





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>.







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   [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>.









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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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   [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>.






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   [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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   [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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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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   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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   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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   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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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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