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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                             IP Infusion
Expires: February 4, 2011                                    E. Nordmark
                                                            Oracle, Inc.
                                                          August 3, 2010


    Neighbor Discovery Optimization for Low-power and Lossy Networks
                        draft-ietf-6lowpan-nd-12

Abstract

   The IETF 6LoWPAN working group defines IPv6 for low-power and lossy
   networks (LLNs) such as IEEE 802.15.4.  This and other similar link
   technologies have limited or no usage of multicast signaling due to
   energy conservation.  In addition, the wireless network may not
   strictly follow traditional concept of IP subnets and IP links.  IPv6
   Neighbor Discovery was not designed for non-transitive wireless
   links.  The traditional IPv6 link concept and heavy use of multicast
   make the protocol inefficient and sometimes impractical in a low
   power and lossy network.  This document describes simple
   optimizations to IPv6 Neighbor Discovery, addressing mechanisms and
   duplicate address detection for 6LoWPAN and similar networks.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on February 4, 2011.



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Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  The Shortcomings of IPv6 Neighbor Discovery  . . . . . . .  5
     1.2.  Mesh-under and Route-over Concepts . . . . . . . . . . . .  6
     1.3.  Applicability, Goals and Assumptions . . . . . . . . . . .  7
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  8
   3.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . . 10
     3.1.  Extensions to RFC4861  . . . . . . . . . . . . . . . . . . 10
     3.2.  Address Assignment . . . . . . . . . . . . . . . . . . . . 11
     3.3.  Host-to-Router Interaction . . . . . . . . . . . . . . . . 12
     3.4.  Router-to-Router Interaction . . . . . . . . . . . . . . . 13
     3.5.  Neighbor Cache Management  . . . . . . . . . . . . . . . . 13
   4.  New Neighbor Discovery Options . . . . . . . . . . . . . . . . 14
     4.1.  Address Registration Option  . . . . . . . . . . . . . . . 14
     4.2.  6LoWPAN Context Option . . . . . . . . . . . . . . . . . . 16
     4.3.  Authoritative Border Router Option . . . . . . . . . . . . 18
   5.  Host Behavior  . . . . . . . . . . . . . . . . . . . . . . . . 19
     5.1.  Forbidden Actions  . . . . . . . . . . . . . . . . . . . . 19
     5.2.  Interface Initialization . . . . . . . . . . . . . . . . . 19
     5.3.  Sending a Router Solicitation  . . . . . . . . . . . . . . 20
     5.4.  Processing a Router Advertisement  . . . . . . . . . . . . 20
       5.4.1.  Address configuration  . . . . . . . . . . . . . . . . 20
       5.4.2.  Storing Contexts . . . . . . . . . . . . . . . . . . . 21
       5.4.3.  Maintaining Prefix and Context Information . . . . . . 21
     5.5.  Registration and Neighbor Unreachability Detection . . . . 22
       5.5.1.  Sending a Neighbor Solicitation  . . . . . . . . . . . 22
       5.5.2.  Processing a Neighbor Advertisement  . . . . . . . . . 22
       5.5.3.  Recovering from Failures . . . . . . . . . . . . . . . 23
     5.6.  Next-hop Determination . . . . . . . . . . . . . . . . . . 23
     5.7.  Address Resolution . . . . . . . . . . . . . . . . . . . . 24
     5.8.  Sleeping . . . . . . . . . . . . . . . . . . . . . . . . . 24



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       5.8.1.  Picking an Appropriate Registration Lifetime . . . . . 24
       5.8.2.  Behavior on Wakeup . . . . . . . . . . . . . . . . . . 24
   6.  Router Behavior for 6LR and 6LBR . . . . . . . . . . . . . . . 25
     6.1.  Forbidden Actions  . . . . . . . . . . . . . . . . . . . . 25
     6.2.  Interface Initialization . . . . . . . . . . . . . . . . . 25
     6.3.  Processing a Router Solicitation . . . . . . . . . . . . . 26
     6.4.  Periodic Router Advertisements . . . . . . . . . . . . . . 26
     6.5.  Processing a Neighbor Solicitation . . . . . . . . . . . . 27
       6.5.1.  Checking for Duplicates  . . . . . . . . . . . . . . . 27
       6.5.2.  Updating the Neighbor Cache  . . . . . . . . . . . . . 27
       6.5.3.  Address Resolution between Routers . . . . . . . . . . 28
       6.5.4.  Neighbor Unreachability Detection  . . . . . . . . . . 29
   7.  Border Router Behavior . . . . . . . . . . . . . . . . . . . . 29
     7.1.  Prefix Determination . . . . . . . . . . . . . . . . . . . 29
     7.2.  Context Configuration and Management . . . . . . . . . . . 30
   8.  Optional Behavior  . . . . . . . . . . . . . . . . . . . . . . 30
     8.1.  Multihop Prefix and Context Distribution . . . . . . . . . 31
       8.1.1.  6LBRs Sending Router Advertisements  . . . . . . . . . 31
       8.1.2.  Routers Sending Router Solicitations . . . . . . . . . 31
       8.1.3.  Routers Processing Router Advertisements . . . . . . . 32
       8.1.4.  Storing the Information  . . . . . . . . . . . . . . . 32
       8.1.5.  Sending Router Advertisements  . . . . . . . . . . . . 33
     8.2.  Multihop Duplicate Address Detection . . . . . . . . . . . 33
       8.2.1.  Special Message Validation . . . . . . . . . . . . . . 35
       8.2.2.  Conceptual Data Structures . . . . . . . . . . . . . . 35
       8.2.3.  6LR Sending a special Neighbor Solicitation  . . . . . 35
       8.2.4.  6LBR Receiving a special Neighbor Solicitation . . . . 36
       8.2.5.  Processing a special Neighbor Advertisement  . . . . . 36
       8.2.6.  Recovering from Failures . . . . . . . . . . . . . . . 37
   9.  Protocol Constants . . . . . . . . . . . . . . . . . . . . . . 37
   10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
     10.1. Message Examples . . . . . . . . . . . . . . . . . . . . . 37
     10.2. Host Bootstrapping Example . . . . . . . . . . . . . . . . 38
     10.3. Router Interaction Example . . . . . . . . . . . . . . . . 40
       10.3.1. Bootstrapping a Router . . . . . . . . . . . . . . . . 40
       10.3.2. Updating the Neighbor Cache  . . . . . . . . . . . . . 40
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 41
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 42
   13. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 42
   14. Changelog  . . . . . . . . . . . . . . . . . . . . . . . . . . 42
   15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 47
     15.1. Normative References . . . . . . . . . . . . . . . . . . . 47
     15.2. Informative References . . . . . . . . . . . . . . . . . . 48
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 49







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1.  Introduction

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

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

   o  Host-initiated interactions to allow for sleeping hosts.

   o  Elimination of multicast-based address resolution for hosts.

   o  Elimination of redirects since they are problematic on links with
      non-transitive connectivity.

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

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

   o  Optional multihop distribution of prefix and 6LoWPAN header
      compression context.

   o  Optional multihop duplicate address detection.

   The document defines three new ICMPv6 message options: the required
   Address Registration option and the optional Authoritative Border
   Router and 6LoWPAN Context options.






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1.1.  The Shortcomings of IPv6 Neighbor Discovery

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

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

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

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

   In a LoWPAN, primarily two types of network topologies are found -
   star networks and mesh networks.  A star network is similar to a
   regular IPv6 subnet with a router and a set of nodes connected to it
   via the same non-transitive link.  But in Mesh networks, the nodes
   are capable of routing and forwarding packets.  Due to the lossy
   nature of wireless communication and a changing radio environment,
   the IPv6-link node-set may change due to external physical factors.
   Thus the link is often unstable and the nodes appear to be moving
   without moving physically.




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

   Considering the above characteristics in a LoWPAN, and the IPv6
   Neighbor Discovery [RFC4861] protocol design center, some
   optimizations and extensions to Neighbor Discovery are useful for the
   wide deployment of IPv6 over low-powered and lossy networks such as
   6LoWPANs.

1.2.  Mesh-under and Route-over Concepts

   In the 6LoWPAN context, often a link-layer mesh routing mechanism is
   referred to as "mesh-under" while routing/forwarding packets using
   IP-layer addresses is referred to as "route-over".  The difference
   between mesh-under and route-over is similar to a bridged-network
   versus IP-routing using Ethernet.  In a mesh-under network all nodes
   are on the same link which is served by one or more routers, which we
   call 6LoWPAN Border Routers (6LBR).  In a route-over network, there
   are multiple links in the 6LoWPAN.  Unlike fixed IP links, these
   link's members may be changing due to the nature of the low-power and
   lossy behavior of wireless technology.  Thus a route-over network is
   made up of a flexible set of links interconnected by interior
   routers, which we call 6LoWPAN Routers (6LR).

   This specification is applicable to both mesh-under and route-over
   networks.  However, in route-over networks, we have two types of
   routers - 6LBRs and 6LRs. 6LoWPAN Border Routers sit at the boundary
   of the 6LoWPAN and the rest of the network while 6LoWPAN Routers are
   inside the LoWPAN. 6LoWPAN Routers are assumed to be running a
   routing protocol.

   In a mesh-under configuration a 6LBR is acting as the IPv6 router
   where all the hosts in the LoWPAN are on the same link, thus they are
   only one IP hop away.  No 6LoWPAN Routers exist in this topology as
   forwarding is handled by a link-layer mesh routing protocol.

   In a route-over configuration, Neighbor Discovery operations take
   place between hosts and 6LRs or 6LBRs.  The 6LR nodes are able to
   send and receive Router Advertisements, Router Solicitations as well
   as forward and route IPv6 packets.  Here packet forwarding happens at
   the routing layer.

   In both types of configurations, hosts do not take part in routing
   and forwarding packets and they act as simple IPv6 hosts.





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1.3.  Applicability, Goals and Assumptions

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

   The document has the following main goals and assumptions

   Goals:

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

   o  Make the host to router interaction the same whether mesh-under or
      route-over is used.

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

   o  Optimize the interfaces between hosts and their default routers.

   o  Support for sleeping hosts.

   o  Minimize the complexity of nodes.

   o  Disseminate context information to hosts as needed by
      [I-D.ietf-6lowpan-hc].

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

   o  Optional duplicate address detection mechanism suitable for route-
      over LoWPANs.

   Assumptions:

   o  EUI-64s are globally unique.

   o  All nodes in the LLN have an EUI-64 interface identifier in order
      to do address auto-configuration and detect duplicate addresses.

   o  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.



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

   o  When using the optional DAD mechanism of Section 8.2 6LRs either
      register with all the 6LBRs, or the network uses some out-of-scope
      mechanism to keep the 6LBRs synchronized.

   o  If IEEE 802.15.4 16-bit short addresses are used, then some
      technique is used to ensure uniqueness of those link-layer
      addresses.  That could be done using DHCPv6 or other techniques
      outside of the scope of this document.  Optionally it can be done
      using the Address Registration Option based duplicate address
      detection (Specified in Section 8.2).

   o  In order to preserve the uniqueness of addresses not derived from
      an EUI-64, they must be either assigned or checked for duplicates
      in the same way throughout the LoWPAN.  This can be done using
      DHCPv6 for assignment and/or using the duplicate address detection
      mechanism Specified in Section 8.2 (or any other protocol
      developed for that purpose).

   o  In order for [I-D.ietf-6lowpan-hc] to operate correctly, the
      compression context must match for all the hosts, 6LRs, and 6LBRs
      that can send, receive, or forward a given packet.  If Section 8.1
      is used to distribute context information this implies that all
      the 6LBRs must coordinate the context information they distribute.

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


2.  Terminology

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

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



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

   6LoWPAN link:
      A wireless link determined by single hop reachability of
      neighboring nodes.  These are considered links with undetermined
      connectivity properties as in
      [I-D.ietf-autoconf-adhoc-addr-model].

   Low-Power and Lossy Network (LLN):
      A network made up of low-power links, often with a high
      probability of packet loss and undetermined connectivity
      properties.  A LoWPAN is such a network for example.

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

   6LoWPAN Router (6LR):
      An intermediate router in the LoWPAN who can communicate with
      other 6LoWPAN routers in the same LoWPAN. 6LoWPAN routers are
      present only in route-over topologies.

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

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

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







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

   Registration:
      The process during which a LoWPAN node sends an Neighbor
      Solicitation message with an Address Registration option to a
      Router creating a Neighbor Cache entry for the LoWPAN node with a
      specific timeout.  Thus for 6LoWPAN Router the Neighbor Cache
      doesn't behave like a cache.  Instead it behaves as a registry of
      all the host addresses that are attached to the Router.


3.  Protocol Overview

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

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

   The protocol also provides for header compression
   [I-D.ietf-6lowpan-hc] by carrying header compression information in a
   new option in Router Advertisement messages.

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

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

3.1.  Extensions to RFC4861

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




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

   o  No Duplicate Address Detection (DAD) is required if EUI-64 based
      IPv6 addresses are used.

   o  DAD is optional if DHCPv6 is used to assign addresses.

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

   o  A new optional Router Advertisement option for Context information
      used by 6LoWPAN header compression.

   o  A new optional mechanism to perform Duplicate Address Detection
      across a route-over 6LoWPAN reusing the above Address Registration
      option.

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

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

3.2.  Address Assignment

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

   Optionally, 6LRs can use the same mechanisms to configure their IPv6
   addresses.




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

3.3.  Host-to-Router Interaction

   A host sends Router Solicitation messages at startup and also when it
   suspects that one of its default routers have become unreachable
   (after Neighbor Unreachability Detection towards the router fails).

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

   When a host has configured a non-link-local IPv6 address, it
   registers that address with one or more of its default routers using
   the Address Registration option (ARO) in an NS message.  The host
   chooses a lifetime of the registration and repeats the ARO option
   periodically (before the lifetime runs out) to maintain the
   registration, even while the host is sleeping.

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

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

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

   As part of the optimizations, Address Resolution is not performed by
   multicasting Neighbor Solicitation messages as in [RFC4861].
   Instead, the routers maintain Neighbor Cache entries for all



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   registered IPv6 addresses.  If the address is not in the Neighbor
   Cache in the router, then the address either doesn't exist, or is
   assigned a host attached to some other router in the 6LoWPAN, or is
   external to the 6LoWPAN.  In a route-over configuration the routing
   protocol is used to route such packets toward the destination.

3.4.  Router-to-Router Interaction

   The optional new router-to-router interaction is only for the route-
   over configuration where 6LRs are present.  It is optional in this
   protocol since the functions it provides might be better provided by
   other protocol mechanisms, be it DHCPv6, link-layer mechanisms, the
   routing protocol, or something else.  Some mechanisms from this
   protocol might be used for router-to-router, while others are
   provided by other protocols.  For instance, context information
   and/or prefix information might be disseminated using this protocol,
   while Duplicate Address Detection is done using some other protocol.

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

   When multihop prefix or context dissemination is used then the 6LRs
   store the ABRO, 6CO and Prefix Information received (directly or
   indirectly) from the 6LBRs and redistribute this information in the
   Router Advertisement they send to other 6LRs or send to hosts in
   response to a Router Solicitations.  There is a version number field
   in the ABRO which is used to limit the flooding of updated
   information between the 6LRs.

   Optionally the 6LRs can perform Duplicate Address Detection against
   one or more 6LBRs using a special form of the Address Registration
   option.  In this case, the Neighbor Solicitation and Advertisement
   messages will be forwarded between the 6LR and 6LBRs and the
   [RFC4861] rule for checking hop-limit=255 is relaxed.  Such multihop
   DAD messages MUST NOT modify any Neighbor Cache entries on the
   routers since we do not have the security benefits provided by the
   hop-limit=255 check.

3.5.  Neighbor Cache Management

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





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

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

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

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

   When a host interacts with a router by sending Router Solicitations
   this results in a Tentative NCE.  Once a node successfully registers
   with a Router the result is a Registered NCE.  As Routers send RAs to
   hosts, and when routers optionally receive RA messages or receive
   multicast NS messages from other Routers the result is Garbage-
   collectible NCEs.

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


4.  New Neighbor Discovery Options

   This section defines new Neighbor Discovery message options used by
   this specification.  The Address Registration Option is mandatory,
   whereas the Authoritative Border Router Option and 6LoWPAN Context
   Option are optional.

4.1.  Address Registration Option

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



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   same option is included in corresponding Neighbor Advertisement (NA)
   messages with a Status field indicating the success or failure of the
   registration.  This option is always host initiated.

   The ARO is reused for the optional multihop Duplicate Address
   Detection from 6LRs to 6LBRs, in which case it has a different
   Length.  In that case one or more AROs can be included in an NS.

   The ARO is required for reliability and power saving.  The lifetime
   field provides flexibility to the host to register an address which
   should be usable (continue to be advertised by the 6LR in the routing
   protocol etc.) during its intended sleep schedule.

   The sender of the NS also includes the EUI-64 of the interface it is
   registering an address from.  This is used as a unique ID for the
   detection of duplicate addresses.  It is used to tell the difference
   between the same node re-registering its address and a different node
   (with a different EUI-64) registering an address that is already in
   use by someone else.

   When the ARO is used by hosts an SLLA option MUST be included and the
   address that is registered MUST be the IPv6 source address for the
   Neighbor Solicitation message.  Thus the Registered Address field is
   omitted and the Length field MUST be two.  When the ARO is used for
   the optional multihop DAD between a 6LR and a 6LBR then there is no
   SLLA option, the Registered Address field is included and the Length
   field MUST be four.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |    Status     |   Reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Reserved            |     Registration Lifetime     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                            EUI-64                             +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                 Registered Address (Optional)                 +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   Fields:

   Type:          TBD1

   Length:        8-bit unsigned integer.  The length of the option in
                  units of 8 octets. 2 without or 4 with the Registered
                  Address.

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

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

   Registration Lifetime:  16-bit unsigned integer.  The amount of time
                  in a unit of 10 seconds that the router should retain
                  the Neighbor Cache entry for the sender of the NS that
                  includes this option.

   EUI-64:        64 bits.  This field is used to uniquely identify the
                  interface of the registered address.

   Registered Address:  128-bit optional field.  MUST NOT be sent by a
                  host.  Used for the optional router-router
                  registrations on behalf of a host.  Carries the host
                  address, which was contained in the IPv6 Source field
                  in the original NS that contained the option sent by
                  the host.

   The Status values used in Neighbor Advertisements are:

          +--------+--------------------------------------------+
          | Status |                 Description                |
          +--------+--------------------------------------------+
          |    0   |                   Success                  |
          |    1   |              Duplicate Address             |
          |    2   |             Neighbor Cache Full            |
          |  3-255 | Allocated using Standards Action [RFC2434] |
          +--------+--------------------------------------------+

                                  Table 1

4.2.  6LoWPAN Context Option

   The optional 6LoWPAN Context Option (6CO) carries prefix information
   for LoWPAN header compression, and is similar to the Prefix
   Information Option of [RFC4861].  However, the prefixes can be remote



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   as well as local to the LoWPAN since header compression potentially
   applies to all IPv6 addresses.  This option allows for the
   dissemination of multiple contexts identified by a Context Identifier
   (CID) for use as specified in [I-D.ietf-6lowpan-hc].  A context may
   be a prefix of any length or an address (/128), and up to 16 6LoWPAN
   Context options may be carried in an Router Advertisement message.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Length    |Context Length | Res |C|  CID  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Reserved           |         Valid Lifetime        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                       Context Prefix                          .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 1: 6LoWPAN Context Option format

   Type:  TBD2

   Length:  8-bit unsigned integer.  The length of the option (including
      the type and length fields) in units of 8 octets.  May be 2 or 3
      depending on the length of the Context Prefix field.

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

   C: 1-bit context compression flag.  This flag indicates if the
      context is valid for use in compression.  A context that is not
      valid MUST NOT be used for compression, but SHOULD be used in
      decompression in case another compressor has not yet received the
      updated context information.  This flag is used to manage the
      context lifecycle based on the recommendations in Section 7.2.

   CID:  4-bit Context Identifier for this prefix information.  CID is
      used by context based header compression specified in
      [I-D.ietf-6lowpan-hc].  The list of CIDs for a LoWPAN is
      configured by on the 6LBR that originates the context information
      for the 6LoWPAN.

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





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   Valid Lifetime:  16-bit unsigned integer.  The length of time in a
      unit of 10 seconds (relative to the time the packet is received)
      that the context is valid for the purpose of header compression or
      decompression.  A value of all one bits (0xffff) represents
      infinity.  A value of all zero bits (0x0) indicates that this
      context entry MUST be removed immediately.

   Context Prefix:  The IPv6 prefix or address corresponding to the
      Context ID (CID) field.  The valid length of this field is
      included in the Context Length field.  This field is padded with
      zeros in order to make the option a multiple of 8-bytes.

4.3.  Authoritative Border Router Option

   The optional Authoritative Border Router Option (ABRO) is needed when
   Router Advertisement (RA) messages are used to disseminate prefixes
   and context information across a route-over topology.  In this case
   6LRs receive Prefix Information options from other 6LRs.  This
   implies that a 6LR can't just let the most recently received RA win.
   In order to be able to reliably add and remove prefixes from the
   6LoWPAN we need to carry information from the authoritative 6LBR.
   This is done by introducing a version number which the 6LBR sets and
   6LRs propagate as they propagate the prefix and context information
   with this Authoritative Border Router Option.  When there are
   multiple 6LBRs they would have separate version number spaces.  Thus
   this option needs to carry the IP address of the 6LBR that originated
   that set of information.

   The Authoritative Border Router option MUST be included in all Router
   Advertisement messages in the case when Router Advertisements are
   used to propagate information between routers (as described in
   Section 8.2.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |  Length = 3   |        Version Number         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Reserved                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                          6LBR Address                         +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   Fields:

   Type:          TBD3

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

   Version Number:  16-bit unsigned integer.  The version number
                  corresponding to this set of information contained in
                  the RA message.  The authoritative 6LBR originating
                  the prefix increases this version number each time its
                  set of prefix or context information changes.

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

   6LBR Address:  IPv6 address of the 6LBR that is the origin of the
                  included version number.


5.  Host Behavior

   Hosts in a LoWPAN use the Address Registration option in the Neighbor
   Solicitation messages they send as a way to maintain the Neighbor
   Cache in the routers thereby removing the need for multicast Neighbor
   Solicitations to do address resolution.  Unlike in [RFC4861] the
   hosts initiate updating the information they receive in Router
   Advertisements by sending Router Solicitations before the information
   expires.  Finally, when Neighbor Unreachability Detection indicates
   that one or all default routers have become unreachable, then the
   host uses Router Solicitations to find a new set of default routers.

5.1.  Forbidden Actions

   A host in a 6LoWPAN MUST NOT accept a Redirect message.  Redirect
   messages are problematic on a link with non-transitive reachability.

   A host would never multicast a Neighbor Solicitation message.

5.2.  Interface Initialization

   When the interface on a host is initialized it follows the
   specification in [RFC4861].  A link-local address is formed based on
   the EUI-64 assigned to the interface, and then the host sends Router
   Solicitation messages as described in [RFC4861] Section 6.3.7.

   There is no need to join the Solicited-Node multicast address since
   nobody multicasts Neighbor Solicitations in this type of network.  A



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   host MUST join the all-nodes multicast address.

5.3.  Sending a Router Solicitation

   The Router Solicitation is formatted as specified in [RFC4861] and
   sent to the IPv6 All-Routers multicast address (see [RFC4861] Section
   6.3.7 for details).  A SLLA option MUST be included to enable unicast
   Router Advertisements in response.  An unspecified source address
   MUST NOT be used in RS messages.

   If the link layer supports a way to send packets to some kind of all-
   routers anycast link-layer address, then that MAY be used to convey
   theses packets to a router.

   Since hosts do not depend on multicast Router Advertisements to
   discover routers, the hosts need to intelligently retransmit Router
   Solicitations whenever the default router list is empty, default
   routers are unreachable, or the lifetime of the prefixes and contexts
   in the previous RA are about to expire.  The RECOMMENDED
   retransmissions it to initially send up to 3 (MAX_RTR_SOLICITATIONS)
   RS messages separated by at least 10 seconds
   (RTR_SOLICITATION_INTERVAL) as specified in [RFC4861], and then
   switch to slower retransmissions.  After the initial retransmissions
   the host SHOULD do binary exponential backoff of the retransmission
   timer for each subsequent retransmission.  However, it is useful to
   have a maximum retransmission timer of 60 seconds
   (MAX_RTR_SOLICITATION_INTERVAL).  In all cases the RS retransmissions
   are terminated when a RA is received.

5.4.  Processing a Router Advertisement

   The processing of Router Advertisements is as in [RFC4861] with the
   addition of handling the 6LoWPAN Context option and triggering
   address registration when a new address has been configured.
   Furthermore, the SLLA option MUST be included in the RA.  Unlike in
   [RFC4861], the maximum value of the RA Router Lifetime field MAY be
   up to 0xFFFF (18 hours).

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

5.4.1.  Address configuration

   Address configuration follows [RFC4862].  For an address not derived
   from an EUI-64, the M flag of the RA determines how the address can
   be configured.  If the M flag is set in the RA, then DHCPv6 MUST be
   used to assign the address.  If the M flag is not set, then the



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   address can be configured by any other means (and duplicate detection
   is performed as part of the registration process).

   Once an address has been configured it will be registered by
   unicasting a Neighbor Solicitation with the Address Registration
   option to one or more routers.

5.4.2.  Storing Contexts

   The host maintains a conceptual data structure for the context
   information it receives from the routers, which is called the Context
   Table.  This includes the Context ID, the prefix (from the Context
   Prefix field in the 6CO), the Compression bit, and the Valid
   Lifetime.  A Context Table entry that has the Compression bit clear
   is used for decompression when receiving packets, but MUST NOT be
   used for compression when sending packets.

   When a 6CO option is received in a Router Advertisement it is used to
   add or update the information in the Context Table.  If the Context
   ID field in the 6CO matches an existing Context Table entry, then
   that entry is updated with the information in the 6CO.  If the Valid
   Lifetime field in the 6CO is zero, then the entry is immediately
   deleted.

   If there is no matching entry in the Context Table, and the Valid
   Lifetime field is non-zero, then a new context is added to the
   Context Table.  The 6CO is used to update the created entry.

   When the 6LBR changes the context information a host might not
   immediately notice.  And in the worst case a host might have stale
   context information.  For this reason 6LBRs use the recommendations
   in Section 7.2 for carefully managing the context lifecycle.

5.4.3.  Maintaining Prefix and Context Information

   The prefix information is timed out as specified in [RFC4861].  When
   the Valid Lifetime for a Context Table entry expires the entry is
   deleted.

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



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   Solicitations is specified in section Section 5.3.

5.5.  Registration and Neighbor Unreachability Detection

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

   Hosts that receive Router Advertisement messages from multiple
   default routers SHOULD attempt to register with more than one of them
   in order to increase the robustness of the network.

   Note that Neighbor Unreachability Detection probes can be suppressed
   if by Reachability Confirmations from transport protocols or
   applications as specified in [RFC4861].

5.5.1.  Sending a Neighbor Solicitation

   The host triggers sending Neighbor Solicitation (NS) messages
   containing an ARO when a new address is configured, when it discovers
   a new default router, or well before the Registration Lifetime
   expires.  Such an NS MUST include a Source Link-Layer Address (SLLA)
   option, since the router needs to record the link-layer address of
   the host.  An unspecified source address MUST NOT be used in NS
   messages.

5.5.2.  Processing a Neighbor Advertisement

   A host handles Neighbor Advertisement messages as specified in
   [RFC4861], with added logic described in this section for handling
   the Address Registration option.

   In addition to the normal validation of a Neighbor Advertisement and
   its options, the Address Registration option is verified as follows
   (if present).  If the Length field is not two, the option is silently
   ignored.  If the EUI-64 field does not match the EUI-64 of the
   interface, the option is silently ignored.

   If the status field is zero, then the address registration was



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   successful.  The host saves the Registration Lifetime from the
   Address Registration option for use to trigger a new NS well before
   the lifetime expires.  If the Status field is not equal to zero, the
   address registration has failed.

5.5.3.  Recovering from Failures

   The procedure for maintaining reachability information about a
   neighbor is the same as in [RFC4861] Section 7.3 with the exception
   that address resolution is not performed.

   The address registration procedure may fail for two reasons: no
   response to Neighbor Solicitations is received (NUD failure), or an
   Address Registration option with a failure Status (Status > 0) is
   received.  In the case of NUD failure the entry for that router will
   be removed thus address registration is no longer of importance.
   When an Address Registration option with a non-zero Status field is
   received this indicates that registration for that address has
   failed.  A failure Status of one indicates that a duplicate address
   was detected and the procedure described in [RFC4862] Section 5.4.5
   is followed.  The host MUST NOT use the address it tried to register.
   If the host has valid registrations with other routers, these MUST be
   removed by registering with each using a zero ARO lifetime.

   A Status code of two indicated that the Neighbor Cache of that router
   is full.  In this case the host SHOULD remove this router from its
   default router list and attempt to register with another router.  If
   the host has no more default routers it needs to revert to sending
   Router Solicitations as specified in section Section 5.3.

   Other failure codes may be defined in future documents.

5.6.  Next-hop Determination

   The IP address of the next-hop for a destination is determined as
   follows.  Destinations to the link-local prefix (FE80::) are always
   sent on the link to that destination.  All other prefixes are assumed
   to be off-link [I-D.ietf-autoconf-adhoc-addr-model].  Anycast
   addresses are always considered to be off-link.  They are therefore
   sent to one of the routers in the Default Router List.

   Multicast addresses are considered to be on-link and are resolved as
   specified in [RFC4944] or the appropriate IP-over-foo document.

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




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5.7.  Address Resolution

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

   Link-layer addresses for neighbors are stored in Neighbor Cache
   entries [RFC4861] In order to achieve LoWPAN compression, most global
   addresses are formed using a link-layer address.  Thus a host can
   minimize memory usage by optimizing for this case and only storing
   link-layer address information if it differs from the link-layer
   address corresponding to the Interface ID of the IPv6 address (i.e.,
   differs in more than the on-link/global bit being inverted).

5.8.  Sleeping

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

5.8.1.  Picking an Appropriate Registration Lifetime

   As all Neighbor Discovery messages are initiated by the hosts, this
   allows a host to sleep or otherwise be unreachable between NS
   messages.  The Address Registration option attached to NS messages
   indicates to a router to keep the Neighbor Cache entry for that
   address valid for the period in the Registration Lifetime field.  A
   host should choose a sleep time appropriate for its energy
   characteristics, and set a registration lifetime larger than the
   sleep time to ensure the registration is renewed successfully
   (considering e.g. clock drift).  A host should also consider the
   stability of the network (how quickly the topology changes) when
   choosing its sleep time (and thus registration lifetime).  A dynamic
   network may require a shorter sleep time.

5.8.2.  Behavior on Wakeup

   When a host wakes up from a sleep period it SHOULD maintain its
   current address registrations that will timeout before the next
   wakeup.  This is done by sending Neighbor Solicitation messages with
   the Address Registration option as described in Section 5.5.1.  The
   host may also need to refresh its prefix and context information by
   sending a new unicast Router Solicitation (the maximum Router



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   Lifetime is 18 hours whereas the maximum Registration lifetime is 7
   days).  If after wakeup the host (using NUD) determines that some or
   all previous default routers have become unreachable, then the host
   will restart doing multicast Router Solicitations to discover new
   default router(s) and restart the address registration process.


6.  Router Behavior for 6LR and 6LBR

   Both 6LRs and 6LBRs maintain the Neighbor Cache [RFC4861] based on
   the Address Registration Options they receive in Neighbor
   Advertisement messages from hosts, Neighbor Discovery packets from
   other nodes, and potentially a routing protocol used in the 6LoWPAN
   as outlined in Section 3.5.  Note that the handling of ARO from other
   routers (with Length=4) is specified in Section 8.

   The routers SHOULD NOT garbage collect Registered Neighbor Cache
   entries (see Section 3.4) since they need to retain them until the
   Registration Lifetime expires.  Similarly, if Neighbor Unreachability
   Detection on the router determines that the a host is UNREACHABLE
   (based on the logic in [RFC4861]), the Neighbor Cache entry SHOULD
   NOT be deleted but be retained until the Registration Lifetime
   expires.  A renewed ARO should mark the cache entry as STALE.  Thus
   for 6LoWPAN Routers the Neighbor Cache doesn't behave like a cache.
   Instead it behaves as a registry of all the host addresses that are
   attached to the Router.

   Routers MAY implement the Default Router Preferences [RFC4191] and
   use that to indicate to the host whether the router is a 6LBR or a
   6LR.  If this is implemented then 6LRs with no route to a border
   router MUST set Prf to (11) for low preference, other 6LRs MUST set
   Prf to (00) for normal preference, and 6LBRs MUST set Prf to (01) for
   high preference.

6.1.  Forbidden Actions

   A router SHOULD NOT send Redirect messages.  Since the link has non-
   transitive reachability the router has no way to determine that the
   recipient of a Redirect message can reach the link-layer address.

   A router MUST NOT set the 'L' (on-link) flag in the Prefix
   Information options, since that might trigger hosts to send multicast
   Neighbor Solicitations.

6.2.  Interface Initialization

   A router initializes its interface more or less as in [RFC4861].
   However, a 6LR might want to wait to make its interfaces advertising



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   (implicitly keeping the AdvSendAdvertisements flag clear) until it
   has received the prefix(es) and context information from its 6LBR.
   That is independent of whether prefixes and context information is
   disseminated using the methods specified in this document, or using
   some other method.

6.3.  Processing a Router Solicitation

   A router processes Router Solicitation messages as specified in
   [RFC4861].  The differences relate to the inclusion of Authoritative
   Border Router options in the Router Advertisement (RA) messages, and
   the exclusive use of unicast Router Advertisements.  If a 6LR has
   received an ABRO from a 6LBR, then it will include that option
   unmodified in the Router Advertisement messages it sends.  And if the
   6LR has received RAs, whether with the same prefixes and context
   information or different, from different 6LBR, then it will need to
   keep those prefixes and context information separately so that the
   RAs the 6LR sends will maintain the association between the ABRO and
   the prefixes and context information.  The router can tell which 6LBR
   originated the prefixes and context information from the 6LBR Address
   field in the ABRO.  When a router has information tied to multiple
   ABROs, a single RS will result in multiple RAs each containing a
   different ABRO.

   A Router Solicitation might be received from a host that has not yet
   registered its address with the router.  Thus the router MUST NOT
   modify an existing Neighbor Cache entry based on the SLLA option from
   the Router Solicitation.  However, a router MAY create a Tentative
   Neighbor Cache entry based on the SLLA option.  Such a Tentative
   Neighbor Cache entry SHOULD be timed out in TENTATIVE_NCE_LIFETIME
   seconds unless a registration converts it into a Registered NCE.

   A 6LR or 6LBR MUST include a Source Link-layer address option in the
   Router Advertisements it sends.  That is required so that the hosts
   will know the link-layer address of the router.  Unlike in [RFC4861],
   the maximum value of the RA Router Lifetime field MAY be up to 0xFFFF
   (18 hours).

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

6.4.  Periodic Router Advertisements

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



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   However, if the routers use Router Advertisements to optionally
   distribute prefix and/or context information across a route-over
   topology, that might require periodic Router Advertisement messages.
   Such RAs are sent using the configurable MinRtrAdvInterval and
   MaxRtrAdvInterval as per [RFC4861].

6.5.  Processing a Neighbor Solicitation

   A router handles Neighbor Solicitation messages as specified in
   [RFC4861], with added logic described in this section for handling
   the Address Registration option.

   In addition to the normal validation of a Neighbor Solicitation and
   its options, the Address Registration option is verified as follows
   (if present).  If the Length field is not two, or if the Status field
   is not zero, then the Neighbor Solicitation is silently ignored.
   Note that Section 8.2 specify optional behavior for a 6LBR for other
   Length field values.

   If the source address of the NS is the unspecified address, or if no
   SLLA option is included, then any included ARO is ignored, that is,
   the NS is processed as if it did not contain an ARO.

6.5.1.  Checking for Duplicates

   If the NS contains a valid ARO, then the router inspects its Neighbor
   Cache on the arriving interface to see if it is a duplicate.  If
   there is no Neighbor Cache entry for the IPv6 source address of the
   NS, then it isn't a duplicate.  If there is such a Neighbor Cache
   entry and the EUI-64 is the same, then it isn't a duplicate either.
   Otherwise it is a duplicate address.  Note that if multihop DAD
   (Section 8.2) is used then the checks are slightly different to take
   into account Tentative Neighbor Cache entries.

   In the case it is a duplicate address then the router responds with a
   unicast Neighbor Advertisement (NA) message sent to the source link-
   layer address (from the Source Link-Layer Address (SLLA) option) of
   the NS.  The NA is formatted with a copy of the ARO from the NS, but
   with the Status field set to one to indicate it was a duplicate.  In
   this case there is no modification to the Neighbor Cache.

6.5.2.  Updating the Neighbor Cache

   If ARO did not result in a duplicate address being detected as above,
   then if the Registration Lifetime is non-zero the router creates (if
   it didn't exist) or updates (otherwise) a Neighbor Cache entry for
   the IPv6 source address of the NS.  If the Neighbor Cache is full and
   a new entry needs to be created, then the router responds with a



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   unicast NA sent to source link-layer address (from the Source Link-
   Layer Address (SLLA) option) of the NS.  The NA is formatted with a
   copy of the ARO from the NS, but with the Status field set to two to
   indicate the router's Neighbor Cache is full.

   The Registration Lifetime and the EUI-64 are recorded in the Neighbor
   Cache entry.  A unicast Neighbor Advertisement (NA) is then sent in
   response to the NS.  This NA SHOULD include a copy of the ARO, with
   the Status field set to zero.  A TLLA option is not required in the
   NA, since the host already knows the router's link-layer address from
   Router Advertisements.

   If the ARO contains a zero Registration Lifetime then any existing
   Neighbor Cache entry for the IPv6 source address of the NS MUST be
   deleted, and a NA sent as above.

   Should the Registration Lifetime in a Neighbor Cache entry expire,
   then the router MUST delete the cache entry.

   The addition and removal of Registered Neighbor Cache entries would
   result in notifying the routing protocol.

   Note: If the optional multihop DAD (Section 8.2) is used, then the
   updating of the Neighbor Cache is slightly different due to Tentative
   NCEs.

6.5.3.  Address Resolution between Routers

   There needs to be a mechanism somewhere for the routers to discover
   each other's link-layer addresses.  If the routing protocol used
   between the routers provides this, then there is no need for the
   routers to use the Address Registration option between each other.
   Otherwise, the routers MAY use the ARO.  When routers use ARO to
   register with each other and the optional multihop DAD Section 8.2 is
   in use, then care should be taken to ensure that there isn't a flood
   of ARO-carrying messages sent to the 6LBR as each router hears an ARO
   from their neighboring routers.  The details for this is out of scope
   of this document.

   Optionally Routers can use multicast Neighbor Solicitations as in
   [RFC4861] to resolve each others link-layer addresses.  Thus Routers
   MAY multicast Neighbor Solicitations for other routers, for example
   as a result of receiving some routing protocol update.  Routers MUST
   respond to multicast Neighbor Solicitations.  This implies that
   Routers MUST join the Solicited-node multicast addresses as specified
   in [RFC4861].





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6.5.4.  Neighbor Unreachability Detection

   Just like in [RFC4861] the use of NUD from routers to hosts is
   required.  NUD may also be used between routers, but is not required
   if an equivalent mechanism is available, for example, as part of the
   routing protocols.


7.  Border Router Behavior

   A 6LBR handles sending of Router Advertisements and processing of
   Neighbor Solicitations from hosts as specified above in section
   Section 6.  A 6LBR SHOULD always include an Authoritative Border
   Router option in the Router Advertisements it sends, listing itself
   as the 6LBR Address.  That requires that the 6LBR maintain the
   version number in stable storage, and increases the version number
   when some information in its Router Advertisements change.  The
   information whose change affects the version are in the Prefix
   Information options (the prefixes or their lifetimes) and in the 6CO
   option (the prefixes, Context IDs, or lifetimes.)

   In addition, a 6LBR is somehow configured with the prefix or prefixes
   that are assigned to the LoWPAN, and advertises those in Router
   Advertisements as in [RFC4861].  Optionally, in the case of route-
   over, those prefixes can be disseminated to all the 6LRs using the
   technique in Section 8.1.  However, there might be mechanisms outside
   of the scope of this document that can be used instead for prefix
   dissemination with route-over.

   If the 6LoWPAN uses Header Compression [I-D.ietf-6lowpan-hc] then the
   6LBR needs to manage the context IDs, and advertise those in Router
   Advertisements by including 6CO options in its Router Advertisements
   so that directly attached hosts are informed about the context IDs.
   Below we specify things to consider when the 6LBR needs to add,
   remove, or change the context information.  Optionally, in the case
   of route-over, the context information can be disseminated to all the
   6LRs using the technique in Section 8.  However, there might be
   mechanisms outside of the scope of this document that can be used
   instead for disseminating context information with route-over.

7.1.  Prefix Determination

   The prefix or prefixes used in a LoWPAN can be manually configured,
   or can be acquired using DHCPv6 Prefix Delegation [RFC3633].  For a
   LoWPAN that is isolated from the network, either permanently or
   occasionally, the 6LBR can assign a ULA prefix using [RFC4193].  The
   ULA prefix should be stored in stable storage so that the same prefix
   is used after a failure of the 6LBR.  If the LoWPAN has multiple



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   6LBRs, then they should be configured with the same set of prefixes.
   The set of prefixes are included in the Router Advertisement messages
   as specified in [RFC4861].

7.2.  Context Configuration and Management

   If the LoWPAN uses Header Compression [I-D.ietf-6lowpan-hc] then the
   6LBR may be configured with context information and related context
   IDs.  If the LoWPAN has multiple 6LBRs, then they MUST be configured
   with the same context information and context IDs.

   The context information carried in Router Advertisement (RA) messages
   originate at 6LBRs and must be disseminated to all the routers and
   hosts within the LoWPAN.  RAs include one 6CO for each context.

   For the dissemination of context information using the 6CO, a strict
   lifecycle SHOULD be used in order to ensure the context information
   stays synchronized throughout the LoWPAN.  New context information
   SHOULD be introduced into the LoWPAN with C=0, to ensure it is known
   by all nodes that may have to decompress based on this context
   information.  Only when it is reasonable to assume that this
   information was successfully disseminated SHOULD an option with C=1
   be sent, enabling the actual use of the context information for
   compression.

   Conversely, to avoid that nodes send packets making use of previous
   values of contexts, resulting in ambiguity when receiving a packet
   that uses a recently changed context, old values of a context SHOULD
   be taken out of use for a while before new values are assigned to
   this specific context.  That is, in preparation for a change of
   context information, its dissemination SHOULD continue for at least
   MIN_CONTEXT_CHANGE_DELAY with C=0.  Only when it is reasonable to
   assume that the fact that the context is now invalid was successfully
   disseminated, should the context ID be taken out of dissemination or
   reused with a different Context Prefix field.  In the latter case,
   dissemination of the new value again SHOULD start with C=0, as above.


8.  Optional Behavior

   Optionally the Router Advertisement messages can be used to
   disseminate prefixes to all the 6LRs in a route-over topology.  This
   is optional because there are likely to be other mechanisms for such
   information distribution.

   There is also the option to reuse the Address Registration option as
   a way for a 6LR to perform DAD (for non-EUI-64 derived IPv6
   addresses) against a 6LBR in a route-over topology.  This is optional



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   because there might be other ways to either allocate unique address,
   such as DHCPv6 [RFC3315], or other future mechanisms for multihop
   DAD.

8.1.  Multihop Prefix and Context Distribution

   The multihop distribution relies on Router Solicitation messages and
   Router Advertisement (RA) messages sent between routers, and using
   the ABRO version number to control the propagation of the information
   (prefixes and context information) that is being sent in the RAs.

   This multihop distribution mechanism can handle arbitrary information
   from an arbitrary number of 6LBRs.  However, the semantics of the
   context information requires that all the 6LNs use the same
   information, whether they send, forward, or receive compressed
   packets.  Thus the manager of the 6LBRs need to somehow ensure that
   the context information is in synchrony across the 6LBRs.  This can
   be handled in different ways.  One possible way to ensure it is to
   treat the context and prefix information as originating from some
   logical or virtual source, which in essence means that it looks like
   the information is distributed from a single source.

   If a set of 6LBRs behave as a single one (using mechanisms out of
   scope of this document) so that the prefixes and contexts and ABRO
   version number will be the same from all the 6LBRs, then those 6LBRs
   can pick a single IP address to use in the ABRO option.

8.1.1.  6LBRs Sending Router Advertisements

   6LBRs supporting multihop prefix and context distribution MUST
   include an ABRO in each of its RAs.  The ABRO Version Number field is
   used to keep prefix and context information consistent throughout the
   LoWPAN along with the guidelines in Section 7.2.  Each time any
   information in the set of PIO or 6CO options change, the ABRO Version
   is increased by one.

   This requires that the 6LBR maintain the PIO, 6CO, and ABRO Version
   Number in stable storage, since an old version number will be
   silently ignored by the 6LRs.

8.1.2.  Routers Sending Router Solicitations

   If multihop distribution is done using Router Advertisement (RA)
   messages, then on interface initialization a router SHOULD send some
   Router Solicitation messages similarly to how hosts do this in
   [RFC4861].  That will cause the routers to respond with RA messages
   which then can be used to initially seed the prefix and context
   information.



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8.1.3.  Routers Processing Router Advertisements

   If multihop distribution is not done using RA messages, then the
   routers follow [RFC4861] which states that they merely do some
   consistency checks and nothing in Section 8.1 applies.  Otherwise the
   routers will check and record the prefix and context information from
   the receive RAs, and use that information as follows.

   If a received RA does not contain a Authoritative Border Router
   option, then the RA MUST be silently ignored.

   The router uses the 6LBR Address field in the ABRO to check if it has
   previously received information from the 6LBR.  If it finds no such
   information, then it just records the 6LBR Address and Version and
   the associated prefixes and context information.  If the 6LBR is
   previously known, then the Version number field MUST be compared
   against the recorded version number for that 6LBR.  The comparison
   MUST be done the same way as TCP sequence number comparisons to
   handle the case when the version number wraps around. if the version
   number received in the packet is greater than the stored version
   number (following [RFC1982] Section 3.2), then the information in the
   RA is silently ignored.  Otherwise the recorded information and
   version number are updated.

   By TCP sequence number comparison we mean that half of the version
   number space is "old" and half is "new".  For example, if the current
   version number is 0x2, then anything between 0x80000003 (0x2-
   0x7fffffff) and 0x1 is old, and anything between 0x3 and 0x80000002
   (0x2+0x8000000) is new.

8.1.4.  Storing the Information

   The router keeps state for each 6LBR that it sees with an ABRO.  This
   includes the version number, and the complete set of Prefix
   Information options and 6LoWPAN Context options.  The prefixes are
   timed out based on the Valid lifetime in the Prefix Information
   Option.  The Context Prefix is timed out based on the Valid lifetime
   in the 6LoWPAN Context option.

   While the prefixes and context information are stored in the router
   their valid and preferred lifetimes are decremented as time passes.
   This ensures that when the router is in turn later advertising that
   information in the Router Advertisements it sends, the 'expiry time'
   doesn't accidentally move further into the future.  For example, if a
   6CO with a Valid lifetime of 10 minutes is received at time T, and
   the router includes this in a RA it sends at time T+5 minutes, the
   Valid lifetime in the 6CO it sends will be only 5 minutes.




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8.1.5.  Sending Router Advertisements

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

   The routers periodically send Router Advertisements as in [RFC4861].
   This is for the benefit of the other routers receiving the prefixes
   and context information.  And the routers also respond to Router
   Solicitations by unicasting RA messages.  In both cases the above
   constraint of keeping the ABRO together with 'its' prefixes and
   context information apply.

   When a router receives new information from a 6LBR, that is, either
   it hears from a new 6LBR (a new 6LBR Address in the ABRO) or the ABRO
   version number of an existing 6LBR has increased, then it is useful
   to send out a few triggered updates.  The recommendation is to behave
   the same as when an interface has become an advertising interface in
   [RFC4861], that is, send up to three RA messages.  This ensures rapid
   propagation of new information to all the 6LRs.

8.2.  Multihop Duplicate Address Detection

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

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




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   The multihop DAD mechanism is used synchronously the first time an
   address is registered with a particular 6LR.  That is, the ARO option
   is not returned to the host until multihop DAD has been completed
   against the 6LBRs.  For existing registrations in the 6LR the
   multihop DAD needs to be repeated against the 6LBRs to ensure that
   the entry for the address in the 6LBRs does not time out, but that
   can be done asynchronously with the response to the hosts.  For
   instance, by tracking how much is left of the lifetime the 6LR
   registered with the 6LBRs and re-registering with the 6LBR when this
   lifetime is about to run out.

   For the synchronous multihop DAD the 6LR performs some additional
   checks to ensure that it has a Neighbor Cache entry it can use to
   respond to the host when it receives a response from a 6LBR.  This
   consists of checking for an already existing (Tentative or
   Registered) Neighbor Cache entry for the registered address with a
   different EUI-64.  If such a Registered NCE exists, then the 6LR
   SHOULD respond that the address is a duplicate.  If such a Tentative
   NCE exists, then the 6LR SHOULD silently ignore the ARO thereby
   relying on the host retransmitting the ARO.  (This is needed to
   handle the case when multiple hosts try to register the same IPv6
   address at the same time.)  If no Neighbor Cache entry exists, then
   the 6LR MUST create a Tentative Neighbor Cache entry with the EUI-64
   and the SLLAO.  This entry will be used to send the response to the
   host when the 6LBR responds.

   When a 6LR receives a Neighbor Solicitation containing an Address
   Registration option with a non-zero Registration Lifetime and it has
   no existing Registered Neighbor Cache entry, then with this mechanism
   the 6LR will invoke synchronous multihop DAD.

   The 6LR will unicast a new Neighbor Solicitation message to one or
   more 6LBRs, where the NS contains an ARO with the host's address in
   the Registered Address field.  This NS will be forwarded by 6LRs
   until it reaches the 6LBR, hence its IPv6 hop limit field might be
   less than 255 when received by the 6LBR.  The 6LBR will respond with
   a Neighbor Advertisement message containing an ARO, which might have
   a hop limit less than 255 when it reaches the 6LR.

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




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   A Tentative Neighbor Cache entry SHOULD be timed out
   TENTATIVE_NCE_LIFETIME seconds after it was created in order to allow
   for another host to attempt to register the IPv6 address.

8.2.1.  Special Message Validation

   Due to the forwarding of the above special NS/NA between the 6LR and
   6LBR the hop limit check on receipt MUST be bypassed for such
   messages that contain a ARO with a Length field of 4.  The receipt of
   such messages MUST NOT modify any state on the router with the
   exception of the DAD table below.

8.2.2.  Conceptual Data Structures

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

8.2.3.  6LR Sending a special Neighbor Solicitation

   When a 6LR that implements the optional multihop DAD receives an NS
   from a host (the ARO has Length = 2) and subject to the above checks,
   the 6LR forms and sends an NS to at least one 6LBR.  The NS contains
   the following information:

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

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

   o  In the IPv6 hop limit, 255 or a smaller number.

   o  In the NS Target Address, the address of the 6LBR.

   o  In the ARO the Status field MUST be set to zero

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

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

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




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   A router MUST NOT modify the Neighbor Cache as a result of receiving
   a Neighbor Solicitation with an ARO of Length=4.

8.2.4.  6LBR Receiving a special Neighbor Solicitation

   When a 6LBR that implements the optional multihop DAD receives an NS
   from a 6LR, that is an NS that contains an ARO with Length = 4, then
   it MUST NOT verify that the hop limit is 255 as specified above.
   Then it proceeds to look for the Registration Address in the DAD
   Table.  If an entry is found and the recorded EUI-64 is different
   than the EUI-64 in the ARO, then it returns an NA with the ARO Status
   set to 1 ('Duplicate Address').  Otherwise it returns an NA with ARO
   Status set to zero.

   If no entry is found in the DAD Table and the Registration Lifetime
   is non-zero, then an entry is created and the EUI-64 and Registered
   Address from the ARO are stored in that entry.

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

   In both of the above cases the ARO Status code is set to zero, and
   the 6LBR forms an NA with the ARO copied from the NS to the NA.  The
   NA is sent back to the 6LR i.e., back to the source of the NS.

8.2.5.  Processing a special Neighbor Advertisement

   When a 6LR that implements the optional multihop DAD receives an NA
   from a 6LBR, that is an NS that contains an ARO with Length = 4, then
   it MUST NOT verify that the hop limit is 255 as specified above.  If
   there is no Tentative Neighbor Cache entry matching the Registered
   address and EUI-64, then NA is silently ignored.  Otherwise, the
   information from the 6LBR is used to form an NA to send to the host.
   The Status code is copied from the ARO received from the 6LBR to the
   ARO that is sent to the host.

   If the Status is non-zero indicating an error, then the Tentative
   Neighbor Cache entry for the Registered Address is removed.
   Otherwise it is made into a Registered Neighbor Cache entry.

   A router MUST NOT modify the Neighbor Cache as a result of receiving
   a Neighbor Advertisement with an ARO of Length=4, unless there is a
   Tentative Neighbor Cache entry matching the IPv6 address and EUI-64.







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8.2.6.  Recovering from Failures

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


9.  Protocol Constants

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

   Additional protocol constants are defined in Section Section 4.

   6LBR Constants:

   MIN_CONTEXT_CHANGE_DELAY+               60 seconds

   6LR Constants:

   MAX_RTR_ADVERTISEMENTS                  3 transmissions

   MIN_DELAY_BETWEEN_RAS*                  10 seconds

   MAX_RA_DELAY_TIME*                      2 seconds

   TENTATIVE_NCE_LIFETIME+                 20 seconds

   Host Constants:

   RTR_SOLICITATION_INTERVAL*              10 seconds

   MAX_RTR_SOLICITATIONS                   3 transmissions

   MAX_RTR_SOLICITATION_INTERVAL+          60 seconds


10.  Examples

10.1.  Message Examples

   The following diagrams show the two-step process of the optimized
   Neighbor Discovery mechanisms for bootstrapping and Address
   Registration.





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   Node                                                     Router

    |                                                          |

    |       ---------- Router Solicitation -------->           |
    |                       [SLLAO]                            |


    |                                                          |

    |       <-------- Router Advertisement ---------           |

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




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




   Node                                                      Router

    |                                                          |

    |       ------- NS with Address Registration ----->        |

    |                     [ARO + SLLAO]                        |

    |                                                          |

    |       <-----NA with Address Registration ---------       |

    |                   [ARO with Status]                      |




             Figure 3: Neighbor Discovery Address Registration

10.2.  Host Bootstrapping Example

   The following example describes the address bootstrapping scenarios
   using the optimized ND mechanisms specified in this document.  It is
   assumed that the 6LN first performs a sequence of operations in order
   to get secure access at the link-layer of the LoWPAN and obtain a key



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   for link-layer security.  The methods of how to establish the link-
   layer security is out of scope of this document.  In this example an
   IEEE 802.15.4 6LN forms a 16-bit short-address based IPv6 addresses
   without using DHCPv6 (i.e., the M flag is not set in the Router
   Advertisements).

   1.  After obtaining link-level security, a 6LN assigns a link-local
   IPv6 address to itself.  A link-local IPv6 address is configured
   based on the 6LN's EUI-64 link-layer address formed as per [RFC4944].

   2.  Next the 6LN determines one or more default routers in the
   network by sending an RS to the all-routers multicast address with
   the SLLA Option set to its EUI-64 link-local address.  If the 6LN was
   able to obtain the link-layer address of a router through its link-
   layer operations then the 6LN may form a link-local destination IPv6
   address for the router and send it a unicast RS.  The 6LR responds
   with a unicast RA to the IP source using the SLLAO from the RS (it
   created a tentative NCE).

   3.  In order to communicate more than one IP hop away the 6LN
   configures a global IPv6 address.  In order to save overhead, this
   6LN wishes to configure its IPv6 address based on a 16-bit short
   address as per [RFC4944].  As the network is unmanaged (M flag not
   set in RA), the 6LN randomly chooses a 16-bit link-layer address and
   forms a tentative IPv6 address from it.

   4.  Next the 6LN registers that address with one or more of its
   default routers by sending a unicast NS message with an ARO
   containing its tentative global IPv6 address to register, the
   registration lifetime and its EUI-64.  An SLLAO is also included with
   the link-layer address corresponding to the address being registered.
   If a successful (status 0) NA message is received the address can
   then be used and the 6LN assumes it has been successfully checked for
   duplicates.  If a duplicate address (status 1) NA message is
   received, the 6LN then removes the temporary IPv6 address and 16-bit
   link-layer address and goes back to step 3.  If a neighbor cache full
   (status 2) message is received, the 6LN attempts to register with
   another default router, or if none, goes back to step 2.

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

   If multihop DAD and multihop prefix and context distribution is used,
   the effect of the 6LRs and hosts following the above bootstrapping is
   a "wavefront" of 6LRs and host being configured spreading from the
   6LBRs.  First the hosts and 6LRs that can directly reach a 6LBR would
   receive one or more RAs and configure and register their IPv6
   addresses.  Once that is done they would enable the routing protocol



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   and start sending out Router Advertisements.  That would result in a
   new set of 6LRs and hosts to receive responses to their Router
   Solicitations, form and register their addresses, etc.  That repeats
   until all of the 6LRs and hosts have been configured.

10.3.  Router Interaction Example

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

   At the initialization of a 6LR, it may choose to bootstrap as a host
   with the help of a parent 6LR if the optional multihop DAD is
   performed with the 6LBR.  The neighbor cache management of a router
   and address resolution among the neighboring routers are described in
   Section 6.5.2 and Section 6.5.3, respectively.  In this example, we
   assume that the neighboring 6lowpan link is secure.

10.3.1.  Bootstrapping a Router

   In this scenario, the bootstrapping 6LR, 'R1', is multiple hops away
   from the 6LBR and surrounded by other 6LR neighbors.  Initially R1
   behaves as a host.  It sends multicast RS and receives an RA from one
   or more neighboring 6LRs.  R1 picks one 6LR as its temporary default
   router and performs address resolution via this default router.
   Note, if multihop DAD is not required (e.g. in a managed network or
   using EUI-64 based addresses) then it does not need to pick a
   temporary default router, however it may still want to send the
   initial RS message if it wants to autoconfigure its address with the
   global prefix disseminated by the 6LBR.

   Based on the information received in the RAs, R1 updates its cache
   with entries for all the neighboring 6LRs.  Upon completion of the
   address registration, the bootstrapping router deletes the temporary
   entry of the default router and the routing protocol is started.

   Also note that R1 may refresh its multihop DAD registration directly
   with the 6LBR (using the nexthop neighboring 6LR determined by the
   routing protocol for reaching the 6LBR).

10.3.2.  Updating the Neighbor Cache

   In this example, there are three 6LRs, R1, R2, R3.  Initially when R2
   boots it sees only R1, and accordingly R2 creates a neighbor cache
   entry for R1.  Now assume R2 receives a valid routing update from
   router R3.  R2 does not have any neighbor cache entry for R3.  If the
   implementation of R2 supports detecting link-layer address from the



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   routing information packets then it directly updates the its neighbor
   cache using that link-layer information.  If this is not possible,
   then R2 should perform multicast NS with source set with its link-
   local or global address depending on the scope of the source IP-
   address received in the routing update packet.  The target address of
   the NS message is the source IPv6 address of the received routing
   update packet.  The format of the NS message is as described in
   Section 4.3 of [RFC4861].

   More generally any 6LR that receives a valid route-updates from a
   neighboring router for which it does not have any neighbor cache
   entry is required to update its neighbor cache as described above.

   The router (6LR and 6LBR) IP-addresses learned via Neighbor Discovery
   are not redistributed to the routing protocol.


11.  Security Considerations

   The security considerations of IPv6 Neighbor Discovery [RFC4861]
   apply.  Additional considerations can be found in [RFC3756].

   This specification expects that the link layer is sufficiently
   protected, for instance using MAC sublayer cryptography.  In other
   words, model 1 from [RFC3756] applies.  In particular, it is expected
   that the LoWPAN MAC provides secure unicast to/from Routers and
   secure broadcast from the Routers in a way that prevents tampering
   with or replaying the Router Advertisement messages.  However, any
   future 6LoWPAN security protocol that applies to Neighbor Discovery
   for 6LoWPAN protocol, is out of scope of this document.

   The multihop DAD mechanisms rely on Neighbor Solicitation and
   Neighbor Advertisement messages that are forwarded by 6LRs, and as a
   result the hop_limit=255 check on the receiver is disabled for such
   messages.  This implies that any node on the Internet could
   successfully send such messages.  We avoid any additional security
   issues due to this by requiring that the routers never modify the
   Neighbor Cache entry due to such messages, and that they reject them
   unless they are received on an interface that has been explicitly
   configured to use these LLN optimizations.

   In some future deployments one might want to use SEcure Neighbor
   Discovery [RFC3971] [RFC3972].  This is possible with the Address
   Registration option as sent between hosts and routers, since the
   address that is being registered is the IPv6 source address of the
   Neighbor Solicitation and SeND verifies the IPv6 source address of
   the packet.  Applying SeND to the optional router-to-router
   communication in this document is out of scope.



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12.  IANA Considerations

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

   o  Address Registration Option (TBD1)

   o  6LoWPAN Context Option (TBD2)

   o  Authoritative Border Router Option (TBD3)

   For the purpose of protocol interoperability testing of this
   specification, the following values are being used temporarily:

   o  TBD1 = 31

   o  TBD2 = 32

   o  TBD3 = 33

   [TO BE REMOVED: This registration should take place at the following
   location: http://www.iana.org/assignments/icmpv6-parameters]


13.  Acknowledgments

   The authors thank Pascal Thubert, Jonathan Hui, Carsten Bormann,
   Richard Kelsey, Geoff Mulligan, Julien Abeille, Alexandru Petrescu,
   Peter Siklosi, Pieter De Mil, Fred Baker, Anthony Schoofs, Phil
   Roberts, Daniel Gavelle, Joseph Reddy, Robert Cragie, Mathilde Durvy
   and Joakim Eriksson for useful discussions and comments that have
   helped shaped and improve this document.

   Additionally, the authors would like to recognize Carsten Bormann for
   the suggestions on the Context Prefix Option and contribution to
   earlier version of the draft, Pascal Thubert for contribution of the
   original registration idea and extensive contributions to earlier
   versions of the draft, Jonathan Hui for original ideas on prefix/
   context distribution and extensive contributions to earlier versions
   of the draft, Geoff Mulligan for suggesting the use of Address
   Registration as part of existing IPv6 Neighbor Discovery messages,
   and Mathilde Durvy for helping to clarify router interaction.


14.  Changelog

   Changes from -11 to -12:




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      o Version field of ABRO moved after Length for 32-bit alignment of
      the reserved space (#90).

      o Several clarifications were made on router interaction,
      including a new section with router interaction examples (#91).

      o Temporary Neighbor Cache Entry created upon host sending NS+ARO,
      and SLLAO removed from multihop DAD NS/NA messages (#87).

   Changes from -10 to -11:

      o Reference to RFC1982 for version number comparison (#80)

      o RA Router Lifetime field use clarified (#81)

      o Make fields 16-bit rather than 32-bit where possible (#83)

      o Unicast RA clarification (#84)

      o Temporary ND option types (#85)

      o SLLA/TLLA clarification (#86)

      o GP16 as source address in initial NS clarification (#87)

   Changes from -09 to -10:

      o Clarifications made to Section 8.2 (#66)

      o Explained behavior of Neighbor Cache (#67)

      o Clarified use of SLLAO in RS and NS messages (#68)

      o Added new term 6LN (#69)

      o Small clarification on 6CO flag (#70)

      o Defined host behavior on ARO failure better (#72)

      o Added bootstrapping example for a host (#73)

      o Added new Neighbor Cache Full ARO error (#74)

      o Added rule on the use of the M flag (#75)

   Changes from -08 to -09:





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      o Clean re-write of the draft (re-use of some introductory
      material)

      o Merged in draft-chakrabarti-6lowpan-ipv6-nd-simple-00

      o Changed address registration to an option piggybacked on NS/NA

      o New Authoritative Border Router option

      o New Address Registration Option

      o Separated Prefix Information and Content Information

      o Optional DAD to the edge

   Changes from -07 to -08:

      o Removed Extended LoWPAN and Whiteboard related sections.

      o Included reference to the autoconf addressing model.

      o Added Optimistic Flag to 6AO.

      o Added guidelines on routers performing DAD.

      o Removed the NR/NC Advertising Interval.

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

   Changes from -06 to -07:

      o Updated addressing and address resolution (#60).

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

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

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

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





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

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

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

      o Some editorial cleanup.

   Changes from -05 to -06:

      o Fixed the Prf codes (#52).

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

      o Corrected an error in Table 1 (#54).

      o Fixed asymmetric and a misplaced transient in the 6LoWPAN
      terminology section.

      o Added Updates RFC4861 to header

   Changes from -04 to -05:

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

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

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

      o Neighbor cache and destination cache removed.

      o IID to link-layer address requirement relaxed.

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

      o Modified 6LoWPAN Information Option (#47).

      o Added a Protocol Constants section (#24)

      o Added the NR processing table (#51)

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




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   Changes from -03 to -04:

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

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

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

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

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

      o Removed S=2 from Address Option (not needed). (#36)

      o Added a section on router dissemination consistency. (#44)

      o Small improvements and extensive editing. (#42, #37, #35)

   Changes from -02 to -03:

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

      o 6LoWPAN and ND terminology separated.

      o Protocol overview explains RFC4861 diff in detail.

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

      o Added NR failure codes.

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

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

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

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





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

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

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

      o Security considerations improved.

      o ICMPv6 destination unreachable supported.

   Changes from -01 to -02:

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

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

      o Terminology cleanup based on Alex's comments.

      o General editing improvements.

   Changes from -00 to -01:

      o Specified the duplicate owner interface identifier procedures.
      A TID lollipop algorithm was sufficient (nonce unnecessary).

      o Defined fault tolerance using secondary bindings.

      o Defined ad-hoc network operation.

      o Removed the E flag from RA and the X flag from RR/RC.

      o Completed message examples.

      o Lots of improvements in text quality and consistency were made.


15.  References

15.1.  Normative References

   [I-D.ietf-autoconf-adhoc-addr-model]
              Baccelli, E. and M. Townsley, "IP Addressing Model in Ad
              Hoc Networks", draft-ietf-autoconf-adhoc-addr-model-03
              (work in progress), March 2010.

   [RFC1982]  Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,



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              August 1996.

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

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

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

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

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

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

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

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

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

15.2.  Informative References

   [I-D.ietf-6lowpan-hc]
              Hui, J. and P. Thubert, "Compression Format for IPv6
              Datagrams in 6LoWPAN Networks", draft-ietf-6lowpan-hc-08
              (work in progress), July 2010.

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

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




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

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

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

   [RFC4919]  Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
              over Low-Power Wireless Personal Area Networks (6LoWPANs):
              Overview, Assumptions, Problem Statement, and Goals",
              RFC 4919, August 2007.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, September 2007.


Authors' Addresses

   Zach Shelby (editor)
   Sensinode
   Hallituskatu 13-17D
   Oulu  90100
   FINLAND

   Phone: +358407796297
   Email: zach@sensinode.com


   Samita Chakrabarti
   IP Infusion
   1188 Arquest Street
   Sunnyvale, CA
   USA

   Email: samitac@ipinfusion.com












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   Erik Nordmark
   Oracle, Inc.
   17 Network Circle
   Menlo Park, CA 94025
   USA

   Email: Erik.Nordmark@Oracle.COM












































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