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RFC 6775

Proposed STD
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Neighbor Discovery Optimization for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)

Part 1 of 3, p. 1 to 15
None       Next RFC Part

Updates:    4944


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Internet Engineering Task Force (IETF)                    Z. Shelby, Ed.
Request for Comments: 6775                                     Sensinode
Updates: 4944                                             S. Chakrabarti
Category: Standards Track                                       Ericsson
ISSN: 2070-1721                                              E. Nordmark
                                                           Cisco Systems
                                                              C. Bormann
                                                 Universitaet Bremen TZI
                                                           November 2012


    Neighbor Discovery Optimization for IPv6 over Low-Power Wireless
                   Personal Area Networks (6LoWPANs)

Abstract

   The IETF work in IPv6 over Low-power Wireless Personal Area Network
   (6LoWPAN) defines 6LoWPANs such as IEEE 802.15.4.  This and other
   similar link technologies have limited or no usage of multicast
   signaling due to energy conservation.  In addition, the wireless
   network may not strictly follow the traditional concept of IP subnets
   and IP links.  IPv6 Neighbor Discovery was not designed for non-
   transitive wireless links, as its reliance on the traditional IPv6
   link concept and its heavy use of multicast make it inefficient and
   sometimes impractical in a low-power and lossy network.  This
   document describes simple optimizations to IPv6 Neighbor Discovery,
   its addressing mechanisms, and duplicate address detection for Low-
   power Wireless Personal Area Networks and similar networks.  The
   document thus updates RFC 4944 to specify the use of the
   optimizations defined here.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6775.

Page 2 
Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................4
      1.1. The Shortcomings of IPv6 Neighbor Discovery ................5
      1.2. Applicability ..............................................6
      1.3. Goals and Assumptions ......................................7
      1.4. Substitutable Features .....................................8
   2. Terminology .....................................................9
   3. Protocol Overview ..............................................11
      3.1. Extensions to RFC 4861 ....................................11
      3.2. Address Assignment ........................................12
      3.3. Host-to-Router Interaction ................................13
      3.4. Router-to-Router Interaction ..............................14
      3.5. Neighbor Cache Management .................................14
   4. New Neighbor Discovery Options and Messages ....................15
      4.1. Address Registration Option ...............................15
      4.2. 6LoWPAN Context Option ....................................17
      4.3. Authoritative Border Router Option ........................19
      4.4. Duplicate Address Messages ................................20
   5. Host Behavior ..................................................22
      5.1. Forbidden Actions .........................................22
      5.2. Interface Initialization ..................................22
      5.3. Sending a Router Solicitation .............................23
      5.4. Processing a Router Advertisement .........................23
           5.4.1. Address Configuration ..............................23
           5.4.2. Storing Contexts ...................................24
           5.4.3. Maintaining Prefix and Context Information .........24
      5.5. Registration and Neighbor Unreachability Detection ........25
           5.5.1. Sending a Neighbor Solicitation ....................25
           5.5.2. Processing a Neighbor Advertisement ................25
           5.5.3. Recovering from Failures ...........................26
      5.6. Next-Hop Determination ....................................26
      5.7. Address Resolution ........................................27

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      5.8. Sleeping ..................................................27
           5.8.1. Picking an Appropriate Registration Lifetime .......27
           5.8.2. Behavior on Wakeup .................................28
   6. Router Behavior for 6LRs and 6LBRs .............................28
      6.1. Forbidden Actions .........................................28
      6.2. Interface Initialization ..................................29
      6.3. Processing a Router Solicitation ..........................29
      6.4. Periodic Router Advertisements ............................30
      6.5. Processing a Neighbor Solicitation ........................30
           6.5.1. Checking for Duplicates ............................30
           6.5.2. Returning Address Registration Errors ..............31
           6.5.3. Updating the Neighbor Cache ........................31
           6.5.4. Next-Hop Determination .............................32
           6.5.5. Address Resolution between Routers .................32
   7. Border Router Behavior .........................................32
      7.1. Prefix Determination ......................................33
      7.2. Context Configuration and Management ......................33
   8. Substitutable Feature Behavior .................................34
      8.1. Multihop Prefix and Context Distribution ..................34
           8.1.1. 6LBRs Sending Router Advertisements ................35
           8.1.2. Routers Sending Router Solicitations ...............35
           8.1.3. Routers Processing Router Advertisements ...........35
           8.1.4. Storing the Information ............................36
           8.1.5. Sending Router Advertisements ......................36
      8.2. Multihop Duplicate Address Detection ......................37
           8.2.1. Message Validation for DAR and DAC .................38
           8.2.2. Conceptual Data Structures .........................39
           8.2.3. 6LR Sending a Duplicate Address Request ............39
           8.2.4. 6LBR Receiving a Duplicate Address Request .........39
           8.2.5. Processing a Duplicate Address Confirmation ........40
           8.2.6. Recovering from Failures ...........................40
   9. Protocol Constants .............................................41
   10. Examples ......................................................42
      10.1. Message Examples .........................................42
      10.2. Host Bootstrapping Example ...............................43
           10.2.1. Host Bootstrapping Messages .......................45
      10.3. Router Interaction Example ...............................46
           10.3.1. Bootstrapping a Router ............................46
           10.3.2. Updating the Neighbor Cache .......................47
   11. Security Considerations .......................................47
   12. IANA Considerations ...........................................48
   13. Interaction with Other Neighbor Discovery Extensions ..........49
   14. Guidelines for New Features ...................................49
   15. Acknowledgments ...............................................52
   16. References ....................................................52
      16.1. Normative References .....................................52
      16.2. Informative References ...................................53

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1.  Introduction

   The IPv6-over-IEEE 802.15.4 [RFC4944] document specifies how IPv6 is
   carried over an IEEE 802.15.4 network with the help of an adaptation
   layer that sits between the Media Access Control (MAC) layer and the
   IP network layer.  A link in a Low-power Wireless Personal Area
   Network (LoWPAN) is characterized as lossy, low-power, low-bit-rate,
   short-range; with many nodes saving energy with long sleep periods.
   Multicast as used in IPv6 Neighbor Discovery (ND) [RFC4861] is not
   desirable in such a wireless low-power and lossy network.  Moreover,
   LoWPAN links are asymmetric and non-transitive in nature.  A LoWPAN
   is potentially composed of a large number of overlapping radio
   ranges.  Although a given radio range has broadcast capabilities, the
   aggregation of these is a complex Non-Broadcast Multiple Access
   (NBMA) [RFC2491] structure with generally no LoWPAN-wide multicast
   capabilities.  Link-local scope is in reality defined by reachability
   and radio strength.  Thus, we can consider a LoWPAN to be made up of
   links with undetermined connectivity properties as in [RFC5889],
   along with the corresponding address model assumptions defined
   therein.

   This specification introduces the following optimizations to IPv6
   Neighbor Discovery [RFC4861] specifically aimed at low-power and
   lossy networks such as LoWPANs:

   o  Host-initiated interactions to allow for sleeping hosts.

   o  Elimination of multicast-based address resolution for hosts.

   o  A host address registration feature using a new option in unicast
      Neighbor Solicitation (NS) and Neighbor Advertisement (NA)
      messages.

   o  A new Neighbor Discovery option to distribute 6LoWPAN header
      compression context to hosts.

   o  Multihop distribution of prefix and 6LoWPAN header compression
      context.

   o  Multihop Duplicate Address Detection (DAD), which uses two new
      ICMPv6 message types.

   The two multihop items can be substituted by a routing protocol
   mechanism if that is desired; see Section 1.4.

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   The document defines three new ICMPv6 message options: the Address
   Registration Option (ARO), the Authoritative Border Router Option
   (ABRO), and the 6LoWPAN Context Option (6CO).  It also defines two
   new ICMPv6 message types: the Duplicate Address Request (DAR) and the
   Duplicate Address Confirmation (DAC).

1.1.  The Shortcomings of IPv6 Neighbor Discovery

   IPv6 Neighbor Discovery [RFC4861] provides several important
   mechanisms used for router discovery, address resolution, Duplicate
   Address Detection, and Redirect messages, along with prefix and
   parameter discovery.

   Following power-on and initialization of the network in IPv6 Ethernet
   networks, a node joins the solicited-node multicast address on the
   interface and then performs Duplicate Address Detection (DAD) for the
   acquired link-local address by sending a solicited-node multicast
   message to the link.  After that, it sends multicast messages to the
   all-routers multicast address to solicit Router Advertisements (RAs).
   If the host receives a valid RA with the A (autonomous address
   configuration) flag, it autoconfigures the IPv6 address with the
   advertised prefix in the RA message.  Besides this, the IPv6 routers
   usually send RAs periodically on the network.  RAs are sent to the
   all-nodes multicast address.  Nodes send Neighbor Solicitation/
   Neighbor Advertisement messages to resolve the IPv6 address of the
   destination on the link.  The Neighbor Solicitation messages used for
   address resolution are multicast.  The Duplicate Address Detection
   procedure and the use of periodic Router Advertisement messages
   assume that the nodes are powered on and reachable most of the time.

   In Neighbor Discovery, the routers find the hosts by assuming that a
   subnet prefix maps to one broadcast domain, and then they multicast
   Neighbor Solicitation messages to find the host and its link-layer
   address.  Furthermore, the DAD use of multicast assumes that all
   hosts that autoconfigure IPv6 addresses from the same prefix can be
   reached using link-local multicast messages.

   Note that the L (on-link) bit in the Prefix Information Option (PIO)
   can be set to zero in Neighbor Discovery, which makes the host not
   use multicast Neighbor Solicitation (NS) messages for address
   resolution of other hosts, but routers still use multicast NS
   messages to find the hosts.

   Due to the lossy nature of wireless communication and a changing
   radio environment, the IPv6-link node-set may change due to external
   physical factors.  Thus, the link is often unstable, and the nodes
   appear to be moving without necessarily moving physically.

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   A LoWPAN can use two types of link-layer addresses: 16-bit short
   addresses and 64-bit unique addresses as defined in [RFC4944].
   Moreover, the available link-layer payload size is on the order of
   less than 100 bytes; thus, header compression is very useful.

   Considering the above characteristics in a LoWPAN, and the IPv6
   Neighbor Discovery [RFC4861] protocol design, some optimizations and
   extensions to Neighbor Discovery are useful for the wide deployment
   of IPv6 over low-power and lossy networks (example: 6LoWPAN and other
   homogeneous low-power networks).

1.2.  Applicability

   In its Section 1, [RFC4861] foresees a document that covers operating
   IP over a particular link type and defines an exception to the
   otherwise general applicability of unmodified [RFC4861].  The present
   specification improves the usage of IPv6 Neighbor Discovery for
   LoWPANs in order to save energy and processing power of such nodes.
   This document thus updates [RFC4944] to specify the use of the
   optimizations defined here.

   The applicability of this specification is limited to LoWPANs where
   all nodes on the subnet implement these optimizations in a
   homogeneous way.  Although it is noted that some of these
   optimizations may be useful outside of 6LoWPANs, for example, in
   general IPv6 low-power and lossy networks and possibly even in
   combination with [RFC4861], the usage of such combinations is out of
   scope of this document.

   In this document, we specify a set of behaviors between hosts and
   routers in LoWPANs.  An implementation that adheres to this document
   MUST implement those behaviors.  The document also specifies a set of
   behaviors (multihop prefix or context dissemination and, separately,
   multihop Duplicate Address Detection) that are needed in route-over
   configurations.  An implementation of this specification MUST support
   those pieces, unless the implementation supports some alternative
   ("substitute") from some other specification.

   The optimizations described in this document apply to different
   topologies.  They are most useful for route-over and mesh-under
   configurations in Mesh topologies.  However, Star topology
   configurations will also benefit from the optimizations due to
   reduced signaling, robust handling of the non-transitive link, and
   header compression context information.

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1.3.  Goals and Assumptions

   The document has the following main goals and assumptions.

   Goals:

   o  Optimize Neighbor Discovery with a mechanism that is minimal yet
      sufficient for the operation in both mesh-under and route-over
      configurations.

   o  Minimize signaling by avoiding the use of multicast flooding and
      reducing the use of link-scope multicast messages.

   o  Optimize the interfaces between hosts and their default routers.

   o  Provide support for sleeping hosts.

   o  Disseminate context information to hosts as needed by 6LoWPAN
      header compression [RFC6282].

   o  Disseminate context information and prefix information from the
      border to all routers in a LoWPAN.

   o  Provide a multihop Duplicate Address Detection mechanism suitable
      for route-over LoWPANs.

   Assumptions:

   o  64-bit Extended Unique Identifier (EUI-64) [EUI64] addresses are
      globally unique, and the LoWPAN is homogeneous.

   o  All nodes in the network have an EUI-64 Interface ID in order to
      do address autoconfiguration and detect duplicate addresses.

   o  The link-layer technology is assumed to be low-power and lossy,
      exhibiting undetermined connectivity, such as IEEE 802.15.4
      [RFC4944].  However, the address registration mechanism might be
      useful for other link-layer technologies.

   o  A 6LoWPAN is configured to share one or more global IPv6 address
      prefixes to enable hosts to move between routers in the LoWPAN
      without changing their IPv6 addresses.

   o  When using the multihop DAD mechanism (Section 8.2), each 6LoWPAN
      Router (6LR) registers with all the 6LoWPAN Border Routers (6LBRs)
      available in the LoWPAN.

Top      ToC       Page 8 
   o  If IEEE 802.15.4 16-bit short addresses are used, then some
      technique is used to ensure the uniqueness of those link-layer
      addresses.  That could be done using DHCPv6, Address Registration
      Option-based Duplicate Address Detection (specified in
      Section 8.2), or other techniques outside of the scope of this
      document.

   o  In order to preserve the uniqueness of addresses (see Section 5.4
      of [RFC4862]) not derived from an EUI-64, they must be either
      assigned or checked for duplicates in the same way throughout the
      LoWPAN.  This can be done using DHCPv6 for assignment and/or using
      the Duplicate Address Detection mechanism specified in Section 8.2
      (or any other protocols developed for that purpose).

   o  In order for 6LoWPAN header compression [RFC6282] to operate
      correctly, the compression context must match for all the hosts,
      6LRs, and 6LBRs that can send, receive, or forward a given packet.
      If Section 8.1 is used to distribute context information, this
      implies that all the 6LBRs must coordinate the context information
      they distribute within a single LoWPAN.

   o  This specification describes the operation of ND within a single
      LoWPAN.  The participation of a node in multiple LoWPANs
      simultaneously may be possible but is out of scope of this
      document.

   o  Since the LoWPAN shares its prefix(es) throughout the network,
      mobility of nodes within the LoWPAN is transparent.  Inter-LoWPAN
      mobility is out of scope of this document.

1.4.  Substitutable Features

   This document defines the optimization of Neighbor Discovery messages
   for the host-router interface and introduces two new mechanisms in a
   route-over topology.

   Unless specified otherwise (in a document that defines a routing
   protocol that is used in a 6LoWPAN), this document applies to
   networks with any routing protocol.  However, because the routing
   protocol may provide good alternate mechanisms, this document defines
   certain features as "substitutable", meaning they can be substituted
   by a routing protocol specification that provides mechanisms
   achieving the same overall effect.

Top      ToC       Page 9 
   The features that are substitutable (individually or in a group):

   o  Multihop distribution of prefix and 6LoWPAN header compression
      context

   o  Multihop Duplicate Address Detection

   Thus, multihop prefix distribution (the ABRO) and the 6LoWPAN Context
   Option (6CO) for distributing header compression contexts go hand in
   hand.  If substitution is intended for one of them, then both of them
   MUST be substituted.

   Guidelines for feature implementation and deployment are provided in
   Section 14.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   This specification requires readers to be familiar with all the terms
   and concepts that are discussed in "Neighbor Discovery for IP
   version 6 (IPv6)" [RFC4861], "IPv6 Stateless Address
   Autoconfiguration" [RFC4862], "IPv6 over Low-Power Wireless Personal
   Area Networks (6LoWPANs): Overview, Assumptions, Problem Statement,
   and Goals" [RFC4919], "Transmission of IPv6 Packets over IEEE
   802.15.4 Networks" [RFC4944], and "IP Addressing Model in Ad Hoc
   Networks" [RFC5889].

   This specification makes extensive use of the same terminology
   defined in [RFC4861], unless otherwise defined below.

   6LoWPAN link:
      A wireless link determined by single IP hop reachability of
      neighboring nodes.  These are considered links with undetermined
      connectivity properties as in [RFC5889].

   6LoWPAN Node (6LN):
      A 6LoWPAN node is any host or router participating in a LoWPAN.
      This term is used when referring to situations in which either a
      host or router can play the role described.

   6LoWPAN Router (6LR):
      An intermediate router in the LoWPAN that is able to send and
      receive Router Advertisements (RAs) and Router Solicitations (RSs)
      as well as forward and route IPv6 packets.  6LoWPAN routers are
      present only in route-over topologies.

Top      ToC       Page 10 
   6LoWPAN Border Router (6LBR):
      A border router located at the junction of separate 6LoWPAN
      networks or between a 6LoWPAN network and another IP network.
      There may be one or more 6LBRs at the 6LoWPAN network boundary.  A
      6LBR is the responsible authority for IPv6 prefix propagation for
      the 6LoWPAN network it is serving.  An isolated LoWPAN also
      contains a 6LBR in the network, which provides the prefix(es) for
      the isolated network.

   Router:
      Either a 6LR or a 6LBR.  Note that nothing in this document
      precludes a node being a router on some interfaces and a host on
      other interfaces as allowed by [RFC2460].

   Mesh-under:
      A topology where nodes are connected to a 6LBR through a mesh
      using link-layer forwarding.  Thus, in a mesh-under configuration,
      all IPv6 hosts in a LoWPAN are only one IP hop away from the 6LBR.
      This topology simulates the typical IP-subnet topology with one
      router with multiple nodes in the same subnet.

   Route-over:
      A topology where hosts are connected to the 6LBR through the use
      of intermediate layer-3 (IP) routing.  Here, hosts are typically
      multiple IP hops away from a 6LBR.  The route-over topology
      typically consists of a 6LBR, a set of 6LRs, and hosts.

   Non-transitive link:
      A link that exhibits asymmetric reachability as defined in
      Section 2.2 of [RFC4861].

   IP-over-foo document:
      A specification that covers operating IP over a particular link
      type, for example, [RFC4944] "Transmission of IPv6 Packets over
      IEEE 802.15.4 Networks".

   Header compression context:
      Address information shared across a LoWPAN and used by 6LoWPAN
      header compression [RFC6282] to enable the elision of information
      that would otherwise be sent repeatedly.  In a "context", a
      (potentially partial) address is associated with a Context
      Identifier (CID), which is then used in header compression as a
      shortcut for (parts of) a source or destination address.

Top      ToC       Page 11 
   Registration:
      The process during which a LoWPAN node sends a Neighbor
      Solicitation message with an Address Registration Option to a
      router creating a Neighbor Cache Entry (NCE) for the LoWPAN node
      with a specific timeout.  Thus, for 6LoWPAN routers, the Neighbor
      Cache doesn't behave like a cache.  Instead, it behaves as a
      registry of all the host addresses that are attached to the
      router.

3.  Protocol Overview

   These Neighbor Discovery optimizations are applicable to both
   mesh-under and route-over configurations.  In a mesh-under
   configuration, only 6LoWPAN Border Routers and hosts exist; there are
   no 6LoWPAN routers in mesh-under topologies.

   The most important part of the optimizations is the evolved host-to-
   router interaction that allows for sleeping nodes and avoids using
   multicast Neighbor Discovery messages except for the case of a host
   finding an initial set of default routers, and redoing such
   determination when that set of routers have become unreachable.

   The protocol also provides for header compression [RFC6282] by
   carrying header compression information in a new option in Router
   Advertisement messages.

   In addition, there are separate mechanisms that can be used between
   6LRs and 6LBRs to perform multihop Duplicate Address Detection and
   distribution of the prefix and compression context information from
   the 6LBRs to all the 6LRs, which in turn use normal Neighbor
   Discovery mechanisms to convey this information to the hosts.

   The protocol is designed so that the host-to-router interaction is
   not affected by the configuration of the 6LoWPAN; the host-to-router
   interaction is the same in a mesh-under and route-over configuration.

3.1.  Extensions to RFC 4861

   This document specifies the following optimizations and extensions to
   IPv6 Neighbor Discovery [RFC4861]:

   o  Host-initiated refresh of Router Advertisement information.  This
      removes the need for periodic or unsolicited Router Advertisements
      from routers to hosts.

   o  No Duplicate Address Detection (DAD) is performed if EUI-64-based
      IPv6 addresses are used (as these addresses are assumed to be
      globally unique).

Top      ToC       Page 12 
   o  DAD is optional if DHCPv6 is used to assign addresses.

   o  A new address registration mechanism using a new Address
      Registration Option between hosts and routers.  This removes the
      need for routers to use multicast Neighbor Solicitations to find
      hosts and supports sleeping hosts.  This also enables the same
      IPv6 address prefix(es) to be used across a route-over 6LoWPAN.
      It provides the host-to-router interface for Duplicate Address
      Detection.

   o  A new Router Advertisement option, the 6LoWPAN Context Option, for
      context information used by 6LoWPAN header compression.

   o  A new mechanism to perform Duplicate Address Detection across a
      route-over 6LoWPAN using the new Duplicate Address Request and
      Duplicate Address Confirmation messages.

   o  New mechanisms to distribute prefixes and context information
      across a route-over network that uses a new Authoritative Border
      Router Option to control the flooding of configuration changes.

   o  A few new default protocol constants are introduced, and some
      existing Neighbor Discovery protocol constants are tuned.

3.2.  Address Assignment

   Hosts in a 6LoWPAN configure their IPv6 addresses as specified in
   [RFC4861] and [RFC4862] based on the information received in Router
   Advertisement messages.  The use of the M (managed address
   configuration) flag in this optimization is, however, more
   restrictive than in [RFC4861].  When the M flag is set, a host is
   assumed to use DHCPv6 to assign any non-EUI-64 addresses.  When the M
   flag is not set, the nodes in the LoWPAN support Duplicate Address
   Detection; thus, a host can then safely use the address registration
   mechanism to check non-EUI-64 addresses for uniqueness.

   6LRs MAY use the same mechanisms to configure their IPv6 addresses.

   The 6LBRs are responsible for managing the prefix(es) assigned to the
   6LoWPAN, using manual configuration, DHCPv6 Prefix Delegation
   [RFC3633], or other mechanisms.  In an isolated LoWPAN, a Unique
   Local Address (ULA) [RFC4193] prefix SHOULD be generated by the 6LBR.

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3.3.  Host-to-Router Interaction

   A host sends Router Solicitation messages at startup and also when
   the Neighbor Unreachability Detection (NUD) of one of its default
   routers fails.

   Hosts receive Router Advertisement messages typically containing the
   Authoritative Border Router Option (ABRO) and may optionally contain
   one or more 6LoWPAN Context Options (6COs) in addition to the
   existing Prefix Information Options (PIOs) as described in [RFC4861].

   When a host has configured a non-link-local IPv6 address, it
   registers that address with one or more of its default routers using
   the Address Registration Option (ARO) in an NS message.  The host
   chooses a lifetime of the registration and repeats the ARO
   periodically (before the lifetime runs out) to maintain the
   registration.  The lifetime should be chosen in such a way as to
   maintain the registration even while a host is sleeping.  Likewise,
   mobile nodes that often change their point of attachment should use a
   suitably short lifetime.  See Section 5.5 for registration details
   and Section 9 for protocol constants.

   The registration fails when an ARO is returned to the host with a
   non-zero Status.  One reason may be that the router determines that
   the IPv6 address is already used by another host, i.e., is used by a
   host with a different EUI-64.  This can be used to support
   non-EUI-64-based addresses such as temporary IPv6 addresses [RFC4941]
   or addresses based on an Interface ID that is an IEEE 802.15.4 16-bit
   short address.  Failure can also occur if the Neighbor Cache on that
   router is full.

   The re-registration of an address can be combined with Neighbor
   Unreachability Detection (NUD) of the router, since both use unicast
   Neighbor Solicitation messages.  This makes things efficient when a
   host wakes up to send a packet and needs to both perform NUD to check
   that the router is still reachable and refresh its registration with
   the router.

   The response to an address registration might not be immediate, since
   in route-over configurations the 6LR might perform Duplicate Address
   Detection against the 6LBR.  A host retransmits the Address
   Registration Option until it is acknowledged by the receipt of an
   Address Registration Option.

   As part of the optimizations, address resolution is not performed by
   multicasting Neighbor Solicitation messages as in [RFC4861].
   Instead, the routers maintain Neighbor Cache Entries for all
   registered IPv6 addresses.  If the address is not in the Neighbor

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   Cache in the router, then the address either doesn't exist, is
   assigned to a host attached to some other router in the 6LoWPAN, or
   is external to the 6LoWPAN.  In a route-over configuration, the
   routing protocol is used to route such packets toward the
   destination.

3.4.  Router-to-Router Interaction

   The new router-to-router interaction is only for the route-over
   configuration where 6LRs are present.  See also Section 1.4.

   6LRs MUST act like a host during system startup and prefix
   configuration by sending Router Solicitation messages and
   autoconfiguring their IPv6 addresses, unlike routers in [RFC4861].

   When multihop prefix and context dissemination are used, then the
   6LRs store the ABRO, 6CO, and prefix information received (directly
   or indirectly) from the 6LBRs and redistribute this information in
   the Router Advertisement they send to other 6LRs or send to hosts in
   response to a Router Solicitation.  There is a Version Number field
   in the ABRO (see Section 4.3), which is used to limit the flooding of
   updated information between the 6LRs.

   A 6LR can perform Duplicate Address Detection against one or more
   6LBRs using the new Duplicate Address Request (DAR) and Duplicate
   Address Confirmation (DAC) messages, which carry the information from
   the Address Registration Option.  The DAR and DAC messages will be
   forwarded between the 6LR and 6LBRs; thus, the [RFC4861] rule for
   checking hop limit=255 does not apply to the DAR and DAC messages.
   Those multihop DAD messages MUST NOT modify any Neighbor Cache
   Entries on the routers, since we do not have the security benefits
   provided by the hop limit=255 check.

3.5.  Neighbor Cache Management

   The use of explicit registrations with lifetimes, plus the desire to
   not multicast Neighbor Solicitation messages for hosts, imply that we
   manage the Neighbor Cache Entries (NCEs) slightly differently than in
   [RFC4861].  This results in three different types of NCEs, and the
   types specify how those entries can be removed:

   Garbage-collectible:  Entries that are subject to the normal rules in
                         [RFC4861] that allow for garbage collection
                         when low on memory.

   Registered:           Entries that have an explicit registered
                         lifetime and are kept until this lifetime
                         expires or they are explicitly unregistered.

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   Tentative:            Entries that are temporary with a short
                         lifetime, which typically get converted to
                         Registered entries.

   Note that the type of the NCE is orthogonal to the states specified
   in [RFC4861].

   When a host interacts with a router by sending Router Solicitations,
   this results in a Tentative NCE.  Once a router has successfully had
   a node register with it, the result is a Registered NCE.  When
   routers send RAs to hosts, and when routers receive RA messages or
   receive multicast NS messages from other routers, the result is
   Garbage-collectible NCEs.  There can only be one kind of NCE for an
   IP address at a time.

   Neighbor Cache Entries on routers can additionally be added or
   deleted by a routing protocol used in the 6LoWPAN.  This is useful if
   the routing protocol carries the link-layer addresses of the
   neighboring routers.  Depending on the details of such routing
   protocols, such NCEs could be either Registered or
   Garbage-collectible.



(page 15 continued on part 2)

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