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rfc:rfc8505

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.

Thubert, et al. Standards Track [Page 1] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

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

Thubert, et al. Standards Track [Page 2] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 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

Thubert, et al. Standards Track [Page 3] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 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

Thubert, et al. Standards Track [Page 4] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 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)

Thubert, et al. Standards Track [Page 5] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

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.

Thubert, et al. Standards Track [Page 6] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

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.

Thubert, et al. Standards Track [Page 7] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

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.

Thubert, et al. Standards Track [Page 8] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 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.

Thubert, et al. Standards Track [Page 9] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 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.

Thubert, et al. Standards Track [Page 10] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 +-------+-----------------------------------------------------------+
 | 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

Thubert, et al. Standards Track [Page 11] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

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.

Thubert, et al. Standards Track [Page 12] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 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

Thubert, et al. Standards Track [Page 13] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 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.

Thubert, et al. Standards Track [Page 14] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 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.

Thubert, et al. Standards Track [Page 15] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

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]).

Thubert, et al. Standards Track [Page 16] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 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.

Thubert, et al. Standards Track [Page 17] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 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.

Thubert, et al. Standards Track [Page 18] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 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

Thubert, et al. Standards Track [Page 19] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 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

Thubert, et al. Standards Track [Page 20] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 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.

Thubert, et al. Standards Track [Page 21] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 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.

Thubert, et al. Standards Track [Page 22] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 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.

Thubert, et al. Standards Track [Page 23] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 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.

Thubert, et al. Standards Track [Page 24] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

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.

Thubert, et al. Standards Track [Page 25] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 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.

Thubert, et al. Standards Track [Page 26] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 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

Thubert, et al. Standards Track [Page 27] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

    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.

Thubert, et al. Standards Track [Page 28] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

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].

Thubert, et al. Standards Track [Page 29] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 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

Thubert, et al. Standards Track [Page 30] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 +--------------+--------------------------------------+------------+
 | 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

Thubert, et al. Standards Track [Page 31] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

9.5. New 6LoWPAN Capability Bits

 IANA has made additions to the "6LoWPAN Capability Bits" subregistry,
 as follows:
           +------+---------------------------+------------+
           | Bit  | Description               | Reference  |
           +------+---------------------------+------------+
           |  10  | EDA Support (D bit)       | RFC 8505   |
           |      |                           |            |
           |  11  | 6LR capable (L bit)       | RFC 8505   |
           |      |                           |            |
           |  12  | 6LBR capable (B bit)      | RFC 8505   |
           |      |                           |            |
           |  13  | Routing Registrar (P bit) | RFC 8505   |
           |      |                           |            |
           |  14  | EARO support (E bit)      | RFC 8505   |
           +------+---------------------------+------------+
                 Table 7: New 6LoWPAN Capability Bits

10. References

10.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <https://www.rfc-editor.org/info/rfc2119>.
 [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
            Architecture", RFC 4291, DOI 10.17487/RFC4291,
            February 2006, <https://www.rfc-editor.org/info/rfc4291>.
 [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
            Control Message Protocol (ICMPv6) for the Internet
            Protocol Version 6 (IPv6) Specification", STD 89,
            RFC 4443, DOI 10.17487/RFC4443, March 2006,
            <https://www.rfc-editor.org/info/rfc4443>.
 [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
            "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
            DOI 10.17487/RFC4861, September 2007,
            <https://www.rfc-editor.org/info/rfc4861>.

Thubert, et al. Standards Track [Page 32] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
            Address Autoconfiguration", RFC 4862,
            DOI 10.17487/RFC4862, September 2007,
            <https://www.rfc-editor.org/info/rfc4862>.
 [RFC4919]  Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
            over Low-Power Wireless Personal Area Networks (6LoWPANs):
            Overview, Assumptions, Problem Statement, and Goals",
            RFC 4919, DOI 10.17487/RFC4919, August 2007,
            <https://www.rfc-editor.org/info/rfc4919>.
 [RFC6282]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
            Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
            DOI 10.17487/RFC6282, September 2011,
            <https://www.rfc-editor.org/info/rfc6282>.
 [RFC6606]  Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem
            Statement and Requirements for IPv6 over Low-Power
            Wireless Personal Area Network (6LoWPAN) Routing",
            RFC 6606, DOI 10.17487/RFC6606, May 2012,
            <https://www.rfc-editor.org/info/rfc6606>.
 [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
            Bormann, "Neighbor Discovery Optimization for IPv6 over
            Low-Power Wireless Personal Area Networks (6LoWPANs)",
            RFC 6775, DOI 10.17487/RFC6775, November 2012,
            <https://www.rfc-editor.org/info/rfc6775>.
 [RFC7400]  Bormann, C., "6LoWPAN-GHC: Generic Header Compression for
            IPv6 over Low-Power Wireless Personal Area Networks
            (6LoWPANs)", RFC 7400, DOI 10.17487/RFC7400,
            November 2014, <https://www.rfc-editor.org/info/rfc7400>.
 [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
            Writing an IANA Considerations Section in RFCs", BCP 26,
            RFC 8126, DOI 10.17487/RFC8126, June 2017,
            <https://www.rfc-editor.org/info/rfc8126>.
 [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in
            RFC 2119 Key Words", BCP 14, RFC 8174,
            DOI 10.17487/RFC8174, May 2017,
            <https://www.rfc-editor.org/info/rfc8174>.

Thubert, et al. Standards Track [Page 33] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

10.2. Informative References

 [Alternative-Ellip-Curve-Reps]
            Struik, R., "Alternative Elliptic Curve Representations",
            Work in Progress, draft-struik-lwip-curve-
            representations-00, October 2017.
 [AP-ND]    Thubert, P., Ed., Sarikaya, B., Sethi, M., and R. Struik,
            "Address Protected Neighbor Discovery for Low-power and
            Lossy Networks", Work in Progress, draft-ietf-6lo-
            ap-nd-08, October 2018.
 [Arch-for-6TiSCH]
            Thubert, P., Ed., "An Architecture for IPv6 over the
            TSCH mode of IEEE 802.15.4", Work in Progress,
            draft-ietf-6tisch-architecture-17, November 2018.
 [Efficient-NPDAO]
            Jadhav, R., Ed., Thubert, P., Sahoo, R., and Z. Cao,
            "Efficient Route Invalidation", Work in Progress,
            draft-ietf-roll-efficient-npdao-09, October 2018.
 [IEEE-802-15-4]
            IEEE, "IEEE Standard for Low-Rate Wireless Networks",
            IEEE Standard 802.15.4, DOI 10.1109/IEEESTD.2016.7460875,
            <https://ieeexplore.ieee.org/document/7460875/>.
 [IPv6-Backbone-Router]
            Thubert, P., Ed. and C. Perkins, "IPv6 Backbone Router",
            Work in Progress, draft-ietf-6lo-backbone-router-08,
            October 2018.
 [IPv6-over-802.11ah]
            Del Carpio Vega, L., Robles, M., and R. Morabito, "IPv6
            over 802.11ah", Work in Progress, draft-delcarpio-6lo-
            wlanah-01, October 2015.
 [IPv6-over-NFC]
            Choi, Y., Ed., Hong, Y-G., Youn, J-S., Kim, D-K., and J-H.
            Choi, "Transmission of IPv6 Packets over Near Field
            Communication", Work in Progress, draft-ietf-6lo-nfc-12,
            November 2018.
 [IPv6-over-PLC]
            Hou, J., Liu, B., Hong, Y-G., Tang, X., and C. Perkins,
            "Transmission of IPv6 Packets over PLC Networks", Work in
            Progress, draft-hou-6lo-plc-05, October 2018.

Thubert, et al. Standards Track [Page 34] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 [Multicast-over-IEEE802-Wireless]
            Perkins, C., McBride, M., Stanley, D., Kumari, W., and JC.
            Zuniga, "Multicast Considerations over IEEE 802 Wireless
            Media", Work in Progress, draft-ietf-mboned-ieee802-mcast-
            problems-03, October 2018.
 [ND-Optimizations]
            Chakrabarti, S., Nordmark, E., Thubert, P., and M.
            Wasserman, "IPv6 Neighbor Discovery Optimizations for
            Wired and Wireless Networks", Work in Progress,
            draft-chakrabarti-nordmark-6man-efficient-nd-07,
            February 2015.
 [Perlman83]
            Perlman, R., "Fault-Tolerant Broadcast of Routing
            Information", North-Holland Computer Networks 7:
            pp. 395-405, DOI 10.1016/0376-5075(83)90034-X, 1983,
            <http://www.cs.illinois.edu/~pbg/courses/cs598fa09/
            readings/p83.pdf>.
 [RFC1958]  Carpenter, B., Ed., "Architectural Principles of the
            Internet", RFC 1958, DOI 10.17487/RFC1958, June 1996,
            <https://www.rfc-editor.org/info/rfc1958>.
 [RFC1982]  Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
            DOI 10.17487/RFC1982, August 1996,
            <https://www.rfc-editor.org/info/rfc1982>.
 [RFC3610]  Whiting, D., Housley, R., and N. Ferguson, "Counter with
            CBC-MAC (CCM)", RFC 3610, DOI 10.17487/RFC3610,
            September 2003, <https://www.rfc-editor.org/info/rfc3610>.
 [RFC3810]  Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
            Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
            DOI 10.17487/RFC3810, June 2004,
            <https://www.rfc-editor.org/info/rfc3810>.
 [RFC3971]  Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
            "SEcure Neighbor Discovery (SEND)", RFC 3971,
            DOI 10.17487/RFC3971, March 2005,
            <https://www.rfc-editor.org/info/rfc3971>.
 [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
            RFC 3972, DOI 10.17487/RFC3972, March 2005,
            <https://www.rfc-editor.org/info/rfc3972>.

Thubert, et al. Standards Track [Page 35] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 [RFC4429]  Moore, N., "Optimistic Duplicate Address Detection (DAD)
            for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006,
            <https://www.rfc-editor.org/info/rfc4429>.
 [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
            Extensions for Stateless Address Autoconfiguration in
            IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
            <https://www.rfc-editor.org/info/rfc4941>.
 [RFC6550]  Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
            Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
            JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
            Low-Power and Lossy Networks", RFC 6550,
            DOI 10.17487/RFC6550, March 2012,
            <https://www.rfc-editor.org/info/rfc6550>.
 [RFC7217]  Gont, F., "A Method for Generating Semantically Opaque
            Interface Identifiers with IPv6 Stateless Address
            Autoconfiguration (SLAAC)", RFC 7217,
            DOI 10.17487/RFC7217, April 2014,
            <https://www.rfc-editor.org/info/rfc7217>.
 [RFC7428]  Brandt, A. and J. Buron, "Transmission of IPv6 Packets
            over ITU-T G.9959 Networks", RFC 7428,
            DOI 10.17487/RFC7428, February 2015,
            <https://www.rfc-editor.org/info/rfc7428>.
 [RFC7668]  Nieminen, J., Savolainen, T., Isomaki, M., Patil, B.,
            Shelby, Z., and C. Gomez, "IPv6 over BLUETOOTH(R) Low
            Energy", RFC 7668, DOI 10.17487/RFC7668, October 2015,
            <https://www.rfc-editor.org/info/rfc7668>.
 [RFC7934]  Colitti, L., Cerf, V., Cheshire, S., and D. Schinazi,
            "Host Address Availability Recommendations", BCP 204,
            RFC 7934, DOI 10.17487/RFC7934, July 2016,
            <https://www.rfc-editor.org/info/rfc7934>.
 [RFC8064]  Gont, F., Cooper, A., Thaler, D., and W. Liu,
            "Recommendation on Stable IPv6 Interface Identifiers",
            RFC 8064, DOI 10.17487/RFC8064, February 2017,
            <https://www.rfc-editor.org/info/rfc8064>.
 [RFC8065]  Thaler, D., "Privacy Considerations for IPv6 Adaptation-
            Layer Mechanisms", RFC 8065, DOI 10.17487/RFC8065,
            February 2017, <https://www.rfc-editor.org/info/rfc8065>.

Thubert, et al. Standards Track [Page 36] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 [RFC8105]  Mariager, P., Petersen, J., Ed., Shelby, Z., Van de Logt,
            M., and D. Barthel, "Transmission of IPv6 Packets over
            Digital Enhanced Cordless Telecommunications (DECT) Ultra
            Low Energy (ULE)", RFC 8105, DOI 10.17487/RFC8105,
            May 2017, <https://www.rfc-editor.org/info/rfc8105>.
 [RFC8163]  Lynn, K., Ed., Martocci, J., Neilson, C., and S.
            Donaldson, "Transmission of IPv6 over Master-Slave/Token-
            Passing (MS/TP) Networks", RFC 8163, DOI 10.17487/RFC8163,
            May 2017, <https://www.rfc-editor.org/info/rfc8163>.
 [RFC8279]  Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
            Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
            Explicit Replication (BIER)", RFC 8279,
            DOI 10.17487/RFC8279, November 2017,
            <https://www.rfc-editor.org/info/rfc8279>.
 [Routing-for-RPL-Leaves]
            Thubert, P., Ed., "Routing for RPL Leaves", Work in
            Progress, draft-thubert-roll-unaware-leaves-05, May 2018.

Thubert, et al. Standards Track [Page 37] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

Appendix A. Applicability and Fulfilled Requirements (Not Normative)

 This specification extends 6LoWPAN ND to provide a sequence number to
 the registration and fulfills the requirements expressed in
 Appendix B.1 by enabling the mobility of devices from one LLN to the
 next.  A full specification for enabling mobility based on the use of
 the EARO and the registration procedures defined in this document can
 be found in subsequent work [IPv6-Backbone-Router] ("IPv6 Backbone
 Router").  The 6BBR is an example of a Routing Registrar that acts as
 an IPv6 ND proxy over a Backbone Link that federates multiple LLNs as
 well as the Backbone Link itself into a single IPv6 subnet.  The
 expected registration flow in that case is illustrated in Figure 6,
 noting that any combination of 6LR, 6LBR, and 6BBR may be collocated.
     6LN              6LR             6LBR            6BBR
      |                |               |                |
      |   NS(EARO)     |               |                |
      |--------------->|               |                |
      |                | Extended DAR  |                |
      |                |-------------->|                |
      |                |               |                |
      |                |               | proxy NS(EARO) |
      |                |               |--------------->|
      |                |               |                | NS(DAD)
      |                |               |                | ------>
      |                |               |                | <wait>
      |                |               |                |
      |                |               | proxy NA(EARO) |
      |                |               |<---------------|
      |                | Extended DAC  |                |
      |                |<--------------|                |
      |   NA(EARO)     |               |                |
      |<---------------|               |                |
      |                |               |                |
                   Figure 6: (Re-)Registration Flow
 [Arch-for-6TiSCH] ("An Architecture for IPv6 over the TSCH mode of
 IEEE 802.15.4") describes how a 6LoWPAN ND host using the
 Time-Slotted Channel Hopping (TSCH) mode of IEEE Std. 802.15.4
 [IEEE-802-15-4] can connect to the Internet via a RPL mesh network.
 Doing so requires additions to the 6LoWPAN ND protocol to support
 mobility and reachability in a secure and manageable network
 environment.  This document specifies those new operations and
 fulfills the requirements listed in Appendix B.2.

Thubert, et al. Standards Track [Page 38] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 The term "LLN" is used loosely in this document and is intended to
 cover multiple types of WLANs and WPANs, including Low-Power IEEE
 Std. 802.11 networking, Bluetooth low energy, IEEE Std. 802.11ah, and
 IEEE Std. 802.15.4 wireless meshes, so as to address the requirements
 discussed in Appendix B.3.
 This specification can be used by any wireless node to register its
 IPv6 Addresses with a Routing Registrar and to obtain routing
 services such as proxy ND operations over a Backbone Link.  This
 satisfies the requirements expressed in Appendix B.4.
 This specification is extended by [AP-ND] to provide a solution to
 some of the security-related requirements expressed in Appendix B.5.
 [ND-Optimizations] ("IPv6 Neighbor Discovery Optimizations for Wired
 and Wireless Networks") suggests that 6LoWPAN ND [RFC6775] can be
 extended to other types of links (beyond IEEE Std. 802.15.4) for
 which it was defined.  The registration technique is beneficial when
 the link-layer technique used to carry IPv6 multicast packets is not
 sufficiently efficient in terms of delivery ratio or energy
 consumption in the end devices -- in particular, to enable
 energy-constrained sleeping nodes.  The value of such an extension is
 especially apparent in the case of mobile wireless nodes, to reduce
 the multicast operations that are related to IPv6 ND [RFC4861]
 [RFC4862] and affect the operation of the wireless medium
 [Multicast-over-IEEE802-Wireless].  This fulfills the scalability
 requirements listed in Appendix B.6.

Appendix B. Requirements (Not Normative)

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

B.1. Requirements Related to Mobility

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

Thubert, et al. Standards Track [Page 39] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 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.

Thubert, et al. Standards Track [Page 40] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 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.

Thubert, et al. Standards Track [Page 41] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 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.

Thubert, et al. Standards Track [Page 42] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 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.

Thubert, et al. Standards Track [Page 43] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

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

B.6. Requirements Related to Scalability

 Use cases from Automatic Meter Reading (AMR) (collection-tree
 operations) and Advanced Metering Infrastructure (AMI) (bidirectional
 communication to the meters) indicate the need for a large number of
 LLN nodes pertaining to a single RPL DODAG (e.g., 5000) and connected
 to the 6LBR over a large number of LLN hops (e.g., 15).
 Related requirements are as follows:
 Req-6.1:  The registration mechanism SHOULD enable a single 6LBR to
           register multiple thousands of devices.
 Req-6.2:  The timing of the registration operation should allow for
           long latency, such as that found in LLNs with ten or
           more hops.

B.7. Requirements Related to Operations and Management

 Guideline 3.8 in Section 3 of [RFC1958] ("Architectural Principles of
 the Internet") recommends the following: "Avoid options and
 parameters whenever possible.  Any options and parameters should be
 configured or negotiated dynamically rather than manually."  This is
 especially true in LLNs where the number of devices may be large and
 manual configuration is infeasible.  Capabilities for dynamic
 configuration of LLN devices can also be constrained by network and
 power limitations.
 A network administrator should be able to validate that the network
 is operating within capacity and that, in particular, a 6LBR does not
 get overloaded with an excessive amount of registrations, so the
 administrator can take actions such as adding a Backbone Link with
 additional 6LBRs and Routing Registrars to the network.

Thubert, et al. Standards Track [Page 44] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

 Related requirements are as follows:
 Req-7.1:  A management model SHOULD be provided that enables access
           to the 6LBR, monitors its usage vs. capacity, and sends
           alerts in the case of congestion.  It is recommended that
           the 6LBR be reachable over a non-LLN link.
 Req-7.2:  A management model SHOULD be provided that enables access
           to the 6LR and its capacity to host additional NCEs.  This
           management model SHOULD avoid polling individual 6LRs in a
           way that could disrupt the operation of the LLN.
 Req-7.3:  Information on successful and failed registrations SHOULD
           be provided, including information such as the ROVR of the
           6LN, the Registered Address, the address of the 6LR, and
           the duration of the registration flow.
 Req-7.4:  In the case of a failed registration, information on the
           failure, including the identification of the node that
           rejected the registration and the status in the EARO,
           SHOULD be provided.

B.8. Matching Requirements with Specifications

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

Thubert, et al. Standards Track [Page 45] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

           |             |                                |
           | 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

Thubert, et al. Standards Track [Page 46] RFC 8505 Registration Extensions for 6LoWPAN ND November 2018

Acknowledgments

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

Authors' Addresses

 Pascal Thubert (editor)
 Cisco Systems, Inc.
 Building D (Regus) 45 Allee des Ormes
 Mougins - Sophia Antipolis
 France
 Phone: +33 4 97 23 26 34
 Email: pthubert@cisco.com
 Erik Nordmark
 Zededa
 Santa Clara, CA
 United States of America
 Email: nordmark@sonic.net
 Samita Chakrabarti
 Verizon
 San Jose, CA
 United States of America
 Email: samitac.ietf@gmail.com
 Charles E. Perkins
 Futurewei
 2330 Central Expressway
 Santa Clara, CA  95050
 United States of America
 Email: charliep@computer.org

Thubert, et al. Standards Track [Page 47]

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