6lowpan Working Group Z. Shelby, Ed.
Internet-Draft Sensinode
Updates: 4944 (if approved) S. Chakrabarti
Intended status: Standards Track IP Infusion
Expires: October 29, 2010 E. Nordmark
Oracle, Inc.
April 27, 2010
Neighbor Discovery Optimization for Low-power and Lossy Networks
draft-ietf-6lowpan-nd-09
Abstract
The IETF 6LoWPAN working group defines IPv6 for low-power and lossy
networks (LLNs) 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 traditional concept of IP subnets and IP links. IPv6
Neighbor Discovery was not designed for non-transitive wireless
links. The traditional IPv6 link concept and heavy use of multicast
make the protocol inefficient and sometimes impractical in a low
power and lossy network. This document describes simple
optimizations to IPv6 Neighbor Discovery, addressing mechanisms and
duplicate address detection for 6LoWPAN and similar networks.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on October 29, 2010.
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Copyright Notice
Copyright (c) 2010 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
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the Trust Legal Provisions and are provided without warranty as
described in the BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. The Shortcomings of IPv6 Neighbor Discovery . . . . . . . 5
1.2. Mesh-under and Route-over Concepts . . . . . . . . . . . . 6
1.3. Applicability, Goals and Assumptions . . . . . . . . . . . 7
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Extensions to RFC4861 . . . . . . . . . . . . . . . . . . 10
3.2. Address Assignment . . . . . . . . . . . . . . . . . . . . 11
3.3. Host-to-Router Interaction . . . . . . . . . . . . . . . . 11
3.4. Router-to-Router Interaction . . . . . . . . . . . . . . . 12
4. New Neighbor Discovery Options . . . . . . . . . . . . . . . . 13
4.1. Address Registration Option . . . . . . . . . . . . . . . 13
4.2. 6LoWPAN Context Option . . . . . . . . . . . . . . . . . . 15
4.3. Authoritative Border Router Option . . . . . . . . . . . . 16
5. Host Behavior . . . . . . . . . . . . . . . . . . . . . . . . 17
5.1. Forbidden Actions . . . . . . . . . . . . . . . . . . . . 18
5.2. Interface Initialization . . . . . . . . . . . . . . . . . 18
5.3. Sending a Router Solicitation . . . . . . . . . . . . . . 18
5.4. Processing a Router Advertisement . . . . . . . . . . . . 18
5.4.1. Address configuration . . . . . . . . . . . . . . . . 18
5.4.2. Storing Contexts . . . . . . . . . . . . . . . . . . . 19
5.4.3. Maintaining Prefix and Context Information . . . . . . 19
5.5. Registration and Neighbor Unreachability Detection . . . . 19
5.5.1. Sending a Neighbor Solicitation . . . . . . . . . . . 20
5.5.2. Processing a Neighbor Advertisement . . . . . . . . . 20
5.5.3. Recovering from Failures . . . . . . . . . . . . . . . 20
5.6. Next-hop Determination . . . . . . . . . . . . . . . . . . 21
5.7. Address Resolution . . . . . . . . . . . . . . . . . . . . 21
5.8. Sleeping . . . . . . . . . . . . . . . . . . . . . . . . . 22
5.8.1. Picking an Appropriate Registration Lifetime . . . . . 22
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5.8.2. Behavior on Wakeup . . . . . . . . . . . . . . . . . . 22
6. Router Behavior for 6LR and 6LBR . . . . . . . . . . . . . . . 22
6.1. Forbidden Actions . . . . . . . . . . . . . . . . . . . . 23
6.2. Interface Initialization . . . . . . . . . . . . . . . . . 23
6.3. Processing a Router Solicitation . . . . . . . . . . . . . 23
6.4. Periodic Router Advertisements . . . . . . . . . . . . . . 24
6.5. Processing a Neighbor Solicitation . . . . . . . . . . . . 24
6.5.1. Checking for Duplicates . . . . . . . . . . . . . . . 24
6.5.2. Updating the Neighbor Cache . . . . . . . . . . . . . 25
6.5.3. Address Resolution between Routers . . . . . . . . . . 25
6.5.4. Neighbor Unreachability Detection . . . . . . . . . . 25
7. Border Router Behavior . . . . . . . . . . . . . . . . . . . . 25
7.1. Prefix Determination . . . . . . . . . . . . . . . . . . . 26
7.2. Context Configuration and Management . . . . . . . . . . . 26
8. Optional Behavior . . . . . . . . . . . . . . . . . . . . . . 27
8.1. Multihop Prefix and Context Distribution . . . . . . . . . 27
8.1.1. Routers Sending Router Solicitations . . . . . . . . . 27
8.1.2. Routers Processing Router Advertisements . . . . . . . 28
8.1.3. Storing the Information . . . . . . . . . . . . . . . 28
8.1.4. Sending Router Advertisements . . . . . . . . . . . . 28
8.2. Duplicate Address Detection . . . . . . . . . . . . . . . 29
8.2.1. Special Message Validation . . . . . . . . . . . . . . 30
8.2.2. Conceptual Data Structures . . . . . . . . . . . . . . 30
8.2.3. 6LR Sending a special Neighbor Solicitation . . . . . 30
8.2.4. 6LBR Receiving a special Neighbor Solicitation . . . . 31
8.2.5. Processing a special Neighbor Advertisement . . . . . 31
8.2.6. Recovering from Failures . . . . . . . . . . . . . . . 31
9. Protocol Constants . . . . . . . . . . . . . . . . . . . . . . 32
10. Message Examples . . . . . . . . . . . . . . . . . . . . . . . 32
11. Security Considerations . . . . . . . . . . . . . . . . . . . 33
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 34
14. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 34
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 38
15.1. Normative References . . . . . . . . . . . . . . . . . . . 38
15.2. Informative References . . . . . . . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 39
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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 which sits between the MAC layer and the IP network layer. A
link in a 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 [RFC4861] is not
desirable in such a wireless low-power and lossy network. Moreover,
LoWPAN links are asymmetric and non-transitive in nature. A LoWPAN
is potentially composed of a large number of overlapping radio
ranges. Although a given radio range has broadcast capabilities, the
aggregation of these is a complex Non-Broadcast MultiAccess (NBMA,
[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
[I-D.ietf-autoconf-adhoc-addr-model], 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.
o Elimination of redirects since they are problematic on links with
non-transitive connectivity.
o A host address registration feature using a new option in unicast
Neighbor Solicitation and Neighbor Advertisement messages.
o A new Neighbor Discovery option to distribute 6LoWPAN header
compression context to hosts.
o Optional multihop distribution of prefix and 6LoWPAN header
compression context.
o Optional multihop duplicate address detection.
The document defines three new ICMPv6 message options: the required
Address Registration option and the optional Authoritative Border
Router and 6LoWPAN Context options.
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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, Redirect, 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-router address to solicit router advertisements. If the host
receives a valid Router Advertisement with the "A" flag, it
autoconfigures the IPv6 address with the advertised prefix in the
Router Advertisement (RA) message. Besides this, the IPv6 routers
usually send router advertisements periodically on the network. RAs
are sent to the all-node multicast address. Nodes send Neighbor
Solicitation/Neighbor Advertisement messages to resolve the IPv6
address of the destination on the link. The Neighbor Solicitation
messages used for address resolution are multicast. The Duplicate
Address Detection procedure and the use of periodic Router
Advertisement messages assumes that the nodes are powered on and
reachable most of the time.
In Neighbor Discovery the routers find the hosts by assuming that a
subnet prefix maps to one broadcast domain, and then multicast
Neighbor Solicitation messages to find the host and its link-layer
address. Furthermore, the DAD of use multicast assumes that all
hosts that autoconfigure IPv6 addresses from the same prefix can be
used using link-local multicast messages.
Note that the 'L' (on-link) bit in the Prefix Information option can
be set to zero in Neighbor Discovery, which makes the host not use
multicast Neighbor Solicitation (NS) messages for address resolution
of other hosts, but routers still use multicast NS messages to find
the hosts.
In a LoWPAN, primarily two types of network topologies are found -
star networks and mesh networks. A star network is similar to a
regular IPv6 subnet with a router and a set of nodes connected to it
via the same non-transitive link. But in Mesh networks, the nodes
are capable of routing and forwarding packets. 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 moving physically.
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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 center, some
optimizations and extensions to Neighbor Discovery are useful for the
wide deployment of IPv6 over low-powered and lossy networks such as
6LoWPANs.
1.2. Mesh-under and Route-over Concepts
In the 6LoWPAN context, often a link-layer mesh routing mechanism is
referred to as "mesh-under" while routing/forwarding packets using
IP-layer addresses is referred to as "route-over". The difference
between mesh-under and route-over is similar to a bridged-network
versus IP-routing using Ethernet. In a mesh-under network all nodes
are on the same link which is served by one or more routers, which we
call 6LoWPAN Border Routers (6LBR). In a route-over network, there
are multiple links in the 6LoWPAN. Unlike fixed IP links, these
link's members may be changing due to the nature of the low-power and
lossy behavior of wireless technology. Thus a route-over network is
made up of a flexible set of links interconnected by interior
routers, which we call 6LoWPAN Routers (6LR).
This specification is applicable to both mesh-under and route-over
networks. However, in route-over networks, we have two types of
routers - 6LBRs and 6LRs. 6LoWPAN Border Routers sit at the boundary
of the 6LoWPAN and the rest of the network while 6LoWPAN Routers are
inside the LoWPAN. 6LoWPAN Routers are assumed to be running a
routing protocol.
In a mesh-under configuration a 6LBR is acting as the IPv6 router
where all the hosts in the LoWPAN are on the same link, thus they are
only one IP hop away. No 6LoWPAN Routers exist in this topology as
forwarding is handled by a link-layer mesh routing protocol.
In a route-over configuration, Neighbor Discovery operations take
place between hosts and 6LRs or 6LBRs. The 6LR nodes are able to
send and receive Router Advertisements, Router Solicitations as well
as forward and route IPv6 packets. Here packet forwarding happens at
the routing layer.
In both types of configurations, hosts do not take part in routing
and forwarding packets and they act as simple IPv6 hosts.
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1.3. Applicability, Goals and Assumptions
The optimizations described in this document 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 minimized signaling, robust handling of the non-
transitive link, and header compression context information.
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 Make the host to router interaction the same whether mesh-under or
route-over is used.
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 Support for sleeping hosts.
o Minimize the complexity of nodes.
o Disseminate context information to hosts as needed by
[I-D.ietf-6lowpan-hc].
o Optionally disseminate context information and prefix information
from the border to all routers in a LoWPAN.
o Optional duplicate address detection mechanism suitable for route-
over LoWPANs.
Assumptions:
o EUI-64s are globally unique.
o All nodes in the LLN have an EUI-64 interface identifier in order
to do address auto-configuration and detect duplicate addresses.
o If IEEE 802.15.4 16-bit short addresses are used, then some
technique is used to ensure uniqueness of those link-layer
addresses. That could be done using DHCPv6 or other techniques
outside of the scope of this document. Optionally it can be done
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using the Address Registration Option based duplicate address
detection (Specified in Section 8.2).
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 with one or more global IPv6 address
prefixes to enable hosts to move between routers in the 6LoWPAN
without changing their IPv6 addresses.
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" [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"
[I-D.ietf-autoconf-adhoc-addr-model].
This specification makes extensive use of the same terminology
defined in [RFC4861] unless otherwise defined below.
6LoWPAN link:
A wireless link determined by single hop reachability of
neighboring nodes. These are considered links with undetermined
connectivity properties as in [I-D.ietf-autoconf-adhoc-addr-model]
Low-Power and Lossy Network (LLN):
A network made up of low-power links, often with a high
probability of packet loss and undetermined connectivity
properties. A LoWPAN is such a network for example.
6LoWPAN Router (6LR):
An intermediate router in the LoWPAN who can communicate with
other 6LoWPAN routers in the same LoWPAN. 6LoWPAN routers are
present only in route-over topologies.
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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 bring a router on some interfaces and a host on
other interfaces as allowed by [RFC2460].
Mesh-under:
A topology where hosts 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 configuration 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.
Registration:
The process during which a LoWPAN node sends an Neighbor
Solicitation message with an Address Registration option to a
Router creating a Neighbor Cache entry for the LoWPAN node with a
specific timeout.
3. Protocol Overview
The Neighbor Discovery optimizations for LLNs 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 optizations 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 those set of routers have become unreachable.
The protocol also provides for header compression
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[I-D.ietf-6lowpan-hc] by carrying header compression information in a
new option in Router Advertisement messages.
In addition, there are optional and 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 RFC4861
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 required if EUI-64 based
IPv6 addresses are used.
o DAD is optional if DHCPv6 is used to assign addresses.
o A New Address Registration mechanism using 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 optional Router Advertisement option for Context information
used by 6LoWPAN header compression.
o A new optional mechanism to perform Duplicate Address Detection
across a route-over 6LoWPAN reusing the above Address Registration
option.
o New optional mechanisms to distribute Prefixes and Context
information across a route-over network which uses a new
Authoritative Border Router option to control the flooding of
configuration changes.
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o A few new default protocol constants are introduced and some
existing Neighbor Discovery protocol constants are tuned for LLN
usage.
3.2. Address Assignment
Hosts in a 6LoWPAN configure their IPv6 address as specified in
[RFC4861] and [RFC4862] based on the information received in Router
Advertisement messages.
Optionally, 6LRs can 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 ULA
[RFC4193] prefix SHOULD be generated by the 6LBR.
3.3. Host-to-Router Interaction
A host sends Router Solicitation messages at startup and also when it
suspects that one of its default routers have become unreachable
(after Neighbor Unreachability Detection towards the router fails).
Hosts receive Router Advertisement messages typically containing the
Authoritative Border Router option (ABRO) and may optionally contain
one or more 6LoWPAN Context options (6CO) in addition to the existing
Prefix Information options (PIO) as described in [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). The host chooses a lifetime
of the registration and repeats the ARO option to maintain the
registration, even while the host is sleeping.
The registration can fail (an ARO option returned to the host with a
non-zero Status) if the router determines that the IPv6 address is
already used by another hosts, that is, is used by a host with a
different EUI-64. This can be used to support non-EUI-64 based
addresses such as temporary IPv6 addresses [RFC4941] or addresses
based on an Interface ID that is a IEEE 802.15.4 16-bit short
addresses.
The re-registration of a address can be combined with Neighbor
Unreachability Detection (NUD) of the router since both use unicast
Neighbor Solicitation messages. This makes things efficient when a
host wakes up to send a packet and both need to perform NUD to check
that the router is still reachable, and refresh its registration with
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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 a
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
Cache in the router, then the address either doesn't exist, or is
assigned 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 optional new router-to-router interaction is only for the route-
over configuration where 6LRs are present. It is optional in this
protocol since the functions it provides might be better provided by
other protocol mechanisms, be it DHCPv6, link-layer mechanisms, the
routing protocol, or something else. Some mechanisms from this
protocol might be used for router-to-router, while others are
provided by other protocols. For instance, context information
and/or prefix information might be disseminated using this protocol,
while Duplicate Address Detection is done using some other protocol.
6LRs can 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 or context dissemination is 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 Advertisements they send to other 6LRs or send to hosts in
response to a Router Solicitations. There is a version number field
in the ABRO which is used to limit the flooding of updated
information between the 6LRs.
Optionally the 6LRs can perform Duplicate Address Detection against
one or more 6LBRs using a special form of the Address Registration
option. In this case, the Neighbor Solicitation and Advertisement
messages will be forwarded between the 6LR and 6LBRs and the
[RFC4861] rule for checking hop-limit=255 is relaxed. Such multihop
DAD messages MUST NOT modify any Neighbor Cache entries on the
routers since we do not have the security benefits provided by the
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hop-limit=255 check.
4. New Neighbor Discovery Options
This section defines new Neighbor Discovery message options used by
this specification. The Address Registration Option is mandatory,
whereas the Authoritative Border Router Option and 6LoWPAN Context
Option are optional.
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 Neighbor Solicitation (NS) messages
sent by hosts. Thus it can be included in the unicast NS messages
that a host sends as part of Neighbor Unreachability Detection to
determine that it can still reach a default router. The ARO is used
by the receiving router to reliably maintain its Neighbor Cache. The
same option is included in corresponding Neighbor Advertisement (NA)
messages with a Status field indicating the success or failure of the
registration. This option is always host initiated.
The ARO is reused for the optional multihop Duplicate Address
Detection from 6LRs to 6LBRs, in which case it has a different
Length. In that case one or more AROs can be included in an NS.
The ARO is required for reliability and power saving. The lifetime
field provides flexibility to the host to register an address which
should be usable (continue to be advertised by the 6LR in the routing
protocol etc.) during its intended sleep schedule.
The sender of the NS also includes the EUI-64 of the interface it is
registering an address from. This is used as a unique ID for the
detection of duplicate addresses. It is used to tell the difference
between the same node re-registering its address and a different node
(with a different EUI-64) registering an address that is already in
use by someone else.
When the ARO is used by hosts the address that is registered MUST be
the IPv6 source address for the Neighbor Solicitation message. Thus
the Registered Address field is omitted and the Length field MUST be
two. When the ARO is used for the optional multihop DAD between a
6LR and a 6LBR then the Registered Address field is included and the
Length field MUST be four.
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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 | Length | Status | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Registration Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| EUI-64 |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Registered Address (Optional) +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type: TBD1
Length: 8-bit unsigned integer. The length of the option in
units of 8 octets. 2 without or 4 with the Registered
Address.
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: 8-bits. This field is unused. It MUST be initialized
to zero by the sender and MUST be ignored by the
receiver.
Registration Lifetime: 32-bit unsigned integer. The amount of time
in seconds that the router should retain the Neighbor
Cache entry 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.
Registered Address: 128-bit optional field. MUST NOT be sent by a
host. Used for the optional router-router
registrations on behalf of a host. Carries the host
address, which was contained in the IPv6 Source field
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in the original NS that contained the option sent by
the host.
The Status values used in Neighbor Advertisements are:
+--------+--------------------------------------------+
| Status | Description |
+--------+--------------------------------------------+
| 0 | Success |
| 1 | Duplicate Address |
| 2-255 | Allocated using Standards Action [RFC2434] |
+--------+--------------------------------------------+
Table 1
4.2. 6LoWPAN Context Option
The optional 6LoWPAN Context Option (6CO) carries prefix information
for LoWPAN header compression, and is similar to the Prefix
Information Option 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 Context Identifier
(CID) for use as specified in [I-D.ietf-6lowpan-hc]. A context may
be a prefix of any length or an address (/128), and up to 16 6LoWPAN
Context options may be carried in an Router Advertisement message.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Valid Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Context Prefix .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: 6LoWPAN Context Option format
Type: TBD2
Length: 8-bit unsigned integer. The length of the option (including
the type and length fields) in units of 8 octets. May be 2 or 3
depending on the length of the Context Prefix field.
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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.
CID: 4-bit Context Identifier for this prefix information. CID is
used by context based header compression specified in
[I-D.ietf-6lowpan-hc]. The list of CIDs for a LoWPAN is
configured by on the 6LBR that originates the context information
for the 6LoWPAN.
Res: 3-bit reserved field. This field is unused. It MUST be
initialized to zero by the sender and MUST be ignored by the
receiver.
Valid Lifetime: 32-bit unsigned integer. The length of time in
seconds (relative to the time the packet is received) that the
context is valid for the purpose of header compression or
decompression. A value of all one bits (0xffffffff) represents
infinity. 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
Context ID (CID) field. The valid length of this field is
included in the Context Length field. This field is padded with
zeros in order to make the option a multiple of 8-bytes.
4.3. Authoritative Border Router Option
The optional Authoritative Border Router Option (ABRO) is needed when
Router Advertisement (RA) messages are used to disseminate prefixes
and context information across a route-over topology. In this case
6LRs receive Prefix Information options from other 6LRs. This
implies that a 6LR can't just let the most recently received RA win.
In order to be able to reliably add and remove prefixes from the
6LoWPAN we need to carry information from the authoritative 6LBR.
This is done by introducing a version number which the 6LBR sets and
6LRs propagate as they propagate the prefix and context information
with this Authoritative Border Router Option. When there are
multiple 6LBRs they would have separate version number spaces. Thus
this option needs to carry the IP address of the 6LBR that originated
that set of information.
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The Authoritative Border Router option MUST be included in all Router
Advertisement messages in the case when Router Advertisements are
used to propagate information between routers (as described in
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 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ 6LBR Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type: TBD3
Length: 8-bit unsigned integer. The length of the option in
units of 8 octets. Always 3.
Reserved: 16-bits. This field is unused. It MUST be
initialized to zero by the sender and MUST be ignored
by the receiver.
Version Number: 32-bit unsigned integer. 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.
6LBR Address: IPv6 address of the 6LBR that is the origin of the
included version number.
5. Host Behavior
Hosts in a LoWPAN use the Address Registration option in the Neighbor
Solicitation messages they send as a way to maintain the Neighbor
Cache in the routers thereby removing the need for multicast Neighbor
Solicitations to do address resolution. Unlike in [RFC4861] the
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hosts initiate updating the information they receive in Router
Advertisements by sending Router Solicitations before the information
expires. Finally, when Neighbor Unreachability Detection indicates
that one or all default routers have become unreachable, then the
host uses Router Solicitations to find a new set of default routers.
5.1. Forbidden Actions
A host in a 6LoWPAN MUST NOT accept a Redirect message. Redirect
messages are problematic on a link with non-transitive reachability.
A host would never multicast a Neighbor Solicitation 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 assigned to the interface, and then the host sends Router
Solicitation messages as described in [RFC4861] Section 6.3.7.
There is no need to join the Solicited-Node multicast address since
nobody multicasts Neighbor Solicitations in this type of network.
5.3. Sending a Router Solicitation
The Router Solicitation is formatted as specified in [RFC4861] and
sent to the IPv6 All-Routers multicast address (see [RFC4861] Section
6.3.7 for details). 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 theses packets to a router.
5.4. Processing a Router Advertisement
The processing of Router Advertisements is as in [RFC4861] with the
addition of handling the 6LoWPAN Context option and triggering
address registration when a new address has been configured.
Should the host erroneously receive a Prefix Information option with
the 'L' (on-link) flag set, then that Prefix Information Option (PIO)
MUST be ignored.
5.4.1. Address configuration
Address configuration follows [RFC4862]. Once an address has been
configured it will be registered by unicasting a Neighbor
Solicitation with the Address Registration option to the router.
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5.4.2. Storing Contexts
The host maintains a conceptual data structure for the context
information it receives from the routers, which is called the Context
Table. This includes the Context ID, the prefix (from the Context
Prefix field in the 6CO), the Compression bit, and the Valid
Lifetime. A Context Table entry that has the Compression bit clear
is used for decompression when receiving packets, but MUST NOT be
used for compression when sending packets.
When a 6CO option is received in a Router Advertisement it is used to
add or update the information in the Context Table. If the Context
ID field in the 6CO matches an existing Context Table entry, then
that entry is updated with the information in the 6CO. If the Valid
Lifetime field in the 6CO is zero, then the entry is immediately
deleted.
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 lifecycle.
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
deleted.
A host should inspect the various lifetimes to determine when it
should next initiate sending a Router Solicitation to ask for any
updates to the information. The lifetimes that matter are the
Default Router lifetime, the Valid Lifetime in the Prefix Information
options, and the Valid Lifetime in the 6CO. The host SHOULD unicast
an Router Solicitation to the router well before the minimum of those
lifetimes (across all the prefixes and all the contexts) expire.
5.5. Registration and Neighbor Unreachability Detection
Unicast Neighbor Solicitation (NS) messages are send by hosts to
register their IPv6 addresses, and also to do NUD to verify that its
default routers are still reachable. The registration is performed
by the host including an ARO in the Neighbor Solicitation it sends.
Even if the host doesn't have data to send, but is expecting others
to try to send packets to the host, the host needs to maintain its
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Neighbor Cache entries in the routers. This is done by sending NS
messages with the ARO to the router well in advance of the
registration lifetime expiring. NS messages are retransmitted up to
MAX_UNICAST_SOLICIT times using a minimum timeout of RETRANS_TIMER
until the host receives an Neighbor Advertisement message with an ARO
option.
Hosts that receive Router Advertisement messages from multiple
default routers SHOULD attempt to register with more than one of them
in order to increase the robustness of the network.
Note that Neighbor Unreachability Detection probes can be suppressed
if by Reachability Confirmations from transport protocols or
applications as specified in [RFC4861].
5.5.1. Sending a Neighbor Solicitation
The host triggers sending Neighbor Solicitation (NS) messages
containing an ARO when a new address is configured, when it discovers
a new default router, or well before the Registration Lifetime
expires. Such an NS MUST include a Source Link-Layer Address (SLLA)
option, since the router needs to record the link-layer address of
the host.
5.5.2. Processing a Neighbor Advertisement
A host handles Neighbor Advertisement messages as specified in
[RFC4861], with added logic described in this section for handling
the Address Registration option.
In addition to the normal validation of a Neighbor Advertisement and
its options, the Address Registration option is verified as follows
(if present). If the Length field is not two, the option is silently
ignored. If the EUI-64 field does not match the EUI-64 of the
interface, the option is silently ignored.
If the status field is zero, then the address registration was
successful. The host saves the Registration Lifetime from the
Address Registration option for use to trigger a new NS well before
the lifetime expires. If the Status field is not equal to zero, the
address registration has failed.
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.
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The address registration procedure may fail for two reasons: no
response to Neighbor Solicitations is received (NUD failure), or an
Address Registration option with a failure Status (Status > 0) is
received. In the case of NUD failure the entry for that router will
be removed thus address registration is no longer of importance.
When an Address Registration option with a non-zero Status field is
received this indicates that registration for that address has
failed. A failure Status of one indicates that a duplicate address
was detected and the procedure described in [RFC4862] Section 5.4.5
is followed. Other failure codes may be defined in future documents.
For all non-zero Status values the host MUST NOT use the address it
tried to register.
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. All other prefixes are assumed
to be off-link [I-D.ietf-autoconf-adhoc-addr-model]. Anycast
addresses are always considered to be off-link. They are therefore
sent to one of the routers in the Default Router List.
Multicast addresses are considered to be on-link and are resolved as
specified in [RFC4944] or the appropriate IP-over-foo document.
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 SLLA option in Router
Advertisement message provide sufficient a priori state in routers
and hosts to resolve an IPv6 address to its associated link-layer
address. As all prefixes but the link-local prefix are always
assumed to be off-link, multicast-based address resolution between
neighbors is not needed.
Link-layer addresses for neighbors are stored in Neighbor Cache
entries [RFC4861] In order to achieve LoWPAN compression, most global
addresses are formed using a link-layer address. Thus a host can
minimize 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).
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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 described further in this section. Routers
may want to cache traffic destined to a host which is sleeping, but
such functionality is out of the scope of this document.
5.8.1. Picking an Appropriate Registration Lifetime
As all Neighbor Discovery messages are initiated by the hosts, this
allows a host to sleep or otherwise be unreachable between NS
messages. The Address Registration option attached to NS messages
indicates to a router to keep the Neighbor Cache entry for that
address valid for the period in the Registration Lifetime field. A
host should choose a sleep time appropriate for its energy
characteristics, and set a registration lifetime larger than the
sleep time to ensure the registration is renewed successfully
(considering e.g. clock drift). A host should also consider the
stability of the network (how quickly the topology changes) when
choosing its sleep time (and thus registration lifetime). A dynamic
network may require a shorter sleep time.
5.8.2. Behavior on Wakeup
When a host wakes up from a sleep period it SHOULD maintain its
current address registrations that will timeout before the next
wakeup. This is done by sending Neighbor Solicitation messages with
the Address Registration option as described in Section 5.5.1. The
host may also need to refresh its prefix and context information by
sending new unicast Router Solicitation. If after wakeup the host
(using NUD) determines that some or all previous default routers have
become unreachable, then the host will restart doing multicast Router
Solicitations to discover new default router(s) and restart the
address registration process.
6. Router Behavior for 6LR and 6LBR
Both 6LRs and 6LBRs maintain the Neighbor Cache [RFC4861] based on
the Address Registration Options they receive in Neighbor
Advertisement messages from hosts. Note that the handling of ARO
from other routers (with Length=4) is specified in Section 8.
The routers SHOULD NOT garbage collect the Neighbor Cache entries
since they need to retain them until the Registration Lifetime
expires. Similarly, if Neighbor Unreachability Detection on the
router determines that the a host is UNREACHABLE (based on the logic
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in [RFC4861]), the Neighbor Cache entry SHOULD NOT be deleted but be
retained until the Registration Lifetime expires. A renewed ARO
should mark the cache entry as STALE.
Routers MAY implement the Default Router Preferences [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
A router SHOULD NOT send Redirect messages. Since the link has non-
transitive reachability the router has no way to determine that the
recipient of a Redirect message can reach the link-layer address.
A router MUST NOT set the 'L' (on-link) flag in the Prefix
Information options, since that might trigger hosts to send multicast
Neighbor Solicitations.
6.2. Interface Initialization
A router initializes its interface more or less as in [RFC4861].
However, a 6LR might want to wait to make its interfaces advertising
(implicitly keeping the AdvSendAdvertisements flag clear) until it
has received the prefix(es) and context information from its 6LBR.
That is independent of whether prefixes and context information is
disseminated using the methods specified in this document, or using
some other method.
6.3. Processing a Router Solicitation
A router processes Router Solicitation messages as specified in
[RFC4861]. The differences relate to the inclusion of Authoritative
Border Router options in the Router Advertisement (RA) messages. If
a 6LR has received an ABRO from a 6LBR, then it will include that
option unmodified in the Router Advertisement messages it sends. And
if the 6LR has received RAs, whether with the same prefixes and
context information or different, from different 6LBR, then it will
need to keep those prefixes and context information separately so
that the RAs the 6LR sends will maintain the association between the
ABRO and the prefixes and context information. The router can tell
which 6LBR originated the prefixes and context information from the
6LBR Address field in the ABRO.
A 6LR or 6LBR MUST include a Source Link-layer address option in the
Router Advertisements it sends. That is required so that the hosts
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will know the link-layer address of the router.
6.4. Periodic Router Advertisements
A router does not need to send any periodic Router Advertisement
messages since the hosts will solicit updated information by sending
Router Solicitations before the lifetimes expire.
However, if the routers use Router Advertisements to optionally
distribute prefix and/or context information across a route-over
topology, that might require periodic Router Advertisement messages.
6.5. Processing a Neighbor Solicitation
A router handles Neighbor Solicitation messages as specified in
[RFC4861], with added logic described in this section for handling
the Address Registration option.
In addition to the normal validation of a Neighbor Solicitation and
its options, the Address Registration option is verified as follows
(if present). If the Length field is not two, or if the Status field
is not zero, then the Neighbor Solicitation is silently ignored.
Note that Section 8.2 specify optional behavior for a 6LBR for other
Length field values.
If the source address of the NS is the unspecified address, or if no
SLLA option is included, then any included ARO is ignored, that is,
the NS is processed as if it did not contain an ARO.
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. If
there is no Neighbor Cache entry for the IPv6 source address of the
NS, then it isn't a duplicate. If there is such a Neighbor Cache
entry and the EUI-64 is the same, then it isn't a duplicate either.
Otherwise it is a duplicate address.
In the case it is a duplicate address then the router responds with a
unicast Neighbor Advertisement (NA) message sent to the source link-
layer address (from the Source Link-Layer Address (SLLA) option) of
the NS. The NA is formatted with a copy of the ARO from the NS, but
with the Status field set to one to indicate it was a duplicate. In
this case there is no modification to the Neighbor Cache.
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6.5.2. Updating the Neighbor Cache
If ARO did not result in a duplicate address being detected as above,
then if the Registration Lifetime is non-zero the router creates (if
it didn't exist) or updates (otherwise) a Neighbor Cache entry for
the IPv6 source address of the NS. The Registration Lifetime and the
EUI-64 are recorded in the Neighbor Cache entry. A unicast Neighbor
Advertisement (NA) is then sent in response to the NS. This NA
SHOULD include a copy of the ARO, with the Status field set to zero.
If the ARO contains a zero Registration Lifetime then any existing
Neighbor Cache entry for the IPv6 source address of the NS MUST be
deleted, and a NA sent as above.
Should the Registration Lifetime in a Neighbor Cache entry expire,
then the router MUST delete the cache entry. This might result in
notifying the routing protocol.
6.5.3. 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 Address Registration option between each other.
Otherwise, the routers MAY use the ARO. When routers use ARO to
register with each other and the optional DAD to the 6LBR Section 8.2
is in use, then care should be taken to ensure that there isn't a
flood of ARO-carrying messages send to the 6LBR as each router hears
an ARO from their neighboring routers. The details for this is out
of scope of this document.
6.5.4. Neighbor Unreachability Detection
Just like in [RFC4861] the use of NUD from routers to hosts is
required. NUD may also be used between routers, but is not required
if an equivalent mechanism is available, for example, as part of the
routing protocols.
7. Border Router Behavior
A 6LBR handles sending of Router Advertisements and processing of
Neighbor Solicitations from hosts as specified above in section
Section 6. A 6LBR SHOULD always include an Authoritative Border
Router option in the Router Advertisements it sends, listing itself
as the 6LBR Address. That requires that the 6LBR maintain the
version number in stable storage, and increases the version number
when some information in its Router Advertisements change.
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In addition, a 6LBR is somehow configured with the prefix or prefixes
that are assigned to the LoWPAN, and advertises those in Router
Advertisements as in [RFC4861]. Optionally, in the case of route-
over, those prefixes can be disseminated to all the 6LRs using the
technique in Section 8.1. However, there might be mechanisms outside
of the scope of this document that can be used instead for prefix
dissemination with route-over.
If the 6LoWPAN uses Header Compression [I-D.ietf-6lowpan-hc] then the
6LBR needs to manage the context IDs, and advertise those in Router
Advertisements by including 6CO options in its Router Advertisements
so that directly attached hosts are informed about the context IDs.
Below we specify things to consider when the 6LBR needs to add,
remove, or change the context information. Optionally, in the case
of route-over, the context information can be disseminated to all the
6LRs using the technique in Section 8. However, there might be
mechanisms outside of the scope of this document that can be used
instead for disseminating context information with route-over.
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 are included in the Router Advertisement messages
as specified in [RFC4861].
7.2. Context Configuration and Management
If the LoWPAN uses Header Compression [I-D.ietf-6lowpan-hc] then the
6LBR may be configured with context information and related context
IDs. If the LoWPAN has multiple 6LBRs, then they MUST be configured
with the same context information and context IDs.
The context information carried in Router Advertisement (RA) messages
originate at 6LBRs and must be disseminated to all the routers and
hosts within the LoWPAN. RAs include one 6CO for each context.
For the dissemination of context information using the 6CO, a strict
lifecycle SHOULD be used in order to ensure the context information
stays synchronized throughout the LoWPAN. New context information
SHOULD be introduced into the LoWPAN with C=0, to ensure it is known
by all nodes that may have to decompress based on this context
information. Only when it is reasonable to assume that this
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information was successfully disseminated SHOULD an option with C=1
be sent, enabling the actual use of the context information for
compression.
Conversely, to avoid that nodes send packets making use of previous
values of contexts, resulting in ambiguity when receiving a packet
that uses a recently changed context, old values of a context SHOULD
be taken out of use for a while before new values are assigned to
this specific context. That is, in preparation for a change of
context information, its dissemination SHOULD continue for at least
MIN_CONTEXT_CHANGE_DELAY with C=0. Only when it is reasonable to
assume that the fact that the context is now invalid was successfully
disseminated, should the context ID be taken out of dissemination or
reused with a different Context Prefix field. In the latter case,
dissemination of the new value again SHOULD start with C=0, as above.
8. Optional Behavior
Optionally the Router Advertisement messages can be used to
disseminate prefixes to all the 6LRs in a route-over topology. This
is optional because there are likely to be other mechanisms for such
information distribution.
There is also the option to reuse the Address Registration option as
a way for a 6LR to perform DAD (for non-EUI-64 derived IPv6
addresses) against a 6LBR in a route-over topology. This is optional
because there might be other ways to either allocate unique address,
such as DHCPv6 [RFC3315], or other future mechanisms for DAD.
8.1. Multihop Prefix and Context Distribution
The multihop distribution relies on Router Solicitation messages and
Router Advertisement (RA) messages sent between routers, and using
the ABRO version number to control the propagation of the information
(prefixes and context information) that is being sent in the RAs.
8.1.1. Routers Sending Router Solicitations
If multihop distribution is done using Router Advertisement (RA)
messages, then on interface initialization a router SHOULD send some
Router Solicitation messages similarly to how hosts do this in
[RFC4861]. That will cause the routers to respond with RA messages
which then can be used to initially seed the prefix and context
information.
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8.1.2. 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. Otherwise the routers will check and record the
prefix and context information from the receive RAs, and use that
information as follows.
If a received RA does not contain a Authoritative Border Router
option, 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 and Version and
the associated prefixes and context information. If the 6LBR is
previously known, then the Version number field MUST be compared
against the recorded version number for that 6LBR. The comparison
MUST be done the same way as TCP sequence number comparisons to
handle the case when the version number wraps around. If the
received version number is older or the same as the recorded version,
then the information in the RA is silently ignored. Otherwise the
recorded information and version number are updated.
8.1.3. Storing the Information
The router keeps state for each 6LBR that it sees with an ABRO. This
includes the version number, and the complete set of Prefix
Information options and 6LoWPAN Context options. The prefixes are
timed out based on the Valid lifetime in the Prefix Information
Option. The Context Prefix is timed out based on the Valid lifetime
in the 6LoWPAN Context option.
While the prefixes and context information are stored in the router
their valid and preferred lifetimes are decremented as time passes.
This ensures that when the router is in turn later advertising that
information in the Router Advertisements it sends the 'expiry time'
doesn't accidentally move further into the future. For example, if a
6CO with a Valid lifetime of 10 minutes is receive at time T, and the
router includes this in a RA it sends at time T+5 minutes, the Valid
lifetime in the 6CO it sends will be only 5 minutes.
8.1.4. Sending Router Advertisements
If multihop distribution is performed using RA messages, then the
routers MUST ensure that the ABRO always stay together with the
prefixes and context information received with that ABRO. Thus if
the router has received prefix P1 with ABRO saying it is from one
6LBR, and prefix P2 from another 6LBR, then the router MUST NOT
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include the two prefixes in the same RA message. Prefix P1 MUST be
in a RA that include a ABRO from the first 6LBR etc.
The routers periodically send Router Advertisements as in [RFC4861].
This is for the benefit of the other routers receiving the prefixes
and context information. And the routers also respond to Router
Solicitations by unicasting RA messages. In both cases the above
constraint of keeping the ABRO together with 'its' prefixes and
context information apply.
When a router receives new information from a 6LBR, that is, either
it hears from a new 6LBR (a new 6LBR Address in the ABRO) or the ABRO
version number of an existing 6LBR has increased, then it is useful
to send out a few triggered updates. The recommendation is to behave
the same as when an interface has become an advertising interface in
[RFC4861], that is, send up to three RA messages. This ensures rapid
propagation of new information to all the 6LRs.
8.2. Duplicate Address Detection
The ARO can be used, in addition to register an address in a 6LR, to
have the 6LR verify that the address isn't used by some other host.
However, that isn't sufficient in a route-over topology 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 an optional and simple technique for 6LRs
and 6LBRs to perform Duplicate Address Detection that reuses the
Address Registration option. 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.
When a 6LR receives a Neighbor Solicitation containing an Address
Registration option with a non-zero Registration Lifetime and there
was no existing Neighbor Cache entry, then with this mechanism the
6LR will unicast a new Neighbor Solicitation message to one or more
6LBRs, where the NS contains an ARO with the host's address in the
Registered Address field. This NS will be forwarded by 6LRs until it
reaches the 6LBR, hence its IPv6 hop limit field might be less than
255 when received by the 6LBR. The 6LBR will respond with a Neighbor
Advertisement message containing an ARO, which might have a hop limit
less than 255 when it reaches the 6LR.
When the 6LR receives the NS from the 6LBR it will respond to the
host, copying the Status field from the ARO it received from the
6LBR.
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8.2.1. Special Message Validation
Due to the forwarding of the above special NS/NA between the 6LR and
6LBR the hop limit check on receipt MUST be bypassed for such
messages that contain a ARO with a Length field of 4. The receipt of
such messages MUST NOT modify any state on the router with the
exception of the DAD table below.
8.2.2. Conceptual Data Structures
A 6LBR implementing the optional multi-hop 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 ARO - and contains the EUI-64 of the host
that is using that address.
8.2.3. 6LR Sending a special Neighbor Solicitation
When a 6LR that implements the optional multihop DAD receives an NS
from a host (the ARO has Length = 2) and the 6LR does not already
have a Neighbor Cache entry for the host's IPv6 address, then the 6LR
forms and sends an NS to at least one 6LBR. The NS 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, 255 or a smaller number.
o In the NS Target Address, the address of the 6LBR.
o No SLLA option - just an Address Registration option with Length 4
o In the ARO the Status field MUST be set to zero
o In the ARO the EUI-64 and Registration lifetime are copied from
the ARO received from the host.
o In the ARO and 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 Neighbor Cache entry for
the host as specified in section Section 6, an NS is sent to the
6LBRs as above.
A router MUST NOT modify the Neighbor Cache as a result of receiving
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a Neighbor Solicitation with an ARO of Length=4.
8.2.4. 6LBR Receiving a special Neighbor Solicitation
When a 6LBR that implements the optional multihop DAD receives an NS
from a 6LR, that is an NS that contains an ARO with Length = 4, then
it MUST NOT verify that the hop limit is 255 as specified above. The
it 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 ARO, then it returns an NA with the ARO Status set to 1
('Duplicate Address'). Otherwise it returns an NA with ARO Status
set to zero.
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 ARO are stored in that entry.
If an entry is found in the DAD Table and the Registration Lifetime
is zero then the entry is deleted from the table.
A router MUST NOT modify the Neighbor Cache as a result of receiving
a Neighbor Advertisement with an ARO of Length=4.
In both of the above cases the 6LBR forms an NA with the ARO Status
set to zero and sends it back to the 6LR.
8.2.5. Processing a special Neighbor Advertisement
When a 6LR that implements the optional multihop DAD receives an NA
from a 6LBR, that is an NS that contains an ARO with Length = 4, then
it MUST NOT verify that the hop limit is 255 as specified above. If
there is no pending ARO from a host for the Registered Address, then
NA is silently ignored. Otherwise, the information from the 6LBR is
used to form an NA to send to the host. The Status code is copied
from the ARO received from the 6LBR to the ARO that is sent to the
host.
If the Status is non-zero indicating an error, then the Neighbor
Cache entry for the Registration Address is removed.
8.2.6. Recovering from Failures
If there is no response from a 6LBR after RETRANS_TIMER [RFC4861]
then the 6LR would retransmit the NS 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 Section 4.
6LBR Constants:
MIN_CONTEXT_CHANGE_DELAY+ 60 seconds
6LR Constants:
MAX_RTR_ADVERTISEMENTS 3 transmissions
MIN_DELAY_BETWEEN_RAS* 10 seconds
MAX_RA_DELAY_TIME* 2 seconds
Host Constants:
MAX_RTR_SOLICITATION_DELAY* 2 second
RTR_SOLICITATION_INTERVAL* 10 seconds
MAX_RTR_SOLICITATIONS 3 transmissions
10. Message Examples
The following diagrams show the two-step process of the optimized
Neighbor Discovery mechanisms for bootstrapping and Address
Registration.
Node Router
| |
| ---------- Router Solicitation --------> |
| |
| <-------- Router Advertisement --------- |
| [PIO + ABRO + SLLAO] |
Figure 2: Basic Router Solicitation/Router Advertisement exchange
between a node and 6LR or 6LBR
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Node Router
| |
| ------- NS with Address Registration -----> |
| [NS + ARO + SLLAO] |
| <-----NA with Address Registration --------- |
| [NA + ARO with Status + SLLAO] |
Figure 3: Neighbor Discovery Address Registration
11. Security Considerations
The security considerations of IPv6 Neighbor Discovery [RFC4861]
apply. Additional considerations can be found in [RFC3756].
This specification expects that the link layer is sufficiently
protected, for instance using MAC sublayer cryptography. In other
words, model 1 from [RFC3756] applies. In particular, it is expected
that the LoWPAN MAC provides secure unicast to/from Routers and
secure broadcast from the Routers in a way that prevents tampering
with or replaying the Router Advertisement messages. However, any
future 6LoWPAN security protocol that applies to Neighbor Discovery
for 6LoWPAN protocol, is out of scope of this document.
The multihop DAD mechanisms rely on Neighbor Solicitation and
Neighbor Advertisement messages that are forwarded by 6LRs, and as a
result the hop_limit=255 check on the receiver is disabled for such
messages. This implies that any node on the Internet could
successfully send such messages. We avoid any additional security
issues due to this by requiring that the routers never modify the
Neighbor Cache entry due to such messages, and that they reject them
unless they are received on an interface that has been explicitly
configured to use these LLN optimizations.
In some future deployments one might want to use SEcure Neighbor
Discovery [RFC3971] [RFC3972]. This is possible with the Address
Registration option as sent between hosts and routers, since the
address that is being registered is the IPv6 source address of the
Neighbor Solicitation and SeND verifies the IPv6 source address of
the packet. Applying SeND to the optional router-to-router
communication in this document is out of scope.
12. IANA Considerations
The document requires three new Neighbor Discovery option types under
the subregistry "IPv6 Neighbor Discovery Option Formats":
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o Address Registration Option (TBD1)
o 6LoWPAN Context Option (TBD2)
o Authoritative Border Router Option (TBD3)
[TO BE REMOVED: This registration should take place at the following
location: http://www.iana.org/assignments/icmpv6-parameters]
13. Acknowledgments
The authors thank Pascal Thubert, Jonathan Hui, Carsten Bormann,
Richard Kelsey, Geoff Mulligan, Julien Abeille, Alexandru Petrescu,
Peter Siklosi, Pieter De Mil, Fred Baker, Anthony Schoofs, Phil
Roberts, Daniel Gavelle and Joakim Eriksson for useful discussions
and comments that have helped shaped and improve this document.
Additionally, the authors would like to recognize Carsten Bormann for
the suggestions on the Context Prefix Option and contribution to
earlier version of the draft, Pascal Thubert for contribution of the
original registration idea and contribution to earlier version of the
draft, Jonathan Hui for contribution to earlier versions of the
draft, and Geoff Mulligan for suggesting the use of Address
Registration as part of existing IPv6 Neighbor Discovery messages.
14. Changelog
Changes from -08 to -09:
o Clean re-write of the draft (re-use of some introductory
material)
o Merged in draft-chakrabarti-6lowpan-ipv6-nd-simple-00
o Changed address registration to an option piggybacked on NS/NA
o New Authoritative Border Router option
o New Address Registration Option
o Separated Prefix Information and Content Information
o Optional DAD to the edge
Changes from -07 to -08:
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o Removed Extended LoWPAN and Whiteboard related sections.
o Included reference to the autoconf addressing model.
o Added Optimistic Flag to 6AO.
o Added guidelines on routers performing DAD.
o Removed the NR/NC Advertising Interval.
o Added assumption of uniform IID formation and DAD throughout a
LoWPAN.
Changes from -06 to -07:
o Updated addressing and address resolution (#60).
o Changed the Address Option to 6LoWPAN Address Option, fixed S
values (#61).
o Added support for classic RFC4861 RA Prefix Information messages
to be processed (#62).
o Added a section on using 6LoWPAN-ND under a hard-wired RFC4861
stack (#63).
o Updated the NR/NC message with a new Router flag, combined the
Code and Status fields into one byte, and added the capability to
carry 6IOs (#64).
o Made co-existence with other ND mechanisms clear (#59).
o Added a new Protocol Specification section with all mechanisms
specified there (#59).
o Removed dependencies and conflicts with RFC4861 wherever
possible (#59).
o Some editorial cleanup.
Changes from -05 to -06:
o Fixed the Prf codes (#52).
o Corrected the OIIO TID field to 8-bits. Changed the Nonce/OII
order in both the OIIO and the NR/NC. (#53)
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o Corrected an error in Table 1 (#54).
o Fixed asymmetric and a misplaced transient in the 6LoWPAN
terminology section.
o Added Updates RFC4861 to header
Changes from -04 to -05:
o Meaning of the RA's M-bit changed to original [RFC4861] meaning
(#46).
o Terms "on-link" and "off-link" used in place of "on-link" and
"off-link".
o Next-hop determination text simplified (#49).
o Neighbor cache and destination cache removed.
o IID to link-layer address requirement relaxed.
o NR/NC changes to enable on-link refresh with routers (#48).
o Modified 6LoWPAN Information Option (#47).
o Added a Protocol Constants section (#24)
o Added the NR processing table (#51)
o Considered the use of SeND on backbone NS/NA messages (#50)
Changes from -03 to -04:
o Moved Ad-hoc LoWPAN operation to Section 7 and made ULA prefix
generation a features useful also in Simple and Extended LoWPANs.
(#41)
o Added a 32-bit Owner Nonce to the NR/NC messages and the
Whiteboard, removed the TID history. (#39)
o Improved the duplicate OII detection algorithm using the Owner
Nonce. (#39)
o Clarified the use of Source and Target link-layer options in
NR/NC. (#43)
o Included text on the use of alternative methods to acquire
addresses. (#38)
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o Removed S=2 from Address Option (not needed). (#36)
o Added a section on router dissemination consistency. (#44)
o Small improvements and extensive editing. (#42, #37, #35)
Changes from -02 to -03:
o Updated terminology, with RFC4861 non-transitive link model.
o 6LoWPAN and ND terminology separated.
o Protocol overview explains RFC4861 diff in detail.
o RR/RC is now Node Registration/Confirmation (NR/NC).
o Added NR failure codes.
o ER Metric now included in 6LoWPAN Summary Option for use in
default router determination by hosts.
o Examples of host data structures, and the Whiteboard given.
o Whiteboard is supported by all Edge Routers for option
simplicity.
o Edge Router Specification chapter re-structured, clarifying
optional Extended LoWPAN operation.
o NS/NA now completely optional for nodes. No address resolution
or NS/NA NUD required.
o link-local operation now compatible with oDAD (was broken).
o Exception to hop limit = 255 for NR/NC messages.
o Security considerations improved.
o ICMPv6 destination unreachable supported.
Changes from -01 to -02:
o Fixed 16 != 0xff bug (ticket closed).
o Specified use of ULAs in ad-hoc LoWPAN section 9 (ticket
closed).
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o Terminology cleanup based on Alex's comments.
o General editing improvements.
Changes from -00 to -01:
o Specified the duplicate owner interface identifier procedures.
A TID lollipop algorithm was sufficient (nonce unnecessary).
o Defined fault tolerance using secondary bindings.
o Defined ad-hoc network operation.
o Removed the E flag from RA and the X flag from RR/RC.
o Completed message examples.
o Lots of improvements in text quality and consistency were made.
15. References
15.1. Normative References
[I-D.ietf-autoconf-adhoc-addr-model]
Baccelli, E. and M. Townsley, "IP Addressing Model in Ad
Hoc Networks", draft-ietf-autoconf-adhoc-addr-model-03
(work in progress), March 2010.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[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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[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.
15.2. Informative References
[I-D.ietf-6lowpan-hc]
Hui, J. and P. Thubert, "Compression Format for IPv6
Datagrams in 6LoWPAN Networks", draft-ietf-6lowpan-hc-07
(work in progress), April 2010.
[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.
[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.
[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.
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Authors' Addresses
Zach Shelby (editor)
Sensinode
Hallituskatu 13-17D
Oulu 90100
FINLAND
Phone: +358407796297
Email: zach@sensinode.com
Samita Chakrabarti
IP Infusion
1188 Arquest Street
Sunnyvale, CA
USA
Email: samitac@ipinfusion.com
Erik Nordmark
Oracle, Inc.
17 Network Circle
Menlo Park, CA 94025
USA
Email: Erik.Nordmark@Oracle.COM
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