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Versions: (draft-shelby-6lowpan-nd) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 RFC 6775

6LoWPAN Working Group                                     Z. Shelby, Ed.
Internet-Draft                                                 Sensinode
Updates: 4944 (if approved)                               S. Chakrabarti
Intended status: Standards Track                                Ericsson
Expires: February 25, 2013                                   E. Nordmark
                                                           Cisco Systems
                                                         August 24, 2012


    Neighbor Discovery Optimization for Low Power and Lossy Networks
                               (6LoWPAN)
                        draft-ietf-6lowpan-nd-21

Abstract

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

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on February 25, 2013.

Copyright Notice




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   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  . . . . . . . . . . . . . . . . . .  6
     1.4.  Substitutable Features . . . . . . . . . . . . . . . . . .  8
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  9
   3.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . . 10
     3.1.  Extensions to RFC4861  . . . . . . . . . . . . . . . . . . 11
     3.2.  Address Assignment . . . . . . . . . . . . . . . . . . . . 12
     3.3.  Host-to-Router Interaction . . . . . . . . . . . . . . . . 12
     3.4.  Router-to-Router Interaction . . . . . . . . . . . . . . . 13
     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 . . . . . . . . . . . . 18
     4.4.  Duplicate Address messages . . . . . . . . . . . . . . . . 20
   5.  Host Behavior  . . . . . . . . . . . . . . . . . . . . . . . . 21
     5.1.  Forbidden Actions  . . . . . . . . . . . . . . . . . . . . 21
     5.2.  Interface Initialization . . . . . . . . . . . . . . . . . 21
     5.3.  Sending a Router Solicitation  . . . . . . . . . . . . . . 22
     5.4.  Processing a Router Advertisement  . . . . . . . . . . . . 22
       5.4.1.  Address configuration  . . . . . . . . . . . . . . . . 23
       5.4.2.  Storing Contexts . . . . . . . . . . . . . . . . . . . 23
       5.4.3.  Maintaining Prefix and Context Information . . . . . . 23
     5.5.  Registration and Neighbor Unreachability Detection . . . . 24
       5.5.1.  Sending a Neighbor Solicitation  . . . . . . . . . . . 24
       5.5.2.  Processing a Neighbor Advertisement  . . . . . . . . . 25
       5.5.3.  Recovering from Failures . . . . . . . . . . . . . . . 25
     5.6.  Next-hop Determination . . . . . . . . . . . . . . . . . . 26
     5.7.  Address Resolution . . . . . . . . . . . . . . . . . . . . 26
     5.8.  Sleeping . . . . . . . . . . . . . . . . . . . . . . . . . 26



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       5.8.1.  Picking an Appropriate Registration Lifetime . . . . . 27
       5.8.2.  Behavior on Wakeup . . . . . . . . . . . . . . . . . . 27
   6.  Router Behavior for 6LR and 6LBR . . . . . . . . . . . . . . . 27
     6.1.  Forbidden Actions  . . . . . . . . . . . . . . . . . . . . 28
     6.2.  Interface Initialization . . . . . . . . . . . . . . . . . 28
     6.3.  Processing a Router Solicitation . . . . . . . . . . . . . 28
     6.4.  Periodic Router Advertisements . . . . . . . . . . . . . . 29
     6.5.  Processing a Neighbor Solicitation . . . . . . . . . . . . 30
       6.5.1.  Checking for Duplicates  . . . . . . . . . . . . . . . 30
       6.5.2.  Returning Address Registration Errors  . . . . . . . . 30
       6.5.3.  Updating the Neighbor Cache  . . . . . . . . . . . . . 31
       6.5.4.  Next-hop Determination . . . . . . . . . . . . . . . . 31
       6.5.5.  Address Resolution between Routers . . . . . . . . . . 31
   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  . . . . . . . . . 34
       8.1.2.  Routers Sending Router Solicitations . . . . . . . . . 35
       8.1.3.  Routers Processing Router Advertisements . . . . . . . 35
       8.1.4.  Storing the Information  . . . . . . . . . . . . . . . 35
       8.1.5.  Sending Router Advertisements  . . . . . . . . . . . . 36
     8.2.  Multihop Duplicate Address Detection . . . . . . . . . . . 36
       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 . . . . . . . . . . . . . . . . . . . . . . 40
   10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
     10.1. Message Examples . . . . . . . . . . . . . . . . . . . . . 41
     10.2. Host Bootstrapping Example . . . . . . . . . . . . . . . . 43
       10.2.1. Host Bootstrapping Messages  . . . . . . . . . . . . . 44
     10.3. Router Interaction Example . . . . . . . . . . . . . . . . 47
       10.3.1. Bootstrapping a Router . . . . . . . . . . . . . . . . 47
       10.3.2. Updating the Neighbor Cache  . . . . . . . . . . . . . 47
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 48
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 49
   13. Interaction with other Neighbor Discovery Extensions . . . . . 50
   14. Guideline for New Features . . . . . . . . . . . . . . . . . . 50
   15. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 52
   16. Changelog  . . . . . . . . . . . . . . . . . . . . . . . . . . 52
   17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 59
     17.1. Normative References . . . . . . . . . . . . . . . . . . . 59
     17.2. Informative References . . . . . . . . . . . . . . . . . . 60
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 61



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1.  Introduction

   The IPv6-over-IEEE 802.15.4 [RFC4944] document specifies how IPv6 is
   carried over an IEEE 802.15.4 network with the help of an adaptation
   layer which sits between the MAC layer and the IP network layer.  A
   link in a 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 [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 [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 and Neighbor Advertisement messages.

   o  A new optional 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 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.

   The document defines three new ICMPv6 message options: the Address
   Registration, Authoritative Border Router, and 6LoWPAN Context
   options.  It also defines two new ICMPv6 message types: the Duplicate
   Address Request and Duplicate Address Confirmation.




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1.1.  The Shortcomings of IPv6 Neighbor Discovery

   IPv6 Neighbor Discovery [RFC4861] provides several important
   mechanisms used for Router Discovery, Address Resolution, Duplicate
   Address Detection, Redirect, along with Prefix and Parameter
   Discovery.

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

   In Neighbor Discovery the routers find the hosts by assuming that a
   subnet prefix maps to one broadcast domain, and then multicast
   Neighbor Solicitation messages to find the host and its link-layer
   address.  Furthermore, the DAD 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 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.

   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.




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   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-powered 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.
   The 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 6LoWPAN, 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) which are needed in route-over
   configurations.  An implementation of this specification MUST support
   those pieces, unless the implementation supports some alternative
   ("substitute") from some some other specification.

   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.

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



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      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  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  Multihop duplicate address detection mechanism suitable for route-
      over LoWPANs.

   Assumptions:

   o  EUI-64 addresses are globally unique and the LoWPAN is
      homogeneous.

   o  All nodes in the network 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.

   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 of Section 8.2 each 6LR
      registers with all the 6LBRs available in the LoWPAN.

   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, the 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,
      [RFC4862]) not derived from an EUI-64, they must be either



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      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.

   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 (ABRO option) and 6LoWPAN Context
   Option (6CO, for distributing Header Compression Contexts) go hand-
   in-hand.  If substitution is intended for one of them, then both of
   them MUST be substituted.

   A guideline for feature implementation and deployment is provided at



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   the end of the 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" [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, Router Solicitations as well as
      forward and route IPv6 packets. 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.







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   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 which 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, which is then used in header compression as a shortcut
      for (parts of) a source or destination address.

   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 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



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   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 between 6LRs and
   6LBRs to perform multihop Duplicate Address Detection and
   distribution of the Prefix and compression Context information from
   the 6LBRs to all the 6LRs, which in turn use normal Neighbor
   Discovery mechanisms to convey this information to the hosts.

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

3.1.  Extensions to RFC4861

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

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

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

   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 for Context information used by
      6LoWPAN header compression.



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

   o  New mechanisms to distribute Prefixes and Context information
      across a route-over network which uses a new Authoritative Border
      Router option to control the flooding of configuration changes.

   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 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 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 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
   Neighbor Unreachability Detection towards 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 (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.  The lifetime should be chosen in such a way as to
   maintain the registration even while a host is sleeping.  Likewise,



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   mobile nodes that change their point of attachment often, 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 option is returned to the host
   with a non-zero Status.  One reason may be that the router determines
   that the IPv6 address is already used by another host, that is, is
   used by a host with a different EUI-64.  This can be used to support
   non-EUI-64 based addresses such as temporary IPv6 addresses [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 an address can be combined with Neighbor
   Unreachability Detection (NUD) of the router since both use unicast
   Neighbor Solicitation messages.  This makes things efficient when a
   host wakes up to send a packet and both need to perform NUD to check
   that the router is still reachable, and refresh its registration with
   the router.

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

   As part of the optimizations, Address Resolution is not performed by
   multicasting Neighbor Solicitation messages as in [RFC4861].
   Instead, the routers maintain Neighbor Cache entries for all
   registered IPv6 addresses.  If the address is not in the Neighbor
   Cache in the router, then the address either doesn't exist, or is
   assigned 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



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   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.

   A 6LR can perform Duplicate Address Detection against one or more
   6LBRs using the new Duplicate Address Request (DAR) and Confirmation
   (DAC) messages, which carry the information from the Address
   Registration option.  The DAR and DAC messages will be forwarded
   between the 6LR and 6LBRs thus the [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 (NCE) 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.

   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



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   the routing protocol carries the link-layer addresses of the
   neighboring routers.  Depending on the details of such routing
   protocols such NCEs could be either Registered or Garbage-
   collectible.


4.  New Neighbor Discovery Options and Messages

   This section defines new Neighbor Discovery message options used by
   this specification.  The Address Registration Option is used by
   hosts, whereas the Authoritative Border Router Option and 6LoWPAN
   Context Option are used in the substitable router-to-router
   interaction.  This section also defines the new router-to-router
   Duplicate Address Request and Confirmation messages.

4.1.  Address Registration Option

   The routers need to know the set of host IP addresses that are
   directly reachable and their corresponding link-layer addresses.
   This needs to be maintained as the radio reachability changes.  For
   this purpose an Address Registration Option (ARO) is introduced,
   which can be included in unicast Neighbor Solicitation (NS) messages
   sent by hosts.  Thus it can be included in the unicast NS messages
   that a host sends as part of Neighbor Unreachability Detection to
   determine that it can still reach a default router.  The ARO is used
   by the receiving router to reliably maintain its Neighbor Cache.  The
   same option is included in corresponding Neighbor Advertisement (NA)
   messages with a Status field indicating the success or failure of the
   registration.  This option is always host initiated.

   The information contained in the ARO is also included in the multihop
   DAR and DAC messages used between 6LRs to 6LBRs, but the option
   itself is not used in those messages.

   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 [EUI64] of the
   interface it is registering an address from.  This is used as a
   unique ID for the detection of duplicate addresses.  It is used to
   tell the difference between the same node re-registering its address
   and a different node (with a different EUI-64) registering an address
   that is already in use by someone else.  The EUI-64 is also used to
   deliver an NA carrying an error Status code to the EUI-64 based link-
   local IPv6 address of the host (see Section 6.5.2).




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   When the ARO is used by hosts an SLLA (Source Link-layer Address)
   option [RFC4861] MUST be included and the address that is to be
   registered MUST be the IPv6 source address of the Neighbor
   Solicitation message.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |   Length = 2  |    Status     |   Reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Reserved            |     Registration Lifetime     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                            EUI-64                             +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Type:          TBD1

   Length:        8-bit unsigned integer.  The length of the option in
                  units of 8 bytes.  Always 2.

   Status:        8-bit unsigned integer.  Indicates the status of a
                  registration in the NA response.  MUST be set to 0 in
                  NS messages.  See below.

   Reserved:      This field is unused.  It MUST be initialized to zero
                  by the sender and MUST be ignored by the receiver.

   Registration Lifetime:  16-bit unsigned integer.  The amount of time
                  in a unit of 60 seconds that the router should retain
                  the Neighbor Cache entry for the sender of the NS that
                  includes this option.

   EUI-64:        64 bits.  This field is used to uniquely identify the
                  interface of the registered address by including the
                  EUI-64 identifier [EUI64] assigned to it unmodified.

   The Status values used in Neighbor Advertisements are:










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          +--------+--------------------------------------------+
          | Status |                 Description                |
          +--------+--------------------------------------------+
          |    0   |                   Success                  |
          |    1   |              Duplicate Address             |
          |    2   |             Neighbor Cache Full            |
          |  3-255 | Allocated using Standards Action [RFC5226] |
          +--------+--------------------------------------------+

                                  Table 1

4.2.  6LoWPAN Context Option

   The optional 6LoWPAN Context Option (6CO) carries prefix information
   for LoWPAN header compression, and is similar to the Prefix
   Information Option of [RFC4861].  However, the prefixes can be remote
   as well as local to the LoWPAN since header compression potentially
   applies to all IPv6 addresses.  This option allows for the
   dissemination of multiple contexts identified by a Context Identifier
   (CID) for use as specified in [RFC6282].  A context may be a prefix
   of any length or an address (/128), and up to 16 6LoWPAN Context
   options may be carried in an Router Advertisement message.

    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 bytes.  May be 2 or 3
      depending on the length of the Context Prefix field.

   Context Length:  8-bit unsigned integer.  The number of leading bits
      in the Context Prefix field that are valid.  The value ranges from
      0 to 128.  If it is more than 64 then the Length MUST be 3.





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   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 [RFC6282].
      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.

   Valid Lifetime:  16-bit unsigned integer.  The length of time in a
      unit of 60 seconds (relative to the time the packet is received)
      that the context is valid for the purpose of header compression or
      decompression.  A value of all zero bits (0x0) indicates that this
      context entry MUST be removed immediately.

   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 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).





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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |  Length = 3   |          Version Low          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Version High         |        Valid Lifetime         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                          6LBR Address                         +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Type:          TBD3

   Length:        8-bit unsigned integer.  The length of the option in
                  units of 8 bytes.  Always 3.

   Version Low, Version High:  Together, Version Low and Version High
                  are a 32-bit unsigned integer, where Version Low is
                  the least significant 16 bits and Version High is the
                  most significant 16 bits.  The version number
                  corresponding to this set of information contained in
                  the RA message.  The authoritative 6LBR originating
                  the prefix increases this version number each time its
                  set of prefix or context information changes.

   Valid Lifetime:  16-bit unsigned integer.  The length of time in a
                  unit of 60 seconds (relative to the time the packet is
                  received) that this set of border router information
                  is valid.  A value of all zero bits (0x0) assumes a
                  default value of 10,000 (~ one week).

   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.








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4.4.  Duplicate Address messages

   For the multihop DAD exchanges between 6LR and 6LBR specified in
   Section 8.2 there are two new ICMPv6 message types called the
   Duplicate Address Request (DAR) and Duplicate Address Confirmation
   (DAC).  We avoid reusing the Neighbor Solicitation and Neighbor
   Advertisement messages for this purpose since these messages are not
   subject to the hop limit=255 check as they are forwarded by
   intermediate 6LRs.  The information contained in the messages are
   otherwise the same as would be in a Neighbor Solicitation carrying a
   Address Registration option, with the message format inlining the
   fields that are in the ARO.

   The DAR and DAC use the same message format with different ICMPv6
   type values, and the Status field is only meaningful in the DAC
   message.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Status     |   Reserved    |     Registration Lifetime     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                            EUI-64                             +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                       Registered Address                      +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IP fields:

   IPv6 source:   A non link-local address of the sending router.

   IPv6 destination:  A non link-local address of the sending router.
                  In a DAC this is just the source from the DAR.

   Hop Limit:     Set to MULTIHOP_HOPLIMIT on transmit.  MUST be ignored
                  on receipt.

   ICMP Fields:



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   Type:          TBD4 for DAR and TBD5 for DAC

   Code:          Set to zero on transmit.  MUST be ignored on receipt.

   Checksum:      The ICMP checksum.  See [RFC4443].

   Status:        8-bit unsigned integer.  Indicates the status of a
                  registration in the DAC.  MUST be set to 0 in DAR.
                  See Table 1.

   Reserved:      This field is unused.  It MUST be initialized to zero
                  by the sender and MUST be ignored by the receiver.

   Registration Lifetime:  16-bit unsigned integer.  The amount of time
                  in a unit of 60 seconds that the router should retain
                  the Neighbor Cache entry for the sender of the NS that
                  includes this option.  A value of 0 indicates in an NS
                  that the neighbor cache entry should be removed.

   EUI-64:        64 bits.  This field is used to uniquely identify the
                  interface of the registered address by including the
                  EUI-64 identifier [EUI64] assigned to it unmodified.

   Registered Address:  128-bit field.  Carries the host address, which
                  was contained in the IPv6 Source field in the NS that
                  contained the ARO option sent by the host.


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 MUST NOT 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



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   the EUI-64 identifier [EUI64] assigned to the interface as per
   [RFC4944] or the appropriate IP-over-foo document for the link, and
   then the host sends 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
   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).  An 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
   these 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, one of its
   default routers becomes unreachable, or the lifetime of the prefixes
   and contexts in the previous RA are about to expire.  The RECOMMENDED
   retransmissions is to initially send up to 3 (MAX_RTR_SOLICITATIONS)
   RS messages separated by at least 10 seconds
   (RTR_SOLICITATION_INTERVAL) as specified in [RFC4861], and then
   switch to slower retransmissions.  After the initial retransmissions
   the host SHOULD do truncated binary exponential backoff [ETHERNET] of
   the retransmission timer for each subsequent retransmission,
   truncating the increase of the retransmission timer at 60 seconds
   (MAX_RTR_SOLICITATION_INTERVAL).  In all cases the RS retransmissions
   are terminated when a RA is received.  See Section 9 for protocol
   constants.

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 (approximately 18 hours).

   Should the host erroneously receive a Prefix Information option with
   the 'L' (on-link) flag set, then that Prefix Information Option (PIO)



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   MUST be ignored.

5.4.1.  Address configuration

   Address configuration follows [RFC4862].  For an address not derived
   from an EUI-64, the M flag of the RA determines how the address can
   be configured.  If the M flag is set in the RA, then DHCPv6 MUST be
   used to assign the address.  If the M flag is not set, then the
   address can be configured by any other means (and duplicate detection
   is performed as part of the registration process).

   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.  Nodes
   should be careful about using header compression in RA messages that
   include 6COs.

5.4.3.  Maintaining Prefix and Context Information

   The prefix information is timed out as specified in [RFC4861].  When
   the Valid Lifetime for a Context Table entry expires the entry is
   placed in a receive-only mode, which is the equivalent of receiving a



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   6CO for that context with C=0.  The entry is held in receive-only
   mode for a period of twice the Default Router Lifetime, after which
   the entry is removed.

   A host should inspect the various lifetimes to determine when it
   should next initiate sending a Router Solicitation to ask for any
   updates to the information.  The lifetimes that matter are the
   Default Router lifetime, the Valid Lifetime in the Prefix Information
   options, and the Valid Lifetime in the 6CO.  The host SHOULD unicast
   one or more Router Solicitations to the router well before the
   minimum of those lifetimes (across all the prefixes and all the
   contexts) expire, and switch to multicast RS messages if there is no
   response to the unicasts.  The retransmission behavior for the Router
   Solicitations is specified in Section 5.3.

5.5.  Registration and Neighbor Unreachability Detection

   Hosts send Unicast Neighbor Solicitation (NS) messages to register
   their IPv6 addresses, and also to do NUD to verify that their default
   routers are still reachable.  The registration is performed by the
   host including an ARO in the Neighbor Solicitation it sends.  Even if
   the host doesn't have data to send, but is expecting others to try to
   send packets to the host, the host needs to maintain its Neighbor
   Cache entries in the routers.  This is done by sending NS messages
   with the ARO to the router well in advance of the registration
   lifetime expiring.  NS messages are retransmitted up to
   MAX_UNICAST_SOLICIT times using a minimum timeout of RETRANS_TIMER
   until the host receives an Neighbor Advertisement message with an ARO
   option.

   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
   by Reachability Confirmations from transport protocols or
   applications as specified in [RFC4861].

   When a host knows it will no longer use a router it is registered to,
   it SHOULD de-register with the router by sending an NS with an ARO
   containing a lifetime of 0.  To handle the case when a host loses
   connectivity with the default router involuntarily, the host SHOULD
   use a suitably low registration lifetime.

5.5.1.  Sending a Neighbor Solicitation

   The host triggers sending Neighbor Solicitation (NS) messages
   containing an ARO when a new address is configured, when it discovers



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   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
   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 indicates that the Neighbor Cache of that router
   is full.  In this case the host SHOULD remove this router from its
   default router list and attempt to register with another router.  If
   the host's default router list is empty, it needs to revert to
   sending Router Solicitations as specified in Section 5.3.



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   Other failure codes may be defined in future documents.

5.6.  Next-hop Determination

   The IP address of the next-hop for a destination is determined as
   follows.  Destinations to the link-local prefix (FE80::) are always
   sent on the link to that destination.  It is assumed that link-local
   addresses are formed as specified in Section 5.2 from the EUI-64, and
   address resolution is not performed.  Packets are sent to link local
   destinations by reversing the procedure in Appendix A of [RFC4291].

   Multicast addresses are considered to be on-link and are resolved as
   specified in [RFC4944] or the appropriate IP-over-foo document.  Note
   that [RFC4944] only defines how to represent a multicast destination
   address in the LoWPAN header.  Support for multicast scopes larger
   than link-local needs an appropriate multicast routing algorithm.

   All other prefixes are assumed to be off-link [RFC5889].  Anycast
   addresses are always considered to be off-link.  They are therefore
   sent to one of the routers in the Default Router List.

   A LoWPAN Node is not required to maintain a minimum of one buffer per
   neighbor as specified in [RFC4861], since packets are never queued
   while waiting for address resolution.

5.7.  Address Resolution

   The address registration mechanism and the SLLA option in Router
   Advertisement messages provide sufficient a priori state in routers
   and hosts to resolve an IPv6 address to its associated link-layer
   address.  As all prefixes, except the link-local prefix and multicast
   addresses, are always assumed to be off-link, multicast-based address
   resolution between neighbors is not needed.

   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 reduce memory usage by optimizing for this case and only storing
   link-layer address information if it differs from the link-layer
   address corresponding to the Interface ID of the IPv6 address (i.e.,
   differs in more than the on-link/global bit being inverted).

5.8.  Sleeping

   It is often advantageous for battery-powered hosts in LoWPANs to keep
   a low duty cycle.  The optimizations described in this document
   enable hosts to sleep as described further in this section.  Routers
   may want to cache traffic destined to a host which is sleeping, but



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   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/NA
   message exchanges.  The Address Registration option attached to NS
   messages indicates to a router to keep the Neighbor Cache entry for
   that address valid for the period in the Registration Lifetime field.
   A host should choose a sleep time appropriate for its energy
   characteristics, and set a registration lifetime larger than the
   sleep time to ensure the registration is renewed successfully
   (considering e.g. clock drift and additional time for potential
   retransmissions of the re-registration).  External configuration of a
   host should also consider the stability of the network (how quickly
   the topology changes) when choosing its sleep time (and thus
   registration lifetime).  A dynamic network requires a shorter sleep
   time so that routers don't keep invalid neighbor cache entries for
   nodes longer than necessary.

5.8.2.  Behavior on Wakeup

   When a host wakes up from a sleep period it SHOULD refresh its
   current address registrations that will timeout before the next
   wakeup.  This is done by sending Neighbor Solicitation messages with
   the Address Registration option as described in 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
   Lifetime is about 18 hours whereas the maximum Registration lifetime
   is about 45.5 days).  If after wakeup the host (using NUD) determines
   that some or all previous default routers have become unreachable,
   then the host will send multicast Router Solicitations to discover
   new default router(s) and restart the address registration process.


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.

   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 host is UNREACHABLE
   (based on the logic in [RFC4861]), the Neighbor Cache entry SHOULD



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   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

   Even if a router in a route-over topology can reach both a host and
   another target, because of radio propagation it generally cannot know
   whether the host can directly reach the other target.  Therefore it
   cannot assume that redirect will actually work from one host to
   another.  Therefore it SHOULD NOT send Redirect messages.  The only
   potential exception to this "SHOULD NOT" is when the deployment/
   implementation has a way to know how the host can reach the intended
   target.  Hence it is RECOMMENDED that the implementation by default
   does not send redirect messages but can be configurable when the
   deployment calls for this.  In contrast, for mesh-under topologies,
   the same considerations about Redirects apply as in 4861.

   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

   The 6LBR router interface initialization behavior is the same as in
   [RFC4861].  However, in dynamic configuration scenario (see
   Section 8.1), a 6LR comes up as a non-router and waits to receive the
   advertisement for configuring its own interface address first before
   making its interfaces advertising and turning into a router.

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



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   information or different, from a different 6LBR, then it will need to
   keep those prefixes and context information separately so that the
   RAs the 6LR sends will maintain the association between the ABRO and
   the prefixes and context information.  The router can tell which 6LBR
   originated the prefixes and context information from the 6LBR Address
   field in the ABRO.  When a router has information tied to multiple
   ABROs, a single RS will result in multiple RAs each containing a
   different ABRO.

   When the ABRO Valid Lifetime associated with a 6LBR times out, all
   information related to that 6LBR MUST be removed.  As an
   implementation note, it is recommend that RAs are sent sufficiently
   more frequently than the ABRO Valid Lifetime so that missing an RA
   does not result in removing all information related to a 6LBR.

   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
   (approximately 18 hours).

   Unlike [RFC4861] which suggests multicast Router Advertisements, this
   specification improves the exchange by always unicasting RAs in
   response to RSs.  This is possible since the RS always includes a
   SLLA option, which is used by the router to unicast the RA.

6.4.  Periodic Router Advertisements

   A router does not need to send any periodic Router Advertisement
   messages since the hosts will solicit updated information by sending
   Router Solicitations before the lifetimes expire.

   However, if the routers use Router Advertisements to 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].






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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.

   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 with the ARO Status field set to one (to
   indicate the address is a duplicate) as described in Section 6.5.2.
   In this case there is no modification to the Neighbor Cache.

6.5.2.  Returning Address Registration Errors

   Address registration errors are not sent back to the source address
   of the NS due to a possible risk of L2 address collision.  Instead
   the NA is sent to the link-local IPv6 address with the IID part
   derived from the EUI-64 field of the ARO as per [RFC4944].  In
   particular, this means that the universal/local bit needs to be
   inverted.  The NA is formatted with a copy of the ARO from the NS,
   but with the Status field set to indicate the appropriate error.

   The error is sent to the link-local address with the IID derived from
   the EUI-64.  Thus if the ARO was from and for a short address, the L2
   destination address for the NA with the ARO error will be the 64-bit
   unique addresses.







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6.5.3.  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
   unicast NA with the ARO Status field set to two (to indicate the
   router's Neighbor Cache is full) as described in Section 6.5.2.

   The Registration Lifetime and the EUI-64 are recorded in the Neighbor
   Cache entry.  A unicast Neighbor Advertisement (NA) is then sent in
   response to the NS.  This NA SHOULD include a copy of the ARO, with
   the Status field set to zero.  A TLLA (Target Link-layer Address)
   option [RFC4861] 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 substitutable multihop DAD (Section 8.2) is used, then
   the updating of the Neighbor Cache is slightly different due to
   Tentative NCEs.

6.5.4.  Next-hop Determination

   In order to deliver a packet destined for a 6LN registered with a
   router, next-hop determination is slightly different for routers than
   hosts (see Section 5.6.  The routing table is checked to determine
   the next hop IP address.  A registered Neighbor Cache Entry (NCE)
   determines if the next hop IP-address is on-link.  It is the
   responsibility of the routing protocol of the router to maintain on-
   link information about its registered neighbors.  Tentative NCEs MUST
   NOT be used to determine on-link status of the registered nodes.

6.5.5.  Address Resolution between Routers

   There needs to be a mechanism somewhere for the routers to discover
   each others' link-layer addresses.  If the routing protocol used
   between the routers provides this, then there is no need for the
   routers to use the Address Registration option between each other.



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   Otherwise, the routers SHOULD use the ARO.  When routers use ARO to
   register with each other and the multihop DAD Section 8.2 is in use,
   then care must be taken to ensure that there isn't a flood of ARO-
   carrying messages sent to the 6LBR as each router hears an ARO from
   their neighboring routers.  The details for this is out of scope of
   this document.

   Routers MAY also 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].


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].  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 as a substitute for prefix
   dissemination in the route-over topology (see Section 1.4).

   If the 6LoWPAN uses Header Compression [RFC6282] with context then
   the 6LBR needs to manage the context IDs, and advertise those in
   Router Advertisements by including 6CO options in its Router
   Advertisements so that directly attached hosts are informed about the
   context IDs.  Below we specify things to consider when the 6LBR needs
   to add, remove, or change the context information.  In the case of
   route-over, the context information is disseminated to all the 6LRs
   using the technique in Section 8 unless a different specification
   provides a substitute for this multihop distribution.





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7.1.  Prefix Determination

   The prefix or prefixes used in a LoWPAN can be manually configured,
   or can be acquired using DHCPv6 Prefix Delegation [RFC3633].  For a
   LoWPAN that is isolated from the network, either permanently or
   occasionally, the 6LBR can assign a ULA prefix using [RFC4193].  The
   ULA prefix should be stored in stable storage so that the same prefix
   is used after a failure of the 6LBR.  If the LoWPAN has multiple
   6LBRs, then they should be configured with the same set of prefixes.
   The set of prefixes are included in the Router Advertisement messages
   as specified in [RFC4861].

7.2.  Context Configuration and Management

   If the LoWPAN uses Header Compression [RFC6282] with context then the
   6LBR must 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.  For [RFC6282],
   maintaining consistency of context information is crucial for
   ensuring packets will be decompressed correctly.

   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.





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8.  Substitutable Feature Behavior

   Normally in a 6LoWPAN multihop network, the Router Advertisement
   messages are used to disseminate prefixes and context information to
   all the 6LRs in a route-over topology.  If all routers are configured
   to use a substitute mechanism for such information distribution, any
   remaining use of the 6LoWPAN-ND mechanisms is governed by the
   substitute specification.

   There is also the option for a 6LR to perform multihop DAD (for non-
   EUI-64 derived IPv6 addresses) against a 6LBR in a route-over
   topology by using the DAR and DAC messages.  This is substitutable
   because there might be other ways to either allocate unique address,
   such as DHCPv6 [RFC3315], or other future mechanisms for multihop
   DAD.  Again in this case, any remaining use of the 6LoWPAN-ND
   mechanisms is governed by the substitute specification.

   To be clear: Implementations MUST support the features describes in
   Section 8.1 and Section 8.2, unless the implementation supports some
   alternative ("substitute") from some some other specification.

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



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   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

   In a 6LoWPAN, unless substituted, multihop distribution is done using
   Router Advertisement (RA) messages.  Thus on interface initialization
   a router (6LR) MUST send Router Solicitation messages following the
   rules specified for hosts in [RFC4861].  That will cause the routers
   to respond with RA messages which then can be used to initially seed
   the prefix and context information.

8.1.3.  Routers Processing Router Advertisements

   If multihop distribution is not done using RA messages, then the
   routers follow [RFC4861] which states that they merely do some
   consistency checks 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, Version, Valid
   Lifetime and the associated prefixes and context information.  If the
   6LBR is previously known, then the Version number field MUST be
   compared against the recorded version number for that 6LBR.  If the
   version number received in the packet is less than the stored version
   number then the information in the RA is silently ignored.  Otherwise
   the recorded information and version number are updated.

8.1.4.  Storing the Information

   The router keeps state for each 6LBR that it sees with an ABRO.  This
   includes the Version number, the Valid Lifetime, and the complete set
   of 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.




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   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.

8.1.5.  Sending Router Advertisements

   When multihop distribution is performed using RA messages, the
   routers MUST ensure that the ABRO always stay together with the
   prefixes and context information received with that ABRO.  Thus if
   the router has received prefix P1 with ABRO saying it is from one
   6LBR, and prefix P2 from another 6LBR, then the router MUST NOT
   include the two prefixes in the same RA message.  Prefix P1 MUST be
   in a RA that include a ABRO from the first 6LBR etc.  Note that
   multiple 6LBRs might advertise the same prefix and context
   information, but they still need to be associated with the 6LBRs that
   advertised them.

   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
   known to the 6LR.  However, that isn't sufficient in a route-over
   topology (or in a LoWPAN with multiple 6LBRs) since some host
   attached to another 6LR could be using the same address.  There might
   be different ways for the 6LRs to coordinate such Duplicate Address
   Detection in the future, or addresses could be assigned using a
   DHCPv6 server that verifies uniqueness as part of the assignment.




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   This specification offers a substitutable simple technique for 6LRs
   and 6LBRs to perform Duplicate Address Detection that reuses the
   information from Address Registration option in the DAR and DAC
   messages.  This technique is not needed when the Interface ID in the
   address is based on an EUI-64, since those are assumed to be globally
   unique.  The technique assumes that the 6LRs either register with all
   the 6LBRs, or that the network uses some out-of-scope mechanism to
   keep the DAD tables in the 6LBRs synchronized.

   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 SLLA
   option.  This entry will be used to send the response to the host
   when the 6LBR responds positively.

   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 Duplicate Address Request message to one or
   more 6LBRs, where the DAR contains the host's address in the
   Registered Address field.  The DAR will be forwarded by 6LRs until it
   reaches the 6LBR, hence its IPv6 hop limit field will not be 255 when
   received by the 6LBR.  The 6LBR will respond with a Duplicate Address
   Confirmation message, which will have a hop limit less than 255 when
   it reaches the 6LR.




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   When the 6LR receives the DAC from the 6LBR, it will look for a
   matching (same IP address and EUI-64) (Tentative or Registered)
   Neighbor Cache entry.  If no such entry is found then the DAC is
   silently ignored.  If an entry is found and the DAC had Status=0 then
   the 6LR will mark the Tentative Neighbor Cache entry as Registered.
   In all cases when an entry is found then the 6LR will respond to the
   host with an NA, copying the Status and EUI-64 fields from the DAC to
   an ARO option in the NA.  In case the status is an error, then the
   destination IP address of the NA is derived from the EUI-64 field of
   the DAC.

   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.  Message Validation for DAR and DAC

   A node MUST silently discard any received Duplicate Address Request
   and Confirmation messages for which at least one of the following
   validity checks is not satisfied:

   o  If the message includes an IP Authentication Header, the message
      authenticates correctly.

   o  ICMP Checksum is valid.

   o  ICMP Code is 0.

   o  ICMP length (derived from the IP length) is 32 or more bytes.

   o  The Registered Address is not a multicast address.

   o  All included options have a length that is greater than zero.

   o  The IP source address is not the unspecified address, nor a
      multicast address.

   The contents of the Reserved field, and of any unrecognized options,
   MUST be ignored.  Future, backward-compatible changes to the protocol
   may specify the contents of the Reserved field or add new options;
   backward-incompatible changes may use different Code values.

   Note that due to the forwarding of the DAR and DAC messages between
   the 6LR and 6LBR there is no hop limit check on receipt for these
   ICMPv6 message types.






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8.2.2.  Conceptual Data Structures

   A 6LBR implementing multihop DAD needs to maintain some state
   separate from the Neighbor Cache.  We call this conceptual data
   structure the DAD table.  It is indexed by the IPv6 address - the
   Registered Address in the DAR - and contains the EUI-64 and the
   registration lifetime of the host that is using that address.

8.2.3.  6LR Sending a Duplicate Address Request

   When a 6LR that implements multihop DAD receives an NS from a host
   and subject to the above checks, the 6LR forms and sends a DAR to at
   least one 6LBR.  The DAR contains the following information:

   o  In the IPv6 source address, a global address of the 6LR.

   o  In the IPv6 destination address, the address of the 6LBR.

   o  In the IPv6 hop limit, MULTIHOP_HOPLIMIT.

   o  The Status field MUST be set to zero

   o  The EUI-64 and Registration lifetime are copied from the ARO
      received from the host.

   o  The Registered Address set to the IPv6 address of the host, that
      is, the sender of the triggering NS.

   When a 6LR receives an NS from a host with a zero Registration
   Lifetime then, in addition to removing the Neighbor Cache entry for
   the host as specified in Section 6, an DAR is sent to the 6LBRs as
   above.

   A router MUST NOT modify the Neighbor Cache as a result of receiving
   a Duplicate Address Request.

8.2.4.  6LBR Receiving a Duplicate Address Request

   When a 6LBR that implements the substitutable multihop DAD receives
   an DAR from a 6LR, it performs the message validation specified in
   Section 8.2.1.  If the DAR is valid the 6LBR proceeds to look for the
   Registration Address in the DAD Table.  If an entry is found and the
   recorded EUI-64 is different than the EUI-64 in the DAR, then it
   returns a DAC NA with the Status set to 1 ('Duplicate Address').
   Otherwise it returns a DAC with Status set to zero and updates the
   lifetime.

   If no entry is found in the DAD Table and the Registration Lifetime



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   is non-zero, then an entry is created and the EUI-64 and Registered
   Address from the DAR are stored in that entry.

   If an entry is found in the DAD Table, the EUI-64 matches, and the
   Registration Lifetime is zero then the entry is deleted from the
   table.

   In both of the above cases the 6LBR forms an DAC with the information
   copied from the DAR and the Status field is set to zero.  The DAC is
   sent back to the 6LR i.e., back to the source of the DAR.  The IPv6
   hop limit is set to MULTIHOP_HOPLIMIT

8.2.5.  Processing a Duplicate Address Confirmation

   When a 6LR implementing multihop DAD receives a DAC message, then it
   first validates the message per Section 8.2.1.  For a valid DAC, if
   there is no Tentative Neighbor Cache entry matching the Registered
   address and EUI-64, then the DAC is silently ignored.  Otherwise, the
   information in the DAC and in the Tentative Neighbor Cache entry is
   used to form an NA to send to the host.  The Status code is copied
   from the DAC to the ARO that is sent to the host.  In case of the DAC
   indicates an error (the Status is non-zero), the NA is returned to
   the host as described in Section 6.5.2 and 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 Duplicate Address Confirmation, unless there is a Tentative
   Neighbor Cache entry matching the IPv6 address and EUI-64.

8.2.6.  Recovering from Failures

   If there is no response from a 6LBR after RETRANS_TIMER [RFC4861]
   then the 6LR would retransmit the DAR to the 6LBR up to
   MAX_UNICAST_SOLICIT [RFC4861] times.  After this the 6LR SHOULD
   respond to the host with an ARO Status of zero.


9.  Protocol Constants

   This section defines the relevant protocol constants used in this
   document based on a subset of [RFC4861] constants. (*) indicates
   constants modified from [RFC4861] and (+) indicates new constants.

   Additional protocol constants are defined in Section 4.

   6LBR Constants:




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   MIN_CONTEXT_CHANGE_DELAY+               300 seconds

   6LR Constants:

   MAX_RTR_ADVERTISEMENTS                  3 transmissions

   MIN_DELAY_BETWEEN_RAS*                  10 seconds

   MAX_RA_DELAY_TIME*                      2 seconds

   TENTATIVE_NCE_LIFETIME+                 20 seconds

   Router Constants:

   MULTIHOP_HOPLIMIT+                      64

   Host Constants:

   RTR_SOLICITATION_INTERVAL*              10 seconds

   MAX_RTR_SOLICITATIONS                   3 transmissions

   MAX_RTR_SOLICITATION_INTERVAL+          60 seconds


10.  Examples

10.1.  Message Examples


   STEP

      6LN                                                        6LR

       |                                                          |

   1.  |       ---------- Router Solicitation -------->           |

       |                       [SLLAO]                            |

       |                                                          |

   2.  |       <-------- Router Advertisement ---------           |

       |              [PIO + 6CO + ABRO + SLLAO]                  |






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     Figure 2: Basic Router Solicitation/Router Advertisement exchange
                      between a node and 6LR or 6LBR




      6LN                                                        6LR

       |                                                          |

   1.  |       ------- NS with Address Registration ------>       |

       |                     [ARO + SLLAO]                        |

       |                                                          |

   2.  |       <----- NA with Address Registration --------       |

       |                   [ARO with Status]                      |




             Figure 3: Neighbor Discovery Address Registration



























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      6LN                           6LR                          6LBR

       |                             |                             |

   1.  | --- NS with Address Reg --> |                             |

       |      [ARO + SLLAO]          |                             |

       |                             |                             |

   2.  |                             | ----------- DAR ----------> |

       |                             |                             |

   3.  |                             | <---------- DAC ----------- |

       |                             |                             |

   4.  | <-- NA with Address Reg --- |                             |

       |      [ARO with Status]      |


    Figure 4: Neighbor Discovery Address Registration with Multihop DAD

10.2.  Host Bootstrapping Example

   The following example describes the address bootstrapping scenarios
   using the improved ND mechanisms specified in this document.  It is
   assumed that the 6LN first performs a sequence of operations in order
   to get secure access at the link-layer of the LoWPAN and obtain a key
   for link-layer security.  The methods of how to establish 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 SLLA option from the RS



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   (it may have created a tentative NCE).  See Figure 2.

   3.  In order to communicate more than one IP hop away the 6LN
   configures a global IPv6 address.  In order to save overhead, this
   6LN wishes to configure its IPv6 address based on a 16-bit short
   address as per [RFC4944].  As the network is unmanaged (M flag not
   set in 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 SLLA option is also
   included with the link-layer address corresponding to the address
   being registered.  If a successful (status 0) NA message is received
   the address can then be used and the 6LN assumes it has been
   successfully checked for duplicates.  If a duplicate address (status
   1) NA message is received, the 6LN then removes the temporary IPv6
   address and 16-bit link-layer address and goes back to step 3.  If a
   neighbor cache full (status 2) message is received, the 6LN attempts
   to register with another default router, or if none, goes back to
   step 2.  See Figure 3.  Note that an NA message returning an error
   would be sent back to the link-local EUI-64 based IPv6 address of the
   6LN instead of the 16-bit (duplicate) address.

   5.  The 6LN now performs maintenance by sending a new NS address
   registration before the lifetime expires.

   If multihop DAD and multihop prefix and context distribution is used,
   the effect of the 6LRs and hosts following the above bootstrapping is
   a "wavefront" of 6LRs and host being configured spreading from the
   6LBRs.  First the hosts and 6LRs that can directly reach a 6LBR would
   receive one or more RAs and configure and register their IPv6
   addresses.  Once that is done they would enable the routing protocol
   and start sending out Router Advertisements.  That would result in a
   new set of 6LRs and hosts to receive responses to their Router
   Solicitations, form and register their addresses, etc.  That repeats
   until all of the 6LRs and hosts have been configured.

10.2.1.  Host Bootstrapping Messages

   This section brings specific message examples to the previous
   bootstrapping process.  When discussing messages, the following
   notation is used:

   LL64: Link-Local Address based on the EUI-64, which is also the
   802.15.4 Long Address.




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   GP16: Global Address based on the 802.15.4 Short Address.  This
   address may not be unique.

   GP64: Global addresses derived from the EUI-64 address as specified
   in [RFC4944].

   MAC64: EUI-64 address used as the link-layer address.

   MAC16: IEEE 802.15.4 16-bit short address.

   Note that some implementations may use LL64 and GP16 style addresses
   instead of LL64 and GP64.  In the following, we will show an example
   message flow as to how a node uses LL64 to register a GP16 address
   for multihop DAD verification.





































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     6LN-----RS-------->6LR
     Src= LL64 (6LN)
     Dst= All-router-link-scope-multicast
     SLLAO= MAC64 (6LN)

    6LR------RA--------->6LN
     Src= LL64 (6LR)
     Dst= LL64 (6LN)

   Note: Source address of RA must be a link-local
   address (Section 4.2, RFC 4861).

    6LN-------NS Reg------>6LR
     Src= GP16 (6LN)
     Dst= LL64 (6LR)
     ARO
     SLLAO= MAC16 (6LN)

    6LR---------DAR----->6LBR
    Src= GP64 or GP16 (6LR)
    Dst= GP64 or GP16 (6LBR)
    Registered Address= GP16 (6LN) and EUI-64 (6LN)

    6LBR-------DAC--------->6LR
    Src= GP64 or GP16 (6LBR)
    Dst= GP64 or GP16 (6LR)
    Copy of information from DAR

    If Status is a Success:

    6LR ---------NA-Reg------->6LN
    Src= LL64 (6LR)
    Dst= GP16 (6LN)
    ARO with Status = 0

    If Status is not a success:

    6LR ---------NA-Reg-------->6LN
    Src= LL64 (6LR)
    Dst= LL64 (6LN) --> Derived from the EUI-64 of ARO
    ARO with Status > 0


                Figure 5: Detailed Message Address Examples







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10.3.  Router Interaction Example

   In the Route-over topology, when a routing protocol is run across
   6LRs the bootstrapping and neighbor cache management are handled a
   little differently.  The description in this paragraph provides only
   a guideline for an implementation.

   At the initialization of a 6LR, it may choose to bootstrap as a host
   with the help of a parent 6LR if the substitutable multihop DAD is
   performed with the 6LBR.  The neighbor cache management of a router
   and address resolution among the neighboring routers are described in
   Section 6.5.3 and Section 6.5.5, respectively.  In this example, we
   assume that the neighboring 6LoWPAN link is secure.

10.3.1.  Bootstrapping a Router

   In this scenario, the bootstrapping 6LR, 'R1', is multiple hops away
   from the 6LBR and surrounded by other 6LR neighbors.  Initially R1
   behaves as a host.  It sends 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 next hop neighboring 6LR determined by the
   routing protocol for reaching the 6LBR).

10.3.2.  Updating the Neighbor Cache

   In this example, there are three 6LRs, R1, R2, R3.  Initially when R2
   boots it sees only R1, and accordingly R2 creates a neighbor cache
   entry for R1.  Now assume R2 receives a valid routing update from
   router R3.  R2 does not have any neighbor cache entry for R3.  If the
   implementation of R2 supports detecting link-layer address from the
   routing information packets then it directly updates the its neighbor
   cache using that link-layer information.  If this is not possible,
   then R2 should perform multicast NS with source set with its link-
   local or global address depending on the scope of the source IP-
   address received in the routing update packet.  The target address of



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   the NS message is the source IPv6 address of the received routing
   update packet.  The format of the NS message is as described in
   Section 4.3 of [RFC4861].

   More generally any 6LR that receives a valid route-update from a
   neighboring router for which it does not have any 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] and
   Address Autoconfiguration [RFC4862] apply.  Additional considerations
   can be found in [RFC3756].

   There is a slight modification to those considerations due to the
   fact that in this specification the M-flag in the Router
   Advertisements disable the use of stateless address autoconfiguration
   for addresses not derived from EUI-64.  Thus a rogue router on the
   link can force the use of only DHCP for short addresses, whereas in
   [RFC4861] and [RFC4862] the rogue router could only cause the
   addition of DHCP and not disable SLAAC for short addresses.

   This specification assumes that the link layer is sufficiently
   protected, for instance using MAC sublayer cryptography.  Thus, its
   threat model is no different from that of IPv6 Neighbor Discovery
   [RFC4861].  The threat model number 1 in section 3 of [RFC3756]
   applies here.  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 DAR and DAC messages that are
   forwarded by 6LRs, and as a result the hop_limit=255 check on the
   receiver does not apply to those messages.  This implies that any
   node on the Internet could successfully send such messages.  We avoid
   any additional security issues due to this by requiring that the
   routers never modify the Neighbor Cache entry due to such messages,
   and that they discard them unless they are received on an interface
   that has been explicitly configured to use these optimizations.

   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



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   the packet.  Applying SeND to the router-to-router communication in
   this document is out of scope.


12.  IANA Considerations

   The document requires three new Neighbor Discovery option types under
   the subregistry "IPv6 Neighbor Discovery Option Formats":

   o  Address Registration Option (TBD1)

   o  6LoWPAN Context Option (TBD2)

   o  Authoritative Border Router Option (TBD3)

   The document requires two new ICMPv6 types under the subregistry
   "ICMPv6 type Numbers":

   o  Duplicate Address Request (TBD4)

   o  Duplicate Address Confirmation (TBD5)

   This document also requests IANA to create a new sub-registry for the
   Status values of the Address Registration Option, under the ICMPv6
   parameters registry.

   Address Registration Option Status Values registry:

      Possible values are 8-bit unsigned integers (0..255).

      Registration procedure is "Standards Action" [RFC5226].

      Initial allocation is as indicated in Table 2:

          +--------+--------------------------------------------+
          | Status |                 Description                |
          +--------+--------------------------------------------+
          |    0   |                   Success                  |
          |    1   |              Duplicate Address             |
          |    2   |             Neighbor Cache Full            |
          |  3-255 | Allocated using Standards Action [RFC5226] |
          +--------+--------------------------------------------+

                                  Table 2







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13.  Interaction with other Neighbor Discovery Extensions

   There are two classes of Neighbor Discovery Extensions that have
   different interaction with this specification.

   One class are extensions to to the Duplicate Address Detection
   mechanisms in [RFC4861] and [RFC4862].  An example of this is
   Optimistic DAD [RFC4429].  Such extensions does not apply when this
   specification is being used, since it uses ARO for DAD (which is
   neither optimistic nor pessimistic - always one roundtrip to the
   router to check DAD).

   All other (non-DAD) Neighbor Discovery extensions, be it path
   selection ones like Default Router Preferences [RFC4191],
   configuration ones like DNS config [RFC5006], or others like DNA
   [RFC6059], are completely orthogonal to this specification, and will
   work as is.


14.  Guideline for New Features

   This section discusses a guideline of new protocol features defined
   in this document.  It also sets some expectations for implementation
   and deployment of these features.  This section is informative in
   nature: It does not override the detailed specifications of the
   previous sections, but summarizes them and presents them in a compact
   form that can be used as a checklist.  The checklist acts as a
   guideline to indicate the possible importance of a feature in terms
   of a deployment as per information available as of the writing of the
   document.  Note that in some cases the deployment is 'SHOULD' where
   the implementation is a 'MUST'.  This is due to the presence of
   substitutable features; the deployment may use alternative methods
   for those.  Therefore, implementing a configuration knob is
   recommended for the substitutable features.  The lists emphasize
   conciseness over completeness.
















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   +----------+---------------------------------+----------+-----------+
   | Section  | Description                     | Deploy   | Implement |
   +----------+---------------------------------+----------+-----------+
   | 3.1      | Host initiated RA               | MUST     | MUST      |
   | 3.2      | EUI-64 based IPv6-address       | MUST     | MUST      |
   |          | 16bit-MAC based address         | MAY      | SHOULD    |
   |          | Other non-unique addresses      | MAY      | MAY       |
   | 3.3      | Host Initiated RS               | MUST     | MUST      |
   |          | ABRO Processing                 | SHOULD   | MUST      |
   | 4.1      | Registration with ARO           | MUST     | MUST      |
   | 4.2, 5.4 | 6LoWPAN Context Option          | SHOULD   | SHOULD    |
   | 5.1      | Re-direct Message Acceptance    | MUST NOT | MUST NOT  |
   |          | Joining Solicited Node          | N/A      | N/A       |
   |          | Multicast                       |          |           |
   |          | Joining all-node Multicast      | MUST     | MUST      |
   |          | Using link-layer indication for | MAY      | MAY       |
   |          | NUD                             |          |           |
   | 5.5      | 6LoWPAN-ND NUD                  | MUST     | MUST      |
   | 5.8.2    | Behavior on wake-up             | SHOULD   | SHOULD    |
   +----------+---------------------------------+----------+-----------+

           Table 3: Guideline for 6LoWPAN-ND features for hosts

   +---------------+-------------------------+------------+------------+
   | Section       | Description             | deploy     | implement  |
   +---------------+-------------------------+------------+------------+
   | 3.1           | Periodic RA             | SHOULD NOT | SHOULD NOT |
   | 3.2           | Address assignment      | SHOULD     | MUST       |
   |               | during Startup          |            |            |
   | 3.3           | Supporting EUI-64 based | MUST       | MUST       |
   |               | MAC Hosts               |            |            |
   |               | Supporting 16-bit MAC   | MAY        | SHOULD     |
   |               | hosts                   |            |            |
   | 3.4, 4.3,     | ABRO Processing/sending | SHOULD     | MUST       |
   | 8.1.3, 8.1.4  |                         |            |            |
   | 8.1           | Multihop Prefix storing | SHOULD     | MUST       |
   |               | and re-distribution     |            |            |
   | 3.5           | Tentative NCE           | MUST       | MUST       |
   | 8.2           | Multihop DAD            | SHOULD     | MUST       |
   | 4.1, 6.5,     | ARO Support             | MUST       | MUST       |
   | 6.5.1 - 6.5.5 |                         |            |            |
   | 4.2           | 6LoWPAN Context Option  | SHOULD     | SHOULD     |
   | 6.3           | Process RS/ARO          | MUST       | MUST       |
   +---------------+-------------------------+------------+------------+

             Table 4: Guideline for 6LR features in 6LoWPAN-ND





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   +--------------+--------------------------+------------+------------+
   | Section      | Description              | deploy     | implement  |
   +--------------+--------------------------+------------+------------+
   | 3.1          | Periodic RA              | SHOULD NOT | SHOULD NOT |
   | 3.2          | Address autoconf on      | MUST NOT   | MUST NOT   |
   |              | Router interface         |            |            |
   | 3.3          | EUI-64 MAC support on    | MUST       | MUST       |
   |              | 6LoWPAN interface        |            |            |
   | 8.1 - 8.1.1, | Multihop Prefix          | SHOULD     | MUST       |
   | 8.1.5        | distribution             |            |            |
   | 8.2          | Multihop DAD             | SHOULD     | MUST       |
   +--------------+--------------------------+------------+------------+

            Table 5: Guideline for 6LBR features in 6LoWPAN-ND


15.  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,
   Colin O'Flynn, Dario Tedeschi, Esko Dijk 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, Colin O'Flynn for useful Error-to suggestions and
   contributions to the Examples section, Geoff Mulligan for suggesting
   the use of Address Registration as part of existing IPv6 Neighbor
   Discovery messages, and Mathilde Durvy for helping to clarify router
   interaction.


16.  Changelog

   Changes from -20 to -21:

      o Clarified the address an address registration error is sent to.

      o Added a new section explaining the interaction with other
      Neighbor Discovery extensions.




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   Changes from -19 to -20:

      o Further clarification on substitutable features.

      o Changed RFC 6282 to a normative reference.

   Changes from -18 to -19:

      o Editorial improvements as a result of IESG comments (#135,
      #142).

      o Extended ABRO with longer version number and valid lifetime,
      while maintaining backward compatibility (#141).

      o Renamed optional features and described them as substitutable
      (#138).

   Changes from -17 to -18:

      o Fixed nits related to IESG submission.

   Changes from -16 to -17:

      o Removed unnecessary normative text from Assumptions.

      o Clarified the next-hop determination of multicast addresses.

      o Editorial improvements from WGLC review.

   Changes from -15 to -16:

      o Added an applicability section (#133)

      o Updated document title to align with HC

      o Minor editing as result of WGLC review (#134)

   Changes from -14 to -15:

      o Changed use of redirect to SHOULD NOT for route-over and MAY for
      mesh-under. (#130)

      o Changed the 16-bit lifetimes to a unit of 60 seconds (#131)

      o Added text to Section 5.4.2 adding a receive-only state to
      context entries that timeout. (#132)

   Changes from -13 to -14:



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      o Introduced the new DAR and DAC ICMPv6 message types for multihop
      DAD to avoid relying on the Length=4 checks for the ARO.  This
      simplifies implementing the hop limit check.

      o Clarified the hop limit values for the multihop DAD messages by
      introducing the MULTIHOP_HOPLIMIT constant set to 64.

      o Clarified when a host should de-register from a router.

      o Added a section on next-hop determination for routers.

      o Removed the infinite lifetime from 6CO.

      o Increased MIN_CONTEXT_CHANGE_DELAY to 300 seconds.

   Changes from -12 to -13:

      o Error-to solution added for returning NA messages carrying an
      error ARO option to the link-local EUI-64 based IPv6 address of
      the host (#126).

      o New examples added.

   Changes from -11 to -12:

      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)




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      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:

      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.





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      o Added guidelines on routers performing DAD.

      o Removed the NR/NC Advertising Interval.

      o Added assumption of uniform IID formation and DAD throughout a
      LoWPAN.

   Changes from -06 to -07:

      o Updated addressing and address resolution (#60).

      o Changed the Address Option to 6LoWPAN Address Option, fixed S
      values (#61).

      o Added support for classic RFC4861 RA Prefix Information messages
      to be processed (#62).

      o Added a section on using 6LoWPAN-ND under a hard-wired RFC4861
      stack (#63).

      o Updated the NR/NC message with a new Router flag, combined the
      Code and Status fields into one byte, and added the capability to
      carry 6IOs (#64).

      o Made co-existence with other ND mechanisms clear (#59).

      o Added a new Protocol Specification section with all mechanisms
      specified there (#59).

      o Removed dependencies and conflicts with RFC4861 wherever
      possible (#59).

      o Some editorial cleanup.

   Changes from -05 to -06:

      o Fixed the Prf codes (#52).

      o Corrected the OIIO TID field to 8-bits.  Changed the Nonce/OII
      order in both the OIIO and the NR/NC. (#53)

      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




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   Changes from -04 to -05:

      o Meaning of the RA's M-bit changed to original [RFC4861] meaning
      (#46).

      o Terms "on-link" and "off-link" used in place of "on-link" and
      "off-link".

      o Next-hop determination text simplified (#49).

      o Neighbor cache and destination cache removed.

      o IID to link-layer address requirement relaxed.

      o NR/NC changes to enable on-link refresh with routers (#48).

      o Modified 6LoWPAN Information Option (#47).

      o Added a Protocol Constants section (#24)

      o Added the NR processing table (#51)

      o Considered the use of SeND on backbone NS/NA messages (#50)

   Changes from -03 to -04:

      o Moved Ad-hoc LoWPAN operation to Section 7 and made ULA prefix
      generation a features useful also in Simple and Extended LoWPANs.
      (#41)

      o Added a 32-bit Owner Nonce to the NR/NC messages and the
      Whiteboard, removed the TID history. (#39)

      o Improved the duplicate OII detection algorithm using the Owner
      Nonce. (#39)

      o Clarified the use of Source and Target link-layer options in
      NR/NC. (#43)

      o Included text on the use of alternative methods to acquire
      addresses. (#38)

      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)




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   Changes from -02 to -03:

      o Updated terminology, with RFC4861 non-transitive link model.

      o 6LoWPAN and ND terminology separated.

      o Protocol overview explains RFC4861 diff in detail.

      o RR/RC is now Node Registration/Confirmation (NR/NC).

      o Added NR failure codes.

      o ER Metric now included in 6LoWPAN Summary Option for use in
      default router determination by hosts.

      o Examples of host data structures, and the Whiteboard given.

      o Whiteboard is supported by all Edge Routers for option
      simplicity.

      o Edge Router Specification chapter re-structured, clarifying
      optional Extended LoWPAN operation.

      o NS/NA now completely optional for nodes.  No address resolution
      or NS/NA NUD required.

      o link-local operation now compatible with oDAD (was broken).

      o Exception to hop limit = 255 for NR/NC messages.

      o Security considerations improved.

      o ICMPv6 destination unreachable supported.

   Changes from -01 to -02:

      o Fixed 16 != 0xff bug (ticket closed).

      o Specified use of ULAs in ad-hoc LoWPAN section 9 (ticket
      closed).

      o Terminology cleanup based on Alex's comments.

      o General editing improvements.

   Changes from -00 to -01:





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      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.


17.  References

17.1.  Normative References

   [ETHERNET]
              "Information technology - Telecommunications and
              information exchange between systems - Local and
              metropolitan area networks - Specific requirements - Part
              3: Carrier sense multiple access with Collision Detection
              (CSMA/CD) Access Method and Physical Layer
              Specifications", IEEE Std 802.3-2008, December 2008, <http
              ://standards.ieee.org/getieee802/download/
              802.3-2008_section1.pdf>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC2491]  Armitage, G., Schulter, P., Jork, M., and G. Harter, "IPv6
              over Non-Broadcast Multiple Access (NBMA) networks",
              RFC 2491, January 1999.

   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, November 2005.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, October 2005.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control



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              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, September 2007.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC6282]  Hui, J. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              September 2011.

17.2.  Informative References

   [EUI64]    IEEE, "Guidelines for 64-bit Global Identifier (EUI-64)
              Registration Authority", <http://standards.ieee.org/
              regauth/oui/tutorials/EUI64.html>.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              December 2003.

   [RFC3756]  Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
              Discovery (ND) Trust Models and Threats", RFC 3756,
              May 2004.

   [RFC3971]  Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
              Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, March 2005.

   [RFC4429]  Moore, N., "Optimistic Duplicate Address Detection (DAD)
              for IPv6", RFC 4429, April 2006.



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   [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.

   [RFC5006]  Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
              "IPv6 Router Advertisement Option for DNS Configuration",
              RFC 5006, September 2007.

   [RFC5889]  Baccelli, E. and M. Townsley, "IP Addressing Model in Ad
              Hoc Networks", RFC 5889, September 2010.

   [RFC6059]  Krishnan, S. and G. Daley, "Simple Procedures for
              Detecting Network Attachment in IPv6", RFC 6059,
              November 2010.


Authors' Addresses

   Zach Shelby (editor)
   Sensinode
   Konekuja 2
   Oulu  90620
   FINLAND

   Phone: +358407796297
   Email: zach@sensinode.com


   Samita Chakrabarti
   Ericsson


   Email: samita.chakrabarti@ericsson.com


   Erik Nordmark
   Cisco Systems


   Email: nordmark@cisco.com






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