datatracker.ietf.org
Sign in
Version 5.6.2.p6, 2014-09-03
Report a bug

Neighbor Discovery Optimization for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)
RFC 6775

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

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

Abstract

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

Status of This Memo

   This is an Internet Standards Track document.

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

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

Shelby, et al.               Standards Track                    [Page 1]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

Copyright Notice

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

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

Table of Contents

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

Shelby, et al.               Standards Track                    [Page 2]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

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

Shelby, et al.               Standards Track                    [Page 3]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

1.  Introduction

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

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

   o  Host-initiated interactions to allow for sleeping hosts.

   o  Elimination of multicast-based address resolution for hosts.

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

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

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

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

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

Shelby, et al.               Standards Track                    [Page 4]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

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

1.1.  The Shortcomings of IPv6 Neighbor Discovery

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

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

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

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

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

Shelby, et al.               Standards Track                    [Page 5]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

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

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

1.2.  Applicability

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

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

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

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

Shelby, et al.               Standards Track                    [Page 6]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

1.3.  Goals and Assumptions

   The document has the following main goals and assumptions.

   Goals:

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

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

   o  Optimize the interfaces between hosts and their default routers.

   o  Provide support for sleeping hosts.

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

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

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

   Assumptions:

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

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

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

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

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

Shelby, et al.               Standards Track                    [Page 7]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

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

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

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

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

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

1.4.  Substitutable Features

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

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

Shelby, et al.               Standards Track                    [Page 8]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

   The features that are substitutable (individually or in a group):

   o  Multihop distribution of prefix and 6LoWPAN header compression
      context

   o  Multihop Duplicate Address Detection

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

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

2.  Terminology

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

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

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

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

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

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

Shelby, et al.               Standards Track                    [Page 9]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

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

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

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

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

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

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

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

Shelby, et al.               Standards Track                   [Page 10]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

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

3.  Protocol Overview

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

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

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

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

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

3.1.  Extensions to RFC 4861

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

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

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

Shelby, et al.               Standards Track                   [Page 11]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

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

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

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

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

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

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

3.2.  Address Assignment

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

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

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

Shelby, et al.               Standards Track                   [Page 12]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

3.3.  Host-to-Router Interaction

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

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

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

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

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

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

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

Shelby, et al.               Standards Track                   [Page 13]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

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

3.4.  Router-to-Router Interaction

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

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

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

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

3.5.  Neighbor Cache Management

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

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

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

Shelby, et al.               Standards Track                   [Page 14]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

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

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

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

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

4.  New Neighbor Discovery Options and Messages

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

4.1.  Address Registration Option

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

Shelby, et al.               Standards Track                   [Page 15]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

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

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

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

   When the ARO is used by hosts, an SLLAO (Source Link-Layer Address
   Option) [RFC4861] MUST be included, and the address that is to be
   registered MUST be the IPv6 source address of the NS message.

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

   Fields:

   Type:                   33

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

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

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

Shelby, et al.               Standards Track                   [Page 16]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

   Registration Lifetime:  16-bit unsigned integer.  The amount of time
                           in units of 60 seconds that the router should
                           retain the NCE for the sender of the NS that
                           includes this option.

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

   The Status values used in NAs are:

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

                                  Table 1

4.2.  6LoWPAN Context Option

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

Shelby, et al.               Standards Track                   [Page 17]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

   Type:            34

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

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

   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 life cycle based on the
                    recommendations in Section 7.2.

   CID:             4-bit Context Identifier for this prefix
                    information.  The CID is used by context-based
                    header compression as specified in [RFC6282].  The
                    list of CIDs for a LoWPAN is configured on the 6LBR
                    that originates the context information for the
                    6LoWPAN.

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

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

   Context Prefix:  The IPv6 prefix or address corresponding to the 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.

Shelby, et al.               Standards Track                   [Page 18]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

4.3.  Authoritative Border Router Option

   The Authoritative Border Router Option (ABRO) is needed when RA
   messages are used to disseminate prefixes and context information
   across a route-over topology.  In this case, 6LRs receive PIOs 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
   that the 6LBR sets and that 6LRs propagate as they propagate the
   prefix and context information with this ABRO.  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 ABRO MUST be included in all RA messages in the case when RAs 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 Low          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Version High         |        Valid Lifetime         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                          6LBR Address                         +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Type:                       35

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

   Version Low, Version High:  Together, Version Low and Version High
                               constitute the Version Number field, a
                               32-bit unsigned integer where Version Low
                               is the least significant 16 bits and
                               Version High is the most significant

Shelby, et al.               Standards Track                   [Page 19]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

                               16 bits.  The version number
                               corresponding to this set of information
                               contained in the RA message.  The
                               authoritative 6LBR originating the prefix
                               increases this version number each time
                               its set of prefix or context information
                               changes.

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

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

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

4.4.  Duplicate Address Messages

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

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

Shelby, et al.               Standards Track                   [Page 20]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

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

   IP fields:

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

   IPv6 Destination:       In a DAR, a non-link-local address of a 6LBR.
                           In a DAC, this is just the source from the
                           DAR.

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

   ICMP Fields:

   Type:                   157 for the DAR and 158 for the DAC.

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

   Checksum:               The ICMP checksum.  See [RFC4443].

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

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

Shelby, et al.               Standards Track                   [Page 21]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

   Registration Lifetime:  16-bit unsigned integer.  The amount of time
                           in units of 60 seconds that the 6LBR should
                           retain the DAD table entry (Section 8.2.2)
                           for the Registered Address.  A value of 0
                           indicates in a DAR that the DAD table entry
                           should be removed.

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

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

5.  Host Behavior

   Hosts in a LoWPAN use the ARO in the NS messages they send as a way
   to maintain the Neighbor Cache in the routers, thereby removing the
   need for multicast NSs to do address resolution.  Unlike in
   [RFC4861], the hosts initiate updating the information they receive
   in RAs by sending RSs before the information expires.  Finally, when
   NUD indicates that one or all default routers have become
   unreachable, then the host uses RSs to find a new set of default
   routers.

5.1.  Forbidden Actions

   A host MUST NOT multicast an NS 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 identifier [EUI64] assigned to the interface as per
   [RFC4944] or the appropriate IP-over-foo document for the link, and
   then the host sends RS messages as described in [RFC4861]
   Section 6.3.7.

   There is no need to join the solicited-node multicast address, since
   nobody multicasts NSs in this type of network.  A host MUST join the
   all-nodes multicast address.

Shelby, et al.               Standards Track                   [Page 22]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

5.3.  Sending a Router Solicitation

   The RS is formatted as specified in [RFC4861] and sent to the IPv6
   all-routers multicast address (see [RFC4861] Section 6.3.7 for
   details).  An SLLAO MUST be included to enable unicast RAs in
   response.  An unspecified source address MUST NOT be used in RS
   messages.

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

   Since hosts do not depend on multicast RAs to discover routers, the
   hosts need to intelligently retransmit RSs whenever the default
   router list is empty, one of its default routers becomes unreachable,
   or the lifetime of the prefixes and contexts in the previous RA is
   about to expire.  The RECOMMENDED rate of retransmissions is to
   initially send up to 3 (MAX_RTR_SOLICITATIONS) RS messages separated
   by at least 10 seconds (RTR_SOLICITATION_INTERVAL) as specified in
   [RFC4861], and then switch to slower retransmissions.  After the
   initial retransmissions, the host SHOULD do truncated binary
   exponential backoff [ETHERNET] of the retransmission timer for each
   subsequent retransmission, truncating the increase of the
   retransmission timer at 60 seconds (MAX_RTR_SOLICITATION_INTERVAL).
   In all cases, the RS retransmissions are terminated when an RA is
   received.  See Section 9 for protocol constants.

5.4.  Processing a Router Advertisement

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

   Should the host erroneously receive a PIO with the L (on-link) flag
   set, then that 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
   address can be configured by any other means (and duplicate detection
   is performed as part of the registration process).

Shelby, et al.               Standards Track                   [Page 23]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

   Once an address has been configured, it will be registered by
   unicasting an NS with an ARO 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.  This structure is called
   the context table.  It includes the CID, 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 is received in an RA, it is used to add or update the
   information in the context table.  If the CID 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 life cycle.  Nodes
   should be careful about using header compression in RA messages that
   include 6COs.

5.4.3.  Maintaining Prefix and Context Information

   The prefix information is timed out as specified in [RFC4861].  When
   the Valid Lifetime for a context table entry expires, the entry is
   placed in a receive-only mode, which is the equivalent of receiving a
   6CO for that context with C=0.  The entry is held in receive-only
   mode for a period of twice the default Router Lifetime, after which
   the entry is removed.

   A host should inspect the various lifetimes to determine when it
   should next initiate sending an RS to ask for any updates to the
   information.  The lifetimes that matter are the default Router
   Lifetime, the Valid Lifetime in the PIOs, and the Valid Lifetime in
   the 6CO.  The host SHOULD unicast one or more RSs to the router well
   before the shortest of those lifetimes (across all the prefixes and
   all the contexts) expires and then switch to multicast RS messages if
   there is no response to the unicasts.  The retransmission behavior
   for the RSs is specified in Section 5.3.

Shelby, et al.               Standards Track                   [Page 24]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

5.5.  Registration and Neighbor Unreachability Detection

   Hosts send unicast NS messages to register their IPv6 addresses, and
   also to do NUD to verify that their default routers are still
   reachable.  The registration is performed by the host including an
   ARO in the NS 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 NCEs in the routers.  This is done by sending
   NS messages with an 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 NA message with an ARO.

   Hosts that receive RA 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 NUD probes can be suppressed by reachability confirmations
   from transport protocols or applications as specified in [RFC4861].

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

5.5.1.  Sending a Neighbor Solicitation

   The host triggers sending 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 an SLLAO, 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 NA messages as specified in [RFC4861], with added
   logic described in this section for handling the ARO.

   In addition to the normal validation of an NA and its options, the
   ARO (if present) is verified as follows.  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.

Shelby, et al.               Standards Track                   [Page 25]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

   If the Status field is zero, then the address registration was
   successful.  The host saves the Registration Lifetime from the ARO
   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 NSs is received (NUD failure), or an ARO 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 ARO with a non-zero Status field is
   received, this indicates that registration for that address has
   failed.  A failure Status of one indicates that a duplicate address
   was detected, and the procedure described in [RFC4862] Section 5.4.5
   is followed.  The host MUST NOT use the address it tried to register.
   If the host has valid registrations with other routers, these MUST be
   removed by registering with each using a zero ARO lifetime.

   A Status code of two indicates that the Neighbor Cache of that router
   is full.  In this case, the host SHOULD remove this router from its
   default router list and attempt to register with another router.  If
   the host's default router list is empty, it needs to revert to
   sending RSs as specified in 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.  It is assumed that link-local
   addresses are formed as specified in Section 5.2 from the EUI-64, and
   address resolution is not performed.  Packets are sent to link-local
   destinations by reversing the procedure in Appendix A of [RFC4291].

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

Shelby, et al.               Standards Track                   [Page 26]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

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

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

5.7.  Address Resolution

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

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

5.8.  Sleeping

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

5.8.1.  Picking an Appropriate Registration Lifetime

   As all ND messages are initiated by the hosts, this allows a host to
   sleep or otherwise be unreachable between NS/NA message exchanges.
   The ARO attached to NS messages indicates to a router to keep the NCE
   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 that the registration is renewed successfully
   (considering, for example, clock drift and additional time for
   potential retransmissions of the re-registration).  External
   configuration of a host should also consider the stability of the
   network (how quickly the topology changes) when choosing its sleep

Shelby, et al.               Standards Track                   [Page 27]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

   time (and thus Registration Lifetime).  A dynamic network requires a
   shorter sleep time so that routers don't keep invalid NCEs for nodes
   longer than necessary.

5.8.2.  Behavior on Wakeup

   When a host wakes up from a sleep period, it SHOULD refresh its
   current address registrations that will time out before the next
   wakeup.  This is done by sending NS messages with an ARO as described
   in Section 5.5.1.  The host may also need to refresh its prefix and
   context information by sending a new unicast RS (the maximum Router
   Lifetime is about 18 hours, whereas the maximum Registration Lifetime
   is about 45.5 days).  If after wakeup the host (using NUD) determines
   that some or all previous default routers have become unreachable,
   then the host will send multicast RSs to discover new default
   router(s) and restart the address registration process.

6.  Router Behavior for 6LRs and 6LBRs

   Both 6LRs and 6LBRs maintain the Neighbor Cache [RFC4861] based on
   the AROs they receive in NA messages from hosts, ND packets from
   other nodes, and, potentially, a routing protocol used in the 6LoWPAN
   as outlined in Section 3.5.

   The routers SHOULD NOT garbage-collect Registered NCEs (see
   Section 3.4), since they need to retain them until the Registration
   Lifetime expires.  Similarly, if NUD on the router determines that
   the host is UNREACHABLE (based on the logic in [RFC4861]), the NCE
   SHOULD NOT be deleted but rather 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 Preference (Prf) extension
   [RFC4191] and use that to indicate to the host whether the router is
   a 6LBR or a 6LR.  If this is implemented, then 6LRs with no route to
   a border router MUST set Prf to (11) for low preference, other 6LRs
   MUST set Prf to (00) for normal preference, and 6LBRs MUST set Prf to
   (01) for high preference.

6.1.  Forbidden Actions

   Even if a router in a route-over topology can reach both a host and
   another target, because of radio propagation it generally cannot know
   whether the host can directly reach the other target.  Therefore, it
   cannot assume that Redirect will actually work from one host to
   another.  Therefore, it SHOULD NOT send Redirect messages.  The only

Shelby, et al.               Standards Track                   [Page 28]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

   potential exception to this "SHOULD NOT" is when the deployment/
   implementation has a way to know how the host can reach the intended
   target.  Hence, it is RECOMMENDED that the implementation by default
   does not send Redirect messages but can be configurable when the
   deployment calls for this.  In contrast, for mesh-under topologies,
   the same considerations about Redirects apply as in [RFC4861].

   A router MUST NOT set the L (on-link) flag in the PIOs, since that
   might trigger hosts to send multicast NSs.

6.2.  Interface Initialization

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

6.3.  Processing a Router Solicitation

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

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

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

Shelby, et al.               Standards Track                   [Page 29]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

   A 6LR or 6LBR MUST include an SLLAO in the RAs it sends; this is
   required so that the hosts will know the link-layer address of the
   router.  Unlike in [RFC4861], the maximum value of the RA Router
   Lifetime field MAY be up to 0xFFFF (approximately 18 hours).

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

6.4.  Periodic Router Advertisements

   A router does not need to send any periodic RA messages, since the
   hosts will solicit updated information by sending RSs before the
   lifetimes expire.

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

6.5.  Processing a Neighbor Solicitation

   A router handles NS messages as specified in [RFC4861], with added
   logic described in this section for handling the ARO.

   In addition to the normal validation of an NS and its options, the
   ARO is verified as follows (if present).  If the Length field is not
   two, or if the Status field is not zero, then the NS is silently
   ignored.

   If the source address of the NS is the unspecified address, or if no
   SLLAO 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.  It
   isn't a duplicate if (1) there is no NCE for the IPv6 source address
   of the NS or (2) there is such an NCE and the EUI-64 is the same.
   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 NCEs.  In the case where it is a
   duplicate address, then the router responds with a unicast NA message
   with the ARO Status field set to one (to indicate that the address is
   a duplicate) as described in Section 6.5.2.  In this case, there is
   no modification to the Neighbor Cache.

Shelby, et al.               Standards Track                   [Page 30]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

6.5.2.  Returning Address Registration Errors

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

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

6.5.3.  Updating the Neighbor Cache

   If the 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) an NCE for the
   IPv6 source address of the NS.  If the Neighbor Cache is full and a
   new entry needs to be created, then the router responds with a
   unicast NA with the ARO Status field set to two (to indicate that the
   router's Neighbor Cache is full) as described in Section 6.5.2.

   The Registration Lifetime and the EUI-64 are recorded in the NCE.  A
   unicast 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
   TLLAO (Target Link-Layer Address Option) [RFC4861] is not required in
   the NA, since the host already knows the router's link-layer address
   from RAs.

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

   Should the Registration Lifetime in an NCE expire, then the router
   MUST delete the cache entry.

   The addition and removal of Registered NCEs would result in notifying
   the routing protocol.

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

Shelby, et al.               Standards Track                   [Page 31]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

6.5.4.  Next-Hop Determination

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

6.5.5.  Address Resolution between Routers

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

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

7.  Border Router Behavior

   A 6LBR handles the sending of RAs and processing of NSs from hosts as
   specified above in Section 6.  A 6LBR SHOULD always include an ABRO
   in the RAs it sends, listing itself as the 6LBR address.  This
   requires that the 6LBR maintain the version number in stable storage
   and increase the version number when some information in its RAs
   changes.  The information whose change affects the version is in the
   PIOs (the prefixes or their lifetimes) and in the 6CO (the prefixes,
   CIDs, or lifetimes).

   In addition, a 6LBR is somehow configured with the prefix or prefixes
   that are assigned to the LoWPAN and advertises those in RAs as in
   [RFC4861].  In the case of route-over, those prefixes can be
   disseminated to all the 6LRs using the technique discussed in

Shelby, et al.               Standards Track                   [Page 32]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

   Section 8.1.  However, there might be mechanisms outside of the scope
   of this document that can be used as a substitute for prefix
   dissemination in the route-over topology (see Section 1.4).

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

7.1.  Prefix Determination

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

7.2.  Context Configuration and Management

   If the LoWPAN uses header compression [RFC6282] with context, then
   the 6LBR must be configured with context information and related
   CIDs.  If the LoWPAN has multiple 6LBRs, then they MUST be configured
   with the same context information and CIDs.  As noted in [RFC6282],
   maintaining consistency of context information is crucial for
   ensuring that packets will be decompressed correctly.

   The context information carried in RA messages originates 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
   life cycle SHOULD be used in order to ensure that the context
   information stays synchronized throughout the LoWPAN.  New context
   information SHOULD be introduced into the LoWPAN with C=0, to ensure
   that it is known by all nodes that may have to perform header
   decompression 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.

Shelby, et al.               Standards Track                   [Page 33]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

   Conversely, to avoid the situation where nodes send packets that make
   use of previous values of contexts -- which would result 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 CID 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.  Substitutable Feature Behavior

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

   There is also the option for a 6LR to perform multihop DAD (for IPv6
   addresses not derived from an EUI-64) against a 6LBR in a route-over
   topology by using the DAR and DAC messages.  This is substitutable
   because there might be other ways to either allocate a unique
   address, such as DHCPv6 [RFC3315], or use other future mechanisms for
   multihop DAD.  Again, in this case, any remaining use of the
   6LoWPAN-ND mechanisms is governed by the substitute specification.

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

8.1.  Multihop Prefix and Context Distribution

   The multihop distribution relies on RS messages and 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 needs 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

Shelby, et al.               Standards Track                   [Page 34]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

   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 the
   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.

8.1.1.  6LBRs Sending Router Advertisements

   6LBRs supporting multihop prefix and context distribution MUST
   include an ABRO in each of their 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 PIOs or 6COs changes, the ABRO version is
   increased by one.

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

8.1.2.  Routers Sending Router Solicitations

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

8.1.3.  Routers Processing Router Advertisements

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

   If a received RA does not contain an ABRO, then the RA MUST be
   silently ignored.

   The router uses the 6LBR Address field in the ABRO to check if it has
   previously received information from the 6LBR.  If it finds no such
   information, then it just records the 6LBR address, Version, Valid
   Lifetime, and the associated prefixes and context information.  If
   the 6LBR is previously known, then the Version Number field MUST be

Shelby, et al.               Standards Track                   [Page 35]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

   compared against the recorded version number for that 6LBR.  If the
   version number received in the packet is less than the stored version
   number, then the information in the RA is silently ignored.
   Otherwise, the recorded information and version number are updated.

8.1.4.  Storing the Information

   The router keeps state for each 6LBR that it sees with an ABRO.  This
   includes the version number, the Valid Lifetime, and the complete set
   of PIOs and 6COs.  The prefixes are timed out based on the Valid
   Lifetime in the PIO.  The Context Prefix is timed out based on the
   Valid Lifetime in the 6CO.

   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 RAs 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 an RA it sends at time T+5 minutes, the Valid
   Lifetime in the 6CO it sends will be only 5 minutes.

8.1.5.  Sending Router Advertisements

   When multihop distribution is performed using RA messages, the
   routers MUST ensure that the ABRO always stays together with the
   prefixes and context information received with that ABRO.  Thus, if
   the router has received prefix P1 with an 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 an RA that includes an 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 RAs as in [RFC4861].  This is for the
   benefit of the other routers receiving the prefixes and context
   information.  The routers also respond to RSs by unicasting RA
   messages.  In both cases, the above constraint of keeping the ABRO
   together with 'its' prefixes and context information applies.

   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 as
   described in [RFC4861], that is, send up to three RA messages.  This
   ensures rapid propagation of new information to all the 6LRs.

Shelby, et al.               Standards Track                   [Page 36]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

8.2.  Multihop Duplicate Address Detection

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

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

   The multihop DAD mechanism is used synchronously the first time an
   address is registered with a particular 6LR.  That is, the ARO is not
   returned to the host until multihop DAD has been completed against
   the 6LBRs.  For existing registrations in the 6LR, 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.  One method to achieve
   this is to track how much is left of the lifetime the 6LR registered
   with the 6LBRs and to re-register with the 6LBR when this lifetime is
   about to run out.

   For synchronous multihop DAD, the 6LR performs some additional checks
   to ensure that it has an NCE 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) NCE 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 NCE exists, then the 6LR MUST create
   a Tentative NCE with the EUI-64 and the SLLAO.  This entry will be
   used to send the response to the host when the 6LBR responds
   positively.

   When a 6LR receives an NS containing an ARO with a non-zero
   Registration Lifetime and it has no existing Registered NCE, then
   with this mechanism the 6LR will invoke synchronous multihop DAD.

Shelby, et al.               Standards Track                   [Page 37]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

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

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

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

8.2.1.  Message Validation for DAR and DAC

   A node MUST silently discard any received DAR and DAC messages for
   which at least one of the following validity checks is not satisfied:

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

   o  ICMP Checksum is valid.

   o  ICMP Code is 0.

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

   o  The Registered Address is not a multicast address.

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

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

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

Shelby, et al.               Standards Track                   [Page 38]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

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

8.2.2.  Conceptual Data Structures

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

8.2.3.  6LR Sending a Duplicate Address Request

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

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

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

   o  In the IPv6 hop limit, MULTIHOP_HOPLIMIT.

   o  The Status field MUST be set to zero.

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

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

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

   A router MUST NOT modify the Neighbor Cache as a result of receiving
   a DAR.

8.2.4.  6LBR Receiving a Duplicate Address Request

   When a 6LBR that implements the substitutable multihop DAD receives a
   DAR from a 6LR, it performs the message validation specified in
   Section 8.2.1.  If the DAR is valid, the 6LBR proceeds to look for
   the Registration Address in the DAD table.  If an entry is found and
   the recorded EUI-64 is different than the EUI-64 in the DAR, then it

Shelby, et al.               Standards Track                   [Page 39]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

   returns a DAC NA with the Status set to 1 ('Duplicate Address').
   Otherwise, it returns a DAC with Status set to zero and updates the
   lifetime.

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

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

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

8.2.5.  Processing a Duplicate Address Confirmation

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

   A router MUST NOT modify the Neighbor Cache as a result of receiving
   a DAC, unless there is a Tentative NCE matching the IPv6 address and
   EUI-64.

8.2.6.  Recovering from Failures

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

Shelby, et al.               Standards Track                   [Page 40]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

9.  Protocol Constants

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

   Additional protocol constants are defined in Section 4.

   6LBR Constants:

   MIN_CONTEXT_CHANGE_DELAY+               300 seconds

   6LR Constants:

   MAX_RTR_ADVERTISEMENTS                  3 transmissions

   MIN_DELAY_BETWEEN_RAS*                  10 seconds

   MAX_RA_DELAY_TIME*                      2 seconds

   TENTATIVE_NCE_LIFETIME+                 20 seconds

   Router Constants:

   MULTIHOP_HOPLIMIT+                      64

   Host Constants:

   RTR_SOLICITATION_INTERVAL*              10 seconds

   MAX_RTR_SOLICITATIONS                   3 transmissions

   MAX_RTR_SOLICITATION_INTERVAL+          60 seconds

Shelby, et al.               Standards Track                   [Page 41]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

10.  Examples

10.1.  Message Examples

   STEP

      6LN                                                        6LR

       |                                                          |

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

       |                       [SLLAO]                            |

       |                                                          |

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

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

     Figure 2: Basic Router Solicitation/Router Advertisement Exchange
                     between a Node and a 6LR or 6LBR

      6LN                                                        6LR

       |                                                          |

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

       |                     [ARO + SLLAO]                        |

       |                                                          |

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

       |                   [ARO with Status]                      |

             Figure 3: Neighbor Discovery Address Registration

Shelby, et al.               Standards Track                   [Page 42]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

      6LN                           6LR                          6LBR

       |                             |                             |

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

       |      [ARO + SLLAO]          |                             |

       |                             |                             |

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

       |                             |                             |

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

       |                             |                             |

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

       |      [ARO with Status]      |

    Figure 4: Neighbor Discovery Address Registration with Multihop DAD

10.2.  Host Bootstrapping Example

   The following example describes the address bootstrapping scenarios
   using the improved ND mechanisms specified in this document.  It is
   assumed that the 6LN first performs a sequence of operations in order
   to get secure access at the link layer of the LoWPAN and obtain a key
   for link-layer security.  The methods of how to establish link-layer
   security are out of scope of this document.  In this example, an IEEE
   802.15.4 6LN forms a 16-bit short IPv6 address without using DHCPv6
   (i.e., the M flag is not set in the RAs).

   1.  After obtaining link-layer 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 SLLAO 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.

Shelby, et al.               Standards Track                   [Page 43]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

       The 6LR responds with a unicast RA to the IP source address using
       the SLLAO from the RS (it may have created a Tentative NCE).  See
       Figure 2.

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

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

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

Shelby, et al.               Standards Track                   [Page 44]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

10.2.1.  Host Bootstrapping Messages

   This section provides specific message examples related to the
   bootstrapping process described above.  When discussing messages, the
   following notation is used:

   LL64:  Link-local address based on the EUI-64, which is also the
      802.15.4 long address.

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

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

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

   MAC16:  IEEE 802.15.4 16-bit short address.

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

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

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

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

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

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

Shelby, et al.               Standards Track                   [Page 45]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

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

    If Status is a success:

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

    If Status is not a success:

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

                Figure 5: Detailed Message Address Examples

10.3.  Router Interaction Example

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

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

10.3.1.  Bootstrapping a Router

   In this scenario, the bootstrapping 6LR, 'R1', is multiple hops away
   from the 6LBR and surrounded by other 6LR neighbors.  Initially, R1
   behaves as a host.  It sends a 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 that 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.

Shelby, et al.               Standards Track                   [Page 46]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

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

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

10.3.2.  Updating the Neighbor Cache

   In this example, there are three 6LRs: R1, R2, and R3.  Initially,
   when R2 boots, it sees only R1, and accordingly R2 creates an NCE for
   R1.  Now assume that R2 receives a valid routing update from router
   R3.  R2 does not have any NCE for R3.  If the implementation of R2
   supports detecting link-layer addresses from the routing information
   packets, then it directly updates its Neighbor Cache using that
   link-layer information.  If this is not possible, then R2 should
   perform multicast NS with the 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 update from a
   neighboring router for which it does not have any NCE is required to
   update its Neighbor Cache as described above.

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

11.  Security Considerations

   The security considerations of IPv6 ND [RFC4861] and address
   autoconfiguration [RFC4862] apply.  Additional considerations can be
   found in [RFC3756].

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

Shelby, et al.               Standards Track                   [Page 47]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

   This specification assumes that the link layer is sufficiently
   protected -- for instance, by using MAC-sublayer cryptography.  Thus,
   its threat model is no different from that of IPv6 ND [RFC4861].  The
   first trust model listed in Section 3 of [RFC3756] applies here.
   However, any future 6LoWPAN security protocol that applies to ND for
   the 6LoWPAN protocol is out of scope of this document.

   The multihop DAD mechanisms rely on DAR and DAC messages that are
   forwarded by 6LRs, and as a result the hop_limit=255 check on the
   receiver does not apply to those messages.  This implies that any
   node on the Internet could successfully send such messages.  We avoid
   any additional security issues due to this by requiring that the
   routers never modify the NCE due to such messages, and that they
   discard them unless they are received on an interface that has been
   explicitly configured to use these optimizations.

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

12.  IANA Considerations

   This document registers three new ND option types under the
   subregistry "IPv6 Neighbor Discovery Option Formats":

   o  Address Registration Option (33)
   o  6LoWPAN Context Option (34)
   o  Authoritative Border Router Option (35)

   The document registers two new ICMPv6 "type" numbers under the
   subregistry "ICMPv6 "type" Numbers":

   o  Duplicate Address Request (157)
   o  Duplicate Address Confirmation (158)

Shelby, et al.               Standards Track                   [Page 48]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

   IANA has also created a new subregistry for the Status values of the
   Address Registration Option, under the ICMPv6 parameters registry.

   Address Registration Option Status Values registry:

   o  Possible values are 8-bit unsigned integers (0..255).
   o  Registration procedure is "Standards Action" [RFC5226].
   o  Initial allocation is as indicated in Table 2:

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

                                  Table 2

13.  Interaction with Other Neighbor Discovery Extensions

   There are two classes of ND extensions that interact with this
   specification in different ways.

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

   All other (non-DAD) ND extensions, be they path selection types like
   default router preferences [RFC4191], configuration types like DNS
   configuration [RFC6106], or other types like Detecting Network
   Attachment [RFC6059], are completely orthogonal to this specification
   and will work as is.

14.  Guidelines for New Features

   This section discusses guidelines of new protocol features defined in
   this document.  It also sets some expectations for implementation and
   deployment of these features.  This section is informative in nature:
   it does not override the detailed specifications of the previous
   sections but summarizes them and presents them in a compact form, to
   be used as checklists.  The checklists act as guidelines to indicate
   the possible importance of a feature in terms of a deployment as per
   information available as of the writing of the document.  Note that
   in some cases the deployment is 'SHOULD' where the implementation is

Shelby, et al.               Standards Track                   [Page 49]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

   a 'MUST'.  This is due to the presence of substitutable features; the
   deployment may use alternative methods for those.  Therefore,
   implementing a configuration knob is recommended for the
   substitutable features.  The lists emphasize conciseness over
   completeness.

   +----------+-----------------------------------+--------+-----------+
   | Section  | Description                       | Deploy | Implement |
   +----------+-----------------------------------+--------+-----------+
   | 3.1      | Host-initiated RA                 | MUST   | MUST      |
   | 3.2      | EUI-64-based IPv6 address         | MUST   | MUST      |
   |          | 16-bit MAC-based address          | MAY    | SHOULD    |
   |          | Other non-unique addresses        | MAY    | MAY       |
   | 3.3      | Host-initiated RS                 | MUST   | MUST      |
   |          | ABRO processing                   | SHOULD | MUST      |
   | 4.1      | Registration with ARO             | MUST   | MUST      |
   | 4.2, 5.4 | 6CO                               | SHOULD | SHOULD    |
   | 5.2      | Joining solicited-node multicast  | N/A    | N/A       |
   |          | Joining all-nodes multicast       | MUST   | MUST      |
   |          | Using link-layer indication for   | MAY    | MAY       |
   |          | NUD                               |        |           |
   | 5.5      | 6LoWPAN-ND NUD                    | MUST   | MUST      |
   | 5.8.2    | Behavior on wakeup                | SHOULD | SHOULD    |
   +----------+-----------------------------------+--------+-----------+

           Table 3: Guideline for 6LoWPAN-ND Features for Hosts

Shelby, et al.               Standards Track                   [Page 50]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

   +---------------+-------------------------+------------+------------+
   | Section       | Description             | Deploy     | Implement  |
   +---------------+-------------------------+------------+------------+
   | 3.1           | Periodic RA             | SHOULD NOT | SHOULD NOT |
   | 3.2           | Address assignment      | SHOULD     | MUST       |
   |               | during startup          |            |            |
   | 3.3           | Supporting EUI-64-based | MUST       | MUST       |
   |               | MAC hosts               |            |            |
   |               | Supporting 16-bit MAC   | MAY        | SHOULD     |
   |               | hosts                   |            |            |
   | 3.4, 4.3,     | ABRO processing/sending | SHOULD     | MUST       |
   | 8.1.3, 8.1.4  |                         |            |            |
   | 8.1           | Multihop prefix storing | SHOULD     | MUST       |
   |               | and redistribution      |            |            |
   | 3.5           | Tentative NCE           | MUST       | MUST       |
   | 8.2           | Multihop DAD            | SHOULD     | MUST       |
   | 4.1, 6.5,     | ARO support             | MUST       | MUST       |
   | 6.5.1 - 6.5.5 |                         |            |            |
   | 4.2           | 6CO                     | SHOULD     | SHOULD     |
   | 6.3           | Process RS/ABRO         | MUST       | MUST       |
   +---------------+-------------------------+------------+------------+

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

   +--------------+--------------------------+------------+------------+
   | Section      | Description              | Deploy     | Implement  |
   +--------------+--------------------------+------------+------------+
   | 3.1          | Periodic RA              | SHOULD NOT | SHOULD NOT |
   | 3.2          | Address autoconf on      | MUST NOT   | MUST NOT   |
   |              | router interface         |            |            |
   | 3.3          | EUI-64 MAC support on    | MUST       | MUST       |
   |              | 6LoWPAN interface        |            |            |
   | 8.1 - 8.1.1, | Multihop prefix          | SHOULD     | MUST       |
   | 8.1.5        | distribution             |            |            |
   | 8.2          | Multihop DAD             | SHOULD     | MUST       |
   +--------------+--------------------------+------------+------------+

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

Shelby, et al.               Standards Track                   [Page 51]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

15.  Acknowledgments

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

   Additionally, the authors would like to recognize Pascal Thubert for
   contributing the original registration idea and for extensive
   contributions to earlier versions of the document, Jonathan Hui for
   original ideas on prefix/context distribution and extensive
   contributions to earlier versions of the document, Colin O'Flynn for
   useful "Error-to" suggestions (Section 6.5.2) and for contributions
   to the Examples section, Geoff Mulligan for suggesting the use of
   address registration as part of existing IPv6 ND messages, and
   Mathilde Durvy for helping to clarify router interaction.

16.  References

16.1.  Normative References

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

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

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

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

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

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

Shelby, et al.               Standards Track                   [Page 52]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

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

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

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

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

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

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

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

16.2.  Informative References

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

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

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

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

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

Shelby, et al.               Standards Track                   [Page 53]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

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

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

   [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.

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

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

   [RFC6106]  Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
              "IPv6 Router Advertisement Options for DNS Configuration",
              RFC 6106, November 2010.

Shelby, et al.               Standards Track                   [Page 54]
RFC 6775              ND Optimization for 6LoWPANs         November 2012

Authors' Addresses

   Zach Shelby (editor)
   Sensinode
   Konekuja 2
   Oulu  90620
   Finland

   Phone: +358407796297
   EMail: zach@sensinode.com

   Samita Chakrabarti
   Ericsson

   EMail: samita.chakrabarti@ericsson.com

   Erik Nordmark
   Cisco Systems

   EMail: nordmark@cisco.com

   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63921
   EMail: cabo@tzi.org

Shelby, et al.               Standards Track                   [Page 55]