    Internet Engineering Task Force (IETF)                   P. Thubert, Ed.
    Request for Comments: 8505                                         Cisco
    Updates: 6775                                                E. Nordmark
    Category: Standards Track                                         Zededa
    ISSN: 2070-1721                                           S. Chakrabarti
                                                                     Verizon
                                                                  C. Perkins
                                                                   Futurewei
                                                               November 2018

                     Registration Extensions for IPv6 over
     Low-Power Wireless Personal Area Network (6LoWPAN) Neighbor Discovery

    Abstract

       This specification updates RFC 6775 -- the Low-Power Wireless
       Personal Area Network (6LoWPAN) Neighbor Discovery specification --
       to clarify the role of the protocol as a registration technique and
       simplify the registration operation in 6LoWPAN routers, as well as to
       provide enhancements to the registration capabilities and mobility
       detection for different network topologies, including the Routing
       Registrars performing routing for host routes and/or proxy Neighbor
       Discovery in a low-power network.

    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 7841.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       https://www.rfc-editor.org/info/rfc8505.

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    Copyright Notice

       Copyright (c) 2018 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
       (https://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 ....................................................3
       2. Terminology .....................................................4
          2.1. Requirements Language ......................................4
          2.2. Related Documents ..........................................4
          2.3. Abbreviations ..............................................4
          2.4. New Terms ..................................................6
       3. Applicability of Address Registration Options ...................7
       4. Extended Neighbor Discovery Options and Messages ................8
          4.1. Extended Address Registration Option (EARO) ................8
          4.2. Extended Duplicate Address Message Formats ................12
          4.3. Extensions to the Capability Indication Option ............13
       5. Updating RFC 6775 ..............................................14
          5.1. Extending the Address Registration Option .................16
          5.2. Transaction ID ............................................17
               5.2.1. Comparing TID Values ...............................17
          5.3. Registration Ownership Verifier (ROVR) ....................19
          5.4. Extended Duplicate Address Messages .......................20
          5.5. Registering the Target Address ............................20
          5.6. Link-Local Addresses and Registration .....................21
          5.7. Maintaining the Registration States .......................22
       6. Backward Compatibility .........................................24
          6.1. Signaling EARO Support ....................................25
          6.2. RFC 6775-Only 6LN .........................................25
          6.3. RFC 6775-Only 6LR .........................................25
          6.4. RFC 6775-Only 6LBR ........................................26
       7. Security Considerations ........................................26
       8. Privacy Considerations .........................................28

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       9. IANA Considerations ............................................29
          9.1. Address Registration Option Flags .........................29
          9.2. Address Registration Option I-Field .......................29
          9.3. ICMP Codes ................................................30
          9.4. New ARO Status Values .....................................31
          9.5. New 6LoWPAN Capability Bits ...............................32
       10. References ....................................................32
          10.1. Normative References .....................................32
          10.2. Informative References ...................................34
       Appendix A. Applicability and Fulfilled Requirements
                   (Not Normative) .......................................38
       Appendix B. Requirements (Not Normative) ..........................39
         B.1. Requirements Related to Mobility ...........................39
         B.2. Requirements Related to Routing Protocols ..................40
         B.3. Requirements Related to Various Low-Power Link Types .......41
         B.4. Requirements Related to Proxy Operations ...................42
         B.5. Requirements Related to Security ...........................42
         B.6. Requirements Related to Scalability ........................44
         B.7. Requirements Related to Operations and Management ..........44
         B.8. Matching Requirements with Specifications ..................45
       Acknowledgments ...................................................47
       Authors' Addresses ................................................47

    1.  Introduction

       IPv6 Low-Power and Lossy Networks (LLNs) support star and mesh
       topologies.  For such networks, "Neighbor Discovery Optimization for
       IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)"
       [RFC6775] (also referred to as "6LoWPAN Neighbor Discovery (ND)")
       defines a registration mechanism and a central IPv6 ND Registrar to
       ensure unique addresses.  The 6LoWPAN ND mechanism reduces the
       dependency of the IPv6 ND protocol [RFC4861] [RFC4862] on
       network-layer multicast and link-layer broadcast operations.

       This specification updates 6LoWPAN ND [RFC6775] to simplify and
       generalize registration in 6LoWPAN Routers (6LRs).  In particular,
       this specification modifies and extends the behavior and protocol
       elements of 6LoWPAN ND to enable the following actions:

       o  Determining the most recent location in the case of node mobility

       o  Simplifying the registration flow for Link-Local Addresses

       o  Support for a routing-unaware leaf node in a route-over network

       o  Proxy registration in a route-over network

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       o  Enabling verification for the registration, using the Registration
          Ownership Verifier (ROVR) (Section 5.3)

       o  Registration to an IPv6 ND proxy (e.g., a Routing Registrar)

       o  Better support for privacy and temporary addresses

       These features satisfy the requirements listed in Appendix B.

    2.  Terminology

    2.1.  Requirements Language

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
       "OPTIONAL" in this document are to be interpreted as described in
       BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
       capitals, as shown here.

    2.2.  Related Documents

       In this document, readers will encounter terms and concepts that are
       discussed in the following documents:

       o  "Neighbor Discovery for IP version 6 (IPv6)" [RFC4861]

       o  "IPv6 Stateless Address Autoconfiguration" [RFC4862]

       o  "IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs):
          Overview, Assumptions, Problem Statement, and Goals" [RFC4919]

       o  "Problem Statement and Requirements for IPv6 over Low-Power
          Wireless Personal Area Network (6LoWPAN) Routing" [RFC6606]

       o  "Neighbor Discovery Optimization for IPv6 over Low-Power Wireless
          Personal Area Networks (6LoWPANs)" [RFC6775]

    2.3.  Abbreviations

       This document uses the following abbreviations:

       6BBR: 6LoWPAN Backbone Router

       6CIO: Capability Indication Option

       6LBR: 6LoWPAN Border Router

       6LN:  6LoWPAN Node

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       6LoWPAN:  IPv6 over Low-Power Wireless Personal Area Network

       6LR:  6LoWPAN Router

       ARO:  Address Registration Option

       DAC:  Duplicate Address Confirmation

       DAD:  Duplicate Address Detection

       DAR:  Duplicate Address Request

       DODAG:  Destination-Oriented Directed Acyclic Graph

       EARO: Extended Address Registration Option

       EDA:  Extended Duplicate Address

       EDAC: Extended Duplicate Address Confirmation

       EDAR: Extended Duplicate Address Request

       LLN:  Low-Power and Lossy Network

       NA:   Neighbor Advertisement

       NCE:  Neighbor Cache Entry

       ND:   Neighbor Discovery

       NS:   Neighbor Solicitation

       RA:   Router Advertisement

       ROVR: Registration Ownership Verifier (pronounced "rover")

       RPL:  IPv6 Routing Protocol for LLNs (pronounced "ripple") [RFC6550]

       RS:   Router Solicitation

       TID:  Transaction ID (a sequence counter in the EARO)

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    2.4.  New Terms

       Backbone Link:  An IPv6 transit link that interconnects two or more
             Backbone Routers.

       Binding:  The association between an IP address, a Media Access
             Control (MAC) address, and other information about the node
             that owns the IP address.

       Registration:  The process by which a 6LN registers an IPv6 Address
             with a 6LR in order to establish connectivity to the LLN.

       Registered Node:  The 6LN for which the registration is performed,
             according to the fields in the EARO.

       Registering Node:  The node that performs the registration.  Either
             the Registered Node or a proxy.

       IPv6 ND Registrar:  A node that can process a registration in either
             NS(EARO) or EDAR messages and consequently respond with an NA
             or EDAC message containing the EARO and appropriate status for
             the registration.

       Registered Address:  An address registered for the Registered Node.

       RFC 6775-only:  An implementation, a type of node, or a message that
             behaves only as specified by [RFC6775], as opposed to the
             behavior specified in this document.

       Route-over network:  A network for which connectivity is provided at
             the IP layer.

       Routing Registrar:  An IPv6 ND Registrar that also provides
             reachability services for the Registered Address, including DAD
             and proxy NA.

       Backbone Router (6BBR):  A Routing Registrar that proxies the 6LoWPAN
             ND operations specified in this document to ensure that
             multiple LLNs federated by a Backbone Link operate as a single
             IPv6 subnetwork.

       updated:  A 6LN, 6LR, or 6LBR that supports this specification, in
             contrast to an RFC 6775-only device.

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    3.  Applicability of Address Registration Options

       The ARO as described in [RFC6775] facilitates DAD for hosts and
       populates NCEs [RFC4861] in the routers.  This reduces the reliance
       on multicast operations, which are often as intrusive as broadcast,
       in IPv6 ND operations (see [Multicast-over-IEEE802-Wireless]).

       This document specifies new status codes for registrations rejected
       by a 6LR or 6LBR for reasons other than address duplication.
       Examples include:

       o  the router running out of space.

       o  a registration bearing a stale sequence number.  This could happen
          if the host moves after the registration was placed.

       o  a host misbehaving and attempting to register an invalid address,
          such as the unspecified address as defined in [RFC4291].

       o  a host using an address that is not topologically correct on
          that link.

       In such cases, the host will receive an error that will help diagnose
       the issue; the host may retry -- possibly with a different address or
       possibly registering to a different router -- depending on the
       returned error.  The ability to return errors to address
       registrations is not intended to be used to restrict the ability of
       hosts to form and use multiple addresses.  Each host may form and
       register a number of addresses for enhanced privacy, using mechanisms
       such as those described in [RFC4941] ("Privacy Extensions for
       Stateless Address Autoconfiguration in IPv6"), e.g., Stateless
       Address Autoconfiguration (SLAAC), and SHOULD conform to [RFC7934]
       ("Host Address Availability Recommendations").

       As indicated in IPv6 ND [RFC4861], a router needs enough storage to
       hold NCEs for all directly connected addresses to which it is
       currently forwarding packets (unused entries may be flushed).  In
       contrast, a router serving the address-registration mechanism needs
       enough storage to hold NCEs for all the addresses that may be
       registered to it, regardless of whether or not they are actively
       communicating.  The number of registrations supported by a 6LR or
       6LBR MUST be clearly documented by the vendor, and the dynamic use of
       associated resources SHOULD be made available to the network
       operator, e.g., to a management console.  Network administrators need
       to ensure that 6LRs/6LBRs in their network support the number and
       types of devices that can register to them, based on the number of
       IPv6 Addresses that those devices require, as well as their address
       renewal rate and behavior.

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    4.  Extended Neighbor Discovery Options and Messages

       This specification does not introduce any new options; it modifies
       existing options and updates the associated behaviors.

    4.1.  Extended Address Registration Option (EARO)

       The ARO is defined in Section 4.1 of [RFC6775].

       This specification introduces the EARO; the EARO is based on the ARO
       for use in NS and NA messages.  The EARO includes a sequence counter
       called the Transaction ID (TID), which is used to determine the
       latest location of a registering mobile device.  A new T flag
       indicates that the presence of the TID field is populated and that
       the option is an EARO.  A 6LN requests routing or proxy services from
       a 6LR using a new R flag in the EARO.

       The EUI-64 field is redefined and renamed "ROVR field" in order to
       carry different types of information, e.g., cryptographic information
       of variable size (see Section 5.3).  A larger ROVR size MAY be used
       if and only if backward compatibility is not an issue in the
       particular LLN.  The length of the ROVR field, expressed in units of
       8 bytes, is the Length value of the option minus 1.  A larger ROVR
       size MAY be used if and only if backward compatibility is not an
       issue in the particular LLN.

       Section 5.1 discusses those changes in depth.

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       The format of the EARO is shown in Figure 1:

          0                   1                   2                   3
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |     Type      |     Length    |    Status     |    Opaque     |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |  Rsvd | I |R|T|     TID       |     Registration Lifetime     |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                                                               |
        ...            Registration Ownership Verifier (ROVR)           ...
         |                                                               |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                               Figure 1: EARO Format

       Option Fields:

       Type:       33

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

       Status:     8-bit unsigned integer.  Indicates the status of a
                   registration in the NA response.  MUST be set to 0 in NS
                   messages.  See Table 1 below.

       Opaque:     An octet opaque to ND.  The 6LN MAY pass it transparently
                   to another process.  It MUST be set to 0 when not used.

       Rsvd (Reserved):
                   This field is unused.  It MUST be initialized to 0 by the
                   sender and MUST be ignored by the receiver.

       I:          2-bit integer.  A value of 0 indicates that the Opaque
                   field carries an abstract index that is used to decide in
                   which routing topology the address is expected to be
                   injected.  In that case, the Opaque field is passed to a
                   routing process with the indication that it carries
                   topology information, and the value of 0 indicates
                   default.  All other values of "I" are reserved and
                   MUST NOT be used.

       R:          The Registering Node sets the R flag to request
                   reachability services for the Registered Address from a
                   Routing Registrar.

       T:          1-bit flag.  Set if the next octet is used as a TID.

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       TID:        1-byte unsigned integer.  A Transaction ID that is
                   maintained by the node and incremented with each
                   transaction of one or more registrations performed at the
                   same time to one or more 6LRs.  This field MUST be
                   ignored if the T flag is not set.

       Registration Lifetime:
                   16-bit integer, expressed in minutes.  A value of 0
                   indicates that the registration has ended and that the
                   associated state MUST be removed.

       Registration Ownership Verifier (ROVR):
                   Enables the correlation between multiple attempts to
                   register the same IPv6 Address.  The ROVR size MUST be
                   64 bits when backward compatibility is needed; otherwise,
                   the size MAY be 128, 192, or 256 bits.

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       +-------+-----------------------------------------------------------+
       | Value | Description                                               |
       +-------+-----------------------------------------------------------+
       |  0-2  | As defined in [RFC6775].  Note: A Status value of 1       |
       |       | ("Duplicate Address") applies to the Registered Address.  |
       |       | If the Source Address conflicts with an existing          |
       |       | registration, "Duplicate Source Address" MUST be used.    |
       |       |                                                           |
       |   3   | Moved: The registration failed because it is not the most |
       |       | recent.  This Status indicates that the registration is   |
       |       | rejected because another more recent registration was     |
       |       | done, as indicated by the same ROVR and a more recent     |
       |       | TID.  One possible cause is a stale registration that has |
       |       | progressed slowly in the network and was passed by a more |
       |       | recent one.  It could also indicate a ROVR collision.     |
       |       |                                                           |
       |   4   | Removed: The binding state was removed.  This Status MAY  |
       |       | be placed in an NA(EARO) message that is sent as the      |
       |       | rejection of a proxy registration to an IPv6 ND           |
       |       | Registrar, or in an asynchronous NA(EARO), at any time.   |
       |       |                                                           |
       |   5   | Validation Requested: The Registering Node is challenged  |
       |       | for owning the Registered Address or for being an         |
       |       | acceptable proxy for the registration.  An IPv6 ND        |
       |       | Registrar MAY place this Status in asynchronous DAC or NA |
       |       | messages.                                                 |
       |       |                                                           |
       |   6   | Duplicate Source Address: The address used as the source  |
       |       | of the NS(EARO) conflicts with an existing registration.  |
       |       |                                                           |
       |   7   | Invalid Source Address: The address used as the source of |
       |       | the NS(EARO) is not a Link-Local Address.                 |
       |       |                                                           |
       |   8   | Registered Address Topologically Incorrect: The address   |
       |       | being registered is not usable on this link.              |
       |       |                                                           |
       |   9   | 6LBR Registry Saturated: A new registration cannot be     |
       |       | accepted because the 6LBR Registry is saturated.  Note:   |
       |       | This code is used by 6LBRs instead of Status 2 when       |
       |       | responding to a Duplicate Address message exchange and is |
       |       | passed on to the Registering Node by the 6LR.             |
       |       |                                                           |
       |   10  | Validation Failed: The proof of ownership of the          |
       |       | Registered Address is not correct.                        |
       +-------+-----------------------------------------------------------+

                            Table 1: EARO Status Codes

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    4.2.  Extended Duplicate Address Message Formats

       The DAR and DAC messages share a common base format as defined in
       Section 4.4 of [RFC6775].  Those messages enable information from the
       ARO to be transported over multiple hops.  The DAR and DAC are
       extended as shown in Figure 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      |CodePfx|CodeSfx|          Checksum             |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |    Status     |     TID       |     Registration Lifetime     |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                                                               |
        ...            Registration Ownership Verifier (ROVR)           ...
         |                                                               |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                                                               |
         +                                                               +
         |                                                               |
         +                       Registered Address                      +
         |                                                               |
         +                                                               +
         |                                                               |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 2: Extended Duplicate Address Message Format

       Modified Message Fields:

       Code:       The ICMP Code [RFC4443] for Duplicate Address messages is
                   split into two 4-bit fields: the Code Prefix and the Code
                   Suffix.  The Code Prefix MUST be set to 0 by the sender
                   and MUST be ignored by the receiver.  A non-null value of
                   the Code Suffix indicates support for this specification.
                   It MUST be set to 1 when operating in a backward-
                   compatible mode, indicating a ROVR size of 64 bits.  It
                   MAY be 2, 3, or 4, denoting a ROVR size of 128, 192, or
                   256 bits, respectively.

       TID:        1-byte integer.  Same definition and processing as the
                   TID in the EARO as defined in Section 4.1.  This field
                   MUST be ignored if the ICMP Code is null.

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       Registration Ownership Verifier (ROVR):
                   The size of the ROVR is known from the ICMP Code Suffix.
                   This field has the same definition and processing as the
                   ROVR in the EARO as defined in Section 4.1.

    4.3.  Extensions to the Capability Indication Option

       This specification defines five new capability bits for use in the
       6CIO as defined by [RFC7400] ("6LoWPAN-GHC: Generic Header
       Compression for IPv6 over Low-Power Wireless Personal Area Networks
       (6LoWPANs)"), for use in IPv6 ND messages.  (The G flag is defined in
       Section 3.3 of [RFC7400].)

       The D flag indicates that the 6LBR supports EDAR and EDAC messages.
       A 6LR that learns the D flag from advertisements can then exchange
       EDAR and EDAC messages with the 6LBR, and it also sets the D flag as
       well as the L flag in the 6CIO in its own advertisements.  In this
       way, 6LNs will be able to prefer registration with a 6LR that can
       make use of new 6LBR features.

       The new L, B, and P flags indicate whether a router is capable of
       acting as a 6LR, 6LBR, or Routing Registrar (e.g., 6BBR) (or some
       combination thereof), respectively.  These flags are not mutually
       exclusive; an updated node can advertise multiple collocated
       functions.

       The E flag indicates that the EARO can be used in a registration.  A
       6LR that supports this specification MUST set the E flag.

          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 = 1  |     Reserved      |D|L|B|P|E|G|
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                           Reserved                            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 3: New Capability Bits in the 6CIO

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       Option Fields:

       Type:  36

       D: The 6LBR supports EDAR and EDAC messages.

       L: The node is a 6LR.

       B: The node is a 6LBR.

       P: The node is a Routing Registrar.

       E: The node is an IPv6 ND Registrar; i.e., it supports registrations
          based on the EARO.

    5.  Updating RFC 6775

       The EARO (see Section 4.1) updates the ARO used within NS and NA
       messages between a 6LN and a 6LR.  The update enables a registration
       to a Routing Registrar in order to obtain additional services, such
       as return routability to the Registered Address by such means as
       routing and/or proxy ND, as illustrated in Figure 4.

                                     Routing
                     6LN            Registrar
                      |                |
                      |   NS(EARO)     |
                      |--------------->|
                      |                |
                      |                | Inject/maintain
                      |                | host route or
                      |                | IPv6 ND proxy state
                      |                | <----------------->
                      |   NA(EARO)     |
                      |<---------------|
                      |                |

                         Figure 4: (Re-)Registration Flow

       Similarly, the EDAR and EDAC update the DAR and DAC messages so as to
       transport the new information between 6LRs and 6LBRs across an LLN
       mesh.  The extensions to the ARO are the DAR and the DAC, as used in
       the Duplicate Address messages.  They convey the additional
       information all the way to the 6LBR.

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       In turn, the 6LBR may proxy the registration to obtain reachability
       services from a Routing Registrar such as a 6BBR, as illustrated in
       Figure 5.  This specification avoids the Duplicate Address message
       flow for Link-Local Addresses in a route-over [RFC6606] topology (see
       Section 5.6).

                                                 Routing
          6LN          6LR            6LBR      Registrar
           |            |              |            |
           |<Link-local>|   <Routed>   |<Link-local>|
           |            |              |            |
           |  NS(EARO)  |              |            |
           |----------->|              |            |
           |            | Extended DAR |            |
           |            |------------->|            |
           |            |              |  proxy     |
           |            |              |  NS(EARO)  |
           |            |              |----------->|
           |            |              |            | Inject/maintain
           |            |              |            | host route or
           |            |              |            | IPv6 ND proxy state
           |            |              |            | <----------------->
           |            |              |  proxy     |
           |            |              |  NA(EARO)  |
           |            | Extended DAC |<-----------|
           |            |<-------------|            |
           |  NA(EARO)  |              |            |
           |<-----------|              |            |
           |            |              |            |

                         Figure 5: (Re-)Registration Flow

       This specification allows multiple registrations, including
       registrations for privacy and temporary addresses, and provides a
       mechanism to help clean up stale registration state as soon as
       possible, e.g., after a movement (see Section 7).

       Section 5 of [RFC6775] specifies how a 6LN bootstraps an interface
       and locates available 6LRs.  A Registering Node SHOULD register to a
       6LR that supports this specification if one is found, as discussed in
       Section 6.1, instead of registering to an RFC 6775-only 6LR;
       otherwise, the Registering Node operates in a backward-compatible
       fashion when attaching to an RFC 6775-only 6LR.

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    5.1.  Extending the Address Registration Option

       The EARO updates the ARO and is backward compatible with the ARO if
       and only if the Length value of the option is set to 2.  The format
       of the EARO is presented in Section 4.1.  More details on backward
       compatibility can be found in Section 6.

       The NS message and the ARO are modified as follows:

       o  The Target Address field in the NS containing the EARO is now the
          field that indicates the address that is being registered, as
          opposed to the Source Address field in the NS as specified in
          [RFC6775] (see Section 5.5).  This change enables a 6LBR to send a
          proxy registration for a 6LN's address to a Routing Registrar and
          in most cases also avoids the use of an address as the Source
          Address before it is registered.

       o  The EUI-64 field in the ARO is renamed "Registration Ownership
          Verifier (ROVR)" and is not required to be derived from a MAC
          address (see Section 5.3).

       o  The option's Length value MAY be different than 2 and take a value
          between 3 and 5, in which case the EARO is not backward compatible
          with an ARO.  The increase in size corresponds to a larger ROVR
          field, so the size of the ROVR is inferred from the option's
          Length value.

       o  A new Opaque field is introduced to carry opaque information in
          cases where the registration is relayed to another process, e.g.,
          to be advertised by a routing protocol.  A new "I" field provides
          a type for the opaque information and indicates the other process
          to which the 6LN passes the opaque value.  A value of 0 for the
          "I" field indicates topological information to be passed to a
          routing process if the registration is redistributed.  In that
          case, a value of 0 for the Opaque field (1) is backward compatible
          with the reserved fields that are overloaded and (2) indicates
          that the default topology is to be used.

       o  This document specifies a new flag in the EARO: the R flag.  If
          the R flag is set, the Registering Node requests that the 6LR
          ensure reachability for the Registered Address, e.g., by means of
          routing or proxy ND.  Conversely, when it is not set, the R flag
          indicates that the Registering Node is a router and that it will
          advertise reachability to the Registered Address via a routing
          protocol (such as RPL [RFC6550]).

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       o  A node that supports this specification MUST provide a TID field
          in the EARO and set the T flag to indicate the presence of the TID
          (see Section 5.2).

       o  Finally, this specification introduces new status codes to help
          diagnose the cause of a registration failure (see Table 1).

       When registering, a 6LN that acts only as a host MUST set the R flag
       to indicate that it is not a router and that it will not handle its
       own reachability.  A 6LR that manages its reachability SHOULD NOT set
       the R flag; if it does, routes towards this router may be installed
       on its behalf and may interfere with those it advertises.

    5.2.  Transaction ID

       The TID is a sequence number that is incremented by the 6LN with each
       re-registration to a 6LR.  The TID is used to determine the recency
       of the registration request.  The network uses the most recent TID to
       determine the most recent known location(s) of a moving 6LN.  When a
       Registered Node is registered with multiple 6LRs in parallel, the
       same TID MUST be used.  This enables the 6LBRs and/or Routing
       Registrars to determine whether the registrations are identical and
       to distinguish that situation from a movement (for example, see
       Section 5.7 and Appendix A).

    5.2.1.  Comparing TID Values

       The operation of the TID is fully compatible with that of the RPL
       Path Sequence counter as described in Section 7.2 of [RFC6550]
       ("RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks").

       A TID is deemed to be more recent than another when its value is
       greater as determined by the operations detailed in this section.

       The TID range is subdivided in a "lollipop" fashion [Perlman83],
       where the values from 128 and greater are used as a linear sequence
       to indicate a restart and bootstrap the counter, and the values less
       than or equal to 127 are used as a circular sequence number space of
       size 128 as mentioned in [RFC1982].  Consideration is given to the
       mode of operation when transitioning from the linear region to the
       circular region.  Finally, when operating in the circular region, if
       sequence numbers are determined to be too far apart, then they are
       not comparable, as detailed below.

       A window of comparison, SEQUENCE_WINDOW = 16, is configured based on
       a value of 2^N, where N is defined to be 4 in this specification.

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       For a given sequence counter,

       1.  Prior to use, the sequence counter SHOULD be initialized to an
           implementation-defined value of 128 or greater.  A recommended
           value is 240 (256 - SEQUENCE_WINDOW).

       2.  When a sequence counter increment would cause the sequence
           counter to increment beyond its maximum value, the sequence
           counter MUST wrap back to 0.  When incrementing a sequence
           counter greater than or equal to 128, the maximum value is 255.
           When incrementing a sequence counter less than 128, the maximum
           value is 127.

       3.  When comparing two sequence counters, the following rules MUST be
           applied:

           1.  When a first sequence counter A is in the interval [128-255]
               and a second sequence counter B is in the interval [0-127]:

               1.  If (256 + B - A) is less than or equal to
                   SEQUENCE_WINDOW, then B is greater than A, A is less than
                   B, and the two are not equal.

               2.  If (256 + B - A) is greater than SEQUENCE_WINDOW, then A
                   is greater than B, B is less than A, and the two are not
                   equal.

               For example, if A is 240 and B is 5, then (256 + 5 - 240) is
               21.  21 is greater than SEQUENCE_WINDOW (16); thus, 240 is
               greater than 5.  As another example, if A is 250 and B is 5,
               then (256 + 5 - 250) is 11.  11 is less than SEQUENCE_WINDOW
               (16); thus, 250 is less than 5.

           2.  In the case where both sequence counters to be compared are
               less than or equal to 127, and in the case where both
               sequence counters to be compared are greater than or equal
               to 128:

               1.  If the absolute magnitude of difference between the two
                   sequence counters is less than or equal to
                   SEQUENCE_WINDOW, then a comparison as described in
                   [RFC1982] is used to determine the relationships
                   "greater than", "less than", and "equal".

               2.  If the absolute magnitude of difference of the two
                   sequence counters is greater than SEQUENCE_WINDOW, then a
                   desynchronization has occurred and the two sequence
                   numbers are not comparable.

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       4.  If two sequence numbers are determined to be not comparable,
           i.e., the results of the comparison are not defined, then a node
           should give precedence to the sequence number that was most
           recently incremented.  Failing this, the node should select the
           sequence number in order to minimize the resulting changes to its
           own state.

    5.3.  Registration Ownership Verifier (ROVR)

       The ROVR field replaces the EUI-64 field of the ARO defined in
       [RFC6775].  It is associated in the 6LR and the 6LBR with the
       registration state.  The ROVR can be a unique ID of the Registering
       Node, such as the EUI-64 address of an interface.  This can also be a
       token obtained with cryptographic methods that can be used in
       additional protocol exchanges to associate a cryptographic identity
       (key) with this registration to ensure that only the owner can modify
       it later, if the proof of ownership of the ROVR can be obtained.  The
       scope of a ROVR is the registration of a particular IPv6 Address, and
       it MUST NOT be used to correlate registrations of different
       addresses.

       The ROVR can be of different types; the type is signaled in the
       message that carries the new type.  For instance, the type can be a
       cryptographic string and can be used to prove the ownership of the
       registration as specified in [AP-ND] ("Address Protected Neighbor
       Discovery for Low-power and Lossy Networks").  In order to support
       the flows related to the proof of ownership, this specification
       introduces new status codes "Validation Requested" and "Validation
       Failed" in the EARO.

       Note regarding ROVR collisions: Different techniques for forming the
       ROVR will operate in different namespaces.  [RFC6775] specifies the
       use of EUI-64 addresses.  [AP-ND] specifies the generation of
       cryptographic tokens.  While collisions are not expected in the
       EUI-64 namespace only, they may happen if [AP-ND] is implemented by
       at least one of the nodes.  An implementation that understands the
       namespace MUST consider that ROVRs from different namespaces are
       different even if they have the same value.  An RFC 6775-only 6LBR or
       6LR will confuse the namespaces; this slightly increases the risk of
       a ROVR collision.  A ROVR collision has no effect if the two
       Registering Nodes register different addresses, since the ROVR is
       only significant within the context of one registration.  A ROVR is
       not expected to be unique to one registration, as this specification
       allows a node to use the same ROVR to register multiple IPv6
       Addresses.  This is why the ROVR MUST NOT be used as a key to
       identify the Registering Node or as an index to the registration.  It
       is only used as a match to ensure that the node that updates a
       registration for an IPv6 Address is the node that made the original

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       registration for that IPv6 Address.  Also, when the ROVR is not an
       EUI-64 address, then it MUST NOT be used as the Interface Identifier
       of the Registered Address.  This way, a registration that uses that
       ROVR will not collide with that of an IPv6 Address derived from
       EUI-64 and using the EUI-64 as the ROVR per [RFC6775].

       The Registering Node SHOULD store the ROVR, or enough information to
       regenerate it, in persistent memory.  If this is not done and an
       event such as a reboot causes a loss of state, re-registering the
       same address could be impossible until (1) the 6LRs and the 6LBR
       time out the previous registration or (2) a management action clears
       the relevant state in the network.

    5.4.  Extended Duplicate Address Messages

       In order to map the new EARO content in the EDA messages, a new TID
       field is added to the EDAR and EDAC messages as a replacement for the
       Reserved field, and a non-null value of the ICMP Code indicates
       support for this specification.  The format of the EDAR and EDAC
       messages is presented in Section 4.2.

       As with the EARO, the EDA messages are backward compatible with the
       RFC 6775-only versions, as long as the ROVR field is 64 bits long.
       Remarks concerning backward compatibility for the protocol between
       the 6LN and the 6LR apply similarly between a 6LR and a 6LBR.

    5.5.  Registering the Target Address

       An NS message with an EARO is a registration if and only if it also
       carries an SLLA Option ("SLLAO") [RFC6775] ("SLLA" stands for "Source
       Link-Layer Address").  The EARO can also be used in NS and NA
       messages between Routing Registrars to determine the distributed
       registration state; in that case, it does not carry the SLLA Option
       and is not confused with a registration.

       The Registering Node is the node that performs the registration to
       the Routing Registrar.  As also described in [RFC6775], it may be the
       Registered Node as well, in which case it registers one of its own
       addresses and indicates its own MAC address as the SLLA in the
       NS(EARO).

       This specification adds the capability to proxy the registration
       operation on behalf of a Registered Node that is reachable over an
       LLN mesh.  In that case, if the Registered Node is reachable from the
       Routing Registrar via a mesh-under configuration, the Registering
       Node indicates the MAC address of the Registered Node as the SLLA in
       the NS(EARO).  If the Registered Node is reachable over a route-over
       configuration from the Registering Node, the SLLA in the NS(ARO) is

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       that of the Registering Node.  This enables the Registering Node to
       attract the packets from the Routing Registrar and route them over
       the LLN to the Registered Node.

       In order to enable the latter operation, this specification changes
       the behavior of the 6LN and the 6LR so that the Registered Address is
       found in the Target Address field of the NS and NA messages as
       opposed to the Source Address field.  With this convention, a TLLA
       Option (Target Link-Layer Address Option, or "TLLAO") indicates the
       link-layer address of the 6LN that owns the address.

       A Registering Node (e.g., a 6LBR also acting as a RPL root) that
       advertises reachability for the 6LN MUST place its own link-layer
       address in the SLLA Option of the registration NS(EARO) message.
       This maintains compatibility with RFC 6775-only 6LoWPAN ND.

    5.6.  Link-Local Addresses and Registration

       LLN nodes are often not wired and may move.  There is no guarantee
       that a Link-Local Address will remain unique among a huge and
       potentially variable set of neighboring nodes.

       Compared to [RFC6775], this specification only requires that a
       Link-Local Address be unique from the perspective of the two nodes
       that use it to communicate (e.g., the 6LN and the 6LR in an NS/NA
       exchange).  This simplifies the DAD process in a route-over topology
       for Link-Local Addresses by avoiding an exchange of EDA messages
       between the 6LR and a 6LBR for those addresses.

       An exchange between two nodes using Link-Local Addresses implies that
       they are reachable over one hop.  A node MUST register a Link-Local
       Address to a 6LR in order to obtain further reachability by way of
       that 6LR and, in particular, to use the Link-Local Address as the
       Source Address to register other addresses, e.g., global addresses.

       If there is no collision with a previously registered address, then
       the Link-Local Address is unique from the standpoint of this 6LR and
       the registration is not a duplicate.  Two different 6LRs might claim
       the same Link-Local Address but different link-layer addresses.  In
       that case, a 6LN MUST only interact with at most one of the 6LRs.

       The exchange of EDAR and EDAC messages between the 6LR and a 6LBR,
       which ensures that an address is unique across the domain covered by
       the 6LBR, does not need to take place for Link-Local Addresses.

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       When sending an NS(EARO) to a 6LR, a 6LN MUST use a Link-Local
       Address as the Source Address of the registration, whatever the type
       of IPv6 Address that is being registered.  That Link-Local Address
       MUST be either an address that is already registered to the 6LR or
       the address that is being registered.

       When a 6LN starts up, it typically multicasts an RS and receives one
       or more unicast RA messages from 6LRs.  If the 6LR can process EARO
       messages, then it places a 6CIO in its RA message with the E flag set
       as required in Section 6.1.

       When a Registering Node does not have an already-registered address,
       it MUST register a Link-Local Address, using it as both the Source
       Address and the Target Address of an NS(EARO) message.  In that case,
       it is RECOMMENDED to use an address for which DAD is not required
       (see [RFC6775]), e.g., derived from a globally unique EUI-64 address;
       using the SLLA Option in the NS is consistent with existing ND
       specifications such as [RFC4429] ("Optimistic Duplicate Address
       Detection (DAD) for IPv6").  The 6LN MAY then use that address to
       register one or more other addresses.

       A 6LR that supports this specification replies with an NA(EARO),
       setting the appropriate status.  Since there is no exchange of EDAR
       or EDAC messages for Link-Local Addresses, the 6LR may answer
       immediately to the registration of a Link-Local Address, based solely
       on its existing state and the SLLA Option that is placed in the
       NS(EARO) message as required in [RFC6775].

       A node registers its IPv6 Global Unicast Addresses (GUAs) to a 6LR in
       order to establish global reachability for these addresses via that
       6LR.  When registering with an updated 6LR, a Registering Node does
       not use a GUA as the Source Address, in contrast to a node that
       complies with [RFC6775].  For non-Link-Local Addresses, the exchange
       of EDAR and EDAC messages MUST conform to [RFC6775], but the extended
       formats described in this specification for the DAR and the DAC are
       used to relay the extended information in the case of an EARO.

    5.7.  Maintaining the Registration States

       This section discusses protocol actions that involve the Registering
       Node, the 6LR, and the 6LBR.  It must be noted that the portion that
       deals with a 6LBR only applies to those addresses that are registered
       to it; as discussed in Section 5.6, this is not the case for
       Link-Local Addresses.  The registration state includes all data that
       is stored in the router relative to that registration, in particular,
       but not limited to, an NCE.  6LBRs and Routing Registrars may store
       additional registration information and use synchronization protocols
       that are out of scope for this document.

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       A 6LR cannot accept a new registration when its registration storage
       space is exhausted.  In that situation, the EARO is returned in an NA
       message with a status code of "Neighbor Cache Full" (Status 2; see
       [RFC6775] and Table 1), and the Registering Node may attempt to
       register to another 6LR.

       If the registry in the 6LBR is full, then the 6LBR cannot decide
       whether a registration for a new address is a duplicate.  In that
       case, the 6LBR replies to an EDAR message with an EDAC message that
       carries a new status code indicating "6LBR Registry Saturated"
       (Table 1).  Note: This code is used by 6LBRs instead of "Neighbor
       Cache Full" when responding to a Duplicate Address message exchange
       and is passed on to the Registering Node by the 6LR.  There is no
       point in the node retrying this registration via another 6LR, since
       the problem is network-wide.  The node may abandon that address,
       de-register other addresses first to make room, or keep the address
       "tentative" [RFC4861] and retry later.

       A node renews an existing registration by sending a new NS(EARO)
       message for the Registered Address, and the 6LR MUST report the new
       registration to the 6LBR.

       A node that ceases to use an address SHOULD attempt to de-register
       that address from all the 6LRs to which it has registered the
       address.  This is achieved using an NS(EARO) message with a
       Registration Lifetime of 0.  If this is not done, the associated
       state will remain in the network until the current Registration
       Lifetime expires; this may lead to a situation where the 6LR
       resources become saturated, even if they were correctly planned to
       start with.  The 6LR may then take defensive measures that may
       prevent this node or some other nodes from owning as many addresses
       as they request (see Section 7).

       A node that moves away from a particular 6LR SHOULD attempt to
       de-register all of its addresses registered to that 6LR and register
       to a new 6LR with an incremented TID.  When/if the node appears
       elsewhere, an asynchronous NA(EARO) or EDAC message with a status
       code of "Moved" SHOULD be used to clean up the state in the previous
       location.  The "Moved" status can be used by a Routing Registrar in
       an NA(EARO) message to indicate that the ownership of the proxy state
       was transferred to another Routing Registrar due to movement of the
       device.  If the receiver of the message has registration state
       corresponding to the related address, it SHOULD propagate the status
       down the forwarding path to the Registered Node (e.g., reversing an
       existing RPL [RFC6550] path as prescribed in [Efficient-NPDAO]).
       Whether it could do so or not, the receiver MUST clean up said state.

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       Upon receiving an NS(EARO) message with a Registration Lifetime of 0
       and determining that this EARO is the most recent for a given NCE
       (see Section 5.2), a 6LR cleans up its NCE.  If the address was
       registered to the 6LBR, then the 6LR MUST report to the 6LBR, through
       a Duplicate Address exchange with the 6LBR, indicating the null
       Registration Lifetime and the latest TID that this 6LR is aware of.

       Upon receiving the EDAR message, the 6LBR determines if this is the
       most recent TID it has received for that particular registry entry.
       If so, then the EDAR is answered with an EDAC message bearing a
       status code of 0 ("Success") [RFC6775], and the entry is scheduled to
       be removed.  Otherwise, a status code of "Moved" is returned instead,
       and the existing entry is maintained.

       When an address is scheduled to be removed, the 6LBR SHOULD keep its
       NCE in a DELAY state [RFC4861] for a configurable period of time, so
       as to prevent a scenario where (1) a mobile node that de-registered
       from one 6LR did not yet register to a new one or (2) the new
       registration did not yet reach the 6LBR due to propagation delays in
       the network.  Once the DELAY time has passed, the 6LBR silently
       removes its entry.

    6.  Backward Compatibility

       This specification changes the behavior of the peers in a
       registration flow.  To enable backward compatibility, a 6LN that
       registers to a 6LR that is not known to support this specification
       MUST behave in a manner that is backward compatible with [RFC6775].
       Conversely, if the 6LR is found to support this specification, then
       the 6LN MUST conform to this specification when communicating with
       that 6LR.

       A 6LN that supports this specification MUST always use an EARO as a
       replacement for an ARO in its registration to a router.  This
       behavior is backward compatible, since the T flag and TID field
       occupy fields that are reserved in [RFC6775] and are thus ignored by
       an RFC 6775-only router.  A router that supports this specification
       MUST answer an NS(ARO) and an NS(EARO) with an NA(EARO).  A router
       that does not support this specification will consider the ROVR as an
       EUI-64 address and treat it the same; this scenario has no
       consequence if the Registered Addresses are different.

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    6.1.  Signaling EARO Support

       [RFC7400] specifies the 6CIO, which indicates a node's capabilities
       to the node's peers.  The 6CIO MUST be present in both RS and RA
       messages, unless the 6CIO information was already shared in recent
       exchanges or pre-configured in all nodes in a network.  In any case,
       a 6CIO MUST be placed in an RA message that is sent in response to an
       RS with a 6CIO.

       Section 4.3 defines a new flag for the 6CIO to signal EARO support by
       the issuer of the message.  New flags are also added to the 6CIO to
       signal the sender's capability to act as a 6LR, 6LBR, and Routing
       Registrar (see Section 4.3).

       Section 4.3 also defines a new flag that indicates the support of
       EDAR and EDAC messages by the 6LBR.  This flag is valid in RA
       messages but not in RS messages.  More information on the 6LBR is
       found in a separate Authoritative Border Router Option (ABRO).  The
       ABRO is placed in RA messages as prescribed by [RFC6775]; in
       particular, it MUST be placed in an RA message that is sent in
       response to an RS with a 6CIO indicating the capability to act as a
       6LR, since the RA propagates information between routers.

    6.2.  RFC 6775-Only 6LN

       An RFC 6775-only 6LN will use the Registered Address as the Source
       Address of the NS message and will not use an EARO.  An updated 6LR
       MUST accept that registration if it is valid per [RFC6775], and it
       MUST manage the binding cache accordingly.  The updated 6LR MUST then
       use the RFC 6775-only DAR and DAC messages as specified in [RFC6775]
       to indicate to the 6LBR that the TID is not present in the messages.

       The main difference from [RFC6775] is that the exchange of DAR and
       DAC messages for the purpose of DAD is avoided for Link-Local
       Addresses.  In any case, the 6LR MUST use an EARO in the reply and
       can use any of the status codes defined in this specification.

    6.3.  RFC 6775-Only 6LR

       An updated 6LN discovers the capabilities of the 6LR in the 6CIO in
       RA messages from that 6LR; if the 6CIO was not present in the RA,
       then the 6LR is assumed to be RFC 6775-only.

       An updated 6LN MUST use an EARO in the request, regardless of the
       type of 6LR -- RFC 6775-only or updated; this implies that the T flag
       is set.  It MUST use a ROVR of 64 bits if the 6LR is RFC 6775-only.

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       If an updated 6LN moves from an updated 6LR to an RFC 6775-only 6LR,
       the RFC 6775-only 6LR will send an RFC 6775-only DAR message, which
       cannot be compared with an updated one for recency.  Allowing
       RFC 6775-only DAR messages to update a state established by the
       updated protocol in the 6LBR would be an attack vector; therefore,
       this cannot be the default behavior.  But if RFC 6775-only and
       updated 6LRs coexist temporarily in a network, then it makes sense
       for an administrator to install a policy that allows this behavior,
       using some method that is out of scope for this document.

    6.4.  RFC 6775-Only 6LBR

       With this specification, the Duplicate Address messages are extended
       to transport the EARO information.  As with the NS/NA exchange, an
       updated 6LBR MUST always use the EDAR and EDAC messages.

       Note that an RFC 6775-only 6LBR will accept and process an EDAR
       message as if it were an RFC 6775-only DAR, as long as the ROVR is
       64 bits long.  An updated 6LR discovers the capabilities of the 6LBR
       in the 6CIO in RA messages from the 6LR; if the 6CIO was not present
       in any RA, then the 6LBR is assumed to be RFC 6775-only.

       If the 6LBR is RFC 6775-only, the 6LR MUST use only the 64 leftmost
       bits of the ROVR and place the result in the EDAR message to maintain
       compatibility.  This way, the support of DAD is preserved.

    7.  Security Considerations

       This specification extends [RFC6775], and the Security Considerations
       section of that document also applies to this document.  In
       particular, the link layer SHOULD be sufficiently protected to
       prevent rogue access.

       [RFC6775] does not protect the content of its messages and expects
       lower-layer encryption to defeat potential attacks.  This
       specification requires the LLN MAC layer to provide secure unicast
       to/from a Routing Registrar and secure broadcast or multicast from
       the Routing Registrar in a way that prevents tampering with or
       replaying the ND messages.

       This specification recommends using privacy techniques (see
       Section 8) and protecting against address theft via methods that are
       outside the scope of this document.  As an example, [AP-ND]
       guarantees the ownership of the Registered Address using a
       cryptographic ROVR.

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       The registration mechanism may be used by a rogue node to attack the
       6LR or 6LBR with a denial-of-service attack against the registry.  It
       may also happen that the registry of a 6LR or 6LBR is saturated and
       cannot take any more registrations; this scenario effectively denies
       the requesting node the capability to use a new address.  In order to
       alleviate those concerns, (1) Section 5.2 provides a sequence counter
       that keeps incrementing to detect and clean up stale registration
       information and that contributes to defeat replay attacks and
       (2) Section 5.7 provides a number of recommendations that ensure that
       a stale registration is removed as soon as possible from the 6LR
       and 6LBR.

       In particular, this specification recommends that:

       o  A node that ceases to use an address SHOULD attempt to de-register
          that address from all the 6LRs to which it is registered.

       o  The registration lifetimes SHOULD be individually configurable for
          each address or group of addresses.  A node SHOULD be configured
          for each address (or address category) with a Registration
          Lifetime that reflects the expectation of how long it will use the
          address with the 6LR to which the address is registered.  In
          particular, use cases that involve mobility or rapid address
          changes SHOULD use lifetimes that are the same order of magnitude
          as the duration of the expectation of presence but that are still
          longer.

       o  The router (6LR or 6LBR) SHOULD be configurable so as to limit the
          number of addresses that can be registered by a single node, but
          as a protective measure only.  In any case, a router MUST be able
          to keep a minimum number of addresses per node.  That minimum
          depends on the type of device and ranges between 3 for a very
          constrained LLN and 10 for a larger device.  A node may be
          identified by its MAC address, as long as it is not obfuscated by
          privacy measures.  A stronger identification (e.g., by security
          credentials) is RECOMMENDED.  When the maximum is reached, the
          router SHOULD use a Least Recently Used (LRU) algorithm to
          clean up the addresses, keeping at least one Link-Local Address.
          The router SHOULD attempt to keep one or more stable addresses if
          stability can be determined, e.g., because they are used over a
          much longer time span than other (privacy, shorter-lived)
          addresses.

       o  In order to avoid denial of registration due to a lack of
          resources, administrators should take great care to deploy
          adequate numbers of 6LRs to cover the needs of the nodes in their
          range, so as to avoid a situation of starving nodes.  It is
          expected that the 6LBR that serves an LLN is a more capable node

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          than the average 6LR, but in a network condition where it may
          become saturated, a particular LLN should distribute the 6LBR
          functionality -- for instance, by leveraging a high-speed Backbone
          Link and Routing Registrars to aggregate multiple LLNs into a
          larger subnet.

       The LLN nodes depend on a 6LBR and may use the services of a Routing
       Registrar for their operation.  A trust model MUST be put in place to
       ensure that only authorized devices are acting in these roles, so as
       to avoid threats such as black-holing or bombing attack whereby an
       impersonated 6LBR would destroy state in the network by using the
       "Removed" status code.  At a minimum, this trust model could be based
       on Layer 2 access control or could provide role validation as well
       (see Req-5.1 in Appendix B.5).

    8.  Privacy Considerations

       As indicated in Section 3, this protocol does not limit the number of
       IPv6 Addresses that each device can form.  However, to mitigate
       denial-of-service attacks, it can be useful as a protective measure
       to have a limit that is high enough not to interfere with the normal
       behavior of devices in the network.  A host should be able to form
       and register any address that is topologically correct in the
       subnet(s) advertised by the 6LR/6LBR.

       This specification does not mandate any particular way for forming
       IPv6 Addresses, but it discourages using EUI-64 for forming the
       Interface Identifier in the Link-Local Address because this method
       prevents the usage of Secure Neighbor Discovery (SEND) [RFC3971],
       Cryptographically Generated Addresses (CGAs) [RFC3972], and other
       address privacy techniques.

       [RFC8065] ("Privacy Considerations for IPv6 Adaptation-Layer
       Mechanisms") explains why privacy is important and how to form
       privacy-aware addresses.  All implementations and deployments must
       consider the option of privacy addresses in their own environments.

       The IPv6 Address of the 6LN in the IPv6 header can be compressed
       statelessly when the Interface Identifier in the IPv6 Address can be
       derived from the lower-layer address.  When it is not critical to
       benefit from that compression, e.g., the address can be compressed
       statefully, or it is rarely used and/or it is used only over one hop,
       privacy concerns should be considered.  In particular, new
       implementations should follow [RFC8064] ("Recommendation on Stable
       IPv6 Interface Identifiers").  [RFC8064] recommends the mechanism
       specified in [RFC7217] ("A Method for Generating Semantically Opaque
       Interface Identifiers with IPv6 Stateless Address Autoconfiguration
       (SLAAC)") for generating Interface Identifiers to be used in SLAAC.

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    9.  IANA Considerations

       IANA has made a number of changes under the "Internet Control Message
       Protocol version 6 (ICMPv6) Parameters" registry, as follows.

    9.1.  Address Registration Option Flags

       IANA has created a new subregistry for "Address Registration Option
       Flags" under the "Internet Control Message Protocol version 6
       (ICMPv6) Parameters" registry.  (See [RFC4443] for information
       regarding ICMPv6.)

       This specification defines eight positions -- bit 0 to bit 7 -- and
       assigns bit 6 for the R flag and bit 7 for the T flag (see
       Section 4.1).  The registration procedure is "IETF Review" or "IESG
       Approval" (see [RFC8126]).

       The initial contents of the registry are shown in Table 2.

                    +-------------+--------------+------------+
                    |  ARO Status | Description  | Reference  |
                    +-------------+--------------+------------+
                    |     0-5     | Unassigned   |            |
                    |             |              |            |
                    |      6      | R Flag       | RFC 8505   |
                    |             |              |            |
                    |      7      | T Flag       | RFC 8505   |
                    +-------------+--------------+------------+

                  Table 2: New Address Registration Option Flags

    9.2.  Address Registration Option I-Field

       IANA has created a new subregistry for "Address Registration Option
       I-Field" under the "Internet Control Message Protocol version 6
       (ICMPv6) Parameters" registry.

       This specification defines four integer values from 0 to 3 and
       assigns value 0 to "Abstract Index for Topology Selection" (see
       Section 4.1).  The registration procedure is "IETF Review" or "IESG
       Approval" [RFC8126].

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       The initial contents of the registry are shown in Table 3.

          +--------+---------------------------------------+------------+
          | Value  | Meaning                               | Reference  |
          +--------+---------------------------------------+------------+
          | 0      | Abstract Index for Topology Selection | RFC 8505   |
          |        |                                       |            |
          | 1-3    | Unassigned                            |            |
          +--------+---------------------------------------+------------+

                   Table 3: New Subregistry for the EARO I-Field

    9.3.  ICMP Codes

       IANA has created two new subregistries of the 'ICMPv6 "Code" Fields'
       registry, which itself is a subregistry of ICMPv6 codes in the
       "Internet Control Message Protocol version 6 (ICMPv6) Parameters"
       registry.

       The new subregistries relate to ICMP Types 157 (Duplicate Address
       Request) (shown in Table 4) and 158 (Duplicate Address Confirmation)
       (shown in Table 5), respectively.  For those two ICMP types, the ICMP
       Code field is split into two subfields: the Code Prefix and the Code
       Suffix.  The new subregistries relate to the Code Suffix portion of
       the ICMP Code.  The range of the Code Suffix is 0-15 in all cases.
       The registration procedure is "IETF Review" or "IESG Approval"
       [RFC8126] for both subregistries.

       The initial contents of these subregistries are as follows:

       +--------------+--------------------------------------+------------+
       | Code Suffix  | Meaning                              | Reference  |
       +--------------+--------------------------------------+------------+
       | 0            | DAR message                          | RFC 6775   |
       |              |                                      |            |
       | 1            | EDAR message with 64-bit ROVR field  | RFC 8505   |
       |              |                                      |            |
       | 2            | EDAR message with 128-bit ROVR field | RFC 8505   |
       |              |                                      |            |
       | 3            | EDAR message with 192-bit ROVR field | RFC 8505   |
       |              |                                      |            |
       | 4            | EDAR message with 256-bit ROVR field | RFC 8505   |
       |              |                                      |            |
       | 5-15         | Unassigned                           |            |
       +--------------+--------------------------------------+------------+

               Table 4: Code Suffixes for ICMP Type 157 DAR Message

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       +--------------+--------------------------------------+------------+
       | Code Suffix  | Meaning                              | Reference  |
       +--------------+--------------------------------------+------------+
       | 0            | DAC message                          | RFC 6775   |
       |              |                                      |            |
       | 1            | EDAC message with 64-bit ROVR field  | RFC 8505   |
       |              |                                      |            |
       | 2            | EDAC message with 128-bit ROVR field | RFC 8505   |
       |              |                                      |            |
       | 3            | EDAC message with 192-bit ROVR field | RFC 8505   |
       |              |                                      |            |
       | 4            | EDAC message with 256-bit ROVR field | RFC 8505   |
       |              |                                      |            |
       | 5-15         | Unassigned                           |            |
       +--------------+--------------------------------------+------------+

               Table 5: Code Suffixes for ICMP Type 158 DAC Message

    9.4.  New ARO Status Values

       IANA has made additions to the "Address Registration Option Status
       Values" subregistry, as follows:

        +-------+--------------------------------------------+------------+
        | Value | Description                                | Reference  |
        +-------+--------------------------------------------+------------+
        |   3   | Moved                                      | RFC 8505   |
        |       |                                            |            |
        |   4   | Removed                                    | RFC 8505   |
        |       |                                            |            |
        |   5   | Validation Requested                       | RFC 8505   |
        |       |                                            |            |
        |   6   | Duplicate Source Address                   | RFC 8505   |
        |       |                                            |            |
        |   7   | Invalid Source Address                     | RFC 8505   |
        |       |                                            |            |
        |   8   | Registered Address Topologically Incorrect | RFC 8505   |
        |       |                                            |            |
        |   9   | 6LBR Registry Saturated                    | RFC 8505   |
        |       |                                            |            |
        |   10  | Validation Failed                          | RFC 8505   |
        +-------+--------------------------------------------+------------+

                          Table 6: New ARO Status Values

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    9.5.  New 6LoWPAN Capability Bits

       IANA has made additions to the "6LoWPAN Capability Bits" subregistry,
       as follows:

                 +------+---------------------------+------------+
                 | Bit  | Description               | Reference  |
                 +------+---------------------------+------------+
                 |  10  | EDA Support (D bit)       | RFC 8505   |
                 |      |                           |            |
                 |  11  | 6LR capable (L bit)       | RFC 8505   |
                 |      |                           |            |
                 |  12  | 6LBR capable (B bit)      | RFC 8505   |
                 |      |                           |            |
                 |  13  | Routing Registrar (P bit) | RFC 8505   |
                 |      |                           |            |
                 |  14  | EARO support (E bit)      | RFC 8505   |
                 +------+---------------------------+------------+

                       Table 7: New 6LoWPAN Capability Bits

    10.  References

    10.1.  Normative References

       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119,
                  DOI 10.17487/RFC2119, March 1997,
                  <https://www.rfc-editor.org/info/rfc2119>.

       [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
                  Architecture", RFC 4291, DOI 10.17487/RFC4291,
                  February 2006, <https://www.rfc-editor.org/info/rfc4291>.

       [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
                  Control Message Protocol (ICMPv6) for the Internet
                  Protocol Version 6 (IPv6) Specification", STD 89,
                  RFC 4443, DOI 10.17487/RFC4443, March 2006,
                  <https://www.rfc-editor.org/info/rfc4443>.

       [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
                  "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
                  DOI 10.17487/RFC4861, September 2007,
                  <https://www.rfc-editor.org/info/rfc4861>.

    Thubert, et al.              Standards Track                   [Page 32]

    RFC 8505         Registration Extensions for 6LoWPAN ND    November 2018

       [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
                  Address Autoconfiguration", RFC 4862,
                  DOI 10.17487/RFC4862, September 2007,
                  <https://www.rfc-editor.org/info/rfc4862>.

       [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, DOI 10.17487/RFC4919, August 2007,
                  <https://www.rfc-editor.org/info/rfc4919>.

       [RFC6282]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
                  Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
                  DOI 10.17487/RFC6282, September 2011,
                  <https://www.rfc-editor.org/info/rfc6282>.

       [RFC6606]  Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem
                  Statement and Requirements for IPv6 over Low-Power
                  Wireless Personal Area Network (6LoWPAN) Routing",
                  RFC 6606, DOI 10.17487/RFC6606, May 2012,
                  <https://www.rfc-editor.org/info/rfc6606>.

       [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
                  Bormann, "Neighbor Discovery Optimization for IPv6 over
                  Low-Power Wireless Personal Area Networks (6LoWPANs)",
                  RFC 6775, DOI 10.17487/RFC6775, November 2012,
                  <https://www.rfc-editor.org/info/rfc6775>.

       [RFC7400]  Bormann, C., "6LoWPAN-GHC: Generic Header Compression for
                  IPv6 over Low-Power Wireless Personal Area Networks
                  (6LoWPANs)", RFC 7400, DOI 10.17487/RFC7400,
                  November 2014, <https://www.rfc-editor.org/info/rfc7400>.

       [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
                  Writing an IANA Considerations Section in RFCs", BCP 26,
                  RFC 8126, DOI 10.17487/RFC8126, June 2017,
                  <https://www.rfc-editor.org/info/rfc8126>.

       [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in
                  RFC 2119 Key Words", BCP 14, RFC 8174,
                  DOI 10.17487/RFC8174, May 2017,
                  <https://www.rfc-editor.org/info/rfc8174>.

    Thubert, et al.              Standards Track                   [Page 33]

    RFC 8505         Registration Extensions for 6LoWPAN ND    November 2018

    10.2.  Informative References

       [Alternative-Ellip-Curve-Reps]
                  Struik, R., "Alternative Elliptic Curve Representations",
                  Work in Progress, draft-struik-lwip-curve-
                  representations-00, October 2017.

       [AP-ND]    Thubert, P., Ed., Sarikaya, B., Sethi, M., and R. Struik,
                  "Address Protected Neighbor Discovery for Low-power and
                  Lossy Networks", Work in Progress, draft-ietf-6lo-
                  ap-nd-08, October 2018.

       [Arch-for-6TiSCH]
                  Thubert, P., Ed., "An Architecture for IPv6 over the
                  TSCH mode of IEEE 802.15.4", Work in Progress,
                  draft-ietf-6tisch-architecture-17, November 2018.

       [Efficient-NPDAO]
                  Jadhav, R., Ed., Thubert, P., Sahoo, R., and Z. Cao,
                  "Efficient Route Invalidation", Work in Progress,
                  draft-ietf-roll-efficient-npdao-09, October 2018.

       [IEEE-802-15-4]
                  IEEE, "IEEE Standard for Low-Rate Wireless Networks",
                  IEEE Standard 802.15.4, DOI 10.1109/IEEESTD.2016.7460875,
                  <https://ieeexplore.ieee.org/document/7460875/>.

       [IPv6-Backbone-Router]
                  Thubert, P., Ed. and C. Perkins, "IPv6 Backbone Router",
                  Work in Progress, draft-ietf-6lo-backbone-router-08,
                  October 2018.

       [IPv6-over-802.11ah]
                  Del Carpio Vega, L., Robles, M., and R. Morabito, "IPv6
                  over 802.11ah", Work in Progress, draft-delcarpio-6lo-
                  wlanah-01, October 2015.

       [IPv6-over-NFC]
                  Choi, Y., Ed., Hong, Y-G., Youn, J-S., Kim, D-K., and J-H.
                  Choi, "Transmission of IPv6 Packets over Near Field
                  Communication", Work in Progress, draft-ietf-6lo-nfc-12,
                  November 2018.

       [IPv6-over-PLC]
                  Hou, J., Liu, B., Hong, Y-G., Tang, X., and C. Perkins,
                  "Transmission of IPv6 Packets over PLC Networks", Work in
                  Progress, draft-hou-6lo-plc-05, October 2018.

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    RFC 8505         Registration Extensions for 6LoWPAN ND    November 2018

       [Multicast-over-IEEE802-Wireless]
                  Perkins, C., McBride, M., Stanley, D., Kumari, W., and JC.
                  Zuniga, "Multicast Considerations over IEEE 802 Wireless
                  Media", Work in Progress, draft-ietf-mboned-ieee802-mcast-
                  problems-03, October 2018.

       [ND-Optimizations]
                  Chakrabarti, S., Nordmark, E., Thubert, P., and M.
                  Wasserman, "IPv6 Neighbor Discovery Optimizations for
                  Wired and Wireless Networks", Work in Progress,
                  draft-chakrabarti-nordmark-6man-efficient-nd-07,
                  February 2015.

       [Perlman83]
                  Perlman, R., "Fault-Tolerant Broadcast of Routing
                  Information", North-Holland Computer Networks 7:
                  pp. 395-405, DOI 10.1016/0376-5075(83)90034-X, 1983,
                  <http://www.cs.illinois.edu/~pbg/courses/cs598fa09/
                  readings/p83.pdf>.

       [RFC1958]  Carpenter, B., Ed., "Architectural Principles of the
                  Internet", RFC 1958, DOI 10.17487/RFC1958, June 1996,
                  <https://www.rfc-editor.org/info/rfc1958>.

       [RFC1982]  Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
                  DOI 10.17487/RFC1982, August 1996,
                  <https://www.rfc-editor.org/info/rfc1982>.

       [RFC3610]  Whiting, D., Housley, R., and N. Ferguson, "Counter with
                  CBC-MAC (CCM)", RFC 3610, DOI 10.17487/RFC3610,
                  September 2003, <https://www.rfc-editor.org/info/rfc3610>.

       [RFC3810]  Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
                  Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
                  DOI 10.17487/RFC3810, June 2004,
                  <https://www.rfc-editor.org/info/rfc3810>.

       [RFC3971]  Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
                  "SEcure Neighbor Discovery (SEND)", RFC 3971,
                  DOI 10.17487/RFC3971, March 2005,
                  <https://www.rfc-editor.org/info/rfc3971>.

       [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
                  RFC 3972, DOI 10.17487/RFC3972, March 2005,
                  <https://www.rfc-editor.org/info/rfc3972>.

    Thubert, et al.              Standards Track                   [Page 35]

    RFC 8505         Registration Extensions for 6LoWPAN ND    November 2018

       [RFC4429]  Moore, N., "Optimistic Duplicate Address Detection (DAD)
                  for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006,
                  <https://www.rfc-editor.org/info/rfc4429>.

       [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
                  Extensions for Stateless Address Autoconfiguration in
                  IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
                  <https://www.rfc-editor.org/info/rfc4941>.

       [RFC6550]  Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
                  Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
                  JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
                  Low-Power and Lossy Networks", RFC 6550,
                  DOI 10.17487/RFC6550, March 2012,
                  <https://www.rfc-editor.org/info/rfc6550>.

       [RFC7217]  Gont, F., "A Method for Generating Semantically Opaque
                  Interface Identifiers with IPv6 Stateless Address
                  Autoconfiguration (SLAAC)", RFC 7217,
                  DOI 10.17487/RFC7217, April 2014,
                  <https://www.rfc-editor.org/info/rfc7217>.

       [RFC7428]  Brandt, A. and J. Buron, "Transmission of IPv6 Packets
                  over ITU-T G.9959 Networks", RFC 7428,
                  DOI 10.17487/RFC7428, February 2015,
                  <https://www.rfc-editor.org/info/rfc7428>.

       [RFC7668]  Nieminen, J., Savolainen, T., Isomaki, M., Patil, B.,
                  Shelby, Z., and C. Gomez, "IPv6 over BLUETOOTH(R) Low
                  Energy", RFC 7668, DOI 10.17487/RFC7668, October 2015,
                  <https://www.rfc-editor.org/info/rfc7668>.

       [RFC7934]  Colitti, L., Cerf, V., Cheshire, S., and D. Schinazi,
                  "Host Address Availability Recommendations", BCP 204,
                  RFC 7934, DOI 10.17487/RFC7934, July 2016,
                  <https://www.rfc-editor.org/info/rfc7934>.

       [RFC8064]  Gont, F., Cooper, A., Thaler, D., and W. Liu,
                  "Recommendation on Stable IPv6 Interface Identifiers",
                  RFC 8064, DOI 10.17487/RFC8064, February 2017,
                  <https://www.rfc-editor.org/info/rfc8064>.

       [RFC8065]  Thaler, D., "Privacy Considerations for IPv6 Adaptation-
                  Layer Mechanisms", RFC 8065, DOI 10.17487/RFC8065,
                  February 2017, <https://www.rfc-editor.org/info/rfc8065>.

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    RFC 8505         Registration Extensions for 6LoWPAN ND    November 2018

       [RFC8105]  Mariager, P., Petersen, J., Ed., Shelby, Z., Van de Logt,
                  M., and D. Barthel, "Transmission of IPv6 Packets over
                  Digital Enhanced Cordless Telecommunications (DECT) Ultra
                  Low Energy (ULE)", RFC 8105, DOI 10.17487/RFC8105,
                  May 2017, <https://www.rfc-editor.org/info/rfc8105>.

       [RFC8163]  Lynn, K., Ed., Martocci, J., Neilson, C., and S.
                  Donaldson, "Transmission of IPv6 over Master-Slave/Token-
                  Passing (MS/TP) Networks", RFC 8163, DOI 10.17487/RFC8163,
                  May 2017, <https://www.rfc-editor.org/info/rfc8163>.

       [RFC8279]  Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
                  Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
                  Explicit Replication (BIER)", RFC 8279,
                  DOI 10.17487/RFC8279, November 2017,
                  <https://www.rfc-editor.org/info/rfc8279>.

       [Routing-for-RPL-Leaves]
                  Thubert, P., Ed., "Routing for RPL Leaves", Work in
                  Progress, draft-thubert-roll-unaware-leaves-05, May 2018.

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    RFC 8505         Registration Extensions for 6LoWPAN ND    November 2018

    Appendix A.  Applicability and Fulfilled Requirements (Not Normative)

       This specification extends 6LoWPAN ND to provide a sequence number to
       the registration and fulfills the requirements expressed in
       Appendix B.1 by enabling the mobility of devices from one LLN to the
       next.  A full specification for enabling mobility based on the use of
       the EARO and the registration procedures defined in this document can
       be found in subsequent work [IPv6-Backbone-Router] ("IPv6 Backbone
       Router").  The 6BBR is an example of a Routing Registrar that acts as
       an IPv6 ND proxy over a Backbone Link that federates multiple LLNs as
       well as the Backbone Link itself into a single IPv6 subnet.  The
       expected registration flow in that case is illustrated in Figure 6,
       noting that any combination of 6LR, 6LBR, and 6BBR may be collocated.

           6LN              6LR             6LBR            6BBR
            |                |               |                |
            |   NS(EARO)     |               |                |
            |--------------->|               |                |
            |                | Extended DAR  |                |
            |                |-------------->|                |
            |                |               |                |
            |                |               | proxy NS(EARO) |
            |                |               |--------------->|
            |                |               |                | NS(DAD)
            |                |               |                | ------>
            |                |               |                | <wait>
            |                |               |                |
            |                |               | proxy NA(EARO) |
            |                |               |<---------------|
            |                | Extended DAC  |                |
            |                |<--------------|                |
            |   NA(EARO)     |               |                |
            |<---------------|               |                |
            |                |               |                |

                         Figure 6: (Re-)Registration Flow

       [Arch-for-6TiSCH] ("An Architecture for IPv6 over the TSCH mode of
       IEEE 802.15.4") describes how a 6LoWPAN ND host using the
       Time-Slotted Channel Hopping (TSCH) mode of IEEE Std. 802.15.4
       [IEEE-802-15-4] can connect to the Internet via a RPL mesh network.
       Doing so requires additions to the 6LoWPAN ND protocol to support
       mobility and reachability in a secure and manageable network
       environment.  This document specifies those new operations and
       fulfills the requirements listed in Appendix B.2.

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    RFC 8505         Registration Extensions for 6LoWPAN ND    November 2018

       The term "LLN" is used loosely in this document and is intended to
       cover multiple types of WLANs and WPANs, including Low-Power IEEE
       Std. 802.11 networking, Bluetooth low energy, IEEE Std. 802.11ah, and
       IEEE Std. 802.15.4 wireless meshes, so as to address the requirements
       discussed in Appendix B.3.

       This specification can be used by any wireless node to register its
       IPv6 Addresses with a Routing Registrar and to obtain routing
       services such as proxy ND operations over a Backbone Link.  This
       satisfies the requirements expressed in Appendix B.4.

       This specification is extended by [AP-ND] to provide a solution to
       some of the security-related requirements expressed in Appendix B.5.

       [ND-Optimizations] ("IPv6 Neighbor Discovery Optimizations for Wired
       and Wireless Networks") suggests that 6LoWPAN ND [RFC6775] can be
       extended to other types of links (beyond IEEE Std. 802.15.4) for
       which it was defined.  The registration technique is beneficial when
       the link-layer technique used to carry IPv6 multicast packets is not
       sufficiently efficient in terms of delivery ratio or energy
       consumption in the end devices -- in particular, to enable
       energy-constrained sleeping nodes.  The value of such an extension is
       especially apparent in the case of mobile wireless nodes, to reduce
       the multicast operations that are related to IPv6 ND [RFC4861]
       [RFC4862] and affect the operation of the wireless medium
       [Multicast-over-IEEE802-Wireless].  This fulfills the scalability
       requirements listed in Appendix B.6.

    Appendix B.  Requirements (Not Normative)

       This appendix lists requirements that were discussed by the
       6lo Working Group for an update to 6LoWPAN ND.  How those
       requirements are matched with existing specifications at the time
       of this writing is shown in Appendix B.8.

    B.1.  Requirements Related to Mobility

       Due to the unstable nature of LLN links, even in an LLN of immobile
       nodes, a 6LN may change its point of attachment from, say, 6LR-a to
       6LR-b but may not be able to notify 6LR-a.  Consequently, 6LR-a may
       still attract traffic that it cannot deliver any more.  When links to
       a 6LR change state, there is thus a need to identify stale states in
       a 6LR and restore reachability in a timely fashion, e.g., by using
       some type of signaling upon detection of the movement or using a
       keep-alive mechanism with a period that is consistent with the needs
       of the application.

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       Req-1.1:  Upon a change of point of attachment, connectivity via a
                 new 6LR MUST be restored in a timely fashion without the
                 need to de-register from the previous 6LR.

       Req-1.2:  For that purpose, the protocol MUST enable differentiating
                 between multiple registrations from one 6LN and
                 registrations from different 6LNs claiming the same
                 address.

       Req-1.3:  Stale states MUST be cleaned up in 6LRs.

       Req-1.4:  A 6LN SHOULD also be able to register its address
                 concurrently to multiple 6LRs.

    B.2.  Requirements Related to Routing Protocols

       The point of attachment of a 6LN may be a 6LR in an LLN mesh.  IPv6
       routing in an LLN can be based on RPL, which is the routing protocol
       that was defined by the IETF for this particular purpose.  Other
       routing protocols are also considered by Standards Development
       Organizations (SDOs) on the basis of the expected network
       characteristics.  It is required that a 6LN attached via ND to a 6LR
       indicate whether or not it (1) participates in the selected routing
       protocol to obtain reachability via the 6LR or (2) expects the 6LR to
       manage its reachability.

       The specified updates enable other specifications to define new
       services such as Source Address Validation Improvement (SAVI) (via
       [AP-ND]), participation as an unaware leaf to a routing protocol
       (such as the protocol described in [RFC6550] (RPL)) (via
       [Routing-for-RPL-Leaves]), and registration to Backbone Routers
       performing proxy ND in an LLN (via [IPv6-Backbone-Router]).

       Beyond the 6LBR unicast address registered by ND, other addresses,
       including multicast addresses, are needed as well.  For example, a
       routing protocol often uses a multicast address to register changes
       to established paths.  ND needs to register such a multicast address
       to enable routing concurrently with discovery.

       Multicast is needed for groups.  Groups may be formed by device type
       (e.g., routers, street lamps), location (geography, RPL subtree),
       or both.

       The Bit Index Explicit Replication (BIER) architecture [RFC8279]
       proposes an optimized technique to enable multicast in an LLN with a
       very limited requirement for routing state in the nodes.

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       Related requirements are as follows:

       Req-2.1:  The ND registration method SHOULD be extended so that the
                 6LR is instructed whether to advertise the address of a 6LN
                 over the selected routing protocol and obtain reachability
                 to that address using the selected routing protocol.

       Req-2.2:  Considering RPL, the ARO that is used in the ND
                 registration SHOULD be extended to carry enough information
                 to generate a DAO message as specified in Section 6.4 of
                 [RFC6550] -- in particular, the capability to compute a
                 Path Sequence and, as an option, a RPLInstanceID.

       Req-2.3:  Multicast operations SHOULD be supported and optimized --
                 for instance, using BIER or the Multicast Protocol for
                 Low-Power and Lossy Networks (MPL).  Whether ND is
                 appropriate for the registration to the Routing Registrar
                 is to be defined, considering the additional burden of
                 supporting Multicast Listener Discovery Version 2 (MLDv2)
                 for IPv6 [RFC3810].

    B.3.  Requirements Related to Various Low-Power Link Types

       6LoWPAN ND [RFC6775] was defined with a focus on IEEE Std.802.15.4
       and, in particular, the capability to derive a unique identifier from
       a globally unique EUI-64 address.  At this point, the 6lo Working
       Group is extending the 6LoWPAN Header Compression (HC) technique
       [RFC6282] to other link types, including ITU-T G.9959 [RFC7428],
       Master-Slave/Token-Passing [RFC8163], Digital Enhanced Cordless
       Telecommunications (DECT) Ultra Low Energy [RFC8105], Near Field
       Communication [IPv6-over-NFC], and IEEE Std. 802.11ah
       [IPv6-over-802.11ah], as well as Bluetooth low energy [RFC7668] and
       Power Line Communication (PLC) Networks [IPv6-over-PLC].

       Related requirements are as follows:

       Req-3.1:  The support of the registration mechanism SHOULD be
                 extended to more LLN links than IEEE Std.802.15.4, matching
                 at least the LLN links for which an "IPv6 over foo"
                 specification exists, as well as low-power Wi-Fi.

       Req-3.2:  As part of this extension, a mechanism to compute a unique
                 identifier should be provided, with the capability to form
                 a Link-Local Address that SHOULD be unique at least within
                 the LLN connected to a 6LBR discovered by ND in each node
                 within the LLN.

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       Req-3.3:  The ARO used in the ND registration SHOULD be extended to
                 carry the relevant forms of the unique identifier.

       Req-3.4:  ND should specify the formation of a site-local address
                 that follows the security recommendations in [RFC7217].

    B.4.  Requirements Related to Proxy Operations

       Duty-cycled devices may not be awake to answer a lookup from a node
       that uses IPv6 ND and may need a proxy.  Additionally, the
       duty-cycled device may rely on the 6LBR to perform registration to
       the Routing Registrar.

       The ND registration method SHOULD defend the addresses of duty-cycled
       devices that are sleeping most of the time and incapable of defending
       their own addresses.

       Related requirements are as follows:

       Req-4.1:  The registration mechanism SHOULD enable a third party to
                 proxy-register an address on behalf of a 6LN that may be
                 sleeping or located deeper in an LLN mesh.

       Req-4.2:  The registration mechanism SHOULD be applicable to a
                 duty-cycled device regardless of the link type and SHOULD
                 enable a Routing Registrar to operate as a proxy to defend
                 the Registered Addresses on its behalf.

       Req-4.3:  The registration mechanism SHOULD enable long sleep
                 durations, on the order of multiple days to a month.

    B.5.  Requirements Related to Security

       In order to guarantee the operations of the 6LoWPAN ND flows,
       spoofing the roles of the 6LR, 6LBR, and Routing Registrar should be
       avoided.  Once a node successfully registers an address, 6LoWPAN ND
       should provide energy-efficient means for the 6LBR to protect that
       ownership even when the node that registered the address is sleeping.

       In particular, the 6LR and the 6LBR should then be able to verify
       whether a subsequent registration for a given address comes from the
       original node.

       In an LLN, it makes sense to base security on Layer 2 security.
       During bootstrap of the LLN, nodes join the network after
       authorization by a Joining Assistant (JA) or a Commissioning Tool
       (CT).  After joining, nodes communicate with each other via secured
       links.  The keys for Layer 2 security are distributed by the JA/CT.

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       The JA/CT can be part of the LLN or be outside the LLN.  In both
       cases, the ability to route packets between the JA/CT and the joining
       node is needed.

       Related requirements are as follows:

       Req-5.1:  6LoWPAN ND security mechanisms SHOULD provide a mechanism
                 for the 6LR, 6LBR, and Routing Registrar to authenticate
                 and authorize one another for their respective roles, as
                 well as with the 6LN for the role of 6LR.

       Req-5.2:  6LoWPAN ND security mechanisms SHOULD provide a mechanism
                 for the 6LR and the 6LBR to validate new registrations of
                 authorized nodes.  Joining of unauthorized nodes MUST be
                 prevented.

       Req-5.3:  The use of 6LoWPAN ND security mechanisms SHOULD NOT result
                 in large packet sizes.  In particular, the NS, NA, DAR, and
                 DAC messages for a re-registration flow SHOULD NOT exceed
                 80 octets so as to fit in a secured IEEE Std.802.15.4
                 [IEEE-802-15-4] frame.

       Req-5.4:  Recurrent 6LoWPAN ND security operations MUST NOT be
                 computationally intensive on the 6LN's CPU.  When
                 calculation of a key hash is employed, a mechanism lighter
                 than SHA-1 SHOULD be used.

       Req-5.5:  The number of keys that the 6LN needs to manipulate SHOULD
                 be minimized.

       Req-5.6:  6LoWPAN ND security mechanisms SHOULD enable (1) the
                 variation of CCM ("Counter with CBC-MAC") [RFC3610] called
                 "CCM*" for use at both Layer 2 and Layer 3 and (2) the
                 reuse of a security code that has to be present on the
                 device for upper-layer security (e.g., TLS).  Algorithm
                 agility and support for large keys (e.g., 256-bit key
                 sizes) are also desirable.

       Req-5.7:  Public key and signature sizes SHOULD be minimized while
                 maintaining adequate confidentiality and data origin
                 authentication for multiple types of applications with
                 various degrees of criticality.

       Req-5.8:  Routing of packets should continue when links pass from the
                 unsecured state to the secured state.

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    RFC 8505         Registration Extensions for 6LoWPAN ND    November 2018

       Req-5.9:  6LoWPAN ND security mechanisms SHOULD provide a mechanism
                 for the 6LR and the 6LBR to validate whether a new
                 registration for a given address corresponds to the same
                 6LN that registered it initially and, if not, determine the
                 rightful owner and deny or clean up the registration if it
                 is a duplicate.

    B.6.  Requirements Related to Scalability

       Use cases from Automatic Meter Reading (AMR) (collection-tree
       operations) and Advanced Metering Infrastructure (AMI) (bidirectional
       communication to the meters) indicate the need for a large number of
       LLN nodes pertaining to a single RPL DODAG (e.g., 5000) and connected
       to the 6LBR over a large number of LLN hops (e.g., 15).

       Related requirements are as follows:

       Req-6.1:  The registration mechanism SHOULD enable a single 6LBR to
                 register multiple thousands of devices.

       Req-6.2:  The timing of the registration operation should allow for
                 long latency, such as that found in LLNs with ten or
                 more hops.

    B.7.  Requirements Related to Operations and Management

       Guideline 3.8 in Section 3 of [RFC1958] ("Architectural Principles of
       the Internet") recommends the following: "Avoid options and
       parameters whenever possible.  Any options and parameters should be
       configured or negotiated dynamically rather than manually."  This is
       especially true in LLNs where the number of devices may be large and
       manual configuration is infeasible.  Capabilities for dynamic
       configuration of LLN devices can also be constrained by network and
       power limitations.

       A network administrator should be able to validate that the network
       is operating within capacity and that, in particular, a 6LBR does not
       get overloaded with an excessive amount of registrations, so the
       administrator can take actions such as adding a Backbone Link with
       additional 6LBRs and Routing Registrars to the network.

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    RFC 8505         Registration Extensions for 6LoWPAN ND    November 2018

       Related requirements are as follows:

       Req-7.1:  A management model SHOULD be provided that enables access
                 to the 6LBR, monitors its usage vs. capacity, and sends
                 alerts in the case of congestion.  It is recommended that
                 the 6LBR be reachable over a non-LLN link.

       Req-7.2:  A management model SHOULD be provided that enables access
                 to the 6LR and its capacity to host additional NCEs.  This
                 management model SHOULD avoid polling individual 6LRs in a
                 way that could disrupt the operation of the LLN.

       Req-7.3:  Information on successful and failed registrations SHOULD
                 be provided, including information such as the ROVR of the
                 6LN, the Registered Address, the address of the 6LR, and
                 the duration of the registration flow.

       Req-7.4:  In the case of a failed registration, information on the
                 failure, including the identification of the node that
                 rejected the registration and the status in the EARO,
                 SHOULD be provided.

    B.8.  Matching Requirements with Specifications

                 +-------------+--------------------------------+
                 | Requirement | Document                       |
                 +-------------+--------------------------------+
                 | Req-1.1     | [IPv6-Backbone-Router]         |
                 |             |                                |
                 | Req-1.2     | [RFC6775]                      |
                 |             |                                |
                 | Req-1.3     | [RFC6775]                      |
                 |             |                                |
                 | Req-1.4     | RFC 8505                       |
                 |             |                                |
                 | Req-2.1     | RFC 8505                       |
                 |             |                                |
                 | Req-2.2     | RFC 8505                       |
                 |             |                                |
                 | Req-2.3     |                                |
                 |             |                                |
                 | Req-3.1     | Technology Dependent           |
                 |             |                                |
                 | Req-3.2     | Technology Dependent           |
                 |             |                                |
                 | Req-3.3     | Technology Dependent           |
                 |             |                                |
                 | Req-3.4     | Technology Dependent           |

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                 |             |                                |
                 | Req-4.1     | RFC 8505                       |
                 |             |                                |
                 | Req-4.2     | RFC 8505                       |
                 |             |                                |
                 | Req-4.3     | [RFC6775]                      |
                 |             |                                |
                 | Req-5.1     |                                |
                 |             |                                |
                 | Req-5.2     | [AP-ND]                        |
                 |             |                                |
                 | Req-5.3     |                                |
                 |             |                                |
                 | Req-5.4     |                                |
                 |             |                                |
                 | Req-5.5     | [AP-ND]                        |
                 |             |                                |
                 | Req-5.6     | [Alternative-Ellip-Curve-Reps] |
                 |             |                                |
                 | Req-5.7     | [AP-ND]                        |
                 |             |                                |
                 | Req-5.8     |                                |
                 |             |                                |
                 | Req-5.9     | [AP-ND]                        |
                 |             |                                |
                 | Req-6.1     | RFC 8505                       |
                 |             |                                |
                 | Req-6.2     | RFC 8505                       |
                 |             |                                |
                 | Req-7.1     |                                |
                 |             |                                |
                 | Req-7.2     |                                |
                 |             |                                |
                 | Req-7.3     |                                |
                 |             |                                |
                 | Req-7.4     |                                |
                 +-------------+--------------------------------+

                   Table 8: Documents That Address Requirements

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    RFC 8505         Registration Extensions for 6LoWPAN ND    November 2018

    Acknowledgments

       Kudos to Eric Levy-Abegnoli, who designed the "First-Hop Security"
       infrastructure upon which the first Backbone Router was implemented.
       Many thanks to Sedat Gormus, Rahul Jadhav, Tim Chown, Juergen
       Schoenwaelder, Chris Lonvick, Dave Thaler, Adrian Farrel, Peter Yee,
       Warren Kumari, Benjamin Kaduk, Mirja Kuehlewind, Ben Campbell, Eric
       Rescorla, and Lorenzo Colitti for their various contributions and
       reviews.  Also, many thanks to Thomas Watteyne for the world's first
       implementation of a 6LN that was instrumental to the early tests of
       the 6LR, 6LBR, and Backbone Router.

    Authors' Addresses

       Pascal Thubert (editor)
       Cisco Systems, Inc.
       Building D (Regus) 45 Allee des Ormes
       Mougins - Sophia Antipolis
       France

       Phone: +33 4 97 23 26 34
       Email: pthubert@cisco.com

       Erik Nordmark
       Zededa
       Santa Clara, CA
       United States of America

       Email: nordmark@sonic.net

       Samita Chakrabarti
       Verizon
       San Jose, CA
       United States of America

       Email: samitac.ietf@gmail.com

       Charles E. Perkins
       Futurewei
       2330 Central Expressway
       Santa Clara, CA  95050
       United States of America

       Email: charliep@computer.org

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