address with the Interface ID
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 address.
6.5.3. Updating the Neighbor Cache
If the 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) an NCE 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 that the
router's Neighbor Cache is full) as described in Section 6.5.2.
The Registration Lifetime and the EUI-64 are recorded in the NCE. A
unicast 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
TLLAO (Target Link-Layer Address Option) [RFC4861] is not required in
the NA, since the host already knows the router's link-layer address
from RAs.
If the ARO contains a zero Registration Lifetime, then any existing
NCE for the IPv6 source address of the NS MUST be deleted and an NA
sent as above.
Should the Registration Lifetime in an NCE expire, then the router
MUST delete the cache entry.
The addition and removal of Registered NCEs would result in notifying
the routing protocol.
Note: If the substitutable multihop DAD (Section 8.2) is used, then
the updating of the Neighbor Cache is slightly different due to
Tentative NCEs.
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6.5.4. Next-Hop Determination
In order to deliver a packet destined for a 6LN registered with a
router, next-hop determination is slightly different for routers than
for hosts (see Section 5.6). The routing table is checked to
determine the next-hop IP address. A Registered 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.
6.5.5. Address Resolution between Routers
There needs to be a mechanism somewhere for the routers to discover
each other's link-layer addresses. If the routing protocol used
between the routers provides this, then there is no need for the
routers to use the ARO between each other. Otherwise, the routers
SHOULD use the ARO. When routers use the ARO to register with each
other and multihop DAD (Section 8.2) 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 scenario are out of scope of this
document.
Routers MAY also use multicast NSs as in [RFC4861] to resolve each
others link-layer addresses. Thus, routers MAY multicast NSs for
other routers, for example, as a result of receiving some routing
protocol update. Routers MUST respond to multicast NSs. This
implies that routers MUST join the solicited-node multicast addresses
as specified in [RFC4861].
7. Border Router Behavior
A 6LBR handles the sending of RAs and processing of NSs from hosts as
specified above in Section 6. A 6LBR SHOULD always include an ABRO
in the RAs it sends, listing itself as the 6LBR address. This
requires that the 6LBR maintain the version number in stable storage
and increase the version number when some information in its RAs
changes. The information whose change affects the version is in the
PIOs (the prefixes or their lifetimes) and in the 6CO (the prefixes,
CIDs, or lifetimes).
In addition, a 6LBR is somehow configured with the prefix or prefixes
that are assigned to the LoWPAN and advertises those in RAs as in
[RFC4861]. In the case of route-over, those prefixes can be
disseminated to all the 6LRs using the technique discussed in
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Section 8.1. 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 Section 1.4).
If the 6LoWPAN uses header compression [RFC6282] with context, then
the 6LBR needs to manage the CIDs and advertise those in RAs by
including 6COs in its RAs so that directly attached hosts are
informed about the CIDs. 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 discussed in Section 8, unless a
different specification provides a substitute for this multihop
distribution.
7.1. Prefix Determination
The prefix or prefixes used in a LoWPAN can be manually configured or
can be acquired using DHCPv6 Prefix Delegation [RFC3633]. For a
LoWPAN that is isolated from the network either permanently or
occasionally, the 6LBR can assign a ULA prefix using [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 is included in the RA messages as specified in
[RFC4861].
7.2. Context Configuration and Management
If the LoWPAN uses header compression [RFC6282] with context, then
the 6LBR must be configured with context information and related
CIDs. If the LoWPAN has multiple 6LBRs, then they MUST be configured
with the same context information and CIDs. As noted in [RFC6282],
maintaining consistency of context information is crucial for
ensuring that packets will be decompressed correctly.
The context information carried in RA messages originates at 6LBRs
and must be disseminated to all the routers and hosts within the
LoWPAN. RAs include one 6CO for each context.
For the dissemination of context information using the 6CO, a strict
life cycle SHOULD be used in order to ensure that the context
information stays synchronized throughout the LoWPAN. New context
information SHOULD be introduced into the LoWPAN with C=0, to ensure
that it is known by all nodes that may have to perform header
decompression 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.
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Conversely, to avoid the situation where nodes send packets that make
use of previous values of contexts -- which would result 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 CID 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.
8. Substitutable Feature Behavior
Normally, in a 6LoWPAN multihop network, the RA 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.
There is also the option for a 6LR to perform multihop DAD (for IPv6
addresses not derived from an EUI-64) 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 a unique
address, such as DHCPv6 [RFC3315], or use other future mechanisms for
multihop DAD. Again, in this case, any remaining use of the
6LoWPAN-ND mechanisms is governed by the substitute specification.
To be clear: Implementations MUST support the features described in
Sections 8.1 and 8.2, unless the implementation supports some
alternative ("substitute") from some other specification.
8.1. Multihop Prefix and Context Distribution
The multihop distribution relies on RS messages and 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.
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 needs 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
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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.
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 the
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.
8.1.1. 6LBRs Sending Router Advertisements
6LBRs supporting multihop prefix and context distribution MUST
include an ABRO in each of their RAs. The ABRO Version Number field
is used to keep prefix and context information consistent throughout
the LoWPAN, along with the guidelines in Section 7.2. Each time any
information in the set of PIOs or 6COs changes, the ABRO version is
increased by one.
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.
8.1.2. Routers Sending Router Solicitations
In a 6LoWPAN, unless substituted, multihop distribution is done using
RA messages. Thus, on interface initialization, a router (6LR) MUST
send RS messages following the rules specified for hosts in
[RFC4861]. This in turn will cause the routers to respond with RA
messages that can then be used to initially seed the prefix and
context information.
8.1.3. Routers Processing Router Advertisements
If multihop distribution is not done using RA messages, then the
routers follow [RFC4861], which states that they merely do some
consistency checks; in this case, nothing in Section 8.1 applies.
Otherwise, the routers will check and record the prefix and context
information from the received RAs, and use that information as
follows.
If a received RA does not contain an ABRO, then the RA MUST be
silently ignored.
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
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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.
8.1.4. Storing the Information
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 PIOs and 6COs. The prefixes are timed out based on the Valid
Lifetime in the PIO. The Context Prefix is timed out based on the
Valid Lifetime in the 6CO.
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 RAs 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 an RA it sends at time T+5 minutes, the Valid
Lifetime in the 6CO it sends will be only 5 minutes.
8.1.5. Sending Router Advertisements
When multihop distribution is performed using RA messages, the
routers MUST ensure that the ABRO always stays together with the
prefixes and context information received with that ABRO. Thus, if
the router has received prefix P1 with an 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 an RA that includes an 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.
The routers periodically send RAs as in [RFC4861]. This is for the
benefit of the other routers receiving the prefixes and context
information. The routers also respond to RSs by unicasting RA
messages. In both cases, the above constraint of keeping the ABRO
together with 'its' prefixes and context information applies.
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 as
described in [RFC4861], that is, send up to three RA messages. This
ensures rapid propagation of new information to all the 6LRs.
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8.2. Multihop Duplicate Address Detection
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.
This specification offers a substitutable simple technique for 6LRs
and 6LBRs to perform DAD that reuses the information from the ARO 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 either the
6LRs register with all the 6LBRs or the network uses some out-of-
scope mechanism to keep the DAD tables in the 6LBRs synchronized.
The multihop DAD mechanism is used synchronously the first time an
address is registered with a particular 6LR. That is, the ARO is not
returned to the host until multihop DAD has been completed against
the 6LBRs. For existing registrations in the 6LR, 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. One method to achieve
this is to track how much is left of the lifetime the 6LR registered
with the 6LBRs and to re-register with the 6LBR when this lifetime is
about to run out.
For synchronous multihop DAD, the 6LR performs some additional checks
to ensure that it has an NCE 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) NCE 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 NCE exists, then the 6LR MUST create
a Tentative NCE with the EUI-64 and the SLLAO. This entry will be
used to send the response to the host when the 6LBR responds
positively.
When a 6LR receives an NS containing an ARO with a non-zero
Registration Lifetime and it has no existing Registered NCE, then
with this mechanism the 6LR will invoke synchronous multihop DAD.
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The 6LR will unicast a DAR 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 DAC message, which will have a hop limit
less than 255 when it reaches the 6LR.
When the 6LR receives the DAC from the 6LBR, it will look for a
matching (same IP address and EUI-64) (Tentative or Registered) NCE.
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 NCE 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 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.
A Tentative NCE 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.
8.2.1. Message Validation for DAR and DAC
A node MUST silently discard any received DAR and DAC messages for
which at least one of the following validity checks is not satisfied:
o If the message includes an IP Authentication Header, the message
authenticates correctly.
o ICMP Checksum is valid.
o ICMP Code is 0.
o ICMP Length (derived from the IP length) is 32 or more bytes.
o The Registered Address is not a multicast address.
o All included options have a length that is greater than zero.
o The IP source address is not the unspecified address, nor is it a
multicast address.
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.
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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.
8.2.2. Conceptual Data Structures
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.
8.2.3. 6LR Sending a Duplicate Address Request
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:
o In the IPv6 source address, a global address of the 6LR.
o In the IPv6 destination address, the address of the 6LBR.
o In the IPv6 hop limit, MULTIHOP_HOPLIMIT.
o The Status field MUST be set to zero.
o The EUI-64 and Registration Lifetime are copied from the ARO
received from the host.
o The Registered Address set to the IPv6 address of the host, that
is, the sender of the triggering NS.
When a 6LR receives an NS from a host with a zero Registration
Lifetime, then, in addition to removing the NCE for the host as
specified in Section 6, a DAR is sent to the 6LBRs as above.
A router MUST NOT modify the Neighbor Cache as a result of receiving
a DAR.
8.2.4. 6LBR Receiving a Duplicate Address Request
When a 6LBR that implements the substitutable multihop DAD receives a
DAR from a 6LR, it performs the message validation specified in
Section 8.2.1. 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
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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.
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.
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.
In both of the above cases, the 6LBR forms a 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.
8.2.5. Processing a Duplicate Address Confirmation
When a 6LR implementing multihop DAD receives a DAC message, then it
first validates the message per Section 8.2.1. For a valid DAC, if
there is no Tentative NCE matching the Registered Address and EUI-64,
then the DAC is silently ignored. Otherwise, the information in the
DAC and in the Tentative NCE 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 the case where the DAC indicates an error (the
Status is non-zero), the NA is returned to the host as described in
Section 6.5.2, and the Tentative NCE for the Registered Address is
removed. Otherwise, it is made into a Registered NCE.
A router MUST NOT modify the Neighbor Cache as a result of receiving
a DAC, unless there is a Tentative NCE matching the IPv6 address and
EUI-64.
8.2.6. Recovering from Failures
If there is no response from a 6LBR after RETRANS_TIMER [RFC4861],
then the 6LR would retransmit the DAR to the 6LBR up to
MAX_UNICAST_SOLICIT [RFC4861] times. After this, the 6LR SHOULD
respond to the host with an ARO Status of zero.
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9. Protocol Constants
This section defines the relevant protocol constants used in this
document based on a subset of [RFC4861] constants. "*" indicates
constants modified from [RFC4861], and "+" indicates new constants.
Additional protocol constants are defined in Section 4.
6LBR Constants:
MIN_CONTEXT_CHANGE_DELAY+ 300 seconds
6LR Constants:
MAX_RTR_ADVERTISEMENTS 3 transmissions
MIN_DELAY_BETWEEN_RAS* 10 seconds
MAX_RA_DELAY_TIME* 2 seconds
TENTATIVE_NCE_LIFETIME+ 20 seconds
Router Constants:
MULTIHOP_HOPLIMIT+ 64
Host Constants:
RTR_SOLICITATION_INTERVAL* 10 seconds
MAX_RTR_SOLICITATIONS 3 transmissions
MAX_RTR_SOLICITATION_INTERVAL+ 60 seconds
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10. Examples
10.1. Message Examples
STEP
6LN 6LR
| |
1. | ---------- Router Solicitation --------> |
| [SLLAO] |
| |
2. | <-------- Router Advertisement --------- |
| [PIO + 6CO + ABRO + SLLAO] |
Figure 2: Basic Router Solicitation/Router Advertisement Exchange
between a Node and a 6LR or 6LBR
6LN 6LR
| |
1. | ------- NS with Address Registration ------> |
| [ARO + SLLAO] |
| |
2. | <----- NA with Address Registration -------- |
| [ARO with Status] |
Figure 3: Neighbor Discovery Address Registration
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6LN 6LR 6LBR
| | |
1. | --- NS with Address Reg --> | |
| [ARO + SLLAO] | |
| | |
2. | | ----------- DAR ----------> |
| | |
3. | | <---------- DAC ----------- |
| | |
4. | <-- NA with Address Reg --- | |
| [ARO with Status] |
Figure 4: Neighbor Discovery Address Registration with Multihop DAD
10.2. Host Bootstrapping Example
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 link-layer
security are out of scope of this document. In this example, an IEEE
802.15.4 6LN forms a 16-bit short IPv6 address without using DHCPv6
(i.e., the M flag is not set in the RAs).
1. After obtaining link-layer 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
[RFC4944].
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 SLLAO 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.
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The 6LR responds with a unicast RA to the IP source address using
the SLLAO from the RS (it may have created a Tentative NCE). See
Figure 2.
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 [RFC4944]. As the network is unmanaged
(M flag not set in the RA), the 6LN randomly chooses a 16-bit
link-layer address and forms a Tentative IPv6 address from it.
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 SLLAO 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 that 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 Figure 3. 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.
5. The 6LN now performs maintenance by sending a new NS address
registration before the lifetime expires.
If multihop DAD and multihop prefix and context distribution are
used, the effect of the 6LRs and hosts following the above
bootstrapping process is a "wavefront" of 6LRs and hosts being
configured, spreading outward from the 6LBRs: First, the hosts and
6LRs that can directly reach a 6LBR would receive one or more RAs and
then configure and register their IPv6 addresses. Once that is done,
they would enable the routing protocol and start sending out RAs.
That would result in a new set of 6LRs and hosts to receive responses
to their RSs, form and register their addresses, etc. That repeats
until all of the 6LRs and hosts have been configured.
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10.2.1. Host Bootstrapping Messages
This section provides specific message examples related to the
bootstrapping process described above. When discussing messages, the
following notation is used:
LL64: Link-local address based on the EUI-64, which is also the
802.15.4 long address.
GP16: Global address based on the 802.15.4 short address. This
address may not be unique.
GP64: Global addresses derived from the EUI-64 address as specified
in [RFC4944].
MAC64: EUI-64 address used as the link-layer address.
MAC16: IEEE 802.15.4 16-bit short address.
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.
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 of 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)
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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
Figure 5: Detailed Message Address Examples
10.3. Router Interaction Example
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.
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
Sections 6.5.3 and 6.5.5, respectively. In this example, we assume
that the neighboring 6LoWPAN link is secure.
10.3.1. Bootstrapping a Router
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 a 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 that 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.
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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.
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).
10.3.2. Updating the Neighbor Cache
In this example, there are three 6LRs: R1, R2, and R3. Initially,
when R2 boots, it sees only R1, and accordingly R2 creates an NCE for
R1. Now assume that R2 receives a valid routing update from router
R3. R2 does not have any NCE for R3. If the implementation of R2
supports detecting link-layer addresses from the routing information
packets, then it directly updates its Neighbor Cache using that
link-layer information. If this is not possible, then R2 should
perform multicast NS with the 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 [RFC4861].
More generally, any 6LR that receives a valid route update from a
neighboring router for which it does not have any NCE is required to
update its Neighbor Cache as described above.
The router (6LR and 6LBR) IP addresses learned via ND are not
redistributed to the routing protocol.
11. Security Considerations
The security considerations of IPv6 ND [RFC4861] and address
autoconfiguration [RFC4862] apply. Additional considerations can be
found in [RFC3756].
There is a slight modification to those considerations, due to the
fact that in this specification the M flag in the RAs disables 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 [RFC4861] and [RFC4862] the
rogue router could only cause the addition of DHCP and not disable
stateless address autoconfiguration for short addresses.
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This specification assumes that the link layer is sufficiently
protected -- for instance, by using MAC-sublayer cryptography. Thus,
its threat model is no different from that of IPv6 ND [RFC4861]. The
first trust model listed in Section 3 of [RFC3756] applies here.
However, any future 6LoWPAN security protocol that applies to ND for
the 6LoWPAN protocol is out of scope of this document.
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 NCE 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.
In some future deployments, one might want to use SEcure Neighbor
Discovery (SEND) [RFC3971] [RFC3972]. This is possible with the ARO
as sent between hosts and routers, since the address that is being
registered is the IPv6 source address of the NS 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.
12. IANA Considerations
This document registers three new ND option types under the
subregistry "IPv6 Neighbor Discovery Option Formats":
o Address Registration Option (33)
o 6LoWPAN Context Option (34)
o Authoritative Border Router Option (35)
The document registers two new ICMPv6 "type" numbers under the
subregistry "ICMPv6 "type" Numbers":
o Duplicate Address Request (157)
o Duplicate Address Confirmation (158)
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IANA has also created a new subregistry for the Status values of the
Address Registration Option, under the ICMPv6 parameters registry.
Address Registration Option Status Values registry:
o Possible values are 8-bit unsigned integers (0..255).
o Registration procedure is "Standards Action" [RFC5226].
o Initial allocation is as indicated in Table 2:
+--------+--------------------------------------------+
| Status | Description |
+--------+--------------------------------------------+
| 0 | Success |
| 1 | Duplicate Address |
| 2 | Neighbor Cache Full |
| 3-255 | Allocated using Standards Action [RFC5226] |
+--------+--------------------------------------------+
Table 2
13. Interaction with Other Neighbor Discovery Extensions
There are two classes of ND extensions that interact with this
specification in different ways.
One class encompasses extensions to the DAD mechanisms in [RFC4861]
and [RFC4862]. An example of this is Optimistic DAD [RFC4429]. Such
extensions do not apply when this specification is being used, since
it uses ARO for DAD (which is neither optimistic nor pessimistic --
always one round trip to the router to check DAD).
All other (non-DAD) ND extensions, be they path selection types like
default router preferences [RFC4191], configuration types like DNS
configuration [RFC6106], or other types like Detecting Network
Attachment [RFC6059], are completely orthogonal to this specification
and will work as is.
14. Guidelines for New Features
This section discusses guidelines 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, to
be used as checklists. The checklists act as guidelines 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
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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.
+----------+-----------------------------------+--------+-----------+
| Section | Description | Deploy | Implement |
+----------+-----------------------------------+--------+-----------+
| 3.1 | Host-initiated RA | MUST | MUST |
| 3.2 | EUI-64-based IPv6 address | MUST | MUST |
| | 16-bit MAC-based address | MAY | SHOULD |
| | Other non-unique addresses | MAY | MAY |
| 3.3 | Host-initiated RS | MUST | MUST |
| | ABRO processing | SHOULD | MUST |
| 4.1 | Registration with ARO | MUST | MUST |
| 4.2, 5.4 | 6CO | SHOULD | SHOULD |
| 5.2 | Joining solicited-node multicast | N/A | N/A |
| | Joining all-nodes multicast | MUST | MUST |
| | Using link-layer indication for | MAY | MAY |
| | NUD | | |
| 5.5 | 6LoWPAN-ND NUD | MUST | MUST |
| 5.8.2 | Behavior on wakeup | SHOULD | SHOULD |
+----------+-----------------------------------+--------+-----------+
Table 3: Guideline for 6LoWPAN-ND Features for Hosts
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+---------------+-------------------------+------------+------------+
| Section | Description | Deploy | Implement |
+---------------+-------------------------+------------+------------+
| 3.1 | Periodic RA | SHOULD NOT | SHOULD NOT |
| 3.2 | Address assignment | SHOULD | MUST |
| | during startup | | |
| 3.3 | Supporting EUI-64-based | MUST | MUST |
| | MAC hosts | | |
| | Supporting 16-bit MAC | MAY | SHOULD |
| | hosts | | |
| 3.4, 4.3, | ABRO processing/sending | SHOULD | MUST |
| 8.1.3, 8.1.4 | | | |
| 8.1 | Multihop prefix storing | SHOULD | MUST |
| | and redistribution | | |
| 3.5 | Tentative NCE | MUST | MUST |
| 8.2 | Multihop DAD | SHOULD | MUST |
| 4.1, 6.5, | ARO support | MUST | MUST |
| 6.5.1 - 6.5.5 | | | |
| 4.2 | 6CO | SHOULD | SHOULD |
| 6.3 | Process RS/ABRO | MUST | MUST |
+---------------+-------------------------+------------+------------+
Table 4: Guideline for 6LR Features in 6LoWPAN-ND
+--------------+--------------------------+------------+------------+
| Section | Description | Deploy | Implement |
+--------------+--------------------------+------------+------------+
| 3.1 | Periodic RA | SHOULD NOT | SHOULD NOT |
| 3.2 | Address autoconf on | MUST NOT | MUST NOT |
| | router interface | | |
| 3.3 | EUI-64 MAC support on | MUST | MUST |
| | 6LoWPAN interface | | |
| 8.1 - 8.1.1, | Multihop prefix | SHOULD | MUST |
| 8.1.5 | distribution | | |
| 8.2 | Multihop DAD | SHOULD | MUST |
+--------------+--------------------------+------------+------------+
Table 5: Guideline for 6LBR Features in 6LoWPAN-ND
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15. Acknowledgments
The authors thank Pascal Thubert, Jonathan Hui, 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 shape and improve this document.
Additionally, the authors would like to recognize Pascal Thubert for
contributing the original registration idea and for extensive
contributions to earlier versions of the document, Jonathan Hui for
original ideas on prefix/context distribution and extensive
contributions to earlier versions of the document, Colin O'Flynn for
useful "Error-to" suggestions (Section 6.5.2) and for contributions
to the Examples section, Geoff Mulligan for suggesting the use of
address registration as part of existing IPv6 ND messages, and
Mathilde Durvy for helping to clarify router interaction.
16. References
16.1. Normative References
[ETHERNET]
"IEEE Standard for 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", IEEE Std 802.3-2008, December 2008,
<http://standards.ieee.org/getieee802/download/
802.3-2008_section1.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC2491] Armitage, G., Schulter, P., Jork, M., and G. Harter, "IPv6
over Non-Broadcast Multiple Access (NBMA) networks",
RFC 2491, January 1999.
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", RFC 4191, November 2005.
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
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RFC 6775 ND Optimization for 6LoWPANs November 2012
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, September 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC6282] Hui, J. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
September 2011.
16.2. Informative References
[EUI64] IEEE, "Guidelines for 64-bit Global Identifier
(EUI-64(TM)) Registration Authority", <http://
standards.ieee.org/regauth/oui/tutorials/EUI64.html>.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
Host Configuration Protocol (DHCP) version 6", RFC 3633,
December 2003.
[RFC3756] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756,
May 2004.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
Shelby, et al. Standards Track [Page 53]
RFC 6775 ND Optimization for 6LoWPANs November 2012
[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.
[RFC4429] Moore, N., "Optimistic Duplicate Address Detection (DAD)
for IPv6", RFC 4429, April 2006.
[RFC4919] Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
over Low-Power Wireless Personal Area Networks (6LoWPANs):
Overview, Assumptions, Problem Statement, and Goals",
RFC 4919, August 2007.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.
[RFC5889] Baccelli, E. and M. Townsley, "IP Addressing Model in Ad
Hoc Networks", RFC 5889, September 2010.
[RFC6059] Krishnan, S. and G. Daley, "Simple Procedures for
Detecting Network Attachment in IPv6", RFC 6059,
November 2010.
[RFC6106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
"IPv6 Router Advertisement Options for DNS Configuration",
RFC 6106, November 2010.
Shelby, et al. Standards Track [Page 54]
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Authors' Addresses
Zach Shelby (editor)
Sensinode
Konekuja 2
Oulu 90620
Finland
Phone: +358407796297
EMail: zach@sensinode.com
Samita Chakrabarti
Ericsson
EMail: samita.chakrabarti@ericsson.com
Erik Nordmark
Cisco Systems
EMail: nordmark@cisco.com
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
EMail: cabo@tzi.org
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