6lowpan Working Group Z. Shelby, Ed.
Internet-Draft Sensinode
Updates: 4944 (if approved) S. Chakrabarti
Intended status: Standards Track IP Infusion
Expires: February 4, 2011 E. Nordmark
Oracle, Inc.
August 3, 2010
Neighbor Discovery Optimization for Low-power and Lossy Networks
draft-ietf-6lowpan-nd-12
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
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on February 4, 2011.
Shelby, et al. Expires February 4, 2011 [Page 1]
Internet-Draft ND Optimization for LLNs August 2010
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
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 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 . . . . . . . . . . . . . . . . . . . . . . 10
3.1. Extensions to RFC4861 . . . . . . . . . . . . . . . . . . 10
3.2. Address Assignment . . . . . . . . . . . . . . . . . . . . 11
3.3. Host-to-Router Interaction . . . . . . . . . . . . . . . . 12
3.4. Router-to-Router Interaction . . . . . . . . . . . . . . . 13
3.5. Neighbor Cache Management . . . . . . . . . . . . . . . . 13
4. New Neighbor Discovery Options . . . . . . . . . . . . . . . . 14
4.1. Address Registration Option . . . . . . . . . . . . . . . 14
4.2. 6LoWPAN Context Option . . . . . . . . . . . . . . . . . . 16
4.3. Authoritative Border Router Option . . . . . . . . . . . . 18
5. Host Behavior . . . . . . . . . . . . . . . . . . . . . . . . 19
5.1. Forbidden Actions . . . . . . . . . . . . . . . . . . . . 19
5.2. Interface Initialization . . . . . . . . . . . . . . . . . 19
5.3. Sending a Router Solicitation . . . . . . . . . . . . . . 20
5.4. Processing a Router Advertisement . . . . . . . . . . . . 20
5.4.1. Address configuration . . . . . . . . . . . . . . . . 20
5.4.2. Storing Contexts . . . . . . . . . . . . . . . . . . . 21
5.4.3. Maintaining Prefix and Context Information . . . . . . 21
5.5. Registration and Neighbor Unreachability Detection . . . . 22
5.5.1. Sending a Neighbor Solicitation . . . . . . . . . . . 22
5.5.2. Processing a Neighbor Advertisement . . . . . . . . . 22
5.5.3. Recovering from Failures . . . . . . . . . . . . . . . 23
5.6. Next-hop Determination . . . . . . . . . . . . . . . . . . 23
5.7. Address Resolution . . . . . . . . . . . . . . . . . . . . 24
5.8. Sleeping . . . . . . . . . . . . . . . . . . . . . . . . . 24
Shelby, et al. Expires February 4, 2011 [Page 2]
Internet-Draft ND Optimization for LLNs August 2010
5.8.1. Picking an Appropriate Registration Lifetime . . . . . 24
5.8.2. Behavior on Wakeup . . . . . . . . . . . . . . . . . . 24
6. Router Behavior for 6LR and 6LBR . . . . . . . . . . . . . . . 25
6.1. Forbidden Actions . . . . . . . . . . . . . . . . . . . . 25
6.2. Interface Initialization . . . . . . . . . . . . . . . . . 25
6.3. Processing a Router Solicitation . . . . . . . . . . . . . 26
6.4. Periodic Router Advertisements . . . . . . . . . . . . . . 26
6.5. Processing a Neighbor Solicitation . . . . . . . . . . . . 27
6.5.1. Checking for Duplicates . . . . . . . . . . . . . . . 27
6.5.2. Updating the Neighbor Cache . . . . . . . . . . . . . 27
6.5.3. Address Resolution between Routers . . . . . . . . . . 28
6.5.4. Neighbor Unreachability Detection . . . . . . . . . . 29
7. Border Router Behavior . . . . . . . . . . . . . . . . . . . . 29
7.1. Prefix Determination . . . . . . . . . . . . . . . . . . . 29
7.2. Context Configuration and Management . . . . . . . . . . . 30
8. Optional Behavior . . . . . . . . . . . . . . . . . . . . . . 30
8.1. Multihop Prefix and Context Distribution . . . . . . . . . 31
8.1.1. 6LBRs Sending Router Advertisements . . . . . . . . . 31
8.1.2. Routers Sending Router Solicitations . . . . . . . . . 31
8.1.3. Routers Processing Router Advertisements . . . . . . . 32
8.1.4. Storing the Information . . . . . . . . . . . . . . . 32
8.1.5. Sending Router Advertisements . . . . . . . . . . . . 33
8.2. Multihop Duplicate Address Detection . . . . . . . . . . . 33
8.2.1. Special Message Validation . . . . . . . . . . . . . . 35
8.2.2. Conceptual Data Structures . . . . . . . . . . . . . . 35
8.2.3. 6LR Sending a special Neighbor Solicitation . . . . . 35
8.2.4. 6LBR Receiving a special Neighbor Solicitation . . . . 36
8.2.5. Processing a special Neighbor Advertisement . . . . . 36
8.2.6. Recovering from Failures . . . . . . . . . . . . . . . 37
9. Protocol Constants . . . . . . . . . . . . . . . . . . . . . . 37
10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
10.1. Message Examples . . . . . . . . . . . . . . . . . . . . . 37
10.2. Host Bootstrapping Example . . . . . . . . . . . . . . . . 38
10.3. Router Interaction Example . . . . . . . . . . . . . . . . 40
10.3.1. Bootstrapping a Router . . . . . . . . . . . . . . . . 40
10.3.2. Updating the Neighbor Cache . . . . . . . . . . . . . 40
11. Security Considerations . . . . . . . . . . . . . . . . . . . 41
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 42
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 42
14. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 42
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 47
15.1. Normative References . . . . . . . . . . . . . . . . . . . 47
15.2. Informative References . . . . . . . . . . . . . . . . . . 48
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 49
Shelby, et al. Expires February 4, 2011 [Page 3]
Internet-Draft ND Optimization for LLNs August 2010
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 for hosts.
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.
Shelby, et al. Expires February 4, 2011 [Page 4]
Internet-Draft ND Optimization for LLNs August 2010
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.
Shelby, et al. Expires February 4, 2011 [Page 5]
Internet-Draft ND Optimization for LLNs August 2010
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.
Shelby, et al. Expires February 4, 2011 [Page 6]
Internet-Draft ND Optimization for LLNs August 2010
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 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.
Shelby, et al. Expires February 4, 2011 [Page 7]
Internet-Draft ND Optimization for LLNs August 2010
o A 6LoWPAN is configured to share one or more global IPv6 address
prefixes to enable hosts to move between routers in the 6LoWPAN
without changing their IPv6 addresses.
o When using the optional DAD mechanism of Section 8.2 6LRs either
register with all the 6LBRs, or the network uses some out-of-scope
mechanism to keep the 6LBRs synchronized.
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
using the Address Registration Option based duplicate address
detection (Specified in Section 8.2).
o In order to preserve the uniqueness of addresses 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 protocol
developed for that purpose).
o In order for [I-D.ietf-6lowpan-hc] 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.
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.
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].
Shelby, et al. Expires February 4, 2011 [Page 8]
Internet-Draft ND Optimization for LLNs August 2010
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 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 who can communicate with
other 6LoWPAN routers in the same LoWPAN. 6LoWPAN routers are
present only in route-over topologies.
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.
Shelby, et al. Expires February 4, 2011 [Page 9]
Internet-Draft ND Optimization for LLNs August 2010
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. Thus for 6LoWPAN Router 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
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
[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]:
Shelby, et al. Expires February 4, 2011 [Page 10]
Internet-Draft ND Optimization for LLNs August 2010
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.
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. The use of the M flag in this optimization
is however more restrictive than in [RFC4861]. When the M flag is
set a host is required to use DHCPv6 to assign any non-EUI-64
addresses. When the M flag is not set, the LoWPAN is required to
support duplicate address detection, thus a host can then safely use
the address registration mechanism to check non-EUI-64 addresses for
uniqueness.
Optionally, 6LRs can use the same mechanisms to configure their IPv6
addresses.
Shelby, et al. Expires February 4, 2011 [Page 11]
Internet-Draft ND Optimization for LLNs August 2010
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) in an NS message. The host
chooses a lifetime of the registration and repeats the ARO option
periodically (before the lifetime runs out) to maintain the
registration, 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. Failure can also occur if the Neighbor Cache on that
router is full.
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
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
Shelby, et al. Expires February 4, 2011 [Page 12]
Internet-Draft ND Optimization for LLNs August 2010
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 Advertisement they send to other 6LRs or send to hosts in
response to a Router Solicitations. There is a version number field
in the ABRO which is used to limit the flooding of updated
information between the 6LRs.
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
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 slightly differently than in
[RFC4861]. This results in three different types of NCEs and the
types specify how those entries can be removed:
Shelby, et al. Expires February 4, 2011 [Page 13]
Internet-Draft ND Optimization for LLNs August 2010
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.
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 node successfully registers
with a Router the result is a Registered NCE. As Routers send RAs to
hosts, and when routers optionally receive RA messages or receive
multicast NS messages from other Routers the result is Garbage-
collectible NCEs.
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
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
Shelby, et al. Expires February 4, 2011 [Page 14]
Internet-Draft ND Optimization for LLNs August 2010
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 an SLLA option MUST be included and 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 there is no
SLLA option, the Registered Address field is included and the Length
field MUST be four.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Registration Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ EUI-64 +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Registered Address (Optional) +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Shelby, et al. Expires February 4, 2011 [Page 15]
Internet-Draft ND Optimization for LLNs August 2010
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: This field is unused. It MUST be initialized to zero
by the sender and MUST be ignored by the receiver.
Registration Lifetime: 16-bit unsigned integer. The amount of time
in a unit of 10 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
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 | Neighbor Cache Full |
| 3-255 | 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
Shelby, et al. Expires February 4, 2011 [Page 16]
Internet-Draft ND Optimization for LLNs August 2010
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | 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.
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 lifecycle based on the recommendations in Section 7.2.
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, Reserved: This field is unused. It MUST be initialized to zero
by the sender and MUST be ignored by the receiver.
Shelby, et al. Expires February 4, 2011 [Page 17]
Internet-Draft ND Optimization for LLNs August 2010
Valid Lifetime: 16-bit unsigned integer. The length of time in a
unit of 10 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 (0xffff) 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.
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 | Version Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ 6LBR Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Shelby, et al. Expires February 4, 2011 [Page 18]
Internet-Draft ND Optimization for LLNs August 2010
Fields:
Type: TBD3
Length: 8-bit unsigned integer. The length of the option in
units of 8 octets. Always 3.
Version Number: 16-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.
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.
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
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. A
Shelby, et al. Expires February 4, 2011 [Page 19]
Internet-Draft ND Optimization for LLNs August 2010
host MUST join the all-nodes multicast address.
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). A SLLA option MUST be included to enable unicast
Router Advertisements 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
theses packets to a router.
Since hosts do not depend on multicast Router Advertisements to
discover routers, the hosts need to intelligently retransmit Router
Solicitations whenever the default router list is empty, default
routers are unreachable, or the lifetime of the prefixes and contexts
in the previous RA are about to expire. The RECOMMENDED
retransmissions it 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 binary exponential backoff of the retransmission
timer for each subsequent retransmission. However, it is useful to
have a maximum retransmission timer of 60 seconds
(MAX_RTR_SOLICITATION_INTERVAL). In all cases the RS retransmissions
are terminated when a RA is received.
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.
Furthermore, the SLLA option MUST be included in the RA. Unlike in
[RFC4861], the maximum value of the RA Router Lifetime field MAY be
up to 0xFFFF (18 hours).
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]. 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
Shelby, et al. Expires February 4, 2011 [Page 20]
Internet-Draft ND Optimization for LLNs August 2010
address can be configured by any other means (and duplicate detection
is performed as part of the registration process).
Once an address has been configured it will be registered by
unicasting a Neighbor Solicitation with the Address Registration
option to one or more routers.
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
one or more Router Solicitations to the router well before the
minimum of those lifetimes (across all the prefixes and all the
contexts) expire, and switch to multicast RS messages if there is no
response to the unicasts. The retransmission behavior for the Router
Shelby, et al. Expires February 4, 2011 [Page 21]
Internet-Draft ND Optimization for LLNs August 2010
Solicitations is specified in section Section 5.3.
5.5. Registration and Neighbor Unreachability Detection
Unicast Neighbor Solicitation (NS) messages are sent 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
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. An unspecified source address MUST NOT be used in NS
messages.
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
Shelby, et al. Expires February 4, 2011 [Page 22]
Internet-Draft ND Optimization for LLNs August 2010
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.
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. 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 indicated 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 has no more default routers it needs to revert to sending
Router Solicitations as specified in section 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. 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.
Shelby, et al. Expires February 4, 2011 [Page 23]
Internet-Draft ND Optimization for LLNs August 2010
5.7. Address Resolution
The address registration mechanism and the SLLA option in Router
Advertisement messages provide sufficient a priori state in routers
and hosts to resolve an IPv6 address to its associated link-layer
address. As all prefixes 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).
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 a new unicast Router Solicitation (the maximum Router
Shelby, et al. Expires February 4, 2011 [Page 24]
Internet-Draft ND Optimization for LLNs August 2010
Lifetime is 18 hours whereas the maximum Registration lifetime is 7
days). 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, Neighbor Discovery packets from
other nodes, and potentially a routing protocol used in the 6LoWPAN
as outlined in Section 3.5. Note that the handling of ARO from other
routers (with Length=4) is specified in Section 8.
The routers SHOULD NOT garbage collect Registered Neighbor Cache
entries (see Section 3.4) 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 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. 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 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
Shelby, et al. Expires February 4, 2011 [Page 25]
Internet-Draft ND Optimization for LLNs August 2010
(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, and
the exclusive use of unicast Router Advertisements. If a 6LR has
received an ABRO from a 6LBR, then it will include that option
unmodified in the Router Advertisement messages it sends. And if the
6LR has received RAs, whether with the same prefixes and context
information or different, from different 6LBR, then it will need to
keep those prefixes and context information separately so that the
RAs the 6LR sends will maintain the association between the ABRO and
the prefixes and context information. The router can tell which 6LBR
originated the prefixes and context information from the 6LBR Address
field in the ABRO. When a router has information tied to multiple
ABROs, a single RS will result in multiple RAs each containing a
different ABRO.
A Router Solicitation might be received from a host that has not yet
registered its address with the router. Thus the router MUST NOT
modify an existing Neighbor Cache entry based on the SLLA option from
the Router Solicitation. However, a router MAY create a Tentative
Neighbor Cache entry based on the SLLA option. Such a Tentative
Neighbor Cache entry SHOULD be timed out in TENTATIVE_NCE_LIFETIME
seconds unless a registration converts it into a Registered NCE.
A 6LR or 6LBR MUST include a Source Link-layer address option in the
Router Advertisements it sends. That is required so that the hosts
will know the link-layer address of the router. Unlike in [RFC4861],
the maximum value of the RA Router Lifetime field MAY be up to 0xFFFF
(18 hours).
Unlike [RFC4861] which suggests multicast Router Advertisements, this
specification optimizes the exchange by always unicasting RAs in
response to RSs. This is possible since the RS always includes a
SLLA option, which is used by the router to unicast the RA.
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.
Shelby, et al. Expires February 4, 2011 [Page 26]
Internet-Draft ND Optimization for LLNs August 2010
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.
Such RAs are sent using the configurable MinRtrAdvInterval and
MaxRtrAdvInterval as per [RFC4861].
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. Note that if multihop DAD
(Section 8.2) is used then the checks are slightly different to take
into account Tentative Neighbor Cache entries.
In the case it is a duplicate address then the router responds with a
unicast Neighbor Advertisement (NA) message 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.
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. If the Neighbor Cache is full and
a new entry needs to be created, then the router responds with a
Shelby, et al. Expires February 4, 2011 [Page 27]
Internet-Draft ND Optimization for LLNs August 2010
unicast NA sent to 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 two to
indicate the router's Neighbor Cache is full.
The Registration Lifetime and the EUI-64 are recorded in the Neighbor
Cache entry. A unicast Neighbor Advertisement (NA) is then sent in
response to the NS. This NA SHOULD include a copy of the ARO, with
the Status field set to zero. A TLLA option is not required in the
NA, since the host already knows the router's link-layer address from
Router Advertisements.
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.
The addition and removal of Registered Neighbor Cache entries would
result in notifying the routing protocol.
Note: If the optional multihop DAD (Section 8.2) is used, then the
updating of the Neighbor Cache is slightly different due to Tentative
NCEs.
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 multihop DAD Section 8.2 is
in use, then care should be taken to ensure that there isn't a flood
of ARO-carrying messages sent to the 6LBR as each router hears an ARO
from their neighboring routers. The details for this is out of scope
of this document.
Optionally Routers can use multicast Neighbor Solicitations as in
[RFC4861] to resolve each others link-layer addresses. Thus Routers
MAY multicast Neighbor Solicitations for other routers, for example
as a result of receiving some routing protocol update. Routers MUST
respond to multicast Neighbor Solicitations. This implies that
Routers MUST join the Solicited-node multicast addresses as specified
in [RFC4861].
Shelby, et al. Expires February 4, 2011 [Page 28]
Internet-Draft ND Optimization for LLNs August 2010
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. The
information whose change affects the version are in the Prefix
Information options (the prefixes or their lifetimes) and in the 6CO
option (the prefixes, Context IDs, or lifetimes.)
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
Shelby, et al. Expires February 4, 2011 [Page 29]
Internet-Draft ND Optimization for LLNs August 2010
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
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
Shelby, et al. Expires February 4, 2011 [Page 30]
Internet-Draft ND Optimization for LLNs August 2010
because there might be other ways to either allocate unique address,
such as DHCPv6 [RFC3315], or other future mechanisms for multihop
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.
This multihop distribution mechanism can handle arbitrary information
from an arbitrary number of 6LBRs. However, the semantics of the
context information requires that all the 6LNs use the same
information, whether they send, forward, or receive compressed
packets. Thus the manager of the 6LBRs need to somehow ensure that
the context information is in synchrony across the 6LBRs. This can
be handled in different ways. One possible way to ensure it is to
treat the context and prefix information as originating from some
logical or virtual source, which in essence means that it looks like
the information is distributed from a single source.
If a set of 6LBRs behave as a single one (using mechanisms out of
scope of this document) so that the prefixes and contexts and ABRO
version number will be the same from all the 6LBRs, then those 6LBRs
can pick a single IP address to use in the ABRO option.
8.1.1. 6LBRs Sending Router Advertisements
6LBRs supporting multihop prefix and context distribution MUST
include an ABRO in each of its RAs. The ABRO Version Number field is
used to keep prefix and context information consistent throughout the
LoWPAN along with the guidelines in Section 7.2. Each time any
information in the set of PIO or 6CO options change, 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
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.
Shelby, et al. Expires February 4, 2011 [Page 31]
Internet-Draft ND Optimization for LLNs August 2010
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 and nothing in Section 8.1 applies. 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 version
number received in the packet is greater than the stored version
number (following [RFC1982] Section 3.2), then the information in the
RA is silently ignored. Otherwise the recorded information and
version number are updated.
By TCP sequence number comparison we mean that half of the version
number space is "old" and half is "new". For example, if the current
version number is 0x2, then anything between 0x80000003 (0x2-
0x7fffffff) and 0x1 is old, and anything between 0x3 and 0x80000002
(0x2+0x8000000) is new.
8.1.4. 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 received at time T, and
the router includes this in a RA it sends at time T+5 minutes, the
Valid lifetime in the 6CO it sends will be only 5 minutes.
Shelby, et al. Expires February 4, 2011 [Page 32]
Internet-Draft ND Optimization for LLNs August 2010
8.1.5. 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
include the two prefixes in the same RA message. Prefix P1 MUST be
in a RA that include a ABRO from the first 6LBR etc. Note that
multiple 6LBRs might advertise the same prefix and context
information, but they still need to be associated with the 6LBRs that
advertised them.
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. 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. However, that isn't sufficient in a route-over topology (or
any 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 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. The technique assumes that the 6LRs
either register with all the 6LBRs, or that the network uses some
out-of-scope mechanism to keep the DAD tables in the 6LBRs
synchronized.
Shelby, et al. Expires February 4, 2011 [Page 33]
Internet-Draft ND Optimization for LLNs August 2010
The multihop DAD mechanism is used synchronously the first time an
address is registered with a particular 6LR. That is, the ARO option
is not returned to the host until multihop DAD has been completed
against the 6LBRs. For existing registrations in the 6LR the
multihop DAD needs to be repeated against the 6LBRs to ensure that
the entry for the address in the 6LBRs does not time out, but that
can be done asynchronously with the response to the hosts. For
instance, by tracking how much is left of the lifetime the 6LR
registered with the 6LBRs and re-registering with the 6LBR when this
lifetime is about to run out.
For the synchronous multihop DAD the 6LR performs some additional
checks to ensure that it has a Neighbor Cache entry it can use to
respond to the host when it receives a response from a 6LBR. This
consists of checking for an already existing (Tentative or
Registered) Neighbor Cache entry for the registered address with a
different EUI-64. If such a Registered NCE exists, then the 6LR
SHOULD respond that the address is a duplicate. If such a Tentative
NCE exists, then the 6LR SHOULD silently ignore the ARO thereby
relying on the host retransmitting the ARO. (This is needed to
handle the case when multiple hosts try to register the same IPv6
address at the same time.) If no Neighbor Cache entry exists, then
the 6LR MUST create a Tentative Neighbor Cache entry with the EUI-64
and the SLLAO. This entry will be used to send the response to the
host when the 6LBR responds.
When a 6LR receives a Neighbor Solicitation containing an Address
Registration option with a non-zero Registration Lifetime and it has
no existing Registered Neighbor Cache entry, then with this mechanism
the 6LR will invoke synchronous multihop DAD.
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 NA from the 6LBR containing a ARO, it will
look for a matching (same IP address and EUI-64) (Tentative or
Registered) Neighbor Cache entry. If no such entry is found then the
ARO is silently ignored. If an entry is found and the ARO had
Status=0 then the 6LR will mark the Tentative Neighbor Cache entry as
Registered. In all cases when an entry is found then the 6LR will
respond to the host with an NA, copying the Status field from the ARO
it received from the 6LBR.
Shelby, et al. Expires February 4, 2011 [Page 34]
Internet-Draft ND Optimization for LLNs August 2010
A Tentative Neighbor Cache entry SHOULD be timed out
TENTATIVE_NCE_LIFETIME seconds after it was created in order to allow
for another host to attempt to register the IPv6 address.
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 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 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 subject to the above checks,
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 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.
Shelby, et al. Expires February 4, 2011 [Page 35]
Internet-Draft ND Optimization for LLNs August 2010
A router MUST NOT modify the Neighbor Cache as a result of receiving
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.
Then 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, 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 ARO Status code is set to zero, and
the 6LBR forms an NA with the ARO copied from the NS to the NA. The
NA is sent back to the 6LR i.e., back to the source of the NS.
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 Tentative Neighbor Cache entry matching the Registered
address and EUI-64, 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 Tentative
Neighbor Cache entry for the Registered Address is removed.
Otherwise it is made into a Registered Neighbor Cache entry.
A router MUST NOT modify the Neighbor Cache as a result of receiving
a Neighbor Advertisement with an ARO of Length=4, unless there is a
Tentative Neighbor Cache entry matching the IPv6 address and EUI-64.
Shelby, et al. Expires February 4, 2011 [Page 36]
Internet-Draft ND Optimization for LLNs August 2010
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.
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
TENTATIVE_NCE_LIFETIME+ 20 seconds
Host Constants:
RTR_SOLICITATION_INTERVAL* 10 seconds
MAX_RTR_SOLICITATIONS 3 transmissions
MAX_RTR_SOLICITATION_INTERVAL+ 60 seconds
10. Examples
10.1. Message Examples
The following diagrams show the two-step process of the optimized
Neighbor Discovery mechanisms for bootstrapping and Address
Registration.
Shelby, et al. Expires February 4, 2011 [Page 37]
Internet-Draft ND Optimization for LLNs August 2010
Node Router
| |
| ---------- Router Solicitation --------> |
| [SLLAO] |
| |
| <-------- Router Advertisement --------- |
| [PIO + 6CO + ABRO + SLLAO] |
Figure 2: Basic Router Solicitation/Router Advertisement exchange
between a node and 6LR or 6LBR
Node Router
| |
| ------- NS with Address Registration -----> |
| [ARO + SLLAO] |
| |
| <-----NA with Address Registration --------- |
| [ARO with Status] |
Figure 3: Neighbor Discovery Address Registration
10.2. Host Bootstrapping Example
The following example describes the address bootstrapping scenarios
using the optimized 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
Shelby, et al. Expires February 4, 2011 [Page 38]
Internet-Draft ND Optimization for LLNs August 2010
for link-layer security. The methods of how to establish the link-
layer security is out of scope of this document. In this example an
IEEE 802.15.4 6LN forms a 16-bit short-address based IPv6 addresses
without using DHCPv6 (i.e., the M flag is not set in the Router
Advertisements).
1. After obtaining link-level security, a 6LN assigns a link-local
IPv6 address to itself. A link-local IPv6 address is configured
based on the 6LN's EUI-64 link-layer address formed as per [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 SLLA Option set to its EUI-64 link-local address. If the 6LN was
able to obtain the link-layer address of a router through its link-
layer operations then the 6LN may form a link-local destination IPv6
address for the router and send it a unicast RS. The 6LR responds
with a unicast RA to the IP source using the SLLAO from the RS (it
created a tentative NCE).
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 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 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.
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 is used,
the effect of the 6LRs and hosts following the above bootstrapping is
a "wavefront" of 6LRs and host being configured spreading from the
6LBRs. First the hosts and 6LRs that can directly reach a 6LBR would
receive one or more RAs and configure and register their IPv6
addresses. Once that is done they would enable the routing protocol
Shelby, et al. Expires February 4, 2011 [Page 39]
Internet-Draft ND Optimization for LLNs August 2010
and start sending out Router Advertisements. That would result in a
new set of 6LRs and hosts to receive responses to their Router
Solicitations, form and register their addresses, etc. That repeats
until all of the 6LRs and hosts have been configured.
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 optional multihop DAD is
performed with the 6LBR. The neighbor cache management of a router
and address resolution among the neighboring routers are described in
Section 6.5.2 and Section 6.5.3, 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 multicast RS and receives an RA from one
or more neighboring 6LRs. R1 picks one 6LR as its temporary default
router and performs address resolution via this default router.
Note, if multihop DAD is not required (e.g. in a managed network or
using EUI-64 based addresses) then it does not need to pick a
temporary default router, however it may still want to send the
initial RS message if it wants to autoconfigure its address with the
global prefix disseminated by the 6LBR.
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 nexthop 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, R3. Initially when R2
boots it sees only R1, and accordingly R2 creates a neighbor cache
entry for R1. Now assume R2 receives a valid routing update from
router R3. R2 does not have any neighbor cache entry for R3. If the
implementation of R2 supports detecting link-layer address from the
Shelby, et al. Expires February 4, 2011 [Page 40]
Internet-Draft ND Optimization for LLNs August 2010
routing information packets then it directly updates the its neighbor
cache using that link-layer information. If this is not possible,
then R2 should perform multicast NS with source set with its link-
local or global address depending on the scope of the source IP-
address received in the routing update packet. The target address of
the NS message is the source IPv6 address of the received routing
update packet. The format of the NS message is as described in
Section 4.3 of [RFC4861].
More generally any 6LR that receives a valid route-updates from a
neighboring router for which it does not have any neighbor cache
entry is required to update its neighbor cache as described above.
The router (6LR and 6LBR) IP-addresses learned via Neighbor Discovery
are not redistributed to the routing protocol.
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.
Shelby, et al. Expires February 4, 2011 [Page 41]
Internet-Draft ND Optimization for LLNs August 2010
12. IANA Considerations
The document requires three new Neighbor Discovery option types under
the subregistry "IPv6 Neighbor Discovery Option Formats":
o Address Registration Option (TBD1)
o 6LoWPAN Context Option (TBD2)
o Authoritative Border Router Option (TBD3)
For the purpose of protocol interoperability testing of this
specification, the following values are being used temporarily:
o TBD1 = 31
o TBD2 = 32
o TBD3 = 33
[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, Joseph Reddy, Robert Cragie, Mathilde Durvy
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 extensive contributions to earlier
versions of the draft, Jonathan Hui for original ideas on prefix/
context distribution and extensive contributions to earlier versions
of the draft, Geoff Mulligan for suggesting the use of Address
Registration as part of existing IPv6 Neighbor Discovery messages,
and Mathilde Durvy for helping to clarify router interaction.
14. Changelog
Changes from -11 to -12:
Shelby, et al. Expires February 4, 2011 [Page 42]
Internet-Draft ND Optimization for LLNs August 2010
o Version field of ABRO moved after Length for 32-bit alignment of
the reserved space (#90).
o Several clarifications were made on router interaction,
including a new section with router interaction examples (#91).
o Temporary Neighbor Cache Entry created upon host sending NS+ARO,
and SLLAO removed from multihop DAD NS/NA messages (#87).
Changes from -10 to -11:
o Reference to RFC1982 for version number comparison (#80)
o RA Router Lifetime field use clarified (#81)
o Make fields 16-bit rather than 32-bit where possible (#83)
o Unicast RA clarification (#84)
o Temporary ND option types (#85)
o SLLA/TLLA clarification (#86)
o GP16 as source address in initial NS clarification (#87)
Changes from -09 to -10:
o Clarifications made to Section 8.2 (#66)
o Explained behavior of Neighbor Cache (#67)
o Clarified use of SLLAO in RS and NS messages (#68)
o Added new term 6LN (#69)
o Small clarification on 6CO flag (#70)
o Defined host behavior on ARO failure better (#72)
o Added bootstrapping example for a host (#73)
o Added new Neighbor Cache Full ARO error (#74)
o Added rule on the use of the M flag (#75)
Changes from -08 to -09:
Shelby, et al. Expires February 4, 2011 [Page 43]
Internet-Draft ND Optimization for LLNs August 2010
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:
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).
Shelby, et al. Expires February 4, 2011 [Page 44]
Internet-Draft ND Optimization for LLNs August 2010
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)
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)
Shelby, et al. Expires February 4, 2011 [Page 45]
Internet-Draft ND Optimization for LLNs August 2010
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)
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.
Shelby, et al. Expires February 4, 2011 [Page 46]
Internet-Draft ND Optimization for LLNs August 2010
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).
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.
[RFC1982] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
Shelby, et al. Expires February 4, 2011 [Page 47]
Internet-Draft ND Optimization for LLNs August 2010
August 1996.
[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.
[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-08
(work in progress), July 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.
Shelby, et al. Expires February 4, 2011 [Page 48]
Internet-Draft ND Optimization for LLNs August 2010
[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.
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
Shelby, et al. Expires February 4, 2011 [Page 49]
Internet-Draft ND Optimization for LLNs August 2010
Erik Nordmark
Oracle, Inc.
17 Network Circle
Menlo Park, CA 94025
USA
Email: Erik.Nordmark@Oracle.COM
Shelby, et al. Expires February 4, 2011 [Page 50]
